Free-machining copper alloy and production method for free-machining copper alloy

Abstract

This free-cutting copper alloy includes specific amounts of Cu, Si, Pb, and P, and a specific amount of Bi as an optional element, with the balance being Zn and inevitable impurities. Among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is less than a specific amount, and the content of A1 is less than a specific amount. The values of the composition relational expressions f1, f2, and f0 that define each constituent element of the composition, the metallographic structure relational expressions f3, f4, and f5 that define the area ratios of the constituent phases of the metallographic structure, and the metallographic structure and composition relational expression f6 that defines the composition and metallographic structure are within respective specific ranges. When etched with a mixed solution of hydrogen peroxide and ammonia water, a grain boundary can be observed inside the β1 phase generated by modification.

Description

TECHNICAL FIELD

[0001] The present invention relates to a free-cutting copper alloy having good corrosion resistance, in particular, dezincification corrosion resistance and resistance to stress corrosion cracking as well as a high strength and a significantly reduced Pb content and a method for producing a free-cutting copper alloy.

[0002] The present invention relates to a free-cutting copper alloy used for components produced by machining such as devices and components used for supply of drinking water that humans and animals ingest daily, those used for sanitary facilities such as kitchen, shower room, or bathroom, water meters, musical instruments, tableware, devices and components for water drainage, industrial plumbing components, electrical or electronic apparatus components, auto parts, mechanical components, stationaries, toys, sliding components, measuring instrument components, precision mechanical components, medical components, and components relating to liquid or gas such as hydrogen, whose specific component names include faucet, mixer tap, tap fitting, shutoff valve, valve, joint, shower head, cock, gear, axle, bearing, shaft, sleeve, spindle, sensor, bolt, nut, flare nut, pen point, insert nut, cap nut, nipple, spacer, and screw, and a method for producing the free-cutting copper alloy.

[0003] The present application claims priority on Japanese Patent Application No. 2023-075579 filed on May 1, 2023 and Japanese Patent Application No. 2023-173342 filed on October 5, 2023, the contents of which are incorporated herein by reference.BACKGROUND ART

[0004] Conventionally, a Cu-Zn-Pb alloy containing 56 to 65 mass% Cu and 1 to 4 mass% Pb with the balance being Zn (so-called a free-cutting brass bar, forging brass, or casting brass) or a Cu-Sn-Zn-Pb alloy containing 80 to 88 mass% Cu, 2 to 8 mass% Sn, and 1 to 8 mass% Pb with the balance being Zn (so-called bronze casting: gunmetal) having excellent machinability and antibacterial property as well as good corrosion resistance has been generally used for devices and components related to drinking water or sanitary facility, water meters, musical instruments, tableware, electrical or electronic apparatus components, home appliance components, auto parts, mechanical components, stationaries, precision mechanical components, medical components, and devices and components relating to liquid or gas such as industrial water, drainage water, or hydrogen including, in terms of component name, faucet, mixer tap, shutoff valve, valve, cock, joint, gear, sensor, nut, and screw.

[0005] However, recently, Pb's influence on human body and the environment is becoming a concern, and momentum to regulate Pb is increasing in various countries. For example, a regulation for reducing the Pb content in drinking water supply devices to be 0.25 mass% or lower came into force in January 2010 in California, the United States. In countries other than the United States also, such regulation is rapidly being established, and development of a copper alloy material that meets the requirements of regulation on Pb content is in demand.

[0006] In addition, in other industrial fields such as those of electrical and electronic apparatuses, automobiles, and machines, free-cutting copper alloys are exceptionally allowed to include maximum 4 mass% Pb by European directives of RoHS and ELV to give an example. However, like in the field of drinking water, strengthening of regulations on Pb content including elimination of exemptions has been actively discussed.

[0007] While there is a trend to strengthen Pb regulations for free-cutting copper alloys, as a replacement of a leaded free-cutting copper alloy including 56 to 65 mass% Cu and 1 to 4 mass% Pb with the balance being Zn, in particular, a free-cutting copper alloy containing 3 mass% Pb that is most widely used in the world (Japan Industrial Standard: C3604; ASTM Standard: C36000; EN Standard: CW614N), (1) a Cu-Zn-Bi or Cu-Zn-Bi-Se alloy including Bi having machinability or, in some cases, also including Se in addition to Bi instead of Pb, (2) a Cu-Zn alloy including a high concentration of Zn in which the amount of β phase is increased to improve machinability, (3) a Cu-Zn-Si alloy including large amounts of γ phase and phase having excellent machinability instead of Pb, and (4) a Cu-Zn-Si-Pb alloy having β phase whose machinability has been improved by the effect of Si contained in the alloy, etc. are proposed.

[0008] In Patent Document 1, for instance, improvement of machinability and dezincification corrosion resistance is devised by adding 0.3 to 4 mass%, preferably 1.8 to 3.2 mass% Bi to a Cu-Zn alloy and annealing the alloy at a temperature between 350°C and 550°C so that β phase is reduced and fragmented by α phase because β phase has poor dezincification corrosion resistance.

[0009] However, alloys including Bi instead of Pb have many problems including the facts that machinability of Bi is inferior to that of Pb, Bi may be harmful to the environment and human body like Pb, Bi has a resourcing problem because it is a rare metal, and Bi embrittles a copper alloy material.

[0010] In addition, as disclosed by Patent Document 1, as β phase of a Cu-Zn alloy has poor dezincification corrosion resistance, reduction of β phase, performance of annealing for causing α phase to fragment β phase, etc. have been practically required as improvement measures.

[0011] Further, even though β phase in a Cu-Zn binary alloy containing a large amount of β phase contributes to improvement of machinability, machinability of β phase is inferior to that of Pb. Further, it has poor dezincification corrosion resistance and resistance to stress corrosion cracking. Therefore, there is no way that such an alloy can be an alternative to a free-cutting copper alloy containing Pb.

[0012] For this reason, in Patent Documents 2 to 8, for example, Cu-Zn-Si alloys including Si instead of Pb are proposed as free-cutting copper alloys.

[0013] Patent Documents 2 to 7 disclose that in alloys containing about 58 to 65 mass% Cu and 0.2 to 1.5 mass% Si, machinability is improved by the effect of Si contained in β phase and the presence of fine phosphorous compounds formed of P and Zn or the like. In these Documents, excellent machinability is realized by defining the area ratios of β phase and γ phase and requiring phosphorous compounds to be present and a small amount of Pb to be contained in the alloys. However, as disclosed by Patent Document 1, it is a well-known fact that β phase of a Cu-Zn alloy has poor dezincification corrosion resistance and also causes deterioration of resistance to stress corrosion cracking. Patent Documents 2 to 7, therefore, disclose no specific data related to dezincification corrosion resistance or resistance to stress corrosion cracking, implying that the technical problems that conventional β phase of a Cu-Zn alloy has, i.e., its dezincification corrosion resistance and resistance to stress corrosion cracking were yet to be improved.

[0014] In Patent Document 8, excellent machinability is realized in an alloy containing 71.5 to 78.5 mass% Cu and 2.0 to 4.5 mass% Si although the Pb content is as small as 0.02 mass% or even less by defining the total area ratio of γ phase and κ phase with excellent machinability that are formed in an alloy containing Cu and Si at high concentrations. In this Patent Document, further improvement of machinability and corrosion resistance is devised by making the alloy contain 0.1 mass% or higher amount of Sn and Al respectively to form a large amount of γ phase.

[0015] In Patent Document 9, excellent machinability is obtained by containing small amounts of Si, Pb, and P or Fe as well as 0.5 mass% or less Pb, and dispersing Pb-rich particles in the matrix and increasing the population density of the Pb-rich particles that are present inside α phase by contriving manufacturing method. For the improvement of dezincification corrosion resistance, Patent Document 9 requires performance of finish heat treatment at a temperature between 400°C and 600°C in effect.

[0016] Patent Document 10 proposes a Cu-Zn-Si-Pb-P copper alloy in which the area ratios of β phase, γ phase, and µ phase are limited and involving a production technique to manufacture hollow hot-forged products having a near-net shape using a hollow material. Patent Document 11 proposes a copper alloy casting made of a Cu-Zn-Zr-P alloy optionally containing Si, Pb, and / or Sn, in which crystal grains are refined by the actions of Zr and P.

[0017] Patent Document 12 proposes a Cu-Zn-Sn-Al copper alloy with excellent color fastness optionally containing Si and / or Pb in which the area ratios of y phase and β phase are limited.

[0018] Patent Document 13 proposes a lead-free copper alloy casting of a Cu-Zn-Si-Sn-Al-P alloy.

[0019] Patent Document 14 discloses that apparent Zn content is important in order to improve corrosion resistance of a Cu-Zn-Si-Sn-Al alloy and proposes a copper alloy whose machinability is improved by containing a large amount of Pb or Bi in effect.

[0020] Patent Document 15 discloses a lead-free copper alloy casting made of a Cu-Zn-Si alloy containing 65 mass% or more Cu with good castability and mechanical strength in which machinability is improved by γ phase. An example containing large amounts of Sn, Al, Mn, Ni, and Sb is presented in this Document.Citation ListPatent Documents

[0021] Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2002-003967 Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2021-042461 Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2021-042459 Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2021-042460 Patent Document 5: PCT International Publication No. WO2020 / 261666 Patent Document 6: Japanese Unexamined Patent Application, First Publication No. 2021-042462 Patent Document 7: PCT International Publication No. WO2021 / 117528 Patent Document 8: PCT International Publication No. WO2007 / 034571 Patent Document 9: Japanese Unexamined Patent Application, First Publication No. 2016-194123 Patent Document 10: PCT International Publication No. WO2013 / 065830 Patent Document 11: PCT International Publication No. WO2006 / 016630 Patent Document 12: PCT International Publication No. WO2015 / 046421 Patent Document 13: Japanese Unexamined Patent Application, First Publication No. 2010-133006 Patent Document 14: Japanese Unexamined Patent Application, First Publication No. 2018-048398 Patent Document 15: Published Japanese Translation No. 2019-508584 of the PCT International Publication SUMMARY OF INVENTIONTechnical Problem

[0022] As indicated in Patent Documents 1 to 15, there is no past record of substantial improvement made in the dezincification corrosion resistance and the resistance to stress corrosion cracking of β phase present in Cu-Zn alloys, which had been a long-standing technical challenge. Further, there is no disclosure of a Cu-Zn alloy containing less than 0.2% Pb or Bi that exhibits low cutting resistance and excellent machinability in high-speed machining exceeding 100 m / min without essentially requiring the alloy to contain Bi.

[0023] The present invention has been made in order to solve the above-described problems in the conventional art, and the objective thereof is to provide a free-cutting copper alloy in which Pb content has been significantly reduced but having excellent hot workability and machinability as well as good dezincification corrosion resistance and resistance to stress corrosion cracking despite a large content of β phase (the β1 phase described later), a high strength, and good balance between strength and ductility, and a method for producing the free-cutting copper alloy.Solution to Problem

[0024] In order to solve the above-described problems and achieve the above-described object, the present inventors conducted thorough investigation and obtained the following findings.

[0025] Incidentally, in this specification, unless specified otherwise, β phase includes β' phase, γ phase includes γ' phase, and α phase includes α' phase. β1 phase refers to modified β phase and is distinguished from β phase and β' phase. β1 phase has a characteristic that a grain boundary pattern, i.e., a boundary of crystal grain can be identified inside the phase when etched with an etching solution made of hydrogen peroxide and ammonia water then observed with a metallographic microscope. In the case of β phase that is present in a common alloy such as a Cu-Zn, Cu-Zn-Bi, or Cu-Zn-Si alloy, no boundary of crystal grains can be observed inside β phase even if it is etched with a solution of hydrogen peroxide and ammonia water. Therefore, β phase and β1 phase can be clearly distinguished. Crystal grain boundary may be simply referred to as grain boundary. Hot-worked material includes hot-extruded material, hot-forged material, and hot-rolled material. Cold workability refers to performance of a material when subjected to working conducted in a cold state such as drawing, wire drawing, rolling, crimping, or bending. Unless specified otherwise, good or excellent machinability stands for low cutting resistance and good or excellent chip breakability when turned with a lathe. Conductivity refers to electric conductivity, thermal conductivity, or electrical conductivity. Cooling rate refers to an average cooling rate in a given temperature range. Twenty-four hours refer to one day. "Manufacturing on the actual production line" refers to production with a mass production facility used for manufacturing products for sale.

[0026] The Patent Document 8 mentioned above discloses that in Cu-Zn-Si alloys, β phase little contributes to machinability of a copper alloy, but rather impairs it. In Patent Documents 10, 11, and 12, the amount of β phase is limited to a very small amount. Patent Document 2 requires a process of annealing at a temperature between 350°C and 550°C to reduce β phase and make alpha phase fragment β phase as a method for improving dezincification corrosion resistance of β phase. Patent Document 9 discloses that in order to improve dezincification corrosion resistance of β phase, it is necessary to include a higher amount of Sn than Si and heat the material to a temperature between 700°C and 850°C, perform hot extrusion in the same temperature range and a finishing heat treatment in which the material is held at a temperature between 400°C and 600°C for 30 minutes or longer then cooled at an average cooling rate of 0.2°C / sec to 10°C / sec in the temperature range from 400°C to 200°C.

[0027] On the other hand, Patent Documents 2 to 7 disclose a discovery that β phase itself exhibits a significant effect on machinability in a Cu-Zn-Si alloy if a certain amount of Si is contained in the β phase. These Patent Documents further disclose that their alloys obtained excellent machinability due to the synergy effect brought by satisfying the three requirements, i.e., fine phosphorous compound is present, a small amount of Pb is contained, and Bi is contained in some cases. However, in order to cause phosphorous compound to be present, these Documents require cooling at an average cooling rate of preferably about 0.1°C / min or higher and 70°C / min or lower in the temperature range from about 530°C to about 450°C after hot working.

[0028] Incidentally, Patent Documents 2 to 7 do not disclose any data related to dezincification corrosion resistance or resistance to stress corrosion cracking. There is no mention of modification of β phase itself, either. This implies that no improvement was made in the dezincification corrosion resistance or the resistance to stress corrosion cracking of β phase that is present in Cu-Zn-Si alloy and no further improvement was made in the machinability.

[0029] The present inventors further addressed modification of β phase itself in a Cu-Zn-Si alloy. As a result of devoted study, machinability of such an alloy was further improved particularly in high-speed cutting without presence of phosphorous compound or essentially containing Bi by the effect of modified β phase, i.e., β1 phase, and significant improvement in the dezincification corrosion resistance and resistance to stress corrosion cracking of β phase, which had been longstanding drawbacks of a Cu-Zn system alloy, was achieved. In addition, strength of an alloy containing β1 phase instead of β phase could be further enhanced without impairing its ductility. In order to obtain this β1 phase, it is necessary to first solid-solubilize specific amounts of Si and P inside the β phase then maintain the state of metallographic structure when it is at a high temperature until and after the alloy's temperature becomes a normal temperature by maintaining the state of β phase when it is at a temperature between 500°C and 670°C and cooling the alloy at a high cooling rate when it is cooled down to normal temperature.

[0030] Apropos, β1 phase can be easily distinguished from β phase of a Cu-Zn system alloy. To see the difference, a surface of a piece of alloy is polished (mirror-polished) then etched with a mixed solution of hydrogen peroxide and ammonia water. Specifically, an aqueous solution prepared by mixing 3 ml of 3 vol% hydrogen peroxide water and 22 ml of 14 vol% ammonia water is used for the etching. The polished metal surface is dipped in the aqueous solution under room temperature between about 15°C and 25°C for about 2 to 10 seconds then the metallographic structure is observed with a metallographic microscope at a magnification of 200X to 1000X. If it is a β1 phase, a grain boundary pattern, i.e., a crystal grain boundary can be found inside the phase, but no crystal grain boundary can be found inside β phase of an ordinary Cu-Zn system alloy. Use of the aforementioned etching solution is disclosed in Patent Documents 2-7, 10, and 11, and pictures of the metallographic structure observed by etching with the solution are also disclosed in the Patent Documents.

[0031] In order to maintain the state of β phase when it is at a temperature between 500°C and 670°C after hot working until the alloy reaches normal temperature, which is a condition to obtain β1 phase, it is necessary to start a cooling treatment that is performed as part of the cooling process after hot working when the temperature of the hot-worked material is lower than 670°C and higher than 500°C, and cool the material at an average cooling rate of higher than 300°C / min in the temperature range from the starting temperature of the cooling treatment to 500°C, then continue to cool at an average cooling rate of at least higher than 300°C / min in the temperature range from 500°C to 300°C in the cooling treatment in the cooling treatment that continues. In this specification, unless specified otherwise, cooling treatment refers to manipulation of cooling rate by water cooling or any other similar method as opposed to natural cooling. In order to obtain fine P-containing compound, Patent Documents 2 to 7 require cooling of hot-worked material at an average cooling rate of about 0.1°C / min or higher and about 70°C / min or lower when the material is in the temperature range from about 530°C to about 450°C after hot working. That is, the present invention and those of Patent Documents 2 to 7 are clearly heading in opposite directions (or providing opposite teachings).

[0032] It should be noted, however, that the present invention may include both β1 phase and phosphorous compounds if, for instance, the cooling rate when the temperature of the material after hot working is around 520°C is 60°C / min and also the average cooling rate in the temperature range from 520°C, which is the starting temperature of the cooling treatment, to 500°C exceeds 300°C / min.

[0033] In the present invention, machinability of a Cu-Zn-Si alloy is significantly improved by the effect of β1 phase particularly in high-speed cutting without requiring phosphorous compounds as disclosed by Patent Documents 2 to 7. Further, by the synergy effect between β1 phase and fine Pb particles or particles containing both Pb and Bi, cutting resistance is decreased and chip breakability is promoted.

[0034] More importantly, dezincification corrosion resistance and resistance to stress corrosion cracking, which were major unsolved problems with conventional β phase, are dramatically improved by the modification of β phase into β1 phase, allowing the problems to be solved. With respect to mechanical characteristics, the present invention retains high strength of conventional β phase which is further enhanced, as well as improved ductility and better balance between strength and ductility. Thus, a copper alloy with machinability equivalent to that of conventional free-cutting brass and better dezincification corrosion resistance and resistance to stress corrosion cracking as well as higher strength than those of conventional free-cutting brass came to be invented.

[0035] A free-cutting copper alloy according to a first aspect of the present invention includes: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, with the balance being Zn and inevitable impurities, in which, among the inevitable impurities, a total content of Fe, Mn, Co, and Cr is lower than 0.40 mass% and a content of A1 is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, the following relationship of 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.5 is satisfied, if Bi is included, the following relationship of 0.003 ≤ f2 = [Pb] + [Bi] < 0.20 is further satisfied, in constituent phases of a metallographic structure excluding non-metallic inclusions, when an area ratio of α phase is represented by (α)%, an area ratio of γ phase is represented by (γ)%, and an area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, the following relationships of 20 ≤ f3 = (a) < 75, 25 < f4 = (β1) ≤ 80, 0 ≤ f5 =(y) < 4, and 27 < f6 = (β1) × ([Si]) 1 / 2< - (γ) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15 are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0036] A free-cutting copper alloy according to a second aspect of the present invention includes: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, with the balance being Zn and inevitable impurities, in which, among the inevitable impurities, a total content of Fe, Mn, Co, and Cr is lower than 0.30 mass% and a content of Al is lower than 0.15 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, the following relationship of 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.2 is satisfied, if Bi is included, the following relationship of 0.004 ≤ f2 = [Pb] + [Bi] < 0.10 is further satisfied, in constituent phases of a metallographic structure excluding non-metallic inclusions, when an area ratio of α phase is represented by (α)%, an area ratio of γ phase is represented by (γ)%, and an area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, the following relationships of 30 ≤ f3 = (α) ≤ 70, 30 ≤ f4 = (β1) ≤ 70, 0 ≤ f5 =(y) < 1, and 35 < f6 = (β1) × ([Si]) 1 / 2< - (γ) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15, are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0037] A free-cutting copper alloy according to a third aspect of the present invention includes: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; higher than 0.05 mass% and lower than 0.90 mass% Sn; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, with the balance being Zn and inevitable impurities, in which, among the inevitable impurities, a total content of Fe, Mn, Co, and Cr is lower than 0.40 mass% and a content of A1 is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, a Bi content is represented by [Bi] mass%, and a Sn content is represented by [Sn], the following relationship of 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≤ 60.5 is satisfied, and the following relationship of f0 = [Sn] / [Si] < 1 is further satisfied, if Bi is included, the following relationship of 0.003 ≤ f2 = [Pb] + [Bi] < 0.20 is satisfied, in constituent phases of a metallographic structure excluding non-metallic inclusions, when an area ratio of α phase is represented by (α)%, an area ratio of γ phase is represented by (γ)%, and an area ratio of β1 phase, i.e., modified β1 phase, is represented by (β1)%, the following relationships of 20 ≤ f3 = (α) < 75, 25 < f4 = (β1) ≤ 80, 0 ≤ f5 =(γ) < 4, and 27 < f6 = (β1) × ([Si]) 1 / 2< - (γ) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15, are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0038] A free-cutting copper alloy according to a fourth aspect of the present invention includes: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; higher than or equal to 0.10 mass% and lower than 0.50 mass% Sn; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, with the balance being Zn and inevitable impurities, in which, among the inevitable impurities, a total content of Fe, Mn, Co, and Cr is lower than 0.30 mass% and a content of A1 is lower than 0.15 mass%, when a Cu content is represented [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, a Bi content Bi is represented by [Bi] mass%, and a Sn content is represented by [Sn], the following relationship of 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≤ 60.2 is satisfied, and the following relationship of f0 = [Sn] / [Si] < 0.6 is further satisfied, if Bi is included, the following relationship of 0.004 ≤ f2 = [Pb] + [Bi] < 0.10 is satisfied, in constituent phases of a metallographic structure excluding non-metallic inclusions, when an area ratio of α phase is represented by (α)%, an area ratio of γ phase is represented by (γ)%, and an area ratio of β1 phase, i.e., modified β1 phase, is represented by (β1)%, the following relationships of 30 ≤ f3 = (α) < 70, 30 ≤ f4 = (β1) ≤ 70, 0 ≤ f5 =(y) < 1, and 35 < f6 = (β1) × ([Si]) 1 / 2< - (γ) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15 are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0039] A free-cutting copper alloy according to a fifth aspect of the present invention is the free-cutting copper alloy according to any one of the first to fourth aspects of the present invention, which is used for a device or component related to drinking water or sanitary facility, a valve, a cock, an industrial plumbing component, a water meter, a musical instrument, an auto part, an electrical or electronic apparatus component, a mechanical component, a stationery, a toy, a sliding component, a measuring instrument component, a precision mechanical component, or a medical component.

[0040] A method for producing a free-cutting copper alloy according to a sixth aspect of the present invention is a method for producing the free-cutting copper alloy according to any one of the first to fourth aspects of the present invention, including one or more hot working steps, in which in the final hot working step among the hot working steps, hot working temperature is higher than 540°C and lower than 750°C, in the cooling process following the final hot working step, the hot-worked material is subjected to a cooling treatment which is started when its temperature is lower than 670°C and higher than 500°C and performed at an average cooling rate of higher than 300°C / min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and that from 500°C to 300°C.

[0041] A method for producing a free-cutting copper alloy according to a seventh aspect of the present invention is a method for producing the free-cutting copper alloy according to any one of the first to fourth aspects of the present invention, including one or more hot working steps and heat treatment steps, in which in the final heat treatment step, annealing is performed with a holding temperature of higher than 520°C and lower than 630°C and a holding time of one minute to five hours, the annealed material is cooled through a cooling treatment which is started after the annealing when the temperature of the material is higher than 500°C and performed at an average cooling rate of higher than 300°C / min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C.Advantageous Effects of Invention

[0042] According to one aspect of the present invention, a free-cutting copper alloy having excellent hot workability, good machinability, dezincification corrosion resistance, and resistance to stress corrosion cracking, a high strength, and a good balance between strength and ductility and containing a significantly reduced amount of Pb, and a method for producing the free-cutting copper alloy can be provided.BRIEF DESCRIPTION OF DRAWINGS

[0043] [Fig. 1A] A picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S01 to Step No. A2. More specifically, Alloy No. S01 has a composition including 63.3 mass% Cu, 0.95 mass% Si, 0.069 mass% P, 0.063 mass% Pb, 0.017 mass% Bi, and Zn as the balance. In Step No. A2, the alloy was subjected to hot extrusion performed at 630°C and a cooling treatment which was started when the temperature of the hot-extruded material became 580°C and performed at an average cooling rate of 1020°C / min in both temperature ranges from 580°C to 300°C and from 500°C to 300°C. [Fig. 1B] A cross-sectional picture of a metallographic structure of the alloy shown in Fig. 1A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509. [Fig. 2A] A picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S11 to Step No. E2. More specifically, Alloy No. S11 has a composition including 62.5 mass% Cu, 0.96 mass% Si, 0.064 mass% P, 0.072 mass% Pb, and Zn as the balance. In Step No. E2, the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm. The rod was heated and hot forged at 680°C to have a thickness of 20 mm when placed horizontally then subjected to a cooling treatment which was started when its temperature became 565°C and performed at an average cooling rate of 900°C / min in both temperature ranges from 565°C to 300°C and from 500°C to 300°C. [Fig. 2B] A cross-sectional picture of the metallographic structure of the alloy shown in Fig. 2A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509. [Fig. 3A] A picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S11 to Step No. E13H. More specifically, Alloy No. S11 has a composition including 62.5 mass% Cu, 0.96 mass% Si, 0.064 mass% P, 0.072 mass% Pb, and Zn as the balance. In Step No. E13H, the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm. The rod was heated and hot forged at 630°C to have a thickness of 20 mm when placed horizontally then cooled at an average cooling rate of 35°C / min. Subsequently, the rod was subjected to a cooling treatment which was started when its temperature became 455°C and performed at an average cooling rate of 800°C / min in the temperature range from 455°C to 300°C. Note that the starting temperatures of the cooling treatment performed on the alloy in Fig. 3A and that in Fig. 2A are different. [Fig. 3B] A cross-sectional picture of the metallographic structure of the alloy shown in Fig. 3A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509. [Fig. 4A] A picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S43 to Step No. E6. More specifically, Alloy No. S43 has a composition including 63.3 mass% Cu, 0.98 mass% Si, 0.084 mass% P, 0.060 mass% Pb, 0.28 mass% Sn, and Zn as the balance. In Step No. E6, a casting produced with a mold having a diameter of 55 mm was machined to a diameter of 50 mm then cut to a length of 200 mm. The casting was heated and hot forged at 630°C to a thickness of 20 mm when placed horizontally then subjected to a cooling treatment which was started when the temperature of the casting became 565°C and performed at an average cooling rate of 900°C / min in both temperature ranges from 565°C to 300°C and from 500°C to 300°C. [Fig. 4B] A cross-sectional picture of the metallographic structure of the alloy shown in Fig. 4A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509. [Fig. 5A] A picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step Nos. A34H and G1. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.98 mass% Si, 0.071 mass% P, 0.071 mass% Pb, and Zn as the balance. In Step No. A34H, an alloy was obtained by hot extrusion at 615°C and cooling at an average cooling rate of 18°C / min in the temperature range from 500°C to 300°C. In Step No. G1, the alloy was further heated to 580°C for 30 minutes then subjected to a cooling treatment which was started when its temperature became 560°C and performed at an average cooling rate of 1800°C / min in both temperature ranges from 560°C to 300°C and from 500°C to 300°C. [Fig. 5B] A cross-sectional picture of the metallographic structure of the alloy shown in Fig. 5A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509. [Fig. 6A] A picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step No. E14H. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.98 mass% Si, 0.071 mass% P, 0.071 mass% Pb, and Zn as the balance. In Step No. E14H, the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm. The rod was heated and hot forged at 630°C to a thickness of 20 mm when placed horizontally then left to cool naturally. The average cooling rate in the temperature range from 500°C to 300°C was 25°C / min. [Fig. 6B] A cross-sectional picture of the metallographic structure of the alloy shown in Fig. 6A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509. DESCRIPTION OF EMBODIMENTS

[0044] Below is a description of free-cutting copper alloys according to the embodiments of the present invention and methods for producing the free-cutting copper alloys. The free-cutting copper alloys according to the embodiments are used for devices and components related to sanitary facilities, musical instruments, tableware, home appliance components, electrical or electronic apparatus components, auto parts, mechanical components, stationaries, precision mechanical components, medical components, and devices and components involving liquid or gas such as industrial water, drainage water, and hydrogen. Examples of specific component names include faucet, mixer tap, shutoff valve, valve, cock, joint, water meter, gear, sensor, nut, and screw.

[0045] Here, in this specification, an element symbol in parentheses such as [Zn] represents the content (mass%) of the element.

[0046] In the embodiments, using this content expressing method, composition relational expressions f1, f2, and f0 are defined as follows.

[0047] Composition relational expression f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P]. In the case Sn is contained in an amount exceeding 0.05 mass%, f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] In the case Bi is contained, composition relational expression f2 = [Pb] + [Bi] In the case Sn is contained, f0 = [Sn] / [Si]

[0048] Further, in the embodiments, in constituent phases of the metallographic structure excluding non-metallic inclusions, the area ratio of α phase is represented by (α)%, the area ratio of γ phase is represented by (γ)%, the area ratio of β phase is represented by (β)%, and the area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%. The area ratio of each of the phases will also be referred to as "the amount of each of the phases", "the proportion of each of the phases", or "the proportion that each of the phases occupies".

[0049] In the embodiments, a plurality of metallographic structure relational expressions are defined as follows. Metallographic Structure Relational Expression f3 = (α) Metallographic Structure Relational Expression f4 = (β1) Metallographic Structure Relational Expression f4A = (β) Metallographic Structure Relational Expression f5 = (γ) Metallographic Structure and Composition Relational Expression f6 = (β1) × ([Si]) 1 / 2< - (y) × 2 + ([Pb] + [Bi] ) 1 / 2< × 20 + ([P] ) 1 / 2< × 15

[0050] A free-cutting copper alloy according to the first embodiment of the present invention includes: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.40 mass%, the content of Al is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, the relationship of 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.5 is satisfied, if Bi is included, the relationship of 0.003 ≤ f2 = [Pb] + [Bi] < 0.20 is further satisfied, in constituent phases of the metallographic structure excluding non-metallic inclusions, when the area ratio of α phase is represented by (α)%, the area ratio of γ phase is represented by (γ)%, and the area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, the relationships of 20 ≤ f3 = (α) < 75, 25 < f4 = (β1) ≤ 80, 0 ≤ f5 =(y) < 4, and 27 < f6 = (β1) × ([Si]) 1 / 2< - (γ) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15 are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0051] A free-cutting copper alloy according to the second embodiment of the present invention includes: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.30 mass%, the content of Al is lower than 0.15 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, the relationship of 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.2 is satisfied, if Bi is included, the relationship of 0.004 ≤ f2 = [Pb] + [Bi] < 0.10 is further satisfied, in constituent phases of the metallographic structure excluding non-metallic inclusions, when the area ratio of α phase is represented by (α)%, the area ratio of γ phase is represented by (γ)%, and the area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, the relationships of 30 ≤ f3 = (α) ≤ 70, 30 ≤ f4 = (β1) ≤ 70, 0 ≤ f5 =(γ) < 1, and 35 < f6 = (β1) × ([Si]) 1 / 2< - (γ) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15 are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0052] A free-cutting copper alloy according to the third embodiment of the present invention includes: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; higher than 0.05 mass% and lower than 0.90 mass% Sn; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.40 mass%, the content of Al is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, a Bi content is represented by [Bi] mass%, and a Sn content is represented by [Sn], the relationship of 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≤ 60.5 is satisfied, the relationship of f0 = [Sn] / [Si] < 1 is further satisfied, if Bi is included, the relationship of 0.003 ≤ f2 = [Pb] + [Bi] < 0.20 is satisfied, in constituent phases of the metallographic structure excluding non-metallic inclusions, when the area ratio of α phase is represented by (α)%, the area ratio of γ phase is represented by (γ)%, and the area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, the relationships of 20 ≤ f3 = (α) < 75, 25 < f4 = (β1) ≤ 80, 0 ≤ f5 =(y) < 4, and 27 < f6 = (β1) × ([Si]) 1 / 2< - (y) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15 are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0053] A free-cutting copper alloy according to the fourth embodiment of the present invention includes: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; higher than or equal to 0.10 mass% and lower than 0.50 mass% Sn; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.30 mass%, the content of Al is lower than 0.15 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, a Bi content is represented by [Bi] mass%, and a Sn content is represented by [Sn], the relationship of 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≤ 60.2 is satisfied, the relationship of f0 = [Sn] / [Si] < 0.6 is further satisfied, if Bi is included, the relationship of 0.004 ≤ f2 = [Pb] + [Bi] < 0.10 is satisfied, in constituent phases of the metallographic structure excluding non-metallic inclusions, when the area ratio of α phase is represented by (α)%, the area ratio of γ phase is represented by (γ)%, and the area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, the relationships of 30 ≤ f3 = (α) ≤ 70, 30 ≤ f4 = (β1) ≤ 70, 0 ≤ f5 =(γ) < 1, and 35 < f6 = (β1) × ([Si]) 1 / 2< - (γ) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15 are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0054] The reasons why the compositional components, the composition relational expressions f0, f1, and f2, the metallographic structure relational expressions f3, f4, and f5, the metallographic structure and composition relational expression f6, the metallographic structure, and the like are defined as described above are explained below.<Compositional Components>(Cu)

[0055] Cu is a main element of the free-cutting copper alloy according to the embodiment. In order to achieve the object of the present invention, the free-cutting copper alloy needs to contain Cu in an amount exceeding 60.5 mass% at least. When the Cu content is 60.5 mass% or lower, the proportion of β1 phase exceeds 80% although depending on the contents of Si, Zn, P, Pb, Bi, and Sn and the production process, and as a result, ductility is poor and β phase fails to be modified sufficiently causing dezincification corrosion resistance, resistance to stress corrosion cracking, and machinability to deteriorate. Accordingly, the lower limit of the Cu content is higher than 60.5 mass%, preferably 61.2 mass% or higher, more preferably 61.5 mass% or higher, and still more preferably 62.0 mass% or higher.

[0056] On the other hand, when the Cu content is 65.0 mass% or higher, the proportion of β1 phase decreases although depending on the contents of Si, Zn, P, Pb, Bi, and Sn and the production process. As a result, excellent machinability cannot be obtained, and strength decreases. Accordingly, the Cu content is lower than 65.0 mass%, preferably 64.8 mass% or lower, and more preferably 64.5 mass% or lower.(Si)

[0057] Si is a main element of the free-cutting copper alloy according to the embodiment. Si contributes to the formation of metallic phases such as κ phase, γ phase, µ phase, β phase, β1 phase, and ζ phase. By performing a specific cooling treatment that is described later on an alloy containing Cu, Zn, Si, and P in the amounts within the respective ranges described above, β phase is modified to form β1 phase. By the effect of β1 phase (modified β phase), significant improvements in dezincification corrosion resistance and resistance to stress corrosion cracking, that were drawbacks of conventional β phase, are realized in addition to improvement in machinability. A representative composition of β1 phase generated by modification would be about 61 mass% Cu, about 1.2 mass% Si, about 37.5 mass% Zn, and about 0.1 mass% P.

[0058] On the other hand, a representative composition of α phase comprise about 66 mass% Cu, about 0.7 mass% Si, and about 33 mass% Zn. In the case of the compositional ranges of the embodiments, inclusion of Si slightly improves machinability of α phase and ameliorate its dezincification corrosion resistance and resistance to stress corrosion cracking. Accordingly, by the effects of β1 phase generated by modification and ameliorated α phase, dezincification corrosion resistance and resistance to stress corrosion cracking of the alloys particularly improve.

[0059] Incidentally, a certain amount of α phase is required for the modification of β phase. For instance, if α phase is not present at all, β phase will not be modified. In order to improve dezincification corrosion resistance, resistance to stress corrosion cracking, ductility, and machinability of the alloys, α phase needs to be 20% or more, preferably 30% or more, in terms of area ratio.

[0060] Si is an essential element required for the modification of β phase into β1 phase. The higher the Si content, the more progresses the modification of β phase, resulting in formation of β1 phase with better properties. In order to modify β phase and generate better modified β1 phase, Si needs to be included in an amount exceeding 0.50 mass%. The Si content is preferably 0.60 mass% or higher, more preferably 0.65 mass% or higher, and still more preferably 0.75 mass% or higher.

[0061] On the other hand, when the Si content reaches a certain amount, the modification to β1 phase peaks. Further, if the Si content is excessive, conductivity is low. In some cases, γ phase appears causing dezincification corrosion resistance, resistance to stress corrosion cracking, and machinability to deteriorate. Speaking of γ phase, Patent Documents 2 to 8 disclose that γ phase improves machinability. However, in the present invention, which is mainly composed of β1 phase and α phase, presence of γ phase rather deteriorates machinability, ductility, dezincification corrosion resistance, and resistance to stress corrosion cracking of the alloy. For this reason, it is preferable to restrict the amounts of Si and γ phase. Accordingly, the Si content is lower than 1.20 mass%, preferably 1.10 mass% or lower. It is 1.0 mass% or lower where conductivity is important.(Zn)

[0062] Zn is a main element of the free-cutting copper alloy according to the embodiment together with Cu and Si and is an element necessary to enhance machinability, strength, high temperature properties, and castability. Incidentally, Zn is described as the balance in the composition, but to be specific, its content is lower than about 38.5 mass% and preferably lower than about 38.0 mass%, and is higher than about 32.0 mass% and preferably higher than 33.0 mass%.(P)

[0063] Like Si, P is an essential element required for the modification of β phase into β1 phase. P solid-solubilizes in β phase during hot working. β phase is modified into β1 phase if the cooling treatment performed in the cooling process after hot working is started when the temperature of the hot-worked material is higher than 500°C and lower than 670°C and the material is cooled at a cooling rate exceeding 300°C / min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C. The modification of β phase into β1 phase significantly improves long-standing drawbacks of conventional β phase, i.e., its dezincification corrosion resistance and resistance to stress corrosion cracking. Presence of β1 phase further makes it possible to decrease cutting resistance during cutting and enhance chip breakability. Aside from that, P-containing compound decreases cutting resistance and improves chip breakability, but if the cooling treatment performed at a cooling rate exceeding 300°C / min is started when the temperature of the material is about 550°C or higher than 530°C, P-containing compound does not appear of even if it does, the amount is small. From a viewpoint of machinability improvement also, the effect brought by the modification of β phase into β1 phase excels that brought by the presence of P-containing compound in β phase. In particular, in high-speed cutting performed at a velocity exceeding 100 m / min, machinability is better improved by the modification of β phase than the presence of P-containing compound in β phase. Incidentally, inclusion of P leads to improvement of the dezincification corrosion resistance and resistance to stress corrosion cracking of α phase and significant improvement of those of alloys consisting of β1 phase and α phase.

[0064] To modify β phase into β1 phase, the lower limit of the P content needs to be at least higher than 0.01 mass%. In view of creation of better β1 phase, the amount of inevitable impurities, the temperature to start the cooling treatment, and the cooling rate after commencement of the cooling treatment, which are described later, the P content is preferably 0.03 mass% or higher, more preferably 0.04 mass% or higher.

[0065] Incidentally, P tends to compound with elements such as Zn, Si, Mn, Fe, Cr, Co, and Al. If the amount of P solid-solubilized inside the β phase that is present in an alloy after hot working is reduced by the formation of such compounds, the modification of β phase into β1 phase is hindered. Zn and Si, which are main elements of the present invention, start to compound with P at around 550°C. If the cooling rate is lowered, the amount of phosphorous compounds increases. Formation of compound between inevitable impurities of Mn, Fe, Cr, and / or Co and P begins at a temperature higher than 550°C. If their amounts are increased, formation of phosphorous compounds is further promoted. That is, presence of Mn, Fe, Cr, and Co hinders the modification into β1 phase, resulting in increased cutting resistance of the alloy and deterioration of chip breakability, dezincification corrosion resistance, and resistance to stress corrosion cracking. Therefore, the total amount of Fe, Mn, Co, and Cr needs to be lower than 0.40 mass% at most, preferably lower than 0.30 mass%.

[0066] Even though the formation of compounds comprising P and Zn and / or Si has a positive effect on machinability, the conditions required to realize the modification of β phase into β1 phase are opposite to those to realize the presence of compounds comprising P and Zn and / or Si. As stated above, for the formation of β1 phase, the cooling treatment performed after hot working needs to be started when the temperature of the hot-worked material is higher than 500°C and lower than 670°C and the cooling rate in the temperature range from the starting temperature of the cooling treatment to 300°C needs to be high whereas for sufficient formation of compounds of P and Zn and / or Si, slow cooling is necessary in the temperature range from about 530°C to about 450°C. Therefore, when a large amount of phosphorous compounds are formed, formation of β1 phase is slightly insufficient, i.e., modification of β phase is slightly insufficient. In some cases, β1 phase is not present at all.

[0067] On the other hand, if 0.18 mass% or more P is contained, ductility and electrical conductivity do not improve but rather deteriorate due to the effect of P solid-solubilized in β1 phase since the effect brought by formation of β1 phase has already peaked. For this reason, the P content is lower than 0.18 mass%, preferably 0.15 mass% or lower, and more preferably 0.12 mass% or lower.(Pb)

[0068] In the embodiment, good machinability is obtained by the effect of β1 phase including Si and P, but it is further improved by containing a small amount of Pb. In the composition of the embodiment, about 0.001 mass% Pb is solid-solubilized in the matrix, and the portion in excess of this amount is present in the form of tiny Pb particle with a diameter of about 0.1 to about 2 µm. It is generally believed that Pb has little effect on improvement of machinability if its content is approximately 0.1 mass%. For instance, ASM Specialty Handbook first edition "Copper and Copper Alloys" discloses a relationship between Pb content and machinability in which the machinability of a Cu-Zn-Pb alloy consisting of 62 to 65 mass% Cu, about 3.2 mass% Pb with the balance being Zn is assumed to be 100% (Fig. 6 on p. 267 of the Handbook). The Fig. 6 indicates that containing 0.1 mass% Pb only has an effect of improving machinability by about 5 percentage points from 25% to 30% in terms of machinability index. In contrast, in the present invention, Pb has a significant effect on machinability even though its content is very small. The effect is exhibited if the Pb content is 0.002 mass% or higher. The Pb content is preferably 0.003 mass% or higher, more preferably 0.01 mass% or higher. When cutting conditions are severe, for example, when cutting speed or feed rate is high, cutting depth is deep, or the diameter of the hole to drill is large, the Pb content is preferably 0.03 mass% or higher. Due to the effects of β1 phase with greatly enhanced machinability and a small amount of Pb contained, machinability of the alloy significantly improves.

[0069] It is well known that Pb improves machinability of copper alloys, but for a Cu-Zn binary alloy, as represented by a free-cutting brass bar C3604, to obtain the effect, about 3 mass% Pb is required. In the embodiment, an alloy with excellent machinability is accomplished by causing β1 phase including Si and P, a tiny amount of Pb particles, and particles composed of Pb and Bi, which will be described later, to be present in the metallographic structure. The upper limit of Pb content is lower than 0.20 mass% since Pb is harmful to human body. The Pb content is preferably lower than 0.10 mass% and, in consideration of its influence on human body and the environment, it is most preferably 0.08 mass% or lower.(Bi)

[0070] Like Pb, Bi is solid-solubilized in the matrix in an amount of approximately 0.0001 mass%, and the portion in excess of this amount is present in the form of particle with a diameter of about 0.1 to about 2 µm. In the case both Pb and Bi are added, large portions of them are present in the form of particles composed of a mixture of Pb and Bi with a diameter of about 0.1 to about 2 µm. The inventors have discovered that if Bi and Pb are both contained in an embodiment where β1 phase is present, machinability equivalent to or better than when Bi or Pb is contained alone can be obtained. The machinability improvement effect of Bi had been known to be weaker than that of Pb, but the inventors revealed that Bi exhibited equivalent or, in some cases, better effect than Pb in in the case of the embodiment. Apropos, Bi deteriorates resistance to stress corrosion cracking of brass, but such an effect disappears almost entirely when it is present in the form of mixed particles of Pb and Bi or when the amount of Bi is small.

[0071] For an alloy containing Bi to obtain good machinability, the Bi content needs to be at least 0.0001 mass% or higher. The Bi content is preferably 0.001 mass% or higher, more preferably 0.002 mass% or higher. The effect of Bi on the human body is presently unknown, but its content is lower than 0.20 mass%, preferably lower than 0.10 mass%, and more preferably 0.08 mass% or lower. Incidentally, as stated above, considering the facts that Bi is well capable of replacing Pb in the embodiments, its effect on the human body is unknown, use of Bi affects the environment since it is a rare metal, and the element is included in the raw material as an inevitable impurity, Bi is included as an optional element, i.e., not required to be contained. Further, considering its influence on human body and the environment as well as stress corrosion cracking, the total content of Pb and Bi (the composition relational expression f2 that is explained later) is lower than 0.20 mass%, preferably lower than 0.10 mass%. Thus, the embodiments are directed to reducing the content of Pb, an element harmful to the human body, in some cases the total content of Pb and Bi, to lower than 0.20 mass% yet obtaining excellent machinability.(Sn)

[0072] Sn further improves the dezincification corrosion resistance of β1 phase by solid solubilizing in the phase, allowing the alloy's dezincification corrosion resistance to improve. To obtain this effect where Sn is contained, its content needs to be higher than 0.05 mass%. The Sn content is preferably 0.10 mass% or higher. Apropos, a higher proportion of Sn is inherently distributed to β and β1 phases than α phase, and Sn is able to exhibit its effect of improving dezincification corrosion resistance even if its content is small. However, when its concentration is high, γ phase is likely to be formed, causing deterioration of ductility. Formation of phase causes deterioration of not only ductility but also machinability and dezincification corrosion resistance of the alloy. It is therefore necessary to limit the Sn content to lower than 0.90 mass% although it depends on the amount of Si contained. The Sn content is preferably lower than 0.70 mass%, more preferably lower than 0.50 mass%.

[0073] As stated above, the inventors have learned that when a large amount of Sn is contained, problem sometimes occurs in the modification of β phase containing Si and P because a larger portion of Sn is distributed to β phase and β1 phase. Specifically, when the Sn content is larger than the Si content, β phase fails to be sufficiently modified, and the effect of improving dezincification corrosion resistance brought by Sn is offset. As described later, the Si content needs to exceed the Sn content.(Inevitable Impurities, in Particular, Fe, Mn, Co, Cr, and Al)

[0074] Examples of the inevitable impurities in the embodiments include Mn, Fe, Al, Ni, Mg, Se, Te, Sn, Bi, Co, Ca, Zr, Cr, Ti, In, W, Mo, B, Ag, and rare earth elements.

[0075] Conventionally, primary raw material of a free-cutting copper alloy, in particular, a free-cutting brass including about 30 mass% or higher Zn is not a quality raw material such as electrolytic copper or electrolytic zinc but recycled copper alloy. In the later steps (downstream steps or processing steps) in this field of art, machining is performed on almost all the parts and components, during which a large amount of copper alloy accounting for 40 to 80% of the material in terms of weight goes to waste. Examples of such waste material include chips, mill ends, burrs, runners, and products with manufacturing defects. These waste copper alloys constitute the primary raw material. If cutting chips, mill ends, and the like are not properly separated, Pb, Fe, Mn, Si, Se, Te, Sn, P, Sb, As, Bi, Ca, Al, Zr, Ni, and / or rare earth elements mix in as part of raw material from leaded free-cutting brass, free-cutting copper alloy free of Pb but containing Bi or the like, or special brass alloy containing Si, Mn, Fe, and / or Al, or other copper alloys. In addition, cutting chips include Fe, W, Co, Mo, and the like which originate from tools. Ni, Cr, or Sn may mix in since disposed products to be recycled include plated products. Further, pure copper-based scrap used instead of electrolytic copper is contaminated with Mg, Sn, Fe, Cr, Ti, Co, In, Ni, Se, and / or Te. Brass-based scraps that are used instead of electrolytic copper or electrolytic zinc often include a material plated with Sn, causing Sn to mix in a recycled material.

[0076] From a viewpoint of reuse of resources and cost reduction, scraps including these elements are used as a raw material to the extent that use of such material does not adversely affect the resultant properties of the alloy at least. Incidentally, in a leaded JIS free-cutting brass bar, C3604 (JIS H 3250), about 3 mass% Pb is essentially contained. Further, as impurities, Fe may be contained up to 0.5 mass% and Fe and Sn may be contained up to 1.0 mass% (maximum total amount of Fe and Sn is 1.0 mass%). As a matter of fact, a high concentration of Fe or Sn is sometimes included in a free-cutting brass bar.

[0077] Fe, Mn, Co, and Cr solid-solubilize in α phase and β phase of a Cu-Zn alloy to a certain concentration. However, if Si and / or P is present then, they tend to compound with Si and / or P. If that happens, Si and P that are required for the modification of β phase could be consumed. Fe, Mn, Co, or Cr compounded with Si forms a Fe-Si compound, an Mn-Si compound, a Co-Si compound, a Cr-Si compound, or the like in the metallographic structure. Likewise, when they compound with P, a Fe-P compound, an Mn-P compound, a Co-P compound, a Cr-P compound, or the like is formed. These intermetallic compounds not only shorten tool life but also increase cutting resistance since they are extremely hard. For this reason, the contents of Fe, Mn, Co, and Cr need to be limited, and each content is preferably lower than 0.30 mass%, more preferably lower than 0.20 mass%, and still more preferably 0.15 mass% or lower. In particular, the total content of Fe, Mn, Co, and Cr is required to be lower than 0.40 mass%, preferably lower than 0.30 mass%, more preferably lower than 0.25 mass%, and still more preferably 0.20 mass% or lower.

[0078] On the other hand, the amount of Al that mixes in from special brass bar, brass casting, or the like needs to be limited because the element affects the modification of β phase if the Al content is large and also forms compounds with P or Si. In an alloy according to an embodiment of the present invention, the Al content needs to be lower than 0.30 mass%, preferably lower than 0.15 mass%, and more preferably 0.10 mass% or lower.

[0079] As other major inevitable impurity elements, empirically speaking, Ni often mixes in from scraps and the like. Even though the effect of Ni on mechanical properties such as machinability is relatively small, its content needs to be restricted in consideration of the impact on human body. Accordingly, the Ni content is preferably lower than 0.20 mass% and more preferably lower than 0.10 mass%. It is not particularly necessary to limit the content of Ag because Ag is commonly considered as Cu and its impact on various properties is negligible, but its preferable content is lower than 0.05 mass%. With respect to Te and Se that inherently have free-cutting properties, they may mix in in large quantity although such a case is rare. In consideration of their influence on ductility and impact properties, each content of Te and Se is preferably lower than 0.10 mass%, more preferably lower than 0.05 mass%, and still more preferably 0.02 mass% or lower. Further, corrosion-resistant brass includes As and / or Sb for the improvement of its corrosion resistance. In consideration of their impact on ductility and human body, each content of As and Sb is preferably lower than 0.05 mass% and more preferably 0.02 mass% or lower.

[0080] Each content of other elements such as Mg, Ca, Zr, Ti, In, W, Mo, B, and rare earth elements is preferably lower than 0.05 mass%, more preferably lower than 0.03 mass%, and still more preferably 0.02 mass% or lower.

[0081] Apropos, the content of the rare earth elements refers to the total amount of one or more of the following elements: Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, and Lu.

[0082] The total content of inevitable impurities excluding Fe, Mn, Co, Cr, and Al is preferably lower than 0.70 mass% and more preferably lower than 0.50 mass%.(Composition Relational Expression f1)

[0083] In composition relational expression f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn], when the Sn content is 0.05 mass% or lower, the value of [Sn] in this expression is 0, and when Bi is not included, the value of [Bi] is 0. Sn is not defined by the composition relational expression f1 if its content is 0.05 mass% or lower since its impact on the expression is minor.

[0084] f1 is an expression that represents a relationship. Even when the amount of each of the elements is within the range defined above, unless this composition relational expression f1 is satisfied, the target properties in embodiments of the present invention cannot be obtained. When the value of the composition relational expression f1 is lower than 57.5, the proportions of β phase and β1 phase are large, causing deterioration of ductility, dezincification corrosion resistance, and resistance to stress corrosion cracking due to insufficient modification of β phase.

[0085] Accordingly, the lower limit of the value of the composition relational expression f1 is 57.5 or higher, preferably 58.0 or higher, and more preferably 58.2 or higher. As the value of the composition relational expression f1 becomes more favorable within the defined range, the proportion of α phase increases and modification of β phase becomes sufficient. As a result, good dezincification corrosion resistance and resistance to stress corrosion cracking as well as good ductility and cold workability can be obtained without impairing excellent machinability.

[0086] On the other hand, the upper limit of the f1 value affects the proportions of β phase and β1 phase. When the value is higher than 60.5, the proportion of β1 phase is small and excellent machinability cannot be obtained. In addition, strength is low. Therefore, the value of the composition relational expression f1 is 60.5 or lower, preferably 60.2 or lower, and more preferably 60.0 or lower.(Composition Relational Expression f2)

[0087] Bi is an optional element that replaces the function of Pb. If Bi is contained, a relational expression of 0.003 ≤ f2 = [Pb] + [Bi] < 0.20 must be satisfied. That is, the contents of Pb and Bi need to be restricted by their total amount considering the facts that the impact of Bi on the human body is unknown at present and Bi affects the environment since it is a rare metal. If Bi is contained, the total content of Pb and Bi is lower than 0.20 mass%, which is the upper limit of the Pb content, and preferably lower than 0.10 mass%. On the other hand, when Bi is contained for the purpose of obtaining good machinability, the total content of Pb and Bi needs to be 0.003 mass% or higher, which is preferably 0.004 mass% or higher, and still more preferably 0.005 mass% or higher.(Composition Relational Expression f0)

[0088] Sn is contained in order to further improve dezincification corrosion resistance. A large portion of Sn is contained in β phase, and the larger the Sn content, the slower the advancement of dezincification corrosion of β1 phase. However, if the Sn content is excessively larger than the Si content, modification of β phase is hindered, which may impair dezincification corrosion resistance of β1 phase.

[0089] That is, providing that f0 = [Sn] / [Si], in order for Sn to exhibit an effect that is commensurate with the amount of Sn contained in consideration of other properties of the alloy, f0 needs to be lower than 1. It is preferably lower than 0.8, more preferably lower than 0.6, and still more preferably lower than 0.5.

[0090] Free-cutting copper alloys according to an embodiment of the present invention have good dezincification corrosion resistance, resistance to stress corrosion cracking, and mechanical properties despite a large content of conventional β phase, which refers to β1 phase in the present invention. They also have machinability that requires sort of brittleness which allows low cutting resistance and generation of finely broken chips as well as ductility, a property entirely opposite to machinability. To obtain good corrosion resistance, machinability, and mechanical properties, the composition relational expressions f0, f1, and f2, the metallographic structure relational expressions f3 to f5, and the metallographic structure and composition relational expressions f6 that are described later need to be satisfied in addition to the requirements regarding each component that constitutes the alloy's composition. By discussing these factors in detail, an alloy that is more suitable for a particular purpose or use can be provided.

[0091] Incidentally, Fe, Mn, Co, Cr, Al, and inevitable impurities that are separately defined are not defined by the composition relational expression f1 because their impact on f1 is small as long as the content is within the range that can be treated as an inevitable impurity.(Comparison with Patent Documents)

[0092] Here, the results of comparison between the compositions of the Cu-Zn-Si alloys described in Patent Documents 1 to 15 and those of alloys according to embodiments of the present invention are shown in Tables 1 to 4. [Table 1]CuSiPPbBiSnAlOthersMetallographicStructureFirst Embodiment60.5 -65.00.50 -1.200.01 -0.180.002 -0.20Optional 0.0001-0.20-<0.30Fe+Mn+Cr+Co <0.4020≤α<75, 25<β1≤80, 0≤γ<4The average cooling rate in the temperature range from 500°C to 300°C after hot working is higher than 300°C / min.Second Embodiment61.2 64.80.65 -1.100.03 -0.150.003 -0.10Optional 0.001-0.10-<0.15Fe+Mn+Cr+Co <0.3030≤α≤70, 30≤β1≤70, 0≤γ<1The average cooling rate in the temperature range from 500°C to 300°C after hot working is higher than 300°C / min.Third Embodiment60.5 -65.00.50 -1.200.01 -0.180.002 -0.20Optional 0.0001-0.200.05 -0.9<0.30Fe+Mn+Cr+Co <0.4020≤α<75, 25<β1≤80, 0≤γ<4The average cooling rate in the temperature range from 500°C to 300°C after hot working is higher than 300°C / min.Fourth Embodiment61.2 -64.80.65 -1.100.03 -0.150.003 -0.10Optional 0.001-0.100.10 -0.50<0.15Fe+Mn+Cr+Co <0.3030≤α≤70, 30≤β1≤70, 0≤y<1The average cooling rate in the temperature range from 500°C to 300°C after hot working is higher than 300°C / min. [Table 2] CuSiPPbBiSnAlOthersMetallographicStructurePatent Document 159-62-0.02 -0.07-0.3-0.4--Fe≤0.3Two phases of α and β phases; β phase is divided by α phaseAnnealed at a temperature between 350°C and 550°CPatent Document 261-651.0-1.50.005 -0.190.003 -0.20Sn+Al+Bi<0.4, Fe+Mn+Cr+Co<0.420≤α≤80, 15≤β≤80, 0≤γ<8The average cooling rate in the temperature range from 530°C to 450°C after hot working is 0.1°C / min or higher and 50°C / min or lower.Patent Document 359.0 -63.50.5-1.00.005 -0.190.003 -0.20Sn+Al+Bi<0.4, Fe+Mn+Cr+Co<0.420≤α≤75, 25≤β≤80, 0≤γ<2The average cooling rate in the temperature range from 530°C to 450°C after hot working is 0.1 °C / min or higher and 50°C / min or lower.Patent Document 458.5 -65.00.4-1.40.003 -0.190.002 0.25Optional 0.001-0.1Sn+Al+Bi<0.45, Fe+Mn+Cr+Co<0.4520≤α≤80, 18≤β≤80, 0≤γ<5The average cooling rate in the temperature range from 530°C to 450°C in the cooling process after casting is 0.1°C / min or higher and 55°C / min or lower. [Table 3] CuSiPPbBiSnAlOthersMetallographicStructurePatent Document 558.5 -64.50.2-1.20.001 -0.20.001 -0.200.1-1.0Sn+Al+Bi<0.45, Fe+Mn+Cr+Co<0.4520≤α≤85, 15≤β≤80, 0≤γ<4The average cooling rate in the temperature range from 530°C to 450°C after hot working is 0.1 °C / min or higher and 50°C / min or lower.Patent Document 658-650.3-1.30.001 -0.20.001 -0.200.02-0.1Sn+Al+Bi<0.45, Fe+Mn+Cr+Co<0.4520≤α<85, 15≤β≤80, 0≤γ<5The average cooling rate in the temperature range from 530°C to 450°C after hot working is 0.1 °C / min or higher and 50°C / min or lower.Patent Document 759.7 -64.70.6-1.30.001 -0.150.001 -0.20.001-0.1Sn+Al+Bi<0.45, Fe+Mn+Cr+Co<0.4517≤β≤75The average cooling rate in the temperature range from 530°C to 440°C after hot working is 0.1°C / min or higher and 70°C / min or lower.Patent Document 871.5 -78.52.0-4.50.01-0.20.005 -0.020.01-0.20.1-1.20.1-2.0-0≤β≤5Patent Document 960-660.01-0.5-0.150.05-0.5-0.2-0.9-0.02≤Fe+PArea ratio of α phase to β phase: 4:1-19:1Finish heat treatment is performed with a heating and holding temperature of 400-600°C and a holding time of 30 minutes or longer.Patent Document 1059-840.05-4.50.01-0.30.003 -0.30.003 -0.30.01-1.50.1-1.0-0≤β+γ+µ≤25; tubular hot-forged copper alloy product with a near-net shape [Table 4] CuSiPPbBiSnAlOthersMetallographicStructurePatent Document 1158-72.50.05-2.00.01 -0.250.01-40.01-30.05-40.01-3Zr: 0.0008 -0.04585≤α+γ → β≤15Patent Document 12Rem.0.01-10.005 -0.090.005 -0.03-0.01 -2.50.1-1.8Zn: 17-34y phase: 0-0.7%β phase: 0-0.9%Patent Document 13Rem.0.1-1.50.01 -0.360.05 -0.50.03 -0.3Ti: 0.01-0.1-Rare earths: 0.01-0.05Patent Document 1455-750.01-1.5-0.150.01 -0.40.01-40.1-0.1---Patent Document 1565-750.5-2.0---0.01-0.20.2-0.5Mn: 0.01-0.25β phase is essentially required.Sb: 0.01-0.55B: 0.001-0.05

[0093] The embodiments and the alloys disclosed by Patent Document 8 are different in the contents of Si and Cu, which are main elements of the alloys. The embodiments and the alloys disclosed by Patent Document 9 are different in the content of Si, which is a main element of the alloys. The embodiments and the alloys disclosed by Patent Document 15 are different in the content of Cu, which is a main element of the alloys.

[0094] Patent Documents 1, 13, and 15 disclose that Pb is not contained. Therefore, the Pb content is different.

[0095] In Patent Documents 8, 10, 11, and 12, β phase in the metallographic structure is significantly limited in view of its effect on machinability, dezincification corrosion resistance, other type of corrosion resistance, and the like. β phase disclosed by these documents and β1 phase in the present invention are two different phases, but for reference, β phase is limited to 5% or lower, 25% or lower, 15% or lower, and 0.9% or lower respectively in these Patent Documents.

[0096] Patent Document 10 relates to a tubular hot-forged part having a near-net shape and discloses that a tubular material is used.

[0097] Patent Documents 1 and 9 disclose that heat treatment is performed at a temperature either between 350°C and 550°C or between 400°C and 600°C in order to reduce or fragment β phase.

[0098] Patent Document 9 discloses that 0.2 mass% or higher Sn is contained, Sn and Si are contained for the improvement of dezincification corrosion resistance of β phase and requires hot extrusion performed at a temperature of 700°C or higher for the improvement of machinability and heat treatment performed at a temperature between 400°C and 600°C for the improvement of corrosion resistance. The proportion of β phase is approximately 5 to 20%, and the Si content is 0.01 to 0.50 mass%, which may be controlled to be 0.2 mass% or lower.

[0099] In Patent Documents 12, 13, and 14, Al is essentially required in order to improve color fastness, castability, and dezincification corrosion resistance.

[0100] Patent Document 14 requires containing at least 0.1 mass% Sn and Al respectively in order to improve dezincification corrosion resistance and large amounts of Pb and Bi in order to obtain excellent machinability.

[0101] Patent Document 15 discloses a corrosion-resistant copper alloy casting that does not contain Pb but requires y phase and have good mechanical properties and castability, which are realized by containing tiny amounts of elements such as Al, Sb, Sn, Mn, Ni, B, and so on in addition to Si and 65 mass% or higher Cu.

[0102] All of Patent Documents 2 to 7 essentially require presence of phosphorous compounds in the metallographic structure that are formed as a result of a treatment performed in the cooling process after hot working at an average cooling rate of about 0.1°C / min or higher and about 70°C / min or lower in the temperature range from about 530°C to about 450°C. Further, the Documents disclose that y phase is effective to obtain good machinability but there is not a word about modification of β phase in these Documents. They are silent about dezincification corrosion resistance and resistance to stress corrosion cracking, and no relevant data is disclosed. In addition, none of the Documents discloses data related to cutting resistance in high-speed machining, either except for Patent Document 7. The present invention is cooled in a manner basically contrary to that disclosed by Patent Documents 2 to 7. Specifically, in the present invention, the cooling treatment performed in the cooling process after hot working is started when the temperature of hot-worked material is lower than 670°C and higher than 500°C, and the average cooling rate is higher than 300°C / min in temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C.

[0103] The most remarkable and clear-cut difference between the present application and Patent Documents 2 to 7 is that none of the prior art documents refers to modification of β phase or modified β phase, that is, β1 phase. β1 phase is formed when the following conditions are met. In the process of hot working, β phase in which certain amounts of Si and P are solid-solubilized is formed; a cooling treatment is started when the temperature of hot-worked material is higher than 500°C; and the average cooling rates in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C are both higher than 300°C / min. β1 phase is thus formed. If the phase formed is β1 phase, a crystal grain boundary is observed inside the phase when etched with a mixed solution of hydrogen peroxide and ammonia water. Before modification to β1 phase, however, even if certain amounts of Si and P are contained, no such boundary is observed inside β phase. Patent Documents 2 to 7 disclose pictures of metallographic structures obtained by etching with a mixed solution of hydrogen peroxide and ammonia water, but no grain boundary is observed inside β phase in any of the pictures. In these prior art documents, starting temperature of the cooling treatment performed after hot working is not defined. However, even if the cooling treatment was started at a temperature lower than 530°C, β1 phase (modified β phase) would not appear since the average cooling rate from the starting temperature of the cooling treatment to 500°C is about 0.1 °C / min or higher and about 70°C / min or lower.

[0104] Unlike conventional β phase, β1 phase is able to improve dezincification corrosion resistance and resistance to stress corrosion cracking significantly, and better improve machinability than β phase. In fact, its machinability improvement effect excels that of the presence of phosphorous compounds. However, for β1 phase to exhibit such effects, its proportion in the metallographic structure needs to be larger than 25% in terms of area ratio. In addition, whereas presence of γ phase impairs machinability in the present invention, γ phase is effective to obtain good machinability in Patent Documents 2 to 7.<Metallographic Structure>

[0105] In a Cu-Zn-Si alloy, 10 or more kinds of phases are present, and they change in a complicated manner. Therefore, desired properties cannot be necessarily obtained by simply satisfying specific composition ranges and relational expressions regarding elements. Desired properties can eventually be obtained by specifying and determining the kinds of phases that need to be present in the metallographic structure as well as the ranges of their respective area ratios. Accordingly, the metallographic structure relational expressions are defined as follows. 20 ≤ f 3 = α < 75 25 < f 4 = β 1 ≤ 80

[0106] Note that in this expression, β1 phase refers to modified β phase. 0 ≤ f 5 = γ < 4(β1 phase)

[0107] Aside from non-metallic inclusions, the metallographic structure of the present invention consists of α phase, β1 phase, and in some cases, a tiny amount of γ phase which may not be present at all.

[0108] The difference between β phase and β1 phase is that in the case of β1 phase, a grain boundary pattern, i.e., boundary of crystal grain is observed inside β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water, but no crystal grain boundary is observed inside β phase.

[0109] β1 phase of an alloy according an embodiment of the present invention is obtained by causing certain amounts or more of Si and P to solid-solubilize in β phase under high temperature during hot working and maintaining the state of β phase when it is between 500°C and 670°C until the temperature of alloy goes down to normal temperature through a cooling treatment (i.e., cooling the alloy at an average cooling rate exceeding 300°C / min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C and continuing to cool it until the temperature of the alloy becomes normal temperature) so that the state of the metallographic structure when its temperature is between 500°C and 670°C is maintained until the temperature of the alloy becomes normal temperature. More specifically, to obtain β1 phase, the alloy is rapidly cooled at a high cooling rate in the temperature range from a high starting temperature of cooling to 300°C then is kept cooling until its temperature goes down to a normal temperature of 100°C or lower. By doing so, the state of metallographic structure under high temperature can be maintained until after it is cooled down to normal temperature. As a result, β1 phase is obtained. Even if the aforementioned cooling treatment is performed on β phase of a Cu-Zn-based alloy not containing both of Si and P in certain amounts, β1 phase cannot be obtained. Likewise, if the cooling treatment is performed on an alloy containing Si and P with a starting temperature below 500°C, e.g., 450°C and a cooling rate exceeding 300°C / min, β1 phase cannot be obtained. Incidentally, for the modification of β phase, α phase is required in a certain amount or more. Therefore, if α phase is not present or the amount of α phase is insufficient, β1 phase cannot be obtained. Further, the degree of the modification is also affected by the amounts of Si, P, and inevitable impurities, the starting temperature of the cooling treatment, and the average cooling rates in temperature ranges from the starting temperature to 500°C and from 500°C to 300°C. If the degree of the modification is improved, that is, β1 phase is better modified, a material with better machinability and good dezincification corrosion resistance and resistance to stress corrosion cracking can be obtained. Modification of β phase generally starts to peak when the amount of Si is about 1 mass% and that of P is about 0.1 mass% although that depends on the amount of inevitable impurities such as Fe. If the amounts of Si and P are excessive, adverse effects such as deterioration in conductivity of ductility, appearance of y phase, and the like may be induced.

[0110] β1 phase (modified β phase) is able to overcome the drawbacks of a Cu-Zn-based alloy, i.e., its dezincification corrosion resistance and resistance to stress corrosion cracking which used to be major challenges. To demonstrate an example in concrete numbers, progress of dezincification corrosion can be reduced by approximately 60% or more, and that of stress corrosion cracking can be delayed by about 50% or more. As disclosed by the referenced patent documents, dezincification corrosion is a significant problem that a Cu-Zn-based alloy containing β phase has. The amount of β phase is limited to 25% or less or 20% or less, and a heat treatment is performed at a temperature between 350°C and 550°C to fragment β phase and reduce the amount of β phase because dezincification corrosion occurs along β phase.

[0111] In terms of strength, it is largely affected by the area ratios of constituent phases. Modified β phase, i.e., β1 phase has a higher strength and better ductility than β phase. Therefore, strength and ductility in an alloy with β1 is well-balanced.(β1 phase; Metallographic Structure Relational Expression f4)

[0112] In a free-cutting Cu-Zn-Si-P-Pb alloy of an embodiment of the present invention, the area ratio of β1 phase needs to be higher than 25% in order to obtain good machinability while minimizing the Pb content. Further, in order to improve machinability and strength, the area ratio is preferably 30% or higher, more preferably 33% or higher. On the other hand, if the proportion of β phase is excessive, for instance, 95%, β phase will not be modified to β1 phase. Modification of β phase occurs where α phase is present. Therefore, a certain amount of α phase is necessary. Further, with respect to dezincification corrosion and stress corrosion cracking, even though β1 phase is able to delay the progress of corrosion much better than β phase, its dezincification corrosion resistance and resistance to stress corrosion cracking are still much weaker than those of α phase. Specifically, in a dezincification corrosion test performed in accordance with ISO 6509, if the metallographic structure consists of α phase and β phase (before modification), β phase is selectively corroded by dezincification, and the depth of such corrosion is as deep as about 500 µm. In the case the metallographic structure consists of α phase and β1 phase (modified β phase), progress of dezincification corrosion is significantly suppressed since even though β1 phase is selectively corroded by dezincification, the depth of corrosion is about 20 µm to about 200 µm depending on the degree of modification and the area ratio of β1 phase. Thus, by modifying β phase into β1 phase, dezincification corrosion resistance improves significantly. Still, dezincification corrosion resistance and ductility of β1 phase are not as good as those of α phase. For this reason, when the area ratio of β1 phase is high, dezincification corrosion resistance and ductility of the alloy are poor. As a result of a devoted study taking these matters into account, the inventors came to conclude that it is necessary to limit the area ratio of β1 phase to 80% or lower. The area ratio is preferably 70% or lower, more preferably 65% or lower.(α phase; Metallographic Structure Relational Expression f3)

[0113] The present invention basically consists of α phase and β1 phase. Although a treatment is performed to modify β phase into β1 phase, this treatment little affects α phase. Further, for the modification of β phase, a certain amount or more of α phase is necessary, and if the amount of β1 phase is excessive, ductility of the alloy is poor. Therefore, an appropriate amount of α phase, a phase that has good ductility, is required. However, if the amount of α phase is excessive, strength of the alloy is low. α phase including Si has only slightly better machinability than that excluding Si. From a standpoint of machinability also, the amount of α phase is limited. Even though the present invention consists of β1 phase having excellent machinability and α phase having somewhat poor machinability, the cutting resistance of the present invention can be maintained to be low, and well-fragmented chips are generated if the proportion of α phase is up to about 75% since α phase functions as a cushioning material and a stress concentration source at boundary with hard β1 phase during machining. Incidentally, it is desirable that α phase is tiny and has a granular shape since it functions as a cushioning material and a stress concentration source at boundary with hard β1 phase during machining.

[0114] As a result of a devoted study on modification of β phase, dezincification corrosion resistance, resistance to stress corrosion cracking, machinability, and mechanical properties of the alloy of the present invention, it was determined that the amount of α phase needs to be 20% or higher, preferably 30% or higher, and more preferably 35% or higher. The upper limit of α phase, on the other hand, is lower than 75%, preferably 70% or lower, and more preferably 67% of lower.(γ Phase; Metallographic Structure Relational Expression f5)

[0115] As disclosed by Patent Documents 8 and 10, γ phase is a phase that contributes to machinability in a Cu-Zn-Si alloy in which the Cu concentration is about 69 mass% to about 80 mass% and the Si concentration is about 2 to about 4 mass%. Patent Document 15 discloses that γ phase is indispensable for a Cu-Zn-Si alloy free of Pb. Further, Patent Documents 2 to 6 disclose that γ phase containing Si has good machinability like β phase containing Si.

[0116] A negative coefficient larger than the coefficient given to the amount of β1 phase is given to γ phase in the metallographic structure relational expression f6 that is described later since γ phase has an adverse effect on the machinability of embodiments of the present invention. Further, γ phase impairs ductility and deteriorates dezincification corrosion resistance because γ phase does not become modified in the embodiments. Appearance of γ phase may not be avoided altogether in a Cu-Zn-Si alloy, but γ phase should be restricted at least. Thus, proportion (area ratio) of γ phase is limited to smaller than 4%. It is preferably smaller than 2%, more preferably smaller than 1%, and optimally, this phase is not present, i.e., f5 = 0.(Metallographic Structure and Composition Relational Expression f6)

[0117] f 6 = β 1 × Si 1 / 2 − γ × 2 + Pb + Bi 1 / 2 × 20 + P 1 / 2 × 15

[0118] f6 is a simple conditional expression for an alloy to obtain good machinability. In f6, the amount of β1 phase and the amounts of Si, Pb, Bi, and P contained in the alloy within the respective compositional ranges defined in the instant specification are put together and regarded to have a positive effect and the amount of γ phase is regarded to have a negative effect. More specifically, the amount of γ phase multiplied by a coefficient of 2 is deducted from the total of the amount of β1 phase containing Si and P multiplied by the amount of Si raised to the power of 1 / 2, the sum of the amounts of Pb and Bi raised to the power of 1 / 2 multiplied by a coefficient of 20, and the amount of P raised to the power of 1 / 2 multiplied by a coefficient of 15. Performance of β1 phase (modified β phase) is directly affected by the concentration of Si as well as that of P, and when a tiny amount of Pb or Bi is contained, its machinability improves. γ phase, on the other hand, hinders machinability in the present invention. The inventors have revealed that the degree of machinability improvement brought by Pb or Bi is deeply related to a total amount of Pb and Bi raised to the power of 1 / 2. Both elements exhibit a significant effect even if the content is very small. As the contents are increased, their machinability improvement effects improve, but intensity of the effect gradually subsides. Due to the presence of β1 phase (modified β phase) in the constituent phases of alloy, Pb or Bi is able to exhibit a significant effect on machinability even if the content is extremely small. Such an effect will never occur to anyone who is familiar with the relationship between machinability of a conventional Cu-Zn-Pb alloy and the amount of Pb contained in the alloy.

[0119] As a result of devoted study, it was revealed that to obtain good machinability, the f6 value is higher than 27, preferably 33 or higher, more preferably higher than 35, and still more preferably 38 or higher. As the f6 value gets higher, machinability becomes closer to that of a free-cutting brass bar containing 3 mass% Pb.

[0120] Now, Figs. 1A to 6B show pictures of the metallographic structures of various alloys and the results of the dezincification corrosion tests performed in accordance with ISO 6509.

[0121] Fig. 1A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S01 to Step No. A2. More specifically, Alloy No. S01 has a composition including 63.3 mass% Cu, 0.95 mass% Si, 0.069 mass% P, 0.063 mass% Pb, 0.017 mass% Bi, and Zn as the balance. In Step No. A2, the alloy was subjected to hot extrusion performed at 630°C and a cooling treatment which was started when the temperature of the hot-extruded material became 580°C and performed at an average cooling rate of 1020°C / min in both temperature ranges from 580°C to 300°C and from 500°C to 300°C. Incidentally, the average cooling rate in the temperature range from 580°C, the temperature at which the cooling treatment was started, to 500°C can be easily calculated from the average cooling rates in temperature ranges from 580°C to 300°C and from 500°C to 300°C.

[0122] Fig. 1B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 1A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.

[0123] Fig. 2A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S11 to Step No. E2. More specifically, Alloy No. S11 has a composition including 62.5 mass% Cu, 0.96 mass% Si, 0.064 mass% P, 0.072 mass% Pb, and Zn as the balance. In Step No. E2, the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm. The rod was heated and hot forged at 680°C to have a thickness of 20 mm when placed horizontally then subjected to a cooling treatment which was started when its temperature became 565°C and performed at an average cooling rate of 900°C / min in both temperature ranges from 565°C to 300°C and from 500°C to 300°C.

[0124] Fig. 2B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 2A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.

[0125] Fig. 3A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S11 to Step No. E13H. More specifically, Alloy No. S11 has a composition including 62.5 mass% Cu, 0.96 mass% Si, 0.064 mass% P, 0.072 mass% Pb, and Zn as the balance. In Step No. E13H, the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm. The rod was heated and hot forged at 630°C to have a thickness of 20 mm when placed horizontally then cooled at an average cooling rate of 35°C / min. Subsequently, the rod was subjected to a cooling treatment which was started when its temperature became 455°C and performed at an average cooling rate of 800°C / min in the temperature range from 455°C to 300°C. Note that the starting temperatures of the cooling treatment performed on the alloy in Fig. 3A and that in Fig. 2A are different.

[0126] Fig. 3B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 3A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.

[0127] Fig. 4A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S43 to Step No. E6. More specifically, Alloy No. S43 has a composition including 63.3 mass% Cu, 0.98 mass% Si, 0.084 mass% P, 0.060 mass% Pb, 0.28 mass% Sn, and Zn as the balance. In Step No. E6, a casting produced with a mold having a diameter of 55 mm was machined to a diameter of 50 mm then cut to a length of 200 mm. The casting was heated and hot forged at 630°C to a thickness of 20 mm when placed horizontally then subjected to a cooling treatment which was started when the temperature of the casting became 565°C and performed at an average cooling rate of 900°C / min in both temperature ranges from 565°C to 300°C and from 500°C to 300°C.

[0128] Fig. 4B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 4A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.

[0129] Fig. 5A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step Nos. A34H and G1. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.98 mass% Si, 0.071 mass% P, 0.071 mass% Pb, and Zn as the balance. In Step No. A34H, an alloy was obtained by hot extrusion at 615°C and cooling at an average cooling rate of 18°C / min in the temperature range from 500°C to 300°C. In Step No. G1, the alloy was further heated to 580°C for 30 minutes then subjected to a cooling treatment which was started when its temperature became 560°C and performed at an average cooling rate of 1800°C / min in both temperature ranges from 560°C to 300°C and from 500°C to 300°C.

[0130] Fig. 5B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 5A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.

[0131] Fig. 6A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step No. E14H. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.98 mass% Si, 0.071 mass% P, 0.071 mass% Pb, and Zn as the balance. In Step No. E14H, the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm. The rod was heated and hot forged at 630°C to a thickness of 20 mm when placed horizontally then left to cool naturally. The average cooling rate in the temperature range from 500°C to 300°C was 25°C / min.

[0132] Fig. 6B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 6A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.

[0133] As shown in the metallographic micrographs of Figs. 1A, 2A, 4A, and 5A, in a hot extruded material, a hot forged material, a hot forged material made of a casting, and a hot extruded material made by subjecting a hot extruded material to an additional heat treatment, a grain boundary pattern, i.e., crystal grain boundary is observed inside β1 phase together with that of α phase crystal grain. Crystal grain boundary here refers to a linear pattern observed inside β1 phase that runs through a crystal grain of β1 phase like the one shown in Fig. 1A.

[0134] As mentioned above, a grain boundary is observed inside modified β phase, i.e., β1 phase, but in Fig. 3A, only something that looks like a faint black line is observed inside β phase, and there is no grain boundary that runs through a crystal grain of β phase. In Fig. 6A, there is not even a trace of grain boundary inside β phase. Instead, black granular precipitates of about 0.5 to 3 µm are present mainly inside β phase and on the phase boundaries between α phase and β phase in Figs 3A and 6A. The granular precipitates are mainly composed of phosphorous compounds, but Pb particles, mixed particles of Pb and Bi, compounds of Fe and the like, oxides, and sulfides are also included. They can be distinguished by microscopic observation, but it is slightly difficult to do the distinguishment from a printed picture. Precipitates which do not fall within the aforementioned size or are present inside α phase are not phosphorous compounds. In each field of view shown in the printed pictures of Figs. 3A and 6A, there are approximately 1000 precipitates that are mainly composed of phosphorous compounds. In Figs. 1A, 2A, 4A, and 5A, nothing like fine granular phosphorous compound is present or the amount is small if present. The number of such compounds is less than 1 / 10 or 1 / 50 of that of the precipitates shown in Figs. 3A and 6A at most. These findings indicate that whether grain boundary is present inside β1 phase or β phase, and whether a large quantity of phosphorous compound is present depend on whether the cooling treatment was started at a temperature higher than 500°C or lower than 500°C.

[0135] Results of the dezincification corrosion tests performed on these alloys in accordance with ISO 6509 are shown in Figs 1B, 2B, 3B, 4B, 5B, and 6B. The maximum corrosion depth of the test pieces in which a grain boundary was observed inside β1 phase (Figs. 1A, 2A, 4A, and 5A) was 120 µm or less, but that of the test pieces in which grain boundary was not observed inside β phase was 350 µm (Fig. 3A) or 460 µm (Fig. 6A), indicating a difference of 3 to 5 times the corrosion depth. The corrosion depth of the test piece containing 0.28 mass% Sn was 40 µm, which was a particularly good result (Fig. 4A). Incidentally, the type of dezincification corrosion was selective corrosion of β1 phase or β phase in all cases. Therefore, dezincification corrosion occurs in these two phases first, but progress of the corrosion in β1 phase is approximately 3 times slower than in β phase.<Properties>(Normal-Temperature Strength and High-Temperature Properties)

[0136] There is a strong demand for reduction in the thickness and weight of parts and components such as those of water supply-related devices, plumbing components, auto parts, and electrical parts that are target applications of embodiments of the present invention. Among the strengths that are required for these applications, tensile strength is important, but the balance between strength and ductility is also important.

[0137] In this regard, it is preferable that hot extruded materials, hot rolled materials, and hot forged materials have a high strength with a tensile strength of 460 N / mm 2< without cold working performed after hot working.

[0138] The tensile strength is more preferably 490 N / mm 2< or higher and still more preferably 520 N / mm 2< or higher. Many valves, joints, and components used for pressure vessel, air conditioner, or freezer are made of a hot-extruded or hot-forged material. As C3604, a leaded copper alloy currently used for these applications, has a tensile strength of about 390 to 420 N / mm 2< and elongation of about 30% to 35%, it is possible to reduce the weights of such components by increasing the strength of the material.

[0139] Aside from strength, it is important that a material to be machined has resistance to fracture since it may be subjected to cold working such as light swaging or bending. To obtain good machinability, some kind of brittleness is required so that well-broken chips are generated, but machinability is a property opposite to cold workability. Likewise, tensile strength and ductility are opposite properties, but it is desirable that tensile strength and ductility (elongation) are very well-balanced. As a yardstick, if a material has a tensile strength of at least 450 N / mm 2< or higher, a ductility of 10% or higher, and the product of the tensile strength (represented by "S") and {(elongation (represented by "E"%) + 100) / 100} raised to the power of 1 / 2, i.e., f7 = S x {(E+100) / 100} 1 / 2< is preferably 540 or higher, the material can be regarded to have both high strength and high ductility. This f7 value is more preferably 570 or higher, and still more preferably 600 or higher. If cold working performed at a cold working ratio of 3-15% is included in the production process, both an elongation of 10% or higher and a tensile strength of 520 N / mm 2< or higher can be obtained. Apropos, the value of f7 of the previously mentioned leaded copper alloy is about 470.(Electrical Conductivity)

[0140] Applications of the embodiments include electrical or electronic apparatus components, components of automobiles that are increasingly powered by electricity, and other parts and components having high conductivity. Currently, phosphor bronzes including 6 mass% or 8 mass% Sn (JIS standard alloys C5191 and C5210) are widely used for these applications, and their electrical conductivities are about 14% IACS and 12% IACS, respectively. Accordingly, if the embodiments have an electric conductivity of 15% or higher, no problem should occur. The upper limit of the electrical conductivity is not particularly defined because improvement of conductivity rarely causes practical problems.(Hot Workability)

[0141] The free-cutting copper alloys of the embodiments are characterized by their excellent deformability when they are at a temperature between 540°C and 750°C. Due to this characteristic, they can be hot-extruded into a bar with a small cross-sectional area or formed into a complex shape by forging. From the viewpoints of energy saving and allowing α phase to have a favorable granular shape, hot working temperature is preferably lower than 750°C and more preferably lower than 720°C. From the viewpoint of thermal deformation resistance, it is preferably higher than 540°C and more preferably higher than 560°C.<Production Process>

[0142] Next, a method for producing the free-cutting copper alloys according to the embodiments will be described.

[0143] The metallographic structures of the alloys according to the embodiments vary depending not only on the composition but also on the production process. They are affected by the average cooling rate in the process of cooling after hot working or heat treatment in addition to hot working temperatures in hot extrusion and hot forging as well as heat treatment conditions. As a result of a devoted study, it was found that the metallographic structures are significantly affected by the starting temperature of the cooling treatment, the average cooling rates in temperature ranges from the starting temperature to 500°C and from 500°C to 300°C in the process of cooling after hot working or heat treatment.(Melting and Casting)

[0144] Melting is performed at a temperature between about 950°C and about 1200°C, a temperature that is about 100°C to about 300°C higher than the melting point (liquidus temperatures) of the alloy according to an embodiment of the present invention. A molten metal is cast into a specific mold when its temperature is about 900°C to about 1100°C, which is about 50°C to about 200°C higher than the melting point, then cooled by several cooling means such as air cooling, slow cooling, and water cooling. After the alloy solidifies, constituent phases of the alloy change in various ways.(Hot Working)

[0145] Examples of hot working include hot extrusion, hot forging, and hot rolling. In the case two or more hot working steps are performed, the final hot working step is performed under the following condition.

[0146] First, regarding hot extrusion, in a preferable embodiment, it is performed so that the material's temperature immediately after it is hot worked (extrusion temperature) is higher than 540°C and lower than 750°C although it depends on the extrusion ratio (hot working ratio) and the capacity of the extrusion facility used. Extruded bar is either coiled, or if the cross-sectional area of the bar is large, placed onto a table in a straight shape. The lower limit of the hot extrusion temperature relates to hot deformation resistance. The lower the extrusion temperature, the finer and granular become the α phase grains and the more improve the dezincification corrosion resistance, resistance to stress corrosion cracking, and machinability. However, when extruded at a high extrusion ratio, the extrusion temperature is preferably 560°C or higher in view of the capacity of the extruder and the starting temperature of the cooling treatment described later. The upper limit of hot extrusion temperature relates to the shape of α phase. By controlling the hot extrusion temperature so that it is within a more preferable temperature range, a stable metallographic structure can be obtained. When hot extrusion is performed at a temperature of 750°C or higher, the proportion of β phase in the metallographic structure once becomes 100% or 90% or larger, and the shape of α phase crystal grains tends to be acicular, or large α phase crystal grains become likely to appear. When acicular and large α phase crystal grains appear, the strength slightly decreases, the balance between strength and ductility slightly deteriorates, and the machinability deteriorates since large α phase crystal grains with a large longer-side hinder machining. They also deteriorate dezincification corrosion resistance and resistance to stress corrosion cracking. Extrusion temperature is preferably 720°C or lower. The shape of α phase crystal grains relates to the composition relational expression f1, and when the value of f1 is 59.0 or lower, preferable extrusion temperature is lower than 720°C. By performing extrusion at a temperature lower than the temperature at which leaded copper alloys are extruded, good machinability and strength can be obtained.

[0147] Further, by adjusting the conditions of the cooling performed after hot extrusion, β phase can be modified and a material having better machinability, good dezincification corrosion resistance, and good resistance to stress corrosion cracking can be obtained. That is, in the cooling process after hot extrusion, the cooling treatment is started when the temperature of the extruded material is lower than 670°C and higher than 500°C then the material is cooled with the average cooling rates in the temperature ranges from the starting temperature to 500°C and from 500°C to 300°C set higher than 300°C / min at least, preferably higher than 600°C / min, and more preferably 900°C / min or higher. It should be noted here that in an ordinary embodiment where a common cooling method is applied, the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C and that in the temperature range from 500°C to 300°C are approximately the same, or the former is slightly higher in some cases. The more preferable the cooling rate, the better modified becomes β1 phase. As a result, a material having improved machinability, good dezincification corrosion resistance, and good resistance to stress corrosion cracking can be obtained. The upper limit of the average cooling rates in temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C is not particularly defined since ordinary production facility is unable to cool hot-extruded material excessively fast, but as a reference, the cooling rate is preferably 9000°C / min or lower. If the cooling is performed at an average cooling rate exceeding 300°C / min, modification of β phase occurs, and a grain boundary can be found inside modified β phase when etched with a mixed solution of hydrogen peroxide and ammonia water and its metallographic structure is observed with a metallographic microscope at a magnification of 500X. Incidentally, if the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C or from 500°C to 300°C is 300°C / min or lower, no such grain boundary can be observed inside β phase. The higher the cooling rate, the better modified (into β1 phase) becomes β phase. The cooling rate in the cooling from a temperature lower than 300°C to normal temperature tends to decline slightly as the temperature approaches to normal temperature, but it is desirable to keep cooling in the same manner as in the temperature range from 500°C to 300°C.

[0148] On the other hand, starting temperature of the cooling treatment also affects the modification of β phase. The starting temperature is preferably 530°C or higher, more preferably 500°C or higher. In the case the cooling treatment is started at a temperature lower than 550°C, if the cooling rate from 550°C to the starting temperature is low, compounds of P and Zn or P and Zn and / or Si begin to form. If the starting temperature is 530°C or lower, formation of phosphorous compounds is further promoted. Although phosphorous compounds are scarcely formed when cooled at a cooling rate exceeding 300°C / min, if the starting temperature of the cooling treatment is lower than 550°C, such compounds are observed in the metallographic structure.

[0149] Incidentally, when the cooling treatment is started at a temperature of 670°C or higher, the proportion of β1 phase becomes excessive, causing deterioration of ductility, dezincification corrosion resistance, and resistance to stress corrosion cracking of the alloy. For this reason, the starting temperature needs to be lower than 670°C, and is preferably lower than 650°C.

[0150] From a perspective of practicability of measurement position, hot working temperature is defined as temperature of a hot worked material which can be measured about two or three seconds after hot extrusion, hot forging, or hot rolling is completed. The metallographic structure is affected by the temperature immediately after working where large plastic deformation occurs.

[0151] The next description is about hot forging. As a material for hot forging, a hot extruded material is mainly used, but a continuously cast bar is also used. There is no need to manipulate the cooling process of the forging material since hot forging is not the final hot working step. Compared with hot extrusion, in hot forging, working speed is faster, and a more complex shape is formed. In some cases, hot forging is performed at a high working ratio up to a thickness of about 3 mm. Further, weight of forged product varies from a few tens of grams to a few kilograms, and small ones cool rapidly immediately after forging and the rapid cooling continues thereafter. Accordingly, forging material is heated to a higher temperature than the temperature to which ingot is heated for hot extrusion. In a preferred embodiment, the temperature of a hot forged product, that is, the material's temperature about two or three seconds immediately after forging is preferably higher than 540°C and lower than 750°C.

[0152] In hot forging, forging temperature is related to the composition relational expression f1, and when the value of f1 is 59.0 or lower, it is preferable that hot forging is performed at a temperature lower than 720°C. Although depending on the working ratio in hot forging, the lower the forging temperature, the smaller becomes the diameter of α phase crystal grain, the more likely to change the shape of α phase crystal grain from acicular to granular, the higher the strength, and the better the balance between strength and ductility, the machinability, the dezincification corrosion resistance, and the resistance to stress corrosion cracking.

[0153] Like in hot extrusion, by appropriately adjusting the cooling rate at which forged material is cooled after hot forging, a material with good dezincification corrosion resistance, resistance to stress corrosion cracking, and machinability can be obtained. That is, in the cooling process after hot forging, a cooling treatment is started when the temperature of the forged material is lower than 670°C and higher than 500°C then the material is cooled with the average cooling rates in the temperature ranges from the starting temperature to 500°C and from 500°C to 300°C set to be higher than 300°C / min at least, preferably higher than 600°C / min, and more preferably 900°C / min or higher. It should be noted here that in an ordinary embodiment where a common cooling method is applied, the average cooling rates in the temperature range from the starting temperature of the cooling treatment to 500°C and that in the temperature range from 500°C to 300°C are approximately the same or the former is slightly higher. By cooling at an average cooling rate exceeding 300°C / min, modification of β phase occurs and a grain boundary can be found inside modified β phase when a cross section of the material is etched with a mixed solution of hydrogen peroxide and ammonia water then the metallographic structure is observed with a metallographic microscope at a magnification of 500X. Incidentally, if the average cooling rate in the temperature range from 500°C to 300°C is 300°C / min or lower, no such grain boundary can be observed inside β phase. The higher the cooling rate, the more progresses the modification of β phase into β1 phase. Like in hot extrusion, the cooling rate in the cooling from a temperature lower than 300°C to normal temperature tends to decline slightly as the temperature approaches to normal temperature, but it is desirable to keep cooling in the same manner as in the temperature range from 500°C to 300°C. The upper limit of the average cooling rates in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C is not particularly defined, but as a reference, the cooling rate is preferably 9000°C / min or lower.

[0154] On the other hand, starting temperature of the cooling treatment also affects the modification of β phase. The starting temperature is preferably higher than 530°C, more preferably higher than 550°C. In the case the cooling treatment is started at a temperature lower than 550°C, if the cooling rate from 550°C to the starting temperature is low, compounds of P and Zn or P and Zn and / or Si begin to form. If the starting temperature is 530°C or lower, formation of phosphorous compounds is further promoted. Although phosphorous compounds are scarcely formed when cooled at a cooling rate exceeding 300°C / min, if the starting temperature of the cooling treatment is lower than 550°C, such compounds are observed in the metallographic structure. On the other hand, when cooling is started at a temperature of 670°C or higher, the proportion of β1 phase becomes excessive, causing deterioration of ductility, dezincification corrosion resistance, and resistance to stress corrosion cracking of the alloy. For this reason, the starting temperature needs to be lower than 670°C, and is preferably lower than 650°C. Incidentally, after hot forging, if the cooling treatment is started after holding the material in a simple furnace in which the atmospheric temperature is set between about 550°C and about 600°C for a period from a few tens of seconds to a few minutes, more uniform and stable forged product can be obtained.

[0155] Further, shot blasting performed upon completion of the aforementioned steps of hot forging and cooling treatment makes an effective mean for the improvement of resistance to stress corrosion cracking. In other copper alloys also, resistance to stress corrosion cracking can be improved by shot blasting or any other way of applying compressional stress on the surface of alloy, but the effect of shot blasting is particularly good in the case of the alloys of the present invention containing β1 phase.

[0156] In hot rolling, an ingot is heated and rolled 5 to 15 times repeatedly. The material's temperature upon completion of the final hot rolling (the material's temperature three or four seconds after completion of the process) is preferably higher than 540°C and lower than 750°C, more preferably lower than 670°C. Upon completion of hot rolling, in the process of cooling the hot-rolled material, like in hot extrusion, a cooling treatment is performed with a starting temperature set to be higher than 500°C and lower than 670°C and an average cooling rate in the temperature ranges from the starting temperature to 500°C and from 500°C to 300°C set to be higher than 300°C / min at least, preferably higher than 600°C / min, and more preferably 900°C / min or higher. The cooling rate in the cooling from a temperature lower than 300°C to normal temperature tends to decline slightly as the temperature approaches to normal temperature, but it is desirable to keep cooling in the same manner as in the temperature range from 500°C to 300°C. The upper limit of the average cooling rates in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C is not particularly defined, but as a reference, the cooling rate is preferably 9000°C / min or lower.(Heat Treatment)

[0157] In the present invention, if the cooling treatment fails to be started when the material's temperature is higher than 500°C after the aforementioned final hot working, or if it is not performed at an average cooling rate exceeding 300°C / min in the temperature range from 500°C to 300°C, β phase cannot be modified. In addition, when wire or bar with a small diameter is manufactured, or if cold working or any step involving heating such as annealing is included in the production process, modification of β phase basically does not occur. Even in such cases, however, β phase is modified to form β1 phase if annealing is performed at a temperature higher than 520°C and lower than 630°C for one minute to five hours and a cooling treatment is started in the cooling step after annealing when the temperature of annealed material is higher than 500°C and performed at an average cooling rate of higher than 300°C / min in both temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C. The aforementioned annealing conditions need to be applied only to the last annealing step if annealing is performed a multiple number of times. From a standpoint of the temperature range in which phosphorous compounds are formed, annealing temperature is preferably 530°C or higher and more preferably 550°C or higher. Further, the average cooling rate in the temperature range from 500°C to 300°C is preferably set to be higher than 600°C / min, and more preferably 900°C / min or higher to enhance the degree of modification into β1 phase. This way of heat treatment is applied to hot-worked materials which cannot be cooled with the previously described conditions after hot working, those which are cold worked after hot working then annealed at least once, or the like. The upper limit of the average cooling rates in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C is not particularly defined, but as a reference, the cooling rate is preferably 9000°C / min or lower.(Cold Working Step)

[0158] To produce hot extruded bar, cold working is sometimes performed on extruded straight or coiled material in order to obtain high strength, improve dimensional accuracy, and straighten or reduce the degree of bending of such material. For example, cold drawing is performed on a hot extruded material at a cold working ratio of about 2% to about 30% which is followed by straightness correction in some cases.

[0159] For slender bars and wires as well as rolled material, cold working and heat treatment are repeatedly performed. After the previously-described final heat treatment, final cold working is performed at a working ratio of 0% to about 30% which is followed by straightness correction. The closer to 0% the cold working ratio, the less likely to crack becomes the material when it is subjected to cold working such as light swaging or bending.(Low-Temperature Annealing)

[0160] In the case of bars, wires, forged products, and rolled materials, for the primary purposes of removal of residual stress, straightness correction of bar (straightening of bar) as well as adjustment and improvement of the metallographic structure, low-temperature annealing is sometimes performed at a temperature lower than or equal to the recrystallization temperature in the final step. However, modification into β1 phase which was achieved by hot working step and heat treatment may be impaired if the alloy is heated. For instance, if it is heated to 300°C for two hours, modification of β phase becomes impaired and β1 phase returns to β phase, which causes the machinability, the dezincification corrosion resistance, and the resistance to stress corrosion cracking once improved by β1 phase to return to those of the original alloy comprising β phase. Therefore, annealing is not recommended, but may be performed if at a temperature below about 150°C.

[0161] Free-cutting alloy of an embodiment of the present invention with the above-described constitution has excellent machinability, good dezincification corrosion resistance and resistance to stress corrosion cracking as well as excellent hot workability, high strength, and balance between strength and ductility even though the Pb content is small since the alloy composition, the composition relational expressions, the metallographic structure, the metallographic structure relational expressions, and the metallographic structure and composition relational expression are defined as described above.

[0162] Embodiments of the present invention are as hereinabove described. It should be noted, however, that the present invention is not limited to the embodiments and can be modified as appropriate within a scope not deviating from the technical requirements of the present invention.

[0163] Examples

[0164] Hereinafter, the results of the experiments that were performed to verify the effects of the embodiments will be described. The following Examples are presented for the purpose of explaining the effects of the embodiments. The constituent elements, the processes, and the conditions stated in the descriptions of the Examples do not limit the technical ranges of the embodiments.

[0165] Using a low-frequency melting furnace and a semi-continuous casting machine used on the actual production line, trial production of copper alloys was performed. Trial production of copper alloys was performed also with laboratory facilities.

[0166] Tables 5 to 7 show the alloy compositions. Tables 8 to 17 show the production steps employed. Regarding composition, "Mm" refers to mischmetal and represents the total content of rare earth elements. Each of the production steps is described below. The cooling rate in hot working, i.e., hot extrusion, hot forging, or hot compression in Tables 8 to 12 and 14 to 16 refers to the average cooling rate between the points of completion of hot working and commencement of cooling treatment. [Table 5]Alloy No.Constituent Component (mass%)Composition Relational ExpressionImpurity (mass%)CuSiPPbBiSnZnf1f2f0FeMnCrCoAlNiAgSbAsMmS0163.30.950.0690.0630.0170.01Rem.58.90.080-0.120.010.000.000.010.030.020.000.000.00S0262.90.980.0710.0710.0000.02Rem.58.40.071-0.080.050.010.000.010.040.010.010.000.00S0364.20.890.0820.0590.0030.01Rem.60.060.062-0.030.010.000.000.000.030.010.000.000.00S1162.50.960.0640.0720.0000.01Rem.58.10.072-0.050.000.020.000.000.020.010.000.000.01S1264.10.930.0740.0680.0020.02Rem.59.80.070-0.110.010.000.000.000.040.020.000.000.00S1363.61.110.0580.0570.0000.01Rem.58.50.057-0.030.000.000.010.000.080.010.000.000.01S2162.80.880.0870.0500.0010.00Rem.58.70.051-0.000.000.000.000.000.000.000.000.000.00S2263.51.000.0720.0800.0000.00Rem.58.90.080-0.000.000.000.000.000.000.000.000.000.00S2362.50.760.0450.0150.0000.00Rem.59.00.015-0.000.000.000.000.000.000.000.000.000.00S2463.81.150.0640.0280.0000.00Rem.58.50.028-0.000.000.000.000.000.000.000.000.000.00S2562.80.900.0630.0380.0260.02Rem.58.60.064-0.190.130.010.000.000.090.010.020.010.01S2662.80.880.0950.0550.0000.03Rem.58.70.055-0.010.000.000.000.200.080.000.000.000.00S2764.01.020.0650.0700.0000.02Rem.59.30.070-0.140.070.000.000.000.000.000.000.010.00S2863.20.900.0950.0710.0000.02Rem.59.00.071-0.120.110.000.000.130.000.000.000.000.02S2962.80.950.0690.0670.0000.00Rem.58.40.067-0.110.000.000.000.080.130.030.010.010.00S3063.00.950.0650.0170.0000.00Rem.58.60.017-0.080.010.000.000.040.100.020.000.000.00 [Table 6] Alloy No.Constituent Component (mass%)Composition Relational ExpressionImpurity (mass%)CuSiPPbBiSnZnf1f2f0FeMnCrCoAlNiAgSbAsMmS3163.30.940.0180.0660.0000.00Rem.59.00.066-0.000.000.000.000.000.000.000.000.000.00S3263.61.020.0440.1030.0080.00Rem.58.90.111-0.000.000.000.000.000.000.000.000.000.00S3363.30.970.0820.0080.0000.00Rem.58.80.008-0.000.000.000.000.000.000.000.000.000.00S3462.20.780.1220.0550.0170.00Rem.58.50.072-0.000.000.000.000.000.000.000.000.000.00S3561.00.710.1000.0660.0000.00Rem.57.70.066-0.000.000.000.000.000.000.000.000.000.00S3664.40.920.0550.0150.0000.00Rem.60.10.015-0.000.000.000.000.000.000.000.000.000.00S3761.20.640.0580.0630.0220.00Rem.58.20.085-0.000.000.000.000.000.000.000.000.000.00S3864.81.130.0580.0710.0000.00Rem.59.60.071-0.000.000.000.000.000.000.000.000.000.00S3963.61.030.0570.0550.0000.61Rem.58.20.0550.590.000.000.000.000.000.000.000.000.000.00S4063.20.880.0950.0710.0000.41Rem.58.70.0710.470.000.000.000.000.000.000.000.000.000.00S4162.70.830.0840.0660.0000.18Rem.58.70.0660.220.000.000.000.000.000.000.000.000.000.00S4263.80.940.0730.0590.0000.32Rem.59.10.0590.340.000.000.000.000.000.000.000.000.000.00S4363.30.980.0840.0600.0000.28Rem.58.50.0600.290.000.000.000.000.000.000.000.000.000.00S4463.20.720.0650.0350.0000.41Rem.59.40.0350.570.000.000.000.000.000.000.000.000.000.00S4563.40.910.0840.0450.0000.28Rem.58.90.0450.280.000.000.000.000.000.000.000.000.000.00S4663.20.950.0820.0610.0090.08Rem.58.70.0700.080.000.000.000.000.000.000.000.000.000.00S4764.80.950.0970.0780.0000.00Rem.60.40.078-0.000.000.000.000.000.000.000.000.000.00S4860.90.550.0580.0890.0000.00Rem.58.40.089-0.000.000.000.000.000.000.000.000.000.00S4963.50.710.0610.0370.0020.00Rem.60.20.039-0.000.000.000.000.000.000.000.000.000.00 [Table 7] Alloy No.Constituent Component (mass%)Composition Relational ExpressionImpurity (mass%)CuSiPPbBiSnZnf1f2f0FeMnCrCoAlNiAgSbAsMmS7162.90.910.0060.0640.0080.00Rem.58.70.072-0.000.000.000.000.000.000.000.000.000.00S7265.51.090.0690.0440.0000.00Rem.60.40.044-0.000.000.000.000.000.000.000.000.000.00S7363.20.810.0670.0010.0000.00Rem.59.40.001-0.000.000.000.000.000.000.000.000.000.00S7460.70.440.0720.0880.0000.00Rem.58.60.088-0.000.000.000.000.000.000.000.000.000.00S7562.91.030.2300.0710.0070.00Rem.58.00.078-0.000.000.000.000.000.000.000.000.000.00S7660.70.800.0560.0460.0110.00Rem.57.00.057-0.000.000.000.000.000.000.000.000.000.00S7760.10.570.0700.0780.0350.00Rem.57.50.113-0.000.000.000.000.000.000.000.000.000.00S7864.91.290.0980.0660.0000.00Rem.58.90.066-0.000.000.000.000.000.000.000.000.000.00S7964.90.900.0630.0200.0000.00Rem.60.70.020-0.000.000.000.000.000.000.000.000.000.00S8059.20.200.0480.1050.0030.00Rem.58.30.108-0.000.000.000.000.000.000.000.000.000.00S8163.10.610.0400.0230.0000.00Rem.60.30.023-0.000.000.000.000.000.000.000.000.000.00S8264.71.110.0930.0580.0001.01Rem.58.50.0580.910.000.000.000.000.000.000.000.000.000.00S8362.50.670.0610.0700.0000.76Rem.58.60.0701.130.000.000.000.000.000.000.000.000.000.00S8463.30.850.0520.0570.0180.03Rem.59.40.075-0.300.130.020.020.000.010.010.000.000.00S8562.00.800.0490.0780.0000.03Rem.58.30.078-0.030.000.000.000.370.010.010.000.000.00X58.80.000.0013.100.0040.27Rem.-3.1-0.230.010.010.000.000.050.010.000.000.00Y58.70.000.0012.100.0010.23Rem.-2.1-0.260.000.000.020.000.030.000.010.000.00Z58.60.000.0012.100.0000.22Rem.-2.1-0.280.010.000.000.000.100.010.000.000.00 [Table 8] Step Group AA with Facilities Used for Products for Sale:Bars (Indirect Extrusion)Hot ExtrusionCooling TreatmentCold Working Ratio (%)Size (mm)Extrusion Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)A1Ø20.9630255609009008.4A2Ø20.963025580102010208.4A3Ø20.9630255607207208.4A4Ø20.9630255504804808.4A5Ø20.9630255209009008.4A11HØ20.963025480-9008.4A12HØ20.963025450-7208.4A13HØ20.9630---158.4A14HØ20.9630255602102108.4 [Table 9] Step Group AB with Facilities Used for Products for Sale:Bars (Indirect Extrusion)Hot ExtrusionCooling TreatmentCold Working Ratio (%)Low-Temperature AnnealingSize (mm)Extrusion Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)A21Ø20.96152557010209608.4A22Ø20.961525550102010208.4A23Ø20.9615255504204208.4A24Ø20.9615255159009008.4A25Ø20.96152557010209608.4130°C*5HA31HØ20.96152557010209608.4300°C*2HA32HØ20.961525475-9008.4A33HØ20.961525450-7808.4A34HØ20.9615---188.4 [Table 10] Step Group AD with Facilities Used for Products for Sale:Bars (Indirect Extrusion)Hot ExtrusionCooling TreatmentCold Working Ratio (%)Size (mm)Extrusion Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)A41Ø20.972030610126012008.4A42Ø20.972030570120012008.4A43Ø20.972030660132012008.4A44Ø20.9720306106606008.4A46HØ20.972030705132012008.4A47HØ20.9720---208.4 [Table 11] Step Group AC with Facilities Used for Products for Sale:Bars (Direct Extrusion)Hot ExtrusionCooling TreatmentCold Working Ratio (%)Size (mm)Extrusion Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)A50Ø50550---200 [Table 12] Extrusion Process B Performed in LaboratoryHot ExtrusionCooling TreatmentSize (mm)Extrusion Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)B1Ø2067055540960960B2Ø50590---25B3HØ20670---40 [Table 13] Step Group C; Casting in LaboratoryCooling Rate After CastingSize (mm)Start-300°C (°C / min)500-300°C (°C / min)C1Ø35-35C2Ø55-23 [Table 14] Hot Forging Step Group D with Mass Production Facilities Used for Products for Sale:Flare NutsHot ForgingCooling TreatmentMaterial from Previous Step Ø20 mmForging Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)D1Bar: A1, A2167010056512001200D2Bar: A13H, A34H, A47H71010060018001800D3Bar: B3H67010055012001200D4Bar: A1, A21670100545420420D5Bar: A13H, A34H67010054518001800D6Casting: C167010054512001200D7Bar: A13H, A34H, B3H, C1, A47H67010054518001800D8Bar: A13H, A47H74010065519801800D12HBar: A13H, A34H, A47H670100475-1080D13HBar: A1, A21670---35D14HBar: A13H, A34H670100560240240D15HBar: A1, A21, A47H77012069519801800 [Table 15] Hot Forging Step Group E with Mass Production Facilities Used for Products for Sale:Flat ForgingHot ForgingCooling TreatmentMaterial from Previous Step Ø50 mmForging Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)E1Bar: A5063035535900900E2Bar: A50, B268035565900900E3Bar: A5070035615900900E4Bar: A5063035515900900E5Bar: A5063035550450450E6Casting: C263035565900900E7Bar: A5072035650960900E11HBar: A5074035690960900E12HBar: A5063035480-900E13HBar: A5063035455-800E14HBar: A50630---25E15HBar: A5063035550200200 [Table 16] Hot Compression Step Group F Performed in LaboratoryHot CompressionCooling TreatmentMaterial from Previous StepCompression Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)F1Casting: C16505056021002100F2Bar: A13H, A34H, A47H6505056021002100F3Bar: B3H6505056021002100F4HBar: A13H, A34H65050560200200F5HBar: A13H, A34H65050--40 [Table 17] Heat Treatment Step Group GAnnealingCooling TreatmentMaterial from Previous StepTemperature (°C)Time (min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)G1Bar: A13H, A34H, A47H5803056018001800G2Forged flare nut: D15803056018001800G3Forged flare nut: D13H5803056018001800G11HBar: A1, A2158060470-1500G12HForged flare nut: D158060470-1500G13HBar: A1, A21, A4158030--25G14HForged flare nut: D158030--25 Production with mass production facilities used for manufacturing products for sale - Step Groups AA, AB, and AD(Steps Nos. A1 to A5, A11H to A14H, A21 to A25, A31H to A34H, A41 to A44, A46H, and A47H)

[0167] A billet with a diameter of 240 mm was produced with the low-frequency melting furnace and the semi-continuous casting machine used for the manufacture on the actual production line. For raw materials, those correspond to ones used for the manufacture on the actual production line were used. The billet was cut to a length of 800 mm then heated. Using an indirect extruder with a nominal capacity of 2750 tons, round bars with a diameter of 20.9 mm were extruded and coiled in a container located in a short distance away from the extruder. In this container, water quantity can be adjusted during water cooling. When the temperature of extruded material reached a specific temperature in the container, the cooling treatment (water cooling) was started with water quantity being adjusted. Incidentally, in some cases, the cooling treatment was started as soon as extruded material was placed in the container. Temperature of extruded material was measured with a radiation thermometer when the material came out of the extruder, when the cooling treatment was started, and when the material's temperature became 500°C and 300°C after that. Apropos, for the measurement of temperature during hot extrusion and hot forging in other steps, IGA 8Pro / MB20, a radiation thermometer manufactured by LumaSense Technologies Inc., and a contact thermometer were both used.

[0168] In Step Group AA, i.e., Steps Nos. A1 to A5 and A11H to A14H, extrusion temperature was 630°C, in Step Group AB, i.e., Steps Nos. A21 to A25 and A31H to A34H, it was 615°C, and in Step Group AD i.e., Steps Nos. A41 to A44, A46H, and A47H, it was 720°C. The cooling treatment was started when coiled extruded bars with a diameter of 20.9 mm reached a certain temperature in the container, and the time it took for the temperature of the material to become 500°C and 300°C were measured. Even though radiation thermometer is unable to measure a temperature below 300°C accurately, the cooling treatment was continued with the same conditions until the temperature of the coiled material became around 100°C or lower. As described in Tables 8 to 10 in detail, the average cooling rate from completion of extrusion to commencement of the cooling treatment, the starting temperature of the cooling treatment, the average cooling rates in temperature ranges from the starting temperature of the cooling treatment to 300°C and from 500°C to 300°C in Steps Nos. A1 to A5. A11H to A14H, A21 to A25, A31H to A34H, A41 to A44, A46H, and A47H are not necessarily the same. When the starting temperature of the cooling treatment was below 500°C, the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 300°C was recorded as the cooling rate in the temperature range from 500°C to 300°C. Incidentally, the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C can be easily calculated from the average cooling rates in temperature ranges from the starting temperature to 300°C and from 500°C to 300°C, and it was confirmed that the cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C is either the same as the cooling rate in the temperature range from 500°C to 300°C or slightly faster than that when the cooling treatment was performed with certain conditions. Upon completion of the cooling process after extrusion, extruded material was formed into straight bar with a diameter of 20 mm by a combined machine through drawing at a working ratio of 8.4% and straightness correction.

[0169] In Steps Nos. A25 and A31H, a bar obtained in Step No. A21 was further subjected to low-temperature annealing in a laboratory at 130°C for five hours then at 300°C for two hours. These materials of Steps Groups AA, AB, and AD were subjected to microscopic observation, cutting tests, dezincification corrosion tests, and tensile tests.Step Group AC; ø50-mm Bars(Step No. A50)

[0170] To trial-manufacture forging material with a machine on the actual production line, ø50-mm bars were prepared through Step Group AC (Step No. A50) described in Table 11., i.e., a billet with a diameter of 240 mm was produced with the low-frequency melting furnace and the semi-continuous casting machine used for the manufacture on the actual production line. The billet was cut to a length of 800 mm then extruded at 550°C into ø50-mm round bars with a direct extruder having a nominal capacity of 3000 tons. The extruded straight bars were placed on a steel table for natural cooling to normal temperature then subjected to straightness correction and provided as material for the flat forging described later. No special cooling treatment was performed since they were to be provided as a material of a product produced by a process in which hot forging is the final hot working step. The average cooling rate from 500°C to 300°C was 20°C / min.Material Extruded in Laboratory; Step Group B(Steps Nos. B1, B2, and B3H)

[0171] In Step Group B, raw material composed of components mixed at a specific ratio was melted in a laboratory as described in Table 12. The molten alloy was poured into a permanent mold with a diameter of 100 mm and a length of 200 mm to cast a billet. Incidentally, impurities such as Fe were further added to some of them intentionally. The concentration of the intentionally-added impurities was approximately the same as or lower than that included in leaded brass available in the market.

[0172] The billet was heated and extruded at 670°C into a ø20-mm round bar in the case of Step No. B1. Following the extrusion, cooling treatment was started when the temperature of the extruded bar was 540°C and performed at an average cooling rate of 960°C / min in the temperature range from 500°C to 300°C. It was confirmed that the average cooling rates in temperature ranges from 540°C to 500°C and from 500°C to 300°C were almost the same. Cooling was continued with the same conditions until the material's temperature became about 100°C or lower. In Step No. B3H, ø20-mm round bars were produced by extrusion with an extrusion temperature of 670°C and cooling at an average cooling rate of 40°C / min from 500°C to 300°C without performing any cooling treatment.

[0173] The round bars produced by Steps Nos. B1 and B3H were subjected to microscopic observation, cutting tests, dezincification corrosion tests, and tensile tests. Part of those produced by Step No. B3H were provided as hot forging material of Step Group D.

[0174] In Step No. B2, the billet was extruded at an extrusion temperature of 590°C into ø50-mm bars to be provided as forging material then cooled from 500°C to 300°C at an average cooling rate of 25°C / min without performing any cooling treatment.Step Group C; material for forging or hot compression: casting(Steps Nos. C1 and C2)

[0175] Raw material composed of components mixed at a specific ratio was melted in a laboratory to produce castings to be used as forging material. The molten alloy was casted in an iron mold with an inner diameter of 35 mm and a depth of 200 mm (Step No. C1) or with an inner diameter of 55 mm and a depth of 200 mm (Step No. C2) when the temperat ure of the molten alloy was 1000°C. The casting was taken out of the mold when its temperature became 700°C and left to cool naturally without being subjected to any cooling treatment.

[0176] As described in Table 13, the average cooling rate from 500°C to 300°C was 35°C / min in Step No. C1 and 23°C / min in Step No. C2. Castings produced with Step No. C1 were lathed until the outer diameter became 20 mm. Those produced with Step No. C2 were lathed until the outer diameter became 50 mm to be provided as forging material for use in Processes "D" and "E" and hot compression material for use in Step Group F.Forged Flare Nut(Step Group D)

[0177] In Step Group D, the extruded bars with a diameter of 20 mm prepared by Step Nos. A1, A13H, A21, A34H, or A47H with an extruder used for manufacturing products for sale, those prepared by Step No. B3H in a laboratory, and castings prepared by Step No. C1 in a laboratory were used as materials for forged flare nuts. The materials were cut into pieces so that each weighed 120±1 grams (119 to 121 grams) then formed into hexagonal flare nut forging materials having a width across flats of 27mm and a length of 26 mm with a 500-ton hot forging press.

[0178] As Table 14 shows, hot forging temperature was 670°C in Steps Nos. D1, D3 to D7, and D12H to D14H, 710°C in Step No. D2, 740°C in Step. No. D8, and 770°C in Step No. D15H, respectively. The average cooling rate from immediately after forging until commencement of the cooling treatment was 100°C / min in all these Steps except Steps Nos. D15H and D13H. In Step No. D15H, it was 120°C / min. In Step No. D13H, the forged material was left to cool naturally instead of subjected to the cooling treatment. The starting temperature of the cooling treatment was between 475°C and 695°C as Table 14 shows. The average cooling rates in the temperature ranges from the starting temperature of the cooling treatment to 300°C and from500°C to 300°C were varied between 240°C / min and 1980°C / min except Step No. D13H. In Steps Nos. D1 to D7 and D14H, the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C and that in the temperature range from 500°C to 300°C were the same, but in D8 and D15H, the former was slightly faster than the latter. In Step No. D12H, the average cooling rate from 475°C to 300°C was recorded as that in the temperature range from 500°C to 300°C.

[0179] In Step No. D7, hot forging and cooling treatment were performed with the same conditions as those in Step No. D5 then shot blasting was additionally performed for 10 minutes using ø0.6-mm steel balls.

[0180] These hot forged products were completed by machining as a single kind of flare nuts having an inner diameter of 5 / 8 inches (= 15.9 mm) and a shape conforming to JIS B 8607. The materials produced by Process D were subjected to microscopic observation, dezincification corrosion tests, and stress corrosion cracking tests.Flat Forging(Step Group E)

[0181] Step Group E is a process for forging products in a laboratory. In this process, the extruded bars prepared by Step No. A50 with an extruder used for manufacturing products for sale, those prepared by Step No. B2 in a laboratory, and castings prepared by Step No. C2 in a laboratory were used as forging materials. Each material was cut to a length of 180 mm, and the 180-mm bars were placed horizontally to be forged to a thickness of 20 mm with a press machine having a hot forging press capacity of 150 tons.(Steps Nos. E1 to E7 and E11H to E15H)

[0182] In Steps Nos. E1, E4 to E6, and E12H to E15H, hot forging temperature was 630°C. It was 680°C in Step No. E2, 700°C in Step No. E3, 720°C in Step No. E7, and 740°C in Step No. E11H. The average cooling rate after hot forging until commencement of the cooling treatment was 35°C / min except Step No. E14H. As Table 15 shows, the cooling treatment was started at a temperature between 690°C and 455°C, and the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 300°C was varied between 900°C / min and 200°C / min. The material was kept cooling with the same cooling conditions until its temperature became normal temperature. In Steps Nos. E1 to E6 and E15H, it was confirmed that the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C and that in the temperature range from 500°C to 300°C were approximately the same. With respect to Steps Nos. E12H and E13H, the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 300°C was recorded as that in the temperature range from 500°C to 300°C. In Step No. E14H, no cooling treatment was performed, and the forged materials were left to cool naturally instead. The average cooling rage from 500°C to 300°C was 25°C / min.

[0183] These flat-forged materials were cut and subjected to cutting tests, mechanical property tests, dezincification corrosion tests, and microscopic observation.Hot Compression Tests(Step Group F)

[0184] The hot extruded bars prepared by Steps Nos. A13H, A34H, and A47H with an extruder used for manufacturing products for sale, those prepared by Step No. B3H in a laboratory, and castings prepared by Step No. C1 were used as materials for test pieces, which were completed by lathing to have a diameter of 15 mm and a height of 27 mm. Then, as Table 16 shows, the materials were hot compressed at 650°C with a strain rate of 0.02 / sec to a height of 8 mm using an Amsler testing machine having a maximum testing force of 100kN and equipped with a heating furnace. In Steps Nos. F1 to F3, the hot-compressed test material was taken out of the heating furnace and cooled at a cooling rate of 50°C / min. Subsequently, a cooling treatment was started when the temperature of the test material was 560°C and performed at an average cooling rate of 2100°C / min in the temperature range from 500°C to 300°C. The treatment was continued after the temperature reached 300°C until normal temperature was reached. In Step No. F4H, the cooling treatment was started when the temperature of the test material was 560°C and performed at an average cooling rate of 200°C / min from the starting temperature of the cooling treatment to 300°C. In Step No. F5H, no special cooling treatment was performed but the material was cooled at an average cooling rate of 40°C / min from 500°C to 300°C. The average cooling rate in the temperature range from the starting temperature of the cooling treatment (560°C) to 500°C and that in the temperature range from 500°C to 300°C were approximately the same. These hot-compressed materials were subjected to microscopic observation and dezincification corrosion tests.Heat Treatment(Step Group G)

[0185] In Step Group G, the bars and the forged products obtained in Steps Nos. A1, A21, A41, A13H, A34H, A47H, D1, and D13H were subjected to heat treatment in a laboratory as Table 17 shows to investigate changes in their properties. In all cases, annealing was performed at 580°C for either 30 or 60 minutes. In Steps Nos. G1, G2, and G3, a cooling treatment was started after annealing when the temperature of the annealed material was 560°C and performed at an average cooling rate of 1800°C / min in the temperature range from the starting temperature of the cooling treatment to 300°C. The average cooling rates in the temperature ranges from the starting temperature of the cooling treatment (560°C) to 500°C and from 500°C to 300°C were approximately the same. In Steps Nos. G11H and G12H, after annealing at 580°C, the material was subjected to a cooling treatment which was started when its temperature was 470°C and performed at an average cooling rate of 1500°C / min from 470°C to 300°C. In Steps Nos. G13H and G14H, no special cooling treatment was performed after annealing at 580°C, but the materials were cooled with an average cooling rate from 500°C to 300°C being 25°C / min. These heat-treated materials were subjected to observation with a metallographic microscope. In the case the material was a bar, cutting tests, dezincification corrosion tests, and tensile tests were additionally performed. In the case it was a forged product, dezincification corrosion tests and stress corrosion cracking tests were additionally performed.(Comparative Material)

[0186] As comparative materials, ø20-mm bars made of C3604, leaded free-cutting brass containing 3% Pb, were prepared. In addition, ø50-mm and ø20-mm bars made of C3771, brass for forging containing 2 mass% Pb were prepared. These were denominated as Alloys X, Y, and Z respectively, and those available on the market were used. Hot forging and cooling treatment were performed on Alloy Y with the same conditions as those applied in Steps Nos. E14H and E1. Hot forging and cooling treatment were performed on Alloy Z with the same conditions as those applied in Steps Nos. D13H and D1. Alloy X was subjected to observation with a metallographic microscope, cutting tests, dezincification corrosion tests, and tensile tests. Alloy Y was subjected to observation with a metallographic microscope, cutting tests, dezincification corrosion tests, and tensile tests. Alloy Z was subjected to observation with a metallographic microscope, dezincification corrosion tests, and stress corrosion cracking tests.

[0187] The above-described test materials were evaluated for the following items. The evaluation results are shown in Tables 18 to 56.

[0188] The area ratio of β phase (before modification) is indicated in column "f4A" of each table separately from that of β1 phase (f4). When f4 = (area ratio of β1 phase) = 0, f6 was calculated applying the value of f4A = (area ratio of β phase) in the term of (β1) instead of the area ratio of β1 phase, and the resultant numerical value is indicated with "*" as a reference value of f6.(Observation of Metallographic Structure)

[0189] Metallographic structure was observed in the method described below then the area ratios (%) of respective phases such as α phase, β phase, and γ phase were measured by image analysis. If any grain boundary was observed inside β phase, the phase was recognized as β1 phase distinguishing from β phase. It was assumed that α' phase, β' phase, and γ' phase were included in α phase, β phase, and γ phase respectively. α phase is granular and oval, and often has twin crystals. β phase and β1 phase are present around granular and oval α phase. These three phases were distinguished based on the definitions stated above.

[0190] Each of the bars and forged products provided as test materials was cut parallel to the longitudinal direction or the flowing direction of the metallographic structure. Subsequently, the cut surface was polished (mirror-polished) and etched with a mixed solution of hydrogen peroxide and ammonia water. Specifically, an aqueous solution prepared by mixing 3 ml of 3 vol% hydrogen peroxide water and 22 ml of 14 vol% ammonia water was used for the etching. The polished metal surface was immersed in the aqueous solution for about 2 to 5 seconds under room temperature between about 15°C to about 25°C. Incidentally, in the case β1 phase was present, the immersion time in the etching process had to be extended to about 5 to 10 seconds, which is slightly longer than usual since corrosion resistance is better when β1 phase is present than when β phase is present.

[0191] The metallographic structure was observed with a metallographic microscope at a magnification of 500X to see the proportion of each phase. It was observed at a magnification of 1000X to check the phases and compounds depending on the state of the metallographic structure. Each phase (α phase, β phase, β1 phase, and γ phase) was manually painted in the micrographs of five visual fields using image processing software "Photoshop CC." With respect to the distinguishment between β phase and β1 phase, if any grain boundary was observed inside β phase at least in one location within one visual field, all the β phases within the same visual field were painted as β1 phase. Subsequently, the micrographs were binarized using image analysis software "WinROOF 2013" to obtain the area ratio of each phase. Specifically, the average area ratio of each phase in the five visual fields was calculated to determine the area ratio of each phase. In this calculation, the aggregate of the area ratio of each and every constituent phase excluding compounds containing P or Si, precipitates, oxides, particles of Pb or Bi, sulfides, and crystallized particles constitutes 100% of the area ratio.

[0192] When modified β phase is examined with a metallographic microscope after being etched with the previously-described etching solution, a grain boundary pattern or a boundary of crystal grain can be observed inside β phase. In the present invention, if such a crystal grain boundary is observed inside β phase, such β phase is regarded as β1 phase to distinguish it from ordinary β phase. Crystal grain boundary here refers to a linear pattern observed inside β1 phase that runs through β1 phase like the one shown in Fig. 1A.

[0193] In metallographic structure, sometimes compounds of P or Si and / or precipitates are present other than the phases. Compounds of P and Zn or Si can be found mainly inside β phase or β1 phase or at phase boundary between β phase and α phase in the form of blackish gray particles of about 0.5 to 2 µm when observed with a 500X metallographic microscope. Compounds of Fe, Mn, or Cr and P or Si can be observed as pale blue particles of about 1 to 5 µm. Particles of Pb or Bi and those in which Pb and Bi are compounded are present in the form of fine black particles of 2 µm or smaller. Most of these compounds and the like can be roughly distinguished with a metallographic microscope.

[0194] When any grain boundary was observed inside β1 phase (β phase) in the metallographic structure, "B" (present) was indicated in the Tables in which evaluation results of alloy's characteristics are recorded, and when none was observed, "D" (absent) was indicated in the Tables.

[0195] With respect to phosphorous compounds, when an alloy containing 0.06 mass% P was cooled at an average cooling rate of 15°C / min to 50°C / min from 500°C to 300°C, approximately 500 to 1000 phosphorous compounds were observed in a field of 500x magnified view (in an area of 80 mm by 120 mm if printed). However, it was hard to distinguish phosphorous compounds from other compounds or particles such as compounds of Fe, Mn, or Cr and P or Si and Pb particles in a printed image of metallographic structure. Therefore, when a printed picture of metallographic structure was used for the distinguishment, if the total number of phosphorous compounds and compounds that could be phosphorous compounds was less than 50 in such a picture of field of view, it was evaluated that phosphorous compounds were not present, and "D" (absent) was indicated as an evaluation result. In the case the number was 50 or more but less than 300, it was evaluated that phosphorous compounds were present but only in a small quantity, and "C" (fair) was indicated as an evaluation result. In the case the number was 300 or more, it was evaluated that phosphorous compounds were present, and "B" was indicated as an evaluation result. Whether phosphorous compounds are present, and the quantity thereof make indicators to know if an appropriate treatment was performed.

[0196] When it was difficult to identify any phases, compounds, or precipitates, an electron backscattering diffraction pattern (FE-SEM-EBSP) method was employed using a field emission scanning electron microscope (FE-SEM) (JSM-7000F, manufactured by JEOL Ltd.) and its accessory EDS with the acceleration voltage set to be 15 kV and the electrical current value set to be 15 to identify them at a magnification of 500X or 2000X.(Electrical Conductivity)

[0197] Electrical conductivity was measured with an electrical conductivity measurement device manufactured by Foerster Japan Ltd. (SIGMATEST; D2.068). Incidentally, in this specification, the terms "electric conductivity" and "electrical conductivity" are meant to have the same meaning. Further, thermal conductivity and electrical conductivity are closely related. The higher the electrical conductivity, the better the thermal conductivity.(Tensile Strength / Elongation)

[0198] Each of the test materials was processed into test piece No. 10 in accordance with JIS Z 2241 to measure their tensile strength and elongation.

[0199] If a hot extruded material or a hot forged material not having undergone any cold working step has a tensile strength of preferably 460 N / mm 2< or higher, more preferably 490 N / mm 2< or higher, and still more preferably 520 N / mm 2< or higher, the material is considered to have the highest level of tensile strength among free-cutting copper alloys. Therefore, reducing wall thickness or weight of components used in various fields, or increasing allowable stress can be attempted by applying these materials. In addition, regarding the balance between strength and elongation, as a yardstick, when the tensile strength is represented by S (N / mm 2< ) and the elongation is represented by E (%), if the value of the characteristic relational expression f7 = S × {(E + 100) / 100} 1 / 2< which indicates the balance between strength and ductility of the alloy is preferably 540 or higher, the alloy can be considered to have high strength and high ductility. The value of f7 is more preferably 570 or higher, and still more preferably 600 or higher. If the f7 value of an alloy is 600 or higher, it can be said that the alloy has a very high standard of balance between strength and elongation among free-cutting copper alloys. Many of the parts of tap fitting, valve, joint, pressure vessel, air conditioner, and freezer are made of a hot-forged or hot-extruded bar, and C3604, a leaded copper alloy currently used for such parts, has a tensile strength of about 390 to 420 N / mm 2< and an elongation of 30 to 35%. The alloy's f7 value is 470. The embodiments of the present invention are able to reduce the weights of such parts since the embodiments have improved strength and good dezincification corrosion resistance and resistance to stress corrosion cracking.<Machinability Testing with Lathe>

[0200] Machinability was evaluated by the cutting test using a lathe as described below.

[0201] To prepare test materials, first, hot extruded bars (Step Groups "AA", "AB", and "B") and hot forged materials (Step Group E) were machined to a diameter of 18 mm. A carbide tool (an insert) of K10 not equipped with chip breaker was attached to a lathe. Using this lathe, the circumferences of the ø18-mm test materials were machined on a dry condition with a rake angle of 0°, a nose radius of 0.4 mm, a clearance angle of 6°, a cutting speed of either 40 m / min or 110 m / min, a cutting depth of 1.0 mm, and a feed rate of 0.11 mm / rev. The effect of cutting speed on the embodiments of the present invention was also studied.

[0202] A signal emitted from a dynamometer (AST tool dynamometer AST-TL1003 manufactured by Mihodenki Co., Ltd.) composed of three parts attached to the tool was converted into an electrical voltage signal and recorded on a recorder. Subsequently, these signals were converted into cutting resistance (principal cutting force, feed force, and thrust in N). In order to suppress the influence of wear on the insert, the cutting test was performed in the order of test pieces A → B → C then C → B → A. This cycle was repeated twice so that each test piece was measured four times. The cutting resistance was calculated by the following expression.

[0203] It should be noted here that principal force is the largest among the components of cutting resistance, and cutting resistance is mostly decided by the size of principal force. Therefore, the mean value of the principal force that was measured four times was calculated and adopted as the cutting resistance of the test piece.

[0204] Assuming that the cutting resistance (principal force) of a commercially available free-cutting brass bar, C3604 (Alloy X; ø20 mm), made of an alloy consisting of 59 mass% Cu, 3 mass% Pb, 0.2 mass% Fe, 0.3 mass% Sn, and Zn as the balance was 100, a relative value of the cutting resistance (machinability index) of each test piece was calculated for relative evaluation. That is, the higher the machinability index, the lower the cutting resistance and the better the machinability.

[0205] Incidentally, the machinability index was calculated as follows.

[0206] Cutting resistance (principal force) of a material depends on the shear strength or the tensile strength of the material, and there is a tendency that the higher the strength, the higher the cutting resistance. For instance, in the case of a copper alloy with chip breakability equivalent to that of C3604 and approximately 1.2 times the strength of C3604, its cutting resistance is about 20% higher than that of C3604 since cutting resistant is proportionate to the strength of material. Thus, if a high strength copper alloy has cutting resistance of approximately 40% higher than that of C3604, the copper alloy is regarded as having no practical problem. As embodiments of the present invention in the form of extruded material have about 1.2 times the tensile strength and the shear strength of C3604, in the evaluation of their machinability, machinability index of about 72 was regarded as the standard (boundary value). Specifically, if the machinability index of a material was 80 or higher, the material was evaluated as having excellent machinability (evaluation: A), i.e., equivalent to that of C3604. When it was 72 or higher and lower than 80, the material was evaluated as having good machinability (evaluation: B). When it was 66 or higher and lower than 72, the material was evaluated as having fair machinability (evaluation: C). When it was lower than 66, the material was evaluated as having poor machinability (evaluation: D). As the present invention aims to obtain good machinability, those with a machinability index of 72 or higher were determined as acceptable.(Dezincification Corrosion Test: Dezincification Corrosion Test of ISO 6509)

[0207] As a test method for the evaluation of dezincification corrosion resistance, the dezincification corrosion test of ISO 6509 was employed. This test method is adopted in many countries and is specified in Japanese Industrial Standards under JIS H 3250 as well.

[0208] In accordance with the procedure of the dezincification corrosion test, first, a test material was imbedded in a phenolic resin material specifically so that its surface to be exposed to test solution was perpendicular to the direction in which the extruded material had been extruded. The surface of the test piece was polished with emery paper No. 1200 then subjected to ultrasonic cleaning in pure water and drying.

[0209] Each test piece was immersed in an aqueous solution containing 1.0% copper (II) chloride dihydrate (CuCl 2 · 2H 2 O) and held at 75°C for 24 hours then taken out of the aqueous solution.

[0210] The test piece was once again imbedded in a phenolic resin material so that its exposed surface maintained a right angle to the extrusion direction, longitudinal direction, or the direction to which crystal grains moved during forging. Subsequently, it was cut so that the cross section included the longest possible corroded portion then polished.

[0211] Corrosion depth was observed at 10 visual fields with a metallographic microscope at a magnification of 100x to 500x. The depth of the deepest point of corrosion was recorded as the maximum corrosion depth.

[0212] Apropos, if the maximum corrosion depth is 200 µm or less, it is considered that there is no problem in corrosion resistance in practical use. It is generally understood that the maximum corrosion depth is preferably 100 µm or less in the case excellent corrosion resistance is required.

[0213] In this testing also, when the maximum corrosion depth exceeded 200 µm, the evaluation was "D" (poor) which stands for inacceptable. When it was over 100 µm but 200 µm or less, the evaluation was "B" (good). When it was 100 or less, the evaluation was "A" (excellent).(Stress Corrosion Cracking Test)

[0214] In order to determine the resistance to stress corrosion cracking in an environment where stress corrosion cracking easily occurs, a stress corrosion cracking test was performed with the following procedure.

[0215] First, to prepare test pieces, ø20-mm bar was cut to weigh 120±1g (119 to 121 grams) and the cut pieces were formed into forged hexagonal flare nut materials having a width across flats of 27 mm and a length of 26 mm using a 500-ton hot forging press with the conditions of Step Group D indicated in Table 14. Further, some of the test pieces were subjected to shot blasting in the same manner as in Step No. D7as indicated in Table 14, and heat treatment was performed on the forged materials with several conditions (Steps Nos. G2, G3, G12H, and G14H) as indicated in Table 17. These forged materials were completed as a single kind of flare nuts having an inner diameter of 5 / 8 inches (= 15.9 mm) and a shape conforming to JIS B 8607 by forming threads and smoothing end surfaces in the process of machining. In order to apply stress to the flare nuts, a base made of C6932 with part of its body machined into a shape of male screw also conforming to JIS B 8607 with a nominal diameter of 5 / 8 inches (= 15.9 mm) was prepared. Test piece was tied to the base until a specific torque was reached which was measured with a torque gauge. Flare nut to which stress had been applied was placed in a desiccator filled with ammonia water containing a certain concentration of ammonia at a location 60 mm away from the water surface. After holding in the ammonia water for a certain period of time, the flare nut was taken out. The testing was performed in a room where the atmospheric temperature was controlled with an air conditioner to 25±1°C (24°C to 26°C).

[0216] The stress corrosion cracking test was performed in the following two methods.(Stress Corrosion Cracking Test -1)

[0217] Ammonia water containing ammonia at a concentration of 14 vol% was put in a desiccator in the quantity of 1000 ml then test pieces with a torque of 70 N·m being applied were exposed to the atmosphere in the desiccator for 48 hours and for 96 hours. The test pieces were washed with a solution containing 10 vol% sulfuric acid and subjected to visual observation for evaluation based on whether any cracks were present. This testing was performed in accordance with JIS H 3250 except exposure time. The exposure time in this testing was 20 times longer than that specified in JIS H 3250 which is two hours. That means the testing condition was quite severe. As the present invention is directed to obtaining good resistance to stress corrosion cracking, when a crack was found after exposure of 48 hours, the evaluation was "D" (poor). If crack was not observed after 48 hours of exposure but was observed after 96 hours, the evaluation was "C" (fair). If no crack was observed after 96 hours, the evaluation was "B" (good). Incidentally, those evaluated as "C" were determined as acceptable.(Stress Corrosion Cracking Test -2)

[0218] In another way of stress corrosion cracking test, 200 ml of ammonia water containing ammonia at a concentration of 28 vol% was put in a desiccator. Test pieces to which a torque of 150 N·m was applied were exposed to the ammonia atmosphere in the desiccator for 75 hours. After that, they were taken out of the desiccator and washed with sulfuric acid solution containing 10 vol% sulfuric acid then subjected to visual examination for cracks. As the concentration of ammonia was higher and the torque applied was larger than those in Stress Corrosion Cracking Test -1, the evaluation criteria were set to be as follows. If the size of the largest crack in a test piece that was clearly visible with eye was half the length of the test piece (26 mm) or larger, i.e., 13 mm or larger, the test piece was evaluated as "D" (poor). If a test piece had any crack with a size of about 1 / 5 the length of the test piece (26 mm) or larger, i.e., 5 mm or larger but smaller than 13 mm that was clearly visible with eye, the evaluation was "C" (fair). If any crack smaller than about 1 / 5 of the length of the test piece (26 mm), i.e., smaller than 5 mm was observed or no crack was visible with eye, the evaluation was "B" (good). Incidentally, those evaluated as "C" were regarded acceptable. [Table 18]Test No.Alloy No.Material No.Step No.MetallographicStructuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase1-A149510059DB2-A246540062DB3-A350500058DB4-A453470056DB5-A557430051CB6-A11H62038047*BD7-A12H66034042*BD8-A13H68032040*BD9-A14H64036045*CD10S01A1D149510059DB11A13HD241590067DB12A1D456440053CB13A13HD554460055DB14A13HD754460055DB15A13HD825750083DB16A13HD12H65035044*BD17A1D13H67033040*BD18A13HD14H64036045*CD19A1D15H13087094*DD20A50E155450053DB [Table 19] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase21A50E249510059DB22A50E337630071DB23A50E459410050CB24A50E553470056DB24.1A50E727730081DB25A50E11H18820089DB26A50E12H64036045*BD27A50E13H67033041*BD28A50E14H69031040*BD29S01A50E15H65035044*CD30A13HF250500058DB31A13HF4H58042051*CD32A13HF5H64036045*BD33A13HG150500058DB34D1G251490057DB35D13HG368320040DB36A1G11H64036045*BD37D1G12H66034043*BD38A1G13H68032040*BD39D1G14H70030039*BD [Table 20] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase41-A2139610070DB42-A2243570065DB43-A2347530062DB44-A2450500059CB45-A2543570065DB46-A31H44056065*DD47-A32H55045054*BD48-A33H58042051*BD49S02-A34H59041050*BD50A21D140600069DB51A34HD232680077DB52A21D447530062DB53A34HD544560064DB54A34HD745550063DB56A34HD12H55045054*BD57A21D13H60040050*BD58A34HD14H55045054*DD59A50E146540063DB [Table 21] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase60A50E240600069DB61A50E328720080DB62A50E450500059CB63A50E544560064DB63.1A50E721790087DB64A50E11H9091099*DD65A50E12H55045054*BD66A50E13H58042051*BD67A50E14H59041050*BD68S02A50E15H56044053*CD69A34HF241590067DB70A34HF4H49051060*CD71A34HF5H55045054*BD72A34HG141590067DB73D1G242580066DB74D13HG359410050DB75A1G11H56044053*BD76D1G12H57043052*BD77A1G13H61039048*BD78D1G14H62038047*BD [Table 22] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase81S11A50E141590067DB82E235650073DB83E324760084DB84E445550064CB85E540600068DB86E11H40960103*DD87E12H50050059*BD88E13H53047055*BD89E14H54046054*BD90E15H51049057*CD91S12A50E169310039DB92E263370045DB93E352480055DB94E472280037CB95E567330042DB95.1E743570065DB96E11H33670074DB97E12H78022031*BD98E13H81019027*BD99E14H82018026*BD100E15H80020028*CD [Table 23] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase101E148520063DB102E241590071DB103E330700082DB104E452480058CB105E546540065DB105.1S13A50E723770089DB106E11H110890102*DD107E12H56044055*CD108E13H60040051*BD109E14H62038049*BD110E15H59041052*CD [Table 24] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase121-A4154460052DB122-A4264360043DB123-A4338620068DB124-A4457430049DB125-A46H19810085DB126-A47H86014022*BD127A47HD256440051DB128A47HD770300037DB129A47HD840600066DB130S03A41D13H84016023*BD131A47HD15H25750080DB132A50E172280036DB133A50E355450052DB134A50E744560062DB135A50E14H85015023*BD136A47HF267330040DB137A47HF5H82018027*BD138A47HG166340041DB139D13HG368320039DB140A41G13H87013022*BD141D2G14H8801209*BD [Table 25] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase201S21B3HD350500056DB202B2E246540060DB203S22-B153470057DB204-B3H68032042*BD205B3HD352480058DB206S23-B157430043DB207S24C1F143570068DB [Table 26] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase208S25-B154460053CB209-B3H67033040*BD210B3HD350500056DB211D3D750500056DB212S26C1F147530059CB213S27C1F157430053DB214S28C1D654460053DB215C2E654460053DB216S29C1F143570065DB217S30C1F145550060DB218S31-B156440050DB219-B3H70030036*CD220B3HF352480054DB221S32-B155450055DB222S33-B150500055DB223S34C1F144560060DB224S35C1F128720071DB225S36C1F171290034DB [Table 27] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase226S37-B143570055DB227-B3H57043044*BD228B3HD342580056DB229D3D741590056DB230S38-B168310140DB231S39C1F139610070DB232S40C1D650500057DB233C2E646540061DB234S41C1F146540058DB235S42-B158420050DB236-B3H72028036*BD237B3HD356440051DB [Table 28] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase238S43C1D646540063DB239C2E642580067DB240S44-B163370039DB241-B3H77023027*BD242B3HD362380040DB243D3D762380040DB244S45C1F150500056DB245S46C1F147530061DB246S47C2E673270036DB247S48B144560051DB248S49B174260029DB [Table 29] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase301S71-B157043048*DD302-B3H71029035*CD303B3HD353047052*DD304D3D753047052*DD305S72-B180200029DB306S73-B163370038DB307S74-B150050043*DD308-B3H64036034*CD309B3HD348052044*DD310S75-B136640077CB311S76C1F116084083*DD312S77-B122078070*DD313S78-B166290535DB314S79C1F180200025DB315S80-B143057035*DD316-B3H57043029*DD317B3HD341059036*DD318D3D741059036*DD [Table 30] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase319S81-B177230024DB320S82-B163330436DB321-B3H75020521*BD322B3HD361360341DB323S83C1F150049147*DD324S84-B170300037CB325-B3H80020027*BD326B3HD367330040CB327S85C1F135065067*CD500X--850150-DD501Y-E14H790210-DD502-E1670330-DD503Z-D13H780220-DD504-D1640360-DD [Table 31] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation1-A117.16232068284A81A2-A217.16381969684A81A3-A317.36142067384A80A4-A417.26172167983A77B5-A517.36122267682A78B6-A11H17.05872365181A71C7-A12H17.45852465179B68C8-A13H17.55542662278B66C9-A14H17.35762464178B65D10S01A1D117.2-------11A13HD217.1-------12A1D417.2-------13A13HD517.3-------14A13HD717.2-------15A18HD817.5-------16A13HD12H17.3-------17A1D13H17.5-------18A13HD14H17.3-------19A13HD15H17.420A50E117.35483463483A80A [Table 32] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation1-A180A--2-A290A--3-A390A--4-A4160B--5-A5150B--6-A11H290D--7-A12H360D--8-A13H450D--9-A14H390D--10S01A1D180ABB11A13HD270ABC12A1D4160BBC13A13HD580ABC14A13HD780ABB15A18HD8130BBC16A13HD12H260DCC17A1D13H430DCC18A13HD14H360DCC19A13HD15H350DCD20A50E1110B-- [Table 33] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation21A50E217.15583364484A82A22A50E317.15602662984A81A23A50E417.05273461082A77B24A50E517.25363361882A77B24.1A50E717.35612462585A76B25A50E11H17.45602161683A71c26A50E12H17.25233560881A71c27A50E13H17.15093759679B68c28A50E14H17.25003858777B66C29S01A50E15H17.35143459577B67c30A13HF217.3-------31A13HF4H17.1-------32A13HF5H17.0-------33A13HG117.2---82A75B34D1G217.3-------35D13HG317.2-------36A1G11H17.1---77B63D37D1G12H17.4-------38A1G13H17.3---73B61D39D1G14H17.2------- [Table 34] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation21A50E280A--22A50E390A--23A50E4160B--24A50E5150B--24.1A50E7150B--25A50E11H200B--26A50E12H280D--27A50E13H390D--28A50E14H440D--29S01A50E15H400D--30A13HF270A--31A13HF4H360D--32A13HF5H410D--33A13HG1110B--34D1G2100ABB35D13HG3120BBB36A1G11H340D--37D1G12H310DCC38A1G13H430D--39D1G14H420DCC [Table 35] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation41-A2116.86411869685A81A42-A2216.86301968785A82A43-A2317.06222068184A75B44-A2417.16232068283A77B45-A2516.86241968184A81A46-A31H16.76061865880A70C47-A32H16.95972265982A71C48-A33H17.05902465781A69C49S02-A34H16.95862365082A69C50A21D116.9-------51A34HD216.8-------52A21D417.1-------53A34HD516.9-------54A34HD716.8-------56A34HD12H16.9-------57A21D13H17.1-------58A34HD14H17.0-------59A50E117.05553263885A82A [Table 36] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation41-A2190A--42-A22100A--43-A23160B--44-A24140B--45-A25100A--46-A31H300D--47-A32H290D--48-A33H380D--49S02-A34H460D--50A21D190ABB51A34HD2110BBC52A21D4160BBC53A34HD590ABC54A34HD790ABB56A34HD12H260DCC57A21D13H440DCC58A34HD14H390DCC59A50E1100A-- [Table 37] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation60A50E217.15633064286A83A61A50E316.85702363285A81A62A50E416.75383161684A77B63A50E516.95473062483A77B63.1A50E716.85682262784A74B64A50E11H16.95651660982A68C65A50E12H17.05303361183A72B66A50E13H16.85273461082A70C67A50E14H16.75183560282A69C68S02A50E15H16.95383261880A68C69A34HF217.0-------70A34HF4H16.8-------71A34HF5H16.7-------72A34HG116.9---82A77B73D1G217.0-------74D13HG316.9-------75A1G11H16.9---79B65D76D1G12H17.0-------77A1G13H17.1---76B62D78D1G14H17.0------- [Table 38] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation60A50E290A--61A50E3130B--62A50E4160B--63A50E5150B--63.1A50E7180B64A50E11H340D--65A50E12H280D--66A50E13H400D--67A50E14H460D--68S02A50E15H420D--69A34HF290A--70A34HF4H360D--71A34HF5H400D--72A34HG1100A--73D1G2120BBB74D13HG3100ABB75A1G11H350D--76D1G12H300DCC77A1G13H450D--78D1G14H430DCc [Table 39] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation81S11A50E117.15602963685A80A82E217.25792865586A82A83E317.35752363884A78B84E417.15413362484A78B85E517.15493263184A76B86E11H17.55801461980A64D87E12H17.15303461483A72B88E13H17.25283360982A71C89E14H17.25223360283A70C90E15H17.15333261282A70C91S12A50E117.45164261580A78B92E217.55294062682A79B93E317.65403562785A81A94E417.54944359177B74B95E517.45104160680A74B95.1E717.75473062486A82A96E11H17.75622763384A80A97E12H17.54754056271C63D98E13H17.44604154667C58D99E14H17.54534454464D56D100E15H17.44634155066C56D [Table 40] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation81S11A50E1120B--82E2110B--83E3140B--84E4150B--85E5170B--86E11H400D--87E12H270D--88E13H350D--89E14H440D--90E15H370D--91S12A50E1110B--92E280A--93E380A--94E4150B--95E5150B--95.1E770A--96E11H120B--97E12H230D--98E13H280D--99E14H430D--100E15H380D-- [Table 41] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation101E115.65613364785A82A102E215.75763165986A82A103E315.85802665185A81A104E415.65493563884A78B105E515.75533464085A76B105.1S13A50E716.05742363785A77B106E11H16.05841662982A68c107E12H15.65433462982A71C108E13H15.75343461882A70C109E14H15.75303361181A70C110E15H15.65403462580A68C [Table 42] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation101E1110B--102E290A--103E3120B--104E4160B--105E5160B--105.1S13A50E7150B--106E11H340D--107E12H260D--108E13H390D--109E14H460D--110E15H400D-- [Table 43] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation121-A4117.66252168886A83A122-A4217.46102367785A81A123-A4317.76321968986A82A124-A4417.56032066182A77B125-A46H17.86171465984A73B126-A47H17.45332860363D54D127A47HD217.6-------128A47HD717.5-------129A47HD817.8-------130S03A41D13H17.4-------131A47HD15H17.9-------132A50E117.54974559880A76B133A50E317.65423863785A82A134A50E717.85563063486A82A135A50E14H17.24504554262D53D136A47HF217.5-------137A47HF5H17.3-------138A47HG117.4---80A75B139D13HG317.5-------140A41G13H17.2---64D55D141D2G14H17.1------- [Table 44] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation121-A4170A--122-A4260A--123-A4380A--124-A44130B--125-A46H190B--126-A47H400D--127A47HD260ABB128A47HD770ABB129A47HD880ABC130A41D13H420DCC131S03A47HD15H140BBC132A50E1110B--133A50E380A--134A50E7100A--135A50E14H440D--136A47HF260A--137A47HF5H410D--138A47HG180A--139D13HG390ABB140A41G13H360D--141D2G14H330DCC [Table 45] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation201S21B3HD317.9-------202B2E217.95403262084A80A203S22-B116.95303361185A82A204-B3H17.14943657678B68C205B3HD317.0-------206S23-B119.15163459780A75B207S24C1F115.2------- [Table 46] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation201S21B3HD390ABB202B2E2100A--203S22-B190A--204-B3H440D--205B3HD380A--206S23-B1130B--207S24C1F190A-- [Table 47] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation208S25-B117.75283160482A76B209-B3H17.74923557277B66c210B3HD317.6-------211D3D717.6-------212S26C1F117.9-------213S27C1F116.5-------214S28C1D617.6-------215C2E617.55333461783A78B216S29C1F117.2-------217S30C1F117.2-------218S31-B117.65053458579B73B219-B3H17.54543853372B62D220B3HF317.7-------221S32-B116.75283360986A81A222S33-B116.95303361180A76B223S34C1F118.5-------224S35C1F119.4-------225S36C1F117.5------- [Table 48] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation208S25-B1170B--209-B3H470D--210B3HD3150BBC211D3D7140BBB212S26C1F1150B--213S27C1F1130B--214S28C1D6140BBC215C2E6130B--216S29C1F1120B--217S30C1F1110B--218S31-B1180B--219-B3H480D--220B3HF3150B--221S32-B1120B--222S33-B1110B--223S34C1F1120B--224S35C1F1160B--225S36C1F190A-- [Table 49] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation226S37-B120.35152958579B74B227-B3H20.54673253775B64D228B3HD320.2-------229D3D720.2-------230S38-B115.45222859180A75B231S39C1F116.4-------232S40C1D617.9-------233C2E617.85433262485A81A234S41C1F118.4-------235S42-B117.35253460883A80A236-B3H17.44603653674B65D237B3HD317.2------- [Table 50] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation226S37-B1140B--227-B3H460D--228B3HD3160BBC229D3D7150BBB230S38-B1150B--231S39C1F160A--232S40C1D640ABB233C2E650A--234S41C1F160A--235S42-B130A--236-B3H400D--237B3HD330ABB [Table 51] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation238S43C1D616.9-------239C2E616.95503263285A82A240S44-B119.55113860080A75B241-B3H19.44453852366C58D242B3HD319.5-------243D3D719.5-------244S45C1F117.6-------245S46C1F117.2-------246S47C2E617.04884157978B74B247S48-B121.24913056078B73B248S49-B119.64724256277B73B [Table 52] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation238S43C1D640ABB239C2E640A--240S44-B160A--241-B3H420D--242B3HD350ABC243D3D750ABB244S45C1F120A--245S46C1F170A--246S47C2E680A--247S48-B1170B--248S49-B1110B-- [Table 53] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation301S71-B117.94943357068C58D302-B3H18.14543853367C57D303B3HD318.0-------304D3D718.0-------305S72-B115.84663954969C64D306S73-B118.55073759374B70c307S74-B122.34723154068c59D308-B3H22.54443651866C57D309B3HD322.3-------310S75-B115.45051754684A80A311S76C1F118.7-------312S77-B121.05211756474B62D313S78-B113.65062356177B70c314S79C1F117.7-------315S80-B124.84503051357D53D316-B3H25.04323450056D51D317B3HD324.9-------318D3D724.9------- [Table 54] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation301S71-B1500D--302-B3H480D--303B3HD3470DCC304D3D7470DCC305S72-B1100A--306S73-B1110B--307S74-B1340D--308-B3H470D--309B3HD3380DCD310S75-B1100A--311S76C1F1330D--312S77-B1510D--313S78-B1310D--314S79C1F1100A--315S80-B1530D--316-B3H490D--317B3HD3520DCD318D3D7520DCD [Table 55] Test No.Alloy No.Material No.Step No.Metal. StructurePropertyMachinabilityf3Electrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation319S81-B17720.74444152767C61D320S82-B16315.65161755876B68c321-B3H7515.74862053270C59D322B3HD36115.4-------323S83C1F15020.0-------324S84-B17018.24982355276B68C325-B3H8018.44513652660D55D326B3HD36718.1-------327S85C1F13518.8-------500X--8526.040333465100A100A501Y-E14H7926.14183248093A92A502-E16725.84302748592A92A503Z-D13H7825.9-------504-D16425.7------- [Table 56] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation319S81-B1160B--320S82-B1250D--321-B3H380D--322B3HD3230DCC323S83C1F1340D--324S84-B1190B--325-B3H470D--326B3HD3210DCC327S85C1F1390D--500X--500D--501Y-E14H480D--502-E1520D--503Z-D13H510DCC504-D1570DCD

[0219] 1) The inventors were able to confirm that a hot-extruded material or a hot-forged material with good machinability, good dezincification corrosion resistance, and good resistance to stress corrosion cracking, high electrical conductivity of 15% IACS or higher, high strength, good ductility, and superb balance between strength and ductility (property relational expression f7) could be obtained by satisfying the composition of an embodiment of the present invention, composition relational expressions f0 to f2, the requirements related to metallographic structure, metallographic structure relational expressions f3 to f5, and metallographic structure and composition relational expression even if the Pb content was small. The inventors were further able to confirm that even though presence of phosphorous compound has an effect of significantly improving machinability, modified β phase, i.e., β1 phase, had even better machinability improvement effect. At the same time, due to the modification of β phase, dezincification corrosion resistance and resistance to stress corrosion cracking that were serious problems of conventional β phase dramatically improve as described below (Alloys Nos. S01, S02, S03, S11, S12, S13, and S21 to S46). 2) β phase became modified by containing more than 0.50 mass% Si, and grain boundary came to be observed inside β1 phase. As a result of the modification, dezincification corrosion resistance and resistance to stress corrosion cracking improved, and cutting resistance decreased (i.e., machinability index increased). When the Si content was 0.65 mass% or higher, better dezincification corrosion resistance, resistance to stress corrosion cracking, and machinability were obtained, suggesting that modification of β phase further advanced. On the other hand, electrical conductivity decreased when the Si content was higher than 1.20 mass% (Alloys Nos. S01, S37, S48, S49, S74, S78, S80, etc.). 3) β phase became modified by containing higher than 0.01 mass% P, and a grain boundary was observed inside β1 phase. As a result of the modification, dezincification corrosion resistance and resistance to stress corrosion cracking improved, and cutting resistance decreased. When the P content was increased to 0.03 mass% or higher, dezincification corrosion resistance, resistance to stress corrosion cracking, and machinability further improved, suggesting that modification of β phase advanced more. When the P content exceeded 0.18 mass%, elongation decreased (Alloys Nos. S01, S31, S71, S75, etc.). 4) β phase became modified by containing more than 60.5 mass% Cu, and a grain boundary was observed inside β1 phase. As a result of the modification, dezincification corrosion resistance and resistance to stress corrosion cracking improved, ductility also improved, and cutting resistance decreased. When a large amount of Cu was contained, proportion of β phase was small and γ phase appeared in some cases, resulting in a decrease in the balance between strength and ductility and poor machinability (Alloys Nos. S01, S35, S38 S47, S48, S72, and S80; Figs. 1A to 6A). 5) When the Pb content was lower than 0.002 mass%, machinability was poor (Alloy No. S73). When the Pb content was increased to 0.003 mass% or higher, or further, 0.01 mass% or higher, machinability improved (Alloys Nos. S33 and S23). It was confirmed that containing Bi had an effect of reducing cutting resistance equivalent to that of Pb (Alloys Nos. S01, S32, etc.). 6) When Sn was contained in excess of 0.05 mass%, dezincification corrosion resistance improved. When the Sn content was increased to higher than 0.10 mass%, dezincification corrosion resistance further improved, and resistance to stress corrosion cracking also improved. On the other hand, when 0.90 mass% or higher Sn was contained, γ phase appeared causing dezincification corrosion resistance to rather deteriorate than improve, ductility to decrease, and cutting resistance to increase (i.e., machinability index decreased). In addition, when the Sn / Si ratio was higher than 1, β phase failed to be modified and dezincification corrosion resistance was poor (Alloys Nos. S01, S39 to S46, S82, and S83). 7) It was confirmed that inclusion of inevitable impurities did not seriously affect various properties of alloy if their contents were approximately the same as those included in the products manufactured on the actual production line (Alloys Nos. S01, S02, S03, S11 to S13, and S27 to S30). 8) When the total content of Fe, Mn, Co, and Cr as inevitable impurities was 0.40 mass% or higher, cutting resistance was high, the balance between strength and ductility was poor, and dezincification corrosion resistance was not so good even though β phase was modified. When the total content was less than 0.40 mass%, cutting resistance decreased, the balance between strength and ductility improved, and dezincification corrosion resistance and resistance to stress corrosion cracking were good. When the total content was less than 0.30 mass%, cutting resistance further decreased, the balance between strength and ductility further improved, and dezincification corrosion resistance and resistance to stress corrosion cracking improved. It is considered that the decreases in cutting resistance, the balance between strength and ductility, and dezincification corrosion resistance occurred due to the following reasons. Fe, Mn, and / or other elements compounded with part of Si and / or P to form intermetallic compounds of Fe, Mn, and / or other elements and Si and / or P. Thus, compounds of Fe and / or other elements and Si and / or P were present, and the concentrations of Si and P required to be effective for the modification of β phase dropped, resulting in deterioration of machinability and corrosion resistance (Alloys Nos. S01, S02, S84, and S25 to S30). 9) When the amount of Al as an inevitable impurity was 0.30 mass% or higher, modification of β phase did not occur, which resulted in poor dezincification corrosion resistance. When the amount of Al was less than 0.30 mass%, β phase was modified and dezincification corrosion resistance was good. When the amount of Al was less than 0.15 mass%, machinability, the balance between strength and ductility, dezincification corrosion resistance, and resistance to stress corrosion cracking were all good (Alloys Nos. S01, S85, S26, and S28 to S30). 10) When the value of the composition relational expression f1 was small, modification of β phase did not occur, and dezincification corrosion resistance was poor. When the value of the composition relational expression f1 was large, the area ratio of β1 phase was small (Alloys Nos. S76 and S79). When the value of the composition relational expression f1 was 57.5 or larger, β phase was modified and dezincification corrosion resistance improved. When the value was 58.0 or larger, dezincification corrosion resistance improved more. On the other hand, when the value of the composition relational expression f1 was 60.5 or smaller, the area ratio of β1 phase increased. When the value was 60.2 or smaller, the area ratio further increased, and the machinability index became higher (e.g., Alloys Nos. S01, S03, S35, and S36). 11) When the area ratio of β1 phase was 25% or smaller, good machinability was not obtained. When the area ratio was larger than 25%, and further, 30% or larger, cutting resistance was low. When the area ratio of β1 phase exceeded 80%, however, β phase often failed to be modified, i.e., no grain boundary was observed inside the β phase. As a result, cutting resistance was high especially when machined at 110 m / min and dezincification corrosion resistance and resistance to stress corrosion cracking were poor. When the area ratio of β1 phase was 80% or smaller, and further, 70% or smaller, β phase was modified and dezincification corrosion resistance and resistance to stress corrosion cracking improved. Even though progress of dezincification corrosion was significantly delayed by the modification of β phase, β1 phase was more corroded by dezincification than α phase. For this reason, when the area ratio of β1 phase was large, dezincification corrosion was slightly expedited (Alloys Nos. S03, S11, S72, S81, and S76; Steps Nos. D1, D11H, E1, and E11H; Fig. 1B, etc.). 12) When the area ratio of γ phase was 4% or larger, elongation was low and cutting resistance was high (Alloys Nos. S78 and S82). 13) When the value of metallographic and composition relational expression f6 was small, machinability was poor (Alloy No. 81). When the f6 value exceeded 35, machinability improved (e.g., Alloy No. S12; Steps Nos. E3 and E4; Alloy No. SO2; Step No. D11H). 14) If a hot-extruded material was cooled after hot extrusion in a cooling treatment which was started when the temperature of the material was higher than 500°C and performed at an average cooling rate exceeding 300°C / min in the temperature ranges from the starting temperature of the cooling treatment to 300°C and from 500°C to 300°C, β phase was modified and a grain boundary was observed inside β1 phase. When β phase was modified, cutting resistance lowered, dezincification corrosion resistance and resistance to stress corrosion cracking significantly improved, strength was high, and the balance between strength and ductility improved. With respect to machinability, in the case β phase was modified, deterioration in machinability due to high-speed machining was very limited (Steps Nos. A1 to A5, D1 to D5, E1 to E5, F1 to F3, and G1 to G3; Figs 1A to 6A, etc.). 15) In the case the starting temperature of the cooling treatment was higher than 500°C and lower than about 530°C and the average cooling rate was between about 400°C / min and about 500°C / min in the temperature range from 500°C to 300°C, a grain boundary was observed inside β1 phase. Also, a small amount of phosphorous compounds were occasionally present. In the case a small amount of phosphorous compounds were present, cutting resistance was slightly higher and dezincification corrosion resistance was a little poorer than when such compounds were absent (Steps Nos. A1 to A5, D1 to D5, E1 to E5, F1 to F3, and G1 to G3; Figs. 1A to 6A and 1B to 6B, etc.). 16) In the case the average cooling rate in the temperature range from 500°C to 300°C was about 200°C / min to 250°C / min, no grain boundary was observed inside β1 phase, and a small amount of phosphorous compounds were present in many cases. As a result of absence of grain boundary inside β1 phase, machinability, especially when machined at a cutting speed of 110 m / min, and dezincification corrosion resistance were poor (Steps Nos. A14H, D14H, etc.). 17) When the starting temperature of the cooling treatment was 550°C or higher and the average cooling rates in the temperature ranges from the starting temperature to 300°C and from 500°C to 300°C were both favorable, machinability index was high and dezincification corrosion resistance and resistance to stress corrosion cracking were good. That is, β phase is considered to become better modified when the cooling treatment is performed with an appropriate starting temperature and a high cooling rate (Steps Nos. A1 to A5, A11H to A14H, D1 to D5, D11H to D14H, E1 to E5, E11H to E15H, F1 to F3, F4H, F5H, G1 to G3, and G11H to G14H; Figs 1B to 6B, etc.). 18) When the starting temperature of the cooling treatment was higher than 670°C, the proportion of β1 phase was larger and cutting resistance was slightly higher than when the starting temperature was 670°C or lower. In particular, when machined at 110 m / min, cutting resistance was high, and dezincification corrosion resistance was not so good. When the proportion of β1 phase was excessive, β phase sometimes failed to be modified. Incidentally, the form of dezincification corrosion was selective corrosion of β1 phase, and when the proportion of β1 phase was large, progression speed of dezincification corrosion appeared to be slightly accelerated (Steps Nos. A1 to A5, D1 to D5, E1 to E5, F1 to F3, G1 to G3, etc.). 19) When β phase was modified and β1 phase was observed, dezincification corrosion resistance was good, and the results in the Stress Corrosion Cracking Test - 1 were all "B" (Steps Nos. D1 to D5). 20) By performing shot blasting after hot working (hot forging), resistance to stress corrosion cracking further improved, and evaluation results of the Stress Corrosion Cracking Tests - 1 and 2 were all "B". In contrast, with respect to the test pieces in which β1 phase was not observed or the Si content was small, resistance to stress corrosion cracking was not improved by shot blasting (Steps Nos. D3 and D7; Alloys Nos. S71 and 80). 21) It was confirmed that when hot working is performed twice, for instance, when a hot-extruded bar is hot forged, as long as the cooling treatment performed after the final hot working, i.e., hot forging in this case, is appropriate, β phase becomes modified regardless of the cooling method after the initial hot working (Steps Nos. D1 to D5 and D7). 22) It was confirmed that even if a casting is used as a hot forging material, if the cooling treatment performed after hot forging is appropriate, β1 phase is observed and the same properties can be obtained just like when an extruded material is used as a hot forging material (Steps Nos. D6, E6, and F1). 23) When an alloy in which β phase had been modified was held at 300°C for two hours, the grain boundary of β1 phase disappeared, resulting in deterioration of machinability and dezincification corrosion resistance. When held at 130°C for five hours instead, a grain boundary was observed inside β1 phase, thus, there was no deterioration of machinability or dezincification corrosion resistance (Steps Nos. A21, A25, and A31H). 24) It was confirmed that even when no cooling treatment is performed with specific conditions in the final hot working process, if a hot-worked material is annealed at a temperature higher than 520°C and lower than 630°C for one minute to five hours, and cooled after annealing with a cooling treatment which is started when the temperature of the annealed material is higher than 500°C and performed at an average cooling rate exceeding 300°C / min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C, β phase becomes modified, resulting in significant improvement particularly in dezincification corrosion resistance and resistance to stress corrosion cracking (Steps Nos. G1 to G3; Figs 5A and 5B).

[0220] As described above, alloys in which the content of each of the alloying elements, values of the composition relational expressions and the metallographic structure relational expressions are within respective appropriate ranges have excellent hot workability and good machinability, mechanical characteristics, dezincification corrosion resistance, and resistance to stress corrosion cracking. Embodiments of the present invention are able to obtain excellent properties by appropriately adjusting the conditions in hot extrusion, hot forging, and heat treatment processes.INDUSTRIAL APPLICABILITY

[0221] The free-cutting copper alloys of the embodiments have excellent hot workability and machinability, high strength, and excellent balance between strength and elongation although they contain only a small amount of Pb. Therefore, the free-cutting copper alloys are suitable for devices and components related to drinking water or sanitary facility, food-related devices, electrical or electronic apparatus components, auto parts, mechanical components, stationaries, toys, musical instruments, sliding components, measuring instrument components, precision mechanical components, medical components, drink-related devices and components, water meters, and components involving liquid or gas such as industrial water, drainage water, or hydrogen.

[0222] Specifically, the free-cutting copper alloys can be suitably applied to the items used in the above-mentioned fields that go by the names including faucet, shutoff valve, mixer tap, shower head, valve, joint, cock, gear, axle, bearing, trumpet, shaft, sleeve, spindle, sensor, bolt, nut, flare nut, pen point, insert nut, cap nut, nipple, spacer, and screw as their material or the like.

Examples

first embodiment

[0050]A free-cutting copper alloy according to the present invention includes: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.40 mass%, the content of Al is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, the relationship of 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.5 is satisfied, if Bi is included, the relationship of 0.003 ≤ f2 = [Pb] + [Bi] i.e.,...

second embodiment

[0051]A free-cutting copper alloy according to the present invention includes: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.30 mass%, the content of Al is lower than 0.15 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, the relationship of 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.2 is satisfied...

third embodiment

[0052]A free-cutting copper alloy according to the present invention includes: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; higher than 0.05 mass% and lower than 0.90 mass% Sn; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.40 mass%, the content of Al is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, a Bi content is represented by [Bi] mass%, and a Sn content is represented by [Sn], the relationship of 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ...

TECHNICAL FIELD

[0001] The present invention relates to a free-cutting copper alloy having good corrosion resistance, in particular, dezincification corrosion resistance and resistance to stress corrosion cracking as well as a high strength and a significantly reduced Pb content and a method for producing a free-cutting copper alloy.

[0002] The present invention relates to a free-cutting copper alloy used for components produced by machining such as devices and components used for supply of drinking water that humans and animals ingest daily, those used for sanitary facilities such as kitchen, shower room, or bathroom, water meters, musical instruments, tableware, devices and components for water drainage, industrial plumbing components, electrical or electronic apparatus components, auto parts, mechanical components, stationaries, toys, sliding components, measuring instrument components, precision mechanical components, medical components, and components relating to liquid or gas such as hydrogen, whose specific component names include faucet, mixer tap, tap fitting, shutoff valve, valve, joint, shower head, cock, gear, axle, bearing, shaft, sleeve, spindle, sensor, bolt, nut, flare nut, pen point, insert nut, cap nut, nipple, spacer, and screw, and a method for producing the free-cutting copper alloy.

[0003] The present application claims priority on Japanese Patent Application No. 2023-075579 filed on May 1, 2023 and Japanese Patent Application No. 2023-173342 filed on October 5, 2023, the contents of which are incorporated herein by reference.BACKGROUND ART

[0004] Conventionally, a Cu-Zn-Pb alloy containing 56 to 65 mass% Cu and 1 to 4 mass% Pb with the balance being Zn (so-called a free-cutting brass bar, forging brass, or casting brass) or a Cu-Sn-Zn-Pb alloy containing 80 to 88 mass% Cu, 2 to 8 mass% Sn, and 1 to 8 mass% Pb with the balance being Zn (so-called bronze casting: gunmetal) having excellent machinability and antibacterial property as well as good corrosion resistance has been generally used for devices and components related to drinking water or sanitary facility, water meters, musical instruments, tableware, electrical or electronic apparatus components, home appliance components, auto parts, mechanical components, stationaries, precision mechanical components, medical components, and devices and components relating to liquid or gas such as industrial water, drainage water, or hydrogen including, in terms of component name, faucet, mixer tap, shutoff valve, valve, cock, joint, gear, sensor, nut, and screw.

[0005] However, recently, Pb's influence on human body and the environment is becoming a concern, and momentum to regulate Pb is increasing in various countries. For example, a regulation for reducing the Pb content in drinking water supply devices to be 0.25 mass% or lower came into force in January 2010 in California, the United States. In countries other than the United States also, such regulation is rapidly being established, and development of a copper alloy material that meets the requirements of regulation on Pb content is in demand.

[0006] In addition, in other industrial fields such as those of electrical and electronic apparatuses, automobiles, and machines, free-cutting copper alloys are exceptionally allowed to include maximum 4 mass% Pb by European directives of RoHS and ELV to give an example. However, like in the field of drinking water, strengthening of regulations on Pb content including elimination of exemptions has been actively discussed.

[0007] While there is a trend to strengthen Pb regulations for free-cutting copper alloys, as a replacement of a leaded free-cutting copper alloy including 56 to 65 mass% Cu and 1 to 4 mass% Pb with the balance being Zn, in particular, a free-cutting copper alloy containing 3 mass% Pb that is most widely used in the world (Japan Industrial Standard: C3604; ASTM Standard: C36000; EN Standard: CW614N), (1) a Cu-Zn-Bi or Cu-Zn-Bi-Se alloy including Bi having machinability or, in some cases, also including Se in addition to Bi instead of Pb, (2) a Cu-Zn alloy including a high concentration of Zn in which the amount of β phase is increased to improve machinability, (3) a Cu-Zn-Si alloy including large amounts of γ phase and phase having excellent machinability instead of Pb, and (4) a Cu-Zn-Si-Pb alloy having β phase whose machinability has been improved by the effect of Si contained in the alloy, etc. are proposed.

[0008] In Patent Document 1, for instance, improvement of machinability and dezincification corrosion resistance is devised by adding 0.3 to 4 mass%, preferably 1.8 to 3.2 mass% Bi to a Cu-Zn alloy and annealing the alloy at a temperature between 350°C and 550°C so that β phase is reduced and fragmented by α phase because β phase has poor dezincification corrosion resistance.

[0009] However, alloys including Bi instead of Pb have many problems including the facts that machinability of Bi is inferior to that of Pb, Bi may be harmful to the environment and human body like Pb, Bi has a resourcing problem because it is a rare metal, and Bi embrittles a copper alloy material.

[0010] In addition, as disclosed by Patent Document 1, as β phase of a Cu-Zn alloy has poor dezincification corrosion resistance, reduction of β phase, performance of annealing for causing α phase to fragment β phase, etc. have been practically required as improvement measures.

[0011] Further, even though β phase in a Cu-Zn binary alloy containing a large amount of β phase contributes to improvement of machinability, machinability of β phase is inferior to that of Pb. Further, it has poor dezincification corrosion resistance and resistance to stress corrosion cracking. Therefore, there is no way that such an alloy can be an alternative to a free-cutting copper alloy containing Pb.

[0012] For this reason, in Patent Documents 2 to 8, for example, Cu-Zn-Si alloys including Si instead of Pb are proposed as free-cutting copper alloys.

[0013] Patent Documents 2 to 7 disclose that in alloys containing about 58 to 65 mass% Cu and 0.2 to 1.5 mass% Si, machinability is improved by the effect of Si contained in β phase and the presence of fine phosphorous compounds formed of P and Zn or the like. In these Documents, excellent machinability is realized by defining the area ratios of β phase and γ phase and requiring phosphorous compounds to be present and a small amount of Pb to be contained in the alloys. However, as disclosed by Patent Document 1, it is a well-known fact that β phase of a Cu-Zn alloy has poor dezincification corrosion resistance and also causes deterioration of resistance to stress corrosion cracking. Patent Documents 2 to 7, therefore, disclose no specific data related to dezincification corrosion resistance or resistance to stress corrosion cracking, implying that the technical problems that conventional β phase of a Cu-Zn alloy has, i.e., its dezincification corrosion resistance and resistance to stress corrosion cracking were yet to be improved.

[0014] In Patent Document 8, excellent machinability is realized in an alloy containing 71.5 to 78.5 mass% Cu and 2.0 to 4.5 mass% Si although the Pb content is as small as 0.02 mass% or even less by defining the total area ratio of γ phase and κ phase with excellent machinability that are formed in an alloy containing Cu and Si at high concentrations. In this Patent Document, further improvement of machinability and corrosion resistance is devised by making the alloy contain 0.1 mass% or higher amount of Sn and Al respectively to form a large amount of γ phase.

[0015] In Patent Document 9, excellent machinability is obtained by containing small amounts of Si, Pb, and P or Fe as well as 0.5 mass% or less Pb, and dispersing Pb-rich particles in the matrix and increasing the population density of the Pb-rich particles that are present inside α phase by contriving manufacturing method. For the improvement of dezincification corrosion resistance, Patent Document 9 requires performance of finish heat treatment at a temperature between 400°C and 600°C in effect.

[0016] Patent Document 10 proposes a Cu-Zn-Si-Pb-P copper alloy in which the area ratios of β phase, γ phase, and µ phase are limited and involving a production technique to manufacture hollow hot-forged products having a near-net shape using a hollow material. Patent Document 11 proposes a copper alloy casting made of a Cu-Zn-Zr-P alloy optionally containing Si, Pb, and / or Sn, in which crystal grains are refined by the actions of Zr and P.

[0017] Patent Document 12 proposes a Cu-Zn-Sn-Al copper alloy with excellent color fastness optionally containing Si and / or Pb in which the area ratios of y phase and β phase are limited.

[0018] Patent Document 13 proposes a lead-free copper alloy casting of a Cu-Zn-Si-Sn-Al-P alloy.

[0019] Patent Document 14 discloses that apparent Zn content is important in order to improve corrosion resistance of a Cu-Zn-Si-Sn-Al alloy and proposes a copper alloy whose machinability is improved by containing a large amount of Pb or Bi in effect.

[0020] Patent Document 15 discloses a lead-free copper alloy casting made of a Cu-Zn-Si alloy containing 65 mass% or more Cu with good castability and mechanical strength in which machinability is improved by γ phase. An example containing large amounts of Sn, Al, Mn, Ni, and Sb is presented in this Document.Citation ListPatent Documents

[0021] Patent Document 1: Japanese Unexamined Patent Application, First Publication No. 2002-003967 Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2021-042461 Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2021-042459 Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2021-042460 Patent Document 5: PCT International Publication No. WO2020 / 261666 Patent Document 6: Japanese Unexamined Patent Application, First Publication No. 2021-042462 Patent Document 7: PCT International Publication No. WO2021 / 117528 Patent Document 8: PCT International Publication No. WO2007 / 034571 Patent Document 9: Japanese Unexamined Patent Application, First Publication No. 2016-194123 Patent Document 10: PCT International Publication No. WO2013 / 065830 Patent Document 11: PCT International Publication No. WO2006 / 016630 Patent Document 12: PCT International Publication No. WO2015 / 046421 Patent Document 13: Japanese Unexamined Patent Application, First Publication No. 2010-133006 Patent Document 14: Japanese Unexamined Patent Application, First Publication No. 2018-048398 Patent Document 15: Published Japanese Translation No. 2019-508584 of the PCT International Publication SUMMARY OF INVENTIONTechnical Problem

[0022] As indicated in Patent Documents 1 to 15, there is no past record of substantial improvement made in the dezincification corrosion resistance and the resistance to stress corrosion cracking of β phase present in Cu-Zn alloys, which had been a long-standing technical challenge. Further, there is no disclosure of a Cu-Zn alloy containing less than 0.2% Pb or Bi that exhibits low cutting resistance and excellent machinability in high-speed machining exceeding 100 m / min without essentially requiring the alloy to contain Bi.

[0023] The present invention has been made in order to solve the above-described problems in the conventional art, and the objective thereof is to provide a free-cutting copper alloy in which Pb content has been significantly reduced but having excellent hot workability and machinability as well as good dezincification corrosion resistance and resistance to stress corrosion cracking despite a large content of β phase (the β1 phase described later), a high strength, and good balance between strength and ductility, and a method for producing the free-cutting copper alloy.Solution to Problem

[0024] In order to solve the above-described problems and achieve the above-described object, the present inventors conducted thorough investigation and obtained the following findings.

[0025] Incidentally, in this specification, unless specified otherwise, β phase includes β' phase, γ phase includes γ' phase, and α phase includes α' phase. β1 phase refers to modified β phase and is distinguished from β phase and β' phase. β1 phase has a characteristic that a grain boundary pattern, i.e., a boundary of crystal grain can be identified inside the phase when etched with an etching solution made of hydrogen peroxide and ammonia water then observed with a metallographic microscope. In the case of β phase that is present in a common alloy such as a Cu-Zn, Cu-Zn-Bi, or Cu-Zn-Si alloy, no boundary of crystal grains can be observed inside β phase even if it is etched with a solution of hydrogen peroxide and ammonia water. Therefore, β phase and β1 phase can be clearly distinguished. Crystal grain boundary may be simply referred to as grain boundary. Hot-worked material includes hot-extruded material, hot-forged material, and hot-rolled material. Cold workability refers to performance of a material when subjected to working conducted in a cold state such as drawing, wire drawing, rolling, crimping, or bending. Unless specified otherwise, good or excellent machinability stands for low cutting resistance and good or excellent chip breakability when turned with a lathe. Conductivity refers to electric conductivity, thermal conductivity, or electrical conductivity. Cooling rate refers to an average cooling rate in a given temperature range. Twenty-four hours refer to one day. "Manufacturing on the actual production line" refers to production with a mass production facility used for manufacturing products for sale.

[0026] The Patent Document 8 mentioned above discloses that in Cu-Zn-Si alloys, β phase little contributes to machinability of a copper alloy, but rather impairs it. In Patent Documents 10, 11, and 12, the amount of β phase is limited to a very small amount. Patent Document 2 requires a process of annealing at a temperature between 350°C and 550°C to reduce β phase and make alpha phase fragment β phase as a method for improving dezincification corrosion resistance of β phase. Patent Document 9 discloses that in order to improve dezincification corrosion resistance of β phase, it is necessary to include a higher amount of Sn than Si and heat the material to a temperature between 700°C and 850°C, perform hot extrusion in the same temperature range and a finishing heat treatment in which the material is held at a temperature between 400°C and 600°C for 30 minutes or longer then cooled at an average cooling rate of 0.2°C / sec to 10°C / sec in the temperature range from 400°C to 200°C.

[0027] On the other hand, Patent Documents 2 to 7 disclose a discovery that β phase itself exhibits a significant effect on machinability in a Cu-Zn-Si alloy if a certain amount of Si is contained in the β phase. These Patent Documents further disclose that their alloys obtained excellent machinability due to the synergy effect brought by satisfying the three requirements, i.e., fine phosphorous compound is present, a small amount of Pb is contained, and Bi is contained in some cases. However, in order to cause phosphorous compound to be present, these Documents require cooling at an average cooling rate of preferably about 0.1°C / min or higher and 70°C / min or lower in the temperature range from about 530°C to about 450°C after hot working.

[0028] Incidentally, Patent Documents 2 to 7 do not disclose any data related to dezincification corrosion resistance or resistance to stress corrosion cracking. There is no mention of modification of β phase itself, either. This implies that no improvement was made in the dezincification corrosion resistance or the resistance to stress corrosion cracking of β phase that is present in Cu-Zn-Si alloy and no further improvement was made in the machinability.

[0029] The present inventors further addressed modification of β phase itself in a Cu-Zn-Si alloy. As a result of devoted study, machinability of such an alloy was further improved particularly in high-speed cutting without presence of phosphorous compound or essentially containing Bi by the effect of modified β phase, i.e., β1 phase, and significant improvement in the dezincification corrosion resistance and resistance to stress corrosion cracking of β phase, which had been longstanding drawbacks of a Cu-Zn system alloy, was achieved. In addition, strength of an alloy containing β1 phase instead of β phase could be further enhanced without impairing its ductility. In order to obtain this β1 phase, it is necessary to first solid-solubilize specific amounts of Si and P inside the β phase then maintain the state of metallographic structure when it is at a high temperature until and after the alloy's temperature becomes a normal temperature by maintaining the state of β phase when it is at a temperature between 500°C and 670°C and cooling the alloy at a high cooling rate when it is cooled down to normal temperature.

[0030] Apropos, β1 phase can be easily distinguished from β phase of a Cu-Zn system alloy. To see the difference, a surface of a piece of alloy is polished (mirror-polished) then etched with a mixed solution of hydrogen peroxide and ammonia water. Specifically, an aqueous solution prepared by mixing 3 ml of 3 vol% hydrogen peroxide water and 22 ml of 14 vol% ammonia water is used for the etching. The polished metal surface is dipped in the aqueous solution under room temperature between about 15°C and 25°C for about 2 to 10 seconds then the metallographic structure is observed with a metallographic microscope at a magnification of 200X to 1000X. If it is a β1 phase, a grain boundary pattern, i.e., a crystal grain boundary can be found inside the phase, but no crystal grain boundary can be found inside β phase of an ordinary Cu-Zn system alloy. Use of the aforementioned etching solution is disclosed in Patent Documents 2-7, 10, and 11, and pictures of the metallographic structure observed by etching with the solution are also disclosed in the Patent Documents.

[0031] In order to maintain the state of β phase when it is at a temperature between 500°C and 670°C after hot working until the alloy reaches normal temperature, which is a condition to obtain β1 phase, it is necessary to start a cooling treatment that is performed as part of the cooling process after hot working when the temperature of the hot-worked material is lower than 670°C and higher than 500°C, and cool the material at an average cooling rate of higher than 300°C / min in the temperature range from the starting temperature of the cooling treatment to 500°C, then continue to cool at an average cooling rate of at least higher than 300°C / min in the temperature range from 500°C to 300°C in the cooling treatment in the cooling treatment that continues. In this specification, unless specified otherwise, cooling treatment refers to manipulation of cooling rate by water cooling or any other similar method as opposed to natural cooling. In order to obtain fine P-containing compound, Patent Documents 2 to 7 require cooling of hot-worked material at an average cooling rate of about 0.1°C / min or higher and about 70°C / min or lower when the material is in the temperature range from about 530°C to about 450°C after hot working. That is, the present invention and those of Patent Documents 2 to 7 are clearly heading in opposite directions (or providing opposite teachings).

[0032] It should be noted, however, that the present invention may include both β1 phase and phosphorous compounds if, for instance, the cooling rate when the temperature of the material after hot working is around 520°C is 60°C / min and also the average cooling rate in the temperature range from 520°C, which is the starting temperature of the cooling treatment, to 500°C exceeds 300°C / min.

[0033] In the present invention, machinability of a Cu-Zn-Si alloy is significantly improved by the effect of β1 phase particularly in high-speed cutting without requiring phosphorous compounds as disclosed by Patent Documents 2 to 7. Further, by the synergy effect between β1 phase and fine Pb particles or particles containing both Pb and Bi, cutting resistance is decreased and chip breakability is promoted.

[0034] More importantly, dezincification corrosion resistance and resistance to stress corrosion cracking, which were major unsolved problems with conventional β phase, are dramatically improved by the modification of β phase into β1 phase, allowing the problems to be solved. With respect to mechanical characteristics, the present invention retains high strength of conventional β phase which is further enhanced, as well as improved ductility and better balance between strength and ductility. Thus, a copper alloy with machinability equivalent to that of conventional free-cutting brass and better dezincification corrosion resistance and resistance to stress corrosion cracking as well as higher strength than those of conventional free-cutting brass came to be invented.

[0035] A free-cutting copper alloy according to a first aspect of the present invention includes: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, with the balance being Zn and inevitable impurities, in which, among the inevitable impurities, a total content of Fe, Mn, Co, and Cr is lower than 0.40 mass% and a content of A1 is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, the following relationship of 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.5 is satisfied, if Bi is included, the following relationship of 0.003 ≤ f2 = [Pb] + [Bi] < 0.20 is further satisfied, in constituent phases of a metallographic structure excluding non-metallic inclusions, when an area ratio of α phase is represented by (α)%, an area ratio of γ phase is represented by (γ)%, and an area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, the following relationships of 20 ≤ f3 = (a) < 75, 25 < f4 = (β1) ≤ 80, 0 ≤ f5 =(y) < 4, and 27 < f6 = (β1) × ([Si]) 1 / 2< - (γ) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15 are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0036] A free-cutting copper alloy according to a second aspect of the present invention includes: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, with the balance being Zn and inevitable impurities, in which, among the inevitable impurities, a total content of Fe, Mn, Co, and Cr is lower than 0.30 mass% and a content of Al is lower than 0.15 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, the following relationship of 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.2 is satisfied, if Bi is included, the following relationship of 0.004 ≤ f2 = [Pb] + [Bi] < 0.10 is further satisfied, in constituent phases of a metallographic structure excluding non-metallic inclusions, when an area ratio of α phase is represented by (α)%, an area ratio of γ phase is represented by (γ)%, and an area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, the following relationships of 30 ≤ f3 = (α) ≤ 70, 30 ≤ f4 = (β1) ≤ 70, 0 ≤ f5 =(y) < 1, and 35 < f6 = (β1) × ([Si]) 1 / 2< - (γ) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15, are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0037] A free-cutting copper alloy according to a third aspect of the present invention includes: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; higher than 0.05 mass% and lower than 0.90 mass% Sn; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, with the balance being Zn and inevitable impurities, in which, among the inevitable impurities, a total content of Fe, Mn, Co, and Cr is lower than 0.40 mass% and a content of A1 is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, a Bi content is represented by [Bi] mass%, and a Sn content is represented by [Sn], the following relationship of 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≤ 60.5 is satisfied, and the following relationship of f0 = [Sn] / [Si] < 1 is further satisfied, if Bi is included, the following relationship of 0.003 ≤ f2 = [Pb] + [Bi] < 0.20 is satisfied, in constituent phases of a metallographic structure excluding non-metallic inclusions, when an area ratio of α phase is represented by (α)%, an area ratio of γ phase is represented by (γ)%, and an area ratio of β1 phase, i.e., modified β1 phase, is represented by (β1)%, the following relationships of 20 ≤ f3 = (α) < 75, 25 < f4 = (β1) ≤ 80, 0 ≤ f5 =(γ) < 4, and 27 < f6 = (β1) × ([Si]) 1 / 2< - (γ) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15, are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0038] A free-cutting copper alloy according to a fourth aspect of the present invention includes: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; higher than or equal to 0.10 mass% and lower than 0.50 mass% Sn; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, with the balance being Zn and inevitable impurities, in which, among the inevitable impurities, a total content of Fe, Mn, Co, and Cr is lower than 0.30 mass% and a content of A1 is lower than 0.15 mass%, when a Cu content is represented [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, a Bi content Bi is represented by [Bi] mass%, and a Sn content is represented by [Sn], the following relationship of 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≤ 60.2 is satisfied, and the following relationship of f0 = [Sn] / [Si] < 0.6 is further satisfied, if Bi is included, the following relationship of 0.004 ≤ f2 = [Pb] + [Bi] < 0.10 is satisfied, in constituent phases of a metallographic structure excluding non-metallic inclusions, when an area ratio of α phase is represented by (α)%, an area ratio of γ phase is represented by (γ)%, and an area ratio of β1 phase, i.e., modified β1 phase, is represented by (β1)%, the following relationships of 30 ≤ f3 = (α) < 70, 30 ≤ f4 = (β1) ≤ 70, 0 ≤ f5 =(y) < 1, and 35 < f6 = (β1) × ([Si]) 1 / 2< - (γ) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15 are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0039] A free-cutting copper alloy according to a fifth aspect of the present invention is the free-cutting copper alloy according to any one of the first to fourth aspects of the present invention, which is used for a device or component related to drinking water or sanitary facility, a valve, a cock, an industrial plumbing component, a water meter, a musical instrument, an auto part, an electrical or electronic apparatus component, a mechanical component, a stationery, a toy, a sliding component, a measuring instrument component, a precision mechanical component, or a medical component.

[0040] A method for producing a free-cutting copper alloy according to a sixth aspect of the present invention is a method for producing the free-cutting copper alloy according to any one of the first to fourth aspects of the present invention, including one or more hot working steps, in which in the final hot working step among the hot working steps, hot working temperature is higher than 540°C and lower than 750°C, in the cooling process following the final hot working step, the hot-worked material is subjected to a cooling treatment which is started when its temperature is lower than 670°C and higher than 500°C and performed at an average cooling rate of higher than 300°C / min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and that from 500°C to 300°C.

[0041] A method for producing a free-cutting copper alloy according to a seventh aspect of the present invention is a method for producing the free-cutting copper alloy according to any one of the first to fourth aspects of the present invention, including one or more hot working steps and heat treatment steps, in which in the final heat treatment step, annealing is performed with a holding temperature of higher than 520°C and lower than 630°C and a holding time of one minute to five hours, the annealed material is cooled through a cooling treatment which is started after the annealing when the temperature of the material is higher than 500°C and performed at an average cooling rate of higher than 300°C / min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C.Advantageous Effects of Invention

[0042] According to one aspect of the present invention, a free-cutting copper alloy having excellent hot workability, good machinability, dezincification corrosion resistance, and resistance to stress corrosion cracking, a high strength, and a good balance between strength and ductility and containing a significantly reduced amount of Pb, and a method for producing the free-cutting copper alloy can be provided.BRIEF DESCRIPTION OF DRAWINGS

[0043] [Fig. 1A] A picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S01 to Step No. A2. More specifically, Alloy No. S01 has a composition including 63.3 mass% Cu, 0.95 mass% Si, 0.069 mass% P, 0.063 mass% Pb, 0.017 mass% Bi, and Zn as the balance. In Step No. A2, the alloy was subjected to hot extrusion performed at 630°C and a cooling treatment which was started when the temperature of the hot-extruded material became 580°C and performed at an average cooling rate of 1020°C / min in both temperature ranges from 580°C to 300°C and from 500°C to 300°C. [Fig. 1B] A cross-sectional picture of a metallographic structure of the alloy shown in Fig. 1A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509. [Fig. 2A] A picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S11 to Step No. E2. More specifically, Alloy No. S11 has a composition including 62.5 mass% Cu, 0.96 mass% Si, 0.064 mass% P, 0.072 mass% Pb, and Zn as the balance. In Step No. E2, the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm. The rod was heated and hot forged at 680°C to have a thickness of 20 mm when placed horizontally then subjected to a cooling treatment which was started when its temperature became 565°C and performed at an average cooling rate of 900°C / min in both temperature ranges from 565°C to 300°C and from 500°C to 300°C. [Fig. 2B] A cross-sectional picture of the metallographic structure of the alloy shown in Fig. 2A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509. [Fig. 3A] A picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S11 to Step No. E13H. More specifically, Alloy No. S11 has a composition including 62.5 mass% Cu, 0.96 mass% Si, 0.064 mass% P, 0.072 mass% Pb, and Zn as the balance. In Step No. E13H, the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm. The rod was heated and hot forged at 630°C to have a thickness of 20 mm when placed horizontally then cooled at an average cooling rate of 35°C / min. Subsequently, the rod was subjected to a cooling treatment which was started when its temperature became 455°C and performed at an average cooling rate of 800°C / min in the temperature range from 455°C to 300°C. Note that the starting temperatures of the cooling treatment performed on the alloy in Fig. 3A and that in Fig. 2A are different. [Fig. 3B] A cross-sectional picture of the metallographic structure of the alloy shown in Fig. 3A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509. [Fig. 4A] A picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S43 to Step No. E6. More specifically, Alloy No. S43 has a composition including 63.3 mass% Cu, 0.98 mass% Si, 0.084 mass% P, 0.060 mass% Pb, 0.28 mass% Sn, and Zn as the balance. In Step No. E6, a casting produced with a mold having a diameter of 55 mm was machined to a diameter of 50 mm then cut to a length of 200 mm. The casting was heated and hot forged at 630°C to a thickness of 20 mm when placed horizontally then subjected to a cooling treatment which was started when the temperature of the casting became 565°C and performed at an average cooling rate of 900°C / min in both temperature ranges from 565°C to 300°C and from 500°C to 300°C. [Fig. 4B] A cross-sectional picture of the metallographic structure of the alloy shown in Fig. 4A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509. [Fig. 5A] A picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step Nos. A34H and G1. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.98 mass% Si, 0.071 mass% P, 0.071 mass% Pb, and Zn as the balance. In Step No. A34H, an alloy was obtained by hot extrusion at 615°C and cooling at an average cooling rate of 18°C / min in the temperature range from 500°C to 300°C. In Step No. G1, the alloy was further heated to 580°C for 30 minutes then subjected to a cooling treatment which was started when its temperature became 560°C and performed at an average cooling rate of 1800°C / min in both temperature ranges from 560°C to 300°C and from 500°C to 300°C. [Fig. 5B] A cross-sectional picture of the metallographic structure of the alloy shown in Fig. 5A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509. [Fig. 6A] A picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step No. E14H. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.98 mass% Si, 0.071 mass% P, 0.071 mass% Pb, and Zn as the balance. In Step No. E14H, the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm. The rod was heated and hot forged at 630°C to a thickness of 20 mm when placed horizontally then left to cool naturally. The average cooling rate in the temperature range from 500°C to 300°C was 25°C / min. [Fig. 6B] A cross-sectional picture of the metallographic structure of the alloy shown in Fig. 6A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509. DESCRIPTION OF EMBODIMENTS

[0044] Below is a description of free-cutting copper alloys according to the embodiments of the present invention and methods for producing the free-cutting copper alloys. The free-cutting copper alloys according to the embodiments are used for devices and components related to sanitary facilities, musical instruments, tableware, home appliance components, electrical or electronic apparatus components, auto parts, mechanical components, stationaries, precision mechanical components, medical components, and devices and components involving liquid or gas such as industrial water, drainage water, and hydrogen. Examples of specific component names include faucet, mixer tap, shutoff valve, valve, cock, joint, water meter, gear, sensor, nut, and screw.

[0045] Here, in this specification, an element symbol in parentheses such as [Zn] represents the content (mass%) of the element.

[0046] In the embodiments, using this content expressing method, composition relational expressions f1, f2, and f0 are defined as follows.

[0047] Composition relational expression f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P]. In the case Sn is contained in an amount exceeding 0.05 mass%, f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] In the case Bi is contained, composition relational expression f2 = [Pb] + [Bi] In the case Sn is contained, f0 = [Sn] / [Si]

[0048] Further, in the embodiments, in constituent phases of the metallographic structure excluding non-metallic inclusions, the area ratio of α phase is represented by (α)%, the area ratio of γ phase is represented by (γ)%, the area ratio of β phase is represented by (β)%, and the area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%. The area ratio of each of the phases will also be referred to as "the amount of each of the phases", "the proportion of each of the phases", or "the proportion that each of the phases occupies".

[0049] In the embodiments, a plurality of metallographic structure relational expressions are defined as follows. Metallographic Structure Relational Expression f3 = (α) Metallographic Structure Relational Expression f4 = (β1) Metallographic Structure Relational Expression f4A = (β) Metallographic Structure Relational Expression f5 = (γ) Metallographic Structure and Composition Relational Expression f6 = (β1) × ([Si]) 1 / 2< - (y) × 2 + ([Pb] + [Bi] ) 1 / 2< × 20 + ([P] ) 1 / 2< × 15

[0050] A free-cutting copper alloy according to the first embodiment of the present invention includes: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.40 mass%, the content of Al is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, the relationship of 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.5 is satisfied, if Bi is included, the relationship of 0.003 ≤ f2 = [Pb] + [Bi] < 0.20 is further satisfied, in constituent phases of the metallographic structure excluding non-metallic inclusions, when the area ratio of α phase is represented by (α)%, the area ratio of γ phase is represented by (γ)%, and the area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, the relationships of 20 ≤ f3 = (α) < 75, 25 < f4 = (β1) ≤ 80, 0 ≤ f5 =(y) < 4, and 27 < f6 = (β1) × ([Si]) 1 / 2< - (γ) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15 are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0051] A free-cutting copper alloy according to the second embodiment of the present invention includes: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.30 mass%, the content of Al is lower than 0.15 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, the relationship of 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.2 is satisfied, if Bi is included, the relationship of 0.004 ≤ f2 = [Pb] + [Bi] < 0.10 is further satisfied, in constituent phases of the metallographic structure excluding non-metallic inclusions, when the area ratio of α phase is represented by (α)%, the area ratio of γ phase is represented by (γ)%, and the area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, the relationships of 30 ≤ f3 = (α) ≤ 70, 30 ≤ f4 = (β1) ≤ 70, 0 ≤ f5 =(γ) < 1, and 35 < f6 = (β1) × ([Si]) 1 / 2< - (γ) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15 are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0052] A free-cutting copper alloy according to the third embodiment of the present invention includes: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; higher than 0.05 mass% and lower than 0.90 mass% Sn; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.40 mass%, the content of Al is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, a Bi content is represented by [Bi] mass%, and a Sn content is represented by [Sn], the relationship of 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≤ 60.5 is satisfied, the relationship of f0 = [Sn] / [Si] < 1 is further satisfied, if Bi is included, the relationship of 0.003 ≤ f2 = [Pb] + [Bi] < 0.20 is satisfied, in constituent phases of the metallographic structure excluding non-metallic inclusions, when the area ratio of α phase is represented by (α)%, the area ratio of γ phase is represented by (γ)%, and the area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, the relationships of 20 ≤ f3 = (α) < 75, 25 < f4 = (β1) ≤ 80, 0 ≤ f5 =(y) < 4, and 27 < f6 = (β1) × ([Si]) 1 / 2< - (y) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15 are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0053] A free-cutting copper alloy according to the fourth embodiment of the present invention includes: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; higher than or equal to 0.10 mass% and lower than 0.50 mass% Sn; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.30 mass%, the content of Al is lower than 0.15 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, a Bi content is represented by [Bi] mass%, and a Sn content is represented by [Sn], the relationship of 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≤ 60.2 is satisfied, the relationship of f0 = [Sn] / [Si] < 0.6 is further satisfied, if Bi is included, the relationship of 0.004 ≤ f2 = [Pb] + [Bi] < 0.10 is satisfied, in constituent phases of the metallographic structure excluding non-metallic inclusions, when the area ratio of α phase is represented by (α)%, the area ratio of γ phase is represented by (γ)%, and the area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, the relationships of 30 ≤ f3 = (α) ≤ 70, 30 ≤ f4 = (β1) ≤ 70, 0 ≤ f5 =(γ) < 1, and 35 < f6 = (β1) × ([Si]) 1 / 2< - (γ) × 2 + ([Pb] + [Bi]) 1 / 2< × 20 + ([P]) 1 / 2< × 15 are satisfied, and a grain boundary can be observed in the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0054] The reasons why the compositional components, the composition relational expressions f0, f1, and f2, the metallographic structure relational expressions f3, f4, and f5, the metallographic structure and composition relational expression f6, the metallographic structure, and the like are defined as described above are explained below.<Compositional Components>(Cu)

[0055] Cu is a main element of the free-cutting copper alloy according to the embodiment. In order to achieve the object of the present invention, the free-cutting copper alloy needs to contain Cu in an amount exceeding 60.5 mass% at least. When the Cu content is 60.5 mass% or lower, the proportion of β1 phase exceeds 80% although depending on the contents of Si, Zn, P, Pb, Bi, and Sn and the production process, and as a result, ductility is poor and β phase fails to be modified sufficiently causing dezincification corrosion resistance, resistance to stress corrosion cracking, and machinability to deteriorate. Accordingly, the lower limit of the Cu content is higher than 60.5 mass%, preferably 61.2 mass% or higher, more preferably 61.5 mass% or higher, and still more preferably 62.0 mass% or higher.

[0056] On the other hand, when the Cu content is 65.0 mass% or higher, the proportion of β1 phase decreases although depending on the contents of Si, Zn, P, Pb, Bi, and Sn and the production process. As a result, excellent machinability cannot be obtained, and strength decreases. Accordingly, the Cu content is lower than 65.0 mass%, preferably 64.8 mass% or lower, and more preferably 64.5 mass% or lower.(Si)

[0057] Si is a main element of the free-cutting copper alloy according to the embodiment. Si contributes to the formation of metallic phases such as κ phase, γ phase, µ phase, β phase, β1 phase, and ζ phase. By performing a specific cooling treatment that is described later on an alloy containing Cu, Zn, Si, and P in the amounts within the respective ranges described above, β phase is modified to form β1 phase. By the effect of β1 phase (modified β phase), significant improvements in dezincification corrosion resistance and resistance to stress corrosion cracking, that were drawbacks of conventional β phase, are realized in addition to improvement in machinability. A representative composition of β1 phase generated by modification would be about 61 mass% Cu, about 1.2 mass% Si, about 37.5 mass% Zn, and about 0.1 mass% P.

[0058] On the other hand, a representative composition of α phase comprise about 66 mass% Cu, about 0.7 mass% Si, and about 33 mass% Zn. In the case of the compositional ranges of the embodiments, inclusion of Si slightly improves machinability of α phase and ameliorate its dezincification corrosion resistance and resistance to stress corrosion cracking. Accordingly, by the effects of β1 phase generated by modification and ameliorated α phase, dezincification corrosion resistance and resistance to stress corrosion cracking of the alloys particularly improve.

[0059] Incidentally, a certain amount of α phase is required for the modification of β phase. For instance, if α phase is not present at all, β phase will not be modified. In order to improve dezincification corrosion resistance, resistance to stress corrosion cracking, ductility, and machinability of the alloys, α phase needs to be 20% or more, preferably 30% or more, in terms of area ratio.

[0060] Si is an essential element required for the modification of β phase into β1 phase. The higher the Si content, the more progresses the modification of β phase, resulting in formation of β1 phase with better properties. In order to modify β phase and generate better modified β1 phase, Si needs to be included in an amount exceeding 0.50 mass%. The Si content is preferably 0.60 mass% or higher, more preferably 0.65 mass% or higher, and still more preferably 0.75 mass% or higher.

[0061] On the other hand, when the Si content reaches a certain amount, the modification to β1 phase peaks. Further, if the Si content is excessive, conductivity is low. In some cases, γ phase appears causing dezincification corrosion resistance, resistance to stress corrosion cracking, and machinability to deteriorate. Speaking of γ phase, Patent Documents 2 to 8 disclose that γ phase improves machinability. However, in the present invention, which is mainly composed of β1 phase and α phase, presence of γ phase rather deteriorates machinability, ductility, dezincification corrosion resistance, and resistance to stress corrosion cracking of the alloy. For this reason, it is preferable to restrict the amounts of Si and γ phase. Accordingly, the Si content is lower than 1.20 mass%, preferably 1.10 mass% or lower. It is 1.0 mass% or lower where conductivity is important.(Zn)

[0062] Zn is a main element of the free-cutting copper alloy according to the embodiment together with Cu and Si and is an element necessary to enhance machinability, strength, high temperature properties, and castability. Incidentally, Zn is described as the balance in the composition, but to be specific, its content is lower than about 38.5 mass% and preferably lower than about 38.0 mass%, and is higher than about 32.0 mass% and preferably higher than 33.0 mass%.(P)

[0063] Like Si, P is an essential element required for the modification of β phase into β1 phase. P solid-solubilizes in β phase during hot working. β phase is modified into β1 phase if the cooling treatment performed in the cooling process after hot working is started when the temperature of the hot-worked material is higher than 500°C and lower than 670°C and the material is cooled at a cooling rate exceeding 300°C / min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C. The modification of β phase into β1 phase significantly improves long-standing drawbacks of conventional β phase, i.e., its dezincification corrosion resistance and resistance to stress corrosion cracking. Presence of β1 phase further makes it possible to decrease cutting resistance during cutting and enhance chip breakability. Aside from that, P-containing compound decreases cutting resistance and improves chip breakability, but if the cooling treatment performed at a cooling rate exceeding 300°C / min is started when the temperature of the material is about 550°C or higher than 530°C, P-containing compound does not appear of even if it does, the amount is small. From a viewpoint of machinability improvement also, the effect brought by the modification of β phase into β1 phase excels that brought by the presence of P-containing compound in β phase. In particular, in high-speed cutting performed at a velocity exceeding 100 m / min, machinability is better improved by the modification of β phase than the presence of P-containing compound in β phase. Incidentally, inclusion of P leads to improvement of the dezincification corrosion resistance and resistance to stress corrosion cracking of α phase and significant improvement of those of alloys consisting of β1 phase and α phase.

[0064] To modify β phase into β1 phase, the lower limit of the P content needs to be at least higher than 0.01 mass%. In view of creation of better β1 phase, the amount of inevitable impurities, the temperature to start the cooling treatment, and the cooling rate after commencement of the cooling treatment, which are described later, the P content is preferably 0.03 mass% or higher, more preferably 0.04 mass% or higher.

[0065] Incidentally, P tends to compound with elements such as Zn, Si, Mn, Fe, Cr, Co, and Al. If the amount of P solid-solubilized inside the β phase that is present in an alloy after hot working is reduced by the formation of such compounds, the modification of β phase into β1 phase is hindered. Zn and Si, which are main elements of the present invention, start to compound with P at around 550°C. If the cooling rate is lowered, the amount of phosphorous compounds increases. Formation of compound between inevitable impurities of Mn, Fe, Cr, and / or Co and P begins at a temperature higher than 550°C. If their amounts are increased, formation of phosphorous compounds is further promoted. That is, presence of Mn, Fe, Cr, and Co hinders the modification into β1 phase, resulting in increased cutting resistance of the alloy and deterioration of chip breakability, dezincification corrosion resistance, and resistance to stress corrosion cracking. Therefore, the total amount of Fe, Mn, Co, and Cr needs to be lower than 0.40 mass% at most, preferably lower than 0.30 mass%.

[0066] Even though the formation of compounds comprising P and Zn and / or Si has a positive effect on machinability, the conditions required to realize the modification of β phase into β1 phase are opposite to those to realize the presence of compounds comprising P and Zn and / or Si. As stated above, for the formation of β1 phase, the cooling treatment performed after hot working needs to be started when the temperature of the hot-worked material is higher than 500°C and lower than 670°C and the cooling rate in the temperature range from the starting temperature of the cooling treatment to 300°C needs to be high whereas for sufficient formation of compounds of P and Zn and / or Si, slow cooling is necessary in the temperature range from about 530°C to about 450°C. Therefore, when a large amount of phosphorous compounds are formed, formation of β1 phase is slightly insufficient, i.e., modification of β phase is slightly insufficient. In some cases, β1 phase is not present at all.

[0067] On the other hand, if 0.18 mass% or more P is contained, ductility and electrical conductivity do not improve but rather deteriorate due to the effect of P solid-solubilized in β1 phase since the effect brought by formation of β1 phase has already peaked. For this reason, the P content is lower than 0.18 mass%, preferably 0.15 mass% or lower, and more preferably 0.12 mass% or lower.(Pb)

[0068] In the embodiment, good machinability is obtained by the effect of β1 phase including Si and P, but it is further improved by containing a small amount of Pb. In the composition of the embodiment, about 0.001 mass% Pb is solid-solubilized in the matrix, and the portion in excess of this amount is present in the form of tiny Pb particle with a diameter of about 0.1 to about 2 µm. It is generally believed that Pb has little effect on improvement of machinability if its content is approximately 0.1 mass%. For instance, ASM Specialty Handbook first edition "Copper and Copper Alloys" discloses a relationship between Pb content and machinability in which the machinability of a Cu-Zn-Pb alloy consisting of 62 to 65 mass% Cu, about 3.2 mass% Pb with the balance being Zn is assumed to be 100% (Fig. 6 on p. 267 of the Handbook). The Fig. 6 indicates that containing 0.1 mass% Pb only has an effect of improving machinability by about 5 percentage points from 25% to 30% in terms of machinability index. In contrast, in the present invention, Pb has a significant effect on machinability even though its content is very small. The effect is exhibited if the Pb content is 0.002 mass% or higher. The Pb content is preferably 0.003 mass% or higher, more preferably 0.01 mass% or higher. When cutting conditions are severe, for example, when cutting speed or feed rate is high, cutting depth is deep, or the diameter of the hole to drill is large, the Pb content is preferably 0.03 mass% or higher. Due to the effects of β1 phase with greatly enhanced machinability and a small amount of Pb contained, machinability of the alloy significantly improves.

[0069] It is well known that Pb improves machinability of copper alloys, but for a Cu-Zn binary alloy, as represented by a free-cutting brass bar C3604, to obtain the effect, about 3 mass% Pb is required. In the embodiment, an alloy with excellent machinability is accomplished by causing β1 phase including Si and P, a tiny amount of Pb particles, and particles composed of Pb and Bi, which will be described later, to be present in the metallographic structure. The upper limit of Pb content is lower than 0.20 mass% since Pb is harmful to human body. The Pb content is preferably lower than 0.10 mass% and, in consideration of its influence on human body and the environment, it is most preferably 0.08 mass% or lower.(Bi)

[0070] Like Pb, Bi is solid-solubilized in the matrix in an amount of approximately 0.0001 mass%, and the portion in excess of this amount is present in the form of particle with a diameter of about 0.1 to about 2 µm. In the case both Pb and Bi are added, large portions of them are present in the form of particles composed of a mixture of Pb and Bi with a diameter of about 0.1 to about 2 µm. The inventors have discovered that if Bi and Pb are both contained in an embodiment where β1 phase is present, machinability equivalent to or better than when Bi or Pb is contained alone can be obtained. The machinability improvement effect of Bi had been known to be weaker than that of Pb, but the inventors revealed that Bi exhibited equivalent or, in some cases, better effect than Pb in in the case of the embodiment. Apropos, Bi deteriorates resistance to stress corrosion cracking of brass, but such an effect disappears almost entirely when it is present in the form of mixed particles of Pb and Bi or when the amount of Bi is small.

[0071] For an alloy containing Bi to obtain good machinability, the Bi content needs to be at least 0.0001 mass% or higher. The Bi content is preferably 0.001 mass% or higher, more preferably 0.002 mass% or higher. The effect of Bi on the human body is presently unknown, but its content is lower than 0.20 mass%, preferably lower than 0.10 mass%, and more preferably 0.08 mass% or lower. Incidentally, as stated above, considering the facts that Bi is well capable of replacing Pb in the embodiments, its effect on the human body is unknown, use of Bi affects the environment since it is a rare metal, and the element is included in the raw material as an inevitable impurity, Bi is included as an optional element, i.e., not required to be contained. Further, considering its influence on human body and the environment as well as stress corrosion cracking, the total content of Pb and Bi (the composition relational expression f2 that is explained later) is lower than 0.20 mass%, preferably lower than 0.10 mass%. Thus, the embodiments are directed to reducing the content of Pb, an element harmful to the human body, in some cases the total content of Pb and Bi, to lower than 0.20 mass% yet obtaining excellent machinability.(Sn)

[0072] Sn further improves the dezincification corrosion resistance of β1 phase by solid solubilizing in the phase, allowing the alloy's dezincification corrosion resistance to improve. To obtain this effect where Sn is contained, its content needs to be higher than 0.05 mass%. The Sn content is preferably 0.10 mass% or higher. Apropos, a higher proportion of Sn is inherently distributed to β and β1 phases than α phase, and Sn is able to exhibit its effect of improving dezincification corrosion resistance even if its content is small. However, when its concentration is high, γ phase is likely to be formed, causing deterioration of ductility. Formation of phase causes deterioration of not only ductility but also machinability and dezincification corrosion resistance of the alloy. It is therefore necessary to limit the Sn content to lower than 0.90 mass% although it depends on the amount of Si contained. The Sn content is preferably lower than 0.70 mass%, more preferably lower than 0.50 mass%.

[0073] As stated above, the inventors have learned that when a large amount of Sn is contained, problem sometimes occurs in the modification of β phase containing Si and P because a larger portion of Sn is distributed to β phase and β1 phase. Specifically, when the Sn content is larger than the Si content, β phase fails to be sufficiently modified, and the effect of improving dezincification corrosion resistance brought by Sn is offset. As described later, the Si content needs to exceed the Sn content.(Inevitable Impurities, in Particular, Fe, Mn, Co, Cr, and Al)

[0074] Examples of the inevitable impurities in the embodiments include Mn, Fe, Al, Ni, Mg, Se, Te, Sn, Bi, Co, Ca, Zr, Cr, Ti, In, W, Mo, B, Ag, and rare earth elements.

[0075] Conventionally, primary raw material of a free-cutting copper alloy, in particular, a free-cutting brass including about 30 mass% or higher Zn is not a quality raw material such as electrolytic copper or electrolytic zinc but recycled copper alloy. In the later steps (downstream steps or processing steps) in this field of art, machining is performed on almost all the parts and components, during which a large amount of copper alloy accounting for 40 to 80% of the material in terms of weight goes to waste. Examples of such waste material include chips, mill ends, burrs, runners, and products with manufacturing defects. These waste copper alloys constitute the primary raw material. If cutting chips, mill ends, and the like are not properly separated, Pb, Fe, Mn, Si, Se, Te, Sn, P, Sb, As, Bi, Ca, Al, Zr, Ni, and / or rare earth elements mix in as part of raw material from leaded free-cutting brass, free-cutting copper alloy free of Pb but containing Bi or the like, or special brass alloy containing Si, Mn, Fe, and / or Al, or other copper alloys. In addition, cutting chips include Fe, W, Co, Mo, and the like which originate from tools. Ni, Cr, or Sn may mix in since disposed products to be recycled include plated products. Further, pure copper-based scrap used instead of electrolytic copper is contaminated with Mg, Sn, Fe, Cr, Ti, Co, In, Ni, Se, and / or Te. Brass-based scraps that are used instead of electrolytic copper or electrolytic zinc often include a material plated with Sn, causing Sn to mix in a recycled material.

[0076] From a viewpoint of reuse of resources and cost reduction, scraps including these elements are used as a raw material to the extent that use of such material does not adversely affect the resultant properties of the alloy at least. Incidentally, in a leaded JIS free-cutting brass bar, C3604 (JIS H 3250), about 3 mass% Pb is essentially contained. Further, as impurities, Fe may be contained up to 0.5 mass% and Fe and Sn may be contained up to 1.0 mass% (maximum total amount of Fe and Sn is 1.0 mass%). As a matter of fact, a high concentration of Fe or Sn is sometimes included in a free-cutting brass bar.

[0077] Fe, Mn, Co, and Cr solid-solubilize in α phase and β phase of a Cu-Zn alloy to a certain concentration. However, if Si and / or P is present then, they tend to compound with Si and / or P. If that happens, Si and P that are required for the modification of β phase could be consumed. Fe, Mn, Co, or Cr compounded with Si forms a Fe-Si compound, an Mn-Si compound, a Co-Si compound, a Cr-Si compound, or the like in the metallographic structure. Likewise, when they compound with P, a Fe-P compound, an Mn-P compound, a Co-P compound, a Cr-P compound, or the like is formed. These intermetallic compounds not only shorten tool life but also increase cutting resistance since they are extremely hard. For this reason, the contents of Fe, Mn, Co, and Cr need to be limited, and each content is preferably lower than 0.30 mass%, more preferably lower than 0.20 mass%, and still more preferably 0.15 mass% or lower. In particular, the total content of Fe, Mn, Co, and Cr is required to be lower than 0.40 mass%, preferably lower than 0.30 mass%, more preferably lower than 0.25 mass%, and still more preferably 0.20 mass% or lower.

[0078] On the other hand, the amount of Al that mixes in from special brass bar, brass casting, or the like needs to be limited because the element affects the modification of β phase if the Al content is large and also forms compounds with P or Si. In an alloy according to an embodiment of the present invention, the Al content needs to be lower than 0.30 mass%, preferably lower than 0.15 mass%, and more preferably 0.10 mass% or lower.

[0079] As other major inevitable impurity elements, empirically speaking, Ni often mixes in from scraps and the like. Even though the effect of Ni on mechanical properties such as machinability is relatively small, its content needs to be restricted in consideration of the impact on human body. Accordingly, the Ni content is preferably lower than 0.20 mass% and more preferably lower than 0.10 mass%. It is not particularly necessary to limit the content of Ag because Ag is commonly considered as Cu and its impact on various properties is negligible, but its preferable content is lower than 0.05 mass%. With respect to Te and Se that inherently have free-cutting properties, they may mix in in large quantity although such a case is rare. In consideration of their influence on ductility and impact properties, each content of Te and Se is preferably lower than 0.10 mass%, more preferably lower than 0.05 mass%, and still more preferably 0.02 mass% or lower. Further, corrosion-resistant brass includes As and / or Sb for the improvement of its corrosion resistance. In consideration of their impact on ductility and human body, each content of As and Sb is preferably lower than 0.05 mass% and more preferably 0.02 mass% or lower.

[0080] Each content of other elements such as Mg, Ca, Zr, Ti, In, W, Mo, B, and rare earth elements is preferably lower than 0.05 mass%, more preferably lower than 0.03 mass%, and still more preferably 0.02 mass% or lower.

[0081] Apropos, the content of the rare earth elements refers to the total amount of one or more of the following elements: Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, and Lu.

[0082] The total content of inevitable impurities excluding Fe, Mn, Co, Cr, and Al is preferably lower than 0.70 mass% and more preferably lower than 0.50 mass%.(Composition Relational Expression f1)

[0083] In composition relational expression f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn], when the Sn content is 0.05 mass% or lower, the value of [Sn] in this expression is 0, and when Bi is not included, the value of [Bi] is 0. Sn is not defined by the composition relational expression f1 if its content is 0.05 mass% or lower since its impact on the expression is minor.

[0084] f1 is an expression that represents a relationship. Even when the amount of each of the elements is within the range defined above, unless this composition relational expression f1 is satisfied, the target properties in embodiments of the present invention cannot be obtained. When the value of the composition relational expression f1 is lower than 57.5, the proportions of β phase and β1 phase are large, causing deterioration of ductility, dezincification corrosion resistance, and resistance to stress corrosion cracking due to insufficient modification of β phase.

[0085] Accordingly, the lower limit of the value of the composition relational expression f1 is 57.5 or higher, preferably 58.0 or higher, and more preferably 58.2 or higher. As the value of the composition relational expression f1 becomes more favorable within the defined range, the proportion of α phase increases and modification of β phase becomes sufficient. As a result, good dezincification corrosion resistance and resistance to stress corrosion cracking as well as good ductility and cold workability can be obtained without impairing excellent machinability.

[0086] On the other hand, the upper limit of the f1 value affects the proportions of β phase and β1 phase. When the value is higher than 60.5, the proportion of β1 phase is small and excellent machinability cannot be obtained. In addition, strength is low. Therefore, the value of the composition relational expression f1 is 60.5 or lower, preferably 60.2 or lower, and more preferably 60.0 or lower.(Composition Relational Expression f2)

[0087] Bi is an optional element that replaces the function of Pb. If Bi is contained, a relational expression of 0.003 ≤ f2 = [Pb] + [Bi] < 0.20 must be satisfied. That is, the contents of Pb and Bi need to be restricted by their total amount considering the facts that the impact of Bi on the human body is unknown at present and Bi affects the environment since it is a rare metal. If Bi is contained, the total content of Pb and Bi is lower than 0.20 mass%, which is the upper limit of the Pb content, and preferably lower than 0.10 mass%. On the other hand, when Bi is contained for the purpose of obtaining good machinability, the total content of Pb and Bi needs to be 0.003 mass% or higher, which is preferably 0.004 mass% or higher, and still more preferably 0.005 mass% or higher.(Composition Relational Expression f0)

[0088] Sn is contained in order to further improve dezincification corrosion resistance. A large portion of Sn is contained in β phase, and the larger the Sn content, the slower the advancement of dezincification corrosion of β1 phase. However, if the Sn content is excessively larger than the Si content, modification of β phase is hindered, which may impair dezincification corrosion resistance of β1 phase.

[0089] That is, providing that f0 = [Sn] / [Si], in order for Sn to exhibit an effect that is commensurate with the amount of Sn contained in consideration of other properties of the alloy, f0 needs to be lower than 1. It is preferably lower than 0.8, more preferably lower than 0.6, and still more preferably lower than 0.5.

[0090] Free-cutting copper alloys according to an embodiment of the present invention have good dezincification corrosion resistance, resistance to stress corrosion cracking, and mechanical properties despite a large content of conventional β phase, which refers to β1 phase in the present invention. They also have machinability that requires sort of brittleness which allows low cutting resistance and generation of finely broken chips as well as ductility, a property entirely opposite to machinability. To obtain good corrosion resistance, machinability, and mechanical properties, the composition relational expressions f0, f1, and f2, the metallographic structure relational expressions f3 to f5, and the metallographic structure and composition relational expressions f6 that are described later need to be satisfied in addition to the requirements regarding each component that constitutes the alloy's composition. By discussing these factors in detail, an alloy that is more suitable for a particular purpose or use can be provided.

[0091] Incidentally, Fe, Mn, Co, Cr, Al, and inevitable impurities that are separately defined are not defined by the composition relational expression f1 because their impact on f1 is small as long as the content is within the range that can be treated as an inevitable impurity.(Comparison with Patent Documents)

[0092] Here, the results of comparison between the compositions of the Cu-Zn-Si alloys described in Patent Documents 1 to 15 and those of alloys according to embodiments of the present invention are shown in Tables 1 to 4. [Table 1]CuSiPPbBiSnAlOthersMetallographicStructureFirst Embodiment60.5 -65.00.50 -1.200.01 -0.180.002 -0.20Optional 0.0001-0.20-<0.30Fe+Mn+Cr+Co <0.4020≤α<75, 25<β1≤80, 0≤γ<4The average cooling rate in the temperature range from 500°C to 300°C after hot working is higher than 300°C / min.Second Embodiment61.2 64.80.65 -1.100.03 -0.150.003 -0.10Optional 0.001-0.10-<0.15Fe+Mn+Cr+Co <0.3030≤α≤70, 30≤β1≤70, 0≤γ<1The average cooling rate in the temperature range from 500°C to 300°C after hot working is higher than 300°C / min.Third Embodiment60.5 -65.00.50 -1.200.01 -0.180.002 -0.20Optional 0.0001-0.200.05 -0.9<0.30Fe+Mn+Cr+Co <0.4020≤α<75, 25<β1≤80, 0≤γ<4The average cooling rate in the temperature range from 500°C to 300°C after hot working is higher than 300°C / min.Fourth Embodiment61.2 -64.80.65 -1.100.03 -0.150.003 -0.10Optional 0.001-0.100.10 -0.50<0.15Fe+Mn+Cr+Co <0.3030≤α≤70, 30≤β1≤70, 0≤y<1The average cooling rate in the temperature range from 500°C to 300°C after hot working is higher than 300°C / min. [Table 2] CuSiPPbBiSnAlOthersMetallographicStructurePatent Document 159-62-0.02 -0.07-0.3-0.4--Fe≤0.3Two phases of α and β phases; β phase is divided by α phaseAnnealed at a temperature between 350°C and 550°CPatent Document 261-651.0-1.50.005 -0.190.003 -0.20Sn+Al+Bi<0.4, Fe+Mn+Cr+Co<0.420≤α≤80, 15≤β≤80, 0≤γ<8The average cooling rate in the temperature range from 530°C to 450°C after hot working is 0.1°C / min or higher and 50°C / min or lower.Patent Document 359.0 -63.50.5-1.00.005 -0.190.003 -0.20Sn+Al+Bi<0.4, Fe+Mn+Cr+Co<0.420≤α≤75, 25≤β≤80, 0≤γ<2The average cooling rate in the temperature range from 530°C to 450°C after hot working is 0.1 °C / min or higher and 50°C / min or lower.Patent Document 458.5 -65.00.4-1.40.003 -0.190.002 0.25Optional 0.001-0.1Sn+Al+Bi<0.45, Fe+Mn+Cr+Co<0.4520≤α≤80, 18≤β≤80, 0≤γ<5The average cooling rate in the temperature range from 530°C to 450°C in the cooling process after casting is 0.1°C / min or higher and 55°C / min or lower. [Table 3] CuSiPPbBiSnAlOthersMetallographicStructurePatent Document 558.5 -64.50.2-1.20.001 -0.20.001 -0.200.1-1.0Sn+Al+Bi<0.45, Fe+Mn+Cr+Co<0.4520≤α≤85, 15≤β≤80, 0≤γ<4The average cooling rate in the temperature range from 530°C to 450°C after hot working is 0.1 °C / min or higher and 50°C / min or lower.Patent Document 658-650.3-1.30.001 -0.20.001 -0.200.02-0.1Sn+Al+Bi<0.45, Fe+Mn+Cr+Co<0.4520≤α<85, 15≤β≤80, 0≤γ<5The average cooling rate in the temperature range from 530°C to 450°C after hot working is 0.1 °C / min or higher and 50°C / min or lower.Patent Document 759.7 -64.70.6-1.30.001 -0.150.001 -0.20.001-0.1Sn+Al+Bi<0.45, Fe+Mn+Cr+Co<0.4517≤β≤75The average cooling rate in the temperature range from 530°C to 440°C after hot working is 0.1°C / min or higher and 70°C / min or lower.Patent Document 871.5 -78.52.0-4.50.01-0.20.005 -0.020.01-0.20.1-1.20.1-2.0-0≤β≤5Patent Document 960-660.01-0.5-0.150.05-0.5-0.2-0.9-0.02≤Fe+PArea ratio of α phase to β phase: 4:1-19:1Finish heat treatment is performed with a heating and holding temperature of 400-600°C and a holding time of 30 minutes or longer.Patent Document 1059-840.05-4.50.01-0.30.003 -0.30.003 -0.30.01-1.50.1-1.0-0≤β+γ+µ≤25; tubular hot-forged copper alloy product with a near-net shape [Table 4] CuSiPPbBiSnAlOthersMetallographicStructurePatent Document 1158-72.50.05-2.00.01 -0.250.01-40.01-30.05-40.01-3Zr: 0.0008 -0.04585≤α+γ → β≤15Patent Document 12Rem.0.01-10.005 -0.090.005 -0.03-0.01 -2.50.1-1.8Zn: 17-34y phase: 0-0.7%β phase: 0-0.9%Patent Document 13Rem.0.1-1.50.01 -0.360.05 -0.50.03 -0.3Ti: 0.01-0.1-Rare earths: 0.01-0.05Patent Document 1455-750.01-1.5-0.150.01 -0.40.01-40.1-0.1---Patent Document 1565-750.5-2.0---0.01-0.20.2-0.5Mn: 0.01-0.25β phase is essentially required.Sb: 0.01-0.55B: 0.001-0.05

[0093] The embodiments and the alloys disclosed by Patent Document 8 are different in the contents of Si and Cu, which are main elements of the alloys. The embodiments and the alloys disclosed by Patent Document 9 are different in the content of Si, which is a main element of the alloys. The embodiments and the alloys disclosed by Patent Document 15 are different in the content of Cu, which is a main element of the alloys.

[0094] Patent Documents 1, 13, and 15 disclose that Pb is not contained. Therefore, the Pb content is different.

[0095] In Patent Documents 8, 10, 11, and 12, β phase in the metallographic structure is significantly limited in view of its effect on machinability, dezincification corrosion resistance, other type of corrosion resistance, and the like. β phase disclosed by these documents and β1 phase in the present invention are two different phases, but for reference, β phase is limited to 5% or lower, 25% or lower, 15% or lower, and 0.9% or lower respectively in these Patent Documents.

[0096] Patent Document 10 relates to a tubular hot-forged part having a near-net shape and discloses that a tubular material is used.

[0097] Patent Documents 1 and 9 disclose that heat treatment is performed at a temperature either between 350°C and 550°C or between 400°C and 600°C in order to reduce or fragment β phase.

[0098] Patent Document 9 discloses that 0.2 mass% or higher Sn is contained, Sn and Si are contained for the improvement of dezincification corrosion resistance of β phase and requires hot extrusion performed at a temperature of 700°C or higher for the improvement of machinability and heat treatment performed at a temperature between 400°C and 600°C for the improvement of corrosion resistance. The proportion of β phase is approximately 5 to 20%, and the Si content is 0.01 to 0.50 mass%, which may be controlled to be 0.2 mass% or lower.

[0099] In Patent Documents 12, 13, and 14, Al is essentially required in order to improve color fastness, castability, and dezincification corrosion resistance.

[0100] Patent Document 14 requires containing at least 0.1 mass% Sn and Al respectively in order to improve dezincification corrosion resistance and large amounts of Pb and Bi in order to obtain excellent machinability.

[0101] Patent Document 15 discloses a corrosion-resistant copper alloy casting that does not contain Pb but requires y phase and have good mechanical properties and castability, which are realized by containing tiny amounts of elements such as Al, Sb, Sn, Mn, Ni, B, and so on in addition to Si and 65 mass% or higher Cu.

[0102] All of Patent Documents 2 to 7 essentially require presence of phosphorous compounds in the metallographic structure that are formed as a result of a treatment performed in the cooling process after hot working at an average cooling rate of about 0.1°C / min or higher and about 70°C / min or lower in the temperature range from about 530°C to about 450°C. Further, the Documents disclose that y phase is effective to obtain good machinability but there is not a word about modification of β phase in these Documents. They are silent about dezincification corrosion resistance and resistance to stress corrosion cracking, and no relevant data is disclosed. In addition, none of the Documents discloses data related to cutting resistance in high-speed machining, either except for Patent Document 7. The present invention is cooled in a manner basically contrary to that disclosed by Patent Documents 2 to 7. Specifically, in the present invention, the cooling treatment performed in the cooling process after hot working is started when the temperature of hot-worked material is lower than 670°C and higher than 500°C, and the average cooling rate is higher than 300°C / min in temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C.

[0103] The most remarkable and clear-cut difference between the present application and Patent Documents 2 to 7 is that none of the prior art documents refers to modification of β phase or modified β phase, that is, β1 phase. β1 phase is formed when the following conditions are met. In the process of hot working, β phase in which certain amounts of Si and P are solid-solubilized is formed; a cooling treatment is started when the temperature of hot-worked material is higher than 500°C; and the average cooling rates in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C are both higher than 300°C / min. β1 phase is thus formed. If the phase formed is β1 phase, a crystal grain boundary is observed inside the phase when etched with a mixed solution of hydrogen peroxide and ammonia water. Before modification to β1 phase, however, even if certain amounts of Si and P are contained, no such boundary is observed inside β phase. Patent Documents 2 to 7 disclose pictures of metallographic structures obtained by etching with a mixed solution of hydrogen peroxide and ammonia water, but no grain boundary is observed inside β phase in any of the pictures. In these prior art documents, starting temperature of the cooling treatment performed after hot working is not defined. However, even if the cooling treatment was started at a temperature lower than 530°C, β1 phase (modified β phase) would not appear since the average cooling rate from the starting temperature of the cooling treatment to 500°C is about 0.1 °C / min or higher and about 70°C / min or lower.

[0104] Unlike conventional β phase, β1 phase is able to improve dezincification corrosion resistance and resistance to stress corrosion cracking significantly, and better improve machinability than β phase. In fact, its machinability improvement effect excels that of the presence of phosphorous compounds. However, for β1 phase to exhibit such effects, its proportion in the metallographic structure needs to be larger than 25% in terms of area ratio. In addition, whereas presence of γ phase impairs machinability in the present invention, γ phase is effective to obtain good machinability in Patent Documents 2 to 7.<Metallographic Structure>

[0105] In a Cu-Zn-Si alloy, 10 or more kinds of phases are present, and they change in a complicated manner. Therefore, desired properties cannot be necessarily obtained by simply satisfying specific composition ranges and relational expressions regarding elements. Desired properties can eventually be obtained by specifying and determining the kinds of phases that need to be present in the metallographic structure as well as the ranges of their respective area ratios. Accordingly, the metallographic structure relational expressions are defined as follows. 20 ≤ f 3 = α < 75 25 < f 4 = β 1 ≤ 80

[0106] Note that in this expression, β1 phase refers to modified β phase. 0 ≤ f 5 = γ < 4(β1 phase)

[0107] Aside from non-metallic inclusions, the metallographic structure of the present invention consists of α phase, β1 phase, and in some cases, a tiny amount of γ phase which may not be present at all.

[0108] The difference between β phase and β1 phase is that in the case of β1 phase, a grain boundary pattern, i.e., boundary of crystal grain is observed inside β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water, but no crystal grain boundary is observed inside β phase.

[0109] β1 phase of an alloy according an embodiment of the present invention is obtained by causing certain amounts or more of Si and P to solid-solubilize in β phase under high temperature during hot working and maintaining the state of β phase when it is between 500°C and 670°C until the temperature of alloy goes down to normal temperature through a cooling treatment (i.e., cooling the alloy at an average cooling rate exceeding 300°C / min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C and continuing to cool it until the temperature of the alloy becomes normal temperature) so that the state of the metallographic structure when its temperature is between 500°C and 670°C is maintained until the temperature of the alloy becomes normal temperature. More specifically, to obtain β1 phase, the alloy is rapidly cooled at a high cooling rate in the temperature range from a high starting temperature of cooling to 300°C then is kept cooling until its temperature goes down to a normal temperature of 100°C or lower. By doing so, the state of metallographic structure under high temperature can be maintained until after it is cooled down to normal temperature. As a result, β1 phase is obtained. Even if the aforementioned cooling treatment is performed on β phase of a Cu-Zn-based alloy not containing both of Si and P in certain amounts, β1 phase cannot be obtained. Likewise, if the cooling treatment is performed on an alloy containing Si and P with a starting temperature below 500°C, e.g., 450°C and a cooling rate exceeding 300°C / min, β1 phase cannot be obtained. Incidentally, for the modification of β phase, α phase is required in a certain amount or more. Therefore, if α phase is not present or the amount of α phase is insufficient, β1 phase cannot be obtained. Further, the degree of the modification is also affected by the amounts of Si, P, and inevitable impurities, the starting temperature of the cooling treatment, and the average cooling rates in temperature ranges from the starting temperature to 500°C and from 500°C to 300°C. If the degree of the modification is improved, that is, β1 phase is better modified, a material with better machinability and good dezincification corrosion resistance and resistance to stress corrosion cracking can be obtained. Modification of β phase generally starts to peak when the amount of Si is about 1 mass% and that of P is about 0.1 mass% although that depends on the amount of inevitable impurities such as Fe. If the amounts of Si and P are excessive, adverse effects such as deterioration in conductivity of ductility, appearance of y phase, and the like may be induced.

[0110] β1 phase (modified β phase) is able to overcome the drawbacks of a Cu-Zn-based alloy, i.e., its dezincification corrosion resistance and resistance to stress corrosion cracking which used to be major challenges. To demonstrate an example in concrete numbers, progress of dezincification corrosion can be reduced by approximately 60% or more, and that of stress corrosion cracking can be delayed by about 50% or more. As disclosed by the referenced patent documents, dezincification corrosion is a significant problem that a Cu-Zn-based alloy containing β phase has. The amount of β phase is limited to 25% or less or 20% or less, and a heat treatment is performed at a temperature between 350°C and 550°C to fragment β phase and reduce the amount of β phase because dezincification corrosion occurs along β phase.

[0111] In terms of strength, it is largely affected by the area ratios of constituent phases. Modified β phase, i.e., β1 phase has a higher strength and better ductility than β phase. Therefore, strength and ductility in an alloy with β1 is well-balanced.(β1 phase; Metallographic Structure Relational Expression f4)

[0112] In a free-cutting Cu-Zn-Si-P-Pb alloy of an embodiment of the present invention, the area ratio of β1 phase needs to be higher than 25% in order to obtain good machinability while minimizing the Pb content. Further, in order to improve machinability and strength, the area ratio is preferably 30% or higher, more preferably 33% or higher. On the other hand, if the proportion of β phase is excessive, for instance, 95%, β phase will not be modified to β1 phase. Modification of β phase occurs where α phase is present. Therefore, a certain amount of α phase is necessary. Further, with respect to dezincification corrosion and stress corrosion cracking, even though β1 phase is able to delay the progress of corrosion much better than β phase, its dezincification corrosion resistance and resistance to stress corrosion cracking are still much weaker than those of α phase. Specifically, in a dezincification corrosion test performed in accordance with ISO 6509, if the metallographic structure consists of α phase and β phase (before modification), β phase is selectively corroded by dezincification, and the depth of such corrosion is as deep as about 500 µm. In the case the metallographic structure consists of α phase and β1 phase (modified β phase), progress of dezincification corrosion is significantly suppressed since even though β1 phase is selectively corroded by dezincification, the depth of corrosion is about 20 µm to about 200 µm depending on the degree of modification and the area ratio of β1 phase. Thus, by modifying β phase into β1 phase, dezincification corrosion resistance improves significantly. Still, dezincification corrosion resistance and ductility of β1 phase are not as good as those of α phase. For this reason, when the area ratio of β1 phase is high, dezincification corrosion resistance and ductility of the alloy are poor. As a result of a devoted study taking these matters into account, the inventors came to conclude that it is necessary to limit the area ratio of β1 phase to 80% or lower. The area ratio is preferably 70% or lower, more preferably 65% or lower.(α phase; Metallographic Structure Relational Expression f3)

[0113] The present invention basically consists of α phase and β1 phase. Although a treatment is performed to modify β phase into β1 phase, this treatment little affects α phase. Further, for the modification of β phase, a certain amount or more of α phase is necessary, and if the amount of β1 phase is excessive, ductility of the alloy is poor. Therefore, an appropriate amount of α phase, a phase that has good ductility, is required. However, if the amount of α phase is excessive, strength of the alloy is low. α phase including Si has only slightly better machinability than that excluding Si. From a standpoint of machinability also, the amount of α phase is limited. Even though the present invention consists of β1 phase having excellent machinability and α phase having somewhat poor machinability, the cutting resistance of the present invention can be maintained to be low, and well-fragmented chips are generated if the proportion of α phase is up to about 75% since α phase functions as a cushioning material and a stress concentration source at boundary with hard β1 phase during machining. Incidentally, it is desirable that α phase is tiny and has a granular shape since it functions as a cushioning material and a stress concentration source at boundary with hard β1 phase during machining.

[0114] As a result of a devoted study on modification of β phase, dezincification corrosion resistance, resistance to stress corrosion cracking, machinability, and mechanical properties of the alloy of the present invention, it was determined that the amount of α phase needs to be 20% or higher, preferably 30% or higher, and more preferably 35% or higher. The upper limit of α phase, on the other hand, is lower than 75%, preferably 70% or lower, and more preferably 67% of lower.(γ Phase; Metallographic Structure Relational Expression f5)

[0115] As disclosed by Patent Documents 8 and 10, γ phase is a phase that contributes to machinability in a Cu-Zn-Si alloy in which the Cu concentration is about 69 mass% to about 80 mass% and the Si concentration is about 2 to about 4 mass%. Patent Document 15 discloses that γ phase is indispensable for a Cu-Zn-Si alloy free of Pb. Further, Patent Documents 2 to 6 disclose that γ phase containing Si has good machinability like β phase containing Si.

[0116] A negative coefficient larger than the coefficient given to the amount of β1 phase is given to γ phase in the metallographic structure relational expression f6 that is described later since γ phase has an adverse effect on the machinability of embodiments of the present invention. Further, γ phase impairs ductility and deteriorates dezincification corrosion resistance because γ phase does not become modified in the embodiments. Appearance of γ phase may not be avoided altogether in a Cu-Zn-Si alloy, but γ phase should be restricted at least. Thus, proportion (area ratio) of γ phase is limited to smaller than 4%. It is preferably smaller than 2%, more preferably smaller than 1%, and optimally, this phase is not present, i.e., f5 = 0.(Metallographic Structure and Composition Relational Expression f6)

[0117] f 6 = β 1 × Si 1 / 2 − γ × 2 + Pb + Bi 1 / 2 × 20 + P 1 / 2 × 15

[0118] f6 is a simple conditional expression for an alloy to obtain good machinability. In f6, the amount of β1 phase and the amounts of Si, Pb, Bi, and P contained in the alloy within the respective compositional ranges defined in the instant specification are put together and regarded to have a positive effect and the amount of γ phase is regarded to have a negative effect. More specifically, the amount of γ phase multiplied by a coefficient of 2 is deducted from the total of the amount of β1 phase containing Si and P multiplied by the amount of Si raised to the power of 1 / 2, the sum of the amounts of Pb and Bi raised to the power of 1 / 2 multiplied by a coefficient of 20, and the amount of P raised to the power of 1 / 2 multiplied by a coefficient of 15. Performance of β1 phase (modified β phase) is directly affected by the concentration of Si as well as that of P, and when a tiny amount of Pb or Bi is contained, its machinability improves. γ phase, on the other hand, hinders machinability in the present invention. The inventors have revealed that the degree of machinability improvement brought by Pb or Bi is deeply related to a total amount of Pb and Bi raised to the power of 1 / 2. Both elements exhibit a significant effect even if the content is very small. As the contents are increased, their machinability improvement effects improve, but intensity of the effect gradually subsides. Due to the presence of β1 phase (modified β phase) in the constituent phases of alloy, Pb or Bi is able to exhibit a significant effect on machinability even if the content is extremely small. Such an effect will never occur to anyone who is familiar with the relationship between machinability of a conventional Cu-Zn-Pb alloy and the amount of Pb contained in the alloy.

[0119] As a result of devoted study, it was revealed that to obtain good machinability, the f6 value is higher than 27, preferably 33 or higher, more preferably higher than 35, and still more preferably 38 or higher. As the f6 value gets higher, machinability becomes closer to that of a free-cutting brass bar containing 3 mass% Pb.

[0120] Now, Figs. 1A to 6B show pictures of the metallographic structures of various alloys and the results of the dezincification corrosion tests performed in accordance with ISO 6509.

[0121] Fig. 1A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S01 to Step No. A2. More specifically, Alloy No. S01 has a composition including 63.3 mass% Cu, 0.95 mass% Si, 0.069 mass% P, 0.063 mass% Pb, 0.017 mass% Bi, and Zn as the balance. In Step No. A2, the alloy was subjected to hot extrusion performed at 630°C and a cooling treatment which was started when the temperature of the hot-extruded material became 580°C and performed at an average cooling rate of 1020°C / min in both temperature ranges from 580°C to 300°C and from 500°C to 300°C. Incidentally, the average cooling rate in the temperature range from 580°C, the temperature at which the cooling treatment was started, to 500°C can be easily calculated from the average cooling rates in temperature ranges from 580°C to 300°C and from 500°C to 300°C.

[0122] Fig. 1B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 1A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.

[0123] Fig. 2A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S11 to Step No. E2. More specifically, Alloy No. S11 has a composition including 62.5 mass% Cu, 0.96 mass% Si, 0.064 mass% P, 0.072 mass% Pb, and Zn as the balance. In Step No. E2, the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm. The rod was heated and hot forged at 680°C to have a thickness of 20 mm when placed horizontally then subjected to a cooling treatment which was started when its temperature became 565°C and performed at an average cooling rate of 900°C / min in both temperature ranges from 565°C to 300°C and from 500°C to 300°C.

[0124] Fig. 2B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 2A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.

[0125] Fig. 3A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S11 to Step No. E13H. More specifically, Alloy No. S11 has a composition including 62.5 mass% Cu, 0.96 mass% Si, 0.064 mass% P, 0.072 mass% Pb, and Zn as the balance. In Step No. E13H, the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm. The rod was heated and hot forged at 630°C to have a thickness of 20 mm when placed horizontally then cooled at an average cooling rate of 35°C / min. Subsequently, the rod was subjected to a cooling treatment which was started when its temperature became 455°C and performed at an average cooling rate of 800°C / min in the temperature range from 455°C to 300°C. Note that the starting temperatures of the cooling treatment performed on the alloy in Fig. 3A and that in Fig. 2A are different.

[0126] Fig. 3B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 3A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.

[0127] Fig. 4A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S43 to Step No. E6. More specifically, Alloy No. S43 has a composition including 63.3 mass% Cu, 0.98 mass% Si, 0.084 mass% P, 0.060 mass% Pb, 0.28 mass% Sn, and Zn as the balance. In Step No. E6, a casting produced with a mold having a diameter of 55 mm was machined to a diameter of 50 mm then cut to a length of 200 mm. The casting was heated and hot forged at 630°C to a thickness of 20 mm when placed horizontally then subjected to a cooling treatment which was started when the temperature of the casting became 565°C and performed at an average cooling rate of 900°C / min in both temperature ranges from 565°C to 300°C and from 500°C to 300°C.

[0128] Fig. 4B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 4A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.

[0129] Fig. 5A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step Nos. A34H and G1. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.98 mass% Si, 0.071 mass% P, 0.071 mass% Pb, and Zn as the balance. In Step No. A34H, an alloy was obtained by hot extrusion at 615°C and cooling at an average cooling rate of 18°C / min in the temperature range from 500°C to 300°C. In Step No. G1, the alloy was further heated to 580°C for 30 minutes then subjected to a cooling treatment which was started when its temperature became 560°C and performed at an average cooling rate of 1800°C / min in both temperature ranges from 560°C to 300°C and from 500°C to 300°C.

[0130] Fig. 5B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 5A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.

[0131] Fig. 6A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step No. E14H. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.98 mass% Si, 0.071 mass% P, 0.071 mass% Pb, and Zn as the balance. In Step No. E14H, the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm. The rod was heated and hot forged at 630°C to a thickness of 20 mm when placed horizontally then left to cool naturally. The average cooling rate in the temperature range from 500°C to 300°C was 25°C / min.

[0132] Fig. 6B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 6A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.

[0133] As shown in the metallographic micrographs of Figs. 1A, 2A, 4A, and 5A, in a hot extruded material, a hot forged material, a hot forged material made of a casting, and a hot extruded material made by subjecting a hot extruded material to an additional heat treatment, a grain boundary pattern, i.e., crystal grain boundary is observed inside β1 phase together with that of α phase crystal grain. Crystal grain boundary here refers to a linear pattern observed inside β1 phase that runs through a crystal grain of β1 phase like the one shown in Fig. 1A.

[0134] As mentioned above, a grain boundary is observed inside modified β phase, i.e., β1 phase, but in Fig. 3A, only something that looks like a faint black line is observed inside β phase, and there is no grain boundary that runs through a crystal grain of β phase. In Fig. 6A, there is not even a trace of grain boundary inside β phase. Instead, black granular precipitates of about 0.5 to 3 µm are present mainly inside β phase and on the phase boundaries between α phase and β phase in Figs 3A and 6A. The granular precipitates are mainly composed of phosphorous compounds, but Pb particles, mixed particles of Pb and Bi, compounds of Fe and the like, oxides, and sulfides are also included. They can be distinguished by microscopic observation, but it is slightly difficult to do the distinguishment from a printed picture. Precipitates which do not fall within the aforementioned size or are present inside α phase are not phosphorous compounds. In each field of view shown in the printed pictures of Figs. 3A and 6A, there are approximately 1000 precipitates that are mainly composed of phosphorous compounds. In Figs. 1A, 2A, 4A, and 5A, nothing like fine granular phosphorous compound is present or the amount is small if present. The number of such compounds is less than 1 / 10 or 1 / 50 of that of the precipitates shown in Figs. 3A and 6A at most. These findings indicate that whether grain boundary is present inside β1 phase or β phase, and whether a large quantity of phosphorous compound is present depend on whether the cooling treatment was started at a temperature higher than 500°C or lower than 500°C.

[0135] Results of the dezincification corrosion tests performed on these alloys in accordance with ISO 6509 are shown in Figs 1B, 2B, 3B, 4B, 5B, and 6B. The maximum corrosion depth of the test pieces in which a grain boundary was observed inside β1 phase (Figs. 1A, 2A, 4A, and 5A) was 120 µm or less, but that of the test pieces in which grain boundary was not observed inside β phase was 350 µm (Fig. 3A) or 460 µm (Fig. 6A), indicating a difference of 3 to 5 times the corrosion depth. The corrosion depth of the test piece containing 0.28 mass% Sn was 40 µm, which was a particularly good result (Fig. 4A). Incidentally, the type of dezincification corrosion was selective corrosion of β1 phase or β phase in all cases. Therefore, dezincification corrosion occurs in these two phases first, but progress of the corrosion in β1 phase is approximately 3 times slower than in β phase.<Properties>(Normal-Temperature Strength and High-Temperature Properties)

[0136] There is a strong demand for reduction in the thickness and weight of parts and components such as those of water supply-related devices, plumbing components, auto parts, and electrical parts that are target applications of embodiments of the present invention. Among the strengths that are required for these applications, tensile strength is important, but the balance between strength and ductility is also important.

[0137] In this regard, it is preferable that hot extruded materials, hot rolled materials, and hot forged materials have a high strength with a tensile strength of 460 N / mm 2< without cold working performed after hot working.

[0138] The tensile strength is more preferably 490 N / mm 2< or higher and still more preferably 520 N / mm 2< or higher. Many valves, joints, and components used for pressure vessel, air conditioner, or freezer are made of a hot-extruded or hot-forged material. As C3604, a leaded copper alloy currently used for these applications, has a tensile strength of about 390 to 420 N / mm 2< and elongation of about 30% to 35%, it is possible to reduce the weights of such components by increasing the strength of the material.

[0139] Aside from strength, it is important that a material to be machined has resistance to fracture since it may be subjected to cold working such as light swaging or bending. To obtain good machinability, some kind of brittleness is required so that well-broken chips are generated, but machinability is a property opposite to cold workability. Likewise, tensile strength and ductility are opposite properties, but it is desirable that tensile strength and ductility (elongation) are very well-balanced. As a yardstick, if a material has a tensile strength of at least 450 N / mm 2< or higher, a ductility of 10% or higher, and the product of the tensile strength (represented by "S") and {(elongation (represented by "E"%) + 100) / 100} raised to the power of 1 / 2, i.e., f7 = S x {(E+100) / 100} 1 / 2< is preferably 540 or higher, the material can be regarded to have both high strength and high ductility. This f7 value is more preferably 570 or higher, and still more preferably 600 or higher. If cold working performed at a cold working ratio of 3-15% is included in the production process, both an elongation of 10% or higher and a tensile strength of 520 N / mm 2< or higher can be obtained. Apropos, the value of f7 of the previously mentioned leaded copper alloy is about 470.(Electrical Conductivity)

[0140] Applications of the embodiments include electrical or electronic apparatus components, components of automobiles that are increasingly powered by electricity, and other parts and components having high conductivity. Currently, phosphor bronzes including 6 mass% or 8 mass% Sn (JIS standard alloys C5191 and C5210) are widely used for these applications, and their electrical conductivities are about 14% IACS and 12% IACS, respectively. Accordingly, if the embodiments have an electric conductivity of 15% or higher, no problem should occur. The upper limit of the electrical conductivity is not particularly defined because improvement of conductivity rarely causes practical problems.(Hot Workability)

[0141] The free-cutting copper alloys of the embodiments are characterized by their excellent deformability when they are at a temperature between 540°C and 750°C. Due to this characteristic, they can be hot-extruded into a bar with a small cross-sectional area or formed into a complex shape by forging. From the viewpoints of energy saving and allowing α phase to have a favorable granular shape, hot working temperature is preferably lower than 750°C and more preferably lower than 720°C. From the viewpoint of thermal deformation resistance, it is preferably higher than 540°C and more preferably higher than 560°C.<Production Process>

[0142] Next, a method for producing the free-cutting copper alloys according to the embodiments will be described.

[0143] The metallographic structures of the alloys according to the embodiments vary depending not only on the composition but also on the production process. They are affected by the average cooling rate in the process of cooling after hot working or heat treatment in addition to hot working temperatures in hot extrusion and hot forging as well as heat treatment conditions. As a result of a devoted study, it was found that the metallographic structures are significantly affected by the starting temperature of the cooling treatment, the average cooling rates in temperature ranges from the starting temperature to 500°C and from 500°C to 300°C in the process of cooling after hot working or heat treatment.(Melting and Casting)

[0144] Melting is performed at a temperature between about 950°C and about 1200°C, a temperature that is about 100°C to about 300°C higher than the melting point (liquidus temperatures) of the alloy according to an embodiment of the present invention. A molten metal is cast into a specific mold when its temperature is about 900°C to about 1100°C, which is about 50°C to about 200°C higher than the melting point, then cooled by several cooling means such as air cooling, slow cooling, and water cooling. After the alloy solidifies, constituent phases of the alloy change in various ways.(Hot Working)

[0145] Examples of hot working include hot extrusion, hot forging, and hot rolling. In the case two or more hot working steps are performed, the final hot working step is performed under the following condition.

[0146] First, regarding hot extrusion, in a preferable embodiment, it is performed so that the material's temperature immediately after it is hot worked (extrusion temperature) is higher than 540°C and lower than 750°C although it depends on the extrusion ratio (hot working ratio) and the capacity of the extrusion facility used. Extruded bar is either coiled, or if the cross-sectional area of the bar is large, placed onto a table in a straight shape. The lower limit of the hot extrusion temperature relates to hot deformation resistance. The lower the extrusion temperature, the finer and granular become the α phase grains and the more improve the dezincification corrosion resistance, resistance to stress corrosion cracking, and machinability. However, when extruded at a high extrusion ratio, the extrusion temperature is preferably 560°C or higher in view of the capacity of the extruder and the starting temperature of the cooling treatment described later. The upper limit of hot extrusion temperature relates to the shape of α phase. By controlling the hot extrusion temperature so that it is within a more preferable temperature range, a stable metallographic structure can be obtained. When hot extrusion is performed at a temperature of 750°C or higher, the proportion of β phase in the metallographic structure once becomes 100% or 90% or larger, and the shape of α phase crystal grains tends to be acicular, or large α phase crystal grains become likely to appear. When acicular and large α phase crystal grains appear, the strength slightly decreases, the balance between strength and ductility slightly deteriorates, and the machinability deteriorates since large α phase crystal grains with a large longer-side hinder machining. They also deteriorate dezincification corrosion resistance and resistance to stress corrosion cracking. Extrusion temperature is preferably 720°C or lower. The shape of α phase crystal grains relates to the composition relational expression f1, and when the value of f1 is 59.0 or lower, preferable extrusion temperature is lower than 720°C. By performing extrusion at a temperature lower than the temperature at which leaded copper alloys are extruded, good machinability and strength can be obtained.

[0147] Further, by adjusting the conditions of the cooling performed after hot extrusion, β phase can be modified and a material having better machinability, good dezincification corrosion resistance, and good resistance to stress corrosion cracking can be obtained. That is, in the cooling process after hot extrusion, the cooling treatment is started when the temperature of the extruded material is lower than 670°C and higher than 500°C then the material is cooled with the average cooling rates in the temperature ranges from the starting temperature to 500°C and from 500°C to 300°C set higher than 300°C / min at least, preferably higher than 600°C / min, and more preferably 900°C / min or higher. It should be noted here that in an ordinary embodiment where a common cooling method is applied, the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C and that in the temperature range from 500°C to 300°C are approximately the same, or the former is slightly higher in some cases. The more preferable the cooling rate, the better modified becomes β1 phase. As a result, a material having improved machinability, good dezincification corrosion resistance, and good resistance to stress corrosion cracking can be obtained. The upper limit of the average cooling rates in temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C is not particularly defined since ordinary production facility is unable to cool hot-extruded material excessively fast, but as a reference, the cooling rate is preferably 9000°C / min or lower. If the cooling is performed at an average cooling rate exceeding 300°C / min, modification of β phase occurs, and a grain boundary can be found inside modified β phase when etched with a mixed solution of hydrogen peroxide and ammonia water and its metallographic structure is observed with a metallographic microscope at a magnification of 500X. Incidentally, if the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C or from 500°C to 300°C is 300°C / min or lower, no such grain boundary can be observed inside β phase. The higher the cooling rate, the better modified (into β1 phase) becomes β phase. The cooling rate in the cooling from a temperature lower than 300°C to normal temperature tends to decline slightly as the temperature approaches to normal temperature, but it is desirable to keep cooling in the same manner as in the temperature range from 500°C to 300°C.

[0148] On the other hand, starting temperature of the cooling treatment also affects the modification of β phase. The starting temperature is preferably 530°C or higher, more preferably 500°C or higher. In the case the cooling treatment is started at a temperature lower than 550°C, if the cooling rate from 550°C to the starting temperature is low, compounds of P and Zn or P and Zn and / or Si begin to form. If the starting temperature is 530°C or lower, formation of phosphorous compounds is further promoted. Although phosphorous compounds are scarcely formed when cooled at a cooling rate exceeding 300°C / min, if the starting temperature of the cooling treatment is lower than 550°C, such compounds are observed in the metallographic structure.

[0149] Incidentally, when the cooling treatment is started at a temperature of 670°C or higher, the proportion of β1 phase becomes excessive, causing deterioration of ductility, dezincification corrosion resistance, and resistance to stress corrosion cracking of the alloy. For this reason, the starting temperature needs to be lower than 670°C, and is preferably lower than 650°C.

[0150] From a perspective of practicability of measurement position, hot working temperature is defined as temperature of a hot worked material which can be measured about two or three seconds after hot extrusion, hot forging, or hot rolling is completed. The metallographic structure is affected by the temperature immediately after working where large plastic deformation occurs.

[0151] The next description is about hot forging. As a material for hot forging, a hot extruded material is mainly used, but a continuously cast bar is also used. There is no need to manipulate the cooling process of the forging material since hot forging is not the final hot working step. Compared with hot extrusion, in hot forging, working speed is faster, and a more complex shape is formed. In some cases, hot forging is performed at a high working ratio up to a thickness of about 3 mm. Further, weight of forged product varies from a few tens of grams to a few kilograms, and small ones cool rapidly immediately after forging and the rapid cooling continues thereafter. Accordingly, forging material is heated to a higher temperature than the temperature to which ingot is heated for hot extrusion. In a preferred embodiment, the temperature of a hot forged product, that is, the material's temperature about two or three seconds immediately after forging is preferably higher than 540°C and lower than 750°C.

[0152] In hot forging, forging temperature is related to the composition relational expression f1, and when the value of f1 is 59.0 or lower, it is preferable that hot forging is performed at a temperature lower than 720°C. Although depending on the working ratio in hot forging, the lower the forging temperature, the smaller becomes the diameter of α phase crystal grain, the more likely to change the shape of α phase crystal grain from acicular to granular, the higher the strength, and the better the balance between strength and ductility, the machinability, the dezincification corrosion resistance, and the resistance to stress corrosion cracking.

[0153] Like in hot extrusion, by appropriately adjusting the cooling rate at which forged material is cooled after hot forging, a material with good dezincification corrosion resistance, resistance to stress corrosion cracking, and machinability can be obtained. That is, in the cooling process after hot forging, a cooling treatment is started when the temperature of the forged material is lower than 670°C and higher than 500°C then the material is cooled with the average cooling rates in the temperature ranges from the starting temperature to 500°C and from 500°C to 300°C set to be higher than 300°C / min at least, preferably higher than 600°C / min, and more preferably 900°C / min or higher. It should be noted here that in an ordinary embodiment where a common cooling method is applied, the average cooling rates in the temperature range from the starting temperature of the cooling treatment to 500°C and that in the temperature range from 500°C to 300°C are approximately the same or the former is slightly higher. By cooling at an average cooling rate exceeding 300°C / min, modification of β phase occurs and a grain boundary can be found inside modified β phase when a cross section of the material is etched with a mixed solution of hydrogen peroxide and ammonia water then the metallographic structure is observed with a metallographic microscope at a magnification of 500X. Incidentally, if the average cooling rate in the temperature range from 500°C to 300°C is 300°C / min or lower, no such grain boundary can be observed inside β phase. The higher the cooling rate, the more progresses the modification of β phase into β1 phase. Like in hot extrusion, the cooling rate in the cooling from a temperature lower than 300°C to normal temperature tends to decline slightly as the temperature approaches to normal temperature, but it is desirable to keep cooling in the same manner as in the temperature range from 500°C to 300°C. The upper limit of the average cooling rates in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C is not particularly defined, but as a reference, the cooling rate is preferably 9000°C / min or lower.

[0154] On the other hand, starting temperature of the cooling treatment also affects the modification of β phase. The starting temperature is preferably higher than 530°C, more preferably higher than 550°C. In the case the cooling treatment is started at a temperature lower than 550°C, if the cooling rate from 550°C to the starting temperature is low, compounds of P and Zn or P and Zn and / or Si begin to form. If the starting temperature is 530°C or lower, formation of phosphorous compounds is further promoted. Although phosphorous compounds are scarcely formed when cooled at a cooling rate exceeding 300°C / min, if the starting temperature of the cooling treatment is lower than 550°C, such compounds are observed in the metallographic structure. On the other hand, when cooling is started at a temperature of 670°C or higher, the proportion of β1 phase becomes excessive, causing deterioration of ductility, dezincification corrosion resistance, and resistance to stress corrosion cracking of the alloy. For this reason, the starting temperature needs to be lower than 670°C, and is preferably lower than 650°C. Incidentally, after hot forging, if the cooling treatment is started after holding the material in a simple furnace in which the atmospheric temperature is set between about 550°C and about 600°C for a period from a few tens of seconds to a few minutes, more uniform and stable forged product can be obtained.

[0155] Further, shot blasting performed upon completion of the aforementioned steps of hot forging and cooling treatment makes an effective mean for the improvement of resistance to stress corrosion cracking. In other copper alloys also, resistance to stress corrosion cracking can be improved by shot blasting or any other way of applying compressional stress on the surface of alloy, but the effect of shot blasting is particularly good in the case of the alloys of the present invention containing β1 phase.

[0156] In hot rolling, an ingot is heated and rolled 5 to 15 times repeatedly. The material's temperature upon completion of the final hot rolling (the material's temperature three or four seconds after completion of the process) is preferably higher than 540°C and lower than 750°C, more preferably lower than 670°C. Upon completion of hot rolling, in the process of cooling the hot-rolled material, like in hot extrusion, a cooling treatment is performed with a starting temperature set to be higher than 500°C and lower than 670°C and an average cooling rate in the temperature ranges from the starting temperature to 500°C and from 500°C to 300°C set to be higher than 300°C / min at least, preferably higher than 600°C / min, and more preferably 900°C / min or higher. The cooling rate in the cooling from a temperature lower than 300°C to normal temperature tends to decline slightly as the temperature approaches to normal temperature, but it is desirable to keep cooling in the same manner as in the temperature range from 500°C to 300°C. The upper limit of the average cooling rates in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C is not particularly defined, but as a reference, the cooling rate is preferably 9000°C / min or lower.(Heat Treatment)

[0157] In the present invention, if the cooling treatment fails to be started when the material's temperature is higher than 500°C after the aforementioned final hot working, or if it is not performed at an average cooling rate exceeding 300°C / min in the temperature range from 500°C to 300°C, β phase cannot be modified. In addition, when wire or bar with a small diameter is manufactured, or if cold working or any step involving heating such as annealing is included in the production process, modification of β phase basically does not occur. Even in such cases, however, β phase is modified to form β1 phase if annealing is performed at a temperature higher than 520°C and lower than 630°C for one minute to five hours and a cooling treatment is started in the cooling step after annealing when the temperature of annealed material is higher than 500°C and performed at an average cooling rate of higher than 300°C / min in both temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C. The aforementioned annealing conditions need to be applied only to the last annealing step if annealing is performed a multiple number of times. From a standpoint of the temperature range in which phosphorous compounds are formed, annealing temperature is preferably 530°C or higher and more preferably 550°C or higher. Further, the average cooling rate in the temperature range from 500°C to 300°C is preferably set to be higher than 600°C / min, and more preferably 900°C / min or higher to enhance the degree of modification into β1 phase. This way of heat treatment is applied to hot-worked materials which cannot be cooled with the previously described conditions after hot working, those which are cold worked after hot working then annealed at least once, or the like. The upper limit of the average cooling rates in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C is not particularly defined, but as a reference, the cooling rate is preferably 9000°C / min or lower.(Cold Working Step)

[0158] To produce hot extruded bar, cold working is sometimes performed on extruded straight or coiled material in order to obtain high strength, improve dimensional accuracy, and straighten or reduce the degree of bending of such material. For example, cold drawing is performed on a hot extruded material at a cold working ratio of about 2% to about 30% which is followed by straightness correction in some cases.

[0159] For slender bars and wires as well as rolled material, cold working and heat treatment are repeatedly performed. After the previously-described final heat treatment, final cold working is performed at a working ratio of 0% to about 30% which is followed by straightness correction. The closer to 0% the cold working ratio, the less likely to crack becomes the material when it is subjected to cold working such as light swaging or bending.(Low-Temperature Annealing)

[0160] In the case of bars, wires, forged products, and rolled materials, for the primary purposes of removal of residual stress, straightness correction of bar (straightening of bar) as well as adjustment and improvement of the metallographic structure, low-temperature annealing is sometimes performed at a temperature lower than or equal to the recrystallization temperature in the final step. However, modification into β1 phase which was achieved by hot working step and heat treatment may be impaired if the alloy is heated. For instance, if it is heated to 300°C for two hours, modification of β phase becomes impaired and β1 phase returns to β phase, which causes the machinability, the dezincification corrosion resistance, and the resistance to stress corrosion cracking once improved by β1 phase to return to those of the original alloy comprising β phase. Therefore, annealing is not recommended, but may be performed if at a temperature below about 150°C.

[0161] Free-cutting alloy of an embodiment of the present invention with the above-described constitution has excellent machinability, good dezincification corrosion resistance and resistance to stress corrosion cracking as well as excellent hot workability, high strength, and balance between strength and ductility even though the Pb content is small since the alloy composition, the composition relational expressions, the metallographic structure, the metallographic structure relational expressions, and the metallographic structure and composition relational expression are defined as described above.

[0162] Embodiments of the present invention are as hereinabove described. It should be noted, however, that the present invention is not limited to the embodiments and can be modified as appropriate within a scope not deviating from the technical requirements of the present invention.

[0163] Examples

[0164] Hereinafter, the results of the experiments that were performed to verify the effects of the embodiments will be described. The following Examples are presented for the purpose of explaining the effects of the embodiments. The constituent elements, the processes, and the conditions stated in the descriptions of the Examples do not limit the technical ranges of the embodiments.

[0165] Using a low-frequency melting furnace and a semi-continuous casting machine used on the actual production line, trial production of copper alloys was performed. Trial production of copper alloys was performed also with laboratory facilities.

[0166] Tables 5 to 7 show the alloy compositions. Tables 8 to 17 show the production steps employed. Regarding composition, "Mm" refers to mischmetal and represents the total content of rare earth elements. Each of the production steps is described below. The cooling rate in hot working, i.e., hot extrusion, hot forging, or hot compression in Tables 8 to 12 and 14 to 16 refers to the average cooling rate between the points of completion of hot working and commencement of cooling treatment. [Table 5]Alloy No.Constituent Component (mass%)Composition Relational ExpressionImpurity (mass%)CuSiPPbBiSnZnf1f2f0FeMnCrCoAlNiAgSbAsMmS0163.30.950.0690.0630.0170.01Rem.58.90.080-0.120.010.000.000.010.030.020.000.000.00S0262.90.980.0710.0710.0000.02Rem.58.40.071-0.080.050.010.000.010.040.010.010.000.00S0364.20.890.0820.0590.0030.01Rem.60.060.062-0.030.010.000.000.000.030.010.000.000.00S1162.50.960.0640.0720.0000.01Rem.58.10.072-0.050.000.020.000.000.020.010.000.000.01S1264.10.930.0740.0680.0020.02Rem.59.80.070-0.110.010.000.000.000.040.020.000.000.00S1363.61.110.0580.0570.0000.01Rem.58.50.057-0.030.000.000.010.000.080.010.000.000.01S2162.80.880.0870.0500.0010.00Rem.58.70.051-0.000.000.000.000.000.000.000.000.000.00S2263.51.000.0720.0800.0000.00Rem.58.90.080-0.000.000.000.000.000.000.000.000.000.00S2362.50.760.0450.0150.0000.00Rem.59.00.015-0.000.000.000.000.000.000.000.000.000.00S2463.81.150.0640.0280.0000.00Rem.58.50.028-0.000.000.000.000.000.000.000.000.000.00S2562.80.900.0630.0380.0260.02Rem.58.60.064-0.190.130.010.000.000.090.010.020.010.01S2662.80.880.0950.0550.0000.03Rem.58.70.055-0.010.000.000.000.200.080.000.000.000.00S2764.01.020.0650.0700.0000.02Rem.59.30.070-0.140.070.000.000.000.000.000.000.010.00S2863.20.900.0950.0710.0000.02Rem.59.00.071-0.120.110.000.000.130.000.000.000.000.02S2962.80.950.0690.0670.0000.00Rem.58.40.067-0.110.000.000.000.080.130.030.010.010.00S3063.00.950.0650.0170.0000.00Rem.58.60.017-0.080.010.000.000.040.100.020.000.000.00 [Table 6] Alloy No.Constituent Component (mass%)Composition Relational ExpressionImpurity (mass%)CuSiPPbBiSnZnf1f2f0FeMnCrCoAlNiAgSbAsMmS3163.30.940.0180.0660.0000.00Rem.59.00.066-0.000.000.000.000.000.000.000.000.000.00S3263.61.020.0440.1030.0080.00Rem.58.90.111-0.000.000.000.000.000.000.000.000.000.00S3363.30.970.0820.0080.0000.00Rem.58.80.008-0.000.000.000.000.000.000.000.000.000.00S3462.20.780.1220.0550.0170.00Rem.58.50.072-0.000.000.000.000.000.000.000.000.000.00S3561.00.710.1000.0660.0000.00Rem.57.70.066-0.000.000.000.000.000.000.000.000.000.00S3664.40.920.0550.0150.0000.00Rem.60.10.015-0.000.000.000.000.000.000.000.000.000.00S3761.20.640.0580.0630.0220.00Rem.58.20.085-0.000.000.000.000.000.000.000.000.000.00S3864.81.130.0580.0710.0000.00Rem.59.60.071-0.000.000.000.000.000.000.000.000.000.00S3963.61.030.0570.0550.0000.61Rem.58.20.0550.590.000.000.000.000.000.000.000.000.000.00S4063.20.880.0950.0710.0000.41Rem.58.70.0710.470.000.000.000.000.000.000.000.000.000.00S4162.70.830.0840.0660.0000.18Rem.58.70.0660.220.000.000.000.000.000.000.000.000.000.00S4263.80.940.0730.0590.0000.32Rem.59.10.0590.340.000.000.000.000.000.000.000.000.000.00S4363.30.980.0840.0600.0000.28Rem.58.50.0600.290.000.000.000.000.000.000.000.000.000.00S4463.20.720.0650.0350.0000.41Rem.59.40.0350.570.000.000.000.000.000.000.000.000.000.00S4563.40.910.0840.0450.0000.28Rem.58.90.0450.280.000.000.000.000.000.000.000.000.000.00S4663.20.950.0820.0610.0090.08Rem.58.70.0700.080.000.000.000.000.000.000.000.000.000.00S4764.80.950.0970.0780.0000.00Rem.60.40.078-0.000.000.000.000.000.000.000.000.000.00S4860.90.550.0580.0890.0000.00Rem.58.40.089-0.000.000.000.000.000.000.000.000.000.00S4963.50.710.0610.0370.0020.00Rem.60.20.039-0.000.000.000.000.000.000.000.000.000.00 [Table 7] Alloy No.Constituent Component (mass%)Composition Relational ExpressionImpurity (mass%)CuSiPPbBiSnZnf1f2f0FeMnCrCoAlNiAgSbAsMmS7162.90.910.0060.0640.0080.00Rem.58.70.072-0.000.000.000.000.000.000.000.000.000.00S7265.51.090.0690.0440.0000.00Rem.60.40.044-0.000.000.000.000.000.000.000.000.000.00S7363.20.810.0670.0010.0000.00Rem.59.40.001-0.000.000.000.000.000.000.000.000.000.00S7460.70.440.0720.0880.0000.00Rem.58.60.088-0.000.000.000.000.000.000.000.000.000.00S7562.91.030.2300.0710.0070.00Rem.58.00.078-0.000.000.000.000.000.000.000.000.000.00S7660.70.800.0560.0460.0110.00Rem.57.00.057-0.000.000.000.000.000.000.000.000.000.00S7760.10.570.0700.0780.0350.00Rem.57.50.113-0.000.000.000.000.000.000.000.000.000.00S7864.91.290.0980.0660.0000.00Rem.58.90.066-0.000.000.000.000.000.000.000.000.000.00S7964.90.900.0630.0200.0000.00Rem.60.70.020-0.000.000.000.000.000.000.000.000.000.00S8059.20.200.0480.1050.0030.00Rem.58.30.108-0.000.000.000.000.000.000.000.000.000.00S8163.10.610.0400.0230.0000.00Rem.60.30.023-0.000.000.000.000.000.000.000.000.000.00S8264.71.110.0930.0580.0001.01Rem.58.50.0580.910.000.000.000.000.000.000.000.000.000.00S8362.50.670.0610.0700.0000.76Rem.58.60.0701.130.000.000.000.000.000.000.000.000.000.00S8463.30.850.0520.0570.0180.03Rem.59.40.075-0.300.130.020.020.000.010.010.000.000.00S8562.00.800.0490.0780.0000.03Rem.58.30.078-0.030.000.000.000.370.010.010.000.000.00X58.80.000.0013.100.0040.27Rem.-3.1-0.230.010.010.000.000.050.010.000.000.00Y58.70.000.0012.100.0010.23Rem.-2.1-0.260.000.000.020.000.030.000.010.000.00Z58.60.000.0012.100.0000.22Rem.-2.1-0.280.010.000.000.000.100.010.000.000.00 [Table 8] Step Group AA with Facilities Used for Products for Sale:Bars (Indirect Extrusion)Hot ExtrusionCooling TreatmentCold Working Ratio (%)Size (mm)Extrusion Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)A1Ø20.9630255609009008.4A2Ø20.963025580102010208.4A3Ø20.9630255607207208.4A4Ø20.9630255504804808.4A5Ø20.9630255209009008.4A11HØ20.963025480-9008.4A12HØ20.963025450-7208.4A13HØ20.9630---158.4A14HØ20.9630255602102108.4 [Table 9] Step Group AB with Facilities Used for Products for Sale:Bars (Indirect Extrusion)Hot ExtrusionCooling TreatmentCold Working Ratio (%)Low-Temperature AnnealingSize (mm)Extrusion Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)A21Ø20.96152557010209608.4A22Ø20.961525550102010208.4A23Ø20.9615255504204208.4A24Ø20.9615255159009008.4A25Ø20.96152557010209608.4130°C*5HA31HØ20.96152557010209608.4300°C*2HA32HØ20.961525475-9008.4A33HØ20.961525450-7808.4A34HØ20.9615---188.4 [Table 10] Step Group AD with Facilities Used for Products for Sale:Bars (Indirect Extrusion)Hot ExtrusionCooling TreatmentCold Working Ratio (%)Size (mm)Extrusion Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)A41Ø20.972030610126012008.4A42Ø20.972030570120012008.4A43Ø20.972030660132012008.4A44Ø20.9720306106606008.4A46HØ20.972030705132012008.4A47HØ20.9720---208.4 [Table 11] Step Group AC with Facilities Used for Products for Sale:Bars (Direct Extrusion)Hot ExtrusionCooling TreatmentCold Working Ratio (%)Size (mm)Extrusion Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)A50Ø50550---200 [Table 12] Extrusion Process B Performed in LaboratoryHot ExtrusionCooling TreatmentSize (mm)Extrusion Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)B1Ø2067055540960960B2Ø50590---25B3HØ20670---40 [Table 13] Step Group C; Casting in LaboratoryCooling Rate After CastingSize (mm)Start-300°C (°C / min)500-300°C (°C / min)C1Ø35-35C2Ø55-23 [Table 14] Hot Forging Step Group D with Mass Production Facilities Used for Products for Sale:Flare NutsHot ForgingCooling TreatmentMaterial from Previous Step Ø20 mmForging Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)D1Bar: A1, A2167010056512001200D2Bar: A13H, A34H, A47H71010060018001800D3Bar: B3H67010055012001200D4Bar: A1, A21670100545420420D5Bar: A13H, A34H67010054518001800D6Casting: C167010054512001200D7Bar: A13H, A34H, B3H, C1, A47H67010054518001800D8Bar: A13H, A47H74010065519801800D12HBar: A13H, A34H, A47H670100475-1080D13HBar: A1, A21670---35D14HBar: A13H, A34H670100560240240D15HBar: A1, A21, A47H77012069519801800 [Table 15] Hot Forging Step Group E with Mass Production Facilities Used for Products for Sale:Flat ForgingHot ForgingCooling TreatmentMaterial from Previous Step Ø50 mmForging Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)E1Bar: A5063035535900900E2Bar: A50, B268035565900900E3Bar: A5070035615900900E4Bar: A5063035515900900E5Bar: A5063035550450450E6Casting: C263035565900900E7Bar: A5072035650960900E11HBar: A5074035690960900E12HBar: A5063035480-900E13HBar: A5063035455-800E14HBar: A50630---25E15HBar: A5063035550200200 [Table 16] Hot Compression Step Group F Performed in LaboratoryHot CompressionCooling TreatmentMaterial from Previous StepCompression Temperature (°C)Cooling Rate (°C / min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)F1Casting: C16505056021002100F2Bar: A13H, A34H, A47H6505056021002100F3Bar: B3H6505056021002100F4HBar: A13H, A34H65050560200200F5HBar: A13H, A34H65050--40 [Table 17] Heat Treatment Step Group GAnnealingCooling TreatmentMaterial from Previous StepTemperature (°C)Time (min)Starting Temperature (°C)Cooling Rate Start-300°C (°C / min)Cooling Rate 500-300°C (°C / min)G1Bar: A13H, A34H, A47H5803056018001800G2Forged flare nut: D15803056018001800G3Forged flare nut: D13H5803056018001800G11HBar: A1, A2158060470-1500G12HForged flare nut: D158060470-1500G13HBar: A1, A21, A4158030--25G14HForged flare nut: D158030--25 Production with mass production facilities used for manufacturing products for sale - Step Groups AA, AB, and AD(Steps Nos. A1 to A5, A11H to A14H, A21 to A25, A31H to A34H, A41 to A44, A46H, and A47H)

[0167] A billet with a diameter of 240 mm was produced with the low-frequency melting furnace and the semi-continuous casting machine used for the manufacture on the actual production line. For raw materials, those correspond to ones used for the manufacture on the actual production line were used. The billet was cut to a length of 800 mm then heated. Using an indirect extruder with a nominal capacity of 2750 tons, round bars with a diameter of 20.9 mm were extruded and coiled in a container located in a short distance away from the extruder. In this container, water quantity can be adjusted during water cooling. When the temperature of extruded material reached a specific temperature in the container, the cooling treatment (water cooling) was started with water quantity being adjusted. Incidentally, in some cases, the cooling treatment was started as soon as extruded material was placed in the container. Temperature of extruded material was measured with a radiation thermometer when the material came out of the extruder, when the cooling treatment was started, and when the material's temperature became 500°C and 300°C after that. Apropos, for the measurement of temperature during hot extrusion and hot forging in other steps, IGA 8Pro / MB20, a radiation thermometer manufactured by LumaSense Technologies Inc., and a contact thermometer were both used.

[0168] In Step Group AA, i.e., Steps Nos. A1 to A5 and A11H to A14H, extrusion temperature was 630°C, in Step Group AB, i.e., Steps Nos. A21 to A25 and A31H to A34H, it was 615°C, and in Step Group AD i.e., Steps Nos. A41 to A44, A46H, and A47H, it was 720°C. The cooling treatment was started when coiled extruded bars with a diameter of 20.9 mm reached a certain temperature in the container, and the time it took for the temperature of the material to become 500°C and 300°C were measured. Even though radiation thermometer is unable to measure a temperature below 300°C accurately, the cooling treatment was continued with the same conditions until the temperature of the coiled material became around 100°C or lower. As described in Tables 8 to 10 in detail, the average cooling rate from completion of extrusion to commencement of the cooling treatment, the starting temperature of the cooling treatment, the average cooling rates in temperature ranges from the starting temperature of the cooling treatment to 300°C and from 500°C to 300°C in Steps Nos. A1 to A5. A11H to A14H, A21 to A25, A31H to A34H, A41 to A44, A46H, and A47H are not necessarily the same. When the starting temperature of the cooling treatment was below 500°C, the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 300°C was recorded as the cooling rate in the temperature range from 500°C to 300°C. Incidentally, the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C can be easily calculated from the average cooling rates in temperature ranges from the starting temperature to 300°C and from 500°C to 300°C, and it was confirmed that the cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C is either the same as the cooling rate in the temperature range from 500°C to 300°C or slightly faster than that when the cooling treatment was performed with certain conditions. Upon completion of the cooling process after extrusion, extruded material was formed into straight bar with a diameter of 20 mm by a combined machine through drawing at a working ratio of 8.4% and straightness correction.

[0169] In Steps Nos. A25 and A31H, a bar obtained in Step No. A21 was further subjected to low-temperature annealing in a laboratory at 130°C for five hours then at 300°C for two hours. These materials of Steps Groups AA, AB, and AD were subjected to microscopic observation, cutting tests, dezincification corrosion tests, and tensile tests.Step Group AC; ø50-mm Bars(Step No. A50)

[0170] To trial-manufacture forging material with a machine on the actual production line, ø50-mm bars were prepared through Step Group AC (Step No. A50) described in Table 11., i.e., a billet with a diameter of 240 mm was produced with the low-frequency melting furnace and the semi-continuous casting machine used for the manufacture on the actual production line. The billet was cut to a length of 800 mm then extruded at 550°C into ø50-mm round bars with a direct extruder having a nominal capacity of 3000 tons. The extruded straight bars were placed on a steel table for natural cooling to normal temperature then subjected to straightness correction and provided as material for the flat forging described later. No special cooling treatment was performed since they were to be provided as a material of a product produced by a process in which hot forging is the final hot working step. The average cooling rate from 500°C to 300°C was 20°C / min.Material Extruded in Laboratory; Step Group B(Steps Nos. B1, B2, and B3H)

[0171] In Step Group B, raw material composed of components mixed at a specific ratio was melted in a laboratory as described in Table 12. The molten alloy was poured into a permanent mold with a diameter of 100 mm and a length of 200 mm to cast a billet. Incidentally, impurities such as Fe were further added to some of them intentionally. The concentration of the intentionally-added impurities was approximately the same as or lower than that included in leaded brass available in the market.

[0172] The billet was heated and extruded at 670°C into a ø20-mm round bar in the case of Step No. B1. Following the extrusion, cooling treatment was started when the temperature of the extruded bar was 540°C and performed at an average cooling rate of 960°C / min in the temperature range from 500°C to 300°C. It was confirmed that the average cooling rates in temperature ranges from 540°C to 500°C and from 500°C to 300°C were almost the same. Cooling was continued with the same conditions until the material's temperature became about 100°C or lower. In Step No. B3H, ø20-mm round bars were produced by extrusion with an extrusion temperature of 670°C and cooling at an average cooling rate of 40°C / min from 500°C to 300°C without performing any cooling treatment.

[0173] The round bars produced by Steps Nos. B1 and B3H were subjected to microscopic observation, cutting tests, dezincification corrosion tests, and tensile tests. Part of those produced by Step No. B3H were provided as hot forging material of Step Group D.

[0174] In Step No. B2, the billet was extruded at an extrusion temperature of 590°C into ø50-mm bars to be provided as forging material then cooled from 500°C to 300°C at an average cooling rate of 25°C / min without performing any cooling treatment.Step Group C; material for forging or hot compression: casting(Steps Nos. C1 and C2)

[0175] Raw material composed of components mixed at a specific ratio was melted in a laboratory to produce castings to be used as forging material. The molten alloy was casted in an iron mold with an inner diameter of 35 mm and a depth of 200 mm (Step No. C1) or with an inner diameter of 55 mm and a depth of 200 mm (Step No. C2) when the temperat ure of the molten alloy was 1000°C. The casting was taken out of the mold when its temperature became 700°C and left to cool naturally without being subjected to any cooling treatment.

[0176] As described in Table 13, the average cooling rate from 500°C to 300°C was 35°C / min in Step No. C1 and 23°C / min in Step No. C2. Castings produced with Step No. C1 were lathed until the outer diameter became 20 mm. Those produced with Step No. C2 were lathed until the outer diameter became 50 mm to be provided as forging material for use in Processes "D" and "E" and hot compression material for use in Step Group F.Forged Flare Nut(Step Group D)

[0177] In Step Group D, the extruded bars with a diameter of 20 mm prepared by Step Nos. A1, A13H, A21, A34H, or A47H with an extruder used for manufacturing products for sale, those prepared by Step No. B3H in a laboratory, and castings prepared by Step No. C1 in a laboratory were used as materials for forged flare nuts. The materials were cut into pieces so that each weighed 120±1 grams (119 to 121 grams) then formed into hexagonal flare nut forging materials having a width across flats of 27mm and a length of 26 mm with a 500-ton hot forging press.

[0178] As Table 14 shows, hot forging temperature was 670°C in Steps Nos. D1, D3 to D7, and D12H to D14H, 710°C in Step No. D2, 740°C in Step. No. D8, and 770°C in Step No. D15H, respectively. The average cooling rate from immediately after forging until commencement of the cooling treatment was 100°C / min in all these Steps except Steps Nos. D15H and D13H. In Step No. D15H, it was 120°C / min. In Step No. D13H, the forged material was left to cool naturally instead of subjected to the cooling treatment. The starting temperature of the cooling treatment was between 475°C and 695°C as Table 14 shows. The average cooling rates in the temperature ranges from the starting temperature of the cooling treatment to 300°C and from500°C to 300°C were varied between 240°C / min and 1980°C / min except Step No. D13H. In Steps Nos. D1 to D7 and D14H, the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C and that in the temperature range from 500°C to 300°C were the same, but in D8 and D15H, the former was slightly faster than the latter. In Step No. D12H, the average cooling rate from 475°C to 300°C was recorded as that in the temperature range from 500°C to 300°C.

[0179] In Step No. D7, hot forging and cooling treatment were performed with the same conditions as those in Step No. D5 then shot blasting was additionally performed for 10 minutes using ø0.6-mm steel balls.

[0180] These hot forged products were completed by machining as a single kind of flare nuts having an inner diameter of 5 / 8 inches (= 15.9 mm) and a shape conforming to JIS B 8607. The materials produced by Process D were subjected to microscopic observation, dezincification corrosion tests, and stress corrosion cracking tests.Flat Forging(Step Group E)

[0181] Step Group E is a process for forging products in a laboratory. In this process, the extruded bars prepared by Step No. A50 with an extruder used for manufacturing products for sale, those prepared by Step No. B2 in a laboratory, and castings prepared by Step No. C2 in a laboratory were used as forging materials. Each material was cut to a length of 180 mm, and the 180-mm bars were placed horizontally to be forged to a thickness of 20 mm with a press machine having a hot forging press capacity of 150 tons.(Steps Nos. E1 to E7 and E11H to E15H)

[0182] In Steps Nos. E1, E4 to E6, and E12H to E15H, hot forging temperature was 630°C. It was 680°C in Step No. E2, 700°C in Step No. E3, 720°C in Step No. E7, and 740°C in Step No. E11H. The average cooling rate after hot forging until commencement of the cooling treatment was 35°C / min except Step No. E14H. As Table 15 shows, the cooling treatment was started at a temperature between 690°C and 455°C, and the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 300°C was varied between 900°C / min and 200°C / min. The material was kept cooling with the same cooling conditions until its temperature became normal temperature. In Steps Nos. E1 to E6 and E15H, it was confirmed that the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C and that in the temperature range from 500°C to 300°C were approximately the same. With respect to Steps Nos. E12H and E13H, the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 300°C was recorded as that in the temperature range from 500°C to 300°C. In Step No. E14H, no cooling treatment was performed, and the forged materials were left to cool naturally instead. The average cooling rage from 500°C to 300°C was 25°C / min.

[0183] These flat-forged materials were cut and subjected to cutting tests, mechanical property tests, dezincification corrosion tests, and microscopic observation.Hot Compression Tests(Step Group F)

[0184] The hot extruded bars prepared by Steps Nos. A13H, A34H, and A47H with an extruder used for manufacturing products for sale, those prepared by Step No. B3H in a laboratory, and castings prepared by Step No. C1 were used as materials for test pieces, which were completed by lathing to have a diameter of 15 mm and a height of 27 mm. Then, as Table 16 shows, the materials were hot compressed at 650°C with a strain rate of 0.02 / sec to a height of 8 mm using an Amsler testing machine having a maximum testing force of 100kN and equipped with a heating furnace. In Steps Nos. F1 to F3, the hot-compressed test material was taken out of the heating furnace and cooled at a cooling rate of 50°C / min. Subsequently, a cooling treatment was started when the temperature of the test material was 560°C and performed at an average cooling rate of 2100°C / min in the temperature range from 500°C to 300°C. The treatment was continued after the temperature reached 300°C until normal temperature was reached. In Step No. F4H, the cooling treatment was started when the temperature of the test material was 560°C and performed at an average cooling rate of 200°C / min from the starting temperature of the cooling treatment to 300°C. In Step No. F5H, no special cooling treatment was performed but the material was cooled at an average cooling rate of 40°C / min from 500°C to 300°C. The average cooling rate in the temperature range from the starting temperature of the cooling treatment (560°C) to 500°C and that in the temperature range from 500°C to 300°C were approximately the same. These hot-compressed materials were subjected to microscopic observation and dezincification corrosion tests.Heat Treatment(Step Group G)

[0185] In Step Group G, the bars and the forged products obtained in Steps Nos. A1, A21, A41, A13H, A34H, A47H, D1, and D13H were subjected to heat treatment in a laboratory as Table 17 shows to investigate changes in their properties. In all cases, annealing was performed at 580°C for either 30 or 60 minutes. In Steps Nos. G1, G2, and G3, a cooling treatment was started after annealing when the temperature of the annealed material was 560°C and performed at an average cooling rate of 1800°C / min in the temperature range from the starting temperature of the cooling treatment to 300°C. The average cooling rates in the temperature ranges from the starting temperature of the cooling treatment (560°C) to 500°C and from 500°C to 300°C were approximately the same. In Steps Nos. G11H and G12H, after annealing at 580°C, the material was subjected to a cooling treatment which was started when its temperature was 470°C and performed at an average cooling rate of 1500°C / min from 470°C to 300°C. In Steps Nos. G13H and G14H, no special cooling treatment was performed after annealing at 580°C, but the materials were cooled with an average cooling rate from 500°C to 300°C being 25°C / min. These heat-treated materials were subjected to observation with a metallographic microscope. In the case the material was a bar, cutting tests, dezincification corrosion tests, and tensile tests were additionally performed. In the case it was a forged product, dezincification corrosion tests and stress corrosion cracking tests were additionally performed.(Comparative Material)

[0186] As comparative materials, ø20-mm bars made of C3604, leaded free-cutting brass containing 3% Pb, were prepared. In addition, ø50-mm and ø20-mm bars made of C3771, brass for forging containing 2 mass% Pb were prepared. These were denominated as Alloys X, Y, and Z respectively, and those available on the market were used. Hot forging and cooling treatment were performed on Alloy Y with the same conditions as those applied in Steps Nos. E14H and E1. Hot forging and cooling treatment were performed on Alloy Z with the same conditions as those applied in Steps Nos. D13H and D1. Alloy X was subjected to observation with a metallographic microscope, cutting tests, dezincification corrosion tests, and tensile tests. Alloy Y was subjected to observation with a metallographic microscope, cutting tests, dezincification corrosion tests, and tensile tests. Alloy Z was subjected to observation with a metallographic microscope, dezincification corrosion tests, and stress corrosion cracking tests.

[0187] The above-described test materials were evaluated for the following items. The evaluation results are shown in Tables 18 to 56.

[0188] The area ratio of β phase (before modification) is indicated in column "f4A" of each table separately from that of β1 phase (f4). When f4 = (area ratio of β1 phase) = 0, f6 was calculated applying the value of f4A = (area ratio of β phase) in the term of (β1) instead of the area ratio of β1 phase, and the resultant numerical value is indicated with "*" as a reference value of f6.(Observation of Metallographic Structure)

[0189] Metallographic structure was observed in the method described below then the area ratios (%) of respective phases such as α phase, β phase, and γ phase were measured by image analysis. If any grain boundary was observed inside β phase, the phase was recognized as β1 phase distinguishing from β phase. It was assumed that α' phase, β' phase, and γ' phase were included in α phase, β phase, and γ phase respectively. α phase is granular and oval, and often has twin crystals. β phase and β1 phase are present around granular and oval α phase. These three phases were distinguished based on the definitions stated above.

[0190] Each of the bars and forged products provided as test materials was cut parallel to the longitudinal direction or the flowing direction of the metallographic structure. Subsequently, the cut surface was polished (mirror-polished) and etched with a mixed solution of hydrogen peroxide and ammonia water. Specifically, an aqueous solution prepared by mixing 3 ml of 3 vol% hydrogen peroxide water and 22 ml of 14 vol% ammonia water was used for the etching. The polished metal surface was immersed in the aqueous solution for about 2 to 5 seconds under room temperature between about 15°C to about 25°C. Incidentally, in the case β1 phase was present, the immersion time in the etching process had to be extended to about 5 to 10 seconds, which is slightly longer than usual since corrosion resistance is better when β1 phase is present than when β phase is present.

[0191] The metallographic structure was observed with a metallographic microscope at a magnification of 500X to see the proportion of each phase. It was observed at a magnification of 1000X to check the phases and compounds depending on the state of the metallographic structure. Each phase (α phase, β phase, β1 phase, and γ phase) was manually painted in the micrographs of five visual fields using image processing software "Photoshop CC." With respect to the distinguishment between β phase and β1 phase, if any grain boundary was observed inside β phase at least in one location within one visual field, all the β phases within the same visual field were painted as β1 phase. Subsequently, the micrographs were binarized using image analysis software "WinROOF 2013" to obtain the area ratio of each phase. Specifically, the average area ratio of each phase in the five visual fields was calculated to determine the area ratio of each phase. In this calculation, the aggregate of the area ratio of each and every constituent phase excluding compounds containing P or Si, precipitates, oxides, particles of Pb or Bi, sulfides, and crystallized particles constitutes 100% of the area ratio.

[0192] When modified β phase is examined with a metallographic microscope after being etched with the previously-described etching solution, a grain boundary pattern or a boundary of crystal grain can be observed inside β phase. In the present invention, if such a crystal grain boundary is observed inside β phase, such β phase is regarded as β1 phase to distinguish it from ordinary β phase. Crystal grain boundary here refers to a linear pattern observed inside β1 phase that runs through β1 phase like the one shown in Fig. 1A.

[0193] In metallographic structure, sometimes compounds of P or Si and / or precipitates are present other than the phases. Compounds of P and Zn or Si can be found mainly inside β phase or β1 phase or at phase boundary between β phase and α phase in the form of blackish gray particles of about 0.5 to 2 µm when observed with a 500X metallographic microscope. Compounds of Fe, Mn, or Cr and P or Si can be observed as pale blue particles of about 1 to 5 µm. Particles of Pb or Bi and those in which Pb and Bi are compounded are present in the form of fine black particles of 2 µm or smaller. Most of these compounds and the like can be roughly distinguished with a metallographic microscope.

[0194] When any grain boundary was observed inside β1 phase (β phase) in the metallographic structure, "B" (present) was indicated in the Tables in which evaluation results of alloy's characteristics are recorded, and when none was observed, "D" (absent) was indicated in the Tables.

[0195] With respect to phosphorous compounds, when an alloy containing 0.06 mass% P was cooled at an average cooling rate of 15°C / min to 50°C / min from 500°C to 300°C, approximately 500 to 1000 phosphorous compounds were observed in a field of 500x magnified view (in an area of 80 mm by 120 mm if printed). However, it was hard to distinguish phosphorous compounds from other compounds or particles such as compounds of Fe, Mn, or Cr and P or Si and Pb particles in a printed image of metallographic structure. Therefore, when a printed picture of metallographic structure was used for the distinguishment, if the total number of phosphorous compounds and compounds that could be phosphorous compounds was less than 50 in such a picture of field of view, it was evaluated that phosphorous compounds were not present, and "D" (absent) was indicated as an evaluation result. In the case the number was 50 or more but less than 300, it was evaluated that phosphorous compounds were present but only in a small quantity, and "C" (fair) was indicated as an evaluation result. In the case the number was 300 or more, it was evaluated that phosphorous compounds were present, and "B" was indicated as an evaluation result. Whether phosphorous compounds are present, and the quantity thereof make indicators to know if an appropriate treatment was performed.

[0196] When it was difficult to identify any phases, compounds, or precipitates, an electron backscattering diffraction pattern (FE-SEM-EBSP) method was employed using a field emission scanning electron microscope (FE-SEM) (JSM-7000F, manufactured by JEOL Ltd.) and its accessory EDS with the acceleration voltage set to be 15 kV and the electrical current value set to be 15 to identify them at a magnification of 500X or 2000X.(Electrical Conductivity)

[0197] Electrical conductivity was measured with an electrical conductivity measurement device manufactured by Foerster Japan Ltd. (SIGMATEST; D2.068). Incidentally, in this specification, the terms "electric conductivity" and "electrical conductivity" are meant to have the same meaning. Further, thermal conductivity and electrical conductivity are closely related. The higher the electrical conductivity, the better the thermal conductivity.(Tensile Strength / Elongation)

[0198] Each of the test materials was processed into test piece No. 10 in accordance with JIS Z 2241 to measure their tensile strength and elongation.

[0199] If a hot extruded material or a hot forged material not having undergone any cold working step has a tensile strength of preferably 460 N / mm 2< or higher, more preferably 490 N / mm 2< or higher, and still more preferably 520 N / mm 2< or higher, the material is considered to have the highest level of tensile strength among free-cutting copper alloys. Therefore, reducing wall thickness or weight of components used in various fields, or increasing allowable stress can be attempted by applying these materials. In addition, regarding the balance between strength and elongation, as a yardstick, when the tensile strength is represented by S (N / mm 2< ) and the elongation is represented by E (%), if the value of the characteristic relational expression f7 = S × {(E + 100) / 100} 1 / 2< which indicates the balance between strength and ductility of the alloy is preferably 540 or higher, the alloy can be considered to have high strength and high ductility. The value of f7 is more preferably 570 or higher, and still more preferably 600 or higher. If the f7 value of an alloy is 600 or higher, it can be said that the alloy has a very high standard of balance between strength and elongation among free-cutting copper alloys. Many of the parts of tap fitting, valve, joint, pressure vessel, air conditioner, and freezer are made of a hot-forged or hot-extruded bar, and C3604, a leaded copper alloy currently used for such parts, has a tensile strength of about 390 to 420 N / mm 2< and an elongation of 30 to 35%. The alloy's f7 value is 470. The embodiments of the present invention are able to reduce the weights of such parts since the embodiments have improved strength and good dezincification corrosion resistance and resistance to stress corrosion cracking.<Machinability Testing with Lathe>

[0200] Machinability was evaluated by the cutting test using a lathe as described below.

[0201] To prepare test materials, first, hot extruded bars (Step Groups "AA", "AB", and "B") and hot forged materials (Step Group E) were machined to a diameter of 18 mm. A carbide tool (an insert) of K10 not equipped with chip breaker was attached to a lathe. Using this lathe, the circumferences of the ø18-mm test materials were machined on a dry condition with a rake angle of 0°, a nose radius of 0.4 mm, a clearance angle of 6°, a cutting speed of either 40 m / min or 110 m / min, a cutting depth of 1.0 mm, and a feed rate of 0.11 mm / rev. The effect of cutting speed on the embodiments of the present invention was also studied.

[0202] A signal emitted from a dynamometer (AST tool dynamometer AST-TL1003 manufactured by Mihodenki Co., Ltd.) composed of three parts attached to the tool was converted into an electrical voltage signal and recorded on a recorder. Subsequently, these signals were converted into cutting resistance (principal cutting force, feed force, and thrust in N). In order to suppress the influence of wear on the insert, the cutting test was performed in the order of test pieces A → B → C then C → B → A. This cycle was repeated twice so that each test piece was measured four times. The cutting resistance was calculated by the following expression.

[0203] It should be noted here that principal force is the largest among the components of cutting resistance, and cutting resistance is mostly decided by the size of principal force. Therefore, the mean value of the principal force that was measured four times was calculated and adopted as the cutting resistance of the test piece.

[0204] Assuming that the cutting resistance (principal force) of a commercially available free-cutting brass bar, C3604 (Alloy X; ø20 mm), made of an alloy consisting of 59 mass% Cu, 3 mass% Pb, 0.2 mass% Fe, 0.3 mass% Sn, and Zn as the balance was 100, a relative value of the cutting resistance (machinability index) of each test piece was calculated for relative evaluation. That is, the higher the machinability index, the lower the cutting resistance and the better the machinability.

[0205] Incidentally, the machinability index was calculated as follows.

[0206] Cutting resistance (principal force) of a material depends on the shear strength or the tensile strength of the material, and there is a tendency that the higher the strength, the higher the cutting resistance. For instance, in the case of a copper alloy with chip breakability equivalent to that of C3604 and approximately 1.2 times the strength of C3604, its cutting resistance is about 20% higher than that of C3604 since cutting resistant is proportionate to the strength of material. Thus, if a high strength copper alloy has cutting resistance of approximately 40% higher than that of C3604, the copper alloy is regarded as having no practical problem. As embodiments of the present invention in the form of extruded material have about 1.2 times the tensile strength and the shear strength of C3604, in the evaluation of their machinability, machinability index of about 72 was regarded as the standard (boundary value). Specifically, if the machinability index of a material was 80 or higher, the material was evaluated as having excellent machinability (evaluation: A), i.e., equivalent to that of C3604. When it was 72 or higher and lower than 80, the material was evaluated as having good machinability (evaluation: B). When it was 66 or higher and lower than 72, the material was evaluated as having fair machinability (evaluation: C). When it was lower than 66, the material was evaluated as having poor machinability (evaluation: D). As the present invention aims to obtain good machinability, those with a machinability index of 72 or higher were determined as acceptable.(Dezincification Corrosion Test: Dezincification Corrosion Test of ISO 6509)

[0207] As a test method for the evaluation of dezincification corrosion resistance, the dezincification corrosion test of ISO 6509 was employed. This test method is adopted in many countries and is specified in Japanese Industrial Standards under JIS H 3250 as well.

[0208] In accordance with the procedure of the dezincification corrosion test, first, a test material was imbedded in a phenolic resin material specifically so that its surface to be exposed to test solution was perpendicular to the direction in which the extruded material had been extruded. The surface of the test piece was polished with emery paper No. 1200 then subjected to ultrasonic cleaning in pure water and drying.

[0209] Each test piece was immersed in an aqueous solution containing 1.0% copper (II) chloride dihydrate (CuCl 2 · 2H 2 O) and held at 75°C for 24 hours then taken out of the aqueous solution.

[0210] The test piece was once again imbedded in a phenolic resin material so that its exposed surface maintained a right angle to the extrusion direction, longitudinal direction, or the direction to which crystal grains moved during forging. Subsequently, it was cut so that the cross section included the longest possible corroded portion then polished.

[0211] Corrosion depth was observed at 10 visual fields with a metallographic microscope at a magnification of 100x to 500x. The depth of the deepest point of corrosion was recorded as the maximum corrosion depth.

[0212] Apropos, if the maximum corrosion depth is 200 µm or less, it is considered that there is no problem in corrosion resistance in practical use. It is generally understood that the maximum corrosion depth is preferably 100 µm or less in the case excellent corrosion resistance is required.

[0213] In this testing also, when the maximum corrosion depth exceeded 200 µm, the evaluation was "D" (poor) which stands for inacceptable. When it was over 100 µm but 200 µm or less, the evaluation was "B" (good). When it was 100 or less, the evaluation was "A" (excellent).(Stress Corrosion Cracking Test)

[0214] In order to determine the resistance to stress corrosion cracking in an environment where stress corrosion cracking easily occurs, a stress corrosion cracking test was performed with the following procedure.

[0215] First, to prepare test pieces, ø20-mm bar was cut to weigh 120±1g (119 to 121 grams) and the cut pieces were formed into forged hexagonal flare nut materials having a width across flats of 27 mm and a length of 26 mm using a 500-ton hot forging press with the conditions of Step Group D indicated in Table 14. Further, some of the test pieces were subjected to shot blasting in the same manner as in Step No. D7as indicated in Table 14, and heat treatment was performed on the forged materials with several conditions (Steps Nos. G2, G3, G12H, and G14H) as indicated in Table 17. These forged materials were completed as a single kind of flare nuts having an inner diameter of 5 / 8 inches (= 15.9 mm) and a shape conforming to JIS B 8607 by forming threads and smoothing end surfaces in the process of machining. In order to apply stress to the flare nuts, a base made of C6932 with part of its body machined into a shape of male screw also conforming to JIS B 8607 with a nominal diameter of 5 / 8 inches (= 15.9 mm) was prepared. Test piece was tied to the base until a specific torque was reached which was measured with a torque gauge. Flare nut to which stress had been applied was placed in a desiccator filled with ammonia water containing a certain concentration of ammonia at a location 60 mm away from the water surface. After holding in the ammonia water for a certain period of time, the flare nut was taken out. The testing was performed in a room where the atmospheric temperature was controlled with an air conditioner to 25±1°C (24°C to 26°C).

[0216] The stress corrosion cracking test was performed in the following two methods.(Stress Corrosion Cracking Test -1)

[0217] Ammonia water containing ammonia at a concentration of 14 vol% was put in a desiccator in the quantity of 1000 ml then test pieces with a torque of 70 N·m being applied were exposed to the atmosphere in the desiccator for 48 hours and for 96 hours. The test pieces were washed with a solution containing 10 vol% sulfuric acid and subjected to visual observation for evaluation based on whether any cracks were present. This testing was performed in accordance with JIS H 3250 except exposure time. The exposure time in this testing was 20 times longer than that specified in JIS H 3250 which is two hours. That means the testing condition was quite severe. As the present invention is directed to obtaining good resistance to stress corrosion cracking, when a crack was found after exposure of 48 hours, the evaluation was "D" (poor). If crack was not observed after 48 hours of exposure but was observed after 96 hours, the evaluation was "C" (fair). If no crack was observed after 96 hours, the evaluation was "B" (good). Incidentally, those evaluated as "C" were determined as acceptable.(Stress Corrosion Cracking Test -2)

[0218] In another way of stress corrosion cracking test, 200 ml of ammonia water containing ammonia at a concentration of 28 vol% was put in a desiccator. Test pieces to which a torque of 150 N·m was applied were exposed to the ammonia atmosphere in the desiccator for 75 hours. After that, they were taken out of the desiccator and washed with sulfuric acid solution containing 10 vol% sulfuric acid then subjected to visual examination for cracks. As the concentration of ammonia was higher and the torque applied was larger than those in Stress Corrosion Cracking Test -1, the evaluation criteria were set to be as follows. If the size of the largest crack in a test piece that was clearly visible with eye was half the length of the test piece (26 mm) or larger, i.e., 13 mm or larger, the test piece was evaluated as "D" (poor). If a test piece had any crack with a size of about 1 / 5 the length of the test piece (26 mm) or larger, i.e., 5 mm or larger but smaller than 13 mm that was clearly visible with eye, the evaluation was "C" (fair). If any crack smaller than about 1 / 5 of the length of the test piece (26 mm), i.e., smaller than 5 mm was observed or no crack was visible with eye, the evaluation was "B" (good). Incidentally, those evaluated as "C" were regarded acceptable. [Table 18]Test No.Alloy No.Material No.Step No.MetallographicStructuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase1-A149510059DB2-A246540062DB3-A350500058DB4-A453470056DB5-A557430051CB6-A11H62038047*BD7-A12H66034042*BD8-A13H68032040*BD9-A14H64036045*CD10S01A1D149510059DB11A13HD241590067DB12A1D456440053CB13A13HD554460055DB14A13HD754460055DB15A13HD825750083DB16A13HD12H65035044*BD17A1D13H67033040*BD18A13HD14H64036045*CD19A1D15H13087094*DD20A50E155450053DB [Table 19] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase21A50E249510059DB22A50E337630071DB23A50E459410050CB24A50E553470056DB24.1A50E727730081DB25A50E11H18820089DB26A50E12H64036045*BD27A50E13H67033041*BD28A50E14H69031040*BD29S01A50E15H65035044*CD30A13HF250500058DB31A13HF4H58042051*CD32A13HF5H64036045*BD33A13HG150500058DB34D1G251490057DB35D13HG368320040DB36A1G11H64036045*BD37D1G12H66034043*BD38A1G13H68032040*BD39D1G14H70030039*BD [Table 20] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase41-A2139610070DB42-A2243570065DB43-A2347530062DB44-A2450500059CB45-A2543570065DB46-A31H44056065*DD47-A32H55045054*BD48-A33H58042051*BD49S02-A34H59041050*BD50A21D140600069DB51A34HD232680077DB52A21D447530062DB53A34HD544560064DB54A34HD745550063DB56A34HD12H55045054*BD57A21D13H60040050*BD58A34HD14H55045054*DD59A50E146540063DB [Table 21] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase60A50E240600069DB61A50E328720080DB62A50E450500059CB63A50E544560064DB63.1A50E721790087DB64A50E11H9091099*DD65A50E12H55045054*BD66A50E13H58042051*BD67A50E14H59041050*BD68S02A50E15H56044053*CD69A34HF241590067DB70A34HF4H49051060*CD71A34HF5H55045054*BD72A34HG141590067DB73D1G242580066DB74D13HG359410050DB75A1G11H56044053*BD76D1G12H57043052*BD77A1G13H61039048*BD78D1G14H62038047*BD [Table 22] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase81S11A50E141590067DB82E235650073DB83E324760084DB84E445550064CB85E540600068DB86E11H40960103*DD87E12H50050059*BD88E13H53047055*BD89E14H54046054*BD90E15H51049057*CD91S12A50E169310039DB92E263370045DB93E352480055DB94E472280037CB95E567330042DB95.1E743570065DB96E11H33670074DB97E12H78022031*BD98E13H81019027*BD99E14H82018026*BD100E15H80020028*CD [Table 23] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase101E148520063DB102E241590071DB103E330700082DB104E452480058CB105E546540065DB105.1S13A50E723770089DB106E11H110890102*DD107E12H56044055*CD108E13H60040051*BD109E14H62038049*BD110E15H59041052*CD [Table 24] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase121-A4154460052DB122-A4264360043DB123-A4338620068DB124-A4457430049DB125-A46H19810085DB126-A47H86014022*BD127A47HD256440051DB128A47HD770300037DB129A47HD840600066DB130S03A41D13H84016023*BD131A47HD15H25750080DB132A50E172280036DB133A50E355450052DB134A50E744560062DB135A50E14H85015023*BD136A47HF267330040DB137A47HF5H82018027*BD138A47HG166340041DB139D13HG368320039DB140A41G13H87013022*BD141D2G14H8801209*BD [Table 25] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase201S21B3HD350500056DB202B2E246540060DB203S22-B153470057DB204-B3H68032042*BD205B3HD352480058DB206S23-B157430043DB207S24C1F143570068DB [Table 26] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase208S25-B154460053CB209-B3H67033040*BD210B3HD350500056DB211D3D750500056DB212S26C1F147530059CB213S27C1F157430053DB214S28C1D654460053DB215C2E654460053DB216S29C1F143570065DB217S30C1F145550060DB218S31-B156440050DB219-B3H70030036*CD220B3HF352480054DB221S32-B155450055DB222S33-B150500055DB223S34C1F144560060DB224S35C1F128720071DB225S36C1F171290034DB [Table 27] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase226S37-B143570055DB227-B3H57043044*BD228B3HD342580056DB229D3D741590056DB230S38-B168310140DB231S39C1F139610070DB232S40C1D650500057DB233C2E646540061DB234S41C1F146540058DB235S42-B158420050DB236-B3H72028036*BD237B3HD356440051DB [Table 28] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase238S43C1D646540063DB239C2E642580067DB240S44-B163370039DB241-B3H77023027*BD242B3HD362380040DB243D3D762380040DB244S45C1F150500056DB245S46C1F147530061DB246S47C2E673270036DB247S48B144560051DB248S49B174260029DB [Table 29] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase301S71-B157043048*DD302-B3H71029035*CD303B3HD353047052*DD304D3D753047052*DD305S72-B180200029DB306S73-B163370038DB307S74-B150050043*DD308-B3H64036034*CD309B3HD348052044*DD310S75-B136640077CB311S76C1F116084083*DD312S77-B122078070*DD313S78-B166290535DB314S79C1F180200025DB315S80-B143057035*DD316-B3H57043029*DD317B3HD341059036*DD318D3D741059036*DD [Table 30] Test No.Alloy No.Material No.Step No.Metallographic Structuref3f4f4Af5f6Presence of P compoundPresence of grain boundary inside β phase319S81-B177230024DB320S82-B163330436DB321-B3H75020521*BD322B3HD361360341DB323S83C1F150049147*DD324S84-B170300037CB325-B3H80020027*BD326B3HD367330040CB327S85C1F135065067*CD500X--850150-DD501Y-E14H790210-DD502-E1670330-DD503Z-D13H780220-DD504-D1640360-DD [Table 31] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation1-A117.16232068284A81A2-A217.16381969684A81A3-A317.36142067384A80A4-A417.26172167983A77B5-A517.36122267682A78B6-A11H17.05872365181A71C7-A12H17.45852465179B68C8-A13H17.55542662278B66C9-A14H17.35762464178B65D10S01A1D117.2-------11A13HD217.1-------12A1D417.2-------13A13HD517.3-------14A13HD717.2-------15A18HD817.5-------16A13HD12H17.3-------17A1D13H17.5-------18A13HD14H17.3-------19A13HD15H17.420A50E117.35483463483A80A [Table 32] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation1-A180A--2-A290A--3-A390A--4-A4160B--5-A5150B--6-A11H290D--7-A12H360D--8-A13H450D--9-A14H390D--10S01A1D180ABB11A13HD270ABC12A1D4160BBC13A13HD580ABC14A13HD780ABB15A18HD8130BBC16A13HD12H260DCC17A1D13H430DCC18A13HD14H360DCC19A13HD15H350DCD20A50E1110B-- [Table 33] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation21A50E217.15583364484A82A22A50E317.15602662984A81A23A50E417.05273461082A77B24A50E517.25363361882A77B24.1A50E717.35612462585A76B25A50E11H17.45602161683A71c26A50E12H17.25233560881A71c27A50E13H17.15093759679B68c28A50E14H17.25003858777B66C29S01A50E15H17.35143459577B67c30A13HF217.3-------31A13HF4H17.1-------32A13HF5H17.0-------33A13HG117.2---82A75B34D1G217.3-------35D13HG317.2-------36A1G11H17.1---77B63D37D1G12H17.4-------38A1G13H17.3---73B61D39D1G14H17.2------- [Table 34] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation21A50E280A--22A50E390A--23A50E4160B--24A50E5150B--24.1A50E7150B--25A50E11H200B--26A50E12H280D--27A50E13H390D--28A50E14H440D--29S01A50E15H400D--30A13HF270A--31A13HF4H360D--32A13HF5H410D--33A13HG1110B--34D1G2100ABB35D13HG3120BBB36A1G11H340D--37D1G12H310DCC38A1G13H430D--39D1G14H420DCC [Table 35] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation41-A2116.86411869685A81A42-A2216.86301968785A82A43-A2317.06222068184A75B44-A2417.16232068283A77B45-A2516.86241968184A81A46-A31H16.76061865880A70C47-A32H16.95972265982A71C48-A33H17.05902465781A69C49S02-A34H16.95862365082A69C50A21D116.9-------51A34HD216.8-------52A21D417.1-------53A34HD516.9-------54A34HD716.8-------56A34HD12H16.9-------57A21D13H17.1-------58A34HD14H17.0-------59A50E117.05553263885A82A [Table 36] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation41-A2190A--42-A22100A--43-A23160B--44-A24140B--45-A25100A--46-A31H300D--47-A32H290D--48-A33H380D--49S02-A34H460D--50A21D190ABB51A34HD2110BBC52A21D4160BBC53A34HD590ABC54A34HD790ABB56A34HD12H260DCC57A21D13H440DCC58A34HD14H390DCC59A50E1100A-- [Table 37] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation60A50E217.15633064286A83A61A50E316.85702363285A81A62A50E416.75383161684A77B63A50E516.95473062483A77B63.1A50E716.85682262784A74B64A50E11H16.95651660982A68C65A50E12H17.05303361183A72B66A50E13H16.85273461082A70C67A50E14H16.75183560282A69C68S02A50E15H16.95383261880A68C69A34HF217.0-------70A34HF4H16.8-------71A34HF5H16.7-------72A34HG116.9---82A77B73D1G217.0-------74D13HG316.9-------75A1G11H16.9---79B65D76D1G12H17.0-------77A1G13H17.1---76B62D78D1G14H17.0------- [Table 38] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation60A50E290A--61A50E3130B--62A50E4160B--63A50E5150B--63.1A50E7180B64A50E11H340D--65A50E12H280D--66A50E13H400D--67A50E14H460D--68S02A50E15H420D--69A34HF290A--70A34HF4H360D--71A34HF5H400D--72A34HG1100A--73D1G2120BBB74D13HG3100ABB75A1G11H350D--76D1G12H300DCC77A1G13H450D--78D1G14H430DCc [Table 39] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation81S11A50E117.15602963685A80A82E217.25792865586A82A83E317.35752363884A78B84E417.15413362484A78B85E517.15493263184A76B86E11H17.55801461980A64D87E12H17.15303461483A72B88E13H17.25283360982A71C89E14H17.25223360283A70C90E15H17.15333261282A70C91S12A50E117.45164261580A78B92E217.55294062682A79B93E317.65403562785A81A94E417.54944359177B74B95E517.45104160680A74B95.1E717.75473062486A82A96E11H17.75622763384A80A97E12H17.54754056271C63D98E13H17.44604154667C58D99E14H17.54534454464D56D100E15H17.44634155066C56D [Table 40] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation81S11A50E1120B--82E2110B--83E3140B--84E4150B--85E5170B--86E11H400D--87E12H270D--88E13H350D--89E14H440D--90E15H370D--91S12A50E1110B--92E280A--93E380A--94E4150B--95E5150B--95.1E770A--96E11H120B--97E12H230D--98E13H280D--99E14H430D--100E15H380D-- [Table 41] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation101E115.65613364785A82A102E215.75763165986A82A103E315.85802665185A81A104E415.65493563884A78B105E515.75533464085A76B105.1S13A50E716.05742363785A77B106E11H16.05841662982A68c107E12H15.65433462982A71C108E13H15.75343461882A70C109E14H15.75303361181A70C110E15H15.65403462580A68C [Table 42] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation101E1110B--102E290A--103E3120B--104E4160B--105E5160B--105.1S13A50E7150B--106E11H340D--107E12H260D--108E13H390D--109E14H460D--110E15H400D-- [Table 43] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation121-A4117.66252168886A83A122-A4217.46102367785A81A123-A4317.76321968986A82A124-A4417.56032066182A77B125-A46H17.86171465984A73B126-A47H17.45332860363D54D127A47HD217.6-------128A47HD717.5-------129A47HD817.8-------130S03A41D13H17.4-------131A47HD15H17.9-------132A50E117.54974559880A76B133A50E317.65423863785A82A134A50E717.85563063486A82A135A50E14H17.24504554262D53D136A47HF217.5-------137A47HF5H17.3-------138A47HG117.4---80A75B139D13HG317.5-------140A41G13H17.2---64D55D141D2G14H17.1------- [Table 44] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation121-A4170A--122-A4260A--123-A4380A--124-A44130B--125-A46H190B--126-A47H400D--127A47HD260ABB128A47HD770ABB129A47HD880ABC130A41D13H420DCC131S03A47HD15H140BBC132A50E1110B--133A50E380A--134A50E7100A--135A50E14H440D--136A47HF260A--137A47HF5H410D--138A47HG180A--139D13HG390ABB140A41G13H360D--141D2G14H330DCC [Table 45] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation201S21B3HD317.9-------202B2E217.95403262084A80A203S22-B116.95303361185A82A204-B3H17.14943657678B68C205B3HD317.0-------206S23-B119.15163459780A75B207S24C1F115.2------- [Table 46] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation201S21B3HD390ABB202B2E2100A--203S22-B190A--204-B3H440D--205B3HD380A--206S23-B1130B--207S24C1F190A-- [Table 47] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation208S25-B117.75283160482A76B209-B3H17.74923557277B66c210B3HD317.6-------211D3D717.6-------212S26C1F117.9-------213S27C1F116.5-------214S28C1D617.6-------215C2E617.55333461783A78B216S29C1F117.2-------217S30C1F117.2-------218S31-B117.65053458579B73B219-B3H17.54543853372B62D220B3HF317.7-------221S32-B116.75283360986A81A222S33-B116.95303361180A76B223S34C1F118.5-------224S35C1F119.4-------225S36C1F117.5------- [Table 48] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation208S25-B1170B--209-B3H470D--210B3HD3150BBC211D3D7140BBB212S26C1F1150B--213S27C1F1130B--214S28C1D6140BBC215C2E6130B--216S29C1F1120B--217S30C1F1110B--218S31-B1180B--219-B3H480D--220B3HF3150B--221S32-B1120B--222S33-B1110B--223S34C1F1120B--224S35C1F1160B--225S36C1F190A-- [Table 49] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation226S37-B120.35152958579B74B227-B3H20.54673253775B64D228B3HD320.2-------229D3D720.2-------230S38-B115.45222859180A75B231S39C1F116.4-------232S40C1D617.9-------233C2E617.85433262485A81A234S41C1F118.4-------235S42-B117.35253460883A80A236-B3H17.44603653674B65D237B3HD317.2------- [Table 50] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation226S37-B1140B--227-B3H460D--228B3HD3160BBC229D3D7150BBB230S38-B1150B--231S39C1F160A--232S40C1D640ABB233C2E650A--234S41C1F160A--235S42-B130A--236-B3H400D--237B3HD330ABB [Table 51] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation238S43C1D616.9-------239C2E616.95503263285A82A240S44-B119.55113860080A75B241-B3H19.44453852366C58D242B3HD319.5-------243D3D719.5-------244S45C1F117.6-------245S46C1F117.2-------246S47C2E617.04884157978B74B247S48-B121.24913056078B73B248S49-B119.64724256277B73B [Table 52] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation238S43C1D640ABB239C2E640A--240S44-B160A--241-B3H420D--242B3HD350ABC243D3D750ABB244S45C1F120A--245S46C1F170A--246S47C2E680A--247S48-B1170B--248S49-B1110B-- [Table 53] Test No.Alloy No.Material No.Step No.PropertyMachinabilityElectrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation301S71-B117.94943357068C58D302-B3H18.14543853367C57D303B3HD318.0-------304D3D718.0-------305S72-B115.84663954969C64D306S73-B118.55073759374B70c307S74-B122.34723154068c59D308-B3H22.54443651866C57D309B3HD322.3-------310S75-B115.45051754684A80A311S76C1F118.7-------312S77-B121.05211756474B62D313S78-B113.65062356177B70c314S79C1F117.7-------315S80-B124.84503051357D53D316-B3H25.04323450056D51D317B3HD324.9-------318D3D724.9------- [Table 54] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation301S71-B1500D--302-B3H480D--303B3HD3470DCC304D3D7470DCC305S72-B1100A--306S73-B1110B--307S74-B1340D--308-B3H470D--309B3HD3380DCD310S75-B1100A--311S76C1F1330D--312S77-B1510D--313S78-B1310D--314S79C1F1100A--315S80-B1530D--316-B3H490D--317B3HD3520DCD318D3D7520DCD [Table 55] Test No.Alloy No.Material No.Step No.Metal. StructurePropertyMachinabilityf3Electrical Conductivity %IACSTensile Strength N / mm 2< Elongation %f7Cutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation319S81-B17720.74444152767C61D320S82-B16315.65161755876B68c321-B3H7515.74862053270C59D322B3HD36115.4-------323S83C1F15020.0-------324S84-B17018.24982355276B68C325-B3H8018.44513652660D55D326B3HD36718.1-------327S85C1F13518.8-------500X--8526.040333465100A100A501Y-E14H7926.14183248093A92A502-E16725.84302748592A92A503Z-D13H7825.9-------504-D16425.7------- [Table 56] Test No.Alloy No.Material No.Step No.ISO Testing of Dezincification CorrosionStress Corrosion CrackingTest-1Test-2Dezincification Corrosion DepthmmEvaluation319S81-B1160B--320S82-B1250D--321-B3H380D--322B3HD3230DCC323S83C1F1340D--324S84-B1190B--325-B3H470D--326B3HD3210DCC327S85C1F1390D--500X--500D--501Y-E14H480D--502-E1520D--503Z-D13H510DCC504-D1570DCD

[0219] 1) The inventors were able to confirm that a hot-extruded material or a hot-forged material with good machinability, good dezincification corrosion resistance, and good resistance to stress corrosion cracking, high electrical conductivity of 15% IACS or higher, high strength, good ductility, and superb balance between strength and ductility (property relational expression f7) could be obtained by satisfying the composition of an embodiment of the present invention, composition relational expressions f0 to f2, the requirements related to metallographic structure, metallographic structure relational expressions f3 to f5, and metallographic structure and composition relational expression even if the Pb content was small. The inventors were further able to confirm that even though presence of phosphorous compound has an effect of significantly improving machinability, modified β phase, i.e., β1 phase, had even better machinability improvement effect. At the same time, due to the modification of β phase, dezincification corrosion resistance and resistance to stress corrosion cracking that were serious problems of conventional β phase dramatically improve as described below (Alloys Nos. S01, S02, S03, S11, S12, S13, and S21 to S46). 2) β phase became modified by containing more than 0.50 mass% Si, and grain boundary came to be observed inside β1 phase. As a result of the modification, dezincification corrosion resistance and resistance to stress corrosion cracking improved, and cutting resistance decreased (i.e., machinability index increased). When the Si content was 0.65 mass% or higher, better dezincification corrosion resistance, resistance to stress corrosion cracking, and machinability were obtained, suggesting that modification of β phase further advanced. On the other hand, electrical conductivity decreased when the Si content was higher than 1.20 mass% (Alloys Nos. S01, S37, S48, S49, S74, S78, S80, etc.). 3) β phase became modified by containing higher than 0.01 mass% P, and a grain boundary was observed inside β1 phase. As a result of the modification, dezincification corrosion resistance and resistance to stress corrosion cracking improved, and cutting resistance decreased. When the P content was increased to 0.03 mass% or higher, dezincification corrosion resistance, resistance to stress corrosion cracking, and machinability further improved, suggesting that modification of β phase advanced more. When the P content exceeded 0.18 mass%, elongation decreased (Alloys Nos. S01, S31, S71, S75, etc.). 4) β phase became modified by containing more than 60.5 mass% Cu, and a grain boundary was observed inside β1 phase. As a result of the modification, dezincification corrosion resistance and resistance to stress corrosion cracking improved, ductility also improved, and cutting resistance decreased. When a large amount of Cu was contained, proportion of β phase was small and γ phase appeared in some cases, resulting in a decrease in the balance between strength and ductility and poor machinability (Alloys Nos. S01, S35, S38 S47, S48, S72, and S80; Figs. 1A to 6A). 5) When the Pb content was lower than 0.002 mass%, machinability was poor (Alloy No. S73). When the Pb content was increased to 0.003 mass% or higher, or further, 0.01 mass% or higher, machinability improved (Alloys Nos. S33 and S23). It was confirmed that containing Bi had an effect of reducing cutting resistance equivalent to that of Pb (Alloys Nos. S01, S32, etc.). 6) When Sn was contained in excess of 0.05 mass%, dezincification corrosion resistance improved. When the Sn content was increased to higher than 0.10 mass%, dezincification corrosion resistance further improved, and resistance to stress corrosion cracking also improved. On the other hand, when 0.90 mass% or higher Sn was contained, γ phase appeared causing dezincification corrosion resistance to rather deteriorate than improve, ductility to decrease, and cutting resistance to increase (i.e., machinability index decreased). In addition, when the Sn / Si ratio was higher than 1, β phase failed to be modified and dezincification corrosion resistance was poor (Alloys Nos. S01, S39 to S46, S82, and S83). 7) It was confirmed that inclusion of inevitable impurities did not seriously affect various properties of alloy if their contents were approximately the same as those included in the products manufactured on the actual production line (Alloys Nos. S01, S02, S03, S11 to S13, and S27 to S30). 8) When the total content of Fe, Mn, Co, and Cr as inevitable impurities was 0.40 mass% or higher, cutting resistance was high, the balance between strength and ductility was poor, and dezincification corrosion resistance was not so good even though β phase was modified. When the total content was less than 0.40 mass%, cutting resistance decreased, the balance between strength and ductility improved, and dezincification corrosion resistance and resistance to stress corrosion cracking were good. When the total content was less than 0.30 mass%, cutting resistance further decreased, the balance between strength and ductility further improved, and dezincification corrosion resistance and resistance to stress corrosion cracking improved. It is considered that the decreases in cutting resistance, the balance between strength and ductility, and dezincification corrosion resistance occurred due to the following reasons. Fe, Mn, and / or other elements compounded with part of Si and / or P to form intermetallic compounds of Fe, Mn, and / or other elements and Si and / or P. Thus, compounds of Fe and / or other elements and Si and / or P were present, and the concentrations of Si and P required to be effective for the modification of β phase dropped, resulting in deterioration of machinability and corrosion resistance (Alloys Nos. S01, S02, S84, and S25 to S30). 9) When the amount of Al as an inevitable impurity was 0.30 mass% or higher, modification of β phase did not occur, which resulted in poor dezincification corrosion resistance. When the amount of Al was less than 0.30 mass%, β phase was modified and dezincification corrosion resistance was good. When the amount of Al was less than 0.15 mass%, machinability, the balance between strength and ductility, dezincification corrosion resistance, and resistance to stress corrosion cracking were all good (Alloys Nos. S01, S85, S26, and S28 to S30). 10) When the value of the composition relational expression f1 was small, modification of β phase did not occur, and dezincification corrosion resistance was poor. When the value of the composition relational expression f1 was large, the area ratio of β1 phase was small (Alloys Nos. S76 and S79). When the value of the composition relational expression f1 was 57.5 or larger, β phase was modified and dezincification corrosion resistance improved. When the value was 58.0 or larger, dezincification corrosion resistance improved more. On the other hand, when the value of the composition relational expression f1 was 60.5 or smaller, the area ratio of β1 phase increased. When the value was 60.2 or smaller, the area ratio further increased, and the machinability index became higher (e.g., Alloys Nos. S01, S03, S35, and S36). 11) When the area ratio of β1 phase was 25% or smaller, good machinability was not obtained. When the area ratio was larger than 25%, and further, 30% or larger, cutting resistance was low. When the area ratio of β1 phase exceeded 80%, however, β phase often failed to be modified, i.e., no grain boundary was observed inside the β phase. As a result, cutting resistance was high especially when machined at 110 m / min and dezincification corrosion resistance and resistance to stress corrosion cracking were poor. When the area ratio of β1 phase was 80% or smaller, and further, 70% or smaller, β phase was modified and dezincification corrosion resistance and resistance to stress corrosion cracking improved. Even though progress of dezincification corrosion was significantly delayed by the modification of β phase, β1 phase was more corroded by dezincification than α phase. For this reason, when the area ratio of β1 phase was large, dezincification corrosion was slightly expedited (Alloys Nos. S03, S11, S72, S81, and S76; Steps Nos. D1, D11H, E1, and E11H; Fig. 1B, etc.). 12) When the area ratio of γ phase was 4% or larger, elongation was low and cutting resistance was high (Alloys Nos. S78 and S82). 13) When the value of metallographic and composition relational expression f6 was small, machinability was poor (Alloy No. 81). When the f6 value exceeded 35, machinability improved (e.g., Alloy No. S12; Steps Nos. E3 and E4; Alloy No. SO2; Step No. D11H). 14) If a hot-extruded material was cooled after hot extrusion in a cooling treatment which was started when the temperature of the material was higher than 500°C and performed at an average cooling rate exceeding 300°C / min in the temperature ranges from the starting temperature of the cooling treatment to 300°C and from 500°C to 300°C, β phase was modified and a grain boundary was observed inside β1 phase. When β phase was modified, cutting resistance lowered, dezincification corrosion resistance and resistance to stress corrosion cracking significantly improved, strength was high, and the balance between strength and ductility improved. With respect to machinability, in the case β phase was modified, deterioration in machinability due to high-speed machining was very limited (Steps Nos. A1 to A5, D1 to D5, E1 to E5, F1 to F3, and G1 to G3; Figs 1A to 6A, etc.). 15) In the case the starting temperature of the cooling treatment was higher than 500°C and lower than about 530°C and the average cooling rate was between about 400°C / min and about 500°C / min in the temperature range from 500°C to 300°C, a grain boundary was observed inside β1 phase. Also, a small amount of phosphorous compounds were occasionally present. In the case a small amount of phosphorous compounds were present, cutting resistance was slightly higher and dezincification corrosion resistance was a little poorer than when such compounds were absent (Steps Nos. A1 to A5, D1 to D5, E1 to E5, F1 to F3, and G1 to G3; Figs. 1A to 6A and 1B to 6B, etc.). 16) In the case the average cooling rate in the temperature range from 500°C to 300°C was about 200°C / min to 250°C / min, no grain boundary was observed inside β1 phase, and a small amount of phosphorous compounds were present in many cases. As a result of absence of grain boundary inside β1 phase, machinability, especially when machined at a cutting speed of 110 m / min, and dezincification corrosion resistance were poor (Steps Nos. A14H, D14H, etc.). 17) When the starting temperature of the cooling treatment was 550°C or higher and the average cooling rates in the temperature ranges from the starting temperature to 300°C and from 500°C to 300°C were both favorable, machinability index was high and dezincification corrosion resistance and resistance to stress corrosion cracking were good. That is, β phase is considered to become better modified when the cooling treatment is performed with an appropriate starting temperature and a high cooling rate (Steps Nos. A1 to A5, A11H to A14H, D1 to D5, D11H to D14H, E1 to E5, E11H to E15H, F1 to F3, F4H, F5H, G1 to G3, and G11H to G14H; Figs 1B to 6B, etc.). 18) When the starting temperature of the cooling treatment was higher than 670°C, the proportion of β1 phase was larger and cutting resistance was slightly higher than when the starting temperature was 670°C or lower. In particular, when machined at 110 m / min, cutting resistance was high, and dezincification corrosion resistance was not so good. When the proportion of β1 phase was excessive, β phase sometimes failed to be modified. Incidentally, the form of dezincification corrosion was selective corrosion of β1 phase, and when the proportion of β1 phase was large, progression speed of dezincification corrosion appeared to be slightly accelerated (Steps Nos. A1 to A5, D1 to D5, E1 to E5, F1 to F3, G1 to G3, etc.). 19) When β phase was modified and β1 phase was observed, dezincification corrosion resistance was good, and the results in the Stress Corrosion Cracking Test - 1 were all "B" (Steps Nos. D1 to D5). 20) By performing shot blasting after hot working (hot forging), resistance to stress corrosion cracking further improved, and evaluation results of the Stress Corrosion Cracking Tests - 1 and 2 were all "B". In contrast, with respect to the test pieces in which β1 phase was not observed or the Si content was small, resistance to stress corrosion cracking was not improved by shot blasting (Steps Nos. D3 and D7; Alloys Nos. S71 and 80). 21) It was confirmed that when hot working is performed twice, for instance, when a hot-extruded bar is hot forged, as long as the cooling treatment performed after the final hot working, i.e., hot forging in this case, is appropriate, β phase becomes modified regardless of the cooling method after the initial hot working (Steps Nos. D1 to D5 and D7). 22) It was confirmed that even if a casting is used as a hot forging material, if the cooling treatment performed after hot forging is appropriate, β1 phase is observed and the same properties can be obtained just like when an extruded material is used as a hot forging material (Steps Nos. D6, E6, and F1). 23) When an alloy in which β phase had been modified was held at 300°C for two hours, the grain boundary of β1 phase disappeared, resulting in deterioration of machinability and dezincification corrosion resistance. When held at 130°C for five hours instead, a grain boundary was observed inside β1 phase, thus, there was no deterioration of machinability or dezincification corrosion resistance (Steps Nos. A21, A25, and A31H). 24) It was confirmed that even when no cooling treatment is performed with specific conditions in the final hot working process, if a hot-worked material is annealed at a temperature higher than 520°C and lower than 630°C for one minute to five hours, and cooled after annealing with a cooling treatment which is started when the temperature of the annealed material is higher than 500°C and performed at an average cooling rate exceeding 300°C / min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C, β phase becomes modified, resulting in significant improvement particularly in dezincification corrosion resistance and resistance to stress corrosion cracking (Steps Nos. G1 to G3; Figs 5A and 5B).

[0220] As described above, alloys in which the content of each of the alloying elements, values of the composition relational expressions and the metallographic structure relational expressions are within respective appropriate ranges have excellent hot workability and good machinability, mechanical characteristics, dezincification corrosion resistance, and resistance to stress corrosion cracking. Embodiments of the present invention are able to obtain excellent properties by appropriately adjusting the conditions in hot extrusion, hot forging, and heat treatment processes.INDUSTRIAL APPLICABILITY

[0221] The free-cutting copper alloys of the embodiments have excellent hot workability and machinability, high strength, and excellent balance between strength and elongation although they contain only a small amount of Pb. Therefore, the free-cutting copper alloys are suitable for devices and components related to drinking water or sanitary facility, food-related devices, electrical or electronic apparatus components, auto parts, mechanical components, stationaries, toys, musical instruments, sliding components, measuring instrument components, precision mechanical components, medical components, drink-related devices and components, water meters, and components involving liquid or gas such as industrial water, drainage water, or hydrogen.

[0222] Specifically, the free-cutting copper alloys can be suitably applied to the items used in the above-mentioned fields that go by the names including faucet, shutoff valve, mixer tap, shower head, valve, joint, cock, gear, axle, bearing, trumpet, shaft, sleeve, spindle, sensor, bolt, nut, flare nut, pen point, insert nut, cap nut, nipple, spacer, and screw as their material or the like.

Examples

first embodiment

[0050]A free-cutting copper alloy according to the present invention includes: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.40 mass%, the content of Al is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, the relationship of 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.5 is satisfied, if Bi is included, the relationship of 0.003 ≤ f2 = [Pb] + [Bi] i.e.,...

second embodiment

[0051]A free-cutting copper alloy according to the present invention includes: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.30 mass%, the content of Al is lower than 0.15 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, the relationship of 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.2 is satisfied...

third embodiment

[0052]A free-cutting copper alloy according to the present invention includes: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; higher than 0.05 mass% and lower than 0.90 mass% Sn; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.40 mass%, the content of Al is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, a Bi content is represented by [Bi] mass%, and a Sn content is represented by [Sn], the relationship of 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ...

Claims

1. A free-cutting copper alloy comprising: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, with the balance being Zn and inevitable impurities, wherein among the inevitable impurities, a total content of Fe, Mn, Co, and Cr is lower than 0.40 mass%, and a content of A1 is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, a relationship of 57.5 ≤ f 1 = Cu − 4.6 × Si + 0.5 × Pb + Bi − P ≤ 60.5 is satisfied, if Bi is included, a relationship of 0.003 ≤ f 2 = Pb + Bi < 0.20 is further satisfied, in constituent phases of a metallographic structure excluding non-metallic inclusions, when an area ratio of α phase is represented by (α)%, an area ratio of γ phase is represented by (y)%, and an area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, relationships of 20 ≤ f 3 = α < 75 , 25 < f 4 = β 1 ≤ 80 , 0 ≤ f 5 = γ < 4 , 27 < f 6 = β 1 × Si 1 / 2 − γ × 2 + Pb + Bi 1 / 2 × 20 + P 1 / 2 × 15 are satisfied, and a grain boundary can be observed inside the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

2. A free-cutting copper alloy comprising: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, with the balance being Zn and inevitable impurities, wherein among the inevitable impurities, a total content of Fe, Mn, Co, and Cr is lower than 0.30 mass%, and a content of A1 is lower than 0.15 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, a relationship of 58.0 ≤ f 1 = Cu − 4.6 × Si + 0.5 × Pb + Bi − P ≤ 60.2 is satisfied, if Bi is included, a relationship of 0.004 ≤ f 2 = Pb + Bi < 0.10 is further satisfied, in constituent phases of a metallographic structure excluding non-metallic inclusions, when an area ratio of α phase is represented by (α)%, an area ratio of γ phase is represented by (y)%, and an area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, relationships of 30 ≤ f 3 = α ≤ 70 , 30 ≤ f 4 = β 1 ≤ 70 , 0 ≤ f 5 = γ < 1 , and 35 < f 6 = β 1 × Si 1 / 2 − γ × 2 + Pb + Bi 1 / 2 × 20 + P 1 / 2 × 15 are satisfied, and a grain boundary can be observed inside the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

3. A free-cutting copper alloy comprising: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; higher than 0.05 mass% and lower than 0.90 mass% Sn; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, with the balance being Zn and inevitable impurities, wherein among the inevitable impurities, a total content of Fe, Mn, Co, and Cr is lower than 0.40 mass%, and a content of A1 is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, a Bi content is represented by [Bi] mass%, and a Sn content is represented by [Sn], a relationship of 57.5 ≤ f 1 = Cu − 4.6 × Si + 0.5 × Pb + Bi − P − Sn ≤ 60.5 is satisfied, and a relationship of f 0 = Sn / Si < 1 is further satisfied, if Bi is included, a relationship of 0.003 ≤ f 2 = Pb + Bi < 0.20 is satisfied, in constituent phases of a metallographic structure excluding non-metallic inclusions, when an area ratio of α phase is represented by (α)%, an area ratio of γ phase is represented by (y)%, and an area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, relationships of 20 ≤ f 3 = α < 75 , 25 < f 4 = β 1 ≤ 80 , 0 ≤ f 5 = γ < 4 , and 27 < f 6 = β 1 × Si 1 / 2 − γ × 2 + Pb + Bi 1 / 2 × 20 + P 1 / 2 × 15 are satisfied, and a grain boundary can be observed inside the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

4. A free-cutting copper alloy comprising: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; higher than or equal to 0.10 mass% and lower than 0.50 mass% Sn; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, with the balance being Zn and inevitable impurities, wherein among the inevitable impurities, a total content of Fe, Mn, Co, and Cr is lower than 0.30 mass% and a content of A1 is lower than 0.15 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, a Bi content is represented by [Bi] mass%, and a Sn content is represented by [Sn], a relationship of 58.0 ≤ f 1 = Cu − 4.6 × Si + 0.5 × Pb + Bi − P − Sn ≤ 60.2 is satisfied, and a relationship of f 0 = Sn / Si < 0.6 is further satisfied, if Bi is included, a relationship of 0.004 ≤ f 2 = Pb + Bi < 0.10 is satisfied, in constituent phases of a metallographic structure excluding non-metallic inclusions, when an area ratio of α phase is represented by (α)%, an area ratio of γ phase is represented by (y)%, and an area ratio of β1 phase, i.e., modified β phase, is represented by (β1)%, relationships of 30 ≤ f 3 = α ≤ 70 , 30 ≤ f 4 = β 1 ≤ 70 , 0 ≤ f 5 = γ < 1 , and 35 < f 6 = β 1 × Si 1 / 2 − γ × 2 + Pb + Bi 1 / 2 × 20 + P 1 / 2 × 15 are satisfied, and a grain boundary can be observed inside the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

5. The free-cutting copper alloy according to any one of claims 1 to 4, which is used for device and component related to drinking water or sanitary facility, valve, cock, industrial plumbing component, water meter, musical instrument, auto part, electrical or electronic apparatus component, mechanical component, stationery, toy, sliding component, measuring instrument component, precision mechanical component, and medical component.

6. A method for producing the free-cutting copper alloy according to any one of claims 1 to 4, the method comprising: one or more hot working steps, wherein in the final hot working step among the hot working steps, hot working temperature is higher than 540°C and lower than 750°C, in the cooling process following the final hot working step, the hot-worked material is subjected to a cooling treatment which is started when the temperature of the material is lower than 670°C and higher than 500°C and performed at an average cooling rate of higher than 300°C / min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C.

7. A method for producing the free-cutting copper alloy according to any one of claims 1 to 4, the method comprising: one or more hot working steps and heat treatment steps, wherein in the final heat treatment step, annealing is performed with a holding temperature of higher than 520°C and lower than 630°C and a holding time of one minute to five hours, the annealed material is cooled with a cooling treatment which is started after annealing when the temperature of the material is higher than 500°C and performed at an average cooling rate of higher than 300°C / min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C.

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