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

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, the total content of Fe, Mn, Co, and Cr is less than a specific amount and the content of Al is less than a specific amount among the inevitable impurities, values of the composition relational expressions f1 and f2 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, and a grain boundary is observed inside the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

Description

TECHNICAL FIELD

[0001] The present invention relates to a free-cutting copper alloy casting having good castability and dezincification corrosion resistance as well as excellent machinability and a significantly reduced Pb content and a method for producing a free-cutting copper alloy casting. The present invention relates to a free-cutting copper alloy casting used for 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, musical instruments, tableware, devices and components for water drainage, industrial plumbing components, mechanical components, sliding components, medical components, auto parts, electrical or home appliance components, pressure vessels, metal fittings for construction, measuring instrument components, daily necessities, stationaries, toys, and components relating to liquid or gas such as drinking water, industrial water, drainage water, or hydrogen, and a method for producing the free-cutting copper alloy casting. Examples of specific component names include tap fitting, faucet, mixer tap, shutoff valve, valve, joint, stem, cock, water meter, drain valve, pressure reducing valve, valve seat, gate valve, fire hydrant, gear, flange, divider tap, faucet valve, ball valve, bend, bearing, sleeve, and connector. The present invention relates to a free-cutting copper alloy casting used for these components that are manufactured by machining and a method for producing the free-cutting copper alloy casting.

[0002] 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

[0003] Conventionally, a Cu-Zn-Pb alloy (so-called a free-cutting brass bar, forging brass, or casting brass) or a Cu-Sn-Zn-Pb alloy (so-called bronze casting: leaded red brass) having excellent machinability has been generally used for devices and components related to drinking water or sanitary facility, auto parts, electrical or home appliance components, mechanical components, stationaries, measuring instrument components, medical components, and devices and components relating to liquid or gas such as industrial water, drainage water, or hydrogen including, in terms of specific name, tap fitting, faucet, mixer tap, shutoff valve, valve, cock, joint, water meter, and connector. Note that composition of a Cu-Zn-Pb alloy comprises, for example, 56 to 70 mass% Cu and 1 to 4 mass% Pb with the balance being Zn. Composition of a Cu-Sn-Zn-Pb alloy comprises, for example, 80 to 88 mass% Cu, 1 to 8 mass% Sn, and 1 to 8 mass% Pb with the balance being Zn.

[0004] 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.

[0005] In addition, in other industrial fields such as those of automobiles, electrical and electronic apparatuses, and machines, free-cutting copper alloys are exceptionally allowed to include maximum 4 mass% Pb by European directives of ELV and RoHS 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.

[0006] While there is a trend to strengthen Pb regulations for free-cutting copper alloys like (1) a Cu-Zn-Bi alloy or Cu-Zn-Bi-Se alloy including Bi having machinability or, in some cases, not only Bi but also Se 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 or Cu-Zn-Sn alloy including large amounts of γ phase and phase having excellent machinability instead of Pb, and (4) a Cu-Zn-Sn-Bi alloy including Bi and a large amount of γ phase, etc. have been conventionally proposed.

[0007] In Patent Document 1, for instance, improvement of corrosion resistance and machinability is devised by adding 0.7 to 2.0 mass% Sn and 0.5 to 2.0 mass% Bi to a Cu-Zn alloy containing 59.5 to 66.0 mass% Cu so that γ phase is precipitated.

[0008] 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 human body like Pb, Bi has a resourcing problem because it is a rare metal, and Bi embrittles a copper alloy material. Further, as indicated in Patent Document 1, even if γ phase were precipitated in a Cu-An-Sn alloy, γ phase containing Sn would have poor machinability, which is demonstrated by the fact that addition of Bi, an element with good machinability, is required to be added together with Sn.

[0009] Further, Patent Document 2 discloses a Cu-Zn alloy containing 59 to 62 mass% Cu to which Bi is added in an amount as large as 0.3 to 4 mass%, preferably 1.8 to 3.2 mass%. Moreover, to improve machinability and dezincification corrosion resistance, annealing is performed at a temperature between 350°C and 550°C so that β phase is reduced and fragmented by α phase since β phase has poor dezincification corrosion resistance. Thus, 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.

[0010] On the other hand, 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.

[0011] For this reason, Cu-Zn-Si alloys including Si instead of Pb are proposed in Patent Documents 3 to 9 as free-cutting copper alloys in recent years.

[0012] Patent Documents 3 to 8 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 2, 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 3 to 8, 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.

[0013] In Patent Document 9, 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.

[0014] In Patent Document 10, 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 10 requires performance of finish heat treatment at a temperature between 400°C and 600°C in effect.

[0015] Patent Document 11 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 12 proposes a copper alloy casting made of a Cu-Zn-Zr-P alloy optionally containing Si, Pb, Sn, and / or Bi, in which crystal grains are refined by the actions of Zr and P.

[0016] Patent Document 13 proposes a Cu-Zn-Sn-Al copper alloy with excellent color fastness optionally containing Si and / or Pb in which the area ratios of γ phase and β phase are limited.

[0017] Patent Document 14 proposes a lead-free copper alloy casting of a Cu-Zn-Si-Sn-Al-P alloy.

[0018] Patent Document 15 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.

[0019] Patent Document 16 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

[0020] Patent Document 1: PCT International Publication No. WO2008 / 081947 Patent Document 2: Japanese Unexamined Patent Application, First Publication No. 2002-003967 Patent Document 3: Japanese Unexamined Patent Application, First Publication No. 2021-042461 Patent Document 4: Japanese Unexamined Patent Application, First Publication No. 2021-042459 Patent Document 5: Japanese Unexamined Patent Application, First Publication No. 2021-042460 Patent Document 6: PCT International Publication No. WO2020 / 261666 Patent Document 7: Japanese Unexamined Patent Application, First Publication No. 2021-042462 Patent Document 8: PCT International Publication No. WO2021 / 117528 Patent Document 9: PCT International Publication No. WO2007 / 034571 Patent Document 10: Japanese Unexamined Patent Application, First Publication No. 2016-194123 Patent Document 11: PCT International Publication No. WO2013 / 065830 Patent Document 12: PCT International Publication No. WO2006 / 016630 Patent Document 13: PCT International Publication No. WO2015 / 046421 Patent Document 14: Japanese Unexamined Patent Application, First Publication No. 2010-133006 Patent Document 15: Japanese Unexamined Patent Application, First Publication No. 2018-048398 Patent Document 16: Published Japanese Translation No. 2019-508584 of the PCT International Publication SUMMARY OF INVENTIONTechnical Problem

[0021] As indicated in Patent Documents 1 to 16, there is no past record of substantial improvement made in the dezincification corrosion resistance 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 mass% 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.

[0022] 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 casting in which Pb content has been significantly reduced but having excellent machinability, excellent castability, good dezincification corrosion resistance despite a large content of β phase (the β1 phase described later), a high strength, and good impact properties, and a method for producing the free-cutting copper alloy casting. Solution to Problem

[0023] 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.

[0024] 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. β1 phase has a characteristic that a pattern of crystal grain boundary 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 grain boundary pattern, i.e., crystal grain boundary 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. For example, in Fig. 1A, which shows a metallographic structure of an alloy of the present invention consisting of two phases of β1 phase, which is generated by modification of β phase, and α phase, and a grain boundary pattern can be observed inside the β1 phase. In the instant specification, the grain boundary pattern shown in Fig. 1A is defined as a crystal grain boundary which may be simply referred to as a grain boundary. On the other hand, in Fig. 2A, which shows a metallographic structure of a comparative alloy example consisting of β phase and α phase, no grain boundary pattern is found inside β phase. That is, no grain boundary is present in the phase. Phosphorous compound refers to a compound composed of P and, in most cases, Zn and / or Si. Cold workability refers to performance of a material when subjected to working conducted in a cold state such as drawing, wire drawing, crimping or bending. Unless specified otherwise, good or excellent machinability stands for low cutting resistance and good 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. Cooling rate at a given temperature, for instance, 500°C refers to the cooling rate when the casting's temperature goes down from a temperature slightly above 500°C until it passes 500°C, i.e., the average cooling rate from a temperature a few tens of degrees Centigrade higher than 500°C to 500°C. "Manufacturing on the actual production line" refers to production with a mass production facility used for manufacturing products for sale.

[0025] The Patent Document 9 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 11, 12, and 13, 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 10 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.

[0026] On the other hand, Patent Documents 3 to 8 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 or casting.

[0027] Incidentally, Patent Documents 3 to 8 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.

[0028] 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 by the effect of modified β phase, i.e., β1 phase, and significant improvement in the dezincification corrosion resistance of β phase, which had been a longstanding drawback 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.

[0029] In order to obtain this β1 phase, it is necessary to first solid-solubilize Si and P in the β phase under high temperature after a casting solidifies then maintain the state of metallographic structure when it is at a high temperature until and after the casting's temperature becomes a normal temperature by maintaining the state of β phase when it is at a high temperature exceeding 500°C, specifically higher than 500°C and lower than 700°C, and cooling the casting 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 grain boundary pattern or 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 3-8, 11, and 12, and pictures of the metallographic structure observed by etching with the solution are also disclosed in the Patent Documents.

[0031] The condition necessary to obtain β1 phase is that the state of β phase when it is at a high temperature exceeding 500°C, specifically, a temperature higher than 500°C and lower than 700°C after the molten metal cast into a mold solidifies is maintained until the casting reaches normal temperature. Specifically, in order to obtain β1 phase, it is necessary to start a cooling treatment in the cooling process after the casting solidifies when its temperature is preferably lower than 700°C and higher than 500°C, and cool it at a cooling rate of higher than 300°C / min when its temperature is 500°C, i.e., when the casting is in the temperature range from 500°C to a temperature only slightly higher than 500°C, then 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 that follows. 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.

[0032] In order to obtain fine P-containing compound, Patent Documents 3 to 8 require slower cooling than what is required for the present invention, i.e., cooling at an average cooling rate of about 0.1°C / min or higher and about 70°C / min or lower when a casting or a hot-worked material is in the temperature range from about 530°C to about 450°C after casting or hot working. That is, the present invention and those of Patent Documents 3 to 8 are clearly heading in opposite directions regarding the cooling process after casting or hot working.

[0033] It should be noted, however, that the present invention may also include both β1 phase and phosphorous compounds if, for instance, the cooling rate at the point the temperature of the casting is around 530°C is about 50°C / min and also the average cooling rate in the temperature range from 530°C, which is the starting temperature of the cooling treatment, to 500°C exceeds 300°C / min in the cooling process.

[0034] The modification of β phase into β1 phase in the present invention allows a significant improvement in the machinability of a Cu-Zn-Si alloy without requiring phosphorous compounds. Further, by the synergy effect between β1 phase and a small amount of fine Pb particles or particles containing both Pb and Bi, cutting resistance is decreased and chip breakability is promoted. As a result, good machinability is maintained in high-speed cutting as well. 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 improved by the modification of β phase into β1 phase without deteriorating ductility or impact resistance.

[0035] In the case of castings, cold working such as swaging is rarely performed. Therefore, castings should not be brittle, i.e., they need to endure impact while in use. Modification of β phase into β1 phase allows a casting to have enhanced strength and prevent deterioration of impact properties. Thus, a copper alloy casting with a free-cutting property equivalent to that of conventional free-cutting brass containing a large amount of Pb and better dezincification corrosion resistance and resistance to stress corrosion cracking as well as higher strength and better impact properties than those of conventional free-cutting brass came to be invented.

[0036] A free-cutting copper alloy casting 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 0.50 mass% and a 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 following relationship of 57.3 ≤ f1 = [Cu] - 4.8 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.8 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 = (α) < 75, 25 < f4 = (β1) ≤ 80, 0 ≤ f5 =(y) < 4, and 28 < f6 = (β1) × (-0.5 × [Si] 2< + 1.5 × [Si]) - 2 × (γ) + ([Pb] + [Bi]) 1 / 2< × 25 + ([P]) 1 / 2< × 12 are satisfied, and a grain boundary is observed inside the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0037] A free-cutting copper alloy casting according to a second aspect of the present invention includes: higher than or equal to 61.5 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 0.35 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] mas %, the following relationship of 58.0 ≤ f1 = [Cu] - 4.8 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.5 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 = (γ) < 1 and 35 < f6 = (β1) × (-0.5 × [Si] 2< + 1.5 × [Si]) - 2 × (γ) + ([Pb] + [Bi]) 1 / 2< × 25 + ([P]) 1 / 2< × 12 are satisfied, and a grain boundary is observed inside the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0038] A free-cutting copper alloy casting 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 0.70 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 0.50 mass% and a 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 following relationship of 57.3 ≤ f1 = [Cu] - 4.8 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≤ 60.8 is satisfied, and the following relationship of f0 = [Sn] / [Si] < 0.8 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 28 < f6 = (β1) × (-0.5 × [Si] 2< + 1.5 × [Si]) - 2 × (γ) + ([Pb] + [Bi]) 1 / 2< × 25 + ([P]) 1 / 2< × 12, are satisfied, and a grain boundary is observed inside the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

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

[0040] A method for producing a free-cutting copper alloy casting according to a fifth aspect of the present invention is a method for producing the free-cutting copper alloy casting according to any one of the first to fourth aspects of the present invention, including casting step(s), in which in the cooling process of the final casting step among the casting steps, the cooling rate at the point the temperature of the casting is 500°C is higher than 300°C / min and the average cooling rate in the temperature range from 500°C to 300°C is higher than 300°C / min.

[0041] A method for producing a free-cutting copper alloy casting according to a sixth aspect of the present invention is a method for producing the free-cutting copper alloy casting according to any one of the first to fourth aspects of the present invention, including casting step(s), in which in the cooling process of the final casting step among the casting steps, a cooling treatment is started at a temperature higher than 500°C and lower than 700°C, the cooling rate at the point the temperature of the casting is 500°C is higher than 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is higher than 300°C / min.

[0042] A method for producing a free-cutting copper alloy casting according to a seventh aspect of the present invention is a method for producing the free-cutting copper alloy casting according to any one of the first to fourth aspects of the present invention, including casting step and heat treatment step, in which in the final heat treatment step, the casting is heated to a temperature higher than 520°C and lower than 650°C and held at the temperature for one minute to five hours, in the process of cooling treatment after the heat treatment, the cooling treatment is started when the temperature of the casting is higher than 520°C, the cooling rate at the point the temperature of the casting is 500°C is higher than 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is higher than 300°C / min.Advantageous Effects of Invention

[0043] According to one aspect of the present invention, a free-cutting copper alloy casting having excellent castability, good machinability, dezincification corrosion resistance, and resistance to stress corrosion cracking, a high strength, and a good balance between strength, impact properties, and ductility and containing a significantly reduced amount of Pb, and a method for producing the free-cutting copper alloy casting can be provided.BRIEF DESCRIPTION OF DRAWINGS

[0044] [Fig. 1A] A picture showing the structure of a copper alloy casting according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step No. A1. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.94 mass% Si, 0.082 mass% P, 0.067 mass% Pb, 0.03 mass% Sn, and Zn as the balance. In Step No. A1, molten alloy heated to 1080°C was poured into a sand mold and a casting was taken out of the mold when its temperature became 650°C then subjected to a cooling treatment which was started when the temperature became 560°C and performed at an average cooling rate of 900°C / min in the temperature ranges from 550°C to 500°C and from 500°C to 300°C. [Fig. 1B] A cross-sectional picture of the metallographic structure of the alloy casting shown in Fig. 1A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the casting in accordance with ISO 6509. [Fig. 2A] A picture showing the structure of a copper alloy casting according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step No. A14H. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.94 mass% Si, 0.082 mass% P, 0.067 mass% Pb, 0.03 mass% Sn, and Zn as the balance. In Step No. A14H, molten alloy heated to 1080°C was poured into a sand mold and a casting was taken out of the mold when its temperature became 650°C then left to cool naturally. The average cooling rate in the temperature range from 500°C to 300°C was 20°C / min. [Fig. 2B] A cross-sectional picture of the metallographic structure of the alloy casting shown in Fig. 2A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the casting in accordance with ISO 6509. [Fig. 3A] A picture showing the structure of a copper alloy casting according to an embodiment, which was obtained by subjecting Alloy No. S12 to Step No. B1. More specifically, Alloy No. S12 has a composition including 63.6 mass% Cu, 1.04 mass% Si, 0.070 mass% P, 0.062 mass% Pb, 0.003 mass% Bi, and Zn as the balance. In Step No. B1, molten alloy heated to 960°C was poured into a permanent mold and a casting was taken out of the mold when its temperature became 750°C then subjected to a cooling treatment which was started when the temperature became 560°C and performed at an average cooling rate of 1020°C / min in the temperature ranges from 550°C to 500°C and from 500°C to 300°C. [Fig. 3B] A cross-sectional picture of the metallographic structure of the alloy casting shown in Fig. 3A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the casting in accordance with ISO 6509. [Fig. 4A] A picture showing the structure of a copper alloy casting according to an embodiment, which was obtained by subjecting Alloy No. S12 to Step No. B11H. More specifically, Alloy No. S12 has a composition including 63.6 mass% Cu, 1.04 mass% Si, 0.070 mass% P, 0.062 mass% Pb, 0.003 mass% Bi, and Zn as the balance. In Step No. B11H, molten alloy heated to at 960°C was poured into a permanent mold and a casting was taken out of the mold when its temperature became 750°C then cooled at an average cooling rate of 40°C / min in the temperature range from 500°C to 300°C. [Fig. 4B] A cross-sectional picture of the metallographic structure of the alloy casting shown in Fig. 4A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the casting in accordance with ISO 6509. [Fig. 5A] A picture showing the structure of a copper alloy casting according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step Nos. A14H and G1. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.94 mass% Si, 0.082 mass% P, 0.067 mass% Pb, 0.03 mass% Sn, and Zn as the balance. In Step No. A14H, molten alloy heated to 1080°C was poured into a sand mold and a casting was taken out of the mold when its temperature became 650°C then left to cool naturally. The average cooling rate in the temperature range from 500°C to 300°C was 20°C / min. The casting obtained in Step No. A14H was subsequently subjected to Step No. G1, in which the casting was heated to 580°C for 20 minutes then subjected to a cooling treatment which was started when its temperature became 575°C and performed at an average cooling rate of 1200°C / min in the temperature ranges from 550°C to 500°C and from 500°C to 300°C. [Fig. 5B] A cross-sectional picture of the metallographic structure of the alloy casting shown in Fig. 5A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the casting in accordance with ISO 6509. DESCRIPTION OF EMBODIMENTS

[0045] Below is a description of free-cutting copper alloy castings according to the embodiments of the present invention and methods for producing the free-cutting copper alloy castings.

[0046] The free-cutting copper alloy castings according to the embodiments are used for devices and components related to drinking water or sanitary facilities, musical instruments, tableware, components, electrical, home appliance, 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, or hydrogen. Examples of specific component names include faucet, mixer tap, shutoff valve, valve, cock, joint, water meter, gear, sensor, nut, screw, and connector.

[0047] Here, in this specification, an element symbol in parentheses such as [Zn] represents the content (mass%) of the element.

[0048] In the embodiments, using this content expressing method, composition relational expressions f1, f2, and f0 are defined as follows.

[0049] In the case Bi is contained, composition relational expression f2 = [Pb] + [Bi]

[0050] In the case Sn is contained, f0 = [Sn] / [Si]

[0051] Further, in the embodiments, in constituent phases of the metallographic structure excluding non-metallic inclusions, precipitates, and Pb or Bi particles, 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".

[0052] 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)

[0053] A free-cutting copper alloy casting 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 as an optional element, higher than or equal to 0.0001 mass% and lower than 0.20 mass% B; 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 0.50 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 57.3 ≤ f1 = [Cu] - 4.8 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.8 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 = (γ) < 4, and 28 < f6 = (β1) × (-0.5 × [Si] 2< + 1.5 × [Si]) - 2 × (γ) + ([Pb] + [Bi]) 1 / 2< × 25 + ([P]) 1 / 2< × 12 are satisfied, and a grain boundary is observed inside the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0054] A free-cutting copper alloy casting according to the second embodiment of the present invention includes: higher than or equal to 61.5 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 0.35 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] - 4.8 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.5 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) × (-0.5 ×[Si] 2< + 1.5 × [Si]) - 2 × (γ) + ([Pb] + [Bi]) 1 / 2< × 25 + ([P]) 1 / 2< × 12 are satisfied, and a grain boundary is observed inside the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0055] A free-cutting copper alloy casting 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 0.70 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 0.50 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 57.3 ≤ f1 = [Cu] - 4.8 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≤ 60.8 is satisfied, f0 = [Sn] / [Si] < 0.8 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 = (γ) < 4, and 28 < f6 = (β1) × (-0.5 × [Si] 2< + 1.5 × [Si]) - 2 × (γ) + ([Pb] + [Bi]) 1 / 2< × 25 + ([P]) 1 / 2< × 12 are satisfied, and a grain boundary is observed inside the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

[0056] The reasons why the compositional components, the composition relational expressions f0, f1, and f2, the metallographic structure relational expressions f3, f4 (f4A), and f5, the metallographic structure and composition relational expression f6 (f6A), the metallographic structure, and the like are defined as described above are explained below.< Compositional Components >(Cu)

[0057] 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, the alloy embrittles due to deterioration in ductility. In addition, β phase fails to be modified sufficiently causing dezincification corrosion resistance, resistance to stress corrosion cracking, machinability, and ductility to deteriorate. Accordingly, the lower limit of the Cu content is higher than 60.5 mass%, preferably 61.0 mass% or higher, more preferably 61.5 mass% or higher, and still more preferably 62.0 mass% or higher.

[0058] 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)

[0059] 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 generated by modification, 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.

[0060] 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, dezincification corrosion resistance, and resistance to stress corrosion cracking, and enhances strength of α phase. Accordingly, by the effects of β1 phase generated by modification and ameliorated α phase, machinability, dezincification corrosion resistance, resistance to stress corrosion cracking, and strength of the alloys improve.

[0061] 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, machinability, impact properties, and ductility of the alloys,α phase needs to be 20% or more, preferably 30% or more, in terms of area ratio.

[0062] Si is an essential element required for the modification of β phase. Si solid-solubilizes in β phase after molten alloy solidifies. The higher the Si content, the more progresses the modification of β phase, resulting in formation of β1 phase with better properties. Si also enhances strength of alloy and improves fluidity of molten alloy in the process of casting. 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 0.70 mass% or higher.

[0063] 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, and ductility and impact properties are poor. In some cases, γ phase appears. Speaking of γ phase, Patent Documents 3 to 9 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, impact properties, 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 1.00 mass% or lower where electrical and thermal conductivities are important.(Zn)

[0064] 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)

[0065] Like Si, P is an essential element required for the modification of β phase into β1 phase. After molten alloy solidifies, P solid-solubilizes in β phase together with Si. If the casting is cooled in the cooling process after the solidification at a cooling rate exceeding 300°C / min at the point the temperature of the casting is 500°C and in the temperature range from 500°C to 300°C, β1 phase is obtained. The modification of β phase into β1 phase enhances machinability and significantly improves long-standing drawbacks of conventional β phase, i.e., its dezincification corrosion resistance and resistance to stress corrosion cracking. Aside from that, compound comprising P and Zn or Si decreases cutting resistance and improves chip breakability, but if the casting is subjected to a cooling treatment which is started when its temperature is about 550°C or higher than 530°C and performed at a cooling rate exceeding 300°C / min, P-containing compound does not appear or even if it does, the amount is very small. With respect to machinability, the effect brought by the modification of β phase into β1 phase excels that brought by the presence of Si-containing β phase and phosphorous compound. In particular, in high-speed cutting, the effect of β1 phase is far better. Incidentally, inclusion of P leads to improvement of the dezincification corrosion resistance of α phase and significant improvement of that of alloys consisting of β1 phase and α phase.

[0066] To modify β phase into β1 phase, the P content needs to be at least higher than 0.01 mass%. In order to obtain better modified β1 phase, the amount of P is preferably 0.03 mass% or higher, more preferably 0.04 mass% or higher. Then dezincification corrosion resistance and resistance to stress corrosion cracking further improve.

[0067] Incidentally, P tend to compound with elements such as Zn, Si, Mn, Fe, Cr, Co, and Al. If P forms compounds with these elements, the amount of P solid-solubilized inside the β phase in an alloy in the cooling process after solidification is reduced, and the modification 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 around 550°C or a temperature higher than that. If the amounts of the elements 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 dezincification corrosion resistance and resistance to stress corrosion cracking. Therefore, the total amount of Fe, Mn, Co, and Cr needs to be lower than 0.50 mass% at most, preferably lower than 0.35 mass%.

[0068] 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 basically 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, casting is cooled after it is solidified at a high cooling rate at the point the temperature of the casting is 500°C and in the temperature range from 500°C to 300°C 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. Thus, the modification from β phase into β1 phase and formation of phosphorous compound are heading in opposite directions, but if the cooling rate is rapidly increased once the temperature reaches, for instance, about 515°C in the process of cooling, phosphorous compounds are formed and β phase is modified at the same time. In that case, however, modification of β phase becomes slightly insufficient.

[0069] 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)

[0070] 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.

[0071] 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)

[0072] 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 the 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.

[0073] For an alloy containing Bi together with a small amount of Pb to obtain good machinability, the Bi content needs to be at least 0.0001 mass% or higher. The amount of Bi is preferably 0.001 mass% or higher, more preferably 0.002 mass% or higher. Further, it is lower than 0.20 mass%, preferably lower than 0.10 mass%, and more preferably 0.08 mass% or lower. In addition, as Bi is more vulnerable to cracking than Pb during casting, if the Bi content is within the range set forth above, cracking is unlikely to occur. Incidentally, considering the facts that Bi is well capable of replacing Pb, use of Bi affects 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 the environment and the problems of stress corrosion cracking and cracking during casting, 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%.(Sn)

[0074] 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 γ 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 0.70 mass% at most although it depends on the amount of Si contained. The Sn content is preferably lower than 0.60 mass%, more preferably lower than or equal to 0.50 mass%.

[0075] 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)

[0076] 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.

[0077] 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, products with manufacturing defects, and disposed products. 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 that is 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.

[0078] 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%).

[0079] Further, in a leaded JIS standard brass casting (JIS H 5120), about 2 mass% Pb is contained as an essential element and the upper limits of the amounts of the components that constitute the balance are defined to be 0.8 mass% for Fe, 1.0 mass% for Sn, 0.5 mass% for Al, and 1.0 mass% for Ni, respectively. In fact, a high concentration of Fe, Sn, Al, or Ni in an amount close to the respective upper limits defined by the JIS is sometimes contained in a free-cutting bar or a brass casting.

[0080] 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, or a Cr-Si compound in the metallographic structure. Likewise, when they compound with P, a Fe-P compound, an Mn-P compound, a Co-P compound, or a Cr-P compound 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 to lower than 0.35 mass% respectively. The content is preferably lower than 0.25 mass%, more preferably 0.15 mass% or lower. In particular, the total content of Fe, Mn, Co, and Cr is required to be lower than 0.50 mass%, preferably lower than 0.35 mass%, more preferably lower than 0.30 mass%, and still more preferably 0.25 mass% or lower.

[0081] 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 its content is large. Further, Al 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%, more preferably lower than 0.15 mass%, and still more preferably 0.10 mass% or lower.

[0082] 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, each content of As and Sb is preferably lower than 0.05 mass% and more preferably 0.02 mass% or lower.

[0083] 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.

[0084] 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.

[0085] 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)

[0086] In composition relational expression f1 = [Cu] - 4.8 × [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.

[0087] The composition relational expression f1 is an expression that represents a relationship between the composition and the metallographic structure. Even when the amount of each of the elements is within the range defined above, unless 57.3 ≤ f1 ≤ 60.8 is satisfied, the target properties in embodiments of the present invention cannot be obtained.

[0088] When the value of the composition relational expression f1 is lower than 57.3, the proportions of β phase and β1 phase are large, causing deterioration of dezincification corrosion resistance, resistance to stress corrosion cracking, ductility, and impact properties due to insufficient modification of β phase. Accordingly, the lower limit of the value of the composition relational expression f1 is 57.3 or higher, preferably 58.0 or higher, and more preferably 58.5 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, ductility, and impact properties can be obtained without impairing excellent machinability.

[0089] 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 60.8, the proportions of β phase and β1 phase are small. As a result, excellent machinability can no longer be obtained, and strength is low. Further, the upper limit of the f1 value is related to castability. Castability and solidification temperature range are closely related. If the f1 value exceeds its upper limit, the solidification temperature range becomes larger than 30°C, resulting in poor castability and appearance of a large number of defects in the portion that solidifies last. Therefore, the upper limit of the value of the composition relational expression f1 is 60.8 or lower, preferably 60.5 or lower, and more preferably 60.1 or lower. As the value of f1 becomes smaller, β1 phase increases, machinability improves, strength enhances, solidification temperature range narrows, and castability improves although that depends on the composition of the alloy and the production process applied.(Composition Relational Expression f2)

[0090] 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 fact that Bi affects the environment since it is a rare metal. Further, both elements of Pb and Bi are metals of low melting point and are present in metallographic structure as metallic particles which may induce cracking during casting. Therefore, their contents need to be limited. In consideration of all these, 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.(Composition Relational Expression f0)

[0091] 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 larger than the Si content, modification of β phase is hindered, which may impair dezincification corrosion resistance and machinability of β1 phase.

[0092] That is, it was revealed that 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 0.8. It is preferably lower than 0.65 and more preferably lower than 0.50.

[0093] Free-cutting copper alloy castings according to an embodiment of the present invention have good dezincification corrosion resistance and resistance to stress corrosion cracking 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 and impact properties that are properties entirely opposite to machinability. To obtain good corrosion resistance, machinability, and mechanical properties, the composition relational expressions f1, f2, and f0, 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.

[0094] 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)

[0095] Here, the results of comparison between the compositions of the Cu-Zn-Si alloys described in Patent Documents 1 to 16 and those of alloys according to embodiments of the present invention are shown in Tables 1 to 4. [Table 1](mass%)CuSiPPbBiSnAlOthersMetallographic StructureFirst Embodiment60.5 -65.00.50 -1.200.01 -0.180.002 -0.20Optional 0.0001-0.2-Fe+Mn+Cr+Co<0.50, Al<0.3020≤α<75, 25<modified β≤80, 0≤γ<4The average cooling rate in the temperature range from 500°C to 300°C after casting or heat treatment is higher than 300°C / min.Second Embodiment61.5 -64.80.65 -1.100.03 -0.150.003 -0.10Optional 0.001-0.1-Fe+Mn+Cr+Co<0.35, A<0.1530≤α≤70, 30≤modified β≤70, 0≤γ<1The average cooling rate in the temperature range from 500°C to 300°C after casting or heat treatment is higher than 300°C / min.Third Embodiment60.5 -65.00.50 -1.200.01 -0.180.002 -0.20Optional 0.0001-0.20.05-0.70Fe+Mn+Cr+Co<0.50, Al<0.3020≤α<75, 25<modified β≤80, 0≤γ<4The average cooling rate in the temperature range from 500°C to 300°C after casting or heat treatment is higher than 300°C / min. [Table 2] (mass%)CuSiPPbBiSnAlOthersMetallographic StructurePatent Document 159.5 -66.00.1 or less0.05 -0.2-0.5-2.00.7-2.5-0.05-0.6Sb*α+γ or α+γ+β structurePatent Document 259-62-0.02 -0.07-0.3-4.0--Fe≤0.3Two-phase structure of α and β phases in which β phase is divided by α phaseAnnealing step at a temperature between 350°C and 550°CPatent Document 361.0 -65.01.0-1.50.005 -0.190.003 -0.20-Sn+Al<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 459.0 -63.50.5-1.00.005 -0.190.003 -0.20-Sn+Al<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 558.5 -65.00.4-1.40.003 -0.190.002 -0.25Optional 0.001-0.1Sn+Al<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.Patent Document 658.5 -64.50.2-1.20.001 -0.20.001 -0.200.1-1.0Sn+Al<0.45, Fe+Mn+Cr+Co<0.4520≤α≤85, 15≤β≤80, 0≤γ<4 [Table 3] (mass%)CuSiPPbBiSnAlOthersMetallographic StructurePatent Document 758-650.3-1.30.001 -0.20.001 -0.200.02-0.1Sn+Al<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 859.7 -64.70.6-1.30.001 -0.150.001 -0.20.001-0.1Sn+Al<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 971.5 -78.52.0-4.50.01-0.20.005 -0.020.01-0.20.1-1.20.1-2.0-0≤β≤5Patent Document 1060-660.01-0.5-0.150.05-0.5-0.2-0.9-0.02≤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. [Table 4] (mass%)CuSiPPbBiSnAlOthersMetallographic StructurePatent Document 1159-840.05-4.50.01-0.30.003 -0.30.003 -0.30.01-1.50.1-1.0-0≤β+γ+µ≤25; tubular hot-forged copper alloy productPatent Document 1258-72.50.05-2.00.01 -0.250.01-40.01-30.05-40.01-3Zr: 0.0008-0.04585 ≤ α + γ → β ≤ 15Patent Document 13Rem.0.01-10.005 -0.090.005 -0.03-0.01 -2.50.1-1.8Zn: 17-34γ phase: 0-0.7%β phase: 0-0.9%Patent Document 14Rem.0.1-1.50.01 -0.36--0.05 -0.50.03 -0.3Ti: 0.01-0.1-Rare earths: 0.01-0.05Patent Document 1555-750.01-1.5-0.150.01 -0.40.01-40.1-0.1---Patent Document 1665-750.5-2.0---0.01-0.20.2-0.5Mn: 0.01-0.25β phase is essentially required.Sb: 0.01-0.55B: 0.001-0.05

[0096] The embodiments and the alloys disclosed by Patent Document 9 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 10 are different in the content of Si, which is a main element of the alloys. The embodiments and the alloys disclosed by Patent Document 16 are different in the content of Cu, which is a main element of the alloys. The alloys disclosed by Patent Documents 1 and 2 do not contain Si.

[0097] Patent Documents 2, 14, and 16 disclose that Pb is not contained. Therefore, the Pb content is different.

[0098] In Patent Documents 9, 11, 12, and 13, 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. β1 phase and β phase are two totally 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 Patent Documents 9, 11, 12, and 13.

[0099] Patent Document 11 relates to a tubular hot-forged part having a near-net shape and discloses that a tubular material is used.

[0100] Patent Documents 2 and 10 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.

[0101] Patent Document 10 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 0.1 to 0.4 mass%, which may be controlled to be 0.2 mass% or lower.

[0102] In Patent Documents 13, 14, and 15, Al is essentially required in order to improve color fastness, castability, and dezincification corrosion resistance.

[0103] Patent Document 15 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.

[0104] Patent Document 16 discloses a corrosion-resistant copper alloy casting that does not contain Pb but requires γ 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.

[0105] All of Patent Documents 3 to 8 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 molten alloy is poured and solidifies or 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 γ 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. The present invention is cooled in a manner basically contrary to that disclosed by Patent Documents 3 to 8. Specifically, in the present invention, the casting is cooled in the cooling process after molten alloy is poured and solidifies at a cooling rate exceeding 300°C / min at the point its temperature is 500°C, i.e., in the temperature range from 500°C to a temperature only slightly higher than 500°C. Also, the casting is subjected to a cooling treatment which is started when its temperature is preferably lower than 700°C and higher than 500°C and performed at an average cooling rate of higher than 300°C / min in the temperature range from 500°C to 300°C. In the present invention, compound composed of P and Zn and / or Si may be present, but the amount of such phosphorous compound is small if present.

[0106] The most remarkable and clear-cut difference between the present application and Patent Documents 3 to 8 is that none of the prior art documents refers to modification of β phase or modified β phase, that is, β1 phase. β phase in which Si and P are solid-solubilized is modified into β1 phase if the casting is cooled in the cooling process after molten metal solidifies at a cooling rate of higher than 300°C / min at the point the temperature of the casting is 500°C and at an average cooling rate of at least higher than 300°C / min in the temperature range from 500°C to 300°C. There is no chance that β phase becomes modified if the casting is cooled 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. A grain boundary is observed inside β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water. Before modification to β1 phase, however, even if specific amounts of Si and P are contained, no such boundary is observed inside β phase. Patent Documents 3 to 8 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.

[0107] Unlike conventional β phase, β1 phase is able to improve dezincification corrosion resistance and resistance to stress corrosion cracking significantly. Further, machinability is better improved by β1 phase than by β phase. In fact, the machinability improvement effect brought by the presence of β1 phase excels that of the combined effect of the presence of phosphorous compounds and β phase containing Si. 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 3 to 8.<Metallographic Structure>

[0108] 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

[0109] Note that in this expression, β1 phase is a phase generated by modification of β phase. 0 ≤ f 5 = γ < 4(β1 phase)

[0110] 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. Only these phases in the metallographic structure of the invention are defined. In the metallographic structure, precipitate such as P-containing compound or Si-containing compound, crystallized particle, particle of Pb or Bi, oxide, sulfide, and / or inclusion may be present other than the constituent phases. In the case of casting, larger amounts of oxide and crystallized particle are often contained compared with hot-extruded material.

[0111] Distinguishment between β phase and β1 phase is easy. If β phase has been modified into β1 phase, a grain boundary pattern, i.e., a 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.

[0112] β1 phase of an alloy according an embodiment of the present invention is obtained by maintaining the state of β phase, in which Si and P are solid-solubilized in certain amounts or more, after molten metal solidifies when it is still at a high temperature exceeding 500°C, i.e., between about 500°C and 700°C, until the temperature of casting goes down to normal temperature through a cooling treatment (i.e., cooling at a cooling rate of higher than 300°C / min at the point the temperature of the casting is 500°C and an average cooling rate of at least higher than 300°C / min in the temperature range from 500°C to 300°C and continuing to cool until the temperature of the casting becomes normal temperature) so that the state of the metallographic structure when its temperature is between 500°C and 700°C is maintained until the temperature of the casting becomes normal temperature. More specifically, to obtain β1 phase, the casting is rapidly cooled at a high cooling rate in the temperature range from the starting temperature of the cooling treatment, which is between 500°C and 700°C, 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 at the temperature between 500°C and 700°C can be maintained after it is cooled down to normal temperature. As a result, β1 phase is obtained. Even if the same 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 an alloy containing certain amounts of Si and P is cooled at a cooling rate of 300°C / min or lower at the point its temperature is 500°C, β1 phase cannot be obtained. If it is cooled at an average cooling rate of 300°C / min or lower in the temperature range from 500°C to 300°C, β1 phase cannot be obtained. If the cooling treatment is started when the alloy's temperature is below 500°C, e.g., 450°C, β1 phase cannot be obtained. In addition, for the modification of β phase, a certain amount of α phase is required. Therefore, if the amount of α phase is insufficient, β1 phase cannot be obtained since modification of β phase does not occur. Further, the degree of the modification is also affected by the amounts of Si, P, and of inevitable impurities such as Fe or Al, the cooling rate at the point the temperature of the casting is 500°C, the average cooling rates in the temperature range from 500°C to 300°C, and the starting temperature of the cooling treatment. 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%. If the amounts of Si and P are excessive, adverse effects such as deterioration in electrical conductivity, thermal conductivity, and / or ductility, appearance of γ phase, and the like may be induced.

[0113] β1 phase obtained by modification 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 delayed by half or more than half. As disclosed by Patent Document 2, 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 first occurs along β phase.

[0114] 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 as well as better impact properties than β phase. Therefore, strength and ductility in an alloy with β1 is well-balanced.(β1 phase; Metallographic Structure Relational Expression f4)

[0115] 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 proportion of β1 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 on a Cu-Zn alloy whose metallographic structure consists of α phase and β phase, β phase is selectively corroded first by dezincification. As the dezincification corrosion advances, the depth of such corrosion finally becomes as deep as about 500 µm to 550 µm. In contrast, in the case of an alloy of the present invention whose metallographic structure consists of α phase and β1 phase, progress of dezincification corrosion is far better suppressed than an alloy whose metallographic structure consists of α phase and β phase since even though β1 phase is selectively corroded by dezincification, the depth of such corrosion is about 20 µm to 240 µm. Thus, although modification of β phase into β1 phase allows significant improvement in dezincification corrosion resistance, dezincification corrosion resistance and impact properties 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 impact properties 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.

[0116] Apropos, non-modified β phase is distinguished from β1 phase and defined by the metallographic structure relational expression f4A.(α phase; Metallographic Structure Relational Expression f3)

[0117] The metallographic structure of the present invention basically consists of α phase and β1 phase. Although a cooling 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 and impact properties of the alloy are 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. In particular, 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 soft 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 β1 phase during machining.

[0118] 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% or lower.(γ Phase; Metallographic Structure Relational Expression f5)

[0119] As disclosed by Patent Documents 9 and 11, 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 80 mass% and the Si concentration is about 2 to 4 mass%. Patent Document 16 discloses that γ phase is indispensable for a Cu-Zn-Si alloy free of Pb. Further, Patent Documents 3 to 7 disclose that β phase containing Si has good machinability and at the same time, γ phase containing Si contributes to improving machinability.

[0120] A negative coefficient larger than the coefficient given to the amount of β 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 impact properties and deteriorates dezincification corrosion resistance because unlike β phase, 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 needs to be 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. f 6 = β 1 × − 0.5 × Si 2 + 1.5 × Si − 2 × γ + Pb + Bi 1 / 2 × 25 + P 1 / 2 × 12

[0121] 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 (-0.5 × [Si] 2< + 1.5 × [Si]), the sum of the amounts of Pb and Bi raised to the power of 1 / 2 multiplied by a coefficient of 25, and the amount of P raised to the power of 1 / 2 multiplied by a coefficient of 12.

[0122] Performance of β1 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 that is generated by modification of 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.

[0123] As a result of devoted study, it was revealed that to obtain good machinability, the value of the metallographic structure and composition relational expression f6 is higher than 28, preferably higher than 35, and more preferably higher than 40. As the f6 value gets higher, machinability becomes closer to that of a free-cutting brass bar containing 3 mass% Pb. On the other hand, machinability peaks when the value of the metallographic structure and composition relational expression f6 is approximately 55 to 80. As the value further increases, machinability rather deteriorates than improves. This is because the effect of β1 phase on machinability peaks when its proportion is about 60% and the effect of Si on machinability peaks when its amount is approximately 1 mass%.

[0124] Incidentally, for alloys composed of non-modified β phase, f6A, an expression formulated based on the expression f6 by replacing f4 (amount of β1 phase) with f4A (amount of β phase) is defined for calculation of reference value.

[0125] Now, Figs. 1A to 5B show pictures of the metallographic structures of various alloy castings and the results of the dezincification corrosion tests performed in accordance with ISO 6509.

[0126] Fig. 1A is a picture showing the structure of a copper alloy casting according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step No. A1. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.94 mass% Si, 0.082 mass% P, 0.067 mass% Pb, 0.03 mass% Sn, and Zn as the balance. In Step No. A1, molten alloy heated to 1080°C was poured into a sand mold and a casting was taken out of the mold when its temperature became 650°C then subjected to a cooling treatment which was started when the temperature became 560°C and performed at an average cooling rate of 900°C / min in the temperature ranges from 550°C to 500°C and from 500°C to 300°C.

[0127] Fig. 1B is a cross-sectional picture of the metallographic structure of the alloy casting shown in Fig. 1A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the casting in accordance with ISO 6509.

[0128] Fig. 2A is a picture showing the structure of a copper alloy casting according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step No. A14H. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.94 mass% Si, 0.082 mass% P, 0.067 mass% Pb, 0.03 maa% Sn, and Zn as the balance. In Step No. A14H, molten alloy heated to 1080°C was poured into a sand mold and a casting was taken out of the mold when its temperature became 650°C then left to cool naturally. The average cooling rate in the temperature range from 500°C to 300°C was 20°C / min.

[0129] Fig. 2B is a cross-sectional picture of the metallographic structure of the alloy casting shown in Fig. 2A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the casting in accordance with ISO 6509.

[0130] Fig. 3A is a picture showing the structure of a copper alloy casting according to an embodiment, which was obtained by subjecting Alloy No. S12 to Step No. B1. More specifically, Alloy No. S12 has a composition including 63.6 mass% Cu, 1.04 mass% Si, 0.070 mass% P, 0.062 mass% Pb, 0.003 mass% Bi, and Zn as the balance. In Step No. B1, molten alloy heated to 960°C was poured into a permanent mold and a casting was taken out of the mold when its temperature became 750°C then subjected to a cooling treatment which was started when the temperature became 560°C and performed at an average cooling rate of 1020°C / min in the temperature ranges from 550°C to 500°C and from 500°C to 300°C.

[0131] Fig. 3B is a cross-sectional picture of the metallographic structure of the alloy casting shown in Fig. 3A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the casting in accordance with ISO 6509.

[0132] Fig. 4A is a picture showing the structure of a copper alloy casting according to an embodiment, which was obtained by subjecting Alloy No. S12 to Step No. B11H. More specifically, Alloy No. S12 has a composition including 63.6 mass% Cu, 1.04 mass% Si, 0.070 mass% P, 0.062 mass% Pb, 0.003 mass% Bi, and Zn as the balance. In Step No. B11H, molten alloy heated to at 960°C was poured into a permanent mold and a casting was taken out of the mold when its temperature became 750°C then cooled at an average cooling rate of 40°C / min in the temperature range from 500°C to 300°C.

[0133] Fig. 4B is a cross-sectional picture of the metallographic structure of the alloy casting shown in Fig. 4A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the casting in accordance with ISO 6509.

[0134] Fig. 5A is a picture showing the structure of a copper alloy casting according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step Nos. A14H and G1. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.94 mass% Si, 0.082 mass% P, 0.067 mass% Pb, 0.03 mass% Sn, and Zn as the balance. In Step No. A14H, molten alloy heated to 1080°C was poured into a sand mold and a casting was taken out of the mold when its temperature became 650°C then left to cool naturally. The average cooling rate in the temperature range from 500°C to 300°C was 20°C / min. The casting obtained in Step No. A14H was subsequently subjected to Step No. G1, in which the casting was heated to 580°C for 20 minutes then subjected to a cooling treatment which was started when its temperature became 575°C and performed at an average cooling rate of 1200°C / min in the temperature ranges from 550°C to 500°C and from 500°C to 300°C.

[0135] Fig. 5B is a cross-sectional picture of the metallographic structure of the alloy casting shown in Fig. 5A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the casting in accordance with ISO 6509.

[0136] As shown in Figs. 1A, 3A, and 5A, a pattern of crystal grain boundary is clearly recognized inside β1 phase in all of the castings; namely, the ones made with a sand mold or a permanent mold, and the one heat-treated after sand casting. 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. In contrast, in either of Figs. 2A or 4A, no pattern that looks like grain boundary is observed inside β phase, indicating a clear-cut difference between them. Instead, many fine black granular precipitates are observed mainly inside β phase and on the phase boundaries between α phase and β phase in Figs 2A and 4A. With respect to black granular precipitates and the like, they are observed in Figs. 1A, 3A, and 5A, but only in a small quantity.

[0137] The black granular precipitates with a size of about 0.5 to 3 µm observed in Figs. 2A and 4A are present in large number mainly inside β phase and on phase boundaries between α phase and β phase. Most of the granular precipitates are compounds between P and Zn, P and Si, or P and Zn and Si, i.e., so-called phosphorous compounds. Further, Pb particles, mixed particles of Pb and Bi, compounds between Fe, Mn and the like and P or Si, those between A1 and P or Si, inclusions such as oxide are also included as part of such precipitates. These compounds, inclusions, and the like can be distinguished by microscopic observation, but it is slightly difficult to do the distinguishment from a printed picture. In each field of view shown in the printed pictures of Figs. 2A and 4A, there are approximately 300 precipitates that are mainly composed of phosphorous compounds. In Figs. 1A, 3A, and 5A, nothing like fine granular phosphorous compound is present or the amount is extremely small if present. The number of such compounds is less than 1 / 10 of that of those shown in Figs. 2A and 4A at most. The cooling record of each test piece tells whether a grain boundary is present inside β1 or β phase and whether a large number of phosphorous compounds are present. Whether the cooling rate at the point the temperature of the casting was 500°C, i.e., in the temperature range between 500°C and a temperature only slightly higher than 500°C, and the average cooling rate in the temperature range from 500°C to 300°C were both higher or lower than 300°C / min decides the outcome. Incidentally, in the instant specification, the "cooling rate at the point the temperature of the casting was 500°C, i.e., in the temperature range between 500°C and a temperature only slightly higher than 500°C" refers to the average cooling rate in the temperature range from 550°C to 500°C. In the case the starting temperature of the cooling treatment is lower than 550°C, however, that refers to the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C instead.

[0138] Results of the dezincification corrosion tests performed on these alloys in accordance with ISO 6509 are shown in Figs 1B, 2B, 3B, 4B, and 5B. Note that the magnification of the images in Figs. 1B, 3B, and 5B is twice as large as that of those in Figs. 2B and 4B. The maximum corrosion depth was 110 µm or less (Figs. 1B, 3B, and 5B) in all of the test pieces in which a grain boundary was observed inside β1 phase (Figs. 1A, 3A, and 5A), but that of the test pieces in which grain boundary was not observed inside β phase (Figs. 2A and 4A) was 480 µm of 420 µm (Figs. 2B and 4B), indicating a difference of 3 times or more in the corrosion depth. It should be noted that the type of dezincification corrosion was selective corrosion of β1 phase or β phase in all cases. Therefore, dezincification corrosion occurs in β1 phase first like in β phase, but progress of the corrosion in β1 phase is at least about twice, or in some cases about 3 times slower than in β phase.<Properties>(Strength, Toughness, and Ductility)

[0139] Generally, segregation of components is more likely to occur in a casting than in a hot-worked material such as a hot-extruded bar. Also, castings have larger crystal grains and contain some inclusions such as oxide and microscopic defects. For this reason, casting is said to be more brittle than hot-worked material and is desired to exhibit a high impact value in the evaluations of toughness and ductility. On the other hand, to have good chip breaking behavior, a material requires sort of brittleness. Impact properties and machinability are opposing properties in certain aspects.

[0140] There is a strong demand for reduction in the thickness and weight of parts and components such as those of drinking water-related devices that are target applications of embodiments of the present invention. For instance, in the case of drinking water-related devices, reduction in the thickness and weight can be achieved by using a material with good corrosion resistance and high strength. Castings are not commonly used for components that will be subject to cold working such as swaging or those that require ductility in use. For the evaluation of the strength of the embodiments, hardness (Vickers hardness and Rockwell hardness) was adopted in consideration of the nature of casting. For the evaluation of toughness and ductility, impact test value (U-notch test) was adopted.

[0141] For a casting to have a high strength, it is preferable if it has a Vickers hardness of 100 Hv or higher and a Rockwell B-scale hardness of 52 or higher at least. It is more preferable if it has a Vickers hardness of 110 Hv or higher and a Rockwell B-scale hardness of 60 or higher. There is an interrelationship between hardness, tensile strength, and proof stress. In the embodiments, Vickers hardness of 100 Hv corresponds to about 400 N / mm 2< of tensile strength and about 120 N / mm 2< of proof stress. Incidentally, brass bar of CAC203 has a Vickers hardness of about 75 Hv and a Rockwell B-scale hardness of about 30. It has a tensile strength of about 350 N / mm 2< and a proof stress of about 80 N / mm 2< .

[0142] As mentioned above, a casting needs to be a material that is not only strong but also non-brittle at least, i.e., a tough material that resists impact. That is, in a Charpy U-notch, the resultant value is preferably 25 J / cm 2< or higher and more preferably 30 J / cm 2< or higher. On the other hand, if the Charpy impact value exceeds about 70 J / cm 2< , for instance, machinability deteriorates since cutting resistance increases and chips are likely to be continuous due to increased viscosity in the material.(Electrical Conductivity)

[0143] Applications of the embodiments include electrical or home appliance components, auto parts, and the like. The embodiments are materials that can replace leaded brass casting of CAC203 and bronze casting of CAC 406. Currently, phosphor bronzes including 6 mass% or 8 mass% Sn (JIS standard alloys C5191 and C5210) are widely used for wrought copper alloy material provided for the above applications, and their electrical conductivities are about 14% IACS and about 12% IACS, respectively, and those of CAC203 and CAC406 are about 24% IACS and about 15% IACS, respectively. Accordingly, if the embodiments have an electric conductivity of 15% IACS or higher, there should be no problem in relation to electrical conductivity. The upper limit of the electrical conductivity is not particularly defined because improvement in conductivity rarely causes a problem in practical use.(Castability)

[0144] The very premise of the free-cutting copper alloy castings of the embodiments is that they can be produced as sound products. Firstly, by the action of Si contained in the alloy, generation of Zn vapor is inhibited in the melting process, which allows smooth operation of melting and casting processes. Secondly, when in the form of molten metal, they have good fluidity due to the action of Si and the limitations of Cu and Zn as well as the restriction by f1, which allows molten metal to reach thin and narrow portion in a casting mold and formation of a complicated shape. In addition, defects that are peculiar to castings such as porous shrinkages cavities are less likely to occur. Further, castings should not have cracking. What affects most as to whether cracking occurs in the casting process is whether a metal of low melting point is present in the form of liquid when the casting is still at a high temperature after it is solidified. In the case a metal of low melting point is present, the quantity of the metal and whether the matrix has ductility under high temperature are the deciding factors.

[0145] In embodiments of the present invention, the total amount of Pb and Bi, which are metals of low melting point present in the matrix in the form of liquid in the solidification and cooling processes of casting, is limited to as low as less than 0.20 mass% so that cracking is less likely to occur during casting. Then, if the composition and various relational expressions of the embodiments are satisfied, the adverse effects caused by the metals of low melting point that are contained in small amounts can be offset since a large amount of β1 phase, a phase that exhibits excellent ductility under high temperature, is contained in the embodiments. Thus, the castings will have no problem of cracking.<Production Process>

[0146] Next, a method for producing the free-cutting copper alloy castings according to the embodiments will be described.

[0147] The metallographic structures of the free-cutting copper alloy castings according to the embodiments vary depending not only on the composition but also on the production process. There are many methods for manufacturing castings including permanent mold casting, sand casting, continuous casting, die casting, and lost wax casting. The cooling rate after molten metal solidifies is largely determined by the thickness and shape of the casting, the material and thickness of the permanent mold or sand mold used, and the like. The cooling rate can be changed by way of heat retention, water cooling, oil cooling, forced air cooling, and the like. On the other hand, various changes occur in metallographic structure during the cooling process after the casting solidifies, and metallographic structure is significantly affected by the cooling rate applied. A change in metallographic structure refers to a significant change in the kinds of constituent phases and the amount (area ratio) of each constituent phase. As a result of a devoted study relating to the cooling process after molten metal solidifies, the inventors discovered that the cooling rate at the point the temperature of the casting is 500°C and the average cooling rate in the temperature range from 500°C to 300°C are most important factors in a cooling treatment and significantly affect dezincification corrosion resistance and machinability.(Melting and Casting)

[0148] Melting is performed at a temperature between about 950°C and 1200°C, a temperature that is about 100°C to 300°C higher than the melting point (liquidus temperature) of the alloy according to an embodiment of the present invention although that depends on the casting method applied. A molten metal is cast into a specific permanent mold, sand mold, or other type of mold when its temperature is about 900°C to 1150°C, which is about 50°C to 250°C higher than the melting point, then cooled. After the alloy solidifies, constituent phases of the alloy change in various ways.(Pouring (Casting) and Solidification)

[0149] The cooling rate after molten metal is poured into a mold and that after the metal solidifies changes in numerous ways depending on the weight and thermal conductivity of the copper alloy poured, the thickness of the casting, the material that the mold such as sand mold or permanent mold is made of.

[0150] In the case molten copper alloy is poured into a permanent mold made of copper alloy or iron to manufacture conventional copper alloy castings, i.e., in the case of permanent mold casting, the casting is taken out of the permanent mold when its temperature becomes about 750°C or lower after the pouring process then subjected to air cooling or slow cooling at an average cooling rate of about 10°C / min to 200°C / min in the temperature range from 500°C to 300°C. As most copper alloy castings contain 1 mass% or more Pb or Bi, they are rarely cooled at a cooling rate of 300°C / min or higher in the temperature range above about 300°C or 250°C, the temperature at which Pb or Bi solidifies. This is because when the temperature of a casting is 300°C or higher, Pb or Bi particles are in a molten state. Therefore, if cooled at an excessively high cooling rate, cracking may occur due to thermal contraction.

[0151] In continuous casting, castings are cooled in a manner almost the same as in permanent mold casting. A casted bar taken out of an iron mold is usually air-cooled at a cooling rate primarily determined by the cross-sectional area of the continuously-cast bar and the casting speed applied. In the case of continuously cast bar with a diameter of 20 mm, it is cooled at a cooling rate of about 40°C / min to 100°C / min in the temperature range from 500°C to 300°C.

[0152] On the other hand, in the case of sand casting, the copper alloy poured into a sand mold is cooled at an average cooling rate of about 0.6°C / min to 60°C / min in the temperature range from 500°C to 300°C in many cases although it depends on the size of the casting, the type of sand used for the mold, and the size of the sand mold. The casting is taken out of the mold when its temperature becomes lower than about 300°C or 250°C then air-cooled, or alternatively, it is left in the mold until its temperature becomes near normal temperature. Apropos, the cooling rate at the point the temperature of the casting is 500°C is approximately the same as or slightly higher than the average cooling rate in the temperature range from 500°C to 300°C.

[0153] Free-cutting copper alloy castings of an embodiment of the present invention have a metallographic structure consisting solely of β phase when its temperature is high, for instance, at 800°C after molten metal is poured and solidifies. Various phases including α phase and γ phase are formed in the process of cooling that follows. In the case of a free-cutting copper alloy casting of an embodiment, β phase is modified into β1 phase by adjusting the cooling rate at the point the temperature of the casting is 500°C to higher than 300°C / min and the average cooling rate from 500°C to 300°C to higher than 300°C / min in the cooling process after the casting is solidified. The cooling treatment is continued after its temperature drops below 300°C. Note that in the case the starting temperature of the cooling treatment is lower than 550°C, the cooling rate at the point the temperature of the casting is 500°C is regarded as the average cooling rate from the starting temperature of the cooling treatment to 500°C.

[0154] In a favorable cooling method, the cooling treatment is started when the temperature of the casting after it is solidified is between 700°C and 500°C. Incidentally, the average cooling rate from the starting temperature of the cooling treatment to 500°C is approximately the same as that from 500°C to 300°C. The treatment is continued even after the temperature of the casting reaches 300°C. The average cooling rate then is slightly lower than that applied to the temperature range from 500°C to 300°C but they are approximately the same.

[0155] Although the starting temperature of the cooling treatment cannot always be identified, in the process of cooling, if the cooling rate at the point the temperature of the casting is 500°C is higher than 300°C / min, the average cooling rage in the temperature range from 500°C to 300°C is higher than 300°C / min, and also the requirements related to metallographic structure are satisfied, β phase becomes modified into β1 phase.

[0156] Through the cooling treatment, β phase becomes modified into β1 phase, and when etched with a mixed solution of hydrogen peroxide and ammonia water, grain boundary appears inside β1 phase, indicating that β phase has been replaced with β1 phase which has different properties than those of conventional β phase. As a result, machinability improves and dezincification corrosion resistance, a long-standing problem of conventional β phase, significantly improves.

[0157] Degree of modification of β phase is affected by the amounts of Si and P, those of Al, Fe, and Sn, the value of the composition relational expression f1, and the amount of α phase. It is also affected by the average cooling rate when the temperature of the casting is between 500°C and 300°C, the cooling rate at the point it is 500°C, and the starting temperature of the cooling treatment, in the process of cooling after the casting is solidified.

[0158] If the starting temperature of the cooling treatment is 700°C or higher, proportion of β phase becomes large, which sometimes causes insufficient modification of β phase. Starting temperature of the cooling treatment is preferably lower than 700°C, more preferably 650°C or lower. On the other hand, if the cooling rate at the point the temperature of the casting is 500°C is 300°C / min or lower, or if the starting temperature of the cooling treatment is 500°C or lower, modification of β phase does not occur. If the cooling treatment is started at a temperature preferably 530°C or higher, more preferably 550°C or higher, β phase is better modified, and dezincification corrosion resistance and machinability of the alloy improves.

[0159] In addition, if the average cooling rate from 500°C to 300°C after commencement of the cooling treatment is 300°C / min or lower, the modification of β phase does not occur. The casting needs to be cooled at an average cooling rate of at least 300°C / min, preferably higher than 600°C / min, and more preferably 900°C / min or higher, in the temperature range from 500°C to 300°C. If cooled at a preferable cooling rate, β phase becomes better-modified into β1 phase, resulting in further improvement in dezincification corrosion resistance and machinability. The upper limits of the cooling rate at the point the temperature of the casting is 500°C and the average cooling rate in the temperature range from 500°C to 300°C are not particularly defined because ordinary production facility is unable to cool castings excessively fast, but as a reference, they are preferably 9000°C / min or lower.

[0160] Incidentally, cast ingot is sometimes used as a casting material, but thermal history of such raw material does not affect the modification of β phase. Whether β phase becomes modified into β1 phase is determined in the cooling process after the final solidification of the casting.(Heat Treatment)

[0161] In the manufacture of castings, in the cooling process after molten metal is poured into a mold, it is sometimes difficult to cool the material at a cooling rate exceeding 300°C / min at the point its temperature is 500°C or at an average cooling rate exceeding 300°C / min in the temperature range from 500°C to 300°C. β phase of such a casting can be modified by performing heat treatment.

[0162] Specifically, β phase can be modified into β1 phase by heating the casting to a temperature higher than 520°C and lower than 650°C for one minute to five hours then subjecting it to a cooling treatment which is started when its temperature is lower than 650°C and higher than 520°C and performed at a cooling rate exceeding 300°C / min at the point its temperature is 500°C, and an average cooling rate exceeding 300°C / min in the temperature range from 500°C to 300°C.

[0163] By polishing (mirror-polishing) a piece of casting after the heat treatment and etching the polished surface with a mixed solution of hydrogen peroxide and ammonia water, grain boundary can be observed inside β1 phase, indicating that β phase has been replaced with β1 phase, a phase having a different nature than that of conventional β phase. Apropos, the heating temperature during the heat treatment and the starting temperature of the cooling treatment are preferably 540°C or higher and more preferably 550°C or higher. On the other hand, if the starting temperature of the cooling treatment is higher than 650°C, the proportion of β1 phase becomes excessively large, causing deterioration in dezincification corrosion resistance and impact properties. Therefore, the starting temperature is preferably 620°C or lower. Further, with respect to the cooling rate at the point the temperature of the casting is 500°C and the average cooling rate from 500°C to 300°C, the faster the better-modified becomes β phase. The starting temperature and the average cooling rate are preferably higher than 600°C / min and more preferably 900°C / min or higher. The upper limits of the cooling rate at the point the temperature of the casting is 500°C and the average cooling rate from 500°C to 300°C are not particularly defined because ordinary production facility is unable to cool castings excessively fast, but as a reference, they are preferably 9000°C / min or lower.

[0164] Incidentally, even if β1 phase is formed by the heat treatment stated above, if any further heat treatment that does not satisfy the above heat treatment conditions is performed or the casting is re-heated to a temperature of 300°C or higher, the modification of β phase becomes nullified, and the β1 phase returns to β phase. When a casting which was subjected to a specific final cooling treatment or to the final heat treatment described above is heated to a temperature of 300°C or higher, the modification of β phase becomes nullified and the β1 phase which was present in the casting returns to ordinary β phase. As a result, the grain boundary that was present inside the β1 phase can no longer be found when the metallographic structure is observed.

[0165] The free-cutting copper alloy castings of the first to third embodiments of the present invention are manufactured by the production method stated above.

[0166] Free-cutting alloy castings of the first to third embodiments of the present invention with the above-described constitution are able to have excellent machinability, good dezincification corrosion resistance, high strength, and good impact properties 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.

[0167] 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.Examples

[0168] 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.

[0169] Using a low-frequency melting furnace that was in use on the actual production line, trial production of copper alloy castings with sand mold was performed. Also, trial production of copper alloy castings was performed with laboratory facilities, and production of bars were performed with a continuous casting machine on the actual production line in which permanent mold is employed. Some of the test pieces were further subjected to heat treatment in a laboratory.

[0170] Tables 5 to 7 show the alloy compositions. Tables 8 to 11 show the production steps employed. Regarding composition, "Mm" refers to mischmetal which represents the total content of rare earth elements. Each of the production steps is described below. [Table 5]Alloy No.Constituent Component (mass%)Composition Relational ExpressionImpurity (mass%)CuSiPPbBiSnZnf1f2f0FeMnCrCoAlNiAgSbAsMmS0164.01.020.0590.0580.0210.04Rem.59.10.079-0.050.010.000.000.020.030.010.000.000.00S0262.90.940.0820.0670.0000.03Rem.58.30.067-0.100.000.010.000.030.070.010.000.010.00S1162.60.980.0740.0580.0010.00Rem.57.90.059-0.000.000.000.000.000.000.000.000.000.00S1263.61.040.0700.0620.0030.00Rem.58.60.065-0.000.000.000.000.000.000.000.000.000.00S1364.50.980.0880.0750.0000.00Rem.59.70.075-0.000.000.000.000.000.000.000.000.000.00S1464.80.940.0670.0590.0000.00Rem.60.30.059-0.000.000.000.000.000.000.000.000.000.00S2164.21.030.1200.0190.0080.03Rem.59.10.027-0.020.070.000.010.000.030.010.010.000.00S2262.80.880.0720.0570.0000.01Rem.58.50.057-0.050.020.010.000.010.020.010.000.010.00S2364.41.120.0490.0260.0000.04Rem.59.00.026-0.010.120.010.000.000.090.010.020.010.01S2462.60.780.1120.0660.0230.01Rem.58.80.089-0.060.010.000.010.000.000.010.000.010.01S2564.60.920.0660.0620.0200.00Rem.60.20.082-0.000.000.000.000.000.000.000.000.000.00S2663.50.990.0170.0730.0000.00Rem.58.80.073-0.000.000.000.000.000.000.000.000.000.00S2763.71.000.0390.0680.0000.00Rem.58.90.068-0.000.000.000.000.000.000.000.000.000.00S2863.70.940.1350.0700.0000.00Rem.59.10.070-0.000.000.000.000.000.000.000.000.000.00 [Table 6] Alloy No.Constituent Component (mass%)Composition Relational ExpressionImpurity (mass%)CuSiPPbBiSnZnf1f2f0FeMnCrCoAlNiAgSbAsMmS2963.60.880.0700.0050.0020.00Rem.59.30.007-0.000.000.000.000.000.000.000.000.000.00S3061.90.660.1020.0860.0000.00Rem.58.70.086-0.000.000.000.000.000.000.000.000.000.00S3164.71.050.0920.0800.0000.00Rem.59.60.080-0.000.000.000.000.000.000.000.000.000.00S3261.10.690.0830.0080.0730.00Rem.57.70.081-0.000.000.000.000.000.000.000.000.000.00S3364.11.060.0790.0550.0000.13Rem.58.80.0550.120.000.000.000.000.000.000.000.000.000.00S3463.70.970.0650.0670.0000.29Rem.58.70.0670.300.000.000.000.000.000.000.000.000.000.00S3563.40.920.0670.0710.0000.44Rem.58.50.0710.480.050.000.000.010.020.130.000.020.000.01S3662.70.870.0800.0760.0000.01Rem.58.50.076-0.010.000.000.000.190.010.010.000.000.00S3763.20.970.0740.0580.0200.00Rem.58.50.078-0.040.000.010.000.080.030.020.000.000.00S3862.80.850.0670.0420.0200.03Rem.58.70.062-0.230.130.000.020.000.020.010.010.010.01S3963.00.900.0710.0690.0000.00Rem.58.60.069-0.120.090.000.000.070.100.000.000.000.02S4062.70.870.0840.0820.0250.00Rem.58.50.107-0.000.000.000.000.000.000.000.000.000.00S4163.80.710.0560.0700.0240.00Rem.60.40.094-0.000.000.000.000.000.000.000.000.000.00S4261.10.540.0750.0790.0280.00Rem.58.50.107-0.000.000.000.000.000.000.000.000.000.00 [Table 7] Alloy No.Constituent Component (mass%)Composition Relational ExpressionImpurity (mass%)CuSiPPbBiSnZnf1f2f0FeMnCrCoAlNiAgSbAsMmS5164.91.300.0800.0700.0000.00Rem.58.60.070-0.000.000.000.000.000.000.000.000.000.00S5261.00.460.0750.0900.0110.00Rem.58.80.101-0.000.000.000.000.000.000.000.000.000.00S5364.90.770.0740.0450.0300.00Rem.61.20.075-0.000.000.000.000.000.000.000.000.000.00S5462.80.870.0040.0810.0110.00Rem.58.70.092-0.000.000.000.000.000.000.000.000.000.00S5565.61.080.0480.0300.0150.00Rem.60.40.045-0.000.000.000.000.000.000.000.000.000.00S5659.30.210.0960.0950.0360.00Rem.58.30.131-0.000.000.000.000.000.000.000.000.000.00S5761.50.960.0700.0500.0000.00Rem.56.80.050-0.000.000.000.000.000.000.000.000.000.00S5863.61.080.2200.0720.0200.00Rem.58.20.092-0.000.000.000.000.000.000.000.000.000.00S5963.70.820.0420.0010.0000.00Rem.59.70.001-0.000.000.000.000.000.000.000.000.000.00S6063.50.660.0890.0280.0330.59Rem.59.70.0610.890.000.000.000.000.000.000.000.000.000.00S6164.61.060.0470.0730.0070.80Rem.58.70.0800.750.000.000.000.000.000.000.000.000.000.00S6262.40.900.0680.0580.0000.03Rem.58.00.058-0.020.000.000.000.380.020.010.000.000.00S6363.10.860.0610.0550.0040.01Rem.58.90.059-0.380.160.030.000.000.010.010.000.000.00S6462.60.580.0270.0130.0000.00Rem.59.80.013-0.000.000.000.000.000.000.000.000.000.00S6560.10.550.0550.0900.0250.00Rem.57.50.115-0.000.000.000.000.000.000.000.000.000.00X58.90.000.0013.1000.0020.29Rem.-3.100-0.240.010.000.000.000.060.010.000.000.00Y60.10.000.0012.1000.0000.33Rem.-2.100-0.290.000.000.020.090.050.000.010.000.00 [Table 8] Production Step Group A with Facilities Used for Manufacturing Products for SaleSand Casting - Pouring temperature: 1080°CRemoval from Sand MoldAverage Cooling Rate until Start of CoolingStart of Cooling TreatmentAverage Cooling Rate from 550°C to 500°CAverage Cooling Rate from 500°C to 300°C(°C)(°C / min)(°C)(°C / min)(°C / min)A165035560900900A275035620900900A365035560660660A465035570420420A565035530780780A675035660900900A11H75035720900900A12H65035460-660A13H65035515210210A14H650---20 [Table 9] Production Step Group B with Laboratory FacilitiesPermanent Mold Casting - Pouring temperature: 960°CRemoval from Permanent MoldAverage Cooling Rate until Start of CoolingStart of Cooling TreatmentAverage Cooling Rate from 550°C to 500°CAverage Cooling Rate from 500°C to 300°C(°C)(°C / min)(°C)(°C / min)(°C / min)B17506056010201020B27506061012001200B375060570480480B47506052510201020B57506067012001200B11H750---40B12H7506073012001200B13H75060470-900B14H75060525240240 [Table 10] Production Step Group C with Laboratory Facilities Castings Manufactured by Continuous Casting - Pouring temperature: 960°CAverage Cooling Rate from 550°C to 500°C (°C / min)Average Cooling Rate from 500°C to 300°C (°C / min)C1900900C2660660C3480480C11H6060 [Table 11] Production Step Group G with Laboratory Facilities: Heat TreatmentPrior Process of Material Prep.Heat TreatmentStart of Cooling Treatment (°C)Average Cooling Rate from 550°C to 500°C (°C / min)Average Cooling Rate from 500°C to 300°C (°C / min)G1A14H580°C× 20 min57512001200G2B11H580°C× 20 min57512001200G3C11H580°Cx 20 min57512001200G11HA14H580°C× 20 min470-900G12HB11H580°C× 20 min470-900G13HA14H580°C× 20 min--25G14HB11H580°C× 20 min--25 Production Step Group A: Production with Facilities Used for Manufacturing Products for Sale - Sand Casting(Steps Nos. A1 to A6 and A11H to A14H)

[0171] Approximately 400 kg of raw material was put into a low-frequency melting furnace with an internal cubic volume of one ton that is used for manufacture on the actual production line. The material was melted at 1080°C then moved to a ladle to be poured into a sand mold (made of silica sand to which resin is added) at about 970°C with which the test piece A described in Section 7.2 of JIS H 5120 can be produced.

[0172] The poured molten metal was cooled at a cooling rate of about 30°C / min and the resultant casting (test piece A) was taken out of the sand mold when its temperature became either 650°C or 750°C and left to cool naturally. The casting taken out of the sand mold was cooled at an average cooling rate of 35°C / min and subsequently subjected to a cooling treatment which was started when the temperature of the casting became between 460°C and 720°C and performed at an average cooling rate adjusted to 20°C / min, 210°C / min, 420°C / min, 660°C / min, 780°C / min, or 900°C / min. The average cooling rate from the starting temperature of the cooling treatment to 500°C was adjusted so that it was approximately the same as that from 500°C to 300°C. In the case the cooling treatment was started when the casting's temperature was 550°C or higher, the average cooling rate from 550°C to 500°C was measured. In the case the starting temperature was lower than 550°C, the average cooling rate from the starting temperature to 500°C was measured.

[0173] It should be noted, however, that in Step No. A12H, the average cooling rate from the point that the casting was taken out of the mold to 460°C was 35°C / min, and that from 460°C to 300°C was 660°C / min.

[0174] In Step No. A14H, the average cooling rate from the point that the casting was taken out of the mold to 500°C was 35°C / min, and that from 500°C to 300°C was 20°C / min.

[0175] Natural cooling, forced air cooling, shower-water cooling, water cooling, simple heat retention, or the like was adopted as a cooling method. Temperature of casting was measured from the moment test piece A was taken out of the sand mold until its temperature became 300°C mainly with a radiation thermometer, but in some cases, a contact thermometer was also used together with radiation thermometer. The radiation thermometer used for the temperature measurement is IGA8Pro / MB20 manufactured by LumaSense Technologies Inc. The test piece A above was subjected to observation with a metallographic microscope, cutting test, dezincification corrosion test, hardness test, and impact test.Production Step Group B: Permanent Mold Casting in a Laboratory(Steps Nos. B1 to B5 and B11H to B14H)

[0176] Raw materials were mixed at a specific ratio and melted in a laboratory. In this process, inevitable impurities such as Fe were intentionally added to some of the test materials to assimilate the testing to actual production. The molten metal was then poured into an iron mold with an inner diameter of 40 mm and a depth of 200 mm when its temperature was about 950°C.

[0177] In consideration of actual casting, when the temperature of the casting became about 750°C, it was taken out of the mold and cooled at a cooling rate of about 60°C / min then a cooling treatment was started when the temperature of the casting was between 730°C and 470°C. Subsequently, the casting was cooled at varied average cooling rates of 1200°C / min, 1020°C / min, 900°C / min, 480°C / min, 240°C / min, or 40°C / min in the temperature range from 500°C to 300°C. The casting was kept cooling until its temperature became room temperature. The cooling rate at the point the temperature of the casting was 500°C, i.e., from 550°C or the starting temperature of the cooling treatment to 500°C was adjusted so that it was approximately the same as the average cooling rate from 500°C to 300°C.

[0178] In Step No. B13H, however, the average cooling rate from the point the casting was taken out until its temperature became 470°C was 60°C / min, and that from 470°C to 300°C was 900°C / min.

[0179] In Step No. B11H, the average cooling rate from the point the casting was taken out until its temperature became 500°C was 45°C / min, and that from 500°C to 300°C was adjusted to 40°C / min.

[0180] With respect to measurement of temperature, the temperature of casting was measured using both a radiation thermometer and a contact thermometer then the average cooling rates in respective temperature ranges were adjusted to specific average cooling rates respectively. The resultant castings made with permanent mold were subjected to observation with a metallographic microscope, cutting test, dezincification corrosion test, hardness test, and impact test.Step Group C: Continuous Casting(Steps Nos. C1 to C3 and C11H)

[0181] To manufacture bars with a diameter of 21 mm, approximately 900 kg of raw material was put into a low-frequency melting furnace with an internal cubic volume of two tons that is used for manufacture on the actual production line and melted at 1080°C. The molten metal was kept at 1010°C in a holding furnace then put through the mold of the continuous casting machine operated at a casting speed of 200 mm / min. With respect to cooling, the mold was water cooled then the continuously cast bars were either air-cooled or directly water-cooled with a shower installed at an appropriate distance from the mold. The location of the shower and the quantity of shower water were adjusted. The temperatures of the bars were measured with both a radiation thermometer and a contact thermometer. The cooling rate at the point the casting was 500°C and the average cooling rate from 500°C and 300°C were adjusted to be the same and varied to 900°C / min, 660°C / min, 480°C / min, or 60°C / min. Incidentally, the starting temperature of the cooling treatment was not particularly specified in this method. The continuously cast bars were subjected to observation with a metallographic microscope, cutting test, dezincification corrosion test, hardness test, and impact test.Step Group G: Heat Treatment(Steps Nos. G1 to G3 and G11H to G14H)

[0182] In Step Group G, heat treatment was additionally performed on some of the castings produced with a sand mold or a permanent mold in Step Group A, B, and C.. The castings made with a sand mold or a permanent mold and the continuously cast bars obtained in Steps Nos. A14H, B11H, and C11H were all heated to 580°C and held at the temperature for 20 minutes.

[0183] In Steps Nos. G1, G2, and G3, the cooling treatment was started following the heating process when the temperature of casting was 575°C and performed at an average cooling rate of 1200°C / min in both temperature ranges from 550°C to 500°C and from 500°C and 300°C.

[0184] In Steps Nos. G11H and G12H, the cooling treatment was started when the temperature of casting was 470°C and performed at an average cooling rates of 50°C / min from 580°C to 470°C and 900°C / min from 470°C to 300°C.

[0185] In Steps Nos. G13H and G14H, the castings were simply cooled at an average cooling rate of 25°C / min from 500°C to 300°C after they were subjected to heat treatment at 580°C without performing any special cooling treatment. The material that underwent a heat treatment were subjected to observation with a metallographic microscope, cutting test, dezincification corrosion test, hardness test, and impact test.(Comparative Material)

[0186] As a comparative material, a ø40-mm bar made of C3604, a leaded free-cutting brass containing 3% Pb, was prepared and denominated as Alloy X. In addition, an ingot made of CAC203, a casting brass containing 2 mass% Pb, was prepared and denominated as Alloy Y. The raw materials were melted and cast in the same manner as in Steps Nos. B1 and B11H to obtain castings with a diameter of 40 mm and a length of 200 mm made in a permanent mold. Alloy X and Alloy Y were both available on the market. These alloys were subjected to observation with a metallographic microscope, cutting tests, dezincification corrosion tests, hardness tests, and impact tests.

[0187] The above-described test materials were evaluated for the following items. The evaluation results are shown in Tables 10 to 21.(Observation of Metallographic Structure)

[0188] 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.

[0189] Incidentally, in Tables 12 to 20, β phase, which is not modified, is indicated as f4A to distinguish itself from β1 phase (A4). Further, in the calculation of f6, if f4 = (β1) = 0, the value of f4A = (β) is applied in the term of (β1) instead of (β1), and the resultant figure is shown in the f6A row for reference.

[0190] Each of the castings provided as test materials was cut parallel to the longitudinal direction. 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 10 seconds under a room temperature between about 15°C and 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 instead of β phase.

[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 area ratio 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%. Non-metal oxides and sulfides were more frequently observed in the castings provided as test material than in hot-extruded bars manufactured with mass production facilities.

[0192] When modified β phase is viewed 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 are mainly found inside β phase or β1 phase or at a 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 microscope.

[0194] When any grain boundary was observed inside β1 phase (β phase) in the metallographic structure, "B" was indicated in Tables 10 to 15, and when none was observed, "D" was indicated in the tables.

[0195] With respect to phosphorous compounds, when a casting containing 0.06 mass% P was cooled at an average cooling rate of 15°C / min to 60°C / min from 500°C to 300°C, approximately 250 to 500 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, Cr, or Al and P or Si, Pb particles, and oxide 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 25 in such a picture of field of view, it was evaluated that phosphorous compounds were not present, and "D" was indicated as an evaluation result. In the case the number was 25 or more but less than 125, it was evaluated that phosphorous compounds were present but only in a small quantity, and "C" was indicated as an evaluation result. In the case the number was 125 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.<Machinability Testing with Lathe>

[0198] Machinability was evaluated by the cutting test using a lathe as described below. To prepare test materials, first, test piece A obtained in Step Group "A" by sand casting, a casting obtained in Step Group "B" by permanent mold casting, a bar obtained in Step Group "C" by continuous casting, and a hot-treated material obtained in Step Group G 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.

[0199] 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 A → B → C then C → B → A, in which each alphabet refers to a test piece name that corresponds to Test No.. This cycle was repeated twice so that each test piece was measured four times. The cutting resistance was calculated by the following expression.

[0200] 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.

[0201] Assuming that the cutting resistance (principal force) of a commercially available free-cutting brass bar, C3604 (Alloy X; ø40 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.

[0202] Incidentally, the machinability index was calculated as follows.

[0203] 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 machinability 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. 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 have about 1.2 times the Vickers hardness of an extruded bar made of C3604 or 1.4 times that of a casting made of CAC203 containing 2% Pb, in the evaluation of their machinability, machinability index of 70 was regarded as the standard (boundary value). Specifically, if the machinability index of a material was 78 or higher, the material was evaluated as having excellent machinability (evaluation: A), i.e., largely equivalent to that of C3604. When it was 70 or higher and lower than 78, the material was evaluated as having good machinability (evaluation: B). When it was 63 or higher and lower than 70, the material was evaluated as having fair machinability (evaluation: C). When it was lower than 63, 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 70 or higher were determined as acceptable.(Dezincification Corrosion Test: Dezincification Corrosion Test of ISO 6509)

[0204] As a test method for the evaluation of dezincification corrosion resistance, the dezincification corrosion test of ISO 6509 was employed. This is a test method which has been adopted in many countries and is specified in Japanese Industrial Standards under JIS H 3250 as well.

[0205] In accordance with the procedure of the dezincification corrosion test, first, a test material was imbedded in a phenolic resin material. The surface of the test piece was polished with emery paper No. 1200 then subjected to ultrasonic cleaning in pure water and drying.

[0206] 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.

[0207] The test piece was once again imbedded in a phenolic resin material. Subsequently, it was cut so that the cross section included the longest possible corroded portion then polished.

[0208] Dezincification 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.

[0209] When ISO 6509 testing is performed on a representative leaded free-cutting brass bar of C3604 and a free-cutting brass casting of CAC203, resultant maximum corrosion depth is about 500 µm or deeper whereas that of the present invention is 50% this maximum corrosion depth of C3604 or CAC203. If the maximum corrosion depth was less than 250 µm, the test piece was evaluated as having good dezincification corrosion resistance in practical use (evaluation: B). If the maximum corrosion depth was 100 µm or less, in particular, it was evaluated as having excellent dezincification corrosion resistance in practical use (evaluation: A).

[0210] On the other hand, if the maximum corrosion depth exceeded 400 µm, the test piece was evaluated as having poor dezincification corrosion resistance (evaluation: "D") since that meant the dezincification corrosion resistance was equivalent to or worse than that of C3604 or CAC203. If the maximum corrosion depth was 250 µm or more and 400 µm or less, it was evaluated as having fair dezincification corrosion resistance (evaluation: C).

[0211] As the present invention aims to obtain good dezincification corrosion resistance, those evaluated as good (evaluation: B) or excellent (evaluation: A) were determined as acceptable.(Mechanical Properties)(Hardness)

[0212] After the observation of metallographic structure, hardness of the test pieces was measured with a Vickers hardness tester applying a pressure of 49 kN. Also, that of those subjected to cutting tests was measured with a Rockwell scale B. It can be said that a casting with a Vickers hardness of preferably 100 Hv or higher, more preferably 110 Hv or higher has a high level of strength among free-cutting copper alloy castings. Likewise, if a casting has a hardness of 52 or higher, more preferably 60 or higher when measured with a Rockwell scale B, it can be said that the casting has a high level of strength among free-cutting copper alloy castings. Use of a casting with good dezincification corrosion resistance and high strength, as described above, leads to a saving of material to be consumed.(Impact Properties)

[0213] The impact test was performed in the following manner. U-notch specimens (notch depth: 2 mm; radius of notch base: 1 mm) that comply with JIS Z 2242 were sampled and subjected to a Charpy impact test using an impact blade of 2 mm in radius to measure their impact values. In order to retain good toughness and impact properties, the Charpy impact test value is preferably 25 J / cm 2< or higher, more preferably 30 J / cm 2< . [Table 12]Test No.Alloy No.Step No.Metallographic Structuref3f4f4Af5f6f6APresence of P compoundPresence of grain boundary inside β phase1A15545-055-DB2A24159-070-DB3A35941-051-DB4A46139-049-DB5A56337-047-CB6S01A63070-080-DB7A11H9-910-102DD8A12H70-300-40BD9A13H67-330-44BD10A14H72-280-38BD11G15149-060-DB12G11H70-300-40BD13G13H73-270-37BD [Table 13] Test No.Alloy No.Step No.Metallographic Structuref3f4f4Af5f6f6APresence of P compoundPresence of grain boundary inside β phase14A14258-067-DB15A23070-077-DB16A34654-063-DB17A44852-061-DB18A55050-058-DB19S02A62377-085-DB20A11H3-970-104DD21A12H58-420-50BD22A13H55-450-54BD23A14H60-400-48BD24G13961-069-DB25G11H57-430-51BD26G13H61-390-47BD [Table 14] Test No.Alloy No.Step No.Metallographic Structuref3f4f4Af5f6f6APresence of P compoundPresence of grain boundary inside β phase101S11B13466-074-DB102B22575-084-DB103B33664-072-DB104B44357-066-DB106B11H51-490-58BD107G23070-079-DB108B14654-064-DB109B23466-077-DB110B34852-062-DB111B45248-058-CB112B52080-091-DB113S12B11H62-380-48BD114B12H2-980-110DD115B13H61-390-49BD116B14H57-430-53CD117G24258-069-DB118G12H61-390-49BD119G14H69-310-41BD [Table 15] Test No.Alloy No.Step No.Metallographic Structuref3f4f4Af5f6f6APresence of P compoundPresence of grain boundary inside β phase120B16634-044-DB121B25545-055-DB122B36931-041-DB123B47228-038-CB124B53763-072-DB125S13B11H82-180-28BD126B12H1783-092-DB127B13H80-200-30BD128B14H76-240-34CD129G26139-049-DB130G14H83-170-27BD131B17426-035-DB132B26238-046-DB133B37525-033-DB135B54456-063-DB136S14B11H88-120-21BD137B12H2476-083-DB138B13H86-140-23BD139B14H83-170-26cD140G26931-039-DB [Table 16] Test No.Alloy No.Step No.Metallographic Structuref3f4f4Af5f6f6APresence of P compoundPresence of grain boundary inside β phase201S21C15050-059-DB202C25347-056-DB203C35842-051-DB204C1H70-300-38BD205G34753-062-DB206S22C14456-062-DB207C24753-059-DB208C1H60-400-47BD209S23B15248-058-DB210B11H68-320-40BD211S24B14951-055-DB212B11H66-340-41BD213S25B17129-038-DB214B11H87-130-22BD215S26B15248-056-DB216B11H69-310-39CD217S27B15248-057-DB218S28B15545-054-DB219S29B16040-043-DB220S30B14654-053-DB [Table 17] Test No.Alloy No.Step No.Metallographic Structuref3f4f4Af5f6f6APresence of P compoundPresence of grain boundary inside β phase221S31B16436-047-DB222S32B13367-064-DB223B11H50-500-50BD224S33B15446-057-DB225B14951-060-DB226B23763-071-DB227B34951-060-DB228B45644-053-CB229S34B52179-087-DB230B11H66-340-43BD231B12H1-990-107DD232B13H64-360-45BD233B14H60-400-49BD234G24555-064-DB [Table 18] Test No.Alloy No.Step No.Metallographic Structuref3f4f4Af5f6f6APresence of P compoundPresence of grain boundary inside β phase235S35B14555-062-DB236S36B14159-065-cB237B11H58-420-49BD238S37B14456-065-DB239B11H61-390-48BD240S38B15347-052-cB241B11H69-310-38BD242S39B15050-057-DB243B11H67-330-41BD244S40B14555-062-DB245S41B17426-031-DB246S42B14555-048-DB [Table 19] Test No.Alloy No.Step No.Metallographic Structuref3f4f4Af5f6f6APresence of P compoundPresence of grain boundary inside β phase501S51B15938-346-DB502S52B150-500-41DD503B11H67-330-31BD504S53B18416-024-DB505S54B149-510-56DD506S55B17723-031-DB507S56B141-590-30DD508S57B11288-095-DB509B11H29-710-78BD510S58B13961-077-DB511S59B16733-033-DB512S60B168-311-31CD [Table 20] Test No.Alloy No.Step No.Metallographic Structuref3f4f4Af5f6f6APresence of P compoundPresence of grain boundary inside β phase513S61B16233-534-DB514B11H76-186-17BD515S62B129-710-76CD516B11H46-540-60BD517S63B16238-044-CB518B11H76-240-32CD519S64B16931-026-DB520S65B126-740-61DD521X-86-140--DD522YB1H94-60--DD523YB183-170--DD [Table 21] Test No.Alloy No.Step No.Electrical ConductivityMachinabilityCutting Speed40 m / min110 m / min% IACSIndexEvaluationIndexEvaluation1A116.582A78A2A216.683A78A3A316.480A76B4A416.479A72B5A516.278A73B6A616.683A76B7S01A11H16.979A65C8A12H16.472B62D9A13H16.471B61D10A14H16.370B58D11G116.579A74B12G11H16.370B61D13G13H16.367C56D [Table 22] Test No.Alloy No.Step No.ISO Dezincification Corrosion TestHardnessImpact TestHVHRBDezincification Corrosion DepthµmEvaluationJ / cm 2< 1A180A11665452A290A12769383A3100A11364474A4170B11263485A5150B11163496A6120B13472337S01A11H350C15077248A12H430D10659539A13H480D108624810A14H480D104565411G180A116644612G11H360C103565713G13H460D1005162 [Table 23] Test No.Alloy No.Step No.Electrical Conductivity % IACSMachinabilityCutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation14A117.384A79A15A217.484A78A16A317.282A78A17A417.381A73B18A517.282A74B19A617.582A73B20S02A11H17.876B63C21A12H17.279A69C22A13H17.374B64C23A14H17.178A65C24G117.380A74B25G11H17.177B65C26G13H17.175B61D [Table 24] Test No.Alloy No.Step No.ISO Dezincification Corrosion TestHardnessImpact TestHVHRBDezincification Corrosion DepthµmEvaluationJ / cm 2< 14A190A124684115A2130B132713416A3110B121674217A4180B120664418A5160B118654319A6160B138733120S02A11H460D152782021A12H440D112634622A13H470D114644523A14H480D111624524G1100A123684125G11H410D110614926G13H470D1076049 [Table 25] Test No.Alloy No.Step No.Electrical Conductivity % IACSMachinabilityCutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation101S11B117.184A79A102B217.383A76B103B317.282A75B104B417.183A74B106B11H16.881A68C107G217.284A78A108B116.485A81A109B216.684A78A110B316.383A76B111B416.282A75B112B516.782A71B113S12B11H16.075B63C114B12H17.177B63C115B13H16.276B64C116B14H16.275B64C117G216.383A77B118G12H16.275B62D119G14H16.471B59D [Table 26] Test No.Alloy No.Step No.ISO Dezincification Corrosion TestHardnessImpact TestHVHRBDezincification Corrosion DepthµmEvaluationJ / cm 2< 101S11B1120B1317234102B2160B1387528103B3180B1307134104B4170B1257037106B11H510D1196740107G2130B1307135108B1110B1267136109B2120B1357430110B3160B1247036111B4160B1216937112B5220B1467922113S12B11H420D1136544114B12H420D1618416115B13H390C1156641116B14H380C1186740117G290A1256939118G12H460D1116248119G14H440D1055854 [Table 27] Test No.Alloy No.Step No.Electrical ConductivityMachinabilityCutting Speed40 m / min110 m / min% IACSIndexEvaluationIndexEvaluation120B116.780A76B121B216.983A78A122B316.778A73B123B416.676B72B124B516.984A79A125S13B11H16.561D53D126B12H17.283A70B127B13H16.564C55D128B14H16.665C55D129G216.781A75B130G14H16.459D51D131B116.976B72B132B217.281A77B133B317.071B66C135B517.384A80A136S14B11H16.748D45D137B12H17.483A73B138B13H16.853D47D139B14H16.961D54D140G217.178A73B [Table 28] Test No.Alloy No.Step No.ISO Dezincification Corrosion TestHardnessImpact TestHVHRBDezincification Corrosion DepthµmEvaluationJ / cm 2< 120B180A1106449121B290A1196743122B3150B1086251123B4140B1066052124B5110B1317234125S13B11H450D1015166126B12H230B1467824127B13H380C1005265128B14H430D1035462129G270A1106251130G14H440D955067131B170A1046054132B280A1126349133B3160B1025756135B590A1256840136S14B11H410D934674137B12H170B1407429138B13H360C954972139B14H390C985063140G260A1076053 [Table 29] Test No.Alloy No.Step No.Electrical ConductivityMachinabilityCutting Speed40 m / min110 m / min% IACSIndexEvaluationIndexEvaluation201S21C116.185A81A202C216.084A78A203C315.981A73B204C1H15.873B62D205G315.984A80A206S22C117.885A80A207C217.983A77B208C1H17.880A68C209S23B115.583A78A210B11H15.374B61D211S24B118.982A76B212B11H18.774B61D213S25B117.378A73B214B11H16.950D45D215S26B116.977B72B216B11H17.072B58D217S27B116.782A76B218S28B116.884A79A219S29B117.876B72B220S30B119.879A72B [Table 30] Test No.Alloy No.Step No.ISO Dezincification Corrosion TestHardnessImpact TestHVHRBDezincification Corrosion DepthµmEvaluationJ / cm 2< 201S21C1100A1256941202C2130B1216842203C3180B1176743204C1H440D1086153205G380A1236745206S22C1120B1236840207C2150B1216742208C1H460D1126448209S23B180A1257036210B11H420D1136543211S24B1110B1186545212B11H480D1055854213S25B190A1056054214B11H430D934871215S26B1170B1206647216B11H500D1086155217S27B1130B1196645218S28B190A1216834219S29B1110B1126450220S30B1140B1156348 [Table 31] Test No.Alloy No.Step No.Electrical Conductivity % IACSMachinabilityCutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation221S31B116.180A76B222S32B120.080A72B223B11H19.976B64C224S33B116.084A80A225B117.184A79A226B217.284A78A227B316.981A74B228B416.982A75B229S34B517.383A72B230B11H16.873B61D231B12H17.877B64C232B13H16.874B63C233B14H16.974B62D234G217.082A76B [Table 32] Test No.Alloy No.Step No.ISO Dezincification Corrosion TestHardnessImpact Test J / cm 2< HVHRBDezincification Corrosion DepthµmEvaluation221S31B180A1136545222S32B1150B1286841223B11H530D1146150224S33B160A1206840225B130A1236841226B250A1317135227B3110B1236840228B4130B1186642229B5160B1437627230S34B11H430D1106348231B12H410D1608318232B13H400C1126347233B14H420D1156546234G230A1236742 [Table 33] Test No.Alloy No.Step No.Electrical Conductivity % IACSMachinabilityCutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation235S35B117.583A77B236S36B118.079A71B237B11H17.774B62D238S37B116.982A76B239B11H16.777B65C240S38B118.077B72B241B11H17.868C59D242S39B117.682A77B243B11H17.374B63C244S40B117.984A80A245S41B119.375B72B246S42B120.476B71B [Table 34] Test No.Alloy No.Step No.ISO Dezincification Corrosion TestHardnessImpact Test J / cm 2< HVHRBDezincification Corrosion DepthµmEvaluation235S35B150A1267037236S36B1170B1276936237B11H500D1146445238S37B1120B1287035239B11H480D1166541240S38B1180B1176643241B11H490D1056149242S39B1140B1226741243B11H460D1106248244S40B1110B1206642245S41B1130B1025563246S42B1180B1146056 [Table 35] Test No.Alloy No.Step No.Electrical Conductivity % IACSMachinabilityCutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation501S51B113.679A70B502S52B122.067C59D503B11H22.061D51D504S53B118.662D58D505S54B118.668C56D506S55B115.673B67C507S56B124.556D51D508S57B117.381A67C509B11H17.282A65C510S58B114.985A78A511S59B118.574B65C512S60B119.872B62D [Table 36] Test No.Alloy No.Step No.ISO Dezincification Corrosion TestHardnessImpact Test J / cm 2< HVHRBDezincification Corrosion DepthµmEvaluation501S51B1220B1337228502S52B1320C1106248503B11H510D985764504S53 S54B1100A934971505B1450D1146260506S55B180A1045658507S56B1470D1035654508S57B1300C1507922509B11H520D1367531510S58B1100A1377620511S59B1110B1065960512S60B1300C1176834 [Table 37] Test No.Alloy No.Step No.Electrical Conductivity % IACSMachinabilityCutting Speed40 m / min110 m / minIndexEvaluationIndexEvaluation513S61B116.175B66C514B11H15.968C54D515S62B117.274B62D516B11H17.173B60D517S63B117.873B63C518B11H17.663C54D519S64B120.671B64C520S65B121.174B63C521X-26.0100A100A522YB1H25.689A88A523YB125.987A88A [Table 38] Test No.Alloy No.Step No.ISO Dezincification Corrosion TestHardnessImpact Test J / cm 2< HVHRBDezincification Corrosion DepthµmEvaluation513S61B1260C1337224514B11H410D1176329515S62B1480D1367330516B11H510D1236837517S63B1330C1146443518B11H460D1056048519S64 S65B1190B1025763520B1500D1266842521X-530D985162522YB1H490D742868523YB1570D823756

[0214] From the above results of measurements, the present inventors learned the followings: 1) The inventors were able to confirm that a casting with good machinability and good dezincification corrosion resistance, good conductivity of 15% IACS or higher, high strength, and good impact properties could be obtained by satisfying the composition of an embodiment of the present invention, composition relational expressions f1, f2, and f0, metallographic structure relational expressions f3 to f5, and metallographic structure and composition relational expression f6 even if the Pb content was small (Alloys Nos. S01, S02, S11 to S14, and S21 to S42). 2) β phase became modified by containing more than 0.50 mass% Si, and a grain boundary was observed inside β1 phase. As a result of the modification, dezincification corrosion resistance and machinability improved. When the Si content was 0.65 mass% or higher, better dezincification corrosion resistance and machinability were obtained. On the other hand, the electrical conductivity decreased when the Si content exceeded 1.20 mass% (Alloys Nos. S01, S30, S32, S42, S51, S52, etc.). 3) β phase became modified by containing more than 0.01 mass% P, and a grain boundary was observed inside β1 phase. As a result of the modification, dezincification corrosion resistance and machinability improved. When the P content was increased to 0.03 mass% or higher, dezincification corrosion resistance and machinability further improved. When the P content exceeded 0.18 mass%, impact properties deteriorated (Alloys Nos. S01, S26 to S28, S54, S58, 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 machinability improved. When the Cu content was increased to 61.5 mass% or higher, dezincification corrosion resistance and machinability further improved. When the Cu content exceeded 65.0 mass%, proportion of β1 phase decreased and γ phase appeared in some cases, resulting in poor machinability (Alloys Nos. S01, S32, S24 S14, S42, S55, S65, etc.). 5) When the Pb content was lower than 0.002 mass%, machinability was poor (Alloy No. S59). When the Pb content was increased to 0.003 mass% or higher, or further, 0.01 mass% or higher, machinability improved (Alloys Nos. S29 and S21). It was confirmed that containing Bi had an effect of improving machinability to the level equivalent to that of Pb (Alloys Nos. S01. S24, etc.). 6) When the Sn content exceeded 0.05 mass%, dezincification corrosion resistance further improved. Dezincification corrosion resistance was further improved by containing more than 0.10 mass% Sn. On the other hand, when 0.70 mass% or higher Sn was contained, γ phase appeared causing dezincification corrosion resistance to rather deteriorate than improve, impact properties to decrease, and machinability index to decrease. In addition, when the Sn / Si ratio was higher than 0.8, β phase failed to be modified and dezincification corrosion resistance and machinability were poor (Alloys Nos. S33 to S35, S60, S61, etc.). 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, S21, S22, S37, S39, etc.). 8) When the total content of Fe, Mn, Cr, and Co as inevitable impurities was 0.50 mass% or higher, machinability was poor, and dezincification corrosion resistance was not so good even though β phase was modified. As the total content of Fe, Mn, Cr, and Co was reduced to less than 0.50 mass%, or further to less than 0.35 mass%, machinability and dezincification corrosion resistance improved. It is considered that the machinability and dezincification corrosion resistance were poor because of the intermetallic compounds formed between parts of Fe, Mn, and / or other elements and Si and / or P. It is considered that the deterioration in machinability and dezincification corrosion resistance was caused by the formation of intermetallic compounds created when part of Si and / or P compounded with Fe, Mn, or other elements. As a result, compounds of Si and / or P and Fe or other elements came to be present, and also the concentrations of Si and P, which are effective to improve machinability and corrosion resistance, decreased, resulting in deterioration of machinability and corrosion resistance (Alloys Nos. S02, S38, S39, S63, etc.). 9) When the amount of A1 as an inevitable impurity was 0.30 mass% or higher, modification of β phase did not occur, which resulted in poor dezincification corrosion resistance, machinability, and impact properties. When the amount of A1 was less than 0.30 mass%, dezincification corrosion resistance and machinability were good. When the amount of A1 was less than 0.15 mass%, its effect was further reduced, resulting in good machinability and dezincification corrosion resistance (Alloys Nos. S01, S62, S36, S37, etc.). 10) When the value of the composition relational expression f1 was smaller than 57.3, modification of β phase did not occur or was insufficient, and dezincification corrosion resistance and machinability in high-speed cutting were poor. When the value of the composition relational expression f1 was larger than 60.8, the area ratio of β1 phase was small and machinability was poor (Alloys Nos. S57 and S53). When the value of the composition relational expression f1 was 57.3 or larger, β phase was modified and dezincification corrosion resistance and machinability improved. When the value was 58.0 or larger, they improved more. On the other hand, when the value of the composition relational expression f1 was 60.8 or smaller, the area ratio of β1 phase increased. When the value was 60.5 or smaller, the area ratio further increased, and the machinability index became higher even in high-speed cutting (e.g., Alloys Nos. S14, S32, and S25). 11) When the area ratio of β1 phase, i.e., the value of f4, was smaller than 25%, good machinability was not obtained. In addition, the hardness index was low. When the area ratio was larger than 25%, and further, 30% or larger, machinability improved even in high-speed cutting. When the area ratio of β1 phase exceeded 80%, however, β phase often failed to be modified. As a result, the machinability index especially in high-speed cutting (110 m / min) was low and dezincification corrosion resistance was poor. In addition, the impact value decreased. When the area ratio of β1 phase was 80% or smaller, and further, 70% or smaller, β phase was better modified and dezincification corrosion resistance and machinability improved. Even though progress of dezincification corrosion was significantly delayed by the modification of β phase, β1 phase was preferentially corroded by dezincification. For this reason, when the area ratio of β1 phase was large, dezincification corrosion was slightly expedited (Alloys Nos. S57, S55, S53, S14, and S11; Steps Nos. A6, B5, A11H, B12H, etc.). 12) When the area ratio of γ phase was 4% or larger, machinability index was low due to the negative effect that γ phase has on machinability even if f4 and f6 were satisfied. In such a case, the impact value was also low (Alloy No. S61). 13) Even if f1 and f4 were satisfied, when the value of the metallographic and composition relational expression f6 was smaller than 28, machinability was poor. When the f6 value was 28 or larger, or further, exceeded 35, machinability improved (Alloys Nos. S64, S41). On the other hand, when the f6 value was between 55 and 80, the highest machinability was obtained. This is considered to be due to the facts that machinability largely peaks when the amount of β1 phase (f4) is 45% to 60% and the Si content is 1 mass% (e.g., Alloys Nos. S02, S12; Steps Nos. A1, A2, etc.). 14) Even when the amount of β phase that is not modified to β1 phase (f4A) was 45 or larger and the f6A value was larger than 40, dezincification corrosion was deep and machinability especially in high-speed cutting was poor (Tests Nos. S22, S106, S223, etc.). 15) In the cooling process after molten metal was poured, when the starting temperature of cooling treatment was lower than 700°C and higher than 500°C and also the average cooling rate from 500°C to 300°C was higher than 300°C / min, β phase was modified and a grain boundary was observed inside β1 phase. When β phase was modified, cutting resistance lowered, dezincification corrosion resistance significantly improved, and strength was high. With respect to machinability, in the case β phase was modified, deterioration in machinability due to high-speed machining was very limited, indicating a significant difference from the test samples in which β phase was not modified (Steps Nos. A1 to A6, A11H to A14H, B1 to B5, B11H to B14H, etc.). 16) In the case the starting temperature of the cooling treatment was higher than 500°C but lower than about 550°C, a grain boundary was observed inside β1 phase, and a small amount of phosphorous compounds were 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, A5, B1, B4, etc.). 17) In the case the starting temperature of the cooling treatment was between 550°C and 650°C, machinability and dezincification corrosion resistance further improved. When the cooling rate at the point the temperature of the casting was 500°C and the average cooling rate from 500°C to 300°C were higher than 600°C / min, or further, higher than 900°C / min, i.e., the higher the cooling rates, the better the machinability, especially in terms of the machinability index at a cutting speed of 110 m / min, and the dezincification corrosion resistance. Thus, it was discovered that the starting temperature of the cooling treatment, the cooling rate at the point the temperature of the casting was 500°C and that from 500°C to 300°C affect the modification of β1 phase. That is, it is considered that β phase was better modified due to the appropriately adjusted conditions of cooling treatment (Steps Nos. A1 to A6, B1 to B5, C1 to C6, etc.). 18) That the cooling rate at the point the temperature of the casting is 500°C is about 300°C / min and the average cooling rate from 500°C to 300°C is about 300°C / min are critical points that decide whether a grain boundary can be observed inside β phase, i.e., whether the modification of β phase occurs. When these conditions were met, machinability particularly in high-speed cutting and dezincification corrosion resistance drastically changed (Steps Nos. A4, A13, B3, B14H, etc.). 19) When the starting temperature of the cooling treatment was higher than 700°C, the proportion of β1 phase was larger and machinability index was slightly lower, machinability particularly in high-speed cutting was lower, and dezincification corrosion resistance was worse than when the starting temperature was 700°C or lower. When the proportion of β1 phase was higher than 80%, β phase sometimes failed to be modified. Incidentally, the form of dezincification corrosion was selective corrosion which occurs in β1 phase. When the proportion of β1 phase was large, the progression speed of dezincification corrosion appeared slightly accelerated (Steps Nos. A1 to A6, A11H, B1 to B5, B12H, etc.). 20) In the case the average cooling rate from 500°C to 300°C was about 200°C / min to 250°C / min, no grain boundary was observed inside β phase, and a small amount of phosphorous compounds were present in many cases. As a result of absence of grain boundary inside β phase, i.e., β phase remaining unmodified, machinability, especially when machined at a cutting speed of 110 m / min, and dezincification corrosion resistance were poor (Steps Nos. A13H, B14H, etc.). 21) It was confirmed that even when the starting temperature of the cooling treatment is unable to be identified in the cooling process after molten metal is poured, if the cooling rate in the temperature range from 550°C to 500°C and the average cooling rate in the temperature range from 500°C to 300°C are both higher than 300°C / min, β phase becomes modified and dezincification corrosion resistance and machinability significantly improve (Steps Nos. C1 to C3 and C11H). 22) Even if the cooling treatment was started when the casting was at a high temperature exceeding 500°C after molten metal was poured and performed at a high cooling rate, no cracking was observed on the casting (Steps Nos. A1 to A6, B1 to B5, and C1 to C3). 23) It was confirmed that if a casting in which β phase remains unmodified is annealed at 580°C for 20 minutes then cooled at a cooling rate exceeding 300°C / min at the point the temperature of the casting is 500°C and an average cooling rate exceeding 300°C / min from 500°C to 300°C, the β phase becomes modified and dezincification corrosion resistance and machinability significantly improve (Steps Nos. G1 to G3 and C11H).

[0215] 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 good machinability, dezincification corrosion resistance, and mechanical characteristics. Embodiments of the present invention are able to obtain excellent properties by appropriately adjusting the cooling conditions after molten metal is poured as well as the heat treatment conditions.INDUSTRIAL APPLICABILITY

[0216] The free-cutting copper alloy castings of the embodiments have excellent machinability, dezincification corrosion resistance, and castability although they contain only a small amount of Pb. The castings also have good impact properties despite their high strength. For this reason, they are suitable for devices and components related to drinking water or sanitary facility, food-related devices, electrical or home appliance components, auto parts, mechanical components, stationaries, toys, musical instruments, sliding components, measuring instrument components, precision mechanical components, medical components, water meters, and components involving liquid or gas such as industrial water, drainage water, or hydrogen. Specifically, the free-cutting copper alloy castings can be suitably applied to the items use d in the above-mentioned fields that go by the names including faucet, hot-and-cold water mixing tap, shutoff valve, water meter, shower head, valve, joint, cock, gear, axle, bearing, shaft, sleeve, spindle, sensor, bolt, nut, and connector as their component or the like.

Claims

1. A free-cutting copper alloy casting 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 0.50 mass% and a 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%, a relationship of 57.3 ≤ f 1 = Cu − 4.8 × Si + 0.5 × Pb + Bi − P ≤ 60.8 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 , and are satisfied, and a grain boundary is observed inside the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

2. A free-cutting copper alloy casting comprising: higher than or equal to 61.5 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 0.35 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%, a relationship of 58.0 ≤ f 1 = Cu − 4.8 × Si + 0.5 × Pb + Bi − P ≤ 60.5 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 are satisfied, and a grain boundary is observed inside the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

3. A free-cutting copper alloy casting 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 0.70 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 0.50 mass%, and a 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] mass%, a relationship of 57.3 ≤ f 1 = Cu − 4.8 × Si + 0.5 × Pb + Bi − P − Sn ≤ 60.8 is satisfied, and a relationship of f 0 = Sn / Si < 0.8 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 are satisfied, and a grain boundary is observed inside the β1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water.

4. The free-cutting copper alloy casting according to any one of claims 1 to 3, which is used for a device or component related to drinking water or sanitary facility, a water meter, a valve, a device or component for water drainage, an industrial plumbing component, an auto part, an electrical or home appliance component, a mechanical component, a stationery, a toy, a musical instrument, a sliding component, a measuring instrument component, or a medical component.

5. A method for producing the free-cutting copper alloy casting according to any one of claims 1 to 3, the method comprising: casting step(s), wherein in the cooling process of the final casting step among the casting steps, the cooling rate at the point the temperature of the casting is 500°C is higher than 300°C / min and the average cooling rate in the temperature range from 500°C to 300°C is higher than 300°C / min.

6. A method for producing the free-cutting copper alloy casting according to any one of claims 1 to 3, the method comprising: casting step(s), wherein in the cooling process of the final casting step among the casting steps, a cooling treatment is started at a temperature higher than 500°C and lower than 700°C, the cooling rate at the point the temperature of the casting is 500°C is higher than 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is higher than 300°C / min.

7. A method for producing the free-cutting copper alloy casting according to any one of claims 1 to 3, the method comprising: casting step and heat treatment step, wherein in the final heat treatment step, the casting is heated to a temperature higher than 520°C and lower than 650°C and held at the temperature for one minute to five hours, in the process of cooling treatment after the heat treatment, the cooling treatment is started when the temperature of the casting is higher than 520°C, the cooling rate at the point the temperature of the casting is 500°C is higher than 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is higher than 300°C / min.

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