Free-machining copper alloy, and method for manufacturing a free-machining copper alloy

Abstract

This free-machining copper alloy contains prescribed amounts of Cu, Si, Pb, and P, and contains a prescribed amount of Bi as an optional element, with the remainder being Zn and unavoidable impurities. Among the unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than a prescribed amount, and the Al content is less than a prescribed amount. The compositional relationships f1, f2, and f0, which are defined from the elemental composition, the structural relationships f3, f4, and f5, which are defined from the surface area ratios of the constituent phases of the metallographic structure, and structural / compositional relationship f6, which is defined from the composition and the metallographic structure, each fall within prescribed ranges. A modified β1 phase has visible grain boundaries when et\ched using a liquid mixture of hydrogen peroxide and aqueous ammonia.

Description

[Technical Field]

[0001] The present invention relates to a free-machining copper alloy that has good corrosion resistance, particularly resistance to dezincification corrosion and stress corrosion cracking, high strength, and a significantly reduced Pb content, as well as a method for producing a free-machining copper alloy. This invention relates to free-machining copper alloys used in equipment and parts used for drinking water consumed daily by humans and animals, equipment and parts used in sanitary facilities such as kitchens, bathrooms, and toilets, water meters, musical instruments, tableware, drainage equipment and parts, industrial piping parts, electrical and electronic equipment parts, automobile parts, machine parts, stationery, toys, sliding parts, instrument parts, precision machine parts, medical parts, and parts related to liquids and gases such as hydrogen.Specific examples of parts include water taps, mixing taps, faucets, stopcocks, valves, fittings, valves, shower heads, cocks, gears, shafts, bearings, shafts, sleeves, spindles, sensors, bolts, nuts, flare nuts, pen tips, insert nuts, cap nuts, nipples, spacers, screws, and other parts that are machined, as well as methods for manufacturing free-machining copper alloys. This application claims priority based on Japanese Patent Application No. 2023-075579, filed in Japan on May 1, 2023, and Japanese Patent Application No. 2023-173342, filed in Japan on October 5, 2023, and the contents thereof are incorporated herein by reference. [Background technology]

[0002] Traditionally, for equipment and parts related to drinking water and sanitary facilities, water meters, musical instruments, tableware, electrical, home appliance and electronic equipment parts, automobile parts, machine parts, stationery, precision machine parts, medical parts, and equipment and parts related to liquids and gases such as industrial water, wastewater, and hydrogen, the commonly used materials for parts such as water taps, mixing taps, stopcocks, valves, cocks, fittings, gears, sensors, nuts, and screws have been Cu-Zn-Pb alloys (so-called free-cutting brass rods, forging brass, casting brass) containing 56-65 masss% Cu, 1-4 masss% Pb, and the remainder being Zn, which have excellent machinability, antibacterial properties, and good corrosion resistance, or Cu-Sn-Zn-Pb alloys (so-called bronze castings: gunmetal) containing 80-88 masss% Cu, 2-8 masss% Sn, 1-8 masss% Pb, and the remainder being Zn, have been commonly used for equipment and parts related to drinking water and sanitary facilities, water meters, musical instruments, tableware, electrical, home appliance and electronic equipment parts, automobile parts, machine parts, stationery, precision machine parts, medical parts, and equipment and parts related to liquids and gases such as industrial water, wastewater, and hydrogen.

[0003] However, in recent years, concerns have arisen regarding the effects of lead (Pb) on human health and the environment, leading to increased regulatory efforts in various countries. For example, in California, USA, regulations limiting the Pb content in drinking water equipment to 0.25 mass% or less have been in effect since January 2010. Regulations are also rapidly advancing in countries outside the US, creating a demand for the development of copper alloy materials that comply with Pb content regulations.

[0004] Furthermore, in other industrial sectors such as electrical and electronic equipment, automobiles, and machinery, for example, the European RoHS and ELV directives exceptionally allow Pb content up to 4 mass% in free-machining copper alloys. However, similar to the drinking water sector, there is active discussion about strengthening regulations on Pb content, including the elimination of these exceptions.

[0005] Amidst this trend of stricter Pb regulations for free-machining copper alloys, several Pb-containing free-machining copper alloys have been proposed as alternatives to free-machining brass containing 3 mass% Pb (JIS standard: C3604, ASTM standard: C36000, EN standard: CW614N), which is the most widely used free-machining brass in the world. These include: (1) Cu-Zn-Bi alloys and Cu-Zn-Bi-Se alloys containing Bi, which has machinability properties, and in some cases, Se in addition to Bi; (2) Cu-Zn alloys containing a high concentration of Zn to increase the β phase and improve machinability; (3) Cu-Zn-Si alloys containing a large amount of γ and κ phases, which have excellent machinability properties, instead of Pb; and (4) Cu-Zn-Si-Pb alloys in which the machinability of the β phase is improved by the inclusion of Si. For example, in Patent Document 1, 0.3 to 4 mass%, preferably 1.8 to 3.2 mass%, of Bi is added to a Cu-Zn alloy, and since the β phase has poor resistance to dezincification corrosion, the β phase is reduced and the alloy is annealed at 350 to 550°C to separate the β phase with the α phase, thereby improving machinability and resistance to dezincification corrosion.

[0006] However, alloys containing Bi instead of Pb have many problems, including the fact that Bi is inferior to Pb in terms of machinability, may be harmful to the environment and human health like Pb, is a rare metal and therefore has resource issues, and makes copper alloy materials brittle. Furthermore, as shown in Patent Document 1, the β phase of Cu-Zn alloys has conventionally had poor resistance to dezincification corrosion, and as a measure to improve this, it is practically necessary to reduce the amount of the β phase and perform annealing to separate the β phase with the α phase.

[0007] Furthermore, while Cu-Zn binary alloys containing a large amount of β phase contribute to improved machinability, the β phase has inferior machinability compared to Pb, and is also inferior in dezincification corrosion resistance and stress corrosion cracking resistance. Therefore, it can hardly be used as a substitute for Pb-containing free-machining copper alloys. Therefore, as a free-machining copper alloy, a Cu-Zn-Si alloy containing Si instead of Pb has been proposed, for example, in Patent Documents 2 to 8.

[0008] Patent documents 2 to 7 generally state that the Cu content is approximately 58 to 65 mass%, and the Si content is approximately 0.2 to 1.5 mass%, and that the machinability is improved by the presence of Si contained in the β phase and fine P compounds formed by P and Zn, and that by specifying the area ratio of the β phase and the presence of P compounds and a small amount of Pb, excellent machinability is achieved. However, it is a well known fact that the β phase of Cu-Zn alloys has poor resistance to dezincification corrosion, as disclosed in Patent Document 1, and further worsens resistance to stress corrosion cracking. Therefore, since no specific data related to resistance to dezincification corrosion and resistance to stress corrosion cracking is found in Patent Documents 2 to 7, it is presumed that the technical problems of the β phase of conventional Cu-Zn alloys, namely resistance to dezincification corrosion and resistance to stress corrosion cracking, have not been improved. Patent Document 8 describes how excellent machinability is achieved with a small amount of Pb (less than 0.02 mass%) by specifying the total area of ​​the γ and κ phases, which have excellent machinability properties and are formed in alloys with high Cu and Si concentrations, where the Cu content is 71.5 to 78.5 mass% and the Si content is 2.0 to 4.5 mass%. Furthermore, by including Sn and Al in amounts of 0.1 mass% or more each, a large amount of γ phase is formed, further improving machinability and corrosion resistance.

[0009] Furthermore, Patent Document 9 describes a Cu-Zn-Sn alloy containing small amounts of Si, Pb, P, or Fe, and containing less than 0.5 mass% of Pb. By devising a manufacturing method, Pb-enriched particles are dispersed in the matrix, and the number density of Pb-enriched particles present in the α phase is increased, thereby obtaining excellent machinability. Patent Document 9 also states that a finish heat treatment of 400 to 600°C is substantially necessary to improve resistance to dezincification corrosion.

[0010] Patent document 10 relates to a technology for manufacturing near-net void hot forged products using a Cu-Zn-Si-Pb-P alloy material, and proposes a copper alloy in which the area ratios of the β phase, γ phase, and μ phase are limited. Patent Document 11 proposes a copper alloy casting in which a Cu-Zn-Zr-P alloy selectively contains Si, Pb, and Sn, and crystal grains are refined by the action of Zr and P. Patent Document 12 proposes a copper alloy with excellent discoloration resistance, which is a Cu-Zn-Sn-Al alloy that selectively contains Si and Pb and limits the area ratio of γ phase and β phase. Patent Document 13 proposes a Pb-free copper alloy casting formed of a Cu-Zn-Si-Sn-Al-P alloy. Patent Document 14 states that an apparent Zn content is important for improving corrosion resistance in a Cu-Zn-Si-Sn-Al alloy, and proposes a copper alloy whose machinability is improved by substantially containing a large amount of Pb or Bi. Patent Document 15 discloses a Pb-free copper alloy casting formed of a Cu-Zn-Si alloy containing 65 mass% or more of Cu, having good castability and mechanical strength, in which machinability is improved by the γ phase, and describes examples containing large amounts of Sn, Al, Mn, Ni, and Sb. [Prior Art Documents] [Patent Documents]

[0011] [Patent Document 1] Japanese Patent Application Laid-Open No. 2002-003967 [Patent Document 2] Japanese Patent Application Laid-Open No. 2021-042461 [Patent Document 3] Japanese Patent Application Laid-Open No. 2021-042459 [Patent Document 4] Japanese Patent Application Laid-Open No. 2021-042460 [Patent Document 5] International Publication No. WO 2020 / 261666 [Patent Document 6] Japanese Patent Application Laid-Open No. 2021-042462 [Patent Document 7] International Publication No. WO 2021 / 117528 [Patent Document 8] International Publication No. WO 2007 / 034571 [Patent Document 9] Japanese Patent Publication No. 2016-194123 [Patent Document 10] International Publication No. 2013 / 065830 [Patent Document 11] International Publication No. 2006 / 016630 [Patent Document 12] International Publication No. 2015 / 046421 [Patent Document 13] Japanese Patent Publication No. 2010-133006 [Patent Document 14] Japanese Patent Publication No. 2018-048398 [Patent Document 15] Special Publication No. 2019-508584 [Overview of the project] [Problems that the invention aims to solve]

[0012] As shown in these Patent Documents 1 to 15, there has been no fundamental improvement in the dezincification corrosion resistance and stress corrosion cracking resistance of the β phase present in Cu-Zn alloys, which have been major technical challenges in the past. Furthermore, there is no disclosure of a Cu-Zn alloy that exhibits low cutting resistance and excellent machinability under high-speed cutting conditions exceeding 100 m / min, even with a Pb and Bi content of less than 0.2%, and without the essential presence of Bi.

[0013] The present invention has been made to solve the problems of the prior art, and aims to provide a free-machining copper alloy that has excellent hot workability and machinability, good resistance to dezincification corrosion and stress corrosion cracking despite containing a large amount of β phase (β1 phase described later), high strength, a good balance between strength and ductility, and a significantly reduced Pb content, as well as a method for producing a free-machining copper alloy. [Means for solving the problem]

[0014] In order to solve the above-mentioned problems and achieve the aforementioned objectives, the inventors diligently conducted research and obtained the following findings. In this specification, unless otherwise specified, the β phase includes the β' phase, the γ phase includes the γ' phase, and the α phase includes the α' phase. The β1 phase is a modified β phase and is distinct from the β and β' phases. The β1 phase is characterized by the observation of grain boundary patterns, i.e., crystal grain boundaries, within the β1 phase when observed with a metallurgical microscope using hydrogen peroxide and ammonia water as etching solutions. In general Cu-Zn alloys, Cu-Zn-Bi alloys, Cu-Zn-Si alloys, etc., no crystal grain boundaries are observed within the β phase even when etched with hydrogen peroxide and ammonia water. Therefore, the β phase and the β1 phase can be clearly distinguished. Crystal grain boundaries are sometimes simply called grain boundaries. Hot-worked materials include hot-extruded materials, hot-forged materials, and hot-rolled materials. Cold workability refers to the performance of cold-worked processes such as drawing, wire drawing, rolling, riveting, and bending. Good or excellent machinability, unless otherwise specified, refers to low cutting resistance and good or excellent chip breaking when cutting the outer circumference using a lathe. Conductivity refers to electrical conductivity, thermal conductivity, and electrical conductivity. Cooling rate refers to the average cooling rate over a certain temperature range. 1 day and 2 nights means one day. Actual operation means manufacturing using actual mass production equipment.

[0015] In the aforementioned Patent Document 8, it is stated that in Cu-Zn-Si alloys, the β phase contributes little to, or rather inhibits, the machinability of the copper alloy. In Patent Documents 10, 11, and 12, the amount of the β phase is also significantly limited. In Patent Document 1, as a method to improve the dezincification corrosion resistance of the β phase, it is necessary to reduce the β phase and perform an annealing process at 350-550°C so that the β phase is separated by the α phase. In Patent Document 9, in order to improve the dezincification corrosion resistance of the β phase, it is necessary to have a higher Sn content than Si, and to heat the alloy to a temperature of 700-850°C, hot extrude it, and then perform a finish heat treatment by holding it at 400-600°C for 30 minutes or more, with an average cooling rate of 0.2-10°C / second from 400 to 200°C.

[0016] On the other hand, Patent Documents 2 to 7 describe the discovery that, in Cu-Zn-Si alloys, first, incorporating a certain amount of Si into the β phase has a significant effect on the machinability of the β phase itself. Furthermore, it is stated that the presence of fine P compounds, the inclusion of small amounts of Pb, and in some cases Bi, work synergistically to obtain excellent machinability in the alloy. However, in order to include P compounds, it is preferably necessary to cool the alloy at an average cooling rate of approximately 0.1°C / min to approximately 70°C / min in the temperature range from approximately 530°C to approximately 450°C after hot working. Incidentally, Patent Documents 2-7 do not disclose any data related to dezincification corrosion resistance or stress corrosion cracking resistance. Furthermore, there is no mention of modifying the β phase itself. In other words, it is presumed that there is no improvement in the dezincification corrosion resistance, stress corrosion cracking resistance, or further improvement in machinability of the β phase present in Cu-Zn-Si alloys.

[0017] The inventors further worked on modifying the β phase itself in Cu-Zn-Si alloys. As a result of diligent research, they found that even without the presence of P compounds, the modified β phase, i.e., the β1 phase, did not require the inclusion of Bi, and further improved machinability, especially during high-speed cutting. They also found that the dezincification corrosion resistance and stress corrosion cracking resistance of the β phase, which had been long-standing issues with Cu-Zn alloys, were significantly improved. In addition, alloys containing the β1 phase instead of the β phase showed further increased strength without impairing ductility. This β1 phase is obtained by first solid-solving a certain amount of Si and P within the β phase, maintaining the β phase state at 500-670°C, and then increasing the cooling rate when cooling to room temperature, thereby bringing the high-temperature metallic structure down to room temperature.

[0018] Incidentally, the β1 phase can be easily distinguished and identified from the β phase of Cu-Zn alloys. The surface is polished to a mirror finish and etched with a mixture of hydrogen peroxide and ammonia water. For etching, an aqueous solution is used, which is a mixture of 3 mL of 3 vol% hydrogen peroxide and 22 mL of 14 vol% ammonia water. When the polished surface of the metal is immersed in this aqueous solution for about 2 to 10 seconds at room temperature (about 15 to 25°C) and observed with a metal microscope at 200 to 1000x magnification, grain boundary patterns, i.e., crystal grain boundaries, can be seen in the β1 phase, but crystal grain boundaries cannot be seen in the β phase of ordinary Cu-Zn alloys. The use of this etching solution is also disclosed in Patent Documents 2 to 7, 10, and 11, and the metal structure is shown.

[0019] To obtain the β1 phase, that is, to maintain the β phase state at 500-670°C after hot working and bring it down to room temperature, it is necessary that the cooling process after hot working starts at a temperature lower than 670°C and higher than 500°C, and that the average cooling rate in the temperature range from the cooling start temperature to 500°C exceeds 300°C / min, and that the average cooling rate in the temperature range from 500°C to 300°C continues to exceed 300°C / min. In this specification, unless otherwise specified, the cooling process refers to controlling the cooling rate by water cooling or a similar method, rather than natural cooling. Patent documents 2-7 state that in order to obtain a compound containing fine P, it is necessary that the hot working material be cooled at an average cooling rate of about 0.1°C / min or more and about 70°C / min or less in the temperature range from about 530°C to about 450°C after hot working. In other words, it is clear that this application and Patent Documents 2-7 are pointing in opposite directions (giving opposite instructions). However, for example, if the cooling rate around 520°C after hot working is 60°C / min, and the average cooling rate in the temperature range from 520°C (the starting temperature of the cooling process) to 500°C exceeds 300°C / min, then alloys containing both the β1 phase and the P compound may exist in this application as well.

[0020] The β1 phase in this application significantly improves the machinability of Cu-Zn-Si alloys, particularly with respect to high-speed cutting, even without the P compounds disclosed in Patent Documents 2 to 7. Furthermore, the synergistic effect of the β1 phase with fine Pb particles, or particles containing Pb and Bi, promotes a reduction in cutting resistance and chip fragmentation. More importantly, the unresolved issues of conventional β-phase, particularly its resistance to dezincification corrosion and stress corrosion cracking, are significantly improved and resolved by the modification from β-phase to β1-phase. Furthermore, its mechanical properties inherit the high strength of conventional β-phase, while its ductility is improved, resulting in even higher strength and a better balance of strength and ductility. As a result, we have invented a copper alloy that has machinability comparable to conventional free-cutting brass, and which exhibits better resistance to dezincification corrosion and stress corrosion cracking, as well as higher strength compared to conventional free-cutting brass.

[0021] The free-machining copper alloy of embodiment 1 of the present invention contains Cu in an amount greater than 60.5 mass% and less than 65.0 mass%, Si in an amount greater than 0.50 mass% and less than 1.20 mass%, Pb in an amount greater than 0.002 mass% and less than 0.20 mass%, and P in an amount greater than 0.01 mass% and less than 0.18 mass%, and contains Bi in an amount greater than 0.0001 mass% and less than 0.20 mass% as an optional element, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co and Cr among the unavoidable impurities is less than 0.40 mass%, and the content of Al is less than 0.30 mass%, and the Cu content is [Cu]mas When the content of s%, Si is [Si]mass%, Pb is [Pb]mass%, P is [P]mass%, and Bi is [Bi]mass%, the relationship 57.5≦f1=[Cu]-4.6×[Si]+0.5×([Pb]+[Bi])-[P]≦60.5 holds, and when Bi is included, the relationship 0.003≦f2=[Pb]+[Bi]<0.20 further holds, and in the constituent phases of the metallic microstructure excluding nonmetallic inclusions, when the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%, the relationship 20≦f3=(α)<75, 25 <f4=(β1)≦80、0≦f5=(γ)<4、27<f6=(β1)×([Si])1 / 2 -(γ)×2+([Pb]+[Bi]) 1 / 2 ×20+([P]) 1 / 2 The β1 phase has a ×15 relationship, and grain boundaries can be observed when etched with a mixture of hydrogen peroxide and aqueous ammonia.

[0022] The free-machining copper alloy of embodiment 2 of the present invention contains 61.2 mass% to 64.8 mass% of Cu, 0.65 mass% to 1.10 mass% of Si, 0.003 mass% to less than 0.10 mass% of Pb, and 0.03 mass% to 0.15 mass% of P, and as an optional element, it contains 0.001 mass% to less than 0.10 mass% of Bi, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co, and Cr among the unavoidable impurities is less than 0.30 mass%, and the content of Al is less than 0.15 mass%, with the Cu content being [Cu] mass%, and the Si content being [Si When the content of copper is [mass%, the content of Pb is [Pb]mass%, the content of P is [P]mass%, and the content of Bi is [Bi]mass%, the relationship 58.0≦f1=[Cu]-4.6×[Si]+0.5×([Pb]+[Bi])-[P]≦60.2 holds, and when Bi is included, the relationship 0.004≦f2=[Pb]+[Bi]<0.10 further holds, and in the constituent phases of the metallic microstructure excluding nonmetallic inclusions, when the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%, the relationships 30≦f3=(α)≦70, 30≦f4=(β1)≦70, 0≦f5=(γ)<1, 35 <f6=(β1)×([Si]) 1 / 2 -(γ)×2+([Pb]+[Bi]) 1 / 2 ×20+([P]) 1 / 2 The β1 phase has a ×15 relationship, and grain boundaries can be observed when etched with a mixture of hydrogen peroxide and aqueous ammonia.

[0023] The free-cutting copper alloy according to aspect 3 of the present invention comprises more than 60.5 mass% and less than 65.0 mass% of Cu, more than 0.50 mass% and less than 1.20 mass% of Si, 0.002 mass% or more and less than 0.20 mass% of Pb, more than 0.01 mass% and less than 0.18 mass% of P, more than 0.05 mass% and less than 0.90 mass% of Sn, optionally contains 0.0001 mass% or more and less than 0.20 mass% of Bi as an optional element, with the balance consisting of Zn and unavoidable impurities. Among the unavoidable impurities, the total content of Fe, Mn, Co and Cr is less than 0.40 mass%, and the content of Al is less than 0.30 mass%. When the content of Cu is defined as [Cu] mass%, the content of Si as [Si] mass%, the content of Pb as [Pb] mass%, the content of P as [P] mass%, the content of Bi as [Bi] mass%, and the content of Sn as [Sn] mass%, the alloy satisfies the relationship of 57.5 ≦ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≦ 60.5, and further satisfies the relationship of f0 = [Sn] / [Si] < 1. When the alloy contains Bi, it satisfies the relationship of 0.003 ≦ f2 = [Pb] + [Bi] < 0.20. In the constituent phases of the metal structure excluding non-metallic inclusions, when the area ratio of the α phase is defined as (α)%, the area ratio of the γ phase as (γ)%, and the area ratio of the β1 phase, which is a modified β phase, as (β1)%, the alloy satisfies the following relationships: 20 ≦ f3 = (α) < 75, 25 < f4 = (β1) ≦ 80, 0 ≦ f5 = (γ) < 4, 27 < f6 = (β1) × ([Si]) 1 / 2 -(γ) × 2 + ([Pb] + [Bi]) 1 / 2 × 20 + ([P]) 1 / 2 × 15, and the β1 phase is characterized in that grain boundaries can be observed when etched with a mixed solution of hydrogen peroxide and aqueous ammonia.

[0024] The free-machining copper alloy of embodiment 4 of the present invention contains 61.2 mass% to 64.8 mass% of Cu, 0.65 mass% to 1.10 mass% of Si, 0.003 mass% to less than 0.10 mass% of Pb, 0.03 mass% to 0.15 mass% of P, and 0.10 mass% to less than 0.50 mass% of Sn, and as an optional element, it contains 0.001 mass% to less than 0.10 mass% of Bi, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co, and Cr among the unavoidable impurities is less than 0.30 mass%, and the content of Al is less than 0.15 mass%, with the Cu content being [Cu] mass%, the Si content being [Si] mass%, and Pb If the content of [Pb] mass%, the content of P is [P] mass%, the content of Bi is [Bi] mass%, and the content of Sn is [Sn] mass%, then the relationship 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≤ 60.2 holds, and further the relationship f0 = [Sn] / [Si] < 0.6 holds, and Bi is If included, the relationship 0.004≦f2=[Pb]+[Bi]<0.10 holds, and in the constituent phases of the metallic microstructure excluding nonmetallic inclusions, if the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%, then 30≦f3=(α)≦70, 30≦f4=(β1)≦70, 0≦f5=(γ)<1, 35 <f6=(β1)×([Si]) 1 / 2 -(γ)×2+([Pb]+[Bi]) 1 / 2 ×20+([P]) 1 / 2 The β1 phase has a ×15 relationship, and grain boundaries can be observed when etched with a mixture of hydrogen peroxide and aqueous ammonia.

[0025] The free-machining copper alloy of aspect 5 of the present invention is characterized in that, as a free-machining copper alloy of any one of aspects 1 to 4 of the present invention, it is used in instruments and parts related to drinking water and sanitary equipment, valves, cocks, industrial piping parts, water meters, musical instruments, automobile parts, electrical and electronic equipment parts, machine parts, stationery, toys, sliding parts, instrument parts, precision machine parts, and medical parts.

[0026] A method for producing a free-machining copper alloy according to aspect 6 of the present invention is a method for producing a free-machining copper alloy according to any one of aspects 1 to 4 of the present invention, comprising one or more hot working steps, wherein in the final hot working step, the hot working temperature is greater than 540°C and less than 750°C, and in the cooling process after hot working, the hot-worked material is cooled at a temperature lower than 670°C and higher than 500°C, and in the cooling process, the average cooling rate in the temperature range from the cooling start temperature to 500°C is greater than 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is greater than 300°C / min.

[0027] A method for producing a free-machining copper alloy according to aspect 7 of the present invention is a method for producing a free-machining copper alloy according to any one of aspects 1 to 4 of the present invention, comprising one or more hot working steps and heat treatment steps, wherein in the final heat treatment step, the alloy is annealed at a temperature above 520°C and below 630°C for 1 minute to 5 hours, and after annealing, a cooling treatment is started at a temperature above 500°C, the average cooling rate in the temperature range from the cooling treatment start temperature to 500°C is above 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is above 300°C / min. [Effects of the Invention]

[0028] According to one aspect of the present invention, it is possible to provide a free-machining copper alloy that has excellent hot workability, good machinability, dezincification corrosion resistance, and stress corrosion cracking resistance, high strength, an excellent balance between strength and ductility, and a significantly reduced Pb content, as well as a method for producing a free-machining copper alloy. [Brief explanation of the drawing]

[0029] [Figure 1A]This is a photograph of the microstructure of the copper alloy in the embodiment, which was obtained by subjecting alloy No. S01 to process No. A2. Specifically, alloy No. S01 has the composition Zn-63.3mass%Cu-0.95mass%Si-0.069mass%P-0.063mass%Pb-0.017mass%Bi. In process No. A2, hot extrusion was performed at 630°C, and the cooling process was started at 580°C. The average cooling rate in the temperature range from 580°C to 300°C and the average cooling rate in the temperature range from 500°C to 300°C were both set to 1020°C / min. [Figure 1B] Figure 1A shows the results of a dezincification corrosion test performed on the alloy according to the ISO 6509 test method, and is a cross-sectional metallographic image including the area showing the maximum corrosion depth. [Figure 2A] This is a photograph of the microstructure of the copper alloy in the embodiment, which was obtained by subjecting alloy No. S11 to process No. E2. Specifically, alloy No. S11 has the composition Zn-62.5mass%Cu-0.96mass%Si-0.064mass%P-0.072mass%Pb. In process No. E2, a rod with a diameter of 50 mm and a length of 200 mm was obtained by hot extrusion at 550°C and an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C. The rod was heated and hot forged at 680°C with the rod placed horizontally to a thickness of 20 mm, and the cooling process was started at 565°C, with both the average cooling rate in the temperature range from 565°C to 300°C and the average cooling rate in the temperature range from 500°C to 300°C being 900°C / min. [Figure 2B] Figure 2A shows the results of a dezincification corrosion test performed on the alloy according to the ISO 6509 test method, and is a cross-sectional metallographic image including the area showing the maximum corrosion depth. [Figure 3A]This is a photograph of the microstructure of the copper alloy in the embodiment, which was obtained by subjecting alloy No. S11 to process No. E13H. Specifically, alloy No. S11 has the composition Zn-62.5mass%Cu-0.96mass%Si-0.064mass%P-0.072mass%Pb. In process No. E13H, a rod with a diameter of 50 mm and a length of 200 mm was obtained by hot extrusion at 550°C and an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C. The rod was heated and hot forged at 630°C with the rod placed horizontally to a thickness of 20 mm, and cooled at an average cooling rate of 35°C / min. Next, the cooling process was started at 455°C, and the average cooling rate in the temperature range from 455°C to 300°C was 800°C / min. The alloy in Figure 3A and the alloy in Figure 2A have different cooling start temperatures. [Figure 3B] Figure 3A shows the results of a dezincification corrosion test performed on the alloy according to the ISO 6509 test method, and is a cross-sectional metallographic image including the area showing the maximum corrosion depth. [Figure 4A] This is a photograph of the microstructure of the copper alloy in the embodiment, which was obtained by subjecting alloy No. S43 to process No. E6. Specifically, alloy No. S43 has the composition Zn-63.3mass%Cu-0.98mass%Si-0.084mass%P-0.060mass%Pb-0.28mass%Sn. In process No. E6, a casting was poured into a mold with a diameter of 55 mm, cut to a diameter of 50 mm, and then cut to a length of 200 mm. The casting was heated, and the casting was hot forged at 630°C with a thickness of 20 mm in a horizontal position. Cooling was started at 565°C, and the average cooling rate in the temperature range from 565°C to 300°C, and the average cooling rate in the temperature range from 500°C to 300°C were both set to 900°C / min. [Figure 4B] Figure 4A shows the results of a dezincification corrosion test performed on the alloy according to the ISO 6509 test method, and is a cross-sectional metallographic image including the area showing the maximum corrosion depth. [Figure 5A]This is a photograph of the microstructure of the copper alloy in the embodiment, which was obtained by subjecting alloy No. S02 to steps A34H and G1. Specifically, alloy No. S02 has the composition Zn-62.9mass%Cu-0.98mass%Si-0.071mass%P-0.071mass%Pb. In step A34H, the alloy was obtained by hot extrusion at 615°C and an average cooling rate of 18°C / min from 500°C to 300°C. In step G1, this alloy was further heated at 580°C for 30 minutes, and the cooling process was started from 560°C, with both the average cooling rate in the temperature range from 560°C to 300°C and the average cooling rate in the temperature range from 500°C to 300°C being 1800°C / min. [Figure 5B] Figure 5A shows the results of a dezincification corrosion test performed on the alloy according to the ISO 6509 test method, and is a cross-sectional metallographic image including the area showing the maximum corrosion depth. [Figure 6A] This is a photograph of the microstructure of the copper alloy in the embodiment, which was obtained by subjecting alloy No. S02 to process No. E14H. Specifically, alloy No. S02 has the composition Zn-62.9mass%Cu-0.98mass%Si-0.071mass%P-0.071mass%Pb. In process No. E14H, a rod with a diameter of 50 mm and a length of 200 mm was obtained by hot extrusion at 550°C and an average cooling rate of 20°C / min from 500°C to 300°C. The rod was heated, placed horizontally, and hot forged at 630°C to a thickness of 20 mm, and then allowed to cool naturally. The average cooling rate in the temperature range from 500°C to 300°C was 25°C / min. [Figure 6B] Figure 6A shows the results of a dezincification corrosion test performed on the alloy according to the ISO 6509 test method, and is a cross-sectional metallographic image including the area showing the maximum corrosion depth. [Modes for carrying out the invention]

[0030] The following describes a free-machining copper alloy and a method for manufacturing the free-machining copper alloy according to an embodiment of the present invention. Examples of products include drinking water, sanitary equipment and fixtures, musical instruments, tableware, electrical, home appliance and electronic equipment components, automobile parts, machine parts, stationery, precision machine parts, medical parts, and equipment and parts related to liquids and gases such as industrial water, wastewater, and hydrogen. Specific examples of parts include water taps, mixing taps, shut-off valves, valves, cocks, fittings, water meters, gears, sensors, nuts, and screws.

[0031] In this specification, element symbols enclosed in parentheses, such as [Zn], indicate the mass percentage of that element. In this embodiment, using this method of indicating the content, the compositional relation formulas f1, f2, and f0 are defined as follows. The compositional relationship is f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P], and when Sn is present in amounts exceeding 0.05 mass%, f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] If Bi is included, the compositional relation is f2 = [Pb] + [Bi]. If Sn is included, then f0 = [Sn] / [Si] That is the case.

[0032] Furthermore, in this embodiment, in the constituent phases of the metallic structure excluding nonmetallic inclusions, the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, the area ratio of the unmodified β phase is (β)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%. The area ratio of each phase is also called the amount of each phase, the proportion of each phase, or the percentage occupied by each phase. In this embodiment, several organizational relational expressions are defined as follows. Organizational relationship formula f3 = (α) Organizational relationship f4 = (β1) Organizational relationship formula f4A=(β) Organizational relationship formula f5 = (γ) The tissue-composition relationship is f6 = (β1) × ([Si]). 1 / 2 -(γ)×2+([Pb]+[Bi]) 1 / 2 ×20+([P]) 1 / 2 ×15

[0033] The free-machining copper alloy according to the first embodiment of the present invention contains Cu in an amount greater than 60.5 mass% and less than 65.0 mass%; Si in an amount greater than 0.50 mass% and less than 1.20 mass%; Pb in an amount greater than 0.002 mass% and less than 0.20 mass%; P in an amount greater than 0.01 mass% and less than 0.18 mass%; Bi in an amount greater than 0.0001 mass% and less than 0.20 mass% as an optional element; the remainder consists of Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co and Cr among the unavoidable impurities is less than 0.40 mass%; the content of Al is less than 0.30 mass%; and the content of Cu is [Cu] When the mass%, Si content is [Si]mass%, Pb content is [Pb]mass%, P content is [P]mass%, and Bi content is [Bi]mass%, the relationship 57.5≦f1=[Cu]-4.6×[Si]+0.5×([Pb]+[Bi])-[P]≦60.5 holds, and when Bi is included, the relationship 0.003≦f2=[Pb]+[Bi]<0.20 further holds, and in the constituent phases of the metallic microstructure excluding nonmetallic inclusions, when the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%, the relationship 20≦f3=(α)<75, 25 <f4=(β1)≦80、0≦f5=(γ)<4、27<f6=(β1)×([Si]) 1 / 2 -(γ)×2+([Pb]+[Bi]) 1 / 2 ×20+([P]) 1 / 2 The β1 phase has a ×15 relationship, and is characterized in that grain boundaries can be observed when etched with a mixture of hydrogen peroxide and aqueous ammonia.

[0034] The free-machining copper alloy according to the second embodiment of the present invention contains 61.2 mass% to 64.8 mass% of Cu, 0.65 mass% to 1.10 mass% of Si, 0.003 mass% to less than 0.10 mass% of Pb, and 0.03 mass% to 0.15 mass% of P, and as an optional element, it contains 0.001 mass% to less than 0.10 mass% of Bi, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co and Cr among the unavoidable impurities is less than 0.30 mass%, and the content of Al is less than 0.15 mass%, and the Cu content is [Cu] mass%, and the Si content is When the content of Si is [mass%], Pb is [mass%], P is [mass%], and Bi is [mass%], the relationship 58.0≦f1=[Cu]-4.6×[Si]+0.5×([Pb]+[Bi])-[P]≦60.2 holds, and when Bi is included, the relationship 0.004≦f2=[Pb]+[Bi]<0.10 further holds, and in the constituent phases of the metallic microstructure excluding nonmetallic inclusions, when the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%, then 30≦f3=(α)≦70, 30≦f4=(β1)≦70, 0≦f5=(γ)<1, 35 <f6=(β1)×([Si]) 1 / 2 -(γ)×2+([Pb]+[Bi]) 1 / 2 ×20+([P]) 1 / 2 The β1 phase has a ×15 relationship, and is characterized in that grain boundaries can be observed when etched with a mixture of hydrogen peroxide and aqueous ammonia.

[0035] The free-machining copper alloy according to the third embodiment of the present invention contains Cu in an amount greater than 60.5 mass% and less than 65.0 mass%; Si in an amount greater than 0.50 mass% and less than 1.20 mass%; Pb in an amount greater than 0.002 mass% and less than 0.20 mass%; P in an amount greater than 0.01 mass% and less than 0.18 mass%; Sn in an amount greater than 0.05 mass% and less than 0.90 mass%; Bi in an amount greater than 0.0001 mass% and less than 0.20 mass% as an optional element; the remainder consists of Zn and unavoidable impurities; of the unavoidable impurities, the total content of Fe, Mn, Co and Cr is less than 0.40 mass%; the content of Al is less than 0.30 mass%; the Cu content is [Cu] mass%; and the Si content is When the amount is [Si]mass%, the P content is [Pb]mass%, the P content is [P]mass%, the Bi content is [Bi]mass%, and the Sn content is [Sn]mass%, the relationship 57.5≦f1=[Cu]-4.6×[Si]+0.5×([Pb]+[Bi])-[P]-[Sn]≦60.5 holds, and the relationship f0=[Sn] / [Si]<1 also holds, and when Bi is included, the relationship 0.003≦f2=[Pb]+[Bi]<0.20 holds, and in the constituent phases of the metallic microstructure excluding nonmetallic inclusions, when the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%, the relationship 20≦f3=(α)<75, 25 <f4=(β1)≦80、0≦f5=(γ)<4、27<f6=(β1)×([Si]) 1 / 2 -(γ)×2+([Pb]+[Bi]) 1 / 2 ×20+([P]) 1 / 2 The β1 phase has a ×15 relationship, and is characterized in that grain boundaries can be observed when etched with a mixture of hydrogen peroxide and aqueous ammonia.

[0036] The free-machining copper alloy according to the fourth embodiment of the present invention contains 61.2 mass% to 64.8 mass% of Cu, 0.65 mass% to 1.10 mass% of Si, 0.003 mass% to less than 0.10 mass% of Pb, 0.03 mass% to 0.15 mass% of P, and 0.10 mass% to less than 0.50 mass% of Sn, and as an optional element, it contains 0.001 mass% to less than 0.10 mass% of Bi, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co, and Cr among the unavoidable impurities is less than 0.30 mass%, and the content of Al is less than 0.15 mass%, with the Cu content being [Cu] mass% and the Si content being [Si] mass%. When the Pb content is [Pb]mass%, the P content is [P]mass%, the Bi content is [Bi]mass%, and the Sn content is [Sn]mass%, the relationship 58.0≦f1=[Cu]-4.6×[Si]+0.5×([Pb]+[Bi])-[P]-[Sn]≦60.2 holds, and the relationship f0=[Sn] / [Si]<0.6 also holds, B If i is included, the relationship 0.004≦f2=[Pb]+[Bi]<0.10 holds, and in the constituent phases of the metallic microstructure excluding nonmetallic inclusions, if the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%, then 30≦f3=(α)≦70, 30≦f4=(β1)≦70, 0≦f5=(γ)<1, 35 <f6=(β1)×([Si]) 1 / 2 -(γ)×2+([Pb]+[Bi]) 1 / 2 ×20+([P]) 1 / 2 The β1 phase has a ×15 relationship, and is characterized in that grain boundaries can be observed when etched with a mixture of hydrogen peroxide and aqueous ammonia.

[0037] The following explains the reasons for defining the component composition, compositional relationships f0, f1, f2, microstructure relationships f3, f4, f5, microstructure-composition relationship f6, and metal structure as described above.

[0038] <Component composition> (Cu) Cu is the main element of the free-machining copper alloy of this embodiment, and in order to overcome the problems of the present invention, it is necessary to contain at least 60.5 mass% of Cu. If the Cu content is 60.5 mass% or less, the proportion of the β1 phase will exceed 80%, depending on the content of Si, Zn, P, Pb, Bi, Sn and the manufacturing process, resulting in low ductility. In addition, the modification of the β phase will not be sufficiently carried out, resulting in poor resistance to dezincification corrosion, resistance to stress corrosion cracking, and machinability. Therefore, the lower limit of the Cu content is greater than 60.5 mass%, preferably 61.2 mass% or more, more preferably 61.5 mass% or more, and even more preferably 62.0 mass% or more. On the other hand, when the Cu content is 65.0 mass% or more, the proportion of the β1 phase decreases, depending on the content of Si, Zn, P, Pb, Bi, and Sn, as well as the manufacturing process. As a result, excellent machinability is not obtained, and the strength also decreases. Therefore, the Cu content is less than 65.0 mass%, preferably 64.8 mass% or less, and more preferably 64.5 mass% or less.

[0039] (Si) Si is the main element of the free-machining copper alloy in this embodiment, and Si contributes to the formation of metallic phases such as the κ phase, γ phase, μ phase, β phase, β1 phase, and ζ phase. By including amounts of Cu, Zn, Si, and P within the aforementioned ranges and adding a process, the β phase is modified, and the β1 phase is generated after a predetermined cooling treatment described later. The β1 phase (modified β phase) improves machinability and significantly improves the dezincification corrosion resistance and stress corrosion cracking resistance, which were drawbacks of the conventional β phase. A typical composition of the modified β1 phase is approximately 61 mass% Cu, 1.2 mass% Si, 37.5 mass% Zn, and 0.1 mass% P.

[0040] On the other hand, the α phase typically has a composition of approximately 66 mass% Cu, 0.7 mass% Si, and 33 mass% Zn. Within the composition range of this embodiment, the machinability of the α phase is slightly improved by the inclusion of Si, and the dezincification corrosion resistance and stress corrosion cracking resistance of the α phase are also improved by the inclusion of Si. Furthermore, the modified β1 phase and the improved α phase result in particularly enhanced dezincification corrosion resistance and stress corrosion cracking resistance as an alloy. Furthermore, a certain amount of α phase is necessary for the modification of the β phase; for example, if there is no α phase at all, the β phase will not be modified. In order to improve the dezincification corrosion resistance, stress corrosion cracking resistance, ductility, and machinability of the alloy, the α phase must be present in an area ratio of 20% or more, preferably 30% or more.

[0041] Si is an essential element for modifying the β phase to the β1 phase. The higher the amount of Si, the more the β phase is modified, resulting in a β1 phase with superior properties. To modify the β phase and create a more modified β1 phase, the Si content must exceed 0.50 mass%. The Si content is preferably 0.60 mass% or more, more preferably 0.65 mass% or more, and even more preferably 0.75 mass% or more. On the other hand, when the amount of Si reaches a predetermined level, the modification of the β1 phase saturates. Furthermore, if the Si content is too high, the conductivity decreases. In some cases, the γ phase appears, worsening dezincification corrosion resistance, stress corrosion cracking resistance, and machinability. Incidentally, Patent Documents 2 to 8 state that the γ phase improves machinability. However, in this application, which mainly consists of the β1 phase and the α phase, the presence of the γ phase actually worsens machinability, reduces the ductility of the alloy, and worsens dezincification corrosion resistance and stress corrosion cracking resistance. For this reason, it is preferable to limit the amount of Si and the amount of the γ phase. From these considerations, the amount of Si is less than 1.20 mass%, preferably 1.10 mass% or less. If conductivity is a priority, the Si content is 1.0 mass% or less.

[0042] (Zn) Zn, along with Cu and Si, is a major constituent element of the free-machining copper alloy of this embodiment, and is an element necessary to improve machinability, strength, high-temperature properties, and castability. Although Zn is described as the remainder, if we were to specify, the Zn content is less than approximately 38.5 mass%, preferably less than 38.0 mass%, more than approximately 32.0 mass%, and preferably more than 33.0 mass%.

[0043] (P) P, like Si, is an essential element for modifying the β phase to the β1 phase. During hot working, P dissolves in the β phase. During cooling after hot working, if the cooling process is started at a temperature above 500°C and below 670°C, and the temperature range from the cooling start temperature to 500°C, and the temperature range from 500°C to 300°C are cooled at a cooling rate exceeding 300°C / min, the β phase is modified to the β1 phase, and the β1 phase is obtained. This modification from the β phase to the β1 phase significantly improves the dezincification corrosion resistance and stress corrosion cracking resistance, which were problems with the conventional β phase. At the same time, the presence of the β1 phase reduces cutting resistance and improves chip fragmentation during cutting. Incidentally, while compounds containing P reduce cutting resistance and improve chip fragmentation, if the cooling process is started at a temperature above approximately 550°C or 530°C at a cooling rate exceeding 300°C / min, compounds containing P are absent or present in small amounts. From the perspective of improving machinability, the effect of modifying the β phase to the β1 phase outweighs the effect of the presence of P-containing compounds in the β phase. In particular, for high-speed cutting exceeding approximately 100 m / min, modifying the β phase to the β1 phase yields better machinability than the presence of P-containing compounds in the β phase. Furthermore, the inclusion of P improves the dezincification corrosion resistance and stress corrosion cracking resistance of the α phase, leading to a significant improvement in the dezincification corrosion resistance and stress corrosion cracking resistance of the alloy composed of the β1 and α phases.

[0044] In order to modify the β phase to the β1 phase, the lower limit of the P content must be at least 0.01 mass%. Considering the superior modification to the β1 phase, as well as the amount of unavoidable impurities described later, the cooling start temperature, and the cooling rate after the start of the cooling process, the P content is preferably 0.03 mass% or more, and more preferably 0.04 mass% or more. Furthermore, P readily forms compounds with elements such as Zn, Si, Mn, Fe, Cr, Co, and Al. When P forms compounds and the amount of P dissolved in the β phase of the alloy after hot working decreases, the modification from the β phase to the β1 phase is inhibited. The formation of compounds between P and Zn and Si, the main elements of the alloy in this application, begins at approximately 550°C, and the amount of P compounds increases as the cooling rate slows down. The formation of compounds between P and the unavoidable impurities Mn, Fe, Cr, and Co begins at temperatures above approximately 550°C, and the formation of P compounds is further promoted as their amounts increase. In other words, the presence of Mn, Fe, Cr, and Co hinders the modification to the β1 phase, resulting in increased cutting resistance of the alloy, poor chip fragmentation, and reduced resistance to dezincification corrosion and stress corrosion cracking. Therefore, the total content of Fe, Mn, Co, and Cr must be kept below 0.40 mass%, preferably below 0.30 mass%.

[0045] The formation of P and Zn,Si compounds is beneficial to machinability, but the modification from the β phase to the β1 phase and the presence of P and Zn,Si compounds are contradictory. As mentioned above, for the formation of the β1 phase, the cooling process must be started at a temperature between 500°C and 670°C after hot working, and the cooling rate must be rapid in the temperature range from the cooling start temperature to 300°C. In contrast, for sufficient formation of P and Zn,Si compounds, slow cooling in the temperature range of approximately 530°C to 450°C is necessary. Therefore, when a large amount of P compounds are formed, the formation of the β1 phase is slightly insufficient, i.e., the modification of the β phase is slightly insufficient. Alternatively, the β1 phase may not be present at all.

[0046] On the other hand, even if P is included in an amount of 0.18 mass% or more, the effect of forming the β1 phase is already saturated, and conversely, the influence of P dissolved in the β1 phase reduces ductility and conductivity. For this reason, the P content is less than 0.18 mass%, preferably 0.15 mass% or less, and more preferably 0.12 mass% or less.

[0047] (Pb) In this embodiment, good machinability is obtained as an alloy by the β1 phase containing Si and P, but the machinability is further improved by the inclusion of a small amount of Pb. In the composition of this embodiment, about 0.001 mass% of Pb is solid-dissolved in the matrix, and any amount of Pb exceeding that exists as fine Pb particles with a diameter of about 0.1 to about 2 μm. Generally, it is considered that the inclusion of about 0.1 mass% of Pb contributes almost nothing to improving machinability. For example, the ASM Specialty Handbook, Copper and Copper Alloys, first edition, published August 2001, p. 267, Fig. 6, shows the relationship between Pb content and machinability, with the machinability of a Cu-Zn-Pb alloy containing 62-65 mass% Cu, approximately 3.2 mass% Pb, and the remainder being Zn, set at 100%. It shows that a Pb content of 0.1 mass% only improves machinability by a mere 5%, from approximately 25% to approximately 30% in terms of machinability index. On the other hand, in this application, Pb has a significant effect on machinability even in trace amounts, exhibiting its effect at a content of 0.002 mass% or more. The Pb content is preferably 0.003 mass% or more, and more preferably 0.01 mass% or more. Furthermore, when cutting conditions become more severe, such as when the cutting speed is high, the feed rate is high, the depth of cut is high, or the drill hole diameter is large, the Pb content is preferably 0.03 mass% or more. The machinability of the β1 phase is greatly improved, and the inclusion of a small amount of Pb significantly enhances the machinability of the alloy. It is a well-known fact that lead (Pb) improves the machinability of copper alloys, and for this purpose, approximately 3 mass% of Pb is required in Cu-Zn binary alloys, as exemplified by the free-cutting brass rod C3604. In this embodiment, an alloy with excellent machinability is completed by incorporating a β1 phase containing Si and P, and a small amount of Pb particles, or Pb and Bi particles as described later, into the metal structure. On the other hand, since Pb is harmful to the human body, the upper limit of Pb is set to less than 0.20 mass%. Furthermore, the Pb content is preferably less than 0.10 mass%, and optimally, it is 0.08 mass% or less, considering the impact on the human body and the environment.

[0048] (Bi) Similar to Pb, Bi is dissolved in the matrix in an amount of approximately 0.0001 mass%, and any amount of Bi exceeding this exists as particles with a diameter of approximately 0.1 to 2 μm. When both Pb and Bi are added, they mostly exist as a mixture of Pb and Bi particles with a diameter of approximately 0.1 to 2 μm. In this embodiment, it was found that by including Bi together with Pb in the presence of the β1 phase, machinability equivalent to or better than that obtained when Pb and Bi were included individually was obtained. Although the function of Bi in improving machinability was considered inferior to that of Pb, in this embodiment it was found to exhibit the same effect as Pb, and in some cases even exceed that of Pb. Incidentally, Bi worsens the stress corrosion cracking resistance of brass, but when it exists as a mixture of Pb and Bi particles, or when the amount of Bi is small, the effect on stress corrosion cracking is almost eliminated.

[0049] When Bi is included, at least 0.0001 mass% of Bi is required for the alloy to have good machinability. The Bi content is preferably 0.001 mass% or more, more preferably 0.002 mass% or more. The effects of Bi on the human body are currently unknown, but the amount of Bi should be less than 0.20 mass%, preferably less than 0.10 mass%, and even more preferably 0.08 mass% or less. In this embodiment, as mentioned above, Bi can adequately substitute for Pb, the effects of Bi on the human body are unknown, Bi is one of the rare metals and has an impact on the environment, and on the other hand, it is included in the raw materials as an unavoidable impurity, so Bi can be included as an optional element or not. Furthermore, considering the effects on the human body and the environment, and the effects on stress corrosion cracking, the total content of Pb and Bi (compositional relationship formula f2 described later) should be less than 0.20 mass%, preferably less than 0.10 mass%. Thus, in this embodiment, the amount of Pb, which is harmful to the human body, is limited to less than 0.20 mass%, including Bi in some cases, while also aiming for excellent machinability.

[0050] (Sn) By solid-solving in the β1 phase, Sn further improves the dezincification corrosion resistance of the β1 phase, thereby enhancing the dezincification corrosion resistance of the alloy. When Sn is included, to obtain this effect, the Sn content must exceed 0.05 mass%, preferably 0.10 mass% or more. Incidentally, Sn is originally distributed more in the β and β1 phases than in the α phase, and even a small amount of Sn improves dezincification corrosion resistance. However, as the Sn concentration increases, the γ phase is more likely to form, and ductility decreases. The formation of the γ phase not only leads to a decrease in ductility of the alloy, but also reduces machinability and worsens dezincification corrosion resistance. Although it is related to the amount of Si, the Sn content should be kept below 0.90 mass%, preferably below 0.70 mass%, and more preferably below 0.50 mass%.

[0051] As mentioned above, Sn is largely distributed in the β phase and β1 phase. As a result, it was found that a high Sn content can cause problems in the modification of the β phase, which contains Si and P. Specifically, it was found that when the amount of Sn exceeds the amount of Si, the modification of the β phase becomes insufficient, and the effect of improving dezincification corrosion resistance due to Sn content is offset. As will be discussed later, the amount of Si must exceed the amount of Sn.

[0052] (Unavoidable impurities, especially Fe, Mn, Co, and Cr / Al) Examples of unavoidable impurities in this embodiment include Mn, Fe, Al, Ni, Mg, Se, Te, Sn, Bi, Co, Ca, Zr, Cr, Ti, In, W, Mo, B, Ag, and rare earth elements. Traditionally, free-cutting copper alloys, particularly free-cutting brass containing approximately 30 mass% or more of Zn, do not primarily use high-quality raw materials such as electrolytic copper or electrolytic zinc, but rather recycled copper alloys as their main raw material. In the downstream processes (processing processes) of this field, most components and parts are machined, resulting in a large amount of discarded copper alloy, at a ratio of 40 to 80 parts by mass per 100 parts by mass of material. Examples include chips, scraps, burrs, sprues, and products containing manufacturing defects. These discarded copper alloys become the main raw material. If cutting chips and scraps are not properly separated, Pb, Fe, Mn, Si, Se, Te, Sn, P, Sb, As, Bi, Ca, Al, Zr, Ni, and rare earth elements may be mixed in as raw materials from free-cutting brass with added Pb, free-cutting copper alloys that do not contain Pb but have Bi added, or special brass alloys containing Si, Mn, Fe, Al, and other copper alloys. Cutting chips also contain Fe, W, Co, Mo, etc., which are mixed in from the tools. Recycled waste products include plated products, so Ni, Cr, and Sn are mixed in. In addition, pure copper scrap used as a substitute for electrolytic copper contains Mg, Sn, Fe, Cr, Ti, Co, In, Ni, Se, and Te. Brass scrap used as a substitute for electrolytic copper or electrolytic zinc is often plated with Sn, so Sn is mixed in.

[0053] From the perspective of resource reuse and cost considerations, scrap containing these elements is used as raw material, at least to the extent that it does not adversely affect its properties. In addition, according to the JIS standard (JIS H 3250), free-cutting brass rod C3604 with added lead contains approximately 3 mass% of the essential element lead, and further impurities such as Fe (up to 0.5 mass%) and Fe+Sn (total amount of Fe and Sn) (up to 1.0 mass%) are permitted. In practice, high concentrations of Fe and Sn are sometimes found in free-cutting brass rods.

[0054] Fe, Mn, Co, and Cr dissolve in the α and β phases of the Cu-Zn alloy up to certain concentrations. However, if Si or P is present, these elements readily combine with Si and P, potentially consuming the Si and P necessary for modifying the β phase. Fe, Mn, Co, and Cr combined with Si form Fe-Si compounds, Mn-Si compounds, Co-Si compounds, Cr-Si compounds, etc., within the metal structure. Similarly, Fe, Mn, Co, and Cr combined with P form Fe-P compounds, Mn-P compounds, Co-P compounds, Cr-P compounds, etc., within the metal structure. These intermetallic compounds are extremely hard, increasing cutting resistance and shortening tool life. Therefore, the amounts of Fe, Mn, Co, and Cr must be limited. Preferably, the content of each is less than 0.30 mass%, more preferably less than 0.20 mass%, and even more preferably 0.15 mass% or less. In particular, the total content of Fe, Mn, Co, and Cr must be less than 0.40 mass%, preferably less than 0.30 mass%, more preferably less than 0.25 mass%, and even more preferably 0.20 mass% or less.

[0055] On the other hand, Al mixed in from special brass rods, brass castings, etc., affects the modification of the β phase if the content is high, and since Al forms compounds with P or Si, it needs to be limited. In the alloy of this embodiment, the Al content needs to be less than 0.30 mass%, preferably less than 0.15 mass%, and more preferably 0.10 mass% or less.

[0056] Among other major unavoidable impurity elements, Ni is empirically often found to be mixed in from scrap and other sources. Although Ni has a relatively small impact on mechanical properties such as machinability, it needs to be limited in consideration of its effects on the human body. The Ni content is preferably less than 0.20 mass%, and more preferably less than 0.10 mass%. As for Ag, Ag is generally considered to be Cu and has almost no impact on various properties, so there is no need to particularly limit it, but the Ag content is preferably less than 0.05 mass%. Te and Se are elements that are free-machining themselves and, although rare, may be mixed in in large quantities. Considering the impact on ductility and impact properties, the content of Te and Se, respectively, is preferably less than 0.10 mass%, more preferably less than 0.05 mass%, and even more preferably 0.02 mass% or less. Furthermore, corrosion-resistant brass contains As and Sb to improve the corrosion resistance of brass. However, considering ductility and effects on the human body, the content of each of As and Sb is preferably less than 0.05 mass%, and more preferably 0.02 mass% or less. The content of other elements such as Mg, Ca, Zr, Ti, In, W, Mo, B, and rare earth elements is preferably less than 0.05 mass%, more preferably less than 0.03 mass%, and even more preferably 0.02 mass% or less. The content of rare earth elements is the total amount of one or more of the following: Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, and Lu. In addition to Fe, Mn, Co, Cr, and Al, the total amount of unavoidable impurities is preferably less than 0.70 mass%, and more preferably less than 0.50 mass%.

[0057] (Compositional relation f1) The compositional relationship f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn]. When the Sn content is 0.05 mass% or less, [Sn] in f1 is 0, and when Bi is not present, [Bi] in f1 is 0. When Sn is 0.05 mass% or less, its effect on the compositional relationship f1 is small, so it is not specified in the compositional relationship f1. f1 is an equation that represents the relationship between composition and metal structure. Even if the amounts of each element are within the range specified above, if this compositional relationship equation f1 is not satisfied, the properties targeted by this embodiment cannot be satisfied. If the compositional relationship equation f1 is less than 57.5, the proportion of β phase and β1 phase increases, the modification of the β phase becomes insufficient, and ductility, dezincification resistance, and stress corrosion cracking resistance deteriorate. Therefore, the lower limit of the composition relation f1 is 57.5 or higher, preferably 58.0 or higher, and more preferably 58.2 or higher. As the composition relation f1 falls within a more preferable range, the proportion of the α phase increases, the β phase is sufficiently modified, and excellent machinability is maintained, along with good resistance to dezincification corrosion and stress corrosion cracking, resulting in good ductility and cold workability. On the other hand, the upper limit of the compositional relationship f1 affects the proportion of the β phase and the β1 phase. If the compositional relationship f1 is greater than 60.5, the proportion of the β1 phase decreases, resulting in poor machinability and lower strength. Therefore, f1 is 60.5 or less, preferably 60.2 or less, and more preferably 60.0 or less.

[0058] (Compositional relation f2) Bi is an optional element that can substitute for the function of Pb, but if Bi is included, the relationship 0.003 ≤ f2 = [Pb] + [Bi] < 0.20 must be satisfied. That is, at this stage, the effects of Bi on the human body are unknown, and considering that Bi is one of the rare metals and has an impact on the environment, it is necessary to limit the total amount of Pb and Bi. If Bi is included, the total amount of Pb and Bi should be less than the upper limit of the amount of Pb, which is 0.20 mass%, and preferably less than 0.10 mass%. On the other hand, in order to obtain good machinability, if Bi is included, the total amount of Pb and Bi must be 0.003 mass% or more, preferably 0.004 mass% or more, and more preferably 0.005 mass% or more.

[0059] (Composition relation f0) Sn is included to further improve dezincification corrosion resistance. Sn is present in large quantities in the β phase, and the higher the amount of Sn, the slower the progression of dezincification corrosion in the β1 phase. On the other hand, if the amount of Sn is too high compared to the amount of Si, it may hinder the modification of the β phase and impair the dezincification corrosion resistance of the β1 phase. In other words, if f0 = [Sn] / [Si], then in order to achieve an effect commensurate with the amount of Sn contained while considering other properties, f0 must be less than 1. Preferably, f0 is less than 0.8, more preferably less than 0.6, and even more preferably less than 0.5.

[0060] The free-machining copper alloy of this embodiment contains a large amount of the conventional β phase, or β1 phase in this application, while exhibiting good resistance to dezincification corrosion, stress corrosion cracking, and mechanical properties. Furthermore, it possesses machinability that requires a certain degree of brittleness, which reduces cutting resistance and finely breaks up chips, and ductility, which are completely contradictory properties. In order to achieve these good corrosion resistance, machinability, and mechanical properties, it is possible to provide an alloy that is better suited to the purpose and application by discussing in detail not only the composition of each component, but also the compositional relationship formulas f0, f1, f2, and the microstructure relationship formulas f3 to f5 and the microstructure-composition relationship formula f6, which will be described later. Furthermore, Fe, Mn, Co, Cr, Al, and other unavoidable impurities as defined separately are not specified in the compositional relationship f1 because their influence on the compositional relationship f1 is small as long as they remain within the range of unavoidable impurities.

[0061] (Comparison with patent documents) Tables 1 to 4 show a comparison of the compositions of the Cu-Zn-Si alloys described in the aforementioned Patent Documents 1 to 15 and the alloy of this embodiment.

[0062] [Table 1]

[0063] [Table 2]

[0064] [Table 3]

[0065] [Table 4]

[0066] This embodiment differs from Patent Document 8 in the content of the main elements Si and Cu, this embodiment differs from Patent Document 9 in the content of the main element Si, and this embodiment differs from Patent Document 15 in the content of the main element Cu. Patent documents 1, 13, and 15 state that Pb is not included, indicating that the Pb content differs. In Patent Documents 8, 10, 11, and 12, the β phase in the metal structure is significantly limited from the viewpoint of machinability, resistance to dezincification corrosion, and corrosion resistance. The β1 phase of this application is different from the β phase in Patent Documents 8, 10, 11, and 12, but the β phase is set to 5% or less, 25% or less, 15% or less, and 0.9% or less, respectively.

[0067] Patent document 10 relates to a near-net-shape tubular hot-forged product and specifies the use of a tubular material. Patent documents 1 and 9 state that heat treatment is performed at temperatures of 350-550°C or 400-600°C in order to reduce or break down the β phase. Patent Document 9 describes a material containing 0.2 mass% or more of Sn, with Sn and Si added to improve the dezincification corrosion resistance of the β phase. It requires hot extrusion at temperatures above 700°C to improve machinability and heat treatment at 400-600°C to improve corrosion resistance. The proportion of the β phase is generally 5-20%, and the Si content is 0.01-0.50 mass%, which can be controlled to 0.2 mass% or less.

[0068] Patent documents 12, 13, and 14 state that aluminum is essential for improving discoloration resistance, castability, and dezincification corrosion resistance. Patent Document 14 states that in order to improve dezincification corrosion resistance, Sn and Al are included in amounts of at least 0.1 mass% each, and that in order to obtain excellent machinability, a large amount of Pb and Bi is required. Patent Document 15 describes a corrosion-resistant copper alloy casting that does not contain Pb, requires a γ phase, and contains 65 mass% or more of Cu and Si, along with trace amounts of Al, Sb, Sn, Mn, Ni, B, etc., thereby possessing good mechanical properties and castability.

[0069] Patent documents 2 to 7 all describe a cooling process after hot working in which the average cooling rate in the temperature range from approximately 530°C to approximately 450°C is between approximately 0.1°C / min and approximately 70°C / min, and that the resulting P compound must be present in the metal structure. Furthermore, they state that the γ phase is effective for machinability, but do not mention the modification of the β phase at all, and remain silent on dezincification corrosion resistance and stress corrosion cracking resistance, with no data disclosed. In addition, with the exception of Patent Document 7, no data on cutting resistance under high-speed cutting conditions is disclosed. In this application, the cooling process after hot working is initiated at a temperature lower than 670°C and higher than 500°C, and the average cooling rate in the temperature range from the cooling start temperature to 500°C, and the average cooling rate in the temperature range from 500°C to 300°C, exceeds 300°C / min, which is fundamentally contrary to the cooling described in Patent Documents 2 to 7.

[0070] The most significant and clear difference between this application and Patent Documents 2-7 is that none of Patent Documents 2-7 mention the modification of the β phase, nor the modified β phase, i.e., the β1 phase. The β1 phase is formed under the following conditions: During hot working, a β phase is formed in which a certain amount of Si and P are dissolved. Then, a cooling process is started at a temperature higher than 500°C, and during the cooling process, the average cooling rate in the temperature range from the cooling start temperature to 500°C is greater than 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is greater than 300°C / min. The β1 phase is formed by the above. When etching is performed with a mixture of hydrogen peroxide and ammonia water, if it is the β1 phase, grain boundaries can be observed within the β1 phase, but if it remains the unmodified β phase, even if it contains a certain amount of Si and P, no grain boundaries can be observed within the β phase. Patent documents 2 to 7 disclose metal structures etched with a mixture of hydrogen peroxide and ammonia water, but in all cases, no grain boundaries are observed within the β phase. Patent documents 2 to 7 do not define the starting temperature for the cooling process after hot working, but even if the starting temperature for the cooling process is below 530°C, the average cooling rate from the starting temperature to 500°C is between approximately 0.1°C / min and approximately 70°C / min, so the β1 phase (modified β phase) is not observed.

[0071] Unlike conventional β phases, the β1 phase exhibits significantly improved resistance to dezincification corrosion and stress corrosion cracking, and further improved machinability, surpassing that of the presence of P compounds. However, for this to be achieved, the β1 phase must be present in the metal structure at an area ratio of more than 25%. Furthermore, while this application states that the presence of the γ phase impairs machinability, Patent Documents 2 to 7 state that the γ phase is beneficial for machinability.

[0072] <Metal structure> Cu-Zn-Si alloys contain more than 10 different phases, resulting in complex phase transitions. Therefore, the desired properties cannot always be obtained solely from the composition range and elemental relationships. Ultimately, the desired properties can be achieved by identifying and determining the types of phases present in the metal structure and their respective area ratios. For this reason, the microstructure relationships are defined as follows. 20≦f3=(α)<75, 25<f4=(β1)≦80, wherein the β1 phase is a modified β phase. 0≦f5=(γ)<4

[0073] (β1 phase) The metal structure of the present application consists of an α phase, a β1 phase, and optionally a small amount or 0% of a γ phase, excluding non-metallic inclusions. The difference between the β phase and the β1 phase is that when the β1 phase is etched with a mixed solution of hydrogen peroxide and aqueous ammonia, a grain boundary pattern, that is, crystal grain boundaries, are observed in the β1 phase, whereas in the case of the β phase, no crystal grain boundaries are observed in the β phase. In this embodiment, the β1 phase is obtained by, during hot working, solid-solving a certain amount or more of Si and P into the β phase at a high temperature, maintaining a temperature of 500-670°C, and then performing a cooling treatment from that temperature range (both the average cooling rate in the temperature range from the cooling treatment start temperature to 500°C and the average cooling rate in the temperature range from 500°C to 300°C exceeding 300°C / min, and continuing to cool to near room temperature), thereby bringing the metallic microstructure at 500-670°C down to room temperature. In detail, the cooling rate is increased in the temperature range from the high-temperature cooling start temperature to 300°C to rapidly cool down to room temperature of 100°C or less. This brings the metallic microstructure at high temperatures down to room temperature. As a result, the β1 phase is obtained. The β1 phase cannot be obtained by performing the above cooling treatment on the β phase of a Cu-Zn alloy that does not contain both the predetermined amounts of Si and P. Similarly, the β1 phase cannot be obtained by cooling an alloy containing Si and P at a temperature lower than 500°C, for example, from 450°C, at a cooling rate exceeding 300°C / min. Furthermore, a certain amount or more of α phase is required for β phase modification; if α phase is absent or insufficient, the β1 phase cannot be obtained. The degree of β phase modification is also influenced by the amounts of Si and P, the amount of unavoidable impurities, the starting temperature of the cooling process, the average cooling rate in the temperature range from the start of the cooling process to 500°C, and the average cooling rate in the temperature range from 500°C to 300°C. Improving the degree of β phase modification, i.e., a more modified β1 phase, results in a material with better machinability, dezincification resistance, and stress corrosion cracking resistance. Depending on the amount of unavoidable impurities such as Fe, the degree of β-phase modification begins to saturate when the amount of Si is approximately 1 mass% and the amount of P is approximately 0.1 mass%. If the amounts of these are too high, it may cause adverse effects such as a decrease in conductivity and ductility, and the appearance of the γ-phase. The β1 phase (modified β phase) overcomes the drawbacks and major challenges of the β phase in Cu-Zn alloys, namely the alloy's resistance to dezincification corrosion and stress corrosion cracking. Specifically, to give an example, the corrosion progression of dezincification corrosion can be reduced by approximately 60% or more, and similarly, the crack progression of stress corrosion cracking can be delayed by approximately 50% or more. As described in the patent documents, dezincification corrosion of Cu-Zn alloys containing the β phase is a major problem, and since dezincification corrosion occurs along the β phase, the amount of β phase is limited to 25% or less or 20% or less, and further heat treatment at 350-550°C is applied to reduce the amount of β phase and fragment the β phase. In terms of strength, it largely depends on the area ratio of the constituent phases, but the modified β phase, or β1 phase, has higher strength and better ductility than the β phase, resulting in an alloy with a good strength-ductility balance.

[0074] (β1 phase, tissue relationship f4) In the Cu-Zn-Si-P-Pb alloy, a free-machining copper alloy of this embodiment, in order to achieve good machinability while minimizing the Pb content, the area ratio of the β1 phase must be at least 25%. Furthermore, in order to improve machinability and strength, the area ratio of the β1 phase is preferably 30% or more, and more preferably 33% or more. On the other hand, if the amount of β1 phase is too high, for example 95%, the β phase will not be modified. Since the modification of the β phase occurs in the presence of the α phase, a certain amount of α phase is necessary. Furthermore, regarding dezincification corrosion and stress corrosion cracking, although the β1 phase significantly slows the progression of dezincification corrosion and stress corrosion cracking compared to the β phase, it is still inferior to the α phase in terms of resistance to dezincification corrosion and stress corrosion cracking. Specifically, regarding dezincification corrosion, when a dezincification corrosion test is performed according to ISO 6509, if the metal structure consists of α phase and β phase (unmodified β phase), the β phase is selectively dezincified, and the depth of this dezincification corrosion reaches approximately 500 μm. If the metal structure consists of α phase and β1 phase (modified β phase), the β1 phase is selectively dezincified, but the depth of this dezincification corrosion is approximately 20 to 200 μm, depending on the degree of modification to the β1 phase and the area ratio of the β1 phase, and the progression of dezincification corrosion is significantly suppressed. Thus, although the dezincification corrosion resistance is greatly improved by modifying the β phase to the β1 phase, even with this improvement, it is still inferior to the α phase in terms of dezincification corrosion resistance and ductility. Therefore, if the area ratio of the β1 phase is high, the dezincification corrosion resistance and ductility of the alloy will be low. Based on the above, and after diligent research, it was found that the area ratio of the β1 phase needs to be set to 80% or less, preferably 70% or less, and more preferably 65% ​​or less.

[0075] (α-phase, tissue relationship formula f3) This invention is basically composed of α phase and β1 phase, and a treatment is carried out to modify the β phase into the β1 phase, but the α phase is hardly affected by this treatment. Furthermore, a certain amount of α phase is required to modify the β phase. If there is too much β1 phase, there will be problems with the ductility of the alloy, and an appropriate amount of ductile α phase is required, while conversely, if there is too much α phase, the strength will be low. Also, the α phase containing Si has only a slight improvement in machinability compared to the α phase without Si, and from the viewpoint of machinability, the amount of α phase is limited. However, even if it is composed of a β1 phase with excellent machinability and an α phase with slightly poor machinability, during cutting the α phase acts as a cushioning material, or acts as a stress concentration source at the boundary with the hard β1 phase during cutting, and even if the amount of α phase is up to approximately 75%, the cutting resistance of the alloy is kept low and the chips are broken up. Furthermore, since the α phase acts as a cushioning material during cutting and as a stress concentration source at the boundary with the hard β1 phase, a fine granular shape is preferable for the α phase. As a result of diligent research into the modification of the β phase, the alloy's resistance to dezincification corrosion, stress corrosion cracking, machinability, and mechanical properties, it has been determined that the amount of α phase must be 20% or more, preferably 30% or more, and more preferably 35% or more. On the other hand, the upper limit of the α phase is less than 75%, preferably 70% or less, and more preferably 67% or less.

[0076] (γ phase, tissue relationship formula f5) As described in Patent Documents 8 and 10, the γ phase is a phase that contributes to machinability in Cu-Zn-Si alloys with a Cu concentration of approximately 69 to 80 mass% and a Si concentration of approximately 2 to 4 mass%. Patent Document 15 states that the γ phase is essential in Cu-Zn-Si alloys that do not contain Pb, and further, Patent Documents 2 to 6 state that both the β phase containing Si and the γ phase containing Si have good machinability. In this embodiment, the γ phase is considered to inhibit the machinability of the alloy, and therefore, in the microstructure-composition relationship formula f6 described later, a negative coefficient larger than the coefficient for the amount of the β1 phase is given. Furthermore, the γ phase impairs ductility and worsens resistance to dezincification corrosion. This is because, in this embodiment, the γ phase is not modified. In Cu-Zn-Si alloys, the appearance of the γ phase is sometimes unavoidable, but at the very least, it must be limited. That is, the proportion (area ratio) of the γ phase should be less than 4%, preferably less than 2%, more preferably less than 1%, and optimally, the γ phase should not be present, i.e., f5 = 0.

[0077] (Organizational and compositional relationship formula f6) f6 = (β1) × ([Si]) 1 / 2 -(γ)×2+([Pb]+[Bi]) 1 / 2 ×20+([P]) 1 / 2 ×15 The structural and compositional relationship formula f6 is a simple conditional formula for obtaining good machinability as an alloy. In f6, the amount of β1 phase, the amount of Si, the amount of Pb and Bi, and the amount of P contained in the alloy within the composition range of this application are organized as positive effects, and the amount of γ phase is organized as a negative effect. That is, for the β1 phase containing Si and P, the amount of β1 phase is multiplied by the square root of the amount of Si, the square root of the sum of the amounts of Pb and Bi is multiplied by a coefficient of 20, the square root of the amount of P is multiplied by a coefficient of 15, and the sum of these is multiplied by the amount of γ phase by a coefficient of 2 and subtracted. The performance of the β1 phase (modified β phase) is directly affected by the Si concentration, and also by the P concentration, and the machinability improves when a very small amount of Pb or Bi is included. On the other hand, in this application, the γ phase inhibits machinability. It was found that the degree of improvement in machinability of Pb or Bi is closely related to the square root of the amounts of Pb and Bi. Both Pb and Bi exert a significant effect even in very small amounts, and the improvement in machinability increases with increasing content, but gradually becomes more gradual. In alloys composed of the β1 phase (modified β phase), trace amounts of Pb or Bi exert a significant effect on machinability, an effect that cannot be achieved from the conventional relationship between Pb content and machinability in Cu-Zn-Pb alloys. Through diligent research, it has been found that in order to obtain good machinability, f6 should be greater than 27, preferably 33 or higher, more preferably greater than 35, and even more preferably 38 or higher, approaching the machinability of a free-cutting brass rod with 3 mass% Pb added.

[0078] Figures 1A to 6B show metallographic images of various alloys and photographs of the results of dezincification corrosion tests according to ISO 6509. Figure 1A is a photograph of the microstructure of the copper alloy in the embodiment, which was obtained by subjecting alloy No. S01 to process No. A2. Specifically, alloy No. S01 has the composition Zn-63.3mass%Cu-0.95mass%Si-0.069mass%P-0.063mass%Pb-0.017mass%Bi. In process No. A2, hot extrusion was performed at 630°C, and the cooling process was started at 580°C. The average cooling rate in the temperature range from 580°C to 300°C and the average cooling rate in the temperature range from 500°C to 300°C were both set to 1020°C / min. The average cooling rate in the temperature range from 580°C to 500°C can be easily calculated from the average cooling rate in the temperature range from 580°C to 300°C and the average cooling rate in the temperature range from 500°C to 300°C. Figure 1B shows the results of a dezincification corrosion test performed on the alloy shown in Figure 1A according to the ISO 6509 test method, and is a cross-sectional metallographic image including the area exhibiting the maximum corrosion depth. Figure 2A is a photograph of the microstructure of the copper alloy in the embodiment, which was obtained by subjecting alloy No. S11 to process No. E2. Specifically, alloy No. S11 has the composition Zn-62.5mass%Cu-0.96mass%Si-0.064mass%P-0.072mass%Pb. In process No. E2, a rod with a diameter of 50 mm and a length of 200 mm was obtained by hot extrusion at 550°C and an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C. The rod was heated and hot forged at 680°C with the rod placed horizontally to a thickness of 20 mm, and the cooling process was started at 565°C, with both the average cooling rate in the temperature range from 565°C to 300°C and the average cooling rate in the temperature range from 500°C to 300°C being 900°C / min. Figure 2B shows the results of a dezincification corrosion test performed on the alloy shown in Figure 2A according to the ISO 6509 test method, and is a cross-sectional metallographic image including the area exhibiting the maximum corrosion depth. Figure 3A is a photograph of the microstructure of the copper alloy in the embodiment, which was obtained by subjecting alloy No. S11 to process No. E13H. Specifically, alloy No. S11 has the composition Zn-62.5mass%Cu-0.96mass%Si-0.064mass%P-0.072mass%Pb. In process No. E13H, a rod with a diameter of 50 mm and a length of 200 mm was obtained by hot extrusion at 550°C and an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C. The rod was heated and hot forged at 630°C with the rod placed horizontally to a thickness of 20 mm, and cooled at an average cooling rate of 35°C / min. Next, the cooling process was started at 455°C, and the average cooling rate in the temperature range from 455°C to 300°C was set to 800°C / min. The alloy in Figure 3A and the alloy in Figure 2A have different cooling start temperatures. Figure 3B shows the results of a dezincification corrosion test performed on the alloy shown in Figure 3A according to the ISO 6509 test method, and is a cross-sectional metallographic image including the area exhibiting the maximum corrosion depth. Figure 4A is a photograph of the microstructure of the copper alloy in the embodiment, which was obtained by subjecting alloy No. S43 to process No. E6. Specifically, alloy No. S43 has the composition Zn-63.3mass%Cu-0.98mass%Si-0.084mass%P-0.060mass%Pb-0.28mass%Sn. In process No. E6, the casting, which was cast into a mold with a diameter of 55 mm, was cut to a diameter of 50 mm and then cut to a length of 200 mm. The casting was heated and hot forged at 630°C with the casting placed horizontally to a thickness of 20 mm, and the cooling process was started at 565°C, with both the average cooling rate in the temperature range from 565°C to 300°C and the average cooling rate in the temperature range from 500°C to 300°C being 900°C / min. Figure 4B shows the results of a dezincification corrosion test performed on the alloy shown in Figure 4A according to the ISO 6509 test method, and is a cross-sectional metallographic image including the area exhibiting the maximum corrosion depth. Figure 5A is a photograph of the microstructure of the copper alloy in the embodiment, which was obtained by subjecting alloy No. S02 to steps A34H and G1. Specifically, alloy No. S02 has the composition Zn-62.9mass%Cu-0.98mass%Si-0.071mass%P-0.071mass%Pb. In step A34H, the alloy was obtained by hot extrusion at 615°C with an average cooling rate of 18°C / min from 500°C to 300°C. In step G1, this alloy was further heated at 580°C for 30 minutes, and the cooling process was started from 560°C, with both the average cooling rate in the temperature range from 560°C to 300°C and the average cooling rate in the temperature range from 500°C to 300°C being 1800°C / min. Figure 5B shows the results of a dezincification corrosion test performed on the alloy shown in Figure 5A according to the ISO 6509 test method, and is a cross-sectional metallographic image including the area exhibiting the maximum corrosion depth. Figure 6A is a photograph of the microstructure of the copper alloy in the embodiment, which was obtained by subjecting alloy No. S02 to process No. E14H. Specifically, alloy No. S02 has the composition Zn-62.9mass%Cu-0.98mass%Si-0.071mass%P-0.071mass%Pb. In process No. E14H, a rod with a diameter of 50 mm and a length of 200 mm was obtained by hot extrusion at 550°C and an average cooling rate of 20°C / min from 500°C to 300°C. The rod was heated, placed horizontally, and hot forged at 630°C to a thickness of 20 mm, and then allowed to cool naturally. The average cooling rate in the temperature range from 500°C to 300°C was 25°C / min. Figure 6B shows the results of a dezincification corrosion test performed on the alloy shown in Figure 6A according to the ISO 6509 test method, and is a cross-sectional metallographic image including the area exhibiting the maximum corrosion depth.

[0079] As shown in Figures 1A, 2A, 4A, and 5A, in hot extruded materials, hot forged materials, hot forged materials made from cast materials, and extruded materials that have undergone additional heat treatment compared to hot extruded materials, grain boundary patterns, i.e., crystal grain boundaries, are observed in metallurgical microscope images, along with α-phase crystal grain boundaries, within the β1 phase. Here, a crystal grain boundary refers to a linear pattern that penetrates within the β1 phase crystal grains, as shown in Figure 1A. As shown above, grain boundaries are recognized within the modified β phase, i.e., the β1 phase. On the other hand, in Figure 3A, slight black streaks are observed within the β phase, but no grain boundaries penetrating the β phase crystal grains are seen. In Figure 6A, no grain boundaries are even visible within the β phase. Instead, in Figures 3A and 6A, black granular precipitates of approximately 0.5 to 3 μm are mainly present within the β phase and at the phase boundary between the β and α phases. The granular precipitates are mainly P compounds, but also include Pb particles, mixed particles of Pb and Bi, compounds such as Fe, oxides, and sulfides. These can be identified under a microscope, but are somewhat difficult to distinguish in printed photographs. Precipitates that fall outside the size range of the aforementioned precipitates, and precipitates present within the α phase, are not P compounds. In Figures 3A and 6A, approximately 1000 mainly P compounds are present within the field of view of the printed photographs. In Figures 1A, 2A, 4A, and 5A, fine granular P compounds are either absent or present in small quantities, at least less than 1 / 10 or 1 / 50 of the number of precipitates in Figures 3A and 6A. From these, it can be concluded that whether or not grain boundaries exist within the β1 phase or within the β phase depends on whether the cooling treatment start temperature is higher or lower than approximately 500°C, and whether or not there are many P compounds depends on whether the cooling treatment start temperature is higher or lower than approximately 500°C. The results of the dezincification corrosion tests on these alloys according to ISO 6509 are shown in Figures 1B, 2B, 3B, 4B, 5B, and 6B. The maximum corrosion depth for samples in which grain boundaries were observed within the β1 phase (Figures 1A, 2A, 4A, and 5A) was 120 μm or less, while the maximum corrosion depth for samples in which no grain boundaries were observed within the β phase was 350 μm (Figure 3A) and 460 μm (Figure 6A), showing a difference of 3 to 5 times in corrosion depth. In particular, the corrosion depth of the sample containing 0.28 mass% of Sn was good at 40 μm (Figure 4A). The form of dezincification corrosion in all cases was selective corrosion of the β1 phase and the β phase. Even with the β1 phase and the β phase, these phases are preferentially subjected to dezincification corrosion, but the β1 phase can slow down the progression of dezincification corrosion to about 1 / 3 or less than that of the β phase.

[0080] <Characteristics> (Strength at room temperature and high-temperature characteristics) There is a strong demand for thinner walls and lighter weights for the components and parts used in this embodiment, such as water supply equipment, piping components, automobiles, and electrical components. Tensile strength is considered important as the required strength, and a balance between strength and ductility is also considered crucial. To achieve this, hot extruded materials, hot rolled materials, and hot forged materials must have a tensile strength of 460 N / mm² in their hot-worked state without cold working. 2 It is preferable that the material is of the same high strength as described above. The tensile strength is more preferably 490 N / mm². 2 More preferably, 520 N / mm 2 That concludes the explanation. Many components used in valves, fittings, pressure vessels, and air conditioning / refrigeration equipment are made by hot extrusion and hot forging. The tensile strength of the currently used lead-added copper alloy, C3604, is approximately 390-420 N / mm². 2 Since the elongation is approximately 30-35%, weight reduction can be achieved by increasing strength. On the other hand, machined materials may undergo light riveting or cold working such as bending, and it is necessary that they do not crack. Machinability requires a certain degree of brittleness in the material because chips are broken up, but this is a property that is contrary to cold workability. Similarly, tensile strength and ductility are contradictory properties, but it is desirable to have a high degree of balance between tensile strength and ductility (elongation). That is, a tensile strength of at least 450 N / mm 2 In summary, the elongation is 10% or more, and the product of the tensile strength (S) and the value of {(elongation (E%) + 100) / 100} to the power of 1 / 2 is f7 = S × {(E + 100) / 100}. 1 / 2 A value of preferably 540 or higher is one measure of a high-strength, high-ductility material. f7 is more preferably 570 or higher, and even more preferably 600 or higher. When cold working is performed with a cold working rate of 3-15%, an elongation of 10% or more and 520 N / mm² are observed. 2 It can possess the above tensile strengths. Incidentally, in the case of the aforementioned Pb-added copper alloy, f7 is approximately 470.

[0081] (conductivity) Applications of this embodiment include electrical and electronic equipment components, automotive parts for the increasingly electrified EV market, and other highly conductive materials and components. Currently, phosphor bronze (JIS standards, C5191, C5210) containing 6 mass% or 8 mass% of Sn is widely used in these applications, with conductivity values ​​of approximately 14% IACS and 12% IACS, respectively. Therefore, if the conductivity of the copper alloy of this embodiment is 15% IACS or higher, there will be no problems regarding electrical conductivity. The upper limit of conductivity is not specifically defined, as it rarely poses practical problems due to improved conductivity.

[0082] (Hot workability) The free-machining copper alloy of this embodiment is characterized by having excellent deformability at 540 to 750°C, allowing it to be hot-extruded into rods with a small cross-sectional area and hot-forged into complex shapes. Due to energy considerations and the favorable granular shape of the α-phase, the hot working temperature is preferably less than 750°C, more preferably less than 720°C, and from the viewpoint of hot deformation resistance, preferably more than 540°C, and more preferably more than 560°C.

[0083] <Manufacturing Process> Next, a method for producing a free-cutting copper alloy according to an embodiment of the present invention will be described. The microstructure of the alloy in this embodiment changes not only with its composition but also with the manufacturing process. It is affected not only by the hot working temperature and heat treatment conditions in hot extrusion and hot forging, but also by the average cooling rate during the cooling process in hot working and heat treatment. Through diligent research, it has been found that the microstructure is greatly influenced by the cooling process in the cooling process of hot working and heat treatment, the cooling start temperature, the average cooling rate in the temperature range from the cooling start temperature to 500°C, and the average cooling rate in the temperature range from 500°C to 300°C.

[0084] (Melting and casting) Melting takes place at a temperature of approximately 950 to 1200°C, which is about 100 to 300°C higher than the melting point (liquidus temperature) of the alloy in this embodiment. Molten metal at a temperature of approximately 900 to 1100°C, which is about 50 to 200°C higher than the melting point, is poured into a predetermined mold and cooled by several cooling methods such as air cooling, slow cooling, and water cooling. After solidification, the constituent phases change in various ways.

[0085] (Hot working) Examples of hot working processes include hot extrusion, hot forging, and hot rolling. When two or more hot working processes are performed, the final hot working process shall be carried out under the following conditions. First, regarding hot extrusion, in a preferred embodiment, depending on the extrusion ratio (hot working rate) and equipment capacity, the material temperature immediately after hot working (extrusion temperature) is between 540°C and 750°C. The extruded rod is either wound into a coil or, if the cross-sectional area of ​​the extruded rod is large, is extruded as a straight rod onto a table. The lower limit of the hot extrusion temperature is related to the deformation resistance at high temperatures; a lower extrusion temperature results in finer, more granular α-phase crystal grains, leading to better resistance to dezincification corrosion, stress corrosion cracking, and machinability. However, when the extrusion ratio is high, the extrusion temperature is preferably 560°C or higher, based on the relationship between equipment capacity and the cooling treatment start temperature described later. The upper limit is related to the shape of the α-phase; a more stable metallic structure can be obtained by controlling the temperature within a narrower range. When hot extrusion is performed at temperatures above 750°C, the metal structure initially consists of a single β phase, or the proportion of the β phase exceeds 90%, and the shape of the α phase crystal grains tends to become needle-shaped, or coarse α phase crystal grains tend to appear. When needle-shaped, coarse α phase crystal grains appear, the strength is slightly reduced, the balance between strength and ductility is slightly worsened, and the large, coarse α phase crystal grains become an obstacle to cutting, resulting in poor machinability. Furthermore, resistance to dezincification corrosion and stress corrosion cracking also deteriorates. The extrusion temperature is preferably 720°C or lower. The shape of the α phase crystal grains is related to the composition relation formula f1, and when the composition relation formula f1 is 59.0 or less, it is preferable that the extrusion temperature be lower than 720°C. By extruding at a lower temperature than copper alloys containing Pb, good machinability and strength can be achieved.

[0086] Furthermore, by optimizing the cooling conditions after hot extrusion, the β phase can be modified, resulting in a material with better machinability, dezincification resistance, and stress corrosion cracking resistance. Specifically, in the cooling process after hot extrusion, the cooling treatment is first started at a temperature lower than 670°C and higher than 500°C. Then, the average cooling rate in the temperature range from the cooling treatment start temperature to 500°C, and the average cooling rate in the temperature range from 500°C to 300°C are set to at least 300°C / min, preferably 600°C / min, and more preferably 900°C / min or higher. In a typical embodiment, if a general cooling method is used, the average cooling rate in the temperature range from the cooling treatment start temperature to 500°C and the average cooling rate in the temperature range from 500°C to 300°C are roughly the same, or in some cases, the former is slightly faster. Cooling at an even more preferable cooling rate further modifies the β1 phase. Further modification of the β1 phase results in a material with better machinability, dezincification resistance, and stress corrosion cracking resistance. The upper limit of the average cooling rate in the temperature range from the cooling treatment start temperature to 500°C and from 500°C to 300°C is sufficient with the cooling rate achievable with normal production equipment and is not specifically defined, but if mentioned, a cooling rate of approximately 9000°C / min or less is preferable. By cooling at an average cooling rate exceeding 300°C / min, the β phase is modified, and when the metal structure is etched with a mixture of hydrogen peroxide and ammonia water and observed with a metallurgical microscope at 500x magnification, grain boundaries can be observed within the modified β phase. Note that if the average cooling rate in the temperature range from the cooling treatment start temperature to 500°C and the average cooling rate in the temperature range from 500°C to 300°C are 300°C / min or less, grain boundaries cannot be observed within the β phase. The degree of modification to the β phase, i.e., to the β1 phase, is greater the faster the cooling rate. When cooling from a temperature below 300°C to room temperature, the cooling rate slows down slightly as it approaches room temperature, but it is desirable to continue cooling in the temperature range from 500°C to 300°C. On the other hand, the starting temperature of the cooling process also affects the modification of the β phase. It is preferable to start at a temperature higher than 530°C, and more preferably at a temperature higher than 550°C. If the cooling process is started at a temperature lower than 550°C, and the cooling rate from 550°C to the cooling process start temperature is slow, compounds of P and Zn, or P and Zn,Si will begin to form, and below 530°C, the formation of P compounds is further promoted. At cooling rates exceeding 300°C / min, almost no P compounds are formed, but if the cooling process start temperature is lower than 550°C, P compounds can be observed in the metal structure. Incidentally, if the cooling treatment start temperature is 670°C or higher, the proportion of the β1 phase becomes too large, which worsens the ductility, dezincification corrosion resistance, and stress corrosion cracking resistance of the alloy. Therefore, the cooling treatment start temperature needs to be lower than 670°C, and preferably lower than 650°C.

[0087] Considering the measurement locations where actual measurements are possible, the hot working temperature is defined as the temperature of the hot-worked material that can be measured approximately 2 or 3 seconds after the end of hot extrusion, hot forging, or hot rolling. The microstructure is affected by the temperature immediately after processing, when significant plastic deformation occurs.

[0088] Next, we will describe hot forging. Hot forging primarily uses hot extruded materials as the raw material, but continuously cast rods are also used. Since the forging material is not the final hot processed material, no cooling measures are necessary. Compared to hot extrusion, hot forging has a faster processing speed, can process into complex shapes, and in some cases can be processed to a wall thickness of approximately 3 mm. Also, the weight of a single forged product ranges from tens of grams to several kilograms, and small forged products are rapidly cooled during forging, and the subsequent cooling rate is also fast. Therefore, the heating temperature of the material during forging is higher than the heating temperature of the ingot during hot extrusion. In a preferred embodiment, the temperature of the hot forged product, i.e., the material temperature from immediately after forging to approximately 2 or 3 seconds later, is preferably above 540°C and below 750°C. In hot forging, the compositional relationship f1 is also relevant, and if the compositional relationship f1 is 59.0 or less, it is preferable that the hot forging temperature be lower than 720°C. Depending on the processing rate of hot forging, the lower the temperature, the smaller the grain size of the α phase crystals, the change in the shape of the α phase crystal grains from needle-like to granular, the higher the strength, the better the balance between strength and ductility, and the better the machinability, dezincification resistance, and stress corrosion cracking resistance.

[0089] Furthermore, similar to hot extrusion, by optimizing the cooling rate after hot forging, it is possible to obtain a material with good resistance to dezincification corrosion, resistance to stress corrosion cracking, and good machinability. Specifically, in the cooling process after hot forging, the cooling treatment is first started at a temperature lower than 670°C and higher than 500°C, and the average cooling rate in the temperature range from the cooling treatment start temperature to 500°C, and the average cooling rate in the temperature range from 500°C to 300°C are set to at least 300°C / min, preferably 600°C / min, and more preferably 900°C / min or higher. In a typical embodiment, if a general cooling method is used, the average cooling rate in the temperature range from the cooling treatment start temperature to 500°C and the average cooling rate in the temperature range from 500°C to 300°C are roughly the same, or the former is slightly faster. By increasing the average cooling rate to over 300°C / min, the β phase is modified. When the material is etched with a mixture of hydrogen peroxide and ammonia water and the metal structure is observed with a 500x metallurgical microscope, grain boundaries can be observed within the modified β phase. However, if the average cooling rate in the temperature range from 500°C to 300°C is 300°C / min or less, grain boundaries cannot be observed within the β phase. The degree of modification to the β phase, i.e., to the β1 phase, is greater with a faster cooling rate. Similar to hot extrusion, when cooling from temperatures below 300°C to room temperature, the cooling rate slows down slightly as it approaches room temperature, but it is desirable to continue cooling in the temperature range from 500°C to 300°C. There is no specific upper limit for the average cooling rate in the temperature range from the cooling start temperature to 500°C and in the temperature range from 500°C to 300°C, but if we were to mention one, a cooling rate of approximately 9000°C / min or less is preferable. On the other hand, the starting temperature of the cooling process also affects the modification of the β phase. It is preferable to start at a temperature higher than 530°C, and more preferably at a temperature higher than 550°C. If the cooling process is started at a temperature lower than 550°C, and the cooling rate from 550°C to the starting temperature is slow, compounds of P and Zn, or P, Zn, and Si will begin to form, and below 530°C, the formation of P compounds will be further promoted. At cooling rates exceeding 300°C / min, almost no P compounds are formed, but if the starting temperature of the cooling process is lower than 550°C, P compounds can be observed in the metal structure. On the other hand, if the starting temperature of the cooling process is 670°C or higher, the proportion of the β1 phase becomes too large, which worsens the ductility, dezincification corrosion resistance, and stress corrosion cracking resistance of the alloy. Therefore, the starting temperature of the cooling process needs to be lower than 670°C, and is preferably lower than 650°C. Furthermore, if, after hot forging, the product is placed in a simple furnace set to an ambient temperature of approximately 550-600°C for several tens of seconds to several minutes, and then the cooling process is started, a more uniform and stable forged product can be obtained. Furthermore, to improve stress corrosion cracking resistance, shot blasting is an effective method after the hot forging-cooling treatment described above is completed. While the stress corrosion cracking resistance of other copper alloys can also be improved by applying compressive stress to the surface, such as through shot blasting, the effect of shot blasting is even greater in the case of the alloy of this invention that contains the β1 phase.

[0090] In hot rolling, the ingot is heated and rolled repeatedly 5 to 15 times. The material temperature at the end of the final hot rolling (material temperature 2 to 3 seconds after the end) is preferably above 540°C and below 750°C, and more preferably below 670°C. After hot rolling is complete, the rolled material is cooled. Similar to hot extrusion, the cooling process is started at a temperature above 500°C and below 670°C. The average cooling rate in the temperature range from the cooling start temperature to 500°C, and the average cooling rate in the temperature range from 500°C to 300°C are set to at least 300°C / min, preferably above 600°C / min, and more preferably above 900°C / min. When cooling from temperatures below 300°C to room temperature, the cooling rate slows down slightly as it approaches room temperature, but it is desirable to continue cooling in the temperature range from 500°C to 300°C. While there are no specific upper limits on the average cooling rate in the temperature range from the cooling start temperature to 500°C and in the temperature range from 500°C to 300°C, if we were to mention one, a cooling rate of approximately 9000°C / min or less is preferable.

[0091] (Heat treatment) In this application, if cooling cannot be started at a temperature higher than 500°C after the final hot working, or if the average cooling rate in the temperature range from 500°C to 300°C cannot exceed 300°C / min, then the β phase cannot be modified. Furthermore, when making small diameter rods, wires, etc., if the process includes cold working and annealing, which involves heat, then the β phase is basically not modified. In these cases, annealing is performed at a temperature above 520°C and below 630°C for 1 minute to 5 hours, and during the cooling after annealing, the cooling process is started at a temperature higher than 500°C, the average cooling rate in the temperature range from the cooling start temperature to 500°C exceeds 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C exceeds 300°C / min, thereby modifying the β phase and forming the β1 phase. The annealing conditions are as follows: if there are multiple annealing steps, only the final annealing step is required. From the viewpoint of the temperature at which the P compound is formed, the annealing temperature is preferably 530°C or higher, and more preferably 550°C or higher. Furthermore, the average cooling rate in the temperature range from 500°C to 300°C is preferably set to over 600°C / min, and more preferably 900°C / min or higher, depending on the degree of modification of the β1 phase. This heat treatment is applied to hot-worked materials that cannot be cooled under predetermined conditions after hot working, and to materials that undergo one or more annealing steps after hot working and cold working. While there are no specific upper limits on the average cooling rate in the temperature range from the cooling start temperature to 500°C and in the temperature range from 500°C to 300°C, if we were to mention one, a cooling rate of approximately 9000°C / min or less is preferable.

[0092] (Cold working process) In the case of hot-extruded rods, cold working may be performed on the hot-extruded material to obtain high strength, improve dimensional accuracy, or to make the extruded rod or coil material a straight shape with minimal curvature. For example, the hot-extruded material may be cold-drawn with a processing rate of approximately 2 to 30%, and in some cases, drawn, followed by straightening. Thin rods, wires, or rolled materials undergo repeated cold working and annealing. After the final heat treatment, they are subjected to cold working and straightening processes with a final processing rate of 0-30%. The closer the final processing rate is to 0%, the less prone the material becomes to cracking when subjected to light cold working processes such as riveting or bending.

[0093] (Low-temperature annealing) In the case of rods, wires, forgings, and rolled materials, low-temperature annealing may be performed as the final step at temperatures below the recrystallization temperature, primarily for the purpose of removing residual stress, straightening the rod (improving the straightness of the rod), and adjusting and improving the metal structure. However, the β1 phase modified by hot working and heat treatment is lost when heat is applied to the alloy. For example, heating at 300°C for 2 hours will destroy the modification of the β phase, causing the modified β1 phase to revert to the original β phase, and the improved machinability, dezincification resistance, and stress corrosion cracking resistance will become the properties of the alloy consisting of the original β phase. Therefore, low-temperature annealing is not recommended, although temperatures below approximately 150°C are acceptable.

[0094] According to the free-machining alloy of the present invention configured as described above, the alloy composition, compositional relationship, metal structure, microstructure relationship, and microstructure-composition relationship are defined as described above, so even with a low Pb content, excellent machinability can be obtained, and it has good resistance to dezincification corrosion, resistance to stress corrosion cracking, excellent hot workability, high strength, and an excellent balance of strength and ductility.

[0095] Although embodiments of the present invention have been described above, the present invention is not limited thereto, and can be modified as appropriate without departing from the technical requirements of the invention. [Examples]

[0096] The following shows the results of verification experiments conducted to confirm the effects of this embodiment. Note that the following examples are for illustrative purposes only, and the constituent elements, processes, and conditions described in the examples do not limit the technical scope of this embodiment.

[0097] We conducted prototype tests of copper alloys using the low-frequency melting furnace and semi-continuous casting machine currently in operation. Furthermore, prototype testing of copper alloys was conducted using laboratory equipment. The alloy compositions are shown in Tables 5-7. The manufacturing processes are shown in Tables 8-17. In the compositions, "Mm" represents mischmetal and indicates the total amount of rare earth elements. Each manufacturing process is described below. In Tables 8-12 and 14-16, the cooling rate for hot working, such as hot extrusion, hot forging, or hot compression, is the average cooling rate from the end of the hot working process to the start of the cooling process.

[0098] [Table 5]

[0099] [Table 6]

[0100] [Table 7]

[0101] [Table 8]

[0102] [Table 9]

[0103] [Table 10]

[0104] [Table 11]

[0105] [Table 12]

[0106] Table 13

[0107] Table 14

[0108] Table 15

[0109] Table 16

[0110] Table 17

[0111] Mechanical Manufacturing Engineering AA, AB, AD (Project Nos. A1~A5, A11H~A14H, A21~A25, A31H~A34H, A41~A44, A46H, A47H) A 240mm diameter billet was manufactured using a low-frequency melting furnace and a semi-continuous casting machine in actual operation. The raw materials used were those similar to those used in actual operation. The billet was cut to a length of 800mm and heated. A 20.9mm diameter round bar was extruded using an indirect extruder with a nominal capacity of 2750 tons, and the extruded material (round bar) was wound into a coil in a tank located a short distance from the extruder. This tank is capable of water cooling with adjustable water volume. When the extruded material reached a predetermined temperature in the tank, the cooling process (water cooling) was started while adjusting the water volume. In some cases, the cooling process was started simultaneously with the placement of the extruded material in the tank. Temperature measurements were taken using an infrared thermometer, measuring the temperature of the extruded material when it was extruded from the extruder, when the cooling process started, and until it reached 500°C and 300°C. For subsequent hot extrusion and hot forging temperature measurements, an infrared thermometer (model IGA8Pro / MB20) and a contact thermometer manufactured by LumaSense Technologies Inc. were used in combination.

[0112] In process AA, i.e., processes A1-A5 and A11H-A14H, the extrusion temperature was 630°C, and in process AB, i.e., processes A21-A25 and A31H-A34H, the extrusion temperature was 615°C. In process AD, i.e., processes A41-A44, A46H, and A47H, the extrusion temperature was 720°C. After the 20.9mm diameter extruded material, wound into a coil, reached a predetermined temperature in the tank, the cooling process was started, and the time it took for the extruded material to reach 500°C and 300°C was measured. Although the radiation thermometer does not accurately measure temperatures below 300°C, the cooling continued under the same conditions, and the material was cooled to a temperature of approximately 100°C or lower. As detailed in Tables 8, 9, and 10, processes A1-A5, A11H-A14H, A21-A25, A31H-A34H, and A41-A44, A46H, and A47H may have different average cooling rates from extrusion to the start of cooling, different starting temperatures for cooling, different average cooling rates in the temperature range from the cooling start temperature to 300°C, and different average cooling rates in the temperature range from 500°C to 300°C. If the cooling start temperature is lower than 500°C, the average cooling rate in the temperature range from the cooling start temperature to 300°C is listed as the cooling rate in the temperature range from 500°C to 300°C. Furthermore, the average cooling rate in the temperature range from the cooling start temperature to 500°C can be easily calculated using the average cooling rate in the temperature range from the cooling start temperature to 300°C, and the average cooling rate in the temperature range from 500°C to 300°C. Therefore, it was confirmed that the cooling rate in the temperature range from the cooling start temperature to 500°C was the same as, or slightly faster under some conditions than, the cooling rate in the temperature range from 500°C to 300°C. After the cooling was completed following hot extrusion, the extruded material was put through a combined machine at a processing rate of 8.4% and drawn and straightened into a φ20mm straight rod.

[0113] In processes A25 and A31H, the rod material obtained in process A21 was further annealed in the laboratory at low temperatures of 130°C for 5 hours and 300°C for 2 hours. These process AA, process AB, and process AD materials were subjected to microscopic observation, cutting tests, dezincification corrosion tests, and tensile tests.

[0114] Process AC, φ50mm rod material (Process No.A50) As a prototype for forging material, a φ50 mm bar was prepared using process AC shown in Table 11 (process No. A50). A billet with a diameter of 240 mm was manufactured using a low-frequency melting furnace and semi-continuous casting machine in actual operation, and the billet was cut to a length of 800 mm. Using the aforementioned billet, it was extruded at 550°C into a 50 mm diameter round bar using a direct extruder with a nominal capacity of 3000 tons. The extruded straight bar was placed on a steel table and allowed to cool naturally to room temperature. It was then straightened and provided as material for the flat forging described later. Since the final hot working was hot forging, no special cooling treatment was performed, and the average cooling rate from 500°C to 300°C was 20°C / min.

[0115] Laboratory extruded material, process B (Process No.B1,B2,B3H) In step B, as shown in Table 12, the raw materials were dissolved in the laboratory at a predetermined ratio. The molten metal was poured into a mold with a diameter of 100 mm and a length of 200 mm to produce a billet. In addition, some billets had impurities such as Fe intentionally added. The concentration of the intentionally added impurities such as Fe was approximately the same level as, or lower than, that of commercially available brass containing Pb. These billets were heated, and in process No. B1, the extrusion temperature was set to 670°C, and the material was extruded into a 20mm diameter round bar. After extrusion, the cooling process began at 540°C, and the material was cooled from 500°C to 300°C at an average cooling rate of 960°C / min. It was confirmed that the average cooling rate from 540°C to 500°C and the average cooling rate from 500°C to 300°C were approximately the same. Subsequently, the cooling continued under the same conditions, and the material was cooled to a temperature of approximately 100°C or lower. In process No. B3H, the extrusion temperature was set to 670°C, and after extrusion into a 20mm diameter round bar, no cooling process was performed, and the material was cooled from 500°C to 300°C at an average cooling rate of 40°C / min. Processes No. B1 and B3H were subjected to microscopic observation, cutting tests, dezincification corrosion tests, and tensile tests, and a portion of process No. B3H was used as the hot forging material for process D. In process No. B2, the extrusion temperature was set to 590°C, the material was extruded to a diameter of 50 mm, and no cooling treatment was performed. The average cooling rate from 500°C to 300°C was set to 25°C / min, resulting in a material for forging.

[0116] Process C, material for forging and hot compression: Castings (Process No.C1,C2) In the laboratory, castings to be used as forging materials were prepared by melting the raw materials in a predetermined ratio and pouring the molten metal at approximately 1000°C into an iron mold with an inner diameter of 35 mm and a depth of 200 mm (Process No. C1), and by pouring it into an iron mold with an inner diameter of 55 mm and a depth of 200 mm (Process No. C2). When the casting reached approximately 700°C, the casting was removed from the mold and allowed to cool naturally without any cooling treatment. As shown in Table 13, the average cooling rate from 500°C to 300°C was 35°C / min in process No. C1 and 23°C / min in process C2. The castings from process No. C1 were machined to an outer diameter of 20 mm using a lathe, and the castings from process No. C2 were machined to an outer diameter of 50 mm, and finished to be used as forging material in processes D and E, and as hot compression material in process F, as described later.

[0117] Flare nut forging (Process D) In process D, φ20mm extruded rods from processes No. A1, A13H, A21, A34H, and A47H, laboratory extruded rods from process No. B3H, and laboratory castings from process C1 were used as the material for the flare nut forgings. These were then cut to a weight of 120±1g (119g~121g) and formed into hexagonal flare nut forging materials with a 27mm across flats and a length of 26mm using a 500-ton hot forging press. As shown in Table 14, the hot forging temperature was 670°C for processes D1, D3-D7, and D12H-D14H, 710°C for process D2, 740°C for process D8, and 770°C for process D15H. The average cooling rate from after forging to the start of the cooling process was 100°C / min, except for processes D15H and D13H, where it was 120°C / min. In process D13H, no cooling process was performed, and natural cooling was allowed. As shown in Table 14, the starting temperature for the cooling process ranged from 475 to 695°C. The average cooling rate in the temperature range from the cooling process start temperature to 300°C, and the average cooling rate in the temperature range from 500°C to 300°C, varied from 240°C / min to 1980°C / min, except for process D13H. In processes D1-D7 and D14H, the average cooling rate in the temperature range from the cooling start temperature to 500°C was the same as the average cooling rate in the temperature range from 500°C to 300°C. In processes D8 and D15H, the former was slightly faster. In process D12H, the average cooling rate from the cooling start temperature of 475°C to 300°C was recorded as the average cooling rate in the temperature range from 500°C to 300°C. In process No. D7, hot forging and cooling were performed under the same conditions as in process No. D5, followed by a shot blasting process. The shot blasting was performed using steel balls with a diameter of 0.6 mm for 10 minutes. These hot-forged products were machined to form flare nuts conforming to JIS B 8607, with an inner diameter of 5 / 8 inch (=15.9 mm). Process D material was subjected to microscopic observation, dezincification corrosion testing, and stress corrosion cracking testing.

[0118] flat forging (Process E) Process E is a laboratory forging process, using extruded rods from process No. A50, laboratory extruded rods from process No. B2, and laboratory castings from process No. C2 as materials, each cut to a length of 180 mm. These round bars were placed horizontally and forged to a thickness of 20 mm using a hot forging press with a capacity of 150 tons.

[0119] (Process No. E1~E7, E11H~E15H) In processes E1, E4-E6, and E12H-E15H, the forging temperature was 630°C; in process E2, it was 680°C; in process E3, it was 700°C; in process E7, it was 720°C; and in process E11H, it was 740°C. After hot forging, except for process E14H, the average cooling rate until the start of the cooling process was 35°C / min. As shown in Table 15, the cooling process was started at temperatures ranging from 690°C to 455°C, and the average cooling rate from the cooling start temperature to 300°C was changed from 900°C / min to 200°C / min. The same cooling method was then used to cool to room temperature. In processes E1-E6 and E15H, it was confirmed that the average cooling rate in the temperature range from the cooling start temperature to 500°C was approximately the same as the average cooling rate in the temperature range from 500°C to 300°C. In processes E12H and E13H, the average cooling rate in the temperature range from the cooling start temperature to 300°C was recorded as the average cooling rate in the temperature range from 500°C to 300°C. In process E14H, no cooling treatment was performed, and natural cooling was allowed, resulting in an average cooling rate of 25°C / min from 500°C to 300°C. These flat forged materials were cut and subjected to cutting tests, mechanical property tests, dezincification tests, and microscopic examinations.

[0120] Hot compression test (Process F) The hot extruded rods from the actual machine in process Nos. A13H, A34H, and A47H, the laboratory hot extruded rod in process No. B3H, and the castings in process C1 were used as materials and finished on a lathe to a diameter of φ15 mm and a height of 27 mm. Then, as shown in Table 16, a 10-ton Amsler-type testing machine with an attached heating furnace was used to hot compress the rods to a height of 8 mm at 650°C under a strain rate of 0.02 / sec. In processes Nos. F1 to F3, the hot-compressed samples were removed from the heating furnace and cooled at a cooling rate of 50°C / min. Next, the cooling process was started at 560°C, and the average cooling rate in the temperature range from 500°C to 300°C was 2100°C / min. The cooling method was continued below 300°C until the samples cooled to room temperature. In process No. F4H, the cooling process was initiated at 560°C, and the average cooling rate from the cooling start temperature to 300°C was 200°C / min. In process No. F5H, no special cooling treatment was performed, and the average cooling rate from 500°C to 300°C was 40°C / min. The average cooling rate in the temperature range from the cooling start temperature (560°C) to 500°C was approximately the same as the average cooling rate in the temperature range from 500°C to 300°C. These hot-compressed materials were subjected to microscopic observation and dezincification corrosion tests.

[0121] heat treatment (Process G) In process G, as shown in Table 17, the changes in properties were investigated by heat-treating the rods and forgings obtained in processes A1, A21, A41, A13H, A34H, A47H, D1, and D13H in the laboratory. Annealing was performed in all cases under conditions of holding at 580°C for 30 or 60 minutes. In processes G1, G2, and G3, cooling was started at 560°C after annealing, and the average cooling rate in the temperature range from the cooling start temperature to 300°C was 1800°C / min. The average cooling rate in the temperature range from the cooling start temperature (560°C) to 500°C and the average cooling rate from 500°C to 300°C were approximately the same. In processes G11H and G12H, after annealing at 580°C, the cooling treatment was initiated at 470°C, and the average cooling rate from 470°C to 300°C was 1500°C / min. In processes G13H and G14H, after annealing at 580°C, no special cooling treatment was applied, and the average cooling rate from 500°C to 300°C was 25°C / min. These heat-treated materials were subjected to metallurgical microscopy observation, and the rods underwent cutting tests, dezincification corrosion tests, and tensile tests, while the forged products underwent dezincification corrosion tests and stress corrosion cracking tests.

[0122] (Comparative material) As comparative materials, a φ20 mm rod made of free-cutting brass C3604 containing 3 mass% Pb was prepared. In addition, φ50 mm and φ20 mm rods made of forging brass C3771 containing 2 mass% Pb were prepared. These were designated as alloys X, Y, and Z, respectively, and commercially available alloys were used. Hot forging and cooling treatment were performed on alloy Y under the same conditions as processes No. E14H and E1. Hot forging and cooling treatment were performed on alloy Z under the same conditions as processes No. D13H and D1. Alloy X was subjected to metallurgical microscopy observation, cutting tests, dezincification corrosion tests, and tensile tests. Alloy Y was subjected to metallurgical microscopy observation, cutting tests, dezincification corrosion tests, and tensile tests. Alloy Z was subjected to metallurgical microscopy observation, dezincification corrosion tests, and stress corrosion cracking tests.

[0123] The above-mentioned test materials were evaluated based on the following criteria. The evaluation results are shown in Tables 18 to 56. The area ratio of the β phase, i.e., the unmodified β phase, is denoted as f4A and distinguished from the area ratio of the β1 phase (f4), and is listed in each table. When f4 = (area ratio of β1) = 0, the (β1) term in the calculation formula for f6 is replaced with the value of f4A = (area ratio of β) and the resulting value is listed as a reference value for f6, marked with an asterisk (*).

[0124] (Observation of metal structure) The metallographic structure was observed using the following method, and the area percentage (%) of each phase, such as the α, β, and γ phases, was measured by image analysis. When grain boundaries were observed within the β phase, it was classified as the β1 phase and distinguished from the β phase. The α', β', and γ' phases were included within the α, β, and γ phases, respectively. The α phase exhibits a granular, elliptical shape and is often accompanied by twinning. The β and β1 phases exist around this granular, elliptical α phase. This is how the α, β, and β1 phases were distinguished. The rods and forged products of each test material were cut parallel to the longitudinal direction or parallel to the flow direction of the metal structure. The surfaces were then polished to a mirror finish and etched with a mixture of hydrogen peroxide and ammonia water. For etching, an aqueous solution was used, which consisted of 3 mL of 3 vol% hydrogen peroxide and 22 mL of 14 vol% ammonia water. The polished metal surface was immersed in this aqueous solution for approximately 2 to 10 seconds at room temperature of approximately 15 to 25°C. In the case of the presence of the β1 phase, corrosion resistance was improved compared to the case where the β phase was present, so the etching immersion time needed to be longer, approximately 5 to 10 seconds.

[0125] Using a metallurgical microscope, the microstructure was observed at 500x magnification, and the proportion of each phase was determined. Depending on the condition of the microstructure, observation was performed at 1000x magnification to confirm the phases and compounds. In the five field-of-view microscope images, each phase (α phase, β phase, β1 phase, γ phase) was manually filled in using the image processing software "Photoshop CC". For the distinction between the β phase and the β1 phase, if one or more β grain boundaries were observed in a single field of view, all β phases in that field of view were filled in as the β1 phase. Next, the images were binarized using the image analysis software "WinROOF2013," and the area percentage of each phase was determined. In detail, the average of the area percentages in the five fields of view was calculated for each phase, and this average was used as the area percentage of each phase. Compounds containing P and Si, precipitates, oxides, Pb or Bi particles, sulfides, and crystals were excluded, and the sum of the area percentages of all constituent phases was set to 100%.

[0126] When the β phase is modified, observation with a metallurgical microscope using the etching solution reveals grain boundary patterns, i.e., crystal grain boundaries, within the β phase. In this application, when crystal grain boundaries are observed within the β phase, that β phase is designated as the β1 phase and distinguished from the normal β phase. Here, a crystal grain boundary refers to a linear pattern that penetrates within the β1 phase crystal grains, as shown in Figure 1A. In addition to the phases, metallographic microstructures may contain P and Si compounds and precipitates. Compounds of P and Zn or Si appear as blackish-gray particles of approximately 0.5 to 2 μm under a 500x metallographic microscope, mainly within the β phase, β1 phase, and at the phase boundary between the β and α phases. Compounds of Fe, Mn, Cr, and P or Si can be observed as light blue particles of approximately 1 to 5 μm. Pb or Bi particles, and particles formed by the combination of Pb and Bi, exist as fine black particles of 2 μm or less. These compounds can be roughly distinguished under a metallographic microscope. In the characterization table, if grain boundaries were observed within the β1 phase (β phase) of the metal structure, it was indicated as "B" (present), and if they were not observed, it was indicated as "D" (absent). Regarding P compounds, when the amount of P was 0.06 mass% and the cooling rate from 500°C to 300°C was an average of 15-50°C / min, approximately 500-1000 P compounds were observed in a field of view at 500x magnification (80mm x 120mm when printed). However, in the printed metal microstructure, it became difficult to distinguish between P compounds, Fe, Mn, Cr, P or Si compounds, Pb particles, etc. Therefore, when determining the presence or absence of P compounds from a printed metal microstructure photograph, if there were fewer than 50 P compounds and compounds that could potentially be P compounds within the field of view of the photograph, it was evaluated as "D" (absent), indicating that P compounds were not present. If there were 50 or more but fewer than 300 P compounds, it was evaluated as "C" (fair), indicating that P compounds were present but in small quantities. If there were 300 or more P compounds, it was evaluated as "B" (present), indicating that P compounds were present. The presence or absence, and the quantity of P compounds, is one indicator of whether appropriate processing was performed.

[0127] When phase identification, compound identification, and precipitate identification were difficult, a field emission scanning electron microscope (FE-SEM) (JSM-7000F, manufactured by JEOL Ltd.) and its attached EDS were used to identify the phases and precipitates using the FE-SEM-EBSP (Electron Back Scattering Diffracton Pattern) method at a magnification of 500x or 2000x, under conditions of an acceleration voltage of 15kV and a current value (set value 15).

[0128] (conductivity) The conductivity was measured using a conductivity measuring device (SIGMATEST D2.068) manufactured by Nippon Förster Co., Ltd. In this specification, the terms "electrical conductivity" and "conductivity" are used interchangeably. Furthermore, since thermal conductivity and electrical conductivity are strongly correlated, higher conductivity indicates better thermal conductivity.

[0129] (Tensile strength / elongation) Each test material was processed into a No. 10 test specimen according to JIS Z 2241, and its tensile strength and elongation were measured. The tensile strength of the hot-extruded or hot-forged material that has not undergone a cold-working process is preferably 460 N / mm². 2 More preferably, 490 N / mm 2 More preferably 520 N / mm 2 If the above is achieved, it represents the highest level among free-machining copper alloys, enabling thinner and lighter components used in various fields, or increasing the allowable stress. Furthermore, in terms of the balance between strength and elongation, the tensile strength is S(N / mm²). 2 If the elongation is E (%), then the characteristic relationship f7 = S × {(E + 100) / 100} represents the balance between strength and ductility. 1 / 2 A value of preferably 540 or higher is one measure of a high-strength, high-ductility material. f7 is more preferably 570 or higher, and even more preferably 600 or higher. This is considered a very high level among free-machining copper alloys. Many parts used in faucets, valves, fittings, pressure vessels, and air conditioning / refrigeration equipment are made by hot forging or hot extrusion, and the tensile strength of the currently used Pb-added copper alloy, C3604, is approximately 390-420 N / mm². 2 While the elongation is 30-35% and f7 is approximately 470, in this embodiment, weight reduction is achieved due to increased strength, as well as good resistance to dezincification corrosion and stress corrosion cracking.

[0130] <Machinability test using a lathe> The machinability was evaluated using a cutting test with a lathe, as described below. Test specimens were prepared by machining hot extruded bar stock (processes AA, AB, and B) and hot forged material (process E) to a diameter of 18 mm. A K10 carbide tool (tip) without a tip breaker was mounted on a lathe. Using this lathe, the circumference of the 18 mm diameter test specimen was machined under dry conditions with a rake angle of 0°, nose radius of 0.4 mm, relief angle of 6°, cutting speed of 40 m / min or 110 m / min, depth of cut of 1.0 mm, and feed rate of 0.11 mm / rev. In this embodiment, the effect of cutting speed was also investigated.

[0131] The signal emitted from a three-part dynamometer (AST-TL1003, manufactured by Miho Electric Works) attached to the tool was converted into an electrical voltage signal and recorded in a recorder. These signals were then converted into cutting resistance (main force component, feed force component, back force component, N). To minimize the effect of chip wear, the cutting test was performed twice in a back-and-forth motion from A→B→C→···C→B→A, and each sample was measured four times. The cutting resistance is calculated using the following formula. Cutting resistance (combinant force of principal force, feed force, and back force) = ((principal force) 2 + (feeding force component) 2 +(back force) 2 ) 1 / 2 Here, the cutting resistance is determined by the main force component, which is the largest component of the cutting force. Therefore, the main force component was used as the cutting resistance, and the average of four measurements was calculated to determine the cutting resistance. The cutting resistance (main component force) of a commercially available free-cutting brass rod C3604 (alloy X, φ20mm) made of Zn-59mass%Cu-3mass%Pb-0.2mass%Fe-0.3mass%Sn alloy was set to 100, and the relative value of the cutting resistance (machinability index) of the sample was calculated and evaluated relative to that sample. In other words, the higher the machinability index, the lower the cutting resistance and the better the machinability. The machinability index was calculated as follows. The index of the cutting test results of the sample (machinability index) = (Cutting resistance of C3604 (main component force) / Cutting resistance of the sample (main component force)) × 100

[0132] Cutting resistance (main component force) depends on the shear strength and tensile strength of the material, and materials with higher strength tend to have higher cutting resistance. For example, in the case of a copper alloy that has good chip breaking properties equivalent to C3604 but is about 1.2 times stronger than C3604, the cutting resistance will be approximately 20% higher than that of C3604 because the cutting resistance is proportional to the material strength. For this reason, in the case of a copper alloy with high strength, a cutting resistance that is about 40% higher than that of C3604 is considered to be practically acceptable. In this embodiment, the tensile strength and shear strength of the extruded material are approximately 1.2 times that of C3604, and therefore, the evaluation criteria for machinability in this embodiment were evaluated with a machinability index of approximately 72 as the boundary value. In detail, if the machinability index is 80 or higher, it is evaluated as having excellent machinability (evaluation: A, excellent) and equivalent machinability to C3604. A machinability index of 72 or higher and less than 80 was evaluated as good machinability (evaluation: B, good). A machinability index of 66 or higher and less than 72 was evaluated as fair machinability (evaluation: C, fair). A machinability index of less than 66 was evaluated as poor machinability (evaluation: D, poor). In this application, since the aim is good machinability, a machinability index of 72 or higher was considered acceptable.

[0133] (Dezincification corrosion test: ISO 6509 dezincification corrosion test) The dezincification corrosion test was performed using the ISO 6509 dezincification corrosion test method. This test method is adopted in many countries and is also specified in the JIS standard JIS H 3250. The procedure for the dezincification corrosion test involved first embedding the test material in a phenolic resin material, specifically, embedding the exposed sample surface perpendicular to the extrusion direction of the extruded material. The sample surface was polished with emery paper up to 1200 grit, and then ultrasonically cleaned in pure water and dried. Each sample was immersed in a 1.0% aqueous solution of cupric chloride dihydrate (CuCl2·2H2O) (12.7 g / L) and kept at 75°C for 24 hours. The samples were then removed from the aqueous solution. The sample was re-embedded in the phenolic resin material so that the exposed surface remained perpendicular to the extrusion direction, longitudinal direction, or forging flow direction. Next, the sample was cut so that the cross-section of the corroded area was obtained as the longest cut section. Subsequently, the sample was polished. Using a metallurgical microscope, corrosion depth was observed at 10 locations in the microscope's field of view, at magnifications ranging from 100 to 500 times. The deepest corrosion point was recorded as the maximum dezincification corrosion depth. Furthermore, when tested according to ISO 6509, a maximum corrosion depth of 200 μm or less is considered to be at a level that does not pose a problem in terms of practical corrosion resistance. When superior corrosion resistance is required, the maximum corrosion depth is preferably 100 μm or less. In this test, a maximum corrosion depth exceeding 200 μm was rated as unacceptable (Rating: D, poor). A maximum corrosion depth between 100 μm and 200 μm was rated as good (Rating: B, good). A maximum corrosion depth of 100 μm or less was rated as excellent (Rating: A, excellent).

[0134] (Stress corrosion cracking test) To determine whether it could withstand stress corrosion cracking environments, a stress corrosion cracking test was conducted using the following procedure. First, a φ20mm rod was cut to a weight of 120±1g (119~121g) as the test material. Using a 500-ton hot forging press, it was formed into a hexagonal flare nut forging material with a side length of 27mm and a length of 26mm under the conditions of process D in Table 14. Furthermore, some samples were shot-peened in the same manner as process No. D7 as shown in Table 14, and the forged material was heat-treated under several conditions as shown in Table 17 (processes No. G2, G3, G12H, G14H). These forged materials were then threaded and the end faces were finished by process cutting to produce a flare nut of type 5 / 8 inch (=15.9mm) with an inner diameter conforming to JIS B 8607. To apply load stress to the flare nut, a base made of C6932 steel, machined to the shape of a male thread for a flare nut fitting with a nominal diameter of 5 / 8 inch (=15.9 mm) in accordance with JIS B 8607, was prepared. The flare nut was tightened to this base using a torque gauge until the specified torque was reached. A desiccator was filled with ammonia water of a specified concentration, and the flare nut, still under load stress, was placed 60 mm above the liquid surface. After being held for a specified time, the flare nut was removed. The test was conducted in a room controlled to 25±1℃ (24~26℃) by an air conditioning system. Stress corrosion cracking tests were conducted using the following two methods.

[0135] (Stress corrosion cracking test - 1) Using ammonia water with an ammonia concentration of 14 vol%, 1000 ml of ammonia water was placed in a desiccator, and a sample subjected to a torque of 70 N·m was exposed in the desiccator for 48 hours and 96 hours. The sample was washed with 10 vol% sulfuric acid and then visually inspected to evaluate for the presence or absence of cracks. This test was conducted in accordance with JIS H 3250 except for the exposure time. While JIS H 3250 specifies an exposure time of 2 hours, this method involved an exposure time more than 20 times longer, making it a rigorous stress corrosion cracking test. In this application, since good resistance to stress corrosion cracking is the goal, samples in which cracks were observed after 48 hours of exposure were judged as "D" (poor). Samples in which no cracks were observed after 48 hours of exposure but cracks were observed after 96 hours of exposure were judged as "C" (fair). Samples in which no cracks were observed after 96 hours of exposure were judged as "B" (good). A judgment of "C" was considered acceptable.

[0136] (Stress corrosion cracking test - 2) As another stress corrosion cracking test, 200 ml of ammonia water with an ammonia concentration of 28 vol% was placed in a desiccator, and a sample subjected to a torque of 150 N·m was exposed to the desiccator and held for 75 hours. The sample was removed from the desiccator, washed with 10 vol% sulfuric acid, and then visually inspected for the presence or absence of cracks. Compared to stress corrosion cracking test-1, the ammonia concentration was higher and the applied torque was larger, so the following criteria were used for evaluation: If the largest visible crack was more than half the length of the sample (26 mm), i.e., a crack of 13 mm or more was clearly observed visually, it was judged as "D" (poor). If the largest visible crack was more than approximately 1 / 5 of the length of the sample (26 mm), i.e., a crack of 5 mm or more but less than 13 mm was clearly observed visually, it was judged as "C" (fair). If the crack was smaller than approximately 1 / 5 of the length of the sample (26 mm), i.e., a crack smaller than 5 mm was observed visually, including no cracks, it was judged as "B" (good). Furthermore, a rating of "C" was deemed acceptable.

[0137] [Table 18]

[0138] Table 19

[0139] Table 20

[0140] Table 21

[0141] Table 22

[0142] Table 23

[0143] Table 24

[0144] Table 25

[0145] Table 26

[0146] Table 27

[0147] Table 28

[0148] Table 29

[0149] Table 30

[0150] Table 31

[0151] Table 32

[0152] Table 33

[0153] Table 34

[0154] Table 35

[0155] Table 36

[0156] Table 37

[0157] Table 38

[0158] Table 39

[0159] Table 40

[0160] Table 41

[0161] Table 42

[0162] Table 43

[0163] Table 44

[0164] Table 45

[0165] Table 46

[0166] Table 47

[0167] Table 48

[0168] Table 49

[0169] Table 50

[0170] Table 51

[0171] Table 52

[0172] Table 53

[0173] Table 54

[0174] Table 55

[0175] Table 56

[0176] 1) By satisfying the composition of this embodiment, the compositional relationship formulas f0 to f2, the requirements for the metallic structure, and the microstructure relationship formulas f3 to f5 and the microstructure-composition relationship formula f6, it was confirmed that hot extruded and hot forged materials can be obtained with a small amount of Pb content, exhibiting good machinability, good resistance to dezincification corrosion and stress corrosion cracking, high electrical conductivity of 15% IACS or higher, high strength, good ductility, and a high balance of strength and ductility (property relationship formula f7). Furthermore, while the presence of P compounds has the effect of significantly improving machinability, the β1 phase, in which the β phase has been modified, further improves machinability. At the same time, it was confirmed that the dezincification corrosion resistance and stress corrosion cracking resistance, which were major problems of the conventional β phase, can be significantly improved by modifying the β phase, as shown below (Alloy Nos. S01, S02, S03, S11, S12, S13, S21~S46).

[0177] 2) By including more than 0.50 mass% of Si, the β phase was modified, and grain boundaries were observed within the β1 phase. As a result, dezincification corrosion resistance and stress corrosion cracking resistance improved, and cutting resistance decreased (machinability index increased). When the amount of Si was 0.65 mass% or more, dezincification corrosion resistance, stress corrosion cracking resistance, and machinability improved further. In other words, it is thought that the β phase was further modified. On the other hand, when the amount of Si exceeded 1.20 mass%, the electrical conductivity decreased. (Alloy Nos. S01, S37, S48, S49, S74, S78, S80, etc.). 3) By including more than 0.01 mass% of P, the β phase was modified, and grain boundaries were observed within the β1 phase. As a result, dezincification corrosion resistance and stress corrosion cracking resistance improved, and cutting resistance decreased. When the amount of P was 0.03 mass% or higher, dezincification corrosion resistance, stress corrosion cracking resistance, and machinability improved further. This suggests that the β phase was further modified. When the amount of P exceeded 0.18 mass%, the elongation value decreased (alloys No. S01, S31, S71, S75, etc.). 4) When the Cu content exceeds 60.5 mass%, the β phase is modified, and grain boundaries are observed within the β1 phase. As a result, dezincification corrosion resistance and stress corrosion cracking resistance are improved, ductility is enhanced, and cutting resistance is reduced. When the Cu content is high, the β phase is less abundant, and in some cases, the γ phase appears, resulting in a lower strength-ductility balance and poor machinability (Alloy Nos. S01, S35, S38, S47, S48, S72, S80, Figures 1A to 6A).

[0178] 5) When the Pb content was less than 0.002 mass%, the machinability was poor (Alloy No. S73). When the Pb content was 0.003 mass% or more, and even 0.01 mass% or more, the machinability improved (Alloy Nos. S33, S23). It was confirmed that the inclusion of Bi lowered the cutting resistance to a level comparable to that of Pb (Alloy Nos. S01, S32, etc.). 6) When Sn content exceeded 0.05 mass%, dezincification corrosion resistance improved. When Sn content exceeded 0.10 mass%, dezincification corrosion resistance improved further. Stress corrosion cracking resistance also improved. On the other hand, when Sn content exceeded 0.90 mass%, the γ phase appeared, which actually worsened dezincification corrosion resistance, reduced ductility, and increased cutting resistance (lower machinability index). Also, when the Sn / Si ratio was 1 or higher, the β phase was not modified, and dezincification corrosion resistance worsened (alloy Nos. S01, S39~S46, S82, S83).

[0179] 7) It was confirmed that even if the alloy contains unavoidable impurities present in actual operation, it does not significantly affect the various properties (alloys No. S01, S02, S03, S11~S13, S27~S30). 8) When the total content of unavoidable impurities Fe, Mn, Co, and Cr was 0.40 mass% or more, the β phase was modified, but the cutting resistance was high, the strength-ductility balance was poor, and the dezincification corrosion resistance was also slightly poor. When the total content of Fe, Mn, Co, and Cr was less than 0.40 mass%, the cutting resistance decreased, the strength-ductility balance improved, and the dezincification corrosion resistance and stress corrosion cracking resistance became good. Furthermore, when the total content of Fe, Mn, Co, and Cr was less than 0.30 mass%, the cutting resistance decreased further, the strength-ductility balance improved, and the dezincification corrosion resistance and stress corrosion cracking resistance became good. The decrease in cutting resistance, strength-ductility balance, and dezincification corrosion resistance are thought to be due to the following reason: It is thought that Fe, Mn, etc. combined with some Si and P, forming intermetallic compounds of Fe, Mn, etc. and Si and P. As a result, compounds of Fe, Si, and P are present, and the Si and P concentrations, which are effective in modifying the β phase, have decreased, leading to a deterioration in machinability and corrosion resistance (alloys No. S01, S02, S84, S25~S30). 9) When the amount of Al, an unavoidable impurity, was 0.30 mass% or more, the β phase was not modified, resulting in poor dezincification corrosion resistance. When the amount of Al was less than 0.30 mass%, the β phase was modified, resulting in good dezincification corrosion resistance. Furthermore, when the amount of Al was less than 0.15 mass%, machinability, strength-ductility balance, dezincification corrosion resistance, and stress corrosion cracking resistance were good (Alloy Nos. S01, S85, S26, S28~S30).

[0180] 10) When the composition relation f1 is small, the β phase is not modified, resulting in poor resistance to dezincification corrosion. When the composition relation f1 is large, the area ratio of the β1 phase decreases (alloys No. S76, S79). When the value of the composition relation f1 is 57.5 or higher, the β phase is modified, improving resistance to dezincification corrosion, and this improvement is further enhanced when the value of the composition relation f1 is 58.0 or higher. On the other hand, when the value of the composition relation f1 is 60.5 or lower, the area ratio of the β1 phase increases, and this increases even further when the value of the composition relation f1 is 60.2 or lower, resulting in a higher machinability index (e.g., alloys No. S01, S03, S35, S36).

[0181] 11) When the area ratio of the β1 phase was 25% or less, good machinability could not be obtained. When the area ratio of the β1 phase exceeded 25%, and further exceeded 30%, the cutting resistance decreased. When the area ratio of the β1 phase exceeded 80%, the β phase was often not modified, meaning that grain boundaries were not observed within the β phase, and as a result, the cutting resistance, especially at 110 m / min, was high, and the dezincification corrosion resistance and stress corrosion cracking resistance were poor. When the area ratio of the β1 phase was 80% or less, and further exceeded 70%, the β phase was modified, and the dezincification corrosion resistance and stress corrosion cracking resistance improved. Although the modification of the β phase significantly slowed the progression of dezincification corrosion, the β1 phase was dezincified more than the α phase. Therefore, a higher area ratio of the β1 phase slightly accelerated the progression of dezincification corrosion (alloys No. S03, S11, S72, S81, S76, processes D1, D11H, E1, E11H, Figure 1B, etc.). 12) When the area ratio of the γ phase was 4% or more, the elongation value decreased. Also, the cutting resistance increased (alloys No. S78, S82). 13) When the microstructure-composition relationship f6 was low, the machinability was poor (alloy No. S81). When f6 was greater than 35, the machinability improved (e.g., alloy No. S12, process No. E3, E4; alloy No. S02, process D11H).

[0182] 14) After hot extrusion, when the starting temperature for the cooling process exceeded 500°C, and the average cooling rate in the temperature range from the starting temperature to 300°C, and the average cooling rate in the temperature range from 500°C to 300°C exceeded 300°C / min, the β phase was modified, and grain boundaries were observed within the β1 phase. When the β phase was modified, the cutting resistance decreased, dezincification corrosion resistance and stress corrosion cracking resistance improved significantly, strength increased, and the strength-ductility balance improved. Regarding machinability, when the β phase was modified, the decrease in machinability was slight even at high cutting speeds (processes No. A1~A5, D1~D5, E1~E5, F1~F3, G1~G3, Figures 1A~6A, etc.). 15) When the cooling start temperature was above 500°C and below approximately 530°C, and when the average cooling rate in the temperature range from 500°C to 300°C was approximately 400 to 500°C / min, grain boundaries were observed within the β1 phase, and a small amount of P compound was sometimes present. When a small amount of P compound was present, the cutting resistance was slightly higher and the resistance to dezincification corrosion was slightly worse than when no P compound was present (processes No. A1-A5, D1-D5, E1-E5, F1-F3, G1-G3, Figures 1A-6A, 1B-6B, etc.).

[0183] 16) When the average cooling rate in the temperature range from 500°C to 300°C was approximately 200-250°C / min, no grain boundaries were observed within the β1 phase, and in most cases, only a small amount of P compound was present. As a result of the absence of grain boundaries within the β1 phase, machinability, especially at 110 m / min, deteriorated, and resistance to dezincification corrosion worsened (process No. A14H, D14H, etc.). 17) When the cooling treatment start temperature was 550°C or higher, and the average cooling rate in the temperature range from the cooling treatment start temperature to 300°C, and the average cooling rate in the temperature range from 500°C to 300°C were favorable cooling rates, the machinability index was high, and the dezincification corrosion resistance and stress corrosion cracking resistance were good. In other words, it is considered that the β phase is further modified when the cooling treatment is started at an appropriate temperature and the cooling rate is fast. (Process Nos. A1~A5, A11H~A14H, D1~D5, D11H~D14H, E1~E5, E11H~E15H, F1~F3, F4H, F5H, G1~G3, G11H~G14H, Figures 1B~6B, etc.). 18) When the starting temperature of the cooling process was higher than 670°C, the proportion of the β1 phase increased, resulting in slightly higher cutting resistance, particularly at 110 m / min, and slightly worse resistance to dezincification corrosion. When the proportion of the β1 phase became too high, the β phase was sometimes not modified. The form of dezincification corrosion was selective corrosion of the β1 phase, and it appeared that a higher proportion of the β1 phase slightly accelerated the rate of dezincification corrosion (process Nos. A1~A5, D1~D5, E1~E5, F1~F3, G1~G3, etc.).

[0184] 19) When the β phase was modified and the β1 phase was observed, the stress corrosion cracking resistance improved, and all samples received a "B" rating in the stress corrosion cracking test-1 (processes No. D1 to D5). 20) After hot working (hot forging), shot blasting further improved stress corrosion cracking resistance, resulting in a "B" rating in both stress corrosion cracking test-1 and stress corrosion cracking test-2. On the other hand, for samples where the β1 phase was not observed, or for samples with low Si content, shot blasting did not improve stress corrosion cracking resistance (process No. D3, D7, alloy No. S71, 80).

[0185] 21) In cases involving two hot working processes, for example, when hot forging a hot-extruded bar, it was confirmed that the β phase is modified regardless of the cooling method after the first hot working process, as long as the cooling treatment of the final hot working process, i.e., hot forging, is appropriate (processes No. D1~D5, D7). 22) Even if the material used for hot forging is a casting, if the cooling treatment after hot forging is appropriate, it was confirmed that the β1 phase can be observed and the same properties can be obtained as with hot extruded materials (process No. D6, E6, F1). 23) When the alloy with the modified β phase was held at 300°C for 2 hours, the grain boundaries of the β1 phase disappeared. As a result, machinability and resistance to dezincification corrosion worsened. Under the conditions of 130°C for 5 hours, the grain boundaries of the β1 phase were observed, and no decrease in machinability or dezincification corrosion resistance was observed (process No. A21, A25, A31H).

[0186] 24) Even if a predetermined cooling treatment is not performed during the final hot working stage, the hot-worked material is annealed at a temperature above 520°C but below 630°C for 1 minute to 5 hours. After annealing, the cooling treatment is started at a temperature above 500°C, and the average cooling rate in the temperature range from the cooling treatment start temperature to 500°C, and the average cooling rate in the temperature range from 500°C to 300°C, exceeds 300°C / min. This modifies the β phase, and it was confirmed that the dezincification corrosion resistance and stress corrosion cracking resistance are significantly improved (Process No. G1~G3, Figure 5A, Figure 5B).

[0187] From the above, the alloy of this embodiment, in which the content and compositional relationships of each additive element and each microstructure relationship are within an appropriate range, exhibits excellent hot workability, good machinability, mechanical properties, resistance to dezincification corrosion, and resistance to stress corrosion cracking. Furthermore, excellent properties can be obtained in the alloy of this embodiment by setting the manufacturing conditions in hot extrusion and hot forging, and the heat treatment conditions within an appropriate range. [Industrial applicability]

[0188] The free-machining copper alloy of this embodiment has a low Pb content, excellent hot workability and machinability, high strength, and an excellent balance of strength and elongation. For this reason, the free-machining copper alloy of this embodiment is suitable for appliances and parts related to drinking water and sanitation equipment, food appliances, electrical and electronic equipment parts, automobile parts, machine parts, stationery, toys, musical instruments, sliding parts, instrument parts, precision machine parts, medical parts, beverage appliances and parts, water meters, and parts related to liquids and gases such as industrial water, wastewater, and hydrogen. Specifically, it can be suitably applied as a component of items used in the aforementioned fields, such as water taps, shut-off taps, mixing taps, shower heads, valves, fittings, cocks, gears, shafts, bearings, trumpets, shafts, sleeves, spindles, sensors, bolts, nuts, flare nuts, pen tips, insert nuts, cap nuts, nipples, spacers, screws, etc.

Claims

1. It contains Cu in amounts exceeding 60.5 mass% but less than 65.0 mass%, Si in amounts exceeding 0.50 mass% but less than 1.20 mass%, Pb in amounts between 0.002 mass% and less than 0.20 mass%, and P in amounts exceeding 0.01 mass% but less than 0.18 mass%, with the remainder being Zn and unavoidable impurities. Of the aforementioned unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than 0.40 mass%, and the content of Al is less than 0.30 mass%, When the Cu content is [Cu]mass%, the Si content is [Si]mass%, the Pb content is [Pb]mass%, and the P content is [P]mass%, The relationship 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb]) - [P] ≤ 60.5 holds true. In the constituent phases of a metallic microstructure excluding nonmetallic inclusions, if the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1), 20≦f3=(α)<75, 25<f4=(β1)≦80, 0≦f5=(γ)<4, 27<f6=(β1)×([Si]) １／２ - (γ) × 2 + ([Pb]) １／２ ×20+([P]) １／２ ×15 They have a relationship, The β1 phase exhibits visible grain boundaries when etched with a mixture of hydrogen peroxide and aqueous ammonia. A free-machining copper alloy characterized by being a hot-extruded material, a hot-forged material, or a hot-rolled material, or a processed material obtained by subjecting these to at least one of cold working and / or heat treatment.

2. It contains Cu in amounts exceeding 60.5 mass% and less than 65.0 mass%, Si in amounts exceeding 0.50 mass% and less than 1.20 mass%, Pb in amounts between 0.002 mass% and less than 0.20 mass%, P in amounts exceeding 0.01 mass% and less than 0.18 mass%, and Bi in amounts between 0.0001 mass% and less than 0.20 mass%, with the remainder being Zn and unavoidable impurities. Of the aforementioned unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than 0.40 mass%, and the content of Al is less than 0.30 mass%, When the Cu content is [Cu]mass%, the Si content is [Si]mass%, the Pb content is [Pb]mass%, the P content is [P]mass%, and the Bi content is [Bi]mass%, The relationship 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.5 holds true. Furthermore, the relationship 0.003 ≤ f² = [Pb] + [Bi] < 0.20 holds, In the constituent phases of a metallic microstructure excluding nonmetallic inclusions, if the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1), 20≦f3=(α)<75, 25<f4=(β1)≦80, 0≦f5=(γ)<4, 27<f6=(β1)×([Si]) １／２ -(c)×2+([Pb]+[B]) １／２ ×20+([P]) １／２ ×15 They have a relationship, The β1 phase exhibits visible grain boundaries when etched with a mixture of hydrogen peroxide and aqueous ammonia. A free-machining copper alloy characterized by being a hot-extruded material, a hot-forged material, or a hot-rolled material, or a processed material obtained by subjecting these to at least one of cold working and / or heat treatment.

3. It contains 61.2 mass% to 64.8 mass% of Cu, 0.65 mass% to 1.10 mass% of Si, 0.003 mass% to less than 0.10 mass% of Pb, and 0.03 mass% to 0.15 mass% of P, with the remainder being Zn and unavoidable impurities. Of the aforementioned unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than 0.30 mass%, and the content of Al is less than 0.15 mass%, When the Cu content is [Cu]mass%, the Si content is [Si]mass%, the Pb content is [Pb]mass%, and the P content is [P]mass%, The relationship 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb]) - [P] ≤ 60.2 holds true. In the constituent phases of a metallic microstructure excluding nonmetallic inclusions, if the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1), 30≦f3=(α)≦70, 30≦f4=(β1)≦70, 0≦f5=(γ)<1, 35<f6=(β1)×([Si]) １／２ - (γ) × 2 + ([Pb]) １／２ ×20+([P]) １／２ ×15 They have a relationship, The β1 phase exhibits visible grain boundaries when etched with a mixture of hydrogen peroxide and aqueous ammonia. A free-machining copper alloy characterized by being a hot-extruded material, a hot-forged material, or a hot-rolled material, or a processed material obtained by subjecting these to at least one of cold working and / or heat treatment.

4. It contains 61.2 mass% to 64.8 mass% of Cu, 0.65 mass% to 1.10 mass% of Si, 0.003 mass% to less than 0.10 mass% of Pb, 0.03 mass% to 0.15 mass% of P, and 0.001 mass% to less than 0.10 mass% of Bi, with the remainder being Zn and unavoidable impurities. Of the aforementioned unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than 0.30 mass%, and the content of Al is less than 0.15 mass%, When the Cu content is [Cu]mass%, the Si content is [Si]mass%, the Pb content is [Pb]mass%, the P content is [P]mass%, and the Bi content is [Bi]mass%, The relationship 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.2 holds true. Furthermore, the relationship 0.004 ≤ f² = [Pb] + [Bi] < 0.10 holds true, In the constituent phases of a metallic microstructure excluding nonmetallic inclusions, if the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1), 30≦f3=(α)≦70, 30≦f4=(β1)≦70, 0≦f5=(γ)<1, 35<f6=(β1)×([Si]) １／２ -(c)×2+([Pb]+[B]) １／２ ×20+([P]) １／２ ×15 They have a relationship, The β1 phase exhibits visible grain boundaries when etched with a mixture of hydrogen peroxide and aqueous ammonia. A free-machining copper alloy characterized by being a hot-extruded material, a hot-forged material, or a hot-rolled material, or a processed material obtained by subjecting these to at least one of cold working and / or heat treatment.

5. It contains Cu in amounts exceeding 60.5 mass% and less than 65.0 mass%, Si in amounts exceeding 0.50 mass% and less than 1.20 mass%, Pb in amounts between 0.002 mass% and less than 0.20 mass%, P in amounts exceeding 0.01 mass% and less than 0.18 mass%, and Sn in amounts exceeding 0.05 mass% and less than 0.90 mass%, with the remainder being Zn and unavoidable impurities. Of the aforementioned unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than 0.40 mass%, and the content of Al is less than 0.30 mass%, When the Cu content is [Cu]mass%, the Si content is [Si]mass%, the Pb content is [Pb]mass%, the P content is [P]mass%, and the Sn content is [Sn]mass%, The relationship 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb]) - [P] - [Sn] ≤ 60.5 holds true, and Furthermore, the relationship f0 = [Sn] / [Si] < 1 holds, In the constituent phases of a metallic microstructure excluding nonmetallic inclusions, if the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1), 20≦f3=(α)<75, 25<f4=(β1)≦80, 0≦f5=(γ)<4, 27<f6=(β1)×([Si]) １／２ - (γ) × 2 + ([Pb]) １／２ ×20+([P]) １／２ ×15 They have a relationship, The β1 phase exhibits visible grain boundaries when etched with a mixture of hydrogen peroxide and aqueous ammonia. A free-machining copper alloy characterized by being a hot-extruded material, a hot-forged material, or a hot-rolled material, or a processed material obtained by subjecting these to at least one of cold working and / or heat treatment.

6. It contains Cu in amounts exceeding 60.5 mass% and less than 65.0 mass%, Si in amounts exceeding 0.50 mass% and less than 1.20 mass%, Pb in amounts between 0.002 mass% and less than 0.20 mass%, P in amounts exceeding 0.01 mass% and less than 0.18 mass%, Sn in amounts exceeding 0.05 mass% and less than 0.90 mass%, and Bi in amounts between 0.0001 mass% and less than 0.20 mass%, with the remainder being Zn and unavoidable impurities. Of the aforementioned unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than 0.40 mass%, and the content of Al is less than 0.30 mass%, When the Cu content is [Cu]mass%, the Si content is [Si]mass%, the Pb content is [Pb]mass%, the P content is [P]mass%, the Bi content is [Bi]mass%, and the Sn content is [Sn]mass%, The relationship 57.5 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≤ 60.5 holds true, and Furthermore, the relationship f0 = [Sn] / [Si] < 1 holds, The relationship 0.003 ≤ f² = [Pb] + [Bi] < 0.20 holds true, In the constituent phases of a metallic microstructure excluding nonmetallic inclusions, if the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1), 20≦f3=(α)<75, 25<f4=(β1)≦80, 0≦f5=(γ)<4, 27<f6=(β1)×([Si]) １／２ -(c)×2+([Pb]+[B]) １／２ ×20+([P]) １／２ ×15 They have a relationship, The β1 phase exhibits visible grain boundaries when etched with a mixture of hydrogen peroxide and aqueous ammonia. A free-machining copper alloy characterized by being a hot-extruded material, a hot-forged material, or a hot-rolled material, or a processed material obtained by subjecting these to at least one of cold working and / or heat treatment.

7. It contains 61.2 mass% to 64.8 mass% of Cu, 0.65 mass% to 1.10 mass% of Si, 0.003 mass% to less than 0.10 mass% of Pb, 0.03 mass% to 0.15 mass% of P, and 0.10 mass% to less than 0.50 mass% of Sn, with the remainder being Zn and unavoidable impurities. Of the aforementioned unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than 0.30 mass%, and the content of Al is less than 0.15 mass%, When the Cu content is [Cu]mass%, the Si content is [Si]mass%, the Pb content is [Pb]mass%, the P content is [P]mass%, and the Sn content is [Sn]mass%, The relationship 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb]) - [P] - [Sn] ≤ 60.2 holds true. Furthermore, the relationship f0 = [Sn] / [Si] < 0.6 holds, In the constituent phases of a metallic microstructure excluding nonmetallic inclusions, if the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1), 30≦f3=(α)≦70, 30≦f4=(β1)≦70, 0≦f5=(γ)<1, 35<f6=(β1)×([Si]) １／２ - (γ) × 2 + ([Pb]) １／２ ×20+([P]) １／２ ×15 They have a relationship, The β1 phase exhibits visible grain boundaries when etched with a mixture of hydrogen peroxide and aqueous ammonia. A free-machining copper alloy characterized by being a hot-extruded material, a hot-forged material, or a hot-rolled material, or a processed material obtained by subjecting these to at least one of cold working and / or heat treatment.

8. It contains 61.2 mass% to 64.8 mass% of Cu, 0.65 mass% to 1.10 mass% of Si, 0.003 mass% to less than 0.10 mass% of Pb, 0.03 mass% to 0.15 mass% of P, 0.10 mass% to less than 0.50 mass% of Sn, and 0.001 mass% to less than 0.10 mass% of Bi, with the remainder being Zn and unavoidable impurities. Of the aforementioned unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than 0.30 mass%, and the content of Al is less than 0.15 mass%, When the Cu content is [Cu]mass%, the Si content is [Si]mass%, the Pb content is [Pb]mass%, the P content is [P]mass%, the Bi content is [Bi]mass%, and the Sn content is [Sn]mass%, The relationship 58.0 ≤ f1 = [Cu] - 4.6 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≤ 60.2 holds true. Furthermore, the relationship f0 = [Sn] / [Si] < 0.6 holds, The relationship 0.004 ≤ f² = [Pb] + [Bi] < 0.10 holds true, In the constituent phases of a metallic microstructure excluding nonmetallic inclusions, if the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1), 30≦f3=(α)≦70, 30≦f4=(β1)≦70, 0≦f5=(γ)<1, 35<f6=(β1)×([Si]) １／２ -(c)×2+([Pb]+[B]) １／２ ×20+([P]) １／２ ×15 They have a relationship, The β1 phase exhibits visible grain boundaries when etched with a mixture of hydrogen peroxide and aqueous ammonia. A free-machining copper alloy characterized by being a hot-extruded material, a hot-forged material, or a hot-rolled material, or a processed material obtained by subjecting these to at least one of cold working and / or heat treatment.

9. A free-machining copper alloy according to any one of claims 1 to 8, characterized in that it is used in equipment and parts related to drinking water and sanitation facilities, valves, cocks, industrial piping parts, water meters, musical instruments, automobile parts, electrical and electronic equipment parts, machine parts, stationery, toys, sliding parts, instrument parts, precision machine parts, and medical parts.

10. A method for producing a free-machining copper alloy as described in any one of claims 1 to 8, Having one or more hot working steps, A method for manufacturing a free-cutting copper alloy, characterized in that, in the final hot working step of the hot working process, the hot working temperature is greater than 540°C and less than 750°C; in the cooling process after hot working, the hot-worked material is cooled at a temperature lower than 670°C and higher than 500°C; and in the cooling process, the average cooling rate in the temperature range from the cooling start temperature to 500°C is greater than 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is greater than 300°C / min.

11. A method for producing a free-machining copper alloy as described in any one of claims 1 to 8, A method for producing a free-machining copper alloy, comprising one or more hot working steps and a heat treatment step, wherein in the final heat treatment step, the alloy is annealed at a temperature above 520°C and below 630°C for 1 minute to 5 hours, after annealing, a cooling process is started at a temperature above 500°C, the average cooling rate in the temperature range from the cooling start temperature to 500°C is above 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is above 300°C / min.

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