Free-machining copper alloy castings, and methods for manufacturing free-machining copper alloy castings

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 and f2, 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. When etched using a liquid mixture of hydrogen peroxide and aqueous ammonia, grain boundaries can be observed in a β1 phase.

Description

[Technical Field]

[0001] This invention relates to a free-machining copper alloy casting that has good castability, resistance to dezincification corrosion, excellent machinability, and a significantly reduced lead content, as well as a method for manufacturing a free-machining copper alloy casting. This invention relates to a free-machining copper alloy casting used in equipment and parts used in drinking water consumed daily by humans and animals, equipment and parts used in sanitary facilities such as kitchens, bathrooms, and toilets, musical instruments, tableware, drainage equipment and parts, industrial piping parts, machine parts, sliding parts, medical parts, automobile parts, electrical and home appliance parts, pressure vessels, building hardware, instrument parts, daily necessities, stationery, toys, and parts related to liquids and gases such as drinking water, industrial water, wastewater, and hydrogen, as well as a method for manufacturing a free-machining copper alloy casting. Specific examples of parts include faucets, water taps, mixing faucets, shut-off valves, valves, fittings, stems, cocks, water meters, drain plugs, pressure reducing valves, valve seats, gate valves, fire hydrants, gears, flanges, branch valves, faucet valves, ball valves, bends, bearings, sleeves, and connectors. The present invention relates to free-machining copper alloy castings used in these machined parts, and to a method for manufacturing free-machining copper alloy castings. 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, Cu-Zn-Pb alloys (so-called free-cutting brass rods, forging brass, casting brass) or Cu-Sn-Zn-Pb alloys (so-called bronze castings (leaded red brass)) have been commonly used for equipment and parts related to drinking water, sanitary facilities, automobile parts, electrical and home appliance parts, machine parts, stationery, instrument parts, medical parts, and equipment and parts related to liquids and gases such as industrial water, wastewater, and hydrogen, specifically including faucets, taps, mixers, stopcocks, valves, cocks, fittings, valves, water meters, and connectors, due to their excellent machinability. Here, the composition of the Cu-Zn-Pb alloy is, for example, 56-70 mass% Cu, 1-4 mass% Pb, and the remainder being Zn. Similarly, the composition of the Cu-Sn-Zn-Pb alloy is, for example, 80-88 mass% Cu, 1-8 mass% Sn, 1-8 mass% Pb, and the remainder being Zn.

[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 automobiles, electrical and electronic equipment, and machinery, for example, the European ELV Directive and RoHS Directive 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 restrictions on free-machining copper alloys, the following have been proposed: (1) Cu-Zn-Bi alloys and Cu-Zn-Bi-Se alloys that contain Bi, which has machinability, instead of Pb, and in some cases, Se together with Bi; (2) Cu-Zn alloys that contain a high concentration of Zn and increase the β phase to improve machinability; (3) Cu-Zn-Si alloys and Cu-Zn-Sn alloys that contain a large amount of γ phase and κ phase, which have excellent machinability instead of Pb; and (4) Cu-Zn-Sn-Bi alloys that contain a large amount of γ phase and also contain Bi. For example, in Patent Document 1, corrosion resistance and machinability are improved by adding 0.7 to 2.0 mass% of Sn and 0.5 to 2.0 mass% of Bi to a Cu-Zn alloy containing 59.5 to 66.0 mass% of Cu, thereby precipitating a γ phase.

[0006] However, alloys containing Bi instead of Pb have many problems, including the fact that Bi is less machinable than Pb, Bi may be harmful to the human body like Pb, Bi is a rare metal and therefore has resource problems, and Bi makes copper alloy materials brittle. Furthermore, as shown in Patent Document 1, even if a γ phase is precipitated in a Cu-Zn-Sn alloy, the γ phase containing Sn has poor machinability, requiring the co-addition of Bi, which has machinability properties.

[0007] Furthermore, in Patent Document 2, a large amount of Bi, preferably 0.3 to 4 mass%, and more preferably 1.8 to 3.2 mass%, is added to a Cu-Zn alloy containing 59 to 62 mass% Cu. In addition, 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. Thus, conventionally, the β phase of Cu-Zn alloys has poor resistance to dezincification corrosion, and measures to improve this have substantially required reducing the β phase and annealing to separate the β phase with the α phase.

[0008] On the other hand, Cu-Zn binary alloys containing a large amount of β phase contribute to improved machinability, but the β phase has inferior machinability compared to Pb, and is also inferior in dezincification corrosion resistance and stress corrosion cracking resistance, so it can hardly be used as a substitute for Pb-containing free-machining copper alloys. Therefore, in recent years, Cu-Zn-Si alloys containing Si instead of Pb have been proposed as free-machining copper alloys in patent documents 3 to 9.

[0009] Patent documents 3 to 8 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 from 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 2, 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 3 to 8, 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.

[0010] Patent Document 9 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.

[0011] Furthermore, Patent Document 10 describes a Cu-Zn-Sn alloy containing small amounts of Si, Pb, P, or Fe, and 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 10 also states that a finish heat treatment of 400 to 600°C is substantially necessary to improve resistance to dezincification corrosion.

[0012] Patent document 11 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 12 proposes a copper-zinc-zirconium-phosphorus alloy copper alloy casting in which Si, Pb, Sn, and Bi are optionally contained, and crystal grains are refined by the action of Zr and P. Patent Document 13 proposes a copper alloy with excellent discoloration resistance in which Si and Pb are optionally contained, and the area ratios of the γ phase and β phase are limited, in a Cu-Zn-Sn-Al alloy. Patent Document 14 proposes a Pb-free copper alloy casting for a Cu-Zn-Si-Sn-Al-P alloy. Patent Document 15 proposes that in a Cu-Zn-Si-Sn-Al alloy, the apparent Zn content is important for improving corrosion resistance, and the machinability is improved by substantially containing a large amount of Pb or Bi. Patent Document 16 describes that a Cu-Zn-Si alloy, which is a Pb-free copper alloy casting containing 65 mass% or more of Cu with good castability and mechanical strength, has improved machinability due to the γ phase, and an example containing a large amount of Sn, Al, Mn, Ni, and Sb is described. [Prior Art Documents] [Patent Documents]

[0013] [Patent Document 1] International Publication No. 2008 / 081947 [Patent Document 2] Japanese Unexamined Patent Application Publication No. 2002-003967 [Patent Document 3] Japanese Unexamined Patent Application Publication No. 2021-042461 [Patent Document 4] Japanese Unexamined Patent Application Publication No. 2021-042459 [Patent Document 5] Japanese Unexamined Patent Application Publication No. 2021-042460 [Patent Document 6] International Publication No. 2020 / 261666 [Patent Document 7] Japanese Unexamined Patent Application Publication No. 2021-042462 [Patent Document 8] International Publication No. 2021 / 117528 [Patent Document 9] International Publication No. 2007 / 034571 [Patent Document 10] Japanese Patent Publication No. 2016-194123 [Patent Document 11] International Publication No. 2013 / 065830 [Patent Document 12] International Publication No. 2006 / 016630 [Patent Document 13] International Publication No. 2015 / 046421 [Patent Document 14] Japanese Patent Publication No. 2010-133006 [Patent Document 15] Japanese Patent Publication No. 2018-048398 [Patent Document 16] Special Publication No. 2019-508584 [Overview of the project] [Problems that the invention aims to solve]

[0014] As shown in these Patent Documents 1 to 16, there has been no fundamental improvement in the dezincification corrosion resistance of the β phase present in Cu-Zn alloys, which has been a major technical challenge in the past. Furthermore, there is no disclosure of a Cu-Zn alloy that exhibits low cutting resistance and excellent machinability at high-speed cutting speeds exceeding 100 m / min, even with a Pb and Bi content of less than 0.2 mass%, and without the essential presence of Bi.

[0015] The present invention has been made to solve the problems of the prior art, and aims to provide a free-machining copper alloy casting that has excellent machinability and castability, good resistance to dezincification corrosion despite containing a large amount of β phase (β1 phase described later), high strength, good impact properties, and a significantly reduced lead content, as well as a method for manufacturing a free-machining copper alloy casting. [Means for solving the problem]

[0016] 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 refers to the modified β phase, and the β phase and the β1 phase are distinguished. The β1 phase is characterized by the observation of grain boundary patterns within the β1 phase when observed with a metallurgical microscope using hydrogen peroxide and ammonia water as etching solutions, and is recognized as a grain boundary. In general Cu-Zn alloys, Cu-Zn-Bi alloys, Cu-Zn-Si alloys, etc., the β phase does not show grain boundary patterns, i.e., grain boundaries, even when etched with hydrogen peroxide and ammonia water. Therefore, the β phase and the β1 phase can be clearly distinguished. For example, Figure 1A, described later, shows the metal structure of the alloy of the present invention consisting of two phases: the modified β1 phase and the α phase, and grain boundary patterns can be observed within the β1 phase. In this application, the grain boundary pattern observed in Figure 1A is referred to as a crystal grain boundary, and is sometimes simply called a grain boundary. On the other hand, Figure 2A is the metallic structure of a comparative alloy consisting of a β phase and an α phase, and no grain boundary pattern is observed in the β phase, i.e., no grain boundaries exist. The P compound refers to a compound of P and mainly Zn and / or Si. Cold workability refers to the performance of cold processing such as drawing, wire drawing, crimping, and bending. Good, excellent machinability, unless otherwise specified, refers to low cutting resistance and good 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. The cooling rate at a certain temperature, for example, 500°C, is the cooling rate from a temperature just above 500°C through 500°C, and is the average cooling rate from a temperature several tens of degrees higher than 500°C to 500°C. Actual operation means manufacturing using actual mass production equipment.

[0017] In the aforementioned Patent Document 9, 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 11, 12, and 13, the amount of the β phase is also significantly limited. In Patent Document 2, as a method to improve the dezincification corrosion resistance of the β phase, it is necessary to reduce the β phase and anneal it at 350-550°C to separate the β phase with the α phase. In Patent Document 10, 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 it 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, and performing a heat treatment at 400-200°C with an average cooling rate of 0.2-10°C / second.

[0018] On the other hand, Patent Documents 3 to 8 describe the discovery that, in Cu-Zn-Si alloys, first, incorporating a certain amount of Si into the β phase significantly improves 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 of approximately 530°C to approximately 450°C after hot working or casting. Incidentally, Patent Documents 3-8 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.

[0019] 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, further improved machinability, especially during high-speed cutting, and significantly improved the dezincification corrosion resistance of the β phase, which had been a long-standing issue with Cu-Zn alloys. In addition, alloys containing the β1 phase instead of the β phase could be made to have even higher strength without impairing ductility. This β1 phase requires, first, solidifying Si and P in the β phase at a high temperature after solidification. Then, by maintaining the β phase at a high temperature exceeding 500°C, i.e., between 500°C and 700°C, and increasing the cooling rate when cooling to room temperature, the high-temperature metallic structure is brought down to room temperature.

[0020] 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 of 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, whereas grain boundary patterns, i.e., 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 3 to 8, 11, and 12, and the metal structure is shown.

[0021] The conditions for obtaining the β1 phase are that the molten metal is poured into the mold, and after solidification, the casting maintains a high temperature of over 500°C, i.e., a β phase state between 500°C and 700°C, until it reaches room temperature. Specifically, to obtain the β1 phase, in the cooling process after solidification, it is preferable that the cooling process is started at a temperature lower than 700°C and higher than 500°C, and at the point when the temperature of the casting reaches 500°C, i.e., in the temperature region just above 500°C, it is necessary that the cooling rate exceeds 300°C / min, and in the subsequent cooling process, the average cooling rate in the temperature region from 500°C to 300°C is at least 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.

[0022] In Patent Documents 3 to 8, in order to obtain a compound containing fine P, the casting or hot-worked material requires slower cooling in the temperature range of approximately 530°C to 450°C after casting or hot working, compared to the present application, which requires an average cooling rate of approximately 0.1°C / min to approximately 70°C / min. In other words, it is clear that the present application and Patent Documents 3 to 8 are moving in opposite directions regarding cooling after solidification or hot working. However, for example, if the cooling rate around 530°C during the cooling process is about 50°C / min, and the average cooling rate in the temperature range from 530°C (the starting temperature of the cooling process) to 500°C exceeds 300°C, then alloys containing both the β1 phase and the P compound may exist in this application as well.

[0023] The modification from the β phase to the β1 phase in this invention significantly improves the machinability of the Cu-Zn-Si alloy, even without the presence of P compounds. Furthermore, the synergistic effect of the β1 phase with a small amount of fine Pb particles, or particles containing Pb and Bi, reduces cutting resistance and promotes chip fragmentation, maintaining good machinability even at high speeds. More importantly, the unresolved issues of dezincification corrosion resistance and stress corrosion cracking resistance of the conventional β phase are significantly improved and resolved by the modification from the β phase to the β1 phase. In addition, the mechanical properties inherit the high strength of the conventional β phase, and the modification from the β phase to the β1 phase provides even higher strength without compromising ductility and impact properties. Furthermore, in the case of castings, cold working such as riveting is rarely performed, and for practical purposes, it is necessary that they are not brittle, that is, that they can withstand impact. By modifying the β phase to the β1 phase, it is possible to achieve higher strength while preventing a decrease in impact properties. As a result, we have invented a copper alloy casting that has machinability comparable to conventional free-cutting brass containing a large amount of Pb, and compared to conventional free-cutting brass, has better resistance to dezincification corrosion and stress corrosion cracking, as well as higher strength and good impact properties.

[0024] The free-machining copper alloy casting according to embodiment 1 of the present invention 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 of 0.002 mass% or more and less than 0.20 mass%, and P in amounts exceeding 0.01 mass% and less than 0.18 mass%, Includes Bi of 0 mass% or more and less than 0.20 mass%, The remainder consists of Zn and unavoidable impurities, and of the unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than 0.50 mass%, and the content of Al is less than 0.30 mass%, and the content of Cu is [Cu] mass%, the content of Si is [Si] mass%, the content of Pb is [Pb] mass%, the content of P is [P] mass%, and the content of Bi is [Bi] mass%, then the relationship 57.3 ≤ f1 = [Cu] - 4.8 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.8 holds. If Bi is present in a concentration of 0.0001 mass% or more,Furthermore, the relationship 0.003≦f2=[Pb]+[Bi]<0.20 exists, 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 20≦f3=(α)<75, 25 <f4=(β1)≦80、0≦f5=(γ)<4、28<f6=(β1)×(-0.5×[Si] 2 +1.5×[Si])-2×(γ)+([Pb]+[Bi]) 1 / 2 ×25+([P]) 1 / 2 It has a ×12 relationship and is characterized in that grain boundaries are observed within the β1 phase when etched with a mixture of hydrogen peroxide and aqueous ammonia.

[0025] The free-machining copper alloy casting according to aspect 2 of the present invention contains 61.5 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. Includes Bi of 0 mass% or more and less than 0.20 mass%, The remainder consists of Zn and unavoidable impurities, and of the unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than 0.35 mass%, and the content of Al is less than 0.15 mass%, and the content of Cu is [Cu] mass%, the content of Si is [Si] mass%, the content of Pb is [Pb] mass%, the content of P is [P] mass%, and the content of Bi is [Bi] mass%, then the relationship 58.0 ≤ f1 = [Cu] - 4.8 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.5 holds. If Bi is present in a concentration of 0.0001 mass% or more, Furthermore, the relationship 0.004≦f2=[Pb]+[Bi]<0.10 exists, 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)×(-0.5×[Si] 2 +1.5×[Si])-2×(γ)+([Pb]+[Bi]) 1 / 2 ×25+([P]) 1 / 2has a relationship of ×12, and when etched with a mixed solution of hydrogen peroxide and aqueous ammonia, grain boundaries are observed within the β1 phase.

[0026] The free-cutting copper alloy casting 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, and more than 0.05 mass% and less than 0.70 mass% of Sn, Includes Bi of 0 mass% or more and less than 0.20 mass%, the balance being Zn and unavoidable impurities, among the unavoidable impurities, the total content of Fe, Mn, Co and Cr is less than 0.50 mass%, and the content of Al is less than 0.30 mass%; when the Cu content is defined as [Cu] mass%, the Si content as [Si] mass%, the Pb content as [Pb] mass%, the P content as [P] mass%, the Bi content as [Bi] mass%, and the Sn content as [Sn] mass%, the relationship 57.3 ≦ f1 = [Cu] - 4.8 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≦ 60.8 is satisfied, and the relationship f0 = [Sn] / [Si] < 0.8 is further satisfied, If Bi is present in a concentration of 0.0001 mass% or more, the relationship 0.003 ≦ f2 = [Pb] + [Bi] < 0.20 is satisfied, and 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 relationships 20 ≦ f3 = (α) < 75, 25 < f4 = (β1) ≦ 80, 0 ≦ f5 = (γ) < 4, 28 < f6 = (β1) × (-0.5 × [Si] 2 + 1.5 × [Si]) - 2 × (γ) + ([Pb] + [Bi]) 1 / 2 × 25 + ([P]) 1 / 2 × 12 is satisfied, and when etched with a mixed solution of hydrogen peroxide and aqueous ammonia, grain boundaries are observed within the β1 phase.

[0027] The free-machining copper alloy casting of embodiment 4 of the present invention is characterized in that, in any one of embodiments 1 to 3 of the present invention, it is used for equipment and parts related to drinking water and sanitation facilities, water meters, valves, drainage equipment and parts, industrial piping parts, automobile parts, electrical and home appliance parts, machine parts, stationery, toys, musical instruments, sliding parts, instrument parts, and medical parts.

[0028] A method for manufacturing a free-machining copper alloy casting according to aspect 5 of the present invention is a method for manufacturing a free-machining copper alloy casting according to any one of aspects 1 to 4 of the present invention, comprising a casting step, wherein in the cooling process of the final casting step, the cooling rate at the point when the temperature of the casting reaches 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.

[0029] A method for manufacturing a free-machining copper alloy casting according to aspect 6 of the present invention is a method for manufacturing a free-machining copper alloy casting according to any one of aspects 1 to 4 of the present invention, comprising a casting step, wherein in the cooling process of the final casting step, the cooling start temperature is set to a temperature higher than 500°C and lower than 700°C, the cooling rate when the casting temperature is 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.

[0030] A method for manufacturing a free-machining copper alloy casting according to aspect 7 of the present invention is a method for manufacturing a free-machining copper alloy casting according to any one of aspects 1 to 4 of the present invention, comprising a casting step and a heat treatment step, wherein in the final heat treatment step, the casting is heated under conditions of being held at a temperature above 520°C and below 650°C for 1 minute to 5 hours, and in the cooling step after heat treatment, the cooling process is started when the temperature of the casting exceeds 520°C, the cooling rate when the temperature of the casting reaches 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. [Effects of the Invention]

[0031] According to one aspect of the present invention, it is possible to provide a free-machining copper alloy casting that has excellent castability, good machinability, resistance to dezincification corrosion and stress corrosion cracking, high strength, an excellent balance of strength, impact properties and ductility, and a significantly reduced Pb content, as well as a method for manufacturing a free-machining copper alloy casting. [Brief explanation of the drawing]

[0032] [Figure 1A] This is a photograph of the microstructure of a copper alloy casting in an embodiment, which was obtained by subjecting alloy No. S02 to process No. A1. Specifically, alloy No. S02 has the composition Zn-62.9mass%Cu-0.94mass%Si-0.082mass%P-0.067mass%Pb-0.03mass%Sn. In process No. A1, the molten metal was poured at 1080°C and cast into a sand mold, removed from the sand mold at 650°C, and the cooling process was started at 560°C. The average cooling rate in the temperature range from 550°C to 500°C was set to 900°C / min, and the average cooling rate in the temperature range from 500°C to 300°C was also set to 900°C / min. [Figure 1B] Figure 1A shows the results of a dezincification corrosion test performed on the alloy casting 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 a copper alloy casting in an embodiment, which was obtained by subjecting alloy No. S02 to process No. A14H. Specifically, alloy No. S02 has the composition Zn-62.9mass%Cu-0.94mass%Si-0.082mass%P-0.067mass%Pb-0.03mass%Sn. In process No. A14H, the molten metal was poured at 1080°C and cast into a sand mold, removed from the sand mold at 650°C, and allowed to cool naturally, with an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C. [Figure 2B] Figure 2A shows the results of a dezincification corrosion test performed on the alloy casting 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 a copper alloy casting in an embodiment, which was obtained by subjecting alloy No. S12 to process No. B1. Specifically, alloy No. S12 has the composition Zn-63.6mass%Cu-1.04mass%Si-0.070mass%P-0.062mass%Pb alloy-0.003mass%Bi. In process No. B1, the molten metal was poured at 960°C and cast into a mold, removed from the mold at 750°C, and the cooling process was started at 560°C. The average cooling rate in the temperature range from 550°C to 500°C was 1020°C / min, and the average cooling rate in the temperature range from 500°C to 300°C was 1020°C / min. [Figure 3B] Figure 3A shows the results of a dezincification corrosion test performed on the alloy casting 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 a copper alloy casting in an embodiment, which was obtained by subjecting alloy No. S12 to process No. B11H. Specifically, alloy No. S12 has the composition Zn-63.6mass%Cu-1.04mass%Si-0.070mass%P-0.062mass%Pb-0.003mass%Bi. In process No. B11H, the molten metal was poured at 960°C and cast into a mold, removed from the mold at 750°C, and the average cooling rate in the temperature range from 500°C to 300°C was set to 40°C / min. [Figure 4B] Figure 4A shows the results of a dezincification corrosion test performed on the alloy casting 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 a copper alloy casting in an embodiment. The copper alloy casting was obtained by subjecting alloy No. S02 to processes A14H and G1. Specifically, alloy No. S02 has the composition Zn-62.9mass%Cu-0.94mass%Si-0.082mass%P-0.067mass%Pb-0.03mass%Sn. In process A14H, the molten metal was poured at 1080°C and cast into a sand mold. It was removed from the sand mold at 650°C and allowed to cool naturally, with an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C. Then, process G1 was applied to the obtained casting. In process No. G1, the casting was heated at 580°C for 20 minutes, and the cooling process was started from 575°C. The average cooling rate in the temperature range from 550°C to 500°C was set to 1200°C / min, and the average cooling rate in the temperature range from 500°C to 300°C was also set to 1200°C / min. [Figure 5B] Figure 5A shows the results of a dezincification corrosion test performed on the alloy casting 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]

[0033] The following describes a free-machining copper alloy casting and a method for manufacturing a free-machining copper alloy casting according to an embodiment of the present invention. Applications of the free-machining copper alloy castings according to the embodiment of the present invention include equipment and fixtures related to drinking water and sanitary facilities, 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 part names include water taps, mixing taps, shut-off valves, valves, cocks, fittings, water meters, gears, sensors, nuts, screws, connectors, and the like.

[0034] 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] - 4.8 × [Si] + 0.5 × ([Pb] + [Bi]) - [P], and when Sn is present in amounts exceeding 0.05 mass%, f1 = [Cu] - 4.8 × [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.

[0035] Furthermore, in this embodiment, in the constituent phases of the metallic structure excluding nonmetallic inclusions, precipitates, and Pb or Bi particles, the area percentage of the α phase is (α)%, the area percentage of the γ phase is (γ)%, the area percentage of the unmodified β phase is (β)%, and the area percentage of the modified β phase, the β1 phase, is (β1)%. The area percentage of each phase is also referred to as 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 f6 = (β1) × (-0.5 × [Si] 2 +1.5×[Si])-2×(γ)+([Pb]+[Bi]) 1 / 2 ×25+([P]) 1 / 2 ×12 The tissue-composition relationship f6A = (β) × (-0.5 × [Si] 2 +1.5×[Si])-2×(γ)+([Pb]+[Bi]) 1 / 2 ×25+([P]) 1 / 2 ×12

[0036] The free-machining copper alloy casting according to the first embodiment of the present invention contains Cu in an amount exceeding 60.5 mass% and less than 65.0 mass%; Si in an amount exceeding 0.50 mass% and less than 1.20 mass%; Pb in an amount of 0.002 mass% or more and less than 0.20 mass%; P in an amount exceeding 0.01 mass% and less than 0.18 mass%; Bi in an amount of 0.0001 mass% or more 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 0.50 mass%; the content of Al is less than 0.30 mass%; and the content of Cu is [Cu When the content of Cu is [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 57.3≦f1=[Cu]-4.8×[Si]+0.5×([Pb]+[Bi])-[P]≦60.8 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、28<f6=(β1)×(-0.5×[Si] 2 +1.5×[Si])-2×(γ)+([Pb]+[Bi]) 1 / 2 ×25+([P]) 1 / 2 It has a ×12 relationship and is characterized by the observation of grain boundaries within the β1 phase when etched with a mixture of hydrogen peroxide and aqueous ammonia.

[0037] The free-machining copper alloy casting according to the second embodiment of the present invention contains 61.5 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 0.35 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%, the content of Pb is mass%, the content of P is mass%, and the content of Bi is mass%, the relationship 58.0≦f1=[Cu]-4.8×[Si]+0.5×([Pb]+[Bi])-[P]≦60.5 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)×(-0.5×[Si] 2 +1.5×[Si])-2×(γ)+([Pb]+[Bi]) 1 / 2 ×25+([P]) 1 / 2 It has a ×12 relationship and is characterized by the observation of grain boundaries within the β1 phase when etched with a mixture of hydrogen peroxide and aqueous ammonia.

[0038] The free-machining copper alloy casting 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 0.70 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 0.50 mass%; the content of Al is less than 0.30 mass%; the Cu content is [Cu] mass%; and the Si content is When the quantity 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 57.3≦f1=[Cu]-4.8×[Si]+0.5×([Pb]+[Bi])-[P]-[Sn]≦60.8 holds, and the relationship f0=[Sn] / [Si]<0.8 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、28<f6=(β1)×(-0.5×[Si] 2 +1.5×[Si])-2×(γ)+([Pb]+[Bi]) 1 / 2 ×25+([P]) 1 / 2 It has a ×12 relationship and is characterized by the observation of grain boundaries within the β1 phase when etched with a mixture of hydrogen peroxide and aqueous ammonia.

[0039] The following explains the reasons for defining the component composition, compositional relationships f0, f1, f2, microstructure relationships f3, f4(f4A), f5, microstructure-composition relationship f6(f6A), and metal structure as described above.

[0040] <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, depending on the content of Si, Zn, P, Pb, Bi, Sn and the manufacturing process, the proportion of the β1 phase will exceed 80%, resulting in low ductility and brittleness. In addition, the modification of the β phase will not be sufficiently carried out, resulting in poor resistance to dezincification corrosion, stress corrosion cracking, and machinability, and poor ductility. Therefore, the lower limit of the Cu content is greater than 60.5 mass%, preferably 61.0 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.

[0041] (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 containing Cu, Zn, Si, and P in the aforementioned ranges and performing a predetermined cooling treatment described later, the β phase is modified and the β1 phase is formed. The modified β1 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.

[0042] 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 inclusion of Si slightly improves the machinability, dezincification corrosion resistance, and stress corrosion cracking resistance of the α phase, and increases its strength. As a result, the modified β1 phase and the improved α phase enhance the machinability, dezincification corrosion resistance, stress corrosion cracking resistance, and strength of the 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, the β phase will not be modified. In order to improve the dezincification corrosion resistance, stress corrosion cracking resistance, machinability, impact properties, and ductility of the alloy, the α phase must be present in an area ratio of 20% or more, preferably 30% or more.

[0043] Si is an essential element for modifying the β phase. After solidification, Si dissolves in the β phase, and the higher the amount of Si, the more the β phase is modified, resulting in a β1 phase with superior properties. Furthermore, the Si content increases the strength of the alloy and improves the flowability of the molten metal during casting. In order to modify the β phase and create a more modified β1 phase, the Si content must exceed 0.50 mass%. Preferably, the Si content is 0.60 mass% or more, more preferably 0.65 mass% or more, and even more preferably 0.70 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, conductivity decreases, ductility decreases, and impact properties worsen. In some cases, the γ phase appears. Incidentally, in Patent Documents 3 to 9, the γ phase is said to improve 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 and impact properties 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. For these reasons, the amount of Si is less than 1.20 mass%, preferably 1.10 mass% or less. If electrical conductivity and thermal conductivity are important, the Si amount is 1.00 mass% or less.

[0044] (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%.

[0045] (P) P, like Si, is an essential element for modifying the β phase to the β1 phase. After solidification, P dissolves in the β phase along with Si, and during the cooling process after solidification, the β1 phase is obtained by cooling at a rate exceeding 300°C / min when the casting temperature reaches 500°C, and again in the temperature range from 500°C to 300°C. The modification from the β phase to the β1 phase improves machinability and significantly improves the resistance to dezincification corrosion and stress corrosion cracking, which were problems with the conventional β phase. Incidentally, compounds of P and Zn or Si reduce cutting resistance and improve chip fragmentation, but when the cooling process is started at a temperature exceeding approximately 550°C or approximately 530°C and cooled at a rate exceeding 300°C / min, compounds containing P are absent or present in only small amounts. Regarding machinability, the effect of modifying from the β phase to the β1 phase surpasses the effect of the presence of a Si-containing β phase and a P compound, and the effect of modifying to the β1 phase is significantly superior, especially during high-speed cutting. Furthermore, the inclusion of P improves the dezincification corrosion resistance of the α phase, leading to a significant improvement in the dezincification corrosion resistance of the alloy composed of the β1 and α phases.

[0046] To modify the β phase into the β1 phase, the P content must be at least 0.01 mass%. To obtain a more modified β1 phase, the amount of P is preferably 0.03 mass% or more, more preferably 0.04 mass% or more, which further improves dezincification corrosion resistance and machinability. Furthermore, P readily forms compounds with elements such as Zn, Si, Mn, Fe, Cr, Co, and Al. When P forms compounds with these elements, the amount of P dissolved in the β phase of the alloy decreases during the cooling process after solidification, hindering the modification to the β1 phase. 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 approximately 550°C or above, and the formation of P compounds is further promoted as the amount of these elements increases. 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 and poor resistance to dezincification corrosion and stress corrosion cracking. Therefore, the total content of Fe, Mn, Co, and Cr must be kept below 0.50 mass%, preferably below 0.35 mass%.

[0047] The formation of compounds of P, Zn, and Si positively impacts machinability, but the modification from the β phase to the β1 phase and the presence of these compounds are fundamentally contradictory. As mentioned above, the formation of the β1 phase requires rapid cooling at 500°C after solidification and in the temperature range from 500°C to 300°C, while sufficient formation of the P, Zn, and Si compounds requires slow cooling in the temperature range of approximately 530°C to 450°C. Thus, the modification from the β phase to the β1 phase and the formation of P compounds proceed in opposite directions. However, if the cooling rate increases sharply, for example, around 515°C, the β phase is modified simultaneously with the formation of the P compounds. In this case, however, the modification of the β phase will be somewhat insufficient.

[0048] 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.

[0049] (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 the 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 little to the improvement of machinability. For example, ASM Specialty Handbook, Copper and Copper Alloys, first edition, published August 2001, p. 267, Fig. 6 shows the relationship between the amount of Pb and machinability, with the machinability of a Cu-Zn-Pb alloy containing 62-65 mass% Cu, about 3.2 mass% Pb, and the remainder being Zn being set to 100%. As shown in Fig. 6 above, the inclusion of 0.1 mass% of Pb 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, and the machinability of the alloy is greatly improved by the inclusion of a small amount of Pb in the β1 phase, which has significantly improved machinability. 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.

[0050] (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.

[0051] When Bi is included, in order for the alloy to have good machinability with a small amount of Pb present, at least 0.0001 mass% of Bi is required. The amount of Bi is preferably 0.001 mass% or more, and more preferably 0.002 mass% or more. On the other hand, 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. Furthermore, since Bi is more sensitive to cracking during casting than Pb, setting it as described above eliminates the risk of cracking during casting. Considering that Bi can adequately substitute for Pb, that Bi is one of the rare metals and has an environmental impact, and that it is included in the raw materials as an unavoidable impurity, Bi can be included as an optional element and may or may not be included. Furthermore, considering the environmental impact, stress corrosion cracking, and cracking during casting, 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%.

[0052] (Sn) By solid-solving in the β1 phase, Sn further improves the dezincification resistance of the β1 phase, thereby enhancing the dezincification 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 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 resistance. Although it is related to the amount of Si, the Sn content should be kept at least below 0.70 mass%, preferably below 0.60 mass%, and more preferably below 0.50 mass%.

[0053] 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.

[0054] (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 zinc, do not primarily use high-quality raw materials such as electrolytic copper or electrolytic zinc, but rather recycled copper alloys. In the downstream processes (processing stages) 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, products containing manufacturing defects, and discarded products. These discarded copper alloys become the main raw materials. 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.

[0055] From the standpoint 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. Furthermore, in the JIS standard (JIS H 3250) for free-cutting brass rod C3604 with added lead, the essential element lead is contained in an amount of approximately 3 mass%, and 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. Furthermore, in brass castings with added lead according to the JIS standard (JIS H 5120), the essential element lead is contained in an amount of approximately 2 mass%, and the permissible limits for residual components are set as follows: Fe: 0.8 mass%, Sn: 1.0 mass%, Al: 0.5 mass%, and Ni: 1.0 mass%. In reality, high concentrations of Fe, Sn, Al, and Ni close to the upper limits of the JIS standard are sometimes found in free-cutting brass rods and brass castings.

[0056] 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 that combine with Si form Fe-Si, Mn-Si, Co-Si, and Cr-Si compounds in the microstructure. Similarly, Fe, Mn, Co, and Cr that combine with P form Fe-P, Mn-P, Co-P, and Cr-P compounds in the microstructure. 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, with each content being less than 0.35 mass%, preferably less than 0.25 mass%, and more preferably 0.15 mass% or less. In particular, the total content of Fe, Mn, Co, and Cr must be less than 0.50 mass%, preferably less than 0.35 mass%, more preferably less than 0.30 mass%, and even more preferably 0.25 mass% or less.

[0057] On the other hand, Al, which is mixed in from special brass rods, brass castings, etc., needs to be limited because a high content affects the modification and properties of the β phase. In addition, Al forms compounds with P or Si. In the alloy of this embodiment, the Al content must be less than 0.30 mass%, more preferably less than 0.15 mass%, and even more preferably 0.10 mass% or less.

[0058] 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, the content of each 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%.

[0059] (Compositional relation f1) The compositional relationship f1 = [Cu] - 4.8 × [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. The composition relation f1 is an equation that represents the relationship between composition and metal structure. Even if the amount of each element is within the range specified above, if f1 does not satisfy the relation 57.3 ≤ f1 ≤ 60.8, the properties targeted by this embodiment cannot be satisfied. If the composition relation f1 is less than 57.3, the proportion of β phase and β1 phase increases, resulting in insufficient modification of the β phase and poor dezincification corrosion resistance, stress corrosion cracking resistance, ductility, and impact properties. Therefore, the lower limit of the composition relation f1 is 57.3 or higher, preferably 58.0 or higher, and more preferably 58.5 or higher. As the composition relation f1 moves into a more preferable range, the proportion of α phase increases, the modification of the β phase becomes sufficient, excellent machinability is maintained, good dezincification corrosion resistance is achieved, and good ductility and impact properties are obtained. On the other hand, the upper limit of the compositional relationship f1 affects the proportion of β phase and β1 phase. If the compositional relationship f1 is greater than 60.8, the proportion of β phase and β1 phase decreases. As a result, excellent machinability is not obtained, and the strength also decreases. Furthermore, the upper limit of f1 is related to castability. Castability and the solidification temperature range are closely related. If the upper limit of f1 is exceeded, the solidification temperature range exceeds 30°C, resulting in poor castability and an increase in defects in the final solidified portion. Therefore, the upper limit of the compositional relationship f1 is 60.8 or less, preferably 60.5 or less, and more preferably 60.1 or less. Depending on the composition and process, as the value of the compositional relationship f1 decreases, the β1 phase increases, improving machinability, increasing strength, narrowing the solidification temperature range, and improving castability.

[0060] (Compositional relation f2) Bi is an optional element that substitutes for the function of Pb, but if Bi is included, the relationship 0.003 ≤ f² = [Pb] + [Bi] < 0.20 must be satisfied. That is, considering that Bi is one of the rare metals and has an impact on the environment, the total amount of Pb and Bi must be limited. Also, both Pb and Bi are low melting point metals and exist as metal particles in the metal structure, so they may cause cracking during casting and must be limited. For these reasons, if Bi is included, the total amount of Pb and Bi should be less than the upper limit of Pb, which is 0.20 mass%, and preferably less than 0.10 mass%. On the other hand, if Bi is included to obtain good machinability, the total amount of Pb and Bi must be 0.003 mass% or more, and preferably 0.004 mass% or more.

[0061] (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 greater than the amount of Si, it may hinder the modification of the β phase, potentially impairing dezincification corrosion resistance and machinability. In other words, assuming f0 = [Sn] / [Si], it was found that in order to achieve an effect commensurate with the amount of Sn content considering other properties, f0 must be less than 0.8. Furthermore, f0 is preferably less than 0.65, and more preferably less than 0.50.

[0062] The free-machining copper alloy casting 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 and stress corrosion cracking. Furthermore, it possesses machinability that requires a certain degree of brittleness, such as low cutting resistance and fine chip fragmentation, while also possessing ductility and impact properties, which are completely contradictory characteristics. In order to achieve these good corrosion resistance, machinability, and mechanical properties, it is possible to provide alloys that are better suited to the purpose and application by discussing in detail not only the composition of each component, but also the compositional relationship formulas f1, f2, f0, and the microstructure relationship formulas f3~f5 and 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.

[0063] (Comparison with patent documents) Tables 1 to 4 show a comparison of the composition of the alloy of this embodiment with the Cu-Zn-Si alloys described in the aforementioned Patent Documents 1 to 16.

[0064] [Table 1] TIFF0007913654000001.tif245170

[0065] [Table 2]

[0066] [Table 3]

[0067] [Table 4]

[0068] This embodiment differs from Patent Document 9 in the content of the main elements Si and Cu, this embodiment differs from Patent Document 10 in the content of the main element Si, and this embodiment differs from Patent Document 16 in the content of the main element Cu. Patent Documents 1 and 2 do not contain Si. Patent documents 2, 14, and 16 state that Pb is not included, indicating that the Pb content differs. Patent documents 9, 11, 12, and 13 severely limit the β phase in the metal structure from the viewpoint of machinability, resistance to dezincification corrosion, and corrosion resistance. Although β1 phase and β phase are completely different, patent documents 9, 11, 12, and 13 specify that the β phase is 5% or less, 25% or less, 15% or less, and 0.9% or less, respectively.

[0069] Patent document 11 relates to a near-net-shape tubular hot-forged product and specifies the use of a tubular material. Patent documents 2 and 10 state that heat treatment is performed at temperatures of 350-550°C or 400-600°C in order to reduce or break up the β phase. Patent Document 10 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 0.1-0.4 mass%, which can be controlled to 0.2 mass% or less.

[0070] Patent documents 13, 14, and 15 state that aluminum is essential for improving discoloration resistance, castability, and dezincification corrosion resistance. Patent Document 15 states that in order to improve resistance to dezincification corrosion, Sn and Al should be 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 should be included. Patent Document 16 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.

[0071] Patent documents 3 to 8 all describe a cooling process after casting and solidification, or 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 it is essential that the resulting P compound is 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, there is no disclosure of data on cutting resistance under high-speed cutting conditions. In this application, during the cooling process after casting and solidification, the casting is cooled at a cooling rate exceeding 300°C / min when the temperature reaches 500°C, i.e., in the temperature range just above 500°C, preferably at a temperature lower than 700°C and higher than 500°C, with the cooling process starting at an average cooling rate of over 300°C / min from 500°C to 300°C, which is fundamentally contrary to the cooling described in patent documents 3 to 8. In this application, compounds of P and Zn or Si may be present, but the amount of P compounds present is small, if any.

[0072] The most significant and clear difference between this application and Patent Documents 3-8 is that none of Patent Documents 3-8 mention the modification of the β phase, nor the modified β phase, i.e., the β1 phase. During cooling after solidification, the β phase containing Si and P is modified into the β1 phase by cooling at a rate exceeding 300°C / min at 500°C, and by cooling in the temperature range from 500°C to 300°C at an average cooling rate of at least 300°C / min. If the average cooling rate is between approximately 0.1°C / min and approximately 70°C / min in the temperature range from approximately 530°C to approximately 450°C, the β phase is not modified. Furthermore, grain boundaries can be observed within the β1 phase when etched with a mixture of hydrogen peroxide and ammonia water, but even if a predetermined amount of Si and P is present, if the β phase remains unchanged from its pre-modification state, grain boundaries cannot be observed within the β phase. Patent documents 3 to 8 disclose metal structures etched with a mixture of hydrogen peroxide and aqueous ammonia, but in all cases, no grain boundaries are observed within the β phase.

[0073] Unlike the conventional β phase, the β1 phase significantly improves dezincification corrosion resistance and stress corrosion cracking resistance. Furthermore, machinability is further improved, and the presence of the β1 phase exceeds the machinability achieved when a phosphorus compound is present in addition to the Si-containing β phase. However, to achieve this effect, the β1 phase needs to be present in the metal structure at an area ratio exceeding 25%. In addition, in the present application, the presence of the γ phase impairs machinability, whereas Patent Documents 3 to 8 state that the γ phase is effective for machinability.

[0074] <Metal Structure> More than 10 types of phases exist in Cu-Zn-Si alloys, and complex phase changes occur therein. Target properties cannot necessarily be obtained only from the composition range and the relational expression of elements. Target properties can be obtained by finally specifying and determining the types of phases present in the metal structure and the range of their area ratios. Accordingly, the structural relational expression is defined as follows. 20 ≤ f3 = (α) < 75, 25 < f4 = (β1) ≤ 80, wherein the β1 phase is a phase obtained by modifying the β phase. 0 ≤ f5 = (γ) < 4

[0075] (β1 Phase) The metal structure of the present application is composed of an α phase, a β1 phase, and optionally a small amount or 0% of a γ phase. Only the aforementioned constituent phases are targeted in the metal structure. In addition to the constituent phases, precipitates such as P-containing compounds and Si-containing compounds, crystallized products, Pb or Bi particles, oxides, sulfides, and inclusions may be present in the metal structure. In the case of castings, oxides and crystallized products are often contained in larger amounts than in hot-extruded materials. Distinguishing between the β phase and the β1 phase is easy: when etching is performed with a mixed solution of hydrogen peroxide and aqueous ammonia, if the β phase is modified into the β1 phase, grain boundary patterns, i.e., crystal grain boundaries, are observed inside the β1 phase, whereas in the case of the β phase, no crystal grain boundaries are observed inside the β phase.

[0076] In this embodiment, the β1 phase is obtained by maintaining a high temperature of over 500°C, i.e., approximately 500-700°C, in a β phase with a certain amount or more of Si and P dissolved in it after solidification, and then cooling it from that temperature range (cooling at a rate of at least 300°C / min, both at 500°C and the average cooling rate from 500°C to 300°C, and continuing to cool it down to near room temperature), thereby bringing the metallic structure at 500-700°C down to room temperature. Specifically, the cooling rate is increased rapidly in the temperature range from the cooling start temperature of 500-700°C to 300°C to cool it down to room temperature of 100°C or less. This brings the metallic structure at 500-700°C down to room temperature. As a result, the β1 phase is obtained. The β1 phase cannot be obtained by performing the same cooling treatment on the β phase of a Cu-Zn alloy that does not contain both predetermined amounts of Si and P. Similarly, if an alloy containing predetermined amounts of Si and P is cooled at a cooling rate of 300°C / min or less at 500°C, the β1 phase will not be obtained. The β1 phase will not be obtained if the temperature range from 500°C to 300°C is cooled at an average cooling rate of 300°C / min or less. The β1 phase will not be obtained if the cooling treatment is performed at a temperature lower than 500°C, for example, from 450°C. Furthermore, a predetermined amount of α phase is required for the modification of the β phase, and if there is insufficient α phase, the β phase will not be modified, and the β1 phase will not be obtained. In addition, the degree of modification of the β phase is also affected by the amount of Si and P, the amount of unavoidable impurities such as Fe and Al, the cooling rate at 500°C, the average cooling rate from 500°C to 300°C, and the starting temperature of the cooling treatment. When the degree of modification of the β phase is improved, that is, when the β1 phase is more modified, a material with better machinability, better resistance to dezincification corrosion, and better resistance to stress corrosion cracking can be obtained. The degree of modification of the β phase 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 impaired thermal and electrical conductivity, ductility, and the appearance of the γ phase.

[0077] The modified β1 phase can overcome 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 a concrete example, the corrosion progression of dezincification corrosion can be slowed down by approximately half, or to less than half. As described in Patent Document 2, dezincification corrosion of Cu-Zn alloys containing the β phase is a major problem, and since dezincification corrosion first occurs along the β phase, it is recommended to limit the amount of β phase to 25% or less or 20% or less, and further reduce the amount of β phase while performing heat treatment at 350-550°C to 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 than the β phase and possesses better ductility and impact properties than the β phase, resulting in an alloy with a good strength-ductility balance.

[0078] (β1 phase, tissue relationship f4) In the Cu-Zn-Si-P-Pb alloy, which is a free-machining copper alloy of this embodiment, in order to achieve good machinability while minimizing the Pb content, the β1 phase is required in an area ratio of at least 25%. Furthermore, in order to improve machinability and strength, the β1 phase is preferably 30% or more, and more preferably 33% or more. On the other hand, if the 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 required. 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, in Cu-Zn alloys whose microstructure consists of α and β phases, the β phase is selectively dezincified first, and the dezincification corrosion progresses, ultimately reaching a dezincification corrosion depth of approximately 500-550 μm. On the other hand, in the present alloy whose microstructure consists of α and β1 phases, although the β1 phase is selectively dezincified, the dezincification corrosion depth is approximately 20-240 μm, and the progression of dezincification corrosion is significantly suppressed compared to when the microstructure consists of α and β phases. Thus, although the β phase is modified into the β1 phase and its resistance to dezincification corrosion is greatly improved, it is inferior to the α phase in terms of resistance to dezincification corrosion and impact properties. Therefore, if the area ratio of the β1 phase is high, the resistance to dezincification corrosion and impact properties of the alloy deteriorate. Based on the above, after diligent research, it has been 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. The unmodified β-phase is distinguished as tissue relationship formula f4A.

[0079] (α-phase, tissue relationship formula f3) The metal structure of this invention is basically composed of α phase and β1 phase. A cooling treatment is performed to modify the β phase into the β1 phase, but the α phase is hardly affected by this cooling 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 and impact properties of the alloy, and an appropriate amount of ductile α phase is required. Conversely, if there is too much α phase, the strength will be low. Also, the α phase containing Si only slightly improves machinability compared to the α phase without Si, and the amount of α phase is limited, especially from the standpoint of machinability. However, even if the alloy is composed of a β1 phase with excellent machinability and an α phase with slightly poor machinability, during cutting, the soft α phase acts as a cushioning material, or acts as a stress concentration source at the boundary with the hard β1 phase during cutting. Even if the alloy contains up to approximately 75% α phase, 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 β1 phase, a fine, granular shape is preferable for the α phase. Based on extensive 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.

[0080] (γ phase, tissue relationship formula f5) As described in Patent Documents 9 and 11, the γ phase is a phase that contributes to machinability in Cu-Zn-Si alloys with a Cu concentration of approximately 69-80 mass% and a Si concentration of approximately 2-4 mass%. Patent Document 16 states that the γ phase is essential in Cu-Zn-Si alloys that do not contain Pb, and further, Patent Documents 3-7 state that while the β phase containing Si has good machinability, the γ phase containing Si also contributes to machinability. In this embodiment, the γ phase is considered to inhibit the machinability of the alloy, and therefore, in the microstructure-composition relation f6 described later, a negative coefficient larger than the coefficient for the amount of the β phase is given. Furthermore, the γ phase impairs impact properties and worsens resistance to dezincification corrosion. This is because, in this embodiment, the β phase is modified, but the γ phase is not. 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 must 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.

[0081] (Organizational and compositional relationship formula f6) f6 = (β1) × (-0.5 × [Si] 2 +1.5×[Si])-2×(γ)+([Pb]+[Bi]) 1 / 2 ×25+([P]) 1 / 2 ×12 The microstructure-composition 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 contained in the alloy within the composition range of this application, the amounts of Pb and Bi, and the amount of P are organized as positive effects, and the amount of γ phase is organized as a negative effect. That is, the β1 phase contains Si and P, and the amount of β1 phase is (-0.5 × [Si] 2 The result is obtained by multiplying (+1.5 × [Si]), multiplying the sum of the amounts of Pb and Bi to the power of 1 / 2 by a coefficient of 25, multiplying the amount of P to the power of 1 / 2 by a coefficient of 12, and then subtracting the sum of these by multiplying the amount of the γ phase by a coefficient of 2.

[0082] The performance of the β1 phase is directly affected by the Si concentration and, similarly, by the P concentration. Furthermore, the presence of even a small amount of Pb or Bi improves machinability. On the other hand, in this application, the γ phase inhibits machinability. It has been found that the degree of improvement in machinability by Pb or Bi is closely related to the square root of the amounts of Pb and Bi. Both Pb and Bi exhibit a significant effect even in very small amounts, and while the improvement in machinability increases with increasing content, it gradually becomes more gradual. In alloys composed of a modified β1 phase, trace amounts of Pb or Bi have a significant effect on machinability, an effect that cannot be achieved through the conventional relationship between Pb content and machinability in Cu-Zn-Pb alloys.

[0083] Through diligent research, it has been found that in order to obtain good machinability, the microstructure-composition relationship f6 should be greater than 28, preferably greater than 35, and even more preferably greater than 40, approaching the machinability of a free-cutting brass rod with 3 mass% Pb added. On the other hand, machinability is highest when the value of the microstructure-composition relationship f6 is approximately 55-80, and as the value of f6 increases further, machinability actually decreases. This is because the effect of the amount of β1 phase on machinability saturates at approximately 60%, and the effect of Si on machinability saturates at approximately 1 mass%. Furthermore, for alloys composed of unmodified β phase, the amount of β phase f4A(β phase) was substituted into the portion f4(β1 phase) representing the amount of β1 phase, and this was applied to the formula for f6 to calculate a reference value, which was then given as f6A.

[0084] Figures 1A to 5B show metallographic images of various alloy castings and photographs of the results of dezincification corrosion tests according to ISO 6509. Figure 1A is a photograph of the microstructure of a copper alloy casting in an embodiment, which was obtained by subjecting alloy No. S02 to process No. A1. Specifically, alloy No. S02 has the composition Zn-62.9mass%Cu-0.94mass%Si-0.082mass%P-0.067mass%Pb-0.03mass%Sn. In process No. A1, the molten metal was poured at 1080°C and cast into a sand mold, removed from the sand mold at 650°C, and the cooling process was started at 560°C. The average cooling rate in the temperature range from 550°C to 500°C was set to 900°C / min, and the average cooling rate in the temperature range from 500°C to 300°C was also set to 900°C / min. Figure 1B shows the results of a dezincification corrosion test performed on the alloy casting 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 a copper alloy casting in an embodiment, which was obtained by subjecting alloy No. S02 to process No. A14H. Specifically, alloy No. S02 has the composition Zn-62.9mass%Cu-0.94mass%Si-0.082mass%P-0.067mass%Pb-0.03mass%Sn. In process No. A14H, the molten metal was poured at 1080°C and cast into a sand mold, removed from the sand mold at 650°C, and allowed to cool naturally, with an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C. Figure 2B shows the results of a dezincification corrosion test performed on the alloy casting 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.

[0085] Figure 3A is a photograph of the microstructure of a copper alloy casting in an embodiment, which was obtained by subjecting alloy No. S12 to process No. B1. Specifically, alloy No. S12 has the composition Zn-63.6mass%Cu-1.04mass%Si-0.070mass%P-0.062mass%Pb alloy-0.003mass%Bi. In process No. B1, the molten metal was poured at 960°C and cast into a mold, removed from the mold at 750°C, and the cooling process was started at 560°C. The average cooling rate in the temperature range from 550°C to 500°C was 1020°C / min, and the average cooling rate in the temperature range from 500°C to 300°C was 1020°C / min. Figure 3B shows the results of a dezincification corrosion test performed on the alloy casting 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 a copper alloy casting in an embodiment, which was obtained by subjecting alloy No. S12 to process No. B11H. Specifically, alloy No. S12 has the composition Zn-63.6mass%Cu-1.04mass%Si-0.070mass%P-0.062mass%Pb-0.003mass%Bi. In process No. B11H, the molten metal was poured at 960°C and cast into a mold, removed from the mold at 750°C, and the average cooling rate in the temperature range from 500°C to 300°C was set to 40°C / min. Figure 4B shows the results of a dezincification corrosion test performed on the alloy casting 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.

[0086] Figure 5A is a photograph of the microstructure of a copper alloy casting in an embodiment, which was obtained by subjecting alloy No. S02 to processes A14H and G1. Specifically, alloy No. S02 has the composition Zn-62.9mass%Cu-0.94mass%Si-0.082mass%P-0.067mass%Pb-0.03mass%Sn. In process A14H, the molten metal was poured at 1080°C and cast into a sand mold, removed from the sand mold at 650°C, and allowed to cool naturally, with an average cooling rate of 20°C / min in the temperature range from 500°C to 300°C. Then, process G1 was applied to the obtained casting. In process No. G1, the casting was heated at 580°C for 20 minutes, and the cooling process was started from 575°C. The average cooling rate in the temperature range from 550°C to 500°C was set to 1200°C / min, and the average cooling rate in the temperature range from 500°C to 300°C was also set to 1200°C / min. Figure 5B shows the results of a dezincification corrosion test performed on the alloy casting 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.

[0087] As shown in Figures 1A, 3A, and 5A, grain boundaries are clearly visible within the β1 phase in sand castings, die castings, and heat-treated sand castings. Here, grain boundaries refer to linear patterns penetrating the β1 phase grains, as shown in Figure 1A. On the other hand, in Figures 2A and 4A, no grain boundary-like patterns are observed within the β phase, and the difference between the two is clear. Instead, in Figures 2A and 4A, many fine black granular precipitates are observed, mainly within the β phase and at the phase boundary between the β and α phases. Conversely, in Figures 1A, 3A, and 5A, only a small amount of black granular precipitates are observed.

[0088] Figures 2A and 4A show numerous black, granular precipitates, approximately 0.5–3 μm in size, primarily within the β phase and at the phase boundary between the β and α phases. These granular precipitates are mainly compounds of P with either Zn or Si, or both, so-called P compounds. In addition, some inclusions such as Pb particles, mixed particles of Pb and Bi, compounds of Fe, Mn, etc., with P or Si, compounds of Al with P or Si, and oxides are also present. While these compounds and inclusions can be identified under a microscope, they are somewhat difficult to distinguish in printed photographs. In Figures 2A and 4A, approximately 300 or more P compounds are present within the field of view of the printed photographs. In Figures 1A, 3A, and 5A, fine granular P compounds are either absent or present in very small quantities, at least less than 1 / 10 of the number of compounds in Figures 2A and 4A. Whether grain boundaries exist within the β1 phase or within the β phase, and whether there is a large amount of P compound, depends on whether the cooling rate at the point when the casting temperature reaches 500°C, i.e., in the temperature region just above 500°C, and the average cooling rate from 500°C to 300°C are faster or slower than 300°C / min. In this application, the cooling rate at the point when the casting temperature reaches 500°C, i.e., in the temperature region just above 500°C, is defined as the average cooling rate from 550°C to 500°C. If the cooling start temperature is lower than 550°C, the average cooling rate from the cooling start temperature to 500°C is used.

[0089] The results of dezincification corrosion tests conducted on these alloys according to ISO 6509 are shown in Figures 1B, 2B, 3B, 4B, and 5B. The magnification of Figures 1B, 3B, and 5B is twice that of Figures 2B and 4B. The maximum corrosion depth of the samples in which grain boundaries were observed within the β1 phase (Figures 1A, 3A, and 5A) was all 110 μm or less (Figures 1B, 3B, and 5B), while the maximum corrosion depth of the samples in which grain boundaries were not observed within the β phase (Figures 2A and 4A) was 480 μm and 420 μm, respectively (Figures 2B and 4B), showing a difference of more than three times in corrosion depth. However, the form of dezincification corrosion in all cases was selective corrosion of the β1 phase and the β phase. Even with the modified β1 phase, dezincification corrosion preferentially occurs in the β phase, similar to the β phase. However, the progression of dezincification corrosion in the β1 phase can be slowed down to at least half, and even to about one-third, of that of the β phase.

[0090] <Characteristics> (Strength, toughness, ductility) Generally, castings, compared to materials that have undergone hot working, such as hot-extruded rods, exhibit segregation of components, larger crystal grains, and contain some inclusions such as oxides and microscopic defects. For this reason, castings are said to be "brittle" compared to hot-worked materials, and a high impact value is desirable in the evaluation of toughness and ductility. On the other hand, materials that exhibit excellent chip breaking properties in cutting require a certain degree of brittleness. Impact properties and machinability are, in some respects, conflicting properties.

[0091] There is a strong demand for thinner and lighter components for the parts and components used in this embodiment, such as equipment related to drinking water. For example, in equipment related to drinking water, thinner and lighter components can be achieved by using materials with good corrosion resistance and high strength. As mentioned above, castings can have component segregation and microscopic defects, and are therefore not often used for parts that undergo cold working such as crimping or for components that require ductility during use. In this embodiment, in accordance with the properties of castings, hardness (Vickers hardness, Rockwell hardness) is used as a method for evaluating strength, and impact test values ​​(U-notch) are used to evaluate toughness and ductility.

[0092] For copper alloy castings to have high strength, it is preferable that they have a Vickers hardness of at least 100 Hv and a Rockwell B-scale hardness of at least 52. More preferably, the Vickers hardness is 110 Hv or higher and the Rockwell B-scale hardness is 60 or higher. Hardness, tensile strength, and yield strength are correlated, and in this embodiment, a Vickers hardness of 100 Hv is approximately 400 N / mm². 2 Its tensile strength is approximately 120 N / mm². 2 This corresponds to the yield strength. Incidentally, brass casting CAC203 containing 1-2 mass% Pb has a Vickers hardness of approximately 75 Hv, a Rockwell B scale hardness of approximately 30, and a tensile strength of approximately 350 N / mm². 2 The yield strength is approximately 80 N / mm². 2 That is the case.

[0093] As mentioned above, castings must not only be high in strength, but also at least not brittle, meaning they must be tough enough to withstand impact. To achieve this, when a Charpy impact test is performed on a U-notch specimen, the Charpy impact value is preferably 25 J / cm². 2 The above is more preferable: 30 J / cm² 2 That concludes the explanation. On the other hand, for example, the Charpy impact test value is approximately 70 J / cm². 2 Beyond a certain point, the material's toughness increases, leading to higher cutting resistance and poorer machinability, such as chip formation.

[0094] (conductivity) The applications of this embodiment include electrical and home appliance components, automotive parts, and other applications, and it is also a substitute material for Pb-added brass castings CAC203 and bronze castings CAC406. Currently, phosphor bronze (JIS standards, C5191, C5210), which contains 6 mass% or 8 mass% of Sn in the wrought copper products, is widely used for the aforementioned applications. Their conductivity is approximately 14% IACS and 12% IACS, respectively, while the conductivity of CAC203 and CAC406 is approximately 24% IACS and 15% 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.

[0095] (Castability) The free-machining copper alloy casting of this embodiment is based on the fundamental premise that a sound casting can be obtained. First, the inclusion of Si suppresses the vapor production of Zn during melting, resulting in good melting and casting. Furthermore, the inclusion of Si, along with the restriction of Cu, Zn, and f1, provides good molten metal flowability, allowing the molten metal to reach even thin-walled sections, enabling casting of complex shapes and minimizing the occurrence of defects such as porous shrinkage cavities. Moreover, the casting must be free from cracks. Regarding casting cracks, the first point is whether or not low-melting-point metals exist as molten material at high temperatures after solidification. If low-melting-point metals are present, the cracks are determined by their amount and whether or not the matrix is ​​ductile at high temperatures. In this embodiment, the total amount of low-melting-point metals Pb and Bi present in the matrix as molten material during the solidification and cooling process of the casting is significantly limited to less than 0.20 mass%, making it less likely to lead to casting cracks. Furthermore, if the composition and various relationships of this embodiment are satisfied, it contains a large amount of β1 phase, which has excellent ductility at high temperatures, so the adverse effects of the small amount of low-melting-point metal can be covered, and there is no problem of casting cracking.

[0096] <Manufacturing Process> Next, a method for producing a free-machining copper alloy casting according to an embodiment of the present invention will be described. The microstructure of the free-machining copper alloy casting in this embodiment changes not only with its composition but also with the manufacturing process. There are various casting methods for manufacturing castings, such as mold casting, sand casting, continuous casting, die casting, and lost-wax casting. The thickness and shape of the casting, as well as the material and thickness of the mold or sand mold, roughly determine the cooling rate of the casting after solidification. The cooling rate can be changed by means of cooling methods such as heat retention, water cooling, oil cooling, and forced air cooling. On the other hand, various changes in the microstructure occur during the cooling process after solidification, and the microstructure changes significantly depending on the cooling rate. Changes in the microstructure refer to significant changes in the types of constituent phases and the amount (area ratio) of those constituent phases. As a result of diligent research on the cooling process after solidification, it was found that the cooling rate at 500°C and the average cooling rate in the temperature range from 500°C to 300°C are the most important factors during the cooling process, and that they greatly affect dezincification corrosion resistance and machinability.

[0097] (Melting and casting) Depending on the casting method, melting is carried out 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 1150°C, which is about 50 to 250°C higher than the melting point, is poured into a predetermined mold such as a metal mold, sand mold, or other mold, and then cooled. After solidification, the constituent phases change in various ways.

[0098] (Casting / Solidification) The cooling rate after casting and solidification varies considerably depending on the weight, thickness, and thermal conductivity of the cast copper alloy, as well as the material of the mold, such as a sand mold or metal mold. In conventional copper alloy casting, when cast into a mold made of copper alloy or iron, i.e., in die casting, the casting is removed from the mold at a temperature of approximately 750°C or lower after casting, and then air-cooled or slowly cooled in a temperature range of 500°C to 300°C at an average cooling rate of approximately 10 to 200°C / min. In the case of copper alloy castings, since they often contain 1 mass% or more of Pb or Bi, they are rarely cooled at a cooling rate of 300°C / min or higher in a temperature range higher than approximately 300°C or 250°C, where Pb or Bi solidify. This is because at temperatures above 300°C, the Pb or Bi particles are in a molten state, and if the cooling rate is too fast, cracks may occur in the casting due to thermal shrinkage.

[0099] In continuous casting, the cooling process is roughly the same as in die casting. The casting rods that emerge from the iron mold are generally air-cooled, and the cooling rate is primarily determined by the cross-sectional area of ​​the continuous casting rod and the casting speed. For a continuous casting rod with a diameter of 20 mm, it is cooled through a temperature range of 500°C to 300°C at an average cooling rate of approximately 40 to 100°C per minute. On the other hand, in sand casting, the copper alloy cast into the sand mold is cooled at an average rate of approximately 0.6 to 60°C per minute in the temperature range of 500°C to 300°C, depending on the size of the casting and the material and size of the sand mold. When the temperature drops below approximately 300°C or 250°C, the casting is either removed from the sand mold and air-cooled, or left in the sand mold until it reaches near room temperature. The cooling rate of these castings at 500°C is roughly the same as, or slightly faster than, the average cooling rate in the temperature range of 500°C to 300°C.

[0100] In the free-machining copper alloy casting of this embodiment, after casting and solidification, the metal structure is a single β phase at a high temperature of, for example, 800°C. Subsequent cooling forms various phases such as α and γ phases. In the free-machining copper alloy casting of this embodiment, during the cooling process after solidification, the cooling rate exceeds 300°C / min when the casting temperature reaches 500°C, and by adjusting the average cooling rate from 500°C to 300°C to exceed 300°C / min, the β phase is modified to a β1 phase. The cooling process continues even at temperatures below 300°C. Here, if the starting temperature for the cooling process is lower than 550°C, the cooling rate at 500°C is used as the average cooling rate from the starting temperature to 500°C.

[0101] A preferred cooling method is to start the cooling process in the temperature range of 700°C to 500°C during the cooling process after solidification. The average cooling rate from the cooling start temperature to 500°C is approximately the same as the average cooling rate from 500°C to 300°C. After the casting temperature reaches 300°C, the cooling process continues even at temperatures below 300°C, and although the average cooling rate is slightly slower than the average cooling rate from 500°C to 300°C, it is approximately the same. In some cases, the starting temperature for the cooling process cannot be determined. However, if, during cooling, the cooling rate exceeds 300°C / min when the casting temperature reaches 500°C, and thereafter the average cooling rate from 500°C to 300°C exceeds 300°C / min, and the requirements for the metal structure are met, then the β phase will be modified to the β1 phase.

[0102] These cooling treatments modify the β phase into the β1 phase. When etched with a mixture of hydrogen peroxide and ammonia, grain boundaries appear within the β1 phase, which is modified and replaced by a β1 phase with different properties than the conventional β phase. As a result, machinability is improved, and the dezincification corrosion resistance, which was a problem with the conventional β phase, is significantly improved.

[0103] The degree of modification of the β phase is influenced by the amounts of Si and P, Al, Fe, and Sn, the value of the compositional relationship f1, and the amount of the α phase. Furthermore, it is influenced by the average cooling rate from 500°C to 300°C, the cooling rate at 500°C, and the cooling start temperature during the cooling process after solidification. If the cooling process starts at a temperature of 700°C or higher, the proportion of the β phase increases, which may result in insufficient modification of the β phase. A preferred cooling process start temperature is lower than 700°C, and more preferably 650°C or lower. On the other hand, if the cooling rate is 300°C / min or lower when the casting temperature is 500°C, or if the cooling process starts at a temperature of 500°C or lower, the β phase will not be modified. If the cooling process starts at a temperature of preferably 530°C or higher, and more preferably 550°C or higher, the β phase will be more modified, improving the alloy's resistance to dezincification corrosion and its machinability.

[0104] Furthermore, if the average cooling rate from 500°C to 300°C is 300°C / min or less after the start of the cooling process, the β phase will not be modified. The average cooling rate in the temperature range from 500°C to 300°C should be at least 300°C / min, preferably 600°C / min, and more preferably 900°C / min or more. Cooling at a preferred rate results in a more modified β phase, which has better resistance to dezincification corrosion and better machinability. The upper limit of the cooling rate at 500°C and the average cooling rate in the temperature range from 500°C to 300°C can be achieved with normal production equipment and is not specifically defined, but if mentioned, a cooling rate of approximately 9000°C / min or less is preferable. Although ingots made from castings are sometimes used as raw materials for castings, the modification of the β phase is not affected by the thermal history of the raw materials used. Rather, whether or not the β phase is modified is determined during the cooling process after the final casting solidifies.

[0105] (Heat treatment) In the production of castings, it can be difficult to achieve a cooling rate exceeding 300°C / min at 500°C, and an average cooling rate exceeding 300°C / min from 500°C to 300°C. Such castings can be modified by heat treatment to alter the β phase. Specifically, the casting is heated at a temperature higher than 520°C and lower than 650°C for 1 minute to 5 hours, and the cooling process is started at a temperature lower than 650°C and higher than 520°C. When the temperature of the casting reaches 500°C, the cooling rate exceeds 300°C / min, and by cooling in the temperature range from 500°C to 300°C at an average cooling rate exceeding 300°C / min, the β phase can be modified to the β1 phase.

[0106] When the heat-treated casting of this embodiment is polished to a mirror finish and etched with a mixture of hydrogen peroxide and ammonia, grain boundaries can be observed within the β1 phase, indicating that it has been replaced by a β1 phase with properties different from the conventional β phase. The heating temperature during heat treatment and the cooling start temperature are preferably 540°C or higher, and more preferably 550°C or higher. On the other hand, if the cooling start temperature is higher than 650°C, the proportion of the β1 phase increases, worsening the dezincification corrosion resistance and impact properties, so 620°C or lower is preferable. Furthermore, a faster cooling rate at 500°C and the average cooling rate from 500°C to 300°C results in greater modification of the β phase, preferably exceeding 600°C / min, and more preferably cooling at 900°C / min or higher. The cooling rate at 500°C and the upper limit of the average cooling rate in the temperature range from 500°C to 300°C can be sufficiently achieved with normal production equipment, and are not specifically defined. However, if we were to mention it, a cooling rate of approximately 9000°C / min or less is preferable. Furthermore, even if the β1 phase is formed by the heat treatment described above, if additional heat treatment that does not satisfy the above conditions is applied, or if the casting is reheated to 300°C or higher, the modification of the β phase is lost, and the β1 phase returns to the normal β phase. If heat of 300°C or higher is applied to the casting that has undergone the final predetermined cooling treatment, or to the casting that has undergone the final predetermined heat treatment described above, the modification of the β phase is lost, and the β1 phase that was present in the predetermined casting returns to the normal β phase. As a result, when the metal structure is observed, the grain boundaries that were present in the β1 phase are no longer observed.

[0107] By this manufacturing method, the free-machining copper alloy castings according to the first to third embodiments of the present invention are produced.

[0108] According to the free-machining copper alloy castings of the first to third embodiments of the present invention, which have the above-described configuration, the alloy composition, compositional relationship formula, metal structure, microstructure relationship formula, and microstructure-composition relationship formula are defined as described above. Therefore, even with a low Pb content, excellent machinability can be obtained, and good dezincification corrosion resistance, high strength, and good impact properties can be obtained.

[0109] 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]

[0110] 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.

[0111] We conducted prototype tests of copper alloy sand castings using a low-frequency melting furnace that is currently in operation. Furthermore, prototype copper alloy castings were produced using laboratory equipment, and continuously cast rods were manufactured using a fully operational continuous die casting machine. In addition, some samples were heat-treated in the laboratory. The alloy compositions are shown in Tables 5-7. The manufacturing process is shown in Tables 8-11. In the composition, "Mm" represents the mischmetal and indicates the total amount of rare earth elements. Each manufacturing process is described below.

[0112] [Table 5]

[0113] [Table 6]

[0114] [Table 7]

[0115] [Table 8]

[0116] [Table 9]

[0117] [Table 10]

[0118] [Table 11]

[0119] Process A: Actual machine manufacturing, sand casting (Process No.A1~A6, A11H~A14H) Using a low-frequency melting furnace with an internal volume of 1 ton that is currently in operation, approximately 400 kg of raw material was charged, melted at 1080°C, transferred to a ladle, and cast at approximately 970°C into a sand mold (made of silica sand with added resin) that yields a Grade A test specimen as described in JIS H 5120, Section 7.2. The cast metal was cooled at a cooling rate of approximately 30°C / min. When the casting temperature reached 650°C or 750°C, the casting of test material A was removed from the sand mold and allowed to cool naturally. The casting removed from the sand mold was cooled at an average cooling rate of 35°C / min. Then, the cooling process was started between 460°C and 720°C, and the average cooling rate between 500°C and 300°C was adjusted to 20°C / min, 210°C / min, 420°C / min, 660°C / min, 780°C / min, or 900°C / min. The average cooling rate from the start temperature to 500°C was adjusted to be the same as the average cooling rate from 500°C to 300°C. If the start temperature of the cooling process was 550°C or higher, the average cooling rate from 550°C to 500°C was measured. If the start temperature of the casting's cooling process was lower than 550°C, the average cooling rate from the start temperature to 500°C was measured.

[0120] However, in process No. A12H, the average cooling rate from the time of casting removal down to 460°C was 35°C / min, and the average cooling rate from 460°C down to 300°C was 660°C / min. In process No. A14H, the average cooling rate from the time of casting removal down to 500°C was 35°C / min, and the average cooling rate from 500°C to 300°C was 20°C / min. As cooling methods, natural air cooling, forced air cooling, shower water cooling, water cooling, and simple heat retention were employed. Temperature measurements were mainly performed using radiation thermometers, with contact thermometers used in some cases. The temperature of the casting was measured from the time the No. A test material was removed from the sand mold until it reached 300°C. The radiation thermometer used for temperature measurement was a LumaSense Technologies Inc. model IGA8Pro / MB20. The obtained No. A test material was then subjected to metallurgical microscopy observation, cutting tests, dezincification corrosion tests, hardness tests, and impact tests.

[0121] Process B: Laboratory manufacturing, mold casting (Process No.B1~B5, B11H~B14H) In the laboratory, the raw materials were dissolved in a predetermined ratio. During this process, unavoidable impurities such as iron were intentionally added to some of the samples, taking into consideration actual operational conditions. The molten metal, at approximately 950°C, was then poured into an iron mold with an inner diameter of 40 mm and a depth of 200 mm. Considering actual casting processes, when the casting reached approximately 750°C, the sample was removed from the mold and cooled at a cooling rate of approximately 60°C / min. Subsequently, the cooling process was started in the temperature range of 730°C to 470°C. Then, the average cooling rate in the temperature range of 500°C to 300°C was varied to 1200°C / min, 1020°C / min, 900°C / min, 480°C / min, 240°C / min, or 40°C / min, and the casting was cooled to room temperature. The cooling rate at 500°C, i.e., the average cooling rate from 550°C or the cooling process start temperature to 500°C, was adjusted to be approximately the same as the average cooling rate from 500°C to 300°C.

[0122] However, in process No. B13H, the average cooling rate from the time of casting removal to 470°C was 60°C / min, and the average cooling rate from 470°C to 300°C was 900°C / min. In process No. B11H, the average cooling rate from the time of casting removal up to 500°C was 45°C / min, and the average cooling rate from 500°C to 300°C was adjusted to 40°C / min. For temperature measurement, the temperature of the castings was measured using both an infrared thermometer and a contact thermometer, and the average cooling rate in each temperature range was adjusted to a predetermined value. The resulting mold castings were then subjected to metallurgical microscopy observation, cutting tests, dezincification corrosion tests, hardness tests, and impact tests.

[0123] Process C: Continuous casting (Process No.C1~C3, C11H) Using a low-frequency melting furnace with an internal volume of 2 tons that is currently in operation, approximately 900 kg of raw material was charged and melted at 1080°C. The molten metal, which was held at 1010°C in a holding furnace, was then cast through a mold in a continuous casting facility at a casting speed of 200 mm / min to produce φ21 mm continuous casting rods. Cooling was performed by water cooling of the mold itself, air cooling, or by directly water cooling the continuous casting rods with a shower placed at an appropriate position from the mold. The shower position and water flow rate were adjusted. For temperature measurement, the temperature of the continuous casting rods was measured using a combination of a radiation thermometer and a contact thermometer. The cooling rate at 500°C was adjusted so that the average cooling rate from 500°C to 300°C was the same, resulting in cooling rates of 900°C / min, 660°C / min, 480°C / min, or 60°C / min. In this method, there is no specific temperature set for starting the cooling process. The resulting continuously cast rods were then subjected to metallurgical microscopy observation, cutting tests, dezincification corrosion tests, hardness tests, and impact tests.

[0124] Process G: Heat treatment (Process No.G1~G3, G11H~G14H) Process G involved further heat treatment of some of the sand castings and die castings produced in processes A, B, and C. Using the sand castings, die castings, and continuous casting rods obtained in processes A14H, B11H, and C11H, heating was carried out in all cases at 580°C for 20 minutes. In processes G1, G2, and G3, after heating, the cooling process was started at 575°C, and the average cooling rate from 550°C to 500°C and from 500°C to 300°C was the same at 1200°C / min. In processes G11H and G12H, the cooling process started at 470°C, the average cooling rate from 580°C to 470°C was 50°C / min, and the average cooling rate from 470°C to 300°C was 900°C / min. In processes G13H and G14H, after heat treatment at 580°C, no special cooling treatment was performed, and the materials were cooled from 500°C to 300°C at an average cooling rate of 25°C / min. These heat-treated materials were subjected to metallurgical microscopy observation, cutting tests, dezincification corrosion tests, hardness tests, and impact tests.

[0125] (Comparative material) As a comparative material, a φ40 mm rod of free-cutting brass C3604 containing 3 mass% Pb was prepared and designated as alloy X. Additionally, an ingot of casting brass CAC203 containing 2 mass% Pb was prepared and designated as alloy Y. Melting and casting were carried out in the same manner as in processes B1 and B11H to obtain mold castings with a diameter of 40 mm and a length of 200 mm. Alloys X and Y are commercially available. These alloys were subjected to metallurgical microscopy observation, cutting tests, dezincification corrosion tests, hardness tests, and impact tests.

[0126] The above-mentioned test materials were evaluated based on the following criteria. The evaluation results are shown in Tables 10 to 21.

[0127] (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. Note that when grain boundaries were observed within the β phase, it was designated 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. In Tables 12 to 20, the β phase, i.e., the unmodified β phase, is listed as f4A to distinguish it from the β1 phase (f4). For f6, when f4=(β1)=0, the (β1) term uses the value of f4A=(β) instead of (β1), and the resulting value is listed as the reference value f6A.

[0128] Each test material casting was cut parallel to its longitudinal direction. The surface was then polished to a mirror finish and etched with a mixture of hydrogen peroxide and ammonia water. For etching, an aqueous solution was used, consisting of 3 mL of 3 vol% hydrogen peroxide and 22 mL of 14 vol% ammonia water. The polished surface of the metal 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, at 5 to 10 seconds.

[0129] 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 fields 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 grain boundaries of the β phase 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 ratio of each phase was determined. In detail, the average of the area ratios in the five fields of view was calculated for each phase, and the average value was taken as the phase ratio 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 ratios of all constituent phases was set to 100%. Compared to hot-extruded rods produced in mass-production equipment, cast test specimens showed a higher frequency of non-metallic oxides and sulfides.

[0130] When the β phase is modified, grain boundary patterns, i.e., crystal grain boundaries, can be observed within the β phase using the etching solution and a metallurgical microscope. 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, metal microstructures may contain P and Si compounds and precipitates. Compounds of P with Zn or Si appear as blackish-gray particles of approximately 0.5-2 μm under a 500x microscope, mainly existing within the β phase, β1 phase, and at the phase boundary between the β and α phases. Compounds of Fe, Mn, Cr with P or Si can be seen as light blue particles of approximately 1-5 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 microscope. In Tables 10 to 15, grain boundaries were indicated as "B" when observed within the β1 phase (β phase) of the metal structure, and as "D" when not observed.

[0131] Regarding P compounds, when the amount of P was 0.06 mass% and the cooling rate was average from 500°C to 300°C at 15-60°C / min, approximately 250-500 P compounds were observed in a field of view at 500x magnification (80mm x 120mm when printed). However, in printed metal microstructures, it becomes difficult to distinguish between P compounds and compounds of Fe, Mn, Cr, Al, P or Si, Pb particles, oxide particles, etc. Therefore, when determining the presence or absence of P compounds from printed metal microstructure images, if there are fewer than 25 P compounds and compounds that could potentially be P compounds within the field of view of the image, it is evaluated as "D" indicating that P compounds do not exist; if there are 25 or more but fewer than 125 P compounds, it is evaluated as "C" indicating that P compounds exist but are few; and if there are 125 or more P compounds, it is evaluated as "B" indicating that P compounds exist. The presence or absence, and the quantity of P compounds, is one indicator of whether appropriate processing has been performed.

[0132] 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).

[0133] (conductivity) 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.

[0134] (Machinability test using a lathe) The machinability was evaluated using a cutting test with a lathe, as described below. Test specimens were prepared by first machining the materials to a diameter of 18 mm using the following conditions: Test specimen A obtained from sand casting in process A, die casting obtained from process B, continuous casting rod obtained from process C, and heat-treated material from process G. 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.

[0135] 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). In the cutting test, to minimize the effect of chip wear, the sample names corresponding to the test numbers were A, B, C, ..., and the test sequence was performed twice in the order of A→B→C→... and ...→C→B→A, with four measurements taken for each sample. 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.

[0136] The cutting resistance (main component force) of a commercially available free-cutting brass rod C3604 (alloy X, φ40mm) 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. 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

[0137] Cutting resistance (main component force) depends on the shear strength or tensile strength of the material, and materials with higher strength tend to have higher cutting resistance. For example, a copper alloy that has the same machinability as C3604 but is about 1.2 times stronger than C3604 will have roughly 20% higher cutting resistance than C3604. For this reason, in the case of a copper alloy with high strength, a cutting resistance that is about 40% higher than C3604 is considered practically acceptable. In this embodiment, the Vickers hardness is approximately 1.2 times and 1.4 times higher than that of C3604 extruded bar stock and CAC203 casting containing 2% Pb, respectively. Therefore, the evaluation criterion for machinability in this embodiment was set with a machinability index of 70 as the boundary value. Specifically, a machinability index of 78 or higher was evaluated as having excellent machinability (evaluation: A, excellent) and approximately the same machinability as C3604. A machinability index of 70 or higher but less than 78 was evaluated as good machinability (evaluation: B, good). A machinability index of 63 or higher but less than 70 was evaluated as fair machinability (evaluation: C, fair). A machinability index of less than 63 was evaluated as poor machinability (evaluation: D, poor). In this application, since the aim is good machinability, a machinability index of 70 or higher was considered acceptable.

[0138] (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. 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 phenolic resin. Next, the sample was cut so that the longest cross-section of the corroded area was obtained. Subsequently, the sample was polished. Using a metallurgical microscope, the dezincification 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.

[0139] When typical Pb-containing free-machining brass rods C3604 and free-machining brass castings CAC203 are tested according to ISO 6509, the maximum corrosion depth is approximately 500 μm or exceeds 500 μm. In this application, if the maximum corrosion depth is less than 250 μm, which is 50% less than the maximum corrosion depth of C3604 or CAC203, it is considered to have good dezincification corrosion resistance in practical use and is evaluated as good (evaluation: B, good). In particular, if the maximum corrosion depth is 100 μm or less, it is considered to have excellent dezincification corrosion resistance in practical use and is evaluated as excellent (evaluation: A, excellent). On the other hand, if the maximum corrosion depth exceeds 400 μm, the dezincification corrosion resistance is equivalent to or worse than that of C3604 or CAC203, and therefore, it is evaluated as "D" (poor) as not retaining dezincification corrosion resistance. If the maximum corrosion depth is between 250 μm and 400 μm, the dezincification corrosion resistance is evaluated as acceptable (evaluation: C, fair). In this application, the aim was to achieve good resistance to dezincification corrosion, so good (evaluation: B) and excellent (evaluation: A) were considered acceptable.

[0140] (mechanical properties) (Hardness) The hardness of each test material was measured using a Vickers hardness tester with a load of 49 kN on samples in which the metallic structure was observed. Furthermore, the hardness of the test pieces after cutting tests was measured using the Rockwell B scale. A Vickers hardness of preferably 100 Hv or higher, and more preferably 110 Hv or higher, indicates a high level of strength among free-machining copper alloy castings. Similarly, a Rockwell B scale score of 52 or higher, and more preferably 60 or higher, indicates a high level of strength among free-machining copper alloy castings. The aforementioned good resistance to dezincification corrosion and high strength lead to a reduction in the amount of material used.

[0141] (Impact properties) The impact test was performed using the following method: A U-notch test specimen (notch depth 2 mm, notch bottom radius 1 mm) conforming to JIS Z 2242 was taken. A Charpy impact test was performed with an impact blade of radius 2 mm, and the impact value was measured. To maintain good toughness and impact characteristics, the Charpy impact test value is preferably 25 J / cm². 2 The above is more preferable: 30 J / cm² 2 That's all.

[0142] [Table 12]

[0143] [Table 13]

[0144] [Table 14]

[0145] [Table 15]

[0146] Table 16

[0147] Table 17

[0148] Table 18

[0149] [Table 19] TIFF0007913654000019.tif103170

[0150] Table 20

[0151] Table 21

[0152] Table 22

[0153] Table 23

[0154] Table 24

[0155] Table 25

[0156] Table 26

[0157] Table 27

[0158] Table 28

[0159] Table 29

[0160] Table 30

[0161] Table 31

[0162] Table 32

[0163] Table 33

[0164] Table 34

[0165] [Table 35] TIFF0007913654000035.tif107170

[0166] [Table 36] TIFF0007913654000036.tif107170

[0167] [Table 37]

[0168] [Table 38]

[0169] From the above measurement results, the following findings were obtained. 1) It has been confirmed that by satisfying the composition of the present embodiment, satisfying the compositional relational expressions f1, f2, f0, the structural relational expressions f3 to f5, and the structure-composition relational expression f6, a casting can be obtained which contains a small amount of Pb, exhibits good machinability, has excellent dezincification corrosion resistance, good conductivity of 15% IACS or higher, high strength, and combines good impact properties (Alloy Nos. S01, S02, S11 to S14, S21 to S42).

[0170] 2) When Si is contained in an amount exceeding 0.50 mass%, the β phase is modified, and grain boundaries are observed within the β1 phase. As a result, both dezincification corrosion resistance and machinability were good. When the Si content is 0.65 mass% or more, dezincification corrosion resistance and machinability are further improved. On the other hand, when Si is contained in an amount exceeding 1.20 mass%, the electrical conductivity decreases (Alloy Nos. S01, S30, S32, S42, S51, S52, etc.). 3) When P is contained in an amount exceeding 0.01 mass%, the β phase is modified, and grain boundaries are observed within the β1 phase. As a result, dezincification corrosion resistance and machinability became good. When the P content is 0.03 mass% or more, dezincification corrosion resistance and machinability are further improved. When the P content exceeds 0.18 mass%, the impact properties deteriorate (Alloy Nos. S01, S26 to S28, S54, S58, etc.). 4) When the Cu content exceeded 60.5 mass%, the β phase was modified, and grain boundaries were observed within the β1 phase. As a result, dezincification corrosion resistance and machinability improved. When the Cu content was 61.5 mass% or higher, dezincification corrosion resistance and machinability improved further. When the Cu content was higher than 65.0 mass%, the β1 phase decreased, and in some cases, the γ phase appeared, resulting in poor machinability (alloys No. S01, S32, S24, S14, S42, S55, S65, etc.).

[0171] 5) When the Pb content was less than 0.002 mass%, the machinability was poor (Alloy No. S59). When the Pb content was 0.003 mass% or more, and even 0.01 mass% or more, the machinability improved (Alloy Nos. S29, S21). It was confirmed that when Bi was included, the alloy had machinability equivalent to that of Pb (Alloy Nos. S01, S24, etc.). 6) When Sn content exceeded 0.05 mass%, dezincification resistance improved further. When Sn content exceeded 0.10 mass%, dezincification resistance improved even more. On the other hand, when Sn content exceeded 0.70 mass%, the γ phase appeared, which actually worsened dezincification resistance, reduced impact properties, and lowered the machinability index. Also, when the Sn / Si ratio was 0.8 or higher, the β phase was not modified, resulting in poor dezincification resistance and poor machinability index (alloys No. S33~S35, S60, S61, etc.).

[0172] 7) It was confirmed that even if the alloy contains unavoidable impurities present in the quantities found in actual operations, it does not significantly affect the various properties (alloys No. S01, S02, S21, S22, S37, S39, etc.). 8) When the total content of unavoidable impurities Fe, Mn, Cr, and Co was 0.50 mass% or more, the β phase was modified, but machinability was poor and dezincification corrosion resistance was also slightly poor. As the total content of Fe, Mn, Cr, and Co decreased to less than 0.50 mass%, and further to less than 0.35 mass%, machinability and dezincification corrosion resistance improved. The decrease in machinability and dezincification corrosion resistance is thought to be due to the combination of Fe, Mn, etc. with some Si and P, forming intermetallic compounds between Fe, Mn, etc. and Si and P. As a result, the Si and P concentrations, which are present and effectively functioning as compounds between Fe, etc. and Si and P, decreased, leading to a deterioration in machinability and corrosion resistance (alloys No. S02, S38, S39, S63, etc.). 9) When the amount of Al, an unavoidable impurity, was 0.30 mass% or more, the β phase was not modified, resulting in poor resistance to dezincification corrosion, poor machinability, and poor impact properties. When the amount of Al was less than 0.30 mass%, resistance to dezincification corrosion and machinability improved, and when the amount of Al was less than 0.15 mass%, the effect of Al was further reduced, resulting in good machinability and resistance to dezincification corrosion (alloys No. S01, S62, S36, S37, etc.).

[0173] 10) When the composition relation f1 was less than 57.3, the β phase was not modified or was only slightly modified, resulting in poor resistance to dezincification corrosion and poor machinability at high speeds. When the composition relation f1 was greater than 60.8, the area ratio of the β1 phase was small, resulting in poor machinability (alloys No. S57, S53). When the value of the composition relation f1 was 57.3 or greater, the β phase was modified, improving resistance to dezincification corrosion and machinability, and further improving at 58.0 or greater. On the other hand, when the value of the composition relation f1 was 60.8 or less, the area ratio of the β1 phase increased, becoming even larger at 60.5 or less, resulting in a higher machinability index, including high-speed cutting (e.g., alloys No. S14, S32, S25).

[0174] 11) When the area ratio of the β1 phase, f4, was less than 25%, good machinability could not be obtained. The hardness index was also low. When the area ratio of the β1 phase exceeded 25%, and even more so, 30% or more, machinability improved, including high-speed cutting. On the other hand, when the area ratio of the β1 phase exceeded 80%, the β phase was often not modified, resulting in a lower machinability index, especially at high-speed cutting (110 m / min), and poor resistance to dezincification corrosion. The impact value also decreased. When the area ratio of the β1 phase was 80% or less, and even more so, 70% or less, the β phase was modified further, improving dezincification corrosion resistance and machinability. Although the progression of dezincification corrosion was significantly slowed as a result of the modification of the β phase, the β1 phase was preferentially subjected to dezincification corrosion. Therefore, a higher area ratio of the β1 phase slightly accelerates the progression of dezincification corrosion (alloys No. S57, S55, S53, S14, S11, process No. A6, No. B5, process No. A11H, No. B12H, etc.).

[0175] 12) When the area ratio of the γ phase is 4% or more, γ negatively affects machinability, resulting in a lower machinability index even when f4 and f6 are satisfied. Additionally, the impact value was low. (Alloy No. S61). 13) Even if f1 and f4 were satisfied, if the value of the microstructure-composition relation f6 was less than 28, the machinability was poor. When the value of f6 was 28 or higher, and even more so when it exceeded 35, the machinability improved (alloys No. S64, S41). On the other hand, the machinability was highest when the value of f6 was between 55 and 80. The machinability is thought to be due to the fact that the β1 phase amount (f4) is 45-60% and the Si amount is 1 mass% (for example, alloys No. S02, S12, process Nos. A1, A2, etc.). 14) Even when the amount of β-phase that has not been modified to β1 phase (f4A) was 45 or more, and f6A exceeded 40, the dezincification corrosion depth was large, and machinability was particularly poor at high speeds (Test Nos. S22, S106, S223, etc.).

[0176] 15) During cooling after casting, if the cooling start temperature was lower than 700°C and exceeded 500°C, and the average cooling rate 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 improved significantly, and strength was increased. Regarding machinability, when the β phase was modified, the decrease in machinability was slight even at high cutting speeds, resulting in a significant difference compared to samples where the β phase was not modified (process No. A1~A6, A11H~A14H, B1~B5, B11H~B14H, etc.). 16) When the cooling start temperature was above 500°C but below approximately 550°C, grain boundaries were observed within the β1 phase, and a small amount of P compound was present. When a small amount of P compound was present, the cutting resistance was slightly higher and the dezincification corrosion resistance was slightly worse than when no P compound was present (processes No. A1, A5, B1, B4, etc.). 17) When the cooling treatment start temperature was between 550°C and 650°C, machinability and dezincification corrosion resistance improved further. Furthermore, when the cooling rate at 500°C and the average cooling rate from 500°C to 300°C exceeded 600°C / min, and even exceeded 900°C / min, the faster the cooling rate, the higher the machinability, especially the machinability index at 110 m / min, and the better the dezincification corrosion resistance. Thus, it was found that the cooling treatment start temperature, the cooling rate at 500°C, and the cooling rate from 500°C to 300°C influenced the modification of the β phase. In other words, it is thought that the β phase was more modified (processes No. A1-A6, B1-B5, C1-C6, etc.).

[0177] 18) The cooling rate at 500°C, approximately 300°C / min, and the average cooling rate from 500°C to 300°C, approximately 300°C / min, are the boundary values ​​for whether or not grain boundaries are observed within the β phase, i.e., whether or not the β phase is modified. In particular, the machinability at high speeds and resistance to dezincification corrosion changed abruptly (processes No. A4, A13, B3, B14H, etc.). 19) When the cooling start temperature was higher than 700°C, the proportion of β1 phase increased, the machinability index was slightly lower, machinability worsened especially at high speeds, and resistance to dezincification corrosion also worsened. When the proportion of β1 phase exceeded 80%, 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 β1 phase slightly accelerated the rate of dezincification corrosion (processes No. A1-A6, A11H, B1-B5, B12H, etc.). 20) When the average cooling rate from 500°C to 300°C was approximately 200-250°C / min, no grain boundaries were observed in the β phase, and in most cases, only a small amount of P compound was present. As a result of the absence of grain boundaries in the β phase, i.e., the β phase was not modified, machinability, especially at 110 m / min, deteriorated, and resistance to dezincification corrosion worsened (process No. A13H, B14H, etc.).

[0178] 21) During cooling after casting, even when the starting temperature for the cooling process is not specifically defined, it was confirmed that the β phase is modified when both the cooling rate from 550°C to 500°C and the average cooling rate in the temperature range from 500°C to 300°C exceed 300°C / min, significantly improving resistance to dezincification corrosion and machinability (processes No. C1-C3, C11H). 22) After casting, no cracks were observed in the castings even when subjected to a rapid cooling process from a high temperature exceeding 500°C (processes A1-A6, B1-B5, C1-C3). 23) When castings in which the β phase was not modified were annealed at 580°C for 20 minutes, it was confirmed that when the cooling rate at 500°C and the average cooling rate from 500°C to 300°C exceeded 300°C / min, the β phase was modified, and the resistance to dezincification corrosion and machinability were significantly improved (Process No. G1~G3, C11H).

[0179] From the above, alloys of this embodiment, in which the content and compositional relationships of each additive element and each microstructure relationship are within an appropriate range, exhibit good machinability, resistance to dezincification corrosion, and mechanical properties. Furthermore, excellent properties can be obtained in alloys of this embodiment by setting the cooling conditions after casting and the heat treatment conditions within an appropriate range. [Industrial applicability]

[0180] The free-machining copper alloy casting of this embodiment has a low Pb content, excellent machinability, dezincification resistance, and castability, and possesses high strength while also having good impact properties. 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 home appliance parts, automobile parts, machine parts, stationery, toys, musical instruments, sliding parts, instrument parts, precision machine parts, medical 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, hot and cold water mixing taps, shut-off valves, water meters, shower heads, valves, fittings, cocks, gears, shafts, bearings, sleeves, spindles, sensors, bolts, nuts, and connectors.

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%, P in amounts exceeding 0.01 mass% but less than 0.18 mass%, Bi in amounts between 0 mass% and less than 0.20 mass%, and the remainder consists of Zn and unavoidable impurities. Of the aforementioned unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than 0.50 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 57.3 ≤ f1 = [Cu] - 4.8 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.8 holds true. If Bi is present in 0.0001 mass% or more, the relationship 0.003 ≤ f2 = [Pb] + [Bi] < 0.20 also 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, 28<f6=(β1)×(-0.5×[Si] ２ +1.5×[Si])-2×(γ)+([Pb]+[Bi]) １／２ ×25 + ([P]) １／２ It has a relationship of ×12, A free-cutting copper alloy casting characterized in that grain boundaries are observed within the β1 phase when etched with a mixture of hydrogen peroxide and aqueous ammonia.

2. It contains 61.5 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 mass% to less than 0.20 mass% of Bi, and the remainder consists of Zn and unavoidable impurities. Of the aforementioned unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than 0.35 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] - 4.8 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] ≤ 60.5 holds true. If Bi is present in a mass of 0.0001% or more, the relationship 0.004 ≤ f² = [Pb] + [Bi] < 0.10 is further established, 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)×(-0.5×[Si] ２ +1.5×[Si])-2×(γ)+([Pb]+[Bi]) １／２ ×25 + ([P]) １／２ It has a relationship of ×12, A free-cutting copper alloy casting characterized in that grain boundaries are observed within the β1 phase when etched with a mixture of hydrogen peroxide and aqueous ammonia.

3. 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 0.70 mass%, Bi in amounts between 0 mass% and less than 0.20 mass%, and the remainder consists of Zn and unavoidable impurities. Of the aforementioned unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than 0.50 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 57.3 ≤ f1 = [Cu] - 4.8 × [Si] + 0.5 × ([Pb] + [Bi]) - [P] - [Sn] ≤ 60.8 holds true, Furthermore, the relationship f0 = [Sn] / [Si] < 0.8 holds. If Bi is present in a mass of 0.0001% or more, 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, 28 <f6 = (β1) × (-0.5 × [Si] ２ + 1.5 × [Si]) - 2 × (γ) + ([Pb] + [Bi]) １／２ × 25 + ([P]) １／２ × 12 has the relationship, A free-cutting copper alloy casting characterized in that grain boundaries are observed within the β1 phase when etched with a mixture of hydrogen peroxide and aqueous ammonia.

4. A free-machining copper alloy casting according to any one of claims 1 to 3, characterized in that it is used in equipment and parts related to drinking water and sanitation facilities, water meters, valves, drainage equipment and parts, industrial piping parts, automobile parts, electrical and home appliance parts, machine parts, stationery, toys, musical instruments, sliding parts, instrument parts, and medical parts.

5. A method for manufacturing a free-machining copper alloy casting according to any one of claims 1 to 3, Having a casting process, A method for manufacturing a free-machining copper alloy casting, characterized in that, in the cooling process of the final casting step of the aforementioned casting process, the cooling rate when the temperature of the casting reaches 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.

6. A method for manufacturing a free-machining copper alloy casting according to any one of claims 1 to 3, Having a casting process, A method for manufacturing a free-machining copper alloy casting, characterized in that, in the cooling process of the final casting step of the aforementioned casting process, the starting temperature for the cooling treatment is set to a temperature higher than 500°C and lower than 700°C, the cooling rate when the casting temperature reaches 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.

7. A method for manufacturing a free-machining copper alloy casting according to any one of claims 1 to 3, It has a casting process and a heat treatment process, A method for manufacturing a free-machining copper alloy casting, characterized in that, in the final heat treatment step, the casting is heated under conditions of being held at a temperature between 520°C and 650°C for 1 minute to 5 hours, and in the cooling step after heat treatment, the cooling process is started when the temperature of the casting exceeds 520°C, the cooling rate when the temperature of the casting reaches 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.

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