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

Abstract

This free-machining copper alloy is characterized in that: prescribed amounts of Cu, Si, Pb, and P are contained, and a prescribed amount of Bi is contained as an optional element, with the remainder including Zn and unavoidable impurities; the total content of Fe, Mn, Co, and Cr among the unavoidable impurities is less than a prescribed amount; the content of Al among the unavoidable impurities is less than a prescribed amount; the composition relational expression f1 defined from the composition of elements and the structure relational expressions f2, f3, and f4 defined from area ratios of constituent phases of the metal structure are respectively within prescribed ranges; and a grain boundary of a modified β1 phase is observable when etched with a liquid mixture of hydrogen peroxide and aqueous ammonia.

Description

Free-cutting copper alloy and method for producing free-cutting copper alloy

[0001] The present invention relates to a free-cutting copper alloy having high corrosion resistance, particularly dezincification corrosion resistance and strength, and significantly reduced Pb content, and a method for producing the free-cutting copper alloy. It relates to a free-cutting copper alloy used for utensils and parts used in drinking water consumed daily by humans and animals, utensils and parts used in sanitary facilities such as kitchens, bathrooms, and toilets, water meters, musical instruments, tableware, drainage utensils and parts, industrial piping parts, electrical and electronic equipment parts, automotive parts, mechanical parts, stationery, toys, sliding parts, instrument parts, precision mechanical parts, medical parts, parts related to liquids and gases such as hydrogen, and specifically, faucet, mixing faucet, faucet fitting, stopcock, valve, joint, valve, shower head, cock, gear, shaft, bearing, shaft, sleeve, spindle, sensor, bolt, nut, flare nut, pen tip, insert nut, bag nut, nipple, spacer, screw, etc. parts to be machined, and a method for producing a free-cutting copper alloy. This application claims priority based on Japanese Patent Application No. 2024-192019 filed in Japan on October 31, 2024, and Japanese Patent Application No. 2024-192034 filed in Japan on October 31, 2024, and incorporates the contents herein.

[0002] Conventionally, for parts such as faucet, mixing faucet, stopcock, valve, cock, joint, gear, sensor, nut, screw, etc. used in utensils and parts related to drinking water, sanitary facilities, water meters, musical instruments, tableware, electrical, household, and electronic equipment parts, automotive parts, mechanical parts, stationery, precision mechanical parts, medical parts, and utensils and parts related to liquids and gases such as industrial water, drainage, and hydrogen, a Cu-Zn-Pb alloy (so-called free-cutting brass bar, forging brass, casting brass) containing 56 to 65 mass% of Cu, 1 to 4 mass% of Pb, and the balance being Zn, or a Cu-Sn-Zn-Pb alloy (so-called bronze casting: gunmetal) containing 80 to 88 mass% of Cu, 2 to 8 mass% of Sn, 1 to 8 mass% of Pb, and the balance being Zn has been generally used, which has excellent machinability, antibacterial properties, and good corrosion resistance.

[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 other than the United States, creating a demand for the development of copper alloy materials that comply with Pb content regulations.

[0004] Furthermore, in other industrial sectors such as electrical and electronic equipment, automobiles, and machinery, for example, the European RoHS Directive and ELV Directive exceptionally allow Pb content up to 4 mass% in free-cutting 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 the trend of stricter Pb regulations for free-cutting copper alloys, as an alternative to Pb-containing free-cutting copper alloys containing 56-65 mass% Cu, 1-4 mass% Pb, and the remainder being Zn, particularly free-cutting brass containing 3 mass% Pb (JIS standard: C3604, ASTM standard: C36000, EN standard: CW614N), which is the most widely used free-cutting copper alloy worldwide, (1) Bi, which has machinability properties, is used instead of Pb, and in some cases Therefore, various alloys have been proposed, including (1) Cu-Zn-Bi alloys and Cu-Zn-Bi-Se alloys containing Se in addition to Bi, (2) Cu-Zn alloys containing a high concentration of Zn and increasing the β phase to improve machinability, (3) Cu-Zn-Si alloys containing a large amount of γ phase and κ phase which have excellent machinability instead of Pb, and (4) Cu-Zn-Si-Pb alloys in which the machinability of the β phase is improved by the inclusion of Si. For example, in Patent Document 1, 0.3 to 4 mass%, preferably 1.8 to 3.2 mass%, of Bi is added to a Cu-Zn alloy, and since the β phase has poor resistance to dezincification corrosion, the β phase is reduced and the β phase is separated by the α phase by annealing at 350 to 550°C to improve machinability and resistance to dezincification corrosion.

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

[0007] Furthermore, while Cu-Zn binary alloys containing a large amount of β phase contribute to improved machinability, the β phase has inferior machinability compared to Pb, and is also inferior in dezincification corrosion resistance and stress corrosion cracking resistance. Therefore, it cannot possibly replace Pb-containing free-machining copper alloys. For this reason, Cu-Zn-Si alloys containing Si instead of Pb have been proposed as free-machining copper alloys, for example, in Patent Documents 2 to 8.

[0008] Patent documents 2 to 7 state that the Cu content is approximately 58 to 65 mass%, and the Si content is approximately 0.2 to 1.5 mass%, and that the machinability is improved by the presence of Si contained in the β phase and fine P compounds formed by P and Zn, and that by specifying the area ratio of the β phase and γ phase, excellent machinability is achieved in combination with the presence of P compounds and a small amount of Pb. However, it is a well known fact that the β phase of Cu-Zn alloys has poor dezincification resistance, as disclosed in Patent Document 1. For this reason, no specific data related to dezincification resistance is disclosed in Patent Documents 2 to 7, and it is presumed that the dezincification resistance, which is a technical problem of the β phase of conventional Cu-Zn alloys, has not been improved.

[0009] Patent Document 8 describes how, by specifying the total surface area of ​​the γ and κ phases, which have excellent machinability properties and are formed in alloys with high Cu and Si concentrations, with a Cu content of 71.5 to 78.5 mass% and a Si content of 2.0 to 4.5 mass%, excellent machinability is achieved with a small amount of Pb content of 0.02 mass% or less. 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.

[0010] Furthermore, Patent Document 9 describes a Cu-Zn-Sn alloy containing small amounts of Si, Pb, P, or Fe, and containing 0.5 mass% or less of Pb. By devising a manufacturing method, Pb-enriched particles are dispersed in the matrix, and the number density of Pb-enriched particles present in the α phase is increased, thereby obtaining excellent machinability. Patent Document 9 also states that a finish heat treatment of 400 to 600°C is substantially necessary to improve resistance to dezincification corrosion.

[0011] Patent document 10 relates to a technology for manufacturing near-net cavity hot forged products using a hollow material in a Cu-Zn-Si-Pb-P alloy, and proposes a copper alloy in which the area ratios of the β phase, γ phase, and μ phase are limited. Patent document 11 proposes a copper alloy casting in which Si, Pb, and Sn are selectively contained in a Cu-Zn-Zr-P alloy, and the crystal grains are refined by the action of Zr and P. Patent document 12 proposes a copper alloy in which Si and Pb are selectively contained in a Cu-Zn-Sn-Al alloy, and the area ratios of the γ phase and β phase are limited, resulting in excellent discoloration resistance. Patent document 13 proposes a copper alloy casting in which Pb is not contained in a Cu-Zn-Si-Sn-Al-P alloy.

[0012] Patent document 14 states that in Cu-Zn-Si-Sn-Al alloys, the apparent Zn content is important for improving corrosion resistance, and proposes a copper alloy in which machinability is improved by substantially containing a large amount of Pb or Bi. Patent document 15 describes a copper alloy casting in a Cu-Zn-Si alloy that contains 65 mass% or more of Cu, has good castability and mechanical strength, does not contain Pb, and whose machinability is improved by the γ phase, and describes an example containing large amounts of Sn, Al, Mn, Ni, and Sb.

[0013] Patent documents 16 and 17 state that the Cu content is approximately 58 to 66 mass%, and the Si content is approximately 0.04 to 1.2 mass%, and that machinability is improved by specifying the size and number of fine P compounds formed by P and Zn, etc., with respect to the Si contained in the alloy. They also state that by specifying the area ratio of the β phase, excellent machinability is achieved in combination with the presence of P compounds. However, it is a well known fact that the β phase of Cu-Zn alloys has poor dezincification resistance, as disclosed in Patent Document 1. Therefore, no specific data related to dezincification resistance is disclosed in Patent Documents 16 and 17, and it is presumed that the dezincification resistance, which is a technical problem of the β phase of conventional Cu-Zn alloys, has not been improved.

[0014] Japanese Patent Publication No. 2002-003967, Japanese Patent Publication No. 2021-042461, Japanese Patent Publication No. 2021-042459, Japanese Patent Publication No. 2021-042460, International Publication No. 2020 / 261666, Japanese Patent Publication No. 2021-042462, International Publication No. 2021 / 117528, International Publication No. 2007 / 034571, Japanese Patent Publication No. 2016-1 Publication No. 94123, International Publication No. 2013 / 065830, International Publication No. 2006 / 016630, International Publication No. 2015 / 046421, Japanese Patent Publication No. 2010-133006, Japanese Patent Publication No. 2018-048398, Japanese Patent Publication No. 2019-508584, DE102022002927B4, DE102022002928B4

[0015] As shown in these Patent Documents 1 to 17, 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.

[0016] The present invention has been made to solve the problems of the prior art, and aims to provide a free-machining copper alloy that has good resistance to dezincification corrosion despite containing a large amount of β phase (β1 phase described later), high strength, and a significantly reduced Pb content, as well as a method for producing a free-machining copper alloy.

[0017] In order to solve the above-mentioned problems and achieve the above-mentioned objectives, the inventors diligently conducted research and obtained the following findings. In this specification, unless otherwise specified, the β phase includes the β' phase, the γ phase includes the γ' phase, and the α phase includes the α' phase. The β1 phase is a modified β phase and is distinct from the β phase and the β' phase. The β1 phase is characterized by the observation of grain boundaries within the β1 phase when observed with a metallurgical microscope using hydrogen peroxide and ammonia water as etching solutions. A grain boundary refers to a linear pattern that penetrates the β1 phase crystal grains observed within the β1 phase, as shown in Figure 1A. In general Cu-Zn alloys, Cu-Zn-Bi alloys, Cu-Zn-Si alloys, etc., no grain boundaries are observed within the β phase even when etched with hydrogen peroxide and ammonia water. Therefore, the β phase and the β1 phase can be clearly distinguished. Hot-worked materials include hot-extruded materials, hot-forged materials, and hot-rolled materials. Cold workability refers to the performance of cold-worked processes such as drawing, wire drawing, rolling, riveting, and bending. Good or excellent machinability, unless otherwise specified, refers to low cutting resistance and good or excellent chip breaking when drilling with a drill. Conductivity refers to electrical conductivity, thermal conductivity, and electrical conductivity. Cooling rate refers to the average cooling rate over a certain temperature range.

[0018] In the aforementioned Patent Document 8, it is stated that in Cu-Zn-Si alloys, the β phase contributes little to, or rather inhibits, the machinability of the copper alloy. In Patent Documents 10, 11, and 12, the amount of the β phase is also significantly limited. In Patent Document 1, as a method to improve the dezincification corrosion resistance of the β phase, it is stated that a process of annealing at 350 to 550°C is necessary to reduce the β phase and to separate the β phase with the α phase. In Patent Document 9, in order to improve the dezincification corrosion resistance of the β phase, it is necessary that the alloy contains more Sn than Si, and that it is necessary to heat it to a temperature of 700 to 850°C, hot extrude it, and then perform a finish heat treatment by holding it at 400 to 600°C for 30 minutes or more, with an average cooling rate of 0.2 to 10°C / second from 400 to 200°C.

[0019] On the other hand, Patent Documents 2 to 7 describe the discovery that, in Cu-Zn-Si alloys, first, incorporating a certain amount of Si into the β phase has a significant effect on the machinability of the β phase itself. Furthermore, it is stated that the presence of fine P compounds, the inclusion of small amounts of Pb, and in some cases Bi, work synergistically to obtain excellent machinability in the alloy. However, in order to include P compounds, it is preferably necessary to cool the alloy at an average cooling rate of approximately 0.1°C / min to approximately 70°C / min in the temperature range from approximately 530°C to approximately 450°C after hot working. Incidentally, Patent Documents 2 to 7 do not disclose any data related to dezincification corrosion resistance. Nor do they mention any modification of the β phase itself. In other words, it is presumed that there is no improvement in the dezincification corrosion resistance of the β phase present in Cu-Zn-Si alloys, nor any further improvement in machinability.

[0020] 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, possesses excellent machinability without requiring the inclusion of Bi, and significantly improves the dezincification corrosion resistance of the β phase, which had been a long-standing issue for Cu-Zn alloys. In addition, alloys containing the β1 phase instead of the β phase showed further increased strength without impairing ductility. This β1 phase is obtained by first solid-solving a certain amount of Si and P within the β phase, maintaining the β phase state at 500-690°C, and increasing the cooling rate when cooling to room temperature, thereby bringing the high-temperature metallic structure down to room temperature (maintaining the high-temperature metallic structure down to room temperature).

[0021] 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 can be seen in the β1 phase, but grain boundaries cannot be seen in the β phase of a normal Cu-Zn alloy. The use of this etching solution is also disclosed in Patent Documents 2 to 7, 10, and 11, and the metal structure is shown.

[0022] To obtain the β1 phase, that is, to maintain the β phase state at 500-690°C after hot working and bring it down to room temperature, it is necessary that the cooling process after hot working starts at a temperature lower than 690°C and higher than 500°C relative to the hot-worked material, and that during the cooling process, the average cooling rate in the temperature range from the cooling start temperature to 500°C exceeds 300°C / min, and subsequently, the average cooling rate in the temperature range from 500°C to 300°C exceeds 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.

[0023] Patent documents 2 to 7 state that in order to obtain a compound containing fine P particles, the hot-worked material needs to be cooled at an average cooling rate of approximately 0.1°C / min to approximately 70°C / min in the temperature range from approximately 530°C to approximately 450°C after hot working. In other words, it is clear that this embodiment and patent documents 2 to 7 are moving in opposite directions (giving opposite instructions). However, for example, if the cooling rate around 520°C after hot working is 60°C / min, and the average cooling rate in the temperature range from 520°C to 500°C (the starting temperature of the cooling process) exceeds 300°C / min, then in this embodiment as well, an alloy containing both the β1 phase and the P compound may exist. However, in this embodiment, the P compound is not important, so the presence of the P compound is not required.

[0024] The β1 phase in this application, even without the P compounds disclosed in Patent Documents 2 to 7, promotes a reduction in cutting resistance and chip fragmentation through the synergistic effect of the β1 phase and fine Pb particles. More importantly, the unresolved and significant problem of dezincification corrosion resistance in conventional β phases is greatly improved and resolved by the modification from β phase to β1 phase. Furthermore, the mechanical properties inherit the high strength of the conventional β phase, resulting in a high-strength alloy. As a result, we have invented a copper alloy that has machinability close to that of conventional free-cutting brass, and which has better dezincification corrosion resistance and higher strength compared to conventional free-cutting brass.

[0025] The free-machining copper alloy of embodiment 1 of the present invention contains Cu in an amount greater than 58.0 mass% and less than 63.5 mass%, Si in an amount greater than 0.05 mass% and less than 0.50 mass%, Pb in an amount greater than 0.002 mass% and less than 0.20 mass%, and P in an amount greater than 0.01 mass% and less than 0.40 mass%, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co and Cr among the unavoidable impurities is less than 0.40 mass%, and the content of Al is less than 0.30 mass%, and the Cu content is [Cu] mass%, the Si content is [Si] mass%, and the Pb content is [ When the Pb content is [P]mass%, the relationship 57.5 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × [Pb] + 0.5 × [P] ≤ 61.5 holds, and in the constituent phases of the metallic structure excluding nonmetallic inclusions, when the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%, the relationships 10 ≤ f2 = (α) < 80, 20 < f3 = (β1) ≤ 90, and 0 ≤ f4 = (γ) < 4 hold, and grain boundaries can be observed inside the β1 phase when etched with a mixture of hydrogen peroxide and ammonia water.

[0026] The free-machining copper alloy of embodiment 2 of the present invention contains 58.8 mass% to 63.0 mass% of Cu, 0.08 mass% to 0.48 mass% of Si, 0.01 mass% to 0.10 mass% of Pb, and 0.03 mass% to 0.20 mass% of P, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co, and Cr among the unavoidable impurities is less than 0.30 mass%, and the content of Al is less than 0.15 mass%, with the Cu content being [Cu] mass%, the Si content being [Si] mass%, and the Pb content being [P When b is mass%, and the P content is [P] mass%, the relationship 58.0 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × [Pb] + 0.5 × [P] ≤ 61.0 exists, and in the constituent phases of the metallic structure excluding nonmetallic inclusions, when the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%, the relationships 15 ≤ f2 = (α) < 75, 25 < f3 = (β1) ≤ 85, 0 ≤ f4 = (γ) < 1 exist, and grain boundaries can be observed inside the β1 phase when etched with a mixture of hydrogen peroxide and ammonia water.

[0027] The free-machining copper alloy of embodiment 3 of the present invention contains Cu in amounts exceeding 58.0 mass% and less than 63.5 mass%, Si in amounts exceeding 0.05 mass% and less than 0.50 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 0.40 mass%, and as an optional element, Sn in amounts exceeding 0.01 mass% and less than 0.90 mass%, and 0.0001 mass It contains Bi in an amount greater than % but less than 0.20 mass%, Sb in an amount of 0.01 mass% or more but less than 0.12 mass%, and As in an amount of 0.01 mass% or more but less than 0.12 mass%, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co and Cr among the unavoidable impurities is less than 0.40 mass%, and the content of Al is less than 0.30 mass%, and the content of Cu is [ When 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%, the content of Bi is [Bi] mass%, and the content of Sn is [Sn] mass, the relationship 57.5 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × ([Pb] + [Bi]) + 0.5 × [P] - 1.1 × [Sn] ≤ 61.5 holds true for nonmetallic inclusions. In the constituent phases of the metal structure excluding the specified phase, 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 following relationships exist: 10 ≤ f2 = (α) < 80, 20 < f3 = (β1) ≤ 90, 0 ≤ f4 = (γ) < 4, and grain boundaries can be observed inside the β1 phase when etched with a mixture of hydrogen peroxide and ammonia water.

[0028] The free-machining copper alloy of embodiment 4 of the present invention contains 58.8 mass% to 63.0 mass% of Cu, 0.08 mass% to 0.48 mass% of Si, 0.01 mass% to 0.10 mass% of Pb, and 0.03 mass% to 0.20 mass% of P, and as an optional element, 0.05 mass% to 0.85 mass% of Sn, and 0.001 mass% or less It contains Bi in an amount of 0.10 mass% or less, Sb in an amount of 0.012 mass% or more and 0.08 mass% or less, As in an amount of 0.025 mass% or more and 0.08 mass% or less, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co and Cr among the unavoidable impurities is less than 0.30 mass%, and the content of Al is less than 0.15 mass%, and the content of Cu is [ When 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%, the content of Bi is [Bi] mass%, and the content of Sn is [Sn] mass, the relationship 58.0 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × ([Pb] + [Bi]) + 0.5 × [P] - 1.1 × [Sn] ≤ 61.0 holds true for nonmetallic inclusions. In the constituent phases of the metal structure excluding the specified phase, 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 following relationships exist: 15 ≤ f2 = (α) < 75, 25 < f3 = (β1) ≤ 85, 0 ≤ f4 = (γ) < 1, and grain boundaries can be observed inside the β1 phase when etched with a mixture of hydrogen peroxide and ammonia water.

[0029] The free-machining copper alloy of embodiment 5 of the present invention is characterized in that, in any one of embodiments 1 to 4 of the present invention, it has an electrical conductivity of 17.0% IACS or higher, a Vickers hardness of 95 HV or higher, and a maximum dezincification corrosion depth of 400 μm or less in the dezincification corrosion test results according to ISO 6509.

[0030] The free-machining copper alloy of embodiment 6 of the present invention is characterized in that, as a free-machining copper alloy of any one of embodiments 1 to 5 of the present invention, it is used in appliances and parts related to drinking water and sanitary equipment, valves, cocks, industrial piping parts, water meters, musical instruments, automobile parts, electrical and electronic equipment parts, machine parts, stationery, toys, sliding parts, instrument parts, precision machine parts, and medical parts.

[0031] The method for producing a free-machining copper alloy according to embodiment 7 of the present invention comprises Cu in an amount exceeding 58.0 mass% and less than 63.5 mass%, Si in an amount exceeding 0.05 mass% and less than 0.50 mass%, Pb in an amount of 0.002 mass% or more and less than 0.20 mass%, and P in an amount exceeding 0.01 mass% and less than 0.40 mass%, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co and Cr among the unavoidable impurities is 0. A copper alloy having a composition of less than 40 mass%, an Al content of less than 0.30 mass%, and with Cu content of [Cu] mass%, Si content of [Si] mass%, Pb content of [Pb] mass%, and P content of [P] mass%, having the relationship 57.5 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × [Pb] + 0.5 × [P] ≤ 61.5, is used, and the process involves one or more hot working steps. A method for producing a free-machining copper alloy by a manufacturing process, or by a manufacturing process having one or more hot working and heat treatment processes, wherein in the former manufacturing process, in the final hot working process, the hot working temperature is greater than 520°C and less than 790°C, and in the cooling process after hot working, the hot-worked material is cooled at a temperature lower than 690°C and higher than 500°C; and in the latter manufacturing process, in the final heat treatment process, the material is annealed at a temperature greater than 520°C and lower than 630°C for 1 minute to 5 hours, and after annealing, the cooling process is started at a temperature higher than 500°C; and in both the former and latter cooling processes, the average cooling rate in the temperature range from the cooling start temperature to 500°C is greater than 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is greater than 300°C / min.

[0032] The method for producing a free-machining copper alloy according to aspect 8 of the present invention comprises 58.8 mass% to 63.0 mass% of Cu, 0.08 mass% to 0.48 mass% of Si, 0.01 mass% to 0.10 mass% of Pb, and 0.03 mass% to 0.20 mass% of P, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co and Cr among the unavoidable impurities is 0.3 A copper alloy having less than 0 mass%, an Al content of less than 0.15 mass%, and with Cu content of [Cu] mass%, Si content of [Si] mass%, Pb content of [Pb] mass%, and P content of [P] mass%, having the relationship 58.0 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × [Pb] + 0.5 × [P] ≤ 61.0, and having one or more hot working steps. A method for producing a free-machining copper alloy by a manufacturing process, or by a manufacturing process having one or more hot working and heat treatment processes, wherein in the former manufacturing process, the final hot working step is a hot working temperature greater than 520°C and less than 790°C, and in the cooling process after hot working, the hot-worked material is cooled at a temperature lower than 690°C and higher than 500°C; and in the latter manufacturing process, the final heat treatment step is annealed at a temperature greater than 520°C and lower than 630°C for 1 minute to 5 hours, and after annealing, the cooling process is started at a temperature higher than 500°C; and in both the former and latter cooling processes, the average cooling rate in the temperature range from the cooling start temperature to 500°C is greater than 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is greater than 300°C / min.

[0033] The method for producing a free-machining copper alloy according to aspect 9 of the present invention comprises Cu in an amount exceeding 58.0 mass% and less than 63.5 mass%, Si in an amount exceeding 0.05 mass% and less than 0.50 mass%, Pb in an amount of 0.002 mass% or more and less than 0.20 mass%, and P in an amount exceeding 0.01 mass% and less than 0.40 mass%, and as an optional element, Sn in an amount exceeding 0.01 mass% and less than 0.90 mass%, Bi in an amount exceeding 0.0001 mass% and less than 0.20 mass%, Sb in an amount of 0.01 mass% or more and 0.12 mass%, and 0.01 When the material contains mass% to 0.12 mass% of As, with the remainder being Zn and unavoidable impurities, and the total content of Fe, Mn, Co, and Cr among the unavoidable impurities is less than 0.40 mass%, and the content of Al is less than 0.30 mass%, and the 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%, the content of Bi is [Bi] mass%, and the content of Sn is [Sn] mass, then the result is 57.5 A method for producing a free-machining copper alloy using a copper alloy having the relationship ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × ([Pb] + [Bi]) + 0.5 × [P] - 1.1 × [Sn] ≤ 61.5, wherein the manufacturing process includes one or more hot working steps, or one or more hot working steps and a heat treatment step, wherein in the former manufacturing process, the final hot working step has a hot working temperature greater than 520°C and less than 790°C, and in the cooling process after hot working, the hot-worked material is cooled to a temperature lower than 690°C. In the latter manufacturing process, the cooling process is started at a temperature higher than 500°C, and in the final heat treatment step, the material is annealed at a temperature above 520°C and below 630°C for 1 minute to 5 hours, after which the cooling process is started at a temperature higher than 500°C. In both the former and latter cooling processes, the average cooling rate in the temperature range from the cooling start temperature to 500°C is greater than 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is greater than 300°C / min.

[0034] The method for producing a free-machining copper alloy according to embodiment 10 of the present invention comprises 58.8 mass% to 63.0 mass% of Cu, 0.08 mass% to 0.48 mass% of Si, 0.01 mass% to 0.10 mass% of Pb, and 0.03 mass% to 0.20 mass% of P, and as optional elements, 0.05 mass% to 0.85 mass% of Sn, 0.001 mass% to 0.10 mass% of Bi, 0.012 mass% to 0.08 mass% of Sb, and 0.025 When the material contains mass% to 0.08 mass% of As, with the remainder being Zn and unavoidable impurities, and the total content of Fe, Mn, Co, and Cr among the unavoidable impurities is less than 0.30 mass%, and the content of Al is less than 0.15 mass%, and the 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%, the content of Bi is [Bi] mass%, and the content of Sn is [Sn] mass, then the ratio is 58.0 A method for producing a free-machining copper alloy using a copper alloy having the relationship ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × ([Pb] + [Bi]) + 0.5 × [P] - 1.1 × [Sn] ≤ 61.0, wherein the manufacturing process includes one or more hot working steps, or one or more hot working steps and a heat treatment step, wherein in the former manufacturing process, the final hot working step has a hot working temperature greater than 520°C and less than 790°C, and in the cooling process after hot working, the hot-worked material is cooled to a temperature lower than 690°C. In the latter manufacturing process, the cooling process is started at a temperature higher than 500°C, and in the final heat treatment step, the material is annealed at a temperature above 520°C and below 630°C for 1 minute to 5 hours, after which the cooling process is started at a temperature higher than 500°C. In both the former and latter cooling processes, the average cooling rate in the temperature range from the cooling start temperature to 500°C is greater than 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is greater than 300°C / min.

[0035] According to one aspect of the present invention, it is possible to provide a free-machining copper alloy that has good resistance to dezincification corrosion despite containing a large amount of β phase (β1 phase described later), high strength, and a significantly reduced Pb content, as well as a method for producing a free-machining copper alloy.

[0036] This is a photograph of the microstructure of a copper alloy in an embodiment, obtained by subjecting alloy No. S01 to process No. B1. In process No. B1, hot compression was performed at 630°C, and the cooling process was started at 570°C. The average cooling rate in the temperature range from 570°C to 300°C and the average cooling rate in the temperature range from 500°C to 300°C were both set to 2100°C / min. This is a photograph of the microstructure of a copper alloy in an embodiment, obtained by subjecting alloy No. S01 to process No. B13H. In process No. B13H, hot compression was performed at 630°C and cooled to room temperature. At that time, the average cooling rate in the temperature range from 500°C to 300°C was set to 70°C / min. Figure 2A shows the results of a dezincification corrosion test performed on the alloy according to the ISO 6509 test method, and is a cross-sectional metallographic photograph including the portion exhibiting the maximum corrosion depth. This is a microstructure observation photograph of the copper alloy in the embodiment, and the copper alloy was obtained by subjecting alloy No. S06 to process No. C1. In process No. C1, a hot-worked material with a thickness of 5 mm was obtained by hot-compressing at 630°C and setting the average cooling rate in the temperature range from 500°C to 300°C to 70°C / min. The hot-worked material was heated in a laboratory heat treatment furnace at 580°C for 30 minutes, and the cooling process was started at 570°C, with both the average cooling rate in the temperature range from 570°C to 300°C and the average cooling rate in the temperature range from 500°C to 300°C being 2100°C / min. Figure 3A shows the results of a dezincification corrosion test performed on the alloy according to the ISO 6509 test method, and is a cross-sectional metallographic photograph including the portion exhibiting the maximum corrosion depth. Figure 4A shows a photograph of the microstructure of a copper alloy in an embodiment. The copper alloy was obtained by subjecting alloy No. S06 to process No. B13H. In process No. B13H, the alloy was hot-compressed at 630°C and cooled to room temperature. The average cooling rate in the temperature range from 500°C to 300°C was set to 70°C / min. Figure 4A shows the results of a dezincification corrosion test performed on the alloy according to the ISO 6509 test method. It is a cross-sectional metallographic photograph including the portion showing the maximum corrosion depth.

[0037] The following describes a free-machining copper alloy and a method for manufacturing the free-machining copper alloy according to an embodiment of the present invention. Applications of the free-machining copper alloy according to an 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 examples of parts include water taps, mixing taps, stopcocks, valves, cocks, fittings, water meters, gears, sensors, nuts, screws, etc.

[0038] Herein, in this specification, element symbols enclosed in parentheses, such as [Zn], indicate the content (mass%) of that element. In this embodiment, using this method of indicating content, the composition relation f1 is defined as follows: Composition relation f1 = [Cu] - 4.7 × [Si] + 0.5 × ([Pb] + [Bi]) + 0.5 × [P] - 1.1 × [Sn]. If Sn is not included, or if Bi is not included, then [Sn] and [Bi] in f1 are 0.

[0039] Furthermore, in this embodiment, in the constituent phases of the metallic microstructure excluding nonmetallic inclusions, the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, the area ratio of the unmodified β phase is (β)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%. The area ratio of each phase is also called the amount of each phase, the proportion of each phase, or the percentage occupied by each phase. In this embodiment, several microstructure relation equations are defined as follows: Microstructure relation equation f2 = (α) Microstructure relation equation f3 = (β1) Microstructure relation equation f4 = (γ)

[0040] The free-machining copper alloy according to the first embodiment of the present invention contains Cu in an amount greater than 58.0 mass% and less than 63.5 mass%, Si in an amount greater than 0.05 mass% and less than 0.50 mass%, Pb in an amount greater than 0.002 mass% and less than 0.20 mass%, and P in an amount greater than 0.01 mass% and less than 0.40 mass%, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co and Cr among the unavoidable impurities is less than 0.40 mass%, and the content of Al is less than 0.30 mass%, with the Cu content being [Cu] mass%, the Si content being [Si] mass%, and the Pb content being When the content of Pb is [Pb] mass%, and the content of P is [P] mass%, the relationship 57.5 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × [Pb] + 0.5 × [P] ≤ 61.5 holds, and in the constituent phases of the metallic structure excluding nonmetallic inclusions, when the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%, the relationships 10 ≤ f2 = (α) < 80, 20 < f3 = (β1) ≤ 90, and 0 ≤ f4 = (γ) < 4 hold, and when etched with a mixture of hydrogen peroxide and ammonia water, grain boundaries can be observed inside the β1 phase.

[0041] The free-machining copper alloy according to the second embodiment of the present invention contains 58.8 mass% to 63.0 mass% of Cu, 0.08 mass% to 0.48 mass% of Si, 0.01 mass% to 0.10 mass% of Pb, and 0.03 mass% to 0.20 mass% of P, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co and Cr among the unavoidable impurities is less than 0.30 mass%, and the content of Al is less than 0.15 mass%, with the Cu content being [Cu] mass%, the Si content being [Si] mass%, and the Pb content being When the P content is [Pb] mass%, and the P content is [P] mass%, the relationship 58.0 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × [Pb] + 0.5 × [P] ≤ 61.0 holds, and in the constituent phases of the metallic structure excluding nonmetallic inclusions, when the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%, the relationships 15 ≤ f2 = (α) < 75, 25 < f3 = (β1) ≤ 85, and 0 ≤ f4 = (γ) < 1 hold, and when etched with a mixture of hydrogen peroxide and ammonia water, grain boundaries can be observed inside the β1 phase.

[0042] The free-machining copper alloy according to the third embodiment of the present invention contains Cu in amounts exceeding 58.0 mass% and less than 63.5 mass%, Si in amounts exceeding 0.05 mass% and less than 0.50 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 0.40 mass%, as optional elements, Sn in amounts exceeding 0.01 mass% and less than 0.90 mass%, and 0.0001 It contains Bi in an amount greater than mass% but less than 0.20 mass%, Sb in an amount of 0.01 mass% or more but less than 0.12 mass%, and As in an amount of 0.01 mass% or more but less than 0.12 mass%, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co and Cr among the unavoidable impurities is less than 0.40 mass%, and the content of Al is less than 0.30 mass%, and the content of Cu is When 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%, the content of Bi is [Bi] mass%, and the content of Sn is [Sn] mass, the relationship 57.5 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × ([Pb] + [Bi]) + 0.5 × [P] - 1.1 × [Sn] ≤ 61.5 holds true for nonmetallic inclusions. In the constituent phases of the metal structure excluding the specified phase, 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 following relationships exist: 10 ≤ f2 = (α) < 80, 20 < f3 = (β1) ≤ 90, 0 ≤ f4 = (γ) < 4. Furthermore, when etched with a mixture of hydrogen peroxide and ammonia water, grain boundaries can be observed within the β1 phase.

[0043] The free-cutting copper alloy according to the fourth embodiment of the present invention contains 58.8 mass% or more and 63.0 mass% or less of Cu, 0.08 mass% or more and 0.48 mass% or less of Si, 0.01 mass% or more and 0.10 mass% or less of Pb, and 0.03 mass% or more and 0.20 mass% or less of P. As optional elements, it contains 0.05 mass% or more and 0.85 mass% or less of Sn, 0.001 mass% or more and 0.10 mass% or less of Bi, 0.012 mass% or more and 0.08 mass% or less of Sb, and 0.025 mass% or more and 0.08 mass% or less of As. The balance consists of Zn and unavoidable impurities. Among the unavoidable impurities, the total content of Fe, Mn, Co, and Cr is less than 0.30 mass%, and the content of Al is less than 0.15 mass%. When the 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%, the content of Bi is [Bi] mass%, and the content of Sn is [Sn] mass%, it has the relationship of 58.0 ≦ f1 = [Cu] - 4.7×[Si] + 0.5×([Pb] + [Bi]) + 0.5×[P] - 1.1×[Sn] ≦ 61.0. In the constituent phases of the metal structure excluding non-metallic 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 β1-phase, which is the modified β-phase, is (β1)%, it has the relationships of 15 ≦ f2 = (α) < 75, 25 < f3 = (β1) ≦ 85, 0 ≦ f4 = (γ) < 1. When etched with a mixed solution of hydrogen peroxide and ammonia water, grain boundaries are observable inside the β1-phase.

[0044] Hereinafter, the reasons for defining the component composition, the compositional relational expression f1, the tissue relational expressions f2, f3, f4, the metal structure, etc. as described above will be explained.

[0045] <Composition> (Cu)Cu is the main element of the free-cutting copper alloy in this embodiment. To overcome the problems of the present invention, it is necessary to contain at least more than 58.0 mass%. When the Cu content is 58.0 mass% or less, depending on the contents of Si, Zn, P, Pb, Sn and the manufacturing process, the proportion of the β1 phase exceeds 90%, and the ductility becomes low. Also, the modification of the β phase becomes insufficient, and the dezincification corrosion resistance deteriorates. Therefore, the lower limit of the Cu content is more than 58.0 mass%, preferably 58.8 mass% or more, more preferably 59.4 mass% or more, and optimally 60.0 mass% or more. On the other hand, when the Cu content is 63.5 mass% or more, depending on the contents of Si, Zn, P, Pb, Sn and the manufacturing process, the proportion of the β1 phase decreases. As a result, excellent machinability cannot be obtained, and the strength also becomes low. Therefore, the Cu content is less than 63.5 mass%, preferably 63.0 mass% or less, more preferably 62.5 mass%, and optimally 62.0 mass% or less.

[0046] (Si) Si is a main element of the free-cutting copper alloy in this embodiment. Si contributes to the formation of metal phases such as κ phase, γ phase, μ phase, β phase, β1 phase, ζ phase. By containing Cu, Zn, Si, and P in the amounts within the above ranges and adding an appropriate process, the β phase is modified, and the β1 phase is generated through a predetermined cooling process described later. The β1 phase (modified β phase) improves the machinability and at the same time significantly improves the dezincification corrosion resistance, which was a drawback of the conventional β phase.

[0047] In the composition range of this embodiment, the machinability of the α phase is slightly improved by the addition of Si, and the dezincification corrosion resistance of the α phase is improved by the addition of Si. The modified β1 phase and the improved α phase improve the dezincification corrosion resistance of the alloy as a whole. Note that a certain amount of the α phase is required for the modification of the β phase. For example, when there is no α phase, the β phase is insufficiently modified. To improve the dezincification corrosion resistance, ductility, and machinability of the alloy, the α phase needs to be at least 10% or more in area ratio, preferably 15% or more, more preferably 20% or more, and optimally 30% or more.

[0048] Si is an essential element for modifying the β phase to the β1 phase. The higher the amount of Si, the more the β phase is modified, resulting in a β1 phase with superior properties. To modify the β phase and create a more modified β1 phase, the Si content must exceed 0.05 mass%. The Si content is preferably 0.08 mass% or more, more preferably 0.13 mass% or more, and optimally 0.20 mass% or more. On the other hand, if the Si content is too high, the conductivity will decrease. For these reasons, the Si content is less than 0.50 mass%, preferably 0.48 mass% or less, and more preferably 0.46 mass%.

[0049] (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 42.0 mass%, preferably less than 41.5 mass%, and more than approximately 35.0 mass%, preferably more than 35.5 mass%.

[0050] (P) Like Si, P is an essential element for modifying the β phase to the β1 phase. During hot working, P dissolves in the β phase. During cooling after hot working, the cooling process is started at a temperature above 500°C but below 690°C. By cooling at a rate exceeding 300°C / min in the temperature range from the cooling start temperature to 500°C, and in the temperature range from 500°C to 300°C, the β phase is modified to the β1 phase, and the β1 phase is obtained. The modification from the β phase to the β1 phase significantly improves the dezincification corrosion resistance, which was a problem with the conventional β phase. At the same time, the presence of the β1 phase reduces cutting resistance and improves chip fragmentation during cutting. Furthermore, the inclusion of P improves the dezincification corrosion resistance of the α phase, which leads to a significant improvement in the dezincification corrosion resistance of alloys composed of the β1 phase and the α phase.

[0051] To modify the β phase to the β1 phase, the lower limit of the P content must be at least 0.01 mass%. Considering the superior modification to the β1 phase, the amount of unavoidable impurities described later, the cooling treatment start temperature, and the cooling rate after the start of the cooling treatment, the P content is preferably 0.03 mass% or more, more preferably 0.04 mass% or more. Note that P readily forms compounds with Zn, Si, Mn, Fe, Cr, Co, Al, etc. When P forms compounds and the amount of P dissolved in the β phase of the alloy after hot working decreases, the modification from the β phase to the β1 phase is inhibited. The formation of compounds between Zn, Si, and P, which are the main elements of the alloy in this embodiment, begins at about 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, Co begins at temperatures above approximately 550°C, and the formation of P compounds is further promoted as their amounts increase. In other words, the presence of Mn, Fe, Cr, and Co hinders the modification from the β phase to the β1 phase, resulting in increased cutting resistance of the alloy, poor chip fragmentation, and reduced resistance to dezincification corrosion. Therefore, the total content of Fe, Mn, Co, and Cr must be kept below 0.40 mass%, preferably below 0.30 mass%.

[0052] 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 contradictory. As mentioned above, the formation of the β1 phase requires starting the cooling process at a temperature between 500°C and 690°C after hot working, and accelerating the cooling rate in the temperature range from the cooling start temperature to 300°C. In contrast, sufficient formation of the P, Zn, and Si compounds requires slow cooling in the temperature range of approximately 530°C to 450°C. Therefore, when a large amount of P compounds are formed, the modification of the β phase is somewhat insufficient. On the other hand, even if P is included in an amount of 0.40 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, cold workability, and electrical conductivity. Therefore, the P content is less than 0.40 mass%, preferably 0.20 mass% or less, more preferably 0.18 mass% or less, and optimally 0.16 mass% or less.

[0053] (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 almost nothing to improving machinability. For example, the ASM Specialty Handbook, Copper and Copper Alloys, first edition, published August 2001, p. 267, Fig. 6, shows the relationship between Pb content and machinability, with the machinability of a Cu-Zn-Pb alloy containing 62-65 mass% Cu, approximately 3.2 mass% Pb, and the remainder being Zn, set at 100%. It shows that a Pb content of 0.1 mass% only improves machinability by a mere 5%, from approximately 25% to approximately 30% in terms of machinability index.

[0054] On the other hand, in this embodiment, even a small amount of Pb has a significant effect on machinability, and exhibits 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 β1 phase with significantly improved machinability and the inclusion of a small amount of Pb.

[0055] It is a well-known fact that Pb improves the machinability of copper alloys, and for this purpose, approximately 3 mass% of Pb is required in a Cu-Zn binary alloy, as exemplified by the free-cutting brass rod C3604. In this embodiment, an alloy with excellent machinability is completed by having a β1 phase containing Si and P, and a small amount of Pb particles, or Pb and Bi particles as described later, within 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 0.10 mass% or less, and optimally, it is 0.08 mass% or less, considering the impact on the human body and the environment.

[0056] (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 are included individually is obtained. 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.

[0057] When Bi is included, at least 0.0001 mass% of Bi is required for the alloy to have good machinability. The Bi content is preferably 0.001 mass% or more, more preferably 0.002 mass% or more. The effects of Bi on the human body are currently unknown, but the amount of Bi is set to less than 0.20 mass%, preferably 0.10 mass% or less, and even more preferably 0.08 mass% or less. In this embodiment, as described above, Bi can adequately substitute for Pb, the effects of Bi on the human body are unknown, Bi is one of the rare metals and has an impact on the environment, and on the other hand, it is included in the raw material as an unavoidable impurity, so Bi is included as an optional element and may or may not be included. Thus, in this embodiment, the amount of Pb, which is harmful to the human body, is limited to less than 0.20 mass%, including Bi in some cases, while aiming for excellent machinability.

[0058] (Sn) When Sn is present, it is found in the β1 phase and, by solid dissolving in the β1 phase, it further improves the dezincification corrosion resistance of the β1 phase and enhances the dezincification corrosion resistance of the alloy. When Sn is present, in order to obtain this effect, the amount of Sn must be greater than 0.01 mass%, preferably 0.05 mass% or more, and more preferably 0.10 mass% or more. Incidentally, Sn is originally distributed more in the β phase and β1 phase than in the α phase, and even a small amount of Sn improves dezincification corrosion resistance, but as the Sn concentration increases, the γ phase is more likely to form, and ductility decreases. The formation of the γ phase not only leads to a decrease in ductility of the alloy, but also reduces machinability and worsens dezincification corrosion resistance. While it is related to the amount of Si, the Sn content must be kept below 0.90 mass%, preferably 0.85 mass% or less, and more preferably 0.80 mass% or less.

[0059] (Sb, As) Both Sb and As, like P and Sn, have the effect of further improving dezincification corrosion resistance in particularly harsh environments, and therefore may be included as optional elements. In order to improve the corrosion resistance of the alloy by including Sb, it is necessary to include 0.01 mass% or more of Sb, and it is preferable to include 0.012 mass% or more of Sb. On the other hand, even if the amount of Sb exceeds 0.12 mass%, the effect of improving corrosion resistance saturates, and the γ phase increases instead, so the Sb content is 0.12 mass% or less, and preferably 0.08 mass% or less. Furthermore, in order to improve the corrosion resistance of the alloy by including As, it is necessary to include 0.01 mass% or more of As, and it is preferable to include 0.025 mass% or more of As. On the other hand, even if the As content exceeds 0.12 mass%, the effect of improving corrosion resistance saturates, so the As content is 0.12 mass% or less, preferably 0.08 mass% or less.

[0060] (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. Conventionally, free-cutting copper alloys, especially free-cutting brass containing about 30 mass% or more of Zn, do not use high-quality raw materials such as electrolytic copper or electrolytic zinc as the main raw material, but rather recycled copper alloys as the main raw material. In the downstream processes (processing processes) of this field, most components and parts are subjected to cutting, and a large amount of copper alloy is discarded at a ratio of 40 to 80 parts by mass per 100 parts by mass of material.

[0061] Examples include chips, scraps, burrs, sprues, and products containing manufacturing defects. These discarded copper alloys become the main raw materials. If the sorting of cutting chips and scraps is insufficient, 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. Since recycled waste products include plated products, Ni, Cr, and Sn may be mixed in. Furthermore, pure copper scrap used as a substitute for electrolytic copper may contain 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, and therefore contains Sn.

[0062] 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. In the case of free-cutting brass rod C3604 with added Pb according to JIS standard (JIS H 3250), the essential element Pb is contained in an amount of approximately 3 mass%, and further, impurities such as Fe are permitted up to 0.5 mass%, and Fe + Sn (total amount of Fe and Sn) up to 1.0 mass%. In practice, high concentrations of Fe and Sn are sometimes found in free-cutting brass rods.

[0063] Fe, Mn, Co, and Cr dissolve in the α and β phases of Cu-Zn alloys up to certain concentrations. However, if Si or P is present, these compounds 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 then form Fe-Si compounds, Mn-Si compounds, Co-Si compounds, Cr-Si compounds, etc., in the metal structure. Similarly, Fe, Mn, Co, and Cr that combine with P form Fe-P compounds, Mn-P compounds, Co-P compounds, Cr-P compounds, etc., in the metal structure. These intermetallic compounds are extremely hard, not only increasing cutting resistance but also shortening tool life. Therefore, the amounts of Fe, Mn, Co, and Cr must be limited, and the content of each is preferably less than 0.30 mass%, more preferably less than 0.20 mass%, and even more preferably 0.15 mass% or less. In particular, the total content of Fe, Mn, Co, and Cr must be less than 0.40 mass%, preferably less than 0.30 mass%, more preferably less than 0.25 mass%, and even more preferably 0.20 mass% or less.

[0064] On the other hand, Al mixed in from special brass rods, brass castings, etc., affects the modification of the β phase if the content is high, and since Al forms compounds with P or Si, it needs to be limited. In the alloy of this embodiment, the Al content needs to be less than 0.30 mass%, preferably less than 0.15 mass%, and more preferably 0.10 mass% or less.

[0065] 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 may be mixed in in large quantities, although rarely. Considering their 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.

[0066] 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 Sc, Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, and Lu. In addition, the total amount of unavoidable impurities other than Fe, Mn, Co, Cr, and Al is preferably less than 0.70 mass%, and more preferably less than 0.50 mass%.

[0067] (Composition relation formula f1) Composition relation formula f1 = [Cu] - 4.7 × [Si] + 0.5 × ([Pb] + [Bi]) + 0.5 × [P] - 1.1 × [Sn]. If Sn is not included or Bi is not included, then [Sn] and [Bi] in f1 are 0. f1 is an expression that represents the relationship between composition and metal structure. Even if the amount of each element is within the range specified above, if this composition relation formula f1 is not satisfied, the properties targeted by this embodiment cannot be satisfied. If the composition relation formula f1 is less than 57.5, the proportion of β phase and β1 phase increases, the modification of the β phase becomes insufficient, and ductility and dezincification resistance deteriorate. Therefore, the lower limit of the composition relation formula f1 is 57.5 or higher, preferably 58.0 or higher, and more preferably 58.2 or higher. As the composition relation f1 falls within a more favorable range, the proportion of the α phase increases, the modification of the β phase becomes sufficient, and excellent machinability is maintained, along with good resistance to dezincification corrosion, resulting in good ductility and cold workability. On the other hand, the upper limit of the composition relation f1 affects the proportions of the β phase and the β1 phase. If the composition relation f1 is greater than 61.5, the proportion of the β1 phase decreases, resulting in poor machinability and lower strength. Therefore, f1 is 61.5 or less, preferably 61.0 or less, and more preferably 60.5 or less.

[0068] Furthermore, if Bi is included, since Bi is an optional element that substitutes for the function of Pb, it is preferable that the relationship 0.003 ≤ [Pb] + [Bi] < 0.20 is satisfied. That is, at this stage, the effects of Bi on the human body are unknown, and considering that Bi is one of the rare metals and has an impact on the environment, it is necessary to limit the total amount of Pb and Bi. If Bi is included, the total amount of Pb and Bi should be less than the upper limit of the amount of Pb, which is 0.20 mass%, and preferably less than 0.10 mass%. On the other hand, in order to obtain good machinability, if Bi is included, the total amount of Pb and Bi must be 0.003 mass% or more, preferably 0.004 mass% or more, and more preferably 0.005 mass% or more.

[0069] The free-machining copper alloy of this embodiment contains a large amount of the conventional β phase, or β1 phase in this embodiment, while exhibiting good resistance to dezincification corrosion and mechanical properties. Furthermore, it possesses machinability that requires a certain degree of brittleness, which allows for fine chip fragmentation during cutting, and ductility, which are completely contradictory properties. In order to achieve these good corrosion resistance, machinability, and mechanical properties, it is possible to provide an alloy that is better suited to the purpose and application by specifying in detail not only the composition of each component, but also the composition relation formula f1, the microstructure relation formulas f2 to f4 described later, and the requirements of the metal structure. Note that Fe, Mn, Co, Cr, Al, and other separately defined unavoidable impurities are not specified in the composition relation formula f1 because their influence on the composition relation formula f1 is small as long as they are within the range treated as unavoidable impurities.

[0070] (Comparison with Patent Documents) Here, Tables 1 to 3 show the results of comparing the composition of the Cu-Zn-Si alloy described in Patent Documents 1 to 17 above with the alloy of this embodiment.

[0071]

[0072]

[0073]

[0074] This embodiment and Patent Document 8 differ in the content of the main elements Si and Cu, this embodiment and Patent Document 9 differ in the content of the main element Si, and this embodiment and Patent Document 15 differ in the content of the main element Cu. Patent Documents 1, 13, and 15 state that Pb is not included, and therefore differ in Pb content. In Patent Documents 8, 10, 11, and 12, the β phase in the metal structure is significantly limited from the viewpoint of machinability, resistance to dezincification corrosion, and corrosion resistance. The β1 phase of this embodiment is different from the β phase of Patent Documents 8, 10, 11, and 12, but the β phase is set to 5% or less, 25% or less, 15% or less, and 0.9% or less, respectively.

[0075] Patent document 10 relates to a near-net-shape tubular hot forged product and specifies the use of a tubular material. Patent documents 1 and 9 state that heat treatment is performed at a temperature of 350 to 550°C or 400 to 600°C in order to reduce or break up the β phase. Patent document 9 contains 0.2 mass% or more of Sn, and contains Sn and Si to improve the dezincification corrosion resistance of the β phase. Hot extrusion at a temperature of 700°C or higher is required to improve machinability, and heat treatment at 400 to 600°C is required to improve corrosion resistance. The proportion of the β phase is generally 5 to 20%, which differs from this embodiment, which requires more than 20% of the β1 phase.

[0076] Patent documents 12, 13, and 14 state that Al is essential to improve discoloration resistance, castability, and dezincification corrosion resistance. Patent document 14 states that in order to improve dezincification corrosion resistance, Sn and Al should be included in amounts of at least 0.1 mass% each, and that a large amount of Pb and Bi should be included to obtain excellent machinability. Patent document 15 states that Pb should not be included, a γ phase should be required, and by including 65 mass% or more of Cu and Si, along with trace amounts of Al, Sb, Sn, Mn, Ni, B, etc., a corrosion-resistant copper alloy casting with good mechanical properties and castability should be obtained.

[0077] Patent documents 2 to 7 all describe a cooling process after hot working in which the average cooling rate in the temperature range from approximately 530°C to approximately 450°C is approximately 0.1°C / min or more and approximately 70°C / min or less. Patent documents 16 and 17 describe a cooling process after hot working in which the average cooling rate in the temperature range from approximately 550°C to approximately 350°C is approximately 30°C / min or more and approximately 60°C / min or less, and require that the resulting P compound be present in the metal structure. Furthermore, they do not mention the modification of the β phase at all, remain silent on dezincification corrosion resistance, and do not disclose any data. In this embodiment, the cooling process after hot working is initiated at a temperature lower than 690°C and higher than 500°C, and the average cooling rate in the temperature range from the cooling start temperature to 500°C, and the average cooling rate in the temperature range from 500°C to 300°C, exceeds 300°C / min, which is basically the opposite of the cooling described in Patent documents 2 to 7, 16 and 17.

[0078] The most significant and clear difference between this embodiment and Patent Documents 2-7, 9, 16, and 17 is that none of the Patent Documents mention the modification of the β phase or the modified β phase, i.e., the β1 phase. The β1 phase is formed under the following conditions: During hot working, a β phase is formed in which a certain amount of Si and P are dissolved. Then, a cooling process is started at a temperature higher than 500°C, and during the cooling process, the average cooling rate in the temperature range from the cooling start temperature to 500°C is greater than 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is greater than 300°C / min. The β1 phase is formed by the above. When etching is performed with a mixture of hydrogen peroxide and ammonia water, grain boundaries are observed within the β1 phase if the β phase is modified into the β1 phase, but even if a certain amount of Si and P is present, if the β phase remains as it was before modification, no grain boundaries are observed within the β phase. Patent documents 2 to 7 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.

[0079] <Metal Structure> Cu-Zn-Si alloys have more than 10 different phases, and complex phase transitions occur. Therefore, the desired properties cannot necessarily be obtained by simply looking at the composition range and elemental relationships. Ultimately, the desired properties can be obtained by identifying and determining the types of phases present in the metal structure and the range of their area ratios. For this reason, the microstructure relationships are defined as follows: 10 ≤ f2 = (α) < 80, 20 < f3 = (β1) ≤ 90, where the β1 phase is the modified β phase. 0 ≤ f4 = (γ) < 4

[0080] (β1 phase) The microstructure of this embodiment, excluding nonmetallic inclusions, consists of α phase, β1 phase, and, in some cases, a small amount or 0% of γ phase. The difference between the β phase and the β1 phase is that when the β1 phase is etched with a mixture of hydrogen peroxide and aqueous ammonia, grain boundary patterns are observed within the β1 phase, whereas in the case of an unmodified, ordinary β phase, no grain boundaries are observed within the β phase.

[0081] In this embodiment, the β1 phase is obtained by, during hot working, solid-solving a certain amount or more of Si and P into the β phase at a high temperature, maintaining a temperature of 500 to 690°C, and then performing a cooling treatment from that temperature range (both the average cooling rate in the temperature range from the cooling treatment start temperature to 500°C and the average cooling rate in the temperature range from 500°C to 300°C exceeding 300°C / min, and continuing to cool to near room temperature), thereby bringing the metallic microstructure at 500 to 690°C down to room temperature. Specifically, the cooling rate is increased in the temperature range from a high cooling start temperature exceeding 500°C to 300°C to cool rapidly, and then cooled to room temperature below 100°C. This brings the metallic microstructure at high temperatures exceeding 500°C down to room temperature. As a result, the β1 phase is obtained. The β1 phase cannot be obtained by performing the above cooling treatment on the β phase of a Cu-Zn alloy that does not contain both predetermined amounts of Si and P. Similarly, the β1 phase cannot be obtained by cooling an alloy containing Si and P at a temperature lower than 500°C, for example, from 450°C, at a cooling rate exceeding 300°C / min. Furthermore, the degree of modification of the β phase is also affected by the amount of Si and P, the amount of unavoidable impurities, the starting temperature of the cooling process, the average cooling rate in the temperature range from the start of the cooling process to 500°C, and the average cooling rate in the temperature range from 500°C to 300°C. Improving the degree of modification of the β phase, that is, a more modified β phase, results in a material with better machinability and better resistance to dezincification corrosion.

[0082] The β1 phase (modified β phase) can overcome the drawback of the β phase in Cu-Zn alloys, which has been a major challenge, specifically in the alloy's resistance to dezincification corrosion. To give a concrete example, it can reduce the progression of dezincification corrosion by approximately 50% or more. As described in the patent documents, dezincification corrosion of Cu-Zn alloys containing the β phase is a major problem, and since dezincification corrosion occurs along the β phase, the amount of β phase is limited to 25% or less or 20% or less, and further heat treatment at 350 to 550°C is applied to reduce the amount of β phase and fragment the β phase.

[0083] (β1 phase, microstructure relationship formula f3) 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 keeping the Pb content to a minimum, the area ratio of the β1 phase must be at least 20%. Furthermore, in order to improve machinability and strength, the area ratio of the β1 phase is preferably 25% or more, and more preferably 30% or more. On the other hand, if the β1 phase exceeds 90%, the modification of the β phase becomes insufficient. If the β1 phase exceeds 90% and the modification of the β phase is insufficient, the alloy's resistance to dezincification corrosion and ductility will be reduced. Based on the above, after diligent research, it has been found that the area ratio of the β1 phase must be set to 90% or less, preferably 85% or less, more preferably 80% or less, and optimally 70% or less.

[0084] (α phase, microstructure relationship formula f2) This embodiment is basically composed of α phase and β1 phase, and a treatment is carried out to modify the β phase into β1 phase, but the α phase is hardly affected by this treatment. If there is too much β1 phase, there is a problem with the ductility of the alloy, and an appropriate amount of ductile α phase is necessary, while conversely, if there is too much α phase the strength will be low. Also, the α phase containing Si has only a slight improvement in machinability compared to the α phase without Si, so the amount of α phase is limited from the viewpoint of machinability as well. As a result of diligent research on the modification of the β phase, the dezincification corrosion resistance of the alloy, machinability and mechanical properties, it has been found that the amount of α phase should be 10% or more, preferably 15% or more, more preferably 20% or more, and optimally 30% or more. On the other hand, the upper limit of the α phase is less than 80%, preferably less than 75%, and more preferably 70% or less.

[0085] (γ phase, microstructure relationship formula f4) As described in Patent Documents 8 and 10, the γ phase is a phase that contributes to machinability in Cu-Zn-Si alloys with a Cu concentration of about 69 to about 80 mass% and a Si concentration of about 2 to about 4 mass%. In Patent Document 15, the γ phase is essential in Cu-Zn-Si alloys that do not contain Pb, and further, in Patent Documents 2 to 6, it is stated that both the β phase containing Si and the γ phase containing Si have good machinability. In this embodiment, the γ phase impairs the machinability and ductility of the alloy and worsens its resistance to dezincification corrosion. In Cu-Zn-Si alloys, the appearance of the γ phase is sometimes unavoidable, but it must be at least limited. That is, the proportion (area ratio) of the γ phase should be less than 4%, preferably less than 2%, more preferably less than 1%, and optimally, the γ phase should not be present.

[0086] As shown in Figures 1A and 3A, in hot-forged materials and samples subjected to additional heat treatment, grain boundary patterns, i.e., crystal grain boundaries, are observed within the β1 phase, along with α phase crystal grain boundaries, in metallurgical microscope images. Here, a crystal grain boundary refers to a linear pattern that penetrates the β1 phase crystal grains, as observed within the β1 phase, as shown in Figures 1A and 3A. Thus, grain boundaries are recognized within the modified β phase, i.e., the β1 phase. On the other hand, in Figures 2A and 4A, no crystal grain boundaries penetrating the β phase crystal grains are observed, indicating that the β phase has not been modified into the β1 phase. Instead, in Figures 2A and 4A, granular precipitates of approximately 0.5 to 3 μm, which appear black, are mainly present within the β phase and at the phase boundary between the β and α phases. In some areas, granular precipitates appear to be intermittently arranged in a line, but these are not grain boundaries of the β1 phase. The granular precipitates are mainly P compounds, but also contain Pb particles, mixed Pb and Bi particles, compounds such as Fe, oxides, and sulfides. These can be identified under a microscope, but are somewhat difficult to distinguish in printed photographs.

[0087] From these findings, whether the β phase is modified into the β1 phase and whether grain boundaries exist in the β1 phase depends on whether the cooling treatment start temperature is higher or lower than approximately 500°C, and whether the average cooling rate in the temperature range from 500°C to 300°C exceeds 300°C / min. The results of the dezincification corrosion test of these alloys according to ISO 6509 are shown in Figures 1B, 2B, 3B, and 4B. The maximum corrosion depth of the samples in which grain boundaries were observed within the β1 phase (Figures 1B and 3B) was less than 250 μm, while the maximum corrosion depth of the samples in which grain boundaries were not observed within the β1 phase, i.e., samples in which the β1 phase was absent (Figures 2B and 4B), exceeded 500 μm, showing a difference of more than twice the corrosion depth. In all cases, the form of dezincification corrosion was selective corrosion of the β1 phase and the β phase. Even in the β1 and β phases, these phases are preferentially subjected to dezincification corrosion, but the β1 phase can slow down the progression of dezincification corrosion to about half or less than that of the β phase.

[0088] <Characteristics> (Strength at room temperature, hardness) Hardness is considered important as a strength required in various fields, including valves, fixtures, fittings, and piping for drinking water, as well as valves and fittings for automobiles. For this reason, it is preferable that hot-worked materials such as hot-extruded materials, hot-rolled materials, and hot-forged materials are high-strength materials with a Vickers hardness of 95 HV or higher at room temperature. The Vickers hardness at room temperature is more preferably 100 HV or higher, even more preferably 110 HV or higher, and optimally 115 HV or higher. There is no particular upper limit to the Vickers hardness at room temperature, but in reality, 250 HV or lower is preferable.

[0089] (Conductivity) Applications of this embodiment include electrical and electronic equipment components, automotive parts for the increasingly electrified EV, and other highly conductive materials and components. Currently, phosphor bronze (JIS standard, C5191, C5210) containing 6 mass% or 8 mass% of Sn is widely used in these applications, and its conductivity is approximately 14% IACS and 12% IACS, respectively. Therefore, if the conductivity of the copper alloy of this embodiment is 17.0% IACS or higher, there will be no problems regarding electrical conductivity. Preferably, it is 18.0% IACS or higher, and more preferably 19.0% IACS or higher. The upper limit of conductivity is not particularly specified because, in practical terms, it rarely causes problems as conductivity improves, but realistically, 28.0% IACS or lower is preferable. %IACS is an abbreviation for International Annealed Copper Standard, and the volume resistivity of annealed standard soft copper is 1.7241 × 10⁻⁶. －２ This value is expressed as a ratio with μΩm representing 100% IACS.

[0090] (Hot workability) The free-machining copper alloy of this embodiment is characterized by having excellent deformability at 520 to 790°C, allowing it to be hot-extruded into rods with a small cross-sectional area and hot-forged into complex shapes. Due to energy considerations and the fact that the α-phase has a granular shape, the hot working temperature is between 520°C and 790°C, preferably 770°C or lower, more preferably 720°C or lower, and from the viewpoint of hot deformation resistance, preferably 540°C or higher, more preferably 560°C or higher.

[0091] (Maximum Dezincification Corrosion Depth) In dezincification corrosion tests conducted according to ISO 6509, it is preferable that the maximum dezincification corrosion depth is 400 μm or less. When tested according to ISO 6509, a maximum corrosion depth of 400 μm or less is considered to be at a level that does not pose a problem in terms of practical corrosion resistance. That is, in dezincification corrosion tests conducted according to ISO 6509, it is preferable that the maximum dezincification corrosion depth is 400 μm or less, more preferably 300 μm or less, and optimally 200 μm or less. It is desirable that the dezincification corrosion depth be as small as possible, and there is no particular lower limit, but the practical range for measurement is 1 μm or more.

[0092] <Manufacturing Process> Next, a method for manufacturing a free-machining copper alloy according to an embodiment of the present invention will be described. The metallic structure of the alloy in this embodiment changes not only with respect to its composition but also with respect to the manufacturing process. It is affected not only by the hot working temperature and heat treatment conditions in hot extrusion and hot forging, but also by the average cooling rate during the cooling process in hot working and heat treatment. Through diligent research, it has been found that the metallic structure is greatly influenced by the cooling start temperature, the average cooling rate in the temperature range from the cooling start temperature to 500°C, and the average cooling rate in the temperature range from 500°C to 300°C during the cooling process in hot working and heat treatment.

[0093] (Melting and Casting) 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 1100°C, which is about 50 to 200°C higher than the melting point, is poured into a predetermined mold and cooled by several cooling methods such as air cooling, slow cooling, and water cooling. After solidification, the constituent phases change in various ways.

[0094] (Hot working) Examples of hot working include hot extrusion, hot forging, and hot rolling. When two or more hot working processes are performed, the final hot working process is performed under the following conditions. First, regarding hot extrusion, in a preferred embodiment, depending on the extrusion ratio (hot working rate) and equipment capacity, the material temperature immediately after hot working (extrusion temperature) is hot extruded at a temperature between 520°C and 790°C. The extruded rod is either wound into a coil or, if the cross-sectional area of ​​the extruded rod is large, is extruded as a straight rod onto a table.

[0095] The lower limit of the hot extrusion temperature is related to the deformation resistance at high temperatures. Lower extrusion temperatures result in finer, more granular α-phase grains, leading to better resistance to dezincification corrosion and machinability. However, when the extrusion ratio is high, the extrusion temperature is preferably 540°C or higher, more preferably 560°C or higher, depending on the equipment capacity and the cooling treatment start temperature described later. The upper limit is related to the shape of the α-phase, and a more stable metal structure can be obtained by controlling the temperature within a narrower range. When hot extrusion is performed at temperatures above 790°C, the metal structure initially consists of a single β phase, or the proportion of the β phase exceeds 90%, and the α-phase grains tend to become needle-shaped, or coarse α-phase grains tend to appear. When needle-shaped, coarse α-phase grains appear, the strength is slightly reduced, the balance between strength and ductility is slightly worse, and the large, coarse α-phase grains become an obstacle to cutting, resulting in poor machinability. Furthermore, resistance to dezincification corrosion also deteriorates. The extrusion temperature is preferably 770°C or lower, more preferably 720°C or lower. The shape of the α-phase crystal grains is related to the compositional relationship formula f1, and when the compositional relationship formula f1 is 59.0 or lower, it is preferable that the extrusion temperature be lower than 720°C. By extruding at a lower temperature than copper alloys containing Pb, good machinability and strength can be achieved.

[0096] Furthermore, by optimizing the cooling conditions after hot extrusion, the β phase can be modified, resulting in a material with better machinability and good resistance to dezincification corrosion. Specifically, in the cooling process after hot extrusion, the cooling treatment is first started at a temperature lower than 690°C and higher than 500°C. Then, the average cooling rate in the temperature range from the cooling treatment start temperature to 500°C, and the average cooling rate in the temperature range from 500°C to 300°C are set to at least 300°C / min, preferably 600°C / min, and more preferably 900°C / min or higher. In a typical embodiment, if a general cooling method is used, the average cooling rate in the temperature range from the cooling treatment start temperature to 500°C and the average cooling rate in the temperature range from 500°C to 300°C are approximately the same, or in some cases, the former is slightly faster. Cooling at an even more preferable cooling rate further modifies the β1 phase. When the β1 phase is further modified, a material with better machinability and good resistance to dezincification corrosion can be obtained.

[0097] The upper limit of the average cooling rate in the temperature range from the cooling start temperature to 500°C and from 500°C to 300°C is sufficient with the cooling rate achievable with normal production equipment and is not specifically defined, but if mentioned, a cooling rate of approximately 12,000°C / min or less is preferable. By cooling at an average cooling rate exceeding 300°C / min, the β phase is modified, and when the metal structure is etched with a mixture of hydrogen peroxide and ammonia water and observed with a metallurgical microscope at 500x magnification, grain boundaries can be observed within the β1 phase (modified β phase). Note that if the average cooling rate in the temperature range from the cooling start temperature to 500°C and the average cooling rate in the temperature range from 500°C to 300°C are 300°C / min or less, grain boundaries cannot be observed within the β phase. The degree of modification to the β phase, i.e., to the β1 phase, is greater the faster the cooling rate. When cooling from a temperature below 300°C to room temperature, the cooling rate slows down slightly as it approaches room temperature, but it is desirable to continue cooling in the temperature range from 500°C to 300°C.

[0098] On the other hand, the starting temperature of the cooling process also affects the modification of the β phase. It is preferable to start at a temperature higher than 530°C, and more preferably at a temperature higher than 550°C. If the cooling process is started at a temperature lower than 550°C, and the cooling rate from 550°C to the starting temperature is slow, compounds of P and Zn, or P, Zn, and Si will begin to form, and below 530°C, the formation of P compounds will be further promoted. At cooling rates exceeding 300°C / min, almost no P compounds are formed, but if the starting temperature of the cooling process is lower than 550°C, P compounds can be observed in the metal structure. However, if the starting temperature of the cooling process is 690°C or higher, the proportion of the β1 phase becomes too large, worsening the ductility and dezincification resistance of the alloy. Therefore, the starting temperature of the cooling process needs to be lower than 690°C, and preferably lower than 670°C.

[0099] Considering the measurement locations where actual measurements are possible, the hot working temperature is defined as the temperature of the hot-worked material that can be measured approximately 2 or 3 seconds after the end of hot extrusion, hot forging, or hot rolling. The microstructure is affected by the temperature immediately after processing, when significant plastic deformation occurs.

[0100] Next, hot forging will be described. Hot forging primarily uses hot extruded materials as the raw material, but continuously cast rods are also used. Since the forging material is not the final hot processed material, no cooling measures are necessary. Compared to hot extrusion, hot forging has a faster processing speed, can process into complex shapes, and in some cases can be strongly processed to a wall thickness of approximately 3 mm. Also, the weight of a single forged product ranges from several tens of grams to several kilograms, and small forged products are rapidly cooled during forging, and the subsequent cooling rate is also fast. Therefore, the heating temperature of the material during forging is higher than the heating temperature of the ingot during hot extrusion. In a preferred embodiment, the temperature of the hot forged product, that is, the material temperature from immediately after forging to approximately 2 or 3 seconds later, is preferably above 520°C and below 790°C. In hot forging as well, there is a relationship with the composition relation formula f1, and when the composition relation formula f1 is 59.0 or less, it is preferable that the hot forging temperature is below 720°C. Depending on the processing rate of hot forging, the lower the temperature, the smaller the grain size of the α-phase crystals, the more the shape of the α-phase crystal grains changes from needle-like to granular, resulting in higher strength, a better balance between strength and ductility, and improved machinability and resistance to dezincification corrosion.

[0101] Furthermore, similar to hot extrusion, by optimizing the cooling rate after hot forging, it is possible to obtain a material with good resistance to dezincification corrosion and good machinability. Specifically, in the cooling process after hot forging, the cooling treatment is first started at a temperature lower than 690°C and higher than 500°C, and the average cooling rate in the temperature range from the cooling treatment start temperature to 500°C, and the average cooling rate in the temperature range from 500°C to 300°C are set to at least 300°C / min, preferably 600°C / min, and more preferably 900°C / min or higher. In a typical embodiment, if a general cooling method is used, the average cooling rate in the temperature range from the cooling treatment start temperature to 500°C and the average cooling rate in the temperature range from 500°C to 300°C are roughly the same, or the former is slightly faster.

[0102] During the cooling process after hot forging, modifying the β phase occurs by increasing the average cooling rate from the cooling start temperature to 500°C, and the average cooling rate from 500°C to 300°C, to over 300°C / min. When the metal structure is etched with a mixture of hydrogen peroxide and ammonia water and observed with a 500x metallurgical microscope, grain boundaries can be observed within the β1 phase (modified β phase). However, if the average cooling rate in the temperature range from 500°C to 300°C is 300°C / min or less, grain boundaries cannot be observed within the β phase. The degree of modification to the β phase, i.e., to the β1 phase, is greater with a faster cooling rate. Similar to hot extrusion, when cooling from temperatures below 300°C to room temperature, the cooling rate slows down slightly as it approaches room temperature, but it is desirable to continue cooling in the temperature range from 500°C to 300°C. While there are no specific upper limits on the average cooling rate in the temperature range from the cooling start temperature to 500°C and in the temperature range from 500°C to 300°C, if we were to mention one, a cooling rate of approximately 12,000°C / min or less is preferable.

[0103] On the other hand, the starting temperature of the cooling process also affects the modification of the β phase. It is preferable to start at a temperature higher than 530°C, and more preferably at a temperature higher than 550°C. If the cooling process is started at a temperature lower than 550°C, and the cooling rate from 550°C to the starting temperature is slow, compounds of P and Zn, or P, Zn, and Si will begin to form, and below 530°C, the formation of P compounds will be further promoted. At cooling rates exceeding 300°C / min, almost no P compounds are formed, but if the starting temperature of the cooling process is lower than 550°C, P compounds can be observed in the metal structure. On the other hand, if the starting temperature of the cooling process is 690°C or higher, the proportion of the β1 phase becomes too large, worsening the ductility and dezincification resistance of the alloy. Therefore, the starting temperature of the cooling process needs to be lower than 690°C, and preferably lower than 670°C. Furthermore, if, after hot forging, the product is placed in a simple furnace set to an ambient temperature of approximately 550 to 600°C for several tens of seconds to several minutes, and then the cooling process is started, a more uniform and stable forged product can be obtained. In addition, after the hot forging-cooling process is completed, shot blasting may be performed to apply compressive stress to the surface.

[0104] In hot rolling, the ingot is heated and rolled repeatedly 5 to 15 times. The material temperature at the end of the final hot rolling (the material temperature 2 to 3 seconds after the end) is preferably above 520°C and below 790°C, and more preferably below 690°C. After hot rolling is completed, the rolled material is cooled. Similar to hot extrusion, the cooling process is started at a temperature above 500°C and below 690°C. The average cooling rate in the temperature range from the cooling start temperature to 500°C, and the average cooling rate in the temperature range from 500°C to 300°C are set to at least 300°C / min, preferably above 600°C / min, and more preferably above 900°C / min. When cooling from a temperature below 300°C to room temperature, the cooling rate slows down slightly as it approaches room temperature, but it is desirable to continue cooling in the temperature range from 500°C to 300°C. While there are no specific upper limits on the average cooling rate in the temperature range from the cooling start temperature to 500°C and in the temperature range from 500°C to 300°C, if we were to mention one, a cooling rate of approximately 9000°C / min or less is preferable.

[0105] (Heat Treatment) In this embodiment, if cooling cannot be started at a temperature higher than 500°C after the final hot working, or if the average cooling rate in the temperature range from 500°C to 300°C does not exceed 300°C / min, the β phase cannot be modified. Also, when making small diameter rods, wires, etc., if the process includes cold working and annealing, which involves heat, the β phase is basically not modified. In such cases, annealing is performed again at a temperature above 520°C and below 630°C for 1 minute to 5 hours, and during cooling after annealing, the cooling process is started at a temperature higher than 500°C, the average cooling rate in the temperature range from the cooling start temperature to 500°C exceeds 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C exceeds 300°C / min, thereby modifying the β phase and forming the β1 phase. If there are multiple annealing steps, only the conditions for the annealing and cooling process in the final annealing step need to be set to appropriate conditions. From the viewpoint of the temperature at which the P compound is formed, the annealing temperature is preferably 530°C or higher, and more preferably 550°C or higher. Furthermore, the average cooling rate in the temperature range from 500°C to 300°C is preferably set to more than 600°C / min, and more preferably 900°C / min or higher, depending on the degree of modification of the β1 phase.

[0106] This heat treatment is applied to hot-worked materials that cannot be cooled under predetermined conditions after hot working, and to materials that undergo one or more annealing processes after hot working and cold working, as described above. There are no specific upper limits on the average cooling rate in the temperature range from the cooling start temperature to 500°C and in the temperature range from 500°C to 300°C, but if we were to mention it, a cooling rate of approximately 12,000°C / min or less is preferable.

[0107] (Cold Working Process) In the case of hot-extruded rods, cold working may be performed on the hot-extruded material to obtain high strength, improve dimensional accuracy, or to make the extruded rod or coil material a straight shape with minimal bending. For example, hot-extruded material may be cold-drawn with a processing rate of approximately 2 to 30%, and in some cases, drawn, followed by straightening. Thin rods, wires, or rolled materials undergo repeated cold working and annealing, and after the final heat treatment mentioned above, cold working and straightening are performed with a final processing rate of 0 to approximately 30%. The closer the final processing rate is to 0%, the less likely the material is to crack when subjected to light cold working such as crimping or bending.

[0108] (Low-Temperature Annealing) In the case of rods, wires, forgings, and rolled materials, low-temperature annealing may be performed in the final process at a temperature below the recrystallization temperature, primarily for the purpose of removing residual stress, straightening the rod (straightness of the rod), and adjusting and improving the metal structure. However, the β1 phase modified by the hot working process and heat treatment will lose its modification when heat is applied to the alloy. For example, heating at 300°C for 2 hours will damage the modification of the β phase, causing the modified β1 phase to revert to the original β phase, and the improved machinability and dezincification resistance will become the properties of the alloy consisting of the original β phase. Therefore, low-temperature annealing is not recommended, but temperatures below approximately 150°C are acceptable.

[0109] According to the free-machining alloy of the present invention configured as described above, the alloy composition, compositional relationship formula, metal structure, and microstructure relationship formula are defined as described above, so that even with a low Pb content, excellent machinability can be obtained, it has good resistance to dezincification corrosion, excellent hot workability, high strength, and an excellent balance between strength and ductility.

[0110] 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.

[0111] 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.

[0112] Prototype tests of copper alloys were conducted using the low-frequency melting furnace and semi-continuous casting machine used in actual operations. Prototype tests of copper alloys were also conducted using laboratory equipment. The alloy compositions are shown in Tables 4 and 5. The manufacturing processes are shown in Tables 6 to 8. Each manufacturing process is described below. In Tables 6 to 8, the cooling rate for hot extrusion and hot forging refers to the average cooling rate from the end of hot working to the start of the cooling process. In Tables 6 to 8, the average cooling rate in the temperature range from the cooling process start temperature to 500°C was equivalent to the average cooling rate in the temperature range from the cooling process start temperature to 300°C.

[0113]

[0114]

[0115]

[0116]

[0117]

[0118] Actual Extrusion Manufacturing Process A (Process No. A1, A12H) A billet with a diameter of 240 mm was manufactured using a low-frequency melting furnace and a semi-continuous casting machine that are currently in operation. Raw materials similar to those used in actual operation were used. The billet was cut to a length of 800 mm and heated. A round bar with a diameter of 20.9 mm was extruded using an indirect extruder with a nominal capacity of 2750 tons, and the extruded material (round bar) was wound into a coil in a tank located a short distance from the extruder. This tank is capable of water cooling with adjustable water volume. When the extruded material reached a predetermined temperature in the tank, the cooling process (water cooling) was started while adjusting the water volume.

[0119] In processes A1 and A12H, extrusion was performed at 630°C, followed by the start of the cooling process at 580°C. The average cooling rates in the temperature range from the cooling start temperature to 300°C were 1350°C / min and 250°C / min, respectively. The average cooling rate in the temperature range from the cooling start temperature (580°C) to 500°C was approximately the same as the average cooling rate from 500°C to 300°C. These extruded materials were cut and subjected to tests for mechanical properties (hardness test), conductivity measurement, dezincification corrosion, cutting test, and microscopic observation. Temperature measurements were performed using an infrared thermometer, measuring the temperature of the extruded material when it was extruded from the extruder, when the cooling process started, and until it reached 500°C and 300°C. For subsequent hot extrusion and hot forging temperature measurements, an infrared thermometer (model IGA8Pro / MB20) and a contact thermometer manufactured by LumaSense Technologies Inc. were used in combination.

[0120] (Process No. A50H) As a prototype for forging material for φ50 mm rods, a 240 mm diameter billet was manufactured using a low-frequency melting furnace and semi-continuous casting machine in actual operation, and the billet was cut to a length of 800 mm. Using the aforementioned billet, it was extruded at 550°C into a 50 mm diameter round bar using a direct extruder with a nominal capacity of 3000 tons. The extruded straight bar was placed on a steel table and allowed to cool naturally to room temperature. No special cooling treatment was performed, and the cooling rate after hot extrusion and the average cooling rate from 500°C to 300°C were approximately 20°C / min. These extruded materials were cut and subjected to tests for mechanical properties (hardness test), conductivity measurement, dezincification corrosion, and microscopic observation.

[0121] Hot Forging Process B Process B is a laboratory process for hot forging. The casting used as the raw material was prepared in the laboratory by melting the raw materials in a predetermined ratio and pouring the molten metal at approximately 1000°C into an iron mold with an inner diameter of 40 mm and a depth of 200 mm. When the casting reached approximately 700°C, it was removed from the mold and allowed to cool naturally without any special cooling treatment. The casting was then cut into rectangular parallelepipeds of lengths of 16 mm, 16 mm, and 25 mm, and hot-compressed at 630°C with a strain rate of 0.05 / second at a 20-ton Amsler-type testing machine equipped with a heating furnace, to a thickness of 25 mm to 5 mm in the length direction.

[0122] (Process Nos. B1, B12H, B13H) In processes B1 and B12H, the hot compression temperature was set to 630°C, and the cooling process was started at 570°C. The average cooling rate in the temperature range from the cooling process start temperature to 300°C was 2100°C / min and 240°C / min, respectively. The average cooling rate in the temperature range from the cooling process start temperature (570°C) to 500°C and the average cooling rate from 500°C to 300°C were approximately the same. In process B13H, the hot compression temperature was set to 630°C, and no cooling process was performed. The cooling rate after hot compression and the average cooling rate from 500°C to 300°C were 70°C / min. These hot-forged materials were cut and subjected to tests for mechanical properties (hardness test), conductivity measurement, dezincification corrosion, and microscopic observation.

[0123] Heat Treatment Process C (Process C1) Process C1 involved heat treatment of the hot-forged material (hot-compressed material) obtained in B13H in the laboratory to investigate changes in its properties. Annealing was performed under conditions of holding at 580°C for 30 minutes. After annealing, cooling was started at 570°C, and the average cooling rate in the temperature range from the cooling start temperature to 300°C was 2100°C / min. The average cooling rate in the temperature range from the cooling start temperature (570°C) to 500°C and the average cooling rate from 500°C to 300°C were approximately the same. These heat-treated materials were subjected to metallurgical microscopy observation, dezincification corrosion testing, mechanical properties (hardness testing), conductivity measurement, and dezincification corrosion testing.

[0124] The above-mentioned test materials were evaluated for the following items. The evaluation results are shown in Tables 9 to 12. The area ratio of the β phase, i.e., the unmodified β phase, was distinguished from the area ratio of the β1 phase (f3) as f3A and is listed in each table.

[0125] (Observation of Metal Microstructure) The metal microstructure was observed using the following method, and the area percentage (%) of each phase, such as the α phase, β phase, and γ phase, was measured by image analysis. 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 in the α, β, and γ phases, respectively. The α phase exhibits a granular, elliptical shape and is often accompanied by twinning. The β and β1 phases exist around this granular, elliptical α phase. This distinguishes the α, β, and β1 phases. The rods and forgings of each test material were cut parallel to the longitudinal direction or parallel to the flow direction of the metal microstructure. The cut surfaces were then polished to a mirror finish and etched with a mixture of hydrogen peroxide and ammonia water. For etching, an aqueous solution of 3 mL of 3 vol% hydrogen peroxide and 22 mL of 14 vol% ammonia water was used. At room temperature of approximately 15 to 25°C, the polished metal surface was immersed in this aqueous solution for approximately 2 to 10 seconds. Note that in the presence of the β1 phase, corrosion resistance was improved compared to the presence of the β phase, so the etching immersion time needed to be longer, approximately 5 to 10 seconds.

[0126] 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 were observed within the β phase in a single field of view, the entire β phase in that field of view was filled in as the β1 phase. Next, the images were binarized using the image analysis software "WinROOF2013," and the area percentage of each phase was determined. In detail, the average of the area percentages in the five fields was calculated for each phase, and this average was used as the area percentage of each phase. Compounds containing P and Si, precipitates, oxides, Pb or Bi particles, sulfides, and crystals were excluded, and the sum of the area percentages of all constituent phases was set to 100%. If phase identification is difficult, the phase may be identified using a field emission scanning electron microscope (FE-SEM) with the EBSD (Electron Back Scattering Diffracton Pattern) method at a magnification of 1000x. In this case, the α phase is identified, and the remaining phase is designated as the β phase or β1 phase. For the areas identified as the β phase or β1 phase by EBSD, the β phase or β1 phase is distinguished by whether or not grain boundaries are observed inside the β phase using a metallurgical microscope. In this embodiment, the presence or absence of grain boundaries in the β phase is not determined by the EBSD method. The β phase or β1 phase is distinguished solely by whether or not grain boundaries (linear patterns penetrating the β phase crystal grains as shown in Figure 1A) are observed inside the β phase when etched with the etching solution using a metallurgical microscope.

[0127] When the metal structure is observed with a metallurgical microscope using the etching solution described above, if the β phase is modified, grain boundary patterns, i.e., crystal grain boundaries, can be observed within the β phase. In this embodiment, 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, compounds and precipitates of P and Si may be present in the metal structure. Compounds of P and Zn or Si can be observed as blackish-gray particles of about 0.5 to 2 μm under a 500x metallurgical microscope, mainly within the β phase, β1 phase, and at the phase boundary between the β phase and α phase. Compounds of Fe, Mn, Cr, and P or Si can be seen as light blue particles of about 1 to 5 μm. Pb or Bi particles, and particles formed by the combination of Pb and Bi, exist as fine black particles of 2 μm or less. These compounds can be roughly distinguished under a metallurgical microscope. In the characterization table, when grain boundaries were observed within the β phase in the metal structure, that β phase was designated as the β1 phase, and the presence of grain boundaries within the β1 phase was indicated as "B" (present). When no grain boundaries were observed within the β phase, it was indicated as "D" (absent).

[0128] (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.

[0129] (Vickers hardness test) The Vickers (HV) hardness (load 49N) of the cross-sections of the hot extruded and hot forged products was measured using a Vickers hardness tester (HV-114) manufactured by Mitutoyo Corporation. The test method was measured in accordance with the method specified in JIS Z2244-1.

[0130] (Dezincification Corrosion Test: ISO 6509 Dezincification Corrosion Test) The dezincification corrosion test was evaluated 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 is as follows: First, the test material is embedded in a phenolic resin material, specifically, the surface of the exposed sample is embedded in the phenolic resin material so that it is perpendicular to the extrusion direction of the extruded material. The surface of the sample is polished with emery paper up to 1200 grit, and then ultrasonically cleaned in pure water and dried.

[0131] The exposed surface of each sample was treated with 1.0% cupric chloride dihydrate (CuCl ２ ・2H ２ The sample was immersed in an aqueous solution of O) (12.7 g / L) and held at a temperature of 75°C for 24 hours. After that, the sample was removed from the aqueous solution. The sample was re-embedded in a phenolic resin material so that the exposed surface remained perpendicular to the extrusion direction, longitudinal direction, or forging flow direction. Next, the sample was cut so that the cross section of the corroded area was obtained as the longest cut section. Subsequently, the sample was polished. Using a metallurgical microscope, the corrosion depth was observed at 10 locations in the field of view of the microscope at a magnification of 100 to 500 times. The deepest corrosion point was recorded as the maximum dezincification corrosion depth.

[0132] Furthermore, when tested according to ISO 6509, a maximum corrosion depth of 400 μm or less is considered to be at a level that does not pose a problem in terms of practical corrosion resistance. When superior corrosion resistance is required, the maximum corrosion depth is preferably 200 μm or less. In this test, a maximum corrosion depth exceeding 400 μm was evaluated as unacceptable (evaluation: D, poor). A maximum corrosion depth between 300 μm and 400 μm was evaluated as acceptable (evaluation: C, fair), and a maximum corrosion depth between 200 μm and 300 μm was evaluated as good (evaluation: B, good). A maximum corrosion depth of 200 μm or less was evaluated as excellent (evaluation: A, excellent).

[0133] (Drill Cutting Test) Using a φ3.5 mm high-speed steel JIS standard drill, a 10 mm deep drill was drilled dry on a drill press under the conditions of rotational speed: 1250 rpm and feed rate: 0.17 mm / rev. Voltage changes were collected in the circumferential and axial directions using an AST type tool dynamometer during drilling, and chips were collected during drilling. Machinability was evaluated based on the chip shape. Problems in practical cutting are chip entanglement with the tool and chip bulkiness. For this reason, a chip shape with an average of less than one turn was evaluated as good ("A"). A chip shape with one or more turns but less than three turns was evaluated as fair ("B"), indicating that drilling is possible, although there are some practical problems. A chip shape with three or more turns was evaluated as poor ("D"). Note that the initial chips generated were excluded. To minimize the effects of drill wear, for sample numbers X, Y, and Z, one back-and-forth pass was performed from X→Y→Z→...Z→Y→X, and each sample was measured twice.

[0134]

[0135]

[0136]

[0137]

[0138] It was confirmed that by satisfying the composition of this embodiment and the composition relation f1, the metal structure relation f2, f3, f4, and the requirements of the metal structure, hot extruded and hot forged materials can be obtained with a small amount of Pb content, exhibiting good machinability, good resistance to dezincification corrosion, a high electrical conductivity of 17.0% IACS or higher, and a strength of HV hardness of 95 or higher. It was confirmed that the dezincification corrosion resistance, which was a major problem with conventional β phases, could be significantly improved by modifying the β phase to the β1 phase, as follows (alloy Nos. S01-S15, S19-S21, S31, S32). On the other hand, even if the composition of this embodiment was satisfied and the composition relation f1 was satisfied, if the metal structure relation f3 = 0, that is, if no grain boundaries were observed inside the β1 phase, which is a requirement of the metal structure, the dezincification corrosion resistance was poor (alloy Nos. S01, S04, processes A12H, B12H, B13H).

[0139] After hot working and appropriate cooling treatment, the β phase was modified to the β1 phase by incorporating more than 0.05 mass% of Si and more than 0.01 mass% of P, and grain boundaries were observed within the β1 phase. As a result, dezincification corrosion resistance improved. Further improvements in dezincification corrosion resistance were observed when the Si content was 0.08 mass% or more and the P content was 0.03 mass% or more. This suggests that the β phase was further modified (alloy No. S07, S10, process B1).

[0140] When Sn content exceeded 0.01 mass%, dezincification corrosion resistance improved. When Sn content exceeded 0.10 mass%, dezincification corrosion resistance improved even further compared to materials without Sn, and dezincification corrosion resistance improved to less than 100 μm in the ISO 6509 test (alloys No. S31, S32).

[0141] When the compositional relation f1 is large, the amount of β phase decreases, and the hardness decreases (alloys No. S19, S21).

[0142] After hot extrusion, when the starting temperature for the cooling process exceeded 500°C, and the average cooling rate in the temperature range from the starting temperature to 300°C, and the average cooling rate in the temperature range from 500°C to 300°C, exceeded 300°C / min, the β phase was modified, and grain boundaries were observed within the β1 phase. When the β phase was modified, the resistance to dezincification corrosion improved (alloy Nos. S01-S15, S19-S21, S31, S32).

[0143] When the average cooling rate in the temperature range from 500°C to 300°C was less than 300°C, no grain boundaries were observed within the β1 phase. As a result, the resistance to dezincification corrosion deteriorated (alloys No. S01, S04, processes A12H, B12H, B13H).

[0144] In the free-machining copper alloy of this embodiment, a P compound is not required. However, even if a P compound is present, the dezincification corrosion resistance is poor, and the presence of a P compound does not improve dezincification corrosion resistance (Figures 4A, 4B, Alloy No. S06, Process B13H).

[0145] Even when the material used for hot forging is a casting, if the cooling treatment after hot forging is appropriate, it was confirmed that the β1 phase can be observed and the same properties can be obtained as with hot extruded materials (alloys No. S01, S04, process No. B1).

[0146] Even when a predetermined cooling treatment is not performed during hot working, it was confirmed that the β phase is modified, and in particular, the resistance to dezincification corrosion is significantly improved, by annealing the hot-worked material at a temperature above 520°C but below 630°C for 1 minute to 5 hours, then starting the cooling treatment at a temperature above 500°C after annealing, and cooling at an average cooling rate of over 300°C / min in the temperature range from the cooling treatment start temperature to 500°C, and an average cooling rate of over 300°C / min in the temperature range from 500°C to 300°C (Alloy No. S01, S04, Process No. C1).

[0147] In Comparative Examples 51 to 56, when etched with a mixture of hydrogen peroxide and ammonia water, the β1 phase was not observed, and the dezincification corrosion depth significantly exceeded 400 μm, indicating insufficient corrosion resistance. In contrast, in Examples 1 to 30 of the present invention, when etched with a mixture of hydrogen peroxide and ammonia water, the β1 phase was observed, and the dezincification corrosion depth was 382 μm or less, demonstrating excellent corrosion resistance.

[0148] 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 mechanical properties and resistance to dezincification corrosion. Furthermore, excellent properties can be obtained in alloys of this embodiment by setting the manufacturing conditions in hot extrusion and hot forging, and the heat treatment conditions within an appropriate range.

[0149] The free-machining copper alloy of this embodiment has a low Pb content, excellent hot workability and machinability, high strength, and an excellent balance of strength and elongation. For this reason, the free-machining copper alloy of this embodiment is suitable for appliances and parts related to drinking water and sanitation equipment, food appliances, electrical and electronic equipment parts, automobile parts, machine parts, stationery, toys, musical instruments, sliding parts, instrument parts, precision machine parts, medical parts, drinking appliances and parts, water meters, and parts related to liquids and gases such as industrial water, wastewater, and hydrogen. Specifically, it can be suitably applied as a component material for items used in the above fields, such as water taps, stopcocks, mixing taps, shower heads, valves, fittings, cocks, gears, shafts, bearings, trumpets, shafts, sleeves, spindles, sensors, bolts, nuts, flare nuts, pen tips, insert nuts, cap nuts, nipples, spacers, and screws.

Claims

1. Containing Cu in an amount greater than 58.0 mass% and less than 63.5 mass%, Si in an amount greater than 0.05 mass% and less than 0.50 mass%, Pb in an amount of 0.002 mass% or more and less than 0.20 mass%, and P in an amount greater than 0.01 mass% and less than 0.40 mass%, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co, and Cr among the unavoidable impurities is less than 0.40 mass%, and the content of Al is less than 0.30 mass%, and when the Cu content is [Cu] mass%, the Si content is [Si] mass%, the Pb content is [Pb] mass%, and the P content is [P] mass%, A free-cutting copper alloy characterized by having the relationship 57.5 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × [Pb] + 0.5 × [P] ≤ 61.5, and in the constituent phases of the metallic structure excluding nonmetallic inclusions, when the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%, the relationships 10 ≤ f2 = (α) < 80, 20 < f3 = (β1) ≤ 90, 0 ≤ f4 = (γ) < 4, and grain boundaries being observable inside the β1 phase when etched with a mixture of hydrogen peroxide and ammonia water.

2. Containing Cu in an amount of 58.8 mass% to 63.0 mass%, Si in an amount of 0.08 mass% to 0.48 mass%, Pb in an amount of 0.01 mass% to 0.10 mass%, and P in an amount of 0.03 mass% to 0.20 mass%, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co, and Cr among the unavoidable impurities is less than 0.30 mass%, and the content of Al is less than 0.15 mass%, and when the Cu content is [Cu] mass%, the Si content is [Si] mass%, the Pb content is [Pb] mass%, and the P content is [P] mass%, A free-cutting copper alloy characterized by having the relationship 58.0 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × [Pb] + 0.5 × [P] ≤ 61.0, and in the constituent phases of the metallic structure excluding nonmetallic inclusions, when the area ratio of the α phase is (α)%, the area ratio of the γ phase is (γ)%, and the area ratio of the modified β phase, the β1 phase, is (β1)%, the relationships 15 ≤ f2 = (α) < 75, 25 < f3 = (β1) ≤ 85, 0 ≤ f4 = (γ) < 1, and grain boundaries being observable within the β1 phase when etched with a mixture of hydrogen peroxide and ammonia water.

3. Containing Cu in amounts exceeding 58.0 mass% and less than 63.5 mass%, Si in amounts exceeding 0.05 mass% and less than 0.50 mass%, Pb in amounts between 0.002 mass% and less than 0.20 mass%, and P in amounts exceeding 0.01 mass% and less than 0.40 mass%, and as optional elements, containing Sn in amounts exceeding 0.01 mass% and less than 0.90 mass%, Bi in amounts exceeding 0.0001 mass% and less than 0.20 mass%, Sb in amounts between 0.01 mass% and 0.12 mass%, and As in amounts between 0.01 mass% and 0.12 mass%, with the remainder being Zn and unavoidable impurities. If the total content of Fe, Mn, Co, and Cr among the aforementioned unavoidable impurities is less than 0.40 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%, the content of Bi is [Bi] mass%, and the content of Sn is [Sn] mass, then the relationship 57.5 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × ([Pb] + [Bi]) + 0.5 × [P] - 1.1 × [Sn] ≤ 61.5 holds. A free-cutting copper alloy characterized in that, in the constituent phases of the metallic structure 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 following relationships exist: 10 ≤ f2 = (α) < 80, 20 < f3 = (β1) ≤ 90, 0 ≤ f4 = (γ) < 4, and grain boundaries can be observed inside the β1 phase when etched with a mixture of hydrogen peroxide and ammonia water.

4. Containing 58.8 mass% to 63.0 mass% of Cu, 0.08 mass% to 0.48 mass% of Si, 0.01 mass% to 0.10 mass% of Pb, and 0.03 mass% to 0.20 mass% of P, and as optional elements, containing 0.05 mass% to 0.85 mass% of Sn, 0.001 mass% to 0.10 mass% of Bi, 0.012 mass% to 0.08 mass% of Sb, and 0.025 mass% to 0.08 mass% of As, with the remainder being Zn and unavoidable impurities. If the total content of Fe, Mn, Co, and Cr among the aforementioned unavoidable impurities is less than 0.30 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%, the content of Bi is [Bi] mass%, and the content of Sn is [Sn] mass, then the relationship 58.0 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × ([Pb] + [Bi]) + 0.5 × [P] - 1.1 × [Sn] ≤ 61.0 holds. A free-cutting copper alloy characterized in that, in the constituent phases of the metallic structure 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 following relationships exist: 15 ≤ f2 = (α) < 75, 25 < f3 = (β1) ≤ 85, 0 ≤ f4 = (γ) < 1, and grain boundaries can be observed inside the β1 phase when etched with a mixture of hydrogen peroxide and ammonia water.

5. The free-machining copper alloy according to any one of claims 1 to 4, characterized in that it has an electrical conductivity of 17.0% IACS or higher, a Vickers hardness of 95 HV or higher, and a maximum dezincification corrosion depth of 400 μm or less in the dezincification corrosion test results according to ISO 6509.

6. The free-machining copper alloy according to any one of claims 1 to 4, characterized in that it is used in appliances and parts related to drinking water and sanitation facilities, valves, cocks, industrial piping parts, water meters, musical instruments, automobile parts, electrical and electronic equipment parts, machine parts, stationery, toys, sliding parts, instrument parts, precision machine parts, and medical parts.

7. Containing Cu in an amount exceeding 58.0 mass% and less than 63.5 mass%, Si in an amount exceeding 0.05 mass% and less than 0.50 mass%, Pb in an amount of 0.002 mass% or more and less than 0.20 mass%, and P in an amount exceeding 0.01 mass% and less than 0.40 mass%, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co and Cr among the unavoidable impurities is less than 0.40 mass%, and the content of Al is less than 0.30 mass%, and Cu A method for producing a free-machining copper alloy using a copper alloy having the relationship 57.5 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × [Pb] + 0.5 × [P] ≤ 61.5, where the content of [Cu] is [Cu] mass%, the content of [Si] mass%, the content of [Pb] mass%, and the content of [P] mass%, wherein the manufacturing process includes one or more hot working steps, or one or more hot working steps and a heat treatment step, A method for manufacturing a free-machining copper alloy, characterized in that, in the former manufacturing process, the final hot working step is performed at a hot working temperature of over 520°C and less than 790°C, and in the cooling process after hot working, the hot-worked material is cooled at a temperature lower than 690°C and higher than 500°C; and in the latter manufacturing process, the final heat treatment step is performed at a temperature of over 520°C and lower than 630°C for 1 minute to 5 hours, and after annealing, the cooling process is started at a temperature higher than 500°C; and in both the former and latter cooling processes, the average cooling rate in the temperature range from the cooling start temperature to 500°C is over 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is over 300°C / min.

8. Containing Cu in an amount of 58.8 mass% to 63.0 mass%, Si in an amount of 0.08 mass% to 0.48 mass%, Pb in an amount of 0.01 mass% to 0.10 mass%, and P in an amount of 0.03 mass% to 0.20 mass%, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co, and Cr among the unavoidable impurities is less than 0.30 mass%, and the content of Al is less than 0.15 mass%, and Cu A method for producing a free-machining copper alloy using a copper alloy having the relationship 58.0 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × [Pb] + 0.5 × [P] ≤ 61.0, where the content of [Cu] mass%, the content of [Si] mass%, the content of [Pb] mass%, and the content of [P] mass%, wherein the manufacturing process includes one or more hot working steps, or one or more hot working steps and a heat treatment step, A method for manufacturing a free-machining copper alloy, characterized in that, in the former manufacturing process, the final hot working step is performed at a hot working temperature of over 520°C and less than 790°C, and in the cooling process after hot working, the hot-worked material is cooled at a temperature lower than 690°C and higher than 500°C; and in the latter manufacturing process, the final heat treatment step is performed at a temperature of over 520°C and lower than 630°C for 1 minute to 5 hours, and after annealing, the cooling process is started at a temperature higher than 500°C; and in both the former and latter cooling processes, the average cooling rate in the temperature range from the cooling start temperature to 500°C is over 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is over 300°C / min.

9. Containing Cu in amounts exceeding 58.0 mass% and less than 63.5 mass%, Si in amounts exceeding 0.05 mass% and less than 0.50 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 0.40 mass%, and as optional elements, containing Sn in amounts exceeding 0.01 mass% and less than 0.90 mass%, Bi in amounts exceeding 0.0001 mass% and less than 0.20 mass%, Sb in amounts of 0.01 mass% or more and less than 0.12 mass%, and As in amounts of 0.01 mass% or more and less than 0.12 mass%, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co and Cr among the unavoidable impurities is 0.40 mass. A method for producing a free-machining copper alloy using a copper alloy having the following relationship: 57.5 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × ([Pb] + [Bi]) + 0.5 × [P] - 1.1 × [Sn] ≤ 61.5, using a copper alloy having one or more hot working steps, or a manufacturing process having one or more hot working steps and a heat treatment step, wherein the copper alloy is less than % and has an Al content of less than 0.30 mass%, and 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 method is to produce a free-machining copper alloy by a manufacturing process having one or more hot working steps, or by a manufacturing process having one or more hot working steps and a heat treatment step. A method for manufacturing a free-machining copper alloy, characterized in that, in the former manufacturing process, the final hot working step is performed at a hot working temperature of over 520°C and less than 790°C, and in the cooling process after hot working, the hot-worked material is cooled at a temperature lower than 690°C and higher than 500°C; and in the latter manufacturing process, the final heat treatment step is performed at a temperature of over 520°C and lower than 630°C for 1 minute to 5 hours, and after annealing, the cooling process is started at a temperature higher than 500°C; and in both the former and latter cooling processes, the average cooling rate in the temperature range from the cooling start temperature to 500°C is over 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is over 300°C / min.

10. Containing Cu in an amount of 58.8 mass% to 63.0 mass%, Si in an amount of 0.08 mass% to 0.48 mass%, Pb in an amount of 0.01 mass% to 0.10 mass%, and P in an amount of 0.03 mass% to 0.20 mass%, and as optional elements, containing Sn in an amount of 0.05 mass% to 0.85 mass%, Bi in an amount of 0.001 mass% to 0.10 mass%, Sb in an amount of 0.012 mass% to 0.08 mass%, and As in an amount of 0.025 mass% to 0.08 mass%, with the remainder being Zn and unavoidable impurities, wherein the total content of Fe, Mn, Co, and Cr among the unavoidable impurities is 0.30 mass. A method for producing a free-machining copper alloy using a copper alloy having the following relationship: 58.0 ≤ f1 = [Cu] - 4.7 × [Si] + 0.5 × ([Pb] + [Bi]) + 0.5 × [P] - 1.1 × [Sn] ≤ 61.0, using a copper alloy having one or more hot working steps, or a manufacturing process having one or more hot working steps and a heat treatment step, wherein the copper alloy is less than % and has an Al content of less than 0.15 mass%, and 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%, then the manufacturing process has one or more hot working steps, or a manufacturing process has one or more hot working steps and a heat treatment step. A method for manufacturing a free-machining copper alloy, characterized in that, in the former manufacturing process, the final hot working step is performed at a hot working temperature of over 520°C and less than 790°C, and in the cooling process after hot working, the hot-worked material is cooled at a temperature lower than 690°C and higher than 500°C; and in the latter manufacturing process, the final heat treatment step is performed at a temperature of over 520°C and lower than 630°C for 1 minute to 5 hours, and after annealing, the cooling process is started at a temperature higher than 500°C; and in both the former and latter cooling processes, the average cooling rate in the temperature range from the cooling start temperature to 500°C is over 300°C / min, and the average cooling rate in the temperature range from 500°C to 300°C is over 300°C / min.

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