Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C9/00—Alloys based on copper
  • C22C9/04—Alloys based on copper with zinc as the next major constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/08—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of copper or alloys based thereon
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working

Definitions

• the present invention relates to a free-cutting copper alloy having good corrosion resistance, in particular, dezincification corrosion resistance and resistance to stress corrosion cracking as well as a high strength and a significantly reduced Pb content and a method for producing a free-cutting copper alloy.
• the present invention relates to a free-cutting copper alloy used for components produced by machining such as devices and components used for supply of drinking water that humans and animals ingest daily, those used for sanitary facilities such as kitchen, shower room, or bathroom, water meters, musical instruments, tableware, devices and components for water drainage, industrial plumbing components, electrical or electronic apparatus components, auto parts, mechanical components, stationaries, toys, sliding components, measuring instrument components, precision mechanical components, medical components, and components relating to liquid or gas such as hydrogen, whose specific component names include faucet, mixer tap, tap fitting, shutoff valve, valve, joint, shower head, cock, gear, axle, bearing, shaft, sleeve, spindle, sensor, bolt, nut, flare nut, pen point, insert nut, cap nut, nipple, spacer, and screw, and a method for producing the free-cutting copper alloy.
• a Cu-Zn-Pb alloy containing 56 to 65 mass% Cu and 1 to 4 mass% Pb with the balance being Zn (so-called a free-cutting brass bar, forging brass, or casting brass) or a Cu-Sn-Zn-Pb alloy containing 80 to 88 mass% Cu, 2 to 8 mass% Sn, and 1 to 8 mass% Pb with the balance being Zn (so-called bronze casting: gunmetal) having excellent machinability and antibacterial property as well as good corrosion resistance has been generally used for devices and components related to drinking water or sanitary facility, water meters, musical instruments, tableware, electrical or electronic apparatus components, home appliance components, auto parts, mechanical components, stationaries, precision mechanical components, medical components, and devices and components relating to liquid or gas such as industrial water, drainage water, or hydrogen including, in terms of component name, faucet, mixer tap, shutoff valve, valve, cock, joint, gear, sensor, nut, and screw.
• Patent Document 1 improvement of machinability and dezincification corrosion resistance is devised by adding 0.3 to 4 mass%, preferably 1.8 to 3.2 mass% Bi to a Cu-Zn alloy and annealing the alloy at a temperature between 350°C and 550°C so that ⁇ phase is reduced and fragmented by ⁇ phase because ⁇ phase has poor dezincification corrosion resistance.
• alloys including Bi instead of Pb have many problems including the facts that machinability of Bi is inferior to that of Pb, Bi may be harmful to the environment and human body like Pb, Bi has a resourcing problem because it is a rare metal, and Bi embrittles a copper alloy material.
• machinability of ⁇ phase is inferior to that of Pb. Further, it has poor dezincification corrosion resistance and resistance to stress corrosion cracking. Therefore, there is no way that such an alloy can be an alternative to a free-cutting copper alloy containing Pb.
• Patent Documents 2 to 8 for example, Cu-Zn-Si alloys including Si instead of Pb are proposed as free-cutting copper alloys.
• Patent Documents 2 to 7 disclose that in alloys containing about 58 to 65 mass% Cu and 0.2 to 1.5 mass% Si, machinability is improved by the effect of Si contained in ⁇ phase and the presence of fine phosphorous compounds formed of P and Zn or the like. In these Documents, excellent machinability is realized by defining the area ratios of ⁇ phase and ⁇ phase and requiring phosphorous compounds to be present and a small amount of Pb to be contained in the alloys.
• Patent Document 1 it is a well-known fact that ⁇ phase of a Cu-Zn alloy has poor dezincification corrosion resistance and also causes deterioration of resistance to stress corrosion cracking.
• Patent Document 8 excellent machinability is realized in an alloy containing 71.5 to 78.5 mass% Cu and 2.0 to 4.5 mass% Si although the Pb content is as small as 0.02 mass% or even less by defining the total area ratio of ⁇ phase and ⁇ phase with excellent machinability that are formed in an alloy containing Cu and Si at high concentrations.
• further improvement of machinability and corrosion resistance is devised by making the alloy contain 0.1 mass% or higher amount of Sn and Al respectively to form a large amount of ⁇ phase.
• Patent Document 9 excellent machinability is obtained by containing small amounts of Si, Pb, and P or Fe as well as 0.5 mass% or less Pb, and dispersing Pb-rich particles in the matrix and increasing the population density of the Pb-rich particles that are present inside ⁇ phase by contriving manufacturing method.
• Patent Document 9 requires performance of finish heat treatment at a temperature between 400°C and 600°C in effect.
• Patent Document 10 proposes a Cu-Zn-Si-Pb-P copper alloy in which the area ratios of ⁇ phase, ⁇ phase, and ⁇ phase are limited and involving a production technique to manufacture hollow hot-forged products having a near-net shape using a hollow material.
• Patent Document 11 proposes a copper alloy casting made of a Cu-Zn-Zr-P alloy optionally containing Si, Pb, and/or Sn, in which crystal grains are refined by the actions of Zr and P.
• Patent Document 12 proposes a Cu-Zn-Sn-Al copper alloy with excellent color fastness optionally containing Si and/or Pb in which the area ratios of y phase and ⁇ phase are limited.
• Patent Document 13 proposes a lead-free copper alloy casting of a Cu-Zn-Si-Sn-Al-P alloy.
• Patent Document 14 discloses that apparent Zn content is important in order to improve corrosion resistance of a Cu-Zn-Si-Sn-Al alloy and proposes a copper alloy whose machinability is improved by containing a large amount of Pb or Bi in effect.
• Patent Document 15 discloses a lead-free copper alloy casting made of a Cu-Zn-Si alloy containing 65 mass% or more Cu with good castability and mechanical strength in which machinability is improved by ⁇ phase.
• An example containing large amounts of Sn, Al, Mn, Ni, and Sb is presented in this Document.
• Patent Documents 1 to 15 there is no past record of substantial improvement made in the dezincification corrosion resistance and the resistance to stress corrosion cracking of ⁇ phase present in Cu-Zn alloys, which had been a long-standing technical challenge. Further, there is no disclosure of a Cu-Zn alloy containing less than 0.2% Pb or Bi that exhibits low cutting resistance and excellent machinability in high-speed machining exceeding 100 m/min without essentially requiring the alloy to contain Bi.
• the present invention has been made in order to solve the above-described problems in the conventional art, and the objective thereof is to provide a free-cutting copper alloy in which Pb content has been significantly reduced but having excellent hot workability and machinability as well as good dezincification corrosion resistance and resistance to stress corrosion cracking despite a large content of ⁇ phase (the ⁇ 1 phase described later), a high strength, and good balance between strength and ductility, and a method for producing the free-cutting copper alloy.
• ⁇ phase includes ⁇ ' phase
• ⁇ phase includes ⁇ ' phase
• ⁇ phase includes ⁇ ' phase
• ⁇ phase includes ⁇ ' phase
• ⁇ 1 phase refers to modified ⁇ phase and is distinguished from ⁇ phase and ⁇ ' phase.
• ⁇ 1 phase has a characteristic that a grain boundary pattern, i.e., a boundary of crystal grain can be identified inside the phase when etched with an etching solution made of hydrogen peroxide and ammonia water then observed with a metallographic microscope.
• Conductivity refers to electric conductivity, thermal conductivity, or electrical conductivity.
• Patent Document 9 discloses that in order to improve dezincification corrosion resistance of ⁇ phase, it is necessary to include a higher amount of Sn than Si and heat the material to a temperature between 700°C and 850°C, perform hot extrusion in the same temperature range and a finishing heat treatment in which the material is held at a temperature between 400°C and 600°C for 30 minutes or longer then cooled at an average cooling rate of 0.2°C/sec to 10°C/sec in the temperature range from 400°C to 200°C.
• Patent Documents 2 to 7 disclose a discovery that ⁇ phase itself exhibits a significant effect on machinability in a Cu-Zn-Si alloy if a certain amount of Si is contained in the ⁇ phase. These Patent Documents further disclose that their alloys obtained excellent machinability due to the synergy effect brought by satisfying the three requirements, i.e., fine phosphorous compound is present, a small amount of Pb is contained, and Bi is contained in some cases.
• these Documents require cooling at an average cooling rate of preferably about 0.1°C/min or higher and 70°C/min or lower in the temperature range from about 530°C to about 450°C after hot working.
• Patent Documents 2 to 7 do not disclose any data related to dezincification corrosion resistance or resistance to stress corrosion cracking. There is no mention of modification of ⁇ phase itself, either. This implies that no improvement was made in the dezincification corrosion resistance or the resistance to stress corrosion cracking of ⁇ phase that is present in Cu-Zn-Si alloy and no further improvement was made in the machinability.
• ⁇ 1 phase can be easily distinguished from ⁇ phase of a Cu-Zn system alloy.
• a surface of a piece of alloy is polished (mirror-polished) then etched with a mixed solution of hydrogen peroxide and ammonia water.
• an aqueous solution prepared by mixing 3 ml of 3 vol% hydrogen peroxide water and 22 ml of 14 vol% ammonia water is used for the etching.
• the polished metal surface is dipped in the aqueous solution under room temperature between about 15°C and 25°C for about 2 to 10 seconds then the metallographic structure is observed with a metallographic microscope at a magnification of 200X to 1000X.
• cooling treatment refers to manipulation of cooling rate by water cooling or any other similar method as opposed to natural cooling.
• Patent Documents 2 to 7 require cooling of hot-worked material at an average cooling rate of about 0.1°C/min or higher and about 70°C/min or lower when the material is in the temperature range from about 530°C to about 450°C after hot working. That is, the present invention and those of Patent Documents 2 to 7 are clearly heading in opposite directions (or providing opposite teachings).
• dezincification corrosion resistance and resistance to stress corrosion cracking which were major unsolved problems with conventional ⁇ phase, are dramatically improved by the modification of ⁇ phase into ⁇ 1 phase, allowing the problems to be solved.
• the present invention retains high strength of conventional ⁇ phase which is further enhanced, as well as improved ductility and better balance between strength and ductility.
• a copper alloy with machinability equivalent to that of conventional free-cutting brass and better dezincification corrosion resistance and resistance to stress corrosion cracking as well as higher strength than those of conventional free-cutting brass came to be invented.
• a free-cutting copper alloy according to a second aspect of the present invention includes: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, with the balance being Zn and inevitable impurities, in which, among the inevitable impurities, a total content of Fe, Mn, Co, and Cr is lower than 0.30 mass% and a content of Al is lower than 0.15 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%,
• a free-cutting copper alloy includes: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; higher than 0.05 mass% and lower than 0.90 mass% Sn; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, with the balance being Zn and inevitable impurities, in which, among the inevitable impurities, a total content of Fe, Mn, Co, and Cr is lower than 0.40 mass% and a content of A1 is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, a Bi content is represented by [Bi] mass%, and a
• a free-cutting copper alloy according to a fifth aspect of the present invention is the free-cutting copper alloy according to any one of the first to fourth aspects of the present invention, which is used for a device or component related to drinking water or sanitary facility, a valve, a cock, an industrial plumbing component, a water meter, a musical instrument, an auto part, an electrical or electronic apparatus component, a mechanical component, a stationery, a toy, a sliding component, a measuring instrument component, a precision mechanical component, or a medical component.
• a method for producing a free-cutting copper alloy according to a seventh aspect of the present invention is a method for producing the free-cutting copper alloy according to any one of the first to fourth aspects of the present invention, including one or more hot working steps and heat treatment steps, in which in the final heat treatment step, annealing is performed with a holding temperature of higher than 520°C and lower than 630°C and a holding time of one minute to five hours, the annealed material is cooled through a cooling treatment which is started after the annealing when the temperature of the material is higher than 500°C and performed at an average cooling rate of higher than 300°C/min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C.
• free-cutting copper alloys according to the embodiments of the present invention and methods for producing the free-cutting copper alloys.
• the free-cutting copper alloys according to the embodiments are used for devices and components related to sanitary facilities, musical instruments, tableware, home appliance components, electrical or electronic apparatus components, auto parts, mechanical components, stationaries, precision mechanical components, medical components, and devices and components involving liquid or gas such as industrial water, drainage water, and hydrogen.
• specific component names include faucet, mixer tap, shutoff valve, valve, cock, joint, water meter, gear, sensor, nut, and screw.
• an element symbol in parentheses such as [Zn] represents the content (mass%) of the element.
• a plurality of metallographic structure relational expressions are defined as follows.
• a free-cutting copper alloy according to the second embodiment of the present invention includes: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.30 mass%, the content of Al is lower than 0.15 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, and a Bi content is represented by [Bi] mass%, the relationship
• a free-cutting copper alloy according to the third embodiment of the present invention includes: higher than 60.5 mass% and lower than 65.0 mass% Cu; higher than 0.50 mass% and lower than 1.20 mass% Si; higher than or equal to 0.002 mass% and lower than 0.20 mass% Pb; higher than 0.01 mass% and lower than 0.18 mass% P; higher than 0.05 mass% and lower than 0.90 mass% Sn; and higher than or equal to 0.0001 mass% and lower than 0.20 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.40 mass%, the content of Al is lower than 0.30 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass%, a Bi content is represented by [Bi] mass%, and a Sn
• a free-cutting copper alloy according to the fourth embodiment of the present invention includes: higher than or equal to 61.2 mass% and lower than or equal to 64.8 mass% Cu; higher than or equal to 0.65 mass% and lower than or equal to 1.10 mass% Si; higher than or equal to 0.003 mass% and lower than 0.10 mass% Pb; higher than or equal to 0.03 mass% and lower than or equal to 0.15 mass% P; higher than or equal to 0.10 mass% and lower than 0.50 mass% Sn; and higher than or equal to 0.001 mass% and lower than 0.10 mass% Bi as an optional element, and the balance comprising Zn and inevitable impurities, in which, among the inevitable impurities, the total content of Fe, Mn, Co, and Cr is lower than 0.30 mass%, the content of Al is lower than 0.15 mass%, when a Cu content is represented by [Cu] mass%, a Si content is represented by [Si] mass%, a Pb content is represented by [Pb] mass%, a P content is represented by [P] mass
• compositional components the composition relational expressions f0, f1, and f2, the metallographic structure relational expressions f3, f4, and f5, the metallographic structure and composition relational expression f6, the metallographic structure, and the like are defined as described above are explained below.
• Si is a main element of the free-cutting copper alloy according to the embodiment. Si contributes to the formation of metallic phases such as ⁇ phase, ⁇ phase, ⁇ phase, ⁇ phase, ⁇ 1 phase, and ⁇ phase.
• ⁇ phase is modified to form ⁇ 1 phase.
• ⁇ 1 phase modified ⁇ phase
• significant improvements in dezincification corrosion resistance and resistance to stress corrosion cracking, that were drawbacks of conventional ⁇ phase, are realized in addition to improvement in machinability.
• a representative composition of ⁇ 1 phase generated by modification would be about 61 mass% Cu, about 1.2 mass% Si, about 37.5 mass% Zn, and about 0.1 mass% P.
• Si is an essential element required for the modification of ⁇ phase into ⁇ 1 phase.
• Si needs to be included in an amount exceeding 0.50 mass%.
• the Si content is preferably 0.60 mass% or higher, more preferably 0.65 mass% or higher, and still more preferably 0.75 mass% or higher.
• Zn is a main element of the free-cutting copper alloy according to the embodiment together with Cu and Si and is an element necessary to enhance machinability, strength, high temperature properties, and castability.
• Zn is described as the balance in the composition, but to be specific, its content is lower than about 38.5 mass% and preferably lower than about 38.0 mass%, and is higher than about 32.0 mass% and preferably higher than 33.0 mass%.
• P is an essential element required for the modification of ⁇ phase into ⁇ 1 phase.
• P solid-solubilizes in ⁇ phase during hot working.
• ⁇ phase is modified into ⁇ 1 phase if the cooling treatment performed in the cooling process after hot working is started when the temperature of the hot-worked material is higher than 500°C and lower than 670°C and the material is cooled at a cooling rate exceeding 300°C/min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C.
• the modification of ⁇ phase into ⁇ 1 phase significantly improves long-standing drawbacks of conventional ⁇ phase, i.e., its dezincification corrosion resistance and resistance to stress corrosion cracking.
• Presence of ⁇ 1 phase further makes it possible to decrease cutting resistance during cutting and enhance chip breakability.
• P-containing compound decreases cutting resistance and improves chip breakability, but if the cooling treatment performed at a cooling rate exceeding 300°C/min is started when the temperature of the material is about 550°C or higher than 530°C, P-containing compound does not appear of even if it does, the amount is small.
• the effect brought by the modification of ⁇ phase into ⁇ 1 phase excels that brought by the presence of P-containing compound in ⁇ phase.
• machinability is better improved by the modification of ⁇ phase than the presence of P-containing compound in ⁇ phase.
• inclusion of P leads to improvement of the dezincification corrosion resistance and resistance to stress corrosion cracking of ⁇ phase and significant improvement of those of alloys consisting of ⁇ 1 phase and ⁇ phase.
• the lower limit of the P content needs to be at least higher than 0.01 mass%.
• the P content is preferably 0.03 mass% or higher, more preferably 0.04 mass% or higher.
• P tends to compound with elements such as Zn, Si, Mn, Fe, Cr, Co, and Al. If the amount of P solid-solubilized inside the ⁇ phase that is present in an alloy after hot working is reduced by the formation of such compounds, the modification of ⁇ phase into ⁇ 1 phase is hindered.
• Zn and Si which are main elements of the present invention, start to compound with P at around 550°C. If the cooling rate is lowered, the amount of phosphorous compounds increases. Formation of compound between inevitable impurities of Mn, Fe, Cr, and/or Co and P begins at a temperature higher than 550°C. If their amounts are increased, formation of phosphorous compounds is further promoted.
• the total amount of Fe, Mn, Co, and Cr needs to be lower than 0.40 mass% at most, preferably lower than 0.30 mass%.
• the conditions required to realize the modification of ⁇ phase into ⁇ 1 phase are opposite to those to realize the presence of compounds comprising P and Zn and/or Si.
• the cooling treatment performed after hot working needs to be started when the temperature of the hot-worked material is higher than 500°C and lower than 670°C and the cooling rate in the temperature range from the starting temperature of the cooling treatment to 300°C needs to be high whereas for sufficient formation of compounds of P and Zn and/or Si, slow cooling is necessary in the temperature range from about 530°C to about 450°C. Therefore, when a large amount of phosphorous compounds are formed, formation of ⁇ 1 phase is slightly insufficient, i.e., modification of ⁇ phase is slightly insufficient. In some cases, ⁇ 1 phase is not present at all.
• the P content is lower than 0.18 mass%, preferably 0.15 mass% or lower, and more preferably 0.12 mass% or lower.
• good machinability is obtained by the effect of ⁇ 1 phase including Si and P, but it is further improved by containing a small amount of Pb.
• Pb in the composition of the embodiment, about 0.001 mass% Pb is solid-solubilized in the matrix, and the portion in excess of this amount is present in the form of tiny Pb particle with a diameter of about 0.1 to about 2 ⁇ m. It is generally believed that Pb has little effect on improvement of machinability if its content is approximately 0.1 mass%.
• ASM Specialty Handbook first edition "Copper and Copper Alloys” discloses a relationship between Pb content and machinability in which the machinability of a Cu-Zn-Pb alloy consisting of 62 to 65 mass% Cu, about 3.2 mass% Pb with the balance being Zn is assumed to be 100% ( FIG. 6 on p. 267 of the Handbook).
• the Fig. 6 indicates that containing 0.1 mass% Pb only has an effect of improving machinability by about 5 percentage points from 25% to 30% in terms of machinability index.
• Pb has a significant effect on machinability even though its content is very small. The effect is exhibited if the Pb content is 0.002 mass% or higher.
• the Pb content is preferably 0.003 mass% or higher, more preferably 0.01 mass% or higher.
• the Pb content is preferably 0.03 mass% or higher. Due to the effects of ⁇ 1 phase with greatly enhanced machinability and a small amount of Pb contained, machinability of the alloy significantly improves.
• Pb improves machinability of copper alloys, but for a Cu-Zn binary alloy, as represented by a free-cutting brass bar C3604, to obtain the effect, about 3 mass% Pb is required.
• an alloy with excellent machinability is accomplished by causing ⁇ 1 phase including Si and P, a tiny amount of Pb particles, and particles composed of Pb and Bi, which will be described later, to be present in the metallographic structure.
• the upper limit of Pb content is lower than 0.20 mass% since Pb is harmful to human body.
• the Pb content is preferably lower than 0.10 mass% and, in consideration of its influence on human body and the environment, it is most preferably 0.08 mass% or lower.
• Bi is solid-solubilized in the matrix in an amount of approximately 0.0001 mass%, and the portion in excess of this amount is present in the form of particle with a diameter of about 0.1 to about 2 ⁇ m.
• Pb and Bi are added, large portions of them are present in the form of particles composed of a mixture of Pb and Bi with a diameter of about 0.1 to about 2 ⁇ m.
• the inventors have discovered that if Bi and Pb are both contained in an embodiment where ⁇ 1 phase is present, machinability equivalent to or better than when Bi or Pb is contained alone can be obtained.
• the Bi content needs to be at least 0.0001 mass% or higher.
• the Bi content is preferably 0.001 mass% or higher, more preferably 0.002 mass% or higher.
• the effect of Bi on the human body is presently unknown, but its content is lower than 0.20 mass%, preferably lower than 0.10 mass%, and more preferably 0.08 mass% or lower.
• use of Bi affects the environment since it is a rare metal, and the element is included in the raw material as an inevitable impurity, Bi is included as an optional element, i.e., not required to be contained.
• the total content of Pb and Bi (the composition relational expression f2 that is explained later) is lower than 0.20 mass%, preferably lower than 0.10 mass%.
• the embodiments are directed to reducing the content of Pb, an element harmful to the human body, in some cases the total content of Pb and Bi, to lower than 0.20 mass% yet obtaining excellent machinability.
• Sn further improves the dezincification corrosion resistance of ⁇ 1 phase by solid solubilizing in the phase, allowing the alloy's dezincification corrosion resistance to improve.
• the Sn content is preferably 0.10 mass% or higher.
• Apropos a higher proportion of Sn is inherently distributed to ⁇ and ⁇ 1 phases than ⁇ phase, and Sn is able to exhibit its effect of improving dezincification corrosion resistance even if its content is small.
• ⁇ phase is likely to be formed, causing deterioration of ductility. Formation of phase causes deterioration of not only ductility but also machinability and dezincification corrosion resistance of the alloy. It is therefore necessary to limit the Sn content to lower than 0.90 mass% although it depends on the amount of Si contained.
• the Sn content is preferably lower than 0.70 mass%, more preferably lower than 0.50 mass%.
• the inventors have learned that when a large amount of Sn is contained, problem sometimes occurs in the modification of ⁇ phase containing Si and P because a larger portion of Sn is distributed to ⁇ phase and ⁇ 1 phase. Specifically, when the Sn content is larger than the Si content, ⁇ phase fails to be sufficiently modified, and the effect of improving dezincification corrosion resistance brought by Sn is offset. As described later, the Si content needs to exceed the Sn content.
• Examples of the inevitable impurities in the embodiments include Mn, Fe, Al, Ni, Mg, Se, Te, Sn, Bi, Co, Ca, Zr, Cr, Ti, In, W, Mo, B, Ag, and rare earth elements.
• primary raw material of a free-cutting copper alloy in particular, a free-cutting brass including about 30 mass% or higher Zn is not a quality raw material such as electrolytic copper or electrolytic zinc but recycled copper alloy.
• machining is performed on almost all the parts and components, during which a large amount of copper alloy accounting for 40 to 80% of the material in terms of weight goes to waste. Examples of such waste material include chips, mill ends, burrs, runners, and products with manufacturing defects. These waste copper alloys constitute the primary raw material.
• cutting chips include Fe, W, Co, Mo, and the like which originate from tools. Ni, Cr, or Sn may mix in since disposed products to be recycled include plated products.
• pure copper-based scrap used instead of electrolytic copper is contaminated with Mg, Sn, Fe, Cr, Ti, Co, In, Ni, Se, and/or Te.
• Brass-based scraps that are used instead of electrolytic copper or electrolytic zinc often include a material plated with Sn, causing Sn to mix in a recycled material.
• scraps including these elements are used as a raw material to the extent that use of such material does not adversely affect the resultant properties of the alloy at least.
• a leaded JIS free-cutting brass bar C3604 (JIS H 3250)
• about 3 mass% Pb is essentially contained.
• Fe may be contained up to 0.5 mass% and Fe and Sn may be contained up to 1.0 mass% (maximum total amount of Fe and Sn is 1.0 mass%).
• a high concentration of Fe or Sn is sometimes included in a free-cutting brass bar.
• Fe, Mn, Co, and Cr solid-solubilize in ⁇ phase and ⁇ phase of a Cu-Zn alloy to a certain concentration.
• Si and/or P are present then, they tend to compound with Si and/or P. If that happens, Si and P that are required for the modification of ⁇ phase could be consumed.
• Fe, Mn, Co, or Cr compounded with Si forms a Fe-Si compound, an Mn-Si compound, a Co-Si compound, a Cr-Si compound, or the like in the metallographic structure.
• a Fe-P compound, an Mn-P compound, a Co-P compound, a Cr-P compound, or the like is formed.
• the amount of Al that mixes in from special brass bar, brass casting, or the like needs to be limited because the element affects the modification of ⁇ phase if the Al content is large and also forms compounds with P or Si.
• the Al content needs to be lower than 0.30 mass%, preferably lower than 0.15 mass%, and more preferably 0.10 mass% or lower.
• f1 is an expression that represents a relationship. Even when the amount of each of the elements is within the range defined above, unless this composition relational expression f1 is satisfied, the target properties in embodiments of the present invention cannot be obtained. When the value of the composition relational expression f1 is lower than 57.5, the proportions of ⁇ phase and ⁇ 1 phase are large, causing deterioration of ductility, dezincification corrosion resistance, and resistance to stress corrosion cracking due to insufficient modification of ⁇ phase.
• the lower limit of the value of the composition relational expression f1 is 57.5 or higher, preferably 58.0 or higher, and more preferably 58.2 or higher.
• the value of the composition relational expression f1 becomes more favorable within the defined range, the proportion of ⁇ phase increases and modification of ⁇ phase becomes sufficient.
• good dezincification corrosion resistance and resistance to stress corrosion cracking as well as good ductility and cold workability can be obtained without impairing excellent machinability.
• the upper limit of the f1 value affects the proportions of ⁇ phase and ⁇ 1 phase.
• the value of the composition relational expression f1 is 60.5 or lower, preferably 60.2 or lower, and more preferably 60.0 or lower.
• the total content of Pb and Bi needs to be 0.003 mass% or higher, which is preferably 0.004 mass% or higher, and still more preferably 0.005 mass% or higher.
• Sn is contained in order to further improve dezincification corrosion resistance.
• a large portion of Sn is contained in ⁇ phase, and the larger the Sn content, the slower the advancement of dezincification corrosion of ⁇ 1 phase.
• the Sn content is excessively larger than the Si content, modification of ⁇ phase is hindered, which may impair dezincification corrosion resistance of ⁇ 1 phase.
• f0 [Sn] / [Si]
• f0 needs to be lower than 1. It is preferably lower than 0.8, more preferably lower than 0.6, and still more preferably lower than 0.5.
• Free-cutting copper alloys according to an embodiment of the present invention have good dezincification corrosion resistance, resistance to stress corrosion cracking, and mechanical properties despite a large content of conventional ⁇ phase, which refers to ⁇ 1 phase in the present invention. They also have machinability that requires sort of brittleness which allows low cutting resistance and generation of finely broken chips as well as ductility, a property entirely opposite to machinability.
• the composition relational expressions f0, f1, and f2 the metallographic structure relational expressions f3 to f5, and the metallographic structure and composition relational expressions f6 that are described later need to be satisfied in addition to the requirements regarding each component that constitutes the alloy's composition.
• Fe, Mn, Co, Cr, Al, and inevitable impurities that are separately defined are not defined by the composition relational expression f1 because their impact on f1 is small as long as the content is within the range that can be treated as an inevitable impurity.
• the average cooling rate in the temperature range from 500°C to 300°C after hot working is higher than 300°C/min.
• Patent Document 1 59-62 - 0.02 -0.07 - 0.3-0.4 - - Fe ⁇ 0.3 Two phases of ⁇ and ⁇ phases; ⁇ phase is divided by ⁇ phase Annealed at a temperature between 350°C and 550°C
• Patent Document 2 61-65 1.0-1.5 0.005 -0.19 0.003 -0.20 Sn+Al+Bi ⁇ 0.4, Fe+Mn+Cr+Co ⁇ 0.4 20 ⁇ 80, 15 ⁇ 80, 0 ⁇ 8
• the average cooling rate in the temperature range from 530°C to 450°C after hot working is 0.1°C/min or higher and 50°C/min or lower.
• Patent Document 3 59.0 -63.5 0.5-1.0 0.005 -0.19 0.003 -0.20 Sn+Al+Bi ⁇ 0.4, Fe+Mn+Cr+Co ⁇ 0.4 20 ⁇ 75, 25 ⁇ 80, 0 ⁇ 2
• the average cooling rate in the temperature range from 530°C to 450°C after hot working is 0.1 °C/min or higher and 50°C/min or lower.
• Patent Document 4 58.5 -65.0 0.4-1.4 0.003 -0.19 0.002 0.25
• the average cooling rate in the temperature range from 530°C to 450°C in the cooling process after casting is 0.1°C/min or higher and 55°C/min or lower.
• Patent Document 5 58.5 -64.5 0.2-1.2 0.001 -0.2 0.001 -0.20 0.1-1.0 Sn+Al+Bi ⁇ 0.45, Fe+Mn+Cr+Co ⁇ 0.45 20 ⁇ 85, 15 ⁇ 80, 0 ⁇ 4
• the average cooling rate in the temperature range from 530°C to 450°C after hot working is 0.1 °C/min or higher and 50°C/min or lower.
• Patent Document 6 58-65 0.3-1.3 0.001 -0.2 0.001 -0.20 0.02-0.1 Sn+Al+Bi ⁇ 0.45, Fe+Mn+Cr+Co ⁇ 0.45 20 ⁇ 85, 15 ⁇ 80, 0 ⁇ 5
• the average cooling rate in the temperature range from 530°C to 450°C after hot working is 0.1 °C/min or higher and 50°C/min or lower.
• Patent Document 7 59.7 -64.7 0.6-1.3 0.001 -0.15 0.001 -0.2 0.001-0.1 Sn+Al+Bi ⁇ 0.45, Fe+Mn+Cr+Co ⁇ 0.45 17 ⁇ 75
• the average cooling rate in the temperature range from 530°C to 440°C after hot working is 0.1°C/min or higher and 70°C/min or lower.
• Patent Documents 1, 13, and 15 disclose that Pb is not contained. Therefore, the Pb content is different.
• ⁇ phase in the metallographic structure is significantly limited in view of its effect on machinability, dezincification corrosion resistance, other type of corrosion resistance, and the like.
• ⁇ phase disclosed by these documents and ⁇ 1 phase in the present invention are two different phases, but for reference, ⁇ phase is limited to 5% or lower, 25% or lower, 15% or lower, and 0.9% or lower respectively in these Patent Documents.
• Patent Document 9 discloses that 0.2 mass% or higher Sn is contained, Sn and Si are contained for the improvement of dezincification corrosion resistance of ⁇ phase and requires hot extrusion performed at a temperature of 700°C or higher for the improvement of machinability and heat treatment performed at a temperature between 400°C and 600°C for the improvement of corrosion resistance.
• the proportion of ⁇ phase is approximately 5 to 20%, and the Si content is 0.01 to 0.50 mass%, which may be controlled to be 0.2 mass% or lower.
• Patent Documents 12, 13, and 14 Al is essentially required in order to improve color fastness, castability, and dezincification corrosion resistance.
• Patent Document 14 requires containing at least 0.1 mass% Sn and Al respectively in order to improve dezincification corrosion resistance and large amounts of Pb and Bi in order to obtain excellent machinability.
• Patent Document 15 discloses a corrosion-resistant copper alloy casting that does not contain Pb but requires y phase and have good mechanical properties and castability, which are realized by containing tiny amounts of elements such as Al, Sb, Sn, Mn, Ni, B, and so on in addition to Si and 65 mass% or higher Cu.
• Patent Documents 2 to 7 essentially require presence of phosphorous compounds in the metallographic structure that are formed as a result of a treatment performed in the cooling process after hot working at an average cooling rate of about 0.1°C/min or higher and about 70°C/min or lower in the temperature range from about 530°C to about 450°C. Further, the Documents disclose that y phase is effective to obtain good machinability but there is not a word about modification of ⁇ phase in these Documents. They are silent about dezincification corrosion resistance and resistance to stress corrosion cracking, and no relevant data is disclosed. In addition, none of the Documents discloses data related to cutting resistance in high-speed machining, either except for Patent Document 7.
• the present invention is cooled in a manner basically contrary to that disclosed by Patent Documents 2 to 7. Specifically, in the present invention, the cooling treatment performed in the cooling process after hot working is started when the temperature of hot-worked material is lower than 670°C and higher than 500°C, and the average cooling rate is higher than 300°C/min in temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C.
• ⁇ 1 phase is formed when the following conditions are met.
• ⁇ phase in which certain amounts of Si and P are solid-solubilized is formed; a cooling treatment is started when the temperature of hot-worked material is higher than 500°C; and the average cooling rates in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C are both higher than 300°C/min.
• ⁇ 1 phase is thus formed.
• Patent Documents 2 to 7 disclose pictures of metallographic structures obtained by etching with a mixed solution of hydrogen peroxide and ammonia water, but no grain boundary is observed inside ⁇ phase in any of the pictures. In these prior art documents, starting temperature of the cooling treatment performed after hot working is not defined.
• ⁇ 1 phase is able to improve dezincification corrosion resistance and resistance to stress corrosion cracking significantly, and better improve machinability than ⁇ phase.
• its machinability improvement effect excels that of the presence of phosphorous compounds.
• ⁇ phase for ⁇ 1 phase to exhibit such effects, its proportion in the metallographic structure needs to be larger than 25% in terms of area ratio.
• ⁇ phase is effective to obtain good machinability in Patent Documents 2 to 7.
• ⁇ 1 phase refers to modified ⁇ phase.
• 0 ⁇ f 5 ⁇ ⁇ 4
• the metallographic structure of the present invention consists of ⁇ phase, ⁇ 1 phase, and in some cases, a tiny amount of ⁇ phase which may not be present at all.
• ⁇ phase and ⁇ 1 phase The difference between ⁇ phase and ⁇ 1 phase is that in the case of ⁇ 1 phase, a grain boundary pattern, i.e., boundary of crystal grain is observed inside ⁇ 1 phase when etched with a mixed solution of hydrogen peroxide and ammonia water, but no crystal grain boundary is observed inside ⁇ phase.
• ⁇ 1 phase of an alloy according an embodiment of the present invention is obtained by causing certain amounts or more of Si and P to solid-solubilize in ⁇ phase under high temperature during hot working and maintaining the state of ⁇ phase when it is between 500°C and 670°C until the temperature of alloy goes down to normal temperature through a cooling treatment (i.e., cooling the alloy at an average cooling rate exceeding 300°C/min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C and continuing to cool it until the temperature of the alloy becomes normal temperature) so that the state of the metallographic structure when its temperature is between 500°C and 670°C is maintained until the temperature of the alloy becomes normal temperature.
• a cooling treatment i.e., cooling the alloy at an average cooling rate exceeding 300°C/min in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C and continuing to cool it until the temperature of the alloy becomes normal temperature
• the alloy is rapidly cooled at a high cooling rate in the temperature range from a high starting temperature of cooling to 300°C then is kept cooling until its temperature goes down to a normal temperature of 100°C or lower.
• the state of metallographic structure under high temperature can be maintained until after it is cooled down to normal temperature.
• ⁇ 1 phase is obtained. Even if the aforementioned cooling treatment is performed on ⁇ phase of a Cu-Zn-based alloy not containing both of Si and P in certain amounts, ⁇ 1 phase cannot be obtained.
• the cooling treatment is performed on an alloy containing Si and P with a starting temperature below 500°C, e.g., 450°C and a cooling rate exceeding 300°C/min, ⁇ 1 phase cannot be obtained.
• ⁇ phase is required in a certain amount or more. Therefore, if ⁇ phase is not present or the amount of ⁇ phase is insufficient, ⁇ 1 phase cannot be obtained.
• the degree of the modification is also affected by the amounts of Si, P, and inevitable impurities, the starting temperature of the cooling treatment, and the average cooling rates in temperature ranges from the starting temperature to 500°C and from 500°C to 300°C.
• Modification of ⁇ phase generally starts to peak when the amount of Si is about 1 mass% and that of P is about 0.1 mass% although that depends on the amount of inevitable impurities such as Fe. If the amounts of Si and P are excessive, adverse effects such as deterioration in conductivity of ductility, appearance of y phase, and the like may be induced.
• ⁇ 1 phase (modified ⁇ phase) is able to overcome the drawbacks of a Cu-Zn-based alloy, i.e., its dezincification corrosion resistance and resistance to stress corrosion cracking which used to be major challenges.
• dezincification corrosion is a significant problem that a Cu-Zn-based alloy containing ⁇ phase has.
• the amount of ⁇ phase is limited to 25% or less or 20% or less, and a heat treatment is performed at a temperature between 350°C and 550°C to fragment ⁇ phase and reduce the amount of ⁇ phase because dezincification corrosion occurs along ⁇ phase.
• Modified ⁇ phase i.e., ⁇ 1 phase has a higher strength and better ductility than ⁇ phase. Therefore, strength and ductility in an alloy with ⁇ 1 is well-balanced.
• the area ratio of ⁇ 1 phase needs to be higher than 25% in order to obtain good machinability while minimizing the Pb content. Further, in order to improve machinability and strength, the area ratio is preferably 30% or higher, more preferably 33% or higher. On the other hand, if the proportion of ⁇ phase is excessive, for instance, 95%, ⁇ phase will not be modified to ⁇ 1 phase. Modification of ⁇ phase occurs where ⁇ phase is present. Therefore, a certain amount of ⁇ phase is necessary.
• ⁇ 1 phase is able to delay the progress of corrosion much better than ⁇ phase
• its dezincification corrosion resistance and resistance to stress corrosion cracking are still much weaker than those of ⁇ phase.
• ⁇ phase is selectively corroded by dezincification, and the depth of such corrosion is as deep as about 500 ⁇ m.
• the metallographic structure consists of ⁇ phase and ⁇ 1 phase (modified ⁇ phase)
• progress of dezincification corrosion is significantly suppressed since even though ⁇ 1 phase is selectively corroded by dezincification, the depth of corrosion is about 20 ⁇ m to about 200 ⁇ m depending on the degree of modification and the area ratio of ⁇ 1 phase.
• dezincification corrosion resistance improves significantly.
• dezincification corrosion resistance and ductility of ⁇ 1 phase are not as good as those of ⁇ phase.
• the area ratio of ⁇ 1 phase is high, dezincification corrosion resistance and ductility of the alloy are poor.
• the area ratio is preferably 70% or lower, more preferably 65% or lower.
• the present invention basically consists of ⁇ phase and ⁇ 1 phase. Although a treatment is performed to modify ⁇ phase into ⁇ 1 phase, this treatment little affects ⁇ phase. Further, for the modification of ⁇ phase, a certain amount or more of ⁇ phase is necessary, and if the amount of ⁇ 1 phase is excessive, ductility of the alloy is poor. Therefore, an appropriate amount of ⁇ phase, a phase that has good ductility, is required. However, if the amount of ⁇ phase is excessive, strength of the alloy is low. ⁇ phase including Si has only slightly better machinability than that excluding Si. From a standpoint of machinability also, the amount of ⁇ phase is limited.
• the cutting resistance of the present invention can be maintained to be low, and well-fragmented chips are generated if the proportion of ⁇ phase is up to about 75% since ⁇ phase functions as a cushioning material and a stress concentration source at boundary with hard ⁇ 1 phase during machining.
• ⁇ phase is tiny and has a granular shape since it functions as a cushioning material and a stress concentration source at boundary with hard ⁇ 1 phase during machining.
• the amount of ⁇ phase needs to be 20% or higher, preferably 30% or higher, and more preferably 35% or higher.
• the upper limit of ⁇ phase is lower than 75%, preferably 70% or lower, and more preferably 67% of lower.
• ⁇ phase is a phase that contributes to machinability in a Cu-Zn-Si alloy in which the Cu concentration is about 69 mass% to about 80 mass% and the Si concentration is about 2 to about 4 mass%.
• Patent Document 15 discloses that ⁇ phase is indispensable for a Cu-Zn-Si alloy free of Pb.
• Patent Documents 2 to 6 disclose that ⁇ phase containing Si has good machinability like ⁇ phase containing Si.
• f6 is a simple conditional expression for an alloy to obtain good machinability.
• the amount of ⁇ 1 phase and the amounts of Si, Pb, Bi, and P contained in the alloy within the respective compositional ranges defined in the instant specification are put together and regarded to have a positive effect and the amount of ⁇ phase is regarded to have a negative effect.
• the amount of ⁇ phase multiplied by a coefficient of 2 is deducted from the total of the amount of ⁇ 1 phase containing Si and P multiplied by the amount of Si raised to the power of 1/2, the sum of the amounts of Pb and Bi raised to the power of 1/2 multiplied by a coefficient of 20, and the amount of P raised to the power of 1/2 multiplied by a coefficient of 15.
• ⁇ 1 phase (modified ⁇ phase) is directly affected by the concentration of Si as well as that of P, and when a tiny amount of Pb or Bi is contained, its machinability improves.
• ⁇ phase hinders machinability in the present invention.
• the inventors have revealed that the degree of machinability improvement brought by Pb or Bi is deeply related to a total amount of Pb and Bi raised to the power of 1/2. Both elements exhibit a significant effect even if the content is very small. As the contents are increased, their machinability improvement effects improve, but intensity of the effect gradually subsides.
• the f6 value is higher than 27, preferably 33 or higher, more preferably higher than 35, and still more preferably 38 or higher. As the f6 value gets higher, machinability becomes closer to that of a free-cutting brass bar containing 3 mass% Pb.
• FIGs. 1A to 6B show pictures of the metallographic structures of various alloys and the results of the dezincification corrosion tests performed in accordance with ISO 6509.
• Fig. 1A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S01 to Step No. A2. More specifically, Alloy No. S01 has a composition including 63.3 mass% Cu, 0.95 mass% Si, 0.069 mass% P, 0.063 mass% Pb, 0.017 mass% Bi, and Zn as the balance.
• Step No. A2 the alloy was subjected to hot extrusion performed at 630°C and a cooling treatment which was started when the temperature of the hot-extruded material became 580°C and performed at an average cooling rate of 1020°C/min in both temperature ranges from 580°C to 300°C and from 500°C to 300°C.
• the average cooling rate in the temperature range from 580°C, the temperature at which the cooling treatment was started, to 500°C can be easily calculated from the average cooling rates in temperature ranges from 580°C to 300°C and from 500°C to 300°C.
• Fig. 1B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 1A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.
• Fig. 2A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S11 to Step No. E2. More specifically, Alloy No. S11 has a composition including 62.5 mass% Cu, 0.96 mass% Si, 0.064 mass% P, 0.072 mass% Pb, and Zn as the balance.
• Step No. E2 the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C/min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm.
• the rod was heated and hot forged at 680°C to have a thickness of 20 mm when placed horizontally then subjected to a cooling treatment which was started when its temperature became 565°C and performed at an average cooling rate of 900°C/min in both temperature ranges from 565°C to 300°C and from 500°C to 300°C.
• Fig. 2B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 2A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.
• Fig. 3A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S11 to Step No. E13H. More specifically, Alloy No. S11 has a composition including 62.5 mass% Cu, 0.96 mass% Si, 0.064 mass% P, 0.072 mass% Pb, and Zn as the balance.
• Step No. E13H the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C/min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm.
• the rod was heated and hot forged at 630°C to have a thickness of 20 mm when placed horizontally then cooled at an average cooling rate of 35°C/min. Subsequently, the rod was subjected to a cooling treatment which was started when its temperature became 455°C and performed at an average cooling rate of 800°C/min in the temperature range from 455°C to 300°C. Note that the starting temperatures of the cooling treatment performed on the alloy in Fig. 3A and that in Fig. 2A are different.
• Fig. 3B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 3A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.
• Fig. 4A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S43 to Step No. E6. More specifically, Alloy No. S43 has a composition including 63.3 mass% Cu, 0.98 mass% Si, 0.084 mass% P, 0.060 mass% Pb, 0.28 mass% Sn, and Zn as the balance.
• Step No. E6 a casting produced with a mold having a diameter of 55 mm was machined to a diameter of 50 mm then cut to a length of 200 mm.
• the casting was heated and hot forged at 630°C to a thickness of 20 mm when placed horizontally then subjected to a cooling treatment which was started when the temperature of the casting became 565°C and performed at an average cooling rate of 900°C/min in both temperature ranges from 565°C to 300°C and from 500°C to 300°C.
• Fig. 4B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 4A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.
• Fig. 5A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step Nos. A34H and G1. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.98 mass% Si, 0.071 mass% P, 0.071 mass% Pb, and Zn as the balance.
• Step No. A34H an alloy was obtained by hot extrusion at 615°C and cooling at an average cooling rate of 18°C/min in the temperature range from 500°C to 300°C.
• the alloy was further heated to 580°C for 30 minutes then subjected to a cooling treatment which was started when its temperature became 560°C and performed at an average cooling rate of 1800°C/min in both temperature ranges from 560°C to 300°C and from 500°C to 300°C.
• Fig. 5B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 5A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.
• Fig. 6A is a picture showing the structure of a copper alloy according to an embodiment, which was obtained by subjecting Alloy No. S02 to Step No. E14H. More specifically, Alloy No. S02 has a composition including 62.9 mass% Cu, 0.98 mass% Si, 0.071 mass% P, 0.071 mass% Pb, and Zn as the balance.
• the alloy was hot extruded at 550°C and cooled at an average cooling rate of 20°C/min in the temperature range from 500°C to 300°C to obtain a rod with a diameter of 50 mm and a length of 200 mm.
• the rod was heated and hot forged at 630°C to a thickness of 20 mm when placed horizontally then left to cool naturally.
• the average cooling rate in the temperature range from 500°C to 300°C was 25°C/min.
• Fig. 6B is a cross-sectional picture of the metallographic structure of the alloy shown in Fig. 6A including the portion where corrosion depth was maximum as a result of a dezincification corrosion test performed on the alloy in accordance with ISO 6509.
• a grain boundary pattern i.e., crystal grain boundary is observed inside ⁇ 1 phase together with that of ⁇ phase crystal grain.
• Crystal grain boundary here refers to a linear pattern observed inside ⁇ 1 phase that runs through a crystal grain of ⁇ 1 phase like the one shown in Fig. 1A .
• a grain boundary is observed inside modified ⁇ phase, i.e., ⁇ 1 phase, but in Fig. 3A , only something that looks like a faint black line is observed inside ⁇ phase, and there is no grain boundary that runs through a crystal grain of ⁇ phase.
• black granular precipitates of about 0.5 to 3 ⁇ m are present mainly inside ⁇ phase and on the phase boundaries between ⁇ phase and ⁇ phase in Figs 3A and 6A .
• the granular precipitates are mainly composed of phosphorous compounds, but Pb particles, mixed particles of Pb and Bi, compounds of Fe and the like, oxides, and sulfides are also included.
• Figs 1B , 2B , 3B , 4B , 5B , and 6B Results of the dezincification corrosion tests performed on these alloys in accordance with ISO 6509 are shown in Figs 1B , 2B , 3B , 4B , 5B , and 6B .
• the maximum corrosion depth of the test pieces in which a grain boundary was observed inside ⁇ 1 phase was 120 ⁇ m or less, but that of the test pieces in which grain boundary was not observed inside ⁇ phase was 350 ⁇ m ( Fig. 3A ) or 460 ⁇ m ( Fig. 6A ), indicating a difference of 3 to 5 times the corrosion depth.
• the corrosion depth of the test piece containing 0.28 mass% Sn was 40 ⁇ m, which was a particularly good result ( Fig. 4A ).
• the type of dezincification corrosion was selective corrosion of ⁇ 1 phase or ⁇ phase in all cases. Therefore, dezincification corrosion occurs in these two phases first, but progress of the corrosion in ⁇ 1 phase is approximately 3 times slower than in ⁇ phase.
• hot extruded materials, hot rolled materials, and hot forged materials have a high strength with a tensile strength of 460 N/mm 2 without cold working performed after hot working.
• the tensile strength is more preferably 490 N/mm 2 or higher and still more preferably 520 N/mm 2 or higher.
• Many valves, joints, and components used for pressure vessel, air conditioner, or freezer are made of a hot-extruded or hot-forged material.
• a leaded copper alloy currently used for these applications has a tensile strength of about 390 to 420 N/mm 2 and elongation of about 30% to 35%, it is possible to reduce the weights of such components by increasing the strength of the material.
• a material to be machined has resistance to fracture since it may be subjected to cold working such as light swaging or bending.
• cold working such as light swaging or bending.
• some kind of brittleness is required so that well-broken chips are generated, but machinability is a property opposite to cold workability.
• tensile strength and ductility are opposite properties, but it is desirable that tensile strength and ductility (elongation) are very well-balanced.
• both an elongation of 10% or higher and a tensile strength of 520 N/mm 2 or higher can be obtained.
• the value of f7 of the previously mentioned leaded copper alloy is about 470.
• Applications of the embodiments include electrical or electronic apparatus components, components of automobiles that are increasingly powered by electricity, and other parts and components having high conductivity.
• phosphor bronzes including 6 mass% or 8 mass% Sn JIS standard alloys C5191 and C5210) are widely used for these applications, and their electrical conductivities are about 14% IACS and 12% IACS, respectively. Accordingly, if the embodiments have an electric conductivity of 15% or higher, no problem should occur.
• the upper limit of the electrical conductivity is not particularly defined because improvement of conductivity rarely causes practical problems.
• the free-cutting copper alloys of the embodiments are characterized by their excellent deformability when they are at a temperature between 540°C and 750°C. Due to this characteristic, they can be hot-extruded into a bar with a small cross-sectional area or formed into a complex shape by forging. From the viewpoints of energy saving and allowing ⁇ phase to have a favorable granular shape, hot working temperature is preferably lower than 750°C and more preferably lower than 720°C. From the viewpoint of thermal deformation resistance, it is preferably higher than 540°C and more preferably higher than 560°C.
• the metallographic structures of the alloys according to the embodiments vary depending not only on the composition but also on the production process. They are affected by the average cooling rate in the process of cooling after hot working or heat treatment in addition to hot working temperatures in hot extrusion and hot forging as well as heat treatment conditions. As a result of a devoted study, it was found that the metallographic structures are significantly affected by the starting temperature of the cooling treatment, the average cooling rates in temperature ranges from the starting temperature to 500°C and from 500°C to 300°C in the process of cooling after hot working or heat treatment.
• Melting is performed at a temperature between about 950°C and about 1200°C, a temperature that is about 100°C to about 300°C higher than the melting point (liquidus temperatures) of the alloy according to an embodiment of the present invention.
• a molten metal is cast into a specific mold when its temperature is about 900°C to about 1100°C, which is about 50°C to about 200°C higher than the melting point, then cooled by several cooling means such as air cooling, slow cooling, and water cooling. After the alloy solidifies, constituent phases of the alloy change in various ways.
• hot working examples include hot extrusion, hot forging, and hot rolling.
• the final hot working step is performed under the following condition.
• hot extrusion in a preferable embodiment, it is performed so that the material's temperature immediately after it is hot worked (extrusion temperature) is higher than 540°C and lower than 750°C although it depends on the extrusion ratio (hot working ratio) and the capacity of the extrusion facility used.
• Extruded bar is either coiled, or if the cross-sectional area of the bar is large, placed onto a table in a straight shape.
• the lower limit of the hot extrusion temperature relates to hot deformation resistance. The lower the extrusion temperature, the finer and granular become the ⁇ phase grains and the more improve the dezincification corrosion resistance, resistance to stress corrosion cracking, and machinability.
• the extrusion temperature is preferably 560°C or higher in view of the capacity of the extruder and the starting temperature of the cooling treatment described later.
• the upper limit of hot extrusion temperature relates to the shape of ⁇ phase.
• Extrusion temperature is preferably 720°C or lower.
• the shape of ⁇ phase crystal grains relates to the composition relational expression f1, and when the value of f1 is 59.0 or lower, preferable extrusion temperature is lower than 720°C.
• ⁇ phase can be modified and a material having better machinability, good dezincification corrosion resistance, and good resistance to stress corrosion cracking can be obtained. That is, in the cooling process after hot extrusion, the cooling treatment is started when the temperature of the extruded material is lower than 670°C and higher than 500°C then the material is cooled with the average cooling rates in the temperature ranges from the starting temperature to 500°C and from 500°C to 300°C set higher than 300°C/min at least, preferably higher than 600°C/min, and more preferably 900°C/min or higher.
• the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C and that in the temperature range from 500°C to 300°C are approximately the same, or the former is slightly higher in some cases.
• the more preferable the cooling rate the better modified becomes ⁇ 1 phase.
• the upper limit of the average cooling rates in temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C is not particularly defined since ordinary production facility is unable to cool hot-extruded material excessively fast, but as a reference, the cooling rate is preferably 9000°C/min or lower. If the cooling is performed at an average cooling rate exceeding 300°C/min, modification of ⁇ phase occurs, and a grain boundary can be found inside modified ⁇ phase when etched with a mixed solution of hydrogen peroxide and ammonia water and its metallographic structure is observed with a metallographic microscope at a magnification of 500X.
• the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C or from 500°C to 300°C is 300°C/min or lower, no such grain boundary can be observed inside ⁇ phase.
• the cooling rate in the cooling from a temperature lower than 300°C to normal temperature tends to decline slightly as the temperature approaches to normal temperature, but it is desirable to keep cooling in the same manner as in the temperature range from 500°C to 300°C.
• starting temperature of the cooling treatment also affects the modification of ⁇ phase.
• the starting temperature is preferably 530°C or higher, more preferably 500°C or higher.
• the cooling treatment is started at a temperature lower than 550°C, if the cooling rate from 550°C to the starting temperature is low, compounds of P and Zn or P and Zn and/or Si begin to form.
• the starting temperature is 530°C or lower, formation of phosphorous compounds is further promoted. Although phosphorous compounds are scarcely formed when cooled at a cooling rate exceeding 300°C/min, if the starting temperature of the cooling treatment is lower than 550°C, such compounds are observed in the metallographic structure.
• the starting temperature needs to be lower than 670°C, and is preferably lower than 650°C.
• hot working temperature is defined as temperature of a hot worked material which can be measured about two or three seconds after hot extrusion, hot forging, or hot rolling is completed.
• the metallographic structure is affected by the temperature immediately after working where large plastic deformation occurs.
• hot forging As a material for hot forging, a hot extruded material is mainly used, but a continuously cast bar is also used. There is no need to manipulate the cooling process of the forging material since hot forging is not the final hot working step. Compared with hot extrusion, in hot forging, working speed is faster, and a more complex shape is formed. In some cases, hot forging is performed at a high working ratio up to a thickness of about 3 mm. Further, weight of forged product varies from a few tens of grams to a few kilograms, and small ones cool rapidly immediately after forging and the rapid cooling continues thereafter. Accordingly, forging material is heated to a higher temperature than the temperature to which ingot is heated for hot extrusion. In a preferred embodiment, the temperature of a hot forged product, that is, the material's temperature about two or three seconds immediately after forging is preferably higher than 540°C and lower than 750°C.
• forging temperature is related to the composition relational expression f1, and when the value of f1 is 59.0 or lower, it is preferable that hot forging is performed at a temperature lower than 720°C.
• the lower the forging temperature the smaller becomes the diameter of ⁇ phase crystal grain, the more likely to change the shape of ⁇ phase crystal grain from acicular to granular, the higher the strength, and the better the balance between strength and ductility, the machinability, the dezincification corrosion resistance, and the resistance to stress corrosion cracking.
• a material with good dezincification corrosion resistance, resistance to stress corrosion cracking, and machinability can be obtained. That is, in the cooling process after hot forging, a cooling treatment is started when the temperature of the forged material is lower than 670°C and higher than 500°C then the material is cooled with the average cooling rates in the temperature ranges from the starting temperature to 500°C and from 500°C to 300°C set to be higher than 300°C/min at least, preferably higher than 600°C/min, and more preferably 900°C/min or higher.
• the average cooling rates in the temperature range from the starting temperature of the cooling treatment to 500°C and that in the temperature range from 500°C to 300°C are approximately the same or the former is slightly higher.
• the average cooling rate in the temperature range from 500°C to 300°C is 300°C/min or lower, no such grain boundary can be observed inside ⁇ phase.
• the cooling rate in the cooling from a temperature lower than 300°C to normal temperature tends to decline slightly as the temperature approaches to normal temperature, but it is desirable to keep cooling in the same manner as in the temperature range from 500°C to 300°C.
• the upper limit of the average cooling rates in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C is not particularly defined, but as a reference, the cooling rate is preferably 9000°C/min or lower.
• starting temperature of the cooling treatment also affects the modification of ⁇ phase.
• the starting temperature is preferably higher than 530°C, more preferably higher than 550°C.
• the cooling treatment is started at a temperature lower than 550°C, if the cooling rate from 550°C to the starting temperature is low, compounds of P and Zn or P and Zn and/or Si begin to form.
• the starting temperature is 530°C or lower, formation of phosphorous compounds is further promoted. Although phosphorous compounds are scarcely formed when cooled at a cooling rate exceeding 300°C/min, if the starting temperature of the cooling treatment is lower than 550°C, such compounds are observed in the metallographic structure.
• the starting temperature needs to be lower than 670°C, and is preferably lower than 650°C.
• the cooling treatment is started after holding the material in a simple furnace in which the atmospheric temperature is set between about 550°C and about 600°C for a period from a few tens of seconds to a few minutes, more uniform and stable forged product can be obtained.
• shot blasting performed upon completion of the aforementioned steps of hot forging and cooling treatment makes an effective mean for the improvement of resistance to stress corrosion cracking.
• resistance to stress corrosion cracking can be improved by shot blasting or any other way of applying compressional stress on the surface of alloy, but the effect of shot blasting is particularly good in the case of the alloys of the present invention containing ⁇ 1 phase.
• the material's temperature upon completion of the final hot rolling is preferably higher than 540°C and lower than 750°C, more preferably lower than 670°C.
• a cooling treatment is performed with a starting temperature set to be higher than 500°C and lower than 670°C and an average cooling rate in the temperature ranges from the starting temperature to 500°C and from 500°C to 300°C set to be higher than 300°C/min at least, preferably higher than 600°C/min, and more preferably 900°C/min or higher.
• the cooling rate in the cooling from a temperature lower than 300°C to normal temperature tends to decline slightly as the temperature approaches to normal temperature, but it is desirable to keep cooling in the same manner as in the temperature range from 500°C to 300°C.
• the upper limit of the average cooling rates in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C is not particularly defined, but as a reference, the cooling rate is preferably 9000°C/min or lower.
• ⁇ phase cannot be modified.
• wire or bar with a small diameter is manufactured, or if cold working or any step involving heating such as annealing is included in the production process, modification of ⁇ phase basically does not occur.
• ⁇ phase is modified to form ⁇ 1 phase if annealing is performed at a temperature higher than 520°C and lower than 630°C for one minute to five hours and a cooling treatment is started in the cooling step after annealing when the temperature of annealed material is higher than 500°C and performed at an average cooling rate of higher than 300°C/min in both temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C.
• the aforementioned annealing conditions need to be applied only to the last annealing step if annealing is performed a multiple number of times.
• annealing temperature is preferably 530°C or higher and more preferably 550°C or higher.
• the average cooling rate in the temperature range from 500°C to 300°C is preferably set to be higher than 600°C/min, and more preferably 900°C/min or higher to enhance the degree of modification into ⁇ 1 phase. This way of heat treatment is applied to hot-worked materials which cannot be cooled with the previously described conditions after hot working, those which are cold worked after hot working then annealed at least once, or the like.
• the upper limit of the average cooling rates in the temperature ranges from the starting temperature of the cooling treatment to 500°C and from 500°C to 300°C is not particularly defined, but as a reference, the cooling rate is preferably 9000°C/min or lower.
• cold working is sometimes performed on extruded straight or coiled material in order to obtain high strength, improve dimensional accuracy, and straighten or reduce the degree of bending of such material.
• cold drawing is performed on a hot extruded material at a cold working ratio of about 2% to about 30% which is followed by straightness correction in some cases.
• cold working and heat treatment are repeatedly performed. After the previously-described final heat treatment, final cold working is performed at a working ratio of 0% to about 30% which is followed by straightness correction. The closer to 0% the cold working ratio, the less likely to crack becomes the material when it is subjected to cold working such as light swaging or bending.
• low-temperature annealing is sometimes performed at a temperature lower than or equal to the recrystallization temperature in the final step.
• modification into ⁇ 1 phase which was achieved by hot working step and heat treatment may be impaired if the alloy is heated. For instance, if it is heated to 300°C for two hours, modification of ⁇ phase becomes impaired and ⁇ 1 phase returns to ⁇ phase, which causes the machinability, the dezincification corrosion resistance, and the resistance to stress corrosion cracking once improved by ⁇ 1 phase to return to those of the original alloy comprising ⁇ phase. Therefore, annealing is not recommended, but may be performed if at a temperature below about 150°C.
• Free-cutting alloy of an embodiment of the present invention with the above-described constitution has excellent machinability, good dezincification corrosion resistance and resistance to stress corrosion cracking as well as excellent hot workability, high strength, and balance between strength and ductility even though the Pb content is small since the alloy composition, the composition relational expressions, the metallographic structure, the metallographic structure relational expressions, and the metallographic structure and composition relational expression are defined as described above.
• Embodiments of the present invention are as hereinabove described. It should be noted, however, that the present invention is not limited to the embodiments and can be modified as appropriate within a scope not deviating from the technical requirements of the present invention.
• Tables 5 to 7 show the alloy compositions.
• Tables 8 to 17 show the production steps employed.
• Mm refers to mischmetal and represents the total content of rare earth elements. Each of the production steps is described below.
• the cooling rate in hot working, i.e., hot extrusion, hot forging, or hot compression in Tables 8 to 12 and 14 to 16 refers to the average cooling rate between the points of completion of hot working and commencement of cooling treatment. [Table 5] Alloy No.
• a billet with a diameter of 240 mm was produced with the low-frequency melting furnace and the semi-continuous casting machine used for the manufacture on the actual production line. For raw materials, those correspond to ones used for the manufacture on the actual production line were used.
• the billet was cut to a length of 800 mm then heated.
• an indirect extruder with a nominal capacity of 2750 tons, round bars with a diameter of 20.9 mm were extruded and coiled in a container located in a short distance away from the extruder. In this container, water quantity can be adjusted during water cooling. When the temperature of extruded material reached a specific temperature in the container, the cooling treatment (water cooling) was started with water quantity being adjusted.
• the cooling treatment was started as soon as extruded material was placed in the container. Temperature of extruded material was measured with a radiation thermometer when the material came out of the extruder, when the cooling treatment was started, and when the material's temperature became 500°C and 300°C after that. Apropos, for the measurement of temperature during hot extrusion and hot forging in other steps, IGA 8Pro/MB20, a radiation thermometer manufactured by LumaSense Technologies Inc., and a contact thermometer were both used.
• Step Group AA i.e., Steps Nos. A1 to A5 and A11H to A14H
• extrusion temperature was 630°C
• Step Group AB i.e., Steps Nos. A21 to A25 and A31H to A34H
• Step Group AD i.e., Steps Nos. A41 to A44, A46H, and A47H
• the cooling treatment was started when coiled extruded bars with a diameter of 20.9 mm reached a certain temperature in the container, and the time it took for the temperature of the material to become 500°C and 300°C were measured.
• the cooling treatment was continued with the same conditions until the temperature of the coiled material became around 100°C or lower.
• the average cooling rate from completion of extrusion to commencement of the cooling treatment, the starting temperature of the cooling treatment, the average cooling rates in temperature ranges from the starting temperature of the cooling treatment to 300°C and from 500°C to 300°C in Steps Nos.
• A1 to A5. A11H to A14H, A21 to A25, A31H to A34H, A41 to A44, A46H, and A47H are not necessarily the same.
• the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 300°C was recorded as the cooling rate in the temperature range from 500°C to 300°C.
• the average cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C can be easily calculated from the average cooling rates in temperature ranges from the starting temperature to 300°C and from 500°C to 300°C, and it was confirmed that the cooling rate in the temperature range from the starting temperature of the cooling treatment to 500°C is either the same as the cooling rate in the temperature range from 500°C to 300°C or slightly faster than that when the cooling treatment was performed with certain conditions.
• extruded material was formed into straight bar with a diameter of 20 mm by a combined machine through drawing at a working ratio of 8.4% and straightness correction.
• Steps Nos. A25 and A31H a bar obtained in Step No. A21 was further subjected to low-temperature annealing in a laboratory at 130°C for five hours then at 300°C for two hours.
• These materials of Steps Groups AA, AB, and AD were subjected to microscopic observation, cutting tests, dezincification corrosion tests, and tensile tests.
• Step Group AC Step No. A50
• a billet with a diameter of 240 mm was produced with the low-frequency melting furnace and the semi-continuous casting machine used for the manufacture on the actual production line.
• the billet was cut to a length of 800 mm then extruded at 550°C into ⁇ 50-mm round bars with a direct extruder having a nominal capacity of 3000 tons.
• the extruded straight bars were placed on a steel table for natural cooling to normal temperature then subjected to straightness correction and provided as material for the flat forging described later. No special cooling treatment was performed since they were to be provided as a material of a product produced by a process in which hot forging is the final hot working step.
• the average cooling rate from 500°C to 300°C was 20°C/min.
• Step Group B raw material composed of components mixed at a specific ratio was melted in a laboratory as described in Table 12.
• the molten alloy was poured into a permanent mold with a diameter of 100 mm and a length of 200 mm to cast a billet.
• impurities such as Fe were further added to some of them intentionally.
• the concentration of the intentionally-added impurities was approximately the same as or lower than that included in leaded brass available in the market.
• Step No. B1H The billet was heated and extruded at 670°C into a ⁇ 20-mm round bar in the case of Step No. B1. Following the extrusion, cooling treatment was started when the temperature of the extruded bar was 540°C and performed at an average cooling rate of 960°C/min in the temperature range from 500°C to 300°C. It was confirmed that the average cooling rates in temperature ranges from 540°C to 500°C and from 500°C to 300°C were almost the same. Cooling was continued with the same conditions until the material's temperature became about 100°C or lower. In Step No. B3H, ⁇ 20-mm round bars were produced by extrusion with an extrusion temperature of 670°C and cooling at an average cooling rate of 40°C/min from 500°C to 300°C without performing any cooling treatment.
• Steps Nos. B1 and B3H were subjected to microscopic observation, cutting tests, dezincification corrosion tests, and tensile tests. Part of those produced by Step No. B3H were provided as hot forging material of Step Group D.
• Step No. B2 the billet was extruded at an extrusion temperature of 590°C into ⁇ 50-mm bars to be provided as forging material then cooled from 500°C to 300°C at an average cooling rate of 25°C/min without performing any cooling treatment.
• Step Group C material for forging or hot compression: casting
• Alloy X was subjected to observation with a metallographic microscope, cutting tests, dezincification corrosion tests, and tensile tests.
• Alloy Y was subjected to observation with a metallographic microscope, cutting tests, dezincification corrosion tests, and tensile tests.
• Alloy Z was subjected to observation with a metallographic microscope, dezincification corrosion tests, and stress corrosion cracking tests.
• test piece was evaluated as "D" (poor). If a test piece had any crack with a size of about 1/5 the length of the test piece (26 mm) or larger, i.e., 5 mm or larger but smaller than 13 mm that was clearly visible with eye, the evaluation was "C” (fair). If any crack smaller than about 1/5 of the length of the test piece (26 mm), i.e., smaller than 5 mm was observed or no crack was visible with eye, the evaluation was "B” (good). Incidentally, those evaluated as "C” were regarded acceptable.
• alloys in which the content of each of the alloying elements, values of the composition relational expressions and the metallographic structure relational expressions are within respective appropriate ranges have excellent hot workability and good machinability, mechanical characteristics, dezincification corrosion resistance, and resistance to stress corrosion cracking.
• Embodiments of the present invention are able to obtain excellent properties by appropriately adjusting the conditions in hot extrusion, hot forging, and heat treatment processes.
• the free-cutting copper alloys of the embodiments have excellent hot workability and machinability, high strength, and excellent balance between strength and elongation although they contain only a small amount of Pb. Therefore, the free-cutting copper alloys are suitable for devices and components related to drinking water or sanitary facility, food-related devices, electrical or electronic apparatus components, auto parts, mechanical components, stationaries, toys, musical instruments, sliding components, measuring instrument components, precision mechanical components, medical components, drink-related devices and components, water meters, and components involving liquid or gas such as industrial water, drainage water, or hydrogen.
• the free-cutting copper alloys can be suitably applied to the items used in the above-mentioned fields that go by the names including faucet, shutoff valve, mixer tap, shower head, valve, joint, cock, gear, axle, bearing, trumpet, shaft, sleeve, spindle, sensor, bolt, nut, flare nut, pen point, insert nut, cap nut, nipple, spacer, and screw as their material or the like.

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