Patent Publication Number: US-2011056591-A1

Title: Brass alloy powder, brass alloy extruded material, and method for producing the brass alloy extruded material

Description:
TECHNICAL FIELD 
     The present invention relates to high strength brass alloys, and more particularly to brass alloy powder and brass alloy extruded materials that are free of lead that is harmful to the environment and the human body. 
     BACKGROUND ART 
     In recent years, environmental issues have attracted considerable attention, and these issues need to be carefully considered in development of alloys. 6/4 brass is not only used as mechanical parts, but also used in a wide range of applications such as gas pipes, water pipes, and valves, due to its reasonable strength, satisfactory mechanical characteristics, and nonmagnetic property. 
     The alloy composition typically includes several percent of lead in order to increase workability of 6/4 brass members. If such lead-containing brass members are used in water pipes, lead can dissolve into the water supply. 
     Leadless brass materials have been developed or under development in order to solve this problem. Conventional development examples include: brass materials containing bismuth instead of lead; brass materials having y-phase precipitated by adding tin, as disclosed in Japanese Patent Publication No. 2000-309835 of unexamined applications (Patent Document 1) and International Patent Publication No. WO98/10106 (Patent Document 2); and brass materials having fine silicon particles dispersed therein. 
     These developed techniques include techniques that not only implement leadless brass materials, but also increase the strength of brass itself to increase the range of applications of the brass materials. 
     At present, however, the strength obtained by adding bismuth is about the same as that obtained by adding lead. Bismuth and lead are both elements that reduce the strength of brass when added to brass materials, and thus do not contribute to an increase in strength of brass members. The method for precipitating γ-phase by adding tin, as disclosed in Japanese Patent Publication No. 2000-309835 of unexamined applications (Patent Document 1) and International Patent Publication No. WO98/10106 (Patent Document 2), increases the proof stress, tensile strength, and the like of brass members, but significantly reduces deformability thereof, thereby reducing workability. Moreover, the method causes brittle fractures originating from the γ-phase. The method for dispersing fine silicon particles contributes to an increase in mechanical strength of brass alloy members, but is disadvantageous in that the machinability of the members is reduced. 
     A method for producing, based on a powder metallurgy method, a free-cutting brass alloy with graphite particles dispersed therein is disclosed in Katsuyoshi Kondoh et al., “Characteristics of Completely Leadless Free-Cutting Brass Alloy by Powder Process,” Collected Abstracts of 46th Technology Conference of Japan Copper and Brass Association (2006),  pp  153-154 (Non-Patent Document 1). Advantages of adding graphite are that completely leadless brass alloys can be obtained, and that graphite can be easily separated upon recycling as graphite floats on molten brass. However, adding graphite is not expected to increase the strength of brass members. Thus, techniques that improve the strength of brass members by using a powder metallurgy method should also be considered when adding graphite. 
     In general, when melting a low melting point metal in a high melting point metal by heating, the low melting point metal rapidly evaporates while being melted due to its high vapor pressure, and it is difficult to control the alloy to a desired alloy composition. 
     Brass is an alloy of copper and zinc. Adding a high melting point metal to brass can be expected to increase strength. However, the boiling point of zinc is as low as 907° C., and it is not easy to add chromium having a melting point of 1,907° C., vanadium having a melting point of 1,902° C., or the like to brass. Increasing the temperature of liquid-phase brass necessarily increases the amount of evaporation of zinc, and the alloy composition rapidly changes toward a copper-rich composition. 
     Examples of a method for melting a high melting point metal include an electron beam melting method, a hydrogen plasma arc melting method, and the like. However, these methods are not suitable for mass production, and are used in small batch processing of rare metals. Moreover, these methods cannot prevent evaporation of low melting point metals. 
     It is possible to add a molten high melting point metal to a low melting point metal, but industrially, melting high melting point metals by heating to their melting points is not economically reasonable, and mass production is difficult. Thus, a method using a thermite reaction of oxides, a method of adding a mother alloy having a lower melting point, and the like are commonly used in the art. 
     Japanese Patent Publication No. H10-168433 of unexamined applications (Patent Document 3) discloses a method for adding an alloy component to zinc. Although this patent publication describes that a mother alloy was used to add chromium, the Zn—Cr thermal equilibrium diagram shows that chromium is hardly solid-solved in zinc. In other words, it can be understood that Zn 17 Cr or Zn l3 Cr as a compound is dispersed in a zinc matrix. Adding this mother alloy to zinc merely increases the ratio of the zinc component, and does not cause any change in the chromium compound. Thus, it is very difficult to melt a high melting point metal as a non-solid-solution element in a low melting point metal, and other methods need to be developed. 
     Techniques of adding chromium to copper are more advanced as compared to techniques of zinc-containing alloys. Representative methods include methods disclosed in Japanese Patent Publications Nos. H11-209835 (Patent Document 4) and 2006-124835 (Patent Document 5) of unexamined applications. In the methods disclosed in these patent publications, chromium, zirconium, tellurium, sulfur, iron, silicon, titanium, or phosphorus is contained in copper. The alloys obtained by each method are precipitation-type copper alloys, and a copper-zirconium compound or the like is precipitated as a strengthening phase. However, unlike zinc-containing alloys, these alloys can be produced even at high temperatures, which facilitates fabrication of these materials. 
     It is known from the development of leadless brass that it is effective to use a powder metallurgy method as a method for adding graphite. This is mainly because graphite and brass can be mixed by using powder. If graphite is added by a normal ingot method, graphite floats on molten brass and cannot be dispersed therein due to the difference in specific gravity therebetween. 
     Related Art Documents 
     Patent Documents 
     Patent Document 1: Japanese Patent Publication No. 2000-309835 of unexamined applications 
     Patent Document 2: International Patent Publication No. 
     WO98/10106 
     Patent Document 3: Japanese Patent Publication No. H10-168533 of unexamined applications 
     Patent Document 4: Japanese Patent Publication No. H11-209835 of unexamined applications 
     Patent Document 5: Japanese Patent Publication No. 2006-124835 of unexamined applications 
     Non-Patent Document 
     Non-Patent Document 1: Katsuyoshi Kondoh et al., Collected Abstracts of 46th Technology Conference of Japan Copper and Brass Association (2006), pp 153-154 
     DISCLOSURE OF THE INVENTION 
     Problems to be Solved by the Invention 
     The inventors of the present application have been developing graphite-containing brass as part of development of leadless brass alloys. However, leadless free-cutting brass alloys having graphite particles dispersed therein have about the same strength as that of lead-containing free-cutting brass alloys, and the strength of such leadless free-cutting brass alloys is not dramatically increased. 
     It is an object of the present invention to provide brass alloy powder that contributes to an increase in strength of brass alloy members. 
     It is another object of the present invention to provide a brass alloy extruded material having high mechanical strength. 
     It is yet another object of the present invention to provide a brass alloy member having high mechanical strength. 
     It is a further object of the present invention to provide a method for producing a brass alloy extruded material having high mechanical strength. 
     Means for Solving the Problems 
     Brass alloy powder according to the present invention has a brass composition formed by a mixed phase of α-phase and (β-phase, and contains 0.5 to 5.0 mass % of chromium. The chromium includes a component that is solid-solved in a mother phase of brass, and a component that is precipitated at crystal grain boundaries. 
     A brass alloy extruded material having high mechanical strength is obtained by extruding an aggregate of the brass alloy powder. The chromium content needs to be 0.5 mass % or more to obtain desired mechanical strength. The chromium content in the brass alloy powder can be increased in order to further increase the mechanical strength of the final brass alloy extruded material. At present, however, the upper limit of the chromium content is 5.0 mass % due to manufacturing reasons. A more preferred chromium content is 1.0 to 2.4 mass %. 
     The chromium component forcibly solid-solved in the mother phase reduces dislocation motion in crystal, and contributes to an increase in proof stress. On the other hand, the chromium component precipitated at the crystal grain boundaries reduces grain boundary sliding to cause extreme work hardening, and contributes to an increase in tensile strength. The component that is solid-solved in the mother phase of the brass includes a component that is solid-solved and dispersed in the mother phase, and a component that is dispersed in the mother phase as precipitates. 
     The brass alloy powder may contain at least one element selected from the group consisting of nickel, manganese, zirconium, vanadium, titanium, silicon, aluminum, and tin. 
     Preferably, the brass alloy powder is rapidly solidified powder, and more preferably, is powder rapidly solidified by a water atomizing method. 
     A brass alloy extruded material according to the present invention is produced by extruding an aggregate of brass alloy powder having a brass composition formed by a mixed phase of α-phase and β-phase, and containing 0.5 to 5.0 mass % of chromium, wherein the chromium includes a component that is solid-solved in a mother phase of brass, and a component that is precipitated at crystal grain boundaries. 
     In one embodiment, the brass alloy extruded material has a 0.2% proof stress of 300 MPa or more. The brass alloy extruded material has a tensile strength of 500 MPa or more. 
     In one embodiment, in order to increase machinability of the brass alloy extruded material, the brass alloy extruded material is produced by adding 0.2 to 2.0 wt % of graphite particles to the brass alloy powder and mixing them together, and extruding the resultant mixed powder aggregate. Preferably, the graphite particles have a particle size of 1 μm to 100 μm. 
     A brass alloy member according to the present invention has a brass composition formed by a mixed phase of α-phase and β-phase, contains 0.5 to 5.0 mass % of chromium, and contains at least one element selected from the group consisting of nickel, manganese, zirconium, vanadium, titanium, silicon, aluminum, and tin. The chromium includes a component that is solid-solved in a mother phase of brass, and a component that is precipitated at crystal grain boundaries. 
     In one embodiment, in order to increase machinability of the brass alloy member, the brass alloy member further contains graphite particles. 
     A method for producing a brass alloy extruded material according to the present invention includes the steps of; producing, by using a rapid solidification method, brass alloy powder having a brass composition formed by a mixed phase of α-phase and β-phase, and containing 0.5 to 5.0 mass % of chromium; and extruding an aggregate of the rapidly solidified brass alloy powder. 
     Preferably, the rapid solidification method is a water atomizing method. Preferably, a heating temperature of the extrusion process is 650° C. or less. 
     In one embodiment, the method further includes the step of; before the extrusion process, adding 0.2 to 2.0 wt % of graphite particles to the brass alloy powder and mixing them together. 
     Functions, effects, and the like provided by the structure of the present invention, including the matters described above, will be described in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows scanning electron microscope (SEM) images of powders produced by a water atomizing method, where  FIG. 1(   a ) shows Cr-free 6/4 brass alloy powder,  FIG. 1(   b ) shows 6/4 brass alloy powder containing 0.5 mass % of Cr, and  FIG. 1(   c ) shows 6/4 brass alloy powder containing 1.0 mass % of Cr. 
         FIG. 2  is a graph showing the result of X-ray diffraction of the produced water atomized powders. 
         FIG. 3  is a graph showing stress-strain curves of extruded materials.  FIG. 4  shows optical microscope images showing the structures of extruded materials, where  FIG. 4(   a ) shows an extruded material formed from a compact billet of a brass alloy containing 1 mass % of Cr,  FIG. 4(   b ) shows an extruded material formed from a compact billet of a brass alloy containing 0.5 mass % of Cr,  FIG. 4(   c ) shows an extruded material formed from a compact billet of a Cr-free brass alloy, and  FIG. 4(   d ) shows an extruded material formed from an ingot billet of a Cr-free brass alloy. 
         FIG. 5  is an SEM image of an extruded material formed from a compact billet of a brass alloy containing 1.0 mass % of Cr. 
         FIG. 6  is a graph showing the relation between the concentration of a chromium component that is solid-solved in a mother phase of brass, and the proof stress. 
         FIG. 7  is a graph showing the relation between the amount of graphite particles added, and machinability. 
     
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     [Novel Method for Producing Brass Alloy Powder] 
     The inventors of the present application studied methods for producing a novel high strength free-cutting brass member by increasing the strength of brass itself as a base material. In general, various additives are added as a method for increasing the strength of brass. For example, high strength brass is obtained by adding iron, aluminum, manganese, or the like to a copper-zinc alloy, and is used for ship propellers and the like due to its tensile strength as high as 460 MPa and its satisfactory corrosion resistance. However, the high strength brass does not necessarily have high workability as its guaranteed elongation is only about 15%. 
     In order to develop alloys in view of adding of graphite, it is necessary to produce novel brass alloy powder and to extrude an aggregate of this powder to increase the strength. Although brass is conventionally produced by ingot methods, the inventors attempted to produce brass alloys having new alloy compositions by using a powder metallurgy method instead of the ingot method. 
     A water atomizing method as one of rapid solidification methods is a method for very rapidly solidifying a molten metal to produce powder, and thus is characterized not only in that a non-equilibrium phase is formed in the powder, but also in that fine crystal grains are obtained. As a new attempt, the inventors added a small amount of chromium (Cr) as a third element to a brass alloy formed by a mixed phase of α-phase and β-phase, to produce powder having different properties from those of conventional brass powder, and obtained a new material by extruding and solidifying an aggregate of this powder by a hot extrusion method. 
     Conventionally, many attempts have been made to improve properties by adding various additives to brass. However, there has been no example in which a transition element is actively added to 6/4 brass by a water atomizing method. 
     The inventors propose a new method for adding chromium as a high melting point metal to 6/4 brass. As described above, in order to melt brass and to melt chromium in the molten brass, the molten metal needs to be heated to the melting point of chromium, but this temperature is higher than the boiling point of zinc. Thus, in view of the vapor pressure of zinc, it is practically impossible to heat the liquid brass to the melting point of chromium. 
     Another possible method for adding chromium to brass is to use a mother alloy containing chromium. However, since a copper-chromium mother alloy also has a high melting point, a method for adding brass to a molten copper-chromium mother alloy cannot maintain a predetermined composition due to evaporation of zinc. 
     The inventors developed a method for producing a brass alloy by using a commercially available Cu-10% Cr mother alloy. In the mother alloy, chromium is dispersed as particles with a particle size of about 10 to 50 μm, and is not solid-solved in copper. This mother alloy is first melted at about 1,200° C. At this temperature, chromium contained in the mother alloy does not melt, and floats as a solid phase in a liquid phase of copper. In this state, copper is gradually added to reduce the chromium concentration. When the chromium concentration reaches about 4%, a single-phase state, namely a liquid phase, is established beyond the solidus and liquidus lines on the phase diagram. A mixed liquid phase of chromium as a high melting point metal with copper was able to be formed in this manner. In this state, a predetermined amount of zinc is added, and the mixture is rapidly solidified by a water atomizing method. Powder having a non-equilibrium phase, in which chromium is forcibly solid-solved in brass, was able to be obtained in this manner. 
     It is also possible to forcibly solid-solve vanadium in brass by a method similar to that described above. However, since the solidus and liquidus lines are located at the vanadium concentration of about 0.5% in the vanadium-copper binary phase diagram, the amount of vanadium is very small. Thus, practically, not only it is technically difficult to add vanadium to brass, but also it is difficult to increase the effect of adding vanadium to brass. 
     According to the method developed by the inventors, alloy composition control can be appropriately performed while reducing evaporation of added zinc as much as possible. It is known that, in 6/4 brass, a slight change in the amount of zinc component changes the ratio of α-phase to β-phase. It is also known that the difference in the ratio of α-phase to β-phase affects the mechanical properties of brass alloys. 
     Thus, it can be seen that the above powder producing method developed by the inventors is an advantageous method for adding a high melting point metal to brass, even in view of composition control of brass alloys. Further adding nickel and manganese having relatively low melting points makes the resultant powder more useful, as this powder can further increase strength. A leadless free-cutting brass alloy having a high strength and an excellent free cutting property can be obtained by adding graphite to the brass alloy powder thus obtained, and extruding the graphite-containing brass alloy powder. As described above, since the present invention can be used in a wide range of applications, it can be said that the inventors have opened the way to development of various kinds of leadless brass having various mechanical characteristics. 
     Conventionally, the crystal grain size can be typically reduced by repeatedly performing plastic working and heat treatment on the workpiece. However, the use of a powder metallurgy method as in the present invention eliminates the need for a special process for reducing the crystal grain size, as powder having a fine crystal structure is already prepared as a starting material. Moreover, since the material composition is already determined in the powder state, the composition of a final product can be known in this stage. In addition to the advantage regarding the production process, the material of the present invention has several excellent characteristics as described below. 
     [Effects of Adding Third Element] 
     Normally, chromium is hardly solid-solved in brass. However, by using a rapid solidification method such as a water atomizing method, chromium melted in a liquid phase state is forcibly solid-solved in a mother phase of brass only by a fixed amount. As crystal grows in the solidification process, part of chromium is condensed at the crystal grain boundaries, and is precipitated as fine crystal grains. Strictly speaking, the component that is solid-solved in the mother phase of brass includes a component that is solid-solved and dispersed in the mother phase, and a component that is dispersed in the mother phase as precipitates. The chromium component forcibly solid-solved in the mother phase and the chromium component precipitated at the crystal grain boundaries act differently on the applied stress. That is, the chromium component forcibly solid-solved in the mother phase reduces dislocation motion in crystal, and contributes to an increase in proof stress of brass alloy members. On the other hand, the chromium component precipitated at the crystal grain boundaries reduces grain boundary sliding to cause extreme work hardening, and contributes to an increase in tensile strength. 
     The effects of adding manganese will be described below. Unlike chromium, manganese is basically solid-solved in brass. Thus, manganese produces no grain boundary precipitate, and causes no extreme work hardening, but acts to increase the proof stress and the tensile strength in a balanced manner. A possible reason for this is that manganese solid-solved in the mother phase causes dislocation pinning. 
     The effects of adding nickel will be described below. Nickel is also completely solid-solved in brass, but facilitates transformation from β-phase to α-phase during hot extrusion of brass alloys to form a fine α-phase in crystal, and thus greatly contributes to an increase in proof stress. However, since nickel does not contribute to work hardening, the maximum tensile stress of a nickel-containing powder extruded material is not much different than that of a nickel-free powder extruded material. 
     Chromium, manganese, and nickel are transition elements in the fourth period of the periodic table, but have different effects when added to brass as described above, and exhibit completely different behaviors. This is because these transition elements strengthen brass by different mechanisms. Thus, adding two or more kinds of elements can produce the respective effects of the elements. 
     The above study result enables estimation of behaviors that are exhibited when other elements are added. Vanadium as a transition element in the fourth period of the periodic table has an equilibrium diagram similar to that of chromium. Thus, if atomized powder is produced by adding vanadium by a method similar to that of adding chromium, there are a vanadium component that is forcibly solid-solved in a mother phase and a vanadium component that is precipitated at crystal grain boundaries, whereby the capability of brass can be improved by a strengthening mechanism similar to that of chromium. 
     Other than the above elements, titanium, silicon, aluminum, tin, and the like, which are commonly known as elements that strengthen brass, are also expected to effectively strengthen brass containing chromium, when added as auxiliary elements. 
     [Rapid Solidification Method] 
     The effects of the present invention are significantly produced because producing brass alloy powder by a rapid solidification method not only produces the non-equilibrium phase and fine crystal grains, but also causes work hardening using grain boundary precipitation of chromium. The inventors used a water atomizing method as an example of the rapid solidification method. Water atomized powder having a 6/4 brass composition is characterized in that β-phase as a non-equilibrium phase is formed. This will be described in more detail below. In the process of rapidly solidifying a 6/4 brass alloy, powder is solidified as β-phase as the region beyond the solidus and liquidus lines is a β-phase region. If this powder is slowly cooled, the powder should have a mixed phase of α-phase and α-phase due to phase transformation. However, this phase transformation hardly occurs due to a high degree of rapid solidification. When the β-phase powder is heated in hot working, phase transformation from β-phase to α-phase occurs, and the powder has a mixed phase. 
     Certain kinds of elements have an effect of stably maintaining a β-phase when added. Chromium and manganese are recognized to have an effect of delaying transformation to α-phase. This is an effect of reducing atomic diffusion in crystal grains, and is highly effective in retaining a non-equilibrium phase formed by rapid solidification. 
     In the present invention, the grain boundary precipitates that are produced during the solidification process reduce grain boundary sliding, thereby causing a remarkable work hardening phenomenon. Preferably, the size of the grain boundary precipitates is controlled to about 100 nm to 500 nm (the maximum length). The dispersed state of the precipitates is also an important factor, and ideally, the precipitates are uniformly dispersed in the structure. Thus, it is desirable that base powder be homogeneous. The use of an atomizing method to produce powder facilitates control of the solidification speed and the powder particle size. 
     [Extrusion Process] 
     The extrusion temperature is a very important factor to increase the strength of a brass alloy extruded material. The lower the extrusion temperature, the more desirable. Powder needs to be heated in order to extrude a powder aggregate. Heating the powder to a high temperature facilitates atomic diffusion, whereby the non-equilibrium phase produced by rapid solidification becomes close to a thermal equilibrium state. Thus, it is important to extrude a brass alloy powder aggregate at the lowest possible temperature for the extrusion process. A preferable extrusion temperature is 650° C. or less. It is difficult to determine the lower limit of the extrusion temperature, because the lower limit temperature is determined by the size of an extrusion billet, the extrusion ratio, the maximum extrusion load of an apparatus, and the like. If extrusion at 500° C. is possible, 500° C. is an appropriate temperature for the extrusion process. In fact, however, a temperature of 550° C. or higher appears to be required to perform the extrusion process. 
     In the extrusion process, an actual extrusion temperature is determined by two factors, namely a temperature drop due to heat dissipation of the billet, and a temperature rise due to the extrusion pressure. Thus, it is impractical to define the extrusion temperature, and it is practical to manage the heating temperature of the billet. In some experiments of brass extrusion, it took 48 seconds until the extrusion was started, when the heating temperature of the billets was controlled to 650° C. In view of data obtained by simulation, the extrusion was started at 577° C. in this case. 
     The inventors found that a higher strength material is obtained by controlling the speed of extruding an aggregate of chromium-containing brass alloy powder. Low temperature extrusion is effective in obtaining a higher strength material, and a further increase in strength is expected by reducing the extrusion speed as well. This will be described later based on the experimental result. 
     In order to increase machinability of a brass alloy extruded material, it is possible to add graphite particles to chromium-containing brass alloy powder and mix them together, and to extrude the mixed powder aggregate. 
     In order to obtain the effect of increasing machinability, it is necessary to add 0.2 to 2.0 wt % of graphite particles to chromium-containing brass alloy powder. The particle size of the graphite particles is preferably in the range of 1 μm to 100 μm. 
     [Amounts of Elements to be Added] 
     Appropriate amounts of third elements that are added vary depending on the type of the element. 
     An increase in proof stress was recognized by adding 0.5 mass % of chromium. When the amount of chromium was increased to 1 mass %, no difference in proof stress was recognized, but the tensile strength was very high. Thus, the amount of chromium is preferably 0.5 mass % or more, and more preferably 1.0 mass % or more. 
     The upper limit of the chromium content is 5.0 mass %. Due to the limitations in the process of producing powder, the upper limit of the chromium concentration is 4% in a copper-chromium liquid phase state. The chromium content becomes 2.4 mass % by adding zinc. It is possible to increase the chromium content by increasing the melting temperature of copper-chromium. For example, if the melting temperature is increased to 1,300° C., chromium can be melted at a concentration of up to 8%, and the chromium content becomes 5.0 mass % by adding zinc. At this temperature, however, the vapor pressure of zinc becomes too high, making composition control difficult. Thus, a more preferable upper limit of the chromium content is 2.4 mass %. 
     Vanadium is precipitated at crystal grain boundaries even if the amount thereof is very small. In view of the fact that the upper limit of the vanadium concentration in a copper-vanadium liquid phase is 0.5%, the amount of vanadium that is added should be close to the upper limit in order to make the most of the effects of vanadium. In this case, the vanadium concentration becomes 0.3 mass % by adding zinc. The melting temperature needs to be increased in order to increase the vanadium concentration to a value higher than 0.3 mass %. However, if the melting temperature is increased to 1,200° C. or higher, the vapor pressure of zinc is too high, making it difficult to produce powder with an optimal composition. Thus, the effect of adding vanadium is necessarily limited, and strengthening needs to be implemented by combination with other elements. 
     There are already many study examples of the effects produced by adding manganese to brass, and brass containing manganese is practically used as high manganese brass. In the present invention, the strength of brass alloys can further be increased by adding manganese as an auxiliary element in combination with addition of chromium or addition of chromium and vanadium. It was verified that adding 0.5 mass % of manganese is effective enough. In conventional study examples, it is also recognized that increasing the amount of manganese significantly reduces material workability. Thus, a preferred upper limit of the amount of manganese is 7 mass % or less, which is a range in which no compound is produced. A more preferred amount of manganese is 1 to 3 mass %. If the amount of manganese exceeds this range, elongation can be reduced, and workability of brass can be reduced. 
     Since nickel is completely solid-solved in copper, it is possible to add any amount of nickel to a Cu—Zn—Ni material to make an alloy. Thus, in the present invention, there is no specific upper limit for the amount of nickel. Adding nickel produces a special effect of increasing only the proof stress, and the proof stress exceeding 300 Mpa can be implemented by adding 1 mass % of nickel. 
     It is obvious that, in practical applications of alloy members, the proof stress is more important than the tensile strength. Although the most significant effect of the present invention is that a predetermined amount of chromium is contained in 6/4 brass, more advantages can be obtained by further adding nickel. Since chromium has a high melting point, it is not easy to add chromium even by a small amount. As described above, the thermal equilibrium state in metallurgy is used to overcome this disadvantage. Naturally, both chromium and nickel should be added in order to simultaneously produce the effects of both elements. In this case, there is an easier way to add chromium and nickel. That is, although the process as described above is performed in order to add only chromium, it is preferable that chromium and nickel be contained in a mother alloy from the beginning in order to add chromium and nickel at the same time. 
     Nickel chromium alloys are commercially available, and their melting point is 1,345° C., which is lower than the melting points of nickel and chromium. It is possible to melt this alloy and copper in a high-frequency furnace. The mixing ratio of nickel to chromium is 1:1, but producing a molten metal by using the nickel-chromium mother alloy is much more easier than producing a molten metal by using a copper-chromium mother alloy. In the case of adding nickel by using this method, a preferred upper limit of the amount of nickel is 2.4 mass % like chromium. 
     The amount of nickel can be increased by changing the mixing ratio of nickel to chromium in the mother alloy. Increasing the amount of chromium in the mother alloy sharply increases the melting point, thereby making production of powder more difficult. However, increasing the ratio of nickel does not significantly increase the melting point, and the melting point does not exceed that of nickel. Thus, it is possible to produce nickel-rich powder, and to increase the amount of nickel. The upper limit of the amount of nickel is not specifically limited, but adding 5 mass % or less of nickel is desirable as this range does not degrade characteristics as brass. With the nickel content in this range, alloys having desired mechanical characteristics can be produced, and such alloys can be used in a wide range of applications. 
     Regarding other elements to be added, the effect of adding the element is produced by adding about several percent, and at least 0.1%, of the element. Appropriate amounts of the elements and combinations thereof vary depending on the desired mechanical characteristics. In terms of increasing the strength, zirconium has an effect of reducing the crystal grain size, and the effect is sufficiently recognized even if 0.1% of zirconium is added. Thus, zirconium can be definitely said to be a strengthening element according to the Hall-Petch law. 
     Since titanium, aluminum, or the like increases the strength of the mother phase by solid solution strengthening, this effect can be produced by adding even a small amount, as small as 1% or less, of the element. 
     Silicon is an element that is commonly used for dispersion strengthening, and an appropriate amount of silicon is about 3%. However, adding silicon does not necessarily result in strengthening, depending on other elements that are added. In particular, in the alloy system of the present invention, no strengthening effect can be obtained if the precipitation sites of chromium are located at the same positions as the dispersion sites of silicon. Thus, the amount of silicon to be added is limited by the amount of chromium to be added, and it is preferable that the total content of chromium and silicon is 3% or less. 
     Tin is solid-solved at about 0.3%, and has an effect as a strengthening element. However, as the amount of tin is increased, y-phase is formed, which causes embrittlement. Thus, it is not preferable to add a large amount of tin, and it is preferable to add 0.1% to 0.5% of tin. 
     [Production of Powder] 
     Cr-free brass powder, brass powder containing 0.5 mass % of Cr, and brass powder containing 1.0 mass % of Cr were produced from Cu-40% Zn brass materials by using a water atomizing method. Table 1 shows the chemical composition of the powders, and  FIG. 1  shows scanning electron microscope (SEM) images showing the appearance of the powders.  FIG. 1(   a ) shows Cr-free 6/4 brass alloy powder,  FIG. 1(   b ) shows 6/4 brass alloy powder containing 0.5 mass % of Cr, and  FIG. 1(   c ) shows 6/4 brass alloy powder having 1.0 mass % of Cr. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 CHEMICAL COMPOSITION OF POWDER 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 Cr 
                 Zn 
                 Pb 
                 Sn 
                 Fe 
                 Ni 
                 Al 
                 O 
                 S 
                 P 
                 Cu 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
               
            
               
                 Cr-FREE POWDER 
                 0 
                 39.6 
                 0.008 
                 0.028 
                 0.002 
                 0.005 
                 0 
                 0.087 
                 0.002 
                 0.012 
                 bal. 
               
               
                 0.5% Cr POWDER 
                 0.52 
                 39.2 
                 0 
                 0.036 
                 0.009 
                 0.011 
                 0.008 
                 0.16 
                 0.006 
                 0.027 
                 bal. 
               
               
                 1.0% Cr POWDER 
                 1.01 
                 40.68 
                 0 
                 0.029 
                 0 
                 0.007 
                 0 
                 0.21 
                 0 
                 0.029 
                 bal. 
               
               
                   
               
            
           
         
       
     
       FIG. 2  shows the result of X-ray diffraction of the produced powders. Only β-phase was detected in the Cr-free brass alloy powder and the brass alloy powder containing 0.5 mass % of Cr. Two phases, namely α-phase and β-phase, were detected in the brass alloy powder containing 1.0 mass % of Cr. In the 6/4 brass composition, β-phase is formed when the brass alloy powder goes beyond the solidus and liquidus lines from the liquid phase, and the rapidly solidified powder is commonly cooled without a transformation. Detailed examination of the brass alloy powder containing 1.0 mass % of Cr showed that this brass alloy powder was in a mixed state of α-phase powder and β-phase powder. A possible reason for this is that the individual powders were cooled at different speeds during the atomizing process, and α-transformed powder was produced. Note that since Cr is present as fine particles, no clear diffraction peak was detected in the X-ray diffraction. 
     [Extrusion of Brass Alloy Powder Containing 1.0 Mass % of Cr] 
     Powder having a composition of 59% of Cu, 40% of Zn, and 1% of Cr, which was produced by a water atomizing method, was compacted at 600 
     MPa into extrusion billets. The billets were extruded by heating in an electric furnace. The heating electric furnace was set to four different temperatures, 650° C., 700° C., 750° C., and 780° C. The billets were extruded into bars by an extruder at an extrusion speed of 3 mm/s and an extrusion ratio of 37. 
     Tensile test pieces having a distance of 10 mm between marks and a circumference of 3 mm were cut out from the bars, and the tensile test was conducted to measure the 0.2% proof stress and the maximum tensile strength. The result is shown in Table 2. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 1% Cr BLASS ALLOY EXTRUDED MATERIAL 
               
            
           
           
               
               
               
            
               
                 HEATING 
                 MAXIMUM TENSILE 
                 0.2% PROOF 
               
               
                 TEMPERATURE 
                 STRENGTH 
                 STRESS 
               
               
                   
               
               
                 650° C. 
                 565 MPa 
                 317 MPa 
               
               
                 700° C. 
                 556 MPa 
                 319 MPa 
               
               
                 750° C. 
                 547 MPa 
                 289 MPa 
               
               
                 780° C. 
                 544 MPa 
                 294 MPa 
               
               
                   
               
            
           
         
       
     
     The result of Table 2 shows that the extruded materials produced by heating the billets to 650° C. have a high maximum tensile strength and a high 0.2% proof stress. These mechanical strength values tend to reduce as the heating temperature increases. Thus, a desirable heating temperature of the billets for the extrusion process is 650° C. or less. 
     [Extrusion of Brass Alloy Powder Containing 0.5 Mass % of Cr] 
     Powder having a composition of 59.5% of Cu, 40% of Zn, and 0.5% of Cr, which was produced by a water atomizing method, was compacted at 600 MPa into extrusion billets. The billets were extruded by heating in an electric furnace. The heating electric furnace was set to four different temperatures, 650° C., 700° C., 750° C., and 780° C. The billets were extruded into bars by an extruder at an extrusion speed of 3 mm/s and an extrusion ratio of 37. 
     Tensile test pieces having a distance of 10 mm between marks and a circumference of 3 mm were cut out from the bars, and the tensile test was conducted to measure the 0.2% proof stress and the maximum tensile strength. The result is shown in Table 3. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 0.5% Cr BLASS ALLOY EXTRUDED MATERIAL 
               
            
           
           
               
               
               
            
               
                   
                 MAXIMUM 
                   
               
               
                   
                 TENSILE 
                 0.2% PROOF 
               
               
                 HEATING TEMPERATURE 
                 STRENGTH 
                 STRESS 
               
               
                   
               
               
                 650° C. 
                 524 MPa 
                 317 MPa 
               
               
                 700° C. 
                 514 MPa 
                 297 MPa 
               
               
                 750° C. 
                 516 MPa 
                 297 MPa 
               
               
                 780° C. 
                 514 MPa 
                 298 MPa 
               
               
                   
               
            
           
         
       
     
     The result of Table 3 shows that the extruded materials produced by heating the billets to 650° C. have a high maximum tensile strength and a high 0.2% proof stress. These mechanical strength values tend to reduce as the heating temperature increases. Thus, a desirable heating temperature of the billets for the extrusion process is 650° C. or less. 
     Comparison with the result of Table 2 shows that, regarding the 0.2% proof stress, the powder containing 0.5 mass % of Cr and the powder containing 1.0 mass % of Cr have substantially the same value. Thus, it is recognized that the proof stress is maintained even if a smaller amount of chromium is added. However, the maximum tensile strength reduces as the amount of chromium decreases. This demonstrates that the proof stress is determined by the amount of forcibly solid-solved chromium, while, regarding the maximum tensile stress, the degree of work hardening is increased by precipitation of excess chromium at the crystal grain boundaries. 
     [Extrusion of Brass Alloy Powder Containing 1.0 Mass % of Ni] 
     Powder having a composition of 59% of Cu, 40% of Zn, and 1.0% of Ni, which was produced by a water atomizing method, was compacted at 600 MPa into extrusion billets. The billets were extruded by heating in an electric furnace. The heating electric furnace was set to four different temperatures, 650° C., 700° C., 750° C., and 780° C. The billets were extruded into bars by an extruder at an extrusion speed of 3 mm/s and an extrusion ratio of 37. 
     Tensile test pieces having a distance of 10 mm between marks and a circumference of 3 mm were cut out from the bars, and the tensile test was conducted to measure the 0.2% proof stress and the maximum tensile strength. The result showed that the extruded materials produced by heating the billets to 650° C. had a 0.2% proof stress of 311 MPa and a maximum tensile strength of 479 MPa. These mechanical strength values tend to reduce as the heating temperature increases. Thus, a desirable heating temperature of the billets for the extrusion process is 650° C. or less. 
     [Extrusion of Brass Alloy Powder Containing 0.7 Mass % of Mn] 
     Powder having a composition of 59% of Cu, 40% of Zn, and 0.7% of Mn, which was produced by a water atomizing method, was compacted at 600 MPa into extrusion billets. The billets were extruded by heating in an electric furnace. The heating electric furnace was set to four different temperatures, 650° C., 700° C., 750° C., and 780° C. The billets were extruded into bars by an extruder at an extrusion speed of 3 mm/s and an extrusion ratio of 37. 
     Tensile test pieces having a distance of 10 mm between marks and a circumference of 3 mm were cut out from the bars, and the tensile test was conducted to measure the 0.2% proof stress and the maximum tensile strength. The result showed that the extruded materials produced by heating the billets to 650° C. had a 0.2% proof stress of 291 MPa and a maximum tensile strength of 503 MPa. These mechanical strength values tend to reduce as the heating temperature increases. Thus, a desirable heating temperature of the billets for the extrusion process is 650° C. or less. 
     [Extrusion of Cr-free Brass Alloy Powder] 
     Powder having a composition of 60% of Cu and 40% of Zn, which was produced by a water atomizing method, was compacted at 600 MPa into extrusion billets. The billets were extruded by heating in an electric furnace. The heating electric furnace was set to four different temperatures, 650° C., 700° C., 750° C., and 780° C. The billets were extruded into bars by an extruder at an extrusion speed of 3 mm/s and an extrusion ratio of 37. 
     Tensile test pieces having a distance of 10 mm between marks and a circumference of 3 mm were cut out from the bars, and the tensile test was conducted to measure the 0.2% proof stress and the maximum tensile strength. The result is shown in Table 4. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Cr-FREE BRASS ALLOY EXTRUDED MATERIAL 
               
            
           
           
               
               
               
            
               
                   
                 MAXIMUM 
                 0.2% PROOF 
               
               
                 HEATING TEMPERATURE 
                 TENSILE STRENGTH 
                 STRESS 
               
               
                   
               
               
                 650° C. 
                 466 MPa 
                 273 MPa 
               
               
                 700° C. 
                 460 MPa 
                 259 MPa 
               
               
                 750° C. 
                 471 MPa 
                 263 MPa 
               
               
                 780° C. 
                 450 MPa 
                 234 MPa 
               
               
                   
               
            
           
         
       
     
     The result of Table 4 shows that the extruded materials produced by heating the billets to 650° C. have a high maximum tensile strength and a high 0.2% proof stress. These mechanical strength values tend to reduce as the heating temperature increases. Thus, a desirable heating temperature of the billets for the extrusion process is 650° C. or less. 
     [Extrusion of Ingot Billets of Cr-free Brass Alloy]Ingot billets having a composition of 60% of Cu and 40% of Zn were extruded by heating in an electric furnace. The heating electric furnace was set to four different temperatures, 650° C., 700° C., 750° C., and 780° C. The billets were extruded into bars by an extruder at an extrusion speed of 3 mm/s and an extrusion ratio of 37. 
     Tensile test pieces having a distance of 10 mm between marks and a circumference of 3 mm were cut out from the bars, and the tensile test was conducted. The result showed that the extruded materials produced by heating the billets to 650° C. had a 0.2% proof stress of 226 MPa and a maximum tensile strength of 442 MPa. 
     [Comparison of Maximum Tensile Strength and 0.2% Proof Stress] 
     Brass alloy extruded materials, which were produced by extruding various billets by heating to 650° C., were compared in terms of the maximum tensile strength and the 0.2% proof stress. Table 5 shows the comparison result.  FIG. 3  shows stress-strain curves of the extruded materials. The following four types of billets were compared: ingot billets of Cr-free brass alloy; compact billets of Cr-free brass alloy; compact billets of brass alloy containing 0.5% of Cr; and compact billets of brass alloy containing 1.0% of Cr. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 STRENGTH COMPARISON AMONG VARIOUS BRASS ALLOY 
               
               
                 BILLETS (650° C. EXTRUSION) 
               
            
           
           
               
               
               
            
               
                   
                 MAXIMUM 
                   
               
               
                   
                 TENSILE 
               
               
                 BILLET TYPE 
                 STRENGTH 
                 0.2% PROOF STRESS 
               
               
                   
               
               
                 Cr-FREE INGOT BILLET 
                 442 MPa 
                 226 MPa 
               
               
                 Cr-FREE COMPACT BILLET 
                 466 MPa 
                 273 MPa 
               
               
                 0.5% Cr COMPACT BILLET 
                 524 MPa 
                 317 MPa 
               
               
                 1.0% Cr COMPACT BILLET 
                 565 MPa 
                 317 MPa 
               
               
                   
               
            
           
         
       
     
     The following can be seen from  FIG. 3  and Table 5. Comparison between the two types of Cr-free brass alloy billets shows that the ingot billets have a higher maximum tensile strength and a higher 0.2% proof stress than those of the compact billets. Specifically, the use of the compact billets increases the maximum tensile strength and the 0.2% proof stress by 5.4% and 20.7%, respectively. It is already apparent from this point that a powder metallurgy method is more advantageous. 
     Moreover, comparison between the compact billets containing 1.0 mass % of chromium, and the Cr-free ingot billets shows that the use of the compact billets containing 1.0 mass % of Cr increases the maximum tensile strength and the 0.2% proof stress of the extruded materials by 27.8% and 40.2%, respectively. A possible reason for this significant increase in 0.2% proof stress is solid-solution strengthening by the forcibly solid-solved chromium. 
     It is recognized that the use of the compact billets containing Cr significantly increases the maximum tensile strength as compared to the Cr-free compact billets. A possible reason for this is that, in the solidification process of the powder producing process, excess chromium that was not able to be solid-solved is concentrated at the crystal grain boundaries to cause grain boundary segregation of chromium, and spherical precipitates having a diameter of about 100 nm to 500 nm are present mainly on the grain boundary triple points and the grain boundaries. Such fine precipitates acted as a high resistance to grain boundary sliding during plastic deformation, and thus exhibited a high degree of work hardening. 
     [Result of Structure Observation] 
       FIG. 4  shows the result of observing, with an optical microscope, the structures of extruded materials produced by heating billets to 650° C.  FIG. 4(   a ) shows an extruded material formed from a compact billet of a brass alloy containing 1 mass % of Cr.  FIG. 4(   b ) shows an extruded material formed from a compact billet of a brass alloy containing 0.5 mass % of Cr.  FIG. 4(   c ) shows an extruded material formed from a compact billet of a Cr-free brass alloy.  FIG. 4(   d ) shows an extruded material formed from an ingot billet of a Cr-free brass alloy. 
     The comparative observation of the images of  FIG. 4  shows that the extruded materials of the compact billets have finer crystal grains than those of the extruded material of the ingot billet. The extruded material of the brass alloy ingot billet has a crystal grain size of 3 to 10 μM, while the extruded material of the Cr-free brass alloy compact billet has a crystal grain size as small as 1 to 6 μm. The extruded materials of the Cr-containing brass alloy compact billets have a crystal grain size of submicron to 5 μm, and thus it is recognized that the crystal grain size is further reduced in the extruded materials of the Cr-containing brass alloy compact billets. 
     As the crystal grain size decreased, the proof stress increased according to the Hall-Petch law. In the structures of the Cr-containing materials, black dot-like fine precipitates having a size of 1 μm or less were observed at the crystal grain boundaries. The energy dispersive X-ray spectroscope (EDS) analysis identified these precipitates as Cr. 
       FIG. 5  shows a SEM image of a compact billet of a brass alloy containing 1 mass % of Cr. 
     Note that although brass alloy powder or extruded materials thereof are mainly described above, the present invention is also applicable to brass alloy members. That is, the brass alloy members have a brass composition formed by a mixed phase of α-phase and β-phase, contain 0.5 to 5.0 mass % of chromium, and contain at least one element selected from the group consisting of nickel, manganese, zirconium, vanadium, titanium, silicon, aluminum, and tin. 
     [Increase in Yield Stress (YS)] 
     It is recognized that adding chromium increases the yield stress of brass alloy members. Of the added chromium, a chromium component that is solid-solved and dispersed in a mother phase of brass especially contributes to the increase in yield stress. Precipitates were quantified by using the result of structure analysis, and the amount of chromium solid-solved in the mother phase was calculated from the amount of chromium added. 
       FIG. 6  is a graph in which the ordinate represents the difference in yield stress between Cr-free brass alloy members, and Cr-containing brass alloy members, and the abscissa represents the concentration (%) of the chromium component solid-solved in the mother phase. The yield stress increased by 34 MPa when the solid-solution amount of chromium was 0.22%, and increased by 54 MPa when the solid-solution amount of chromium was 0.35%. Thus, it is recognized that the yield stress increases in proportion to the concentration of chromium that is solid-solved in the mother phase of brass. 
     [Improvement in Free-Cutting Property by Adding Graphite Particles] 
     In the production of brass alloy extruded materials by powder extrusion, adding graphite particles can reduce adverse effects on the environment as lead-free materials can be implemented. There have been examples in which graphite is added to commonly used brass. However, there has been no example in which graphite is added to brass alloys having their strength increased by adding chromium. The inventors attempted to increase machinability by adding graphite to brass having its strength increased by adding chromium. 
     Graphite particles used have an average particle size of 5 μm. Chromium-containing brass powder produced by a water atomizing method was mixed with the graphite particles by a mechanical stirring method. The mixed powder thus obtained was formed into compact billets by a method similar to that described above, and the compact billets were extruded into bars by a hot extrusion process. Three different amounts of graphite particles, namely 0.5 wt %, 0.75 wt %, and 1.0 wt % of graphite particles, were added to the chromium-containing brass alloy powder. 
       FIG. 7  is a graph showing the relation between the amount of graphite particles and machinability. It is recognized that the machinability is dramatically increased by adding graphite particles to the chromium-containing brass alloy powder and extruding the resultant brass alloy powder. The machinability was evaluated by measuring the test time of a drilling test. Test pieces were round bars cut with a length of 5 cm, and the drilling test was conducted with a drill diameter of 4.5 mm. A load of 1.3 kgf was applied to the drill, and the rotating speed of the spindle was 900 rpm. The test was conducted ten times, and an average value of the times required for the drill to penetrate the extruded material is shown in the graph of  FIG. 7 . 
     The drill did not penetrate the test pieces containing no graphite, even if the cutting process was performed for 180 seconds or longer. Since the cutting progress seemed to be stopping, the test was stopped if the drill didn&#39;t penetrate the test piece within 180 seconds. 
     The relation between the amount of graphite added and the time required for the drill to penetrate the test piece was examined. Regarding the brass alloys containing 0.5% of chromium, it took 180 seconds or more for the drill to penetrate the test piece when no graphite was added, but the drill penetrated the test piece in an average of 28 seconds when 0.5% of graphite was added. The time required for the drill to penetrate the test piece was reduced to 20 second or less by adding 0.75% or more of graphite, and a dramatic increase in machinability was recognized. Thus, regarding the brass alloys containing 0.5% of chromium, adding 0.75% or more of graphite is preferable in order to significantly increase the machinability. 
     Regarding the brass alloys containing 1.0% of chromium, it took 180 seconds or more for the drill to penetrate the test piece even if 0.5% of graphite was added. The drill penetrated the test piece in an average of 38 seconds when the amount of graphite was increased to 0.75%. The time required for the drill to penetrate the test piece was reduced to 20 second or less by adding 1.0% of graphite. Thus, regarding the brass alloys containing 1.0% of chromium, adding 1.0% of graphite is preferable in order to significantly increase the machinability. 
     [Increase in Strength by Slow Extrusion] 
     The inventors found that higher strength materials are obtained by controlling the extrusion speed of chromium-containing brass alloys. Low temperature extrusion is effective in obtaining high strength materials, and the strength can further be increased by reducing the extrusion speed as well. The actual measurement values are as follows. Regarding the brass alloys containing 1.0% of chromium, the proof stress was 317 MPa and the maximum tensile strength was 565 MPa when the extrusion process was performed at a normal speed (ram speed: 3 mm/s). However, the proof stress was increased to 467 MPa and the maximum tensile strength was increased to 632 MPa when the extrusion process was performed at one tenth the normal extrusion speed (ram speed: 0.3 mm/s). 
     Although the embodiment of the present invention has been described with reference to the drawings, the present invention is not limited to the illustrated embodiment. Various modifications and variations can be made to the illustrated embodiment within a scope that is the same as, or equivalent to the present invention. 
     INDUSTRIAL APPLICABILITY 
     The present invention can be advantageously used to manufacture 6/4 brass alloy members having excellent mechanical characteristics.