WO2017047101A1

WO2017047101A1 - Iron-based sintered compact and method for producing same

Info

Publication number: WO2017047101A1
Application number: PCT/JP2016/004259
Authority: WO
Inventors: 拓也高下; 小林　聡雄; 中村　尚道; 伊都也佐藤
Original assignee: Ｊｆｅスチール株式会社
Priority date: 2015-09-18
Filing date: 2016-09-16
Publication date: 2017-03-23
Also published as: JP6428909B2; SE1850423A1; US20180178291A1; JPWO2017047101A1; CA2990561A1; SE542547C2; KR20180031749A; CN108026614B; CA2990561C; CN108026614A

Abstract

Provided is an iron-based sintered compact having excellent mechanical properties. In this sintered compact, the surface area ratio of pores does not exceed 15%, and the median diameter D50 of the pores in terms of surface area does not exceed 20μm.

Description

Iron-based sintered body and method for producing the same

The present invention relates to an iron-based sintered body, particularly an iron-based sintered body suitable for the production of high-strength sintered parts for automobiles, having a high sintering density, and carburizing, quenching, and tempering the sintered body. The present invention relates to an iron-based sintered body that reliably improves the tensile strength and toughness (impact value) after the treatment. The present invention also relates to a method for producing the iron-based sintered body. And a manufacturing method thereof.

Powder metallurgy technology is a technology that enables a drastic reduction in cutting costs because parts with complicated shapes can be manufactured in a shape very close to the product shape (so-called near net shape) and with high dimensional accuracy. For this reason, powder metallurgy products are used in various fields as various machines and parts.

Recently, there has been a strong demand for powder metallurgy products to improve strength for miniaturization and weight reduction of parts and toughness from the viewpoint of safety. In particular, for powder metallurgy products (iron-based sintered bodies) frequently used for gears and the like, there is a strong demand for higher hardness from the viewpoint of wear resistance in addition to higher strength and higher toughness. Since the strength and toughness of iron-based sintered bodies vary depending on their components, structure, density, etc., it is necessary to develop an iron-based sintered body appropriately controlled in order to meet the above-mentioned demands. .

In general, the compact before sintering is a mixed powder obtained by mixing an iron-based powder, an alloy powder such as copper powder or graphite powder, and a lubricant such as stearic acid or lithium stearate. It is manufactured by filling in and pressure molding.
The density of the molded body obtained by a normal powder metallurgy process is generally about 6.6 to 7.1 Mg / m ³ . The formed body is subsequently subjected to a sintering process to be a sintered body, and further subjected to sizing and cutting as necessary to obtain a powder metallurgy product. When higher strength is required, carburizing heat treatment or bright heat treatment may be performed after sintering.

The iron-based powder used here is classified into iron powder (for example, pure iron powder) and alloy steel powder according to the components. Moreover, as classification according to the manufacturing method of iron-based powder, there are atomized iron powder and reduced iron powder. Iron powder in the classification according to this manufacturing method is used in a broad sense including pure iron powder and alloy steel powder.

In order to obtain a sintered body having high strength and high toughness, it is advantageous to promote both alloying and maintain high compressibility, particularly in an iron-based powder as a main component.
First, as means for alloying iron-based powders,
(1) Mixed powder in which each alloy element powder is mixed with pure iron powder,
(2) Pre-alloyed steel powder in which each alloy element is completely alloyed,
(3) Partially diffused alloy steel powder (also called composite alloy steel powder) in which each alloy element powder is partially adhered and diffused on the surface of pure iron powder or prealloyed steel powder
Etc. are known.

The mixed powder (1) has the advantage of having high compressibility comparable to that of pure iron powder. However, at the time of sintering, each alloy element does not sufficiently diffuse into Fe to form a heterogeneous structure, and as a result, the strength of the finally obtained sintered body may be inferior. Also, when Mn, Cr, V, Si, etc. are used as alloy elements, these elements are oxidized more easily than Fe, so that the sintered body finally obtained by oxidation during sintering There was a problem that the strength of the steel was lowered. In order to suppress the oxidation and reduce the oxygen content of the sintered body, it is necessary to strictly control the atmosphere during sintering and, when carburizing after sintering, the CO ₂ concentration and dew point in the carburizing atmosphere. There is. For this reason, the mixed powder of the above (1) cannot meet the recent demand for high strength and has not been used.

On the other hand, if the prealloyed steel powder (2), which is completely alloyed with each element, is used, the segregation of the alloy elements is completely prevented and the structure of the sintered body can be made uniform, so that the mechanical properties are stabilized. To do. In addition, when Mn, Cr, V, Si, or the like is used as an alloy element, there is an advantage that the oxygen content of the sintered body can be reduced by limiting the type and amount of the alloy element. However, when pre-alloyed steel powder is produced from molten steel by the atomizing method, oxidation in the atomizing process of molten steel and solid solution hardening of the steel powder due to complete alloying are likely to occur. There was a problem that it was difficult to increase. When the density of the molded body is low, the toughness of the sintered body is low when the molded body is sintered. Therefore, even when pre-alloyed steel powder is used, it cannot meet the recent demands for high strength and high toughness.

The partially diffused alloy steel powder (3) above is prepared by mixing each alloy element powder with pure iron powder or prealloyed steel powder and heating it in a non-oxidizing or reducing atmosphere to obtain pure iron powder or prealloyed steel powder. Each alloy element powder is partially diffusion bonded to the surface of the alloy steel powder particles. Therefore, the advantages of the iron-based mixed powder (1) and the prealloyed steel powder (2) can be obtained.

Therefore, by using partially diffused prealloyed steel powder, it is possible to obtain a low oxygen content in the sintered body and a high compressibility in the compacted body similar to pure iron powder. As a result, the strength of the sintered body increases.

Ni and Mo are frequently used as basic alloy components used in the partially diffused alloy steel powder.
Ni has the effect of improving the toughness of the sintered body. This is because the addition of Ni stabilizes austenite, and as a result, more austenite remains as retained austenite without being transformed into martensite after quenching. Moreover, Ni has the effect | action which strengthens the matrix of a sintered compact by solid solution strengthening.

In contrast, Mo has the effect of improving hardenability. Therefore, Mo strengthens the matrix of the sintered body by suppressing the formation of ferrite during the quenching process and facilitating the formation of bainite or martensite. Mo has both the effect of solid solution strengthening by solid solution in the matrix and the effect of precipitation strengthening the matrix by forming fine carbides.

As an example of mixed powder for high-strength sintered parts using the above-mentioned partially diffused alloy steel powder, for example, Patent Document 1 discloses an alloy in which Ni: 0.5 to 4 mass% and Mo: 0.5 to 5 mass% are partially alloyed. Further disclosed is a mixed powder for high-strength sintered parts in which Ni: 1 to 5 mass%, Cu: 0.5 to 4 mass%, and graphite powder: 0.2 to 0.9% mass% are further mixed with steel powder. The sintered material described in Patent Document 1 contains at least 1.5 mass% Ni, and in the examples, it substantially contains 3 mass% or more of Ni. That is, it means that a large amount of Ni such as 3 mass% or more is required to obtain a high strength of 800 MPa or more in the sintered body. Furthermore, in order to obtain a high-strength material of 1000 MPa or higher by carburizing, quenching, and tempering the sintered body, a large amount of Ni such as 3 mass% or 4 mass% is similarly required.

However, Ni is a disadvantageous element from the viewpoint of dealing with recent environmental problems and recycling, and it is desirable to avoid using it as much as possible. In terms of cost, the addition of several mass% of Ni is extremely disadvantageous. Furthermore, when Ni is used as an alloy element, there is also a problem that long-time sintering is required to sufficiently diffuse Ni into iron powder and steel powder. Furthermore, when the diffusion of Ni, which is an austenite phase stabilizing element, is insufficient, the high Ni region is stabilized as an austenite phase (hereinafter also referred to as γ phase), and the Ni dilute region is stabilized by other phases. As a result, the metal structure of the sintered body becomes non-uniform.

As a technique that does not contain Ni, Patent Document 2 discloses a technique related to Mo partially-diffused alloy steel powder that does not contain Ni. That is, by optimizing the amount of Mo, a sintered body having high ductility and toughness that can withstand re-pressurization after sintering can be obtained.

Further, regarding high-density sintered bodies not containing Ni, Patent Document 3 discloses that iron powder having an average particle diameter of 1 to 18 μm and copper powder having an average particle diameter of 1 to 18 μm are 100: (0.2 to 5 ) Are mixed and molded and sintered at a weight ratio. In the technique described in Patent Document 3, an extremely high density sintered body having a sintered body density of 7.42 g / cm ³ or more is obtained by using an iron-based powder having an average particle diameter extremely smaller than usual. Is possible.

In Patent Document 4, a high-strength and high-toughness sintered body is obtained by using Ni-free powder in which Mo is diffused and adhered to the surface of an iron-based powder and the specific surface area is 0.1 m ² / g or more. It is described.

Furthermore, Patent Document 5 describes that a sintered body having high strength and high toughness is obtained by using a powder obtained by diffusing and adhering Mo to an iron-based powder containing reduced iron powder.

In Patent Document 6, Fe-Mn-Si powder is added to fine iron powder and warm forming is performed under mold lubrication, thereby reducing the maximum pore length of the sintered body and increasing the strength and strength. It is described that a tough sintered body is obtained.

Japanese Patent No. 3663929 Japanese Patent No. 36551420 JP-A-4-285141 WO 2015/045273 A1 Japanese Patent Laying-Open No. 2015-14048 Japanese Patent Laid-Open No. 2015-4098

However, it has been found that the sintered materials obtained according to the descriptions of Patent Document 2, Patent Document 3, Patent Document 4, Patent Document 5 and Patent Document 6 have the following problems.

The technique described in Patent Document 2 assumes that high strength is obtained by re-compression after sintering, and it is difficult to achieve both sufficient strength and toughness when manufactured by a normal powder metallurgy process.

In addition, in the sintered material described in Patent Document 3, the average particle size of the iron-based powder used is 1 to 18 μm, which is smaller than usual. When the particle size is small in this way, the fluidity of the mixed powder is deteriorated, and the coarseness of the powder when filling the mold induces cracks and chips in the molded body, resulting in a sintered body having sufficient strength and toughness. It is difficult.

In addition, since the powder described in Patent Document 4 has a very large specific surface area, when such a powder is used, the fluidity of the powder is lowered, and the powder density at the time of filling the mold causes cracks in the molded body. As a result of inducing cracks and the like, it is difficult to obtain a sintered body having sufficient strength and toughness.

Also in the sintered body described in Patent Document 5, similar to the technique described in Patent Document 4, since reduced iron powder having a large specific surface area is used, the fluidity of the powder decreases, and the powder at the time of mold filling As a result of the coarse and dense inducing cracks and chips in the molded body, it is difficult to obtain a sintered body having sufficient strength and toughness.

The sintered body described in Patent Document 6 mainly enhances toughness by regulating the maximum pore length, but it is difficult to achieve both strength and toughness only by regulating the maximum pore length. Is required.

An object of the present invention is to provide an iron-based sintered body having excellent mechanical properties in combination with its manufacturing method.

In order to achieve the above object, the inventors have made various studies for obtaining a sintered body having high strength and high toughness. As a result, the following knowledge was obtained.
In other words, in an iron-based sintered body obtained by sintering a mixed powder composed of iron-based powder and additive, after sintering, controlling the average pore diameter is an impact caused by dispersion of stress-concentrated parts in the structure. It came to find out that it contributed to the improvement of the value.

The present invention has been completed based on such findings and further studies. That is, the gist of the present invention is as follows.
1. An iron-based sintered body having an area fraction of pores of 15% or less and a median diameter D50 based on the area of the pores of 20 μm or less.

2. 2. The iron-based sintered body according to 1 above, which contains Mo, Cu and C.

3. 3. The iron-based sintered body according to 2 above, which contains Mo: 0.2 to 1.5 mass%, Cu: 0.5 to 4.0 mass%, and C: 0.1 to 1.0 mass%.

4). An iron-based sintered body obtained by carburizing, quenching, and tempering the iron-based sintered body according to any one of 1 to 3 above.

5. After forming a mixed powder for powder metallurgy, in which at least Cu powder and graphite powder are mixed with partially diffused alloy steel powder in which Mo is diffused and adhered to the particle surface of the iron-based powder, at a pressure of 400 MPa or more, 1000 ° C or more and 10 minutes A method for producing an iron-based sintered body, comprising performing the above sintering.

6). A method for producing an iron-based sintered body, comprising carburizing, quenching, and tempering the iron-based sintered body produced by the method 5 above.

7). 7. The method for producing an iron-based sintered body according to 5 or 6 above, wherein the mixed powder for powder metallurgy contains Mo: 0.2 to 1.5 mass%, and the balance has components of Fe and inevitable impurities.

8). The partially diffused alloy steel powder has an average particle size of 30 to 120 μm, a specific surface area of less than 0.10 m ² / g, and a circularity of particles having a diameter in the range of 50 to 100 μm is 0.65 or less. The method for producing an iron-based sintered body according to any one of 5 to 7 above.

9. 9. The method for producing an iron-based sintered body according to any one of 5 to 8, wherein the mixed amount of the Cu powder is 0.5 to 4.0 mass% of the mixed powder for powder metallurgy.

According to the present invention, an iron-based sintered body having both high strength and high toughness can be provided.

Hereinafter, the present invention will be specifically described.
The iron-based sintered body of the present invention is characterized in that the area fraction of pores in the sintered body is 15% or less and the median diameter D50 based on the area of the pores is 20 μm or less.

The iron-based sintered body, which is formed by sintering a compact that has been pressure-formed from an alloy steel powder for powder metallurgy, inevitably generates pores, and controlling the pores improves the strength and toughness of the sintered body. It is important to do. That is, since pores with smaller diameters are less likely to become crack initiation points, it is essential that the area-based median diameter D50 is 20 μm or less. More preferably, it is 15 μm or less. When the median diameter D50 exceeds 20 μm, the toughness is significantly lowered.

Here, the median diameter D50 of the pores can be measured according to the following.
First, the sintered body is embedded in a thermosetting resin. Thereafter, the cross section is mirror-polished, and an image of 843 μm × 629 μm per field of view is taken with an optical microscope at a magnification of 100 times. The cross-sectional area A of all the pores in 20 visual fields arbitrarily extracted from the obtained cross-sectional photograph by image analysis is obtained. The circle equivalent diameter d _c is the diameter of circle having the same surface area as the cross-sectional area obtained determined according to the following equation (I). Next, the areas are integrated in ascending order of the equivalent circle diameter, and the equivalent circle diameter at which the integrated value is 50% of the total pore area is defined as the area reference median diameter D50.

As described above, the median diameter D50 of the pores in the sintered body is set to 20 μm or less because when the median diameter D50 exceeds 20 μm, irregular pores increase and such voids are deformed. This is because the strength and toughness are reduced.
Here, in order to set the area fraction of pores in the sintered body to 15% or less and the median diameter D50 of the pores to 20 μm or less, the partial diffusion alloy steel powder of the mixed powder for powder metallurgy, which is a raw material of the sintered body, Particles with an average particle size of 30 to 120 μm and specific surface area of less than 0.10 m ² / g, with a diameter in the range of 50 to 100 μm, have a circularity of 0.65 or less. By using the material adhered to the surface, sintering is promoted in the production of the sintered body described later, and as a result, an intended sintered body is obtained.

In addition, since it is better that the number of pores is small, the area fraction of pores in the sintered body is limited to 15% or less. Because, if the pores exceed 15% in area fraction, the metal content in the sintered body will decrease, so even if the pore diameter is reduced, sufficient strength and toughness cannot be obtained. It is. It should be noted that enormous labor is required to make the voids in the sintered body 0%, which is not realistic. The porosity of the sintered body obtained by the method described later is at least about 5%.

Here, the area fraction of the pores in the sintered body can be obtained by the following method.
In the same manner as described above, it obtains all pores of the cross-sectional area A in 20 fields, by summing them, to obtain a total pore area A _t in all field observed. The A _t, divided by the sum of the areas of all of the field of observation, pore area fraction is obtained.

Furthermore, it is more preferable that the pore length contained in the sintered body is small. The “average maximum pore length”, which is an index of the pore length, is calculated as follows. First, the maximum value of the distance between two points on the periphery of each pore included in the field of view of the cross-sectional photograph is obtained by image analysis, and this is set as the “pore length” of each pore. The “maximum pore length” is the maximum among the “pore lengths” of all pores included in one field of view of the cross-sectional photograph. Further, the “average maximum pore length” is an arithmetic average value of the maximum pore lengths measured in 20 arbitrarily extracted visual fields. In order to obtain sufficient mechanical properties, the average maximum pore length is preferably less than 100 μm.

The above-mentioned sintered body preferably contains Mo, Cu and C. That is, Mo has an effect of improving hardenability. Cu has an effect of promoting solid solution strengthening and hardenability improvement of the iron-based powder. C has the effect of increasing the strength of the iron-based sintered body by being precipitated in iron as a solid solution or fine carbide. The preferred ranges of the respective elements contained in the iron-based sintered body of the present invention are Mo: 0.2 to 1.5 mass%, Cu: 0.5 to 4.0 mass%, and C: 0.1 to 1.0 mass%. When any element is less than the above range, a sufficient strength-increasing effect cannot be obtained, and when it is added more than the above range, the structure is excessively hardened and the toughness is impaired.

Next, a method for obtaining the above sintered body will be described. The method described below is an example, and the iron-based sintered body of the present invention may be obtained by a method other than the following method.
That is, when a sintered compact is produced by sintering a compact obtained by pressure-molding a powder mixture for powder metallurgy, when the mixed powder is formed into a compact by pressing the punch, the punch is pressurized. This is done by a method of molding while applying rotation around the direction. By this method, more shear strain is applied to the mixed powder than in normal molding, the plastic deformation of the mixed powder is facilitated, and the pore diameter in the sintered body can be reduced.

Next, a method for producing a sintered body that is particularly suitable when the sintered body contains Mo, Cu, and C will be described.
That is, an iron-based sintered body can be obtained by converting a powder mixture for powder metallurgy containing an iron-based powder and an additive into a formed body by conventional pressure molding and further performing conventional sintering. At this time, in the molded body, a thickened portion of Mo is formed in the sintered neck portion between the particles of the iron-based powder, and the entanglement between the powders at the time of molding is strongly enhanced by using the iron-based powder with low circularity. In order to increase the density of the sintered body, it is preferable to promote the sintering and to advance the sintering while suppressing the Cu expansion. As the density of the sintered body increases, both strength and toughness improve, but unlike sintered bodies using Ni as in conventional materials, the mechanical properties of the sintered body obtained by this manufacturing method are uniform in the metal structure. Therefore, the variation is small and stable.

In order to obtain such a sintered body, it is preferable that the sintered body is manufactured by using the iron-based powder of the above mixed powder for powder metallurgy as the partial diffusion alloy steel powder shown below.
That is, the mixed powder for powder metallurgy suitably used in the present invention is a partial diffusion alloy steel powder (hereinafter referred to as partial alloy) in which Mo is diffused and adhered to the surface of an iron-based powder having an appropriate average particle size, circularity and specific surface area. (Also referred to as steel powder), graphite powder is mixed with an appropriate amount of Cu powder having an average particle size range described later.

Hereinafter, the mixed powder for powder metallurgy according to the present invention will be described in detail. The “%” shown below means “mass%” unless otherwise specified, and the Mo amount, Cu amount and graphite powder amount represent the respective ratios in the entire powder mixture for powder metallurgy (100% mass%). ing.

(Iron-based powder)
As described above, the partially diffused alloy steel powder has Mo diffused on the surface of the iron-based powder, has an average particle size of 30 to 120 μm, a specific surface area of less than 0.10 m ² / g, and a diameter of 50 It is preferable that the circularity (cross-sectional circularity) of the powder in the range of ˜100 μm is 0.65 or less. Here, when the iron-based powder is partially alloyed, the particle size and the circularity hardly change. Accordingly, an iron-based powder within the same range as the average particle diameter and circularity of the partially diffused alloy steel powder is used.

First, it is preferable that the iron-based powder has an average particle diameter of 30 to 120 μm and a powder having a diameter of 50 to 100 μm having a circularity (cross-sectional circularity) of 0.65 or less. That is, for the reasons described later, it is necessary that the average particle size of the partially alloyed steel powder is 30 to 120 μm and the circularity of the powder in the range of 50 to 100 μm is 0.65 or less. It is necessary to satisfy these conditions.

Here, the average particle diameter of the iron-base powder and the partially alloyed steel powder is the median diameter D50 of the weight cumulative distribution, and is obtained by measuring the particle size distribution using a sieve specified in JIS Z 8801-1. When the cumulative particle size distribution is created from the obtained particle size distribution, the particle size is such that the weight above and below the sieve is 50%.

Further, the circularity of the iron-based powder and the partially alloyed steel powder can be determined according to the following. In the following description, iron-based powder will be described as an example, but in the case of partially alloyed steel powder, the circularity is obtained in the same procedure.
First, iron-based powder is embedded in a thermosetting resin. At this time, the iron-based powder is uniformly embedded in the thermosetting resin with a thickness of 0.5 mm or more so that a sufficient amount of iron-based powder cross section can be observed on the observation surface where the embedded resin is polished and exposed. Thereafter, a cross section of the iron-based powder is revealed by polishing, the cross section is mirror-polished, and the cross section is magnified with an optical microscope and photographed. From the obtained cross-sectional photograph, the cross-sectional area A and the outer peripheral length Lp of each iron-based powder in the cross-sectional photograph are obtained by image analysis. As software capable of such image analysis, for example, Image J (Open Source, National Institutes of Health) is available. The equivalent circle diameter dc is calculated from the obtained cross-sectional area A. Here, dc is obtained by the same formula (I) as in the case of pores.

Next, the circular approximate outer circumference Lc is calculated by multiplying the particle diameter dc by the circumference ratio π. The circularity C is calculated from the obtained Lc and the outer peripheral length Lp of the iron-based powder cross section. Here, the circularity C is a value defined by the following formula (II).
When the circularity C is 1, the cross-sectional shape is a perfect circle, and as the value C decreases, the cross-section becomes irregular.

Note that the iron-based powder means a powder having an Fe content of 50% or more. Examples of the iron-based powder include atomized raw powder (atomized iron powder as atomized), atomized iron powder (reduced atomized raw powder in a reducing atmosphere), reduced iron powder, and the like. In particular, the iron-based powder used in the present invention is preferably atomized raw powder or atomized iron powder. This is because the reduced iron powder contains a large number of pores in the particles, so that there is a possibility that a sufficient density cannot be obtained during pressure molding. Further, the reduced iron powder contains more inclusions in the particles as starting points of fracture than the atomized iron powder, and there is a risk that fatigue strength, which is an important mechanical property of the sintered body, is reduced.

That is, a suitable iron-based powder used in the present invention is atomized raw powder that is obtained by atomizing molten steel, drying, classifying, and not performing heat treatment for deoxidation treatment (reduction treatment) or decarburization treatment, or Any atomized iron powder obtained by reducing atomized raw powder in a reducing atmosphere.
The iron-based powder according to the circularity described above can be obtained by appropriately adjusting the spraying conditions during atomization and the conditions of additional processing performed after spraying. Further, iron-base powders having different circularities may be mixed and adjusted so that the circularity of the iron-base powder having a particle diameter in the range of 50 to 100 μm falls within the above range.

(Partial diffusion alloy steel powder)
Partially diffused alloy steel powder is obtained by diffusing and adhering Mo to the surface of the iron-based powder described above, having an average particle size of 30 to 120 μm, a specific surface area of less than 0.10 m ² / g, and a diameter of 50 to 100 μm. It is necessary that the circularity of the powder in the range is 0.65 or less.

That is, the partial diffusion alloy steel powder is produced by diffusing and adhering Mo to the above-mentioned iron-based powder. The amount of Mo at that time shall be a ratio of 0.2 to 1.5% in the entire powder mixture for powder metallurgy (100%). When the amount of Mo is less than 0.2%, in the sintered body produced using the powder mixture for powder metallurgy, the effect of improving the hardenability is small and the effect of improving the strength is also small. On the other hand, if it exceeds 1.5%, the effect of improving the hardenability in the sintered body is saturated, and rather the non-uniformity of the structure of the sintered body is increased, so that high strength and high toughness cannot be obtained in the sintered body. Therefore, the amount of Mo to be diffused is 0.2 to 1.5%. Preferably it is 0.3 to 1.0%, more preferably 0.4 to 0.8%.

Here, examples of the Mo supply source include Mo-containing powder. Examples of the Mo-containing powder include Mo pure powder, Mo oxide powder, and Mo alloy powder such as Fe-Mo (ferromolybdenum) powder. As the Mo compound, Mo carbide, Mo sulfide, Mo nitride, and the like can be used as suitable Mo-containing powders. These may be used alone or as a mixture of a plurality of substances.

Specifically, the iron-based powder and the Mo-containing powder are mixed at the above-described ratio (Mo amount is 0.2 to 1.5% in the entire powder mixture for powder metallurgy (100%)). There is no restriction | limiting in particular about the mixing method, For example, it can carry out in accordance with a conventional method using a Henschel mixer, a corn type mixer, etc.

Next, the mixed powder of the iron-based powder and the Mo-containing powder is heated, and Mo is diffused into the iron-based powder through the contact surface between the iron-based powder and the Mo-containing powder to join Mo to the iron-based powder. To do. By this heat treatment, partial alloy steel powder containing Mo is obtained.
As the atmosphere for the heat treatment, a reducing atmosphere or a hydrogen-containing atmosphere is suitable, and a hydrogen-containing atmosphere is particularly suitable. Alternatively, heat treatment may be applied under vacuum.
The heat treatment temperature is preferably in the range of 800 to 1100 ° C. when a Mo compound such as oxidized Mo powder is used as the Mo-containing powder. When the heat treatment temperature is less than 800 ° C., the Mo compound is not sufficiently decomposed, and Mo does not diffuse into the iron-based powder, making it difficult to attach Mo. On the other hand, when the temperature exceeds 1100 ° C., sintering between the iron-based powders during the heat treatment proceeds, and the circularity of the iron-based powder exceeds the specified range. On the other hand, when a metal and an alloy such as Mo pure metal or Fe—Mo are used as the Mo-containing powder, a preferable heat treatment temperature is in the range of 600 to 1100 ° C. When the temperature of the heat treatment is less than 600 ° C., the diffusion of Mo into the iron-based powder is insufficient and it becomes difficult to adhere the Mo. On the other hand, when the temperature exceeds 1100 ° C., the sintering of the iron-based powders during the heat treatment proceeds, and the circularity of the partially alloyed steel powder exceeds the specified range.

When heat treatment, that is, diffusion adhesion treatment is performed as described above, normally, the partially alloyed steel powders are in a sintered and solidified state. Do. That is, if necessary, the pulverization conditions are strengthened or coarse powder is removed by classification with a sieve having a predetermined opening so that the prescribed particle size is obtained. Furthermore, you may anneal as needed.

That is, it is important that the average particle size of the partially alloyed steel powder is in the range of 30 to 120 μm. Preferably, the lower limit of the average particle diameter is 40 μm, more preferably 50 μm. On the other hand, the upper limit of the average particle diameter is 100 μm, more preferably 80 μm.
The average particle size of the partially alloyed steel powder is the median diameter D50 of the weight cumulative distribution as described above, and is obtained by measuring the particle size distribution using a sieve specified in JIS Z 8801-1. When the cumulative particle size distribution is created from the measured particle size distribution, the particle diameter is such that the weight of the sieve top and the sieve bottom is 50%.
Here, if the average particle size of the partial alloy steel powder is less than 30 μm, the fluidity of the partial alloy steel powder is deteriorated, which hinders the production efficiency at the time of compression molding in a mold. On the other hand, if the average particle diameter of the partial alloy steel powder exceeds 120 μm, the driving force during the sintering becomes weak, and coarse pores are formed around the coarse partial alloy steel powder in the sintering process, and the sintering is performed. This results in a decrease in the density of the sintered body and causes a decrease in strength and toughness after carburizing, quenching, and tempering the sintered body. The maximum particle size of the partially alloyed steel powder is preferably 180 μm or less.

Further, from the viewpoint of compressibility, the specific surface area of the partially alloyed steel powder is set to less than 0.10 m ² / g. Here, the specific surface area of the partial alloy steel powder refers to the specific surface area of the powder of the partial alloy steel powder excluding additives (Cu powder, graphite powder, lubricant).

When the specific surface area of the partially alloyed steel powder exceeds 0.10 m ² / g, the fluidity of the mixed powder for powder metallurgy is lowered. There is no particular lower limit, but about 0.010 m ² / g is the limit that can be obtained industrially. The specific surface area can be arbitrarily controlled by adjusting the particle size of coarse particles of more than 100 μm and fine particles of less than 50 μm after diffusing adhesion treatment by sieving. That is, the specific surface area decreases by reducing the proportion of fine particles or increasing the proportion of coarse particles.

Furthermore, the degree of circularity of particles having a partial alloy steel powder diameter of 50 to 100 μm needs to be 0.65 or less. The circularity is preferably 0.60 or less, more preferably 0.58 or less. In other words, by reducing the circularity, the entanglement between the powders during pressure molding is strengthened and the compressibility of the powder mixture for powder metallurgy is improved, so that coarse pores in the compact and sintered body are eliminated. Decrease. On the other hand, if the circularity is excessively reduced, the compressibility of the powder mixture for powder metallurgy is reduced, so the circularity is preferably 0.40 or more.

Here, the circularity of the particles having a partial alloy steel powder diameter of 50 to 100 μm can be measured as follows. First, with the particle diameter of the partial alloy steel powder calculated in the same manner as the iron-based powder described above as dc, the partial alloy steel powder having this dc in the range of 50 to 100 μm is extracted. At this time, an optical microscope image sufficient to extract at least 150 particles of partially alloyed steel powder in the range of 50 to 100 μm is performed. And the degree of circularity is calculated about the extracted partial alloy steel powder similarly to the case of the above-mentioned iron base powder.
The reason why the particle diameter of the partially alloyed steel powder is limited to 50 to 100 μm is that reducing the circularity of the powder in the left range is most effective for promoting the sintering. That is, since particles smaller than 50 μm are fine particles, the sintering promoting effect is originally high, and even if the circularity of the particles smaller than 50 μm is lowered, the sintering promoting effect is small. In addition, particles having a particle diameter exceeding 100 μm are extremely coarse, and even if the circularity is lowered, the sintering promoting effect is small.

In the present invention, the remaining composition in the partially alloyed steel powder is iron and inevitable impurities. Here, examples of impurities contained in the partial alloy steel powder include C (excluding graphite), O, N, S, and the like. %: O: 0.3% or less, N: 0.004% or less, S: 0.03% or less, Si: 0.2% or less, Mn: 0.5% or less, P: 0.1% or less, there is no particular problem, but O is 0.25 % Or less is more preferable. In addition, when the amount of inevitable impurities exceeds these ranges, the compressibility in forming using the partially alloyed steel powder is lowered, and it becomes difficult to form a compact having a sufficient density.

In the present invention, for the purpose of obtaining a tensile strength of 1000 MPa or more after further carburizing, quenching, and tempering a sintered body produced using a powder mixture for powder metallurgy, Cu powder is added to the partial alloy steel powder obtained above. And add graphite powder.

(Cu powder)
Cu is a useful element that enhances the solid solution strengthening and hardenability of the iron-based powder and increases the strength of the sintered part, and is added in the range of 0.5% to 4.0%. In other words, if the amount of Cu powder added is less than 0.5%, the above-mentioned useful effects of Cu addition hardly appear, while if it exceeds 4.0%, not only the strength improvement effect of sintered parts is saturated, but also sintering. It causes a drop in body density. Therefore, the amount of Cu powder added is limited to the range of 0.5 to 4.0%. Preferably it is 1.0 to 3.0% of range.

In addition, when Cu powder with a coarse particle size is used, when sintering a compact of a powder mixture for powder metallurgy, the molten Cu infiltrates between the particles of the partial alloy steel powder, and the volume of the sintered body after sintering. May be expanded and the density of the sintered body may be reduced. In order to suppress such a decrease in the density of the sintered body, the average particle size of the Cu powder is preferably 50 μm or less. More preferably, it is 40 μm or less, and further preferably 30 μm or less. In addition, although there is no restriction | limiting in particular in the minimum of the average particle diameter of Cu powder, In order not to raise the manufacturing cost of Cu powder unnecessarily, about 0.5 micrometer is preferable.

Here, the average particle diameter of Cu powder can be obtained by the following method.
Since powder having an average particle size of 45 μm or less is difficult to measure the average particle size by sieving, the particle size is measured with a laser diffraction / scattering particle size distribution analyzer. As a laser diffraction / scattering type particle size distribution measuring apparatus, there is LA-950V2 manufactured by Horiba. Of course, other laser diffraction / scattering particle size distribution measuring devices may be used, but in order to perform accurate measurement, the lower limit of the measurable particle diameter range is 0.1 μm or less and the upper limit is 45 μm or more. Is preferred. In the apparatus, the solvent in which the Cu powder is dispersed is irradiated with laser light, and the particle size distribution and average particle diameter of the Cu powder are measured from the diffraction and scattering intensity of the laser light. As the solvent for dispersing the Cu powder, it is preferable to use ethanol, which has good particle dispersibility and is easy to handle. Use of a solvent having a high van der Waals force such as water and low dispersibility is not preferable because particles aggregate during measurement and a measurement result coarser than the original average particle diameter is obtained. Therefore, it is preferable to carry out an ultrasonic dispersion treatment on the ethanol solution into which Cu powder has been added before measurement.
In addition, since the appropriate dispersion treatment time varies depending on the target powder, the dispersion treatment time is carried out in 7 steps of 10 min intervals between 0 to 60 min, and the average particle diameter of Cu powder is measured after each dispersion treatment. Do. During each measurement, measurement is performed while stirring the solvent in order to prevent particle aggregation. And the smallest value is used as an average particle diameter of Cu powder among the particle diameters obtained by seven measurements performed by changing the dispersion treatment time at intervals of 10 min.

(Graphite powder)
Since graphite powder is effective in increasing strength and fatigue strength, it is added to the partially alloyed steel powder within a range of 0.1 to 1.0% and mixed. The above effects cannot be obtained unless the amount of graphite powder added is less than 0.1%. On the other hand, if it exceeds 1.0%, it becomes hypereutectoid, so that cementite is precipitated and the strength is lowered. Therefore, the amount of graphite powder added is limited to the range of 0.1 to 1.0%. Preferably, it is 0.2 to 0.8%. The average particle size of the graphite powder to be added is preferably in the range of about 1 to 50 μm.

In the present invention, the above-mentioned Cu powder and graphite powder are mixed with the partial diffusion alloy steel powder to which Mo is diffused and adhered to form a mixed powder for powder metallurgy of the Fe—Mo—Cu—C system. What is necessary is just to perform the mixing method according to the conventional method of powder mixing.

In addition, when it is necessary to create a part shape by cutting or the like at the stage of the sintered body, it is possible to appropriately add a machinability improving powder such as MnS to the mixed powder for powder metallurgy according to a conventional method. it can.

Next, molding conditions and sintering conditions suitable for production of a sintered body using the powder metallurgy mixed powder of the present invention will be described.
In the pressure molding using the above mixed powder for powder metallurgy, a powdery lubricant can be further mixed. It can also be molded by applying or adhering a lubricant to the mold. In any case, as the lubricant, any of metal soaps such as zinc stearate and lithium stearate, amide waxes such as ethylenebisstearic acid amide, and other known lubricants can be suitably used. . In the case of mixing the lubricant, the amount is preferably about 0.1 to 1.2 parts by mass with respect to 100 parts by mass of the mixed powder for powder metallurgy.

In producing a compact by pressure-molding the powder metallurgy mixed powder of the present invention, the pressure-molding is preferably performed at a pressure of 400 to 1000 MPa. When the applied pressure is less than 400 MPa, the density of the obtained molded body is lowered, and the properties of the sintered body are deteriorated. On the other hand, if it exceeds 1000 MPa, the life of the mold becomes extremely short, which is economically disadvantageous. The pressure molding temperature is preferably in the range of room temperature (about 20 ° C.) to about 160 ° C.

In addition, it is preferable to sinter the molded body in a temperature range of 1100 to 1300 ° C. If the sintering temperature is less than 1100 ° C., the sintering does not proceed and it becomes difficult to obtain a desired tensile strength of 1000 MPa or more. On the other hand, if it exceeds 1300 ° C, the life of the sintering furnace is shortened, which is economically disadvantageous. The sintering time is preferably in the range of 10 to 180 minutes.

With this procedure, the sintered compact obtained using the powder metallurgy mixed powder according to the present invention under the above sintering conditions has the same compact density as compared with the case where the alloy steel powder out of the above range is used. However, a high sintered body density can be obtained after sintering.

In addition, the obtained sintered body can be subjected to strengthening treatment such as carburizing quenching, bright quenching, induction quenching, carbonitriding treatment, etc., if necessary. However, the sintered body using the mixed powder for powder metallurgy according to the present invention has improved strength and toughness as compared with the conventional sintered body not subjected to the strengthening treatment. In addition, each reinforcement | strengthening process should just be performed according to a conventional method.

The iron-based sintered body of the present invention thus obtained preferably contains Mo: 0.2 to 1.5 mass%, Cu: 0.5 to 4.0 mass%, and C: 0.1 to 1.0 mass%. That is, the C content is preferably in the range of 0.1 to 1.0% at which the effects of increasing strength and increasing fatigue strength are maximized. In other words, the above effect cannot be obtained unless C is less than 0.1%. On the other hand, if it exceeds 1.0%, it becomes hypereutectoid, so that cementite is precipitated and the strength is lowered. Therefore, the amount of C contained in the sintered body is limited to a range of 0.1 to 1.0%. Preferably, it is 0.2 to 0.8%. Moreover, about the suitable content of Mo and Cu, it is the same as the reason in the above-mentioned mixed powder for powder metallurgy.

In addition, when manufacturing a sintered compact, when mixing a lubricant etc. with the above-mentioned mixed powder for powder metallurgy, the powder is adjusted so that the content of Mo, Cu and C in the sintered compact is within the above range. The amount of Mo, Cu and C in the mixed powder for metallurgy is adjusted.

Also, the amount of C contained in the sintered body may vary from the amount of graphite added depending on the sintering conditions (temperature, time, atmosphere, etc.). Therefore, by adjusting the addition amount of graphite powder within the above range according to the sintering conditions, the C content in the preferred range of the present invention (0.1 to 1.0%, more preferably 0.2 to 0.8%) is contained. An iron-based sintered body can be manufactured.

EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not limited only to the following examples.
[Example 1]
As the iron-based powder, atomized raw powder having different circularity was used. The roundness of the atomized raw powder was adjusted in various ways by pulverization using a high-speed mixer (LFS-GS-2J, manufactured by Fukae Pautech Co., Ltd.).
To this iron-based powder, oxidized Mo powder (average particle size: 10 μm) was added at a predetermined ratio, mixed for 15 minutes with a V-type mixer, and then heat-treated in a hydrogen atmosphere with a dew point of 30 ° C. (holding temperature: 880). A partial alloy steel powder in which a predetermined amount of Mo shown in Table 1 was diffused and adhered to the particle surface of the iron-based powder was produced at a temperature of 1 ° C. for 1 hour. The amount of Mo was variously changed as shown in Sample Nos. 1 to 8 in Table 1.

The produced partial alloy steel powder was embedded in a resin and polished so that a cross section of the partial alloy steel powder was exposed. The partial alloy steel powder was evenly embedded in the thermosetting resin with a thickness of 0.5 mm or more so that a sufficient amount of the partial alloy steel powder cross section could be observed on the polished surface, that is, the observation surface. After polishing, the polished surface was magnified with an optical microscope and photographed, and the circularity was calculated by image analysis according to the above.
In addition, specific surface area measurement by BET method was performed on partially alloyed steel powder. All the partial alloy steel powders were confirmed to have a specific surface area of less than 0.10 m ² / g.

Subsequently, the average particle size and amount of Cu powder shown in Table 1 and the amount of graphite powder (average particle size: 5 μm) shown in Table 1 were added to these partially alloyed steel powders, and further obtained. Alloy steel powder for powder metallurgy: After adding 0.6 parts by mass of ethylenebisstearic acid amide to 100 parts by mass, the mixture was mixed for 15 minutes with a V-type mixer.
Incidentally, sample Nos. 9 to 25 use partial alloy steel powder equivalent to sample No. 5, and various amounts of Cu powder and graphite powder to be added are changed. Samples Nos. 26 to 31 are based on the partially alloyed steel powder of Sample No. 5 and the average particle size is adjusted by sieving. Samples Nos. 32-38 have different degrees of circularity of the partially alloyed steel powder.
After that, the mixed powder was pressure-molded at a density of 7.0 g / cm ³ , and rod-shaped compacts (length: 55 mm, width: 10 mm and thickness: 10 mm each), outer diameter: 38 mm, inner diameter: 25 mm And the ring-shaped molded object of thickness: 10mm was produced. The molding pressure at this time was all 400 MPa or more.

The rod-shaped molded body and the ring-shaped molded body were sintered to obtain a sintered body. This sintering was performed in a propane modified gas atmosphere under conditions of sintering temperature: 1130 ° C. and sintering time: 20 minutes.
For the ring-shaped sintered body, the outer diameter, inner diameter and thickness were measured and the mass was measured, and the sintered body density (Mg / m ³ ) was calculated. Further, according to the method described above, the median diameter, area fraction, and average maximum pore length of the pores in the sintered body were investigated.
For the rod-shaped sintered bodies, 5 pieces each were processed into round bar tensile test pieces (JIS No. 2) with a parallel part diameter of 5 mm in order to be subjected to the tensile test specified in JIS Z2241, and 5 pieces each. To be subjected to the Charpy impact test specified in JIS Z2242, in the form of a sintered bar with the size specified in JIS Z2242 (no notch), both have a carbon potential of 0.8mass% gas carburization (holding temperature) : 870 ° C., holding time: 60 minutes), followed by quenching (60 ° C., oil quenching) and tempering (holding temperature: 180 ° C., holding time: 60 minutes).
These carburized, quenched, and tempered round bar tensile test pieces and Charpy impact test bar-shaped test pieces are subjected to a tensile test specified by JIS Z2241 and a Charpy impact test specified by JIS Z2242, The thickness (MPa) and impact value (J / cm ² ) were measured, and the average value was obtained for the number of tests n = 5.

The above measurement results are also shown in Table 1. The criteria for determination are as follows.
(1) Mixed powder flowability Mixed powder for powder metallurgy: 100g passed through a nozzle with a diameter of 2.5mmφ, which passed within 80s without stopping, passed (○), which required more time Or the whole quantity or a part of it stopped and it did not flow, and it was determined as rejected (x).
(2) Sintered body density The sintered body density is equal to or greater than the conventional 4Ni material (4Ni-1.5Cu-0.5Mo, maximum particle size of raw material powder: 180 μm), 6.95 Mg / m ³ or more The case was determined to be acceptable.
(3) Tensile strength A case where the tensile strength of a round bar tensile specimen subjected to carburizing, quenching, and tempering treatment was 1000 MPa or more was determined to be acceptable.
(4) Impact value A case where the impact value of a Charpy impact test bar-shaped specimen subjected to carburizing, quenching and tempering treatment was 14.5 J / cm ² or more was judged to be acceptable. Note that this impact value test was also performed on the sintered body before carburizing, quenching, and tempering.

Here, Sample Nos. 1, 8, 9, 14, 19, 26, 38 and 38 * are examples in which the median diameter D50 of the pores in the sintered body exceeds 20 μm, and all have low impact values. The toughness is insufficient and the tensile strength is low.
Further, regarding the influence of the components in the sintered body, the sample Nos. 1 to 8 compare the Mo amount, the Nos. 9 to 14 the Cu amount, and the Nos. 15 to 19 the graphite amount. Similarly, No. 20 to 25 examined the effect of the Cu particle size, No. 26 to 31 the effect of the alloy particle size, and No. 32 to 38 examined the effect of the circularity and average particle size of the partially alloyed steel powder. It is a result. Table 1 also shows the results of a 4Ni material (4Ni-1.5Cu-0.5Mo, maximum particle size of raw material powder: 180 μm) as a conventional material. It turns out that the example of an invention can obtain the characteristic more than the conventional 4Ni material.
As shown in Table 1, all the inventive examples are sintered bodies having high tensile strength and toughness.

[Example 2]
Three types of atomized iron powders having different specific surface areas and roundnesses were prepared. The specific surface area and circularity can be adjusted by applying a high-speed mixer (LFS-GS-2J type, manufactured by Fukae Powtech Co., Ltd.) to atomized iron powder, and blending coarse powder with a particle size of 100 μm or more and fine powder with a particle size of 45 μm or less. This was done by adjusting the ratio.

To this iron-based powder, oxidized Mo powder (average particle size: 10 μm) was added at a predetermined ratio, mixed for 15 minutes with a V-type mixer, and then heat-treated in a hydrogen atmosphere with a dew point of 30 ° C. (holding temperature: 880). A partial alloy steel powder in which a predetermined amount of Mo shown in Table 2 was diffused and adhered to the particle surface of the iron-based powder was produced by heating at 1 ° C. for 1 hour. These partial alloy steel powders were embedded in a resin and polished so that the cross section of the partial alloy steel powder was exposed, and then an enlarged upper photograph was taken with an optical microscope, and the circularity was calculated by image analysis. Moreover, the specific surface area was measured for the partially alloyed steel powder by the BET method.

Next, 2 mass% of Cu powder with an average particle size of 35 μm and 0.3 mass% graphite powder (average particle size: 5 μm) were added to these partially alloyed steel powders, and the obtained alloy for powder metallurgy was further obtained. Steel powder: After adding 0.6 parts by mass of ethylenebisstearic acid amide to 100 parts by mass, the mixture was mixed for 15 minutes with a V-type mixer. These mixed powders were molded at a molding pressure of 686MPa. Length: 55mm, width: 10mm and thickness: 10mm rod-shaped compacts (10 pieces each), outer diameter: 38mm, inner diameter: 25mm, and thickness: 10mm A ring-shaped molded body was produced.

The rod-shaped molded body and the ring-shaped molded body were sintered to obtain a sintered body. This sintering was performed in a propane modified gas atmosphere under conditions of sintering temperature: 1130 ° C. and sintering time: 20 minutes.
For the ring-shaped sintered body, the outer diameter, inner diameter and thickness were measured and the mass was measured, and the sintered body density (Mg / m ³ ) was calculated. Further, according to the method described above, the median diameter, area fraction, and average maximum pore length of the pores in the sintered body were investigated.

As for the rod-shaped sintered bodies, 5 pieces each were processed into round bar tensile test pieces (JIS No. 2) with a parallel part diameter of 5 mm in order to be subjected to the tensile test specified in JIS Z2241, and 5 pieces each were JIS Z2242. In order to be subjected to the Charpy impact test specified in the above, as-sintered rod shape (no notch), carbon carburization: 0.8 mass% gas carburization (holding temperature: 870 ° C, holding time: 60 minutes), Subsequently, quenching (60 ° C., oil quenching) and tempering (holding temperature: 180 ° C., holding time: 60 minutes) were performed.
These carburized, quenched, and tempered round bar tensile test pieces and Charpy impact test bar-shaped test pieces are subjected to a tensile test specified by JIS Z2241 and a Charpy impact test specified by JIS Z2242, The thickness (MPa) and impact value (J / cm ² ) were measured, and the average value was obtained for the number of tests n = 5.
The measurement results are also shown in Table 2. The acceptance criteria for various characteristic values are the same as those in the first embodiment.

As can be seen from Table 2, if the median diameter D50 of the pores in the sintered body is 20 μm or less, the impact value is high, the toughness is excellent, and the tensile strength is also high. Furthermore, as a result of manufacturing using the partially alloyed steel powder whose circularity and specific surface area are within the scope of the invention, the sintered body density, tensile strength and impact value have achieved the targets.

Claims

An iron-based sintered body having an area fraction of pores of 15% or less and a median diameter D50 based on the area of the pores of 20 μm or less.
2. The iron-based sintered body according to claim 1, comprising Mo, Cu, and C. 3.
The iron-based sintered body according to claim 2, containing Mo: 0.2 to 1.5 mass%, Cu: 0.5 to 4.0 mass%, and C: 0.1 to 1.0 mass%.
An iron-based sintered body obtained by carburizing, quenching, and tempering the iron-based sintered body according to any one of claims 1 to 3.
After forming a mixed powder for powder metallurgy, in which at least Cu powder and graphite powder are mixed with partially diffused alloy steel powder in which Mo is diffused and adhered to the particle surface of the iron-based powder, at a pressure of 400 MPa or more, 1000 ° C or more and 10 minutes A method for producing an iron-based sintered body, comprising performing the above sintering.
A method for producing an iron-based sintered body, comprising carburizing, quenching, and tempering the iron-based sintered body produced by the method according to claim 5.
The method for producing an iron-based sintered body according to claim 5 or 6, wherein the mixed powder for powder metallurgy contains Mo: 0.2 to 1.5 mass%, and the balance has components of Fe and inevitable impurities. .
The partially diffused alloy steel powder has an average particle size of 30 to 120 μm, a specific surface area of less than 0.10 m 2 / g, and a circularity of particles having a diameter in the range of 50 to 100 μm is 0.65 or less. A method for producing an iron-based sintered body according to any one of claims 5 to 7.
The method for producing an iron-based sintered body according to any one of claims 5 to 8, wherein the mixed amount of the Cu powder is 0.5 to 4.0 mass% of the mixed powder for powder metallurgy.