Mixed powder for powder metallurgy and method for producing sintered component

Abstract

To provide a mixed powder for powder metallurgy containing a graphite powder which is hardly scattered.SOLUTION: A mixed powder for powder metallurgy comprises a metal powder and a graphite powder. The graphite powder comprises a spherical graphite powder. Each particle of the spherical graphite powder has an aspect ratio of 4 or less, which is represented by a ratio of a length of a major axis to a length of a minor axis. A content ratio of the spherical graphite powder in the graphite powder is 70% or more. An apparent density of the graphite powder is 0.2 g / cm3 or more. A tap density of the graphite powder is 0.5 g / cm3 or more.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present disclosure relates to powder mixtures for powder metallurgy and methods for producing sintered parts. [Background technology]

[0002] Patent Document 1 discloses a mixed powder for powder metallurgy, which is a mixture of powder raw materials including iron powder, copper powder, and graphite powder and a binder. In this mixed powder, a specific fatty acid ester is used as a binder to prevent segregation of the graphite powder due to poor dispersion and to reduce dust when handling the mixed powder.

[0003] Patent Document 2 discloses a mixed powder for powder metallurgy containing an iron-based powder, copper powder, and graphite powder. In this mixed powder, the Pb content in the copper powder is limited to reduce the dimensional variation of a sintered body obtained using the mixed powder for powder metallurgy. The graphite powder is a flake-like natural graphite powder. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 8-176606 [Patent Document 2] Japanese Patent Application Publication No. 9-287002 Summary of the Invention [Problem to be solved by the invention]

[0005] There is a demand for a mixed powder for powder metallurgy in which graphite powder is less likely to be unevenly distributed even without the use of a binder. Graphite powder has a lower specific gravity than metal powders such as iron powder and copper powder, and is therefore more likely to scatter. The composition of graphite powder itself that is less likely to scatter has not been fully studied.

[0006] An object of the present disclosure is to provide a mixed powder for powder metallurgy containing graphite powder that is less likely to scatter. [Means for solving the problem]

[0007] The mixed powder for powder metallurgy of the present disclosure contains a metal powder and a graphite powder. The graphite powder includes spherical graphite powder. Each particle of the spherical graphite powder has an aspect ratio, expressed as the ratio of the length of the major axis to the length of the minor axis, of 4 or less. The content of the spherical graphite powder in the graphite powder is 70% or more. The apparent density of the graphite powder is 0.2 g / cm. 3 The graphite powder has a tap density of 0.5 g / cm 3 That's all. [Effects of the Invention]

[0008] In the mixed powder for powder metallurgy of the present disclosure, the graphite powder contains spherical graphite powder at a predetermined content ratio, so that the graphite powder is less likely to scatter. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 is a schematic view showing an example of a mixed powder for powder metallurgy according to an embodiment. [Figure 2] FIG. 2 is a diagram showing an SEM image of graphite powder contained in the mixed powder for powder metallurgy according to the embodiment, taken with a scanning electron microscope. [Figure 3] FIG. 3 is a schematic diagram illustrating a method for producing a sintered part according to an embodiment. [Figure 4] FIG. 4 is a diagram showing an SEM image of the graphite powder of Sample No. 1-11 taken with a scanning electron microscope. [Figure 5] FIG. 5 is a graph showing the relationship between the average particle size and the apparent density of the graphite powder in Test Example 1. [Figure 6] FIG. 6 is a graph showing the relationship between the average particle size and tap density of the graphite powder in Test Example 1. [Figure 7] FIG. 7 is a schematic diagram illustrating a method for evaluating the dispersibility of graphite powder in Test Example 1. [Figure 8] FIG. 8 is a graph showing the evaluation results of the dispersibility of graphite powder in Test Example 1. [Figure 9]FIG. 9 is a schematic diagram illustrating a method for evaluating the dimensional variation of the sintered parts in Test Example 2. [Figure 10] FIG. 10 is a graph showing the evaluation results of the dimensional variation of the sintered parts in Test Example 2. DETAILED DESCRIPTION OF THE INVENTION

[0010] [Description of the embodiments of the present disclosure] It has been believed that if the metal powder and graphite powder are uniformly mixed in the powder metallurgy mixture before press molding, the graphite powder will be uniformly dispersed in the resulting compact. However, as a result of extensive research by the present inventors, it has been found that if the graphite powder contains primarily flake-like graphite powder, the graphite powder will scatter when the powder metallurgy mixture is fed into a press die, and the scattered graphite powder will accumulate on the powder filled in the die cavity. In other words, it has been found that the scattering of graphite powder causes uneven distribution of the graphite powder in the powder metallurgy mixture in the cavity. "Uneven distribution" refers to uneven distribution of graphite powder in the powder metallurgy mixture. "Uneven distribution" refers not only to uneven distribution of graphite powder in the powder metallurgy mixture due to poor dispersion, but also to uneven distribution of graphite powder in a compact produced using the powder metallurgy mixture. Pressing with unevenly distributed graphite powder results in uneven distribution of graphite powder in the resulting compact. Therefore, when a molded body was produced using spherical graphite powder, it was found that the graphite powder was uniformly dispersed in the produced molded body. First, embodiments of the present disclosure will be listed and described.

[0011] (1) A mixed powder for powder metallurgy according to an embodiment of the present disclosure includes a metal powder and a graphite powder. The graphite powder includes spherical graphite powder. Each particle of the spherical graphite powder has an aspect ratio, expressed as the ratio of the length of the major axis to the length of the minor axis, of 4 or less. The content of the spherical graphite powder in the graphite powder is 70% or more. The apparent density of the graphite powder is 0.2 g / cm. 3 The graphite powder has a tap density of 0.5 g / cm 3 That's all.

[0012] When the graphite powder contains spherical graphite powder, the apparent density and tap density of the graphite powder tend to be large. When the content of spherical graphite powder in the graphite powder is 70% or more, the apparent density of the graphite powder is 0.2 g / cm 3 and the tap density of the graphite powder is 0.5 g / cm 3 The above requirements are easily met. Apparent density is 0.2g / cm 3 or more, and tap density is 0.5 g / cm 3 The graphite powder described above is less likely to scatter. Therefore, the graphite powder is less likely to scatter when the mixed powder for powder metallurgy is supplied to a press die, and the graphite powder is less likely to be unevenly distributed in a compact produced using the mixed powder for powder metallurgy. If the graphite powder is less likely to be unevenly distributed in a compact, variations in carbon concentration are less likely to occur in a sintered part produced using the mixed powder for powder metallurgy. Sintered parts that are less likely to have variations in carbon concentration are less likely to have variations in dimensional accuracy.

[0013] Since the graphite powder itself is unlikely to scatter, there is no need to add a binder to prevent the graphite powder from scattering. In other words, since the graphite powder itself is unlikely to scatter, it is sufficient to simply mix the metal powder and the graphite powder.

[0014] As described above, graphite powder may scatter when the mixed powder for powder metallurgy is supplied to a press die. If the graphite powder scatters, there is a risk that equipment around the die may become contaminated. A mixed powder for powder metallurgy containing graphite powder that is less likely to scatter is less likely to adhere to equipment other than the device to which the mixed powder for powder metallurgy is supplied, and therefore it is easy to prevent the equipment from becoming contaminated.

[0015] (2) In the mixed powder for powder metallurgy of (1) above, the metal powder may include an iron-based powder, each particle of which contains more than 50 mass % iron.

[0016] The mixed powder for powder metallurgy containing the iron-based powder is suitable as a raw material powder for sintered parts. The sintered parts produced using the mixed powder for powder metallurgy as a raw material powder have high strength.

[0017] (3) In the mixed powder for powder metallurgy of (2) above, the content of the iron-based powder in the mixed powder for powder metallurgy may be 90 mass % or more.

[0018] If the content of the iron-based powder in the mixed powder for powder metallurgy is 90 mass % or more, the strength of a sintered part produced using this mixed powder for powder metallurgy as a raw material powder is likely to be further improved.

[0019] (4) In the mixed powder for powder metallurgy of (3) above, the metal powder may further contain copper powder. The content of the copper powder in the mixed powder for powder metallurgy may be more than 0 mass% and not more than 8.0 mass%. The content of the graphite powder in the mixed powder for powder metallurgy may be 0.1 mass% or more and not more than 2.0 mass%.

[0020] The copper powder and graphite powder contribute to improving the strength of sintered parts manufactured using the powder metallurgy mixed powder as raw material powder. When the copper powder and graphite powder are contained within the above ranges, the strength of the sintered parts is likely to be further improved.

[0021] (5) In the mixed powder for powder metallurgy according to any one of (1) to (4) above, the graphite powder may have an average particle size of 3 μm or more and 40 μm or less.

[0022] When the average particle size of the graphite powder is 3 μm or more, the graphite powder is less likely to aggregate.When the average particle size of the graphite powder is 40 μm or less, when a sintered part is produced using the mixed powder for powder metallurgy as a raw material powder, the graphite powder is more likely to undergo solid-state diffusion into the metal powder, particularly the iron-based powder, depending on the sintering temperature.

[0023] (6) A method for manufacturing a sintered part according to an embodiment of the present disclosure includes the steps of preparing a powder metallurgy mixed powder according to any one of (1) to (5) above, pressing a raw material powder containing the powder metallurgy mixed powder to produce a compact, and sintering the compact.

[0024] A sintered part manufactured using the mixed powder for powder metallurgy according to the embodiment of the present disclosure as a raw material powder has excellent dimensional accuracy.

[0025] [Details of the embodiments of the present disclosure] Specific examples of the powder mixture for powder metallurgy and the method for manufacturing a sintered part according to the present disclosure will be described with reference to the drawings. The same reference numerals in the drawings indicate the same or corresponding parts. In the drawings, some components may be exaggerated or simplified for ease of explanation. The dimensional proportions of the various parts in the drawings may also differ from the actual proportions. The present invention is not limited to these examples, but is defined by the claims, and all modifications within the meaning and scope of the claims are intended to be included.

[0026] <Mixed powder for powder metallurgy> A mixed powder 1 for powder metallurgy according to an embodiment will be described with reference to Figures 1 and 2. As shown in Figure 1, the mixed powder 1 for powder metallurgy contains a metal powder 2 and a graphite powder 5. One of the features of the mixed powder 1 for powder metallurgy is that the graphite powder 5 contains spherical graphite powder 6, as shown in Figure 2. For ease of understanding, each particle of the metal powder 2 and each particle of the graphite powder 5 is shown as a circle in Figure 1.

[0027] ≪Metal powder≫ The metal powder 2 is the main powder of the mixed powder for powder metallurgy 1. The metal powder 2 includes, for example, an iron-based powder 3. In this example, the metal powder 2 includes the iron-based powder 3 and copper powder 4.

[0028] [Iron-based powder] The iron-based powder 3 is the main powder of the metal powder 2. In other words, the iron-based powder 3 is the main powder of the mixed powder for powder metallurgy 1. The mixed powder for powder metallurgy 1 containing the iron-based powder 3 is suitable as a raw material powder for sintered parts, which will be described later.

[0029] Each particle of the iron-based powder 3 contains more than 50% by mass of iron. The content ratio of elements contained in each particle is the ratio relative to the total mass of the elements contained in that particle, which is 100% by mass. Each particle of the iron-based powder 3 is made of pure iron or an iron alloy. Pure iron is made of 99.9% by mass or more of iron and unavoidable impurities. Iron alloys contain additive elements, with the remainder being iron and unavoidable impurities. The iron content ratio of the iron alloy may be 80% by mass or more, or 90% by mass or more. The additive element is, for example, chromium. The iron-based powder 3 may be a pure iron powder made of pure iron, an iron alloy powder made of an iron alloy, or a mixed powder of pure iron powder and iron alloy powder. The iron alloy powder may be a powder obtained by dissolving iron and additive elements to form a pre-alloy, or a powder obtained by partially diffusing the additive elements into pure iron powder to form an alloy. The iron alloy powder may be a powder obtained by dissolving iron and additive elements to form a pre-alloy, and then further partially diffusing the additive elements into the pre-alloy. The method for producing the iron-based powder 3 is not particularly limited. The iron-based powder 3 can be produced by, for example, water atomization, gas atomization, or reduction.

[0030] The content of the iron-based powder 3 in the mixed powder for powder metallurgy 1 is, for example, 90% by mass or more. If the content of the iron-based powder 3 is 90% by mass or more, a sintered part manufactured using this mixed powder for powder metallurgy 1 as a raw material powder will have high strength. The content of the iron-based powder 3 in the mixed powder for powder metallurgy 1 may be 93% by mass or more, or 95% by mass or more. The content of the iron-based powder 3 in the mixed powder for powder metallurgy 1 is, for example, 99% by mass or less, or 98% by mass or less. The content of the iron-based powder 3 in the mixed powder for powder metallurgy 1 is, for example, 90% by mass or more and 99% by mass or less, 90% by mass or more and 98% by mass or less, 93% by mass or more and 98% by mass or less, or 95% by mass or more and 98% by mass or less.

[0031] The average particle size of the iron-based powder 3 is, for example, 20 μm or more and 250 μm or less. When the average particle size of the iron-based powder 3 is 20 μm or more, the iron-based powder 3 has good fluidity and is easy to press. When the average particle size of the iron-based powder 3 is 250 μm or less, a sintered part manufactured using the mixed powder for powder metallurgy 1 containing this metal powder 2 as a raw material powder has a dense structure. The average particle size of the iron-based powder 3 may be 30 μm or more and 200 μm or less, 50 μm or more and 200 μm or less, or 50 μm or more and 150 μm or less. The average particle size of the iron-based powder 3 is the median diameter D50, which is the cumulative 50% from the smallest diameter side in the number-based particle size distribution measured using a laser diffraction particle size distribution analyzer.

[0032] [Copper powder] The copper powder 4 contributes to improving the strength of a sintered part manufactured using the mixed powder for powder metallurgy 1 as a raw material powder. At least a portion of the copper powder 4 melts during sintering when manufacturing the sintered part.

[0033] Each particle of the copper powder 4 contains more than 50% by mass of copper. Each particle of the copper powder 4 is made of pure copper or a copper alloy. Pure copper is made of 99.5% by mass or more of copper and inevitable impurities. Copper alloys contain additional elements, with the remainder being copper and inevitable impurities. The copper content in the copper alloy may be 80% by mass or more, or 90% by mass or more. The additional element is, for example, phosphorus. The copper powder 4 may be pure copper powder made of pure copper, copper alloy powder made of a copper alloy, or a mixed powder of pure copper powder and copper alloy powder.

[0034] The content of copper powder 4 in the mixed powder for powder metallurgy 1 is, for example, more than 0% by mass and not more than 8.0% by mass. Although copper powder 4 contributes to improving the strength of the sintered part, it can also contribute to reducing the dimensional accuracy of the sintered part. When the content of copper powder 4 in the mixed powder for powder metallurgy 1 is 8.0% by mass or less, the dimensional accuracy of the sintered part is less likely to reduce. The content of copper powder 4 in the mixed powder for powder metallurgy 1 may be 0.1% by mass or more and 6.0% by mass or less, 0.3% by mass or more and 5.0% by mass or more, or 0.5% by mass or more and 3.0% by mass or less.

[0035] The average particle size of the copper powder 4 is, for example, 5 μm or more and 90 μm or less. When the average particle size of the copper powder 4 is 5 μm or more, the copper powder 4 has good fluidity and is easy to press. When the average particle size of the copper powder 4 is 90 μm or less, a sintered part manufactured using the mixed powder for powder metallurgy 1 containing this metal powder 2 as a raw material powder has a dense structure. The average particle size of the copper powder 4 may be 10 μm or more and 80 μm or less, 10 μm or more and 60 μm or less, or 20 μm or more and 50 μm or less. The average particle size of the copper powder 4 is, for example, smaller than the average particle size of the iron-based powder 3. The average particle size of the copper powder 4, like the average particle size of the iron-based powder 3, is the median diameter D50 in the number-based particle size distribution.

[0036] 〔others〕 Although not shown, the mixed powder for powder metallurgy 1 may further contain one or more powders selected from the group consisting of nickel powder, molybdenum powder, and manganese sulfide powder. The content of the nickel powder in the mixed powder for powder metallurgy 1 is, for example, more than 0% by mass and not more than 10% by mass, or 0.1% by mass to 4% by mass. The content of the molybdenum powder in the mixed powder for powder metallurgy 1 is, for example, more than 0% by mass and not more than 10% by mass, or 0.1% by mass to 1% by mass. The content of the manganese sulfide powder in the mixed powder for powder metallurgy 1 is, for example, more than 0% by mass and not more than 5% by mass, or 0.1% by mass to 2.0% by mass.

[0037] The metal powder 2 may mainly contain a metal powder other than the iron-based powder 3, as long as it is sinterable. The metal powder 2 does not necessarily need to contain the copper powder 4.

[0038] <Graphite powder> The graphite powder 5 contributes to improving the strength of a sintered part manufactured using the mixed powder for powder metallurgy 1 as a raw material powder. At least a portion of the graphite powder 5 undergoes solid-state diffusion into the metal powder 2, particularly the iron-based powder 3, during sintering to manufacture the sintered part.

[0039] As shown in FIG. 2, the graphite powder 5 includes spherical graphite powder 6. Each particle of the spherical graphite powder 6 has an aspect ratio, which is the ratio of the major axis length to the minor axis length, of 4 or less. Particles with an aspect ratio of 4 or less have a shape that is closer to a sphere. The smaller the aspect ratio, the closer the particle is to a sphere. The aspect ratio of each particle of the spherical graphite powder 6 can be measured by observing the graphite powder 5 with a scanning electron microscope (SEM). An SEM image is acquired so that it contains 50 or more particles. The acquired SEM image is a two-dimensional image. The major and minor axes of each particle in this SEM image are extracted by image processing. The major axis is the longest line segment that passes through the area center of gravity of the particle and intersects the outline of the particle. The minor axis is the shortest line segment that is perpendicular to the major axis. Using the major and minor axes of each extracted particle, the aspect ratio of each particle is calculated by dividing the length of the major axis by the length of the minor axis, i.e., (major axis length / minor axis length). The aspect ratio of each particle of the spherical graphite powder 6 may be 3.5 or less, 3.0 or less, 2.5 or less, or 2.0 or less. The aspect ratio of each particle of the spherical graphite powder 6 is, for example, 1 or more.

[0040] The content of the spherical graphite powder 6 in the graphite powder 5 is 70% or more. The content of the spherical graphite powder 6 in the graphite powder 5 is the ratio of the number of particles of the spherical graphite powder 6 to the number of particles of the graphite powder 5. If the content of the spherical graphite powder 6 in the graphite powder 5 is 70% or more, the apparent density and tap density of the graphite powder 5 tend to be large. The apparent density and tap density will be described later. The content of the spherical graphite powder 6 in the graphite powder 5 can be determined from the SEM image. In the SEM image, 50 or more particles are randomly selected, and the percentage of the number of particles having an aspect ratio of 4 or less divided by the number of the randomly selected particles is the content of the spherical graphite powder 6 in the graphite powder 5. The content of the spherical graphite powder 6 in the graphite powder 5 may be 75% or more, 80% or more, 85% or more, 90% or more, or 95% or more. The content ratio of the spherical graphite powder 6 in the graphite powder 5 may be 100%. When the content ratio of the spherical graphite powder 6 in the graphite powder 5 is 100%, the aspect ratio of all particles of the graphite powder 5 is 4 or less. When the content ratio of the spherical graphite powder 6 in the graphite powder 5 is less than 100%, the graphite powder 5 contains particles with an aspect ratio of more than 4, for example, flaky particles 6s.

[0041] The apparent density of graphite powder 5 is 0.2 g / cm 3 The apparent density is 0.2 g / cm 3 Graphite powder 5 having a density of 0.2 g / cm or more is less likely to scatter. The apparent density of graphite powder 5 is a density measured by applying the procedure described in JIS Z 2504:2020 "Metal powder - Apparent density measurement method." The funnel used for the measurement has an orifice diameter of 5.5 ± 0.1 mm and a funnel angle of 60°. The apparent density of graphite powder 5 is 0.2 g / cm. 3 Super, 0.3g / cm 3 or more, or 0.4 g / cm 3 More than that is fine.

[0042] The tap density of graphite powder 5 is 0.5 g / cm 3 That's all. The tap density is 0.5 g / cm 3Graphite powder 5 having a tap density of 0.6 g / cm or more is less likely to scatter. The tap density of graphite powder 5 is a density measured according to the procedure described in JIS Z 2512:2012 "Metal powder - Tap density measurement method." The tap density of graphite powder 5 is 0.6 g / cm 3 More than 0.7g / cm 3 or more, or 0.8 g / cm 3 More than that is fine.

[0043] The graphite powder 5 has an average particle size of, for example, 3 μm or more and 40 μm or less. When the graphite powder 5 has an average particle size of 3 μm or more, the graphite powder 5 is less likely to aggregate. When the graphite powder 5 has an average particle size of 40 μm or less, the graphite powder 5 is more likely to undergo solid-state diffusion into the metal powder 2, particularly the iron-based powder 3, depending on the sintering temperature when a sintered part is produced using the mixed powder for powder metallurgy 1 as a raw material powder. The graphite powder 5 may have an average particle size of 5 μm or more and 35 μm or less, 5 μm or more and 30 μm or less, or 10 μm or more and 30 μm or less. The average particle size of the graphite powder 5 is, for example, smaller than the average particle size of the metal powder 2. The average particle size of the graphite powder 5, like the average particle size of the iron-based powder 3, is the median diameter D50 in the number-based particle size distribution.

[0044] The content of graphite powder 5 in the mixed powder for powder metallurgy 1 is, for example, 0.1% by mass or more and 2.0% by mass or less. When the content of graphite powder 5 in the mixed powder for powder metallurgy 1 is 0.1% by mass or more, the strength of the sintered part is easily improved. When the content of graphite powder 5 in the mixed powder for powder metallurgy 1 is 2.0% by mass or less, the dimensional accuracy of the sintered part is easily maintained or improved. The content of graphite powder 5 in the mixed powder for powder metallurgy 1 may be 0.5% by mass or more and 2.0% by mass or less, 0.5% by mass or more and 1.5% by mass or less, 0.6% by mass or more and 1.3% by mass or less, or 0.8% by mass or more and 1.1% by mass or less.

[0045] <Manufacturing method for sintered parts> A method for manufacturing a sintered part according to an embodiment will be described with reference to Fig. 3. The method for manufacturing a sintered part includes step A of preparing a mixed powder for powder metallurgy, step B of pressing a raw material powder containing the mixed powder for powder metallurgy to produce a compact, and step C of sintering the compact. One of the features of the method for manufacturing a sintered part is that the mixed powder for powder metallurgy 1 according to the embodiment described above is used as the mixed powder for powder metallurgy.

[0046] ≪Process A≫ In step A, the mixed powder for powder metallurgy 1 according to the embodiment described above is prepared. The prepared mixed powder for powder metallurgy 1 is used as raw powder 7 for a sintered part. The raw powder 7 may contain a lubricant (not shown). The lubricant serves to enhance lubrication between the raw powder 7 and a die 91 during press molding, which will be described later. The lubricant improves moldability and facilitates densification of the compact 8. Known lubricants used in press molding can be used. Examples of lubricants that can be used include metal soaps such as lithium stearate or zinc stearate, and amides such as ethylene bisstearamide. The lubricant may be applied to the die 91. When a lubricant is included, the content of the lubricant in the raw powder 7 is, for example, 0.1% by mass to 1.2% by mass, or 0.4% by mass to 1.0% by mass.

[0047] ≪Process B≫ In step B, the raw material powder 7 prepared in step A is press-molded. The press-molding is performed using a die 91. The raw material powder 7 is filled into the cavity of the die 91 and pressurized. When the raw material powder 7 is press-molded, a molded body 8 corresponding to the shape of the cavity is produced.

[0048] The graphite powder 5 contained in the mixed powder for powder metallurgy 1 may scatter when the raw material powder 7 is supplied to the die 91. As described above, graphite powder 5 containing spherical graphite powder 6 is less likely to scatter. Therefore, the graphite powder 5 is less likely to scatter when the mixed powder for powder metallurgy 1 is supplied to the die 91, and the graphite powder 5 is less likely to be unevenly distributed in the molded body 8 produced using the mixed powder for powder metallurgy 1. If the graphite powder 5 is less likely to be unevenly distributed in the molded body 8, differences in carbon concentration are less likely to occur in sintered parts produced using the raw material powder 7 containing the mixed powder for powder metallurgy 1. Sintered parts that are less likely to have differences in carbon concentration are less likely to have variations in dimensional accuracy. Furthermore, the mixed powder for powder metallurgy 1 containing graphite powder 5 that is less likely to scatter is less likely to adhere to equipment around the die 91, making it easier to prevent the equipment from becoming dirty.

[0049] ≪Process C≫ In step C, the press-molded compact 8 is sintered. FIG. 3 shows an example in which the compact 8 is sintered in a furnace 92. By sintering, the particles of the mixed powder for powder metallurgy 1 are bonded together. When the mixed powder for powder metallurgy 1 contains an iron-based powder 3, a copper powder 4, and a graphite powder 5, by sintering, the carbon of the graphite powder 5 diffuses into the iron-based powder 3 in a solid phase, and the copper of the copper powder 4 melts and spreads into the gaps between the particles.

[0050] During sintering, the compacts 8 are heated at a temperature not exceeding the melting point of the alloy forming the sintered part, for example, 900°C to 1300°C. A sintering temperature of 900°C or higher can improve the bonding strength between particles in the powder metallurgy mixture 1. The sintering temperature may be 1000°C to 1300°C, 1050°C to 1300°C, or 1100°C to 1300°C. Sintering may be performed as a batch process in which a predetermined number of compacts 8 are heated at once in a furnace 92. While FIG. 3 shows an example in which one compact is heated, multiple compacts 8 may be heated at once, as shown in FIG. 9 (described later). The sintering atmosphere may be, for example, vacuum, nitrogen, hydrogen, or argon. Sintering may also be performed as a continuous process in which the compacts 8 are continuously heated.

[0051] [Test Example 1] In Test Example 1, a mixed powder for powder metallurgy containing graphite powder mainly composed of spherical graphite powder and a mixed powder for powder metallurgy containing graphite powder mainly composed of flake graphite powder were prepared, and the dispersibility of the graphite powders was evaluated.

[0052] <Sample description> Seven samples were prepared: Samples No. 1-1 to No. 1-5, Sample No. 1-11, and Sample No. 1-12. Each sample contained a powder metallurgy mixture containing 2 mass% copper powder, 1 mass% graphite powder, and the remainder being an iron-based powder. The iron-based powder was a pure iron powder made of pure iron and was JIP301A manufactured by JFE Steel Corporation. The iron-based powder had an average particle size of 95 μm. The copper powder was a pure copper powder made of pure copper and was Cu-AtW-250 manufactured by Fukuda Metal Foil & Powder Co., Ltd. The average particle size of the copper powder was 35 μm. Each sample also contained a lubricant. In each sample, the raw powder containing the powder metallurgy mixture and the lubricant served as the raw powder for the sintered part. The lubricant comprised 0.8 mass% of the raw powder, based on 100 mass% of the raw powder. The lubricant was a powder made of ethylenebisstearamide and was BAP-1 manufactured by Dainichi Chemical Industry Co., Ltd. The average particle size of the lubricant was 25 μm. Samples No. 1-1 to No. 1-5, No. 1-11, and No. 1-12 differed in the graphite powder.

[0053] In Samples No. 1-1 to No. 1-5, the graphite powder mainly contains spherical graphite powder. The graphite powder in Samples No. 1-1 to No. 1-5 is CGB series graphite powder manufactured by Nippon Graphite Industries Co., Ltd. The content of spherical graphite powder in Samples No. 1-1 to No. 1-5 is 70% or more. Samples No. 1-1 to No. 1-5 differ in the average particle size of the graphite powder. The average particle size of the graphite powder is the median diameter D50, which is the cumulative 50% from the smallest diameter side in the number-based particle size distribution measured using a laser diffraction particle size analyzer. The average particle size of the graphite powder is 6 μm for Sample No. 1-1, 8 μm for Sample No. 1-2, 12 μm for Sample No. 1-3, 15 μm for Sample No. 1-4, and 20 μm for Sample No. 1-5. The SEM image shown in Figure 2 is of graphite powder sample No. 1-1.

[0054] In Samples No. 1-11 and No. 1-12, the graphite powder mainly contains flake graphite powder. The graphite powder in Sample No. 1-11 is J-CPB manufactured by Nippon Graphite Industries Co., Ltd. The graphite powder in Sample No. 1-12 is CPB manufactured by Nippon Graphite Industries Co., Ltd. In Samples No. 1-11 and No. 1-12, the content of flake graphite powder in the graphite powder is 70% or more. The average particle size of the graphite powder differs between Samples No. 1-11 and No. 1-12. The average particle size of the graphite powder is 5 μm for Sample No. 1-11 and 23 μm for Sample No. 1-12. The SEM image shown in Figure 4 is the graphite powder of Sample No. 1-11.

[0055] <Apparent density of graphite powder> The apparent density of the graphite powder in each sample was measured. The apparent density of the graphite powder was measured based on JIS Z 2504:2020, "Metal Powders - Apparent Density Measurement Method." The funnel used for the measurement had an orifice diameter of 5.5±0.1 mm and a funnel angle of 60°. The results are shown in Figure 5. In the graph shown in Figure 5, the horizontal axis represents the average particle size of the graphite powder, and the vertical axis represents the apparent density. Filled circles represent the results for Samples No. 1-1, No. 1-2, No. 1-3, and No. 1-5. Open circles represent the results for Samples No. 1-11 and No. 1-12.

[0056] As shown in Figure 5, Samples No. 1-1, No. 1-2, No. 1-3, and No. 1-5 have higher apparent densities than Samples No. 1-11 and No. 1-12. These results show that when the graphite powder contains mainly spherical graphite powder, the apparent density is higher than when the graphite powder contains flake graphite powder.

[0057] <Tap density of graphite powder> The tap density of the graphite powder in each sample was measured. The tap density of the graphite powder was measured based on JIS Z 2512:2012 "Metal Powders - Tap Density Measurement Method." The measuring device used was a Tap Denser KYT-5000 manufactured by Seishin Enterprise Co., Ltd. The tap stroke was 10 mm, the tapping speed was 180 times per minute, and the number of taps was 700 or more. The results are shown in Figure 6. In the graph shown in Figure 6, the horizontal axis represents the average particle size of the graphite powder and the vertical axis represents the tap density. Filled circles indicate the results for Samples No. 1-1 to No. 1-5. Open circles indicate the results for Samples No. 1-11 and No. 1-12.

[0058] As shown in Figure 6, Samples No. 1-1 to No. 1-5 have higher tap densities than Samples No. 1-11 and No. 1-12. This result shows that when the graphite powder contains mainly spherical graphite powder, the tap density is higher than when the graphite powder contains flake graphite powder.

[0059] <Graphite powder dispersibility> The graphite powder dispersibility of each sample was investigated. The graphite powder dispersibility was evaluated by measuring the number of dispersed graphite powder particles using a test apparatus 10 shown in Figure 7. The test apparatus 10 includes a case 11, an orifice 12, and a dust meter 13. The case 11 is an acrylic case. The case 11 has a width W of 500 mm, a depth D of 350 mm, and a height H1 of 400 mm. The depth D is the length perpendicular to the plane of the paper in Figure 7. The orifice 12 is attached to the top plate of the case 11. The dust meter 13 is located adjacent to the side panel of the case 11. The dust meter 13 is placed on a base 14. The height H2 of the base 14 is 50 mm. The height H3 of the dust meter 13 is 178 mm. The length L between the side panel of the case 11 adjacent to the dust meter 13 and the center of the orifice 12 is 350 mm. The dust meter 13 used was a PM2.5 dust monitor dust meter DC110PRO manufactured by Satotec. This dust meter 13 measures the number of particles using a laser light scattering method. The measurement started in an environment where the number of particles of 0.5 μm or larger was 500 or less.

[0060] The raw material powder 7 of each sample was allowed to fall freely into the case 11 from the orifice 12, and the maximum count of the scattered raw material powder 7 was measured with the dust meter 13. In this example, for reference, measurement was also carried out on sample No. 100, which consisted only of an iron-based powder. The results are shown in Figure 8. In the graph shown in Figure 8, the horizontal axis represents the sample number, and the vertical axis represents the maximum count measured with the dust meter 13. For ease of understanding, the scattered powder is shown by a dashed line in Figure 7.

[0061] As shown in Figure 8, the number of scattered powder particles in Sample No. 100 was less than 600. Sample No. 100 was composed of only iron-based powder and did not contain graphite powder, making it difficult to scatter. Comparing Sample No. 1-1 and Sample No. 1-11, which have similar average particle sizes, the number of scattered powder particles in Sample No. 1-1 was approximately 60% less than in Sample No. 1-11. It is believed that Sample No. 1-1's graphite powder mainly contains spherical graphite powder, which makes it difficult for the graphite powder to scatter, and therefore the entire powder. Similarly, comparing Sample No. 1-5 and Sample No. 1-12, which have similar average particle sizes, the number of scattered powder particles in Sample No. 1-5 was approximately 50% less than in Sample No. 1-12. It is believed that Sample No. 1-5's graphite powder mainly contains spherical graphite powder, which makes it difficult for the graphite powder to scatter, and therefore the entire powder. These results show that when the graphite powder contains mainly spherical graphite powder, the powder as a whole is less likely to scatter.

[0062] As shown in Figure 8, when comparing Sample No. 1-1, Sample No. 1-2, and Sample No. 1-5, in which the graphite powder mainly contains spherical graphite powder, the larger the average particle size of the graphite powder, the fewer the scattered powder particles. In other words, the smaller the average particle size of the graphite powder, the greater the scattered powder particles. Even if the scattered powder particles are large, it can be seen that when the graphite powder mainly contains spherical graphite powder, the graphite powder is less likely to scatter, and ultimately the entire powder is less likely to scatter, compared to when the graphite powder has a similar average particle size and mainly contains flake graphite powder.

[0063] [Test Example 2] In Test Example 2, sintered parts were produced using the above-mentioned Sample No. 1-3 and Sample No. 1-12, and the dimensional variations of the sintered parts were evaluated.

[0064] The raw material powder of sample No. 1-3 was pressed into a compact, which was then sintered to produce a sintered part. Similarly, the raw material powder of sample No. 1-12 was pressed into a compact, which was then sintered to produce a sintered part. The press-forming and sintering conditions were the same for both samples. The compact shape was cylindrical, with an outer diameter of 70 mm and an inner diameter of 45 mm. Sintering was carried out in a continuous sintering furnace, with three compacts stacked vertically and arranged five horizontally and five vertically on a mounting table. The sintering temperature was 1100°C or higher, and the sintering temperature was maintained for 15 minutes or more. The sintering atmosphere was nitrogen.

[0065] FIG. 9 shows multiple sintered parts 20 arranged on a mounting table 21 after sintering. For ease of understanding, each sintered part 20 is shown cylindrical in shape in FIG. 9. To evaluate the dimensional variation of the sintered parts 20, two sintered part groups 20A located near the center of the mounting table 21 and two sintered part groups 20B located near the periphery of the mounting table 21 were used. The dimensions of each sintered part 20 in the two sintered part groups 20A and each sintered part 20 in the two sintered part groups 20B were measured, and the dimensional variation of a total of 12 sintered parts 20 was calculated. The results are shown in FIG. 10. In the graph shown in FIG. 10, the horizontal axis represents sample number and the vertical axis represents the dimensional variation of the sintered parts 20.

[0066] As shown in Figure 10, Sample No. 1-3, in which the graphite powder mainly contained spherical graphite powder, had smaller dimensional variation than the sample containing flake graphite powder. Sample No. 1-3 contains mainly spherical graphite powder, so the graphite powder is less likely to scatter when the powder metallurgy mixed powder is fed into a press die. Therefore, it is thought that the graphite powder is less likely to be unevenly distributed in the compact, which makes it less likely for differences in carbon concentration to occur in the sintered parts. It is thought that the less differences in carbon concentration to occur makes it less likely for variations in dimensional accuracy to occur. [Explanation of symbols]

[0067] 1 Mixed powder for powder metallurgy 2 Metal powder 3 Iron-based powder 4 copper powder 5. Graphite powder 6. Spherical graphite powder 6s Flake graphite powder 7 Raw material powder 8 Molded body 91 Mold 92 Furnace 10 Test equipment 11 cases 12 Orifice 13 Dust meter 14 Foundation W width L length H1, H2, H3 height 20 Sintered parts 20A, 20B Sintered parts group 21 Mounting table

Claims

1. Contains metal powder and graphite powder, The graphite powder includes spherical graphite powder, Each particle of the spherical graphite powder has an aspect ratio, expressed as the ratio of the length of the major axis to the length of the minor axis, of 4 or less; The content of the spherical graphite powder in the graphite powder is 70% or more, The apparent density of the graphite powder is 0.2 g / cm ３ That's all, The graphite powder has a tap density of 0.5 g / cm ３ That's all. Mixed powder for powder metallurgy.

2. the metal powder comprises an iron-based powder; 2. The mixed powder for powder metallurgy according to claim 1, wherein each particle of the iron-based powder contains more than 50% by weight of iron.

3. 3. The mixed powder for powder metallurgy according to claim 2, wherein a content ratio of the iron-based powder in the mixed powder for powder metallurgy is 90 mass % or more.

4. The metal powder further comprises copper powder; a content ratio of the copper powder in the mixed powder for powder metallurgy is more than 0% by mass and 8.0% by mass or less, 4. The mixed powder for powder metallurgy according to claim 3, wherein a content of the graphite powder in the mixed powder for powder metallurgy is 0.1% by mass or more and 2.0% by mass or less.

5. 2. The mixed powder for powder metallurgy according to claim 1, wherein the graphite powder has an average particle size of 3 μm or more and 40 μm or less.

6. A step of preparing the mixed powder for powder metallurgy according to any one of claims 1 to 5; a step of pressing a raw material powder containing the mixed powder for powder metallurgy to produce a compact; and sintering the compact. Manufacturing methods for sintered parts.

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