JPS62278201A - Production of metallic powder for powder metallurgy - Google Patents

Abstract

PURPOSE:To prevent the segregation of fine particles and to improve the compressibility and formability of metallic powder for powder metallurgy by separately handling coarse particles and fine particles of metallic powder and uniformly sticking the fine particles to the outsides of the coarse particles to form aggregates of particles. CONSTITUTION:Coarse particles and fine particles of metallic powder are separately obtd. by production in separate production lines or by classification after production in the same production line. The coarse particles are mixed with the fine particles in a proper ratio and the fine particles are uniformly stuck to the outsides of the coarse particles to form aggregates of particles.

Description

Detailed Description of the Invention 3. Detailed Description of the Invention (Field of Industrial Application) The present invention is directed to a method for producing metal powder for powder metallurgy, in particular, to prevent segregation of fine powder in the metal powder and to improve compression and formability. The present invention also relates to a method for producing metal powder for powder metallurgy that has excellent sinterability.

(Prior art) Metal powder manufacturing methods, such as water atomization and oil atomization, produce powders with −i of 32 mesos (500 μI) or less, but metal powder produced by such liquid atomization methods , generally has a particle size distribution close to a normal distribution.

When using such atomized powder for sintering,
These powders are (atomized) - (solid-liquid separation) - (drying) - (reduction/decarburization/annealing) = (pulverization) = (classification) = (
blend) and processed according to this flow and used as sintering powder.

However, conventional techniques that include heat treatment (hereinafter simply referred to as heat treatment) for reducing, decarburizing, etc. do not fully utilize the characteristics of powder. In other words, this heat treatment is generally a reduction or decarburization treatment at a high temperature of 900 to 1150°C, but in the conventional method, coarse grain powder and fine grain powder are mixed in the same ratio at the time of atomization. Therefore, powder of sufficient quality and yield cannot be obtained.

For example, heat treatment of powder involves forming a powder layer several millimeters to several tens of millimeters thick on a moving bed of a roller hearth furnace, belt furnace, etc., and heating it to 900°C or higher using a heating method such as a radiant tube. However, if coarse grain powder and fine grain powder are mixed at random, that is, if the ratio of coarse grain powder and fine grain powder is not controlled, and if it is charged without any preliminary processing, coarse grain powder and fine-grained powder are likely to cause segregation, and particularly fine-grained powder forms strong clumps. Fine-grained powder is easier to sinter than coarse-grained powder, and because it is strongly bonded, it is difficult to crush it even in the subsequent crushing process. Because it is,
Coarse particles must be classified and removed, which ultimately reduces the yield of the powder manufacturing process. Moreover, the presence of segregation causes deterioration in quality as a metal powder for sintering.

This is due to the fact that the smaller the particle size, the easier it is to sinter at the same temperature. For example, 32/60 mesh starts sintering at 1050°C, while -350 mesh starts sintering at 750°C.

Table 1 shows a comparison between the particle size distribution before heat treatment and the particle size distribution of the particles after heat treatment and light pulverization. It can be seen that after heat treatment, the amount of fine particles is extremely reduced and the proportion of coarse particles increases.

Table 1 (Note) Heat treatment: Heat to 980°C for 30 minutes.

Furthermore, when the fine powders adhere to each other, they become porous particles, and when formed into a compact, the density and further the density of the sintered product decreases, resulting in a decrease in the strength of the sintered product.

On the other hand, in the premix method in which fine grain powder, which is an additive alloying element such as Ni, Mo, or Cu, is added to coarse grain iron powder, mixed, and compacted, the powder physical properties of coarse grain powder and fine grain powder ( Due to the large difference in density, particle shape, particle size, etc., there is a tendency for the premix to be classified during handling, such as during transportation before being sent to the next process, and segregation is likely to occur.

Note that Japanese Patent Publication No. 36046/1983 proposes a treatment for agglomeration of fine particles. In order to improve the apparent high density of atomized iron powder, it is made into irregularly shaped particles. However, since the entire atomized iron powder is subjected to agglomeration treatment, the excellent compressibility inherent in the atomized iron powder inevitably deteriorates. In addition, in order to prevent the fine powder from becoming coarse, the heat treatment process must be strictly controlled, and the crushing process after treatment must also be strictly controlled, so there is a concern that processing costs will increase. be.

(Problems to be Solved by the Invention) Thus, an object of the present invention is to solve the above-mentioned problems that occur in metal powder in which coarse powder and fine powder are randomly mixed, and to prevent segregation of fine powder. It is also an object of the present invention to provide a method for producing metal powder for powder metallurgy with improved sintering properties.

(Means for Solving the Problem) In order to achieve the above-mentioned object, the inventors of the present invention have conducted various studies and have found that the coarse particle fraction and the fine particle fraction of the metal powder are handled separately, and the characteristics of each are treated separately. I learned that by taking advantage of this, it is possible to relatively easily produce excellent raw material powder for powder metallurgy.

Here, the gist of the present invention is to divide metal powder into coarse particles and fine particles, mix the two in an appropriate mixing ratio, and create a particle aggregate in which fine particles are uniformly attached around coarse particles. This is a method for producing metal powder for powder metallurgy, which is characterized by granulating it into powder metallurgy.

First, the physical properties of the powder aggregate, which is the basis of the present invention, will be explained with reference to FIG. Figure 1 (a) shows a state in which only fine powder is aggregated. This has excellent sinterability, but poor compressibility and moldability. (B) is an aggregate consisting only of coarse powder, which has excellent compressibility and formability, but has extremely poor sinterability, making it impossible to obtain a sintered product with sufficient strength. (c) and (d) are mixtures of fine powder and coarse powder. In this case, the fine powder particles are unevenly distributed and aggregated in one part, so that the part with excellent sinterability (the part of fine powder) is separated from the part with excellent compressibility and moldability. When such powder is used for sintering, there are parts where the bond between the powders (so-called neck) is insufficient to grow, resulting in a decrease in strength.

(E) in FIG. 1 is a mixture of coarse powder and fine powder, and its aggregation state is ideal. That is, in the state (e), coarse powder with excellent compressibility and formability is uniformly covered with fine powder with excellent sinterability, and when this is compression molded and sintered, it becomes a dense powder. A product with high strength can be obtained.

For example, in metal powder produced by the atomization method, coarse powder and fine powder are randomly mixed, so if this is processed in the process described above (page 3), it will usually be from Figure 1 (a) to
The result is a mixture of particle aggregates (d). Some special measures must be taken to make most of the powder into the form shown in (e).

In the present invention, the powder obtained in the powder manufacturing process (any method such as an atomization method, a reduction method, an electrolytic method, or a pulverization method may be used) is subjected to a subsequent process, that is, a process such as reduction, decarburization, annealing, etc.
A new method was adopted in which coarse particles and fine particles were once separated and then blended at an appropriate mixing ratio. In general, the metal powder obtained in the powder manufacturing process is shown in Figure 2 (a).
As shown in , the particle size distribution is ＼゛0, which is close to the normal distribution.
The coarse particle fraction and fine particle fraction in the present invention include the following two definitions. (2) In FIG. 2 (a), when dividing before and after the boundary shown by the broken line, for example, if the boundary is above and below 250 mesh, the area above the sieve of 250 mesh is called the coarse particle fraction, and the area below the sieve is called the fine particle fraction. ■As shown in Figure 2 (b), powders (A,
In B), A is referred to as the coarse grain fraction and B is referred to as the fine grain fraction. ■
In this case, A and B may have the same component composition or may be different from each other.

The powders separated in this way are mixed at an appropriate mixing ratio so that the agglomeration state shown in Figure 1 (e) is obtained, that is, each coarse powder is covered with fine powder as evenly as possible. Ru. Although the mixing ratio cannot be determined uniquely, it can be calculated individually from the average particle size of the coarse particles and the average particle size of the fine particles, depending on the desired characteristics of the sintered body. can.

When the components or compositions of coarse and fine metal powders are the same, the odor is generally 50 to 80% of the coarse powder (weight, 20% of the fine powder).
~50% (by weight) is preferred. When the components or compositions of the coarse powder and the fine powder of the metal powder are different from each other, the coarse powder is 80 to 99% (heavy N), and the fine powder is 1 to 20% (heavy N).
Weight) is preferred.

As mentioned above, it is sufficient to distinguish between fine grain powder and coarse grain powder on a relative basis, but depending on the purpose, the boundaries may be divided at 250 mesh, 350 mesh, or 700 mesh.

Coarse powder only needs to be mainly coarse powder, and there is no problem even if some fine powder is mixed into the coarse powder. Figure 2 (b)
When using 82 types of powder A as shown in Figure 1, the fine part of A (A゛) acts as a fine particulate powder, while the coarse part of B (B'') acts as a coarse part of powder. , I3 is known, it is not difficult to appropriately select their blending ratio.

According to a preferred embodiment of the present invention, in the case of oil atomized copper powder,
The coarse powder can be 160 mesosh, and the fine powder can be -250 mesosh.

In addition, for example, coarse grain powder may be classified into several types, and fine grain powder may be blended and mixed for each to make granules.There are many ways to differentiate and combine fine grain powder and coarse grain powder. However, any of them is within the scope of the present invention as long as they exhibit the effect of adhering fine powder to coarse powder. However, the boundary is preferably 350 mesh in practical terms, and 700 mesh in terms of quality, and 350 mesh is the boundary that is easy to industrialize including peripheral equipment technology.

When the components or compositions of the coarse metal powder and the fine metal powder are different from each other, for example, when premixing iron powder and Ni, MO% Cu-based additive alloy powder, the coarse iron powder can be prepared by atomizing. Fine powder (N1%)
Additive alloy optical element powder such as Mos Cu) may be produced by any method. For example, reduction method, electrolytic method, pulverization method, etc. are not particularly limited. Also, coarse powder and fine powder may be produced by any method. It is not necessary to classify the base powder and the additive alloy powder as they may be relative, but it is preferable to use a powder finer than 250 mesh as the fine powder.

The granulation process in the present invention consists of (1) heating during the powder manufacturing process (
heating for reduction, decarburization, annealing, etc.);
■There is a case where it is carried out after the powder manufacturing process and in the product manufacturing process using the sintering method.

In the premix method of different powders, ■ is adopted.

In either case, a desired particle aggregate can be obtained by blending coarse powder and fine powder in an appropriate ratio and mixing well. However, it is preferred to use a suitable binder in the granulation process. In this case, coarse powder whose surface is wetted with an adhesive binder and dried fine powder are blended and mixed. Simply mixing coarse powder and fine powder
Fine grain powder may not necessarily be uniformly sprinkled around coarse grain powder, and it may also separate during the conveyance process after mixing, so to ensure that these defects are prevented I let them do it.

In that case, simply using an adhesive binder will not work.
For example, if the binder permeates the entire particle, fine particles tend to stick to each other.

Therefore, according to the present invention, by first wetting the coarse powder that is difficult to adhere to, and then allowing dry fine powder to coexist with this, the surface tension of the binder can be used to spread the surface of the coarse powder as uniformly as possible. It is attached to the surface. This can also prevent undesirable classification due to differences in specific gravity during the subsequent conveyance process.

The binder is removed during the subsequent heat treatment, and at the same time, the fine powder is adhered to the surface of the coarse powder.

(Operation) The third part is a flowchart showing an example of the method of the present invention. Coarse powder and fine powder are first prepared, mixed in appropriate proportions, sent to the granulation process with or without a binder, mixed and granulated, and the fine powder is sprinkled around the coarse particles. and granulate it to the required particle size. This is then sent to a conventional heating process for reduction, decarburization, or annealing. Generally, the processing conditions at this time are 600~
The treatment time is about 15 to 60 minutes at 1200°C. In this step, diffusion sintering occurs at the interface between the coarse powder and the fine powder, strengthening the bond between the two.

The raw material powder, which has become partially lumpy due to the above heat treatment, is further lightly pulverized and classified to remove excessively large portions. In order to use this as a sintering raw material powder, the particle size distribution is adjusted to the required one by blending particles with appropriate particle sizes depending on the application.

FIG. 4 schematically shows changes in the shape of powder particles in each of the above steps when the raw material powders are obtained from the same production line. For example, the starting metal powder, which is liquid atomized powder, is classified in advance to prepare coarse powder and fine powder.
and (b)), blend and mix these in appropriate proportions, and granulate with or without a binder,
It looks like fine particles are sprinkled around large particles (see Figure 4, fcl), and then heat-treated to solidify the adhesion of the fine particles (see Figure 4, fdl). ).

Here, the classification and granulation steps, which are the characteristics of the present invention, will be further explained.

The classification of coarse powder and fine powder differs depending on the particle size structure and purpose of the required sintering raw material powder, but for general water atomized and oil atomized powders as shown in Table 1 above, 250 mesh or 350 mesh is used. Depending on the case, the powder is divided into coarse powder and fine powder at 700 mesh.

It is advantageous to attach a binder to the coarse powder in advance, because by blending and mixing the fine powder with it, the fine powder is just sprinkled around the coarse powder and granulation is performed. be.

In this case, the adhesive binder used is mineral oil, vegetable oil, animal oil, or water, which is the atomizing medium of the liquid atomization method that is used in large quantities as steel powder for powder metallurgy, and is industrially inexpensive. .

When coarse powder is produced by a liquid atomization method, by adjusting the removal (drying) of the spray medium, the same state as when the surface of the coarse powder is wetted with a binder can be obtained. Further, as another adhesive binder, there is a hydrocarbon compound I# (eg, paraffin, resin, etc.), and if this is used, post-treatment is easy and it is advantageous in terms of cost.

When wetting the surface of coarse powder with adhesive binder, 400℃
The melting point is preferably 400° C. or lower because it must become liquid at a temperature of 400° C. or lower. On the other hand, in the debinding process of granulated powder, at temperatures above 600°C, adhesion between coarse and fine powder begins to occur, resulting in phenomena such as cranking, carburization, and the presence of residue, which increases the hardness of the powder. At the same time, it causes a decrease in compressibility. Therefore, the boiling point of the binder is preferably at most 600°C.

The optimum amount of adhesive binder to be added differs depending on the type of binder, but in general, it is 1 to 1% of the total amount of powder.
An increase in tensile strength was observed when the content was 0% by weight, especially when the content was 0% by weight.
The effect is remarkable for 5% debt. If the amount added exceeds 10% by weight, the adhesive binder will penetrate into the fine powder, and as a result, the surface tension of the binder will tend to cause granules of the fine powder to form, inevitably resulting in a decrease in compressibility.

However, if the amount is less than 1%, the binder cannot sufficiently wet the surface of the coarse grain powder, and therefore cannot exhibit its effect.

The blending of such a binder and the subsequent mixing and granulation method of coarse powder and fine powder are sufficient as long as the binder is uniformly adhered to the surface of the coarse powder, and as long as this is not limited, it is preferable to use a machine. For example, there are mechanical stirring methods such as mechanical stirring by sprinkling fine powder over coarse powder while applying chopping and vibration, and mixing using a stirring blade. The mixing time does not need to be very long; a few minutes to an hour is sufficient.

Classification and granulation in the case where oil, which is a spray medium for oil atomization, is used as a binder will be explained in more detail.

The atomized powder obtained by oil atomization has a particle size distribution of 60 mensider and an average particle size of 65 to 80 μm. Before these powder groups are forcibly deoiled using a centrifugal M machine, etc., the metal powders are suspended in oil and subjected to wet classification using oil as a spray medium.
After separating into coarse powder and fine powder at 0 mesh, the coarse powder is forcibly deoiled to about 10%, and the fine powder is dried after forced oil removal.

A fixed amount of dry fine powder is supplied to the coarse powder to which a suitable amount of oil has adhered, and then granulated using a granulator set of Peresotaida. The blending ratio of coarse grain powder and fine grain powder was 6:4 (heavy M). After granulation, 0.3 to 0.5% of carbonized carbon content was added during oil atomization, which also served as heat treatment. In order to reduce the amount of carbon, it was charged into a decarburization furnace and heat treated. At this time, after the fine powder is firmly adhered to the coarse powder due to the sintering phenomenon observed during the temperature rise and decarburization process, the loose adhesion between the granulated particles is broken up by light pulverization, and the particles are classified again. , blended to form a finished powder.

Next, FIGS. 5 to 7 are process diagrams of various modified examples for carrying out the method according to the present invention.

In FIG. 5, the liquid atomized powder is first separated into solid and liquid, then dried, and then classified into coarse powder and fine powder, which are again blended and mixed in appropriate proportions and granulated. After granulation, heat treatment is used to strengthen the adhesion between coarse and fine powder, and then a light pulverization process is carried out, and the metal for powder metallurgy is processed through the classification and blending processes again to become a sintering raw material. The powder is 20. The light pulverization process is carried out to break loose bonds between coarse particles, and can be carried out as necessary depending on the degree of sintering of the particles during the heat treatment.

FIG. 6 shows a method in which a part of the process shown in FIG. 5 is changed, and the liquid atomized powder is dried, reduced and decarburized, and then pulverized and classified. In the case of water atomization, a reduction process can be used, and in the case of oil atomization, a decarburization process can be used. The light crushing step is to break up adhesion and adhesion between granules.

FIG. 7 shows an example in which wet classification is performed prior to solid-liquid separation. In this case, water is used as a binder for water atomized powder, and oil is used for oil atomized powder as a binder in the granulation process. Generally, before solid-liquid separation, the material is wet classified, liquid separated to the required liquid content (for example, liquid separated using a centrifugal separator), then granulated, dried, and then heated.

Part 8 specifically shows the case where the present invention is applied to a premix of different types of powders. Coarse powder (for example, pure iron powder) and fine powder (Nis
Additive alloy powders such as Cu and Mo are mixed in a predetermined ratio and granulated. Also at this time, addition of a binder is optional. If these powders have already been subjected to heat treatments such as reduction and annealing, heating after granulation is not necessarily necessary, and they may be compressed, molded, and sent to the sintering process as they are.

Next, the present invention will be explained in more detail with reference to Examples.

Example 1 In this example, molten steel having the composition shown in Table 2 was produced by a normal oil atomization method according to the steps shown in FIG. Ta.

After solid-liquid separation of the oil atomized powder obtained in this way using a centrifuge, it is divided into coarse powder and fine powder at 350 Mekosh, and then 100/150.150/200.2
The above-mentioned fine powder was mixed with coarse powder of 00/250 and 250/350 mesh in a ratio of approximately 4:1 to 2:1 and granulated without a binder. The heat treatment was performed at 900°C for 60 minutes. The processing conditions are summarized in Table 3.

For comparison, a conventional method is also shown.

Table 3 The heat-treated powder obtained was further subjected to light pulverization, classification, and blending steps to obtain powder having the particle size distribution shown in Table 4. A microscopic photograph (400 magnification) of the granulated powder obtained at this time is shown in FIG. The yield and powder characteristics are summarized in Table 5.

Table 4 Table 5 The yield of powder of 60 mesh or less, which can be used as a standard raw material for powder metallurgy, is improved by about 6% by adopting the method of the present invention, which is good because it is classified before decarburization. This is due to particle size control. From the apparent density and fluidity, it appears that the powder produced by the method of the present invention has a slightly irregular shape.

Next, green compacts were formed and sintered using the powders produced by the present invention and the conventional method, each having the particle size distribution shown in Table 4 and the powder properties shown in Table 5.

The results are summarized in Tables 6 and 7, and these data show that the method of the present invention is an excellent method for improving green compact properties such as compressibility and formability, and sintered compact properties such as tensile and elongation. I understand that there is something.

Table 7 (Note) Test conditions Example 2 Using as-atomized steel powder obtained by the oil atomization method, -60 mesh, -100 mesh, -250
The 160 mesh and -100 mesh are respectively classified into coarse powder, and mineral oil (Super Highland 40, manufactured by Nippon Oil Co., Ltd.) is blended into this coarse powder as an adhesive binder. I went grain. The composition of the oil-atomized steel powder used is summarized in Table 8, and the granulation conditions are summarized in Table 9.

Table 8 Table 9 Minute mixing.

Next, after being blended and granulated in this way, the mixture was heated at 600°C for 20 minutes in a hydrogen atmosphere, then heated at 900°C for 13 minutes in an Hto-containing hydrogen atmosphere, and then heated at 970°C for 13 minutes.
It was decarburized by heating in an O-hydrogen atmosphere and pulverized. The powder properties, green compact properties, and sintered compact properties of the pulverized powder were investigated in the following manner.

Green compact properties: Zn-5t 0.8% addition, 5 Ton/a
J sintered body properties: 0.5%C-0.8%Zn-5t7,
Adjusted to Log/cj, sintered at 1250°C x 30 minutes under Ng atmosphere, heated at 850°C x 30 minutes under h atmosphere, then oil quenched, 200°C x 60 minutes N! Tempered under atmosphere.

The results are summarized in Table 10.

As can be seen, if only the tensile strength and Charpy impact value were to be improved compared to the conventional method, the sample light 1-7
It is possible to improve it to some extent by increasing the amount of fine powder as shown in 1-12.

However, as mentioned above, in that case, the compressibility is significantly reduced. In comparison, according to the present invention, improvements in compressibility and moldability are observed even though the amount of fine powder is increased.

Further, as shown in Table 10, the yield of -60 Mesosier is also significantly improved.

Furthermore, the difference in the mixing method according to the invention also has a great influence on its properties. For example, as can be seen by comparing samples 1-3 and 1-5, the difference is due to the mixing method,
In other words, it is based on the difference in particle morphology. Preferably, the use of mixing method A is good for improving characteristics.

Example 3 Using commercially available steel powder obtained by the oil atomization method and using wax (lost wax -21, manufactured by Kato Yoko ■) as an adhesive binder, the characteristics of the present invention were evaluated in comparison with the conventional method. did. The composition and particle size distribution of the atomized steel powder used are summarized in Tables 11 and 12, respectively.

Table 11 Table 13 Next, using the powder granulated in this way, 950
After heat treatment in an atmosphere of water and water for 60 minutes at
Table 14 summarizes the various properties of the powder classified by 60 mesosius, which were investigated in the same manner as in Example 2.

From these results, it is recognized that compressibility and sinterability are improved.

As in sample No. 2-4, if a binder is merely added to a mixture of coarse powder and fine powder without separating them into coarse powder and fine powder, the strength and toughness of the sintered body are significantly reduced.

Table 14 FIG. 10 is a graph showing the relationship between the amount of binder added and the tensile strength of the sintered body. The strength is improved by adding a binder, and this effect is seen when the binder is added in an amount of 1 weight % or more, whereas if it exceeds 10 weight %, the strength actually decreases. This is a common tendency regardless of the type of binder. Here, the tensile strength was measured on a sintered body obtained in the same manner as in Example 2.

Example 4 In this example, a case will be described in which pure iron powder is blended as coarse powder and N1% MO% CLI powder as an added alloying element is blended as fine powder.

Table 15 shows the properties of the powder used.

Table 15 The powder thus prepared was then mixed according to the mixing method shown in Table 16 and sintered according to Example 2. The mixing method and characteristics of the sintered body at that time are summarized in Table 16. Note that mineral oil (Super Highland 40, manufactured by Nippon Oil Corporation) was used as the binder.

As is clear from these results, when N1% MO is added, the tensile strength is improved by about 10 kgf/mm2, and the impact value is improved by about 0.5 kgf/ms+''.

Although no significant improvement was observed in Cu compared to these,
Improvement in characteristics is recognized. This is because Cu is a substance with a low melting point of 1083° C., so even if it is not coated in the initial green compact state, the difference is minimized by melting. Compared to that, Ni, Mo, especially Mo
Since it is difficult to diffuse, its effect is more pronounced.

Table 16 1.

Thoughts

[Brief explanation of the drawing]

Fig. 1 is a diagram schematically showing the state of aggregation of powder; Fig. 2 is a diagram explaining the definition of coarse powder and fine powder; Fig. 3 is a flowchart showing each step of the method according to the present invention. Sheet; Figure 4 is an explanatory diagram schematically showing changes in particle morphology during the classification, granulation, and twisting processing steps in the present invention; Figures 5 to 8 are diagrams showing each embodiment of the method of the present invention; ##≠ Figure 9 is a microscopic photograph of the metal stand obtained in the example of the present invention - OJv'' Figure 10 shows the relationship between the amount of binder added and the tensile strength of the sintered body. This is a graph showing the ice 3 concave 57th door, ro 7 7th yu δ diagram.

Claims (8)

[Claims] (1) The metal powder is divided into coarse particles and fine particles, and the two are mixed in an appropriate mixing ratio to form a particle aggregate in which fine particles are uniformly attached around the coarse particles. Features: A method for producing metal powder for powder metallurgy. (2) The method according to claim 1, wherein coarse powder and fine powder produced by separate production lines are blended, mixed, and granulated. (3) The method according to claim 1, wherein metal powders produced on the same production line are classified, and the obtained coarse powder and fine powder are blended, mixed, and granulated. (4) The method according to claim 2 or 3, wherein the metal powder manufacturing method is a liquid atomization method. (5) After wetting the surface of the coarse powder with an adhesive binder of 1 to 10% by weight of the total amount of powder, blending the coarse powder and fine powder;
5. A method according to any one of claims 1 to 4, which comprises mixing. (6) the adhesive binder is one selected from the group consisting of water, vegetable oil, mineral oil, paraffin and resin;
A method according to claim 5. (7) The method according to any one of claims 1 to 6, wherein the metal powder is oil atomized steel powder. (8) The method according to any one of claims 1 to 7, wherein the coarse powder and the fine powder of the metal powder have different components or compositions.