JP2000144206A - Compression molding method of powder for powder metallurgy - Google Patents

Abstract

PROBLEM TO BE SOLVED: To surely obtain a compression molded body of high density by filling a powder for powder metallurgy with a lubricant blended therein in a forming die to an inner wall surface the lubricant is applied, and setting the forming temperature in the compression molding to be not lower than the melting point of the lubricant blended in the powder. SOLUTION: In compression molding a powder for powder metallurgy such as iron powder and iron-based alloy powder, the forming temperature is controlled to >= Tm, more preferably, >= Tm and <= [Tm×3] where Tm is the melting point of the lubricant blended in raw powder. The lubricant is melted and exuded to a surface of a molded body, and naturally removed from pores between the raw powders, and the exuded lubricant effectively works to reduce the friction between the forming die and the powder, and the compression molded body can highly be densified. Thus, the mechanical characteristics and the magnetic characteristics of the final molded body can be improved.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for compressively molding powder for powder metallurgy such as iron powder or iron-based alloy powder. And a method capable of improving magnetic characteristics and the like.

[0002]

2. Description of the Related Art In compression molding powders for powder metallurgy such as iron powders and iron-base alloy powders, for example, a lubricant is added to a raw material powder for the compression molding in advance to increase the fluidity of the powder and to form the powder. A method of reducing friction with the mold has been adopted. The compounding amount of the lubricant is generally in the range of 0.2 to 10% by weight based on the powder to be sintered (for example, JP-A-2-156002), and Metal powder is used.
er report, Vol. 42. No. 11, p7
81-786 (1987), the compounding amount of the lubricant is 0.1.
It is reported that the maximum compression density can be obtained at 5%, and even in the present practical examples, most of them are 0.5 to 1.0% by weight.
Range.

In this case, even if the molding pressure is increased in order to increase the density of the compact, the lubricant is filled in the gaps between the raw material powders and the improvement in the density is hindered. . On the other hand, if the amount of the lubricant is reduced, the friction between the powder and the mold becomes large, so that high-pressure densification cannot be performed, and the problem of shortening the life of the mold arises.

[0004] On the other hand, it is also known that when a lubricant is applied to the inner wall surface of a mold, friction between the powder and the mold is reduced. However, since the lubricant is not blended with the raw material powder, the fluidity and the filling property of the powder are poor, and it is difficult to obtain a high-density molded product even by compression molding under high pressure.

[0005] In order to increase the density of the compression molded body, the raw material powder and the molding die are set at a temperature lower than the melting point of the lubricant (usually 70 ° C to 12 ° C).
U.S. Pat. No.
o. No. 4,9555,798. Also,
Japanese Patent Application Laid-Open No. Hei 5-271709 discloses that pressure molding is performed by heating to a temperature lower than the temperature at which the lubricant is completely melted (specifically, about 370 ° C. or lower). These methods are all based on the finding that when the lubricant is melted, the fluidity of the powder is significantly reduced.

However, even in this case, the lubricant remains in the molded body with the usual amount of the lubricant, so that it is not a fundamental measure for increasing the density.

[0007] Japanese Patent Application Laid-Open No. 9-272901 discloses that a lubricant-free powder is used, a lubricant is applied to the inner wall surface of the mold, and the mold is heated to 150 to 400 ° C. A method of increasing the density of a compact by pressure molding has been proposed. However, in this method, since no lubricant is blended in the raw material powder, the fluidity of the powder is poor, and as a result, a sufficient high density cannot be achieved. In addition, in this method, there is also a problem that the density variation inside the molded body increases.

[0008]

SUMMARY OF THE INVENTION The present invention solves the problems of the prior art as described above, in particular, the problem of insufficient fluidity of powder during compression molding and the problem of friction with a compression mold. It is an object of the present invention to establish a technique for reliably obtaining a compression molded body.

[0009]

The compression molding method according to the present invention, which can solve the above-mentioned problems, is a powder metallurgy in which a lubricant is compounded in a mold in which a lubricant is applied to an inner wall surface. In filling and compressing the powder, the molding temperature is set at the melting point of the lubricant compounded in the powder.

In carrying out the above method, the amount of the lubricant to be added to the powder is 0.01 to 0.20% by weight.
The molding temperature is preferably not less than the melting point (Tm) of the lubricant compounded in the powder and [Tm ×
3] The range is as follows.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted various studies from various angles in order to increase the density of a compression molded article.
As described above, a lubricant is applied to the inner wall surface of the mold, and the powder for powder metallurgy containing a relatively small amount of lubricant is filled into the mold for compression molding. Was controlled to be equal to or higher than the melting point of the lubricant.

Hereinafter, the configuration and preferred embodiments of the present invention will be described in detail. In the present invention, the powder for powder metallurgy is used to produce a molded article of an arbitrary shape through a process such as sintering after applying powder to a predetermined shape by pressure molding. Powder is generically referred to. Also, in this specification, friction between the molding die and the powder, including those in which a lubricant or the like is blended to reduce friction between the powders,
Sometimes referred to as powder for powder metallurgy.

Specific examples thereof include metal powders and ceramic powders, and the present invention is very effectively applied to metal powders which undergo plastic deformation during molding. The most typical example is pure iron powder (impurity). Small amounts of C, Mn, S
including those containing i, P, S, Cr, O, N, etc.)
And Ni, Mo, Mn, C to improve the strength after sintering.
Alloy powder (pre-alloy type, diffusion type, hybrid type thereof and the like) to which r, Si and other elements are intentionally added.

However, it should be noted that if the amount of the alloying element is too large, the iron powder hardens and the compressibility decreases, which may hinder the high density of powder metallurgy products.

In order to enhance the properties after sintering, various alloying elements, for example, graphite, Cu, Ni, Mo, etc. may be used alone or in combination of two or more.
Further, a composite powder in which graphite or the like is adhered to the surface of iron powder using a small amount of a binder may be used.

In the present invention, what is extremely important is that the temperature at the time of compression molding is specified. Therefore, the following description will be made mainly on the molding temperature.

In the present invention, based on the melting point (Tm) of the lubricant compounded in the raw material powder, the molding temperature is set to Tm or more,
It is more preferable to control the temperature to be not less than Tm and not more than [Tm × 3]. By adopting such a molding temperature, it is possible to remarkably increase the density of the compression-molded body. Becomes The reason is as follows.

That is, when the molding temperature in the above range is adopted,
When the lubricant is melted during compression molding, the lubricant oozes out on the surface of the molded product and is naturally removed from the gaps between the raw material powders, and the oozed lubricant effectively works to reduce the friction between the molding die and the powder. Thus, the density of the compression molded body can be increased.

As described above, in the prior art, the molding temperature is preferably lower than the melting point of the lubricant from the viewpoint of improving the fluidity of the powder. However, in the present invention, the amount of the lubricant is reduced to a small extent to reduce the lubricant. Such problems can be avoided. However, if the molding temperature is too high, the thermal deterioration of the lubricant becomes severe, causing problems such as seizure with the molding die. Therefore, the melting point (Tm) of the lubricant does not exceed [Tm × 3]. Should be adjusted accordingly.

The molding temperature is also important for lowering the deformation resistance of the powder and increasing the density, and the most suitable temperature condition is selected from the above-mentioned preferable temperature range according to the type of the raw material powder to be used. It is desired.

The heating means is not particularly limited, and a method in which the powder itself is preheated to an appropriate temperature, or a method in which the powder is filled into a mold and then heated by utilizing heat transfer from the mold, and the like can be employed. If the temperature of the mold is low, the temperature of the powder decreases during compression molding and the compressibility decreases, so it is essential to employ a heating method that can appropriately maintain the temperature during molding.

As an example, when iron powder is used as the powder, at a low temperature of less than 80 ° C., the deformation resistance of the iron powder is high, so that the plastic deformation ability becomes insufficient and the density is increased. On the other hand, the increase in density caused by increasing the molding temperature saturates at about 500 ° C., and increasing the molding temperature beyond that is not only economically wasteful, but rather impedes the shortening of the life of the mold. Appears. Taking the above reasons into consideration, a more preferable lower limit of the molding temperature is 100 ° C., and a more preferable upper limit temperature is 250 ° C.

If the amount of the lubricant is not added at all (0%), the lubricating effect during pressurization cannot be obtained, so that the density of the molded body cannot be improved, and the density variation inside the molded body becomes large. The shrinkage at the time of knotting becomes partially non-uniform, causing dimensional variations. When a small amount of the lubricant is blended, the variation in the density of the molded body is eliminated. However, when the blending amount is too large, the lubricant is left in the molded body, and the densification is rather hindered.

Therefore, in order to avoid the above-mentioned difficulties caused by excessive blending while effectively exhibiting the lubricating effect, the amount of the lubricant blended should be 0.01% by weight or more, more preferably 0.02% by weight or more, and 0.8% by weight. Wt% or less, more preferably 0.2 wt%
Below, it is more preferable to select from the range of 0.10% or less.

There are no particular restrictions on the type of lubricant, and all lubricants can be used as long as they can be melted in a moldable temperature range. The most common type is a metal salt of a higher fatty acid such as stearic acid. Alternatively, it is a wax-based lubricant.

The type of the lubricant applied to the inner wall surface of the mold is not particularly limited. For example, metal salts of higher fatty acids such as metal stearate, waxes, molyaden disulfide, BN
Any lubricant known for powder metallurgy can be used, such as a system or a graphite system. The above lubricants can be used alone or in combination of two or more. Also, it is preferable to select and use an optimal lubricant in consideration of the type of the raw material powder, the heating temperature during molding, and the like.

The method of applying the lubricant to the inner wall surface of the mold includes a method of attaching the lubricant in a solid state, a method of dissolving or dispersing in a solvent to apply a brush or spray, and a method of applying the lubricant by heating and melting the lubricant. Should be adopted.

As a method for heating the mold, any method such as external heater heating, Joule heating by energization, high frequency heating, or infrared heating can be employed. In addition, since it takes some time for the powder filled in the mold to be warmed by the heated mold, in order to complete the compression molding in a shorter time, the powder must be appropriately filled before filling into the mold. Preheating to a suitable temperature is also effective. However, it should be noted that if the preheating temperature is too high, the iron powder may be oxidized or sintered.

The preferred pressure during compression molding is 5 ton / cm
² above, in the range 15 t / cm ² or less, plastic deformation due to pressure when the molding pressure using a raw material powder of example iron powder mainly less than the above range does not proceed sufficiently, hate becomes insufficient compaction In addition, since the high pressure densification is saturated at the above upper limit pressure, excessively increasing the molding pressure exceeding 15 tons / cm ² is wasteful from the viewpoint of economy and equipment.

[0030]

EXAMPLES Hereinafter, the present invention will be described more specifically with reference to Examples. However, the following Examples are not intended to limit the present invention, and should be appropriately performed within a range that can conform to the purpose described above and below. Modifications can be made and implemented, all of which are included in the technical scope of the present invention.

Example Using a V-type mixer, raw material powders are mixed for 30 minutes with the composition shown in Table 1. About 20 g of each obtained mixed powder was weighed and heated at a predetermined temperature in a mold (diameter 31.5 mm).
× 12.5 mm deep) and pressure molded under the conditions shown in Tables 1 and 2. The density of the obtained molded body was measured by the following method, and the results shown in Table 2 were obtained. The density of the compact was calculated from the volume and weight of the compact.

[0032]

[Table 1]

[0033]

[Table 2]

From Tables 1 and 2, the following can be considered. No. Nos. 1 to 8 were obtained by using Li stearate as both a lubricant in the mold and a powdered lubricant, and examining the effect on the density of the compact when variously changing the temperature during press molding. N whose temperature is higher than the melting point of the lubricant
o. At 5 to 8, the molding temperature is lower than the melting point of the lubricant.
o. Higher compact densities are obtained as compared to Nos. 1 to 4 (see FIG. 1).

No. Nos. 9 to 16 were obtained by examining the effects on the compact density when various temperatures were changed during pressure molding using a graphite-based lubricant as a lubricant in the mold and zinc stearate as a powdered lubricant. No., in which the molding temperature was higher than the melting point of the powder-containing lubricant. In Nos. 12 to 16, the molding temperature is lower than the melting point of the lubricant. 9-11
A higher molded body density is obtained as compared to (see FIG. 2).

[0036]

According to the present invention, the temperature at the time of press molding is set to be equal to or higher than the melting point of the lubricant based on the melting point of the lubricant blended in the metallurgical powder. Thereby, a high-density compression-molded body can be easily and reliably obtained.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the temperature during compression molding and the density of a compact.

FIG. 2 is a graph showing the relationship between the temperature during compression molding and the density of a compact.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yoshikazu Seki 2-3-1, Shinhama, Araimachi, Takasago City, Hyogo Prefecture Inside Kobe Steel, Ltd. Takasago Works (72) Inventor Masaaki Sato 2-3-3, Araimachi Shinama, Takasago City, Hyogo Prefecture No. 1 Inside Kobe Steel, Ltd. Takasago Works (72) Inventor Noriaki Akagi 2-3-1, Shinama, Araimachi, Takasago City, Hyogo Prefecture Inside Kobe Steel Works, Takasago Works (72) Inventor Tetsuya Sawayama Araimachi, Takasago City, Hyogo Prefecture 2-3-1 Niihama Kobe Steel, Ltd. Takasago Works F-term (reference) 4K018 CA02 CA16

Claims (3)

[Claims] In filling a powder metallurgy powder containing a lubricant into a molding die having a lubricant applied to an inner wall surface thereof and performing compression molding, the molding temperature is adjusted to the lubricant contained in the powder. Compression molding method for powder for powder metallurgy, wherein the melting point is not less than the melting point of the powder. 2. The compression molding method according to claim 1, wherein the molding temperature is controlled to be not less than the melting point (Tm) of the lubricant compounded in the powder and not more than [Tm × 3]. 3. The compression molding method according to claim 1, wherein the compounding amount of the lubricant compounded in the powder is 0.01 to 0.20% by weight.