JPH06316704A - Production of metallic sintered body - Google Patents

Abstract

PURPOSE:To obtain a sintered body excellent in corrosion resistance, magnetic properties or the like, at the time of obtaining a sintered body by subjecting the injection molded body of a mixture of metal powder and a thermoplastic binder to degreasing and sintering, by specifying the degreasing ratio of carbon. CONSTITUTION:An injection molded body is produced from a mixture obtd. by mixing metal powder and a thermoplastic binder in a prescribed ratio. In the stage of degreasing the thermoplastic binder from the same molded body, the degreasing is executred so as to regulate the degreasing ratio of carbon to be >=99%. The degreasing stage can be executed by various methods, but, degreasing by a degreasing furnace in an oxidizing atmosphere is preferable, thereby, oxidized films are formed on the surface of the metal powder, the films are mutually bonded to increase the strength of the metallic molded body, and the subsequent treating is made easy. According to the kinds of the metal powder to be applied, degreasing can be executed in air, but, in a gaseous mixture of oxygen and an inert gas, degreasing can be executed in such a manner that the mixing ratio of both is changed in accordance with the affinity of the metal powder and oxygen. Furthermore, the state of the pressure in the atmosphere is regulated to the ordinary one or reduced one.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention provides an injection molded body from a mixture of metal powder and a thermoplastic binder at a predetermined ratio, and a degreasing step of degreasing the thermoplastic binder from the injection molded body. The present invention relates to a method for producing a metal sintered body, which is then sintered to obtain a metal sintered body.

[0002]

2. Description of the Related Art Mechanical components can be manufactured or manufactured by various methods, but are generally manufactured from metal powder as follows. That is, to the metal element powder or alloy powder, a thermoplastic binder is added in order to impart fluidity, plasticizing action, infiltrating action, hardening action, etc. and kneaded, and the obtained mixture is injection-molded into a desired shape, for example. A metal injection molded body is molded and the thermoplastic binder is removed from the injection molded body and then sintered. By the way, as is clear from the above description, the thermoplastic binder has various functions, but the main function is to knead the metal powder and perform injection molding with an injection machine, and obtain the obtained metal injection molded article. The function is to keep the injection-molded body in a predetermined shape until it is sintered in the furnace. If the thermoplastic binder remains in the injection-molded body, it reacts with or mixes with the metal powder during sintering, and the metal-sintered body Adversely affect. Therefore, a degreasing step of removing the thermoplastic binder from the injection molded body is performed before sintering.

A degreasing method for removing a thermoplastic binder from a metal injection-molded article is well known in the art without mentioning a literature name. A method of degreasing by heating in an inert atmosphere or a vacuum atmosphere, a degreasing in a solvent And a method of decomposing the thermoplastic binder by irradiating infrared rays, and these methods are industrially practiced. The reason for degreasing in an inert atmosphere is to prevent the metal injection-molded body from being oxidized during degreasing, and in order to prevent oxidation, a method of degreasing in a reducing atmosphere such as hydrogen gas may also be used. A method of degreasing in a solvent in a low temperature range in which a metal injection-molded body is hard to oxidize is also practiced. On the other hand, Japanese Patent Application Laid-Open No. 4-285102 proposes a degreasing method of degreasing in an atmosphere containing oxygen.

On the other hand, a thermoplastic binder used for molding a metal injection molded body is, for example, Japanese Patent Laid-Open No. 61-96001.
As described in JP-A No. 62-87234 and the like, it is generally a mixture of polyoxyethylene, wax, butyl stearate and the like, and contains a thermoplastic organic substance as a main component. Therefore, the constituent elements of the thermoplastic binder are carbon, hydrogen, nitrogen, oxygen and the like.

[0005]

As described above, various degreasing methods for removing the thermoplastic binder before obtaining the sintered body from the metal injection-molded body have been proposed, and by appropriately adopting these methods. Thermoplastic binders harmful to the sintered metal products are removed. By the way, the constituent elements of the thermoplastic binders currently used are carbon, hydrogen, nitrogen, oxygen, etc., and if these elements remain in the injection molded body,
As described above, although it is an element that significantly impairs the metal characteristics of the sintered body, carbon has a particularly large effect. Therefore, there is also a method in which the residual amounts of [O] and [C] in the injection-molded body after degreasing are stoichiometrically controlled, and the reaction of C + 2O → CO2 is performed at the time of sintering to simultaneously perform deoxidation and decarburization. It has been implemented. However, it is technically extremely difficult to arbitrarily control [O] and [C] in the injection molded body,
The balance of both elements may be lost, and the amount of one of [O] and [C] in the injection-molded product may become abnormally high. In particular,
When [O] is at a low level and the amount of [C] is large, [C] cannot be removed by post-treatment, and loss due to defective components of the sintered body cannot be avoided.

As described above, degreasing is performed in order to obtain a predetermined quality metal sintered body from the metal injection molded body.
As disclosed in Japanese Patent Application Laid-Open No. 4-285102, it is the actual situation that 5 to 10% of the thermoplastic binder remains. Leaving such an amount of thermoplastic binder,
About 06 to 0.08% of carbon remains in the green body. As a result, a predetermined quality metal sintered body is not obtained. In particular, when 0.06% or more of carbon remains in the stainless steel, the corrosion resistance is significantly reduced, and when such amount of carbon is contained in the magnetic material, the magnetic properties are similarly deteriorated. The present invention is intended to solve the above-mentioned conventional drawbacks. Specifically, even if the currently used thermoplastic binder is used, a predetermined quality metal sintered body, that is, corrosion resistance and magnetic properties are obtained. It is an object of the present invention to provide a method for producing a metal sintered body, which is capable of obtaining a metal sintered body having excellent properties.

[0007]

In order to achieve the above object, the present invention provides an injection-molded article from a mixture of metal powder and a thermoplastic binder in a predetermined ratio, and the injection-molded article is used to After the degreasing process of degreasing the thermoplastic binder,
Then, when the metal sintered body is obtained by sintering, the thermoplastic binder is degreased in the degreasing step so that the degreasing rate of carbon is 99% or more.

[0008]

The metal powder in the present invention refers to a metal element or alloy element alone or a mixture of one or more metal elements or alloy elements. Practical examples include alloy steel powders for nickel-based magnetic materials represented by PB-48 and PC-78, alloy steel powders for cobalt-based magnetic materials represented by 50Co-Fe, Invar, Super Invar and Kovar. The alloy steel powder represented by the above, the stainless steel powder, the high speed steel powder such as SKH61 and SKH57, and the like can be applied. In all of these metal powders, when a degreasing step of removing the thermoplastic binder from the metal injection molded body is carried out in an oxidizing atmosphere, an oxide film is formed on the surface of the metal powders and the mutual bonding of the metal powders is promoted. It is a metal element or alloy powder.

These metal powders can be obtained, for example, by a reduction method, a gas atomization method, a vacuum atomization method, a rotating consumable electrode method, etc. as disclosed in JP-A-56-108802. However, when applying a water atomizing method in which mechanical energy is applied to molten metal through water to crush it, and the crushed molten metal is cooled and solidified to obtain metal powder, the shape of the obtained metal powder is irregular. Since the bonding force between the metal powders is strong, the strength of the metal injection molded body to be molded becomes high. Therefore, degreasing can be performed almost completely.

The particle size of the powder metal applied to the present invention is not particularly limited as long as it can be sintered. However, considering the strength of the injection-molded body after degreasing, the cost for obtaining the metal powder, etc., 1 to 70 μm, preferably 2 to 45 μm
It is preferable to apply the metal powder of Further, the type of metal powder does not matter. That is, the metal powder may be a single metal or an alloy, or one or a mixture of two or more thereof.

As the thermoplastic binder, conventionally used thermoplastic binders can be applied as long as they are excellent in moldability and degreasing action. For example, a mixture of wax, polyvinyl butyral using ethyl alcohol as a solvent, vinyl chloride resin using toluene as a solvent, and vinyl chloride resin using butyl acetate as a solvent is applied. Alternatively, a mixture of paraffin wax, ethylene acrylate, polyethylene and the like can be applied. In any case, a thermoplastic binder that exerts a fluidizing action, a plasticizing action, a wetting action, a hardening action and the like on the metal powder is applied.

A metal powder selected as described above,
The thermoplastic binder is put into a well-known mixer, kneaded at a predetermined temperature for a predetermined time, pelletized by a pelletizer, and made into a compound. The temperature at the time of kneading is 80 to 160 ° C., depending on the kind of the thermoplastic binder,
Kneading time can be shortened by raising the temperature. As an injection machine and a mold device necessary for injection molding, conventionally known ones are applied. Further, according to the present invention, it is possible to manufacture a product having a complicated shape such as a rotor of a gyroscope motor and an impeller for a small motor.

The degreasing step of degreasing the metal injection-molded article obtained as described above can be carried out by the various methods described above, but it is desirable to degrease in a degreasing furnace in an oxidizing atmosphere. This is because when degreasing is performed in an oxidizing atmosphere, an oxide film is formed on the surface of the metal powder, the films are bonded to each other, and the strength of the metal injection-molded article is increased, and the subsequent handling becomes easy. The oxygen concentration in the oxidizing atmosphere is adjusted to 20 to 70% according to the difference in the affinity of the metal powder, which is an element of the metal injection-molded article, for oxygen. Since air contains about 21% oxygen, it can be degreased in air, depending on the type of metal powder applied. By degreasing in air, the cost of the degreasing process can be significantly reduced.
In this way, since it can be degreased even in air, the oxidizing atmosphere in the present embodiment usually means in air, but of course oxygen and nitrogen gas, argon, and an inert gas such as helium. This also means in a mixed gas atmosphere. That is, the oxygen concentration in the atmosphere can be appropriately adjusted by changing the mixing ratio of oxygen and the inert gas according to the affinity between the metal powder and oxygen. The pressure in the atmosphere is either normal pressure or reduced pressure.

When degreasing is performed in the degreasing furnace, the temperature inside the degreasing furnace is controlled and the temperature is raised with a predetermined temperature gradient. Although the temperature gradient depends on the kind of the thermoplastic binder, it is desirable to prevent the deformation of the metal injection-molded article by reducing the gradient up to near the melting point of the thermoplastic resin. Then, in order to further promote the formation of an oxide film on the surface of the metal powder, degreasing may be performed at a relatively high temperature of 500 ° C.

As described above, even if degreasing is performed in an air atmosphere, the amount of carbon in the air is as small as about 0.03% in terms of carbon dioxide gas, so that the effect of carbon on the metal sintered body is small.
However, when a mixed gas obtained by adding 20 to 70% oxygen to an inert gas such as nitrogen gas or argon gas is used, the effect of carbon from the atmospheric gas can be eliminated.

[0016]

【Example】

Example 1: [Preparation of metal powder] This was carried out for 3% Si-Fe, which is a typical steel type of magnetic material. The metal powder was obtained by dissolving 3% Si-Fe and using a water atomizing method. That is, high-pressure water was sprayed on the melt, which was crushed and cooled and solidified to obtain a metal powder. The main component composition, average particle size and maximum particle size of the obtained 3% Si-Fe metal powder were measured. The results are shown in Table 1. Table 1 Component composition Element ratio (Wt%) Si 3.05 C 0.028
Mn 0.010 S 0.0005 P 0.0010 Fe Bal Average particle size 9.46 μm Maximum particle size 38 μm

[Preparation of compound] The above metal powder 8
To 9.6% by weight, 10.4% by weight of a thermoplastic binder having the composition shown in Table 2 below is added, and 100 to 100% in a kneader is added.
The mixture was kneaded for 45 minutes under the condition of 130 ° C, and then pelletized with a pelletizer. A compound was obtained by cooling with cold air. Table 2 Component ratio (Wt%) Polyoxyethylene-polyoxypropylene condensate-based polyether 60 Montane wax 20 Butyl stearate 10 Paraffin wax 10 [Injection molding] The above compound is injection molded to manufacture a dot printer yoke. It was injected into a mold by a machine to obtain a green body. [Degreasing] This green body was put in a commercially available degreasing furnace and degreased according to the heating pattern shown in Table 3. Atmosphere gas, that is, air was used. The pressure at this time is 15-6
It was 0 mmAq. Table 3 Temperature time Temperature rising rate Normal temperature to 80 degrees C 49 degrees C / Hr 80 to 160 degrees C 157 degrees C / Hr 160 to 450 degrees C 22 14 degrees C / Hr

Degreasing was performed under the above conditions, and the amounts of carbon and oxygen were measured. The results are shown in Table 4. Table 4 Green body Degreased injection molded body of this example Carbon (%) 6.4 0.05 Oxygen (ppm) 3.900 16.200 As is apparent from Table 4, according to Example 1, thermoplastic bonding Despite degreasing 99% or more of carbon in the agent,
Even if the metal injection-molded body after degreasing was taken out of the degreasing furnace and moved, it did not break. It is considered that the reason is that an oxide film was formed on the surface of the metal powder and the films were bonded to each other, and the strength did not decrease particularly because the reaction between SiO ₂ and Fe ₂ O ₃ proceeded. [Sintering] The metal injection-molded body after degreasing was sintered in a vacuum sintering furnace at a temperature of 1280 ° C for 3 hours to obtain a product. The component composition of the obtained product is shown in Table 5. Table 5 Component composition (Wt%) Element ratio Si 3.04 C 0.028
Mn 0.010 S 0.0005 P 0.0010 Fe Bal The sintered density was 98.2%.

In Table 5 showing the composition of the product of this example and Table 1 showing the composition of the raw material powder, paying attention to the composition, particularly carbon C, since degreasing is almost completely performed in this example, that is, heat Since 99% or more of the carbon in the plastic binder has been degreased, no increase in carbon harmful to the magnetic properties of the product is observed. FIG. 1 is a diagram showing the relationship between μm, which is an important magnetic property value, and carbon content. From this diagram, when 10% of the thermoplastic binder is left as in the conventional case, the carbon content is 0.
It can be seen that the content of 06% or more significantly reduces the μm. On the other hand, in this example, no increase in carbon in the product was observed, and it was found that μm maintains a high value because it is as low as 0.028%.

Example 2: [Preparation of metal powder] This was carried out for SUS304L, which is a typical stainless steel type. That is, SUS304L was dissolved and a metal powder was obtained by the water atomizing method in the same manner as in Example 1. The main component composition and particle size of the obtained SUS304L powder were measured. The results are shown in Table 6. Table 6 Component composition Element ratio (Wt%) C 0.012 Ni 11.25 Cr 18.31 Si 1.00 Mn 0.18 S 0.004 P 0.028 Fe Bal Average particle size 9.24 μm Maximum particle size 41 0.0 μm [Injection molding], [Degreasing], [Sintering] The compounds were adjusted in the same manner as in Example 1, and the obtained compound was injected into a mold by an injection molding machine to produce a green body of an impeller for a small motor. Got Degreasing and sintering were performed under substantially the same conditions as in Example 1. Table 7 shows the component composition of the obtained product after sintering. Table 7 Composition components Element ratio (Wt%) C 0.011 Ni 11.23 Cr 18.30 Si 1.01 Mn 0.17 S 0.004 P 0.028 Fe Bal The sintered density is 98.3%. Met.

Comparing the component composition of the product with the component composition of the raw metal powder, particularly for carbon, as in Example 1, degreasing was performed almost completely in this example, that is, the thermoplastic binder. Since the carbon contained therein is degreased by 99% or more, no increase in carbon harmful to the corrosion resistance of the product is observed. FIG. 2 is a diagram showing the effect of SUS304L stainless steel on pitting corrosion. In this example, no increase in carbon in the product was observed, and the corrosion resistance was extremely excellent because it was as low as 0.011%. I understand.

[0022]

As described above, according to the invention, a degreasing step of making an injection molded body from a mixture of metal powder and a thermoplastic binder in a predetermined ratio and degreasing the thermoplastic binder from the injection molded body. And then sintering to obtain a metal sintered body, the thermoplastic binder is degreased in the degreasing step so that the degreasing rate of carbon is 99% or more. Does not increase. Therefore,
It is possible to obtain a metal sintered body having excellent magnetic properties and corrosion resistance.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between carbon content and magnetic characteristics in 3% Si—Fe.

FIG. 2 is a graph showing the relationship between the carbon content and corrosion resistance in SUS304L stainless steel.

Claims (1)

[Claims] 1. An injection molded body is made from a mixture of metal powder and a thermoplastic binder in a predetermined ratio, and a degreasing step of degreasing the thermoplastic binder from the injection molded body is performed,
Then, when sintering to obtain a metal sintered body, in the degreasing step, the thermoplastic binder is degreased so that the degreasing rate of carbon is 99% or more. .