JPH02290901A - Metal fine powder for compacting and manufacture of sintered body thereof - Google Patents

Abstract

PURPOSE:To provide fine powder for compacting which can manufacture a sintered stainless steel having a little impurity and good corrosion resistance by specifying particle diameter and specific surface area of the metal fine powder for manufacturing the sintered stainless steel. CONSTITUTION:This fine powder for compacting is the metal fine particles which are used to manufacture the sintered stainless steel and have 10+ or -5mu average practicle diameter and <=0.7m<2>/g specific surface area. After compacting by adding and mixing binder to the fine powder for compacting, by heating this green compact under non- oxidizing atmosphere, the binder is removed and C/O mol ratio is regulated to 0.5-3.0. After that, this green compact is sintered at 1000-1350 deg.C and <=0.1Torr pressure and successively, by sintering at 1200-1350 deg.C under non-oxidizing atmosphere, the sintered stainless steel having excellent corrosion resistance and sufficient mechanical characteristic is obtd. When the average particle diameter of the above fine powder is less than 5mu, packing property and degreasing property are reduced and further, the corrosion resistance and mechanical characteristic are reduced, and when this is more than 15mu, the sinterability and compatibility are reduced and dimensional accuracy in the sintered body is deteriorated. When the specific surface area is more than 0.7m<2>/g, the degreasing property is reduced.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a molding metal fine powder used in the production of sintered stainless steel with excellent corrosion resistance, and a sintered body using the molding metal fine powder. It is about the method.

[Prior Art] In recent years, the production of sintered parts using powder metallurgy has shown remarkable growth, and the scope of application of sintered parts is expanding. Among these, automobile parts, electronic/electrical parts, etc. using stainless steel,
As the shapes of office parts become more complex, the manufacturing method is also replacing the cutting method with the powder metallurgy method.

However, sintered alloys manufactured by powder metallurgy have pores, and these pores have the disadvantage of impairing corrosion resistance and mechanical properties. Therefore, the density of the sintered alloy must be as high as possible, and the density ratio (ratio of bulk density to true density) is 92
% or more is desired.

In conventional mold molding, the raw material thickness is several tens to 150 um.
Because of the large size, the density ratio was 80 to 90% only by molding and sintering, and a sufficiently high density could not be obtained. In particular, since the raw material is coarse, there are large gaps between particles and pores with a diameter of 50 μm or more. This is because the gaps do not shrink and disappear even during sintering, but remain in the sintered structure, resulting in a noticeable deterioration in corrosion resistance. Therefore, in order to improve corrosion resistance, sintered alloys have been developed in which other alloying elements are added to stainless steel to create a liquid phase and increase the density. For example, JP-A-58-2 1
As shown in No. 3859, there is a sintered material in which by adding Co or B, a liquid phase containing Co or B is generated during sintering, and this liquid phase is dispersed in the base so as to fill the pores. However, CO is an expensive powder that increases the cost of the product.
The economical advantage of powder metallurgy is lost.

Also, as shown in Japanese Patent Application Laid-Open No. 61-253349,
Sintered stainless steel has also been proposed, which is made densified by adding P and causing the appearance of a liquid phase. However, the liquid phase in which P is dissolved remains as a brittle phase after cooling, which deteriorates the mechanical properties. Therefore, methods of increasing density by adding such alloying elements and performing liquid phase sintering must be avoided. Furthermore, there are methods such as recompacting or resintering the sintered material, or hot forging or hot isostatically treating the sintered material to reduce residual porosity that directly affects corrosion resistance as much as possible. In this case, there were problems such as the process became complicated, special equipment was required, and the work became complicated. Furthermore, recently, a method for sintering sintered bodies using fine metal powder has been proposed. Although this material has excellent sinterability, it has the disadvantage that it cannot be formed into molds. This is because the molded product has cracks and lamination defects, and it also gets into the mold clearance and damages the mold. For this reason, injection molding using a binder is advantageous as a processing method suitable for mass production as it is possible to manufacture parts with complex shapes. However, since the resin and wax used as the binder are not completely removed by the debinding agent treatment and remain, impurities in the final sintered body increase, causing deterioration in corrosion resistance.

[Problems to be Solved by the Invention] The present invention solves the above-mentioned problems of the prior art, and provides a molding metal fine powder that can produce sintered stainless steel with few impurities and good corrosion resistance, and a sintering method using this metal fine powder. The aim is to provide a method for manufacturing the body.

[Means to solve the problem]

The present inventors have arrived at the present invention as a result of various studies to solve the above-mentioned problems. 1O±5μm and a specific surface area of 0.7rn'/g or less, and after adding and mixing a binder to the metal fines and molding, the molded body is made into a non-oxidizing material. The binder is removed by heating in an atmosphere and the C/0 molar ratio is reduced to 0. 5-3. After that, the temperature was adjusted to 1000 to 1350°C and the pressure was adjusted to 0. Sintering below 1 Torr, followed by sintering at 1200 to 13 Torr in a non-oxidizing atmosphere.
The present invention provides a method for producing a sintered body characterized by sintering at 50°C.

[Effect]

In the present invention, the average particle size of the metal fine powder is limited to lO±5 gm. The reason is shown below.

If the average particle size is less than 5 μm, the particle size becomes too small and the filling properties deteriorate, so the amount of binder added becomes large. As a result, the degreasing performance is reduced, which not only increases the amount of residual carbon in the molded product but also causes it to have an uneven carbon distribution, impairing its corrosion resistance and mechanical properties. Therefore,
The lower limit was set at 5 μm. If the average particle size exceeds 15 μm, the particle size becomes too large and sinterability deteriorates. Furthermore, the kneading properties of the binder are poor, resulting in an unevenly mixed state, resulting in poor moldability and uneven density distribution in the molded product. As a result, the final sintered body has poor dimensional accuracy, such as deformation and warpage. Therefore, the upper limit is 15μ
It was set as m.

Even if the average particle size of fine metal powder is limited as described above, the specific surface area (BET method) is 0. 7 If r11"/g is exceeded, the sinterability improves, but the amount of binder covering the surface of the metal fine powder increases. As a result, the degreasing performance decreases and the mechanical and chemical properties of the final sintered body are adversely affected. Therefore, the upper limit of the specific surface area was set to 0.7rn2/g.

In the present invention, Cr-containing powder such as water atomized powder having a stenite composition (SUS316) is used as the metal fine powder to obtain stainless steel by sintering. Since the present invention uses fine powder with an average particle size of lO±5 μm, defects such as lamination and cracking occur during molding if only this fine powder is used. Therefore, in order to prevent these defects from occurring, a binder is added and mixed before molding. The binder is thermoplastic resin,
It can be molded using wax or a mixture of both. Of course, the present invention is not limited to these binders. The amount of binder used varies depending on the molding method. In addition to conventional mold press machines, molding machines include extrusion molding machines, powder rolling machines,
Injection molding machines are used. The amount of binder used in ordinary mold presses is approximately 0.5 to 3.0% by weight, but the injection molding method, which allows products with complex shapes to be molded, requires approximately 10% by weight of binder. After molding, the temperature is raised and maintained at a constant rate in a non-oxidizing atmosphere to remove the binder. If the temperature increase rate is too fast, the product will crack or swell, so it should be 5℃/h to 20℃.
/h to reach ambient temperature. The binder remains without being completely removed, but by promoting the reaction between the carbon in this residual binder and the oxygen in the oxide film on the surface of the fine metal powder,
In order to reduce as much as possible the amount of 0.0 impurities in the final sintered body, the C/O molar ratio is adjusted to an optimal value.

The metal fine powder in the present invention is a fine powder containing Cr, which is a hardly reducible element. In the present invention, compared to reduction in a hydrogen atmosphere used in a normal sintering process, reduced pressure sintering allows reduction to be easily promoted by the action of contained C, and as a result, a high-density sintered body can be produced. Obtainable. The sintering action begins at the point of contact between particles and proceeds by the solid-state diffusion of atoms. However, if the powder surface is covered with oxides, the diffusion of atoms is blocked and densification does not proceed, resulting in the formation of a sintered body. High heat level is not achieved. In other words, in order to obtain high density, it is necessary to reduce the Cr-based oxide, and for that purpose, the pressure of 0. Sinter at less than I T o r r. 0,
If the pressure exceeds 1 Torr, the reduction reaction of Cr-based oxides will be difficult to proceed, so the upper limit should be set at 0. I T o r r.

As mentioned above, the reduction reaction of Cr-based oxides can be easily promoted by containing C, but in this case, it is necessary to appropriately adjust the C/O molar ratio in the compact before vacuum sintering. be. This is because the reduction of 0.0 in the sintered body is achieved by the progress of the reaction C+0→CO C+20−CO2. Therefore, the molar ratio of C and O reduced by the reduction reaction is 0.5 to 1. Therefore, regarding the amount of C,0 in the compact before sintering,
The amount must be the sum of the above low 11tc, 0 amount and the final sintered body C, 0 amount. In addition, if the C/O molar ratio does not take into account operational C and 0 fluctuation factors such as the contamination status in the sintering furnace, the sintered body will have an excess of C or O, and C≦0.06 weight. % 0≦0.3% by weight cannot be obtained. The present inventors have learned from experience the appropriate range of the C/O molar ratio. When the lower limit of the C/0 molar ratio is less than 0.5%, the C content in the sintered body exceeds 0.06% by weight, and no increase in sintered density is observed. On the other hand, the C/O molar ratio is 3.0
If the content exceeds 0.3% by weight, the sintered body will contain more than 0.3% by weight, and the pores will become coarser due to the appearance of a liquid phase, resulting in poor corrosion resistance and loss of shape. Therefore, C in the compact before vacuum sintering
/O molar ratio was defined in the range of 0.5 to 3.0.

The reason why the temperature range for vacuum sintering is set to 1000-1350°C is that at temperatures lower than 1000°C, the Cr-based oxide is not sufficiently reduced, so the oxide remains and inhibits subsequent sintering. Therefore, the lower limit was set at 1000°C. On the other hand, 13
When sintered at temperatures exceeding 50°C, not only does the amount of Cr evaporate from the surface increase and the concentration distribution becomes non-uniform, but defects such as the appearance of a liquid phase and loss of shape are observed. Therefore, the upper limit was set at 1350°C.

1200-1350 in a non-oxidizing atmosphere to achieve densification and homogenization of alloying elements by diffusion.
Sinter at °C. The previous step of vacuum sintering creates contact points between the particles and sintering begins, but by raising the temperature even higher, diffusion is promoted and sintering progresses, making the remaining pores smaller and more spheroidal. The reason why the atmosphere was non-oxidizing was to prevent Cr evaporation at high temperatures of 1200° C. or higher.

Note that the gas used for the non-oxidizing atmosphere here is Ar%He.
% Inert gas such as N2, H2, Co, CH4 . C3
H. or combustion exhaust gas.

Sintering in the range of 1200-1350℃ is 1200
At temperatures lower than °C, the diffusion of elements is slow, making it impossible to increase the density and make the alloy composition of the sintered body uniform. If the temperature exceeds 1350°C, a liquid phase will appear, causing the shape to collapse and a brittle phase to remain, resulting in a decrease in strength. For this reason, the lower limit was set at 1200°C and the upper limit was set at 1350°C. [Example] As fine metal powder, stainless steel composition (S
Using US316) water atomized material, it was classified to have an average particle size of 3.8, 8.5, ■3.0, and 19.2.
It was adjusted to a micron-sized powder. The specific surface area of these fine powders is BE
When measured by T method, they were 0.8, 0.6, respectively.
They were 0.4 and 0.3rn2/g.

Thermoplastic resin and wax were mixed with these fine powders and kneaded using a pressurizer. After cooling the kneaded material, it is crushed into granules to form pellets, and the pellets are molded into a length of 40 mm using an injection molding machine. Medium 20mm. It was molded into a rectangular parallelepiped with a thickness of 3 mm. The molded body was heated to 600°C at a heating rate of 10°C/h in a nitrogen atmosphere.
heating until the C/0 molar ratio in the molded body is 1.0 to 2.
The binder was removed so that it became 0. These were prepared under reduced pressure (<
1 0-3 Torr) 1 1 50"C for 1 hour, and then held at 1300°C for 3 hours in an Ar gas atmosphere at normal pressure. After cooling, the density ratio was determined from the density and true density using the Archimedes method. In addition, the C.0 content of the sintered body was analyzed.In addition, in order to evaluate the corrosion resistance, the sintered body was left in artificial sweat for 24 hours,
Afterwards, we checked using a stereomicroscope to see if there was any rust.
If there is no rust at all, it is considered good, and if there is even a little rust or discoloration, it is considered rusted. Average particle size of metal fine powder and C.I. of sintered body Figure 1 shows the relationship between O content and density ratio. If the average particle size is less than 5 μm, the degreasing performance is poor, and the finer the powder, the better the sinterability, which leads to more pore clogging, and the gas inside the sintered body is trapped in the pores, resulting in stagnation of carbon removal and zero removal. Therefore, rust occurred in the corrosion test using artificial sweat test. On the other hand, when the thickness exceeds 15 μm, the moldability is poor due to decreased fluidity. Furthermore, the roughness of the material resulted in poor sinterability and large residual pores, resulting in poor corrosion resistance. In metal fine powder whose average particle size and specific surface area are within the limited range of the present invention, decarbonization and de0ization reactions proceed sufficiently, and the sintered density shows no rust at all in a corrosion test using an artificial sweat test. A stainless steel with excellent density and corrosion resistance was obtained.

[Effects of the Invention] The sintered stainless steel produced by the method for producing metal fine powder and sintered body of the present invention has excellent corrosion resistance and sufficient mechanical properties, and can be used as a material under severe conditions. Can be used widely.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing the relationship between the average particle size of fine metal powder and the density ratio, C content, and 0 content of the final sintered body in Examples.

Claims (1)

[Claims] 1. Fine metal powder used for manufacturing sintered stainless steel, with an average particle size of 10±5 μm and a specific surface area of 0.7 m^
2/g or less. 2. After adding and mixing a binder to the fine metal powder according to claim 1 and molding, the molded body is heated in a non-oxidizing atmosphere to remove the binder and adjust the C/O molar ratio to 0.5 to 0.5. 3.0
After that, the temperature is 1000-1350℃ and the pressure is 0.
．． A method for producing a sintered body, comprising sintering at 1 Torr or less, and then sintering at 1200 to 1350°C in a non-oxidizing atmosphere.