JPH0257660A - Manufacture of sintered alloy steel having excellent corrosion resistance - Google Patents

Abstract

PURPOSE:To manufacture the title sintered alloy by adding a binder to steel powder contg. specific wt.% of Cr and Ni, compacting it, sintering the compact in a nonoxidizing atmosphere at specific temp., sintering it under specific reduced pressure and specific temp. and furthermore sintering the same in a nonoxidizing atmosphere at specific temp. CONSTITUTION:A binder is added and mixed into steel powder contg., by weight, 15 to 25% Cr and 8 to 24% Ni and the mixture is compacted. In the compact, the binder is removed by heating in a nonoxidizing atmosphere. The compact is successively sintered at 1100 to 1370 deg.C in a nonoxidizing atmosphere. The sintered body is then sintered under <=0.1Torr reduced pressure at 1100 to 1370 deg.C; it is furthermore sintered at 1250 to 1370 deg.C in a nonoxidizing atmosphere. To the steel powder, <=10% Mo may be incorporated as well. By this method, the sintered alloy having excellent corrosion resistance can be manufactured.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a method for producing inexpensive sintered alloy steel with excellent corrosion resistance produced by a powder metallurgy method.

<Prior Art> In recent years, the production of sintered parts by powder metallurgy has shown remarkable growth, and the range of applications of sintered parts is expanding. Among these, as the shapes of automobile parts, electronic/electrical parts, and office parts using stainless steel become more complex, the manufacturing method is also being replaced from cutting methods to powder metallurgy methods.

However, sintered alloys manufactured by powder metallurgy have pores, and these pores have the disadvantage of impairing corrosion resistance and mechanical properties. Therefore, it is necessary that the density of the sintered alloy is as high as possible, and a density ratio of 92% or more is desired.

<Problems to be Solved by the Invention> When manufacturing sintered parts using powder metallurgy, conventional mold press molding has a large particle size of raw material powder ranging from 150 to several tens of micrometers, so it is difficult to achieve a density ratio by just molding and sintering. The density was 80 to 90%, and a sufficiently high density could not be obtained. In particular, since the raw material is coarse-grained powder, the gaps between particles are large, creating pores with a diameter of 50 μm or more. Although these pores shrink during sintering, they do not disappear and remain in the structure of the sintered alloy. However, the corrosion resistance was significantly deteriorated due to this.

Therefore, in order to increase the density and improve the corrosion resistance of sintered alloys,
Various methods have been proposed.

For example, by performing liquid phase sintering to achieve high density,
There are ways to improve corrosion resistance at the same time.

In this method, other alloying elements are added to stainless steel powder to improve corrosion resistance, and a liquid phase appears to increase the density.

Specifically, as shown in JP-A-58-213859, CO and B are dispersed in the dough so that a liquid phase containing Co and B is generated during sintering and fills the pores. This method uses a binding material. However, since CO is an expensive metal, it increases the cost of the product and impairs the economy, which is an advantage of powder metallurgy.

Furthermore, as shown in JP-A No. 61-253349, a sintered stainless steel has been proposed in which P is added to similarly cause a liquid phase to appear and to increase the density. However, since the liquid phase in which P is dissolved remains as a brittle phase after cooling, the mechanical properties deteriorate.

Therefore, a sintered alloy having desired characteristics cannot be obtained simply by adding such alloying elements and increasing the density by liquid phase sintering.

There is also a method of re-sintering the secondary sintered body at high temperature and high pressure (hot isostatic pressing, HIP) to increase the density and at the same time improve corrosion resistance.

This method was developed as a method for producing sintered ceramic bodies, but in recent years it has also been applied to the production of sintered alloys. Specifically, as the pressure medium, Ar gas is usually used, and the pressure is 1000 to 2000 atmospheres, 1300 to
This is a method in which the primary sintered body is re-sintered under conditions of 1500°C to form a dense sintered alloy.

Although this method can increase the density of the sintered alloy and improve its corrosion resistance, special equipment is required to obtain a pressure of 1,000 to 2,000 atmospheres, resulting in a very high cost.

Furthermore, the present applicant has proposed in Japanese Patent Application No. 63-156841 a method for manufacturing a sintered alloy steel having excellent corrosion resistance and mechanical properties.

According to this method, a sintered alloy steel with excellent corrosion resistance and mechanical properties can be obtained. However, fine powder with an average particle size of 15 μm or less must be used as the raw material steel powder. Since such fine powder is obtained by high-pressure water atomization or by classifying steel powder obtained by water atomization, the cost is extremely high.

As described above, the various methods proposed for increasing the density and improving the corrosion resistance of sintered alloys have drawbacks such as increased costs.

The present invention uses a powder having a normal particle size (average particle size 16 to
The purpose of the present invention is to provide a method for producing a sintered alloy steel that has excellent corrosion resistance because the density ratio is 92% or more even when alloyed steel powder (40 μm) is used, and there is no decrease in the amount of Cr on the surface of the steel powder.

Means for Solving the Problems> The present inventors have completed the present invention as a result of extensive research regarding the firing conditions for sintered alloy steel.

The present invention uses steel powder containing Cr: 18-25% by weight, Ni: 8-24% by weight, or contains Cr: 16-25% by weight, Ni: 8-24% by weight, Motto weight% or less. Using steel powder, after adding and mixing a binder to the steel powder and molding, the binder in the molded body is removed by heating in a non-oxidizing atmosphere. ~137
Sintered at 0°C, then cooled once to room temperature or without cooling, sintered at 1100 to 1370°C under reduced pressure of 0.1 Torr or less, and then cooled once to room temperature or without cooling. The present invention provides a method for producing a sintered alloy steel having excellent corrosion resistance, characterized in that sintering is carried out at 1250 to 1370 tons in a non-oxidizing atmosphere without oxidizing.

The present invention will be explained in detail below.

The raw material steel powder used in the method for producing sintered alloy steel with excellent corrosion resistance of the present invention contains 16 to 25% by weight of Cr and 8% by weight of Ni.
~24% by weight, containing 0 or less than 10% MO.
These components are important elements that affect corrosion resistance.
The content of each is limited by the following reasons.

Cr: The higher the Cr content, the better the corrosion resistance. If the content is less than 16% by weight, the desired corrosion resistance cannot be obtained; on the other hand, if it is added in excess of 25% by weight, no further significant improvement in the effect is observed, which is disadvantageous in terms of cost. .

Furthermore, when the Cr content is high, problems such as sigma embrittlement and 475° C. embrittlement occur.

Ni: Ni is an element necessary to stabilize the austenite phase. When the austenite phase is stabilized, mechanical properties such as corrosion resistance and toughness are improved. If the content is less than 8% by weight, the ability to form a stable austenite phase is poor and corrosion resistance deteriorates. On the other hand, even if it is added in an amount exceeding 24% by weight, no more significant effect will be observed, and this will be disadvantageous in terms of cost.

Mo: Mo is an element effective in improving corrosion resistance and oxidation resistance. Even if it is added in an amount exceeding 10% by weight, no further significant improvement in the effect is observed, which is disadvantageous in terms of cost.

The raw steel powder used in the present invention does not need to contain Mo, but as mentioned above, since MO is a metal effective in improving corrosion resistance and oxidation resistance, raw steel powder containing Mo may be used. By using powder, a sintered body with better corrosion resistance and oxidation resistance can be obtained.

The average grain size of the steel powder is one of the factors that influences the density ratio of the sintered body, and the smaller the average grain size, the higher the density ratio.
- If steel powder with an average particle size of more than 15 μm is used in the shell, the gaps between the particles that occur during molding will also become large, making it impossible to obtain the desired corrosion resistance. However, in the present invention, even if raw steel powder with a normal particle size (average particle size 16 to 40 μm) is used, by going through the sintering process described below, the gaps between the particles are small and the sintered bond has corrosion resistance. Gold steel is obtained.

Therefore, the raw material steel powder may be obtained by a conventional method such as an atomization method or a powder mixing method.

In the present invention, the steel powder described above is first molded, but since the average particle size is about 16 to 40 μm, liquid phase sintering does not proceed with the steel powder alone. Therefore, molding is performed after adding and mixing the binder.

The binder used is one mainly composed of a thermoplastic resin and/or wax, and a plasticizer, lubricant, degreasing accelerator, etc. are added as necessary.

Thermoplastic resins include acrylic, polyethylene,
There are polypropylene-based and polystyrene-based waxes, and waxes include natural waxes such as beeswax, Japanese wax, and montan wax, and low-molecular polyethylene, microcrystalline wax, paraffin wax, etc. There are synthetic waxes, and one or more selected from these waxes are used.

The plasticizer is selected depending on the combination with the main resin or wax, but specifically, di-2-ethylhexyl phthalate (DOP), diethyl phthalate (DEP)
Examples include di-n-butyl phthalate (DHP).

Examples of the lubricant include higher fatty acids, fatty acid amides, fatty acid esters, etc. In some cases, waxes are also used as the lubricant.

Moreover, a sublimable substance such as camphor can also be added as a degreasing accelerator.

Incidentally, the amount of the binder varies depending on the molding method in the post-process, and is 0.5 to 3.0% by weight in normal mold press molding, and about 10% by weight in injection molding.

Batch type or continuous type kneaders can be used to mix and knead steel powder and binder.Patch type kneaders include pressure kneaders and Banbury mixers, and continuous type kneaders include twin screw extrusion. Each type of machine is advantageously suited to the other.

After kneading, the mixture is granulated using a pelletizer or a pulverizer, if necessary, to obtain a molding compound.

Molding is performed by conventional mold press molding, extrusion molding, powder rolling molding, injection molding, etc., but injection molding is preferred.

Injection molding may be carried out using an injection molding machine used for ordinary injection molding, such as an ordinary injection molding machine for plastics, or an injection molding machine for various ceramics or metal powders. At this time, the injection pressure is usually 0.5 to 2
．． 5t/cm 2 degrees, temperature 100~180℃
That's about it.

After molding, heating is performed in a non-oxidizing atmosphere, preferably in a N atmosphere or in an Ar aeration atmosphere, to remove the binder.

Note that, as is known, the processing atmosphere at this time may be in a pressurized or reduced pressure state.

The maximum heating temperature is about 450 to 650°C, and
The holding time is about 0 to 6 hours.

At this time, if the heating rate is too high, cracks and bulges will occur in the obtained molded product, so it is preferable to heat the molded product at a temperature of 5 to 50%.

After removing the binder, sintering is performed. At the end of the heat treatment in the previous process, some of the binder remains, but due to sintering,
The reaction between the carbon in the residual binder and the oxygen in the oxide film present on the surface of the stainless steel powder is promoted to reduce the amount of C in the final sintered body. Therefore, sintering is performed after adjusting the degree of removal of the binder or performing an oxidation treatment after removal to adjust the C10 molar ratio to an optimum value, preferably 0.3 to 3.0.

The first stage sintering is performed at 1100-1370°C in a non-oxidizing atmosphere, with a holding time of 0.5-8 hours.

The first stage of sintering is liquid phase sintering that utilizes the presence of C derived from a binder and the like. This step significantly increases the density ratio of the sintered alloy steel.

Here, sintering is performed in a non-oxidizing atmosphere because if sintering is performed in an atmosphere containing oxygen, c+o-*co, CO
This is because the +O−+CO reaction progresses, carbon is consumed, and liquid phase sintering is not performed satisfactorily.

In addition, if the temperature is less than 1100°C, liquid phase sintering will not be possible and the density will be insufficient, while if it exceeds 1370°C, the liquid phase will be too large, causing the shape to collapse or a brittle phase to occur. This is not preferable because it causes a decrease in strength and also causes deterioration of the surface quality of the sintered alloy steel. When sintering is performed within this temperature range, liquid phase sintering is possible in the presence of C, and the density ratio of the sintered alloy steel is improved even if steel powder with a normal particle size is used as the raw material powder. do.

If the holding time is less than 0.5 hours, a sufficient increase in the density ratio cannot be expected, and if it exceeds 8 hours, the effect of gradually increasing the density ratio will be lost, which is not preferable.

After the first stage sintering, the material is cooled to room temperature or the second stage sintering step is performed without cooling. here,
The presence or absence of cooling depends on the equipment used for sintering.

The second stage sintering is performed under reduced pressure of 0.1 Torr or less.
The temperature is 100-1370°C and the holding time is 0.5-8 hours.

The second stage of sintering is reduced pressure sintering, and this process reduces the amount of C in the sintered alloy steel, which impairs corrosion resistance, and also reduces the amount of Cr.
The reduction of system oxides is promoted.

Compared to sintering under a hydrogen atmosphere, which is used in a normal sintering process, reduced pressure sintering can easily remove carbon due to the effect of the carbon content.
It is possible to promote the reduction of r-based oxides, and as a result,
High-density sintered alloy steel can be obtained.

The sintering process begins at the point of contact between steel powders and progresses through solid-state diffusion of metal atoms. However, if the surface of the steel powder is covered with oxides, the diffusion of metal atoms is blocked and densification progresses. First, densification of the sintered alloy steel cannot be achieved. That is, in order to obtain high density, it is necessary to reduce the Cr-based oxide, and for this purpose, sintering is performed under reduced pressure (below 0, I Torr). When the pressure exceeds 0.1 Torr,
The reduction reaction of Cr-based oxides is difficult to proceed.

Furthermore, the temperature during sintering also affects the reduction reaction of the Cr-based oxide.

If the temperature is less than 1100° C., sufficient decarburization cannot be expected and the Cr-based oxide is not sufficiently reduced, so the oxide remains and inhibits subsequent sintering. When the temperature exceeds 1370°C, sintering progresses before decarburization progresses, resulting in an increase in the amount of C in the sintered alloy steel, resulting in excess Ch<
The corrosion resistance of the sintered alloy steel deteriorates due to the formation of carbides of Fe and Cr, resulting in a low Cr band, or due to the presence of Cr carbides or C itself. Furthermore, C causes the appearance of a liquid phase, but if the amount of C is large, the liquid phase portion becomes too large, which causes the shape to collapse and a brittle phase to remain, resulting in a decrease in strength. This is undesirable because it causes deterioration.

If the holding time is less than 0.5 hours, sufficient decarburization and reduction of Cr-based oxides cannot be expected, and if it exceeds 8 hours, no gradual effect can be expected, which is not preferable.

After the second stage sintering, similarly to the first stage sintering, depending on the equipment used, the material is cooled to room temperature or the third stage sintering step is performed without cooling.

The third stage sintering is carried out at 1250-1370°C in a non-oxidizing atmosphere with a holding time of 0.5-8 hours.

The third stage of sintering serves both to increase the density of the sintered alloy steel and to perform solution treatment. Through this step, the density increases step by step, and the Cr on the surface of the steel powder, which was reduced in the second step, is compensated for by diffusion from the inside, and the amount of Cr becomes uniform.

Sintering up to the previous stage has created contact points between the steel powder, but by sintering at an even higher temperature, solid-state diffusion of metal atoms is promoted and sintering progresses, resulting in finer residual pores and a spherical shape. Achieve high density. The temperature at this time is 125
At temperatures below 0°C, the improvement in the density of the sintered alloy steel is not remarkable, and the Cr on the surface of the steel powder, which evaporated and decreased during the previous step of sintering under reduced pressure, is compensated for by diffusion from the inside. On the other hand, when sintering exceeds 1370°C, the amount of Cr evaporated from the surface of the steel powder increases, and not only does the Cr concentration distribution become uneven, but the liquid phase portion becomes too large. This is not preferable because it causes deterioration of the shape or leaves a brittle phase, resulting in a decrease in strength, and also causes deterioration of the surface quality of the sintered alloy steel.

If the retention time is less than 0.5 hours, sufficient high-density thinning and Cr
Diffusion is not performed for more than 8 hours, which is not preferable because a gradual effect can no longer be expected.

Note that the reason why this step is performed in a non-oxidizing atmosphere is to maintain the amount of C in the sintered alloy steel up to the previous stage and the amount of zero.

Furthermore, in the steps up to this point, gases used to create a non-oxidizing atmosphere include inert gases such as Ar and He, reducing gases such as CH4 and C3Ha, N2, combustion exhaust gas, and the like.

<Example> The present invention will be specifically explained based on examples.

(Example ■) Raw material powder: Cr: 18.7% by weight, Ni: 12.4% by weight, C: 0
．． A water atomized steel powder with an average particle size of 32 μm (powder A) having a composition of 0.02% by weight, 0.06% by weight of O, and the balance consisting of Fe and unavoidable impurity elements;
r: 24.6% by weight, Ni: 8.8% by weight, MO28.
Water atomized steel powder with an average particle size of 36 μm (B powder) was prepared.

In addition to these, thermoplastic resin 10 mainly composed of acrylic resin
% by weight was added and kneaded using a pressure kneader. This was injection molded at 160°C and it/cm2, and the
It was made into a molded body of 20 mm x 3 mm. Next, the temperature was raised to 600°C at a temperature increase rate of 10'C/h in an Ar atmosphere and held for 3 hours to remove the binder.

Next, as a first-stage sintering process, the temperature shown in Table 1 was maintained in Ar for 60 minutes, and then released in the atmosphere, and then, as a second-stage sintering process, a 0. After holding at 1250° C. for 5 hours under reduced pressure of 0.04 Torr, it was cooled to room temperature, and as a third stage sintering treatment, it was held at 1300° C. for 60 minutes in Ar to obtain sintered alloy steel.

After cooling, the density ratio is determined from the density and true density by the Archimedes method, and the concentration distribution of Cr in the sintered alloy steel is calculated by
Analyzed with MA device. When the Cr content on the surface of the sintered alloy steel was 80% or more of the internal Cr content, it was considered uniform, and when it was less than 80%, it was considered non-uniform.

Furthermore, in order to evaluate the corrosion resistance, sintered alloy steel was
After being left in artificial sweat at 0° C. for 24 hours, the presence or absence of rust was observed using a stereomicroscope.

If no rust was observed, it was considered good, and if there was even a little rust or discoloration, it was considered rusted.

The results are shown in Table 1.

(Example ir) Using powder A, a molded body was produced in the same manner as in Example ■,
The binder was removed. Next, as the first stage sintering process,
After being held at 1250°C for 60 minutes in Ar, it was allowed to cool in the atmosphere, and then as a second sintering process, 0.07To
After holding for 5 hours at the temperature shown in Table 2 under reduced pressure of rr,
Subsequently, as the third stage sintering process, 1300 ml of sintering was performed in Ar.
℃ for 60 minutes to obtain a sintered alloy steel.

Thereafter, the density ratio was determined in the same manner as in Example (2), and the C content and Cr concentration distribution in the sintered alloy steel were analyzed using an XMA apparatus.

Furthermore, similarly to Example I, corrosion resistance was evaluated.

The results are shown in Table 2.

(Example m) Using B powder, molding was performed in the same manner as in Example I, and the binder was removed. Next, as the first stage sintering process, 1
After being held at 250°C for 60 minutes, it was allowed to cool in the atmosphere, and then, as a second stage M treatment, it was held at 1250°C for 5 hours under a reduced pressure of 0.05 Torr, and then the third As the sintering treatment of the stage, the steps were heated to 60°C at the temperature shown in Table 3 in Ar.
The mixture was held for a minute to obtain a sintered alloy steel.

Thereafter, in the same manner as in Example I, the density ratio, C' concentration distribution, and corrosion resistance were measured and evaluated.

The results are shown in Table 3.

Example 2 in Table 1 examines the influence of temperature during the first stage sintering on the density ratio, Cr concentration distribution, and corrosion resistance of sintered alloy steel.

The present invention examples all have a density ratio of 92% or more, and Cr
The concentration distribution was uniform, indicating good corrosion resistance. On the other hand, in the comparative example, it is thought that sufficient liquid phase sintering was not achieved due to the low temperature during the first stage sintering, and the density ratio was 90.
The corrosion resistance was as low as ~91%, and therefore the corrosion resistance was poor.

Example! (2) examines the influence of temperature during second stage sintering on the density ratio, C content, Cr concentration distribution, and corrosion resistance of sintered alloy steel.

All of the examples of the present invention had a density ratio of 93% or more, a C content of 0.03% or less, a uniform Cr concentration distribution, and good corrosion resistance.

On the other hand, in the comparative example, it is thought that sufficient decarburization and reduction of Cr-based oxides were not achieved due to the low temperature during the second stage sintering, and for this reason, the amount of C was 0.08-0. The number was 0.9%. Therefore, the corrosion resistance was poor.

Example! ! ! investigated the influence of temperature during the third stage sintering on the density ratio, Cr concentration distribution, and corrosion resistance of sintered alloy steel.

The present invention examples all have a density ratio of 93% or more, and Cr
The concentration distribution was uniform, indicating good corrosion resistance. On the other hand, in the comparative example, since the temperature during the third stage sintering was low, the Cr on the surface of the steel powder, which evaporated and decreased during the second stage sintering, could not be sufficiently compensated for by diffusion from the inside, and the Cr The concentration distribution became uneven, and deterioration of corrosion resistance was observed, which was thought to be due to a partial decrease in corrosion resistance.

A sintered alloy steel with excellent corrosion resistance can be easily produced without the need for special equipment.

In the present invention, raw material steel powder with a normal particle size can be used, so the raw material cost is extremely low, and in addition, no special equipment is required during production, so sintered alloy steel with excellent corrosion resistance can be produced. Manufacturing costs are reduced. for that,
Sintered alloy steel can now be used in a wider range of fields.

<Effects of the Invention> According to the present invention, the normal particle size (average particle size 1
Even if alloy steel powder of 6 to 4 oAtm) is used, the density ratio is 92
% or more, and there is no decrease in the amount of Cr on the surface of the steel powder, thereby providing a method for producing a sintered alloy steel that has excellent corrosion resistance.

Claims (2)

[Claims] (1) Using steel powder containing Cr: 16 to 25% by weight and Ni: 8 to 24% by weight, after adding and mixing a binder to the steel powder and molding, the binder in the molded body is removed. removed by heating in an oxidizing atmosphere, followed by 1100-1 in a non-oxidizing atmosphere.
Sintered at 370°C, then cooled once to room temperature or without cooling, sintered at 1100 to 1370°C under reduced pressure of 0.1 Torr or less, and then cooled once to room temperature or without cooling. 1. A method for producing a sintered alloy steel having excellent corrosion resistance, which comprises sintering at 1250 to 1370° C. in a non-oxidizing atmosphere. (2) Using steel powder containing Cr: 16 to 25% by weight, Ni: 8 to 24% by weight, and Mo: 10% by weight or less, adding and mixing a binder to the steel powder and molding, and then forming the molded body. The binder inside is removed by heating in a non-oxidizing atmosphere, followed by 1100-1
Sintered at 370°C, then cooled once to room temperature or without cooling, sintered at 1100 to 1370°C under reduced pressure of 0.1 Torr or less, and then cooled once to room temperature or without cooling. 1. A method for producing a sintered alloy steel having excellent corrosion resistance, which comprises sintering at 1250 to 1370° C. in a non-oxidizing atmosphere.