JPH02170902A - Manufacture of corrosion-resistant sintered stainless steel - Google Patents

Abstract

PURPOSE:To manufacture a high density and corrosion-resistant sintered stainless steel by kneading stainless steel powder with binder of organic compound, injection-molding and sintering the obtd. green body after degreasing to the specific carbon content. CONSTITUTION:The powder of stainless steel of SUS 303, 304, 304L, 316, 316L, etc., is kneaded with the binder mainly composed of the organic compound and injection-molded to form the green body. This green body is sintered after degreased, to obtain the sintered body. For this purpose a green body having the binder contact of 8-15 wt.% is degreased at >=3 deg.C/hr temp. rising speed, <=350 deg.C max. temp. and <=8 hr max. temp. holding time under the atmosphere; or >=3 deg.C/hr temp. rising speed, 500-600 deg.C max. temp. and <=8 hr max. temp. holding time under vacuum. By this method, the degreased body having 0.05-1.0% carbon content is made. By sintering this degreased body, the development of chromium carbide obstructing the corrosion resistance is retrained to obtain a high density and corrosion resistant sintered stainless steel whose remaining porosity ratio is slight.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Field of Application] The present invention relates to a method for manufacturing a highly corrosion-resistant stainless steel sintered body used for watch parts and the like.

[Prior art] Powder metallurgy, in which metal powder is shaped and sintered using a pressure press, has drawbacks such as limitations on the shape of the product and insufficient density.There are methods to overcome these drawbacks. Injection molding methods for metal powders have been attracting attention as a result.

In the injection molding method, a binder mainly consisting of an organic compound that becomes fluid when heated is used. After mixing with metal powder and heating and kneading, it is heated and fluidized using an injection molding machine, and then injected into a mold to solidify and mold it as a green molded product.Next, the green molded product is heated in an oven to evaporate the organic binder. Alternatively, it is decomposed and volatilized to degrease it, and then fired in a firing furnace at a high temperature below the melting point of the metal. The sintered body obtained in this way has a three-dimensionally complex shape and has the advantage of reaching a density close to the true specific gravity compared to a sintered body made by powder metallurgy, so the range of applications is rapidly expanding. However, the corrosion resistance was not always satisfactory.

Issues that the C invention aims to solve Among these, injection molded sintered bodies using stainless steel as metal powder are attracting attention as a promising material for near-net shapes because stainless steel materials are difficult to machine. However, they have the disadvantage of being more prone to rust than steel or cast products.

Corrosion resistance, which is a major feature of stainless steel, generally decreases in injection molded sintered products because the sintered products tend to leave pores, and the organic binder used in molding is not completely removed by degreasing. , remains as carbon, and the carbon combines with chromium in the stainless steel during firing, reducing the anticorrosion effect of chromium.

Therefore, in order to improve the corrosion resistance of injection molded sintered bodies, it is necessary to increase the sintered density as much as possible to reduce pores as much as possible, and to reduce the amount of residual carbon contained in the degreased body. be.

However, the carbon in the degreased body itself functions to lower the temperature required for sintering the stainless steel powder and to reduce metal oxides produced during the degreasing process during firing. That is, since carbon has the function of facilitating sintering, increasing the sintered density, and reducing the residual rate of pores, it is not generally advisable to reduce the amount.

Furthermore, when the binder is volatilized by degreasing, the stainless steel powder comes into direct contact with the outside air at high temperatures, making it particularly likely that the iron in the stainless steel will oxidize. Stainless steel powder once oxidized during degreasing generally becomes extremely difficult to sinter, making it difficult to increase the density of the sintered body. That is, if degreasing is simply strengthened in order to reduce the amount of carbon contained in the degreased body, there is a problem that the stainless steel will oxidize, making it difficult to increase the density of the sintered body, and making it easy for pores to remain.

In this way, it is more difficult than expected to improve the corrosion resistance of injection molded stainless steel sintered bodies.
Therefore, in reality, there is hesitation in applying it to products that are exposed to environments such as high humidity and salt water.

In view of the above problems, an object of the present invention is to provide a method for injection molding stainless steel powder and manufacturing a sintered product, which
The object of the present invention is to provide a high-density sintered product that suppresses the formation of chromium carbide that impedes corrosion resistance and has a low porosity residual rate.

[Means for Solving the Problems] The inventor of the present application has conducted a detailed study on the influence of the amount of residual carbon contained in an injection-molded degreased body of stainless steel powder on the density of the sintered body, the amount of chromium carbide produced, and its corrosion resistance. At the same time, we are conducting intensive research on effective degreasing methods for binders, which are generally organic compounds with relatively high molecular weights and are difficult to volatilize in a cylinder due to tar formation. The present invention was achieved as a result of many efforts to suppress the oxidation of metals.

The means adopted by the present invention is to knead stainless steel powder with a binder mainly consisting of an organic compound, injection mold this to form a green body, degrease the green body, and then sinter the sintered product. In the manufacturing process, it is characterized in that it is fired into a degreased body containing 0.05 to 1 and 0% residual carbon content.

As a method for obtaining the above degreased body, the amount of binder is 8 to 8.
Using a 15% by weight green body, the green body was heated in the air at a heating rate of 3°C/hour or more and at a maximum temperature of 350°C.
Hereinafter, it is desirable to degrease within the maximum temperature holding time of 8 hours.

Furthermore, as another method for obtaining the above-mentioned degreased body, a green body having a binder amount of 8 to 15% by weight is used, and the green body is heated in a vacuum at a heating rate of 3° C./hour or more at a maximum temperature of 50° C.
This is possible by degreasing at 0 to 600°C for a maximum temperature holding time of 8 hours or less.

[Effects] When the amount of carbon in the degreased body is 0.05% or less, chromium carbide is hardly produced during sintering, but oxidation of the metal during degreasing may be accelerated, and oxidized metal may be removed during firing. Due to the lack of reducing power, sintering becomes difficult and the increase in density becomes insufficient. Furthermore, when the carbon content is 1.0% or more, the sintering temperature is lowered, but the sintering increases the formation of chromium carbide, which impairs corrosion resistance.

Although the degreased body production conditions for obtaining such a defatted body can be selected from a wide range, preferably, the amount of binder contained in the green body used is 8 to 15% by weight.

If the amount of binder is less than this, the structure of the green body will not be uniform and it will be difficult to obtain a good sintered product because a composition with poor fluidity will be forced to be injection molded, and the amount of binder will be 15% by weight. When the amount exceeds that amount, the degreasing load increases and the amount of residual carbon in the degreased body increases, and at the same time, it becomes difficult to control the amount.

The first condition for degreasing is that the atmosphere during degreasing is an atmosphere containing oxygen. The oxidative decomposition of the binder is promoted by oxygen in the atmosphere, and the binder is removed smoothly, so the amount of residual carbon can be kept low and its amount can be easily controlled.

However, at the same time, the stainless steel is oxidized by oxygen, so to prevent this as much as possible, the temperature conditions for degreasing are set at a heating rate of 3° C./hour or more, a maximum temperature of 350° C. or less, and a maximum temperature holding time of 8 hours or less. If a large amount of thermal history is applied in excess of this range, oxidation of the metal in the stainless steel will be promoted, making it difficult to sinter the degreased body and insufficiently increasing the density of the sintered body.

The second condition for degreasing is to use a vacuum furnace. In that case, the heating rate should be 3℃/hour or more, and the maximum temperature should be 500℃ or more.
The temperature is set at 600°C and the maximum temperature holding time is within 8 hours.

When degreasing in a vacuum, there is no risk of stainless steel being oxidized, but if the maximum temperature is low, the binder tends to remain in the form of tar, so it is important to heat the decomposition at a maximum temperature of 500 to 600°C. .

If degreasing is carried out in an inert gas, carbon tends to remain in excess of the required amount, but if a vacuum furnace is used, the cracked gas is efficiently exhausted, so that the amount of residual carbon falls within a predetermined range.

In fact, the detailed conditions for degreasing depend somewhat on the substances that make up the binder, but of course the temperature must be high enough to decompose and remove the binder, and the heating rate must be low enough to prevent cracks from occurring. is selected within the range of the above temperature conditions.

The optimal firing conditions for the degreased body naturally vary depending on the type of stainless steel, and also vary slightly depending on the properties and composition of the degreased body, but in the case of SUS304L and SUS316L (7), the maximum temperature generally used is around 1000°C to 1390°C. .

The stainless steel powder used in the present invention can be used in various particle sizes, but it is desirable that the average particle size is 20 μm or less. Powder with an average particle size larger than 20 μm has a large interval between particles, so There is a tendency for holes to remain after the molding, and rust is likely to occur. In addition, although various types of stainless steel are used in the present invention, in particular, austenitic 5O53
03, 304, 304L, and 316.316L are effective.

〔Example〕

Example 1 SUS304L-20 μm (average particle size Jogm) and an organic binder for metal injection molding were weighed so that the binder compounding ratio was as shown in Table 1.

(The following are blank spaces) Table 1 Table 2 The weighed metal powder and organic binder were placed in a tabletop pressurized Negoo and kneaded for 30 minutes at 140°C, then kneaded for 90 minutes at 120″C, and then the kneaded product was coarsely ground.

The coarsely pulverized kneaded material was put into an injection molding machine to mold a watch case. The molding conditions are as shown in Table 2.

The watch case molded body was placed in an air circulation degreasing furnace, and
Heat up to 10℃ at 10℃/Hr to 310℃.
After being left at ℃ for 1 hour, it is cooled in a furnace and degreased. At this time, the amount of remaining carbon, including the carbon contained in the powder, is 0.38%.

Place the degreased watch case body into a vacuum pressure sintering furnace and heat it to 1330°C.
After sintering at X2.5 hours, the product is cooled in a vacuum to 850° C., and then cooled in a furnace at 1 atm of N2 gas.

At this time, the amount of carbon remaining is 0.001%, making it ultra-low carbon stainless steel. This amount is a low carbon content that is difficult to reach with ingots (the standard carbon content of ingot SUS304L is 0.03 or less), and the density of the obtained sintered body is easily reached by conventional methods where ρ = 97% or more. It is a high density that cannot be obtained.

This watch case was subjected to accelerated salt spray test (JISD0201).
-1971), no rust was observed at all.

Example? A watch case is molded by kneading SUS304L-20 (average particle size: 10 m) and an organic binder for metal injection molding.

The molded body is placed in an atmospheric degreasing furnace and heated at 10°C to 340°C in the atmosphere.
After heating and degreasing at /Hr, cool in the furnace without leaving. The amount of residual carbon at this time was 0.12%.

Put the degreased body into a vacuum pressure sintering furnace and heat it at 1340℃ x 2.58℃.
After sintering at R, the product is cooled in a vacuum to 850° C., and then cooled in a furnace at 1 atm of N2 gas. The amount of residual carbon at this time is 0
．． 003%, and is an ultra-low carbon stainless steel.

When the density of the obtained sintered body was ρ=97% or higher and an accelerated salt spray test was conducted, it was found that no rust occurred and the corrosion resistance was extremely good.

Example 3 SUS304L-20μm (average particle size lOILm) and the organic binder shown in Table 1 were heated and kneaded using a pressure kneader.
This kneaded product was injection molded into a watch case under the conditions shown in Table 2.

The molded body was placed in a vacuum furnace (10 Torr) and heated at a temperature increase rate of 320° C./hour, held at 550° C. for 2 hours, and then cooled in the furnace.

The amount of residual carbon at this time was 0.015%. The degreased body was placed in a vacuum pressure sintering furnace and heated at 1340°C for 2.5 hours.
After sintering at R, the product is cooled in a vacuum to 850° C., and then cooled in a furnace at 1 atm of N2 gas. The amount of residual carbon at this time is 0
．． 00396, an ultra-low carbon stainless steel.

The density of the obtained sintered body was ρ=97% or more, and when an accelerated salt spray test was conducted, no rust was generated and the corrosion resistance was extremely good.

[Effects of the Invention] As described above, the manufacturing method of the present invention can manufacture an injection molded sintered product with excellent corrosion resistance using stainless steel powder. Conventionally, sintered products were prone to rust, so their uses were limited, but the sintered products of the present invention have little risk of rusting even when used in environments with high humidity or salt water. It has the excellent effect of being able to be used as a component for wristwatches, etc.

Claims (1)

[Claims] 1) A sintered product in which stainless steel powder is kneaded with a binder mainly consisting of an organic compound, the mixture is injection molded to form a green body, and the green body is degreased and then fired. In the manufacturing method, the carbon content is 0.05 to 1.0%
A method for producing corrosion-resistant sintered stainless steel, the method comprising firing a degreased body of. 2) The amount of binder in the green body is 8 to 15% by weight, and the conditions for degreasing the green body are as follows: the temperature rise rate is 3℃/
more than 1 hour, maximum temperature 350℃ or less, maximum temperature holding time 8
2. The method of manufacturing a corrosion-resistant sintered stainless steel according to claim 1, wherein the manufacturing time is within hours. 3) The amount of binder in the green body is 8 to 15% by weight, and the conditions for degreasing the green body are: in a vacuum, at a heating rate of 3°C/hour or more, at a maximum temperature of 500 to 600°C, and at a maximum temperature holding time of 8 hours or less. A method for producing a corrosion-resistant sintered stainless steel according to claim 1. 4) The method for producing corrosion-resistant sintered stainless steel according to claim 1, wherein the average particle size of the stainless steel powder is 20 μm or less. 5) Stainless steel is SUS303, 304, 304L,
5. The method for producing a corrosion-resistant sintered stainless steel according to any one of claims 1 to 4, wherein the stainless steel is 316 or 316L.