JPH04276041A - Production of sintered martensitic stainless steel - Google Patents

Abstract

PURPOSE:To provide a method for producing a sintered martensitic stainless steel by means of injection molding, capable of precisely controlling the C content in a sintered body while maintaining the high density, low O content, and uniformity of contained Cr level in the internal and the external surface of the sintered body. CONSTITUTION:The method is constituted of a stage where powders whose components except C have a composition of stainless steel are used as raw materials and an organic binder is added to the above powdered raw materials and the resulting mixture is kneaded so as to be formed into a molding material, a stage where injection molding is applied to this molding material to form a molded body, a stage where the above organic binder is removed from this molded body to form a degreased body, and a sintering stage where a sintered body is obtained by passing this degreased body successively through a first step where it is held under a reduced pressure of <=30Torr at 900-1300 deg.C, a second step where it is held in an hydrocarbon-containing gaseous atmosphere at 900-1200 deg.C, and a third step where it is held in a nonoxidizing gaseous atmosphere at 1250-1400 deg.C.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing sintered alloy steel using injection molding.

0002

2. Description of the Related Art A method using injection molding as a method for producing sintered materials is suitable for producing parts with complex shapes. Furthermore, injection molding allows the use of fine powder (average particle size of about 10 μm), which has the advantage of providing a sintered material with high density and high properties. Among these, stainless steel sintered materials obtained by injection molding are being used more and more because they exhibit superior corrosion resistance compared to sintered materials obtained by conventional powder metallurgy using powder with an average particle size of about 100 μm. be.

[0003] The present inventors have already disclosed a stainless steel sintered material having excellent corrosion resistance and a method for manufacturing the same (see Japanese Patent Laid-Open No. 2-138435). However, conventional stainless steel sintered steel is made with special emphasis on its corrosion resistance.
．． It is limited to low carbon materials with a carbon content of 0.6% or less, and the manufacturing method focuses on reducing the amount of carbon as much as possible in the sintering process. Therefore, depending on the conventional manufacturing method, it is difficult to manufacture materials containing about 0.1% to 1% of C, such as martensitic stainless steel. In particular, in the sintering process, since the amount of C contained changes, it has been difficult to accurately control the amount of C in the sintered body within a narrow range.

0004

[Problem to be solved by the invention] In view of the above actual situation,
The present invention is a method for manufacturing a martensitic stainless steel sintered material using injection molding, in which the density of the sintered body is kept high, the O content is kept low, and the surface Cr content and internal Cr content are kept at the same level. The purpose of the present invention is to provide a method for manufacturing sintered martensitic stainless steel that can precisely control the amount of sintered C, which is very important when used as a sintered part, while maintaining the sintered stainless steel.

[0005]

[Means for Solving the Problems] In order to achieve the above object of the present invention, the present invention uses powder having a stainless steel composition excluding C as a raw material, adds an organic binder to this raw powder, and kneads the powder. a step of obtaining a molding raw material by injection molding this molding material, a step of obtaining a molded body by removing the organic binder from this molded body, and a step of sintering this degreased body. The sintering process is performed under a reduced pressure of 30 Torr or less,
The first stage is held at 900-1300°C, the second stage is held at 900-1200°C in a hydrocarbon-containing gas atmosphere, and the third stage is held at 1250-1400°C in a non-oxidizing gas atmosphere. The present invention provides a method for manufacturing sintered martensitic stainless steel characterized by the following.

[0006]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be explained in detail below. The method for producing sintered martensitic stainless steel according to the present invention uses a powder whose components excluding C have the composition of stainless steel as a raw material, adds an organic binder to this raw material powder, and kneads them to form a forming material. That is, a kneading step to obtain an injection molding compound, an injection molding step to obtain an injection molded object by injection molding this molding raw material, a degreasing step to remove an organic binder from this molded object to obtain a degreased object, and the present invention. This is the most characteristic part of the process, and consists of three steps.The degreased body is sintered to produce a sintered body with low O, a uniform Cr content inside and outside, and an extremely well-controlled Cr content, that is, sintered body. It consists of a sintering process to obtain martensitic stainless steel.

First, the sintering process, which is the most characteristic feature of the present invention, will be explained. In the present invention, the sintering process consists of three stages,
The first stage focuses on promoting the reduction of oxides contained in the raw material powder and suppressing the evaporation of Cr, and the second stage focuses on controlling the amount of C contained to the target amount. The third stage focuses on repairing the decrease in surface Cr concentration that inevitably occurred in the first stage and sintering and densification.

[0008] The first stage of sintering is performed at a temperature of 900 to 130°C.
It is carried out under the conditions of 0° C. and a pressure of 30 Torr or less. Reduction can also be carried out in a hydrogen atmosphere, but in a composition containing a large amount of Cr, which is a difficult to reduce element, like the sintered steel of the present invention, a significantly large amount of high-purity hydrogen gas is required. Economically unfavorable. On the other hand, as in the present invention, 30T
When using a reduced pressure atmosphere of orr or less, the reduction reaction can be carried out economically and efficiently by direct reaction between the C content and the O content of the sintered body.

According to chemical equilibrium theory, the higher the temperature and the lower the pressure,
The reduction reaction progresses, and at the same time, the reduction in Cr concentration on the surface of the sintered body due to Cr evaporation is promoted. On the other hand, in terms of reaction kinetics, the reduction reaction is dominated by the diffusion of CO gas, which is a reaction product, and the decrease in the Cr concentration at the surface of the sintered body is dominated by the atomic diffusion of Cr. Furthermore, as sintering progresses, the gas flow path inside the sintered body is blocked, so the diffusion rate of CO gas decreases significantly, but it was experimentally confirmed that the effect on the diffusion rate of Cr is small.

[0010] The temperature range of the first stage sintering is 900 to 1
The temperature was 300°C. If the temperature is less than 900°C, the reduction reaction can occur in equilibrium, but the reaction rate is slow and it takes a long time to obtain a low O sintered body, which is not preferable. Therefore, the first stage of sintering is preferably performed at 900°C or higher.

On the other hand, when the temperature exceeds 1300°C, sintering progresses rapidly and the diffusion rate of CO gas decreases significantly, so that the simultaneous reduction and decarburization reactions do not proceed efficiently, resulting in a low O sintered body. I can't get it. Furthermore, since both the Cr vapor pressure and the Cr diffusion rate are sufficiently high, the Cr concentration significantly decreases over a deep range from the surface of the sintered body. Therefore, the upper limit temperature of the first stage sintering was set to 1300°C.

[0012] However, the temperature at which sintering and densification becomes faster differs depending on the diameter of the raw material powder, and if the average particle size is small, the temperature is lower, and if the average particle size is larger, the temperature is higher. You can choose.

Furthermore, the first stage of sintering is carried out in a vacuum heating furnace without introducing gas into the furnace from the outside.
When only evacuation is performed using a vacuum pump, the temperature is 0.1 Torr or less, and when a non-oxidizing gas is introduced into the furnace from the outside and evacuation is performed using a vacuum pump, the temperature is 30 Torr or less. conduct. In the former case, 0.1T
In the latter case, if it exceeds 30 Torr, the reduction reaction of Cr oxide will not proceed efficiently, which is not preferable.

[0014] To explain in more detail, what governs the reduction reaction of Cr oxide is the total partial pressure of the reaction product CO or CO2 gas (hereinafter abbreviated as product gas pressure). outside the reaction system (outside the sintering furnace) so that the product gas pressure can always be maintained below the oxidation/reduction equilibrium pressure.
It is an essential condition that it be discharged to Methods to satisfy this condition include: firstly, using a vacuum atmosphere; secondly, using high-purity non-oxidizing gas such as Ar, N2, H2, etc.; and thirdly, using both in combination. .

In the first case, the total pressure in the furnace can be maintained at 0.1 Torr or less in a highly dense heating furnace in which the product gas pressure is substantially equal to the total pressure in the sintering furnace. This can be done in a vacuum sintering furnace equipped with a vacuum pump with sufficient pumping speed. In the second case, the pressure inside the furnace is set to atmospheric pressure, and in order to reduce the product gas pressure to 0.1 Torr or less, fresh high-purity gas that does not contain product gas is Then, 759.9 Torr or more is required. As described above, it is industrially extremely disadvantageous to supply about 10,000 times as much non-oxidizing gas as the produced gas during the reaction, so the second case is not preferred.

In the third case, fresh high-purity non-oxidizing gas containing no product gas is introduced into the vacuum sintering furnace shown in the first case through the pressure regulating valve. It is said to have some effect on suppressing Cr evaporation in
The total pressure inside the furnace is preferably 30 Torr or less. In this method, the total pressure in the furnace is expressed as the sum of the product gas pressure and the introduced non-oxidizing gas pressure, but if the pumping speed of the vacuum pump is constant, regardless of the presence or absence of the introduced gas,
The rate at which the product gas is pumped out of the furnace is constant. However, when the total pressure inside the furnace exceeds 30 Torr, the pumping speed of the vacuum pump (especially when a mechanical booster and an oil rotary pump are combined) decreases rapidly, and
Due to the decrease in the rate of separation of the product gas from the surface of the sintered body, the exhaust rate of the product gas decreases, resulting in a decrease in the reduction reaction rate. Therefore, the upper limit of the total pressure in the furnace is set to 30 Torr.

The second stage of sintering must be carried out in a hydrocarbon-containing gas atmosphere. The reason why the hydrocarbon-containing gas is used here is to control the amount of C in the sintered body by utilizing the equilibrium reaction of hydrocarbon decomposition (for example, CH4 → 2H2 +C). Other gases that can supply carbon include CO, but this is not preferred because oxidation (reverse reaction during the first stage of sintering) cannot be suppressed unless the pressure is reduced.

[0018] This hydrocarbon-containing gas is supplied as a mixed gas of hydrogen and hydrocarbon which is equilibratedly determined to have a target carbon potential at the sintering holding temperature. However, such a mixed gas only needs to be supplied to the final stage of the second stage of sintering, and if the amount of C in the sintered body is expected to be low in the initial stage, the amount of hydrocarbons may be added to the reverse. In addition, if the amount of C is predicted to be high, the processing time can be shortened by adding extra hydrogen. In addition, as predicted from equilibrium theory, in addition to hydrocarbons and hydrogen,
Similar processing can be performed by diluting with an inert gas or lowering the total pressure by vacuum evacuation. Here, the hydrocarbons that can be used are preferably methane, propane, and butane in view of their price and availability, but are not particularly limited to these.

[0019] The sintering temperature in the second stage is 900 to 120
Preferably, the temperature is 0°C. In this second stage, the sintering temperature determines the diffusion rate of C. Therefore,
If the sintering temperature is lower than 900°C, it will take too much time for C to diffuse, so the sintering temperature here is preferably 900°C or higher. Moreover, when this sintering temperature is high, the carbon potential of the atmosphere is sensitively influenced by the gas composition and temperature. Therefore, if the sintering temperature exceeds 1200°C, it becomes difficult to accurately control the carbon potential, so the sintering temperature is preferably 1200°C or lower.

[0020] Subsequently, the third stage of sintering is carried out at 1250 to 1400°C in a non-oxidizing atmosphere in order to achieve high density and uniformity of alloying elements by diffusion. The reason why the atmosphere was non-oxidizing was to suppress evaporation of Cr. Note that the gas used for the non-oxidizing atmosphere here is Ar, He, etc.
, inert gas such as nitrogen, reducing gas such as hydrogen, carbon monoxide, methane, propane, or combustion exhaust gas. By making the pressure of these gases sufficiently higher than the vapor pressure of Cr, and further reducing or eliminating the flow rate in the heating furnace, evaporation of Cr on the surface of the sintered body can be more effectively suppressed. As a result, the slope of the decrease in Cr concentration from the inside of the sintered body to the surface of the sintered body, which is inevitably generated in the first stage of sintering, is the driving force, and the Cr concentration decreases from the inside of the sintered body to the surface of the sintered body while sintered. Cr atoms diffuse into the low Cr concentration area, and as a result, the Cr concentration on the surface of the sintered body can be restored to 80% or more of the Cr concentration inside the sintered body while the sintered body remains sintered.

[0021] The present inventors also found that if the sintering temperature is constant (corresponding to a constant Cr diffusion rate) in the first and third stages of sintering, the low Cr portions on the surface can be repaired. has experimentally confirmed that it requires a longer time than it took to generate it. Therefore, in order to effectively repair the low Cr portion on the surface in a short time, it is preferable that the sintering temperature in the third stage is higher than the sintering temperature in the first stage. Furthermore, the temperature is preferably higher than that in the first stage in order to achieve sintering densification and to promote the refinement and spheroidization of residual sintered pores.

Therefore, in the present invention, the sintering temperature in the third stage of sintering is set at 1250 to 1400°C, which is higher than that in the first stage.
It is limited to. In other words, if the temperature is lower than 1250°C, not only will it not be possible to effectively repair the low Cr portion on the surface of the sintered body, but also a sintered body with insufficient sintering densification (low density) will be obtained. , the sintering temperature in the third stage is 1250
℃ or higher is preferable.

On the other hand, if the temperature exceeds 1400° C., excessive liquid phase is generated, causing problems such as the shape of the sintered body being destroyed, and a brittle phase remaining, causing a decrease in the strength of the sintered body. Therefore,
The sintering temperature in the third stage is preferably 1400°C or lower.

As described above, by the three-step sintering process that is the most characteristic feature of the present invention, a comprehensively superior martensitic stainless steel sintered body with precisely controlled C content can be obtained from a degreased injection molded body. be able to. The method for producing the degreased injection molded article will be described below.

The raw material powder used in the present invention contains C and O
It is an alloy with the same composition as martensitic stainless steel and/or powder obtained by mixing, with the main components being Cr: 11-18%, Ni: 2.5
% or less, Mo: less than 0.6%, C: 3
%, O: less than 2%, and the remainder consists of Fe and unavoidable components such as Si, Mn, P, and S. From the viewpoint of availability, preferred powders include water atomized powder, gas atomized powder, oil atomized powder, carbonyl powder, pulverized powder, and the like alone or in combination. The particle size of the raw material powder is preferably about 20 μm or less on average from the viewpoint of ease of injection molding.

In the present invention, first, an organic binder is added and mixed to these raw material powders, and the mixture is kneaded to obtain an injection molding raw material (injection molding compound). As the organic binder that can be used here, any known binder mainly consisting of thermoplastic resins, waxes, plasticizers, or mixtures thereof can be used, and if necessary, lubricants, degreasing accelerators, etc. can be used. May be added.

[0027] As the thermoplastic resin, one or more of acrylic, polyethylene, polypropylene, polystyrene, etc. can be used as a mixture and/or copolymerized. As waxes, one or a mixture of two or more of natural waxes such as beeswax, wood wax, and montan wax, and synthetic waxes such as low-molecular polyethylene, microcrystalline wax, and paraffin wax can be used.

The plasticizer may be appropriately selected depending on the main components of the binder, and may be selected from di-2-ethylhexyl phthalate (DO
P), diethyl phthalate (DEP), di-n-phthalate
An example is butyl (DBP). Further, waxes can also be used as plasticizers.

As the lubricant, higher fatty acids, fatty acid amides, fatty acid esters, etc. can be used, and in some cases, waxes may also be used as the lubricant. Further, if necessary, a sublimable substance such as camphor may be added as a degreasing accelerator.

[0030] The mixing ratio of such binder and raw material powder is usually about 30:70 to 70:30 vol%. The method of kneading the raw material powder and the binder is not particularly limited, and various kneaders such as a pressure kneader, Banbury mixer, twin-screw extruder, etc. may be used. The injection molding compound (molding raw material) prepared in this manner may be granulated using a pelletizer, a pulverizer, etc., as necessary, to form pellets.

Next, the obtained injection molding compound (molding raw material) is injection molded to produce a molded article. A normal injection molding machine for plastics can be used for this injection molding process. At this time, the injection molding pressure is 500-2500k
gf/cm2, the cylinder temperature of the injection molding machine is 10
The temperature is about 0 to 250°C. The mold temperature is about 5 to 50°C.

The obtained injection molded article is degreased in a heating furnace by a known method. The atmosphere in the heating furnace can be air, an inert gas such as nitrogen or argon, a reducing gas such as hydrogen, or a vacuum. At low temperatures of less than about 200° C., which affect the shape of the degreased body, it is preferable to use a vacuum atmosphere in order to promote binder removal. More preferably, a method of introducing an inert gas or the like while evacuation is performed is more preferable. While removing the binder, the temperature is raised to a maximum temperature of 400 to 800°C, and maintained as necessary to complete degreasing. The temperature increase rate at this time is usually about 10°C/hour to 10°C/minute.

The degreased body thus obtained is subjected to the three-step sintering process of the present invention as described above, resulting in high density, low O content, uniform Cr content inside and outside, and well-controlled C content. sintered martensitic stainless steel can be obtained.

[0034]

EXAMPLES The present invention will be specifically explained below based on examples. (Example 1) Average particle size 9.7 μm as raw material powder, SU
Water atomized powder having a composition of S416 was prepared. The amounts of C and O in this raw material powder are 0.15% and 0.55%, respectively.
%Met. To 100 g of this raw material powder, 10.2 g of an organic binder was added and kneaded to obtain a molding raw material. The organic binder is 30% acrylic resin as thermoplastic resin,
25% EVA (ethylene-vinyl acetate copolymer) resin,
It consists of 15% DOP (dioctyl phthalate) as a plasticizer and 30% paraffin wax as a wax. This molding raw material was injection molded using an injection molding machine at 150°C, and the width was 20 mm, the length was 20 mm, and the thickness was 2.5 mm.
A plate-shaped test piece was prepared as an injection molded article. This molded body (plate-shaped test piece) was heated to a maximum temperature of 600° C. over 48 hours in a heating furnace under nitrogen gas flow to perform degreasing. The obtained degreased body had no defects in appearance and was sound.

[0035] Next, this degreased body was heated to 0.0001 Torr.
The most characteristic feature of the present invention is to carry out the calcination process by holding at 1100°C for 1 hour in a vacuum of I made a conclusion. Here, the methane-containing gas is 0
．． A mixture consisting of 27% methane and residual hydrogen was used. The temperature was raised and lowered at a rate of 10° C./min during the test. For comparison, sintering was performed in the same manner except that the second step of the sintering process of the present invention, which was the 3-hour treatment in a methane-containing gas, was omitted. The properties of the sintered body thus obtained are summarized in Table 1.

[0036]

[Table 1]

As is clear from Table 1, a stable low O content (%) and a stable sintered density can be obtained by any sintering method, and the difference in Cr concentration between the surface and the inside can be reduced. A sintered body with no However, it is clear from the standard deviation of the C content of the sintered bodies of the examples of the present invention that the obtained C content can be precisely controlled only by the present invention. Furthermore, in Comparative Example 1, the C content of some of the sintered bodies exceeded 0.15%, which was out of specification. The number of samples used to calculate the average and standard deviation is indicated in the table as n.

(Example 2) A degreased body was prepared in exactly the same manner as in Example 1. As shown in Table 2, various sintered bodies were obtained from the degreased body by varying the sintering conditions in the three stages of the sintering process of the present invention, and the characteristics of the obtained sintered bodies were investigated. The effects of the invention were investigated. Conditions not specified in Table 2 are as follows:
This is similar to Example 1.

[0039]

[Table 2]

As is clear from Table 2, all of the materials obtained by the sintering method of the present invention (examples of the present invention) stably have higher density and lower O content than the comparative examples. Not only was the surface Cr concentration of the sintered body not significantly different from the internal Cr concentration, but the standard deviation of the C content (%) was small. On the other hand, when the first stage of sintering was performed in a low vacuum (Comparative Example 2), the reaction between C and O in the degreased body was insufficient, so the C content (%) Both the O content (%) was high, and the density was low due to the sintering inhibiting effect of these impurities. Furthermore, when the temperature in the first stage was too high (Comparative Example 3), the reaction between C and O in the degreased body was insufficient, resulting in the same problem as Comparative Example 2. On the other hand, if the temperature is too high (Comparative Example 4), the surface Cr concentration will be significantly lower than that inside.

Furthermore, when the temperature in the second stage was low (Comparative Example 5), the rate of C diffusion was not sufficiently high, so the amount of C was low. On the other hand, when the temperature in the second stage is too high (Comparative Example 6), the standard deviation of the C content (%) of the obtained sintered body becomes large because the carbon potential of the atmosphere fluctuates rapidly. Ta. Furthermore, when the sintering temperature in the third stage is low (Comparative Example 7), the surface Cr concentration is low, and when the sintering temperature in the third stage is high (Comparative Example 8), the surface Cr concentration is low.
), the shape was distorted because a liquid phase was generated during sintering.

As is clear from the above results, it can be seen that by the method of the present invention, a sound martensitic stainless steel sintered material can be manufactured with high precision for the first time.

[0043]

[Effects of the Invention] As detailed above, according to the present invention,
When producing a martensitic stainless steel sintered body using injection molding, the sintering process consists of three steps in which the injection molded body is degreased and then sintered. In order to promote reduction and suppress Cr evaporation, in the second stage, the C content is controlled in a hydrocarbon-containing gas atmosphere, and in the third stage, the decrease in Cr concentration that occurs in the first stage is repaired and sintering is densified. By performing sintering while aiming for the following, we can produce sintered martensite that not only has high density and low O content, and has surface Cr at the same level as the inside, but also has a sintered body C content precisely controlled as desired. Can produce stainless steel.

Claims (1)

[Claims] Claim 1: A process in which a powder having a stainless steel composition excluding C is used as a raw material, an organic binder is added to this raw powder and kneaded to obtain a molding raw material, and a molded body is obtained by injection molding this molding raw material. a step of removing the organic binder from this molded body to obtain a degreased body, and a sintering process of sintering this degreased body to obtain a sintered body, and the sintering process is performed sequentially at a temperature of 30 Torr. Under reduced pressure of less than 900
a first stage held at ~1300°C; a second stage held at 900-1200°C in a hydrocarbon-containing gas atmosphere;
A method for producing sintered martensitic stainless steel, comprising a third step of maintaining the temperature at 1250 to 1400°C in a non-oxidizing gas atmosphere.