JP2674715B2 - Method for manufacturing porous mold - Google Patents

Description

TECHNICAL FIELD The present invention relates to a method for producing a porous mold used for casting.

(Prior Art) Porous molds are attracting attention as casting molds because they have excellent gas release properties and heat retention properties, and even when manufacturing complex shapes, good quality cast products can be obtained. There is.

Conventionally, the production of this porous mold is mainly performed by a powder metallurgical method. As the powder used, for example, iron powder, ferritic or austenitic stainless steel powder, etc. are known, but in the molds obtained from these powders, the micro Vickers hardness is as low as Hv250 at most and casting metal is low. The mold had insufficient hardness and had a short life.

Therefore, in order to obtain a high hardness porous mold having a hardness of Hv400 or higher desired as a casting mold, a high carbon steel that can obtain a high hardness by quenching, for example, SUS440C, using a SKD-based steel grade. A method of increasing the hardness, or a method of increasing the hardness by mixing carbon powder with a stainless steel powder raw material that is not quenched and integrating them by sintering is conceivable.

(Problems to be Solved by the Invention) However, in the conventional method of using a medium-high carbon steel that can obtain high hardness by quenching as described above, the quenchable steel type is rapidly cooled in a spraying step for pulverization and quenched. However, since the powder itself has a high hardness and the moldability is deteriorated, it is necessary to anneal once in order to improve the moldability, and there is a problem that the manufacturing process is increased. Further, the method of mixing the carbon powder with the non-quenching stainless steel powder described above is because the carbon powder is not always uniformly mixed and the hardness becomes non-uniform, or the sintered body becomes high in hardness. There is a problem in that the workability of the union is reduced.

The present invention has been made in order to solve such a problem, and makes it possible to manufacture a mold having a complicated shape by improving the workability of the raw material and the workability of the sintered body, and the mold has high strength. It is an object of the present invention to provide a method for manufacturing a porous mold that has excellent durability.

(Means for Solving the Problems) Therefore, a method for manufacturing a porous mold is low C, low N-
A raw material containing Cr-based stainless steel powder as a main raw material is molded, the molded body is sintered in a vacuum or in an atmosphere gas, and the obtained sintered body is machined as necessary, and then the sintered body is formed. It is characterized by nitriding by heating in a temperature range of 800 to 1100 ° C in a nitrogen atmosphere.

It is desirable that the C content and N content of the low C, low N—Cr stainless steel powder as the main raw material be C ≦ 0.15 wt% and N ≦ 0.10 wt%. The C content and N content of the low C, low N—Cr stainless steel powder powder according to claim 1 are in the range of C ≦ 0.15 wt% and N ≦ 0.10 wt%. If these values are exceeded, the powder will have high hardness and the formability will decrease, and the sintered body obtained will also have high hardness and the workability will decrease.

Further, as the Cr amount of the low N-Cr type stainless steel powder,
In order not to impair the corrosion resistance of the mold, it is desirable that the content be 11 wt% or more. Furthermore, Cu and Mo can be added.

Furthermore, it is possible to add a lubricant, a binder or the like for improving the moldability to the raw material, if necessary. Other characteristics such as a small amount of metal powder or metal fiber, or ceramic powder or ceramic fiber, etc.
It is also possible to adjust the porosity).

The molding method is not particularly limited,
Various molding methods such as press molding, cold isostatic pressing (CIP) and slip casting can be used.

Sintering is preferably performed in a vacuum, but may be performed in an atmosphere gas unless significant oxidation, nitriding, carburizing, or the like. The sintering temperature and sintering time are selected in consideration of the strength and porosity of the sintered body. For example, it may be carried out at 1100 to 1200 ° C. for about 2 hours.

The nitriding of the sintered body is carried out after mechanical processing if necessary, but if processing is not necessary, it can be carried out subsequent to sintering in a vacuum or an atmosphere gas.

The heating temperature during nitriding was set to 800 ° C or higher because it was 800 ° C.
This is because if the amount is less than this, it is not possible to sufficiently nitride the inside of the porous sintered body. 1100 ℃ or less is 1100
This is because if the temperature exceeds ℃, it will enter the sintering temperature range of the previous step.

It is desirable to perform nitriding using a vacuum furnace, introduce N ₂ gas introduced into the furnace, and heat the N ₂ gas at a pressure of about 3.00 to 900 torr. In this case, in order to promote nitriding, it is also possible to mix H ₂ and Ar gas. Further, if no oxidation that hinders nitriding occurs, an atmosphere furnace may be used.

Further, during nitriding, it is possible to carry out a slight carburization at the same time.

In addition, the hardening mechanism by nitriding is a solid solution hardening of N, precipitation hardening of fine nitrides, and an increase in nitriding causes the austenite of the high temperature phase to generate quenching ability. Although a mechanism may be considered, it is preferable that the heating temperature is 900 ° C. or higher in order to promote austenitization when accelerating the hardening of.

Further, the amount of nitrogen at the time of nitriding is preferably about 0.6 to 1.3 wt%. If it is less than 0.6 wt%, nitriding will be insufficient, and if it exceeds 1.3 wt%, it will be brittle.

(Function) A normal nitriding method for hardening a steel material is performed at about 600 ° C. to harden a surface layer of about 0.1 to 0.3 mm from the surface. According to the present invention, by setting the nitriding temperature to 800 ° C. or higher, the inside of the porous sintered body can be nitrided and hardened.

(Example) Hereinafter, an example of the present invention will be described.

-100 mesh SUS430L powder (0.019wt% C, 0.02wt%
2% by weight of zinc stearate as a lubricant to N),
Molded with a mold at a pressure of 3 ton / cm ² , thickness 15 mm × diameter 40
A cylindrical molded body of mm was obtained.

This compact was heated in a vacuum furnace at 1150 ° C for 2 hours to
A sintered body of 6.2 g / cm ³ was obtained. Further, N ₂ gas was introduced into the vacuum furnace and heated at 950 ° C. for 1 hour under N ₂ gas pressure of 800 torr to nitride the sintered body.

What was obtained by heating and then cooling the furnace was designated as Test Example 1.
What was obtained by rapid cooling under N ₂ gas at atmospheric pressure was used as Test Example 2 in the test described later. The N ₂ contents of Test Example 1 and Test Example 2 after cooling were 0.75% and 0.71%.

Hardness Test and Durability Test Next, the micro Vickers hardness measurement and the durability test were performed on these Test Examples 1 and 2.

The hardness was measured by cutting out a sintered body into a size of a test piece, polishing, and then applying a load of 50 g.

In the durability test, after the hardness was measured, it was used as a part of a mold used for low-pressure casting of Al, and the usable number of castings of this mold was investigated.

Further, in contrast to Test Example 1 and Test Example 2, as Comparative Example 1, a product that was not nitrided after sintering in vacuum was manufactured, and hardness measurement and durability test were performed in the same manner as in Test Example 1 and Test Example 2. I went.

Test Results Hardness measurement results, Test Example 1, Test Example 2 and Comparative Example 1
Were Hv430, Hv490, and Hv210, respectively.

As a result of the durability test, in the test example 1, the number of injections was 8000 times, in the test example 2 it was possible to use 12000 times, and in the comparative example 1, cracking occurred after 5000 times.

(Effects of the Invention) As described above, according to the method for manufacturing a porous mold of the present invention, since low C, low N—Cr-based stainless steel powder is used, moldability and workability are good and There is an effect that it is possible to manufacture a complex-shaped product and obtain a porous mold having high strength and excellent durability.

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Claims (1)

(57) [Claims] 1. A raw material containing low-C, low-N--Cr type stainless steel powder as a main raw material is shaped, and this shaped body is sintered in a vacuum or an atmosphere gas, and the obtained sintered body is required. After machining according to the above, the sintered body is heated in a nitrogen atmosphere in a temperature range of 800 to 1100 ° C. for nitriding, thereby nitriding the porous die.