JPH0716610A - Method for working high-carbon martensitic stainless steel - Google Patents

Abstract

PURPOSE:To manufacture stainless steel in which eutectic carbide having probability of causing the deterioration or the like of workability is not present substantially by making a base stock into an intermediate size by hot plastic work, next executing cold plastic work of a specified reduction of area and executing specified hot work. CONSTITUTION:The base stock of high-carbon martensitic stainless steel of SUS-404C or the like is made into the intermediate size by hot plastic work. Next, cold plastic working of the reduction of area of >=30% is executed. After that, the plastic work of the reduction of area of >=60% is executed at a high temp. of >=850 deg.C. Further, by executing, as necessary, finishing work, the desired size is made. The cold plastic work is executed by die drawing. Before die drawing, spheroidizing annealing is executed. In this way, a wire is not broken even when the base stock is drawn into a thin wire as a secondary work and the stainless steel is manufactured without the generation of void due to the eutectic carbide.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for processing a high carbon martensitic stainless steel material.

[0002]

2. Description of the Related Art For example, a conventional method for processing a high carbon martensitic stainless steel material, which is a raw material for manufacturing bearings, blades, etc., is to heat a steel ingot or a slab obtained by hammer agglomeration / rolling, etc. After heating to a predetermined temperature in a furnace, hot rolling was performed to obtain a steel material having a desired size.

[0003]

High carbon martensitic stainless steels are eutectic carbides (primary carbides) during solidification.
To crystallize. It is known that the crystal grains of the primary carbides are significantly larger than the secondary carbides precipitated in the subsequent steps, which causes the workability in the subsequent primary and secondary processing steps to be impaired.

In the conventional processing method for high carbon martensitic stainless steel, primary carbides are fragmented and refined due to the wrought effect accompanying hot working, but this wrought effect also has a limit, and finally several Sometimes eutectic carbides of the order of 10 μm remained. If such a large amount of carbide remains, it becomes a cause of impairing workability and also causing a surface peeling of the product. In particular, the mechanical properties of the product may be deteriorated, such as wire breakage during wire drawing in the secondary processing step, strength deterioration and ductility deterioration due to the occurrence of microscopic defects such as carbide cracks and voids.

[0005]

The gist of the present invention made for the purpose of solving such a problem is that a high carbon martensitic stainless steel material is first made into an intermediate size by hot plastic working, Cold-working with a surface reduction rate of 30% or more, and then subjecting to a hot-working temperature of 850 ° C or more with a surface reduction rate of 60% or more, and further finishing if necessary. According to the above, there is a method for processing a high carbon martensitic stainless steel material, which is characterized in that a target dimension is obtained.

In adopting the processing method of the present invention, first, a target dimension is set. Next, the dimension after cold plastic working is set so as to ensure a surface reduction rate of 60% or more in hot plastic working by back-calculating from the set target dimensions. Further, the intermediate dimension is set so that the area reduction rate of 30% or more in the cold plastic working can be ensured by back-calculating from the set dimension after the cold working. Therefore, the intermediate dimension is
It needs to be determined in consideration of the target size, the area reduction rate in hot plastic working and the area reduction rate in cold plastic working.
It should be noted that when the finishing process is performed, the area reduction rate here must also be taken into consideration.

The present invention divides a eutectic carbide by subjecting a high carbon martensitic stainless steel intermediate material (hereinafter, also simply referred to as an intermediate material) having an intermediate size obtained by hot plastic working to cold plastic working, The eutectic carbides are divided by the subsequent hot plastic working by refining, and micro defects of the carbides generated by the refining are improved, and the present invention is cold working as compared with hot working. Machining reduces the machinability of the matrix, fragmenting eutectic carbides,
Based on the finding that miniaturization is promoted, and on the other hand, in hot working, when the size of the carbide becomes less than a certain size, it becomes difficult to divide and miniaturize while the compression of the carbide and the steel matrix is performed. It was done by.

[0008]

[Operation] An intermediate material having an intermediate size by hot plastic working is subjected to cold plastic working with a surface reduction rate of 30% or more. By this cold plastic working, the eutectic carbide remaining in the intermediate material is broken and divided, and the carbide thus generated is finely dispersed in the steel matrix. At this time, if the area reduction rate is less than 30%, the eutectic carbides will not be destroyed, divided, and the dispersion of the carbides will be insufficient. Therefore, it is necessary to set the area reduction rate to 30% or more.

[0009] Incidentally,

[0010]

[Equation 1]

[0011] When the cold plastic working is performed by die drawing according to the second aspect of the present invention, the eutectic carbide is destroyed and fragmented and the carbide is dispersed when the intermediate material is drawn out from the die. If the spheroidizing annealing according to claim 3 is carried out prior to die drawing, the deformation resistance during die drawing becomes small and there are few problems such as die wear.

By the hot plastic working after the cold plastic working, the carbides that have been fractured and divided and finely dispersed in the steel matrix are pressed to the steel matrix and voids and the like disappear. In order to obtain such an effect, it is necessary to perform hot plastic working at a temperature of 850 ° C. or higher and to reduce the area reduction rate at that time to 60% or higher. This is because when the processing temperature is less than 850 ° C. or the area reduction rate is less than 60%, the pressure-bonding between the carbides that have been broken or divided and finely dispersed in the steel matrix and the steel matrix is poor. The temperature of this hot plastic working is 9
If the temperature is lower than 00 ° C, the die is easily worn, so that the temperature of hot plastic working is preferably 900 ° C or higher. Further, if the hot plastic working temperature is 1000 ° C. or higher, the die wear hardly poses a problem.
More preferably, the temperature is 0 ° C or higher.

[0013]

EXAMPLES Examples will be described in detail below in order to further clarify the characteristics of the present invention. The example shown below is SUS4.
It is an example in which a wire was obtained by processing a 40C (high carbon martensitic stainless steel) material. A comparative example is also shown. [Example 1] Example 1 is an example of obtaining a wire rod having a target dimension of φ6.5 mm.

An intermediate material having a diameter of 15 mm and SUS440C was obtained by hot rolling. The average size of the eutectic carbide of this intermediate material was 7.5 μm. After subjecting this intermediate material to spheroidizing annealing and coating treatment, it was subjected to die wire drawing to obtain a wire having a diameter of 12.5 mm. The area reduction rate associated with the die wire drawing is 30.5%. When the density of this wire was measured and compared with the density of the intermediate material, the density reduction rate due to the die wire drawing was 2.1%.

Here, the density reduction rate is expressed by the following equation, where ρ _{o is} the density before processing and ρ is the density after processing.

[0016]

[Equation 2]

It has been confirmed that the density decrease rate increases as the number of micro defects in the carbide increases. Next, this die wire drawn wire rod (diameter 12.5 mm) 11
After heating to 00 ° C., hot rolling was performed to obtain a wire rod having a target dimension of 6.5 mm in diameter. Area reduction rate due to hot rolling is 7
3%. The average size of the carbide of the wire rod after the hot rolling was 4.2 μm, and the density reduction rate due to the hot rolling was 0.02%. [Example 2] Example 2 is an example in which cold drawing was performed twice.

An intermediate material having a diameter of 8.0 mm and SUS440C was obtained by hot rolling. The average size of the eutectic carbide of this intermediate material was 7.2 μm. This intermediate material was subjected to spheroidizing annealing and coating treatment, and then subjected to die wire drawing to obtain a wire material having a diameter of 6.2 mm. After that, spheroidizing annealing and coating treatment were performed again, and then die drawing was performed to obtain a wire rod having a diameter of 5.0 mm. The area reduction rate due to the two times of die wire drawing is 61%. The area reduction rate associated with the die wire drawing is 40% in the first time and 35% in the second time, and is 30% or more in each time. This wire (diameter 5.0
(mm) density was measured and compared with the density of the intermediate material, the density reduction rate due to the two times die wire drawing was 3.3%.
Met.

Next, this die wire-drawn wire rod (diameter 5.0 mm) was heated to 1000 ° C. and hot-rolled to obtain a wire rod having a target dimension of 3.0 mm in diameter. The area reduction rate associated with hot rolling is 64%. The average size of the carbide of the wire rod after the hot rolling was 3.5 μm, and the density reduction rate due to the hot rolling was 0.07%. [Comparative Example 1] In Comparative Example 1, an area reduction rate of cold plastic working is less than 30%, a temperature of hot plastic working is less than 850 ° C, and an area reduction rate of hot plastic working is less than 60%. Is.

An intermediate material having a diameter of 5.5 mm and SUS440C was obtained by hot rolling. The average size of the eutectic carbide of this intermediate material was 7.3 μm. After subjecting this intermediate material to spheroidizing annealing and coating treatment, it was subjected to die wire drawing to obtain a wire having a diameter of 4.9 mm. The area reduction rate associated with the die wire drawing is 20.5%. When the density of this wire was measured and compared with the density of the intermediate material, the density reduction rate due to the die wire drawing was 1.5%.

Next, this die wire-drawn wire (diameter 4.9 mm) was heated to 700 ° C. and then hot-rolled,
A wire rod having a diameter of 3.5 mm was obtained. The area reduction rate associated with hot rolling is 49%. The average size of the carbide of the wire rod after the hot rolling was 6.3 μm, and the density reduction rate due to the hot rolling was 0.5%. [Comparative example 2] This comparative example 2 is an example of processing by a conventional technique.

A cast steel of SUS440C was hot-rolled to obtain a wire rod of 5.5 mm. The average size of the carbides in this wire was 7.0 μm. This wire (diameter 5.5m
m) was heated to 1100 ° C. and then hot rolled to obtain a wire rod having a diameter of 3.5 mm. The area reduction rate associated with hot rolling is 49%. The average size of the carbide of this wire (diameter 3.5 mm) was 6.5 μm.

As described above, in the wire rods obtained in Examples 1 and 2, the average size of carbides is fine. Further, the rate of decrease in density due to hot rolling is extremely small, and microscopic defects such as voids have disappeared. On the other hand, in the wire rod obtained in Comparative Example 1, the decrease in the average size of the carbides is slight, and the refining of the carbides is insufficient. Also, the rate of decrease in density due to hot rolling is large, indicating that microscopic defects such as voids have not disappeared. The wire rod obtained in Comparative Example 2 showed a slight decrease in the average size of the carbides, and the refinement of the carbides was insufficient.

Table 1 shows the manufacturing process of the wire rods of the above examples and comparative examples.

[0025]

[Table 1]

The processing method of the high carbon martensitic stainless steel material of the present invention has been described above according to the embodiments.
The present invention is not limited to such an embodiment,
Various implementations are possible without departing from the scope of the present invention. For example, the above embodiment is an example of processing a high carbon martensitic stainless steel material to obtain a wire rod, but the present invention is not limited to the processing of the wire rod and can be applied to the processing of bar steel, steel plate and the like. Further, although the die wire drawing is adopted as the cold plastic working in the above-mentioned embodiment, other means such as cold rolling may be adopted. When performing cold plastic working a plurality of times as in Example 2, cold plastic working is performed while sandwiching spheroidizing annealing for reducing deformation resistance.
It is preferable that the surface reduction rate be 30% or more at least once.

Further, although not shown in the above embodiment,
As a finishing process, for example, cold drawing may be performed. Since the carbide has already been made finer, the wire will not be broken even if the finishing work of the thin wire is carried out.

[0028]

As described above, according to the method of the present invention, it is possible to obtain a high carbon martensitic stainless steel material which is substantially free of eutectic carbides that cause deterioration of workability and surface peeling of products. . This high-carbon martensitic stainless steel material does not break even if it is drawn to a small diameter as a secondary working, and does not cause voids due to eutectic carbides, and therefore does not cause deterioration in quality. .

[Brief description of drawings]

FIG. 1 is a graph showing the average size and density reduction rate of carbides in Examples 1, 2 and Comparative Example 1.

Claims (3)

[Claims] 1. A high carbon martensitic stainless steel material is first made into an intermediate size by hot plastic working, and then subjected to cold plastic working with an area reduction rate of 30% or more, and then at a temperature of 850 ° C. or more. A method for processing a high-carbon martensitic stainless steel material, which comprises subjecting the steel to a target dimension by subjecting it to a plastic working with a surface reduction rate of 60% or more in a hot state, and further finishing if necessary. 2. The method for processing a high carbon martensitic stainless steel material according to claim 1, wherein the cold plastic working is die wire drawing. 3. The method for processing a high carbon martensitic stainless steel material according to claim 2, wherein spheroidizing annealing is performed prior to the die wire drawing.