JP3845722B2 - Superplastic stainless steel manufacturing method and stainless steel superplastic working method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing a stainless steel material for superplastic processing, and more particularly to a superplastic processing method for a stainless steel material that can be processed with a small load and is unlikely to crack, and a superplastic processing method for a stainless steel material.
[0002]
[Prior art]
For example, as shown in FIG. 3, the present inventors perform 85% or more plastic working below the temperature (hereinafter referred to as Md point) at which martensitic transformation occurs when plastic working is applied to austenitic stainless steel below that temperature. In addition, almost all the amount is martensite (hereinafter referred to as α '), and a method is proposed in which this is directly deformed above the temperature at which α' reversely transforms to austenite (hereinafter referred to as γ) above that temperature (hereinafter referred to as the As point). (Patent No. 2916619).
[0003]
In addition, it has been clarified that superplastic behavior is manifested when it is deformed above the As point after annealing at the As point or higher (Iron & Steel, Vol. (1994), No. 3, see pages 67-71).
[0004]
[Problems to be solved by the invention]
In the conventional technology, cold strong working is required in the manufacturing process of superplastic stainless steel, and it is difficult to manufacture a large member because a large load needs to be applied to the material (Problem 1).
[0005]
Furthermore, there is a problem that cracking occurs due to the strong processing of martensite, which is relatively brittle, and the material yield is poor (Problem 2).
[0006]
Accordingly, an object of the present invention is to solve these problems and to provide an efficient method for producing a superplastic austenitic stainless steel material and a superplastic working method for a stainless steel material.
[0007]
[Means for Solving the Problems]
In order for an austenitic stainless steel to exhibit superplastic behavior, conventionally, it has been necessary to add about 90% or more of strong work to the austenitic stainless steel at a temperature below the Md point. This is because α ′ is generated by processing below the Md point and a large strain is applied, and when this is held above the As point, recrystallization accompanied by reverse transformation behavior from α ′ to γ This is because in order to generate a very fine crystal grain structure having a grain size of about 1 μm or less, it is considered that processing and α ′ generation proceed simultaneously.
[0008]
However, the present inventors anticipate that even if α ′ generation and processing are performed separately, the recrystallization behavior with reverse transformation will be equal if the content of α ′ is equal to the total amount of processing rate added. Was confirmed by experiments.
[0009]
Because of this fact and the fact that metal materials generally have higher deformation loads, the deformation load decreases, so there is a method in which martensite is generated by the minimum necessary cold working and then heated to a temperature below the As point to apply strain. Invented. Thereby, the said subject 1 was able to be solved.
[0010]
Furthermore, paying attention to the fact that γ is more ductile than α ′, the majority of the processing is added to the austenitic stainless steel at the Md point and below the recrystallization temperature, and then the minimum required cold working or deep cooling is performed. We have invented a method of generating martensite by treatment and substituting for the cold working of about 90%, which is a conventional method for producing stainless steel for superplastic working. Thereby, the said subject 2 was able to be solved.
[0011]
More specifically, the invention according to claim 1 is characterized in that the austenitic stainless steel is martensitic transformed and then heated to a temperature not lower than the Ms point and not higher than the As point to perform plastic working. This is a method for producing a plastic stainless steel material.
[0012]
The invention according to claim 2 is characterized in that the austenitic stainless steel is martensitic transformed, then heated to a temperature not lower than the Md point and not higher than the As point, subjected to plastic working, and then annealed not lower than the As point. This is a method for producing a superplastic stainless steel material.
[0013]
In the invention according to claim 3, the austenitic stainless steel is subjected to martensitic transformation, then heated to a temperature not lower than the Md point and not higher than the As point, and subjected to plastic working, and this is superplastically processed at a temperature not lower than the As point. This is a superplastic processing method for stainless steel materials.
[0014]
The invention according to claim 4 is a method for producing a superplastic stainless steel material, characterized in that austenitic stainless steel is subjected to plastic working at a temperature not lower than the Md point and not higher than the recrystallization temperature, and is subsequently subjected to martensitic transformation. It is.
[0015]
Further, the invention according to claim 5 is characterized in that austenitic stainless steel is subjected to plastic working at a temperature not lower than the Md point and not higher than the recrystallization temperature, then subjected to martensitic transformation, and then annealed at the As point or higher. This is a method for producing a plastic stainless steel material.
[0016]
In addition, the invention according to claim 6 is that plastic processing is applied to austenitic stainless steel at a temperature not lower than the Md point and not higher than the recrystallization temperature, and then martensitic transformation is performed, and this is superplastically processed at a temperature higher than the As point. This is a characteristic superplastic processing method for stainless steel.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 shows an outline of a manufacturing process to which the superplastic working method for stainless steel material according to the present invention is applied. Cold working is performed at a temperature (Md) or less at which the martensitic transformation occurs when plastic working is performed at or below the temperature. Then, martensite is generated (step 1). Next, this is heated and subjected to strong processing in the warm (step 2). In that case, strong processing can be performed with a small load. Thereafter, annealing or processing is performed at a temperature (As) above which the martensite is transformed back to austenite above that temperature (step 3).
[0018]
FIG. 2 further shows an outline of another manufacturing process to which the superplastic working method of the stainless steel material according to the present invention is applied. When plastic working is applied below that temperature, the temperature is higher than the temperature (Md) above which the martensitic transformation is applied. Inter-process (step 1). In the processing at this time, cracks do not occur. This is subjected to minimum cold working at a temperature below the Md point (step 2). In this processing, the occurrence of cracks is minimized. Thereafter, annealing or processing is performed at a temperature (As) above which the martensite is transformed back to austenite above that temperature (step 3).
(Example)
[0019]
The following shows an example in which the austenitic stainless steel SUS304 is adjusted to a superplastic working stainless steel material according to the present invention and confirmed to exhibit superplastic behavior similar to the conventional method.
[0020]
The material used was a commercially available SUS304 plate (plate pressure 12 mm) that had been subjected to solution treatment. From this, a 10 mm-thick rolling piece was cut out and subjected to a comparative experiment.
[0021]
The conventional superplastic stainless steel was manufactured by rolling the small pieces for rolling to 1 mm thickness (cooling rate = 90%) while cooling with ice water (hereinafter referred to as conventional material).
[0022]
Among the present inventions, the one according to FIG. 1 is rolled to a thickness of 2.5 mm (rolling rate = 75%) while cooling the rolling pieces with ice water, and then heated to 300 ° C. to a thickness of 1 mm. (Integrated rolling reduction = 90%) (hereinafter referred to as Example 1).
[0023]
In the present invention, the one according to FIG. 2 is rolled to a thickness of 4 mm while heating the rolling pieces to 300 ° C., and then rolled to a thickness of 1 mm while cooling the material with ice water (cold rolling reduction) Rate = 75%, integrated reduction rate = 90%) (hereinafter referred to as Example 2).
[0024]
Further, as a comparative material, a rolling piece having a thickness of 4 mm was cut out and cooled to 1 mm in thickness (cooling rate = 75%) while cooling with ice water (hereinafter referred to as a comparative material).
[0025]
When the appearance of the material after rolling was observed, a relatively large number of ear cracks were generated in the conventional material, but there were few in the present invention.
[0026]
These four materials were annealed at 700 ° C. × 3600 seconds, and tensile test pieces were cut out from these materials and subjected to a tensile test at a strain rate of 1/10000 per second at 700 ° C. using JIS No. 7 test pieces.
[0027]
As a result, it is considered that the conventional material exhibits 312% elongation and exhibits superplastic behavior.
[0028]
Example 1 showed an elongation of 383%, and Example 2 showed an elongation of 364%. From this, it can be confirmed that both Example 1 and Example 2 are superplastic stainless steel materials.
[0029]
Furthermore, since the comparative material shows only 228% elongation, it can be proved that the warm working performed in Example 1 and Example 2 showed an effect in producing the superplastic stainless steel material.
[0030]
Thereby, it can be confirmed that the processing load required for the production can be reduced, and a method for producing a superplastic stainless steel material having a high material yield can be provided with less cracking during the production.
[0031]
In Examples 1 and 2, warm plastic working was added only before or after martensitic transformation, but both were also subjected to warm working within the scope of the present invention. Of course it is included.
[0032]
In addition, this method can be applied directly to the superplastic working method of the patent No. 2916619 proposed by the present inventors as described above, without performing annealing before superplastic working, and directly working above the As point. It goes without saying that is possible.
[0033]
【The invention's effect】
Since this invention was comprised as mentioned above, it can be processed with a small load, and it can be set as the manufacturing method of the superplastic austenitic stainless steel material which is hard to generate | occur | produce a crack, and the superplastic processing method of stainless steel material.
[Brief description of the drawings]
FIG. 1 is a diagram showing an aspect of a first manufacturing process in a method for manufacturing a superplastic stainless steel material according to the present invention.
FIG. 2 is a diagram showing an aspect of a second manufacturing step in the method for manufacturing a superplastic stainless steel material according to the present invention.
FIG. 3 is a diagram showing an embodiment of a manufacturing process of a conventional method.

Claims (6)

A method for producing a superplastic stainless steel material, characterized in that austenitic stainless steel is subjected to martensitic transformation, and then subjected to plastic working by heating to a temperature not lower than Ms and not higher than As. A method of producing a superplastic stainless steel material, characterized in that austenitic stainless steel is transformed into martensite, heated to a temperature not lower than the Md point and not higher than the As point, subjected to plastic working, and then annealed above the As point. Austenitic stainless steel is martensitic transformed, then heated to a temperature above the Md point and below the As point, and then subjected to plastic working, which is then superplastically processed at a temperature above the As point. Processing method. A method for producing a superplastic stainless steel material, characterized by subjecting austenitic stainless steel to plastic working at a temperature not lower than the Md point and not higher than the recrystallization temperature, and subsequently to martensitic transformation. A method for producing a superplastic stainless steel material, characterized in that austenitic stainless steel is subjected to plastic working at a temperature not lower than the Md point and not higher than the recrystallization temperature, then transformed into a martensite, and then annealed at the As point or higher. A superplastic working method for stainless steel, characterized in that austenitic stainless steel is subjected to plastic working at a temperature not lower than the Md point and not higher than the recrystallization temperature, followed by martensitic transformation and superplastic working at a temperature higher than the As point. .

Images