Austenitic stainless steel excellent in intergranular corrosion resistance and stress corrosion cracking resistance, and method for producing austenitic stainless steel material

Abstract

An austenitic stainless steel excellent in intergranular corrosion resistance and stress corrosion cracking resistance, comprising: C: 0.005 wt % or less; Si: 0.5 wt % or less; Mn: 0.5 wt % or less; P: 0.005 wt % or less; S: 0.005 wt % or less; Ni: 15.0 to 40.0 wt %, Cr: 20.0 to 30.0 wt %, N: 0.01 wt % or less; O: 0.01 wt % or less; and the balance of Fe and inevitable impurities, wherein the content of B included in the inevitable impurities is 3 wt ppm or less.

Description

TECHNICAL FIELD[0001]The present invention relates to an austenitic stainless steel excellent in intergranular corrosion resistance and stress corrosion cracking resistance even under severe intergranular corrosion environments such as a corrosion environment for a boiling heat transfer surface in a high-concentration nitric acid solution containing highly oxidizing metal ions and an environment in high-temperature high-pressure water under neutron irradiation, and relates to a method for producing an austenitic stainless steel material excellent in intergranular corrosion resistance and stress corrosion cracking resistance even under the severe intergranular corrosion environments.BACKGROUND ART[0002]It has been well known that austenitic stainless steels generally show corrosion resistance even in environments containing strong oxidizing acids such as nitric acid by forming a passive film on the surface. The austenitic stainless steels have been used, for example: as a structural ...

AI Technical Summary

Benefits of technology

[0038]Specifically, the methods described in Patent Documents 8 to 10 do not reduce the content of impurities, a cause of intergranular corrosion together with Cr-depletion layer, for prevention of the intergranular stress corrosion cracking by component adjustment. It is thus impossible in principle to solve the problem of stress corrosion cracking generated under the irradiation environment.

[0040]In Patent Document 12, the lower limit value of the B content is restricted to 0.0005 wt % for improvement in hot processing efficiency and the higher limit value to 0.005 wt % for prevention of deterioration in intergranular corrosion resistance, but it is obvious that the limitation above is not sufficiently effective in improving the corrosion resistance.

Problems solved by technology

Thus, the corrosion environment to the austenitic stainless steel is in a boiling-inducing heat transfer surface corrosion.

It is a severe environment in which the corrosion rate is higher than that by immersion corrosion at the same metal surface temperature and the corrosion rate increases gradually over time.

For that reason, even if the austenitic stainless steel materials described in Patent Documents 1 to 7 or the preparation methods thereof are used, there still remains a possibility of severe intergranular corrosion.

In addition, Patent Documents 4 and 5 only consider economy, and the stainless steels cannot be considered to be resistance to nitric acid corrosion consistently.

The test is an evaluation test simulating the corrosion environment containing highly oxidizing metal ions that is used in reprocessing plants for spent nuclear fuels, and is not suited for evaluation of advantages and disadvantages in corrosion resistance of stainless steels.

Thus, after the cold working, Cr based carbides, a possible cause of intergranular corrosion, are dispersed uniformly, but the Cr-depletion layers formed around the Cr based carbides precipitating in great amounts lead to acceleration of corrosion.

In addition, the heat treatment is not effective at all for removal of the impurity elements, such as P, S, N and O, segregating in grain boundaries.

Accordingly, the method unlikely gives desired corrosion resistance.

Images