冷間圧延したFe-Cr-Ni系ステンレス鋼の疲れ強さに対する組織と化学組成の影響

Abstract

In order to investigate the influence of the amount of martensite and the chemical composition on the fatigue strength of cold rolled Fe-Cr-Ni stainless steels, saturation flux densities were measured to estimate the amount of martensite and bending fatigue tests were carried out. In these measurements 75% cold rolled stainless steel sheets with 4∼12%Ni and 17∼21%Cr (16.5∼29%Ni equivalent) were used as the specimens. The results obtained are summarized as follows:(1) In the fatigue strength, the strain-induced martensite structure is highest, the athermal martensite structure is not so high as the former, and the least high fatigue strength belongs to the austenite structure.(2) The fatigue strength of the martensite structure is remarkably lowered by a mixed austenite structure; especially athermal martensite is sensitive to the presence of the other phases. It seems that the marked variation in fatigue strength of Fe-Cr-Ni stainless steel is due largely to the coexistence of these three structures, namely, strain-induced martensite, athermal martensite and austenite.(3) The fatigue strength of 75% cold-rolled stainless steel varies according to the chemical composition. Over 23%Ni equivalent the fatigue strength is comparatively low owing to the large amount of austenite. In the range of Ni equivalent from 18% to 23%, especially at about 20%, the upper limit of the fatigue strength is high because of the large amount of strain-induced martensite, but the fatigue strength is subject to the influence of retained austenite and its variation is remarkable, under 17%Ni equivalent the fatigue strength takes an intermediate value between strain-induced martensite and austenite as the structure is approximately 100% athermal martensite.