18-8 ステンレス鋼および 18Ni マルエージ鋼オーステナイトの動的再結晶とその関連現象

Abstract

The dynamic recrystallization behavior of austenite in an 18-8 stainless steel and an 18 Ni maraging steel was studied mainly by microstructural observations of the specimens which were water-quenched immediately after the tensile deformation to various strains at temperatures ranging from 800 to 1 200°C and strain rates of 10-310-1s-1. The changes in austenite grain size and hardness at room temperature due to the dynamic recrystallization were also studied.The deformation conditions for the occurrence of dynamic recrystallization in both steels were made clear as a function of deformation temperature (T), strain rate (ε) and strain (ε). Dynamic recrystallization takes place under the deformation conditions with Z (Zener-Hollomon parameter) less than the critical value of Zc. Zc is increased with an increase in strain or with a decrease in initial austenite grain size. The addition of Cr seems to have the retarding effect on the occurrence of dynamic recrystallization of austenite. The dynamic recrystallized grain size (D-) is determined only by the deformation condition Z, and does not depend on the strain and the initial grain size. Present result indicates that the dynamic recrystallization is effective for refining austenite grain size. The room temperature hardness of dynamically recrystallized austenite in 18-8 stainless steel and the martensite formed from the dynamically re-crystallized austenite in 18 Ni maraging steel is higher than that of conventionally heat-treated specimens. However, such an increment of hardness due to dynamic recrystallization is not so large.