準安定オーステナイト系ステンレス鋼におけるせん断型逆変態オーステナイトの細粒化機構に及ぼす前加工の影響

Abstract

Effect of prior deformation on the martensite shear reversion mechanism of the lath martensite to austenite and the grain refining of the reversed austenite has been investigated in a metastable austenitic 14.25%Cr-10.94%Ni steel in terms of the microstructures. The steel used transforms to almost fully lath martensitic structure through 60% cold rolling at room temperature. Further cold rolling above 60% results in an additional deformation to the lath martensite. Martensite lath structures are significantly damaged on and around the slip bands and dislocation cells are formed in the damaged areas. The volume fraction of damaged areas increases with increasing deformation rate.The lath martensite reverts to austenite abruptly with a martensitic shear between 800 K and 900 K, independing on prior deformation. The reversed austenite in the undamaged areas receives the microstructural characteristics of martensite after the reversion. Lath austenitic structure, which is constructed by austenitic, laths and blocks, are formed in the undamaged areas and the traces of slip bands exist in the damaged areas of reversed austenite as a line of dislocation cells. In the undamaged areas, the lath austenitic structure recovers very slowly. Dislocation forms cell structure in the austenitic blocks and the cell structure changes to subgrains. In the damaged areas, recovery of the reversed austenite proceeds very fast and fine recrystallized austenite grains are formed along the traces of slip bands in a short time annealing. In order to obtain uniform fine austenite grains, a lot of slip bands should previously be introduced by heavy cold rolling in the lath martensitic structure.