Influence of rolling of TRIP steel in the intercritical region on the stability of retained austenite

Deformation in the intercritical region-or two-phase (alpha + gamma) area-has a substantial effect on the microstructures and mechanical properties of TRIP-aided steel. It indeed increases dislocation density, arranging it in a subgrain substructure which in its turn affects the mechanisms occurring during the ageing treatment that is given on the run-out table. The first purpose of the present work is to investigate the effect of increasing amounts of prior rolling deformation at a given temperature in the intercritical area upon the resulting microstructure. Rolling in the intercritical region stabilizes retained austenite against strain induced transformation on room temperature straining. Increased dislocation density, grain refinement and carbon enrichment are the main factors that govern the retained austenite stability, and consequently the strength and work-hardening behaviour of the material. The second purpose of the study, hence, is the understanding of these stabilising effects. It was found that the morphology of the retained austenite and of the surrounding phase should also be involved as factors that dictate the retained austenite transformation to martensite. These two factors are related to the internal stress state. (C) 1999 Elsevier Science S.A. All rights reserved.