Structure and warm press-formability of TRIP-aided dual-phase sheet steels

Retained austenite properties, roles of the retained austenite on deformation and warm press formability of TRIP-aided dual-phase (TDP) sheet steels are dealt with in this paper. The retained austenite particles of 5 to 20 vol% are relatively stable against straining, and the deformation-transformation behavior is considerably affected by forming temperature. The retained austenite particles are proposed to play two important roles in deformation of the TDP steel; namely, (i) increasing a long range internal stress as a hard phase and (ii) relaxing a local stress concentration and martensite-hardening due to the strain-induced transformation. Warm forming between 50 and 250 degrees C improves considerably total elongation, stretch-formability and stretch-flangeability of the TDP steels, and achieves the best combination of these formabilities among several commercial 600 to 1000 MPa grade high-strength sheet steels. The excellent warm formabilities and optimum forming temperatures are related to the deformation-transformation behavior of the retained austenite particles.