Effects of structure and stability of retained austenite on deep drawability of TRIP-aided dual-phase sheet steels

Effects of retained austenite parameters (volume fraction and stability) an d second phase morphology ("a network structure" and "an isolated fine and acicular one") on the deep drawability in high-strength TRLP-aided dual-pha se (TDP) sheet steels with different silicon and manganese contents were in vestigated. The deep drawability was evaluated with a limiting drawing rati o (LDR=D-0/d(p)), where the D-0 and the d(p) are a maximum blank diameter d rawn out and a punch diameter, respectively. The deep drawability was affec ted by the volume fraction of the retained austenite rather than by its sta bility (carbon concentration) and morphology. Namely, the higher the volume fraction of the retained austenite, the larger the strength-deep drawabili ty balance, i.e., the product of tensile strength and LDR. Such excellent d eep drawability was caused by large local necking resistance at the cup wal l just above the punch bottom due to "the transformation hardening" and "th e stress relaxation" resulting from the strain-induced martensite transform ation, as well as a low drawing resistance of the shrinking flange. Further more, the earing behavior of drawn cup was preferably decreased by an acicu lar type of retained austenite that was mainly isolated in the ferrite matr ix, away from bainite phase.