Abstract
Effects of retained austenite parameters (volume fraction and stability) and second phase morphology ("a network structure" and "an isolated fine and acicular one") on the deep drawability in high-strength TRIP-aided dual-phase (TDP) sheet steels with different silicon and manganese contents were investigated. The deep drawability was evaluated with a limiting drawing ratio (LDR=Do/dp), where the Do and the dp are a maximum blank diameter drawn out and a punch diameter, respectively. The deep drawability was affected by the volume fraction of the retained austenite rather than by its stability (carbon concentration) and morphology. Namely, the higher the volume fraction of the retained austenite, the larger the strength-deep drawability balance, i.e., the product of tensile strength and LDR. Such excellent deep drawability was caused by large local necking resistance at the cup wall just above the punch bottom due to "the transformation hardening" and "the stress relaxation" resulting from the strain-induced martensite transformation, as well as a low drawing resistance of the shrinking flange. Furthermore, the earing behavior of drawn cup was preferably decreased by an acicular type of retained austenite that was mainly isolated in the ferrite matrix, away from bainite phase.
