Effect of carbon content on deep drawability of TRIP-aided dual-phase sheet steels

Effect of retained austenite on the deep drawing in high strength TRIP-aided dual-phase (TDP) sheet steels with different carbon content were investigated. The deep drawability based on the limiting drawing ratio (LDR=D-0/d(p)), where the D-0 and the d(p) are a maximum blank diameter and a punch diameter respectively, was affected by the volume fraction of the retained austenite and by its stability (carbon concentration). Namely, the higher the volume fraction of the retained austenite and its stability, the larger strength-deep drawability balance, i.e., the product of tensile strength and LDR. Furthermore, the higher blank folding force, the larger the strength-deep drawability balance of high carbon TDP steel. The excellent deep drawability was due to 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.