Effects of second phase morphology on warm stretch-flangeability of TRIP-aided dual-phase sheet steels

摘要

To improve the stretch-flangeability of high strength TRIP-aided dual phase (TDP) sheet steels, the effects of second phase morphology ("a network structure: TYPE I" and "an isolated fine and acicular one: TYPE II") on the warmstretch-flangeability were investigated. Excellent stretch-flangeability was achieved in the TDP steels with TYPE II morphology, in which an acicular type of retained austenite was mainly isolated in the ferrite matrix, away from bainite phase. In thissteel, void formation in a punched surface layer was considerably suppressed, with relatively large work-hardening. Further, the transformation induced plasticity (TRIP) due to untransformed retained austenite enhanced the localized ductility onhole-expanding. Significant stretch flangeability improvement of the TDP steel with TYPE II morphology was obtained by warm punching at 150-200°C and the successive warm expanding at 50-200°C. The latter temperatures increased linearly with increasing isof the retained austenite.