TRIP型複合組織鋼の温間伸びフランジ性に及ぼす第2相形態の影響

抄録

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 warm stretch-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 this steel, 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 on hole-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 M_s of the retained austenite.