EFFECTS OF 2ND-PHASE MORPHOLOGY ON WARM S TRETCH-FLANGEABILITY OF TRIP-AIDED DUAL-PHASE SHEET STEELS

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: T YPE II'') on the warm stretch-flangeability were investigated. Excelle nt stretch-flangeability was achieved in the TDP steels with TYPE II m orphology, 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 suppresse d, with relatively large work-hardening. Further, the transformation-i nduced plasticity (TRIP) due to untransformed retained austenite enhan ced the localized ductility on hole-expanding. Significant stretch-fla ngeability improvement of the TDP steel with TYPE II morphology was ob tained by warm punching at 150-300 degrees C and the successive warm e xpanding at 50-200 degrees C. The latter temperatures increased linear ly with increasing M-s of the retained austenite.