Effects of Retained Austenite Parameters on Warm Stretch-flangeability in TRIP-aided Dual-phase Sheet Steels

摘要

Effects of volume fraction and carbon concentration of retained austenite on warm stretch-flangeability in high-strength TRIP-aided dual-phase (TDP) sheet steels with different silicon and manganese contents were investigated. A significant improvement of the stretch-flangeability was obtained by warm holepunching at temperatures between 150 and 200 deg C and the successive hole-expanding at temperatures between 50 and 200 deg C, relating to martensite-start temperature of the retained austenite. The warm stretch-flangeability was affected by carbon concentration of the retained austenite rather than by the volume fraction of retained austenite. Namely, the higher the carbon concentration of the retaind austenite, the larger the hole-expanding ration of the steel. Such a large hole-expanding ratio was resulting from the following two reasons; (1) smaller surface damage and a large amount of retained austenite untransformed on hole-punching and (2) large localized ductility due to the TRIP effect on hole-expanding.