Effects of retained austenite parameters an warm stretch-flangeability in TRIP-aided dual-phase sheet steels

Effects of volume fraction and carbon concentration of retained austenite o n warm stretch-flangeability in high-strength TRIP-aided dual-phase (TDP) s heet steels with different silicon anti manganese contents were investigate d. A significant improvement of the stretch-flangeability was obtained by w arm hole-punching at temperatures between 150 and 200 degrees C and the suc cessive hole-expanding at temperatures between 50 and 200 degrees C, relati ng to martensite-start temperature of the retained austenite. The warm stre tch-flangeability was affected by carbon concentration of the retained aust enite rather than by the volume fraction of retained austenite. Namely, the higher the carbon concentration of the retaind austenite, the larger the h ole-expanding ratio of the steel. Such a large hole-expanding ratio was res ulting from the following two reasons; (1) smaller surface damage and a lar ge amount of retained austenite untransformed on hole-punching and (2) larg e localized ductility due to the TRIP effect on hole-expanding.