Warm stretch-formability of TRIP-aided dual-phase sheet steels

The effects of retained austenite parameters (stability and volume fraction) and second phase morphology (''a network structure: TYPE I'' and ''an isolated fine and acicular one: TYPE II'') on a warm stretch-formability of high-strength TRIP-aided dual-phase sheet steels were investigated. Good warm stretch-formability, ie., large stretch-height and low stretching load, was achieved particularly in the steels with the second phase morphology of TYPE II, in which the retained austenite islands were nearly isolated in the ferrite matrix away from bainite islands. Significant improvement of the stretch-height by warm forming was obtained in the steels containing a large amount of retained austenite with low carbon concentration. Optimum forming temperature T-p corresponding to the maximum stretch-height was between 20 and 225 degrees C and increased linearly with increasing the M(s) of the retained austenite. The T-p rose by 10-30 degrees C compared to that of total elongation due to a high applied mean normal stress, except for some steels with the TYPE II morphology.