Development of high strength low alloy TRIP-aided steels with annealed martensite matrix

Formable high-strength low-alloy TRIP-aided sheet steels with annealed martensite matrix or TRIP-aided annealed martensitic steel were developed for automotive applications. The steels possessed a large amount of plate-like retained austenite along annealed martensite lath boundary, the stability of which against the strain-induced transformation was higher than that of the conventional TRIP-aided dual-phase steel with polygonal ferrite matrix. In a tensile strength range between 600 and 1000 MPa, the TRIP-aided annealed martensitic steels exhibited superior large elongation and reduction of area. In addition, the steels possessed the same excellent stretch-flangeability and bendability as TRIP-aided bainitic steel with bainitic ferrite matrix. These properties were discussed by matrix structure, a strength ratio of second phase to matrix, retained austenite stability, internal stress in matrix and so on.