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

Abstract

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.