Microstructure, mechanical properties and fracture behavior of ultra-high strength dual-phase steel

An ultra-high strength dual-phase steel with main components 0.16C–1.38Si–3.20Mn was produced by cold rolling followed by intercritical heat treatment. Influences of different annealing temperatures on the mechanical properties were investigated, focusing on the development of localized deformation as well as crack initiation and propagation. The work hardening behavior of tested steels after annealing was analyzed in terms of the modified C–J analysis. The experimental results indicate that the microstructures of all the tested steels contain ferrite and martensite. Annealing at 800°C, the best comprehensive performance with a yield strength of 873MPa, tensile strength of 1483MPa, total elongation of 11% and yield ratio of 0.58 was achieved. Besides, the mechanisms of damage taking place in the ferrite and crack initiating at the interface between the ferrite and martensite have been observed. The crack initiating perpendicular to the lath of the martensite and then following at or near the narrowest part of a martensite ligament have also been observed. The original crack propagation exhibits a transgranular fracture feature.