Ductile Fracture Behavior of Bainite-MA Dual Phase Steels

In order to clarify ductile fracture behavior such as void nucleation, crack initiation and crack propagation in bainite-MA dual phase steels, instrumented Charpy impact tests and static bending tests with Charpy specimens were conducted (here, bainite has the same meaning as bainitic ferrite). In bainite-MA steels with smaller MA volume fractions and finer MA sizes, the sample showed higher Charpy absorbed energy with higher crack initiation energy and crack propagation energy. From SEM observation of the instrumented Charpy fracture specimens, it was found that void nucleation was enhanced around the boundaries between bainitic ferrite and coarser MA. Based on the results of the static bending tests with Charpy specimens, bainite-MA steels with smaller MA volume fractions and finer MA sizes showed higher void nucleation strain and crack initiation strain. From a finite element analysis of local plastic strain around the boundaries between bainitic ferrite and MA, it was concluded that the equivalent plastic strain decreases in the case of finer MA and longer distance between two MAs compared to coarser MA and shorter distance between two MAs.