Study on Fracture Mechanism and Microstructural Evolution in 780 MPa Grade FB Steel with a High Hole Expanding Ratio

FB780 steel with a high hole-expanding ratio, as ferrite–bainite dual-phase steel, is an indispensable material to manufacture different automobile parts. In the present study, uniaxial tension tests and metallographic analyses were conducted to reveal the fracture mechanism in FB780 steel. TiN particles ruptured and formed micron-sized crack sources due to stress concentrations, and voids were generated by dislocation pile-ups in Fe3C particles. Intragranular dislocations mainly accumulated around second-phase particles and grain boundaries. And these dislocations started to manifest intertwining characteristics with the prolonged deformation and formed serious stress concentrations, thus finally, turned into the crack sources. Hence, the aggregation and growth of crack sources and voids resulted in ductile fracture. In addition, the fraction of low-angle grain boundaries in ferrite grains increased with the prolonged deformation. Moreover, the γ fiber texture and the {001} texture in the tensile deformation area facilitated the propagation of cracks. Small-sized newborn grains appearing in the tensile fracture mainly accumulated at low-angle grain boundaries and facilitated center separation during fracture.