Hot plasticity and fracture mechanism of the third generation of automobile steel

Tensile tests at different temperatures were performed on a Gleeble-1500 hot simulator to investigate the hot plasticity of the third generation of automobile steel (TG steel). The fracture morphology and fracture mechanism were analyzed by optical microscopy (OM) and scanning electron microscopy (SEM). It is found that the strength of the steel decreases with temperature rise. The curve of hot plasticity consists of three segments:the first brittle zone, the high-temperature plastic zone and the third brittle zone. The curve in the third brittle zone contains two relative minimum points. In the temperature range of 800 to 1300℃, the microstructure of the steel is austenite, and the fracture mode is ductile fracture. Due to dynamic recrystallization, large plastic deformation occurs before the material breaks off, big and deep dimples form on the fracture surface of samples. At 750℃ there is a uneven distribution of ferrite along austenite grain boundaries, the fracture mode is interface fracture, and the fracture surface includes both holes caused by cohesive failure and the part like stones caused by crack enlargement along austenite grain boundaries. The plasticity of the steel at 650℃ deteriorates, and the quasi-cleavage of ferrite leads to a minimal value in the curve of hot plasticity again.