Experimental investigation and modeling of ductile fracture behavior of TRIP780 steel in hot working conditions

In this research, the fracture and hardening behaviors of TRIP780 steel in hot working conditions are explored and studied via the uniaxial tensile test and using a newly proposed necking correction method. The hot working conditions used in the experiment include the forming temperature falling in 550-850°C and the strain rate range of 0.005-5 s-1. It is revealed that the strain rate has a less effect on the formability of material at the testing temperature and the variation of hardening rate is less apparent at the higher working temperature. From the fracture point of view, the fracture strain is increased from 0.005 to 0.1 s-1, due to the nucleation and growth of voids and the dynamic equilibrium between recovery and strain hardening. However, an apparent decrease of fracture strain at the strain rate of 0.1-5 s-1indicates that the decrease of ductility is caused by the dominating strain hardening mechanism, viz., dislocations pile-up. Furthermore, a set of damage-viscoplasticity constitutive model based on the nucleation growth of voids and dislocation density was proposed for in-depth understanding of the hardening and fracture behaviors of TRIP780 in hot working conditions.