PREDICTION OF FORMING LIMIT OF BORON STEEL AT ELEVATED TEMPERATRUE BASED ON CDM THEORY

To meet the requirement of energy saving and automobile safety, more and more attentions are paid to the development and application of high strength steels. Hot stamping of ultra high strength steel is an important way to reduce the weight of body-in-white, improve the anti-impact and anti-collision performance of the vehicle, and has become a hot technology in the world wide. The forming limit is a very important parameter in boron steel hot stamping process. Due to the complexity of the Nakazima test at elevated temperature, there are a lot of problems to be solved. In the paper, the ductile damage model and finite element simulation technology were combined to predict the forming limit of boron steel at elevated temperature. Based on the continuum damage mechanics (CDM), the Lemaitre ductile damage model considering the e ect of e ective stress and e ective plastic strain on evolution of microvoid was established. In order to accurately predict the formability of boron steel at elevated temperature, a modified dissipation potential coupling the e ect of plastic strain was introduced. Genetic Algorithm-II (NSGA-II) was integrated with finite element software FORGETM to find the optimized ductile damage parameters. The obtained damage parameters can accurately predict the sheet instability and fracture behaviour in tensile test. The established ductile damage model is introduced in the simulation of Nakazima test with boron steel. The comparison between the predicted forming limit and that of experiment witnesses the reliability of the forming limit prediction at high temperature using the proposed ductile damage model.