Numerical and experimental study on ductile fracture of quenchable boron steels with different microstructures

Five sets of boron steel sheet blanks are heat treated and their microstructures are close to mixture of ferritic and pearlitic, upper bainitic, lower bainitic and fully martensitic, respectively. It shows that the micro-hardness of the blank that quenched in the water-cooled die is of 527 HV0.1, while intermediate hardness of 401 HV0.1, 322 HV0.1 and 280 HV0.1 maintained in the heated die as well as 181 HV0.1 cooled in the open furnace. Various constitutive models such as Ghosh, extend Swift, modified Voce, et al., are calibrated and compared using the stress-strain data of the five material grades. The constitutive parameters of each grade are calibrated using the standard uni-axial tensile tests at quasi-static strain rates and at room temperature. The inverse analysis method coupled with optimization algorithm is used to calibrate the parameters of the hardening laws. It is concluded that the Voce + Voce model can accurately predict the stress-strain relation of the five kinds of materials at the same time. The Gurson-Tvergaard-Needleman (GTN) model and the Johnson-Cook model are used to describe the damage behavior of the ductile boron steel B1500HS grades. The simulated force-displacement curves are compared with those of the experiments, showing considerable good coincidence.