Numerical modeling and experimental verification of ductile damage in boron steel hot stamping process

Abstract High Strength Steel (HSS) sheets formed by hot stamping is more and more widely used in automotive industry. Compared to cold formed components, hot stamped parts show significantly improved material formability as well as the desired microstructure and mechanical property distribution. However, the description of the material formability by means of Forming Limit Diagrams (FLDs), which are usually implemented in FEM dedicated codes, appears strongly limiting when applied to hot stamping, as the material behavior is dependent on temperature and strain rate. A general Lemaitre based damage model was developed and implemented in the FE model of the hot stamping process to describe the fracture onset of 22MnB5 sheets. Tensile tests were carried out at elevated temperatures, in a range between 550 oC and 750 oC, at different strain rates, to identify the necessary parameters for the calibration of the Lemaitre damage model. Nakajima-type experiments were conducted and numerically simulated to test the validity of the proposed criterion. The comparison between numerical and experimental results in terms of critical load at crack growth initiation shows that the proposed criterion is able to predict the ductile fracture onset in hot stamping.