Ultra-high Strength Steel Plate Thermal Deformation Dynamic Damage Degradation Assessment Model And Forming Limit Prediction

With the improvement of automobile lightweight and safety performance, ultra-high strength steel was widely used in vehicle body structures, then the hot stamping emerged at the right moment. Damage and fracture was the most common defects in sheet metal stamping process. In this paper, in order to study the damage evolution of high strength steel at hot deformation process, the uniaxial tests were carried out on the BR1500 HS ultra-high strength steel which was austenization and non- austenization at different temperature and strain rate. The fracture mechanism and flow behavior of the metal were analyzed under different deformation conditions. The constitutive relationship and normalized damage model were established. The forming limit diagrams of BR1500 HS ultra-high strength steel were mapped by digital simulation at different temperature. In this paper, the main research contents and conclusions were as follows:(1) The appearance and fracture morphology of non-isothermal tensile samples were studied. The localized necking phenomenon appeared for all of the samples. At 600?,700? and 800?, the deformation gave priority ferrite structure at the center of samples. The localized necking was only appeared the center of the samples, and the fracture were also here. However, at 850?,900? and 950?, the center of samples were only austenitic, and ferrite distributed at the both ends. The samples taken place double-neck, which were center symmetry. The fracture occurred in the ferrite area by the reason of the ferrite was softer than austenitic at this temperature, and the fracture occurred in one of the necking.(2) Based on the true stress-strain curves at different temperature and strain rate, dynamic recovery only occurred in ferrite structure area. Nevertheless, austenitic structure discontinuous happened dynamic recrystallization deformation mechanism. The modified Johnson-Cook models were proposed under two conditions by taking account of softening mechanism caused by dynamic recovery and dynamic recrystallization. Through comparative analysis, the correlation coefficients under two conditions were 0.997981 and 0.996518 between the predicted values of this constitutive equation and experimental values, and the average absolute relative error distributed-2.7232% to 6.2162% and 3.354% to 3.740% respectively.(3) The modified constitutive equation and Normalized Cockroft-Latham ductile fracture criterion were coupled into the finite element software by secondary development for ABAQUS software. The normalized ductile damage model that considered the influences by temperature and strain rate was established using regression analysis.(4) The finite model of forming limit test was established and the modified constitutive equation and normalized ductile damage of BR1500 HS ultra-high strength steel were coupled into the finite element model. The forming limit diagram was obtained by simulate at different temperature under two conditions.