The Study On Fracture Failure Criterion Of Advanced High Strength Dual-phase Steel Body Panel During Warm Forming

Advanced high strength dual-phase steel has gradually become an important lightweight material for automobile body due to its excellent performance such as high strength and high toughness.However,advanced high-strength dual-phase steels not only have high strength characteristics but also have poor plasticity at room temperature,so it is easy to rupture in the process of cold stamping.The research shows that the warm stamping technology can effectively improve the problem that the poor formability of advanced high strength double-phase steel body plate at room temperature,and the energy consumption is less than that of hot stamping,so it gradually became popular.The deformation law and fracture characteristics of advanced high-strength dual-phase steel sheet at warm forming conditions are different from those at room temperature.Although the plasticity of sheet is improved,the sheet is still prone to rupture at the edge of the small fillet die in the process of warm stamping.Advanced high strength dual-phase steel DP780 is taken as the research object,and the warm deformation and fracture behavior of DP780 sheet metal under certain temperature and strain rate range are studied.The constitutive model and rupture failure prediction criteria for DP780 sheet metal are considered.Combined with typical stamping process,the engineering practice is carried out.（1）The temperature tensile stress-strain curve and related mechanical properties of advanced high strength dual-phase steel sheet were obtained by unidirectional temperature tensile test with temperature range of 573～873K and strain rate range of1×10^-4～1×10^-3s^-1.The Grosman constitutive model was used to accurately describe the warm forming stress-strain curves of DP780 steel plate,and the reliability of the constitutive model was verified by numerical simulation.（2）Analyzing the effect of strain rate conditions and temperature conditions on the mechanical performance,and then a response surface model was established to reflect the relationship between product of strength and elongation and temperature and strain rate.By optimizing the response surface model,the temperature and strain rate corresponding to the maximum strong plastic volume were obtained,which were taken as the optimal forming conditions for the temperature and strain rate range.（3）Through combine the multiple sets of shear test under different temperature with numerical simulation to obtain the stress and strain state parameters of sample ligament area,and then using the equations solving method to calibrate the different temperature Modified Mohr-Coulomb（MMC）failure criterionparameters,and through the simulation tensile shear test to verify the reliability of fracture criterion parameters,analyzes the law of fracture criterion parameters change with temperature.（4）The MMC fracture criterion was applied to the simulation of DP780 sheet metal groove parts with warm stamping,and the error between the numerical simulation results and the actual test results was little.Therefore,it was verified that the MMC fracture criterion could be used to predict the shear fracture behavior of DP780 sheet metal at the radius of small rounded die.The temperature bulging test of DP780 sheet metal was simulated,and the forming limit curve under the optimal temperature forming condition was obtained,and compared with other experimental forming curves in this temperature range,it was verified that the forming effect under the optimal temperature forming condition was better,and the prediction method of the optimal temperature forming condition had certain reference value.