Research On Calculation Model And Test Method Of Nonlinear Forming Limit Of DP780 Dual Phase Steel

In the automobile manufacturing industry,the lightweighting of body-in-white was an important development trend.Due to its excellent mechanical properties,DP dual-phase steel was widely used in body-in-white structural parts.However,the DP dual-phase steel was relatively poor in plasticity and had a complex stress-strain relationship,which leads to the failure of conventional forming limit curves(FLC)under nonlinear strain paths.In this study,1.6mm thick DP780 was used as the research material,and the calculation model and experimental device of nonliner forming limit cruve(N-FLC)were studied:First,according to the as-received stress-strain curves under pre-strain,the effect of pre-strain on the mechanical properties and hardening behavior of DP780 was studied.A P-E hardening model with excellent fitting performance was proposed.And the functional relationship between the P-E parameters and the pre-strain parameters was established.The resulting pre-strained P-E hardening model could accurately reflect the evolution trend of the stress-strain curve with the change of the pre-strain.Second,the pre-strained P-E hardening model was used to significantly modify the existing calculation method.And comparing the applicability of different yield criteria and combining the proposed multi-step pre-strain normalization method,a nonlinear forming limit calculation model was obtained.The developed program could quickly realize the calculation of N-FLC.The stamping test of T-shaped parts was carried out to preliminarily verify the accuracy of the N-FLC calculation model.An improved bidirectional drawing mold was given.The numerical simulation model was established and the optimized design of the punch and the sheet profile was carried out.The experiments were carried out on a hydraulic platform and the results showed that the device could achieve diverse nonlinear strain paths.By comparing the experimental and simulation results,the influence of the experimental parameters on the strain path was discussed.The ultimate strain under the nonlinear strain path was obtained using the digital image correlation(DIC)system,which finally verifies the practicability of the developed N-FLC calculation model.