Research On The Forming Limit Of DP780 Dual-phase Steel Under Different Pre-strain Loading

With the rapid increase of cars in the world,there will be problems in the areas of energy,environment and automotive safety.Thus,automobile lightweight is the key way to solve these problems.In the development of automotive lightweight materials technology,dual-phase steel is an ideal material for lightweight because of its low yield ratio and good impact energy absorption effect,but its low plasticity leads to local necking and rupture during the forming process.In recent years,the prediction of forming limit of dual-phase steel under nonlinear loading has attracted more and more attention.Research has shown that the material exhibits different mechanical properties and forming properties especially under pre-strain.At present,the material stress-strain relationship and limit criterion of dual-phase steel under plastic deformation theory are complicated and the forming limit is seriously affected by the nonlinear strain path and preload in the stamping process.Therefore,the forming limit rule of dual-phase steel can be obtained quickly,cheaply and accurately,which has important significance for the selection of workpiece materials.The main research contents of this paper are as follows:Firstly,the mechanical properties of DP780 dual-phase steel were studied by standard tensile test.DP780 dual-phase was used for unidirectional tensile test of calibration distance and proportion samples,using Digital Image Correlation(DIC).The Austrian conversion relationship of elongation after fracture was obtained.It provided the basis for the establishment of the constitutive model of DP780dual-phase steel and the pre-strain loading test,and the thickness anisotropy coefficient in equivalent prestrain calculation formula was obtained.Secondly,the important experimental parameters of DP780 dual-phase under different equivalent prestrains were obtained.According to the calculated equivalent strain results,pre-test and secondary tensile test were carried out under different types of pre-strain,different equivalent pre-strain degrees and different rolling directions.Then,the stress-strain curve and the influence law of performance parameters of DP780 dual-phase steel under pre-strain loading were determined by post-processing of the strain measurement data.The stress-strain curve of secondary tension within the experimental error range of uniaxial tension and equal biaxial tension after various degrees of preloading the same;Thirdly,A mathematical model of equivalent prestrain correlative forming limit for DP780 dual-phase steel was proposed.Based on IFU-FLC local necking criterion and forming limit strain calculation method,a modified traditional forming limit calculation model was obtained.The calculated results are consistent with the experimental FLC.The influences of various factors on the forming limit through experimental data were analyzed,and it was found that the path type in the 8%equivalent prestrain range was less dependent on the forming limit.As the pre-strain degree increases,the subsequent loading forming limit Significantly reduced,the forming limit of 45° to the rolling direction is the lowest.Then,a mathematical model of forming limit related to equivalent pre-strain was established by fitting to predict the forming limit of 8% equivalent pre-strain degree;Finally,the accuracy of the prediction model was verified by finite element simulation and and stamping process application experiments.By comparing the experimental strain measurement with the simulation of accurate material constitutive parameters in the finite element analysis software Dynaform,the accuracy of finite element simulation was proved.The finite element analysis and stamping process experiments application of nonlinear characteristic T-shape part were carried out were carried out.The accuracy and reasonable application value of the equivalent pre-strain related forming limit model was verified.