Large Strain Springback Of High Strength Steel Based On Two-surface Hardening Model

With the increase of environmental protection requirements and collision regulations,the weight of automobiles has continued to increase.Due to the high application costs and limited advanced technologies of carbon fiber and composite polymer materials,the development and application of high strength steel sheets,especially advanced high strength steels,has become an important part of automotive lightweight material solutions.However,its high yield strength makes the problem of stamping springback increasingly prominent.Springback is a phenomenon of reverse yielding in a small range after large deformations,which will affect the assembly and appearance quality of the part,it is difficult to control in actual production.Therefore,a high-precision material model was established to accurately predict the springback of part,and a series of springback experiments was established to analyze the impact comprehensively,which is a crucial method in controlling springback.Due to mechanical characteristics such as the Bauschinger effect,work hardening,permanent softening,and smooth transition of the elastic-plastic interval during cyclic loading of high strength steel plates,the use of traditional material models makes accurate springback prediction particularly difficult.At the same time,the research on the influencing factors of high strength steel plate bending springback is incomplete.Based on the above problems,the paper aimed to establish a accurate and efficient cyclic loading platform,construct various material hardening models and apply it to the springback simulation of various U-bending and automobile stamping parts-threshold inner panel,and establish bending springback experiments that was used to explore the influence of material model on the accuracy of springback prediction and analyze the effect of influence factors on springback.The main contents of this article are as follows:(1)Studying high-precision two-surface hardening material constitutiveIn order to describe the stamping deformation mechanism of high strength steel accurately,improve the calculation accuracy of forming simulation and reduce the deviation of springback prediction,a two-surface hardening constitutive theory that fully considers the mechanical properties of high strength steel under cyclic loading is analyzed and studied deeply,which can simulate large plastic deformations under complex strain paths and reverse yielding in small strain ranges effectively.(2)Build an tensile and compression cyclic loading system for thin sheetTo obtain precise and effective two-surface hardened material constitutive,an control method of low-friction cross-comb-type and anti-axial strong extrusion based on optical strain was proposed,an anti-warping clamping device is designed,which can meet the cyclic tensile compression of 980 MPa class ultra-high strength steel.And an tensile and compression cyclic loading system for thin sheet is established,which can achieve a maximum symmetrical strain of 11% and a maximum compressive strain of 22%.The effective cyclic stress-strain curves of DP590,DP980 and QP980 high strength steel under different pre-strains are obtained.(3)Establishing U-bend experiments and numerical simulation of springbackThe experiments of low alloy steel,DP steel and QP steel under different u-shaped bending methods and process parameters was designed,and the effects of material thickness,properties,forming gap,blank holding force,bending angle and bending methods on springback were mainly explored.For DP590,DP980 and QP980 high strength steel sheets,the effects of various hardening models on the numerical simulation accuracy of U-bend springback such as free bending,bending with blank holder force,and bending with middle force were discussed.The springback analysis data shows that: under any forming process,the springback can be predicted by the two-surface hardening model with a Hill48 yield criterion more accurately,and the springback angle improvement effect is 5%-25%.(4)Establishing a numerical simulation about springback of automobile stamping parts-threshold inner panelThe springback finite element simulation of threshold inner panel based on different material models was established,and the applicability of the two-surface hardening model to the springback prediction of complex automobile parts was verified.The springback data shows that the established high-precision two-surface hardening model(Hill48)can not only improve the springback prediction trend of the threshold inner panel effectively,but also improve the accuracy of springback prediction greatly.The model's springback prediction improvement effect can reach more than 45%.