Experimental Research And Numerical Simulation On Cold Bending And Springback Characteristics Of Ultra-high Strength Steel

With the demand of high strength and weight reduction, high strength steel hasbecome the first choice of auto-body material. However, ultra-high strengthsteel(UHSS), above1000MPa level, represented by MS Steel, has high yield stressand low elongation, which determines no large deformation. Bending is a typicalforming method of these steels and bending cracks appeare. By now, technical studieson forming problems of UHSS(above1000MPa) have fewly reported and experimentdata about local forming, especially forming limit are few published. On the otherhand, due to ultra-high flow stress, springback after unloading is much greater thanordinary HSS. It is difficult to accurately predict the springback behavior duringreverse loading of DP steel because the Bauschinger effect should be considered. ForBauschinger effect, researchers have proposed various kinds of theoretical models,which precision increase are offen offset by the complexity of these models.Aiming at above problems, with the financial support of national natural sciencesfund(51075269) and Bao Steel, this study accomplishes the experimental study onbending property of1000MPa UHSS and springback prediction consideringBauschinger effect. Major research work is as follows:Three-point bending experiment on7different steels, including MS steel, DPsteel, QP steel. The online observation of outer surface micro crack was designed toinstantly capture the critical fracture. The minimum bending radius experiment resultsand maximum bending angle under specified conditions. The effect of thickness,strength, elongation, anisotropic coefficient on bending property were summarized.The springback behavior was simulated by Dynaform and Autoform and theprecision with the experiment results was compared. The isotropic hardening rule wasselected because the absence of cyclic loading during three point bending. Theinfluence of Barlat’89and Hill’48yield criterion on springback was compared.Commonly, springback is difficult to control during bend-unbend process whenblank flows over draw-bead. Therefore, U bending experiments with variousdraw-beads were designed. Bend-unbend experiments with various levels wereperformed by changing draw-bead height and blank geometry. The section shape afterspringback was obtained by laser scanning. The U bending process with twononlinear haedening models(Yoshida-Uemori and NSK Swift model) were simulated.NSK Swift model has equal predictive precision with Yoshida-Uemori model bycomparing spring-back angle, and both of them fit the experiment result well. Due tothe control function proposed in NSK theory, it can reflect the material property very well and all the model parameters can be simply obtained from uniaxial tension data.