Abstract
The effect of bottoming load on the reduction of springback was investigated by performing U-bending experiments on a 590MPa level high-strength steel sheet for three different forming gaps (i.e., the gaps between the punch and die, which were 0, 5 and 10% less than the sheet thickness) at the punch corner. From the experiment, it was found that springback decreases with increasing bottoming load to some extent but a certain amount of springback remains even under a higher load. In the 3D Finite Element (FE) simulation of the U-bending/bottoming, it was clarified that bending stresses at a punch R-corner are markedly reduced by bottoming; however, those in the vicinity of the end of the corner cannot be eliminated. This is why it is so difficult to completely eliminate the springback by bottoming in U-bending. Thus, it is recommended in real press forming operations to apply a certain amount of bottoming load to reduce springback, but it should be very large. One of the important conclusions for an accurate FE simulation of springback is that the proper choice of a material model is essential, and furthermore, consideration of the elastic deformation of tools will give better results. The best combination, found in the present study, is the use of the Yoshida-Uemori kinematic hardening law for the material model and 3D deformable solid model for tools.
