Bulge bottoming process for reducing springback in U-bending of 980 MPa high-strength steel

Abstract

The objective of this study is to propose a bulged bottom process as a means of reducing the amount of springback from a U-shaped channel in advanced high-strength steel sheets. The recently proposed method is based on the U-bending process, but it employs modified tooling, specifically a punch head with a shallow groove and a bottom die plate with a bulgy shape. Two distinct types of steel sheets, each exhibiting an ultimate tensile strength of 980 MPa and a thickness of 1.2 mm, were subjected to investigation. The efficacy of the process in reducing springback was examined by comparing it to the springback observed in the conventional U-bending process. A finite element analysis was conducted to evaluate the proposed processing technique, considering the effects of plastic anisotropy and the elastic modulus degradation with increased plastic deformation. Furthermore, the anisotropic hardening law was employed to account for the Bauschinger effect and the associated strain hardening behavior during loading path changes. The results of the experiments and simulations were evaluated and examined to gain insight into the effect of anisotropic hardening on springback under specific loading conditions and to interpret the mechanisms of springback reduction.