Finite element based analysis of two-stage forming for advanced high strength steel part

Abstract

Advanced high strength (AHS) steel sheets have been increasingly used for producing various automotive structural parts. The components of new generation vehicles mostly exhibit very complex shapes, for which multiple step forming procedures are required in order to achieve the desired geometry. By sheet metal forming of such parts with large deformation, fracture and springback phenomenon are significantly crucial, especially, in case of the AHS steels. In this work, a two-stage forming process of an automotive part made of AHS steel grade 780 was investigated. Initially, the forming limit of steel was experimentally determined according to the Nakajima test. Subsequently, the specific two-step forming process and its corresponding FE simulations were conducted. In the simulations, the forming limit curve (FLC) was used as failure criterion and calculated formabilities of deformed parts were evaluated. Additionally, the Yoshida-Uemori kinematic hardening model and that coupled with the anisotropic Yld89 yield criterion were applied. Then, the springback occurrences of parts formed after each forming step were examined, in which the measured and computed results were compared. It was thus found that the obtained FLC could precisely describe the material formability, while the Yoshida-Uemori model more acceptably predicted the springback angles of AHS parts in the two-stage forming process.