Improvement of Flange-Up Formability by Controlling Sheet Bending Behavior

Flange-up forming in the longitudinal edge of press-formed parts is one of the most difficult problems for materials with poor ductility, such as ultrahigh-strength steel. The authors investigated the effects of the shape and material properties on flange-up formability in previous studies, and it was clarified that the bending behavior at the flange root during forming was the key to improving the stretch flange formability. In this study, we propose a new pad, which is called the lower pad and which can control the bending behavior at the root, and the effect of the lower pad structure on the flange-up formability is examined numerically as well as experimentally. It is found that the lower pad improves the formability, especially in the case of materials with poor ductility, and enables flange-up forming even for 980-MPa-grade ultrahigh-strength steels. Furthermore, it is clarified that an optimum lower pad stroke exists. The formability is improved as the lower pad stroke becomes large. However, too large a stroke causes localized large curvature at the root and excessive thinning at the edge.