The dimensional defects in low stiffness automotive panels with convex shape such as waving are severe problem in the press forming of ultrahigh-strength sheet steels. The main countermeasure to dimensional defects is the reduction in springback. However, conventional countermeasures to springback are often ineffective for the waving of low stiffness panels. In this study, the dimensional defects in low stiffness model panels with convex shape were investigated by press forming experiments and numerical simulations. The results indicate that there are four main factors for the occurrence of waving in low stiffness panels: (1) bending moment resulting from the stress difference between circumferential compression around the convex shape and radial tension in the convex bottom, (2) out-of-plane deformation caused by circumferential compression around the convex shape, (3) bending moment resulting from the stress distribution in the thickness direction at punch and die shoulders, and (4) excessive material around the convex shape. On the basis of these results, it can be considered that preforming in which the four factors can be reduced by applying tensile stresses to an entire panel is effective in reducing waving.
