We investigated the punch damage in the piercing of a 1470-MPa-class ultrahigh-strength steel sheet and discussed the effects of PVD coating properties on the punch damage behavior. Sequential piercing experiments were conducted using SKD11 punches with various types of PVD coating. A maximum of 240 shots of piercing were performed with a small clearance (5% of the pierced material thickness). The friction coefficient μ between the coatings and 1470 MPa steel was evaluated using a plate drawing tester at a high contact load without lubrication. For 120 shots of piercing, the higher the μ of coatings, the larger the damage area. For 240 shots, the effects of the heat resistance and adhesion of coatings additionally appeared. For the coatings with lower heat resistance, such as TiN and TiCN, the damage was greater and the effect of μ was more significant than for the coatings with high heat resistance, such as CrN, TiAlN, and TiCrAlN. In addition, among the low-heat-resistance coatings, the damage was smaller for the high-adhesion type. It was concluded that the frictional property, heat resistance, and coating adhesion are the major factors affecting punch damage in the piercing of ultrahigh-strength steel sheets.

Tailored blanks with in-plane differential thickness are required to reduce automobile body weight and satisfy two or more strength requirements in one panel. We developed a method of partial thinning in the width direction of steel sheets by bending-assisted compression drawing. This method reduces shear deformation energy by combining bending deformation and compression deformation, which can lead to a reduction in forming load. In this study, we conducted an experiment to partially reduce the thickness about 10 mm from the width edge of the mild steel sheet by the proposed method. We found that this method enables us to partially reduce the mild steel sheet thickness by 30% on the blank edge side. Furthermore, we combined the proposed method with a continuous stretch-bending method that can partially reduce the thickness in the longitudinal direction (drawing direction). As a result, a tailored blank with in-plane differential thickness was demonstrated experimentally.

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.