The formability of steel sheets must be improved to form complex car parts. Multistep forming is one way of improving this formability. However, for stretch forming, the relationship between preforming shape and formability in the main forming or how to design the preforming shape has not yet been reported. In this study, we showed that the forming height limit in the main forming was determined by the central cross-sectional line length of the shape after preforming in the case of forming a circular truncated cone shape in two steps as indicated in a previous report. Moreover, we also investigated the preformed shape other than the truncated cone shape studied, and the optimization of the preformed shape to improve the formability by FEM analysis and verified it in a pressing trial.

In high-strength steel (HSS), shear edge failure due to stretch-flange formability is a serious problem. Shear edge failure is strongly affected by cutting methods. As countermeasures, cut-off punching and double punching of the entire circumference have conventionally been considered. However, it has not been shown whether these processes can be applied to the stretch-flange forming part of the outer circumference of automotive parts. In this study, the effect of the punching allowance and the method of improving the double punching process on the outer circumferences of automotive parts were investigated using high-strength hot-rolled steel sheets. The results of hole expansion tests and observations of shear edge faces and metal flow show that partial double punching reduces the rigidity of the cut-off side and increases the collapse probability. Also, the crack propagation caused by high tensile stress near the upper blade is accelerated. In comparison with double punching of the entire circumference of a 590 MPa or 780 MPa-class high-strength steel sheet, the effect of improving the stretch-flange formability is approximately 1.5 times the hole expansion test value, as indicated by the index, up to the area where the punching exceeds the size of the sheet thickness. However, it is considered that the stiffness of the cut-off side varies depending on the curvature and the thickness of the plane, and the effect of such variations will be confirmed in a wide range of the curvature and the thickness in the future.