Effects of Conical Punch Shape and Thickness on Stretch Flangeability of Hot-Rolled High-Tensile-Strength Steel Sheet

Abstract

The hole expansion test using a conical punch was carried out to clarify the effects of geometries such as vertical angle, tip shape of the punch and sheet thickness on the stretch flangeability. The results showed that the hole expansion ratio depended on the fracture limit strain and the localized deformation behavior on the hole edge accompanied by the geometries. The fracture limit strain was influenced by the work hardening around the hole edge with contact between the punch and the hole edge. In particular, with little work hardening, the fracture limit strain depended only on the sheet thickness, independent of the vertical angle. Moreover, the localized deformation behavior was influenced by the gradient of the circumferential stress along the radial direction around the hole edge. This gradient changed with the vertical angle. When the vertical angle was sharp, the gradient of the circumferential stress showed a steep slope up to the late stage of the hole expansion process, and the deformation was slowly localized. Consequently, the hole edge was deformed more uniformly and the hole expansion ratio became large. When the hole edge deformed uniformly, the hole expansion ratio significantly depended on the fracture limit strain.