The Effects of Piercing Methods on Burring Formability under Practical Hole Diameter

Abstract

Advanced High Strength Steels (AHSS) sheets are used for automobile bodies to comply with the demands for weight reduction and collision performance. However, as the tensile strength is increasing, the ductility of steel sheets is decreasing. Therefore the forming defects such as fractures may occur. In case of chassis components, it is needed to prevent stretch flange fractures from burring edge. There are following methods to improve burring formability. 1) Double Punching Method (DPM): punching blank coaxially with a pre-punched hole. 2) Humped bottom Punch Method (HPM): punching blank under tension using a punch with a humped part. However, there isn’t much previous evidence evaluating the burring formability of DPM and HPM under practical punching diameter for chassis components. In this study, the burring formability using DPM and HPM has been evaluated under 30mm punching diameters, through experiments and FEM analysis. The results showed that: 1) DPM: deforming mainly occurred in scrap side and equivalent plastic strain was reduced on the punched edge surface. 2) HPM: the burring formability requires both increasing stress triaxiality in blank and reducing equivalent plastic strain applied by the preceding bumped part. Under identical conditions, the same improvement effect of DPM was obtained with HPM.