Effects of Punch Configuration on the AHSS Edge Stretchability

The hole piercing process is a simple but important task in manufacturing processes. The quality requirement of the pierced hole varies between different applications. It can be either the size or the edge quality of the hole. Furthermore, the pierced hole is often subject to a secondary forming process, in which the edge stretchability is of a main concern. The recently developed advanced high strength steels (AHSS) and ultra high strength steels (UHSS) have been widely used for vehicle weight reduction and safety performance improvements. Due to the higher strength nature of these specially developed sheet steels, the hole piercing conditions are more extreme and challenging, and the quality of the pierced hole can be critical due to their relatively lower edge stretching limits than those for the conventional low and medium strength steels. The stretchability of the as-sheared edge inside the hole can be influenced by the material property, die condition and processing parameters. Previous studies showed that the as-pierced edge stretchability can be improved by implementing the bevel shape punch head with an optimal die clearance. In this study, production punches are fabricated with different configurations and surface treatments to study the as-pierced edge stretchability of AHSS. The hole piercing experiments are conducted on DP600, DP780 and DP980 steels using a computer controlled punching system. The hole expansion test is used to evaluate the effect of pierced edge conditions on the edge stretchability. Results indicate that a selection of the 15% die clearance (per side) and a conical shape punch results in a less damaged edge, which significantly delays edge fracture in the forming process and increases the edge stretchability of AHSS.