Effect of mechanical and fractographic properties on hole expandability of various automobile steels during hole expansion test

Abstract

Hole expandability is a vital formability parameter for automobile body parts that are subjected to deep drawing conditions. In this paper, the influence of mechanical and fractographic properties on hole expansion ratio has been studied and reported. The hole expansion test has been carried out for seven different automotive steel sheets of varying thicknesses. Hole expansion ratio expressed in terms of hole expansion percentage is strongly influenced by the microstructure of the sheet metal. Hole expansion test experiments were performed on flat circular plates with a hole in the center to investigate the fracture behaviors of various automobile steels such as microalloyed, C–Mn, high-strength I.F. (SPRC-35) of three grades, extra galvannealed I.F., and HSLA (E-36) steel sheets. In the hole expansion test, deformation by lip is caused when the punch expands the hole. Fracture by petalling occurs when the holes in the sheets are completely pierced by the punch. Large circumferential strains are accommodated in the deforming sheet material. The mechanical properties, namely, strain hardening exponent (n), normal anisotropy ( $$ \overline{r} $$ ), formability parameter (n $$ \overline{r} $$ ), and other properties, namely, Mohr's circle shear strains (γ 31 and γ 12), strain triaxiality factor (T), and stress triaxiality factor (T o), affected the hole expansion ratio of different steels tested. Similarly, the fractographic factors, such as void size in micrometers, void area fraction, and d-factor, affect the hole expansion ratio. Among the steel sheets tested, extra galvannealed I.F. steel possesses the highest hole expansion ratio.