Comparison of hole expansion ratios in a dual phase steel for holes prepared by conventional punching and fine piercing

Dual phase steels generally exhibit poorer stretch flangeability compared to lower strength steels. Stretch flangeability is normally evaluated using hole expansion testing, wherein a specimen with a 10 mm central hole is expanded using a conical punch till onset of cracking at the edge of the hole. The percentage increase in hole diameter is referred to as the hole expansion ratio (HER). In this study, hole expansion ratios of a DP 600 steel were evaluated. Holes were prepared on a standard 90 mm x 90 mm specimen using two different hole preparation conditions: i) conventional punching where the hole edge exhibits both fracture and shear regions and ii) fine piercing where the hole edge exhibits almost completely smooth shear. Hole expansion testing was carried out on these specimens as per standard ISO 16630. The specimens with holes produced by fine piercing exhibited lower values of HER in spite of having a hole edge with a smooth surface. The specimens with holes prepared by conventional punching exhibited higher HER values. The influence of strain hardening of the cut edge during hole preparation and the surface roughness of the cut edge on the HER are discussed.