50 巻 (2009) 580 号 p. 414-418
The effect of the quality of a sheared edge on the formability of hole expansion of an ultra high strength steel sheet having low ductility was examined. The quality of the sheared edge in piercing was changed by the clearance between the punch and the die, and thus the formability of hole expansion after piercing was influenced. From a tensile test on a specimen having pierced holes, it was observed that cracks are initiated around the large macroscopic roughness of the sheared edge owing to the notch effect. The limiting expansion ratio of the pierced ultra high strength steel sheet was influenced by the roughness and not by microscopic roughness. In addition, the limiting expansion ratio decreased with an increase in the hardness of the fracture surface. Thus the finishing of a sheared edge with a conical punch is effective in improving the formability of the hole expansion of ultra high strength steel sheets.