Abstract
A punching process including thickening of a hole edge of an ultra-high strength steel sheet with a taper punch and step die was developed to improve fatigue strength of the punched sheet. In this process, the sheet was punched by the bottom of the punch, and subsequently the hole edge was thickened by the taper of the punch and the corner step of the die. The taper angle of the punch and the step height of the die were optimised to increase the amount of thickening. The quality of the sheared edge for the thickened punched sheet was improved by ironing with the taper of the punch, and compressive residual stress was generated around the hole edge. Seizure on the surface of the punch was prevented by VC-coating for repeated punching and realistic punching speed. It was found that the present punching process including thickening is effective in improving the fatigue strength of the punched ultra-high strength steel sheet and in preventing occurrence of delayed fracture.
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