Abstract
The punch with a step was applied to improve the punch life in punching of ultra-high strength steel sheets. The contact pressure in punching with the step was reduced due to the early separation of the sheet under tension. Punching with the step punch was carried out for 7000 times in stroke, whereas chipping in the punch occurred at 3000 times in punching without the step. Then, punching with the step punch was applied to the quenched steel sheet to prevent the occurrence of the delayed cracking of the sheared edge. The delayed cracking occurred, although the residual stress in the sheared edge decreased with increasing the step height. Furthermore, shearing with the step punch was applied to improve the stretch-flangeability of the ultra-high strength steel and high strength steel sheets.
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