Prevention of delayed cracking of punched 1.5 GPa ultra-high strength steel sheets by ironing with punched slug

Abstract

Hydrogen-induced delayed cracking of punched 1.5 GPa ultra-high strength steel sheets was prevented by ironing with a slug ejected from punching, because the risk of delayed cracking for sheared 1.5 GPa sheets is very high due to high tensile residual stress and large plastic deformation. To prevent the occurrence of delayed cracking, the sheet was punched, and then, the punched hole was ironed by passing the punched slug. Although the residual stress around the punched edge of the 1.5 GPa sheet was tensile, the stress was turned to compressive stress by slug ironing. In addition, the fracture surface of the sheared edge was changed to a smooth ironed surface. A cathode hydrogen charging test of the punched edge for delayed cracking was performed. Although delayed cracks were caused at the punched hole by hydrogen charging, no cracks occurred at the ironed hole for charging. It was found that slug ironing of the punched hole is effective in preventing delayed cracking.