55 巻 (2014) 636 号 p. 50-54
Hot semi-punching and cold scrap removing processes were developed to make holes in hot-stamped steel parts having high strength. For hot semi-punching, additional channels for taking punching scraps out of dies are not required. Achieving the minimum amount of residue without detachment of punching scraps and no clearance between the die and punch were the best conditions for the hot semi-punching process. The hot punching and cold removing loads were much smaller than the cold punching load of quenched sheets, the quality of the sheared edge was high and the delayed fracture around the sheared edge was prevented. In addition, quenching of punching portions was prevented by sandwiching these portions between ceramic tools during rapid resistance heating to facilitate cold punching of hot-stamped parts. The cold punching load for the local prevention of quenching was half of that without local prevention and the delayed fracture was also prevented, whereas the drop in hardness around the sheared edge became larger than that measured for laser cutting.