49 巻 (2008) 564 号 p. 56-60
The features of sheared edges in warm and hot punching of an ultra high strength steel sheet using resistance heating were examined. As the heating temperature increased, the depth of the shiny burnished surface on the sheared edge increased and that of the rough fracture surface decreased. The rollover depth and burr height of the sheared edge markedly increased from about 800°C. The roughness level of the fracture surface became high above 650°C, whereas that of the burnished surface was almost constant. The punching load was considerably reduced by the heating, i.e., 40% of the cold punching load at 650°C and 15% at 1070°C. The effects of the punching speed and the clearance between the punch and the die on the surface quality of the sheared edge became marked at high temperatures. It was found that the warm and hot shearing of ultra high strength steel sheets is effective for improving the surface quality of the sheared edge and in reducing the shearing load.