Punching of ultra-high strength steel sheets using local resistance heating of shearing zone

Abstract

A punching process using local resistance heating of a shearing zone was developed to shear ultra-high strength steel sheets. The shearing zone was heated by passing electric current between the sheet holder and the knockout in order to decrease the flow stress in the shearing, and the heating of the die and punch was prevented by no contact with the sheet during the heating. Electrode pins having an individual spring were employed to attain uniform heating of the shearing zone. The welding resistance of the heads of the electrode pins to the sheet by the heating was examined for Ag–W, Cu–W, Ag + WC and W. The Cu–W pins having the highest welding resistance were employed in a punching experiment of 980 MPa level ultra-high strength steel sheets. The punching load was considerably reduced by the heating, e.g., about 1/5 of the cold punching load at 800 °C. 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.