Punching of Ultra High Strength Steel Sheets Using Local Resistance Heating

Abstract

A punching process using local resistance heating near a circular shearing band was developed to shear ultra high strength steel sheets. The shearing band was heated by the electrification 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 eliminating contact with the sheet during the electrification. Electrode pins having a spring were employed to attain uniform heating of the shearing band. The degree of welding of the head of a Cu-W electrode pin to the sheet by the electrification was smaller than those of Ag-W, Ag+WC and W pins, and thus the Cu-W pins 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 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.