Tailored die quenching of steel parts having strength distribution using bypass resistance heating in hot stamping

A tailored die quenching process of steel parts having a strength distribution using bypass resistance heating in hot stamping was developed. In the tailored die quenching process, zones requiring high strength in a quenchable steel sheet were heated, and then were quenched. In the bypass resistance heating, zones in contact with copper bypasses having a low resistance and large cross-sectional area were not heated due to the passage of the current though the copper bypasses. The bypass resistance heating was stable even for the heavy current in rapid heating of the steel sheets because of passage of current in one direction, and the electrical power loss was small. The hardenability for the bypass resistance heating was first examined by sandwiching a partially heated sheet between large steel blocks without deformation. Next, the tailored die quenching process using bypass resistance heating in the hat-shaped bending of the steel sheet was performed to form a part having high strength around the corners. A hat-shaped part having a tensile strength of approximately 1.5 GPa around the corners was formed, and the input energy and punching load in the bottom of the bent sheet were considerably smaller than those for whole heating.