52 巻 (2011) 606 号 p. 801-805
A hot stamping process of quenchable steel sheets using bypass resistance heating for forming ultra-high strength steel products having a strength distribution was developed. In the resistance heating of the sheet, non-electrified portions by contact with copper bypasses with a low resistance were not heated, whereas portions not in contact with the bypasses were heated by the electrification and were quenched by holding tools at the bottom dead center. The bypass resistance heating was stable as partial heating, 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, hot hat-shaped bending using the bypass resistance heating was performed to form a product having a high strength around the corners. A hat-shaped product having a tensile strength of approximately 1.5 GPa at the corners was formed, and the input energy and punching load in the bottom portion were considerably smaller than those for the whole heating.