50 巻 (2009) 586 号 p. 1049-1053
Local die quenching in the hot stamping of quenchable steel sheets was developed to form ultra high strength steel products having a strength distribution. Local portions of the heated sheet were quenched by holding grooved tools at the bottom dead center during the stamping. Non-contact portions without the quenching were generated in the sheet by grooving the tools, i.e. the strength in the contact portions is high owing to the quenching and that in the non-contact portions is low owing to the lack of the quenching. The hardenability for the local die quenching was first examined by sandwiching limited portions of the heated sheet between a large steel die and sheet holder without deformation. Next, hot hat-shaped bending with grooved tools was performed to form products having high strength around corners in contact with the tools. In addition, quenching in the flange portions of the bent sheet was prevented by steatite plates having low conductivity placed in the flange portions of the tools. Hat-shaped products having a tensile strength of approximately 1.5 GPa only at the corners were formed, and the punching loads in the bottom and flange portions were considerably lower than those for the non-grooved tools.