48 巻 (2007) 553 号 p. 140-144
The die quenching of ultrahigh-strength steel sheets in hot stamping using rapid resistance heating was experimentally investigated. A 1.2 mm-thick steel sheet with an ultimate tensile strength of 980 MPa was used as an experimental material. A test piece 130mm long and 20mm wide was resistance-heated at various temperatures above the Ac1 point with an AC power supply and, immediately after the completion of the electrification, the heated test piece was quenched by sandwiching with cold steel blocks 115mm long, 65mm wide and 30mm thick and holding for a predetermined time. The quenched test piece was evaluated by optical microstructure observation and hardness and tensile tests. The influence of material conditions on the quenching performance of the annealed sheets was also examined. Our findings are as follows. (1) The sheets resistance-heated above the Ac3 point and properly die-quenched showed a high hardness value of 495 HV, which is 1.6 times as large as the 312 HV of the original sheet, and an ultimate tensile strength of above 1400 MPa. (2) The use of an annealed sheet with coarse grain structures is not suitable for die quenching using rapid resistance heating. (3) The die quenching conducted on a trial basis in hot hat-shaped bending was successful and the hardness of the formed part increased by approximately 1.6 times that of the blank sheet.