| 摘要 |
<p>PURPOSE:To provide ultrahigh strength steel very excellent in toughness, furthermore high in yield ratio and showing high impact value even in severe conditions such as a low temp. one by obtaining ultrafine austenitic crystalline grains uniform both in the surface layer and the inside of the steel. CONSTITUTION:A solid steel {test sheet (a)} having the same sectional form in a longitudinal direction is inserted into an induction heating coil 1. While the relative places between the steel {test sheet (a)} and the induction heating coil 1 are moved in a longitudinal direction, the ac frequency, electric power and feed rate of the electric current to be passed through the induction heating coil 1 are regulated, the difference in temps. between the surface of the heated steel {test sheet (a)} and the center of the inside is held to <=50 deg.C and rapid heating is executed to directly above the austenitizing temp. through the transformation temps. from the Ac1 transformation point to the Ac3 transformation point. After that, the rapidly heated steel {test sheet (a)} is immediately subjected to rapid cooling to a temp. at which hardening is completed. After the completion of the hardening, tempering is executed to obtain the ultrahigh strength steel homogeneous from the surface layer to the central part.</p> |
