| 摘要 |
PURPOSE:To prepare a low yield ratio and high strength steel plate, by applying specific heat treatment to low carbon steel with reduced contents of N and Al to convert the structure thereof to a two-phase structure of martensite and ferrite. CONSTITUTION:A steel plate containing less than 0.10% C, 0.05-1.5% Mn, 1-80ppm N, less than 0.10% sol. Al is hot rolled and, after taken up at 500- 750 deg.C, the hot rolled steel material is subjected to cold rolling to obtain a plate material. This cold rolled steel material is annealed at 800-950 deg.C for 1sec-3min and cooled to 780-680 deg.C from that temp. at a cooling speed of 10 deg.C/sec or more. Succeedingly, said steel material is rapidly cooled to a temp. of the Ms point or less of said steel at a cooling speed of 50 deg.C/sec to apply annealing treatment at 250-350 deg.C within 5min. By this method, the structure of the steel material is converted to a two-phase structure of ferrite and martensite and a high strength steel plate having a yield ratio of 60% or less and excellent in moldability is obtained. |
