| 摘要 |
PURPOSE:To produce a low-yield ratio high-tensile strength thin steel sheet excellent in ductility and resistance to secondary work embrittlement at a low cost, by heating and cooling a steel sheet, containing a fixed ratio each of C, Mn, P and Al, under a specified condition. CONSTITUTION:A steel sheet containing, by wt%, 0.02-0.15% C, 0.8-3.5% Mn, 0.03-0.15% P, up to 0.10% Al and the balance Fe and inevitable impurities is cold-rolled and then heated 10sec-10min in a temperature range of Ac1 transition point -950 deg.C. Thereafter, the steel sheet is cooled in a manner such that an averaged cooling speed in a range of 600-300 deg.C is held above a critical cooling speed CR deg.C/min defined by the formula but in the range of 15-200 deg.C/ min. In this way, a high-tensile strength steel sheet especially having tensile strength of about 50kg/mm.<2> or more is obtained at a low cost. |
