| 摘要 |
<p>PURPOSE:To obtain a high strength steel sheet having a soft surface layer and a hard central layer and excellent in weldability and workability by cold- rolling a hot rolled Mn-containing steel plate with a specific composition, applying decarburizing annealing to the resulting steel sheet, holding the steel sheet in a specific temp. region, and further controlling cooling conditions. CONSTITUTION:A hot rolled steel plate having a composition containing, by weight, 0.15-0.60% C, 0.50-2.50% Si, 0.50-3.50% Mn, <=0.040% P, <=0.010% S, <=0.10% Al, and <=0.005% N is pickled and cold-rolled, and the cold rolled steel sheet is subjected to decarburizing. annealing. Subsequently, the above sheet is held in a temp. region between Ac1+50 deg.C and Ac3+50 deg.C for 10-120sec and then cooled down to 350-500 deg.C at 20-200 deg.C/dec cooling rate. Successively, the above temp. is maintained for 30-300sec, and then the above sheet is cooled down to room temp. at >=10 deg.C/sec cooling rate. By this method, the high strength cold rolled steel sheet which has a 10vol.% at least, of soft surface layer of <=0.10% C content and a >=10vol.% of hard central layer containing retained austenite can be easily manufactured.</p> |
