| 摘要 |
PROBLEM TO BE SOLVED: To improve hole expandability and machinability in a high tensile strength steel sheet. SOLUTION: This steel sheet has a composition containing, by weight, 0.02 to 0.20% C, <=2.5% Si, 0.9 to 2.2% Mn, <=0.10% P, <=0.010% S, 0.003 to 1.0% Al, 0.0010 to 0.020% N, 0.01 to 0.40% Ti and 0.0003 to 0.010% Sn and also satisfying the condition of Sn>=N/15 and, as a metallic structure, contains bainite having hardness of >=150 HV by Vickers hardness in >=15% by volume ratio. In its production, a slab is heated to >=1,050 deg.C, is hot-rolled so as to be finished at the finishing temperature of 780 to 1,000 deg.C, is subsequently cooled at a average cooling rate of 5 deg.C/s and is coiled at 300 to 650 deg.C.
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