| 摘要 |
PURPOSE:To provide a high-strength hot-rolled steel sheet having improved DC butt weldability and formability by adding proper amounts of Nb and Ti to a steel contg. specified amounts of C, Si, Mn, Al, P and S and by specifying the carbon equiv. CONSTITUTION:The composition of a high-strength hot-rolled steel sheet of 55kgf/mm<2> level is composed of, by weight, 0.04-0.10% C, <=0.3% Si, 0.8-1.4% Mn, 0.01-0.05% Al, <=0.03% P, <=0.01% S, <=0.06% in total of 0.01-0.04% Nb and/or 0.03-0.05% Ti and the balance essentially Fe, and the carbon equiv. Ceq represented by a formula Ceq=C+Si/24+Mn/6 is regulated to 0.25-0.34. By the composition, the maximum hardness Hv of the weld zone of the steel sheet can be restricted to <=about 300, superior total elongation is ensured, and when the steel sheet is formed into wheel rims, etc., rejects are produced at a lowered rate.
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