| 摘要 |
PURPOSE: To provide superior spot weldability and nonaging characteristic without adding Ti and Nb by combinedly adding P and B to an ultralow carbon steel and controlling the draft at temper rolling. CONSTITUTION: This steel has a composition consisting of, by weight ratio, 0.0001-0.0026% C, <=1.2% Si, 0.03-3.0% Mn, 0.02-0.15% P, 0.0010-0.020% S, 0.005-0.1% Al, 0.0001-0.0080% N, 0.0001-0.0030% B, and the balance Fe with inevitable impurities and satisfying B/N> 1. This steel is hot-rolled at (Ar3 -100 deg.C, and the resultant plate is cooled, within 1.5sec after hot rolling, down to <=750 deg.C at a rate of 50 deg.C/sec and coiled at 500-750 deg.C. Then, the plate is cold-rolled at >=70% draft and subjected to plating and, if necessary, to alloying treatment in inline-annealing-type continuous galvanizing equipment. Subsequently, according to C content in the steel sheet, temper rolling is done at a draft of >=1.5×(1-400×C)% and >=2080×(C-0.0015)%.
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