| 摘要 |
PROBLEM TO BE SOLVED: To improve the weldability and welded formability by hot-rolling, cold-rolling and then annealing a raw steel material contg. specified amts. of C, Mn, S, N, Nb, Ti and V and having specified average crystal grain diameter and area ratio under specified conditions to impart a specified elongation. SOLUTION: A raw steel material contg. 0.001-0.20% C, 0.05-3.5% Mn, <=0.02% S, <=0.02% N, at least one kind among 0.005-0.20% Nb, 0.005-0.30% Ti and 0.005-0.20% V and the balance Fe with inevitable impurities and having a parent phase of <=10μm average crystal grain diameter or further a second phase of <=20% area ratio is hot-rolled at a finishing temp. of >=800 deg.C, then cold-rolled, annealed at 750-850 deg.C, hence recrystallized and cooled at a rate of 5-70 deg.C sec to obtain a cold-rolled sheet having >=10% uniform elongation and >=5% local elongation.
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