| 摘要 |
PROBLEM TO BE SOLVED: To provide steel which has excellent weldability, and a production method therefor. SOLUTION: The steel has a composition containing, by mass, <=0.06% C, <=0.6% Si, 0.1 to 2.0% Mn, <=0.02% P, <=0.01% S, <=0.06% Al, <=0.006% N, and containing Nb alone or both of Nb and Ti in the ranges of 0.01 to 0.50% Nb and 0.005 to 0.10% Ti so that Excess Nb defined by Excess Nb= Nb-7.8×[C-(Ti-3.4N)/4] satisfies +0.01 or higher, and the balance iron with inevitable impurities. In its metallic structure at a position of the sheet thickness of 1/4 in the cross-section in the rolling direction, the areal fraction of polygonal or pseudo-polygonal-ferrite is <50%. In its production method, after reheating to 1,000 to 1,300 deg.C, hot rolling is finished. Subsequently, the steel sheet is retained at a surface temperature of >=900 deg.C for >=30 sec, and is thereafter allowed to cool, or is subjected to accelerated cooling from 700 deg.C to an optional temperature of <=600 deg.C.
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