| 摘要 |
PROBLEM TO BE SOLVED: To provide steel which has excellent ductile fracture resistance in a high strain loaded state caused by a big earthquake or the like, and has excellent fire resistance, and a production method therefor. SOLUTION: The steel has a composition containing, by mass, 0.03 to 0.20% C, 0.01 to 1.0% Si, 0.5 to 2.0% Mn and 0.1 to 0.7% Mo, and, if required, further containing one or more kinds selected from Cu, Ni, Cr, V, Nb and Ti, and the balance substantially Fe with inevitable impurities. The steel has a metallic structure of ferrite in 20 to 90% by area fraction, a yield ratio in a nominal stress-nominal strain curve of <=0.77, and a work hardening exponent (n) of >=0.15. A slab having the above composition is heated at 1,000 to 1,200 deg.C, and is thereafter hot-rolled. Immediately after that, or after air-cooling to Ar3 -50 to Ar3 -150 deg.C, the steel is subjected to accelerated cooling, is further reheated to a two phase region, and is subsequently cooled.
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