| 摘要 |
PROBLEM TO BE SOLVED: To provide a (400 to 510)N/mm<2> class low yield ratio steel material of 700 deg.C specification, used mainly for a skeleton of building and used in an uncoated or minimally coated state, and its production. SOLUTION: This steel material has a steel composition composed essentially of, by weight ratio, 0.01-0.05% C, 0.3-1.0% Mo, 0.05-0.1% Nb, and 0.02-0.1% V and further satisfies Nb+V combined precipitation parameters represented by respective contents of C, Nb, and V, heating temp. T at rolling, Ac point temp., and postrolling treatment temp. TP, NVP=100×(V/Nb)×(C-Nb/7-V /5)>=0.01 and EP=(T-1000)×(Ac1 -TP)>0. Moreover, the fraction of ferritic structure in this steel is regulated to >=0.8.
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