| 摘要 |
PROBLEM TO BE SOLVED: To provide steel for molded structure which is small in the hardening and softening of the heat-affected zone and to provide its producing method. SOLUTION: This steel has a composition containing, by mass, 0.01 to 0.1% C, <=0.6% Si, 0.4 to 2.0% Mn, <=0.025% P, <=0.008% S, <=0.06% Al, 0.05 to 0.2% Nb, 0.005 to 0.035% Ti and 0.001 to 0.005% N, and further containing, at need, Cu, Ni, Cr, Mo, V, Mg, Ca and rare earth metals, and the balance iron, and PCM defined by the following formula (1) satisfies <=0.18%: PCM=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10...(1). In this producing method, a slab having the above composition is reheated to a temperature in which Nb enters into solid solution, is rolled by >=30% in theγphase unrecrystallized temperature region so as to be finished at >=750 deg.C, is cooled from >=700 to 550 deg.C at a cooling rate above that of air cooling and is subsequently air-cooled.
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