| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for producing low yield ratio fire resistant shape steel capable of largely and stably producing fire resistant shape steel having <=75% yield ratio used for multistoried buildings at a low cost. SOLUTION: Steel contg., by weight, 0.04 to 0.2% C, 0.05 to 1% Si, 0.5 to 2% Mn, 0.2 to 0.8% Mo, <=0.05% P, <=0.01% S, 0.002 to 0.1% Al and <=0.02% N, and the balance Fe with inevitable impurities is heated to >=1,000 deg.C, is thereafter subjected to hot rolling of >=50% reduction of area in the temp. region of >=Ar3 and is subsequently subjected to accelerated cooling from >=Ar3 to the temp. region of (Ar3-100) deg.C to (Ar3-20) deg.C at a cooling velocity of >=1 deg.C/sec, and waiting is executed for (t) sec satisfying the following inequality 1 in the waiting temp. T( deg.C)=(Ar3-100) deg.C to (Ar3-20) deg.C. Next, it is subjected to accelerated cooling to the temp. region of 400 to 600 deg.C at a cooling velocity (v) ( deg.C/sec) satisfying the following inequality II: the inequality I: 101.2-0.01XΔT<=t<=150 sec (ΔT=Ar3( deg.C)-T( deg.C)), and the inequality II: 0.9-0.4 logh<=logv<=2.6-0.9 logh (h: the thickness (mm) of the steel).
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