| 摘要 |
PROBLEM TO BE SOLVED: To inexpensively and stably mass-produce low yield ratio high tensile strength steel used for multistoried buildings or the like without deteriorating the uniformity of the material in the plate thickness direction. SOLUTION: Steel contg., as steel components, by weight, 0.02 to 0.18% C, 0.05 to 0.5% Si, 0.6 to 1.7% Mn and <=0.08% Al is heated at >=1000 deg.C and is thereafter subjected to hot rolling of >=50% draft at >=Ar3, after that, accelerated cooling is started from >=Ar3 at >=2 deg.C/sec, at the point of time in which the surface temp. reaches <=(Ar3-200) deg.C, the cooling is temporarily interrupted, and it is reheated to 650 deg.C to <=Ac1, is again subjected to accelerated cooling to from the average temp. Ar3 to (Ar3-100) deg.C at >=2 deg.C/sec, is subjected to waiting for (t) sec, i.e. the waiting time satisfying the following inequality, is thereafter subjected to accelerated cooling to <400 deg.C average temp. at >=2 deg.C/sec and is subsequently tempered at >630 deg.C to the Ac1 point: the inequality: 101.3-0.006×ΔT<=t<=150 (whereΔT( deg.C) denotes the temp. difference between Ar3 and the average temp. T( deg.C) at waiting (=Ar3-T), and (t) (sec) denotes the waiting time).
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