| 摘要 |
PROBLEM TO BE SOLVED: To provide a method for inexpensively and stably mass-producing a low yield ratio high tensile strength steel used for multistoried buildings or the like without deteriorating the uniformity of the material in the thickness direction. SOLUTION: A steel contg., by weight, 0.02 to 0.18% C, 0.05 to 0.5% Si, 0.6 to 1.7% Mn and <=0.08% Al, and the balance Fe with inevitable impurities is heated to >=1000 deg.C, is thereafter subjected to hot rolling at a draft of >=50% in the temp. region of Ar3 or above, is then cooled at a cooling rate of >=2 deg.C/sec to the steel surface temp. of from Ar3 to (Ar3 -200) deg.C and is reculerated to the surface temp. of 650 deg.C to Ac1 . After that, it is again cooled at a cooling rate of >=2 deg.C/sec to the steel average temp. region of Ar3 to (Ar3 -100) deg.C, is held in this temp. region T for a time: t (sec) satisfying the following inequality and is thereafter cooled at a cooling rate of 2 to 15 deg.C/sec to the steel average temp. region of 400 to 600 deg.C, where 10<1.3-0.006> XΔT<=t<=150 (ΔT: Ar3 ( deg.C)-T( deg.C)).
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