| 摘要 |
PROBLEM TO BE SOLVED: To provide low yield ratio steel for low temperature use which has stabilized strength and yield ratio, and a method for advantageously producing the steel. SOLUTION: The steel has a composition containing 0.02 to 0.16% C, 0.10 to 0.5% Si, 0.70 to 1.6% Mn and 0.01 to 0.08% Al, and the balance Fe with inevitable impurities, and a structure consisting of the three phases of ferrite, bainite and martensite, and in which the volume ratio of the martensite is 1 to 15%. For obtaining the above structure, rolling and cooling conditions are controlled so that MVF=0.114.Rnx -0.00616.Tfin -0.576.CRX-500 +8 (Rnx is the draft (%) in theγrecrystallization region; Tfin is the rolling finishing temperature ( deg.C); and CRX-500 is the cooling rate ( deg.C/s) at X to 500 deg.C; wherein, X=MIN(Tfin , 800) reaches 1 to 15.
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