| 摘要 |
PROBLEM TO BE SOLVED: To produce a steel sheet which has excellent low temperature toughness, and has yield strength of >=690 N/mm2 added with less amount of expensive alloy elements. SOLUTION: Steel containing, by mass, 0.03 to 0.09% C, 0.05 to 0.5% Si, 1.5 to 2.0% Mn, <=0.02% P, <=0.005% S, 0.005 to 0.05% Nb, 0.005 to 0.02% Ti and <=0.05% Al is rolled at a rolling finishing temperature of >=650 deg.C, is thereafter cooled to <=400 deg.C at a cooling rate of >=10 deg.C/sec, and is next tempered under the condition in which a tempering parameter TP defined by the following formula (1) reaches a range of 0 to 2,000; wherein, in the formula (1), T is a tempering temperature ( deg.C), t is holding time (sec), and r is a heating rate ( deg.C/sec): TP=(T-300)×[20+5 log[(t+(T-300)/r)/3600]]...(1).
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