| 摘要 |
PROBLEM TO BE SOLVED: To obtain a steel sheet which has a specific tensile strength and excellent toughness and is optimum for a large structure by composing it of a specified compsn., allowing it to have residual austenite in a specified volume ratio and allowing MSO value to satisfy a specified temp. range. SOLUTION: This steel sheet excellent in toughness is the one having a compsn. respectively contg., by mass, 0.03 to 0.23% C, <=3% (not including 0%) Mn and <=1% (not including 0%) Mo and moreover contg. Si and/or Al by 0.08 to 0.5% in total and Ni and/or Cu by 0.1 to 1.8% in total, in which the volume ratio of residual austenite is controlled to∞2 vol%, and moreover, MSO value ( deg.C) shown in the formula in the formula, [C%], [Mn%], [Al%], [Mo%], [Ni%] and [C%] respectively denote the contents (mass%) of C, Mn, Al, Mo, Ni and Cu, and Vαdenotes the ferrite fractional ratio (the total fractional ratio of polygonal ferrite and ferrite in bainite)} satisfies the range of -100 to 100 deg.C.
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