| 摘要 |
PROBLEM TO BE SOLVED: To produce steel having >=800 MPa tensile strength and combining excellent toughness, weldability and cost effectiveness, to provide a method for producing same, to provide a continuously cast slab and to provide a method for casting same. SOLUTION: This steel has a chemical compsn. contg., by weight, >0.1 to 0.2% C, >1.5 to 2.5% Mn, 0.005 to 0.06% Nb, 0.0005 to 0.0025% B, <=0.07% Al and <=0.006% N and also satisfying the following inequality, in which the aspect ratio of oldγgrains is >=2 in the center part of the thickness of the steel and >=3 in the 1/4 part of the thickness, and >=90% of the metallic structure are formed of a mixed structure of tempered martensite and lower bainite: the inequality: 0.05%<=Vs<=0.25%, where Vs=C+0.2×Mn-0.1×Ni-0.7×Mo+0.1×Cu.
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