| 摘要 |
PROBLEM TO BE SOLVED: To provide high toughness and high strength steel which has excellent shade high weather resistance, i.e., forms stable rust having sufficient protective properties and exhibits weather resistance even in the severest environment from the viewpoint of weather resistance without depending on the addition of extremely large quantities of alloy elements causing the increase of the cost and deterioration in its weldability. SOLUTION: The high strength and high toughness weather resistant steel having excellent shade weather resistance has a composition containing, by mass, 0.01 to 0.2% C, 0.01 to 1% Si, 0.1 to 2% Mn, 0.1 to 1.5% Cu, 0.1 to 9% Ni, 0.001 to 0.1% Al, 0.001 to 0.05% P, 0.001 to 0.01% N and <=0.015% S, and the balance iron with inevitable impurities. The steel has a superfine-grained ferritic structure in which the average ferritic grain size is <=3μm, and the area ratio of a second phase other than ferrite occupied in the structure is <=25% in the range of 10 to 50% of the sheet thickness in the sheet thickness direction from each face of the front face and back face in the steel sheet.
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