| 摘要 |
PROBLEM TO BE SOLVED: To provide a low yield ratio, high strength and high toughness steel sheet producible with high production efficiency at low cost without increasing the stock cost by the addition of large quantities of alloy elements or the like and without deteriorating weld heat affected zone toughness, and to provide its production method. SOLUTION: The low yield ratio, high strength and high toughness steel sheet comprises, by mass, 0.03 to 0.1% C, 0.01 to 0.5% Si, 1.2 to 2.5% Mn and≤0.08% Al, and comprises at least two or more metals selected from 0.005 to 0.04% Ti, 0.005 to 0.07% Nb and 0.005 to 0.1% V, and the balance substantially Fe. In the steel sheet, C/(Ti+Nb+V) as a ratio between the C content and the total content of Ti, Nb and V by atomic% is 1.2 to 3, the metallic structure is substantially composed of the three phases of ferrite, bainite and insular martensite, and the area fraction of the insular martensite is 3 to 20%. COPYRIGHT: (C)2005,JPO&NCIPI
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