| 摘要 |
PROBLEM TO BE SOLVED: To provide a low yield ratio, high strength and high toughness steel sheet mainly suitable for use in a line pipe field, and to provide a method for producing the same. SOLUTION: Disclosed is a steel sheet having a component comprising, by mass, 0.03 to 0.08% C, 0.01 to 0.5% Si, 1.2 to 2.0% Mn, 0.05 to 0.4% Mo,≥0.1% Cu+Ni, 0.005 to 0.04% Ti, 0.005 to 0.07% Nb and≤0.08% Al, and, if necessary, comprising one or two or more kinds selected from V, Cr, Ca and B, and the balance Fe with inevitable impurities, and having a three phase metallic structure of ferrite, bainite and insular martensite in which the volume fraction of the insular martensite is 3 to 15% and the volume fraction of retained austenite is≥2%, and uniform elongation in the longitudinal direction is≥12%. The steel having the component is heated at 1,000 to 1,300°C, is hot-rolled at a rolling finishing temperature of an Ar<SB>3</SB>temperature or above, is thereafter subjected to accelerated cooling to 500 to 650°C at a cooling rate of≥5°C/s, and is immediately reheated to 550 to 750°C at a temperature rising rate of≥0.5°C/s. COPYRIGHT: (C)2009,JPO&INPIT
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