| 摘要 |
PROBLEM TO BE SOLVED: To provide a low-yield-ratio and high-strength thick steel plate which has a yield strength of 650 MPa or more and a yield ratio of 85% or lower, and is superior in toughness at a heat-affected zone in a high-heat-input weld, and to provide a manufacturing method therefor. SOLUTION: This steel includes, by mass%, 0.03 to 0.10% of C, 0.05 to 0.50% of Si, 1.4 to 3.0% of Mn, 0.02% or less of P, 0.0050% or less of S, 0.005 to 0.1% of Al, 0.004 to 0.03% of Ti, 0.0025 to 0.0065% of N, one or more elements of Cu, Ni, Cr, Mo, Nb, V, Ca, a REM, Mg and B as needed, while controlling Ceq to 0.50 to 0.68% and Ti/N to more than 2.0 but less than 4.4, and the balance Fe with unavoidable impurities; wherein a microstructure that contains island-shaped martensites has an average equivalent circle diameter of 1 to 10μm and an average aspect ratio of 4.0 or less, in an area fraction of 5 to 18%. The method for manufacturing the steel plate includes the steps of: heating the steel having the above component composition to 1,000 to 1,250°C; hot-rolling the steel at 800°C or higher; cooling the plate from the Ar<SB>3</SB>point or higher to Ar<SB>3</SB>-350°C to Ar<SB>3</SB>-100°C at 5 to 100°C/s; temporarily stopping cooling; subsequently re-heating it to the Ac<SB>1</SB>point or lower at a heating rate of 0.5°C/s or higher; air-cooling it; and tempering it at 400°C or higher but the Ac<SB>1</SB>point or lower, as needed. COPYRIGHT: (C)2009,JPO&INPIT
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