| 摘要 |
PROBLEM TO BE SOLVED: To provide a high tensile strength thick steel plate having excellent weldability and a low yield ratio, and to provide its production method. SOLUTION: A steel stock having a composition comprising 0.045 to 0.18% C, 0.05 to 0.50% Si and 0.6 to 2.0% Mn, comprising proper amounts of P, S, Al and N, and further comprising Mo and W in such a manner that (Mo+W/2) satisfies 0.08 to 0.70%, and the balance Fe with inevitable impurities, and having a weld crack sensitivity parameter Pcm of≤0.22% is subjected to: hot rolling where rolling finishing temperature reaches the one in the range of 800 to 950°C at the surface; and cooling treatment where, immediately after the hot rolling, accelerated cooling is performed at a cooling rate in the range of 0.5 to 50°C/s at the average cooling rate, and is stopped at a temperature in the range of 500 to 670°C. By the above process, the thick steel plate simultaneously satisfying a high strength of≥590 MPa by a tensile strength and a low yield ratio of≤80%, and having excellent weldability and toughness as well can be obtained. Further, for reducing the variation of the material in the plate thickness direction, it is preferable that the content of C is controlled to 0.045 to 0.08%, and Mo and W are comprised in such a manner that (Mo+W/2) satisfies 0.08 to 0.20%, and accelerated cooling stop temperature is controlled to≥580°C. COPYRIGHT: (C)2007,JPO&INPIT
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