| 摘要 |
PROBLEM TO BE SOLVED: To provide thick, high tensile-strength hot-rolled steel sheets that have high strength of TS: 510 MPa or higher and excellent low temperature toughness. SOLUTION: A steel raw material having a composition containing C: 0.02 to 0.08%, Nb: 0.01 to 0.10% and Ti: 0.001 to 0.05%, and wherein C, Ti and Nb satisfy (Ti+(Nb/2))/C<4, is heated, and after hot-rolling, is subjected to cooling in such a manner that the average cooling rate at the position of 1 mm in the sheet thickness direction from the surface of the steel sheet is controlled to≥100°C/s and the average cooling rate at the central position of the sheet thickness of the steel sheet is controlled to≥10°C/s to below a specified cooling end temperature dependent on the amounts of the alloy elements and the cooling rate, and is then taken up at≥300°C and also at below a specified take up temperature dependent on the amounts of the alloy elements. It thereby becomes a thick, high tensile-strength hot-rolled steel sheet having a structure in which the minimum lath intervals in bainite or tempered martensite in the surface of the steel sheet is≥0.1μm, and structural variation in the sheet thickness direction is reduced, and having low temperature toughness and particularly excellent DWTT characteristics and CTOD characteristics. COPYRIGHT: (C)2010,JPO&INPIT |
