| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet which has an excellent strength-ductility balance, and excellent fatigue resistance. SOLUTION: A steel stock having a composition comprising, by mass, 0.05 to 0.3% C, 0.01 to 2.5% Si and 0.5 to 3.0% Mn is heated, and is then successively subjected to: a hot rolling stage; a primary cooling stage where cooling is performed to a temperature range of 650 to 950°C at a cooling rate of 5 to 500°C/s; a strain impartation stage where repeated bending and rebending working by a leveller is performed and a strain of 0.05 to 2.0 is imparted to the outermost surface part of the hot rolled steel sheet; and a secondary cooling stage where cooling is performed to a temperature of≤650°C at a cooling rate of 10 to 300°C. Thus, the high strength hot rolled steel sheet is obtained having a crystal grain size gradient structure in which the volume fraction of polygonal ferrite as the main phase is≥70%, the volume fraction of a secondary phase other than that is≥5%, and the average crystal grain size of the polygonal ferrite gradually decreases from the center of the sheet thickness toward the surface layer. COPYRIGHT: (C)2004,JPO&NCIPI
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