| 摘要 |
<p>A high strength hot rolled steel sheet having excellent fatigue resistance is provided. A steel having a composition that includes, in terms of mass%, C: 0.05 to 0.15%, Si: 0.2 to 1.2%, Mn: 1.0 to 2.0%, Al: 0.005 to 0.10%, N: 0.006% or less, and at least one selected from Ti: 0.03 to 0.13%, Nb: 0.02 to 0.10%, and V: 0.02 to 0.15% is subjected to rough rolling at a reduction of 80% or more and finish rolling at a finish rolling delivery temperature in the range of 800 to 950°C. Immediately after completion of the finish rolling, the finish rolled sheet is subjected to cooling in two stages including a process of cooling the finish rolled sheet from the finish rolling delivery temperature to a cooling end temperature in the range of 550 to 610°C at an average cooling rate of 25°C/sec. or more and a process of cooling the finish rolled sheet from the cooling end temperature of the previous process to a coiling temperature at an average cooling rate of 100°C/sec. or more. Then the sheet is coiled at 350 to 550°C. In this manner, the microstructure of a surface layer portion having a depth of 500 µm from the surface in the sheet thickness direction comes to contain 50% or more of a fine bainite phase in terms of area fraction and the microstructure of a sheet thickness center portion that extends from a position located at a depth of 1/4 of the sheet thickness to a position located at a depth of 3/4 of the sheet thickness comes to contain 90% or more of a fine bainite phase in terms of area fraction. As a result, a high strength hot rolled steel sheet having a tensile strength TS of 780 MPa or more and excellent fatigue resistance is obtained.</p> |
