| 摘要 |
<p>PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet having improved fatigue strength without bringing about its cost increase and deterioration in total elongation, and a production method thereof.SOLUTION: There is provided the high strength hot rolled steel sheet excellent in fatigue characteristics having composition containing, by mass%, C:0.03 to 0.09%, Si:0.01 to 2.20%, Mn:0.30 to 2.20%, P≤0.100%, S≤0.010%, Al:0.005 to 0.050%, N≤0.0100% and the balance iron with inevitable impurities. In the high strength hot rolled steel sheet excellent in fatigue characteristics, a micro structure in an area of thickness corresponding to at least 10% of sheet thickness from surface and rear face of the steel sheet consists of a composite structure of ferrite and bainite or a bainite single structure, a micro structure in the area of thickness corresponding to at least 50% of sheet thickness of a center part is a composite structure of ferrite and pearlite and the hardness of the surface and rear face is 1.10 times or more as an average hardness of the area of the center part of the sheet thickness.</p> |
