Patent ID: 7485194

Claim:
A high-strength hot-rolled steel sheet having excellent anti-die-galling property and anti-fatigue property, comprising C: not less than 0.02 mass % but not more than 0.2 mass %, Si: not less than 0.2 mass % but not more than 1.2 mass %, Mn: not less than 1.0 mass % but not more than 3.0 mass %, Mo: not less than 0.1 mass % but not more than 1.0 mass %, Al: not less than 0.01 mass % but not more than 0.1 mass %, P: not more than 0.03 mass % and S: not more than 0.01 mass % and the remainder being substantially Fe and inevitable impurities, and has a tensile strength of not less than 590 MPa, a yield ratio of less than 70%, a steel microstructure containing not less than 55 vol % of ferrite and not less than 10 vol % but not more than 40 vol % of martensite provided that a total of both is not less than 95 vol %, and a ratio ds/dc of an average crystal grain size ds of the ferrite in a region ranging from a surface of the steel sheet to a position corresponding to a quarter-thickness in the steel sheet to an average crystal grain size dc of the ferrite in a region ranging from the position corresponding to the quarter-thickness in the steel sheet to a center of a thickness in the steel sheet is 0.3<ds/dc≦1.0, and a surface roughness is not more than 1.5 μm as an arithmetic mean roughness Ra.