| 摘要 |
<p>A high tensile hot-rolled still sheet excellent in the resistance to occurrence of scuffs on a mold and in fatigue characteristics, which has a chemical composition, in mass %: C: 0.02 to 0.2 %, Si: 0.2 to 1.2 %, Mn: 1.0 to 3.0 %, Mo: 0.1 to 1.0 %, Al: 0.01 to 0.1 %, P: 0.03 % or less, S: 0.01 % or less and balance: Fe and inevitable impurities, has a steel structure wherein a ferrite and martensite phase account for 55 vol % or more and 10 to 40 vol %, respectively, and the sum of the two phases accounts for 95 vol % or more, and wherein the ratio of the average crystal grain diameter of the ferrite present in the surface portion of the steel sheet (ds) to that in the central portion of the steel sheet (dc), ds/dc, is controlled to have a range of 0.3 < ds/dc ≤ 1.0, and has a surface roughness of 1.5 μm or less in terms of an arithematic mean roughness Ra; and a method for producing the steel sheet.</p> |
