| 摘要 |
PROBLEM TO BE SOLVED: To provide a hot-rolled steel sheet having excellent tensile strength, shape-fixability, ductility, stretch flange formability and surface properties, and to provide a method for producing the same. SOLUTION: The hot-rolled steel sheet has a chemical composition comprising, by mass, 0.01 to 0.15% C, 0.2 to 1.5% Si, 0.5 to 2.5% Mn, 0.003 to 0.03% P,≤0.02% S, 0.005 to 1.0% Al and≤0.01% N, and further satisfying inequality; 0.39×Si-5×P≤Al (wherein, the symbols of the elements denote the contents (unit:mass%) of the respective elements in the steel), and the balance Fe with impurities, has a steel structure comprising, by area%, 75 to 90% polygonal ferrite, 5 to 15% martensite and 5 to 20% bainite, and the balance <5%, and in which the average crystal grain size of the polygonal ferrite is 3 to 20μm, has surface properties that the ratio of Si scale flaws with the maximum length of≥5 mm in the steel sheet surface is≤10 area%, and has mechanical properties that tensile strength is≥590 MPa, yield ratio is≤70%, TS×El value is≥18,000 MPa*%, and also, TS×λvalue is≥50,000 MPa*%. COPYRIGHT: (C)2010,JPO&INPIT
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