| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength hot rolled steel sheet having excellent ductility, stretch flange formability and surface properties while having high tensile strength of≥590 MPa, and to provide a method for producing the same. SOLUTION: The hot rolled steel sheet has a chemical composition comprising, by mass, 0.02 to 0.05% C, 0.6 to 1.0% Si, 0.8 to 1.5% Mn,≤0.05% P,≤0.005% S, 0.10 to 1.0% Al,≤0.01% N, 0.01 to 0.10% Ti, 0.01 to 0.10% Nb and 0.001 to 0.005% Ca, and the balance Fe with impurities, has a steel structure comprising ferrite of≥93% by area ratio, has mechanical properties in which tensile strength is≥590 MPa, a product (TS×El value) between the tensile strength (MPa) and the total elongation (%):≥17,500 MPa×% and a product (TS×λvalue) between the tensile strength (MPa) and hole expansion ratio(%):≥72,000 MPa×%, and has surface properties in which the area ratio of insular scale flaws with the maximum length of≥5 mm is≤5% in the surface of the steel sheet. COPYRIGHT: (C)2010,JPO&INPIT
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