| 摘要 |
PROBLEM TO BE SOLVED: To provide a composite structure type high-tensile strength cold-rolled steel sheet having a trace amount of Si content and having a tensile strength of≥590 MPa, and in which TS×El satisfies 19,000 MPa×% or above. SOLUTION: The cold rolled steel sheet has a composition comprising 0.03 to 0.20% C,≤0.4% Si and 1.0 to 3.0% Mn, and comprising≤0.02% Al and 0.008 to 0.025% N so as to satisfy N/Al≥0.5, further comprising N in a solid solution state of≥0.005%, and the balance Fe with inevitable impurities, and has a structure comprising, by a volume ratio, a ferrite phase of 60 to 94%, a martensitic phase of 3 to 30%, and a retained austenitic phase of≥3.0%. In this way, the high tensile strength cold rolled steel sheet is obtained whose strength-ductility balance is excellent while maintaining the fineness in the surface. Further, one or two selected from Cr and Mo and/or one or two selected from Cu and Ni and/or one or more selected from Nb, Ti, V and B may be incorporated therein so as to satisfy N/(Al+Nb+Ti+V+B)≥0.5. COPYRIGHT: (C)2009,JPO&INPIT
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