| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength cold-rolled steel sheet which has a low yield ratio and excellent ductility and hole expansibility without requiring special hot rolling and the addition of alloy elements such as Nb and Ti.SOLUTION: The cold-rolled steel sheet has: a chemical composition containing, by mass, 0.01 to 0.15% C, 0.01 to 1.5% Si, 1.5 to 3.5% Mn, ≤0.1% P, ≤0.01% S, >0.10 to 1.5% Al and ≤0.010% N, wherein α value defined by formula (1) of α=Mn+Si×0.5+Al×0.4 is ≥1.9; a steel structure where the volume ratio of ferrite at a depth position of 1/4 of the sheet thickness from the surface of the steel sheet is ≥40% and the volume ratio of martensite is ≥3%; and mechanical properties in which a yield ratio YR is ≤70%, tensile strength TS(MPa) and hole expansion ratio HER(%) satisfy inequality (2) of TS×HER≥0.9×10, and a tensile strength TS(MPa) and the total elongation El(%) satisfy inequality (3) of TS×El≥5.0×10. |
