| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength cold-rolled steel sheet excellent in elongation and stretch-flanging properties.SOLUTION: Provided is a cold rolled steel sheet having a chemical composition comprising, by mass, 0.06 to 0.3% of C, 0.6 to 2.5% of Si, 0.6 to 3.5% of Mn, 0.1% or less of P, 0.05% or less of S, 0 to 0.08% of Ti, 0 to 0.04% of Nb, the total of Ti and Nb by 0 to 0.10%, 0 to 1.0% solAl, 0 to 1.0% Cr, 0 to 0.3% Mo, 0 to 0.3% V, 0 to 0.005% B, 0 to 0.003% Ca, 0 to 0.003% REM, and the remainder made up by Fe and impurities, in which, at the 1/4 depth position of the sheet thickness, in the microstructure, the total of tempered martensite and bainite as the main phases≥50 area%, also, bainite≥10 area%, as the second phase, ferrite with the average particle diameter of 4.0 μm or less prescribed at the large-angle grain boundary of ≥ 15° inclination≥5 area%, the ratio of the nano-hardness of the other structure to the ferrite ≤2.5, and, at the 1/2 depth position of the sheet thickness, in the texture, the average of the X-ray intensities in the orientation group from {100}<011> to {211}<011> being below 6 by a ratio to the average of the X-ray intensities of the structure having no texture. |
