| 摘要 |
<p>PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet having a high strength satisfying a tensile strength of 440 MPa or higher, also having the total elongation of 20% or higher and a hole expansion percentage of 70% or higher, and also having satisfactory bendability.SOLUTION: Provided is a cold rolled steel sheet comprising 0.03 to 0.20% C, 0.005 to 2.0% Si, 0.3 to 2.0% Mn, 0.0004 to 0.1% P, 0.0029% or lower S, 0.0002 to 2.0% or lower sol.Al, 0.0080% or lower N, above 0.12 to 0.30% or lower Ti, and 0.0005 to 0.0100% B, and comprising, by area%, above 5 to 95% or lower ferrite and on or more kinds of superhard phase groups consisting of martensite, retained austenite and cementite by 2.0 to 20% in total, and in which the average grain size of the ferrite is 15.0μm or lower, the average grain size of the superhard phase groups is 2.0μm or lower, the numerical density of Ti-B based precipitates with a diameter equivalent to a circle of 1μm is 500 pieces/mm, the average spacing of the superhard phases is 2.0μm or lower, and the numerical density of cracks with a depth of 3μm or higher on the surface of the steel sheet is 150 pieces/mm or lower.</p> |
