| 摘要 |
<p>PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet having low yield ratio and excellent in ductility and hole expandability without requiring special hot rolling or addition of alloy elements such as Nb and Ti.SOLUTION: A cold rolled steel sheet has a chemical composition containing, by mass%, C:0.01 to 0.15%, Si:0.01 to 1.5%, Mn:1.5 to 3.5%, P:0.1% or less, S:0.01% or less, Al:0.005 to 1.5% and N:0.010% and havingαvalue (=Mn+Si) of 2.3% or more, and a steel structure having a volume fraction of ferrite of 40% or more and the volume fraction of martensite of 3% or more at a 1/4 depth position of sheet thickness from a surface of the steel sheet, and mechanical characteristic containing a yield ratio YR of 70% or less, a tensile strength TS (MPa) and a hole expansion ratio HER (%) satisfying TS×HER≥0.80×10and TS/(C×100)≥250.</p> |
