| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength cold rolled steel sheet excellent in elongation and stretch-flangeability and having high yield ratio, and to a production method thereof.SOLUTION: There is provided the high yield ratio cold rolled steel sheet that has a composition containing, by mass%, C:0.15 to 0.27%, Si:0.8 to 2.4%, Mn:2.3 to 3.5%, P:0.08% or less, S:0.005% or less, Al:0.01 to 0.08%, N:0.010% or less and the balance Fe with inevitable impurities, and that has micro structure which includes: ferrite having an average crystal particle diameter of 5 μm or less and having a volume fraction of 3 to 20%; retained austenite having a volume fraction of 5 to 20%; martensite having a volume fraction of 5 to 20%; and the balance including bainite and/or tempered martensite, and in which the number of the total of the retained austenite, martensite or a mixture phase thereof each having a crystal particle diameter of 2 μm or less is 150 or more per 2000 μmin a sheet thickness cross-section parallel to a rolling direction of the steel sheet. |
