| 摘要 |
PROBLEM TO BE SOLVED: To provide a high strength and high ductility cold rolled steel sheet which has no generation of surface defects in a slab, and has excellent ductility, spot weldability and press formability as well. SOLUTION: The steel sheet has a composition containing 0.05 to 0.40% C, 0.5 to 3.0% Si, 0.5 to 3.0% Mn, <=0.05% P, <=0.005% S, <0.10% Al, <=0.0100% N and <=0.20% Ti, and further containing one or two kinds of Ca and rare earth metals, and has a structure containing, by volume, >=30% ferrite and retainedγof >=3% by the average in the total sheet thickness direction, and in which the content of residualγin the region from the steel sheet surface to a position of 1/16 in the sheet thickness is <=50% to the content of residualγin the central part in the sheet thickness direction. Ti/N>=2.5 is preferably satisfied. The steel sheet may contain one or two kinds of Nb and V, and one or more kinds selected from Cr, Mo and B.
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