HIGH STRENGTH COLD ROLLED STEEL SHEET AND METHOD FOR PRODUCTION THEREOF

摘要

A high strength cold rolled steel sheet which has a structure comprising ferrite grains having an average grain diameter of 10 mum or less, wherein, in the ferrite grain, Nb(C,N)'s having a diameter of 50 nm or more are present in an average number per unit area (average area density) of 7.0 x 10<-2> pieces/mum<2>, and along the grain boundary of the ferrite grain, a region is formed which has a width of 0.2 to 2.4 mum and has an average area density of NbC being 60 % or less of an average area density of NbC deposited in the central portion of the ferrite grain; and, for example, the high strength cold rolled steel sheet which has a chemical composition, in mass %, C: 0.004 to 0.02 %, Si: 1.5 % or less, Mn: 3 % or less, P: 0.15 % or less, S: 0.02 % or less, sol.Al: 0.1 to 1.5 %, N: 0.001 to 0.007 %, Nb: 0.03 to 0.2 %, and the balance: Fe and inevitable impurities. The above high strength cold rolled steel sheet has a tensile strength of 340 MPa or more, and is excellent in the resistance to planar strain and in bulging characteristics, and thus is most suitable for panel parts of an automobile.