| 摘要 |
The present invention relates to a high strength cold rolled steel sheet composed of ferrite grains having an average grain diameter of 10 .mu.m or less, in which the average number per unit area of Nb(C, N) precipitates having a diameter of 50 nm or more is 7. 0 X 10 -2/ .mu. m2 or less, and a zone having a width of 0.2 to 2.4 .mu. m and an average area density of NbC precipitates of 60 % or less of that of the central portion of the ferrite grains is formed along grain boundaries of the ferrite grains, for example, the steel sheet consisting of 0.004 to 0.02 % of C, 1.5 % or less of Si, 3 % or less of Mn, 0.15 % or less of P, 0.02 % or less of S, 0.1 to 1.5 % of sol.Al, 0.001 to 0.007 % of N, 0.03 to 0.2 % of Nb, by mass, and the balance of Fe and inevitable impurities. The steel sheet of the present invention is most preferably used for automobile panel parts since it has the TS of 340 MPa or more and the superior surface strain resistance and press formability.
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