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AssigneeJfe Steel Corporation
InventorsYoshihiko Ono, Yasunobu Nagataki, Yasushi Tanaka, Kozo Harada, Hisanori Ando
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This disclosure relates to a high strength cold rolled steel sheet composed of ferrite grains having an average grain diameter of 10 μm or less, in which the average number per unit area of Nb(C, N) precipitates having a diameter of 50 nm or more is 7.0×10−2/μm2 or less, and a zone having a width of 0.2 to 2.4 μm 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 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.
A high strength cold rolled 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, 0.001 to 0.007% of N, 0.03 to 0.2% of Nb, 0.2 to 0.6% of sol.Al by mass, and the balance of Fe and inevitable impurities and composed of ferrite grains having an average grain diameter of 10 μm or less, in which the average number per unit area (hereinafter referred to as “average area density”) of Nb(C, N) precipitates having a diameter of 50 nm or more in the ferrite grains is 7.0×10−2/μm2 or less, and a zone having a width of 0.2 to 2.4 μm 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.
Dependencies are inferred from the claim text.