Patent ID: 7608156
Filing Date: 2009-10-27
Classification: C21D,C22C

Abstract:
1. 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 /μm 2 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.