Patent ID: 6638371
Filing Date: 2003-10-28
Classification: C21D,C22C

Abstract:
A cold-rolled steel sheet having an ultrafine grain structure including a ferrite phase, the cold-rolled steel sheet comprising:0.03 to 0.16 mass percent of C; 2.0 mass percent or less of Si; at least one of 3.0 mass percent or less of Mn and 3.0 mass percent or less of Ni; at least one of 0.2 mass percent or less of Ti and 0.2 mass percent or less of Nb; 0.01 to 0.1 mass percent of Al; 0.1 mass percent or less of P; 0.02 mass percent or less of S; 0.005 mass percent or less of N; and Fe and incidental impurities, wherein the ferrite phase has a content of 65 percent by volume or more and an average grain size of 3.5 &mgr;m or less, and the C, Si, Mn, Ni, Ti, and Nb satisfy expressions (1), (2), and (3): 637.5+4930(Ti*+(48/93)Â·[%Nb])>A1â€ƒâ€ƒ(1) A3<860â€ƒâ€ƒ(2) [%Mn]+[%Ni]>1.3â€ƒâ€ƒ(3) whereTi*=[%Ti]âˆ’(48/32)Â·[%S]âˆ’(48/14)Â·[%N]; A1=727+14[%Si]âˆ’28.4[%Mn]âˆ’21.6[%Ni]; A3=920+612.8[%C]2âˆ’507.7[%C]+9.8[%Si]3âˆ’9.5[%Si]2+68.5[%Si]+2[%Mn]2âˆ’38[%Mn]+2.8[%Ni]2âˆ’38.6[%Ni]+102[%Ti]+51.7[%Nb]; and [%M] represents element M content (mass %).