Patent ID: 7252722
Filing Date: 2007-08-07
Classification: C21D,C22C,Y10T

Abstract:
1. A steel sheet consisting essentially of 0.004 to 0.02% C, 1.0% or less Si, 0.7 to 3.0% Mn, 0.02 to 0.15% P, 0.02% or less S, 0.01 to 0.1% Al, 0.004% or less N, 0.2% or less Nb, by mass %, optionally Ti, Bi or at least one element selected from the group consisting of Cr, Mo, Ni and Cu, and the balance being Fe; the Nb content satisfies a formula of wherein Nb*=Nb−(93/14)×N, and wherein C, N and Nb designate the content in mass % of carbon, nitrogen and niobium, respectively; and a yield strength and an average grain size of the ferritic grains which satisfy a formula of wherein YP designates yield strength in MPa, and d designates an average size of ferritic grains in μm.