HIGH STRENGTH STEEL SHEET AND PROCESS FOR PRODUCING SAME

摘要

Provided are a high strength steel sheet having excellent shape fixability and a process for producing the steel sheet. The high strength steel sheet has a composition containing in terms of mass% 0.08 0.20% C up to 0.3% Si 0.1 3.0% Mn up to 0.10% P up to 0.030% S up to 0.10% Al up to 0.010% N and 0.20 0.80% V with the remainder comprising Fe and unavoidable impurities and has a structure which comprises 95% or more ferrite phase in terms of areal proportion and in which fine grains have been precipitated. The fine grains have been dispersedly precipitated so that precipitate grains having a grain diameter less than 10 nm are present at a population density of 1.0×10 grains/µm or higher and so as to result in a distribution in which the standard deviation of the natural logarithms of the grain diameters of the precipitate grains having a grain diameter less than 10 nm is 1.5 or less. Thus a high strength steel sheet combining high strength with shape fixability is stably obtained the steel sheet having such a high strength that the yield strength YP is 1 000 MPa or higher and having a structure in which a large number of fine grains having a grain diameter less than 10 nm have been precipitated so as to form a narrow size distribution.