POSCO Holdings Inc.
ULTRAHIGH-STRENGTH STEEL HAVING EXCELLENT COLD WORKABILITY AND SSC RESISTANCE, AND MANUFACTURING METHOD THEREFOR

Abstract:

One embodiment of the present invention provides an ultrahigh-strength steel having excellent cold workability and SSC resistance, comprising, by wt %, carbon (C) in an amount of more than 0.08% and equal to or less than 0.2%, 0.05-0.5% of silicon (Si), 0.5-2% of manganese (Mn), 0.005-0.1% of aluminum (Al), 0.01% or less of phosphorus (P), 0.0015% or less of sulfur (S), 0.001-0.03% of niobium (Nb), 0.001-0.03% of vanadium (V), 0.001-0.03% of titanium (Ti), 0.01-1% of chromium (Cr), 0.01-0.15% of molybdenum (Mo), 0.01-0.5% of copper (Cu), 0.05-4% of nickel (Ni), 0.0005-0.004% of calcium (Ca), and the balance of Fe and other inevitable impurities, wherein the microstructure of a surface layer part, which is the region from the surface to 10% of the total thickness, comprises 90 area % or more of polygonal ferrite, the microstructure of the region excluding the surface layer part comprises 90 area % or more of tempered martensite or 90 area % or more of a mixed structure of tempered martensite and tempered bainite, and the dislocation density of the surface layer part is 3.times.10 14/m 2 or less.