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*10 14 / m 2 or less.

Description

technical field [0001] The present disclosure relates to ultra-high strength steels having excellent cold workability and SSC resistance and methods of manufacturing the same, and more particularly, to ultrahigh strength steels having excellent cold workability and SSC resistance suitable for offshore structures and the like such as oil drilling ships or wind power installation ships. SSC-resistant ultra-high-strength steels and methods for their manufacture. Background technique [0002] Recently, facilities have become lightweight and are increasingly used in environments that require acid resistance or corrosion resistance, therefore, steel used for offshore structures used in oil drilling facilities, etc., is required to have ultra-high strength and hydrogen resistance to cracking. In particular, there is an increasing need for sulfide stress cracking (SSC) resistance associated with resistance to hydrogen generated in corrosive environments under stress. [0003] Sinc...

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Problems solved by technology

[0005] However, the uniform elongation value of low-temperature transformed structures such as martensite or bainite is significantly smaller than that of soft structures, causing surface cracks during cold working

In addition, when corrosion occurs on the surface layer part due to the high dislocation density of the surface layer part, hydrogen can easily migrate into the steel, and crack growth resistance may become weak, resulting in a decrease in SSC resistance

[0006] Therefore, the above-mentioned conventional methods have limitations in manufacturing ultra-high-strength steel having excellent cold workability and SSC resistance for offshore structures, the steel having a thickness of 6 mm to 100 mm and a yield strength of 690 MPa or more

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