High strength steel for structure with excellent corrosion resistance and manufacturing method for same
One aspect of the present invention may provide steel having high strength characteristics and excellent corrosion resistance, which is suitable for a structure, and a method for manufacturing same.
1 . A steel for a structure having excellent corrosion resistance, the steel comprising, by weight percentage (wt %), carbon (C): 0.03 to 0.12%, silicon (Si): 0.01 to 0.8%, manganese (Mn): 1.6 to 2.4%, phosphorus (P): 0.02% or less, sulfur(S): 0.01% or less, aluminum (Al): 0.005 to 0.5%, niobium (Nb): 0.005 to 0.1%, boron (B): 10 ppm or less, titanium (Ti): 0.005 to 0.1%, nitrogen (N): 15 to 150 ppm, calcium (Ca): 60 ppm or less, and a balance of iron (Fe) and inevitable impurities,
the steel further comprising at least one or two or more selected from the group consisting of, by wt %, chromium (Cr): 1.0% or less (including 0%), molybdenum (Mo): 1.0% or less (including 0%), nickel (Ni): 2.0% or less (including 0%), copper (Cu): 1.0% or less (including 0%), and vanadium (V): 0.3% or less (including 0%),
wherein a corrosion index (CI) represented by Equation 1 is 3.0 or less,
the steel comprising a surface layer portion, disposed externally on the steel, and a central portion, disposed internally in the steel, the surface layer portion and the central portion being microstructurally divided in a thickness direction of the steel,
wherein the surface layer portion consists of a tempered bainite as a matrix structure, a fresh martensite as a second structure and a remainder of austenite,
wherein the central portion consists of acicular ferrite as a matrix structure,
wherein an average grain diameter of a microstructure of the surface layer portion is 3 μm or less (excluding 0 μm),
wherein an average grain diameter of a microstructure of the central portion is 5 to 20 μm, and
wherein weight loss per unit area in a general corrosion acceleration test based on ISO 14993 cyclic corrosion test (CCT) is 1.2 g/cm 2 or less,
CI=26.01*[Cu]+3.88*[Ni]+1.20*[Cr]+1.49*[Si]+17.28*[P]−7.29*[Cu]*[Ni]−9.1*[Ni]*[P]−33.39*[Cu] 2 [Equation 1]
where [Cu], [Ni], [Cr], [Si], and [P] refer to weight % of Cu, Ni, Cr, Si, and P, respectively, and refer to 0 when corresponding alloy composition is not included.
2 . The steel of claim 1 , wherein the surface layer portion comprises an upper surface layer portion, disposed on an upper side of the steel, and a lower surface layer portion disposed on a lower side of the steel, and
wherein each of the upper surface layer portion and the lower surface layer portion is provided to have a thickness of 3 to 10% compared with a thickness of the steel.
3 . The steel of claim 1 ,
wherein the tempered bainite and the fresh martensite are included in the surface layer portion in a total fraction of 95 area % or more.
4 . The steel of claim 1 ,
wherein the austenite is included in the surface layer portion in a fraction of 5 area % or less.
5 . The steel of claim 1 , wherein tensile strength of the steel is 570 MPa or more.