High strength steel for structure with excellent corrosion resistance and manufacturing method for same

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/cm2 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.