High-strength structural steel having excellent cold bendability, and manufacturing method therefor
A high-strength structural steel having excellent cold bendability, according to one embodiment of the present invention, comprises, by wt %, 0.02-0.1% of C, 0.01-0.6% of Si, 1.7-2.5% of Mn, 0.005-0.5% of Al, 0.02% or less of P, 0.01% or less of S, 0.0015-0.015% of N, and the balance of Fe and other inevitable impurities, wherein an outer surface layer part and an inner central part thereof are microstructurally divided in a thickness direction, the surface layer part can comprise tempered austenite as a matrix structure, and the central part can comprise bainitic ferrite as a matrix structure.
1. A structural steel, comprising:
by weight %, 0.02-0.1% of C, 0.01-0.6% of Si, 1.7-2.5% of Mn, 0.005-0.5% of Al, 0.02% or less of P, 0.01% or less of S, 0.0015-0.015% of N, a balance of Fe and other unavoidable impurities, wherein the structural steel is microstructurally divided into an outer surface layer part and an inner central part in a thickness direction,
wherein the surface layer part comprises tempered bainite as a matrix structure,
the central part comprises bainitic ferrite as a matrix structure, and
an average grain size of a microstructure of the surface layer part is 3 μm or less, excluding 0 μm.
2. The structural steel of claim 1 , wherein the surface layer part comprises an upper surface layer portion on an upper side of the steel, and a lower surface-layer portion on a lower side of the steel,
wherein the upper surface layer portion and the lower surface layer portion each have a thickness of 3 to 10% of a thickness of the steel.
3. The structural steel of claim 1 , wherein the surface layer part further comprises fresh martensite as a second structure, and
wherein the tempered bainite and the fresh martensite are included in the surface layer part in a fraction of 95 area % or more.
4. The structural steel of claim 3 , wherein the surface layer part further comprises austenite as a residual structure,
wherein the austenite is included in the surface layer part in a fraction of 5 area % or less.
5. The structural steel of claim 1 , wherein the bainitic ferrite is included in the central part in a fraction of 95 area % or more.
6. The structural steel of claim 1 , wherein an average grain size of a microstructure of the central part is 5 to 20 μm.
7. The structural steel of claim 1 , further comprising, by weight %, one or two or more of Ni: 0.01-2.0%, Cu: 0.01-1.0%, Cr: 0.05-1.0%, Mo: 0.01-1.0%, Ti: 0.005-0.1%, Nb: 0.005-0.1%, V: 0.005-0.3%, B: 0.0005-0.004%, and Ca: 0.006% or less.
8. The structural steel of claim 1 , wherein a tensile strength of the steel is 800 MPa or more, and a high angle grain boundary fraction of the surface layer part is 45% or more.
9. The structural steel of claim 1 , wherein in a cold bending test, in which a plurality of cold bending jigs having various tip curvature radii (r) are applied to cold-bending the steel by 180° and then whether cracks occur in the surface layer part of the steel occur is observed, and the cold bending jig is applied such that the tip curvature radii (r) are sequentially decreased, a critical curvature ratio (r/t) is 1.0 or less, the critical curvature ratio (r/t) being a ratio of the tip curvature radii (r) of the cold bending jig at a time when the cracks occur in the surface layer part of the steel, with respect to a thickness (t) of the steel.