HIGH-STRENGTH STEEL WITH EXCELLENT DURABILITY AND METHOD FOR MANUFACTURING SAME

1 . A high-strength steel material having excellent durability, comprising, by weight, carbon (C): 0.05 to 0.15%, silicon (Si): 0.01 to 1.0%, manganese (Mn): 1.0 to 2.3%, aluminum (Al): 0.01 to 0.1%, chromium (Cr): 0.005 to 1.0%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.001 to 0.01%, nitrogen (N): 0.001 to 0.01%, niobium (Nb): 0.005 to 0.07%, titanium (Ti): 0.005 to 0.11%, a balance of Fe, and other inevitable impurities,

wherein a sum of a fraction of a ferrite phase and a fraction of a bainite phase in a microstructure is 90% or more, and

a fraction of a crystal grain, in which an aspect ratio (a ratio of short side/long side) of the crystal grain in a central portion (a portion ranging a t/4 point to a t/2 point in a thickness direction) is 0.3 or less, is less than 50%, and a length of a grain boundary observed in a unit area (1 mm 2 ) in the central portion is 700 mm or more.

2 . The high-strength steel material of claim 1 , wherein a fraction of an MA phase (a martensite-austenite mixed structure) is less than 3%.

3 . The high-strength steel material of claim 1 , wherein a combined area fraction of an MA phase (a martensite-austenite mixed structure) and a martensite phase is 1 to 10%.

4 . The high-strength steel material of claim 1 , wherein a tensile strength is 650 MPa or more, and a ratio of fatigue limit and yield strength (fatigue limit/yield strength) is 0.25 or more.

5 . A method of manufacturing a high-strength steel material having excellent durability, comprising:

reheating a steel slab including, by weight, carbon (C): 0.05 to 0.15%, silicon (Si): 0.01 to 1.0%, manganese (Mn): 1.0 to 2.3%, aluminum (Al): 0.01 to 0.1%, chromium (Cr): 0.005 to 1.0%, phosphorus (P): 0.001 to 0.05%, sulfur (S): 0.001 to 0.01%, nitrogen (N): 0.001 to 0.01%, niobium (Nb): 0.005 to 0.07%, titanium (Ti): 0.005 to 0.11%, a balance of Fe, and other inevitable impurities, in a temperature within a range of 1200 to 1350° C.;

hot-rolling the heated steel slab to prepare a hot-rolled steel sheet;

cooling the hot-rolled steel sheet to a temperature within a range of 400 to 500° C. and then coiling (CT); and

air-cooling to a temperature within a range of room temperature to 200° C. after coiling,

wherein the hot-rolling includes finish hot-rolling performed at a temperature (FDT (° C.)) satisfying the following Relationship 1, and

the cooling includes first cooling and second cooling, wherein the first cooling is performed at a cooling rate (CR 1 ) satisfying the following Relationship 2, and the second cooling is performed at a cooling rate (CR 2 ) satisfying the following Relationship 3:

Tn−50 ≤FDT (hot-rolling end temperature (° C.)) ≤Tn   [Relationship 1]

where Tn=730+92×[C]+70×[Mn]+45×[Cr]+650×[Nb]+410×[Ti]−80×[Si]−1.4×(t−5) (where, an element refers to wt % of the element, and t refers to a thickness (mm) of the final hot-rolled steel sheet)

CR 1 ≥196−300×[C]+4.5×[Si]−71.8×[Mn]−59.6×[Cr]+187×[Ti]+852×[Nb]  [Relationship 2]

where an element refers to wt % of the element

CR Min ≤CR 2 ≤CR Max   [Relationship 3]

where CR Max =76.6−157×[C]−25.2×[Si]−14.1×[Mn]−27.3×[Cr]+61×[Ti]+448×[Nb], CR Min =27.4−45.3×[C]+5.28×[Si]−11×[Mn]−7.33×[Cr]+42.3×[Ti]+82×[Nb] (where, an element refers to wt % of the element).

6 . The method of claim 5 , wherein the first cooling ends at 600° C.

7 . The method of claim 5 , wherein the second cooling ends at a coiling temperature (CT (° C.)).

8 . The method of claim 5 , further comprising pickling and oiling the cooled steel sheet, after the cooling.

9 . The method of claim 8 , further comprising heating the pickled and oiled steel sheet to a temperature within a range of 450 to 740° C., after the pickling and oiling, and then hot-dip galvanizing.

10 . The method of claim 9 , wherein the hot-dip galvanizing is performed using a plating bath containing, by weight, magnesium (Mg): 0.01 to 30%, aluminum (Al): 0.01 to 50%, a balance Zn, and inevitable impurities.