High-strength steel with excellent durability and method for manufacturing same

Abstract

The present invention relates to steel used for members of chassis parts and wheel discs of commercial vehicles, or the like and, more specifically, to a high-strength steel with excellent durability and a 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.