Bearing wire rod and manufacturing method therefor

Abstract

Disclosed is a bearing wire rod includes, in percent by weight (wt %), 0.8 to 1.2% of C, 0.01 to 0.6% of Si, 0.1 to 0.6% of Mn, 1.0 to 2.0% of Cr, 0.01 to 0.06% of Al, 0.02% or less (exclusive of 0) of N, and the balance of Fe and inevitable impurities, wherein a prior austenite grain size of a microstructure is from 3 to 10 μm, and a sum of lengths of high angle grain boundaries having a misorientation angle of 15° or more per unit area is from 1,000 to 4,000 mm/mm2.

1 . A bearing wire rod comprising, in percent by weight (wt %), 0.8 to 1.2% of C, 0.01 to 0.6% of Si, 0.1 to 0.6% of Mn, 1.0 to 2.0% of Cr, 0.01 to 0.06% of Al, 0.02% or less (exclusive of 0) of N, and the balance of Fe and inevitable impurities,
 wherein a prior austenite grain size of a microstructure is from 3 to 10 μm, and   a sum of lengths of high angle grain boundaries having a misorientation angle of 15° or more per unit area is from 1,000 to 4,000 mm/mm 2 .   
     
     
         2 . The bearing wire rod according to  claim 1 , wherein a sum of lengths of low angle grain boundaries having a misorientation angle of 15° or less per unit area is from 250 to 800 mm/mm 2 , and a ratio of grain boundaries having a misorientation angle of 5° or less to the low angle grain boundaries is from 40 to 80%. 
     
     
         3 . The bearing wire rod according to  claim 1 , wherein the microstructure comprises reticulated proeutectoid cementite in grain boundaries and pearlite in grains. 
     
     
         4 . The bearing wire rod according to  claim 3 , wherein an interlamellar spacing in the pearlite is from 0.05 to 0.2 μm. 
     
     
         5 . The bearing wire rod according to  claim 1 , wherein a tensile strength is 1,200 MPa or more and a reduction in area (RA) is 20% or more. 
     
     
         6 . The bearing wire rod according to  claim 1 , wherein an average aspect ratio of cementite is 2.5 or less after performing softening heat treatment once. 
     
     
         7 . The bearing wire rod according to  claim 1 , wherein a tensile strength is 750 MPa or less after performing softening heat treatment once. 
     
     
         8 . A method for manufacturing a bearing wire rod, the method comprising:
 heating a billet comprising, in percent by weight (wt %), 0.8 to 1.2% of C, 0.01 to 0.6% of Si, 0.1 to 0.6% of Mn, 1.0 to 2.0% of Cr, 0.01 to 0.06% of Al, 0.02% or less (exclusive of 0) of N, and the balance of Fe and inevitable impurities, in a temperature range of 950 to 1,050° C.;   preparing a wire rod by finish hot rolling the billet in a temperature range of Ael to Acm ° C. with a critical deformation, represented by Expression (1) below, or more; and   cooling the wire rod to a temperature range of 500 to 600° C. at a rate of 3° C./sec or more, and cooling the wire rod at a rate of 1° C./sec or less:
   −1.6Ceq 2 +3.11Ceq−0.48  Expression (1):
 
   (wherein Ceq=C+Mn/6+Cr/5, and C, Mn, and Cr denote wt % of respective elements).   
     
     
         9 . The method according to  claim 8 , wherein the wire rod satisfies Expression (2) below:
     T   pf   −T   f ≤50° C.  Expression (2):
   (wherein T pf  is an average surface temperature of the wire rod before the finish hot rolling, and T f  is an average surface temperature of the wire rod after the finish hot rolling).   
     
     
         10 . The method according to  claim 8 , wherein a heating time is 90 minutes or less. 
     
     
         11 . The method according to  claim 8 , wherein an average austenite grain size (AGS) before the finish hot rolling is from 5 to 20 μm. 
     
     
         12 . The method according to  claim 8 , further comprising performing softening heat treatment by heating the wire rod to a temperature range of Ael to Ael+40° C. after the cooling and maintaining the temperature for 5 to 8 hours. 
     
     
         13 . The method according to  claim 12 , further comprising cooling the wire rod to 660° C. at a rate of 20° C./hr or less after the softening heat treatment.