Wire rod, hypereutectoid bainite steel wire, and method for manufacturing thereof

Abstract

A wire rod according to the present invention includes a predetermined chemical composition, wherein a metal structure includes 90 area % to 100 area % of bainite, wherein when eight test pieces having a length of 400 mm, which are obtained by dividing a wire rod having a length of 3200 mm into eight components having a same length, are manufactured, an average tensile strength TS of each test pieces satisfies a relation of “[TS]≦810×[C]+475” by N/mm 2 , wherein a difference between a maximum value and a minimum value of the tensile strengths of each test pieces is 50 N/mm 2 or less, and wherein an average reduction of area RA of each test pieces satisfies a relation of “[RA]≧−0.083×[TS]+154” by %.

1 . A wire rod comprising, as a chemical composition, by mass %:
 C: more than 0.80% to 1.20%,   Si: 0.10% to 1.50%,   Mn: 0% to 1.00%,   P: 0% to 0.02%,   S: 0% to 0.02%,   Cr: 0% to 1.00%,   Ni: 0% to 1.00%,   Cu: 0% to 1.00%,   Mo: 0% to 0.50%,   Ti: 0% to 0.20%,   Nb: 0% to 0.20%,   V: 0% to 0.20%,   B: 0% to 0.0050%,   Al: 0% to 0.10%,   Ca: 0% to 0.05%, and   a remainder including Fe and impurities,   wherein a metal structure includes 90 area % to 100 area % of a bainite;   wherein when eight test pieces having a length of 400 mm, which are obtained by dividing a wire rod having a length of 3200 mm into eight components having a same length, are manufactured, an average tensile strength TS of each test pieces satisfies a following equation 1, by N/mm 2 ,   wherein a difference between a maximum value and a minimum value of the tensile strengths of each test pieces is 50 N/mm 2  or less, and   wherein an average RA of reduction of area of each test pieces satisfies a following equation 2, by %,
   [ TS]≦ 810×[ C]+ 475  (equation 1),
 
   [ RA]≧− 0.083×[ TS]+ 154  (equation 2),
 
   Here, [C] means C content in the wire rod by mass %, [TS] means the average TS of the tensile strengths by N/mm 2 , and [RA] means the average RA of the reduction of area by %.   
     
     
         2 . A hypereutectoid bainite steel wire,
 which is obtained by the wire drawing the wire rod according to  claim 1 .   
     
     
         3 . A method for manufacturing the wire rod according to  claim 1 , the method comprising:
 rolling a billet to obtain a wire rod,   wherein the billet includes, as a chemical composition, by mass %:   C: more than 0.80% to 1.20%,   Si: 0.10% to 1.50%,   Mn: 0% to 1.00%,   P: 0% to 0.02%,   S: 0% to 0.02%,   Cr: 0% to 1.00%,   Ni: 0% to 1.00%,   Cu: 0% to 1.00%,   Mo: 0% to 0.50%,   Ti: 0% to 0.20%,   Nb: 0% to 0.20%,   V: 0% to 0.20%,   B: 0% to 0.0050%,   Al: 0% to 0.10%,   Ca: 0% to 0.05%, and   a remainder including Fe and impurities;   immersing the wire rod of 850° C. to 1050° C. into a first molten salt bath or a first molten lead bath of 350° C. to 450° C., subsequently taking out the wire rod from the first molten salt bath or the first molten lead bath;   immersing the wire rod into a second molten salt bath or a second molten lead bath of 530° C. to 600° C. at a time which is 5 seconds or less from the take-out and which is t s  seconds before to t s  seconds after start of bainite transformation of the wire rod; and   taking out the wire rod from the second molten salt bath or the second molten lead bath after the bainite transformation is completely finished,
     t   s =0.05× t   complete   (equation 3),
 
   t complete  represents a time from a start to a finish of the bainite transformation of the wire rod in seconds, when continuing an immersion of the wire rod into the first molten salt bath or the first molten lead bath.   
     
     
         4 . The method for manufacturing the wire rod according to  claim 3 ,
 wherein an elapsed time between a time when the wire rod is immersed into the first molten salt bath or the first molten lead bath and a time when the wire rod is immersed into the second molten salt bath or the second molten lead bath is 10 seconds to 40 seconds.   
     
     
         5 . The method for manufacturing the wire rod according to  claim 3 ,
 wherein the time when the bainite transformation starts in the wire rod in the first molten salt bath or the first molten lead bath is determined by detecting a reheat of the wire rod.   
     
     
         6 . A method for manufacturing the hypereutectoid bainite steel wire according to  claim 2 , the method comprising:
 rolling a billet to obtain a wire rod,   wherein the billet includes, as a chemical composition, by mass %:   C: more than 0.80% to 1.20%,   Si: 0.10% to 1.50%,   Mn: 0% to 1.00%,   P: 0% to 0.02%,   S: 0% to 0.02%,   Cr: 0% to 1.00%,   Ni: 0% to 1.00%,   Cu: 0% to 1.00%,   Mo: 0% to 0.50%,   Ti: 0% to 0.20%,   Nb: 0% to 0.20%,   V: 0% to 0.20%,   B: 0% to 0.0050%,   Al: 0% to 0.10%,   Ca: 0% to 0.05%, and   a remainder including Fe and impurities;   immersing the wire rod of 850° C. to 1050° C. into a first molten salt bath or a first molten lead bath of 350° C. to 450° C., subsequently taking out the wire rod from the first molten salt bath or the first molten lead bath;   immersing the wire rod into a second molten salt bath or a second molten lead bath of 530° C. to 600° C. at a time which is 5 seconds or less from the take-out and which is t s  seconds before to t s  seconds after start of bainite transformation of the wire rod;   taking out the wire rod from the second molten salt bath or the second molten lead bath after the bainite transformation is completely finished; and   wire drawing to the wire rod taken out from the second molten salt bath or the second molten lead bath,
     t   s =0.05× t   complete  
 
   t complete  represents a time from a start to a finish of the bainite transformation of the wire rod in seconds, when continuing an immersion of the wire rod into the first molten salt bath or the first molten lead bath.   
     
     
         7 . The method for manufacturing the hypereutectoid bainite steel wire according to  claim 6 ,
 wherein the immersion time that the wire rod is immersed into the first molten salt bath or the first molten lead bath is 10 seconds to 40 seconds.   
     
     
         8 . The method for manufacturing the hypereutectoid bainite steel wire according to  claim 6 ,
 wherein the time when the bainite transformation starts in the wire rod in the first molten salt bath or the first molten lead bath is determined by detecting a reheat of the wire rod.   
     
     
         9 . A method for manufacturing the hypereutectoid bainite steel wire according to  claim 2 , the method comprising:
 wire drawing a wire rod obtained by rolling a billet to obtain a steel wire,   wherein the billet includes, as a chemical composition, by mass %:   C: more than 0.80% to 1.20%,   Si: 0.10% to 1.50%,   Mn: 0% to 1.00%,   P: 0% to 0.02%,   S: 0% to 0.02%,   Cr: 0% to 1.00%,   Ni: 0% to 1.00%,   Cu: 0% to 1.00%,   Mo: 0% to 0.50%,   Ti: 0% to 0.20%,   Nb: 0% to 0.20%,   V: 0% to 0.20%,   B: 0% to 0.0050%,   Al: 0% to 0.10%,   Ca: 0% to 0.05%, and   a remainder including Fe and impurities;   immersing the steel wire of 850° C. to 1050° C. into a first molten salt bath or a first molten lead bath of 350° C. to 450° C., subsequently taking out the steel wire from the first molten salt bath or the first molten lead bath;   immersing the steel wire into a second molten salt bath or a second molten lead bath of 530° C. to 600° C. at a time which is 5 seconds or less from the take-out and which is t s  seconds before to t s  seconds after start of bainite transformation of the steel wire; and   taking out the steel wire from the second molten salt bath or the second molten lead bath after the bainite transformation is completely finished,
     t   s =0.05× t   complete  
 
   t complete  represents a time from a start to a finish of the bainite transformation of the steel wire in seconds, when continuing an immersion of the steel wire into the first molten salt bath or the first molten lead bath.   
     
     
         10 . The method for manufacturing the hypereutectoid bainite steel wire according to  claim 9 ,
 wherein an elapsed time between a time when the steel wire is immersed into the first molten salt bath or the first molten lead bath and a time when the steel wire is immersed into the second molten salt bath or the second molten lead bath is 10 seconds to 40 seconds.   
     
     
         11 . The method for manufacturing the hypereutectoid bainite steel wire according to  claim 9 ,
 wherein the time when the bainite transformation starts in the steel wire in the first molten salt bath or the first molten lead bath is determined by detecting a reheat of the steel wire.   
     
     
         12 . The method for manufacturing the hypereutectoid bainite steel wire according to  claim 9 , the method further comprising:
 a wire drawing to the steel wire taken out from the second molten salt bath or the second molten lead bath.   
     
     
         13 . The method for manufacturing the hypereutectoid bainite steel wire according to  claim 10 , the method further comprising:
 a wire drawing to the steel wire taken out from the second molten salt bath or the second molten lead bath.   
     
     
         14 . The method for manufacturing the hypereutectoid bainite steel wire according to  claim 11 , the method further comprising:
 a wire drawing to the steel wire taken out from the second molten salt bath or the second molten lead bath.