High Carbon steel wire material having excellent wire drawability and manufacturing process thereof

Abstract

A high carbon steel wire material which is made of high carbon steel as a raw material for wire products such as steel cords, bead wires, PC steel wires and spring steel, allows for these wire products to be manufactured efficiently at a high wire drawing rate and has excellent wire drawability and a manufacturing process thereof. This high carbon steel wire material is made of a steel material having specific contents of C, Si, Mn, P, S, N, Al and O, and the Bcc-Fe crystal grains of its metal structure have an average crystal grain diameter (D ave ) of 20 μm or less and a maximum crystal grain diameter (D max ) of 120 μm or less, preferably an area ratio of crystal grains having a diameter of 80 μm or more of 40% or less, an average sub grain diameter (d ave ) of 10 μm or less, a maximum sub grain diameter (d max ) of 50 μm or less and a (D ave /d ave ) ratio of the average crystal grain diameter (D ave ) to the average sub grain diameter (d ave ) of 4.5 or less.

1 . A high carbon steel wire material having excellent wire drawability, comprising 
 0.6 to 1.1% by mass of C,    0.1 to 2.0% by mass of Si,    0.1 to 1.0% by mass of Mn,    0.020% or less by mass of P,    0.020% or less by mass of S,    0.006% or less by mass of N,    0.03% or less by mass of Al and 0.0030% or less by mass of O, the balance being Fe and unavoidable impurities, wherein    the Bcc-Fe crystal grains of its metal structure having an average crystal grain diameter (D ave ) of 20 μm or less and a maximum crystal grain diameter (D max ) of 120 μm or less.    
   
   
       2 . The high carbon steel wire material according to  claim 1 , wherein the bcc-Fe crystal grains of the metal structure have an area ratio of crystal grains having a diameter of 80 μm or more of 40% or less.  
   
   
       3 . The high carbon steel wire material according to  claim 1 , wherein the bcc-Fe crystal grains of the metal structure have an average sub grain diameter (d ave ) of 10 μm or less and a maximum sub grain diameter (d max ) of 50 μm or less.  
   
   
       4 . The high carbon steel wire material according to  claim 1 , wherein the (D ave /d ave ) ratio of the average crystal grain diameter (D ave ) to the average sub grain diameter (d ave ) of the bcc-Fe crystal grains of the metal structure is 4.5 or less.  
   
   
       5 . The high carbon steel wire material according to  claim 1 , wherein when the tensile strength of the steel wire material is represented by TS and the content of C in the steel wire material is represented by Wc, they satisfy the relationship of the following expression (1):  
         TS≦ 1240 ×Wc   0.52   (1).  
   
   
       6 . The high carbon steel wire material according to  claim 1 , wherein steel further contains at least one element selected from 1.5% or less (not including 0%) by mass of Cr, 1.0% or less (not including 0%) by mass of Cu and 1.0% or less (not including 0%) by mass of Ni.  
   
   
       7 . The high carbon steel wire material according to  claim 1 , wherein steel further contains at least one element selected from 5 ppm or less (not including 0 ppm) of Mg, 5 ppm or less (not including 0 ppm) of Ca and 1.5 ppm or less (not including 0 ppm) of REM.  
   
   
       8 . The high carbon steel wire material according to  claim 1 , wherein the total decarbonization of the surface layer (D m-T ) is 100 μm or less and the adhesion of scale is 0.15 to 0.85% by mass.  
   
   
       9 . A process for manufacturing a high carbon steel wire material having excellent wire drawability, comprising the steps of 
 heating a steel wire material made of steel having composition, specified in  claim 1 , at 730 to 1,050° C.,    cooling it to 470 to 640° C. (T 1 ) at an average cooling rate of 15° C./sec or more, and    heating it to 550 to 720° C. (T 2 ) at an average temperature elevation rate of 3° C./sec or more, where (T 2 ) is higher than the above temperature (T 1 ).    
   
   
       10 . A process for manufacturing a high carbon steel wire material having excellent wire drawability, comprising the steps of 
 heating a steel wire material made of steel having composition, specified in  claim 6 , at 730 to 1,050° C.,    cooling it to 470 to 640° C. (T 1 ) at an average cooling rate of 15° C./sec or more, and    heating it to 550 to 720° C. (T 2 ) at an average temperature elevation rate of 3° C./sec or more, where (T 2 ) is higher than the above temperature (T 1 ).    
   
   
       11 . A process for manufacturing a high carbon steel wire material having excellent wire drawability, comprising the steps of 
 heating a steel wire material made of steel having composition, specified in  claim 7 , at 730 to 1,050° C.,    cooling it to 470 to 640° C. (T 1 ) at an average cooling rate of 15° C./sec or more, and    heating it to 550 to 720° C. (T 2 ) at an average temperature elevation rate of 3° C./sec or more, where (T 2 ) is higher than the above temperature (T 1 ).    
   
   
       12 . A process for manufacturing a high carbon steel wire material having excellent wire drawability, comprising the steps of 
 heating a steel material having composition, specified in  claim 1 , at 900 to 1,260° C.,    hot rolling it at a temperature of 740° C. or higher to subject it to finish rolling at a temperature of 1, 100° C. or lower,    cooling it with water to 750 to 950° C. to wind it on a conveyor device,    cooling it at an average cooling rate of 15° C./sec or more to 500 to 630° C. (T 3 ) within 20 seconds after winding, and    reheating it to 580 to 720° C. (T 4 ) within 45 seconds after winding, where (T 4 ) is higher than the above minimum value (T 3 ).    
   
   
       13 . A process for manufacturing a high carbon steel wire material having excellent wire drawability, comprising the steps of 
 heating a steel material having composition, specified in  claim 6 , at 900 to 1,260° C.,    hot rolling it at a temperature of 740° C. or higher to subject it to finish rolling at a temperature of 1, 100° C. or lower,    cooling it with water to 750 to 950° C. to wind it on a conveyor device,    cooling it at an average cooling rate of 15° C./sec or more to 500 to 630° C. (T 3 ) within 20 seconds after winding, and    reheating it to 580 to 720° C. (T 4 ) within 45 seconds after winding, where (T 4 ) is higher than the above minimum value (T 3 ).    
   
   
       14 . A process for manufacturing a high carbon steel wire material having excellent wire drawability, comprising the steps of 
 heating a steel material having composition, specified in  claim 7 , at 900 to 1,260° C.,    hot rolling it at a temperature of 740° C. or higher to subject it to finish rolling at a temperature of 1, 100° C. or lower,    cooling it with water to 750 to 950° C. to wind it on a conveyor device,    cooling it at an average cooling rate of 15° C./sec or more to 500 to 630° C. (T 3 ) within 20 seconds after winding, and    reheating it to 580 to 720° C. (T 4 ) within 45 seconds after winding, where (T 4 ) is higher than the above minimum value (T 3 ).