Wire rod for springs with excellent corrosion fatigue resistance, steel wire, and manufacturing method thereof

Abstract

An aspect of the present invention relates to a wire rod for springs with high strength and excellent corrosion fatigue resistance, in which a combination of Cr, Cu, and Ni content is controlled to an appropriate level, the maximum depth of corrosion pits is set to be below a certain level, and fine carbides containing Mo are set to be at a certain level or greater.

1 . A wire rod for springs with excellent corrosion fatigue resistance, comprising:
 C: 0.40 to 0.70%, Si: 1.30 to 2.30%, Mn: 0.20 to 0.80%, Cr: 0.20 to 0.80%, Cu: 0.01 to 0.40%, Ni: 0.10 to 0.60%, Mo: 0.01 to 0.40%, P: 0.02% or less, S: 0.015% or less, N: 0.01% or less, and a balance of Fe and inevitable impurities by weight %,   wherein the wire rod satisfies equation 1 below:
   −0.14≤0.70[Cr]−0.76[Cu]−0.24[Ni]≤0.47   Equation 1
 
   where each element symbol is a value of a content of each element measured by weight %,   wherein a microstructure comprises 50 area % or less of ferrite and a balance of pearlite, and   wherein the wire rod comprises 8.0×10 4  count/mm 2  or higher of Mo-based carbides.   
     
     
         2 . The wire rod for springs of  claim 1 , further comprising:
 one or more elements selected from among V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% by weight %.   
     
     
         3 . The wire rod for springs of  claim 1 , wherein the Mo-based carbide includes 5 weight % of higher of Mo based on carbide. 
     
     
         4 . A method of manufacturing a wire rod for springs with excellent corrosion fatigue resistance, comprising:
 heating a billet to 900 to 1100° C., the billet comprising C: 0.40 to 0.70%, Si: 1.30 to 2.30%, Mn: 0.20 to 0.80%, Cr: 0.20 to 0.80%, Cu: 0.01 to 0.40%, Ni: 0.10 to 0.60%, Mo: 0.01 to 0.40%, P: 0.02% or less, S: 0.015% or less, N: 0.01% or less, and a balance of Fe and inevitable impurities by weight % and satisfying equation 1 below:
   −0.14≤0.70[Cr]−0.76[Cu]−0.24[Ni]≤0.47   Equation 1
 
   where each element symbol is a value of a content of each element measured by weight %;   obtaining a wire rod by finishing-hot-rolling the heated billet at 800 to 1000° C.; and   coiling the wire rod and cooling the wire rod such that the time for maintaining the wire rod at a temperature in a range of 600 to 700° C. is to be 31 seconds or longer.   
     
     
         5 . The method of  claim 4 , wherein the billet further comprises one or more elements selected from among V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% by weight %. 
     
     
         6 . A steel wire for springs with excellent corrosion fatigue resistance, comprising:
 C: 0.40 to 0.70%, Si: 1.30 to 2.30%, Mn: 0.20 to 0.80%, Cr: 0.20 to 0.80%, Cu: 0.01 to 0.40%, Ni: 0.10 to 0.60%, Mo: 0.01 to 0.40%, P: 0.02% or less, S: 0.015% or less, N: 0.01% or less, and a balance of Fe and inevitable impurities by weight %,   wherein the steel wire satisfies equation 1 below:
   −0.14≤0.70[Cr]−0.76[Cu]−0.24[Ni]≤0.47   Equation 1
 
   where each element symbol is a value of a content of each element represented by weight %,   wherein a microstructure is tempered martensite, and   wherein the steel wire comprises 8.0×10 4  count/mm 2  or higher of Mo-based carbides.   
     
     
         7 . The steel wire for springs of  claim 6 , further comprising:
 one or more elements selected from among V: 0.01 to 0.20%, Ti: 0.01 to 0.15%, and Nb: 0.01 to 0.10% by weight %.   
     
     
         8 . The steel wire for springs of  claim 6 , wherein the Mo-based carbide comprises 5 weight % or higher of Mo based on carbide. 
     
     
         9 . The steel wire for springs of  claim 6 , wherein a maximum depth of a corrosion pit of the steel wire is 120 μm or less. 
     
     
         10 . The steel wire for springs of  claim 6 , wherein tensile strength of the steel wire is 1800 MPa or higher. 
     
     
         11 . A method of manufacturing a steel wire for springs with excellent corrosion fatigue resistance, comprising:
 obtaining the steel wire by drawing a wire rod manufactured by the method in  claim 4 ;   austenitizing the steel wire by heating the steel wire to 850˜1000° C. and maintaining the heated steel wire for 1 minute or longer; and   oil-cooling the austenitized wire rod to 25˜80° C. and tempering the wire rod at 350˜500° C.