Coil spring, and method for manufacturing same

Abstract

To provide a coil spring having excellent fatigue resistance. Disclosed is a coil spring made of steel, including (in % by mass, the same shall apply for a chemical composition): C: 0.40 to 0.70%; Si: 1.50 to 3.50%; Mn: 0.30 to 1.50%; Cr: 0.10 to 1.50%; V: 0.50 to 1.00%, and Al: 0.01% or less (excluding 0%), with the balance being iron and inevitable impurities, wherein an average crystal grain size number of prior austenite crystals in a depth of 0.3 mm from a surface is 11.0 or more, while a difference in grain size number between the respective prior austenite crystals is in a range of less than 3 from a grain size number observed at the maximum frequency, and wherein a carburized layer is provided in a depth of 0.30 to 1.00 mm from the surface, while an average Vickers hardness is 600 or higher at a position in a depth of (¼)×diameter in the depth direction from the surface.

1 . A coil spring made of steel, comprising:
 C: 0.40 to 0.70% by mass;   Si: 1.50 to 3.50% by mass;   Mn: 0.30 to 1.50% by mass;   Cr: 0.10 to 1.50% by mass;   V: 0.50 to 1.00% by mass;   Al: 0.01% by mass or less (excluding 0%), with the balance being iron; and   inevitable impurities,   wherein:   an average crystal grain size number of prior austenite crystals in a depth of 0.3 mm from a surface is 11.0 or more, while a difference in grain size number between the respective prior austenite crystals is in a range of less than 3 from a grain size number observed at the maximum frequency; and   a carburized layer is provided in a depth of 0.30 to 1.00 mm from the surface, while an average Vickers hardness is 600 or higher at a position in a depth of (¼)×diameter in the depth direction from the surface.   
     
     
         2 . The coil spring according to  claim 1 , further comprising:
 Ni: 1.50% by mass or less (excluding 0%), and/or   Nb: 0.50% by mass or less (excluding 0%).   
     
     
         3 . A method for manufacturing the coil spring of  claim 1 , the method comprising performing a vacuum carburization process at 1,000° C. or higher. 
     
     
         4 . A method for manufacturing the coil spring of  claim 2 , the method comprising performing a vacuum carburization process at 1,000° C. or higher.