| abstract |
Provided is a coil spring that is excellent in fatigue resistance while reducing material costs and simplifying a manufacturing process without reducing yield. SOLUTION: In mass%, C: 0.5 to 0.7%, Si: 1.0 to 2.0%, Mn: 0.1 to 1.0%, Cr: 0.1 to 1.0 %, P: 0.035% or less, S: 0.035% or less, the balance being a component composed of iron and inevitable impurities, bainite 65% or more by area ratio in any wire cross section, residual austenite In the residual austenite, the average C concentration in the retained austenite is 0.65 to 1.7%, and in the residual stress distribution of any strand cross section, the cross section of the strand cross section When the equivalent circle diameter is D (mm), the compressive residual stress layer is formed in the range of 0.35 mm to D / 4 from the surface, and the maximum compressive residual stress is 800 to 2000 MPa. In the surface Vickers hardness distribution, the center hardness is 550 to 650 HV, The range of the depth 0.05~0.3mm terms are 50~500HV large high hardness layer than the hardness of the center is formed, and both end faces are flat ground. [Selection figure] None |