| 摘要 |
PROBLEM TO BE SOLVED: To provide a coil spring which achieves the reduction in material cost and the simplification of a production process without lowering a yield rate and is excellent in fatigue resistance.SOLUTION: A coil spring has a composition comprising, by mass, 0.5-0.7% C, 1.0-2.0% Si, 0.1-1.0% Mn, 0.1-1.0% Cr, 0.035% or less P, and 0.035% or less S, with the balance comprising iron and unavoidable impurities; and has a structure, in an arbitrary wire cross section, containing, by area ratio, 65% or more of bainite and 4-13% of retained austenite. The average C concentration in the retained austenite is 0.65-1.7%. In the residual stress distribution in an arbitrary wire cross section, when the cross section circle-equivalent diameter of a cross section of the wire is D (mm), the compressed residual stress layer is formed in a range of from the surface to 0.35 mm to D/4, and the maximum compressive residual stress is 800-2,000 MPa. In the Vickers hardness distribution in an arbitrary wire cross section, the hardness at the center is 550-650 HV, and a high-hardness layer harder by 50-500 HV than the center is formed from the surface to a depth of 0.05-0.3 mm. Both ends of the wire are ground to be flat. |
