Spring and manufacture method thereof

A production method for a spring, comprising:

• a step of preparing a wire rod consisting of, by mass %, 0.5 to 0.7% of C, 1.0 to 2.0% of Si, 0.1 to 1.0% of Mn, 0.1 to 1.0% of Cr, not more than 0.035% of P, not more than 0.035% of S, and the balance of Fe and inevitable impurities;
• a step of cold forming the wire rod into a coil spring;
• a heat treatment step after the cold forming;
• and
• a shot peening step of shooting shot at the coil spring after the heat treatment step,
• wherein in the heat treatment step, the coil spring is austenitized at a temperature of Ac3 point to (Ac3 point+250° C.) and is cooled at a cooling rate of not less than 20° C./second, and then the coil spring is held at a temperature of (Ms point−20° C.) to (Ms point+60° C.) for not less than 400 seconds and is cooled to room temperature, thereby obtaining a coil spring having a structure including not less than 65% of bainite and 4 to 13% of residual austenite by area ratio in a cross section, the residual austenite containing carbon at an average concentration of 0.65 to 1.7%,
• the coil spring having a compressive residual stress layer in a cross section from a surface to a depth of 0.35 mm to D/4, in which D (mm) is a circle-equivalent diameter of the cross section, the compressive residual stress layer having maximum compressive residual stress of 800 to 2000 MPa, and
• the coil spring having a center portion with Vickers hardness of 550 to 650 HV in a cross section and having a high hardness layer with greater hardness than the center portion by 50 to 500 HV from a surface to a depth of 0.05 to 0.3 mm, wherein the residual austenite has an average circle-equivalent grain diameter of not more than 3 μm.