Method for manufacturing a coil spring for vehicle suspension

A method for manufacturing a coil spring for vehicle suspension, comprising:

• performing a bending process of forming a spring wire comprising a spring steel into a helical shape;
• performing a first shot peening process configured to impart compressive residual stress to the spring wire by applying to the spring wire a first shot of a shot size of at least 1.0 mm;
• and
• performing a second shot peening process configured to apply to the spring wire a second shot that is smaller in shot size than the first shot, after performing the first shot peening process;
• wherein, by performing the first shot peening process and the second shot peening process, there are generated in the spring wire (i) a residual stress increase part in which an absolute value of the compressive residual stress increases from a surface of the spring wire in a depth direction, (ii) a first residual stress peak in which an absolute value of the compressive residual stress is at a maximum, (iii) a second residual stress peak formed between the residual stress increase part and the first residual stress peak, wherein a magnitude of an absolute value of the compressive residual stress of the second residual stress peak is higher than a magnitude of an absolute value of the compressive residual stress at the surface of the spring wire, and (iv) a residual stress decrease part in which an absolute value of the compressive residual stress decreases from the first residual stress peak in the depth direction of the spring wire;
• and
• wherein the residual stress decrease part includes a region at a depth exceeding a permissible pit depth of the spring wire at which a magnitude of the compressive residual stress is equivalent to the magnitude of the compressive residual stress at the surface of the spring wire.