PURPOSE: To obtain the titled cold-formed coil spring having high fire resistance and low permanent strain property, by forming a large-sized wire material, obtd. by rapid heating with induction heat, quenching and then heat treatment under a specified condition, provided with high strength, ductility and toughness in combination.

CONSTITUTION: A wire material containing C≥0.3% and having a wire diameter above 10mm is subjected to heat treatment comprising rapid heating with high- frequency induction heat or the like and rapid cooling, to form a fine-grained structure having a grain size above ASTM No.9. After this high-strength wire material is heated at about 300W600°C relatively higher than a conventional tempering temp., said heating is stopped. Thereafter, it is held at said temp. for a very short time, i.e. 60sec or shorter, and then cooled. Thus, the wire material having high strength, i.e. tensile strength above 150kg/mm2, elongation above 9% and drawing above 49%, and abound in cold workability such as ductility and toughness is obtained. By cold-working said wire material, a coil spring having a fatigue limit above twisting shear stress τw=60±45kgf/mm2 at 2,000,000 cycles and very small permanent strain property is obtained.