Process of preparing steel coil spring

A process of preparing a coil spring, which comprises heating a steel wire, having a diameter of more than 10 mm, of a hardenable carbon steel containing at least 0.27% C, 0.15-2.2% Si and 0.60-1.65% Mn, by high-frequency induction at a rate of greater than 100.degree. C./second to a temperature within the austenitic range for the hardenable carbon steel, quenching the thus heated steel wire, heating the quenched steel wire by high-frequency induction at a rate of greater than 100.degree. C./second to between 300.degree. and 600.degree. C., stopping said heating, holding the steel wire for 10 to 60 seconds and then cooling the thus heated steel wire at a cooling rate of greater than 50.degree. C./second to yield a cold plastic workable steel wire having a tensile strength of greater than 150 Kgf/mm.sup.2, subjecting the cold plastic workable steel wire to cold plastic working to shape the wire into the form of a coil spring, and heating the resultant shaped wire to between 300.degree. and 500.degree. C. for from 30 to 60 minutes to temper said shaped wire and produce a coil spring product, with the proviso that the maximum temperature to which said shaped wire is heated is less than the maximum temperature to which said quenched steel is heated.