Patent ID: 7011720

Claim:
A method for continuously heat treating a double tapered steel wire (S), characterized by comprising the steps of: continuously detecting a diameter of said steel wire (S) in heat treatment of said steel wire (S), wherein said steel wire (S) has a constant large-diameter portion ( 21 ), a constant small-diameter portion ( 24 ) and a tapered end portions ( 22 , 23 ) disposed therebetween in its longitudinal direction; controlling the amount of energy of induction heating supplied to said steel wire (S), wherein the amount of said energy is proportional to a wire diameter of said steel wire (S) having been detected so that said steel wire (S) is evenly heated over the entire length of said steel wire (S); and at least one of quenching and tempering said steel wire (S) in a manner such that the tensile strength of the small-diameter portion and the tensile strength of the large-diameter portion of said steel wire (S) are substantially equal; wherein said steel wire (S) substantially keeps its original shape unchanged throughout in its heat treatment.