The method for the heat treatment of a steel reinforcing element (F) for a tire comprises a transformation of the steel microstructure and in which the temperature of the reinforcing element (F) is reduced during the transformation of the steel microstructure by simultaneously extracting heat from the reinforcing element (F) and supplying heat to the reinforcing element (F).

A filament according to an aspect of the present invention includes a predetermined chemical composition, in which a diameter r of the filament is 0.15 mm to 0.35 mm, a soft portion is formed along an outer circumference of the filament, the Vickers hardness of the soft portion is lower than that of a position of the filament at a depth of of the diameter r by Hv 50 or higher, the thickness of the soft portion is 1 m to 0.1r mm, the metallographic structure of a center portion of the filament contains 95% to 100% of pearlite by area %, the average lamellar spacing of pearlite in a portion from a surface of the filament to a depth of 1 m is less than that of pearlite at the center of the filament, the difference between the average lamellar spacing of pearlite in the portion from the surface of the filament to the depth of 1 m and the average lamellar spacing of pearlite at the center of the filament is 2.0 nm or less, and the tensile strength is 3200 MPa or higher.