Super high tensile steel wire for rubber product reinforcement, steel cord using this steel wire and radial tire using this steel cord

A radial tire including a reinforcing material comprising a plurality of steel cords, each of said steel cords being made by a process comprising the steps of twisting together three steel wires with a bunching-type twisting machine;

• and
• wherein each of said steel wires has a surface, an interior with a central portion, a wire diameter, a tensile strength, Y in N/mm.sup.2, such that Y.gtoreq.-1960d+3920, said d being the wire diameter in mm, and a flat Vickers hardness distribution in a cross-section perpendicular to a length direction thereof from the surface to the interior, but excluding the central portion having a central portion diameter corresponding to 1/4 of said wire diameter of said steel wire;
• and each of said steel wires is made by a method comprising the steps of:
• a) wet drawing a carbon steel wire rod material containing 0.80 to 0.89% by weight carbon to a predetermined intermediate diameter and subsequently heat-treating and plating to form a final raw material;
• b) then wet drawing the final raw material to form the steel wire having the wire diameter,
• wherein said wet drawing steps are performed with a plurality of drawing dies, each of the drawing dies is provided with a drawing hole having a drawing hole diameter d.sub.1 and the drawing die has an approach angle 2.alpha. equal to from 8.degree. to 10.degree. and a bearing length of 0.3 d.sub.1, and the wet drawing of the final raw material includes a final drawing step performed by wet drawing the final raw material through a double die, said double die consisting of an upstream one of said drawing dies and a downstream one of said drawing dies arranged in series, of which the downstream one of said drawing dies of said double die performs a skin pass of area percentage reduction of from 1.2 to 3.9%, and the double die of the final drawing step and at least one of said drawing dies upstream of the double die have sintered diamond nibs and the steel wire immediately after passing through said double die of the final drawing step has a temperature controlled so as to be less than 150.degree. C.;
• and
• wherein two of the three steel wires are substantially parallel to each other in each of said steel cords, while a third of the three wires is wound around said two steel wires parallel to each other in a spiral at a pitch of 45 to 65 times the wire diameter.