A contour of a cross-section perpendicular to a longitudinal direction of a wire 2 is a circle having a diameter of D. An imaginary circle 4 which is concentric with the circle of the contour of the wire 2 and has a diameter that is ¾ of the diameter D, is assumed. On the imaginary circle 4, a first measurement point M1, a second measurement point M2, a third measurement point M3, a fourth measurement point M4, a fifth measurement point M5, a sixth measurement point M6, a seventh measurement point M7, and an eighth measurement point M8 are assumed. A Vickers hardness (Hv) is measured at each of the eight measurement points. A standard deviation σ of the eight measurement values is not greater than 10. The average of the eight measurement values is preferably not less than 670 and preferably not greater than 770.
