Abstract: The present invention provides a leaf spring material superior in mechanical characteristics and a manufacturing method of the leaf spring material capable of reliably achieving the same, utilizing induction hardening. The manufacturing method of the leaf spring material comprises the steps of imparting tensile stress on a first surface along the longitudinal direction of the first surface and compressive stress on a second surface along the longitudinal direction of the second surface of a substantially strip-shaped steel plate, and subjecting the first surface to induction hardening. With this induction hardening, an induction-hardened structure having a higher average hardness than that of a parent material structure in the vicinity of the second surface and comprising martensite and finely and evenly dispersed austenite is imparted on a surface layer in the vicinity of the first surface.

Abstract: The present invention provides a leaf spring material superior in mechanical characteristics and a manufacturing method of the leaf spring material capable of reliably achieving the same, utilizing induction hardening. The leaf spring material comprises a substantially strip-shaped steel plate having a first surface and a second surface. The leaf spring material has in the vicinity of the first surface a hardened layer comprising an induction-hardened structure having a higher average hardness than that of the parent material structure in the vicinity of the second surface, and a compressive residual stress layer of a thickness at least not smaller than that of the thickness of the hardened layer in the vicinity of the first surface. Such a characteristic structure is achieved by stress-induced hardening that imparts a predetermined tensile stress or higher.