Steel wire for spring
A steel wire for a spring includes a steel wire that has Ca or Na adhered thereto in an amount of 0.2 g/m 2 or less. The steel wire has an oxide film on a surface thereof, and the oxide film has a thickness of, for example, from 2.0 μm to 20 μm.
1. A steel wire for a spring, having a non-zero amount of Ca adhered thereto in an amount of 0.2 g/m 2 or less or having a non-zero amount of Na adhered thereto in an amount of 0.2 g/m 2 or less, wherein
the steel wire has an oxide film on a surface thereof, and
the oxide film has a thickness of from 2.0 μm to 20 μm.
2. The steel wire for a spring according to claim 1 , wherein the oxide film has a surface roughness Rz of 10 μm or less.
3. The steel wire for a spring according to claim 1 , wherein an area fraction of a ferrite phase in a cross-section taken in a direction orthogonal to an axial direction of the steel wire for a spring is 30% or less in a surface region extending from a surface toward a center of the steel wire for a spring to a depth of 1.0% of a diameter of the steel wire for a spring.
4. The steel wire for a spring according to claim 3 , wherein C B -C A satisfies 0.01% by mass or less, where C A is a C content of the surface region, and C B is a C content of a central region inside the surface region.
5. The steel wire for a spring according to claim 1 , wherein H B -H A satisfies 30 or less in terms of Vickers hardness, where H A is a surface hardness at a depth of 1.0% of a diameter of the steel wire for a spring from a surface toward a center of the steel wire for a spring in a cross-section taken in a direction orthogonal to an axial direction of the steel wire for a spring, and H B is an internal hardness at a depth of 25% of the diameter.