High-strength spring steel and spring steel wire

A high-strength spring steel characterized by:

• containing, in mass,
• C:
• 0.4 to 0.8%,
• Si:
• 0.9 to 3.0%,
• Mn:
• 0.1 to 2.0%,
• P:
• 0.015% or less,
• S:
• 0.015% or less,
• Cr:
• 1.5% or less, and
• N:
• 0.001 to 0.007%,
• with the balance consisting of Fe and unavoidable impurities;
• and, in the microstructure of the steel after hot rolling, the density of globular cementite carbides 0.2 to 3 μm in circle-equivalent diameter being 0.5 piece/μm2 or less and the density of globular cementite carbides over 3 μm in circle-equivalent diameter being 0.005 piece/μm2 or less.

A high-strength spring steel characterized by:

• containing, in mass,
• C:
• 0.8 to 1.0%,
• Si:
• 0.9 to 3.0%,
• Mn:
• 0.1 to 2.0%,
• P:
• 0.015% or less,
• S:
• 0.015% or less,
• Cr:
• 0.5% or less,
• N:
• 0.001 to 0.007%, and
• further one or both of W:
• 0.05 to 1.0% and Co:
• 0.05 to 5.0%,
• with the balance consisting of Fe and unavoidable impurities;
• and, in the microstructure of the steel after hot rolling, the density of globular cementite carbides 0.2 to 3 μm in circle-equivalent diameter being 0.5 piece/μm2 or less and the density of globular cementite carbides over 3 μm in circle-equivalent diameter being 0.005 piece/μm2 or less.

A high-strength spring steel characterized by containing, in mass,

• C:
• 0.8 to 1.2%,
• Si:
• 0.9 to 3.0%,
• Mn:
• 0.1 to 2.0%,
• P:
• 0.015% or less,
• S:
• 0.015% or less,
• Cr:
• 0.5 to 1.5%,
• N:
• 0.001 to 0.015%, and
• W:
• 0.05 to 1.0%,
• with the balance consisting of Fe and unavoidable impurities.

A high-strength heat-treated steel wire for springs characterized by:

• containing, in mass,
• C:
• 0.4 to 1.0%,
• Si:
• 0.9 to 3.0%,
• Mn:
• 0.1 to 2.0%,
• P:
• 0.015% or less,
• S:
• 0.015% or less,
• Cr:
• 2.5% or less, and
• N:
• 0.001 to 0.007%,
• with the balance consisting of Fe and unavoidable impurities;
• the tensile strength (TS) being 1,900 MPa or more;
• the percentage of the area occupied by the globular cementite carbides 0.2 μm or more in circle-equivalent diameter on a microscopic observation plane being 7% or less;
• the density of the globular cementite carbides 0.2 μm or more in circle-equivalent diameter being 1 piece/μm2 or less;
• the density of the globular cementite carbides over 3 μm in circle-equivalent diameter being 0.001 piece/μm2 or less;
• the grain size number of prior austenite being 10 or higher;
• the maximum grain size of carbides being 15 μm or less;
• and the grain size of oxides being 15 μm or less.