Valve spring

A valve spring, comprising:

• a nitrided layer formed in an outer layer, and
• a core portion that is further inward than the nitrided layer;
• wherein:
• a chemical composition of the core portion consists of, in mass %,
• C:
• 0.53 to 0.59%,
• Si:
• 2.51 to 2.90%,
• Mn:
• 0.70 to 0.85%,
• P:
• 0.020% or less,
• S:
• 0.020% or less,
• Cr:
• 1.40 to 1.70%,
• Mo:
• 0.17 to 0.53%,
• V:
• 0.23 to 0.33%,
• Ca:
• 0.0001 to 0.0050%,
• Cu:
• 0.050% or less,
• Ni:
• 0.050% or less,
• Al:
• 0.0050% or less,
• Ti:
• 0.050% or less,
• N:
• 0.0070% or less, and
• Nb:
• 0 to 0.020%,
• with the balance being Fe and impurities;
• in the core portion, a number density of V-based precipitates having a maximum diameter ranging from 2 to 10 nm is 500 to 8000 pieces/μm2;
• and
• when, among inclusions in the core portion,
• inclusions in which, in mass %, an O content is 10.0% or more are defined as oxide-based inclusions,
• inclusions in which, in mass %, an S content is 10.0% or more and the O content is less than 10.0% are defined as sulfide-based inclusions, and
• among the sulfide-based inclusions, inclusions in which, in mass %, a Ca content is 10.0% or more, the S content is 10.0% or more, and the O content is less than 10.0% are defined as Ca sulfides,
• in the core portion, a numerical proportion of the Ca sulfides with respect to a total number of the oxide-based inclusions and the sulfide-based inclusions is 0.20% or less.