Age-hardening steel for cold forging use

Age-hardening steel for cold forging use, a chemical composition of the age-hardening steel consisting of, by mass %,

• C:
• 0.02 to 0.13%,
• Si:
• 0.01 to 0.50%,
• Mn:
• 0.20 to 0.70%,
• P:
• 0.020% or less (including 0%),
• S:
• 0.005 to 0.020%,
• Al:
• 0.005 to 0.050%,
• Cr:
• 0.02 to 1.50%,
• V:
• 0.02 to 0.50%,
• Nb:
• 0.005 to 0.050%,
• Cu:
• 0.20% or less,
• Ni:
• 0.20% or less,
• Mo:
• 0.20% or less,
• N:
• 0.003 to 0.030% and
• a balance of Fe and unavoidable impurities,
• wherein
• a content of solute Nb (mass %) is 25% or more with respect to the total content of Nb,
• a content of solute V (mass %) is 50% or more with respect to the total content of V,
• fn1 expressed by the following formula (1) is 0.03 or more,
• fn2 expressed by the following formula (2) is 13.5 or less, and
• the metal structure contains, by area ratio,
• ferrite:
• 85% or more and
• a total of bainite and martensite:
• 5% or less (including 0%):
• fn1=[Nb]/[V] (1)
• fn2=125×C−13×V−4×Nb (2)
• where in formula (1) and formula (2), [V] indicates the mass % of solute V, [Nb] indicates the mass % of solute Nb, C indicates the mass % of C which the steel contains, V indicates the mass % of V which the steel contains, and Nb indicates the mass % of Nb which the steel contains.