Steel for cold forging and manufacturing method thereof

A steel for cold forging comprising, as a chemical composition, by mass %,

• C:
• 0.05% to 0.30%,
• Si:
• 0.05% to 0.45%,
• Mn:
• 0.40% to 2.00%,
• S:
• 0.008% to less than 0.040%,
• Cr:
• 0.01% to 3.00%,
• Al:
• 0.010% to 0.100%,
• Bi:
• 0.0001% to 0.0050%,
• Mo:
• 0% to 1.00%,
• Ni:
• 0% to 1.00%,
• V:
• 0% to 0.30%,
• B:
• 0% to 0.0200%,
• Mg:
• 0% to 0.0035%,
• Ti:
• 0% to 0.060%,
• Nb:
• 0% to 0.080%, and
• a remainder of Fe and impurities,
• wherein N, P, and O contained in the impurities are as follows
• N:
• 0.0250% or less,
• P:
• 0.050% or less, and
• O:
• 0.0020% or less,
• 1200/mm2 or more of sulfides having an equivalent circle diameter of 1.0 to 10.0 μm are contained in a microstructure,
• an average distance between the sulfides is less than 30.0 μm, and
• the sulfides in the microstructure satisfy Expression (1) and Expression (2):
• d+3σ≤10.0 . . . (1)
• SA/SB<0.30 . . . (2),
• in Expression (1), d is an average value of equivalent circle diameters of the sulfides having an equivalent circle diameter of 1.0 μm or more, σ is a standard deviation of the equivalent circle diameters of the sulfides having the equivalent circle diameter of 1.0 μm or more, in Expression (2), SA is a number of sulfides having the equivalent circle diameter of 1.0 μm or more and less than 3.0 μm, and SB is a number of sulfides having an equivalent circle diameter of 1.0 μm or more.