Steel for cold forging and manufacturing method thereof

1. 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/mm 2 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.

2. The steel for cold forging according to claim 1 , comprising, as the chemical composition, by mass %, one or more of:

Mo: 0.02% to 1.00%,

Ni: 0.10% to 1.00%,

V: 0.03% to 0.30%,

B: 0.0005% to 0.0200%, and

Mg: 0.0001% to 0.0035%.

3. The steel for cold forging according to claim 1 , comprising, as the chemical composition, by mass %, one or two of:

Ti: 0.007% to 0.060%, and

Nb: 0.010% to 0.080%.

4. The steel for cold forging according to claim comprising, as the chemical composition, by mass %, one or two of:

Ti: 0.002% to 0.060%, and

Nb: 0.010% to 0.080%.

5. A manufacturing method of a steel for cold forging, comprising:

casting a cast piece having the chemical composition according to claim 1 , and having a dendrite primary arm spacing of less than 600 μm in a range of 15 mm from a surface of the cast piece;

performing a hot working on the cast piece to obtain a steel material; and

annealing the steel material.

6. The manufacturing method of a steel for cold forging according to claim 5 ,

wherein, in the casting, an average cooling rate in a temperature range from a liquidus temperature to a solidus temperature at a depth of 15 mm from the surface of the cast piece is set to 120° C./min to 500° C./min.