Hot forging steel and hot forged product
There is provided a hot forging steel including, by mass %, C: more than 0.30% and less than 0.60%, Si: 0.10% to 0.90%, Mn: 0.50% to 2.00%, S: 0.010% to 0.100%, Cr: 0.01% to 1.00%, Al: more than 0.005% and 0.100% or less, N: 0.0030% to 0.0200%, Bi: more than 0.0001% and 0.0050% or less, Ti: 0% or more and less than 0.040%, V: 0% to 0.30%, Ca: 0% to 0.0040%, Pb: 0% to 0.40%, and a remainder including Fe and impurities, in which P and O in the impurities are respectively P: 0.050% or less and O: 0.0050% or less, an expression d+3σ<20 is satisfied, and a presence density of MnS having an equivalent circle diameter of smaller than 2.0 μm is 300 pieces/mm 2 or more in a cross section parallel to a rolling direction of a steel.
1. A hot forging steel comprising, by mass %,
C: more than 0.30% and less than 0.60%,
Si: 0.10% to 0.90%,
Mn: 0.50% to 2.00%,
S: 0.010% to 0.100%,
Cr: 0.01% to 1.00%,
Al: more than 0.005% and 0.100% or less,
N: 0.0030% to 0.0200%,
Bi: more than 0.0001% and 0.0050% or less,
Ti: 0% or more and less than 0.040%,
V: 0% to 0.30%,
Ca: 0% to 0.0040%,
Pb: 0% to 0.40%, and
a remainder comprising Fe and impurities,
wherein P and O in the impurities are respectively
P: 0.050% or less and
O: 0.0050% or less,
a following Expression (1) is satisfied, and
a presence density of MnS having an equivalent circle diameter of smaller than 2.0 μm is 300 pieces/mm 2 or more in a cross section parallel to a rolling direction of a steel,
d +3σ<20 (1)
where,
d in the Expression (1) represents an average equivalent circle diameter of the MnS in an unit of μm having the equivalent circle diameter of 1.0 μm or greater, and
σ in the Expression (1) represents a standard deviation of the equivalent circle diameter of the MnS having the equivalent circle diameter of 1.0 μm or greater.
2. The hot forging steel according to claim 1 comprising, by mass %,
Ti: 0.001% or more and less than 0.040%.
3. The hot forging steel according to claim 2 comprising, by mass %,
V: 0.03% to 0.30%.
4. The hot forging steel according to claim 2 comprising, by mass %, one or more selected from the group consisting of
Ca: 0.0003% to 0.0040%, and
Pb: 0.05% to 0.40%.
5. The hot forging steel according to claim 4 comprising, by mass %,
P: 0.020% or less.
6. The hot forging steel according to claim 3 comprising, by mass %, one or more selected from the group consisting of
Ca: 0.0003% to 0.0040%, and
Pb: 0.05% to 0.40%.
7. The hot forging steel according to claim 6 comprising, by mass %,
P: 0.020% or less.
8. The hot forging steel according to claim 3 comprising, by mass %,
P: 0.020% or less.
9. The hot forging steel according to claim 2 comprising, by mass %,
P: 0.020% or less.
10. The hot forging steel according to claim 1 comprising, by mass %,
V: 0.03% to 0.30%.
11. The hot forging steel according to claim 10 comprising, by mass %, one or more selected from the group consisting of
Ca: 0.0003% to 0.0040%, and
Pb: 0.05% to 0.40%.
12. The hot forging steel according to claim 11 comprising, by mass %,
P: 0.020% or less.
13. The hot forging steel according to claim 10 comprising, by mass %,
P: 0.020% or less.
14. The hot forging steel according to claim 1 comprising, by mass %, one or more selected from the group consisting of
Ca: 0.0003% to 0.0040%, and
Pb: 0.05% to 0.40%.
15. The hot forging steel according to claim 14 comprising, by mass %,
P: 0.020% or less.
16. The hot forging steel according to claim 1 comprising, by mass %,
P: 0.020% or less.
17. The hot forging steel according to claim 1 comprising, by mass %,
Al: more than 0.005% to 0.041%.
18. A hot forged product comprising, by mass %,
C: more than 0.30% and less than 0.60%,
Si: 0.10% to 0.90%,
Mn: 0.50% to 2.00%,
S: 0.010% to 0.100%,
Cr: 0.01% to 1.00%,
Al: more than 0.005% and 0.100% or less,
N: 0.0030% to 0.0200%,
Bi: more than 0.0001% and 0.0050% or less,
Ti: 0% or more and less than 0.040%,
V: 0% to 0.30%,
Ca: 0% to 0.0040%,
Pb: 0% to 0.40%, and
a remainder comprising Fe and impurities,
wherein P and O in the impurities are respectively
P: 0.050% or less and
O: 0.0050% or less,
a following Expression (2) is satisfied, and
a presence density of MnS having an equivalent circle diameter of smaller than 2.0 μm is 300 pieces/mm 2 or more in a cross section parallel to a rolling direction of a steel,
d +3σ<20 (2)
where,
d in Expression (2) represents an average equivalent circle diameter of the MnS in an unit of μm having the equivalent circle diameter of 1.0 μm or greater, and
σ in Expression (2) represents a standard deviation of the equivalent circle diameter of the MnS having the equivalent circle diameter of 1.0 μm or greater.