Steel for induction hardening, roughly shaped material for induction hardening, producing method thereof, and induction hardening steel part

1. A roughly shaped material for an induction hardening comprising, by mass %,

C: more than 0.75% to 1.20%,

Si: 0.002 to 3.00%,

Mn: 0.20 to 2.00%,

S: 0.002 to 0.100%,

Al: more than 0.050% to 3.00%,

P: limited to 0.050% or less,

N: limited to 0.0200% or less,

O: limited to 0.0030% or less,

at least one of

V: 0.005 to less than 0.20%,

Nb: 0.005 to 0.10%, and

Ti: 0.005 to 0.10%, and

the balance of iron and unavoidable impurities,

wherein an Al content and a N content, by mass %, satisfy Al−(27/14)×N>0.050%, and wherein the roughly shaped material has a number of graphite grains with an average grain size of 0.5 μm or more, which is included in the roughly shaped material for induction hardening is 40 pieces/mm 2 or less.

2. The roughly shaped material for induction hardening according to claim 1 , further comprising, by mass %,

B: 0.0005 to 0.0050%.

3. The roughly shaped material for induction hardening according to claim 1 , further comprising at least one of, by mass %,

Cr: 0.05% to less than 0.30%,

Mo: 0.01 to 1.00%,

Cu: 0.05 to 1.00%, and

Ni: 0.05 to 2.00%.

4. The roughly shaped material for induction hardening according to claim 1 , further comprising at least one of, by mass %,

Ca: 0.0005 to 0.0030%,

Zr: 0.0005 to 0.0030, and

Mg: 0.0005 to 0.0030%.

5. The roughly shaped material for induction hardening according to claim 2 , further comprising at least one of, by mass %,

Cr: 0.05% to less than 0.30%,

Mo: 0.01 to 1.00%,

Cu: 0.05 to 1.00%, and

Ni: 0.05 to 2.00%.

6. The roughly shaped material for induction hardening according to claim 2 , further comprising at least one of, by mass %,

Ca: 0.0005 to 0.0030%,

Zr: 0.0005 to 0.0030%, and

Mg: 0.0005 to 0.0030%.

7. The roughly shaped material for induction hardening according to claim 3 , further comprising at least one of, by mass %,

Ca: 0.0005 to 0.0030%,

Zr: 0.0005 to 0.0030%, and

Mg: 0.0005 to 0.0030%.

8. The roughly shaped material for induction hardening according to claim 5 , further comprising at least one of, by mass %,

Ca: 0.0005 to 0.0030%,

Zr: 0.0005 to 0.0030%, and

Mg: 0.0005 to 0.0030%.

9. A producing method of a roughly shaped material for induction hardening, the method comprising the following successive steps:

warm working or hot working, cooling, and annealing a steel for induction hardening having a chemical composition of the roughly shaped material for induction hardening according to any one of claims 1 - 3 , 4 , 5 and 6 - 8 ,

wherein the annealing is performed at an annealing temperature of 680 to 800° C. and an annealing time of 10 to 360 minutes, and wherein an average cooling rate in a temperature range of 750 to 650° C. during the cooling is 300° C./hour or less.

10. A producing method of a roughly shaped material for induction hardening, the method comprising the following successive steps:

hot working and cooling a steel for induction hardening having a chemical composition of the roughly shaped material for the induction hardening according to any one of claims 1 - 3 , 4 , 5 and 6 - 8 ,

wherein an average cooling rate in a temperature range of 750 to 650° C. during the cooling is 300° C./hour or less.

11. An induction hardening steel part which is produced using the roughly shape material according to any one of claims 1 - 3 , 4 , 5 and 6 - 8 ,

wherein a hardness of a hardened surface layer at a depth of 50 nm from a topmost surface of the induction hardening steel part is HV650 or more,

wherein a hardness of a non-induction hardening area is HV180 or more, and

wherein a number of graphite grains with an average grain size of 0.5 μm or more which exist in the non-induction hardening area is 40 pieces/mm 2 or less.