Steel for machine structure exhibiting excellent machinability

1. A steel for a machine structure, the steel comprising, in mass %:

C: 0.40% to less than 0.75%;

Si: 0.01% to 3.0%;

Mn: 072% to 1.8%;

S: 0.001% to 0.1%;

Al: more than 0.1% and not more than 1.0%;

N: 0.001% to 0.02%; and

P: limited to not more than 0.05%,

with a balance including Fe and inevitable impurities, wherein

an amount of C: [C], an amount of Si: [Si], an amount of Mn: [Mn], and an amount of Al: [Al] satisfy the following Expression (1) and Expression (2):

139.38≦214×[C]+30.6×[Si]+42.8×[Mn]−14.7×[Al]≦177  (1)

0.72≦[C]+1/7×[Si]+1/5×[Mn]<1.539  (2).

2. The steel for a machine structure according to claim 1 , wherein the steel further satisfies the following Expression (3):

113−135×[C]−27×[Mn]≦13  (3).

3. The steel for a machine structure according to claim 1 , wherein the steel further satisfies the following Expression (4):

55≦33+31×[C]+4.5×[Si]+1.5×[Mn]<72.45  (4).

4. The steel for a machine structure according to claim 2 , wherein the steel further satisfies the following Expression (4):

55≦33+31×[C]+4.5×[Si]+1.5×[Mn]<72.45  (4).

5. The steel for a machine structure according to claim 1 , wherein the steel further satisfies the following Expression (5):

1.5<[Si]+1.8×[Mn]<6.24  (5).

6. The steel for a machine structure according to claim 1 , wherein the steel further comprises B: 0.0001% to 0.015% in mass %.

7. The steel for a machine structure according to claim 4 , wherein the steel further comprises, in mass %, one or more elements of Cr: 0.01% to 0.8%, Mo: 0.001% to 1.0%, Ni: 0.001% to 5.0%, and Cu: 0.001% to 5.0%, and

in a case where the steel comprises Cr: 0.01% to 0.8%, the following Expression (6) is satisfied instead of Expression (1), the following Expression (7) is satisfied instead of Expression (2), the following Expression (8) is satisfied instead of Expression (3), and the following Expression (9) is satisfied instead of Expression (4):

139.38≦214×[C]+30.6×[Si]+42.8×[Mn]+23.8×[Cr]−14.7×[Al]≦177  (6)

0.72≦[C]+1/7×[Si]+1/5×[Mn]+1/9×[Cr]<1.627  (7)

113−135×[C]−27×[Mn]−18×[Cr]≦13  (8)

55≦33+31×[C]+4.5×[Si]+1.5×[Mn]+2.4×[Cr]<74.37  (9).

8. The steel for a machine structure according to claim 1 , wherein the steel further comprises, in mass %, one or more elements of Ca: 0.0001% to 0.02%, Mg: 0.0001% to 0.02%, Zr: 0.0001% to 0.02%, and Rem: 0.0001% to 0.02%.

9. The steel for a machine structure according to claim 1 , wherein the steel further comprises, in mass %, one or more elements of Ti: 0.005% to 0.5%, Nb: 0.0005% to 0.5%, W: 0.0005% to 0.5%, V: 0.0005% to 0.5%, Ta: 0.0001% to 0.2%, and Hf: 0.0001% to 0.2%.

10. The steel for a machine structure according to claim 1 , wherein the steel further comprises, in mass %, one or more elements of Sb: 0.0001% to 0.015%, Sn: 0.0005% to 2.0%, Zn: 0.0005% to 0.5%, Te: 0.0003% to 0.2%, Se: 0.0003% to 0.2%, Bi: 0.001% to 0.5%, and Pb: 0.001% to 0.5%.

11. The steel for a machine structure according to claim 1 , wherein the steel further comprises, in mass %, one or more elements of Li: 0.00001% to 0.005%, Na: 0.00001% to 0.005%, K: 0.00001% to 0.005%, Ba: 0.00001% to 0.005%, and Sr: 0.00001% to 0.005%.

12. The steel for a machine structure according to claim 2 , wherein the steel further satisfies the following Expression (5):

1.5<[Si]+1.8×[Mn]<6.24  (5).

13. The steel for a machine structure according to claim 3 , wherein the steel further satisfies the following Expression (5):

1.5<[Si]+1.8×[Mn]<6.24  (5).

14. The steel for a machine structure according to claim 4 , wherein the steel further satisfies the following Expression (5):

1.5<[Si]+1.8×[Mn]<6.24  (5).

15. The steel for a machine structure according to claim 2 , wherein the steel further comprises B: 0.0001% to 0.015% in mass %.

16. The steel for a machine structure according to claim 3 , wherein the steel further comprises B: 0.0001% to 0.015% in mass %.

17. The steel for a machine structure according to claim 4 , wherein the steel further comprises B: 0.0001% to 0.015% in mass %.

18. A steel for a machine structure, the steel comprising, in mass %:

C: 0.40% to less than 0.75%;

Si: 0.01% to 3.0%;

Mn: 0.72 to 1.8%;

S: 0.001% to 0.1%;

Al: more than 0.1% and not more than 1.0%;

N: 0.001% to 0.02%;

at least one element selected from the group consisting of B: 0.0001% to 0.015%, Cr: 0.01% to 0.8%, Mo: 0.001% to 1.0%, Ni: 0.001% to 5.0%, Cu: 0.001% to 5.0%, Ca: 0.0001% to 0.02%, Mg: 0.0001% to 0.02%, Zr: 0.0001% to 0.02%, Rem: 0.0001% to 0.02%, Ti: 0.005% to 0.5%, Nb: 0.0005% to 0.5%, W: 0.0005% to 0.5%, V: 0.0005% to 0.5%, Ta: 0.0001% to 0.2%, Hf: 0.0001% to 0.2%, Sb: 0.0001% to 0.015%, Sn: 0.0005% to 2.0%, Zn: 0.0005% to 0.5%, Te: 0.0003% to 0.2%, Se: 0.0003% to 0.2%, Bi: 0.001% to 0.5%, Pb: 0.001% to 0.5%, Li: 0.00001% to 0.005%, Na: 0.00001% to 0.005%, K: 0.00001% to 0.005%, Ba: 0.00001% to 0.005%, and Sr: 0.00001% to 0.005%; and

P: limited to not more than 0.05%,

with a balance including Fe and inevitable impurities, wherein

an amount of C: [C], an amount of Si: [Si], an amount of Mn: [Mn], and an amount of Al: [Al] satisfy the following Expression (1) and Expression (2):

139.38≦214×[C]+30.6×[Si]+42.8×[Mn]−14.7×[Al]≦177  (1)

0.72≦[C]+1/7×[Si]+1/5×[Mn]<1.539  (2), and

in a case where the steel comprises Cr: 0.01% to 0.8%, the following Expression (6) is satisfied instead of Expression (1), and the following Expression (7) is satisfied instead of Expression (2):

139.38≦214×[C]+30.6×[Si]+42.8×[Mn]+23.8×[Cr]−14.7×[Al]≦177  (6)

0.72≦[C]+1/7×[Si]+1/5×[Mn]+1/9×[Cr]<1.627  (7).

19. The steel for a machine structure according to claim 18 , wherein the steel further satisfies the following Expression (3):

113−135×[C]−27×[Mn]≦13  (3), and

in a case where the steel comprises Cr: 0.01% to 0.8%, the following Expression (8) is satisfied instead of Expression (3):

113−135×[C]−27×[Mn]−18×[Cr]≦13  (8).

20. The steel for a machine structure according to claim 18 , wherein the steel further satisfies the following Expression (4):

55≦33+31×[C]+4.5×[Si]+1.5×[Mn]<72.45  (4), and

in a case where the steel comprises Cr: 0.01% to 0.8%, the following Expression (9) is satisfied instead of Expression (4):

55≦33+31×[C]+4.5×[Si]+1.5×[Mn]+2.4×[Cr]<74.37  (9).

21. The steel for a machine structure according to claim 19 , wherein the steel further satisfies the following Expression (4):

55≦33+31×[C]+4.5×[Si]+1.5×[Mn]<72.45  (4), and

in a case where the steel comprises Cr: 0.01% to 0.8%, the following Expression (9) is satisfied instead of Expression (4):

55≦33+31×[C]+4.5×[Si]+1.5×[Mn]+2.4×[Cr]<74.37  (9).

22. The steel for a machine structure according to claim 18 , wherein the steel further satisfies the following Expression (5):

1.5<[Si]+1.8×[Mn]<6.24  (5).

23. The steel for a machine structure according to claim 19 , wherein the steel further satisfies the following Expression (5):

1.5<[Si]+1.8×[Mn]<6.24  (5).

24. The steel for a machine structure according to claim 20 , wherein the steel further satisfies the following Expression (20):

1.5<[Si]+1.8×[Mn]<6.24  (5).

25. The steel for a machine structure according to claim 21 , wherein the steel further satisfies the following Expression (5):

1.5<[Si]+1.8×[Mn]<6.24  (5).