Steel for nitriding and nitrided part

1. A steel for nitriding with a composition comprising, by mass %:

C: 0.10% to 0.20%;

Si: 0.01% to 0.7%;

Mn: 0.2% to 1.29%;

Cr: 0.2% to 1.3%;

Al: 0.01% to less than 0.10%;

V: over 0.2% to 1.0%;

Mo: 0% to 0.54%;

N: 0.0034 to 0.02%;

P limited to not more than 0.05%;

S limited to not more than 0.20%; and

a balance including Fe and inevitable impurities, the composition satisfying Expression 1, where [V] is an amount of V by mass %, and [C] is an amount of C by mass %, wherein

the steel for nitriding has a microstructure containing bainite of not less than 50% in terms of an area percentage,

2≦[V]/[C]≦10  (Expression 1).

2. The steel for nitriding according to claim 1 , wherein

[C], [Mn], [Si], [Cr], and [Mo] satisfy Expression 2, where [C], [Mn], [Si], [Cr], and [Mo] are an amount of C, an amount of Mn, an amount of Si, an amount of Cr, and an amount of Mo, respectively, by mass %,

65≦8.65×[C] 1/2 ×(1+4.1×[Mn])×(1+0.64×[Si])×(1+2.33×[Cr])×(1+3.14×[Mo])≦400  (Expression 2).

3. The steel for nitriding according to claim 1 , wherein the composition further comprises B: 0.0003% to 0.005% by mass %, and

[C], [Mn], [Si], [Cr], and [Mo] satisfy Expression 3, where [C], [Mn], [Si], [Cr], and [Mo] are an amount of C, an amount of Mn, an amount of Si, an amount of Cr, and an amount of Mo, respectively, by mass %,

65≦8.65×[C] 1/2 ×(1+4.1×[Mn])×(1+0.64×[Si])×(1+2.33×[Cr])×(1+3.14×[Mo])×(1+1.5×(0.9−[C]))≦400  (Expression 3).

4. The steel for nitriding according to claim 1 , wherein an amount of Mn is not less than 0.2% and not more than 1.0% by mass %.

5. The steel for nitriding according to claim 1 , wherein

an amount of Mo is not less than 0.05% and not more than 0.2% by mass %, and

an amount of V is not less than 0.3% and not more than 0.6% by mass %.

6. The steel for nitriding according to claim 1 , wherein

[C], [Mn], [Cr], [Mo], and [V] satisfy Expression 4, where [C], [Mn], [Cr], [Mo], and [V] are an amount of C, an amount of Mn, an amount of Cr, an amount of Mo, and an amount of V, respectively, by mass %,

0.50≦[C]+{[Mn]/6}+{([Cr]+[Mo]+[V])/5}≦0.80  (Expression 4).

7. The steel for nitriding according to claim 1 , wherein the content of C is 0.10% to 0.19% by mass.

8. The steel for nitriding according to claim 1 , wherein the content of Mn is 0.5% to 1.29% by mass.

9. The steel for nitriding according to claim 1 , wherein

the composition further contains at least one element of Ti and Nb, and

a total amount of Ti and Nb is not less than 0.01% and not more than 0.4% by mass %.

10. The steel for nitriding according to claim 9 , wherein

[C], [Mn], [Si], [Cr], and [Mo] satisfy Expression 2, where [C], [Mn], [Si], [Cr], and [Mo] are an amount of C, an amount of Mn, an amount of Si, an amount of Cr, and an amount of Mo, respectively, by mass %,

65≦8.65×[C] 1/2 ×(1+4.1×[Mn])×(1+0.64×[Si])×(1+2.33×[Cr])×(1+3.14×[Mo])≦400  (Expression 2).

11. The steel for nitriding according to claim 9 , wherein

the composition further comprises B: 0.0003% to 0.005% by mass %, and

[C], [Mn], [Si], [Cr], and [Mo] satisfy Expression 3, where [C], [Mn], [Si], [Cr], and [Mo] are an amount of C, an amount of Mn, an amount of Si, an amount of Cr, and an amount of Mo, respectively, by mass %,

65≦8.65×[C] 1/2 ×(1+4.1×[Mn])×(1+0.64×[Si])×(1+2.33×[Cr])×(1+3.14×[Mo])×(1+1.5×(0.9−[C]))≦400  (Expression 3).

12. The steel for nitriding according to claim 9 , wherein

an amount of Mn is not less than 0.2% and not more than 1.0% by mass %.

13. The steel for nitriding according to claim 9 , wherein

an amount of Mo is not less than 0.05% and not more than 0.2% by mass %, and

an amount of V is not less than 0.3% and not more than 0.6% by mass %.

14. The steel for nitriding according to claim 9 , wherein

[C], [Mn], [Cr], [Mo], and [V] satisfy Expression 4, where [C], [Mn], [Cr], [Mo], and [V] are an amount of C, an amount of Mn, an amount of Cr, an amount of Mo, and an amount of V, respectively, by mass %,

0.50≦[C]+{[Mn]/6}+{([Cr]+[Mo]+[V])/5}≦0.80  (Expression 4).