Heat-resistant martensite alloy excellent in high-temperature creep rupture strength and ductility and process for producing the same

1. A martensitic heat resistant alloy having a composition (A) comprising, % by weight:

0.03 to 0.15% of C;

0.2 to 0.9% of Si;

0.01 to 1.5% of Mn;

8.0 to 13.0% of Cr;

0.0005 to 0.015% of Al;

no more than 2.0% of Mo;

no more than 4.0% of W;

0.05 to 0.5% of V;

0.01 to 0.2% of Nb;

0.1 to 5.0% of Co;

0.008 to 0.03% of B;

less than 0.005% of N: and

Fe and inevitable impurities as the remainder,

wherein (B) the contents (% by weight) of Mo, W, B and N satisfy the following formulae (1) and (2),

B−0.772N>0.007  (1)

W+1.916Mo−16.99B>2.0  (2).

2. A martensitic heat resistant alloy having a composition (A) comprising, % by weight:

0.03 to 0.15% of C;

0.2 to 0.9% of Si;

0.01 to 1.5% of Mn;

8.0 to 13.0% of Cr;

0.0005 to 0.015% of Al;

no more than 2.0% of Mo;

no more than 4.0% of W;

0.05 to 0.5% of V;

0.01 to 0.2% of Nb;

0.1 to 5.0% of Co;

0.008 to 0.03% of B;

less than 0.005% of N: and

Fe and inevitable impurities as the remainder,

wherein (B) the mole-based ratio of the content of B with respect to the content of Al, (B/Al), is no smaller than 2.5.

3. The martensitic heat resistant alloy having a composition (A) comprising, % by weight:

0.03 to 0.15% of C;

0.2 to 0.9% of Si;

0.01 to 1.5% of Mn;

8.0 to 13.0% of Cr;

0.0005 to 0.015% of Al;

no more than 2.0% of Mo;

no more than 4.0% of W;

0.05 to 0.5% of V;

0.01 to 0.2% of Nb;

0.1 to 5.0% of Co;

0.008 to 0.03% of B;

less than 0.005% of N: and

Fe and inevitable impurities as the remainder,

wherein (B) the contents (% by weight) of Mo, W, B and N satisfy the following formulae (1) and (2),

B−0.772N>0.007  (1)

W+1.916Mo−16.99B>2.0  (2), and

the mole-based ratio of the content of B with respect to the content of Al, (B/Al), is no smaller than 2.5.

4. The martensitic heat resistant alloy according to claim 1 , the composition thereof further comprising, % by weight, at least one type of element selected from the group consisting of: no more than 0.1% of Ni; and no more than 0.1% of Cu.

5. The martensitic heat resistant alloy according to claim 1 , the composition thereof further comprising, % by weight, no more than 0.03% of P; no more than 0.0 1% of S; and no more than 0.02% of O.

6. A method for producing a martensitic heat resistant alloy, comprising:

subjecting the alloy material having the composition according to claim 1 , to a normalizing process in which the alloy material is heated to a temperature in a range of 1050 to 1200° C., retained therein and cooled; and

then subjecting the alloy material to a tempering process in which the alloy material is heated to a temperature in a range of 750 to 850° C., retained therein and cooled.

7. The martensitic heat resistant alloy according to claim 2 , the composition thereof further comprising, % by weight, at least one type of element selected from the group consisting of: no more than 0.1% of Ni; and no more than 0.1% of Cu.

8. The martensitic heat resistant alloy according to claim 3 , the composition thereof further comprising, % by weight, at least one type of element selected from the group consisting of: no more than 0.1% of Ni; and no more than 0.1% of Cu.

9. The martensitic heat resistant alloy according to claim 2 , the composition thereof further comprising, % by weight, no more than 0.03% of P; no more than 0.01% of S; and no more than 0.02% of O.

10. The martensitic heat resistant alloy according to claim 3 , the composition thereof further comprising, % by weight, no more than 0.03% of P; no more than 0.0 1% of S; and no more than 0.02% of O.

11. The martensitic heat resistant alloy according to claim 4 , the composition thereof further comprising, % by weight, no more than 0.03% of P; no more than 0.01% of S; and no more than 0.02% of O.

12. A method for producing a martensitic heat resistant alloy, comprising:

subjecting the alloy material having the composition according to claim 2 , to a normalizing process in which the alloy material is heated to a temperature in a range of 1050 to 1200° C., retained therein and cooled; and

then subjecting the alloy material to a tempering process in which the alloy material is heated to a temperature in a range of 750 to 850° C., retained therein and cooled.

13. A method for producing a martensitic heat resistant alloy, comprising:

subjecting the alloy material having the composition according to claim 3 , to a normalizing process in which the alloy material is heated to a temperature in a range of 1050 to 1200° C., retained therein and cooled; and

then subjecting the alloy material to a tempering process in which the alloy material is heated to a temperature in a range of 750 to 850° C., retained therein and cooled.

14. A method for producing a martensitic heat resistant alloy, comprising:

subjecting the alloy material having the composition according to claim 4 , to a normalizing process in which the alloy material is heated to a temperature in a range of 1050 to 1200° C., retained therein and cooled; and

then subjecting the alloy material to a tempering process in which the alloy material is heated to a temperature in a range of 750 to 850° C., retained therein and cooled.

15. A method for producing a martensitic heat resistant alloy, comprising:

subjecting the alloy material having the composition according to claim 5 , to a normalizing process in which the alloy material is heated to a temperature in a range of 1050 to 1200° C., retained therein and cooled; and

then subjecting the alloy material to a tempering process in which the alloy material is heated to a temperature in a range of 750 to 850° C., retained therein and cooled.