Heat-resistant Ti alloy and process for producing the same

1. A heat-resistant Ti alloy having a composition consisting of, in terms of % by mass:

5.0-7.0% of Al;

3.0-5.0% of Sn;

2.5-6.0% of Zr;

2.0-4.0% of Mo;

0.05-0.80% of Si;

0.001-0.200% of C;

0.05-0.20% of O; and

0.3-2.0% in total of at least one kind selected from the group consisting of Nb and Ta;

with the balance being Ti and unavoidable impurities,

wherein the heat-resistant Ti alloy has a composite structure having an equiaxed α phase and β grains containing an acicular α phase inside thereof, and

the equiaxed α phase has an average grain diameter of 5 μm to 20 μm and an average aspect ratio of 5.0 or less, and is contained in an amount of 5-35% in terms of sectional areal proportion to the composite structure.

2. The heat-resistant Ti alloy according to claim 1 , wherein the β grains have an average grain diameter of 10 μm to 200 μm.

3. A heat-resistant Ti alloy having a composition consisting of, in terms of % by mass:

5.0-7.0% of Al;

3.0-5.0% of Sn;

2.5-6.0% of Zr;

2.0-4.0% of Mo;

0.05-0.80% of Si;

0.001-0.200% of C;

0.05-0.20% of O;

0.3-2.0% in total of at least one kind selected from the group consisting of Nb and Ta; and

at least one selected from the group consisting of:

0.005-0.200% of B;

0.2% or less of N;

0.2% or less of Fe;

0.2% or less of Ni; and

0.2% or less of Cr,

with the balance being Ti and unavoidable impurities,

wherein the heat-resistant Ti alloy has a composite structure having an equiaxed α phase and β grains containing an acicular α phase inside thereof, and

the equiaxed α phase has an average grain diameter of 5 μm to 20 μm and an average aspect ratio of 5.0 or less, and is contained in an amount of 5-35% in terms of sectional areal proportion to the composite structure.

4. The heat-resistant Ti alloy according to claim 3 , wherein the β grains have an average grain diameter of 10 μm to 200 μm.

5. A process for producing a heat-resistant Ti alloy having a composite structure having an equiaxed α phase and β grains containing an acicular α phase inside thereof, the process comprising:

a step of preparing a bulk alloy having a composition consisting of, in terms of % by mass:

5.0-7.0% of Al;

3.0-5.0% of Sn;

2.5-6.0% of Zr;

2.0-4.0% of Mo;

0.05-0.80% of Si;

0.001-0.200% of C;

0.05-0.20% of O; and

0.3-2.0% in total of at least one kind selected from the group consisting of Nb and Ta;

with the balance being Ti and unavoidable impurities,

a first heat treatment step in which the alloy is heated and held at a temperature that is within a β-single-phase temperature region and is higher than a β transformation point T β ,

an adjustment forging step in which the alloy is hot-forged at a temperature that is within an (α+β)-two-phase temperature region and is lower than the β transformation point T β , thereby adjusting an equiaxed α phase,

a second heat treatment step in which the alloy is heated and held at a temperature that is within the (α+β)-two-phase temperature region and is higher than the temperature in the adjustment forging step, followed by cooling to precipitate an acicular α phase, and

an aging heat treatment step which is performed at 570-650° C.,

wherein, prior to the first heat treatment step, the alloy is subjected to a pre-forging step in which the alloy is hot-forged in the β-single-phase temperature region and further hot-forged in the (α+β)-two-phase temperature region, whereby the acicular α phase is formed in the β-grains, and the equiaxed α phase has an average grain diameter of 5 μm to 20 μm and an average aspect ratio of 5.0 or less and is contained in an amount of 5-35% in terms of sectional areal proportion to the composite structure.

6. The process for producing a heat-resistant Ti alloy according to claim 5 , wherein the β grains have an average grain diameter of 10 μm to 200 μm.

7. The process for producing a heat-resistant Ti alloy according to claim 5 , wherein the first heat treatment step is a step in which the alloy is heated and held at a temperature that is within a β-single-phase temperature region of [higher than T β and (T β +80° C.) or lower].

8. The process for producing a heat-resistant Ti alloy according to claim 7 , wherein, in the first heat treatment step, the alloy is held at a constant temperature and then gradually cooled at a cooling rate corresponding to or lower than in air cooling.

9. The process for producing a heat-resistant Ti alloy according to claim 5 , wherein the pre-forging step is a step in which the alloy is hot-forged in the β-single-phase temperature region and further hot-forged in an (α+β)-two-phase temperature region of [(β−100° C.) or higher and lower than T β ] so as to result in a total forming ratio in the forging of 3 or higher.

10. The process for producing a heat-resistant Ti alloy according to claim 9 , wherein the adjustment forging step is a step in which the alloy is hot-forged at a strain rate of 0.1-10/sec in the (α+β)-two-phase temperature region of [(T β −100° C.) or higher and lower than T β ] so as to result in a total forming ratio in the forging of 3 or higher, and

the second heat treatment step is a step in which the alloy is held at a temperature in an (α+β)-two-phase temperature region of [(T β −50° C.) or higher and lower than T β ].

11. The process for producing a heat-resistant Ti alloy according to claim 10 , further including, after the adjustment forging step, an upset forging step in which the alloy is subjected to hot upset forging at a strain rate of 0.1-10/sec in the (α+β)-two-phase temperature region of [(T β −100° C.) to (T β −30° C.)] so as to result in a total forming ratio in the upset forging of 3 or higher.

12. A process for producing a heat-resistant Ti alloy having a composite structure having an equiaxed α phase and β grains containing an acicular α phase inside thereof, the process comprising:

a step of preparing a bulk alloy having a composition consisting of, in terms of % by mass:

5.0-7.0% of Al;

3.0-5.0% of Sn;

2.5-6.0% of Zr;

2.0-4.0% of Mo;

0.05-0.80% of Si;

0.001-0.200% of C;

0.05-0.20% of O;

0.3-2.0% in total of at least one kind selected from the group consisting of Nb and Ta; and

at least one selected from the group consisting of:

0.005-0.200% of B;

0.2% or less of N;

0.2% or less of Fe;

0.2% or less of Ni; and

0.2% or less of Cr,

with the balance being Ti and unavoidable impurities,

a first heat treatment step in which the alloy is heated and held at a temperature that is within a β-single-phase temperature region and is higher than a β transformation point T β ,

an adjustment forging step in which the alloy is hot-forged at a temperature that is within an (α+β)-two-phase temperature region and is lower than the β transformation point T β , thereby adjusting an equiaxed α phase,

a second heat treatment step in which the alloy is heated and held at a temperature that is within the (α+β)-two-phase temperature region and is higher than the temperature in the adjustment forging step, followed by cooling to precipitate an acicular α phase, and

an aging heat treatment step which is performed at 570-650° C.,

wherein, prior to the first heat treatment step, the alloy is subjected to a pre-forging step in which the alloy is hot-forged in the β-single-phase temperature region and further hot-forged in the (α+β)-two-phase temperature region, whereby the acicular α phase is formed in the β-grains, and the equiaxed α phase has an average grain diameter of 5 μm to 20 μm and an average aspect ratio of 5.0 or less and is contained in an amount of 5-35% in terms of sectional areal proportion to the composite structure.

13. The process for producing a heat-resistant Ti alloy according to claim 12 , wherein the β grains have an average grain diameter of 10 μm to 200 μm.

14. The process for producing a heat-resistant Ti alloy according to claim 12 , wherein the first heat treatment step is a step in which the alloy is heated and held at a temperature that is within a β-single-phase temperature region of [higher than T β and (T β +80° C.) or lower].

15. The process for producing a heat-resistant Ti alloy according to claim 14 , wherein, in the first heat treatment step, the alloy is held at a constant temperature and then gradually cooled at a cooling rate corresponding to or lower than in air cooling.

16. The process for producing a heat-resistant Ti alloy according to claim 12 , wherein the pre-forging step is a step in which the alloy is hot-forged in the β-single-phase temperature region and further hot-forged in an (α+β)-two-phase temperature region of [(T β −100° C.) or higher and lower than T β ] so as to result in a total forming ratio in the forging of 3 or higher.

17. The process for producing a heat-resistant Ti alloy according to claim 16 , wherein the adjustment forging step is a step in which the alloy is hot-forged at a strain rate of 0.1-10/sec in the (α+β)-two-phase temperature region of [(T β −100° C.) or higher and lower than T β ] so as to result in a total forming ratio in the forging of 3 or higher, and

the second heat treatment step is a step in which the alloy is held at a temperature in an (α+β)-two-phase temperature region of [(T β −50° C.) or higher and lower than T β ].

18. The process for producing a heat-resistant Ti alloy according to claim 17 , further including, after the adjustment forging step, an upset forging step in which the alloy is subjected to hot upset forging at a strain rate of 0.1-10/sec in the (α+β)-two-phase temperature region of [(T β −100° C.) to (T β −30° C.)] so as to result in a total forming ratio in the upset forging of 3 or higher.