Heat-resistant Ti alloy and process for producing the same

Patent metadata

Field	Value
Publication number	US-10526689-B2
Application number	US-201715836574-A
Country	US
Kind code	B2
Filing date	Dec 8, 2017
Priority date	Dec 15, 2016
Publication date	Jan 7, 2020
Grant date	Jan 7, 2020

Abstract

The present invention relates to a heat-resistant Ti alloy having excellent high-temperature strength and a process for producing the same. More particularly, the present invention relates to a heat-resistant Ti alloy having a composite structure having an equiaxed α phase and β grains containing an acicular α phase inside thereof, and a process for producing the same.

First claim

What is claimed is:

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.

Assignees

• Daido Steel Co Ltd

Inventors

• Koyanagi Yoshihiko
• Okajima Takuma

Classifications

• C22F1/183 Primary

  of titanium or alloys based thereon · CPC title

• B21J1/025

  affecting grain orientation · CPC title

• C22C14/00

  Alloys based on titanium · CPC title

• B21J1/06

  Heating or cooling methods or arrangements specially adapted for performing forging or pressing operations {(B21J5/063 takes precedence)} · CPC title