JP3297012B2 - High strength titanium alloy with excellent cold rollability - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a novel titanium alloy having high strength and excellent cold workability, and more particularly, to a titanium alloy which is comparable to or most commonly used as a Ti-6Al-4V alloy which is most widely used among titanium alloys. The present invention relates to a titanium alloy having higher strength and improved drawback of cold workability.

[0002]

2. Description of the Related Art Titanium alloys are lightweight, strong, tough,
Due to its excellent corrosion resistance, it has recently been widely used in the fields of the aerospace industry and the chemical industry. However, titanium alloys are inherently poor workable materials, and therefore have a major drawback in that the cost for forming is significantly higher than other materials. For example, typical α +
The Ti-6Al-4V alloy, which is a β-type titanium alloy, is a difficult-to-work material and has extremely poor cold workability, so that it is virtually impossible to manufacture a sheet material by the cold rolling method.

[0003] When a Ti-6Al-4V alloy is processed into a plate shape, a technique called pack rolling is adopted. That is, pack rolling refers to Ti obtained by hot rolling.
This is a method in which a -6Al-4V alloy plate is superposed in layers and placed in a mild steel box, and a thin plate is produced by hot rolling while keeping the temperature below a predetermined temperature.

However, this method requires a mild steel cover and pack welding for manufacturing the pack, and a method of applying a release agent for preventing diffusion bonding of the titanium alloy plate.
Compared to cold rolling, the operation is extremely complicated and requires a great deal of cost. In addition, since the temperature range suitable for hot rolling is limited, there are many restrictions on processing.

On the other hand, JP-A-3-274238 and JP-A-3-166350 disclose Al, Ti in a base metal.
The content of V and Mo is specified and Fe, Ni, C
By containing an appropriate amount of at least one alloy element selected from o and Cr, Ti-6Al-4V has a strength comparable to that of the Ti-6Al-4V alloy, and also has a superplastic workability and a hot workability at the time of production. It is stated that a titanium alloy superior to the 4V alloy is obtained.

[0006] However, depending on the processing temperature range, generally, superior superplastic workability means that deformation resistance is low at the processing temperature, and the strength of the titanium alloy disclosed in the above publication is Ti-6Al-. It is feared that it is insufficient compared with the 4V alloy.

[0007]

SUMMARY OF THE INVENTION Under the above circumstances, it is necessary to have a strength comparable to or exceeding that of a Ti-6Al-4V alloy, and to further enhance cold workability in order to reduce working costs. Has recently been a major challenge in the field of titanium alloys.

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object to have strength equal to or higher than that of Ti-6Al-4V and to have excellent cold workability. To provide an improved titanium alloy.

[0009]

The high-strength titanium alloy according to the present invention, which can solve the above-mentioned problems, is as follows: Al: 3.5 to 5.0% by mass%, Mo: 2.0 to 4.0%. 0%, C
r: 2.0 to 4.0%, or O: 0.25% or less or Si: 0.5% or less as another element, with the balance being substantially composed of Ti. This titanium alloy can be effectively utilized, for example, as a plate material for a heat exchanger, utilizing its excellent workability and strength characteristics.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION As described above, the present invention provides a titanium alloy having excellent strength and excellent cold workability. Specifically, the type of alloy element contained in Ti And each content is strictly defined.

As is well known, an α + β titanium alloy has an α phase having a hexagonal crystal structure and a β phase having a body-centered cubic crystal structure.
Composed of a two-phase mixed structure with
As for the phase, the sliding surface at the time of plastic deformation by processing is less than the β phase,
Poor cold workability. The reason why the cold workability of the Ti-6Al-4V alloy is inferior is that the volume ratio of the α phase is high,
Therefore, it seems that the cold workability can be improved by relatively increasing the volume ratio of the β phase.

Therefore, the present inventors have reduced the amount of the α-stabilizing element to reduce the volume ratio of the α phase in the titanium alloy,
In addition, research was conducted in the direction of increasing β-stabilizing elements in order to increase the volume ratio of β-phase in titanium. As a result, the amount of Al and O, which are α-stabilizing elements, and the amount of M, which is β-stabilizing elements,
By properly controlling the contents of o and Cr, cold workability and ductility can be enhanced, and these elements can be dissolved in a Ti base material to exhibit a solid solution strengthening effect. As a result, Ti- The present inventors have found that a titanium alloy having both strength equal to or higher than that of 6Al-4V and excellent cold workability can be obtained, and the present invention has been conceived.

In the present invention, the content of the alloying element is defined from the above viewpoints. The preferred content range of each of the contained elements will be described in more detail as follows.

Al: 3.5 to 5.0% Al is an element that contributes to strength improvement as an α-stabilizing element. If the Al content is less than 3.5%, the titanium alloy has insufficient strength. However, when the Al content exceeds 5.0%, the cold workability decreases, and the number of cold rolling and annealing until rolling to a predetermined thickness increases, which leads to an increase in cost.

Mo: 2.0 to 4.0% Mo is a β-stabilizing element as described above, which increases the volume ratio of the β phase and forms a solid solution with the β phase to contribute to an increase in strength. In addition, it has the property of forming a fine equiaxed crystal structure by forming a solid solution in titanium, and is an extremely important element from the viewpoint of improving the strength-ductility balance. 2.0% or more, more preferably 2.
It should be contained at least 5%.

However, if the amount of Mo is too large, the corrosion resistance of the oxide scale formed during annealing and the oxygen solid solution layer called α-case increases, and it becomes difficult to remove it. That is, in cold rolling the titanium alloy of the present invention, before or during the cold rolling, softening annealing is performed in an air atmosphere, and in the annealing step, an oxide scale or α
Cases are formed, and these not only cause surface cracks and the like in the subsequent rolling and processing steps, but also cause deterioration in appearance, so they must be removed in any case. If the amount is too large, the corrosion resistance of the surface layer of the titanium alloy becomes too high, so that it is difficult to remove the oxide scale and the α case, and it is difficult to perform descaling by pickling or the like, thereby causing deterioration in workability.

Further, if the Mo content is too large, the density of the entire titanium alloy is increased, and the high specific strength characteristic inherent in the titanium alloy is impaired, so that it is 4.0% or less, more preferably 3. Should be kept below 5%.

Cr: 2.0-4.0% Cr is also a β-phase stabilizing element and contributes to an increase in the volume ratio of the β-phase, and also has a function of increasing the strength mainly by forming a solid solution in the β-phase. It is necessary to have these effects effectively 2.
It must be contained at least 0%, more preferably at least 2.5%. However, if the Cr content is too large, the ductility decreases, and the excellent strength / ductility balance intended in the present invention cannot be maintained. Therefore, the Cr content should be suppressed to 4.0% or less, more preferably 3.5% or less. .

The essential alloying elements in the titanium alloy according to the present invention are the above four kinds, and the balance is substantially Ti. However, a small amount of Si or O ( It is also effective to contain oxygen).

O: 0.25% or less O is an α-stabilizing element and forms a solid solution in the α-phase and contributes to an increase in strength. However, if too much, cold workability is deteriorated.
It should be suppressed to 0.25% by weight or less, more preferably about 0.15% or less.

When the titanium alloy of the present invention contains a small amount of Si as another element, the oxidation resistance at high temperatures can be increased and the high-temperature strength can be improved without substantially affecting the cold workability. When applied to high temperature applications, it is desirable to contain an appropriate amount of Si. However, Si
If the amount is too large, adverse effects on the cold workability appear. Therefore, the content should be suppressed to 0.5% or less, more preferably to about 0.3% or less.

The titanium alloy of the present invention may contain a small amount of an element other than the above as an element or an unavoidable impurity element within a range that does not impair the workability and strength characteristics described above. The inclusion of trace amounts of these elements is permissible as long as the properties of the titanium alloy are not impaired.

The titanium alloy of the present invention thus obtained is
It has a strength comparable to or higher than that of Ti-6Al-4V alloy, and also has excellent cold workability indicating a critical cold rolling reduction of 60% or more. By performing cold rolling after cold rolling,
It can be formed into an arbitrary shape such as a thin plate, corrugated plate, rod, or tube, and can be widely used for various applications.

The cold working is usually carried out at a rolling reduction of 4 times.
The processing is performed at about 0 to 80%, and at a processing rate of this level, processing can be performed to a target thickness by one processing. If it is desired to further increase the working ratio, softening annealing at about 700 to 850 ° C. for about 3 to 120 minutes is performed once or plural times during the cold working, so that the working is performed to the target thickness.
That is, since the titanium alloy of the present invention has excellent cold workability as described above, it is possible to reach the target thickness and size by cold rolling without the need for complicated pack rolling and the like unlike conventional titanium alloys. Processing can be performed, processing cost can be greatly reduced, and processing efficiency can be dramatically increased.

Therefore, the titanium alloy of the present invention can be used very effectively as a material for a golf head, a plate fin material for a heat exchanger, and the like. Especially as a material for heat exchanger,
Conventionally, stainless steel and aluminum alloys having excellent corrosion resistance and good workability have been widely used, and titanium alloys have not been practically used because they are difficult to process. It has properties superior to stainless steel and aluminum alloys in terms of high temperature strength, corrosion resistance, specific strength, etc., so it can be used as a heat exchanger material especially for high temperature and high pressure applications where high temperature strength characteristics are required. It is possible to provide a more excellent heat exchanger such as strength and life.

[0026]

EXAMPLES Hereinafter, the structure and operation and effect of the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples, and the present invention is applicable to the above and following points. The present invention can be appropriately modified and implemented within the scope of the invention, and all of them are included in the technical scope of the present invention.

Example A titanium alloy having the composition shown in Table 1 was melted by vacuum arc melting and then cast to produce an ingot having a diameter of 100 mm. Next, after slab rolling to a 15 mm thick plate in the β temperature range (1000 to 1050 ° C.),
After holding at 1050 ° C. for 30 minutes and air-cooling, hot rolling was performed in an α + β temperature range (850 ° C.) below the β transformation point to produce a hot-rolled sheet having a thickness of 5.7 mm. Then again α + β
After annealing in a temperature range (750 ° C.) for 5 minutes, shot blasting and pickling are performed to remove an oxide layer on the surface,
This was used as a cold rolled material. The cold rolling was performed at a rolling reduction of 0.2 mm per pass, and the cold rolling was continued until the edge of the sheet was cracked, and the cold rolling property was evaluated. During this time, a sample was taken at a rolling reduction of 40%, annealed in an α + β region (720 ° C.) for 5 minutes, and then subjected to a room temperature tensile test. The test piece was machined on the surface of the test plate, and the distance between the gauges was 50 mm.
The width of the parallel part was processed to 12.5 mm.

When performing the above test, the cold rolling limit was 4
As for the test plate less than 0%, the plate at the time of reaching the cold rolling limit was diverted. In addition, Ti-6Al-
A commercially available plate was used as the 4V alloy. Table 2 shows the results.

[0029]

[Table 1]

[0030]

[Table 2]

The following can be considered from Tables 1 and 2. Alloy No. 1: a conventional Ti-6Al-4V alloy,
It is used as a standard for performance evaluation.

Alloy No. 2,3: Examples satisfying the requirements of the present invention, having excellent cold rolling properties with a cold rolling limit of 60% or more, and excellent strength properties after cold rolling and annealing (excellent strength) TS) -elongation (El) balance.

Alloy No. 4: A comparative material having an insufficient Al content and having good cold rolling properties, but having insufficient strength after cold rolling and annealing.

Alloy No. 5: Comparative material with too much Al content, good strength but low elongation and poor cold rolling.

Alloy No. 6: The effect of solid solution strengthening is insufficient due to insufficient Mo content, and the strength after cold rolling and annealing is insufficient.

Alloy No. 7: Comparative material containing too much Mo, having good strength but low elongation and poor cold workability.

Alloy No. 8: Comparative material with insufficient Cr content, and insufficient strength after cold rolling and annealing.

Alloy No. 9: Comparative material with too much Cr content, good strength, but low elongation and poor cold workability.

Alloy No. 10: Comparative material having too much oxygen content, good strength, but a little low elongation and poor cold workability.

Alloy No. 11: a more preferred embodiment of the present invention containing an appropriate amount of Si, having a high elongation and It has a higher strength than a few.

Alloy No. 12: Comparative example with too much Si content, high strength but low elongation and poor cold workability.

[0042]

The present invention is constituted as described above, and regulates the respective contents of Al, Mo, and Cr as alloying elements to be contained in titanium, or furthermore, an appropriate amount of O (oxygen) or Si.
In addition to having the strength characteristics not inferior to the Ti-6Al-4V alloy, which is the most widely used titanium alloy, improving the cold rolling property lacking in the alloy, forming workability and strength characteristics Thus, it is possible to provide a titanium alloy having the above. And this titanium alloy,
It can be widely used in various applications by taking advantage of its excellent cold workability and mechanical properties.However, by taking advantage of its excellent corrosion resistance and light weight, and utilizing its excellent cold rolling properties, for example, heat exchange It can be used very effectively as a dexterous plate fin material.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-56-3645 (JP, A) JP-A-1-201433 (JP, A) JP-A-47-29212 (JP, A) (58) Investigation Field (Int.Cl. ⁷ , DB name) C22C 1/00-49/14 F28F 21/08

Claims (5)

(57) [Claims] 1. A mass% of Al: 3.5 to 5.0% , Mo: 2.0 to 4.0%, and Cr: 2.0 to 4.0%, with the balance being substantially Ti A high-strength titanium alloy with excellent cold-rolling properties, comprising: 2. The titanium alloy according to claim 1, further comprising O: 0.25% or less (excluding 0%) as another element. 3. The titanium alloy according to claim 1, further containing 0.5% or less (excluding 0%) of Si as another element. 4. The method according to claim 1, wherein the critical cold rolling reduction is 60% or more.
4. The titanium alloy according to any one of claims 1 to 3. 5. Use as a plate material for a heat exchanger
The high-strength titanium according to any one of claims 1 to 4,
Alloy.