JPH059629A - High touchness titanium alloy and production thereof - Google Patents

Abstract

PURPOSE:To stably provide a Ti alloy having high strength and satisfactory toughness necessary for a structural material. CONSTITUTION:A Ti alloy having a compsn. consisting of 4.0-7.0% Al, 3.0-5.0% V, 0.5-5.0% Fe, 0.1-5.0% Mn and the balance Ti with inevitable impurities is subjected to soln. heat treatment by heating to 750-950 deg.C and cooling to room temp. at <=10 deg.C/sec cooling rate. The alloy is then aged by heating and holding at 400-700 deg.C for 15min-8hr.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention relates to a high toughness titanium alloy (hereinafter referred to as "Ti alloy") which exhibits excellent performance as chemical industrial equipment / equipment, energy development equipment / equipment, structural material for general industry, etc. Note) and its manufacturing method.

[0002]

2. Description of the Related Art In general, Ti alloys have excellent corrosion resistance and are also known as materials with high specific strength (strength / specific gravity). Utilizing the characteristics of high strength and low specific gravity Demand for structural materials has also grown significantly. As a Ti alloy, it has excellent high temperature strength and weldability.
“Type”, “β type” which is excellent in normal temperature strength, fracture toughness, cold workability and heat treatment property, and “α + β type” which is said to have both characteristics, are known as structural materials. The above α + β type alloys have become the mainstream of applicable materials because of their superiority in practicality, and it is "Ti-6" that is the most manufactured and most used among the α + β type Ti alloys.
"Al-4V alloy" is mentioned, and it has established a typical position as a practical Ti alloy.

In order to obtain high strength, these Ti alloys are usually subjected to heat treatment during use. As the heat treatment, "annealing treatment" may be adopted when the case of the Ti-6Al-4V alloy material is taken as an example, but generally, "solution treatment" and "aging treatment" are subsequently performed. "Two-step heat treatment (solution treatment aging treatment: STA)" is applied for use. For example, in the American standard ASM4967E,
For Ti-6Al-4V alloy, "5 hours after quenching in cooling water stirred at 955 ° C ± 15 ° C for 1-2 hours.
"Aging treatment is performed by holding at 38 ° C ± 8 ° C for 4 to 8 hours." This is because Ti-6Al is formed by forming a martensite phase by rapidly cooling it from a high temperature region and then decomposing it into fine α and β phases in the subsequent aging treatment.
It is defined in order to stably impart high strength to the -4V alloy.

On the other hand, the α + β type Ti alloy, which is expected to be hot as a structural material, does not have sufficient toughness on the other hand. The resistance was weak. " That is, when the above-mentioned Ti alloy is subjected to the solution aging treatment that has been conventionally performed, it is possible to strengthen the alloy, but at the same time, the toughness value is also greatly decreased, and there is another problem as a structural material. There wasn't.

Therefore, when a Ti alloy is used as a structural material, it has been necessary not only to secure high strength but also to give a good toughness value. However, no Ti alloy has been found that exhibits both excellent strength and toughness that are fully satisfactory.

Therefore, the object of the present invention is to obtain room temperature (25
Strength) at 0.2% yield strength of 105.0 kgf / mm ² or more, and Charpy impact value at room temperature: 3.0 kgm / cm ² or more, to realize a high strength / high toughness Ti alloy. .

[0006]

The inventors of the present invention have made extensive studies in order to achieve the above-mentioned objects, and as a result, have made new studies as follows. That is, as described above, among the various Ti alloys, the α + β type Ti alloy is considered to be the most suitable as the structural material from the viewpoint of various performances.
The solution aging treatment, which has been applied as an effective heat treatment for strengthening Ti alloys, is the aging treatment that produces a supersaturated solid solution (martensite) by quenching from quenching from high temperature, as mentioned earlier. By this, fine α phase and β phase are aged and precipitated from this supersaturated solid solution to strengthen. However, heat treatment that incorporates a method of rapidly cooling a high-temperature Ti alloy to form a supersaturated solid solution can inevitably reduce the toughness value even if other conditions are devised. Absent.

This is due to the following reason. That is, α + β type Ti
It was clarified by observing the fracture process in the solution-age-aged material of the alloy in detail, but when the alloy material fractures, "fine cracks" appear in front of the main crack that propagates first.
Occurs, and there is a phenomenon in which the cracks are connected and develop into a complete crack. Moreover, the place where the "fine cracks" occur is the interface between the α phase and the β phase. Then, according to the conventional solution aging treatment conditions, when the supersaturated solid solution is formed and the fine α phase is precipitated by the subsequent aging treatment, the number of interfaces between the α phase and the β phase increases, so that the crack occurrence point is Since there are a large number of cracks, the propagation of cracks is facilitated, and the toughness value of the solution-age-aged material is reduced.

Therefore, in order to improve the toughness value of the α + β type Ti alloy, it is necessary to strengthen the interface between the α phase and the β phase, but addition of Fe as an alloy component is an effective means to meet this requirement. obtain. In other words, when Fe, which is known as a β-stabilizing element, is added to an α + β-type Ti alloy and the heat treatment is appropriately performed, the effect of strengthening the interface between the α phase and the β phase appears, and crack propagation resistance, that is, toughness. A noticeable improvement in the value is recognized.

However, although Fe is also known as an element for improving the strength of Ti alloys, the goal is to add only the amount of Fe required to achieve the toughness improvement target of α + β type Ti alloys. It is difficult to achieve such strength. However,
If the amount of addition of Fe is further increased to achieve the desired strength target, TiFe intermetallic compound precipitates to cause embrittlement of the alloy, and on the contrary, the target of toughness cannot be satisfied.

However, when an appropriate amount of Mn is added together with an appropriate amount of Fe, the strength of the α + β type Ti alloy can be sufficiently improved without adversely affecting the toughness improving action of the α + β type Ti alloy. It was found that the desired strength and toughness targets could be achieved.

Further, in the heat treatment of the alloy, when the martensite phase is decomposed by aging as described above, the toughness value tends to decrease, so that the conventional "heating in a high temperature region followed by rapid cooling to form a martensite phase, It is important not to generate the martensite phase after heating at high temperature without following the condition of `` aging this '', but even if the martensite phase is not generated in this way, after cooling to room temperature It was also found that the target strength and target toughness values can be compatible with each other if the aging temperature is properly selected.

The present invention has been completed on the basis of the above findings and the like. "Titanium alloy is Al: 4.0 to 7.0% (hereinafter,% representing a component ratio is a weight%), V: 3.0%. ~ 5.0%, Fe: 0.5 ~ 5.0%, Mn:
It is characterized in that it has a high toughness value as well as sufficient strength by being composed of a component composition containing 0.1 to 5.0% with the balance being Ti and inevitable impurities. : 4.0-7.0%, V: 3.0-5.0%, Fe:
An alloy containing 0.5 to 5.0% and Mn: 0.1 to 5.0%, with the balance consisting of Ti and unavoidable impurities, heated to 750 to 950 ° C, and then cooled to room temperature at a cooling rate of 10 ° C / sec or less. "Temperature treatment" followed by "aging treatment" of heating and holding at 400 to 700 ° C for 15 minutes to 8 hours enables stable production of a titanium alloy having high strength and high toughness. It also has a great feature.

As described above, in the present invention, the composition of the Ti alloy is carefully devised, and by combining this with the predetermined heat treatment conditions, excellent strength and toughness are provided.
Although a Ti alloy has been realized, the reason why the chemical composition of the alloy and the processing conditions are limited as described above in the present invention will be explained below together with its action.

[0014]

(Function) (A) Chemical composition of alloy Al and V Ti alloy, Al is an α-phase stabilizing element and the most commonly used additive element. On the other hand, V is a β-stabilizing element. These elements are added for the purpose of solid solution strengthening. However, the Al content is 4.0% or more, and the V content is
If 3.0% or more is not secured, the desired strength cannot be secured. However, when the Al content exceeds 7.0%, an intermetallic compound called α ₂ is precipitated in the α phase, which causes remarkable embrittlement. Also, the V content is 5.0%
If it exceeds the above, the strength of the alloy is remarkably increased, but the toughness value is greatly reduced. Therefore, the Al content is 4.0-
The V content was limited to 7.0% and 3.0 to 5.0%, respectively.

Fe Fe has an effect of increasing the strength of the alloy and an effect of strengthening the interface between the α phase and the β phase to improve the toughness, but if the content is less than 0.5%, it is desirable. The effect of improving the toughness value cannot be secured. On the other hand, when Fe is contained in excess of 5.0%, TiFe intermetallic compounds are precipitated and the alloy becomes brittle, so that improvement in toughness value cannot be expected. Therefore, the Fe content is determined to be in the range of 0.5 to 5.0%.

Mn Mn exhibits an action of increasing the strength of the alloy together with Fe, but if the content thereof is less than 0.1%, the desired effect due to the above action cannot be obtained, while if it exceeds 5.0% Mn
If the content of Mn is increased, the alloy strength is remarkably increased and the toughness value is decreased, so the Mn content was set to 0.1 to 5.0%.

In addition, as the inevitable impurities of the Ti alloy according to the present invention, C, H, O, N, Y and the like can be mentioned. Usually, the inclusion of these within the following ranges is allowed. C: 0.10% or less, H: 0.0125% or less, O: 0.20% or less, N: 0.05% or less, Y: 0.005% or less.

(B) Heat treatment conditions Solution treatment conditions If the solution treatment temperature is less than 750 ° C., the desired strength cannot be secured in the Ti alloy after the solution aging treatment. On the other hand, if the solution treatment is performed at a temperature higher than 950 ° C., the β phase is transformed into the martensite phase during cooling, and the desired good toughness value cannot be obtained. Further, when the cooling rate to room temperature after the high temperature holding in the solution treatment becomes faster than 10 ° C./sec, quenching occurs in the cooling process and transformation to the martensite phase occurs, so that the toughness value of the alloy is also high. I can't hope for improvement. Therefore, the solution treatment temperature is 750
~ 950 ° C, and the cooling rate after holding at high temperature is 10 ° C /
Limited to less than a second.

Aging treatment conditions If the aging treatment temperature is less than 400 ° C., the aging effect is not recognized, so that desired strength cannot be imparted to the alloy. On the other hand, if the aging treatment is carried out at a temperature higher than 700 ° C., it will be in an over-aged state and, conversely, the alloy strength will be lowered. Therefore,
The aging treatment temperature was limited to 400 to 700 ° C. If the heating and holding time in the aging treatment is less than 15 minutes, the desired aging effect cannot be obtained. Although it is not necessary to set an upper limit on the aging time from the viewpoint of product performance, long-term aging is disadvantageous from the viewpoint of productivity and economic efficiency, and aging treatment for more than 8 hours is unnecessary. Therefore, the aging time was set to 15 minutes to 8 hours.

Next, the present invention will be described more specifically by way of examples.

EXAMPLES Example 1 First, a Ti alloy ingot having a chemical composition shown in Table 1 (outer diameter 50 mm ×
A height of 110 mm: 1 kg each was prepared. Next, each of the obtained ingots is heated to 1100 ° C. and then “width 50 mm ×
After forging to a thickness of 30 mm, it was reheated to 900 ° C. again and hot-rolled to a width of 50 mm and a thickness of 7 mm in the α + β range.

[0021]

[Table 1]

The test material after rolling was heated to 850 ° C. for 1
After holding for a period of time, it was cooled to room temperature by air cooling. At this time, the cooling rate of the central portion of the thickness of the test material was measured with a thermocouple, and it was confirmed to be "5 ° C / sec". After the solution treatment of the Ti alloy test material, 60
The sample was subjected to an aging treatment in which it was heated and held at 0 ° C. for 6 hours, and was air-cooled.

Next, a plate-shaped test piece having a wall thickness of 3 mm, a width of 6.25 mm, and a gauge length of 25 mm parallel to the rolling longitudinal direction was taken from each of the test materials after the heat treatment and stretched at room temperature. It was submitted to the test. In order to evaluate the toughness, a JIS No. 4 half size Charpy impact test piece (V notch) having a width of 5 mm was also taken from each of the test materials after heat treatment at room temperature (25 ° C.). An impact test was conducted at.

The results of these tests are also shown in Table 1. The overall evaluation of the test results was "0.2% proof stress" and "Charpy
When the 0.2% proof stress is 105.0 kgf / mm ² or more and the Charpy impact value is 3.0 kgm / cm ² or more, the product is marked as “○” and the proof stress and the Charpy impact When the value did not reach the above value, it was indicated by "x". From the results shown in Table 1, it can be confirmed that the Ti alloys whose chemical composition satisfies the conditions specified in the present invention have achieved the targets of strength and toughness.

Example 2 Ti-6.2% Al-4.1% V- manufactured by vacuum arc melting
0.6% Fe-2.0% Mn alloy ingot (outer diameter 300mm: 150
(kg) is heated to 1150 ° C and forged to a width of 50 mm x
It was a hot rolled material with a thickness of 30 mm. Next, after heating this hot-rolled material to 900 ° C., it is hot-rolled to have a width of 50 mm and a thickness of 12
mm ". Length 1 from this rolled Ti alloy material
A plurality of heat treatment test materials of 00 mm were cut out and heat treated under the conditions shown in Table 2.

[0026]

[Table 2]

Then, from each test material after the heat treatment,
In the same manner as in (1), a plate-shaped tensile test piece and a JIS No. 4 half size Charpy impact test piece (V notch) were sampled and subjected to a tensile test and an impact test at room temperature (25 ° C). The results of these tests are also shown in Table 2. The comprehensive evaluation of the test results was performed in the same manner as in Example 1. From the results shown in Table 2, it is confirmed that the Ti alloys treated according to the conditions specified in the present invention both satisfy the targets of strength and toughness.

[0028]

[Summary of Effects] As described above, according to the present invention, the 0.2% proof stress at room temperature is 105.0 kgf / mm ² or more, and the Charpy
It is possible to stably provide a Ti alloy with a high impact strength of 3.0 kgm / cm ² or more and excellent toughness, and use it as a structural material in the chemical industry, energy industry and other industrial fields. Even in such a case, it is possible to expect a sufficiently satisfactory performance, which brings industrially useful effects.

Claims (2)

[Claims] 1. A weight ratio of Al: 4.0 to 7.0%, V: 3.0 to 5.0%, Fe:
A titanium alloy containing 0.5 to 5.0% and Mn: 0.1 to 5.0%, with the balance being Ti and unavoidable impurities and having a high toughness value. 2. A weight ratio of Al: 4.0 to 7.0%, V: 3.0 to 5.0%, Fe:
An alloy containing 0.5 to 5.0%, Mn: 0.1 to 5.0% and the balance of Ti and unavoidable impurities is heated to 750 to 950 ° C. and then cooled to room temperature at a cooling rate of 10 ° C./sec or less. A method for producing a titanium alloy having a high toughness value, which comprises performing "treatment" and then performing "aging treatment" of heating and holding at 400 to 700 ° C for 15 minutes to 8 hours.