JPH0819503B2 - Titanium alloy excellent in superplastic workability, method for producing the same, and superplastic workability method for titanium alloy - Google Patents

Description

TECHNICAL FIELD The present invention relates to a titanium alloy having high strength and excellent superplastic workability, and a method for producing the same.

[Prior Art] Titanium alloys are lightweight and tough, and have recently been actively used as materials for aerospace equipment such as airplanes and rockets. However, the titanium alloy is a difficult-to-work material, and there are problems that the product yield is extremely low and the manufacturing cost is significantly high when manufacturing a complicated upper member.

Superplastic working is known as a working method effective for solving such a problem. Superplasticity is a processing method utilizing superplasticity phenomenon, and it is industrially important especially to utilize the superfine grain plasticity found in fine crystal grains. Even in the Ti-6Al-4V alloy, which is the most widely used titanium alloy, a material having a fine grain structure of 5 to 10 μm is superplastically worked, but the working temperature is 875 to 9
It has many problems in terms of equipment and operation, such as high temperature of 50 ° C, short life of processing jig, and use of expensive material having high temperature strength as a jig.

Therefore, having excellent superplasticity characteristics than Ti-6Al-4V alloy, and, for the purpose of lowering the superplastic working temperature, Fe, Co, or Fe, Co, Ni in Ti-6Al-4V alloy Added alloys have been developed (US Pat. No. 4,299,626).
issue).

[Problems to be Solved by the Invention] However, such Ti-6Al-4V-Fe-Co-Ni
Even in alloys, the superplastic working temperature is 5% higher than that of Ti-6Al-4V.
It is only 0 to 80 ° C lower, which is not enough. Also, the superplastic elongation is not sufficient.

On the other hand, even if the superplastic working temperature can be lowered sufficiently, if the deformation resistance during superplastic working is large, the working becomes difficult.

The present invention has been made in view of the above circumstances, the superplastic working temperature is low, the deformation resistance during superplastic working is small, and the superplastic elongation is greater than that of the conventional titanium alloy. An object of the present invention is to provide a titanium alloy, a method for producing the same, and a superplastic working method for such a titanium alloy.

[Means and Actions for Solving the Problems] The titanium alloy excellent in superplastic workability according to the present invention is
% By weight, Al: 5.5 to 6.75%, V: 3.5 to 4.5%, O: 0.2% or less, Cr: 0.85 to 3.15%, Mo: 0.85 to 3.15%, and 3% ≦ {1.8 × ( Cr%) + (Mo%)} ≦ 8%, the balance consists of Ti and unavoidable impurities,
It is characterized in that the average grain size of α crystals is 6 μm or less.

The method for producing a titanium alloy excellent in superplastic workability according to the present invention is, by weight%, Al: 5.5 to 6.75%, V: 3.5 to 4.5%,
O: 0.2% or less, Cr: 0.85 to 3.15%, Mo: 0.85 to 3.15%, and satisfy the condition of 3% ≦ {1.8 × (Cr%) + (Mo%)} ≦ 8% , A titanium alloy, the balance of which is Ti and unavoidable impurities, is heated at a temperature of (β transformation point −200 ° C.) or higher and lower than the β transformation point, and then rolled down to a rolling ratio of 3 or higher at a temperature lower than the β transformation point. It is characterized by applying.

The superplastic working method of the titanium alloy according to the present invention, by weight%, Al: 5.5 to 6.75%, V: 3.5 to 4.5%, O: 0.2% or less, C
Contains r: 0.85 to 3.15%, Mo: 0.85 to 3.15%, and satisfies the condition of 3% ≦ {1.8 × (Cr%) + (Mo%)} ≦ 8% with the balance Ti and unavoidable impurities. The titanium alloy consisting of (β transformation point −200 ° C.) or higher is heated at a temperature lower than the β transformation point, and subsequently subjected to reduction at a reduction ratio of 3 or more at a temperature lower than the β transformation point, The recrystallization heat treatment is performed at a temperature of −200 ° C. or higher and lower than the β transformation point to perform superplastic working.

The present inventors have conducted various studies from the viewpoints described below to develop a titanium alloy having the above-mentioned characteristics,
The present invention having the above structure has been completed.

In order to obtain a titanium alloy having high strength and capable of superplastic working, its microstructure must be a structure having fine equiaxed α-crystals. Further, in order for the superplastic phenomenon of the titanium alloy to appear, it is necessary that the volume fraction of the α phase in the microstructure is 40 to 60%.
Therefore, in order to lower the superplastic working temperature than the Ti-6Al-4V alloy, an element that lowers the β transformation point, that is,
Cr and Mo may be added.

However, although Cr contributes greatly to the increase in strength, if the content of these is too large, an intermetallic compound that is an embrittlement phase is formed with Ti, and a segregation phase called β-flec during melting is formed. It is not preferable because it is generated and, as a result, mechanical properties are deteriorated. Mo similarly contributes to the strength increase, but if the addition amount is too large, the specific gravity of the titanium alloy is increased, the characteristics of the titanium alloy as a high specific strength material are impaired, and the diffusion rate in the β phase is small. Therefore, the deformation resistance during superplastic working is increased, which is not preferable. Therefore, it is necessary to regulate the content of these in a certain range where these disadvantages do not occur.

The β phase stability of titanium alloy with these elements is 1.8
It is shown by x (Cr%) + (Mo%). If this value is small, the β transformation point is high, and conversely, if it is large, the β transformation point is low. The optimum superplasticity temperature of the titanium alloy, that is, the temperature at which the superplasticity phenomenon can be expressed is the temperature at which the volume fraction of the α phase becomes 40 to 60% as described above, and this temperature has a close relationship with the β transformation point. is there. That is, if this value is too small, the advantage that the superplasticity development temperature is low cannot be obtained, and if it is too large, the temperature at which the volume fraction of the α phase becomes 40 to 60% becomes too low, and at that temperature atomic diffusion occurs. It becomes insufficient and sufficient superplastic elongation cannot be obtained. Therefore, in order to obtain excellent superplastic workability, it is necessary to define this value in an appropriate range.

Also, the fine-grain superplasticity depends largely on the crystal grain size, and the smaller the grain size, the better the properties can be obtained. Therefore, it is necessary to specify the upper limit of the crystal grain size to such an extent that the superplastic property is not impaired. Such refinement of crystal grains is achieved by performing the final hot working under the condition that a fine recrystallized structure of equiaxed α crystal can be obtained by the subsequent recrystallization heat treatment. That is, if the final hot working conditions are not appropriate, a fine recrystallized structure of equiaxed α crystal cannot be obtained by the subsequent recrystallization heat treatment.

Furthermore, the recrystallization heat treatment after the final hot rolling is a prerequisite for performing superplastic working, and by appropriately performing this treatment, subsequent superplastic working can be favorably performed.

Next, the reason why each component of the titanium alloy according to the present invention is limited to the above range in the present invention will be described.

Al: Al is added as an α-phase stabilizing element for obtaining an α + β structure and contributes to an increase in strength. However, if its content is less than 5.5%, it is insufficient to obtain the desired strength. On the other hand, if the content exceeds 6.75%, the embrittlement phase α ₂ phase (Ti ₃ Al) precipitates and mechanical properties deteriorate, which is not preferable. Therefore, the amount of Al is specified in the range of 5.5 to 6.75%.

V: V is added as a β-phase stabilizing element for obtaining an α + β structure, and contributes to the strength increase without forming an intermetallic compound which is an embrittlement phase with Ti. However, the content is 3.
If it is less than 5%, it is insufficient to obtain the desired strength,
Further, if the content exceeds 4.5%, the superplastic elongation is reduced and the deformation resistance during superplastic working is increased. Therefore, the V amount is specified to be 3.5 to 4.5%.

O: O forms a solid solution in the α phase and contributes to the strength increase. But,
If its content exceeds 0.2%, the β transformation point is raised, and the mechanical properties at room temperature, especially the ductility are deteriorated. Therefore, the O content is specified to be 0.2% or less.

Cr: Cr is added as a β-phase stabilizing element and contributes to the improvement of superplastic properties (increased superplastic elongation and reduced deformation resistance) by lowering the β-transformation point, and mainly forms a solid solution in the β-phase. And contributes to the increase in strength at room temperature. However, if the content is less than 0.85%, the contribution to the improvement of these superplastic properties and the increase of room temperature strength is insufficient. On the other hand, if it exceeds 3.15%, an intermetallic compound, which is an embrittlement phase, is formed between Ti and β-fleck is generated during melting, and as a result, ductility is deteriorated. Therefore, the Cr content is specified in the range of 0.85 to 3.15%.

Mo: Mo is also added as a β-phase stabilizing element, which contributes to the improvement of superplasticity characteristics (reduction of superplasticity development temperature) by lowering the β transformation point, and mainly solid solution in β phase to increase the strength. Contribute to. However, if the content is less than 0.85%, these effects are insufficient. Further, if it exceeds 3.15%, since Mo is a heavy metal, the density of the alloy is increased and the characteristic of the titanium alloy having high specific strength is impaired. This will increase the deformation stress during plastic forming. Therefore, the amount of Mo is 0.85 to 3.15.
%.

1.8 × (Cr%) + (Mo%) indicates the β phase stability of the titanium alloy. As mentioned above, the smaller the value, the higher the β transformation point, and the larger the value, the lower the β transformation point. . The optimum superplasticity temperature of the titanium alloy, that is, the temperature at which the superplasticity phenomenon can be expressed is the temperature at which the volume fraction of the α phase becomes 40 to 60% as described above, and this temperature has a close relationship with the β transformation point. is there. If this value is less than 3%, the characteristic of the present invention that the superplasticity development temperature is low is impaired, and if it exceeds 8%, the temperature at which the volume fraction of the α phase becomes 40 to 60% becomes too low, and the temperature becomes too low. In that case, atomic diffusion becomes insufficient and sufficient superplastic elongation cannot be obtained.

Therefore, set this value to 3% ≤ {1.8 x (Cr%) + (Mo
%)} ≦ 8%.

 Next, the reason for limiting the α-phase grain size will be described.

The α-crystal grain size is closely related to the superplastic property, and the smaller the grain size, the better the superplastic property. In the titanium alloy according to the present invention, if the average grain size of α crystals exceeds 6 μm, not only the superplastic elongation decreases but also the deformation stress increases, which is not preferable. Therefore, the α crystal grain size is specified to be 6 μm or less.

The following method is employed to reduce the α crystal grain size to 6 μm or less.

First, hot forging, or hot rolling, or both, to process and manufacture a titanium alloy ingot into a slab,
The slab is then reheated and hot worked to final dimensions. In the final hot working, it is necessary to satisfy the following three conditions. The heating temperature is set to (β transformation temperature −200 ° C.) or higher and lower than the β transformation point. Make sure that the temperature of the work piece does not exceed the β transformation point during processing.
The reduction ratio during processing is set to 3 or more.

 The reason for defining the conditions in this way is as follows.

With respect to the above, when the heating temperature is at or above the β transformation point, even if recrystallization annealing is performed after working, a rod-shaped α crystal structure is formed instead of an equiaxed structure suitable for superplastic forming, and α Crystals are formed, deteriorating superplastic properties and ductility at room temperature. Further, if the heating temperature is less than the β transformation point −200 ° C., the temperature is too low and results such as cracking occur during processing.

With regard to the above, if the temperature of the work piece becomes equal to or higher than the β transformation point during processing, a rod-shaped α structure will be formed as in the case of.

With respect to the above, when the reduction ratio is as small as less than 3, sufficient strain cannot be accumulated in the recrystallization of the α crystal, and even if the recrystallization heat treatment is performed, it is not a fine grain equiaxed structure suitable for superplastic forming, but a rod-shaped one. An α crystal structure or a coarse α structure is not preferable.

Next, the reasons for limiting the recrystallization heat treatment conditions performed prior to superplastic working will be described.

If the recrystallization heat treatment is less than (β transformation point −200 ° C.), the temperature is too low to perform recrystallization sufficiently, and equiaxed grains cannot be obtained, which is not preferable. Further, if the heat treatment temperature is equal to or higher than the β transformation point, the equiaxed α crystal disappears to become a β single phase, and superplasticity cannot be obtained, which is not preferable. Therefore, the recrystallization heat treatment temperature is defined as (β transformation point −200 ° C.) or higher and lower than the β transformation point.

The superplastic working may be performed subsequent to the recrystallization heat treatment, or may be performed simultaneously with the recrystallization heat treatment.

[Examples] Examples of the present invention will be described in detail below.

For alloys having the compositions shown in Table 1, ingots were melted in an arc furnace in an argon atmosphere and hot forged to manufacture slabs having various thicknesses.

Then, each of these slabs was reheated to the temperature shown in Table 1, and subsequently, each of these slabs was rolled at the rolling reduction ratio shown in Table 1 to finish a plate material having a thickness of 5 mm.
Then, the plate material thus finished was subjected to a recrystallization heat treatment. This recrystallization heat treatment was performed in a temperature range of (β transformation point −200 ° C.) or more and less than β transformation point except for Experiment No. C15. Following this recrystallization heat treatment, a superplastic tensile test was performed. Further, the α crystal grain size (average grain size) of each plate material subjected to the recrystallization heat treatment and the room temperature tensile test were performed together. The α crystal grains were measured by the line segment method, and the grain size was not measured for those having a rod-shaped structure with an aspect ratio (ratio of major axis and minor axis) of 3 or more.

The temperature of the recrystallization heat treatment and the α crystal grain size of each plate material are shown together in Table 1, and the results of the room temperature tensile test and the superplastic tensile test are shown in Table 2.

In Table 1, experimental numbers A1 to A7 represent examples within the scope of the present invention, and B1 to B4 represent conventional examples ((1) in the conventional example is
Ti-6Al-4V alloy, (2) is Ti-6Al-4V-Co- (Ni) alloy, and C1 to C15 are comparative examples outside the scope of the present invention. In addition, the β transformation points in the compositions of the respective experiment numbers are also shown in Table 1.

In the superplastic tensile test, the parallel part is 5 mm wide, 5 mm long,
The test piece having a thickness of 4 mm was used and the test was performed in a vacuum of 5 × 10 ⁻⁶ Torr or less. Further, the maximum deformation stress was obtained by dividing the maximum load by the initial cross-sectional area.

As shown in Table 1 and Table 2, the experiment numbers A1 to A7, which are examples, have alloy compositions within the scope of the present invention.
Moreover, since the production conditions are in accordance with the method according to the present invention, the α-crystal grain size is extremely fine and sufficiently satisfies 6 μm or less. As for room temperature tensile properties, the tensile strength is 105 kgf / mm ² or more and the elongation is 17% or more.
It was confirmed that the value was better than that of the −4V alloy.

Next, the superplastic tensile properties shown in Table 2 will be described with reference to FIGS. 1 to 6.

FIG. 1 is a graph showing the relationship between the value of 1.8 × (Cr%) + (Mo%) and the superplastic elongation. As is clear from this figure, this value is within the range of 3 to 8%, which is within the range of the present invention.
It was confirmed that a large elongation of 1500% or more could be obtained.

FIG. 2 is a graph showing the relationship between the Cr content and the superplastic elongation. As is clear from this figure, it was confirmed that a large elongation of 1500% or more can be obtained in the range of 0.85 to 3.15%, which is within the range of the present invention.

FIG. 3 is a graph showing the relationship between the Mo content and superplastic elongation. As is clear from this figure, it was confirmed that a large elongation of 1500% or more can be obtained in the range of 0.85 to 3.15%, which is within the range of the present invention.

FIG. 4 is a graph showing the relationship between α crystal grain size and superplastic elongation. As is clear from this figure, it was confirmed that a large elongation of 1500% or more can be obtained if this value is 6 μm or less, which is within the range of the present invention.

As described above, it was confirmed that excellent superplasticity characteristics can be obtained in the composition range and the grain size range defined in the present invention.

The test numbers A1 to A7 which are within the scope of the present invention are not only good in that the superplastic elongation is 1500% or more as described above, but the temperature at which the maximum superplastic elongation is 800 ° C. or lower and Ti-6Al-4V alloy 75 to 100 ℃ lower than that, and despite its extremely low temperature, the deformation stress at that temperature is 1.41 kgf / mm
It was confirmed to be as small as ² or less.

On the other hand, it was confirmed that C1 to C14, which are comparative examples, all had lower superplastic elongation values than the examples.

Among the comparative examples, C1 to C10 have a composition outside the scope of the present invention, and among them, C3, C5 and C9 show a relatively large elongation of 1320% or more, but the maximum superplastic elongation was obtained. It was confirmed that the temperature was 825 ° C. or higher, which was 25 to 50 ° C. higher than that of the example, and the superplastic property was obviously inferior to that of the example. The temperatures at which the maximum superplastic elongations were obtained for C7 and C10 were extremely low at 750 ° C, but the superplastic elongations were 130% each.
It remains at 0% and 780%, and the deformation stress is 2.85 kgf / mm ² and 2.8, respectively.
It was an extremely large value of 0 kgf / mm ² . Also in C1, C2, C4 and C6, the superplastic elongation was insufficient at around 1000%.
Since C8 has a small elongation at room temperature of 6% and cannot withstand practical use, a superplastic tensile test was not performed.

C11 to C15 are those whose manufacturing conditions and α crystal grain size are out of the range of the present invention, C11 to C13 are reduction ratios outside the range of the present invention, and C14 is the final hot rolling heating temperature from the range of the present invention. Deviated, C15 is because the recrystallization heat treatment temperature is out of the range of the present invention, does not become a fine grain equiaxed structure, plastic elongation is confirmed to be inferior to the value of the example of 550 to 1380%. It was The grain size of C12, C14 and C15 was not measured because the α crystals did not become equiaxed grains but became coarse rod-shaped crystals with an aspect ratio of 3 or more.

EFFECTS OF THE INVENTION According to the present invention, the superplasticity such that the superplasticity processing temperature is low, the deformation resistance during superplasticity processing is small, and the superplastic elongation is larger than that of the conventional titanium alloy while maintaining excellent strength. A titanium alloy excellent in workability and a method for producing the same can be provided.

Further, the titanium alloy according to the present invention can be widely used as a high-strength titanium alloy excellent in superplastic workability, including materials for aerospace equipment, by utilizing these excellent properties.

Further, the titanium alloy according to the present invention is excellent not only in the processing method utilizing superplasticity but also in the processing method such as isothermal forging, normal hot forging, or warm forging because of the characteristic that the deformation resistance is small. It is obvious that it has workability and can be applied in a very wide range of applications.

[Brief description of drawings]

Fig. 1 is a graph showing the relationship between the value of 1.8 x (Cr%) + (Mo%) and superplastic elongation, Fig. 2 is a graph showing the relationship between Cr content and superplastic elongation, and Fig. 3 is FIG. 4 is a graph showing the relationship between the Mo content and superplastic elongation, and FIG. 4 is a graph showing the relationship between α crystal grain size and superplastic elongation.

Claims (3)

[Claims] 1. By weight%, Al: 5.5 to 6.75% V: 3.5 to 4.5% O: 0.2% or less Cr: 0.85 to 3.15% Mo: 0.85 to 3.15% and 3% ≦ {1.8 × (Cr%) + (Mo%)} ≦ 8%, the balance consists of Ti and unavoidable impurities,
A titanium alloy excellent in superplastic workability, characterized in that the average grain size of α crystals is 6 μm or less. 2. In weight%, Al: 5.5 to 6.75% V: 3.5 to 4.5% O: 0.2% or less Cr: 0.85 to 3.15% Mo: 0.85 to 3.15% and 3% ≦ {1.8 × A titanium alloy satisfying the condition of (Cr%) + (Mo%) ≤ 8% and the balance of Ti and unavoidable impurities is heated at a temperature of (β transformation point -200 ° C) or more and less than β transformation point. Next, a method for producing a titanium alloy having excellent superplastic workability, which is characterized by continuously performing a reduction at a reduction ratio of 3 or more at a temperature below the β transformation point. 3. In weight%, Al: 5.5 to 6.75% V: 3.5 to 4.5% O: 0.2% or less Cr: 0.85 to 3.15% Mo: 0.85 to 3.15% and 3% ≦ {1.8 × A titanium alloy satisfying the condition of (Cr%) + (Mo%) ≤ 8% and the balance of Ti and unavoidable impurities is heated at a temperature of (β transformation point -200 ° C) or more and less than β transformation point. Continuously, reduction at a reduction ratio of 3 or more at a temperature lower than the β transformation point is performed, and recrystallization heat treatment is performed at a temperature of (β transformation point −200 ° C.) or higher and lower than the β transformation point to perform superplastic working. A superplastic working method for a titanium alloy, characterized by: