JP2001288518A - High strength and high toughness titanium alloy member and its producing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high strength and high toughness titanium alloy member having good strength characteristics and fracture toughness. SOLUTION: In this high strength and high toughness titanium alloy member, comprising an α+β type titanium alloy in which the Mo equivalent obtained by the following formula, Mo equivalent = Mo%+1/5Ta%+1/3.6Nb%+1/1.5V %+1.25Cr%+1.25Ni%+1.7Mn%+1.7Co%+2.5Fe% (% denotes mass %) satisfies 6.0 to 12.0, the volume fractional ratio of the intergranular α structure formed on the old β grain boundary is <=5.0%, also, the size of the minor axis in the acicular α phase formed in the old β grains is >=0.75 μm, and the aspect ratio is >=7.0.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength, high-toughness titanium alloy member useful for, for example, a jet engine member of an aircraft or a material for a golf club head, and a method for producing the same.

[0002]

2. Description of the Related Art Materials used for compressor disks of jet engines and the like are required to have high mechanical properties due to the severe use environment, and not only strength but also resistance to crack propagation, that is, fracture toughness value. High is desired. In contrast, the use of titanium alloys has been expanding in recent years due to its high specific strength, and among titanium alloys,
It is becoming mainstream to use α + β type titanium alloys which have an excellent balance between strength and toughness and can be controlled to desired properties by thermomechanical treatment.

Further, regarding the above α + β type titanium alloy,
As a method of improving the strength-toughness balance, there is a method called a β process, which is being recognized as a method of manufacturing a titanium alloy member having an excellent strength-toughness balance. β
The process is to heat the α + β type titanium alloy to a temperature above the β transformation point, and then perform hot working before the α phase precipitates to introduce precipitation sites into the old β crystal grains, Prevents preferential precipitation at the old β grain boundary during α phase precipitation, forms acicular α phase from within the grains, develops acicular structure in subsequent solution treatment, and further develops between acicular α laths in subsequent aging treatment It is completed by producing fine precipitation α.

[0004] However, completely suppressing the grain boundary α phase preferentially and continuously precipitated at the old β grain boundary and growing the acicular α phase in the old β grain is fundamentally contradictory. ,
Tissue control was difficult.

[0005] In this regard, various production methods have been proposed. For example, JP-A-8-2696
According to No. 56, by performing rapid cooling after performing a finish forging in a β single phase region higher by 30 ° C. than β Transus, it is possible to suppress continuous precipitation of an α phase at a β grain boundary in a direction perpendicular to a processing direction, A method for improving fracture toughness regardless of the finishing ratio has been proposed.

[0006] As described above, the titanium alloy is generally subjected to a solution treatment and an aging treatment as a post-treatment after the working for strengthening. The present inventors have studied and found that, according to the above-described method, in the as-forged member, even though the precipitation of the α phase at the old β grain boundary can be suppressed, the α phase remains at the old β grain boundary during the subsequent solution treatment. Is generated and grown, and as a result, an α phase is continuously precipitated at the grain boundary, and it has been found that the strength characteristics, particularly the strength and the elongation, are deteriorated.

[0007] When the cooling rate after processing is further increased, the lath of the acicular α phase formed in the old β grains becomes finer.
This resulted in degradation of fracture toughness.

[0008] As described above, there are still difficult points in imparting good strength properties and fracture toughness to members even by the β process, and there has been a strong demand for improvement.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and has as its object to provide a high-strength, high-toughness titanium alloy member having good strength characteristics and fracture toughness. And a method for reliably manufacturing the same.

[0010]

In order to solve the above-mentioned problems, a high-strength, high-toughness titanium alloy member according to the present invention (claim 1) has a Mo equivalent obtained by the following Mo equivalent formula of 6.0 to 6.0. An α + β type titanium alloy that satisfies 12.0, the grain boundary α structure formed in the old β grain boundary is 5.0% or less in volume fraction, and the needle-like shape formed in the old β grain The α-phase has a minor axis diameter of 0.75 μm or more and an aspect ratio of 7.0 or more, and has excellent strength properties and fracture toughness. However, Mo equivalent = Mo% + ／
Ta% + 1 / 3.6Nb% + 1 / 1.5V% + 1.25
Cr% + 1.25Ni% + 1.7Mn% + 1.7Co%
+ 2.5Fe% (% represents mass%)

[0011] In the high-strength, high-toughness titanium alloy member, the minor axis diameter of the aged precipitate α phase existing between the acicular α laths is 0.10 μm or less and the aspect ratio is 5.0.
And the phase fraction of all α phases is 40% in volume fraction
It is preferable that the above is attained, and by doing so, more favorable strength can be secured.

Further, according to the present invention (claim 4), high strength,
The method for producing a high toughness titanium alloy member is such that the Mo equivalent obtained by the following Mo equivalent equation satisfies 6.0 to 12.0.
After heating the + β-type titanium alloy to the β region, processing is started at an arbitrary temperature in the β region, and (β Transus -150
After finishing the processing in an arbitrary temperature range of not less than β) and less than β transus, after cooling at a cooling rate of 0.2 to 2.0 ° C./sec from the end point, solution treatment and aging treatment are performed in the α + β2 phase region. By doing so, it is possible to reliably obtain the titanium alloy having excellent strength characteristics and fracture toughness. However, Mo equivalent = Mo% + ／ Ta% + 1 / 3.6Nb
% + 1 / 1.5V% + 1.25Cr% + 1.25Ni%
+ 1.7Mn% + 1.7Co% + 2.5Fe% (% represents% by mass)

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below. In order to improve the strength properties, especially strength and elongation, after the solution treatment, the grain boundary α generated in the old β grain boundary is completely suppressed, and in order to improve the fracture toughness, the It is desirable to grow the acicular α phase generated in the β grains, but as described above, it is not easy to achieve them simultaneously. The present inventors have conducted various studies on the tissue morphology and mechanical properties in order to establish clear tissue control guidelines.

First, the strength characteristics (strength and elongation) will be described. The present inventors have studied the correlation between the structure and the strength / elongation, and after the solution treatment, the volume fraction of 5.0 was obtained.
% If it can be suppressed to not more than%. It should be noted that the grain boundary α fraction described in the present invention is calculated by image analysis of a microstructure photograph of at least two visual fields at a magnification of 400 times.

[0015] Further, means for realizing this have been studied. The above-mentioned structure characteristics describe the structure of the structure after the hot working and the subsequent solution treatment, and each condition has an influence, but particularly the hot working condition has a great effect. This is because the structure morphology formed by the solution treatment in the α + β2 phase region basically follows the structure after processing. Therefore, the study was repeated focusing on the metallurgical phenomenon that occurs during hot working.

When the α + β type titanium alloy is heated to the β region and cooled, first, the reworked β is recrystallized, and subsequently the recrystallized β
Grain growth occurs, and a grain boundary α phase and an intragranular α phase are generated. If recrystallization occurs, the area of the old β grain boundary increases, and the fraction of the α phase in the grain boundary increases, which is not preferable. In addition, the processed structure (dislocation / subgrain), which is the preferential nucleation site of the intragranular acicular α phase, disappears, and the production of the acicular α phase is delayed.

For this reason, it is necessary to generate the α phase without complete recrystallization, and as a result of examining the conditions, the processing is completed at an arbitrary temperature lower than β transus regardless of the processing rate, and the solution treatment temperature is reduced. It has been found that the cooling may be performed at a cooling rate of 0.2 ° C./sec or more. By doing so, recrystallization does not proceed during cooling up to α phase precipitation,
The grain boundary α phase does not exceed 5.0% in volume fraction, and good strength characteristics can be secured.

The reason for defining the cooling rate is as follows. There is a correlation between the β transus and the optimum value of the cooling rate.In the case of alloys with a high β transus, the diffusion rate of elements increases, and the growth rate of the α phase at the grain boundary after nucleation increases. It is necessary to increase the cooling rate in order to suppress the cooling. As a result of intensive studies on this point, the alloy included in the Mo equivalent range presented in the present invention, even if the alloy having the highest β transus, was 0.2 ° C. /
Cooling in seconds or more ensures good strength properties.

On the other hand, when the cooling is performed at a cooling rate lower than 0.2 ° C./sec, since the grain boundary α phase is generated at a volume fraction exceeding 5.0%, good strength characteristics are obtained. I can't get it.

In the present invention, the conditions for the solution treatment are not specified, but if the treatment is performed at least in a temperature range of β transus or less to form the above-mentioned structure, good strength characteristics can be secured.

Next, improvement in fracture toughness will be described. In this regard, it is known that branching a crack from a crack propagation surface is effective, and it is known that formation of an acicular α-structure is desirable. As a result of intensive studies focusing on the needle-like structure, it was found that good toughness can be ensured when the minor axis diameter of the needle-like α-phase is 0.75 μm or more and the aspect ratio is 7.0 or more. .

The acicular α-phase described in the present invention is an acicular α-phase formed in old β grains, and has an area of 0.5 μm ² or more. The aspect ratio is the ratio of the major axis diameter to the minor axis diameter of the needle-shaped α, and is obtained by analyzing a microstructure photograph at a magnification of 300 times and calculating an average value over two visual fields.

Next, means for realizing this will be described. In controlling the morphology of the needle-like structure, as a result of repeated investigations focusing on the nucleation and growth processes, the processing end temperature and cooling rate are very important, specifically, β transus-a temperature higher than 150 ° C. It was found that if the working was terminated at 2.0 ° C./second or less and the cooling was performed at 2.0 ° C./second or less, the acicular α phase generated in the β grains could grow to a shape sufficient to secure fracture toughness.

When processing is completed at a temperature lower than β-transus-150 ° C., the nucleation rate is excessively increased, and
Since the diffusion rate decreases, the α-phase microstructure in the old β grains is not acicular but equiaxed, so that the fracture toughness deteriorates. For this reason, the processing may be terminated in a temperature range higher than β Transus-150 ° C., and more preferably β Transus-10
0 ° C.

With respect to the cooling rate, as described above, there is a correlation between the β transus and the optimum value of the cooling rate. In a component type alloy having a low β transus, the diffusion rate of the element decreases, so that the acicular α grows. It is difficult. In this regard, in the alloys included in the component ranges presented in the present invention, even if the alloy has the lowest β transus, it is 2.0%.
If cooled at a rate of less than ℃ / sec, the needle-shaped α can grow,
Fracture toughness can be secured.

The structural characteristics described above can be achieved at the same time, and it has been found that a titanium alloy provided with both characteristics has an excellent balance between strength characteristics and toughness, and the present invention has been completed. The processing temperature range characteristic of the present invention has been determined as described above, but other details of the present invention will be described below.

The present inventors have found that, in addition to the structure characteristics described above, by appropriately controlling the structure after the aging treatment, it is possible to further improve the strength characteristics, particularly the proof stress. . The yield strength is determined by the form of the precipitated α-phase formed after the aging treatment. The minor axis diameter of the aged precipitated α-phase existing between the acicular α laths is 0.10 μm or less, and the aspect ratio is 5.0 or less. And the total α phase fraction is 40
It has been found that good strength characteristics can be ensured when the structure is formed in excess of%.

The aging precipitation α described in the present invention
The phase refers to an α phase precipitated between needle-like α and having an area of less than 0.5 μm ² . The aspect ratio of the aged precipitated α phase is obtained by image analysis of a microstructure photograph taken at a magnification of 10,000 times and calculating an average value for two or more visual fields.

With respect to the aged precipitate α-phase form, the minor axis diameter is 0.1
If the aspect ratio exceeds 0 μm and the aspect ratio exceeds 5.0, the strength cannot be secured because the amount of precipitation strengthening is insufficient. Also, when the total α-phase fraction does not contain 40% in volume fraction, it becomes difficult to secure good strength for the same reason as described above. The upper limit of the total α-phase fraction is determined by the component system of the titanium alloy, and the present invention does not particularly define the upper limit.

The present inventors have found that the form of the precipitated α can be controlled by the temperature and the holding time of the aging treatment, and that the total α-phase fraction is also determined by the aging temperature. However, not only that, but also affected by the structure morphology and alloy composition after the previous stage processing and solution treatment, the appropriate temperature and time can not be described unconditionally, in the present invention aging treatment Although the conditions are not specified, it is possible to obtain excellent proof stress when the above-mentioned structure is formed.

If the structure morphology proposed in the present invention is realized, a high strength and high toughness α + β type titanium alloy can be obtained, but the mechanical properties are naturally affected by the alloy components. . The following equation determined by the added alloy component: Mo equivalent = Mo% + ／ Ta%
+ 1 / 3.6Nb% + 1 / 1.5V% + 1.25 Cr%
+ 1.25Ni% + 1.7Mn% + 1.7Co% + 2.
It is demonstrated that when an α + β type titanium alloy whose Mo equivalent determined by 5Fe% (% represents mass%) falls within the range of 6.0 to 12.0, that is, a so-called Nearβ titanium alloy, it exhibits extremely excellent properties. The present inventors have found.
As long as the Mo equivalent is in the range of 6.0 to 12.0, the effects of the present invention can be expected without limiting the other components in detail.

Further, the titanium alloy member shown in the present invention is:
Since it exhibits good mechanical properties, it can be applied to uses other than jet engine disc materials. Particularly, it can be effectively used as a material for a golf club head.

[0033]

The present invention will be described below in detail with reference to examples. Representative titanium alloys having the component compositions shown in Table 1 were used as test materials. The test material was melted by a vacuum arc melting furnace, cut into a shape having a diameter of 150 mm and a length of 240 mm, processed in various temperature ranges, and cooled. This processed material was subjected to a solution treatment and an aging treatment, cut out to an appropriate size, polished and etched, and various values were calculated by image analysis from the obtained structure photograph. The results are shown in Table 2. The conditions of the solution treatment and the aging treatment in this example are as shown in Tables 3 and 4. The morphology was controlled by changing the respective cooling start temperatures.

[0034]

[Table 1]

[0035]

[Table 2]

[0036]

[Table 3]

[0037]

[Table 4]

The mechanical properties were evaluated by a tensile test and a fracture toughness test. ASTM E for tensile test
8 and the measurement of fracture toughness was performed according to ASTM E399. Table 5 shows the results of these evaluation tests.

[0039]

[Table 5]

In Table 5, Sample No. Examples 1 to 14 are examples in which the processing is completed in the temperature range suggested in the present invention and air-cooled, and all the structural conditions are satisfied. on the other hand,
Sample No. Nos. 15 to 26 are comparative examples in which any of the processing conditions and the structure constituting conditions do not satisfy the present invention. Here, Comparative Examples 15 and 21 were cooled without processing, and are listed as Comparative Examples in order to evaluate the importance of introducing intragranular α-nucleation sites by processing.

Regarding the strength characteristics, the proof stress was 1100MP.
a and elongation of 8% or more were evaluated as good (○), and fracture toughness of 60 MPa√m or more was evaluated as good (○). Any one of the strength characteristics and the fracture toughness, which was inferior, was evaluated as poor (x). According to the results in Table 5, sample N
o. It can be seen that the samples of Examples 1 to 14 of the present invention have excellent strength properties and fracture toughness in any of the alloy systems as compared with Comparative Examples 15 to 26 having the same components.

[0042]

As described above, the high-strength, high-toughness titanium alloy member according to the present invention is excellent in strength characteristics and fracture toughness, and requires strict mechanical properties as well as jet engine disk materials. Can be applied to applications. Further, according to the method for producing a high-strength, high-toughness titanium alloy member according to the present invention, it is possible to reliably provide a titanium alloy member having excellent strength characteristics and fracture toughness.

Continuation of the front page (51) Int.Cl. ⁷ Identification symbol FI Theme coat II (Reference) C22F 1/00 630 C22F 1/00 630A 630B 673 673 683 683 691 691B 692 692A 694 694B (72) Inventor Hideto Oyama Hyogo 2-3-1, Shinhama, Arai-machi, Takasago City, Kobe Prefecture Inside Kobe Steel, Ltd. Takasago Works (72) Inventor Shinya Ishigai 2-3-1, Shinama, Araimachi, Takasago City, Hyogo Prefecture, Kobe Steel Works, Takasago Works

Claims (4)

[Claims] 1. A titanium alloy comprising an α + β type titanium alloy having a Mo equivalent determined by the following Mo equivalent formula satisfying 6.0 to 12.0, and a grain boundary α structure formed at an old β grain boundary has a volume fraction of 5%. 0.0% or less, and the short axis diameter of the acicular α phase formed in the old β grains is 0.75 μm or more and the aspect ratio is 7.0 or more, high strength, High toughness titanium alloy member. However, Mo equivalent = Mo% + ／ T
a% + 1 / 3.6Nb% + 1 / 1.5V% + 1.25C
r% + 1.25 Ni% + 1.7 Mn% + 1.7Co% +
2.5Fe% (% represents mass%) 2. The aging precipitated α phase existing between the acicular α laths has a minor axis diameter of 0.10 μm or less and an aspect ratio of 5.
2. The high-strength and high-toughness titanium alloy member according to claim 1, wherein the titanium alloy member has a volume fraction of 40% or more in terms of a volume fraction of all α phases. 3. A high-strength, high-toughness titanium alloy member, wherein the high-strength, high-toughness titanium alloy member according to claim 1 or 2 is used as a material for a golf club head. 4. An α + β type titanium alloy whose Mo equivalent obtained by the following Mo equivalent formula satisfies 6.0 to 12.0,
After heating to the β region, the processing is started at an arbitrary temperature in the β region, and the processing is terminated in an arbitrary temperature region of (β transus-150 ° C.) or more and less than β transus, and then 0.2 mm from the end point. After cooling at a cooling rate of ~ 2.0 ° C / sec, α +
A method for producing a high-strength, high-toughness titanium alloy member, comprising performing solution treatment and aging treatment in a β2 phase region. However, Mo
Equivalent = Mo% + ／ Ta% + 1 / 3.6Nb% + 1 /
1.5V% + 1.25Cr% + 1.25Ni% + 1.7
Mn% + 1.7Co% + 2.5Fe% (% represents mass%)