JP3252596B2 - Method for producing high strength and high toughness titanium alloy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength,
The present invention relates to a method for producing a tough and homogeneous titanium alloy.

[0002]

2. Description of the Related Art Titanium alloys are light in weight but have high strength.The specific strength standardized by specific gravity is the highest among metal materials, and it has very good corrosion resistance. At present, it is a metal material that is used as a lightweight and high-strength material in many fields, mainly in the aerospace industry. Recently, however, further weight reduction and higher speeds have been required in the aircraft field. It is expected that a titanium alloy having high strength and high toughness will be required.

In recent years, "Near β-type titanium alloys" have been known which provide a metastable β single-phase structure without martensitic transformation even when rapidly cooled from the β single-phase region. Since it has higher strength and toughness as compared with α + β alloys represented by −4V alloy, it has attracted much attention from the field of aircraft and the like. And this Nearβ
The type alloy has a smaller amount of alloying elements and lower hot deformation resistance than the β type alloy, so it has excellent forgeability, and the conventional Ti
It is expected to replace -6Al-4V alloy.

[0004] A commercially available Near β has been developed so far.
Titanium alloys such as Ti-10V-2Fe-3Al alloy and Ti-
5Al-2Sn-2Zr-4Mo-4Cr alloy and the like,
It is known that the metastable β phase of these Near β type alloys contains a large amount of ω phase and exhibits stress-induced transformation in which martensitic transformation occurs by processing. Near β type titanium alloy is a material whose strength is improved by precipitation hardening when subjected to aging treatment after processing.However, in the case of low-temperature aging or short-time aging, an aging ω phase is generated, It is also known that, after aging for a long time, a structure in which an α phase finally precipitates in a β phase which is a parent phase.

[0005] The strengthening mechanism of the near β type alloy as described above is that “a fine α phase precipitates in the β phase by solution treatment in the two phase region below the β transformation point and subsequent aging”. The measures to improve the properties of Near β type titanium alloys that have been adopted so far are mainly to control the strengthening mechanism to further improve the properties of high strength and high toughness which are considered to be the characteristics of this alloy system. Was something.

[0006] For example, referring to AMS standard 4983, as a means for securing the standard characteristics of the Near β type titanium alloy, the temperature was maintained in a temperature range of [[β transformation point -40 ° C] to [β transformation point -15 ° C] for 30 minutes or more. A "two-step solution treatment method" is disclosed in which the solution is cooled to room temperature by furnace cooling or air cooling, and then further heated and held in the same manner, and then water-cooled, cooled to room temperature and aged. JP-A-63-105954 discloses that “Near β type titanium alloy is cooled while cooling from β range.
By processing in the + β region and then reheating to the α + β region, high strength and toughness can be secured without causing anisotropy in fracture toughness. ” Further, Japanese Patent Application Laid-Open No. 2-217452 discloses "Near
Applying “first-stage solution treatment performed at high temperature of α + β” and “second-stage solution treatment performed at lower temperature than this” to β-type titanium alloy, followed by low-temperature aging A method of ensuring good fracture toughness together with high strength "has been disclosed.

[0007] However, despite several proposals for improving the strength and fracture toughness of the Near β type titanium alloy as described above, titanium is said to "increase in strength and conversely decrease fracture toughness". Due to the common characteristics of alloys, it is difficult to improve both strength and fracture toughness sufficiently by the means proposed so far, and it is also satisfactory from the viewpoint of "elongation" and "homogeneity of properties". It was difficult to obtain an alloy.

[0008] In view of the above, an object of the present invention is to improve the excellent properties of the near β type titanium alloy such as high strength and high toughness in a well-balanced manner and to obtain a homogeneous titanium alloy exhibiting sufficient elongation. Alloy, specifically 0.2% proof stress: 110 kgf / mm ² or more, fracture toughness: 180 kgf / mm ^3/2
Accordingly, it is an object of the present invention to provide a homogeneous titanium alloy having an elongation of 10% or more.

[0009]

Means for Solving the Problems Accordingly, the present invention has made intensive studies to achieve the above object, and has obtained the following findings. That is, in the near β type titanium alloy, a structure composed of a proeutectic α phase and a β phase is formed in a state where processing is performed in the α + β region, which is a general process. When a certain “solution treatment in the α + β region” is performed, the strength and fracture toughness increase with an increase in temperature, but if the temperature is too close to the β transformation point or exceeds the β transformation point, the ductility significantly decreases. However, the reliability as an industrial material is greatly reduced.

[0010] This is because, as the solution treatment temperature approaches the β transformation point, a phenomenon occurs in which sub-grains of the β phase generated by the solution treatment partially abnormally coarsen. This is because a single-phase structure composed of very coarse β grains is generated when the temperature exceeds the transformation point. Therefore, in order to sufficiently improve both strength and fracture toughness while securing sufficient ductility, it is important to realize a structure in which subgrains are uniformly coarsened to such an extent that ductility does not decrease significantly. Is a constant temperature forging in the lower temperature range that is not too close to the β transformation point for the Near β type titanium alloy, and at the highest possible temperature within that range, and then the solution is forged around the forging temperature in the regulated temperature range. It is very effective to further perform the aging treatment after the aging treatment.

The present invention has been completed as a result of further research based on the above findings and the like. "The β transformation point of -60 ° C. to [β transformation point of -10 ° C.] ] In a temperature range of 30% or more, and then within this temperature range and at a constant temperature forging temperature of -20 ° C.
Solution treatment is performed for 30 minutes or more in the temperature range that does not deviate from the range of [constant temperature forging temperature + 20 ° C], and then 400 to 60
0.2% by aging at 0 ℃ for more than 30 minutes
Strength: 110 kgf / mm ² or more, elongation: 10% or more, fracture toughness: 180 kgf / mm ^3/2 or more. are doing.

The “Near β type titanium alloy” is
It is needless to say that among alloys in which the α phase does not precipitate and the β phase remains when rapidly cooled from the β region to room temperature (β-type titanium alloy), the titanium alloy generates the ω phase.
V-2Fe-3Al alloy, Ti-17V alloy, Ti-5Al-2Sn-
2Zr-4Mo-4Cr alloy, Ti-11.5V-2Al-2Sn-11Zr
Alloys, Ti-12V-2.3 Al-2Sn-6Zr alloys, and the like.

As the form of the Near β type titanium alloy to be subjected to the constant temperature forging, it is desirable to use a billet, a slab, or the like formed by forging in the α + β region. This is because it is difficult to obtain a homogeneous and fine structure in the β forging material unless the working degree in the constant temperature forging is sufficiently secured. Of course,
If there is no problem in increasing the degree of working in constant temperature forging, it is possible to use a β forging material, and in this case, it is thought that the toughness will be improved.
It is not preferable because problems such as an increase in anisotropy of toughness appear.

Hereinafter, the reason why the manufacturing conditions of the high-strength and toughness titanium alloy in the present invention are limited as described above will be described in detail together with the operation thereof.

[Action]

A) Constant Temperature Forging Conditions In the present invention, the reason why the processing of the Near β type titanium alloy is particularly “constant temperature forging” is as follows. In other words, in "normal forging", the structure is inhomogeneous due to non-uniform temperature and the dispersion of mechanical properties such as strength, toughness and elongation is large, whereas in "constant temperature forging", the surface of the workpiece is This is because both the center and the center are processed at the same temperature, so that the homogeneity of the structure is improved. Further, rolling or extrusion is not suitable because the structure tends to be layered and the anisotropy of mechanical properties increases. Furthermore, the present invention is characterized in that the isothermal forging and the solution treatment are performed at substantially the same temperature. However, by performing the isothermal forging, the alloy structure can approach the structure formed by the solution treatment, and the uniform structure can be obtained. Another advantage is that it is advantageous in improving the properties (ie, the homogeneity of the mechanical properties).

[0015] The reason for setting the temperature of the constant temperature forging to [β transformation point -60 ° C] to [β transformation point -10 ° C] is as follows. That is, if the temperature is higher than [β transformation point −10 ° C.], the sub-grains become too coarse in the α + β region, and if the temperature is in the β region, the sub-grains disappear and a very coarse β single-phase structure is formed. Even so, the ductility of the material is significantly reduced. On the other hand, the constant temperature forging temperature is [β transformation point -60
C.], the growth of sub-grains becomes insufficient, and the strength and toughness of the material decrease. Preferably,
The constant temperature forging temperature is [β transformation point -50 ° C.] to [β transformation point −1
5 ° C.]. On the other hand, if the degree of working in the constant temperature forging is less than 30%, the development of the sub-grain of the β phase is insufficient, and the structure of the material becomes inhomogeneous, so that the material cannot have high fracture toughness. Was limited to 30% or more.

B) Solution Treatment Conditions In the present invention, the solution treatment is performed at a temperature within the constant temperature forging temperature range and within the range of [constant temperature forging temperature ± 20 ° C.] following the constant temperature forging. The most important point of the present invention is that the heat treatment is performed at substantially the same temperature as that of the constant temperature forging.
That is, in the isothermal forging, the material is maintained at a uniform temperature for a longer time than in the normal forging, and thus the structure after the isothermal forging is in a stable state at that temperature. If the temperature during the next solution treatment is in the range of [constant temperature forging temperature −20 ° C.] to [constant temperature forging temperature + 20 ° C.], the structure generated by constant temperature forging is stably maintained even in this solution treatment state. As a result, good homogeneity of the tissue is obtained, so that a material having very high homogeneity in mechanical properties is realized.

Here, when the solution treatment temperature exceeds the β transformation point, a coarse β single phase structure is formed and the ductility of the material is significantly reduced, but the solution treatment temperature is higher than [constant temperature forging temperature + 20 ° C.]. Even in this case, the diffusion of the alloy element occurs, and the structure generated by the isothermal forging changes. Then, remarkable coarsening of the sub-grains occurs. At this time, since the coarsening occurs inhomogeneously, the tissue becomes inhomogeneous, and the ductility decreases. On the other hand, the solution treatment temperature is [constant temperature forging temperature -20.
C]], the α phase increases in the structure formed by the isothermal forging, and the β phase supporting strength and fracture toughness is undesirably reduced. Therefore, the solution treatment temperature is preferably basically the same as the isothermal forging temperature. If the solution heat treatment time is less than 30 minutes, a homogeneous α + β structure is not obtained in the solution heat treatment state, and the mechanical properties of the material become inhomogeneous.

C) Aging treatment condition Lastly, the aging treatment is performed.
At 600 ° C., the processing time is 30 minutes or more. Because
If the aging treatment temperature is less than 400 ° C., high strength is obtained, but precipitation of the aged α phase that contributes to fracture toughness and ductility does not sufficiently occur, and if it exceeds 600 ° C., the aged precipitation α phase becomes coarse and high. This is because strength cannot be obtained. Further, if the aging time is less than 30 minutes, the precipitation of the aging α phase is inhomogeneous, and the mechanical properties of the material are also inhomogeneous. Therefore, the aging time needs to be 30 minutes or more. The specific aging treatment temperature and treatment time may be set according to the alloy composition and the solution treatment temperature.

Next, the effects of the present invention will be described more specifically with reference to examples.

EXAMPLES First, Ti-10 obtained by double vacuum arc melting was used.
A V-2Fe-3Al alloy ingot (diameter 420 mm) was heated to the β temperature range (the β transformation point of this alloy was about 800 ° C.), and after β forging into a 200 mm diameter round bar, Further, the slab was heated to 750 ° C. in the α + β region, and a slab having a thickness of 100 mm and a width of 100 mm was obtained by forging.

Next, the slab is 150 mm long × 1 width wide.
A plurality of blocks having a size of 00 mm x a thickness of 100 mm were sampled, subjected to constant temperature forging under the conditions shown in Table 1, held at a solution treatment temperature shown in Table 1 for 2 hours, and then subjected to a solution treatment of water cooling. After maintaining at the indicated aging temperature for 8 hours, an aging treatment of air cooling was performed.

[0021]

[Table 1]

Next, from each material after the aging treatment, a diameter of 6.25 was obtained.
A round bar tensile test specimen having a length of 32 mm and a parallel portion of 32 mm was sampled from a direction perpendicular to the forging direction, and a hand was used for measuring fracture toughness.
Full-size CT specimens were taken such that the crack surface was the same as the thickness direction and the cracks propagated perpendicular to the forging direction, and each was subjected to a tensile test and a fracture toughness test.
In the tensile test conducted at room temperature, the strain rate was 0.5% / min up to 0.2% proof stress, and 15% / min after 0.2% proof stress.
On the other hand, the fracture toughness tests were performed in accordance with ASTM-E399. The test results are shown in Table 1.

As is clear from the results shown in Table 1, when the conditions specified in the present invention are followed, a Near β type titanium alloy material having an excellent balance of strength, elongation and fracture toughness can be obtained stably. On the other hand, if the manufacturing conditions do not satisfy the requirements of the present invention, at least one of strength, elongation and fracture toughness is inferior, and it can be seen that a sufficiently satisfactory material cannot be obtained.

[0024]

[Summary of Effects] As described above, according to the present invention, it is possible to manufacture a homogeneous Near β type titanium alloy member having excellent strength, elongation, and fracture toughness, and to achieve particularly high reliability for aircraft and the like. Titanium alloy material that can meet the strict demands of the required field can be supplied stably,
An industrially useful effect is provided.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP 4-57734 (JP, B2) Materials and Processes, Vol. 2, No. 2, p. 326 (1989) Materials and Processes, Vol. 3, No. 5, p. 1718 (1990) (58) Field surveyed (Int. Cl. ⁷ , DB name) C22F 1/18 B21J 5/00

Claims (1)

(57) [Claims] [1] A β-transformation point-
60 ° C.] to [β transformation point-10 ° C.]
After performing the constant temperature forging of 0% or more, it is within this temperature range and [the constant temperature forging temperature -20 ° C] to [the constant temperature forging temperature +2].
0 ° C.] in a temperature range that does not deviate from the range of 30 ° C. or more, followed by aging treatment at 400 to 600 ° C. for 30 minutes or more. Production method.