JP2017002390A - Titanium alloy forging material - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a titanium alloy forging material excellent in strength and ductility and excellent in homogeneity of mechanical strength.SOLUTION: There is provided a titanium alloy forging material consisting of a titanium alloy having Mo equivalent [Mo]represented by the following formula of 10 or more and less than 13, where content of an element X (mass%) is [X], [Mo]=[Mo]+[Ta]/5+[Nb]/3.6+[W]/2.5+[V]/1.5+1.25[Cr]+1.25[Ni]+1.7[Mn]+1.7[Co]+2.5[Fe] and having a particle diameter of 5 μm or more, area percentage of a primary α phase with an aspect ratio of 2 or more of 1% or less, area percentage of a primary α phase with a particle diameter of 0.5 μm or more of over 0% and 20% or less and average spacing of a secondary α phase of 100 to 200 nm.SELECTED DRAWING: None

Description

  The present invention relates to a titanium alloy forging, and more particularly to a near β-type titanium alloy forging.

  Aircraft parts are required to have high ductility, high toughness, etc. in addition to being lightweight and high in strength, so α + β type titanium alloys and near β type titanium alloys are often used. . α + β-type titanium alloy has excellent balance of strength, ductility, etc. because the α phase of dense hexagonal crystal (hcp structure), which is the main phase, and β phase of body-centered cubic crystal (bcc structure) coexist stably at room temperature. Moreover, it becomes a β phase single phase in a temperature range equal to or higher than the β transformation point (Tβ). The near β-type titanium alloy has an intermediate metal structure between the α + β-type titanium alloy and the high-strength β-type titanium alloy, and the α-phase and the β-phase coexist in the same manner as the α + β-type titanium alloy. These titanium alloy forgings include α + β forging that heats and forges to a temperature range below Tβ (α + β two-phase region) so as not to reach a temperature above Tβ, and a temperature range above Tβ (β single unit). There is a thing by the β forging which heats and forges to a phase region. It is known that the α + β forged material and the β forged material have completely different material structures and have different material properties.

  Among titanium alloys, Ti-10V-2Fe-3Al alloy is known as a high-strength near β-type titanium alloy. In order to further improve the properties of the Ti-10V-2Fe-3Al alloy, several improved techniques have been developed. For example, Patent Document 1 discloses a manufacturing method for obtaining a near β-type titanium alloy having excellent strength, toughness, and ductility. Patent Document 2 discloses a production method for obtaining a near β-type titanium alloy having excellent strength and toughness.

Japanese Patent No. 3252596 Japanese Patent No. 3334354

  However, aircraft parts are required to be further improved in strength, ductility and the like. The manufacturing methods disclosed in Patent Document 1 and Patent Document 2 still have room for improvement in strength and ductility, and have large variations in characteristics and insufficient reproducibility.

  This invention is made | formed in view of the said problem, and it aims at providing the titanium alloy forging material excellent in intensity | strength and ductility, and excellent in the homogeneity of mechanical strength.

  As a result of diligent research, the present inventors have adopted a forging method by α + β forging, and as a metal structure, the above-mentioned problems can be solved by making the shape and area ratio of the α phase have specific requirements. It was found and the present invention could be reached.

That is, in the titanium alloy forging according to the present invention, when the content (mass%) of the element X is [X], the Mo equivalent [Mo] _eq represented by the following formula (1) is 10 or more and less than 13 A titanium alloy forging made of a titanium alloy,
[Mo] _eq = [Mo] + [Ta] / 5 + [Nb] /3.6+ [W] /2.5+ [V] /1.5+1.25 [Cr] +1.25 [Ni] +1.7 [ Mn] +1.7 [Co] +2.5 [Fe] (1)
The area ratio of the primary α phase having a particle diameter of 5 μm or more and an aspect ratio of 2 or more is 1% or less, the area ratio of the primary α phase having a particle diameter of 0.5 μm or more is more than 0% and 20% or less, The average interval of the next α phase is assumed to be 100 to 200 nm.

  The titanium alloy forged material having such a structure is one in which the α phase is present in a specific manner in the β phase, and is excellent in strength, ductility, and homogeneity thereof.

  In the titanium alloy forging according to the present invention, the titanium alloy contains V: 9.0 to 11.0% by mass, Al: 2.6 to 3.4% by mass, Fe: 1.6 to 2.22. It is preferable that it contains mass% and the balance is Ti and inevitable impurities. The titanium alloy forged material having such a structure is excellent in strength and ductility.

  The titanium alloy forged material according to the present invention has excellent strength and ductility, and excellent mechanical strength homogeneity.

It is a schematic diagram which shows the method of calculating the average space | interval of a secondary alpha phase.

Hereinafter, embodiments of the present invention will be described in detail.
[Titanium alloy forging]
The titanium alloy forging according to the present invention is a titanium alloy forging that can be used for aircraft parts and the like, and has high strength, high ductility and homogeneous mechanical properties by controlling the metal structure by forging or heat treatment. It has composition which has.

In the titanium alloy forging according to the present invention, when the content (mass%) of the element X is [X], the Mo (molybdenum) equivalent [Mo] _eq represented by the following formula (1) is 10 or more and 13 It is a titanium alloy forging material made of a titanium alloy that is less than.
[Mo] _eq = [Mo] + [Ta] / 5 + [Nb] /3.6+ [W] /2.5+ [V] /1.5+1.25 [Cr] +1.25 [Ni] +1.7 [ Mn] +1.7 [Co] +2.5 [Fe] (1)
Mo equivalent is generally used as an index indicating the stability of each phase of the titanium alloy. For details of Mo equivalent, see G. Lutjering & JC Williams, "Titanium", Second Edition, Springer-Verlag, Berlin, 2010, p30 or Furuhara, Maki, Metal, vol.66 (1996), No.4, p289, etc. Is explained.
The Mo equivalent must have a value of 10 or more, more preferably 10.5 or more, in order to ensure strength. On the other hand, in order to make hot forgeability and ductility favorable, it is necessary to control to less than 13, More preferably, it is 12.5 or less.

[Titanium alloy]
There is a Ti-10V-2Fe-3Al alloy defined in AMS4984 as a titanium alloy satisfying the above-mentioned definition of Mo equivalent. The alloy composition of Ti-10V-2Fe-3Al alloy contains V: 9.0 to 11.0 mass%, Al: 2.6 to 3.4 mass%, Fe: 1.6 to 2.22 mass% The balance is Ti and inevitable impurities. As unavoidable impurities, for example, C: 0.05 mass% or less, N: 0.05 mass% or less, O: 0.13 mass% or less, H: 0.015 mass% or less, Y: 0.005 mass % Or less. Here, the Mo equivalent is calculated as a content of 0 for an element not contained in the Ti-10V-2Fe-3Al alloy in the formula (1).

  In the case of Ti-10V-2Fe-3Al alloy, V: 9.0% by mass or more and Fe: 1.6% by mass or more are necessary for solid solution strengthening of β phase and stabilization of β phase. In order to strengthen the solid solution strengthening of the phase and stabilize the α phase, Al: 2.6% by mass or more is necessary. Moreover, since excessive addition may impair hot forgeability and ductility, it controls to V: 11.0 mass% or less, Al: 3.4 mass% or less, and Fe: 2.22 mass% or less. In addition, if the inevitable impurities increase, the material may become brittle, so the upper limit is controlled as described above.

  Other examples of the titanium alloy having an Mo equivalent of 10 or more and less than 13 include Ti-5Al-5V-5Mo-3Cr alloy.

[Metal structure]
The metal structure of the titanium alloy is generally composed of an α phase and a β phase, and the α phase is further classified into a primary α phase and a secondary α phase. The metal structure of the forged titanium alloy of the present invention has a particle size of 5 μm or more, an area ratio of primary α phase of 2 or more in aspect ratio of 1% or less, and an area ratio of primary α phase of particle diameter of 0.5 μm or more. It is more than 0% and not more than 20%, and the average interval between secondary α phases is 100 to 200 nm. Hereinafter, each characteristic will be sequentially described.

  The metal structure of the titanium alloy forged material of the present invention substantially consists of an α phase and a β phase. The α phase is composed of a primary α phase and a secondary α phase. The secondary α phase is an α phase precipitated in the aging process, and the primary α phase is an α phase other than the secondary α phase. The structure other than the α phase and the β phase may contain a small amount of carbides, inclusions, and the like.

  When the area ratio of the primary α phase (hereinafter sometimes referred to as “coarse α phase”) having a particle size of 5 μm or more and an aspect ratio of 2 or more is present in a titanium alloy metal structure exceeding 1%, The elongation may decrease or the numerical value of the elongation may vary. This is presumably because when the α phase particle size is coarse and the aspect ratio is large, strain tends to concentrate on the primary α phase and ductility tends to decrease. Therefore, the metal structure of the forged titanium alloy of the present invention controls the area ratio of the coarse α phase to 1% or less. The area ratio of the coarse α phase is preferably 0.9% or less. Here, the coarse α phase is intended only for the primary α phase and not the secondary α phase. The area ratio of the coarse α phase can be measured by image analysis of an optical micrograph. As a method of controlling the area ratio of the coarse α phase to 1% or less, there is a method of performing forging under predetermined conditions, details of which will be described later.

  When the area ratio of the primary α phase of the titanium alloy having a particle size of 0.5 μm or more is 20% or less, the strength can be increased while ensuring ductility. In order to ensure ductility, a certain amount of the primary α phase is necessary, but if the area ratio exceeds 20%, the strength is lowered. Further, when the α phase is 0%, it is difficult to ensure ductility. Therefore, the metal structure of the titanium alloy forged material of the present invention controls the area ratio of the primary α phase having a particle size of 0.5 μm or more to more than 0% and 20% or less. The area ratio of the primary α phase having a particle size of 0.5 μm or more is preferably 3% or more, less than 15%, and more preferably less than 10%. The area ratio of the primary α phase can be measured by image analysis of an optical micrograph. As a method for controlling the area ratio of the primary α phase, there is a method in which the temperature of the solution treatment is performed under a predetermined condition, details of which will be described later. The area ratio of the primary α phase with a particle size of 0.5 μm or more is counted for the primary α phase with a particle size of 5 μm or more and an aspect ratio of 2 or more, and the primary particle size of 5 μm is independent of the aspect ratio. α phase is counted.

  When the average microstructure of the secondary α phase is 200 nm or less, the metal structure of the titanium alloy can increase the strength while ensuring ductility. On the other hand, when the average interval of the secondary α phase is as small as less than 100 nm, there is a concern that the material becomes brittle. Therefore, the metal structure of the titanium alloy forged material of the present invention controls the average interval of the secondary α phase to 100 to 200 nm. The average interval between secondary α phases is preferably 110 to 190 nm. The average interval between secondary α phases can be calculated as described later with reference to SEM photographs. As a method for controlling the average interval of the secondary α phase, there is a method in which the cooling rate after the solution treatment and the aging treatment temperature are performed under predetermined conditions, details of which will be described later.

[Production method of titanium alloy forging]
The titanium alloy forging material having the above-described metal structure can be manufactured by applying the method for manufacturing a titanium alloy forging material described below. The method for producing a titanium alloy forged material of the present invention is characterized in that processing is performed under the specific processing conditions described below in each of the forging step, the solution treatment step, and the aging step.

  The titanium alloy forged material according to the present invention is manufactured into a desired product shape by forging an ingot made of a titanium alloy having the above composition into a billet under the following conditions, followed by solution treatment and aging treatment. In addition, about a manufacturing process and manufacturing conditions other than the manufacturing conditions described below, a titanium alloy forging material can be obtained by performing well-known conditions suitably.

(Forging process)
In the forging process, forging is performed by heating to the β transformation region, and the shape as a forging material is adjusted. Thereafter, the steel is heated to the α + β temperature region and forged so that the equivalent strain amount in the α + β region becomes 2 to 10 (hereinafter, the accumulated equivalent strain amount is referred to as “accumulated strain amount ε”). . By increasing the cumulative strain amount ε to 2 or more, the area ratio of the coarse α phase can be reduced to 1% or less. In order to increase the cumulative strain amount ε, heating and forging may be repeated in a plurality of times. The cumulative strain amount ε may exceed 10, but is set to 2 to 10 because the effect is saturated. The cumulative strain amount ε is preferably 3 to 9. In order to set the cumulative strain amount ε to 2, for example, when uniform deformation is assumed by uniaxial compression, the rolling reduction (working degree) may be about 86%. Similarly, in order to set the cumulative strain amount ε to 10, the reduction rate may be approximately 100%. Here, the cumulative strain amount ε was calculated as ε = ln (t0 / t), to: test piece height before compression, and t: test piece height after compression.

  On the other hand, since the area ratio of the coarse α phase increases as the heating time increases, the total cumulative heating time (holding time at 700 ° C. or higher) is controlled to 90 hours or shorter. The cumulative heating time is preferably 80 hr or less. Thus, forging under the condition that the cumulative strain amount ε is as large as possible and the cumulative heating time is as small as possible, that is, while increasing the cumulative strain amount ε while shortening the cumulative heating time, It is important to suppress the area ratio of the coarse α phase to 1% or less and ensure the elongation as a titanium alloy forged material.

  Here, the equivalent strain amount is also referred to as an equivalent plastic strain amount. The amount of equivalent strain in the α + β region at the specimen collection position can be measured by analysis using commercially available FEM analysis software (for example, analysis software “FORGE” manufactured by TRANSVALOR). Similarly, the accumulated strain amount ε can be measured by analyzing the accumulated equivalent plastic strain amount when forging is performed a plurality of times using commercially available FEM analysis software.

(Solution process)
After forging, a solution treatment is performed by heating and holding in a temperature range of (Tβ-60) ° C. to (Tβ-10) ° C. Here, Tβ is a β transformation point. After maintaining at a predetermined temperature, cooling is performed by controlling the cooling rate to 0.5 ° C./sec to 20 ° C./sec. The holding time of the solution treatment is 0.5 to 5 hours.

  By maintaining the heating temperature in the temperature range of (Tβ-60) ° C. to (Tβ-10) ° C., the area ratio of the primary α phase having a particle size of 0.5 μm or more can be appropriately controlled. That is, in order to prevent the disappearance of the primary α phase having a particle size of 0.5 μm or more (area ratio 0%), the heating temperature is set to (Tβ−10) ° C. or lower. Further, in order to set the area ratio of the primary α phase having a particle size of 0.5 μm or more to 20% or less, the heating temperature is set to (Tβ-60) ° C. or more. The heating time is preferably 1.5 to 4 hours. Moreover, the cooling rate after heating and holding is controlled to 0.5 ° C./sec to 20 ° C./sec. Furthermore, when the temperature of the solution treatment is T (° C.) and the treatment time is t (minutes), the heat treatment parameter T × logt is preferably less than 1400.

(Aging process)
After cooling in the solution treatment step, aging treatment is performed by heating and holding in a temperature range of 480 ° C to 520 ° C. The aging treatment temperature is preferably 485 to 515 ° C. The holding time of the aging treatment is 4 to 12 hours.

  By appropriately performing both the cooling rate after heating and holding in the solution treatment step and the heating and holding in the temperature range of 480 ° C. to 520 ° C. in the aging step, the average interval of the secondary α phase is controlled to an appropriate interval. be able to. That is, when the cooling rate after solution treatment is higher than 20 ° C./sec or when the aging treatment temperature is less than 480 ° C., the average interval between secondary α phases tends to be less than 100 nm. On the other hand, when the cooling rate after solution treatment is slower than 0.5 ° C./sec or when the aging treatment temperature exceeds 520 ° C., the average interval between secondary α phases tends to exceed 200 nm.

  Examples in which the effects of the present invention have been confirmed will be specifically described below in comparison with comparative examples that do not satisfy the requirements of the present invention. In addition, this invention is not limited to a following example.

[Production of test materials]
Using a billet made of a Ti-10V-2Fe-3Al alloy (Tβ: 810 ° C., Mo equivalent of 11.7) defined by AMS 4984, after forging at a temperature of 810 ° C. or higher of the β transformation point, listed in Table 1 Under each condition, heating holding and hot working assuming finish forging in the α + β region were performed. Table 1 shows the heating temperature, cumulative strain amount ε, and cumulative heating time in finish forging in the α + β region.

  Using the forged material, solution treatment and aging treatment were performed under the conditions shown in Table 1. The temperature increase rate of the solution treatment was about 0.2 ° C./sec. After heating and holding the solution, the cooling rate was changed as shown in Table 1 by adjusting the cooling method (water cooling, air cooling, air cooling) and the specimen collection position. The cooling rate was measured by inserting a thermocouple at the same depth as the specimen collection position. On the other hand, in the aging treatment, after heating and holding at a predetermined temperature, all were cooled to room temperature by air cooling. The time for solution heat treatment and aging treatment was the time after placing in the furnace at the heating temperature shown in the table.

[Evaluation of test material]
About the obtained test material, various physical properties were measured and evaluated under the evaluation conditions described below.
(Tensile test)
The test piece was collected so that the forging direction and the load axis direction of the tensile test piece were parallel. Four test pieces were collected per condition. At this time, each test piece was sampled from a position where the distance from the forged material surface to the test piece sampling position was equivalent in order to equalize the strain amount and the cooling rate at the four test piece positions. The specimen size was ASTM E8 Specimen2. From the test results, a sample having a 0.2% proof stress exceeding 1050 MPa, an elongation exceeding 10%, and an elongation standard deviation of 1.5 or less was regarded as acceptable.

(Tissue observation)
(1) Average interval between secondary α phases Immediately after the tensile specimen is collected so that a cross-section parallel to the L direction of the forged material (which can be identified by the direction of β crystal grain extension when observed with an optical microscope) can be observed. A block for tissue observation was cut out from the adjacent location. Resin embedding, polishing, and corrosion (fluoric nitric acid solution) were performed to obtain a sample for SEM observation. Thereafter, observation was performed at a magnification of 400 times, and the grains determined to have an equivalent circle diameter of 0.5 μm or more were defined as the primary α phase, and the regions of less than 0.5 μm were defined as other regions such as the secondary α phase and β phase. Magnified photograph (magnification) using FE-SEM (Field Emission Scanning Electron Microscope) (Hitachi, Ltd., SU-70) in the region containing only β phase and secondary α phase (region not including primary α phase) 30,000 times) was obtained per condition. Based on the photograph, five line segments were drawn at equal intervals from the end of the photograph in the horizontal and vertical directions, and the points where the line segments intersected the secondary α phase were counted. Thereafter, the average interval of the secondary α phase was calculated from (total length of line segments) ÷ (total number of counts). FIG. 1 is a schematic diagram showing a method for calculating the average interval between secondary α phases. The intersection points X1 to X5 between the line segment 1 and the secondary α phase P were counted. In addition, in FIG. 1, the state about one of the line segments drawn five is shown.

  In rare cases, there may be an extremely fine α phase or an extremely fine β phase region at the time of measurement. However, the α phase / β whose equivalent circle diameter is counted as 5 nm or more by image analysis software. Phases were counted. Here, in calculating the equivalent circle diameter, in the case of a secondary α phase passing through the intersections X4 and X5 in FIG. 1, the region of the β phase (white) included in the secondary α phase has an equivalent circle diameter. Not calculated (ie, the equivalent circle diameter was determined from the area of the black region only)

(2) Measurement of area ratio of coarse α phase The forged material sample was observed with an optical microscope (OLYMPUS, GX71). In the observation, 10 photographs were taken at random with 400 times magnification under each condition. Thereafter, the particle size and aspect ratio of each primary α phase contained in 10 photographs were determined by image analysis (image analysis software; Image-Pro Plus, manufactured by Nippon Roper). The primary α phase is gradually constricted (dented) and further divided by forging and heat treatment, but the primary α phases that overlap each other even if constricted are counted as one primary α phase. Whether or not the area ratio of the primary α phase having a particle diameter of 5 μm or more and an aspect ratio of 2 or more was 1% or less was determined by the image analysis described above.

(3) Measurement of area ratio of primary α-phase The area ratio of the primary α-phase was determined by image analysis (image analysis software; Image-Pro Plus, manufactured by Nippon Roper Co., Ltd.) for the 400 × optical micrograph. The primary α phase was targeted for those with an equivalent circle diameter of 0.5 μm or more. Whether or not the area ratio of the primary α phase having a particle diameter of 0.5 μm or more exceeds 0% and is 20% or less was determined by the image analysis described above.
As described above, the primary α phase has an equivalent circle diameter of 0.5 μm or more in an optical micrograph at a magnification of 400 times.

In an actual forging product, at the thickest place of the forged product (that is, the place where the maximum inscribed circle is drawn), the surface layer side of the inscribed circle (position 10mm to 20mm deep from the outermost layer of the test piece) And the central part must be evaluated and both must satisfy the provisions of the present invention. That is, since the mechanical characteristics and material structure may vary from the surface layer side to the center of the maximum inscribed circle, the surface layer side and the center part are evaluated and judged.
The evaluation results are shown in Table 2.

Test material No. 1-2 and 8-10 satisfy the Mo equivalent of the present invention, and are produced using the production conditions described above. This satisfied the provisions of the present invention, and was excellent in strength, ductility, and homogeneity of mechanical strength.
Test material No. In No. 3, the cumulative strain amount is as small as 0.3 in the forging process in the α + β region, so that the area ratio of the coarse α phase exceeds 1%, the elongation is reduced, and the standard deviation of the elongation is large. became.
Test material No. No. 4 in the forging process in the α + β region, the cumulative strain amount is as small as 0.3 and the cumulative heating time is longer than 90 hours. Even larger than 3, the elongation was reduced, and the standard deviation of the elongation was further increased.

Test material No. In No. 5, since the heating temperature in the solution treatment was less than (Tβ-60) ° C., the area ratio of the primary α phase exceeded 20% and the yield strength was inferior in 0.2%.
Test material No. In No. 6, the cooling rate was slower than 0.5 ° C./sec, and the aging treatment was performed at a temperature exceeding 520 ° C., so the average interval between the secondary α phases exceeded 200 nm, and the 0.2% yield strength was inferior. It was.
Test material No. In No. 7, since the cooling rate was faster than 20 ° C./sec and the aging treatment was performed at a temperature of less than 480 ° C., the average interval between the secondary α phases was less than 100 nm, and the elongation was inferior.

P Secondary α phase X1, X2, X3, X4, X5 Intersection l Line segment

Claims (2)

When the content (mass%) of the element X is [X], it is a titanium alloy forging made of a titanium alloy having a Mo equivalent [Mo] _eq represented by the following formula (1) of 10 or more and less than 13. And
[Mo] _eq = [Mo] + [Ta] / 5 + [Nb] /3.6+ [W] /2.5+ [V] /1.5+1.25 [Cr] +1.25 [Ni] +1.7 [ Mn] +1.7 [Co] +2.5 [Fe] (1)
The area ratio of the primary α phase having a particle size of 5 μm or more and an aspect ratio of 2 or more is 1% or less,
The area ratio of the primary α phase having a particle size of 0.5 μm or more is more than 0% and 20% or less,
A titanium alloy forging material, wherein the average interval between secondary α phases is 100 to 200 nm.   The titanium alloy contains V: 9.0 to 11.0% by mass, Al: 2.6 to 3.4% by mass, Fe: 1.6 to 2.22% by mass, the balance being Ti and inevitable The titanium alloy forging material according to claim 1, which is an impurity.

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