JP6696202B2 - α + β type titanium alloy member and manufacturing method thereof - Google Patents

Description

  The present invention relates to an α + β type titanium alloy member and a method for manufacturing the same.

  Titanium alloys are used in various fields because they are lightweight, have high strength, and have excellent corrosion resistance. In particular, Ti-6Al-4V alloy (Ti alloy containing 6 mass% Al and 4 mass%), which is the most general-purpose alloy, has been used in the aerospace field for a long time, and in recent years, it has also begun to be applied to the automobile field and the like. ..

  On the other hand, titanium alloys such as Ti-6Al-4V alloys are poor in workability and machinability and difficult to process, and parts having a complicated shape have to be manufactured by shaving, resulting in low yield. Due to this, there is a problem that the manufacturing cost is high.

  One of the methods to solve this problem is the superplastic processing method that utilizes the transformation superplasticity phenomenon caused by the phase transformation or the fine crystal grain (structure) superplasticity phenomenon caused by the fine crystal grains. Be done.

  Superplasticity is a property in which a metal material exhibits a breaking elongation of several 100 to several 1000% without necking while maintaining a low flow stress when the material is processed under a specific condition. The superplastic working method utilizes this property to precisely plastically work a part having a complicated shape.

However, in order to process a Ti-6Al-4V alloy by a superplastic working method, it is generally necessary to select a working condition of a high temperature of about 900 ° C. or higher and a low strain rate of 1 × 10 ⁻³ s ⁻¹ or lower. .. Therefore, there are still many problems such as shortening the life of the mold used for superplastic working and low productivity.

  Further, the material to be subjected to superplastic working also needs to have a fine equiaxed structure of 5 to 10 μm, but it is difficult to obtain this fine equiaxed structure in the Ti-6Al-4V alloy, which is a factor that reduces productivity. Has become one of.

  Further, in the Ti-6Al-4V alloy, since expensive V is used as the β-phase stabilizing element, there is a problem that the material cost is high.

  Patent Document 1 contains, by mass%, Al: 4.4% or more and less than 5.5%, Fe: 1.4% or more and less than 2.1%, Mo: 1.5% or more and less than 5.5%. However, it is a titanium alloy having a chemical composition of Si: less than 0.1% and C: less than 0.01% with the balance being Ti and impurities, and has room temperature strength equal to or higher than Ti-6Al-4V alloy. , An α + β type titanium alloy having room temperature ductility and fatigue strength as well as excellent hot workability and cold workability is disclosed.

  In Patent Document 2, Al: 3.0% or more and 5.0% or less, V: 2.1% or more and 3.7% or less, Mo: 0.85% or more and 3.15% or less, and O in mass%. : 0.15% or less, further containing one or more of Fe, Ni, Co and Cr, and 0.85% ≦ Fe + Ni + Co + 0.9 × Cr ≦ 3.15%, and 7 % High-strength titanium alloy satisfying% ≦ 2 × Fe + 2 × Ni + 2 × Co + 1.8 × Cr ≦ 13%, consisting of balance Ti and impurities, and exhibiting superplasticity at a lower temperature than Ti-6Al-4V alloy is disclosed. There is.

JP, 2005-320618, A JP-A-3-274238

CAMP-ISIJ, Vol. 26, (2013), p. 1065 CAMP-ISIJ, Vol. 26, (2013), p. 440

  The invention disclosed in Patent Document 1 provides an α + β-type titanium alloy having room temperature strength, room temperature ductility and fatigue strength equal to or higher than Ti-6Al-4V alloy and having excellent hot workability and cold workability. However, there is no disclosure regarding superplasticity of the α + β type titanium alloy.

  The titanium alloy disclosed in Patent Document 2 contains expensive V as a β-phase stabilizing element in an amount of 2.1 to 3.7% by mass similarly to the Ti-6Al-4V alloy, and the material cost is high. Further, in order to produce this titanium alloy, in order to develop superplasticity characteristics, after hot working with a reduction amount of 50% or more (β transformation point −250 ° C.) or more and at a temperature of less than β transformation point. It is necessary to perform a complicated work heat treatment of performing heat treatment, which also increases the manufacturing cost.

  The present invention has been made in view of such problems of the conventional technique, and does not use expensive element V, does not require complicated thermomechanical treatment, and performs simple heat treatment or hot working. By heat treatment, it has room temperature strength equal to or higher than that of Ti-6Al-4V alloy, and does not cause constriction even at high strain rate at relatively low temperature, and can develop superplastic properties that can obtain plastic elongation of several 100% or more. Another object of the present invention is to provide an α + β type titanium alloy member and a method for manufacturing the same.

The present inventors, as a result of repeated studies to solve the above problems,
(A) When a titanium alloy (representative composition: Ti-5Al-2Fe-3Mo alloy) having the chemical composition disclosed in Patent Document 1 is heated and held in a temperature range above the β transformation point and then cooled at high speed, it becomes extremely fine. It becomes a needle-shaped structure, and this fine needle-shaped structure changes to a fine equiaxed structure during the subsequent processing, although it is different from the fine equiaxed structure that is generally regarded as a condition for developing superplasticity. Exhibiting plastic properties, and
(B) It was found that the superplasticity characteristics of this Ti-5Al-2Fe-3Mo alloy are expressed at a lower temperature and a higher strain rate than the Ti-6Al-4V alloy, and further investigation based on these findings A and B. By repeating the above, the present invention was completed. The present invention is as listed below.

(1) Chemical composition, in mass%, Al: 4.4% or more and less than 5.5%, Fe: 1.4% or more and less than 2.1%, Mo: 1.5% or more and less than 5.5%, The balance: Ti and impurities, of which Si: less than 0.1% and C: less than 0.01%, and the total amount of impurities: less than 0.3%,
The metal structure has a fine needle-like structure in which the widths of acicular martensite grains and acicular α grains in the short axis direction are 3 μm or less on average, and fine equiaxed grains in which equiaxed α grains have an average particle size of 5 μm or less. An α + β type titanium alloy member having one or both of the tissues.

  (2) The α + β type titanium alloy member according to item 1, which is for superplastic working.

  (3) The α + β type titanium alloy member according to item 1 or 2, wherein the volume ratio of the grain boundary α phase is 1% or less, and the width of the grain boundary α phase in the short axis direction is 3 μm or less on average.

  (4) In mass%, Al: 4.4% or more and less than 5.5%, Fe: 1.4% or more and less than 2.1%, Mo: 1.5% or more and less than 5.5%, balance: Ti and A material having a chemical composition of impurities, Si: less than 0.1% and C: less than 0.01%, and a total amount of the impurities: less than 0.3%, is β-transformed. The method for producing an α + β-type titanium alloy member according to any one of items 1 to 3, wherein the cooling rate at a β transformation point is 50 to 100 ° C./sec from a temperature range equal to or higher than the point.

ADVANTAGE OF THE INVENTION According to this invention, although it does not use V which is an expensive additive element and does not require complicated thermomechanical processing, it has room temperature strength equivalent to or more than Ti-6Al-4V alloy, and Ti-6Al-4V alloy. The α + β type titanium alloy member that exhibits superplasticity characteristics at a working temperature lower than the usual conditions (working temperature 900 ° C. or more, strain rate 1 × 10 ⁻³ s ⁻¹ or less) and a high strain rate that can obtain superplasticity in Provided.

  The α + β type titanium alloy member according to the present invention has a long life of the die due to manifestation of superplasticity at a low temperature, a reduction in production cost, an improvement in productivity due to manifestation of superplasticity at a high strain rate, and an inexpensive general-purpose element. The effects on the industry, such as the reduction of material costs by utilizing, are immeasurable.

FIG. 1 is an example of an optical microscope photograph of an α + β type titanium alloy member according to the present invention, showing a fine needle-like structure. FIG. 2 is an example of an optical micrograph of an α + β type titanium alloy member according to the present invention, showing a mixed structure of a fine needle-like structure and a fine equiaxed structure.

  Hereinafter, the present invention will be described in detail. In the following description, “%” relating to chemical composition or concentration means “mass%” unless otherwise specified.

1. Α + β type titanium alloy member according to the present invention

(1-1) Chemical composition

(1-1-1) Al: 4.4% or more and less than 5.5% Al is an α-phase stabilizing element having a high solid solution strengthening ability, and as the Al content increases, the tensile strength at room temperature increases. .. Although Al is an inexpensive element, its solid solution strengthening ability is large, and in order to obtain a tensile strength (1000 MPa or more) equal to or higher than that of Ti-6Al-4V alloy at room temperature, the Al content is 4.4%. It is above, preferably at least 4.7%, more preferably at least 4.8%.

  On the other hand, if Al is contained excessively, ductility and cold workability at high temperature and room temperature are deteriorated. The reason that the room temperature ductility and the cold workability are lowered is that Al raises the stacking fault energy and suppresses twin crystal deformation. When the Al content is 5.5% or more, twin crystal deformation is significantly suppressed. become. Therefore, the Al content is less than 5.5%, preferably 5.3% or less, and more preferably 5.1% or less.

(1-1-2) Fe: 1.4% or more and less than 2.1% Fe is a relatively inexpensive β-phase stabilizing substitution type solid solution element, and the tensile strength increases according to the Fe content. Further, since Fe is an element showing a high β-phase stabilizing ability, it is possible to reduce the content thereof. In order to obtain a tensile strength of 1000 MPa or more at room temperature, the Fe content is 1.4% or more, preferably 1.6% or more, more preferably 1.8% or more.

  On the other hand, Fe is liable to be solidified and segregated in Ti, and in a large ingot of several hundred kg or more, when the content of Fe is 2.1% or more, the segregation of Fe becomes remarkable. Therefore, the Fe content is less than 2.1%, preferably 2.0% or less.

(1-1-3) Mo: 1.5% or more and less than 5.5% Mo is a β-phase stabilizing substitution type solid solution element, and like Fe, not only improves room temperature strength but also hot work. Improves workability and cold workability. In order to improve cold workability, the Mo content is 1.5% or more, preferably 2.4% or more, and more preferably 2.9% or more.

  On the other hand, if Mo is contained in an amount of 5.5% or more, solidification segregation in a large ingot becomes a problem, so the Mo content is less than 5.5%, preferably 4.9% or less, and more preferably 4 It is 0.0% or less.

(1-1-4) Remainder The balance other than the above is Ti and impurities, and among the impurities, Si: less than 0.1%, C: less than 0.01%, and the total amount of the impurities: 0. It is less than 0.3%.

  When 0.1% or more of Si as an impurity and 0.01% or more of C as an impurity are contained, room temperature ductility, hot workability and cold workability are adversely affected. Therefore, the Si content is less than 0.1% and the C content is less than 0.01%.

  Other impurity elements other than Si and C may be contained as long as the effect is not impaired. Examples of other impurity elements include O, N, H, P, S, Cl, Mg, Cr, Ni and Sn.

  Further, the total amount of impurity elements including Si and C is less than 0.3% from the viewpoint of maintaining room temperature ductility, hot workability and cold workability.

(1-1-5) Mo Equivalent In the present invention, it is desirable that the Mo equivalent, which is an index of β-phase stability and calculated by the following equation (1), is in the range of 0.5 to 7.0.
[Mo] eq = [Mo] +2.9 [Fe]-[Al] (1)

  When the Mo equivalent is less than 0.5, the hardenability is low, and the β → α phase transformation may occur at the high temperature during the thermomechanical treatment, resulting in coarsening of needle-shaped α grains and equiaxed α grains. On the other hand, when the Mo equivalent exceeds 7.0, the β phase fraction increases, and the strength at room temperature may decrease to less than 1000 MPa. Therefore, the Mo equivalent is preferably 0.5 or more and 7.0 or less.

(1-2) Metallographic structure

  FIG. 1 is an example of an optical microscope photograph of an α + β type titanium alloy member according to the present invention, showing a fine needle-like structure. FIG. 2 is an example of an optical micrograph of an α + β type titanium alloy member according to the present invention, showing a mixed structure of a fine needle-like structure and a fine equiaxed structure.

  The metal structure is a fine needle-shaped structure in which the widths of acicular martensite grains and acicular α grains in the short axis direction are 3 μm or less on average, and fine equiaxed grains in which equiaxed α grains have an average particle size of 5 μm or less. Either or both of the organizations.

  In the case of an acicular structure, if the width of the acicular martensite grains and acicular α grains in the minor axis direction exceeds 3 μm on average, the acicular α grains are less likely to be fragmented during processing after being kept at high temperature, resulting in a large elongation. I can't get it. Therefore, the width of the acicular martensite grains and acicular α grains in the short axis direction is 3 μm or less on average.

  On the other hand, in the case of equiaxed α particles, the surroundings are covered with β phase, and the β particles are deformed, so that they are easier to deform than the acicular α particles. Therefore, in the case of equiaxed α grains, the average grain size is allowed up to 5 μm. Therefore, the average grain size of equiaxed α grains is 5 μm or less.

  Further, the presence of the grain boundary α phase may reduce the elongation, and when the volume ratio of the grain boundary α phase exceeds 1%, the elongation at the time of processing is greatly reduced. Therefore, it is desirable that the volume ratio of the grain boundary α phase is 1% or less.

  When the average width of the grain boundary α phase in the minor axis direction exceeds 3 μm, the workability may be significantly reduced. Therefore, the average width of the grain boundary α phase in the minor axis direction is 3 μm or less. Is desirable.

  The average width of the acicular martensite grains and the acicular α grains (acicular grains), the average grain size of the equiaxed α grains (equiaxial grains), and the average width of the grain boundary α phase in the minor axis direction are all optical values. A test piece for microscopic observation is taken, an embedded polishing sample having a C or T cross section (cross section perpendicular to the longitudinal direction) as an observation surface is prepared, and a nitric hydrofluoric acid aqueous solution (nitric acid concentration: about 12%, hydrofluoric acid concentration: (About 1.5%) at room temperature, then randomly measured at 10 locations from each visual field at a magnification of 500 times, and calculating an average value when measuring a total of 20 visual fields.

  Furthermore, for the material in which the grain boundary α phase was confirmed, the area ratio was measured by performing image analysis at a magnification of 500 times from the embedded sample for optical microscope observation, and the average value of the three visual fields was calculated as the average value of the grain boundary α phase. Area ratio.

(1-3) Shape A round bar, a square bar, and a plate are illustrated.

(1-4) Mechanism of Superplasticity Development As described above, superplasticity is roughly classified into two types: fine grain superplasticity and transformation superplasticity. The superplasticity developed in the α + β type titanium alloy member according to the present invention is fine grain superplasticity. Hereinafter, a mechanism of developing fine grain superplasticity will be described.

Fine grain superplasticity is a phenomenon that occurs when a material having a fine equiaxed microstructure before processing is deformed at a relatively low strain rate at a constant temperature of about 0.5 _Tm or higher. Regarding the fine equiaxed structure, a dual-phase alloy is more suitable for superplasticity as the structure becomes finer and superplasticity is more likely to occur. In the α + β type titanium alloy according to the present invention, the closer the α / β phase ratio is to 1, the more easily superplasticity is exhibited. Therefore, it is preferable to set the chemical composition of the alloy so that the α / β phase ratio becomes close to 1 at the processing temperature.

  Recently, for example, in Non-Patent Documents 1 and 2, when an α + β type titanium alloy-based Ti-5Al-2Fe-3Mo alloy according to the present invention is heat-treated under specific heat-treatment conditions, the width in the minor axis direction is several tens nm. It was reported that a fine acicular structure composed of acicular martensite grains and acicular α grains of ˜several μm can be obtained.

  Heretofore, it has been considered that a fine equiaxed structure is required for the development of superplasticity. However, the Ti-5Al-2Fe-3Mo alloy described in this document, that is, the needle-like structure of the α + β-type titanium alloy according to the present invention is extremely fine. Furthermore, even if it is held for several tens of minutes at 700 to 900 ° C., which is the superplastic working temperature range suitable for the α + β type titanium alloy according to the present invention, the α + β two-phase range results, so that the needle-shaped α-grains (needle-shaped martensite) The grain growth of the site grains is also maintained at high temperature and transformed into the α phase), and the width of the acicular α grains in the minor axis direction is maintained at 3 μm or less on average. For this reason, when a processing strain is introduced during processing of the α + β type titanium alloy according to the present invention, needle-shaped α-grains undergo structural dissection or dynamic recrystallization, and the needle-shaped α-grains change to equiaxed α-grains. As a result, the β phase surrounding the α phase is connected to improve the plastic fluidity, and a large plastic elongation can be obtained.

  As described above, the α + β-type titanium alloy according to the present invention exhibits superplasticity characteristics by changing from a fine acicular structure to a fine equiaxed structure during processing. By satisfying both the chemical composition defined in the present invention and the thermo-mechanical treatment conditions described later, the fine needle-like structure can be obtained and superplasticity can be exhibited.

  Therefore, it may be a structure composed of only the fine needle-shaped structure as described above, or may be a mixed structure in which the fine needle-shaped structure and the fine equiaxed structure are mixed. The grain boundary α phase generated in the old β grain boundary is apt to have a larger width in the minor axis direction than the acicular α grains, and is continuously formed in the old β grain boundary portion. Therefore, it is difficult to cause tissue division, if not sufficiently divided, the deformation is concentrated in the β phase adjacent to the α phase, a void occurs when the ductility limit is reached, leading to fracture by connecting it, You may not be able to get a big stretch.

  Since the actual working condition of superplastic working is elongation of about 200%, in the present invention, the case where elongation of 200% or more is expressed is defined as superplasticity.

2. Manufacturing Method According to the Present Invention The α + β-type titanium alloy member according to the present invention is made of a material having the above-described chemical composition from a temperature range equal to or higher than the β transformation point and a cooling rate at the β transformation point of 50 to 100 ° C./sec. It is manufactured by cooling so that

(2-1) Thermomechanical treatment conditions An example of the thermomechanical treatment that can achieve the effects of the present invention will be described below.

  In the present invention, a very fine structure can be obtained by a relatively simple thermomechanical treatment. For example, by heating to a β transformation point or higher, transforming the entire structure of the test piece into a β phase, and then performing high-speed cooling such as water cooling (cooling rate at the β transformation point is 50 to 100 ° C./sec), It can be a fine needle-like structure.

  With this method, a fine structure can be obtained regardless of the amount of processing before heat treatment. Alternatively, a mixed structure of a fine needle-like structure and a fine equiaxed structure can be obtained by similarly high-speed cooling after the β region heating and rolling.

(2-2) Range of processing conditions In the present invention, the processing conditions under which superplastic characteristics are obtained are generally: processing temperature: 700 ° C or higher (preferably 700 to 900 ° C), strain rate: 1 x 10 ^-2 s ^{−. It is 1} or less (preferably 1 × 10 ⁻⁴ to 1 × 10 ⁻² s ⁻¹ ), and normal conditions for obtaining superplasticity in a titanium alloy such as Ti-6Al-4V alloy (processing temperature 900 ° C., strain rate 1 × 10 ⁻³ s ⁻¹ or less), superplasticity can be exhibited at a lower processing temperature and a higher strain rate.

  Therefore, according to the present invention, it is possible to extend the life of the mold by expressing superplasticity at a low temperature, reduce the production cost, and further improve the productivity by expressing superplasticity at a high strain rate.

  The present invention will be described more specifically with reference to Examples.

  Chemical composition No. shown in Table 1. Titanium alloy Nos. 1 to 11 The ingots in which 1 to 11 were plasma-melted were subjected to β-region heating forging, and then β-region heating rolling was performed to obtain a round bar having a diameter of 20 mm.

  The obtained material as it is, or after being heated to 1050 ° C. above the β transformation point and held for 30 minutes, is rapidly cooled at a cooling rate at the β transformation point: 70 ° C./sec, and then the parallel portion has a diameter of 3 mm and a length. A 6 mm test piece was prepared.

  In addition, at the time of collecting the tensile test piece, a test piece for optical microscope observation was taken from the vicinity of the site, and an embedded polishing sample having a C cross section (cross section perpendicular to the longitudinal direction) as an observation surface was prepared. It was observed after etching at room temperature using (nitric acid concentration: about 12%, hydrofluoric acid concentration: about 1.5%).

  At this time, while confirming the morphology of the metal structure, the average width of the acicular martensite grains and acicular α grains (acicular grains), the average grain size of equiaxed α grains (equiaxial grains), and the grain boundary α phase The average width in the direction of the short axis (all were randomly measured from each visual field at a magnification of 500 times at 10 locations, and an average value was calculated when a total of 20 visual fields were measured) was measured.

  When the width of the acicular grains or the average grain size of the equiaxed grains is less than 1 μm, accurate numerical values could not be measured with an optical microscope having a low resolution, and therefore the table describes it as less than 1 μm. Furthermore, for the material in which the grain boundary α phase was confirmed, the area ratio of the grain boundary α phase was measured from the embedded sample for optical microscope observation (the area ratio was measured by image analysis at a magnification of 500 times, and the The average value was used as the area ratio).

The sampled tensile test piece was heated to 700 to 800 ° C. at a temperature rising rate of 45 ° C./minute, held for 10 minutes, and then pulled at a strain rate of 1 × 10 ⁻³ to 1 × 10 ⁻² s ^−1. The properties (tensile strength, drawing, butt elongation) were evaluated.

Table 2 shows titanium alloy No. The results obtained by processing the raw materials 1 to 11 as they are into a tensile test piece and performing a tensile test under the conditions of a test temperature of 800 ° C. and a strain rate of 1 × 10 ⁻² s ⁻¹ and an observation result of the metal structure of the raw material are shown.

  Alloy No. All of A-1 to 11 had a mixed structure of acicular grains and equiaxed grains, and no grain boundary α phase could be confirmed.

  In each of the inventive examples of Alloys A-1 to A-7, the average width of the acicular grains was 3 μm or less, the average grain size of the equiaxed grains was 5 μm or less, and the butt elongation was 200% or more. Further, no void was confirmed in the cross-sectional structure near the fractured part of the fractured test piece.

  In the comparative example of alloy A-8, since the Al, Fe, and Mo contents are all below the lower limit of the range of the present invention, the average width of the acicular grains in the minor axis direction becomes large, and the butt elongation becomes poor. It was

  In the comparative example of alloy A-9, the Mo content was less than the lower limit of the range of the present invention, so that the average width of the acicular grains in the minor axis direction was large and the butt elongation was poor.

  In the comparative example of alloy A-10, the content of Si and C as impurities exceeded the upper limit of the range of the present invention, so that the butt elongation was poor.

  Furthermore, the comparative example of Alloy A-10 has a high alloy cost because it is a chemical composition system (Ti-6Al-4V alloy) different from the chemical composition system (Ti-5Al-2Fe-3Mo alloy) of the present invention.

In Table 3, similarly to Table 2, titanium alloy No. The results of tensile testing of raw materials 1 to 11 directly processed into tensile test pieces under the conditions of a test temperature of 700 ° C. and a strain rate of 1 × 10 ⁻³ s ⁻¹ are shown.

  In each of the invention examples A-12 to A18, the butt elongation was 200% or more. Further, no void was confirmed in the cross-sectional structure near the fractured part of the fractured test piece.

  In the comparative example of alloy A-19, the Al, Fe, and Mo contents are all below the lower limit of the range of the present invention, so that the average width of the acicular grains in the minor axis direction becomes large and the butt elongation becomes poor. It was

  In the comparative example of alloy A-20, the Mo content was less than the lower limit of the range of the present invention, so that the average width of the acicular grains in the minor axis direction was large and the butt elongation was poor.

  In the comparative example of the alloy A-21, the Si and C contents of impurities exceeded the upper limit of the range of the present invention, so that the butt elongation became poor.

  Furthermore, the comparative example of alloy A-22 has a high alloy cost because it is a chemical composition system (Ti-6Al-4V alloy) different from the chemical composition system (Ti-5Al-2Fe-3Mo alloy) of the present invention.

Table 4 shows titanium alloy No. The materials 1 to 11 were heated to a β transformation point or higher and then rapidly cooled, then processed into tensile test pieces, and the tensile test was performed under the conditions of a test temperature of 800 ° C. and a strain rate of 1 × 10 ⁻² s ^−1. , And the observation results of the metal structure of the material.

  Alloy No. All of B-1 to B-11 were needle-shaped structures composed of needle-shaped grains, and equiaxed grains could not be confirmed.

  Alloy No. In each of Examples B-1 to 7 of the present invention, the average width of the acicular grains and the grain boundary α phase is 3 μm or less, the volume ratio of the grain boundary α phase is 1% or less, and the butt elongation is 200%. That was all. Further, no void was confirmed in the cross-sectional structure near the fractured part of the fractured test piece.

  In the comparative example of alloy B-8, the Al, Fe, and Mo contents were all below the lower limit of the range of the present invention, so that the area ratio of the grain boundary α phase was large and the butt elongation was poor.

  In the comparative example of Alloy B-9, the Mo content was less than the lower limit of the range of the present invention, so that the area ratio of the grain boundary α phase was large and the butt elongation was poor.

  In the comparative example of the alloy B-10, the content of impurities Si and C exceeded the upper limit of the range of the present invention, so that the butt elongation was poor.

  Further, the comparative example of the alloy B-11 is a chemical composition system (Ti-6Al-4V alloy) different from the chemical composition system (Ti-5Al-2Fe-3Mo alloy) of the present invention, and therefore, under these processing conditions, Inability to develop plasticity, poor butt elongation, and high alloy cost.

In Table 5, similarly to Table 4, titanium alloy No. The materials of Nos. 1 to 11 were heated to a β transformation point or higher and then rapidly cooled, then processed into tensile test pieces, and the results of the tensile test under the conditions of a test temperature of 700 ° C. and a strain rate of 1 × 10 ⁻³ s ⁻¹ were shown. Show.

  Alloy No. In all of B-12 to 18, the butt elongation was 200% or more. Further, no void was confirmed in the cross-sectional structure near the fractured part of the fractured test piece.

  In the comparative example of alloy B-19, the butt elongation was poor because the Al, Fe, and Mo contents were all below the lower limit of the range of the present invention.

  In the comparative example of the alloy B-20, the Mo content was below the lower limit of the range of the present invention, so that the butt elongation was poor.

  In the comparative example of the alloy B-21, the content of Si and C as impurities exceeds the upper limit of the range of the present invention, so that the butt elongation becomes poor.

Furthermore, since the comparative example of B-22 is a chemical composition system (Ti-6Al-4V alloy) different from the chemical composition system (Ti-5Al-2Fe-3Mo alloy) of the present invention, the butt elongation is poor. became.

Claims (3)

The chemical composition is mass%, Al: 4.4% or more and less than 5.5%, Fe: 1.4% or more and less than 2.1%, Mo: 1.5% or more and less than 5.5%, balance: Ti And Si: less than 0.1% and C: less than 0.01% of the impurities, and the total amount of the impurities: less than 0.3%,
The metal structure has a fine needle-like structure in which the widths of acicular martensite grains and acicular α grains in the short axis direction are 3 μm or less on average, and fine equiaxed grains in which equiaxed α grains have an average particle size of 5 μm or less. Ri either or both der tissue, further 1% or less the volume ratio of the grain boundary alpha phase, and the width in the minor axis direction of the grain boundary alpha phase is Ru der average 3μm or less, alpha + beta titanium Alloy material.   The α + β type titanium alloy member according to claim 1, which is for superplastic working. In mass%, Al: 4.4% or more and less than 5.5%, Fe: 1.4% or more and less than 2.1%, Mo: 1.5% or more and less than 5.5%, balance: Ti and impurities , Si among the impurities is less than 0.1% and C is less than 0.01%, and the total amount of the impurities is less than 0.3%. the temperature range, the cooling rate at the beta transus is cooled so that 50 to 100 ° C. / sec, according to claim 1 or 2, alpha + beta type method for producing a titanium alloy member.