JP2017145429A - α+β TYPE TITANIUM ALLOY MEMBER AND MANUFACTURING METHOD THEREFOR - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an α+β type titanium alloy member having room temperature strength equal to or more than that of a Ti-6Al-4V alloy and exhibiting super plasticity property at low temperature and high strain rate without using V which is an expensive additive element nor needs for complex thermomechanical treatments.SOLUTION: There is provided an α+β type titanium alloy member having a chemical composition of 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% and the balance:Ti with impurities, where Si:less than 0.1% and C:less than 0.01% and total amount of the impurities:less than 0.3%, a metallic structure including one or both of fine acicular structure with width in a short axis direction of acicular martensite particles and acicular α particles of average 3 μm or less and fine isometric structure with average particle diameter of isometric α particles of 5 μm. It is manufactured by cooling a raw material from a temperature range of the β transformation point or more at a cooling rate at the β transformation point of 50 to 100°C/sec.SELECTED DRAWING: Figure 1

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 and have high strength and excellent corrosion resistance. In particular, Ti-6Al-4V alloy (6 mass% Al and 4 mass% containing Ti alloy), which is the most general-purpose alloy, has been used in the aerospace field for a long time, and has recently begun to be applied to the automotive field and the like. .

  On the other hand, titanium alloys such as Ti-6Al-4V alloy are difficult to process due to poor workability and machinability, and parts having complicated shapes must be manufactured by machining, so that the yield is low. As a result, there is a problem that the manufacturing cost is high.

  As one of the methods for solving this problem, there is known a superplastic processing method utilizing a transformation superplastic phenomenon caused by phase transformation or a fine grain (structure) superplasticity phenomenon caused by fine crystal grains. It is done.

  Superplasticity is a property of exhibiting elongation at break ranging from several hundreds to several thousand% in a metal material without causing necking while maintaining a low flow stress when the material is processed under a specific condition. The superplastic processing method uses this property to precisely plastically process parts having complicated shapes.

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

  In addition, the material to be used for superplastic processing needs to have a fine equiaxed structure of 5 to 10 μm. However, it is difficult to obtain this fine equiaxed structure in the Ti-6Al-4V alloy. It has become one of

  Furthermore, the Ti-6Al-4V alloy uses expensive V as the β-phase stabilizing element, and 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% And a titanium alloy having a chemical composition consisting of impurities Si: less than 0.1% and C: less than 0.01%, the balance being Ti and impurities, and a room temperature strength equal to or higher than that of a Ti-6Al-4V alloy Further, an α + β type titanium alloy having room temperature ductility and fatigue strength and excellent in hot workability and cold workability is disclosed.

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

Japanese Patent Laying-Open No. 2005-320618 JP-A-3-274238

CAMP-ISIJ, Vol. 26, (2013), page 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 better than those of a Ti-6Al-4V alloy and excellent in hot workability and cold workability. However, there is no disclosure regarding the superplasticity of this α + β type titanium alloy.

  The titanium alloy disclosed by patent document 2 contains 2.1-3.7 mass% of expensive V as a beta phase stabilization element like a Ti-6Al-4V alloy, and its material cost is high. In order to produce this titanium alloy, in order to express the superplastic properties, after hot working at a reduction amount of 50% or more (β transformation point−250 ° C.) at a temperature not lower than the β transformation point. It is necessary to perform a complicated heat treatment to perform heat treatment, and the manufacturing cost increases from this point.

  The present invention has been made in view of such problems of the prior art, and does not use V, which is an expensive element, and does not require complicated machining heat treatment, and can be carried out such as simple heat treatment and hot working. By heat treatment, it has room temperature strength equal to or better than that of Ti-6Al-4V alloy, and can exhibit superplastic properties that can produce plastic elongation of several hundred% or more without causing constriction even at a relatively low temperature and high strain rate. An object of the present invention is to provide an α + β type titanium alloy member and a method for producing the same.

As a result of intensive studies to solve the above problems, the present inventors have
(A) When a titanium alloy having a chemical composition disclosed in Patent Document 1 (representative composition: Ti-5Al-2Fe-3Mo alloy) is heated and held in a temperature range equal to or higher than the β transformation point and then cooled at a high speed, it is extremely fine. It becomes a needle-like structure, and this fine needle-like structure is different from a fine equiaxed structure, which is generally regarded as a condition for developing superplastic properties, but changes to a fine equiaxed structure during subsequent processing. Developing plastic properties; and
(B) It has been found that the superplastic properties of this Ti-5Al-2Fe-3Mo alloy are manifested at a lower temperature and higher strain rate than Ti-6Al-4V alloy, and further examination based on these findings A and B. As a result, the present invention was completed. The present invention is listed below.

(1) 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%, The balance: consisting of Ti and impurities, Si among the impurities: less than 0.1% and C: less than 0.01%, and the total amount of the impurities: less than 0.3%,
The metal structure is a fine acicular structure in which the width in the minor axis direction of acicular martensite grains and acicular α grains is 3 μm or less on average, and a fine equiaxed shape in which the average grain diameter of equiaxed α grains is 5 μm or less An α + β type titanium alloy member that is 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 1 or 2, wherein the grain boundary α phase has a volume ratio of 1% or less and the average width of the grain boundary α phase in the minor axis direction is 3 μm or less.

  (4) 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%, balance: Ti and A material composed of impurities, having a chemical composition of Si: less than 0.1% and C: less than 0.01% of the impurities, and a total amount of the impurities: less than 0.3%, β transformation The manufacturing method of the alpha + beta type titanium alloy member in any one of Claims 1-3 cooled so that the cooling rate in a beta transformation point may be 50-100 degree-C / sec from the temperature range more than a point.

According to the present invention, an expensive additive element V is not used, and a complex heat treatment is not required, but the room temperature strength is equal to or higher than that of a Ti-6Al-4V alloy, and a Ti-6Al-4V alloy. An α + β type titanium alloy member that exhibits superplastic properties at a lower processing temperature and higher strain rate than the normal conditions (processing temperature 900 ° C. or higher, strain rate 1 × 10 ⁻³ s ⁻¹ or lower) at which superplasticity can be obtained in Provided.

  The α + β-type titanium alloy member according to the present invention is a low-cost general-purpose element that increases the service life of the mold due to the development of superplasticity at a low temperature, reduces the production cost, improves the productivity due to the development of superplasticity at a high strain rate. The industrial effects such as reduction of material costs by utilizing the technology are immense.

FIG. 1 is an example of an optical micrograph of an α + β type titanium alloy member according to the present invention, and shows a fine acicular structure. FIG. 2 is an example of an optical micrograph of an α + β type titanium alloy member according to the present invention, and shows a mixed structure of a fine acicular structure and a fine equiaxed structure.

  The present invention will be described in detail below. In the following description, “%” regarding chemical composition or concentration means “% by 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 the tensile strength at room temperature increases as the Al content increases. . Al is an inexpensive element, but 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 the Ti-6Al-4V alloy at room temperature, the Al content is 4.4%. Or more, preferably 4.7% or more, and more preferably 4.8% or more.

  On the other hand, when Al is contained excessively, ductility and cold workability at high temperature and room temperature are lowered. The reason why room temperature ductility and cold workability are reduced is that Al increases stacking fault energy and suppresses twin deformation. When the Al content is 5.5% or more, suppression of twin deformation is significant. 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-stabilized substitutional solid solution element, and the tensile strength increases according to the Fe content. Further, since Fe is an element that exhibits high β-phase stabilization ability, its content can be reduced. 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, and more preferably 1.8% or more.

  On the other hand, Fe is easy to solidify and segregate in Ti, and if it contains 2.1% or more in a large ingot of several hundred kg or more, the segregation of Fe becomes remarkable. For this reason, Fe content is less than 2.1%, Preferably it is 2.0% or less.

(1-1-3) Mo: 1.5% or more and less than 5.5% Mo is a β-phase-stabilized substitutional solid solution element and, like Fe, not only improves room temperature strength, but also hot Improve 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, when 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, more preferably 4 0.0% or less.

(1-1-4) Balance The balance other than the above is Ti and impurities, of which Si is less than 0.1%, C is less than 0.01%, and the total amount of the impurities is 0. Less than 3%.

  Containing 0.1% or more of Si as an impurity and 0.01% or more of C as an impurity adversely affects room temperature ductility, hot workability, and cold workability. For this reason, 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 effects are not impaired. Examples of other impurity elements include O, N, H, P, S, Cl, Mg, Cr, Ni, and Sn.

  Furthermore, 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 this invention, it is desirable to make Mo equivalent which is a parameter | index of (beta) phase stability and calculated | required by the following (1) formula into the range of 0.5-7.0.
[Mo] eq = [Mo] +2.9 [Fe]-[Al] (1)

  When the Mo equivalent is less than 0.5, the hardenability is low, and β → α phase transformation occurs at high temperature during the heat treatment process, and acicular α grains and equiaxed α grains may be coarsened. On the other hand, when the Mo equivalent exceeds 7.0, the β phase fraction increases, and the strength at room temperature may be reduced to less than 1000 MPa. For this reason, it is preferable that Mo equivalent is 0.5 or more and 7.0 or less.

(1-2) Metallographic structure

  FIG. 1 is an example of an optical micrograph of an α + β type titanium alloy member according to the present invention, and shows a fine acicular structure. FIG. 2 is an example of an optical micrograph of an α + β type titanium alloy member according to the present invention, and shows a mixed structure of a fine acicular structure and a fine equiaxed structure.

  The metal structure is a fine acicular structure having an average width of 3 μm or less in the minor axis direction of acicular martensite grains and acicular α grains, and a fine equiaxed shape in which the average grain diameter of equiaxed α grains is 5 μm or less. Either one or both of the organizations.

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

  On the other hand, in the case of equiaxed α-grains, the periphery is covered with a β-phase, and deformation of the β-grains is easier than that of acicular α-grains. For this reason, in the case of equiaxed α-grains, an average particle size of 5 μm is allowed. Therefore, the average particle diameter of the equiaxed α grains is 5 μm or less.

  In addition, when the grain boundary α phase is present, the elongation may decrease. When the volume ratio of the grain boundary α phase exceeds 1%, the elongation during processing increases greatly. For this reason, the volume ratio of the grain boundary α phase is desirably 1% or less.

  Further, when the width in the minor axis direction of the grain boundary α phase exceeds 3 μm on the average, the reduction of the processing rate may be noticeable in the same manner, so that the width in the minor axis direction of the grain boundary α phase is 3 μm or less on average. It is desirable.

  The average width of acicular martensite grains and acicular alpha grains (acicular grains), the average grain diameter of equiaxed alpha grains (equiaxial grains), and the mean width in the minor axis direction of the grain boundary alpha phase are all optical. A specimen for microscopic observation was collected, and an embedded polished sample having a C or T cross section (cross section perpendicular to the longitudinal direction) as an observation surface was prepared. An aqueous nitric hydrofluoric acid solution (nitric acid concentration: about 12%, hydrofluoric acid concentration: After etching at room temperature using about 1.5%), 10 points are randomly measured from each visual field at a magnification of 500 times, and the average value when measuring 20 visual fields in total is calculated.

  Furthermore, for the material in which the grain boundary α phase was confirmed, the area ratio was measured by analyzing the image at a magnification of 500 times from the embedded sample for observation with an optical microscope, and the average value of the three fields of view was determined as the grain boundary α phase. The area ratio.

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

(1-4) Superplasticity manifestation mechanism 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 crystal superplasticity. Hereinafter, the mechanism of manifestation of fine crystal superplasticity will be described.

Fine grain superplasticity is a phenomenon that occurs when a material having a fine equiaxed structure in a pre-working structure is deformed at a relatively low strain rate at a constant temperature of about 0.5 _Tm or higher. As for the fine equiaxed structure, the finer the structure, the easier the superplasticity occurs, and the two-phase alloy that can easily maintain the fine structure is more suitable for the superplasticity. In the α + β type titanium alloy according to the present invention, the closer the α / β phase ratio is to 1, the easier it is to develop superplasticity. Therefore, it is preferable to set the chemical composition of the alloy so that the α / β phase ratio is 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 subjected to heat treatment under specific heat treatment conditions, the width in the minor axis direction is several tens of nm. It has been reported that a fine needle-like structure composed of needle-like martensite grains and needle-like α grains having a size of several μm can be obtained.

  Conventionally, it has been said that a fine equiaxed structure is required for the development of superplastic characteristics. However, the needle-like structure of the Ti-5Al-2Fe-3Mo alloy described in this document, that is, 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 a superplastic processing temperature range suitable for the α + β type titanium alloy according to the present invention, it becomes an α + β two-phase region. The grain growth of the site grains is also transformed to the α phase by holding at a high temperature, and the width of the acicular α grains in the minor axis direction is maintained at an average of 3 μm or less. For this reason, when processing strain is introduced during the processing of the α + β type titanium alloy according to the present invention, the structure of the acicular α grains is divided and dynamic recrystallization occurs, and the acicular α grains change into equiaxed α grains. As a result, the β phase surrounding the α phase is connected to improve the plastic fluidity, thereby obtaining a large plastic elongation.

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

  For this reason, the structure which consists only of the above fine acicular structures may be sufficient, but the mixed structure | tissue which the fine acicular structure | tissue and the fine equiaxed structure | tissue mixed may be sufficient. The grain boundary α phase generated in the old β grain boundary is likely to be wider in the minor axis direction than the needle-like α grain, and is continuously generated in the old β grain boundary portion. For this reason, it is difficult to cause structural division, and if it is not sufficiently divided, deformation concentrates in the β phase adjacent to the α phase, a void is generated when the ductility limit is reached, and it leads to fracture when connected, Large elongation may not be obtained.

  In addition, since the actual processing condition of superplastic processing is about 200% in elongation, in the present invention, a case where elongation of 200% or more is expressed is defined as superplasticity.

2. Production method according to the present invention The α + β type titanium alloy member according to the present invention is a material having the above-described chemical composition, and the cooling rate at the β transformation point is 50 to 100 ° C./second from the temperature range above the β transformation point. It manufactures by cooling so that it may become.

(2-1) Thermomechanical processing conditions An example of thermomechanical processing 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 heat treatment. For example, by heating to the β transformation point or higher and transforming the entire structure of the test piece to the β phase, by performing high-speed cooling such as water cooling (the cooling rate at the β transformation point is 50 to 100 ° C./second), It can be a fine needle-like structure.

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

(2-2) Range of processing conditions In the present invention, processing conditions for obtaining superplastic characteristics are generally processing temperature: 700 ° C. or higher (preferably 700 to 900 ° C.), strain rate: 1 × 10 ⁻² s ^{−. 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 a Ti-6Al-4V alloy (processing temperature 900 ° C., strain rate) 1 × 10 ⁻³ s ⁻¹ or less), it is possible to develop superplasticity at a lower processing temperature and a higher strain rate.

  For this reason, according to the present invention, it is possible to extend the life of the mold due to the development of superplasticity at a low temperature, to reduce the production cost, and to improve the productivity due to the development of 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 No. 1-11 The ingot in which 1 to 11 was plasma-melted was forged in the β region and then heated in the β region to obtain a round bar having a diameter of 20 mm.

  The obtained material is heated as it is or heated to 1050 ° C. above the β transformation point and held for 30 minutes, and then cooled at the β transformation point: 70 ° C./sec. A 6 mm test piece was prepared.

  At the time of collecting the tensile test piece, a test piece for observation with an optical microscope was taken from the vicinity of the tensile test piece to prepare an embedded polished sample having a C cross section (cross section perpendicular to the longitudinal direction) as an observation surface. Observation was performed after etching at room temperature using (nitric acid concentration: about 12%, hydrofluoric acid concentration: about 1.5%).

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

  When the width of the acicular grains and the average grain diameter of the equiaxed grains are less than 1 μm, an accurate numerical value could not be measured with an optical microscope having a low resolution, so that it is described in the table 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 an embedded sample for observation with an optical microscope (by analyzing the image at a magnification of 500 times, The average value was taken as the area ratio).

About the extract | collected tensile test piece, after heating up to 700-800 degreeC with the temperature increase rate of 45 degreeC / min and hold | maintaining for 10 minutes, it tension | ^{tensiles on} the conditions of the strain rate of 1 * 10 ^{< -3} > ^-1 * 10 ^<-2 > s ^{< -1} >. The properties (tensile strength, drawing, butt elongation) were evaluated.

Table 2 shows titanium alloy no. The materials of 1 to 11 are processed into tensile test pieces as they are, the results of a tensile test 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 materials are shown.

  Alloy No. A-1 to 11 were all mixed structures of acicular grains and equiaxed grains, and no grain boundary α phase could be confirmed.

  In all of the inventive examples of Alloys A-1 to A-7, the average width of the needle-like grains was 3 μm or less, the average grain diameter of the equiaxed grains was 5 μm or less, and the butt elongation was 200% or more. Moreover, no void was confirmed even in the cross-sectional structure near the fractured portion of the fracture 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 is increased, and the butt elongation is unsatisfactory. It was.

  In Comparative Example of Alloy A-9, since the Mo content was below the lower limit of the range of the present invention, the average width of the acicular grains in the minor axis direction was increased, and the butt elongation was unsatisfactory.

  In 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 unsatisfactory.

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

In Table 3, as in Table 2, the titanium alloy No. The raw material of 1-11 is processed into a tensile test piece as it is, and the result of having carried out the tension test on conditions with a test temperature of 700 degreeC and a strain rate of 1 * 10 <^-3> s < ^-1 > is shown.

  In all of the inventive examples A-12 to 18, the butt elongation was 200% or more. Moreover, no void was confirmed even in the cross-sectional structure near the fractured portion of the fracture test piece.

  In the comparative example of Alloy A-19, 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 is increased and the butt elongation is unsatisfactory. It was.

  In Comparative Example of Alloy A-20, since the Mo content was below the lower limit of the range of the present invention, the average width of the acicular grains in the minor axis direction was increased, and the butt elongation was unsatisfactory.

  In the comparative example of Alloy A-21, 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 unsatisfactory.

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

Table 4 shows titanium alloy no. The materials 1 to 11 were heated to the β transformation point or higher and then cooled at high speed, and then processed into a tensile test piece, and a tensile test was performed at a test temperature of 800 ° C. and a strain rate of 1 × 10 ⁻² s ^−1. The observation result of the metal structure of the material is shown.

  Alloy No. B-1 to 11 are all acicular structures composed of acicular grains, and no equiaxed grains could be confirmed.

  Alloy No. In the present invention examples of B-1 to 7, 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. Moreover, no void was confirmed even in the cross-sectional structure near the fractured portion of the fracture test piece.

  In the comparative example of Alloy B-8, since the Al, Fe, and Mo contents were all below the lower limit of the range of the present invention, the area ratio of the grain boundary α phase was increased and the butt elongation was unsatisfactory.

  In Comparative Example of Alloy B-9, the Mo content was below the lower limit of the range of the present invention, so the area ratio of the grain boundary α phase was increased, and the butt elongation was unsatisfactory.

  In the comparative example of Alloy B-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 unsatisfactory.

  Furthermore, since the comparative example of alloy B-11 is a chemical component system (Ti-6Al-4V alloy) different from the chemical component system (Ti-5Al-2Fe-3Mo alloy) of the present invention, it is super The plasticity cannot be expressed, the butt elongation becomes unsatisfactory, and the alloy cost is high.

In Table 5, as in Table 4, titanium alloy No. The materials 1 to 11 were heated to the β transformation point or higher and then cooled at high speed, and then processed into a tensile test piece. The result of a tensile test under the conditions of a test temperature of 700 ° C. and a strain rate of 1 × 10 ⁻³ s ⁻¹ was obtained. Show.

  Alloy No. In any of B-12 to 18, the butt elongation was 200% or more. Moreover, no void was confirmed even in the cross-sectional structure near the fractured portion of the fracture test piece.

  In Comparative Example of Alloy B-19, the butt elongation was unsatisfactory because the Al, Fe, and Mo contents were all below the lower limit of the range of the present invention.

  In Comparative Example of Alloy B-20, the butt elongation was unsatisfactory because the Mo content was below the lower limit of the range of the present invention.

  In the comparative example of Alloy B-21, 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 unsatisfactory.

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

Claims (4)

Chemical composition is mass%, Al: 4.4% to less than 5.5%, Fe: 1.4% to less than 2.1%, Mo: 1.5% to less than 5.5%, balance: Ti And, among the impurities, Si: less than 0.1% and C: less than 0.01%, and the total amount of the impurities: less than 0.3%,
The metal structure is a fine acicular structure in which the width in the minor axis direction of acicular martensite grains and acicular α grains is 3 μm or less on average, and a fine equiaxed shape in which the average grain diameter of equiaxed α grains is 5 μm or less An α + β type titanium alloy member that is one or both of the tissues.   The α + β type titanium alloy member according to claim 1, which is for superplastic working.   The α + β-type titanium alloy member according to claim 1 or 2, wherein the volume ratio of the grain boundary α phase is 1% or less and the width of the minor axis direction of the grain boundary α phase is 3 µm or less on average. 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. Of the impurities, Si: less than 0.1% and C: less than 0.01%, and the total amount of the impurities: a material having a chemical composition of less than 0.3%, a material having a β transformation point or more The manufacturing method of the alpha + beta type titanium alloy member in any one of Claims 1-3 cooled so that the cooling rate in a beta transformation point may be 50-100 degree-C / sec from a temperature range.