JP6673121B2 - α + β type titanium alloy rod and method for producing the same - Google Patents

Description

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

  BACKGROUND ART In recent years, titanium alloys have been used in a wide range of fields such as structural materials and biomaterials because of their characteristics of being lightweight and having high strength. There is disclosed an invention for improving the properties (for example, strength and corrosion resistance) of a titanium alloy in accordance with the use of the titanium alloy. Also disclosed is an invention for reducing the production cost of a titanium alloy.

  For example, in Patent Document 1, 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% by mass%. And has a room temperature strength, a room temperature ductility, and a fatigue strength superior to a Ti-6Al-4V-based alloy which is an α + β-type titanium alloy which has a chemical composition of a balance of Ti and impurities and is generally used, and An α + β-type titanium alloy excellent in hot workability, cold workability and corrosion resistance and low in cost is disclosed.

  In addition, an invention that focuses on the Young's modulus of a titanium alloy from the viewpoint of increasing rigidity is also disclosed. The reason for this is that the required Young's modulus differs for each member in which titanium is used, and thus a titanium alloy having a Young's modulus suitable for the required Young's modulus is selected. Young's modulus is an important factor in material design.

  The Young's modulus of titanium at room temperature is about 100 to 120 GPa for industrial pure titanium and α-type titanium alloy mainly composed of α-phase (hexagonal close-packed structure), and α + β-type titanium alloy composed of α-phase and β-phase. , Β-phase (face-centered cubic lattice structure) is about 70 to 90 GPa for β-type titanium alloy.

  The hexagonal close-packed structure (hcp structure) has fewer slip systems related to deformation than the face-centered cubic lattice structure (bcc structure). In general, an α phase having a small slip system is hardly deformed and tends to have a high Young's modulus. On the other hand, a β phase having many slip systems tends to be deformed, and the Young's modulus tends to be low. For this reason, α-type titanium alloys and α + β-type titanium alloys are often selected for members requiring high Young's modulus.

  However, even in the case of a β-type titanium alloy, the Young's modulus increases to 100 to 120 GPa, as in the case of the α-type titanium alloy or the α + β-type titanium alloy, by aging heat treatment in the α + β two-phase region to precipitate the α phase. There is. As described above, it is known that the Young's modulus is increased by controlling the precipitation structure by aging heat treatment or the like.

  Patent Document 2 contains, by mass%, Al: 4.4% or more and less than 5.5%, Fe: 1.4% or more and less than 2.1%, Mo: 2.5% or more and less than 5%, The area ratio (quantity ratio) of the primary α-phase is 5 by aging heat treatment in which the remaining Ti and the α + β-type titanium alloy having the chemical composition of impurities are cooled from a temperature of over 810 ° C to 940 ° C at a cooling rate of water cooling or more. An invention is disclosed in which an α + β-type titanium alloy member is manufactured by controlling the Young's modulus to 75 GPa or more and less than 100 GPa by controlling the modulus to at least 75% and less than 49%.

  Patent Document 3 contains, by mass%, Al: 4.7 to 5.5% and Fe: 0.8 to 2.1%, and an oxygen equivalent [O] eq = [O] +2.77 [N] ]: A main metal structure by heating a titanium alloy having a chemical composition of 0.06 to 0.35%, balance Ti and impurities to a temperature equal to or higher than the β transformation point, and then cooling at a cooling rate equivalent to air cooling or more. The invention discloses an α + β-type titanium alloy member having a high Young's modulus of 120 to 140 GPa by setting the width of the needle-like α grains in the minor axis direction to 10 μm or less.

  Further, Patent Document 4 discloses not only the aging heat treatment and the heat treatment process such as the cooling rate as described above, but also, by mass%, Al: 4.7 to 5.5%, Fe: 0.5 to 1.4%, N: 0.03% or less, Si: 0.15 to 0.40%, oxygen equivalent [O] eq = [O] +2.77 [N]: 0.13% or more and less than 0.25%, In addition to reducing the β-phase fraction at room temperature by having the chemical composition of the balance of Ti and impurities, by performing unidirectional hot rolling, the relative intensity of (0002) reflection of X-rays by crystal grains can be reduced. Although the c-axis orientation of the (0001) plane of the α phase falls within a range of 30 ° from the normal direction (ND direction) of the hot rolled sheet, the strongest strength XND and ± in the sheet width direction (TD direction) of the hot rolled sheet. The ratio with the strongest intensity XTD that falls within 10 degrees / ± 10 degrees, and XTD / XND is 4 The invention discloses that the Young's modulus of the α + β type titanium alloy hot-rolled sheet in the sheet width direction is increased to 135 GPa or more by controlling the texture so as to be 0.0 or more.

JP 2005-320618 A JP 2007-314834 A JP-A-2015-4100 JP 2014-224301 A

  Generally, when the Young's modulus of a metal material used for an industrial member or the like is low, it is necessary to increase the sectional area of the member in order to secure rigidity. Therefore, it is desired that such an industrial member has a higher Young's modulus from the viewpoint of manufacturing cost and the like.

  In addition, titanium alloys are used after being processed into plates, bars, pipes, wires, and the like. In particular, for the bars, titanium alloy bars having a high Young's modulus in the axial direction are required.

  The invention disclosed in Patent Document 1 aims at improving mechanical properties such as room temperature strength, room temperature ductility, and fatigue strength, and therefore cannot increase the Young's modulus of a titanium alloy rod in the axial direction.

  The invention disclosed in Patent Document 2 adjusts the Young's modulus without adjusting the content of elements with a relatively inexpensive chemical composition, but the maximum value of the obtained Young's modulus is 125 GPa, which is higher in the axial direction. A titanium alloy rod having a Young's modulus cannot be provided.

  The invention disclosed in Patent Document 3 secures a high Young's modulus by appropriately selecting the elements contained in the alloy design stage, but it has been sufficiently studied to increase the Young's modulus in the axial direction of the titanium alloy rod. I can't say that.

  The invention disclosed in Patent Document 4 relates to a hot-rolled α + β titanium alloy sheet, and it is difficult to increase the Young's modulus of a titanium alloy rod in the axial direction by this invention.

  More specifically, the α phase of the titanium alloy has a strong anisotropy due to the hcp structure. Further, the Young's modulus greatly differs depending on the crystal orientation. For this reason, as disclosed in Patent Document 4, in a hot-rolled titanium alloy sheet, the crystal orientation is controlled by a hot rolling method such that the c-axis of the hcp structure is directed in the width direction of the hot-rolled sheet. In addition, it is possible to increase the Young's modulus of the hot-rolled sheet in the sheet width direction.

  However, in hot rolling of a titanium alloy rod, it is difficult to control the c-axis of the hcp structure so as to point in the axial direction of the titanium alloy rod. Therefore, it is difficult to develop a high Young's modulus in the axial direction of the titanium alloy rod. difficult.

  As described above, the conventional techniques disclosed in Patent Documents 1 to 4 cannot increase the Young's modulus of the titanium alloy rod in the axial direction.

  An object of the present invention is to provide an α + β type titanium alloy rod having a relatively inexpensive alloy composition and having a high Young's modulus in the axial direction, and a method for producing the same.

  The present inventors have conducted intensive studies on α + β type titanium alloy rods having a high Young's modulus in the axial direction, and as a result, obtained the findings A to C listed below and completed the present invention.

  (A) In titanium, it is known that a β phase which is stable at a high temperature has a lower Young's modulus than an α phase which is stable at a low temperature. The average Young's modulus of the α phase is about 120 GPa, while the Young's modulus of the β phase is 80 GPa. For this reason, the Young's modulus decreases as the ratio of the β phase increases. If the β phase has a chemical composition at which the Ms point (martensite transformation temperature) becomes close to room temperature, the β phase becomes unstable, and the Young's modulus may further decrease to about 60 GPa.

  (B) In general, α + β-type titanium alloys include Fe, Ni, Cr, Mn as eutectoid β-phase stabilizing elements in addition to Al, Zn, and O as α-phase stabilizing elements, It has high strength by containing a β-phase stabilizing element such as V and Mo. By containing these β-phase stabilizing elements, the β-phase is stabilized even at room temperature, and as the content increases, the proportion of the β-phase increases and the Young's modulus tends to decrease.

  (C) Based on a relatively inexpensive Ti-Al-Fe-based titanium alloy component based on a Ti-Al-Fe-Mo-based α + β-type titanium alloy rod containing Mo and then cooled by performing a solution heat treatment. By subjecting the α + β type titanium alloy rod to aging heat treatment with elastic strain imparted in the axial direction, the transformed α phase precipitated during the aging heat treatment is composed of a layered part oriented in the axial direction of the α + β type titanium alloy rod. The α + β type titanium alloy rod having a high Young's modulus of 125 GPa or more in the axial direction can be obtained.

The present invention is as listed below.
(1) An α + β type titanium alloy rod having a chemical composition in mass% of Al: 4.4% or more and less than 5.5%, Fe: 1.4% or more and less than 2.5%, Mo: 1.%. 5% to less than 5.5%, Ni: 0% to less than 0.15%, Cr: 0% to less than 0.25%, Mn: 0% to less than 0.25%, Si: less than 0.1%, C: less than 0.01%, the balance being Ti and impurities, and the metal structure has a proeutectoid α phase, a β phase, and a transformed α phase, and the volume fraction of the β phase is less than 15%; The phase is composed of a layered portion, the average of the angle between the axial direction of the alloy rod and the direction of the layered portion is 45 degrees or less, and the standard deviation of the direction of the layered portion is 20 degrees or less, α + β type titanium alloy rod.

  (2) The α + β type titanium alloy rod according to (1), wherein the Young's modulus in the axial direction is 125 GPa or more.

(3) After subjecting the titanium alloy rod to a solution heat treatment for cooling from a temperature of 850 to 920 ° C. at a cooling rate of 30 ° C./sec or more,
The method for producing an α + β type titanium alloy rod according to the above mode (1) or (2), wherein the titanium alloy rod is subjected to a heat treatment at a temperature of 300 to 700 ° C. in a state where elastic strain is present in the axial direction.

  According to the present invention, it is possible to provide an α + β type titanium alloy rod having a relatively inexpensive alloy composition and having a high Young's modulus in the axial direction.

FIG. 1 is a photograph of the metallographic structure after the strain aging heat treatment. FIG. 2 is a photograph of a metal structure when an aging heat treatment is performed without giving a strain. FIG. 3 is a schematic diagram of a metal structure after a strain aging heat treatment.

  The present invention will be described with reference to the accompanying drawings. In the following description, “%” regarding the chemical composition means “% by mass” unless otherwise specified. In the following description, an α + β type titanium alloy round bar is taken as an example, but the present invention is also applied to an α + β type titanium alloy square bar.

1. Titanium alloy round bar according to the present invention (1) Chemical composition First, essential elements will be described.

(1-1) Al: 4.4% or more and less than 5.5% Al is an α-phase stabilizing element, and increases the strength of a titanium alloy round bar by solid solution strengthening. If the Al content is less than 4.4%, this effect cannot be obtained, and a sufficient strength of 1000 MPa or more cannot be obtained. For this reason, the Al content is at least 4.4%, preferably at least 4.5%, and more preferably at least 4.6%.
On the other hand, when the Al content is 5.5% or more, ductility and cold workability at high temperature and room temperature may be reduced. Therefore, the Al content is less than 5.5%, preferably 5.4% or less, more preferably 5.3% or less.

(1-2) Fe: 1.4% or more and less than 2.5% Fe, as a β-phase stabilizing element, increases the strength of a titanium alloy round bar by solid solution strengthening. If the Fe content is less than 1.4%, this effect cannot be obtained, and a sufficient strength of 1000 MPa or more cannot be obtained. For this reason, the Fe content is at least 1.4%, preferably at least 1.5%, and more preferably at least 1.6%.
On the other hand, if the Fe content is 2.5% or more, segregation tends to occur during solidification, and segregation becomes remarkable in a large ingot of several hundred kg or more. Therefore, the Fe content is less than 2.5%, preferably not more than 2.4%, and more preferably not more than 2.3%.

(1-3) Mo: 1.5% or more and less than 5.5% Mo is a β-phase stabilized substitution type solid solution element, and improves room temperature strength, high temperature strength, room temperature ductility, and fatigue strength similarly to Fe. And improve hot workability and cold workability. Further, as described later, in the present invention, after a certain amount of the β phase remains after the solution treatment, the β phase is subjected to isothermal transformation, so that the transformed α phase is formed in a certain direction in the β phase, that is, the crystal orientation is aligned, thereby forming a layered structure. It is formed into a needle-like structure composed of a part, and although this is not completely parallel to the axial direction of the α + β-type titanium alloy round bar, the structure is aligned in a direction close to that, and the axial direction of the α + β-type titanium alloy round bar To increase the Young's modulus.
If the Mo content is less than 1.5%, the above effects and an appropriate amount of β phase cannot be obtained. Therefore, the Mo content is 1.5% or more, preferably 1.7% or more, and more preferably 1.9% or more.
On the other hand, if the Mo content is 5.5% or more, a problem of solidification segregation occurs. Therefore, the Mo content is less than 5.5%, preferably 5.3% or less, and more preferably 5.0% or less.

  Next, the optional elements will be described.

(1-4) One or more of Ni: 0 to less than 0.15%, Cr: 0 to less than 0.25% or Mn: 0 to less than 0.25% These elements are β-phase stable similarly to Fe. It is an inexpensive element that enhances strength by solid solution strengthening as a chemical element. In the present invention, if necessary, a part of Fe is replaced by at least one of Ni of less than 0.15%, Cr of less than 0.25%, or Mn of less than 0.25%, thereby reducing the amount of Fe. Cost can be reduced.

On the other hand, when these elements are contained in excess of the above upper limits, intermetallic compounds (Ti ₂ Ni, TiCr ₂ , TiMn) which are equilibrium phases are formed, and the fatigue strength and the room temperature ductility are deteriorated.
The total content of Ni, Cr, Mn, and Fe is preferably 1.4% or more and less than 2.5%. If the total content is less than 1.4%, the content of the same β-phase stabilizing element Mo may be increased to obtain a metal structure after cold working, which will be described later, and the production cost may be increased. is there. It is preferably at least 1.5%, more preferably at least 1.6%. On the other hand, when the total content is 2.5% or more, segregation during the production of a large ingot becomes remarkable. It is preferably at most 2.4%, more preferably at most 2.3%.

  Next, impurities will be described. Some of the impurities are contained in the raw material or others are mixed in the production process.

(1-5) Si: less than 0.1% Si is an impurity, and if contained in a large amount, may deteriorate room temperature ductility, cold workability, and hot workability. Therefore, the Si content is less than 0.1%, preferably 0.08% or less, and more preferably 0.05% or less.
On the other hand, excessively reducing the Si content leads to an increase in manufacturing cost. Therefore, the Si content is usually 0.005% or more, preferably 0.007% or more, and more preferably 0.01% or more. % Or more.

(1-6) C: less than 0.01% C is an impurity, and when contained in a large amount, may deteriorate the room temperature ductility, cold workability, and hot workability. Therefore, the C content is less than 0.01%, preferably 0.009% or less, and more preferably 0.008% or less.

(1-7) O: 0.2% or less, N: 0.05% or less, H: 0.015% or less These elements are impurities and are the same as 60 kinds of JIS H 4600 (Ti-6Al-4V). Preferably, the O content is 0.2% or less, the N content is 0.05% or less, and the H content is 0.015% or less. Further, in order to enhance the room temperature ductility and cold workability, it is more preferable that the O content is 0.15% or less, the N content is 0.02% or less, and the H content is 0.01% or less.

(1-8) Remainder The remainder other than the above is Ti.

(2) Metal structure (2-1) Form of α phase The metal structure has a primary α phase, a β phase, and a transformed α phase. In an α + β type titanium alloy, when an aging heat treatment is performed after the solution treatment, a transformed α phase transformed from the β phase precipitates in the β phase. At this time, the precipitation direction of the transformed α phase, that is, the crystal orientation is random as shown in FIG. Note that the structure photograph of FIG. 2 shows only the transformed α phase and does not include the primary α phase.

  FIG. 1 is a photograph of the metallographic structure after the strain aging heat treatment, and FIG. 3 is a schematic view of the metallic structure at that time.

  On the other hand, in the present invention, as will be described later, in the present invention, the aging heat treatment is performed in a state where stress is applied in the axial direction of the α + β type titanium alloy round bar (the strain is imparted during the aging heat treatment), so that FIG. As shown in FIG. 3, a needle-like structure (a needle-like structure aligned in one direction) is formed in the β-phase in a certain direction, that is, the crystal orientation is aligned, and the transformed α-phase is formed of a layered portion. Since the transformed α-phase has a different crystal orientation in each β-phase crystal grain, the β-phase crystal grains are oriented in the direction of the needle-like structure composed of the layered portions and in the axial direction (stress-loading direction) of the titanium alloy round bar. Although a slight angle difference occurs, the layer portions are aligned in a direction close to, but not completely parallel to, the stress loading direction, that is, the axial direction of the α + β type titanium alloy round bar.

  Also, the normal direction of the transformed α-phase (0001) plane, that is, the c-axis direction of hcp becomes an angle close to the axial direction (45 degrees or less), and the texture becomes uniform with the crystal orientation. For this reason, according to the present invention, the Young's modulus of the α + β type titanium alloy round bar in the axial direction can be increased.

  Specifically, the transformed α phase is composed of a needle-like structure composed of a layered portion, and the deviation in the directivity direction of the layered portion is 20 degrees or less, preferably 18 degrees or less, and more preferably 15 degrees or less, The average angle formed by the axial direction of the α + β type titanium alloy round bar and the directivity direction of the layer portion is 45 degrees or less, preferably 30 degrees or less, and more preferably 15 degrees or less.

  As described above, the mechanism in which the direction of precipitation of the transformed α phase that precipitates by applying strain during the aging heat treatment is not clear, but β in forming the layered portion under a stressed state is not clear. It is presumed that this is because variant selection (selection of crystal orientation) occurs during the phase transformation from the phase to the α phase.

(2-2) Volume Ratio of β Phase In the present invention, the Young's modulus of the α + β type titanium alloy round bar in the axial direction is increased by controlling the structure form (metal structure) of the α phase. Unlike other metal materials, in a titanium alloy, the Young's modulus is not determined only by the chemical composition, but greatly varies depending on the metal structure such as the area ratio of the β phase and the martensite phase.

  For example, also in the Ti-Al-Fe-Mo component system of the present invention, the Young's modulus can be changed by changing the aging heat treatment temperature and other conditions.

  As described above, in the present invention, the Young's modulus in the axial direction of the α + β type titanium alloy round bar is increased by controlling the volume ratio and the microstructure of the α phase. As described above, the Ti-Al-Fe-Mo component system of the present invention contains Fe and Mo, which are β-phase stabilizing elements, in a certain amount or more. It is present close to volume%.

  For this reason, in the present invention, the volume fraction of the β phase is made smaller than the equilibrium state by a method described later, specifically, the volume fraction of the β phase is less than 15%, preferably 10% or less, more preferably By setting the content to 5% or less, the proportion of the β phase having a small Young's modulus is reduced, thereby increasing the Young's modulus of the α + β type titanium alloy round bar in the axial direction.

(2-3) Measurement method of volume ratio of β phase The volume ratio of β phase can be measured by X-ray diffraction. That is, after the surface of the material is mirror-polished, the volume ratio of the β phase is measured by wide angle X-ray diffraction measurement using a Cu tube at 40 kV, 150 mA, and a diffraction angle 2θ of 20 to 100 °. It can be measured by analyzing the diffraction intensity of the generated diffraction peak.

(2-4) Confirmation Method of Metal Structure and Each Phase The needle-like structure constituted by the layered portion of the α-phase in the β-phase is observed by using a transmission electron microscope or EBSD (Electron Backsccaterd Diffraction pattern). You can check. A sample for observation is prepared from the L cross section of the round bar, and by observing, it can be confirmed whether or not a layered portion (unidirectional needle-like structure) is formed. When observing with a transmission electron microscope, a sample for observation with a transmission electron microscope is prepared from the L section by the FIB method and observed at an acceleration voltage of 200 kV. When observing by EBSD, an observation sample is prepared from the L section by polishing the colloidal silica and observed.
Here, a method of measuring the angle difference and the deviation between the needle-shaped tissue constituted by the layered portion and the directing direction will be described. About 10 to 20 β-phase structures are observed with a transmission electron microscope or EBSD in an L section of the bar (a cross section parallel to the axial direction of the bar). The angle difference between the axial direction of the bar material of each β grain and the directivity direction of the needle-like structure of the transformed α-phase formed in a layer is obtained, and the average is defined as the angle difference between the axial direction of the bar material and the layer-like needle-like structure . Further, the standard deviation of the directivity direction of the layered portion of each β grain is obtained from the average angle difference, and is determined as the deviation.

2. Method for producing titanium alloy round bar The titanium alloy round bar according to the present invention is, for example, a solution formed by cooling a titanium alloy rod having the above chemical composition from a temperature of 850 to 920 ° C at a cooling rate of 30 ° C / sec or more. An α + β-type titanium alloy rod having the above metal structure, which has been subjected to heat treatment and aging heat treatment at a temperature of 300 to 700 ° C. in a state where elastic strain is present in the axial direction of the titanium alloy rod.

(1) Casting, hot forging and hot rolling Using titanium dioxide TiO ₂ as a raw material by a crawl method to produce titanium metal at a high temperature of about 900 to 1000 ° C., and taking one to several days to produce Mg and MgCl ₂ . After evaporating and removing titanium sponge, sponge titanium is pressed and connected, combined with a consumable electrode, and vacuum alloy melting (VAR: Vacuum Arc Remelting) method of several to several tens tons of titanium alloy Produce an ingot, and further perform electron beam melting in a high vacuum to obtain a titanium alloy ingot having the above-described chemical composition, and further manufacture a titanium alloy round bar by performing hot forging and hot rolling. .

(2) Solution treatment In the present invention, the α + β titanium alloy round bar having the above-mentioned chemical composition is cooled at a cooling rate of water cooling or higher from a temperature of 850 ° C. or more and 920 ° C. or less in the annealing step. In the present invention, utilizing the isothermal transformation at the time of aging heat treatment after the solution treatment, the volume fraction of the β phase is made lower than the equilibrium state, and further, the needle constituted by the layered portion of the transformed α phase precipitated in the β phase A high Young's modulus is obtained by aligning the texture.

  Here, the isothermal transformation is a transformation similar to the martensitic transformation. By utilizing the isothermal transformation, it is possible to obtain a β-phase fraction lower than the normal equilibrium state. Since the isothermal transformation occurs in a specific chemical component system, the composition of the β phase after the solution treatment is important. If the solution treatment is performed in the above temperature range, isothermal transformation can be caused during the subsequent aging heat treatment.

  If the treatment temperature is higher than 920 ° C., martensitic transformation occurs during cooling after the solution treatment, and even if an α phase is generated by isothermal transformation in the subsequent aging heat treatment, the needle-like structure constituted by the layered portion is not removed. The Young's modulus cannot be increased because they are not aligned in the direction. On the other hand, if the treatment temperature is lower than 850 ° C., diffusion transformation occurs in the aging heat treatment after cooling, and a needle-like structure composed of layered portions aligned in one direction cannot be formed. For this reason, the temperature of the solution treatment is 850 to 920 ° C. Since this solution treatment temperature is in the two-phase region of the α phase and the β phase, all of the pro-eutectoid α phase does not disappear, and a part remains even after the solution treatment.

(3) Cooling Cooling after solution treatment is performed at a cooling rate higher than water cooling (for example, 30 ° C./sec or more). If the cooling is air cooling (cooling rate: 1 ° C./sec or less), a fine transformation α phase precipitates in β phase grains during cooling, and no isothermal transformation occurs during the subsequent aging heat treatment, and a Young's modulus of 125 GPa or more Can not be obtained. For this reason, the cooling rate after the solution treatment is a cooling rate higher than water cooling.

(4) Aging heat treatment and imparting distortion α + β titanium alloy is subjected to aging heat treatment at a temperature of 300 ° C. or more and 700 ° C. or less after solution treatment in a state where stress is applied in the axial direction of the α + β titanium alloy round bar. The Young's modulus of the round bar in the axial direction can be increased to 125 GPa or more. The aging time varies depending on the aging temperature, and the aging temperature of 300 ° C. or more and 700 ° C. or less can be set to 1 minute to 8 hours to optimize the β phase volume ratio.

  In the present invention, the α + β type titanium alloy round bar is strained in the axial direction of the α + β type titanium alloy round bar during the aging heat treatment. The distortion amount at this time is set to 0.1 to 8%.

  After the solution treatment, if the aging heat treatment is performed without applying stress in the above temperature range, the α phase (transformed α phase) generated by the isothermal transformation is not aligned in one direction as described above. Has an Young's modulus in the axial direction of less than 125 GPa.

  On the other hand, when the aging heat treatment is performed in a state where a stress is applied in the axial direction of the α + β type titanium alloy round bar, the transformed α phase (needle structure constituted by the layered portion) is aligned in one direction during isothermal transformation. And the Young's modulus in the axial direction of the α + β type titanium alloy round bar is 125 GPa or more.

  The stress applied to the α + β type titanium alloy round bar may be obtained by applying a stress while pulling the round bar during the aging heat treatment, or by applying a tensile process in the axial direction first, thereby forming the shaft of the round bar. A residual stress may be generated in the direction, and then aging heat treatment may be performed. In any case, it suffices that the α + β type titanium alloy round bar at the time of the aging heat treatment is strained.

  Titanium alloy ingots having the chemical composition shown in Table 1 were produced by a vacuum arc melting (VAR) method, and these were used as titanium alloy round bars produced by hot forging and hot rolling.

  Examples of the present invention and comparative examples shown in Table 2 correspond to No. 1 of Table 1. The materials 1 to 10 were subjected to a solution treatment at 900 ° C. for 1 hour, then cooled with water, and then subjected to an aging heat treatment while applying a stress in the axial direction of the round bar. The distortion amount at this time was set to 3%.

  With respect to the material after the aging heat treatment, the volume fraction of the β phase and the Young's modulus in the axial direction of the round bar were measured. The Young's modulus of the round bar in the axial direction was measured by preparing an ASTM E8M subsize (diameter of the parallel portion is 6.25 mm and length of 25 mm) such that the parallel portion is in the axial direction of the round bar, and attached a strain gauge. Using the data, up to half the stress of the proof stress was measured by linear approximation.

  No. of Table 2 In the present invention examples A-1 to A-7, the aging heat treatment was performed in the range of 300 to 700 ° C. while applying a tensile stress in the axial direction. In the metal structure of the material after aging, the volume fraction of the β phase was less than 15%, and a unidirectional layered portion was formed in the β phase. Therefore, the axial Young's modulus of the round bar showed a high value of 125 GPa or more.

  No. of Table 2 In the present invention examples of A-8 to A-9, after the tensile processing was performed in the axial direction to generate the residual stress in the axial direction, the aging heat treatment was performed in the range of 300 to 700 ° C. In the metal structure of the material after aging heat treatment, the volume fraction of the β phase was less than 15%, and a unidirectional layered portion was formed in the β phase. Therefore, the axial Young's modulus of the round bar shows a high value of 125 GPa or more.

  No. of Table 2 In the comparative example of A-10, the aging heat treatment was performed in the range of 300 to 700 ° C. while applying a tensile stress in the axial direction. However, since the chemical composition is out of the range of the present invention, no unidirectional layered portion is formed in the β phase, and the Young's modulus in the axial direction of the round bar is less than 125 GPa.

  Examples and comparative examples shown in Table 3 correspond to No. Solution 1 was subjected to solution treatment and aging heat treatment at various temperatures and cooling rates. With respect to the material after the aging heat treatment, the volume fraction of the β phase and the Young's modulus in the axial direction of the round bar were measured. The distortion amount at this time was set to 3%.

  The Young's modulus of the round bar in the axial direction was measured by preparing an ASTM E8M subsize (diameter of the parallel portion is 6.25 mm and length of 25 mm) such that the parallel portion is in the axial direction of the round bar, and attached a strain gauge. Using the data, up to a stress of up to half of the proof stress was measured by linear approximation.

  No. 3 in Table 3. In the comparative example of A-11, the solution treatment temperature was as low as 800 ° C. Therefore, aging heat treatment is performed in the range of 300 to 700 ° C. while applying a tensile stress in the axial direction. However, in the metal structure, the volume fraction of the β phase is 15% or more, and one direction is contained in the β phase. , The axial Young's modulus of the round bar is as low as 125 GPa or less.

  No. 3 in Table 3. In the present invention examples of A-12 to A-13, the solution treatment is in the range of 850 to 920 ° C, and the aging heat treatment is performed in the range of 300 to 700 ° C while applying tensile stress in the axial direction. . In the metal structure of the material after the aging heat treatment, the volume ratio of the β phase is less than 15%, and a unidirectional layered portion is formed in the β phase. Therefore, the axial Young's modulus of the round bar shows a high value of 125 GPa or more.

  No. 3 in Table 3. In the comparative example of A-14, the solution treatment temperature was as high as 940 ° C., and the unidirectional layered portion was not formed in the metal structure after the aging heat treatment. Therefore, the axial Young's modulus of the round bar was less than 125 GPa. .

  No. 3 in Table 3. In the comparative example of A-15, since the cooling rate of the solution heat treatment temperature was slow as air cooling, the β-phase fraction was as high as 15% or more in the metal structure after aging, and no unidirectional layered portion was formed. . Therefore, the axial Young's modulus of the round bar is less than 125 GPa.

Further, in Table 3, No. In the comparative example of A-16, since no stress was applied at the time of the aging heat treatment, and the unidirectional layered portion was not formed in the metal structure, the axial Young's modulus of the round bar was less than 125 GPa.

Claims (3)

α + β type titanium alloy rod,
Chemical composition in mass%
Al: 4.4% or more and less than 5.5%,
Fe: 1.4% or more and less than 2.5%,
Mo: 1.5% or more and less than 5.5%,
Ni: 0% or more and less than 0.15%,
Cr: 0% or more and less than 0.25%,
Mn: 0% or more and less than 0.25%,
Si: less than 0.1%,
C: less than 0.01%,
The remaining Ti and impurities,
The metal structure has a proeutectoid α phase, a β phase, and a transformed α phase, the volume ratio of the β phase is less than 15%, the transformed α phase is constituted by a layered portion, and An α + β type titanium alloy rod, wherein the average of the angles formed by the direction of the layer portion and the direction of the layer is 45 degrees or less, and the standard deviation of the direction of the direction of the layer portion is 20 degrees or less.   The α + β-type titanium alloy rod according to claim 1, wherein the Young's modulus in the axial direction is 125 GPa or more. After subjecting the titanium alloy rod to a solution heat treatment for cooling from a temperature of 850 to 920 ° C. at a cooling rate of 30 ° C./sec or more,
The method for producing an α + β-type titanium alloy rod according to claim 1 or 2, wherein heat treatment is performed at a temperature of 300 to 700 ° C in a state where elastic strain exists in the axial direction of the titanium alloy rod.