JP6540179B2 - Hot-worked titanium alloy bar and method of manufacturing the same - Google Patents

Description

The present invention relates to a manufacturing method between excellent thermal fatigue properties machining titanium alloy rod material and its.

Titanium is lightweight, has high specific strength, and has excellent heat resistance. For this reason, titanium or titanium alloys are used in a wide range of fields such as aircraft and automobiles.
As titanium or a titanium alloy, for example, techniques described in Patent Literatures 1 to 5 have been proposed.

In Patent Document 1, a crystal having a c-axis inclination of 15 ° or less and 75 ° or more, which is an angle between a direction perpendicular to the cutting surface and the c-axis of the crystal lattice, and a crystal grain diameter of 100 μm or more There has been described α-type titanium member having a crystal orientation distribution is 10 pieces / mm ² or less.
Further, Patent Document 2 describes a forging method of a titanium alloy which is forged in a temperature range equal to or lower than a temperature 65 ° C. lower than the β transformation point after β forging a Ti-Al-V-Mo alloy.

  Patent Document 3 describes a method for producing an α + β-type titanium alloy plate having excellent mechanical properties and reduced ultrasonic noise, and the α + β-type titanium alloy slab in a heated state is 0.5 or more than the β single phase region. After cooling at a cooling rate of ° C / s or more, it is heated in the α + β temperature range to perform hot forging with a height ratio of 10% or more, then hot rolling in the α + β temperature range and heat treatment in the α + β temperature range It is proposed to apply sequentially.

According to Patent Document 4, α is within a range of ± 30 ° in the longitudinal direction of the billet toward the direction of strong pressure, and in the range of ± 40 ° to 90 ° in the direction perpendicular to the longitudinal direction from the longitudinal direction of the billet. A titanium alloy billet is described in which the c-axis direction of the phase is accumulated and the degree of accumulation is 3 or more.
In Patent Document 5, the average particle diameter of granular α-titanium is 6 μm to 15 μm, the maximum size of the colony which is an aggregate of granular α-titanium is 120 μm or less, and α is within ± 40 ° from the longitudinal direction of the titanium alloy billet. A titanium alloy billet is described in which there is c-axis accumulation of titanium phase, the degree of accumulation being greater than 1.5 at the center of the cross section D perpendicular to the longitudinal direction.

JP, 2013-1961, A JP-A-59-104233 JP-A-8-49053 JP, 2012-224935, A JP, 2014-65967, A

  However, when the conventional titanium or titanium alloy is used for, for example, a fan blade of an aircraft engine, an engine valve of a car, etc., the fatigue characteristics are insufficient and it is required to improve the fatigue characteristics.

The present invention has been made in view of the above circumstances, and an object thereof is to provide a method for producing a hot-working the titanium alloy rod material having excellent fatigue properties and their.

In order to solve the above problems, the inventor conducted a fatigue test of a two-phase hot-worked titanium alloy bar having a metal structure composed of an α phase and a β phase, and focused on the origin of a crack at the time of fracture. I examined it earnestly. As a result, it has been found that a hot-worked titanium alloy bar having excellent fatigue properties can be obtained by sufficiently reducing coarse pro-eutectoid α-grains having a specific crystal orientation which is likely to be a crack origin. I thought of the invention.
The gist of the present invention is as follows.

(1) A hot-worked titanium alloy bar ,
The components of the hot-worked titanium alloy bar are Al: 5.50 to 6.75 mass%, V: 3.50 to 4.50 mass%, Fe: 0.05 to 0.30 mass%, O: Containing 0.05 to 0.20% by mass, the balance being Ti and unavoidable impurities,
And has a metal structure consisting of an α phase consisting of pro-eutectoid α grains and needle-like α grains, and a β phase,
The area ratio of the pro-eutectoid α particles in the metal structure is 30% or more,
Among the pro-eutectoid α-grains, an angle (inclination of c-axis) between c-axis direction of dense hexagonal structure and length direction of hot-worked titanium alloy bar is 25 ° or more and 55 ° or less, and equivalent to a circle A hot-worked titanium alloy bar characterized in that an area ratio of pro-eutectoid α particles having a diameter of 20 μm or more in the metal structure is 2.0% or less.

(2) A hot-worked titanium alloy bar,
The components of the hot-worked titanium alloy bar are Al: 5.50 to 6.50 % by mass, Sn: 1.75 to 2.25% by mass, Zr: 3.50 to 4.50% by mass, Mo: 5.50 to 6.50 wt%, Fe: 0.02 to 0.15 wt%, O: 0.02 to 0.15 and containing mass%, Ri balance Ti and unavoidable impurities der,
And has a metal structure consisting of an α phase consisting of pro-eutectoid α grains and needle-like α grains, and a β phase,
The area ratio of the pro-eutectoid α particles in the metal structure is 30% or more,
Among the pro-eutectoid α-grains, an angle (inclination of c-axis) between c-axis direction of dense hexagonal structure and length direction of hot-worked titanium alloy bar is 25 ° or more and 55 ° or less, and equivalent to a circle A hot-worked titanium alloy bar characterized in that an area ratio of pro-eutectoid α particles having a diameter of 20 μm or more in the metal structure is 2.0% or less .

( 3 ) A method of manufacturing a hot-worked titanium alloy bar made of an α + β two-phase titanium alloy manufactured by hot working an ingot,
Β processing step of hot working the ingot in a β single phase temperature range,
a first α + β processing step of hot working so that the wrought ratio is 1.7 or more in the α + β two-phase temperature range;
a heat treatment step of heating to a β single phase temperature range and then water cooling;
The hot- worked titanium according to claim 1 or 2, characterized in that a second α + β processing step of hot working so that the wrought ratio is 3.0 or more in the α + β two-phase temperature range is performed in this order. Method of manufacturing alloy bar.

The hot-worked titanium alloy bar according to the present invention has a metal structure consisting of an α phase and a β phase, and the area ratio of coarse pro-eutectoid α grains having a specific crystal orientation in the metal structure is 2.0% Since it is the following, the outstanding fatigue characteristic is obtained.
According to the method of manufacturing a hot-worked titanium alloy rod of the present invention, the area ratio of coarse pro-eutectoid α-grains in metal structure having a specific crystal orientation having a metal structure consisting of α phase and β phase The hot-worked titanium alloy bar excellent in the fatigue characteristic which is 2.0% or less is obtained.

It is the graph which showed the relationship between the size of the pro-eutectoid alpha particle which became the starting point of the crack, and the inclination of c axis. It is a schematic diagram for demonstrating angle (theta) (inclination of c axis | shaft) of c axis direction of pro-eutectoid (alpha) particle which has a close-packed hexagonal structure, and the length direction of a titanium alloy bar to make. It is a microscope picture of an example of a titanium alloy bar of the present invention. No. It is a microscope picture of a titanium alloy bar of 1. No. It is a microscope picture of five titanium alloy bars.

Hereinafter, the hot-worked titanium alloy bar of the present invention and the method for producing the same will be described in detail.
In the following, the hot-worked titanium alloy bar may be simply referred to as a titanium alloy bar.
In order to solve the above-mentioned problems, the present inventor conducted a low cycle fatigue test on a dual phase titanium alloy bar having a metal structure consisting of an α phase and a β phase, and determined the fatigue life (the number of repetitions until breakage) Asked for). Test pieces for fatigue test were taken from a round bar formed of Ti-6Al-4V (a titanium alloy containing 6% by mass of Al and 4% by mass of V) manufactured by hot working an ingot. . And the thing whose repetition frequency until it fractures | ruptures is 80,000 times or more was evaluated as favorable, and what was less than 80,000 times was evaluated as defect.

  In addition, the fracture surface of the test piece after the fatigue test was observed by SEM (scanning electron microscope), and the starting point at which the crack was generated was specified. As a result, the origin of the crack was coarse pro-eutectoid α particles in any of the test pieces. Therefore, the inventor of the present invention has made the EBSD (electrons) of the size (equivalent circle diameter (μm)) and the crystal orientation (the inclination θ (°) of the c axis) of the pro-eutectoid α particles that became the origin of the crack. It was investigated using line backscattering diffraction (Electron Backscatter Diffraction). The results are shown in FIG.

  FIG. 1 is a graph showing the relationship between the size of a pro-eutectoid α grain that is the starting point of a crack and the inclination of the c-axis. In FIG. 1, the symbol は indicates the result of a test piece in which the number of repetitions until breakage in the fatigue test is 80,000 or more, and the symbol x indicates the result of a test piece in which the number of repetitions is less than 80,000. Further, as shown in FIG. 2, the inclination of the c-axis of the pro-eutectoid α-grains shown in FIG. 1 is between the c-axis direction of the pro-eutectoid α-grains having a dense hexagonal structure and the length direction of the titanium alloy bar It means the angle θ.

As shown in FIG. 1, in the test piece in which the pro-eutectoid α particles have a crystal orientation (inclination of c axis) of 25 ° or more and 55 ° or less and a size (equivalent circle diameter) of 20 μm or more, The characteristics were poor.
From the above results, coarse pro-eutectoid α-grains with an equivalent circle diameter of 20 μm or more with a c-axis inclination of a close-packed hexagonal structure of 25 ° or more and 55 ° or less cause low cycle fatigue characteristics to deteriorate. I understand.

The reason why the pro-eutectoid α-grains having the c-axis inclination and the equivalent circle diameter in the above range cause the low cycle fatigue characteristics to be reduced is presumed to be as follows.
The base slip of a close-packed hexagonal crystal is more likely to occur as the crystal orientation (indicated by symbol “θ” in FIG. 2) approaches 45 °, and becomes active when the crystal orientation is 25 ° or more and 55 ° or less. In addition, the larger the size of the equiaxed pro-eutectoid α-grains contained in the metallographic structure, the easier it is for the stress applied to the test piece to be concentrated, and the stress concentration becomes remarkable if the equivalent circle diameter is 20 μm or more. . Accordingly, pro-eutectoid α-grains having a c-axis inclination of 25 ° to 55 ° and a circle-equivalent diameter of 20 μm or more are prone to bottom slippage of dense hexagonal crystals and stress is easily concentrated, so the fatigue life is It is thought that it became short.

Next, in order to obtain a titanium alloy bar having excellent fatigue properties, the present inventor manufactured a titanium alloy bar having a small amount of pro-eutectoid α-grains, the inclination of the c axis and the equivalent circle diameter being in the above range. We studied earnestly about. Specifically, using a free forging method as hot working, titanium alloy bars were made on trial under various conditions.
As a result, (1) β processing step of hot working the ingot in a β single phase temperature region, and (2) hot working of a ingot in a wrought ratio of 1.7 or more in an α + β two phase temperature region (2) a second α + β processing which is hot-worked to have a wrought ratio of 3.0 or more in a processing step, (3) a β heat treatment step of heating to a β single phase temperature range and then water cooling, It has been found that the steps and the steps should be performed in this order.

That is, according to the hot working method of performing the above (1) to (4), coarse pro-eutectoid α-grains having a c-axis inclination of 25 ° or more and 55 ° or less and a circle equivalent diameter (particle diameter) of 20 μm or more It can be reduced to
Furthermore, when the test piece was extract | collected from the titanium alloy bar manufactured using said hot-working method and the low cycle fatigue test was done, the outstanding fatigue life was obtained. Specifically, the area ratio of the pro-eutectoid α grains in the metal structure of which the inclination of the c axis and the equivalent circle diameter fall within the above range is reduced to 2.0% or less, and the number of repetitions until breakage occurs in the low cycle fatigue test A titanium alloy rod having good fatigue properties, having a strength of at least 80,000 times, was obtained.

"Titanium alloy bar"
Next, the titanium alloy bar of the present embodiment will be described in detail.
FIG. 3 is a photomicrograph of a titanium alloy rod of the present embodiment which is an example of the present invention. The titanium alloy rod of the present embodiment is manufactured by hot working an ingot, and has a crystal structure of a dense hexagonal (hcp) structure α phase and a body centered cubic (bcc) structure β phase It consists of two phases.

  The titanium alloy rod of the present embodiment has a metal structure composed of an α phase consisting of pro-eutectoid α grains and needle-like α grains and a β phase. As shown in FIG. 3, the pro-eutectoid α particles are equiaxed. The acicular α particles are generated from the β phase when the temperature is lowered from the β single phase temperature range to the α + β two phase temperature range in hot working. As shown in FIG. 3, the beta particles forming the beta phase are needle-like. The metal structure of the titanium alloy rod according to the present embodiment is sufficiently hot-worked in the α + β two-phase temperature range, whereby needle-like α particles and β particles alternate between equiaxed pro-eutectoid α particles. It is a so-called bimodal organization that is layered on top of the other.

In the titanium alloy rod of the present embodiment, the area ratio of α phase in the metal structure (sum of pro-eutectoid α grains and needle-like α grains) can be 80% or more, even 90% or more Good.
In addition, the area ratio of pro-eutectoid α-grains in the metal structure is 30% or more. When the area ratio of pro-eutectoid α-grains in the metal structure is 30% or more, excellent fatigue strength can be obtained by the ductility improvement effect by including the equiaxed structure.

In the titanium alloy bar of the present embodiment, an angle (inclination of the c axis) between the c axis direction of the dense hexagonal structure and the length direction of the titanium alloy bar of the proeutectoid α grains is 25 ° or more and 55 ° or less And the area ratio of the pro-eutectoid α particles having a circle equivalent diameter of 20 μm or more in the metal structure is 2.0% or less. Since pro-eutectoid α-grains having a c-axis inclination of 25 ° or more and 55 ° or less and a circle-equivalent diameter of 20 μm or more cause deterioration in fatigue characteristics, the smaller the better, the less it is contained in the metal structure preferable. However, even if the inclination of the c-axis and the equivalent circle diameter are included in the above range, a good fatigue life can be obtained if the area ratio in the metal structure is 2.0% or less.
For this reason, the area ratio of the pro-eutectoid α particles in the metal structure of which the inclination of the c-axis and the equivalent circle diameter are in the above-mentioned range is 2.0% or less, preferably 1.5% or less. More preferably, it is 0% or less, and it is most preferable that the pro-eutectoid α-grains having the inclination of c axis and the equivalent circle diameter in the above range are not included (0.0%).

Next, the components of the α + β two-phase titanium alloy forming the titanium alloy rod of the present embodiment will be described.
The component of the α + β two-phase titanium alloy preferably contains 5.00 to 7.00% by mass of Al, and in addition to Ti and Al, V, Sn, Zr, Mo, Si, Fe, O, etc. It may be included. When the Al content is 5.00% by mass or more, a titanium alloy bar having excellent fatigue properties in steel strength can be obtained. In addition, when the Al content is 7.00% by mass or less, the titanium alloy rod can be prevented from becoming brittle due to the formation of an intermetallic compound such as TiAl.

  The α + β two-phase titanium alloy may be formed of, for example, components defined by AMS4928. That is, Al: 5.50 to 6.75 mass%, V: 3.50 to 4.50 mass%, Fe: 0.05 to 0.30 mass%, O: 0.05 to 0.20 mass%. And the balance is Ti and unavoidable impurities. As unavoidable impurities, for example, N: 0.08% by mass or less, C: 0.08% by mass or less, H: 0.015% by mass or less.

  Further, the α + β two-phase titanium alloy may be formed of, for example, a component specified by AMS 4981. That is, Al: 5.50 to 6.50% by mass, Sn: 1.75 to 2.25% by mass, Zr: 3.50 to 4.50% by mass, Mo: 5.50 to 6.50% by mass, Fe: 0.02-0.15 mass%, O: 0.02-0.15 mass% are contained, and remainder is Ti and an unavoidable impurity. As unavoidable impurities, for example, N: 0.08% by mass or less, C: 0.08% by mass or less, H: 0.015% by mass or less.

The shape of the titanium alloy bar of the present embodiment is not particularly limited. For example, the titanium alloy bar of the present embodiment may be a round bar having a circular cross-sectional shape orthogonal to the longitudinal direction, or may be a square bar having a polygonal shape such as a square or an octagon. When the titanium alloy bar is a round bar, the cross-sectional shape orthogonal to the longitudinal direction may be a perfect circle, but it does not have to be a perfect circle, and it may be an approximate circle.
The size (equivalent circle diameter obtained from the area of the cross section orthogonal to the length direction) of the titanium alloy bar of this embodiment is not particularly defined, and can be determined according to the application of the titanium alloy bar, The size can be made a predetermined size by adjusting the size of the ingot obtained by melting and the wrought ratio in the manufacturing process described later.

"Method of manufacturing titanium alloy bar"
Next, a method of manufacturing a titanium alloy bar of the present embodiment will be described.
The titanium alloy bar of the present embodiment is manufactured by hot working an ingot (ingot) having a predetermined component.
A conventionally known method can be used as a method of producing an ingot. The size of the ingot is not particularly limited, and may be, for example, a cylindrical shape having a diameter of 600 mm to 750 mm.
As a method of hot working, a conventionally known method can be used, and for example, forging, rolling, extrusion and the like can be mentioned.

In the method of manufacturing a titanium alloy rod according to this embodiment, (1) β processing step, (2) first α + β processing step, (3) β heat treatment step, and (4) second α + β processing step are performed in this order .
In the present embodiment, the wrought ratio, which is the degree to which the material to be processed is hot-worked in each of the steps (1), (2) and (4), is the area before processing and the cross-section perpendicular to the length direction The ratio to the area (area before processing / area after processing) is meant.
Hereinafter, each process of (1)-(4) is demonstrated in detail.

(1) β Processing Step In the β processing step of this embodiment, the ingot obtained by melting is heated to a β single phase temperature range in a heating furnace, and subjected to hot processing such as forging. In the β-processing step, the ingot, which is a material to be processed, is hot-processed in the state of β single phase.

  In the β processing step, the temperature in the heating furnace for heating the ingot is 30 ° C. or more higher than β transformation temperature and 200 ° C. or more higher than β transformation temperature (β transformation temperature + 30 ° C. to β transformation temperature It is preferable to set it as +200 degreeC temperature range). If the temperature in the heating furnace is 30 ° C. higher than the β transformation point temperature, the temperature in the heating furnace may be uneven, or the size of the ingot may be large. It is preferable because the whole mass is heated to the β transformation temperature or more. Moreover, since the oxidation of the surface layer of the ingot is suppressed and the coarsening of the metal structure in the ingot is suppressed when the temperature in the heating furnace is equal to or lower than the temperature higher by 200 ° C. than the β transformation point temperature, A high quality titanium alloy bar is obtained.

The wrought ratio in the β processing step is preferably 1.05 or more in order to collapse the solidified structure in the ingot. The upper limit of the wrought ratio can be appropriately determined in accordance with the size of the ingot and the size of the target titanium alloy bar, and the wrought ratio in the first α + β processing step and the second α + β processing step described later.
The number of heats in the β processing step (the number of repetitions of heating and hot working of the material to be processed) may be once or plural times, and the temperature for heating the ingot which is the material to be processed, hot It can determine suitably according to the conditions of hot processing, such as the method of processing, and wrought ratio.

  By performing the β processing step on the ingot, a billet, which is an intermediate material of a titanium alloy bar, is obtained. The obtained billet may be placed in a heating furnace heated to an α + β two-phase temperature range to perform the first α + β processing step, or may be allowed to cool as it is (air cooling) to room temperature.

(2) 1st alpha + beta processing process At the 1st alpha + beta processing process of this embodiment, the billet obtained by performing beta processing process is heated to alpha + beta two phase temperature range in a heating furnace, and hot working such as forging is performed . In the first α + β processing step, the billet, which is the material to be processed, is processed in the state of the α + β two-phase.

  In the first α + β processing step, the temperature in the heating furnace for heating the billet is not less than 150 ° C. lower than the β transformation temperature and not more than 30 ° C. lower than the β transformation temperature (β transformation temperature −150 ° C. to β transformation point It is preferable to set it as temperature -30 degreeC temperature range). When the temperature in the heating furnace is 150 ° C. lower than the β transformation temperature or higher, the deformation resistance of the billet during hot working can be prevented from becoming too large, and hot working can be performed easily and efficiently. be able to. Further, if the temperature in the heating furnace is equal to or lower than the temperature lower by 30 ° C. than the β transformation temperature, the α phase is sufficiently precipitated in the metal structure of the billet, so hot working is performed in the α + β two-phase temperature range The effect by is sufficiently obtained.

  In the first α + β processing step, the surface temperature of the billet during hot working is preferably in the range of the β transformation point temperature of −270 ° C. to the temperature in the above heating furnace, and the β transformation point temperature of −250 ° C. It is more preferable to set it in the range of the temperature in the heating furnace. Sufficient strain can be applied to the billet by setting the surface temperature of the billet during hot working to a temperature at which the β transformation point temperature is −270 ° C. or higher. As a result, the crystal orientation in the metal structure of the billet is dispersed, and the number of pro-eutectoid α particles having a crystal orientation (inclination of c axis) of 25 ° or more and 55 ° or less according to steps (3) and (4) described later. A reduced metallographic structure is easily obtained. In addition, since sufficient strain can be applied to the billet, fine pro-eutectoid α-particles can be easily obtained by the steps (3) and (4) described later. In addition, by setting the surface temperature of the billet during hot working to a temperature at which the β transformation point temperature is −270 ° C. or higher, the deformation resistance of the billet does not become too large, and efficient hot working can be performed.

The surface temperature of the billet gradually decreases during hot working. Therefore, if the surface temperature of the billet during hot working becomes less than the β transformation point temperature −270 ° C. before the hot working at the predetermined working ratio is finished, the hot working is temporarily interrupted, and the billet is again operated. It is preferable to heat and then perform hot working.
The hot working in the first α + β working process is preferably performed while measuring the surface temperature of the billet using a thermometer such as a radiation thermometer in order to control the surface temperature of the billet during the hot working.

  In the first α + β processing step, it is more preferable to perform hot working so that the forging ratio is 1.7 or more, and perform hot processing so that the forging ratio is 2.0 or more. By setting the wrought ratio in the first α + β processing step to 1.7 or more, it is sufficient for colonies consisting of needle-like α particles and β phase generated from β particles generated in β processing step present in the metal structure of the billet Strain can be applied to cause recrystallization by the β heat treatment process, and the crystal orientation in the billet metal structure is dispersed. As a result, a metal structure with a reduced number of pro-eutectoid α-grains having a crystal orientation (inclination of c axis) of 25 ° or more and 55 ° or less is easily obtained by the steps (3) and (4) described later. Fine pro-eutectoid alpha particles are easily obtained. If the wrought ratio in the first α + β processing step is less than 1.7, pro-eutectoid α-grains having the c-axis inclination and the equivalent circle diameter in the above ranges can not be sufficiently reduced. The upper limit of the wrought ratio is not particularly limited, but if it is too large, it becomes difficult to secure the wrought ratio in the second α + β processing step, so it is desirable to be 4.0 or less.

The number of heats in the first α + β processing step may be one or more than one, and can be appropriately determined according to the temperature of heating the billet, the method of hot working, the conditions of hot working such as wrought ratio .
In the present embodiment, sufficient strain is given to the billet by performing the first α + β processing step. For this reason, while it becomes easy to be obtained by the process of (3) and (4) mentioned later, the metal structure in which the crystal orientation (inclination of c axis) is 25 degrees or more and 55 degrees or less and in which the number of proeutectoid alpha particles is reduced Fine pro-eutectoid alpha particles are easily obtained.

  The billet after completion of the first α + β processing step may be placed in a heating furnace heated to a β single phase temperature range to perform a β heat treatment step, or may be allowed to cool to room temperature by leaving it to cool (air cooling) .

(3) β heat treatment step In the β heat treatment step of the present embodiment, after heating the billet after the first α + β processing step to a β single phase temperature range in a heating furnace, the billet is cooled at a sufficiently fast cooling rate by water cooling. Cooling. In the β heat treatment step, the billet is quenched after being in the β single phase state. The heating in the β heat treatment step is only once.
In the β heat treatment step, after heating to a β single phase temperature range, it may be water cooled immediately or may be subjected to hot working and then water cooled and quenched.

  As a means for water cooling the billet, a method of immersing the billet in a sufficient amount of water can be used. The means for water cooling the billet is not limited to the above method, as long as it can cool the billet at a sufficiently fast cooling rate, for example, even if a method of spraying water on the billet is used Good.

  The temperature in the heating furnace in the β heat treatment step is not less than 20 ° C. higher than the β transformation temperature and not more than 100 ° C. higher than the β transformation temperature (β transformation temperature + 20 ° C. to β transformation temperature + 100 ° C.) It is preferable to If the temperature in the heating furnace is 20 ° C. higher than the β transformation temperature, the entire billet will surely reach the β transformation temperature even if there is a nonuniform temperature portion in the heating furnace. Moreover, the coarsening of the metal structure of a billet can be suppressed as the temperature in a heating furnace is below 100 degreeC temperature higher than (beta) transformation point temperature.

  In this embodiment, by quenching the billet heated to the β single phase temperature range in the β heat treatment step, fine needle-like grain boundary α grains are generated at the interface of small β grains generated in the β single phase temperature range. Ru. As a result, in the subsequent (4) second α + β processing step, colonies consisting of grain boundary α particles and / or needle-like α particles and the β phase are easily divided.

(4) Second α + β Processing Step In the second α + β processing step of the present embodiment, the billet after the β heat treatment step is heated to the α + β two-phase temperature range in a heating furnace and subjected to hot processing such as forging and / or rolling. In the second α + β processing step, the billet which is the material to be processed is processed in the state of the α + β two-phase.

  In the second α + β processing step, the temperature in the heating furnace for heating the billet is not less than 150 ° C. lower than the β transformation point temperature and not more than 30 ° C. lower than the β transformation point temperature for the same reason as the first α + β processing step (β It is preferable to set it as the transformation point temperature -150 degreeC-(beta) transformation point temperature-30 degreeC temperature range).

  In the second α + β processing step, as in the first α + β processing step, the surface temperature of the billet during hot processing is preferably in the range of the β transformation point temperature −270 ° C. to the temperature in the above heating furnace, β It is more preferable to set the transformation temperature to -250 ° C. to the above temperature in the heating furnace. Sufficient strain can be applied to the billet by setting the surface temperature of the billet during hot working to a temperature at which the β transformation point temperature is −270 ° C. or higher. As a result, the crystal orientation in the billet metal structure is dispersed, and a metal structure with a reduced number of pro-eutectoid α-grains having a crystal orientation (inclination of c axis) of 25 ° or more and 55 ° or less is easily obtained. In addition, since sufficient strain can be applied to the billet, fine pro-eutectoid α-particles can be easily obtained. In addition, by setting the surface temperature of the billet during hot working to a temperature at which the β transformation point temperature is −270 ° C. or higher, the deformation resistance of the billet does not become too large, and efficient hot working can be performed.

The surface temperature of the billet gradually decreases during hot working. Therefore, also in the hot working in the second α + β working process, the surface temperature of the billet during the hot working is β transformation before the hot working at the predetermined forging ratio is finished, as in the first α + β working process. In the case where the point temperature is lower than -270 ° C., it is preferable to temporarily interrupt hot working and heat the billet again before hot working.
Moreover, it is preferable to carry out the hot processing in the second α + β processing step while measuring the surface temperature of the billet using a thermometer such as a radiation thermometer as in the first α + β processing step.

  In the second α + β processing step, it is more preferable to carry out hot working so that the forging ratio is 3.0 or more and hot working so that the forging ratio is 3.5 or more. A sufficient strain can be applied to the billet by setting the wrought ratio in the second α + β processing step to 3.0 or more. As a result, the colony consisting of the grain boundary α grain and / or needle-like α grain produced from the small β grain generated in the β heat treatment step and the β phase is sufficiently divided, and the grain boundary α grain and / or needle divided The crystallographic orientation is dispersed as well as the fine α-grains are finely equiaxed. Therefore, a metallographic structure having an area ratio of 2.0% or less in the metallographic structure of proeutectoid α particles having an inclination of c axis of 25 ° to 55 ° and a circle equivalent diameter (particle diameter) of 20 μm or more can be obtained. . If the wrought ratio in the second α + β processing step is less than 3.0, pro-eutectoid α-grains having the c-axis inclination and the equivalent circle diameter in the above ranges can not be sufficiently reduced. The upper limit of the wrought ratio is not particularly limited, but if it is too large, it is necessary to increase the number of heats and the productivity is poor.

The number of heats in the second α + β processing step may be one or more than one, and can be appropriately determined according to the temperature of heating the billet, the method of hot working, the conditions of hot working such as wrought ratio .
In the present embodiment, the titanium alloy rod of the present embodiment having a predetermined shape and metal structure is obtained by performing the second α + β processing step.

  In the present embodiment, annealing may be performed on the titanium alloy bar manufactured in this manner as necessary, or the surface oxide layer may be removed or machined by machining the surface. You may adjust the

Next, examples of the present invention will be described.
Example 1
The titanium alloy bar was manufactured and evaluated by the method shown below.
(Β processing process)
A columnar ingot with a diameter of about 750 mm having the following composition obtained by melting is heated to a β single phase temperature range in a heating furnace heated to 1050 ° C. or more and 1200 ° C. or less, and then taken out from the heating furnace Forged. Thereafter, heating and forging in a heating furnace are repeated several times again, and a round bar-shaped No. 1 having a circular cross-sectional shape perpendicular to the longitudinal direction is shown in Table 1. 1 to No. I got 10 billets.

  For some ingots, heating and forging in a heating furnace heated to 1050 ° C. or more and 1200 ° C. or less are repeated several times, and the cross-sectional shape orthogonal to the longitudinal direction has a width of 600 mm and a thickness of 500 mm shown in Table 1 No. 2 of square bar which is a square of 11 billets.

"composition"
Al: 6.20% by mass, V: 4.10% by mass, Fe: 0.18% by mass, O (oxygen): 0.19% by mass, the balance being Ti and unavoidable impurities (β transformation point temperature 1000 ° C).

(1st α + β processing step)
No. 1 to No. After heating the billet of 11 in the heating furnace of the heating temperature shown in Table 1, it took out from the heating furnace and forged. Thereafter, heating and forging in the heating furnace are repeated several times again to obtain the wrought ratio shown in Table 1, and the cross-sectional shape orthogonal to the longitudinal direction has a circle with a diameter shown in Table 1 or a width shown in Table 1 And a billet which is a square of thickness and thickness. In the first α + β processing step, the surface temperature of the billet during hot working was in the range of 770 ° C. or more and the heating temperature or less shown in Table 1.

(Β heat treatment process)
No. 1 after the first α + β processing step. 1 to No. No. 11 billets were heated in the heating furnace at the heating temperature shown in Table 1, then removed from the heating furnace. The billets other than 7 were immediately immersed in a water tank containing a sufficient amount of water and water cooled. No. The billet No. 7 was removed from the heating furnace and allowed to cool.

(2nd α + β processing step)
No. 1 to No. After heating the billet of 11 in the heating furnace of the heating temperature shown in Table 1, it took out from the heating furnace and forged. Thereafter, heating and forging in the heating furnace are repeated several times again to obtain the wrought ratio shown in Table 1, and the cross-sectional shape orthogonal to the longitudinal direction has a circle with a diameter shown in Table 1 or a width shown in Table 1 No. which is a square of thickness and thickness 1 to No. Eleven titanium alloy bars were obtained. In the second α + β processing step, the surface temperature of the billet during hot working was in the range of 750 ° C. or more and the heating temperature or less shown in Table 1. Also, no. 1 to No. The 11 titanium alloy rod was annealed at 705 ° C. after the second α + β processing step.

"Metal structure"
No. 1 to No. About the titanium alloy rod of 11, the test piece which makes a section in the length direction the observation surface was taken from the central part in the length direction of the titanium alloy rod, and the observation surface was observed using SEM (scanning electron microscope) .

FIG. It is a microscope picture of a titanium alloy bar of 1. FIG. It is a microscope picture of five titanium alloy bars. Also, FIG. It is a microscope picture of a 3 titanium alloy bar.
As shown in FIGS. 1, No. 3, No. The metal structure of the titanium alloy rod of No. 5 had a metal structure consisting of an α phase consisting of equiaxed pro-eutectoid α grains and acicular α grains and a β phase.
No. 2 and No. 4, no. 6 to No. Also for No. 11 titanium alloy bars, no. As a result of carrying out microscope observation similarly to titanium alloy rod material of No. 1, no. Similar to 1, it had a metal structure consisting of an α phase consisting of equiaxed primary α grains and acicular α grains and a β phase.

Also, no. 1 to No. The metal structure of each of the eleven titanium alloy bars was examined using EBSD (Electron Backscatter Diffraction) according to the method described below.
First, a test piece having a cross section in the length direction as an observation surface was collected from the center in the length direction of the titanium alloy bar. Next, the observation surface of the test specimen was measured using EBSD at a measurement interval of 2.3 μm and an acceleration voltage of 15 kV, with a rectangular area of 1.2 mm long and 0.9 mm wide as a field of view.

From the obtained measurement results, a PQ (pattern quality) map and a phase map were created from Kikuchi pattern analysis, and the α phase was extracted. In addition, the Kikuchi pattern analysis excluded the beta phase and performed only the alpha phase.
Next, pro-eutectoid α-grains are determined with an angle difference (misorientation angle) of the orientation (c-axis direction) of adjacent EBSD measurement points being 5 ° or less, and each pro-eutectoid α-grain is determined from the measurement points of the pro-eutectoid α-grains The area ratio of pro-eutectoid α-grains in the metal structure was determined. The results (area ratio of pro-eutectoid α particles) are shown in Table 1.

In addition, the circle equivalent diameter of each pro-eutectoid α particle was calculated from the EBSD measurement score of each pro-eutectoid α particle.
In addition, the average value in the c-axis direction at the EBSD measurement point in each pro-eutectoid α grain is calculated, and using it, the c-axis direction of pro-eutectoid α grain and the length of titanium alloy rod for each pro-eutectoid α grain The angle (inclination of the c axis) θ with respect to the direction was calculated.
Then, among the proeutectoid α grains, the area ratio of the proeutectoid α grains in the metal structure having an inclination θ of c axis of 25 ° to 55 ° and a circle equivalent diameter of 20 μm or more was determined. The results (area ratio of coarse pro-eutectoid α particles) are shown in Table 1.

"Fatigue characteristics"
No. 1 to No. Low cycle fatigue tests were performed on the 11 titanium alloy bars by the method described below to evaluate the fatigue characteristics.
From the center of the titanium alloy bar, a test piece having a diameter of 5.08 mm and a length of 15.24 mm was taken parallel to the length direction. The fatigue test was a low cycle fatigue test in which the strain range was 0 to 0.8% in axial force and one swing. The fatigue life was taken as the number of times until the test piece ruptures, and the fatigue characteristics were evaluated as good (○) when the number of times before fracture was 80,000 or more and defective (×) less than 80,000 times. The results (fatigue characteristics) are shown in Table 1.

  As shown in Table 1, no. 1 to No. The titanium alloy rod of No. 5 is an example of the present invention, and the area ratio of coarse pro-eutectoid α grains was 2.0% or less, and had good fatigue properties.

On the other hand, no. In the titanium alloy bars 6 and 10, since the wrought ratio in the first α + β processing step is small, the area ratio of coarse pro-eutectoid α-grains exceeds 2.0%, and good fatigue characteristics are not obtained.
Also, no. 8 and No. Since the wrought ratio in the second α + β processing step was small, the area ratio of coarse pro-eutectoid α-grains exceeded 2.0%, and good fatigue characteristics were not obtained.
Also, no. Since the titanium alloy bar 7 was allowed to cool after heating in the β heat treatment step, the area ratio of coarse pro-eutectoid α grains exceeded 2.0%, and good fatigue characteristics were not obtained.
Also, no. The area ratio of coarse pro-eutectoid α-grains exceeds 2.0% because the wrought ratio in the 1st α + β processing step and the 2nd α + β processing step is small, and good fatigue characteristics are obtained. It was not.

Example 2
The titanium alloy bar was manufactured and evaluated by the method shown below.
(Β processing process)
After heating a cylindrical ingot with a diameter of about 600 mm having the following composition to a β single phase temperature range in a heating furnace heated to 1000 ° C. or more and 1150 or less, it was taken out from the heating furnace and forged. Thereafter, heating and forging in the heating furnace are repeated again several times, and a round bar-shaped No. 1 having a circular cross-sectional shape orthogonal to the longitudinal direction is shown in Table 2. A to No. I got a billet of G.

"composition"
Al: 6.20% by mass, Mo: 6.30% by mass, Sn: 2.02% by mass, Zr: 3.70% by mass, Fe: 0.12% by mass, O (oxygen): 0.13% by mass And the balance is Ti and unavoidable impurities (β transformation temperature 950 ° C.).

(1st α + β processing step)
No. A to No. After heating the billet of G in the heating furnace at the heating temperature shown in Table 2, it was removed from the heating furnace and forged. Thereafter, heating and forging in the heating furnace were repeated several times again to obtain the wrought ratio shown in Table 2, and a billet having a circular cross-sectional shape perpendicular to the longitudinal direction shown in Table 2 was obtained. . In the first α + β processing step, the surface temperature of the billet during hot working was in the range of 740 ° C. or more and the heating temperature or less shown in Table 2.

(Β heat treatment process)
No. 1 after the first α + β processing step. A to No. After heating the billet of G in the heating furnace at the heating temperature shown in Table 2, it was taken out of the heating furnace and immediately immersed in a water tank containing a sufficient amount of water to be water-cooled.

(2nd α + β processing step)
No. A to No. After heating the billet of G in the heating furnace at the heating temperature shown in Table 2, it was removed from the heating furnace and forged. Thereafter, heating and forging in the heating furnace are repeated several times again to obtain the wrought ratio shown in Table 2, and the cross-sectional shape orthogonal to the longitudinal direction is No. A to No. G titanium alloy bar was obtained. In the second α + β processing step, the surface temperature of the billet during hot working was in the range of 720 ° C. or more and the heating temperature or less shown in Table 2. Also, no. A to No. The G titanium alloy rod was annealed at 705 ° C. after the second α + β processing step.

"Metal structure"
No. A to No. Regarding the titanium alloy bar of G, No. 1 of <Example 1>. 1 to No. In the same manner as in the titanium alloy rod of No. 11, a test piece having a cross section in the longitudinal direction as an observation surface was collected from the central portion in the longitudinal direction of the titanium alloy rod, and the observation surface was observed with a microscope.
As a result, no. A to No. The titanium alloy rod of G had a metal structure consisting of an α phase consisting of equiaxed pro-eutectoid α grains and acicular α grains and a β phase.

No. A to No. Regarding the titanium alloy bar of G, No. 1 of <Example 1>. 1 to No. In the same manner as in 11, using EBSD, the metal structure was examined, and the area ratio of primary α particles in the metal structure was determined. The results (area ratio of pro-eutectoid α particles) are shown in Table 2.
Also, no. A to No. Regarding the titanium alloy bar of G, No. 1 of <Example 1>. 1 to No. In the same manner as in 11, the area ratio of the pro-eutectoid α particles in the metal structure having an inclination θ of 25 ° to 55 ° and an equivalent circle diameter of 20 μm or more among the proeutectoid α particles was determined. . The results (area ratio of coarse pro-eutectoid α particles) are shown in Table 2.

As shown in Table 2, no. A to No. The titanium alloy bar of E is an example of the present invention, and the area ratio of coarse pro-eutectoid α grains was 2.0% or less, and had good fatigue properties.
On the other hand, no. Since the wrought ratio in the first α + β processing step is small, the area ratio of coarse pro-eutectoid α-grains exceeds 2.0%, and good fatigue characteristics can not be obtained.

Claims (3)

Hot-worked titanium alloy bar ,
The components of the hot-worked titanium alloy bar are Al: 5.50 to 6.75 mass%, V: 3.50 to 4.50 mass%, Fe: 0.05 to 0.30 mass%, O: Containing 0.05 to 0.20% by mass, the balance being Ti and unavoidable impurities,
And has a metal structure consisting of an α phase consisting of pro-eutectoid α grains and needle-like α grains, and a β phase,
The area ratio of the pro-eutectoid α particles in the metal structure is 30% or more,
Among the pro-eutectoid α-grains, an angle (inclination of c-axis) between c-axis direction of dense hexagonal structure and length direction of hot-worked titanium alloy bar is 25 ° or more and 55 ° or less, and equivalent to a circle A hot-worked titanium alloy bar characterized in that an area ratio of pro-eutectoid α particles having a diameter of 20 μm or more in the metal structure is 2.0% or less. Hot-worked titanium alloy bar ,
The components of the hot-worked titanium alloy bar are Al: 5.50 to 6.50% by mass, Sn: 1.75 to 2.25% by mass, Zr: 3.50 to 4.50% by mass, Mo: 5.50 to 6.50% by mass, Fe: 0.02 to 0.15% by mass, O: 0.02 to 0.15% by mass, the balance being Ti and unavoidable impurities,
And has a metal structure consisting of an α phase consisting of pro-eutectoid α grains and needle-like α grains, and a β phase,
The area ratio of the pro-eutectoid α particles in the metal structure is 30% or more,
Among the pro-eutectoid α-grains, an angle (inclination of c-axis) between c-axis direction of dense hexagonal structure and length direction of hot-worked titanium alloy bar is 25 ° or more and 55 ° or less, and equivalent to a circle A hot-worked titanium alloy bar characterized in that an area ratio of pro-eutectoid α particles having a diameter of 20 μm or more in the metal structure is 2.0% or less. A method of manufacturing a hot-worked titanium alloy bar made of an α + β two-phase titanium alloy manufactured by hot working an ingot,
  Β processing step of hot working the ingot in a β single phase temperature range,
  a first α + β processing step of hot working so that the wrought ratio is 1.7 or more in the α + β two-phase temperature range;
  a heat treatment step of heating to a β single phase temperature range and then water cooling;
  The hot-worked titanium according to claim 1 or 2, characterized in that a second α + β processing step of hot working so that the wrought ratio is 3.0 or more in the α + β two-phase temperature range is performed in this order. Method of manufacturing alloy bar.