JP2013189708A - Titanium alloy forged material and method for producing the same, and ultrasonic flaw-detection testing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a titanium alloy forged material which reduces the noise when the test is applied with an ultrasonic flaw-detection while keeping the mechanical characteristic of the β forged material needed to aircraft parts such as fatigue-strength; and a method for producing the same.SOLUTION: This titanium alloy forged material having mixed granular structure of flat grains of the priorβ grain, in which the diameter in the thickness direction is 50-500 μm and the aspect ratio exceeds 3, and non-flat grains of the prior β grain, in which the diameter of the direction is 30-200 μm and the aspect ratio is 1-3 has such peculiarity that the flat grains are 40-99% and the non-flat grains are 1-60%, and the sum total of the flat grains and the non-flat grains exisits in ≥90%.

Description

  The present invention relates to an α + β-type titanium alloy β-forged material to be inspected for defects by ultrasonic inspection and a method for producing the same.

Α + β type titanium alloy, represented by Ti-6Al-4V alloy, has various characteristics such as weldability, superplasticity, and diffusion bondability in addition to light weight, high strength, and high corrosion resistance. Is used a lot. In the α + β type titanium alloy, the α phase of the dense hexagonal crystal (hcp structure) as the main phase and the β phase of the body-centered cubic crystal (bcc structure) coexist stably at room temperature, and the β transformation point (T _β ) or higher It becomes β phase single phase in the temperature range. The forging material of α + β type titanium alloy includes α + β forging which is heated to a temperature range (α + β two-phase region) below T _β so as not to reach a temperature _{equal to} or higher than T _β and is forged in this temperature range, and T _β It is known that there are those by β forging in which the above temperature range (β single phase range) is heated and forged, the material structure formed is completely different, and the material properties are accordingly different.

The titanium alloy forged material has a needle-like α phase structure according to β forging. Specifically, the organization is formed as follows. That is, it becomes a β-phase single phase in a temperature range equal to or higher than T _β , and after the equiaxed β phase is flattened by forging, it is cooled to a temperature range lower than T _β and held in this temperature range, The α phase is deposited in the form of a film at the grain boundary of the β phase (β grain), and then the α phase is deposited in the form of needles in the crystal grains of the β grain (shown in white in FIG. 2 (a)). Is α phase). In β forging, forging is completed in the β single-phase region, forging continues after the temperature drops outside the β single-phase region (α + β two-phase region), and the temperature drops in the α + β two-phase region. Some start forging after that. Furthermore, the β forging material changes the form and thickness of the α phase of the grain boundaries of the old β grains, and the length and thickness of the acicular α phase in the grains, depending on the forging conditions and the subsequent cooling conditions. There may be one in which no grain boundary α phase exists. On the other hand, the titanium alloy forged material has a granular α structure according to α + β forging (see FIG. 2B). In general, forging materials with α + β-type titanium alloy, fracture toughness is better for forged materials with β-forging than forged materials with α + β-forging, and conversely, fatigue strength characteristics are for forged materials with α + β-forging. It is known that this method is superior to the forged material that has been β-forged.

  Since aircraft engine parts are required to have high reliability, the presence or absence of defects is inspected by ultrasonic flaw detection. In ultrasonic flaw detection, the ultrasonic wave transmitted (transmitted) from the probe is incident from the surface of the object to be inspected, and the reflected wave reflected by the defect such as a flaw is received by the probe. This is an inspection for determining the presence or absence of internal defects. However, α + β-type titanium alloys in which α and β phases coexist, regardless of whether they are α + β forged or β forged, have high noise due to the material structure during ultrasonic flaw detection. Or the noise caused by the material structure is mistaken as a defect. Therefore, engine parts and the like formed of an α + β type titanium alloy (hereinafter, titanium alloy) are required to reduce noise during ultrasonic flaw detection and improve ultrasonic flaw detection performance.

  Therefore, as an α + β type titanium alloy material with reduced noise, for example, before hot rolling in the α + β two-phase region, the structure is refined by rapid cooling from the β single-phase region, and then heat in the α + β two-phase region. A titanium alloy rolled sheet having an equiaxed α structure by hot rolling and heat treatment has been proposed (Patent Document 1).

Japanese Patent No. 2988269

  However, the prior art described above relates to an α + β type titanium alloy material by α + β forging. On the other hand, as for β forging, as described above, since the formation process of the material structure and the form finally formed are greatly different from α + β forging, it is considered that the cause of noise during ultrasonic flaw detection is different. Since the improvement methods are also different, it is impossible to reduce noise by applying the above technique.

  The present invention has been made in view of the above problems, and for β-forged materials of α + β-type titanium alloys, while maintaining mechanical properties required for aircraft parts such as fatigue strength properties, at the time of ultrasonic flaw detection An object of the present invention is to provide a titanium alloy forged material excellent in ultrasonic flaw detection with reduced noise and a method for producing the same.

  As a result of earnest research, the inventors of the present invention easily transmit the reflected wave regularly at the grain boundary of the old β grains that have been crushed into a flat shape having a wide surface perpendicular to the incident direction of the transmitted wave by β forging. It has been clarified that it is received by the probe and is the main cause of noise. Furthermore, it has been clarified that by appropriately controlling the material structure, the ultrasonic flaw detection property of the β-forged material can be improved while maintaining the fatigue strength characteristics and the like.

  That is, the titanium alloy forged material according to the present invention is a forged material that has been β-forged to be subjected to ultrasonic flaw detection, and has a diameter in the thickness direction of 50 μm to 500 μm and an old β grain having an aspect ratio of more than 3. It has a mixed grain structure of flat grains that are crystal grains and non-flat grains that are crystal grains of old β grains having a diameter in the direction of 30 μm to 200 μm and an aspect ratio of 1 to 3. In the titanium alloy forging, the flat particles are 40% or more and 99% or less, the non-flat particles are 1% or more and 60% or less, and the flat particles and the non-flat particles are 90% or more in total. It is characterized by.

  Since the titanium alloy forging material having such a structure has a structure in which old β grains having a predetermined size that is not flat are mixed in a predetermined range, the non-flat old β grains are not reduced in strength as the β forging material. Reflecting the transmitted wave at the grain boundary does not receive the probe, and noise is reduced, so that the ultrasonic flaw detection is excellent.

Further, the forged titanium alloy according to the present invention is preferably made of a titanium alloy having a Mo equivalent [Mo] eq represented by the following formula (1) of more than 2.7 and less than 15.
[Mo] eq = [Mo] + [Ta] / 5 + [Nb] /3.6+ [W] /2.5+ [V] /1.5+1.25 [Cr] +1.25 [Ni] +1.7 [ Mn] +1.7 [Co] +2.5 [Fe] (1)
However, [X] in the formula (1) is the content (mass%) of the element X in the titanium alloy.

  With this configuration, the titanium alloy forged material becomes an α + β type titanium alloy, and the influence of the shape of the old β grains becomes strong, and mechanical characteristics and ultrasonic flaw detection properties can coexist.

  Further, the forged titanium alloy according to the present invention may have a thickness of at least 50 mm. With such a configuration, even if the thickness is increased, ultrasonic flaw detection can be performed to a deep portion with high accuracy, and a highly reliable product can be obtained.

The titanium alloy forging according to the present invention is manufactured by performing β forging. In the method for producing a titanium alloy forging according to the present invention, when the β forging is represented by T _{β as the} β transformation point, the β forging is heated to T _β + 10 ° C. or more, and the β crystal grain size is 400 μm or more and 1000 μm or less. It is held until it reaches the range, forged in a temperature range of T _β −30 ° C. or more, and is 20 seconds or longer in the temperature range and less than the limit holding time (second) t _max represented by the following formula (2) after holding, characterized in that it immediately cooled to T _beta -150 ° C. or lower.
t _max = [11.64 (1-T _H / 1425)] ^4.35 (2)
However, T _H in the formula (2) is a temperature (℃) during holding after the forging.

  With this procedure, the titanium alloy forged material manufacturing method is maintained for a predetermined time in a temperature range equal to or higher than the β transformation point after forging, and the non-flat old β grains grow appropriately, and the flat old β grains generated by forging And a titanium alloy forged material having a mixed grain structure.

  The ultrasonic flaw detection inspection method for a titanium alloy forging according to the present invention uses the probe having a probe diameter in the range of 5 to 30 mm and ultrasonic waves in the frequency range of 1 to 20 MHz. It includes a step of flaw detection in a direction parallel to the direction in which the forging reduction amount of the forging material is the largest.

  By such a method, the ultrasonic flaw detection inspection method can scan a wide area of the titanium alloy forging material with a probe because the noise is sufficiently low even if flaw detection is performed in a direction that results in relatively high noise. Inspection is facilitated and high-precision inspection can be performed.

  According to the titanium alloy forging according to the present invention, defects can be detected with high accuracy by ultrasonic flaw detection, and the reliability of products such as aircraft engine parts is improved. And according to the manufacturing method of the titanium alloy forging material which concerns on this invention, the titanium alloy forging material which has the said effect can be manufactured easily. Moreover, according to the ultrasonic inspection method according to the present invention, the titanium alloy forged material can be inspected with high accuracy.

It is a schematic diagram which shows the structure | tissue state of (beta) forging material of a titanium alloy, and is a model explaining the relationship with the noise in an ultrasonic inspection. It is an image photograph of the structure of a titanium alloy forging material, (a) is an example of β forging material, and (b) is an example of α + β forging material.

Hereinafter, embodiments of the present invention will be described in detail.
[Titanium alloy forging]
The titanium alloy forging according to the present invention is applied to aircraft engine parts, like the conventional β forging, and is particularly suitable for those that require inspection of internal defects by ultrasonic flaw detection. . Specifically, it can be applied to a titanium alloy forged material used for a disk or a shaft, and the thickness (length in the forging direction) can be 50 mm or more even at the thinnest part.

  The titanium alloy forged material according to the present invention is made of an α + β type titanium alloy (hereinafter referred to as titanium alloy). Like the conventional β forged material, the old β grains (β phase) and the grain boundaries and crystals of the old β grains And α phase precipitated in the grains. However, the titanium alloy forged material according to the present invention is a flat grain having a diameter in the thickness direction of 50 μm or more and 500 μm or less and an aspect ratio of 3 or more, and a diameter in the direction of 30 μm or more and 200 μm or less. It has a mixed grain structure including non-flat grains having an aspect ratio of 1 or more and 3 or less. Further, in the titanium alloy forged material, the flat particles are 40% or more and 99% or less, the non-flat particles are 1% or more and 60% or less, and the flat particles and the non-flat particles are 90% or more in total. Hereinafter, the old β grains will be described in detail.

(Mixed grain structure)
In the present invention, among the old β grains, those having an aspect ratio of more than 3 are defined as flat grains, and those having 3 or less (1 or more and 3 or less) are defined as non-flat grains. In β-forging, the titanium alloy material is heated and held in a temperature range (β single-phase region) that is equal to or higher than the β transformation point (T _β ), thereby becoming a β-phase single-phase state. A flat β phase (β crystal grains) is formed and grows. Then, the β crystal grains are crushed and deformed into a flat shape extending perpendicularly to the forging direction (the reduction direction) by the forging process, and the β crystal grains having a pancake shape are stacked. Since the conventional β forging material is immediately cooled after forging in the β single-phase region and falls to a sufficiently low temperature range (α + β two-phase region) below T _β , as shown in FIG. Almost all β crystal grains are flat grains. On the other hand, the titanium alloy forged material according to the present invention has a mixed grain structure of flat grains and non-flat grains as shown in FIG. 1 (a) and 1 (b), the α phase formed during cooling is present in the grain boundaries and grains of β crystal grains (old β grains), but the α phase is not shown. To do.

(Flat particles: 50 to 500 μm in diameter in the thickness direction, abundance 40 to 99%)
The titanium alloy forging according to the present invention has high fracture toughness and fatigue strength due to the polycrystalline structure of flat β crystal grains (former β grains), similar to the conventional β forging. The reason why the old β grains are defined as having an aspect ratio exceeding 3 is that crystal grains having an aspect ratio of 3 or less do not contribute to the improvement of the fatigue strength of the titanium alloy forged material. On the other hand, the upper limit of the aspect ratio of the flat particles of the old β grains is not particularly specified, but is 30 or less under general forging conditions. In other words, in order to obtain crystal grains having an aspect ratio exceeding 30, it is necessary to forge at a rolling reduction of about 90% or more, which is not practical. In addition, when the diameter of the old β grain is smaller than 50 μm in the thickness direction (diameter in the minimum direction), the number of grain boundaries in the ultrasonic flaw detection direction of the titanium alloy forged material increases, and there is a risk of noise increase. On the other hand, if the diameter in the thickness direction exceeds 500 μm, the fatigue strength of the titanium alloy forged material decreases. Although the thickness direction of the flat grains depends on the shape of the titanium alloy forged material, it often coincides with the forging direction (the reduction direction). In the present invention, the aspect ratio means a diameter in a direction perpendicular to this direction with respect to a diameter in the thickness direction (the same applies to non-flat particles).

  And as for the titanium alloy forging material, fatigue strength improves, so that there are many rates of the flat grain of the magnitude | size of the said range. The titanium alloy forged material has a flat grain of less than 40%, and the effect of improving fatigue strength due to the flat grain cannot be sufficiently obtained. Therefore, the flat grain is set to 40% or more, preferably 50% or more, more preferably 60% or more. Most preferably, it is 70% or more. On the other hand, in the titanium alloy forged material, when flat grains exceed 99%, noise in ultrasonic flaw detection increases, so the flat grains are 99% or less, preferably 97% or less, more preferably 95% or less, and still more preferably. Is 90% or less. Therefore, in the titanium alloy forged material according to the present invention, flat β grains of old β grains having a diameter in the thickness direction of 50 μm or more and 500 μm or less and an aspect ratio exceeding 3 are present in an amount of 40% or more and 99% or less. The old β-grained flat grains of such shape and size are sufficiently grown by adjusting the holding temperature and time when heated to the β-single phase region before forging, and growing the β-grains to an appropriate size. It is obtained by deforming to an aspect ratio exceeding 3 by forging at a low reduction rate. Furthermore, the abundance ratio of the old β grains can be controlled by the holding time after forging.

(Non-flat particles: 30-200 μm in the thickness direction, 1-60% abundance)
In the β forged material, ultrasonic waves incident from the surface to the inside are easily reflected by the grain boundaries of the old β grains. When ultrasonic waves are incident in a direction parallel to the forging direction, as shown in FIG. 1B, in the conventional β forging material in which the old β grains are mainly flat grains, most of the grain boundaries are perpendicular to the forging direction. Therefore, the incident wave is regularly reflected, and many of the reflected waves are received by the probe and become noise. As shown in FIG. 1 (a), the titanium alloy forging according to the present invention has a mixed grain structure in which non-flat grains are mixed with flat grains of old β grains, thereby dispersing reflected waves and generating noise. Do not become. The non-flat particles of the old β grains have a small reflected wave dispersion effect unless the aspect ratio is 3 or less, and a noise reduction effect cannot be obtained. Further, when the non-flat particles of the old β-grains have a diameter in the thickness direction (a diameter in the minimum direction) exceeding 200 μm, there is a possibility that fatigue strength and fracture toughness are reduced. On the other hand, when the non-flat particles of the old β grains have a diameter in the thickness direction of less than 30 μm, the area of the grain boundaries of the non-flat grains is narrow, and a noise reduction effect cannot be obtained.

  The titanium alloy forged material has a non-flat grain size of 1% or more, preferably 3% or less because the noise reduction effect of the non-flat grain is not sufficiently obtained when the non-flat grain size is less than 1%. % Or more, more preferably 5% or more, and most preferably 10% or more. On the other hand, as the titanium alloy forged material increases in the proportion of non-flat grains, the number of flat grains is relatively reduced and the fatigue strength is lowered. Specifically, when the non-flat grain exceeds 60%, the titanium alloy forged material has insufficient fatigue strength. Therefore, the non-flat grain is 60% or less, preferably 50% or less, more preferably 40% or less, and still more preferably. Is 30% or less, most preferably 20% or less. Therefore, in the titanium alloy forged material according to the present invention, non-flat grains of old β grains having a diameter in the thickness direction of 30 μm or more and 200 μm or less and an aspect ratio of 3 or less are 1% or more and 60% or less. The size and existence ratio of the non-flat grains of the old β grains having such shapes and sizes can be controlled by the holding temperature in the β single-phase region after forging and the holding time corresponding thereto.

(The total proportion of old β flat and non-flat particles is 90% or more)
Titanium alloy forgings, if the total of flat and non-flat grains with the aspect ratio and diameter described above with respect to the old β grains is less than 90%, there is an excess of fine β grains or coarse β grains outside these ranges As a result, fatigue strength and fracture toughness are insufficient, and noise in ultrasonic inspection is increased. Therefore, in the titanium alloy forged material according to the present invention, the total amount of the old β flat particles and non-flat particles is 90% or more, preferably 92% or more, more preferably 94% or more.

  In the present invention, the abundance ratios of the old β-grained flat particles and non-flat particles of the titanium alloy forged material indicate the area ratio in the cross section. The aspect ratio, diameter, and area ratio of the old β grains of titanium alloy forged materials are determined by cutting the titanium alloy forged material in a plane parallel to the forging direction and corroding the cross section after polishing (mechanical polishing, electrolytic polishing). This can be determined based on the result of observing the surface. For example, a plurality of visual fields of about 1 to several mm square are selected from the cross section, and the cross-sectional structure is observed with an optical microscope. Then, the length (diameter) of the old β grains in each of the forging direction of the cross section and the direction orthogonal to the forging direction is measured, and flat grains and non-flat grains are determined based on the diameter and aspect ratio in the forging direction (thickness direction). And the respective area ratios in the field of view may be calculated.

(Titanium alloy: Mo equivalent 2.7 and less than 15)
The titanium alloy for forming the titanium alloy forging according to the present invention can be applied as long as it is an α + β type titanium alloy, but the Mo equivalent [Mo] eq represented by the following formula (1) exceeds 2.7. The composition is preferably less than 15. As the Mo equivalent increases, the volume content of the α phase decreases and the influence of the shape of the old β grain boundary increases, and the fracture toughness and fatigue strength due to the flat particles of the old β grain are increased. The improvement effect is further obtained. The Mo equivalent of the titanium alloy is more preferably 3.5 or more, and further preferably 4.5 or more. On the other hand, the titanium alloy is preferably less than 15 because the alloy element is easily segregated and the structure may vary as the Mo equivalent [Mo] eq increases. The Mo equivalent of the titanium alloy is more preferably 12 or less, and still more preferably 10 or less.
[Mo] eq = [Mo] + [Ta] / 5 + [Nb] /3.6+ [W] /2.5+ [V] /1.5+1.25 [Cr] +1.25 [Ni] +1.7 [ Mn] +1.7 [Co] +2.5 [Fe] (1)
However, [X] in the formula (1) is the content (mass%) of the element X in the titanium alloy.

  Specific examples of such a titanium alloy include titanium alloys specified by AMS4981 and AMS4995. Titanium alloys (Ti-6Al-2Sn-4Zr-6Mo alloy, Ti-6246 alloy) specified by AMS4981 are Al: 5.50-6.50 mass%, Sn: 1.75-2.25 mass%, Zr: 3.50 to 4.50% by mass, Mo: 5.50 to 6.50% by mass, the balance being Ti and inevitable impurities, and the Mo equivalent calculated from the average value of each element is 6 .0. The inevitable impurities generally include N: 0.04 mass%, C: 0.08 mass%, H: 0.015 mass%, Fe: 0.15 mass%, and O: 0.15 mass%. To do.

  Titanium alloy (Ti-5Al-2Sn-2Zr-4Cr-4Mo alloy, Ti-17 alloy) specified by AMS4995 is Al: 4.5 to 5.5 mass%, Sn: 1.5 to 2.5 mass %, Zr: 1.5 to 2.5% by mass, Cr: 3.5 to 4.5% by mass, Mo: 3.5 to 4.5% by mass, O: 0.08 to 0.12% by mass The balance is Ti and inevitable impurities, and the Mo equivalent calculated from the average value of each element is 9.5. The inevitable impurities generally include Fe: 0.03% by mass, C: 0.05% by mass, N: 0.04% by mass, and H: 0.0125% by mass.

[Production method of titanium alloy forging]
The titanium alloy forged material according to the present invention is formed by forging an ingot made of a titanium alloy having a desired composition into a billet by a known method (referred to as a billet forging step), performing machining as necessary, and then performing β forging. To produce the desired product shape. The billet forging step is performed, for example, in the order of β forging → α + β forging → β heat treatment → stress relief annealing → α + β forging → annealing. α + β forging is heated to a temperature range about 10 to 200 ° C. lower than the β transformation point (appropriately expressed as T _β ), and β forging is heated to a temperature range about 10 to 150 ° C. higher than T _β. Forging is performed at a ratio (area ratio after forging of the cross section perpendicular to the forging direction to before forging, for example, 1.5), and cooled to room temperature. Whether the forging in the billet forging process is α + β forging or β forging may be set according to the characteristics required for the product, and the number of forgings may be determined according to the desired billet diameter and the like. Further, the two annealings may be performed as necessary. For example, the second annealing is performed to facilitate subsequent machining. Furthermore, by machining the titanium alloy billet, the surface oxide film, wrinkles and burrs can be removed, the surface roughness can be adjusted, and subsequent forging (β forging in the production of titanium alloy forgings) is easy. . And in order to manufacture the titanium alloy forged material which concerns on this invention, a titanium alloy billet is beta forged by the following method. Prior to β forging, the titanium alloy billet may be subjected to rough ground forging in an α + β two-phase region and finished to a desired shape. Note that the titanium alloy forged material before β forging is referred to as a titanium alloy material, and here, a titanium alloy billet is applied as the titanium alloy material.

In the method for producing a titanium alloy forged material according to the present invention, a titanium alloy material (titanium alloy billet) is heated to T _β + 10 ° C. or higher, and the β crystal grain size (average particle size) is in the range of 400 μm to 1000 μm. Until forging in a temperature range of T _β −30 ° C. or higher, and holding for 20 seconds or longer in this temperature range and less than the limit holding time (seconds) t _max represented by the following formula (2) , immediately cooled to T _beta -150 ° C. or lower.
t _max = [11.64 (1-T _H / 1425)] ^4.35 (2)
However, T _H in the formula (2) is the temperature at the time of holding after forging (° C.).

(Heating temperature before forging: T _β + 10 ° C or more)
The heating before forging is performed in order to heat the titanium alloy billet to the β single-phase region to form the β-phase single phase before forging, as in general β forging. The β single-phase region is a temperature range _{equal to} or higher than the β transformation point (T _β ), and T _β is the lowest temperature at which the entire titanium alloy billet (100%) becomes the β phase, and the titanium alloy billet (titanium alloy forged material) ) Varies depending on the composition of the titanium alloy forming. For example, the T _beta titanium alloy as defined in AMS4981 (Ti-6246 alloy) is about 960 ° C., the T _beta titanium alloy as defined in AMS4995 (Ti-17 alloy) is about 890 ° C.. In the present invention, the titanium alloy billet is surely made into a β-phase single phase to the deep part, forging is completed in a temperature range of T _β −30 ° C. or higher, and thereafter, the titanium alloy billet is held in the same temperature range for a certain time. On the other hand, as the titanium alloy billet becomes higher in the β single phase region, the growth rate of the β phase crystal grains becomes faster, so it becomes difficult to control the crystal grain size, and when T _β + 150 ° C. is exceeded, the surface becomes thicker. Since the oxide scale is easily formed and needs to be removed after forging, the heating temperature is preferably T _β + 150 ° C. or less. Further, if the heating temperature before forging is excessively high, the temperature at the completion of forging becomes high, and after the forging, it is cooled to a temperature outside T _β −30 ° C. or higher (less than T _β −30 ° C.). As such, β crystal grains (non-flat grains) may grow excessively.

After heating the titanium alloy billet to reach the β single-phase region, the titanium alloy billet is held for a certain period of time before forging starts, and the β crystal grains are grown to an appropriate size, specifically, a diameter of 400 μm to 1000 μm. . The holding time varies depending on the holding temperature of the titanium alloy billet, but may be held, for example, at 1000 ° C. for about 60 to 480 minutes. Once the desired β crystal grain structure is formed, the temperature of the titanium alloy billet may be lowered to less than T _β + 10 ° C. before forging. However, as described later, forging is completed and further thereafter It is set so that a temperature range of T _β −30 ° C. or higher can be held until the holding time.

(Forging temperature: _Tβ- 30 ° C or more)
A titanium alloy billet that has been heated and held for a certain period of time is forged into a product shape. The mold used for forging is preferably heated to 400 ° C. or higher, and more preferably heated to a forging temperature (temperature of a titanium alloy billet). By using such a heated mold, without the surface of the titanium alloy billet to be forged too is cooled early with respect to the internal, near surface be maintained above T _beta -30 ° C. Forging Can be completed. When temperature below T _beta -30 ° C. to complete the forging descends, as described later, not subsequently to the non-flattened grains formed. The forging completion temperature is preferably T _β −10 ° C. or higher, and more preferably exceeds T _β . It should be noted that the product portion of the titanium alloy forged material may be held in the temperature range of T _β −30 ° C. or higher until the completion of forging and the end of holding described later, such as the surface layer to be removed after forging (after cooling) (non-product part) the excess thickness may be cooled during said during or after the holding forging temperature outside (T _beta less than -30 ° C.).

The processing rate (reduction rate) in forging is not particularly specified, and forging can be performed under the same conditions as in general finish forging. In order to make β crystal grains into flat grains having an aspect ratio of more than 3, when forging a cylindrical billet by a mold having a flat surface, for example, processing with a rolling reduction of 45% or more, preferably 55% or more, or It is preferable to add a corresponding process. Moreover, it is preferable that the moving speed of the mold with respect to the titanium alloy billet is 10 ⁻³ to 10 (1 / s).

(Temperature after forging (° C.) T _H (≧ T _β −30 ° C.) holding time (seconds): 20 or more [11.64 (1-T _H / 1425)] less than ^4.35
After forging a titanium alloy billet, subsequently maintained for a predetermined time T _beta -30 ° C. or higher. In this way, by keeping the forged titanium alloy billet in a temperature range of T _β −30 ° C. or higher, aside from the β crystal grains that have become flat grains by forging, new non-flat β crystal grains (non- Flat particles) are formed. Since the formation and growth of non-flat β crystal grains almost stops when the titanium alloy is cooled below the above temperature range, the effect of the present invention can be achieved even if the titanium alloy is kept below T _β −30 ° C. after forging the titanium alloy billet. In contrast, a thick and continuous α phase is precipitated at the grain boundaries of the old β grains, which may deteriorate the fatigue strength. The holding temperature after forging is preferably T _β −10 ° C. or more, and more preferably exceeds T _β . On the other hand, the upper limit of the holding temperature after forging is preferably 1160 ° C. or less, because the rate at which non-flat grains are formed and the size and the existing ratio are difficult to control, and T _β is less than 1010 ° C. In this case, T _β + 150 ° C. or lower is more preferable.

If the holding time after forging is less than 20 seconds, the size (diameter in the thickness direction) and the existence ratio of the non-flat particles are insufficient, and the noise reduction effect in the ultrasonic flaw detection by the non-flat particles cannot be obtained. Therefore, the holding time at T _β −30 ° C. or higher after forging is 20 seconds or longer, preferably 30 seconds or longer, more preferably 40 seconds or longer. On the other hand, as the retention time elapses, the proportion of non-flat grains increases and the proportion of flat grains decreases relatively, so if retained for an excessively long time in this temperature range, the fatigue strength of titanium alloy forgings Decreases.

Here, the growth rate of the β crystal grains depends on the holding temperature, and increases as the temperature increases. Such velocity behavior is estimated based on the diffusion behavior of atoms. Therefore, a relational expression between the temperature (° C.) T and the time (seconds) t until the existence ratio of the β crystal grains is grown based on the diffusion equation representing the ease of atom diffusion is expressed by the following equation: Expressed in (3). Note that a, b, and n are constants.
t = [b (1-T / a)] ⁿ (3)

The present inventors have experimentally, by changing the holding temperature (° C.) T _H titanium alloy billet after forging, the non-flat particles is existing ratio in the titanium alloy forging according to the present invention the upper limit (60%) The retention time (limit retention time) t _max until exceeding was measured and inserted into the equation (3) to determine the constants a, b, and n. As a result, a = 1425, b = 11.64, and n = 4.35, and Equation (2) for calculating the limit holding time _tmax was obtained.
t _max = [11.64 (1-T _H / 1425)] ^4.35 (2)

Therefore, the holding time at T _β −30 ° C. or higher after forging is set to be less than the limit holding time t _max seconds represented by the formula (2) based on the holding temperature T _H. Incidentally, the equation (2), the holding temperature T _H is the shorter limit holding time t _max higher, so as to calculate the t _max on the basis of the temperature during the highest forging completed as the holding temperature T _H.

The titanium alloy billet after forging is cooled to T _β −150 ° C. or less immediately after the holding time has elapsed, thereby stopping the growth of non-flat β crystal grains outside the β single-phase region (α + β two-phase region), In addition, the precipitation of a thick and continuous α phase at the grain boundaries of the old β grains is suppressed, and deterioration of the fatigue strength of the obtained titanium alloy forging is prevented. Therefore, the cooling rate after holding is preferably 10 ° C./min or more, more preferably 50 ° C./min or more. On the other hand, the upper limit of the cooling rate is not particularly defined, but 500 ° C./min or less is practical, and is preferable because the acicular α phase in the grains is lengthened to improve fracture toughness. The cooling method may be a known method such as air cooling, air blowing, water cooling, hot water cooling, or oil cooling. In addition, the cooling rate in particular below T ( _beta) -150 degreeC is not prescribed | regulated, What is necessary is just to set according to another required characteristic.

  The obtained titanium alloy forged material is subjected to tempering heat treatment by solution treatment and aging treatment by a known method, if necessary, and further machined to remove oxide film and surplus, Ultrasonic flaw detection is performed. Specifically, it is preferable to perform ultrasonic flaw detection after removing a thickness of 1 mm or more from the surface and smoothing the surface to a surface roughness of 6.3S or more. The titanium alloy forged material is then machined again as necessary to become a product such as an engine part. These processes may be performed by a known method.

[Ultrasonic flaw detection method]
The ultrasonic flaw detection inspection for the titanium alloy forging according to the present invention can be performed by a known method. The probe is selected from a probe having a diameter of 5 to 30 mm, and the ultrasonic wave (transmitted wave) is a frequency. The range of 1-20 MHz is used. The probe diameter is preferably 10 mm or more, and the ultrasonic frequency is preferably 15 MHz or less. Moreover, it is preferable to perform inspection by a water immersion flaw detection method having high defect detection resolution. The titanium alloy forged material according to the present invention can be subjected to ultrasonic flaw detection inspection in which flaw detection is performed in a direction including a direction parallel to the direction in which the amount of rolling reduction is greatest. The direction of ultrasonic flaw detection inspection refers to the traveling direction of the transmission wave (the direction in which the inside of the titanium alloy forging is transmitted) (see FIG. 1). Titanium alloy forgings tend to have the most noise in the direction with the largest forging reduction, but the titanium alloy forgings according to the present invention are sufficiently noise-free and perform high-accuracy inspection even if flaw detection is performed in these directions. be able to. In addition, titanium alloy forgings often have the smallest (thin) thickness in this direction, so that inspection can be performed accurately to the deep part, and the surface area perpendicular to this direction for scanning the probe Is often wide and easy to inspect. Moreover, it is preferable to inspect for a total of two or more times by flaw detection in the one direction or further changing the direction according to the shape of the titanium alloy forged material (product). Further, depending on the thickness of the titanium alloy forged material (the length in the traveling direction of the transmission wave), the transmission wave may be incident from the opposite direction.

  As mentioned above, although the form for implementing this invention has been described, the Example which confirmed the effect of this invention is demonstrated concretely compared with the comparative example which does not satisfy | fill the requirements of this invention below. It should be noted that the present invention is not limited by this embodiment, and can be implemented with modifications within the scope shown in the claims, all of which are included in the technical scope of the present invention.

[Test specimen preparation]
As titanium alloy material, Ti-6246 alloy defined by _{AMS4981 (T β: 960 ℃,} Mo eq: 6.0) billet a length of φ120mm consisting (axial direction) was used and cut into 180 mm.

(Β forging)
The titanium alloy billet was maintained at 920 ° C. for 2 hours in a furnace so that the temperature distribution inside the titanium alloy billet was constant, and then heated to the forging temperatures shown in Table 1. The titanium alloy billet was removed from the furnace after being held at the forging temperature for 30 to 480 minutes depending on the forging temperature until the β crystal grain size shown in Table 1 was reached, and heated to the forging temperature with a low-frequency heating device in advance. Forged using a mold. Forging was performed using a pair of flat surface-shaped molds at a mold movement speed shown in Table 1, with the deformation direction (rolling direction) as the billet axis direction and a rolling reduction of 67% (the length of the material after forging). 60 mm). Note that the forging temperature is less than T _beta specimen No. For No. 10, the titanium alloy billet was heated to 1010 ° C., held in the same manner as the other test specimens until the β crystal grain size shown in Table 1, and then removed from the furnace and allowed to air-cool to the forging temperature shown in Table 1. Forged later.

After forging, the load on the mold is gradually reduced, the upper and lower surfaces of the titanium alloy billet are sandwiched between the molds heated to the forging temperature, and the side surfaces of the titanium alloy billet are covered with a heat insulating material. Was held until the holding time after forging passed, and then immediately taken out and cooled to room temperature to obtain a titanium alloy forged material. In addition, the specimen No. 6 was cooled by air, and the other specimens were cooled by water. In addition, a titanium alloy billet is 1 / 2H, 1 / 4D position (H: thickness of a forging material, D: diameter of a forging material) at the time of heating, holding, and forging, that is, the thickness direction and the radial direction of the forging material, respectively. The forging temperature was controlled by measuring the temperature at the intermediate position with a thermocouple. Moreover, the cooling rate after forging described in Table 1 was measured by a preliminary experiment. That is, prepare a titanium alloy material having the same shape as the titanium alloy forging, insert a thermocouple at the 1 / 2H, 1 / 4D position, heat and hold at 1050 ° C., perform air cooling and water cooling, and set the cooling curve I got it. Thereafter, the cooling rate was calculated assuming that the cooling rate from when it reached 900 ° C. to when it reached 700 ° C. was constant. Further, with the forging temperature as the holding temperature T _H after forging, the limit holding time t _max is calculated from the equation (2) and is also shown in Table 1.

(refining)
Titanium alloy forging cooling to room temperature, after solution treatment cooling with T _beta less than (alpha + beta two-phase region) of was heated to 935 ° C. and held for 2 hours 30 ° C. / min, 8 h holding at 595 ° C. Then, an aging treatment for cooling to room temperature at 35 ° C./min was performed to obtain a test specimen.

(Observation of material structure)
A 15 mm square cube small sample including 1 / 2H and 1 / 4D positions in the test specimen was cut out from the test specimen. And in order to make observation of an old beta grain boundary easy, the heat processing which heats to 900 degrees C less than T _beta (alpha + beta two phase area), air-cools after holding for 30 minutes was performed. In this way, heat treatment in the α + β two-phase region reduces the area ratio of the acicular α phase in the old β grains while maintaining the shape of the old β grains without causing recrystallization or grain growth of β grains. This makes it easy to observe the old β grain boundary. A cross section parallel to the forging direction and the radial direction of the specimen is cut out from the heat-treated small piece, and the cross section is mechanically polished with emery paper. Corrosion was performed with a nitric acid solution, and the structure was observed. Tissue observation is performed with an optical microscope, and a field of view of 3200 μm × 2000 μm is observed in a panorama shape at a magnification of 100 times, and the aspect ratio and the diameter in the thickness direction (axial direction) are obtained for the old β grains to satisfy the requirements of the present invention. Flat and non-flat particles were detected. And the area ratio of a flat grain and a non-flat grain was calculated | required. The results are shown in Table 1.

[Evaluation]
The test specimen was subjected to an ultrasonic flaw detection by a water immersion flaw detection method as an evaluation of the ultrasonic flaw detection property of the titanium alloy forged material, and fatigue strength was evaluated as a mechanical characteristic. In any evaluation, specimen No. Table 1 shows the results normalized by 8 (ie, divided by the value of specimen No. 8).

(Ultrasonic flaw detection)
A 53 mm-square cubic test piece was cut out from the test specimen, and an ultrasonic flaw detection inspection was performed by a water immersion flaw detection method. A probe having a probe diameter of 19.05 mm and a focal length of 152.4 mm was used, an ultrasonic wave having a frequency of 5 MHz was used as a transmission wave, and a water distance (distance from the probe to the surface of the test piece) was 160 mm. After adjusting the sensitivity so that the reflection intensity from a flat bottom hole with a diameter of 0.62 mm is 80% using a standardized test piece, the center 40 mm × 40 mm on the test piece surface (surface perpendicular to the forging direction) is inspected. As a region, an ultrasonic flaw detection test was performed in the direction parallel to the axial direction (forging in one direction) of the specimen as the direction in which the forging reduction amount was the largest while moving and scanning the probe, and the C scope was obtained. .

  The C scope moves the probe along the surface of the object to be inspected at a constant water distance, and extracts the maximum noise intensity value in the flaw detection depth range detected by the probe for each surface scanning point. The flaw detection results are displayed two-dimensionally. With respect to the maximum noise detected by the probe moved and scanned in each specimen, the specimen No. On the basis of 8, 0.8 or less is accepted.

(Fatigue properties)
A fatigue test piece in which the circumferential (tangential) direction of the test body was parallel to the load axis was cut out from the 1 / 2H and 1 / 4D positions of the test body. A low cycle fatigue test in accordance with ASTM standard E466 was performed at room temperature under load control under conditions of a maximum load of 1000 MPa, a stress ratio of 0, and a trapezoidal wave until the fatigue test piece broke. Regarding the number of fracture cycles, the test specimen No. On the basis of 8, 0.3 or more is accepted.

  As shown in Table 1, it was confirmed that the fatigue strength increased as the proportion of the old β-grained flat particles (area ratio) increased, while the noise in the ultrasonic flaw detection increased. Specimen No. 8 and 9 were cooled without maintaining the temperature after forging in the β single-phase region, so they corresponded to conventional β forging materials. Most of the old β grains were flat and non-flat grains were insufficient. Although the strength was high, the ultrasonic flaw detection property was poor.

  In contrast, the test specimen No. 1 to 7 and 13 to 15 are the production method according to the present invention, that is, by maintaining the temperature for a predetermined range after forging in the β single phase region, the non-flat particles of the old β grains grow appropriately, The presence ratio of each of the flat grains and the flat grains became an example satisfying the range of the titanium alloy forged material according to the present invention. As a result, the test specimen No. 1 to 7 and 13 to 15 are specimen Nos. Although the fatigue strength was slightly lower than that of No. 8, it exhibited excellent ultrasonic flaw detection properties while maintaining the mechanical properties required for aircraft engine parts and the like. In particular, specimen No. 6, 7, 13, and 14 were able to effectively reduce noise during ultrasonic flaw detection while maintaining fatigue characteristics at a high level. From this, about the titanium alloy forging material which consists of a titanium alloy of the composition used in the present Example, the abundance ratio of flat grains is 90 to 98%, and the abundance ratio of non-flat grains is 1.5 to 10%. It can be seen that this is particularly preferable.

On the other hand, the specimen No. Since Nos. 11 and 12 were held for an excessively long time after forging, non-flat grains grew excessively. Therefore, although the noise is low, the fatigue strength is greatly reduced. Specimen No. No. 10 was heated in the β single phase region before forging and then forged after being cooled to less than T _β −30 ° C. (α + β two phase region), so that forging was started in the supercooled β phase state (β Fatigue strength was particularly high because a fine α phase was formed. However, the specimen No. No. 10 was already T _β −30 ° C. at the start of forging and was not maintained in the β single-phase region after that, so the non-flat grains of the old β grains were not formed at all, and the noise was particularly high.

[Test specimen preparation]
As titanium alloy material, Ti-17 alloys defined by _{AMS4995 (T β: 890 ℃,} Mo eq: 9.5) billets φ105mm consisting used by cutting in the length (axial) 175mm.

(Β forging)
After maintaining in the furnace at 850 ° C. for 2 hours so that the temperature distribution inside the titanium alloy billet was constant, the titanium alloy billet was heated to the forging temperatures shown in Table 2. A mold in which the titanium alloy billet is removed from the furnace after being held at the forging temperature for 150 to 200 minutes according to the forging temperature until the β crystal grain size reaches 550 μm, and is heated to the forging temperature with a low-frequency heating device in advance. Forged using Forging was performed using a pair of flat surface-shaped molds at a mold moving speed of 1800 mm / min, with the deformation direction (rolling direction) as the billet axis direction and a rolling reduction of 67% (material length after forging 58 mm ). Note that the forging temperature was near T _beta specimen No. For No. 18, in order to obtain a β single-phase structure with certainty, the titanium alloy billet was heated to 940 ° C. and held in the same manner as the other specimens until the β crystal grain size was 550 μm. Forging was carried out after air cooling to the forging temperature shown in FIG.

After forging, in the same manner as in Example 1, while controlling the forging temperature and the like, the upper and lower surfaces of the titanium alloy billet were sandwiched between the molds heated to the forging temperature, and the retention time after forging shown in Table 2 elapsed from the completion of forging. Until then, taken out immediately and cooled to room temperature by blowing with air to obtain a titanium alloy forged material. Moreover, the cooling rate after forging described in Table 2 was measured by a preliminary experiment in the same manner as in Example 1 except for the heating temperature and the like. That is, a titanium alloy material having the same shape as the titanium alloy forged material is prepared, a thermocouple is inserted at the 1 / 2H, 1 / 4D position, and heated and held at 960 ° C., and then cooled by blowing to obtain a cooling curve. did. Thereafter, the cooling rate was calculated assuming that the cooling rate from when it reached 900 ° C. to when it reached 750 ° C. was constant. Further, the limit holding time t _max is calculated from the formula (2) using the forging temperature as the holding temperature T _H after forging, and is also shown in Table 2.

(refining)
Titanium alloy forging cooling to room temperature, after solution treatment cooling with T _beta less than (alpha + beta two-phase region) of was heated to 805 ° C. and held for 2 hours 445 ° C. / min, 8 h holding at 610 ° C. Then, an aging treatment for cooling to room temperature at 60 ° C./min was performed to obtain a test specimen.

(Observation of material structure)
A 15 mm square cubic small sample including 1 / 2H and 1 / 4D positions in the test specimen was cut out from the test specimen, and a cross section parallel to the forging direction and the radial direction of the test specimen was cut out. From this small piece, a cross section that is parallel to the forging direction and the radial direction of the specimen is cut out, and the cross section is mechanically polished with emery paper. After final polishing with diamond abrasive grains, it is corroded with hydrofluoric acid solution. And subjected to tissue observation. Tissue observation was performed in the same manner as in Example 1, and the area ratio of flat particles and non-flat particles was determined. The results are shown in Table 2.

[Evaluation]
The test specimen was subjected to an ultrasonic flaw detection by a water immersion flaw detection method as an evaluation of the ultrasonic flaw detection property of the titanium alloy forged material, and fatigue strength was evaluated as a mechanical characteristic. In any evaluation, specimen No. Table 2 shows the results normalized by 18 (ie, divided by the value of specimen No. 18).

(Ultrasonic flaw detection)
Similarly to Example 1, a 53 mm square test piece was cut out from the test specimen, and an ultrasonic flaw detection test was performed to obtain a C scope. Specimen No. for maximum noise. On the basis of 18, 0.8 or less is accepted.

(Fatigue properties)
A fatigue test piece in which the circumferential (tangential) direction of the test body was parallel to the load axis was cut out from the 1 / 2H and 1 / 4D positions of the test body. A low cycle fatigue test in accordance with ASTM standard E466 was performed at room temperature under the conditions of maximum load of 1030 MPa, stress ratio of 0, and trapezoidal wave under load control until the fatigue test piece broke. Regarding the number of fracture cycles, the test specimen No. On the basis of 18, 0.3 or more is accepted.

  Similar to Example 1 in which the Ti-6246 alloy was applied to the test specimen, as shown in Table 2, the fatigue strength increased as the proportion of the old β-grained flat particles (area ratio) increased, while the ultrasonic wave It was confirmed that noise in the flaw detection inspection increased. Specimen No. No. 18 was cooled without maintaining the temperature after forging in the β single-phase region, so it corresponds to the conventional β forging material. Most of the old β grains are flat grains and non-flat grains are insufficient. High, but inferior in ultrasonic flaw detection.

  In contrast, the test specimen No. Nos. 16 and 17 are the production method according to the present invention, that is, the temperature is maintained for a predetermined range after forging in the β single-phase region, so that the non-flat particles of the old β grains grow appropriately, and the non-flat grains, flat Each abundance ratio of the grains became an example satisfying the range of the titanium alloy forging according to the present invention. As a result, the test specimen No. 16 and 17 are specimen Nos. Although the fatigue strength was slightly lower than that of No. 18, it exhibited excellent ultrasonic flaw detection properties while maintaining the mechanical properties required for aircraft engine parts and the like.

Claims (5)

It is a forged titanium alloy forged material that undergoes ultrasonic flaw detection,
A flat grain which is a crystal grain of an old β grain having a diameter in the thickness direction of 50 μm or more and 500 μm or less and an aspect ratio exceeding 3, and an old β grain having a diameter in the direction of 30 μm or more and 200 μm or less and an aspect ratio of 1 or more and 3 or less Having a mixed grain structure of non-flat grains that are crystal grains of
The titanium alloy forging material, wherein the flat grains are 40% to 99%, the non-flat grains are 1% to 60%, and the flat grains and the non-flat grains are 90% or more in total. 2. The titanium alloy forging according to claim 1, comprising a titanium alloy having a Mo equivalent [Mo] eq represented by the following formula (1) of more than 2.7 and less than 15.
[Mo] eq = [Mo] + [Ta] / 5 + [Nb] /3.6+ [W] /2.5+ [V] /1.5+1.25 [Cr] +1.25 [Ni] +1.7 [ Mn] +1.7 [Co] +2.5 [Fe] (1)
However, [X] in the formula (1) is the content (mass%) of the element X in the titanium alloy.   The titanium alloy forged material according to claim 1 or 2, wherein the thickness is at least 50 mm. A production method for producing a titanium alloy forged material according to any one of claims 1 to 3 by performing β-forging,
In the β forging, when the β transformation point is expressed by T _β , the β forging is heated to T _β + 10 ° C. or higher and held until the β crystal grain size is in a range of 400 μm to 1000 μm, and T _β −30 ° C. or higher. After forging in the temperature range and holding for less than 20 seconds in the temperature range and less than the limit holding time (seconds) t _max represented by the following formula (2), the temperature is immediately increased to T _β −150 ° C. or less. A method for producing a titanium alloy forging material, characterized by cooling.
t _max = [11.64 (1-T _H / 1425)] ^4.35 (2)
However, T _H in the formula (2) is a temperature (℃) during holding after the forging. An ultrasonic flaw detection inspection method for a titanium alloy forged material according to any one of claims 1 to 3,
Using a probe having a probe diameter in the range of 5 to 30 mm, flaw detection is performed in the direction parallel to the direction in which the forging reduction amount of the titanium alloy forging material is maximum with ultrasonic waves having a frequency in the range of 1 to 20 MHz. An ultrasonic flaw detection method comprising the steps of: