JP5827165B2 - Titanium alloy forging, its manufacturing method, and its ultrasonic flaw detection inspection method - Google Patents

Description

  The present invention relates to a technology related to a β-forged material of α + β-type titanium alloy that is inspected for the presence of defects by ultrasonic inspection.

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 at a temperature range of T _β or more, and after the equiaxed β phase (β grains) is flattened by forging, it is cooled to a temperature range below T _β and held at this temperature range. Then, the α phase precipitates in the form of a film along the grain boundaries of the β grains, and subsequently the α phase precipitates in the form of needles in the crystal grains of the β grains (shown in white in FIG. 2 (a)). 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.

  Aircraft engine parts are required to have high fatigue strength characteristics as well as high reliability, and therefore are inspected for defects 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, conventionally, as an α + β type titanium alloy material with reduced noise, for example, before hot rolling in the α + β two-phase region, the β single-phase region is rapidly cooled to refine the structure, and the subsequent α + β two-phase region A titanium alloy rolled sheet having an equiaxed α structure obtained by hot rolling and heat treatment is 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 property with reduced noise, a manufacturing method thereof, and an ultrasonic flaw detection inspection method thereof.

  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, a titanium alloy forging according to the present invention, a flat particle is the old β grains of crystal grains the aspect ratio exceeds 3, the aspect ratio of the old β grains of the crystal grains is 1 to 3 non-flat The non-flat particles having a mixed grain structure with a diameter in the thickness direction of 10 μm or more have an average equivalent circle diameter of 100 μm or less. The titanium alloy forged material has a thickness direction diameter of 20 μm or more and 500 μm or less of flat grains of 40% or more and 98% or less, and a thickness direction diameter of 10 μm or more and 150 μm or less of non-flat grains of 2% or more and 50% or less, 90% or more of the flat grains and the non-flat grains having the diameters in the respective thickness directions are present in total.

  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 (X: Mo, Ta, Nb, W, V, Cr, Ni, Mn, Co, Fe) in the titanium alloy. And

  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, the β forging is heated to (T _β + 10 ° C.) or more when the β transformation point is expressed by T _β , and the β crystal grain size is 100 μm or more and 400 μm. It keeps until it becomes the range of less than, and it forges in the temperature range (T ( _beta ) -30 degreeC) or more, It is 15 seconds or more in the said temperature range, and the limit holding time (second) t represented by following Formula (2) t after the holding time of less than _max, characterized by cooling to immediately _(T β -150 ℃) or lower.
t _max = [8.0 (1-T _H / 1500)] ^4.76 (2)
However, T _H in the formula (2) is a temperature (℃) during holding after the forging.

  With such a procedure, the titanium alloy forging manufacturing method is maintained for a predetermined time in a predetermined temperature range in the vicinity of the β transformation point after β forging, and non-flat old β grains grow moderately, resulting in flatness generated by forging. A titanium alloy forging material having a mixed grain structure with the former β grains is obtained.

  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 has a mixed grain structure of flat grains having an aspect ratio exceeding 3 and non-flat grains having an aspect ratio of 1 or more and 3 or less with respect to crystal grains of the old β grains. Further, in the titanium alloy forged material, the average equivalent circle diameter of the non-flat particles having a diameter in the thickness direction of 10 μm or more is 100 μm or less. Further, the titanium alloy forged material has a flat particle with a diameter in the thickness direction of 20 μm or more and 500 μm or less of 40% or more and 98% or less, and a non-flat particle with a diameter of 10 μm or more and 150 μm or less in the thickness direction of 2% or more and 50% or less. Furthermore, there are a total of 90% or more of flat particles and non-flat particles having a diameter in the thickness direction in each of these ranges.

(Mixed grain structure)
In the present invention, among the old β grains, crystal grains having an aspect ratio exceeding 3 are defined as flat grains, and crystal grains having 3 or less (1 or more and 3 or less) are defined as non-flat grains. In the present invention, the aspect ratio refers to a diameter in a direction perpendicular to the diameter in the thickness direction. The diameter in the thickness direction (hereinafter referred to as the thickness direction diameter) refers to the length in the minimum direction in the crystal 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 with a thickness direction diameter of 20 to 500 μm: abundance ratio of 40 to 98%)
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. Further, if the diameter of the old β grain is small, the number of grain boundaries in the ultrasonic flaw detection direction of the titanium alloy forged material increases, and if it is less than 20 μm, noise may increase. On the other hand, flat grains having a thickness direction diameter exceeding 500 μm reduce the fatigue strength of the titanium alloy forged material. In addition, although the thickness direction (direction where a diameter becomes the minimum) of a flat grain depends on the shape of the titanium alloy forged material, it often coincides with the forging direction (reduction direction).

  And as for the titanium alloy forging material, fatigue strength improves, so that there are many proportions of the flat grain whose thickness direction diameter is the range of 20-500 micrometers. The titanium alloy forged material, if this flat grain is less than 40%, the effect of improving the fatigue strength due to the flat grain is not sufficiently obtained, so the flat grain size in the above range is 40% or more, preferably 50% or more, More preferably, it is 60% or more, and most preferably 70% or more. On the other hand, if the titanium alloy forged material exceeds 98% even if the flat grain has a diameter in the thickness direction of 20 μm or more, noise in the ultrasonic flaw detection increases, the flat grain should be 98% or less, preferably 95% or less. More preferably, it is 90% or less, and most preferably 84% or less. Therefore, in the titanium alloy forged material according to the present invention, flat particles of old β grains having a thickness direction diameter of 20 μm or more and 500 μm or less are present in an amount of 40% or more and 98% or less.

  As described above, by increasing the diameter in the thickness direction of flat grains, the number of grain boundaries in the ultrasonic flaw detection direction of the titanium alloy forged material is reduced, noise is suppressed, and fracture toughness and fatigue strength are improved. Therefore, the average thickness direction diameter is preferably 30 μm or more, and more preferably 70 μm or more. On the other hand, when the flat particles become large, the area of the grain boundary decreases, and it becomes difficult to secure the existence ratio of non-flat particles as will be described later. Therefore, the diameter in the thickness direction may be 170 μm or less on average. Preferably, it is 130 μm 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 with a thickness direction diameter of 10 to 150 μm: abundance 2 to 50%)
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 of less than 10 μm, the area of the grain boundaries of the non-flat grains is narrow, and a noise reduction effect cannot be obtained. On the contrary, non-flat grains having a thickness direction diameter exceeding 150 μm may reduce the fatigue strength and fracture toughness of the titanium alloy forged material.

  The titanium alloy forged material has a non-flat grain having a diameter in the range of 10 to 150 μm in a range of less than 2%, and a noise reduction effect due to the non-flat grain cannot be sufficiently obtained. The content is preferably 5% or more, more preferably 8% or more, still more preferably 12% or more, and most preferably 16% 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 50%, the titanium alloy forged material has insufficient fatigue strength, so the non-flat grain is 50% or less, preferably 40% or less, more preferably 30% or less. Therefore, in the titanium alloy forged material according to the present invention, non-flat particles of old β grains having a diameter in the thickness direction of 10 μm or more and 150 μm or less are 2% or more and 50% or less.

(Total existence ratio of 90% or more of flat grains having a thickness direction diameter of 20 to 500 μm and non-flat grains having a thickness direction diameter of 10 to 150 μm)
Titanium alloy forging material is a fine β which is out of these ranges if the total of flat particles and non-flat particles of the above-mentioned predetermined range of thickness direction diameters with limited abundance ratio is less than 90% with respect to the old β grains. Crystal grains or coarse β crystal grains are excessive, resulting in insufficient fatigue strength and fracture toughness, and increased noise in ultrasonic inspection. Therefore, the titanium alloy forged material according to the present invention has a total of 90% or more of flat grains having a thickness direction diameter of 20 to 500 μm and non-flat grains having a thickness direction diameter of 10 to 150 μm, preferably 92% or more. More preferably, it is 94% or more.

(Non-flat particles with a diameter in the thickness direction of 10 μm or more: equivalent circle diameter average of 100 μm or less)
As described above, non-flat particles having a large particle size lower the fatigue strength and fracture toughness of the titanium alloy forged material. Therefore, the forged titanium alloy according to the present invention defines an average size of non-flat grains excluding fine particles having a thickness direction diameter of less than 10 μm. Specifically, the non-flat particles having a diameter in the thickness direction of 10 μm or more are average, including those having a diameter in the thickness direction exceeding 150 μm, and the equivalent circle diameter (diameter of the circle having the same area as the cross section) is 100 μm or less, preferably 90 μm or less. More preferably, it is 80 μm or less. As a result, the forged titanium alloy has a non-flat particle having a thickness direction diameter of 150 μm or less as a whole or a large particle size, or a flat particle and a non-flat particle having a thickness direction diameter outside the predetermined range are 10% or less. However, most of them are limited so that they are not coarse non-flat particles having a thickness direction diameter of more than 150 μm. 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. Furthermore, when heated to the β single-phase region before forging, the holding temperature and time are adjusted to grow the β crystal grains to an appropriate size, thereby suppressing the size of the flat grains obtained by forging. Thus, when the forging is completed, a sufficient area of the grain boundaries of the flat grains can be secured, a sufficient number of non-flat grains can be formed in the vicinity of the grain boundaries, and the existence ratio can be controlled while suppressing the grain size. .

  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 obtained 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 this 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). Should be defined. Thereby, each area ratio in a visual field and the average value of the circle equivalent diameter of a non-flat grain can 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 each content (mass%) of the element X (X: Mo, Ta, Nb, W, V, Cr, Ni, Mn, Co, Fe) 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 forging according to the present invention, a titanium alloy material (titanium alloy billet) is heated to (T _β + 10 ° C.) or more, and a β crystal grain size (average grain size) is in a range of 100 μm or more and less than 400 μm. Until it becomes, and forging in a temperature range of ( _Tβ- 30 ° C) or more, and for 15 seconds or more in this temperature range and less than the limit holding time (seconds) _tmax expressed by the following formula (2) after time held immediately _(T β -150 ℃) cooled to a temperature below.
t _max = [8.0 (1-T _H / 1500)] ^4.76 (2)
However, T _H in the formula (2) is the temperature at the time of holding after forging (° C.).

(Heating temperature before forging: ≧ _Tβ + 10 ° C.)
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 up to the deep part, and forging is completed in a temperature range of (T _β −30 ° C.) or higher, and thereafter, kept in the same temperature range for a certain time. . On the other hand, as the titanium alloy billet becomes a high temperature in the β single phase region, the growth rate of the β phase crystal grains increases, so it becomes difficult to control the crystal grain size, and when it exceeds (T _β + 150 ° C.), Since a thick oxide scale is easily formed on the surface and needs to be removed after forging, the heating temperature is preferably (T _β + 150 ° C.) or less. Further, when the heating temperature before forging is excessively high, up to higher temperatures during forging completion is cooled to below after forging _(T β -30 ℃) temperatures above outside i.e. _(T β -30 ℃) Furthermore, there is a possibility that β crystal grains (non-flat grains) grow excessively as described later.

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 100 μm or more and less than 400 μm. . Although holding time changes with holding temperature of a titanium alloy billet, what is necessary is just to hold | maintain for about 5 to 60 minutes at 1000 degreeC, for example. Note that the titanium alloy billet, once the desired β crystal grain structure is formed, may drop in temperature to less than (T _β + 10 ° C.) before forging. It is set so that the forging can be held in a temperature range equal to or higher than ( _Tβ- 30 ° C) until the forging time is further completed.

(Forging temperature: ≧ _Tβ- 30 ° C)
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, the surface of the titanium alloy billet to be forged is not cooled too quickly with respect to the inside, and the vicinity of the surface is maintained at ( _Tβ- 30 ° C) or higher. Forging can be completed. When the titanium alloy billet is forged in a temperature range of less than ( _Tβ- 30 ° C), non-flat particles are not formed thereafter, as described later. Completion temperature of the forging is preferably not less than _(T β -10 ℃), it is more preferably more than T _beta. Incidentally, the forging completed, further being held in below until the end of the hold (T _beta -30 ° C.) or higher temperature region is removed well product portion of the titanium alloy forging, after forging (after cooling) surface The temperature in the surplus meat (other than the product part) is not specified.

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.) (second): 15 or more [8.0 (1-T _H / 1500)] less than ^4.76 )
After forging the titanium alloy billet, the titanium alloy billet is continuously held at a temperature of ( _Tβ- 30 ° C) or higher for a predetermined time. In this way, by keeping the forged titanium alloy billet in a temperature range of (T _β −30 ° C.) or more, separately from the β crystal grains that have become flat grains by forging, new non-flat β crystal grains (Non-flat particles) are formed. When the titanium alloy is cooled below the above temperature range, the formation and growth of β crystal grains almost cease. Therefore, even if the titanium alloy is kept in a temperature range below (T _β -30 ° C.) after forging of the titanium alloy billet, the present invention On the contrary, a thick and continuous α phase is precipitated at the grain boundary of the old β grains, which may deteriorate the fatigue strength. Holding temperature after forging is preferably at least _(T β -10 ℃), it is more preferably more than T _beta. On the other hand, when the holding temperature after forging becomes high, the rate at which non-flat grains are formed becomes high and it is difficult to control the size and the existence ratio, so that it is preferably 1150 ° C. or less, and T _β is less than 1000 ° C. In this case, ( _Tβ + 150 ° C.) or less is more preferable.

When the holding time after forging is less than 15 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 using the non-flat particles cannot be obtained. Accordingly, the holding time at ( _Tβ- 30 ° C) or higher after forging is 15 seconds or longer, preferably 20 seconds or longer, more preferably 30 seconds or longer. On the other hand, as the retention time elapses, the particle size of the non-flat grains increases, the existence ratio increases, and the existence ratio of the flat grains relatively decreases. The fatigue strength of the alloy forging is reduced.

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, the holding temperature of the titanium alloy billet after forging (° C.) T _H a varied, retention time to the non-flat particles reach the circle equivalent diameter 100μm in average (marginal retention time) t _max was measured and inserted into equation (3) to determine constants a, b, n. As a result, a = 1500, b = 8.0, n = 4.76, and Equation (2) for calculating the limit holding time t _max was obtained.
t _max = [8.0 (1-T _H / 1500)] ^4.76 (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.

By cooling the forged titanium alloy billet to (T _β -150 ° C) or less immediately after the holding time has elapsed, growth of non-flat β crystal grains outside the β single phase region (α + β two phase region) is stopped. And suppressing the precipitation of the thick and continuous α phase at the grain boundaries of the old β grains, thereby preventing the fatigue strength of the obtained titanium alloy forging from deteriorating. 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. The cooling rate in the temperature range below _(T β -150 ℃) is not specifically defined, may be set according to other required properties.

  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 can 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 reduction in the forging is the largest. 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, which is defined by AMS4981: billet length of φ120mm consisting _(T β 960 ℃) (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. After maintaining the time until the β crystal has the average grain size shown in Table 1 at the forging temperature, the titanium alloy billet is taken out of the furnace and forged using a die that has been heated to the forging temperature with a low-frequency heating device in advance. . Preliminary experiments at each holding temperature (forging temperature) were performed using a titanium alloy material having the same shape as the titanium alloy forging material to observe the average grain size of β crystals shown in Table 1, and observed with an optical microscope. The average particle diameter of the β crystal was measured from the tissue photograph by the section method, and the retention time was determined. 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. 3, the titanium alloy billet was heated to 1000 ° 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 air-cooled 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. Microscopic observation is performed with an optical microscope, and the field of view of 3200 μm × 2000 μm is observed in a panoramic shape at a magnification of 100 times, and the aspect ratio and the thickness direction (axial direction) diameter of the old β grains are obtained. And non-flat particles were detected. Then, the area ratio of the flat particles having the thickness direction diameter and the non-flat particles satisfying the requirements of the present invention is obtained, and the average value of the thickness direction diameters of the flat particles having the thickness direction diameter of 20 μm or more and the non-flat size of 10 μm or more. The average value of the equivalent circle diameter of the flat particles was calculated. These values are shown in Table 1.

[Evaluation]
(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 a direction parallel to the forging direction (axial direction of the specimen) while moving and scanning the probe, and a 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. The maximum noise detected by the probe moved and scanned in each test piece is shown in Table 1, and 35% or less is accepted.

(Fatigue properties)
Fatigue strength was evaluated as a mechanical property of the titanium alloy forging. 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. A value normalized with 7 as a reference (1) is calculated (divided by the number of fracture cycles of specimen No. 7), shown in Table 1, and 0.5 or more is regarded as acceptable.

  As shown in Table 1, the test specimen No. No. 7 is a sufficiently large old β grain because the β crystal is forged in the β single phase region before the forging and then forged, and the retention time after forging is further reduced. Since it was within the predetermined range, the existence ratio (area ratio) was sufficiently maintained. As a result, the test specimen No. 7 has high fatigue strength, but on the other hand, since the area of the grain boundaries of the flat grains is small, the number of non-flat grains is small and the area ratio is insufficient, so the noise reduction effect in ultrasonic flaw detection is insufficient. Met.

  In contrast, the test specimen No. 1 to 6, the production method according to the present invention, that is, by growing the β crystal in a β single phase region while suppressing the grain size within a predetermined range before forging, and keeping the temperature for a predetermined range after forging, The non-flat grains of the old β grains grew to an appropriate size, and the existence ratios of the non-flat grains and the flat grains satisfied the range of the titanium alloy forged material according to the present invention. As a result, the test specimen No. 1 to 3 and 6 are specimen Nos. No. 7 fatigue strength, test specimen No. Nos. 4 and 5 are specimen Nos. The fatigue strength is slightly lower than that of No. 7, and even if the non-flat particles of the old β grains are small as described above, they retain the necessary mechanical characteristics as aircraft engine parts, etc. It showed flaw detection.

  On the other hand, the specimen No. Since Nos. 8 and 9 were held for an excessively long time after forging, non-flat grains grew excessively, the diameter in the thickness direction exceeded the range of the present invention, and the average equivalent circle diameter was excessive. Therefore, specimen No. Nos. 8 and 9 have low noise, but the test specimen No. In the same manner as in No. 7, despite the fact that β crystals were grown larger than the range of the present invention before forging to form large flat grains, the test specimen No. The fatigue strength was significantly lower than 7. In particular, specimen No. In No. 9, the fatigue strength was remarkably lowered because the existence ratio of flat grains was greatly reduced. In addition, the specimen No. 10 is a specimen No. As in the case of No. 7, the flat grains were large and the retention time after forging was insufficient, so that there were particularly few non-flat grains and a lot of noise.

Claims (5)

Has a aspect ratio is flattened grains is the old β grains of crystal grains exceeds 3, non-a flat grain aspect ratio is the old β grains of the crystal grains is 1 to 3, the mixed grain structure of,
The non-flat particles having a diameter in the thickness direction of 10 μm or more have an average equivalent circle diameter of 100 μm or less,
The flat particles having a diameter in the thickness direction of 20 μm or more and 500 μm or less are 40% or more and 98% or less, and the non-flat particles having a diameter in the thickness direction of 10 μm or more and 150 μm or less are 2% or more and 50% or less. The titanium alloy forging material, wherein 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 more and held until the β crystal grain size is in the range of 100 μm or more and less than 400 μm, (T _β -30 Forging in a temperature range of 15 ° C. or higher, and holding in the temperature range for 15 seconds or longer and a limit holding time (second) represented by the following formula (2) that is less than t _max , immediately (T _β -150 ° C) A method for producing a titanium alloy forging, characterized by cooling to the following temperature.
t _max = [8.0 (1-T _H / 1500)] ^4.76 (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: