JP5952689B2 - Titanium alloy forged material, method for producing the same, and method for producing titanium alloy forged parts - 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. First, 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. 3 (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 in the form and thickness of the α phase on the grain boundary 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. May not have a film-like α phase on the grain boundary. On the other hand, the titanium alloy forged material has a granular α structure according to α + β forging (see FIG. 3B). In general, in α + β-type titanium alloy forgings, fracture toughness is better for forged materials that are β-forged than forged materials for which α + β-forged, and conversely, fatigue strength characteristics are that of forged materials that are α + β-forged. 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 with reduced noise, a method for producing the same, and a method for producing a titanium alloy forged part.

  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 was clarified that by appropriately controlling the shape of the old β grain, 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 having a flat surface, has a needle-like α phase structure, and has an average aspect ratio of 2 to 10 on average. crystal structure Tona is shall. Then, a titanium alloy forging, the average value of the old β grains in the grain boundary is a line parallel to the angle in the normal direction of the flat surface in a cross section parallel to the normal direction of the flat surface at less than 80 ° And the average length of the α phase formed on the grain boundaries of the old β grains in the grain boundary direction is 15 μm or less. Furthermore, in the titanium alloy forged material according to the present invention, the average diameter of the old β grains in the normal direction of the flat surface is 60 μm or more and 700 μm or less. Further, in the titanium alloy forging according to the present invention, the old β grains having an aspect ratio of less than 3 and a diameter in the normal direction of the flat surface of 30 μm or more and 200 μm or less are in a cross section parallel to the normal direction of the flat surface . It is assumed that the area ratio does not exist 1% or more.

  The titanium alloy forged material having such a structure is defined such that the aspect ratio of the old β grains, that is, the degree of flatness is defined within a predetermined range, and the grain boundary of the old β grains is defined not to be perpendicular or substantially perpendicular to the forging direction. , Ultrasonic flaw detection because the transmission wave incident in parallel to the forging direction is reflected on the grain boundary of the old β grains and is not received by the probe without reducing the strength of the β forging material, and noise is reduced. Excellent in properties.

  Further, the forged titanium alloy according to the present invention has a width of the grain boundary α band which is a region where the α phase formed on the grain boundary of the old β grain and the α phase formed along the grain boundary exist. The average is preferably 10 μm or less.

  The titanium alloy forged material having such a configuration has good fracture toughness because most of the α phase in the old β grains is needle-like.

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.

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 300 μm or more and 1000 μm. held until the following range, after forging in one direction under the condition that the _(T β -150 ℃) or (T β ₊ 200 ℃) at a temperature range, equivalent strain of 0.45 or more 2.1 or less, characterized by cooling to a temperature lower than (T β -150 _℃).

  By this procedure, the titanium alloy forging material can be obtained by a forging method in which the old β grains are moderately flat in β forging.

The method for producing a titanium alloy forged part according to the present invention includes a step of producing a titanium alloy forged material by the method for producing a titanium alloy forged material, and a parallel to the titanium alloy forged material in a direction in which the reduction amount in the forging is the largest. And flaw detection by irradiating ultrasonic waves in various directions. By this method for manufacturing a titanium alloy forged part, a part used for an aircraft engine can be manufactured.

  With this method, the titanium alloy forged part manufacturing method has a large area in the titanium alloy forging because the ultrasonic flaw detection has a sufficiently low noise even when ultrasonic flaw detection is performed in a direction of relatively high noise. The surface can be scanned with a probe, and inspection can be performed easily and with high accuracy.

  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. Further, according to the method for manufacturing a titanium alloy forged part according to the present invention, it is possible to manufacture a titanium alloy forged part that has been subjected to high-accuracy ultrasonic flaw inspection on the titanium alloy forged material.

It is a cross-sectional schematic diagram explaining the state of the structure | tissue of (beta) forging material of a titanium alloy. It is a schematic diagram for demonstrating the shape of the (alpha) phase in (beta) forging material of a titanium alloy, and is an expanded sectional view of the grain boundary vicinity of an old beta grain. 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 80 mm or more on average, and 50 mm or more even at the thinnest part.

  The titanium alloy forging according to the present invention is made of an α + β type titanium alloy (hereinafter referred to as titanium alloy), and like the conventional β forging, the old β grains (β phase) and the crystal grain boundaries of the old β grains And α phase precipitated in the crystal grains. However, in the titanium alloy forged material according to the present invention, the average aspect ratio of the old β grains is 2 or more and 10 or less, and the diameter of the old β grains in the forging direction is preferably 700 μm or less on average. It is preferable that 1% or more of old β grains having a diameter of 30 μm or more and 200 μm or less in the forging direction are not present. In addition, in the titanium alloy forged material according to the present invention, the angle between the grain boundaries of the old β grains and the line parallel to the forging direction is 80 ° or less on average. Further, in the titanium alloy forged material according to the present invention, the α phase formed on the grain boundaries of the old β grains has an average length in the grain boundary direction of 15 μm or less, on the grain boundaries, and It is preferable that the grain boundary α band, which is a region where the α phase is formed, along the grain boundary has an average width of 10 μm or less.

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. Β-phase crystal grains (β crystal grains, β grains) having a ratio close to 1 are formed and grown. And the β crystal grains are crushed by the forging process, deformed into a flat shape spreading perpendicularly to the forging direction (reduction direction), and the polycrystalline structure in which the β crystal grains (former β grains) in a pancake shape are stacked (See FIG. 1). When cooled after forging drops to a sufficiently low temperature range of less than T _β (α + _β two-phase region), alpha-phase is precipitated in the former beta grain of grain boundaries on and intragranular (see Figure 3 (a)). Therefore, in the β forged material, the diameter of the old β grain is often minimized in the forging direction. In addition, when the titanium alloy forged material is cooled slowly and the temperature is in the β single-phase region for a long time after forging, new equiaxed β grains grow. In FIG. 1, the α phase formed in a film shape on the grain boundary of the old β grain is shown in an enlarged view in a circle indicated by a broken line with respect to the α phase, and the other illustrations are omitted.

(Aspect ratio of old β grains: average 2 to 10)
In the present invention, the aspect ratio of the old β grains refers to the ratio of the diameter in the direction perpendicular to this direction to the diameter in the forging direction. That is, in FIG. 1, the ratio of the diameter in the left-right direction to the diameter in the up-down direction. Moreover, the diameter of a crystal grain refers to the maximum length in a predetermined direction. 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. Titanium alloy forged material has a flat aspect ratio with a large aspect ratio as the amount of strain applied to the former β grains, which were equiaxed with a small aspect ratio (close to 1) before forging, increases as the amount of strain applied during forging increases. Thus, the contribution of improving the fatigue strength of the old β grains increases.

Titanium alloy forgings have an average aspect ratio of the old β grains of less than 2, so that there are many equiaxed β grains, and these β grains do not contribute to the improvement of fatigue strength, so the fatigue strength is insufficient. To do. Therefore, the titanium alloy forged material has an average aspect ratio of the old β grains of 2 or more and preferably 3 or more. Further, as will be described later, the old β grains having an aspect ratio of less than 3 should be as few as possible. More preferred. The aspect ratio of the old β grains is due to the fact that the β grains did not deform sufficiently due to lack of strain during forging or because they were not immediately cooled to a temperature lower than T _β after forging. It becomes smaller when a lot of new equiaxed β grains are produced. On the other hand, as the aspect ratio of the old β grains increases, that is, as the flatness becomes flatter, the inclination of the grain boundaries of the old β grains decreases and approaches the plane perpendicular to the forging direction, and the plane becomes larger and super- Noise in sonic flaw inspection (when ultrasonic waves are irradiated parallel to the forging direction; the same applies hereinafter) increases. If the aspect ratio of the old β grains exceeds 10 on average, there is a risk that measurement accuracy may be reduced due to noise in ultrasonic flaw detection. Accordingly, the titanium alloy forged material has an average aspect ratio of the old β grains of 10 or less, preferably 8 or less, more preferably 7 or less, and even more preferably 6 or less. The aspect ratio of such old beta grains, the amount of strain during forging can be controlled by (rolling reduction), further, after the forging is completed in a temperature well below a short time T _beta cooled rapidly It is preferable to make it reach so that new β grains do not grow.

(An angle between the old β grain boundary and a line parallel to the forging direction: average of 80 ° or less)
As described above, the titanium alloy forged material has a structure in which flat old β grains spreading perpendicularly to the forging direction are stacked in the forging direction. Therefore, as shown in a cross-sectional view in FIG. 1, the grain boundaries of the old β grains (old beta grain boundaries) form a surface that is relatively close to the forging direction. In the titanium alloy forged material, the inclination of the grain boundary is closer to the plane perpendicular to the forging direction, and as the grain boundary becomes wider and larger, the noise in the ultrasonic flaw inspection increases. Specifically, the angle between the grain boundaries of the old β grains and the plane perpendicular to the forging direction is less than 10 ° on average, in other words, the angles formed with lines parallel to the forging direction (θ ₁ and θ _{2 in} FIG. 1). , Θ ₃ , θ ₄ ) exceeds 80 ° on average, it may be difficult to measure due to noise in ultrasonic flaw detection. Therefore, the angle between the old β grain boundary and the line parallel to the forging direction is 80 ° or less on average. This angle is determined by an acute angle (absolute value of a maximum of 90 °). In addition, since the grain boundaries of the old β grains have many curved surfaces, as will be described later, the cross section of the titanium alloy forged material may be observed and measured at the grain boundary tangent lines appearing in a line (curve) shape in the cross section. Such an angle of the grain boundary of the old β grains can be obtained by controlling the old β grains to the aspect ratio defined above, that is, the old β grains do not become too flat.

(The diameter of the old β grains in the forging direction: average 700 μm or less)
Since the fatigue strength of the titanium alloy forged material decreases when the old β grains are too large, specifically, the average diameter in the forging direction is preferably 700 μm or less. On the other hand, the lower limit of the diameter of the old β grain is not particularly defined, but it is difficult to make it smaller than 60 μm in practice. The diameter of the old β grains of the titanium alloy forged material depends on the diameter of the β grains grown by heating before forging, and can be controlled by the heating temperature and the holding time. In other words, in order to reduce the size of the old β grains, it is necessary to make them fine β grains by heating. However, the larger titanium alloy material (titanium alloy forging material before forging) has a temperature rise rate in the heating and the surface layer. Since there is a large difference in the deep part, it is difficult to make fine and uniform β grains. Specifically, it is realistic that the β grains before forging have a diameter of 300 μm or more. Therefore, the diameter in the forging direction after forging (titanium alloy forged material) is 60 μm or more, and the β grains before forging have a diameter of 400 μm or more. However, it is preferable that the diameter after forging is 80 μm or more.

(Existence ratio of old β grains having an aspect ratio of less than 3 and a diameter of 30 μm or more and 200 μm or less in the forging direction: less than 1%)
In the titanium alloy forged material, when β grains newly grow after forging, the presence ratio of the old β grains flattened by forging is reduced due to the equiaxed β grains, and the fatigue strength is lowered. Further, if the forged titanium alloy has an average aspect ratio of the old β grains of 2 or more and is small overall and has a small diameter, the old β grains have a small effect of improving the fatigue strength. Specifically, 1% or more of old β grains having an aspect ratio of less than 3 and a diameter in the forging direction of 30 μm or more and 200 μm or less (hereinafter referred to as non-flat β grains as appropriate) are not present (the existence ratio is less than 1%). Is preferred. Such tissue, a beta particle with heating before forging grown to an appropriate size, forged under conditions sufficient strain is applied, further, after the forging is completed in a short time by cooling rapidly T _beta It is obtained by reaching a temperature sufficiently lower than that, specifically, a temperature lower than ( _Tβ- 30 ° C) so that new β grains do not grow. In addition, this existence ratio in a titanium alloy forging material points out the area rate in a cross section.

(Length of α phase on grain boundary in grain boundary direction: average 15 μm or less)
The α phase precipitated on the grain boundaries of the old β grains (see FIG. 1) is in the form of a film along the grain boundaries and usually grows from the grain boundaries into the old β grains after depositing on the grain boundaries. To do. In addition, the α phase which begins to precipitate on the old β grain boundary after completion of forging usually has a film thickness (the length of the old β grain boundary in the normal direction, t _α in FIG. 1) of about 2 μm. In the titanium alloy forged material, when the α phase is continuously formed long along the grain boundary, the fatigue strength deteriorates. Accordingly, the length of the α phase on the old β grain boundary in the direction of the old β grain boundary (l _{α in} FIG. 1) is 15 μm or less on average, preferably 10 μm or less, and more preferably 7 μm or less. . The length l _α of the α phase is determined by observing the cross section of the titanium alloy forging as described later, and the α phase formed on the old β grain boundary appearing linearly in the cross section. What is necessary is just to measure the length. If the titanium alloy forging material has a small strain applied by forging, the α phase spreads easily along the grain boundary during subsequent cooling, so that sufficient strain is applied as described in the manufacturing method described later. It is preferable to forge so that it may be added.

(Width of grain boundary α band: average 10 μm or less)
Here, when the forged titanium alloy is forged at a temperature range in which the α phase is precipitated, the α phase already precipitated on the grain boundaries and in the grains of the old β grains is finely divided. In addition, even in a supercooled region where the α phase is not precipitated during forging, the α phase precipitates immediately after the forging is completed, so that the α phase precipitation density increases. In normal β forging, needles are formed in the old β grains. When it grows, it remains granular (equal axis). This equiaxed α phase refers to an aspect ratio (the ratio of the length in the normal direction of the old β grain boundary to the length in the direction of the old β grain boundary) of 2 or less, and the needle formed in the grain It is clearly distinguished from the α-phase (intra-granular α, see FIG. 2). Such an equiaxed α phase increases as the formed region gradually spreads from the grain boundary to the periphery thereof, that is, is formed along the grain boundary (see FIG. 2). As described above, when the forged titanium alloy has a lot of α phase formed along the grain boundary around the grain boundary of the old β grain, in other words, if it exists in a wide range, the fracture toughness deteriorates. There is a fear. Therefore, the titanium alloy forging material has a width (hereinafter referred to as a grain boundary α-band) in which the α phase formed along the grain boundary is present in the periphery including the former β grain boundary. (Length of normal β grain boundary in the normal direction) w _α (see FIG. 2) is preferably 10 μm or less on average. Note that the grain boundary α band also includes a region formed on the former β grain boundary where an α phase (see FIG. 1) that is not equiaxed is formed, in other words, a needle shape (inside the grain in FIG. 2). A zone where an α phase other than α) is formed. Therefore, in a titanium alloy forging material in which an equiaxed α phase is not formed around the old β grain boundary, the film-like α phase in which the width w _α of the grain boundary α band is formed on the old β grain boundary This titanium alloy forged material is more preferable because it matches the film thickness t _α (see FIG. 1). As will be described later, the width w _α of the grain boundary α band is obtained by observing the cross section of the titanium alloy forging, and the α phase formed on both sides of the old β grain boundary appearing linearly in the cross section is equiaxed. What is necessary is just to measure the perpendicular | vertical direction length of the said former beta grain boundary of the area | region (grain boundary alpha zone) in which this equiaxed alpha phase is contained. Titanium alloy forging is completed by forging in a predetermined temperature range as described in the manufacturing method described later so that the formation of equiaxed α phase around the grain boundary of the old β grain is suppressed. Is preferred.

  The aspect ratio and diameter of the former β grain of the titanium alloy forging according to the present invention, the angle of the grain boundary, the area ratio of the non-flat β grain, the length of the α phase on the former β grain boundary, and the grain boundary α band The width of can be a value in one or a plurality of visual fields in a cross section parallel to the forging direction of the titanium alloy forged material. That is, the titanium alloy forged material is cut along a plane parallel to the forging direction, and the cross section is corroded after polishing (mechanical polishing, electrolytic polishing), and one or more fields of about 1 to several mm square, for example, are viewed from this cross section. Select and observe the cross-sectional structure 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 thereto is measured, the aspect ratio is calculated, and the non-flat β grains are defined based on the diameter and the aspect ratio. Further, in the field of view, with respect to the old β grain boundaries that intersect one or more straight lines parallel to the forging direction (one-dot chain line in FIG. 1), the respective intersection angles (90 ° or less), more specifically, the old β grains at the intersections The angle between the tangent of the field and the straight line is measured. Further, the length in the grain boundary direction of the α phase formed on the old β grain boundary and the width of the region where the equiaxed α phase is formed around the old β grain boundary are measured. Thereby, the aspect ratio and diameter of the old β grain in the field of view of the cross section of the titanium alloy forging, the angle of the grain boundary, the length of the α phase on the grain boundary, the average value of the width of the grain boundary α band, and The area ratio of non-flat β grains 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 the β crystal grain size (average grain size) is in the range of 300 μm to 1000 μm. And forging in a temperature range of (T _β −150 ° C.) or more and (T _β + 200 ° C.) or less with an equivalent strain of 0.45 or more and 2.1 or less, (T _β -150 ° C. ) Cool to a lower temperature.

(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 to the deep part, and forging is completed in a temperature range of (T _β −150 ° C.) or higher. 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 it exceeds (T _β + 250 ° 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 + 250 ° C.) or less.

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 300 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, until the forging is completed (T _β It is set so that it can be maintained in a temperature range of −150 ° C. or higher.

(Forging temperature (T _F ): T _β −150 ° C. ≦ T _F ≦ T _β + 200 ° C.)
A titanium alloy billet that has been heated and held for a certain period of time is forged into a product shape. Here, the titanium alloy billet, at a temperature range below T _beta, on the beta crystal grains of the grain boundaries, more start to precipitate the α phase in the grains, the temperature range of less than (T _β -150 ℃) Then, precipitation of these α phases becomes remarkable. If many of these α phases are formed during forging, the fracture toughness may deteriorate. Therefore, the temperature at the completion of the forging of titanium alloy billet was set to _(T β -150 ℃) or higher, preferably at least (T β -110 _℃). 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). Such use a heated mold, without the surface of the titanium alloy billet to be forged too is cooled early against the inner surface near also held in _(T β -150 ℃) or higher Forging can be completed. On the other hand, if the temperature of forging is excessively high, by take some time to be cooled to below after the forging finished (T _β -150 ℃), and new beta grain growth or old beta grain grain boundary on the There is a possibility that the α phase is deposited thick (thick) and the fatigue strength of the titanium alloy forged material is lowered. Therefore, the forging temperature (the temperature from the start to the completion of forging) is set to (T _β + 200 ° C.) or less. It should be noted that the product portion of the titanium alloy forged material may be maintained in a temperature range of not less than the completion of forging (T _β -150 ° C.), and the surplus surface such as the surface layer to be removed after forging (after cooling) (product The temperature in other than the part is not particularly specified.

(Equivalent strain: 0.45 to 2.1)
Forging can be performed under the same conditions as general finish forging. Here, as the strain applied to the titanium alloy billet by forging increases, the β grains are flattened more and the aspect ratio of the old β grains increases. When the equivalent strain is less than 0.45, the old β grains have an aspect ratio that is too low to contribute to the improvement of fatigue strength, and the α phase continues along the grain boundaries of the old β grains by cooling after forging. Since it is formed long, the fatigue strength of the titanium alloy forged material deteriorates. Therefore, forging is performed so that the equivalent strain applied to the titanium alloy material is 0.45 or more, preferably 0.5 or more. On the other hand, when the equivalent strain exceeds 2.1, the old β grain is excessively flattened and the aspect ratio becomes excessive, and the old β grain boundary becomes close to a plane perpendicular to the forging direction. In this case, measurement accuracy decreases due to noise. Therefore, the titanium alloy material is forged so that the equivalent strain applied to the titanium alloy material is 2.1 or less, preferably 2.0 or less, more preferably 1.8 or less, and even more preferably 1.7 or less. The equivalent strain can be obtained by the finite element method. In order to make the old β grains have an average aspect ratio of 2 or more with such an equivalent strain, for example, forging a cylindrical billet with a mold having a flat surface, the rolling reduction is 33% or more and 80% or more. It is preferable to add the following processing or processing corresponding thereto. 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).

The titanium alloy billet is cooled to a temperature lower than (T _β −150 ° C.) after completion of forging, thereby stopping the growth of new β grains outside the β single phase region (α + β two phase region), and the old β Suppressing the precipitation (thickness) of the α phase on the grain boundaries of the grains is prevented, thereby preventing the fatigue strength of the obtained titanium alloy forging from deteriorating. Accordingly, after forging is completed starts cooling without leaving as much as possible time, specifically, it is preferable to reach a temperature lower than within 300 seconds from the time of the forging finished (T β -150 _℃). Therefore, the cooling rate after forging is preferably 10 ° C./min or more, and 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 lower than _(T β -150 ℃) is not specifically defined, it may be set according to other required properties.

[Production method of titanium alloy forged parts]
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 inspection is carried out to produce a titanium alloy forged part. 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 parts are then machined again as necessary to become products such as engine parts such as disks and shafts. These processes can be performed by a known method.

(Ultrasonic flaw detection method)
The ultrasonic flaw detection in the method for manufacturing a titanium alloy forged part according to the present invention can be performed by a known method, and the probe is selected from those having a probe diameter in the range of 5 to 30 mm, and ultrasonic waves (transmitted waves) are selected. ) Preferably uses a frequency range of 1 to 20 MHz. 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 an ultrasonic flaw inspection in which flaw detection is performed in a direction including the direction parallel to the forging direction (see FIG. 1), that is, the direction in which the reduction amount in forging is the largest. The direction in which the amount of reduction in forging is the largest is before and after forging (titanium alloy material and titanium alloy forged material), and is the direction in which the dimensional reduction rate is maximum, and is the forging direction shown in FIG. The forging direction can also be estimated from the shape of the old β grains in the structure after forging (titanium alloy forged material). In addition, the direction of ultrasonic flaw detection refers to the traveling direction of the transmission wave (the direction in which the inside of the titanium alloy forged material is transmitted). 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 are easy to inspect because the surface area perpendicular to this direction of scanning the probe is often wide. Moreover, it is preferable to inspect for a total of two or more times by changing the flaw detection in the one direction or further changing the direction according to the shape of the titanium alloy forged material (titanium alloy forged part, 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: a billet consisting of (T _β 960 ℃), was used after forging into a predetermined shape by alpha + beta two-phase region. The shape of the titanium alloy material was determined according to the reduction amount (reduction ratio) at the time of forging so that the forging direction thickness after forging (β forging) (titanium alloy forging material) was 45 mm.

(Β forging)
The titanium alloy billet is held in a furnace at 930 ° C. for 2 hours so that the internal temperature distribution is constant, and then heated to 1010 ° C. and held until the β grains have an average particle size of 500 to 600 μm, After being taken out of the furnace and air-cooled to the forging temperature shown in Table 1, forging was performed using a mold heated to the forging temperature in advance by a low frequency heating device. Forging was performed using a pair of flat surface-shaped molds, with a mold moving speed of 1800 mm / min, with the deformation direction (rolling direction) as the billet axis direction and the rolling reduction shown in Table 1 (the length of the material after forging) 45 mm).

  The titanium alloy billet was immediately removed (within 15 seconds) from the mold after forging, and cooled to room temperature to obtain a titanium alloy forged material. At this time, the specimen No. 4 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.

(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]
(Aspect ratio and diameter of old β grains, angle of old β grain boundaries, area ratio of non-flat β grains)
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 was performed with an optical microscope, and a field of view of 3200 μm × 2000 μm was observed in a panoramic manner at a magnification of 100 times. For the old β grains, find the diameter and aspect ratio in the forging direction (axial direction), calculate the average value for all the old β grains in the field of view, and detect non-flat β grains based on the aspect ratio and diameter, The area ratio in the visual field was obtained. In addition, straight lines parallel to the forging direction are drawn at 300 μm intervals, and the angle between the tangent at the intersection and the straight line (≦ 90 °) is measured for the grain boundary of the old β grain intersecting the straight line. The average value of angles was calculated. These values are shown in Table 1.

(Length of α phase on old β grain boundary, width of grain boundary α band)
A 15 mm square cubic small sample including 1 / 2H and 1 / 4D positions in the test specimen was cut out of the test specimen, and a cross section parallel to the forging direction and the radial direction of the test specimen was scanned using a scanning electron microscope (SEM). Ten fields of view of 65 μm × 60 μm were observed at 2000 magnification. The length l _α (see FIG. 1) in the grain boundary direction of the α phase formed on the grain boundaries of the old β grains was measured, and the average value was calculated. In addition, the width w _α (see FIG. 2) of a zone (grain boundary α-band) in which an equiaxed (aspect ratio of 2 or less) α phase is formed on both sides of the grain boundary was measured, and an average value was calculated. . For specimens in which no equiaxed α phase was observed, the film thickness t _α (see FIG. 1) of the α phase formed on the grain boundaries was measured. These values are shown in Table 1.

[Evaluation]
(Ultrasonic flaw detection)
A 45 mm square cubic test piece was cut out from the test specimen and subjected to ultrasonic flaw detection by the 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 50% or less is acceptable.

(Mechanical properties)
Fatigue strength evaluation and fracture toughness value evaluation (K _IC ) were performed as evaluation of the mechanical properties of the titanium alloy forging. A test piece in which the circumferential (tangential) direction of the test specimen was parallel to the load axis was cut out from the 1 / 2H and 1 / 4D positions of the test specimen and prepared separately for each evaluation.

  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 test piece broke. Regarding the number of fracture cycles, the test specimen No. Values normalized with 8 as a reference (1) are calculated (divided by the number of fracture cycles of specimen No. 8), and are shown in Table 1. A fatigue strength of 0.5 or more is considered acceptable.

A fracture toughness test in accordance with ASTM standard E399 was conducted at room temperature to obtain K _IC, which is shown in Table 1.

  As shown in Table 1, the test specimen No. Since No. 8 was forged by applying sufficient strain in the β single phase region as normal β forging, it became an old β grain structure with a large flatness (aspect ratio) exceeding the range of the present invention. As a result, the test specimen No. No. 8 had high fatigue strength, but on the other hand, the grain boundary of the old β grains was nearly perpendicular to the forging direction, and therefore no noise reduction effect was obtained in ultrasonic flaw detection.

  In contrast, the test specimen No. Nos. 1 to 5 are production methods according to the present invention, that is, before the forging, the β crystal is controlled to grow within a predetermined grain size, forged so as to apply a predetermined range of strain in the β single phase region, and immediately cooled. As a result, an α phase having a predetermined length is formed on the grain boundaries of the old β grains, and only an acicular α phase is formed in the grains, and the old β grains are appropriately flattened flatly. The average of the β grain aspect ratio and the grain boundary angle became an example satisfying the range of the titanium alloy forging according to the present invention. As a result, the test specimen No. 1 to 4 are specimen Nos. Although the fatigue strength is lower than that of the specimen No. 8, the test specimen No. 1 retains the mechanical properties necessary for aircraft engine parts and the like, and has an increased aspect ratio of the old β grains within the scope of the present invention. No. 5 is a specimen No. 8 exhibited mechanical properties equivalent to those of No. 8, and all exhibited excellent ultrasonic flaw detection properties.

  Specimen No. Since No. 6 is an example that satisfies the range of the titanium alloy forged material according to the present invention with respect to the structure, that is, the α phase on the old β grains and the old β grain boundaries, the fatigue strength and the ultrasonic flaw detection property were good. However, specimen No. No. 6, because the forging temperature was low and the α phase was precipitated during forging, the acicular α phase in the old β grain was divided, and an equiaxed α phase was formed in a wide area around the old β grain boundary. As a result, the fracture toughness value deteriorated and became less than 50 MPa√m, which is lower than the other examples (test bodies No. 1 to 5).

  On the other hand, the specimen No. In No. 7, since the strain during forging was insufficient, the old β grains were not sufficiently crushed and the aspect ratio was too low. As a result, although the noise was low, the fatigue strength was reduced.

Claims (6)

A forged titanium alloy material having a flat surface ,
Has a needle-like α-phase structure, it aspect ratio from the old β grains of the polycrystalline structure is 2 to 10 on average,
The old β grains have an average diameter in the normal direction of the flat surface of 60 μm or more and 700 μm or less,
In the cross section parallel to the normal direction of the flat surface, the grain boundary of the old β grains has an average angle of 80 ° or less with the line parallel to the normal direction of the flat surface ,
The α phase formed on the grain boundaries of the old β grains has an average length in the grain boundary direction of 15 μm or less,
The old β grains having an aspect ratio of less than 3 and a diameter in the normal direction of the flat surface of 30 μm or more and 200 μm or less do not exist in an area ratio of 1% or more in a cross section parallel to the normal direction of the flat surface. A featured titanium alloy forging. The width of the grain boundary α band, which is the region where the α phase formed on the grain boundary of the old β grain and the α phase formed along the grain boundary exists, is 10 μm or less on average. The titanium alloy forging material according to claim 1 . The titanium alloy forging material according to claim 1 or 2 , 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. 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 300 μm to 1000 μm, and (T _β −150 ° C.) or higher (T β ₊ 200 ℃) at a temperature range, after forging in one direction under the condition that the equivalent strain of 0.45 or more 2.1 or less, is cooled to a temperature lower than _(T β -150 ℃) A method for producing a forged titanium alloy material. Producing a titanium alloy forged material by the method for producing a titanium alloy forged material according to claim 4 ;
Performing a flaw detection process by irradiating the titanium alloy forged material with ultrasonic waves in a direction parallel to a direction in which the amount of reduction in the forging is the largest. A method for producing a titanium alloy forged part, comprising producing a part for use in an aircraft engine by the method for producing a titanium alloy forged part according to claim 5 .