WO2018181937A1 - Titanium alloy material - Google Patents

Abstract

Provided is a titanium alloy material having a composition generally classified as a near α type and/or α+β type, the titanium alloy material being characterized by having an outer shell region, located on the surface side and having a Vickers hardness in the range of at least 400 HV and less than 450 HV, and a central region, located inside the outer shell region and having a Vickers hardness of at least 320 HV and less than 400 HV, wherein the boundary between the outer shell region and the central region is located, in the range of 1/200-1/40 of the length or diameter in the minor axis direction in a transverse plane, inward from the surface.

Description

Titanium alloy material

The present invention relates to a titanium alloy material.
This application claims priority on March 31, 2017 based on Japanese Patent Application No. 2017-072832 filed in Japan, the contents of which are incorporated herein by reference.

Titanium alloys are lightweight, resistant to rust, and have high corrosion resistance, and are therefore used as materials for aircraft parts, buildings (roofs), eyeglass frames, and the like. Titanium alloys have excellent biocompatibility and are also used as materials for bone plates (bone joining materials). Furthermore, although there are not many applications, it is also used as a material for parts of automobiles and two-wheeled vehicles such as automobile intake engine valves and two-wheeled connecting rods. These titanium alloy products are manufactured by subjecting a titanium alloy material to plastic working. Examples of plastic working include rolling, drawing, extrusion, forging, bending, and drawing. Examples of the titanium alloy material include a wire and a plate.

In Patent Document 1, as a titanium alloy having high strength and good workability, the composition is generally classified into near α-type and / or α + β-type titanium alloys, and the average crystal grain size is less than 1000 nm. A configuration consisting of a structure in which axial crystals are uniformly dispersed is described.

JP 2011-68955 A

By the way, in order to improve the strength of the titanium alloy product, it is effective to increase the strength of the titanium alloy material as the raw material. In addition, it is known that the durability of titanium alloy products improves as the strength increases. According to the gigacycle fatigue property data sheets (No. 98, No. 111) of titanium alloys published by NIMS (National Institute for Materials Science), titanium alloys have a tensile strength up to 1250 MPa. It is said that a linear relationship is established between the strength and the durability limit, and the durability is improved along with the tensile strength.

When a titanium alloy wire is processed, wire drawing or swaging (rotary forging) is generally performed. It is known that the titanium alloy wire processed by these processing methods has the maximum processing strain on the surface and the hardness increases from the center toward the surface. Therefore, a titanium alloy material that has been subjected to wire drawing or swaging is less likely to be deformed as its hardness increases, and a large amount of processing power may be required for plastic processing. In addition, partial cracking is likely to occur on the surface during plastic processing, and the strength and durability of the titanium alloy product may be reduced.

The present invention has been made in view of the above-mentioned circumstances, and provides a titanium alloy material that can produce a titanium alloy product having high strength and excellent durability with relatively little processing power. With the goal.

The present inventor studied the strength and workability of titanium alloy materials having compositions generally classified into near α type and / or α + β type. As a result, the outer shell region located on the surface side has a Vickers hardness in the range of 400 HV or more and less than 450 HV, and the central region located inside the outer shell region has a Vickers hardness of less than 400 HV, and the outer shell region The titanium alloy material in which the boundary between the center and the central region is located in the range of 1/200 to 1/40 of the length or diameter in the minor axis direction in the cross section from the surface to the inside is the conventional wire drawing The present inventors have found that plastic processing can be performed with less processing power compared to a titanium alloy material whose surface Vickers hardness is improved to about 400 HV by processing and swaging. And the titanium alloy product manufactured using said titanium alloy raw material has high intensity | strength, and it confirmed that it was excellent in durability, and completed this invention.
In a titanium alloy material subjected to conventional wire drawing and swaging, the strength increases depending on the amount of processing strain. For this reason, if the strength is increased until the surface Vickers hardness is 400 HV or higher, the range of the region having a high Vickers hardness becomes too wide, and the processing power during plastic processing increases. On the other hand, in the titanium alloy material, the boundary between the outer shell region and the central region is 1/200 to 1/40 of the length or diameter in the minor axis direction in the cross section from the surface to the inside. Since the outer shell region is narrow, it is considered that plastic processing can be performed with a small processing power even if the Vickers hardness of the outer shell region is as high as 400 HV or higher.

The present invention has been made on the basis of the above-mentioned knowledge, and the titanium alloy material of the present invention is a titanium alloy material having a composition generally classified into near α type and / or α + β type, and has a cross-sectional view. An outer shell region having a Vickers hardness of 400 HV or more and less than 450 HV, and a central region located inside the outer shell region and having a Vickers hardness of 320 HV or more and less than 400 HV, And the boundary between the outer shell region and the central region is located in the range of 1/200 to 1/40 of the length or diameter in the minor axis direction in the cross section from the surface to the inside. It is characterized by.

Here, in the titanium alloy material of the present invention, the outer shell region preferably has a structure containing α-phase crystal grains having a crystal grain size of 1 μm or less in an area ratio of 90% or more.
In this case, the outer shell region is fine with a crystal grain size of 1 μm or less and contains α-phase crystal particles having a high strength in an area ratio of 90% or more, so that the strength of the outer shell region becomes higher.
In the central region, when the area ratio of low-strength β-phase crystal particles and α-phase crystal particles having a crystal grain size larger than 1 μm increases to exceed 20%, the strength and durability decrease. It is desirable to have a structure containing 80% or more α-phase crystal particles having a crystal grain size of 1 μm or less in terms of area ratio.

In the titanium alloy material of the present invention, the α-phase crystal particles contained in the outer shell region have a C-plane along the outer surface and (10-10) a primary column surface along the transverse plane. It is preferable to accumulate so that it becomes. In the present specification, “−1” represents a 1 with a bar (−).
In this case, the α-phase crystal particles contained in the outer shell region are oriented so that the (0001) plane = the C plane is along the outer surface and the (10-10) primary column surface is along the cross section. Therefore, even if pressure is applied in a direction perpendicular to the cross section, it is difficult to slip. Therefore, the strength of the outer shell region is further increased. In addition, by using the high strength of the outer shell region and performing compression processing such as shot peening, compression residual stress can be applied deeper than conventional materials, and improvement in durability can be expected (unlike the present invention). Another example in which compressive residual stress is deeply introduced by shot peening if the surface is hard: Journal of the Japan Institute of Metals, Vol. 78, No. 2 (2014) 75-81).

Furthermore, in the titanium alloy material of the present invention, the central region has a structure including α-phase crystal particles having a crystal grain size of 1 μm or less in an area ratio of 80% or more, and the α-phase crystal particles ( 10-10) The ratio of the degree of integration of the primary column surface to the degree of integration of the (10-10) primary column surface of the α-phase crystal particles in the central region is preferably 2 or more.
In this case, the α-phase crystal particles included in the central region have a (10-10) primary column surface although the C-plane is along the outer surface as compared with the α-phase crystal particles included in the outer shell region. Since the degree of integration is low, the central region is low in hardness and is easily plastically deformed.

According to the present invention, it is possible to provide a titanium alloy material that can produce a titanium alloy product having high strength and excellent durability with relatively little processing power.

It is a photograph of the transverse section of the titanium alloy material concerning one embodiment of the present invention. It is an enlarged photograph of FIG. It is a conceptual diagram explaining the arrangement | sequence of the alpha phase crystal particle contained in the titanium alloy raw material of this embodiment. It is the positive electrode point figure and reverse pole figure in the cross section in the titanium alloy material of this embodiment, Comprising: (a) is the positive electrode point figure and reverse pole figure of an outer shell area | region, (b) is the positive electrode point of a center area | region. It is a reverse pole figure with a figure. It is a conceptual diagram explaining the mechanism in which an outer shell area | region produces | generates in a titanium alloy material by the strong rolling using a groove roll. It is a conceptual diagram explaining the typical slip surface and slip direction in the alpha phase crystal particle (dense hexagonal crystal) contained in the titanium alloy material of this embodiment. It is drawing explaining the measurement position of the Vickers hardness in the long titanium alloy raw material obtained in Example 1, and is a side view explaining the cutout position of the cross section of the long titanium alloy raw material. It is drawing explaining the measurement position of the Vickers hardness in the long titanium alloy raw material obtained in Example 1, and is sectional drawing which shows the measurement position of the Vickers hardness of a cross section. It is a measurement result of the Vickers hardness in the cross section of the long titanium alloy raw material of Example 1, and the long titanium alloy raw material of the comparative example 1. 4 is a cross-sectional photograph of a long titanium alloy sheet obtained in Example 2. FIG. It is an enlarged photograph of FIG. It is a measurement result of the Vickers hardness of the long titanium alloy plate material obtained in Example 2, and is the Vickers hardness in the thickness direction of the long titanium alloy plate material. It is a measurement result of the Vickers hardness of the long titanium alloy sheet material obtained in Example 2, and is the Vickers hardness in the width direction of the long titanium alloy sheet material.

Below, the titanium alloy raw material which is embodiment of this invention is demonstrated with reference to the attached figure.
The titanium alloy material according to the present embodiment is a long wire rod having a circular cross section perpendicular to the longitudinal direction or a long plate member having a rectangular cross section perpendicular to the longitudinal direction. The titanium alloy material is manufactured by rolling a titanium alloy in the longitudinal direction. A method for manufacturing the titanium alloy material will be described later.

The titanium alloy material of the present embodiment is formed of a titanium alloy having a composition generally classified into near α type and / or α + β type. These titanium alloys can form a titanium alloy material having a high hardness since the content of the α phase, which is relatively stronger than the β phase, is large.
Here, the α + β type titanium alloy means a titanium alloy in which the β phase becomes an area ratio of 10 to 50% at room temperature at a cooling rate such as normal casting. The near α-type titanium alloy is a titanium alloy containing 0.1 to 2% by mass of a β-phase stabilizing element such as V, Cr, or Mo. The β-phase has an area ratio of 0% at room temperature at the same cooling rate. It means a titanium alloy that exceeds 10%.

Examples of titanium alloys having an α + β type composition include Ti-6Al-4V (the numerical value means mass%, the same shall apply hereinafter), Ti-8Mn, Ti-3Al-2.5V, Ti-6Al-6V- 2Sn, Ti-7Al-1Mo, Ti-6Al-2Sn-4Zr-6Mo, Ti-5Al-2Cr-1Fe, Ti-6Al-2Sn-4Zr-2Mo, Ti-6Al-2Sn-4Zr-2Mo-0.1Si, etc. Is mentioned. Examples of titanium alloys having a near α type composition include Ti-6Al-5Zr-0.5Mo-0.25Si, Ti-5.5Al-3.5Sn-3Zr-1Nb-0.25Mo-0.3Si, Ti-6Al-2.7Sn-4Zr-0.4Mo-0.45Si and Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si-0.06C.

The titanium alloy material of the present embodiment is preferably formed of a titanium alloy having a composition of an α + β type titanium alloy. A titanium alloy having an α + β type composition contains Al in a range of 4 mass% to 9 mass%, V in a range of 2 mass% to 10 mass%, with the balance being Ti and inevitable impurities. It is preferable that A particularly preferred titanium alloy is Ti-6Al-4V.

FIG. 1 is a cross-sectional photograph of a titanium alloy material according to an embodiment of the present invention, and FIG. 2 is an enlarged photograph of FIG.
As shown in FIGS. 1 and 2, the titanium alloy material 1 of the present embodiment includes an outer shell region 2 located on the surface side and a central region 3 located inside the outer shell region 2.
The outer shell region 2 has a Vickers hardness of 400 HV or more and less than 450 HV.
The center region 3 has a Vickers hardness of 320 HV or more and less than 400 HV.

The boundary 4 between the outer shell region 2 and the central region 3 of the titanium alloy material 1 is 1/200 to 1/40 of the diameter in a cross section perpendicular to the longitudinal direction from the surface of the titanium alloy material 1 to the inside. It is located in the range. That is, the outer shell region 2 is a region having a diameter up to 1/200 of the diameter in the cross section from the surface to the inside, and a region having a diameter up to 1/40 of the diameter in the cross section from the surface to the inside. is there.
In addition, when the shape of the cross section perpendicular | vertical with respect to the longitudinal direction of a titanium alloy raw material is a noncircle, such as a rectangle and an ellipse, the position of the boundary 4 is based on the length of the short-axis direction in a cross section. For example, when the titanium alloy material 1 is a plate having a rectangular cross section perpendicular to the longitudinal direction, the position of the boundary 4 is 1 of the length in the short axis direction in the cross section from the surface of the plate toward the inside. / 200 to 1/40.

It can be confirmed, for example, as follows that the boundary 4 between the outer shell region 2 and the central region 3 of the titanium alloy material 1 is in the above position.
The titanium alloy material 1 is cut at a predetermined position in the longitudinal direction in a direction perpendicular to the longitudinal direction to obtain a sample piece for confirming the boundary 4. The obtained sample piece is embedded in resin, and the cross section of the cut piece is polished with emery paper or buff to finish to a mirror surface. Next, the cross section is immersed in hydrofluoric acid (2 wt.% Hydrofluoric acid 21 ml, 4 wt.% Nitric acid 33 ml, pure water 446 ml) for 60 seconds, and then washed with pure water and ethanol. Then, the structure of the cross section after the treatment with hydrofluoric acid is observed with an optical microscope. The position of the boundary 4 between the outer shell region 2 and the central region 3 is confirmed by utilizing the fact that the outer shell region 2 is hardly dissolved by the treatment with hydrofluoric acid and is observed as a white layer with an optical microscope. Then, the Vickers hardness of the outer shell area | region 2 and the center area | region 3 is measured, and it confirms that it exists in said hardness range.

The measurement of Vickers hardness is based on JIS Z 2244: 2009, and a weight of 2.94N (300 gf) in the direction perpendicular to the cross section of the titanium alloy material 1 (longitudinal direction of the titanium alloy material 1) ( (Test force) is given.

The variation in the Vickers hardness of the outer shell region 2 and the central region 3 in the rolling direction can be measured, for example, as follows.
An interval is formed in the longitudinal direction from a titanium alloy material that is a long wire (that is, the titanium alloy is rolled and manufactured before being formed into a product by cutting and plastic working). Cut out cross sections perpendicular to the longitudinal direction at three locations. The cut-out position of the cross section can be appropriately set in consideration of the position where plastic working is performed.

Next, the Vickers hardness of the outer shell region 2 and the central region 3 is measured for the three cut out cross sections. The measurement position of the Vickers hardness of the outer shell region 2 is a half position of the depth of the outer shell region, and four locations are equally spaced around the center of the cross section with respect to one cross section. Position. The measurement position of the Vickers hardness in the central region 3 is the center of the cross section at a distance of d / 8 from the surface to the inside, where d is the diameter in the cross section of the titanium alloy material 1 with respect to one cross section. There are a total of 8 positions: 4 places that are equally spaced around the center and 4 places that are equally spaced around the center of the cross section at a distance of d / 8 outward from the center.

In the titanium alloy material 1 of the present embodiment, the average value of the Vickers hardness in the central region 3 when the average value of the Vickers hardness in the outer shell region 2 is 100 is 80 or more and less than 100 in order to maintain the strength. In view of workability, it is more preferably in the range of 80 to 95. Therefore, the average value of the Vickers hardness in the central region 3 is preferably 320 HV or more and less than 400 HV, and more preferably 320 HV or more and 380 or less. The average value of the Vickers hardness of the outer shell region 2 is the average of the 12 Vickers hardnesses measured as described above, and the average value of the Vickers hardness of the central region 3 is as described above. This is the average of the 24 Vickers hardness values measured.

In the titanium alloy material 1 of the present embodiment, the standard deviations of the Vickers hardness of the outer shell region 2 and the central region 3 measured as described above are each preferably 10 HV or less. The titanium alloy product manufactured using the titanium alloy material 1 having a small standard deviation of the Vickers hardness in the outer shell region 2 has a uniform strength in the outer shell region, and the durability improves more stably. Further, the titanium alloy material 1 having a small standard deviation of the Vickers hardness in the central region 3 has uniform workability, and can stably manufacture a titanium alloy product with a small processing power.

The titanium alloy material 1 of the present embodiment has a structure including α-phase crystal particles having a particle size of 1 μm or less.
FIG. 3 is a conceptual diagram illustrating the arrangement of α-phase crystal particles contained in the titanium alloy material of the present embodiment.

In the titanium alloy material 1 of the present embodiment, the outer shell region 2 has a structure including α-phase crystal grains having a crystal grain size of 1 μm or less in an area ratio of 90% or more. Central region 3 has a structure containing α-phase crystal grains having a crystal grain size of 1 μm or less in an area ratio of 80% or more. The α-phase crystal particles of the titanium alloy have a dense hexagonal lattice structure.
As shown in FIG. 3, the α-phase crystal particles 5 included in the outer shell region 2 have a (0001) bottom surface = C plane (that is, a bottom surface of a dense hexagonal lattice) in a direction along the outer surface and (10 -10) The primary column surface (that is, the side surface of the dense hexagonal lattice) is oriented in the direction along the cross section. On the other hand, in the central region 3, although the C plane is oriented in the direction along the outer surface, the degree of integration of the (10-10) primary column surface is not as high as the outer shell region.

4A and 4B are a positive pole figure and a reverse pole figure in a cross section of the titanium alloy material of the present embodiment, wherein FIG. 4A is a positive pole figure and a reverse pole figure of the outer shell region, and FIG. It is a positive pole figure and a reverse pole figure of a field.
The positive dot diagram shows a particular crystal plane and shows how the normal direction of that plane is oriented with respect to the sample plane. The inverted pole figure focuses on a specific direction of the sample and shows how many crystal planes are parallel to the sample with that direction as the normal. By combining the positive pole figure and the reverse pole figure, it can be seen how many faces of the a-phase crystal grains are oriented (accumulated) with respect to the cross section.

From the positive pole figure and the reverse pole figure of FIG. 4, the titanium alloy material of the present embodiment shows that in the outer shell region 2, the C-plane is along the outer surface (10-10) and the primary column surface is along the cross section. In contrast to the accumulation in the direction (integration degree 8.613), in the central region 3, the accumulation degree of the (10-10) primary column surface is low (integration degree 2.357). The degree of integration is the X-ray diffraction intensity when the crystal plane is randomly oriented, and the intensity ratio with respect to a specific plane is obtained. The higher the value, the more the crystal plane is oriented. It shows that you are doing. The ratio of the accumulation degree of the (10-10) primary column surface of the α phase crystal particles in the outer shell region 2 to the accumulation degree of the (10-10) primary column surface of the α phase crystal particles in the central region 3 is 2 or more It is preferable that

The titanium alloy material of the present embodiment can be manufactured by using, for example, α ′ processing. Specifically, the titanium alloy ingot is subjected to a solution treatment to obtain an α ′ martensitic titanium alloy ingot, and the obtained α ′ martensitic titanium alloy ingot is converted into the α ′ martensitic titanium. And a second step of performing strong rolling by heating to a temperature of −300 ° C. or higher and −100 ° C. or lower with respect to the β transformation point of the alloy ingot. In this α 'processing, a material with a basic lattice of dense hexagonal crystals (α' martensite titanium alloy ingot) is hot-rolled hot, so a large strain is generated in the cross-section of the material with a small number of passes of strong rolling. Can be introduced at a speed.

The titanium alloy ingot used for manufacturing the titanium alloy material of the present embodiment is made of a Ti alloy generally classified into near α type and / or α + β type. Examples of the near α-type Ti alloy and the α + β-type Ti alloy are as described above. The shape of the titanium alloy ingot is not particularly limited, but is a prismatic shape in the present embodiment.

In the first step, the solution treatment means that the titanium alloy ingot is heated to a temperature equal to or higher than the β transformation point of the titanium alloy to generate and hold the β phase, and then the quenching treatment is performed to convert the β phase into the α ′ martensite. This is a treatment for transforming into a site phase to form an α ′ martensitic titanium alloy ingot. The α ′ martensite phase in the α ′ martensite titanium alloy ingot obtained in this first step has a dense hexagonal lattice structure, but is unstable in energy due to stacking faults or accumulation of dislocations, and is recrystallized. Has a large number of nucleation sites. For this reason, it is possible to generate fine α-phase crystal particles having a particle size of 1 μm or less by dynamic recrystallization by performing strong rolling (second step) described later on the α ′ martensite titanium alloy ingot. It becomes possible.

When the titanium alloy ingot is made of a Ti-6Al-4V alloy (β transformation point: 995 ° C.), the solution treatment is performed by holding the titanium alloy ingot at a temperature of 1000 ° C. or more for 1 second or more and then a temperature of the β transformation point or more. It is preferable to carry out by cooling to room temperature under the condition that the average cooling rate from is 20 ° C / second or more. If the heating temperature is less than 1000 ° C., the amount of α ′ martensite phase produced may be insufficient. Further, if the holding time is less than 1 second, the diffusion of atoms becomes insufficient, and the alloy element may not be uniformly dissolved. Furthermore, if the average cooling rate is less than 20 ° C./second, structural defects such as stacking faults and dislocations in the α ′ martensite phase may be reduced. Moreover, there is a possibility that a Widmanstatten structure, which is a slowly cooled structure with few structural defects, may develop.

In the second step, the α ′ martensitic titanium alloy ingot is strongly rolled by heating to a temperature of −300 ° C. or higher and −100 ° C. or lower with respect to the β transformation point of the α ′ martensitic Ti alloy material. For example, when the α ′ martensite titanium alloy ingot is made of a Ti-6Al-4V alloy, the heating temperature is 700 ° C. or higher and 900 ° C. or lower, preferably 700 ° C. or higher and 750 ° C. or lower. Strong rolling produces fine α-phase crystal particles (dense hexagonal lattice) with a crystal grain size of 1 μm or less, and the α-phase crystal particles are oriented along the outer surface of the C-plane (that is, the bottom surface of the dense hexagonal lattice). And (10-10) the primary column surface (that is, the side surface of the dense hexagonal lattice) is oriented in the direction along the cross section. Here, in order to produce 80% or more of fine α-phase crystal particles by area ratio, it is preferable to apply a strain of 0.8 or more to the α ′ martensitic titanium alloy ingot at a time. A normal rolling process, drawing process, forging process, and the like performed in the manufacture of a titanium alloy material have a non-deformable region, so that a region with a strain of less than 0.8 tends to remain in the cross section. For this reason, it is difficult to uniformly generate fine α-phase crystal particles. For example, fine α phase crystal particles may be generated only in the center portion of the plate thickness in rolling or forging, and only in the surface portion in drawing.

In the method for manufacturing a titanium alloy material according to this embodiment, strong rolling is performed twice or more. In the first strong rolling, a prismatic α ′ martensitic titanium alloy ingot is pressed and rolled from above and below to form a horizontally long elliptical column having a horizontally long elliptical shape (also called an oval). By the first strong rolling, a titanium alloy material having a mixed structure including fine α-phase crystal particles having a particle diameter of 1 μm or less and α ′ martensite needle-like particles is obtained. In the first strong rolling, it is preferable to press the α ′ martensite titanium alloy ingot so that the content of fine α phase crystal particles is 60% or more in terms of area ratio.

In the second strong rolling, the horizontally elongated elliptical columnar titanium alloy material obtained by the first strong rolling is rotated 90 degrees around the center of the transverse section perpendicular to the longitudinal direction, and the transverse section (horizontally long elliptical) ) In the state in which the long axis is in the vertical direction, pressurizing and rolling from the vertical direction to make the cross section circular, and then cooling to obtain a cylindrical titanium alloy wire. Or after pressing and rolling from the up-down direction and making the cross section into a quadrangle with rounded corners, cooling is performed to obtain a rounded columnar titanium alloy material.

In the second strong rolling, by pressing the surface of the titanium alloy material in a direction different from the first strong rolling, a larger strain can be applied to the entire titanium alloy material with a smaller number of strong rolling operations. For this reason, in the titanium alloy material after the second strong rolling, fine equiaxed grains (α-phase crystal grains of the titanium alloy) having a grain size of 1 μm or less are formed in an oriented state. Strong rolling may be performed three times or more. Further, if necessary, a wire rod or a plate with a further reduced surface area can be produced by adding a process after the second strong rolling.

FIG. 5 shows the generation mechanism of the outer shell region by the above-described two strong rollings. In FIG. 5, the solid arrow indicates the direction of shear, and the white arrow indicates the direction of compression.
Here, the cross-sectional shape of the raw material before rolling (α 'martensitic titanium alloy ingot) and the material after rolling is shown. In this example, a square material with rounded corners is used as a raw material 11 before rolling, and rolled into a horizontally long oval shaped material 12 by the first strong rolling by the first rolls 21a and 21b, and the second strong rolling by the second rolls 22a and 22b. Then, the horizontally long oval shaped material 12 is rotated 90 ° and rolled to the circular cross-sectional shaped material 13. Thus, in the rolling process, the surface where the roll and the material are in contact with each other while being compressed in the cross section is subject to a large shear due to the influence of friction. In the present application, since rolling is performed in a state where the material is rotated 45 ° to 90 ° for each rolling, the sheared region spreads over the entire circumference of the rolled material. That is, the central region is subjected only to compressive deformation, whereas the outer shell region is subjected to compressive deformation and shear strain, and thus receives large strain. Further, as described in FIG. 6 below, an outer shell region is formed by a slip surface and a slip direction peculiar to dense hexagonal crystals such as α phase crystal grains of a titanium alloy.

FIG. 6 shows a typical slip plane and slip direction in a dense hexagonal crystal such as an α-phase crystal grain of a titanium alloy. At normal temperature, (0001) bottom surface = C-plane slip is mainly used, but in the case of processing at a high temperature such as strong rolling in this embodiment, (10-10) primary column surface slip and (10− 12) Twin deformation easily occurs.
First, the reason why the C-plane is oriented in the direction along the outer surface in the central region will be described. In the raw material before rolling, the crystal lattice is oriented in an arbitrary direction, but when the C-plane is orthogonal to the compression direction, it is hardly deformed and is rolled as it is. On the other hand, when the C plane is parallel or inclined to the compression direction, (10-12) twin deformation occurs, and the C plane deforms in a direction perpendicular to the compression direction. It becomes a texture accumulated in the direction along the surface (SR Angnewel .: Acta Mater., Vol. 49 (2001), 4277-4289).

Next, the reason why the C-plane is aligned along the outer surface and the (10-10) primary column surface is aligned along the cross section in the outer shell region will be described.
As shown in Fig. 5, when shear strain acts in the circumferential direction in the outer shell region of the cross section, first, C-plane slip occurs, but (10-10) the primary column surface slip is more than the case where it slips independently by cross slip. Since it easily occurs, the primary column surface slip also follows the C-plane slip. For this reason, if the rolling compression direction is changed every 45 ° to 90 ° for each rolling, the C-plane is uniformly along the circumferential direction in the outer shell region of the cross-section, and the (10-10) primary column surface is the cross-section. To come along. That is, in the outer region, the degree of integration is high, and the Vickers hardness is high. On the other hand, the central region of the titanium alloy material has only a compressive deformation and no shear deformation. Therefore, the deformation amount is smaller than that of the outer region, and the degree of integration of the (10-10) primary column surface is low. For this reason, the Vickers hardness is relatively low in the central region of the cylindrical titanium alloy wire.

In the first strong rolling, the shear strain applied to the surface layer portion of the horizontally long elliptical shape material 12 increases as the temperature of the horizontally long elliptical shape material 12 increases under high pressure (high strain), high speed rolling (high strain rate), and the horizontally long elliptical shape material 12. Become. Therefore, the Vickers hardness of the outer shell region of the horizontally long oval shape member 12 is increased. However, if the temperature of the horizontally long elliptical shape material 12 becomes too high, there is a risk of promoting recrystallization or precipitating the β phase.

Each condition of the maximum rolling reduction, the area reduction rate, the average strain, and the average strain rate when performing the first strong rolling and the second strong rolling using the groove roll mill is the conditions shown in Table 1 below. It is preferable. That is, in hot rolling at a rolling temperature of 700 to 750 ° C., the cumulative strain until the second strong rolling is preferably 0.8 or more and the processing speed (strain rate) is preferably 40 / s or more. In the second strong rolling, preferable conditions differ between when the cross section is changed from an oval to a circle and when the cross section is changed from an oval to a rounded corner. The conditions shown in Table 1 are preferable conditions when rolling at an average roll diameter of 250 mm, a rolling speed of 300 rpm (≈3.9 m / s), and a rolling start temperature of 700 ° C.
The cumulative strain is calculated by adding the average strain values in each rolling process shown in Table 1.
Table 1 shows preferable conditions for the first strong rolling and the second strong rolling.

As described above, it is possible to obtain a long titanium alloy material in which α-phase crystal particles of a fine titanium alloy having a particle size of 1 μm or less are generated in the entire cross section. Cooling of the titanium alloy wire after the strong rolling may be performed by air cooling or water cooling. However, if the cooling is performed for a long time, β in the equilibrium phase is precipitated and the mechanical properties are deteriorated. In the long titanium alloy material obtained by this manufacturing method, the longitudinal direction is the rolling direction (RD).

As described above, the titanium alloy material 1 of the present embodiment has a high Vickers hardness of the outer shell region 2 located on the surface side in the range of 400 HV or more and less than 450 HV, so the titanium alloy material of the present embodiment is used. The manufactured titanium alloy product has a high Vickers hardness in the outer shell region where the load is most applied during use. For this reason, this titanium alloy product has high strength, can deepen the compressive residual stress when subjected to shot peening, and has excellent durability.

Further, the titanium alloy material 1 of the present embodiment has a Vickers hardness of the central region 3 as low as less than 400 HV, and the boundary 4 between the outer shell region 2 and the central region 3 extends in the longitudinal direction from the surface toward the inside. On the other hand, since it is located in the range of 1/200 to 1/40 of the length or diameter in the minor axis direction in the vertical cross section, the surface Vickers hardness is reduced to about 400 HV by conventional wire drawing or swaging. Compared with a titanium alloy material improved to a maximum, plastic working can be performed with less processing power.

Further, in the titanium alloy material of the present embodiment, the outer shell region 2 has a structure in which the particle size is as fine as 1 μm or less and the α-phase crystal particles having high strength are included in an area ratio of 90% or more. Strength becomes higher.

Further, in the titanium alloy material of the present embodiment, the α-phase crystal particles in the outer shell region in the cross section perpendicular to the longitudinal direction are such that the C-plane is along the outer surface and (10-10) primary column The surfaces are integrated so as to be in a direction along the transverse plane, and it is difficult to slip even when pressure is applied in a direction perpendicular to the transverse plane as well as the direction perpendicular to the outer surface. Therefore, the strength of the outer shell region is further increased.

Furthermore, the titanium alloy material of the present embodiment has a structure in which the central region has α-phase crystal particles having a crystal grain size of 1 μm or less in an area ratio of 80% or more, and the α-phase crystal particles ( 10-10) The ratio of the accumulation degree of the primary column surface to the accumulation degree of the (10-10) primary column surface of the α-phase crystal particles in the central region is 2 or more, and the α-phase crystal included in the central region Compared with the α-phase crystal particles contained in the outer shell region, the particles have a smaller degree of (10-10) primary column surface accumulation and are softer, so that the central region is easily plastically deformed.

As mentioned above, although embodiment of this invention was described, this invention is not limited to this, It can change suitably in the range which does not deviate from the technical idea of the invention.
For example, in the present embodiment, the titanium alloy material has been described as a wire having a circular cross section, but the titanium alloy material of the present invention may be a plate material. In other words, in rolling, the final groove roll shape can be formed into a flat or square groove roll to form a plate material as shown in Example 2 described later. In addition, wire and plate materials can be manufactured even if warm processing (drawing, swaging, rolling, etc.) of 600 ° C or less is performed using a material with finer crystal grains and cold processing or recrystallization does not occur. can do.

The titanium alloy material of the present embodiment can be used as a material for aircraft parts, buildings (roofs), eyeglass frames, bone plates, intake engine valves, and connecting rods, for example, as with conventional titanium alloy materials. The titanium alloy material of the present embodiment can also be used as a material for various spring products such as suspension springs, stabilizers, and torsion bars.

[Example 1]
A prismatic Ti-6Al-4V alloy (vertical: 23 mm, horizontal: 23 mm, corner radius of curvature R: 5 mm, length: 1000 mm) was prepared. This Ti-6Al-4V alloy was held at a temperature of 1100 ° C. for 30 minutes and then quenched by water cooling to obtain an α ′ martensitic titanium alloy material. The average cooling rate by water cooling was 40 ° C./second.

The obtained α ′ martensite titanium alloy material was held at a temperature of 750 ° C. for 20 minutes. Then, the strong rolling of 6 times was implemented as follows using the groove roll mill.

First strong rolling: A prismatic α ′ martensite titanium alloy ingot (α ′ martensite titanium alloy material) is pressed from above and below and rolled into a horizontally long elliptical columnar material having a horizontally long elliptical cross section. . As a result, a titanium alloy material having a mixed structure including fine α-phase crystal particles having a particle size of about 60% in area ratio of 1 μm or less and the remaining α ′ martensite needle-like particles was obtained. Here, the average strain rate by strong rolling was 79.2 / s, and the average strain of the α ′ martensitic titanium alloy ingot was 0.75.

Second strong rolling: The horizontally long elliptical columnar titanium alloy material obtained by the first strong rolling is rolled by pressing from above and below in the state of being rotated 90 degrees around the center of the cross section. The surface was formed into a rounded columnar shape with a rounded corner (however, the diagonal line was in the vertical direction). By repeating the rolling in this way, the area ratio of the α-phase crystal particles was increased. Moreover, the average strain rate by strong rolling was 64.5 / s, and the cumulative strain until the second strong rolling was 1.44.
Third strong rolling: The rounded columnar titanium alloy material obtained by the second strong rolling is rolled by pressing from above and below in a state where it is rotated 45 degrees around the center of the cross section, and then crossed again. The surface was shaped into a horizontally long elliptical column having a horizontally long elliptical shape.
Fourth strong rolling: The horizontally long elliptical columnar titanium alloy material obtained by the third strong rolling is pressed and rolled from above and below in the state of being rotated 90 degrees around the center of the cross section, and again The cross section was formed into a rounded columnar shape with a rounded corner (however, the diagonal line was in the vertical direction).
Fifth strong rolling: The round and round columnar titanium alloy material obtained by the fourth strong rolling is pressed and rolled from above and below in a state where it is rotated 45 degrees around the center of its cross section, and again, A horizontally long elliptical column having a horizontally long elliptical cross section was used.
Sixth strong rolling: The horizontally long elliptical columnar titanium alloy material obtained by the fifth strong rolling is rolled by pressing from above and below in a state where it is rotated 90 degrees around the center of the cross section. The surface was formed into a circular column.

The conditions of the first strong rolling and the second strong rolling in Example 1 and Example 2 are shown in Table 2 below.

After the above-mentioned strong rolling, a cylindrical long titanium alloy material (wire) having a diameter of 12 mm and a length of 3000 mm was obtained by water cooling or air cooling.

For the obtained long titanium alloy material, the position of the boundary between the outer shell region and the central region was confirmed by the following method in order to see the variation in the longitudinal direction.

(How to check the boundary position)
As shown in FIG. 7A, the obtained long titanium alloy material is spaced apart by a length of 1450 [= (3000-100) / 2] mm in the longitudinal direction except for the front and rear end portions of 50 mm. A cross section was cut out to prepare three (front, center, and rear) sample pieces. The rolling start side of the titanium alloy material was the front.
Each of the three sample pieces was embedded in a resin, and the cross section of the sample piece was polished with a polishing paper and a buff to finish to a mirror surface, and then the cross section of the sample piece was hydrofluoric acid (2 wt.% Hydrofluoric acid 21 ml, 4 wt.% Nitric acid 33 ml and pure water (446 ml) for 60 seconds, and then washed with pure water and ethanol. Then, the cross section after the nitric acid treatment was observed with an optical microscope, and the depth of the outer shell region (white layer portion) was measured. As a result, there is no large variation in the longitudinal direction, and the depth of the outer shell region is shallow and 60 μm from the surface (that is, 1/200 of the diameter = 12 mm / 200 = 60 μm) and deep and 300 μm from the surface (that is, the diameter) 1/40 = 12 mm / 40 = 300 μm).

(Confirm boundary by measuring Vickers hardness)
Then, as shown in FIG. 7B, four locations (upper, lower, right, left) were set at equal intervals around the center of the cross section of each sample piece.
The Vickers hardness at the position of 1/2 of the depth of the outer shell region was measured from the surface of the titanium alloy material toward the inside from the four locations set for each sample piece. As a result, it was confirmed that the Vickers hardness was 400 HV or more and less than 450 HV at all measurement positions of each sample piece.
Next, the Vickers hardness was measured at a position exceeding the depth of the outer shell region. As a result, it was confirmed that the Vickers hardness was 320HV or more and less than 400HV. That is, it was confirmed that the obtained long titanium alloy material has a boundary between the outer shell region and the central region located within a range of 1/200 to 1/40 of the diameter from the surface to the inside. It was.

About the outer shell region and the central region of the obtained long titanium alloy material, the Vickers hardness, the area ratio of α-phase crystal particles having a particle size of 1 μm or less, and (10-10) the degree of integration of the primary column surface are as follows: Measured by the method. The results are shown in Table 3.

(Measurement method of Vickers hardness)
Three sample pieces produced by the method for checking the boundary position were used as measurement samples.
As shown in FIG. 7B, the Vickers hardness in the outer shell region was measured at four locations (upper, lower, right, left) set for each sample piece.
As shown in FIG. 7B, the Vickers hardness of the central region is a position of d / 8 from the surface, where d is the diameter of the cross section at four locations (upper, lower, right, left) set for each sample piece. (= 1500 μm) and the position of d / 8 from the center of the cross section (= 1500 μm).

(Method for measuring the area ratio of α-phase crystal particles)
Three sample pieces produced by the method for checking the boundary position were used as measurement samples.
Using an SEM (Scanning Electron Microscope: Scanning Electron Microscope) / EBSD (Electron Back Scatter Diffraction Pattern: Electron Backscattering Diffraction) apparatus (JSM-7000F, manufactured by JEOL Ltd.), an Inverse Pole: Fig., Orientation map (with crystal orientation difference of 3 ° or more as the grain boundary) is created, and α phase crystal grains that are equiaxed grains with a grain size of 1 µm or less in the IPF map of α phase that is the main constituent phase The ratio (area ratio) was calculated. Note that the area ratio of the α-phase crystal particles was measured at one place in each of the outer shell region and the central region of each sample piece.

((10-10) Measuring method of degree of integration of primary column surface)
Three sample pieces produced by the method for checking the boundary position were used as measurement samples.
The degree of integration was determined from the reverse pole figure obtained using the SEM / EBSD apparatus. This degree of accumulation indicates how many times the existence probability of a crystal grain having a specific orientation is compared to a structure having a completely random orientation distribution (degree of accumulation 1). Using the texture analysis of the reverse pole figure used, it was determined under the conditions of calculation order = 16 and Gaussian half-value width = 5 ° under the condition that inversion symmetry was taken into account and the symmetry of the sample was not forced. Note that (10-10) the degree of integration of the primary column surface was measured at one location for the outer shell region and the central region of each sample piece.

As shown in Table 3, the long titanium alloy material (wire) produced in Example 1 has a high average value of Vickers hardness in the outer shell region of 413 HV and a small standard deviation of 8.0 HV. In addition, the area ratio of α-phase crystal particles with a particle size of 1 μm or less is 90% or more in the outer shell region and 80% or more in the central region, and the (10-10) primary column surface in the outer shell region is accumulated. The degree is more than twice the central area. For this reason, by using this long titanium alloy material, it is possible to manufacture a titanium alloy product having high strength in the outer shell region and excellent durability. The maximum Vickers hardness at an arbitrary position in the outer shell region was 447 HV. The maximum Vickers hardness at an arbitrary position in the outer shell region was 447 HV.

In the central region, the average value of Vickers hardness is 360 HV, and when the average value of Vickers hardness in the outer shell region is 100, the average value of Vickers hardness in the central region is as low as 87. Deviation is as small as 9.3HV. For this reason, by using this long titanium alloy material, a titanium alloy product can be stably manufactured with less processing power.

[Comparative Example 1]
As Comparative Example 1, a commercially available long titanium alloy material prepared by cold-drawing a Ti-6Al-4V alloy was prepared.

[Evaluation]
(Measurement of Vickers hardness)
For the titanium alloy material of Example 1 and the long titanium alloy material of Comparative Example 1, a cross section was cut out and a straight line passing through the center of the cut out circular cross section was drawn. Next, there are a total of 13 points, 3 points at both ends of the straight line (position of 1/100 of the diameter from the surface) and the center, and 5 points at equal intervals between each end and the center. The Vickers hardness was measured. The result is shown in FIG. In addition, the value of Vickers hardness shown in FIG. 8 is a relative value with the maximum value of Vickers hardness measured with the long titanium alloy material of Example 1 being 100.

8 shows that the long titanium alloy material of Example 1 has higher Vickers hardness than the long titanium alloy material of Comparative Example 1 at both ends and the center. Moreover, it turns out that the long titanium alloy raw material of the comparative example 1 has a Vickers hardness higher than the long titanium alloy raw material of Example 1 in the relatively wide range except both ends and a center part.

[Example 2]
A prismatic Ti-6Al-4V alloy (vertical: 26 mm, horizontal: 26 mm, angular radius of curvature R: 6 mm, length: 970 mm) was prepared. This Ti-6Al-4V alloy was kept at a temperature of 1100 ° C. for 30 minutes and then quenched by water cooling to obtain an α ′ martensitic titanium alloy material. The average cooling rate by water cooling was 40 ° C./second.

The obtained α ′ martensite titanium alloy material was held at a temperature of 725 ° C. for 30 minutes. Then, the strong rolling of 6 times was implemented as follows using the groove roll mill.

First strong rolling: A prismatic α ′ martensite titanium alloy ingot (α ′ martensite titanium alloy material) is pressed from above and below and rolled into a horizontally long elliptical columnar material having a horizontally long elliptical cross section. . As a result, a titanium alloy material having a mixed structure including fine α-phase crystal particles having a particle size of about 60% in area ratio of 1 μm or less and the remaining α ′ martensite needle-like particles was obtained. Here, the average strain rate by strong rolling was 79.2 / s, and the strain of the α ′ martensitic titanium alloy ingot was 0.75.

Second strong rolling: The horizontally long elliptical columnar titanium alloy material obtained by the first strong rolling is rolled by pressing from above and below in the state of being rotated 90 degrees around the center of the cross section. The surface was formed into a rounded columnar shape with a rounded corner (however, the diagonal line was in the vertical direction). By repeating the rolling in this way, the area ratio of the α-phase crystal particles was increased. Moreover, the average strain rate by strong rolling was 64.5 / s, and the cumulative strain until the second strong rolling was 1.44.
Third strong rolling: The rounded columnar titanium alloy material obtained by the second strong rolling is rolled by pressing from above and below in a state where it is rotated 45 degrees around the center of the cross section, and then crossed again. The surface was shaped into a horizontally long elliptical column having a horizontally long elliptical shape.
Fourth strong rolling: The horizontally long elliptical columnar titanium alloy material obtained by the third strong rolling is pressed and rolled from above and below in the state of being rotated 90 degrees around the center of the cross section, Was formed into a round, substantially rectangular plate.
Fifth strong rolling: The rectangular titanium alloy sheet obtained by the fourth strong rolling was rotated 90 degrees, pressed from above and below, rolled, and rounded to have a rounded rectangular shape.
Sixth strong rolling: The horizontally long plate-like titanium alloy sheet obtained by the fifth strong rolling is pressed and rolled from above and below in the state of being rotated 90 degrees around the center of the cross section, and again It was molded into a substantially rectangular plate with rounded sides.

After the above strong rolling, the obtained substantially rectangular plate-like titanium alloy plate was water-cooled to obtain a long titanium alloy plate having a thickness of 4 mm, a width of 32 mm, and a length of 4800 mm.
The cross section of the obtained long titanium alloy sheet was observed using an optical microscope. FIG. 9 shows an overall photograph of the cross section of the long titanium alloy sheet obtained in Example 2, and FIG. 10 shows an enlarged photograph. From the photographs of FIGS. 9 and 10, it was confirmed that the long titanium alloy sheet 31 includes an outer shell region 32 located on the surface side and a central region 33 located inside the outer shell region 32.

The position of the boundary 34 between the outer shell region 32 and the central region 33 of the long titanium alloy sheet was measured in the same manner as in Example 1. As a result, the depth of the outer shell region is shallow and 20 μm from the surface (that is, 1/200 of the thickness of 4 mm), and deep and 100 μm from the surface (that is, 1/40 of the thickness of 4 mm). Was confirmed.

Further, with respect to the cross section of the long titanium alloy sheet, the Vickers hardness was measured at equal intervals on the central axis in the thickness direction (4 mm) and the width direction (32 mm). The result is shown in FIG.
FIG. 11A shows the Vickers hardness in the thickness direction, and FIG. 11B shows the Vickers hardness in the width direction.
From the results shown in FIGS. 11A and 11B, it is confirmed that the long titanium alloy sheet of Example 2 has a Vickers hardness of 400 HV or more in the outer shell region and a Vickers hardness of 320 HV or more and less than 400 HV in the central region. It was.

Furthermore, when the area ratio of the α-phase crystal particles having a crystal grain size of 1 μm or less was obtained from EBSD analysis, it was confirmed that both the outer shell region and the central region were 90% or more, and the entire region was composed of fine particles of 1 μm or less. It was.

DESCRIPTION OF SYMBOLS 1 Titanium alloy raw material 2 Outer shell area | region 3 Center area | region 4 Boundary 11 Raw material before rolling 12 Horizontally long oval shape material 13 Circular cross-section shape material 21a, 21b 1st roll 22a, 22b 2nd roll 31 Long titanium alloy board | plate material 32 Outer shell area | region 33 Central region 34 border

Claims (4)

1. A titanium alloy material having a composition generally classified as near α type and / or α + β type,
   An outer shell region located on the surface side and having a Vickers hardness in the range of 400 HV to less than 450 HV,
   A central region located inside the outer shell region and having a Vickers hardness of 320 HV or more and less than 400 HV,
   The boundary between the outer shell region and the central region is located in the range of 1/200 to 1/40 of the length or diameter in the minor axis direction in the cross section from the surface to the inside. Titanium alloy material.
2. 2. The titanium alloy material according to claim 1, wherein the outer shell region has a structure containing α-phase crystal particles having a particle size of 1 μm or less in an area ratio of 90% or more.
3. The α-phase crystal particles contained in the outer shell region are accumulated so that a C-plane is along the outer surface and a (10-10) primary column surface is along the transverse plane. 2. The titanium alloy material according to 2.
4. The central region has a structure including α phase crystal particles having a crystal grain size of 1 μm or less in an area ratio of 80% or more, and the degree of integration of the (10-10) primary column surface of the α phase crystal particles in the outer shell region 4. The titanium alloy material according to claim 1, wherein the ratio of the α-phase crystal grains in the central region to the degree of integration of the (10-10) primary column surface is 2 or more.

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