titanium plate

Abstract

A titanium plate according to one embodiment of the present invention has a chemical composition comprising, in mass%, O: 0.02% to 0.15%, Fe: 0.02% to 0.20%, N: 0 to 0.08%, C: 0 to 0.100%, and H: 0 to 0.013%, with the remainder being Ti and impurities; wherein the crystal orientation of the α-phase is represented by the Euler angle g=(φ1, Φ, φ2), and the area ratio of the α-phase is defined as X1 for a crystal orientation where the absolute value of the orientation difference with respect to (0°, 90°, 0°) is within 30°, and the area ratio of the α-phase is defined as X2 for a crystal orientation where the absolute value of the orientation difference with respect to (0°, 90°, 30°) is within 30°, and the orientation difference with respect to both (0°, 90°, 0°) and (0°, 90°, 30°) When the area ratio of the above α phase, in which the absolute values ​​are all within 30° of the crystal orientation, is defined as X1&2, X1+X2-X1&2 is 0.075 or less.

Description

Technology Field

[0001] The present invention relates to a titanium plate. Background Technology

[0002] Titanium constituting titanium plates is classified into pure titanium and titanium alloys based on its composition. Pure titanium has a lower content of alloy components compared to titanium alloys, and in terms of crystal structure, it has a crystal structure called a hexagonal close-packed structure (hcp) at room temperature. The hexagonal close-packed structure is also called the hexagonal phase or α phase.

[0003] The hcp crystals constituting pure titanium have poor symmetry as a crystal structure. Consequently, the properties of pure titanium change significantly depending on the crystal orientation, that is, the texture. When a titanium sheet is manufactured through the general manufacturing methods for titanium sheets—hot rolling (optional hot-rolled sheet annealing), cold rolling, and final annealing—a texture is formed mainly with a crystal orientation (Split-TD) in which the c-axis (

[0001] axis) of the hcp is tilted approximately 35° from the ND (plane normal direction) toward the TD (sheet width direction). A texture mainly with Split-TD orientation exhibits excellent plane strain extrusion formability with TD as the main deformation. In this regard, when performing strict forming on a pure titanium sheet, the design is sometimes made so that the deformation mode of the part where deformation is most concentrated becomes plane strain in the sheet width direction.

[0004] Patent Document 1 discloses a titanium plate that is pure titanium containing a predetermined composition, wherein the ratio of crystal grains in which the angle formed by the RD (rolling direction) and the c-axis is 0 to 50° or 70 to 90° is 90.0% or more and 95% or less. Prior art literature

[0005] Japanese Patent Publication No. 2017-226858 The problem to be solved

[0006] However, in recent years, the demand for the processability of titanium plates has been steadily increasing. Specifically, there is a demand for titanium plates with significantly improved planar deformation extrusion formability, where TD is the primary deformation.

[0007] The present invention aims to provide a titanium plate with superior planar deformation extrusion formability, in which TD is the main deformation, compared to conventional plates. means of solving the problem

[0008] The gist of the present invention is as follows.

[0009] (1) A titanium plate according to one embodiment of the present invention is a titanium plate having a chemical composition comprising, in mass%, O: 0.02% to 0.15%, Fe: 0.02% to 0.20%, N: 0 to 0.0800%, C: 0 to 0.1000% and H: 0 to 0.0130%, with the remainder being Ti and impurities, wherein the crystal orientation of the α phase is represented by the Euler angle g=(φ1, Φ, φ2), and the area ratio of the α phase is defined as X1 for a crystal orientation where the absolute value of the orientation difference with respect to (0°, 90°, 0°) is within 30°, and the area ratio of the α phase is defined as X2 for a crystal orientation where the absolute value of the orientation difference with respect to (0°, 90°, 30°) is within 30°, and (0°, 90°, 0°) and (0°, 90°, When the area ratio of the above α phase, where the absolute value of the orientation difference for both sides of 30° is within 30°, is defined as X1&2, X1+X2-X1&2 is 0.075 or less.

[0010] (2) In the titanium plate described in (1) above, the area ratio of the α phase that has a crystal orientation with an absolute value of the orientation difference with respect to (0°, 35°, 0°) within 15° is defined as X3, the area ratio of the α phase that has a crystal orientation with an absolute value of the orientation difference with respect to (0°, 35°, 30°) within 15° is defined as X4, and the area ratio of the α phase that has a crystal orientation with an absolute value of the orientation difference with respect to both (0°, 35°, 0°) and (0°, 35°, 30°) within 15° is defined as X3&4, and (X3+X4-X3&4) / (X1+X2-X1&2) may be 5.0 or greater.

[0011] (3) In the titanium plate described in (1) or (2) above, the average crystal grain size of the α phase may be 70.0 μm or less.

[0012] (4) In the titanium plate described in (1) or (2) above, the average crystal grain size of the α phase may be 20.0 μm or less.

[0013] (5) In the titanium plate described in any one of (1) to (4) above, the coefficient of variation of the crystal grain size of the α phase, which is the value obtained by dividing the standard deviation of the crystal grain size of the α phase by the average crystal grain size of the α phase, may be 0.75 or less. Effects of the invention

[0014] The present invention provides a titanium plate with superior planar deformation extrusion formability, with TD as the main deformation, compared to conventional plates. Brief explanation of the drawing

[0015] Figure 1a is a diagram illustrating a notation method using Euler angles to represent crystal orientation in three dimensions. FIG. 1b is a diagram illustrating a notation method using Euler angles to represent crystal orientation in three dimensions. FIG. 1c is a diagram illustrating a notation method using Euler angles to represent crystal orientation in three dimensions. FIG. 1d is a diagram illustrating a notation method using Euler angles to represent crystal orientation in three dimensions. FIG. 2 is a drawing showing the crystal orientation distribution function ODF in Euler angle space for a titanium plate according to the present embodiment. FIG. 3 is a diagram showing the crystal orientation distribution function ODF in Euler angle space for a conventional titanium plate. Specific details for implementing the invention

[0016] The inventors have discovered that in pure titanium produced by conventional manufacturing methods, in addition to the Split-TD mentioned above, there is also a slight crystal orientation (T-texture) in which the c-axis is oriented parallel to the plate width direction (TD). As a result of the inventors' investigation, it was found that the presence of this T-texture deteriorates the planar deformation extrusion formability of the titanium plate with TD as the main deformation. Accordingly, the inventors further investigated methods to suppress the occurrence of T-texture and found that by introducing deformation during hot rolling of pure titanium and coiling and cooling the titanium plate to maintain this deformation, the occurrence of T-texture can be suppressed, and the planar deformation extrusion formability of the titanium plate with TD as the main deformation can be dramatically improved. A titanium plate according to the present embodiment will be described in detail below.

[0017] Chemical Composition of Titanium Plate

[0018] The titanium plate according to the present invention may include, for example, Type 1 and Type 2 as defined in JIS H4600 (2012) and the corresponding Grade 1 and 2 as defined in ASTM B265, and titanium as defined in DIN 17850, 3·7025, 3·7035, and 3·7055 (also referred to as industrial pure titanium). The chemical composition of the titanium plate according to the present embodiment is described in detail as follows. In addition, the unit of elemental content “%” means mass%.

[0019] O: 0.02% or more, 0.15% or less

[0020] O is an element that is necessarily included in titanium and improves yield strength by 0.2%. However, if the amount of O becomes too high, ductility decreases, and plane deformation extrusion formability, where TD is the main deformation, deteriorates. To obtain the effect of improving yield strength by 0.2% through the addition of O, the lower limit of the O content is 0.02%, preferably 0.03%. In addition, from the perspective of formability, the upper limit of the O content is 0.15%, preferably 0.10%, and more preferably 0.08%.

[0021] Fe: 0.02% or more and 0.20% or less

[0022] Fe is an element that is necessarily included in titanium and has the effect of improving yield strength by 0.2%. However, if the amount of Fe becomes too high, the β phase precipitates during final annealing and has an adverse effect on formability. Since the effect of improving yield strength by 0.2% is obtained, the lower limit of the Fe content is 0.02%, and preferably 0.03%. On the other hand, from the perspective of formability, the upper limit of the Fe content is 0.20%, preferably 0.10%, and more preferably 0.08%.

[0023] N: 0 to 0.0800%

[0024] N does not need to be included in the titanium plate. In addition, N reduces the processability of the titanium plate. Therefore, the lower limit of the N content is 0%. In addition, the upper limit of the N content is 0.080%. However, considering the cost of reducing the N content, the N content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more. In addition, the N content may be 0.0700% or less, 0.0600% or less, or 0.0500% or less.

[0025] C: 0 to 0.1000%

[0026] C does not need to be included in the titanium plate. Therefore, the lower limit of the C content is 0%. In addition, the upper limit of the C content is 0.1000%. However, considering the cost of reducing the C content, the C content may be 0.0001% or more, 0.0003% or more, or 0.0005% or more. In addition, the C content may be 0.0800% or less, 0.0500% or less, or 0.0300% or less.

[0027] H: 0 to 0.0130%

[0028] H does not need to be included in the titanium plate. In addition, H causes embrittlement of the titanium plate. Therefore, the lower limit of the H content is 0%. In addition, the upper limit of the H content is 0.0130%. However, considering the cost of reducing the H content, the H content may be 0.0001% or more, 0.0002% or more, or 0.0003% or more. In addition, the H content may be 0.0100% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0030% or less.

[0029] The remainder of the chemical composition of the titanium plate according to the present embodiment may be Ti and impurities. Specifically, impurities include Cl, Na, Mg, Si, Ca introduced during the refining process, and Al, Zr, Sn, Mo, Nb, Ta, V, etc. introduced from scrap. When these impurities are present, the content is, for example, 0.1% or less for each, and a total amount of 0.5% or less is a level that is not problematic.

[0030] Crystal structure of titanium plates

[0031] The titanium plate according to the present embodiment is industrially referred to as pure titanium in that it has the compositional properties described above. In pure titanium, the α-phase (hcp) is the dominant crystal structure due to its compositional properties. A crystal structure dominated by the α-phase means that the fraction of the α-phase occupying the entire evaluation surface is 95% or more in terms of area percentage. This fraction is preferably 97% or more, and more preferably 99% or more. In addition, the crystal structure other than the α-phase is the β-phase.

[0032] Description of Set Organization by Euler Angles

[0033] In the titanium plate according to the present embodiment, a notation method based on Euler angles is used to represent the crystal orientation in the texture in three dimensions. Below, a notation method based on Euler angles (Bunge's notation method) will be explained.

[0034] As a premise for expressing crystal orientation by Euler angles, first, three coordinate axes RD, TD, and ND are assumed to be orthogonal to each other as the sample coordinate system (coordinate system of the plate material). RD is the rolling direction of the titanium plate, TD is the plate width direction of the titanium plate, and ND is the plate plane normal direction of the titanium plate. RD and TD can be determined based, for example, the elongation direction of the roll marks formed on the surface of the titanium plate, the dimensions of the titanium plate, or the elongation direction of the crystal in the titanium plate.

[0035] Next, three coordinate axes, X, Y, and Z, are assumed to be mutually orthogonal to each other as a crystal coordinate system (in the case of titanium α phase, a coordinate system based on the direction of the hcp structure). In the crystal coordinate system, the Z axis is

[0001] in terms of mirror index. In the crystal coordinate system, the X axis may be [10-10] in the direction (normal direction of the columnar plane) or [1-210] in the direction. In the titanium plate according to the present embodiment, the X axis is [1-210] in the direction. In this case, the Y axis becomes [10-10] in the direction (normal direction of the columnar plane).

[0036] (A) In the notation method based on Euler angles, as shown in Fig. 1a, the sample coordinate system and the crystal coordinate system are in alignment (the X-axis is in alignment with RD, the Y-axis is in alignment with TD, and the Z-axis is in alignment with ND).

[0037] (B) As shown in FIG. 1b, the crystal coordinate system of this reference crystal is rotated by φ1° around the Z-axis. The crystal coordinate system of the crystal after rotation is denoted as (X', Y', Z) in FIG. 1b.

[0038] (C) Next, as shown in FIG. 1c, it is assumed that this crystal is rotated Φ° around the X(X') axis after rotating φ1°. The axes of the crystal coordinate system of the crystal after rotation are denoted as (X', Y', Z') in FIG. 1c.

[0039] (D) Finally, as shown in FIG. 1d, it is assumed that this crystal is rotated φ2° around the Z(Z') axis after a φ1° rotation and a Φ° rotation. The axes of the crystal coordinate system of the crystal after rotation are denoted as (X", Y''', Z') in FIG. 1d.

[0040] In the notation method using Euler angles, the crystal orientation (such as the direction of the c-axis) of any crystal grain is expressed using these three angles: φ1°, Φ°, and φ2°. That is, the crystal orientation of the crystal shown in FIG. 1a is expressed as (0, 0, 0) using Euler angles, the crystal orientation of the crystal shown in FIG. 1b is expressed as (φ1, 0, 0) using Euler angles, the crystal orientation of the crystal shown in FIG. 1c is expressed as (φ1, Φ, 0) using Euler angles, and the crystal orientation of the crystal shown in FIG. 1d is expressed as (φ1, Φ, φ2) using Euler angles. For the α-phase crystal structure, which is the columnar structure of the titanium plate, it can be expressed by φ1 (0 to 90°), Φ (0 to 90°), and φ2 (0 to 60°) from the perspective of being hexagonal.

[0041] In addition, regarding the mirror indices described in FIGS. 1a to 1d, the values ​​to which an overline is applied are negative values. Meanwhile, in the specification, a minus sign is used instead of an overline to indicate negative values ​​in the mirror indices. Accordingly, the notation of some determination orientations in the drawings is rewritten in the specification as follows.

[0042]

[0043] The crystal orientation distribution of a polycrystalline material is represented by a function f(φ1, Φ, φ2) using the aforementioned Euler angles (φ1, Φ, φ2), and this function is called the Orientation Distribution Function (ODF). If the crystal orientation (φ1, Φ, φ2) is denoted as g, the ODF can be expressed as f(g). As the value of f(g) increases, it means that there are more crystals oriented at the corresponding crystal orientation g.

[0044] The typical texture of a titanium plate manufactured by conventional methods, which is a texture with a c-axis inclined to TD at approximately 35° as the preferred orientation (Split-TD), is hereinafter expressed using Euler angles (φ1, Φ, φ2). In order to represent the three-dimensional crystal orientation distribution function on a two-dimensional plane, f(g) at a specific φ2 is represented as a contour line in a space with the horizontal axis: φ1 (0 to 90°) and the vertical axis: Φ (0 to 90°). In titanium, representative orientations are indicated as φ2=0° and 30° (Figs. 2 and 3). As is clear from Figs. 2 and 3, for any φ2, it is evident that a peak with a high value of f(g) exists near Φ=35° and φ1=0°. All of these crystal orientations exhibit an orientation in which the c-axis of the hcp structure is inclined 35° from ND toward TD, and when φ2=0°, [1-210] is directed toward RD, and when φ2=30°, [0-110] is directed toward RD. Among these orientations, the crystal orientations that appear as the principal orientations (0°, 35°, 0°) or (0°, 35°, 30°) exhibit excellent plane deformation extrusion formability with TD as the principal deformation, so press processing utilizing this characteristic is performed.

[0045] FIG. 3 is a drawing of a pure titanium plate manufactured by a conventional manufacturing method. In FIG. 3, the crystal orientation is observed from φ1=0° to Φ=35 to 90°. Among these, Φ=90° represents a crystal orientation in which the c-axis of hcp is parallel to TD. These crystal orientations (T-texture) represented by (0°, 90°, 0°) or (0°, 90°, 30°) worsen the plane strain extrusion formability with TD as the main deformation. In a conventional manufacturing method, it is difficult to reduce the crystal orientations represented by (0°, 90°, 0°) or (0°, 90°, 30°), and there were cases where the plane strain extrusion formability with TD as the main deformation was insufficient.

[0046] Presence ratio of T-texture

[0047] In the titanium plate according to the present embodiment, the proportion of T-texture in the titanium plate is defined as follows. That is, when the crystal orientation of the α phase is represented by the Euler angle g=(φ1, Φ, φ2), the area ratio of the α phase for a crystal orientation where the absolute value of the orientation difference with respect to (0°, 90°, 0°) is within 30° is defined as X1, and the area ratio of the α phase for a crystal orientation where the absolute value of the orientation difference with respect to (0°, 90°, 30°) is within 30° is defined as X2. In addition, the area ratio of the α phase for a crystal orientation where the absolute value of the orientation difference with respect to both (0°, 90°, 0°) and (0°, 90°, 30°) is within 30° is defined as X1&2. X1&2 is the area ratio of the α phase included in both X1 and X2. And, X1+X2-X1&2 is used as the ratio of the existence of T-texture.

[0048] Crystal orientations (T-texture) represented by (0°, 90°, 0°) and (0°, 90°, 30°) worsen the plane deformation extrusion formability with TD as the main deformation. Therefore, the area ratio of crystals with these crystal orientations is preferably small. In order to obtain sufficient formability, in the titanium plate according to the present embodiment, the upper limit of the sum of the two (X1+X2-X1&2) is 0.075, and preferably 0.070.

[0049] FIG. 2 is a drawing showing f(g) at φ2=0° and 30° for a titanium plate according to the present embodiment, and FIG. 3 is a drawing showing a conventional titanium plate, with f(g) at φ2=0° and 30° in a space with the horizontal axis: φ1 (0 to 90°) and the vertical axis: Φ (0 to 90°). (X1+X2-X1&2), which represents the ratio of the existence (area ratio) of crystals having a crystal orientation (T-texture) in which the absolute value of the orientation difference is within 30° centered on (0°, 90°, 0°) and (0°, 90°, 30°), was 0.082 in the case of FIG. 3 of the conventional example, whereas it was reduced to 0.062 in FIG. 2 of the titanium plate according to the present embodiment.

[0050] Prevalence of Split-TD

[0051] In addition, the presence ratio of Split-TD in the titanium plate is defined as follows. That is, X3 is defined as the area ratio of the α-phase with a crystal orientation in which the absolute value of the orientation difference with respect to (0°, 35°, 0°) is within 15°, and X4 is defined as the area ratio of the α-phase with a crystal orientation in which the absolute value of the orientation difference with respect to (0°, 35°, 30°) is within 15°. Furthermore, X3&4 is defined as the area ratio of the α-phase with a crystal orientation in which the absolute value of the orientation difference with respect to both (0°, 35°, 0°) and (0°, 35°, 30°) is within 15°. X3&4 is the area ratio of the α-phase included in both X3 and X4. Then, X3+X4-X3&4 is defined as the presence ratio of Split-TD.

[0052] Since the crystal orientations (Split-TD) represented by (0°, 35°, 0°) or (0°, 35°, 30°) exhibit excellent plane deformation extrusion formability with TD as the main deformation, it is preferable for the crystal area ratio of these crystal orientations to be large. Therefore, considering the area ratio (X1+X2-X1&2) of the aforementioned (0°, 90°, 0°) and (0°, 90°, 30°), it is preferable for (X3+X4-X3&4) / (X1+X2-X1&2) to be large. In order to obtain sufficient formability in the titanium plate according to the present embodiment, the lower limit of (X3+X4-X3&4) / (X1+X2-X1&2) is preferably 5.0, and more preferably 5.5.

[0053] Method for evaluating crystal area ratios X1 to X4, X1&2, and X3&4 of α-phases having a predetermined crystal orientation

[0054] The area ratios X1 to X4 of the crystal orientations mentioned above can be measured as follows. A plane perpendicular to the plate width direction of a titanium plate (hereinafter also referred to as the "L cross-section") was polished to serve as the measurement surface, and an electron beam was scanned with a scanning electron microscope (SEM) over a field of view of 15 mm × the total plate thickness of this plane. The EBSD pattern was measured using the electron backscatter diffraction (EBSD) method with a step size of 10 μm. Based on the data, the respective area ratios were calculated using OIM Analysis ver 7.31 software manufactured by TSL Solutions. For example, by specifying close-packed hexagonal as the crystal structure and specifying "simultaneously evaluate the area ratio of crystal orientations with an absolute value of orientation difference within 30° centered on (0°, 90°, 0°) and the area ratio of crystal orientations with an absolute value of orientation difference within 30° centered on (0°, 90°, 30°)," X1+X2-X1&2 can be obtained as the evaluation result. X3+X4-X3&4 can be obtained through the same procedure. For the analysis, only analysis data with a CI (Confidence of Index) value of 0.1 or higher were used. In addition, considering the symmetry of rolling deformation, calculations were performed as co-orientations for crystal orientations that are line-symmetric with respect to the plate thickness direction, rolling direction, and plate width direction, respectively. Furthermore, the measurement conditions were as follows.

[0055] · Heating voltage: 15 kV

[0056] · Current: Approx. 35 nA

[0057] · Magnification: 200x

[0058] Average crystal grain size of the α phase

[0059] If the average crystal grain size of the α-phase is too large, there is a possibility that wrinkles may occur during press forming. In addition, strength increases as the average crystal grain size of the α-phase decreases. Therefore, the average crystal grain size of the α-phase is preferably 70.0 μm or less, 50.0 μm or less, 25.0 μm or less, and more preferably 20.0 μm or less. On the other hand, if the average crystal grain size of the α-phase is less than 2 μm, unrecrystallized grains remain, which may adversely affect formability. Therefore, the average crystal grain size of the α-phase is preferably 2.0 μm or more.

[0060] Variation in crystal grain size of the α-phase

[0061] The value obtained by dividing the standard deviation of the crystal grain size (circle equivalent diameter) of the α-phase by the average value of the crystal grain size of the α-phase is defined as the coefficient of variation of the crystal grain size of the α-phase. If the coefficient of variation of the crystal grain size of the α-phase is too large, the distribution of the crystal grain size becomes non-uniform, and there is a possibility that formability will deteriorate as deformation concentrates on specific crystal grains. Therefore, it is desirable for the coefficient of variation of the crystal grain size of the α-phase to be 0.75 or less, and more desirable for it to be 0.70 or less.

[0062] Evaluation Method for Average Crystalline Grain Size and Coefficient of Variation of Phase α

[0063] The average crystal grain size of the α-phase is determined at an arbitrary cross-section of the titanium plate and can be measured by observing the plane perpendicular to the plate width direction (L-section). The plane perpendicular to the plate width direction (L-section) of the titanium plate was polished to serve as the measurement surface. An electron beam was scanned using a scanning electron microscope (SEM) over a field of view of this plane equal to the total plate thickness × 1000 μm, and the EBSD pattern was measured using the electron backscatter diffraction (EBSD) method with a step size of 0.5 μm. Based on this data, the average crystal grain size of the α-phase was calculated using OIM Analysis ver 7.31 software manufactured by TSL Solutions. Boundaries with an orientation difference of 15° or greater are recognized as grain boundaries, and the region enclosed by these grain boundaries is defined as a grain. Additionally, there are approximately 1000 or more grains within this field of view. The average crystal grain size was evaluated as the arithmetic mean of the average values ​​of the circle equivalent diameters. Furthermore, the coefficient of variation was calculated by dividing the standard deviation of the α-phase crystal grain size (circle equivalent diameter) by the average crystal grain size. The measurement conditions of the EBSD pattern, other than the measurement field of view and step size, are the same as the conditions for measuring X1 to X4, etc.

[0064] When the crystal grain size of the α-phase is relatively coarse, during the grain growth process after the completion of recrystallization, crystal grains having Split-TD grow preferentially, and the aggregation of crystal grains having T-texture tends to decrease. However, when the crystal grain size of the α-phase is coarse, wrinkles form during forming processing as described above. Meanwhile, conventional manufacturing methods had a problem where T-texture increased when the crystal grain size of the α-phase became fine. In contrast, the titanium plate according to the present embodiment has almost no T-texture even when the crystal grain size of the α-phase is relatively fine. Therefore, the inventors were finally able to realize a titanium plate that does not exist in the prior art, which combines both the characteristic that the α-phase is fine and unlikely to develop wrinkles, and the characteristic of excellent plane deformation extrusion formability with TD as the main deformation. When the average crystal grain size of the α phase is 70.0 μm or less, T-texture is present in the conventional case, whereas the titanium plate according to the present embodiment has almost no T-texture, so the effect can be fully exerted.

[0065] Method for manufacturing titanium plates

[0066] The titanium plate according to the present embodiment is characterized by a texture with almost no T-texture and has excellent plane deformation extrusion formability with TD as the main deformation. The manufacturing method is not particularly limited, but for example, the texture is achieved by controlling hot rolling to retain specific dislocations and suppressing T-texture formation by a manufacturing method shown below, and then increasing the cold rolling rate to increase the density of Split-TD, which is a preferred orientation, while decreasing the density of T-texture.

[0067] As described above, by controlling the hot rolling end temperature and subsequent cooling rate of pure titanium and performing cold rolling at a large reduction rate, a texture is developed in which the T-texture, which is considered to be the main factor of anisotropy, is almost non-existent even if the grain is fine after final annealing, and no wrinkles occur during forming, thereby enabling the production of a titanium thin sheet with excellent plane deformation extrusion formability with TD as the main deformation.

[0068] Hereinafter, an example of a method for manufacturing a titanium plate according to the present embodiment will be described. As a process flow, a melting process, a breaking process, a hot rolling process, a cold rolling process, and a final annealing process are performed in sequence.

[0069] "Dissolution Process"

[0070] Titanium raw material manufactured to a predetermined purity is melted by a conventionally known method and formed into a predetermined ingot. Specifically, a vacuum arc melting method (VAR method) or an electron beam melting method (EB method) may be applied.

[0071] "Incrementation Process"

[0072] The titanium slab is processed into a slab shape using conventionally known breakdown rolling or forging. The titanium slab obtained through the above breakdown process may be subjected to cutting or cleaning treatments by known methods as necessary. Additionally, this process may be omitted if necessary.

[0073] Hot rolling (heating and rolling process)

[0074] In the titanium plate according to the present embodiment, it is important to control the reduction rate during hot rolling, the end temperature during hot rolling, the coiling temperature after hot rolling, and the average cooling rate from the end temperature up to 300°C.

[0075] Regarding heating prior to hot rolling, conventionally known methods may be used, for example, heating the slab to 700 to 1000°C. However, from the perspective of accurately controlling the hot rolling end temperature, the heating temperature is preferably lower than the β transformation point temperature. Furthermore, the "β transformation point temperature" refers to the boundary temperature at which the α phase begins to form when pure titanium or a titanium alloy is cooled from the β phase single-phase region. The β transformation point temperature can be obtained from a phase diagram. The phase diagram can be obtained, for example, by the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) method, and for this reason, the integrated thermodynamic calculation system Thermo-Calc and a specified database (TI3) from Thermo-Calc Software AB can be used.

[0076] [Hot Rolled]

[0077] "Average reduction rate per pass in finish rolling" 10% or more

[0078] "Inter-pass time in finish rolling" 2.0 seconds or less

[0079] In the finishing rolling of hot-rolled titanium, it is desirable to have an average reduction rate of 10% or more per pass and a time between passes of 2.0 seconds or less. By doing so, the extinction of dislocations due to recovery can be suppressed, allowing a large amount of dislocations to be introduced into the titanium plate. As described below, dislocations have the effect of suppressing the formation of T-texture during the cooling of the titanium plate after hot rolling. Therefore, by performing finishing rolling under these conditions, a texture with less T-texture can be developed.

[0080] "Hot Rolling Termination Temperature" 600℃ or higher, 750℃ or lower

[0081] In titanium, as many as five types of sliding systems exist as plastic deformation mechanisms, and the type that is active varies depending on the temperature. It was found that if dislocations generated by the activity of sliding systems active at 600°C or higher remain, the formation of T-texture can be suppressed during the final annealing after cold rolling. The hot rolling end temperature is controlled so that these dislocations remain.

[0082] If the hot rolling end temperature is below 600°C, the density of dislocations that suppress T-texture formation is low, and a large amount of T-texture is formed during final annealing, resulting in poor plane deformation extrusion formability with TD as the main deformation. Therefore, the lower limit of the hot rolling end temperature is 600°C, preferably 625°C, and more preferably 650°C. On the other hand, if the hot rolling end temperature exceeds 750°C, recovery or recrystallization occurs during processing, and the above dislocations disappear, resulting in a large amount of T-texture being formed during final annealing, and poor plane deformation extrusion formability with TD as the main deformation. Therefore, the upper limit temperature is 750°C, preferably 730°C, and more preferably 720°C. In addition, the hot rolling end temperature is measured with a radiation thermometer. The coiling temperature, etc. described below, is also a value measured with a radiation thermometer.

[0083] "Winding temperature" 450℃ or lower

[0084] "Average cooling rate from hot rolling end temperature to 300℃" 5.0℃ / s or higher

[0085] By accelerating the cooling rate after hot rolling, recovery or recrystallization during cooling is suppressed, thereby retaining the dislocation and suppressing the formation of T-texture during final annealing.

[0086] When a titanium plate is coiled after hot rolling, the cooling rate of the titanium plate decreases. If the coiling temperature exceeds 450°C, recovery or recrystallization occurs in the coiled titanium plate, and the dislocations disappear, resulting in the formation of a large amount of T-texture during final annealing and poor plane deformation extrusion formability with TD as the main deformation. Therefore, the coiling temperature is 450°C or lower.

[0087] In addition, if the cooling rate is lower than 5°C / s, recovery or recrystallization occurs during cooling, and the dislocations disappear, resulting in the formation of a large amount of T-texture during final annealing and poor plane deformation extrusion formability with TD as the main deformation. Therefore, the lower limit of the cooling rate is 5.0°C / s, preferably 10.0°C / s, and more preferably 20.0°C / s. There is no particular restriction on the upper limit, but it may be set to 100°C / s based on equipment capacity, etc.

[0088] "Hot-rolled plate annealing and cold-rolled intermediate annealing" must not be performed.

[0089] If hot-rolled sheet annealing and cold-rolled intermediate annealing are performed, the aforementioned dislocations disappear, resulting in the formation of a large amount of T-texture during final annealing and poor plane deformation formability where TD is the main deformation. Therefore, hot-rolled sheet annealing and cold-rolled intermediate annealing must not be performed after hot rolling and before final annealing. Furthermore, hot-rolled sheet annealing refers to annealing performed on a titanium sheet after coiling and before the start of cold rolling, while cold-rolled intermediate annealing refers to annealing performed on a titanium sheet between passes when cold rolling is performed in multiple passes.

[0090] The obtained hot-rolled plate may be subjected to removal of oxide scale, etc. by pickling or cutting using known methods, or to cleaning treatment.

[0091] [Cold Rolled]

[0092] "Total Cold Rolling Rate" 78% or higher

[0093] If the total cold rolling rate of cold rolling is less than 78%, the degree of integration into Split-TD decreases, and the integration into T-texture increases. Therefore, the total cold rolling rate is set to 78% or higher. More preferably, the total cold rolling rate is 81% or higher. Regarding the upper limit of the total cold rolling rate, 95% is preferred from the perspective of edge cracking, etc. The plate thickness of the titanium plate that has undergone hot rolling and cold rolling under the above conditions is, for example, within the range of 0.3 mm to 1.0 mm.

[0094] "Final Annealing" An annealing temperature of 500°C or higher and 750°C or lower, a combination of temperature and time satisfying Equation (1).

[0095]

[0096] P in the above equation (1) is a function called the Larson-Miller parameter, T is the annealing temperature (°C), and t is the annealing time (s).

[0097] If the annealing temperature is below 500°C, recrystallization is not completed, and there is a possibility that the plane deformation extrusion formability with TD as the main deformation will be poor. Therefore, the lower limit of the annealing temperature is 500°C, and preferably 550°C. On the other hand, if the annealing temperature is higher than 750°C, the crystal grain size changes significantly with slight variations in time, and the crystal grain size becomes too coarse, which may cause wrinkles during forming. Therefore, the upper limit of the annealing temperature is 750°C, and more preferably 700°C. In addition, if the Larson-Miller parameter P is less than 18500, recrystallization is not completed, and there is a possibility that the plane deformation extrusion formability with TD as the main deformation will be poor. Therefore, the lower limit of P is 18500, and preferably 19000.

[0098] Meanwhile, as the value of the Larson-Miller parameter P decreases, the average crystal grain size of the titanium plate can be reduced. If the Larson-Miller parameter P is 23,000 or less, the average crystal grain size becomes 70.0 μm or less, and the occurrence of wrinkles during forming can be suppressed. If the Larson-Miller parameter P is 21,500 or less, the average crystal grain size becomes 25.0 μm or less, and it is possible to reliably prevent the occurrence of wrinkles during forming. Therefore, it is desirable to set the upper limit of P to 21,500.

[0099] (Manufacturing method for making the coefficient of variation of the crystal grain size of the α-phase 0.75 or less)

[0100] When the cold rolling rate is low, the state of deformation accumulation in each grain of the cold-rolled sheet becomes uneven, and there is a possibility that the distribution of grain sizes after annealing will become uneven. Therefore, it is better to have a high cold rolling rate, and preferably 80% or more.

[0101] Examples

[0102] First, a titanium ingot, which serves as the material for the titanium sheet shown in Table 2, was manufactured by vacuum arc remelting (VAR). Subsequently, a slab with a thickness of 150 mm × a width of 800 mm × a length of 5000 mm was manufactured by breakdown rolling or forging. In addition, "Bal." in Table 2 indicates the remainder. Also, in Table 2, the content of impurity elements was omitted.

[0103] The method for measuring the chemical composition of the slab was as follows. The Fe content was measured by IPC emission spectroscopic analysis using a sequential ICP emission analyzer. The O and N content was measured by thermal conductivity and infrared absorption methods using an oxygen-nitrogen simultaneous analyzer. The C content was measured by infrared absorption methods using a carbon-sulfur analyzer. H was measured by thermal conductivity methods using a hydrogen analyzer. The chemical composition of each manufactured hot-rolled plate was equivalent to the chemical composition of the titanium slab shown in Table 2. In addition, for titanium materials A to K shown in Table 2, the phase diagram of the titanium alloy was obtained by the CALPHAD method using Thermo-Calc, an integrated thermodynamic calculation system from Thermo-Calc Software AB, and a specified database (TI3), and the β transformation point Tβ was calculated.

[0104] Next, these slabs were subsequently hot-rolled under the conditions shown in Table 3 or Table 5. After hot-rolling, the titanium plates (hot-rolled plates) were subjected to hot-rolled plate annealing as necessary, followed by shot blasting and pickling. Furthermore, in the manufacture of all inventive examples and comparative examples, the average reduction rate per pass during the finishing hot-rolling was set to 10% or more, and the time between passes was set to 2.0 seconds or less. Additionally, the term "hot-rolled reduction rate" as described in Tables 3 and 5 refers to the total reduction rate during hot-rolling. Furthermore, the term "hot-rolled plate thickness" as described in Tables 3 and 5 refers to the thickness of the hot-rolled plate after the completion of hot-rolling.

[0105] Next, the obtained hot-rolled plate was cold-rolled and annealed under the conditions shown in Table 4 or Table 6 to produce a cold-rolled plate with a thickness of 0.8 mm. In addition, in the table, "Tβ" is the β transformation point, and "Larson-Miller parameter" is the value of P=(T+273.15)×(Log10(t)+20). T is the annealing temperature (°C), and t is the annealing time (s).

[0106] The method for evaluating the crystal area ratios X1+X2-X1&2 and X3+X4-X3&4, which are crystal orientations in a titanium plate, was as follows. First, to measure the area ratios X1+X2-X1&2 and X3+X4-X3&4, a plane perpendicular to the plate width direction of the titanium plate (hereinafter also referred to as the "L cross-section") was polished to serve as the measurement surface. On this measurement surface, an observation field of view with a width of 15 mm was established along the plate surface, covering the entire plate thickness direction. Within this observation field of view, while scanning electron beams with a scanning electron microscope (SEM), EBSD patterns were measured using the electron backscatter diffraction (EBSD) method with a step size of 10 μm. For the data, each area ratio was calculated using OIM Analysis ver 7.31 software manufactured by TSL Solutions. At this time, only analysis data with a CI value (Confidence of Index value) of 0.1 or higher were used for the analysis. In addition, considering the symmetry of the rolling deformation, calculations were performed by assuming that crystal orientations that are line-symmetric with respect to the plate thickness direction, rolling direction, and plate width direction, respectively, are the same orientation. The measurement conditions for the EBSD pattern were as follows.

[0107] · Heating voltage: 15 kV

[0108] · Current: Approx. 35 nA

[0109] · Magnification: 200x

[0110] In addition, the method for evaluating the average crystal grain size and coefficient of variation of the α-phase was as follows. First, to measure the crystal grain size of the α-phase, a plane (L cross-section) perpendicular to the plate width direction of the titanium plate was polished to serve as the measurement surface. On this measurement surface, an observation field of view with a width of 1000 μm was established along the plate surface, covering the entire plate thickness direction. Within this observation field of view, while scanning electron beams with a scanning electron microscope (SEM), the EBSD pattern was measured using the electron backscatter diffraction (EBSD) method with a step size of 0.5 μm. Regarding the data, OIM Analysis ver 7.31 software manufactured by TSL Solutions was used to recognize boundaries with an orientation difference of 15° or more as grain boundaries, define the region surrounded by these grain boundaries as grains, and calculate the crystal grain size of the α-phase as the equivalent diameter of the circle. The average crystal grain size was taken as the arithmetic mean of the equivalent diameters of the circles. In addition, the coefficient of variation was calculated by dividing the standard deviation of the crystal grain size (circle equivalent diameter) of the α phase by the average crystal grain size.

[0111] The formability of the titanium plate was evaluated based on the plane deformation elongation height in the plate width direction. Formability was evaluated using a die with a hole diameter of φ44 mm, a shoulder radius of R6 mm, and a φ70 mm bead, and a φ40 mm spherical head punch, with a crimping force of 7 ton and a punch lifting speed of 20 mm / min. A blank was cut from a cold-rolled plate with a side length of 60 mm parallel to the rolling direction and a side length of 90 mm parallel to the plate width direction. For lubrication, a poly sheet (Nichias Co., Ltd. Nafron Tape 9001 (0.05 t)) and a high-viscosity oil (Nihon Kosaku Oil Co., Ltd. Crafting Oil #660) were used. The displacement at the maximum load, read from the load-displacement curve obtained from the test, was considered as the elongation height. A titanium plate with an elongation height of 24.0 mm or more was evaluated as a titanium plate with good formability, that is, a titanium plate with excellent planar deformation elongation formability with TD as the main deformation compared to conventional plates.

[0112] The evaluation of wrinkles during the forming of the titanium plate was based on the surface roughness after the tensile test. A JIS 13B tensile test specimen was taken from the titanium plate such that the rolling direction was the tensile direction. This specimen was subjected to a tensile test up to a nominal deformation of 20% at a strain rate of 30% / min. After the tensile test, the surface roughness of the parallel section of the specimen was measured using a laser microscope. Using a laser microscope manufactured by Keyence Inc., three fields of view measurements were performed at a magnification of 500x, a field of view of 213㎛×284㎛, with measurement mode: surface shape, measurement area: surface, measurement quality: high precision, and Z measurement pitch of 0.01㎛. Using VK Analyzer software ver. 2.5.0.1, the data was automatically corrected for the surface inclination of the sample. Then, in accordance with JIS B0601:2001, the cutoff values ​​were set to λc = 0.8 mm and λs = 2.5 μm. Under the condition that 11 lines were drawn parallel to the rolling direction to divide the plate width direction into 12 equal parts, the arithmetic mean roughness Ra in the rolling direction was calculated for each field of view, and the result was evaluated based on the average of three fields of view. An average Ra of 0.25 μm or less was considered "very good," and 0.65 μm or less was considered "good." However, cases where the elongation height evaluation result was good even though the wrinkling exceeded 0.65 μm were considered to be titanium plates with superior plane deformation elongation formability, where TD is the main deformation, compared to conventional plates. Furthermore, the same results regarding surface roughness are obtained for the parallel section of the test specimen. This test was conducted by measuring at the center of the parallel section.

[0113]

[0114]

[0115]

[0116]

[0117]

[0118] Examples 1 to 36 in Tables 3 and 4 are examples of the present invention. Additionally, Examples 1 to 25 in Tables 5 and 6 are comparative examples that do not satisfy one or more of the requirements of the present invention. Furthermore, in Tables 2 to 6, numerical values ​​that fall outside the scope of the present invention, or numerical values ​​and items that fall outside the suitable range of the manufacturing method, are underlined.

[0119] In Invention Examples 1 to 36, the chemical composition and X1+X2-X1&2 were within the scope of the invention. These invention examples were evaluated as titanium plates with superior planar deformation extrusion formability with TD as the main deformation compared to conventional ones.

[0120] For Comparative Examples 1 to 18, 24, and 25 in Tables 5 and 6, X1+X2-X1&2 was outside the scope of the invention. This is presumed to be because the manufacturing methods of Comparative Examples 1 to 18, 24, and 25 were outside the suitable range of the present invention. Comparative Examples 1 to 18 and 25 had inferior planar deformation extrusion formability with TD as the main deformation compared to the invention. Furthermore, regarding Comparative Example 24, since it was not cold-rolled and the plate thickness was very large, an evaluation of the extrusion height was not performed. However, since X1+X2-X1&2 of Comparative Example 24 was outside the scope of the invention, it is presumed that Comparative Example 24 had inferior planar deformation formability, similar to the other Comparative Examples.

[0121] Specifically, in the manufacture of Comparative Example 1, Comparative Example 9, and Comparative Example 14, the hot rolling end temperature was excessive.

[0122] In the manufacture of Comparative Example 2, Comparative Example 10, and Comparative Example 15, the hot rolling end temperature was insufficient.

[0123] In the manufacture of Comparative Examples 3, 11, and 16, the average cooling rate from the hot rolling end temperature to 300°C was insufficient.

[0124] In the manufacture of Comparative Examples 4, 12, 14, and 18, hot-rolled plate annealing was performed.

[0125] In the manufacture of Comparative Examples 5, 13, and 17, the final cold rolling rate was insufficient.

[0126] In the manufacture of Comparative Example 6, intermediate annealing was performed.

[0127] In the manufacture of Comparative Example 7, the annealing temperature during the final annealing was insufficient.

[0128] In the manufacture of Comparative Example 8, the Larson-Miller parameter P was insufficient.

[0129] In the manufacture of Comparative Example 24, cold rolling was omitted.

[0130] In the manufacture of Comparative Example 25, the winding temperature was excessive.

[0131] Comparative Examples 19 to 23 in Tables 5 and 6 had chemical compositions outside the scope of the invention. Comparative Examples 19 to 23 also failed one or both of the wrinkle evaluation results and the elongation height.

Claims

Claim 1 A titanium plate having a chemical composition comprising, in mass %, O: 0.02% to 0.15%, Fe: 0.02% to 0.20%, N: 0 to 0.0800%, C: 0 to 0.1000%, and H: 0 to 0.0130%, with the remainder being Ti and impurities; wherein the crystal orientation of the α-phase is represented by the Euler angle g=(φ1, Φ, φ2), and the area ratio of the α-phase is defined as X1 for a crystal orientation where the absolute value of the orientation difference with respect to (0°, 90°, 0°) is within 30°, and the area ratio of the α-phase is defined as X2 for a crystal orientation where the absolute value of the orientation difference with respect to (0°, 90°, 30°) is within 30°, and the absolute values ​​of the orientation difference with respect to both (0°, 90°, 0°) and (0°, 90°, 30°) are all A titanium plate in which, when the area ratio of the α phase having a crystal orientation within 30° is defined as X1&2, X1+X2-X1&2 is 0.075 or less. Claim 2 A titanium plate according to claim 1, wherein the area ratio of the α-phase having a crystal orientation with an absolute value of the orientation difference with respect to (0°, 35°, 0°) within 15° is defined as X3, the area ratio of the α-phase having a crystal orientation with an absolute value of the orientation difference with respect to (0°, 35°, 30°) within 15° is defined as X4, and the area ratio of the α-phase having a crystal orientation with an absolute value of the orientation difference with respect to both (0°, 35°, 0°) and (0°, 35°, 30°) within 15° is defined as X3&4, wherein (X3+X4-X3&4) / (X1+X2-X1&2) is 5.0 or greater. Claim 3 A titanium plate according to claim 1 or 2, wherein the average crystal grain size of the α phase is 2.0 μm or more and 70.0 μm or less. Claim 4 A titanium plate according to claim 1 or 2, wherein the average crystal grain size of the α phase is 20.0 μm or less. Claim 5 A titanium plate according to claim 1 or 2, wherein the coefficient of variation of the crystal grain size of the α-phase, which is the value obtained by dividing the standard deviation of the crystal grain size of the α-phase by the average crystal grain size of the α-phase, is 0.75 or less. Claim 6 A titanium plate according to paragraph 3, wherein the coefficient of variation of the crystal grain size of the α-phase, which is the value obtained by dividing the standard deviation of the crystal grain size of the α-phase by the average crystal grain size of the α-phase, is 0.75 or less. Claim 7 A titanium plate according to claim 4, wherein the coefficient of variation of the crystal grain size of the α-phase, which is the value obtained by dividing the standard deviation of the crystal grain size of the α-phase by the average crystal grain size of the α-phase, is 0.75 or less.

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