Titanium plate

Abstract

A titanium plate according to one aspect of the present invention has a chemical composition comprising, in % by mass, 0.02% to 0.15% of O, 0.02% to 0.20% of Fe, 0 to 0.08% of N, 0 to 0.100% of C, 0 to 0.013% of H and a remainder comprising Ti and impurities, in which the value of X1+X2-X1&2 is 0.075 or less wherein the crystal orientation of an α-phase is expressed in an Euler angle g = (φ1,Φ,φ2), the area ratio of the α-phase in such a crystal orientation that the absolute value of the misorientation relative to (0°,90°,0°) falls within 30° is defined as X1, the area ratio of the α-phase in such a crystal orientation that the absolute value of the misorientation relative to (0°,90°,30°) falls within 30° is defined as X2, and the area ratio of the α-phase in such a crystal orientation that the absolute value of the misorientation relative to each of (0°,90°,0°) and (0°,90°,30°) falls within 30° is defined as X1&2.

Description

[Technical Field]

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

[0002] The titanium that makes up titanium plates is classified into pure titanium and titanium alloys depending on its chemical composition. Pure titanium has a lower alloying element content than titanium alloys, and at room temperature has a crystalline structure known as a hexagonal close-packed (hcp) structure. The hcp structure is also known as the hexagonal crystal or α phase.

[0003] The hcp crystals that make up pure titanium have poor symmetry. Therefore, the properties of pure titanium vary significantly depending on the crystal orientation, i.e., texture. When titanium sheet is produced through the typical titanium sheet manufacturing process of hot rolling (optional hot-rolled sheet annealing), cold rolling, and final annealing, a texture is formed that is primarily a crystal orientation (Split-TD), in which the hcp c-axis (the 0001 axis) is tilted approximately 35° from the ND (normal direction of the sheet surface) toward the TD (transverse direction). A texture primarily based on the Split-TD orientation offers excellent plane-strain stretch formability, with the TD being the principal strain. For this reason, when pure titanium sheet is subjected to severe forming, it is sometimes designed so that the deformation mode in the area with the greatest strain concentration is plane strain in the transverse direction.

[0004] Patent Document 1 discloses a titanium plate made of pure titanium containing predetermined components, in which the proportion of crystal grains in which the angle between 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 documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Publication No. 2017-226858 Summary of the Invention [Problem to be solved by the invention]

[0006] However, in recent years, there has been an increasing demand for titanium sheets with improved workability. Specifically, there is a demand for titanium sheets with improved plane strain stretch formability, with TD as the principal strain.

[0007] An object of the present invention is to provide a titanium sheet that is superior to conventional titanium sheets in plane strain stretch formability with TD as the principal strain. [Means for 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 has a chemical composition, in mass%, of 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 balance being Ti and impurities; The average grain size of the α phase is 2.0 μm or more and 70.0 μm or less, When the crystal orientation of the α phase is expressed by Euler angles g = (φ1, Φ, φ2), and the area fraction of the α phase having a crystal orientation whose absolute value of the misorientation from (0°, 90°, 0°) is within 30° is defined as X1, the area fraction of the α phase having a crystal orientation whose absolute value of the misorientation from (0°, 90°, 30°) is within 30° is defined as X2, and the area fraction of the α phase having a crystal orientation whose absolute value of the misorientation from both (0°, 90°, 0°) and (0°, 90°, 30°) is within 30° is defined as X1&2, X1 + X2 - X1&2 is 0.075 or less. (2) In the titanium plate described in (1) above, when the area fraction of the α phase having a crystal orientation whose absolute value of the misorientation from (0°, 35°, 0°) is defined as X3, the area fraction of the α phase having a crystal orientation whose absolute value of the misorientation from (0°, 35°, 30°) is defined as X4, and the area fraction of the α phase having a crystal orientation whose absolute value of the misorientation from both (0°, 35°, 0°) and (0°, 35°, 30°) is defined as X3&4, (X3+X4-X3&4) / (X1+X2-X1&2) may be 5.0 or more. ( 3In the titanium plate described in (1) or (2) above, the average grain size of the α phase may be 20.0 μm or less. ( 4 )From (1) above to ( 3 In the titanium plate described in any one of the above items 1 to 4, the coefficient of variation of the grain size of the α phase, which is the value obtained by dividing the standard deviation of the grain size of the α phase by the average grain size of the α phase, may be 0.75 or less. [Effects of the Invention]

[0010] The present invention can provide a titanium sheet that is superior to conventional titanium sheets in plane strain stretch formability with TD as the principal strain. [Brief explanation of the drawings]

[0011] [Figure 1A] FIG. 1 is a diagram illustrating a notation method using Euler angles to express crystal orientation in three dimensions. [Figure 1B] FIG. 1 is a diagram illustrating a notation method using Euler angles to express crystal orientation in three dimensions. [Figure 1C] FIG. 1 is a diagram illustrating a notation method using Euler angles to express crystal orientation in three dimensions. [Figure 1D] FIG. 1 is a diagram illustrating a notation method using Euler angles to express crystal orientation in three dimensions. [Figure 2] FIG. 1 is a diagram showing the crystal orientation distribution function ODF of the titanium plate according to the present embodiment, expressed as contour lines in the space of Euler angles. [Figure 3] FIG. 1 is a diagram showing the crystal orientation distribution function ODF of a conventional titanium plate as contour lines in the space of Euler angles. DETAILED DESCRIPTION OF THE INVENTION

[0012] The present inventors have found that pure titanium produced by conventional manufacturing methods exhibits, in addition to the Split-TD crystal orientation, a small amount of T-texture, in which the c-axis is oriented parallel to the sheet width direction (TD). The inventors' investigations revealed that the presence of this T-texture deteriorates the plane strain stretch formability of titanium sheets with the TD as the principal strain. The inventors then conducted further investigations into methods for suppressing the occurrence of T-texture and discovered that by introducing strain during hot rolling of pure titanium and maintaining this strain during coiling and cooling of the titanium sheet, the occurrence of T-texture can be suppressed and the plane strain stretch formability of titanium sheets with the TD as the principal strain can be dramatically improved. The titanium sheet according to this embodiment will be described in detail below.

[0013] <Chemical composition of titanium plate> Examples of titanium plates according to the present invention include titanium (also called commercially pure titanium) specified as 3·7025, 3·7035, or 3·7055 according to DIN 17850, and types 1 and 2 specified in JIS H4600 (2012), corresponding to those specified in ASTM B265. The chemical composition of the titanium plate according to the present embodiment is specifically described below. The unit "%" for the content of elements means % by mass.

[0014] O: 0.02% or more, 0.15% or less O is an element that is always contained in titanium and improves the 0.2% proof stress. However, if the O content is too high, ductility decreases and plane strain stretch formability with TD as the principal strain deteriorates. To obtain the effect of improving the 0.2% proof stress by adding O, the lower limit of the O content is 0.02%, preferably 0.03%. Furthermore, from the viewpoint of formability, the upper limit of the O content is 0.15%, preferably 0.10%, more preferably 0.08%.

[0015] Fe: 0.02% or more and 0.20% or less Fe is an element that is always contained in titanium and has the effect of improving 0.2% proof stress. However, if the Fe content is too high, β phase precipitates during final annealing, adversely affecting formability. To obtain the effect of improving 0.2% proof stress, the lower limit of the Fe content is 0.02%, preferably 0.03%. On the other hand, from the viewpoint of formability, the upper limit of the Fe content is 0.20%, preferably 0.10%, more preferably 0.08%.

[0016] N: 0 to 0.0800% N does not necessarily have to be contained in the titanium plate. Furthermore, N reduces the workability of the titanium plate. Therefore, the lower limit of the N content is 0%. The upper limit of the N content is 0.080%. However, taking into account 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. The N content may also be 0.0700% or less, 0.0600% or less, or 0.0500% or less.

[0017] C: 0 to 0.1000% C may not be contained in the titanium plate. Therefore, the lower limit of the C content is 0%. The upper limit of the C content is 0.1000%. However, taking into consideration 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. The C content may also be 0.0800% or less, 0.0500% or less, or 0.0300% or less.

[0018] H: 0 to 0.0130% The titanium plate does not necessarily contain H. Furthermore, H causes embrittlement of the titanium plate. Therefore, the lower limit of the H content is 0%. The upper limit of the H content is 0.0130%. However, taking into consideration 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. The H content may also be 0.0100% or less, 0.0080% or less, 0.0060% or less, 0.0040% or less, or 0.0030% or less.

[0019] The remainder of the chemical composition of the titanium plate according to this embodiment may be Ti and impurities. Specific examples of impurities include Cl, Na, Mg, Si, and Ca that are mixed in during the refining process, and Al, Zr, Sn, Mo, Nb, Ta, and V that are mixed in from scrap. When these impurities are contained, their content is, for example, 0.1% or less each, and a total content of 0.5% or less is not a problem.

[0020] <Crystal structure of titanium plate> The titanium plate according to this embodiment has the above-described chemical composition and is therefore industrially referred to as pure titanium. In pure titanium, the α-phase (hcp) is the predominant crystalline structure due to its chemical composition. A crystalline structure in which the α-phase is the predominant phase means that the α-phase fraction of the entire evaluation surface is 95% or more in terms of area fraction. This fraction is preferably 97% or more, more preferably 99% or more. Furthermore, the crystalline structure other than the α-phase is the β-phase.

[0021] 《Description of texture using Euler angles》 In the titanium plate according to this embodiment, the crystal orientation in the texture is expressed three-dimensionally using a notation method using Euler angles. The notation method using Euler angles (Bunge's notation method) will be described below.

[0022] As a premise for expressing crystal orientation using Euler angles, first, three mutually orthogonal coordinate axes, RD, TD, and ND, are assumed 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 direction normal to the plate surface. RD and TD can be identified based on, for example, the extension direction of the roll marks formed on the surface of the titanium plate, the dimensions of the titanium plate, or the extension direction of the crystals in the titanium plate.

[0023] Next, assume three mutually orthogonal coordinate axes, the X-axis, the Y-axis, and the Z-axis, as a crystal coordinate system (in the case of titanium α-phase, a coordinate system based on the direction of the hcp structure). The Z-axis in the crystal coordinate system is the

[0001] direction when expressed in Miller indices. The X-axis in the crystal coordinate system may be taken as the [10-10] direction (normal direction of the cylindrical surface) or the [1-210] direction. In the titanium plate according to this embodiment, the X-axis is taken as the [1-210] direction. In this case, the Y-axis is taken as the [10-10] direction (normal direction of the cylindrical surface). (A) In the notation method using Euler angles, the state in which the sample coordinate system and the crystal coordinate system coincide (X axis coincides with RD, Y axis coincides with TD, and Z axis coincides with ND) is used as the basis, as shown in Figure 1A. (B) Assume that the reference crystal is rotated by φ1° around the Z axis as shown in Figure 1B. The crystal coordinate system of the rotated crystal is denoted as (X', Y', Z) in Figure 1B. (C) Next, as shown in Figure 1C, assume that the crystal is rotated by Φ° around the X (X') axis after the φ1° rotation. The axes of the crystal coordinate system of the rotated crystal are denoted as (X', Y", Z') in Figure 1C. (D) Finally, as shown in Figure 1D, assume that the crystal is rotated by φ2° around the Z (Z') axis after the φ1° and Φ° rotations. The axes of the crystal coordinate system of the rotated crystal are denoted as (X", Y''', Z') in Figure 1D.

[0024] In the Euler angle notation method, these three angles, φ1°, Φ°, and φ2°, are used to express the crystal orientation (such as the direction of the c-axis) of any crystal grain. That is, the crystal orientation of the crystal shown in Figure 1A is expressed as (0,0,0) using Euler angles; the crystal orientation of the crystal shown in Figure 1B is expressed as (φ1,0,0) using Euler angles; the crystal orientation of the crystal shown in Figure 1C is expressed as (φ1,Φ,0) using Euler angles; and the crystal orientation of the crystal shown in Figure 1D is expressed as (φ1,Φ,φ2) using Euler angles. The α-phase, which is the main phase of titanium plate, has a hexagonal structure, so it can be expressed as φ1 (0-90°), Φ (0-90°), and φ2 (0-60°).

[0025] In the Miller indices shown in Figures 1A to 1D, values ​​with an overline are negative values. However, in the specification, a minus sign is used instead of an overline to indicate a negative value in the Miller indices. Therefore, some of the crystal orientations in the drawings are rewritten in the specification as follows:

[0026] [Table 1]

[0027] The crystal orientation distribution of a polycrystalline body is expressed 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 g, the ODF can be expressed as f(g). The higher the value of f(g), the more crystals there are that are oriented in the crystal orientation g.

[0028] The texture common in titanium sheets manufactured using conventional methods, in which the c-axis is tilted approximately 35° to the TD as the preferred orientation (Split-TD), is expressed below using Euler angles (φ1, Φ, φ2). To represent the three-dimensional crystal orientation distribution function on a two-dimensional paper, f(g) at a specific φ2 is expressed as contour lines in a space with the horizontal axis being φ1 (0–90°) and the vertical axis being Φ (0–90°). In titanium, typical orientations are represented by φ2 = 0° and 30° (Figures 2 and 3). As is clear from Figures 2 and 3, there are clearly peaks of high f(g) values ​​near Φ = 35° and φ1 = 0° at all φ2 values. In all of these crystal orientations, the c-axis of the hcp structure is tilted 35° from ND to TD, with [1-210] pointing to RD when φ2 = 0° and [0-110] pointing to RD when φ2 = 30°. Among these orientations, the crystal orientations indicated by the main orientations (0°, 35°, 0°) and (0°, 35°, 30°) are extremely suitable for plane strain stretch forming with TD as the principal strain, and press processing that takes advantage of this feature is carried out.

[0029] Figure 3 shows a pure titanium sheet manufactured using a conventional manufacturing method. In Figure 3, crystal orientation is observed at φ1 = 0° and from φ = 35 to 90°. Of these, φ = 90° indicates a crystal orientation in which the hcp c-axis is parallel to TD. These crystal orientations (T-texture) represented by (0°, 90°, 0°) and (0°, 90°, 30°) deteriorate plane strain stretch formability with TD as the principal strain. With conventional manufacturing methods, it is difficult to reduce the crystal orientations represented by (0°, 90°, 0°) and (0°, 90°, 30°), and plane strain stretch formability with TD as the principal strain is sometimes insufficient.

[0030] <<Ratio of T-texture>> In the titanium plate according to this embodiment, the abundance ratio of T-texture in the titanium plate is defined as follows. Specifically, when the crystal orientation of the α-phase is expressed by the Euler angle g = (φ1, Φ, φ2), the area ratio of the α-phase, whose crystal orientation has an absolute misorientation value of within 30° from (0°, 90°, 0°), is defined as X1, and the area ratio of the α-phase, whose crystal orientation has an absolute misorientation value of within 30° from (0°, 90°, 30°), is defined as X2. Furthermore, the area ratio of the α-phase, whose crystal orientation has an absolute misorientation value of within 30° from both (0°, 90°, 0°) and (0°, 90°, 30°), is defined as X1&2. X1&2 is the area ratio of the α-phase included in both X1 and X2. The abundance ratio of T-texture is then calculated as X1 + X2 - X1&2.

[0031] The crystal orientations (T-texture) represented by (0°, 90°, 0°) and (0°, 90°, 30°) deteriorate plane strain stretch formability with TD as the principal strain. Therefore, the smaller the area ratio of crystals with these crystal orientations, the better. To ensure sufficient formability, in the titanium plate according to this embodiment, the upper limit of the sum of the two (X1 + X2 - X1&2) is 0.075, and preferably 0.070. Figure 2 shows f(g) at φ2 = 0° and 30° for the titanium plate according to the present embodiment, and Figure 3 shows f(g) at φ2 = 0° and 30° for a conventional titanium plate, expressed as contour lines in a space with a horizontal axis of φ1 (0 to 90°) and a vertical axis of Φ (0 to 90°). (X1 + X2 - X1&2), which indicates the abundance ratio (area fraction) of crystals having a crystal orientation (T-texture) whose absolute value of the misorientation is within 30° around (0°, 90°, 0°) and (0°, 90°, 30°), was 0.082 in the case of the conventional example shown in Figure 3, but is reduced to 0.062 in Figure 2 for the titanium plate according to the present embodiment.

[0032] <Split-TD Existence Ratio> The abundance ratio of Split-TD in a titanium plate is defined as follows: X3 is the area ratio of the α phase, whose crystal orientation has an absolute misorientation value of within 15° from (0°, 35°, 0°), and X4 is the area ratio of the α phase, whose crystal orientation has an absolute misorientation value of within 15° from (0°, 35°, 30°). Furthermore, X3&4 is the area ratio of the α phase, whose crystal orientation has an absolute misorientation value of within 15° from both (0°, 35°, 0°) and (0°, 35°, 30°). X3&4 is the area ratio of the α phase included in both X3 and X4. The abundance ratio of Split-TD is defined as X3 + X4 - X3&4.

[0033] The crystal orientations (Split-TD) indicated by (0°, 35°, 0°) and (0°, 35°, 30°) are extremely excellent in plane strain stretch formability with TD as the principal strain, so the larger the area fraction of the crystals with these crystal orientations, the better. Therefore, taking into account the area fractions (X1 + X2 - X1&2) of the aforementioned (0°, 90°, 0°) and (0°, 90°, 30°), the larger the (X3 + X4 - X3&4) / (X1 + X2 - X1&2) ratio, the better. In the titanium plate according to this embodiment, in order to obtain sufficient formability, the lower limit of (X3 + X4 - X3&4) / (X1 + X2 - X1&2) is preferably 5.0, more preferably 5.5.

[0034] <Evaluation method for area ratios X1 to X4, X1&2, and X3&4 of α-phase crystals with specified crystal orientation> The area fractions X1 to X4 of the crystals with the above crystal orientations can be measured as follows. A surface perpendicular to the width direction of the titanium plate (hereinafter referred to as the "L-section") was polished to create the measurement surface. The EBSD pattern was measured using a scanning electron microscope (SEM) with a step size of 10 μm, scanning an electron beam across a field of view of the entire plate thickness × 15 mm on this surface. The EBSD pattern was then measured using the electron beam backscatter diffraction (EBSD) method. The data was then used to calculate the respective area fractions using TSL Solutions' OIM Analysis version 7.31 software. For example, by specifying a close-packed hexagonal crystal structure and specifying "simultaneously evaluate the area fraction of crystals with a crystal orientation with an absolute misorientation value of within 30° around (0°, 90°, 0°) and the area fraction of crystals with a crystal orientation with an absolute misorientation value of within 30° around (0°, 90°, 30°)," the evaluation result X1 + X2 - X1&2 can be obtained. X3 + X4 - X3&4 can also be obtained using a similar procedure. Only analytical data with a CI (Confidence of Index) value of 0.1 or higher was used for the analysis. In addition, taking into account the symmetry of rolling deformation, calculations were performed assuming that crystal orientations that are symmetrical with respect to the thickness direction, rolling direction, and width direction are the same. The measurement conditions were as follows: Heating voltage: 15kV ·Current amount: approx. 35nA ·Magnification: 200x

[0035] Average grain size of the α phase If the average grain size of the α phase is too large, wrinkles may occur during press forming. Furthermore, the smaller the average grain size of the α phase, the greater the strength. Therefore, the average 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 grain size of the α phase is less than 2 μm, unrecrystallized grains may remain, which may adversely affect formability. Therefore, the average grain size of the α phase is preferably 2.0 μm or more.

[0036] <<Fluctuation in α-phase grain size>> The coefficient of variation of the α-phase grain size is defined as the standard deviation of the α-phase grain size (circle equivalent diameter) divided by the average value of the α-phase grain size. If the coefficient of variation of the α-phase grain size is too large, the distribution of grain size becomes non-uniform, and deformation may concentrate on specific grains, resulting in poor formability. Therefore, the coefficient of variation of the α-phase grain size is preferably 0.75 or less, and more preferably 0.70 or less.

[0037] <Method for evaluating the average grain size and coefficient of variation of the α phase> The average grain size of the α phase was measured at any cross section of the titanium plate by observing a plane perpendicular to the plate width direction (L-section). The plane perpendicular to the plate width direction (L-section) was polished to create the measurement surface. The EBSD pattern was measured using a scanning electron microscope (SEM) with a scanning electron beam at a step size of 0.5 μm over a field of view of 1000 μm x the total plate thickness. The average grain size of the α phase was calculated using TSL Solutions' OIM Analysis version 7.31 software. Boundaries with a misorientation of 15° or more were considered grain boundaries, and the areas surrounded by these grain boundaries were considered grains. Note that there were approximately 1000 or more grains in this field of view. The average grain size was evaluated as the arithmetic mean of the average equivalent circle diameter. The coefficient of variation was calculated by dividing the standard deviation of the α phase grain size (equivalent circle diameter) by the average grain size. The measurement conditions for 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.

[0038] When the α-phase grain size is relatively coarse, grains with Split-TD tend to grow preferentially during the grain growth process after recrystallization, and the concentration of grains with T-texture tends to decrease. However, when the α-phase grain size is coarse, wrinkles occur during forming, as described above. On the other hand, conventional manufacturing methods have had the problem of increasing T-texture when the α-phase grain size becomes fine. In contrast, the titanium sheet according to the present embodiment has almost no T-texture, even when the α-phase grain size is relatively fine. Therefore, the inventors have realized, for the first time, a titanium sheet not found in the prior art, which combines the properties of wrinkle resistance due to the fine grains of the α-phase and excellent plane strain stretch formability with TD as the principal strain. When the average grain size of the α-phase is 70.0 μm or less, T-texture is present in conventional titanium sheets, but the titanium sheet according to the present embodiment has almost no T-texture, allowing it to fully demonstrate its effects.

[0039] <<Titanium plate manufacturing method>> The titanium sheet according to this embodiment is characterized by a texture that is almost free of T-texture and has excellent plane strain stretch formability with TD as the principal strain. While the manufacturing method is not particularly limited, for example, the texture can be achieved by controlling hot rolling to retain specific dislocations and suppressing T-texture formation, and then increasing the cold rolling rate to increase the concentration of the preferred orientation, Split-TD, and reduce the concentration of T-texture, using the manufacturing method described below. As described above, by controlling the hot-rolling finish temperature and the subsequent cooling rate of pure titanium and performing cold rolling at a large reduction, it is possible to develop a texture that is almost completely free of T-texture, which is thought to be the main cause of anisotropy, even if the grains are fine after final annealing.This makes it possible to produce titanium sheet that exhibits excellent plane strain stretch formability with TD as the principal strain and that does not develop wrinkles during forming.

[0040] An example of a method for manufacturing a titanium plate according to this embodiment will be described below. The process flow involves performing a melting step, a blooming step, a hot rolling step, a cold rolling step, and a final annealing step in this order.

[0041] "Dissolution process" Titanium raw material produced to a predetermined purity is melted by a conventionally known method to produce a predetermined ingot. Specifically, vacuum arc melting (VAR) or electron beam melting (EB) can be applied.

[0042] "Bulking process" The titanium slab is processed into a slab shape by conventional blooming or forging. The titanium slab obtained through the blooming process may be subjected to cutting or cleaning treatment by known methods, if necessary. This step may be omitted if necessary.

[0043] "Hot rolling (heating and rolling process)" For the titanium sheet according to this embodiment, it is important to control the reduction rate during hot rolling, the finishing temperature during hot rolling, the coiling temperature after hot rolling, and the average cooling rate from the finishing temperature to 300°C.

[0044] Heating before hot rolling can be performed using a conventional method, for example, by heating the slab to 700 to 1000°C. However, from the perspective of accurately adjusting the hot rolling end temperature, the heating temperature is preferably below the β transformation temperature. The "β transformation 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 single-phase region. The β transformation 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 purpose, Thermo-Calc, an integrated thermodynamic calculation system from Thermo-Calc Software AB, and a specified database (TI3) can be used.

[0045] [Hot rolling] "Average reduction rate per pass in finish rolling" 10% or more "Interpass time in finish rolling" 2.0 seconds or less In the finish rolling of the hot rolling, it is preferable to set the average reduction rate per pass to 10% or more and the time between passes to 2.0 seconds or less. This makes it possible to suppress the annihilation of dislocations due to recovery and introduce a large amount of dislocations into the titanium sheet. As will be described later, dislocations have the function of suppressing the formation of T-texture when the titanium sheet is cooled after hot rolling. Therefore, by performing the finish rolling under these conditions, a texture with less T-texture can be developed. "Hot rolling end temperature" 600℃ to 750℃ Titanium has five types of slip systems that function as plastic deformation mechanisms, and the types that become active depend on the temperature. It has been found that if dislocations generated by the activity of slip systems active at 600°C or higher remain, the formation of T-texture can be suppressed during final annealing after cold rolling. The hot rolling end temperature is controlled so that these dislocations remain.

[0046] If the hot rolling end temperature is less than 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 strain stretch formability with TD as the principal strain. 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 and recrystallization occur during processing, the above-mentioned dislocations disappear, and a large amount of T-texture is formed during final annealing, resulting in poor plane strain stretch formability with TD as the principal strain. Therefore, the upper limit of the hot rolling end temperature is 750°C, preferably 730°C, and more preferably 720°C. The hot rolling end temperature is measured with a radiation thermometer. The coiling temperature and other temperatures described below are also values ​​measured with a radiation thermometer.

[0047] "Winding temperature" 450℃ or less "Average cooling rate from hot rolling end temperature to 300°C" 5.0°C / s or more By increasing the cooling rate after hot rolling, recovery and recrystallization during cooling are suppressed, the dislocations remain, and the formation of T-texture during final annealing is suppressed. When a titanium sheet is coiled after hot rolling, the cooling rate of the titanium sheet decreases. If the coiling temperature exceeds 450°C, recovery and recrystallization occur in the coiled titanium sheet, the dislocations disappear, and a large amount of T-texture is formed during final annealing, which deteriorates the plane strain stretch formability with TD as the principal strain. For this reason, the coiling temperature is 450°C or less. Furthermore, if the cooling rate is less than 5°C / s, recovery and recrystallization occur during cooling, the dislocations disappear, and a large amount of T-texture is formed during final annealing, resulting in poor plane strain stretch formability with TD as the principal strain. 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 upper limit, but it may be 100°C / s depending on the equipment capacity, etc.

[0048] "Hot-rolled sheet annealing and cold-rolled intermediate annealing" must not be performed If hot-rolled sheet annealing and cold-rolled intermediate annealing are performed, the above dislocations disappear, and a large amount of T-texture is formed during final annealing, resulting in poor plane strain stretch formability with TD as the principal strain. Therefore, hot-rolled sheet annealing and cold-rolled intermediate annealing should not be performed after hot rolling until final annealing. Note that hot-rolled sheet annealing refers to the annealing performed on titanium sheets after coiling and before the start of cold rolling, while cold-rolled intermediate annealing refers to the annealing performed on titanium sheets between passes when cold rolling is performed in multiple passes.

[0049] The obtained hot-rolled sheet may be subjected to a known method such as pickling or cutting to remove oxide scale or the like, or may be subjected to a cleaning treatment.

[0050] [Cold rolling] "Total cold rolling rate" 78% or more If the total cold rolling reduction is less than 78%, the concentration in Split-TD decreases and the concentration in T-texture increases. Therefore, the total cold rolling reduction is set to 78% or more. A more preferable total cold rolling reduction is 81% or more. The upper limit of the total cold rolling reduction is preferably 95% from the viewpoint of edge cracking. The thickness of the titanium sheet that has undergone the hot rolling and cold rolling under the above conditions is, for example, within the range of 0.3 mm to 1.0 mm.

[0051] "Final annealing" is an annealing temperature of 500°C or more and 750°C or less, and the temperature-time combination satisfies formula (1). 18500≦P=(T+273.15)×(Log(t)+20) Equation (1) In the above formula (1), P is a function called the Larson-Miller parameter, T is the annealing temperature (°C), and t is the annealing time (s).

[0052] If the annealing temperature is less than 500°C, recrystallization may not be completed, and plane strain stretch formability with TD as the principal strain may be deteriorated. Therefore, the lower limit of the annealing temperature is 500°C, preferably 550°C. On the other hand, if the annealing temperature is higher than 750°C, the grain size may change significantly with a small change in time, and the grain size may become too coarse, which may cause wrinkles during forming. Therefore, the upper limit of the annealing temperature is 750°C, more preferably 700°C. Furthermore, if the Larson-Miller parameter P is less than 18,500, recrystallization may not be completed, and plane strain stretch formability with TD as the principal strain may be deteriorated. Therefore, the lower limit of P is 18,500, preferably 19,000.

[0053] On the other hand, the smaller the value of the Larson-Miller parameter P, the smaller the average crystal grain size of the titanium plate can be. If the Larson-Miller parameter P is 23,000 or less, the average crystal grain size will be 70.0 μm or less, and the occurrence of wrinkles during molding can be suppressed. If the Larson-Miller parameter P is 21,500 or less, the average crystal grain size will be 25.0 μm or less, and the occurrence of wrinkles during molding can be reliably prevented. Therefore, it is preferable to set the upper limit of P to 21,500.

[0054] (Manufacturing method in which the coefficient of variation of the α phase grain size is 0.75 or less) If the cold rolling rate is small, the accumulation of strain in each grain in the cold rolled sheet may become uneven, which may result in an uneven distribution of grain size after annealing. Therefore, the cold rolling rate should be high, preferably 80% or more. [Example]

[0055] First, titanium ingots were produced by vacuum arc remelting (VAR) to serve as the raw material for the titanium thin plates shown in Table 2. Slabs measuring 150 mm thick, 800 mm wide, and 5000 mm long were then produced by blooming or forging. In Table 2, "Bal." represents the balance. Also, in Table 2, the contents of impurity elements are omitted. The chemical composition of the slabs was measured as follows. The Fe content was measured by ICP optical emission spectroscopy using a sequential ICP optical emission spectrometer. The O and N contents were measured by thermal conductivity and infrared absorption spectroscopy using an oxygen and nitrogen simultaneous analyzer. The C content was measured by infrared absorption spectroscopy using a carbon and sulfur analyzer. H was measured by thermal conductivity spectroscopy using a hydrogen analyzer. The chemical composition of each hot-rolled sheet was identical to that of the titanium slabs listed in Table 2. Furthermore, for titanium materials A to K listed in Table 2, phase diagrams of titanium alloys were obtained using 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 temperature, Tβ, was calculated.

[0056] Next, these slabs were hot rolled under the conditions shown in Table 3 or Table 5. The titanium sheets (hot-rolled sheets) after hot rolling were subjected to hot-rolled sheet annealing as necessary, followed by shot blasting and pickling. In the production of all invention examples and comparative examples, the average reduction per pass in the finish hot rolling was set to 10% or more, and the time between passes was set to 2.0 seconds or less. The term "hot-rolling reduction" listed in Tables 3 and 5 refers to the total reduction in hot rolling. The term "hot-rolled sheet thickness" listed in Tables 3 and 5 refers to the thickness of the hot-rolled sheet after hot rolling is completed. The hot-rolled sheets were then cold-rolled and annealed under the conditions shown in Table 4 or Table 6 to produce cold-rolled sheets with a thickness of 0.8 mm. In the tables, "Tβ" is the β transformation point, and the "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).

[0057] The evaluation method for the crystal area ratios X1+X2-X1&2 and X3+X4-X3&4, which are the crystal orientations in titanium plates, was as follows. First, to measure the area ratios X1+X2-X1&2 and X3+X4-X3&4, a surface perpendicular to the width direction of the titanium plate (hereinafter also referred to as the "L cross section") was polished to create a measurement surface. A 15 mm wide observation field was set on this measurement surface, including the entire thickness direction and along the plate surface. In this observation field, an electron backscatter diffraction (EBSD) pattern was measured with a step size of 10 μm using a scanning electron microscope (SEM) while scanning with an electron beam. The area ratios were calculated using the OIM Analysis ver. 7.31 software from TSL Solutions. Only analysis data with a CI (Confidence of Index) value of 0.1 or higher was used for the analysis. In addition, taking into account the symmetry of rolling deformation, calculations were performed assuming that crystal orientations that are symmetrical with respect to the thickness direction, rolling direction, and width direction are the same. The EBSD pattern measurement conditions were as follows: Heating voltage: 15kV ·Current amount: approx. 35nA ·Magnification: 200x

[0058] The average grain size and coefficient of variation of the α phase were evaluated as follows. First, to measure the grain size of the α phase, a surface perpendicular to the width direction of the titanium plate (L-section) was polished to create a measurement surface. A 1000 μm-wide observation field was set on this measurement surface, encompassing the entire thickness direction and along the plate surface. Electron backscatter diffraction (EBSD) patterns were measured in this observation field using a scanning electron microscope (SEM) with a step size of 0.5 μm. Using the data, TSL Solutions' OIM Analysis ver. 7.31 software was used to recognize boundaries with a misorientation of 15° or more as grain boundaries, and the regions surrounded by these grain boundaries were considered as grains. The grain size of the α phase was calculated as the equivalent circle diameter. The average grain size was the arithmetic mean of the equivalent circle diameters. The coefficient of variation was calculated by dividing the standard deviation of the α phase grain size (equivalent circle diameter) by the average grain size.

[0059] The formability of titanium sheets was evaluated by measuring the plane strain overhang height in the sheet width direction. Formability evaluation was performed using a die with a hole diameter of 44 mm, a shoulder radius of 6 mm, and a 70 mm diameter bead, and a 40 mm diameter ball-nosed punch, with a blank holder force of 7 ton and a punch rise rate of 20 mm / min. Blanks with sides parallel to the rolling direction of 60 mm and sides parallel to the sheet width direction of 90 mm were cut from the cold-rolled sheets. Polysheet (Naflon Tape 9001 (0.05 t) from Nichias Corporation) and high-viscosity oil (Nippon Engineering Oil Co., Ltd. Engineering Oil #660) were used for lubrication. The displacement at the maximum load, as determined from the load-displacement curve obtained in the test, was considered to be the overhang height. Titanium sheets with an extension height of 24.0 mm or more were evaluated as titanium sheets with good formability, that is, titanium sheets with superior plane strain extension formability with TD as the principal strain compared to conventional titanium sheets.

[0060] Wrinkles during forming of titanium plates were evaluated based on the roughness after tensile testing. JIS13B tensile test specimens were taken from the titanium plates so that the rolling direction was the tensile direction. These specimens were subjected to a tensile test at a strain rate of 30% / min up to a nominal strain of 20%. After the tensile test, the parallel surface of the specimen was subjected to roughness measurement using a laser microscope. Using a Keyence Corporation laser microscope, measurements were taken at a magnification of 500x, covering a field of view of 213 μm × 284 μm, with measurement mode: surface profile, measurement area: surface, measurement quality: high resolution, and a Z measurement pitch of 0.01 μm. The data was analyzed using VK Analyzer software version 2.5.0.1. After automatic correction for sample surface inclination, 11 lines were drawn parallel to the rolling direction, dividing the sheet width into 12 equal parts, in accordance with JIS B0601:2001, with cutoff values ​​of λc = 0.8 mm and λs = 2.5 μm. The average arithmetic mean roughness (Ra) in the rolling direction for each field was calculated, and the average value for three fields was used for evaluation. An average Ra of 0.25 μm or less was considered excellent, and an average Ra of 0.65 μm or less was considered good. However, even if the wrinkling was greater than 0.65 μm, a good result in the overhang height evaluation was considered to represent a titanium sheet with superior plane strain formability with TD as the principal strain. Note that the same surface roughness results can be obtained for any parallel portion of the test specimen. Measurements were taken at the center of the parallel portion.

[0061] [Table 2]

[0062] [Table 3]

[0063] [Table 4]

[0064] [Table 5]

[0065] [Table 6]

[0066] Examples 1 to 36 in Tables 3 and 4 are examples of the present invention. 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. In Tables 2 to 6, values ​​outside the range of the present invention or values ​​or items outside the preferred range of the production method are underlined.

[0067] In Examples 1 to 36, the chemical compositions and X1 + X2 - X1&2 were within the ranges of the present invention. These examples were evaluated as titanium sheets with superior plane strain stretch formability with TD as the principal strain compared to conventional examples.

[0068] In Comparative Examples 1 to 18, 24, and 25 in Tables 5 and 6, X1 + X2 - X1&2 was outside the range of the present invention. This is presumably because the manufacturing methods of Comparative Examples 1 to 18, 24, and 25 were outside the preferred range of the present invention. In Comparative Examples 1 to 18, and 25, the plane strain stretch formability with TD as the principal strain was inferior to that of the present invention. Note that Comparative Example 24 was not cold-rolled and had an extremely large plate thickness, so the stretch height was not evaluated. However, because X1 + X2 - X1&2 in Comparative Example 24 was outside the range of the present invention, it is presumed that, like the other Comparative Examples, Comparative Example 24 had poor plane strain formability. Specifically, in the production of Comparative Examples 1, 9, and 14, the hot rolling finish temperature was excessive. In the production of Comparative Examples 2, 10, and 15, the hot rolling finish temperature was insufficient. In the production of Comparative Examples 3, 11, and 16, the average cooling rate from the hot rolling finish temperature to 300°C was insufficient. In the production of Comparative Examples 4, 12, 14, and 18, hot-rolled sheet annealing was performed. In the production of Comparative Examples 5, 13, and 17, the final cold rolling rate was insufficient. In the production of Comparative Example 6, intermediate annealing was performed. In the production of Comparative Example 7, the annealing temperature in the final annealing was insufficient. In the production of Comparative Example 8, the Larson-Miller parameter P was insufficient. In the production of Comparative Example 24, cold rolling was omitted. In the production of Comparative Example 25, the winding temperature was excessive.

[0069] The chemical compositions of Comparative Examples 19 to 23 in Tables 5 and 6 were outside the range of the invention. Comparative Examples 19 to 23 also failed either or both of the wrinkle evaluation results and the overhang height.

Claims

1. In mass%, O: 0.02% to 0.15%, Fe: 0.02% to 0.20%, N: 0-0.0800%, C: 0 to 0.1000%, and H: 0 to 0.0130%, and the balance being Ti and impurities, The average grain size of the α phase is 2.0 μm or more and 70.0 μm or less, The crystal orientation of the α phase is defined as the Euler angle g = (φ １ , Φ, φ ２ ), where the area fraction of the α phase having a crystal orientation whose absolute value of misorientation from (0°, 90°, 0°) is within 30° is defined as X1, the area fraction of the α phase having a crystal orientation whose absolute value of misorientation from (0°, 90°, 30°) is within 30° is defined as X2, and the area fraction of the α phase having a crystal orientation whose absolute value of misorientation from both (0°, 90°, 0°) and (0°, 90°, 30°) is within 30° is defined as X1&2, A titanium plate in which X1 + X2 - X1&2 is 0.075 or less.

2. The area ratio of the α phase, which has a crystal orientation whose absolute value of the misorientation with respect to (0°, 35°, 0°) is within 15°, is defined as X3, The area ratio of the α phase, which has a crystal orientation whose absolute value of the orientation difference from (0°, 35°, 30°) is within 15°, is defined as X4, When the area ratio of the α phase having a crystal orientation whose absolute value of the misorientation from both (0°, 35°, 0°) and (0°, 35°, 30°) is within 15° is defined as X3&4, 2. The titanium plate according to claim 1, wherein (X3 + X4 - X3&4) / (X1 + X2 - X1&2) is 5.0 or more.

3. 3. The titanium plate according to claim 1, wherein the average grain size of the α phase is 20.0 μm or less.

4. 4. The titanium plate according to claim 1, wherein the coefficient of variation of the crystal grain size of the α phase, which is the standard deviation of the crystal grain size of the α phase divided by the average crystal grain size of the α phase, is 0.75 or less.

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