Titanium alloy sheet, titanium alloy tube, titanium alloy component, and hollow component

Abstract

Provided is a titanium alloy sheet excellent in high temperature strength and workability at room temperature. This titanium alloy sheet has a chemical composition containing, by mass%, more than 1.40% and 2.10% or less of Cu, 0.50-1.50% of Sn, 0.10-0.60% of Si, 0.10-1.00% of Nb, and 0.08% or less of O with the balance being Ti and impurities, and the number density of the precipitated phase is 0.15 / μm2 or more.

Description

Titanium alloy plates, titanium alloy tubes, titanium alloy parts, and hollow parts

[0001] The present invention relates to titanium alloy plates, titanium alloy tubes, titanium alloy parts, and hollow parts.

[0002] Exhaust system parts are required to have high-temperature strength because they are used in an environment where they come into contact with high-temperature exhaust gases. In addition, exhaust system parts often have complex shapes, so they also require workability at room temperature. In recent years, titanium alloy materials with excellent high-temperature strength and workability at room temperature have been developed.

[0003] For example, Patent Document 1 discloses a titanium alloy sheet having a Cu content of 0.5 to 1.5%. Patent Document 2 discloses a titanium alloy material having a Cu content of 0.7 to 1.4% and in which the area fraction and average grain size of the α phase and the area fraction and average grain size of intermetallic compounds are controlled. Patent Document 3 discloses a titanium alloy material having a Cu content of 0.7 to 1.5% and in which the average grain size of the α phase and the number density of the secondary phase are controlled.

[0004] International Publication No. WO 2011 / 081077 International Publication No. WO 2019 / 155553 International Publication No. WO 2022 / 157844

[0005] As exhaust gas temperatures tend to rise, titanium alloy materials for exhaust system components are desired to have higher high-temperature strength. Complex and highly precise shapes are also desired. However, improving high-temperature strength generally increases room-temperature strength, making it difficult to ensure room-temperature workability. From the perspective of achieving both high-temperature strength and room-temperature workability, the inventions of Patent Documents 1 to 3 leave room for improvement.

[0006] The present invention aims to solve the above-mentioned problems and to provide a titanium alloy plate, a titanium alloy pipe, a titanium alloy part, and a hollow part that have excellent high-temperature strength and workability at room temperature.

[0007] The present invention relates to the following titanium alloy plate, titanium alloy pipe, titanium alloy part, and hollow part.

[0008] (1) The chemical composition is, in mass%, Cu: more than 1.40% and not more than 2.10%, Sn: 0.50 to 1.50%, Si: 0.10 to 0.60%, Nb: 0.10 to 1.00%, O: 0.08% or less, the balance: Ti and impurities, and the number density of the precipitate phase is 0.15 / μm ２ That's it, titanium alloy plate.

[0009] (2) The titanium alloy plate according to (1) above, wherein the average crystal grain size of the α phase is 5.0 μm or more.

[0010] (3) A steel sheet having a base material and a welded portion, wherein the base material has a chemical composition, in mass%, of Cu: more than 1.40% and not more than 2.10%, Sn: 0.50 to 1.50%, Si: 0.10 to 0.60%, Nb: 0.10 to 1.00%, O: 0.08% or less, and the balance: Ti and impurities, and the number density of the precipitate phase in the base material is 0.15 / μm ２ That's it, titanium alloy tube.

[0011] (4) The titanium alloy pipe according to (3) above, wherein the average grain size of the α phase in the base material is 5.0 μm or more.

[0012] (5) A titanium alloy part using the titanium alloy plate described in (1) or (2) above.

[0013] (6) A hollow part using the titanium alloy tube described in (3) or (4) above.

[0014] According to the present invention, it is possible to obtain titanium alloy plates, titanium alloy pipes, titanium alloy parts, and hollow parts that are excellent in high-temperature strength and workability at room temperature. Furthermore, when these parts are kept in a high-temperature environment for a long period of time, the high-temperature strength can be further improved.

[0015] Fig. 1 is a diagram for explaining a method for taking a test piece from a titanium alloy pipe, and Fig. 2 is a schematic diagram for explaining a method for counting crystal grains.

[0016] In order to solve the above-mentioned problems, the present inventors have conducted detailed investigations into high-temperature strength and room-temperature workability, and have come to the following findings.

[0017] Cu contained in a titanium alloy material improves the strength and high-temperature strength of the titanium alloy material by dissolving in the α phase. In order to further improve the high-temperature strength of a titanium alloy material, it is effective to increase the Cu content and make greater use of solid-solution strengthening by Cu. However, when the Cu content is high, the deterioration of workability due to solid-solution strengthening also becomes significant.

[0018] As disclosed by the present inventors in Patent Document 2, annealing is performed to convert a part of Cu into Ti. ２ By precipitating Cu or the like as a precipitate phase, workability at room temperature is improved, and attempts have been made to dissolve the precipitate phase in solid solution to increase high-temperature strength when used in a high-temperature environment.

[0019] However, as a result of further investigations by the present inventors, it was found that, particularly when the Cu content is increased for the purpose of further improving the high-temperature strength, the effect of improving the workability at room temperature is not sufficiently obtained, and further increase in the Cu content does not improve the high-temperature strength.

[0020] Therefore, the present inventors further improved the manufacturing conditions and investigated the precipitation state of the precipitate phase that is more suitable for workability at room temperature and high-temperature strength.

[0021] As a result, Ti ２ It has been discovered that controlling the number density of the Cu-containing precipitate phase and maintaining this number density at a certain level or higher is important for improving workability at room temperature and high-temperature strength. This further suppresses solid-solution strengthening at room temperature compared to conventional techniques, ensuring workability. Furthermore, at high temperatures, Cu is supplied from the precipitate phase, promoting solid-solution strengthening and further improving high-temperature strength.

[0022] Further investigation by the present inventors has revealed that a titanium alloy material having a chemical composition within a predetermined range and a precipitate phase with a certain or higher number density exhibits improved high-temperature strength when used for a long period of time in a high-temperature environment. It is believed that the high number density of the precipitate phase allows Cu to continue to be supplied from the precipitate phase to the α phase even during long-term use in a high-temperature environment, further promoting solid-solution strengthening. This reduces the risk of breakage of the titanium alloy material, even when used for a long period of time in a high-temperature environment.

[0023] They also found that in order to increase the number density of the precipitate phase, it is necessary to increase the heating temperature before hot rolling compared to conventional techniques and to perform cold-rolled sheet annealing under conditions of heating at a lower temperature for a longer period of time.

[0024] The present invention was made based on the above findings. Each of the features of the present invention will be described in detail below.

[0025] (A) Chemical Composition In the chemical composition of the titanium alloy plate according to the present invention and the chemical composition of the base material of the titanium alloy pipe according to the present invention, the reasons for limiting the content of each element are as follows. In the following description, "%" in the content means "mass %".

[0026] Cu: More than 1.40% and Not More than 2.10% Cu has a large solid solubility limit and is effective in improving high-temperature strength and room-temperature strength. In particular, when applied to exhaust system components, Cu redistributes from the precipitate phase to the α phase during high-temperature use, resulting in solid-solution strengthening of the α phase, further increasing high-temperature strength. If the Cu content is 1.40% or less, the above effects are not fully achieved, even if the contents of elements other than Cu are within the range of this embodiment. Furthermore, the number density of the precipitate phase becomes insufficient. On the other hand, if the Cu content exceeds 2.10%, workability at room temperature is impaired due to solid-solution strengthening. Furthermore, in high-temperature environments, the area fraction of the low-strength β phase increases, resulting in a decrease in high-temperature strength compared to when the Cu content is 1.4% or less. Therefore, the Cu content is set to be more than 1.40% and not more than 2.10%.

[0027] The Cu content is preferably 1.41% or more, 1.43% or more, 1.45% or more, or more than 1.50%. This further strengthens the steel during use at high temperatures, specifically, improves the tensile strength at 700°C after holding at 750°C for 100 hours. The Cu content is preferably 2.08% or less, more preferably 2.07% or less, 2.05% or less, 2.03% or less, or 2.00% or less.

[0028] Sn: 0.50 to 1.50% Sn is an element that has a large solid solubility limit and improves high-temperature strength. In addition, Sn is an element that dissolves more easily in the α phase than Cu, so Sn is preferentially distributed in the α phase and Ti ２ Cu is more likely to precipitate. If the Sn content is less than 0.50%, the above effect cannot be sufficiently obtained even if the contents of elements other than Sn are within the range of this embodiment. On the other hand, if the Sn content exceeds 1.50%, workability decreases due to solid solution strengthening. Therefore, the Sn content is set to 0.50 to 1.50%. The Sn content is preferably 0.60% or 0.70% or more, and more preferably 0.80% or more. Furthermore, the Sn content is preferably 1.40% or less, or 1.30% or less, and more preferably 1.20% or less.

[0029] Si: 0.10 to 0.60% Si is an element that improves high-temperature strength and oxidation resistance. If the Si content is less than 0.10%, the above effects are not sufficiently achieved, even if the contents of elements other than Si are within the ranges of this embodiment. On the other hand, if the Si content exceeds 0.60%, a large amount of silicide precipitates, reducing workability at room temperature, even if the contents of elements other than Si are within the ranges of this embodiment. Therefore, the Si content is set to 0.10 to 0.60%. The Si content is preferably 0.15% or more, and more preferably 0.20% or more. Furthermore, the Si content is preferably 0.55% or less, or 0.50% or less, and more preferably 0.45% or less.

[0030] Nb: 0.10 to 1.00% Nb is an element that improves oxidation resistance. To achieve this effect, the Nb content must be 0.10% or more. Since Nb is expensive, costs increase, so the Nb content is set to 1.00% or less. Therefore, the Nb content is set to 0.10 to 1.00%. The Nb content is preferably 0.20% or more, and more preferably 0.30% or more. Furthermore, the Nb content is preferably 0.90% or less, or 0.80% or less, and more preferably 0.70% or less.

[0031] O: 0.08% or less O is an element inevitably contained in titanium alloy materials, and improves strength at room temperature but reduces ductility. Since it hardly contributes to strength at high temperatures, a small content is preferable. Therefore, the O content is set to 0.08% or less. The O content is preferably set to 0.07% or less.

[0032] In the chemical composition of the titanium alloy plate according to the present invention and the chemical composition of the base material of the titanium alloy pipe according to the present invention, the balance is Ti and impurities. Here, "impurities" refer to components that are mixed in due to various factors in the raw materials and manufacturing process when the titanium alloy plate and titanium alloy pipe are industrially manufactured, and are acceptable within a range that does not adversely affect the present embodiment.

[0033] Impurities include, but are not limited to, Al, Fe, C, N, Ni, Mn, Cr, H, Cl, Na, Mg, Ca, B, Zr, Mo, Ta, V, W, Hf, and Ge. The content of each of these elements is preferably less than 0.1%, and the total content of these elements is preferably less than 0.5%. It is more preferable that the content of each of these elements is 0.05% or less, and the total content of these elements is less than 0.3%. It is more preferable that the Fe content is 0.08% or less, and the H content is preferably 150 ppm or less. It is preferable that the B content be minimized, and the B content is preferably 0.01% or less.

[0034] (B) Metallographic Structure The metallographic structure of the titanium alloy plate according to the present invention preferably consists of a precipitated phase in addition to the α-phase, which is the main phase. The metallographic structure of the base material of the titanium alloy pipe according to the present invention preferably consists of a precipitated phase in addition to the α-phase, which is the main phase. The precipitated phase is mainly Ti ２ Cu, but also contains β phase, silicide, etc. The area ratio of the precipitated phase is preferably 15% or less. The amount of β phase, silicide, etc. is Ti ２ Ti is less than Cu ２ The amount is such that the effect of Cu is not impaired. Therefore, the area ratio is calculated by including the β phase and silicide in the precipitate phase. The α phase is equiaxed.

[0035] <Number Density of Precipitate Phase> In the titanium alloy plate according to the present invention, the number density of the precipitate phase is 0.15 / μm ２ In addition, in the base material of the titanium alloy pipe according to the present invention, the number density of the precipitated phase is 0.15 / μm ２ That's all.

[0036] Even if the chemical composition according to the present invention is satisfied, the number density of the precipitated phase is 0.15 / μm ２ If the number density of the precipitate phase is less than 0.15 / μm, the amount of Cu dissolved in the base material is large, resulting in high strength, but workability at room temperature tends to decrease. As described above, by setting the number density of the precipitate phase to a certain amount or more, it is possible to achieve both high-temperature strength and workability at room temperature. In addition, it is possible to sufficiently improve high-temperature strength when used for a long period of time in a high-temperature environment. Therefore, the number density of the precipitate phase should be 0.15 / μm ２ The number density of the precipitate phase is 0.16 / μm ２ More preferably, 0.17 / μm ２ The upper limit of the number density of the precipitate phase is not particularly limited. However, if the number density of the precipitate phase is excessive, the workability at room temperature may be deteriorated. Therefore, the number density of the precipitate phase is preferably 1.00 / μm or less. ２ The number density of the precipitate phase is preferably 0.80 / μm or less. ２ More preferably, 0.60 / μm or less ２ Below, 0.40 / μm ２ or less, or 0.35 / μm ２ The following is even more preferred:

[0037] The number density of precipitated phases is measured using the following method. For titanium alloy plates and titanium alloy parts, samples are collected so that the central portion of the plate thickness in a cross section parallel to the plate thickness direction serves as the observation surface. The observation surface is then etched, and a region 213 μm in the plate thickness direction and 284 μm in the direction perpendicular to the plate thickness direction is observed under an optical microscope at 500x magnification. The etching is performed as follows: absorbent cotton is soaked in an etching solution (a mixture of nitric acid and hydrofluoric acid) and the observation surface is rubbed 20 times over a period of 20 to 60 seconds. If the metal structure cannot be fully confirmed when observed under an optical microscope, additional etching is performed. Excessive etching can result in the formation of pits on the observation surface. If pits with a diameter of 8 μm or more are observed, the sample is polished again, and the etching time is shortened as necessary. The etching solution contains 3% by mass of nitric acid and 5% by mass of hydrogen fluoride. The optical microscope photograph is taken at a resolution of 1440 pixels in the thickness direction and 1920 pixels in the direction perpendicular to the thickness direction.

[0038] In the optical microscope photographs obtained as described above, the α phase is observed as white (high brightness), while the grain boundaries of the α phase and the precipitate phase are observed as black (low brightness). Furthermore, the grain boundaries of the α phase are observed as linear, while the precipitate phase is observed as rounded. Based on these differences, the precipitate phase is identified and its number density is calculated. Specifically, the optical microscope photographs are analyzed using the image analysis software FIJI as follows. First, the image is binarized using Auto Threshold using Method: "Li," and the black and white are inverted so that the α phase appears black. Next, the overlapping precipitate phases are separated using Watershed, and then the Circularity is set to 0.5 to 1.0, and the number and area ratio of the precipitate phases are measured using Analyze Particles... Finally, the number of precipitate phases is divided by the area of ​​the observation field to calculate the number density of the precipitate phases. The area fraction of the α phase is calculated by subtracting the area fraction of the precipitate phase calculated as described above from 100%. In the analysis, the minimum size of the precipitate phase is 0.147 μm, i.e., 1 pixel, and a precipitate phase is measured when its maximum diameter exceeds 0.147 μm.

[0039] In the case of titanium alloy pipes and hollow parts, the above observations and measurements are performed at the center of the wall thickness of a cross section parallel to the longitudinal direction and wall thickness direction of the titanium alloy pipe in an area other than the weld metal and heat-affected zone. The area fraction of the α phase is calculated by subtracting the area fraction of the precipitate phase calculated as described above from 100%.

[0040] <Average grain size of the α phase> In the titanium alloy plate according to the present invention, the average grain size of the α phase is preferably 5.0 to 50.0 μm. Furthermore, in the base material of the titanium alloy pipe according to the present invention, the average grain size of the α phase is preferably 5.0 to 50.0 μm. It is believed that by setting the average grain size of the α phase to 5.0 μm or more, the workability described below can be maintained. Furthermore, by setting the average grain size of the α phase to 50.0 μm or less, non-uniform deformation during processing can be suppressed, resulting in the suppression of the occurrence of surface irregularities and a good appearance. Therefore, the average grain size of the α phase is preferably 5.0 to 50.0 μm. The average grain size of the α phase is more preferably 30.0 μm or less, more preferably 20.0 μm or less, and even more preferably 15.0 μm or less.

[0041] The average grain size of the α phase is measured by the following method. In the case of a titanium alloy plate, a sample is taken so that the center of the plate thickness in a cross section parallel to the plate thickness direction serves as the observation surface. The observation surface is then mirror-polished and etched with an etching solution (a mixture of nitric acid and hydrofluoric acid), and a 400 μm × 400 μm area is observed under an optical microscope at 200x magnification. The etching solution contains 3% by mass of nitric acid and 5% by mass of hydrogen fluoride.

[0042] Then, using the structural photograph, the average crystal grain size of the α phase is determined by the intercept method. Specifically, the observation area is divided into five equal parts in the thickness direction of the plate to divide it into five small regions, and a line segment of length Ln (400 μm) is drawn in the center of each small region in a direction perpendicular to the thickness direction of the plate, and the number Xn of crystal grains separated by the line segment is measured. Here, n is a code that identifies the line segment and is an integer from 1 to 5. Then, the crystal grain size Dn of each line segment is determined by formula (I). After that, the average crystal grain size D of the five line segments is determined by formula (II). Dn (μm) = Ln / Xn (I) D (μm) = (D １ +D ２ +D ３ +D ４ +D ５ ) / 5 ... (II)

[0043] Figure 2 is a schematic diagram for explaining how to count crystal grains. Figure 2 shows a part of an observation field with a length of 400 μm in the direction perpendicular to the plate thickness direction, and one of five line segments is shown by a dashed line. The numerical values ​​"0.5" and "1" mean that "0.5" and "1" are counted as "0.5" and "1" crystal grains, respectively.

[0044] As shown in Figure 2, when a line segment completely crosses a crystal grain, it is counted as one crystal grain. Furthermore, when the crystal grain is interrupted at the end of the observation area, and the start or end point of the line segment is included within the crystal grain, it is counted as 0.5 crystal grain. For titanium alloy pipes and hollow parts, the above observations and measurements are performed on the center of the wall thickness of a cross section parallel to the longitudinal and thickness directions of the titanium alloy pipe, in an area other than the weld metal and heat-affected zone.

[0045] (C) Properties <Workability> Although it depends on the part shape after forming, it is required that the titanium alloy plate can be formed and welded into a tubular shape. Furthermore, there are cases where the tube needs to be bent after that. Therefore, the titanium alloy plate and titanium alloy tube according to the present invention are required to have sufficient workability when forming parts. In the present invention, workability is evaluated by total elongation, and the titanium alloy plate and titanium alloy tube according to the present invention preferably have a total elongation of 25.0% or more. It is not necessary to set an upper limit for total elongation, but approximately 50.0% is a practical upper limit for industrial applications.

[0046] The total elongation is measured by the following method. First, in the case of titanium alloy plate, a No. 13B test piece specified in JIS Z 2241:2022 is taken so that the longitudinal direction of the test piece coincides with the width direction of the titanium alloy plate. Then, the total elongation (%) is measured by a room temperature tensile test. Here, the strain rate is 0.5% / min up to 2% strain, and 30% / min after the strain amount exceeds 2%.

[0047] In the case of titanium alloy parts, the above-mentioned No. 13B test specimens are taken from the parts, avoiding processed parts (hereinafter also referred to as "processed parts"), for example, parts that have been subjected to a large degree of processing such as pressing, and the tests are conducted in the same manner. However, when a No. 13B test specimen cannot be taken from a titanium alloy part due to its shape, the dimensions of the test specimen shall be reduced by the same proportion as the dimensions of the above-mentioned No. 13B test specimen, and a test specimen that can be taken from the part and has a shape similar to the No. 13B test specimen shall be used as a substitute. Using such a substitute test specimen, the tests are conducted in the above-mentioned manner.

[0048] In the case of titanium alloy pipes, the following circular arc-shaped test pieces are taken and tested in the same manner. Figure 1 is a diagram illustrating a method for taking test pieces from titanium alloy pipes. In Figure 1(a), the drawing direction of the titanium alloy pipe 1 is indicated by an arrow. The gripping portion 4 and the welded portion 5 are indicated by a dashed line and a dot, respectively. In Figure 1(b), the circumferential direction of the titanium alloy pipe 1 is indicated by an arrow. (a) is a perspective view of the titanium alloy pipe 1, and (b) is a view of the titanium alloy pipe 1 as seen from the drawing direction.

[0049] 1(a) and 1(b), the test piece 2 is arc-shaped and has a parallel portion 3 and a gripped portion 4. The width of the parallel portion 3 of the test piece 2 is 60% of the outer diameter R of the titanium alloy pipe 1. The length of the parallel portion 3 of the test piece 2 is 35 mm in a direction parallel to the elongation direction of the titanium alloy pipe 1. The thickness of the test piece 2 is the same as that of the titanium alloy pipe 1. In 1(b), the test piece 2 is taken so that the circumferential center of the parallel portion 3 and the circumferential center of the welded portion 5 face each other in the radial direction of the titanium alloy pipe 1. The shape of the gripped portion 4 of the test piece 2 is not particularly limited. The welded portion 5 includes a weld metal and a heat-affected zone, not shown.

[0050] In the case of hollow parts, test pieces shown in Figure 1 above are taken from the hollow part, avoiding heavily worked portions such as bent portions of titanium alloy pipes and / or pressed portions of titanium alloy plates, and the test is carried out in the same manner. However, if the shape of a hollow part makes it impossible to take a test piece shown in Figure 1, the dimensions of the test piece should be reduced by the same proportion as the dimensions of the test piece shown in Figure 1 above, and a test piece that can be taken from the hollow part and has a shape similar to the test piece shown in Figure 1 should be used instead. Using such a substitute test piece, the test is carried out in the manner described above.

[0051] <High-temperature strength> In anticipation of application to exhaust system parts that can withstand higher exhaust gas temperatures, the titanium alloy plate and titanium alloy pipe according to the present invention preferably have a tensile strength of 55 MPa or more at 700°C. Furthermore, the tensile strength at 700°C after holding at 750°C for 100 hours is preferably 72 MPa or more, and more preferably 75 MPa or more. By being heated to high temperatures during use, the high-temperature strength increases, thereby reducing the risk of damage to parts.

[0052] Measurement of tensile strength at 700°C is performed as follows. First, in the case of titanium alloy plates, a test piece with a parallel portion width of 10 mm, a parallel portion length of 35 mm, and a parallel portion thickness equal to the plate thickness, and a gripping portion is taken. Then, the temperature is raised to 700°C at a heating rate of 45°C / min, and the test piece is held in the test atmosphere for 10 minutes to ensure that the test temperature is reached. Thereafter, a tensile test is performed at a test temperature of 700°C and a strain rate of 7.5% / min, and the tensile strength at 700°C is determined.

[0053] In the case of titanium alloy parts, test pieces of the above dimensions are taken from the part, avoiding the processed parts, and the test is carried out in the same manner. However, if the shape of a titanium alloy part makes it impossible to take a test piece of the above dimensions, the dimensions of the test piece shall be reduced by the same proportion as the above dimensions of the test piece, and a test piece that can be taken from the part and has a similar shape to the above test piece shall be used as a substitute. Using such a substitute test piece, the test shall be carried out in the above manner.

[0054] In the case of titanium alloy tubes, test pieces shown in Figure 1 are taken and tested in the manner described above. In the case of hollow parts, test pieces having the dimensions shown in Figure 1 are taken from the hollow part, avoiding processed portions, and tests are conducted in the same manner. However, if a test piece having the dimensions shown in Figure 1 cannot be taken from a hollow part due to the part's shape, the dimensions of the test piece are reduced by the same proportion as the dimensions of the test piece shown in Figure 1, and a test piece that can be taken from the hollow part and has a shape similar to the test piece shown in Figure 1 is used as a substitute. Tests are conducted in the manner described above using such substitute test pieces.

[0055] Measurement of tensile strength at 700°C after holding at 750°C for 100 hours is performed using the following method. Test specimens of the above-described shape are held in air at 750°C for 100 hours, then air-cooled, shot-blasted and pickled to remove oxide scale, and then subjected to a tensile test under the above-described conditions to determine tensile strength. In the case of titanium alloy parts, test specimens of the above-described dimensions are taken from the part, avoiding processed areas, and the test is performed in the same manner. However, if the shape of a titanium alloy part makes it impossible to take test specimens of the above-described dimensions, the dimensions of the test specimen are reduced by the same proportion as the above-described test specimen, and a test specimen of a similar shape that can be taken from the part is used instead. The test is performed using such a substitute test specimen using the above-described method.

[0056] In the case of titanium alloy pipes, test pieces shown in Figure 1 are taken and tested in the manner described above. In the case of hollow members, test pieces of the dimensions shown in Figure 1 are taken from the hollow member, avoiding processed parts, and tests are conducted in the same manner. However, if a test piece of the dimensions shown in Figure 1 cannot be taken from a hollow member due to its shape, the dimensions of the test piece are reduced by the same proportion as the dimensions of the test piece described above, and a test piece of a similar shape to the test piece described above is used as a substitute. Using such a substitute test piece, tests are conducted in the manner described above.

[0057] <Oxidation Resistance> The titanium alloy plate and titanium alloy pipe according to the present invention have a weight change of 6.50 mg / cm3 before and after being held at 800°C for 200 hours in an air atmosphere, as an indicator of oxidation resistance. ２ It is preferable that:

[0058] The weight change is measured by the following method. First, in the case of a titanium alloy plate, a test piece of 20 mm width x 20 mm length without changing the thickness is taken from the titanium alloy plate. Then, the surface of the test piece is wet-polished with emery paper #400, and the weight of the test piece is measured. Thereafter, the test piece is exposed to a still atmosphere at 800°C for 200 hours, and the mass after exposure is measured. The increase in mass of the test piece before and after exposure is divided by the surface area of ​​the test piece ((increased mass (mg) / surface area of ​​test piece (cm) ２ If scale peeling occurs during the test, the peeled scale shall be included in the mass after exposure.

[0059] In the case of titanium alloy parts, test pieces of the above dimensions are taken from the part, avoiding the processed parts, and the test is carried out in the same manner. However, if the shape of a titanium alloy part makes it impossible to take a test piece of the above dimensions, the dimensions of the test piece shall be reduced by the same proportion as the above dimensions of the test piece, and a test piece that can be taken from the part and has a similar shape to the above test piece shall be used as a substitute. Using such a substitute test piece, the test shall be carried out in the above manner.

[0060] In the case of titanium alloy pipes, tubular test pieces with a length of 20 mm and no change in thickness are taken from a portion of the titanium alloy pipe other than the weld metal and heat-affected zone, and the test is conducted in the manner described above. In the case of hollow components, test pieces of the above dimensions are taken from a portion of the hollow component other than the weld metal and heat-affected zone, and other than the processed portion, and the test is conducted in the same manner. However, in the case of hollow components, if test pieces of the above dimensions cannot be taken due to the shape of the component, the dimensions of the test piece are reduced by the same proportion as the above-mentioned test pieces, and a test piece that can be taken from the hollow component and has a shape similar to the above-mentioned test piece is used as a substitute. The test is conducted in the manner described above using such substitute test pieces.

[0061] (D) Dimensions The dimensions of the titanium alloy plate and titanium alloy pipe according to the present invention are not particularly limited. However, when used as a material for automobile exhaust system parts, the outer diameter of the titanium alloy pipe is preferably 5 to 100 mm, and the wall thickness is preferably 0.4 to 2.0 mm. The thickness of the titanium alloy plate is preferably 0.4 to 2.0 mm, and more preferably 0.5 to 1.5 mm.

[0062] (E) Uses The titanium alloy plate, titanium alloy pipe, titanium alloy part, and hollow part according to the present invention can be used as exhaust system parts, such as main mufflers, exhaust manifolds, exhaust pipes, catalytic converters, and mufflers for automobiles such as four-wheeled automobiles and two-wheeled automobiles.

[0063] The titanium alloy plate according to the present invention has excellent high-temperature strength and workability at room temperature, and can be used as a titanium alloy pipe after processing and welding. That is, the titanium alloy pipe is made by forming the titanium alloy plate into a tubular shape and welding it. In the titanium alloy pipe, the base material excluding the welded portion has the above-mentioned properties.

[0064] The titanium alloy plate according to the present invention has excellent high-temperature strength and workability at room temperature, so it can be used as a titanium alloy part after various processing. That is, the titanium alloy part is the titanium alloy plate processed as described above. In the titanium alloy part, the base material excluding the processed part has the above-mentioned characteristics.

[0065] The titanium alloy tube according to the present invention has excellent high-temperature strength and workability at room temperature, and can be used as a hollow part after various processing. That is, the hollow part is obtained by processing the titanium alloy tube described above.

[0066] Furthermore, the above-mentioned titanium alloy plates and titanium alloy pipes may be subjected to various processing and welding processes to be used as hollow parts. The hollow parts may have multiple processed parts and welded parts. In the hollow parts, the base material excluding the welded parts and processed parts has the above-mentioned properties. The welded parts refer to the weld metal and the heat-affected zone.

[0067] (F) Manufacturing Method The manufacturing method of the titanium alloy plate according to the present invention is not particularly limited. However, for example, it is possible to manufacture the titanium alloy plate by sequentially carrying out a hot rolling process, a hot-rolled sheet annealing process, a cold rolling process, and a cold-rolled sheet annealing process under the conditions shown below for a titanium material having the above-mentioned chemical composition. Furthermore, intermediate annealing may be carried out in the cold rolling process as needed. Each condition will be explained in detail below.

[0068] <Hot Rolling Process> First, a titanium material adjusted to the above-mentioned chemical composition is produced, for example, by vacuum arc melting or electron beam melting, and then hot forged into a slab. The slab is then heated to a heating temperature of 880 to 930°C, and then hot rolled to produce a hot-rolled material. By setting the heating temperature to 880 to 930°C and allowing the precipitated phases already precipitated at the slab stage to solid-solve, the number density of the precipitated phases is reduced to 0.15 / μm in the cold-rolled sheet annealing process. ２ The precipitation phase can be precipitated so as to satisfy the above conditions. In addition, an increase in production costs due to excessive heating can be suppressed. The thickness of the hot-rolled material is preferably 2.5 to 15 mm.

[0069] <Hot-rolled sheet annealing process> In the hot-rolled sheet annealing process, the hot-rolled material is annealed for 5 to 60 minutes in a temperature range of 700 to 830°C. By annealing the hot-rolled sheet at such a temperature and for such a time, strain in the hot-rolled material is reduced, making it easier to perform cold rolling. In addition, excessive oxidation can be suppressed. In the chemical composition specified in the present invention, the temperature range of 700 to 830°C is below the β transformation point. The method for calculating the β transformation point is as described below. The annealing may be air annealing or vacuum annealing. After the hot-rolled sheet annealing process, it is preferable to water-cool the material and remove oxide scale by shot blasting and / or pickling.

[0070] The β transformation point can be obtained from a phase diagram. The phase diagram is obtained by the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) method. Specifically, a phase diagram of a titanium alloy is obtained by the CALPHAD method using Thermo-Calc, an integrated thermodynamic calculation system from Thermo-Calc Software AB, and a predetermined database (TI3), and the β transformation point is calculated.

[0071] <Cold Rolling Step> In the cold rolling step, the hot rolled material after the hot rolled sheet annealing step is cold rolled to obtain a cold rolled material. The reduction in the cold rolling step may be 60 to 85%. When intermediate annealing, which will be described later, is performed, the reduction may be 60 to 85% when comparing the thickness before cold rolling with the thickness after intermediate annealing, when cold rolling is started again, and when cold rolling is completed. The thickness of the cold rolled material is preferably 0.4 to 2.0 mm.

[0072] In the cold rolling step, the cold rolling may be interrupted before the final reduction to perform intermediate annealing. When intermediate annealing is performed, it is preferable that the annealing temperature is above 750°C and below the β transformation point, the annealing time is 1 to 5 minutes, and the average cooling rate from the annealing temperature to 700°C is 5°C / s or more.

[0073] <Cold-rolled sheet annealing step> In the cold-rolled sheet annealing step, the cold-rolled material is annealed for 20 to 200 hours in a temperature range of 600 to 710°C to obtain a titanium alloy sheet. If the cold-rolled sheet annealing temperature is less than 600°C or the cold-rolled sheet annealing time is less than 20 hours, the precipitation of the precipitate phase is insufficient, and the workability at room temperature is reduced. In addition, the average crystal grain size of the α phase may be less than 5.0. If the cold-rolled sheet annealing temperature exceeds 710°C or the cold-rolled sheet annealing time exceeds 200 hours, the precipitate phase becomes coarse, and the number density of the precipitate phase becomes 0.15 / μm ２ In addition, the α phase may become coarse, and the average grain size of the α phase may exceed 50.0 μm. In this way, by performing cold-rolled sheet annealing in the temperature range of 600 to 710°C for 20 to 200 hours, the number density of the precipitated phase can be reduced to 0.15 / μm. ２ The average grain size of the α phase can be 5.0 to 50.0 μm. Annealing can be air annealing or vacuum annealing.

[0074] The titanium alloy pipe is a welded titanium alloy pipe. To manufacture a titanium alloy pipe, the titanium alloy plate manufactured as described above may be formed into a tubular shape and welded to form a welded titanium alloy pipe. The manufacturing method of the titanium alloy pipe is not particularly limited, but it can be manufactured, for example, by press-forming the titanium alloy plate described above into a tubular shape and welding the ends. The welding method is also not particularly limited, but methods such as TIG welding, MIG welding, and laser welding can be used. Furthermore, during welding, a filler metal such as CP-Ti (commercially pure titanium) or an alloy wire of the same type as the titanium alloy plate may be used. In this manner, welded pipes such as electric resistance welded pipes and spiral pipes can be manufactured.

[0075] When producing a titanium alloy part, the production method is not particularly limited, but for example, the titanium alloy plate produced as described above may be press-formed into a hat shape or the like.

[0076] When producing hollow parts, the production method is not particularly limited, but for example, they can be produced by subjecting the titanium alloy pipe produced as described above to bending, pressing, expanding, shrinking, etc.

[0077] Furthermore, the manufacturing method for hollow parts made from titanium alloy plates and titanium alloy pipes is not particularly limited. For example, a titanium alloy plate that has been subjected to press forming or the like and a titanium alloy pipe may be welded by the above-mentioned welding method. Also, a titanium alloy plate that has been subjected to press forming or the like and a titanium alloy pipe that has been processed by the above-mentioned processing method may be welded by the above-mentioned welding method.

[0078] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0079] Titanium materials having the compositions shown in Table 1 were melted by electron beam melting, hot forged into slabs, heated to 860 to 930°C, and then hot-rolled into 3.5 mm thick strips in a hot continuous rolling mill. The hot-rolled strips were annealed in air at 800 to 830°C for 3 to 10 minutes, and oxide scale was removed by shot blasting and pickling. Subsequently, cold-rolled strips having a thickness of 1.2 mm were obtained in a cold rolling mill, and then vacuum annealed at 530 to 700°C for 4 to 24 hours with furnace cooling to obtain titanium alloy sheets Nos. 1 to 22.

[0080] Table 2 lists the β transformation point of each titanium alloy plate. The β transformation point was obtained from a phase diagram. The phase diagram was obtained by the CALPHAD method. Specifically, the titanium alloy phase diagram was obtained by the CALPHAD method using Thermo-Calc, an integrated thermodynamic calculation system from Thermo-Calc Software AB, and a predetermined database (TI3), and the β transformation point of each titanium alloy plate was calculated.

[0081] In addition, titanium alloy pipe No. 23 was produced using titanium alloy plate No. 6 (length 300 mm, width 200 mm, thickness 1.2 mm). Specifically, the titanium alloy plate was press-formed into a tubular shape so that the rolling direction of the titanium alloy plate coincided with the circumferential direction of the titanium alloy pipe, and the titanium alloy pipe was obtained by TIG welding without using a filler metal.

[0082]

[0083] The number density of the precipitated phase, the area ratio of the α phase, and the average grain size of the α phase of each titanium alloy plate and titanium alloy pipe were measured by the following methods. In addition, the workability, high-temperature strength, and oxidation resistance of each titanium alloy plate and titanium alloy pipe were evaluated by the following methods.

[0084] <Number Density of Precipitate Phases> The number density of precipitate phases was measured using the following method. A sample was taken from the above-described titanium alloy plate so that the central portion of the plate thickness in a cross section parallel to the plate thickness direction served as the observation surface. The observation surface was then etched, and a region measuring 213 μm in the plate thickness direction and 284 μm in a direction perpendicular to the plate thickness direction was observed under an optical microscope at 500x magnification. The etching was performed using the following procedure. An etchant (a mixture of nitric acid and hydrofluoric acid) was soaked in absorbent cotton, and the observation surface was rubbed 20 times over a period of 20 to 60 seconds. If the metal structure could not be adequately confirmed when observed under an optical microscope, additional etching was performed. If pits with a diameter of 8 μm or more were observed, the sample was polished again, and the etching time was shortened as necessary. The above-described etching solution contained 3% by mass of nitric acid and 5% by mass of hydrogen fluoride. The optical microscope photographs were taken at a resolution of 1440 pixels in the thickness direction and 1920 pixels in the direction perpendicular to the thickness direction.

[0085] In the optical microscope photographs obtained as described above, the α phase was observed as white, while the grain boundaries of the α phase and the precipitate phase were observed as black. Furthermore, the grain boundaries of the α phase were observed as linear, while the precipitate phase was observed as rounded. Based on these differences, the precipitate phase was identified and its number density was calculated. Specifically, the optical microscope photographs were analyzed using the image analysis software FIJI as follows. First, the image was binarized using Auto Threshold using Method: "Li," and the black and white were inverted so that the α phase appeared black. Next, the overlapping precipitate phases were separated using Watershed, and then the Circularity was set to 0.5 to 1.0, and the number and area ratio of the precipitate phases were measured using Analyze Particles... Finally, the number of precipitate phases was divided by the area of ​​the observation field to calculate the number density of the precipitate phases. The area fraction of the α phase was calculated by subtracting the area fraction of the precipitate phase calculated as described above from 100%. The results are shown in Table 2. In the analysis, the minimum size of the precipitate phase was 0.147 μm, i.e., 1 pixel, and a precipitate phase was measured when the maximum diameter exceeded 0.147 μm.

[0086] For the titanium alloy pipe of Test No. 23, the above-mentioned observations and measurements were carried out at the center of the wall thickness of a cross section parallel to the longitudinal direction and the wall thickness direction in a region other than the weld metal and the heat-affected zone. The area fraction of the α phase was calculated by subtracting the area fraction of the precipitate phase calculated as described above from 100%. The results are shown in Table 3.

[0087] <Average grain size of α phase> The average grain size of the α phase was measured by the following method. A sample was taken from the above-mentioned titanium alloy material so that the observation surface was the center of the thickness of a cross section parallel to the thickness direction. The observation surface was then mirror-polished and etched with an etching solution (a mixed solution of nitric acid and hydrofluoric acid), and a 400 μm × 400 μm region was observed under an optical microscope at 200x magnification. The etching solution contained 3% by mass of nitric acid and 5% by mass of hydrogen fluoride.

[0088] Then, the average grain size of the α phase was determined by the intercept method using the structural photograph. Specifically, the observation area was divided into five equal parts in the thickness direction, and each small area was divided into five small areas. A line segment with a length Ln (400 μm) was drawn in the center of each small area in a direction perpendicular to the thickness direction, and the number of crystal grains Xn separated by the line segment was measured. Here, n is a code for identifying the line segment and is an integer between 1 and 5. The grain size Dn of each line segment was then calculated using formula (I). The average grain size D of the five line segments was then calculated using formula (II). In measuring Xn, as described above with reference to FIG. 2, a grain where a line segment completely crossed a crystal grain was counted as 1, and a grain where the start or end point of the line segment was included within a crystal grain was counted as 0.5. The results are shown in Table 2. Dn (μm) = Ln / Xn (I) D (μm) = (D １ +D ２ +D ３ +D ４ +D ５ ) / 5 ... (II)

[0089] For the titanium alloy pipe of Test No. 23, the average grain size of the α phase was determined by carrying out the above-mentioned observations and measurements in the region other than the weld metal and the heat-affected zone at the center of the wall thickness of the cross section parallel to the longitudinal direction and the wall thickness direction. The results are shown in Table 3.

[0090] <Workability> The workability was evaluated by the following method. First, a No. 13B test piece specified in JIS Z 2241:2022 was taken from the above-mentioned titanium alloy plate so that the longitudinal direction of the test piece coincided with the width direction of the titanium alloy plate. Then, the total elongation (%) was measured by a room temperature tensile test. Here, the strain rate was 0.5% / min up to 2% strain, and 30% / min after the strain amount exceeded 2%. The results are shown in Table 2 as "total elongation."

[0091] For the titanium alloy pipe of Test No. 23, the circular arc-shaped test piece shown in Fig. 1 was taken and the above-mentioned measurement was carried out. The results are shown in Table 3 as "total elongation".

[0092] <High-Temperature Strength> Evaluation of high-temperature strength was performed as follows. First, a test piece with a parallel portion width of 10 mm, a parallel portion length of 35 mm, and a parallel portion thickness remaining the same as the plate thickness, and having a gripping portion, was taken from the above-mentioned titanium alloy plate. Then, the temperature was raised to 700°C at a heating rate of 45°C / min, and the test piece was held in a test atmosphere for 10 minutes so that the test temperature was sufficiently reached. Thereafter, a tensile test was performed at a test temperature of 700°C and a strain rate of 7.5% / min, and the tensile strength at 700°C was determined. The results are shown in Table 2 as "strength before heating and holding."

[0093] For the titanium alloy pipe of Test No. 23, the circular arc-shaped test piece shown in Fig. 1 was taken and the above-mentioned measurements were carried out. The results are shown in Table 3 as "strength before heating and holding."

[0094] Measurement of tensile strength at 700°C after holding at 750°C for 100 hours was carried out in the following manner. Test pieces having the above-described shapes were held in the air at 750°C for 100 hours, then air-cooled, shot-blasted and pickled to remove oxide scale, and then subjected to a tensile test under the above-described conditions to determine the tensile strength. The results are shown in Table 2 as "strength after heating and holding."

[0095] For the titanium alloy pipe of Test No. 23, the circular arc-shaped test piece shown in Fig. 1 was taken and the above-mentioned measurements were carried out. The results are shown in Table 3 as "strength after heating and holding."

[0096] <Oxidation Resistance> The oxidation resistance was evaluated by the following method. First, a test piece having a width of 20 mm and a length of 20 mm was taken from the above-mentioned titanium alloy plate without changing the thickness. The surface of the test piece was then wet-polished with emery paper #400, and the weight of the test piece was measured. Thereafter, the test piece was exposed to static air at 800°C for 200 hours, and the mass after exposure was measured. The increase in mass of the test piece before and after exposure was divided by the surface area of ​​the test piece ((increased mass (mg) / surface area of ​​test piece (cm) ２ )) was taken as the weight change. If scale spalling occurred during the test, the spalled scale was included in the mass after exposure. The results are shown in Table 2 as "oxidation weight gain."

[0097] For the titanium alloy pipe of Test No. 23, a tubular test piece with a length of 20 mm and the same thickness was taken from the titanium alloy pipe and tested in the above-mentioned manner. The results are shown in Table 3 as "oxidation weight gain."

[0098]

[0099]

[0100] Test Nos. 1 to 7, 9, 16 to 20, and 23, which are examples of the present invention, all satisfied the requirements defined in the present invention, and therefore had good total elongation and high-temperature strength, and were also able to sufficiently improve high-temperature strength after heating and holding at 700°C. Furthermore, the oxidation weight gain also showed good results.

[0101] On the other hand, in Test Nos. 8, 10 to 15, 21, and 22, which did not satisfy the requirements of the present invention, the results showed deterioration in any of the properties of total elongation, high-temperature strength, or high-temperature strength after heating and holding at 750°C. Specifically, in Test No. 8, the Cu content exceeded 2.10%, resulting in deterioration of workability at room temperature due to solid solution strengthening. In addition, the high-temperature strength did not meet the target value, and the high-temperature strength after heating and holding could not be increased to the target value. In Test No. 10, the Si content was less than 0.10%, resulting in failure to increase the high-temperature strength and the high-temperature strength after heating and holding to the target values. Furthermore, the oxidation resistance was insufficient. In Test No. 11, the Si content exceeded 0.60%, resulting in deterioration of workability at room temperature.

[0102] In Test No. 12, the Sn content was less than 0.50%, so the high-temperature strength did not meet the target value, and the high-temperature strength after heating could not be increased to the target value. In Test No. 13, the Sn content exceeded 1.50%, so workability at room temperature was reduced. In Test No. 14, the heating temperature before hot rolling was less than 880 °C, so the precipitate phase that had already precipitated at the slab stage could not be solid-dissolved. In addition, the cold-rolled sheet annealing time was less than 20 h, so precipitation of the precipitate phase was insufficient and the number density of the precipitate phase was low. As a result, workability at room temperature was reduced, and the high-temperature strength after heating could not be increased to the target value. In Test No. 15, the cold-rolled sheet annealing time was less than 20 h, so precipitation of the precipitate phase was insufficient and the number density of the precipitate phase was low. As a result, workability at room temperature was reduced.

[0103] In Test No. 21, the Cu content was 1.40% or less and the O content was more than 0.08%. Furthermore, the heating temperature before hot rolling and the cold-rolled sheet annealing conditions were outside the preferred conditions, and hot-rolled sheet annealing was not performed. Therefore, the number density of the precipitated phase decreased. Furthermore, although the high-temperature strength met the target value, the high-temperature strength after heating and holding could not be increased to the target value. In Test No. 22, the Cu content was 1.40% or less, so the number density of the precipitated phase decreased, and although the high-temperature strength met the target value, the high-temperature strength after heating and holding could not be increased to the target value.

[0104] The titanium alloy plates, titanium alloy pipes, titanium alloy parts, and hollow parts of the present invention have high high-temperature strength and excellent workability at room temperature. Therefore, they can be used in main muffler sections, exhaust manifolds, exhaust pipes, catalytic converters, and waste disposal equipment components such as mufflers for automobiles such as four-wheeled automobiles and two-wheeled automobiles. As a result, the weight of automobiles such as four-wheeled automobiles and two-wheeled automobiles will be reduced, making a significant contribution to industry.

Claims

1. The chemical composition is, in mass%, Cu: over 1.40% and not more than 2.10%, Sn: 0.50 to 1.50%, Si: 0.10 to 0.60%, Nb: 0.10 to 1.00%, O: 0.08% or less, and the balance: Ti and impurities, and the number density of the precipitate phase is 0.15 / μm ２ That's it, titanium alloy plate.

2. The titanium alloy plate according to claim 1, wherein the average grain size of the α phase is 5.0 μm or more.

3. A steel sheet having a base material and a welded portion, the base material having a chemical composition, in mass%, of Cu: more than 1.40% and not more than 2.10%, Sn: 0.50 to 1.50%, Si: 0.10 to 0.60%, Nb: 0.10 to 1.00%, O: not more than 0.08%, and the balance: Ti and impurities, and the number density of the precipitated phase in the base material is 0.15 / μm ２ That's it, titanium alloy tube.

4. A titanium alloy tube according to claim 3, wherein the average grain size of the α phase in the base material is 5.0 μm or more.

5. A titanium alloy part made from the titanium alloy plate according to claim 1 or 2.

6. A hollow part made from the titanium alloy tube according to claim 3 or 4.

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