Titanium alloy sheet and automotive exhaust system component

Abstract

To provide a titanium alloy sheet excellent in high temperature strength and formability.SOLUTION: 0.5 to 2.0% of Cu, 0.010 to 0.045% of Fe, 0 to 1.0% of Nb, 0.03 to 0.10% of O, 0.08% or less of N, 0.05% or less of C, 0.015% or less of H, and a balance of Ti and impurities, in which an average crystal grain size of an α phase in a cross section parallel to a sheet thickness direction is 3 to 50 μ m, and an area ratio of an α phase in which an angle between a c-axis and the sheet thickness direction is in a range of 60 ° to 90 ° in the α phase is less than 25%, A titanium alloy sheet in which, in a (0001) pole figure from the sheet thickness direction, the maximum integration degree of the (0001) plane in the range where the angle formed with the sheet thickness direction is 25 to 45 ° is ≥ 4.5 is adopted.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to a titanium alloy plate and an automotive exhaust system part. [Background technology]

[0002] The exhaust systems of motorcycles and automobiles (hereafter referred to as automobiles) are made up of exhaust system components such as exhaust manifolds, exhaust pipes, catalytic mufflers, and mufflers (pre-mufflers or silencers (main mufflers)). In order to withstand high-temperature exhaust gases and accommodate complex shapes, stainless steel, which has excellent corrosion resistance, high-temperature strength, and workability, has been widely used for these components.

[0003] However, in recent years, pure titanium, which has corrosion resistance that surpasses stainless steel, is lightweight and easy to process, has a low thermal expansion coefficient and excellent thermal fatigue properties, and has excellent design features such as a unique color and texture, has been used in some automobile exhaust system parts.

[0004] Automotive exhaust system parts are exposed to relatively high-temperature exhaust gases, so titanium sheets used as materials for exhaust system parts are required to have excellent high-temperature strength at 600 to 700°C.

[0005] Furthermore, because automotive exhaust system parts have complex shapes, titanium sheets used as the raw material for exhaust system parts must have excellent formability when formed into the part shape by cold working such as pressing.

[0006] Patent Document 1 describes a heat-resistant titanium alloy plate with excellent cold workability, which is composed, by mass%, of 0.3 to 1.8% Cu, 0.18% or less oxygen, 0.30% or less Fe, the balance being Ti and less than 0.3% impurity elements.

[0007] Patent Document 2 describes a titanium alloy with excellent high-temperature oxidation resistance that contains 0.15 to 2 mass % of Si, restricts Al to less than 0.30 mass %, and is made up of the remainder titanium and unavoidable impurities. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] Japanese Patent Application Laid-Open No. 2005-298970 [Patent Document 2] Japanese Patent Application Laid-Open No. 2007-270199 Summary of the Invention [Problem to be solved by the invention]

[0009] Recently, there has been a demand for titanium alloy sheets with superior high-temperature strength and cold formability. Therefore, an object of the present invention is to provide a titanium alloy sheet and an automotive exhaust system part with excellent high-temperature strength and formability. [Means for solving the problem]

[0010] In order to solve the above problems, the following configuration is adopted. [1] In mass %, Cu: 0.5 to 2.0% Fe: 0.010~0.045%, Nb: 0 to 1.0% O: 0.03 to 0.10%, N: 0.08% or less, C: 0.05% or less, H: 0.015% or less, The balance is Ti and impurities. The average grain size of the α phase in a cross section parallel to the plate thickness direction is 3 to 50 μm, The area ratio of the α phase in which the angle between the c axis and the sheet thickness direction is within the range of 60° to 90° is less than 25%, and A titanium alloy plate in which, in a (0001) pole figure from the plate thickness direction, the maximum density of the (0001) plane within an angle range of 25 to 45° with respect to the plate thickness direction is 4.5 or more. [2] The titanium alloy sheet according to [1], wherein the area ratio of the second phase other than the α phase in a cross section parallel to the sheet thickness direction is 0.1% or more and 3.5% or less. [3] In the (0001) pole figure from the thickness direction, the angle between the thickness direction and the (0001) plane is in the range of 25 to 45°, and the angle between the thickness direction and the rolling direction is in the range of -10° to 10°, and the maximum integration degree of the (0001) plane is 4.5 or more. Titanium alloy sheet according to [1]. [4] The titanium alloy plate according to [1], which satisfies the following formula (1): 6.5<[%Nb]+2.5×[%Cu]+15×[%Fe]+45×[%O]+12×D -0.5 <14.5 …(1) In the formula (1), [%Nb], [%Cu], [%Fe] and [%O] are the contents (mass%) of Nb, Cu, Fe and O contained in the titanium alloy plate, respectively, and D is the average crystal grain size (μm) of the α phase. [5] An automotive exhaust system part made of a titanium alloy plate, The titanium alloy plate is In mass%, Cu: 0.5 to 2.0% Fe: 0.010~0.045%, Nb: 0 to 1.0% O: 0.03 to 0.10%, N: 0.08% or less, C: 0.05% or less, H: 0.015% or less, The balance is Ti and impurities. The average grain size of the α phase in a cross section parallel to the plate thickness direction is 3 to 50 μm, The area ratio of the α phase in which the angle between the c axis and the sheet thickness direction is within the range of 60° to 90° is less than 25%, and An automotive exhaust system part having a texture in which, in a (0001) pole figure from the thickness direction, the angle formed with the thickness direction is in the range of 25 to 45° and the maximum integration degree of the (0001) plane is 4.5 or more. [6] The automotive exhaust system part according to [5], wherein the area ratio of the second phase other than the α phase in a cross section parallel to the plate thickness direction is 0.1% or more and 3.5% or less. [7] The automotive exhaust system part according to [5], which satisfies the following formula (1): 6.5<[%Nb]+2.5×[%Cu]+15×[%Fe]+45×[%O]+12×D -0.5 <14.5 …(1) In the formula (1), [%Nb], [%Cu], [%Fe] and [%O] are the contents (mass%) of Nb, Cu, Fe and O contained in the titanium alloy plate, respectively, and D is the average crystal grain size (μm) of the α phase. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a titanium alloy plate and an automotive exhaust system part having excellent high-temperature strength and formability. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 2 is a (0001) pole figure of a titanium alloy plate according to an embodiment of the present invention, viewed in the thickness direction. DETAILED DESCRIPTION OF THE INVENTION

[0013] The present inventors have conducted extensive research to obtain a titanium alloy sheet with excellent high-temperature strength and formability, and have found that adding a certain amount of Cu to titanium significantly improves high-temperature strength in the 500-700°C temperature range in which automotive exhaust system parts and the like are used. They have also found that by performing hot rolling and cold rolling on the titanium alloy sheet under more precisely controlled rolling conditions, the texture can be optimized and the formability of the titanium alloy sheet can be improved.

[0014] The relationship between the texture and formability of titanium alloy sheets is described in detail. In the case of pure titanium, hot rolling causes the c-axis of the α phase to tilt in the width direction (TD), which is perpendicular to the rolling direction (RD), and the c-axis accumulates near a 35° inclination with respect to the thickness direction (ND), forming a texture known as Split-TD, which improves formability.

[0015] On the other hand, in titanium alloys containing Cu, a β-stabilizing element, to improve high-temperature strength, a texture called T-texture is formed, and the proportion of α-phase, in which the c-axis is oriented almost parallel to the sheet width direction, increases. Titanium alloys with a pronounced T-texture have significantly reduced formability.

[0016] Therefore, in the present invention, it was discovered that in a titanium alloy containing Cu, both high-temperature strength and formability can be achieved by reducing the T-texture and aligning and accumulating the c-axis at an angle of approximately 35° to the plate thickness direction.

[0017] Hereinafter, a titanium alloy plate and an automotive exhaust system part according to an embodiment of the present invention will be described.

[0018] The titanium alloy sheet of this embodiment is a titanium alloy sheet containing, by mass%, Cu: 0.5 to 2.0%, Fe: 0.010 to 0.045%, Nb: 0 to 1.0%, O: 0.03 to 0.10%, N: 0.08% or less, C: 0.05% or less, H: 0.015% or less, and the balance: Ti and impurities. The average grain size of the α phase in a cross section parallel to the thickness direction is 3 to 50 μm. The area ratio of the α phase in which the angle between the c axis and the thickness direction is within a range of 60 to 90° is less than 25%. In addition, in a (0001) pole figure from the thickness direction, the angle with the thickness direction is within a range of 25 to 45°, and the maximum integration degree of the (0001) plane is 4.5 or more. In the titanium alloy plate of this embodiment, the area ratio of the second phase other than the α phase in a cross section parallel to the plate thickness direction is preferably 0.1% or more and 3.5% or less. The titanium alloy plate of this embodiment preferably has a texture in which, in the (0001) pole figure from the plate thickness direction, the angle formed with the plate thickness direction is in the range of 25 to 45°, the angle formed with the rolling direction is in the range of -10° to 10°, and the maximum integration degree of the (0001) plane is 4.5 or more. The titanium alloy plate of this embodiment preferably satisfies the following formula (1).

[0019] 6.5<[%Nb]+2.5×[%Cu]+15×[%Fe]+45×[%O]+12×D -0.5 <14.5 …(1) In the formula (1), [%Nb], [%Cu], [%Fe] and [%O] are the contents (mass%) of Nb, Cu, Fe and O contained in the titanium alloy plate, respectively, and D is the average crystal grain size (μm) of the α phase.

[0020] In this embodiment, excellent high-temperature strength means that the 0.2% yield strength at 700°C is 20 MPa or more. In this embodiment, excellent formability means that when a titanium alloy plate of a predetermined shape is stretched at room temperature using a punch and die of a predetermined shape, stretching is possible without causing cracks until the plate reaches a predetermined forming height or more. In this embodiment, room temperature refers to a temperature range of 10 to 35°C.

[0021] <Chemical composition> The chemical components of the titanium alloy plate of this embodiment will be described.

[0022] Cu: 0.5 to 2.0% Cu is an element that has excellent solid-solution strengthening ability at high temperatures, but does not suppress twinning at room temperature, making it a preferable element for achieving both high-temperature strength and formability. In order to obtain the high-temperature strength required in this embodiment, a Cu content of 0.5% or more is required, and this is set as the lower limit. Preferably, it is 0.6% or more, and more preferably, it is 0.7% or more. On the other hand, if Cu is excessive, particularly above the solid solubility limit of the α phase, the amount of TiCu precipitated increases, making it impossible to sufficiently increase the average crystal grain size, resulting in a decrease in both high-temperature strength and formability. Therefore, the upper limit is set to 2.0% or less. It is preferably 1.9% or less, and more preferably 1.8% or less.

[0023] Fe: 0.010 to 0.045% Fe is a useful element because it is a β-stabilizing element and can also increase strength. However, because its solid solubility limit in the α-phase is very small, excessive Fe content causes the β-phase to precipitate, increasing the amount of the β-phase (second phase). This causes Cu, an element that tends to concentrate in the β-phase, to concentrate in the β-phase, reducing the amount of Cu that can dissolve in the α-phase, which is necessary to improve high-temperature strength, and thus reducing high-temperature strength and formability. Therefore, the Fe content must be in the range of 0.010% to 0.045%, preferably 0.015 to 0.040%.

[0024] O: 0.03 to 0.10% O is an element that can increase strength through solid solution strengthening, but an excessive O content suppresses twinning deformation and significantly reduces formability. Since 0.03% or more O ensures sufficient strength at room temperature, this is the lower limit. On the other hand, if the O content exceeds 0.10%, formability significantly decreases, so this is the upper limit. The O content is preferably 0.08% or less.

[0025] Nb: 0 to 1.0% Nb is a completely soluble β-stabilizing element, but its solid solution amount in the α phase is relatively large. Therefore, solid solution strengthening can be expected from room temperature to high temperatures. In addition, the addition of Nb can improve oxidation resistance at high temperatures, particularly at 600 to 800°C. Therefore, Nb may be added when high-temperature oxidation is required. The preferred Nb content is 0.1% or more, more preferably 0.3% or more, and even more preferably 0.4% or more. On the other hand, if Nb is added in excess, even though Nb has a large solid solubility limit, the total amount of Nb and other β-stabilizing elements such as Cu and Fe becomes large, resulting in an increased amount of β-phase formation. This results in a decrease in high-temperature strength, so the upper limit is set to 1.0% or less. The preferred Nb content is 0.9% or less, and more preferably 0.8% or less.

[0026] N: 0.08% or less C: 0.05% or less H:0.015% or less N and C are elements that reduce the ductility and workability of titanium alloy sheets. Furthermore, H is an element that causes embrittlement. Therefore, the N content is limited to 0.08% or less, the C content to 0.05% or less, and the H content to 0.015% or less. The N, C, and H contents are preferably low, and even 0% is acceptable. However, since N, C, and H may be mixed in as impurities from the raw material sponge titanium, scrap, and alloying element raw materials, the N, C, and H contents may each be set to 0.0001% or more.

[0027] Remainder: Ti and impurities The chemical composition of the titanium alloy plate according to this embodiment contains the above elements, with the remainder being Ti and impurities. Examples of impurities include metal elements derived from scrap used as a raw material. Specifically, promoting the use of scrap as a raw material results in the inclusion of elements other than the above elements (Cu, Fe, Nb, O, C, N, and H). While strict management can prevent the inclusion of these elements, the processing costs increase. Therefore, in the titanium alloy plate according to this embodiment, the inclusion of elements derived from scrap (as impurities) is permitted to the extent that the effects of the titanium alloy plate according to this embodiment are not impaired. Examples of elements derived from scrap include Si, Cr, Ni, Mo, V, Sn, Co, Zr, Mn, Ta, W, Hf, Pd, Ru, and B. When these elements are contained, their contents are, for example, 0.1% or less each, and a total of 0.3% or less is acceptable.

[0028] <Average grain size: 3~50μm> The larger the average grain size of the α phase of a Ti alloy sheet, the higher the formability and high-temperature strength, but the lower the strength in the medium temperature range from room temperature to less than 500°C. Therefore, it is necessary to appropriately control the grain size of the α phase along with the chemical composition. If the average grain size of the α phase is less than 3 μm, the formability will be extremely low, so the average grain size is set to 3 μm or more. Preferably, it is set to 5 μm or more. On the other hand, if the average grain size of the α phase exceeds 50 μm, although ductility at room temperature can be ensured, the number of grains may decrease depending on the sheet thickness, and formability may be reduced. Furthermore, if the average grain size is too large, the required strength at room temperature may not be ensured. Therefore, the average grain size is set to 50 μm or less. Preferably, it is set to 40 μm or less, and more preferably, it is set to 30 μm or less. The average grain size is the average grain size in a cross section parallel to the sheet thickness direction. The cross section parallel to the plate thickness direction includes a cross section parallel to the plate longitudinal direction (the direction corresponding to the rolling direction) or a cross section parallel to the plate width direction, but in this embodiment either cross section is acceptable, and preferably a cross section parallel to the plate longitudinal direction.

[0029] <Collective organization> When titanium alloy plate is used as a material for automobile exhaust system parts, it is necessary to form the titanium alloy plate into the part shape, and for this purpose, the titanium alloy plate needs to have excellent formability at room temperature. In order to improve the formability of the titanium alloy plate, it is necessary to have the following texture.

[0030] That is, the titanium alloy sheet of this embodiment has an α-phase area ratio of less than 25% in which the angle between the c-axis and the sheet thickness direction is in the range of 60° to 90°, and has a texture called Split-TD in which the maximum integration degree of the (0001) plane in the range of the angle between the sheet thickness direction and the sheet thickness direction is 4.5 or more in the (0001) pole figure in the sheet thickness direction. Figure 1 shows the (0001) pole figure in the sheet thickness direction of the titanium alloy sheet of this embodiment.

[0031] [Area ratio of α phase where the angle between the c axis and the thickness direction is within the range of 60° to 90°: less than 25%] Titanium with an α-phase hcp structure exhibits very little deformation in the c-axis direction of the hcp. Therefore, controlling the c-axis direction of the α-phase is important. During hot rolling, the c-axis of the α-phase of titanium is oriented in a wide range, split between a direction parallel to the T direction (sheet width direction) and a direction parallel to the sheet thickness direction. Among these, T-texture, in which the c-axis is parallel to the T direction, remains in large quantities when the reduction ratios in hot rolling and cold rolling are not appropriately controlled. While this T-texture increases the Young's modulus of titanium alloy sheets, it significantly reduces formability in the T direction.

[0032] To ensure high formability, it is necessary to minimize this T-texture. If the area ratio of the α phase, in which the angle between the c-axis and the thickness direction is within the range of 60° to 90°, is less than 25%, the proportion of T-texture is sufficiently small to ensure high formability. It is preferably 22% or less, and more preferably 20% or less.

[0033] [Split-TD type texture with a maximum density of 4.5 or more for the (0001) plane within the angle of 25 to 45° with respect to the thickness direction] As mentioned above, reducing the proportion of T-texture texture is important to ensure high formability. However, because Split-TD texture provides the highest formability, the greater the deviation from this orientation of textures other than T-texture, the worse the formability. Therefore, as shown in Figure 1, it is important to form Split-TD texture and increase the maximum density. In particular, because T-texture develops more easily in titanium alloys than in pure titanium, appropriate control of cold rolling and subsequent annealing is extremely important for achieving the maximum density.

[0034] In this embodiment, in a (0001) pole figure from the sheet thickness direction obtained by EBSD measurement, the maximum density portion exists in the sheet thickness direction at an angle of 25 to 45° and at an angle of -10° to 10° with respect to the rolling direction, and sufficient formability can be ensured if the maximum density is 4.5 or more. Preferably, it is 5.0 or more. On the other hand, since a higher density improves formability, no upper limit is set, but the practical upper limit is 30 or less.

[0035] <Area ratio of second phase> The structure of the titanium alloy sheet of this embodiment consists of an α phase and a second phase. The second phase is either TiCu or a β phase, or both. Since the β phase has lower high-temperature strength than the α phase, the β phase is minimized to increase high-temperature strength. On the other hand, if the structure is a single α phase, the pinning effect is not exerted during annealing when manufacturing the titanium alloy sheet, and the α phase may become coarser than necessary, resulting in reduced formability. Furthermore, if the proportion of the second phase is too small, the grain growth of the α phase may become uneven, resulting in mixed grains, which may reduce formability. Therefore, the area proportion of the second phase is desirably 0.1% or more and 3.5% or less, and preferably 3.0% or less. The area fraction of the second phase is the area fraction in a cross section parallel to the sheet thickness direction. As in the case of the average grain size, a cross section parallel to the sheet thickness direction includes a cross section parallel to the sheet longitudinal direction (the direction corresponding to the rolling direction) or a cross section parallel to the sheet width direction. In this embodiment, either cross section is acceptable, and a cross section parallel to the sheet longitudinal direction is preferred.

[0036] <Formula (1)> Mechanical properties such as strength and ductility vary depending on the chemical composition, the average grain size of the α phase, etc., and therefore, these must be systematically controlled. In this embodiment, if the value of the middle part of the following formula (1) is greater than 6.5 and less than 14.5, mechanical properties such as strength and ductility can be further improved, and formability can also be further improved.

[0037] 6.5<[%Nb]+2.5×[%Cu]+15×[%Fe]+45×[%O]+12×D -0.5 <14.5 …(1)

[0038] In the formula (1), [%Nb], [%Cu], [%Fe] and [%O] are the contents (mass%) of Nb, Cu, Fe and O contained in the titanium alloy plate, respectively, and D is the average crystal grain size (μm) of the α phase.

[0039] If the value of the middle part of formula (1) exceeds 6.5, high-temperature strength and formability can be further improved. It is preferably 7.0 or more. On the other hand, if the value of the middle part of formula (1) is less than 14.5, the strength at room temperature does not become excessively high, and formability does not decrease. It is preferably 14.0 or less.

[0040] The titanium alloy plate of the present embodiment can ensure formability and strength in the temperature range of room temperature, medium temperature and high temperature range of 500 to 700 ° C. Hereinafter, the formability, strength at room temperature and high temperature strength will be described.

[0041] <Moldability> The titanium alloy sheet of this embodiment exhibits high formability, particularly in the width direction perpendicular to the rolling direction. Specifically, two rectangular test pieces were prepared: one measuring 90 mm in the width direction and 30 mm in the longitudinal direction (rolling direction), and the other measuring 90 mm in the width direction and 60 mm in the longitudinal direction (rolling direction). The test piece was then placed between a hemispherical punch with a 40 mm diameter tip and a die with a circular recess with a 44 mm diameter. A blank holder with a 40.5 mm inner diameter through-hole was used to apply a blank holder force of 7 tons to the test piece. The punch was raised at a rate of 20 mm / min, and poly sheet (Naflon Tape) and high-viscosity oil (#660, Nippon Kosakuyu Co., Ltd.) were used as lubricants. The maximum bulge height at break was measured. A maximum bulge height of 20 mm or greater was considered to be good formability in the width direction.

[0042] <Strength and ductility at room temperature> Since the titanium alloy plate of this embodiment is a structural material, it is preferable that it has high strength at room temperature. Therefore, it is preferable that the 0.2% proof stress in the rolling direction is 220 MPa or more. On the other hand, if the 0.2% proof stress exceeds 400 MPa, the load on the equipment during forming at room temperature is too high, which is not preferable. Therefore, the 0.2% proof stress in the rolling direction of the titanium alloy plate is set to 220 MPa or more and 400 MPa or less. The upper limit of the 0.2% proof stress is preferably 380 MPa or less, and more preferably 370 MPa or less.

[0043] A ductility of 25% or more at room temperature ensures sufficient workability for processing such as tensile deformation, so this is set as the lower limit. It is preferably 27% or more. While there is no upper limit, the practical upper limit is 45% or less.

[0044] <High temperature strength> The titanium alloy plate of this embodiment preferably has high high-temperature strength. In particular, when used in mufflers, the plate may be exposed to high temperatures due to exhaust gas, and it is preferable that the plate has sufficient strength to withstand such temperatures. Specifically, the 0.2% yield strength at 700°C is preferably 20 MPa or more, more preferably 25 MPa or more.

[0045] <Method for measuring average crystal grain size> The average grain size can be measured and calculated as follows. Specifically, a titanium alloy plate is cut parallel to the plate thickness direction, and the exposed cross section is chemically polished. An electron backscattering diffraction (EBSD) pattern is used to measure 2 to 10 fields of view in a 500 to 1000 μm × 500 to 1000 μm area of ​​the exposed cross section in 1 μm increments. Regarding the grain size, the boundaries with a misorientation of 5° or more measured by EBSD are defined as grain boundaries, and the areas surrounded by these grain boundaries are defined as grains. The circle-equivalent grain size (area A = π × (grain size D / 2)2) is calculated from the area of ​​the grains, and the average value based on the number of these grains is defined as the average grain size.

[0046] <Method for measuring texture> The area ratio and maximum accumulation degree of the α phase in which the angle between the c axis and the sheet thickness direction is in the range of 60° to 90° are calculated using the following method.

[0047] For the area ratio of the α phase within the 60° to 90° range, the titanium alloy plate is cut parallel to the plate thickness direction and the exposed cross section is chemically polished. Using EBSD, an area of ​​500 to 1000 μm x 500 to 1000 μm on the exposed cross section is measured in 1 μm increments, covering 2 to 10 fields of view. From the measurement data, TSL Solutions' OIM Analysis software is used to extract data from measurement points where the angle between the ND and c axes is between 60° and 90°, and the percentage divided by the total number of data points is used to determine the percentage within that angle.

[0048] The peak position of the concentration of a specific orientation in the (0001) pole figure was calculated using the data from TSL Solutions' OIM Analysis™ software (Ver. 8.1.0) through texture analysis of the inverse pole figure using the spherical harmonics method. The position with the highest contour line was the peak concentration position, and the highest concentration among the peak positions was taken as the maximum concentration. The concentration of a specific orientation in the (0001) pole figure indicates how many times the frequency of crystal grains with that orientation is compared to a structure with a completely random orientation distribution (concentration of 1). In the above, the L-section at the center of the width was used as the observation surface. However, since the crystal orientation of titanium alloy sheets is uniformly distributed across the width, the L-section at any width position can also be used as the observation surface.

[0049] <Method for measuring the area ratio of the second phase> The area fraction of the second phase in a titanium alloy plate is measured and calculated using an EPMA (Electron Probe Microanalyzer) (SEM / EPMA) attached to a SEM (Scanning Electron Microscopy). Specifically, a cross section of the titanium alloy plate parallel to the plate thickness direction is mirror-polished, and a 500 μm × 500 μm area of ​​the cross section, centered at a position 1 / 4 of the plate thickness from the surface, is measured at a magnification of 100 to 200 times, with 2 to 5 fields of view measured in 1 to 2 μm increments, to measure the concentration distribution of Fe and Cu. Since Fe and Cu are concentrated in the β phase or Ti2Cu region, points (concentrated areas) where the Fe or Cu concentration is 1 mass % or more higher than the average concentration in the measurement range are defined as second phases, and the area fraction is calculated.

[0050] <Methods for measuring room temperature strength, ductility, and high temperature strength> Room temperature strength and ductility are measured by the following method. A No. 13B test piece, as specified in JIS Z 2241:2022, is taken from the titanium alloy plate so that the longitudinal direction of the test piece coincides with the longitudinal direction of the plate. Then, a room temperature tensile test is performed to measure the 0.2% proof stress and total elongation (%). Here, the strain rate is 0.5% / min up to 2% strain, and 30% / min after the strain exceeds 2%. The obtained total elongation is taken as the ductility.

[0051] The high-temperature strength (tensile strength) at 700°C is measured using the following method. A test specimen with a parallel section width of 10 mm, a parallel section length of 35 mm, and a parallel section thickness equal to the plate thickness is taken from the titanium alloy plate so that the longitudinal direction of the test specimen coincides with the longitudinal direction of the plate. The test specimen is then heated to 700°C at a heating rate of 45°C / min, and then held in the test atmosphere for 10 minutes to ensure that the test specimen reaches the test temperature. A tensile test is then performed at a test temperature of 700°C and a strain rate of 7.5% / min to determine the high-temperature strength at 700°C.

[0052] When the rolling direction of the titanium alloy plate can be confirmed, the rolling direction is taken as the longitudinal direction of the plate. When the rolling direction cannot be confirmed, the RD direction can be estimated from the (0001) pole figure obtained when measuring the texture, and this can be taken as the longitudinal direction of the plate.

[0053] <Automotive exhaust system parts> Next, an automotive exhaust system part (hereinafter referred to as "exhaust system part") according to this embodiment will be described. The exhaust system component of this embodiment is obtained by forming a titanium alloy plate, and has the same chemical composition, average grain size, and texture as the titanium alloy plate.

[0054] However, with regard to the texture, when a (0001) pole figure is measured from the sheet thickness direction, it is sufficient that a split-TD texture is observed and that a maximum density part with a maximum density of 4.5 or more exists within the angle range of -10° to 10° with respect to the rolling direction. As with the texture of titanium alloy sheets, it is not necessarily clear that the maximum density part exists within the angle range of -10° to 10° with respect to the rolling direction; this point may be inferred from the observation of a split-TD texture. In this case, the direction in which the two split maximum density parts are aligned can be inferred to be the direction corresponding to the sheet width direction.

[0055] The average crystal grain size, texture, and area ratio of the second phase of the exhaust system component may be measured in accordance with various measurement methods used for titanium alloy plates.

[0056] <Manufacturing method> Next, a method for manufacturing the titanium alloy plate of this embodiment will be described. The titanium alloy plate of this embodiment can be manufactured by a hot rolling process in which a titanium alloy material is hot rolled, a cold rolling process in which the hot-rolled titanium alloy plate (hereinafter referred to as a hot-rolled plate) is cold-rolled, and a final annealing process in which the cold-rolled titanium alloy plate (hereinafter referred to as a cold-rolled plate) is annealed. A hot-rolled plate annealing process in which the hot-rolled plate is annealed may be performed between the hot rolling process and the cold rolling process, and a straightening and temper rolling process may be performed after the final annealing process. Each process will be described below.

[0057] <Hot rolling process> It is known that when titanium material is unidirectionally rolled in the temperature region of the β single-phase region or the α + β two-phase region, T-texture develops strongly. In this embodiment, however, in order to develop the desired texture, i.e., to minimize the development of T-texture, it is necessary to control the rolling conditions in the cold rolling process as well as to suppress the development of T-texture in processes other than the cold rolling process while developing Split-TD-texture. Therefore, the hot rolling in this embodiment must be controlled as follows.

[0058] First, the heating temperature must be controlled to 800°C or higher and lower than (β transformation point + 100)°C. If the temperature exceeds (β transformation point + 100)°C, the majority of the rolling process will be performed in a region with a high β phase content, resulting in the development of a strong T-texture. Therefore, even if subsequent cold rolling or annealing is controlled, the T-texture cannot be reduced, resulting in poor formability. Therefore, the heating temperature must be lower than (β transformation point + 100)°C. On the other hand, if the heating temperature is lower than 800°C, the reaction force of the material at the beginning of hot rolling will be large, and if the rolling speed is increased, the material may heat up to near the β transformation point due to processing heat. This may accelerate the development of T-texture. Therefore, the heating temperature is set to 800°C or higher.

[0059] The β transformation point refers to the boundary temperature at which the α phase begins to form when a titanium alloy is cooled from the β single-phase region. The β transformation point can be obtained from a phase diagram. A phase diagram can be obtained, for example, using the CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) method. Specifically, the β transformation point can be calculated from a titanium alloy phase diagram obtained using the CALPHAD method using Thermo-Calc, an integrated thermodynamic calculation system from Thermo-Calc Software AB, and a specified database (TI3).

[0060] In the hot rolling process of this embodiment, it is important to control the rolling ratio. The higher the rolling ratio, the more likely it is that the desired texture will be formed. If the rolling ratio in the hot rolling process is 95% or more, the desired texture will be obtained. Therefore, the rolling ratio is set to 95% or more.

[0061] After the hot rolling is completed, the hot-rolled sheet is preferably cooled until the surface temperature becomes 300° C. or less. The cooling may be performed by water cooling, for example.

[0062] <Hot-rolled sheet annealing process> Hot-rolled sheet annealing may or may not be performed. If hot-rolled sheet annealing is not performed, strain can be accumulated in the sheet, allowing for more texture development. On the other hand, if hot-rolled sheet annealing is performed, most of the second phase can be solid-dissolved once, and the α phase will be formed uniformly during annealing after cold rolling, resulting in a stable average grain size of the α phase.

[0063] When hot-rolled sheet annealing is performed, it is recommended to perform the annealing at a soaking temperature of 730°C or higher and 830°C or lower, preferably 820°C or lower. If the soaking temperature is lower than 730°C or higher than 830°C, the solid solubility limit of Cu becomes smaller, and combined with the final annealing, the area ratio of the second phase may become larger than necessary. Furthermore, if the soaking temperature is higher than 830°C, the area ratio of the β phase increases, and when the β phase transforms during subsequent cooling, the crystal orientation changes, and the desired texture may not be formed.

[0064] The soaking time for annealing the hot-rolled sheet is preferably, for example, 1 to 10 minutes (60 to 600 seconds).

[0065] <Cold rolling process> In this embodiment, how to control the cold rolling is very important. As described above, T-texture is formed by hot rolling, but it is very difficult to reduce the T-texture by controlling the hot rolling alone. Therefore, it is necessary to control the cold rolling conditions.

[0066] Specifically, the cold rolling may be performed in one pass without intermediate annealing, or in two passes with intermediate annealing in between. The cold rolling may be performed in one pass or in two or more passes.

[0067] The total reduction in cold rolling must be 70% or more. If intermediate annealing is performed, the reduction in the second cold rolling after intermediate annealing must be 60% or more. If the total reduction is less than 70%, a large amount of T-texture remains, resulting in reduced formability. Furthermore, even if the total reduction is 70% or more, if intermediate annealing is performed and the reduction in the second cold rolling is less than 60%, a large amount of T-texture will remain, resulting in reduced formability. Therefore, the cold rolling process must be performed at the above reductions. The higher the reduction, the better the texture and the less T-texture there is, so there is no particular upper limit; however, due to the characteristics of cold rolling equipment, 92% or less is the practical upper limit.

[0068] <Intermediate annealing> Intermediate annealing, which may be performed during cold rolling, may or may not be performed. Not performing intermediate annealing is preferable because it allows for a higher cold rolling rate and improves the texture. However, even if intermediate annealing is performed, the desired texture can be obtained by performing hot rolling and cold rolling within the above ranges.

[0069] The conditions for the intermediate annealing are not particularly limited, and may be the same as those for the final annealing.

[0070] <Final annealing process> Final annealing increases the amount of dissolved Cu in the α phase, improving high-temperature strength. The final annealing step is performed in a vacuum batch heat treatment furnace or continuous annealing equipment. In either case, annealing is performed in a non-oxidizing atmosphere.

[0071] When using a vacuum batch type heat treatment furnace, the coiled cold-rolled sheet is placed in the furnace, the soaking temperature is set to 600°C or higher and lower than 700°C, and the holding time for the temperature of the outermost part of the coil within the soaking temperature range is set to 5 hours or more (18,000 seconds or more). There is no particular upper limit to the holding time, but from the viewpoint of productivity, it is set to 30 hours or less.

[0072] When annealing using continuous annealing equipment, the annealing temperature is set to 750°C or higher and the β transformation point or lower, and the time held in this temperature range is 1 minute or longer but less than 10 minutes (60 seconds or longer but less than 600 seconds).

[0073] When using a vacuum batch-type heat treatment furnace, the maximum temperature is set to less than 700°C due to issues such as seizure. In this temperature range, the diffusion rate of elements within the titanium alloy sheet is not very fast. Therefore, in order to maximize the amount of Cu dissolved in solid solution at the target temperature (maximizing the solid solution strengthening ability), it is necessary to hold the temperature for a long time. In addition, in batch-type annealing, the temperature varies depending on the location within the coil. Therefore, holding the temperature for a long time is also very important in order to make the entire coil homogeneous. To satisfy the above, it is necessary to hold the temperature of the outer periphery of the coil at the specified temperature for at least 5 hours, preferably at least 6 hours.

[0074] When annealing using continuous annealing equipment, the annealing temperature can be high, but it must be controlled to a temperature at which Cu can easily dissolve. Therefore, the temperature is set to 750°C or higher and below the β transformation point. On the other hand, the holding time can be short, as long as the holding time in the same temperature range is 1 minute or more and less than 10 minutes.

[0075] <Straightening and temper rolling process> In this embodiment, after final annealing, tempering may be performed using a tension leveler or skin pass, but this is not necessary. By performing such rolling, it is possible to improve the strength at room temperature while correcting the shape. However, formability such as ductility may be reduced. Therefore, the reduction amount (elongation amount of the entire coil) in temper rolling should be set to 0.2% or more and 2% or less. If the reduction amount is 0.2% or more, the target room temperature strength can be obtained, and if the reduction amount is 2% or less, a reduction in formability can be avoided. [Example]

[0076] Titanium materials having the compositions of alloy numbers 1 to 15 shown in Table 1 were melted by vacuum arc melting and hot forged to form slabs. The obtained slabs were successively subjected to a hot rolling process, a cold rolling process, and a final annealing process. Cooling after hot rolling was performed by water cooling to 300°C or less. Intermediate annealing in the cold rolling was performed by a batch method. Some of the slabs were subjected to a hot-rolled sheet annealing process between the hot rolling process and the cold rolling process. In addition, some of the slabs were subjected to a straightening and temper rolling process. The conditions for each process are shown in Table 2. Titanium alloy sheets Nos. 1 to 27 were produced in this manner.

[0077] The average grain size and texture of the obtained titanium alloy sheets were measured. The area ratio of the second phase was also measured. Furthermore, room temperature strength, ductility, and high temperature strength were also measured. The measurement methods were as described above. However, the average grain size, EBSD measurement, and area ratio of the second phase were measured on cross sections parallel to the thickness direction and longitudinal direction of the titanium alloy sheets. The field of view when measuring the average grain size and texture was 1000 μm × 1000 μm. The results are shown in Table 3.

[0078] As shown in Tables 1 to 3, the titanium alloy sheets Nos. 1 to 10 and Nos. 16 to 21 had chemical compositions and textures that satisfied the ranges of the present invention, and thus were excellent in formability, room temperature mechanical properties (strength, ductility), and high temperature strength.

[0079] Titanium alloy sheets Nos. 11 to 15 had chemical compositions outside the range of the present invention, and therefore were insufficient in formability or room temperature strength.

[0080] Although the titanium alloy sheets Nos. 22 to 27 had chemical compositions that met the range of the present invention, the manufacturing conditions were outside the preferred ranges, so that the average grain size or texture did not meet the range of the present invention, resulting in insufficient formability or room temperature strength. In addition, some of the sheets also had reduced ductility.

[0081] [Table 1]

[0082] [Table 2]

[0083] [Table 3]

Claims

1. In mass%, Cu: 0.5-2.0%, Fe: 0.010-0.045%, Nb: 0 to 1.0%, O: 0.03 to 0.10%, N: 0.08% or less, C: 0.05% or less, H: 0.015% or less, The balance is Ti and impurities. The average grain size of the α phase in a cross section parallel to the plate thickness direction is 3 to 50 μm, The area ratio of the α phase in which the angle between the c axis and the plate thickness direction is within the range of 60° to 90° is less than 25%, and A titanium alloy plate having a (0001) pole figure from the plate thickness direction in which the maximum degree of integration of the (0001) plane within an angle range of 25 to 45 degrees with respect to the plate thickness direction is 4.5 or more.

2. 2. The titanium alloy plate according to claim 1, wherein the area ratio of the second phase other than the α phase in a cross section parallel to the plate thickness direction is 0.1% or more and 3.5% or less.

3. Titanium alloy plate according to claim 1, wherein in the (0001) pole figure from the plate thickness direction, the angle between the plate thickness direction and the (0001) plane is in the range of 25 to 45 degrees, and the angle between the plate thickness direction and the rolling direction is in the range of -10 degrees to 10 degrees, the maximum integration degree of the (0001) plane is 4.5 or more.

4. The titanium alloy plate according to claim 1, which satisfies the following formula (1): 6.5 < [% Nb] + 2.5 × [% Cu] + 15 × [% Fe] + 45 × [% O] + 12 × D －０．５ <14.5 …(1) In the formula (1), [%Nb], [%Cu], [%Fe] and [%O] are the contents (mass%) of Nb, Cu, Fe and O contained in the titanium alloy plate, respectively, and D is the average crystal grain size (μm) of the α phase.

5. An automotive exhaust system part made of a titanium alloy plate, The titanium alloy plate is In mass%, Cu: 0.5-2.0%, Fe: 0.010-0.045%, Nb: 0 to 1.0%, O: 0.03 to 0.10%, N: 0.08% or less, C: 0.05% or less, H: 0.015% or less, The balance is Ti and impurities. The average grain size of the α phase in a cross section parallel to the plate thickness direction is 3 to 50 μm, The area ratio of the α phase in which the angle between the c axis and the plate thickness direction is within the range of 60° to 90° is less than 25%, and An automotive exhaust system part having a texture in which, in a (0001) pole figure from the thickness direction, the angle formed with the thickness direction is in the range of 25 to 45° and the maximum integration degree of the (0001) plane is 4.5 or more.

6. 6. The automotive exhaust system part according to claim 5, wherein the area ratio of the second phase other than the α phase in a cross section parallel to the plate thickness direction is 0.1% or more and 3.5% or less.

7. 6. The automotive exhaust system part according to claim 5, which satisfies the following formula (1): 6.5<[%Nb]+2.5×[%Cu]+15×[%Fe]+45×[%O]+12×D−0.5<14.5…(1) In the formula (1), [%Nb], [%Cu], [%Fe] and [%O] are the contents (mass%) of Nb, Cu, Fe and O contained in the titanium alloy plate, respectively, and D is the average crystal grain size (μm) of the α phase.

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