Titanium alloy sheet and exhaust system component for motor vehicles

Abstract

A titanium alloy plate having a prescribed chemical composition, a metallographic structure composed of an α phase and a second phase, the α phase having an average grain diameter of 3.0 to 10.0 μm, in the α phase, a proportion of the number of grains having a grain diameter within a range of ±2 μm from the average grain diameter being 25% or more, in the α phase, a proportion of the number of grains having a grain diameter within a range of ±4 μm from the average grain diameter being 45% or more, in a case where 100 regions of 10 μm x 10 μm obtained by equally dividing a region of 100 μm x 100 μm in a cross section are set as measurement regions, and a number density of the second phase is found for each of the measurement regions, a number of measurement regions in which 5 or more and 15 or less of the second phase are observed within the measurement regions is 80 or more.

Description

Technical Field

[0001] This disclosure relates to titanium alloy plates and exhaust system components for motor vehicles. Background Technology

[0002] The exhaust system of four-wheeled motor vehicles and two-wheeled motor vehicles (hereinafter referred to as motor vehicles, etc.) includes an exhaust manifold and an exhaust pipe. Exhaust gases discharged from the engine and collected in the exhaust manifold are discharged to the outside through the exhaust port at the rear of the vehicle via the exhaust pipe. A catalytic converter and an exhaust muffler are located in the middle of the exhaust pipe to purify the exhaust gases and eliminate exhaust noise. In this manual, the entire system from the exhaust manifold to the exhaust pipe and then to the exhaust port is referred to as the "exhaust system". Furthermore, the components constituting the exhaust system, such as the exhaust manifold, exhaust pipe, catalytic converter, and exhaust muffler, are referred to as "exhaust system components".

[0003] Previously, stainless steel, known for its excellent corrosion resistance, high strength, and machinability, was used in the components of exhaust systems for motor vehicles. However, in recent years, titanium, which is lighter, stronger, and also offers superior corrosion resistance compared to stainless steel, has gradually gained popularity. For example, JIS 2 type industrial pure titanium is used in the exhaust systems of two-wheeled vehicles. Furthermore, titanium alloys with higher heat resistance are increasingly replacing JIS 2 type industrial pure titanium.

[0004] In particular, there has been a recent trend of rising exhaust gas temperatures. Therefore, exhaust gas temperatures in the exhaust pipe can sometimes reach around 800°C, requiring sufficient high-temperature resistance within this temperature range. Furthermore, it is desirable to suppress high-temperature oxidation (excellent oxidation resistance at high temperatures) in exhaust system components.

[0005] Furthermore, even if titanium plates exhibit excellent strength and oxidation resistance at high temperatures, poor machinability can make machining the components difficult. Therefore, when applying titanium plates to exhaust system components, good machinability during forming is also required.

[0006] Patent document 1 describes a titanium alloy with excellent high-temperature oxidation resistance, which contains 0.15 to 2% by mass of Si and limits Al to less than 0.30% by mass, with the balance being titanium and unavoidable impurities.

[0007] In addition, Patent Document 2 describes a titanium alloy with excellent high-temperature oxidation resistance and corrosion resistance, characterized in that it contains Al: 0.30-1.50% and Si: 0.10-1.0% by mass.

[0008] In addition, Patent Document 3 describes a heat-resistant titanium alloy for exhaust device components with excellent cold workability, which contains, by mass %, Cu: more than 2.1% and less than 4.5%, O: less than 0.04% and Fe: less than 0.06%, with the balance being Ti and unavoidable impurities.

[0009] In addition, Patent Document 4 describes a titanium alloy material for exhaust system components with excellent oxidation resistance, which contains, by mass %: Si: 0.1-0.6%, Fe: 0.04-0.2% and O: 0.02-0.15%, and the total content of Fe and O is more than 0.1% and less than 0.3%, with the balance being Ti and unavoidable impurities with a content of less than 0.04%.

[0010] However, the titanium alloys described in Patent Documents 1-4 aim to ensure high-temperature strength by limiting the chemical composition, without improving grindability.

[0011] Titanium plates used in exhaust system components are sometimes required to have a glossy surface. In such cases, the titanium plate is processed to achieve the required glossy surface by grinding its surface. As described in Patent Documents 1-4, although techniques for improving high-temperature oxidation resistance, corrosion resistance, and cold workability while ensuring heat resistance are known, further improving the grindability of the product after processing has not been studied.

[0012] Existing technical documents

[0013] Patent documents

[0014] Patent Document 1: Japanese Patent Application Publication No. 2007-270199

[0015] Patent Document 2: Japanese Patent Application Publication No. 2005-290548

[0016] Patent Document 3: Japanese Patent Application Publication No. 2009-030140

[0017] Patent Document 4: Japanese Patent Application Publication No. 2013-142183 Summary of the Invention

[0018] The problem the invention aims to solve

[0019] This disclosure was made in view of the above circumstances, and its object is to provide a titanium alloy sheet and an exhaust system component for motor vehicles with excellent workability, grindability, oxidation resistance at high temperatures and high temperature strength.

[0020] Solution for solving the problem

[0021] [1] One aspect of the titanium alloy plate disclosed herein has the following chemical composition: containing, by mass %: Cu: 0.7%–1.5%, Sn: 0.5%–1.5%, Si: 0.10%–0.60%, Nb: 0.1%–1.0%, Zr: 0%–1.0%, Cr: 0%–0.5%, Mo: 0%–0.5%, and Al: 0%–1.0%, and Fe and O are limited to Fe: less than 0.08% and O: less than 0.07%, respectively, with the balance being Ti and impurities. The metallographic structure consists of an α phase and a second phase, wherein the average grain diameter of the α phase is 3.0–1 mm. 0.0μm, in the α phase, the proportion of grains with a grain diameter within the range of ±2μm of the average grain diameter is 25% or more, in the α phase, the proportion of grains with a grain diameter within the range of ±4μm of the average grain diameter is 45% or more, and when 100 regions of 10μm×10μm are obtained by dividing a 100μm×100μm region in the cross section into 100 equal parts and set as measurement regions, and the number density of the second phase is calculated for each measurement region, the number of measurement regions in which 5 or more and 15 or fewer of the second phase are observed in the measurement regions is 80 or more.

[0022] [2] According to the titanium alloy plate described in [1] above, the area ratio of the second phase can be 1.0% or more.

[0023] [3] Another aspect of this disclosure relates to a motor vehicle exhaust system component comprising the titanium alloy plate described in [1] or [2] above.

[0024] [4] Another aspect of this disclosure relates to a motor vehicle exhaust system component obtained by forming the titanium alloy sheet described in [1] or [2] above.

[0025] The effects of the invention

[0026] According to the above-described manner of this disclosure, a titanium alloy sheet and an exhaust system component for motor vehicles with excellent machinability, grindability, and oxidation resistance at high temperatures can be provided. Detailed Implementation

[0027] Motor vehicle exhaust system components are manufactured by processes such as stamping of titanium alloy sheets. Furthermore, these components are used in high-temperature environments. Additionally, the titanium alloy sheets used for exhaust system components sometimes require a high degree of surface gloss. This gloss is sometimes improved by grinding the surface of the titanium alloy sheet; therefore, the titanium alloy sheet must have good machinability.

[0028] To improve the grindability of titanium alloy plates, it is necessary to (1) ensure a flat surface before grinding and (2) reduce the generation of grain-based cracks during grinding.

[0029] Regarding (1) above, for example, if there are unevenness or other irregularities on the surface of the titanium alloy plate caused by welding, the grinding time required to eliminate these irregularities becomes longer. That is, making the plate with a flat surface is one way to improve its grindability.

[0030] Regarding point (2) above, one reason for the impaired appearance after grinding is that the hardness varies depending on the crystal orientation, resulting in different grinding states for each grain. Since it is difficult to control the crystal orientation to be perfectly uniform, grain refinement is usually used to minimize the difference in grinding states between grains. However, sufficient grindability can only be achieved by reducing the grain diameter and ensuring uniform grain size. Furthermore, excessive grain refinement deteriorates formability, thus there are limits to grain refinement.

[0031] As mentioned above, considering the non-uniformity in actual manufacturing, it is not easy to control the grain diameter within the minimum range that ensures formability while simultaneously reducing the grain size distribution. Therefore, the inventors conducted research on reducing the differences in grain grinding conditions caused by crystal orientation. The results showed that by forming intermetallic compounds with a specified number density or higher as a second phase within the grains or at grain boundaries, the intermetallic compounds constrain the grains, suppress deformation during grinding, and result in more uniform grinding.

[0032] Furthermore, exhaust system components reach high temperatures during use, necessitating the addition of alloying elements to ensure the required high-temperature strength for these components. The inventors have investigated a chemical composition to ensure high-temperature strength at 800°C.

[0033] Based on the above viewpoints, in-depth research was conducted, resulting in the titanium alloy plate of this embodiment.

[0034] Hereinafter, a titanium alloy plate (the titanium alloy plate of this embodiment) according to one embodiment of the present disclosure and a motor vehicle exhaust system component (the motor vehicle exhaust system component of this embodiment) according to one embodiment of the present disclosure will be described.

[0035] The titanium alloy plate of this embodiment has the following chemical composition: by mass % Cu: 0.7%–1.5%, Sn: 0.5%–1.5%, Si: 0.10%–0.60%, Nb: 0.1%–1.0%, Zr: 0%–1.0%, Cr: 0%–0.5%, Mo: 0%–0.5%, and Al: 0%–1.0%, with Fe and O limited to Fe: 0.08% or less and O: 0.07% or less, respectively, and the balance being Ti and impurities. Furthermore, in the titanium alloy plate of this embodiment, the metallographic structure consists of an α phase and a second phase. The average grain diameter of the α phase is 3.0–10.0 μm. In the α phase, the proportion of grains with a grain diameter within ±2 μm of the average grain diameter is 25% or more, and the proportion of grains with a grain diameter within ±4 μm of the average grain diameter is 45% or more. Furthermore, for the titanium alloy plate of this embodiment, when 100 (parts) of 10μm×10μm area obtained by dividing the 100μm×100μm area in the cross section into 100 equal parts are set as measurement areas, and the number density of the second phase is calculated for each measurement area, the number of measurement areas in which 5 or more and 15 or less of the second phase are observed in the measurement areas is 80 or more.

[0036] Furthermore, the area ratio of the second phase of the titanium alloy plate in this embodiment is preferably 1.0% or more.

[0037] Secondly, the exhaust system component for motor vehicles in this embodiment includes the aforementioned titanium alloy plate.

[0038] First, the chemical composition of the titanium alloy plate of this embodiment will be explained. The unit "%" for the content of each element constituting the chemical composition means "mass %". In addition, the range indicated by "~" includes the values ​​at both ends as the lower limit and the upper limit.

[0039] Cu: 0.7–1.5%

[0040] To ensure sufficient high-temperature strength, the Cu content needs to be 0.7% or higher. Preferably, the Cu content is 0.8% or higher.

[0041] On the other hand, when the Cu content is greater than 1.5%, the processability decreases. Furthermore, the possibility of Cu segregation during ingot manufacturing increases. Therefore, the Cu content is set to 1.5% or less. The Cu content is preferably 1.4% or less, more preferably 1.3% or less, and even more preferably 1.2% or less.

[0042] Sn: 0.5-1.5%

[0043] To ensure sufficient high-temperature strength, the Sn content needs to be 0.5% or more. The Sn content is preferably 0.6% or more, more preferably 0.8% or more, and even more preferably 0.9% or more.

[0044] On the other hand, since Sn is difficult to form intermetallic compounds, it can be present in relatively large quantities. However, if the Sn content is too high, processability decreases, and the solid solution limits of Cu and Si in the α phase also decrease. Therefore, the Sn content needs to be 1.5% or less. Furthermore, Sn is a high-density element, and even with large additions, the amount is not significant when compared to the atomic number ratio, thus contributing little to solid solution strengthening. This is also the reason for limiting the upper limit of its content. The Sn content is preferably 1.4% or less, more preferably 1.3% or less, and even more preferably 1.2% or less.

[0045] Si: 0.10–0.60%

[0046] To ensure oxidation resistance and high-temperature strength, the Si content needs to be 0.10% or more. The Si content is preferably 0.15% or more, and more preferably 0.20% or more.

[0047] On the other hand, when the Si content is greater than 0.60%, silicides will form, which will not only significantly hinder grain growth but also reduce processability. Therefore, the Si content is set to 0.60% or less. The Si content is preferably 0.50% or less, more preferably 0.40% or less, even more preferably 0.35% or less, and even more preferably 0.30% or less.

[0048] Nb: 0.1–1.0%

[0049] To ensure oxidation resistance, the Nb content needs to be 0.1% or more. The Nb content is preferably 0.2% or more, and more preferably 0.3% or more.

[0050] On the other hand, the higher the Nb content, the higher the oxidation resistance, but in addition to the increase in raw material costs, the improvement in oxidation resistance will also reach its limit. Therefore, the Nb content is set to 1.0% or less. The Nb content is preferably 0.7% or less, more preferably 0.5% or less, and even more preferably 0.4% or less.

[0051] Zr: 0~1.0%

[0052] Zr is an element that readily forms intermetallic compounds with Si and Ti. Zr is also present in the formed intermetallic compounds. The presence of Zr facilitates a pinning effect and suppresses grain growth through a solute drag-like effect. Therefore, Zr can be included as needed. To achieve the aforementioned effects, the Zr content is preferably 0.1% or more.

[0053] On the other hand, the presence of Zr leads to a decrease in the β phase transition point, and the formation of intermetallic compounds is promoted, while the solute dragging effect weakens as the content decreases. Therefore, in the case of Zr content, the Zr content is set to 1.0% or less. The Zr content is preferably 0.8% or less, more preferably 0.6% or less, even more preferably 0.5% or less, and even more preferably 0.4% or less.

[0054] Zr is an optional element, therefore the lower bound is 0%.

[0055] Cr: 0% to 0.5%

[0056] Mo: 0% or more and 0.5% or less

[0057] Cr and Mo are optional elements, and their content can be 0%, but by including Cr and Mo, grain growth is suppressed by the solute dragging effect, and high-temperature strength is improved. Therefore, they can be included as needed. To obtain the above effects, it is preferable that the Cr content and Mo content are both 0.05% or more. More preferably, they are both 0.1% or more.

[0058] On the other hand, when the Cr and Mo contents increase, there is an excess of β phase at high temperatures, leading to decreased oxidation resistance. Furthermore, excessive Mo content reduces processability. Therefore, in cases containing Cr and Mo, the Cr and Mo contents are each 0.5% or less. Preferably, the Cr and Mo contents are each 0.4% or less, more preferably 0.3% or less.

[0059] Al: 0–1.0%

[0060] Al is an optional element; it can be 0%, but it can also be included to ensure high-temperature strength. To achieve the aforementioned effects, the Al content is preferably 0.1% or higher.

[0061] On the other hand, as the Al content increases, the α phase is stabilized and the formation of the β phase is suppressed, further improving high-temperature strength and oxidation resistance, but reducing processability, which is therefore undesirable. Furthermore, cold-rollability also decreases significantly. Therefore, in the case of Al content, the Al content is set to 1.0% or less. The Al content is preferably 0.8% or less, more preferably 0.6% or less, and even more preferably 0.5% or less.

[0062] Fe: below 0.08%

[0063] When the Fe content is excessive, the β phase is easily formed in the low-temperature range. Therefore, adding elements to the β phase leads to a decrease in the amount of solid-solubilized elements in the α phase, and the high-temperature strength decreases due to the increased β phase ratio. Furthermore, there are cases where the increased β phase ratio deteriorates oxidation resistance. Additionally, in the presence of Cr and Mo, the appropriate content range of Cr and Mo becomes narrower, making it difficult to control the chemical composition of Cr and Mo. Therefore, the lower the Fe content, the better, and it needs to be limited to 0.08% or less. The Fe content is preferably 0.06% or less, more preferably 0.04% or less.

[0064] O: below 0.07%

[0065] Oxygen (O) is an element that increases room temperature strength but hardly improves high-temperature strength. That is, when the O content increases, high-temperature strength does not improve; only springback increases, while processability decreases. Therefore, the lower the O content, the better. However, it is difficult to reduce oxygen (O) industrially, and extreme reductions would increase raw material costs. Therefore, the O content is limited to around 0.04%, although deviations can sometimes reach around 0.07%. Thus, the O content is restricted to below 0.07%.

[0066] One or more of Ni, V, Mn, Co, Ta, W, C, and N are present at 0 to 0.05% each, and their total content is less than 0.30%.

[0067] Ni, V, Mn, Co, Ta, and W all have significant β-phase stabilizing effects. Therefore, as in this embodiment, in titanium alloy plates where Nb, Cr, and Mo control the α and β phases, the lower the content of these elements, the better. Furthermore, when the content of N and C is excessive, the α phase is stabilized, and due to increased strength at room temperature, workability deteriorates. Therefore, the content of N and C is also better to be as low as possible. Therefore, whether these elements are intentionally included or included as impurities, it is preferable that the content of each element is 0.05% or less, and the total content of these elements is 0.30% or less.

[0068] Since the fewer of these elements the better, the lower limit for the content of each element and the total content is 0%.

[0069] The balance of the titanium alloy plate in this embodiment is Ti and other impurities besides those mentioned above.

[0070] Other impurities include, for example, hydrogen (H) and boron (B). H is an element that forms hydrides with Ti, and the titanium alloy sheet can sometimes become brittle when hydrides are formed. Therefore, even when present as impurities, it is preferable to minimize the H content. In the titanium alloy sheet of this embodiment, the H content is preferably 0.013% or less. Boron (B) can form coarse precipitates within the ingot. Therefore, even when present as impurities, it is preferable to minimize the B content. In the titanium alloy sheet of this embodiment, the B content is preferably 0.01% or less.

[0071] Next, the microstructure of the titanium alloy plate of this embodiment will be described.

[0072] The titanium alloy plate of this embodiment contains an α phase and a second phase with an average grain diameter of 3.0 μm or more and 10.0 μm or less. The second phase is the structure other than the α phase and is mainly composed of intermetallic compounds. The intermetallic compounds in this embodiment mainly include Ti₂Cu and silicides. It is also possible that a β phase is contained in the second phase, but even if a β phase is contained, it is in extremely small amounts (e.g., less than 0.2%), so the second phase can also be considered as an intermetallic compound. The α phase constitutes the majority (e.g., more than 95%) of the metallographic structure, and the remainder of the metallographic structure is the second phase.

[0073] Average grain diameter of the α phase: 3.0–10.0 μm

[0074] In the titanium alloy plate of this embodiment, a small average grain diameter of the α phase means that unrecrystallized portions remain. Therefore, when the average grain diameter of the α phase is small, the machinability decreases. The unrecrystallized portions also contribute to uneven grinding, resulting in poor grindability. Therefore, in order to avoid the formation of unrecrystallized portions, the average grain diameter of the α phase is set to 3.0 μm or more.

[0075] On the other hand, if the average grain diameter of the α phase is too large, the abrasiveness deteriorates. Therefore, it is necessary to keep the average grain diameter of the α phase below 10.0 μm.

[0076] The average grain diameter of the α phase can be determined using EBSD by the following method.

[0077] The α-phase grains are uniformly dispersed. Therefore, measurements can be performed at any position along the width direction. However, for example, at the center of the plate thickness in a section perpendicular to the width direction (L-section) at half the plate width (a distance of half the plate width from the end of the plate along the width direction), the accelerating voltage is set to 15 kV, the measurement interval is set to 0.2 μm at a magnification of 500x or higher, and the measurement is performed only on the α-phase. The measurement field of view is set to contain more than 300 grains in one field of view or more than 400 grains in total across multiple fields of view. The average CI value of the measured sample is adjusted to 0.2 or higher. OIM-analysis is used in the measurement analysis software. TM (Version 7.3.1) Boundaries with an orientation difference of 15° or more are considered grain boundaries. The grain diameter of each grain is approximated by the area of ​​the grain divided by this boundary using a circular equivalent diameter approximation. Grains with a diameter of 1.0 μm or less and grains not fully contained within the field of view are excluded from the grain diameter calculation. Grains whose grain boundaries are divided by the boundary of the measured field of view are considered grains not fully contained within the field of view.

[0078] When the width direction of the sheet is unknown, it can be determined by measuring the surface. Since the blank has a Split-TD type texture, (0001) will be strongly oriented at an angle of 30 to 40° in the width direction of the sheet. Therefore, in the measurement performed from the surface, the direction axis of the position where (0001) is strongly oriented becomes the width direction of the sheet.

[0079] Grain size distribution of the α phase: the proportion of grains with a diameter of ±2 μm from the average grain diameter is greater than 25%, and the proportion of grains with a diameter of ±4 μm from the average grain diameter is greater than 45%.

[0080] Even with an average grain diameter of less than 10.0 μm, a small number of coarse grains may still be present. If the grain diameters differ, the grinding state of the grains will also differ; therefore, if grains with large diameter differences are present, a sufficiently aesthetically pleasing result cannot be achieved after grinding. For this reason, in addition to controlling the average grain diameter of the α phase, it is preferable to control the particle size distribution of the α phase.

[0081] In the titanium alloy plate of this embodiment, the proportion of α-phase grains (grains constituting the α-phase) with a grain diameter within the range of ±2 μm of the average grain diameter (from -2 μm to +2 μm) is 25% or more of the total, and the proportion of grains with a grain diameter within the range of ±4 μm of the average grain diameter (from -4 μm to +4 μm) is 45% or more of the total. This reduces the likelihood of the titanium alloy plate containing coarse grains and improves its grindability.

[0082] There are no particular restrictions on the upper limits of the proportion of α-phase grains with a grain diameter in the range of ±2 μm of the average grain diameter and the proportion of grains with a grain diameter in the range of ±4 μm of the average grain diameter. The proportion can be 100% or less.

[0083] The particle size distribution of the α phase was determined using the following method.

[0084] The α-phase grains are uniformly dispersed. Therefore, measurements can be performed at any location along the width direction. However, for example, a measurement region for grain size distribution is set at the center of the plate thickness in a cross-section (L-section) perpendicular to the plate width direction at half the plate width. This region is a rectangular area with a side length of 100 μm or more, containing more than 100 grains. The crystal orientation of the α-phase in the measurement region is analyzed by electron backscatter diffraction (EBSD). OIM-analysis is used in the analysis software. TM (version 7.3.1) Boundaries with an orientation difference of more than 15° are considered as grain boundaries. The grain diameter of each grain is calculated by approximating the area of ​​the divided grains using the circular equivalent diameter.

[0085] Next, the grain diameter of each obtained grain, the proportion of grains with grain diameters within ±2 μm of the average grain diameter obtained by the method, and the proportion of α-phase grains with grain diameters within ±4 μm of the average grain diameter are determined.

[0086] Therefore, the grain size distribution of the crystals in the measurement area can be determined.

[0087] When the width direction of the sheet is unknown, it can be determined by measuring the surface. Since the blank has a Split-TD type texture, (0001) will be strongly oriented at an angle of 30 to 40° in the width direction of the sheet. Therefore, in the measurement performed from the surface, the direction axis of the position where (0001) is strongly oriented becomes the width direction of the sheet.

[0088] Distribution of the number density of the second phase: In 100 measurement regions, the number of measurement regions in which 5 to 15 of the second phase were observed was greater than 80.

[0089] Next, the distribution of the second phase will be explained.

[0090] The titanium alloy plate of this embodiment has a metallographic structure dominated by the α phase, with the remainder being a second phase. The second phase primarily consists of various intermetallic compounds. When using pinning or solute dragging effects to suppress α phase grain growth, a uniform presence of the second phase or alloying elements is necessary. By uniformly containing the second phase, the α phase grain growth at high temperatures during use is prevented from becoming uneven, thus improving high-temperature strength. Conversely, when the second phase is unevenly distributed in the metallographic structure, the degree of α phase grain growth varies locally in the high-temperature region, leading to a decrease in high-temperature strength. Furthermore, the uneven distribution of the second phase contributes to the formation of a mixed-grain structure during annealing; if this mixed-grain structure forms, abrasiveness and fatigue properties deteriorate. Even without the formation of a mixed-grain structure, abrasiveness is reduced due to the uneven distribution of the second phase. Therefore, a uniform distribution of the second phase in the metallographic structure is essential.

[0091] In the titanium alloy plate of this embodiment, the proportion of the region containing a predetermined number of second phases is used as an indicator of the uniformity of the distribution of the second phase, obtained by measuring the number of second phases in multiple measurement regions of the cross-section of the titanium alloy plate.

[0092] Specifically, in a cross-section perpendicular to the width direction of the titanium alloy plate at half the plate width, each side of a 100μm (100μm × 100μm) region is divided into 10 equal parts (100 regions with 10μm sides). These 10 regions are designated as number density measurement areas. The number of second phases in each measurement area is calculated. Furthermore, the number of measurement areas in which 5 to 15 second phases are observed within the 100 regions is determined. If 80 or more measurement areas in which 5 to 15 second phases are observed within the measurement area are considered to have a uniform distribution of the second phase. Alternatively, 5 to 15 second phases may be observed throughout the entire measurement area. That is, the upper limit for the number of measurement areas in which 5 to 15 second phases are observed within the measurement area is 100. When calculating the number density of the second phase, if the second phase exists at the boundary of the divided region, it is divided by the number of adjacent regions. For example, if the two regions exist across two regions, the two regions are added together as 0.5 each.

[0093] When the distribution of the second phase meets this condition, the grindability is improved. In addition, it is difficult to produce a mixed crystal structure of the α phase when heated at high temperature, thus improving the fatigue strength.

[0094] The number of regions where more than 5 but less than 15 second phases were observed can be determined using the following method.

[0095] Since the α phase and the second phase are uniformly dispersed, the measurement location can be any position in the width direction. However, for example, the location of a region with one side of 100 μm is the center of the plate thickness at a cross-section (L-section) perpendicular to the width direction of the titanium alloy plate, located at 1 / 2 the length of the plate width. This region is divided into 10 equal parts along each side, defining 100 measurement regions. Each measurement region is observed using a scanning electron microscope (SEM), and the α phase and the second phase are distinguished based on the backscattered electron images. The second phase, which is an intermetallic compound, is white or black and consists of tiny precipitates compared to the parent phase α phase; therefore, it can be identified as the second phase based on this characteristic. The number of second phases within each measurement region is counted. This is performed on 100 measurement regions, and the number of measurement regions with 5 to 15 second phases is counted.

[0096] When the width direction of the sheet is unknown, it can be determined by measuring the surface. Since the blank has a Split-TD type texture, (0001) will be strongly oriented at an angle of 30 to 40° in the width direction of the sheet. Therefore, in the measurement performed from the surface, the direction axis of the position where (0001) is strongly oriented becomes the width direction of the sheet.

[0097] Area ratio of the second phase

[0098] The area fraction of the second phase in the metallographic structure is preferably 0.01% or more, more preferably 0.05% or more, even more preferably 0.1% or more, and even more preferably 1.0% or more. The presence of the second phase at an area fraction of 0.1% or more improves abrasiveness. In particular, abrasiveness can be further improved by making the area fraction of the second phase 1.0% or more. On the other hand, to ensure sufficient processability, it is preferable that the upper limit of the area fraction of the second phase is 3.0% or less, more preferably 2.0% or less.

[0099] The area fraction of the second phase was measured in the same region as the number density of the second phase. Specifically, a region with one side measuring 100 μm was observed using a scanning electron microscope (SEM) (for example, a section perpendicular to the width direction of the titanium alloy plate, taken at half the length of the plate, at the center of the plate thickness, called the L-section). The α-phase and the second phase were distinguished based on the backscattered electron image. The second phase, being an intermetallic compound, is white or black and consists of minute precipitates compared to the parent phase α-phase; therefore, it can be identified as the second phase based on this characteristic. Furthermore, the area of ​​the second phase within the region was measured, and the area fraction (%) of the second phase was determined.

[0100] Furthermore, the microstructure of the titanium alloy sheet in this embodiment is preferably equiaxed. In acicular microstructure, due to the dense concentration of regions with the same macroscopic crystal orientation, the abrasiveness deteriorates. Specifically, the average aspect ratio (major axis length / minor axis length) of the α phase, which constitutes the majority of the microstructure, is preferably 3.0 or less. As described later, although acicular grains are temporarily formed during hot-rolled sheet annealing or intermediate annealing when heated to above 830°C and above the β phase transformation point, recrystallization occurs through subsequent cold rolling and final annealing, forming an equiaxed α phase. The aspect ratio is the ratio of the (major axis length / minor axis length) of the α grains on the L-section of the titanium alloy sheet, i.e., the aspect ratio, and is the average aspect ratio of 10 grains.

[0101] The titanium alloy plate of this embodiment preferably has the following characteristics.

[0102] Total elongation: ≥25.0%

[0103] While it also depends on the shape of the part after forming, it is necessary to be able to at least form and weld the titanium alloy sheet into a tubular shape. Furthermore, the tube needs to be bent afterward. Therefore, to ensure sufficient processability during part forming, the total elongation of the titanium alloy sheet in this embodiment is preferably 25.0% or more. It is not necessary to limit the upper limit of the total elongation, but approximately 50.0% is a substantially acceptable upper limit in industrial applications.

[0104] Total elongation was determined by performing a room temperature tensile test. Small-sized ASTM tensile test specimens (parallel section width: 6.25 mm, parallel section length: 32 mm, mark spacing: 25 mm) were collected from the aforementioned titanium alloy plate, with the length direction parallel to the rolling direction. The strain rate was set to 30% / min, and tensile tests were conducted at room temperature. The test temperature ranged from 10 to 35°C.

[0105] Eriksen Cup spur value: 9.5mm or higher

[0106] The Erichsen cupping test is a test to evaluate factors important in deep drawing and stretching processes other than tube shape forming. In this embodiment, the titanium alloy sheet preferably has an Erichsen cupping value of 9.5 mm or higher, taking into account a balance with improved abrasiveness.

[0107] The Erikson cupping value was determined according to the Erikson cupping test method specified in JIS Z 2247 (2006). The thickness of the test sample ranged from 0.1 to 2.0 mm, and the width was 90 mm or more. The testing machine was as described in JIS B 7729 (2005). The fixture dimensions were those of the standard test piece. The lubricant used was a 50 μm thick polytetrafluoroethylene (PTFE) sheet (registered trademark).

[0108] Oxidation resistance: The oxidation increment after being exposed to air at 800°C for 100 hours is 5.0 mg / cm³. 2 the following

[0109] The oxidation increment in commonly used exhaust system components is approximately 5.0 mg / cm³. 2 In the titanium alloy plate of this embodiment, it is desirable to achieve this even assuming use at 800°C. Therefore, as an indicator of oxidation resistance, it is preferable that the oxidation increment after being kept at 800°C for 100 hours in the atmosphere meets a value of 5.0 mg / cm³. 2 the following.

[0110] A 20mm × 20mm test piece was taken from the aforementioned titanium alloy plate. The surface was wet-polished with #400 sandpaper and exposed to still air at 800℃ for 100 hours. The increase in mass after exposure was measured, and the value was obtained by dividing the increase in mass by the surface area of ​​the tensile test piece (increase in mass (mg) / surface area of ​​test piece (cm²)). 2 As an oxidation increment. When oxide scale peeling occurs through oxidation testing, the peeled oxide scale should also be included in the post-exposure mass.

[0111] High-temperature strength (tensile strength): above 26 MPa at 800℃

[0112] As a material, high-temperature strength is required. In this embodiment, considering that high-temperature strength within the envisioned temperature range is important, and considering that it is suitable for exhaust system components that can withstand high-temperature exhaust gas temperatures, the titanium alloy plate in this embodiment preferably has a tensile strength of 26 MPa or higher at 800°C.

[0113] The high-temperature strength (tensile strength) at 800℃ was determined by conducting a high-temperature tensile test. Tensile test specimens (parallel section width 10 mm, parallel section length and mark distance 35 mm) were collected from the aforementioned titanium alloy plate, with the length direction parallel to the rolling direction. The high-temperature tensile test was conducted at a strain rate of 7.5% / min. The test atmosphere was set to 800℃ in the atmosphere, and the specimens were kept in the test atmosphere for 10 minutes to allow them to fully reach the test temperature before the test was performed.

[0114] abrasiveness

[0115] Wet grinding was performed using #1500 sandpaper, followed by alumina polishing for 60 minutes. The gloss level after grinding was used to evaluate the grindability.

[0116] The polishing slurry used for alumina polishing was a solution of 250g of alumina powder with an average particle size of 3μm added to 1 liter of water. In the polishing test, the sample was embedded in epoxy resin with a diameter of 28mm, and six samples were placed on the holder of an automatic polishing device, polished under a pressure of 60N. Gloss was determined according to the specular gloss determination method of JIS Z 8741 (1997).

[0117] The incident angle and the light receiving angle are set to 20° to measure the gloss. From the viewpoint of abrasiveness, the gloss (Gs20) is preferably 920 or higher.

[0118] The titanium alloy sheet of this embodiment can be used as a blank for automotive exhaust system components. That is, by forming the titanium alloy sheet of this embodiment into a predetermined shape and welding it, various automotive exhaust system components can be manufactured. Examples of automotive exhaust system components of this embodiment include exhaust manifolds, exhaust pipes, catalytic converters, and exhaust mufflers; the titanium alloy sheet of this embodiment can be used as the blank for these components. These exhaust system components can be used not only in four-wheeled vehicles but also in two-wheeled vehicles.

[0119] The thickness of the titanium alloy plate in this embodiment is not limited, but when used as a blank for exhaust system components for motor vehicles, it is preferably 0.5 to 2.0 mm; more preferably 0.6 to 1.5 mm.

[0120] Next, the manufacturing method of the titanium alloy plate of this embodiment will be described.

[0121] In the traditional manufacturing process of titanium alloy plates, ingots with a specified chemical composition, produced by electron beam melting or vacuum arc melting, are first rolled (forging or rolling) at temperatures in the β single-phase domain to fracture the solidification structure, and then hot-rolled into coils. These coils are then annealed as needed, descaled, and repeatedly cold-rolled and annealed as required.

[0122] Generally, a microstructure composed of equiaxed grains exhibits an excellent balance between strength and workability. To obtain a microstructure composed of equiaxed grains and to ensure excellent cold rollability, annealing is typically performed after hot rolling at a temperature below the β phase transformation point. However, below the β phase transformation point, a state exists where both the α and second phases exist, leading to elemental partitioning between the α and second phases. In particular, the higher the temperature, the faster the elemental partitioning occurs. When elemental partitioning occurs, the distribution of the second phase becomes non-uniform.

[0123] Regarding the distribution of alloying elements in titanium alloys, the distribution state that occurs during solidification (ingot manufacturing) is homogenized to some extent during the initial rolling process. Although the initial rolling process heats the material to the β single-phase domain temperature, it may sometimes fall below the β phase transformation point at the end of the process. Furthermore, even if the temperature does not fall below the β phase transformation point, the cooling rate is very slow, resulting in elemental distribution during cooling. Even if water cooling is performed after initial rolling to increase the cooling rate, the difference in cooling rate between the interior and the surface is significant, inevitably leading to some degree of elemental distribution in the interior where the cooling rate is low.

[0124] Furthermore, even if the billet is heated above the β-transformation point before hot rolling to eliminate element distribution, the temperature drop during hot rolling will cause it to fall below the β-transformation point, and element distribution will still occur during hot rolling. Additionally, to raise the internal temperature of the billet above the β-transformation point, it needs to be maintained for a long time, thus leading to the formation of a surface solidified layer through oxidation, reducing cold rollability.

[0125] In this embodiment, conventionally, at least one of the annealing and intermediate annealing of the hot-rolled sheet, which is below the β phase transformation point, is performed above the β phase transformation point to reduce the previous element distribution. Furthermore, cooling is performed at an average cooling rate of 5°C / second or more from the annealing temperature to 700°C, thereby obtaining a titanium alloy sheet with reduced element distribution. Unlike the initial rolling process, after producing a thinner hot-rolled sheet, heating it above the β phase transformation point can suppress element distribution in both the surface and interior layers. When both the annealing and intermediate annealing of the hot-rolled sheet are performed above the β phase transformation point, the alloying elements are more evenly distributed.

[0126] In the case of the titanium alloy plate in this embodiment, the β phase transformation point is greater than 830°C in most cases.

[0127] That is, the titanium alloy plate of this embodiment can be manufactured by a manufacturing method including the following steps.

[0128] (I) Hot rolling process, hot rolling is performed on an ingot made of titanium alloy having the above chemical composition to produce a hot rolled plate.

[0129] (II) Hot-rolled plate annealing process: The hot-rolled plate is annealed as needed (hot-rolled plate annealing).

[0130] (III) Cold rolling process: cold rolling of hot-rolled plate with a reduction rate of more than 60%. Intermediate annealing may also be performed before final rolling if necessary.

[0131] (IV) Final annealing process: The titanium alloy sheet after the cold rolling process is subjected to final annealing at a homogenization temperature of 550°C or higher and less than 670°C for 1 minute to 24 hours.

[0132] Among them, at least one of the hot-rolled plate annealing and the intermediate annealing of the cold rolling process is carried out, and the annealing temperature is greater than 830°C and is above the β phase transformation point.

[0133] The following describes each process of the manufacturing conditions.

[0134] <Hot Rolling Process>

[0135] In the hot rolling process, a hot-rolled plate is produced by hot rolling an ingot made of a titanium alloy having the above-mentioned chemical composition.

[0136] There are no special restrictions on the hot rolling conditions; they can be common conditions.

[0137] There are no particular restrictions on the processes preceding the hot rolling process. For example, for ingots with a specified chemical composition manufactured by electron beam melting or vacuum arc melting, a hot rolling process can be performed in the β single-phase domain for the purpose of fracture of the solidification structure (forging or rolling) to produce hot-rolled plates.

[0138] <Hot-rolled sheet annealing process>

[0139] When annealing the hot-rolled sheet obtained by hot rolling, it is preferable to set the annealing temperature to be greater than 830°C and above the β phase transformation point, set the annealing time to 1 to 5 minutes, and set the average cooling rate from the annealing temperature to 700°C to be greater than 5°C / second.

[0140] By setting the annealing temperature to above 830°C (above the β-phase transformation point) and the annealing time to at least 1 minute, elemental distribution can be suppressed, resulting in a more uniform distribution of alloying elements and a more uniform distribution of the second phase. On the other hand, annealing times exceeding 5 minutes can lead to a decrease in yield due to oxidation or a decrease in manufacturability due to prolonged annealing, which is therefore undesirable. While there is no upper limit to the annealing temperature, from the viewpoint of reduced yield due to oxidation, an annealing temperature of 1000°C or lower is preferred.

[0141] Furthermore, if the average cooling rate from the annealing temperature to 700°C is slow, elemental partitioning will occur during the cooling process. Therefore, the average cooling rate from the annealing temperature to 700°C is set to 5°C / second or higher. Even if the average cooling rate is increased, the degree of elemental partitioning will not have a significant impact on the distribution of the second phase, so there is no need to limit the upper limit of the average cooling rate, but it can be set to 300°C / second or lower.

[0142] When intermediate annealing is performed under the following conditions, with the annealing temperature set to above 830°C and above the β phase transformation point, the annealing time set to 1 to 5 minutes, and the average cooling rate from the annealing temperature to 700°C set to above 5°C / second, the hot-rolled plate annealing process may be omitted or may be performed under conditions other than those described above.

[0143] <Cold rolling process>

[0144] In the cold rolling process, hot-rolled sheets after the hot rolling process or after the hot-rolled sheet annealing process are cold-rolled. When cold-rolling hot-rolled sheets, since it is necessary to obtain small equiaxed particles after final annealing, the reduction rate during cold rolling (cumulative reduction rate in multi-pass applications) is set to 60% or more. To prevent cracking, the reduction rate during cold rolling can be below 90%. When performing intermediate annealing (described later), the reduction rate of cold rolling before intermediate annealing is set as the intermediate cold rolling rate, and the reduction rate of cold rolling after intermediate rolling is set as the final cold rolling rate, which is set to 60% or more.

[0145] Furthermore, in the annealing of hot-rolled sheets, if annealing is carried out at a temperature greater than 830°C and above the β-phase transformation point, the metallographic structure becomes acicular, reducing cold rollability. Therefore, in this case, the preferred cold rolling conditions are a reduction rate of 10% or less for the first and second passes, and 15% or less thereafter. By processing with a low reduction rate up to the second pass, stable cold rolling can be performed without cracking. Afterward, due to the heat generated during processing, the temperature rises, so even if the reduction rate is increased, cracking is less likely to occur.

[0146] When hot-rolled sheet annealing is not performed, intermediate annealing is performed during the cold rolling process, interrupting the cold rolling process before final pressing. When performing intermediate annealing, it is preferable to set the annealing temperature to be greater than 830°C and above the β phase transformation point, the annealing time to be 1 to 5 minutes, and the average cooling rate from the annealing temperature to 700°C to be greater than 5°C / second.

[0147] The reason for setting this condition is the same as that explained in the section on hot-rolled plate annealing.

[0148] Even when hot-rolled plates are annealed, the above-mentioned intermediate annealing can be performed.

[0149] As described above, by setting either the annealing temperature of the hot-rolled sheet or the final intermediate annealing to a temperature greater than 830°C and above the β-phase transformation point, element distribution can be suppressed, resulting in a more uniform distribution of alloying elements and a more uniform distribution of the second phase. In particular, by setting the intermediate annealing temperature to at least a temperature above the β-phase transformation point, heating the sheet to a temperature above the β-phase transformation point while maintaining a thinner sheet thickness, element distribution on the surface and within the sheet can be suppressed. Furthermore, by performing annealing at a temperature greater than 830°C and above the β-phase transformation point in both the hot-rolled sheet annealing and intermediate annealing processes, element distribution can be suppressed, resulting in a more uniform distribution of alloying elements.

[0150] <Final Annealing Process>

[0151] Titanium alloy sheets after cold rolling undergo final annealing in a temperature range above 550°C and below 670°C to facilitate recrystallization. Annealing temperatures below 550°C result in the formation of a large amount of intermetallic compounds, hindering recrystallization and creating unrecrystallized regions, thus reducing abrasiveness. Furthermore, annealing temperatures above 670°C may lead to α-phase grain growth, resulting in coarse grains. Therefore, the final annealing temperature is set below 670°C.

[0152] The final annealing time can be set from 1 minute to 24 hours. Setting the annealing time to more than 1 minute ensures thorough recrystallization. Conversely, setting the annealing time to less than 24 hours prevents the formation of coarse grains. There are no particular limitations on the cooling rate after final annealing (cooling rate in the temperature range below 550°C).

[0153] Next, the exhaust system components for motor vehicles according to this embodiment will be described.

[0154] The exhaust system component for a motor vehicle according to this embodiment includes the aforementioned titanium alloy sheet (sometimes composed of the aforementioned titanium alloy sheet). The exhaust system component for a motor vehicle according to this embodiment is obtained, for example, by forming the titanium alloy sheet of this embodiment using stamping. Since the chemical composition does not change due to forming, the chemical composition of the exhaust system component for a motor vehicle is the same as that of the titanium alloy sheet of this embodiment. When the titanium alloy sheet is formed into an exhaust system component for a motor vehicle, due to twinning deformation caused by forming, the grain diameter becomes small in the deformed portion. Twinning deformation can be determined by OIM analysis. However, when the degree of machining increases, the crystal orientation difference between the parent phase and the twinning deformation changes, making analysis difficult. Therefore, in order to determine the grain diameter of the α phase, it is necessary to prepare a test sample from a portion of the exhaust system component with an appropriate degree of machining.

[0155] Example

[0156] Titanium alloys with the chemical composition shown in Table 1 were melted into ingots using vacuum arc melting. The ingots were then hot-rolled at 1000°C to produce hot-rolled sheets with a thickness of 10 mm. These sheets were then hot-rolled again at 860°C to obtain hot-rolled sheets with a thickness of 4.0 mm. In Table 1, the individual contents of Ni, V, Mn, Co, Ta, W, C, and N are omitted; the total contents of these elements are recorded in the "Other" column. The individual contents of each element are all 0.05% or less. Furthermore, the H content among the impurities is all 0.013% or less.

[0157] Then, a descaling process is performed, or a descaling process is performed after hot-rolled sheet annealing at the temperatures and times specified in Table 2 as needed. Afterwards, intermediate annealing is performed along with cold rolling as needed, followed by final cold rolling. Finally, final annealing is performed. In this way, titanium alloy sheets No. 1 to No. 48 are manufactured.

[0158] The obtained titanium alloy plate was then subjected to a grinding process. The grinding was performed using wet grinding with #1500 sandpaper, followed by alumina polishing for 60 minutes. The polishing slurry used for alumina polishing was a solution of 250g of alumina powder with an average particle size of 3μm added to 1 liter of water. During the grinding process, the sample was embedded in 28mm diameter epoxy resin, and six samples were placed on the holder of the automatic grinding device, where grinding was performed under a pressure of 60N.

[0159] Various evaluations were performed on the ground titanium alloy plates.

[0160] As described above, regarding the average grain diameter of the α phase, using EBSD, at the center of the plate thickness in a section perpendicular to the plate width direction (L section) at half the plate width length, the accelerating voltage was set to 15 kV, and the measurement interval was set to 0.2 μm at a magnification of 500x. Measurements were performed only on the α phase. The measurement field of view was set to contain more than 300 grains in one field of view or more than 400 grains in total across multiple fields of view. The average CI value of the measured sample was adjusted to 0.2 or higher. OIM-analysis was used in the measurement analysis software. TM (Version 7.3.1) Boundaries with an orientation difference of 15° or more are considered grain boundaries. The grain diameter of each grain is calculated by approximating the area of ​​the grains divided by these boundaries using a circular equivalent diameter. Grains with a diameter of less than 1.0 μm and grains not fully contained within the field of view are excluded from the grain diameter calculation.

[0161] As described above, regarding the grain size distribution of the α phase, the measurement region (a rectangular region with one side length of 100 μm or more, constructed from the center of the plate thickness at a cross-section perpendicular to the plate width direction at half the plate width (L-section)) set during the average grain diameter measurement was used as the measurement region for grain size distribution. The crystal orientation of the α phase in the measurement region was analyzed using electron backscatter diffraction (EBSD). In the analysis software, OIM-analysis was used. TM (Version 7.3.1) Boundaries with an orientation difference of 15° or more are considered grain boundaries. The grain diameter of each grain is approximated using a circular equivalent diameter approximation based on the area of ​​the grains defined by these boundaries. Then, based on the obtained grain diameters and the average grain diameter obtained using the above method, the proportions of α-phase grains with grain diameters within ±2 μm of the average grain diameter and the proportions within ±4 μm of the average grain diameter are determined.

[0162] As described above, regarding the distribution of the second phase, at the center of the plate thickness of a section (L section) perpendicular to the plate width direction at 1 / 2 of the plate width length, a region with one side of 100 μm is divided into 10 × 10 equal parts and designated as a measurement region (100 regions with one side of 10 μm are used as measurement regions). For each measurement region, the number of second phases per unit area is calculated, and the number of measurement regions in which 5 or more but less than 15 second phases are observed is determined.

[0163] The measurement area was observed using a scanning electron microscope (SEM), and the α phase and the second phase were distinguished based on the backscattered electron image. The second phase, which is an intermetallic compound, is white or black and consists of minute precipitates compared to the parent phase α; therefore, it can be identified as the second phase based on this characteristic.

[0164] The area fraction of the second phase was measured in the same region as the number density of the second phase. A region with one side measuring 100 μm (the central portion of the plate thickness at the midpoint of the plate width, perpendicular to the plate width direction (L section)) was observed using a scanning electron microscope (SEM). The α phase and the second phase were identified based on the backscattered electron image. Furthermore, the area of ​​the second phase within this region was measured, and the area fraction (%) of the second phase was determined.

[0165] Total elongation was determined by performing a room temperature tensile test. For the room temperature tensile test, small ASTM tensile test specimens (parallel section width: 6.25 mm, parallel section length: 32 mm, mark spacing: 25 mm) were collected from the aforementioned titanium alloy plate, with the length direction parallel to the rolling direction. The strain rate was set to 30% / min. The test temperature ranged from 10 to 35°C.

[0166] The Erikson cupping value was determined according to the Erikson cupping test method specified in JIS Z 2247 (2006). The width of the test sample was 90 mm or more. The testing machine was as described in JIS B 7729 (2005). The lubricant was a 50 μm thick polytetrafluoroethylene (PTFE) sheet. The fixture dimensions were those of the standard test piece.

[0167] If the total elongation is above 25.0% and the Eriksen cup value is above 9.5mm, it is judged to have excellent machinability.

[0168] A 20mm × 20mm test piece was taken from the aforementioned titanium alloy plate. The surface was wet-polished with #400 sandpaper and exposed to still air at 800℃ for 100 hours. The increase in mass after exposure was measured, and the value was obtained by dividing the increase in mass by the surface area of ​​the tensile test piece (increase in mass (mg) / surface area of ​​test piece (cm²)). 2 This is considered as an oxidation increment. When oxide scale peels off during an oxidation test, the peeled oxide scale is also included in the post-exposure mass. If the oxidation increment is 5.0 mg / cm³... 2 The following indicates that it has excellent oxidation resistance at high temperatures.

[0169] The high-temperature strength (tensile strength) at 800℃ was determined by taking tensile test specimens (parallel section width: 10 mm, parallel section length and distance between markings: 35 mm) from the aforementioned titanium alloy plate, with the length direction parallel to the rolling direction, and conducting tensile tests at a strain rate of 7.5% / min. The test atmosphere was set in an atmosphere of 800℃, and the specimens were kept in the test atmosphere for 10 minutes to allow them to fully reach the test temperature before the test was conducted. If the high-temperature strength (tensile strength) at 800℃ is above 26 MPa, it is considered to have excellent high-temperature strength.

[0170] Gloss (Gs20) was determined according to the specular gloss measurement method in JIS Z 8741 (1997). In the gloss measurement, the incident angle and the light receiving angle were set to 20°. A gloss Gs20 of 920 or higher was considered to indicate excellent abrasiveness.

[0171] The evaluation results for each are shown in Table 3.

[0172] Table 1

[0173]

[0174] The underlined part indicates that it is outside the scope of this invention.

[0175] Table 2

[0176]

[0177] The underlined part indicates that it is outside the scope of the invention or outside the scope of the preferred manufacturing conditions.

[0178] ※1 The cooling rate for hot-rolled sheet annealing and intermediate annealing is the average cooling rate from the annealing temperature to 700°C. ※2 In the case of intermediate annealing, it is the final cold rolling ratio.

[0179] Table 3

[0180]

[0181] An underlined value indicates that the value is outside the scope of the invention or outside the range of preferred characteristic values.

[0182] As shown in Table 1, Nos. 1 to 8, 11 to 13, 15, 17, 19, 21 to 25, 27, 28, 31 to 33, 36, 38, 39, and 45 to 48 are titanium alloy plates within the scope of this disclosure, exhibiting excellent properties.

[0183] Furthermore, in these invention examples, the metallographic structure is equiaxed. That is, the average aspect ratio (major axis length / minor axis length) of the α phase is 3.0 or less.

[0184] On the other hand, in No. 9, the Si content is low, and the average grain diameter of the α phase becomes coarser. Furthermore, the distribution of the second phase also deteriorates. Consequently, the oxidation increment increases, resulting in low oxidation resistance at high temperatures. Additionally, the high-temperature strength is also low. Moreover, due to the increased average grain diameter of the α phase, the gloss and abrasiveness after polishing are low.

[0185] In No. 10, the excessive Si content leads to a high Eriksen cup value and low machinability.

[0186] In No. 14, the Al content is too high, resulting in a high Eriksen cup value and low processability.

[0187] In No. 16, the excessive Zr content leads to a smaller average grain diameter of the α phase, and also leaves behind a significant amount of unrecrystallized microstructure. Consequently, the overall elongation decreases, the Eriksen cupping value becomes lower, and the machinability and grindability are also reduced.

[0188] In No. 18, the Cr content is too high, and a β phase is formed when heated at high temperature, resulting in low oxidation resistance.

[0189] In No. 20, the excessive Mo content leads to a smaller average grain diameter of the α phase, and also leaves behind a significant amount of unrecrystallized microstructure. Consequently, the overall elongation and Eriksen cupping value are low, resulting in poor processability. Furthermore, its grindability is also low. Additionally, the formation of the β phase upon high-temperature heating contributes to its low oxidation resistance.

[0190] In No. 26, the annealing temperature of the hot-rolled plate was lower than the β-phase transformation point, and no intermediate annealing was performed, resulting in a poorer distribution of the second phase. The observed result was the presence of coarse grains, leading to low abrasiveness.

[0191] In No. 29, both the annealing temperature and the intermediate annealing temperature of the hot-rolled plate are lower than the β-phase transformation point, resulting in a poorer distribution of the second phase. The observed result is the presence of coarse grains, leading to low abrasiveness.

[0192] In No. 30, the cooling rate after hot-rolled plate annealing was low, and no intermediate annealing was performed, resulting in an increased grain diameter distribution of the α phase. The observed result was the presence of coarse grains. Consequently, the abrasiveness was low.

[0193] In No. 34, the final annealing temperature is low, resulting in a smaller average grain diameter of the α phase. Furthermore, a significant amount of unrecrystallized microstructure remains. Consequently, the total elongation and Eriksen cupping value are lower, leading to poor machinability. Additionally, the grindability is also low.

[0194] In No. 35, due to the low reduction rate of the final cold rolling, the strain introduction is insufficient, resulting in an expanded distribution of the α phase grain diameter, containing coarse grains and low abrasiveness.

[0195] In No. 37, the high final annealing temperature resulted in coarser average α-grain size and a wider distribution of α-phase grain diameter. Furthermore, the distribution of the second phase was also uneven. Consequently, the grindability was low.

[0196] In No. 40, the Cu content is low, resulting in low high-temperature strength.

[0197] In No. 41, the Cu content is too high, resulting in a low Eriksen cup value and poor machinability.

[0198] In No.42, the Sn content is low and the high-temperature strength is low.

[0199] In No. 43, the Sn content is too high, resulting in lower total elongation and Erikson cup value, and lower processability.

[0200] In No.44, the Nb content is low, resulting in poor oxidation resistance at high temperatures.

[0201] Industrial availability

[0202] According to this disclosure, a titanium alloy sheet and an exhaust system component for motor vehicles with excellent machinability, grindability, and oxidation resistance at high temperatures can be provided.

Claims

1. A titanium alloy plate, characterized in that, It has the following chemical composition: Contains, by mass % Cu: 0.7%–1.5% Sn: 0.5%–1.5% Si: 0.10%~0.60% Nb: 0.1%–1.0% Zr：0％～1.0％、 Cr：0％～0.5％、 Mo: 0%–0.5%, and Al：0％～1.0％， Furthermore, Fe and O are limited to Fe: below 0.08% and O: below 0.07%, respectively. The balance consists of Ti and impurities. The metallographic structure consists of an α phase and a second phase. The average grain diameter of the α phase is 3.0–10.0 μm. In the α phase, the proportion of grains with a grain diameter within the range of ±2 μm of the average grain diameter is greater than 25%. In the α phase, the proportion of grains with a grain diameter within the range of ±4 μm of the average grain diameter is greater than 45%. If a 100 μm × 100 μm region in the cross section is divided into 100 equal parts and 100 10 μm regions are set as measurement regions, and the number density of the second phase is calculated for each measurement region, the number of measurement regions in which 5 or more and 15 or fewer of the second phase are observed in the measurement regions is 80 or more.

2. The titanium alloy plate according to claim 1, characterized in that, The area fraction of the second phase is greater than 1.0%.

3. A motor vehicle exhaust system component comprising the titanium alloy plate as described in claim 1 or 2.

4. An exhaust system component for a motor vehicle, which is obtained by forming the titanium alloy sheet as described in claim 1 or 2.

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