JP7541255B2 - Titanium alloy plate and automotive exhaust system parts - Google Patents

Description

The present disclosure relates to titanium alloy plates and automotive exhaust system components.

The exhaust system of a four-wheeled vehicle or two-wheeled vehicle (hereafter referred to as "vehicles, etc.") is equipped with an exhaust manifold and an exhaust pipe. Exhaust gas discharged from the engine and collected by the exhaust manifold is passed through the exhaust pipe and discharged to the outside from an exhaust port at the rear of the vehicle body. A catalytic device and a muffler (silencer) are arranged midway through the exhaust pipe to purify the exhaust gas and silence exhaust noise. In this specification, the entire system from the exhaust manifold to the exhaust pipe and exhaust port is referred to as the "exhaust system." In addition, the parts that make up the exhaust system, such as the exhaust manifold, exhaust pipe, catalytic device, and muffler, are referred to as "exhaust system parts."

Traditionally, stainless steel, which has excellent corrosion resistance, high strength and workability, has been used for the components of exhaust systems for automobiles, etc., but in recent years, titanium material, which is lighter than stainless steel and has high strength and excellent corrosion resistance, is being used more and more. For example, JIS Class 2 commercially pure titanium material is used for the exhaust systems of motorcycles. Furthermore, titanium alloys, which have higher heat resistance, are now being used in place of JIS Class 2 commercially pure titanium material.

In particular, exhaust gas temperatures have been rising in recent years. As a result, the exhaust gas temperature in exhaust pipes can reach about 800°C, and sufficient high-temperature strength is required even in this temperature range. In addition, it is also desirable for exhaust system parts to be able to suppress high-temperature oxidation (to have excellent oxidation resistance at high temperatures).
Furthermore, even if a material has excellent strength and oxidation resistance at high temperatures, if its workability is poor, it will be difficult to process it into parts. Therefore, when using titanium sheet for exhaust system parts, it is also required that the material has good workability during forming.

Patent Document 1 describes a titanium alloy having excellent resistance to high-temperature oxidation, which contains 0.15 to 2 mass % Si, has Al restricted to less than 0.30 mass %, and is made up of the remainder titanium and unavoidable impurities.
Furthermore, Patent Document 2 describes a titanium alloy having excellent high-temperature oxidation resistance and corrosion resistance, which is characterized by containing, on a mass basis, 0.30 to 1.50% Al and 0.10 to 1.0% Si.
Furthermore, Patent Document 3 describes a heat-resistant titanium alloy for exhaust device components that has excellent cold workability and contains, by mass%, Cu: more than 2.1% to 4.5%, oxygen: 0.04% or less, Fe: 0.06% or less, with the balance being Ti and unavoidable impurities.
Patent Document 4 describes a titanium alloy material for exhaust system parts having excellent oxidation resistance, which contains, by mass%, Si: 0.1 to 0.6%, Fe: 0.04 to 0.2%, O: 0.02 to 0.15%, the total content of Fe and O being 0.1% or more and 0.3% or less, with the balance being Ti and unavoidable impurities whose individual content is less than 0.04%.

However, the titanium alloys described in Patent Documents 1 to 4 aim to ensure high-temperature strength by limiting the chemical composition, but do not improve polishability.

Titanium plates for exhaust system parts are sometimes required to have a glossy surface. In such cases, the titanium plate is polished to achieve a surface texture that provides the required gloss. As described in Patent Documents 1 to 4, there are known techniques for improving high-temperature oxidation resistance, corrosion resistance, and cold workability while maintaining heat resistance, but no studies have been conducted on further improving polishability after product processing.

Japanese Patent Application Publication No. 2007-270199 Japanese Patent Application Publication No. 2005-290548 Japanese Patent Application Publication No. 2009-030140 Japanese Patent Application Publication No. 2013-142183

The present disclosure has been made in consideration of the above circumstances, and aims to provide titanium alloy plates and automotive exhaust system parts that have excellent processability, polishability, oxidation resistance at high temperatures, and high-temperature strength.

[1] A titanium alloy plate according to one embodiment of the present disclosure contains, by mass%, Cu: 0.7% to 1.5%, Sn: 0.5% to 1.5%, Si: 0.10% to 0.60%, Nb: 0.1% to 1.0%, Zr: 0% to 1.0%, Cr: 0% to 0.5%, Mo: 0% to 0.5%, Al: 0% to 1.0%, Fe: 0.08% or less, O: 0.07% or less, and the balance is Ti and impurities. The metal structure is composed of an α phase and a second phase, and the average crystal grain size of the α phase is 3.0 to 10.0 μm. the proportion of crystal grains in the α phase whose grain size is in the range of the average grain size ±2 μm is 25% or more, the proportion of crystal grains in the α phase whose grain size is in the range of the average grain size ±4 μm is 45% or more, and in a cross section, a 100 μm × 100 μm region is divided into 100 equal 10 μm × 10 μm regions as measurement regions, and when the number density of the second phase is determined for each measurement region, the number of measurement regions in which 5 to 15 particles of the second phase are observed within the measurement region is 80 or more.
[2] The titanium alloy plate according to the above item [1] may have an area ratio of the second phase of 1.0% or more.
[3] An automobile exhaust system part according to another embodiment of the present disclosure includes the titanium alloy plate according to the above [1] or [2].
[4] An automobile exhaust system part according to another embodiment of the present disclosure is obtained by forming the titanium alloy plate described in [1] or [2] above.

According to the above aspects of the present disclosure, it is possible to provide titanium alloy plates and automotive exhaust system parts that have excellent processability, polishability, and oxidation resistance at high temperatures.

Automobile exhaust system parts are obtained by, for example, press forming titanium alloy plate, and automobile exhaust system parts are used in high temperature environments. In addition, the titanium alloy plate for exhaust system parts may be required to have a glossy surface. Since the glossiness of titanium alloy plate may be increased by polishing the surface, titanium alloy plate is required to have good polishability.
In order to improve the polishability of a titanium alloy plate, it is necessary that (1) the surface state before polishing is flat, and (2) the occurrence of scratches originating from crystal grains during polishing is reduced.

Regarding (1) above, for example, if there are irregularities on the surface of a titanium alloy plate caused by seizure, the polishing time required to remove the irregularities will be long. In other words, one way to improve polishability is to make the surface of the plate flat.

Regarding (2) above, one of the reasons why the appearance after polishing is impaired is that the hardness differs depending on the crystal orientation, resulting in different polishing conditions for each crystal grain. Since it is not easy to control the crystal orientation to be extremely uniform, crystal grains with different polishing conditions are generally made less noticeable by making the grains finer. However, sufficient polishing properties cannot be obtained unless the grain size is made uniform, not just made smaller. Also, if the crystal grains are made too fine, formability deteriorates, so there is a limit to how fine the grains can be made.

In this way, it is not easy to reduce the grain size distribution while controlling the grain size to a range that ensures a minimum level of formability, taking into account the non-uniformity that occurs in actual manufacturing. The inventors therefore conducted research to reduce the difference in the polishing state of grains due to crystal orientation. As a result, they discovered that by forming intermetallic compounds with a predetermined number density or higher as second phases within grains and at grain boundaries, the intermetallic compounds can restrain the grains, suppress deformation during polishing, and achieve more uniform polishing.

In addition, because exhaust system parts reach high temperatures during use, it is necessary to add alloy elements to ensure the high-temperature strength required for exhaust system parts. The inventors have investigated chemical compositions that ensure high-temperature strength at 800°C.

After careful consideration from the above perspectives, we have completed the titanium alloy plate of this embodiment.

Hereinafter, a titanium alloy plate according to one embodiment of the present disclosure (the titanium alloy plate of the present embodiment) and an automotive exhaust system part according to one embodiment of the present disclosure (the automotive exhaust system part of the present embodiment) will be described.
The titanium alloy plate of this embodiment has a chemical composition that contains, in mass%, Cu: 0.7% to 1.5%, Sn: 0.5% to 1.5%, Si: 0.10% to 0.60%, Nb: 0.1% to 1.0%, Zr: 0% to 1.0%, Cr: 0% to 0.5%, Mo: 0% to 0.5%, Al: 0% to 1.0%, Fe: 0.08% or less, O: 0.07% or less, and the balance is Ti and impurities. In addition, the titanium alloy plate of this embodiment has a metal structure consisting of an α phase and a second phase, the average grain size of the α phase is 3.0 to 10.0 μm, the number ratio of the grains of the α phase whose grain size is in the range of the average grain size ± 2 μm is 25% or more, and the number ratio of the grains of the α phase whose grain size is in the range of the average grain size ± 4 μm is 45% or more. In addition, in the cross section of the titanium alloy plate of this embodiment, when a 100 μm×100 μm area is divided into 100 equal parts, and 100 (places) of 10 μm×10 μm areas are measured, and the number density of the second phase is obtained for each of the measurement areas, the number of measurement areas in which 5 to 15 of the second phase are observed within the measurement area is 80 or more.
In addition, the titanium alloy plate of this embodiment preferably has an area ratio of the second phase of 1.0% or more.
Next, the automobile exhaust system part of the present embodiment includes the above-mentioned titanium alloy plate.

First, the chemical composition of the titanium alloy plate of this embodiment will be described. The unit of content of each element constituting the chemical composition, "%", means "mass %". In addition, the range indicated with "~" includes the values at both ends as the lower and upper limits.

Cu: 0.7-1.5%
In order to ensure sufficient high temperature strength, the Cu content must be 0.7% or more, and preferably 0.8% or more.
On the other hand, if the Cu content exceeds 1.5%, the workability decreases. Also, the possibility of Cu segregating during the production of the ingot 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 further preferably 1.2% or less.

Sn: 0.5-1.5%
In order to ensure sufficient high-temperature strength, the Sn content must be 0.5% or more. The Sn content is preferably 0.6% or more, more preferably 0.8% or more, and even more preferably 0.7% or more. It is preferably 0.9% or more.
On the other hand, since Sn does not easily form intermetallic compounds, it can be contained in a relatively large amount. However, if the Sn content is excessive, the workability decreases and the solubility limit of Cu and Si in the α phase is exceeded. Therefore, the Sn content must be 1.5% or less. In addition, Sn is an element with a large specific gravity, and even if a large amount is added, the atomic ratio is not so large. The Sn content is preferably 1.4% or less, more preferably 1.3% or less, and even more preferably 1.0% or less. . Less than 2%.

Si: 0.10~0.60%
In order to ensure oxidation resistance and high-temperature strength, the Si content must be 0.10% or more. The Si content is preferably 0.15% or more, and more preferably 0.20% or more. be.
On the other hand, if the Si content exceeds 0.60%, silicide is formed, which significantly inhibits grain growth and reduces workability. Therefore, the Si content is set to 0.60% or less. 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.

Nb: 0.1-1.0%
In order to ensure oxidation resistance, the Nb content must be 0.1% or more, preferably 0.2% or more, and more preferably 0.3% or more.
On the other hand, the more Nb is contained, the more the oxidation resistance improves, but the raw material cost increases and the effect of improving the oxidation resistance reaches a plateau. 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 further preferably 0.4% or less.

Zr: 0 to 1.0%
Zr is an element that facilitates the formation of an intermetallic compound between Si and Ti. Zr is also present in the intermetallic compounds that are formed. By including Zr, it becomes easier to obtain the pinning effect, and the grain growth can be suppressed by the solute drag effect. Therefore, Zr may be included as necessary. To obtain the above effects, it is preferable that the Zr content is 0.1% or more.
On the other hand, the inclusion of Zr lowers the β transformation point, and the promotion of intermetallic compound formation and the solute drag effect become smaller relative to the content. Therefore, when Zr is included, 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.
Since Zr is an optional element, the lower limit is 0%.

Cr: 0% or more and 0.5% or less Mo: 0% or more and 0.5% or less Cr and Mo are optional elements, and their contents may be 0%, but by including Cr and Mo, grain growth is suppressed by the solute drag effect and high-temperature strength is improved. Therefore, they may be included as necessary. In order to obtain the above effects, it is preferable that the Cr content and Mo content are each 0.05% or more. More preferably, each is 0.1% or more.
On the other hand, if the Cr or Mo content is high, the amount of β phase increases at high temperatures, which reduces the oxidation resistance. Moreover, if the Mo content is excessive, the workability decreases. Therefore, if they are contained, the Cr content and the Mo content are each set to 0.5% or less. The Cr content and the Mo content are each preferably set to 0.4% or less, and more preferably set to 0.3% or less.

Al: 0 to 1.0%
Al is an optional element and may be 0%, but may be contained in order to ensure high temperature strength. In order to obtain the above effects, the Al content is preferably 0.1% or more.
On the other hand, if the Al content is increased, the α phase is stabilized and the formation of the β phase is suppressed, and the high-temperature strength and oxidation resistance are further improved, but the workability is reduced, which is not preferable. In addition, the cold rollability is also significantly reduced. Therefore, when Al is contained, 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.

Fe: 0.08% or less If the Fe content is too high, the β phase is likely to occur from a low temperature region. Therefore, the amount of solid solution elements in the α phase decreases due to the concentration of the added elements in the β phase, and the high temperature strength decreases due to the increase in the β phase ratio. In addition, the oxidation resistance may deteriorate due to the increase in the β phase ratio. In addition, when Cr and Mo are contained, the appropriate content range of Cr and Mo becomes narrow, making it difficult to control the chemical components of Cr and Mo. Therefore, the Fe content is preferably as low as possible, and it is necessary to limit it to 0.08% or less. The Fe content is preferably 0.06% or less, more preferably 0.04% or less.

O: 0.07% or less O is an element that increases room temperature strength but hardly improves high temperature strength. In other words, if the O content is high, the high temperature strength is not improved, and the springback amount increases and workability is reduced. Therefore, the lower the O content, the better. However, it is difficult to reduce oxygen (O) industrially, and if it is reduced too much, the raw material cost increases. Therefore, about 0.04% is contained, and when considering the variation, it may be about 0.07%. Therefore, the O content is limited to 0.07% or less.

Ni, V, Mn, Co, Ta, W, C, N, one or more of 0 to 0.05% each, and 0.30% or less in total. Ni, V, Mn, Co, Ta, and W all have the effect of stabilizing the β phase to a certain extent. Therefore, in the titanium alloy plate in which the α phase and the β phase are controlled by Nb, Cr, and Mo, as in this embodiment, the contents of these elements are preferably small. In addition, if N and C are contained in excess, the α phase is stabilized and the strength at room temperature is increased, so that the workability is deteriorated. Therefore, the contents of N and C are also preferably small. Therefore, whether these elements are intentionally contained or contained as impurities, it is preferable that the contents of each element are 0.05% or less, and the total content of these elements is 0.30% or less.
Since the contents of these elements are preferably small, the lower limit of each content and the total content is 0%.

The balance of the titanium alloy plate of this embodiment is Ti and other impurities other than those mentioned above.
Other impurities include, for example, H and B. H is an element that forms hydrides together with Ti, and when hydrides are formed, the titanium alloy plate may become embrittled. Therefore, even if it is contained as an impurity, it is preferable to suppress the H content as much as possible. In the titanium alloy plate of this embodiment, the H content is preferably 0.013% or less. There is a concern that B will become coarse precipitates in the ingot. Therefore, even if it is contained as an impurity, it is preferable to suppress the B content as much as possible. In the titanium alloy plate of this embodiment, the B content is preferably 0.01% or less.

Next, the structure of the titanium alloy plate of this embodiment will be described.
The titanium alloy plate of this embodiment contains an α phase having an average grain size of 3.0 μm or more and 10.0 μm or less, and a second phase in the structure. The second phase is a structure other than the α phase, and is mainly an intermetallic compound. The intermetallic compound of this embodiment mainly contains Ti ₂ Cu and silicide. The second phase may contain a β phase, but even if the β phase is contained, it is a very small amount (e.g., 0.2% or less), so the second phase may be considered to be an intermetallic compound. The α phase is a structure that occupies the majority (e.g., 95% or more) of the metal structure, and the remainder of the metal structure is the second phase.

Average grain size of α phase: 3.0 to 10.0 μm
In the titanium alloy plate of this embodiment, the small average grain size of the α phase means that unrecrystallized parts remain. Therefore, when the average grain size of the α phase is small, the workability is reduced. The unrecrystallized parts also cause uneven polishing, so the polishability is poor. Therefore, in order to prevent the unrecrystallized parts from occurring, the average grain size of the α phase is set to 3.0 μm or more.
On the other hand, if the average crystal grain size of the α phase becomes too large, the polishability deteriorates, so the average crystal grain size of the α phase must be 10.0 μm or less.

The average grain size of the α phase can be determined by the following method using EBSD.
The α-phase crystal grains are uniformly distributed. Therefore, the measurement may be performed at any position in the width direction. For example, the measurement is performed at the center of the thickness of the cross section (L cross section) perpendicular to the width direction at the 1/2 position of the sheet width length (the position of 1/2 of the sheet width from the end of the sheet width direction in the sheet width direction), with an acceleration voltage of 15 kV, a magnification of 500 times or more, a measurement pitch of 0.2 μm, and only the α-phase to be measured. The measurement field is set to a size that includes 300 or more crystal grains in one field, or a total of 400 or more crystal grains in multiple fields. The measurement sample is adjusted so that the average CI value is 0.2 or more. The measurement analysis software uses OIM-analysis ^TM (version 7.3.1), and the boundaries with a crystal orientation difference of 15° or more are regarded as grain boundaries, and the grain size of each crystal grain is calculated by approximating the circle equivalent diameter from the area of the crystal grains divided by this boundary. When calculating the crystal grain size, crystal grains with a crystal grain size of 1.0 μm or less and crystal grains that are incompletely included in the field of view are excluded. A crystal grain whose grain boundary is divided by the boundary of the measurement field of view is determined to be a crystal grain that is incompletely included in the field of view.
If the sheet width direction is unknown, it can be determined by measuring the surface. Since the material has a split-TD type texture, (0001) is strongly oriented at an angle of 30 to 40 degrees to the sheet width direction. Therefore, the directional axis where there is a position where (0001) is strongly oriented in the measurement from the surface is the sheet width direction.

The crystal grain size distribution of the α phase: The number ratio of crystal grains with a crystal grain size of the average crystal grain size ±2 μm is 25% or more, and the number ratio of crystal grains with a crystal grain size of the average crystal grain size ±4 μm is 45% or more. Even if the average crystal grain size is 10.0 μm or less, there is a possibility that a small amount of coarse crystal grains are included. Since the polishing state of the crystal grains differs depending on the crystal grain size, if crystal grains with a large difference in crystal grain size are included, a satisfactory appearance cannot be obtained after polishing. Therefore, in addition to the average crystal grain size of the α phase, it is preferable to control the grain size distribution of the α phase.

In the titanium alloy plate of this embodiment, the proportion of α-phase grains (of the grains constituting the α-phase) whose grain size is within the range of the average grain size ±2 μm (average grain size -2 μm to average grain size +2 μm) is 25% or more of the total, and the proportion of grains whose grain size is within the range of the average grain size ±4 μm (average grain size -4 μm to average grain size +4 μm) is 45% or more of the total. Such a titanium alloy plate is less likely to contain coarse grains, and has improved polishability.
The upper limit of the number ratio of α-phase crystal grains having an α-phase crystal grain size in the range of the average crystal grain size ±2 μm and the number ratio of crystal grains having an α-phase crystal grain size in the range of the average crystal grain size ±4 μm is not particularly limited and may be 100%, but may also be less than 100%.

The particle size distribution of the α phase is determined by the following method.
The α-phase crystal grains are uniformly distributed. Therefore, the measurement may be performed at any position in the width direction. For example, the measurement region of the grain size distribution is set so that a rectangular region with a side length of 100 μm or more is present in the center of the sheet thickness of a cross section (L cross section) perpendicular to the sheet width direction at the 1/2 position of the sheet width length, and 100 or more crystal grains are present in the region. The crystal orientation of the α-phase is analyzed by the electron backscatter diffraction (EBSD) method in the measurement region. The analysis software used is OIM-analysis ^TM (version 7.3.1), and the boundaries with a crystal orientation difference of 15° or more are regarded as grain boundaries, and the grain size of each crystal grain is calculated by approximating the circle equivalent diameter from the area of the divided crystal grains.
Next, the grain size of each of the obtained grains, the percentage of the number of grains whose grain size is within ±2 μm of the average grain size obtained by the above method, and the percentage of the number of α-phase grains whose grain size is within ±4 μm of the average grain size are obtained.
This allows the grain size distribution of the crystal grains within the measurement area to be determined.
If the sheet width direction is unknown, it can be determined by measuring the surface. Since the material has a split-TD type texture, (0001) is strongly oriented at an angle of 30 to 40 degrees to the sheet width direction. Therefore, the directional axis where there is a position where (0001) is strongly oriented in the measurement from the surface is the sheet width direction.

Distribution of number density of second phases: out of 100 measurement regions, the number of measurement regions in which 5 to 15 particles of second phases are observed is 80 or more. Next, the distribution state of the second phase will be described.
In the titanium alloy plate of this embodiment, the metal structure is mostly composed of the α phase, and the remainder is composed of the second phase. The main components of the second phase are various intermetallic compounds. When the pinning effect or the solute drag effect is used to suppress the grain growth of the α phase, the second phase or alloying elements must be present uniformly. By uniformly containing the second phase, the grain growth of the α phase in a high-temperature environment during use is not uneven, and high-temperature strength is improved. However, if the second phase is distributed unevenly in the metal structure, the degree of grain growth of the α phase in the high-temperature range varies locally, and high-temperature strength is reduced. In addition, uneven second phase distribution causes the formation of a duplex structure during annealing, and the formation of the duplex structure deteriorates the polishability and fatigue properties. In addition, even if the duplex structure is not formed, the polishability is reduced due to the uneven second phase distribution. Therefore, the second phase must be distributed uniformly in the metal structure.

In the titanium alloy plate of the present embodiment, the ratio of the area containing a predetermined number of second phases is used as an index of the uniformity of distribution of the second phase, which is obtained by measuring the number of second phases in a plurality of measurement areas of the cross section of the titanium alloy plate.
Specifically, in a cross section perpendicular to the plate width direction of a titanium alloy plate at the plate width 1/2 position, each side length of a region with a side length of 100 μm (100 μm x 100 μm) is divided into 10 equal parts, and each region (100 regions with a side length of 10 μm) is used as a number density measurement region, and the number of second phases is calculated for each measurement region. Then, among the 100 regions, the number of measurement regions in which 5 to 15 second phases are observed (exist) in the measurement region (10 x 10 μm) is calculated. If the number of measurement regions in which 5 to 15 second phases are observed in the measurement region is 80 or more, the second phases are considered to be uniformly distributed. In all measurement regions, 5 to 15 second phases may be observed. In other words, the upper limit of the number of measurement regions in which 5 to 15 second phases are observed in the measurement region is 100. When calculating the number density of the second phase, if the second phase exists at the boundary between divided regions, the number is divided by the number of adjacent regions. For example, if the second phase exists across two regions, 0.5 particles is added to each of the two regions.
When the distribution state of the second phase satisfies this condition, the polishability is improved, and the formation of a mixed grain structure of the α phase during high temperature heating is suppressed, thereby improving the fatigue strength.

The number of regions in which 5 to 15 second phases are observed is determined by the following method.
Since the α phase and the second phase are uniformly dispersed, the measurement position may be any position in the width direction. For example, the position of the region with one side of 100 μm is the center of the plate thickness of the cross section (L cross section) perpendicular to the plate width direction at the position of 1/2 the plate width length of the titanium alloy. This region is divided into 10 equal parts by the length of each side to determine 100 measurement regions. Each measurement region is observed with a scanning electron microscope (SEM), and the α phase and the second phase are distinguished from the backscattered electron image. The second phase, which is an intermetallic compound, is white or black compared to the α phase, which is the parent phase, and is a fine precipitate, so that it can be distinguished from the second phase from this characteristic. Then, the number of the second phases in the measurement region is counted to obtain the number. This is performed for 100 measurement regions, and the number of measurement regions with the number of the second phases of 5 to 15 is counted.
If the sheet width direction is unknown, it can be determined by measuring the surface. Since the material has a split-TD type texture, (0001) is strongly oriented at an angle of 30 to 40 degrees to the sheet width direction. Therefore, the directional axis where there is a position where (0001) is strongly oriented in the measurement from the surface is the sheet width direction.

Area ratio of the second phase The area ratio of the second phase in the metal 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. When the second phase exists at an area ratio of 0.1% or more, the polishability can be improved. In particular, by setting the area ratio of the second phase to 1.0% or more, the polishability can be further improved. On the other hand, in order to ensure sufficient processability, the upper limit of the area ratio of the second phase is preferably 3.0% or less, more preferably 2.0% or less.

The area ratio of the second phase is measured in the same region as the measurement of the number density of the second phase. That is, the above-mentioned region with a side length of 100 μm (for example, created from the center of the plate thickness of a cross section (L cross section) perpendicular to the plate width direction at 1/2 the plate width length of a titanium alloy) is observed with a scanning electron microscope (SEM), and the α phase and the second phase are distinguished from each other from the backscattered electron image. The second phase, which is an intermetallic compound, is white or black compared to the parent α phase and is a fine precipitate, so that it can be distinguished from the second phase from this characteristic. The area of the second phase in the region is then measured to determine the area ratio (%) of the second phase.

In addition, the metal structure of the titanium alloy plate of this embodiment is preferably an equiaxed structure. In an acicular structure, regions with the same crystal orientation are densely packed macroscopically, which deteriorates the polishability. Specifically, the average aspect ratio (long axis length/short axis length) of the α phase, which occupies most of the structure, is preferably 3.0 or less. As described later, in hot-rolled sheet annealing or intermediate annealing, needle-shaped crystal grains are formed once by heating to above 830 ° C and above the β transformation point, but recrystallization occurs by subsequent cold rolling and final annealing to form an equiaxed α phase. The aspect ratio is calculated as the ratio of (long axis length/short axis length) of α crystal grains on the L cross section of the titanium alloy plate, and the average aspect ratio of 10 crystal grains is calculated.

The titanium alloy plate of this embodiment preferably has the following characteristics:

Total elongation: 25.0% or more Although it depends on the part shape after forming, it is necessary that the titanium alloy plate can be formed and welded into a pipe shape. In addition, the pipe needs to be bent after that. Therefore, the titanium alloy plate of this embodiment preferably has a total elongation of 25.0% or more in order to ensure sufficient workability during part forming. There is no need to limit the upper limit of total elongation, but about 50.0% is the practical upper limit in industry.

Total elongation is measured by conducting a room temperature tensile test. The room temperature tensile test is performed by taking an ASTM sub-size tensile test piece (parallel section width: 6.25 mm, parallel section length: 32 mm, gauge length: 25 mm) from the above titanium alloy plate with the longitudinal direction parallel to the rolling direction, and at a strain rate of 30%/min. The test temperature is within the range of 10 to 35°C.

Erichsen value: 9.5 mm or more The Erichsen test is a test for evaluating the important factors of deep drawing and stretching in forming shapes other than tubular shapes. In the titanium alloy plate of this embodiment, the Erichsen value is preferably 9.5 mm or more, taking into consideration the balance with the improvement of polishability.

The Erichsen value is measured in accordance with the Erichsen test method specified in JIS Z 2247 (2006). The thickness of the measurement sample shall be in the range of 0.1 to 2.0 mm, and the width shall be 90 mm or more. The test machine shall be as specified in JIS B 7729 (2005). The jig dimensions shall be those used for testing with standard test pieces. However, a 50 μm thick Teflon (registered trademark) sheet shall be used as the lubricant.

Oxidation resistance: oxidation mass gain after 100 hours at 800°C in air is 5.0mg/ ^cm2 or less The oxidation mass gain of most exhaust system parts generally used is 5.0mg/ ^cm2 or less, and it is desirable to achieve this in the titanium alloy plate of this embodiment even when it is assumed to be used at 800°C. Therefore, as an indicator of oxidation resistance, it is preferable to satisfy the oxidation mass gain of 5.0mg/ ^cm2 or less after 100 hours at 800°C in air.

The oxidation mass gain is determined by taking a 20 mm x 20 mm test piece from the above titanium alloy plate, wet-polishing the surface with #400 emery paper, exposing it to still air at 800°C for 100 hours, measuring the mass increase after exposure, and dividing the mass increase by the surface area of the tensile test piece ((mass increase (mg)/surface area of test piece ( ^cm2 ))).If scale spalling occurs due to the oxidation test, the spalled scale must also be included in the mass after exposure.

High-temperature strength (tensile strength): 26 MPa or more at 800 ° C. It is necessary to ensure high-temperature strength as a material. In this embodiment, high-temperature strength in the temperature range expected to be used is considered important, and assuming application to exhaust system parts that can handle high exhaust gas temperatures, the titanium alloy plate of this embodiment preferably has a tensile strength of 26 MPa or more at 800 ° C.

The high-temperature strength (tensile strength) at 800°C is measured by conducting a high-temperature tensile test. A high-temperature tensile test is performed by taking a tensile test piece (parallel section width 10 mm, parallel section length and gauge length 35 mm) from the above titanium alloy plate with the longitudinal direction parallel to the rolling direction, and using a strain rate of 7.5%/min. The test atmosphere is 800°C in air, and the test piece is held in the test atmosphere for 10 minutes to allow it to reach the test temperature sufficiently, and then the test is performed.

Polishability Polishability was evaluated by wet polishing with emery paper #1500 and polishing with an alumina buff for 60 minutes, and then measuring the glossiness.
The polishing solution used for alumina buff polishing is a solution in which 250 g of alumina powder with an average particle size of 3 μm is added to 1 liter of water. For the polishing test, the sample is embedded in epoxy resin with a diameter of 28 mm, six samples are set in the holder of an automatic polishing device, and polished with a pressure of 60 N. The gloss level is measured in accordance with the measurement method for specular gloss level in JIS Z 8741 (1997).
The glossiness is measured with the incident angle and the receiving angle being 20°. From the viewpoint of polishability, the glossiness (Gs20) is preferably 920 or more.

The titanium alloy plate of this embodiment can be used as a material for automobile exhaust system parts. That is, the titanium alloy plate of this embodiment can be formed into a predetermined shape and welded to produce various automobile exhaust system parts. Examples of automobile exhaust system parts of this embodiment include parts such as exhaust manifolds, exhaust pipes, catalytic converters, and mufflers, and the titanium alloy plate of this embodiment can be used as a material for these parts. These exhaust system parts can be used not only for four-wheeled automobiles but also for two-wheeled automobiles.

The thickness of the titanium alloy plate of this embodiment is not limited, but when used as a material for automotive exhaust system parts, it is preferably 0.5 to 2.0 mm. More preferably, it is 0.6 to 1.5 mm.

Next, a method for manufacturing the titanium alloy plate according to this embodiment will be described.
In the conventional manufacturing process of titanium alloy plate, an ingot having a predetermined chemical composition is produced by electron beam melting or vacuum arc melting, etc., and then a blooming process (forging or rolling) is performed at a temperature in the β single phase region to destroy the solidification structure, and then a coil is produced by hot rolling. The coil is annealed as necessary, and after descaling, cold rolling and annealing are repeated as necessary.

Generally, a metal structure composed of equiaxed grains has an excellent balance between strength and workability. In order to obtain a metal structure composed of equiaxed grains and to achieve excellent cold rollability, annealing is generally performed below the β transformation point after hot rolling. However, below the β transformation point, α phase and a second phase exist, and element distribution occurs between the α phase and the second phase. The higher the temperature, the shorter the element distribution. When element distribution occurs, the distribution of the second phase becomes non-uniform.

The distribution of alloying elements in titanium alloys is homogenized to a certain extent during the blooming process, with the distribution state occurring during solidification (when the ingot is produced); however, although the blooming process involves heating in the β single-phase region, the temperature may fall below the β transformation point upon completion of the process. Even if the temperature does not fall below the β transformation point, the cooling rate is very slow, and distribution will occur during cooling. Even if the cooling rate is increased by, for example, water cooling after blooming, the difference in cooling rate between the interior and surface layer is large, and some degree of element distribution will inevitably occur in the interior, where the cooling rate is slow.

Even if the slab is heated to above the β transformation point before hot rolling in order to eliminate element distribution, the temperature drops below the β transformation point during hot rolling, and element distribution progresses during hot rolling. Also, a long holding time is required to raise the temperature to above the β transformation point all the way to the inside of the slab, which causes oxidation to form a surface hardened layer, resulting in reduced cold rollability.

In this embodiment, at least one of the annealing of the hot-rolled sheet and the intermediate annealing, which was conventionally performed below the β transformation point, is performed at the β transformation point or higher, thereby reducing the element distribution up to that point, and cooling is performed so that the average cooling rate from the annealing temperature to 700°C is 5°C/sec or higher, thereby obtaining a titanium alloy sheet with reduced element distribution. Unlike the blooming process, it is possible to suppress element distribution both in the surface layer and inside by heating to the β transformation point or higher after forming a thin hot-rolled sheet. Annealing to the β transformation point or higher in both the annealing of the hot-rolled sheet and the intermediate annealing can distribute the alloy elements more uniformly.
In the case of the titanium alloy plate of this embodiment, the β transformation point is more than 830°C in most cases.

That is, the titanium alloy plate of this embodiment can be manufactured by a manufacturing method including the following steps.
(I) A hot rolling step in which an ingot made of a titanium alloy having the above-mentioned chemical composition is hot rolled to form a hot rolled sheet.
(II) A hot-rolled sheet annealing process, in which the hot-rolled sheet is annealed (hot-rolled sheet annealing) as necessary.
(III) A cold rolling step in which the hot rolled sheet is cold rolled at a rolling reduction of 60% or more. However, if necessary, intermediate annealing may be performed before the final rolling.
(IV) A final annealing process, in which the titanium alloy plate after the cold rolling process is subjected to final annealing at a soaking temperature of 550 ° C. or more and less than 670 ° C. for 1 minute to 24 hours.
However, at least one of hot-rolled sheet annealing and intermediate annealing in the cold rolling process is performed, and the annealing temperature is set to be higher than 830° C. and equal to or higher than the β transformation point.
Each step of the manufacturing process will be described below.

<Hot rolling process>
In the hot rolling process, an ingot made of a titanium alloy having the above-mentioned chemical composition is hot rolled to form a hot-rolled sheet.
The hot rolling conditions are not particularly limited and may be any known conditions.

There are no particular restrictions on the processes prior to the hot rolling process. For example, an ingot having a given chemical composition produced by electron beam melting or vacuum arc melting may be subjected to a blooming process (forging or rolling) in the β single-phase region in order to destroy the solidification structure, and then hot-rolled to produce a hot-rolled sheet.

<Hot-rolled sheet annealing process>
When hot-rolled sheet annealing is performed on the hot-rolled sheet obtained by hot rolling, the annealing temperature is preferably set to more than 830°C and to the β transformation point or higher, the annealing time is set to 1 to 5 minutes, and the average cooling rate from the annealing temperature to 700°C is preferably set to 5°C/sec or higher.
By setting the annealing temperature to more than 830°C and the β transformation point or higher, and setting the annealing time to 1 minute or more, element distribution can be suppressed, alloy elements can be distributed more uniformly, and the distribution of the second phase can be made uniform. On the other hand, if the annealing time exceeds 5 minutes, the yield will decrease due to oxidation, and the manufacturability will decrease due to the long time, which is not preferable. There is no need to limit the upper limit of the annealing temperature, but from the viewpoint of the decrease in yield due to oxidation, the annealing temperature is preferably 1000°C or less.
Furthermore, if the average cooling rate from the annealing temperature to 700° C. is slow, element partitioning occurs during cooling. Therefore, the average cooling rate from the annealing temperature to 700° C. is set to 5° C./sec or more. Since increasing the average cooling rate does not significantly affect the degree of element partitioning or the distribution of the second phase, there is no need to specify an upper limit, but the rate may be set to 300° C./sec or less.
However, when the intermediate annealing described later is performed under conditions in which the annealing temperature is higher than 830°C and is equal to or higher than the β transformation point, the annealing time is 1 to 5 minutes, and the average cooling rate from the annealing temperature to 700°C is 5°C/sec or more, the hot-rolled sheet annealing step may not be performed or may be performed under conditions other than those described above.

<Cold rolling process>
In the cold rolling process, cold rolling is performed on the hot rolled sheet after the hot rolling process or the hot rolled sheet after the hot rolled sheet annealing process. When cold rolling is performed on the hot rolled sheet, it is necessary to obtain fine equiaxed grains after the final annealing, so the reduction ratio in the cold rolling (the cumulative reduction ratio in the case of multiple passes) is set to 60% or more. The reduction ratio in the cold rolling may be set to 90% or less to prevent cracks. When intermediate annealing, which will be described later, is performed, the reduction ratio of the cold rolling before the intermediate annealing is set to the intermediate cold rolling ratio, and the reduction ratio of the cold rolling after the intermediate rolling is set to the final cold rolling ratio, and the final cold rolling ratio is set to 60% or more.
In addition, when hot-rolled sheet annealing is performed at a temperature of more than 830°C and at or above the β transformation point, the metal structure becomes acicular, and the cold rollability decreases. Therefore, in this case, the cold rolling conditions are preferably a reduction ratio of 10% or less from the first pass to the second pass, and 15% or less thereafter. By working at a low reduction ratio up to the second pass, stable cold rolling can be performed without causing cracks. After that, the temperature rises due to processing heat, so even if the reduction ratio is increased, cracks are less likely to occur.

When hot-rolled sheet annealing is not performed, in the cold rolling process, the cold rolling is interrupted before the final reduction and intermediate annealing is performed. When intermediate annealing is performed, it is preferable that the annealing temperature is more than 830° C. and is equal to or higher than the β transformation point, the annealing time is 1 to 5 minutes, and the average cooling rate from the annealing temperature to 700° C. is 5° C./second or more.
The reason for this condition is the same as that explained in the case of hot-rolled sheet annealing.
Even when hot-rolled sheet annealing is performed, the above-mentioned intermediate annealing may be performed.

As described above, by setting either the annealing temperature of the hot-rolled sheet annealing or the final intermediate annealing to a temperature above 830°C and above the β transformation point, element distribution can be suppressed, alloy elements can be distributed more uniformly, and the distribution of the second phase can be made uniform. In particular, by setting the annealing temperature of at least the intermediate annealing to a temperature above the β transformation point, the sheet is heated to the β transformation point or above in a thinner state, and element distribution on the surface and inside of the sheet can be suppressed. Furthermore, by performing annealing at a temperature above 830°C and above the β transformation point in both the hot-rolled sheet annealing and the intermediate annealing, element distribution can be suppressed and alloy elements can be distributed more uniformly.

<Final annealing process>
The titanium alloy plate after the cold rolling process is subjected to final annealing at a temperature range of 550°C or more and less than 670°C for recrystallization. If the annealing temperature is less than 550°C, a large amount of intermetallic compounds are generated, so recrystallization does not proceed sufficiently, and unrecrystallized regions are generated, resulting in reduced polishability. If the annealing temperature is 670°C or more, the α phase may grow into grains, resulting in coarse crystal grains. Therefore, the final annealing temperature is set to less than 670°C.
The annealing time of the final annealing is 1 minute to 24 hours. By setting the annealing time to 1 minute or more, recrystallization proceeds sufficiently. Furthermore, by setting the annealing time to 24 hours or less, the formation of coarse crystal grains is prevented. There is no particular restriction on the cooling rate after the final annealing (cooling rate in the temperature range below 550°C).

Next, the automobile exhaust system part of this embodiment will be described.
The automotive exhaust system part of this embodiment includes the titanium alloy plate. (In some cases, it is made of the titanium alloy plate.) The automotive exhaust system part of this embodiment is obtained by forming the titanium alloy plate of this embodiment, for example, by press forming. Since the chemical composition does not change by forming, the chemical composition of the automotive exhaust system part is equivalent to that of the titanium alloy plate of this embodiment. When the titanium alloy plate is formed into an automotive exhaust system part, twin deformation occurs by forming, so that the grain size becomes fine in the part deformed by forming. The twin deformation can be identified by OIM analysis. However, as the degree of processing increases, the crystal orientation difference between the parent phase and the twin deformation changes, making the analysis difficult. Therefore, in order to identify the grain size of the α phase, it is necessary to prepare a measurement sample from a part of the automotive exhaust system part that has a moderate degree of processing.

A titanium alloy having the chemical composition shown in Table 1 was made into an ingot by vacuum arc button melting. The produced ingot was hot rolled at 1000°C to make a 10 mm thick hot rolled sheet. Then, hot rolling was performed at 860°C to obtain a 4.0 mm thick hot rolled sheet. In Table 1, the contents of Ni, V, Mn, Co, Ta, W, C, and N are omitted, and the total contents of these elements are listed in the "others" column. The contents of each of these elements were all 0.05% or less. Furthermore, among the impurities, the H content was all 0.013% or less.

After that, a descaling process was performed or, if necessary, hot-rolled sheet annealing was performed at the temperature and time shown in Table 2, followed by a descaling process, followed by cold rolling and intermediate annealing if necessary, followed by final cold rolling. Furthermore, final annealing was performed. In this manner, titanium alloy sheets No. 1 to No. 48 were manufactured.

The obtained titanium alloy plate was then subjected to a polishing process. The polishing process consisted of wet polishing with emery paper #1500 and alumina buff polishing for 60 minutes. The polishing solution used for the alumina buff polishing was a solution in which 250 g of alumina powder with an average particle size of 3 μm was added to 1 liter of water. The polishing process involved embedding the sample in epoxy resin with a diameter of 28 mm, setting six samples in the holder of an automatic polishing device, and polishing with a pressure of 60 N.

Various evaluations were performed on the polished titanium alloy plates.

The average grain size of the α phase was measured at the center of the plate thickness of the cross section (L cross section) perpendicular to the plate width direction at the position of 1/2 the plate width length, using EBSD, as described above, with an acceleration voltage of 15 kV, a magnification of 500 times, and a measurement pitch of 0.2 μm, targeting only the α phase. The measurement field was set to a size that included 300 or more grains in one field, or a total of 400 or more grains in multiple fields, and the measurement sample was adjusted so that the average CI value was 0.2 or more. The measurement analysis software used was OIM-analysis ^TM (version 7.3.1), and the boundaries with a crystal orientation difference of 15° or more were regarded as grain boundaries, and the grain size of each grain was calculated by approximating the circle equivalent diameter from the area of the grains divided by this boundary. When calculating the grain size, grains with a grain size of 1.0 μm or less and grains incompletely included in the field were excluded.

As described above, the grain size distribution of the α phase was measured using the measurement area (rectangular area with a side length of 100 μm or more: created from the center of the plate thickness of the cross section (L cross section) perpendicular to the plate width direction at the position of 1/2 the plate width length) set when measuring the average crystal grain size. The crystal orientation of the α phase was analyzed by electron backscatter diffraction (EBSD) method for the measurement area. The analysis software OIM-analysis ^TM (version 7.3.1) was used, and the boundaries with a crystal orientation difference of 15° or more were regarded as grain boundaries, and the grain size of each grain was obtained by approximating the circle equivalent diameter from the area of the divided grains. Next, from the grain size of each obtained grain and the average grain size obtained by the above method, the number ratio of α phase grains whose grain size was in the range of the average grain size ±2 μm and the number ratio of α phase grains whose grain size was in the range of the average grain size ±4 μm were obtained.

As described above, the distribution state of the second phase was determined by dividing an area with a side length of 100 μm into 10 x 10 equal parts at the center of the sheet thickness of a cross section (L cross section) perpendicular to the sheet width direction at 1/2 the sheet width length position, and determining the number of second phases per unit area for each measurement area, and determining the number of measurement areas in which 5 to 15 second phases were observed.
The measurement area was observed with a scanning electron microscope (SEM), and the α-phase and the second phase were distinguished from the backscattered electron image. The second phase, which is an intermetallic compound, is white or black compared to the α-phase, which is the parent phase, and is a fine precipitate, so it could be distinguished from the second phase from these characteristics.

The area ratio of the second phase was measured in the same region as the number density of the second phase. The above-mentioned region with a side length of 100 μm (the center of the plate thickness of the cross section (L cross section) perpendicular to the plate width direction at the position of 1/2 the plate width length) was observed with a scanning electron microscope (SEM), and the α phase and the second phase were distinguished from the backscattered electron image. The area of the second phase in the region was then measured, and the area ratio (%) of the second phase was calculated.

Total elongation was measured by conducting a room temperature tensile test. The room temperature tensile test was performed by taking an ASTM sub-size tensile test piece (parallel part width: 6.25 mm, parallel part length: 32 mm, gauge length: 25 mm) from the titanium alloy plate with the longitudinal direction parallel to the rolling direction, and at a strain rate of 30%/min. The test temperature was in the range of 10 to 35°C.

The Erichsen value was measured according to the Erichsen test method specified in JIS Z 2247 (2006). The plate width of the measurement sample was 90 mm or more. The test machine was as described in JIS B 7729 (2005). A 50 μm thick Teflon (registered trademark) sheet was used as the lubricant. The jig dimensions were the same as those used for tests using standard test pieces.

It was determined that a material had excellent processability if its total elongation was 25.0% or more and its Erichsen value was 9.5 mm or more.

The oxidation mass gain was determined by taking a 20 mm x 20 mm test piece from the titanium alloy plate, wet-polishing the surface with emery paper #400, exposing it to still air at 800°C for 100 hours, measuring the mass increase after exposure, and dividing the mass increase by the surface area of the tensile test piece ((mass increase (mg)/surface area of test piece ( ^cm2 ))). If scale spalling occurred during the oxidation test, the spalled scale was also included in the mass after exposure. If the oxidation mass gain was 5.0 mg/ ^cm2 or less, it was determined that the material had excellent oxidation resistance at high temperatures.

The high-temperature strength (tensile strength) at 800°C was measured by taking a tensile test piece (parallel part width: 10 mm, parallel part length and gauge length: 35 mm) from the above titanium alloy plate with the longitudinal direction parallel to the rolling direction, and conducting a tensile test at a strain rate of 7.5%/min. The test atmosphere was 800°C in air, and the test piece was held in the test atmosphere for 10 minutes to ensure that it had sufficiently reached the test temperature, after which the test was conducted. If the high-temperature strength (tensile strength) was 26 MPa or more at 800°C, it was determined that the high-temperature strength was excellent.

The glossiness (Gs20) was measured according to the method for measuring specular glossiness of JIS Z 8741 (1997). The angle of incidence and the angle of reception were set to 20°. If the glossiness Gs20 was 920 or more, it was determined that the polishability was excellent.
The evaluation results are shown in Table 3.

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 range of the present disclosure and showed excellent properties.
In addition, all of these inventive examples had an equiaxed metal structure, i.e., the average aspect ratio (major axis length/minor axis length) of the α phase was 3.0 or less.

On the other hand, No. 9 had a low Si content, and the average crystal grain size of the α phase was coarse. In addition, the distribution state of the second phase was also deteriorated. As a result, the oxidation weight gain increased, and the oxidation resistance in a high-temperature environment was low. In addition, the high-temperature strength was also low. Furthermore, because the average crystal grain size of the α phase was large, the glossiness after polishing was low and the polishability was low.
No. 10 had an excessively large Si content, and thus had a high Erichsen value and poor workability.
No. 14 had an excessively high Al content, and thus had a high Erichsen value and poor workability.
No. 16 had an excessive Zr content, which resulted in a small average crystal grain size of the α phase and a large amount of unrecrystallized structure remaining. As a result, the total elongation and the Erichsen value were low, resulting in poor workability and poor polishability.
In No. 18, the Cr content was excessive, and the β phase was formed when heated at high temperatures, resulting in low oxidation resistance.
No. 20 had an excessive Mo content, which resulted in a small average crystal grain size of the α phase and a large amount of unrecrystallized structure remaining. As a result, the total elongation and Erichsen value were low, and the workability was poor. The polishability was also poor. Furthermore, the β phase was formed during high-temperature heating, resulting in low oxidation resistance.
In No. 26, the annealing temperature of the hot-rolled sheet was lower than the β transformation point, and no intermediate annealing was performed, so the distribution state of the second phase was deteriorated. As a result of observation, it was found to contain coarse crystal grains. As a result, the polishability was low.
In No. 29, the annealing temperature of the hot-rolled sheet annealing and the intermediate annealing temperature were both below the β transformation point, so the distribution state of the second phase was deteriorated. As a result of observation, it was found to contain coarse crystal grains. As a result, the polishability was low.
In No. 30, the cooling rate after hot-rolled sheet annealing was low and no intermediate annealing was performed, so the distribution of the α-phase crystal grain size was broad. As a result of observation, it was found to contain coarse crystal grains. As a result, the polishability was low.
In No. 34, the final annealing temperature was low, the average crystal grain size of the α phase was small, and a large amount of unrecrystallized structure remained. Therefore, the total elongation and the Erichsen value were low, and the workability was low. The polishability was also low.
In No. 35, the reduction ratio in the final cold rolling was low, and the introduction of strain was insufficient, so the distribution of the crystal grain size of the α phase was broadened and coarse crystal grains were included, resulting in low polishability.
In No. 37, the final annealing temperature was high, the average grain size of the α grains became coarse, and the distribution of the grain size of the α phase crystal was broadened. In addition, the distribution of the second phase was not uniform. As a result, the polishability was low.
No. 40 had a low Cu content and was low in high-temperature strength.
No. 41 had an excessive Cu content, and thus had a low Erichsen value and poor workability.
No. 42 had a low Sn content and was low in high-temperature strength.
No. 43 had an excessive Sn content, and thus had a low total elongation and Erichsen value, resulting in poor workability.
No. 44 had a low Nb content and was poor in oxidation resistance at high temperatures.

According to the present disclosure, it is possible to provide titanium alloy plates and automotive exhaust system parts that have excellent processability, polishability, and oxidation resistance at high temperatures.

Claims (4)

In mass percent,
Cu: 0.7% to 1.5%,
Sn: 0.5% to 1.5%,
Si: 0.10% to 0.60%,
Nb: 0.1% to 1.0%,
Zr: 0% to 1.0%,
Cr: 0% to 0.5%,
Mo: 0% to 0.5%,
Al: 0% to 1.0%;
Fe: 0.08% or less, O: 0.07% or less,
The balance is Ti and impurities.
The metal structure is composed of an α phase and a second phase,
The average grain size of the α phase is 3.0 to 10.0 μm,
The proportion of crystal grains having a grain size in the range of an average grain size ±2 μm in the α phase is 25% or more;
The proportion of crystal grains having a grain size in the range of an average grain size ±4 μm in the α phase is 45% or more;
A titanium alloy plate characterized in that, in a cross section, a 100 μm×100 μm area is divided into 100 equal parts, each of which is a 100 μm×10 μm area, and when the number density of the second phase is calculated for each of the measurement areas, the number of measurement areas in which 5 to 15 pieces of the second phase are observed within the measurement area is 80 or more. The titanium alloy plate according to claim 1, characterized in that the area fraction of the second phase is 1.0% or more. An exhaust system part for an automobile comprising the titanium alloy plate according to claim 1 or 2. An automobile exhaust system part obtained by forming the titanium alloy plate according to claim 1 or 2.