Titanium alloy plate, method for manufacturing titanium alloy component, and method for manufacturing titanium alloy plate

Abstract

The purpose of the present invention is to provide a titanium alloy plate capable of forming a titanium alloy component having excellent high-temperature durability even when strain is applied during processing while sufficiently maintaining strength in a high-temperature environment. A titanium alloy plate according to one embodiment of the present invention contains, as elements, 0.4-0.6% by mass of Al, 0.3-0.6% by mass of Si, and at least one element selected from the group consisting of Mo, Ta, Nb, W, V, Mn, and Co, with the remainder being Ti and unavoidable impurities, has a tensile strain of 5.5-6.5% in a direction perpendicular to the thickness of the plate, and after the plate is held at 800 DEG C for 30 minutes, the tensile strain of the plate is less than 5.5% and less than 6.5%. An alternating load in the plate thickness direction is applied at a stress amplitude of 80 MPa, a stress ratio of-1, and a frequency of 25 Hz, and the average crystal grain diameter of the titanium alloy at a position 1 / 2 of the plate thickness at a time when the initial load stress is reduced to 50% is 30 [mu] m or less.

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a titanium alloy plate, a titanium alloy part, and a method for manufacturing a titanium alloy plate. Background Art

[0002] From the viewpoints of light weight and improved designability, etc., titanium materials or titanium alloy materials are used for exhaust pipes of internal combustion engines, etc. Parts such as exhaust pipes are formed by performing bending processing or extrusion processing on plate-shaped titanium materials or titanium alloy materials. Therefore, it is required that the titanium material or titanium alloy material can be formed by rolling and has sufficient ductility for processing.

[0003] The part is designed to withstand the vibration of the internal combustion engine or the vibration caused by vehicle running such as motorcycles and four-wheel trucks. In addition, since the part is exposed to high temperatures due to the exhaust of the internal combustion engine, it is required to have high oxidation resistance (see Patent Document 1).

[0004] Prior Art Documents

[0005] Patent Documents

[0006] Patent Document 1: Japanese Unexamined Patent Application Publication No. 2007-270199 Summary of the Invention

[0007] Problems to be Solved by the Invention

[0008] In parts using titanium materials or titanium alloy materials, the portion where strain is applied during processing is likely to promote grain growth. As a result, the fatigue strength is reduced and the high-temperature durability is likely to be reduced.

[0009] In Patent Document 1, it is described that by containing a specific amount of Si and actively restricting Al, the high-temperature oxidation resistance of the titanium alloy can be improved. However, Patent Document 1 does not study the relationship between the strain applied during processing and the high-temperature durability.

[0010] The present invention is proposed based on such circumstances, and its object is to provide a titanium alloy plate that can sufficiently maintain its strength in a high-temperature environment and, at the same time, can form a titanium alloy part with excellent high-temperature durability even when strain is applied during processing. That is, the object of the present invention is to provide a titanium alloy plate that can form a titanium alloy part in which high-temperature strength and high-temperature durability coexist.

[0011] Means for Solving the Problems

[0012] A titanium alloy plate according to one embodiment of the present invention contains, as elements, Al: 0.4% by mass or more and 0.6% by mass or less, Si: 0.3% by mass or more and 0.6% by mass or less, and further contains at least one selected from the group consisting of Mo, Ta, Nb, W, V, Mn, and Co. The balance is Ti and unavoidable impurities. A tensile strain of 5.5% or more and 6.5% or less is imparted in the direction perpendicular to the plate thickness, and after holding at 800 °C for 30 minutes, an alternating load in the plate thickness direction is applied with a stress amplitude of 80 MPa, a stress ratio of -1, and a frequency of 25 Hz. The average grain diameter of the titanium alloy at the position of 1 / 2 of the plate thickness at the moment when the initial load stress is reduced to 50% is 30 μm or less.

[0013] Effects of the Invention

[0014] The titanium alloy plate according to one embodiment of the present invention can sufficiently maintain the strength in a high-temperature environment, and at the same time, has excellent high-temperature durability even when strain is applied during processing. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 It is a flowchart showing a method for manufacturing a titanium alloy plate according to one embodiment of the present invention.

[0016] Figure 2 It is a flowchart showing a method for manufacturing a titanium alloy part according to one embodiment of the present invention.

[0017] Figure 3 It is an optical microscope photograph showing a cross-sectional structure including a fracture part in the titanium alloy plate of No. 1.

[0018] Figure 4 It is an optical microscope photograph showing a cross-sectional structure including a fracture part in the titanium alloy plate of No. 5. DETAILED DESCRIPTION OF THE INVENTION

[0019] [Description of Embodiments of the Present Invention]

[0020] First, embodiments of the present invention will be listed and described.

[0021] (1) A titanium alloy plate according to one embodiment of the present invention contains, as elements, Al: 0.4% by mass or more and 0.6% by mass or less, Si: 0.3% by mass or more and 0.6% by mass or less, and further contains at least one selected from the group consisting of Mo, Ta, Nb, W, V, Mn, and Co. The balance is Ti and unavoidable impurities. A tensile strain of 5.5% or more and 6.5% or less is imparted in a direction perpendicular to the plate thickness, and after holding at 800 °C for 30 minutes, an alternating load in the plate thickness direction is applied with a stress amplitude of 80 MPa, a stress ratio of -1, and a frequency of 25 Hz. The average grain diameter of the titanium alloy at the position of half the plate thickness at the moment when the initial load stress is reduced to 50% is 30 μm or less.

[0022] Since this titanium alloy plate contains Al and Si within the above ranges, further contains at least one selected from the group consisting of Mo, Ta, Nb, W, V, Mn, and Co, imparts a tensile strain of 5.5% or more and 6.5% or less in a direction perpendicular to the plate thickness, holds at 800 °C for 30 minutes, then applies an alternating load in the plate thickness direction with a stress amplitude of 80 MPa, a stress ratio of -1, and a frequency of 25 Hz, and the average grain diameter of the titanium alloy at the position of half the plate thickness at the moment when the initial load stress is reduced to 50% is 30 μm or less, it can sufficiently maintain strength in a high-temperature environment. At the same time, even when strain is applied during processing, its high-temperature durability is excellent.

[0023] In the above (1), the Mo equivalent [Mo]eq represented by the following formula 1 can be 0.12 or more.

[0024] [Mo]eq = [Mo] + [Ta] / 5 + [Nb] / 3.6 + [W] / 2.5 + [V] / 1.5 + 1.25[Cr] + 1.25[Ni] + 1.7[Mn] + 1.7[Co] + 2.5[Fe] …1

[0025] Here, [X] in the above formula 1 represents the content [% by mass] of element X in the titanium alloy plate.

[0026] Thus, by satisfying the above formula 1, this titanium alloy plate can easily suppress the rapid coarsening of the metal structure at high temperatures when a strain of about 5% is applied. As a result, the high-temperature durability can be more surely improved.

[0027] (3) In the above (1) or (2), Nb: 0.1% by mass or more and 0.5% by mass or less can be contained. By containing Nb within the above range, the oxidation resistance and the high-temperature durability when strain is applied can be improved simultaneously.

[0028] (4) In any one of the above (1) to (3), the following formula 2 can be further satisfied.

[0029] 9 ≤ 3.4[Al] + 13.4[Si] + 13.9[Mo]eq ≤ 14…2

[0030] In this way, by further satisfying the above formula (2), the ductility for processing, high-temperature strength, and high-temperature durability during strain application of the titanium alloy plate can be further improved as a whole.

[0031] (5) A method for manufacturing a titanium alloy part according to another aspect of the present invention uses the titanium alloy plate according to any one of (1) to (4) above and includes a processing step of applying a strain of 3% or more and 8% or less. Here, the "strain" means the nominal strain ε. In addition, the "applying a strain of 3% or more and 8% or less" means that when ε = ΔL / L0 (ΔL: amount of change in length, L0: length before change), there is a portion where 3 ≤ ε × 100 ≤ 8.

[0032] The method for manufacturing the titanium alloy part can manufacture a titanium alloy part that can sufficiently maintain strength in a high-temperature environment and has excellent high-temperature durability.

[0033] (6) In the above (5), the titanium alloy part may be an exhaust pipe. The method for manufacturing the titanium alloy part is suitable as a method for manufacturing an exhaust pipe that can sufficiently maintain strength in a high-temperature environment and has excellent high-temperature durability.

[0034] (7) A method for manufacturing a titanium alloy plate according to another aspect of the present invention uses a plate containing 0.4% by mass or more and 0.6% by mass or less of element Al, 0.3% by mass or more and 0.6% by mass or less of Si, and further containing at least one selected from the group consisting of Mo, Ta, Nb, W, V, Mn, and Co, with the balance being Ti and unavoidable impurities, and includes: a hot rolling step of hot rolling the above plate; and a cold rolling step of cold rolling the hot rolled plate obtained through the above hot rolling step, and satisfies at least any one of the following (a) and (b).

[0035] (a) In the above hot rolling step, it includes a step of rolling at a rolling ratio of 0.6 or more and 0.9 or less.

[0036] (b) In the above cold rolling step, it includes a step of rolling at a rolling ratio of 0.5 or more and 0.9 or less.

[0037] The method for manufacturing the titanium alloy plate can manufacture a titanium alloy plate that can sufficiently maintain strength in a high-temperature environment and has excellent high-temperature durability even when a strain is applied during processing.

[0038] Further, in the present invention, the so-called "rolling ratio" means a value calculated by (h1 - h2) / h1, where h1 is the plate thickness before rolling and h2 is the plate thickness after rolling.

[0039] [Details of Embodiments of the Present Invention]

[0040] Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. Also, regarding the numerical values described in this specification, only one of the upper limit value and the lower limit value described may be adopted, or the upper limit value and the lower limit value may be arbitrarily combined.

[0041] <Titanium Alloy Plate>

[0042] In this titanium alloy plate, as elements, it contains Al: 0.4 mass% or more and 0.6 mass% or less, Si: 0.3 mass% or more and 0.6 mass% or less, and also contains at least one selected from the group consisting of Mo, Ta, Nb, W, V, Mn, and Co, with the balance being Ti and unavoidable impurities. Further, in this titanium alloy plate, a tensile strain of 5.5% or more and 6.5% or less is imparted in the direction perpendicular to the plate thickness, and after holding at 800°C for 30 minutes, an alternating load in the plate thickness direction is applied with a stress amplitude of 80 MPa, a stress ratio of -1, and a frequency of 25 Hz, and the average grain diameter of the titanium alloy at the position of half of the plate thickness at the moment when the initial load stress is reduced to 50% is 30 μm or less.

[0043] This titanium alloy plate is plate-shaped. This titanium alloy plate is formed into titanium alloy parts such as the exhaust pipe of an internal combustion engine, for example. As the above exhaust pipe, vehicle exhaust pipes such as those of motorcycles and light-duty trucks can be cited, including muffler components such as exhaust manifolds, exhaust pipes, catalytic mufflers, pre-mufflers, and mufflers (main mufflers).

[0044] In the titanium alloy plate for forming titanium alloy parts such as exhaust pipes, excellent ductility for processing is desired, and the strength and high-temperature durability in a high-temperature environment can be balanced. The amount of deformation caused by processing varies depending on the part, but in the part where a strain higher than 10% is applied, since recrystallization occurs at high temperature, the rapid coarsening of the metal structure does not occur. On the other hand, in the part where a strain of about 5% is applied, the metal structure rapidly coarsens at high temperature, and the durability is likely to decrease. Therefore, in order to obtain sufficient high-temperature durability in this titanium alloy plate, it is desired to suppress the rapid coarsening of the metal structure even when a strain of about 5% is applied.

[0045] In such a view, the titanium alloy plate contains Al and Si within the above range, and also contains at least one selected from the group consisting of Mo, Ta, Nb, W, V, Mn, and Co. After applying a tensile strain of 5.5% or more and 6.5% or less in the direction perpendicular to the plate thickness and holding at 800 °C for 30 minutes, an alternating load in the plate thickness direction is applied with a stress amplitude of 80 MPa, a stress ratio of -1, and a frequency of 25 Hz. The average grain diameter of the titanium alloy at the position of 1 / 2 of the plate thickness at the moment when the initial load stress is reduced to 50% is 30 μm or less. Therefore, it can sufficiently maintain the strength in a high-temperature environment, has excellent ductility for processing, and has excellent high-temperature durability even when strain is applied during processing.

[0046] (Al)

[0047] Al (aluminum) is mainly dissolved in the α phase in the titanium alloy plate, and the high-temperature strength is improved by solid-solution strengthening. As the lower limit of the Al content in the titanium alloy plate, as described above, it is 0.4 mass%, preferably 0.45 mass%, and more preferably 0.5 mass%. On the other hand, as the upper limit of the Al content in the titanium alloy plate, as described above, it is 0.6 mass%, preferably 0.55 mass%. If the above content is lower than the above lower limit, the high-temperature strength may become insufficient. Conversely, if the above content is higher than the above upper limit, the ductility of the titanium alloy plate becomes insufficient, and it may be difficult to process the exhaust pipe of an internal combustion engine, etc.

[0048] (Si)

[0049] Si (silicon) improves the creep characteristics and oxidation resistance of the titanium alloy plate. As the lower limit of the Si content in the titanium alloy plate, as described above, it is 0.3 mass%, preferably 0.4 mass%. On the other hand, as the upper limit of the Si content in the titanium alloy plate, as described above, it is 0.6 mass%, preferably 0.5 mass%. If the above content is lower than the above lower limit, the improvement effect of oxidation resistance may not be obtained sufficiently. Conversely, if the above content is higher than the above upper limit, the ductility of the titanium alloy plate becomes insufficient, and it may be difficult to process the exhaust pipe of an internal combustion engine, etc.

[0050] (Mo equivalent)

[0051] As described above, the titanium alloy plate is processed into a titanium alloy part through processing such as extrusion processing or bending processing. However, in order to obtain sufficient high-temperature durability, it is desired to suppress the rapid coarsening of the metal structure even when a strain of about 5% is applied. In this regard, the titanium alloy plate can easily suppress the rapid coarsening of the metal structure at high temperatures by controlling the Mo equivalent [Mo]eq represented by the following formula 1.

[0052] [Mo]eq = [Mo] + [Ta] / 5 + [Nb] / 3.6 + [W] / 2.5 + [V] / 1.5 + 1.25[Cr] + 1.25[Ni] + 1.7[Mn] + 1.7[Co] + 2.5[Fe] …1

[0053] Wherein, [X] in the above formula (1) means the content [mass%] of element X in the titanium alloy plate.

[0054] The above Mo equivalent is used as an index to represent the stability of the β phase. In other words, in this titanium alloy plate, the Mo equivalent has the meaning of the content of the β phase stabilizing element. When the inventors of the present invention conducted intensive research, they obtained the following knowledge: by controlling the Mo equivalent, even when a strain of about 5% is applied, the rapid coarsening of the metal structure at high temperature can be suppressed.

[0055] As the lower limit of the above Mo equivalent, it is preferably 0.12, more preferably 0.35, and further preferably 0.4. On the other hand, as the upper limit of the above Mo equivalent, it is preferably 0.8, more preferably 0.6. If the above Mo equivalent is lower than the lower limit, when a strain caused by processing (especially a strain of about 5%) is applied, the grain growth cannot be suppressed, the metal structure coarsens, and thus the high-temperature durability may become insufficient. Conversely, if the above Mo equivalent is higher than the above upper limit, the ductility of this titanium alloy plate may become insufficient.

[0056] In addition, this titanium alloy plate preferably satisfies the following formula (2).

[0057] 9 ≤ 3.4[Al] + 13.4[Si] + 13.9[Mo]eq ≤ 14 …2

[0058] By satisfying the above formula (2), this titanium alloy plate can improve the ductility for processing, high-temperature strength, and high-temperature durability when a strain is applied as a whole. To explain in more detail, this titanium alloy plate can improve the high-temperature strength by increasing [Al] and [Si] in the above formula (2). In addition, this titanium alloy plate can easily maintain the high-temperature strength by increasing [Mo]eq in the above formula (2). Therefore, when this titanium alloy plate becomes 9 ≤ 3.4[Al] + 13.4[Si] + 13.9[Mo]eq in the above formula (2), the high-temperature strength can be improved, and this strength can be fully maintained, thereby improving the fatigue characteristics at high temperature. As the lower limit value of the above formula (2), it is more preferably 10. In addition, when this titanium alloy plate becomes 3.4[Al] + 13.4[Si] + 13.9[Mo]eq ≤ 14 in the above formula (2), the insufficiency of ductility can be suppressed.

[0059] For this titanium alloy plate, when the above Mo equivalent is within the desired range, the types of elements (β-phase stabilizing elements) for stabilizing the β-phase are not particularly limited. That is, as the β-phase stabilizing element, any of Mo (molybdenum), Ta (tantalum), Nb (niobium), W (tungsten), V (vanadium), Cr (chromium), Ni (nickel), Mn (manganese), Co (cobalt), Fe (iron) can be contained. However, it is preferred that this titanium alloy plate contains Nb as the β-phase stabilizing element. Nb can improve the oxidation resistance of this titanium alloy plate by adding a relatively small amount. In addition, even when the content of Nb is relatively large, a decrease in the ductility of this titanium alloy plate can be suppressed.

[0060] When Nb is contained as the above β-phase stabilizing element, as the lower limit of the Nb content, it is preferably 0.1% by mass, more preferably 0.2% by mass. On the other hand, as the upper limit of the Nb content, it is preferably 0.5% by mass, more preferably 0.3% by mass. If the above content is lower than the above lower limit, it may be difficult to effectively improve the oxidation resistance of this titanium alloy plate. Conversely, if the above content is higher than the above upper limit, the ductility of this titanium alloy plate may become insufficient.

[0061] (inevitable impurities)

[0062] In this titanium alloy plate, the other elements are Ti and inevitable impurities. In this titanium alloy plate, due to the reasons of ore and manufacturing method, inevitable impurities are usually contained.

[0063] As the inevitable impurities that may be contained in this titanium alloy plate, for example, C (carbon), Fe, Ni, Cr, Pd (palladium), N (nitrogen), O (oxygen), etc. can be cited. When C is contained as the above inevitable impurity, as the content of C, for example, it can be 0.05% by mass or less. When Fe is contained as the above inevitable impurity, as the content of Fe, for example, it can be 0.1% by mass or less. When Ni, Cr or Pd is contained as the above inevitable impurity, as the content of these elements, for example, for each element, it can be 0.01% by mass or less. When N is contained as the above inevitable impurity, as the content of N, for example, it can be 0.01% by mass or less. When O is contained as the above inevitable impurity, as the content of O, for example, it can be 0.2% by mass or less.

[0064] Among the above inevitable impurities, Fe, Ni and Cr are included in the above β-phase stabilizing elements. Therefore, this titanium alloy plate can increase the Mo equivalent by containing one or more of Fe, Ni and Cr as the above inevitable impurities. Also, from the viewpoint of increasing the Mo equivalent, this titanium alloy plate can also actively add one or more of Fe, Ni and Cr.

[0065] Among the above-mentioned inevitable impurities, Fe and O can be used to adjust the balance between strength and ductility. In addition, as long as O can ensure ductility at room temperature, it will not affect the effects of the present invention. The titanium alloy plate can use raw materials with high concentrations of Fe, O, etc. to increase strength. On the contrary, it can also use high-purity raw materials to obtain high formability that can meet more stringent forming requirements.

[0066] (Average grain diameter)

[0067] The titanium alloy plate is given a tensile strain of 5.5% or more and 6.5% or less in a direction perpendicular to the plate thickness, and after holding at 800 °C for 30 minutes, an alternating load in the plate thickness direction is applied with a stress amplitude of 80 MPa, a stress ratio of -1, and a frequency of 25 Hz. The average grain diameter of the titanium alloy at the position of half of the plate thickness at the moment when the initial load stress is reduced to 50% is 30 μm or less. In other words, the titanium alloy plate is given an arbitrary tensile strain within the range of 5.5% or more and 6.5% or less in a direction perpendicular to the plate thickness, and after holding at 800 °C for 30 minutes, for the part where the tensile strain has been applied, an alternating load in the plate thickness direction is applied with a stress amplitude of 80 MPa, a stress ratio of -1, and a frequency of 25 Hz. The average grain diameter of the titanium alloy at the position of half of the plate thickness at the moment when the initial load stress is reduced to 50% is 30 μm or less. As the upper limit of the above-mentioned average grain diameter, from the viewpoints of improving fatigue strength and high-temperature durability, it is preferably 25 μm, more preferably 20 μm, and further preferably 15 μm. On the other hand, as the lower limit of the above-mentioned average grain diameter, there is no particular limitation, but from the viewpoint of ensuring formability during part processing, it can be 3 μm or 6 μm. Also, the so-called "average grain diameter of the titanium alloy" means a value measured by the cutting method in a field of view centered on the position of half of the plate thickness (measurement position) near the fracture part (part within 500 μm from the fracture part) and including 20% of the plate thickness.

[0068] The average grain diameter of the above-mentioned titanium alloy can be adjusted by, for example, controlling the rolling conditions during the manufacturing process of the titanium alloy plate to reduce the average grain diameter of the raw material. For example, the titanium alloy plate is formed into a plate shape through a hot rolling process and a cold rolling process. The average grain diameter of the above-mentioned titanium alloy can be adjusted by controlling the rolling ratio of one or both of the above-mentioned hot rolling process and the above-mentioned cold rolling process.

[0069] The above hot rolling process preferably includes a process of rolling at a rolling rate of 0.6 or more and 0.9 or less. In the above hot rolling process, for example, a rough rolling process and a finish rolling process are included. The above rough rolling process and the above finish rolling process are carried out, for example, after the blooming process. In the above hot rolling process, the rolling rate in any one of the above rough rolling process and the above finish rolling process can be controlled within the above range. However, in the above hot rolling process, it is preferable to control the rolling rate of the above finish rolling process within the above range.

[0070] From the viewpoint of making the structure more uniform and improving the suppression effect of grain growth, as the lower limit of the above rolling rate, as described above, it is preferably 0.6, more preferably 0.65, and further preferably 0.7. In addition, from the viewpoint of preventing the yield rate from decreasing as the oxygen-affected layer on the surface is removed, as the upper limit of the above rolling rate, as described above, it can be 0.9. That is, this titanium alloy plate can be obtained by rolling at a rolling rate of 0.6 or more and 0.9 or less, or can be obtained by rolling at a rolling rate of 0.65 or more and 0.9 or less, or can be obtained by rolling at a rolling rate of 0.7 or more and 0.9 or less in the above hot rolling process.

[0071] The above cold rolling process preferably includes a process of rolling at a rolling rate of 0.5 or more and 0.9 or less. From the viewpoint of making the structure more uniform and improving the suppression effect of grain growth, as the lower limit of the above rolling rate, as described above, it is preferably 0.5, more preferably 0.6, and further preferably 0.65. In addition, from the viewpoint of preventing the yield rate from decreasing as the crack at the end is removed, as the upper limit of the above rolling rate, as described above, it can be 0.9. That is, this titanium alloy plate can be obtained by rolling at a rolling rate of 0.5 or more and 0.9 or less, or can be obtained by rolling at a rolling rate of 0.6 or more and 0.9 or less, or can be obtained by rolling at a rolling rate of 0.65 or more and 0.9 or less in the above cold rolling process.

[0072] <Manufacturing method of titanium alloy plate>

[0073] The manufacturing method of this titanium alloy plate uses the following plate, which contains element Al: 0.4% by mass or more and 0.6% by mass or less, Si: 0.3% by mass or more and 0.6% by mass or less, and also contains at least one selected from the group consisting of Mo, Ta, Nb, W, V, Mn, and Co, and the balance is Ti and inevitable impurities. As Figure 1As shown, the manufacturing method of the titanium alloy plate has the following processes: a hot rolling process S2 for hot rolling the above plate; a cold rolling process S3 for cold rolling the hot rolled plate obtained in the hot rolling process S2. In addition, the manufacturing method of the titanium alloy plate has the following processes: a blooming process S1 for forming the above plate by blooming a raw material having a coarse metal structure during solidification of an ingot; a heat treatment process S4 for heat treating the cold rolled plate obtained in the cold rolling process S3.

[0074] The manufacturing method of the titanium alloy plate satisfies at least any one of the following requirements (a) and (b). In addition, the manufacturing method of the titanium alloy plate preferably satisfies both requirements (a) and (b).

[0075] (a) The hot rolling process S2 includes a process of rolling at a rolling rate of 0.6 or more and 0.9 or less.

[0076] (b) The cold rolling process S3 includes a process of rolling at a rolling rate of 0.5 or more and 0.9 or less.

[0077] By satisfying at least any one of the above requirements (a) and (b), the manufacturing method of the titanium alloy plate can control the average grain diameter of the titanium alloy in the obtained titanium alloy plate. More specifically, according to the manufacturing method of the titanium alloy plate, a titanium alloy plate can be manufactured as follows: a tensile strain of 5.5% or more and 6.5% or less is imparted in a direction perpendicular to the plate thickness, and after holding at 800 °C for 30 minutes, an alternating load in the plate thickness direction is applied at a stress amplitude of 80 MPa, a stress ratio of -1, and a frequency of 25 Hz, and the average grain diameter of the titanium alloy at the position of 1 / 2 of the plate thickness at the moment when the initial load stress is reduced to 50% is 30 μm or less. Therefore, according to the manufacturing method of the titanium alloy plate, a titanium alloy plate that can sufficiently maintain strength in a high temperature environment and has excellent high temperature durability even when strain is applied during processing can be manufactured.

[0078] (Blooming process)

[0079] In the blooming process S1, the raw material having a coarse metal structure during solidification of the ingot is subjected to blooming forging or blooming rolling, or both, to form a plate for the hot rolling process S2. In the blooming process S1, the above raw material is processed at a temperature higher than that of the hot rolling process S2 (for example, a temperature of 900 °C or more and 1100 °C or less).

[0080] (Hot rolling process)

[0081] The hot rolling process S2 includes, for example, a rough rolling process and a finish rolling process. The above-mentioned finish rolling process is carried out as the final rolling step in the hot rolling process S2. In the hot rolling process S2, the rolling ratio of any one of the rough rolling process and the finish rolling process can be controlled in a manner that satisfies the requirement of the above-mentioned (a). However, in the hot rolling process S2, it is preferable to control the rolling ratio of the finish rolling process in a manner that satisfies the requirement of the above-mentioned (a). Regarding the requirement of the above-mentioned (a), as a more preferable rolling ratio, it is the same as the range described for this titanium alloy plate.

[0082] (Cold rolling process)

[0083] The cold rolling process S3 can be carried out after annealing, air cooling, surface grinding, etc. of the hot rolled plate obtained by the hot rolling process S2. Regarding the requirement of the above-mentioned (b), as a more preferable rolling ratio, it is the same as the range described for this titanium alloy plate.

[0084] (Heat treatment process)

[0085] In the heat treatment process S4, for example, final annealing is carried out on the cold rolled plate obtained by the cold rolling process S3. As the heat treatment temperature of the heat treatment process S4, it can be, for example, 600 °C or more and 700 °C or less. In addition, as the heat treatment time of the heat treatment process S4, it can be, for example, 10 hours or more and 40 hours or less.

[0086] <Manufacturing method of titanium alloy parts>

[0087] Refer to Figure 2 A manufacturing method of a titanium alloy part according to an embodiment of the present invention will be described.

[0088] The manufacturing method of this titanium alloy part uses the above-mentioned titanium alloy plate. The manufacturing method of this titanium alloy part includes a processing step S11 of applying a strain of 3% or more and 8% or less.

[0089] The manufacturing method of this titanium alloy part can manufacture a titanium alloy part that can sufficiently maintain strength in a high-temperature environment and has excellent high-temperature durability.

[0090] The manufacturing method of this titanium alloy part uses the above-mentioned titanium alloy plate to manufacture, for example, an exhaust pipe of an internal combustion engine. That is, as the titanium alloy part manufactured by the manufacturing method of this titanium alloy part, an exhaust pipe is preferably used. As the above-mentioned exhaust pipe, for example, exhaust pipes for vehicles such as motorcycles and trucks can be cited, including muffler components such as exhaust manifolds, exhaust pipes, catalytic mufflers, pre-mufflers, and mufflers (main mufflers). The manufacturing method of this titanium alloy part is suitable as a manufacturing method of an exhaust pipe that can sufficiently maintain strength in a high-temperature environment and has excellent high-temperature durability.

[0091] (Processing step)

[0092] In the processing step S11, a strain of 3% or more and 8% or less is applied to the titanium alloy plate to process it into a desired shape. In other words, in the processing step S11, the titanium alloy plate is processed into a desired shape in such a manner that a strain of 3% or more and 8% or less is applied to at least a part of the titanium alloy plate. For example, when bending the titanium alloy plate in the processing step S11, the part where the strain within the above range is applied may be a region of a part in the plate thickness direction (radial direction). As described above, even when a strain of about 5% is applied to the titanium alloy plate, a sharp coarsening of the metal structure at high temperatures can be suppressed. Therefore, the manufacturing method of this titanium alloy part can manufacture a titanium alloy part with a desired shape, excellent high-temperature strength, and excellent oxidation resistance and high-temperature durability by including the processing step S11.

[0093] [Other Embodiments]

[0094] The above embodiments do not limit the configuration of the present invention. Therefore, based on the description in this specification and common technical knowledge, the constituent elements of each part of the above embodiments can be omitted, replaced, or added, and all of these should be interpreted as belonging to the scope of the present invention.

[0095] For example, the titanium alloy plate can also be formed into a titanium alloy part other than an exhaust pipe.

[0096] Examples

[0097] Hereinafter, the present invention will be described in detail based on examples, but the present invention is not limitedly interpreted based on the description of these examples.

[0098] <Test Example 1>

[0099] (Fabrication of Specimens)

[0100] As the titanium alloy plate, a titanium alloy plate having the composition shown in Table 1 was used. Also, in Table 1, the content of each element represents the blending ratio during specimen fabrication. In addition, "-" in Table 1 means not blended.

[0101] In Test Example 1, as test pieces, those formed through the following manufacturing process were used.

[0102] After heating the ingot with the composition shown in Table 1 at 1000 °C for 30 minutes, it was forged to an initial thickness of 20 mm, and 1 mm was cut from the surface to remove the oxide layer. Then, it was heated at 850 °C for 30 minutes and rolled until the thickness reached 4 mm from 18 mm. The obtained hot-rolled sheet was annealed at 750 °C for 5 minutes, air-cooled, surface ground to remove the scale, and then cold-rolled until the thickness reached 0.9 mm from 3.5 mm. Thereafter, as the final process, vacuum annealing was performed at 660 °C for 28 hours (holding time) to obtain a test piece.

[0103]

Table 1

[0104]

[0105] (High-temperature durability)

[0106] For the titanium alloy plates numbered 1 to 5, after applying a tensile strain of more than 5.5% and less than 6.5% by rolling in the direction perpendicular to the plate thickness (rolling direction), a bending fatigue test was conducted in the atmosphere using a Schenk type plane bending fatigue testing machine "PTF-160" manufactured by Tokyo Koki Testing Machine Co., Ltd. This bending fatigue test started after holding the test piece at 800 °C for 30 minutes. The test conditions were an alternating load in the plate thickness direction with a stress amplitude of 80 MPa and a stress ratio of -1, a frequency of 25 Hz, and the moment when the initial load stress decreased to 50% was judged as fracture. Before the start of the bending fatigue test, the average grain diameter of the titanium alloys numbered 1 to 5 was about 10 μm. The number of repetitions at fracture is shown in Table 2. In addition, the cross-sectional microstructure including the fracture part at the time of fracture of No. 1 is shown in Figure 3 and the cross-sectional microstructure including the fracture part at the time of fracture of No. 5 is shown in Figure 4

[0107] (Grain growth)

[0108] For Nos. 1 to 5, in the above bending fatigue test, the average grain diameter of the titanium alloy at the position of half the plate thickness at the moment when the initial load stress decreased to 50% was measured. The average grain diameter of the titanium alloy was measured using the sectioning method in the vicinity of the fracture part (within 500 μm from the fracture part) with the position of half the plate thickness as the center (measurement position) and in a field of view including 20% of the plate thickness. The measurement results are shown in Table 2.

[0109]

Table 2

[0110]

[0111] [Evaluation results]

[0112] ​As shown in Table 2, among No.1 to No.3, by controlling the average grain diameter of the titanium alloy to be below 30 μm, the number of repetitions can be increased and the high-temperature durability is excellent. In contrast, in No.5, since the average grain diameter of the titanium alloy is as large as more than 30 μm, the coarsening of the metal structure causes the number of repetitions to decrease. As Figure 4 shown, in No.5, the cracks generated by the bending fatigue test propagate along the grain boundaries, and the coarsening of the metal structure leads to a reduction in the fatigue life. In addition, it is speculated that in No.4, due to the insufficient Al content in the titanium alloy plate, the high-temperature strength is insufficient and the number of repetitions is small.

[0113] <Test Example 2>

[0114] Based on Table 1, Table 2 and Figure 3 when further investigated, the inventors found that the reason why the metal structure in the titanium alloy plate of No.1 remained fine was the pinning effect of the β phase of titanium. Based on this view, the following tests were conducted on No.1 to No.5, and the quality of the titanium alloy plates was evaluated. In addition, for No.1 to No.5, the value of 3.4[Al] + 13.4[Si] + 13.9[Mo]eq was calculated. The results are shown in Table 3.

[0115] (Fabrication of Specimens)

[0116] Titanium alloy plates with the composition shown in Table 1 and a plate thickness of 0.9 mm were fabricated.

[0117] (Elongation)

[0118] For the titanium alloy plates of No.1 to No.5, small-sized tensile test pieces were cut according to ASTM E8 / E8M with the tensile load axis parallel to the rolling direction. At room temperature, the strain rate was 0.5% / min until the 0.2% yield strength was obtained, and 12.8 mm / min from the 0.2% yield strength to fracture to conduct a tensile test, and the elongation [%] was obtained. The results are shown in Table 3.

[0119] (Oxidation Weight Increase)

[0120] For the titanium alloy plates of No.1 to No.5, test pieces with a length of 50 mm and a width of 20 mm were cut respectively, bent at 90° with a curvature radius of 6 mm and the length direction perpendicular to the bending line, and then air-cooled after being kept at a temperature within 800°C ± 14°C in the atmosphere for 100 hours. The weights of the test pieces were measured before and after heating, and the oxidation weight increase per unit surface area [mg / cm 2 ²] was calculated. The results are shown in Table 3.

[0121] (High-Temperature Strength)

[0122] For titanium alloy plates numbered from No.1 to No.5, test pieces with a gauge length (G.L., the effective distance where stress is applied) of 23 mm were cut. In the atmosphere at 800 °C, a tensile test was conducted with a tensile speed of 0.13 mm / min until the 0.2% yield strength was obtained and 1.25 mm / min from the point of obtaining the 0.2% yield strength until fracture, and the high-temperature strength (tensile strength at 800 °C) [MPa] was determined. The results are shown in Table 3.

[0123]

Table 3

[0124]

[0125] [Evaluation Results]

[0126] As shown in Table 3, Nos. 1 to 3 with an Al content of 0.4 mass% or more and 0.6 mass% or less, an Si content of 0.3 mass% or more and 0.6 mass% or less, and containing at least one selected from the group consisting of Mo, Ta, Nb, W, V, Mn, and Co, and a grain growth of 30 μm or less have preferable values for elongation, oxidation weight gain, and high-temperature strength.

[0127] Industrial Applicability

[0128] As described above, the titanium alloy plate of one embodiment of the present invention is suitable for forming titanium alloy parts with excellent high-temperature strength and excellent high-temperature durability even when strain is applied.

Claims

1. A titanium alloy plate, wherein, As an element, it contains: Al: 0.4% by mass or more and 0.6% by mass or less, Si: 0.3% by mass or more and 0.6% by mass or less, It also contains at least one selected from the group consisting of Mo, Ta, Nb, W, V, Mn, and Co, The balance is Ti and unavoidable impurities, A tensile strain of 5.5% or more and 6.5% or less is imparted in the direction perpendicular to the plate thickness. After holding at 800 °C for 30 minutes, an alternating load in the plate thickness direction is applied with a stress amplitude of 80 MPa, a stress ratio of -1, and a frequency of 25 Hz. The average grain diameter of the titanium alloy at the position of 1 / 2 of the plate thickness at the moment when the initial load stress is reduced to 50% is 30 μm or less.

2. The titanium alloy plate according to claim 1, wherein The Mo equivalent [Mo]eq represented by the following formula 1 is 0.12 or more, [Mo]eq = [Mo] + [Ta] / 5 + [Nb] / 3.6 + [W] / 2.5 + [V] / 1.5 + 1.25[Cr] + 1.25[Ni] + 1.7[Mn] + 1.7[Co] + 2.5[Fe] …1 Wherein, the meaning of [X] in the above formula 1 is the content of element X in the titanium alloy plate in mass%.

3. The titanium alloy plate according to claim 1, wherein It contains Nb: 0.1% by mass or more and 0.5% by mass or less.

4. The titanium alloy plate according to claim 1, wherein, It satisfies the following formula 2, 9 ≤ 3.4[Al] + 13.4[Si] + 13.9[Mo]eq ≤ 14…2.

5. A manufacturing method for a titanium alloy part, wherein, The titanium alloy plate described in any one of claims 1 to 4 is used, and a processing step of applying a strain of 3% or more and 8% or less is provided.

6. The manufacturing method of the titanium alloy part according to claim 5, wherein, The titanium alloy part is an exhaust pipe.

7. A method for manufacturing a titanium alloy plate, wherein, A plate material containing, as elements, Al: 0.4% by mass or more and 0.6% by mass or less, Si: 0.3% by mass or more and 0.6% by mass or less, and also containing at least one selected from the group consisting of Mo, Ta, Nb, W, V, Mn, and Co, with the balance being Ti and unavoidable impurities is used, The method for manufacturing the titanium alloy plate includes: A hot rolling step of hot rolling the plate material; A cold rolling step of cold rolling the hot rolled plate obtained by the hot rolling step, And it satisfies at least one of the following requirements (a) and (b): (a) The hot rolling step includes a step of rolling at a rolling rate of 0.6 or more and 0.9 or less; (b) The cold rolling step includes a step of rolling at a rolling rate of 0.5 or more and 0.9 or less.

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