Titanium alloy plate and method for producing the same

Abstract

To provide a titanium alloy plate having excellent strength, moldability and economical efficiency and a method for producing the same.SOLUTION: A titanium alloy plate has a chemical composition consisting of in mass%, Cu: 0.70% or less, Cr: 0.03-0.30%, Si: 0.03-0.15%, Fe: 0.06% or less, O: 0.15% or less, N: 0.15% or less, C: 0.05% or less, H: 0.013% or less, with the balance being: Ti and impurities, satisfying [Cu+1.2Cr+3.4Si+5O≥0.80]. In the metallographic structure, the average grain size d is 0.020-0.150 mm, and the relationship between a plate thickness t (mm) and the average grain size d (mm) satisfies [t / d≥3.0].SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a titanium alloy plate and a method for manufacturing the same.

Background Art

[0002] Titanium alloys are excellent in strength, corrosion resistance, etc., and are thus utilized in various applications such as the aerospace field and transportation equipment. On the other hand, titanium alloys have low formability and are difficult to process into complex part shapes. For this reason, for example, Patent Documents 1 to 3 disclose titanium alloy plates with improved formability.

Prior Art Documents

Patent Documents

[0003]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0004] Incidentally, strength and formability are mutually contradictory characteristics. That is, when formability is improved, strength decreases. For this reason, there is a problem that it is difficult to improve both strength and formability in a well-balanced manner. Further, when alloying elements are added to improve strength and formability, there is a problem that the manufacturing cost increases.

[0005] The titanium alloy plates disclosed in Patent Documents 1 to 3 described above have a relatively high content of additive elements, resulting in an increase in manufacturing cost. Therefore, there is room for further improvement from the perspective of economic efficiency. Thus, there is a problem that it is difficult to obtain a titanium alloy plate that is excellent in all of strength, formability, and economic efficiency.

[0006] Based on the above, an object of the present invention is to provide a high-strength titanium alloy sheet that solves the above problems and has excellent economic efficiency while maintaining formability. Specifically, as described later, high strength preferably means a 0.2% proof stress of 200 MPa or more, and maintaining formability preferably means a uniform elongation of 25% or more.

Means for Solving the Problems

[0007] The present invention has been made to solve the above problems, and its gist lies in the following titanium alloy sheet and its manufacturing method.

[0008] (1) The chemical composition is in mass%, Cu: 0.70% or less, Cr: 0.03 to 0.30%, Si: 0.03 to 0.15%, Fe: 0.06% or less, O: 0.15% or less, N: 0.15% or less, C: 0.05% or less, H: 0.013% or less, The balance: Ti and impurities, satisfies the following formula (i), in the metallographic structure, the average crystal grain size d is 0.020 to 0.150 mm, a titanium alloy sheet in which the relationship between the plate thickness t and the average crystal grain size d satisfies the following formula (ii). Cu + 1.2Cr + 3.4Si + 5O ≥ 0.80 ···(i) t / d ≥ 3.0 ···(ii) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the titanium alloy, and is zero if not contained. Each symbol in the above formula (ii) is defined as follows. t: plate thickness (mm) d: average crystal grain size (mm)

[0009] (2) The titanium alloy sheet according to (1) above, wherein the plate thickness is 0.3 to 1.5 mm.

[0010] (3) In the metallic structure, the total boundary length of deformation twins present per unit crystal grain is more than 0 mm and 1.0 mm or less, the titanium alloy plate according to (1) or (2) above.

[0011] (4) A method for manufacturing the titanium alloy plate according to (1) or (2) above, After heating a titanium slab or ingot having the chemical composition according to (1) above in a temperature range of 800 to 1100 °C, rolling, and completing the rolling in a temperature range of 700 °C or higher, a hot rolling step; After the hot rolling step, quenching to a temperature range of 600 °C or lower to obtain a hot rolled sheet, a cooling step; Cold rolling the hot rolled sheet to obtain a cold rolled sheet, a cold rolling step; After the cold rolling step, finish annealing the cold rolled sheet in a temperature range of 600 to 700 °C to obtain a cold rolled annealed sheet, an annealing step. A method for manufacturing a titanium alloy plate.

[0012] (5) A method for manufacturing the titanium alloy plate according to any one of (1) to (3) above, After the annealing step, a step of applying pre-strain to the cold rolled annealed sheet at an elongation rate of less than 5.0%, The method for manufacturing a titanium alloy plate according to (4) above.

Advantages of the Invention

[0013] According to the present invention, a titanium alloy plate excellent in strength, formability, and economy can be obtained.

Embodiments for Carrying Out the Invention

[0014] The present inventors have studied a titanium alloy plate excellent in strength, formability, and economy, and obtained the following findings (a) to (c).

[0015] (a) To improve the strength, it is desirable to increase the contents of Cu, Cr, and Si. On the other hand, these elements combine with Ti to form fine intermetallic compounds. Such intermetallic compounds refine the crystal grains. When the crystal grains are fine, the formability decreases. Therefore, it is desirable to form as few intermetallic compounds as possible and have coarse crystal grains. Specifically, the average crystal grain size is preferably in the range of 0.020 to 0.150 mm.

[0016] (b) Therefore, when containing Cu, Cr, and Si, it is necessary to prevent the formation of intermetallic compounds that reduce the formability while increasing the strength. Thus, it is effective to control the contents of these elements within a predetermined range. Thereby, while setting an upper limit for the content of each element, they can be contained in a well-balanced manner. As a result, the strength of the titanium alloy plate can be improved without forming intermetallic compounds, and a titanium alloy plate with excellent economic efficiency can be obtained. Also, by appropriately controlling the cooling conditions after hot rolling during manufacturing, etc., the formation of intermetallic compounds can be suppressed, and the formability can also be ensured.

[0017] (c) In addition, it is desirable to apply prestress after finish annealing performed after cold rolling. By applying prestress, the strength of the formed product can be efficiently improved.

[0018] One embodiment of the present invention has been made based on the above findings. Hereinafter, each requirement of this embodiment will be described in detail.

[0019] 1. Chemical Composition The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%". Also, the content of each element described in the following paragraphs is the average analysis value of the entire alloy plate. For the analysis sample, the surface layer of 0.05 mm (0.1 mm in total thickness) where uneven distribution of O, N, etc. is a concern may be removed, and the sample may be evenly collected from the entire plate thickness for analysis.

[0020] Cu: 0.70% or less Cu has the effect of improving strength. However, if Cu is contained in excess, Cu forms an intermetallic compound with Ti, inhibiting the growth of crystal grains during annealing. In particular, in batch annealing, when the content exceeds 0.70%, due to the formation of the intermetallic compound, the crystal grains become fine. As a result, the formability decreases. Therefore, the Cu content should be 0.70% or less. The Cu content is preferably 0.65% or less, and more preferably 0.60% or less. On the other hand, in order to obtain the above effect, the Cu content is preferably 0.10% or more.

[0021] Cr: 0.03 - 0.30% Cr, like Cu, has the effect of improving strength. Therefore, the Cr content should be 0.03% or more. The Cr content is preferably 0.05% or more, and more preferably 0.10% or more. However, Cr is a β-stabilizing element. Therefore, if Cr is contained in excess, a β-phase is formed, inhibiting the growth of crystal grains during annealing and making the crystal grains fine. As a result, the formability decreases. Therefore, the Cr content should be 0.30% or less. The Cr content is preferably 0.25% or less, and more preferably 0.20% or less.

[0022] Si: 0.03 - 0.15% Si also has the effect of improving strength, like Cu and Cr. Therefore, the Si content should be 0.03% or more. The Si content is preferably 0.05% or more. However, if Si is contained in excess, it inhibits the growth of crystal grains. Also, Si forms an intermetallic compound with Ti, which also inhibits the growth of crystal grains during annealing. Therefore, the Si content should be 0.15% or less. The Si content is preferably 0.12% or less, and more preferably 0.10% or less.

[0023] Fe: 0.06% or less Fe is a β-stabilizing element and an impurity contained in the titanium alloy. If the Fe content is excessive, a β-phase is formed, which inhibits the growth of crystal grains during annealing. As a result, the formability decreases. Therefore, the Fe content should be 0.06% or less. Preferably, the Fe content is 0.04% or less. Regarding the lower limit of the Fe content, there is no particular limitation, but substantially, it is preferably 0.001% or more.

[0024] O: 0.15% or less O is an impurity element contained in the titanium alloy and has the effect of improving strength, similar to N described later. However, if O is contained in excess, the formability significantly decreases. Therefore, the O content should be 0.15% or less. Preferably, the sum of the O content and the N content is 0.15% or less. On the other hand, in order to obtain the above-described effect, preferably, the sum of the O content and the N content is 0.05% or more.

[0025] While Cu, Cr, Si, and O have the effect of improving strength, they easily form intermetallic compounds that reduce formability. Therefore, the relationship between the contents of these elements needs to satisfy equation (i).

[0026] Cu + 1.2Cr + 3.4Si + 5O ≥ 0.80 ··· (i) However, each element symbol in the above equation (i) represents the content (mass%) of each element contained in the titanium alloy, and is zero if not contained.

[0027] If the value on the left side of equation (i) is less than 0.80, sufficient strength cannot be obtained. Therefore, the value on the left side of equation (i) should be 0.80 or more. Preferably, the value on the left side of equation (i) is 0.90 or more, and more preferably 1.0 or more. Regarding the upper limit of the value on the left side of equation (i), there is no particular limitation. Considering the upper limit values of each element, the value on the left side of equation (i) is about 2.32.

[0028] N: 0.15% or less N is an impurity element contained in the titanium alloy and, like O described above, has the effect of improving strength. However, if N is contained in excess, the formability will be significantly reduced. Therefore, the N content should be 0.15% or less. The N content is preferably 0.15% or less in total with the O content. On the other hand, in order to obtain the above-described effect, the O content is preferably 0.05% or more in total with the N content.

[0029] C: 0.05% or less C, like O and N described above, has the effect of improving strength. However, if C is contained in excess, the formability will be reduced. Therefore, the C content should be 0.05% or less. The C content is preferably 0.02% or less. Regarding the lower limit of the C content, there is no particular limitation, but it is preferably substantially 0.001% or more.

[0030] H: 0.013% or less H is an element contained as an impurity in the titanium alloy and causes embrittlement. Therefore, the H content should be 0.013% or less. The H content is more preferably 0.008% or less. Regarding the lower limit of the H content, there is no particular limitation, but it is preferably substantially 0.0001% or more.

[0031] In the chemical composition of this embodiment, the balance is Ti and impurities. Here, "impurities" means components that are mixed in due to raw materials such as ores and scraps and various factors in the manufacturing process when the titanium alloy is industrially manufactured, and are allowed within a range that does not adversely affect this embodiment.

[0032] As elements that may be mixed in as impurities, for example, there are elements such as Al, Sn, Ni, Mn, Zr, Nb, Mo, V, and other elements of the platinum group. These elements may be mixed in when using raw materials such as scraps and low-grade sponge titanium, but even if they are mixed in, the content of each element is preferably 0.05% or less. Also, when these elements are contained, the total content is preferably less than 0.3%.

[0033] 2. Average crystal grain size In the metal structure of the titanium alloy plate of this embodiment, the average crystal grain size d shall be in the range of 0.020 to 0.150 mm. When the average crystal grain size d is less than 0.020 mm, the crystal grains are fine and a titanium alloy plate with sufficient formability cannot be obtained. Therefore, the average crystal grain size d shall be 0.020 mm or more. Preferably, the average crystal grain size d shall be 0.030 mm or more, and more preferably 0.040 mm or more.

[0034] On the other hand, when the average crystal grain size d exceeds 0.150 mm, the surface unevenness increases due to the deformation of the crystal grain unit, and conversely, the formability decreases. Also, the inspection of surface defects becomes difficult. Therefore, the average crystal grain size d shall be 0.150 mm or less. Preferably, the average crystal grain size d shall be 0.120 mm or less, and more preferably 0.100 mm or less.

[0035] Here, the above-mentioned average crystal grain size d can be measured by the following method. Specifically, the central part (position) of the plate thickness of the L cross-section of the alloy plate is used as the measurement surface, etched with an appropriate etching solution, and the average crystal grain size d is calculated by performing measurement using EBSD. For calculating the average crystal grain size d, data analysis software (OIM Analysis) may be used.

[0036] Note that the crystal grain size may be calculated with the crystal grain as the equivalent circle diameter. Also, the measurement conditions of EBSD can be changed according to the crystal grain size of the material for the measurement magnification and the number of fields of view. In principle, it is desirable to perform the measurement in a range where 100 or more crystal grains are included in one field of view. Even if the number of crystal grains included in one field of view is less than 100 due to the apparatus or other observation conditions, the measurement should be performed so that 20 or more crystal grains are included, and the measurement of multiple fields of view should be used so that 100 or more crystal grains are included. Only the crystal grains completely included in the measurement field of view are used for the measurement. That is, when the grain boundary divides the boundary of the field of view, that crystal grain is excluded from the measurement.

[0037] The step size at that time is set to be 1 / 10 or less of the average crystal grain size. It is more preferable that the step size is about 1 / 30 to 1 / 20 of the average crystal grain size. For example, when the average crystal grain size is about 30 μm, the magnification is 300 times, the field of view area is 350 μm × 400 μm, one field of view, and the step size may be set to 1 μm.

[0038] 3. Plate thickness In the titanium alloy plate of the present embodiment, the plate thickness t is preferably in the range of 0.3 to 1.5 mm. This is because when the plate thickness t is 0.3 to 1.5 mm, both strength and formability are likely to be required.

[0039] 4. Relationship between plate thickness and average crystal grain size In the titanium alloy plate of the present embodiment, the relationship between the plate thickness t and the average crystal grain size d needs to satisfy the following formula (ii). t / d ≧ 3.0 ···(ii) However, each symbol in the above formula (ii) is defined as follows. t: Plate thickness (mm) d: Average crystal grain size (mm)

[0040] Generally, when the average crystal grain size d increases, the formability improves. However, when the left side value of formula (ii), t / d, is less than 3.0, the number of crystal grains in the plate thickness direction becomes excessively small, and the uniform elongation decreases instead. For this reason, the formability deteriorates. Also, it becomes difficult to stably obtain good formability. Therefore, t / d should be 3.0 or more. It is preferably 3.5 or more, and more preferably 4.0 or more. The upper limit of t / d is not particularly limited, but usually becomes about 10.0.

[0041] 5. Total length of deformation twins The titanium alloy plate is used after being formed. At this time, the strength varies depending on the degree of processing applied. This is because the degree of work hardening is different. And the strength improvement in the initial stage of work hardening has a small decrease in ductility and is effective for the strength improvement after forming. Therefore, it is desirable to apply pre-strain, that is, uniform processing to such an extent that the ductility is not significantly reduced, as described later. By applying pre-strain, deformation twins are formed within the crystal grains, and the strength is improved. And in the metal structure of the titanium alloy plate of this embodiment, the total boundary length of the deformation twins existing per unit crystal grain (hereinafter simply referred to as "total twin boundary length") is preferably more than 0 mm and 1.0 mm or less.

[0042] Here, the total twin boundary length existing per unit crystal grain is the length around the deformation twin measured by the method described later, and is an index representing the degree of the introduced pre-strain.

[0043] If the total twin boundary length per unit crystal grain is more than 0 mm, initial work hardening has occurred. Therefore, it is preferable that the total twin boundary length existing per unit crystal grain is more than 0 mm, and more preferably 0.1 mm or more. On the other hand, if the total twin boundary length per unit crystal grain exceeds 1.0 mm, the work hardening progresses too much, and conversely, the formability decreases. Therefore, the total twin boundary length per unit crystal grain is preferably 1.0 mm or less, more preferably 0.9 mm or less, and even more preferably 0.8 mm or less.

[0044] Incidentally, the total twin boundary length per unit grain can be measured by the following procedure. Specifically, similar to the average crystal grain size described above, it may be measured by EBSD. However, the measurement is performed with a step size of 0.2 to 0.5 μm. Other conditions are the same as those for the measurement of the average crystal grain size. After measurement, using data analysis software (OIM Analysis), only the crystal grains completely included in the measurement field of view are separated. For deformation twins, three types of twins, {11-22} twin, {10-12} twin, and {11-21} twin, may be considered. The analysis is performed with an allowable range of 10° for the K1 plane and axis of these twins to identify the deformation twins. For the twins identified from these three orientations, the sum L of the lengths of the entire perimeter of their boundaries is measured and divided by the number of crystal grains included in the field of view to obtain the total twin boundary length per unit crystal grain. Incidentally, even when measuring in multiple fields of view, the sum L of all the twin boundary lengths measured in the same way may be divided by the total number of crystal grains included in the fields of view.

[0045] 6. Target Strength and Formability In the titanium alloy sheet of the present embodiment, the strength is in a range that is difficult to obtain with a titanium alloy excellent in economy that does not contain alloying elements as described above, that is, the 0.2% proof stress is preferably 200 MPa or more, and this range is evaluated as high strength. The strength is more preferably 220 MPa or more and even more preferably 230 MPa or more in terms of the 0.2% proof stress. Regarding formability, the uniform elongation is preferably 25% or more, and this range is evaluated as having good formability. The uniform elongation is preferably 26% or more and more preferably 27% or more.

[0046] 7. Manufacturing Method A preferred manufacturing method for the titanium alloy sheet according to the present embodiment will be described. The titanium alloy sheet according to the present embodiment can be stably manufactured, for example, by the following manufacturing method.

[0047] 7-1. Blocking Step It is preferable to manufacture the ingot having the above-described chemical composition by methods such as electron beam melting, vacuum arc melting (also referred to as "VAR"), and electron beam melting (also referred to as "EB melting"). The shape of the ingot is not particularly limited. It may be rectangular or cylindrical. Further, a step of cutting the surface of the ingot may be performed as necessary, or a step of dividing the ingot by hot forging or the like (hereinafter simply referred to as the "dividing step") may be performed.

[0048] In the above-described dividing step, it is preferable to heat in the β single-phase region (1300 °C or higher and 1300 °C or lower above the β transformation point) and perform processing with a cross-sectional reduction rate of 20% or more. This dividing step is performed for the purpose of eliminating solidification defects and solidification structures and improving hot workability. Therefore, if there is no problem in production by hot rolling or the like, the dividing step may not be performed.

[0049] When the dividing step is not performed on a rectangular ingot, the ingot skin causes surface defects during hot rolling. Therefore, it is desirable to cut the surface to eliminate the ingot skin. Further, when the dividing step is performed, depending on the ingot skin, it may not be necessary to cut the surface. In this case, after completion of the dividing step, the surface may be cut together with scale removal. That is, the surface layer scale and the hardened layer may be removed at the end of the dividing step to obtain a slab for hot rolling. In the case of a cylindrical ingot, hot rolling is difficult in its original shape. Therefore, similar to the rectangular ingot, the dividing step may be performed to obtain a slab for hot rolling.

[0050] 7-2. Hot Rolling Step Subsequently, it is preferable to heat the above ingot or the slab for hot rolling in a temperature range of 800 to 1100 °C and perform hot rolling. If the heating temperature during hot rolling is less than 800 °C, it becomes difficult to cool in a preferable temperature range after hot rolling. Therefore, the heating temperature during hot rolling is preferably 800 °C or higher. On the other hand, if the heating temperature during hot rolling exceeds 1100 °C, oxidation proceeds excessively, the yield decreases, and surface defects occur due to the formation of a hardened layer or the like. Therefore, the heating temperature during hot rolling is preferably 1100 °C or lower. In hot rolling, it is sufficient to ensure a reduction rate of 70% or more in one heating.

[0051] Hot rolling is preferably completed in a temperature range of 700 °C or higher. That is, the completion temperature of hot rolling is preferably 700 °C or higher. In the production of the titanium alloy plate of the present embodiment, as will be described later, it is necessary to rapidly cool to a predetermined temperature range in order to suppress the formation of intermetallic compounds after hot rolling.

[0052] In the case of a thin plate with a relatively thin thickness, it is generally manufactured in a coil shape. In order to make it into a coil shape, it needs to be wound up. In this case, since it is difficult to dissipate heat from the plate surface, the cooling becomes slow. Therefore, it is necessary to cool sufficiently before winding up into a coil shape.

[0053] Here, in the chemical composition of the titanium alloy plate of the present embodiment, intermetallic compounds containing Ti and Si are likely to precipitate at a temperature near 700 °C. Also, intermetallic compounds containing Ti and Cu are likely to precipitate at a temperature near 600 °C. That is, in the range of 600 to 700 °C, intermetallic compounds are likely to precipitate. As a result of the formation of such intermetallic compounds, the crystal grains after hot rolling become fine and the formability decreases.

[0054] In addition, this intermetallic compound is likely to precipitate not only in the above temperature range but also as long as the processing strain introduced by hot rolling remains. Therefore, after hot rolling, it is necessary to cool sufficiently to a temperature range where the above intermetallic compound is difficult to form and then wind it up into a coil.

[0055] As will be described later, when batch annealing is performed, finish annealing after cold rolling may be performed in a temperature range of 600 to 700 °C. Also in this case, if intermetallic compounds are precipitated, the crystal grains become fine and the formability decreases. Further, the formation of intermetallic compounds reduces the solid solution amount of the added elements in the matrix phase, making it difficult to improve the strength by solid solution strengthening.

[0056] Therefore, it is preferable to rapidly cool the material to a temperature range of 600°C or lower before coiling after the hot rolling process to obtain a hot rolled sheet. More preferably, it is rapidly cooled to a temperature range of 550°C or lower before coiling. The above rapid cooling means cooling at a cooling rate of 10°C / s or more, and as a cooling method, for example, water cooling can be considered. Also, the above-mentioned temperature refers to the surface temperature of the sheet, which is the temperature measured by a radiation thermometer, and the same applies to the paragraphs described later.

[0057] After hot rolling, descaling may be performed as necessary. Also, annealing after hot rolling, so-called hot rolled sheet annealing, is not preferably performed if there is no problem during cold rolling because intermetallic compounds are likely to form.

[0058] In the case of performing hot rolled sheet annealing, it is preferably performed in a temperature range of 700 - 800°C. When performing hot rolled sheet annealing in a temperature range of less than 700°C, intermetallic compounds are likely to form. On the other hand, when performing hot rolled sheet annealing in a temperature range of 800°C or higher, a β phase is formed, and Cu and Si are concentrated in the β phase, making it easier for intermetallic compounds to precipitate. Therefore, when performing hot rolled sheet annealing, it is preferably performed in a temperature range of 700 - 800°C. Also, the annealing time is preferably 30 s or more and 5 min or less because the shorter the time, the higher the productivity.

[0059] When performing hot rolled sheet annealing, it is preferably annealed by continuous annealing. This is because in batch annealing, it is not possible to sufficiently anneal while avoiding the precipitation of intermetallic compounds. The annealing atmosphere is not particularly limited, but it may be performed in an air atmosphere.

[0060] Also, regarding the cooling rate after hot rolled sheet annealing, in this case, the sheet thickness is usually 6 mm or less, and in this case, the cooling rate may be 1°C / s or more corresponding to air cooling because it can suppress the precipitation of intermetallic compounds.

[0061] When the plate thickness exceeds 6 mm, cooling becomes difficult to progress. For this reason, the cooling rate becomes slow, and depending on the plate thickness, a cooling rate of 1 °C / s or more may not be obtained. In that case, it is better to perform water cooling that can cool faster. By performing water cooling, although it depends on the plate thickness, a cooling rate of 10 °C / s or more can be achieved, and a cooling rate of 1 °C / s or more can be sufficiently obtained. After hot-rolled plate annealing and cooling, descaling may be performed. The method of descaling is not particularly limited, and for example, it may be performed by mechanical methods such as shot blasting, pickling, polishing, or chemical methods.

[0062] 7-3. Cold rolling process Subsequently, it is preferable to perform cold rolling on the hot-rolled plate to obtain a cold-rolled plate. The cold rolling may be performed in multiple steps. Also, at this time, intermediate annealing may be performed as necessary between cold rolling and cold rolling. When performing intermediate annealing, for example, it is preferably performed in a temperature range of 750 to 800 °C for 30 seconds to 2 minutes. The intermediate annealing may be a continuous annealing similar to the hot-rolled plate annealing. Also, the annealing atmosphere in the intermediate annealing is not particularly limited. Annealing may be performed in an air atmosphere. Also, when intermediate annealing is performed, it is preferable to cool at a cooling rate corresponding to the above-described air cooling or water cooling. Similar to the hot-rolled plate annealing, when the plate thickness is 6 mm or less and the plate thickness is thin, cooling may be performed by air cooling or the like with a slow cooling rate, but when the plate thickness exceeds 6 mm and the plate thickness is thick, it is better to cool by water cooling.

[0063] In cold rolling, the cold rolling rate in the final cold rolling performed immediately before finish annealing described later is preferably 50% or more. If the above-described cold rolling rate is less than 50%, uniform equiaxed grains cannot be obtained, and a mixed grain structure in which coarse grains and fine grains are mixed is likely to occur.

[0064] When a mixed grain structure occurs, coarse grains exist locally. In such a part, the number of crystal grains in the plate thickness direction decreases. As a result, the uniform elongation decreases, and the formability is likely to decrease. For this reason, the cold rolling rate in the final cold rolling may be adjusted according to the desired plate thickness, but the cold rolling rate is preferably 50% or more.

[0065] 7-4. Finish annealing After the cold rolling process, it is preferable to perform finish annealing on the cold rolled sheet. Finish annealing is the final annealing performed after all the cold rolling processes are completed, and is also referred to as final annealing. In finish annealing, it is preferable to anneal at an annealing temperature in the temperature range of 600 to 700 °C to obtain a cold rolled annealed sheet. If the annealing temperature of the finish annealing is less than 600 °C, the crystal grains become fine. As a result, the formability decreases. Therefore, the annealing temperature in the finish annealing is preferably 600 °C or higher. On the other hand, if the annealing temperature in the finish annealing exceeds 700 °C, surface quality deterioration due to sticking inside the coil is likely to occur. Therefore, the annealing temperature in the finish annealing is preferably 700 °C or lower. The annealing temperature in the finish annealing is more preferably in the range of 630 to 680 °C.

[0066] Also, the annealing time in the finish annealing is preferably 1 h or more. If the annealing time of the finish annealing is less than 1 h, the crystal grains do not grow sufficiently and the formability decreases. Therefore, the above annealing time is preferably 1 h or more, and more preferably 4 h or more. The upper limit of the annealing time of the finish annealing is not particularly limited, but is usually about 20 h from the viewpoint of productivity.

[0067] In finish annealing, preheating may be performed before annealing. This is because it is easy to reduce the temperature variation inside the coil by performing preheating. Also when performing preheating, in order to suppress the precipitation of intermetallic compounds, the heating temperature is preferably 550 °C or lower. Also, the finish annealing may be batch annealing. This is because annealing at a low temperature for a long time can be performed. Also, the annealing atmosphere of the finish annealing is preferably vacuum or Ar gas in order to suppress oxidation.

[0068] 7-5. Pre-strain After finish annealing, that is, after annealing in the temperature range of 600 to 700 °C, pre-strain may be applied as necessary to improve the strength of the cold-rolled annealed sheet. Pre-strain means performing processing with a small amount of processing in advance before processing the product. In the titanium alloy sheet of this embodiment, it is preferable to apply a pre-strain with an elongation rate of less than 5.0%. When the elongation rate is 5.0% or more, it is considered that, among other factors, the total length of deformation twin boundaries exceeds 1.0 mm, resulting in a decrease in formability. The elongation rate is preferably 4.0% or less, and more preferably 3.0% or less. As a method of applying pre-strain, it is preferably performed by a tension leveler or skin pass rolling used for shape correction. The elongation rate is calculated using the following formula (a).

[0069] Elongation rate (%) = (l - l0) / l0 × 100 ···(a) However, each symbol in the above formula is defined as follows. l0: Gauge length of the test piece before pre-strain processing l: Gauge length of the test piece after pre-strain processing

[0070] Hereinafter, the titanium alloy sheet according to this embodiment will be described more specifically by way of examples, but this embodiment is not limited to these examples.

Examples

[0071] A block of a titanium alloy with a thickness of 60 mm having the chemical composition shown in Table 1 was hot-rolled under the conditions shown in Table 2 and water-cooled or air-cooled to 600 °C or lower to obtain a hot-rolled sheet with a thickness of 4 mm. In the chemical composition of Table 1, the O content is the analysis value by the inert gas fusion infrared absorption method. The N and H contents are the analysis values by the inert gas fusion thermal conductivity method. The C content is the analysis value by the high-frequency combustion infrared absorption method. The contents of elements other than these are the analysis values by inductively coupled plasma (ICP) emission spectrometry. In Table 2, "WQ" represents "water cooling", "AC" represents "air cooling", and "FC" represents "furnace cooling".

[0072] Thereafter, for examples other than No. 17, 18, 21, and 22, descaling was performed by shot blasting and pickling with hydrofluoric acid without performing hot-rolled sheet annealing. On the other hand, for examples No. 17, 18, 21, and 22, hot-rolled sheet annealing was performed, and then the above-described descaling was carried out. Cold rolling was performed one or two times, and for some of No. 19 to 22, intermediate annealing was carried out in the atmosphere during cold rolling. Note that the cold rolling rate (cold reduction rate) in the item of intermediate cold rolling / annealing among No. 19 to 22 in the table is the cold rolling rate of cold rolling before intermediate annealing.

[0073] The conditions of the intermediate annealing performed are as shown in Table 2. For No. 19 to 22, after intermediate annealing, descaling was carried out by shot blasting and pickling with hydrofluoric acid. Thereafter, final cold rolling and finish annealing were performed and cooled under the conditions shown in Table 2 for all examples of titanium alloys. The finish annealing was performed in a vacuum and furnace-cooled. After cooling, for No. 2, 24 to 26, 32 to 36, pre-strain was imparted by temper rolling at the elongation rate described in Table 2.

[0074]

Table 1

[0075]

Table 2

[0076] Regarding the titanium alloy sheets thus obtained, the average crystal grain size and the total twin boundary length were measured by the following procedure.

[0077] (Average crystal grain size) The average crystal grain size was calculated by measuring the central part (position) of the plate thickness of the L-section of the alloy plate as the measurement surface, etching with an appropriate etching solution, and performing measurements using EBSD. To calculate the average crystal grain size d, data analysis software (OIM Analysis) was used. Note that the crystal grain size was calculated with the crystal grains as the equivalent circle diameter. In addition, the EBSD measurement conditions were set within a range that included a total of 130 to 150 crystal grains in two fields of view. For other conditions, as described above, they were appropriately adjusted according to the size of the average crystal grain size.

[0078] (Total twin boundary length) The total twin boundary length was measured by EBSD in the same manner as the above-described average crystal grain size. However, the measurement was performed with a step size of 0.2 to 0.5 μm. Other conditions were the same as those for the measurement of the average crystal grain size. After measurement, using data analysis software (OIM Analysis), only the crystal grains completely included in the measurement field of view were separated. For deformation twins, three types, namely {11-22} twins, {10-12} twins, and {11-21} twins, were considered. The analysis was performed with the allowable range of the K1 plane and axis of these twins set at 10° to identify the deformation twins. By dividing the sum L of the boundary lengths of these three twins by the number of crystal grains included in the measured field of view, the total twin boundary length per unit crystal grain was obtained.

[0079] In addition, a tensile test was performed on the obtained titanium alloy plate, and the 0.2% proof stress and uniform elongation were measured. When the 0.2% proof stress was 200 MPa or more, it was evaluated that the strength was good. Also, when the uniform elongation was 25% or more, it was evaluated that the formability was good.

[0080] For the tensile test specimens, JIS 13B sub-size test specimens were used. The specimens had a parallel part length of 32 mm and a width of 6.25 mm, and those with a gauge length of 25 mm were used. Also, the tensile speed was controlled by stroke, with a strain of 2% or less at 0.13 mm / min and 7.5 mm / min from then until fracture. Below, the results are summarized and shown in Table 3.

[0081]

Table 3

[0082] Titanium alloy plates of Test Nos. 4, 5, 11 to 15, 17 to 22, 24 to 29, and 31 to 36 that satisfy the requirements of this embodiment had high strength and good formability. Here, for Test Nos. 24, 26, 32 to 36, since pre-strains within a preferable range were applied, not only was the strength improved, but good formability was also maintained. On the other hand, for Test No. 25, since the amount of pre-strain applied was large, although the strength was improved, the uniform elongation was smaller compared to other examples of the present invention, resulting in somewhat inferior formability.

[0083] On the other hand, titanium alloy plates of Test Nos. 1 to 3, 6 to 10, 16, 23, and 30 that do not satisfy the requirements of this embodiment resulted in inferiority in at least one of strength or formability. For Test Nos. 1 to 3 where the chemical composition deviated from the requirements of this embodiment, mainly the strength was inferior. On the other hand, for Test Nos. 6 to 10 that deviated from the same requirements, the formability was inferior. Note that Test No. 9 was particularly high in strength and was considered to have inferior formability, and ear cracks occurred in a part of the width end during cold rolling. Also, for the examples of Test Nos. 16, 23, and 30, since they were not manufactured under preferable manufacturing conditions, the average crystal grain size and the like were outside the range of this embodiment, resulting in inferior strength or formability.

Claims

1. The chemical composition is, by mass%, Cu: 0.70% or less, Cr: 0.03 - 0.30%, Si: 0.03 - 0.15%, Fe: 0.06% or less, O: 0.15% or less, N: 0.15% or less, C: 0.05% or less, H: 0.013% or less, The balance: Ti and impurities, satisfies the following formula (i), in the metallographic structure, the average crystal grain size d is 0.020 - 0.150 mm, a titanium alloy plate in which the relationship between the plate thickness t and the average crystal grain size d satisfies the following formula (ii). Cu + 1.2Cr + 3.4Si + 5O ≥ 0.80... (i) t / d ≥ 3.0... (ii) However, each element symbol in the above formula (i) represents the content (mass%) of each element contained in the titanium alloy, and is taken as zero if not contained. Each symbol in the above formula (ii) is defined as follows. t: plate thickness (mm) d: average crystal grain size (mm)

2. The titanium alloy plate according to Claim 1, wherein the plate thickness is 0.3 - 1.5 mm.

3. The titanium alloy plate according to Claim 1 or 2, wherein in the metallographic structure, the total boundary length of deformation twins present per unit crystal grain is more than 0 mm and 1.0 mm or less.

4. A method for manufacturing the titanium alloy plate according to Claim 1 or 2, comprising: a hot rolling step of heating a titanium slab or titanium ingot having the chemical composition according to Claim 1 to a temperature range of 800 - 1100°C, then rolling, and completing the rolling in a temperature range of 700°C or higher; a cooling step of rapidly cooling to a temperature range of 600°C or lower after the hot rolling step to obtain a hot rolled sheet; a cold rolling step of cold rolling the hot rolled sheet to obtain a cold rolled sheet; an annealing step of finish annealing the cold rolled sheet in a temperature range of 600 - 700°C after the cold rolling step to obtain a cold rolled annealed sheet. A method for manufacturing a titanium alloy plate.

5. A method for manufacturing the titanium alloy plate according to Claim 3, comprising: a hot rolling step of heating a titanium slab or titanium ingot having the chemical composition according to Claim 1 to a temperature range of 800 - 1100°C, then rolling, and completing the rolling in a temperature range of 700°C or higher; a cooling step of rapidly cooling to a temperature range of 600°C or lower after the hot rolling step to obtain a hot rolled sheet; a cold rolling step of cold rolling the hot rolled sheet to obtain a cold rolled sheet; an annealing step of finish annealing the cold rolled sheet in a temperature range of 600 - 700°C after the cold rolling step to obtain a cold rolled annealed sheet; a step of applying pre-strain to the cold rolled annealed sheet at an elongation rate of less than 5.0% after the annealing step. Method for manufacturing a titanium alloy plate.

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