JP2023040457A - Titanium alloy plate and method for producing the same - Google Patents

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 The present invention relates to a titanium alloy plate and a method for producing the same.

Titanium alloys are excellent in strength, corrosion resistance, etc., and are used in various applications such as the field of aviation and transportation. On the other hand, titanium alloys have low moldability and are difficult to process into complex shapes. For this reason, for example, Patent Documents 1 to 3 disclose titanium alloy plates with improved formability.

Japanese Patent Application Laid-Open No. 2005-298970 WO2019/043882 JP 2010-31314 A

By the way, strength and formability are properties that conflict with each other. That is, when formability improves, strength decreases. Therefore, there is a problem that it is difficult to improve both strength and formability in a well-balanced manner. Also, in order to improve strength and formability, alloying elements are sometimes added, but in this case, there is a problem that the manufacturing cost increases.

The titanium alloy plates disclosed in Patent Documents 1 to 3 mentioned above have a relatively high content of additive elements, which increases the manufacturing cost. Therefore, there is room for further improvement from the viewpoint of economy. Thus, there is a problem that it is difficult to obtain a titanium alloy sheet excellent in all of strength, formability and economy.

Based on the above, an object of the present invention is to solve the above problems and to provide a high-strength titanium alloy sheet which maintains formability and is excellent in economic efficiency, and a method for producing the same. Specifically, as will be described later, high strength is preferably 200 MPa or more at 0.2% yield strength, and maintaining formability is preferably uniform elongation of 25% or more.

The present invention has been made to solve the above problems, and the gist thereof is the following titanium alloy plate and method for producing the same.

(1) chemical composition, 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,
balance: Ti and impurities,
satisfying the following formula (i),
In the metal structure, the average crystal grain size d is 0.020 to 0.150 mm,
A titanium alloy plate, wherein 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 (% by mass) of each element contained in the titanium alloy, and if it is not contained, it is zero, and each symbol in the above formula (ii) is as follows. is defined as
t: plate thickness (mm)
d: average grain size (mm)

(2) The titanium alloy plate according to (1) above, which has a plate thickness of 0.3 to 1.5 mm.

(3) The titanium alloy plate according to (1) or (2) above, wherein the sum of boundary lengths of deformation twins existing per unit crystal grain in the metal structure is more than 0 mm and 1.0 mm or less.

(4) A method for producing a titanium alloy plate according to (1) or (2) above,
A hot rolling step in which a titanium slab or titanium ingot having the chemical composition described in (1) above is heated to a temperature range of 800 to 1100° C. and then rolled, and the rolling is completed in a temperature range of 700° C. or higher;
After the hot rolling step, a cooling step of quenching to a temperature range of 600 ° C. or less to form a hot rolled sheet;
A cold-rolling step of cold-rolling the hot-rolled sheet to obtain a cold-rolled sheet;
After the cold rolling step, the cold rolled sheet is finish annealed in a temperature range of 600 to 700 ° C. to obtain a cold rolled annealed sheet.
A method for manufacturing a titanium alloy plate.

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

According to the present invention, a titanium alloy sheet excellent in strength, formability and economy can be obtained.

The present inventors have studied titanium alloy sheets that are excellent in strength, formability and economy, and have obtained the following findings (a) to (c).

(a) In order 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 an intermetallic compound makes crystal grains finer. If the crystal grains are fine, the moldability is lowered. For this reason, it is desirable that the crystal grains are coarse grains while minimizing the formation of intermetallic compounds. Specifically, the average crystal grain size is preferably in the range of 0.020 to 0.150 mm.

(b) Therefore, it is necessary to contain Cu, Cr, and Si to prevent the formation of intermetallic compounds that reduce formability while increasing strength. Therefore, it is effective to control the content of these elements within a predetermined range. Thereby, it is possible to contain each element in a well-balanced manner while setting the upper limit of the content of each element. As a result, it is possible to improve the strength of the titanium alloy plate without forming an intermetallic compound, so that it is possible to obtain a titanium alloy plate excellent in economic efficiency. In addition, the formation of intermetallic compounds can be suppressed and formability can be ensured by appropriately controlling the cooling conditions after hot rolling during production.

(c) In addition, it is desirable to apply prestrain after the final annealing which is performed after cold rolling. By applying pre-strain, the strength of the molded product can be efficiently improved.

An embodiment of the present invention has been made based on the above findings. Each requirement of this embodiment will be described in detail below.

1. Chemical composition The reasons for limiting each element are as follows. In addition, in the following description, "%" about content means "mass %." Also, the content of each element described in the following paragraphs is the average analytical value of the entire alloy plate. A sample for analysis may be obtained by removing a surface layer of 0.05 mm (0.1 mm in the total thickness) where O, N and the like are likely to be unevenly distributed, and sampling the sample evenly from the entire plate thickness for analysis.

Cu: 0.70% or less Cu has the effect of improving the strength. However, when Cu is contained excessively, Cu forms an intermetallic compound with Ti, which inhibits the growth of crystal grains during annealing. In particular, in batch annealing, if the content exceeds 0.70%, the crystal grains become fine due to the formation of intermetallic compounds. This results in poor moldability. Therefore, the Cu content is set to 0.70% or less. The Cu content is preferably 0.65% or less, more preferably 0.60% or less. On the other hand, in order to obtain the above effects, the Cu content is preferably 0.10% or more.

Cr: 0.03-0.30%
Cr, like Cu, has the effect of improving strength. Therefore, the Cr content is set to 0.03% or more. The Cr content is preferably 0.05% or more, more preferably 0.10% or more. However, Cr is a β-stabilizing element. For this reason, when Cr is contained excessively, a β phase is formed, inhibiting the growth of crystal grains during annealing and making the crystal grains finer. This results in poor moldability. Therefore, the Cr content is set to 0.30% or less. The Cr content is preferably 0.25% or less, more preferably 0.20% or less.

Si: 0.03-0.15%
Si, like Cu and Cr, also has the effect of improving strength. Therefore, the Si content should be 0.03% or more. The Si content is preferably 0.05% or more. However, excessive Si content inhibits the growth of crystal grains. Si also inhibits the growth of crystal grains during annealing by forming an intermetallic compound with Ti. Therefore, the Si content should be 0.15% or less. The Si content is preferably 0.12% or less, more preferably 0.10% or less.

Fe: 0.06% or less Fe is a β-stabilizing element and an impurity contained in the titanium alloy. Excessive Fe content results in the formation of β-phase, which inhibits grain growth during annealing. This results in poor moldability. Therefore, the Fe content is set to 0.06% or less. The Fe content is preferably 0.04% or less. Although the lower limit of the Fe content is not particularly limited, it is substantially preferably 0.001% or more.

O: 0.15% or less O is an impurity element contained in the titanium alloy, and has the effect of improving the strength, like N, which will be described later. However, when O is contained excessively, the moldability is remarkably lowered. Therefore, the O content is set to 0.15% or less. The sum of the O content and the N content is preferably 0.15% or less. On the other hand, in order to obtain the effect described above, the sum of the O content and the N content is preferably 0.05% or more.

Cu, Cr, Si and O have the effect of improving strength, but tend to form intermetallic compounds that reduce formability. Therefore, the relationship between the contents of these elements must satisfy the formula (i).

Cu+1.2Cr+3.4Si+5O≧0.80 (i)
However, each element symbol in the above formula (i) represents the content (% by mass) of each element contained in the titanium alloy, and is zero when not contained.

(i) If the value of the left side of the formula is less than 0.80, sufficient strength cannot be obtained. Therefore, the left-side value of formula (i) is set to 0.80 or more. (i) The left-side value of the formula is preferably 0.90 or more, more preferably 1.0 or more. Note that the upper limit of the left-side value of expression (i) is not particularly limited. Considering the upper limit of each element, the left side value of the formula (i) is about 2.32.

N: 0.15% or less N is an impurity element contained in the titanium alloy, and has the effect of improving the strength, like O described above. However, when N is contained excessively, the formability is remarkably lowered. Therefore, the N content is set to 0.15% or less. The total N content and O content is preferably 0.15% or less. On the other hand, in order to obtain the effect described above, the sum of the O content and the N content is preferably 0.05% or more.

C: 0.05% or less C, like O and N described above, has the effect of improving the strength. However, when C is contained excessively, moldability is lowered. Therefore, the C content should be 0.05% or less. The C content is preferably 0.02% or less. Although the lower limit of the C content is not particularly limited, it is substantially preferably 0.001% or more.

H: 0.013% or less H is an element that is contained as an impurity in titanium alloys and causes embrittlement. Therefore, the H content should be 0.013% or less. The H content is more preferably 0.008% or less. Although the lower limit of the H content is not particularly limited, it is substantially preferably 0.0001% or more.

In the chemical composition of this embodiment, the balance is Ti and impurities. Here, the term "impurities" refers to components mixed in by various factors in raw materials such as ores, scraps, and manufacturing processes during the industrial production of titanium alloys, as long as they do not adversely affect the present embodiment. means acceptable.

Elements that may be mixed as impurities include, for example, Al, Sn, Ni, Mn, Zr, Nb, Mo, V, and other platinum group elements. These elements may be mixed in when raw materials such as scrap or low-grade sponge titanium are used, but even if they are mixed, the content of each element should be 0.05% or less. preferable. Moreover, when these elements are contained, the total content is preferably less than 0.3%.

2. Average Grain Size In the metal structure of the titanium alloy plate of the present embodiment, the average grain size d is in the range of 0.020 to 0.150 mm. If the average grain size d is less than 0.020 mm, a titanium alloy sheet with fine crystal grains and sufficient formability cannot be obtained. Therefore, the average crystal grain size d is set to 0.020 mm or more. The average crystal grain size d is preferably 0.030 mm or more, more preferably 0.040 mm or more.

On the other hand, if the average crystal grain size d exceeds 0.150 mm, the deformation of the crystal grain unit increases the surface unevenness, and conversely, the formability decreases. Also, inspection of surface defects becomes difficult. Therefore, the average crystal grain size d is set to 0.150 mm or less. The average crystal grain size d is preferably 0.120 mm or less, more preferably 0.100 mm or less.

Here, the average crystal grain size d can be measured by the following method. Specifically, the thickness central portion (position) of the L cross section of the alloy plate is used as a measurement surface, etching is performed with an appropriate corrosive liquid, and the average grain size d is calculated by performing measurement using EBSD. Data analysis software (OIM Analysis) may be used to calculate the average grain size d.

Note that the crystal grain size may be calculated by taking the crystal grain as a circle-equivalent diameter. As for the EBSD measurement conditions, the measurement magnification and the number of fields of view can be changed depending on the crystal grain size of the material, and in principle, it is desirable to carry out the measurement in a range in which one field of view contains 100 or more crystal grains. Even if 100 or more crystal grains are not included in one field of view due to the equipment and other observation conditions, measure so that 20 or more crystal grains are included, and measure 100 or more crystals by measuring multiple fields of view. Grains should be included. Only crystal grains that are completely included in the field of view for measurement are used. That is, if a grain boundary divides the boundary of the field of view, that grain is excluded from the measurement.

The step size at that time is 1/10 or less of the average crystal grain size. More preferably, 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 should be 300×, the field area should be 350 μm×400 μm, one field of view, and the step size should be 1 μm.

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 moldability are likely to be required.

4. Relationship Between Plate Thickness and Average Grain Size In the titanium alloy plate of the present embodiment, the relationship between the plate thickness t and the average grain size d must 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 grain size (mm)

Usually, when the average crystal grain size d increases, the formability improves, but when t/d, which is the left side value of the formula (ii), is less than 3.0, the number of crystal grains in the plate thickness direction is excessively small. , and uniform elongation decreases. For this reason, moldability deteriorates. Moreover, it becomes difficult to stably obtain good moldability. Therefore, t/d is set to 3.0 or more. t/d is preferably 3.5 or more, more preferably 4.0 or more. Although the upper limit of t/d is not particularly limited, it is usually about 10.0.

5. Sum of Lengths of Deformation Twins A titanium alloy plate is used after forming. At this time, the strength varies depending on the degree of processing applied. This is because the degrees of work hardening are different. Further, the improvement in strength at the initial stage of work hardening is effective in improving the strength after molding because the reduction in ductility is small. For this reason, it is desirable to apply a pre-strain described later, that is, a process that is uniform and does not significantly reduce the ductility. By applying pre-strain, deformation twins are formed in the grains and the strength is improved. In the metal structure of the titanium alloy plate of the present embodiment, the sum of the boundary lengths of deformation twins existing per unit grain (hereinafter simply referred to as "total twin boundary length") is 0 mm. It is preferably more than 1.0 mm or less.

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

If the total twin boundary length per unit grain exceeds 0 mm, initial work hardening occurs. Therefore, the total twin boundary length existing per unit crystal grain is preferably more than 0 mm, 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, work hardening will proceed excessively, resulting in a decrease in moldability. Therefore, the total twin boundary length per unit grain is preferably 1.0 mm or less, more preferably 0.9 mm or less, and even more preferably 0.8 mm or less.

The total twin boundary length per unit crystal grain may be measured by the following procedure. Specifically, as with the average crystal grain size described above, the measurement may be performed by EBSD. However, the step size is measured at 0.2 to 0.5 μm. Other conditions are the same as those for the measurement of the average crystal grain size. After measurement, data analysis software (OIM Analysis) is used to separate only crystal grains that are completely included in the measurement field. Three deformation twins, {11-22} twins, {10-12} twins, and {11-21} twins, should be considered. The K1 plane and axis of these twins are analyzed with an allowable range of 10° to identify deformation twins. For the twins identified from these three orientations, the sum of the lengths of the perimeters of the boundaries L is measured and divided by the number of grains included in the field of view to obtain the total twin boundaries per unit grain. be the length. Even when measuring in a plurality of fields of view, the total sum L of all twin boundary lengths similarly measured may be divided by the total sum of crystal grains included in the field of view.

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 an economical titanium alloy that does not contain alloying elements as described above, that is, 0.2% yield strength. is preferably 200 MPa or more, and this range is evaluated as high strength. The strength, at 0.2% yield strength, is preferably 220 MPa or more, more preferably 230 MPa or more. Further, with respect to formability, the uniform elongation is preferably 25% or more, and this range is evaluated as good formability. The uniform elongation is preferably 26% or more, more preferably 27% or more.

7. Manufacturing Method A preferred method for manufacturing the titanium alloy plate according to the present embodiment will be described. The titanium alloy plate according to this embodiment can be stably produced by, for example, the following production method.

7-1. Blooming Process The ingot having the chemical composition described above is preferably produced by a method such as electron beam melting, vacuum arc melting (also referred to as "VAR"), or electron beam melting (also referred to as "EB melting"). The shape of the ingot is not particularly limited. It may be rectangular or cylindrical. If necessary, a step of cutting the surface of the ingot, or a step of blooming by hot forging or the like (hereinafter simply referred to as "blooming step") may be performed.

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

If the blooming process is not performed on a rectangular ingot, the casting surface causes surface defects during hot rolling. Therefore, it is desirable to remove the casting surface by cutting the surface. Further, when the blooming process is performed, it is not necessary to cut the surface although it depends on the casting surface. In this case, after the blooming process is completed, the surface may be cut together with scale removal. That is, at the end of the blooming process, the scale and hardened layer on the surface layer are removed to obtain a slab for hot rolling. In the case of a columnar ingot, it is difficult to hot-roll the ingot as it is. Therefore, as with the rectangular ingot, a blooming process may be performed to obtain a slab for hot rolling.

7-2. Hot Rolling Step Subsequently, the ingot or slab for hot rolling is preferably heated to a temperature range of 800 to 1100° C. and hot rolled. If the heating temperature during hot rolling is less than 800° C., it becomes difficult to cool within the preferred 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. Therefore, the heating temperature during hot rolling is preferably 1100° C. or lower. In addition, in hot rolling, it suffices if a rolling reduction of 70% or more can be secured in one heating.

Hot rolling is preferably completed in a temperature range of 700°C or higher. That is, the hot rolling completion temperature is preferably 700° C. or higher. This is because, as will be described later, the production of the titanium alloy sheet of the present embodiment requires rapid cooling to a predetermined temperature range after hot rolling in order to suppress the formation of intermetallic compounds.

In the case of a thin plate with a relatively thin plate thickness, it is generally manufactured in a coil shape. In order to form the coil shape, it must be wound, but in this case, it becomes difficult to dissipate heat from the plate surface, which slows down the cooling. Therefore, it must be sufficiently cooled before being wound into a coil shape.

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 temperatures around 700°C. At temperatures around 600° C., intermetallic compounds containing Ti and Cu are likely to precipitate. That is, in the range of 600 to 700° C., intermetallic compounds tend to precipitate. As a result of the formation of such an intermetallic compound, the crystal grains after hot rolling become finer and formability is lowered.

In addition, the intermetallic compound is more likely to precipitate not only within the above temperature range, but also as the working strain introduced by hot rolling remains. Therefore, after hot rolling, it is necessary to wind the steel sheet into a coil after sufficiently cooling it to a temperature range in which the intermetallic compound is difficult to form.

As will be described later, when batch annealing is performed, finish annealing may be performed in a temperature range of 600 to 700° C. after cold rolling. Also in this case, if the intermetallic compound is precipitated, the crystal grains become finer and the formability is lowered. In addition, the formation of intermetallic compounds reduces the solid-solution amount of additive elements in the matrix, making it difficult to improve the strength by solid-solution strengthening.

For this reason, after the hot rolling process, it is preferable to rapidly cool the steel sheet to a temperature range of 600° C. or lower before winding it into a coil, to form a hot rolled sheet. It is more preferable to rapidly cool to a temperature range of 550° C. or less before winding into a coil. The rapid cooling means cooling at a cooling rate of 10° C./s or more, and water cooling can be considered as a cooling method, for example. Moreover, the temperature mentioned above is the surface temperature of the plate, which is the temperature measured by the radiation thermometer, and the same applies to the paragraphs described later.

After hot rolling, descaling may be performed as necessary. Annealing after hot rolling, so-called hot-rolled sheet annealing, tends to form intermetallic compounds.

When hot-rolled sheet annealing is performed, it is desirable to perform it in a temperature range of 700 to 800°C. When the hot-rolled sheet is annealed in a temperature range of less than 700°C, intermetallic compounds are likely to be formed. On the other hand, when the hot-rolled sheet is annealed in a temperature range of 800° C. or higher, a β phase is formed, and Cu and Si are concentrated in the β phase, thereby facilitating the precipitation of intermetallic compounds. Therefore, when hot-rolled sheet annealing is performed, it is desirable to perform it in the temperature range of 700 to 800°C. Further, the shorter the annealing time is, the more the productivity is improved.

When performing hot-rolled sheet annealing, it is preferable to anneal by continuous annealing. This is because batch annealing cannot sufficiently anneal while avoiding precipitation of intermetallic compounds. Although the annealing atmosphere is not particularly limited, the annealing may be performed in an air atmosphere.

In addition, regarding the cooling rate after hot-rolled sheet annealing, at this time, the sheet thickness is usually 6 mm or less, and in this case, the cooling rate may be 1 ° C./s or more, which corresponds to air cooling. . It is because it can suppress that an intermetallic compound precipitates.

If the plate thickness exceeds 6 mm, the cooling will be difficult to proceed. Therefore, the cooling rate becomes slow, and depending on the plate thickness, it may not be possible to obtain a cooling rate of 1° C./s or more. In that case, it is better to perform water cooling that can cool faster. This is because water cooling provides a cooling rate of 10° C./s or more, depending on the thickness of the plate, and a sufficiently high cooling rate of 1° C./s or more can be obtained. The hot-rolled sheet may be annealed and then descaled after cooling. The descaling method is not particularly limited, but may be performed by, for example, a mechanical method such as shot blasting, pickling, or polishing, or a chemical method.

7-3. Cold Rolling Step Subsequently, the hot-rolled sheet is preferably cold-rolled to obtain a cold-rolled sheet. Cold rolling may be performed in multiple steps. In this case, intermediate annealing may be performed between cold rollings, if necessary. When intermediate annealing is performed, it is preferable to perform it in a temperature range of 750 to 800° C. for 30 seconds to 2 minutes, for example. Intermediate annealing may be continuous annealing, like hot-rolled sheet annealing. Moreover, the annealing atmosphere in the intermediate annealing is not particularly limited. Annealing may be performed in an air atmosphere. Moreover, when intermediate annealing is performed, it is preferable to cool at a cooling rate corresponding to the air cooling or water cooling described above. As with hot-rolled sheet annealing, if the sheet thickness is 6 mm or less and the sheet thickness is thin, it may be cooled by air cooling with a slow cooling rate, but if the sheet thickness is over 6 mm and the sheet thickness is thick, water cooling may be used. It is better to cool.

In the cold rolling, the cold rolling reduction in the final cold rolling performed immediately before the finish annealing, which will be described later, is preferably 50% or more. If the above-mentioned 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.

In a mixed grain structure, coarse grains are present locally. In such portions, the number of crystal grains decreases in the plate thickness direction. As a result, the uniform elongation is lowered, and the formability tends to be lowered. Therefore, the cold rolling reduction in the final cold rolling may be adjusted according to the desired plate thickness, but the cold rolling reduction is preferably 50% or more.

7-4. Finish Annealing After the cold rolling step, the cold-rolled sheet is preferably subjected to finish annealing. Finish annealing is the final annealing performed after all the cold rolling steps are completed, and is also called final annealing. In the final annealing, the steel is preferably annealed at an annealing temperature of 600 to 700° C. to form a cold-rolled annealed sheet. Crystal grains become fine as the annealing temperature of finish annealing is less than 600 degreeC. This results in poor moldability. Therefore, the annealing temperature in the final annealing is preferably 600° C. or higher. On the other hand, if the annealing temperature in the final annealing is higher than 700° C., the deterioration of the surface properties is likely to occur due to seizure inside the coil. Therefore, the annealing temperature in the final annealing is preferably 700° C. or lower. The annealing temperature in the final annealing is more preferably in the range of 630-680°C.

Also, the annealing time in the final annealing is preferably 1 hour or more. If the annealing time of the final annealing is less than 1 hour, the crystal grains will not grow sufficiently and the formability will deteriorate. Therefore, the annealing time is preferably 1 hour or longer, more preferably 4 hours or longer. Although the upper limit of the annealing time of the finish annealing is not particularly limited, it is usually about 20 hours from the viewpoint of productivity.

In the final annealing, preheating may be performed before annealing. This is because preheating tends to reduce temperature variations in the coil. Also when preheating, the heating temperature is preferably 550° C. or less in order to suppress precipitation of intermetallic compounds. Also, the finish annealing may be batch annealing. This is because annealing can be performed at a low temperature for a long time. The annealing atmosphere for the final annealing is preferably vacuum or Ar gas in order to suppress oxidation.

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 to improve the strength of the cold-rolled annealed sheet, if necessary. Pre-straining refers to performing processing with a small amount of processing in advance before processing the product. The titanium alloy plate of the present embodiment is preferably pre-strained with an elongation of less than 5.0%. This is because when the elongation is 5.0% or more, the sum of deformation twin boundary lengths exceeds 1.0 mm, and the formability is lowered. The elongation rate is preferably 4.0% or less, more preferably 3.0% or less. As a method for applying pre-strain, a tension leveler used for shape correction or skin pass rolling is preferably used. The elongation rate is calculated using the following formula (a).

Elongation rate (%) = (l−l ₀ )/l ₀ ×100 (a)
However, each symbol in the above formula is defined as follows.
l ₀ : Gauge length of test piece before pre-straining l: Gauge length of test piece after pre-straining

EXAMPLES Hereinafter, the titanium alloy plate according to the present embodiment will be described in more detail with reference to Examples, but the present embodiment is not limited to these Examples.

A 60 mm thick titanium alloy block 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 less to obtain a hot-rolled sheet having a thickness of 4 mm. . In addition, in the chemical composition of Table 1, the O content is an analytical value by an inert gas fusion infrared absorption method. The N and H contents are analytical values obtained by the inert gas fusion thermal conductivity method. Also, the C content is an analytical value obtained by a high-frequency combustion infrared absorption method. Contents of elements other than these are analytical values obtained by inductively coupled plasma (ICP) emission spectrometry. Further, "WQ" in Table 2 represents "water cooling", "AC" represents "air cooling", and "FC" represents "furnace cooling".

After that, No. For Examples other than 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, No. For Examples 17, 18, 21 and 22, hot-rolled sheet annealing was performed, followed by descaling as described above. Cold rolling is performed once or twice, and some No. For 19-22, intermediate annealing was performed in air between cold rolling. In addition, No. in the table. The cold rolling rate (cold rolling rate) in the item of intermediate cold rolling/annealing in 19 to 22 is the cold rolling rate of cold rolling before intermediate annealing.

The conditions of the intermediate annealing performed are as shown in Table 2. No. For Nos. 19 to 22, after intermediate annealing, descaling was performed by shot blasting and pickling with hydrofluoric acid. Thereafter, final cold rolling and finish annealing were performed under the conditions shown in Table 2 for all titanium alloy examples, followed by cooling. Finish annealing was performed in a vacuum and furnace cooled. After cooling, no. 2, 24 to 26, and 32 to 36 were pre-strained by temper rolling at the elongation percentages shown in Table 2.

The average grain size and the total twin boundary length of the titanium alloy sheets thus obtained were measured by the following procedure.

(Average grain size)
For the average crystal grain size, the central portion (position) of the plate thickness of the L cross section of the alloy plate is used as a measurement surface, etching is performed with an appropriate corrosive liquid, and the average crystal grain size d is calculated by performing measurement using EBSD. . Data analysis software (OIM Analysis) was used to calculate the average grain size d. In addition, the crystal grain size was calculated by taking the crystal grain as a circle-equivalent diameter. The EBSD measurement conditions were such that a total of 130 to 150 crystal grains were included in two fields of view. Other conditions were appropriately adjusted according to the average crystal grain size, as described above.

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

Also, the obtained titanium alloy plate was subjected to a tensile test to measure the 0.2% yield strength and uniform elongation. When the 0.2% proof stress was 200 MPa or more, the strength was evaluated as good. Moreover, when the uniform elongation was 25% or more, the formability was evaluated as good.

A JIS 13B subsize test piece was used as the test piece for the tensile test. The test piece used had a parallel portion length of 32 mm, a width of 6.25 mm, and a gauge length of 25 mm. In addition, the tensile speed was set to 0.13 mm/min up to 2% strain by stroke control, and thereafter set to 7.5 mm/min until breakage. The results are summarized in Table 3 below.

Test No. that satisfies the requirements of this embodiment. 4, 5, 11-15, 17-22, 24-29 and 31-36 titanium alloy plates had high strength and good formability. Here, No. In Nos. 24, 26, and 32-36, since prestrain was applied within a preferable range, the strength was improved and good moldability was maintained. On the other hand, No. In No. 25, since the amount of prestrain applied was large, although the strength was improved, the uniform elongation was smaller than that of the other invention examples, resulting in slightly inferior formability.

On the other hand, Test No. which does not satisfy the requirements of this embodiment. Titanium alloy sheets Nos. 1 to 3, 6 to 10, 16, 23 and 30 were inferior in at least one of strength and formability. Test No. whose chemical composition deviates from the requirements of this embodiment. As for 1 to 3, the strength was mainly inferior. On the other hand, test no. With respect to 6 to 10, the moldability was inferior. In addition, test No. No. 9 had particularly high strength and was considered to have poor formability, and edge cracks occurred in part of the width end portion during cold rolling. Also, No. Examples 16, 23, and 30 were not manufactured under preferable manufacturing conditions, so the average crystal grain size and the like were outside the range of the present embodiment, resulting in poor strength or moldability.

Claims (5)

The chemical composition, 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,
balance: Ti and impurities,
satisfying the following formula (i),
In the metal structure, the average crystal grain size d is 0.020 to 0.150 mm,
A titanium alloy plate, wherein 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 (% by mass) of each element contained in the titanium alloy, and if it is not contained, it is zero, and each symbol in the above formula (ii) is as follows. is defined as
t: plate thickness (mm)
d: average grain size (mm) The titanium alloy plate according to claim 1, having a plate thickness of 0.3 to 1.5 mm. 3. The titanium alloy plate according to claim 1 or 2, wherein the sum of boundary lengths of deformation twins existing per unit crystal grain in the metal structure is more than 0 mm and not more than 1.0 mm. A method for producing a titanium alloy plate according to claim 1 or 2,
A hot rolling step in which a titanium slab or titanium ingot having the chemical composition according to claim 1 is heated to a temperature range of 800 to 1100° C. and then rolled, and the rolling is completed in a temperature range of 700° C. or higher;
After the hot rolling step, a cooling step of quenching to a temperature range of 600 ° C. or less to form a hot rolled sheet;
A cold-rolling step of cold-rolling the hot-rolled sheet to obtain a cold-rolled sheet;
After the cold rolling step, the cold rolled sheet is finish annealed in a temperature range of 600 to 700 ° C. to obtain a cold rolled annealed sheet.
A method for manufacturing a titanium alloy plate. A method for producing a titanium alloy plate according to any one of claims 1 to 3,
After the annealing step, the step of pre-straining the cold-rolled annealed sheet at an elongation rate of less than 5.0%.
A method for producing a titanium alloy plate according to claim 4.