KR101773602B1 - Method for improving formability of pure titanium sheet and pure titanium sheet prepared thereby - Google Patents

Abstract

According to the present invention, a method of improving formability of a pure titanium sheet comprises: (a) a first cold-rolling step of rolling a pure titanium sheet; (b) a step of performing a first heat treatment on the first cold-rolled sheet; (c) a second cold-rolling step of rolling the first heat-treated sheet; and (d) a step of performing a second heat treatment on the second cold-rolled sheet. The method of improving formability of a pure titanium sheet performs the first heat treatment after performing the first cold-rolling step to form a recrystallized microstructure capable of easily forming twin crystals in the titanium sheet. The final heat treatment is performed after performing the second cold-rolling step at an optimal reduction ratio to maximize an effect of texture dispersion by the twin crystals after the first heat treatment to remarkably increase formability of the pure titanium sheet by the effect of texture dispersion by the twin crystals.

Description

TECHNICAL FIELD The present invention relates to a method for improving the formability of a pure titanium sheet material and a pure titanium sheet material produced thereby,

The present invention relates to a method for improving the formability of a pure titanium sheet material and a pure titanium sheet material produced thereby.

Titanium (Ti) has excellent corrosion resistance and specific strength, and is used for manufacturing various products. In particular, pure titanium is used for manufacturing seawater heat exchanger since it is not corroded at all in seawater, It is also used as a material for heat exchangers and chemical plants.

When the pure titanium plate material is used for manufacturing a plate type heat exchanger, which is a type of heat exchanger, high formability of the material is required because the plate is formed into a complicated pattern in order to increase the heat transfer efficiency of the heat exchanger.

Generally, titanium is formed by Hexagonal Closed Packing (HCP) and has a limited deformation mechanism, whereby the texture of titanium makes the plastic deformation unstable and lowers the formability of the titanium plate. Since the c axis of the HCP is inclined at an angle of 25 DEG to 35 DEG in the transverse direction (TD) of the sheet material in the vertical direction (ND) of the sheet material in most of the crystal grains, Even if slippage of the prism surface which is easiest to be generated in the titanium occurs, it is not possible to effectively receive deformation in the thickness direction of the plate material and to cause cracks .

Accordingly, various studies for improving the moldability without affecting the mechanical strength of the titanium plate have been conducted.

For example, Patent Document 1 relates to a "titanium or titanium alloy plate excellent in balance between press formability and strength", wherein a lubricating coating is applied to a surface of a titanium plate rolled in one direction, and a coefficient of dynamic friction of the lubricating coating surface is 0.15 And the moldability is improved by minimizing the heterogeneity of the deformation due to the texture during the molding process. However, the above-described method has a limitation in use because additional facilities for applying the lubricant coating must be constructed.

As another example, Non-Patent Document 1 relates to a method for improving the formability of a pure titanium plate for business use through aggregate organization control, wherein a texture is controlled through a different speed rolling method at a high temperature, A description has been given of a method for improving the moldability. However, the above-described method has a problem that warping of the plate material occurs during the rapid rolling.

As another example, Non-Patent Document 2 relates to a method capable of simultaneously alloying a small amount of iron source with pure titanium to ensure mechanical strength and moldability, and it is possible to optimize the heat treatment process so that the particles are finely dispersed and homogeneously distributed A description has been given of a method for minimizing the degradation of moldability accompanying alloying. However, the titanium alloy produced by the above method has a disadvantage in that it is difficult to process it into a thin plate material.

On the other hand, twinning, which is another important deformation mechanism of pure titanium with slip, is very effective in dispersing texture of the material by rapidly changing crystal orientation of crystal grains, but it is difficult to utilize the advantages of twinning in general cold rolling process of titanium The reason is that the twinning occurs only with the slip in the middle of the deformation, and the twinning structure is destroyed by the slip-induced deformation in the latter half of the deformation.

Accordingly, there is a need to develop a new cold rolling process that can utilize the advantages of effective twinning of twin rolls by improving the conventional cold rolling process.

Korean Patent No. 10-1325364 (Registered on Oct. 29, 2013) Korean Patent Laid-Open No. 10-2014-0004793 (Publication date: 2014.01.13) Korean Patent No. 10-1522799 (Registered on May 5, 2015) Korean Patent Publication No. 10-2014-0070102 (Publication date: 2014.06.10)

 Takeshi Kudo, Shogo Murakami, Yoshio Itsumi. Influence of microstructure on formability in Ti-Fe alloy, 2010, Vol. 60, pp. 33-36  Xinsheng Huang, Kazutaka Suzuki, Yasumasa Chino. Improvement of stretch formability of pure titanium sheet by differential speed rolling, 2010, Vol. 63, pp. 473-476

SUMMARY OF THE INVENTION The present invention has been made to solve the problems of the prior art as described above, and it is an object of the present invention to provide a novel cold rolling method for improving the moldability of a pure titanium plate material and a pure titanium plate material do.

(A) a first cold rolling step of rolling a pure titanium sheet material; (b) subjecting the first cold-rolled plate to a first heat treatment; (c) a second cold rolling step of rolling the first heat treated plate material; And (d) subjecting the second cold-rolled plate to a second heat treatment, thereby improving the formability of the pure titanium plate.

The pure titanium plate is characterized by an ASTM grade of 1-3.

And rolling at least 10 passes in the step (a).

And the step (b) is performed at 700 to 900 ° C for 50 to 70 minutes.

And the reduction rate of the step (c) is not less than 1% and less than 40%.

And rolling at a reduction ratio of 18.2% in the step (c).

The step (c) is characterized by forming the {11-22} twin in the titanium sheet material.

In the step (c), the basal plane of the {11-22} twin crystal is perpendicular to the rolled surface.

And the step (d) is performed at 700 to 900 ° C for 50 to 70 minutes.

In addition, the present invention provides a pure titanium sheet produced by the above-described method.

The pure titanium plate material produced by the above-described method has an average strain anisotropy coefficient value of 2.31 or less.

The present invention also provides a heat exchanger made of a pure titanium plate manufactured by the above-described method.

A method of improving the formability of a pure titanium plate according to the present invention includes the steps of forming a recrystallized microstructure capable of easily forming a twin crystal on a titanium plate by performing a first heat treatment after a first cold rolling step, After the heat treatment, the second cold rolling step is carried out at an optimum reduction rate capable of maximizing the effect of aggregate structure dispersion by twinning, and then the final heat treatment is performed. As a result, the formability of the titanium plate is remarkably increased .

In addition, the titanium plate material improved in moldability by the above method is easy to process and can be effectively used for manufacturing various products such as members having complex shapes.

Fig. 1 is a process drawing showing each step of the method for improving the moldability of the pure titanium plate material of the present invention.
2 is a diagram schematically illustrating process steps according to an embodiment and a comparative example.
3 is a (0001) pole figure obtained through electron backscattering diffraction (EBSD) analysis for a first heat treated plate material by the method according to Examples 1 to 4.
Fig. 4 is a view showing the microstructure of the sheet subjected to the second cold-rolling treatment by the method according to Examples 1 to 4 (note that the {11-22} twin formed after the second cold rolling treatment is green and Blue, {10-12} twin is displayed in red).
5 is a graph showing the area fraction of {11-22} twinning and {10-12} double twinning formed by the second cold rolling treatment according to the methods of Examples 1 to 4.
6 is a (0001) pole figure obtained by electron backscattering diffraction (EBSD) analysis for a plate subjected to a second cold-rolling by the method according to Examples 1 to 4.
FIG. 7 is a (0001) pole figure obtained through electron backscattering diffraction (EBSD) analysis for the plate subjected to the second heat treatment and the plate according to the comparative example by the method according to the first to fourth embodiments.
8 is a diagram schematically showing an outline and a specimen of an apparatus for measuring the limit dome height, which is performed in Experimental Example 2. FIG.
By the method according to Examples 1 to 4 and Comparative Example
FIG. 3 is a schematic view showing an outline and a specimen of an apparatus for measuring the limit dome height according to the embodiment. FIG.
9 is a graph showing the height of a limit dome of a pure titanium plate manufactured by the method according to Examples 1 to 4 and Comparative Example.

In the following description of the present invention, a detailed description of known functions and configurations incorporated herein will be omitted when it may make the subject matter of the present invention rather unclear.

Embodiments in accordance with the concepts of the present invention can make various changes and have various forms, so that specific embodiments are illustrated in the drawings and described in detail in this specification or application. It should be understood, however, that the embodiments according to the concepts of the present invention are not intended to be limited to any particular mode of disclosure, but rather all variations, equivalents, and alternatives falling within the spirit and scope of the present invention.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The singular expressions include plural expressions unless the context clearly dictates otherwise. In this specification, the terms "comprises ", or" having ", or the like, specify that there is a stated feature, number, step, operation, , Steps, operations, components, parts, or combinations thereof, as a matter of principle.

Hereinafter, the present invention will be described in detail.

As shown in FIG. 1, a method of improving the formability of a pure titanium plate according to the present invention includes: (a) a first cold rolling step of rolling a pure titanium plate; (b) subjecting the first cold-rolled plate to a first heat treatment; (c) a second cold rolling step of rolling the first heat treated plate material; And (d) subjecting the second cold-rolled plate to a second heat treatment.

The pure titanium plate material may be a commercially available pure titanium plate material, or may be a pure titanium plate subjected to a heat-recrystallization heat treatment after hot rolling, and the composition of the plate material is not particularly limited. For example, a plate having a grade of 1 to 3 according to the ASTM standard and a plate having a grade of 1 to 3 of the standard grade of JIS H4600 may be used.

The step (a) is a first cold rolling step of rolling a pure titanium plate, and is a step of rolling a pure titanium plate by one or more passes.

In this step, the pure titanium sheet material can be rolled according to a target small thickness amount (mm). In order to roll the pure titanium sheet material with a desired thickness reduction amount, rolling can be performed in one pass or more, Or more.

Further, in the method for improving the formability according to the present invention, the pure steel sheet is rolled by separately dividing the thickness reduction amount into each step of the first cold rolling step and a second cold rolling step to be described later, And the number of cold rolling passes can be adjusted according to the amount of reduction.

As a concrete example, when the final target thickness is 16.2 mm, the thickness reduction amount in the first cold rolling step may be set to 16 mm after setting the small thickness reduction amount in the second cold rolling step to 0.2 mm. In the case of setting the total reduction to 88.8% in the cold rolling process, it is possible to set to perform 16 passes of rolling in order to reduce the thickness of 16 mm (note that in one pass, the number of times of rolling once Quot;).

Next, step (b) is a step of first heat-treating the first cold-rolled plate, and recrystallizing the titanium crystal grains in the titanium plate through heat treatment to facilitate twinning.

In the general titanium cold rolling process, by the activation of the slip and the twinning until the middle of the second half, the texture that acts as a factor for concentrating the deformation during the molding process is dispersed. In the latter half, however, only the slip is activated. Such a slip-induced deformation leads to the destruction of the twin structure and reinforcement of the aggregate structure. Accordingly, in a general titanium cold rolling process requiring a lot of thickness reduction, there is no dispersion effect of the twinning.

Therefore, since the first cold-rolled sheet material has deformation due to rolling, the activity of the twin crystal becomes gradually lower. Therefore, in the present invention, the step (c) to be described below by recrystallizing the crystal grains of the first cold- The first cold-rolled plate can be heat-treated to facilitate twinning.

When the first heat treatment is performed at a temperature of less than 700 ° C, a sufficient driving force for recrystallization can not be provided. As a result, twinning bands and dislocations remain, resulting in a decrease in formability of the final plate. , More preferably 700 ° C or higher and a beta potential temperature of 890 ° C or lower, and then cooled to recrystallize the microstructure of the titanium plate material.

When the first heat treatment is performed for less than 50 minutes, the recrystallized grains are not uniform. If the first recrystallization is performed for more than 70 minutes, recrystallization proceeds sufficiently, but there is a problem involving an increase in the surface layer oxide layer. Min or less, more preferably 60 minutes.

In addition, the heat treatment in this step can be performed in an atmospheric, nitrogen or argon atmosphere.

Next, step (c) is a second cold rolling step for rolling the first heat treated plate material, and through this step, the {11-22} twin Can be formed. At this time, {10-12} double twinning can also be formed.

The formation of the {11-22} twin crystal may vary depending on the rolling speed and the content of interstitial elements of titanium, so that a small amount of thickness reduction and a reduction ratio can be set.

In this step, a small amount of thickness reduction in step (a) can be set after setting an optimal thickness reduction amount for forming {11-22} twinning. For example, when the final target thickness reduction amount is 16.2 mm, It is possible to constitute cold rolling by setting a small amount of thickness reduction in the cold rolling step to 0.4 mm and then setting a small amount of thickness reduction in the first cold rolling step to 15.8 mm.

The {11-22} twinning and {10-12} twinning that occur during the rolling of the titanium sheet material in this step cause a sharp azimuthal change of the crystal grains, unlike the slip, and the twin crystals have about 64 ° and 85 ° It has an angle relation and shows directionality. The twinning is selectively formed in the texture and the stress direction, and the {11-22} twin and the {10-12} twin occur in a stressed state in which the c-axis is compressed and stretched, respectively. In addition, the twinning can occur not only within the initial grain but also within the twinned twin that occurs first and also in the form of a double twin.

The reduction rate can be adjusted to form {11-22} twinning effective for dispersion of the texture in the twin. When the reduction rate is less than 1%, the {11-22} twinning is not sufficiently formed, which makes it difficult to improve the final formability of the formed titanium plate. When the reduction rate is more than 40%, the occurrence of {11-22} twinning is suppressed It is difficult to further improve the moldability and a {10-12} double twinning may occur, which may cause a problem of reduction in formability. Therefore, a reduction of from 1% to less than 40%, more preferably from 10% It is possible to cold-roll the titanium sheet at a rate of 10%. In particular, the plate of pure titanium produced by rolling at a reduction ratio of 18.2% in this step has a {11-22} twin area fraction of 40%, which is excellent in formability.

In addition, in order to roll the titanium plate at the optimum reduction rate, the cold rolling process may be performed by one or more passes.

The basal plane of the {11-22} twin formed in this step may be perpendicular to the rolled surface, so that the texture can be effectively dispersed.

Next, step (d) is a step of subjecting the second cold-rolled plate to a second heat treatment, and recrystallizing the particles to increase the activity of the twin crystal.

When the second heat treatment is performed at a temperature of less than 700 캜, a sufficient driving force for recrystallization may not be provided, and the twinning band and the dislocation remain. Therefore, the moldability of the final plate is decreased. , More preferably 700 ° C or higher and a beta potential temperature of 890 ° C or lower.

When the first heat treatment is performed for less than 50 minutes, the recrystallized grains are not uniform. If the first recrystallization is performed for more than 70 minutes, recrystallization proceeds sufficiently, but there is a problem involving an increase in the surface layer oxide layer. Min or less, more preferably 60 minutes.

In addition, the heat treatment in this step can be performed in an atmospheric, nitrogen or argon atmosphere.

In addition, this step can be configured to improve the surface quality of the sheet material by removing the oil remaining on the surface by heat-treating the second cold-rolled titanium sheet material.

Accordingly, the method of improving the formability of the pure titanium plate of the present invention is characterized in that after the first cold rolling step, a first heat treatment is performed to form a recrystallized microstructure which can easily form twinning in the titanium plate, After the first heat treatment, the second cold rolling step is carried out at an optimum reduction rate which maximizes the effect of twinning, and then the final heat treatment is performed. Thus, the formability of the titanium plate due to the twinning- Can be significantly increased.

The present invention provides a pure titanium sheet produced by the above-described method and having improved moldability.

The final pure titanium sheet material may have an average value of the strain anisotropy coefficient of 2.31 or less, which is an index of the deformation capacity in the thickness direction. Thus, the pure titanium sheet produced by the rolling process consisting of the first cold rolling and the first- It has a lower value than the average deformation modulus value 2.55 of the plate material and can exhibit improved formability.

In addition, the pure titanium plate manufactured by the manufacturing method of the present invention using the ASTM standard grade 2 plate material can be manufactured by the conventional first cold rolling and pure heat treatment only by the first heat treatment, The limit dome height of the plate can be increased by 20% compared to the titanium plate.

The pure titanium sheet obtained by the method of improving moldability according to the present invention described above has a high area fraction of {11-22} twin so as to effectively disperse the texture, and the average modulus of anisotropy is low, Therefore, it can be effectively used for manufacturing various products such as the absence of a complicated formation, and in particular, it can be manufactured and used as a heat exchanger having a complicated pattern.

Hereinafter, the present invention will be described in more detail with reference to examples.

The embodiments presented are only a concrete example of the present invention and are not intended to limit the scope of the present invention.

≪ Example 1 >

 After the hot-rolling, a commercial titanium sheet of 18 mm in length consisting of 0.21% of oxygen, 0.007% of carbon, 0.004% of nitrogen, 0.001% of hydrogen, 0.04% of iron and the rest of titanium was recrystallized. For the production of the final thickness of 1.8 mm plate, the smallest total thickness is set to 16.2 mm, and the reduction rate of the second cold rolling step is set to 10% as shown in Table 1, The reduction rate of the rolling step was set at 88.8%.

Thereafter, as shown in Fig. 2, the sheet material was subjected to 16 pass rolling and first cold rolling. The first cold-rolled sheet material was subjected to a first heat treatment at 700 ° C for 60 minutes, and the sheet material was subjected to one-pass rolling and second cold-rolling. The second cold-rolled sheet was subjected to a second heat treatment at 700 ° C for 60 minutes to produce a titanium sheet.

[Table 1]

≪ Example 2 >

As shown in Table 1, except that the reduction ratio in the second cold rolling step was set to 18.2% and the reduction ratio in the first cold rolling step was set to 87.8%, a titanium plate material .

≪ Example 3 >

As shown in Table 1, except that the reduction ratio in the second cold rolling step was set to 28% and the reduction rate in the first cold rolling step was set to 86.1%, a titanium plate material .

<Example 4>

As in Table 1, except that the reduction ratio in the second cold rolling step was set to 35.7% and the reduction ratio in the first cold rolling step was set to 84.4%, a titanium plate was prepared in the same manner as in Example 1 .

<Comparative Example>

After hot rolling, recrystallized commercial titanium sheet of 0.21% oxygen, 0.007% carbon, 0.004% nitrogen, 0.001% hydrogen, 0.04% iron and the balance of titanium was prepared. For the final thickness of 1.8 mm, the total thickness was 16.2 mm and the reduction rate was 90%.

As shown in Fig. 2, the sheet material was subjected to 17 pass rolling, first cold pressing, and first heat treatment at 700 캜 for 60 minutes to produce a titanium plate.

<Experimental Example 1> Microstructure analysis of a titanium sheet

(1) Microstructure analysis after the first heat treatment

In order to analyze the microstructure of the plate after the first heat treatment, according to Examples 1 to 4 of the present invention, electron back scattered diffraction (EBSD) analysis was performed on the plate subjected to the first heat treatment And the results are shown in Fig.

As shown in Fig. 3, it can be seen that in the titanium plate according to Examples 1 to 4, the basal plane shows a rolled texture which is distributed at an angle of 25 DEG to 35 DEG with respect to the rolled surface of the work.

(2) Microstructure analysis after the second cold rolling step

In order to analyze the microstructure of the sheet after the second cold rolling, EBSD was performed on the second cold-rolled sheet according to Examples 1 to 4 of the present invention, and the results are shown in Figs. 4 to 6 Respectively.

In FIG. 4, the twinning formed after the second cold rolling is indicated by green and blue in the {11-22} twin, and the red in the {10-12} twin. As shown in FIG. 4, a {11-22} twin was effectively formed by dispersing the texture in the inside of the titanium plate by the second cold rolling, and {10-12} twin was formed inside the {11-22} twin It can be confirmed that it is formed as a double.

5 shows the area fraction of {11-22} initial twin and {10-12} double twin according to the reduction ratio (%) in the second cold rolling. As shown in FIG. 5, the initial twin of {11-22} exhibits an area fraction of 40% which is the highest at a reduction rate of 18% in the second rolling of Example 2, and the area fraction decreases at a further reduction rate, 10-12} It can be seen that the double twin increases gradually as the reduction rate increases.

Figure 6 is also a (0001) pole figure showing the development of the texture. As shown in FIG. 6, the {11-22} twin formed in the second cold rolling was rearranged with an angular relationship of about 65 degrees with respect to the orientation of the initial crystal grains so that the bases of the {11-22} And 90 °, and it is distributed perpendicularly to the RD-TD surface. It can be seen that the texture most developed in the plate according to the embodiment 2 is most developed. In addition, since the {10-12} double twin is rearranged with an angle of about 85 ° with respect to the {11-22} twin, the basal plane is distributed horizontally with the RD-TD plane. , The {11-12} twinning weakens the texture and {10-12} twinning enhances the horizontal texture of the basal plane with the RD-TD.

(3) Microstructure analysis after the second heat treatment

In order to analyze the microstructure of the sheet after the second heat treatment, EBSD analysis was performed on the sheet subjected to the second heat treatment according to Examples 1 to 4 and Comparative Example of the present invention, Respectively.

As shown in FIG. 7, it can be confirmed that the titanium plate material having an improved texture can be produced by dispersing the texture of the aggregate structure due to the twinning of {11-22} remaining in the titanium plate material even after the second heat treatment.

In comparison with the titanium plate according to the comparative example, it can be seen that the titanium plate according to Examples 1 to 4 disperses the rolled steel texture and weakens the strength at the same time.

<Experimental Example 2> Moldability analysis of titanium sheet

(1) Measuring the limit dome height

In order to analyze the moldability of the titanium plate manufactured according to Examples 1 to 4 and Comparative Examples of the present invention, the height limit dome was measured using an Erichsen cupping test.

As shown in Fig. 8, a test piece (1 mm in thickness, 50 mm in diameter) was produced from the plate materials according to Examples 1 to 4 and Comparative Example, and the test piece was tested at a punching speed of 10 mm / To determine the limit dome height (LDH). The limit dome height is an average value obtained by performing three or more times under the same test conditions, and the results are shown in FIG.

As shown in FIG. 9, the titanium plate according to Examples 1 to 4 showed a limit dome height improved by 20% as compared with the titanium plate according to the comparative example, and the moldability was improved.

(2) Analysis of tensile properties

In order to analyze the tensile properties of the titanium plate produced according to Examples 1 to 4 and Comparative Example of the present invention, the yield strength, elongation at break, elongation at break, elongation at break in the RD direction, The index and strain anisotropy coefficients were measured and calculated.

For this purpose, a specimen (1.5 mm in diameter, 30 mm in length) was fabricated and tested using a tensile tester. The specimens were attached at room temperature with an extensometer at a strain rate of 10 ^-3 s ^-1 and tested at least three times under the same conditions for the accuracy of the experiment. From the test results, the yield strength, tensile strength, elongation and work hardening index (n-value) were derived, and the strain anisotropy coefficient (r-value) . For reference, the average value was calculated using the following equation (1).

[Formula 1]

[Table 2]

As shown in Table 2, it can be seen that there is a difference in the result value, which can be judged to be the result of different group organization.

 Referring to the modified anisotropy coefficient, it can be confirmed that the modified anisotropy coefficient is low. The lower the deformation anisotropy coefficient is, the better the deforming capacity of the plate in the thickness direction is, so that the titanium plate according to the embodiment of the present invention has excellent formability .

In the case of the titanium plate according to Example 2 having the lowest strain anisotropy coefficient, the base surface has a relationship of about 90 with the rolled surface, and the texture distribution perpendicular to the RD-TD surface is strongest in the final plate material compared to other titanium plates And RD-TD surfaces can be most effectively accommodated in the thickness direction, and thus the formability is increased due to the low strain anisotropy coefficient.

In addition, in the plate materials other than the example 2, the aggregate structure which facilitates deformation in the thickness direction as described above was weakly developed, and thus it can be confirmed that the plate material according to the example 2 has the best formability.

In addition, it can be seen that the plate material produced according to the manufacturing method of the present invention has a low main rolling structure texture strength, thereby alleviating the concentration of deformation during the forming process and improving the formability.

In addition, as shown in Table 2, it can be confirmed that the yield strength of the titanium plate according to Example 2 is improved by 16% in the RD direction as compared with the titanium plate according to the comparative example.

Therefore, the titanium sheet material produced by the method of improving the moldability of the titanium sheet material of the present invention has the {11-22} twinning formed inside the sheet material, and the rolled aggregate structure is dispersed by the {11-22} Of the total population.

Claims (12)

(a) a first cold rolling step of rolling a pure titanium plate of ASTM grade 1 to 3 at a reduction ratio of 87.8%;
(b) subjecting the first cold-rolled plate to a first heat treatment;
(c) a second cold rolling step of rolling the first heat-treated plate material at a reduction ratio of 18.2%; And
(d) subjecting the second cold-rolled plate to a second heat treatment. delete The method according to claim 1,
Wherein the rolling of at least 10 passes is performed in the step (a). The method according to claim 1,
Wherein the step (b) is performed at 700 to 900 ° C for 50 to 70 minutes. delete delete The method according to claim 1,
Wherein the step (c) comprises forming a {11-22} twin crystal within the titanium plate material. 8. The method of claim 7,
Wherein the basal plane of the {11-22} twin crystal in the step (c) is perpendicular to the rolled surface. The method according to claim 1,
Wherein the step (d) is performed at 700 to 900 DEG C for 50 to 70 minutes. A pure titanium sheet produced by the method according to any one of claims 1, 3, 4 and 7 to 9. 11. The method of claim 10,
Wherein the pure titanium plate has an average value of an anisotropic coefficient of strain of 2.31 or less. A heat exchanger formed of a pure titanium plate produced by the method according to any one of claims 1, 3, 4 and 7 to 9.

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