WO2022131483A1

WO2022131483A1 - High-strength pure titanium board having good formability at room temperature, and method for producing same

Info

Publication number: WO2022131483A1
Application number: PCT/KR2021/011696
Authority: WO
Inventors: 김재혁; 이상원; 홍재근; 박찬희; 염종택; 원종우; 최성우
Original assignee: 한국재료연구원
Priority date: 2020-12-15
Filing date: 2021-08-31
Publication date: 2022-06-23
Also published as: KR20230088655A; KR102582659B1; KR20220085856A

Abstract

The present invention relates to a pure titanium board and a method for producing same, the pure titanium board being characterized in that contours, on a pole figure, with the same (002) texture intensity value are symmetric with respect to both the rolling direction (RD) and transverse direction (TD) axes.

Description

High-strength pure titanium plate with excellent room temperature formability and manufacturing method thereof

The present invention relates to a high formability titanium sheet material with excellent strength and advantageous texture for forming through the production of a sheet material using a new rolling process and a subsequent processing heat treatment process, and thus the press formability is greatly improved compared to the conventional titanium sheet material will be.

Titanium alloy is an alloy for structural materials having low density, high strength, excellent specific strength and corrosion resistance. Recently, demand is increasing as an industrial material requiring weight reduction such as blades for power generation and heat exchangers.

Among them, commercially pure titanium (hereinafter, titanium or pure titanium means commercially pure titanium) has excellent corrosion resistance and biocompatibility.

However, since pure titanium has a low yield strength due to the characteristics of a hexagonal close packed (HCP) single phase crystal structure, industrial applications have been limited.

In order to increase the strength of the pure titanium, a grain refinement method has been mainly applied in the prior art.

However, in order to refine grains, a rolling process with a high reduction ratio must be performed several times, and the rolling process with a high reduction ratio develops a strong texture in a pure titanium sheet having hexagonal crystals.

The development of texture in a pure titanium plate has a problem of lowering the formability in the case of titanium with a small slip system.

In addition, the pure titanium sheet material manufactured by the conventional manufacturing method such as the rolling process of the high reduction ratio has a problem in that it is difficult to process it into a complex shape due to poor formability, particularly, press workability.

Therefore, it is required to develop a pure titanium plate with improved formability and high strength.

The present invention is to solve the problems of the conventional pure titanium plate described above, and the combination of formability and strength through the rolling process and subsequent heat treatment process design is greatly improved compared to the conventional pure titanium plate. And it aims to provide the manufacturing method.

More specifically, an object of the present invention is to provide a pure titanium plate material capable of achieving high strength and excellent formability by forming a texture advantageous for molding while having an ultrafine grain microstructure, and a method for manufacturing the same.

In addition, an object of the present invention is to be able to economically provide a high-strength pure titanium plate having excellent room temperature formability only by a general rolling process without a separate plasticizing process.

In a titanium plate according to an embodiment of the present invention for achieving the above object, a trajectory having the same value of {002} texture strength on a pole figure is a rolling direction (RD) and a rolling side It is characterized in that it is simultaneously symmetrical about each axis of the direction (TD).

At this time, it is preferable that the trajectory (contour) of the maximum value of the {002} texture strength on the pole figure intersects the axes of the rolling direction (RD) and the rolling side direction (TD) at the same time.

In this case, it is preferable that the {002} texture strength is at most 5.2 or less.

At this time, the difference between the yield strength and/or tensile strength in the rolling direction (RD) and the yield strength and/or tensile strength in the rolling lateral direction (TD) of the plate material is preferably within 5%.

At this time, it is preferable that the aspect ratio of the crystal grains of the plate material is 0.4 or more.

At this time, it is preferable that the Erichson value (index of Erichsen) of the plate material is 15 mm or more.

A method of manufacturing a titanium plate according to an embodiment of the present invention for achieving the above object, (a) rolling the titanium plate in the rolling direction (RD) and the rolling side direction (TD); (b) annealing the rolled sheet material; characterized in that the ratio of the rolling reduction in the rolling direction to the rolling reduction in the rolling side direction in step (a) is 1: 1.25 to 1: 1.75 do it with

In this case, the total reduction in step (a) is preferably 60% or more.

At this time, the annealing in step (b) is preferably maintained at 600 ℃ or less.

According to the present invention, through the control of grain refinement and texture through the design of the rolling process and the subsequent heat treatment process, the formability as well as the room temperature strength is greatly improved compared to the conventional pure titanium plate material, so that it can be widely applied to industries requiring weight reduction. It is possible to obtain a high formability pure titanium sheet material.

In addition, according to the method of manufacturing a titanium plate material according to the present invention, since improved room temperature press formability can be obtained, it is possible to manufacture titanium parts at a lower cost compared to the conventional pure titanium plate material.

In addition to the above-described effects, the specific effects of the present invention will be described together while describing specific details for carrying out the invention below.

1 shows a method of manufacturing a titanium plate material using a conventional general rolling process.

(a) of FIG. 2 shows the microstructure of the pure titanium plate made by the general rolling process of FIG. 1, and (b) shows the microstructure of the pure titanium plate made by the general rolling process {002} Show the pole figure (PF) of the plane.

3 shows a method of manufacturing a pure titanium plate according to an embodiment of the present invention.

4 (a) shows the microstructure of a pure titanium plate made by the controlled rolling process of FIG. 3, and (b) shows the microstructure of the pure titanium plate made by the controlled rolling process {002} Show the pole figure (PF) of the plane.

Figure 5 is a schematic diagram showing the Erichsen (Erichsen) test method for evaluating the formability of the pure titanium plate material of the present invention.

6 (a) to (h) are, respectively, raw material (mill-annealed), general rolling process (RD), RD:TD reduction ratio = 1:1, RD:TD reduction ratio = 1:1.25, RD:TD The reduction ratio=1:1.5, RD:TD reduction ratio=1:1.75, RD:TD reduction ratio=1:2, and TD-rolled pure titanium sheet are shown as the measurement results of pole viscosity.

FIG. 7 shows the “RD/TD texture intensity ratio” of FIG. 6 .

8 (a) and (b) show electron backscatter diffraction images of a pure titanium plate rolled by a general rolling process (RD) and a controlled rolling process in which the RD:TD reduction ratio is 1:1.5, respectively. do.

9 (a) to (d) is a pure titanium sheet rolled under the conditions of general rolling process (RD), TD rolling process, RD:TD reduction ratio = 1:1, and RD:TD reduction ratio = 1:1.5, respectively. The measurement results of the evaluation of the mechanical properties are shown.

Figure 10 shows the results of the Erichsen (Erichsen) test for evaluating the formability of the pure titanium plate material of the present invention.

Hereinafter, with reference to the drawings, embodiments of the present invention will be described in detail so that those of ordinary skill in the art to which the present invention pertains can easily implement them. The present invention may be embodied in many different forms and is not limited to the embodiments described herein.

In order to clearly explain the present invention, parts irrelevant to the description are omitted, and the same reference numerals are assigned to the same or similar components throughout the specification. Further, some embodiments of the present invention will be described in detail with reference to exemplary drawings. In adding reference numerals to components of each drawing, the same components may have the same reference numerals as much as possible even though they are indicated in different drawings. In addition, in describing the present invention, if it is determined that a detailed description of a related known configuration or function may obscure the gist of the present invention, the detailed description may be omitted.

In describing the components of the present invention, terms such as first, second, A, B, (a), (b), etc. may be used. These terms are only for distinguishing the elements from other elements, and the nature, order, order, or number of the elements are not limited by the terms. When it is described that a component is “connected”, “coupled” or “connected” to another component, the component may be directly connected or connected to the other component, but other components may be interposed between each component. It should be understood that each component may be “interposed” or “connected”, “coupled” or “connected” through another component.

The plate-shaped titanium plate is rolled a plurality of times (n times) in the rolling direction (longitudinal direction of the plate, rolling direction, refer to the arrow in FIG. 1) at room temperature until the final desired reduction ratio in the general rolling process. Thereafter, the rolled titanium sheet is annealed to remove the internal energy accumulated therein. The annealing process is typically performed at a temperature of half or more of a melting point for at least 1 hour. The final annealed titanium plate has a new recrystallized microstructure with significantly reduced defects such as dislocations.

However, even if a new recrystallized microstructure is formed by the annealing, a processing method for creating a flow of material in a specific direction, such as rolling, results in a texture in which crystal grains are arranged in a specific crystal direction (rolling direction). In particular, in the case of a rolled sheet material, the direction to the crystal plane as well as the crystal direction can be defined.

The microstructure having the texture is to create mechanical anisotropy. In other words, the rolled sheet having a microstructure of the texture has a characteristic that the mechanical properties are different depending on the direction.

The mechanical anisotropy again adversely affects the workability of the sheet material, such as press formability. For example, if the mechanical properties in the rolling direction (RD) and the transverse direction (TD) of the sheet material are different from each other, the poor workability in the weak direction makes it difficult to uniformly process the sheet material. can cause

As shown in Fig. 2(a), when rolling is performed at a high rolling reduction for grain refinement, even if a recrystallized microstructure in which internal defects are removed by annealing is formed, the pure titanium sheet has crystal grains in the rolling direction. It can be seen that it has a stretched anisotropic microstructure.

It was measured that a strong texture was formed in the rolling lateral direction during the rolling process due to the characteristics of the pure titanium plate having a hexagonal closed packed (HCP) crystal structure (FIG. 2(b)). Referring to FIG. 2(b) , it can be seen that the {002} planes are strongly oriented in a direction perpendicular to the transverse rolling direction.

The above results in FIGS. 1 and 2 mean that when a pure titanium plate is processed through a general rolling process at a high reduction ratio for grain refinement, the anisotropy of mechanical properties is necessarily accompanied. And the above result means that although it is possible to improve the mechanical properties through grain refinement in the general rolling process, the improvement of the isotropic mechanical properties cannot be achieved.

The plate material manufacturing method of FIG. 3 has a feature of controlling the rolling direction in the rolling process, compared to the conventional general rolling process of FIG. 1 . More specifically, the plate material manufacturing method of FIG. 3 is characterized in that the rolling direction is mixed in the rolling direction and the rolling transverse direction during the rolling process. In other words, in the plate material manufacturing method of FIG. 3 , the rolling reduction in the rolling direction and the rolling reduction in the rolling transverse direction during rolling were controlled at specific ratios, respectively.

Meanwhile, in the sheet material manufacturing method of FIG. 3 , the annealing process after the controlled rolling process is substantially the same as the annealing process after the general rolling process of FIG. 1 . However, the annealing process is preferably performed at 600° C. or less. If the annealing exceeds 600° C., grain growth may occur excessively after recrystallization, thereby reducing mechanical strength.

Figure 4 (a) shows the microstructure of the scanning electron microscope observing the microstructure of the pure titanium plate made of 73.3% of the total cumulative rolling reduction (3.0t > 0.8t) by the controlled rolling process of Figure 3, (b) is The pole figure (PF) of the {002} plane of the pure titanium plate by the controlled rolling process is shown.

As shown in Fig. 4(a), when controlled rolling is performed with a high reduction amount for grain refinement, the pure titanium sheet material does not have an anisotropy tendency in which the grains are stretched in the rolling direction, but rather has an isotropy tendency. can be known In addition, it can be seen that the grain size of the microstructure produced by the controlled rolling process is finer than the grain size of the microstructure produced by the general rolling process.

Furthermore, although pure titanium having a hexagonal closed packed (HCP) crystal structure has anisotropic characteristics in plastic deformation, the texture of a pure titanium plate manufactured by a controlled rolling process is in the rolling direction and transverse direction. As a result, it has a more balanced shape, and the strength of the collective tissue is also significantly weakened (Fig.

The above results in FIGS. 3 and 4 mean that when a pure titanium plate is processed through a controlled rolling process at a high rolling reduction for grain refinement, the anisotropy of mechanical properties can be suppressed. And the above result means that the controlled rolling process can achieve not only the improvement of mechanical properties through grain refinement, but also the achievement of isotropic mechanical properties.

On the other hand, the result of FIG. 4 shows that the total reduction amount is 73.3%, but when the total reduction amount is about 60% or more, it was confirmed that the microstructure and the texture as shown in FIG. 4 were obtained.

The improvement of the isotropy of the pure titanium plate according to the present invention greatly affects the formability of the plate. In the present invention, in order to evaluate the formability of the plate, an Ericsson test was performed according to EN ISO 20482 and KS B 5529 standards.

In the Ericsson test, a disk-shaped specimen having a diameter of 50 mm and a thickness of 0.7 mm is prepared, the specimen is inserted between the upper die and the lower die, and the specimen is fixed with a force of 10 kN. At this time, the lubricating oil may be a known press oil. Thereafter, a spherical punch having a diameter of 20 mm is used to apply deformation at a rate of 0.1 mm/sec, and as shown in FIG. 5 , the punch is inserted until the specimen is broken, and then the deformation height at break is measured.

Hereinafter, the present invention will be described in more detail through experimental examples of the present invention.

6 shows the change in pole figure (PF) according to the change in the reduction ratio of various RD and TD when the total reduction amount is 60% or more.

First, in the case of a raw material (mill-anneaed), it was measured to have a strong {002} texture in the rolling transverse direction and the strength (or strength) of the texture was also very high (FIG. 6 (a)).

In the case of the general rolling process and the case where the reduction ratio of RD:TD is 1:1, it has a strong {002} texture in the rolling transverse direction, and the strength (or strength) of the texture is still very high (Fig. b) and (c)).

On the other hand, when the reduction ratio of RD:TD was 1:1.25, although it had a strong {002} texture in the rolling transverse direction, the strength (or strength) of the texture was measured to be more reduced (FIG. 6 (d)). In particular, when the reduction ratio of RD:TD is 1:1.25, the {002} texture on the pole figure shows a substantially symmetrical shape at the same time in the rolling direction and the rolling transverse axis. The texture as described above means that the anisotropy of the pure titanium plate material is reduced on a plane including the rolling direction and the rolling transverse direction.

On the other hand, when the reduction ratio of RD:TD was 1:1.5, a very dramatic change was observed. Specifically, when the reduction ratio of RD:TD is 1:1.5, the {002} texture on the pole figure is substantially symmetrical with respect to the rolling direction and the rolling transverse axis at the same time and the texture on the pole figure The trajectory of the maximum value of the strength has a texture that intersects the axes of the rolling direction (RD) and the rolling lateral direction (TD) at the same time, and the strength (or strength) of the texture also varies depending on the overall reduction ratio, but all of them are 4 or less. It was measured to have a low value (Fig. 6 (e)).

The result of the texture when the reduction ratio of RD:TD of FIG. 6(e) is 1:1.5 is very unusual and encouraging. Because the texture {002} on the pole figure is substantially symmetrical at the same time to the rolling direction and the rolling transverse axis, the locus of the maximum value of the texture strength on the pole figure is the rolling direction ( RD) and the rolling lateral direction (TD) intersecting at the same time means that the anisotropy of the pure titanium sheet material on the plane including the rolling direction and the rolling transverse direction has disappeared.

Furthermore, the decrease in the strength of the texture to 3 or less (FIG. 6 (e)) means that the directionality of grains in the pure titanium plate material is greatly reduced, which means that the isotropy of the pure titanium plate material is improved.

On the other hand, in the case of the RD:TD rolling reduction ratios of 1:1.75, 1:2, and TD rolling, the {002} texture was more strongly expressed again (FIGS. 6 (f) to (h)). Specifically, under the rolling conditions, unlike the case of RD rolling, the texture appeared along the rolling direction and the strength of the texture also showed a tendency to increase.

However, when the rolling reduction ratio of RD:TD was 1:1.75, the {002} texture maintained substantially symmetrical characteristics in the rolling direction and the rolling transverse axis at the same time on the pole figure, but the rolling reduction of RD:TD When the ratio was 1:2.0, it was measured that the symmetry of the texture was completely disappeared.

Table 1 below is a table summarizing the results of FIG. 6 .

[Table 1]

"Agglomerate strength" in Table 1 means the maximum value of the texture measured in RD (rolling direction) or TD (rolling transverse direction).

"RD/TD texture intensity ratio" in Table 1 is a ratio obtained by dividing the maximum values in RD and TD measured in each experimental example. Here, from the general rolling process to the RD:TD reduction ratio of 1:1.25, the TD measured value is greater than the RD measured value (see FIG. 6), so the “RD/TD texture strength ratio” in Table 1 is the TD measured value/RD Means the measured value. On the other hand, from the RD:TD rolling reduction ratio of 1:1.75 to the TD rolling, the RD measured value is larger than the TD measured value (see Fig. 6), so Table 1 "RD/TD texture strength ratio" is the RD measured value/TD measured value. it means.

In the end, when the "RD/TD texture strength ratio" in Table 1 is close to 1, it means that the strength of the texture in the rolling direction and the rolling lateral direction in the pure titanium sheet is similar to each other, which is again the case of the pure titanium sheet. It means that the isotropy of mechanical properties is improved.

FIG. 7 shows the “RD/TD texture intensity ratio” of FIG. 6 .

As shown in FIG. 7 , the texture strength ratio for each rolling direction starts to increase rapidly with an inflection point between the RD:TD reduction ratio of 1:1 and 1:1.25, and the RD:TD reduction ratio is 1:1.5 days. was measured to have a maximum value when After that, the tissue strength ratio again started to decrease rapidly, and it was measured that the RD:TD reduction ratio slowly decreased with an inflection point between 1:1.75 and 1:2.

The tissue strength ratio result of FIG. 7 can be said to quantitatively prove that the RD:TD reduction ratio has a critical significance between 1:1.25 and 1:1.75.

As shown in Fig. 8 (a), the pure titanium sheet rolled by the general rolling process (RD) first has crystal grains elongated in a specific direction due to the developed texture. As a result, it was confirmed that the grain aspect ratio in the general rolled pure titanium plate under the same total reduction condition showed a low value of 0.2, and the average grain size was measured to be about 3.6 μm. .

On the other hand, it can be seen that the pure titanium plate rolled by the controlled rolling process has a relatively weak texture development, so it is generally uniform and has fine grains mainly, elongated in a specific direction, and has a microstructure in which coarse grains are part of it. (Fig. 8(b)). As a result, it was confirmed that the grain aspect ratio in the control-rolled pure titanium plate under the same total reduction condition had a very high value as 0.43, and the average grain size was about 0.8㎛, which was sub-micro (sub-micron) level was measured.

9 shows the evaluation results of the mechanical properties of the pure titanium plate material according to the various RD and TD changes in the reduction ratio.

First, in the case of the general rolling process (RD) (FIG. 9 (a)) and in the case of a pure titanium sheet rolled with an RD:TD reduction ratio = 1:1 (FIG. 9 (c)), a tensile test in the rolling direction (RD) The tensile strength was better than the result of the tensile test in the rolling lateral direction (TD) direction, while the yield strength was measured to be lower, and the elongation was measured to be approximately the same. On the other hand, the tensile test results in the 45 degree direction showed that both strength and elongation were significantly increased compared to the RD and TD directions. The tensile test result means that the pure titanium sheet rolled by the general rolling process (RD) and RD:TD reduction ratio = 1:1 has very high mechanical anisotropy, and the tensile test result is the result of the pole figure of FIG. also fits very well.

On the other hand, in the case of a pure titanium plate rolled in the rolling side direction (TD) (FIG. 9 (b)), the mechanical anisotropy was also measured to be very high as a result of the tensile test, and the tensile test result was very well with the result of the pole figure of FIG. match

On the other hand, the pure titanium plate material (FIG. 9 (d)) rolled with RD:TD reduction ratio = 1:1.5 according to an embodiment of the present invention was measured to have substantially the same or similar yield strength and tensile strength in each direction. The tensile test result agrees very well with the result of the pole figure of FIG. 6 , and it means that the anisotropy of the pure titanium sheet material can be greatly improved through the control of the rolling process.

Table 2 below is a table summarizing the mechanical property evaluation results of FIGS. 9 (a) and (d).

[Table 2]

(YS, UTS: MPa)

As described in Table 2 above, the pure titanium plate (FIG. 9 (d)) rolled with RD: TD reduction ratio = 1:1.5 according to an embodiment of the present invention has yield strength according to RD, 45 degrees, and TD directions ( YS) and the difference in tensile strength (UTS) are both within 3%, and it can be seen that they have substantially isotropic properties in terms of strength.

In addition, the difference in yield strength (YS) and tensile strength (UTS) for each RD, 45 degree and TD direction was measured to be within 5% of the pure titanium sheet rolled by control rolling even if the total rolling reduction was changed. This is considered to be because the texture produced by the controlled rolling in the present invention has better isotropy than the texture produced by the conventional general rolling.

Table 3 below shows

grades

1 and 2 commercial pure titanium plate (mill annealed), general rolling process (RD), rolling transverse rolling process (TD), RD:TD reduction ratio = 1:1 and RD:TD reduction ratio = 1:1.5 Shows the Erikson values of the control-rolled pure titanium sheet. Here, I.E. is the Erichsen value (index Erichsen), a numerical value expressed in mm that indicates the distance (punch stroke) that the punch tip moves from the lower die surface until a crack that reaches the back surface of at least one part of the test piece occurs (until fracture) , and it means that the larger the I. E. value, the better the stretching properties.

[Table 3]

From the results of Table 3, it can be seen that the RD:TD reduction ratio = 1:1.5 control-rolled pure titanium plate has an Erickson value of 15 mm or more. In particular, it was measured that the RD:TD reduction ratio = 1:1.5 control-rolled pure titanium plate contained the least impurities such as oxygen, and thus had better workability than the grade 1 pure titanium plate, which had excellent elongation and workability.

Table 4 shows the rolling direction (RD) of mill annealed

grades

1 and 2 pure titanium sheet, normal rolled (RD) pure titanium sheet, and controlled rolled (RD:TD reduction ratio = 1:1.5) pure titanium sheet. This is a summary of the evaluation results of the mechanical properties of

[Table 4]

It can be seen that the controlled-rolled pure titanium plate according to an embodiment of the present invention has a higher yield strength and Erikson value than

conventional grades

1 and 2 commercial titanium plates as well as a general rolled pure titanium plate. In particular, it can be confirmed that the control-rolled pure titanium sheet material of the present invention has better workability (high Ericsson value) than the commercial grade 1 titanium sheet material due to the isotropy of the improved mechanical properties unique to the present invention.

As described above, the present invention has been described with reference to the illustrated drawings, but the present invention is not limited by the embodiments and drawings disclosed in the present specification. It is obvious that variations can be made. In addition, although the effects of the configuration of the present invention are not explicitly described and described while describing the embodiments of the present invention, it is natural that the effects predictable by the configuration should also be recognized.

Claims

A titanium sheet, characterized in that the trajectory having the same value of the {002} texture strength on the pole figure is simultaneously symmetrical with respect to the respective axes of the rolling direction (RD) and the rolling side direction (TD).
The method of claim 1,

Titanium sheet material, characterized in that the trajectory of the maximum value of the {002} texture strength on the pole figure intersects the axes of the rolling direction (RD) and the rolling side direction (TD) at the same time.
The method of claim 1,

The {002} texture strength is a titanium plate, characterized in that the maximum is 5.2 or less.
The method of claim 1,

A titanium sheet material, characterized in that the difference between the yield strength and/or tensile strength in the rolling direction (RD) and the yield strength and/or tensile strength in the rolling lateral direction (TD) of the sheet material is within 5%.
The method of claim 1,

A titanium plate, characterized in that the aspect ratio (aspect ratio) of the crystal grains of the plate is 0.4 or more.
The method of claim 1,

Titanium plate, characterized in that the Erichson value (index of Erichsen) of the plate is 15 mm or more.
(a) rolling the titanium plate in the rolling direction (RD) and the rolling side direction (TD);

(b) annealing the rolled plate material; including,

In the step (a), the ratio of the rolling reduction in the rolling direction to the rolling reduction in the rolling side direction is 1: 1.25 to 1: 1.75.
8. The method of claim 7,

The method of manufacturing a titanium plate material, characterized in that the total reduction in step (a) is 60% or more.
8. The method of claim 7,

The annealing in step (b) is a method of manufacturing a titanium plate, characterized in that maintained at 600 ℃ or less.