KR102178434B1 - Method for manufacturing of titanium plate having excellent quality of surface - Google Patents

Abstract

An aspect of the present invention is to provide a method of effectively removing an oxide layer and an oxygen-enriched layer formed on a surface while minimizing surface defects in manufacturing a titanium plate.

Description

Manufacturing method of titanium plate with excellent surface quality {METHOD FOR MANUFACTURING OF TITANIUM PLATE HAVING EXCELLENT QUALITY OF SURFACE}

The present invention relates to a method of manufacturing a titanium plate having excellent surface quality.

Due to its high specific strength and excellent corrosion resistance, titanium is widely used not only as a material for aircraft, power generation, and petrochemical facilities, but also as a material for living organisms such as medical implants because of its excellent human affinity.

Titanium plates using such titanium are usually forged ingots cast by the method of VAR (Vacuum Arc Remelting), PACHM (Plasma Arc Cold Hearth Melting) or EBCHM (Electron Beam Cold Hearth Melting). After manufacturing in the form of a slab through a rolling process, the slab is manufactured into a final sheet coil through the processes of hot rolling, annealing, pickling, cold rolling, and vacuum annealing (FIG. 1).

A series of processes for manufacturing the titanium plate will be described as follows.

First of all, the slab manufacturing process is a process of removing the ingot structure by demolition forging or rolling a heated ingot, and forming a shape of a slab suitable for a subsequent hot rolling process.

In the hot rolling process, the slab is heated to a transformation temperature below the transformation temperature (for example, 850 to 890°C) and then rolled to form a thin coil (hot rolled coil) having a thickness of 3 to 4 mm, and at this time, a solid oxide layer ( Oxide layer) and a high hardness oxygen-rich layer having a thickness of several tens of µm due to oxygen solidification may be formed under it.

The annealing process of the hot-rolled material produced therefrom is a process of softening a material through recrystallization of a structure hardened by rolling deformation, and is a process for obtaining a soft material to facilitate subsequent cold rolling.

In the subsequent cold rolling process, a thin sheet coil having a final target thickness is manufactured, and the formed cold rolled coil is subjected to a vacuum annealing process at a temperature above the recrystallization temperature and below the transformation temperature to prevent the formation of an oxide layer on the surface, while preventing the formation of a strain hardened structure. It loosens and secures workability.

On the other hand, the annealing process of the titanium hot-rolled material is usually performed by a continuous method in which the hot-rolled coil is passed through a furnace heated in advance in an atmospheric atmosphere of 750 to 800°C. After such an annealing process, the oxide layer and the high hardness oxygen-rich layer generated on the surface of the hot-rolled material can be removed through mechanical shot blasting and chemical pickling processes.

However, even by the above-described method, removal is insufficient, so that some oxygen-enriched layers remain, and during mechanical shot blasting, a work hardened layer of high hardness may be generated on the surface. The high-hardness layer on the surface of the hot-rolled material is a major cause of linear defects occurring during the subsequent cold-rolling process due to the difference in material from the base material.

Unlike iron (Fe) materials, titanium tends to form a relatively coarse oxygen-enriched layer, which is a high-hardness structure due to oxygen solidification, directly under the oxide layer as well as an oxide layer on the surface when heated.

In the oxygen-enriched layer, since oxygen atoms are dissolved in the titanium crystal structure to increase hardness and strength, if the oxygen-enriched layer generated during the hot process is not completely removed through mechanical and chemical processing before the subsequent cold processing, the base material during the cold working process There is a high possibility that surface defects may occur due to the material difference between the and.

Since such an oxygen-enriched layer is generated by solid solution of a small amount of oxygen atoms in the base material, it is difficult to distinguish by observing the structure of the surface or the cross-section, and it is usually possible to measure the microhardness of the cross-sectional structure. Therefore, it is difficult to determine the amount of titanium to be processed in order to remove the oxygen-enriched layer.

In addition, if the surface of the hot-rolled material is excessively removed for the removal of the oxygen-enriched layer, production yield may be lost. On the other hand, if the oxygen-enriched layer is removed to a minimum, the oxygen-enriched layer remains and defects during the subsequent cooling process. It is likely to cause.

Therefore, in order to obtain a titanium plate with minimized surface defects, there is a need for a method to effectively remove the highly hardened layer (oxygen enriched layer) that has not been completely removed from the surface of the hot-rolled material that has been put into the cold working process. .

An aspect of the present invention is to provide a method of effectively removing an oxide layer and an oxygen-enriched layer formed on a surface while minimizing surface defects in manufacturing a titanium plate.

The subject of the present invention is not limited to the above description. Anyone of ordinary skill in the art to which the present invention pertains will not have difficulty in understanding the additional subject of the present invention from the contents throughout the present specification.

One aspect of the present invention, the step of preparing a titanium hot-rolled plate; Shot blasting and pickling the titanium hot-rolled sheet; Subjecting the shot blasted and pickled titanium hot-rolled sheet to a first annealing heat treatment; Cold rolling the first annealed heat-treated titanium hot-rolled sheet; And performing a second annealing heat treatment after the cold rolling, wherein the first annealing heat treatment is performed in a vacuum atmosphere of 700 to 850° C. for 8 to 10 hours. .

According to the present invention, it is possible to provide a titanium plate material having excellent surface quality by suppressing linear defects in a cold rolling process.

1 schematically shows a series of processes for manufacturing a conventional titanium plate.
2 is a schematic view showing a series of processes for manufacturing a titanium plate according to an aspect of the present invention.
3 shows a photograph of observing the microstructure of a titanium hot-rolled coil immediately after hot rolling (arrow 2 is an indented particle station generated during hardness measurement).
4 shows a photograph of observing the microstructure of a titanium hot-rolled sheet manufactured by a conventional process (arrow 2 is an indented particle station generated during hardness measurement).
5 shows a photograph of observing the microstructure of a titanium hot-rolled sheet manufactured by a process according to an aspect of the present invention.
6 shows a photograph of observing the surface of a titanium cold-rolled sheet manufactured without performing a vacuum annealing heat treatment process.
7 is a graph showing the result of the number of surface defects of the titanium cold-rolled sheet according to whether the vacuum annealing heat treatment process is performed.

The present inventors have a problem that surface defects occur as the oxide layer and the oxygen-enriched layer formed on the surface of the titanium plate manufactured through the conventional hot rolling process and the continuous annealing process in an atmospheric atmosphere are not sufficiently removed from mechanical and chemical processing. Was confirmed.

Accordingly, the present inventors have studied in depth a method for manufacturing a titanium plate having excellent surface quality by effectively removing the oxide layer formed on the surface of the titanium plate and minimizing the generation of an oxygen-enriched layer of a high-hardness structure.

As a result, the present invention is technical to provide a titanium plate having desired physical properties through vacuum annealing after first removing the oxide layer of the hot-rolled material while omitting the existing hot-rolled material annealing process in manufacturing a titanium plate material. It is meaningful.

Hereinafter, the present invention will be described in detail.

A titanium plate having excellent surface quality according to an aspect of the present invention can be manufactured through a series of processes as follows (FIG. 2).

Specifically, preparing a titanium hot-rolled sheet; Shot blasting and pickling the titanium hot-rolled sheet; Subjecting the shot blasted and pickled titanium hot-rolled sheet to a first annealing heat treatment; Cold rolling the first annealed heat-treated titanium hot-rolled sheet; And performing a second annealing heat treatment after the cold rolling.

First, the titanium hot-rolled plate may be a hot-rolled coil obtained by heating and hot rolling the titanium slab after manufacturing a titanium slab by split forging or rolling a titanium ingot.

At this time, the process of heating and hot rolling the titanium slab is under normal conditions, for example, the slab heated at 900°C or higher is rolled at a temperature below the transformation temperature, such as 850 to 890°C, and the thickness is 3 to 4 mm. A thin hot-rolled coil of about the same can be obtained.

In the present invention, by shot blasting and pickling the titanium hot-rolled sheet obtained by the above method, the oxide layer formed on the surface of the titanium hot-rolled sheet can be removed.

Since the shot blasting and the pickling treatment process can be performed under normal conditions, the conditions are not particularly limited.

However, for example, a short blaster may be used for the shot blasting, and a mixed pickling solution containing nitric acid and hydrofluoric acid may be used for the pickling process. Specifically, it may be based on the method presented in Korean Patent Registration No. 10-1611754.

In the present invention, a process of vacuum annealing the titanium hot-rolled sheet material from which the oxide layer has been removed according to the above can be performed, and the annealing process at this time is referred to as a first annealing heat treatment.

Specifically, the first annealing heat treatment may be performed by charging the hot-rolled coil from which the oxide layer is removed into a vacuum annealing furnace heated to a temperature above the recrystallization temperature of pure titanium and below the starting temperature of transformation. Preferably, it can be carried out for 8 to 10 hours in a vacuum atmosphere of 700 to 850°C, more preferably in a temperature range of 800 to 850°C, and still more preferably at a temperature exceeding 800°C.

If the temperature during the first annealing heat treatment is less than 700° C. or the heat treatment time is less than 8 hours, the recrystallization treatment of the deformed structure formed by hot rolling is insufficient, and thus the target level of material softening cannot be achieved. On the other hand, if the temperature exceeds 850℃ or the heat treatment time exceeds 10 hours, the material softening effect is sufficient, but the productivity will be greatly reduced. There is concern.

The first annealed, heat-treated titanium hot-rolled sheet may be cold-rolled to manufacture a titanium cold-rolled sheet, and in this case, a person skilled in the art may determine the reduction ratio according to the target thickness.

Thereafter, the residual stress of the titanium cold-rolled sheet may be removed, and annealing heat treatment may be performed for the purpose of securing workability, and the annealing heat treatment process at this time is referred to as a second annealing heat treatment.

In the present invention, the second annealing heat treatment may be performed under the same conditions as the aforementioned first annealing heat treatment, but the vacuum atmosphere may be changed to a non-oxidizing atmosphere.

In the conventional titanium hot-rolled sheet, the hot-rolled hot-rolled coil is continuously annealed in an atmospheric atmosphere and then pickled by mechanical and chemical methods, whereas in the present invention, the continuous annealing process is excluded, while the pickling process is preferentially performed and then vacuum By going through an annealing process, a titanium plate with excellent surface quality can be obtained.

It will be described in detail with reference to the following Examples that the surface quality of the titanium plate can be improved by the process according to the present invention.

In particular, in the case of the present invention, the surface hardness value of the titanium hot-rolled sheet material subjected to the first annealing heat treatment is 135 Hv or less, and the surface hardness value may be lowered compared to before the first annealing heat treatment is performed. More preferably, it may be 130Hv or less.

This is, as vacuum annealing is performed in the first annealing heat treatment process after the shot blasting and pickling process, the surface hardened structure formed by shot blasting is recrystallized to remove the deformed twin structure, and the oxide layer is minimized as oxygen in the oxygen-enriched layer diffuses It is due to being.

Here, the surface of the titanium hot-rolled sheet refers to an area from the surface to the thickness direction of 10 μm to 100 μm, and the surface hardness value is expressed as an average value after measuring five times within the area corresponding to the surface.

Hereinafter, the present invention will be described in more detail through examples. However, it should be noted that the following examples are for illustrative purposes only and are not intended to limit the scope of the present invention. This is because the scope of the present invention is determined by matters described in the claims and matters reasonably inferred therefrom.

( Example )

Titanium slab containing 0.03%Fe-0.05%O-0.003%C-0.003%N-0.001%H-balance Ti in wt% is heated for at least 4 hours in an atmospheric atmosphere at 860~880℃ and then hot rolled to a thickness of about 3mm. A hot-rolled coil was manufactured.

As a result of observing the microstructure of the hot-rolled coil obtained by completing the hot rolling, it can be seen that the oxide layer 1 was formed on the surface of the hot-rolled coil in several μm as shown in FIG. 3.

In addition, the microhardness was measured in a surface layer area of 10 to 100 μm deep below the oxide layer and an inner area of 100 to 1500 μm depth directly below the oxide layer. At this time, the test load of the ultra-small hardness was set to 50 mN, the indentation speed was set to 10 mN/s, and the load holding time was set to 10 seconds, and the average value was calculated after measuring 5 times at the same position.

As a result, the average value of ultra-microhardness in the inner region of 100 to 1500 µm in depth was measured to be about 100 Hv, while the average value of ultra-micro-hardness in the surface region of 10 to 100 µm in depth was measured to be 160 Hv. As described above, the reason why the hardness is higher in the surface layer than in the inside is because the oxygen-enriched layer is formed directly under the oxide layer by heating in a high-temperature atmospheric atmosphere.

[Conventional titanium hot-rolled sheet manufacturing (comparative example)]

After annealing and heat treatment of the hot-rolled coil in a continuous annealing furnace at a temperature of 750 to 770 ℃, shot blasting using a shot blaster, and then chemically in a mixed pickling solution containing nitric acid (100 g/l) and hydrofluoric acid (25 g/l). The surface oxide layer was removed through pickling.

As a result of observing the microstructure of the titanium hot-rolled sheet material (Comparative Example) manufactured through the annealing heat treatment and oxidation layer removal process, as shown in FIG. 4, the oxide layer (see FIG. 3) was removed to a few μm formed on the surface of the hot-rolled coil. , It can be seen that the deformed twin tissue (3) was generated.

In addition, the ultra-micro hardness was measured in the surface layer area 10 to 100 μm deep in the thickness direction from the surface of the titanium hot rolled sheet material, and the inner area 100 to 1500 μm deep below it, and the measurement conditions were the same as those of the hot rolled coil. .

As a result, it was found that the average value of ultra-microhardness in the inner region was measured as 100 Hv, while the average value of ultra-micro-hardness in the surface region was 170 Hv, which was still a high hardness. This is believed to be due to the fact that the work hardened structure (deformed twin structure) 3 formed by shot blasting after annealing heat treatment remains, and the oxygen-enriched layer is not completely removed.

[Manufacture of titanium hot-rolled sheet material of the present invention (invention example]

The hot rolled coil was shot blasted using a shot blaster, and then the surface oxide layer was removed by chemical pickling in a mixed pickling solution containing nitric acid (100 g/l) and hydrofluoric acid (25 g/l).

As a result of observing the microstructure after removing the surface oxide layer as described above, it was confirmed that the same structure as in FIG. 4, that is, the surface oxide layer was removed, and a modified twin structure was generated.

Thereafter, the hot rolled coil from which the surface oxide layer was removed was subjected to vacuum annealing heat treatment at 800° C. for 10 hours.

As a result of observing the microstructure of the titanium hot-rolled sheet material (inventive example) manufactured through the vacuum annealing heat treatment process, as shown in Fig. 5, no oxide layer remains on the surface, and a work hardened structure formed by shot blasting (deformed twin crystal It can be seen that the structure) was recrystallized and removed by vacuum annealing heat treatment.

In addition, the ultra-micro hardness was measured in the surface layer area 10 to 100 μm deep in the thickness direction from the surface of the titanium hot rolled sheet material, and the inner area 100 to 1500 μm deep below it, and the measurement conditions were the same as those of the hot rolled coil. .

As a result, it was confirmed that the average value of ultra-microhardness in the inner region was measured as about 102 Hv, while the average value of ultra-micro-hardness in the surface region was about 131 Hv, which was significantly lower than the comparative example. This is due to the effect that the work hardened structure (deformed twin structure) formed by shot blasting as mentioned above is recrystallized by vacuum annealing heat treatment, and the oxygen-enriched layer is reduced as oxygen diffuses in the oxygen-enriched layer.

[Manufacture of titanium cold rolled sheet material]

On the other hand, a titanium hot-rolled sheet material (Comparative Example) subjected to annealing heat treatment followed by shot blasting using a shot blaster and chemical pickling in a nitric and hydrofluoric acid mixed pickling solution, and shot blasting and chemical pickling according to the present invention, followed by vacuum annealing. Titanium hot-rolled sheet material (invention example) subjected to the heat treatment process was cold-rolled from an initial thickness of 3 mm to a final thickness of 0.7 mm using a reversible cold rolling machine, respectively, to prepare a titanium cold-rolled sheet material.

For each of the titanium cold-rolled sheets, 5 specimens each having a length of 1000 mm in width and length were collected, and the number of surface defects (linear defects) visible with the naked eye was counted.

As a result, as shown in Fig. 7, the titanium cold-rolled sheet material (B) not subjected to the vacuum annealing heat treatment appeared as eight horizontal linear defects in the rolling direction (see Fig. 6), whereas the titanium cold-rolled sheet material (A) according to the present invention ), it can be seen that the number is greatly reduced to only two.

From the above results, it can be seen that when an oxygen-enriched layer or work hardened structure of higher hardness than the inside remains on the surface of the titanium hot-rolled sheet, horizontal linear defects occur in the rolling direction during subsequent cold rolling, resulting in inferior surface defects. I can.

Further, in the case of performing the vacuum annealing heat treatment process after first performing the shot blasting and chemical pickling process on the titanium hot-rolled sheet material as in the present invention, the oxygen portion and the hardened structure of the high hardness can be effectively removed, thereby reducing surface defects. Can be suppressed and a good quality surface can be obtained.

Claims (5)

Preparing a titanium hot-rolled sheet;
Shot blasting and pickling the titanium hot-rolled sheet;
Subjecting the shot blasted and pickled titanium hot-rolled sheet to a first annealing heat treatment;
Cold rolling the first annealed heat-treated titanium hot-rolled sheet; And
Including the step of performing a second annealing heat treatment after the cold rolling,
The first annealing heat treatment method for producing a titanium plate having excellent surface quality, characterized in that performed for 8 to 10 hours in a vacuum atmosphere of 800 ~ 850 ℃.
delete The method of claim 1,
After the first annealing heat treatment, a method of manufacturing a titanium plate having excellent surface quality having a surface hardness value of 135 Hv or less.
The method of claim 1,
The method of manufacturing a titanium plate having excellent surface quality, wherein the deformed twin structure is removed by recrystallization of the surface structure of the titanium hot-rolled sheet during the first annealing heat treatment.
The method of claim 1,
The second annealing heat treatment is performed for 8 to 10 hours in a vacuum or non-oxidizing atmosphere of 700 to 850°C. Method for producing a titanium plate having excellent surface quality.

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