KR102025628B1 - Descaler - Google Patents

Abstract

Embodiments include an apparatus for removing a scale formed on a surface of a steel sheet, the apparatus comprising: a first spraying apparatus including a plurality of first nozzles for injecting a fluid toward the steel sheet; And a second injection device including a plurality of second nozzles for injecting particles toward the fluid, wherein the plurality of second nozzles are spaced apart from the plurality of first nozzles in a first direction. The direction is parallel to the moving direction of the steel sheet, the injection angle of the second nozzle is smaller than the injection angle of the first nozzle, the injection pressure of the second nozzle is 30% to 60% of the injection pressure of the first nozzle And the height of the second nozzle is 50% or more and 90% or less of the height of the first nozzle, and the height of the first nozzle and the second nozzle is the height at the surface of the steel sheet.

Description

Descaling Equipment {DESCALER}

Embodiments relate to a descaling device.

In general, molten metal dissolved in a blast furnace or an electric furnace of a steel mill is injected into a mold to produce a rolled material (steel sheet) having a predetermined shape. The steel sheet is then heated in a furnace and rolled according to product characteristics.

When the steel sheet heated above the recrystallization temperature in the furnace reacts with oxygen in the atmosphere, an oxide film, that is, a scale, is formed on the surface thereof. Since the scale causes a defect in rolling, the scale is removed before the rolling operation, which is called a descale operation.

In general, descale operations are performed by a descaling device (descaler).

The descaling apparatus is installed in a conveying apparatus (for example, a table roller, etc.) which draws out and transports the heated steel sheet, and removes the scale by spraying high pressure cooling water on the surface of the steel sheet conveyed along the conveying apparatus.

1A and 1B are views showing a conventional descaling apparatus.

1A and 1B, a conventional descaling device is defined along a length direction of a high pressure water jet header 4 and a high pressure water jet header 4 which are respectively disposed on an upper part and a lower part of a steel sheet 1 to be transferred. It comprises a plurality of nozzles 2 spaced apart at intervals.

The nozzle 2 receives the high pressure water from the high pressure water jet header 4 and injects the high pressure water 5 to the surface of the steel sheet 1. At this time, each nozzle (2) is disposed at a predetermined interval (p) along the longitudinal direction of the high-pressure water jet header 4, and spaced apart from the steel sheet (1) by a predetermined distance (d) to the surface of the steel sheet (1) High pressure water (5) is to be injected.

The nozzle 2 is usually provided with a fan type linear nozzle. Accordingly, the high pressure water 5 extends in the width direction (the direction perpendicular to the conveying direction of the steel sheet) of the steel sheet 1 as it approaches the surface of the steel sheet 1, and has a thin line shape having a thickness of usually 2 to 20 mm. Is sprayed on. At this time, the high pressure water 5 to be sprayed has a certain spray angle (A) according to the width and shape of the outlet of the nozzle (2).

The high pressure water 5 injected by the nozzle 2 collides with the steel plate 1 to remove the scale attached to the surface of the steel plate 1. At this time, the high pressure water 5 is sprayed to have a predetermined lead angle (B) and the surface of the steel sheet 1, to prevent the removed scale from flowing back to the steel sheet (1) side.

That is, since the high pressure water 5 is obliquely sprayed by the set lead angle B, the scale removed by the high pressure water 5 does not fall back to the already descaled region, and the outside of the steel sheet 1 Can be moved to.

In addition, the high pressure water 5 is sprayed to have an offset angle C inclined with respect to the width direction of the steel plate 1, avoiding interference between the high pressure water 5, and removing the removed scale It will be exported outside of (1). In addition, in the high pressure water 5, an overlap region O is set to ensure sufficient collision pressure for removing the scale.

However, due to the lead angle B, the offset angle C, and the overlap region O, the collision pressure nonuniformity of the high pressure water 5 and the cooling deviation of the steel sheet 1 occur.

2A and 2B are diagrams for explaining the impingement pressure unevenness and cooling deviation of the high pressure water 5 due to the conventional descaling apparatus.

Referring first to FIG. 2A, when the high-pressure water 5 sprayed from each nozzle 2 has an offset angle C with respect to the width direction of the steel plate 1, the steel plate 1 according to the arrangement of the nozzle 2. The high pressure water 5a of the front end and the high pressure water 5b of the rear end exist based on this coming direction.

Since the high-pressure water 5 is injected in an oblique state to have a predetermined lead angle B as described above, the water 5c that has been thrown off after colliding with the steel plate 1 is sheared at the overlap region O. It interferes with the high pressure water 5 at the rear stage.

In particular, when the lead angle B is formed in the same direction as that of FIG. 1A, the water 5c splashed by the high pressure water 5b at the rear end interferes with the high pressure water 5a at the front end.

Since such interference mainly occurs in the overlap region O, the collision pressure of the high-pressure water 5 in the overlap region O is reduced.

In addition, as shown in Figure 2b, the longitudinal axis of the high-pressure water jet header 4 is arranged to match the width direction of the steel sheet 1, the lead angle (B) is set so that the high-pressure water (5) is a steel sheet ( When the offset angle (C) is set for each nozzle (2) in the state of being sprayed obliquely with respect to the surface of 1), the distance between the high-pressure water (5) on both sides of the spray pattern of the nozzle (2) to contact the steel sheet ( 5d, 5e) will be different. Due to this distance difference, even in the case of the high-pressure water 5 sprayed from the same nozzle 2, the collision pressure is bound to vary depending on the area.

Such impingement pressure non-uniformity may cause a deviation even in the cooling of the steel sheet 1, and the cooling deviation may also occur due to the multiple injection of the high pressure water 5 in the overlap region O.

The impingement pressure nonuniformity and the cooling deviation as described above cause the quality of the steel sheet 1 itself by causing strips and material deviations for each region, which appear as the scale of the surface of the steel sheet 1 is partially removed. In addition, when rolling the steel plate 1 which is not completely descaled, there exists a problem that surface quality falls.

The embodiment provides a scale removing apparatus with improved scale peeling performance.

Descaling apparatus according to an embodiment of the present invention, the first injection device including a plurality of first nozzles for injecting a fluid toward the steel sheet; And a second injection device including a plurality of second nozzles for injecting particles toward the fluid, wherein the plurality of second nozzles are spaced apart from the plurality of first nozzles in a first direction. The direction is a direction parallel to the moving direction of the steel sheet.

The injection pressure of the second nozzle may be smaller than the injection pressure of the first nozzle.

The injection pressure of the second nozzle may be 30% to 60% of the injection pressure of the first nozzle.

The height of the second nozzle may be 50% or more and 90% or less of the height of the first nozzle, and the height of the first nozzle and the second nozzle may be the height of the surface of the steel sheet.

An angle formed by the first virtual line extending the first nozzle and the second virtual line extending the second nozzle may be 30 degrees to 90 degrees.

The particle may comprise sodium hydrogen carbonate.

The first nozzle may have a first spray angle, and the second nozzle may have a second spray angle.

An injection angle of the second nozzle may be smaller than an injection angle of the first nozzle.

The plurality of first nozzles may have a first offset angle, and the first offset angle may be an angle at which the discharge port of the first nozzle is inclined based on the width direction of the steel sheet.

The plurality of second nozzles may have a second offset angle, and the second offset angle may be an angle at which the discharge port of the second nozzle is inclined based on the width direction of the steel sheet.

The first offset angle and the second offset angle may be symmetrical with respect to the width direction of the steel sheet.

The fluid mixed with the particles may have the first offset angle when contacting the steel sheet.

In another embodiment of the present invention, the first injection device including a plurality of first nozzles for injecting a fluid toward the steel sheet; A second injection device including a plurality of second nozzles for injecting particles toward the fluid; And it may include a controller for controlling the second injection value according to the type of the steel sheet.

The plurality of second nozzles may be spaced apart from the plurality of first nozzles in a first direction, and the first direction may be parallel to a moving direction of the steel sheet.

The controller may control whether the particles are sprayed and the amount of spraying according to the type of the steel sheet.

According to an embodiment, the descaling performance may be improved.

In addition, it is possible to solve the problem of reducing the collision pressure of the overlap area, the problem of deterioration of the sheet flow due to the cooling deviation and the supercooling.

Various and advantageous advantages and effects of the present invention are not limited to the above description, and will be more readily understood in the course of describing specific embodiments of the present invention.

1A and 1B show a conventional descaling device
2a and 2b is a view for explaining the problem of the conventional descaling device
3 is a diagram of an apparatus for removing a scale according to an embodiment of the present invention;
4 is a side view of FIG. 3
5 and 6 are views for explaining the process of mixing the high pressure water and particles
7 is a photograph of the scale removed by controlling the injection pressure of the first nozzle to 3Mpa
8 is a graph measuring scale peelability by adjusting the injection pressure of the first nozzle;
9 is a view for explaining an arrangement angle of the first nozzle and the second nozzle;
FIG. 10 is a photograph in which the first nozzle and the second nozzle are adjusted to a reference angle and the scale is removed.
11 is a graph measuring scale peelability by adjusting an arrangement angle of a first nozzle and a second nozzle;
12 is a view for explaining the height relationship between the first nozzle and the second nozzle;
FIG. 13 is a photograph in which the heights of the first nozzle and the second nozzle are set to the first stage and the scale is removed
14 is a graph measuring scale peelability by adjusting heights of a first nozzle and a second nozzle;
15 and 16 illustrate offset angles of a first nozzle and a second nozzle.

As the inventive concept allows for various changes and numerous embodiments, particular embodiments will be illustrated and described in the drawings. However, this is not intended to limit the present invention to specific embodiments, it should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope of the present invention.

The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limiting of the invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In this application, the terms "comprise" or "have" are intended to indicate that there is a feature, number, step, operation, component, part, or combination thereof described in the specification, and one or more other features. It is to be understood that the present invention does not exclude the possibility of the presence or the addition of numbers, steps, operations, components, components, or a combination thereof.

Unless defined otherwise, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art. Terms such as those defined in the commonly used dictionaries should be construed as having meanings consistent with the meanings in the context of the related art, and are not construed in ideal or excessively formal meanings unless expressly defined in this application. Do not.

Hereinafter, exemplary embodiments will be described in detail with reference to the accompanying drawings, and the same or corresponding components will be given the same reference numerals regardless of the reference numerals, and redundant description thereof will be omitted.

3 is a view of a scale removing apparatus according to an embodiment of the present invention, Figure 4 is a side view of FIG.

3 and 4, a scale removing apparatus according to an embodiment includes a first injection device including a plurality of first nozzles 11 for injecting a fluid (high pressure water) 2 toward the steel plate 1 ( 10) and a second injection device 20 including a plurality of second nozzles 21 for injecting particles 3 toward the fluid.

The steel sheet 1 may move along the first direction D1. The first injector 10 may be disposed on both surfaces of the steel sheet 1, respectively. The first injector 10 supplies a first nozzle 11 for supplying the high pressure water 2 to the plurality of first nozzles 11 and the first nozzles 11 for spraying the high pressure water 2 toward the surface of the steel sheet 1. It may include a header (12). The first header 12 may be connected to an external high pressure water tank.

The first injection device 10 may use an existing high pressure water injection device as it is. As described with reference to FIGS. 1 and 2, the plurality of first nozzles 11 may have an injection angle, a lead angle, an offset angle, and the like.

The second spray device 20 supplies a plurality of second nozzles 21 for injecting the particles 3 toward the high pressure water 2 and a second header 22 for supplying the particles 3 to the second nozzles 21. ) May be included. The second header 22 may be connected to an external particle supply device.

The plurality of second nozzles 21 may be spaced apart from the plurality of first nozzles 11 in the first direction D1. In this case, the first direction may be a direction parallel to the moving direction of the steel sheet 1. The plurality of second nozzles 21 may be disposed in front of the plurality of first nozzles 11 in the first direction.

The particles 3 injected from the second nozzle 21 may be mixed with the high pressure water 2 injected from the first nozzle 11. According to the embodiment, since the high pressure water 2 and the particle 3 are mixed and injected, the amount of impact applied to the surface of the steel sheet 1 can be improved compared to the conventional technology of spraying only the high pressure water.

The controller 40 may control on / off, injection pressure, etc. of the first injection device 10 and the second injection device 20. The controller 40 may obtain the information of the steel sheet 1 and control the first spraying device 10 and the second spraying device 20 according to the type of the steel sheet 1.

For example, the controller 40 may not output the injection control signal to the second injection device 20 when the scale peelability of the steel sheet 1 is good and a large collision pressure is not required. That is, the scale may be removed by spraying only the high pressure water of the first injection device 10.

In addition, when the scale peeling property of the said steel plate 1 is not good, a control signal may be output to the 2nd injection device 20 so that the injection amount of the particle 3 may be improved. In this case, information about the various steel sheets 1 may be stored in a memory.

According to an embodiment, the particles 3 may be mixed by spraying the high pressure water 2 injected into the air. Therefore, the high pressure water 2 can maintain the offset angle, the spray angle, the lead angle, and the like, even in the state in which the particles 3 are mixed.

In this case, the number of first nozzles 11 and the number of second nozzles 21 may be the same. Therefore, since the particles 3 are sprayed equally to the high pressure water 2 sprayed from the plurality of first nozzles 11, uniformity may be improved.

If the high pressure water 2 and the particle 3 are mixed and sprayed in one nozzle in advance, excessive pressure may be generated inside the nozzle by the mixing process. Therefore, there is a problem that it is difficult to control the injection angle and the offset angle of the high pressure water 2 finally injected. In addition, there is a problem that the wear is accelerated in the mixing process to shorten the life.

Referring to FIG. 5, the particles 3 injected from the second nozzle 21 may be sprayed onto the surface of the high pressure water 2 and then sprayed toward the surface of the steel sheet 1 together with the high pressure water 2. have. In this case, the injection pressure of the second nozzle 21 may be 30% to 60% of the injection pressure of the first nozzle 11.

If the injection pressure is less than 30%, there is a problem that the particles (3) do not penetrate into the inside of the high pressure water (2) and bounces off, and when the injection pressure is greater than 60%, the dense particles (3) are high pressure water There is a problem of passing through (2). That is, when the injection pressure of the second nozzle 21 is 30% to 60% of the injection pressure of the first nozzle 11, most of the particles 3 may be mixed with the high pressure water 2 to improve the scaling performance. have.

The high pressure water 2 may increase the moving speed of the particle 3 by transmitting momentum to the particle 3 introduced into the inside. The faster particles 3 may collide with the scale on the surface of the steel sheet 1. Therefore, since the impact pressure larger than the pressure generated by the collision of water is formed, the scale peelability can be improved.

Referring to FIG. 6, the particles 3 injected into the high pressure water 2 may be evenly spread along the injection region of the high pressure water 2. Therefore, the uniform impact amount can be transmitted to the region where the high pressure water 2 is in contact with the steel sheet 1. The spraying area P1 of the particle 3 may be circular, but is not limited thereto, and the spraying area P1 may be elliptical.

Existing descaling apparatus has been used to increase the collision pressure by increasing the injection pressure. However, the present embodiment can increase the collision pressure by using particles having a high density. Therefore, the injection pressure can be reduced and the amount of water used can also be reduced.

The particle 3 may remain inside the scale after generating a crack on the scale surface. The remaining particles 3 may be thermally decomposed to generate gas (eg, carbon dioxide). The gas can expand to promote the growth of cracks. Thus, an additional scale peeling effect can be obtained.

The particle 3 may comprise sodium hydrogen carbonate. However, the present invention is not limited thereto, and any particle capable of increasing the physical impact by increasing the density or promoting crack growth by pyrolysis can be used without limitation.

Sodium hydrogen carbonate may be sodium carbonate, water, carbon dioxide by the thermal decomposition of the hot-rolled steel sheet 1 in the following thermal decomposition reaction formula.

Scheme 1

₂ NaHCO ₃ = Na ₂ CO ₃ + H ₂ O + CO ₂

In addition, since sodium carbonate is very water soluble in water, there is an advantage that does not require a separate after-treatment equipment. For example, a separate washing process may be performed, or may be dissolved in the remaining water remaining on the steel sheet.

According to the embodiment, since the chemical treatment is accompanied by the physical treatment, the header pressure may be lowered, and the problem of deterioration of the mail flow due to cooling may be solved by reducing the amount of water used. In addition, it is possible to improve the surface quality of the steel sheet 1 by removing the scale that is not removed due to lack of collision pressure.

7 is a photograph of the scale removed by controlling the injection pressure of the first nozzle to 3Mpa, Figure 8 is a graph measuring the scale peelability by adjusting the injection pressure of the first nozzle.

7 and 8, it can be seen that the erosion depth t and the width W1 and W2 increase as the injection pressure of the first nozzle 11 increases at 3Mpa. However, it can be seen that there is little improvement in the erosion depth t and the widths W1 and W2 when the injection pressure is 5 Mpa or more.

The experimental result is a value on the manufactured small module. In practice, the injection pressure of the first nozzle 11 arranged in the line may be 10Mpa to 30Mpa. If it is smaller than 10 Mpa, it is difficult to have sufficient impact pressure, and if it is larger than 30 Mpa, the efficiency does not increase.

FIG. 9 is a view illustrating an arrangement angle of the first nozzle and the second nozzle, FIG. 10 is a photograph in which the first nozzle and the second nozzle are adjusted to a reference angle and the scale is removed, and FIG. 11 is the first nozzle. It is a graph which measured the scale peelability by adjusting the arrangement angle of a nozzle and a 2nd nozzle.

Referring to FIG. 9, an arrangement angle θ1 formed by the first virtual line extending the first nozzle 11 and the second virtual line extending the second nozzle 21 may be 30 degrees to 90 degrees. 30 degrees or less, it is sprayed in the same direction there is a problem that the particles 3 do not dig into the inside of the high-pressure water 2, most of them bounce out. If it is 90 degrees or more, there is a problem of causing a large interference with the advancing direction of the high-pressure water 2 to reduce the collision force.

Referring to FIGS. 10 and 11, the experiment adjusts the placement angle θ1 of the first nozzle 11 and the second nozzle 21 by 30 degrees, and then adjusts the angle to the erosion depth t and the width W1. , W2) was tested. As the arrangement angle θ1 of the first nozzle 11 and the second nozzle 21 decreases (+ direction), the erosion effect decreases, and the effect increases as the placement angle θ1 increases.

FIG. 12 is a view for explaining the height relationship between the first nozzle and the second nozzle, FIG. 13 is a picture in which the heights of the first nozzle and the second nozzle are set to the first stage and the scale is removed, and FIG. 14 is the first nozzle. It is a graph which measured scale peelability by adjusting the height of a nozzle and a 2nd nozzle.

Referring to FIG. 12, the height H2 of the second nozzle 21 may be 50% or more and 90% or less of the height H1 of the first nozzle 11. If it is higher than 90%, there is a problem of being thrown out by the strong injection pressure. In addition, when less than 50%, there is a problem that some empty space is generated in the triangular area where the high pressure water is injected, so that the particles 3 are not mixed or the particles 3 pass through the high pressure water 2.

Referring to FIGS. 13 and 14, the experiment was conducted in a fifth step while lowering the height by 10%. The first stage is a point where the second nozzle 21 is 90% of the height of the first nozzle 11. As the height of the second nozzle 21 decreases, the erosion depth t and the widths W1 and W2 decrease.

15 and 16 are diagrams for describing offset angles of the first nozzle and the second nozzle.

Referring to FIG. 15, the first nozzle 11 sprays the high pressure water 2 at the first spray angle θ2 on the plane of the steel sheet, and the second nozzle 21 particles at the second spray angle θ3. (3) can be sprayed.

Since the particles 3 are advantageously sprayed on the upper end of the high-pressure water 2, the second spray angle θ3 may be smaller than the first spray angle θ2. However, the present invention is not necessarily limited thereto, and the second injection angle θ3 may be appropriately adjusted according to the distance between the first nozzle 11 and the second nozzle 21.

The high pressure water 2 injected from the first nozzle 11 may have a first offset angle θ5 inclined in the width direction D2 of the steel sheet. The first offset angle θ5 may be an angle formed between the end surface 2c of the high pressure water 2 and the width direction D2. The offset angle can be adjusted by rotating the discharge port of the nozzle clockwise. At this time, the discharge port of the nozzle may be formed long in the width direction of the steel sheet. Therefore, if there is no offset angle, the extension direction of the discharge port of the nozzle and the width direction of the steel sheet may be parallel.

In the embodiment, since the particles 3 are mixed with the high pressure water 2 in the air, the high pressure water 2 may have a first offset angle θ5 even when the particles 3 are mixed. The first offset angle θ5 may have an appropriate range to increase the scale removing performance.

Accordingly, the second nozzle 21 may also have a second offset angle θ4. The second offset angle θ4 may be an angle formed between the end surface 3c of the sprayed particle and the width direction D2. Since the first nozzle 11 has the first offset angle θ5, when the second nozzle 21 sprays the particles horizontally in the width direction, the spraying distances are different from both side surfaces 3a and 3b so that the uniform particles are uniform. (3) Mixing may be difficult.

The first offset angle θ5 and the second offset angle θ4 may be the same and may be symmetrical with respect to the width direction of the steel sheet 1. According to such a structure, the distance which the particle 3 is sprayed on the surface of the high pressure water 2 becomes uniform, and the collision pressure may become uniform.

The second offset angle θ4 of the second nozzle 21 may be rotated opposite to the first offset angle θ5 of the first nozzle 11. For example, if the first nozzle 11 is rotated 15 degrees clockwise, the second nozzle 21 may be rotated 15 degrees counterclockwise. Here, 15 degrees in the clockwise direction may be an angle rotated clockwise about the axis of the nozzle on the steel plate after the nozzle is disposed perpendicular to the steel plate.

That is, if there is no offset control, one side 2b of the high pressure water 2 may be separated from one side 3b of the particle 3 by the first offset angle θ5. On the contrary, the other side 2a of the high pressure water 2 may be closer to the other side 3a of the particle 3. However, according to the embodiment, the second offset angle θ4 of the second nozzle 21 may be set opposite to the first nozzle 11 to reduce the difference between the left and right separation distances.

However, the present invention is not limited thereto, and as shown in FIG. 16, the second offset angle of the second nozzle 21 may be rotated in the same direction as the first offset angle of the first nozzle 11 to reduce the difference between the left and right separation distances. . That is, the first offset angle and the second offset angle may be the same. For example, if the first nozzle 11 is rotated 15 degrees counterclockwise around the axis, the second nozzle 21 can also be rotated 15 degrees counterclockwise around the axis. Therefore, the high pressure water injected from the nozzle and the end surfaces 2c and 3c of the particles may be parallel to each other. As a result, the deviation caused by the difference in length between the two jets can be reduced.

2: high pressure water
3: particles
10: first injection device
11: Nozzle 1
12: first header
20: second injection device
21: first nozzle
22: second header
40: controller

Claims (10)

In the apparatus for removing the scale formed on the surface of the steel sheet,
A first injection device comprising a plurality of first nozzles for injecting fluid toward the steel sheet; And
A second injection device including a plurality of second nozzles for injecting particles toward the fluid;
The plurality of second nozzles are spaced apart from the plurality of first nozzles in a first direction,
The first direction is parallel to the moving direction of the steel sheet,
The spray angle of the second nozzle is smaller than the spray angle of the first nozzle,
The injection pressure of the second nozzle is 30% to 60% of the injection pressure of the first nozzle,
The height of the second nozzle is 50% or more and 90% or less of the height of the first nozzle,
And a height of the first nozzle and the second nozzle is a height at the surface of the steel sheet. The method of claim 1,
The angle between the first virtual line extending the first nozzle and the second virtual line extending the second nozzle is 30 to 90 degrees. The method of claim 1,
The particle removing device comprises sodium hydrogen carbonate. The method of claim 1,
The plurality of first nozzles have a first offset angle,
And the first offset angle is an angle at which the discharge port of the first nozzle is inclined with respect to the width direction of the steel sheet. The method of claim 4, wherein
The plurality of second nozzles have a second offset angle,
And the second offset angle is an angle at which the discharge port of the second nozzle is inclined with respect to the width direction of the steel sheet. The method of claim 5,
And the first offset angle and the second offset angle are symmetrical with respect to the width direction of the steel sheet. The method of claim 6,
And the fluid in which the particles are mixed has the first offset angle upon contact with the steel sheet. In the apparatus for removing the scale formed on the surface of the steel sheet,
A first injection device comprising a plurality of first nozzles for injecting fluid toward the steel sheet;
A second injection device including a plurality of second nozzles for injecting particles toward the fluid; And
A controller for controlling the second injection value according to the type of the steel sheet,
The spray angle of the second nozzle is smaller than the spray angle of the first nozzle,
The injection pressure of the second nozzle is 30% to 60% of the injection pressure of the first nozzle,
The height of the second nozzle is 50% or more and 90% or less of the height of the first nozzle,
And a height of the first nozzle and the second nozzle is a height at the surface of the steel sheet. The method of claim 8,
The plurality of second nozzles are spaced apart from the plurality of first nozzles in a first direction,
And said first direction is parallel to the moving direction of said steel sheet. The method of claim 8,
The controller removes the scale according to the type of the steel sheet, and the descaling apparatus for controlling the injection amount.

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