JP7052931B2 - Secondary cooling method for continuously cast slabs - Google Patents

Description

The present invention relates to a secondary cooling method for continuously cast slabs.

A general method for manufacturing a continuously cast slab will be described with reference to FIGS. 4 and 5 by taking a vertical bending type continuous casting facility as an example.

The molten steel injected into the mold 3 from the tundish (not shown) is primarily cooled by the mold 3 to form a flat plate-shaped slab 5 forming a solidified shell, which is flat and descends from the vertical band 7 to the curved band 11. Proceed to. Then, at the bent portion 9 on the entry side of the curved band 11, the slab 5 is bent while being guided by a plurality of rolls (not shown) so as to maintain a constant radius of curvature.

After that, the straightening portion 13 is bent back (corrected) while gradually increasing the radius of curvature, and when the straightening portion 13 is exited, the slab 5 becomes flat again and proceeds to the horizontal band 15. After solidification is completed in the horizontal band 15, the slab 5 is cut to a predetermined length by the gas cutting machine 17 installed on the outlet side of the continuous casting machine 1.

After leaving the mold 3, the slab 5 is secondary using a water spray (water one-fluid spray or water-air two-fluid mixed mist spray) to complete solidification from the vertical band 7 to the horizontal band 15 to the center. Cooling is being carried out.

Normally, in the secondary cooling, the strength of the shell is secured by injecting a large flow rate of water in the vertical band 7 directly under the mold to perform strong cooling. On the curved zone 11 and later, on the contrary, the cooling is weakened, and the surface temperature is raised (reheated) by heat conduction from the high temperature portion inside. Then, the surface temperature of the straightening portion 13 is adjusted to be equal to or higher than the embrittlement temperature range to avoid the occurrence of lateral cracking.

In addition, depending on the type of steel, the casting speed is increased for the purpose of improving production efficiency, straightening is performed while the center of the slab is not solidified, and solidification is completed by performing strong cooling in the horizontal band 15 at the end of continuous casting. The method of making it is also adopted. When the cooling capacity is uneven in these strong cooling zones, a temperature deviation occurs on the surface of the slab, and the thermal stress caused by the temperature deviation causes surface cracking. In addition, when strong cooling is performed at the end of the continuous casting process, the solidification completion position at the center of the slab becomes non-uniform due to uneven cooling, which affects the internal quality. Therefore, in order to stably realize high cooling capacity in the strong cooling zone, it is desirable that the cooling water maintains the nucleate boiling state on the surface of the slab.

A plurality of guide rolls 19 are installed in the secondary cooling zone, and the cooling water is sprayed into the gaps between these guide rolls 19 (see FIG. 5).

Observing the injection state of the cooling water (example of the horizontal band 15) from the short side of the slab, as shown in FIG. 5, in the cooling by the spray nozzle 21, the cooling water is directly injected onto the surface of the slab. , The contact portion with the guide roll 19 and the guide roll 19 block the cooling water to generate a non-direct irradiation region Y in which the cooling water does not directly hit.

In the direct irradiation region X, the cooling water is continuously supplied from the nozzle, so that the high cooling capacity is maintained. As a result, when the slab moves from the direct irradiation region X to the non-direct irradiation region Y, the slab surface temperature rises significantly (reheat). At this time, the slab enters the direct irradiation region X between the next rolls and does not quickly reach the nucleate boiling state, the boiling state changes unstable in the casting direction, and a large temperature fluctuation occurs. Further, since the same unstable transition of the boiling state can occur in the slab width direction, a large temperature difference occurs in the slab width direction as well. Due to these temperature fluctuations, thermal stress is generated on the surface of the slab, causing surface cracks, and the solidification completion position becomes non-uniform in the slab width direction, resulting in deterioration of internal quality and other quality problems. ..

As a method for improving the uniformity of the local cooling capacity in the secondary cooling of the continuous casting process as described above, for example, Patent Document 1 defines the ratio between the direct irradiation range length of the water spray in the casting direction and the distance between the guide rolls. , A technique for improving the uniformity of cooling capacity has been proposed.

Further, Patent Document 2 proposes a technique of providing a refrigerant guide plate close to the slab surface between guide rolls to distribute cooling water to the slab surface.

Japanese Unexamined Patent Publication No. 2003-136205 Japanese Unexamined Patent Publication No. 2018-15781

In the technique of Patent Document 1, the cooling uniformity in the casting direction is improved by widening the area of the direct irradiation portion of the spray water, but there is no mention of the boiling state in the direct irradiation portion, and the above strong cooling conditions are used. It is unknown whether stable nuclear boiling can be achieved and maintained.

Further, although the spray pattern of the spray water to be used in the width direction of the slab is not described, it can be presumed to have a two-row elliptical shape. At this time, since the spray width and the water amount density of the end portion in the width direction of the spray water are lower than those of the central portion, the target uniformity of the cooling capacity cannot be realized. In addition, it is preferable that the spray nozzle to be used has a plurality of injection ports, but the nozzle shape becomes complicated, the risk of nozzle clogging increases, and there is a high possibility that the ideal spray thickness cannot be secured.

On the other hand, in the technique of Patent Document 2, it is stated that a non-boiling to nucleate boiling state can be realized by bringing the refrigerant guide plate close to the slab surface and forming a fast-flowing water film between the guide plate and the slab surface.

However, since the guide plate and the surface of the slab are very close to each other, there is a high risk of collision, and it is possible that scratches may remain on the surface of the slab or the guide plate may be damaged.

In addition, since a water supply port with a small diameter is installed near the slab, even if it is not collided or damaged, it may be clogged with a scale piece or the like when it is used continuously. If the formation of the water film becomes non-uniform due to damage to the guide plate or clogging of the holes, the nucleate boiling state cannot be realized and the cooling becomes non-uniform. Therefore, it is important to maintain the soundness of the equipment to ensure the uniformity of cooling capacity, but since the guide plate is installed so as to close the gap between the rolls, it cannot be easily attached and detached at the time of inspection. .. Therefore, a large equipment management cost is required to perform uniform cooling as claimed.

As described above, the spraying conditions of the water spray that can stably realize and maintain the nucleate boiling state in both the casting direction and the slab width direction have not been clarified.

The present invention has been made to solve such a problem, and stably realizes and maintains the nucleate boiling state in both the casting direction and the width direction of the slab, and as a result, the equipment maintenance is easy and the cooling capacity is increased. It is an object of the present invention to provide a secondary cooling method for continuously cast slabs which can enhance uniformity.

In order to solve the above problems, the present invention has the following features.
[1] Between guide rolls having a radius d (unit: mm) installed at an inter-axis distance P (unit: mm) in all or part of the casting direction of the horizontal zone in the secondary cooling zone of the continuous casting machine. In addition, it is a secondary cooling method for continuously cast slabs in which spray nozzles having a square injection pattern are arranged in the slab width direction to cool the slabs.
The distance L (unit) between points A and B, which are two points where the water density of the cooling water sprayed from each of the spray nozzles is 50% of the maximum value of the water density in the casting direction. : Mm) and the inter-axis distance P satisfy the following equation (1) and
A method for secondary cooling of continuously cast slabs, which comprises cooling while maintaining the nucleate boiling state in the range from the point A to the point B.

L / P ≧ 0.70 ・ ・ ・ (1)
[2] The angle θ (unit: degree) formed by the straight line connecting the nozzle injection port of the spray nozzle and the point A and the straight line connecting the nozzle injection port and the point B satisfies the equation (2). , Secondary cooling of the continuously cast slab according to [1], wherein the nozzle height h (unit: mm), which is the height of the nozzle injection port from the slab, satisfies the formula (3). Method.

180-4tan ^-1 [3P / (20d)] ≤θ≤100 ... (2)
7P / [20tan (θ / 2)] ≦ h ≦ [P-2dtan {(180-θ) / 4}] / [2tan (θ / 2)] ... (3)
[3] The water volume density of the cooling water sprayed by each of the spray nozzles is 400 (L / m ² ) / min or more and 2000 (L / m) per unit surface area of the slab in the cooling section by the spray nozzles. ² ) The secondary cooling method for continuously cast slabs according to [1] or [2], which is characterized by having a value of 2) / min or less.

According to the present invention, in the secondary cooling zone of the continuous casting machine, spray nozzles having a square injection pattern are arranged in the width direction of the slab, and the maximum value of the distribution of the amount of cooling water sprayed from each spray nozzle in the casting direction is 50. The guide roll and the spray nozzle are arranged so that the relationship between the distance L (unit: mm) connecting the two points A and B, which is%, and the distance P between the axes satisfies L / P ≧ 0.70. By cooling while maintaining the nuclear boiling state in the range of points A to B, it becomes possible to stably realize and maintain the nuclear boiling in a wide range of the slab surface, and the slab is stably of high quality. Will be able to be manufactured.

FIG. 1 is an explanatory diagram of an injection pattern and a flow rate distribution of a spray nozzle according to an embodiment of the present invention. FIG. 2 is an explanatory diagram illustrating the arrangement relationship between the spray nozzle and the guide roll according to the embodiment of the present invention. FIG. 3 is an explanatory diagram of the injection pattern and the flow rate distribution of the spray nozzle of Comparative Example 1 in the description of the embodiment. FIG. 4 is an explanatory diagram illustrating an outline of a conventional general continuous casting facility. FIG. 5 is an explanatory diagram of an arrangement of a guide roll and a spray nozzle and an injection state in a conventional general continuous casting facility.

The secondary cooling method for the continuously cast slab according to the present embodiment is continuous casting composed of a vertical band 7, a bent portion 9, a curved band 11, a straightening section 13, and a horizontal band 15 from the upstream side in the casting direction. A radius d installed with an inter-axis distance P (unit: mm) in a part of the casting direction section of the horizontal band 15 in the secondary cooling zone of the machine 1 (see FIG. 4) or the entire casting direction section of the horizontal band 15. A method for secondary cooling of continuously cast slabs in which spray nozzles 21 having a square injection pattern are arranged in the slab width direction between the guide rolls 19 (unit: mm) to cool the slab 5. The relationship between the distance L (unit: mm) connecting the two points A and B, which is 50% of the maximum value of the water volume distribution in the casting direction of the cooling water sprayed from the spray nozzle 21, and the distance P between the shafts is the following equation (1). ) Is arranged so that the guide roll 19 and the spray nozzle 21 are arranged, and the cooling is performed while maintaining the nuclear boiling state in the range of points A to B.

L / P ≧ 0.70 ・ ・ ・ (1)
In this embodiment, as shown in FIG. 1, a spray nozzle 21 having a quadrangular injection pattern is used. The reason for using the spray nozzle 21 having such a quadrangular injection pattern is as follows.

When the spray nozzle 21 is arranged in the gap of the guide roll 19 to cool the slab surface, the shape of the portion where the slab surface is exposed (cooled surface) is elongated (long in the slab width direction and in the casting direction). It becomes a rectangle (short). In order to cover the range of the elongated rectangle as much as possible and to sprinkle the cooling water uniformly, it is preferable to arrange the spray nozzles 21 having the rectangular injection pattern side by side in the slab width direction. By doing so, the cooling water can be uniformly directly applied to the surface to be cooled without any gap, nucleate boiling can be uniformly achieved, and local reheat does not occur.

Further, when the widthwise water density distributions of the spray nozzles 21 adjacent to each other in the width direction of the slab are overlapped, the water density of the wrap portion is 50% or more and 100% or less of the maximum value of the water density when a single spray is sprayed. It is desirable to set the lap allowance of the injection regions of the adjacent spray nozzles 21 so as to be.

If the water density of the wrap portion is less than 50% of the maximum value, the water density of the wrap portion is insufficient and the nucleate boiling state is not reached at the time of cooling, and temperature unevenness occurs in the width direction. On the other hand, if it is larger than 100%, the lap range is too wide, so the cooling water of the adjacent spray nozzles 21 interferes with each other, and there is a concern that the water amount density distribution will not be as expected when actually spraying, and the cooling will be uneven. Will increase.

Further, in the present embodiment, the distance L (unit: mm) connecting the two points A and B, which is 50% of the maximum value of the water amount distribution in the casting direction of the cooling water sprayed from each spray nozzle 21, and the shaft distance. The guide roll 19 and the spray nozzle 21 are arranged so that the relationship of the distance P satisfies L / P ≧ 0.70.

The reason for arranging in this way is as follows.

When strong cooling is performed using nucleate boiling, the difference in cooling capacity between the direct and non-direct parts of the cooling water of the spray nozzle 21 becomes significantly large. Therefore, the temperature change between the direct and non-direct parts becomes large, which causes defects such as cracks. Further, when the flow rate of the cooling water is throttled, if the reheat in the non-directly exposed portion is too large, nucleate boiling is not quickly realized even in the directly exposed portion, which may cause temperature unevenness.

In this respect, if L / P ≧ 0.70, the region to be the non-directly shining part is narrow, and the cooling water flowing from the direct shining part to the non-directly struck part is sufficient enough not to hinder the cooling of the slab, so that the temperature unevenness is caused. Does not occur.

In addition, the cooling water that collides with the slab flows from the direct irradiation portion toward the surroundings. At this time, the flow in the casting direction is dammed by the gap between the guide roll and the slab, and a flow in the slab width direction is formed and drained. Therefore, when the water density is high and the range of the non-direct irradiation portion is too small, the flow during rolling and the direct irradiation portion may interfere with each other. Therefore, it is desirable that the relationship between the distance L connecting the two points A and B and the distance P between the axes satisfies L / P ≦ 0.90.

Further, since the spray pattern of the spray nozzle 21 of the present embodiment is quadrangular, the spray thickness does not change in the slab width direction, and the L / P can be kept within the specified range on the entire width direction.

In this regard, when the spray pattern is elliptical as in the spray nozzle of Patent Document 1, the spray thickness of the direct irradiation portion becomes small at the end portion in the slab width direction, and the L / P value is obtained in the entire width direction. Is difficult to keep within the specified range.

Further, in the present embodiment, it is a requirement to realize and maintain the nucleate boiling state in order to carry out stable strong cooling.

In addition to the length of the direct irradiation part of the cooling water, the water density is also an important factor in realizing and maintaining this nucleate boiling state. If the water density is not sufficient, even if the slab 5 enters the cooling water direct irradiation portion, the nucleate boiling state is not immediately reached, and the temperature drops due to the membrane boiling and then the nucleate boiling occurs.

At this time, the cooling rate differs depending on the width direction position (center of slab width, slab corner), and the transition point from film boiling to nucleate boiling is affected by the surface texture, so the starting point of nucleate boiling is the slab. It varies in the width direction. Therefore, a large temperature deviation occurs in the width direction, surface cracking due to thermal stress and variation in the internal solidification completion position in the width direction occur, which causes surface and internal defects.

Therefore, as a result of examining the water density in which the nucleate boiling state is quickly realized and maintained in the cooling water direct irradiation part, the inventors have found that 400 (L / m ² ) / min or more is required.

The reason why the water density is required to be 400 (L / m ² ) / min or more is as follows.

When the slab surface temperature is high, the cooling water becomes a film boiling state on the slab surface and a steam film is generated. When the jet water density is less than 400 (L / m ² ) / min, the water density is small, so that the steam film does not collapse immediately due to the collision of cooling water, and the film boiling state is maintained until the slab surface temperature drops to some extent. To. After that, the surface temperature drops, and when the transition from membrane boiling to nucleate boiling occurs, cooling progresses rapidly.

Therefore, once the surface temperature varies depending on the slab surface position, the boiling state also differs depending on the slab surface position, and as a result, the temperature unevenness further increases.

On the other hand, when the water volume density is 400 (L / m ² ) / min or more, even if a steam film is generated on the surface of the slab, the steam film immediately collapses due to the collision of the cooling water, so that the nucleate boiling state is rapidly entered. Therefore, the boiling state is made uniform depending on the position of the slab surface, and temperature unevenness does not occur.

On the other hand, if nucleate boiling is realized, the cooling by boiling becomes dominant, and the dependence of the cooling capacity on the water density becomes smaller. Therefore, if the water volume density is higher than 2000 (L / m ² ) / min, the cooling capacity cannot be expected to improve significantly, and the total amount of cooling water used becomes excessive and the capital investment of water treatment equipment increases. It is appropriate that the water content density of the water is in the range of 400 (L / m ² ) / min or more and 2000 (L / m ² ) / min or less.

However, in the present invention, the water density is in the range of 400 (L / m ² ) / min or more and 2000 (L / m ² ) / min or less depending on the operating conditions (slab surface temperature, collision pressure of cooling water, etc.). It is not essential to do so, and the water density should be set so that the nucleate boiling state occurs.

For example, if the specified water density cannot be achieved due to some reason such as equipment abnormality such as water leakage from the piping and the nucleate boiling state is not reached immediately after entering the strong cooling section, the boiling state should be monitored. It is necessary to increase the amount of water while doing so to surely achieve and maintain the nucleate boiling state.

Here, when the cooling water comes into contact with the surface of the slab and boils, it vaporizes into steam. Steam (water smoke) in which this steam condenses in the air is observed. Here, in the nucleate boiling state, the cooling water in contact with the surface of the slab foams violently and a large amount of water vapor is generated, so that the amount of water smoke generated increases. On the other hand, in the membrane boiling state, the amount of water vapor and water smoke generated is also small because the boiling cooling water is less foamed.

Therefore, a camera is installed in each section to monitor the amount of water smoke generated by visual observation or measurement by a transmissometer. By conducting an experiment in advance to obtain a threshold value for the amount of water smoke generated to distinguish between nucleate boiling and membrane boiling, and confirming whether or not the amount of water smoke generated exceeds the threshold value, the nucleate boiling state can be determined in a predetermined section. You can check if you have achieved it. Then, if the nucleate boiling state has not been achieved, the amount of cooling water is adjusted to be increased. This ensures that the nucleate boiling state can be achieved and maintained.

Further, in convection heat transfer including boiling, the fluid temperature and the solid temperature are locally equal at the point where they come into contact with each other. Since the temperature of water in a liquid state rises only to the boiling point under atmospheric pressure, it is considered that the surface temperature of the slab is also about 100 ° C. if nucleate boiling is realized. Therefore, the nucleate boiling state is achieved by measuring the temperature of the slab surface and the surrounding cooling water using a contact-type thermometer with a small probe and confirming that the temperature is stable at around 100 ° C. Can be confirmed if has been achieved. Then, if the nucleate boiling state has not been achieved, the amount of cooling water is adjusted to be increased. This ensures that the nucleate boiling state can be achieved and maintained.

As described above, in the present embodiment, in the region where strong cooling is performed in the secondary cooling zone, a water spray having a rectangular injection pattern is used, and the length of the cooling water direct irradiation portion between the guide rolls 19 is the roll interval. By setting the injection angle and injection height so that it is 70% or more of the above, and cooling while maintaining the nuclear boiling state in the cooling water direct irradiation part, it is possible to suppress large temperature fluctuations on the surface of the slab. It is possible to stably manufacture high-quality slabs 5 by preventing surface and internal defects such as surface cracks and non-uniform solidification completion positions.

The effect of this embodiment is demonstrated in Examples described later.

As shown in FIG. 2, the angle (injection angle) θ (unit: degree) formed by the straight line CA and the straight line CB with the center of the nozzle injection port as the point C is 100 degrees or less in order to maintain the uniformity of the water amount distribution. Is desirable.

Further, the distance L (hereinafter referred to as “direct irradiation portion length L”) connecting the two points A and B, which is 50% of the maximum value of the water amount distribution in the casting direction of the cooling water sprayed from the spray nozzle 21, is the formula (1). ), It is necessary to set the injection angle θ. Hereinafter, the conditions to be satisfied for the injection angle θ will be described.

As shown in FIG. 2, the relationship of the following equation (4) holds for the length of P / 2-L / 2 = Y (referred to as a non-direct portion).

Further, the injection angle θ needs to be set in a range in which the straight lines CA and CB do not come into contact with the guide roll 19. Therefore, when the straight line CA (or the straight line CB) circumscribes the guide roll 19, the following equation (5) holds for the triangle DAE.

From the above relationship, it is desirable to set the injection angle θ within the range of equation (2).

When the injection angle θ is determined so as to satisfy the equation (2), the range of the height h (unit: mm) from the slab surface is also determined in the same manner. This point will be described below.

Since the direct irradiation portion length L can be described by the equation (6) for a certain injection angle θ, by substituting this into the equation (1), the lower limit of the height h is expressed by the equation (7).

Further, since the upper limit of the height h is the position where the straight lines CA and CB come into contact with the guide roll 19, the equation (8) holds. Therefore, when the equation (6) is substituted into the equation (8) and deformed with respect to the height h, the upper limit of the height h is expressed as the equation (9). Therefore, the range of the height h is as shown in the equation (3).

By setting the injection angle θ and the injection height h of the spray nozzle 21 so as to satisfy the above equations (2) and (3), the size of the direct irradiation portion length L becomes 70% or more of the guide roll interval P, and the direct irradiation portion The range can be wide enough to prevent local fluctuations in the slab surface temperature.

Since the secondary cooling method was carried out in order to confirm the effect of the present invention, this will be described below.

Among the secondary cooling zones of the vertical bending type continuous casting machine 1 (see FIG. 4), the cooling device (see FIGS. 1 and 2) according to the embodiment of the present invention is used to perform strong cooling in the horizontal zone 15. The slab 5 was manufactured.

The machine length of the continuous casting machine 1 is 45 m, and a thermometer for measuring the temperature distribution on the surface of the slab and a gas cutting machine 17 are installed at the machine end. The slab is manufactured by changing the radius and spacing of the guide roll 19, the injection angle of the spray nozzle 21 to be used, the pitch of the spray nozzle in the slab width direction, the spray nozzle installation height and casting speed, and the water volume density, and the temperature during cooling. The surface texture, internal defects, and manufacturing cost of the slab after unevenness and casting were evaluated.

For the evaluation, the thickness of the cast slab was unified to 235 mm.

The casting conditions and results are shown in Table 1.

Comparative Example 1 and Examples 1 and 2 are examples of casting by applying the conditions of the prior art and the technique of the present invention, respectively. In Comparative Example 1, a water spray having an elliptical shape of the injection pattern (see FIG. 3) was used. The injection angle of this spray in the casting direction was as small as 30 °, and L / P = 0.21. For this reason, the temperature fluctuations between the direct and non-direct rays of the cooling water became large, and when the slab after production was inspected, surface cracks due to the temperature fluctuation were confirmed on the surface of the slab.

In addition, since the water density was as small as 100 (L / m ² ) / min, it was not possible to quickly realize the nucleate boiling state with the slabs. As a result, it could not be cooled efficiently and the casting speed was limited to 1.5 m / s. In addition, the solidification completion position at the center of the slab became non-uniform, and internal defects such as bias of central segregation and internal cracking also occurred.

On the other hand, in Example 1, the technique of the present invention is applied, a water spray having a quadrangular injection pattern is used, and L / P = 0.72 is set by appropriately setting the relationship between the injection angle and the nozzle installation height. It was realized. The water density was 400 (L / m ² ) / min, and the casting speed was increased to 3.0 m / s.

As a result, temperature fluctuations in the casting direction could be suppressed, and a boiling state could be quickly realized and maintained in the slab width direction. When the slabs after casting were inspected, no defects were found on the surface or inside, and high-quality slabs could be manufactured with high efficiency.

Further, in Example 2, the equipment arrangement is the same as in Example 1, and the water amount density of the cooling water is 2000 (L / m ² ) / min. As a result, temperature fluctuations in the casting direction could be suppressed, and a boiling state could be quickly realized and maintained in the slab width direction. When the slabs after casting were inspected, no defects were found on the surface or inside, and high-quality slabs could be manufactured with high efficiency.

In Comparative Example 2 and Examples 3 and 4, a water spray having a quadrangular injection pattern was used, and the water density was 400 (L / m ² ) / min. As a result, in each of the examples, the nucleate boiling state could be quickly realized and maintained from the inlet of the strong cooling zone in the direct irradiation part of the cooling water.

However, in Comparative Example 2, since the injection angle θ was 70 ° and L / P = 0.65, the temperature fluctuation between the direct and non-direct parts of the cooling water became large, and the slab after casting was confirmed. Surface cracks were confirmed.

On the other hand, in Example 3, a nozzle having a smaller injection angle (84 °) than in Example 1 was used, but L / P = 0.70 was realized by adjusting the nozzle height, and the temperature fluctuated in the casting direction. Was able to be suppressed. When the slabs after casting were inspected, no defects were found on the surface or inside, and high-quality slabs could be manufactured with high efficiency.

Further, in Example 4, a nozzle having a larger injection angle (100 °) than in Example 1 is used, and L / P = 0.73 is realized by adjusting the nozzle height, and the temperature fluctuates in the casting direction. Was able to be suppressed. Then, when the slab after casting was inspected, no defects were confirmed on the surface and inside as in Example 3, and a high-quality slab could be produced with high efficiency.

In Comparative Examples 3 and 4 and Examples 5 and 6, the injection height is changed based on the conditions of Example 1. When the injection angle of the nozzle used is 95 °, the range of the injection height h from the equation (3) is 97 to 101 mm. In Examples 5 and 6, the lower limit and the upper limit of the injection height h are set, respectively, and L / P ≧ 0.70 is satisfied under both conditions. No defects were found inside, and high-quality slabs could be manufactured with high efficiency.

On the other hand, in Comparative Example 3, the injection height h was below the lower limit (h = 90 mm), and L / P = 0.66 was below 0.70, so that the slab surface temperature fluctuated greatly and the slab after casting fluctuated greatly. When the above was confirmed, surface cracks were confirmed.

Further, in Comparative Example 4, when the upper limit of the injection height h was exceeded (h = 105 mm), a part of the injected cooling water was blocked by the guide roll 19. As a result, the direct irradiation portion length was 0.70 or more at L / P = 0.72 due to the cooling water passing between the guide rolls 19, but the water density was reduced to 380 (L / m ² ) / min. A stable nucleate boiling state could not be achieved, and when the slab after casting was confirmed, surface cracks and internal defects were confirmed.

Comparative Example 5 is an example in which the water density is reduced to 350 (L / m ² ) / min by using the same spray nozzle 21 as in Example 1. At this time, since the nucleate boiling state could not be stably realized as in Comparative Example 4, when the slab after casting was confirmed, surface cracks and internal defects were confirmed.

Comparative Example 6 and Example 7 are examples in which the radius d and the interval P of the guide roll 19 are changed to 80 mm and 250 mm by using the same spray nozzle 21 as in Example 1.

In Comparative Example 6, since the nozzle height h is set to the same as that of the first embodiment, the height h exceeds the upper limit (86 mm) of the radius d and the interval P, and a part of the cooling water is blocked by the guide roll 19. rice field. As a result, the direct irradiation portion length was 0.70 or more at L / P = 0.71 due to the cooling water passing between the guide rolls 19, but the water density was reduced to 330 (L / m ² ) / min. A stable nucleate boiling state could not be achieved, and when the slab after casting was confirmed, surface cracks and internal defects were confirmed.

On the other hand, in Example 7, by adjusting the nozzle installation height to 85 mm, all the cooling water can be sprayed onto the slab, and the water density is 400 (L / m ² ) / min as set, and L / P = 0.74. Since 0.70 or more could be achieved, temperature fluctuations on the surface of the slab could be suppressed and a rapid nucleate boiling state could be realized and maintained. As a result, when the slabs after casting were inspected, no defects were confirmed on the surface or inside, and high-quality slabs could be manufactured with high efficiency.

As described above, by secondary cooling under the condition that L / P ≧ 0.70 and the nucleate boiling state can be maintained, high quality slabs can be produced with high quality without any defects on the surface and inside of the slabs. It was demonstrated that it can be manufactured efficiently.

In Examples 1 to 6, the spray nozzles 21 were arranged in a straight line in parallel with the rolls at intervals of 250 mm (width pitch 250 mm) in the gaps between the support rolls of the secondary cooling zone (without staggered arrangement). Further, in Example 7, the spray nozzles 21 were arranged at intervals of 210 mm. Under these conditions, the water density of the lap portion was within the range of 50% or more and 100% or less of the maximum value in all cases, and no defect was observed as described above.

In Comparative Example 7, only the width pitch of the spray nozzle 21 was changed to 275 mm with respect to Example 1, the water density of the lap portion was 40% of the maximum value, and the nucleate boiling state was stably maintained. It has not been realized. In Comparative Example 7, temperature unevenness in the width direction was clearly observed along the arrangement of the spray nozzle 21. In addition, vertical cracks were generated on the surface of the slab, which was considered to be caused by the temperature unevenness in the width direction.

From this, it can be seen that it is preferable to arrange the spray nozzle 21 so that the water density of the wrap portion is in the range of 50% or more and 100% or less of the maximum value.

1 Continuous casting machine 3 Mold 5 Cast piece 7 Vertical band 9 Bending part 11 Curved band 13 Straightening part 15 Horizontal band 17 Gas cutting machine 19 Guide roll 21 Spray nozzle A, B Casting direction water volume distribution of cooling water sprayed from spray nozzle Point where is 50% of the maximum value C Nozzle injection port θ The angle formed by the straight line AB and the straight line BC P The distance between the axes of the guide roll d The radius of the guide roll

Claims (3)

Injecting between guide rolls of radius d (unit: mm) installed at the inter-axis distance P (unit: mm) in all or part of the casting direction of the horizontal zone in the secondary cooling zone of the continuous casting machine. It is a secondary cooling method for continuously cast slabs that cools the slabs by arranging spray nozzles with a square pattern in the slab width direction.
The distance L (unit) between points A and B, which are two points where the water density of the cooling water sprayed from each of the spray nozzles is 50% of the maximum value of the water density in the casting direction. : Mm) and the inter-axis distance P satisfy the following equation (1) and
The spray nozzles are arranged side by side in the slab width direction so that the water density of the wrap portion in the injection region in the slab width direction is 50% or more and 100% or less of the maximum value of the water density when a single spray is made. ,
A method for secondary cooling of continuously cast slabs, which comprises cooling while maintaining the nucleate boiling state in the range from the point A to the point B.
L / P ≧ 0.70 ・ ・ ・ (1) The angle θ (unit: degree) formed by the straight line connecting the nozzle injection port of the spray nozzle and the point A and the straight line connecting the nozzle injection port and the point B satisfies the following equation (2), and the above. The secondary cooling method for continuously cast slabs according to claim 1, wherein the nozzle height h (unit: mm), which is the height of the nozzle injection port from the slab, satisfies the following formula (3). ..
180-4tan ^-1 [3P / (20d)] ≤θ≤100 ... (2)
7P / [20tan (θ / 2)] ≦ h ≦ [P-2dtan {(180-θ) / 4}] / [2tan (θ / 2)] ... (3) The water volume density of the cooling water sprayed by each of the spray nozzles is 400 (L / m ² ) / min or more and 2000 (L / m ² ) / per unit surface area of the slab in the cooling section by the spray nozzles. The secondary cooling method for continuously cast slabs according to claim 1 or 2, wherein the amount is min or less.

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