Continuous casting process of steel

Abstract

To provide a continuous casting process of steel capable of suppressing poor opening at an upper nozzle provided at a bottom of a ladle by suitably setting the time a tip of a long nozzle is immersed in a molten steel within a tundish.SOLUTION: A continuous casting process of steel includes, since when the tip of a long nozzle 9 comes in contact with slug within a tundish 2 until a molten-steel flow controller 5 is brought into an open state, bringing the molten-steel flow controller 5 into the open state after a lapse of depressurization time (t1+t2+t3) to reduce the internal pressure of the long nozzle 9 through communicating between internal and external of the long nozzle 9 by atmospheric-pressure communicating means 10, where the depressurization time (t1+t2+t3) is defined based on a thickness (h1) of a layer of slug in the tundish 2, an immersion depth (h2) of the tips of nozzles 3, 9 from a boundary between slug and molten steel in the tundish 2, and a descending speed of a ladle 1 by a lifting-lowering device.SELECTED DRAWING: Figure 6

Description

[Technical field]

[0001] The present invention relates to a method for continuously casting steel, and more particularly to a method for continuously casting steel when starting continuous casting in a continuous casting machine. [Background technology]

[0002] An example of a method for continuous casting of steel is described in Patent Document 1. In the continuous casting machine described in Patent Document 1, an upper nozzle is provided at the bottom of a ladle to allow molten steel in the ladle to flow down. A sliding nozzle is connected below the upper nozzle to communicate between the ladle and the long nozzle, to block the communication between them, and to control the flow rate of molten steel flowing from the ladle to the long nozzle. The sliding nozzle is provided with atmospheric pressure communication means that communicates between the inside and outside of the long nozzle in a closed state in which the communication between the ladle and the long nozzle is blocked. A long nozzle is connected below the sliding nozzle to inject molten steel in the ladle into a tundish.

[0003] When the ladle is replaced in the above-mentioned continuous casting machine and continuous casting is to be started using the replaced ladle, the outlet hole of the upper nozzle of the replaced ladle is filled with a filler material mainly composed of silica sand for initial opening. Thus, the upper nozzle is blocked. The sliding nozzle is set in a closed state, and outflow of the filler material from the upper nozzle is suppressed. In addition, the inside and outside of the long nozzle are communicated by the atmospheric pressure communication means. When continuous casting is to be started using the replaced ladle, the ladle is lowered to immerse the tip of the long nozzle in the molten steel in the tundish.

[0004] When the tip of the long nozzle is immersed in the molten steel in the tundish, the molten steel penetrates into the inside of the long nozzle from the tip, reducing the volume of the long nozzle and compressing the air in the long nozzle. In addition, the air in the long nozzle is heated by the molten steel and expands, increasing the pressure in the long nozzle. Therefore, in the continuous casting method described in Patent Document 1, the tip of the long nozzle is immersed in the molten steel in the tundish for 10 seconds or more, during which time the inside and outside of the long nozzle are communicated by an atmospheric pressure communication means so that the pressure inside the long nozzle is approximately equal to atmospheric pressure. After that, the sliding nozzle is set to an open state that communicates the ladle and the long nozzle. As a result, the filler material filled in the upper nozzle is pushed out of the upper nozzle by the weight of the molten steel, and the molten steel in the ladle is poured into the tundish through the long nozzle. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2006-231398 A Summary of the Invention [Problem to be solved by the invention]

[0006] In the continuous casting method described in Patent Document 1, the tip of the long nozzle is immersed in the molten steel in the tundish for 10 seconds or more to reduce the pressure inside the long nozzle. However, even if the tip of the long nozzle is immersed in the molten steel in the tundish for 10 seconds or more, the pressure inside the long nozzle may not be approximately equal to atmospheric pressure. The pressure inside the long nozzle resists a load corresponding to the weight of the molten steel in the ladle that pushes the filler out from the upper nozzle. Therefore, when the pressure inside the long nozzle is higher than atmospheric pressure, the pressure inside the long nozzle makes it difficult to push the filler out from the upper nozzle due to the load corresponding to the weight of the molten steel in the ladle. As a result, opening failure may occur at the upper nozzle. In addition, if the time for which the tip of the long nozzle is immersed in the molten steel in the tundish is excessively long, skinning may occur inside the long nozzle, which may significantly increase the opening failure. Note that skinning means that the molten steel solidifies at the liquid surface of the molten steel that has entered the inside of the long nozzle, causing the long nozzle to close.

[0007] The present invention has been made to solve the above-mentioned problems, and has an object to provide a continuous casting method for steel that can suppress poor opening of an upper nozzle provided at the bottom of a ladle by appropriately setting the time for which the tip of a long nozzle is immersed in molten steel in a tundish. [Means for solving the problem]

[0008] The means for solving the above problems are as follows. [1] A method for continuous casting of steel, comprising the steps of: closing a molten steel flow control device and filling a molten steel outflow hole on the upper side of the molten steel flow control device with a filler material; using a ladle provided at the bottom of the molten steel flow control device, the molten steel flow control device being provided with an atmospheric pressure communicating means for communicating the inside and outside of a long nozzle when a long nozzle connected to the lower part of the molten steel flow control device is immersed in molten steel in a tundish; and pouring molten steel from the ladle into the tundish from the ladle; opening a depressurization time during which the atmospheric pressure communicating means communicates the inside and outside of the long nozzle to reduce the pressure inside the long nozzle from the time when the tip of the long nozzle comes into contact with the slag inside the tundish until the molten steel flow control device is opened; and the depressurization time is determined based on the thickness of the slag layer inside the tundish, the immersion depth of the tip of the long nozzle in the molten steel below the slag inside the tundish, and the descent speed of the ladle by a lifting device. [2] The continuous casting method of steel described in [1], wherein the depressurization time includes a passing time for the tip of the long nozzle to pass through the layer of slag inside the tundish, a time for the tip of the long nozzle to pass through the molten steel inside the tundish from the boundary between the slag and molten steel inside the tundish until it reaches the immersion depth, and a lower limit waiting time for keeping the tip of the long nozzle waiting at a depth equal to the combined thickness of the slag layer and the immersion depth. [3] The method for continuous casting steel according to [1] or [2], wherein when the depressurization time exceeds a predetermined threshold value, the depressurization time is set to the threshold value. Effect of the Invention

[0009] According to the present invention, the time for which the tip of the nozzle is immersed in the molten steel in the tundish can be appropriately set, and poor opening of the upper nozzle provided at the bottom of the ladle can be suppressed. [Brief description of the drawings]

[0010] [Figure 1]1 is a diagram showing a portion of an example of a continuous casting machine to which the continuous steel casting method according to the present embodiment can be applied. FIG. [Diagram 2] FIG. 2 is an enlarged view of a portion of the ladle. [Diagram 3] FIG. 13 is a diagram for explaining the depressurization time of a long nozzle. [Figure 4] FIG. 13 is a diagram showing the relationship between depressurization time and the incidence rate of opening failure. [Diagram 5] FIG. 1 is a diagram showing a conventional depressurization time in a continuous casting machine before the application of the continuous steel casting method according to the present embodiment, and a depressurization time in the continuous casting machine after the application of the continuous steel casting method according to the present embodiment. [Figure 6] FIG. 1 is a diagram showing the incidence rate of opening defects in a continuous casting machine before the application of the method for continuous casting of steel according to the present embodiment, and the incidence rate of opening defects in a continuous casting machine after the application of the method for continuous casting of steel according to the present embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] An example of a method for continuously casting steel according to an embodiment of the present invention (hereinafter, referred to as the present embodiment) will be described below. A continuous casting machine to which the method for continuously casting steel according to the present embodiment can be applied has a plurality of ladles. In the continuous casting machine, molten steel is poured into a tundish from at least one of the plurality of ladles. When the pouring of molten steel from the ladle is completed, the ladle into which the pouring of molten steel has been completed is separated from the tundish. In addition, other ladles for storing molten steel of the next heat (sometimes referred to as the next charge) are arranged above the tundish so as to overlap each other, and molten steel is poured into the tundish from the other ladles. In this way, in the continuous casting machine to which the method for continuously casting steel according to the present embodiment can be applied, molten steel is continuously poured into the tundish while repeatedly replacing the ladles.

[0012] Fig. 1 is a diagram showing a part of an example of a continuous casting machine to which the method for continuous casting of steel according to this embodiment can be applied. A ladle 1 shown in Fig. 1 is supported by a swing tower (not shown), which corresponds to the lifting device in this embodiment, and stores molten steel to be poured into a tundish 2 inside the swing tower. The swing tower is configured to arrange the ladles 1 on top of each other above the tundish 2, and to be able to lift and lower the ladle 1. The swing tower is configured so that the ladle 1 is brought close to the tundish 2 by lowering it, and is also configured so that the ladle 1 is separated from the tundish 2 by lifting it. The swing tower described above may be a conventionally known one.

[0013] FIG. 2 is an enlarged view of a part of the ladle 1. As shown in FIG. 2, a through hole is formed at the bottom of the ladle 1. An upper nozzle 3 for flowing out the molten steel in the ladle 1 is connected to the through hole. Before starting pouring the molten steel from the ladle 1 into the tundish 2, the upper nozzle 3 is filled with a filler 4 to block the nozzle in order to prevent the molten steel in the ladle 1 from cooling and solidifying and causing nozzle clogging. FIG. 2 shows this state. The filler 4 may be, for example, a conventionally known filler mainly composed of silica sand. The filler 4 is filled into the upper nozzle 3 before the molten steel is poured into the ladle 1. A sliding nozzle 5 is provided below the upper nozzle 3. The sliding nozzle 5 is a nozzle configured to control the flow and stop of the molten steel from the ladle 1 and the amount of molten steel poured from the ladle 1 into the tundish 2. The through hole formed at the bottom of the ladle 1 and the upper nozzle 3 correspond to the molten steel outflow hole on the upper side of the molten steel flow rate control device of this embodiment. The sliding nozzle 5 corresponds to the molten steel flow rate control device of this embodiment.

[0014] The sliding nozzle 5 shown in Fig. 2 has a fixed plate 6, a sliding plate 7, and a flow straightening nozzle 8. An opening penetrating the fixed plate 6 is formed in the fixed plate 6. The opening diameter is set to be approximately the same as the inner diameter of the upper nozzle 3. The fixed plate 6 is fixed to the lower end of the upper nozzle 3 with the opening of the fixed plate 6 and the upper nozzle 3 approximately aligned in position.

[0015] The sliding plate 7 is configured to be able to slide relative to the fixed plate 6. An opening is formed in the sliding plate 7, penetrating the sliding plate 7. The diameter of the opening is set to be approximately the same as the inner diameter of the upper nozzle 3 and the inner diameter of the opening of the fixed plate 6. An actuator (not shown) is connected to the sliding plate 7. By operating the actuator, the sliding plate 7 slides relative to the fixed plate 6, and the opening of the fixed plate 6 can be opened and closed. By controlling the amount of operation of the actuator and the amount of movement of the sliding plate 7 by the actuator, the area of ​​overlap between the opening of the fixed plate 6 and the opening of the sliding plate 7 is changed. In this way, the flow rate of molten steel passing through the opening of the fixed plate 6 and the opening of the sliding plate 7 is changed.

[0016] The flow straightening nozzle 8 is a nozzle that suppresses the fluid flow of the molten steel flowing out from the sliding nozzle 5 from becoming turbulent, and may be a conventionally known nozzle. The flow straightening nozzle 8 is fixed to the underside of the sliding plate 7 with the positions of the flow straightening nozzle 8 and the opening of the sliding plate 7 approximately aligned. The inner diameter of the flow straightening nozzle 8 shown in FIG. 2 is set to be approximately the same as the inner diameter of the opening of the sliding plate 7.

[0017] A long nozzle 9 is connected to the tip of the flow straightening nozzle 8. When the ladle 1 is placed above the tundish 2 so that they overlap, the long nozzle 9 extends from the bottom of the ladle 1 to the tundish 2. Therefore, the long nozzle 9 functions to suppress contact between the molten steel and the atmosphere when the molten steel is poured from the ladle 1 to the tundish 2. More specifically, it suppresses contact between the molten steel and oxygen. The long nozzle 9 is also connected to the bottom of the sliding nozzle 5 in this manner.

[0018] In the example shown in FIG. 2, the sliding nozzle 5 is provided with the atmospheric pressure communication means of this embodiment. The atmospheric pressure communication means is configured to communicate the inside and outside of the long nozzle 9 when the sliding nozzle 5 is in a closed state. Also, when the sliding nozzle 5 is in an open state, it is configured to block communication between the inside and outside of the long nozzle 9. Specifically, as shown in FIG. 2, when the opening of the fixed plate 6 is closed by the sliding plate 7, that is, when the sliding nozzle 5 is in a closed state, a groove 10 that communicates the opening of the sliding plate 7 with the outside is formed in the fixed plate 6. The groove 10 is not connected to the opening of the sliding plate 7 when the opening of the fixed plate 6 and the opening of the sliding plate 7 overlap each other, and is formed so as not to communicate the inside and outside of the long nozzle 9. The above-mentioned groove 10 corresponds to the atmospheric pressure communication means.

[0019] Returning to the explanation of Fig. 1, an immersion nozzle 11 is provided at the bottom of the tundish 2. The immersion nozzle 11 is a nozzle for injecting molten steel into the mold 12, and extends to the inside of the mold 12. Therefore, the immersion nozzle 11 prevents the molten steel injected from the tundish 2 into the mold 12 from coming into contact with the atmosphere. The immersion nozzle 11 and the mold 12 may be of conventional types.

[0020] The molten steel poured into the mold 12 is cooled by contact with the mold 12, and a solidified shell (not shown) is formed on the surface of the mold. A slab 13 having a solidified shell on the outside and unsolidified molten steel on the inside is continuously withdrawn from the mold 12 below the mold 12. The slab 13 is then further cooled until it is completely solidified to its center. The level of the molten steel in the mold 12 is controlled to be substantially constant while the slab 13 is being withdrawn from the mold 12.

[0021] Here, a case will be described in which a ladle that has finished pouring molten steel into a tundish 2 is replaced with another ladle in which molten steel is stored, and molten steel is poured from the replaced ladle 1 into the tundish 2. The other ladle mentioned above corresponds to the ladle indicated by the reference numeral "1" in Fig. 1 and Fig. 2. Hereinafter, it will be simply referred to as ladle 1. The ladle that has finished pouring molten steel into the tundish 2 is separated from the tundish 2 by a swing tower. At the same time, the ladle 1 in which molten steel is stored is placed at a predetermined position above the tundish 2. The upper nozzle 3 of the ladle 1 placed above the tundish 2 is blocked by a filler as described above. The sliding nozzle 5 is in a closed state. The inside and outside of the long nozzle 9 are connected by a groove 10, which is an atmospheric pressure communication means.

[0022] Next, the ladle 1 is lowered toward the tundish 2 by the swing tower, and the tip of the long nozzle 9 is immersed in the molten steel in the tundish 2. This is to prevent the molten steel from coming into contact with air when the molten steel is poured from the ladle 1 into the tundish 2. Also, this is to prevent the molten steel in the tundish 2 from being excessively stirred by the fluid flow of the molten steel poured into the tundish 2.

[0023] A slag layer may be formed above the molten steel in the tundish 2. When a slag layer is formed, when the ladle 1 is lowered by the swing tower, the tip of the long nozzle 9 first comes into contact with the slag and passes through the slag layer as the ladle 1 is lowered. Finally, the tip of the long nozzle 9 comes into contact with the molten steel below the slag layer. When the ladle 1 is further lowered and the tip of the long nozzle reaches a preset depth (hereinafter, sometimes referred to as the molten steel depth h2), the lowering of the ladle 1 by the swing tower is stopped.

[0024] A pressure corresponding to the depth h2 of the molten steel acts on the tip of the long nozzle 9, and this pressure causes molten steel to flow into the long nozzle 9 from the tip. The internal volume of the long nozzle 9 decreases by the amount of molten steel that flows into the long nozzle 9. In addition, the air inside the long nozzle 9 is heated by the molten steel and expands. As a result, the pressure inside the long nozzle 9 increases.

[0025] The air expanded inside the long nozzle 9 is discharged to the outside through the atmospheric pressure communication means due to the pressure difference between the inside and outside of the long nozzle 9. This is to prevent poor opening of the upper nozzle 3 caused by the pressure inside the long nozzle 9. By doing this, the pressure inside the long nozzle 9 gradually decreases (hereinafter, this may be referred to as depressurization). The amount of air that can flow through the groove 10, which is the atmospheric pressure communication means, is determined approximately by the size of the groove 10 and the pressure difference between the inside and outside of the long nozzle 9. For this reason, it takes a certain amount of time to depressurize (hereinafter, the time required for depressurization may be referred to as depressurization time).

[0026] 3 is a diagram for explaining the depressurization time of the long nozzle 9. As shown in FIG. 3, the pressure P inside the long nozzle 9 when the tip of the long nozzle 9 is immersed in a depth (h1+h2) that is the sum of the thickness h1 of the slag layer and the preset molten steel depth h2 is ASP can be expressed as follows using atmospheric pressure P:

number

[0027] Pressure P inside the long nozzle 9 ASPThe volume V of air discharged from inside the long nozzle 9 (sometimes referred to as depressurization volume) in order to make the pressure in the long nozzle 9 substantially the same as the external pressure, that is, atmospheric pressure, can be expressed by the following formula.

number

[0028] The depressurization time t required from the time when the tip of the long nozzle 9 comes into contact with the slag until the pressure inside and outside the long nozzle 9 become substantially equal can be expressed by the following equation using the flow rate Q of the air flowing out of the groove 10, which is the atmospheric pressure communication means.

number

[0029] When the pressure relief time t calculated by the above-mentioned formulas (1) to (3) has elapsed from the time when the tip of the long nozzle 9 contacted the slag, the pressure inside the long nozzle 9 and the pressure outside the long nozzle 9 become almost the same. Therefore, if the sliding nozzle 5 is set to the open state after the pressure relief time t has elapsed, poor opening of the upper nozzle 3 caused by the pressure inside the long nozzle 9 can be suppressed.

[0030] More specifically, when the depressurization time for the internal and external pressures of the long nozzle 9 to become substantially the same in a conventional continuous casting machine is calculated using equations (1) to (3), it is approximately 51 seconds. Table 1 below shows the parameters used to calculate the depressurization time in the conventional continuous casting machine. [Table 1]

[0031] In addition, in the conventional operating method of a continuous casting machine, the sliding nozzle 5 is opened when the tip of the long nozzle 9 reaches a preset molten steel depth h2 in the molten steel below the slag. That is, the depressurization time in the conventional operating method of a continuous casting machine (referred to as the conventional depressurization time) is the sum of the slag layer passing time t1 and the molten steel passing time t2. The slag layer passing time t1 is the time for the tip of the long nozzle 9 to pass through the slag layer. The molten steel passing time t2 is the time for the tip of the long nozzle 9 to pass through the molten steel below the slag until it reaches the above-mentioned molten steel depth h2. Specifically, the conventional depressurization time (t1+t2) is set to about 15 to 25 seconds. The sliding nozzle is immediately set to the open state when the conventional depressurization time (t1+t2) has elapsed. In this way, pouring of molten steel from the ladle 1 to the tundish 2 begins.

[0032] Furthermore, as the number of times molten steel is charged into the tundish 2 increases, the amount of slag that accumulates in the tundish 2 increases accordingly, and the thickness h1 of the slag layer increases. Meanwhile, the depth h2 of the molten steel at the tip of the long nozzle 9 in the molten steel below the slag remains almost constant. As a result, in a conventionally known operating method of a continuous casting machine, the time t1 required to pass through the slag layer becomes longer as the number of charges increases, and the conventional depressurization time (t1+t2) becomes longer accordingly.

[0033] Figure 4 shows the relationship between the depressurization time t and the occurrence rate of opening failure at the upper nozzle 3. The speed at which the ladle 1 is lowered by the swing tower is almost constant. Opening failure at the upper nozzle 3 means that when the sliding nozzle 5 is opened, the weight of the molten steel in the ladle 1 does not push out the filler material in the upper nozzle 3, so that the upper nozzle 3 remains closed and does not open. Alternatively, it means that even if the upper nozzle 3 opens, the amount of molten steel poured from the ladle 1 to the tundish 2 is less than the specified amount.

[0034] As shown in FIG. 4, the occurrence rate of opening failure at the upper nozzle 3 decreases with an increase in the depressurization time t. Moreover, when the depressurization time t exceeds 25 seconds, which is the conventional depressurization time, the occurrence rate of opening failure at the upper nozzle 3 further decreases. In other words, it was found that the occurrence rate of opening failure at the upper nozzle 3 further decreases when the tip of the long nozzle 9 is made to wait for a certain period of time at a preset molten steel depth h2 in the molten steel below the slag. Therefore, in this embodiment, a lower limit waiting time t3 is set for making the tip of the long nozzle 9 wait for a certain period of time at the above-mentioned molten steel depth h2. When the tip of the long nozzle 9 is made to wait in the molten steel, the sliding nozzle 5 is maintained in a closed state. By doing so, unlike the conventional method, the time to exhaust the air inside the long nozzle 9 is secured, and the occurrence of opening failure at the upper nozzle 3 is suppressed. Therefore, the depressurization time t in this embodiment is the sum of the slag layer passing time t1, the molten steel passing time t2, and the lower limit waiting time t3.

[0035] The depressurization time t including the lower limit waiting time t3 can be calculated based on the thickness h1 of the slag layer, the molten steel depth h2 of the tip of the long nozzle 9 in the molten steel below the slag, and the descending speed of the ladle 1 by the swing tower. As described above, the thickness h1 of the slag layer increases with an increase in the number of charges. In contrast, the molten steel depth h2 at which the tip of the long nozzle 9 is immersed in the molten steel by lowering the long nozzle 9 by the swing tower is almost determined by design. Therefore, the immersion depth (h1+h2) of the tip of the long nozzle 9 from the slag surface increases with an increase in the number of charges. In this embodiment, in order to keep the total immersion time, that is, the depressurization time t (t1+t2+t3) including the lower limit waiting time t3 constant, the lower limit waiting time t3 at the above-mentioned immersion depth (h1+h2) is reduced with an increase in the number of charges. In this embodiment, for example, maps or tables are prepared in advance that define the relationships among the number of charges, the molten steel temperature, the thickness h1 of the slag layer, the depth h2 of the molten steel at the tip of the long nozzle 9 in the molten steel below the slag, and the descending speed of the ladle 1 by the swing tower. The lower limit waiting time t3 is calculated using these maps and tables. The number of charges means the number of times the ladle 1 is replaced.

[0036] On the other hand, if the depressurization time t becomes excessively long, there is a possibility that skinning, in which the surface of the molten steel that has entered the inside of the long nozzle 9 solidifies, may occur. For this reason, in this embodiment, a threshold value for the depressurization time t is set. When the depressurization time t exceeds the threshold value, the depressurization time t is set to the same value as the threshold value. The threshold value can be obtained in advance by experiment. In this embodiment, as an example, the threshold value is 29 seconds. Therefore, when the depressurization time t exceeds the threshold value of 29 seconds, the depressurization time t is set to 29 seconds. After the depressurization time t has elapsed, the sliding nozzle 5 is opened. Note that the threshold value for the depressurization time t described above varies depending on the size of the continuous casting machine, the inner diameter of the long nozzle, the molten steel temperature, the steel type, and the cross-sectional area of ​​the groove 10, which is the atmospheric pressure communication means. In other words, the threshold value varies depending on the equipment conditions and the operating conditions, and the threshold value can be determined, for example, by experiment.

[0037] FIG. 5 is a diagram showing the depressurization time of the continuous casting machine before the application of the continuous casting method of steel according to the present embodiment, and the depressurization time of the continuous casting machine after the application of the continuous casting method of steel according to the present embodiment. The open bar graph in FIG. 5 shows the depressurization time of the continuous casting machine before the application of the continuous casting method of steel according to the present embodiment. The bar graph combining the open bar graph and the dotted bar graph in FIG. 5 shows the depressurization time of the present embodiment. As shown in FIG. 5 and as described above, in the continuous casting machine before the application of the continuous casting method of steel according to the present embodiment, the lower limit waiting time t3 is not set. Therefore, the sliding nozzle 5 is opened immediately after the time that is the sum of the slag layer passing time t1 and the molten steel passing time t2 passing through the molten steel under the slag has elapsed. On the other hand, in the continuous casting machine after the application of the continuous casting method of steel according to the present embodiment, the depressurization time t is set based on the slag layer passing time t1, the molten steel passing time t2, and the lower limit waiting time t3, and a threshold value of the depressurization time t is set. As a result, the time from when the long nozzle 9 is immersed in the molten steel until the sliding nozzle 5 is opened is 28 to 29 seconds.

[0038] FIG. 6 is a diagram showing the incidence of opening defects at the upper nozzle 3 in a continuous casting machine before the application of the continuous casting method of steel according to the present embodiment, and the incidence of opening defects at the upper nozzle 3 in a continuous casting machine after the application of the continuous casting method of steel according to the present embodiment. As shown in FIG. 6, in the continuous casting machine after the application of the continuous casting method of steel according to the present embodiment, the incidence of opening defects at the upper nozzle 3 can be reduced compared to the incidence of opening defects at the upper nozzle 3 in the continuous casting machine before the application of the continuous casting method of steel according to the present embodiment. That is, in this embodiment, since the pressure release time t can be appropriately set, the incidence of opening defects at the upper nozzle 3 can be reduced. In addition, according to this embodiment, it is possible to suppress an increase in the number of steps and defects in product quality caused by the occurrence of opening defects at the upper nozzle 3 when replacing the ladle 1.

[0039] In addition, in the continuous casting machine before applying the continuous steel casting method according to this embodiment in FIG. 6, the sliding nozzle was operated after 10 seconds or more had elapsed since the tip of the long nozzle 9 was immersed in the molten steel in the tundish 2.

[0040] The results shown in FIG. 6 indicate that simply setting the conventional pressure relief time, which is the sum of the slag layer passing time t1 and the molten steel passing time t2, to 10 seconds or more may not be enough to appropriately set the pressure relief time t for each operating condition of the continuous casting machine and suppress poor opening.

[0041] As described above, the depressurization time t in this embodiment includes the slag layer passing time t1, the molten steel passing time t2, and the lower limit waiting time t3 that takes into consideration the number of ladle changes and the molten steel temperature. Therefore, according to this embodiment, the depressurization time t can be appropriately set for each operating condition of the continuous casting machine to reduce the occurrence rate of opening failure. EXAMPLES

[0042] The present invention will be described more specifically with reference to examples. The present invention is not limited to the examples described below. The examples described here are examples in which the method for continuous casting of steel according to this embodiment is applied to a continuous casting machine configured in a manner similar to that shown in FIG. 1. Specifically, at least one groove is formed in the fixed plate to communicate the opening of the sliding plate with the outside when the opening of the fixed plate is closed by the sliding plate of the sliding nozzle. The groove extends from the inside of the long nozzle to the outside. In the examples, the width of the above-mentioned groove is set to 40 mm and the depth is set to 4 mm. The cross-sectional shape of the groove is rectangular.

[0043] Then, based on the thickness of the slag layer in the tundish, the immersion depth of the tip of the long nozzle, and the descending speed of the ladle by the swing tower, the depressurization time for immersing the tip of the long nozzle in the molten steel in the tundish is calculated. The above-mentioned depressurization time is compared with a preset threshold value, and if the depressurization time exceeds the threshold value, the depressurization time is set to the same value as the threshold value. In the embodiment, the depressurization time is about 28 to 29 seconds. When the depressurization time has elapsed since the tip of the long nozzle was immersed in the molten steel in the tundish, the sliding nozzle is set to an open state, and pouring from the ladle into the tundish is started.

[0044] In the comparative example, the sliding nozzle was set to the open state immediately after the swing tower reached the lower limit of its descent, i.e., the tip of the long nozzle reached the preset molten steel depth h2 described above. When the swing tower reached the lower limit of its descent, the tip of the long nozzle was immersed in the molten steel in the tundish.

[0045] Then, 1000 or more charges of opening were performed for each of the embodiment and the comparative example, and the natural opening rate of the embodiment and the comparative example were investigated. The natural opening rate of the embodiment and the comparative example are summarized in Table 2. The natural opening means that when the sliding nozzle is opened, the filler material blocked at the upper nozzle is pushed out by the molten steel in the ladle 1, and the upper nozzle opens. When the upper nozzle is opened naturally, the continuous casting machine is not operated to start pouring the molten steel, except for opening the sliding nozzle. The natural opening can be determined, for example, based on the time from when the sliding nozzle is opened to when the molten steel is poured from the ladle into the tundish, the amount of molten steel poured from the ladle into the tundish, etc. The natural opening rate means the ratio of the natural opening to the number of charges.

[0046] In contrast, the opening failure means that when the sliding nozzle is opened, the filler material blocked at the upper nozzle is not pushed out by the molten steel in the ladle, and the upper nozzle does not open. Therefore, when the opening failure occurs, the time from the time when the sliding nozzle is opened to the time when the molten steel is poured from the ladle into the tundish becomes longer than in the case of natural opening. Alternatively, the amount of molten steel poured from the ladle into the tundish decreases. When the opening failure occurs, for example, the swing tower is raised to increase the load with which the molten steel in the ladle presses the filler material, that is, the potential energy of the molten steel in the ladle. By doing so, the filler material is pushed out from the upper nozzle. Alternatively, oxygen gas is blown into the upper nozzle or the long nozzle to clean it.

[0047] [Table 2]

[0048] As shown in Table 2, the natural opening rate of the embodiment was 97.9%, which was improved by 1.6% compared to 96.3% of the comparative example. [Explanation of symbols]

[0049] 1 ladle 2 Tundish 3 Upper nozzle 4 Filling material 5 Sliding nozzle 6 Fixed plate 7 Sliding plate 8. Flow straightening nozzle 9 Long Nozzle 10 Groove (atmospheric pressure communication means) 11 Submerged nozzle 12 Mold 13 Casting

Claims

1. When the molten steel flow control device is in a closed state and a molten steel outflow hole on the upper side of the molten steel flow control device is filled with a filler, and when the long nozzle connected to the lower part of the molten steel flow control device is immersed in molten steel in a tundish, the molten steel flow control device uses a ladle provided at the bottom of the molten steel flow control device and is provided with an atmospheric pressure communicating means for communicating between the inside and the outside of the long nozzle, and pours molten steel from the ladle into the tundish, the molten steel flow rate control device is opened after a depressurization time has elapsed during which the inside and outside of the long nozzle are communicated by the atmospheric pressure communication means to reduce the pressure inside the long nozzle from the time when the tip of the long nozzle comes into contact with the slag inside the tundish until the molten steel flow rate control device is opened, a pressure relief time determined based on a thickness of the slag layer inside the tundish, an immersion depth of the tip of the long nozzle in the molten steel below the slag inside the tundish, and a descending speed of the ladle by a lifting device.

2. 2. The continuous steel casting method according to claim 1, wherein the depressurization time includes a passing time for the tip of the long nozzle to pass through the layer of slag inside the tundish, a passing time for the tip of the long nozzle to pass through the molten steel inside the tundish from the boundary between the slag and the molten steel inside the tundish until it reaches the immersion depth, and a lower limit waiting time for making the tip of the long nozzle wait at a depth equal to the sum of the thickness of the slag layer and the immersion depth.

3. The method for continuous casting steel according to claim 1 or 2, wherein, when the depressurization time exceeds a preset threshold value, the depressurization time is set to the threshold value.

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