Method for continuous casting of steel

Abstract

To provide a method for continuously casting a steel with which the natural opening ratio of a long nozzle can be improved by restraining the pressure rise in the long nozzle at the time of changing a ladle.SOLUTION: A molten steel flow rate control device (5) for controlling a flow rate of molten steel poured from a ladle (1) into a tundish (2) is closed, and a tip portion of a long nozzle (9) for pouring the molten steel from the ladle (1) into the tundish (2) is immersed in the molten steel in the tundish (2) with a filler (4) filled in a molten steel outflow hole (3) of the ladle (1).SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to a method for continuous casting of steel, and more particularly to a method for continuous casting of steel in which molten steel is poured from a ladle into a tundish using a long nozzle while being shielded from air. [Background technology]

[0002] Patent Document 1 discloses an example of a method for continuous casting of steel. The continuous casting machine of Patent Document 1 has an upper nozzle at the bottom of a ladle for causing molten steel in the ladle to flow downward. A sliding nozzle is connected below the upper nozzle, which connects the ladle and the long nozzle, blocks the communication between them, and controls the flow rate of molten steel flowing from the ladle into the long nozzle. An atmospheric pressure communication groove is formed in the fixing plate of the sliding nozzle, which connects the inside and outside of the long nozzle when the communication between the ladle and the long nozzle is blocked. The long nozzle is connected below the sliding nozzle.

[0003] When a ladle is replaced in a continuous casting machine and continuous casting is not yet 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, and the upper nozzle is closed. The sliding nozzle is closed to prevent the filler material from flowing out of the upper nozzle, and the inside and outside of the long nozzle are connected by an atmospheric pressure communication groove. When continuous casting is to begin using the replaced ladle, the ladle is lowered and the tip of the long nozzle is immersed in the molten steel in the tundish.

[0004] When the tip of the long nozzle is immersed in molten steel in the tundish, the molten steel enters the interior of the long nozzle from the tip, reducing the volume of the long nozzle and compressing the air inside the long nozzle. The air inside the long nozzle is also heated by the molten steel and expands, increasing the pressure inside the long nozzle. Therefore, in the continuous casting method disclosed in Patent Document 1, an atmospheric pressure communication groove is used to connect the inside and outside of the long nozzle, thereby reducing the pressure inside the long nozzle. This facilitates the outflow of filler material when the sliding nozzle is opened, improving the natural opening rate (sometimes referred to as the natural tapping rate). [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Japanese Patent Application Laid-Open No. 2006-231398 Summary of the Invention [Problem to be solved by the invention]

[0006] In continuous casting, the stationary plate is subjected to repeated thermal loads, and therefore must be strong enough to withstand such thermal loads. Therefore, in the continuous casting method disclosed in Patent Document 1, the air communication groove is formed in the stationary plate to a degree that does not impair the strength of the stationary plate, which may result in an air communication groove of a size sufficient to adequately depressurize the long nozzle. Furthermore, repeated thermal loads may gradually deform or clog the air communication groove, potentially preventing adequate depressurization of the long nozzle via the air communication groove. As a result, the continuous casting method disclosed in Patent Document 1 has the problem of a low natural opening rate due to the high pressure inside the long nozzle.

[0007] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a continuous steel casting method that can suppress a pressure increase in the long nozzle during ladle replacement and improve the natural hole opening rate of the long nozzle. [Means for solving the problem]

[0008] The means for solving the above-mentioned problems are as follows. [1] A method for continuous casting of steel, comprising: closing a molten steel flow rate control device that controls the flow rate of molten steel poured from a ladle into a tundish; filling a molten steel outflow hole of the ladle with a filler material; immersing the tip of a long nozzle that pours the molten steel from the ladle into the tundish in the molten steel; setting the immersion depth from the liquid surface of the molten steel to the tip within a predetermined immersion depth range; and opening the molten steel flow rate control device within a predetermined time from the time the tip comes into contact with the molten steel. [2] The method for continuous casting steel according to [1], wherein the upper limit of the predetermined range of immersion depth is 440 mm. [3] The method for continuous casting steel according to [1], wherein the upper limit of the predetermined range of immersion depth is 415 mm. [4] The method for continuous casting steel according to any one of [1] to [3], wherein the lower limit of the predetermined range of immersion depth is 50 mm. [5] A method for continuous casting steel according to any one of [1] to [3], wherein the lower limit of the predetermined range of immersion depth is 280 mm. [6] The method for continuous casting steel according to any one of [1] to [5], wherein the predetermined time is 30 seconds. [Effects of the Invention]

[0009] According to the present invention, the pressure increase in the long nozzle during ladle replacement can be suppressed, thereby improving the natural opening rate of the long nozzle. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is a diagram showing a part of a continuous casting machine to which the continuous steel casting method according to the present embodiment can be applied. [Figure 2] FIG. 2 is an enlarged view of a portion of the ladle. [Figure 3]1 is a diagram for explaining the operation of a continuous casting machine to which the continuous steel casting method according to the present embodiment is applied. FIG. [Figure 4] FIG. 2 is a diagram for explaining pressure changes inside a long nozzle in a continuous casting machine to which the continuous steel casting method according to the present embodiment is applied. [Figure 5] 1 is a diagram showing the number of charges and the rate of occurrence of non-open holes for Example 1, Example 2, Comparative Example 1, and Comparative Example 2. FIG. [Figure 6] FIG. 1 is a diagram showing the number of charges and the rate of occurrence of non-open holes for Example 3, Example 4, and Comparative Example 3. [Figure 7] FIG. 1 is a diagram for explaining the relationship between the immersion depth of the tip of the long nozzle and the rate of occurrence of inclusion defects in steel. DETAILED DESCRIPTION OF THE INVENTION

[0011] A continuous steel casting method 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 continuous steel casting method according to the present embodiment can be applied has a plurality of ladles. In the continuous casting machine, molten steel is poured from at least one of the plurality of ladles into a tundish, and when pouring of molten steel from that ladle is completed, that ladle is separated from the tundish. In addition, another ladle for storing molten steel of the next heat (sometimes referred to as the next charge) is positioned above the tundish, and molten steel is poured from that other ladle into the tundish. In this way, in a continuous casting machine to which the continuous steel casting method according to the present embodiment can be applied, molten steel is continuously poured into the tundish while repeatedly replacing the ladle.

[0012] Fig. 1 is a diagram showing part of a continuous casting machine to which the continuous steel casting method according to this embodiment can be applied. The ladle 1 shown in Fig. 1 stores molten steel to be poured into a tundish 2 and is supported by a swing tower (not shown). The swing tower is configured to position the ladle 1 above the tundish 2 and to be able to raise and lower the ladle 1. That is, the ladle 1 is configured to be moved closer to the tundish 2 by lowering it, and to be moved away from the tundish 2 by raising it. The swing tower may be a conventionally known type.

[0013] Figure 2 is an enlarged view of a portion of the ladle 1. As shown in Figure 2, a through hole is formed in the bottom of the ladle 1. An upper nozzle 3 is connected to the through hole, which allows the molten steel inside the ladle 1 to flow out. Before the start of pouring molten steel from the ladle 1 into the tundish 2, the upper nozzle 3 is filled with a filler material 4 to block the nozzle, preventing the molten steel in the ladle 1 from cooling and solidifying and causing nozzle clogging. Figure 2 shows this state.

[0014] The filler 4 may be, for example, a conventionally known filler containing silica sand as a main component. The filler 4 is filled into the upper nozzle 3 before the molten steel is poured into the ladle 1.

[0015] A sliding nozzle 5 is connected to the lower side of the upper nozzle 3. The sliding nozzle 5 is configured to control the outflow and stopping of molten steel from the ladle 1, and the amount of molten steel poured from the ladle 1 into the tundish 2. The sliding nozzle 5 corresponds to the molten steel flow rate control device of this embodiment, and the through hole connected to the upper side of the sliding nozzle 5 and the upper nozzle 3 correspond to the molten steel outflow hole of this embodiment.

[0016] 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 is formed in the fixed plate 6, penetrating the fixed plate 6. The diameter of the opening 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 positioned approximately in line.

[0017] The sliding plate 7 is configured to be able to slide in the left-right direction in Figure 2 relative to the fixed plate 6. An opening is formed in the sliding plate 7 that passes through 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 in 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, allowing the opening in the fixed plate 6 to be opened or closed.

[0018] The overlapping area between the opening of the fixed plate 6 and the opening of the sliding plate 7 is changed by controlling the amount of movement of the actuator and the amount of movement of the sliding plate 7 by the actuator. 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, thereby controlling the amount of molten steel poured from the ladle 1 into the tundish 2.

[0019] The flow straightening nozzle 8 is a nozzle that prevents the fluid flow of 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 below 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.

[0020] A long nozzle 9 is connected to the tip of the straightening nozzle 8. In other words, the long nozzle 9 is connected to the bottom of the sliding nozzle 5. When the ladles 1 are arranged above the tundish 2 so that they overlap each other, the long nozzle 9 extends from the bottom of the ladle 1 to the tundish 2. Therefore, the long nozzle 9 functions to prevent the molten steel from coming into contact with the atmosphere when the molten steel is poured from the ladle 1 into the tundish 2. Specifically, the long nozzle 9 prevents the molten steel from coming into contact with oxygen.

[0021] In the example shown in FIG. 2, the sliding nozzle 5 is provided with atmospheric pressure communication means. 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. Furthermore, the atmospheric pressure communication means is configured to block communication between the inside and outside of the long nozzle 9 when the sliding nozzle 5 is in an open state. 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 openings of the fixed plate 6 and the sliding plate 7 overlap each other, and is configured not to communicate the inside and outside of the long nozzle 9. The groove 10 described above corresponds to the atmospheric pressure communication means.

[0022] Returning to the explanation of Figure 1, an immersion nozzle 11 is provided at the bottom of the tundish 2. The immersion nozzle 11 is a nozzle for pouring molten steel into the mold 12, and extends to the interior of the mold 12. Therefore, the immersion nozzle 11 prevents the molten steel poured 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.

[0023] The molten steel poured into the mold 12 is cooled upon contact with the mold 12, forming a solidified shell (not shown) on its surface. A slab 13, which has a solidified shell as its outer shell and unsolidified molten steel inside, is continuously withdrawn from the mold 12 downward. The slab 13 is then further cooled until it is completely solidified to its center, thereby achieving continuous casting of steel. Note that while the slab 13 is being withdrawn from the mold 12, the surface level of the molten steel in the mold 12 is controlled to be approximately constant.

[0024] This section explains the timing for starting pouring molten steel from the replaced ladle 1 into the tundish 2 after replacing a ladle that has finished pouring molten steel into the tundish 2 with another ladle (corresponding to the ladle designated by the symbol "1" in Figures 1 and 2; in the following explanation, simply referred to as ladle 1) that contains molten steel. The ladle that has finished pouring molten steel into the tundish 2 is separated from the tundish 2 by a swing tower, and at the same time, the ladle 1 containing molten steel is placed at a predetermined location above the tundish 2. The upper nozzle 3 of the ladle 1 is blocked by a filler material 4, and the sliding nozzle 5 is in a closed state. The inside and outside of the long nozzle 9 are connected by a groove 10.

[0025] The ladle 1 is lowered toward the tundish 2 by the swing tower, immersing the tip of the long nozzle 9 in the molten steel in the tundish 2. This is to prevent contact between the molten steel and air. In this embodiment, the immersion depth of the tip of the long nozzle 9 is set within a predetermined depth range by controlling the amount of descent of the ladle 1 by the swing tower. When the tip of the long nozzle 9 reaches the predetermined depth range, the descent of the ladle 1 by the swing tower is stopped. At approximately the same time, the sliding nozzle 5 is opened. The descent speed of the ladle 1 by the swing tower is kept almost constant. Continuous casting is continued using the continuous casting machine shown in Figure 1. Therefore, the molten steel surface in the tundish 2 gradually drops as continuous casting continues. If the immersion depth of the tip of the long nozzle 9 falls outside the predetermined depth range, the ladle 1 is lowered by the swing tower to bring the immersion depth of the tip of the long nozzle 9 within the predetermined depth range.

[0026] Here, a method for adjusting the immersion depth of the tip of the long nozzle 9 within a predetermined range of immersion depth will be described. For example, the position of the tip of the long nozzle 9 in the height direction is calculated based on the height of the swing tower, the level of the molten steel in the tundish 2, the weight of the molten steel stored in the tundish 2, and the length of the long nozzle 9. Whether or not the tip of the long nozzle 9 is located within the predetermined range of immersion depth is determined based on the calculated position of the tip of the long nozzle 9 in the height direction and the level of the molten steel in the tundish 2. Alternatively, a mark may be attached to the long nozzle 9, and whether or not the tip of the long nozzle 9 is located within the predetermined range of immersion depth may be determined visually based on the position of the mark in the height direction and the level of the molten steel in the tundish 2. If the tip is not located within the predetermined range of immersion depth, the ladle 1 is lowered by the swing tower to adjust the immersion depth of the tip of the long nozzle 9 within the predetermined range of immersion depth. The liquid level of the molten steel in the tundish 2 and the weight of the molten steel stored in the tundish 2 can be calculated based on the amount of molten steel poured from the ladle 1, the size of the tundish 2, and the amount of cast pieces 13 produced by continuous casting.

[0027] The reason why the immersion depth of the tip of the long nozzle 9 is set within a predetermined range of immersion depth will be explained. Pressure corresponding to the immersion depth of the tip acts on the tip of the long nozzle 9, and this pressure causes molten steel to penetrate into the interior of the long nozzle 9 from the tip. As the molten steel penetrates into the interior of the long nozzle 9, the volume of the long nozzle 9 decreases and the air inside the long nozzle 9 is compressed, causing the pressure inside the long nozzle 9 to increase. In addition, the air inside the long nozzle 9 is heated by the molten steel and expands, which also causes the pressure inside the long nozzle 9 to increase.

[0028] If the immersion depth of the tip of the long nozzle 9 becomes excessively deep, the amount of molten steel that penetrates into the long nozzle 9 increases, resulting in a corresponding decrease in the volume of the long nozzle 9 and an increase in the amount of air compression. Furthermore, the amount of heat that the air in the long nozzle 9 receives from the molten steel increases, resulting in a greater expansion of the air. As a result, the pressure inside the long nozzle 9 may become excessively high, and this is to be avoided. On the other hand, if the immersion depth of the tip of the long nozzle 9 becomes excessively shallow, the fluid flow of the molten steel injected from the long nozzle 9 into the tundish 2 stirs the surface of the molten steel in the tundish 2. This may result in slag entrainment, in which slag accumulated on top of the molten steel becomes mixed into the molten steel, and this is to be avoided.

[0029] The upper limit of the predetermined immersion depth range, i.e., the maximum immersion depth, is preferably 440 mm or less, and more preferably 415 mm or less. A maximum immersion depth of 440 mm or less or 415 mm or less means that the tip of the long nozzle 9 is located shallower than 440 mm or 415 mm below the surface of the molten steel in the tundish 2. The lower limit of the predetermined immersion depth range, i.e., the minimum immersion depth, is preferably 50 mm or more when producing general-purpose steel, and is preferably 280 mm or more when producing high-quality steel. A minimum immersion depth of 50 mm or more or 280 mm or more means that the tip of the long nozzle 9 is located deeper than 50 mm or 280 mm below the surface of the molten steel in the tundish 2.

[0030] In this embodiment, the sliding nozzle 5 is opened within a predetermined time from the time when the tip of the long nozzle 9 comes into contact with the molten steel. The reason for opening the sliding nozzle 5 within the predetermined time will be explained. When the tip of the long nozzle 9 is immersed in the molten steel, the long nozzle 9 is closed, and the air inside is heated by the heat of the molten steel and expands, causing the pressure inside the long nozzle 9 to increase. The air inside the long nozzle 9 flows to the outside through the groove 10 due to the pressure difference between the inside and outside of the long nozzle 9 (hereinafter, this may be referred to as pressure relief). However, the amount of air that can flow through the groove 10 is largely determined by the width and depth of the groove 10 and the pressure difference between the inside and outside of the long nozzle 9. Therefore, it takes a certain amount of time for the pressure inside the long nozzle 9 and the pressure outside to become approximately equal. If the time taken for depressurization (hereinafter referred to as depressurization time) becomes excessively long, the surface of the molten steel that has penetrated into the long nozzle 9 may cool and solidify, causing the long nozzle 9 to become clogged (this may be referred to as skinning).

[0031] Therefore, in this embodiment, a threshold value for the depressurization time is set so that the sliding nozzle 5 is opened within a predetermined time from the time when the tip of the long nozzle 9 comes into contact with the molten steel in the tundish 2. When the depressurization time exceeds the threshold value, the sliding nozzle 5 is immediately opened. In this embodiment, the threshold value is set to 30 seconds.

[0032] Here, a description will be given of how to determine whether the tip of the long nozzle 9 has come into contact with the molten steel in the tundish 2. For example, the position of the tip of the long nozzle 9 in the height direction is calculated based on the height of the swing tower, the level of the molten steel in the tundish 2, the weight of the molten steel stored in the tundish 2, and the length of the long nozzle 9. Whether or not the tip of the long nozzle 9 has come into contact with the molten steel in the tundish 2 is determined based on the calculated value of the position of the tip of the long nozzle 9 in the height direction and the level of the molten steel in the tundish 2. Alternatively, a mark may be attached to the long nozzle 9, and whether or not the tip of the long nozzle 9 has come into contact with the molten steel in the tundish 2 may be determined visually based on the position of the mark in the height direction and the level of the molten steel in the tundish 2.

[0033] The above-mentioned threshold value for the depressurization time may be changed depending on the size of the continuous casting machine, the inner diameter of the long nozzle 9, the molten steel temperature, the steel type, and the width and depth of the groove 10. In other words, the threshold value for the depressurization time varies depending on the equipment conditions and operating conditions. The threshold value for the depressurization time can be determined, for example, by experiment. The point in time when the molten steel comes into contact with the tip of the long nozzle 9 may be determined visually or by analyzing an image taken by a camera (not shown). Then, a timer (not shown) starts measuring time, and the sliding nozzle 5 is opened within the above-mentioned time.

[0034] (Actions and Effects) FIG. 3 is a diagram illustrating the operation of a continuous casting machine incorporating the continuous steel casting method according to this embodiment. After pouring molten steel into a tundish 2, the ladle is separated from the tundish 2 by a swing tower. At the same time, the ladle 1 containing the molten steel and the long nozzle 9 are positioned above the tundish 2, as shown in FIG. 3(A). The swing tower then lowers the ladle 1 toward the tundish 2. As shown in FIG. 3(B), a timer begins measuring the depressurization time when the tip of the long nozzle 9 contacts the molten steel. The swing tower continues to lower the ladle 1. When the immersion depth of the tip of the long nozzle 9 falls within a predetermined range, the swing tower stops lowering the ladle 1. At approximately the same time, the sliding nozzle 5 is opened, as shown in FIG. 3(C). In this embodiment, regardless of the immersion depth of the tip of the long nozzle 9, the sliding nozzle 5 is immediately opened when the depressurization time exceeds a threshold.

[0035] Figure 4 is a diagram illustrating the pressure change inside the long nozzle 9 in a continuous casting machine to which the continuous steel casting method according to this embodiment is applied. The solid line in Figure 4 shows the pressure change inside the long nozzle 9 in a continuous casting machine to which the continuous steel casting method according to this embodiment is applied. The dashed line in Figure 4 shows the pressure change inside the long nozzle when the immersion depth of the long nozzle is deeper than in this embodiment. In this embodiment, regardless of the amount of molten steel in the tundish 2, the immersion depth of the tip of the long nozzle 9 is within a predetermined range of immersion depth.

[0036] Therefore, in this embodiment, the area of ​​the long nozzle 9 that comes into contact with the molten steel is smaller than when the immersion depth of the long nozzle is deeper than in this embodiment, and the amount of heat received from the molten steel is also smaller. As a result, the amount of air expansion inside the long nozzle 9 in this embodiment is smaller than when the immersion depth of the long nozzle is deeper than in this embodiment. Accordingly, the pressure inside the long nozzle 9 in this embodiment is also lower than when the immersion depth of the long nozzle is deeper than in this embodiment. In other words, this embodiment is designed to suppress a pressure increase inside the long nozzle 9 during ladle replacement. In this embodiment, the sliding nozzle 5 is opened immediately when the immersion depth of the tip of the long nozzle 9 reaches a predetermined immersion depth. In this embodiment, the sliding nozzle 5 is opened when the pressure inside the long nozzle 9 is lower than in the conventional example. Therefore, in this embodiment, the pressure inside the long nozzle 9 is less likely to obstruct the outflow of the filler material, making it easier for the filler material filled in the upper nozzle 3 to flow out, and improving the natural opening rate of the long nozzle 9 during ladle replacement. Furthermore, in this embodiment, the thermal load on the long nozzle 9 and the sliding nozzle 5 can be reduced compared to when the immersion depth of the long nozzle is deeper than in this embodiment. Therefore, it is possible to prevent the groove 10 from gradually deforming or becoming clogged due to repeated thermal loads. [Example]

[0037] The present invention will be described more specifically with reference to examples. 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 similarly to the continuous casting machine shown in Fig. 1. At least one groove is formed in the fixed plate, which connects the opening of the sliding plate to the outside when the opening of the sliding plate is closed by the sliding plate of the sliding nozzle.

[0038] Hole drilling work by ladle exchange was carried out for a total of 1,400 charges or more in Examples 1 to 4 and Comparative Examples 1 to 3, and the incidence of non-drilling was investigated for each of Examples 1 to 4 and Comparative Examples 1 to 3. The number of charges and incidence of non-drilling for each of Examples 1 to 4 and Comparative Examples 1 to 3 are summarized in Figures 5 and 6.

[0039] In Example 1, the immersion depth of the tip of the long nozzle in the molten steel in the tundish was set to 280 mm or more and 440 mm or less, and the sliding nozzle was opened within 30 seconds from the time the tip of the long nozzle came into contact with the molten steel.

[0040] Example 2 was the same as Example 1, except that the immersion depth of the tip of the long nozzle in the molten steel in the tundish was set to 280 mm or more and 415 mm or less.

[0041] Example 3 was the same as Example 1, except that the immersion depth of the tip of the long nozzle in the molten steel in the tundish was set to 150 mm or more and 280 mm or less.

[0042] Example 4 was the same as Example 1, except that the immersion depth of the tip of the long nozzle in the molten steel in the tundish was set to 50 mm or more and 150 mm or less.

[0043] Comparative Example 1 was the same as Example 1, except that the immersion depth of the tip of the long nozzle in the molten steel in the tundish was set to 441 mm or more.

[0044] Comparative Example 2 is an example in which the immersion depth of the tip of the long nozzle in the molten steel in the tundish is set to 280 mm or more and 415 mm or less, and the sliding nozzle is opened after 35 seconds or more have passed since the tip of the long nozzle came into contact with the molten steel.

[0045] In Comparative Example 3, the immersion depth of the tip of the long nozzle in the molten steel in the tundish was set to 49 mm or less, and the sliding nozzle was opened 35 seconds or more after the tip of the long nozzle came into contact with the molten steel.

[0046] (evaluation) As shown in FIG. 5 , the non-hole occurrence rates for Example 1 and Example 2 were 2.98% and 0.89%, respectively, lower than the non-hole occurrence rates of 8.25% for Comparative Example 1 and 5.71% for Comparative Example 2. It was found that Examples 1 and 2 were more likely to form natural pores than Comparative Examples 1 and 2. Furthermore, as shown in FIG. 6 , the non-hole occurrence rates for Example 3 and Example 4 were 3.05% and 1.16%, respectively, lower than the non-hole occurrence rate of 7.69% for Comparative Example 3. It was found that Examples 3 and 4 were more likely to form natural pores than Comparative Example 3. These results demonstrate that the shallower the immersion depth of the tip of the long nozzle, the lower the non-hole occurrence rate when the sliding nozzle is opened within 30 seconds of contacting the tip of the long nozzle with the molten steel.

[0047] The term "non-opening" refers to a situation in which, when the sliding nozzle is opened, the molten steel in the ladle does not push out the filler material blocking the upper nozzle, causing the upper nozzle, i.e., the long nozzle, to not open or to be difficult to open. Therefore, when non-opening occurs, the time from when the sliding nozzle is opened until molten steel is poured from the ladle into the tundish becomes longer, or the amount of molten steel poured from the ladle into the tundish decreases, compared to the case of natural opening. In this example, a threshold value was set for the time from when the sliding nozzle is opened until molten steel is poured from the ladle into the tundish. If the time exceeded the threshold, it was determined to be non-opening, and the occurrence rate was calculated. The occurrence rate of non-opening refers to the proportion of non-openings in the number of charges. The term "natural opening" refers to a situation in which, when the sliding nozzle is opened, the molten steel in the ladle pushes out the filler material blocking the upper nozzle, causing the upper nozzle to open.

[0048] Next, the relationship between the immersion depth of the tip of the long nozzle and the incidence of inclusion defects in steel due to slag entrainment was investigated. Hole-making work involving ladle exchange was performed for a total of 95 charges in Examples 5 to 8, and the relationship between the immersion depth of the tip of the long nozzle and the incidence of inclusion defects was investigated. The relationship between the number of charges, the immersion depth of the tip of the long nozzle, and the incidence of inclusion defects in steel in Examples 5 to 8 is summarized in Figure 7. Inclusion defects refer to steel defects that occur when slag is entrained in molten steel during steel production using a continuous casting machine. The incidence of inclusion defects refers to the proportion of inclusion defects occurring among the number of charges.

[0049] In Example 5, the immersion depth of the tip of the long nozzle in the molten steel in the tundish was set to 230 mm, and the sliding nozzle was opened within 30 seconds from the time the tip of the long nozzle came into contact with the molten steel.

[0050] Example 6 was the same as Example 5, except that the immersion depth of the tip of the long nozzle in the molten steel in the tundish was set to 280 mm.

[0051] Example 7 was the same as Example 5, except that the immersion depth of the tip of the long nozzle in the molten steel in the tundish was set to 330 mm.

[0052] Example 8 was the same as Example 5, except that the immersion depth of the tip of the long nozzle in the molten steel in the tundish was set to 380 mm.

[0053] (evaluation) As shown in Figure 7, the incidence of steel inclusion defects due to slag entrapment decreased in the order of Examples 5 to 8. In other words, it was found that the incidence of steel inclusion defects due to slag entrapment increased as the immersion depth of the tip of the long nozzle became shallower. From these results, when producing high-quality steel, it is preferable that the tip of the long nozzle be located more than 280 mm below the surface of the molten steel in the tundish. [Explanation of symbols]

[0054] 1 ladle 2 tundishes 3 Upper nozzle 4 Filling material 5 Sliding Nozzle 6 Fixed plate 7 Sliding plate 8. Straightening nozzle 9 Long Nozzle 10 Groove (atmospheric pressure communication means) 11 Submerged Entry Nozzle 12 Mold 13 Castings

Claims

1. A method for continuous casting steel, comprising: closing a molten steel flow rate control device that controls the flow rate of molten steel poured from a ladle into a tundish; and, with a filler material filled in a molten steel outflow hole of the ladle, immersing a tip of a long nozzle that pours the molten steel from the ladle into the tundish in the molten steel in the tundish; setting the immersion depth from the liquid surface of the molten steel to a predetermined range of the immersion depth of the tip; and opening the molten steel flow rate control device within a predetermined time from the time the tip comes into contact with the molten steel.

2. 2. The method for continuous casting steel according to claim 1, wherein the upper limit of the predetermined range of immersion depth is 440 mm.

3. 2. The method for continuous casting steel according to claim 1, wherein the upper limit of the predetermined range of immersion depth is 415 mm.

4. 4. The method for continuous casting steel according to claim 1, wherein the lower limit of the predetermined range of immersion depth is 50 mm.

5. 4. The method for continuous casting steel according to claim 1, wherein the lower limit of the predetermined range of immersion depth is 280 mm.

6. 4. The method for continuous casting of steel according to claim 1, wherein the predetermined time is 30 seconds.

7. 5. The method of continuous casting steel according to claim 4, wherein said predetermined time is 30 seconds.

8. 6. The method of continuous casting steel according to claim 5, wherein said predetermined time is 30 seconds.

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