Method for suppressing backflow of inert gas

Abstract

To disclose a method for suppressing backflow of an inert gas in continuous casting of a molten steel.SOLUTION: A method of the present disclosure for suppressing backflow of an inert gas into an intermediate vessel in continuous casting of a molten steel when the molten steel is supplied from the intermediate vessel to a mold through a sliding gate and a submerged nozzle, and when the inert gas is blown into the submerged nozzle includes: obtaining in advance a relation (1) among an opening degree of the sliding gate, a throughput of the molten steel, and a critical flow rate at which the backflow of the inert gas occurs; and blowing the inert gas into the submerged nozzle at a flow rate lower than the critical flow rate based on the opening degree of the sliding gate, the throughput of the molten steel, and the relation (1) obtained in advance, in the continuous casting of the molten steel.SELECTED DRAWING: Figure 10

Description

[Technical Field]

[0001] This application discloses a method for suppressing the backflow of inert gas into an intermediate vessel during continuous casting of molten steel, when molten steel is supplied from an intermediate vessel to a mold via a sliding gate (hereinafter sometimes abbreviated as "SG") and an immersion nozzle, and the inert gas is blown into the immersion nozzle. Note that, as used herein, "backflow" refers to the phenomenon in which gas bubbles rise against the vertically downward flow of liquid. [Background technology]

[0002] Patent Document 1 discloses a method for adjusting the flow rate of inert gas blown into the submerged entry nozzle by controlling a valve that adjusts the flow rate of the inert gas based on a change in the opening degree of the sliding gate, when molten steel is supplied from a tundish to a mold via a sliding gate and a submerged entry nozzle and an inert gas is blown into the submerged entry nozzle during continuous casting of molten steel. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 6-031413 Summary of the Invention [Problem to be solved by the invention]

[0004] According to the inventor's new findings, in continuous casting of molten steel, a portion of the inert gas injected into the submerged entry nozzle may flow back up into the intermediate vessel. When the inert gas flows back up into the intermediate vessel, the surface of the molten steel in the intermediate vessel is disturbed by the boiling of the molten steel, causing agitation of slag and molten steel at the surface of the molten steel, exposing the surface of the molten steel to the atmosphere, which may promote re-oxidation of the molten steel. Until now, sufficient research has not been conducted on how to prevent such flow back up of the inert gas. [Means for solving the problem]

[0005] The present application discloses the following aspects as one of means for solving the above problems. <Aspect 1> A method for suppressing inert gas from flowing back into an intermediate vessel during continuous casting of molten steel, in which the molten steel is supplied from the intermediate vessel to a mold through a sliding gate and an immersion nozzle, and an inert gas is blown into the immersion nozzle, comprising: A relationship (1) between the opening degree of the sliding gate, the throughput of the molten steel, and a critical flow rate at which the backflow of the inert gas occurs is obtained in advance; and In the continuous casting of the molten steel, injecting the inert gas into the submerged entry nozzle at a flow rate below the critical flow rate based on the opening of the sliding gate, the throughput of the molten steel, and the previously determined relationship (1); A method comprising: <Aspect 2> determining in advance a relationship (2) between an actual flow rate of the inert gas supplied to the molten steel in the submerged nozzle and an opening degree of the sliding gate in accordance with a throughput of the molten steel; and In the continuous casting of molten steel, estimating an actual flow rate of the inert gas to be supplied to the molten steel in the submerged entry nozzle based on an opening degree of the sliding gate, a throughput of the molten steel, and the previously determined relationship (2); further comprising blowing the inert gas into the submerged nozzle so that the estimated actual flow rate is below the critical flow rate; The method of embodiment 1. <Aspect 3> The relationship (2) is determined in advance through a model experiment using a low-melting-point metal. The method of embodiment 2. <Aspect 4> The relationship (1) is determined in advance through a model experiment using a low-melting-point metal. The method of any one of aspects 1 to 3. <Aspect 5> The cross-sectional average flow velocity x of the molten steel in the submerged entry nozzle and the void fraction y in the submerged entry nozzle are expressed by the following formula (I): y≦0.003×e 0.6x (I) (where y is dimensionless and x is in m / s) The inert gas is blown into the submerged nozzle so as to satisfy the above. The method of any one of aspects 1 to 4. [Effects of the Invention]

[0006] According to the method disclosed herein, in continuous casting of molten steel, when molten steel is supplied from an intermediate vessel to a mold via a sliding gate and an immersion nozzle, and inert gas is blown into the immersion nozzle, the inert gas can be prevented from flowing upstream into the intermediate vessel. [Brief explanation of the drawings]

[0007] [Figure 1] 1 shows a schematic diagram of a state in which molten steel is supplied from an intermediate vessel to a mold through a sliding gate and an immersion nozzle during continuous casting of molten steel, and an inert gas is blown into the immersion nozzle. [Figure 2] 1 illustrates the flow of the method of the present disclosure. [Figure 3] 1 is a diagram showing a relationship between the opening degree of the sliding gate and the supply flow rate of the inert gas at a predetermined molten steel throughput. [Figure 4] The diagram shows the schematic configuration of a model experimental device using low-melting-point metals. [Figure 5] FIG. 10 is a schematic diagram for explaining measurement of pressure loss. [Figure 6] 10 is a schematic diagram for explaining the concept of a method for estimating an actual flow rate of an inert gas supplied to a flow path. FIG. [Figure 7] 10 is a schematic diagram for explaining the concept of a method for estimating an actual flow rate of an inert gas supplied to a flow path. FIG. [Figure 8] The relationship between the SG opening and pressure loss, and the relationship between the inert gas flow rate and pressure loss are shown. [Figure 9] FIG. 10 is a diagram for explaining an example of a change in SG opening degree due to inert gas injection. [Figure 10] This shows an example of the relationship between the sliding gate opening, the throughput of molten steel in an actual machine (actual machine equivalent TP), and the critical flow rate of Ar upstream in an actual machine (upstream critical actual machine equivalent Ar flow rate). [Figure 11] The figure shows the relationship between the cross-sectional average flow velocity x of molten steel in the SEN and the runup critical void fraction y. DETAILED DESCRIPTION OF THE INVENTION

[0008] 1. Background According to the inventors' findings, when inert gas is injected into molten steel flowing through various channels between the ladle and tundish and between the tundish and mold, it is carried downstream if the flow rate is low. However, if the flow rate is high, not only is it carried downstream but some of the gas also flows upstream. When inert gas flows back up, the gas boils up onto the surface of the molten steel in the upper vessel (the ladle or tundish), stirring the slag and molten steel. Exposing the molten steel surface to the atmosphere promotes reoxidation, potentially reducing the purity of the molten steel. This phenomenon is unusual from the perspective of gas-liquid multiphase flow, and no detailed studies have been found. While reverse annular flow and flooding are examples of phenomena similar to the above-mentioned backflow, they are not strictly speaking the same phenomenon. Therefore, no models exist for determining the conditions under which backflow occurs or for estimating the resulting pressure loss (Reference 1). Reference 1: Revised Handbook of Gas-Liquid Two-Phase Flow, edited by the Japan Society of Mechanical Engineers, Corona Publishing

[0009] The inventors conducted investigations using liquid metal model experiments, water model tests, and full-scale tests. As a result, they found that the above-mentioned run-up phenomenon occurs under the following conditions: (1) the specific gravity of the gas phase is sufficiently smaller than that of the liquid phase, (2) a vessel is present upstream of the vertical pipe where the gas-liquid two-phase flow exists, and (3) the gas phase does not disperse but takes the form of large bubbles or a gas film. That is, the above-mentioned run-up phenomenon can be said to occur in a gas-liquid two-phase flow that satisfies these conditions, for example, a gas-liquid two-phase flow of inert gas and molten steel between an intermediate vessel and a mold in continuous casting.

[0010] On the other hand, it is difficult to understand the above-mentioned run-up phenomenon in detail from only the operation of the actual plant, and it is also difficult to carry out various studies, such as determining the inert gas flow rate that will prevent run-up. When trying to prevent run-up of inert gas in an actual plant, ad hoc measures are required, such as "injecting inert gas while increasing the flow rate of the inert gas, and then decreasing the flow rate when run-up begins," and the current situation is that it is difficult to sufficiently and reliably suppress run-up of inert gas.

[0011] 2. Methods for suppressing inert gas backflow The present inventors have conducted extensive research to solve the above-mentioned problem of inert gas backflow, and as a result, have found that by reproducing the backflow phenomenon in advance through experiments (which can be model experiments or actual tests) and comprehensively setting conditions, for example, on the sliding gate opening, throughput, and inert gas flow rate, it is possible to determine the critical value (critical flow rate) for inert gas backflow, and by controlling the inert gas flow rate in the actual plant based on the determined critical flow rate, it is possible to suppress inert gas backflow in the actual plant.

[0012] The method of the present disclosure will be described with reference to Figures 1 and 2. As shown in Figure 1, the method S10 of the present disclosure is a method for suppressing the inert gas 60 from flowing upstream into the intermediate vessel 20 when the molten steel 10 is supplied from the intermediate vessel 20 to a mold 50 via a sliding gate 30 and an immersion nozzle 40 during continuous casting of the molten steel 10, and the inert gas 60 is blown into the immersion nozzle 40. Also, as shown in Figure 2, the method S10 of the present disclosure is a method for suppressing the inert gas 60 from flowing upstream into the intermediate vessel 20. Step S1: determining in advance the relationship (1) between the opening degree of the sliding gate 30, the throughput of the molten steel 10, and the critical flow rate at which the inert gas 60 flows back; Step S2: In the continuous casting of the molten steel 10 (in actual continuous casting operation), injecting the inert gas 60 into the submerged entry nozzle 40 at a flow rate below the critical flow rate based on the opening of the sliding gate 30, the throughput of the molten steel 10, and the previously determined relationship (1); Includes.

[0013] 2.1 Process S1 In step S1, the relationship (1) between the opening of the sliding gate 30, the throughput of the molten steel 10, and the critical flow rate at which backflow of the inert gas 60 occurs is determined under conditions corresponding to the actual conditions in step S2. For example, conditions other than the opening and throughput (such as the shape of the submerged entry nozzle) are fixed to conditions corresponding to the actual conditions, and the relationship (1) between the opening, throughput, and critical flow rate can be determined by comprehensively adjusting the conditions for the opening, throughput, and inert gas flow rate. Alternatively, in step S1, the relationship (1) between the opening, throughput, critical flow rate, and other conditions may be determined. According to the inventor's new findings, there is a predetermined correlation between the opening of the sliding gate 30, the throughput, and the critical flow rate. For example, there is a relationship between the opening and critical flow rate such that the larger the opening, the smaller the critical flow rate, and there is a relationship between the throughput and critical flow rate such that the smaller the throughput, the smaller the critical flow rate. In other words, the larger the opening and the smaller the throughput, the smaller the critical flow rate, and the smaller the opening and the larger the throughput, the larger the critical flow rate. By summarizing these, the relationship (1) between the opening, throughput, and critical flow rate can be obtained.

[0014] The relationship (1) may be determined through various model experiments or numerical calculations. In particular, the relationship (1) may be determined in advance through model experiments using a low-melting-point metal (a metal with a melting point lower than that of steel, such as Sn or a Sn alloy, whose melting point is 300°C or less). In model experiments using a low-melting-point metal, dimensionless numbers such as the Froude number, modified Froude number, Reynolds number, and Weber number can be made to correspond to those in a molten steel system (Patent No. 6750533). This allows for accurate simulation of the flow state of gas and molten steel in a molten steel system. Specifically, the relationship between the sliding gate opening, throughput, and critical flow rate can be determined through model experiments using a low-melting-point metal, and then converted to a molten steel system to determine the relationship (1). The relationship (1) may be expressed mathematically as a function of the opening, throughput, and critical flow rate. The mathematical expression may be, for example, a polynomial. A specific form of the model experiment using a low melting point metal will be described in detail in the Examples.

[0015] The relationship (1) may be the relationship between the cross-sectional average flow velocity of the molten steel 10 in the submerged entry nozzle 40 and the void fraction in the submerged entry nozzle 40. According to the new findings of the present inventors, there is a relationship expressed by the following formula (I') between the backflow critical void fraction y' in the submerged entry nozzle 40 and the cross-sectional average flow velocity x of the molten steel 10 in the submerged entry nozzle 40. The following formula (I') can be said to be one specific example of "the relationship (1) between the opening degree of the sliding gate 30, the throughput of the molten steel 10, and the critical flow rate at which backflow of the inert gas 60 occurs" in step S1.

[0016] y'=0.003×e 0.6x (I') (where y' is dimensionless and x is in m / s.)

[0017] The "cross-sectional average flow velocity x of the molten steel 10 in the submerged entry nozzle 40" corresponds to the opening degree of the sliding gate 30 and the throughput of the molten steel 10, and can be determined based on the opening degree of the sliding gate 30, the throughput of the molten steel 10, the shape of the submerged entry nozzle 40, and the like. Specifically, the cross-sectional average flow velocity x is determined by dividing the throughput of the molten steel 10 by the cross-sectional area of ​​the passage through which the molten steel 10 passes. The position at which the cross-sectional average flow velocity x of the molten steel 10 in the submerged entry nozzle 40 is determined may be any position at which the cross-sectional average flow velocity x is maximum. For example, it may be a position at which the cross-sectional average flow velocity x is maximum upstream (toward the intermediate vessel) of the position at which the inert gas 60 is injected. In this embodiment, the maximum value x of the cross-sectional average flow velocity may be determined, and the runup critical void fraction y' may be determined from the maximum value x and the above formula (I'). The cross-sectional average flow velocity x may be, for example, 1 m / s or more and 6 m / s or less. Furthermore, the void fraction y in the submerged entry nozzle 40 corresponds to the flow rate of the inert gas 60 injected into the submerged entry nozzle 40; the greater the flow rate of the inert gas 60, the greater the void fraction y. The greater the void fraction y, the more likely the inert gas 60 to rise up. The "critical void fraction y'" refers to the maximum value (critical value) of the void fraction y at which the inert gas 60 does not rise up; when the void fraction y exceeds the critical void fraction y', the inert gas 60 will rise up. Note that the "void fraction" refers to the proportion of the gas flow rate to the total flow rate in the flow path, and is calculated assuming a homogeneous flow. The "void fraction" is the same value in the model experimental system as it is in the actual equipment conditions it simulates.

[0018] 2.2 Process S2 In step S2, in continuous casting of molten steel 10 (continuous casting using an actual machine), inert gas 60 is injected into the submerged entry nozzle 40 at a flow rate below the critical flow rate based on the opening of the sliding gate 30, the throughput of the molten steel 10, and the previously determined relationship (1). More specifically, in continuous casting using an actual machine, the critical flow rate is determined from the opening of the sliding gate 30, the throughput of the molten steel 10, and the above relationship (1) (a flow rate at which the upstream flow of the inert gas 60 can be suppressed is determined based on the critical flow rate), and the inert gas 60 is actually injected at a flow rate at which the upstream flow can be suppressed.

[0019] As described above, the flow rate at which the inert gas 60 is injected may be below the critical flow rate (a flow rate at which the inert gas 60 can be prevented from flowing back into the intermediate vessel 20). This prevents the inert gas 60 from flowing back into the intermediate vessel 20, thereby preventing the exposure and reoxidation of the molten steel surface in the intermediate vessel 20 due to boiling. The method and position at which the inert gas 60 is injected are not particularly limited. According to the method S10 of the present disclosure, the inert gas 60 can be prevented from flowing back regardless of the method and position at which the inert gas 60 is injected.

[0020] As described above, there is a relationship shown in the above formula (I') between the upstream critical void fraction y' in the SEN 40 and the cross-sectional average flow velocity x of the molten steel 10 in the SEN 40. In step S2, it can be said that by blowing the inert gas 60 into the SEN 40 so that the void fraction y in the SEN 40 is equal to or less than the upstream critical void fraction y', it is possible to suppress the upstream of the inert gas 60. Specifically, in step S2, the cross-sectional average flow velocity x of the molten steel 10 in the SEN 40 and the void fraction y in the SEN 40 are related by the following formula (I): y≦0.003×e 0.6x (I) (where y is dimensionless and x is in m / s) For example, when the cross-sectional average flow velocity x of the molten steel 10 in the submerged nozzle 40 is 5 m / s, the void fraction y is 0.003×e 3.0 An inert gas 60 may be blown into the submerged nozzle 40 so that the pore size becomes equal to or smaller than 0.060257 (=6.0257%).

[0021] 2.3 Other processes According to the findings of the present inventors, in actual continuous casting operations, the injection rate of the inert gas 60 may not coincide with the actual flow rate of the inert gas 60 actually supplied to the molten steel 10 in the submerged nozzle 40. This is because, in actual casting, some of the inert gas 60 may leak from gaps at the joints between components. In the method S10 of the present disclosure, it is preferable to accurately estimate the actual flow rate of the inert gas supplied into the submerged nozzle 40 and then ensure that the actual flow rate is below the critical flow rate. This is believed to more accurately and reliably suppress the inert gas 60 from flowing back up. For example, in the method S10 of the present disclosure, it is preferable to estimate the actual flow rate of the inert gas as in the following steps S3 and S4, and then inject the inert gas 60 into the submerged nozzle 40 so that the estimated actual flow rate is below the critical flow rate. Note that the following steps S3 and S4 are a preferred example of a method for estimating the actual flow rate of the inert gas. In the method S10 of the present disclosure, the actual flow rate of the inert gas may be estimated by a method different from the following steps S3 and S4.

[0022] That is, the method S10 of the present disclosure includes, in addition to the above steps S1 and S2: Step S3: determining in advance the relationship (2) between the actual flow rate of the inert gas 60 supplied to the molten steel 10 in the submerged entry nozzle 40 and the opening degree of the sliding gate 30 in accordance with the throughput of the molten steel 10; and Step S4: In the continuous casting of the molten steel 10 (in actual continuous casting operation), an actual flow rate of the inert gas 60 to be supplied to the molten steel 10 in the submerged entry nozzle 40 is estimated based on the opening degree of the sliding gate 30, the throughput of the molten steel 10, and the previously determined relationship (2). may further comprise The inert gas 60 may be blown into the submerged nozzle 40 so that the estimated actual flow rate is below the critical flow rate. Note that the step S3 may be performed before the step S1, after the step S1, or simultaneously with the step S1. The step S4 may be performed, for example, during the step S2.

[0023] 2.3.1 Process S3 In step S3, the relationship (2) between the actual flow rate of the inert gas 60 supplied to the molten steel 10 in the submerged entry nozzle 40 and the opening degree of the sliding gate 30 is determined in advance according to the throughput of the molten steel 10. The "actual flow rate of the inert gas supplied to the molten steel in the submerged entry nozzle" is the flow rate of the inert gas blown into the submerged entry nozzle minus the amount of gas leaking out of the submerged entry nozzle, and refers to the flow rate actually supplied to the molten steel in the submerged entry nozzle.

[0024] 1 and 3(A), in continuous casting of molten steel 10, the opening degree of the sliding gate 30 can be controlled so that the throughput (TP) of the molten steel 10 becomes a target value while maintaining a height H between a level M1 of the molten steel 10 in the intermediate vessel 20 and a level M2 of the molten steel 10 in the mold 50. For example, by maintaining the opening degree of the sliding gate 30 while maintaining the height H, the throughput of the molten steel 10 can be kept constant. When it is desired to increase the throughput of the molten steel 10, the opening degree of the sliding gate 30 is increased while maintaining the height H, and when it is desired to decrease the throughput of the molten steel 10, the opening degree of the sliding gate 30 is decreased while maintaining the height H.

[0025] In such a case, as shown in Fig. 3(B), when the inert gas 60 is injected into the submerged entry nozzle 40, a pressure loss occurs corresponding to the amount of the inert gas 60 that is actually supplied to the molten steel 10 without leaking to the outside. That is, in order to maintain the throughput of the molten steel 10 when the inert gas 60 is injected into the submerged entry nozzle 40, it is necessary to increase the opening degree of the sliding gate 30 to overcome the pressure loss caused by the inert gas 60 supplied to the molten steel 10. In other words, there is a predetermined correlation between the actual flow rate of the inert gas 60 supplied to the molten steel 10 in the submerged entry nozzle 40 and the opening degree of the sliding gate 30, depending on the throughput of the molten steel 10.

[0026] The relationship between the actual flow rate of the inert gas 60 supplied to the molten steel 10 in the SEN 40 and the opening degree of the sliding gate 30 may be determined through various model experiments, numerical calculations, and the like. In particular, the relationship (2) may be determined in advance through model experiments using low-melting-point metals. As described above, model experiments using low-melting-point metals allow dimensionless numbers such as the Froude number, modified Froude number, Reynolds number, and Weber number to be matched with those in the molten steel system. Furthermore, the equipment used in the model experiments can also create an ideal state in which there is virtually no leakage of inert gas. This allows the relationship between the flow rate of the inert gas actually supplied to the low-melting-point metal in the SEN (= the flow rate of the inert gas injected into the SEN) and the opening degree of the sliding gate to be determined. Converting this to a molten steel system allows the relationship (2) between the actual flow rate of the inert gas 60 supplied to the molten steel 10 in the SEN 40 and the opening degree of the sliding gate 30 to be determined. The relationship (2) may be expressed by a mathematical formula as a function of the opening degree of the sliding gate 30 and the actual flow rate of the inert gas. A specific form of the model experiment using a low-melting-point metal will be described in detail in the Examples.

[0027] 2.3.2 Process S4 In step S4, during continuous casting of the molten steel 10, the actual flow rate of the inert gas 60 supplied to the molten steel 10 in the submerged nozzle 40 is estimated based on the opening degree of the sliding gate 30, the throughput of the molten steel 10, and the previously determined relationship (2).

[0028] In step S4, for example, in continuous casting of molten steel 10, the actual flow rate of the inert gas 60 supplied to the molten steel 10 in the submerged entry nozzle 40 can be estimated simply by obtaining information on the opening degree of the sliding gate 30 and information on the throughput of the molten steel 10. The opening degree of the sliding gate 30 and the throughput of the molten steel 10 can be measured or determined directly or indirectly by known methods that are obvious to those skilled in the art.

[0029] For example, if the opening degree of the sliding gate 30 required to maintain a predetermined molten steel throughput during continuous casting of molten steel 10 without injecting inert gas ( FIG. 3(A) ) is assumed to be the “initial opening degree,” then, when inert gas is injected while maintaining that throughput, the opening degree of the sliding gate 30 can be increased by a predetermined amount from the initial opening degree in response to the actual flow rate of the inert gas supplied to the molten steel 10 in the submerged entry nozzle 40 ( FIG. 3(B) ). Furthermore, if the actual flow rate of the inert gas supplied to the molten steel 10 in the submerged entry nozzle 40 changes due to a change in the amount of inert gas injected or leakage, for example, the opening degree of the sliding gate 30 also changes in accordance with the change in the actual flow rate. In other words, according to steps S3 and S4, the actual flow rate of the inert gas supplied to the molten steel 10 in the submerged entry nozzle 40 and the amount of change therein can be estimated from the opening degree of the sliding gate 30 and the amount of change therein while maintaining the throughput of the molten steel 10. That is, "estimating the actual flow rate" in the present application includes not only a form of estimating the actual flow rate itself, but also a form of estimating the amount of change in the actual flow rate.

[0030] 3. Supplementary Information As described above, the continuous casting machine and continuous casting conditions employed in the method S10 of the present disclosure may be any conditions capable of suppressing the inert gas 60 from flowing back into the intermediate vessel 20. The type (steel grade) of the molten steel 10 is not particularly limited, as long as it is an alloy containing iron. Furthermore, the configurations of the intermediate vessel 20 (e.g., a tundish), the sliding gate 30, the submerged entry nozzle 40, and the mold 50 are not particularly limited. The mechanism and injection position of the inert gas 60 are also not particularly limited. For example, a flow path (hole) for introducing the inert gas 60 may be provided in the side wall of the sliding gate 30 or the submerged entry nozzle 40, and the inert gas 60 may be injected through this path. Alternatively, the inert gas 60 may be injected upstream of the sliding gate 30. The type of inert gas 60 is also not particularly limited, and may be, for example, argon (Ar). According to the method S10 of the present disclosure, it is possible to suppress the inert gas 60 from flowing back into the intermediate vessel 20 without introducing any new equipment into a conventional continuous casting machine.

[0031] 4. Variations A modified example based on the knowledge regarding the above formula (I) will be described. The method according to the modified example of the present disclosure is a method for suppressing the inert gas 60 from flowing upstream into the intermediate vessel 20 when the molten steel 10 is supplied from the intermediate vessel 20 to the mold 50 through the sliding gate 30 and the submerged entry nozzle 40 during continuous casting of the molten steel 10 and the inert gas 60 is blown into the submerged entry nozzle 40, the method comprising: The cross-sectional average flow velocity x of the molten steel 10 in the submerged entry nozzle 40 and the void fraction y in the submerged entry nozzle 40 are expressed by the following formula (I): y≦0.003×e 0.6x (I) (where y is dimensionless and x is in m / s) The inert gas 60 is blown into the submerged nozzle 40 so as to satisfy the following condition. The meaning of formula (I) is as described above. The method according to this modification can also appropriately suppress the inert gas from flowing upstream.

[0032] In the method according to the above modification, the above steps S3 and S4 may also be performed. Step S3: determining in advance the relationship (2) between the actual flow rate of the inert gas 60 supplied to the molten steel 10 in the submerged entry nozzle 40 and the opening degree of the sliding gate 30 in accordance with the throughput of the molten steel 10; and Step S4: In the continuous casting of the molten steel 10 (in actual continuous casting operation), an actual flow rate of the inert gas 60 to be supplied to the molten steel 10 in the submerged entry nozzle 40 is estimated based on the opening degree of the sliding gate 30, the throughput of the molten steel 10, and the previously determined relationship (2). may further comprise The inert gas 60 may be blown into the submerged nozzle 40 so that the void fraction y based on the estimated actual flow rate satisfies the above formula (I). Details of steps S3 and S4 are as described above. [Example]

[0033] The present invention will be further explained below with reference to model experiments using low-melting-point metals, but the present invention is not limited to the following examples. The present invention allows various conditions to be adopted as long as the object is achieved without departing from the gist of the invention.

[0034] 1. Consideration of methods for estimating the actual flow rate of inert gas (steps S3 and S4 above) 1.1 Experimental conditions Figure 4 shows an overview of the equipment used in the model experiment to study a method for estimating the actual inert gas flow rate. The position of the sliding gate (SG), the submerged entry nozzle, and the mold vessel shape were modeled after those of a real continuous casting machine. In the experiment, the entire equipment shown in Figure 4 was heated and maintained with a heater. A vacuum was drawn in the simulated tundish (supply tank), filling it with molten tin (melting point 231.9°C). The pressure was maintained constant by operating the suction of the vacuum pump VP via a manual valve. The molten tin was circulated by the electromagnetic pump MP, and the circulating flow rate was measured with an electromagnetic flow meter MFM. The molten tin level in the simulated mold was measured with a laser level meter LL, and the molten tin level in the simulated mold was maintained constant by operating various manual valves. The pressure in the circulation system was measured with a pressure gauge. The molten tin level in the circulation system was measured with a microwave level meter ML.

[0035] Argon (Ar) was blown into the SG structure from an inlet above the immersion nozzle. The SG structure reproduced a flow path with the SG opening fixed at a constant SG opening condition (%), as described below.

[0036] Here, the "SG opening" can be determined based on the stroke amount of the middle sliding plate among the three plates, as shown in FIG. 3, for example. Specifically, if the stroke rate of the sliding plate is SR (see formula (I) below), the SG opening (%) can be determined as 100-SR. That is, when the circular hole in the sliding plate is fully open to the flow path, the stroke rate is 0% and the SG opening is 100%. When the circular hole in the sliding plate is fully closed to the flow path, the stroke rate is 100% and the SG opening is 0%. Similarly, for example, when the stroke rate is 75%, the SG opening is 25%. In this experiment, the SG opening was determined based on the stroke rate. However, the SG opening may also be determined based on the area opening. That is, the SG opening may be determined as the ratio (a / A × 100%) of the area a of the portion facing the flow path (the portion functioning as an opening) to the total area (A) of the circular hole in the sliding plate. SR=X / D0×100% …(I) D0: Diameter of the circular hole in the sliding plate (mm) X: The stroke amount (mm) of the sliding plate, which is 0 when fully open and D0 when fully closed.

[0037] Eight 0.5 mm diameter holes were evenly spaced around the circumference of the inner wall of the tube above the SG structure. An air storage chamber was installed outside the submerged entry nozzle so as to cover all of the 0.5 mm diameter holes. After filling the air storage chamber, Ar was blown into the tube evenly through each hole without a break. The Ar flow rate was kept constant using a mass flow controller.

[0038] The circulation flow rate, pressure in the tundish, and molten metal surface height in the tundish and mold were measured while changing the SG opening, the Ar flow rate, and the electromagnetic pump output.

[0039] 1.2 Measurement of pressure loss In the above experimental system, the state before the electromagnetic pump is operated is as shown in Figure 5(A). The pressure inside the tundish at this time is P, and the height difference between the molten metal surface in the tundish and the molten metal surface in the mold is H. Next, Figure 5(B) shows the state after the electromagnetic pump is operated. The circulation flow rate of Sn circulated by the pump is Q, and the representative speed calculated from this is v. The increase or decrease in the pressure inside the tundish at this time is ΔP, and the increase or decrease in the height difference is ΔH. Using Bernoulli's equation with a focus on the tundish-mold relationship, the fluid energy E can be expressed as shown in equation (1) below.

[0040] E=(1 / 2)Pv 2 +ΔP+ρgΔH+ε0(v) (1)

[0041] where ρ is the density of the fluid, g is the acceleration due to gravity, and ε0 is the pressure loss caused by the flow path structure, such as elbows, expansions and contractions, and wall friction, and is generally expressed as a function of velocity v.

[0042] Next, when the SG is closed or Ar is being blown in, the following equation (2) shows the case where the same circulation flow rate Q (i.e., the same representative velocity v) as in equation (1) is maintained. As an example, the state when the SG is closed is shown in Figure 5(C).

[0043] E+ΔE=(1 / 2)Pv 2 +ΔP'+ρgΔH'+ε0(v) ···(2)

[0044] In this case, the pressure loss due to SG or Ar injection is taken into account, so the required fluid energy increases by ΔE. Therefore, the increase or decrease in pressure and the increase or decrease in height difference may not match the values ​​shown in equation (1). Therefore, the respective notations are ΔP' and ΔH'. Considering the difference between equations (1) and (2), the increase in fluid energy, i.e., the pressure loss, can be expressed as equation (3) below.

[0045] ΔE=ΔP-ΔP'+ρg(ΔH-ΔH') ···(3)

[0046] In the above experimental system, all values ​​on the right side of equation (3) are known or measurable, so it is possible to measure the pressure drop due to SG or Ar injection.

[0047] 1.3 Concept of the method for estimating the actual flow rate of Ar supplied to the flow path The actual flow rate of Ar in an actual machine can be estimated using the pressure loss due to SG and Ar injection calculated by the above formula (3). In an actual continuous casting machine, the mold and tundish are both open to the atmosphere, so the pressure is always constant. In this state, the throughput TP and the molten metal surface position in the mold and the molten metal surface position in the tundish are kept constant by automatic control of the SG opening. In other words, the only variable term in the above formula (2) is the pressure loss term, and if the other terms are constant, the pressure loss term will also be constant. In this case, for example, if the pressure loss due to the SG is ΔE sg = a [kPa] (Fig. 6, left side). Here, the same pressure loss ΔE ar Consider a situation where Ar is injected, causing a pressure drop of 0.01kPa. In this case, the SG is expected to be fully open due to automatic control of the SG opening to keep the throughput TP, the molten metal level in the mold, the molten metal level in the tundish, and pressure loss constant (right side of Fig. 6).

[0048] Next, let us consider a case where the actual flow rate of Ar supplied is unknown, similar to the actual situation in an actual machine. When a certain throughput TP is maintained and the SG opening is 50%, the pressure loss due to the SG is ΔE sg Let us assume a casting situation where the pressure is equal to a [kPa] (Fig. 7, left side). Here, we consider a situation where an unknown amount of Ar is injected. As a result, the SG opening is changed to 25% by automatic control. At this time, the pressure loss due to the SG is ΔE sg ' = c [kPa], the pressure loss due to the Ar blown in is ΔE ar= ac [kPa]. In other words, the Ar flow rate corresponding to this pressure loss was injected as the actual flow rate (right side of Figure 7). Note that the above approach to estimating the actual flow rate requires the SG opening and pressure loss due to Ar injection at each throughput as reference values, but this can be achieved by measuring the pressure loss under each condition in a model experiment.

[0049] 1.4 Experimental results 1.4.1 Pressure loss due to SG opening and Ar injection measured in the experimental system Based on the above idea, the pressure loss due to SG opening and Ar injection was measured in the above model experiment. The results are shown in Figures 8(A) and (B). As shown in Figures 8(A) and (B), for both SG opening and Ar injection, the pressure loss was on the order of several kPa to several tens of kPa, which is within a comparable numerical range for each pressure loss.

[0050] As an example of applying the above concept, Figure 9 shows the predicted change in SG opening when Ar is injected at 0.3 NL / min under the initial conditions (throughput TP = 4.9 ton / min and SG opening = 45%). The values ​​in the figure are based on the measurement results shown in Figures 8(A) and 8(B). In the initial state, the pressure loss is only 20 kPa due to the SG opening (Figure 9(1)). From this, it can be seen that if the throughput TP and the molten surface position between the tundish and the mold are maintained by automatic control even after Ar injection, the pressure loss in the system will be maintained at 20 kPa (Figure 9(2)). On the other hand, the pressure loss due to an Ar flow rate of 0.3 NL / min is 9 kPa (Figure 9(3)). Consequently, the SG opening is changed to 52% (Figure 9(4)), resulting in a pressure loss of 11 kPa, so that the total pressure loss due to the SG opening and Ar injection becomes 20 kPa.

[0051] 1.4.2 Summary As described above, model experiments using low-melting-point metals enable the measurement of pressure loss during SG operation and Ar injection. This is because the experimental system allows accurate measurement of the molten metal level in the tundish, the molten metal level in the mold, the pressure in the tundish, the Ar flow rate, and the SG opening. Furthermore, the pressure loss during Ar injection from near the top of the SG was measured in the model experiments, as well as the pressure loss of the SG alone. The results showed that there was no significant difference in the order of pressure loss due to SG opening and Ar injection. However, the SG pressure loss was significant depending on the SG opening, demonstrating the validity of estimating the actual Ar flow rate as described above. When these results are applied to an actual process, the actual Ar flow rate supplied to the molten steel in the SEN can be estimated from the change in SG opening during Ar injection. This is based on the idea that the pressure loss in the system is maintained constant by controlling the SG opening in the actual process. Reference values ​​for the SG opening and pressure loss due to Ar injection should be determined in advance, such as through model experiments using low-melting-point metals.

[0052] 1.5 Examination on actual equipment The validity of the above estimation of the actual Ar flow rate was indirectly confirmed by conducting a continuous casting test in an actual machine.

[0053] First, prior to the continuous casting test, a molten metal model experiment (using the above-mentioned low-melting-point metal) was conducted to simulate the continuous casting test, and the relationship between the Ar flow rate and the sliding gate opening degree at each throughput was clarified.

[0054] Next, a continuous casting test was conducted using an actual machine corresponding to the above model experiment. Here, Ar was injected only through the three-plate sliding gate during continuous casting. The actual flow rate of Ar injected from the sliding gate into the molten steel in the SEN was estimated based on the SG opening and the relationship obtained from the above model experiment. The results of (1) nozzle clogging and (2) the amount of pore defects on the slab surface were investigated for cases where Ar injection was controlled so that the estimated value was a predetermined value, and cases where such control was not performed.

[0055] In the following continuous casting tests, Ar was injected at a rate of 10 NL / min at a casting speed of Vc = 1.5 m / min, and the estimated actual flow rate of Ar was 1 NL / min, which was used as the "reference." Note that the Ar injection flow rate (10 NL / min) here refers to the flow rate set as a constant flow rate by a mass flow controller installed upstream of the Ar injection unit. The same applies below.

[0056] (1) Nozzle blockage After the completion of continuous casting, the inner wall of the submerged nozzle was observed, and the thickness was converted into an index, where the thickness of the layer adhering to the inner wall in the above-mentioned "criteria" was assigned a value of "1" and the absence of a layer adhering to the inner wall was assigned a value of "0," and this was evaluated as the "clogging index."

[0057] (2) Pore defects on the surface of the slab In the above "standard," the number of cellular defects present in the surface 10 mm of the cast piece obtained after continuous casting was indexed as "1," and the absence of any cellular defects was indexed as "0," and this was evaluated as the "cellular defect index."

[0058] 1.5.1 Example 1 This is an example of the "reference" mentioned above. In the above continuous casting test, when Ar was injected at 10 NL / min for a casting speed of Vc = 1.5 m / min, the estimated actual flow rate of Ar was 1 NL / min. In other words, it was estimated that only 1 / 10 of the injected amount of Ar was actually injected.

[0059] 1.5.2 When sealing performance is reduced 1.5.2.1 Example 2 After intentionally reducing the Ar sealing property in the continuous casting machine, a continuous casting test was conducted in the same manner as in Example 1. In this case, when Ar was injected at 10 NL / min at a casting speed of Vc = 1.5 m / min, the estimated actual flow rate of Ar was 0.5 NL / min. In other words, it was estimated that Ar was only injected at half the actual flow rate of Example 1. Continuous casting was conducted while maintaining this condition.

[0060] 1.5.2.2 Example 3 In a continuous casting machine similar to that of Example 2, the Ar flow rate was increased to optimize the estimated value of the actual Ar flow rate. By increasing the Ar flow rate to 15 NL / min, the estimated value became 1.0 NL / min. Continuous casting was continued while maintaining the estimated value at 1.0 NL / min.

[0061] 1.5.3 When sealing performance is improved 1.5.3.1 Example 4 After intentionally strengthening the Ar sealing performance in the continuous casting machine, a continuous casting test was conducted in the same manner as in Example 1. In this case, when Ar was injected at 10 NL / min at a casting speed of Vc = 1.5 m / min, the estimated actual flow rate of Ar was 2 NL / min. In other words, it was estimated that Ar was being injected at an actual flow rate twice that of Example 1. Continuous casting was conducted while maintaining this condition.

[0062] 1.5.3.2 Example 5 In a continuous casting machine similar to that of Example 4, the Ar injection flow rate was reduced to optimize the estimated value of the actual Ar flow rate. By reducing the Ar injection flow rate to 5 NL / min, the estimated value became 1.0 NL / min. Continuous casting was continued while maintaining the estimated value at 1.0 NL / min.

[0063] 1.5.4 Evaluation Results Table 1 below shows the evaluation results of the clogging index and the bubble defect index for Examples 1 to 5.

[0064] [Table 1]

[0065] As is clear from the results shown in Table 1, regardless of the sealing performance of the Ar in the continuous casting machine, by changing the set flow rate of Ar injection so that the estimated value of the actual Ar flow rate was 1 NL / min, the same results as in Example 1, which was the reference, were obtained for both the blockage index and the bubble defect index (Examples 3 and 5). This proves that the estimation of the actual Ar flow rate was valid.

[0066] On the other hand, in Example 2, where the estimated value of the actual Ar flow rate was maintained at 0.5 NL / min, the bubble defect index decreased and the blockage index increased compared to the reference Example 1. This can be said to be the result of a decrease in the actual flow rate of Ar supplied to the molten steel, which reduced the number of bubbles in the molten steel while also reducing the effectiveness of floating and removing inclusions, etc., and can be said to prove that the estimation of the actual Ar flow rate was valid.

[0067] In addition, in Example 4, where the estimated value of the actual Ar flow rate was maintained at 2 NL / min, the clogging index decreased but the bubble defect index increased compared to the reference Example 1. This can be said to be the result of an increase in the actual flow rate of Ar supplied to the molten steel, which improved the effect of floating and removing inclusions, etc., while also increasing the number of bubbles in the molten steel, and it can be said to prove that the estimation of the actual Ar flow rate was valid.

[0068] From the above results, it can be said that by determining in advance the relationship between the actual flow rate of inert gas supplied to molten steel in the SEN and the SG aperture depending on the molten steel throughput based on low-melting-point metal model experiments, etc., it is possible to obtain a reasonable estimate of the actual flow rate of inert gas supplied to molten steel in the SEN during actual continuous casting operation based on the SG aperture, the molten steel throughput, and the previously determined relationship. In other words, it is possible to appropriately estimate the actual state of inert gas injection into molten steel in the SEN, enabling appropriate injection depending on, for example, the use of the cast slab and the conditions of continuous casting.

[0069] 2. Consideration of methods to suppress inert gas backflow (steps S1 and S2 above) The critical condition at which the inert gas backflow occurs under actual test conditions was determined in advance by liquid metal model experiments.

[0070] 2.1 Experimental conditions This study also used the apparatus shown in Figure 4 for the experiment. The position of the sliding gate, the submerged entry nozzle, and the mold vessel shape were all modeled after those of a real continuous casting machine. In the experiment, the entire apparatus shown in Figure 4 was heated and maintained with a heater. A vacuum was drawn in the simulated tundish (supply tank) to fill it with molten Sn (melting point 231.9°C). The pressure was maintained constant by operating the suction of the vacuum pump VP via a manual valve. The molten Sn was circulated by the electromagnetic pump MP, and the circulating flow rate was measured with an electromagnetic flow meter MFM. The molten Sn level in the simulated mold was measured with a laser level meter LL, and the molten Sn level in the simulated mold was maintained constant by operating various manual valves. The pressure in the circulation system was measured with a pressure gauge. The molten Sn level in the circulation system was measured with a microwave level meter ML. In this experiment, the simulated object was an actual submerged entry nozzle with an inner diameter of 83 mm and an inner diameter of the sliding gate, a distance from the molten metal surface in the mold to the sliding gate middle plate of 600 mm, and a distance from the molten metal surface in the mold to the Ar injection position of 700 mm, but this is merely an example. The technology disclosed herein can also identify the predetermined relationship for other simulated objects using a method similar to that described below.

[0071] During circulation operation in this experimental system, the flow rate of Ar injected through the submerged entry nozzle was gradually increased, and the Ar injection flow rate at which run-up began was measured. When run-up occurred, Ar bubbles were observed rising on the surface of the molten metal in the simulated tundish. This was accompanied by a recovery of the pressure inside the simulated tundish, causing large fluctuations in the circulation flow rate. The minimum Ar injection flow rate at which this phenomenon occurred was identified as the critical flow rate for Ar run-up. The critical flow rate can also be determined based on the void fraction. For example, the void fraction is defined as the ratio of the gas flow rate to the total flow rate in the channel, assuming a homogeneous flow. The void fraction is the same for the model experimental system and the actual conditions it simulates. Therefore, the Ar flow rate in the actual machine can also be calculated based on the void fraction.

[0072] 2.2 Experimental results Figure 10 shows the experimentally calculated throughput and the calculated critical flow rate for each sliding gate opening. In the liquid metal model experiment, the molten metal surface position and pressure inside the simulated tundish vessel can be adjusted, so different throughputs can be generated even with the same sliding gate opening. Utilizing this feature, as shown in Figure 10, a throughput-critical flow rate curve for each opening was obtained. By overlaying the sliding gate opening and molten steel throughput conditions assumed in actual operation on the throughput-critical flow rate curve graph, the critical flow rate assumed in actual operation could be obtained.

[0073] As described above, through the liquid metal model experiment, we were able to determine the relationship (1) between the SG opening, the molten steel throughput, and the critical flow rate at which inert gas backflow occurs. Furthermore, in actual operation, the actual Ar flow rate was estimated using steps S3 and S4, and then Ar was injected so that the estimated actual flow rate was below the critical flow rate determined based on the SG opening, the molten steel throughput, and the relationship (1). It was confirmed that Ar backflow did not occur. However, in actual operation, steps S3 and S4 are optional. The actual Ar flow rate may be estimated using a method other than these, or estimation of the actual Ar flow rate may be omitted. However, by estimating the actual Ar flow rate using steps S3 and S4 and injecting Ar so that the estimated actual flow rate is below the critical flow rate determined above, Ar backflow can be more reliably suppressed.

[0074] It is expected that the critical flow rate will change if conditions such as the nozzle inner diameter, Ar injection position, molten metal surface position in the intermediate vessel, and molten metal surface position in the mold are changed. However, even in this case, by conducting liquid metal model experiments while changing the conditions in the simulated object, it is possible to determine the relationship (1) between the sliding gate opening, throughput, and the critical flow rate at which inert gas backflow occurs (for example, a graph of the critical flow rate curve).

[0075] 2.3 Beneficial effects of suppressing run-up of inert gas In an actual operation of continuous casting of molten steel, the actual flow rate of Ar was estimated as in steps S3 and S4 above, and Ar was injected so that the estimated actual flow rate was below the critical flow rate determined as in steps S1 and S2 above (Example 6), and the properties of the resulting cast slab were compared between two cases where Ar was injected so that the estimated actual flow rate was above the critical flow rate (Example 7). The continuous casting machine used in Examples 6 and 7 had a tundish capacity of 60 tons, was a two-strand machine, and had a steady-state throughput of 3.5 ton / min.

[0076] In Example 6, as described above, no Ar flow-up occurred, and reoxidation due to exposure of the molten steel surface in the tundish did not occur. In contrast, in Example 7, Ar flow-up occurred, and the molten steel surface in the tundish was exposed due to boiling, resulting in reoxidation of the molten steel. As a result, the slab obtained in Example 7 contained more oxide-based inclusions than the slab obtained in Example 6, and the slab in Example 7 was made of molten steel with lower cleanliness. Specifically, elemental analysis of the slab samples obtained in Examples 6 and 7 revealed that the oxygen content in the sample in Example 6 was reduced by 5 ppm compared to Example 7. Since it is believed that almost all of the oxygen introduced by reoxidation exists in the molten steel as inclusions, the decrease in the oxygen content can be interpreted as a decrease in the amount of inclusions, and the slab in Example 6 was made of molten steel with higher cleanliness.

[0077] 3. Summary From the above examples, it can be said that the method satisfying the following (A) can suppress the backflow of inert gas during continuous casting of molten steel. Furthermore, it can be said that in the method satisfying the above (A), it is preferable that the following (B) is further satisfied. (A) A method for suppressing backflow of an inert gas into an intermediate vessel during continuous casting of molten steel, when the molten steel is supplied from the intermediate vessel to a mold via a sliding gate and an immersion nozzle and an inert gas is injected into the immersion nozzle, the method comprising: determining in advance a relationship (1) between the opening of the sliding gate, the throughput of the molten steel, and a critical flow rate at which backflow of the inert gas occurs; and, during the continuous casting of the molten steel, injecting the inert gas into the immersion nozzle at a flow rate below the critical flow rate based on the opening of the sliding gate, the throughput of the molten steel, and the previously determined relationship (1). (B) determining in advance a relationship (2) between the actual flow rate of the inert gas supplied to the molten steel in the submerged entry nozzle and the opening degree of the sliding gate in accordance with the throughput of the molten steel, and estimating the actual flow rate of the inert gas supplied to the molten steel in the submerged entry nozzle in the continuous casting of the molten steel based on the opening degree of the sliding gate, the throughput of the molten steel, and the previously determined relationship (2), and injecting the inert gas into the submerged entry nozzle so that the estimated actual flow rate is below the critical flow rate.

[0078] 4. Study on the relationship between the average flow velocity in the SEN and the critical void fraction for run-up in the SEN In addition to the above examples, further investigation was conducted into the relationship between the average flow velocity of molten steel in the SEN and the critical void fraction for backflow in the SEN. Experiments conducted under various conditions revealed that the critical void fraction for backflow y' (the void fraction at which backflow occurs) varies depending on the average cross-sectional flow velocity x in the SEN (the average flow velocity in a cross section perpendicular to the longitudinal direction of the SEN). Figure 11 shows the relationship between the average cross-sectional flow velocity x and the critical void fraction for backflow y'. As shown in Figure 11, the larger the average cross-sectional flow velocity x, the larger the critical void fraction for backflow y'. The results shown in Figure 11 reveal that there is a relationship between the critical void fraction for backflow y' in the SEN 40 and the average cross-sectional flow velocity x of the molten steel 10 in the SEN 40, as expressed by the following formula (I'):

[0079] y'=0.003×e 0.6x (I') (where y' is dimensionless and x is in m / s)

[0080] From the above results, it can be said that it is possible to suppress the backflow of the inert gas by injecting the inert gas into the submerged entry nozzle 40 so that the void fraction y in the submerged entry nozzle 40 is equal to or less than the above-mentioned critical backflow void fraction y'. That is, when the cross-sectional average flow velocity x of the molten steel in the submerged entry nozzle and the void fraction y in the submerged entry nozzle are expressed by the following formula (I): y≦0.003×e 0.6x (I) (where y is dimensionless and x is in m / s) It can be said that by blowing inert gas into the submerged entry nozzle so as to satisfy the above condition, it is possible to prevent the inert gas from flowing back up. [Explanation of symbols]

[0081] 10 Molten Steel 20 Intermediate container 30 Sliding Gate 40 Submerged Entry Nozzle 50 Mold 60 Inert Gas

Claims

1. A method for suppressing inert gas from flowing back into an intermediate vessel during continuous casting of molten steel, in which the molten steel is supplied from the intermediate vessel to a mold through a sliding gate and an immersion nozzle, and an inert gas is blown into the immersion nozzle, comprising: A relationship (1) between the opening degree of the sliding gate, the throughput of the molten steel, and a critical flow rate at which the backflow of the inert gas occurs is obtained in advance; and In the continuous casting of the molten steel, injecting the inert gas into the submerged entry nozzle at a flow rate below the critical flow rate based on the opening of the sliding gate, the throughput of the molten steel, and the previously determined relationship (1); A method comprising:

2. determining in advance a relationship (2) between an actual flow rate of the inert gas supplied to the molten steel in the submerged nozzle and an opening degree of the sliding gate in accordance with a throughput of the molten steel; and In the continuous casting of the molten steel, estimating an actual flow rate of the inert gas to be supplied to the molten steel in the submerged entry nozzle based on an opening degree of the sliding gate, a throughput of the molten steel, and the previously determined relationship (2). further comprising blowing the inert gas into the submerged nozzle so that the estimated actual flow rate is below the critical flow rate; The method of claim 1.

3. The relationship (2) is determined in advance by a model experiment using a low-melting-point metal. The method of claim 2.

4. The relationship (1) is determined in advance by a model experiment using a low-melting-point metal. The method according to any one of claims 1 to 3.

5. The cross-sectional average flow velocity x of the molten steel in the submerged entry nozzle and the void fraction y in the submerged entry nozzle are expressed by the following formula (I): y≦0.003×e ０．６ｘ ・・・(I) (where y is dimensionless and x is in m / s) The inert gas is blown into the submerged nozzle so as to satisfy the above. The method according to any one of claims 1 to 3.

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