Molten steel continuous casting method and continuous casting apparatus

Abstract

To disclose a method for purifying molten steel by utilizing backflow of an inert gas in continuous casting of molten steel.SOLUTION: A molten steel continuous casting method according to the present disclosure includes supplying the molten steel from a tundish to a mold through a nozzle, and blowing an inert gas into the nozzle. The inert gas is blown into the nozzle to satisfy the following relationship (1): 0.003×e0.69x≤y≤0.018×e0.39x, where y is the void ratio (-) in the nozzle, and x is the cross-sectional average flow velocity (m / s) of the molten steel in the nozzle.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present application discloses a method and apparatus for continuously casting molten steel. [Background technology]

[0002] Patent Document 1 discloses a method for adjusting the flow rate of inert gas blown into the nozzle by controlling a valve that adjusts the flow rate of 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 nozzle, and inert gas is blown into the 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] In continuous casting of molten steel, to obtain a cast piece with few defects due to inclusions, it is important to prevent the formation of inclusions in the molten steel, prevent the inclusions from being mixed into the molten steel, and remove the inclusions from the molten steel. For example, by ensuring as long a flow path as possible from the molten steel poured into the intermediate vessel to the nozzle, or by equalizing the flow velocity difference within the intermediate vessel, known as a plug flow, it is thought that inclusions can float or settle due to the difference in specific gravity with the molten steel. If a so-called short pass occurs in the intermediate vessel, in which the poured molten steel flow preferentially flows down the surface of the molten steel or the bottom of the intermediate vessel, the floating and settling effects of inclusions will be insufficient. Conventional continuous casting methods and continuous casting apparatuses for purifying molten steel and reducing inclusion defects in the cast piece still have room for improvement. [Means for solving the problem]

[0005] The present application discloses the following aspects as means for solving the above problems. <Aspect 1> A method for continuous casting of molten steel, comprising the steps of: The molten steel is supplied from the intermediate vessel to a mold through a nozzle, blowing an inert gas into the nozzle; Including, The following relationship (1): 0.003×e 0.69x ≦y≦0.018×e 0.39x (1) y: Void fraction in the nozzle (-) x: average cross-sectional flow velocity of the molten steel in the nozzle (m / s) The inert gas is blown into the nozzle so that A method for continuous casting of molten steel. <Aspect 2> A method for continuous casting of molten steel according to aspect 1, comprising: estimating the actual flow rate of the inert gas injected into the nozzle; Estimating a void fraction y in the nozzle based on the estimated actual flow rate; and injecting the inert gas into the nozzle so that the estimated void fraction y and the cross-sectional average flow velocity x of the molten steel in the nozzle satisfy the relationship (1); A method for continuous casting of molten steel, comprising: <Aspect 3> A continuous casting apparatus for molten steel, comprising an intermediate vessel, a nozzle, a mold, and an inert gas blowing means, The following relationship (1): 0.0025×e 0.6931x ≦y≦0.0176×e 0.3971x (1) y: Void fraction in the nozzle (-) x: average cross-sectional flow velocity of the molten steel in the nozzle (m / s) Inert gas is blown into the nozzle from the inert gas blowing means so that A continuous casting apparatus for molten steel, comprising: [Effects of the Invention]

[0006] According to the method and apparatus for continuous casting of molten steel disclosed herein, it is possible to purify the molten steel and reduce defects in the physical properties of inclusions in the cast slab. [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 nozzle during continuous casting of molten steel, and an inert gas is blown into the nozzle. [Figure 2] 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 3] The diagram shows the schematic configuration of a model experimental device using low-melting-point metals. [Figure 4] FIG. 10 is a schematic diagram for explaining measurement of pressure loss. [Figure 5] 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 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] The relationship between the SG opening and pressure loss, and the relationship between the inert gas flow rate and pressure loss are shown. [Figure 8] FIG. 10 is a diagram for explaining an example of a change in SG opening degree due to inert gas injection. [Figure 9] The graph shows the relationship between the cross-sectional average flow velocity x of the molten steel in the nozzle, the void fraction y in the nozzle, and the cleanliness of the molten steel (○: good, △: slightly poor, ×: significantly poor). DETAILED DESCRIPTION OF THE INVENTION

[0008] 1. Background According to the inventors' findings, when inert gas is injected into molten steel flowing through the ladle-tundish and tundish-mold channels, if the flow rate is low, it is carried downstream. However, if the flow rate is high, not only is it carried downstream but some of the gas flows upstream. "Runup" refers to the rising of gas bubbles against the vertical downward flow of the liquid phase. The inventors conducted investigations using liquid metal model experiments, water model tests, and full-scale tests. As a result, they found that the runup 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 piping 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 runup phenomenon can be said to occur in a gas-liquid two-phase flow that satisfies these conditions, such as a gas-liquid two-phase flow of inert gas and molten steel between an intermediate vessel and a mold (especially between the intermediate vessel and a nozzle) in continuous casting.

[0009] In continuous casting of molten steel, it is believed that the following advantages are obtained by allowing an inert gas to flow upstream from the nozzle to the intermediate vessel. (A) By flowing inert gas from the nozzle to the intermediate vessel, an upward flow can be generated in the intermediate vessel that counteracts the short pass toward the nozzle. In other words, by flowing inert gas from the nozzle to the intermediate vessel, it is thought that the short pass in the intermediate vessel can be suppressed. This makes it difficult for inclusions in the molten steel to be carried into the nozzle, purifying the molten steel and reducing defects in the inclusion properties of the slab. (B) By sending inert gas upstream from the nozzle to the intermediate vessel, it is possible to cause fluctuations in the molten metal surface immediately above the nozzle, which is thought to suppress the generation of vortices in the intermediate vessel. This is thought to suppress the inclusions from being mixed into the molten steel, and to reduce defects in the inclusion properties of the slab. (C) By flowing the inert gas upstream from the nozzle to the intermediate vessel, it is believed that the inclusions in the molten steel can be adsorbed by the bubbles of the inert gas, and the inclusions can be raised and removed together with the bubbles. This is believed to purify the molten steel and reduce defects in the inclusion properties of the slab.

[0010] On the other hand, in continuous casting of molten steel, allowing the inert gas to flow upstream from the nozzle to the intermediate vessel is thought to have the following disadvantages. (a) If the inert gas is excessively allowed to ascend from the nozzle to the intermediate vessel, the surface of the molten steel in the intermediate vessel is excessively disturbed, the slag on the surface of the molten steel is stirred, and the bare metal is exposed to the atmosphere, which is thought to cause excessive generation of inclusions and make it easier for the inclusions to be carried into the molten steel. As a result, it is thought that defects in the inclusion properties are more likely to occur in the cast slab. (b) If the inert gas is excessively allowed to flow upstream from the nozzle to the intermediate vessel, the amount of inert gas brought into the nozzle and the mold will also increase, and the molten steel surface in the mold will be disturbed by the inert gas. As in (a) above, it is thought that this may result in excessive generation of inclusions and a decrease in the cleanliness of the molten steel.

[0011] Considering the above, when inert gas is introduced from the nozzle to the intermediate vessel during continuous casting of molten steel, if the amount of upstream gas is below a certain level, it can lead to purification of the molten steel. However, if the amount of upstream gas is excessive, the purity of the molten steel may actually decrease. In other words, controlling the amount of upstream gas from the nozzle to the intermediate vessel during continuous casting of molten steel is thought to purify the molten steel and reduce the occurrence of inclusion defects in the cast slab. However, the above-mentioned phenomenon of upstream gas flow is rare from the perspective of gas-liquid multiphase flow, and no detailed studies have been found. Furthermore, reverse annular flow and flooding are examples of phenomena similar to the above-mentioned phenomenon of upstream gas flow, but they are not strictly speaking the same phenomenon. Therefore, there are no models for determining the conditions under which upstream gas flow occurs or for estimating the pressure loss caused by it (Reference 1). Reference 1: Revised Handbook of Gas-Liquid Two-Phase Flow, edited by the Japan Society of Mechanical Engineers, Corona Publishing

[0012] The present inventors have conducted extensive research into the above-mentioned phenomenon of inert gas backflow during continuous casting of molten steel, and as a result have discovered the following. (I) In continuous casting of molten steel, the amount of inert gas going back up from the nozzle to the intermediate vessel correlates with the void fraction y(-) in the nozzle and the cross-sectional average flow velocity x (m / s) of the molten steel in the nozzle. The amount of inert gas going back up can be controlled by controlling y and x. Specifically, when the void fraction y in the nozzle is constant, the amount of inert gas going back up increases as the cross-sectional average flow velocity x of the molten steel in the nozzle decreases. Furthermore, when the cross-sectional average flow velocity x of the molten steel in the nozzle is constant, the amount of inert gas going back up increases as the void fraction y in the nozzle increases.

[0013] Based on the above findings, the present inventors have confirmed the relationship between the amount of inert gas flowing back up during continuous casting of molten steel, the cleanliness of the molten steel, and the amount of inclusion defects in the cast slab. As a result, they have discovered the following. (II) The void fraction y in the nozzle and the cross-sectional average flow velocity x of the molten steel in the nozzle satisfy the following relationship (1a): y≧0.003×e 0.69x (1a) When the above condition is satisfied, the inert gas flows back up, and the molten steel is purified due to the above-mentioned advantages, and inclusion defects in the cast slab are easily suppressed. (III) The void fraction y in the nozzle and the cross-sectional average flow velocity x of the molten steel in the nozzle satisfy the following relationship (1b): y≦0.018×e 0.39x (1b) When the above requirement is satisfied, the amount of inert gas flowing upstream becomes appropriate, the above-mentioned disadvantages are eliminated, the molten steel is purified, and inclusion defects in the slab are easily suppressed.

[0014] The continuous casting method and continuous casting apparatus for molten steel according to the present disclosure have been completed based on the above findings. Hereinafter, one embodiment of the continuous casting method and continuous casting apparatus for molten steel according to the present disclosure will be described, but the technology of the present disclosure is not limited to the following embodiment.

[0015] 2. Continuous casting method of molten steel As shown in FIG. 1 , a method for continuously casting molten steel 10 according to one embodiment includes the following steps: Molten steel 10 is supplied from an intermediate vessel 20 to a mold 40 through a nozzle 30, blowing an inert gas 50 into the nozzle 30; Including, The following relationship (1): 0.003×e 0.69x ≦y≦0.018×e 0.39x (1) y: Void fraction in the nozzle 30 (-) x: average cross-sectional flow velocity of the molten steel 10 in the nozzle 30 (m / s) Inert gas 50 is blown into the nozzle 30 so that It is characterized by:

[0016] Here, the left and middle terms of the relationship (1) correspond to the relationship (1a), and the middle and right terms of the relationship (1) correspond to the relationship (1b). That is, according to the method for continuous casting of molten steel according to this embodiment, by injecting inert gas into the nozzle so as to satisfy the relationship (1), the amount of inert gas that flows upstream from the nozzle to the intermediate vessel becomes appropriate, the molten steel is purified, and inclusion defects in the slab can be suppressed.

[0017] 2.1 Void fraction in the nozzle The void fraction y in the nozzle 30 corresponds to the flow rate of the inert gas 50 injected into the nozzle 30, and the greater the flow rate of the inert gas 50, the greater the void fraction y. Also, as mentioned above, the greater the void fraction y, the more likely the inert gas 50 will backflow. "Void fraction" refers to the ratio of the gas flow rate to the total flow rate in the flow path (gas flow rate / total flow rate), and is calculated assuming a homogeneous flow. The "void fraction" is the same value in the model experimental system as it is under the conditions of the actual equipment it simulates.

[0018] In this embodiment, the specific value of the void fraction can be any value as long as the above relationship (1) is satisfied. The void fraction y(-) may be, for example, greater than 0 and equal to or less than 0.23, or may be 0.01 or greater and equal to or less than 0.15. More specifically, the void fraction y(-) may be, for example, When the throughput (Ton / min) is 1.25 and the cross-sectional average flow velocity x (m / s) described below is 3.00, the ratio may be 0.025 or more and 0.05 or less, When the throughput (Ton / min) is 3.15 and the cross-sectional average flow velocity x (m / s) described below is 5.11, the ratio may be 0.09 or more and 0.13 or less.

[0019] 2.2 Cross-sectional average velocity of molten steel in the nozzle The cross-sectional average flow velocity x of the molten steel 10 in the nozzle 30 can be determined based on the throughput of the molten steel 10, the shape of the nozzle 30, and the like. When the nozzle 30 has a sliding gate 31 as shown in FIG. 1 , the cross-sectional average flow velocity x can be determined by taking into consideration the opening degree of the sliding gate 31. Specifically, the cross-sectional average flow velocity x is determined by dividing the throughput (TP) 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 nozzle 30 is determined may be any position at which the cross-sectional average flow velocity x is maximum. For example, the position at which the cross-sectional average flow velocity x is maximum is upstream (toward the intermediate vessel) of the position at which the inert gas 50 is injected.

[0020] In this embodiment, the specific value of the cross-sectional average flow velocity x can be any value as long as the magnitude relationship between the left and right sides of the above relationship (1) is satisfied. In the above relationship (1), when the cross-sectional average flow velocity x exceeds a predetermined value, the value of the right side becomes smaller than the value of the left side, so it is obvious that the cross-sectional average flow velocity x is equal to or less than the predetermined value. The cross-sectional average flow velocity x may be, for example, greater than 0 m / s and equal to or less than 6.5 m / s, or may be equal to or greater than 1 m / s and equal to or less than 6 m / s.

[0021] 2.3 Inert gas flow rate at the nozzle As described above, the void fraction y correlates with the flow rate of the inert gas 50 in the nozzle 30. Here, according to the knowledge of the present inventors, in actual continuous casting operations, the amount of inert gas 50 injected may not coincide with the actual flow rate of the inert gas 50 actually supplied to the molten steel 10 in the nozzle 30. This is because in actual equipment, a portion of the inert gas 50 may leak from gaps at the joints between the components. In such cases, it is advisable to accurately estimate the actual flow rate of the inert gas 50 injected into the nozzle 30 and estimate the void fraction y based on this estimated value. That is, the method for continuous casting molten steel according to this embodiment is as follows: estimating the actual flow rate of the inert gas 50 injected into the nozzle 30; Estimating the void fraction y in the nozzle 30 based on the estimated actual flow rate; and The inert gas 50 is injected into the nozzle 30 so that the estimated void fraction y and the cross-sectional average flow velocity x of the molten steel 10 in the nozzle 30 satisfy the above-mentioned relationship (1). It may also include.

[0022] The following method can be given as an example of a method for estimating the actual flow rate of the inert gas 50 blown into the nozzle 30. The following method is a preferred example of a method for estimating the actual flow rate of the inert gas 50 blown into the nozzle 30. In this embodiment, the actual flow rate of the inert gas 50 may be estimated by a method different from the following method.

[0023] The method for estimating the actual flow rate of the inert gas 50 injected into the nozzle 30 is as follows: Step 1: determining in advance the relationship between the actual flow rate of the inert gas 50 supplied to the molten steel 10 in the nozzle 30 and the opening degree of the sliding gate 31 in accordance with the throughput of the molten steel 10; and Step 2: In the continuous casting of the molten steel 10 (in actual continuous casting operation), an actual flow rate of the inert gas 50 to be supplied to the molten steel 10 in the nozzle 30 is estimated based on the opening degree of the sliding gate 31, the throughput of the molten steel 10, and a previously determined relationship; It may include:

[0024] In step 1, the relationship between the actual flow rate of the inert gas 50 supplied to the molten steel 10 in the nozzle 30 and the opening degree of the sliding gate 31 is determined in advance according to the throughput of the molten steel 10. As described above, the "actual flow rate of the inert gas supplied to the molten steel in the nozzle" is the flow rate of the inert gas injected into the nozzle minus the amount of gas leaking out of the nozzle, and refers to the flow rate actually supplied to the molten steel in the nozzle.

[0025] 1 and 2(A), in continuous casting of molten steel 10, the opening degree of the sliding gate 31 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 40. For example, by maintaining the opening degree of the sliding gate 31 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 31 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 31 is decreased while maintaining the height H.

[0026] 2(B), when the inert gas 50 is injected into the nozzle 30, a pressure loss occurs corresponding to the amount of the inert gas 50 that is actually supplied to the molten steel 10 without leaking to the outside. That is, when the inert gas 50 is injected into the nozzle 30, in order to maintain the throughput of the molten steel 10, it is necessary to increase the opening degree of the sliding gate 31 to overcome the pressure loss caused by the inert gas 50 supplied to the molten steel 10. In other words, there is a predetermined correlation between the actual flow rate of the inert gas 50 supplied to the molten steel 10 in the nozzle 30 and the opening degree of the sliding gate 31, depending on the throughput of the molten steel 10.

[0027] The relationship between the actual flow rate of the inert gas 50 supplied to the molten steel 10 in the nozzle 30 and the opening degree of the sliding gate 31 may be determined by various model experiments, numerical calculations, or the like. In particular, this relationship may be determined in advance by a model experiment 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 a model experiment using a low-melting-point metal, it is possible to match dimensionless numbers such as the Froude number, modified Froude number, Reynolds number, and Weber number with those in a molten steel system (Patent No. 6750533). Furthermore, the apparatus used in the model experiment can also create an ideal state in which there is virtually no leakage of inert gas. This makes it possible to determine the relationship between the flow rate of the inert gas actually supplied to the low-melting-point metal in the submerged entry nozzle (= the flow rate of the inert gas injected into the submerged entry nozzle) and the opening degree of the sliding gate, and by converting this to a molten steel system, it is possible to determine the relationship between the actual flow rate of the inert gas 50 supplied to the molten steel 10 in the nozzle 30 and the opening degree of the sliding gate 31. This relationship may be expressed by a mathematical formula as a function of the opening degree of the sliding gate 31 and the actual flow rate of the inert gas 50. A specific form of the model experiment using a low-melting-point metal will be described in detail in the Examples.

[0028] In step 2, during continuous casting of molten steel 10, the actual flow rate of inert gas 50 supplied to molten steel 10 in nozzle 30 is estimated based on the opening degree of sliding gate 31, the throughput of molten steel 10, and a predetermined relationship. In step 2, for example, during continuous casting of molten steel 10, the actual flow rate of inert gas 50 supplied to molten steel 10 in nozzle 30 can be estimated simply by obtaining information related to the opening degree of sliding gate 31 and information related to the throughput of molten steel 10. The opening degree of sliding gate 31 and the throughput of 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, when the opening degree of the sliding gate 31 required to maintain a predetermined molten steel throughput during continuous casting of molten steel 10 without injecting inert gas ( FIG. 2(A) ) is assumed to be an "initial opening degree," if inert gas is injected while maintaining the throughput, the opening degree of the sliding gate 31 can be increased by a predetermined amount from the initial opening degree in accordance with the actual flow rate of the inert gas 50 supplied to the molten steel 10 in the nozzle 30 ( FIG. 2(B) ). Furthermore, if the actual flow rate of the inert gas 50 supplied to the molten steel 10 in the nozzle 30 changes due to a change in the injection rate or leakage rate of the inert gas 50, the opening degree of the sliding gate 31 also changes in accordance with the change in the actual flow rate. In other words, according to Steps 1 and 2, the actual flow rate of the inert gas 50 supplied to the molten steel 10 in the nozzle 30 and the amount of change in the opening degree can be estimated from the opening degree of the sliding gate 31 and the amount of change in the opening degree 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. Continuous casting equipment for molten steel The technique of the present disclosure has an aspect as a continuous casting apparatus for molten steel in addition to an aspect as a continuous casting method for molten steel. As shown in FIG. 1 , a continuous casting apparatus 100 for molten steel 10 according to one embodiment includes: An intermediate vessel (20), a nozzle (30), a mold (40), and an inert gas blowing means (60); The following relationship (1): 0.003×e 0.69x ≦y≦0.018×e 0.39x (1) y: Void fraction in the nozzle 30 (-) x: average cross-sectional flow velocity of the molten steel 10 in the nozzle 30 (m / s) Inert gas 50 is blown into the nozzle 30 from the inert gas blowing means 60 so that It is structured as follows.

[0031] In the continuous casting apparatus 100, the flow rate of the inert gas 50 injected into the nozzle 30 from the inert gas injection means 60 is controlled to satisfy the relationship (1). The continuous casting apparatus 100 may have the same configuration as a conventional continuous casting apparatus, except for the inert gas injection means 60. The inert gas injection means 60 may be configured, for example, by directly or indirectly connecting an inert gas supply source (e.g., a container filled with high-pressure inert gas) to the nozzle 30 via piping, a valve, or the like. The amount of inert gas 50 injected into the nozzle 30 from the inert gas injection means 60 may be controlled, for example, by controlling the valve opening of the inert gas injection means 60. This control may be performed manually by an operator or automatically and mechanically by a control unit (not shown). In either case, it is sufficient that the inert gas 50 is injected into the nozzle 30 from the inert gas injection means 60 so as to satisfy the relationship (1).

[0032] 4. Supplementary Information As described above, the continuous casting machine and continuous casting conditions employed in this embodiment may be any machine capable of utilizing the upstream flow of the inert gas 50 from the nozzle 30 to the intermediate vessel 20. The type (steel grade) of the molten steel 10 is not particularly limited, as long as it is an iron-containing alloy. Furthermore, the configurations of the intermediate vessel 20 (e.g., a tundish), the nozzle 30 having the sliding gate 31, and the mold 40 are not particularly limited. The mechanism and injection position for injecting the inert gas 50 are not particularly limited. For example, a flow path (hole) for introducing the inert gas 50 may be provided in the side wall of the nozzle 30, and the inert gas 50 may be injected through this flow path. The type of inert gas 50 is also not particularly limited, and may be, for example, argon (Ar). In this embodiment, the molten steel can be purified by utilizing the upstream flow of the inert gas 50 to the intermediate vessel 20 simply by controlling the amount of inert gas 50 injected from the inert gas injection means 60 to the nozzle 30, without introducing any new equipment into a conventional continuous casting machine. In the continuous casting of molten steel 10 according to this embodiment, the above relationship (1) is satisfied for at least a portion of the period during continuous casting. In the continuous casting of molten steel 10, there are cases where the continuous casting conditions change, such as a change in the diameter of the nozzle 30 due to adhesion of inclusions, etc. When the relationship (1) is no longer satisfied due to such a change in the casting conditions, it is advisable to change the amount of inert gas 50 being blown, etc., so that the relationship (1) is satisfied. [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. Study on the method to estimate the actual flow rate of inert gas 1.1 Experimental conditions Figure 3 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 3 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. 2, 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 4(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 4(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 4(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. 5, left side). Here, the same pressure loss ΔE arConsider 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 Figure 5).

[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. 6, 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 6). 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 7(A) and (B). As shown in Figures 7(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.

[0050] As an example of applying the above concept, Figure 8 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 7(A) and 7(B). In the initial state, the pressure loss is only 20 kPa due to the SG opening (Figure 8(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 8(2)). On the other hand, the pressure loss due to an Ar flow rate of 0.3 NL / min is 9 kPa (Figure 8(3)). Consequently, the SG opening is changed to 52% (Figure 8(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. The validity of the above estimation of the actual Ar flow rate in an actual machine has been confirmed by conducting a continuous casting test in an actual machine.

[0052] 2. Investigation of molten steel purification using inert gas upstream In a continuous casting apparatus such as that shown in Figure 1, molten steel was continuously cast while injecting Ar as an inert gas through a three-plate sliding gate 31. Multiple cast pieces were produced while varying the amount of Ar injected, and the oxygen concentration in the steel was evaluated by analyzing the components obtained from each cast piece. For example, if the cleanliness of the molten steel is improved by allowing Ar to flow appropriately upstream from the nozzle 30 to the intermediate vessel 20 (tundish), the amount of oxide-based inclusions decreases, resulting in a lower oxygen concentration in the steel. Each experimental result was indexed, with the lowest oxygen concentration in the steel being assigned a value of "0" and the highest being assigned a value of "1," and this was evaluated as the "oxygen concentration index in the steel."

[0053] Table 1 below shows the "void fraction y(-)" in the nozzle calculated based on the amount of Ar injected, the "throughput (Ton / min)" of molten steel during continuous casting, the "average cross-sectional flow velocity (m / s)" of molten steel in the nozzle, the "flow rate (NL / min) of Ar injected into the nozzle," and the "oxygen concentration index in steel" indexed as described above.

[0054] [Table 1]

[0055] FIG. 9 shows the relationship between "void fraction y," "cross-sectional average flow velocity x," and "molten steel cleanliness (○: good (oxygen concentration index in steel is 0.2 or less), △: slightly poor (oxygen concentration index in steel is more than 0.2 and 0.8 or less), ×: very poor (oxygen concentration index in steel is more than 0.8)"). The "○," "△," and "×" plotted in FIG. 9 are based on the results in Table 1 above, respectively.

[0056] As is clear from the results shown in Table 1 and FIG. 9, in the continuous casting of molten steel, the following relationship (1): 0.003×e 0.69x ≦y≦0.018×e 0.39x (1) y: Void fraction in the nozzle (-) x: average cross-sectional flow velocity of the molten steel in the nozzle (m / s) It can be seen that when inert gas is injected into the nozzle so as to satisfy the relation (Tests Nos. 3 and 4), the oxygen concentration index in the steel is significantly reduced and the cleanliness of the molten steel is improved. When the above relation (1) is satisfied, it is thought that a portion of the inert gas injected into the nozzle flows back up to the intermediate vessel to purify the molten steel through the following mechanisms (A) to (C).

[0057] (A) It is believed that a portion of the inert gas injected into the nozzle flows upstream to the intermediate vessel, creating an upward flow in the intermediate vessel that counteracts the short-pass flow toward the nozzle, thereby suppressing the short-pass flow in the intermediate vessel. This makes it difficult for inclusions in the molten steel to be carried into the nozzle, purifying the molten steel, reducing inclusion defects in the slab, and lowering the oxygen concentration index in the steel.

[0058] (B) It is believed that a portion of the inert gas injected into the nozzle flows upstream into the intermediate vessel, causing fluctuations in the molten steel surface immediately above the nozzle and suppressing the generation of vortices in the intermediate vessel. This in turn suppresses the inclusions from being mixed into the molten steel, reduces defects in the inclusion properties of the slab, and lowers the oxygen concentration index in the steel.

[0059] (C) It is believed that a portion of the inert gas injected into the nozzle flows upstream to the intermediate vessel, allowing the inclusions in the molten steel to be adsorbed by the inert gas bubbles, which then rise to the surface and are removed. This is believed to have resulted in the purification of the molten steel, the reduction of inclusion defects in the slab, and the reduction of the oxygen concentration index in the steel.

[0060] On the other hand, when the void fraction y was less than the value of the left side of the above relationship (1) (Tests No. 1 and 2), the oxygen concentration index in the steel was slightly larger. When the void fraction y was less than the value of the left side of the above relationship (1), it is thought that the inert gas did not flow backward, and the effects of the above mechanisms (A) to (C) were not obtained.

[0061] Furthermore, when the void fraction exceeded the value on the right side of the above relationship (1) (Tests No. 5 and 6), the oxygen concentration index in the steel increased significantly. When the void fraction y exceeded the value on the right side of the above relationship (1), it is believed that excessive inert gas flowed upstream, causing inclusions to be introduced into the molten steel by the following mechanisms (a) and (b).

[0062] (a) When the inert gas blown into the nozzle excessively ascended into the intermediate vessel, the surface of the molten steel in the intermediate vessel was excessively disturbed, stirring the slag on the surface of the molten steel. Furthermore, the bare molten steel was exposed to the atmosphere, which led to the excessive generation of inclusions, making it easier for the inclusions to be carried into the molten steel. As a result, defects in the inclusion properties occurred in the slab, and the oxygen concentration index in the steel increased.

[0063] (b) When the amount of inert gas blown into the nozzle was excessively increased, the amount of inert gas brought into the nozzle and mold also increased, disturbing the molten steel surface in the mold, and similar to (a) above, excessive inclusions were generated, which is thought to have reduced the cleanliness of the molten steel. As a result, defects in the inclusion properties occurred in the cast slab, and the oxygen concentration index in the steel increased.

[0064] Similar continuous casting tests were conducted under various conditions other than those described above, and it was found that when the above relationship (1) was satisfied, the oxygen concentration index in the steel was significantly reduced, whereas when the above relationship (1) was not satisfied, the oxygen concentration index in the steel was relatively increased. [Explanation of symbols]

[0065] 10 Molten Steel 20 Intermediate container 30 nozzles 31 Sliding Gate 40 Mold 50 Inert Gas 60 Inert gas injection means

Claims

1. A method for continuous casting of molten steel, comprising the steps of: The molten steel is supplied from the intermediate vessel to a mold through a nozzle, blowing an inert gas into the nozzle; Including, The following relationship (1): 0.003×e ０．６９ｘ ≦y≦0.018×e ０．３９ｘ (1) y: void fraction in the nozzle (-) x: cross-sectional average flow velocity of the molten steel in the nozzle (m / s) The inert gas is blown into the nozzle so that A method for continuous casting of molten steel.

2. 2. The method for continuous casting of molten steel according to claim 1, estimating the actual flow rate of the inert gas injected into the nozzle; Estimating a void fraction y in the nozzle based on the estimated actual flow rate; and injecting the inert gas into the nozzle so that the estimated void fraction y and the cross-sectional average flow velocity x of the molten steel in the nozzle satisfy the relationship (1); A method for continuous casting of molten steel, comprising:

3. A continuous casting apparatus for molten steel, comprising an intermediate vessel, a nozzle, a mold, and an inert gas blowing means, The following relationship (1): 0.003×e ０．６９ｘ ≦y≦0.018×e ０．３９ｘ (1) y: void fraction in the nozzle (-) x: cross-sectional average flow velocity of the molten steel in the nozzle (m / s) Inert gas is blown into the nozzle from the inert gas blowing means so that A continuous casting apparatus for molten steel, comprising:

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