Continuous casting method for steel

Abstract

To provide a steel continuous casting method which can suppress clogging of nozzles.SOLUTION: There is provided a steel continuous casting method using nozzles in molten steel injection from a tundish to a mold, wherein the nozzles have an upper nozzle, an immersion nozzle, and a sliding gate arranged between the upper nozzle and the immersion nozzle, and as a condition that high-purity Ar gas is flowed in a molten steel flow channel of the nozzles and the high-purity Ar gas is flowed into the molten steel flow channel of the nozzles, (A) the high-purity Ar gas is flowed in the molten steel channel of the nozzle from the upper nozzle, the high-purity Ar gas is not flowed in the molten steel channel of the nozzles from the sliding gate, a flow rate of the high-purity Ar gas supplied from the upper nozzle is 1 NL / min or more and 25 NL / min or less, or (B) the high-purity Ar gas is flowed in the molten steel channel of the nozzles from the upper nozzle and the sliding gate, and a total flow rate of the high-purity Ar gas supplied from the upper nozzle and the sliding gate is 1 NL / min or more and 25 NL / min or less.SELECTED DRAWING: Figure 2

Description

Technical Field

[0005]

[0001] This application relates to a method for continuous casting of steel.

Background Art

[0002] In a method for continuous casting of steel using a nozzle to inject molten steel from a tundish into a mold, there is a problem that non-metallic oxides (inclusions) in the molten steel adhere and accumulate on the inner wall of the nozzle, blocking the flow path.

[0003] When the flow path is blocked during casting, the productivity decreases because the casting has to be stopped once to remove the deposits. Also, the yield deteriorates because the ingots deposited together with the deposits are removed. Furthermore, it has a great impact on the quality of the cast slab.

[0004] Conventionally, to prevent nozzle blockage, blowing of Ar gas into the nozzle has been carried out. The Ar gas blown into the nozzle flattens along the inner wall and forms a curtain-like gas film. The presence of the gas film on the inner wall of the immersion nozzle reduces the chance of the molten steel contacting the inner wall, making it difficult for inclusions to adhere, and suppressing nozzle blockage. Also, by purifying the Ar gas to a high purity, the interfacial tension between the molten steel and the gas film can be increased, so it is considered that a gas film can be formed stably and nozzle blockage can be further suppressed.

[0005] Patent Document 1 and Patent Document 2 describe that by adjusting the purity, oxygen concentration, dew point, flow rate, and blowing position of the Ar gas blown into the immersion nozzle, blockage of the immersion nozzle can be suppressed. Patent Document 3 describes that in addition to high-purity Ar gas, by blowing Ar gas onto the outer peripheral part of the sliding surface of the sliding gate or using a stopper without a sliding surface instead of the sliding gate, a better effect of suppressing blockage of the immersion nozzle can be achieved.

Prior Art Documents

Patent Documents

[0006] [Patent Document 1] Japanese Patent Publication No. 2014-8530 [Patent Document 2] Japanese Patent Publication No. 2014-184462 [Patent Document 3] Japanese Patent Publication No. 2021-151660 [Overview of the project] [Problems that the invention aims to solve]

[0007] When using a nozzle with an upper nozzle, a sliding gate, and an immersion nozzle for continuous casting of steel, there is a problem in which outside air is drawn into the nozzle from the fitting parts of each component and the sliding surface of the sliding gate. When outside air is drawn into the nozzle, the purity of the Ar gas decreases, the gas film formed on the inner wall of the nozzle becomes unstable, the molten steel is re-oxidized, alumina-based inclusions become coarser, and nozzle blockage progresses.

[0008] The methods described in Patent Documents 1 and 2 do not take into account the intake of outside air into the nozzle. Therefore, outside air is inevitably drawn in from the sliding surface of the sliding gate and the mating parts of each component of the nozzle, which is thought to reduce the purity of the Ar gas and make it impossible to stably maintain the gas film. The method described in Patent Document 3 blows Ar gas onto the outer circumference of the sliding surface of the sliding gate, but outside air is still drawn in from the mating parts of each component in addition to the sliding surface, so there is still room for improvement. For example, in the method described in Patent Document 3, Ar gas is not supplied from the upper nozzle, and there is no gas film at the mating part between the upper nozzle and the sliding gate, so the intake of outside air from this mating part is a particular problem.

[0009] Furthermore, while the methods described in Patent Documents 1 to 3 primarily address the reduction of contact opportunities between molten steel and immersion nozzles, suppressing the intake of outside air from mating parts and sliding surfaces is also an important issue for preventing nozzle blockage.

[0010] Therefore, the main objective of this disclosure is to provide a continuous casting method for steel that can suppress nozzle blockage, in view of the above circumstances. [Means for solving the problem]

[0011] This disclosure provides, as one embodiment for solving the above problem, a continuous casting method for steel using a nozzle for injecting molten steel from a tundish into a mold, wherein the nozzle has an upper nozzle, an immersion nozzle, and a sliding gate positioned between the upper nozzle and the immersion nozzle, and high-purity Ar gas having a purity of 99.99% or more, an oxygen concentration of 2 ppm or less, and a dew point of -65°C or less is flowed into the molten steel flow path of the nozzle, and the conditions for flowing high-purity Ar gas into the molten steel flow path of the nozzle but, above High-purity Ar gas is flowed into the molten steel flow path of the nozzle from the nozzle and sliding gate, and the total flow rate of high-purity Ar gas supplied from the upper nozzle and sliding gate is between 1 NL / min and 25 NL / min. the law of nature, The flow rate of the high-purity Ar gas supplied from the upper nozzle is greater than the flow rate of the high-purity Ar gas supplied from the sliding gate. Characterized by, A method for continuous casting of steel is provided.

[0012] Ma Furthermore, after casting, the FeO concentration of alumina-based inclusions adhering to the inner wall of the immersion nozzle may be 5 atom% or less, and the average particle size of the alumina-based inclusions may be 50 μm or less. [Effects of the Invention]

[0013] According to the continuous casting method for steel of this disclosure, nozzle blockage can be suppressed. [Brief explanation of the drawing]

[0014] [Figure 1] This is a schematic diagram illustrating the process of continuous steel casting. [Figure 2] This is a schematic cross-sectional view of nozzle 100. [Modes for carrying out the invention]

[0015] A continuous casting method of steel according to the present disclosure will be described in detail using an embodiment. A schematic diagram showing the state of continuous casting of steel is shown in FIG. 1. Further, FIG. 2 shows a schematic cross-sectional view of a nozzle 100 which is an example of a nozzle that can be used in an embodiment.

[0016] One embodiment is a continuous casting method of steel using a nozzle 100 for injecting molten steel 2 from a tundish 1 into a mold 3. The nozzle 100 has an upper nozzle 10, a submerged nozzle 20, and a sliding gate 30 disposed between the upper nozzle 10 and the submerged nozzle 20. High purity Ar gas having a purity of 99.99% or more, an oxygen concentration of 2 ppm or less, and a dew point of -65°C or less is flowed into the molten steel flow path of the nozzle. The conditions for flowing the high purity Ar gas into the molten steel flow path of the nozzle are (A) flowing high purity Ar gas from the upper nozzle 10 into the molten steel flow path of the nozzle 100, not flowing high purity Ar gas from the sliding gate into the molten steel flow path of the nozzle, and the flow rate of the high purity Ar gas supplied from the upper nozzle 10 is 1 NL / min or more and 25 NL / min or less, or Or, (B) flowing high purity Ar gas from the upper nozzle 10 and the sliding gate 30 into the molten steel flow path of the nozzle 100, and the total flow rate of the high purity Ar gas supplied from the upper nozzle 10 and the sliding gate 30 is 1 NL / min or more and 25 NL / min or less, which is a continuous casting method of steel.

[0017] First, a typical continuous casting method of steel will be briefly described using FIG. 1. As shown in FIG. 1, the molten steel 2 supplied from a ladle (not shown) to the tundish 1 passes through the nozzle 100, is discharged from the discharge hole 21 of the submerged nozzle 20, and is injected into the mold 3. At this time, the flow rate of the molten steel 2 passing through the inside of the nozzle 100 is adjusted according to the opening degree of the sliding gate 30. Then, the molten steel 2 supplied to the mold 3 is drawn downward while being cooled to form a slab.

[0018] Typically, Ar gas is flowed into the nozzle 100 as the molten steel 2 passes through it. As shown in Figure 2, this forms a curtain-like gas film A along the inner wall of the nozzle 100, reducing the opportunities for the molten steel 2 to come into contact with the inner wall of the nozzle 100 and making it difficult for inclusions to adhere. Furthermore, by flowing high-purity Ar gas, the interfacial tension between the molten steel 2 and the gas film A is increased, stabilizing the gas film A. In addition, forming a stable gas film A further suppresses the intake of outside air.

[0019] In one embodiment, it is essential to flow high-purity Ar gas from the upper nozzle 10. This allows a gas film A to exist above the nozzle 100, suppressing the intake of outside air from the fitting portions 51-53 of each component of the nozzle 100 and the sliding surfaces 54 and 55 of the sliding gate 30. In particular, in one embodiment, it is possible to suppress the intake of outside air from the fitting portion between the upper nozzle 10 and the sliding gate 30, which has not been considered in Patent Documents 1-3. By suppressing the intake of outside air, the re-oxidation of molten steel is suppressed, alumina-based inclusions are dispersed and maintained, and nozzle blockage is suppressed. Furthermore, since the decrease in the purity of the high-purity Ar gas can be suppressed, the gas film A can be stably maintained. In addition, in one embodiment, the flow rate of the high-purity Ar gas is set within a predetermined range. This ensures that a gas film exists where outside air intake may occur, and does not increase the number of bubble defects in the cast slab.

[0020] Thus, in one embodiment, by stably maintaining a film A of high-purity Ar gas on the inner wall of the nozzle (from the Ar gas supply section 11 of the upper nozzle 10 to the discharge hole 21 of the immersion nozzle 20), the inhalation of outside air from the fitting sections 51-53 of each component of the nozzle 100 and the sliding surfaces 54 and 55 of the sliding gate 30 can be suppressed. As a result, alumina-based inclusions are dispersed and maintained, and the adhesion of inclusions to the inner wall of the immersion nozzle 30 is suppressed. Therefore, according to this embodiment, blockage of the nozzle 100 can be significantly suppressed.

[0021] The following describes in detail one embodiment of a continuous casting method for steel.

[0022] <Nozzle 100> The nozzle 100 is responsible for injecting molten steel 2 from the tundish 1 into the mold 3. The nozzle 100 has a nozzle 10, an immersion nozzle 20, and a sliding gate 30 positioned between the upper nozzle 10 and the immersion nozzle 20. The nozzle 100 also has a lower nozzle 40 positioned between the sliding gate 30 and the immersion nozzle 20. However, the nozzle that can be used in one embodiment is not limited to this, and the nozzle only needs to have an upper nozzle, an immersion nozzle, and a sliding gate.

[0023] The upper nozzle 10 is connected to the tundish 1 and the sliding gate 30, and is the part that receives the molten steel 2 supplied from the tundish 1.

[0024] The immersion nozzle 20 is the part that injects the molten steel 2 supplied from the tundish 1 into the mold 3, and its lower part is positioned to be immersed in the molten steel 2 in the mold 3. In addition, a discharge hole 21 for injecting the molten steel 2 into the mold 3 is provided at the lower part (mold 3 side) of the immersion nozzle 20. The number of discharge holes 21 is not particularly limited, and at least one is sufficient.

[0025] The sliding gate 30 is a component for adjusting the flow rate of molten steel 2 supplied to the immersion nozzle 20 (mold 3). The sliding gate 30 has a three-layer structure and comprises an upper fixing plate 31, a lower fixing plate 32, and an intermediate plate 33 positioned between the upper fixing plate 31 and the lower fixing plate 32. The intermediate plate 33 is slidably positioned between the upper fixing plate 31 and the lower fixing plate 32. The intermediate plate 33 is also called a sliding plate, and its sliding motion constricts the flow path.

[0026] The lower nozzle 40 is provided between the sliding gate 30 and the immersion nozzle 20, and serves to connect these components. The lower nozzle 40 is an optional component. Therefore, the nozzle 100 may connect the sliding gate 30 and the immersion nozzle 20 without using the lower nozzle 40.

[0027] During continuous casting, there is a problem in which outside air is drawn into the nozzle through the fitting parts of each component and the sliding surface of the sliding gate 30. In nozzle 100, the upper nozzle 10 and the sliding gate 30 (upper fixing plate 31) fit together to form a fitting part 51, the sliding gate 30 (lower fixing plate 32) and the lower nozzle 40 fit together to form a fitting part 52, and the lower nozzle 40 and the immersion nozzle 20 fit together to form a fitting part 53. Since these fitting parts 51 to 53 are not completely sealed, outside air is drawn into the nozzle 100. Also, since the sliding surface of the sliding gate 30 is not completely sealed, there is a risk of outside air being drawn in. The "sliding surface" refers to the sliding surface between the fixing plate of the sliding gate 30 and the intermediate plate 33. Specifically, it refers to the sliding surface 54 between the upper fixing plate 31 and the intermediate plate 33, and the sliding surface 55 between the lower fixing plate 32 and the intermediate plate 33. Such sliding surfaces 54 and 55 are prone to drawing in outside air through the joint between the sliding surfaces.

[0028] When outside air is drawn into the nozzle 100, the molten steel 2 is oxidized, causing the nozzle to become clogged. Therefore, in one embodiment, high-purity Ar gas is flowed into the molten steel passage of the nozzle. This forms a curtain-like gas film A that covers the surface of the inner wall of the nozzle 100, suppressing the drawing in of outside air from the fitting portions 51-53 of each component and the sliding surfaces 54 and 55 of the sliding gate 30.

[0029] <High-purity argon gas> The high-purity Ar gas used in one embodiment is Ar gas with a purity of 99.99% or higher, an oxygen concentration of 2 ppm or lower, and a dew point of -65°C or lower. By using such high-purity Ar gas, the interfacial tension between the curtain-like gas film A formed along the inner wall of the nozzle 100 and the molten steel 2 can be increased, thereby stabilizing the gas film A. Furthermore, by forming a stable gas film, the intake of outside air can be further suppressed.

[0030] The purity of high-purity Ar gas should be 99.99% or higher, but from the viewpoint of forming a more stable gas film, the purity may be 99.999% or higher. The oxygen concentration of high-purity Ar gas should be 2 ppm or less, but from the viewpoint of forming a more stable gas film A, it may be 1 ppm or less. The dew point of high-purity Ar gas should be -65°C or lower, but from the viewpoint of forming a more stable gas film A, the dew point may be -80°C or lower.

[0031] <Location for supplying high-purity Ar gas> In one embodiment, the conditions for flowing high-purity Ar gas into the molten steel flow path of nozzle 100 are either (A) or (B) below. (A) High-purity Ar gas is flowed from the upper nozzle 10 into the molten steel flow path of nozzle 100, and high-purity Ar gas is not flowed from the sliding gate 30 into the molten steel flow path of nozzle 100, and the flow rate of high-purity Ar gas supplied from the upper nozzle 10 is 1 NL / min or more and 25 NL / min or less. (B) High-purity Ar gas is flowed into the molten steel flow path of nozzle 100 from the upper nozzle 10 and the sliding gate 30, and the total flow rate of high-purity Ar gas supplied from the upper nozzle 10 and the sliding gate 30 is 1 NL / min or more and 25 NL / min or less.

[0032] First, let's explain condition (A). In condition (A), high-purity Ar gas is flowed from the upper nozzle 10 into the molten steel flow path of nozzle 100, but high-purity Ar gas is not flowed from the sliding gate 30 into the molten steel flow path of nozzle 100. That is, of the upper nozzle 10 and the sliding gate 30, high-purity Ar gas is flowed into the molten steel flow path of nozzle 100 only from the upper nozzle 10. This makes it possible to form a gas film A from above the fitting portion 51 of nozzle 100, and in particular, it is possible to suppress the suction of outside air from the fitting portion 51.

[0033] The method for supplying high-purity Ar gas from the upper nozzle 10 into the molten steel flow path of the nozzle 100 is not particularly limited. For example, multiple Ar gas supply units 11 may be provided circumferentially around the upper nozzle 10, and high-purity Ar gas may be supplied from the Ar gas discharge holes 11 into the molten steel flow path of the nozzle 100. The form of the Ar gas supply unit 11 is not particularly limited, but it may be a hole through which Ar gas can be supplied. Alternatively, it may be a porous refractory material through which Ar gas can be supplied. The number of Ar gas supply units 11 is not particularly limited, but may be set appropriately so that a curtain-like gas film A is formed along the inner wall of the nozzle 100. The position of the Ar gas supply unit 11 is not particularly limited. It may be positioned at any location within the flow path of the upper nozzle 10.

[0034] Furthermore, under condition (A), the flow rate of high-purity Ar gas supplied from the upper nozzle 10 is 1 NL / min or more and 25 NL / min or less. This allows a stable gas film to exist on the inner wall of the nozzle 100. Therefore, under condition (A), the inhalation of outside air from the fitting portions 51-53 of each component and the sliding surfaces 54 and 55 of the sliding gate 30 can be suppressed.

[0035] Here, when high-purity Ar gas is supplied to the upper nozzle 10 from multiple Ar gas supply units 11, it means that the total flow rate is between 1 NL / min and 25 NL / min. If the flow rate of high-purity Ar gas supplied from the upper nozzle 10 is less than 1 NL / min, a gas film will not stably exist on the inner wall of the nozzle 100, and outside air will be sucked in from either the fitting portions 51-53 or the sliding surfaces 54 and 55 of the sliding gate 30. If the flow rate of high-purity Ar gas supplied from the upper nozzle 10 exceeds 25 NL / min, it will lead to an increase in bubble defects in the cast slab. In addition, there is a risk that the gas film A will float to the tundish 1 against the flow of molten steel 2, entraining the flux and causing contamination of the molten steel.

[0036] From the viewpoint of ensuring a stable gas film exists from above to below the nozzle 100 (from the Ar gas supply section 11 of the upper nozzle 10 to the discharge hole 21 of the immersion nozzle 20), the flow rate of high-purity Ar gas may be 3 NL / min or more, 5 NL / min or more, 10 NL / min or more, less than 25 NL / min, 22 NL / min or less, 21 NL / min or less, or 20 NL / min or less.

[0037] Next, we will explain condition (B). In condition (B), high-purity Ar gas is flowed from the upper nozzle 10 and the sliding gate 30 into the molten steel flow path of nozzle 100. The method for flowing high-purity Ar gas from the upper nozzle 10 into the molten steel flow path of nozzle 100 is as described above.

[0038] The method for supplying high-purity Ar gas from the sliding gate 30 into the molten steel flow path of the nozzle 100 is not particularly limited. For example, an Ar gas supply unit 34 capable of supplying high-purity Ar gas may be provided on the sliding gate 30. The Ar gas supply unit 34 may be provided on the upper fixing plate 31 or on the lower fixing plate 32. From the viewpoint of further suppressing the intake of outside air from the sliding surface 54 between the upper fixing plate 31 and the intermediate plate 33, the Ar gas supply unit 34 may be provided on the upper fixing plate 31. The form of the Ar gas supply unit 34 is not particularly limited, but it may be a hole capable of supplying Ar gas. Also, similar to the upper nozzle 10, the sliding gate 30 may supply high-purity Ar gas from a plurality of Ar gas supply units 34 provided in the circumferential direction into the molten steel flow path of the nozzle 100. The number of Ar gas discharge holes 34 is not particularly limited, but may be set appropriately so that a curtain-like gas film A is formed along the inner wall of the nozzle 100.

[0039] Furthermore, under condition (B), the flow rate of high-purity Ar gas supplied from the upper nozzle 10 and the sliding gate 30 is 1 NL / min or more and 25 NL / min or less. This allows for the stable formation of a gas film A from the top to the bottom of the nozzle 100 (from the Ar gas supply section 11 of the upper nozzle 10 to the discharge hole 21 of the immersion nozzle 20). Therefore, under condition (B), the inhalation of outside air from the fitting sections 51-53 of each component and the sliding surfaces 54 and 55 of the sliding gate 30 can be suppressed.

[0040] Here, when high-purity Ar gas is supplied from multiple Ar gas supply units 11 and 34 to the upper nozzle 10 and sliding gate 30, it means that the total flow rate is between 1 NL / min and 25 NL / min. If the total flow rate of high-purity Ar gas supplied from the upper nozzle 10 and sliding gate 30 is less than 1 NL / min, a gas film will not stably exist on the fitting portions 51-53 of the nozzle 100 and the sliding surfaces 54 and 55 of the sliding gate 30, and outside air will be drawn in. If the total flow rate of high-purity Ar gas supplied from the upper nozzle 10 and sliding gate 30 exceeds 25 NL / min, it will lead to an increase in bubble defects in the cast slab. In addition, there is a risk that gas film A will float to the tundish 1 against the flow of molten steel 2, entraining flux and causing contamination of the molten steel.

[0041] From the viewpoint of ensuring a stable gas film exists from above to below the nozzle 100 (from the Ar gas supply section 11 of the upper nozzle 10 to the discharge hole 21 of the immersion nozzle 20), the flow rate of high-purity Ar gas may be 3 NL / min or more, 5 NL / min or more, 10 NL / min or more, less than 25 NL / min, 22 NL / min or less, 21 NL / min or less, or 20 NL / min or less.

[0042] Furthermore, the flow rate of the high-purity Ar gas supplied from the upper nozzle 10 may be greater than the flow rate of the high-purity Ar gas supplied from the sliding gate 30. This allows a stable gas film A to be formed above the fitting portion 51 between the upper nozzle 10 and the sliding gate 30, and in particular, the inhalation of outside air from the fitting portion 51 can be further suppressed.

[0043] As described above, in one embodiment, either condition (A) or (B) may be adopted. However, from the viewpoint of forming gas film A more stably, condition (B) may be adopted as the condition for flowing high-purity Ar gas into the molten steel flow path of the nozzle.

[0044] In one embodiment, under both conditions (A) and (B), it is permitted to preemptively flow Ar gas from the immersion nozzle 20 into the molten steel flow path of the nozzle 100 (immersion nozzle 20). By flowing Ar gas from the immersion nozzle 20, the stability of the gas film A formed on the inner wall of the immersion nozzle 20 can be improved, and nozzle blockage can be further suppressed. The method of flowing Ar gas from the immersion nozzle 20 into the molten steel flow path of the nozzle 100 is not particularly limited. In Figure 2, an Ar gas supply unit 22 is provided on the immersion nozzle 20. The Ar gas supply unit 22 may be appropriately configured to be applicable to the upper nozzle 10. Here, the purity of the Ar gas supplied from the immersion nozzle 20 is not particularly limited. However, from the viewpoint of enhancing the nozzle blockage suppression effect, the high-purity Ar gas described above may be used.

[0045] <Alumina-based inclusions adhering to the immersion nozzle 20 after casting> According to the present inventors, the blockage of the nozzle 100 is due to the coarsening of alumina-based inclusions adhering to the inner wall of the immersion nozzle 20. The estimated mechanism of nozzle blockage due to alumina-based inclusions is as follows: First, the alumina-based inclusions aggregate and coalesce via FeO in the molten steel, becoming coarser, and these coarser alumina-based inclusions adhere to the inner wall of the immersion nozzle 20. Alternatively, the alumina-based inclusions adhering to the inner wall of the immersion nozzle 20 become coarser. It is believed that nozzle blockage progresses through this mechanism. Here, the main cause of FeO generation in the molten steel is thought to be the re-oxidation of the molten steel by outside air drawn in from the fitting portions 51-53 and sliding surfaces 54, 55 of the nozzle 100.

[0046] Therefore, in one embodiment, by flowing high-purity Ar gas into the molten steel flow path of the nozzle 100 under predetermined conditions, a gas film is created on the fitting portions 51-53 and the sliding surfaces 54 and 55, thereby suppressing the intake of outside air. This lowers the FeO concentration of the alumina inclusions and reduces the particle size of the alumina inclusions, thus maintaining the dispersibility of the alumina inclusions in the molten steel 2. Accordingly, according to one embodiment, in addition to the effect of suppressing the blockage of the nozzle 100, it also has the effect of suppressing the coarsening of alumina inclusions adhering to the immersion nozzle 20.

[0047] Therefore, one embodiment may have the following features from the viewpoint of clearly defining the effect of suppressing the coarsening of alumina inclusions. That is, in one embodiment, after casting, the FeO concentration of alumina-based inclusions adhering to the inner wall of the immersion nozzle 20 may be 5 atom% or less, and the average particle size may be 50 μm or less. This further suppresses the clogging of the nozzle 100.

[0048] "After casting" refers to the period after the completion of continuous casting of steel. The conditions for continuous casting of steel are not particularly limited to those specified in one embodiment, and any conditions may be used. Examples of conditions include mold thickness, mold width, casting speed, and steel composition. Furthermore, "alumina-based inclusions" refers to inclusions whose main component is alumina.

[0049] Here, the lower the FeO concentration of the alumina-based inclusions, the greater the effect of suppressing nozzle clogging. Therefore, the FeO concentration of the alumina-based inclusions may be 4 atom% or less, 3 atom% or less, or 2 atom% or less. The lower limit of the FeO concentration of the alumina-based inclusions is not particularly limited, but for example, it may be 0.1 atom% or more.

[0050] Furthermore, the smaller the average particle size of the alumina-based inclusions, the greater the effect of suppressing nozzle clogging. Therefore, the average particle size of the alumina-based inclusions may be 45 μm or less, 30 μm or less, or 25 μm or less. The lower limit of the average particle size of the alumina-based inclusions is not particularly limited, but for example, it may be 1 μm or more.

[0051] The FeO concentration and average particle size of alumina inclusions can be obtained from image analysis using a scanning electron microscope (SEM). Specifically, the procedure is as follows: First, the immersion nozzle is cut lengthwise after casting, and samples of alumina inclusions are cut out from five arbitrary locations on the cross-section. Then, the obtained samples are observed using a SEM. In the SEM image analysis, spot composition analysis and particle size measurement are performed at 30 locations for each sample. The average value of the obtained results is then used as the FeO concentration and average particle size of the alumina inclusions.

[0052] The continuous casting method for steel according to the present disclosure has been described above using one embodiment. According to the continuous casting method for steel according to the present disclosure, nozzle blockage can be suppressed. [Examples]

[0053] The present disclosure will be further explained below with reference to examples.

[0054] A continuous casting test of steel was conducted under the conditions shown in Table 1. The nozzle used was nozzle 100, as shown in Figure 2. The casting test was carried out under the conditions of a mold thickness of 0.25 m, a mold width of 1.0 to 1.6 m, and a casting speed of 1.0 m / min to 1.2 m / min. The composition of the steel is shown in Table 2. The method for measuring the FeO concentration and average particle size of the alumina inclusions in Table 1 is as described above.

[0055] The method for calculating the nozzle occlusion index in Table 1 is explained below. The nozzle occlusion index is a numerical representation of the degree of occlusion relative to the diameter of the immersed nozzle. The calculation method is as follows: First, the immersed nozzle is recovered after the casting test, and the average adhesion thickness of the entire inner surface is calculated from its longitudinal cross-section. Next, the average adhesion thickness of Comparative Example 4 is set to 10 and indexed, and the nozzle occlusion index of the other test examples is calculated. In this example, a nozzle occlusion index of 3 or less was evaluated as a good result.

[0056] [Table 1]

[0057] [Table 2]

[0058] Table 1 shows that Examples 1-6 had lower FeO concentrations in alumina inclusions and smaller average particle sizes compared to Comparative Examples 1-4. Furthermore, Examples 1-6 exhibited significantly lower nozzle clogging indices compared to Comparative Examples 1-4. These results are discussed below.

[0059] First, let's consider the purity of the Ar gas. Comparing Example 2 with Comparative Example 1, Example 1 used high-purity Ar gas, while Comparative Example 1 used lower-purity Ar gas. As a result, the FeO concentration and average particle size of the alumina inclusions, as well as the nozzle clogging index, were significantly better in Example 2 than in Comparative Example 1. From this, it can be concluded that using high-purity Ar gas is important for suppressing nozzle clogging.

[0060] This is thought to be due to the following reasons. The Ar gas used in Example 2 had a purity of 99.99% or higher, an oxygen concentration of 2 ppm or less, and a dew point of -65°C or lower, making it extremely high purity. Therefore, it was possible to stably form an Ar gas film on the inner wall of the nozzle, allowing a film to be present on the inner walls of the fitting parts of each component and the sliding gate, thereby suppressing the intake of outside air from the sliding surfaces of the fitting parts and the sliding gate. On the other hand, the Ar gas used in Comparative Example 1 had a purity of 99.9% or higher, an oxygen concentration of 5 ppm or less, and a dew point of -55°C or lower, which is lower purity than the Ar gas in Example 2. Therefore, it is thought that the interfacial tension between the film and the molten steel decreased, making it impossible to stably form a film on the inner walls of the fitting parts of each component and the sliding gate. As a result, it is believed that outside air was drawn in from the fitting portion and the sliding surface of the sliding gate, which increased the FeO concentration of the alumina inclusions adhering to the inner wall of the immersion nozzle, causing the particles to coarseen and increasing the nozzle occlusion index.

[0061] Next, we consider the location where the Ar gas is supplied. Comparing Examples 1-6 with Comparative Example 3, we find that in Examples 1-6, the Ar gas is supplied from the upper nozzle, while in Comparative Example 3, the Ar gas is not supplied from the upper nozzle. From these results, it can be concluded that supplying the Ar gas from the upper nozzle is important.

[0062] This is thought to be due to the following reasons. The nozzle clogging suppression effect is thought to be improved by suppressing the intake of outside air from the fitting parts of each nozzle component and the sliding surface of the sliding gate. Therefore, it is thought that the nozzle clogging suppression effect can be improved by supplying Ar gas from the upper nozzle, which is above the fitting parts of each nozzle component and the sliding surface of the sliding gate. In particular, it is thought that the intake of outside air from the fitting part between the upper nozzle and the sliding gate can be suppressed. Furthermore, it is thought that supplying Ar gas from the sliding gate in addition to the upper nozzle can further enhance the stability of the vapor film and improve the nozzle clogging suppression effect. On the other hand, the immersion nozzle is located below the fitting parts of each nozzle component and the sliding surface of the sliding gate. Therefore, it is thought that supplying Ar gas from the immersion nozzle had little effect on the nozzle clogging suppression effect.

[0063] Now, let's further examine Comparative Example 3. Comparative Example 3 uses high-purity Ar gas, but the Ar gas is not supplied from the upper nozzle. Therefore, it is thought that the inhalation of outside air from the fitting portion between the upper nozzle and the sliding gate could not be suppressed. Consequently, it is thought that in Comparative Example 3, the FeO concentration of alumina inclusions adhering to the inner wall of the immersion nozzle increased, the particles became coarser, and the nozzle occlusion index increased.

[0064] Next, we will examine the flow rate of Ar gas. In Examples 1 to 6, the flow rate of Ar gas supplied from the upper nozzle, or the total flow rate of Ar gas supplied from the upper nozzle and the sliding gate, is in the range of 1 NL / min to 25 NL / min. On the other hand, in Comparative Example 2, the flow rate of Ar gas supplied from the upper nozzle is 27 NL / min. As a result, the nozzle occlusion index was significantly low in Examples 1 to 6, while it was high in Comparative Example 2.

[0065] This is thought to be due to the following reasons: When the Ar gas flow rate is in the range of 1 NL / min to 25 NL / min, a stable Ar gas film is thought to form. On the other hand, when the Ar gas flow rate is 27 NL / min, the buoyancy of the formed film becomes large, causing it to float to the tundish against the flow of molten steel, entraining the flux and causing contamination of the molten steel. In addition, according to Comparative Example 4, Ar gas is supplied from the upper nozzle, but the flow rate is 0.5 NL / min. Therefore, a stable film was not formed, and outside air was drawn in.

[0066] For the reasons stated above, it is believed that a stable Ar gas film can be formed when the Ar gas flow rate is in the range of 1 NL / min to 25 NL / min. In fact, according to the inventors' findings, it is believed that a stable Ar gas film can be formed when the flow rate is between 1 NL / min and 25 NL / min.

[0067] Based on these results, the following conditions are considered important for suppressing nozzle blockage. • Use high-purity Ar gas with a purity of 99.99% or higher, an oxygen concentration of 2 ppm or less, and a dew point of -65°C or lower. The conditions for flowing high-purity Ar gas into the molten steel flow path of the nozzle are (A) or (B) below. (A) High-purity Ar gas is supplied from the upper nozzle to the molten steel flow path of the nozzle, and high-purity Ar gas is not supplied from the sliding gate to the molten steel flow path of the nozzle, and the flow rate of high-purity Ar gas supplied from the upper nozzle is 1 NL / min or more and 25 NL / min or less. (B) High-purity Ar gas is flowed into the molten steel flow path of the nozzle from the upper nozzle and the sliding gate, and the total flow rate of the high-purity Ar gas supplied from the upper nozzle and the sliding gate is 1 NL / min or more and 25 NL / min or less.

[0068] By applying these conditions, it is believed that after casting, the FeO concentration of alumina-based inclusions adhering to the inner wall of the immersion nozzle can be reduced to 5 atom% or less, and the average particle size of the alumina-based inclusions can be reduced to 50 μm or less. As a result, it is believed that clogging of the immersion nozzle can be prevented. [Explanation of Symbols]

[0069] 1 Tan Dish 2 Molten steel 3. Mold 10 Upper nozzle 11 Ar gas supply unit 20 Immersion nozzles 21 Discharge hole 22 Ar gas supply section 30 Sliding Gates 31 Upper fixing plate 32 Lower fixing plate 33 Intermediate plate 34 Ar gas supply unit 40 Lower nozzle 51-53 Fitting part 54, 55 Sliding surface 100 nozzles A air membrane

Claims

1. A continuous casting method for steel that uses a nozzle to inject molten steel from a tundish into a mold, The nozzle comprises an upper nozzle, an immersion nozzle, and a sliding gate positioned between the upper nozzle and the immersion nozzle. High-purity Ar gas with a purity of 99.99% or higher, an oxygen concentration of 2 ppm or lower, and a dew point of -65°C or lower is flowed into the molten steel flow path of the nozzle. The conditions for flowing the high-purity Ar gas into the molten steel flow path of the nozzle are: The high-purity Ar gas is flowed from the upper nozzle and the sliding gate into the molten steel flow path of the nozzle, and the total flow rate of the high-purity Ar gas supplied from the upper nozzle and the sliding gate is 1 NL / min or more and 25 NL / min or less. The flow rate of the high-purity Ar gas supplied from the upper nozzle is greater than the flow rate of the high-purity Ar gas supplied from the sliding gate. Characterized by, A continuous casting method for steel.

2. After casting, the FeO concentration of the alumina-based inclusions adhering to the inner wall of the immersion nozzle is 5 atom% or less, and the average particle size of the alumina-based inclusions is 50 μm or less. The method for continuous casting of steel according to claim 1.

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