Continuous casting method for steel

Abstract

To provide a continuous casting method for steel that can produce cast pieces with excellent surface and internal quality.SOLUTION: Provided is a continuous casting method for steel in which when molten steel is fed into a casting mold from a sliding nozzle at the bottom of a tundish through an immersion nozzle, the immersion nozzle is made to have a transverse cross-sectional shape of the outside of an immersion part into the molten steel in the casting mold as an ellipse or streamlined shape, and a length ratio D1 / D2 of one axis D1 approximately parallel to a casting mold long side and the other axis D2 perpendicular to the one axis is in the range of 1.00 to 3.00, a ratio S1 / S0 of a cross-sectional area S1 at a smallest cross-sectional area part of the inner hole to a cross-sectional area S0 of a nozzle hole of the sliding nozzle is in the range of 0.96 to 1.30, and a ratio N1 / S1 of an area N1 of one of the discharge holes to the cross-sectional area S1 is in the range of 0.96 to 1.20.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a method for continuous casting of steel in which molten steel is supplied into a mold from a sliding nozzle provided at the bottom of a tundish through a submerged nozzle. [Background technology]

[0002] A commonly used continuous steel casting method is to supply molten steel from a ladle to a mold through a tundish, which is an intermediate vessel. In this method, a submerged entry nozzle installed at the bottom of the tundish is immersed in the molten steel in the mold for casting. For example, when pouring molten steel into a mold for casting a slab with a rectangular cross section, a submerged entry nozzle having one discharge hole on each side of the mold is usually used. A stopper installed in the tundish or a sliding nozzle installed at the bottom of the tundish is used as a device for controlling the flow rate of molten steel passing through such a submerged entry nozzle.

[0003] Generally, sliding nozzles have two or three plates. In the case of two plates, the sliding direction is the width direction (parallel to the long side of the mold), so drift occurs in the width direction. In the case of three plates, the sliding direction is the thickness direction (perpendicular to the long side of the mold), so drift occurs in the thickness direction. On the other hand, in the case of stopper control, drift is said to be less likely to occur because the molten metal falls evenly from the periphery of the stopper. However, since the stopper is a refractory material and has buoyancy in the molten steel, the axis of the stopper and the axis of the upper nozzle are not necessarily coaxially arranged, and drift cannot be prevented.

[0004] Various techniques have been developed in the past to stabilize the discharge flow of molten steel from the submerged nozzle and produce slabs with good surface and internal quality. Patent Document 1 discloses a continuous casting method in which the angle between the sliding nozzle and the discharge flow in the horizontal plane is set to 80 to 90° in order to prevent the one-sided flow phenomenon of molten steel in the mold. Patent Document 2 discloses an injection method in which the submerged nozzle has a rectangular cross section and the injection flow from the injection nozzle to the mold is maintained as a uniform low-speed downward flow during casting. Patent Document 3 discloses a continuous casting method in which the discharge hole is slit-shaped to disperse and homogenize the molten steel flow discharged from the submerged nozzle, thereby producing slabs without surface or internal defects.

[0005] Patent Document 4 discloses a submerged entry nozzle equipped with twisted tape-shaped swirling blades inside. Patent Document 5 discloses a continuous casting method in which an inert gas is introduced into the submerged entry nozzle to control the internal pressure, thereby preventing drift in the flow of molten steel from the discharge hole.

[0006] Patent Document 6 discloses a technique in which the cross-sectional shape of the inner bore of the submerged entry nozzle is a flat shape such as an ellipse, the major axis of which is parallel to the long side of the mold, and the sliding direction of the sliding nozzle is perpendicular to the major axis, thereby stabilizing the flow of molten steel in the submerged entry nozzle. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] JP 2002-301549 A [Patent Document 2] Japanese Patent Application Publication No. 58-74257 [Patent Document 3] Japanese Patent Application Publication No. 9-285852 [Patent Document 4] JP 2000-237852 A [Patent Document 5] Japanese Patent Application Publication No. 9-225604 [Patent Document 6] JP 2007-69222 A Summary of the Invention [Problem to be solved by the invention]

[0008] However, even with the above-mentioned conventional methods, it is still not possible to stabilize the flow of molten steel discharged into the mold and to sufficiently prevent drifting, and it is not possible to sufficiently prevent surface defects called slivers caused by inclusions that occur on the coil surface after rolling, and bubble defects called blowholes caused by argon blown into the submerged entry nozzle.

[0009] The present invention has an object to provide a method for continuous casting of steel which solves the above-mentioned problems of the conventional method, and which can produce slabs with excellent surface and internal quality by stabilizing the discharge flow from the submerged entry nozzle, thereby preventing the entrainment of non-metallic inclusions such as alumina which cause slivers and argon bubbles which cause blowholes. [Means for solving the problem]

[0010] The method for continuous casting of steel according to the present invention which advantageously solves the above-mentioned problems is characterized in that, when molten steel is supplied into a mold from a sliding nozzle provided at the bottom of a tundish via an immersed nozzle, the immersed nozzle has an elliptical or streamlined cross-sectional shape of an outer shape of a portion immersed in the molten steel in the mold, the length ratio D1 / D2 of one axis D1 of the cross-sectional shape of the inner bore which is approximately parallel to a long side of the mold and another axis D2 perpendicular to the one axis is in the range of 1.00 to 3.00, the ratio S1 / S0 of a cross-sectional area S1 at the minimum cross-sectional area part of the inner bore to a cross-sectional area S0 of a nozzle hole of the sliding nozzle is in the range of 0.96 to 1.30, and the ratio N1 / S1 of an area N1 of one of the discharge holes to the cross-sectional area S1 is in the range of 0.96 to 1.20.

[0011] The method for continuous casting steel according to the present invention comprises the steps of: a. The submerged nozzle is arranged so that the direction of the one axis is approximately parallel to the long side of the mold, and at least a pair of discharge holes are provided on both side surfaces in the one axial direction so as to discharge molten steel toward the opposing short side direction of the mold; b. The submerged nozzle is arranged so that the direction of one axis is approximately parallel to the long side of the mold, and at least one pair of discharge holes is provided on both side surfaces so that the discharge hole discharges molten steel while being deflected at an angle of 60° or less from the opposing long side of the mold; c. The submerged nozzle is arranged so that the distance between the outer surface of the other shaft side and the inner wall of the long side of the mold is 80 mm or more; d. Casting is performed while imparting rotation to the molten steel in the mold using an electromagnetic stirrer; This is thought to be a more preferable solution. Effect of the Invention

[0012] According to the present invention, the outer shape of the submerged nozzle in the molten steel in the mold is elliptical or streamlined, the length ratio of the major axis to the minor axis of the cross-sectional shape of the inner hole is specified, and the major axis is arranged parallel to the long side of the mold. This allows the submerged nozzle to be immersed without disturbing the flow of molten steel in the mold near the submerged nozzle, and ensures the surface flow rate of the meniscus. In addition, the ratio of the cross-sectional area of ​​the inner hole of the submerged nozzle to the cross-sectional area of ​​the nozzle hole of the sliding nozzle is specified, and the ratio of the area of ​​the discharge hole of the submerged nozzle to the cross-sectional area of ​​the inner hole is specified. This makes it possible to prevent drift without causing nozzle clogging due to air being sucked into the submerged nozzle.

[0013] Furthermore, it is preferable that the major axis of the immersed nozzle is arranged approximately parallel to the major side of the mold, and at least one pair of discharge holes is provided on both sides in the major axis direction, thereby making it possible to prevent the discharged molten steel flow from penetrating deeply into the unsolidified portion from the meniscus.

[0014] It is also preferable that the submerged nozzle is arranged so that the major axis of the submerged portion is approximately parallel to the long side of the mold, and at least one pair of discharge holes is provided on both sides so that the discharge holes are deflected within a predetermined angle from the opposing long side of the mold to discharge molten steel. In this way, a swirling flow can be imparted to the molten steel in the mold. This makes it possible to prevent nonmetallic inclusions and the like from being captured by the solidified shell and produce a cast strip with excellent surface properties.

[0015] Furthermore, it is preferable to optimize the distance between the outer surface of the submerged entry nozzle on the minor axis side and the inner wall of the mold on the major axis side, so that the molten steel can be cast while ensuring a sufficient flow rate in the vicinity of the submerged entry nozzle.

[0016] In addition, it is preferable to perform casting while imparting a swirling force to the molten steel in the mold using an electromagnetic stirrer, which can prevent nonmetallic inclusions from being captured in the solidified shell and produce a cast slab with excellent surface properties. [Brief description of the drawings]

[0017] [Figure 1] FIG. 1 is a vertical sectional view of a mold and a submerged nozzle suitable for a method for continuously casting steel according to one embodiment of the present invention. [Diagram 2] 1A and 1B are diagrams showing the cross-sectional shapes of the minimum cross-sectional area part of the submerged nozzle, in which (a) shows an elliptical shape and (b) shows an axisymmetric streamline shape. [Diagram 3] 1A and 1B are schematic top views showing the positional relationship between the mold and the submerged entry nozzle inside the mold, where (a) shows the case where the discharge hole is oriented parallel to the long side of the mold, and (b) shows the case where the discharge hole is biased toward the long side. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0018] Hereinafter, the embodiments of the present invention will be described in detail. Note that the drawings are schematic and may differ from the actual ones. Furthermore, the following embodiments are merely examples of devices and methods for embodying the technical idea of ​​the present invention, and are not intended to limit the configuration to the following. In other words, the technical idea of ​​the present invention can be modified in various ways within the technical scope described in the claims.

[0019] The inventors analyzed the flow of molten steel in a submerged entry nozzle and a mold, and obtained the following findings, which led to the completion of the present invention.

[0020] In the case of a conventional submerged entry nozzle, whose inner bore has a perfectly circular cross section, when the sliding nozzle slides, a drift in the sliding direction of the sliding nozzle occurs inside the submerged entry nozzle because the opening is biased to one side. This drift increases the variation in the flow rate of molten steel from the discharge hole of the submerged entry nozzle, and increases the maximum discharge flow rate.

[0021] The increase in maximum discharge flow velocity increases the depth to which the discharge flow penetrates into the unsolidified portion. As a result, inclusions such as alumina, which is a product of deoxidation, and continuous casting powder, as well as argon bubbles injected into the SEN penetrate deep into the unsolidified portion of the molten steel and remain there without rising to the surface. This leads to surface defects and internal defects such as cracks. In addition, if the cross-sectional shape of the SEN's outer shape is a perfect circle, the resistance of the SEN's immersed portion to the flow of molten steel in the mold increases, and the meniscus surface flow velocity decreases. It was found that when the meniscus surface flow collides with the SEN, the flow velocity becomes uneven and vortexes are formed around it, cutting the continuous casting powder and leading to surface defects.

[0022] In order to prevent these problems, it has been found that it is effective to cast the nozzle inner bore with a flat cross-sectional shape such as an ellipse or oval, or a streamlined shape, with its major axis oriented approximately parallel to the long side of the mold.

[0023] FIG. 1 is a schematic vertical cross-sectional view showing a schematic configuration of a continuous casting facility for carrying out a continuous casting method of steel according to this embodiment, as seen from the short side of a slab. A sliding nozzle 1 is provided at the bottom of a tundish (not shown). A submerged nozzle 2 is connected to the lower side of the sliding nozzle 1. Molten steel is poured into a mold 3 from the submerged nozzle 2. In the example of FIG. 1, an electromagnetic stirring coil 4 for stirring the molten steel in the mold is installed on the mold 3. The sliding nozzle 1 has a nozzle hole 11 with a cross-sectional area of ​​S0. In the example of FIG. 1, the sliding nozzle 1 is a three-plate type that is sandwiched between an upper plate 5 and a lower plate 6 and has a sliding direction SD perpendicular to the long side of the mold 3. The solidified shell that is cooled and solidified in the mold 3 is pulled out in the casting direction CD.

[0024] In this embodiment, the sliding nozzle 1 has a nozzle hole 11 in a perfect circle. The submerged nozzle 2 has an upper inner hole 21 in a perfect circle. The submerged nozzle 2 has a lower inner hole 21 in an elliptical shape as shown in FIG. 2(a) or an axially symmetric streamlined shape as shown in FIG. 2(b). Since the thickness of the nozzle refractory is almost constant, the cross-sectional shape of the inner hole governs the outer shape. Here, the elliptical shape includes an oblong shape and does not include a perfect circle. Instead of an elliptical shape, an elliptical shape having a parallel part in which the short side of a rectangle is replaced with an arc can be used. The streamlined shape refers to a shape that does not generate vortexes around it when placed in a flow and is composed of curves that receive the least resistance from the flow. In a uniform flow, the shape is rounded at the front and pointed at the rear.

[0025] As shown in Fig. 2(a), an ellipse or oval has a major axis having a length (major axis) D1 and a minor axis having a length (minor axis) D2 perpendicular to the major axis. In the axisymmetric streamlined shape shown in Fig. 2(b), the length of the axis parallel to the flow D1 and the length of a perpendicular line drawn from the peripheral point farthest from the axis to the axis are defined as half the length D2 of the other axis. As will be described later, it is preferable that the molten steel between the immersion nozzle installed at the center (in the width direction and thickness direction) of a rectangular mold in top view and the opposing long side of the mold be a swirling flow, that is, molten steel flows in opposite directions on either side of the mold center, and by making the outer shape of the immersion part of the immersion nozzle an axisymmetric streamlined shape, the generation of vortexes on the surface of the molten steel in the mold can be reduced.

[0026] In this embodiment, the submerged nozzle 2 is immersed in the molten steel in the mold in an arrangement as shown in FIG. 3(a) or (b). The cross-sectional shape of the minimum cross-sectional area portion 23 of the submerged nozzle 2 is elliptical. In this embodiment, the major axis direction LA of the submerged nozzle 2 is arranged parallel or substantially parallel to the long side of the mold 3 as one axis direction, and the minor axis direction SA is the other axis direction. When a horizontal swirling flow is given to the molten steel in the rectangular mold in top view, the molten steel near the submerged nozzle flows parallel to the long side, so if the nozzle is arranged as shown in FIG. 3(a) or (b), the flow resistance can be reduced. In this case, the minor axis direction SA is perpendicular or substantially perpendicular to the long side of the mold. Here, "arranged substantially parallel to the long side" means that the installation error accuracy of the equipment and devices is allowed, and a deviation of 5° or less is allowed.

[0027] In the example of Fig. 3(a), the submerged entry nozzle 2 has two discharge holes 22, one on each side in the longitudinal direction LA, so that the molten steel can be discharged in the direction of the opposing short side of the mold. At least one pair of discharge holes 22 is sufficient, and two pairs are also acceptable. This arrangement can prevent the discharge flow of molten steel from penetrating deep into the casting direction CD from the meniscus.

[0028] In the example of FIG. 3(b), a pair of discharge holes 22 are provided so as to discharge molten steel by deflecting the holes at an angle θ with the opposing long sides of the mold, so that a swirling flow can be formed in the molten steel in the mold, and the solidified shell near the meniscus can be washed to prevent inclusions from adhering. There is at least one pair of discharge holes 22, and two pairs may be provided. The deflection angle θ is preferably 60° or less, and more preferably 30° or less. If the deflection angle θ is too large, the discharge flow from the submerged entry nozzle collides with the solidified shell on the long side, which may cause operational accidents such as internal cracks and breakouts due to delayed solidification of the solidified shell. In addition, the deflection angle θ does not include 0°, and is more preferably 5° or more from the viewpoint of effectively generating a swirling flow of molten steel in the mold.

[0029] In this embodiment, it is preferable that the sliding direction of the sliding nozzle 1 is perpendicular to the longitudinal direction LA of the inner hole of the submerged nozzle 2. The width of the direction in which the molten steel drifts inside the submerged nozzle 2 can be reduced, so that the molten steel flows evenly in the longitudinal direction LA. Therefore, drift of the molten steel inside the submerged nozzle 2 that occurs when the sliding nozzle 1 slides can be reduced.

[0030] In the submerged nozzle 2 having the inner hole 21 of the above-mentioned shape, the length ratio D1 / D2 between one axial length D1 and the other axial length D2 must be in the range of 1.00 to 3.00 at the minimum cross-sectional area portion 23 directly above the discharge hole 22. If the length ratio D1 / D2 is less than 1.00, the flatness of the inner hole 21 is reversed, and it is not possible to effectively prevent the occurrence of drift in the sliding direction of the sliding nozzle 1. In addition, there is a risk that the inner hole cross-sectional area S1 of the submerged nozzle 2 becomes too small compared with the nozzle hole cross-sectional area S0 of the sliding nozzle 1. On the other hand, if the length ratio D1 / D2 exceeds 3.00, the shape becomes too flat, increasing the risk of the submerged nozzle being broken, and in the event of alumina clogging during casting, it is difficult to ensure a sufficient molten steel flow rate, making it impossible to control the molten steel level in the mold, and there is a risk that stable operation cannot be continued due to fluctuations in the molten steel level. In the case of the submerged nozzle 2 having a plurality of discharge holes 22 on the top and bottom, the minimum cross-sectional area portion 23 is located at the upper end position of the uppermost discharge hole 22.

[0031] As described above, the inner hole 21 of the submerged nozzle 2 not only changes its shape from the top to the bottom, but also reduces its cross-sectional area. The ratio S1 / S0 of the cross-sectional area S1 of the minimum cross-sectional area portion 23 directly above the discharge hole 22 to the cross-sectional area S0 of the nozzle hole 11 of the sliding nozzle 1 must be in the range of 0.96 to 1.30. If the ratio S1 / S0 is less than 0.96, the supply of molten steel will be insufficient when the opening of the sliding nozzle increases in the non-steady state portion where the height of the molten steel in the tundish decreases. As a result, the molten steel level in the mold cannot be controlled, and there is a risk that a situation will occur in which stable operation cannot be continued due to fluctuations in the molten steel level. On the other hand, if the ratio S1 / S0 exceeds 1.30, the effect of the flatness of the inner hole will be reduced, and the effect of suppressing drift in the sliding direction will be reduced. In addition, there is a concern that the nozzle itself will become larger, resulting in an increase in running costs. In addition, negative pressure may be generated at the step between the lower nozzle 6 and the submerged nozzle 2, which may cause air to be sucked in. In addition, as the sliding nozzle 1 slides, there is a high possibility that uneven flow of the molten steel in the submerged nozzle may occur.

[0032] At the start of casting, the molten steel poured from the ladle fills the tundish and is poured into the mold through the submerged entry nozzle. During this time, the molten steel temperature drops as heat is absorbed by each refractory. If the temperature drops significantly, the temperature may fall below the freezing point, causing the molten steel to solidify in the submerged entry nozzle, resulting in a so-called hanging state. In such a case, "manual intervention at the start of casting" is performed, for example, by inserting an oxygen pipe through the outlet hole of the submerged entry nozzle and blowing oxygen onto the solidified surface to melt it. This manual intervention at the start of casting is dangerous for workers and is an obstacle to stable operation of continuous casting, as it is undesirable in terms of quality.

[0033] In addition, the ratio N1 / S1 of the area N1 of one of the discharge holes 22 to the cross-sectional area S1 must be in the range of 0.96 to 1.20. When the discharge holes 22 are provided in pairs on both sides of the submerged nozzle 2, the area N1 of one of the discharge holes 22 is the total area of ​​the discharge holes 22 on one side. When the discharge holes 22 are provided on the bottom of the submerged nozzle 2, the area N1 of one of the discharge holes 22 is calculated by adding up the areas of the discharge holes 22 on the bottom. When the surface quality of the product is important, it is preferable to not provide the discharge holes 22 on the bottom of the submerged nozzle, but to provide the discharge holes 22 only on the side. If the ratio N1 / S1 is less than 0.96, nozzle clogging is likely to occur, the molten steel level fluctuations will increase, and the amount of molten steel discharged will be insufficient relative to the required amount, which may prevent stable operation. On the other hand, when the ratio N1 / S1 exceeds 1.20, the balance of the discharge rates between the opposing nozzles is lost, and the flow rate of one of the nozzles becomes larger, which is called drift. This drift can send inclusions and other materials deep into the unsolidified part of the molten steel in the casting direction CD. Furthermore, nozzle clogging can occur in the area of ​​the outlet where the flow rate is reduced, which can increase the fluctuation of the molten steel surface. In addition, if the drift becomes extremely large, there is a risk of an operational accident such as a breakout.

[0034] As shown in FIG. 3(a), the distance L between the outer surface of the short axis side of the submerged nozzle 2 and the inner wall of the long side of the mold 3 is S It is preferable that the distance L is 80 mm or more. S Distance L is the smaller of the distances between the two opposing long sides of the mold. S If the gap is less than 80 mm, a sufficient molten steel flow rate cannot be obtained when a swirling flow is generated in the mold, for example, when the molten steel is electromagnetically stirred, and there is a high possibility that the solidified shell will capture inclusions that cause surface defects.

[0035] As shown in Fig. 1 and Figs. 3(a) and (b), it is preferable to install an electromagnetic stirring coil 4 for electromagnetic stirring in the mold 3. Casting can be performed while imparting swirling force to the molten steel in the mold 3. By electromagnetically stirring the molten steel, it is possible to prevent inclusions and the like from being captured by the solidified shell, and to produce a cast piece with excellent surface properties. EXAMPLES

[0036] Next, examples of the present invention will be described, but the present invention is not limited to the following examples.

[0037] 300 tons of molten ultra-low carbon steel was produced in a converter-RH degassing process. The temperature of the molten steel in the tundish was set to 1560-1580°C, and the molten steel was poured into the mold using a three-layer sliding nozzle, a two-layer sliding nozzle, and a submerged nozzle, producing a slab 250 mm thick and 1200-1600 mm wide at a casting speed of 1.6-2.0 mm / min. The molten steel was rotated horizontally by electromagnetic stirring during casting. The steel plate was a cold-rolled steel plate with a thickness of 0.7-1.2 mm. The effect was evaluated based on the coil defect rate (number of defects / coil length (m)) and the presence or absence of nozzle clogging during casting. The results of tests conducted under various conditions for continuous casting are shown in Tables 1-1 and 1-2. In the table, the shape of the inner hole of the submerged nozzle is the shape of the minimum cross-sectional area part 23 directly above the upper discharge hole, which is elliptical except for treatment No. 19, which is streamlined. In addition, D1 is the length (mm) of one axis parallel to the long side of the mold, D2 is the length (mm) of the other axis perpendicular to the one axis, and S1 is the cross-sectional area (mm) of the minimum cross-sectional area part 23 of the submerged nozzle 2. 2 ), S0 is the cross-sectional area of ​​the nozzle hole 11 of the sliding nozzle 1 (mm 2 ), θ is the angle (°) between the direction of the discharge hole of the submerged entry nozzle and the long side of the mold, n is the number of discharge holes 22 of the submerged entry nozzle 2, N1 / S1 is the ratio of the area of ​​one of the discharge holes to the cross-sectional area of ​​the minimum cross-sectional area 23 of the submerged entry nozzle 2, L S is the distance (mm) between the outer surface of the short axis side of the submerged entry nozzle 2 and the inner wall of the long side of the mold, and ηD is the coil defect occurrence rate, which is the percentage (% / m) of the number of defects per coil length (m). The success rate at the start of casting is the manual intervention rate at the start of casting, expressed as a percentage of the number of manual interventions to the total number of casting starts. The smaller the manual intervention rate, the more stable the operation. The rate of abnormality occurrence due to molten metal level fluctuations is expressed as a percentage of the number of molten metal level fluctuations to the number of castings. In Tables 1-1 and 1-2, SD represents the relationship between the sliding direction of the sliding nozzle 1 and the long side of the mold, with "orthogonal" indicating a three-layer sliding nozzle and "parallel" indicating a two-layer sliding nozzle.

[0038] [Table 1-1]

[0039] [Table 1-2]

[0040] For Process Nos. 1 to 19, the results are from casting tests carried out within the scope of the present invention. The coil defect occurrence rate ηD was 1.9% or less, and the quality results were very good. In addition, no nozzle blockage occurred during casting. The manual intervention rate at the start of casting was low, and the rate of abnormalities due to fluctuations in the molten metal level was also low, resulting in stable operation.

[0041] In the case of No. 20, the nozzle flatness ratio D1 / D2 was large. In this case, it is assumed that the refractory itself was difficult to manufacture and prone to cracking, and the resistance to the passage of molten steel was large, causing nozzle clogging. In addition, the D1 / D2 ratio was too large, so the inner hole shape became flat, and when alumina clogging occurred during casting, the molten steel flow rate was not sufficient, and the occurrence of abnormalities due to fluctuations in the molten steel level was high. Process No. 21 is a case where the ratio S1 / S0 was small. In this case, nozzle clogging occurred when pouring into the mold. It is presumed that this was because the nozzle inner diameter cross-sectional area was small compared to the nozzle hole cross-sectional area of ​​the sliding nozzle 1. When S1 became small, that is, even when S0 was fully open, the flow resistance of molten steel was large in the S1 part, the molten steel flow rate decreased, and the required amount of molten steel was not supplied. In other words, a hanging state occurred in the S1 part, and not only did the controllability of the molten steel level fluctuation decrease significantly, but the molten steel temperature drop at the start of casting was also significant, and the manual intervention rate at the start of casting became extremely high, making it impossible to perform stable operation. In the case of No. 22, the ratio S1 / S0 was large. In this case, the flow straightening effect was not obtained, and the occurrence rate of abnormalities due to the fluctuation of the molten metal level during casting was high. As a result, the coil quality deteriorated and the nozzle was clogged at the end of the casting process. In process No. 23, the nozzle flatness D1 / D2 was smaller than 1.00, and the length of the major axis and the minor axis were reversed. In addition, the ratio S1 / S0 was small. As a result, the resistance of the flow caused by the submerged entry nozzle increased, and the quality deteriorated. The small S1, in other words, even when S0 was fully open, the flow resistance of the molten steel was large in the S1 part, the molten steel flow rate decreased, and the required amount of molten steel was not supplied. In other words, a hanging state occurred in the S1 part, and not only did the controllability of the molten steel level fluctuation decrease significantly, but the molten steel temperature drop at the start of casting was also significant, and the manual intervention rate at the start of casting became extremely high, making it impossible to perform stable operation. In case No. 24, the area N1 of the discharge hole was too small compared to the minimum cross-sectional area S1 of the nozzle. The coil performance improved, but nozzle blockages were observed occasionally during casting. When N1 was smaller than S1, the flow resistance of the molten steel at the discharge hole was large, the molten steel flow rate decreased, and the required amount of molten steel was not supplied. As a result, not only was the controllability of the molten steel level fluctuation significantly reduced, but the molten steel temperature drop at the start of casting was also significantly large, and the manual intervention rate at the start of casting was extremely high, making it impossible to perform stable operation. In the case of treatment No. 25, the area N1 of the discharge hole was too large compared to the minimum cross-sectional area S1 of the nozzle. Because the discharge hole was large, the flow straightening effect was not confirmed, and the occurrence rate of abnormalities due to fluctuations in the molten metal level during casting was also high. Therefore, the coil quality did not improve.

[0042] From the above results, it is clear that stable operation of continuous casting can be achieved from the start to the end by setting appropriate values ​​for all of the features of the present invention, namely, the length ratio D1 / D2 of the perpendicular axes of the cross-sectional shape of the submerged nozzle and the inner bore, the cross-sectional area ratio S1 / S0 of the inner bore of the submerged nozzle to the nozzle hole of the sliding nozzle, and the area ratio N1 / S1 of the discharge hole of the submerged nozzle to the inner bore. If any of the conditions of the features of the present invention are not met, stable operation is possible for a short period of time, but any of the factors that affect stable operation, such as controllability during molten metal level fluctuations, starting stability, and alumina clogging, will deteriorate, and stable operation of continuous casting will be impaired.

[0043] In this specification, the unit of mass "ton" is 10 3Expressed in kg. [Industrial Applicability]

[0044] INDUSTRIAL APPLICABILITY The method for continuous casting of steel according to the present invention enables stable operation without clogging of the submerged entry nozzle and reduces the occurrence of defects in the products, and is therefore industrially useful. [Explanation of symbols]

[0045] 1 Sliding nozzle 2 Submerged nozzle 3. Mold 4. Electromagnetic Stirring Coil 5 Upper plate 6 Lower Plate 11 Nozzle hole 21 Inner hole 22 Discharge hole 23 Minimum cross-sectional area SD Sliding direction CD Casting direction LA One axis direction (long axis direction) SA Other axis direction (minor axis direction) D1: Length of one axis, major axis D2 other axis (length), minor axis L S Distance between the outer surface of the submerged entry nozzle and the long side of the mold

Claims

1. When molten steel is supplied into a mold from a sliding nozzle provided at the bottom of a tundish through a submerged nozzle, The submerged nozzle has an outer cross-sectional shape of a portion that is immersed in the molten steel in the mold, the outer cross-sectional shape being elliptical or streamlined, One axis D approximately parallel to the long side of the mold in the cross-sectional shape of the inner hole １ and another axis D perpendicular to the one axis ２ Length ratio D １ / D ２ is in the range of 1.00 to 3.00, The cross-sectional area S of the inner hole at its smallest cross-sectional area １ and the cross-sectional area S of the nozzle hole of the sliding nozzle ０ Comparison with S １ / S ０ is in the range of 0.96 to 1.30, One side area of ​​the discharge hole N １ and the cross-sectional area S １ Ratio to N １ / S １ A method for continuous casting of steel,

2. 2. The method for continuous casting of steel according to claim 1, wherein the submerged nozzle is arranged such that a direction of the one axis is approximately parallel to a long side of the mold, and at least a pair of discharge holes are provided on both side surfaces in the direction of the one axis so as to discharge molten steel toward a direction of an opposing short side of the mold.

3. 2. The method for continuous casting of steel according to claim 1, wherein the submerged nozzle is arranged so that a direction of the one axis is approximately parallel to a long side of the mold, and at least a pair of discharge holes are provided on both side surfaces so that the discharge hole discharges molten steel while being deflected at an angle of within 60° with respect to the opposing long side of the mold.

4. The method for continuous casting steel according to any one of claims 1 to 3, wherein the submerged nozzle is arranged so that a distance between the other shaft-side outer surface and an inner wall of a longer side of the mold is 80 mm or more.

5. The method for continuous casting of steel according to any one of claims 1 to 3, wherein casting is carried out while imparting a swirling force to the molten steel in the mold by an electromagnetic stirrer.

6. 5. The method for continuous casting of steel according to claim 4, wherein casting is carried out while imparting a swirling force to the molten steel in the mold by an electromagnetic stirrer.

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