Immersion nozzle for continuous casting, and continuous casting method for steel

Abstract

Provided is an immersion nozzle for continuous casting capable of suppressing entrainment of mold powder into molten steel. Provided is an immersion nozzle for injecting molten steel from a tundish into a mold of a continuous casting machine, the immersion nozzle being characterized by comprising: a molten steel flow straightening part that is immersed in the molten steel, is exposed above the molten steel surface formed inside the mold, and is provided on the molten steel surface; and a tubular part that penetrates the molten steel flow straightening part in the vertical direction and discharges the molten steel to the inside of the mold, the molten steel flow straightening part having a continuous shape in which, at least in a prescribed range in the casting direction of the molten steel from the molten steel molten metal surface, the cross-sectional shape of the outer periphery of the molten steel flow straightening part is elliptical or streamlined having a major axis in the width direction of the mold, and the ratio of the minor axis b to the major axis a in the cross-sectional shape of the outer periphery of the molten steel flow straightening part is 0.95 or less, and the cross-sectional shape of the outer periphery of the tubular part being circular or elliptical in the width direction of the mold, the ratio of β to α in said shape being 0.90-1.1, where α is the length of one axis substantially parallel to the long side of the mold, and β is the length of another axis orthogonal to said one axis.

Description

Submerged nozzle for continuous casting and method for continuous casting of steel

[0001] The present invention relates to a continuous casting submerged entry nozzle and a continuous steel casting method. More specifically, the present invention relates to a continuous casting submerged entry nozzle that injects molten steel into a mold during continuous casting of molten steel, and a continuous steel casting method using the continuous casting submerged entry nozzle. More specifically, the present invention relates to a continuous casting submerged entry nozzle and a continuous steel casting method that can suppress entrainment of mold powder, which is used for purposes such as lubrication during continuous casting of molten steel, into the molten steel.

[0002] When continuous casting molten steel using a continuous casting machine, the molten steel is first poured from a tundish into a mold. As the molten steel poured into the mold is cooled, an initial solidified shell is formed on the surface of the molten steel. Then, as the molten steel is cooled in the secondary cooling zone that follows after the formation of the initial solidified shell, the solidification of the molten steel progresses to the interior of the molten steel.

[0003] During the casting of molten steel, a composite oxide called mold powder is constantly supplied into the mold to improve lubrication between the initial solidified shell and the mold. After being supplied into the mold, the mold powder becomes molten and penetrates between the mold and the initial solidified shell. Meanwhile, molten mold powder also floats on the surface of the molten steel (meniscus) of the mold. This mold powder floating on the surface of the molten steel (meniscus) of the mold is entrained by the molten steel flow generated in the mold. If this mold powder entrained by the molten steel flow is captured by the initial solidified shell, it will cause surface defects when the cast molten steel is made into a steel product, significantly impairing the quality of the steel product.

[0004] Various studies have been conducted on the phenomenon of mold powder being entrained in the molten steel flow generated in the mold. The main causes of this phenomenon are known to be the scraping of the molten steel surface (meniscus) of the mold by the molten steel flow and entrainment by the vortex of the molten steel flow. From this perspective, methods using electromagnetic brakes and methods that optimize the discharge hole and inner shape of the submerged entry nozzle have been proposed to prevent mold powder from being entrained in the molten steel flow.

[0005] For example, Patent Document 1 (Japanese Patent No. 3491099) proposes a method of applying a static magnetic field to the meniscus to apply a braking force to the molten steel, reducing the meniscus flow rate and preventing mold powder entrainment. Furthermore, Patent Document 2 proposes an immersion nozzle that prevents mold powder entrainment by optimizing the aspect ratio and area of ​​the discharge hole to suppress inclusion adhesion to the discharge hole and prevent drift caused by discharge hole clogging. Patent Document 3 (Japanese Patent No. 6963192) also proposes an immersion nozzle that suppresses the occurrence of suction in the discharge hole by optimizing the inner tube shape of the immersion nozzle, thereby preventing mold powder entrainment in the molten steel. Patent Document 4 (Japanese Patent No. 7175513) proposes an immersion nozzle that prevents mold powder entrainment in the molten steel by uniformly distributing the molten steel flow rate from the discharge hole by making the inner circumferential surface of the discharge hole spherical.

[0006] Japanese Patent Application Laid-Open No. 07-314100 Japanese Patent Application Laid-Open No. 2001-129645 Japanese Patent Application Laid-Open No. 2021-094585 Japanese Patent Application Laid-Open No. 2021-126663

[0007] However, the above-mentioned conventional techniques still have the following problems to be solved: The continuous steel casting method using a static magnetic field described in Patent Document 1 requires equipment for applying a static magnetic field, which is extremely expensive, and in addition, although the average meniscus flow velocity is suppressed, when a sudden high-velocity flow occurs at the meniscus due to nozzle clogging or the like, the flow velocity cannot be sufficiently suppressed, and the effect of suppressing mold powder entrainment is insufficient.

[0008] Furthermore, the techniques described in Patent Documents 2 to 4, which involve optimizing the discharge hole and inner surface shape of the submerged entry nozzle, are expected to be effective in suppressing mold powder entrainment in the early stages of casting. However, the submerged entry nozzles and other devices that prevent mold powder entrainment described in Patent Documents 2 to 4 experience significant clogging and melting damage to the submerged entry nozzle due to the discharge flow, which causes the submerged entry nozzle to deviate from its proper shape in the later stages of the casting process, resulting in the sudden generation of a high-velocity flow of molten steel at the meniscus. Thus, the submerged entry nozzles and other devices described in Patent Documents 1 to 4 have the problem of being unable to achieve a sufficient effect in suppressing mold powder entrainment in molten steel.

[0009] The present invention has been made in view of the above circumstances, and an object of the present invention is to provide an immersion nozzle for continuous casting, which is used to inject molten steel into a continuous casting mold in the continuous casting of steel, and a method for continuous casting of steel, which are capable of suppressing the entrainment of mold powder into molten steel.

[0010] The inventors conducted various experiments to solve the above problems and found that by employing a continuous casting submerged entry nozzle having a specific configuration, it is possible to suppress the entrainment of mold powder into molten steel and to obtain a steel product with little mold powder remaining in the slab. The present invention was made based on the above findings and is summarized as follows.

[0011] The present invention advantageously solves the above-mentioned problems by providing an immersion nozzle for continuous casting used to inject molten steel from a tundish into a mold of a continuous casting machine, the immersion nozzle comprising: a molten steel rectifying section that is immersed in the molten steel and is provided on the surface of the molten steel formed inside the mold so as to be exposed above the surface of the molten steel; and a tubular section that passes through the molten steel rectifying section in the vertical direction and discharges the molten steel into the mold, the molten steel rectifying section having a continuous shape in which the cross-sectional shape of the outer periphery is an ellipse or streamlined shape with a major axis in the width direction of the mold at least within a predetermined range from the surface of the molten steel in the casting direction of the molten steel, and the molten steel rectifying section has a ratio (b / a) of a minor axis b to a major axis a of 0.95 or less in the cross-sectional shape of the outer periphery The immersion nozzle for continuous casting according to the present invention is characterized in that the cross-sectional shape of the outer periphery of the tubular portion is circular or elliptical in the width direction of the mold, and the value of (β / α), which is the ratio of the length α of one axis substantially parallel to the long side of the mold to the length β of another axis perpendicular to the one axis, is 0.90 or more and 1.1 or less. Note that, in the immersion nozzle for continuous casting according to the present invention, it is considered that a more preferable solution can be provided in that (a) the molten steel rectifying portion has the continuous shape in a range of 50 mm or more from the surface of the molten steel in the casting direction of the molten steel.

[0012] Furthermore, the method for producing steel according to the present invention, which advantageously solves the above-mentioned problems, is characterized in that molten steel in a tundish is poured into a mold of a continuous casting machine using the above-mentioned submerged entry nozzle for continuous casting.

[0013] The submerged entry nozzle for continuous casting according to the present invention can suppress the generation of vortices in the vicinity of the submerged entry nozzle even when a fast flow of molten steel occurs in the meniscus due to drift or the like, and therefore can suppress the entrainment of mold powder into molten steel not only in the early stages of the casting process but also in the later stages of the casting process without the need for large-scale equipment.

[0014] Fig. 1 is a schematic diagram showing an example of an immersion nozzle for continuous casting according to the present invention, and is a perspective view of the immersion nozzle for continuous casting. Fig. 2 is a plan view of the immersion nozzle for continuous casting. Fig. 3 is a longitudinal sectional view taken along a plane passing through the center line of the discharge hole of the tubular part of the immersion nozzle for continuous casting. Fig. 4 is a diagram schematically showing the flow state of molten steel in a mold when a conventional immersion nozzle is used. Fig. 5 is a diagram schematically showing the flow state of molten steel in a mold when a continuous casting immersion nozzle according to the present invention is used.

[0015] [First embodiment] A description will be given of a continuous casting submerged entry nozzle according to the first embodiment. The continuous casting submerged entry nozzle according to this embodiment is a submerged entry nozzle for pouring molten steel from a tundish into a mold of a continuous casting machine, and comprises: a molten steel rectifying section that is immersed in the molten steel and is exposed above the molten steel surface formed inside the mold, and is provided on the molten steel surface; and a tubular section that passes through the molten steel rectifying section in the vertical direction and discharges the molten steel into the mold, and the molten steel rectifying section has a continuous shape in which, at least within a predetermined range from the molten steel surface in the casting direction of the molten steel, the outer periphery of the molten steel rectifying section has an elliptical or streamlined cross section with a major axis in the width direction of the mold, and the ratio (b / a), which is the ratio of the minor axis b to the major axis a of the outer periphery of the molten steel rectifying section, is 0.95 or less. The cross-sectional shape of the outer periphery of the tubular portion is circular or elliptical in the width direction of the mold, and the value of (β / α), which is the ratio of the length α of one axis approximately parallel to the long side of the mold to the length β of another axis perpendicular to the one axis, is 0.90 or more and 1.1 or less.

[0016] Generally, a semi-finished product called a slab is produced by continuous casting by pouring molten steel into a mold of a continuous casting machine. The width of the slab is approximately twice its thickness, meaning that the width is significantly greater than the thickness of the slab. The mold into which the molten steel is poured has a pair of long sides facing each other in the front-to-rear direction and a pair of short sides facing each other in the left-to-right direction. The mold is configured so that the short sides can move inside the long sides.

[0017] A continuous casting submerged entry nozzle that injects molten steel into a mold has a plurality of discharge holes for injecting the molten steel into the mold. The plurality of discharge holes may be formed as a pair of discharge holes. A pair of discharge holes may be formed toward each of the short sides of the mold that face each other in the left-right direction. The continuous casting submerged entry nozzle injects molten steel into the mold from each of the discharge holes formed toward the short sides of the mold.

[0018] The inventors then analyzed the flow state of molten steel inside the mold. Specifically, to analyze the flow state of molten steel inside the mold, they performed water model experiments on molten steel, numerical simulations, measurements using sensors installed inside the mold, and analysis of cast pieces (slabs) produced from the molten steel. As a result, the following mechanism regarding the flow of molten steel inside the mold was clarified.

[0019] First, most mold powder entrainment phenomena are caused by vortices of molten steel generated near the submerged entry nozzle. Second, when the flow velocity of molten steel at the meniscus near the submerged entry nozzle becomes too high, a vortex of molten steel is generated near the submerged entry nozzle, causing mold powder to be entrained in the molten steel. Third, when inclusions present in the molten steel cause nozzle clogging in the submerged entry nozzle, resulting in an excessive molten steel flow from one of the pair of discharge holes formed in the submerged entry nozzle, the flow velocity of molten steel at the meniscus becomes even more excessive. Fourth, even when the pair of discharge holes formed in the submerged entry nozzle are not clogged, the flow velocity and direction of the molten steel discharged from the submerged entry nozzle change over time. Therefore, when the flow velocity of molten steel is excessive and its direction is directed toward the meniscus, the flow velocity of molten steel at the meniscus becomes even more excessive.

[0020] Based on this mechanism, the inventors have furthered their research and development and discovered that the cross-sectional shape of the SEN and the shape of the SEN in the casting direction are important for suppressing the generation of vortices near the SEN. In other words, the reason vortices are generated near the SEN is that the flow velocity of the molten steel around the SEN becomes excessively high, causing the flow of molten steel to separate from the SEN, generating vortices next to the SEN on the downstream side of the mold.

[0021] From this technical viewpoint, the inventors have discovered that by making the outer periphery of the submerged entry nozzle have a streamlined or elliptical cross-sectional shape that is wide in the mold width direction, the flow of molten steel near the submerged entry nozzle becomes smooth and the generation of vortices can be suppressed. Below, we will explain the structure of the submerged entry nozzle for continuous casting according to this embodiment, which makes the flow of molten steel near the submerged entry nozzle smooth and suppresses the generation of vortices.

[0022] <Schematic diagram of continuous casting submerged entry nozzle> Figures 1A, 1B, and 1C are schematic diagrams showing an example of a continuous casting submerged entry nozzle according to the present invention. Figure 1A is a perspective view of the continuous casting submerged entry nozzle, Figure 1B is a plan view of the continuous casting submerged entry nozzle, and Figure 1C is a longitudinal sectional view of the continuous casting submerged entry nozzle taken along a plane passing through the center line of the discharge hole. 1A to 1C , reference numeral 100 denotes a continuous casting immersion nozzle, 123 denotes a discharge hole (on the left short side of the mold), 124 denotes a discharge hole (on the right short side of the mold), 101 denotes a molten steel rectifying portion, 103 denotes a molten steel flow path, 122 denotes the bottom of the tubular portion 102 (the bottom surface of the portion below the molten steel rectifying portion), a denotes the major axis of the molten steel rectifying portion 101, b denotes the minor axis of the molten steel rectifying portion 101, α denotes the major axis of the tubular portion 102 below the molten steel rectifying portion, β denotes the minor axis of the tubular portion 102 below the molten steel rectifying portion, and h denotes the length of the molten steel rectifying portion 101 continuing in the casting direction from the position of a molten steel surface (meniscus) 201, i.e., the immersion depth of the molten steel rectifying portion 101 immersed in molten steel 200.

[0023] 1A to 1C, the continuous casting submerged entry nozzle 100 according to this embodiment is a submerged entry nozzle for pouring molten steel 200 from a tundish (not shown) into a mold 300 of a continuous casting machine. The continuous casting submerged entry nozzle 100 according to this embodiment has a basic structure of a tubular portion 102 having a substantially straight tubular shape, an inner bore 121, and a plurality of discharge holes near its bottom. The continuous casting submerged entry nozzle 100 has a technical feature in that a molten steel flow straightening portion 101 is added to the tubular portion 102 to reduce the generation of vortices formed from the molten steel flow on the molten steel surface (meniscus) 201 formed inside the mold 300.

[0024] The continuous casting submerged entry nozzle 100 shown in Figures 1A to 1C is an example of a submerged entry nozzle in which the outer periphery of the molten steel rectifying section 101 at the position of the molten steel surface (meniscus) 201 has an elliptical cross-sectional shape. That is, the continuous casting submerged entry nozzle 100 is a submerged entry nozzle that includes the molten steel rectifying section 101 and a tubular section 102, and has a pair of discharge holes that are symmetrical about the axis of the submerged entry nozzle above and below the vertical direction of the tubular section 102. The mold 300 is composed of a mold short side 301, a mold long side 302, and a mold bottom side 303. The molten steel rectifying section 101 and the tubular section 102 that the continuous casting submerged entry nozzle 100 is equipped with will be described below.

[0025] <Molten Steel Straightening Section of Continuous Casting Submerged Entry Nozzle> The continuous casting submerged entry nozzle 100 is equipped with a molten steel rectifying section 101. This molten steel rectifying section 101 is immersed in the molten steel 200 and is provided on the molten steel surface 201 so as to be exposed above the molten steel surface 201 formed inside the mold 300. That is, a portion of the molten steel rectifying section 101 is immersed in the molten steel 200 inside the mold 300, and another portion of the molten steel rectifying section 101 is exposed toward the tundish from the molten steel surface (meniscus) 201 formed inside the mold 300.

[0026] It is sufficient that a portion of the molten steel rectifying portion 101 is immersed in the molten steel 200, and the other portion is exposed toward the tundish from the molten steel surface (meniscus) 201 formed inside the mold 300. Taking into consideration the flow rate of the molten steel flow of the molten steel 200, fluctuations in the molten steel surface (meniscus) 201, and the like, the molten steel rectifying portion 101 may be provided so that the other portion is exposed, for example, in a range of 0.1 to 50 mm above the molten steel surface (meniscus) 201. In particular, taking into consideration expected fluctuations in the molten steel surface (meniscus) 201, the molten steel rectifying portion 101 is preferably provided 1.0 mm or more above the molten steel surface (meniscus) 201 toward the tundish.

[0027] The molten steel rectifying portion 101 provided in the continuous casting submerged entry nozzle 100 has a cross-sectional shape of an outer periphery of the molten steel rectifying portion 101 that is elliptical or streamlined, with the major axis in the width direction of the mold 300, at least within a predetermined range from the molten steel surface 201 in the casting direction of the molten steel 200. Here, the elliptical shape includes an oblong shape. Alternatively, an elliptical shape may be used, in which the shorter sides of a rectangle are replaced with arcs to form an oval shape with parallel portions. The streamlined shape refers to a shape formed by curves that do not generate vortices around the molten steel rectifying portion 101 when placed in the flow of molten steel 200 and that minimizes the resistance it receives from the flow of molten steel 200.

[0028] For example, in the case of a uniform flow of molten steel 200, the tip of the streamlined shape of molten steel rectifying portion 101 may be formed into a pointed shape. The cross-sectional shape of the outer periphery of molten steel rectifying portion 101 may be formed into a point-symmetric or line-symmetric streamlined shape. By forming the cross-sectional shape of the outer periphery of molten steel rectifying portion 101 into a point-symmetric or line-symmetric streamlined shape, it is possible to reduce the generation of vortices formed from the molten steel flow generated on the molten steel surface (meniscus) 201 inside the mold 300, which is preferable.

[0029] The molten steel rectifying portion 101 has a continuous shape in which the value (b / a), which is the ratio of the minor axis b to the major axis a in the cross-sectional shape of the outer periphery of the molten steel rectifying portion 101, is 0.95 or less. By having the value (b / a), which is the ratio of the minor axis b to the major axis a in the cross-sectional shape of the outer periphery of the molten steel rectifying portion 101, of 0.95 or less, the molten steel rectifying portion 101 has a shape that is less likely to cause fluid separation even if the flow rate of the molten steel 200 becomes excessively high, and the flow of the molten steel 200 does not separate from the molten steel rectifying portion 101, and vortices do not occur next to the nozzle downstream in the casting direction, which is preferable. From this technical viewpoint, the value (b / a), which is the ratio of the minor axis b to the major axis a in the cross-sectional shape of the outer periphery of the molten steel rectifying portion 101, may further be 0.85 or less, preferably 0.65 or less, and more preferably 0.50 or less.

[0030] A continuous shape in which the ratio (b / a), which is the ratio of the minor axis b to the major axis a in the cross-sectional shape of the outer periphery of the molten steel rectifying portion 101, is 0.95 or less is formed at least in a predetermined range below the molten steel surface (meniscus) 201 in the casting direction of the molten steel 200. The predetermined range in which this continuous shape is formed can be appropriately set in consideration of the height of the molten steel surface (meniscus) 201 formed inside the mold 300, the flow state of the molten steel 200, etc., and is set so that the flow of the molten steel 200 does not separate from the molten steel rectifying portion 101 and vortexes do not occur on the molten steel surface beside the nozzle.

[0031] Note that, since the outer shape of the cross-sectional shape of the outer periphery of the molten steel rectifying portion 101 may be worn away by mold powder, it is desirable to set the outer diameter shape under the above conditions after taking into consideration in advance the amount of wear of the molten steel rectifying portion 101. Furthermore, a method is often adopted in which the depth to which the molten steel rectifying portion 101 is immersed in the molten steel 200 is changed over the casting time, so that the same position of the molten steel rectifying portion 101 does not come into contact with the mold powder. Even when such a method is adopted, it is desirable to set the above conditions in accordance with the depth to which the molten steel rectifying portion 101 is immersed in the molten steel at the expected start and end of casting.

[0032] <Tubular portion of continuous casting submerged entry nozzle> The continuous casting submerged entry nozzle 100 has a tubular portion 102 that penetrates the molten steel rectifying portion 101 in the vertical direction. The tubular portion 102 corresponds to a submerged entry nozzle having a substantially straight tubular shape. The molten steel rectifying portion 101 and the tubular portion 102 may be molded integrally, or the molten steel rectifying portion 101 and the tubular portion 102 may be molded separately and then molded by combining these components. The tubular portion 102 discharges the molten steel 200 transported from the tundish into the mold 300. The tubular portion 102 has an upper end that extends toward the tundish and a lower end that extends toward the mold bottom 303.

[0033] Here, the tubular portion 102 has a substantially straight shape, and the shape of its upper end and the shape of its lower end may be substantially the same. Furthermore, the tubular portion 102 may have a slightly different shape, as long as the molten steel 200 transferred from the tundish flows from the upper end of the tubular portion 102 to the plurality of discharge holes formed near the bottom surface. An inner hole 121 is formed inside the tubular portion 102. A tubular portion bottom surface 122 is formed at the bottom of the tubular portion 102. The inner hole 121 formed inside the tubular portion 102 serves as a molten steel flow path 103 for the molten steel 200, through which the molten steel 200 poured from the upper end of the tubular portion 102 flows to the vicinity of the tubular portion bottom surface 122 of the tubular portion 102.

[0034] Furthermore, the tubular portion 102 has, at its lower end, a pair of discharge holes, namely, discharge hole 123 and discharge hole 124, which are used to discharge the molten steel 200 into the mold 300. Discharge hole 123 faces the mold short side 301 located on the left side in the width direction of the mold 300. Discharge hole 124 faces the mold short side 301 located on the right side in the width direction of the mold 300.

[0035] Molten steel 200 is poured from an inner hole 121 formed at the upper end of a tubular portion 102 that protrudes from the center of an upper surface 112 of the molten steel rectifying portion 101 toward the tundish. The inner hole 121 of the tubular portion 102 penetrates the interior 111 of the molten steel rectifying portion 101 and communicates from the upper end of the tubular portion 102 to the lower end of the tubular portion 102. Therefore, the molten steel 200 poured from the inner hole 121 formed at the upper end of the tubular portion 102 is transported in the casting direction toward a tubular portion bottom surface 122 of the tubular portion 102 via the inner hole 121 formed at the lower end of the tubular portion 102 that is formed inside the molten steel rectifying portion 101.

[0036] Molten steel 200 transported in the casting direction through inner bore 121 formed in tubular portion 102 reaches tubular portion bottom surface 122. The molten steel 200 that has reached tubular portion bottom surface 122 is discharged into the mold 300 as a discharge flow 202 from discharge holes 123 and 124 that tubular portion 102 has. Thereafter, the molten steel 200 that has been discharged into the mold 300 accumulates, forming a molten steel surface (meniscus) 201 inside the mold 300.

[0037] Next, the shape of the tubular portion 102 and the discharge holes 123 and 124 formed below the tubular portion 102 will be described. If the cross-sectional shape of the outer periphery of the tubular portion 102 is made elliptical or streamlined, which is substantially the same as the cross-sectional shape of the outer periphery of the molten steel rectifying portion 101 up to the vicinity of the discharge holes 123 and 124, the lengths of the discharge holes 123 and 124 in the discharge direction will inevitably be longer than those of a normal submerged entry nozzle. This significantly increases the risk of clogging of the submerged entry nozzle, which promotes not only defects caused by mold powder but also other steelmaking-related defects, thereby impeding productivity.

[0038] In addition, when continuous casting of molten steel 200 is performed with the same casting width, the discharge position of molten steel 200 becomes closer to the mold short side 301 of the mold 300 than when continuous casting of molten steel 200 is performed using a normal submerged entry nozzle. As a result, the speed at which the discharge flow of molten steel 200 discharged from discharge holes 123 and 124 formed in tubular portion 102 collides with mold short side 301 becomes excessively high, causing the initial solidified shell formed on the surface of molten steel 200 to re-melt, and causing the molten steel 200 to leak out when it leaves mold 300 due to the insufficient thickness of the initial solidified shell, resulting in a so-called "breakout."

[0039] From the above, in the continuous casting submerged entry nozzle 100 according to this embodiment, the cross-sectional shape of the outer periphery of the tubular portion 102 is circular or elliptical in the width direction of the mold 300, and the value of (β / α), which is the ratio of the length α of one axis substantially parallel to the long sides of the mold 300 to the length β of another axis perpendicular to the one axis, is 0.90 or more and 1.1 or less. In other words, near the discharge holes 123 and 124 of the tubular portion 102, the cross-sectional shape of the outer periphery of the tubular portion 102 is circular or elliptical in the width direction of the mold 300, and the value of (β / α), which is the ratio of the length α of one axis substantially parallel to the long sides of the mold 300 to the length β of the other axis perpendicular to the one axis, needs to be set to 0.90 or more.

[0040] A ratio (β / α) of the length α of one axis approximately parallel to the long side of the mold 300 to the length β of the other axis perpendicular to the axis α is preferably 0.90 or greater, since this does not increase the risk of clogging the submerged entry nozzle and does not promote defects caused by the mold powder or other steelmaking defects. On the other hand, if the value of (β / α) is greater than 1.1, the thickness of the discharge hole flow path becomes too small, making it impossible to adjust the discharge flow in the vertical direction. Therefore, the upper limit of the value of (β / α) is set to 1.1 or less.

[0041] In the continuous casting submerged entry nozzle 100 shown in Figures 1A to 1C, the angles of the discharge holes 123 and 124 are approximately 90 degrees with respect to the axial direction of the submerged entry nozzle, but these discharge hole angles, discharge hole shapes, and pool depth and shape can be changed to appropriate values ​​depending on the casting conditions and the quality required of the slab.

[0042] As described above, the continuous casting submerged entry nozzle 100 according to this embodiment is provided with the molten steel rectifying section 101 and the tubular section 102 each having a predetermined shape, and thereby the flow of the molten steel 200 separates from the molten steel rectifying section 101, preventing the generation of vortices of the molten steel 200 next to the nozzle on the downstream side in the casting direction, thereby reducing the risk of clogging of the submerged entry nozzle and adhesion of inclusions.

[0043] As described above, according to the present embodiment of the invention, in an immersion nozzle used for pouring molten steel into a continuous casting mold in continuous casting of steel, it is possible to suppress entrainment of mold powder into molten steel.

[0044] Second Embodiment An explanation will be given of a continuous casting submerged entry nozzle according to a second embodiment. The continuous casting submerged entry nozzle according to this embodiment is characterized in that, in the continuous casting submerged entry nozzle according to the first embodiment, the molten steel flow straightening portion has the continuous shape in a range of 50 mm or more from the molten steel surface in the casting direction of the molten steel. In other words, the continuous shape of the continuous casting submerged entry nozzle according to this embodiment is formed so as to continue for at least 50 mm or more from the molten steel surface (meniscus) 201 in the casting direction of the molten steel 200.

[0045] If this continuous shape is 50 mm or more, a vortex generated by separation of the flow of molten steel 200 occurring in a portion below the continuous shape in the casting direction, which does not have this continuous shape, can reach the molten steel surface (meniscus) 201, and there is no possibility that mold powder will be entrained in the molten steel 200. Note that the continuous shape of the molten steel flow rectifying portion 101 is formed continuously so as to satisfy the condition that it does not overlap with the discharge holes 123 and 124.

[0046] In this way, with the continuous casting submerged entry nozzle 100 according to this embodiment, vortices generated by separation of the flow of the molten steel 200 do not reach the molten steel surface (meniscus) 201. As a result, the continuous casting submerged entry nozzle 100 according to this embodiment can further suppress the entrainment of mold powder into the molten steel 200.

[0047] As described above, according to the invention related to this embodiment, it is possible to suppress vortices generated by separation of the flow of molten steel occurring in the lower part in the casting direction that does not have a continuous shape, from reaching the surface of the molten steel (meniscus), and to prevent the mold powder from being entrained in the molten steel.

[0048] Third Embodiment A method for producing steel according to a third embodiment will now be described. The method for producing steel according to this embodiment is characterized in that steel is produced using the continuous casting submerged entry nozzle according to the above embodiment. First, in the method for producing steel according to this embodiment, the continuous casting submerged entry nozzle 100 according to this embodiment is installed at the bottom of a tundish. Next, the tundish is installed above the mold 300 so that the continuous casting submerged entry nozzle 100 is positioned approximately in the center of the space formed by the mold 300. In this way, in the method for producing steel according to this embodiment, the tundish, continuous casting submerged entry nozzle 100, and continuous casting mold 300, which are components necessary for continuously casting molten steel, are prepared and installed.

[0049] Furthermore, in the steel manufacturing method according to this embodiment, the molten steel 200 is poured into a tundish from a ladle that contains the molten steel 200 produced in a refining furnace such as a converter, and the molten steel 200 is poured from the tundish into a mold 300 through a continuous casting submerged entry nozzle 100. As the molten steel 200 is poured into the mold 300 through the continuous casting submerged entry nozzle 100, the poured molten steel 200 accumulates inside the mold 300. As the poured molten steel 200 accumulates inside the mold 300, a molten steel surface (meniscus) 201 is formed inside the mold 300.

[0050] In the continuous casting of molten steel 200, mold powder is supplied to the molten steel 200 poured into the mold 300. The mold powder supplied to the mold 300 and turned to a molten state penetrates between the initial solidification shells formed by the cooling of the molten steel 200 inside the mold 300. Meanwhile, a portion of the mold powder supplied to the mold 300 and turned to a molten state floats on the molten steel surface (meniscus) 201 of the mold 300. Furthermore, when the mold powder is poured into the mold 300, an inert gas such as argon gas or nitrogen gas may be blown into the molten steel 200 that has been supplied to the molten steel 200 and is flowing down the molten steel flow path 103 of the continuous casting submerged entry nozzle 100 via a sliding nozzle, an upper nozzle, or the like.

[0051] Figure 2 is a schematic diagram showing the flow state of molten steel in a mold when molten steel is cast using a conventional submerged entry nozzle. In Figure 2, reference numeral 301 denotes the short side of the continuous casting mold, reference numeral 201 denotes the molten steel surface (corresponding to a meniscus), reference numeral 202 denotes the discharge flow from the discharge hole 123, reference numeral 203 denotes a branched upward flow formed when the discharge flow 202 from the discharge hole branches, reference numeral 204 denotes the meniscus flow, reference numeral 205 denotes a separated flow generated from a separation point along the submerged entry nozzle, and reference numeral 206 denotes a vortex of molten steel 200 formed in the separated flow.

[0052] 2 , the discharge flow of molten steel 200 discharged from the submerged entry nozzle 400 fluctuates over time, with the discharge flow 202 of molten steel 200 from the right-side discharge hole 124 becoming stronger and penetrating deeper diagonally to the casting direction. On the other hand, compared to the discharge flow 202 from the right-side discharge hole 124, the discharge flow 202 from the left-side discharge hole 123 is directed upward (toward the tundish) and, after colliding with the narrow side 301 of the continuous casting mold, becomes a flow directed toward the molten steel surface (meniscus) 201. After reaching the molten steel surface (meniscus) 201, the discharge flow 202 from the left-side discharge hole 123 becomes a meniscus flow 204 directed toward the submerged entry nozzle 400. At this time, no meniscus flow 204 is generated on the right side of the discharge hole 124, but a meniscus flow 204 is generated from the left side to the right side of the submerged nozzle 400.

[0053] 2, when molten steel 200 is cast using the conventional submerged entry nozzle 400 shown in Figure 2, the discharge flow 202 from the discharge hole 123 of the submerged entry nozzle 400 separates at the separation point of the meniscus flow 204, generating a separated flow 205. As a result, the separated flow 205 causes turbulence in the discharge flow 202 on the right side of the submerged entry nozzle 400, generating a vortex 206 formed from the molten steel 200. Mold powder floating on the molten steel surface (meniscus) 201 is caught up in this vortex 206, deteriorating the quality of the steel product.

[0054] Figure 3 is a diagram schematically illustrating the flow state of molten steel in a mold when molten steel is cast using a continuous casting submerged entry nozzle. The reference symbols in Figure 3 are the same as those in Figure 2, and reference symbol 207 denotes a rearward flow of molten steel 200 that forms behind the nozzle without separating from the flow along the nozzle. As shown in Figure 3, when molten steel 200 is cast using the continuous casting submerged entry nozzle 100 according to this embodiment, no separation from the submerged entry nozzle occurs in the discharge flows 202 that flow on the left and right sides of the submerged entry nozzle 100.

[0055] Furthermore, even if separation from the molten steel 200 occurs in the flow of molten steel 200 flowing on the left and right sides of the continuous casting submerged entry nozzle 100 due to various conditions during continuous casting of the molten steel 200, it is minimal, and the frequency of occurrence of vortices 206 is drastically reduced. Furthermore, when the molten steel 200 is cast using the continuous casting submerged entry nozzle 100, the elliptical portion in the casting direction is also formed into a continuous shape with a predetermined length, so it is possible to suppress the occurrence of separated flows 205 and vortices 206 that are caused by the flow of molten steel 200 that occurs below the molten steel surface (meniscus) 201 and that flows in the same direction as the molten steel surface (meniscus) 201.

[0056] Furthermore, in the continuous casting of molten steel 200, a phenomenon is known in which the discharge holes 123 and 124 of the continuous casting submerged entry nozzle 100 become clogged with inclusions and the like as the casting time passes. If clogging occurs near one of the left and right discharge holes 123 and 124 of the continuous casting submerged entry nozzle 100, a large amount of molten steel 200 will be discharged from only the discharge holes on that side. Discharge of a large amount of molten steel 200 from only the discharge holes on one side increases the flow rate of the molten steel toward the molten steel surface (meniscus) 201, which can cause molder powder entrainment.

[0057] However, when the molten steel 200 is cast using the continuous casting submerged entry nozzle 100, even if the discharge holes 123 and 124 of the continuous casting submerged entry nozzle 100 are clogged with inclusions or the like, it is possible to suppress the entrainment of mold powder into the molten steel 200. In other words, by continuously casting the molten steel 200 using the continuous casting submerged entry nozzle 100 according to the above embodiment, it is possible to obtain steel products such as cast pieces with extremely good surface quality in which the entrainment of mold powder is suppressed.

[0058] In recent continuous casting operations of molten steel, a magnetic field has been applied to the molten steel in a continuous casting mold to control the flow of the molten steel in the mold in order to prevent inclusions from being captured in the cast slab. Even when the molten steel 200 is cast using the continuous casting submerged entry nozzle 100 according to this embodiment, a static magnetic field may be applied to the discharge flow to prevent inclusions from sinking in, or a traveling magnetic field may be applied to impart a swirling flow aimed at a cleaning effect of washing away inclusions, even if this does not affect the effects of the present invention, and therefore these methods may be used in combination.

[0059] As described above, according to the method for producing steel according to this embodiment, by continuously casting the molten steel 200 using the continuous casting submerged entry nozzle 100 according to the above embodiment, mold powder entrained by the molten steel flow is not captured in the initial solidification shell, and when the cast molten steel 200 is made into a steel product, no surface defects occur, and the quality of the steel product is not impaired.

[0060] As described above, according to the present embodiment of the invention, it is possible to suppress the vortexes that occur on the sides of the continuous casting submerged entry nozzle, which determine the majority of mold powder entrainment. Therefore, according to the steel manufacturing method of this embodiment, it is possible to stably prevent mold powder entrainment and to obtain a cast slab with extremely good surface quality.

[0061] [Other Embodiments] The present invention has been described above with reference to the embodiments, but the present invention is not limited to the above embodiments. Various modifications that can be understood by those skilled in the art can be made to the configuration and details of the present invention within the technical scope of the present invention. Furthermore, systems or devices that combine separate features included in each embodiment in any way are also included in the technical scope of the present invention.

[0062] The effects of the present invention will be specifically explained below based on examples, but the present invention is not limited to these examples.

[0063] Example 1: Continuous casting was carried out in a commercial continuous slab casting machine using the continuous casting submerged entry nozzle according to the present invention. The cross-sectional dimensions of the continuously cast slab (slab) were 220-260 mm in thickness and 1000-2000 mm in width. Argon gas was used as the inert gas blown into the continuous casting submerged entry nozzle, and an optimum mold powder was selected and added to the meniscus depending on the slab withdrawal speed and steel type.

[0064] (Design Specifications of Continuous Casting Immersion Nozzle) A continuous casting Immersion Nozzle according to the present invention was manufactured, and steel was produced using the manufactured continuous casting Immersion Nozzle. In Example 1, the cross-sectional shape of the outer tube of the continuous casting Immersion Nozzle, which is the portion immersed in molten steel, was set to an elliptical shape. Furthermore, in Example 1, the continuous casting Immersion Nozzle was manufactured in such a manner that, when the major axis of the outer periphery of the elliptical outer tube is a and the minor axis thereof is b, the ratio (b / a) was 0.91, the value of h, which is the distance from the meniscus position to the elliptical shape continuing in the casting direction of the molten steel, was 50 mm, and the major axis of the outer tube below the molten steel straightening section was α and the minor axis thereof is β, the ratio (β / α) was 1.0.

[0065] (Evaluation of Steel Products and Manufacturing Process) Using the continuous casting submerged nozzle manufactured in Example 1, molten steel was cast at a molten steel flow rate of 3.4 tons / min to manufacture steel products. The continuous casting mold used in Example 1 was not equipped with a magnetic field generator. Slab cast pieces manufactured using the continuous slab casting machine were hot-rolled to produce hot-rolled steel sheets. Surface defects caused by mold powder on these hot-rolled steel sheets were investigated, and the mold powder remaining in the cast pieces was evaluated based on these surface defects. In other words, the lower the defect index of the steel product, the less mold powder remaining in the cast pieces was evaluated.

[0066] In addition, a mechanism is adopted in which the solidified shell thickness in the mold is estimated from the temperature behavior of the thermocouples on the narrow sides of the mold during casting of molten steel, and if the thickness falls below a threshold, an alarm sounds indicating an increased risk of breakout. Therefore, in Example 1 of the present invention, the presence or absence of a breakout alarm was also evaluated as an evaluation of the operability of continuous casting.

[0067] In this way, the steel product was evaluated based on the defect index of the steel product. Furthermore, the steel manufacturing method using the continuous casting submerged entry nozzle manufactured in Example 1 was evaluated based on the presence or absence of a breakout alarm. Table 1 shows the evaluation results of the design of the continuous casting submerged entry nozzle of Example 1, the steel product, and the manufacturing process.

[0068] Examples 2 to 4 of the Invention Continuous casting submerged entry nozzles were produced in the same manner as in Example 1 of the invention, except that the cross-sectional shape of the outer tube of the continuous casting submerged part into molten steel, which is the outer tube equipped with the submerged entry nozzle; the value of (b / a), which is the ratio between the major axis of the outer tube of the cross-sectional shape and its minor axis, where a is the major axis and b is the minor axis; the value of h, which is the distance from the meniscus position to the elliptical shape that continues in the casting direction of the molten steel; and the value of (β / α), which is the ratio between the major axis of the outer periphery of the tubular part and its minor axis, where α is the major axis and β is the minor axis, were changed.

[0069] Furthermore, using each of the continuous casting submerged entry nozzles manufactured in Invention Examples 2 to 4, molten steel was cast to produce steel products in the same manner as Invention Example 1, except that the molten steel flow rate was changed. The steel products manufactured in Invention Examples 2 to 4 and the steel manufacturing methods using the manufactured continuous casting submerged entry nozzles were evaluated in the same manner as Invention Example 1. Table 1 shows the evaluation results of the continuous casting submerged entry nozzle designs, steel products, and manufacturing processes of Invention Examples 2 to 4.

[0070] <Comparative Examples 1 to 4> For comparison with Invention Examples 1 to 4, the design of the continuous casting submerged entry nozzle was changed, and continuous casting submerged entry nozzles of Comparative Examples 1 to 4 were manufactured. Casting was carried out using the continuous casting submerged entry nozzles manufactured in Comparative Examples 1 to 4. That is, molten steel was cast under the same conditions as the Invention Examples, except that casting was carried out using the continuous casting submerged entry nozzles manufactured in Comparative Examples 1 to 4. In addition, evaluation of the steel products manufactured in Comparative Examples 1 to 4 and evaluation of the steel manufacturing methods using the manufactured continuous casting submerged entry nozzles were carried out in the same manner as Invention Example 1. Table 1 shows the evaluation results of the continuous casting submerged entry nozzle designs, steel products, and manufacturing processes of Comparative Examples 1 to 4.

[0071] As shown in Table 1, the specific design specifications of the continuous casting submerged entry nozzles manufactured in Comparative Examples 1 to 4 are as follows: Comparative Example 1: Continuous casting submerged entry nozzle satisfying the relationship (b / a) > 0.95 Comparative Example 2: Continuous casting submerged entry nozzle satisfying the relationships (b / a) ≦ 0.95, h < 50 mm Comparative Example 3: Continuous casting submerged entry nozzle satisfying the relationships (b / a) ≦ 0.95, h ≧ 50 mm, (β / α) < 0.9 Comparative Example 4: Continuous casting submerged entry nozzle satisfying the relationships (b / a) ≦ 0.95, h ≧ 50 mm, (β / α) > 1.1

[0072]

[0073] Table 1 shows the operating conditions and results. Comparing Inventive Examples 1 to 4 with Comparative Examples 1-1 to 1-3, the defect index of the steel products of Inventive Examples 1 to 4 was significantly improved. In Comparative Examples 2-1 and 2-2, improvements were observed compared to Comparative Examples 1-1 to 1-3, but the improvement was not sufficient. In Comparative Examples 3-1 and 3-2, the quality of the steel products was good, but a breakout alarm was sounded in the latter half of the casting, so casting was stopped.

[0074] Furthermore, in Comparative Example 3-2, a breakout alarm was sounded early in the casting process, which forced the casting to be halted. This meant that a sufficient number of cast pieces were not obtained to produce a steel product, and the quality of the steel product could not be evaluated. In Comparative Examples 4-1 and 4-2, good steel product quality comparable to that of Invention Examples 1 to 4 was obtained, but the inner tube of the submerged entry nozzle became clogged with inclusions midway through casting, causing the casting to be halted. In other words, it was found that the application of the submerged entry nozzle for continuous casting according to the present invention significantly reduced the defect index of steel products and also enabled stable production operations.

[0075] The continuous casting submerged entry nozzle according to the present invention can suppress the generation of vortices in the vicinity of the submerged entry nozzle and inhibit the entrainment of mold powder, thereby making it possible to obtain steel products such as cast pieces with extremely high surface quality and to achieve stable operation of continuous casting of molten steel, thereby contributing to the development of related industries such as the steel manufacturing industry and being industrially useful.

[0076] 100 Continuous casting submerged entry nozzle 101 Molten steel rectifying section 102 Tubular section (section below the molten steel rectifying section) 103 Molten steel flow path 111 Lower surface of rectifying section 112 Upper surface of rectifying section 121 Inner hole 122 Bottom surface of tubular section (bottom surface of section below the molten steel rectifying section) 123 Discharge hole (left short side of mold) 124 Discharge hole (right short side of mold) 200 Molten steel 201 Molten steel surface (meniscus) 202 Discharge flow 203 Branched upward flow 204 Meniscus flow 205 Separation flow 206 Vortex (molten steel) 207 Backward flow 300 Mold 301 Short side of mold 302 Long side of mold 303 Bottom side of mold 400 Submerged entry nozzle (conventional product) a Long diameter of molten steel straightening part b Short diameter of molten steel straightening part α Diameter of tubular part (one axis) β Diameter of tubular part (other axis) h Length of straightening part continuing from the molten steel surface (meniscus) position in the casting direction

Claims

1. An immersion nozzle for injecting molten steel from a tundish into a mold of a continuous casting machine, the immersion nozzle comprising: a molten steel rectifying section that is immersed in the molten steel and is exposed above the molten steel surface formed inside the mold and is provided on the molten steel surface; and a tubular section that penetrates the molten steel rectifying section in the vertical direction and discharges the molten steel into the mold; the molten steel rectifying section has an outer circumferential cross-sectional shape that is an ellipse or streamlined shape with a major axis in the width direction of the mold at least within a predetermined range from the molten steel surface in the casting direction of the molten steel, and has a continuous shape in which the ratio (b / a) of the minor axis b to the major axis a in the outer circumferential cross-sectional shape of the molten steel rectifying section is 0.95 or less; a cross-sectional shape of an outer periphery of the tubular portion is circular or elliptical in the width direction of the mold, and a value of (β / α), which is a ratio of a length α of one axis approximately parallel to a long side of the mold to a length β of another axis perpendicular to the one axis, is 0.90 or more and 1.1 or less.

2. The continuous casting submerged nozzle according to claim 1, characterized in that the molten steel straightening portion has the continuous shape within a range of 50 mm or more from the surface of the molten steel in the casting direction of the molten steel.

3. A method for continuous casting of steel, comprising the step of pouring molten steel in a tundish into a mold for continuous casting using the submerged nozzle for continuous casting as claimed in claim 1 or 2.

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