Immersion nozzle for continuous casting and method for continuously casting steel

Abstract

To provide an immersion nozzle for continuous casting capable of preventing a mold powder from being drawn into molten steel.SOLUTION: An immersion nozzle for injecting a molten steel from a tundish into a mold of a continuous casting machine includes: a molten steel flow-regulating part, which is immersed in the molten steel and provided on a surface of the molten steel so as to be exposed upwardly in a direction above the molten steel surface formed inside the mold; and a tubular part, which penetrates the molten steel flow-regulating part in a vertical direction and discharges the molten steel into the interior of the mold. The molten steel flow-regulating part has a continuous shape in which, in a predetermined range in a casting direction of the molten steel from the surface of the molten steel: a cross-sectional shape of an outer periphery is elliptical or streamlined having a major axis in a width direction of the mold, and a ratio of a short axis b to a major axis a of the cross-sectional shape of an outer periphery of the molten steel flow-regulating part is 0.95 or less; and a cross-sectional shape of an outer periphery of the tubular part is circular or elliptical in the width direction of the mold, and a ratio of the length α of one axis approximately parallel to a 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.SELECTED DRAWING: Figure 1a

Description

Technical Field

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

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

[0003] During the casting of molten steel, a composite oxide called mold powder is constantly supplied into the mold for the purpose of improving the lubricity between the initial solidification shell and the mold. The mold powder that has become molten after being supplied into the mold penetrates between the mold and the initial solidification shell. On the other hand, molten mold powder also floats on the molten steel surface (meniscus) of the mold. The mold powder floating on the molten steel surface (meniscus) of this mold is entrained by the molten steel flow generated in the mold. When the mold powder entrained by this molten steel flow is captured by the initial solidification shell, it becomes a surface defect when the cast molten steel becomes a steel product, significantly deteriorating the quality of the steel product.

[0004] Meanwhile, various studies have been conducted on the phenomenon in which mold powder is entrained in the molten steel flow generated in the mold. It is known that the main causes of this phenomenon are 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 point of view, methods using electromagnetic brakes and methods using optimization of the discharge hole and its inner shape of the submerged entry nozzle have been proposed to prevent the mold powder from being entrained in the molten steel flow.

[0005] For example, Patent Document 1 (Patent No. 3491099) proposes a method of applying a static magnetic field to the meniscus to impart a braking force to the molten steel, thereby reducing the meniscus flow speed 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 adhesion of inclusions to the discharge hole and inhibit drift caused by clogging of the discharge hole. Furthermore, Patent Document 3 (Patent Publication No. 6,963,192) proposes an immersion nozzle in which the shape of the inner tube of the immersion nozzle is optimized to suppress the occurrence of the suction phenomenon in the discharge hole and prevent the entrainment of mold powder into the molten steel. Patent Document 4 (Patent No. 7175513) proposes an immersion nozzle in which the inner circumferential surface of the discharge hole is made spherical, thereby making the distribution of the molten steel flow rate from the discharge hole uniform and preventing the mold powder from being entrained in the molten steel. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 07-314100 [Patent Document 2] JP 2001-129645 A [Patent Document 3] Patent Publication No. 2021-094585 [Patent Document 4] Japanese Patent Application Laid-Open No. 2021-126663

SUMMARY OF THE INVENTION

PROBLEMS TO BE SOLVED BY THE INVENTION

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

[0008] Further, the method by optimizing the discharge holes and the inner surface shape of the immersion nozzle described in Patent Documents 2 to 4 is expected to have an effect of suppressing mold powder entrainment at the initial stage of casting. However, the immersion nozzle or the like for preventing mold powder entrainment described in Patent Documents 2 to 4 causes at least some clogging and erosion of the immersion nozzle due to the discharge flow, so that in the latter stage of the casting process, it deviates from the proper immersion nozzle shape, and a sudden high-speed flow of molten steel occurs in the meniscus. Thus, the immersion nozzles described in Patent Documents 1 to 4 have a problem that a sufficient effect of suppressing mold powder entrainment into the molten steel cannot be obtained.

[0009] The present invention has been made in view of the above circumstances, and an object thereof is to provide a continuous casting immersion nozzle and a continuous casting method of steel that can suppress the entrainment of mold powder into molten steel in an immersion nozzle used for injecting molten steel into a continuous casting mold in the continuous casting of steel.

MEANS FOR SOLVING THE PROBLEMS

[0010] As a result of conducting various experiments to solve the above problems, the inventors have found that by adopting a submerged nozzle for continuous casting having a specific form, it is possible to suppress the entrainment of mold powder into the molten steel, and to obtain a steel product with less mold powder remaining in the slab. The present invention has been made based on the above findings, and the gist thereof is as follows.

[0011] The submerged nozzle for continuous casting according to the present invention that advantageously solves the above problems is a submerged nozzle for injecting molten steel from a tundish into a mold of a continuous casting machine, The submerged nozzle includes a molten steel rectifying portion that is immersed in the molten steel and exposed upward from the molten steel surface formed inside the mold and provided on the molten steel surface, and a tubular portion that penetrates the molten steel rectifying portion in the vertical direction and discharges the molten steel into the mold. At least in a predetermined range in the casting direction of the molten steel from the molten steel surface, the cross-sectional shape of the outer periphery of the molten steel rectifying portion is an elliptical shape or a streamline shape having a major axis in the width direction of the mold, and has a continuous shape in which the value of (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, 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 β of the other axis perpendicular to one axis to the length α of the one axis substantially parallel to the long side of the mold, is 0.90 or more and 1.1 or less. It is considered that the submerged nozzle for continuous casting according to the present invention, (a) the molten steel rectifying portion having the continuous shape in a range of 50 mm or more in the casting direction of the molten steel from the molten steel surface, etc., can be a more preferable solution means.

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

Effects of the Invention

[0013] According to the submerged nozzle for continuous casting according to the present invention, even when a molten steel flow with a high flow velocity is generated in the meniscus due to uneven flow or the like, the generation of vortices in the vicinity of the submerged nozzle can be suppressed. Therefore, not only in the initial stage of the casting process, but also in the later stage of the casting process, the entrainment of mold powder into the molten steel can be suppressed without the need for large-scale equipment.

Brief Description of the Drawings

[0014]

Figure 1a

Figure 1b

Figure 1c

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0015] [First Embodiment] The submerged nozzle for continuous casting according to the first embodiment will be described. The submerged nozzle for continuous casting according to the present embodiment is a submerged nozzle for injecting molten steel from a tundish into a mold of a continuous casting machine, and the submerged nozzle is immersed in the molten steel and exposed upward from the molten steel surface formed inside the mold and provided on the molten steel surface. A molten steel rectifying portion, and a tubular portion that penetrates the molten steel rectifying portion in the vertical direction and discharges the molten steel into the mold. The molten steel rectifying section has a continuous shape such that at least in a predetermined range in the casting direction of the molten steel from the molten steel surface, the cross-sectional shape of the outer periphery is an elliptical shape or a streamline shape having a major axis in the width direction of the mold, and the value of (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 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 β of the other axis orthogonal to one axis to the length α of the one axis substantially parallel to the long side of the mold, is 0.90 or more and 1.1 or less.

[0016] Generally, a slab, which is a semi-finished product cast piece called a slab produced by continuously casting by pouring molten steel into a mold of a continuous casting machine, has a width that is about twice or more the thickness, so the width of the cast piece is significantly larger than the thickness of the cast piece. The mold has a pair of mold long sides facing in the front-rear direction and a pair of mold short sides facing in the left-right direction. The mold is configured such that the mold short sides can move inside the mold long sides.

[0017] The immersion nozzle for continuous casting for injecting molten steel into the mold has a plurality of discharge holes for injecting molten steel into the mold. The plurality of discharge holes may be formed from a pair of discharge holes. The pair of discharge holes may be formed toward the respective mold short sides facing in the left-right direction. The immersion nozzle for continuous casting injects molten steel into the mold from the respective discharge holes formed toward the mold short sides.

[0018] Then, the inventors analyzed the flow state of the molten steel inside the mold. Specifically, a water model experiment of the molten steel, numerical simulation, measurement by sensors installed inside the mold, and analysis of the cast piece (slab) produced from the molten steel were performed to analyze the flow state of the molten steel inside the mold. As a result, the following mechanism regarding the flow of the molten steel inside the mold became clear.

[0019] First, most of the mold powder entrainment phenomenon is caused by the vortex of molten steel generated near the immersion nozzle. Second, when the flow velocity of the molten steel at the molten steel surface (meniscus) near the immersion nozzle becomes too large, a vortex of molten steel is generated near the immersion nozzle, and the mold powder is entrained into the molten steel. Third, when nozzle clogging occurs in the immersion nozzle due to inclusions present in the molten steel and the molten steel flow from one of the discharge holes on one side of the pair of discharge holes formed in the immersion nozzle becomes excessive, the flow velocity of the molten steel at the molten steel surface (meniscus) becomes even more excessive. Fourth, even when nozzle clogging has not occurred in the pair of discharge holes formed in the immersion nozzle, the flow velocity and direction of the discharged molten steel from the immersion nozzle change with time. For this reason, when the flow velocity of the molten steel is excessive and its direction is towards the molten steel surface (meniscus), the flow velocity of the molten steel at the molten steel surface (meniscus) becomes even more excessive.

[0020] Based on such a mechanism, the inventors further advanced the research and development, and newly found that in order to suppress the generation of vortices near the immersion nozzle, the cross-sectional shape of the immersion nozzle and the shape of the immersion nozzle in the casting direction are important. That is, the reason why vortices are generated near the immersion nozzle is that the flow velocity of the molten steel around the immersion nozzle becomes excessive, the flow of the molten steel peels off from the immersion nozzle, and vortices are generated beside the immersion nozzle on the downstream side of the mold.

[0021] From such a technical perspective, the inventors found that by making the cross-sectional shape of the outer periphery of the immersion nozzle a wide streamline shape or an elliptical shape in the mold width direction, the molten steel flow near the immersion nozzle becomes smooth, and the generation of vortices can be suppressed. Hereinafter, the structure of the immersion nozzle for continuous casting according to the present embodiment in which the molten steel flow near the immersion nozzle becomes smooth and the generation of vortices can be suppressed will be described.

[0022] <Schematic diagram of the immersion nozzle for continuous casting> This is a schematic diagram showing an example of a submerged nozzle for continuous casting according to the present invention. Fig. 1(a) is a perspective view of the submerged nozzle for continuous casting, Fig. 1(b) is a plan view of the submerged nozzle for continuous casting, and Fig. 1(c) is a longitudinal sectional view when cut along a plane passing through the center line of the discharge holes of the submerged nozzle for continuous casting. In addition, in Figs. 1(a) to 1(c), reference numeral 100 denotes a submerged nozzle for continuous casting, 123 denotes a discharge hole (left mold short side), 124 denotes a discharge hole (right mold short side), 101 denotes a molten steel rectifying section, 103 denotes a molten steel flow path, 122 denotes the bottom of the tubular section 102 (the bottom surface of the part below the molten steel rectifying section), a denotes the major axis of the molten steel rectifying section 101, b denotes the minor axis of the molten steel rectifying section 101, α denotes the major axis of the tubular section 102 below the molten steel rectifying section, β denotes the minor axis of the tubular section 102 below the molten steel rectifying section, and h denotes the length of the molten steel rectifying section 101 continuous in the casting direction from the position of the molten steel surface (meniscus) 201, that is, the immersion depth at which the molten steel rectifying section 101 is immersed in the molten steel 200.

[0023] As shown in Figs. 1(a) to 1(c), the submerged nozzle 100 for continuous casting according to the present embodiment is a submerged nozzle for injecting molten steel 200 from a tundish (not shown) into the mold 300 of a continuous casting machine. The submerged nozzle 100 for continuous casting according to the present embodiment has a substantially straight tubular shape and has a basic structure of a tubular section 102 having an inner hole 121 and a plurality of discharge holes near its bottom surface. And the submerged nozzle 100 for continuous casting has a technical feature in that a molten steel rectifying section 101 for reducing the generation of vortices formed from the molten steel flow generated on the molten steel surface (meniscus) 201 formed inside the mold 300 is added to the tubular section 102.

[0024] The submerged nozzle 100 for continuous casting shown in Figs. 1(a) to 1(c) is an example of a submerged nozzle in which the cross-sectional shape of the outer periphery of the molten steel rectifying section 101 at the position of the molten steel surface (meniscus) 201 is elliptical. That is, the submerged nozzle 100 for continuous casting includes a molten steel rectifying section 101 and a tubular section 102, and is a submerged nozzle having a pair of discharge holes that are symmetric about the axis of the submerged nozzle in the vertical direction of the tubular section 102. Note that the mold 300 is composed of a mold short side 301, a mold long side 302, and a mold bottom side 303. Hereinafter, the molten steel rectifying part 101 and the tubular part 102 provided in the immersion nozzle 100 for continuous casting will be described.

[0025] <Molten Steel Rectifying Part of Immersion Nozzle for Continuous Casting> The immersion nozzle 100 for continuous casting is provided with a molten steel rectifying part 101. This molten steel rectifying part 101 is immersed in the molten steel 200 and is provided on the molten steel surface 201, exposed upward from the molten steel surface 201 formed inside the mold 300. That is, a part of the molten steel rectifying part 101 is immersed in the molten steel 200 inside the mold 300, and the other part of the molten steel rectifying part 101 is exposed in the tundish direction from the molten steel surface (meniscus) 201 formed inside the mold 300.

[0026] It is sufficient that a part of the molten steel rectifying part 101 is immersed in the molten steel 200 and the other part is exposed in the tundish direction from the molten steel surface (meniscus) 201 formed inside the mold 300. Considering the flow velocity of the molten steel flow of the molten steel 200, the fluctuation of the molten steel surface (meniscus) 201, etc., for example, the other part of the molten steel rectifying part 101 may be provided so as to be exposed in the range of 0.1 to 50 mm upward from the molten steel surface (meniscus) 201. In particular, the molten steel rectifying part 101 is preferably provided 1.0 mm or more in the tundish direction above the assumed molten steel surface (meniscus) 201 in consideration of the surface fluctuation of the molten steel surface (meniscus) 201.

[0027] The molten steel rectifying part 101 provided in the immersion nozzle 100 for continuous casting has an elliptical shape or a streamline shape with a major axis in the width direction of the mold 300 in the cross-sectional shape of the outer periphery of the molten steel rectifying part 101 at least in a predetermined range in the casting direction of the molten steel 200 from the molten steel surface 201. Here, the ellipse includes an oblong. Further, instead of the ellipse, it can be an oval having a parallel part in which the short side of the rectangle is replaced by an arc. The streamline shape means a shape composed of a curve that does not generate a vortex around it when the molten steel rectifying part 101 is placed in the flow of the molten steel 200 and has the smallest resistance received from the flow of the molten steel 200.

[0028] For example, in the case of a uniform flow of molten steel 200, the tip of the streamline shape of the molten steel rectifying section 101 may be sharpened. The cross-sectional shape of the outer periphery of the molten steel rectifying section 101 may also be a streamline shape that is point-symmetric and line-symmetric. By making the cross-sectional shape of the outer periphery of the molten steel rectifying section 101 a streamline shape that is point-symmetric and line-symmetric, 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 section 101 has a continuous shape in which the value of (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 section 101, is 0.95 or less. By the value of (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 section 101, being 0.95 or less, even if the flow velocity of the molten steel 200 in the molten steel rectifying section 101 becomes excessive, the shape is such that fluid separation is difficult, and the flow of the molten steel 200 from the molten steel rectifying section 101 does not separate, and vortices do not occur beside the nozzle on the downstream side in the casting direction, which is preferable. From such a technical perspective, further, the value of (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 section 101, may be 0.85 or less, preferably 0.65 or less, and more preferably 0.50 or less.

[0030] The continuous shape in which the value of (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 section 101, is 0.95 or less is formed in at least 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 from the molten steel rectifying section 101 does not separate and vortices do not occur on the molten steel surface beside the nozzle.

[0031] Note that since the outer shape of the cross-section of the molten steel rectifying section 101 may be worn by the mold powder, it is desirable to set the outer diameter shape under the above conditions after considering the wear amount of the molten steel rectifying section 101 in advance. Further, by changing the depth at which the molten steel rectifying section 101 is immersed in the molten steel 200 with the casting time, a method is often adopted in which the same position of the molten steel rectifying section 101 is not brought into contact with the mold powder. Even in the case of adopting such a method, it is preferable to set the above conditions in accordance with the depth at which the molten steel rectifying section 101 is immersed in the molten steel at the beginning and end of the assumed casting.

[0032] <Tubular part of the immersion nozzle for continuous casting> The continuous casting immersion nozzle 100 includes a tubular part 102 penetrating the molten steel rectifying section 101 in the vertical direction inside. The tubular part 102 corresponds to an immersion nozzle having a substantially straight tube shape. The molten steel rectifying section 101 and the tubular part 102 may be integrally formed, or the molten steel rectifying section 101 and the tubular part 102 may be separately formed and these members may be combined and formed. The tubular part 102 discharges the molten steel 200 conveyed from the tundish into the mold 300. The tubular part 102 has an upper end extending in the tundish direction and a lower end extending in the direction of the mold bottom 303.

[0033] Here, the tubular part 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. Also, the tubular part 102 may be slightly different as long as the molten steel 200 conveyed from the tundish flows into a plurality of discharge holes formed near the bottom surface from the upper end of the tubular part 102. An inner hole 121 is formed inside the tubular part 102. A tubular part bottom surface 122 is formed at the bottom of the tubular part 102. The inner hole 121 formed inside the tubular part 102 serves as a molten steel flow path 103 for the molten steel 200 in which the molten steel 200 injected from the upper end of the tubular part 102 flows to near the tubular part bottom surface 122 of the tubular part 102.

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

[0035] The molten steel 200 is injected from the inner hole 121 formed at the upper end of the tubular portion 102 that protrudes from the central portion of the upper surface 112 of the molten steel rectifying portion 101 and is provided in the tundish direction. The inner hole 121 of the tubular portion 102 penetrates the inside of the molten steel rectifying portion 101 and communicates from the upper end to the lower end of the tubular portion 102. Therefore, the molten steel 200 injected from the inner hole 121 formed at the upper end of the tubular portion 102 is conveyed in the casting direction toward the bottom surface 122 of the tubular portion 102 of the tubular portion 102 via the inner hole 121 formed at the lower end of the tubular portion 102 formed inside the molten steel rectifying portion 101.

[0036] The molten steel 200 conveyed in the casting direction in the inner hole 121 formed in the tubular portion 102 reaches the bottom surface 122 of the tubular portion. The molten steel 200 that has reached the bottom surface 122 of the tubular portion is discharged as a discharge flow 202 into the mold 300 from the discharge hole 123 and the discharge hole 124 that the tubular portion 102 has. Thereafter, as the molten steel 200 discharged into the mold 300 accumulates, a molten steel meniscus 201 is formed 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 into an elliptical or streamline shape that 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 discharge direction length of the discharge holes 123 and 124 will inevitably become longer than that of a normal immersion nozzle. For this reason, the risk of clogging of the immersion nozzle increases significantly, which not only promotes defects caused by mold powder but also other defects related to steelmaking, thereby inhibiting productivity.

[0038] In addition, when continuously casting the molten steel 200 with the same casting width, the discharge position of the molten steel 200 approaches the short side 301 of the mold 300 of the mold more than when continuously casting the molten steel 200 using a normal immersion nozzle. As a result, the speed at which the discharge flow of the molten steel 200 discharged from the discharge holes 123 and 124 formed in the tubular portion 102 collides with the short side 301 of the mold becomes excessive, the initial solidification shell formed on the surface of the molten steel 200 is remelted, and when passing through the mold 300, the thickness of the initial solidification shell is insufficient and the molten steel 200 leaks out, that is, a so-called "breakout" occurs.

[0039] From the above, in the immersion nozzle 100 for continuous casting according to the present 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 ratio (β / α) of the length β of the other axis orthogonal to one axis substantially parallel to the long side of the mold 300 to the length α of the one axis is 0.90 or more and 1.1 or less. That is, in the vicinity of 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 ratio (β / α) of the length β of the other axis orthogonal to one axis substantially parallel to the long side of the mold 300 to the length α of the one axis needs to be set to 0.90 or more.

[0040] If the ratio (β / α) of the length β of the other axis orthogonal to one axis substantially parallel to the long side of the mold 300 to the length α of the one axis is 0.90 or more, it is preferable because the risk of nozzle clogging does not increase, and not only defects caused by mold powder but also other steelmaking property defects are not promoted. On the other hand, when the value of (β / α) becomes larger than 1.1, the wall thickness of the discharge hole flow path becomes too small, and the discharge flow cannot be adjusted in the vertical direction. Therefore, the upper limit of the (β / α) value is set to 1.1 or less.

[0041] In the immersion nozzle 100 for continuous casting described in FIGS. 1(a) to 1(c), the angles of the discharge holes 123 and 124 are approximately 90 degrees with respect to the axial direction of the immersion nozzle. However, regarding these discharge hole angles, discharge hole shapes, pool depth, and shape, appropriate values may be changed according to the casting conditions and the quality required for the cast slab.

[0042] As described above, the immersion nozzle 100 for continuous casting according to the present embodiment includes the molten steel rectifying portion 101 having a predetermined shape and the tubular portion 102. As a result, the flow of the molten steel 200 is separated from the molten steel rectifying portion 101, and no vortex of the molten steel 200 is generated beside the nozzle on the downstream side in the casting direction. Thus, the risk of nozzle clogging and inclusion adhesion can be suppressed.

[0043] As described above, according to the invention according to the present embodiment, in the immersion nozzle used for injecting molten steel into a continuous casting mold in the continuous casting of steel, the entrainment of mold powder into the molten steel can be suppressed.

[0044] [Second Embodiment] The immersion nozzle for continuous casting according to the second embodiment will be described. The immersion nozzle for continuous casting according to the present embodiment is characterized in that it 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 the immersion nozzle for continuous casting according to the first embodiment. That is, the continuous shape of the immersion nozzle for continuous casting according to the present embodiment is formed so as to be continuous 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, the vortex generated by the separation of the flow of the molten steel 200 occurring in the lower part in the casting direction without the continuous shape does not reach the molten steel surface (meniscus) 201, and it is preferable because there is no possibility that the molten steel 200 entrains the mold powder. Note that the continuous shape of the molten steel rectifying portion 101 is continuously formed so as to satisfy the condition of not overlapping the discharge holes 123 and 124.

[0046] In this way, in the immersion nozzle 100 for continuous casting according to the present embodiment, the vortex generated by the separation of the molten steel flow does not reach the molten steel surface (meniscus) 201. As a result, the immersion nozzle 100 for continuous casting according to the present embodiment can further suppress the entrainment of the mold powder into the molten steel 200.

[0047] As described above, according to the invention of the present embodiment, the vortex generated by the separation of the molten steel flow occurring in the lower part in the casting direction that does not have a continuous shape is suppressed from reaching the molten steel surface (meniscus), and the entrainment of the mold powder into the molten steel can be prevented.

[0048] [Third Embodiment] A method for manufacturing steel according to the third embodiment will be described. The method for manufacturing steel according to the present embodiment is characterized by manufacturing steel using the immersion nozzle for continuous casting according to the above embodiment. First, in the method for manufacturing steel according to the present embodiment, the immersion nozzle 100 for continuous casting according to the present embodiment is installed at the bottom of the tundish. Next, the tundish is installed above the mold 300 so that the immersion nozzle 100 for continuous casting is located substantially at the center of the space formed by the mold 300. In this way, in the method for manufacturing steel according to the present embodiment, the tundish, which is a member necessary for continuous casting of molten steel, the immersion nozzle 100 for continuous casting, and the mold 300 for continuous casting are prepared and installed.

[0049] Furthermore, in the method for manufacturing steel according to the present embodiment, while injecting the molten steel 200 from the ladle that stores the molten steel 200 melted in a refining furnace such as a converter into the tundish, the molten steel 200 is injected from the tundish into the mold 300 through the immersion nozzle 100 for continuous casting. When the molten steel 200 is injected into the mold 300 through the immersion nozzle 100 for continuous casting, the molten steel 200 injected into the inside of the mold 300 accumulates. As the molten steel 200 injected into the inside of the mold 300 accumulates, 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 injected into the inside of the mold 300. The mold powder supplied to the inside of the mold 300 and melted penetrates between the initial solidification shells formed by the cooling of the molten steel 200 inside the mold 300. On the other hand, a part of the mold powder supplied to the inside of the mold 300 and melted floats on the molten steel surface (meniscus) 201 of the mold 300. Furthermore, when injecting the mold powder into the inside of the mold 300, an inert gas such as argon gas or nitrogen gas may be blown through a sliding nozzle, an upper nozzle, etc. into the molten steel 200 flowing down the molten steel flow path 103 of the continuous casting immersion nozzle 100 and supplied to the molten steel 200.

[0051] Figure 2 is a diagram schematically showing the flow state of molten steel in the mold when casting molten steel using a normal immersion nozzle. Here, in Figure 2, reference numeral 301 is the short side of the continuous casting mold, reference numeral 201 is the molten steel surface (corresponding to the meniscus), reference numeral 202 is the discharge flow from the discharge hole 123, reference numeral 203 is the branched upward flow formed by the branching of the discharge flow 202 from the discharge hole, reference numeral 204 is the meniscus flow, reference numeral 205 is the peeling flow generated from the peeling point along the immersion nozzle, and reference numeral 206 is the vortex of the molten steel 200 formed in the peeling flow.

[0052] As shown in Figure 2, the discharge flow of the molten steel 200 discharged from the immersion nozzle 400 varies with time, and the discharge flow 202 of the molten steel 200 from the right discharge hole 124 has become stronger, and it is understood that the discharge flow 202 of the molten steel 200 is in a state of deeply diving obliquely in the casting direction. On one hand, compared with the discharge flow 202 from the right discharge hole 124, the discharge flow 202 from the left discharge hole 123 is directed upward (towards the tundish side) and becomes a flow directed towards the molten steel meniscus 201 after colliding with the short side 301 of the continuous casting mold. And after the discharge flow 202 from the left discharge hole 123 reaches the molten steel meniscus 201, it becomes the meniscus flow 204 directed towards the immersion nozzle 400. At this time, on the right side of the discharge hole 124, the meniscus flow 204 does not occur, and the meniscus flow 204 from the left side to the right side of the immersion nozzle 400 occurs.

[0053] In this way, when casting the molten steel 200 using the normal immersion nozzle 400 shown in FIG. 2, at the separation point of the meniscus flow 204, the discharge flow 202 from the discharge hole 123 of the immersion nozzle 400 separates, resulting in the generation of the separation flow 205. As a result, turbulence of the discharge flow 202 is generated by the separation flow 205 on the right side of the immersion nozzle 400, and a vortex 206 is generated from the molten steel 200. Due to this vortex 206, the mold powder floating on the molten steel meniscus 201 is entrained, deteriorating the quality of the steel product.

[0054] FIG. 3 is a diagram schematically showing the flow state of the molten steel in the mold when casting the molten steel using the immersion nozzle for continuous casting. Each reference numeral shown in FIG. 3 is the same as each reference numeral shown in FIG. 2, and the reference numeral 207 is the rear flow of the molten steel 200 formed behind the nozzle without the flow along the nozzle separating. As shown in FIG. 3, when casting the molten steel 200 using the immersion nozzle 100 for continuous casting according to this embodiment, separation from the immersion nozzle does not occur in the discharge flow 202 flowing on the left and right sides of the immersion nozzle 100.

[0055] Furthermore, even if detachment occurs from the immersion nozzle 100 in the molten steel flow of the molten steel 200 flowing left and right of the immersion nozzle 100 for continuous casting due to various conditions during the continuous casting of the molten steel 200, it is slight, and the generation frequency of the vortex 206 drastically decreases. Furthermore, when the molten steel 200 is cast using the immersion nozzle 100 for continuous casting, since the elliptical portion in the casting direction also has a continuous shape ensuring a predetermined length, the detachment flow 205 and the generation of the vortex 206 caused by the molten steel flow of the molten steel 200 in the same direction as the molten steel meniscus 201 generated below the molten steel meniscus 201 can also be suppressed.

[0056] Also, in the continuous casting of the molten steel 200, as the casting time elapses, a phenomenon is known in which the discharge holes 123 and 124 of the immersion nozzle 100 for continuous casting are blocked by inclusions or the like. If clogging occurs near one of the left and right discharge holes 123 and 124 of the immersion nozzle 100 for continuous casting, a large amount of the molten steel 200 will be discharged only from one discharge hole. When a large amount of the molten steel 200 is discharged only from one discharge hole, it causes an increase in the flow rate of the molten steel to the molten steel meniscus 201, which is a factor in mold powder entrainment.

[0057] However, when the molten steel 200 is cast using the immersion nozzle 100 for continuous casting, even if the phenomenon occurs in which the discharge holes 123 and 124 of the immersion nozzle 100 for continuous casting are blocked by inclusions or the like, it is possible to suppress the entrainment of the mold powder into the molten steel 200. That is, by continuously casting the molten steel 200 using the immersion nozzle 100 for continuous casting according to the above embodiment, it is possible to obtain steel products such as slabs with extremely good surface quality with suppressed entrainment of the mold powder.

[0058] In recent continuous casting operations of molten steel, a magnetic field is applied to the molten steel in a continuous casting mold to control the flow of the molten steel in the mold in order to suppress the capture of inclusions in the cast slab. Even when the molten steel 200 is cast using the submerged entry nozzle 100 for continuous casting according to this embodiment, a static magnetic field may be applied to the discharge flow to suppress the inclusions from sinking in, and a traveling magnetic field may be applied to impart a swirling flow aimed at a cleaning effect of washing away the inclusions, without affecting the effects of the present invention, and therefore there is no problem in using these methods 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 submerged entry nozzle for continuous casting 100 according to the above embodiment, the mold powder entrained by the molten steel flow is not captured by the initial solidified shell, and when the cast molten steel 200 becomes a steel product, no surface defects are formed, 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 vortexes that occur on the sides of the continuous casting submerged entry nozzle, which determines the majority of the entrainment of mold powder. Therefore, according to the method for producing steel according to the present embodiment, it is possible to stably prevent the entrainment of mold powder and to obtain a cast slab with extremely good surface quality.

[0061] [Other embodiments] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above-mentioned 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. In addition, systems or devices that combine the separate features included in each embodiment in any way are also included in the technical scope of the present invention. EXAMPLES

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

[0063] <Example 1> In an actual slab continuous casting machine, continuous casting operation was carried out using the immersion nozzle for continuous casting according to the present invention. The cross-sectional dimensions of the continuously cast slab are 220 - 260 mm in thickness and 1000 - 2000 mm in width. Also, argon gas was used as the inert gas blown into the immersion nozzle for continuous casting, and an optimal mold powder was selected and added to the meniscus according to the slab withdrawal speed and steel type.

[0064] (Design specifications of the immersion nozzle for continuous casting) The immersion nozzle for continuous casting according to the present invention was manufactured, and steel was produced using the manufactured immersion nozzle for continuous casting. In Invention Example 1, the cross-sectional shape of the molten steel immersion part, which is the outer tube provided in the immersion nozzle for continuous casting, was set to an elliptical shape. Further, in Invention Example 1, when the major axis of the outer circumference of the outer tube having an elliptical shape was a and the minor axis was b, the value of (b / a), which is the ratio of these, was 0.91, the value of h, which is the distance of the elliptical shape continuous in the casting direction of the molten steel from the meniscus position, was 50 mm, when the major axis of the outer tube of the tubular part below the molten steel rectifying part was α and the minor axis was β, the immersion nozzle for continuous casting was manufactured by setting the value of (β / α), which is the ratio of these, to be 1.0.

[0065] (Evaluation of steel products and manufacturing process) Using the immersion nozzle for continuous casting manufactured in Invention Example 1, molten steel was cast to produce steel products by setting the molten steel flow rate to 3.4 ton / min. In Invention Example 1, the continuous casting mold used was a mold without a magnetic field generator installed. The slab cast produced by the slab continuous casting machine was hot-rolled into a hot-rolled steel sheet, and the surface defects caused by the mold powder were investigated in this hot-rolled steel sheet, and the mold powder remaining in the slab was evaluated from these surface defects. That is, it was evaluated that the lower the defect index of the steel product, the less the mold powder remaining in the slab.

[0066] During the casting of molten steel, the solidification shell thickness in the mold is estimated from the temperature behavior of the thermocouple on the short side of the mold. When it falls below a threshold value, an alarm is sounded on the assumption that the risk of breakout increases. Therefore, in Invention Example 1, the presence or absence of a breakout alarm was also evaluated as an assessment of the operability of continuous casting.

[0067] In this way, the evaluation of steel products was carried out based on the defect indicators of steel products. Furthermore, in Invention Example 1, the evaluation of the steel manufacturing method using the continuous casting immersion nozzle produced was carried out based on the presence or absence of a breakout alarm. Table 1 shows the design of the continuous casting immersion nozzle of Invention Example 1, the evaluation results of steel products and the manufacturing process.

[0068] <Invention Examples 2 to 4> For the cross-sectional shape of the molten steel immersion part, which is the outer pipe of the continuous casting immersion nozzle, when the major axis of the outer pipe with the cross-sectional shape is a and the minor axis is b, the value of (b / a), which is the ratio of these, the value of h, which is the distance of the elliptical shape continuous in the casting direction of the molten steel from the meniscus position, the major axis of the outer circumference of the tubular part is α, and the minor axis is β, when the value of (β / α), which is the ratio of these, was changed, a continuous casting immersion nozzle was manufactured in the same manner as in Invention Example 1.

[0069] Furthermore, using each of the continuous casting immersion nozzles produced in Invention Examples 2 to 4, molten steel was cast in the same manner as in Invention Example 1 except that the molten steel flow rate was changed to produce steel products. Also, the evaluation of the steel products produced in Invention Examples 2 to 4 and the evaluation of the steel manufacturing method using the continuous casting immersion nozzles produced were carried out in the same manner as in Invention Example 1. Table 1 shows the design of the continuous casting immersion nozzles of Invention Examples 2 to 4, the evaluation results of steel products and the manufacturing process.

[0070] <Comparative Examples 1 to 4> For comparison with Invention Examples 1 to 4, the design of the immersion nozzle for continuous casting was changed, and the immersion nozzles for continuous casting of Comparative Examples 1 to 4 were manufactured. Casting was performed using the immersion nozzles for continuous casting manufactured in Comparative Examples 1 to 4. That is, steel casting was performed under the same conditions as in the invention examples except that casting was performed using the immersion nozzles for continuous casting manufactured in Comparative Examples 1 to 4. In addition, the evaluation of the steel products manufactured in Comparative Examples 1 to 4 and the evaluation of the steel manufacturing method using the immersion nozzles for continuous casting manufactured were performed in the same manner as in Invention Example 1. Table 1 shows the design of the immersion nozzles for continuous casting of Comparative Examples 1 to 4, and the evaluation results of the steel products and the manufacturing process.

[0071] As shown in Table 1, the design specifications of the immersion nozzles for continuous casting manufactured in Comparative Examples 1 to 4 are specifically as follows. Comparative Example 1: An immersion nozzle for continuous casting satisfying the relationship of (b / a) > 0.95 Comparative Example 2: An immersion nozzle for continuous casting satisfying the relationships of (b / a) ≤ 0.95 and h < 50 mm Comparative Example 3: An immersion nozzle for continuous casting satisfying the relationships of (b / a) ≤ 0.95, h ≥ 50 mm, and (β / α) < 0.9 Comparative Example 4: An immersion nozzle for continuous casting satisfying the relationships of (b / a) ≤ 0.95, h ≥ 50 mm, and (β / α) > 1.1

[0072]

Table 1

[0073] Table 1 shows the operating conditions and operating results. Comparing Invention Examples 1 to 4 with Comparative Examples 1-1 to 1-3, the defect index of the steel products of the present Invention Examples 1 to 4 could be significantly improved. In addition, in Comparative Examples 2-1 and 2-2, although improvement was seen compared to Comparative Examples 1-1 to 1-3, sufficient improvement was not achieved. In Comparative Examples 3-1 and 3-2, the quality of the steel products was good, but a breakout warning was issued in the latter half of the casting, so the casting was stopped.

[0074] Furthermore, in Comparative Example 3-2, a breakout alarm was issued at the initial stage of casting, so casting was stopped, and sufficient cast slabs could not be obtained to become steel products, and the quality of the steel products could not be evaluated. In Comparative Examples 4-1 and 4-2, although good quality steel products were obtained with little difference from those of Invention Examples 1 to 4, the inner pipe portion of the immersion nozzle was clogged with inclusions during casting, resulting in the stoppage of casting. That is, it was found that by applying the immersion nozzle for continuous casting according to the present invention, the defect index of steel products can be significantly reduced, and stable production can also be achieved in terms of operation.

Industrial Applicability

[0075] According to the immersion nozzle for continuous casting according to the present invention, generation of vortices in the vicinity of the immersion nozzle can be suppressed, and entrainment of mold powder can be suppressed. Therefore, steel products such as cast slabs with extremely good surface quality can be obtained, and stable operation of continuous casting of molten steel can be realized, contributing to the development of related industries such as the iron and steel industry and being industrially useful.

Explanation of Signs

[0076] 100 Immersion nozzle for continuous casting 101 Molten steel rectifying section 102 Tubular section (section below the molten steel rectifying section) 103 Molten steel flow path 111 Lower surface of the rectifying section 112 Upper surface of the rectifying section 121 Inner hole 122 Bottom surface of the tubular section (bottom surface of the section below the molten steel rectifying section) 123 Discharge hole (short side of the left mold) 124 Discharge hole (short side of the right mold) 200 Molten steel 201 Molten steel surface level (meniscus) 202 Discharge flow 203 Branch ascending flow 204 Meniscus flow 205 Peeling flow 206 Vortex (molten steel) 207 Rearward flow 300 Mold 301 Short side of the mold 302 Long side of the mold 303 Bottom side of the mold 400 Immersion nozzle (conventional product) a Major diameter of the molten steel rectifying section b Minor diameter of the molten steel rectifying section α Diameter of the tubular part (one axis) β Diameter of the tubular part (the other axis) h Length of the rectifying section continuous in the casting direction from the position of the molten steel surface (meniscus)

Claims

1. A submerged nozzle for injecting molten steel from a tundish into a mold of a continuous casting machine, wherein the submerged nozzle is immersed in the molten steel and has a molten steel rectifying portion provided on the molten steel surface, which is exposed upward from the molten steel surface formed inside the mold, and a tubular portion that penetrates the molten steel rectifying portion in the vertical direction and discharges the molten steel into the mold, wherein the molten steel rectifying portion has an elliptical shape or a streamline shape with a major axis in the width direction of the mold at least in a predetermined range in the casting direction of the molten steel from the molten steel surface, and has a continuous shape in which the value of (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, is 0.95 or less, wherein 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 β of the other axis perpendicular to one axis to the length α of the one axis substantially parallel to the long side of the mold, is 0.90 or more and 1.1 or less. A submerged nozzle for continuous casting characterized by this.

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

3. A method for continuous casting of steel, characterized by injecting molten steel in a tundish into a mold of a continuous casting machine using the submerged nozzle for continuous casting according to claim 1 or 2.

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