Immersion nozzle

Abstract

The present invention is directed to a flat immersion nozzle capable of stabilizing a bath level in a mold, etc. The present invention provides an immersion nozzle X comprising a tubular body lhaving a bottom 2, an upper end serving as an inlet 11 of molten steel, and a molten steel flow passage 15 internally formed to extend downwardly from the inlet, at least a lower portion 13 of the tubular body 1 being formed such that an outer shape thereof and the molten steel flow passage 15 therein have a rectangular flat cross-section, the tubular body 1 having: two opposed sidewalls 131 each defining a short side of the rectangular flat cross-section of the lower portion13 and extending parallel to an up-down directional central axis 151 of the molten steel flow passage 15; and a pair of discharge ports 16 formed in respective lower ends of the short side-defining sidewalls 131 of the lower portion 13, in opposed relation, and communicated with the molten steel flow passage 15, wherein the immersion nozzle X comprises a pair of guide plates 3 in the vicinity of respective ones of the pair of discharge ports 16, each of the pair of guide plates 3 being formed to connect together two opposed sidewalls 132 each defining a long side of the rectangular flat cross-section of the lower portion 13 and to guide molten steel flowing through the molten steel flow passage 15, toward a respective one of the pair of discharger ports 16, wherein a ratio A / B is from 0.25 to 0.9, where A represents a shortest distance between the pair of guide plates 3 in a direction along the long side, and B represents a length of the molten steel flow passage 15 in the lower portion 13 in the direction along the long side.

Description

TECHNICAL FIELD

[0001] The present invention relates to an immersion nozzle for use in continuous casting to pour molten steel from a tundish into a mold, and more particularly to an immersion nozzle having a rectangular flat horizontal section in the vicinity of a discharge port thereof, such as those used for thin slabs, medium-thick slabs or the like.BACKGROUND ART

[0002] In a continuous casting process for forming a slab having a predetermined shape by continuously subjecting molten steel to cooling and solidification, molten steel is poured into a mold via an immersion nozzle installed in the bottom of a tundish. Generally, the immersion nozzle is composed of a tubular body which has a bottom, an upper end serving as an inlet of molten steel, and a molten steel flow passage internally formed to extend downwardly from the molten steel inlet, wherein a pair of discharge ports communicated with the molten steel flow passage are formed in a sidewall of a lower portion of the tubular body in opposed relation to each other. The immersion nozzle is used in a state in which the lower portion thereof is immersed in molten steel in a mold. This is intended to prevent scattering of poured molten steel, and further block contact of the molten steel with the atmosphere, thereby preventing oxidation thereof. Further, the use of the immersion nozzle is intended to allow the flow of molten steel in the mold to be straightened, thereby preventing impurities such as slag or non-metal inclusions floating on the bath level of the molten steel from being entrained into the molten steel.

[0003] In recent years, there has been a growing tendency toward manufacturing thinned slabs, such as a thin slab and a medium-thick slab, during continuous casting. In order to cope with a thin mold for this type of continuous casting, the immersion nozzle needs to be formed in a flat shape. For example, Patent Document 1 discloses a flat immersion nozzle with a rectangular flat cross-section having long sides and short sides, wherein the flat immersion nozzle has two opposed short side-defining sidewalls each provided with a discharge port, and Patent Document 2 discloses a flat immersion nozzle having a lower end wall additionally provided with a discharge port. Generally, in these flat immersion nozzles, a molten steel flow passage is formed such that the width (which is synonymous with the length in a direction along the long side (long-side direction); the same applies hereinafter) thereof is expanded in a direction from a molten steel inlet to the discharge ports connecting to a mold.

[0004] However, in the flat immersion nozzle having such a molten steel flow passage whose width is expanded and whose cross-sectional shape is a rectangular flat shape, the flow of molten steel inside the immersion nozzle becomes more likely to be disordered, and thus a discharge flow toward the mold also becomes more likely to be disordered. Resulting turbulence of the molten steel flow or discharge flow becomes a factor causing defective quality of slabs, an increase in danger during casting operation, etc., such as an increase in fluctuation of a bath level (molten steel surface) in the mold, entrainment of a mold powder into slabs, or unevenness in temperature. Therefore, it is necessary to stabilize the molten steel flow inside the immersion nozzle and the discharge flow discharged toward the mold.

[0005] With a view to stabilizing such a molten steel flow and discharge flow, Patent Document 3 discloses an immersion nozzle formed with at least two bending facets each extending from a point (center) on a plane in a lower region of an elongated bore, which is a molten steel flow passage, toward a lower edge of a respective one of two discharge ports. The Patent Document 3 also discloses an immersion nozzle comprising a pair of baffles for dividing a molten steel flow into two separate streams (FIGS. 23 and 28). In the flat immersion nozzle disclosed in the Patent Document 3, the stability of the molten steel flow inside the immersion nozzle are enhanced, as compared with the immersion nozzles as disclosed in the Patent Documents 1 and 2, in which there is not any means to change a flow direction / pattern in an internal space of the molten steel flow passage.PRIOR ART DOCUMENTS[Patent Document]

[0006] Patent Document 1: JP H11-5145 A Patent Document 2: JP H11-47897 A Patent Document 3: JP 4583508 B2 SUMMARY OF INVENTION[Technical Problem]

[0007] However, as will be described in detail later, when the present inventors evaluated the fluctuation of a bath level in a mold by a water model test, with regard to the immersion nozzle disclosed in Patent Document 3, they found that there was a problem with a large fluctuation of the bath level in the mold.

[0008] Therefore, the technical problem to be solved by the present invention is to provide a flat immersion nozzle capable of stabilizing a bath level in a mold, etc., i.e., capable of reducing the fluctuation of the bath level in the mold.[Solution to Technical Problem]

[0009] The present inventors conducted a detailed analysis the cause of the situation where, in the mold in the immersion nozzle disclosed in Patent Document 3, the fluctuation of the bath level is large. As a result, the present inventors have found that in the immersion nozzle disclosed in Patent Document 3, since two bending facets are formed in an outwardly and downwardly-flared shape, i.e., the outer shape of a lower portion of a tubular body, which is the body of the immersion nozzle, and a molten steel flow passage, are formed in an outwardly and downwardly-flared shape, discharge flows discharged from the pair of discharge ports in an approximately horizontal direction become stronger, particularly under high throughput conditions, and consequently reverse flows are increased in the mold, leading to larger fluctuation of the bath level in the mold.

[0010] Based on this finding, with regard to the shape of a tubular body, which is the body of an immersion nozzle, the present inventors decided to employ a so-called straight shape in which two opposite short side-defining sidewalls in a lower portion of the tubular body, each having a respective one of a pair of discharge ports, are formed parallel to an up-down directional central axis of a molten steel flow passage, and then the inventors conducted repeated tests and researches in order to stabilize the molten steel flow inside the molten steel flow passage and optimize the discharge flow toward the mold, in the straight shape-type immersion nozzle. As a result, the inventors have found that it is effective to provide, in the vicinity of the pair of discharge ports, a pair of guide plates for guiding molten steel flowing through the molten steel flow passage of the immersion nozzle toward the pair of discharge ports, and that it is possible to stabilize the molten steel flow inside the molten steel flow passage of the immersion nozzle and optimize the discharge flow toward the mold, by providing the pair of guide plates to connect two opposite long side-defining sidewalls in the lower portion of the tubular body, and setting the long-side directional shortest distance between the pair of guide plates to fall within an appropriate range, and the inventors have completed the present invention.

[0011] Specifically, according to one aspect of the present invention, the following immersion nozzle is provided.

[0012] An immersion nozzle comprising a tubular body having a bottom, an upper end serving as an inlet of molten steel, and a molten steel flow passage internally formed to extend downwardly from the inlet, at least a lower portion of the tubular body being formed such that an outer shape thereof and the molten steel flow passage therein have a rectangular flat cross-section, the tubular body having: two opposed sidewalls each defining a short side of the rectangular flat cross-section of the lower portion and extending parallel to an up-down directional central axis of the molten steel flow passage; and a pair of discharge ports formed in respective lower ends of the short side-defining sidewalls of the lower portion, in opposed relation, and communicated with the molten steel flow passage, wherein the immersion nozzle comprises a pair of guide plates in the vicinity of respective ones of the pair of discharge ports, each of the pair of guide plates being formed to connect together two opposed sidewalls each defining a long side of the rectangular flat cross-section of the lower portion and to guide molten steel flowing through the molten steel flow passage, toward a respective one of the pair of discharger ports, wherein a ratio A / B is from 0.25 to 0.9, where A represents a shortest distance between the pair of guide plates in a direction along the long side, and B represents a length of the molten steel flow passage in the lower portion in the direction along the long side.[Advantageous Effects of Invention]

[0013] According to the present invention, it becomes possible to provide a flat immersion nozzle capable of stabilizing a molten steel flow inside a molten steel flow passage and optimizing a discharge flow toward a mold, thereby reducing the fluctuation of a bath level in the mold.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 illustrates an immersion nozzle according to one embodiment of the present invention, wherein: FIG. 1(a) is a vertical sectional view taken along a width direction, i.e., a long-side direction; FIG. 1(b) is a vertical sectional view taken along a thickness direction, i.e., a short-side direction; FIG. 1(c) is a cross-sectional view taken along and viewed in a direction of the arrowed line C-C of FIG. 1(a); and FIG. 1(d) is a bottom view viewed in a direction of the arrowed line D-D of FIG. 1(a). FIG. 2 is a perspective view of a relevant part of the immersion nozzle illustrated in FIG. 1. FIG. 3 is a diagram of a fluid analysis (CFD) model. FIG. 4 illustrates diagrams showing results obtained by analyzing the flow state of a molten steel flow inside the immersion nozzle by fluid analysis (CFD). FIG. 5 illustrates a graph showing the relationship between a ratio "A / B" and the mass flow rate of a discharge flow from a discharge port, obtained by fluid analysis (CFD). FIG. 6 illustrates a graph showing the relationship between the ratio "A / B" and a difference in mass flow rate between discharge flows from a pair of discharge ports, obtained by fluid analysis (CFD). FIG. 7 is a vertical sectional view taken along the width direction, i.e., the long-side direction, showing a conventional immersion nozzle disclosed in Patent Document 3. FIG. 8 illustrates results of water model tests, wherein FIG. 8(a) shows the result in an inventive example, and FIG. 8(b) shows the result in a conventional example. DESCRIPTION OF EMBODIMENTS

[0015] FIG. 1 illustrates an immersion nozzle X according to one embodiment of the present invention, wherein: FIG. 1(a) is a vertical sectional view taken along a width direction, i.e., a long-side direction; FIG. 1(b) is a vertical sectional view taken along a thickness direction, i.e., a short-side direction; FIG. 1(c) is a cross-sectional view taken along and viewed in a direction of the arrowed line C-C of FIG. 1(a); and FIG. 1(d) is a bottom view viewed in a direction of the arrowed line D-D of FIG. 1(a). Further, FIG. 2 shows a relevant part of the immersion nozzle illustrated in FIG. 1 in the form of a perspective view.

[0016] As appearing in FIG. 1(a) and 1(b), the immersion nozzle X is roughly composed of a tubular body 1 having a bottom 2, wherein the tubular body 1 comprises: a cylindrical upper portion 12 having an inlet 11 of molten steel at an upper end thereof; a lower portion 13 formed to have a rectangular flat cross-section; and an intermediate portion 14 connecting the upper portion 12 and the lower portion 13 and having an outwardly and downwardly flared-shape in FIG. 1(a), and wherein the tubular body 1 is internally formed with a molten steel flow passage 15 extending downwardly from the inlet 11. In other words, the tubular body 1 is the body of the immersion nozzle X.

[0017] The lower portion 13 of the tubular body 1 has a rectangular flat cross-section, as mentioned above. Specifically, as appearing in FIG. 1(c), the cross-section (horizontal section) of the lower portion 13 is formed in a rectangular flat shape having short sides each defined, respectively, by a first pair of opposed sidewalls (two short side-defining sidewalls) 131 of the lower portion 13, and long sides each defined, respectively, by a second pair of opposed sidewalls (two long side-defining sidewalls) 132 of the lower portion 13. In addition, as appearing in FIG. 1(a), the vertical section of the lower portion 13 along the width direction, i.e., the long-side direction, is formed in a so-called straight shape in which each of the two short side-defining sidewalls 131 extends parallel to an up-down directional center axis 151 of the molten steel flow passage 15. Further, a pair of discharge ports 16 communicated with the molten steel flow passage 15 are formed in respective lower ends of the two short side-defining sidewalls 131, in opposed relation.

[0018] As just described, the immersion nozzle X has a flat shape in which the lower portion 13 of the tubular body 1 is formed such that an outer shape thereof and the molten steel flow passage 15 therein have a rectangular flat cross-section, and each of the two short side-defining sidewalls 131 has a straight shape extending parallel to the up-down directional central axis151 of the molten steel flow passage 15. In this basic shape, the immersion nozzle X has a pair of guide plates 3 in the vicinity of respective ones of the pair of discharge ports 16. Each of the pair of guide plates 3 is formed to connect together the two long side-defining sidewalls 132 and to guide molten steel flowing through the molten steel flow passage 15, toward a respective one of the pair of discharge ports 16. Specifically, in the immersion nozzle X, the pair of guide plates 3 are arranged in an inverted V shape in the vertical section along the long-side direction, as appearing in FIG. 1(a), and are arranged symmetrically with respect to the up-down directional center axis 151 of the molten steel flow passage 15, i.e., an up-down directional center axis of the tubular body 1.

[0019] The pair of guide plates 3 will be described in more detail. When the shortest distance between the pair of guide plates 3 in the long-side direction is represented by A, and the length of the molten steel flow passage 15 in the lower portion 13 in the long-side direction is represented by B, as shown in FIG. 1(a), the ratio A / B is from 0.25 to 0.9. As will be described in detail later, if the ratio A / B is less than 0.25, a negative pressure is generated inside the tubular body 1, which is the body of the immersion nozzle X, particularly inside the lower portion 13. This causes large turbulence in the molten steel flow inside the tubular body 1, and the discharge flows from the pair of discharge ports 16 become unstable due to pulsation or the like caused by the turbulence, leading to a significant fluctuation of a bath level in a mold. On the other hand, if the ratio A / B exceeds 0.9, the function of guiding the molten steel flowing through the molten steel flow passage 15 toward the pair of discharge ports 16, i.e., the function of diverting the molten steel flow to generate a discharge flow in an approximately horizontal direction, cannot be fully brought out. From a viewpoint of suppressing the bias of the molten steel flow inside the tubular body 1 and the discharge flow from the pair of discharge ports 16, i.e., biased flow, the ratio A / B is preferably set to 0.4 or more. Therefore, the ratio A / B is preferably set in the range of 0.4 to 0.9.

[0020] In this embodiment, a diverter 4 is provided at the bottom 2. As appearing in FIGS. 1(a) and 2, this diverter 4 is formed in a triangular shape which has an apex on the up-down directional center axis 151 and two sides each inclining downwardly from the apex toward a respective one of the pair of discharge ports 16. Thus, the diverter 4 diverts the molten steel flow inside the tubular body 1, particularly inside the lower portion 13, toward the pair of discharge ports 16.

[0021] The embodiment of the present invention has been described by taking the immersion nozzle X as an example. However, the present invention is not limited to this immersion nozzle X. For example, in the immersion nozzle X, the pair of guide plates 3 are formed in an inverted V shape, i.e., each of them is formed in a straight shape. Alternatively, each of the pair of guide plates 3 may be formed in a curved or bent shape. Further, in the immersion nozzle X, each of the pair of guide plates 3 is provided such that a lower end thereof extends to a respective one of the pair of discharge ports 16. Alternatively, the lower end of each of the pair of guide plates 3 does not necessarily extend to a respective one of the pair of discharge ports 16 but may be located upstream of the respective one of the pair of discharge ports 16. In its essence, it is only necessary for each of the pair of guide plates 3 to be configured to connect the two long side-defining sidewalls 132 together and to guide the molten steel flowing through the molten steel flow passage 15 toward a respective one of the pair of discharge ports 16.

[0022] Further, in the immersion nozzle X, the triangular diverter 4 is provided at the bottom 2. However, the shape of the diverter 4 is not limited to a triangular shape but may be any other suitable shape, such as a trapezoidal shape, a conical shape, or a truncated cone shape. In its essence, it is only necessary for the diverter 4 to be configured to divert the molten steel flow inside the tubular body 1, toward at least the pair of discharge ports 16. It should be noted that since the bottom 2 itself can function as a diverter 4, the diverter 4 is not an essential component in the present invention.EXAMPLES

[0023] The influence of the above-mentioned ratio "A / B" was verified by fluid analysis, i.e., Computational Fluid Dynamics (CFD). In the CFD, as shown in FIG. 3, each of the pair of discharge ports 16 was divided into four sections, and the mass flow rate of the discharge flow from each of the eight discharge ports 16a to 16h was obtained, and the flow state of the molten steel flow inside the tubular body 1 was analyzed. It should be noted that in FIG. 3, the four discharge ports 16a to 16d correspond to one of the pair of discharge ports 16, and although illustration is omitted in FIG. 3, one of the pair of guide plates 3 is actually present between the discharge port 16b and the discharge port 16c. Further, in FIG. 3, the four discharge ports 16e to 16h correspond to the other of the pair of discharge ports 16, and although illustration is omitted in FIG. 3, the other of the pair of guide plates 3 is actually present between the discharge port 16f and the discharge port 16g. Further, a set of the discharge ports 16a to 16d and a set of the discharge ports 16e to 16h are arranged symmetrically with respect to the up-down directional center axis 151 of the molten steel flow passage 15, i.e., the up-down directional center axis of the tubular body 1.

[0024] FIG. 4 shows the results of analysis about the flow state of a molten steel flow inside the tubular body 1 by the CFD. FIG. 5 shows the relationship between the ratio "A / B" and the mass flow rate of a discharge flow from the discharge port 16f, obtained by the CFD. FIG. 6 shows the relationship between the ratio "A / B" and a difference in mass flow rate between discharge flows discharged from the pair of discharge ports 16d, 16e ([the mass flow rate of a discharge flow discharged from the discharge port 16d] - [the mass flow rate of a discharge flow discharged from the discharge port 16e]), obtained by the CFD.

[0025] From FIG. 5, it can be seen that a negative pressure is generated when the ratio A / B is 0.2. It is considered that this is because when A / B is 0.2, the flow state of the molten steel flow inside the tubular body 1 becomes unstable, as can be seen from FIG. 4(a). In contrast, when the ratio A / B is 0.25 or more, no negative pressure is generated, and the flow state of the molten steel flow inside the tubular body 1 is gradually stabilized along with an increase in the ratio A / B. In particular, when the ratio A / B is 0.4 or more, the flow state of the molten steel flow inside the tubular body 1 is further stabilized, and the bias of the molten steel flow and the discharge flow, i.e., biased flow, is significantly suppressed. From these fluid analysis results, it can be said that it is effective to set the ratio A / B to 0.25 or more, and it is preferable to set the ratio A / B to 0.4 or more. Further, for the above-mentioned reason, the upper limit of the ratio A / B is set to 0.9.

[0026] Next, the fluctuation of a bath level in a mold was evaluated by a water model test. To the water model test, the immersion nozzle X (see FIG. 1) in which the ratio A / B is set to 0.8 was provided as an inventive example, and an immersion nozzle Y (see FIG. 7) experimentally produced based on the disclosure of the aforementioned Patent Document 3was provided as a conventional example. In the immersion nozzle Y, the shortest distance A' between the pair of baffles 5 in the long-side direction was 60 mm, and the length B' of the molten steel flow passage immediately above the pair of baffles 5, in the long-side direction, was 185 mm, i.e., the ratio A' / B' is 0.3.

[0027] FIG. 8 shows the results of the water model test, wherein FIG. 8(a) shows the result in the inventive example, and FIG. 8(b) shows the result in the conventional example. From comparison between FIG. 8(a) and FIG. 8(b), it can be seen that the present invention can reduce the fluctuation of the bath level in the mold. This is because the present invention made it possible to stabilize the molten steel flow inside the molten steel flow passage and optimize the discharge flow toward the mold. In other words, in the present invention, the tubular body, which is the body of the immersion nozzle, is formed in a straight shape, and then in order to stabilize the molten steel flow inside the molten steel flow passage and optimize the discharge flow toward the mold, each of the pair of guide plates is provided to connect together the two long side-defining sidewalls of the lower portion of the tubular body, and the shortest distance between the pair of guide plates in the long-side direction is set in an appropriate range, so that it becomes possible to reduce the fluctuation of the bath level in the mold.LIST OF REFERENCE SIGNS

[0028] X, Y: immersion nozzle 1: tubular body (body of immersion nozzle) 11: inlet of molten steel 12: upper portion 13: lower portion 131: short side-defining sidewall 132: long side-defining sidewall 14: intermediate portion 15: molten steel flow passage 16, 16a-16h: discharge port 2: bottom 3: guide plate 4: flow diverter 5: baffle

Examples

examples

[0023]The influence of the above-mentioned ratio "A / B" was verified by fluid analysis, i.e., Computational Fluid Dynamics (CFD). In the CFD, as shown in FIG. 3, each of the pair of discharge ports 16 was divided into four sections, and the mass flow rate of the discharge flow from each of the eight discharge ports 16a to 16h was obtained, and the flow state of the molten steel flow inside the tubular body 1 was analyzed. It should be noted that in FIG. 3, the four discharge ports 16a to 16d correspond to one of the pair of discharge ports 16, and although illustration is omitted in FIG. 3, one of the pair of guide plates 3 is actually present between the discharge port 16b and the discharge port 16c. Further, in FIG. 3, the four discharge ports 16e to 16h correspond to the other of the pair of discharge ports 16, and although illustration is omitted in FIG. 3, the other of the pair of guide plates 3 is actually present between the discharge port 16f and the discharge port 16g. Furth...

TECHNICAL FIELD

[0001] The present invention relates to an immersion nozzle for use in continuous casting to pour molten steel from a tundish into a mold, and more particularly to an immersion nozzle having a rectangular flat horizontal section in the vicinity of a discharge port thereof, such as those used for thin slabs, medium-thick slabs or the like.BACKGROUND ART

[0002] In a continuous casting process for forming a slab having a predetermined shape by continuously subjecting molten steel to cooling and solidification, molten steel is poured into a mold via an immersion nozzle installed in the bottom of a tundish. Generally, the immersion nozzle is composed of a tubular body which has a bottom, an upper end serving as an inlet of molten steel, and a molten steel flow passage internally formed to extend downwardly from the molten steel inlet, wherein a pair of discharge ports communicated with the molten steel flow passage are formed in a sidewall of a lower portion of the tubular body in opposed relation to each other. The immersion nozzle is used in a state in which the lower portion thereof is immersed in molten steel in a mold. This is intended to prevent scattering of poured molten steel, and further block contact of the molten steel with the atmosphere, thereby preventing oxidation thereof. Further, the use of the immersion nozzle is intended to allow the flow of molten steel in the mold to be straightened, thereby preventing impurities such as slag or non-metal inclusions floating on the bath level of the molten steel from being entrained into the molten steel.

[0003] In recent years, there has been a growing tendency toward manufacturing thinned slabs, such as a thin slab and a medium-thick slab, during continuous casting. In order to cope with a thin mold for this type of continuous casting, the immersion nozzle needs to be formed in a flat shape. For example, Patent Document 1 discloses a flat immersion nozzle with a rectangular flat cross-section having long sides and short sides, wherein the flat immersion nozzle has two opposed short side-defining sidewalls each provided with a discharge port, and Patent Document 2 discloses a flat immersion nozzle having a lower end wall additionally provided with a discharge port. Generally, in these flat immersion nozzles, a molten steel flow passage is formed such that the width (which is synonymous with the length in a direction along the long side (long-side direction); the same applies hereinafter) thereof is expanded in a direction from a molten steel inlet to the discharge ports connecting to a mold.

[0004] However, in the flat immersion nozzle having such a molten steel flow passage whose width is expanded and whose cross-sectional shape is a rectangular flat shape, the flow of molten steel inside the immersion nozzle becomes more likely to be disordered, and thus a discharge flow toward the mold also becomes more likely to be disordered. Resulting turbulence of the molten steel flow or discharge flow becomes a factor causing defective quality of slabs, an increase in danger during casting operation, etc., such as an increase in fluctuation of a bath level (molten steel surface) in the mold, entrainment of a mold powder into slabs, or unevenness in temperature. Therefore, it is necessary to stabilize the molten steel flow inside the immersion nozzle and the discharge flow discharged toward the mold.

[0005] With a view to stabilizing such a molten steel flow and discharge flow, Patent Document 3 discloses an immersion nozzle formed with at least two bending facets each extending from a point (center) on a plane in a lower region of an elongated bore, which is a molten steel flow passage, toward a lower edge of a respective one of two discharge ports. The Patent Document 3 also discloses an immersion nozzle comprising a pair of baffles for dividing a molten steel flow into two separate streams (FIGS. 23 and 28). In the flat immersion nozzle disclosed in the Patent Document 3, the stability of the molten steel flow inside the immersion nozzle are enhanced, as compared with the immersion nozzles as disclosed in the Patent Documents 1 and 2, in which there is not any means to change a flow direction / pattern in an internal space of the molten steel flow passage.PRIOR ART DOCUMENTS[Patent Document]

[0006] Patent Document 1: JP H11-5145 A Patent Document 2: JP H11-47897 A Patent Document 3: JP 4583508 B2 SUMMARY OF INVENTION[Technical Problem]

[0007] However, as will be described in detail later, when the present inventors evaluated the fluctuation of a bath level in a mold by a water model test, with regard to the immersion nozzle disclosed in Patent Document 3, they found that there was a problem with a large fluctuation of the bath level in the mold.

[0008] Therefore, the technical problem to be solved by the present invention is to provide a flat immersion nozzle capable of stabilizing a bath level in a mold, etc., i.e., capable of reducing the fluctuation of the bath level in the mold.[Solution to Technical Problem]

[0009] The present inventors conducted a detailed analysis the cause of the situation where, in the mold in the immersion nozzle disclosed in Patent Document 3, the fluctuation of the bath level is large. As a result, the present inventors have found that in the immersion nozzle disclosed in Patent Document 3, since two bending facets are formed in an outwardly and downwardly-flared shape, i.e., the outer shape of a lower portion of a tubular body, which is the body of the immersion nozzle, and a molten steel flow passage, are formed in an outwardly and downwardly-flared shape, discharge flows discharged from the pair of discharge ports in an approximately horizontal direction become stronger, particularly under high throughput conditions, and consequently reverse flows are increased in the mold, leading to larger fluctuation of the bath level in the mold.

[0010] Based on this finding, with regard to the shape of a tubular body, which is the body of an immersion nozzle, the present inventors decided to employ a so-called straight shape in which two opposite short side-defining sidewalls in a lower portion of the tubular body, each having a respective one of a pair of discharge ports, are formed parallel to an up-down directional central axis of a molten steel flow passage, and then the inventors conducted repeated tests and researches in order to stabilize the molten steel flow inside the molten steel flow passage and optimize the discharge flow toward the mold, in the straight shape-type immersion nozzle. As a result, the inventors have found that it is effective to provide, in the vicinity of the pair of discharge ports, a pair of guide plates for guiding molten steel flowing through the molten steel flow passage of the immersion nozzle toward the pair of discharge ports, and that it is possible to stabilize the molten steel flow inside the molten steel flow passage of the immersion nozzle and optimize the discharge flow toward the mold, by providing the pair of guide plates to connect two opposite long side-defining sidewalls in the lower portion of the tubular body, and setting the long-side directional shortest distance between the pair of guide plates to fall within an appropriate range, and the inventors have completed the present invention.

[0011] Specifically, according to one aspect of the present invention, the following immersion nozzle is provided.

[0012] An immersion nozzle comprising a tubular body having a bottom, an upper end serving as an inlet of molten steel, and a molten steel flow passage internally formed to extend downwardly from the inlet, at least a lower portion of the tubular body being formed such that an outer shape thereof and the molten steel flow passage therein have a rectangular flat cross-section, the tubular body having: two opposed sidewalls each defining a short side of the rectangular flat cross-section of the lower portion and extending parallel to an up-down directional central axis of the molten steel flow passage; and a pair of discharge ports formed in respective lower ends of the short side-defining sidewalls of the lower portion, in opposed relation, and communicated with the molten steel flow passage, wherein the immersion nozzle comprises a pair of guide plates in the vicinity of respective ones of the pair of discharge ports, each of the pair of guide plates being formed to connect together two opposed sidewalls each defining a long side of the rectangular flat cross-section of the lower portion and to guide molten steel flowing through the molten steel flow passage, toward a respective one of the pair of discharger ports, wherein a ratio A / B is from 0.25 to 0.9, where A represents a shortest distance between the pair of guide plates in a direction along the long side, and B represents a length of the molten steel flow passage in the lower portion in the direction along the long side.[Advantageous Effects of Invention]

[0013] According to the present invention, it becomes possible to provide a flat immersion nozzle capable of stabilizing a molten steel flow inside a molten steel flow passage and optimizing a discharge flow toward a mold, thereby reducing the fluctuation of a bath level in the mold.BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG. 1 illustrates an immersion nozzle according to one embodiment of the present invention, wherein: FIG. 1(a) is a vertical sectional view taken along a width direction, i.e., a long-side direction; FIG. 1(b) is a vertical sectional view taken along a thickness direction, i.e., a short-side direction; FIG. 1(c) is a cross-sectional view taken along and viewed in a direction of the arrowed line C-C of FIG. 1(a); and FIG. 1(d) is a bottom view viewed in a direction of the arrowed line D-D of FIG. 1(a). FIG. 2 is a perspective view of a relevant part of the immersion nozzle illustrated in FIG. 1. FIG. 3 is a diagram of a fluid analysis (CFD) model. FIG. 4 illustrates diagrams showing results obtained by analyzing the flow state of a molten steel flow inside the immersion nozzle by fluid analysis (CFD). FIG. 5 illustrates a graph showing the relationship between a ratio "A / B" and the mass flow rate of a discharge flow from a discharge port, obtained by fluid analysis (CFD). FIG. 6 illustrates a graph showing the relationship between the ratio "A / B" and a difference in mass flow rate between discharge flows from a pair of discharge ports, obtained by fluid analysis (CFD). FIG. 7 is a vertical sectional view taken along the width direction, i.e., the long-side direction, showing a conventional immersion nozzle disclosed in Patent Document 3. FIG. 8 illustrates results of water model tests, wherein FIG. 8(a) shows the result in an inventive example, and FIG. 8(b) shows the result in a conventional example. DESCRIPTION OF EMBODIMENTS

[0015] FIG. 1 illustrates an immersion nozzle X according to one embodiment of the present invention, wherein: FIG. 1(a) is a vertical sectional view taken along a width direction, i.e., a long-side direction; FIG. 1(b) is a vertical sectional view taken along a thickness direction, i.e., a short-side direction; FIG. 1(c) is a cross-sectional view taken along and viewed in a direction of the arrowed line C-C of FIG. 1(a); and FIG. 1(d) is a bottom view viewed in a direction of the arrowed line D-D of FIG. 1(a). Further, FIG. 2 shows a relevant part of the immersion nozzle illustrated in FIG. 1 in the form of a perspective view.

[0016] As appearing in FIG. 1(a) and 1(b), the immersion nozzle X is roughly composed of a tubular body 1 having a bottom 2, wherein the tubular body 1 comprises: a cylindrical upper portion 12 having an inlet 11 of molten steel at an upper end thereof; a lower portion 13 formed to have a rectangular flat cross-section; and an intermediate portion 14 connecting the upper portion 12 and the lower portion 13 and having an outwardly and downwardly flared-shape in FIG. 1(a), and wherein the tubular body 1 is internally formed with a molten steel flow passage 15 extending downwardly from the inlet 11. In other words, the tubular body 1 is the body of the immersion nozzle X.

[0017] The lower portion 13 of the tubular body 1 has a rectangular flat cross-section, as mentioned above. Specifically, as appearing in FIG. 1(c), the cross-section (horizontal section) of the lower portion 13 is formed in a rectangular flat shape having short sides each defined, respectively, by a first pair of opposed sidewalls (two short side-defining sidewalls) 131 of the lower portion 13, and long sides each defined, respectively, by a second pair of opposed sidewalls (two long side-defining sidewalls) 132 of the lower portion 13. In addition, as appearing in FIG. 1(a), the vertical section of the lower portion 13 along the width direction, i.e., the long-side direction, is formed in a so-called straight shape in which each of the two short side-defining sidewalls 131 extends parallel to an up-down directional center axis 151 of the molten steel flow passage 15. Further, a pair of discharge ports 16 communicated with the molten steel flow passage 15 are formed in respective lower ends of the two short side-defining sidewalls 131, in opposed relation.

[0018] As just described, the immersion nozzle X has a flat shape in which the lower portion 13 of the tubular body 1 is formed such that an outer shape thereof and the molten steel flow passage 15 therein have a rectangular flat cross-section, and each of the two short side-defining sidewalls 131 has a straight shape extending parallel to the up-down directional central axis151 of the molten steel flow passage 15. In this basic shape, the immersion nozzle X has a pair of guide plates 3 in the vicinity of respective ones of the pair of discharge ports 16. Each of the pair of guide plates 3 is formed to connect together the two long side-defining sidewalls 132 and to guide molten steel flowing through the molten steel flow passage 15, toward a respective one of the pair of discharge ports 16. Specifically, in the immersion nozzle X, the pair of guide plates 3 are arranged in an inverted V shape in the vertical section along the long-side direction, as appearing in FIG. 1(a), and are arranged symmetrically with respect to the up-down directional center axis 151 of the molten steel flow passage 15, i.e., an up-down directional center axis of the tubular body 1.

[0019] The pair of guide plates 3 will be described in more detail. When the shortest distance between the pair of guide plates 3 in the long-side direction is represented by A, and the length of the molten steel flow passage 15 in the lower portion 13 in the long-side direction is represented by B, as shown in FIG. 1(a), the ratio A / B is from 0.25 to 0.9. As will be described in detail later, if the ratio A / B is less than 0.25, a negative pressure is generated inside the tubular body 1, which is the body of the immersion nozzle X, particularly inside the lower portion 13. This causes large turbulence in the molten steel flow inside the tubular body 1, and the discharge flows from the pair of discharge ports 16 become unstable due to pulsation or the like caused by the turbulence, leading to a significant fluctuation of a bath level in a mold. On the other hand, if the ratio A / B exceeds 0.9, the function of guiding the molten steel flowing through the molten steel flow passage 15 toward the pair of discharge ports 16, i.e., the function of diverting the molten steel flow to generate a discharge flow in an approximately horizontal direction, cannot be fully brought out. From a viewpoint of suppressing the bias of the molten steel flow inside the tubular body 1 and the discharge flow from the pair of discharge ports 16, i.e., biased flow, the ratio A / B is preferably set to 0.4 or more. Therefore, the ratio A / B is preferably set in the range of 0.4 to 0.9.

[0020] In this embodiment, a diverter 4 is provided at the bottom 2. As appearing in FIGS. 1(a) and 2, this diverter 4 is formed in a triangular shape which has an apex on the up-down directional center axis 151 and two sides each inclining downwardly from the apex toward a respective one of the pair of discharge ports 16. Thus, the diverter 4 diverts the molten steel flow inside the tubular body 1, particularly inside the lower portion 13, toward the pair of discharge ports 16.

[0021] The embodiment of the present invention has been described by taking the immersion nozzle X as an example. However, the present invention is not limited to this immersion nozzle X. For example, in the immersion nozzle X, the pair of guide plates 3 are formed in an inverted V shape, i.e., each of them is formed in a straight shape. Alternatively, each of the pair of guide plates 3 may be formed in a curved or bent shape. Further, in the immersion nozzle X, each of the pair of guide plates 3 is provided such that a lower end thereof extends to a respective one of the pair of discharge ports 16. Alternatively, the lower end of each of the pair of guide plates 3 does not necessarily extend to a respective one of the pair of discharge ports 16 but may be located upstream of the respective one of the pair of discharge ports 16. In its essence, it is only necessary for each of the pair of guide plates 3 to be configured to connect the two long side-defining sidewalls 132 together and to guide the molten steel flowing through the molten steel flow passage 15 toward a respective one of the pair of discharge ports 16.

[0022] Further, in the immersion nozzle X, the triangular diverter 4 is provided at the bottom 2. However, the shape of the diverter 4 is not limited to a triangular shape but may be any other suitable shape, such as a trapezoidal shape, a conical shape, or a truncated cone shape. In its essence, it is only necessary for the diverter 4 to be configured to divert the molten steel flow inside the tubular body 1, toward at least the pair of discharge ports 16. It should be noted that since the bottom 2 itself can function as a diverter 4, the diverter 4 is not an essential component in the present invention.EXAMPLES

[0023] The influence of the above-mentioned ratio "A / B" was verified by fluid analysis, i.e., Computational Fluid Dynamics (CFD). In the CFD, as shown in FIG. 3, each of the pair of discharge ports 16 was divided into four sections, and the mass flow rate of the discharge flow from each of the eight discharge ports 16a to 16h was obtained, and the flow state of the molten steel flow inside the tubular body 1 was analyzed. It should be noted that in FIG. 3, the four discharge ports 16a to 16d correspond to one of the pair of discharge ports 16, and although illustration is omitted in FIG. 3, one of the pair of guide plates 3 is actually present between the discharge port 16b and the discharge port 16c. Further, in FIG. 3, the four discharge ports 16e to 16h correspond to the other of the pair of discharge ports 16, and although illustration is omitted in FIG. 3, the other of the pair of guide plates 3 is actually present between the discharge port 16f and the discharge port 16g. Further, a set of the discharge ports 16a to 16d and a set of the discharge ports 16e to 16h are arranged symmetrically with respect to the up-down directional center axis 151 of the molten steel flow passage 15, i.e., the up-down directional center axis of the tubular body 1.

[0024] FIG. 4 shows the results of analysis about the flow state of a molten steel flow inside the tubular body 1 by the CFD. FIG. 5 shows the relationship between the ratio "A / B" and the mass flow rate of a discharge flow from the discharge port 16f, obtained by the CFD. FIG. 6 shows the relationship between the ratio "A / B" and a difference in mass flow rate between discharge flows discharged from the pair of discharge ports 16d, 16e ([the mass flow rate of a discharge flow discharged from the discharge port 16d] - [the mass flow rate of a discharge flow discharged from the discharge port 16e]), obtained by the CFD.

[0025] From FIG. 5, it can be seen that a negative pressure is generated when the ratio A / B is 0.2. It is considered that this is because when A / B is 0.2, the flow state of the molten steel flow inside the tubular body 1 becomes unstable, as can be seen from FIG. 4(a). In contrast, when the ratio A / B is 0.25 or more, no negative pressure is generated, and the flow state of the molten steel flow inside the tubular body 1 is gradually stabilized along with an increase in the ratio A / B. In particular, when the ratio A / B is 0.4 or more, the flow state of the molten steel flow inside the tubular body 1 is further stabilized, and the bias of the molten steel flow and the discharge flow, i.e., biased flow, is significantly suppressed. From these fluid analysis results, it can be said that it is effective to set the ratio A / B to 0.25 or more, and it is preferable to set the ratio A / B to 0.4 or more. Further, for the above-mentioned reason, the upper limit of the ratio A / B is set to 0.9.

[0026] Next, the fluctuation of a bath level in a mold was evaluated by a water model test. To the water model test, the immersion nozzle X (see FIG. 1) in which the ratio A / B is set to 0.8 was provided as an inventive example, and an immersion nozzle Y (see FIG. 7) experimentally produced based on the disclosure of the aforementioned Patent Document 3was provided as a conventional example. In the immersion nozzle Y, the shortest distance A' between the pair of baffles 5 in the long-side direction was 60 mm, and the length B' of the molten steel flow passage immediately above the pair of baffles 5, in the long-side direction, was 185 mm, i.e., the ratio A' / B' is 0.3.

[0027] FIG. 8 shows the results of the water model test, wherein FIG. 8(a) shows the result in the inventive example, and FIG. 8(b) shows the result in the conventional example. From comparison between FIG. 8(a) and FIG. 8(b), it can be seen that the present invention can reduce the fluctuation of the bath level in the mold. This is because the present invention made it possible to stabilize the molten steel flow inside the molten steel flow passage and optimize the discharge flow toward the mold. In other words, in the present invention, the tubular body, which is the body of the immersion nozzle, is formed in a straight shape, and then in order to stabilize the molten steel flow inside the molten steel flow passage and optimize the discharge flow toward the mold, each of the pair of guide plates is provided to connect together the two long side-defining sidewalls of the lower portion of the tubular body, and the shortest distance between the pair of guide plates in the long-side direction is set in an appropriate range, so that it becomes possible to reduce the fluctuation of the bath level in the mold.LIST OF REFERENCE SIGNS

[0028] X, Y: immersion nozzle 1: tubular body (body of immersion nozzle) 11: inlet of molten steel 12: upper portion 13: lower portion 131: short side-defining sidewall 132: long side-defining sidewall 14: intermediate portion 15: molten steel flow passage 16, 16a-16h: discharge port 2: bottom 3: guide plate 4: flow diverter 5: baffle

Examples

examples

[0023]The influence of the above-mentioned ratio "A / B" was verified by fluid analysis, i.e., Computational Fluid Dynamics (CFD). In the CFD, as shown in FIG. 3, each of the pair of discharge ports 16 was divided into four sections, and the mass flow rate of the discharge flow from each of the eight discharge ports 16a to 16h was obtained, and the flow state of the molten steel flow inside the tubular body 1 was analyzed. It should be noted that in FIG. 3, the four discharge ports 16a to 16d correspond to one of the pair of discharge ports 16, and although illustration is omitted in FIG. 3, one of the pair of guide plates 3 is actually present between the discharge port 16b and the discharge port 16c. Further, in FIG. 3, the four discharge ports 16e to 16h correspond to the other of the pair of discharge ports 16, and although illustration is omitted in FIG. 3, the other of the pair of guide plates 3 is actually present between the discharge port 16f and the discharge port 16g. Furth...

Claims

1. An immersion nozzle comprising a tubular body having a bottom, an upper end serving as an inlet of molten steel, and a molten steel flow passage internally formed to extend downwardly from the inlet, at least a lower portion of the tubular body being formed such that an outer shape thereof and the molten steel flow passage therein have a rectangular flat cross-section, the tubular body having: two opposed sidewalls each defining a short side of the rectangular flat cross-section of the lower portion and extending parallel to an up-down directional central axis of the molten steel flow passage; and a pair of discharge ports formed in respective lower ends of the short side-defining sidewalls of the lower portion, in opposed relation, and communicated with the molten steel flow passage, wherein the immersion nozzle comprises a pair of guide plates in the vicinity of respective ones of the pair of discharge ports, each of the pair of guide plates being formed to connect together two opposed sidewalls each defining a long side of the rectangular flat cross-section of the lower portion and to guide molten steel flowing through the molten steel flow passage, toward a respective one of the pair of discharger ports, wherein a ratio A / B is from 0.25 to 0.9, where A represents a shortest distance between the pair of guide plates in a direction along the long side, and B represents a length of the molten steel flow passage in the lower portion in the direction along the long side.

2. The immersion nozzle as claimed in claim 1, wherein the ratio A / B is from 0.4 to 0.9.

3. The immersion nozzle as claimed in claim 1 or 2, comprising a flow diverter at the bottom, the flow divider being formed to divert a molten steel flow toward at least the pair of discharge ports.

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