Patent Publication Number: US-8113391-B2

Title: Immersion nozzle for continuous casting

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2008-84166 filed on Mar. 27, 2008, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a continuous casting immersion nozzle for pouring molten steel from a tundish into a mold. More specifically, the present invention relates to an immersion nozzle used for high-speed casting of medium-thickness slabs (about 70 mm to about 150 mm thick). 
     2. Description of the Related Art 
     With the trend toward faster continuous casting aimed at increasing productivity of slabs, Japanese Unexamined Patent Application Publication No. 57-106456, for example, discloses as an immersion nozzle that advantageously fits increasing throughputs of casting steel products, an immersion nozzle having a plurality of small holes disposed in the bottom (See  FIG. 15 ). The immersion nozzle may be used with no difficulty in continuous casting when the throughput of cast slabs (pouring rate) is 1 m/min to 1.5 m/min. 
     Japanese Unexamined Patent Application Publication No. 7-232247 discloses an immersion nozzle for continuous casting including a cylindrical body, the body having a pair of outlets disposed in the sidewall of a lower section thereof and a slit opening formed in a downwardly tapered lower section thereof. The outlets and slit opening are designed to decrease defects in the cast steel products caused by entrapment of inclusions (See  FIG. 16A ,  FIG. 16B ). In this immersion nozzle, the bottom is fully opened with the slit opening to make a large open area. 
     International Publication No. 2005/049249 discloses an immersion nozzle including a tubular body, the body having a pair of opposing lateral outlets in the sidewall of a lower section thereof. The lateral outlets each are divided by one or two inward horizontal projections into two or three vertically arranged portions to make a total of four or six outlets (See  FIG. 17A ,  FIG. 17B ). The publication describes that the immersion nozzle permits inhibition of clogging and generation of more stable and controlled exit-streams which are more uniform in velocity and in which spin and swirl are significantly reduced. 
     In the conventional immersion nozzles that have a pair of outlets disposed in the lower sidewall of the tubular body, larger amounts of the exit-streams issue from the lower portions of the outlets, which results in imbalance in amounts between the exit-streams that issue from the lower portions and the exit-streams that issue from the upper portions of the outlets. With a rise in the throughput, this imbalance increases to form negative pressure in the upper portions of the outlets, thereby possibly allowing the molten steel in the mold to flow into the nozzle through the upper portions of the outlets. This leads to excessive velocities of part of the molten steel streams impinging on the narrow sidewalls of the mold, which in turn causes increased velocities of the reverse flows that impinge on the narrow sidewalls and turn back. The increased velocities of the reverse flows raise the level fluctuation at the surface of the molten steel in the mold, resulting in asymmetric streams on the right- and left-hand sides of the immersion nozzle. 
     The present invention has been made in view of the above circumstances, and it is an object of the present invention to provide an immersion nozzle for continuous casting, particularly for high-speed continuous casting of medium-thickness slabs, which nozzle permits a reduction in the drift of molten steel flow in the mold and a reduction in the level fluctuation at the surface of the molten steel to improve the quality and productivity of slabs. 
     SUMMARY OF THE INVENTION 
     The present invention provides an immersion nozzle for continuous casting. The immersion nozzle has a tubular body with a bottom. The tubular body has an inlet for entry of molten steel disposed at an upper end and a passage to extend downward from the inlet. The tubular body is depressed in cross section at least at a lower section. The lower section has two narrow sidewalls and two broad sidewalls. A pair of opposing first outlets are disposed in the narrow sidewalls of the lower section so as to communicate with the passage. The lower section has ridges horizontally projecting into the passage from inner surfaces of the broad sidewalls between the pair of first outlets. Additionally, a pair of second outlets are disposed in the bottom so as to communicate with the passage, and are disposed symmetrically about an axis of the tubular body. The axes of the pair of second outlets cross each other in the passage. 
     In the immersion nozzle according to the present invention, it is preferable that a/a′ ranges from 0.1 to 0.25 and b/b′ ranges from 0.15 to 0.35, where a′ is a horizontal width of the first outlets; b′ is a vertical length of the first outlets; a is a projection height of the ridges; and b is a vertical width of the ridges. 
     Also, it is preferable that f/a′ ranges from 0.75 to 0.9, e/e′ ranges from 0.1 to 0.17, and α ranges from 40° to 60°, where f is a length of the second outlets along the narrow sidewalls; α is an angle formed between each of the axes of the second outlets and the horizontal plane; e is a minimum internal measurement between the pair of second outlets; and e′ is a width of the passage, along the broad sidewalls, immediately above the first outlets. 
     Further, the immersion nozzle according to the present invention may further include slits for allowing communication between the first outlets and the second outlets to make the exit-streams more balanced. In this respect, it is preferable that d/a′ ranges from 0.2 to 1, where d is the width of the slits. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  shows an immersion nozzle for continuous casting according to one embodiment of the present invention. 
         FIG. 1B  is a cross-sectional view taken on line  1 B- 1 B of FIG. A. 
         FIG. 1C  is a bottom view of the immersion nozzle for continuous casting. 
         FIG. 1D  is a cross-sectional view taken on line  1 D- 1 D of  FIG. 1B . 
         FIG. 2  is a partial side view of the immersion nozzle. 
         FIG. 3  is a partial vertical sectional view of the immersion nozzle, taken along the broad sidewall of a lower section thereof. 
         FIG. 4A  is a bottom view of the immersion nozzle. 
         FIG. 4B  is a cross-sectional view taken on line  4 B- 4 B of  FIG. 3 . 
         FIG. 5  is a schematic view for explaining water model tests performed using models of the immersion nozzle according to the embodiment of the present invention. 
         FIG. 6  shows a graph of the relationship between a/a′ and Δσ of the immersion nozzle according to the embodiment of the present invention. 
         FIG. 7  shows a graph of the relationship between b/b′ and Δσ of the immersion nozzle according to the embodiment of the present invention. 
         FIG. 8  shows a graph of the relationship between f/a′ and Δσ of the immersion nozzle according to the embodiment of the present invention. 
         FIG. 9  shows a graph of the relationship between e/e′ and Δσ of the immersion nozzle according to the embodiment of the present invention. 
         FIG. 10  shows a graph of the relationship between d/a′ and Lσ+Rσ 0  of the immersion nozzle according to the embodiment of the present invention. 
         FIG. 11A  is a view explaining a simulation model, used in fluid analysis, of the immersion nozzle according to the embodiment of the present invention. 
         FIG. 11B  is a view explaining a simulation model, used in fluid analysis, of an immersion nozzle according to prior art. 
         FIG. 12A  is a view showing the results of fluid analysis performed using the simulation model of the immersion nozzle according to the embodiment of the present invention, the flow rate being 4.0 m/min. 
         FIG. 12B  is a view showing the results of fluid analysis performed using the simulation model of the immersion nozzle according to the prior art, the flow rate being 4.0 m/min. 
         FIG. 13A  is a view showing the results of fluid analysis performed using the simulation model of the immersion nozzle according to the embodiment of the present invention, the flow rate being 4.4 m/min. 
         FIG. 13B  is a view showing the results of fluid analysis performed using the simulation model of the immersion nozzle according to the prior art, the flow rate being 4.4 m/min. 
         FIG. 14A  is a view showing the results of fluid analysis performed using the simulation model of the immersion nozzle according to the embodiment of the present invention, the flow rate being 4.8 m/min. 
         FIG. 14B  is a view showing the results of fluid analysis performed using the simulation model of the immersion nozzle according to the prior art, the flow rate being 4.8 m/min. 
         FIG. 15  is a cross sectional view of an immersion nozzle for continuous casting according to Japanese Unexamined Patent Application Publication No. 57-106456. 
         FIG. 16A  and  FIG. 16B  are cross sectional views of an immersion nozzle for continuous casting according to Japanese Unexamined Patent Application Publication No. 7-232247. 
         FIG. 17A  and  FIG. 17B  are cross sectional views of an immersion nozzle for continuous casting according to International Publication No. 2005/049249. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1A  shows an immersion nozzle  10  for continuous casting according to one embodiment of the present invention. Throughout the specification, the directions are set with the immersion nozzle  10  arranged upright. 
     The immersion nozzle  10  according to the present embodiment includes a tubular body  11  with a bottom  20 . The tubular body  11  has a cylindrical upper section  11   a , a lower section  11   c  of a depressed cross section, and a taper section  11   b  that is tapered when seen in side view and that connects the upper section  11   a  and the lower section  11   c . The upper section  11   a  has at the upper end an inlet  12  from which a passage  13  extends downward through the tubular body  11 . 
     The lower section  11   c  of a depressed cross section has opposing narrow sidewalls  18 ,  18  and opposing broad sidewalls  19 ,  19 . The narrow sidewalls  18 ,  18  have respectively opposing first outlets  14 ,  14  disposed at positions close to the bottom  20  so as to communicate with the passage  13 . The first outlets  14 ,  14  are vertically elongated slots. 
     The broad sidewalls  19 ,  19  have respectively opposing horizontal ridges  15 ,  15  that project from inner surfaces thereof into the passage  13  between the pair of first outlets  14 ,  14 . The ridges  15 ,  15  are of a substantially rectangular cross section. The term “substantially rectangular cross section” is intended to cover a rectangular cross section with rounded corners. When seen in a view showing the narrow sidewall  18  in front, the first outlet  14  is constricted in the middle. 
     The ridges  15 ,  15  reduce the excessive velocities of streams of molten steel in the lower portions of the first outlets  14 ,  14 , and also the ridges  15 ,  15  significantly reduce the amount of the molten steel that flows from a mold into the immersion nozzle  10  through the upper portions of the first outlets  14 ,  14 . Further, the ridges  15 ,  15  lower the maximum velocities of molten steel streams that impinge on the narrow sidewalls of the mold, and thus decreases the velocities of the reverse flows thereby to reduce the level fluctuation at the surface of the molten steel, providing more symmetric streams on the right- and left-hand sides of the immersion nozzle  10 . 
     The tubular body  11  has a pair of second outlets  16 ,  16  disposed in the bottom  20  so as to communicate with the passage  13 . The second outlets  16 ,  16  are arranged symmetrically about the axis of the tubular body  11  such that the axes  24 ,  24  of the respective second outlets  16 ,  16  cross each other within the passage  13 . The second outlets  16 ,  16  are in a truncated inverted V arrangement when the tubular body  11  is vertically cut along the broad sidewall of the lower section thereof. 
     In the immersion nozzle  10  according to the present embodiment, the first outlets  14 ,  14  are allowed to communicate with the second outlets  16 ,  16  by vertically extending slits  17 ,  17  disposed in the narrow sidewalls  18 ,  18 , respectively. 
     Water model tests were performed using models of the immersion nozzle  10  in order to determine the optimum configurations of the first outlets  14 ,  14 , the second outlets  16 ,  16 , and the slits  17 ,  17 . The water model tests performed will be described in the below. 
     Parameters used to determine the optimum configurations of the outlets and slits are denoted as follows. The horizontal width of the first outlets  14 ,  14  is denoted as a′, the vertical length of the first outlets  14 ,  14  is denoted as b′, the projection height of the ridges  15 ,  15  is denoted as a, and the vertical width of the ridges  15 ,  15  is denoted as b (See  FIG. 2 ). The length of the second outlets  16 ,  16  in a direction of the short side is denoted as f, the angle formed between each of the axes  24 ,  24  of the second outlets  16 ,  16  and the horizontal plane is denoted as α, the minimum internal measurement between the second outlets  16 ,  16  is denoted as e, and the width of the passage  13  in a direction of the long side immediately above the first outlets  14 ,  14  is denoted as e′ (See  FIG. 3 ,  FIG. 4B ). The width of the slits  17 ,  17  is denoted as d (See  FIG. 2 ,  FIG. 4B ). 
       FIG. 5  is a schematic view for explaining the water model tests. 
     A 1/1 scale mold  21  was made of an acrylic resin. The mold  21  was dimensioned such that the length of the long sides (in  FIG. 5 , in the left-right direction) was 1300 mm and that the length of the short sides (in  FIG. 5 , in a direction perpendicular to the paper surface) was 100 mm. Water was circulated through the immersion nozzle  10  and the mold  21  by means of a pump at a rate equivalent to a throughput of 4.4 m/min. 
     The immersion nozzle  10  was placed in the center of the mold  21  such that the long sides of the depressed cross section were parallel to the long sides of the mold  21 . Propeller-type flow speed detectors  22 ,  22  were installed 325 mm (¼ of the length of the long sides of the mold  21 ) off narrow sidewalls  23 ,  23 , respectively, of the mold  21  and 30 mm deep from the water surface. Then, the velocities of the reverse flows Fr, Fr were measured. 
     The results of the water model tests will be described below. For the tests, an envisaged basic model was dimensioned as follows. In each test, only a dimension serving as a target parameter was varied and the other dimensions were made to have the fixed values of corresponding dimensions of the basic model.
     Dimensions of the basic model: a=5 mm, a′=26 mm, b=25 mm, b′=115 mm, f=23 mm, e=26 mm, e′=143 mm, α=60°, d=10 mm   

       FIG. 6  shows a graph that represents the correlation between a/a′ and Δσ. Here, Δσ is a difference between standard deviations, of the velocities of the right- and left-hand reverse flows Fr, Fr, calculated using data obtained by measuring the velocities of the reverse flows Fr, Fr for three minutes by means of the flow speed detectors  22 ,  22 , as shown in  FIG. 5 . As Δσ increases, the difference becomes wider between the velocities of the right- and left-hand reverse flows Fr, Fr. In the present invention, either 4 cm/sec or 2 cm/sec was taken as the critical value of Δσ. When Δσ was less than 4 cm/sec, it was confirmed through visual observation in the water model tests that the discharge angles of the respective right- and left-hand exit-streams to the horizontal plane were substantially the same. When Δσ was less than 2 cm/sec, not only the discharge angles of the respective right- and left-hand exit-streams to the horizontal plane were substantially the same, but Karman vortexes did not occur which would have otherwise periodically generated between the broad sidewalls of the mold  21  and the immersion nozzle  10 . Karman vortexes induce local entrapment of mold powder, giving rise to problems. 
       FIG. 6  indicates that Δσ was 2 cm/sec or less when a/a′ ranged from 0.1 to 0.25, and that the exit-streams in the mold were balanced. When a/a′ was less than 0.1, the ridges did not fully exhibit the effect of interrupting the flow, and the exit-streams in the lower portions of the first outlets had excessive velocities, to make the right- and left-hand streams in the mold  21  extremely asymmetric. On the other hand, when a/a′ was beyond 0.25, the exit-streams in the lower portions of the first outlets had slightly too low velocities, namely, the exit-streams in the upper portions of the first outlets had excessive velocities, to increase the velocities of the reverse flows Fr, Fr at the water surface in the mold  21 , thereby causing adverse effects such as entrapment of mold powder. 
       FIG. 7  shows the correlation between b/b′ and Δσ.  FIG. 7  indicates that Δσ was 4 cm/sec or less when b/b′ ranged from 0.15 to 0.35. When b/b′ was less than 0.15, the ridges did not fully exhibit the effect of interrupting the flow, and the exit-streams in the lower portions of the first outlets had excessive velocities, to form extremely asymmetric right- and left-hand streams in the mold  21 . On the other hand, when b/b′ was beyond 0.35, the exit-streams in the lower portions of the first outlets had slightly too low velocities, namely, the exit-streams in the upper portions of the first outlets had excessive velocities, to increase the velocities of the reverse flows Fr, Fr at the water surface in the mold  21  and to give adverse effects such as entrapment of mold powder. It is desirable to dispose the ridges at positions to divide the first outlets each into two equal portions vertically arranged in order to balance the velocities of the exit-streams from the lower portions of the first outlets and the velocities of the exit-streams from the upper portions of the first outlets. 
       FIG. 8  shows a graph that represents the correlation between f/a′ and Δσ.  FIG. 8  indicates that Δσ was 2 cm/sec or less when f/a′ ranged from 0.75 to 0.9. When f/a′ was less than 0.75, the width f of the second outlets  16 ,  16  was too small relative to the length a′ of the first outlets  14 ,  14 , and thus insufficient amounts of the exit-streams were discharged from the second outlets to result in excessive velocities of the reverse flows Fr, Fr at the water surface in the mold  21 , thereby causing adverse effects such as entrapment of mold powder. On the other hand, when f/a′ was beyond 0.9, excessive amounts of the exit-streams were discharged from the second outlets, namely, insufficient amounts of the exit-streams were discharged from the first outlets, to make the entire flow in the mold  21  unstable. This results in the level fluctuation at the water surface and the asymmetric right- and left-hand streams in the mold  21 . 
       FIG. 9  shows a graph that represents the correlation between e/e′ and Δσ.  FIG. 9  indicates that Δσ was 4 cm/sec or less when e/e′ ranged from 0.1 to 0.17. When e/e′ was less than 0.1, excessive amounts of the exit-streams were discharged from the second outlets, and insufficient amounts of the exit-streams were discharged from the first outlets, to make the entire flows in the mold  21  unstable. This results in the level fluctuation at the water surface and the asymmetric right- and left-hand streams in the mold  21 . On the other hand, when e/e′ was beyond 0.17, the length of the second outlets  16 ,  16  was too short relative to the width e′ of the passage  13 , and thus insufficient amounts of the exit-streams were discharged from the second outlets, which caused excessive velocities of the reverse flows Fr, Fr at the water surface in the mold  21 , thereby causing adverse effects such as entrapment of mold powder. 
     Though there is no presentation in the drawings on the test results about the angle α formed between each of the axes of the second outlets  16 ,  16  and the horizontal plane, it was confirmed that Δσ was minimum when α was 40° to 60°. When α was less than 40°, the exit-streams from the second outlets were synchronized with the exit-streams from the first outlets to increase the velocities of the reverse flows Fr, Fr at the water surface in the mold  21 , thereby causing adverse effects such as entrapment of mold powder. Further, since the dimensions of the second outlets were relatively decreased, the exit-streams from the second outlets had increased velocities to raise the velocities of the reverse flows Fr, Fr and thereby to extremely increase the level fluctuation at the water surface. On the other hand, when α was beyond 60°, the exit-streams from the pair of second outlets joined together to make a flow that wandered unstably like a pendulum, resulting in Δσ of beyond 4 cm/sec, which was not desirable. 
       FIG. 10  shows a graph that represents the correlation between d/a′ and Lσ+Rσ. In this graph, Lσ is a standard deviation of the velocity of the left-hand reverse flow Fr; Rσ is a standard deviation of the velocity of the right-hand reverse flow Fr; and Lσ+Rσ is the sum of the standard deviations of the velocities of the right- and left-hand reverse flows Fr, Fr. Throughout the tests performed, all the values of Δσ obtained were below 2 cm/sec, and thus Lσ+Rσ was used as an evaluation criterion.  FIG. 10  indicates that Lσ+Rσ was 30 cm/sec or less when d/a′ ranged from 0.2 to 1. When d/a′ was less than 0.2, the reverse flows Fr, Fr had excessive velocities to cause adverse effects such as entrapment of mold powder. On the other hand, there occurred problems such as cracks at the lower end of the immersion nozzle due to strength poverty when d/a′ was beyond 1. 
     A description will be made regarding the fluid analyses on the amounts of exit-streams from the immersion nozzle for continuous casting according to the embodiment of the present invention and those from an immersion nozzle according to prior art. 
     The fluid analyses were performed by using FLUENT (fluid analysis software) manufactured by Fluent Asia Pacific Co., Ltd (i.e., ANSYS Japan K.K. at present).  FIGS. 11A and 11B  show simulation models used for the fluid analyses.  FIG. 11A  shows a simulation model of the nozzle according to the embodiment of the present invention, while  FIG. 11B  shows a simulation model of a nozzle according to prior art.  FIGS. 12A ,  13 A and  14 A show the results of fluid analyses performed using the model shown in  FIG. 11A , while  FIGS. 12B ,  13 B and  14 B show the results of fluid analyses performed using the model shown in  FIG. 11B . The model according to the prior art includes a tubular body having a passage inside and depressed in cross section at least at a lower section thereof. In this model, a pair of first opposing outlets are disposed in the narrow sidewalls of the lower section and communicate with the passage, and a second outlet which communicate with the passage is formed in the bottom of the tubular body in a manner to fully open the bottom. Table 1 presents the parameters of each simulation model. 
     The analyses were performed on the assumption that the mold was 1300 mm long and 100 mm wide; the throughputs were 4.0 m/min ( FIG. 12A ,  FIG. 12B ), 4.4 m/min ( FIG. 13A ,  FIG. 13B ) and 4.8 m/min ( FIG. 14A ,  FIG. 14B ); and the nozzle immersion depth was 303 mm. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Embodiment 
                   
               
               
                 Parameter 
                 of Present Invention 
                 Prior Art 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
            
               
                 a/a′ 
                 0.19 
                 — 
               
               
                 b/b′ 
                 0.20 
                 — 
               
               
                 f/a′ 
                 0.88 
                 — 
               
               
                 e/e′ 
                 0.14 
                 1.00 
               
               
                 α 
                 55° 
                 — 
               
               
                 d/a′ 
                 0.4 
                 — 
               
               
                   
               
            
           
         
       
     
       FIGS. 12A ,  12 B,  13 A,  13 B,  14 A, and  14 B present the results of the analyses. These figures indicate the following. 
     In the case of the immersion nozzle according to the prior art, the right- and left-hand streams were asymmetric and the reverse flows had high velocities, causing the risk of the entrapment of mold powder and the level fluctuation at the molten steel surface. On the other hand, in the case of the immersion nozzle according to the embodiment of the present invention, the right- and left-hand streams were substantially symmetric and the reverse flows had velocities in a desirable range to reduce the level fluctuation at the molten steel surface and to improve the quality and productivity of the slabs. 
     While preferred embodiments of the invention have been described and shown above, it should be understood that these are exemplary of the invention and are not to be considered as limiting. Additions, omissions, substitutions, and other modifications can be made without departing from the spirit or scope of the present invention. Accordingly, the invention is not to be considered as being limited by the foregoing description, and is only limited by the scope of the appended claims.