Patent Publication Number: US-2002005267-A1

Title: Electromagnetic braking device for continuous casting mold and method of continuous casting by using the same

Description:
TECHNICAL FIELD  
       [0001] The present invention relates to magnetic or solenoid brake apparatuses for continuous casting molds and continuous casting methods using the same. The present invention particularly relates to a magnetic brake apparatus for a continuous casting mold which is suitably applied when a static magnetic field is generated in molten steel in a mold used in continuous casting to control the flow of the molten steel, and to a continuous casting method using the same.  
       BACKGROUND ART  
       [0002] In general, in continuous casting of slabs, molten steel reserved in a tundish is introduced into a continuous casting mold via an sub-entry nozzle connected to the bottom of the tundish, although no drawing is shown. In this case, the flow rate of the molten steel discharged from the discharging opening of the sub-entry nozzle is significantly higher than the casting rate. Thus, when inclusions or/and bubbles in the molten steel are deeply penetrated and captured by solidified shells, these inevitably cause defects of the product. When the upward flow is dominant in the jet stream of the molten steel, the rise of the mold meniscus promotes fluctuation of the melt surface, resulting in adverse effects on the slab quality and casting operation.  
       [0003] In order to avoid such a problem, for example, Japanese Patent Laid-Open No. 3-142049 discloses a continuous casting technology for preventing the occurrence of the above-mentioned problem, in which a static magnetic field is applied to the molten steel in the casting mold to brake the flow of the molten steel in the casting mold.  
       [0004]FIG. 6A is a cross-sectional view of a main portion of a casting apparatus disclosed in the above-mentioned patent, and FIG. 6B is an enlarged longitudinal cross-sectional view of a part of FIG. 6A. In the drawings, numeral  101  represents a continuous casting mold comprising a pair of short side walls  101 A and a pair of long side walls  101 B, its inside being cooled by water. Numeral  102  represents an sub-entry nozzle for supplying the molten steel from a tundish (not shown in the drawing) to the casting mold  101 . Numerals  103 A and  103 B represent iron core bodies for forming a magnetic path. Numerals  104 A,  104 B,  105 A and  105 B represent upper and lower magnetic poles (iron cores) which are connected to the iron core bodies  103 A and  103 B and extend along the width direction of the casting mold  101 . Numeral  106  represents a magnetic field controlling means for controlling the intensity of the static magnetic field generated between the magnetic poles. The magnetic field controlling means  106  comprises a bracket  107  fixed to a support, a bracket  108  fixed to the iron core body  103 B, a pivot pin connecting the two brackets  107  and  108 , and a hydraulic cylinder  110  fixed to the support in which the tip of the rod is engaged with the iron core body. Numeral  102 B in the drawings represents a discharging opening of the sub-entry nozzle  102 .  
       [0005] When the upper magnetic pole  104 A at the left or A side in FIG. 6A is; an N pole and the upper magnetic pole  104 B at the B side is an S pole in the continuous casting mold  101 , an A-to-B magnetic field is generated in the upper magnetic pole whereas a B-to-A magnetic field is generated in the lower magnetic pole. When molten steel is supplied into such a magnetic field, the upward flow is decelerated by the upper magnetic field while the downward flow is decelerated by the lower magnetic field. When the intensity of the static magnetic field is modified between the upper magnetic pole and the lower magnetic pole in the casting mold  101 , the hydraulic cylinder  110  is operated by the magnetic field controlling means  106  so that the iron core body rotates around the pivot pin  109  to change the inter-pole distance of the upper magnetic poles.  
       DISCLOSURE OF THE INVENTION  
       [0006] In the technology disclosed in the above-mentioned patent, a position sensor for exactly adjusting the distance, in addition to the hydraulic: cylinder  110  and the pivot pin  109 , is required. Thus, a wide space and many devices are required for a facility for adjusting the intensity of the static magnetic field. The patent also discloses another method for adjusting the magnetic field in which a nonmagnetic material is inserted in a part of the iron core. This method, however, has disadvantages, that is, the type, width of the slab and the intensity of the magnetic field in response to the casting speed cannot be changed without limitation in the casting process. Since exchange of the nonmagnetic material requires long periods of time, operation efficiency is significantly low.  
       [0007] The present invention has been accomplished for solving these problems, and it is a first object to provide a technology which can readily change the intensity of the magnetic field during casting without expensiveness and limitation.  
       [0008] It is a second object of the present invention to produce a high-quality cast product by achieving the first object. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0009]FIG. 1 is a cross-sectional view of a main portion which illustrates an outlined configuration of an embodiment in accordance with the present invention.  
     [0010]FIG. 2 is a schematic view of a combination of poles of magnetic fields.  
     [0011]FIG. 3 is a line graph illustrating the quality of a slab prepared in an example.  
     [0012]FIG. 4 is another line graph illustrating the quality of a slab prepared in an example.  
     [0013]FIG. 5 is a cross-sectional view of a main portion which illustrates an outlined configuration of another embodiment in accordance with the present invention.  
     [0014]FIG. 6 is an outlined cross-sectional view of a conventional casting mold.  
     [0015]FIG. 7 is a cross-sectional view of a main portion which illustrates an outlined configuration of another embodiment in accordance with the present invention.  
     [0016]FIG. 8 is a schematic view of another combination of poles of magnetic fields.  
     [0017]FIG. 9 is a schematic view of another combination of poles of magnetic fields. 
    
    
     REFERENCE NUMERALS  
     [0018] 10  continuous casting mold  
     [0019] 12  sub-entry nozzle  
     [0020] 14 A upper iron core at the free side  
     [0021] 14 B upper iron core at the fixed side  
     [0022] 16 A upper coil at the free side  
     [0023] 16 B upper coil at the fixed side  
     [0024] 17 A first upper electromagnet  
     [0025] 17 B second upper electromagnet  
     [0026] 18 A lower iron core at, the free side  
     [0027] 18 B lower iron core at the fixed side  
     [0028] 20 A lower coil at the free side  
     [0029] 20 B lower coil at the fixed side  
     [0030] 21 A first lower electromagnet  
     [0031] 21 B second lower electromagnet  
     [0032] 22 A connecting iron core  
     [0033] 22 B connecting iron core  
     [0034] 24 A current controlling unit  
     [0035] 24 B current controlling unit  
     [0036] 24 C current controlling unit  
     [0037] 24 D current controlling unit  
     [0038] Sm molten steel  
     BEST MODE FOR CARRYING OUT THE INVENTIOIN  
     [0039] The embodiments of the present invention will now be described in detail with reference to the drawings.  
     [0040]FIGS. 1 and 7 are cross-sectional views of a main portion illustrating outlined configurations of embodiments in accordance with the present invention. The magnetic brake apparatus in these embodiments in accordance with the present invention is applied to a continuous casting mold shown by reference numeral  10  in the drawings. The continuous casting mold  10  is substantially the same as that shown in FIG. 6. Cooling water circulates through the interior of the side wall, and molten steel Sm is supplied to the continuous casting mold  10  through a discharging opening (not shown in the drawings) of an sub-entry nozzle  12 . The magnetic brake apparatus in these embodiments has a first upper electromagnet  17 A comprising an upper iron core  14 A which is placed near the rear face of the side wall of the continuous casting mold  10  at the free side (the left side in the drawings) and lies slightly above the discharging opening of the sub-entry nozzle  12 , and an upper magnetic coil  16 A wound around the electromagnet; and a second upper electromagnet  17 B at the fixed side (the right side in the drawings) in the position of the same height comprising an upper iron core  14 B and an upper magnetic coil  16 B. The first and second upper electromagnets  17 A and  17 B are oppositely placed with the continuous casting mold  10  intervening therebetween.  
     [0041] In FIG. 1, a first lower electromagnet  21 A at the free side comprising a lower iron core  18 A and a lower magnetic coil  20 A, and a second lower electromagnet  21 B at the fixed side comprising a lower iron core  18 B and a lower magnetic coil  20 B are provided below the upper electromagnet. These two electromagnets  21 A and  21 B are also oppositely placed. The upper iron cores  14 A and  14 B and the lower iron cores  18 A and  18 B are integrally formed with connecting iron cores  22 A and  22 B provided therebetween, and are magnetically connected to each other. In this embodiment, a current is supplied to these two upper magnetic coils  16 A and  16 B being constituents of the first and second upper electromagnets through an upper current controlling unit  24 A, and independently, a current is supplied to these two lower magnetic coils  20 A and  20 B being constituents of the first and second lower electromagnets through a lower current controlling unit  24 B. These currents are independently controllable.  
     [0042] That is, a current of a given ampere is applied to the two upper magnetic coils  16 A and  16 B, whereas a current of another ampere is applied to the two lower magnetic coils  20 A and  20 B. The intensities of the static magnetic fields between the upper electromagnets  17 A and  17 B and between the lower electromagnets  21 A and  21 B are independently adjustable.  
     [0043] In FIG. 7, a first lower electromagnet  21 A at the free side comprising a lower iron core  18 A and a lower magnetic coil  20 A and a second lower electromagnet  21 B at the fixed side comprising a lower iron core  18 B and a lower magnetic coil  20 B are provided below the upper electromagnets. These two electromagnets are also oppositely placed. The upper iron cores  14 A and  14 B and the lower iron cores  18 A and  18 B are integrally formed with connecting iron cores  22 A and  22 B provided therebetween and are magnetically connected to each other. Different currents are independently supplied to the four magnetic coils  16 A,  16 B,  20 A and  20 B through current controlling units  24 A to  24 D.  
     [0044] The operation of the embodiments will now be described.  
     [0045] In FIG. 1, when normal static magnetic fields are generated at the upper and lower portions, two current controlling units  24 A and  24 B independently control the currents for the upper electromagnets  17 A and  17 B and the lower electromagnets  21 A and  21 B. Thus, as shown in the relationship of the magnetic poles of the upper and lower electromagnets in FIG. 2, when the upper magnetic pole at the free side is an S pole, the opposing upper magnetic pole at the fixed side is an N pole, the lower magnetic pole at the free side is an N pole, and the lower magnetic pole at the fixed side is an S pole. That is, poles opposing each other across the molten steel and the upper and lower poles on the same side are different from each other. In this embodiment, in order to prevent capture of mold powder at the meniscus section of the molten steel, the upper magnetic field may be enhanced to moderate the fluctuation of the molten surface. In order to prevent penetration of nonmetallic inclusions into the deep interior of the molten steel, the lower magnetic field may be lowered to suppress the downward flow of the molten steel in the casting mold. The upper and lower electromagnets can appropriately control the intensities of the magnetic fields to adequately control the flow of the molten steel depending on the purposes.  
     [0046] Thus, the quality of the cast slab is improved by casting while adequately controlling the intensities of the static magnetic fields by the upper and lower electromagnets in response to the width and type of the slab and the casting speed using the magnetic brake apparatus of this embodiment.  
     [0047] In FIG. 7, when normal static magnetic fields are generated at the upper and lower portions, the four current controlling units  24 A to  24 D independently control the currents for the corresponding electromagnets. Thus, as shown in the relationship of the magnetic poles of the upper and lower electromagnets in FIG. 2, poles opposing each other across the molten steel and the upper and lower poles on the same side are different from each other. In this case, the most effective results are achieved when the currents of the magnetic coils for the opposing poles are the same. In order to prevent capture of mold powder at the meniscus section of the molten steel, the upper magnetic field may be enhanced to moderate the fluctuation of the molten surface. In order to prevent penetration of nonmetallic inclusions into the deep interior of the molten steel, the lower magnetic field may be lowered to suppress the downward flow of the molten steel in the casting mold.  
     [0048] In conventional apparatuses, it is impossible to make the intensity of the upper or lower magnetic field zero even when the current to the magnetic coil is zero, because the upper and lower iron cores are magnetically connected to each other through the connecting iron core. In contrast, in this embodiment, the direction of the current of one magnetic coil between the two opposing electrodes is inverted by the current controlling units  24 A to  24 D so that the opposing magnetic poles are the same as shown in FIGS. 8 and 9. The intensity of the magnetic field thereby becomes zero.  
     [0049] Thus, in order to prevent inclusion of non-metallic impurities into the solid shell at the meniscus section for the purpose of securing the quality below the skin rather than capture of powder by the fluctuation of the molten surface, or in order to prevent capture of bubbles of argon gas blown into the steel so that the discharging opening of the sub-entry nozzle is not clogged, a magnetic field of zero between the upper electromagnets is effective when the flow of the molten steel is required at the meniscus section. This embodiment can readily perform such a control.  
     EXAMPLE  
     [0050] An example of the embodiment will now be described.  
     [0051] Continuous casting was performed under the following conditions using a mold having a magnetic brake apparatus in accordance with the embodiment shown in FIG. 1 or  7  to produce a cast slab of low-carbon aluminum-killed steel. Its surface and internal quality was examined. FIG. 3 shows the results when the intensity of the lower magnetic field was fixed to 2,400 gauss while the intensity of the upper magnetic field was varied. On the other hand, FIG. 4 shows the results when the intensity of the upper magnetic field was fixed to 2,500 gauss.  
                               [Casting Conditions]                                                    Casting speed:   2.5   m/min           Width of slab:   1,400   mm           Thickness of slab   220   mm                      
 
     [0052] Intensity of lower magnetic field: 2,000 to 3,000 gauss  
     [0053] Intensity of upper magnetic field: 2,000 to 3,000 gauss  
     [0054] The results shown in FIGS. 3 and 4 illustrate that adjustment of the intensity of the magnetic field in response to the operational conditions is significantly effective.  
     [0055] As described above, since the flow of the molten steel can be appropriately controlled in the casting mold in this embodiment, inclusion of non-metallic impurities into the molten steel pool by the jet stream of the molten steel and capture of mold powder into the molten steel by the fluctuation of the molten surface at the meniscus section are prevented. Accordingly, a high-quality slab can be produced with high efficiency.  
     [0056] Another embodiment in accordance with the present invention will now be described.  
     [0057]FIG. 5 is a cross-sectional view, which corresponds to FIG. 1, of an outlined configuration of a magnetic brake apparatus in accordance with the present invention. The magnetic brake apparatus in this embodiment has no connecting iron cores  22 A and  22 B, shown in FIG. 1, for magnetically connecting the upper and lower iron cores at the free and fixed sides, and thus upper and lower iron cores  14 A,  14 B,  18 A and  18 B are magnetically independent of each other. Other configurations are substantially the same as those in the first embodiment.  
     [0058] Since the upper and lower iron cores at the same side are not magnetically connected to each other in this embodiment, the input current generates a magnetic field with a lower intensity than that in the above-mentioned embodiment. Similar control can, however, be performed and the static magnetic field of either the upper or lower electromagnet: can be set to be near zero.  
     [0059] Although the present invention has been described in detail, the present invention is not limited to the above-mentioned embodiments and includes various modifications within a scope without departing from the gist of the present invention.  
     INDUSTRIAL APPLICABILITY  
     [0060] According to the present invention as described above, the intensity of the magnetic field between the magnetic poles of the upper and lower electromagnets can be readily and inexpensively varied during casting without restriction.