Abstract:
An apparatus for providing a magnetic field in a casting mold to slow and redirect in a controllable fashion the flow of liquid steel exiting from a submerged entry nozzle into the casting mold uses selectable removable ferromagnetic and non-magnetic laminar elements stackable on the ends of core fingers in the vicinity of the poles of an electromagnetic yoke positioned adjacent the mold face. By selecting the type and location of the stackable elements on the ends of the fingers, one can modify the properties of the magnetic field permeating the interior of the mold. Optionally, independent field coils may be provided for energizing selected portions of the magnetic field core structure to provide further magnetic field control without having to remove and replace laminar elements.

Description:
RELATED APPLICATION  
       [0001]    This is a continuation-in-part application of application Ser. No. 09/108,466 filed on Jul. 1, 1998. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    The present invention relates to a magnetic field apparatus for controlling the flow of molten steel in a casting mold, and more particularly to an apparatus for providing an adjustable magnetic field in a casting mold to impede and redirect in a controllable fashion the flow of liquid steel exiting from a submerged entry nozzle that discharges into the casting mold.  
         BACKGROUND  
         [0003]    It is known in the art of steelmaking to continuously cast molten steel using an oscillating mold, typically a water-cooled copper-faced mold having a straight or curved channel. The mold typically has a rectangular horizontal cross-sectional forming conduit as thick and wide as the slab to be cast. Liquid steel in the upper portion of the mold is cooled as it moves downward through the water cooled mold, generating a steel shell as it passes through the mold before exiting the mold at the bottom. The molten steel enters the mold from a tundish through an entry nozzle submerged in the liquid steel in the mold. The submerged entry nozzle is normally located generally centrally of the mold cross-section, and is provided with opposed exit ports that direct liquid steel generally horizontally outwardly toward the narrow sides of the mold. Some nozzles have a bottom port as well.  
           [0004]    The flow of liquid steel out of the submerged entry nozzle varies in direction and velocity due to various external conditions (such as the ferrostatic head of steel above the nozzle, and steel chemistry). This can create disturbances in the steel flow that adversely affect both the surface quality and internal quality of the casting. These disturbances tend to generate undesired temperature imbalances that interfere with uniform solidification of the steel as it passes through the mold and downstream thereof, and also increase the tendency of the steel to incorporate unwanted inclusions from the mold powder/slag/impurities mixture at the meniscus of the liquid steel at the top of the mold. A conventional magnetic brake inhibits these disturbances by reducing the velocity of liquid steel emanating from the submerged entry nozzle, thereby tending to constrict the eddies and prevent them from reaching the end edges of the mold and the upper surface of the pool of liquid steel at the top of the mold.  
           [0005]    A conventional magnetic brake includes a magnetic circuit energized by direct or slowly varying electric current passing through windings around an iron core forming part of the magnetic circuit. The magnetic circuit passes through the wide faces of the mold so as to provide a magnetic field through the interior of the mold. Normally, in a conventional magnetic brake, the magnetic circuit passes through the mold about mid-way along the longitudinal length of the mold and overlaps the point of entry of liquid steel into the mold from the submerged entry nozzle, but does not extend up to the top of the liquid steel pool nor down to the bottom of the mold.  
           [0006]    Although the magnetic field in a conventional magnetic brake can be varied (by varying the amount of current flowing through the windings around the iron core of the magnetic circuit) there is, nevertheless, typically no fine control over the manner in which the magnetic field is applied. Such fine control would improve the ability to control the flow characteristics of the steel as it exits from the submerged entry nozzle, in the interest of generating uniform solidification of the shell of cast steel emerging from the mold and in the interest of reducing unwanted inclusion and non-uniform surface effects.  
           [0007]    Attempts have been made by various prior workers in the field to provide some variation in the magnetic field applied through the mold.  
           [0008]    Representative such attempts are disclosed, for example, in U.S. Pat. No. 5,404,933, issued Apr. 11, 1995 to Andersson et. al. (the Andersson patent), and U.S. Pat. No. 5,613,548 issued Mar. 25, 1997 to Streubel et. al. (the Streubel patent). The Andersson patent discloses an apparatus for controlling the flow of molten metal by applying a static or periodic low-frequency magnetic field across the area through which the molten metal flows. The Streubel Patent discloses an apparatus that accomplishes a similar result by attaching partial cores to a principal core surrounded by an electrical core, thereby influencing the magnetic field applied.  
         SUMMARY OF THE INVENTION  
         [0009]    The present invention is directed generally to an apparatus for providing a magnetic field in molten steel inside a mold for casting molten steel, which magnetic field can be reconfigured so as to modify the flow characteristics of molten steel exiting from a submerged entry nozzle in the mold both by the use of (1) removable ferromagnetic or non-magnetic laminar elements positioned in the magnetic circuit adjacent the mold face, to accommodate changes in the chemistry and other characteristics of the steel to be cast in the mold, or (2) discrete individually energizable coils in the magnetic circuit during the casting of molten steel, in response to changing conditions in the molten steel, or both. It is contemplated that a suitable selection of ferromagnetic and non-magnetic laminar elements in a matrix array immediately adjacent the mold face will accommodate the more major and persistent changes in steel characteristics (e.g., steel chemistry), while the use of the individually engergizable coils (which may also be arranged in a matrix array adjacent the array of laminar elements) is intended to accommodate transient variations in the characteristics of the molten steel (e.g., ferrostatic head).  
           [0010]    In the aspect of the present invention directed to providing a magnetic field that may be reconfigured between casting runs, there is provided a pair of magnetic poles comprising at least a pair of magnetic field cores, each core being energized by at least one discrete coil located in the vicinity of a discrete opposed wide face of the mold. The cores are connected by a yoke so that the cores and the yoke together with the mold containing molten steel form a complete magnetic circuit. When the coils are energized, the magnetic field extends generally horizontally from one wide face of the mold to the other. Each magnetic field core has one or more horizontal rows of generally horizontally disposed closely packed “fingers” in proximity to the proximate wide face of the casting mold. (The term “fingers” is used herein to identify a physically discrete projecting portion of the core adjacent the mold face, but it is to be understood that spaces between fingers is undesirable, although frequently necessary because of the need to accommodate opposed projections such as strengthening ribs on the surface of the mold.) The fingers protrude from the ends of their respective cores in two parallel, generally symmetrical generally horizontal arrays, each array abutting a respective face of the mold. (While the benefit of the invention as contemplated by the inventor is best obtained by having two generally identical matching arrays of fingers, one on either side of the mold, there may be circumstances in which the arrays are chosen not to be identical, or the fingers are provided on one side of the mold only.) The individual fingers in each array may abut one another, or some fingers may be slightly spaced apart so as to avoid interfering with other structural elements in the vicinity of the mold faces.  
           [0011]    The fingers are comprised of removable ferromagnetic laminar elements and optionally spacers or non-magnetic laminar elements. These laminar elements for each finger are arrangeable in a vertically stacked array extending into proximity with the proximate wide mold face at a selected location. For continuity of the magnetic circuit, each finger should be positioned as close as possible to the adjacent mold face. The local magnetic field in the molten steel in the casting mold near each finger (each selected location) may be varied independently of the local magnetic field in the molten steel in the casting mold near the other selected locations by the addition or removal (effected between casting runs) of ferromagnetic or non-magnetic laminar elements to or from selected fingers, so as to modify flow characteristics of molten steel exiting from the submerged entry nozzle into the casting mold during casting runs. As it is desirable to have a generally uniform magnetic field across the entire transverse width of the array of fingers, fingers near the center of the array may have fewer ferromagnetic laminar elements attached than do fingers at the periphery of the array, to compensate for the natural tendency of the magnetic field to be stronger in the center. It may also be desirable to substitute non-magnetic laminar elements for ferromagnetic laminar elements in portions of the central fingers, or to provide spacers between selected successive ferromagnetic laminar elements, thereby creating air gaps in the magnetic field that serve essentially the same function as non-magnetic laminar elements.  
           [0012]    To increase the degree of control of the magnetic field in the vertical direction, more than one horizontal array of fingers may be provided on each side of the mold face, or the capacity of each finger to accept laminar elements may be increased so that the vertical span of each finger is increased. If the first alternative is selected, additional rows of generally horizontally disposed closely packed fingers may be stacked vertically, creating a two-dimensional matrix of fingers, the amount and position of magnetic material in each finger being determined by selectively stacking ferromagnetic and nonmagnetic laminar elements. It may be desirable to provide an increased capacity to apply a magnetic field over the vertical dimension, such as by increasing the number of energizing coils and arranging them in a corresponding two-dimensional matrix, so as to accommodate any changes in the magnetic field distribution that the operator wishes to make.  
           [0013]    Another aspect of the present invention is the provision of a magnetic field that may be reconfigured during casting. In this aspect, the magnetic field is created by a number of opposed pairs of magnetic field cores, each of which cores is energized by a discrete energizing coil. One core in each pair is located on one side of the wide face of the mold and its mating core on the other side of the mold directly opposite the first core. The terminal faces of each pair of opposed cores comprise poles of a component magnetic circuit, the overall magnetic circuit for the electromagnetic brake comprising the total of the component magnetic circuits. Each core is coupled within the magnetic circuit by an encircling yoke made from a magnetic material. A discrete individually controllable electrical current may be passed through each coil. When the mold contains molten steel, a composite magnetic circuit is formed, each component of which passes through one core of one discrete pair of cores, the yoke, the other core of that pair of cores, and the adjacent selected portion of the mold and the molten steel contained therein, so that when the coils are energized, the magnetic field extends from one wide face of the mold to the other. The local magnetic field in any one of the selected portions of the mold may be varied by varying the electrical currents passing through the pairs of coils associated with the pairs of magnetic field cores near that selected portion of the mold, so as to modify flow characteristics of molten steel exiting from the submerged entry nozzle into the casting mold. As each component magnetic circuit pole is provided with a discrete energizing coil, each pole pair may be energized independently of the other pole pairs, thereby providing control of the local magnetic field in the molten steel in the casting mold during casting.  
           [0014]    In this further aspect of the invention each coil preferably energizes a portion of the core associated with at least one discrete finger having removable ferromagnetic and non-magnetic laminar elements. Note that the array of pole pairs and counterpart array of energizing coils may desirably correspond to the array of fingers, but need not do so.  
           [0015]    The cores, including at least some of the removable ferromagnetic laminar elements, and the yoke should be made of iron or an alloy chiefly composed of iron. The removable non-magnetic laminar elements may be made of a heat resistant ceramic material. The ferromagnetic and nonmagnetic laminar elements may be stackable rectangular parallelepiped plates, and they may be of varying heights and widths. If desired, some of the laminar elements may be dimensioned to span more than one finger. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]    In the drawings, which illustrate embodiments of the invention:  
         [0017]    [0017]FIG. 1 is a schematic bottom isometric view of an apparatus suitable for embodying a magnetic brake in conformity with the present invention.  
         [0018]    [0018]FIG. 2 is a simplified schematic plan view of one magnetic pole of the apparatus of FIG. 1 and an associated casting mold.  
         [0019]    [0019]FIG. 3 is a schematic end elevation section view of a finger of the magnetic pole of FIG. 2 taken along the line  3 - 3  of FIG. 2, illustrating a vertically stackable series of removable plates (laminar elements) in conformity with one aspect of the invention.  
         [0020]    [0020]FIG. 4 is a schematic side elevation section view of a finger of the magnetic pole of FIG. 2 taken along the line  4 - 4  of FIG. 2, and illustrating the vertically stackable series of removable plates seen also in FIG. 3, in conformity with one aspect of the invention.  
         [0021]    [0021]FIG. 5 is a schematic side elevation section view of a finger of the magnetic pole of FIG. 2 taken along the line  4 - 4  of FIG. 2, and illustrating an alternative embodiment of the vertically stackable series of removable plates wherein the fixed end piece of the illustrated finger is replaced by a removable end piece.  
         [0022]    [0022]FIG. 6 is a schematic isometric view of one polar finger array of an embodiment of the present invention showing stackable laminar elements spanning more than one finger, in conformity with one aspect of the invention.  
         [0023]    [0023]FIG. 7 is a schematic plan view of one polar array of a multipole variant of an apparatus embodying the present invention, illustrating the multiple energizing coil feature of one aspect of the invention.  
         [0024]    [0024]FIG. 8 is a schematic isometric view of a multipolar array of a partial embodiment of a magnetic brake according to an embodiment of the invention that combines options illustrated in preceding figures.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    A magnetic field apparatus embodying the present invention is generally indicated by numeral  10  in FIG. 1. Apparatus  10  is comprised of two magnetic cores  12 , each surrounded by a discrete coil  14 . The cores  12  are connected together by a yoke  15  leaving a gap  25  for a casting mold (not shown in FIG. 1, but discussed below). In use, the casting mold and liquid steel in it complete a magnetic circuit including the yoke  15  and the cores  12 .  
         [0026]    On either side of the gap  25 , the cores  12  are split into separate fingers, which are indicated generally by reference numeral  16 . Ideally there would be no space between the fingers  16 , and the fingers  16  would come into close proximity with the casting mold, so that with the mold in place receiving liquid steel, there would be two minimal gaps in the magnetic circuit.  
         [0027]    [0027]FIG. 1 illustrates a pair of discrete magnetic poles  11  each comprised of one core  12  surrounded by an associated coil  14  and ending in fingers  16 . In FIG. 2, one of the magnetic poles  11  of the apparatus  10  is shown close to one wide face of a casting mold  24  having a mold cavity  26  and a submerged entry nozzle  28 . The end of the magnetic core  12  close to the casting mold  24  is split into several protruding fingers  16  which are shown in further detail in FIGS. 3 and 4. As discussed above, the empty horizontal spacing between the fingers  16  could be eliminated where possible. The spacing is needed only when there are obstructions associated with the external water jacket and any other structural features (not shown) of the mold itself which must pass between the magnetic core  12  and the casting mold  24 . One such possible structural feature is one or more strengthening ribs (not shown) that extend down the the wide faces of the mold. Such ribs can be accommodated by insetting the appropriate fingers relative to such ribs. By way of example, the centralmost pair of fingers is inset relative to the other fingers shown in FIG. 1. In FIG. 2, the schematically uniform spacing between the fingers  16  is shown for ease of illustration only.  
         [0028]    The vertical position of the yoke relative to the mold is determined by the operator, taking into account factors such as the ferrostatic head of liquid steel above the submerged entry nozzle  28 , the expected wear on the submerged entry nozzle  28 , the size of the mold  26 , and the chemical and physical properties of the steel.  
         [0029]    In the embodiment illustrated in FIGS. 3 and 4, each finger  16  has a fixed lowermost end piece  20  which is an extension of the magnetic core  12 . Each fixed end piece  20  is provided with bores  17  threaded for receiving bolts  18 . Removable upper end pieces (stackable laminar elements)  22  in the form of relatively small rectangular parallelepiped plates made from ferromagnetic or non-magnetic material, three of which are illustrated by way of example but not by way of limitation, are secured to the fixed lower end piece  20  using bolts  18 , so as to build up a laminated structure having a selected amount of magnetic material. The amount and position of magnetic material in a particular finger  16  directly affects the structure and strength of the magnetic field in the casting mold  24  in the vicinity of that finger  16 ; decreasing the amount of magnetic material by substituting non-magnetic stackable elements for ferromagnetic stackable elements decreases the magnetic field locally. Note that the magnetic field in the casting mold  24  may be quickly and easily varied by selecting the number, type (usually, ferromagnetic or non-magnetic), and position of removable upper end pieces  22  for each finger  16  (as well as the current flow through any associated coil; see the discussion of FIG. 7 below) to produce the desired flow pattern in the molten steel.  
         [0030]    [0030]FIG. 5 shows an alternative embodiment of the structure of the finger  16  in FIGS. 3 and 4. A removable lower end piece  21  is provided in order to allow for the positioning of a non-magnetic end piece at the bottom of a stack. The removable lower end piece is provided with threaded bores  17  and attached to the core using bolts  18 . Other bolts  18  are used to attach removable upper end pieces  22  to the removable lower end piece  21 . The number of layers of removable upper end pieces  22  shown is merely an example, and should not be taken as a limitation of this embodiment.  
         [0031]    [0031]FIG. 6 illustrates how the removable end pieces (stackable laminar elements)  22  may span horizontally more than one finger  16 . In places where it is desirable to have a strong magnetic field, the gaps between the fingers  16  may be eliminated entirely by the use of removable upper end pieces  22  which are two or more times the width of a finger  16 . FIG. 6 shows this embodiment with removable lower end pieces  21 , but fixed lower end pieces could also be used. The bolts  18  holding the fingers  16  together are in the same position as in FIG. 5. The particular arrangement shown is for illustrative purposes only. The laminar elements  21 ,  22  may be made of materials with varying degrees of ferromagnetic properties, depending on the magnetic field requirements.  
         [0032]    Additional control over the magnetic field in a casting mold  24  may be achieved by use of more than one magnetic pole as illustrated in FIG. 7. Reference numeral  30  in FIG. 7 schematically indicates an exemplary five-pole system, each pole  31  terminating a core  32  (only one core of each pole pair is shown in FIG. 7). A discrete energizing coil  34  is associated with each core  32 , and, in this illustration, one finger  16  per core  32 . The coils  34  are arranged in a manner such that no two adjacent coils are at the same longitudinal position on the cores  32  so as to avoid physical interference between coils associated with adjacent cores and so as to maintain minimal spacing between adjacent cores. More than one finger  16  per pole  31  may be provided if necessary. FIG. 7 illustrates an idealized case in which there are no interfering obstructions. However, for even better control it may be advantageous to use more than one finger per pole (preferably with no spacing between fingers) even in the absence of obstructions. Each finger  16  preferably has the structure illustrated in one of FIGS. 3, 4 or  5  and described above for the single pole case, namely, a fixed or removable lower end piece  20  or  21  to which replaceable upper end pieces  22  may be bolted to build up a laminated structure having a selected amount of magnetic material and non-magnetic material in selected locations.  
         [0033]    By independently controlling electrical current passing through the coils  34 , the configuration of the magnetic field in the casting mold  24  may be controlled as casting proceeds. For example, a selected replaceable upper end piece  22  on a selected finger may have been removed or replaced to produce a particular magnetic field emanating from the pole associated with that finger when the current passing through the coils  34  is set at a selected set of values, but during casting, a somewhat weaker magnetic field associated with that finger may become advantageous. A weaker magnetic field from that finger may then be obtained without stopping the casting process by reducing the current to the associated energizing coil  34 . The particular changes to be made in the various energization currents for all the coils  34  may be determined empirically, and may be expected to depend upon such factors as the type of steel being cast, the dimensions of the mold  24 , the temperature distribution of the molten steel in the mold  24 , and the rate and the temperature at which molten steel is flowing into the mold  24  through the submerged entry nozzle  28 .  
         [0034]    [0034]FIG. 8 shows an embodiment of the present invention in which the five-pole array  30  of FIG. 7 is expanded in the vertical direction, creating a two-dimensional matrix of fingers for greater control over the magnetic field distribution. The illustration shows five such five-pole arrays stacked vertically, resulting in a 25-pole matrix  40 , each pole having one or more fingers. The coils  34  are arranged in a manner such that no two adjacent coils interfere with one another. Long bolts  19 , which have a length approximately equal to the height of the 25-pole matrix  40 , may be used in place of the shorter bolts  18  shown in previous illustrations. Removable lower end pieces  21  are shown by way of example only. The illustrated arrangement of the end pieces  21 ,  22  is merely one possible such arrangement, and is not intended to limit this embodiment of the invention.  
         [0035]    While particular elements, embodiments and applications of the present invention have been shown and described, it will be understood, of course, that the invention is not limited thereto, since modifications may be made by those skilled in the applicable technologies, particularly in light of the foregoing description. The appended claims include within their ambit such modifications and variants of the exemplary embodiments of the invention described herein as would be apparent to those skilled in the applicable technologies.