Patent Publication Number: US-2011048669-A1

Title: Electromagnetic stirrer arrangement with continuous casting of steel billets and bloom

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. provisional patent application No. 61/238,347 filed on Aug. 31, 2009, which is hereby incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The field is the electromagnetic stirring of continuously cast steel and more particularly to an arrangement of an electromagnetic stirrer and a continuous casting mold assembly. 
     DESCRIPTION OF THE PRIOR ART 
     In the production of continuously cast billets and blooms, two types of electromagnetic stirrer (EMS) arrangements with respect to continuous casting mold are commonly used, namely, internal and external. 
     In the internal EMS arrangement, the stirrer is positioned inside of a mold housing. The stirrer is thus in relatively close proximity to the casting mold the solidifying steel contained therein. With reference now to  FIG. 1 , a sectional elevation view of an exemplary internal EMS continuous casting mold assembly is shown. The assembly includes a mold  1 , water jacket  2 , electromagnetic stirrer (EMS)  3 , and the mold housing  4 . The stirrer  3  is of a rotary type, multi-phase device commonly used for the application for stirring liquid steel (not shown) within the mold  1  during continuous casting operations. The stirrer  3  can be enclosed in a separate housing  5  and is commonly cooled either by water supplied from a closed circuit, or the water used for mold cooling. As seen from  FIG. 1 , the stirrer  3  is positioned in relatively close proximity to the water jacket  2  and mold  1 , which provides the most efficient and effective utilization of the A.C. magnetic field produced by the stirrer  3 . 
     In an external EMS arrangement, the stirrer is installed on the caster within its own enclosure which is arranged around the mold housing. The stirrer internal diameter is sized to accommodate the largest section size of the outside housing of the caster and remains installed on the caster during casting. With reference now to  FIG. 2 , a sectional elevation view of an exemplary external EMS continuous casting mold assembly is shown. As can be seen, the stirrer  6  is enclosed in the stirrer housing  1  which is installed on a continuous casting machine (not shown). The mold housing  2 , which includes the mold  3 , water jacket  4 , and the foot rolls  5 , is inserted inside the inner diameter of the stirrer housing  1  (direction of insertion is indicated by arrow A). Foot rolls are not always required, but are typically necessary for casting practice with increased casting speed and/or larger sections of the casting strand, to provide shell support immediately below the mold. 
     In order to accommodate the mold housing  2  and the attached foot rolls  5 , the internal diameter of stirrer housing  1  should be substantially large in comparison with that of the stirrer  3  arranged internally within the mold housing  4 , as shown in  FIG. 1 . 
     Though the above described molding assemblies have proven to be adequate, drawbacks persist. Thus, there is a need in the art for a new continuous casting arrangement with EMS. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, an electromagnetic stirrer arrangement is disclosed. The electromagnetic stirrer arrangement includes a housing having a bottom opening and a top opening. An electromagnetic stirrer is positioned inside the housing. A modular mold assembly includes a mold, a top plate, a bottom plate and a plurality of rods connecting the top and the bottom plates. The mold has open top and bottom ends. The top plate is positioned proximate to the open top end of the mold and the bottom plate is positioned proximate to the open bottom end of the mold. The connecting rods extend between and securing together the top and bottom plate. The modular mold assembly is designed to allow replacement of a mold (insertion into or removal from) in the housing. 
     According to another aspect of the present invention, an electromagnetic stirrer arrangement is disclosed. The electromagnetic stirrer arrangement includes a housing having a bottom opening and a top opening. An electromagnetic stirrer is positioned inside the housing. A modular mold assembly includes a mold, a top plate, a bottom plate and a water jacket. The mold has open top and bottom ends. The top plate is positioned proximate the open top of the mold and the bottom plate is positioned proximate the open bottom of the mold. The water jacket is positioned around the mold to form a channel therebetween for directing cooling fluid over an exterior surface of the mold. The modular mold assembly is insertable and removable from the housing. 
    
    
     
       DESCRIPTION OF THE DRAWING 
         FIG. 1  is a sectional elevation view of a prior art internal EMS arrangement in a continuous caster mold housing assembly. 
         FIG. 2  is a sectional elevation view of a prior art external EMS arrangement with the continuous caster mold housing assembly. 
         FIG. 3  is a graph showing the effect of stirrer internal diameter (ID) on magnetic flux density at a constant kVA input. 
         FIG. 4  is a graph showing the effect of stirrer ID on stirring velocity within the casting mold. 
         FIG. 5  is a graph showing the effect of stirrer ID on the kVA input required to maintain a constant level of magnetic flux density. 
         FIG. 6  is a sectional elevation of a hybrid EMS arrangement in a continuous caster mold housing assembly. 
         FIG. 7  is a sectional elevation of the casting mold modular assembly. 
     
    
    
     DETAILED DESCRIPTION 
     The internal diameter of an electromagnetic stirrer (EMS) affects the magnetic flux density value in a continuous casting mold. In turn, this affects stirring velocity in the melt induced by the magnetic field created by the EMS. At a constant apparent power input to the EMS (in kVA), magnetic flux density declines as the EMS internal diameter increases. This phenomenon is shown in the graph of  FIG. 3 . For illustrative purposes, the curve indicates values of flux density between the typical diameter of an internal EMS, and external EMS without a foot roll attached to the mold housing assembly and an external EMS with a foot roll attached to the mold housing assembly of the same section size mold. 
     As seen from  FIG. 3 , EMS diameter has a marked effect on magnetic flux density when power input and operating frequency are held constant. The intensity of stirring in the melt is one of the main defining factors of EMS metallurgical performance and quantitatively, stirring intensity is commonly determined by melt stirring velocity. 
     With reference now to  FIG. 4 , the effect of EMS internal diameter on stirring velocity is shown, where the power input (kVA) and operating frequency are held constant. For clarity, the same input values are used in  FIG. 4  as those in  FIG. 3 . Similar to the effect on magnetic flux density, the effect of stirrer internal diameter on stirring velocity is very strong. The marks on the curve shown in  FIG. 4  while not identified in that figure are for the same types of stirrer arrangements as the corresponding marks shown in  FIG. 3 . 
     The effect of an EMS internal diameter increase on the decline of magnetic flux density and stirring velocity may be counteracted by increasing power input to the EMS. However, due to practical limitations, this requirement often cannot be fulfilled, as the required power input is exponentially related to EMS internal diameter, as illustrated by  FIG. 5 . 
     With reference now to  FIG. 6 , a sectional elevation of a continuous casting arrangement is shown and generally indicated by the numeral  100 .  FIG. 7  shows a sectional elevation of a casting mold modular assembly  120  removed from the casting arrangement  100 . It should be appreciated that like reference numerals are used to identify like elements in both  FIG. 6  and  FIG. 7 . The casting arrangement  100  can be used with any continuous casting molds, i.e. vertical or curved type, employed for the production of steel billets and blooms of different cross-section sizes and geometry. The casting arrangement  100  includes an exterior mold housing  101  which surrounds a casting mold  102  when the casting mold modular assembly  120  is installed. Housing  101  is open at the top and bottom so that it can receive casting mold modular assembly  120 . An electromagnetic stirrer (EMS)  105  is arranged within mold housing  101  and it may also be cooled by the same mold cooling water or by water supplied through a designated cooling system. If a designated cooling system is employed, specially prepared cooling water may be used to cool EMS  105 . In such an instance, a separator  106  may be positioned within housing  101  and interior to EMS  105  to keep separate the mold cooling water from the EMS coolant. The EMS  105  is a multi-phase electrical device similar to the stator of an asynchronous motor which is comprised of an iron-made stator that has mounted on it electrical windings. The EMS  105  operates at a low frequency A.C. current, typically within the range of 2 to 8 Hz, producing a rotating A.C. magnetic field which sets up the stirring motion of the melt (not shown) within the mold  102 . 
     The modular assembly  120  includes the casting mold  102  which is fabricated from a copper alloy and has an open end on each side for delivery of liquid steel through the top opening T and withdrawal of the cast strand with solidifying core (not shown) through the bottom opening B. A water jacket  103  surrounds the mold  102  and forms a channel  104  between the outer surface of mold  102  and the inner surface of water jacket  103 . Cooling water is directed through channel  104  to cool the mold  102 . 
     The casting mold modular assembly  120  shown in  FIG. 7  further includes an upper plate  107  and a lower plate  108  that are positioned proximate to the top T and bottom B opening of mold  102 . Plates  107  and  108  include a central opening  121  and  122  respectively (see  FIG. 6 ), that is generally sized to receive mold  102  therein. Plates  107  and  108  are rigidly attached together by a plurality of rods  109  that extend parallel to and are spaced from mold  102 . According to one embodiment, at least four (4) rods  109  extend between top and bottom plates  107  and  108  in an evenly spaced arrangement. According to another embodiment eight (8) rods extend between top and bottom plates  107  and  108  in an evenly spaced arrangement. Each rod  109  may be secured to the respective plate  107  and  108  with screws  110 . In this manner, a rigid, modular, cage-like structure is formed. 
     As shown in  FIG. 6 , top plate  107  includes a groove  112  on central opening  121  and bottom plate  108  includes a groove  111  on central opening  122 . Grooves  111  and  112  are adapted to receive o-rings  123  that provide a water tight seal between the mold  102  and the plates  107  and  108 . Bottom plate  108  further includes a second groove  125  on the outer facing surface thereof. Groove  125  is adapted to receive an o-ring  126  that provides a water tight seal between plate  108  and housing  101 . 
     A plate  116  extends outwardly from the top end of water jacket  103 , circumferentially around mold  102 . Plate  116  segments the interior of housing  101  to prevent mixture of incoming and outgoing cooling water flows (water flows represented by arrows). Upper plate  107  supports mold  102  by preventing axial displacement. Further supporting mold  102  is a plate  117  positioned below and flush with top plate  107 . Plate  117  is coupled to the plate  107 , surrounds mold  102  and is received in a groove  125  on the outer surface of mold  102 . A protecting plate  113  of the mold housing  101  includes an opening substantially the same size as mold  102  and is secured to top plate  107 . Protecting plate  113  protects top plate  107  from damage in event of liquid steel spillage. 
     With reference now to  FIG. 7 , it can be seen that modular assembly  120  can be easily removed from the mold housing  101  and replaced. This is achieved by releasing the bolts  114  (see  FIG. 6 ) securing the upper plate  107  of modular assembly  120  to a flange  115  of mold housing  101 . 
     It should be appreciated that the casting arrangement  100  described herein includes the features and advantages found in both internal and external EMS arrangements. Specifically, the “hybrid” arrangement provides the benefits of an internal EMS arrangement in terms of energy efficiency and metallurgical effectiveness while also enabling convenient and speedy casting mold changes, similar to an external EMS arrangement. 
     It should further be appreciated that, compared to an internal EMS arrangement, the casting arrangement  100  minimizes capital costs of equipment installation by reducing the number of mold housings and stirrers when the used with a multiple strand section caster. Further, operating costs are reduced compared to external EMS due to the smaller relative internal diameter of the EMS. Smaller internal diameter leads to reduced power requirements to attain operating values of magnetic flux density and frequency. Operating costs savings become especially significant when cast strand section sizes are within a wide range, e.g. 100 mm sq. to 200 mm sq. or greater. 
     It should also be appreciated that modular mold assembly  120  is configured to be fixed within the mold housing with mold section sizes based on stirrer design and operating parameters in order to assure maximal stirring effectiveness. 
     It should also be further appreciated that the casting arrangement  100 , allows for convenient and relatively rapid replacement of casting mold  102  in accordance with the production schedule of casting operations by using the removable modular assembly  120 . This assembly does not include the mold housing  101  and EMS  105  as they are common to the existing mold housing assemblies. Each mold modular assembly  120  can be exchanged for another one, if required, within the mold housing  101  equipped with the EMS  105 . 
     The replacement procedure takes place at a mold preparation shop, in accordance with production schedule requirements. The amount of time and labor required for replacement of the modular mold assembly  120  is markedly reduced in comparison with that required for changing a mold in a typical internal EMS arrangement. Further, in applications with a multiple section size caster, the amount of mold housings to be used with the modular mold assembly is also drastically reduced in comparison with that required for prior art internal and external EMS arrangements. These advantages, combined with greatly reduced operating costs over the external EMS arrangement, result in substantial economic benefits in comparison with the existing conventional EMS arrangements. 
     It is to be understood that the description of the foregoing exemplary embodiment(s) is (are) intended to be only illustrative, rather than exhaustive, of the present invention. Those of ordinary skill will be able to make certain additions, deletions, and/or modifications to the embodiment(s) of the disclosed subject matter without departing from the spirit of the invention or its scope, as defined by the appended claims.