Abstract:
A method for reducing vortex formation in molten metal when bottom tapping the molten metal from a metallurgical vessel in a metal making process. The method includes the steps of tapping the molten metal via a tapping hole in the metallurgical vessel, and providing a flow of the molten metal in the metallurgical vessel while tapping via a time-varying electromagnetic field applied to the metallurgical vessel, the flow of the molten metal being such that it constantly moves vortices in the molten metal away from a tapping hole region during the tapping to thereby prevent accumulation of the vortices for vortex formation over the tapping hole. It is also presented an arrangement for carrying out the method.

Description:
FIELD OF THE INVENTION 
     The present disclosure generally relates to a metal making process and in particular to vortex reduction during tapping operations in the metal making process. 
     BACKGROUND OF THE INVENTION 
     In a metal making process molten metal is during various stages of the process tapped from tapping holes of metallurgical vessels such as electric arc furnaces, tundishes or ladles. The molten metal is thereby transported to the next stage in the process. 
     When tapping the molten metal from a metallurgical vessel, vortex formation normally occurs above the tapping hole. When the vortex is formed, slag on top of the melt is carried over into the next metallurgical vessel below the tapping hole via the vortex. The slag carry-over has detrimental effects on the metal quality. 
     EP0192991 discloses a method of operating a metallurgical melting furnace whose furnace vessel is provided with at least one tapping opening. According to the disclosure, vortices are counteracted in the melt in the area of the tapping opening by means of an electromagnet generating an electromagnetic field which acts on the melt. The vortex formation is counteracted by controlling the electromagnet such that it produces electromagnetic fields providing a counter rotation relative the vortex flow in the molten metal. 
     SUMMARY OF THE INVENTION 
     There are however drawbacks with the above method of generating a counter-rotational motion in the melt by means of an electromagnetic field. It is for instance difficult to determine the rotational speed of the vortex, which is necessary for determining a correct counter-rotational motion in the melt by means of the electromagnetic field. 
     Moreover, the above-described method is arranged to counteract a vortex which has already been formed in the tapping hole region, but it does not prevent the formation of a vortex in the tapping hole area. 
     In view of the above, a general object of the present disclosure is to provide a simplified method and arrangement for reducing vortex formation in molten metal during tapping of the molten metal from a metallurgical vessel. 
     Another object is to provide a method and arrangement for preventing or at least delaying the on-set of vortex formation above the tapping hole during tapping of the molten metal from the metallurgical vessel. 
     To this end, in a first aspect of the present disclosure, there is provided a method for reducing vortex formation in molten metal when bottom tapping the molten metal from a metallurgical vessel in a metal making process, wherein the method comprises: tapping the molten metal via a tapping hole in the metallurgical vessel; and providing a flow of the molten metal in the metallurgical vessel while tapping by means of a time-varying electromagnetic field applied to the metallurgical vessel, the flow of the molten metal being such that it constantly moves vortices in the molten metal away from a tapping hole region during the tapping to thereby prevent accumulation of the vortices for vortex formation over the tapping hole. 
     The tapping hole region is herein defined as an area extending axially from the tapping hole, centred around the central axis of the tapping hole, through the metallurgical vessel. 
     By constantly moving the molten metal such that vortices which naturally arise in the volume of the molten metal constantly move, the vortices are not allowed to accumulate in the tapping hole region, i.e. the region around the central axis of the tapping hole. Thereby vortex formation is prevented or the probability of vortex formation above the tapping hole is at least reduced. Beneficially, by preventing the formation of a vortex above the tapping hole, slag on top of the molten metal will not be carried over into the next metallurgical vessel during tapping operations, and hence the metal quality of the slab, billet, bloom or other metal product may be improved. 
     The molten metal may for instance be molten steel, molten aluminium or molten copper. 
     In one embodiment the time-varying electromagnetic field provides a forced convection of the molten metal in the metallurgical vessel. Hence, instead of providing a counter rotational motion of the molten metal as in EP0192991v, the molten metal moves according to a forced convectional motion in the metallurgical vessel during tapping. 
     In one embodiment the molten metal flow is transverse to a central axis of the tapping hole. In particular, the molten metal flows in a direction transverse to the central axis of the tapping hole at any given depth of the molten metal in the metallurgical vessel. Hence, at substantially any depth in the molten metal above the tapping hole, the molten metal essentially flows perpendicularly in relation to the central axis of the tapping hole. To this end the molten metal flows essentially parallel with the bottom surface of the metallurgical vessel at any depth in the molten metal above the tapping hole. Thus the molten metal flows in such a way that close to the bottom surface of the metallurgical vessel molten metal is pushed to discharge quickly through the tapping hole, while closer to the surface of the molten metal the molten metal is continually carried away from the central axis of the tapping hole, and thus from the tapping hole region. Thereby at any depth above the tapping hole the molten metal is either moved away from the region around the central axis of the tapping hole or pushed through the tapping hole for discharging the molten metal. Thus, vortices are carried away from the tapping hole region, and as a result, vortex formation above the tapping hole is prevented. 
     In one embodiment the molten metal flows towards a first inner wall portion of the metallurgical vessel at the bottom of the metallurgical vessel and towards a second inner wall portion opposite the first inner wall portion at the surface of the molten metal. 
     In one embodiment the time-varying electromagnetic field has such strength that a flow rate of the molten metal is in the range 0.1-1 m/s. 
     In one embodiment the flow rate is in the range 0.1-0.6 m/s. By providing a time-varying electromagnetic field which generates a molten metal flow rate in the range 0.1-0.6 m/s, energy for powering e.g. an electromagnetic stirrer for the generation of the time-varying electromagnetic field can be saved. In particular, the range 0.1-0.6 m/s is a lower flow rate than the flow rate utilised when the electromagnetic stirrer stirs the molten metal during meltdown and stirring of the melt in the metallurgical vessel. Furthermore, the lower flow rate does not disturb the metal mix e.g. steel mix, obtained during for instance the melting process by means of providing additives to the metal and the stirring thereof. 
     In one embodiment the time-varying electromagnetic field is provided by an electromagnetic stirrer. 
     According to a second aspect of the present disclosure there is provided an arrangement for a metal making process, the arrangement comprising a metallurgical vessel for accommodating molten metal, the metallurgical vessel having a tapping hole for bottom tapping the molten metal from the metallurgical vessel; and an electromagnetic field emitting device arranged to generate a time-varying electromagnetic field in molten metal arranged in the metallurgical vessel, wherein the electromagnetic field emitting device is arranged to induce a time-varying electromagnetic field in the molten metal during tapping of the molten metal from the metallurgical vessel to thereby generate a flow of the molten metal in the metallurgical vessel, the electromagnetic field being such that the flow constantly moves vortices away from a tapping hole region in the molten metal during the tapping to thereby prevent accumulation of the vortices for vortex formation over the tapping hole. 
     In one embodiment the electromagnetic field emitting device is an electromagnetic stirrer. 
     In one embodiment the metallurgical vessel is an electric arc furnace. Alternatively the metallurgical vessel may be a tundish or a ladle. 
     In one embodiment the time-varying electromagnetic field is such that it provides a forced convection of the molten metal in the metallurgical vessel. 
     In one embodiment the time-varying electromagnetic field is such that it provides flow of molten metal which transverses a central axis of the tapping hole. 
     In one embodiment the time-varying electromagnetic field is such that it provides a flow of molten metal towards a first inner wall portion of the metallurgical vessel at the bottom of the metallurgical vessel and towards a second inner wall portion opposite the first inner wall portion at the surface of the molten metal. 
     In one embodiment the electromagnetic field has such strength that a flow rate of the molten metal is in the range 0.1-1 m/s. 
     In one embodiment the flow rate is in the range 0.1-0.6 m/s. 
     Generally, all terms used in the claims are to be interpreted according to their ordinary meaning in the technical field, unless explicitly defined otherwise herein. All references to “a/an/the element, apparatus, component, means, step, etc.” are to be interpreted openly as referring to at least one instance of the element, apparatus, component, means, step, etc., unless explicitly stated otherwise. The steps of any method disclosed herein do not have to be performed in the exact order disclosed, unless explicitly stated. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The inventive concept will now be described, by way of example, with reference to the accompanying drawings, in which: 
         FIG. 1  is a schematic perspective view of an example of an arrangement for metal making; 
         FIG. 2 a    is a top view of a metallurgical vessel in which vortices are formed above a tapping hole during tapping; 
         FIG. 2 b    is a top view of a metallurgical vessel in which a vortex has been formed from a plurality of vortices above the tapping hole of the metallurgical vessel; and 
         FIG. 3  is a schematic perspective view of the arrangement in  FIG. 1  during tapping. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The inventive concept will now be described more fully hereinafter with reference to the accompanying drawings, in which certain embodiments are shown. It is to be noted, however, that the metallurgical vessel disclosed herein may be embodied in many different forms and should not be construed as limited to the embodiments set forth hereinafter; rather, these embodiments are provided by way of example so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Like numbers refer to like elements throughout the description. 
     Metallurgical vessels are used in metal production e.g. in steel or metal works. Such metallurgical vessels may for instance be ladles, electric arc furnaces or tundishes. Whenever referred to in the following, a metallurgical vessel is to be understood to mean an electric arc furnace, a ladle, a tundish or any other refractory metallurgical vessel having a tapping hole at its bottom. 
       FIG. 1  shows an arrangement  1  for metal making. The arrangement  1  comprises a metallurgical vessel  3  and an electromagnetic field emitting device, in the following exemplified by an electromagnetic stirrer  5 . The electromagnetic stirrer  5  comprises a coil arrangement  6 , a frequency converter  7  for operating the coil arrangement  6  and a control unit  9  for controlling the frequency converter  7 . The electromagnetic stirrer  5  is arranged below the metallurgical vessel  3 . It is however to be noted that, depending on the shape of a metallurgical vessel, the electromagnetic stirrer could also be positioned at one of the sides of the metallurgical vessel. 
     The metallurgical vessel  3  has walls  11 - 1  and  11 - 2  presenting first and a second inner wall portions, respectively. The first and the second inner wall portions are opposite each other. The metallurgical vessel  3  further has a bottom  13  presenting an inner bottom surface  15 , and a tapping hole  17  extending through the bottom  13 . The tapping hole  17  provides a through opening from the interior of the metallurgical vessel  3  to its exterior. The tapping hole  17  is typically provided off-centre with respect to a centre point C of the bottom surface  15 , but a centrally located tapping hole is also envisaged in some embodiments. The tapping hole  17  has a central axis A extending axially through the tapping hole  17 . 
     Whether the metallurgical vessel  3  is arranged to receive scrap or molten metal depends on where in the metal making process the metallurgical vessel  3  is to be used. If the metallurgical vessel  3  is an electric arc furnace, it is arranged to receive scrap for meltdown of the scrap to molten metal. If the metallurgical vessel  3  is a tundish or a ladle it is arranged to receive molten metal for instance from an electric arc furnace. In either case, the molten metal is tapped from the metallurgical vessel  3  through the tapping hole  17  in the bottom  13 . 
     When tapping molten metal from the metallurgical vessel  3 , the molten metal is typically tapped into another metallurgical vessel  19 . 
     In cases when the metallurgical vessel  3  is an electric arc furnace, the tapping hole  17  is typically filled with a refractory material such as refractory sand when loaded with scrap for meltdown. As a result, molten metal resulting from the meltdown of the scrap is held within the metallurgical vessel  3  until tapping is desired. When subsequently tapping of the molten metal is to be performed, the refractory material is removed from the tapping hole  17 , thereby allowing the molten metal to be tapped from the metallurgical vessel  3  through the tapping hole  17 . 
     The metallurgical vessel  3  may in some variations be pivotable for performing tapping of the molten metal from the metallurgical vessel  3 . The metallurgical vessel  3  may for instance be pivotable when embodied as an electric arc furnace. The bottom tapping through the tapping hole can thereby be facilitated. 
     The principles of vortex formation in a metallurgical vessel will now shortly be described with reference to  FIGS. 2 a    and  2   b.    
       FIGS. 2 a - b    show top views of the metallurgical vessel  3  accommodating a molten metal  21 . The tapping hole  17  is shown in both  FIG. 2 a    and  FIG. 2 b    to simplify the understanding of the vortex formation process. In reality the molten metal covers the tapping hole  17  and is hence not visible from above. 
     During tapping of the molten metal  21  from the metallurgical vessel  3  a plurality of vortices such as vortices V 1 , V 2 , V 3 , V 4 , and V 5  are formed in the molten metal  21 . The vortices V 1 , V 2 , V 3 , V 4 , and V 5  move towards the tapping hole  17  in the volume of the molten metal  21 , as shown by arrows  23 . 
     The vortices V 1 , V 2 , V 3 , V 4 , and V 5  accumulate above the tapping hole in a region around the central axis A of  FIG. 1 . As illustrated in  FIG. 2 b    the accumulated vortices V 1 , V 2 , V 3 , V 4 , and V 5  form a larger vortex V tot . The vortex V tot  is undesirable as it carries over slag from the surface of the molten metal  21  into e.g. the next metallurgical vessel in the process. 
     With reference to  FIG. 3  a method of preventing or at least reducing the formation of the vortex V tot  above the tapping hole  17  will now be described. 
       FIG. 3  shows the arrangement  1 , which has already been described structurally in  FIG. 1 , during tapping. The metallurgical vessel  3  depicted in  FIG. 3  contains molten metal  21  and the refractory material in the tapping hole  17  has been removed in order to allow tapping of the molten metal  21 . Moreover, the metallurgical vessel  3  is slightly pivoted to facilitate tapping of the molten metal  21  through the tapping hole  17 . 
     The control unit  9  controls the frequency converter  7  such that the electromagnetic stirrer  5  generates a time-varying electromagnetic field which is applied to the metallurgical vessel  3  and which generates a time-varying electromagnetic field in the molten metal  21 . The time-varying electromagnetic field is preferably a linear electromagnetic field in the sense that it gives rise to a linear force in the molten metal. To this end the linear electromagnetic field affects essentially the entire molten metal in the metallurgical vessel, i.e. essentially the entire molten metal is moved in the metallurgical vessel by the linear force generated by the linear electromagnetic field. Hereto, the time-varying electromagnetic field in the molten metal provides a flow F of the molten metal  21  in the metallurgical vessel  3 . The flow F is of a forced convection-type, circulating the molten metal  21  in the metallurgical vessel  3 . In particular, the generated flow F is non-rotational and the flow F is transverse to, or crosses, the central axis A of the tapping opening  17  to thereby move the molten metal away from the central axis A along an upper portion of the depth d of the molten metal  21  while pushing the molten metal  21  which is close to the inner bottom surface  15  to discharge through the tapping hole  17 . Thus, the flow F is such that the molten metal  21  flows towards the first inner wall portion of the metallurgical vessel  3  at the bottom  13  of the metallurgical vessel  3  and towards the second inner wall portion opposite the first inner wall portion at the surface of the molten metal  21 . Any vortices V 1 , V 2 , V 3 , V 4 , and V 5  formed in the volume of the molten metal  21  and moving towards the central axis A due to the tapping through the tapping hole  17  are hence constantly moved away from the central axis A, thereby preventing the accumulation of the vortices V 1 , V 2 , V 3 , V 4 , and V 5  above the tapping hole around the central axis A and thus preventing the formation of an accumulated vortex such as vortex V tot  of  FIG. 2   b.    
     The time-varying electromagnetic field generated in the molten metal  21  may be of such strength that a flow rate of the flow F of molten metal  21  is greater than 0.1 m/s. In one embodiment, the flow rate of the flow F of molten metal  21  may be in the range 0.1-0.7 m/s, and preferably in the range 0.1 m/s to below 0.7 m/s. In one embodiment the flow rate of the flow F of molten metal  21  may be in the range 0.1-0.6 m/s. 
     In one embodiment where the metallurgical vessel is an electric arc furnace, the time-varying electromagnetic field may have the same strength as when stirring the molten metal during meltdown. It is however preferred to generate a lower flow rate of the molten metal than when stirring the molten metal during meltdown. 
     The time-varying electromagnetic field to be generated by the electromagnetic stirrer  5  and applied to the metallurgical vessel  3  may be determined by empirical studies based on the type of metal to be melted, the shape and structure of the metallurgical vessel, the specific use of the metallurgical vessel e.g. as an electric arc furnace, tundish or ladle, or the specific compositions added to the metal during the meltdown, or a combination thereof. A control scheme most suitable for the specific application can thereby be determined and used in the control unit  9  for control of the frequency converter  7 . 
     The time-varying electromagnetic field may continuously be applied to the metallurgical vessel  3  from meltdown to tapping, e.g. when the metallurgical vessel  3  is an electric arc furnace. In this case the strength of the time-varying electromagnetic field may be adjusted for the tapping, as has been described above. Alternatively, the time-varying electromagnetic field may be applied to the metallurgical vessel  3  essentially simultaneously as tapping of the molten metal  21  commences. 
     The inventive concept has mainly been described above with reference to a few embodiments. However, as is readily appreciated by a person skilled in the art, other embodiments than the ones disclosed above are equally possible within the scope of the inventive concept, as defined by the appended patent claims. For instance, the movement of the molten metal can be changed from a forward flowing direction to a backward flowing direction in the metallurgical vessel by modifying the time-varying electromagnetic field.