Patent Publication Number: US-8973791-B2

Title: Frame for a device for holding and replacing casting plates and assembly

Description:
BACKGROUND OF THE INVENTION 
     (a) Field of the Invention 
     The present invention relates to the technical field of the continuous casting of molten metal. 
     (b) Description of Related Art 
     A device for holding and replacing plates or tubes for transferring molten metal contained in a metallurgical vessel is already known in the art. The device can be arranged immediately below a metallurgical vessel and is used for transferring the molten metal from an upper metallurgical vessel to a lower metallurgical vessel for instance from a ladle to a tundish or from a tundish to a casting mould. 
     The plate generally consists of a metallic casing surrounding or cladding a refractory material. The plate is used to transfer the molten metal, either in the form of a free flow, or through a tube rigidly connected to the plate. In the latter case, the plate is frequently referred to as a “casting tube”, “outer nozzle”, submerged nozzle or pouring nozzle. Hereinafter, the term plate will be used to refer both to the plate for transferring molten metal in free flow form and to the plate provided with a tube referred to as a casting tube. 
     The devices for holding and replacing plates or tubes may have different names on the market such as tube changer device, device for the insertion and/or removal of a pouring nozzle, calibrated nozzle changer, tube exchange device or others. 
     A device for holding and replacing a plate for casting molten metal out of a vessel generally comprises a frame with a casting opening, said frame being configured for being fixed to the lower side of a metal casting vessel and comprising a first, upper portion and a second, lower portion, joining at a middle section plane defining the plane where an upper refractory element and a lower refractory element form a sliding contact,
         the upper side portion of the frame comprising n upper refractory element receiver, or means for receiving and clamping in place at its pouring position the upper refractory element, such that the through bore of the upper refractory element is in fluid communication with the casting opening and   the lower side portion of the frame comprising:   a passage extending along a first axis of first direction (X) between an inlet opening and an outlet opening configured for receiving and moving a plate from said inlet to said outlet, passing by a casting position in registry with the casting opening of the frame;   a displacing element, or means for displacing, and a guide, or means for guiding, the plate from a standby position to a casting position in registry with the casting opening of the frame, and optionally for guiding it to the outlet, said guide or guiding means running substantially parallel to the first direction (X),   substantially parallel to the first direction (X) and extending from the guide or guiding means at the level of the pouring nozzle casting position, means for pressing up the plate at its casting position in the direction of the upper portion of the frame (in the direction of the metallurgical vessel).       

     More specifically, the device generally consists of a frame comprising two guiding rails and rockers arms or thrusters for cooperation with a plate or a plate of a casting tube. The means for displacing the plate generally consist of a mechanical, pneumatic or hydraulic arm or cylinder. 
     The frame of the device for holding and replacing a plate is generally cast and is unlikely subjected to wear. However, the parts such as the clamping means, the guiding means and the thrusting means like the clamps, the rails, the rockers or the springs are wearing parts of the device. These parts are checked at each maintenance operation of the device and replaced if necessary. 
     The plate arranged below the vessel is worn in the course of metal casting, for example due to slag erosion. The casting orifice may also become clogged or obstructed over time. It is thus necessary to replace the plate during casting, using a device for holding and replacing the plate. Such devices are known particularly from the document EP 0 192 019 A1 relating to a device for replacing casting tubes and the document U.S. Pat. No. 6,019,258 relating to a device for replacing calibrated plates. The plate is replaced in the casting position by sliding a new plate which was in standby forwards, said new plate pushing the worn plate, so as to eject and replace said plate in the casting position. The devices generally comprise guiding means such as rails or slides and thrusting means or pushing means such as springs. The guiding and displacing means are used to guide and move the plate to the operating position thereof or remove the same from the operating position thereof. The thrusting or pushing means are used to hold the plate in tight contact with the refractory element located upstream when the plate is in the operating position. 
     Providing a sealing surface or shut-off surface or blank surface on the plate, arranged behind the plate casting orifice is known from the document WO 20041065041. This sealing surface is intended to seal the casting channel of the metallurgical vessel if required, for example in the event of an accident. Indeed, it may be necessary to stop (interrupt) the metal casting in the event of an emergency. For this purpose, it is simply necessary to push (move forward) the plate in the casting position by a distance greater than or equal to the casting channel diameter, so that the sealing surface blocks (closes) the channel. 
     In a device for holding and replacing a plate suitable to interrupt the cast in case of an emergency, the displacing means can adopt two successive positions:
         a casting position wherein the plate is in fluid communication with the casting channel   a sealing position wherein the sealing surface of the plate is facing the casting channel       

     Such devices generally require the use of a so-called double stroke jack or cylinder, the short stroke displacing the plate to the casting position and the long stroke displacing the plate to the sealing position. The sealing position is also called shut-off position or closure position. 
     By convention, the forward direction of a plate, frame or device for replacing plates is defined with reference to the direction of plate replacement in the device for replacing plates, the plate being moved forwards to adopt the following successive positions: standby position, casting position (when the casting orifice extends from the casting channel), sealing position (when the sealing surface seals the casting channel) and ejection (evacuation or exit) position (when the casting plate is released from the device). 
     One difficulty lies in that it is possible to arrange a new plate by mistake in the wrong direction in the device for holding and replacing plates. In this case, the sealing surface is not arranged behind but in front of the casting orifice. As a result, when the new plate is pushed to the casting position, the casting orifice thereof does not extend exactly from the casting channel and, furthermore, if the new plate is then pushed to the sealing position in the event of an emergency, the sealing surface is not opposite the casting channel, such that the casting is not completely discontinued. This may have serious consequences for the metal casting facility and for those working on the casting site, in that it is no longer possible to discontinue the casting. 
       FIG. 1  represents an example wherein a plate  10  according to the prior art has been inserted the wrong direction in a device  90  for holding and replacing plates. The device is used for transferring molten metal in a continuous casting facility, for example steel, for example from a tundish to a casting mould. The plate  10  replaces a worn plate  12 , by sliding the plate  10  in the direction  14  corresponding to the first translation axis X, under the thrust of the displacing means, for example a hydraulic cylinder. In  FIG. 1 , the plate  10  is in a position which should have been a casting position if it had been inserted in the right direction. 
     The plate  10  comprises a sliding face  16 , in contact with an upstream refractory element, with reference the direction of molten metal flow corresponding to the axis Z. More specifically, the face  16  is in contact with an inner nozzle  18  of the vessel, arranged in the bottom of the vessel, said inner nozzle  18  comprising a casting channel  20 . 
     The sliding face  16  comprises a casting orifice  22 , intended to extend from the channel  20  when the plate  10  is arranged in the right direction in the casting position, and a sealing (shut-off) surface  24 , for sealing (closing) the channel  20  when the plate moves to the sealing (shut-off) position. 
     As can be seen in  FIG. 1 , when the plate  10  is in the casting position and in the wrong direction, only one gap  26  is generated between the casting channel  20  and the casting orifice  22 . Therefore, although a maximum molten metal flow rate would be required, only the gap  26  allows the molten metal to pass through. Furthermore, if, for exceptional reasons, it is desired to discontinue casting, by pushing the plate  10  to the sealing position, the gap  26  widens and the casting channel  20  is not sealed by the sealing surface  24  allowing molten metal to pass through. This gap may even be the cause of leakage liable to allow the metal to infiltrate the device for replacing plates, and cause non-negligible damage in the casting facility. 
     The aim of the present invention is particularly that of improving the safety in the continuous casting facility in a simple manner. 
     The fact that an operator can arrange a plate in the wrong direction has been discussed in the documents U.S. Pat. No. 5,211,857 or U.S. Pat. No. 5,011,050. The devices described in these documents present two directions perpendicular to one another. One plate insertion direction or loading direction and one plate replacement direction or firing direction. The loading direction is perpendicular to the plate exchange direction. The plate exchange direction is parallel to the casting mould. The insertion of the plate into the device is made by sliding it onto the loading rails. The safety system comprises a pre-position guide defining a gauging opening therethrough complementary to the plate to require a single plate orientation to pass through said gauging opening as the plate is loaded into the device. In the loading area, the loading rails are asymmetrical or present a locating step which in cooperation with the pre-position guide prevent the plate from reaching the loading position if the plate orientation is not correct. In particular, the above documents describe a device having two different loading rails. For instance, one of the rails has a projection engaging with a groove arranged on the plate sliding surface. In the plate replacement direction, the two edges of the plate are identical and are devoid of asymmetry. This way, the core part of the device which operates the plate replacement is not modified internally and is substantially identical to the other known devices. 
     The devices describe in the documents U.S. Pat. No. 5,211,857 or U.S. Pat. No. 5,011,050 involve certain drawbacks. They require the use of a pre-position guide unit and the proper mounting of this last. The projection, the groove or the locating step have to be properly mounted on one of the loading rails. Furthermore, the projections and grooves have relatively small dimensions. It is thus possible that the operator would not realise that the plate has been arranged in the wrong direction. The rails are worn over time and the projections are also worn; it is possible that, after a certain period of use, the projection no longer fulfils the role thereof. The rails are also wearing parts requiring regular replacement. During the assembly or the maintenance of the device, the operator could easily make a mistake during the mounting of the loading rails and/or the pre-guide unit. For instance, he could position the left rail on the right or vice versa or forget to add the locating step. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention particularly relates to a device making it possible to avoid the abovementioned drawbacks. The operations of assembly and maintenance of the device being also foolproof for the operator. 
     To this end, the invention relates to a frame for a device for holding and replacing a plate for casting molten metal out of a vessel which renders the operation of maintenance and assembly foolproof for the operator. 
     The invention relates to a frame for a device for holding and replacing plates for casting molten metal out of a metallurgical vessel having a casting channel, the axis of the casting channel defining the casting axis (Z); 
     the frame comprising a casting opening arranged to be in registry with the casting channel of the vessel in operating position; the frame configured for being fixed to the lower side of a metallurgical vessel; 
     the frame comprising a first, upper portion and a second lower portion, joining at a middle section plane defining the plane where an upper refractory element and a plate form a sliding contact; the plane being substantially perpendicular to the casting axis (Z);
         the upper portion of the frame comprising a receiver or means for receiving the upper refractory element when the device is assembled, in the operating position in the vicinity of the casting channel of the metallurgical vessel:   the lower portion of the frame comprising;   a passage extending between an inlet opening and an outlet along a first, translation axis (X) corresponding to the plate replacement direction, said passage being arranged to enable the introduction of the plate into the frame and the extraction of the plate from the frame by translation along the plate replacement direction; and wherein the translation axis (X) is parallel to the middle section plane and, together with the casting axis (Z) define a central longitudinal plane;   a housing (located in said passage between inlet and outlet openings, configured for receiving and holding a plate, when the device is assembled, in the operating position in the vicinity of the casting channel of the metallurgical vessel, said housing comprising a first and opposite second sides substantially parallel to and located on either sides of the central longitudinal plane, each of the first and second sides of the housing comprising recesses for receiving pressing elements or means for pressing up the plate, in the operating position, in the direction of the upper portion of the frame;       

     Characterised in that the orthogonal projection onto the central longitudinal plane of the recesses situated on the first side of the housing are spaced apart vertically from the orthogonal projection onto said central longitudinal plane of the recesses situated on the second side of the housing. 
     More specifically, the orthogonal projections onto the central longitudinal plane of the recesses respectively situated on either side of the housing are spaced apart vertically. In other words the recesses situated on either side of the housing are located at different levels or are offset. In certain embodiments, the orthogonal projections of each set of recesses could slightly overlap while still be offset. 
     The recesses are designed to match the pressing elements or means. The pressing elements or means are known to the person skilled in the art and usually consist of pushers including springs and rockers. 
     In certain embodiments, the recesses comprise one or any combination of any of the following features:
         (a) holes for receiving compression means or compression elements, such as for receiving springs   (b) grooves for mating pushers, such as for mating rocker arms,   (c) slots having a slot axis for articulating the pushers       

     In certain embodiments, the recesses are substantially identical on each side of the housing. This way, the same pressing element or pressing means parts can be used equivalently on either side of the housing. 
     Advantageously, the frame further comprises at least two fixing notches located at the inlet opening and situated on either side of the housing for attaching the rails for guiding the plates into the housing. Similarly, the orthogonal projections onto the central longitudinal plane of the fixing notches situated on either side of the housing are spaced apart vertically. The fixing notches are substantially identical. This way, the same rails can be used equivalently on either side of the housing. 
     The frame may also comprise at least two fixing notches ( 116 , 117 ) located at the outlet opening and situated on either side of the housing for attaching the rails ( 66 ,  68 ) for guiding the plates out of the housing. As for the fixing notches located at the inlet, the orthogonal projections onto the central longitudinal plane of the notches situated on either side of the housing are spaced apart vertically. The fixing notches being substantially identical, the same rails can be used indifferently on either side of the housing. 
     In certain embodiments, the fixing notches are located so as the rails attached to the fixing notches extend from the pushers received in the recesses situated on the same side of the housing. This way, the plate is displaced from the standby position to the operating position and from the operating position to the exit position along a substantially horizontal plane. 
     The invention provides a foolproof system ensuring that an inattentive operator would not assemble the parts of the device in the wrong direction, due to the fact that all the parts are identical. The asymmetry is created by the frame, the frame being designed for receiving standard identical parts in specific area. 
     As it is possible to fit the frame with the same pressing element or means and the same rails on either side of the housing, the stock management is also simplified. 
     The invention thus requires the use of a plate wherein the thrust edges, i.e. the portions to receive the thrust from the pressing element or means, are not symmetrical. Such a plate can thus only be inserted into the housing of the frame in one sole orientation ensuring the proper functioning thereof, both for metal casting and for interrupting said casting if required. 
     For matching the pushers of the device, the plate comprising a pair of opposed thrust edges spaced apart vertically. 
     In certain embodiments, the plate comprises a pair of opposed plate edges, one of which having a first thickness and the second of which having a second thickness greater than said first thickness; the bottom surface of the plate edges corresponding to the trust edges. 
     In certain embodiments, the second thickness is at least 5 mm greater then the first thickness, or at least 10 mm greater. 
     The term “plate edge thickness” refers to the distance, in the vertical direction, between the top surface and the bottom surface of the plate edge. Generally, the top surface of the edge is flush with the sliding face of the plate, and the bottom surface consists of a surface engaging by sliding with a bottom wall of a guiding rail provided on the device for holding and replacing plates. For example, both plate edges each have a substantially rectangular cross-section, the height of one of the two rectangles being smaller than that of the other. 
     In the case illustrated by the figures, the bottom surface of the plate edge corresponds to the sliding surface and the trust surface. 
     The invention also relates to an assembly of pressing element or means and of a frame wherein the pressing element or means are assembled into the recesses of each side of the housing. 
     The assembly further comprises a plate having a pair of opposed thrust edges mating the pressing element or means in operating position; typically a plate as described above. Due to the asymmetrical edges thereof, the plate can only be arranged in a device for replacing plates along a single direction, the asymmetrical edges performing a keying role. Indeed, since the two thrust edges do not match symmetrically, a simple way to distinguish them is provided and the insertion of one thrust edge instead of the other in the device for replacing plates can advantageously be prohibited. Also, if a new plate in the standby position is arranged in the wrong direction, the asymmetrical thrust edges indicate that the direction is incorrect. For example, the operator may observe that the arrangement is incorrect by noting that the sliding face of the plate in the standby position is not arranged correctly in a housing or that the casting tube is not perpendicular (vertical). According to a further example, the incorrectly positioned asymmetrical edges in relation to the device for replacing plates may prevent any insertion of the plate in the device. The asymmetrical edges may also prevent the insertion of a plate due to the interaction of the edges of the plate with the pushers of the frame. 
     Generally, the plate comprises a refractory element, the refractory element comprising a sliding face and a casting orifice, and a metallic casing cladding a portion of the refractory element but the sliding face. In certain embodiments, the metallic casing comprises the thrust edges. 
     The refractory element may comprise a casting tube opening onto the casting orifice and projecting from the metallic casing. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The invention will be understood more clearly on reading the following description, given merely as a non-limitative example of the scope of the invention with reference to the figures, wherein: 
         FIG. 1  is a longitudinal sectional view of a device for holding and replacing plates according to the prior art, illustrating the scenario wherein the plate is inserted in the wrong direction; 
         FIG. 2  is a perspective view of a frame of a device for holding and replacing plates, illustrating a plate in the casting position according to the invention; 
         FIG. 3  is a schematic cross-sectional view of  FIG. 2  along the axis III-III; 
         FIG. 4  is a perspective view of a plate of an assembly according to the invention; 
         FIG. 5  is a perspective view of a metallic casing of a plate according to  FIG. 4 ; 
         FIGS. 6 and 7  are similar views to  FIGS. 2 and 3  representing impossible insertions of a plate in a frame according to the invention; 
         FIG. 8  is a longitudinal sectional view of a device for holding and replacing plates, illustrating a plate in the casting position and a plate in the standby position; 
         FIG. 9  illustrate the pressing means; 
         FIG. 10  is a perspective bottom view of a frame according to the invention; 
         FIG. 11  is a perspective bottom view of a frame according to the invention partly assembled with the pressing means. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The vertical direction is defined as the direction of flow of the molten metal at the metallurgical vessel outlet. Furthermore, the longitudinal direction of the casing, plate, frame or device for holding and replacing plates is defined as the direction wherein the plate is replaced from a standby position to a casting position. Finally, the transverse direction is defined as the direction perpendicular to the two other vertical and longitudinal directions, such that the longitudinal, transverse and vertical directions define a three dimensional orthogonal coordinate system. It should be noted that the longitudinal and transverse directions are defined with reference to the direction of movement of the plates during the replacement thereof in the device, these directions may particularly be applied to plates wherein the sliding face has a square or rectangular general shape, regardless of the orientation of the rectangle. The central longitudinal axis corresponds to the longitudinal axis of the sliding face of the plate or the casting opening of the frame. This longitudinal axis passes through the centre of the plate casting orifice, the orifice possibly having a circular or oblong shape, and through the centre of the sealing surface thereof, corresponding to the centre merging with the centre of the casting channel when the plate is in the sealing position. 
     Hereinafter, the vertical direction, corresponding to the casting direction, is referred to as the Z direction, the longitudinal direction, corresponding to the plate replacement direction, is referred to as the X direction, and the transverse direction is referred to as the Y direction. The X, Y, Z directions are orthogonal with respect to each other. In the case of the present invention, the plate replacement direction is also referred to as the plate insertion direction. The flow is carried out from the upper refractory element to the lower refractory element, in particular from the inner nozzle  18  to the plate  34 . 
     In the case of a plate with a generally rectangular outline, the central longitudinal plane may be defined as the plane comprising the vertical axis passing through the centre of the casting orifice and the median of the two longest sides of the rectangle circumscribing the plate. The central longitudinal axis corresponds to the XZ plane in the operating position. 
     In the case of a plate with a generally square outline wherein the casting orifice is off-centre, the central longitudinal axis is the axis comprising the centre of the casting orifice and the intersection of the diagonals of the square circumscribing the plate. The longitudinal axis corresponds to the X axis when the plate is in the operating position. 
     As can be seen in  FIG. 10 , the frame  30  of the device  90  (not shown) comprises a casting opening  21  configured to be in registry with the casting channel of the vessel in operation. The three dimensional orthogonal coordinate system has been positioned in the centre of the casting opening  21  to facilitate the understanding of the invention. The translation axis X corresponds to the plate replacement direction also illustrated by the arrow  14 . The axis Z corresponds to the casting direction and the axis Y corresponds to the transverse direction which is perpendicular to the other two axes. 
     The frame comprises a first upper portion and a second lower portion joining at a middle section plane  51  defining the plane where the inner nozzle  18  and the plate  34  form a sliding contact. The middle section plane  51  is represented  FIG. 3 . The upper portion of the frame is located above the plane  51  and the lower portion of the frame is located below plane  51 . The sliding face of the inner nozzle  18  and the sliding face  16  of the plate  34  join at the middle section plane  51 . The upper portion of the frame comprises receiving and clamping elements, or means for receiving and clamping the inner nozzle in operation position. The lower portion of the frame is described in relation to  FIG. 10 . 
     The frame  30  represented in  FIGS. 2 and 3  defines a housing  32  for receiving a plate  34  and holding it in the casting position against a metallurgical vessel (not shown) situated above said plate. The central longitudinal plane  50  of the frame is parallel with the XZ plane, or merges therewith. 
     As can be seen in  FIGS. 10 and 11  which is a bottom view of the lower part of the frame, the lower portion of the frame comprises a passage extending along the first translation axis (X) between an inlet opening and an outlet opening corresponding to the plate replacement direction  14 . The plate  34  is introduced into the frame  30  at the inlet and is moved in operating position by translation along the plate replacement direction  14 . When a new plate is introduced into the frame, the worn plate  34  is extracted from the frame  30  towards the outlet. The XZ plane corresponds to the central longitudinal plane  50  and the plane XY is parallel to the middle section plane  51 . In operating position, the plate  34  is received and held in a housing  32  in the vicinity of the casting opening  21 . The housing  32  comprises two sides  100 , 101  substantially parallel to the plate insertion direction  14 , each side  100 , 101  of the housing  32  comprises recesses  110  for receiving means  120  for pressing up the plate, in the direction of the upper portion of the frame. The recesses of the side  100  are not at the same level as the recesses of the side  101 . They are offset by a distance d along the Z axis. 
     As explained above, on either side of the housing  32 , in relation to the central longitudinal plane  50 , the frame  30  comprises recesses for receiving pressing means  120  intended, when the device is assembled, to apply a force on the plate  34  in the direction of the upper portion of the frame. The pressing means  120  comprises pushers  54 , for instance a rocker arm  56 , traversed by a longitudinal axis  58 , pivotably mounted about said axis  58 . The arm  56  comprises an end or rocker arm extremity  60  for supporting a compression means  62 , in this case, a compression spring  62 . The springs  62  applying downward pressure on the end  60 , which applies upward pressure parallel with Z on the opposite end  64 . The pressing means are represented schematically in  FIG. 3  and in detail in  FIG. 9 . 
     The recesses  110  comprise holes  111  and grooves  112  configured for receiving the springs  62  and the rocker arms  56  as well as slots  113  having a slot axis ( 58 ) for articulating the rockers ( 56 ) 
     The recesses for receiving the pressing means  120  on either side of the central longitudinal plane  50  are so disposed that the orthogonal projection onto the central longitudinal plane ( 50 ) of the recesses ( 110 ) situated on the first side ( 100 ) of the housing ( 32 ) are spaced apart vertically from the orthogonal projection onto said central longitudinal plane ( 50 ) of the recesses ( 110 ) situated on the second side ( 101 ) of the housing ( 32 ). Indeed, the pushers  54  are, in this case, positioned such that the height of the housing  32  next to the pushers on one side of the housing is different to the height of the housing  32  next to the pushers on the other side of the housing. The pushers  54  situated on either side of the housing  32  of the frame  30  are not at the same height along the Z axis. This creates asymmetry of the housing  32  along the central longitudinal plane  50 , thus the plate insertion direction. 
     This asymmetry of the housing  32  makes it possible to produce a keying device ensuring that an inattentive operator would not introduce the plate  34  in the wrong direction into the housing  32 , due to the fact that the recesses for the pressing element or means  120 , in particular for the pushers,  54  situated on either side of the central longitudinal plane  50  of the housing  32  are not symmetrical. 
     As can be seen in  FIGS. 10 and 11 , the frame  30  also comprises notches  114 , 115  at the inlet opening for receiving a first  66  and a second  68  guiding rails. The notches are spaced apart vertically (along the axis Z). In fact, the notches are offset from a distance d. The rails are assembled into the notches by means known in the art. Once assembled, the first  66  and second  68  rails are asymmetrical in relation to the central longitudinal plane  50 . They are also offset from a distance d. The distance d is illustrated  FIG. 7 . 
     The notches are positioned on the frame  30  so as the rails extend from the pushers  54  received in recesses situated on the same side of the housing  32 . For the rails, the term “extending from” the thrusters refers to the fact that a plate  34  inserted in the device for holding and replacing plates can slide on the rails  66  and  68  into the housing  32  where it is then thrust towards the inner nozzle  18  by the pushers  54 . The guiding rails  66  and  68  can thus be slightly offset in relation to the pushers  54 . 
     The frame  30  may also comprise similar rails  116 , 117  situated at the outlet opening ( FIG. 10 ). These rails are used to guide the worn plate in the ejection or exit position. As for the rails  66  and  68 , these rails extend from the pushers  54  received in recesses situated on the same side of the housing  32 . 
     In the case illustrated, the rails  66  and  68  are identical and standard but are positioned on the frame at different heights along the Z axis. During assembly of the frame  30  or during maintenance operations, an inattentive operator would not be able to assemble the rails incorrectly as all the rails are identical and fit the notches. The rails  66 ,  68 , in this embodiment, are attached to the frame  30  by known means, for example screws ( FIG. 11 ). 
       FIG. 11  represents a frame partly assembled with the pressing element or means and the rails. As can be seen in  FIG. 11 , the pressing element or means and the rails are identical for both sides of the frame. The asymmetry is given by the location of the recesses and the location of the notches. 
     It can be seen in  FIG. 4  that the plate  34  according to the invention comprises a refractory element  46  and a metallic casing  52  for encasing the refractory  46 . The refractory element  46  comprises a casting tube  47 , extending from the casting channel  20  to lateral outlets or ports  48  through which the molten metal flows. The casting tube projects downstream from the metallic casing  52 , with reference to the direction of flow of the molten metal. However, it could be envisaged that the element  46  with the casing  52 , form a basic plate, without or with a short tubular extension  47 . 
     The plate  34 , more specifically the refractory element  46 , comprises a sliding face  16 . In the casting position, the sliding face  16  is in contact with an upstream refractory element, with reference to the direction of flow of the molten metal. More specifically, the face  16  is in contact with an inner nozzle  18  partly embedded into the bottom wall of a metallurgical vessel, said inner nozzle  18  comprising a casting channel  20 . 
     The sliding face  16  comprises a casting orifice  22  centred on a geometric axis  70  and intended to extend from the channel  20  when the plate  34  is in the casting position. Furthermore, the sliding face  16  comprises, to the rear of the orifice  22 , a sealing surface or shut-off surface  24  for sealing the channel  20  when the plate  34  moves to the sealing position. The orifice  22  is aligned with the sealing surface  24 , along a longitudinal axis  72  which, with the geometric axis  70  of the casting orifice  22 , defines a central plane ( 70 , 72 ). The central plane corresponds to the central longitudinal plane  50  of the housing  32  when the plate  34  is inserted in the device. 
     This plate  34  comprises, on either side of the casting orifice in relation to the central plane, thrust edges  74 ,  76 , intended to be subjected to a force applied by the pushers  54  when the plate  34  is inserted into the device. The thrust edges  74 ,  76 , do not match in the planar symmetry defined by the central plane. In the case illustrated, the plate sliding edges enabling the plate to slide in the device for holding and replacing plates merge with the thrust edges  74 ,  76 . 
     These thrust edges  74 ,  76  are thus asymmetrical in relation to the central plane or the central longitudinal plane  50  such that a single direction is possible for introducing the plate  34  into the device for replacing plates. More specifically, in this example, the plate edges  78 ,  80  are asymmetrical along the vertical direction Z, in that they have a different thickness, along the entire guiding length thereof. Indeed, each edge  78 ,  80 , comprises three adjacent surfaces respectively orthogonal to one another, i.e. a horizontal top surface  78   a ,  80   a , slightly recessed with respect to the sliding face  16  of the refractory element  46 , a substantially vertical lateral surface  78   b ,  80   b , parallel to the central plane and a horizontal bottom surface  78   c ,  80   c , in this case merged with the thrust edges  74 ,  76 . The thickness  84 , or height  84 , of the first edge  80  is greater than the thickness  82  of the second edge  78 . In other words, the distance in the Z direction of the orthogonal projection of the edge  82  on the central plane is less than that of the edge  80 , by a value d. For improved comprehension, the references have been indicated in  FIG. 5 . 
     It can be seen in  FIG. 4  that the lateral outlets  48  are aligned along the longitudinal axis  72  substantially parallel with the thrust and sliding edges  74 ,  76  of the plate  34 . 
     The metallic casing  52  illustrated in  FIG. 5  is made of cast iron and is thick but it could be made of another material. It is intended to clad the plate portion of the refractory element  46 , seen in  FIG. 4 . The assembly of the casing  52  and the element  46  forms a plate  34  for transferring the liquid metal. The casing  52  is particularly used to stiffen the element  46 . 
     The casing  52  is much more resistant than the refractory element  46  to the molten metal casting conditions. Therefore, it can be considered to reuse the casing for assembling a new refractory element  46  into it. As mentioned above, the refractory element projects from the metallic casing. The surfaces  78   a  and  80   a  are thus slightly recessed in relation to the sliding surface  16 . 
     Due to the asymmetry of the edges  78 ,  80  of the plate  34  and the asymmetry of the pushers  54  and the rails  66 ,  68 , it is not possible to insert the plate  34  in the wrong direction in a device for replacing and holding plates, as illustrated in  FIGS. 6 and 7  where it can be seen that if an operator tries to insert the plate  34  in the wrong direction, i.e. by positioning the sealing surface  24  at the front, the edge  78  would not be able to enter the housing  32  as the thickness  84  thereof is greater than the height of the housing  32  at this point. Furthermore, if the frame  30  comprises guiding rails  66 ,  68 , to the rear of the housing  32 , the operator may be able to slide the plate  34  on these rails, but he will quickly notice the error as the axis of the casting tube would not be aligned with the casting direction Z and the plate  34  would not be able to enter the housing  32 . 
     In the example shown  FIG. 3 , the asymmetry is provided on the metallic casing. The casing  52  has a pair of opposed side edges of different thickness. but the refractory element  46  is of the standard type, i.e. having no asymmetry in relation to the central plane. However, it is also possible to use a refractory  46  itself asymmetrical in relation to the central plane. 
     The operation of the device  90  will now be described with reference to  FIG. 8 . 
     When a plate  12  is in the casting position, a new plate  10  is moved to the standby position on the device  90 . To replace the plate  12 , the plate  10  is pushed in the X direction, which moves the plate  12 . The plate  12  first moves to the sealing position and, then, under the effect of an additional driving force, moves to its exit position. Once the plate  10  has replaced the plate  12  in casting position, a new plate can be fed again in standby position 
     It is understood that, due to the asymmetry of the thrust edges  74 ,  76  of the plate  34  and the asymmetry of the frame  30  (leading to the asymmetry of the pressing element or means  120  and of the guiding element or means ( 66 ,  68 )), the plate  10  is guaranteed to be inserted in the correct direction in the device  90 . 
     It should be noted that the invention is not limited to the embodiments described above. 
     REFERENCES 
     
         
           10 . New plate 
           12  Worn plate 
           14  Sliding direction 
           16  Sliding face 
           18  Inner nozzle 
           20  Casting channel 
           21  Casting opening 
           22  Casting orifice 
           24  Sealing surface or shut-off surface 
           26  Gap 
           30  Frame 
           32  Housing 
           34  Plate 
           46  Refractory element 
           47  casting tube of the Refractory element 
           48  outlets or ports 
           50  central longitudinal plane 
           51  middle section plane 
           52  Metallic casing 
           54  Thrusters or pushers 
           56  Arm or Rocker arm 
           58  slot axis 
           60 ,  64  arm extremity 
           62  Compression means 
           66 , 68  Rails for guiding the plate 
           70  Geometric axis (=axis of the casting orifice) 
           72  Longitudinal axis 
           74 ,  76  Thrust edge 
           78 ,  80  Plate edge 
           78   a ,  80   a  Top surface of edge 
           78   b ,  80   b  Lateral surface of edge 
           78   c ,  80   c  Bottom surface of edge 
           82 ,  84  Edge thickness 
           90  Device 
           100 , 101  Side of housing 
           110  Recesses 
           111  Hole 
           112  Groove 
           113  Slot 
           114 , 115 ,  116 , 117  Notch 
           120  Pressing means