Abstract:
Molten steel is conducted by a tubular shroud interconnecting a slide gate at a bottom tap hole of a ladle with the molten steel in an underlying tundish of a continuous caster. The flow path is confirmed to be isolated from contaminants in atmospheric air by applying a source of partial vacuum to the internal cavity of tubular shroud to allow prevailing atmospheric pressure acting on molten steel in a tundish to push molten steel upwardly in the internal cavity of the tubular shroud. A measure of the partial vacuum in the cavity of the shroud is used to assess the integrity of the gas tight seal. Before and after the integrity of the gas tight seal is determined, a three way valve is used to apply an inert gas to the volume in the cavity of the shroud.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   Not applicable 
   BACKGROUND OF THE INVENTION 
   The present invention relates to the continuous casting of molten steel transferred to a tundish of a continuous caster by a succession of ladles where molten steel flows from a ladle into the tundish and then into one or more continuous casting molds. Maintaining a steady flow of molten steel into the one or more continuous casting molds is essential, so as not to disturb a delicate balance of sufficient cooling for the necessary containment of a liquid steel core in the newly formed solid shell, and casting speed required for proper metal solidification. The temperature of the molten steel is critical to the casting process, and so it is imperative that a ladle is tapped for the casting process according to a schedule established to prevent the cooling of the molten steel below a desired casting temperature and to avoid the need to return the molten steel to the steel making furnace for reprocessing and the consequential stoppage of the casting process. The volume of molten steel in the tundish is selected to always maintain an operating level even when floating out of slag impurities and during an interruption to the flow of molten steel from one ladle during sequencing of ladles to the caster to reestablish the flow of molten steel by a second ladle. 
   A typical sequencing of ladles is started by first increasing the flow of molten steel from a first ladle before the ladle is empty to raise the liquid steel level in the tundish above an operating level. When the flow from the ladle changes from molten steel to slag, the slidegate is closed to stop the flow of slag. A pouring shroud is disconnected from the ladle slidegate and the ladle is moved away from the casting position. At the same time a second ladle is brought into the casting position and a pouring shroud is connected to the slidegate of the second ladle whereupon opening of the slidegate initiates the flow of molten steel into the tundish. The entire sequence, from the stoppage of the flow of molten steel in one ladle to the establishment of a flow of molten steel in a replacement ladle, must be completed before the liquid level in the tundish has been depleted to a certain critical level, below which the quality of the cast steel strand is adversely affected. The sequence of changing the supply of molten steel from one ladle to another is normally accomplished within a very safe time margin. Two typical devices employed in the efficient exchange of ladles are ladle cars and a ladle turret. 
   The present invention is addressed to the management of a ceramic pouring shroud having the general form of a tube arranged to isolate the stream of molten steel from contamination, for example, with oxygen and nitrogen in the atmosphere while passing from the ladle to the tundish. Such a ceramic pouring shroud is supported and moved by operation of a manipulator that includes synchronous movement of the ladle and ceramic pouring shroud after pressing the upper end of the ceramic pouring shroud into sealing contact the with the slide gate of the ladle. The sealing contact is maintained while the ladle is moved with into the casting position wherein the lower end of the ceramic pouring shroud is partly submerged in the molten steel in the tundish. The environment and conditions wherein these operations by the manipulator are carried out are very adverse because of the extreme temperature and the very limited amount of time available to establish and maintain the essential gaseous sealed relation between the ladle and the tundish. A need therefore exists for verifying the sufficiency of the gaseous tight seal between the ceramic pouring shroud and the ladle to prevent the ingress of atmospheric gaseous while molten steel is flowing in the shroud from the ladle to the tundish. A need also exists to establish and maintain an inert atmosphere in the cavity of the ceramic pouring shroud at least until the flow of molten steel commences in the ceramic pouring shroud but preferable continuously while molten steel flows from the ladle to the tundish. 
   Accordingly, it is an object of the present invention to provide a method and apparatus for verifying the sufficiency of the gaseous tight seal between the ceramic pouring shroud and a ladle for molten steel incident to the operations of a manipulator employed to hold the ceramic pouring shroud against a ladle containing molten steel. 
   It is a further object of the present invention to provide a method and apparatus to supply establish and maintain an inert atmosphere in the cavity of the ceramic pouring shroud at least until the flow of molten steel commences in the ceramic pouring shroud from a ladle to a tundish. 
   SUMMARY OF THE INVENTION 
   According to the present invention there is provided in a continuous caster for molten steel conducted from a slide gate at a bottom tap hole of a ladle into a tundish along a flow path confirmed to be isolated from contaminants in atmospheric air, apparatus for establishing the flow path including the combination of a ladle lift actuator to position such a ladle between a shroud assembling position and a ladle taping position for delivering molten steel into a tundish, a generally tubular shroud to communicate with such a slide gate for conducting molten steel from the bottom tap hole in the ladle, a manipulator including a shroud support moveable by manipulator actuators to displace the shroud into a gaseous sealing relation with the slide gate at the shroud assembling position, the manipulator actuators being operative to displace the generally tubular shroud while supported by the manipulator in the gaseous sealing relation with the slide gate as a unit with the ladle by operation of the lift actuator in a direction for establishing the ladle taping position wherein molten steel is conducted by the tubular shroud into the tundish beneath a surface of molten steel therein, a control for gaseous mediums to selectively connect a metal flow path in the shroud with a supply of an inert gas for purging the metal flow path of atmospheric air contaminants prior to receiving molten steel from the ladle and while conducting molten steel into the tundish, a source of partial vacuum, the control being operable to apply the source of partial vacuum to the metal flow path in the shroud, and a sensor responsive to the prevailing gas pressure of the applied partial vacuum by the control in the metal flow path for monitoring the integrity of the gaseous sealing relation with the slide gate. 
   According to another aspect of the present invention, there is provided 7. A method for supplying molten steel from a slide gate at a bottom tap hole of a ladle into a tundish along a flow path isolated from contaminants in atmospheric air, the method including the steps of assembling a tubular shroud in a manipulator, moving a ladle containing molten steel into a shroud assembling position, operating the manipulator to press the tubular shroud against the slide gate of the ladle to form a gas tight seal, introducing an inert gas into an internal cavity of the tubular shroud to purge atmospheric air and maintain an inert gas atmosphere therein, moving the ladle and tubular shroud in unison to ladle taping position wherein the open end of the tubular shroud is submerge in molten steel in a tundish for delivering molten steel into a tundish, terminating the supply of inert gas into an internal cavity of the tubular shroud, applying a partial vacuum to the internal cavity of tubular shroud to allow prevailing atmospheric pressure acting on molten steel in a tundish to push molten steel upwardly in the internal cavity of the tubular shroud, using a measure of the partial vacuum in the cavity of the shroud to assess the integrity of the gas tight seal, and operating the slide gate when the integrity of the gas tight seal is adequate to isolate the internal cavity of the shroud from contamination by atmospheric air while delivering molted steel into the tundish. 
   The method of the present invention may also be performed by the steps of engaging a tubular shroud with a manipulator, operating the manipulator to press a shroud seal at one end of the tubular shroud against a slide gate arranged for controlling hot metal flow from a bottom tap hole in a ladle, purging the molten metal flow path in the shroud with argon gas at least until the manipulator moves the shroud into a position where the shroud permeates the molten steel level in a tundish, and thereafter applying only vacuum to the molten metal flow path in the shroud, determining the integrity of the connection between the shroud seal and the slide gate as a measure of the vacuum generated in the molten metal flow path in the shroud, terminating the vacuum and applying a supply of argon gas to the molten metal flow path when the measure of the magnitude of vacuum is sufficient to verify the integrity of the shroud seal, and operating the slide gate to supply molten steel form the ladle to the tundish. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The present invention will be more fully understood when the following description is read in light of the accompanying drawings in which: 
       FIG. 1  is an elevation view illustrating a ladle turret incorporating a ladle lift bearing system of the present invention for positioning of a ladle to discharged liquid metal to a tundish of a continuous caster; 
       FIG. 2  is a plan view of the ladle turret for the arrangement shown in  FIG. 1 ; 
       FIG. 3  is illustrates a manipulator for a tubular shroud forming part of the apparatus according to the present invention; and 
       FIGS. 4A ,  4 B and  4 C are schematic illustrations of a sequence of operation of the manipulator apparatus illustrated in  FIG. 3  for installing a tubular shroud on a slide gate of a ladle according to the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIGS. 1 and 2  illustrate a ladle turret arrangement for handling ladles of molten steel in a continuous casting installation. This ladle turret arraignment is suitable to practice the present invention although other well known ladle handling facilities can be used without departing from the present invention. Molten steel is delivered in ladles  11  and  12  supported by a ladle turret  13  used to rotate the ladles into and from a position directly above a tundish  14 . The tundish delivers a stream of molten steel controlled by a stopper  15  to a mold  16  of a continuous caster  17 . Each ladle is provided with a pair of trunnions  18  at diametrically opposite sites lying along a horizontal axis located above the center of gravity of the ladle. The trunnions are engaged by J-hooks, not shown, of a hot metal crane to transport the ladle to and from the ladle turret. Each ladle is also provided with rectangular ladle seats  19  located below the trunnions  18  for supporting the ladle on spaced apart L-shaped support arms  21  forming part of the ladle turret  13 . The construction and operation of the ladles, tundish and continuous caster are per se well known in the art. The ladle support arms  21  include a horizontal leg  22  formed with an elongated recess  23  providing horizontal stop surfaces for positive retention of the ladle seats  19 . Movements of the ladle support arms are mechanically synchronized for precise controlled lifting of the ladles by the use of two identical parallelogram linkages for each ladle. The parallelogram linkages for each ladle are located in parallel vertical planes containing the ladle support arms  21 . The L-shaped configuration of each ladle support arm further includes a vertical leg  24  extending upward from the horizontal leg  22  and provided with vertically spaced bores joined by pivot pins  25  and  26  to a lifting arm  27  located above and parallel with a stabilizing arm  28 . The free ends of the arms  27  and  28  are joined to a vertical frame  29  by pivot pins  30 . The frame  29  is centrally located on a circular plate  31  and rotated on a base  31 A provided with gear teeth to mesh with teeth of a drive gear  32  driven by a motor  33 . The lifting arm  27  for supporting each ladle is joined by an upper link frame  34  at a location approximately midway between the pivot pins  25  and  30 . The upper link frame  34  is lifted vertically by an actuator assembly  35 . It is necessary to raise a ladle to an elevation suitable for installing a tubular shroud  36  to a ladle nozzle  37  forming part of a ladle side gate  38  and when necessary to a selected elevation to allow the use of an oxygen lance to melt solidified steel in the ladle exit port. After the tubular shroud is installed, the actuator assembly  35  is operated to lower the ladle until the discharge end of the tubular shroud  36  is submerged in molten steel in the tundish  14 . 
   The present invention incorporates a manipulator  40  and associated apparatus to install the ceramic pouring shroud  36  and to asses the integrity of a gaseous sealed connection between the tubular shroud and a slide gate  38 . Referring to  FIG. 3 , the shroud  36  which is made of high temperature resistant ceramic material has an enlarged, cup shaped end portion  42  on which a seal  43  of high temperature resistance sealant material such as ceramic fiber is placed for forming a gas impervious seal with the ladle nozzle  37  so that molten steel can pass to an internal duct  44  in an elongated tubular section  46  of the shroud. The shroud has an extended length sufficient to submerge an end portion  48  in a volume of molten steel  50  beneath a protective layer  52  of slag in the tundish  14 . A carrier sleeve  54  has a central opening dimensioned to seat the elongated tubular section  46  and allow the cup shaped end portion  42  to engage in supporting contact with the carrier sleeve  54 . The carrier sleeve  54  is provided with trunnions  56  extending from opposite lateral sides for pivotal supported in cradle arms  58  that are part of a fixture  59  mounted on the extended end of a manipulator arm  60 . Opposed pairs guide rollers  62  support the manipulator arm  60  for movement in the direction of the extended length of the arm by an actuator  64  such as a motor. The rollers  62  are mounted onto a frame  66  connected by a pivot pin  67  to a base  68  that can be raised and lowered by an actuator  70  such as a piston and cylinder assembly. The base  68  includes an upstanding lever  68 A joined by a clevis to the rod end of a piston and cylinder assembly  72  or other forms of and actuator which can be operated to adjust the angular relation between the manipulator arm  60  relative the base  68 . The actuators  64 ,  70  and  72  are controlled by a programmed controller interacting with the control for actuator assembly  35  so that the ladle  12  and tubular shroud  36  move as a unit from the assembly site as illustrated diagrammatically in  FIG. 4A  to a generally lower position where the end portion  48  of the shroud is submerged into molten steel as illustration diagrammatically in  FIG. 4B . During the time required for this unitary movement, a three way valve  80  is operated to supply an inert gas by duct  82  such as argon gas to the internal duct  44  from a source  84  sufficient to purge the volume of the duct  44  of atmospherics air. The arrangement is such that by the time of the lower end of the shroud is in close proximity to the upper surface of the layer of slag  52 . The entire volume of the internal duct  44  will contain essentially only inert gas. After the end portion  48  is lowered to the desired submerged location in the molten steel, the three way valve  80  is operated to terminate the supply of inert gas and moved to a position to apply a predetermined partial vacuum previously created in a storage vessel  86 . A supply of pressure resistant vessels in a serial fashion to create the needed partial vacuum in the shroud each time the integrity of the flow path in the shroud seal is confirmed to be isolated from contaminates in the atmospheric air. The applied partial vacuum is of a predetermined magnitude so that a column of molten steel is drawn upwardly in the internal duct  44  of the shroud  36  in response to the atmospheric pressure applied across the surface of the layer of slag and thus also the underlying layer of molten steel. The duct  82  is coupled to a pressure transducer  88  which supplies a corresponding electrical signal in a feed back loop by electric lines to a monitor, not shown, where data received is used to determine the length of column of molten steel and thus also the integrity of the gaseous seal between the cup-shaped end portion  42  and the ladle nozzle  37 . The length of the column of molten steel in the shroud depends on the magnitude of the partial vacuum that can be generated. Thus a negative gage static pressure prevails in metal flow path in the shroud between slide gate and the upper surface of the molten steel column residing in the shroud. As shown in  FIG. 4C , when the magnitude of the partial vacuum that can be generated corresponds to a predetermined value, a controller supplies a control signal to an actuator used to move the ladle slide gate  38  to an open position and at the same time the controller supplies a control signal to the three way valve  80  to shift the three way valve to a position to again to start the flow of an inert gas into the duct  44  while molten steel is in transit down the duct in the tubular shroud  36  to the tundish  14 . If the required magnitude of the partial vacuum is not achieved, it is assumed the shroud  36  and/or seal  43  must be quickly replaced. For this purpose, the turret and manipulator are operated to reposition the ladle and the shroud to the relative location shown the diagram of  FIG. 4A . After a replaced shroud  35  and seal  43  are installed by the manipulator on the slide gate of the ladle, the ladle and the shroud are again reposition toward the tundish as shown in  FIG. 4B  and explained here in before. In this procedure the seal between the tubular shroud and the nozzle if it is not correct, corrective action can be taken before starting the flow of molten steel into the tundish. 
   While the present invention has been described in connection with the preferred embodiments of the various figures, it is to be understood that other similar embodiments may be used or modifications and additions may be made to the described embodiment for performing the same function of the present invention without deviating there from. Therefore, the present invention should not be limited to any single embodiment, but rather construed in breadth and scope in accordance with the recitation of the appended claims.