Patent Publication Number: US-7721785-B2

Title: Continuously casting steel strip

Description:
This application is a continuation of co-pending application Ser. No. 11/005,722, now U.S. Pat. No. 7,191,819, the disclosure of which is hereby incorporated by reference. 

   BACKGROUND AND SUMMARY OF THE INVENTION 
   This invention relates to continuous casting of thin steel strip in a strip caster, particularly a twin roll caster. 
   In a twin roll caster, molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are internally cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a thin cast strip product, delivered downwardly from the nip between the casting rolls. The term “nip” is used herein to refer to the general region at which the casting rolls are closest together. 
   The molten metal may be poured from a ladle through a metal delivery system comprised of a tundish and a core nozzle located above the nip, to form a casting pool of molten metal supported on the casting surfaces of the rolls above the nip and extending along the length of the nip. This casting pool is usually confined between refractory side plates or dams held in sliding engagement with the end surfaces of the rolls so as to dam the two ends of the casting pool against outflow. 
   When casting steel strip in a twin roll caster, the thin cast strip leaves the nip at very high temperatures, of the order of 1400° C. If exposed to normal atmosphere, it will suffer very rapid scaling due to oxidation at such high temperatures. A sealed enclosure is therefore provided beneath the casting rolls to receive the hot cast strip, and through which the strip passes away from the strip caster, which contains an atmosphere that inhibits oxidation of the strip. The oxidation inhibiting atmosphere may be created by injecting a non-oxidizing gas, for example, an inert gas such as argon or nitrogen, or combustion exhaust reducing gases. Alternatively, the enclosure may be sealed against ingress of an ambient oxygen-containing atmosphere during operation of the strip caster, and the oxygen content of the atmosphere within the enclosure reduced, during an initial phase of casting, by allowing oxidation of the strip to extract oxygen from the scaled enclosure as disclosed in U.S. Pat. Nos. 5,762,126 and 5,960,855. 
   The length of the casting campaign has been generally determined in the past by the wear cycle on the core nozzle, tundish and side dams. Multi-ladle sequences can be continued so long as the source of hot metal supplies ladles of molten steel, by use of a turret on which multiple ladles can be transferred to operating position. Therefore, the focus of attention in the casting campaign has been extending the life cycle of the core nozzle, tundish and side dams. When a nozzle, tundish or side dam would wear to the point that it had to be replaced, the casting campaign would have to be stopped, and the worn out component replaced. This would generally require removing unworn components as well since otherwise the length of the next campaign would be limited by the remaining useful life of the worn but not replaced refractory components, with attendant waste of useful life of refractories and increased cost of casting steel. Further, all of the refractory components, both replaced and continued components, would have to be preheated the same as starting the original casting campaign before the next casting could be done. Graphitized alumina, boron nitride and boron nitride-zirconia composites are examples of suitable refractory materials for this purpose. Since the core nozzle, tundish and side dams all have to be preheated to very high temperatures approaching that of the molten steel to withstand contact with the molten steel over long periods, considerable waste of casting time between campaigns resulted. See U.S. Pat. Nos. 5,184,668 and 5,277,243. 
   The disclosed apparatus and method limits down time in changes of worn refractory components, decreases waste of useful life of refractory components, reduces energy needs in casting, and increases casting capacity of the caster. Useful life of refractories can be increased, and reheating of unreplaced refractory components can be avoided or minimized. The core nozzle must be put in place before the tundish, and conversely the tundish must be removed before core nozzle can be replaced, and both of these refractory components wear independently of each other. Similarly, the side dams wear independently of the core nozzles and tundish, and independently of each other, because the side dams must initially be urged against the ends of the casting rolls under applied forces, and “bedded in” by wear so as to ensure adequate sealing against outflow of molten steel from the casting pool. The forces applied to the side dams may be reduced after an initial bedding-in period, but will always be such that there is significant wear of the side dams throughout the casting operation. For this reason, the core nozzle and tundish in the metal delivery system can have a longer life than the side dams, and can normally continue to be operated through several more ladles of molten steel supplied in a campaign. Nevertheless, the duration of a casting campaign is often determined by the rate of wear of the side dams because tundish and core nozzle, which still have useful life, are often changed when the side dams are changed to increase casting capacity of the caster. No matter which refractory component wears out first, a casting run will need to be terminated to replace the worn out component. Since the cost of thin cast strip production is directly related to the length of the casting time, unworn components in the metal delivery system are generally replaced before the end of their useful life as a precaution to avoid further disruption of the next casting campaign, with attendant waste of useful life of refractory components. 
   By the disclosed apparatus and method, it is possible to replace in a minimal period of time anyone or more of the refractory components, for example, the core nozzle, tundish and/or side dams, without replacing any of the other refractory components, to avoid the need for reheating the unreplaced refractory components, and in turn, to extend casting campaign lengths, reduce waste of refractory components, and reduce operating costs and increase casting time. 
   The second tundish and/or second side dam or dams, or portions thereof, are generally preheated and replaced as singular refractory components, and the core nozzle is generally preheated and replaced as a singular or two part refractory component, but in particular embodiments these refractory components may be preheated and replaced in parts or pieces as desired. In any event, the refractory component or portion thereof may be preheated to a temperature near the temperature of molten steel in the casting pool. Typically, the preheat temperature is greater than about 1200° C. The preheating of rapidly transferring of the second core nozzle may be done for at least about 2 hours before transfer to the operating position, the preheating of rapidly transferring of the second tundish may be done for at least about 2 hours before transfer to the operating position, and the preheating of rapidly transferring of the second side dams may be done for at least about 0.5 hours before transfer to the operating position. If only a portion of one of these refractory components is to be replaced, the preheating of that portion of the component will normally be done for the same time period as for the preheating of the entire refractory component unless that portion is such that it can be preheated to the desired preheat temperature in less time. The preheat temperature is also normally the same if more than one core nozzle, one tundish or two side dams is used in the particular embodiment. 
   An apparatus for producing thin cast strip by continuous casting may be comprised of: 
   a) a pair of casting rolls having a nip therebetween; 
   b) a metal delivery system comprising a first core nozzle and a first tundish for delivering molten metal into a casting pool between the casting rolls above the nip, and first side dams adjacent the ends of the nip to confine said casting pool; 
   c) a casting roll drive capable of counter-rotating the casting rolls to form solidified metal shells on casting surfaces of the casting rolls and to cast solidified thin steel strip through the nip between the casting rolls from said solidified shells; 
   d) a plurality of preheating chambers removed from an operating position for casting capable of preheating at least a portion of at least one refractory component selected from the group consisting of a second core nozzle, a second tundish and at least one second side dam to a temperature to avoid thermal shock when contacted by molten steel while casting continues; 
   e) a gate capable of interrupting the flow of molten metal to the casting pool, and capable of resuming flow of molten steel to reform the casting pool; 
   f) a first transfer device capable of removing from an operating position at least portions of at least one component selected from the group consisting of at least a portion of the first core nozzle, the first tundish and at least one of said first side dams desired to be replaced while leaving other components in an operating position; and 
   g) a second transfer device capable of rapidly transferring at least portions of at least one preheated component selected from the group consisting of the second core nozzle, the second tundish and at least one second side dam for replacement from the preheating chamber to the operating position for casting while leaving other components in an operating position. 
   Again, in the apparatus, the second tundish and/or second side dam or dams, or portions thereof, are generally preheated and replaced as singular refractory components, and the core nozzle is generally preheated and replaced as a singular or two part refractory component, but in particular embodiments these refractory components may be preheated and replaced in parts or pieces as desired. In any event, at least one component from the group consisting of the second core nozzle, the second tundish or the second side dams may be preheated to a temperature near the temperature of molten steel in the casting pool. Again, typically the component or components, or portion thereof, to be replaced is/are preheated to 1200° C. The preheating of the second core nozzle may be done for at least about 2 hours before transfer to the operating position, the preheating of rapidly transferring of the second tundish may be done for at least about 2 hours before transfer to the operating position, and the preheating of rapidly transferring of the second side dams may be done for at least about 0.5 hours before transfer to the operating position. Again, if only a portion of one of these refractory components is to be replaced, the preheating of that portion of the refractory component will normally be done for the same time period as for the preheating of the entire refractory component unless that portion is such that it can be preheated to the desired preheat temperature in less time. The preheat temperature is also normally the same if more than one core nozzle, one tundish or two side dams is used in the particular embodiment. 
   The apparatus may further comprise a sensor, such as an optical sensor or an electrical sensor, to monitor the wear of the first core nozzle, the first tundish and/or the first side dams. The method may further comprise the step of monitoring the wear of at least a portion of one refractory component from the group consisting of the first core nozzle, the first tundish and the first side dams. The first core nozzle, first tundish or first side dams may be removed one at a time, or in pieces, when the sensor reveals that the refractory component is worn to a specified limit. Note again that when a refractory component is replaced in parts as worn, a separate sensor will normally be provided for each piece of the refractory component to be replaced as worn. 
   The apparatus may also be automated by including in addition a control system, typically including a computerized circuit, so that, when a given level of wear is detected by the sensor(s) in a particular worn first core nozzle, first tundish and/or first side dames), or portion thereof, the worn refractory component or portion thereof is automatically replaced. Note that when a refractory component is replaced in parts as worn, a separate sensor will normally be provided for each portion of the refractory component to be replaced as worn. 
   Alternatively, the apparatus may have a preheating chamber or chambers removed from an operating position for casting thin cast strip capable of preheating one or both of the second side dams, or portions thereof, to a temperature to avoid thermal shock when contacted by molten steel. In this embodiment, the core nozzle or the tundish, or both, (or a part thereof) may be replaced independently of the side dams. It should be noted that the apparatus can be embodied if more than two side dams are desired to be utilized in a particular embodiment. 
   The molten steel may be introduced between the casting rolls through a metal delivery system comprising a tundish and a core nozzle, in one or more pieces, disposed above the nip, and the interruption of the flow of molten steel to the casting pool may be achieved by interrupting flow to the metal delivery system by closing the slide gate. The preheating of the replacement side dams in the preheat chamber(s) is initiated while continuing casting of the strip. The wear of the side dams may be monitored by a sensor or sensors, and the removal and replacement of the side dams may be accomplished when the sensor indicates that the dams or portion thereof is (are) worn to specified limits. 
   In order to ensure the components in the metal delivery system do not suffer thermal shock on resumption of casting and also to ensure that steel does not solidify within the flow passages of the metal delivery system, it is desirable that the time interval between interrupting and resuming the flow of molten steel in either the method or the apparatus be less than about 15 minutes. The change time will depend on the number and nature of the component or components being replaced, and typically will be less than about 5 minutes. 
   More specifically, the replacement of the replacement one or more side dams, tundish and/or core nozzles, or portions thereof, may be carried out so that this time interval is about 5 minutes or less, or about 2 minutes or less. 
   It should be noted that the tundish here that is replaced is a replaceable tundish above the core nozzle, and may be sometimes called the transition piece or delivery vessel. 
   There may be another tundish above the replaceable tundish, which is also part of the metal delivery system that is not replaced in the disclosed method and apparatus as discussed below. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The operation of an illustrative twin roll installation in accordance with the disclosed method and apparatus will now be described with reference to the accompanying drawings in which: 
       FIG. 1  is a schematic illustrating the operation of the disclosed method and apparatus; 
       FIG. 2  is a vertical cross-section through an illustrative twin roll strip caster installation operable in accordance with the system shown in  FIG. 1 ; 
       FIG. 3  illustrates a metal delivery system for the caster; 
       FIG. 4  is an enlarged view depicting an illustrative caster sealed enclosure to receive the cast strip; 
       FIG. 5  is an enlarged vertical cross-section through an end part of the twin roll caster. 
       FIG. 6  is a cross-section taken generally along the line  6 - 6  in  FIG. 5 ; and 
       FIG. 7  is a cross-section taken generally along the line  7 - 7  in  FIG. 5 . 
   

   DETAILED DESCRIPTION OF THE DRAWINGS 
   The illustrative twin roll caster comprises a twin roll caster denoted generally as  11  producing a cast steel strip  12  which passes within a sealed enclosure  10  to a guide table  13 , which guides the strip to a pinch roll stand  14  through which it exits the sealed enclosure  10 . The seal of the enclosure  10  may not be complete, but appropriate to allow control of the atmosphere within the enclosure and access of oxygen to the cast strip within the enclosure as hereinafter described. After exiting the sealed enclosure  10 , the strip may pass through other sealed enclosures and may be subjected to in-line hot rolling and cooling treatment forming no part of the claimed invention. 
   Twin roll caster  11  comprises a pair of laterally positioned casting rolls  22  forming a nip  15  therebetween, to which molten metal from a ladle  23  is delivered through a metal delivery system  24 . Metal delivery system  24  comprises a tundish  25 , a removable tundish  26  and one or more core nozzles  27  which are located above the nip  15 . The molten metal delivered to the casting rolls is supported in a casting pool  16  on the casting surfaces of the casting rolls  22  above the nip  15 . 
   The casting pool of molten steel supported on the casting rolls is confined at the ends of the casting rolls  22  by a pair of first side dams  35 , which are applied to stepped ends of the rolls by operation of a pair of hydraulic cylinder units  36  acting through thrust rods  50  connected to side plate holders  37 . 
   The casting rolls  22  are internally water cooled by coolant supply  17  and driven in counter rotational direction by drives  18 , so that metal shells solidify on the moving casting roll surfaces as the casting surfaces move through the casting pool  16 . These metal shells are brought together at the nip  15  to produce the thin cast strip  12 , which is delivered downwardly from the nip  15  between the rolls. 
   Tundish  25  is fitted with a lid  28 . Molten steel is introduced into the tundish  25  from ladle  23  via an outlet nozzle  29 . The tundish  25  is fitted with a stopper rod  33  and a slide gate valve  34  to selectively open and close the outlet  31  and effectively control the flow of metal from the tundish to the removable tundish  261 . The molten metal flows from tundish  25  through the outlet  31  through an outlet nozzle  32  to removable tundish  26 , (also called the distributor vessel or transition piece), and then to core nozzles  27 . At the start of a casting operation a short length of imperfect strip is produced as the casting conditions stabilize. 
   After continuous casting is established, the casting rolls are moved apart slightly and then brought together again to cause this leading end of the strip to break away so as to form a clean head end of the following cast strip to start the casting campaign. The imperfect material drops into a scrap box receptacle  40  located beneath caster  11  and forming part of the enclosure  10  as described below. At this time, swinging apron  38 , which normally hangs downwardly from a pivot  39  to one side in enclosure  10 , is swung across the strip outlet from the nip  15  to guide the head end of the cast strip onto guide table  13 , which feeds the strip to the pinch roll stand  14 . Apron  38  is then retracted back to its hanging position to allow the strip to hang in a loop beneath the caster, as shown in  FIGS. 2 and 4 , before the strip passes to the guide table  13  where it engages a succession of guide rollers. 
   The twin roll caster illustratively may be of the kind which is illustrated in some detail in U.S. Pat. Nos. 5,184,668 and 5,277,243, and reference may be made to those patents for appropriate constructional details which form no part of the claimed invention. 
   Enclosure  10  is formed by a number of separate wall sections which fit together at various seal connections to form a continuous enclosure wall. These comprise a first wall section  41  which is formed at twin roll caster  11  to enclose casting rolls  22 , and a wall enclosure  42  which extends downwardly beneath first wall section  41 , to form an opening which is closed by sealing engagement with the upper edges of a scrap box receptacle  40  as described below. 
   A seal  43  between the scrap box receptacle  40  and the enclosure wall  42  may be formed by a knife and sand seal around the opening in the enclosure wall, which can be established and broken by vertical movement of scrap box receptacle  40  relative to the enclosure wall  42 . More particularly, the upper edge of the scrap box receptacle may be formed with an upwardly facing channel which is filled with sand and which receives a knife flange depending downwardly around the opening on the enclosure wall. A seal is formed by raising the scrap box receptacle to cause the knife flange to penetrate the sand in the channel to establish the seal  43  This seal can be broken by lowering the scrap box receptacle  40  from its operative position preparatory to movement away from the caster to a scrap discharge position (not shown). 
   Scrap box receptacle  40  is mounted on a carriage  45  fitted with wheels  46 , which run on rails  47 , whereby the scrap box receptacle can be moved to the scrap discharge position. Carriage  45  is fitted with a set of powered screw jacks  48  operable to lift the scrap box receptacle  40  from a lowered position, in which it is spaced from the enclosure wall  42 , to a raised position where the knife flange penetrates the sand to form seal  43  between the two. 
   Sealed enclosure  10  further may have a third wall section  61  disposed about guide table  13  The third wall section  61  is also connected to the frame of pinch roll stand  14 , which includes a pair of pinch rolls  62 , against which the enclosure  10  is sealed by sliding seals  63 . 
   Most of the enclosure wall sections  41  and  61 , together with wall enclosure  42 , may be lined with fire brick. Scrap box receptacle  40  may be lined either with fire brick or with a castable refractory lining. 
   The first enclosure wall section  41  surrounds the casting rolls  22  and is formed with side plates  64  provided with notches  65  shaped to snugly receive the side dam plate holders  37  when the pair of side dams  35  are pressed against the ends of casting rolls  22  by the cylinder units  36 . The interfaces between the side plate holders  37  and the enclosure side wall sections  41  are sealed by sliding seals  66  to maintain sealing of the enclosure  10 . Seals  66  may be formed of ceramic fiber rope or other suitable sealing material. 
   The cylinder units  36  extend outwardly through the enclosure wall section  41 , and at these locations the enclosure is sealed by sealing plates  67  fitted to the cylinder units so as to engage with the enclosure wall section  41  when the cylinder units are actuated to press the pool closure plates against the ends of the casting rolls. Cylinder units  36  also move refractory slides  68  which are moved by the actuation of the cylinder units to close slots  69  in the top of the enclosure, through which the side dams  35  are initially inserted into the enclosure  10  and into the holders  37  for application to the casting rolls. The top of the sealed enclosure  10  is closed by the tundish  26 , the side plate holders  37  and the slides  68  when the cylinder units are actuated to urge the side dams  35  against the casting rolls  22 . In this way, the complete enclosure  10  is sealed prior to a casting operation, thereby limiting the supply of oxygen to the strip  12  as it passes from casting rolls  22  to the pinch roll stand  14 . Initially the strip may take up all of the oxygen from enclosure  10  space to form heavy scale on the strip. 
   However, the sealing of space of enclosure  10  limits the ingress of oxygen containing atmosphere below the amount of oxygen that could be taken up by the strip. Thus, after an initial start-up period, the oxygen content in the enclosure  10  will remain depleted so limiting the availability of oxygen for oxidation of the strip  12 . In this way, the formation of scale is controlled without the need to continuously feed a reducing or non-oxidizing gas into the enclosure  10 . 
   Of course, a reducing or non-oxidizing gas may be fed into the enclosure  10 . 
   However, in order to avoid the heavy scaling during the start-up period, the enclosure can be purged immediately prior to the commencement of casting, so as to reduce the initial oxygen level within the enclosure  10 . In this way, the time is reduced that is needed to stabilized the oxygen level as a result of the interaction of oxygen in the sealed enclosure due to oxidation of strip  12  passing through it. Thus, illustratively, the enclosure  10  may conveniently be purged with, for example, nitrogen gas. It has been found that reduction of the initial oxygen content to levels of between 5% to 10% will limit the scaling of the strip at the exit from the enclosure  10  to about 10 microns to 17 microns even during the initial start-up phase. 
   When it is determined that a change has to be made in the side dams  35 , core nozzle  27  of removable tundish  26  due to wear or any another reason, preheating is commenced of a second refractory component identified to be in need of replacement. This preheating of the second tundish  26 ′ or second core nozzle  27 ′ is started while casting is continuing at least 2 hours before transfer to the operating position, and the preheating of the second side dams  35 ′ is started at least 0.5 hours before transfer to the operating position. 
   This preheating is done in a preheating heater  58   54  or  57 , typically a preheating chamber, in a location convenient to the caster  11 , but removed from the operating position of the refractory components during casting. 
   During this preheating of the replacement refractory component, casting typically continues without interruption. When the refractory component is ready to be replaced, namely, the tundish  26 , the core nozzle  27  or the side dams  35 , the slide gate  34  is closed and the tundish  26 , the core nozzle  27  and the casting pool  16  are drained of molten metal. 
   Typically, the tundish  26 ′, and side dam  35 ′ are preheated and replaced as singular refractory components, and the core nozzle  27 ′ is preheated and replaced as a singular or two piece refractory component, but in particular embodiments may be preheated and replaced in pieces or parts as those portions of the refractory component are worn 
   If the first tundish  26  is to be replaced, typically transfer car  49  comes in and removes the tundish  26  from the operating position, and then the second tundish  26 ′ is taken from a preheating chamber  58  to the operating position by transfer car  51 . The details of the transfer cars  49  and  51  are not shown since they are essentially fork lifts on rails that move from the preheating position to the operating position, with hydraulic lifts to raise and lower the tundish into either the preheating position or the operating position. Note that transfer cars  49  and  51  may be the same transfer car if there is a place for the car transfer to rapidly set the removed first tundish  26  as shown in  FIG. 1 ; however, to save time in removing the first tundish  26  and positioning the second tundish  26 ′ in the operating position, two transfer cars  49  and  51  may be employed. Following positioning of the second tundish  26 ′ in the operating position, the gate  34  is opened to fill the tundish  26 ′ and core nozzles  27  and continue the casting operation by fining the tundish  26  and core nozzle  27  and forming casting pool  16  with molten metal. 
   If the first core nozzles  27 , typically in two parts, are to be replaced, transfer car  49  comes in and removes the first tundish  26  from the operating position, and then a pair of transfer robots  52  take the first core nozzle  27  from the operating position, and a pair of transfer robots  53  transfer the second core nozzle  27 ′, again typically in two parts, from preheating chambers  54  to the operating position. Note that the core nozzle  27  may be in one or two pieces, or multiple pieces, and may be replaced in whole or in pieces as worn to specified limits, depending on the particular embodiment of the metal delivery system. Note also that transfer robots  52  and  53  may be the same as shown in  FIG. 1  if there is a place for the robots to rapidly set down the removed first core nozzle  27 ; however, to save time in removing the first core nozzle  27  and positioning the second core nozzle  27 ′ in the operating position, separate transfer robots  52  and  53 , typically in pairs, may be employed. Following positioning of the second core nozzle  27 ′ in the operating position, transfer car  49  then repositions the tundish  26  in the operating position and the slide gate  34  is opened to fill the tundish  26  and core nozzle  27 ′ and continue the casting campaign by filling the tundish  26 , core nozzle  27 ′ and casting pool  16  with molten metal. Note that if desired, core nozzle  27 ′ and removable tundish  26 ′ may be replaced at the same time, as described in more detail below. 
   When it is determined that a change has to be made in the side dams  35  due to wear or any another reason, preheating is begun of one or more second side dams  35 ′ identified to be in need of replacement as casting continues. This preheating of the second side dams  35 ′ is started at least 0.5 hours before transfer to the operating position. During this preheating of the replacement refractory component, casting is typically continued without interruption. When the preheating is completed and the change in side dams is to take place, the slide gate  34  is closed and the tundish  26 , core nozzle  27  and casting pool  16  are drained and the casting is interrupted. A pair of transfer robots  55  remove the first side dams  35  from the operating position, and then a pair of transfer robots  56  transfer the second side dams  35 ′ from the preheating chamber  57  to the operating position. Note that transfer robots  55  and  56  may be the same as shown in  FIG. 1  if there is a place for the transfer robots to rapidly set aside the removed first side dams  35 ; however, to save time in removing the side dams  35  and positioning the second side dams  35 ′ in the operating position, two pairs of transfer robots  55  and  56  may be employed. Following positioning of the second side dams  35 ′ in the operating position, the slide gate  34  is opened to fill the tundish  26  and core nozzle  27  and form casting pool  16 , and continue casting. Note that transfer robots  55  and  56  may be the same transfer robots  52  and  53 , used to transfer the core nozzles, fitted with a second set gripper arms  71 . 
   In each case, there is a premium on the speed with which the transfer of the tundish, core nozzles and/or side dams is completed to minimize the interruption of the casting operation. The transfer is completed within 15 minutes and typically within 5 minutes or even 2 minutes to avoid thermal shock to the refractories. 
   Each transfer robot  52 ,  53 ,  55  and  56  is a robot device known to those skilled in the art with gripping arms  71  to grip the core nozzle  27  or  27 ′ typically in two parts, or side dams  35  or  35 ′. They can be raised and lowered and also moved horizontally along overhead tracks to move the core nozzle  27 ′ or the side dams  35 ′ from a preheating chamber  54  or  57  at a separate location from the operating position to the caster for downward insertion of the plates through the slots  69  into the holders  37 . Gripper arms  71  are also operable to remove at least portions of worn core nozzle  27  or side dams  35 . The step of removing the worn side dam  35  is done by operating cylinder unit  36  to withdraw the thrust rod sufficiently to open the slot  69  and to bring side dam  35  into position directly beneath that slot, after which the gripping arm  71  of the transfer robot  55  can be lowered through the slot to grip the side dam  35  and then raised to withdraw the worn side dam. The side dams  35  may be removed when they become worn to specified limits as will be explained further below, and may be removed one at a time as worn to a specified limit. During a casting run and at a time interval before the side dams  35  have worn down to an unserviceable level, the wear rate of the side dams  35  may be monitored by sensors, and the preheating of replacement side dams  35 ′ is commenced in preheat furnaces at preheating chamber  57  separate from the caster  11 . This time interval may be of at least about 0.5 hours for normal preheating in conventional preheat furnaces, although longer preheat times may be necessary and accommodated according to the particular equipment used. If only a portion of core nozzle  27  or the side dams  35  are/is to be replaced, the preheating of that portion of the refractory component will normally be done for the same time period as for the preheating of the entire refractory component unless that portion is such that it can be preheated to the desired preheat temperature in less time. 
   In each case, when the replacement tundish  26 ′, core nozzles  27 ′ or side dams  35 ′ have been preheated to service temperatures approaching the temperature of the molten metal, the procedure is initiated for replacement of that refractory component. To avoid thermal stock, generally the preheating should be to at least 1200° C. The caster operator actuates slide gate  34  to interrupt casting by interrupting the flow of molten steel to removable tundish  26  (also called a delivery vessel or transition piece) while allowing casting to proceed to drain molten steel from tundish  26 , core nozzle  27  and casting pool  16 . 
   To change the side dams  35 , when the molten steel has drained from the metal delivery system and casting pool, cylinder units  36  are operated to retract the side plate holders  37  and to bring the side dams  35  directly beneath the slots  69  which are opened by the retraction movement of the slides  68 . Transfer robots  55  may then be lowered such that their gripping arms  71  can grip the side dams  35  and raised and remove those worn side dams, which can then be dumped for scrap or refurbishment. The transfer robots  56  are then moved to the preheat chambers where they pick up the replacement side dams  35 ′ and move them into position above the slots  69  and the retracted side plate holders  37 . Side dams  35 ′ are then lowered by the transfer robots  56  into the plate holders, the transfer robots  56  are raised and the cylinder units  36  operated to urge the preheated replacement side dams  35 ′ against the end of the casting rolls  22  and to move the slides  68  to close the enclosure slots  69 . The operator then actuates slide gate  34  to initiate resumption of casting by pouring molten steel into tundish  26  and core nozzle  27 , to initiate a normal casting operation in a minimum of time. 
   The tundish  26 , core nozzle  27  or side dams  35  at any desired time may be replaced as described herein. The core nozzle  27  may be replaced as a singular refractory component or in parts. The side dams  35  may be replaced one at a time, in pairs or in a plurality of parts. The illustrated apparatus and the above described method has made it possible for tundish, core nozzle and/or side dam replacement to be carried out in less than about 15 minutes, and typically in 5 minutes or less, or 2 minutes or less. The other refractory components, which are not replaced, can continue to be used in the caster without reheating. 
   It has been found that refractory components that remain in the casting system retain sufficient heat to avoid thermal shock on resumption of casting and to ensure that steel does not solidify within the flow passages of the metal delivery system if the replacement is done in the way described. 
   It may be desirable to replace a side dam or dams  35  when worn to specified limits, such as when the dams become or will become unserviceable. For example, the wear of the side dams may be monitored by means of load/displacement transducers mounted on cylinders  36 . The cylinders will generally be operated so as to impose a relatively high-force on the side dams  35  during an initial bedding-in period in which there will be a higher wear rate after which, the force may be reduced to a normal operating force. The output of the displacement transducers on cylinders  36  can then be analyzed by a control system, usually including a computerized circuit, to establish a progressive wear rate and to estimate a time at which the wear will reach a level at which the side plates become unserviceable. 
   The control system is responsive to the sensors to determine the time at which preheating of replacement side darns must be initiated prior to interrupting the cast for replacement of the side dams. 
   Wear of tundish  26  and core nozzles  27  also can be monitored by sensors positioned sense the areas of these refractories components most likely to wear first. In this way, the entire apparatus can be automated so that the change of the side dams, core nozzles and tundish is done automatically by the control system (not shown) which monitors the sensors on the side dams, core nozzles and tundish, and automatically initiate the preheating and subsequent change out of the refractory components identified that is in need of placement. If the refractory components are to be replaced in pieces as wear is detected, a sensor will typically be positioned at the place most likely to wear of each portion of the refractory component to be replaced. 
   Although the invention has been illustrated and described in detail in the foregoing drawings and description with reference to several embodiments, it should be understood that the description is illustrative and not restrictive in character, and that the invention is not limited to the disclosed embodiments. Rather, the present invention covers all variations, modifications and equivalent structures that come within the scope and spirit of the invention. Additional features of the invention will become apparent to those skilled in the art upon consideration of the detailed description, which exemplifies the best mode of carrying out the invention as presently perceived. Many modifications may be made to the present invention as described above without departing from the spirit and scope of the invention.