Abstract:
A method of continuously casting thin strip dynamically controlling roll casting surface configuation by controlling the temperature of water flowing through the longitudinal water flow passages in a cyclindrical tube thickness of no more than 80 millimeters of counter rotated casting rolls, and varying the speed of the casting rolls with attenuation of the ends of the casting rolls with a casting roll drive system responsive to electrical signals received from sensors during a casting campaign.

Description:
BACKGROUND AND SUMMARY 
     This invention relates to the casting of metal strip by continuous casting in a twin roll caster. 
     In a twin roll caster, molten metal is introduced between a pair of counter-rotated horizontal casting rolls that are cooled so that metal shells solidify on the moving roll surfaces and are brought together at a nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term “nip” is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel or series of smaller vessels from which it flows through a metal delivery nozzle located above the nip, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow. 
     Further, the twin roll caster may be capable of continuously producing cast strip from molten steel through a sequence of ladles. Pouring the molten metal from the ladle into smaller vessels before flowing through the metal delivery nozzle enables the exchange of an empty ladle with a full ladle without disrupting the production of cast strip. 
     In casting thin strip by twin roll caster, the unpredictability of the crown in the casting surfaces of the casting rolls during a casting campaign is a difficulty. The crown of the casting surfaces of the casting rolls determines the thickness profile, i.e., cross-sectional shape, of thin cast strip produced by the twin roll caster. Casting rolls with convex (i.e., positive crown) casting surfaces produced cast strip with a negative (depressed) cross-sectional shape, and casting rolls with concave (i.e., negative crown) casting surfaces produced cast strip with a positive (i.e., raised) cross-sectional shape. The casting rolls generally are formed of copper or copper alloy with internal passages for circulation of cooling water usually coated with chromium or nickel to form the casting surfaces, which undergo substantial thermal deformation with exposure to the molten metal. 
     In thin strip casting, there is a desired roll crown to produce a desired strip cross-sectional profile under typical casting conditions. It is usual to machine the casting rolls with an initial crown when cold based on the projected crown in the casting surfaces of the casting rolls under typical casting condition. However, the differences between the crown shape of the casting surfaces between cold and casting conditions is difficult to predict. Moreover, the actual crown of the casting surfaces during the casting campaign can vary significantly from that projected crown under typical conditions, since the crown of the casting surfaces of the casting rolls can change even during typical casting due to changes in the temperature of molten metal supplied to the casting pool of the caster, changes in casting speed and other casting conditions, and even with slight changes in the composition of the molten metal as occurs during casting. 
     Accordingly, there has been a need for a reliable and effective way to directly and closely control the shape of the crown in the casting surfaces of the casting rolls during casting, and in turn, the cross-sectional profile of the thin cast strip produced by the twin roll caster. Previous proposals for casting roll crown control have relied on mechanical devices to physically deform the casting roll, e.g., by the movement of deforming pistons or other elements within the casting roll or by applying bending forces to the support shafts of the casting rolls. Yet, there has not been an effective way to dynamically control the roll crown to produce the desired profile of the cast strip until now. 
     We have determined that reliable and effective control of the casting roll crown and, in turn, cross-sectional strip profile can be achieved by providing a casting roll of such configuration to enable control of the crown in the casting surfaces by varying casting parameters. 
     Disclosed is a method of continuously casting thin strip dynamically controlling roll crown comprising the steps of: 
     a. assembling a caster having a pair of counter rotating casting rolls with a nip there between capable of delivering cast strip downwardly from the nip, where each casting roll has a casting surface formed by a cylindrical tube of a material selected from the group consisting of copper and copper alloy optionally with a coating thereon and having a plurality of longitudinal water flow passages extending through the tube having a thickness of no more than 80 millimeters, the cylindrical tube capable of changing crown of the casting surface with changes in temperature of water flowing through the passages during casting, 
     b. assembling a metal delivery system capable of forming a casting pool supported on the casting surfaces of the casting rolls above the nip with side dams adjacent ends of the nip to confine the casting pool, 
     c. positioning at least one sensor capable of sensing thickness profile of the cast strip downstream of the nip and generating electrical signals indicative of the thickness profile of the cast strip, 
     d. controlling the temperature of the water flowing through the longitudinal water flow passages in the tube thickness, 
     e. counter rotating the casting rolls and varying the speed of the casting rolls with a casting roll drive system, and 
     f. controlling the casting roll drive to vary the speed of rotation of the casting rolls and varying the temperature of the water flow circulated through the water flow passages by a control system responsive to electrical signals received from the sensors to control roll crown of the casting rolls during a casting campaign. 
     The cylindrical tube of each casting roll is of a circumferential thickness that, by varying the casting speed and controlling the temperature of the water circulated through the casting rolls, the crown in the casting surfaces of the casting can reliably be varied to achieve and maintain a desired cross-sectional profile of the cast strip. The thickness of the cylindrical tube may range between 40 and 80 millimeters in thickness or between 60 and 80 millimeters in thickness. The casting rolls may have a cavity internal of the cylindrical tube to define the thickness of the cylindrical tube and facilitate flexure of the cylindrical tube to provide crown control with changes in casting speed and temperature of water circulated through the casting rolls. Water may be circulated through the water flow passages and the cavities of the casting rolls in series. Alternatively, water may be circulated through the water flow passages and then through the cavity of at least one of the casting rolls, or water may be circulated through the cavity and then through the water flow passages of at least one of the casting rolls. 
     Also disclosed is an apparatus for continuously casting thin strip by dynamically controlling roll crown comprising: 
     a. a caster having a pair of counter rotating casting rolls with a nip there between capable of delivering cast strip downwardly from the nip where each casting roll has a casting surface formed by a cylindrical tube of a material selected from the group consisting of copper and copper alloy optionally with a coating thereon and has a plurality of longitudinal water flow passages extending through the tube having a thickness of no more than 80 millimeters, the cylindrical tube capable of changing crown of the casting surface with changes in temperature of water flowing through the passages during casting, 
     b. a metal delivery system capable of forming a casting pool supported on the casting surfaces of the casting rolls above the nip with side dams adjacent ends of the nip to confine the casting pool, 
     c. at least one sensor capable of sensing thickness profile of the cast strip downstream of the nip and generating electrical signals indicative of the thickness profile of the cast strip, 
     d. a water flow controller capable of controlling the temperature of the water flowing through the longitudinal water flow passages in the tube thickness, 
     e. a casting roll drive system capable of counter rotating the casting rolls and varying the speed of the casting rolls during casting, and 
     f. a control system responsive to electrical signals received from the sensors capable of controlling the casting roll drive to vary the speed of rotation of the casting rolls and controlling the water flow controller to vary the temperature of the water flow circulated through the water flow passages to control roll crown of the casting rolls during a casting campaign. 
     Again, the cylindrical tube may have an internal cavity to define the cylindrical tube and provide for flexure thereof as described above. Tube may be between 40 and 80 millimeters in thickness or between 60 and 80 millimeters in thickness. 
     The longitudinal water flow passages in the tube thickness may be arranged in three pass sets round the cylindrical tube thickness, so that the cooling water circulates through the three passages of the set in series before exiting the casting roll either directly or through the internal cavity. Alternatively, the longitudinal water flow passages in the tube thickness may be arranged in single pass sets round the cylindrical tube thickness so that the cooling water circulates through one passage before exiting the casting roll either directly or through the internal cavity. 
     At least one sensor capable of sensing thickness profile of the cast strip may be adjacent to pinch rolls through which the strip first passes after casting. A plurality of sensors capable of sensing thickness profile of the cast strip may be positioned laterally across the strip. 
     Various aspects of the invention will become apparent to those skilled in the art from the following detailed description, drawings and claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is described in more detail in reference to the accompanying drawings in which: 
         FIG. 1  is a diagrammatical side view of a twin roll caster of the present disclosure; 
         FIG. 2  is an enlarged partial sectional view of a portion of the twin roll caster of  FIG. 1  including a strip inspection device for measuring strip profile; 
         FIG. 2A  is a schematic view of a portion of twin roll caster of  FIG. 2 ; 
         FIG. 3A  is a cross sectional view longitudinally through a portion of one of the casing rolls of  FIG. 2 ; 
         FIG. 3B  is a cross sectional view longitudinally through the remaining portion of the casing roll of  FIG. 3A  joined on line A-A; 
         FIG. 4  is an end view of the casting roll of  FIG. 3A  on line  4 - 4  shown in partial interior detail in phantom; 
         FIG. 5  is a cross sectional view of the casting roll of  FIG. 3A  on line  5 - 5 ; 
         FIG. 6  is a cross sectional view of the casting roll of  FIG. 3A  on line  6 - 6 ; 
         FIG. 7  is a cross sectional view of the casting roll of  FIG. 3A  on line  7 - 7 ; 
         FIG. 8  is a schematic illustration of the twin casting rolls of  FIG. 2  with a water supply system; 
         FIG. 9  is a schematic illustration similar to  FIG. 8  with the water supply in an alternative configuration; 
         FIG. 10  is a graph illustrating maximum roll surface temperature to water inlet temperature for three different flows rates; 
         FIG. 11  is a graph illustrating strip crown to roll surface temperature for two different casting speeds; 
         FIG. 12  is a graph illustrating roll surface temperature across a part of the width of a casting roll; 
         FIG. 13  is a graph illustrating heat flux to edge distance for the casting roll of  FIG. 12 ; 
         FIG. 14  is a graph illustrating thermal crown to edge distance for the casting roll of  FIG. 12 ; 
         FIG. 15  is a graph illustrating heat flux attenuation to casting speed; 
         FIG. 16  is a graph illustrating water flow rate and water temperature at an inlet to time; 
         FIG. 17  is a graph illustrating strip gauge and roll crown to edge distance for a casting roll; and 
         FIG. 18  is a graph illustrating strip gauge and roll crown to edge distance for another casting roll. 
     
    
    
     DETAILED DESCRIPTION 
     Referring now to  FIGS. 1 ,  2 , and  2 A, a twin roll caster is illustrated that comprises a main machine frame  10  that stands up from the factory floor and supports a pair of counter-Rotatable casting rolls  12  mounted in a module in a roll cassette  11 . The casting rolls  12  are mounted in the roll cassette  11  for ease of operation and movement as described below. The roll cassette  11  facilitates rapid movement of the casting rolls  12  ready for casting from a setup position into an operative casting position in the caster as a unit, and ready removal of the casting rolls  12  from the casting position when the casting rolls  12  are to be replaced. There is no particular configuration of the roll cassette  11  that is desired, so long as it performs that function of facilitating movement and positioning of the casting rolls  12  as described herein. 
     The casting apparatus for continuously casting thin steel strip includes the pair of counter-Rotatable casting rolls  12  having casting surfaces  12 A laterally positioned to form a nip  18  there between. Molten metal is supplied from a ladle  13  through a metal delivery system to a metal delivery nozzle  17 , core nozzle, positioned between the casting rolls  12  above the nip  18 . Molten metal thus delivered forms a casting pool  19  of molten metal above the nip  18  supported on the casting surfaces  12 A of the casting rolls  12 . This casting pool  19  is confined in the casting area at the ends of the casting rolls  12  by a pair of side closure plates, or side dams  20 , (shown in dotted line in  FIGS. 2 and 2A ). The upper surface of the casting pool  19  (generally referred to as the “meniscus” level) may rise above the lower end of the delivery nozzle  17  so that the lower end of the delivery nozzle  17  is immersed within the casting pool  19 . The casting area includes the addition of a protective atmosphere above the casting pool  19  to inhibit oxidation of the molten metal in the casting area. 
     The ladle  13  typically is of a conventional construction supported on a rotating turret  40 . For metal delivery, the ladle  13  is positioned over a movable tundish  14  in the casting position to fill the tundish  14  with molten metal. The movable tundish  14  may be positioned on a tundish car  66  capable of transferring the tundish  14  from a heating station (not shown), where the tundish  14  is heated to near a casting temperature, to the casting position. A tundish guide, such as rails  39 , may be positioned beneath the tundish car  66  to enable moving the movable tundish  14  from the heating station to the casting position. 
     The movable tundish  14  may be fitted with a slide gate  25 , actuable by a servo mechanism, to allow molten metal to flow from the tundish  14  through the slide gate  25 , and then through a refractory outlet shroud  15  to a transition piece or distributor  16  in the casting position. From the distributor  16 , the molten metal flows to the delivery nozzle  17  positioned between the casting rolls  12  above the nip  18 . 
     The side dams  20  may be made from a refractory material such as zirconia graphite, graphite alumina, boron nitride, boron nitride-zirconia, or other suitable composites. The side dams  20  have a face surface capable of physical contact with the casting rolls  12  and molten metal in the casting pool  19 . The side dams  20  are mounted in side dam holders (not shown), which are movable by side dam actuators (not shown), such as a hydraulic or pneumatic cylinder, servo mechanism, or other actuator to bring the side dams  20  into engagement with the ends of the casting rolls  12 . Additionally, the side dam actuators are capable of positioning the side dams  20  during casting. The side dams  20  form end closures for the molten pool of metal on the casting rolls  12  during the casting operation. 
       FIG. 1  shows the twin roll caster producing the cast strip  21 , which passes across a guide table  30  to a pinch roll stand  31 , comprising pinch rolls  31 A. Upon exiting the pinch roll stand  31 , the thin cast strip  21  may pass through a hot rolling mill  32 , comprising a pair of work rolls  32 A, and backup rolls  32 B, forming a gap capable of hot rolling the cast strip  21  delivered from the casting rolls  12 , where the cast strip  21  is hot rolled to reduce the strip to a desired thickness, improve the strip surface, and improve the strip flatness. The work rolls  32 A have work surfaces relating to the desired strip profile across the work rolls  32 A. The hot rolled cast strip  21  then passes onto a run-out table  33 , where it may be cooled by contact with a coolant, such as water, supplied via water jets  90  or other suitable means, and by convection and radiation. In any event, the hot rolled cast strip  21  may then pass through a second pinch roll stand  91  to provide tension of the cast strip  21 , and then to a coiler  92 . The cast strip  21  may be between about 0.3 and 2.0 millimeters in thickness before hot rolling. 
     At the start of the casting operation, a short length of imperfect strip is typically produced as casting conditions stabilize. After continuous casting is established, the casting rolls  12  are moved apart slightly and then brought together again to cause this leading end of the cast strip  21  to break away forming a clean head end of the following cast strip  21 . The imperfect material drops into a scrap receptacle  26 , which is movable on a scrap receptacle guide. The scrap receptacle  26  is located in a scrap receiving position beneath the caster and forms part of a sealed enclosure  27  as described below. The enclosure  27  is typically water cooled. At this time, a water-cooled apron  28  that normally hangs downwardly from a pivot  29  to one side in the enclosure  27  is swung into position to guide the clean end of the cast strip  21  onto the guide table  30  that feeds it to the pinch roll stand  31 . The apron  28  is then retracted back to its hanging position to allow the cast strip  21  to hang in a loop beneath the casting rolls  12  in enclosure  27  before it passes to the guide table  30  where it engages a succession of guide rollers. 
     An overflow container  38  may be provided beneath the movable tundish  14  to receive molten material that may spill from the tundish  14 . As shown in  FIG. 1 , the overflow container  38  may be movable on rails  39  or another guide such that the overflow container  38  may be placed beneath the movable tundish  14  as desired in casting locations. Additionally, an optional overflow container (not shown) may be provided for the distributor  16  adjacent the distributor  16 . 
     The sealed enclosure  27  is formed by a number of separate wall sections that fit together at various seal connections to form a continuous enclosure wall that permits control of the atmosphere within the enclosure  27 . Additionally, the scrap receptacle  26  may be capable of attaching with the enclosure  27  so that the enclosure  27  is capable of supporting a protective atmosphere immediately beneath the casting rolls  12  in the casting position. The enclosure  27  includes an opening in the lower portion of the enclosure  27 , lower enclosure portion  44 , providing an outlet for scrap to pass from the enclosure  27  into the scrap receptacle  26  in the scrap receiving position. The lower enclosure portion  44  may extend downwardly as a part of the enclosure  27 , the opening being positioned above the scrap receptacle  26  in the scrap receiving position. As used in the specification and claims herein, “seal,” “sealed,” “sealing,” and “sealingly” in reference to the scrap receptacle  26 , enclosure  27 , and related features may not be a complete seal so as to prevent leakage, but rather is usually less than a perfect seal as appropriate to allow control and support of the atmosphere within the enclosure  27  as desired with some tolerable leakage. 
     A rim portion  45  may surround the opening of the lower enclosure portion  44  and may be movably positioned above the scrap receptacle  26 , capable of sealingly engaging and/or attaching to the scrap receptacle  26  in the scrap receiving position. The rim portion  45  may be movable between a sealing position in which the rim portion  45  engages the scrap receptacle  26 , and a clearance position in which the rim portion  45  is disengaged from the scrap receptacle  26 . Alternately, the caster or the scrap receptacle  26  may include a lifting mechanism to raise the scrap receptacle  26  into sealing engagement with the rim portion  45  of the enclosure  27 , and then lower the scrap receptacle  26  into the clearance position. When sealed, the enclosure  27  and scrap receptacle  26  are filled with a desired gas, such as nitrogen, to reduce the amount of oxygen in the enclosure  27  and provide a protective atmosphere for the cast strip  21 . 
     The enclosure  27  may include an upper collar portion  43  supporting a protective atmosphere immediately beneath the casting rolls  12  in the casting position. When the casting rolls  12  are in the casting position, the upper collar portion  43  is moved to the extended position closing the space between a housing portion  53  adjacent the casting rolls  12 , as shown in  FIG. 2 , and the enclosure  27 . The upper collar portion  43  may be provided within or adjacent the enclosure  27  and adjacent the casting rolls  12 , and may be moved by a plurality of actuators (not shown) such as servo-mechanisms, hydraulic mechanisms, pneumatic mechanisms, and rotating actuators. 
     The casting rolls  12  are internally water cooled as described below so that as the casting rolls  12  are counter-rotated, shells solidify on the casting surfaces  12 A, as the casting surfaces  12 A move into contact with and through the casting pool  19  with each revolution of the casting rolls  12 . The shells are brought close together at the nip  18  between the casting rolls  12  to produce a thin cast strip product  21  delivered downwardly from the nip  18 . The thin cast strip product  21  is formed from the shells at the nip  18  between the casting rolls  12  and delivered downwardly and moved downstream as described above. 
     The construction of each of the two casting rolls  12  is generally the same as described with reference to  FIGS. 3A ,  3 B, and  4 - 7 . Each casting roll  12  includes a cylindrical tube  120  of a metal selected from the group consisting of copper and copper alloy, optionally with a coating thereon, e.g., chromium or nickel, to form the casting surfaces  12 A. Each cylindrical tube  120  may be mounted between a pair of stub shaft assemblies  121  and  122 . The stub shaft assemblies  121  and  122  have end portions  127  and  128 , respectively (shown in  FIGS. 4-6 ), which fit snugly within the ends of cylindrical tube  120  to form the casting roll  12 . The tube cylindrical  120  is thus supported by end portions  127  and  128  having flange portions  129  and  130 , respectively, to form internal cavity  163  therein, and support the assembled casting roll between the stub shaft assemblies  121  and  122 . 
     The outer cylindrical surface of each cylindrical tube  120  is a roll casting surface  12 A. The cylindrical thickness of the cylindrical tube  120  may be no more than 80 millimeters thick so that crown of the outer surface of the cylindrical tube  120  can be controlled by controlling the casting speed and the temperature of the cooling water circulates through the casting roll as described below. The thickness of the tube  120  may range between 40 and 80 millimeters in thickness or between 60 and 80 millimeters in thickness. 
     Each cylindrical tube  120  is provided with a series of longitudinal water flow passages  126 , which may be formed by drilling long holes through the circumferential thickness of the cylindrical tube  120  from one end to the other. The ends of the holes are subsequently closed by end plugs  141  attached to the end portions  127  and  128  of stub shaft assemblies  121  and  122  by fasteners  171 . The water flow passages  126  are formed through the thickness of the cylindrical tube  120  with end plugs  141 . The number of stub shaft fasteners  171  and end plugs  141  may be selected as desired. End plugs  141  may be arranged to provide, with water passage in the stub shaft assemblies described below, in single pass cooling from one end to the other of the roll  12 , or alternatively, to provide multi-pass cooling where, for example, the flow passages  126  are connected to provide three passes of cooling water through adjacent flow passages  126  before returning the water to the water supply directly or through the cavity  163 . 
     The water flow passages  126  through the thickness of the cylindrical tube  120  may be connected to water supply in series with the cavity  163 . The water passages  126  may be connected to the water supply so that the cooling water first passes through the cavity  163  and then the water supply passages  126  to the return lines, or first through the water supply passages  126  and then through the cavity  163  to the return lines. 
     The cylindrical tube  120  may be provided with circumferential steps  123  at end to form shoulders  124  with the working portion of the roll casting surface  12 A of the roll  12  there between. The shoulders  124  are arranged to engage the side dams  20  and confine the casting pool  19  as described above during the casting operation. 
     End portions  127  and  128  of stub shaft assemblies  121  and  122 , respectively, typically sealingly engage the ends of cylindrical tube  120  and have radially extending water passages  135  and  136  shown in  FIGS. 4-6  to deliver water to the water flow passages  126  extending through the cylindrical tube  120 . The radial flow passages  135  and  136  are connected to the ends of at least some of the water flow passages  126 , for example, in threaded arrangement, depending on whether the cooling is a single pass or multi-pass cooling system. The remaining ends of the water flow passages  126  may be closed by, for example, threaded end plugs  141  as described where the water cooling is a multi-pass system. 
     As shown in detail by  FIG. 7 , cylindrical tube  120  may be positioned in annular arrays in the thickness of cylindrical tube  120  either in single pass or multi-pass arrays of water flow passages  126  as desired. The water flow passages  126  are connected at one end of the casting roll  12  by radial ports  160  to the annular gallery  140  and in turn radially flow passages  135  of end portion  127  in stub shaft assembly  120 , and are connected at the other end of the casting roll  12  by radial ports  161  to annular gallery  150  and in turn radial flow passages  136  of end portions  128  in stub shaft assembly  121 . Water supplied through one annular gallery,  140  or  150 , at one end of the roll  12  can flow in parallel through all of the water flow passages  126  in a single pass to the other end of the roll  12  and out through the radial passages,  135  or  136 , and the other annular gallery,  150  or  140 , at that other end of the cylindrical tube  120 . The directional flow may be reversed by appropriate connections of the supply and return line(s) as desired. Alternatively or additionally, selective ones of the water flow passages  126  may be optionally connected or blocked from the radial passages  135  and  136  to provide a multi pass arrangement, such as a three pass. 
     The stub shaft assembly  122  may be longer than the stub shaft assembly  121 , and the stub shaft assembly  122  provided with two sets of water flow ports  133  and  134 . Water flow ports  133  and  134  are capable of connection with rotary water flow couplings  131  and  132  by which water is delivered to and from the casting roll  12  axially through stub shaft assembly  122 . In operation, cooling water passes to and from the water flow passages  126  in the cylindrical tube  120  through radial passages  135  and  136  extending through end portions  127  and  128  of the stub shaft assemblies  121  and  122 , respectively. The stub shaft assembly  121  is fitted with axial tube  137 , to provide fluid communication between the radial passages  135  in end portions  127  and the central cavity within the casting roll  12 . The stub shaft assembly  122  is fitted with axial space tube  138 , to separate a central water duct  138 , in fluid communication with the central cavity  163 , and from annular water flow duct  139  in fluid communication with radial passages  136  in end portion  122  of stub shaft assembly  122 . Central water duct  138  and annular water duct  139  are capable of providing inflow and outflow of cooling water to and from the casting roll  12 . In operation, incoming cooling water may be supplied through supply line  131  to annular duct  139  through ports  133 , which is in turn in fluid communication with the radial passages  136 , gallery  150  and water flow passages  126 , and then returned through the gallery  140 , the radial passages  135 , axial tube  137 , central cavity  163 , and central water duct  138  to outflow line  132  through water flow ports  134 . Alternatively, the water flow to, from and through the casting roll  12  may be in the reverse direction as desired. As discussed in more detail below, the water flow ports  133  and  134  may be connected to water supply and return lines so that water may flow to and from water flow passages  126  in the cylindrical tube  120  of the casting roll  12  in either direction, as desired. Depending on the direction of flow, the cooling water flows through the cavity  163  either before or after flow through the water flow passages  126 . 
       FIG. 8  illustrates one arrangement in which cooling water may be supplied to the casting rolls  12  in a closed loop system. A pump  151  delivers water through a supply line  152  to the ports  133  of one casting roll  12 , and to the ports  134  of the other casting roll  12 . By this arrangement, water is delivered to the radial passages  135  at one end of one casting roll  12  and to the radial passages  136  at the other end of the second casting roll  12 . Water flows from the other ports,  134  and  133  respectively, through a discharge line  153  to a heat exchanger  154  and back to the pump  151  through a return line  155 . Both of the casting rolls  12  may receive cooling water from the common supply pump  151  at essentially the same temperature, although such is not required. However, water is delivered to the flow passages  126  of one casting roll  12  through cavity  163 , and discharge from the flow passages  126  of the other casting roll  12  through cavity  163 . By this arrangement, differential expansion due to a temperature difference across one casting roll  12  tends to be offset by differential expansion of the other casting roll  12  due to the mutual reversal of the flow direction to the two rolls  12 . 
     It is understood, however, that the water flow pattern and direction may be chosen as desired. For example, the direction of water flow may be the same in both casting rolls  12  by connection of the water supply in an arrangement illustrated in  FIG. 9 . Components illustrated in  FIG. 9  that are similar to  FIG. 8 . However, in  FIG. 9 , the water supply line  152  is connected to the ports  133  of both rolls  12  and the discharge line  153  is connected to the ports  134  of both rolls  12 . 
     The systems illustrated in  FIGS. 8 and 9  may be operated to control the crown of the casting surfaces  12 A of the casting rolls  12 . In operation, deformation of the crown of the casting surfaces  12 A may be controlled by regulating the temperature of the cooling water flowing through the water flow passages  126  of the cylindrical tube  120  or controlling the speed of rotation of the casting rolls  12  with heat flux attenuation of the ends of the casting roll. In turn, the thickness profile of cast strip  21  can be controlled with the control of the crown of the casting surfaces  12 A of the casting rolls  12 . Since the circumferential thickness of the cylindrical tube  120  is made to a thickness of no more than 80 mm, the crown of the casting surfaces  12 A may be made to deform responsive to changes in the temperature of the cooling water or change in speed of the casting rolls with heat flux attenuation of the ends of the casting roll. As previously explained, the thickness of the cylindrical tube  120  may range between 40 and 80 millimeters in thickness or between 60 and 80 millimeters in thickness. 
     To control the temperature of the cooling water and casting speed to achieve a desired strip thickness profile, a strip thickness profile sensor  71  may be positioned downstream to detect the thickness profile of the cast strip  21  as shown in  FIGS. 2 and 2A . The strip thickness sensor  71  is provided typically between the nip  18  and the pinch rolls  31 A to provide for direct control of the casting roll  12 . The sensor may be an x-ray gauge or other suitable device capable of directly measuring the thickness profile across the width of the strip periodically or continuously. Alternatively, a plurality of non-contact type sensors are arranged across the cast strip  21  at the roller table  30  and the combination of thickness measurements from the plurality of positions across the cast strip  21  are processed by a controller  72  to determine the thickness profile of the strip periodically or continuously. The thickness profile of the cast strip  21  may be determined from this data periodically or continuously as desired. 
       FIGS. 10-18  are a series of graphs obtained from a twin roll caster similar to that illustrated in  FIGS. 1-9 . In several runs, the caster was operated at different set casting speeds, and with cooling water supplied at different inlet temperatures during the course of a casting run at each casting speed. In the twin roll caster utilized in these runs, the casting rolls comprised a cylindrical tube of copper alloy having an outer peripheral diameter of 489.6 mm, a length of 1400 mm and a circumferential thickness of 64.5 mm. 
       FIG. 10  is a graph illustrating the maximum measured roll surface temperature increases with increasing water inlet temperature at three different water flow rates.  FIG. 10  also shows that the maximum measured roll surface temperature at a given water inlet temperature increases with decreasing water flow rate. 
       FIG. 11  is a graph of strip thickness profile (strip crown) versus average measured roll surface temperature (i.e. the average roll surface temperature measured across the width of the roll) at two casting roll speeds.  FIG. 11  shows that strip thickness profile reduces with increasing average measured roll temperature, as roll crown increases. Thus, strip thickness profile can be varied and controlled with the casting roll temperature and correlated water inlet temperature.  FIG. 11  also shows that at a given casting roll temperature, the thickness profile (strip crown) markedly decreases with decreasing casting speed and heat flux attenuation of the ends of the casting roll as discussed below in relation to  FIGS. 12-14 . 
       FIG. 12  is a graph of roll surface temperature across a part of the casting roll width in millimeters from one end of the casting roll, with the casting roll operating at a substantially constant casting speed. The graph illustrates that there is a substantial increase in casting roll surface temperature, of the order of 30° C., from the end of the casting roll to a position approximately 150 mm inboard of the end of the casting roll. 
       FIG. 13  shows heat flux versus distance from the end of the casting roll. The variable heat flux curve is derived from calculations of the data set forth in the graph in  FIG. 12 . The constant heat flux curve is the theoretical limit which the heat flux approaches at the end of the strip with increase in casting speed. The variable heat flux curve in  FIG. 13  illustrates significant attenuation of the heat flux at the ends of the casting roll with actual casting. 
       FIG. 14  illustrates the effect of the end heat flux attenuation shown in  FIG. 13 .  FIG. 14  is a graph of change in casting surface configuration (roll crown) with distance from the end of the casting roll for the roll operation that generated the data illustrated in  FIGS. 12 and 13 , i.e., for variable heat flux across the width of the roll, and for casting roll operation with a constant heat flux generated across the width of the roll.  FIG. 14  shows the difference between the casting roll crown in the central section of a casting roll operating under variable heat flux compared to a constant heat flux. We have also found that with the heat flux lower at the end of a casting roll compared to 150 millimeters from the ends of the roll, more constraint to the overall axial expansion of the casting roll and greater radial expansion results at the center of the casting roll, i.e., greater roll crown, in the central section of the casting roll and a reduced thickness profile of the strip. In other runs, similar results have been obtained with different casting speeds, with the results showing greater heat flux attenuation with decreasing casting speed. 
       FIG. 15  is a graph of heat flux attenuation versus casting speed. The graph illustrates our finding that when casting occurs at lower casting speeds, the temperature profile of the crown in the surface of a casting roll over the last 150 millimeters from the side edge increases (even though the average temperature of the casting roll is lower). This has the effect of constraining the cylindrical tube of the casting roll, increasing diameters in the central section of the casting roll, and thus causing the casting roll to “belly out” or “crown up” more for a given heat flux than when the casting roll was rotating faster. This results in a corresponding decrease in the strip cross-sectional profile due to the increased roll crown. 
       FIG. 16  is a graph illustrating a cooling water temperature increase from 27° C. to 32° C. during the course of particular casting run carried out at a constant casting speed. The graph of  FIG. 16  also shows an analysis of the strip produced by the caster before and after the water inlet temperature change. Coil # 1  was cast strip at a selected time in the casting run before the water inlet temperature change, and Coil # 2  was cast strip at a selected time in the casting run after the water inlet temperature change. In both cases the cast strip was analyzed to determine the thickness profile at that point in the casting run. 
       FIGS. 17 and 18  show the strip thickness profiles for the two tested sections of strip identified as Coil # 1  and Coil # 2  in  FIG. 16 . The graphs in  FIGS. 17 and 18  illustrate that with a relatively higher cooling water temperature (Coil # 2 ) the magnitude of the thickness perturbations, e.g. ridges, is lower than for a relatively lower cooling water temperature (Coil # 1 ). The graphs in  FIGS. 17 and 18  also illustrate that there is significant localized variations in strip thickness profile in strip produced by the caster prior to the increase in water temperature, which was significantly reduced with increase in water temperature. The localized variations in strip thickness are evident from the series of ridges (which indicate local thickness variations) across the width of the strip in each of the graphs in  FIGS. 17 and 18 . Controlling the temperature of the casting roll with change of the water inlet temperature demonstrates control for the shape of the roll crown and the strip thickness profile, as well as control over the range of localized variations in strip thickness profile. At a relatively higher cooling water temperature, the casting rolls expand more than at a relatively lower cooling water temperature and thus “crown up” more, thereby bringing the two cast shells of the thin cast strip closer together and reduce strip thickness profile. In this example, there is less molten metal being carried between the two shells in the cast strip with higher water temperature, than was the case with lower water temperatures where the two cast shells were farther apart and had greater bulging and different magnitude of ridges. 
     These examples illustrate control of the casting speed and the temperature of cooling water can control the crown of the casting surfaces of the casting rolls. 
     While principles and modes of operation have been explained and illustrated with regard to particular embodiments, it must be understood, however, that the invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.