Patent Publication Number: US-6213193-B1

Title: Caster including a gas delivery means to resist backflowing and freezing of molten metal to the tip of a nozzle and an associated method

Description:
This application is a divisional of U.S. Ser. No. 08/823,915, filed on Mar. 25, 1997 and now issued as U.S. Pat. No. 5,967,220. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a caster including a gas delivery means to resist backflowing and freezing of molten metal to the tip of a nozzle and an associated method. 
     Casters for casting molten metal, such as molten aluminum, into metal products are known. Molten metal is typically introduced into the caster from a trough that is fed from a furnace. Typically, a nozzle introduces the molten metal into the mold of the caster. In a twin belt caster, the mold is formed by a pair of opposed movable belts and a pair of opposed side dams. A metal product, such as a slab, is formed in the mold by solidifying the molten metal. An example of a twin belt caster is described in U.S. Pat. No. 4,964,456. 
     A recurring problem with casters utilizing a nozzle is that molten metal can freeze at the nozzle tip. This freezing of molten metal at the nozzle tip causes undesirable surface qualities in the cast slab. In addition, freezing of molten metal at the nozzle tip can cause nozzle damage. 
     Also, despite the known devices to seal the belt to the nozzle (see, e.g., U.S. Pat. No. 4,785,873) a space can form between the nozzle and the belt, and molten metal can enter this space, and thereafter freeze to the nozzle tip. 
     What is needed, therefore, is a caster that includes means for resisting freezing and backflowing of molten metal to the nozzle tip. By resisting this freezing and backflowing of molten metal to the nozzle tip, a higher quality cast metal product can be produced in the caster. 
     SUMMARY OF THE INVENTION 
     The invention has met or exceeded the above-mentioned needs as well as others. The caster of the invention comprises means for defining a mold to receive molten metal therein and a nozzle for delivering the molten metal into the mold. The nozzle includes a tip. The caster further includes means for delivering a gas to a space defined by the mold defining means and the nozzle. In this way, freezing and backflowing of the molten metal near the tip is resisted. 
     The method of the invention includes providing a caster substantially as described above and solidifying the molten metal into a metal product in the mold of the caster. The method further comprises introducing a gas into the space defined by the nozzle and the mold defining means while the molten metal is solidifying in the mold. Once again, the introduction of the gas into the space resists freezing and backflowing of the molten metal to the tip of the nozzle. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A full understanding of the invention can be gained from the following description of the preferred embodiment when read in conjunction with the accompanying drawings in which: 
     FIG. 1 is a partially schematic and partially cutaway elevational view of a twin belt caster. 
     FIG. 2 is a cross-sectional view of a nozzle and belt showing the problem of freezing of molten metal to the tip of the nozzle. 
     FIG. 3 is a detailed partially schematic view of one embodiment of the invention. 
     FIG. 4 is a cross-sectional view taken along line  4 — 4  of FIG.  3 . 
     FIG. 5 is a schematic view, partially in section, of another embodiment of the invention showing one version of the automatic, self-adjusting gas pressure control means. 
     FIG. 6 is a schematic view, partially in section, of another embodiment of the invention showing another version of the automatic, self-adjusting gas pressure control means. 
    
    
     DETAILED DESCRIPTION 
     As used herein, the term “metal product” means primarily clad or unclad strip or slab made substantially of one or more metals, including without limitation, aluminum and aluminum alloys and can also include, in a broader sense, clad or unclad bar, foil or rod. 
     Referring now particularly to FIG. 1, a partially schematic and partially cutaway elevational view of a twin belt caster  20  is shown. The caster  20  is supplied with molten metal from a holding furnace  22 . The molten metal is delivered from the holding furnace  22  by a trough  24  to the tundish  26  of the caster  20 . The molten metal then is directed by the tundish  26  into a plurality of tubes  28  and then into the nozzle  30 . The nozzle  30  introduces the molten metal  32  into the mold  34  of the caster  20 . The mold  34  includes a center portion  36  and, because this mold  34  is generally rectangular in cross-section in order to form slabs, a pair of outside edge portions  38  and  40 . The mold  34  is defined by a pair of opposed movable side dams  44  and  46  and a pair of opposed movable belts, only one of which, belt  48 , can be seen in FIG.  1 . It will be appreciated that a stationary side dam can also be provided. The molten metal  32  solidifies into a metal product  50  in the mold  34  and is then moved out of the mold  34  at casting speed. Although a twin belt caster has been shown, it will be appreciated that the invention is not so limited, and can be used with other types of casters, such as block casters and roll casters. 
     For a more detailed description of a twin belt caster, reference is made to U.S. Pat. No. 4,964,456. For a more detailed description of the tundish  26 , tubes  28  and nozzle  30  reference is made to U.S. Pat. No. 4,798,315. Finally, for a more detailed description of the movable side dams  44  and  46 , reference is made to U.S. Pat. No. 4,794,978. All of the above three United States Patents are expressly incorporated by reference herein. 
     The above-mentioned problem of molten metal freezing to the tip of a nozzle of a twin belt caster will be explained with reference to FIG.  2 . FIG. 2 shows a very detailed cross-sectional view of a nozzle  70 , similar to nozzle  30  shown in FIG.  1 . The nozzle  70  defines a passageway  72  for the flow of molten metal  74  into the mold  76  of a caster. The molten metal  74  is solidified into a metal product  78  in the mold  76 . The mold  76  is defined by a pair of opposed movable belts  80 ,  82  and a pair of movable or stationary side dams (not shown in this view). 
     Despite the devices and methods known in the prior art, a space  90 ,  92  can form between the outside surfaces  70   a  and  70   b  of the nozzle  70  and the respective belts  80  and  82 . This space  90 ,  92  is shown exaggerated on FIG. 2 in order to clearly illustrate the problem. It has been determined that this space  90 ,  92  can range from 0 to 0.25 mm in width. 
     The ambient pressure in this space  90 ,  92  is approximately the atmospheric pressure of the environment where the caster is located. Because molten metal  74  is introduced into the mold  76 , a metallostatic pressure “MP” is created, and thus, the molten metal has a tendency to flow into the space  90 ,  92 . Because of this backflow, the molten metal  74  will freeze on the tips  70   c  and  70   d  of the nozzle  70  creating frozen metal product  94  and  96  disposed thereon. 
     Referring to FIGS. 3 and 4, a first embodiment of the invention will be described. This first embodiment involves introducing a gas into the space  100 ,  101  (FIG. 4) defined by (i) the mold defining means, which in the case of a twin belt caster includes the belts  102 ,  104  (FIG. 4) and the side dams  106 ,  108  (FIG. 3) and (ii) the outside surfaces  110   a ,  110   b  of the nozzle  110  (FIG.  4 ). Referring now particularly to FIG. 3, the nozzle  110  includes a pair of metal tubes  112 ,  114  which are interposed between the nozzle  110  and the opposed side dams  106 ,  108 . The tubes  112 ,  114  provide a wearing surface  112   a ,  114   a  for the side dams  106 ,  108  and thus protect the nozzle  110  from excess wear. The tubes  112 ,  114  each define respective gas passageways  120 ,  122 . For a more detailed description of this arrangement reference is made to commonly owned U.S. patent application Ser. No. 08/566,776 and now issued as U.S. Pat. No. 5,787.968, the disclosure of which is expressly incorporated by reference herein. 
     Referring again to FIGS. 3 and 4, the embodiment shown includes a gas supply, such as tank  140  with a gas supply line  142  attached thereto. The gas supply line  142  has two branches, line  144  and line  146  which feed gas into respective gas passageways  120  and  122  of tubes  112  and  114 . Gas supply line  142  also includes a valve  150  and a pressure meter  152  for controlling the flow of the gas into the branch lines  144  and  146 . The passageways  120 ,  122  each have an opening  160 ,  162  through which the gas exits the passageways  120 ,  122 . As can be seen in FIGS. 3 and 4, nozzle  110  includes grooves  170 ,  172  defined therein. After the gas exits openings  160 ,  162 , it enters the grooves  170 ,  172 . From there the gas flows into spaces  100 ,  101  as indicated by the arrows labeled “GF” on FIGS. 3 and 4. This gas flow, which preferably has a pressure that is slightly less than the metallostatic pressure MP, resists backflow of the molten metal  180  into spaces  100 ,  101  and thus in turn resists freezing of the molten metal to the tips  110   c  and  110   d  of the nozzle  110 . Seals  181  and  182  (such as those disclosed in U.S. Pat. No. 4,785,873, the disclosure of which is incorporated by reference herein) are provided in order for the gas to flow towards the tips  110   c  and  110   d  and not out the upper part of spaces  100  and  101 . 
     In practice, for each ten and one half inches of molten metal head, a metallostatic pressure of 1 psi is created. Thus, the pressure of the gas flow GF into space  100 ,  101  can be regulated to provide enough pressure to resist backflow and molten metal freezing to the nozzle tips  110   c ,  110   d . As mentioned above, it is preferred that the pressure of the gas flow GF be slightly less than the metallostatic pressure MP. If the pressure of the gas flow GF is greater than the metallostatic pressure, the gas may enter the nozzle  110 , which is undesirable because bubbles are created which can cause voids in the as-cast slab. 
     FIGS. 5 and 6 show alternate embodiments of the invention which involve automatic control of the pressure of the gas flow GF which is responsive to the metallostatic pressure MP. Referring now to FIG. 5, a schematic drawing of a gas pressure control means  200  is shown. The gas control means  200  includes a gas supply which preferably is a tank  202  containing an inert gas, preferably argon. A gas supply line  204  is connected to the tank  202 . Gas supply line  204  then branches at node point “NP” into a nozzle gas tube  206  and a mold gas tube  208 . Mold gas tube  208  includes a valve  210  and a pressure meter  212  and nozzle gas tube  206  includes a valve  214  controlled by a motor  216  which in turn is controlled by a relay circuit means  218 , which will be explained in further detail below. The nozzle gas tube  206  also includes a pressure meter  220 . 
     The mold gas tube  208  extends through the passageway  222  defined by the nozzle  224  and into the mold  226  of the twin belt caster, the mold  226  being defined by a pair of opposed belts  230 ,  232  and a pair of side dams (not shown in this view). As with the embodiment shown in FIGS. 3 and 4, the nozzle  224  has two grooves  236 ,  238 . In addition, as was described with respect to FIGS. 3 and 4, spaces  240 ,  242  (again, exaggerated to clearly illustrate the point) are created between belts  230 ,  232  and the nozzle outside surfaces  224   a  and  224   b.    
     The nozzle gas tube  206 , after the pressure meter  220 , also branches into two branch supply lines  244 ,  246 . These branch supply lines  244 ,  246  are then connected to tubes (not shown in this view) similar to tubes  112  and  114  of FIGS. 3 and 4. In this way, the gas in the branch supply lines  244 ,  246  is introduced into passageways (similar to passageways  120 ,  122  in FIGS. 3 and 4, but not shown in FIG. 5) through openings (similar to openings  160 ,  162  in FIGS. 3 and 4, but not shown in FIG. 5) into grooves  236 ,  238  and then into spaces  240 ,  242 . 
     This embodiment of the invention provides an automatic, self-adjusting gas pressure control means. Referring again to FIG. 5, balance means  250  responsive to the gas pressure in the mold gas tube  208  and the nozzle gas tube  206  is provided in order to insure that the right amount of gas pressure is maintained at the nozzle tips  224   c ,  224   d . The balance means  250  include a balance rod  252 , a first piston  254  operatively associated with the balance rod  252  and a second piston  255  operatively associated with the balance rod  252 . 
     A first piston gas supply tube  256  is provided having a first end in communication with the first piston  254  and a second end in communication with the mold gas tube  208 . The second piston gas supply tube  258  has a first end in communication with the second piston  255  and a second end in communication with the nozzle gas tube  206 . 
     The balance rod  252  is connected to a balance  260  having a fulcrum  262 , a weighted end  264  and a contact end  266 . The balance  260  can pivot about the fulcrum  262  when the balance rod  252  is moved by the first piston  254  or the second piston  256 . The contact end  266  includes an upper surface  266   a  and a lower surface  266   b . The contact end  266  can move between the space  268  created by an upper contact  270  and a lower contact  272 . Upper contact  270  is connected by line  274  to a first relay coil  276 . The lower contact  272  is connected by line  277  to a second relay coil  278 . The relay circuit  218  includes a power source, such as a battery  280 , to energize the circuit upon contact of the contact end  266  with either the upper contact  270  or the lower contact  272 . The first relay coil  276  has a pair of first relay contacts  281 ,  282  and the second relay coil  278  has a pair of second relay contacts  284 ,  286 . The relay circuit  218  controls the motor  216  of the valve  214  via lines  288 ,  290 . 
     The operation of this embodiment of the invention will now be explained. Initially, gas from the tank  202  is introduced into supply line  204 . This gas flows into the mold gas tube  208  but not nozzle gas tube  206  as the valve  214  is initially in a closed position. Preferably, ½ cc/sec of gas is introduced into the mold gas tube  208  before introducing molten metal into the mold  226 . Once molten metal  292  is introduced into the mold  226 , the metallostatic pressure MP of the molten metal  292  will cause the pressure in the mold gas tube  208  to increase. Also, because of the metallostatic pressure, the molten metal  292  will backflow into spaces  240  and  242 . The automatic gas pressure control means  200  of the invention provides a countervailing gas pressure, indicated by GF to resist this backflow. Seals  294  and  296 , similar to seals  180  and  182  in FIG. 4, are also provided to insure that the gas does not flow out of the upper portion of spaces  240  and  242  without reaching the tips of the nozzle. The automatic gas pressure control means also provides a mechanism to stop the flow of gas into the space  240 ,  242  when the gas pressure therein is greater than the metallostatic pressure MP. 
     Referring again to FIG. 5, the increased pressure in the mold gas tube  208  will be introduced into first piston gas supply tube  256 , thus causing first piston  254  to move to the left as shown by the arrow L 1  in FIG.  5 . This movement of the first piston  254  moves the balance rod toward the left as indicated by arrow L 2 , thus pivoting the balance upward, as shown by the arrow L 3  in FIG.  5 . When the upper surface  266   a  of the contact end  266  of the balance  260  contacts upper contact  270 , the first relay coil  276  is energized, which in turn causes switch SW 1  to move from contact A to contact B, as shown in phantom in FIG.  5 . When this occurs, the circuit is completed, and motor  216  causes valve  214  to move from its initial closed position to an open position. This allows gas to flow into the nozzle gas tube  206  and eventually into spaces  240  and  242  to resist molten metal from freezing on the nozzle tips  224   c ,  224   d . At the same time, gas flows into the second piston gas supply tube  258  and into the second piston  255  to cause second piston  255  to move to the right as shown by arrow R 1  of FIG.  5 . This will in turn move the balance rod  252  to the right (arrow R 2 ) causing the balance  260  to now pivot downwardly (arrow R 3 ) so that upper contact surface  266   a  no longer makes contact with upper contact  270 . This deenergizes the first relay coil  276  which in turn causes switch SW 1  to move from contact point B to A. This turns off the motor  216 , which still leaves the valve  214  in an open position. The balance  260  continues to move downwardly until lower contact surface  266   b  of the balance  260  contacts lower contact  272 . Once contact is made, second relay coil  278  is energized causing switch SW 2  to move from contact C to D which then energizes the motor  216  to close the valve  214  and thus discontinue gas flow into the nozzle gas tube  206 . Again, the pressure will increase in mold gas tube  208  causing the first piston to move to the left (arrow L 1 ), balance rod  252  to move to the left (arrow L 2 ) which will pivot balance  260  upwardly (arrow L 3 ). Once contact between the lower contact surface  226   b  of balance and the lower contact  272  is broken, second relay  278  is deenergized, causing switch SW 2  to move from contact D to contact C. This will leave the valve in the closed position. In order to avoid hysteresis, a timer or a dead band mechanism can be used. 
     This back and forth movement continues in order to control precisely the gas pressure at the nozzle tips  224   c ,  224   d.    
     It will be appreciated that the balancing means provides an automatic self-adjusting method of controlling the gas pressure near the tips of the nozzle. This control will insure that molten metal is resisted from freezing to the nozzle tips. 
     FIG. 5 shows a specific embodiment (i.e., balance means  250 ) which is responsive to the gas pressure in the mold gas tube  208  and nozzle gas tube  206 . It will be appreciated, however, that the invention is not limited to the balance means  250  shown in FIG. 5 but can be any sensor means that is responsive to the gas pressure in mold gas tube  206  and nozzle gas tube  208 , for example, a diaphragm or a mercury sensor switch. 
     FIG. 6 shows another embodiment of the gas pressure control means  300 . In this embodiment a mold rod  320  is disposed in the molten metal passageway  322  formed by the nozzle  324 . The mold rod  320  is preferably made of nickel alloy, coated with a ceramic material. The mold rod  320  is connected to a piston  326 . A balance  330 , including a weighted end  332 , a fulcrum  334  and a piston attachment end  336  is also provided. Similar to the embodiment shown in FIG. 5, a balance rod  340  is connected to the balance  330 . The balance rod  340  can move between two contacts  350 ,  352 , with a left surface  340   a  of the balance rod  340  adapted to contact left contact  350  and a right surface  340   b  of the balance rod  340  adapted to contact right contact  352 . The structure of the remainder of the relay circuit is similar to the relay circuit  218  shown in FIG.  5  and will not be set forth in detail at this point. 
     In operation, when molten metal  370  is introduced into the mold  372 , the metallostatic pressure MP will tend to create a backflow into spaces  374 ,  376  which is defined by belts  378 ,  380  and the nozzle surfaces  324   a  and  324   b . The metallostatic pressure MP will also cause the mold rod  320  to move upwardly (arrow U 1 ), thus pivoting balance in the direction of arrow U 2  on FIG.  6 . This, in turn, will cause balance rod  340  to move towards the right (in the direction of arrow U 3 ). Once balance rod surface  340   b  contacts the right contact  352 , first relay  382  is energized which in turn (as was explained above with respect to FIG. 5) opens the valve  384  allowing gas to flow into the nozzle gas tube  386  and eventually into space  374 ,  376 . Seals  396  and  398 , similar to seals  294  and  296  in FIG. 5, are again provided to insure that the gas does not flow out of the upper portion of spaces  374  and  376  without reaching the tips of the nozzle. The gas flowing into nozzle gas tube  386  will also flow into pivot gas tube  390  in order to counteract the upward movement of the piston  326 . This gas pressure may eventually move the balance  330  downward, in the direction of arrow D 1 . This, in turn, will move the balance rod to the left in FIG. 6 as shown by arrow D 2 . When the surface  340   a  of the balance rod  340  contacts left contact  350 , the second relay  292  is energized which in turn closes the valve  384 , as was described above with respect to FIG.  5 . 
     It will be appreciated that a caster has been disclosed including a gas delivery means to resist freezing of molten metal to the tip of a nozzle and an associated method. 
     An associated method of the invention is also provided. The method comprises providing a caster, such as (but not limited to) a twin belt caster and solidifying molten metal in the mold of the caster. While the molten metal is solidified, a gas, preferably argon, is introduced into the space between the mold defining means and the nozzle so that freezing of the molten metal to the tip of the nozzle is resisted. 
     While specific embodiments of the invention have been disclosed, it will be appreciated by those skilled in the art that various modifications and alterations to those details could be developed in light of the overall teachings of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention which is to be given the full breadth of the appended claims and any and all equivalents thereof.