Abstract:
The invention describes a submerged sensor for use in a metallurgical vessel, such as a tundish. The sensor is positioned so that in use it is below a surface of a molten metal bath within the vessel. The sensor conveniently extends through the vessel wall and projects into the volume of the vessel. An upper end of the sensor is encased in a protective assembly. The protective assembly of the invention can be shaped so as to control the pattern of molten metal flow in the vessel or measure more accurately the molten metal exiting the vessel. Advantageously, the invention can combine the functionality required for flow pattern modification such as provided by a dam, impact pad or baffle, with the functionality required for a sensor. This combination reduces cost and complexity.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a submerged sensor adapted for use in a metallurgical vessel and, more specifically, to a submerged temperature sensor located in a tundish for continuous casting.  
       BACKGROUND OF THE INVENTION  
       [0002]     The ability to continuously monitor the status of molten metal in a metallurgical vessel is highly desirable for both economic and quality reasons. Accurate monitoring can, for example, greatly reduce energy consumption caused by overheating. Other benefits of continuously monitoring molten metal conditions include the ability to measure high temperature phase changes, chemical reactions, and other related phenomena.  
         [0003]     Certain problems exist when measuring conditions of molten metal within a metallurgical vessel. These problems include extremely aggressive slags, thermal shock, violent turbulence from material additions, argon stirring, splashing and skulling, and difficulties in fixing the sensor in the vessel.  
         [0004]     Sensors are typically encased in a protective assembly. Sensors include chemical sensors, such as oxygen sensors, and temperature sensors, such as thermocouples. Thermocouples for measuring molten metal temperatures have been described in, for example, U.S. Pat. No. 4,749,416. Such prior art arrangements permanently fix the thermocouple in a sidewall or bottom of the vessel. Alternatively, thermocouples may include a moveable protective assembly as described in U.S. Pat. No. 4,721,533, U.S. Pat. No. 4,746,534, U.S. Pat. No. 5,071,258 and U.S. Pat. No. 6,139,180. Movable protective assemblies typically insert the thermocouple through a floating slag layer into a bath of molten metal. Slag is very corrosive and a thermocouple protective assembly, which is in contact with slag, must combine good thermal conductivity and the ability to withstand chemical attack by the slag layer. A minimum one hundred-hour service life is desired.  
         [0005]     Refractory materials with good thermal conductivity typically include carbon, Unfortunately, carbon detracts from slag corrosion resistance and may lead to premature failure after as few as ten hours of operation. Premature failure can also occur due to wire fracture, “emf drift,” and high temperature chemical reactions. Carbon in the protective assembly may accentuate such failures by participating in chemical reactions damaging to the sensor. Encasing the thermocouple inside a refractory metal sheath within the protective assembly has been effective at reducing undesirable chemical reactions. Unfortunately, such metal sheaths are expensive and are not entirely effective. Even a few milligrams of contaminants are sufficient to destroy the thermocouple by fracturing the wire, particularly along grain boundaries, or causing emf drift as a result of reaction product around the thermocouple.  
         [0006]     As a further complication, an operator is often interested in the status of the molten metal actually exiting the vessel. In continuous casting, metal exists the vessel through a bottom outlet. The prior art is deficient in that the sensor is typically located near the surface of the molten metal bath. The sensor records, therefore, only near the surface and not near the outlet. Gradients in the vessel ensure that measurements near the surface of the bath are unlikely to be identical with measurements near the outlet. For example, the temperature near the top of a bath will often differ substantially from the temperature at the outlet. Sensors used in the prior art can be lengthened to extend closer to the outlet but this increases both the buoyant and inertial forces acting on the sensor. High buoyancy or inertial forces can cause dangerous conditions if control over the sensor is lost. The sensor may then eject violently from the bath. Additionally, a long sensor is more costly and more subject to premature failure.  
         [0007]     A further deficiency of a prior art sensor descending through the surface of the bath is the need for an external support. The external support is highly inconvenient and subject to damage by heat rising from the molten metal and by collision with surrounding structures or equipment during movement of the vessel. The external support must not obstruct vessel movement. Consequently, the support must be placed so as not to obstruct movement or it must be disconnected, repositioned and reconnected each time the vessel is moved. The obstructive nature of the external support often leads to damage of the sensor or the external support.  
         [0008]     A need exists for a sensor that is easy to install and operate while permitting measurement near the outlet of a metallurgical vessel. Preferably, the sensor should avoid high buoyancy and inertial forces. The sensor should be unaffected by slag corrosion, and should not interfere with movement of the metallurgical vessel. Preferably, an external support is unnecessary. The sensor should not include refractory materials that deleteriously affect the sensor.  
       SUMMARY OF THE PRESENT INVENTION  
       [0009]     The present invention describes a submerged sensor for use in a metallurgical vessel, such as a tundish. The sensor is positioned so that in use it is below a surface of a molten metal bath within the vessel. The sensor conveniently extends through the vessel wall and protrudes into the volume of the vessel. A terminal end of the sensor is encased in a protective assembly. A refractory material may surround the protective assembly.  
         [0010]     An object of the invention is to provide a metallurgical vessel with a sensor that either has no buoyancy in a metal bath or resists the buoyancy force without an external support to secure the sensor. The absence of an external support eliminates the possibility of breakage of the support during movement of the vessel, and permits movement of the vessel without manipulation, repositioning, connecting or disconnecting of an external support. A second object of the invention is to provide a sensor suitable for measuring the lower portion of the metal bath where an outlet for the metallurgical vessel is typically located. Another object of the invention is to position the sensor away from the corrosive effects of slag. A further object is to provide a vessel with a sensor that projects into the molten metal, thereby altering and controlling the pattern of metal flow in the vessel.  
         [0011]     In one embodiment, the sensor may be a temperature sensor, such as a thermocouple. The temperature sensor can be simple, easy to install and remove, and is preferably disposable. The sensor permits continuous temperature measurement of molten metal in a vessel. In a second embodiment, the sensor is placed adjacent to an outlet of the vessel, such as that used in continuous casting, so that the sensor actually measures the outflow from the vessel. In another embodiment, the sensor may be placed along the walls of the vessel or within dams, weirs or baffles inside the volume of the vessel.  
         [0012]     In still another embodiment, the protective assembly of the invention can be shaped so as to control the pattern of molten metal flow in the vessel or measure more accurately molten metal exiting the vessel. The invention can combine the functionality required for flow pattern modification such as provided by a tundish with a dam, impact pad or baffle, with the functionality required for molten metal temperature measurement. This combination reduces cost and complexity of the tundish.  
         [0013]     Other advantages of the invention include the need for a relatively short protective assembly, thereby reducing cost, and a configuration that imparts substantially only compressive forces on the assembly, which forces a ceramic assembly most easily resists. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0014]      FIG. 1 . shows a side view of a prior art metallurgical vessel with a sensor.  
         [0015]      FIG. 2  shows a side view of a prior art metallurgical vessel with a sensor and wier.  
         [0016]      FIG. 3   a  shows a side view of a metallurgical vessel with a sensor of the present invention.  
         [0017]      FIG. 3   b  shows a top view of the metallurgical vessel of  FIG. 3   a.    
         [0018]      FIG. 3   c  shows a side view of a sensor of the present invention.  
         [0019]      FIG. 4   a  shows a top view of a vessel with a sensor that projects into the volume of the vessel.  
         [0020]      FIG. 4   b  shows a cross-section of the projecting sensor of  FIG. 4   a.   
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0021]      FIG. 1  is a sectional view of a prior art metallurgical vessel  3  with an external sensor  2 . The sensor  2  enters the molten metal bath  8  through the top surface  9  of the bath  8 . Frequently, the top surface  9  includes a layer of slag floating on the bath  8 . The sensor may be, for example, a temperature-measuring device such as a thermocouple. The sensor is at least partially enclosed within a protective assembly  1  and is typically at least partially immersed within the bath  8  through the top surface  9 . Immersion is necessary to measure conditions in the bath. The protective assembly often comprises a refractory ceramic and may also include an internal sheath into which the sensor is placed. The protective assembly of the prior art must resist corrosion by slag and may include a slag-resistant sleeve. The sheath may comprise a refractory ceramic or metal, such as molybdenum or high purity alumina.  
         [0022]     In the continuous casting of metal, an upstream metal flow  11  from an upstream vessel (not shown) enters a pouring tube  6  and then into the metallurgical vessel as an incoming flow  12 . The pouring tube typically discharges the incoming stream  12  below the top surface  9  of the bath  8 . A nozzle  7  defines an outlet through the bottom of the vessel  3  and enables molten metal to drain from the vessel. An accelerating flow  14  accelerates to the entrance of the nozzle  7 . An outflow  13  leaves the metallurgical vessel  3  through a nozzle  7 . The highest velocities of molten metal flow in the metallurgical vessel are typically found in two areas; near the exit end of the pouring tube  6  as illustrated by the incoming flow  12  and at the point where the accelerating flow  14  enters the nozzle  7 .  
         [0023]     A sensor entering the molten metal bath through the top surface must be affixed to some external support to remain immersed and in place. The external support must fix the sensor position despite both inertial and buoyancy forces. Inertial forces are induced by molten metal flow in the bath. Buoyancy forces arise because the sensor is typically substantially less dense than the molten metal it displaces. Generally, the buoyancy force is much greater than the inertial forces.  FIG. 1  illustrates an external support  4 , which is affixed to the sensor assembly and to some convenient external rigid fixture  5  such as a metallurgical vessel cover or metallurgical vessel car or other neighboring structural fixture.  
         [0024]     As shown in  FIG. 2 , the prior art may include a dam  21  within a metallurgical vessel  3 . The dam  21  is used to deflect upward the internal metal flow  22 . The deflection reduces stagnation in the upper part of the vessel, and permits the sensor  2  to measure more accurately conditions of the outflow  13  and accelerating flow  14  entering the nozzle. Of course, the prior art sensor  2  does not actually measure the outflow  13  or the accelerating flow  14 . The dam  21  only directs more of the flow past the sensor  2 . The dam  21  is only one type of projection that could be used to direct the internal flow  22 . Other examples include baffles and impact pads. In any case, these devices are separate from the protective assembly  1  and result in additional cost and complexity.  
         [0025]     Desirably, a sensor measures an outflow from a vessel. The prior art is deficient in that sensors are inserted into the top of the bath, whereas the molten metal is exiting from the bottom of the bath.  FIGS. 3   a ,  3   b  and  3   c  illustrate a first embodiment of the invention.  FIG. 3   a  is a sectional view of a sensor of the invention as used in a metallurgical vessel  3 .  FIG. 3   b  is a top view of the same.  FIG. 3   c  is a view showing the sensor assembly  36  comprising a sensor  2 , a protective assembly  1  surrounding an upper portion of the sensor and optionally a protective sleeve  31  surrounding at least a part of a lower portion of the sensor. The sensor may be a temperature measuring device, chemcial sensor, or any other type of sensor known to one skilled in the art. The protective assembly comprises a first refractory composition. Conveniently, the composition comprises a refractory ceramic such as alumina or zirconia. The protective sleeve comprises a second refractory composition, which may or not be the same as the first refractory composition. The first and second compositions may be pressed, cured or fired to instill sufficient mechanical strength to protect the sensor from damage.  
         [0026]      FIG. 3   a  shows the vessel  3  having a bottom  32  defining a hole  33  for the sensor assembly. The vessel may be any type of metallurgical vessel but is commonly a tundish or other vessel which, during operation, a substantial volume of the vessel outflows over a relevant time period. The sensor assembly extends through hole  33 . Conveniently, the protective assembly  1  rests on the inner surface  34  of the bottom  32 . A protective sleeve  31  may be used to protect the sensor from damage. Preferably, a refractory material  35  surrounds the protective assembly  1 . The refractory material may comprise one or more refractory compositions, including castable or vibratable materials. More preferably, the refractory material  35  adheres to the protective assembly I and to the inner surface  34 . Conveniently, the refractory material may also be used beneath the protective assembly  1  in order to fill the hole  33  and reduce the likelihood of an escape of molten metal through the hole  33 . The refractory material may also stabilize the protective assembly  1  against inertial forces resulting from molten metal flow.  
         [0027]     A vessel of the invention may be made-up in accordance with the above description before movement of the vessel into operating position. After service, the vessel can be moved out of operating position. Residual metal can be dumped from the tundish. After casting, and typically during or after dumping residual metal, the sensor is discarded or recovered. The vessel is then again made ready for service.  
         [0028]     The sensor of the invention exposes substantially upward facing or substantially vertical surfaces to the molten metal. Such upward or vertical surfaces are subject to little or no buoyancy forces. In fact, the molten metal exerts mainly compressive forces on the protective assembly. Refractory ceramic materials, which typically comprise the protective assembly, are strongest under compression, so the sensor assembly needs no external support.  
         [0029]     Placement of the sensor at the bottom of the vessel also permits conditions in the lower portion of the vessel especially of the outflow to be measured more conveniently. The sensor assembly may be placed close to the nozzle  7 . Unlike prior art that extend through the top surface of the molten metal bath, placement near the nozzle does not require lengthening the sensor. Inertial forces are not increased and the amount of material required to make the sensor assembly is unchanged.  
         [0030]     Referring to  FIG. 3   a , the incoming flow  12  enters the vessel  3  through a pouring tube  6  submerged in the molten metal bath. The outflow  13  leaves the vessel through a nozzle  7 . Immediately around the nozzle  7 , an accelerating flow  14  moves toward the nozzle  7 . Referring to  FIG. 3   b , the protective assembly projects into the volume of the vessel and partially deflects the accelerating flow  14 . The accelerating flow  14  enters the nozzle  7  and becomes the outflow  13 . In other words, the location of protective assembly  1  in the lower portion of the vessel near the nozzle  7  ensures the sensor is measuring conditions of the flow just prior to its entrance into the nozzle  7 . “Near” means a position around the nozzle from which a sensor is capable of measuring the accelerating flow during casting. Placing the sensor near the outlet reduces differences between the sensor measurement and the actual condition of the outflow.  
         [0031]      FIGS. 4   a  and  4   b  show a second embodiment of the invention.  FIG. 4   a  is a top view of a sensor assembly of the present invention in a metallurgical vessel.  FIG. 4   b  is a cross-sectional view of the sensor assembly  36 . The assembly includes the protective assembly  1 , the sensor  2  and, optionally, the protective pipe  31 . This embodiment shows an alternative geometric configuration of the protective assembly  1 , in which the protective assembly  1  is in the shape of a dam. In this embodiment, the shape of the protective assembly  1  is used to further alter, deflect, and control the accelerating flow  14 . Of course, other geometries for a protective assembly  1  may be used, including baffle shapes, impact pad shapes, weirs, and other shapes, which are known to those skilled in the art.  
         [0032]     Obviously, numerous modifications and variations of the present invention are possible. It is, therefore, to be understood that within the scope of the following claims, the invention may be practiced otherwise than as specifically described.