Abstract:
Microwave transmission and reception, and analysis thereof, are used to monitor the level of molten metal, paradigmatically steel, in a tundish receiving molten metal (steel) from a ladle or other vessel and passing it to the head of a continuous caster, while the steel in the tundish is covered with slag. The control factors may include level, weight or volume limits in the tundish derived from historical or real-time data, including data representing a predictable or developing vortex in the well feeding the continuous caster. The system is particularly useful in minimizing transition mix during change from one metal specification or grade to another.

Description:
RELATED APPLICATION  
       [0001]     This application claims the full benefit under 35USC119 of Provisional Application No. 60/531,959 filed Dec. 23, 2003. 
     
    
     TECHNICAL FIELD  
       [0002]     In the continuous casting of steel and other metals, the level of metal in a tundish is controlled to inhibit vortexing and consequent entrainment of slag into the cast metal. Control is effected with the aid of microwave transmissions directed toward the molten material in the tundish, advantageously toward the well or tap hole in the tundish, optionally combined with other factors and data representing optimum levels of steel and slag in the tundish, product specifications, or disturbances in the mold. The process is particularly useful for transitioning between products of different specifications; the formation of intermix can be minimized.  
       BACKGROUND OF THE INVENTION  
       [0003]     Good control of the continuous casting of steel requires good control of the level of steel in the mold. See the early Pat. No. 3,537,505 to Uster et al, as an example of the control of the level of steel in the mold. During continuous steel casting operations, the mold is fed by a tundish and it is desirable to know and control the level of the liquid steel in the tundish, and the thickness of the slag layer on the steel in the tundish. Both quality and economic factors are involved, and accurate knowledge and control of the amounts of steel and slag in the tundish can lead simultaneously to improvements in quality and reductions in cost.  
         [0004]     The tundish delivers molten steel to a mold at the top or head of the continuous cast strand at a rate that is dictated by the movement of the strand, i.e. the production rate of the caster. This in turn is governed by numerous factors including major factors such as the cross section of the strand, the cooling and solidification rates in the caster, and the heat-removal capacity of the mold. Disturbances in the mold, at the top of the strand, provide erratic temperature and level inputs to the gate control system. See Asano et al U.S. Pat. No. 5,311,924 for a method of stabilizing metal level in the mold, using a predetermined level maintenance value. A vibration sensor helps control disturbances in Ardell et al U.S. Pat. No. 5,042,700.  
         [0005]     The level of liquid steel above the tundish well is the important parameter in determining when to close the tundish gate during drainage of the tundish for a low tundish practice intermix transition, for a tundish change operation, or for termination of the cast. If the level of liquid steel is too low, there is a danger of formation of a vortex above the tundish well, resulting in the entrainment of slag into the caster mold, a highly undesirable event. Furthermore, the presence of a vortex may cause turbulence within the caster mold, resulting in mold level fluctuation and steel quality deterioration. If the level is too high, the amount of intermix will be large, or the final solid tundish skull will be larger, negatively affecting acceptable yield.  
         [0006]     Accurate measurement of steel level in the tundish allows an accurate determination of when to close the tundish gate or when to open a new ladle as a function of tundish weight. This can facilitate both an improvement in steel yield and better predictions of the quantity of intermix. A continuous or relatively frequent measurement of steel level in the tundish allows precise control of the caster operation to maximize yield, maintain acceptable steel quality, and minimize cost.  
         [0007]     The thickness of the slag layer in the tundish is an indication of how well ladle slag is prevented from entering the tundish as a ladle is drained. In general, the amount of ladle slag entering the tundish should be minimized, because the ladle slag is emulsified when it pours into the tundish and may not be separated completely before the steel enters the mold. Some ladle slags are also very oxidizing with respect to aluminum-deoxidized steel, and promote reoxidation of the steel in the tundish. In order to maximize yield, however, the ladle is drained as far as possible without excessive slag carryover, so the transfer of some slag from the ladle to the tundish is commonly tolerated. This amount may vary considerably from heat to heat in the casting sequence.  
         [0008]     There are three basic tundish operating modes: (1) steady state operation, whereby the steel levels in the tundish and mold are held substantially constant by matching tundish inlet and outlet flows, (2) a transition, desirably smooth, from one ladle or heat to another as the source of molten metal (ladle exchange), and (3) a transition from casting one grade of steel to another grade. Good tundish management during all three of these general operating modes will be facilitated by monitoring or awareness of: 
        1. Amount of slag in the tundish;     2. Amount of iron oxide in the tundish slag;     3. Quality requirements of the grade being cast;     4. Value of steel being cast;     5. Cost of disposition of intermix volume or downgraded steel;     6. Casting conditions such as nozzle obstructions or mold turbulence.        
 
         [0015]     Many of these factors are complex and/or interrelated, and a heuristic approach is required for determination of the optimum tundish steel level for a given procedure. Maintaining the molten metal at an optimum level will enhance yield and reduce the cost of steel production while maintaining quality.  
         [0016]     It is desirable to utilize the tundish in a steady manner—that is, for as long as is practical to retain steady state conditions before the tundish must be changed. As different steel grades will display different rates of corrosion or other damage to the refractory, the grade of steel being fed to the caster will be a factor in determining when to change the tundish. Data should be accumulated correlating steel grade to refractory life. Also a tendency to include slag in the steel entering the caster is higher after several heats have passed through the tundish; while we don not wish to be bound by any particular theory, this appears to be because slag residue is more likely to be present in damaged or eroded refractory. Generally, it is also desirable to maintain a molten steel at a comfortable level above a predetermined minimum (such as the level at which a vortex may form), so that (1) any included slag will tend to float rather than be poured along with the steel, (2) vortexing will be avoided, (3) control of steel flow will be more manageable because the hydrostatic head (or heel weight) is within a predetermined range, and, importantly also (4) transition from one heat to another can be smooth and trouble free. For various reasons mentioned above, the tundish is likely to be replaced after about ten to about twenty or more heats are passed through it. Accumulated slag is dumped and the remains of molten steel are generally recycled to the steelmaking facilities. Tundish maintenance is expensive and accordingly it is desirable to extend the number of heats passing through a tundish as long as possible; also there is an economic cost to the entire production line for removing a tundish and replacing it with another. Reducing the frequency of replacement is also desirable for this reason.  
         [0017]     The accurate measurement of slag depth in the tundish can assist in the dispositioning of steel in relation to quality. Significant slag carryover from ladle to tundish is known to cause defects in certain products with stringent steel cleanliness demands. Knowledge of the slag amount can assist the quality engineer in deciding how to apply steel more efficiently to orders with different quality requirements.  
         [0018]     It is known in the art generally to relate slag depth to weight in a ladle or other container, since the surface area of the ladle and slag density can be approximated with reasonable accuracy.  
         [0019]     The measurement of depth of slag disposed on molten steel is known and practiced in the art. One method involves inserting a steel rod, either manually or utilizing a pantograph, through the slag and into the steel. The steel rod is held at constant height for a time sufficient to allow that portion of the rod submerged in the steel to melt. The rod is then withdrawn, and the length of the portion glowing red with heat is measured. This is representative of the slag depth at the point at which the rod was inserted. This method of slag depth measurement is labor intensive, inaccurate, discontinuous, and not necessarily representative of the average slag depth. A related method involves insertion of a tube having a sacrificial cap—see Speranza U.S. Pat. No. 5,888,324.  
         [0020]     A microwave method of slag thickness measurement is described in Meszaros et al U.S. Pat. Nos. 6,130,637, 6,166,681, and 6,255,983, in which pulsed radar time of flight measurements are used to determine the levels of slag and the steel under it. The measurement is remote, is more indicative of the average slag depth as opposed to depth at a single point, and can be repeated frequently and less dangerously. This method of slag depth measurement is more suitable for the object of the present invention, which is described more fully below. Radar has been used to detect wake turbulence generated by aircraft—see Ooga U.S. Pat. No. 6,424,408 and Rubin Pat. No. 6,480,142.  
         [0021]     The measurement and control of metal level in a continuous casting mold is known in the art. One method, described in Baumert U.S. Pat. No. 4,175,612, involves the use of a coil suspended near the top of the mold. This method and others like it are not suitable for control of tundish level, since the steel is not in communication with a mold, but rather is disposed within a tundish with a refractory lining. Furthermore, such a method gives no information with respect to the thickness of the slag layer disposed on top of the steel.  
         [0022]     Another problem associated with low-level tundish operation is that the steel flow rate through a given nozzle opening continuously decreases with the steel level in the tundish. This decrease requires frequent adjustments to the tundish discharge nozzle opening and may cause excessive mold turbulence. The mold turbulence can cause quality deterioration and downgrading of the steel.  
         [0023]     The design of the continuous casting tundish takes into consideration the depth of steel required over the exit nozzle for optimum fluid dynamics and steel quality. It is known in the art to control the level of steel in the tundish to provide a sufficient depth over the exit nozzles. During ladle exchange, grade change, tundish exchange, or termination of the cast it is not practical to maintain the depth of steel over the nozzles that is maintained during steady-state operation. As described, excessive drainage of the tundish can result in slag entrainment and vortex formation that can be problematic. It is known in the art to apply vortex inhibitor apparatus, such a those described in Vassilicos U.S. Pat. No. 5,171,513 and Labate U.S. Pat. No. 4,799,650 to reduce the likelihood of slag entrainment into the casting mold during low level conditions in the tundish. The usefulness of these tools is arguable and, in any case, the problematic effects of low-level tundish operation often remain.  
         [0024]     It is known in the art to detect the outflow of slag as steel drains from one vessel into another, as described for example in Thiessen U.S. Pat. No. 4,816,758 and Hatono et al Pat. No. 4,693,614. Furnace tilt angles are controlled by various factors in Koffron&#39;s U.S. Pat. No. 6,280,499. See also Ryan et al U.S. Pat. No. 5,375,816, which uses an electrically conductive element to detect the presence of slag. It is also known to detect the onset of vortex formation that may be followed by slag entrainment, as described in Ardell et al U.S. Pat. No. 5,042,700 and in Koffron et al U.S. Pat. No. 6,074,598, employing an electromagnetic device to detect the slag. Vortex formation has also been controlled by slag-control “shapes” which are usually objects having a specific gravity between those of molten steel and slag; such objects are placed directly above the discharge opening where a vortex is likely to form—see Koffron U.S. Pat. No. 4,601,415, Forte et al Pat. Nos. 4,968,007 and 5,421,560 and patents reviewed therein. A lance for injection of inert gas has also been suggested to inhibit vortex formation—Patrushka et al U.S. Pat. No. 5,203,909. Inert gas is also injected in Sao et al U.S. Pat. No. 6,346,212 to inhibit vortex formation. The slag control body of DeMarco U.S. Pat. No. 6,153,146 need not have a specific gravity between that of the steel and that of the slag. The importance of vortex formation as a problem in the art is further illustrated in Dainton U.S. Pat. No. 5,766,543 and Sankaranaryanan et al Pat. No. 5,382,003, both discussing devices for combatting slag entrainment.  
         [0025]     An object of the present invention is to facilitate low-level tundish operation for application to ladle exchange, transition control, tundish exchange, and termination of a cast strand. A further object of this invention is to improve yield and quality in steel strand casting by accurate prediction of the optimum level to which a tundish should be drained during the above mentioned low-level operations, and to control the tundish level to the optimum level.  
       SUMMARY OF THE INVENTION  
       [0026]     Our invention includes a method of managing a tundish to control the transition between a first grade of molten steel and a second grade of molten steel to be solidified sequentially in a continuous caster by draining the molten steel through a well in the tundish to a mold at the head of the continuous caster, wherein the first grade of molten steel is covered by a layer of slag in the tundish, comprising draining the first grade of steel to a level in the tundish slightly higher than the level at which a vortex is expected to form above the well, the level of the first grade of steel being determined by directing microwave radiation into the tundish from above the layer of slag and analyzing reflections of the microwave radiation from the slag and the steel, and adding the second grade of molten steel to the tundish at a rate at least that of the rate of draining the first grade of steel. Slag inclusions in the steel fed to the continuous caster are not only reduced because the steel level is not allowed to become too low, but also because the more or less continuously held level of steel provides time for any emulsified slag to become de-emulsified.  
         [0027]     Our invention further includes a method of maintaining the level of molten steel within desired limits in a tundish feeding a continuous caster including a mold comprising monitoring the level of molten steel in the tundish by microwave analysis and controlling the flow of molten steel out of the tundish to maintain the level within the predetermined limits. A predetermined steel level range is maintained in continuous use of the tundish through the passage of from about ten to twenty or more heats of steel; the level is monitored by microwave intermittently or continuously throughout such a term of use. The term of use may include one or more steel heats as supplied to the tundish from a steelmaking vessel—that is, the method may be used substantially continuously throughout several heats. As will be seen below, one may monitor the steel volume or weight in the tundish as well as the level, and the control factors may vary with the specifications of the steel as well as real time conditions in the caster.  
         [0028]     In another aspect, our invention includes a method of managing a tundish containing molten steel and slag during a transition between a grade A of steel and a grade B of steel to be cast sequentially in a continuous caster fed by the tundish comprising (a) establishing a data-base on steel product characteristics affected by at least one of (i) molten steel level in the tundish, (ii) slag depth in the tundish, (iii) flow rates into the tundish, (iv) flow rates out of the tundish, (v) iron content of the slag and (vi) patterns of steel flow which are indicative of the formation of an incipient vortex, (b) determining a lowest level for molten steel of grade A in the tundish at which the probability of a vortex occuring above a well in the tundish, and the probability of slag entrainment into the mold, are deemed acceptably low (c) monitoring for formation of an incipient vortex above a well in the tundish, (d) controlling flow of the molten steel of grade A into and out of the tundish as a function of at least one of (i) a desired product characteristic of the steel, (ii) the lowest level of molten steel of grade A at which the probability of vortex formation is acceptably low, and (iii) formation of an incipient vortex and (e) thereafter adding molten steel of grade B to the tundish. By way of example, in some tundishes, a vortex is considered to be a danger when the level of steel is less than 10 inches, in practice typically less than 12 inches.  
         [0029]     Also, our invention includes a method of inhibiting the formation of a vortex over a well in a vessel containing molten steel covered by slag comprising (a) directing microwave radiation toward the slag in an area over the well, (b) receiving reflections of the microwave radiation, (c) analyzing the reflections to identify patterns thereof indicative of the incipient formation of a vortex in the well, (d) establishing a reference base of the patterns for comparison with a real time pattern of microwave reflections over the well, and (e) adding additional molten steel to the vessel when the real time pattern resembles the reference base. Perhaps more succinctly, our invention includes a method of inhibiting the formation of a vortex over a well in a vessel containing molten steel covered by slag comprising (a) directing microwave radiation toward said slag in an area over said well, (b) receiving reflections of said microwave radiation, (c) analyzing said reflections to identify a pattern thereof indicative of the incipient formation of a vortex in said well as predictable from a reference base, and (d) adding additional molten steel to said vessel when said pattern is identified. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]      FIG. 1  is a side sectional view of a tundish during normal operation of a typical steel continuous caster, showing placement of a microwave transceiver for purposes of our invention.  
         [0031]      FIG. 2  shows the same tundish as in  FIG. 1  having a low level of molten steel, leading to the undesirable formation of a vortex over the well. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0032]     Referring first to  FIG. 1 , the tundish  2  has a refractory lining  1 . It contains molten steel  12  on which floats a layer of slag  14 . Molten steel  8  in refractory-lined shroud  98  is more or less continuously provided from a ladle not shown. Shroud  9  penetrates through the slag  14  to minimize the turbulence and exposure of steel to the air. Tundish  2  has a cover  10  having an aperture  11  for shroud  9 . Another aperture  13  in cover  10  permits microwaves  22  to be transmitted into the tundish and strike the surface of slag  14 . Microwaves  22  emanate from transmitter  20  placed preferably directly above aperture  13 . Microwave transmitter  20  and aperture  13  are both directly over well  15  in the bottom of tundish  2 . Well  15  is fitted with a sliding gate  27  which may be controlled to be closed or in various open positions. When sliding gate  27  is open, molten steel  12  drains through well  15  and conduit  6 , through flux layer  18  and into mold  4 , which forms the head  16  of a continuously moving strand of steel in the caster as is known in the art.  
         [0033]     Microwave transmitter  20  is preferably also a microwave receiver and hence a transceiver, typically using the same antenna for sending and receiving reflected microwaves, as is known in the art of radar or microwave transmission and receiving. See the discussion of rod and horn antennas in Fehrenbach et al U.S. Pat. No. 6,404,382, and the level calculators described by Kleib in U.S. Pat. No. 6,300,897 and Diede in U.S. Pat. No. 6,320,532. The transceiver  20  is connected to computer  28 , which is enabled to analyze the time-of-flight data to determine not only the upper level of the slag but also the upper level of the molten steel even though it is covered by the slag, as explained in Meszaros et al U.S. Pat. Nos. 6,130,637, 6,166,681, and 6,255,983, all of which are hereby incorporated by reference into this disclosure in their entireties. Unless otherwise noted herein, our techniques for detection of the levels of slag and steel in the tundish follow those described in the Meszaros et al patents. The microwaves may be pulsed.  
         [0034]     Elaborating on the application of microwave measurement technology to the present invention, first, it should be understood that the microwave transmitter  20  is preferably placed a known or measured distance from the bottom of tundish  2 , thus giving the system a reference distance for microwave reflections. As explained in the Meszaros et al patents referenced above, interpretation of the reflected microwaves is aided by the quite different conductivities of the slag and the molten steel. During normal continuous casting, therefore, measurement of the level of the steel, and the thickness of the slag, in the tundish are similar to those described in the Meszaros et al patents. Time-of-flight analysis of the reflected microwaves will demonstrate two recognizable patterns, representing reflections from the steel and from the slag; these are correlated to the respective levels of the steel and the slag. As will be seen below, however, such readings may be supplemented by inputs of several other kinds and actions may be taken based on deviation from historical data of these and other kinds.  
         [0035]      FIG. 2  shows the same equipment as in  FIG. 1  except that here the level of molten steel  12  is low. This means that the incoming steel  26  at the lower end of shroud  98  is undesirably exposed to the air, and a vortex  24  has been formed above well  15 . The formation of vortex  24  means that the slag  14  will tend to go down the well  15  along with the steel, an undesirable event. Computer  28  is programmed to recognize the reflected pattern of an incipient vortex, and will generate a signal to alert the system prior to the maturation of the vortex  24 . When an incipient vortex is detected, the flow of molten steel to the tundish is increased and the conditions of  FIG. 2  are averted.  
         [0036]     Persons skilled in the art will recognize that our method and apparatus may act to prevent the entrainment of slag into the continuous caster alternatively (a) when a vortex is actually detected, (b) when a vortex is anticipated imminently as detected in the movement, or patterns of movement, of the steel and slag, or (c) when a vortex is anticipated based on historical data correlating vortex formation to the level of molten steel in the tundish. In addition, our method of level control in a tundish may be based entirely on measurement, by radar, of the levels of steel and slag in the tundish independently of the possibility of vortex formation. That is, our invention includes a method of maintaining the level of molten steel within desired limits in a tundish feeding a continuous caster including a mold, comprising monitoring the level of molten steel in the tundish by microwave analysis continuously or intermittently throughout a plurality of heats, and controlling the flow of molten steel out of the tundish to maintain the level within the predetermined limits, also throughout the plurality of heats. This method may be practiced while also monitoring the well of the tundish, using microwave or other means (such as vibration monitoring) for vortex formation or flow patterns characteristic of incipient vortex formation.  
         [0037]     Table 1 contains data generated by a computer model, which correlates molten steel height in a specific commercially used tundish with the volume and weight of steel in the tundish. The data are for a single grade of steel.  
                                             TABLE 1                           Steel volume               Steel Height   (cubic feet)   Weight (pounds)   Weight (tons)                                0.00   0.00   0.00   0.00       2.54   11.56   5086   2.54       15.82   71.93   31,649   15.82       29.00   131.83   58,005   29.00       43.07   195.79   86,148   43.07       58.06   263.89   116,112   58.06                  
 
         [0038]     It will be seen that by monitoring the steel level it is possible to generate, for a specific tundish and a specific grade of steel, control limits for any of the factors listed in Table 1.  
         [0039]     Persons skilled in the art may also assume that the acceptable level of probability of vortex formation may vary somewhat as a function of the specifications of the particular steel. Our invention may take this into account also, and accordingly our invention includes a method wherein the level of steel in the tundish is controlled within predetermined limits by monitoring the level of steel in the tundish by microwave analysis and controlling the level (either incoming or outgoing flow of molten steel, or both) with control signals based upon the level so determined possibly modified by slag level, volume, or weight, or the level, volume or weight of steel in the tundish, together with various factors in the mold of the continuous caster, such as the rate of solidification, the strand production rate, turbulence, and vibrations.  
         [0040]     In another aspect, our invention may be seen as a method of managing a tundish to control the transition between a first grade of molten steel and a second grade of molten steel to be solidified sequentially in a continuous caster by draining the first and second grades of molten steel through a well in the tundish to a mold at the head of the continuous caster, wherein the first grade of molten steel is covered by a layer of slag in the tundish comprising (a) draining the first grade of steel to a level in the tundish slightly higher than the predetermined level at which a vortex is expected to form above the well, (b) while draining the first grade of steel, monitoring the level of the first grade of steel in the tundish by directing microwave radiation into the tundish from above the layer of slag and analyzing reflections of the microwave radiation from the slag and the steel, and, (c) when step (a) is completed, adding the second grade of molten steel to the tundish at a rate at least that of the rate of draining the first grade of steel. By way of example, in a common industrial tundish, the level of molten steel which is likely to lead to the formation a vortex is ten inches above the bottom of the tundish, and a useful limit slightly higher than the level at which a vortex is likely to form is from 1% to 10% higher than the level at which a vortex is likely to form, as determined by analysis of historical data.  
         [0041]     Our invention also includes a method of managing a tundish to control the transition between a first grade of molten steel and a second grade of molten steel to be solidified sequentially in a continuous caster by draining the first and second grades of molten steel through a well in said tundish to a mold at the head of the continuous caster, wherein the first grade of molten steel is initially in the tundish and covered by a layer of slag in the tundish, comprising (a) draining the first grade of steel to a level in the tundish as monitored in step (b) which level is (i) slightly higher than the predetermined level at which a vortex is expected to form above the well, or (ii) where vortex formation is indicated as in step (b), whichever is the first level to be reached; (b) while draining the first grade of steel, monitoring the level of the first grade of steel in the tundish by directing microwave radiation into the tundish from above the layer of slag and analyzing reflections of the microwave radiation from the slag and the steel, (i) to determine the level of the steel and (ii) to detect a microwave pattern indicative of vortex formation in the tundish, and (c) when step (a) is completed, adding the second grade of molten steel to the tundish at a rate at least that of the rate of draining the first grade of steel.  
         [0042]     Another aspect of the invention may be seen as a method of maintaining the level of molten steel within predetermined limits in a tundish, the molten steel being beneath a layer of slag, while the tundish substantially continuously feeds the molten steel to a continuous caster including a mold comprising monitoring the level, volume, or weight of the molten steel in the tundish by microwave analysis and controlling the flow of the molten steel into and out of the tundish to maintain the level, volume or weight within the predetermined limits. Controlling the flow of molten metal out of the tundish may be modified by at least one of (a) steel level in the mold, (b) based on historical records, whether a vortex is likely to form at the level, volume or weight of steel in the tundish, (c) slag level in the tundish, (d) slag volume in the tundish, (e) slag weight in the tundish, (f) iron oxide content in the slag, (g) the composition of steel in the tundish, (h) the rate of solidification of the steel in the mold (i) vibration in the tundish or tundish nozzle, (j) turbulence in the mold of the continuous caster, (k) casting rate, and (l) value of steel being cast.  
         [0043]     As described in the above referenced Meszaros et al U.S. Pat. No. 6,166,681, time-of-flight radar microwave analysis may be used to determine the level of various materials covered by other materials. And, as it is known to handle molten aluminum in a tundish to move it from a ladle or other vessel to a caster or similar device, our invention may be used in aluminum processing as well as in steel processing. As in steel casting, the molten aluminum in the tundish is covered by a second phase material, called dross, analagous to the slag on the molten steel. Slag, whether in steelmaking or in aluminum processing, is made up of many discrete particles and phases, thus having large numbers of surfaces and interfaces, which should be taken into account when analyzing the radar reflectance data to determine the level of molten metal under the blanket of molten slag. While the makeup of the materials in the tundish will generate different specific data for aluminum and steel, and the compositions of their respective slags will also affect the specific data gathered, the analytical process described in the three Meszaros et al U.S. Patents incorporated by reference above is essentially identical for both aluminum and steel. We intend for our invention to include control of a tundish handling any kind of molten metal covered by slag, where microwave analysis can determine the level of the molten metal. Accordingly, for our purposes the dross of aluminum processing is a type of slag and is included in the term “slag.” Where a tundish or similar vessel includes a nonferrous metal such as copper, covered by a matte as opposed to a slag, use of our invention is not recommended, since the matte (containing various sulfides) is likely to have a conductivity too similar to the molten metal, resulting in difficulty in detecting the difference between microwave reflections from the matte and from the underlying molten metal.