Description of the Prior Art In the process of continuously casting steel, molten steel is transferred from one metallurgical vessel into another and finally into a mold. Throughout the process, shielding the molten steel from air is important to prevent deleterious oxidation. While in metallurgical vessels, a floating a layer of slag on the surface of the molten steel reduces contact of the steel with air, thereby reducing oxidation. Slag comprises a mixture of impurities, which separate from the steel during the refining process, and non-metallic fluxes, which are intentionally added to facilitate casting and reduce oxidation. At casting temperatures, slag itself exists as both a solid and a liquid. Solid slag includes less

dense compounds and floats on top of the molten slag, which in turn floats on the molten steel. The chemical and physical properties of the liquid slag, including its depth, may significantly affect final steel product by influencing the actual steel meniscus within the metallurgical vessel.

Metallurgical vessels typically have a bottom aperture through which the molten steel may flow. The slag floats on the steel surface as the steel exits the aperture. In this fashion, slag continues to shield the molten steel from oxidation throughout the casting process. Importantly, the floating slag must not contaminate the steel exiting the bottom aperture. Slag contamination can seriously degrade steel quality, interfere with casting, and even generate explosive reactions. Slag must, therefore, be kept from draining through the bottom aperture.

Knowing the depth or meniscus of slag or molten steel during the draining of a metallurgical vessel is essential to maximizing steel output, and reducing oxidation and slag contamination. In normal operation, the depth of steel in the metallurgical vessel can be kept high enough that slag contamination is not problematic. Occasionally, however, the metallurgical

vessel may need to be taken out of service for maintenance or repairs. In such situations, it is desirable to drain the molten steel from the vessel to avoid waste. Care must be taken to avoid slag contamination as the steel level decreases.

The depth or meniscus of molten steel beneath the slag may be ascertained visually. Visual observation is, at best, a crude approximation. Further exacerbating the problem, slag thickness is not necessarily constant throughout the casting process. Allowing for these approximations and the danger of permitting slag to contaminate the steel, an operator must leave more than a minimum amount of steel in the vessel.

Technological solutions to this problem include measuring impedance changes in conductive coils within a metallurgical vessel. Eddy currents in the coils create a fluctuating magnetic field that distinguishes between steel and slag. Unfortunately, such coils are inaccurate, relatively expensive and fragile devices.

U. S. Pat. No. 5, 650, 117 describes first and second conductive pins mounted near one another in the wall of a ladle shroud. The first pin electrically contacts the flow of molten metal through the shroud. The second pin is insulated within the shroud wall. An electrical

potential is measured across the first and second pins.

Abrupt changes in electrical potential indicate the presence of slag in the shroud. The flow of steel through the shroud can then be stopped, but often not before significant amounts of slag escape from the vessel.

U. S. Pat. No. 4, 365, 788 describes a plurality of combination electrodes embedded in a wall of a metallurgical vessel. Measuring the electrical resistance of the electrodes can distinguish between steel and slag. Such combination electrodes contain multiple components that must be assembled and then imbedded in a vessel wall. Failure of an electrode requires shutting down the casting. These limitations obviously increase the cost of production.

U. S. Pat. No. 5, 827, 474 teaches an electrically conductive ceramic probe disposed in a metallurgical vessel. The probe is moved vertically in the vessel while measuring the electrical potential of the probe.

Electrical potential changes as the probe is lowered through the slag and into the steel. Alternatively, the probe may be fixed while the molten steel level changes.

A defective probe can easily be changed. An operator recognizes the steel-slag interface as a discontinuous

change in the measured electrical properties. This probe cannot readily distinguish between solid slag and liquid slag.

A need exists for an easily replaceable probe capable of more accurately determining the menisci and depths of steel and slag, differentiating between solid slag and liquid slag, and identifying the slag-steel interface. Prior art probes can detect the steel-slag interface but do so via a discontinuous change in measured electrical properties. Such a change gives an operator no warning of the proximity of the steel-slag interface. Prior art probes also do not differentiate between solid slag and liquid slag. None reliably measure the thickness of slag floating on the steel surface.

SUMMARY OF THE INVENTION The present invention relates to a device and a method for determining the depth or meniscus of steel or slag in a metallurgical vessel. In a broad sense, the invention comprises at least one electrically conductive probe, a means for measuring the impedance and electrical potential of the probe, and a means for vertically positioning the probe within a metallurgical vessel.

The device is described as a probe comprising a refractory ceramic material with a tapered distal end that is immersed in the metallurgical vessel. The probe also has a proximal end for connecting to the measuring means. The tapered distal end of the electrically conductive ceramic probe is described as improving sensitivity and response to impedance changes in the probe. The tapered end is also described as more accurately determining the menisci of steel and slag, and of distinguishing solid slag from liquid slag.

Another embodiment of the invention describes a plurality of stationary, electrically conductive ceramic probes, where each one is disposed at different vertical heights within a metallurgical vessel. A plurality of stationary probes may be used in place of a vertically positionable probe.

The method of the invention is described as measuring the electrical output from at least one electrically conductive ceramic probe having a tapered distal end. In one aspect, the method describes using changes in the impedance and electrical potential of the probe to determine the menisci and depths of steel, liquid slag and solid slag. In another aspect, the method includes vertically moving the probe while

measuring electrical properties. In still another aspect of the invention, the method uses a plurality of stationary probes disposed at various elevations in place of a vertically movable probe.

DESCRIPTION OF THE DRAWINGS FIG. 1 shows a metallurgical vessel having a tapered probe of the present invention attached to a means for detecting electrical potential and impedance.

FIG. 2 shows a metallurgical vessel having a prior art probe attached to a means for detecting electrical potential and impedance.

FIG. 3 shows a metallurgical vessel having a plurality of probes of the present invention, each probe attached to a means for detecting electrical potential and impedance. Dashed lines indicate circuits omitted for clarity.

FIG. 4 compares the change in electrical potential between a probe of the present invention and a probe of the prior art, in particular, the figure shows a continuous and a discontinuous transition for the present invention and the prior art, respectively.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a device of the present invention. An electrically conductive ceramic probe 1 having a

conically shaped distal end 2 is immersed in a metallurgical vessel 3 containing molten steel 4 and a slag layer 5. The distal end 2 is defined by a cone angle 13. An electrical lead 6 is attached to the proximal end 7 of the probe 1, and connected to at least one meter 8. The meter measures impedance and electrical potential, and comprises a voltmeter 14 with an impedance meter 15 connected in parallel to the input and output terminal of the volt meter 14. In the process of continuous casting, the molten steel 4 exits through a nozzle 9 located at the bottom of the metallurgical vessel 3. The slag layer 5 includes a solid slag layer 10 floating on a liquid slag layer 11.

An interface 12 exists between the slag and the molten steel. Not shown is a means for vertically positioning the probe 1 within the metallurgical vessel 3.

A prior art probe is shown in FIG. 2. An electrically conductive ceramic probe 21 having a blunt distal end 22 is immersed in a metallurgical vessel 3 containing molten steel 4 and a slag layer 5. An electrical lead 6 is attached to the proximal end 27 of the probe 21, and connected to meter 8, capable of measuring impedance and electrical potential, as described above. As in FIG. 1, the slag layer 5

includes a solid layer 10 floating on a liquid layer 11 with an interface 12 between the slag and the steel. A means for vertically positioning the probe 21 within the metallurgical vessel 3 is not shown.

Broadly speaking, the device of the present invention comprises a probe having distal and proximal ends, an electrical conductor, and a meter or meters for measuring impedance and electrical potential. The interrelationship between the various system components is described in U. S. Pat. No. 5, 827, 474, the entire specification of which is incorporated herein by reference. The probe comprises an electrically conductive ceramic with an elongated body having a tapered distal end, which may be immersed in molten steel. Preferably, the distal end will taper continuously, and most preferably will be substantially conical. The electrical conductor connects the proximal end of the probe to the electrical meter.

The ceramic comprises an electrically conductive refractory material, typically a refractory oxide and graphite. The refractory oxide may be a metal oxide or semimetallic oxide, including, for example, alumina, silica, magnesia, zirconia and other refractory ceramic oxides. Such oxides are rendered electrically conductive

by mixing with between 5 weight percent and 50 wt. % graphite or other form of carbon. Probes comprising zirconia/graphite are effective where highly erosive slags are present. Most commonly, the probe will comprise alumina/graphite, with 30 wt. % to 95 wt. % alumina and 5 wt. % to 50 wt. % graphite, and will be isostatically pressed to render the resulting composition more erosion resistant to the effects of slag and molten steel.

Electrical potential and impedance measurements will vary depending on the type of material into which the probe is immersed and the depth of immersion of the probe in the material. Molten steel, solid slag and liquid slag will all produce different electrical measurements from the probe. Electrical potential and impedance will, therefore, vary depending on where the probe is disposed in a metallurgical vessel, that is, in the molten steel, or solid or liquid slag. The integrity of the measurements may be checked utilizing the same techniques disclosed and claimed in U. S. Patent Nos.

5, 549, 280, the entire specification of which is incorporated herein by reference.

The tapered end of the probe may be of any shape so long as the cross-sectional area of the end is

substantially decreasing to the tip. Typically, the cross-sectional area will continuously decrease, and most preferably the area will continuously decrease at a constant rate so that the distal end is conical. The particular shape of the tapered end and the degree of taper will affect the abruptness of the measured changes in electrical properties.

For example, a conical distal end of the probe should have a cone angle between 5° and 45° with a cone length of greater than about 2 inches, and preferably between about 6 inches and 12 inches. A smaller cone angle, corresponding to a longer conical end, will produce more gradual changes in electrical properties.

As shown in FIG. 4, the electrical potential as a function of the depth of the probe as measured with a conical probe changes more gradually near the steel-slag interface than a comparable blunt end probe. The blunt end probe experiences a discontinuous change in electrical potential upon transecting the steel-slag interface. Using a conical probe, an operator may anticipate the steel-slag interface or the interface between the solid slag and liquid slag. Prior art blunt end probes do not warn of an impending interface.

Consequently, tapered probes permit greater confidence in operating near the steel-slag interface.

The probe of the present invention will also have a proximal end to which a first end of an electrical conductor may be fixed, typically with an electrically conductive refractory mortar. The second end of the electrical conductor is attached to a means for measuring electrical potential and impedance. Typically, the means comprises a voltmeter with an impedance meter connected in parallel to the voltmeter. The meters may be connected to data storage and computer circuitry, which automatically determine the menisci or depths of steel, liquid slag or solid slag remaining in the metallurgical vessel. Additionally, the circuitry may be connected to a vertical positioning means that controls the height of the probe in the vessel. For example, as the depth of the steel decreases, the probe may be lowered to track the height of the steel or slag level. Such a vertical positioning means is well known and includes an articulated arm and an arm moving assembly.

In practice, an operator attaches a probe to a vertical positioning means. The operator then calibrates the height of the probe above the bottom of a metallurgical vessel with readings from the positioning

means. The steel meniscus or depth of a slag layer in a metallurgical vessel, for example, may be determined by lowering the device into the molten steel so that the tapered end is affirmatively below the slag. The electrical properties are tracked as a function of height of the probe in the vessel. An analogous measurement includes a plot of electrical properties as a function of time, where time correlates with the height of the probe in the vessel or the decrease in material in the metallurgical vessel. Most commonly, the probe is slowly raised until the electrical meters register a change in slope. This corresponds to the top of the tapered end and the beginning of the steel/slag interface. The operator will then determine the height of this interface above the vessel's bottom. As the probe continues to be raised, the electrical readings will change until the tapered end is completely above the molten steel.

Another change in slope will be observed as the probe transects the solid slag/liquid slag interface. At each of these changes, the operator may determine the height of the interface above the bottom of the vessel, and correspondingly, the thickness of the slag layers.

Depending on the thickness of the liquid slag and the geometry of the tapered end, the probe could transect

several interfaces simultaneously. This situation could complicate a plot of electrical properties versus height by overlapping several curves at the same time.

Appropriate deconvoluting routines, which are well known in the art, can separate the contributions from the various layers and determine thickness of solid and liquid slag and the depth of the molten steel. The routines are conveniently performed as part of the computer circuitry.

The probe may also be held stationary near a bottom aperture of a vessel. At this position, the probe will signal the proximity of the slag to the bottom of the vessel. As steel is drained from the vessel, the taper of the distal end will alert an operator to the steel- slag interface in time to shut off the flow before slag can exit the vessel. Electrical signals from prior art, blunt-end probes change abruptly leaving an operator little time to shut off the flow of molten steel.

Another alternative embodiment of the invention, as shown in FIG. 3, uses a plurality of vertically displaced probes, 1A, 1B and 1C, to obtain simultaneous impedance readings at various heights above the bottom of the vessel 3. In the figure, a first probe 1A penetrates into the liquid steel 4, a second probe 1B is in the slag

layer 5, and a third probe 1C is positioned in the solid slag layer 10. Advantageously, this embodiment permits the simultaneous determination of the menisci of steel, liquid slag and solid slag. Benefits of this embodiment include a decrease in mechanical complexity by removing the vertically positionable means, innate redundancy, and the ability to determine slag depth instantly across a broad height range.

Obviously, numerous modifications and variations of the present invention are possible in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described herein.