Metallurgical apparatus – Process
Reexamination Certificate
2000-11-15
2003-05-13
Kastler, Scott (Department: 1742)
Metallurgical apparatus
Process
C266S100000
Reexamination Certificate
active
06562285
ABSTRACT:
FIELD OF APPLICATION
This invention relates to a method and system for the detection of impurities in a flow of molten metal.
BACKGROUND OF THE INVENTION
In the art of refining steel there exists the well known problem of minimizing slag carryover from one vessel to another, for example from a tundish to a mould, or as in the example noted below from a basic oxygen furnace to a ladle. In the refining process, raw steel, with certain impurities, is heated in a basic oxygen furnace (BOF) as pure oxygen is introduced. The impurities in the molten metal oxidize and form slag, leaving the purified molten metal. One of the properties of the slag is that it is less dense than the molten steel and as such it floats to the surface of the container. Traditionally, for pouring the molten metal, the BOF is tilted upon an axis and the container is tapped below a point where the slag should lie. In this manner, the refined steel can be poured into the ladle without the slag. As the amount of steel in the BOF is reduced, there is a greater probability that turbulent flow will cause slag carryover into the ladle. This is undesirable because of the great expense that must be incurred to remove the slag from the ladle, or to condition the metal so that the remaining slag is less reactive in later processes.
To prevent slag carryover, a method of slag detection is required so that when the detected slag exceeds a predetermined amount the pouring process is halted. Typically the pouring process is halted through closing the tap and the restoration of the BOF to an upright position. The process of detecting slag however still poses problems as a result of the environmental conditions of a steel foundry. The conditions in a steel mill do not easily lend themselves to human intervention in the pouring process, unless it is intervention from a distance. Another problem is that as a result of the high temperatures, molten steel appears similar to molten slag to the unaided eye of an operator.
Many techniques have been applied in an attempt to solve these problems, but the solutions are either ineffective or costly to implement, and in some cases both. Despite the fact that slag has a different consistency than molten steel, the high temperatures that are present heat both compounds and render the visual identification of the differences difficult for a human operator.
One currently used system is described in U.S. Pat. No. 5,968,227, to Goldstein et al, entitled “System and Method for Minimizing Slag Carryover During the Tapping of a BOF Converter in the Production of Steel”. This patent teaches the use of an infrared detection system to determine if slag has entered the stream between the BOF and ladle. The range of IR wavelengths selected for use in this invention is only minimally blocked by the water and carbon dioxide gasses that surround the steel making process. This allows the IR camera to be located far enough from the source of heat that background heat does not interfere with the imaging. When the camera images the stream, it converts the stream into a collection of discretized elements. Typically, each element is a pixel, each with an intensity that is related to its thermal levels. A microprocessor is then able to analyze the distinct elements and compare their intensity values with known temperatures from a lock-up table. This allows a digitized image to be constructed for display on a monitor. The images shown on the monitor, having been through the digitization process, are able to clearly shown an operator the presence of slag, due to its different colouration. An embodiment is also described whereby a computer would be able to analyze the contrast of the image and determine the slag content so that automated termination of the pouring can be achieved. Though this method is effective it is also expensive. As noted above, to avoid the problems caused by the omnipresent carbon dioxide and water vapours, IR wavelengths are the chosen radiation for analysis. This requires expensive and delicate imaging equipment, and also requires a delicate calibration process that, if performed incorrectly, will produce results far from the ideal. In addition, as with all electrical equipment in environments as hostile as a steel mill or foundry, there is a need for periodic recalibration, which may require the temporary cessation of activities, which is highly undesirable.
Another prior art method is illustrated in U.S. Pat. No. 4,222,506 to Sakashita et al, entitled “Molten Steel Outflow Automatically Controlling Device”. This patent is specifically related to the detection of slag in the stream between the ladle and a tundish. The patent teaches the use of an infrared camera connected to a colour monitor, where the infrared image values are mapped to visible parts of the spectrum for display purposes. As the image is displayed it is analysed for the presence of yellow colouration in what should be the red stream of the steel. If more yellow appears than is defined in a threshold, an indication is given that the pouring should stop. This technique would work poorly in the BOF-to-ladle environment due to the prevalence of particulate matter, which blocks much of the information that the infrared camera is supposed to detect. In addition, gaseous matter, such as water and carbon dioxide, would interfere with the imaging to such an extent that the method would be ineffective for proper control of slag carryover. A further problem with this method is that the equipment has to be calibrated carefully so that there is a clear delineation between the molten metal and the slag. Miscalibration of the equipment will lead to performance greatly below the ideal. In addition, as a result of the environment to which the electrical equipment is subjected, periodic recalibration of the equipment is necessary, which results in additional costs.
It would be desirable to provide a method to accurately detect and display the presence of slag in the stream between two vessels, such as between the BOF and the ladle, or between the ladle and the tundish, while avoiding the use of unnecessarily expensive equipment that has a limited service life. It would further be desirable to provide such a method wherein the results of the detection process are displayed in such manner that an operator can reliably be alerted to the presence of slag so that pouring can be terminated when deemed necessary to prevent excess slag carryover.
SUMMARY OF THE INVENTION
It is an object of the invention to obviate or mitigate at least one disadvantage of previous methods and systems. It is a further object to provide a method and system for the detection of impurities in a flow of molten metal.
The present invention provides a method of representing slag in a molten metal stream comprising at least the following steps. The first step is to obtain and store a plurality of pairs of first and second digital images of the molten metal stream. The second step is to compare one or more selected properties of said stored first and second digital images of each pair which are representative of the presence or absence of slag in said stream. The third step is to obtain a molten metal flow delta on the basis of the above comparison, the delta representing the amount of slag in the stream and the rate of change in the amount of slag. The fourth step is to generate output signals which are representative of the molten metal flow delta.
The output signals may be displayed on level meters and numerical displays or used to initiate alarms indicative of the slag content exceeding a threshold or used to distinguish slag from the molten metal flow through the colourization of slag on a display.
The first and second digital images may be characterized as first and second histograms which are compared to identify changes between the histograms that representative of the presence or absence of slag. These histograms may be based on pixel intensity levels which may be adjusted on the basis of automatic gain control levels read from the digital imaging equipment.
The
Andreales, Sceales, Starke & Sawall
Kastler Scott
Metallurgical Sensors, Inc.
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