Metallurgical apparatus – Process – Plugging or tapping
Reexamination Certificate
2001-03-09
2004-05-18
Kastler, Scott (Department: 1742)
Metallurgical apparatus
Process
Plugging or tapping
C266S090000, C222S590000
Reexamination Certificate
active
06737014
ABSTRACT:
CROSS-REFERENCE TO RELATED PATENT APPLICATION
This patent application claims priority based upon United Kingdom patent application No. 0006609.2, filed Mar. 17, 2000.
FIELD OF THE INVENTION
A process for determining the presence of a slag phase in molten steel.
BACKGROUND OF THE INVENTION
Steel making is considered a batch process. A unit of steel is melted or made with oxygen in a primary steelmaking vessel. The steel is then transferred to a ladle where it is alloyed and refined. Then the steel is transferred to a distribution vessel called a tundish from which it is distributed to one or more molds for solidification. In each of the batch vessels, a slag is present on the steel, comprised of liquid and solid oxides. The properties of the slag are quite different in each batch operation and it is not desirable to allow the transfer of the slag from one vessel to the next in the production sequence.
Slag from the primary steelmaking vessel should not be carried into the ladle, slag formed in the ladle should not be carried into the tundish, and slag from the tundish should not be carried into the molds. At the same time, it is desirable to maximize the yield of metal during transfer operations. Ideally, all the steel and none of the slag should be transferred from one batch operation to the next. Practically, this is not possible since slag and steel tend to form an emulsion or mixture, particularly near the end of a transfer operation. In that case, either some steel must be left untransferred, or some slag must be transferred to the next operation. An object of the present invention is to provide the operator with a tool that will minimize the duration of two phase flow, and to help him to choose by grade the optimum condition for steel retention and slag transfer.
It is known that the degree of mixing of slag and steel during a transfer operation increases with the rate of steel flow. At high flow rates, a vortex may develop well before the end of the transfer operation. In that case, steel and slag may flow together for some time, causing an unacceptable amount of slag transfer. It is the object of steel transfer operations to maintain flow conditions that prevent the mixing and co-transfer of steel and slag. An object of the present invention is to indicate the onset of a vortex and to cause a change in the transfer operation to dissipate the vortex, either automatically or by informing the operator of a recommended course of action.
In the prevention of slag transfer from one vessel to the next, detection of slag flow is important. In many cases, the detection of slag flow is visual and after the fact. For example, in the tapping of steel from an oxygen steel making vessel, the operator will watch the tap stream and the surface of steel in the ladle for indications of slag flow. A significant amount of slag flow causes the stream to brighten and flare due to the higher emissivity and lower surface tension of slag in relation to steel. Also, the lower density of slag causes it to flow across the surface of the steel in the ladle whereas the steel stream penetrates the surface. These indications cause the operator to stop the transfer operation to prevent the further flow of slag into the ladle, but this is usually after significant slag volume has transferred from the steel making vessel into the ladle. The operators vary in level of skill, experience, and attention to detail, causing the amount of slag carry-over to be quite variable from heat to heat. It is therefore desirable to have an operator independent system that can detect the onset of slag flow during the furnace tapping sequence and cause the modification or end of the tapping sequence to minimize the inflow of slag to the ladle.
In another example, when teeming steel from the ladle to the tundish, the operator may watch the pour box area of the tundish for signs of slag flow, such as a brightening of the slag surface around the pouring tube, or a welling up of slag around the pouring tube. Upon seeing these signs, the operator will cause the end of the teeming operation to prevent further flow of slag. Once again, significant slag flow from ladle to tundish may have occurred by that time.
Several aids have been developed to detect slag flow from the ladle. One is based on the difference in conductivity between slag and steel and a resistance is continuously measured between two contact points within the nozzle. This method cannot detect vortexing which often precedes slag flow. Also this method fails if the steel fails to contact one of the probes. Additionally, this method fails if the slag flow is in the center of the stream, allowing the steel to contact both probes and the slag to go undetected.
U.S. Pat. No. 4,140,300 of Gruner et al teaches a method of slag detection that monitors the radiation intensity of the stream of steel flowing through a discharge tube. A lateral side duct is inserted in the ladle shroud, or discharge tube, through which the steel stream can be observed. A change in radiation intensity signals the onset of slag flow. This method is intrusive and requires side duct modification for each shroud, so it has not found acceptance. Additionally, slag flow through the center of the steel stream would go undetected.
Another method of slag detection relies on an indirect method of conductivity measurement using a magnetic field. An electromagnetic coil is placed around the flowing stream of steel. When slag begins to flow, the field properties are changed by the lower conductivity of the stream. The percentage of high conductivity to low conductivity stream area is set at a predetermined rate; and, when this falls below a given threshold, then an alarm signals the operator to shut the ladle stream. Alternatively, the ladle stream can be caused to automatically shut off when the given threshold value is reached. A disadvantage of this method is that vortical flow is not detected. Slag may form only a small percentage of the area of a vortexing stream, and this may not be enough to trigger the alarm to shut the ladle. A further disadvantage of this method is that the electromagnetic coils must be embedded into the refractory bottom of each ladle. These coils require periodic replacement and are a costly maintenance item. Replacement of a damaged coil is usually done when a ladle is scheduled to be relined with new refractory. Until that time, a ladle may go without slag detection ability for several batches of steel.
Yet another method of slag detection relies on the operator's ability to detect a difference in the vibration of a ladle shroud as slag flow begins. Steel has about twice the density of slag, and it causes the ladle shroud to vibrate significantly as it flows from the ladle into the tundish. This vibration tends to increase in strength during vortexing and decrease in strength during slag flow. Thus, a skilled operator can place a hand on the ladle shroud manipulator arm and sense the vibration during the latter part of a ladle pouring operation. The vibration will abruptly diminish as slag begins to flow through the shroud, at which time the operator causes the termination of the ladle draining operation. A vortex is more difficult to detect by hand, but a skilled operator may also sometimes detect the onset of vortexing flow and may cause the termination of the ladle draining operation at that point. While this method of slag detection is somewhat effective, it relies greatly on the skill and attentiveness of the operator and is thus inconsistent. Also, the operator does not have the ability to discern the various vibration frequencies associated with operations and activities around the casting machine. Some of these may influence his ability to accurately detect slag. In addition, the operator is influenced by his knowledge of approximate weight of steel left in the ladle. His level of sensitivity in slag detection may be low if he perceives that a significant amount of steel remains, and he may miss the early onset of slag. Conversely, his level of sensitivity may be
Davidkhanian Alex
Kemeny Frank L.
Langari Ali
Walker David I.
Howard J Greenwald P.C.
Kastler Scott
Nupro Corporation
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