Nozzle clogging is a serious issue that can interfere with castability but also can substantially reduce productivity by lowering down the casting throughout and casting speed (meant to compensate for chocking), unplanned tundish change, early ladle change over and frequent nozzle change. The buildup of solid or semi-solid material on a refractory surface can affect the stream dynamics and cause large agglomerated particles to be sporadically released into the liquid steel stream. Almost all solid inclusions ( except titania) are not wet by liquid steel and thus will agglomerate if they come in contact with other similar solid inclusions or another refractory surface. Liquid inclusions, if they contact a refractory surface, will spread on that surface, as all slags wet oxide surfaces. Further, if the liquid inclusion is not saturated with respect to the refractory that it contacts, it will react with the refractory (leading to perfect wetting). Liquid inclusion films on refractory surfaces also begin to cool due to heat transfer to the refractory. This leads either to erosion of the nozzle if the liquid film remains fluid and is transported downward by the steel flow, or to buildup if the liquid inclusion begins to precipitate solid material due to the decrease in temperature when it contacts the refractory surface. Thus, the phenomenon of material buildup on refractory surfaces is a function of heat transfer through the nozzle, steel temperature, steel chemistry, and the nozzle design and material. The condition for precipitation of a solid or semi-solid inclusion is a must within the liquid steel for clogging to be possible. If the primary inclusion is solid, some buildup of material will always occur. If the primary inclusion is liquid, the buildup will occur if the liquid inclusion precipitates solid material during processing due to thermal or chemical change. Apart from thermodynamic conditions, inclusion transport to the interface, i.e., where inclusions precipitated are necessary for the causes for build-up.
Excessive clogging leads to the closing of the strand which will hamper the overall output of the machine. Unplanned early tundish change will break the normal sequence of the tundish leading to non-utilization of refractory capability and returning of metal due to early ladle change over leads to re-blowing or dumping of metal. Nozzle chocking in open type of casting poses other threats turbulent and inclined metal stream which can cause breakouts and bad surface quality, jam formation on mold walls (if not removed can lead to sticker type of breakouts) apart from frequent nozzle change. At the same time, the quality can be affected when the clog gets dislodged increasing inclusion count in the strand and can even alter the mould slag composition. Mould level fluctuation leading to mold slag entrapment, as a consequence of clogging, results in bad surface quality of the solidified surface. All these issues can lead to the loss of quality and productivity which is an aggravatingly costly affair for the company.
Clogging can be divided into three classes and is grouped according to their sources:
Class 1: Oxide formation :
Category -1.1 by air aspiration, 1.2 by reaction between nozzle refractory and steel.
Class 2: Transportation of oxides to the nozzle wall :
Category – 2.1 deoxidation products, 2.2 reoxidation products, 2.3 exogenous inclusions, 2.4 products of inclusion modification.
Class 3: low temperature chilling of steel :
Category – 3.1 low superheat or high rate of heat loss, 3.2 locally increased liquidus due to solute segregation.
1.1- Air aspiration: As steel flows from the tundish through a nozzle system, a negative pressure is created at the nozzle wall due to the venturi effect and thus each joint becomes susceptible to air infiltration. The oxygen in aspirated air will quickly react with dissolved aluminum to form an aluminate inclusion. This reaction will happen first at the surface of the steel stream, giving the nascent inclusion immediate access to the nozzle wall which ultimately leads to build-up due to agglomeration of aluminates. One indicator of this Category 1.1 clogging is an observed nitrogen pickup from tundish to mold. However, the negative pressure created by steel flow through the nozzle assembly can be counteracted by the introduction of argon through channels or porous refractory. The inert gas may be applied to several locations, such as the upper nozzle, upper plate, lower plate, collector nozzle, and SEN. It is very difficult to prevent air aspirations and strict measures is to be taken to prevent such occurrences. Real-time monitoring of the argon back-pressure in each location is useful to indicate a problem. A drop in back-pressure is normally an indication of short-circuiting, whereas a rise in back-pressure may indicate clogged channels or pores. The flow rate of argon is essential in preventing air aspiration and excessive argon flow may cause mold turbulence, leading to surface defects in the product and can even lead to agitation of steel plume for inclined ladle shroud generating a tendancy towards tundish open eye formation and more oxygen pickup. Argon flow rates are to be adjusted to optimal levels using back-pressure, mold turbulence and nitrogen pickup as the supporting indicators. The study of flow rates and pressure fluctuations can be useful in relating to nozzle clogging.
Use of ladle covering compound, argon and gasket sealing between shroud and ladle collector nozzle, use of ladle shroud and/or trumpet-shaped shroud meant for submerged pouring, argon sealing in slide gate mechanism, submerged entry nozzle and mold powder are effective in preventing air infiltration. However, entrapment can lead to the formation of oxides (category 2.2). Depending on the types of oxides formed, the temperature of liquid steel and the melting point of the compound it can be determined whether the compound will precipitate on the nozzle walls.
The first step in determining the cause of nozzle clogging is to analyze the clog material by examining the frozen remnants in the nozzle well block, gate or SEN. A polished macroscopic examination of a section of clog will reveal the relative percentages of nonmetallic materials and steel and will thus determine the cause for chocking. Energy-dispersive x-ray analysis may reveal the chemistry or composition of the inclusion in the clog material.
If the analysis shows little amount of steel or few inclusions in the tundish well nozzle, the reason for clogging is attributed to the blockage of the nozzle which prevents the metal transfer from the tundish to the mold. For the case where steel percentage is predominant, clogging takes place due to the freezing of cold steel commonly referred to as thermal clogs and is caused due to insufficient superheat or a high rate of heat loss through the nozzle refractory ( Category 3.1 clogging). However, this problem can be solved by increasing steel superheat, better nozzle insulation, better insulation by adding tundish covering compound, use of ladle covers, grouting and tundish preheating.
The most common type of clog contains significant fractions of both steel and nonmetallic material which indicates that the primary clogging mechanism was the deposition of nonmetallic material in a loose network at the nozzle wall commonly referred to as inclusional clogs. The combination of thermal and inclusional clogging is also possible, and many clogs contain solidified metal. As steel flows through this network, it is slowed down sufficiently such that the specific heat loss to the nozzle becomes high enough for freezing to take place. In the case of steel with high alloy content, in particular grade of steel, the freezing rate may be increased due to solute segregation taking place during the freezing process. This condition will tend to increase the ratio of steel to nonmetallic material in the clog.
The progression of nozzle clogging can be considered a dynamic phenomenon where cluster of inclusions may form at or are transported to the nozzle wall. Sintering will then take place over time.
The impact of the steel flow and argon purging (for some tundish where argon gas flows through the porous refractory stopper rod dislodges some of the clusters) may have sufficient force to knock some of the inclusion clusters free. While the inclusion clusters may attain sufficient strength to slow down the steel and trap it as it cools to the liquidus temperature. Depending on prevailing conditions, the size of the clog may progress or regress at variable unpredictable speed.
Reoxidation in the Tundish: Chances for re reoxidation is high during tundish filling during the first heat of a sequence. This may be reduced with argon purging of the tundish and the use of tundish sealing covers.
Reoxidation can also occur during ladle exchanges or delay in connecting when the tundish level falls low enough for open pouring and the generation of surface turbulence leading to slag entrapment and generation of exogeneous type of inclusions (category 2.3) . During reoxidation periods, the tundish slag oxygen potential may increase leading to alumina formation and category 2.2 nozzle clogging. The clogging may continue for some time, even after steady conditions in the tundish have resumed. Reoxidation may also occur under certain steady-state casting conditions where the tundish slag layer is depleted which will allow the containation of steel with air, forming inclusions that may lead to category 2.2 nozzle clogging. Discontinuity of tundish slag coverage may occur due to insufficient application of tundish covering compound and bath turbulence especially around the ladle shroud for open pouring practices during ladle change over period or by the upward flow of liquid caused due to improper installation of flow control modifiers. Some tundish coverings, such as those containing very high MgO, form a solid crusty layer that contain cracks through which air may come into contact with steel. The signs of this kind of reoxidation include increasing contents of aluminum, manganese and iron oxides in the tundish slag, and nitrogen pickup in the bath.
Practices for minimizing reoxidation in the tundish: 1. flow control design to minimize surface turbulence; 2. Sufficient, efficient, periodical application of a tundish flux that quickly forms a contiguous liquid slag layer; 3. low reducible oxide content in the tundish flux (minimizing FeO, MnO, and alkalis); 4. argon purging in the tundish, particularly during start sequence; 5. a tightly sealing tundish cover which can maintain inert atmosphere, 6. Practicing submerged ladle opening; 7. minimizing ladle slag carryover by adopting advanced detection methods; and 8. controlled tundish fill to avoid excessive turbulence, achieved through either fill rate or turbulence suppressing devices.
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