Nozzle Design and its effect: Nozzle geometry and clogging are closely related. Category 2 clog material consists of solid inclusions that are present in the steel as it flows through the nozzle system which is transported to the nozzle wall, which is unlikely to occur for laminar flow throughout the nozzle. The rate of inclusion separation increases with the degree of turbulence inside the nozzle. Local eddy transport is responsible for the collision of inclusions with the nozzle wall and buildup. The regions of low flow are more susceptible to clogging. The centripetal force aids the movement of inclusions toward the nozzle wall as steel flows around an outside radius, such as a sharp entry point. The rate of clogging can increase for asymmetrical back flow in case of misaligned nozzles. Despite all precautions, some inherent eddy transport of inclusions to the nozzle wall will occur. Argon purging through channeled or porous refractory surfaces can act to push away inclusions and prevent their adherence to the nozzle wall. The argon flow should be sufficient to create a slightly positive pressure on the inside wall of the nozzle. However, excessive flow rate may result in mold turbulence and even result in surface defects in the final product. Thus special care is to be taken for designing the nozzle geometry and proper installation is to be ensured to minimize turbulence and asymmetrical backflow.
If the inclusions precipitate away from the refractory surface, then the problem is one of transport to the surface. Fluid dynamics and the details of nozzle geometry can be manipulated to slow down the buildup rate of the transported and agglomerated material. Turbulence in fluid flow can also be manipulated to attempt to remove buildup, and the final buildup rate is given by the sum of the inclusions transported to the interface minus those removed from the interface.
Incase of inclusions precipitating on the refractory surface the problem is because of inclusional growth and is a function of heat transfer, fluid flow, steel chemistry and the heterogeneous nucleating ability of the surface.
The following practices are recommended:
1. Using large radii and smooth surfaces in all flow transition areas; 2. minimizing the number of components, and joints in the nozzle assembly to prevent air aspirations;
3. Ensuring tight tolerances between all nozzle components, gaskets, and mortars and grouts. However, should not be tight enough to hinder the expansion of the refractory; 4. proper nozzle component alignment to minimize asymmetrical flow, and 5. removing all foreign material from the bore of the assembled nozzle such as excess spray lining or mortar and ensuring a clean and smooth bore surface.
If the refractory material is reacting with the liquid steel, then another mechanism for precipitation takes place due to local steel chemistry change. But air aspiration through joints can cause local chemistry change and the potential for precipitation on or close to the refractory surface increases.
Deposition of Indigenous Alumina Inclusions: According to the research studies the inclusions deposited at the nozzle orifice did not form in situ but were formed elsewhere and deposited at the orifice.
Precipitation of Alumina on the Refractory Surface : The growth of the alumina deposit is by the precipitation of the dissolved aluminum and oxygen. which can have four origins listed below:
- Air aspiration through the refractory: air aspiration through the refractory pores due to leakage through the gas injection ports and between the refractory joints can cause oxidation of the dissolved aluminum in the steel. Alumina will be formed as a product which will subsequently grow as a deposit on the refractory wall
- Oxygen Provided by the Refractory Materials: When the molten steel comes in contact with refractory, dissolved aluminum in the steel can reduce solid or gaseous oxides to form alumina, which gradually grows or is deposited on the nozzle wall . Poirier proposed that the oxygen from the refractory is transported to the refractory/steel interface as CO, which oxidizes aluminum. They concluded that the deposition takes place by in situ nucleation of alumina from the steel/refractory reaction and clogging would still occur even if the steel is perfectly clean.
- Temperature drop at the Nozzle: Due to the heat loss from the nozzle to the atmosphere, the temperature of the steel drops when it passes through the nozzle. This drop in temperature supersaturates the steel with respect to aluminum and oxygen, resulting in the precipitation of alumina. In addition, due to the lower local temperature, steel can solidify in the inter-particulate spaces and reinforce the clog.
- Supersaturation of Oxygen in Liquid Steel: The reoxidation rate is greater than the alumina growth rate, leading to a transient supersaturation that heterogeneously dissipates on the refractory or clog surface.
Studies suggest that the major cause of buildups is the transport and agglomeration of indigenous inclusion. It is clear that buildups are related to the precipitation and transport of solids or liquids that can precipitate solids in contact with a refractory surface. which can be either due to heat transfer or the nature of the surface chemistry of the inclusion.
Clogged material in aluminum-killed carbon steels: In the case of aluminum killed steel, the matrix of oxide deposit is a three-dimensional network of small alumina particles sintered to each other. The bulk of the deposit is composed of white bulk alumina particles. The white powdery alumina deposit has a porosity of 80% or greater. which consists of empty voids or filled with steel. The deposit is preferentially present at the sites where flow eddies are present.
A bulk deposit is heavily present in the areas submerged inside the melt pool in the mold. The straight part of the SEN is essentially clean. It can be thought that the presence of steel in the form of a pool at the bottom of the SEN will cause recirculation patterns in the steel, leading to accelerated inclusion deposition. In the case of tundish well nozzles, the deposit is heavy near the entry of metal from the tundish into the nozzle.
For the tundish well block nozzle, the deposit was penetrated by fully dense steel. It is possible that the steel penetrated into the clog material after the casting was stopped, since these specimens were in a location above the slide gate. But it is also possible that the steel was present at the time casting was occurring. This is because, unlike in the case of an SEN, where the pressure of the flowing steel may even be below atmospheric level, in the case of a well nozzle there is a sufficient pressure head from the steel present in the tundish. The refractory particles forming the deposit are in the size range of a few microns.
This indicates that the morphology of alumina is a function not only of the particles that agglomerate or grow on the refractory interface but also of the time that the alumina spends at high temperatures.
The most common type of inclusion in aluminum-killed steels is because of the presence of pure alumina. Depending on the oxygen source, the resulting clog can be category 1.1, 1.2, 2.1 or 2.2. Other inclusions found in clogged nozzles include calcium sulfide, calcium aluminates and magnesium aluminates. A clog made up primarily of calcium sulfide indicates an ineffective calcium treatment meant for inclusion modification which generally is the case for high sulphur content more than 0.015%. High-melting-point calcium aluminates is a result of incomplete inclusion shape control by calcium addition. Calcium aluminates formation can also be due to exogenous sources like the entrainment of tundish slag( 2.3). The presence of magnesium aluminate spinel may be a result of very low oxygen potential due to fully killed slag and high dissolved aluminum content.
Summary of the compounds responsible for nozzle clogging
Al2O3– These types of inclusions are found primarily in the casting of aluminium killed steels. It can be caused by inclusion agglomeration, precipitation, reaction between the steel and the nozzle or air aspiration through the nozzle. FeO-Al2O3– known as Hercynite is normally formed due to reoxidation of the clog after it is removed from the caster. The solidified iron oxidizes and reacts with alumina. However, this phase is not stable in aluminum-killed steels except in very oxidizing conditions.
MgO-Al2O3– (spinel) Increasing levels of soluble magnesium contents in steels leads to the formation of the magnesium aluminate spinel phase in the liquid steel.
CaO-Al2O3– (solid or semi-solid) These are found during calcium treatment of aluminum killed steels, used mainly for inclusions modifications, due to insufficient calcium being added or reoxidation after calcium addition.
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