The primary reasons for nozzle clogging in continuous casting of steel is because of the buildup due to de-oxidation product such as Alumina, liquid steel freezing, complex oxides such as spinel formation, and reaction products such as CaS. Deposit of clogging material on the sidewalls of the nozzle can cause irregular flow from the tundish into the mold even resulting in re-oxidation of the steel and surface turbulence leading to slag entrapment and bad surface quality in the cast product. Nozzle clogging affects productivity and profitability of the industry by limiting the tundish life, early ladle change over, strand closures and lower strand casting speed.
The most effective way to prevent nozzle clogging in the continuous casting of steels is as follows: inclusion count reduction, inclusion modification by calcium addition, preventing re-oxidation of the steel, proper tundish geometry/ nozzle design, and proper tundish and nozzle refractories.
Inclusions modifications: Modifying the inclusions in the steel to a liquid state rather than a solid at casting temperatures. This is done by the calcium treatment of steel at the end of the refining process. Deoxidation products formed in silicon-killed steel are typically manganese silicates that pose no threats to nozzle clogging being liquid at casting temperatures.
Calcium treatment: The products of aluminum deoxidation give rise to the formation of solid alumina particles with a dendritic morphology having the liquidus temperature much higher than the steel casting temperatures which is the main cause for nozzle clogging. The degree of nozzle clogging has generally been found to increase with increasing alumina population/density. In aluminum-killed steels, the inclusion size distribution, population, and morphology can be influenced by the timing, order, and quantity of additions. If lime is present when the aluminum is added, a complex deoxidation product is formed that grows, agglomerates and separates from the steel more readily than alumina.
The addition of the right quantity of Ca may help in lowering the inclusion’s liquidus temperature below steel casting temperatures due to the formation of compounds such as 12CaO.7Al2O3. When correctly applied, liquid calcium aluminates are formed that do not adhere to casting nozzles. Calcium addition to the melt takes place by means of one of three ways; by CaSi powder addition, CaSi wire addition, or calcium injection with argon. Casi powder has poor recovery owing to calcium’s vapor temperature being lower than that of steelmaking temperatures. Therefore the addition of calcium powder on top of the melt will lead to vaporization of the major portion of the calcium into gas and leaving the system without getting absorbed into the steel. The deeper the calcium can be inserted into the melt, the greater will be the pressure generated and higher will be the vapor temperature for calcium. That’s why CaSi wire is used. CaSi wire consists of calcium powder encased in a steel tube. When the wire is injected into the melt the calcium is protected from melting by the presence of steel shell by not exposing it to high melt temperatures until it goes deep enough into the melt and the presence of enough pressure prevents the calcium from vaporizing. The same principle is applied for Calcium injection by sticking a lance into the melt and inserting it deep enough to avoid vaporization and calcium is blown into the melt by the making use of inert argon gas. Calcium treatment becomes expensive due to low recoveries and high reagent cost, although the improved castability and quality justify this cost. Depending on the specifications of the product being cast, Ca-wire is preferred over Ca-Si wire to avoid excessive Si pickup.
The desired alumina modification phase is the low melting point 12CaO•7Al2O3 phase. This compound is also known as C12A7— where C refers to the lime units and A to the alumina units. Insufficient calcium addition results in partial modification of alumina, creating compounds that are solid at steelmaking temperatures, such as, for example, CaO•6Al2O3 (CA6). Whereas excess calcium will lead to the formation of C3A type of compounds that are solid at casting temperatures. Solid calcium aluminates have lower acicularity compared to pure alumina and therefore have a lower apparent density than pure alumina, which makes them more apt to reach the nozzle wall by backflow and eddy transport, and thus contribute to clogging. The correct amount of calcium for castability is determined by the thermodynamic relationships between oxygen, aluminum, sulfur and calcium in the steel.
Knowledge of the conditions in the melt and the amount of calcium recovery is required prior to calcium addition. In practice, there is a reliance on reproducible recoveries to attain the correct residual calcium for castability. However, the conditions in the steel may be different from that expected due to reoxidation by FeO in slag, air aspiration or excessive argon stirring. In these cases, inclusion modification may be unsuccessful, and nozzle clogging may result. It is, therefore, best to operate with tight control of oxygen and steel chemistry and with a window of residual calcium acceptable for effective inclusion modification. This will require minimizing the total oxygen, aluminum and sulfur in the steel, and the control of the reducible oxides in the slag.
However the potential cause for clogging varies depending on grades, for example, re-sulfurized free machining steel can have issues due to the formation of calcium sulfides preferentially over calcium aluminate formation. The sulfide formed is a fine, needle-shaped precipitate that will result in severe nozzle clogging, even in low-sulfur steels. Calcium treatment of resulfurized steel requires a modified steelmaking process of desulfurization, killing, alumina modification and finally resulfurization, all the while maintaining a low total oxygen content.
Magnesium aluminate, or spinel : Even parts-per-million levels of Mg in the steel can cause the formation of magnesium aluminate spinel that will lead to nozzle clogging. The first step in preventing spinel formation in low-oxygen-potential systems is to eliminate metallic magnesium sources. Preference should be given to primary aluminum rather than recycled aluminum. Residual magnesium levels as low as 0.02% can lead to spinel formation.
Ladle slag line refractories also may contain metallic magnesium or metallic aluminum that contains magnesium. Once the metallic magnesium sources have been eliminated, attention must be focused on the oxygen potential of the slag/metal system. Total oxygen should be maintained at under 20 ppm to minimize the calcium injection required for shape control. Under certain conditions, liquid calcium magnesium aluminates may form with sufficiently low levels of calcium in the steel.
Reduction in the number of inclusions present in the steel: The number of inclusions in the steel can be reduced by increasing the size of the inclusions. By Stokes law, larger inclusions will have a greater upward velocity to float out of steel and get absorbed into the slag enabling to not being cast through the nozzle. If the mechanism of growth is the transport of existing inclusions, then the level of existing inclusions is a factor in the growth rate, and clean steel practices must be incorporated.
Use of proper tundish geometry or tundish furniture/flow modifiers: By adding tundish furniture or flow modifiers such as dams and weirs inclusion flow can be directed to give optimum exposure to the slag. Weirs are used to direct steel flow downwards whereas dams are used to direct flows upwards. By having two sets of weir-dam combinations between the ladle shroud and nozzle, the inclusions in the steel can be made to travel upwards and at the same time minimize bath turbulence.
