Preventing re-oxidation from atmospheric oxygen: ladle shroud is used between the ladle and the tundish which transports the liquid steel from the ladle to under the slag layer in the tundish whereas an impact pad suppresses the turbulence in the tundish. Increased turbulence can disrupt the slag surface in the tundish and expose the liquid steel to the atmosphere/air causing re-oxidation and slag entrapment. The submergence depth of the shroud into the melt should be optimum in order not to cause tundish level turbulence. Moreover, the inclined shroud can sweep off the top slag and lead to a tundish open eye causing reoxidation. The verticality of the shroud along with argon sealing and use of gasket reduces the chances of air aspiration. To prevent metal-slag interaction, baffles are used to slow down steel flow but allow steel to flow through the holes. Tundish fluxes are added as a covering compound that acts as a protective barrier between the steel and atmosphere thereby preventing exposure to the atmosphere and reoxidation of steel.
Submerged entry nozzles or submerged entry shrouds (SEN or SES) are used as the nozzle which will provide a barrier between the steel and atmosphere during the course of steel movement from tundish to mold. SEN are typically made of alumina graphite where the added graphite prevents wetting of the inclusions onto the nozzle walls. Argon purging on the top and/or bottom slide gate plates prevents air infiltrations.
Development of a reactive nozzle: The solid inclusional material in the steel will react with the nozzle to form a liquid at the steel/nozzle interface. This approach can be found successful as long as the inclusion flux to the nozzle is less than the capacity of the nozzle to transform the solid inclusion to a fully liquid inclusion. Thus, thermal conditions at the nozzle/steel interface and steel cleanliness must be controlled for the success of this type of technology.
Casting Steels Containing a Solid Inclusion: If solid inclusions are to be cast, the solution that remain is to slow down the buildup rate by minimizing the number of inclusions and optimizing the dynamics of fluid flow. Thus, clean steel practices to minimize inclusion production, insulation and nozzle preheat to minimize the thermal effect of the nozzle, and proper nozzle design to control fluid flow all act together to minimize buildup rates.
Reducing Thermal Problems: Buildups that contain only steel or inclusion that precipitates due to the reduced temperature near a refractory interface often are thermally induced or affected. These are solved by introducing heat through increased preheating or superheat or minimizing heat loss by insulation.
Inclusion Transport and Agglomeration: The first issue in clogs that are primarily caused by transport and agglomeration is to determine whether the clogging is caused by a transient condition or a continuous problem. Thus, a determination of the buildup rate or the frequency of clogging must be conducted. For example, if the majority of casting situations do not cause a problem and the issue is one of a few sporadic but catastrophic clogs then one can assume that there is a sporadic local increase in the production rate of inclusions due to a processing issue. Most common sporadic problems are related to reoxidation in the tundish, aspiration through a slide gate, variable ladle metallurgy practices leading to variable incoming steel cleanliness and problems with refractory integrity.
Initiation of Clogging
Initiation of clogging can occur by direct reaction between the steel and the refractory, precipitation on the refractory or transport of inclusions to the refractory. Thus, these are some of the causes for clog initiation:
1.Refractory Compatibility: The nozzle surface that is in contact with the steel should be inert. Often reaction between the nozzle and the glazes applied to nozzle surfaces can initiate buildup in aluminum-killed steels.
2.Refractory Integrity: All refractories must be gastight, and all joints between different types of refractories must also be gastight. Cracking due to thermal shock must be avoided to avoid external reoxidation.
3.Fluid Flow at the Steel/Refractory Interface: Transport of inclusions from the bulk steel to the interface can lead to the initiation of clogging. Thus, a refractory design that promotes smooth fluid flow and a defined laminar flow near the interface does not necessarily promote clog initiation. Details of fluid-flow design to avoid bulk eddies and the development of a very smooth refractory/steel interface to promote a laminar layer next to the interface are keys to extending the initiation time of clogging.
Clog Growth Rate
The clog growth rate is due to the transport of inclusions from the bulk. Once the clog is initiated, the interfaces immediately roughen and allow the continued contact of turbulent eddies and an increased transport rate of inclusions. There are a number of issues in controlling the clog growth rate.
1.Fluid-Flow Control: The areas of low liquid steel flow rate are more prone to a faster buildups rate. The nozzle design is important to avoid this problem. In SENs the bottom of the nozzle is always problematic, as there is no simple fluid-flow solution to ensure that the turbulence level is low. Proper nozzle design can, however, significantly reduce buildup rates by eliminating local problems.
Bulk turbulence or flow instability can dislodge the clogging material to get re-entrained into the flowing stream. Increased bulk turbulence, as induced by gas injection, can lead to an increase in the clog break-up rate and decrease the overall buildup rate but can cause other quality issues.
Minimizing Thermal Problems: Nozzle preheat and insulation practices are vitally important. Often buildups are stabilized by solidifying the steel that is trapped within the buildup. Thus, insulation and preheat can prevent the initiation of the buildup and also lead to a decrease in the buildup rate. In the same manner, increased superheat also helps.
Precipitation
In buildups that are caused by precipitation, one must affect either the temperature field or change the steel chemistry. Increased superheat and improved nozzle preheat and insulation are always helpful. Often thermal conditions at cast start initiate precipitating buildups and fluid-flow conditions within the nozzle, then control the transport of material to the interface. The growth rate is then a function of the local temperature at the buildup/steel interface and turbulence within the nozzle. For example, often in titanium-treated steels, titanium nitride precipitates immediately on the nozzle during nozzle fill; however, after that transient in temperature is overcome, the thermodynamic condition for precipitation is not met, and the layer starts to dissolve until thermodynamic equilibrium is reached. If the pouring temperature is too low, and the condition for precipitation is always satisfied at the buildup/steel interface, the buildup continues to grow at the rate at which the elements are transported from the bulk liquid
If the buildup is initiated by the reaction between the nozzle and the steel and then grow by transport and agglomeration of already present inclusions. Under this condition, changes in nozzle design and chemistry can have a remarkable effect on the time to clogging. Sometimes there can be precipitation and agglomeration occurring simultaneously. For example, in the case of stainless steels containing magnesium aluminate spinel inclusions that also are titanium stabilized, a multi-part buildup is found that can be minimized by eliminating the magnesium source or turning the spinels from a solid inclusion to a liquid inclusion through calcium treatment.
Inadvertent additions of reactive elements can even change the liquid inclusion to a solid or semi-solid inclusion. In case of manganese silicon steels one must avoid the inadvertent addition of aluminum to the steel either from the ferroalloys or the slag modifiers. Additions of ppm levels of magnesium and more than 50 ppm of aluminum make the liquid manganese silicates precipitate alumina or magnesium aluminate spinel inclusions and become semi-solid or fully solid. Whereas insufficient calcium treatment for Aluminium killed steel causes reoxidation or the inadvertent additions of magnesium through ferroalloy additions, extensive ladle reheating, degassing or long holding times and arcing time in the ladle at higher temperature can lead to the buildup of soluble magnesium in the steel and problems with material buildup during casting.
Titanium-Treated Steels: Some stainless steels treated with titania at a level below where precipitation of TiN is possible are susceptible to thermal clogging. This is due to the reaction of titanium with the nozzle refractory and the precipitation of titania on the nozzle surface. Titania is wet by steel and leads to no contact resistance between the ceramic and the steel. It also leads to an increased heat transfer rate between the nozzle and the steel. This leads to an increased buildup growth rate. The reaction of titanium also increases penetration of steel into the refractory and causes a larger area of heat dissipation. Thus thermal clogging can be seen in titanium-treated steels where little thermally-induced clogging is measured if the same grade is cast with no titanium.
High Phosphorus-Containing Steels: Aluminum-killed, low-carbon steels that are rephosphorized have shown to have accelerated buildups compared to the identical grade that is not rephosphorized. The buildup is found to be solely alumina. In these steels, either the phosphorus addition leads to increased inclusional mass that is not removed before casting or the phosphorus content leads to an interfacial effect similar to that of titanium.
