Mold Slag Entrainment
Mold slag entrainment is likely to occur for surface turbulence and vortexing high-velocity flow which shears off the slag from the surface. The entrainment of mold slag takes place for very high meniscus surface velocity which shears off the mold slag fingers down into the flow. If these get broken into particles and get dispersed into the flow internal defect results. The surface velocity must be kept below a critical value in order to prevent slag entrainment which again is a function of viscosity. The critical velocity also depends on the relative densities of the steel and flux phases and the mold geometry Entrainment is easier for deeper slag layers, lower slag viscosity, and lower slag surface tension. High surface velocity can cause emulsification of the slag, leading to foam formation due to steel-slag intermix. Too much argon gas flow allows easy capture of particles via vortexing or surface shearing flow.
Vortexing most often occurs during conditions of asymmetrical flow, where steel flows rapidly through the narrow passage between the SEN and the mold leading to the creation of swirling just beside the SEN which draws mold slag downward, near the sides of the nozzle. If it gets entrained with the jets exiting the nozzle ports, this slag will be dispersed everywhere and create defects. In addition to the vortex, slag may also be drawn downward by the recirculation pattern that accompanies flow from the nozzle ports. Thus, slag entrainment is most likely to take place with shallow nozzle submergence and high casting speed.
Level Fluctuation
Surface defects can be observed due to fluctuation in the meniscus level. If the steel jets entering the mold cavity move too close to the meniscus, slag entrainment and surface defects from level fluctuations may result. These variations take two forms: steady variation across the mold width known as a “standing wave,” and“level fluctuations,” where the local level changes with time where the later can cause the most serious surface defects. The critical maximum surface velocity has been estimated to be 0.3 m/s or 0.4 m/s.
Defects:
A sudden increase or fall in the liquid level is responsible for the serious problem. A sudden jump in the local level can cause molten steel to overflow the meniscus. In worst-case scenario, the steel can stick to the mold wall leading to sticker type of breakout. Alternatively, a jump in level can cause an irregular extended frozen meniscus shape, or “hook.” This extended meniscus can capture mold powder or possibly bubbles or inclusions.
Variations of more than the oscillation stroke over a time interval is the most detrimental. Even low-frequency variations (period > 60 s) may cause defects if the meniscus overflows and the solid slag rim is imprinted on the shell or captured.
A sudden severe drop in liquid level exposes the inside of the solidifying shell to the mold slag and also leads to surface depressions. Relaxing the temperature gradient causes cooling and bending of the top of the shell toward the liquid steel. When the liquid level rises back, the solidification of new hot solid against this cool solid surface layer leads to even more bending and stresses when the surface layer reheats. When liquid steel finally overflows the meniscus to continue with ordinary solidification, a surface depression is left behind.
The associated problems include surface cracks and segregation. Surface cracks allow air to penetrate beneath the steel surface, where it forms iron oxide, leading to line defects in the final product.
If the steel jet is directed too deeply or has too little superheat, then the liquid surface will have very little motion and will become too cold. This stagnant and cold meniscus can cause inadequate melting of the powder, which relies on fluid flow to generate convective heat transfer to help transport heat across the liquid slag layer to melt the powder. In addition, this can lead to the freezing of the steel meniscus, which will aggravate the formation of hooks and associated defects.
For example, surface velocity below 0.3 m/s has shown evidence of increased surface pinhole defects. To avoid these problems, the flow pattern should be designed to exceed a critical minimum velocity across the top surface. Higher surface velocity results from increased casting speed, higher argon gas flow rate, application of electromagnetic forces or reduced submergence depth. Steady, controlled oscillation of the mold level does not pose a threat to the quality as the liquid adjacent to the mold wall tends to move with the wall.
Flow System Design
It is important to maintain a constant liquid steel level in the mold, periodic optimum powder feeding rate (to keep a constant liquid slag layer thickness), casting speed, gas injection rate, slide gate opening, and nozzle position (alignment and submergence). The steady mold flow pattern must be designed and controlled. It is affected by both nozzle design and operating conditions. Nozzle geometry influences the flow pattern in the mold, which is greatly responsible for controlling surface turbulence and the associated surface defects.
Jet Impingement
Problems like breakouts and freezing can result from an impingement of too hot or too cold molten steel jets onto the solidifying shell Sticker type of breakouts can result from severe level fluctuation, oscillation, and slag lubrication problem. Local thin and hot regions of the solidifying shell, which can result from high superheat dissipation at the region where an excessively hot jet impinges on the inside of the shell. The problems get even more aggravated when high superheat dissipation is coupled with slow heat extraction from the corresponding shell exterior which is likely to occur in the off-corner regions due to air gap formation.
The coldest liquid is found deep in the caster, at the meniscus, and at the top surface beside the inlet nozzle. The fluid is coldest here because it is both far from the inlet and stagnant. When a cold nozzle is inserted during an SEN exchange, the cool steel in this region is susceptible to the skull formation, meniscus freezing and even “bridging,” where steel or slag freezes across the shortest distance between the nozzle and meniscus of the wide face, often leading to a breakout.
Almost all of the superheat is dissipated to the shell in the mold or just below, In slab casting with bifurcated nozzles, the impingement region on the narrow face absorbs the most superheat. This region extends to the off-corner region of the wide face. Jet impingement produces a thinner shell on the narrow face, compared with classic parabolic shell growth on the wide face. The importance of this effect increases with higher casting speeds, higher superheats and lower heat transfer due to gap formation. Asymmetric flow, such as that caused by nozzle clogging, also aggravates this effect.
The flow pattern also depends on parameters that generally cannot be adjusted to accommodate the flow pattern, such as the position of the flow control device (slide gate or stopper rod), nozzle clogging, casting speed, strand width, and strand thickness. However other parameters such as injection of argon gas, nozzle submergence depth, and the application of electromagnetic forces can be adjusted to maintain an optimal flow pattern.
