Factors such as the meniscus surface contours, level fluctuations, entrainment of the top surface mold slag, dissipation of superheat, temperature at the meniscus, entrapment of subsurface inclusions and gas bubbles have a significant effect not only on the quality of the final product but also on the productivity as well.
Superheat Dissipation
Superheat is the excess heat present in the steel represented by the difference between the steel liquidus temperature and actual steel temperature entering the mold. The fluid pattern would be such that the liquid steel delivers enough heat to the meniscus during the initial stages of solidification. The thermal conductivity of the liquid depends on the viscosity which again gets influenced by the turbulence in the liquid. Most of the superheat is dissipated in the mold and the hottest region along the top meniscus is found midway between the SEN and narrow face whereas the coldest regions are the top corners at the meniscus near the narrow face and near the SEN. If these cooler regions become too cold, it can lead to meniscus freezing facilitating the formation of thick slag rim. These can cause quality problems such as deep oscillation marks, which can later initiate transverse cracks and can also hinder proper infiltration of liquid mold flux into the gap, which can be a cause for longitudinal cracks and other surface defects. The steel surface can solidify against the cold region near the SEN to form a solid bridge between the SEN and the shell against the mold wall, which can result in breakout. Meniscus freezing problems are avoided by allowing the flow to reach the surface quickly and thus deep nozzle submergence is not recommended.
The jet of liquid steel coming out of the submerged entry nozzle delivers most of its superheat to the inside of the shell solidifying against the narrow face. The large temperature gradients found part-way down the domain gives an indication that the maximum heat flux delivery to the inside of the solidifying shell takes place at the mold exit. Good contact between the solidifying shell and the mold will cause adequate heat transfer so as not to cause catastrophic consequences but if a gap forms between the shell and the mold, heat extraction gets reduced which makes this superheat flux sufficient to slow down shell growth which can result in breakout in extreme conditions. Thus breakouts are common at mold exit just off the corners, due to less contact between mold and shell and the problem gets worse with higher flow rates and non-uniform flow from the nozzle.
Top-Surface Contour and Level Fluctuations
Meniscus behavior is greatly affected by the level fluctuations with time and the shape of the top surface of the liquid steel. The top surface of the liquid steel represents the interface between the steel and the lowest molten flux layer. When the surface waves are stable, then the interface shape can be estimated from the pressure distribution along the interface calculated from a simulation with a fixed boundary.
The condition of the meniscus during solidification directly impact the final quality of the steel product. It has been observed that at the time of casting with low argon and without electromagnetics in a wide mold, the interface gets raised by about 25 mm near the narrow face meniscus, relative to the lower interface near the SEN. The rise in the interface level is caused by the vertical momentum of the jet flowing up the narrow face, which again depends greatly on the flow pattern and flow rate. The rise in level increases with the decrease in the density difference between the fluids.
The kinetic energy contained in the moving fluid corresponds to the time-averaged velocity fluctuations and is converted temporarily to potential energy in the form of a rise or fall in level. For typical conditions, the most severe level fluctuations are observed near the narrow face, which has the highest turbulence and interface level. These level fluctuations can be reduced by changing the flow pattern and increasing the submergence depth or by employing a downward angled port that can direct the jet deeper into the mold. Increasing argon injection shifts the location of maximum level fluctuations/turbulence towards the SEN at the central region of the mold.
Throughput is less for smaller mold and especially at a lower speed which corresponds to a higher volume fraction of argon for argon injection rate being kept constant. It is likely that the higher argon fraction increases bubble concentration near the SEN, which can lift the jet forcing the interface level near the SEN to increase, thereby producing the highest level fluctuations.
Sudden fluctuations in the meniscus level are detrimental as they disrupt initial solidification and can vortex mold flux in the solidifying steel, resulting in surface defects in the final product. Level fluctuations can even deflect the meniscus and hinder the infiltration of the mold flux into the gap between the solidifying shell and the mold, resulting in the build-up of a thick flux rim which ultimately leads to the formation of air gaps between the shell and the mold. Together, this can lead to the formation of deep, nonuniform oscillation marks, surface depressions, laps, bleeds, and many other defects. The thermal stress created in the tip of the solidifying shell resulting from severe level fluctuation can cause distortion of the shell, which further contributes to surface depressions.
Entrapment of Inclusions and Gas Bubbles
The jets of molten steel coming out of the nozzle carry inclusions and argon bubbles into the mold cavity. If these particles are carried deep into the liquid pool and become trapped in the solidifying shell, they lead to the formation of internal defects in the final products. Defects such as sliver, blisters, and cracks are the manifestation of inclusions getting trapped in solidifying shell. The trapped argon bubbles elongate during rolling and may expand during subsequent annealing processes to create surface blisters and pencil pipes in case of steel with lower strength.
Inclusion size and shape have an effect on their drag and flotation velocities. Most of the argon bubbles circulate in the upper mold area and float out to the slag layer owing to the higher flotation velocities for 0.3 – 1.0 mm bubbles. It has been observed that the bubbles circulating in the upper recirculation region without random turbulent motion eventually float out whereas bubbles with turbulent motion are seen to touch against the solidification front and this takes place on both the inside and outside radius at almost equal frequency, particularly near the narrow faces high in the mold, where the shell is thin.
