The production of metal strip by pouring molten steel between two rotating cylinders was first attempted in the 19th century. But it has taken the development of new materials and coatings, and newly acquired expertise in cooling and automation to make the process viable. The technology is now developing at a fast pace worldwide, and pioneer plants are providing first results. Economically, the process is extremely attractive, since smaller amounts of sheet metal can be produced locally and, with a minimum amount of subsequent working, very cost-efficiently.

The process employs two counter-rotating parallel rolls, against who's cooled surfaces the molten steel solidifies. Solidification begins just below the meniscus and shell growth continues as it moves downwards through the melt pool. At the kissing point, the two shells are essentially fused together forming a continuous strip, which then exits the caster downwards. Since the melt pool is small compared to that in conventional casters, the metal delivery has to be treated very carefully and good flow conditions in the melt pool are essential for faultless strip quality. Conventional delivery nozzles cannot be used, as inhomogeneity of flow in the melt pool is reflected in the strip quality. Similarly, the meniscus of the liquid steel must be stable, as any disturbance, wave or vortex at the free surface leads to unstable solidification and defects in the strip.

Different inlet nozzle types are being tested and optimized using CFX-TASCflow, in simulations which encompass both the internal flow in the nozzles as well as that in the melt pool. Whilst the flow exiting the delivery system is turbulent, it becomes laminar as the exit is approached, and for this reason the flow was simulated both as laminar and as turbulent. Transient simulations have focused on the detection of flow instabilities, and heat transfer calculations, which include the modeling of the cooled rolls and solidification, are providing understanding of the effects of buoyancy-induced secondary flow in the melt pool. Step by step, weaknesses such as recirculation zones, vortices, development of waves and uneven distribution of temperature are being identified and eliminated, and the flow quality improved.

Work is still progressing and in future stages electromagnetic braking, which is used to control the behavior of the nozzle jets, will be included in the model and optimized.

Schematic of the process and simulation of the solidification.

CFX calculation of static pressure distribution at the surface.

Buoyancy-driven vortical surface flow.