Slab quality improvement by controlling mould fluid flow

Abstract

Cleanliness of steel is a primary requirement for flat products. It is obtained with minimum of defects by controlling the liquid flow characteristics in the mould and fluctuations over the meniscus surface. Liquid flow in the mould region is due to the momentum of the pouring stream which in turn is related to the clogging of submerged entry nozzle and argon flow in the mould. This makes control of liquid steel flow dynamics in the mould important. The mould level fluctuation index, flow fraction or clogging percentage and optimised gas flow models have been developed and are correlated for minimised surface fluctuations throughout the casting sequence. Tundish weight, casting speed, casting section and immersion depth of the nozzle which primarily change the flow profile inside the mould are the key operational variables considered for model calculations. The operational parameters were adjusted to follow the developed models criteria for different casting conditions. Online application of these operational control models contributed to stabilise the mould fluid flow and have helped in decision making for pumping, flushing and tube changing. The present paper describes the mathematical approach adopted in calculation of optimum casting parameters for controlling flow of liquid steel, nozzle clogging and gas injection rate at JSW Steel Ltd. This has resulted in considerable reduction in mould level fluctuations and production of superior quality slabs even at higher casting speeds.

Analysis of the Depth of Immersion of the Submerged Entry Nozzle on the Oscillations of the Meniscus in a Continuous Casting Mold

“…For Case III the jet develops without alterations, losing kinetic energy uniformly by viscous dissipation as it approaches the narrow wall. As has been reported [32], the quality of steel-among other variables-depends on the solidification process and the flow feeding by the SEN. In other work [29] it was reported that the fluid dynamic structure depends exclusively on the design of the SEN, but due to the results found in this study it is possible to add that it also depends on the depth of immersion in the mold.…”

“…However, as indicated [39] a static free surface can cause problems of overcooling and inhibition of steel-slag contact, causing some quality problems in the product. The level oscillations in a single plane in the mold have been reported [32]; however, being a three-dimensional and transient phenomenon, the oscillation profile changes significantly along such thickness. Figure 8 shows the level oscillations in PII, PIII and PIV at the simulation times of 640, 530 and 720 s for the three cases analyzed.…”

“…To establish the limits on the permissible oscillation range of the steel level in the mold without affecting the surface quality of the slab, the F index has been developed; which is a level fluctuation factor in the meniscus area, which relates the momentum of the steel flow that rises to the surface once it impinges with the narrow wall of the mold [31,32].…”

Knowledge discovery and predictive accuracy comparison of different classification algorithms for mould level fluctuation phenomenon in thin slab caster

“…This finding is also in accordance with literature where Zhang et al (2007) also found increase in metal slag inclusion during ladle change. Also, Kumar et al 2007 reported deviation of the mould level fluctuation index from optimum value during ladle change for operational plant. So, these reported cases of mould level fluctuation or slag entrapment defects during ladle change operation gives us confidence in our finding.…”

“…Kumar et al (2007) developed an mould level fluctuation index in an operational plant and the plant data suggests deviation of the index from the optimum values during ladle change and width change due to asymmetry in the flow. Gursoy (2014) investigates effect of different flow controllers on the mould fluid flow including during ladle change.…”