Slidegate Dithering Effects on Transient Flow and Mold Level Fluctuations

Rui Liu (top row, left) research assistant; Brian G. Thomas (top row, right), C.J. Gauthier Professor of Mechanical Engineering (bgthomas@illinois. edu), Department of Mechanical Science and Engineering, University of Illinois at UrbanaChampaign, Urbana, Ill., USA; Love Kalra (bottom row, left), senior engineer (love.kalra@ arcelormittal.com), ArcelorMittal No. 3 Steel Producing, East Chicago, Ind., USA; Tathagata Bhattacharya (bottom row, right), research engineer (tathagata.bhattacharya@arcelormittal. com), ArcelorMittal Global R&D – East Chicago, East Chicago, Ind., USA; and Aloka Dasgupta (not pictured), senior process engineer, ArcelorMittal No. 3 Steel Producing, East Chicago, Ind., USA Slidegate dithering has been implemented in the continuous casting of steel to improve the control of flowrate by oscillating the slidegate back and forth at a particular stroke and frequency. The continuous motion prevents sticking and may reduce clogging. However, slidegate dithering also causes transient variations in fluid flow inside the submerged entry nozzle (SEN) as well as in the mold region.1 This has a direct impact on mold level f luctuations, which are crucial to the quality of the final products. Mold level fluctuations are one of the most significant mechanisms responsible for mold slag entrainment and the formation of surface defects and other quality problems in the final product, such as slivers.2 Previous work on dithering has generally focused on measurement of mold level.3–5 Control systems must be altered to maintain constant average flowrate and stable mold level, and special control algorithms have been developed to accomplish this.3–5 Recently, dithering has been investigated using computational models,1 calculating mold level using a simple pressure method with a fixed wall condition imposed at the slag/steel interface. With a low-frequency dithering practice (0.4 Hz) and a 1,840-mm mold width, the periodic mold level change was found to be dominated by mass conservation of the liquid steel. The predictions of mold level fluctuations using the pressure method were compared favorably with plant measurements from the dithering trial. This method has thus been found to be reasonably accurate for relatively small surface waves. For larger waves capable of causing sloshing, the effects of gravity need to be incorporated, so a free-surface tracking/capturing algorithm, such as the volume of fluid (VOF) method,6 has to be implemented in the model. In this work, models were developed, validated and combined together as a system to predict 3D transient turbulent flow in the nozzle and mold region, together with argon gas effects using an efficient new free-surface tracking of this complicated flow system. In addition, a simple analytical model based on mass conservation of the liquid was developed and validated. The detailed mechanism of how mold flow activates the mold sloshing was revealed, and parametric studies were conducted to study the effects of casting speed, mold width and dithering stroke on mold level fluctuations.