Modeling of Continuous-Casting Defects Related to Mold Fluid Flow

The quality of continuous-cast steel is greatly influenced by fluid flow in the mold, particularly at the meniscus. Recent examples of computational model applications at the University of Illinois are presented to investigate the formation of several different types of defects related to flow phenomena. The amount of gas injection into the tundish nozzle to avoid air aspiration is quantified by modeling. Computational model calculations of superheat transport and surface level fluctuations are presented. Meniscus defects, such as subsurface hooks and their associated inclusions, may form if the superheat contained in the steel is too low, or if top-surface level fluctuations are too large. A thermal stress model has been used to compute the distortion of the meniscus during a level fluctuation. Gas bubbles and inclusion particles may enter the mold with the steel flowing through the submerged nozzle. In addition, mold slag may be entrained from the top surface. These particles may be removed safely into the slag layer, or may become entrapped into the solidifying shell, to form sliver or blister defects in the rolled product. Transient, turbulent flow models have been applied to simulate the transport and entrapment of particles from both of these sources. The insights gained by these modeling efforts aid greatly in the development of processing conditions to avoid the formation of these defects. INTRODUCTION In the continuous casting of steel, the task of the flow system is to transport molten steel at a desired flow rate from the ladle into the mold cavity and to deliver steel to the meniscus area that is neither too cold nor too turbulent. In addition, the flow conditions should minimize exposure to air, avoid the entrainment of slag or other foreign material, aid in the removal of inclusions into the slag layer and encourage uniform solidification. Achieving these somewhat contradictory tasks needs careful optimization. Fluid flow in the mold is controlled by many design parameters and operating conditions. Nozzle geometry is the most important, and includes the bore size, port angle, port opening size, nozzle wall thickness, port shape (round, oval, square), number of ports (bifurcated or multiport), and nozzle bottom design). The flow pattern also depends on parameters which 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. Fortunately, other parameters besides nozzle geometry can be adjusted to maintain an optimal flow pattern. These include the injection of argon gas, nozzle submergence depth, and the application of electromagnetic forces. In choosing optimal settings for these parameters, it is important to understand how they all act together to determine the flow characteristics. An increase in casting speed, for example, might be compensated by a simultaneous increase in submergence depth (or electromagnetic force), in order to maintain the same surface flow intensity. Thus, all of the flow-control parameters must be optimized together as a system. In designing the flow system, it is important to consider transients. Sudden changes are the main cause of the flow instabilities which generate surface turbulence and other problems. Because flow parameters are more easily optimized only for steady operation, each of the parameters which affects fluid flow must be carefully controlled. It is especially important to keep nearly constant the liquid steel level in the mold, powder feeding rate (to keep a constant liquid slag layer thickness), casting speed, gas injection rate, slide gate