Transport and Entrapment of Particles in Continuous Casting of Steel

The entrapment of inclusions, bubbles, slag, and other particles into solidified steel products is a critically-important quality concern. During continuous casting, particles may enter the mold with the steel flowing through the submerged nozzle. In addition, mold slag may be entrained from the top surface. A computational model has been developed to simulate the transport and entrapment of particles from both of these sources. The model first computes transient turbulent flow in the mold region using Large Eddy Simulation (LES), with the sub-grid-scale (SGS) k model. Next, the transport and capture of over 30,000 particles are simulated using a Lagrangian approach to track the trajectories. A new criterion was developed to model particle pushing and capture by a dendritic interface and was incorporated into the particle transport model. Particles smaller than the primary dendrite arm spacing are entrapped if they enter the boundary layer region and touch the solidifying steel shell. Larger particles are entrapped only if they remain stable while the shell grows around them. The new criterion models this by considering a balance of ten different forces which act on a particle in the boundary layer region, including the bulk hydrodynamic forces (lift, pressure gradient, stress gradient, Basset, and added mass forces), transverse drag force, (caused by fluid flow across the dendrite interface), gravity (buoyancy) force, and the forces acting at the interface (Van der Waals interfacial force, lubrication drag force, and surface energy gradient force). The criterion was validated by reproducing experimental results in two different systems. Finally, the model was used to predict the entrapment distributions, removal rates, and fractions of different sized particles in a straight-walled thin slab caster. Although more large particles are safely removed than small ones, the capture rate as defects is still very high. INTRODUCTION The entrapment of inclusions, bubbles, slag, and other particles during solidification of steel products is a critically-important quality concern. These particles require the finished product to undergo expensive inspection, surface grinding and even rejection. Furthermore, if undetected, large particles lower the fatigue life, while captured bubbles and inclusion clusters cause slivers, blisters, and other surface defects in rolled products. During continuous casting, particles may enter the mold with the steel flowing through the submerged nozzle. In addition, mold slag may be entrained from the top surface. The fraction of these particles which ultimately end up entrapped in the solidified shell has not previously been quantified. A computational model has been developed to simulate the transport and entrapment of particles from both of these sources. The model first computes transient turbulent flow in the mold region using Large Eddy Simulation (LES), with a sub-grid-scale (SGS) k model. Next, the transport and capture of over 30,000 particles are simulated using a Lagrangian approach to track the trajectories. The entrapment of particles which touch the boundaries representing the solidifying shell is determined by evaluating a force balance on each particle that resides in the fluid boundary layer at the dendritic interface. A schematic of the steel continuous casting process is depicted in Fig. 1 , with a close-up of the simulated regions of the nozzle and liquid-pool of the continuous casting mold and upper strand given in Fig. 2. Steel flows from the ladle, through the tundish and into the mold through a submerged netry nozzle. Jets of molten steel exit the nozzle ports and traverse across the mold cavity to impinge on the solidifying steel shell near the narrow faces. These jets carry bubbles and inclusion particles into the mold cavity. In addition, high speed flow across the top surface may shear droplets of liquid mold slag into the flow, where they may become entrained in the liquid steel . If the flow pattern enables the particles to reach the top surface, they should be harmlessly removed into the liquid slag layer, so long as the slag is not saturated and the surface tension forces are not excessive. However, when the flow pattern is detrimental, particles become entrapped in the solidifying steel shell, where they cause serious quality problems and costly rejects. Particle trajectories and removal depend on particle size, which is further complicated by collisions and attachment to bubbles. Particles that become trapped near the meniscus generate surface delamination defects, and may initiate surface cracks. This problem is more likely when there are rapid fluctuations in the level of the top surface. It is also more likely when the meniscus partially freezes to form meniscus “hooks”, which entrap particles into the solidifying meniscus before they can enter the liquid slag. Meniscus hooks are more prevalent there is insufficient liquid temperature at the meniscus. The local superheat of the molten steel near the meniscus depends on the flow pattern in the mold, as the jets also transport superheat. Fig. 1. Schematic of Steel Processing including ladle, tundish, and continuous casting 24 00 m m