Первоисточник материала: Feature Article from JOM
TRANSPORT AND ENTRAPMENT OF PARTICLES IN CONTINUOUS CASTING OF STEEL
Quan Yuan and Brian G. Thomas
University of Illinois at Urbana-Champaign,
Department of Mechanical and Industrial Engineering,
1206 West Green Street, Urbana, IL USA, 61801
Keywords: inclusions, steel, modeling, turbulent flow, transport, particle capture
Abstract
The entrapment of inclusions, bubbles, slag, and other particles into solidified steel products is a critically-important quality concern. These particles require expensive inspection, surface grinding and rejection of steel. 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. 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 k sub-grid-scale (SGS) 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 different systems. It was then applied to predict the entrapment of slag particles into the solidification front in molten steel.
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 schematic of the steel continuous casting process is depicted in Fig. 1 [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 entry 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 carrybubbles 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 [2]. If the flow pattern brings the particles to the top surface, they are harmlessly removed into the liquid slag layer, so long as the slag is not saturated and the surface tension forces can be overcome. When the flow pattern is detrimental, however, 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 complicated by collisions and attachment to bubbles. Particles trapped near the meniscus generate surface delamination defects, and may initiate surface cracks. This problem is aggravated by 1) rapid fluctuations in the top surface level and 2) partial freezing of the meniscus to form “hooks”, which entrap particles before they can enter the liquid slag.
Figure 1: Schematic of Steel Processing including ladle,tundish, and continuous casting steel.
Figure 2: Schematic of the computational domain of the thin-slab caster, including tundish nozzle.
Particles which become trapped in the solidifying front deep inside the product,[3, 4] lead to internal cracks, slivers in the final rolled product, and blisters. These intermittent defects are particularly costly because they are often not detected until after many subsequent finishing steps. There is clearly a great incentive to understand how to control the mold flow pattern in order to minimize particle entrapment and the associated quality problems. As a first step, this work presents a new computational model to simulate particle transport and entrapment in a continuous slab caster, in order to quantify these phenomena.
Model Description
A computational model has been developed to simulate the transport and entrapment of particles entering the molten steel pool both from the nozzle and from the bottom of the mold slag layer. 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.
Application to Inclusion Entrapment in a Slab Caster
The model was applied to simulate fluid flow,
Conclusions
Lagrangian computations of particle transport during continuous casting of steel slabs were performed, based on time-dependent fluid velocity fields obtained from Large Eddy Simulations of the three-dimensional fluid flow. A new capture criterion based on a balance of the important forces acting on a particle near a solidification front has been developed, validated with test problems and applied to simulate particle capture in the solidification front. This criterion depends on many factors including the particle size and density, transverse fluid velocity, sulfur concentration gradient, solidification front velocity, and primary dendrite arm spacing. The results reveal that most of the inclusions entering the mold are captured, especially for small particles. The model makes several other practical findings and is a useful tool for understanding and improving mold flow to avoid particle entrapment.
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