Modeling of Multiphase Flow, Superheat Dissipation, Particle Transport, and Capture in a Vertical and Bending Continuous Caster

Abstract

A new model of particle entrapment during continuous casting of steel is presented, which includes the effects of multiphase flow from argon gas injection and thermal buoyancy from superheat in the strand. The model simulates three different capture mechanisms, including capture by solidified hooks at the meniscus, entrapment between dendrites, and engulfment by the surrounding of large particles. The fluid flow and bubble capture results are validated with plant measurements, including nail board dipping tests and ultrasonic tests, respectively, and good agreement is seen. Results suggest that the superheat has a negligible effect on the flow in the mold region. However, higher (30 K) superheat causes a more complex flow in the lower strand by creating multiple recirculation zones due to the thermal buoyancy effects. This causes less penetration deep into the strand, which leads to fewer and shallower particle captures. Lower (10 K) superheat may enable significant top surface freezing, leading to very large internal defect clusters. Lower superheat also leads to deeper meniscus hooks, which sometimes (0.003%) capture large (1 mm) bubbles. Capture bands occur near the transition line from vertical to curved, due to the downward fluid velocity balancing the particle terminal velocity, enabling capture in the relative stagnation region beneath the longitudinal recirculation zone. These findings agree with plant observations.

Model of multiphase flow, superheat transport, and particle capture in a steel continuous caster

“…A system of computational models was applied in this work to simulate turbulent multiphase flow, heat transfer, and particle entrapment during continuous casting of steel, as described in recent work by the same authors [1]. A flowchart of the submodels making up this system is shown in figure 1 [1]. The flow model considered the effects of argon gas injection and thermal buoyancy on the flow.…”

“…First, the mean bubble size needed by the multiphase flow model was predicted using a model of pressure distribution and flow rate within a slide-gate flow delivery system, PFSG [5], to find the local pressure, and a two-stage bubble size model for gas injection into downward flowing metal [6]. The results were implemented into a Rosin-Rammler distribution, based on water model measurements, in order to estimate the bubble size distribution for the particle transport model in the steel caster [1].…”

“…Next, the solidification heat-transfer model CON1D was calibrated to match plant measurements and applied to determine the shell thickness profile that defines the liquid pool domain, and the primary dendrite arm spacing (PDAS) profile [1]. The shell thickness profile was then implemented into the multiphase thermal -fluid flow model via UDFs for mass, momentum, and energy sinks.…”

“…Multiphase flow was computed with a steady Eulerian-Eulerian (EE) model of the Reynolds-Averaged Navier Stokes (RANS) equations for the molten steel and gas phases in Ansys Fluent, including the standard 𝑘-ɛ model for turbulence with standard wall function boundary conditions [1]. Effects of thermal buoyancy were included with a fully-coupled heat transfer model of superheat transport.…”

“…Internal defects arise where particles are captured deeper in the caster. Small particles can be entrapped between dendrites, while larger particles can be surrounded by engulfment, leading to slivers in the rolled product [1]. Large particles are especially problematic.…”