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Performance Comparison of Three Kinds of Submerged Entry Nozzles for Bloom Mold
Abstract
It is significant to research the metallurgical performance of mold because it's the heart of the continuous caster. Many high-grade steels are made in bloom mold, so this paper set a coupled mathematical model including fluid flow, heat transfer with solidification, inclusion movement in bloom mold with area of 380mm*280mm. Based on this model, the effect of three kinds of submerged entry nozzles with one-port, two-port and four-port on the fluid flow field, temperature field, solidified shell thickness profile and inclusions movement was investigated respectively. Results show that one-port nozzle makes the thickness of solidified shell increase gradually and uniformly because there is no impingement on shell, but it causes a deeper impinging jet which results in an only 7% removal rate of Al2O3 inclusions with size of 50 mu m and a higher inclusion concentration within 25-35mm beneath the bloom. Two-port nozzle causes a strong impingement on the narrow face which results in a thinner and worse shell growing there, but causes a lower impinging depth which results in a 17% removal rate of inclusions and a higher inclusion concentration within 10-20mm beneath the bloom. Four-port nozzle can weaken the jet impingement on shell and promote the flotation of inclusions because of the formation of several spiral rising flows in mold. The removal rate of inclusions is 25% by using four-port nozzle which is worth to popularize in practice.
Se ha realizado una revisión de la literatura para identificar qué se sabe en relación con los mecanismos de transferencia de calor, comportamiento termofluidodinámico, características de la solidificación, factores que influyen en el origen de defectos en el acero y uso de estrategias que impactan en una reducción de los defectos que se originan, principalmente, en el molde de la colada continua de acero. La metodología consistió en colectar y sintetizar conocimientos fragmentados, comparar la información encontrada en diferentes fuentes, y dar una respuesta, clara y actualizada, sobre el comportamiento termofluidodinámico del acero en el molde de colada. Como resultado de esta revisión se puede concluir que los defectos graves, como grietas y depresiones, están relacionados con el comportamiento termomecánico; las grietas se asocian al flujo turbulento, variación en el nivel del menisco, alta velocidad de colada y comportamiento inadecuado del polvo colador y la segregación se relaciona con la contracción del acero, temperatura y velocidad de colada y el flujo de calor en el contorno de la pieza. También se ha encontrado que, a pesar de la complejidad de los fenómenos que ocurren en el molde, se puede lograr la formación de una costra de acero adecuada y reducir la aparición de defectos, realizando las acciones que propicien un ajuste adecuado de los parámetros del molde. Además, es imprescindible aplicar prácticas de conicidad y oscilación del molde, configuración de buza y aplicación de campos electromagnéticos, para producir un acero de calidad.
Turbulent flow, the transport of inclusions and bubbles, and inclusion removal by fluid flow transport and by bubble flotation in the strand of the continuous slab caster are investigated using computational models, and validated through comparison with plant measurements of inclusions. Steady 3-D flow of steel in the liquid pool in the mold and upper strand is simulated with a finite-difference computational model using the standard k-ɛ turbulence model. Trajectories of inclusions and bubbles are calculated by integrating each local velocity, considering its drag and buoyancy forces. A “random walk” model is used to incorporate the effect of turbulent fluctuations on the particle motion. The attachment probability of inclusions on a bubble surface is investigated based on fundamental fluid flow simulations, incorporating the turbulent inclusion trajectory and sliding time of each individual inclusion along the bubble surface as a function of particle and bubble size. The change in inclusion distribution due to removal by bubble transport in the mold is calculated based on the computed attachment probability of inclusions on each bubble and the computed path length of the bubbles. The results indicate that 6%–10% inclusions are removed by fluid flow transport, 10% by bubble flotation, and 4% by entrapment to the submerged entry nozzle (SEN) walls. Smaller bubbles and larger inclusions have larger attachment probabilities. Smaller bubbles are more efficient for inclusion removal by bubble flotation, so long as they are not entrapped in the solidifying shell. A larger gas flow rate favors inclusion removal by bubble flotation. The optimum bubble size should be 2–4 mm.
Particle motion and capture in continuous steel casters were simulated using a Lagrangian trajectory-tracking approach, based
on time-dependent flow fields obtained from large-eddy simulations (Part I of this article). A computation was first conducted
on a water model of a full-scale standard slab caster, where measurements were available. It simulated the transport of 15,000
plastic particles and their removal by a screen positioned near the mold top surface. The computation shows the screenremoval
fractions to be 27±5 pct for 0 to 10 seconds and 26±2 pct for 10 to 100 seconds, which agrees with previous measurements.
The flow exiting the nozzle was relatively uniform, and turbulent motion in the domain was very chaotic, so particle removal
did not depend on the initial location of particles introduced in the nozzle port. A computation of motion and capture of
40,000 small inclusions (10 and 40 µm) was then performed in an actual thin-slab steel caster. The particles moved through
the mold region with an asymmetrical distribution, which was caused by transients in fluid turbulence in the lower recirculation
region, rather than by inlet variations at the nozzle port. Only about 8 pct of these small particles were removed to the
top surface. This removal fraction was independent of both particle size and density, likely because all the simulated particles
were too small to deviate significantly from the surrounding fluid flow. Finally, the computational results were further processed
to predict the ultimate distribution of impurity particles in the solid thin slab after a short burst of inclusions entered
the mold. They were reprocessed to reveal the distribution of total oxygen content for a steady inclusion supply from the
nozzle. The results of this work confirm the important role of flow transients in the transport and capture of particles during
continuous casting and can serve as a benchmark for future simplified models.
Fundamentally based computational models are developed and applied to quantify the removal of inclusions by bubbles during
the continuous casting of steel. First, the attachment probability of inclusions on a bubble surface is investigated based
on fundamental fluid flow simulations, incorporating the turbulent inclusion trajectory and sliding time of each individual
inclusion along the bubble surface as a function of particle and bubble size. Then, the turbulent fluid flow in a typical
continuous casting mold, trajectories of bubbles, and their path length in the mold are calculated. The change in inclusion
distribution due to removal by bubble transport in the mold is calculated based on the computed attachment probability of
inclusions on each bubble and the computed path length of the bubbles. In addition to quantifying inclusion removal for many
different cases, the results are important to evaluate the significance of different inclusion-removal mechanisms. The modeling
approach presented here is a powerful tool for investigating multiscale phenomena in steelmaking and casting operations to
learn how to optimize conditions to lower defects.