Toward Better Control of Inclusion Cleanliness in a Gas Stirred Ladle Using Multiscale Numerical Modeling

Abstract

The industrial objective of lowering the mass of mechanical structures requires continuous improvement in controlling the mechanical properties of metallic materials. Steel cleanliness and especially control of inclusion size distribution have, therefore, become major challenges. Inclusions have a detrimental effect on fatigue that strongly depends both on inclusion content and on the size of the largest inclusions. Ladle treatment of liquid steel has long been recognized as the processing stage responsible for the inclusion of cleanliness. A multiscale modeling has been proposed to investigate the inclusion behavior. The evolution of the inclusion size distribution is simulated at the process scale due to coupling a computational fluid dynamics calculation with a population balance method integrating all mechanisms, i.e., flotation, aggregation, settling, and capture at the top layer. Particular attention has been paid to the aggregation mechanism and the simulations at an inclusion scale with fully resolved inclusions that represent hydrodynamic conditions of the ladle, which have been specifically developed. Simulations of an industrial-type ladle highlight that inclusion cleanliness is mainly ruled by aggregation. Quantitative knowledge of aggregation kinetics has been extracted and captured from mesoscale simulations. Aggregation efficiency has been observed to drop drastically when increasing the particle size ratio.

Numerical simulation of modification of non-metallic inclusions by calcium treatment in the argon-stirred ladle

“…However, improvements of the model are possible if a more accurate simulation of the complex behavior of inclusion population is desired. Of course, among those improvements, consideration of the actual kinetics of the inclusion modification process, account of inclusion removal by settling or flotation into the slag [24], and a more realistic description of the top slag layer most likely represent the greatest issues [25]. To do this special interface tracking methods and mesh refining would for sure be needed but leading to an increase of the calculation cost.…”

Aggregation kernel of globular inclusions in local shear flow: application to aggregation in a gas-stirred ladle

“…According to equation (16), the following data are required for estimating the global aggregation frequency f ij : the collision kernel b ij as a function of the shear rate ; the shear distribution in volume ( ∂V ∂ _ g ) ; the shear rate distribution seen by each particles ( ∂N i ∂ _ g ). The first data are provided by the present work through equations (7) and (8):…”

“…Among them, a recent numerical simulation shows that aggregation plays the most significant role since the flotation rate increases with the particle size. The authors conclude that the inclusion removal in the gas-stirred ladle can be described as the aggregation of small particle sizes to larger aggregates which are finally removed by flotation [8]. For the hydrodynamic conditions prevailing in the liquid metal ladle and for the usual size of NMI (in the range between one micron and a few tens of microns), the aggregation is mainly induced by turbulence.…”

Multiscale Simulation of Non-Metallic Inclusion Aggregation in a Fully Resolved Bubble Swarm in Liquid Steel

“…An industrially suitable approach for numerical simulations at process scale is to couple particle modeling by a population balance equation with computational fluid dynamics (CFD). Since aggregation is a critical process, such an approach strongly depends on the quality and realism of aggregation kernels which have to properly represent local particle dynamics [4][5][6][7]. Since inclusion sizes can be considered smaller than the Kolmogorov length scale in these applications, most authors use the model by Saffman and Turner [8] for turbulent aggregation of inclusions and introduce a collision efficiency accounting for the short distance effects between particles such as adhesive forces usually following Van der Waals law and hydrodynamic interaction.…”

“…The present work builds upon earlier work by the present authors [7,16,17] and develops a novel multiscale approach in which simulations are conducted both at inclusion scale and bubble swarm scale, coupled by a statistical model to derive process quantities that apply to liquid metal flotation processes.…”