Modelling shell and oscillation mark formation during continuous casting via explicit incorporation of slag infiltration

The development of reliable numerical models is vital to improve the quality of continuously cast products and to increase the productivity of the casting machine. In order to provide accurate predictions, these models must include detailed descriptions of the physical phenomena occurring inside the mould, such as metal flow, heat transfer and solidification. However, these topics are often treated separately during modelling due to their complexity. This has a negative impact on the accuracy of the predictions. To address this issue, a numerical model capable of coupling the flow dynamics with both the heat transfer to the mould walls and solidification has been developed. The 2‐dimenional model is based on a commercial CFD code that solves the Navier‐Stokes Equations coupled with a Volume of Fluid interface tracking technique for the multiphase system slag‐steel‐air under transient conditions within a conventional slab mould. The use of an extremely fine mesh in the meniscus region (~50 μm) allows, for the first time, the explicit calculation of liquid slag infiltration into the shell‐mould gap. Heat transfer through the solid mould faces and mould oscillation were also included in the model to provide a more realistic representation of the process. The model developed was tested in two case studies. In the first case, the predicted values were compared to prior numerical models and laboratory experiments directed to casting of conventional slabs. Excellent agreement was found for characteristics such as slag film development and heat flux variations during mould oscillation. Furthermore, predicted values for shell thickness, consumption and heat flux were also found to be in good agreement with plant measurements. The findings of this case study provided improved, fundamental understanding of the mechanisms involved in slag infiltration and solidification inside the mould and how these affect key process parameters, such as powder consumption and shell growth. The second case study consisted of a sensitivity study, where casting conditions (e.g. casting speed, mould cooling, steel/slag properties and oscillation settings) were varied in the simulations to determine their effect on both powder consumption and the formation of defects. The simulations predicted the initial formation of typical casting defects known as oscillation marks, without the aid of any external data fitting. The key result drawn from the sensitivity study was the determination of simple rules for the calculation of consumption, heat flux and defect formation as a function of the casting conditions. This opens the possibility of using the model as a diagnostic tool and for process optimisation

Asymmetric Flow Control in a Slab Mold through a New Type of Electromagnetic Field Arrangement

This research aims to investigate the control effect of asymmetric flow in a slab mold using a novel magnetic field arrangement: freestanding adjustable combination electromagnetic brake (FAC-EMBr). Three scenarios (submerged entry nozzle moves to the narrow face, wide face of the slab mold, and rotates 10°) were studied using three-dimensional numerical simulation. The results show that the magnetic field generated by the FAC-EMBr system can effectively cover three key zones in mold and that the magnetic flux density in the zone cover by a vertical magnetic pole can be adjusted according to the actual flow condition. The FAC-EMBr can effectively improve the asymmetric flow in a mold and near the narrow surface caused by the asymmetric arrangement of the nozzle and can effectively inhibit the occurrence of the flow deviation phenomenon and stabilize the steel/slag interface fluctuation. At the same time, FAC-EMBr has obvious inhibition effects on the surface velocity and can optimize the asymmetric distribution of the surface velocity and the upper reflux velocity caused by the asymmetric arrangement of the nozzle. This study can provide theoretical evidence for the development and utilization of a new electromagnetic brake technology

Liquid metal flows in continuous casting molds: A numerical study of electromagnetic flow control, turbulence and multiphase phenomena

Der Effekt eines externen Magnetfeldes auf die mehrphasige und turbulente Strömung in Stranggußkokillen und deren Wechelspiel führt in den wissenschaftlichen Arbeiten zu widersprüchlichen Aussagen. Die verschiedenen Prozessparameter können innerhalb eines kleinen Varianzbereichs entscheidenden Einfluss auf die Aussage haben, ob ein Magnetfeld begünstigend oder schädigend auf die Qualität des Produkts wirkt. Um wichtige Einflussfaktoren zu identifizieren, werden daher numerische Strömungssimulationen des Prozesses durchgeführt. Dazu wird zunächst ein mehrphasiger und inkompressibler Mehrregionen-CFD-Löser für magnetohydrodynamische Strömungen entwickelt und validiert, um die komplexe Strömung in einer Stranggußkokille mit hoher Genauigkeit simulieren zu können. Darauf aufbauend wird das numerische Setup anhand einer Modellkokille mit aktuellen Messdaten validiert. Durch die neuartige Kombination Lagrange'scher Lösungsmethoden mit angepassten Termen für die Magnetohydrodynamik sowie der skalenaufgelösten magnetohydrodynamischen Turbulenz, können erstmals Aussagen zur optimalen Magnetfeldverteilung im Hinblick auf Strömungsstabilität, Turbulenzmodulation und Blasenverteilung getroffen werden. Mit Hilfe dieses Wissens können neuartige Konzepte elektromagnetischer Bremssysteme für den Stranggußprozess entwickelt werden

Argon bubble transport and capture in continuous casting with an external magnetic field using GPU-based large eddy simulations

Continuous casting produces over 95% of steel in the world today, hence even small improvements to this important industrial process can have large economic impact. In the continuous casting of steel process, argon gas is usually injected at the slide gate or stopper rod to prevent clogging, but entrapped bubbles may cause defects in the final product. Many defects in this process are related to the transient fluid flow in the mold region of the caster. Electromagnetic braking (EMBr) device is often used at high casting speed to modify the mold flow, reduce the surface velocity and fluctuation. This work studies the physics in continuous casting process including effects of EMBr on the motion of fluid flow in the mold region, and transport and capture of bubbles in the solidification processes. A computational effective Reynolds-averaged Navier-Stokes (RANS) model and a high fidelity Large Eddy Simulation (LES) model are used to understand the motion of the molten steel flow. A general purpose multi-GPU Navier-Stokes solver, CUFLOW, is developed. A Coherent-Structure Smagorinsky LES model is implemented to model the turbulent flow. A two-way coupled Lagrangian particle tracking model is added to track the motion of argon bubbles. A particle/bubble capture model based on force balance at dendrite tips is validated and used to study the capture of argon bubbles by the solidifying steel shell. To investigate the effects of EMBr on the turbulent molten steel flow and bubble transport, an electrical potential method is implemented to solve the magnetohydrodynamics equations. Volume of Fluid (VOF) simulations are carried out to understand the additional resistance force on moving argon bubbles caused by adding transverse magnetic field. A modified drag coefficient is extrapolated from the results and used in the two-way coupled Eulerian-Lagrangian model to predict the argon bubble transport in a caster with EMBr. A hook capture model is developed to understand the effects of hooks on argon bubble capture

Enhanced steel product quality and productivity through improved flux performance in the mould by optimising the multiphase flow conditions and with special regard to melting and entrapment

Optimum melting of flux and behaviour of the molten flux pool situated above the steel melt surface as well as the avoidance of flux entrapment are important requirements to be achieved simultaneously for a stable casting process and a high product quality. The mould casting powder has to satisfy various requirements like lubrication, promotion of uniform heat-transfer, inclusion absorption, thermal insulation and chemical insulation. The aim of the project performed by steel producers (Arcelor España, CAS, Sidenor and TKN) and research institutions (BFI and CSM) was to provide precise information on the necessary optimum constructive and process engineering measures to adjust casting conditions and as a result flow conditions in the mould in order to avoid flux entrapment into the mould and also to guarantee a sufficient thickness and behaviour of the flux layer floating on the steel melt. The main objectives were: • Provision of more detailed information on the interrelation between melting conditions of the flux floating on the steel melt, especially the dynamic behaviour of this flux layer and steel flow. Focus had to be given especially on what flow conditions are needed in the mould to guarantee a sufficient layer and behaviour of the flux pool and simultaneously to avoid entrapment of flux into the steel melt. • Provision of a data base concerning optimised casting parameters (casting velocity, SEN immersion depth), SEN design to adjust the necessary flow conditions for given steel grades and selected casting powders. Also the influence of gas injection and of electromagnetic forces (EMS) had to be considered. Improvement of casting powders was also aim where necessary. • Verification of this data base in the operational praxis.European commision Contract No RFSR-CT-2003-00027 01/09/2003 - 28/02/200

Control of Continuous Casting Process Based on Two-Dimensional Flow Field Measurements

Two-dimensional flow field measurement allows us to obtain detailed information about the processes inside the continuous casting mould. This is very important because the flow phenomena in the mould are complex, and they significantly affect the steel quality. For this reason, control based on two-dimensional flow monitoring has a great potential to achieve substantial improvement over the conventional continuous casting control. Two-dimensional flow field measurement provides large amounts of measurement data distributed within the whole cross-section of the mould. An experimental setup of the continuous casting process called Mini-LIMMCAST located in Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Dresden, Germany, is used for this thesis. This thesis examines two alternatives of flow measurement sensors: Ultrasound Doppler Velocimetry (UDV) and Contactless Inductive Flow Tomography (CIFT). Both sensor variants can obtain information on the velocity profile in the mould. Two approaches were considered to create the process model needed for model-based control: a spatially discretized version of a model based on partial differential equations and computational fluid dynamics and a model obtained using system identification methods. In the end, system identification proved to be more fruitful for the aim of creating the model-based controller. Specific features of the flow were parametrized to obtain the needed controlled variables and outputs of identified models. These features are mainly related to the exiting jet angle and the meniscus velocity. The manipulated variables considered are electromagnetic brake current and stopper rod position. Model predictive control in several versions was used as the main control approach, and the results of simulation experiments demonstrate that the model predictive controller can control the flow and achieve the optimum flow structures in the mould using UDV. CIFT measurements can provide similar velocity profiles. However, further technical developments in the CIFT sensor signal processing, such as compensating for the effects of the strong and time-varying magnetic field of the electromagnetic brake on CIFT measurements, are necessary if this sensor is to be used for closed-loop control.Two-dimensional flow field measurement allows us to obtain detailed information about the processes inside the continuous casting mould. This is very important because the flow phenomena in the mould are complex, and they significantly affect the steel quality. For this reason, control based on two-dimensional flow monitoring has a great potential to achieve substantial improvement over the conventional continuous casting control. Two-dimensional flow field measurement provides large amounts of measurement data distributed within the whole cross-section of the mould. An experimental setup of the continuous casting process called Mini-LIMMCAST located in Helmholtz-Zentrum Dresden-Rossendorf (HZDR), Dresden, Germany, is used for this thesis. This thesis examines two alternatives of flow measurement sensors: Ultrasound Doppler Velocimetry (UDV) and Contactless Inductive Flow Tomography (CIFT). Both sensor variants can obtain information on the velocity profile in the mould. Two approaches were considered to create the process model needed for model-based control: a spatially discretized version of a model based on partial differential equations and computational fluid dynamics and a model obtained using system identification methods. In the end, system identification proved to be more fruitful for the aim of creating the model-based controller. Specific features of the flow were parametrized to obtain the needed controlled variables and outputs of identified models. These features are mainly related to the exiting jet angle and the meniscus velocity. The manipulated variables considered are electromagnetic brake current and stopper rod position. Model predictive control in several versions was used as the main control approach, and the results of simulation experiments demonstrate that the model predictive controller can control the flow and achieve the optimum flow structures in the mould using UDV. CIFT measurements can provide similar velocity profiles. However, further technical developments in the CIFT sensor signal processing, such as compensating for the effects of the strong and time-varying magnetic field of the electromagnetic brake on CIFT measurements, are necessary if this sensor is to be used for closed-loop control.

A multiscale approach for fluid flow effect on microstructure and segregation

Maintaining competitiveness in steel manufacturing requires improving process efficiency and production volume whilst enhancing product quality and performance. This is particularly demanding for producing value-added advanced steel grades. In today’s world of high quality steels, cast in near net shape where the ability to control microstructure through thermo-mechanical processes is limited, understanding of the dependence of the solidification structure on the process parameters like fluid flow is of technical importance. Variations of phase evolution across different length scales during solidification resulting from a continuous casting process define the macrosegregation (at the scale of casting) and hence the final properties of the solid steel. Macro scale (100 to 10-3 m) fluid flow during continuous casting washes away the rejected solute ahead of the micro scale (10-6 to 10-5 m) solid/liquid interface giving rise to different undercooling levels at different positions of the moving solidification front. With the progress of solidification, the intensity of the washing effect will decrease and the influence of diffusion will come into play, thereby contributing to the macro scale solute profile. Understanding the competition between the crystallographic growth direction and solute transport with casting parameters during the progress of casting will provide an important perspective towards reducing the macrosegregation in the cast product. Stringent quality requirements for the present generation steel grades for automotive applications demand more information into the growing micro scale solute profile mechanism and how it relates to the phenomenon occurring at the macro scale. In order to address the translation of micro scale information into the macro scale, a combined theoretical and experimental approach had been undertaken. Starting with a single component system, open source phase-field method based solidification model coupled with fluid flow have been developed. Quantitative validation of the solidification model for single component system with experimental results in literature have been done. The developed micro scale model in presence of fluid flow gives an account of the preferred solid/liquid interface growth direction. At different degrees of undercooling, the model predicts the transient nature of the evolving solute profile. The effects of flow velocity and dendrite growth speed on the interface growth direction were separated. Improved theoretical formulations for estimation of the bending angle (defined as deviation from the original growth direction of primary dendrite in absence of fluid flow) were put forward which extends the current knowledge available from literature. On the experimental part, dendrite bending angle measurements were made in the industrial steel slab samples from a conventional slab caster at Tata Steel in IJmuiden, The Netherlands. The dendrites were found to undergo a change in the growth direction indicating the transition in the fluid flow profile occurring within the mould. Also, the magnitude of the bending angle was found to decrease away from the slab surface. Through the proposed approach of micro-macro coupling an attempt was made to correlate the macro scale fluid flow profile within the continuous casting mould with that of the developed micro scale bending angle formulation. The proposed formulation based on the anisotropy in solid/liquid interface energy was found to fit the experimental deflection angles better than the few available empirical correlations in literature. Line scanning measurements were performed to determine the composition profile of industrial slab samples proving the influence of fluid flow on macrosegregation

Computational Fluid Dynamic (CFD) Simulation for Continuous Casting Process of Steels

A comprehensive numerical simulation of flow behavior of molten steel as well as slag inside tannish and mold of continuous casting machine has been performed using Computational Fluid Dynamics (CFD). Modeling of continuous casting process of steels is very important as fluid flow behavior inside tannish, sub-entry nozzle (SEN) and mold directly influence the quality of steel product. Investigations on inclusion entrapment and the chances of SEN clogging study during continuous casting process for steel are objectives of this thesis work. The outcrop of the developed mathematical model is beneficial to study effect of flow controller (e.g. dam and weir) putting inside tundish on the fluid flow behavior throughout tannish, sub-entry nozzle (SEN) and mold of continuous casting machine. In addition to that, a massless particle is injected from the inlet and the trajectories of the particle inside tundish have been investigated using Discrete Phase Model (DPM) along with k-e turbulence models. All the numerical simulation of fluid flow has been performed using ANSYS 15.0 software. This CFD simulation work demonstrates the correlation of flow controller’s shape, size and position with inclusion flotation possibility and path in tundish. This work also elaborates and shows the significance of flow controller position inside tundish on fluid flow behavior in tundish, sub-entry nozzle (SEN) and mold and quality of steel. The extract and underlying theory for this developed CFD model can be extended to different kinds of CCM process for various metal or metallic alloys to reveal the interrelation between inclusion removal kinetics and fluid flow behavior of molten steel and sla

Refining and Casting of Steel

Steel has become the most requested material all over the world during the rapid technological evolution of recent centuries. As our civilization grows and its technological development becomes connected with more demanding processes, it is more and more challenging to fit the required physical and mechanical properties for steel in its huge portfolio of grades for each steel producer. It is necessary to improve the refining and casting processes continuously to meet customer requirements and to lower the production costs to remain competitive. New challenges related to both the precise design of steel properties and reduction in production costs are combined with paying special attention to environmental protection. These contradictory demands are the theme of this book

Modelling temperature profile for the continuous casting billet with a linear final electromagnetic stirrer

LAUREA MAGISTRALENegli ultimi anni con la forte domanda sulla produzione di acciai puliti, ci sono requisiti più elevati per la microstruttura e l'omogeneizzazione della composizione del prodotto di fusione. L'applicazione di tecnica elettromagnetica di agitazione (EMS electromagnetic stirrer) promuove la formazione di una zona equiassica di cristalli nel filamento. Provoca il raffinamento della struttura di solidificazione, la riduzione del contenuto di inclusioni e il miglioramento della qualità della superficie, della sotto-superficie e della struttura interna del prodotto fuso. Questo lavoro di tesi ha lo scopo di modellare la distribuzione della temperatura nelle diverse sezioni della billetta lungo il lingotto e le zone degli spray. È stato realizzato per applicare la tecnica EMS nella fase finale di solidificazione per migliorare la qualità del prodotto di fusione. La simulazione del campo termico viene eseguita nel software di calcolo SCILAB. Varie simulazioni vengono eseguite cambiando il numero di spruzzi applicati alle pareti della billetta al di fuori del lingotto.In recent years with the stress on the production of clean steels, there are higher requirements for the microstructure and the composition homogenization of the cast product. The application of electromagnetic stirring (EMS) technique promotes the formation of an equiaxed crystal zone in the strand. It causes the refinement of the solidification structure, the reduction in the content of inclusions and improvement in the quality of the surface, sub surface and the inner structure of the cast product. This thesis work is aimed at modelling the temperature distribution at the different sections of the billet along the ingot and the sprays zones. It’s done in order to apply the EMS technique at the final stage of solidification to improve the quality of the cast product. Simulation for thermal field is carried out in the SCILAB computation software. Various simulations are performed by changing the number of sprays applied of the billet’s walls outside the ingot