<p>Secondary cooling during continuous casting is a delicate process because the cooling rate of water spray directly affects the slab surface and internal quality. Undercooling may lead to slab surface bulging or even breakout, whereas overcooling can cause deformation and crack of slabs due to excessive thermal residual stresses and strains. Any slab which does not meet the required quality will be downgraded or scrapped and remelted. In order to remain competitive and continuously produce high-quality and high-strength steel at the maximum production rate, the secondary cooling process must be carefully designed and controlled. Efficient and uniform heat removal without deforming or crack the slab is a significant challenge during secondary cooling. In the meantime, the on-site thermal measurement techniques are limited due to the harsh environment. In contrast, experimental measurements are only valid for the tested conditions, and the measurement process is not only labor-intensive, but the result might be inapplicable when changes in the process occur. On the other hand, the high-performance computing (HPC)-powered computational fluid dynamics (CFD) approach has become a powerful tool to gain insights into complex fluid flow and heat transfer problems. Yet, few successful numerical models for heat transfer phenomena during secondary cooling have been reported, primarily due to complex phenomena. </p> <p> </p> <p>Therefore, the current study has proposed two three-dimensional continuum numerical models and a three-step coupling procedure for the transport of mass, momentum, and energy during the secondary cooling process. The first numerical model features the simulation of water spray impingement heat and mass transfer on the surface of a moving slab considering atomization, droplet dispersion, droplet-air interaction, droplet-droplet interaction, droplet-wall impingement, the effect of vapor film, and droplet boiling. The model has been validated against five benchmark experiments in terms of droplet size prior to impingement, droplet impingement pressure, and heat transfer coefficient (HTC) on the slab surface. The validated model has been applied to a series of numerical simulations to investigate the effects of spray nozzle type, spray flow rate, standoff distance, spray direction, casting speed, nozzle-to-nozzle distance, row-to-row distance, arrangement of nozzles, roll and roll pitch, spray angle, spray water temperature, slab surface temperature, and spray cooling on the narrow face. Furthermore, the simulation results have been used to generate a mathematically simple HTC correlation, expressed as a function of nine essential operating parameters. A graphic user interface (GUI) has been developed to facilitate the application of correlations. The calculated two-dimensional HTC distribution is stored in the universal comma-separated values (csv) format, and it can be directly applied as a boundary condition to on-site off-line/on-line solidification calculation at steel mills. The proposed numerical model and the generic methodology for HTC correlations should benefit the steel industry by expediting the development process of HTC correlations, achieving real-time dynamic spray cooling control, supporting nozzle selection, troubleshooting malfunctioning nozzles, and can further improve the accuracy of the existing casting control systems.</p> <p> </p> <p>In the second numerical model, the volume-averaged Enthalpy-Porosity method has been extended to include the slurry effect at low solid fractions through a switching function. With the HTC distribution on the slab surface as the thermal boundary condition, the model has been used to investigate the fluid flow, heat transfer, and solidification inside a slab during the secondary cooling process. The model has been validated against the analytical solution for a stationary thin solidifying body and the simulation for a moving thin solidifying body. The effects of secondary dendrite arm spacing, critical solid fraction, crystal constant, switching function constant, cooling rate, rolls, nozzle-to-nozzle distance, and arrangement of nozzles have been evaluated using the validated model. In addition, <a>the solidification model has been coupled with the predictions from the HTC correlations, and the results have demonstrated the availability of the correlations other than on-site continuous casting control. </a>Moreover, the model, along with the three-step coupling procedure, has been applied to simulate the initial solidification process in continuous casting, where a sufficient cooling rate is required to maintain a proper solidification rate. Otherwise, bulging or breakout might occur. The prediction is in good agreement with the measured shell thickness, which was obtained from a breakout incident. With the help of HPC, such comprehensive simulations will continue to serve as a powerful tool for troubleshooting and optimization.</p>

<p> The automotive industry is using advanced high strength steels (AHSS) to improve the fuel efficiency of passenger vehicles by lightweighting strategy. The higher strength of AHSS allows vehicle manufacturers to implement thinner and lighter components while still meet the safety requirements. Press hardened steels (PHS) exhibit the highest tensile strength among AHSS and are widely used for manufacturing crash relevant automotive parts. Boron-containing steel with enhanced hardenability is the most commonly used grade of steel for press hardening. The addition of a small amount of boron, 0.002 – 0.005 wt.%, can effectively increase hardenability. However, the boron addition also causes problems in commercially production of steel slabs by continuous casting. Defects including transverse corner cracks, surface cracks, and internal halfway cracks are sometimes found in continuously-cast boron steel slabs during or after the final cooling process. These problems can arise during the post-casting cooling process because boron addition changes the phase transformation behavior of steel.</p> <p>The cooling of slabs during and after continuous casting is a multiphysics process including coupled heat transfer, solidification, solid-solid phase transformations, and deformation. Numerical models are helpful for a better understanding of the cooling process and the interaction of different physical phenomena in it. In this work, a 3-D thermomechanical finite volume model (FVM) with coupled heat transfer, stress, and phase transformation calculations is developed to investigate the temperature history, phase evolution, and stress development during cooling.</p> <p>The model is used to simulate the cooling process of continuous cast steel slabs at different post-casting stages. The effect of boron addition on stress development and phase evolution during cooling of a single slab is investigated via simulation of both boron-containing and non-boron steels. The results show the slab with boron consists of mostly bainite, in contrast to the non-boron grade which is mostly ferrite and pearlite after cooling. Higher tensile stresses, both peak and residual, and plastic strains, which could lead to cracking, are observed at the edge of slab in the boron-containing grade. The effect of slowing the cooling rate by using a radiation shield is studied for the boron-containing steel. The reduced thermal gradient and the increased ferrite formation reduce the stresses in the slab. The cooling process of a stack of multiple slabs is also simulated to study the influence of slabs stacking on cooling rate and slab deformation. A slower cooling rate can be achieved in stacked slabs and the compressive load provided by slabs above the slab can prevent large deformation and flatten the slab during cooling. The combination of slab stacking and radiation shield is modeled to study the stress development under a slow cooling rate that is feasible in practice. Boron addition also affects the water quenching process of steel strips on the runout table after hot rolling. Simulations of strips with and without boron show different cooling curves, residual stress and phase distributions as austenite decomposition does not occur for boron-containing steel due to the fast cooling rate. Therefore, the cooling strategy on the runout table should be adjusted accordingly to control the coiling temperature and improve strip quality.</p>

Das Stranggießen von Stahl ist mit 96 % Marktanteil das weltweit wichtigste Verfahren zur Stahlherstellung. Im Gießprozess beeinflusst das Strömungsprofil in der Kokille entscheidend die Qualität des erstarrten Stahls. Um eine möglichst optimale Kokillenströmung einzustellen, werden Aktuatoren eingesetzt, die die sich bewegende Schmelze kontaktlos mithilfe der Lorentzkraft beeinflussen. Diese Aktuatoren würden auch eine Strömungsregelung ermöglichen, wenn eine geeignete Messtechnik für heiße Schmelzen vorhanden wäre. Allerdings lösen bislang verfügbare Messverfahren vor allem die Strömung im Randgebiet der Kokille auf und sind häufig in ihrer zeitlichen Auflösung limitiert. Eine neue infrage kommende Messtechnik ist die berührungslose induktive Strömungstomographie (contactless inductive flow tomography, CIFT), die aus der gemessenen strömungsinduzierten Verzerrung eines angelegten Magnetfeldes die dreidimensionale Strömung rekonstruieren kann. In dieser Arbeit wird anhand eines 1:8-Labormodells einer Stranggusskokille und numerischen Simulationen untersucht, ob CIFT bei Anlagen mit elektromagnetischen Bremsen eingesetzt werden kann. Besondere Herausforderungen entstehen aufgrund der Verzerrung des CIFT-Anregungsmagnetfeldes durch die ferromagnetische Bremse, der großen Dynamik von 6 Größenordnungen zwischen dem Magnetfeld der Bremse und dem strömungsinduzierten Magnetfeld sowie intrinsischen Strömungsoszillationen mit einer charakteristischen Frequenz im Bereich der üblicherweise verwendeten CIFT-Anregungsfrequenzen. Es wird dargelegt, dass sich CIFT in derartigen Aufbauten einsetzen lässt, wenn (a) eine geeignete Anregungsmagnetfeldstruktur erzeugt werden kann, (b) gradiometrische Induktionsspulen als Magnetfeldsensoren eingesetzt werden und (c) die Anregungsfrequenz in einem optimalen, schmalen Bereich gewählt wird. Diese Messungen werden erst durch in dieser Arbeit dargelegte theoretische und experimentell validierte Analysen der Induktionsspulen möglich, wofür Schwerpunkte auf deren Modellierung, Design und Messunsicherheit gelegt wurden. Außerdem werden für dieses Stranggussmodell erstmals experimentelle Ergebnisse mit horizontal anstatt vertikal orientierten Anregungsmagnetfeldern präsentiert. Um die Skalierbarkeit von CIFT in Richtung industrieller Anlagen zu demonstrieren, werden zum einen CIFT-Strömungsrekonstruktionen in einem heißen 1:2-Labormodell einer Kokille vorgestellt. Eine weitere Herausforderung für CIFT ist die in industriellen Kokillen typischerweise aufgebrachte ferromagnetische Nickelschicht, die eine verzerrende und abschirmende Wirkung auf umgebende Magnetfelder hat. Diese Beschichtung stellt aufgrund ihrer zeitlich und räumlich schwankenden Permeabilität eines der größten Hindernisse für die Anwendung von CIFT im industriellen Stahlguss dar. Die Auswirkung dieser Beschichtung auf CIFT wird mit numerischen Simulationen quantifiziert. Dabei werden neue, im Rahmen dieser Arbeit patentierte Anregungsgeometrien untersucht und erste Strömungsrekonstruktionen gezeigt. / Continuous casting of steel is the most relevant steel casting method worldwide, with a 96 % market share. During the casting process, the flow profile in the mold has a decisive impact on the quality of the solidified steel. In order to obtain an optimal flow, electromagnetic actuators are employed to influence the melt by Lorentz forces in a contactless manner. Yet, tailored flow control is difficult, since conventional flow measurement techniques provide information with limited temporal or spatial resolution when applied to hot melts. One novel measurement technique that comes into question is contactless inductive flow tomography (CIFT), which relies on measuring the flow-induced perturbations to an applied magnetic field, and subsequent reconstructions of the three-dimensional flow. Using a 1:8 lab model of a casting mold and numerical simulations, this thesis examines if CIFT can be applied to continuous casters with electromagnetic brakes. Specific challenges arise from the distortion of the CIFT excitation field by the ferromagnetic brake, the dynamic range of 6 orders of magnitude between the magnetic field of the brake and the flow-induced magnetic field, as well as intrinsic flow oscillations with a characteristic frequency close to the CIFT excitation frequency. It is demonstrated that CIFT can be applied to these kind of setups, if (a) a proper structure of the excitation field can be achieved, (b) gradiometric induction coil sensors are employed and (c) the excitation frequency is chosen within an optimum narrow range. These measurements were enabled by novel theoretical analyses of the induction coils with corresponding experimental validation. Furthermore, for the first time, experimental results using a horizontally applied magnetic field are presented, in contrast to older experiments relying on a vertical magnetic excitation field. Additionally, this thesis presents CIFT flow reconstructions for a hot 1:2 model of a continuous casting mold, in order to demonstrate the scalability of CIFT towards industrial facilities. Moreover, the effects of ferromagnetic nickel coatings on CIFT, which are typically found in industrial casting molds, are quantified using numerical simulations. These coatings pose one of the most challenging obstacles for the industrial application of CIFT, since their temperature-dependent permeability shows temporal and spatial variations. For this scenario, new excitation geometries—developed and patented within the scope of this thesis—are investigated and first flow reconstructions are presented.

The thesis deals with optimization of the continuous slab casting process. The thesis summarizes the basic analytical and empirical findings concerning to the solidification process, the numerical modeling and the selected optimization techniques. Physical conditions and factors that affect the quality of steel including their relationships are also described. The basis of the solution strategy is the original numerical model of the temperature field in its off-line version. The numerical model was verified by the real historical data. The optimization part is based on the fuzzy logic implemented above the numerical model. The optimization algorithm is used for the optimal control of the casting process. The universal usage of the optimization model is demonstrated on several cases, e.g. the finding of optimal casting parameters that ensure the high quality of products, the optimal reactions on breakdown situations, the determination of an optimal relationship between casting parameters, etc. Based on optimization results, the suitable caster modification to increase the surface temperature at the unbending point was proposed. The whole concept of the numerical and optimization model is general and it can be applied to arbitrary slab or billet continuous casting.

This paper presents an investigation on using the variation of ferrostatic pressure exerted by the molten steel in the strand to the support rolls during continuous casting to detect the location of the final solidification. The final solidification point is of high importance for applying soft reduction during continuous casting in order to ensure inner quality and achieve high productivity. The measurement was conducted at one of the casters at SSAB Oxelösund, of which all the support rolls are separately mounted. The project finds that this method shows promising results and can help to narrow down the range of the location due to certain interferences and noises. The ferrostatic pressure decrease during tail-out and the variation of whether a pressure drop exists is the dividing line of full and partial solidification. With the promising results achieved, more effort should be put to further improve the method.

<p> Continuous Casting (CC) is an essential process in the steel industry to transform molten steel into solid product. This process begins with primary cooling (PC) where the molten steel is cooled, and the initial solidification begins. It is important to monitor the process of PC as defects such as thinning of the shell in the mold can lead to breakouts. Key parameters in PC are the mold design, casting condition, and steel composition. In the research conducted, key parameters for PC are investigated to analyze the impact on flow formation and solidification. To optimize mold design, angular taper to the narrow face can be employed to accommodate for any shell shrinkage. Utilizing computational fluid dynamics, a range of mold taper is simulated per the developed solidification model with defined temperature-dependent material properties. When simulated without a taper, significant air gap formation in the corners of the mold is visible due to thermal shrinkage of the shell. This air gap decreases the cooling rate due to the shell’s lack of contact with the cooling mold wall. A parametric study of mold taper ranging from no taper to 3° as well as change in casting conditions, superheat and casting speed, are conducted to analyze the impact of taper with respect to the casting conditions. Per the conditions applied, angular taper between 1° and 2° resulted into reduction of undercooling and overcooling in the corner of the mold which is subjected to cooling from the broad face and narrow face of the cool mold wall. The turbulent flow in the mold region was found to drastically influences the quality of steel produced during continuous casting. The flow itself can lead to surface defects or slag entrainment based on the formation. A high surface wave due to turbulence of the injected melt lead to fluctuations and the instability compromised the quality of the steel produced as well as entrained the slag. To regulate the flow, electromagnetic forces can be applied in the mold, dampening the local turbulent flow. As the electrically conductive molten steel interacts with the induced magnetic field, it reduced the velocity of the steel jet released from the ports of the submerged entry nozzle. Per the simulation-based study conducted increasing the EMBr strength from 2975G to 4350G reduced the peak surface wave height by 59.47% and volume of flux rate of decrease by 4.25%. Additionally, increasing the SEN depth from 110 mm to 350 mm increased the average wave height by 19% and volume of flux rate of decrease by 2.6%. Lastly, increasing the mold width from 1.067 m to 1.50m increased average wave height by 8.71% and volume of flux rate of decrease by 0.9%. </p>

The present dissertation describes part of the efforts directed towards the development of computational tools to support process modeling. This work is also a further development of the Thermoslag software developed in the Division of Materials Process Science, KTH. The essential parts of the thesis are a) development of a semi-empirical model for the estimation of the molar volumes/densities of multicomponent slags with a view to incorporate the same in the model for viscosities and b) further development of the viscosity model for application towards fluorid- containing slags, as for example, mould flux slags. The model for the estimation of molar volume is based on a correlation between the relative integral molar volume of a slag system and the relative integral molar enthalpies of mixing of the same system. The integral molar enthalpies of the relevant systems could be evaluated from the Gibbs energy data available in the Thermoslag software. The binary parameters were evaluated from experimental measurements of the molar volumes. Satisfactory correlations were obtained in the case of the binary silicate and aluminate systems. The model was extended to ternary and multi component systems by computing the molar volumes using the binary parameters. The model predictions showed agreements with the molar volume data available in literature. The model was used to estimate the molar volumes of industrial slags as well as to trace the trends in molar volume due to compositional variations. The advantage of the present approach is that it would enable prediction of molar volumes of slags that are compatible with the thermodynamic data available. With a view to extend the existing model for viscosities to F--containing slags, the viscosities of mould flux slags for continues casting in steel production have been investigated in the present work. The measurements were carried out utilizing the rotating cylinder method. Seven mould fluxes used in the Swedish steel industry and the impact of Al2O3 pick up by mould flux slags on viscosities were included in the study. The results showed that even relatively small additions of Al2O3 are related with a significant increase in viscosity / QC 20101123

The current demands on Swedish steel industry to produce low quantity batches of specialized products requires research on steel casting processes. There are several physical processes that need be taken into account for this problem to be viewed in full light such as thermal-processes, solidification and fluid dynamics. This work focuses on the fluid-dynamics part; more specifically, the dependence of flow quality within the caster on nozzle and mould geometry. The simulations are carried out using a scale-resolving method, in specific LES (Large Eddy Simulation) which is coupled with a DPM (Discrete Phase Model) to model Argon behaviour. The results of these simulations are presented and validated against physical experiment and data from industrial trials. Conclusions are drawn regarding optimal nozzle types in respect to different mould geometries. The mould eigenfrequencies are shown to exhibit a connection with the casting velocity. This results in so called sweet spots in casting velocity where flow irregularities due to sloshing is minimal. It is shown that the mountain type nozzle is preferable for smaller geometries whilst comparatively larger geometries benefit from a cup type. / FLOWFLEX CC

In this study, the goal is to optimize the process and to reduce the clogging tendency during the continuous casting process. The focus is on clogging when the refractory base material (RBM) in the SEN is in contact with the liquid steel. It is difficult or impossible to avoid non-metallic inclusions in the liquid steel, but by a selection of a good RBM in the SEN clogging can be reduced.   Different process steps were evaluated during the casting process in order to reduce the clogging tendency. First, the preheating of the SEN was studied. The results showed that the SEN can be decarburized during the preheating process. In addition, decarburization of SEN causes a larger risk for clogging. Two types of plasma coatings were implemented to protect the RBM, to prevent reactions with the RBM, and to reduce the clogging tendency. Calcium titanate (CaTiO3) mixed with yttria stabilized zirconia (YSZ) plasma coatings were tested in laboratory and pilot plant trials, for casting of aluminium-killed low-carbon steels. For casting of cerium alloyed stainless steels, YSZ plasma coatings were tested in laboratory, pilot plant and industrial trials. The results showed that the clogging tendency was reduced when implementing both coating materials.   It is also of importance to produce clean steel in order to reduce clogging. Therefore, the steel cleanliness in the tundish was studied experimentally. The result showed that inclusions originated from the slag, deoxidation products and tundish refractory and that they were present in the tundish as well as in the final steel product. / <p>QC 20170227</p> / VINNOVA

O presente trabalho tem como objetivo explorar o uso de técnicas de controle avançados na indústria siderúrgica. Propõe-se uma estratégia de controle do nível do aço da piscina formada entre os rolos de um sistema lingotamento contínuo de tiras (LCT) utilizando a tecnologia twin-roll (rolos duplos). O processo LCT rolos duplos tem por finalidade a produção de tiras solidificadas de espessura constante sob uma força de separação entre os rolos também constante. O nível de aço bem como a força de separação são as variáveis mais críticas para a produção de tiras de aço de alta qualidade. O nível pode ser controlado usando a entrada de aço ou a velocidade de laminação. Entretanto, a velocidade de laminação é usualmente utilizada para regular a força de separação entre os rolos. A estratégia de controle proposta inclui a incorporação de um tundish intermediário submerso na piscina. O controle do nível é então feito a partir da saída de aço do tundish intermediário. Consideramos as técnicas de controle linearizante por realimentação de estado e de controle fuzzy usando ambos os modelos Takagi-Sugeno (T-S) e Mamdani. Resultados de simulação são apresentados para uma planta instalada no Instituto de Pesquisa Tecnológica (IPT) de São Paulo, divisão de metalurgia (DIMET). / The aim of this work is to explore the use of advanced control techniques in the metallurgical industry. A control strategy to regulate the molten steellevel of a strip-casting process is proposed. The process produces a solidified strip of constant thickness given by the roll gap under a constant roll separation force. Along with the molten steel level the rool separation force are the most criticaI process variables. The molten steel level may be controlled using the tundish output flow or the casting speed. However, the casting speed is usually used to control the roll force separation. In the control strategy proposed it is incorporated an intermediary tundish submerse into the pool between the rotating rolls to improve the strip thickness uniformity. The molten steel level is thus controlled by the intermediary tundish output flow. Conventional PI, feedback linearizing plus a fuzzy control term and a fuzzy controller in a cascade configuration are considered. Simulation results are presented considering the real system parameters of a plant installed at the Instituto de Pesquisa Tecnológica (IPT) de São Paulo, Divisão de Metalurgia (DIMET).