Le second chapitre introduit les différents couplages présents dans une analyse mécanique thermique métallurgique. Il rappelle ensuite les notions de métallurgie nécessaires à la compréhension de ce travail. Le troisième chapitre décrit la modélisation adoptée pour l'analyse thermique métallurgique non couplée au problème mécanique. Le modèle choisi est celui de FERNANDES (CP30). Le code développé par ce dernier était limité à l'étude de cylindres infiniment longs, c'est-à-dire à un problème unidimensionnel. L'intégration de ce modèle, dans le code LAGAMINE apporte une grande liberté quant aux formes géométriques traitées. Actuellement, les cas bidimensionnels (état plan ou axisymétrique) peuvent être traités, une extension aux cas tridimensionnels ne pose pas de problème si ce n'est celui des temps de calcul. Du point de vue théorique, nous avons proposé une formulation différentielle de la modélisation des transformations par diffusion. Cette approche, indépendante de celle de SJOSTROM (CP38) a permis de vérifier cette dernière par une voie différente. En outre, la comparaison de cette approche avec celle plus classique de FERNANDES constitue un élément nouveau intéressant. Pour terminer, des tests de validation des développements correspondants dans le code LAGAMINE sont présentés. Le chapitre quatre est consacré aux problèmes numériques à résoudre pour réaliser une simulation de refroidissement d'une pièce métallique. Une bibliographie des schémas d'intégration temporelle des problèmes thermiques linéaires et non linéaires est présentée. A ce niveau, nous ne pouvons parler de contribution originale puisqu'il s'agit de schémas devenus classiques. Cependant, il est assez rare de trouver un résumé de ces notions souvent dispersées dans de nombreux articles. A ce titre, nous considérons ce point comme un apport intéressant de cette thèse. La prise en compte des changements de phase a occasionné un grand nombre de recherches. Après avoir fait le point sur les méthodes proposées par la littérature, nous décrivons les développements introduits dans le code LAGAMINE. Le calcul des flux et de la matrice tangente est détaillé. Ce dernier point constitue un apport personnel original, surtout au niveau de la contribution des transformations de phase à la matrice tangente. Le chapitre cinq est consacré aux interactions existant entre les phénomènes mécaniques, thermiques et métallurgiques. Une présentation approfondie de chaque couplage est réalisée et la notion de plasticité de transformation est introduite. Notre contribution personnelle et originale réside dans :  une loi élasto-plastique thermique couplée pour un matériau monophasique,  une loi élasto-plastique thermique métallurgique pour un matériau multiphasique. L'intégration des lois élasto-plastiques thermiques, métallurgiques ou non, dans le code LA GAMINE constitue le chapitre six. Les points suivants constituent un apport original de cette thèse:  schéma d'intégration temporelle de base pour ces lois couplées,  apport des sous-intervalles d'intégration,  étude de la matrice tangente couplée d'un élément isoparamétrique à 8 nuds,  amélioration du schéma d'intégration temporelle de base pour assurer la stabilité en cas de transformation martensitique,  développement de la notion d'état plan généralisé pour l'étude du refroidissement d'une poutrelle métallique. Le chapitre sept présente le problème du remaillage. Après une description succincte des solutions proposées par la littérature, nous justifions notre choix d'un remaillage avec maillage indépendant. Ce dernier semble en effet le mieux adapté aux problèmes de forgeage et d'estampage que nous désirons résoudre. Le chapitre huit est consacré à la détermination de l'instant opportun pour réaliser un remaillage. Nous distinguerons deux types de critères: a. les critères avec estimation de l'erreur, développés dans le cadre de l'optimisation des maillages d'éléments finis lors d'une analyse linéaire, ou adaptés aux problèmes non linéaires de formage des métaux, b. les critères sans estimation de l'erreur, basés sur des mesures de distorsion des éléments, les valeurs propres, les mesures des interpénétrations de la pièce forgée et de la matrice. Concernant les critères du type a, notre contribution personnelle est la suivante:  étude bibliographique traduisant dans un vocabulaire d'ingénieur, les développements mathématiques souvent présents lors des études d'estimateurs d'erreur,  adaptation du critère proposé par ZIENKIEWICZ aux lois élastoviscoplastiques présentes dans LAGAMINE et essai d'une variante basée sur une idée personnelle. Concernant les critères de type b, nous proposons des mesures de distorsions originales pour un élément isoparamétrique plan à 8 nuds et nous présentons une analyse des valeurs propres de la matrice tangente. Pour terminer, nous présentons une comparaison des critères de type a et b programmés dans LAGAMINE. A notre connaissance, une telle comparaison n'avait encore jamais été réalisée. Elle nous permet de vérifier la concordance des deux approches. Les avantages et inconvénients de chacune de ces méthodes sont établis. Ils permettent de réaliser un choix des critères à conserver dans l'optique d'un remaillage automatique. Le chapitre neuf présente succinctement la création du nouveau maillage. Les exigences relatives à un mailleur automatique adapté au problème de remaillage sont précisées. Une première approche fut réalisée par nos soins grâce à un programme interactif REMDATA. Les développements originaux de ce programme sont détaillés en annexe 3. Le chapitre dix traite le transfert des informations de l'ancien maillage au nouveau maillage. A ce niveau, deux types d'informations sont considérés: a. les informations relatives aux éléments de solide, b. les informations relatives aux éléments modélisant le contact Le transfert des informations de type a est une opération souvent traitée par la littérature. Après une revue bibliographique des méthodes proposées, nous présentons une méthode originale et personnelle de transfert. Ses qualités principales sont sa précision, sa facilité de mise en uvre et sa souplesse d'utilisation. Une comparaison de cette méthode avec deux autres méthodes basées respectivement sur les moindres carrés et sur l'extrapolation nodale des valeurs aux points d'intégration est présentée. Concernant les informations de type b, nous n'avons trouvé aucune référence qui traitait leur transfert d'un maillage à l'autre. Ce point est cependant important, comme le montrent des essais de remaillage réalisés sans interpolation de ces variables. Les deux méthodes de transfert présentées dans cette thèse montrent l'évolution de notre approche de ce problème. Toutes deux constituent des apports personnels et originaux. Le chapitre onze présente cinq exemples d'application des développements réalisés: 1. l'analyse thermique métallurgique d'une poutrelle métallique refroidie par arrosage et refroidissement naturel, 2. l'analyse thermique métallurgique mécanique d'une poutrelle métallique refroidie par arrosage et refroidissement naturel, 3. l'analyse thermique métallurgique mécanique de la trempe d'un cylindre en acier 60NCD11, 4. la simulation de l'estampage d'une aube de turbine (analyse mécanique avec remaillage), 5. la simulation de l'estampage d'un pignon de machine (analyse mécanique avec remaillage). Hormis l'application 3 qui constitue l'application du code à un cas test déjà traité par DENIS et SJOSTROM (CP31), ces exemples constituent des contributions originales. Quelques conclusions sont proposées au chapitre douze. Parmi nos contributions originales, certaines nous semblent d'une ampleur et d'un intérêt plus grands :  la loi élastoplastique thermique métallurgique et son schéma d'intégration prennent en compte les couplages existant entre les différents phénomènes. Le nombre de codes réalisant une telle analyse est assez réduit alors que des calculs réalistes de contraintes résiduelles de trempe, de soudure ne peuvent être obtenus par des analyses découplées. De plus, l'adaptation des modèles développés à d'autres traitements thermiques (revenu n.) constitue de nouvelles perspectives de recherches pour l'équipe de M.S.M.,  une présentation des estimateurs d'erreur disponibles dans le cadre de la méthode des éléments finis est une information précieuse pour les ingénieurs numériciens comme pour les industriels qui ont recours à ces calculs,  les différents avantages et inconvénients des critères de remaillage basés respectivement sur les estimateurs d'eITeur et les mesures des distorsions des éléments constituent un élément important pour ceux qui doivent faire un choix entre ces deux types de critères,  l'importance du transfert des informations relatives au contact a été mis en évidence, alors que ce sujet n'est pas abordé dans la littérature.

This thesis work concerns about the importance of the cast surfaces, surface phenomenon such as the formation of the oscillation marks and exudation and related defects including cracks and segregation that happened during the continuous casting. All of the investigated materials were collected during the plant trials while an in-depth analysis on these materials was performed at the laboratory scale with certain explanations supported by the schematic and theoretical models. The work consists on different material classes such as steels and phosphor bronzes with a focus on the surface defects and their improvements. In order to facilitate the theoretical analysis which could be capable of explaining the suggested phenomenon in the thesis, a reduced model is developed which required lesser computational resources with lesser convergence problems. / <p>QC 20160527</p> / Oscilation mark formation during continous casting of steel

Non-metallic inclusions in molten steel have received worldwide attention due to their serious influence on both the steel product quality and the steel production process. These inclusions may come from the de-oxidation process, the re-oxidation by air and/or slag due to an entrainment during steel transfer, and so on. The presence of some inclusion types can cause a termination of a casting process by clogging a nozzle. Thus, a good knowledge of the inclusion behavior and deposition rate in steel flows is really important to understand phenomena such as nozzle clogging. In this thesis, inclusion behaviors and deposition rates in steel flows were investigated by using mathematical simulations and validation by experiments. A ladle teeming process was simulated and Ce2O3 inclusion behavior during a teeming stage was studied. A Lagrangian method was used to track the inclusions in a steel flow and to compare the behaviors of inclusions of different sizes. In addition, a statistical analysis was conducted by the use of a stochastic turbulence model to investigate the behaviors of different-sized inclusions in different nozzle regions. The results show that inclusions with a diameter smaller than 20 μm were found to have similar trajectories and velocity distributions in the nozzle. The inertia force and buoyancy force were found to play an important role for the behavior of large-size inclusions or clusters. The statistical analysis results indicate that the region close to the connection region of the straight pipe and the expanding part of the nozzle seems to be very sensitive for an inclusion deposition. In order to know the deposition rate of non-metallic inclusions, an improved Eulerian particle deposition model was developed and subsequently used to predict the deposition rate of inclusions. It accounts for the differences in properties between air and liquid metals and considers Brownian and turbulent diffusion, turbophoresis and thermophoresis as transport mechanisms. A CFD model was firstly built up to obtain the friction velocity caused by a fluid flow. Then, the friction velocity was put into the deposition model to calculate the deposition rate. For  the  case  of  inclusion/particle  deposition  in  vertical  steel  flows,  effects  on  the deposition rate of parameters such as steel flow rate, particle diameter, particle density, wall roughness and temperature gradient near a wall were investigated. The results show that the steel flow rate/friction velocity has a very important influence on the rate of the deposition of large particles, for which turbophoresis is the main deposition mechanism. For small particles, both the wall roughness and thermophoresis have a significant influence on the particle deposition rate. The extended Eulerian model was thereafter used to predict the inclusion deposition rate in a submerged entry nozzle (SEN). Deposition rates of different-size inclusions in the SEN were obtained. The result shows that the steel flow is non-uniform in the SEN of the tundish. This leads to an uneven distribution of the inclusion deposition rates at different locations of the inner wall of the SEN. A large deposition rate was found to occur at the regions near the SEN inlet, the SEN bottom and the upper region of two SEN ports. For the case of an inclusion/particle deposition in horizontal straight channel flows, the deposition rates of particles at different locations of a horizontal straight pipe cross- section were found to be different due to the influence of gravity and buoyancy. For small particles with a small particle relaxation time, the gravity separation is important for their deposition  behaviors  at  high  and  low  parts  of  the  horizontal  pipe  compared  to  the turbophoresis. For large particles with a large particle relaxation time, turbophoresis is the dominating deposition mechanism. / <p>QC 20150326</p>

Abstract This thesis studies the factors that govern transverse cracking during continuous casting of low carbon, niobium microalloyed and boron microalloyed steels. Crack susceptibility in the thick slab, billet and thin slab casting processes are compared by using typical conditions in laboratory hot ductility tests. There is limited published literature on hot ductility in aluminium-killed and siliconkilled boron microalloyed steels and the proposed mechanisms of failure by transverse cracking are contradictory. Few published papers specifically compare hot ductility behaviour of any steels between thick slab, billet and thin slab continuous casting processes. Thus, the basis of this research is to assess the influence of casting parameters and compositional variations on hot ductility behaviour in low carbon steels, niobium microalloyed steels, aluminium-killed boron microalloyed steels and silicon-killed, boron microalloyed steels. The typical temperature ranges, cooling rate and strain rate conditions of the continuous casting processes were used in reheated and in situ melted hot tensile tests performed on steel specimens. Solidification, transformation and precipitation temperatures were calculated using solubility equations and modelled using the Thermo-CalcTM thermodynamics program. Scanning electron microscopy and transmission electron microscopy were used to determine the modes of failure in the tested specimens. In the low carbon steels, hot ductility was improved by increasing the strain rate; by calcium treatment, which minimises copper sulphide and iron sulphide formation; and by maintaining a nickel to copper ratio of 1:1. It was shown that thin slab casting conditions provided the best hot ductility results for the low carbon steels. All the niobium steels showed poor ductility in the single-phase austenite temperature region, indicating that intergranular precipitation of fine niobium carbonitrides was the cause of the poor ductility. It was shown that the hot ductility was greatly improved by calcium treatment, by decreasing the cooling rate and by increasing the strain rate. Slow iv thin slab and thick slab casting conditions provided the best hot ductility results for the niobium steels. Hot ductility was substantially improved in the aluminium-killed boron steels by increasing the boron to nitrogen ratio from 0.19 to 0.75. The results showed that, at cooling rates generally associated with thick slab, bloom and slow thin slab casting, a boron to nitrogen ratio of ≥0.47 was sufficient to avoid a ductility trough altogether. However, under conditions typically experienced in fast thin slab and billet casting, a boron to nitrogen ratio of 0.75 was required to provide good hot ductility. The mechanism of the ductility improvement with increasing boron to nitrogen ratio was found to be enhanced precipitation of boron nitride, leading to a decrease in nitrogen available for aluminium nitride precipitation. In the silicon-killed boron steels, it was found that the boron to nitrogen ratio had the overriding influence on hot ductility and hence on crack susceptibility. Excellent hot ductility was found for boron to nitrogen ratios above 1. Additionally, analysis of industrial casting data showed that the scrap percentage due to transverse cracking increased significantly at manganese to sulphur ratios below fourteen. An exponential decay relationship between the manganese to sulphur ratio and the average scrap percentage due to transverse cracking was determined as a tool to predict scrap levels in the casting plant.

O fluxante é uma escória sintética que influencia na qualidade superficial do aço e na estabilidade do processo de lingotamento contínuo. Este produto é aplicado diretamente sobre o aço líquido na região do molde de cobre refrigerado a água e atua diretamente no resfriamento primário do aço. O fluxante tem as propriedades físico-químicas adaptadas para cada tipo de aço e também para as condições de lingotamento. Na superfície do aço líquido, o fluxante funde e forma uma poça líquida, atuando como isolante térmico, protegendo o aço da reoxidação e absorvendo inclusões principalmente de Al2O3. A poça líquida escoa, lubrificando e controlando a transferência de calor na interface entre o molde e a pele de aço em solidificação. O problema de qualidade superficial do aço, quando relacionado ao fluxante, se resume a alarmes de colamento, trincas de quina, marcas de oscilação profundas e trincas longitudinais, sendo este último um problema particular do aço médio carbono. Neste trabalho, foram analisados diferentes fluxantes baseados inicialmente no fluxante comercial aplicado no lingotamento contínuo de placas de aço médio carbono. Todos os fluxantes foram desenvolvidos com composições químicas similares. O objetivo foi avaliar o impacto da substituição de fontes de matérias-primas em diferentes composições de modo a avaliar as propriedades físico-químicas com base no fluxante comercial de referência. Como resultado, de todas as propriedades físico-químicas, foi a temperatura de cristalização que sofreu a alteração mais significativa. / The mould flux is a synthetic slag which influences the quality of steel surface and also the stability of the continuous casting process. This product is applied in direct contact with the surface of liquid steel in the region of the water cooled cupper mold and acts directly on the primary cooling of steel. The mould flux has the physical and chemical properties adapted for each type of steel as weel as casting conditions. On the surface of liquid steel, the mould flux melts and forms the liquid pool, working as an insulator, protecting the steel against reoxidation and also absorbing mainly Al2O3 inclusions. The liquid pool flows, lubricating and controlling the heat transfer in the interface between the mold and the steel shell during solidification. The surface quality issues when related with mould flux can cause alarms of breakout, corner cracks, deep oscillation marks, longitudinal cracks that is a particular problem in medium carbon steel grade. In this study, different mould fluxes were investigated and compared to a commercial mould flux applied to the continuous casting of slabs in the medium carbon steel grade. All mould fluxes were developed with a similar chemical composition. The main goal was to evaluate the impact of changing raw material sources on the physicochemical properties of the different mould fluxes and the results was compared with a commercial standard mould flux. As result, variations in the crystallization temperatures were observed more than others properties analyzed.

The most common way of producing steel worldwide is with the continuous casting process, where a strand of steel is continuously cast. Further optimization of this process requires data from measurements, measurements which must take place in a hostile high-temperature and corrosive environment such as liquid steel. At Swerim, a Continuous Casting Simulator has been constructed to replicate the flow conditions in the process. A eutectic bismuth-tin alloy with properties similar to steel is used instead to make measurements in liquid metal possible. In this report pressure, velocity and vibration measurements were made using multiple sensors under different flow conditions.

An in-depth study has been carried out to investigate refractory degradation during continuous steel casting processes. Slag/refractory interactions have been investigated through a study of the wetting behaviour and gas generation phenomena at the slag/refractory interface at 1550oC using sessile drop experiments. The influence of carbon content and slag composition was investigated. Two different steel casting slags (slag1: MnO 50%, SiO2 25%, Al2O3 25%; and slag2: MnO 40%, SiO2 60%) have been studied along with yttria stabilized zirconia refractory substrates respectively containing 10%, 15% and 20% carbon. The gas generation has been measured using the Infrared gas analyser, and its influence has also been investigated by taking optical images at varying times. The total amount of gases emitted has shown the dependence on the compositions of refractory substrates and the existence of casting slags. Under the investigation of zirconia-carbon refractory, a new phenomenon that the gas generated can push the liquid slags away and minimise the contact of slags and refractory substrates has been proven.

The thesis deals with a numerical model based on the finite element method applied to the continuous casting process. This model has been developed and implemented in the LAGAMINE finite element code and two industrial applications of steel casting are presented to illustrate the possibilities of the model. The first chapter of the thesis introduces the continuous casting process from the industrial point of view, allowing the definition of the essential terminology and the industrial challenges. Then, the objectives of the model are detailed: the model is applied to the upper part of the caster (in the mould region) as well as the bending and straightening zones. Chapters two, three and four are dedicated to the theoretical developments of thermal aspects (phase transformations, thermal contraction, heat flow and thermal boundary conditions), mechanical aspects (generalized plain strain state, material constitutive law, ferrostatic pressure and mechanical contact) and the staggered analysis for thermal / mechanical coupling. In the two next chapters, some numerical results are shown. The model has been first applied to a 125mm square billet in the mould region. The industrial goal was to evaluate the effect of the mould taper on the cooling of the billet (temperature evolution, solid shell growing, stress and strain development). The second application focuses on the risk of transverse cracking during straightening of steel slabs and the effect of some local defects (reduction in secondary cooling, rolls locking or misalignment). The thesis ends with some conclusions and perspectives for the model.

Conventional one-point turbulence closures have beenextended with an additional transported scalar for modeling ofmagnetohydrodynamic (MHD) turbulence. The new scalar, α ,captures the length scale anisotropy and tendency towardstwo-dimensionality, which is characteristic feature of MHDturbulence, and allows accurate modeling of the Jouledissipation of turbulence. The concept has been used for both afull Reynolds stress closure, and a three-equationK-ε -αmodel. An exact transport equation forαwas derived from the governing equations. All terms inthe equation require modeling, however. The proposed modeltransport equation for α includes terms for magneticdissipation, nonlinear energy transfer, and effects of meanshear and strain. Modeling of the magnetic and strain-relatedterms was based on rapid distortion analysis of the linearizedequations, while modeling of nonlinear effects isphenomenological in nature. For homogeneous turbulence, themodel was compared with linear theory, direct numericalsimulations and experiments. For turbulence subjected to astrong magnetic field, the model reproduces the energy andlength scale evolution predicted by linear theory. Whennonlinear effects are of importance, it predicts energy decayand length scale evolution in agreement with experiments. Theeddy viscosity and Reynolds stress versions of the modelcoincide with the respective conventional models in the absenceof a magnetic field. The objective of this project has been todevelop efficient MHD turbulence models for engineeringapplications, especially for modeling of continuous steelcasting. The novel MHD turbulence models appear to benumerically robust, and they have been implemented in acommercial flow solver, together with electromagnetic equationsfor the Lorentz forces in the mean momentum equations. <b>Keywords:</b>Turbulence model, magnetohydrodynamics, MHD,magnetohydrodynamic turbulence, computational fluid dynamics,continuous casting, dimensionality, Reynolds stresses, eddyviscosity

The theoretical stability analysis of mold level control system for slab continuous casting machine is presented in this thesis. In the procedure of analyzing the stability of the mold level control system, the PLC program written for the control system is studied first in order to obtain the mathematical model of a PID controller. Then the mathematical models of servo-amplifier, servo-valve, electro hydraulic system to the output of mold level are established. A simulative control system using Matlab software is constructed in accordance with these mathematical models so that not only the results of stability analysis can be verified but also the dynamic response of controlled system can be studied. Finally, the effects of some potential disturbance on system¡¦s dynamics, stability, and control accuracy are also analyzed.