WE43 magnesium foils (thickness ≤ 200 μm) were successfully produced via hot rolling. The initially extruded material was heat treated at 450 °C for 2 h to achieve a more homogenous microstructure. Afterwards the sheets were hot rolled at 480 °C in two to five rolling passes to achieve a thickness less than 200 μm and finally heat treated (T5 and T6 heat treatment). Microstructural und texture evolution after foil rolling and the final heat treatment were investigated and the resulting mechanical properties were also evaluated. Therefore, the samples were quenched directly after foil rolling and the final heat treatment. The foil rolling led depending on the number of the rolling passes either to a deformation microstructure (two and three passes) or globular grains (four and five passes).

WE43 magnesium foils (thickness ≤ 200 μm) were successfully produced via hot rolling. The initially extruded material was heat treated at 450 °C for 2 h to achieve a more homogenous microstructure. Afterwards the sheets were hot rolled at 480 °C in two to five rolling passes to achieve a thickness less than 200 μm and finally heat treated (T5 and T6 heat treatment). Microstructural und texture evolution after foil rolling and the final heat treatment were investigated and the resulting mechanical properties were also evaluated. Therefore, the samples were quenched directly after foil rolling and the final heat treatment. The foil rolling led depending on the number of the rolling passes either to a deformation microstructure (two and three passes) or globular grains (four and five passes).

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A nonlinear finite element model of the hot and cold rolling processes has been developed for flat rolling stock with rectangular cross section. This model can be used to analyze the flat rolling of cold and hot steel rectangular strips under a series of different parameters, providing the rolling designer with a tool that he can use to understand the behavior of the steel as it flows through the different passes. The models developed, take into account all of the non-linearities present in the rolling problem: material, geometric, boundary, and heat transfer. A coupled thermal-mechanical analysis approach is used to account for the coupling between the mechanical and thermal phenomena resulting from the pressure-dependent thermal contact resistance between the steel slab and the steel rolls. The model predicts the equivalent stress, equivalent plastic strain, maximum strain rate, equivalent total strain, slab temperature increase, increase in roll temperature, strip length increase, slab thickness % reduction (draft), and strip's velocity increase, for both the cold and hot rolling processes. The FE model results are an improvement over the results obtained through the classical theory of rolling. The model also demonstrates the role that contact, plastic heat generation and friction generated heat plays in the rolling process. The analysis performed shows that the steel in cold rolling can be accurately modeled using the elastic-plastic (solid Prandtl-Reuss) formulation, with a von Mises yield surface, the Praguer kinematic hardening rule, and the Ramberg-Osgood hardening material model. The FE models also demonstrate that the steel in hot rolling can be modeled using the rigid-viscoplastic (flow Levy-Mises) formulation, with a von Mises yield surface, and Shida's material model for high temperature steel where the flow stress is a function of the strain, strain rate, and the temperature. Other important contributions of this work are the demonstration that in cold rolling, plane sections do not remain plane as the classic theory of rolling assumes. As a consequence, the actual displacements, velocity, and stress distributions in the workpiece are compared to and shown to be an improvement over the distributions derived from the classical theory. Finally, the stress distribution in the rolls during the cold rolling process is found, and shown to be analogous to the stress distribution of the Hertz contact problem. / Master of Science

The steel industry has been producing flat plates through the process of hot rolling since the late 1600s. Hot rolling uses a series of rolls to progressively thin a strip of steel to a desired thickness. In deforming the strip, the rolling process causes variations in thickness across the width of the strip. These variations are commonly referred to as crown, which is specifically the difference in thickness between the center and edge of a strip. For most applications steel mill clients require flat products, or products with little variation in thickness. Therefore, variations represent wasted material which must be removed before the plate or sheet can be used in consumer products. Controlling the flatness of the metal strip is a high priority for the hot rolling business. The purpose of this work was to develop a 3-D dynamic model of the rolling process to simulate the behaviour of a strip while being rolled and predict its profile. To accomplish this task, much of the rolling process needed to be modeled. The profile of the strip is a product of the deformation of the rolls and frame within a mill stand. Therefore, not only did the geometry of these components need to be modeled, but the material properties and dynamic motion were required as well. The dynamic nature of the process necessitated the modeling of the rotation of the rolls and translation of the strip, aspects of rolling which are not typically simulated. Five models were developed during the project. The purpose of the first two models was to find the stiffnesses of the roll-stack and stand frame. The roll-stack refers to the rolls and their arrangement. The reference mill from which data was provided used a four-high roll-stack with two rolls above the strip and two below. The frame that holds the roll-stack, while massive, stretches when the strip is deformed between the rolls. This stretch changes the position of the rolls affecting the load and deformation of the strip. A lumped-mass model was created to simulate the dynamics of the roll-stack and frame. When the strip enters the gap between the rolls, there is a large impact force which causes the rolls to vibrate. The lumped-mass model was used to determine parameters to bring the system to steady state. The final two models simulated the entire rolling process with rotating rolls and moving strip. The 3-D dynamic rolling model was capable of predicting the strip profile due after exiting the rolls. Two calibrations were used to reduce model error before running a validation. The rolling causes thickness variation across the width of the metal strips; therefore, strips are intentionally rolled thick to meet a minimum thickness. In modern steel mills, specialized control systems are used to adjust parameters as the steel strip passes through each stand of rolls. Varying the parameters allows the thickness and profile of the strip to be controlled. Each stand may have several rolls in different configurations. These rolls are either work rolls, which directly contact the strip, or backup rolls, which contact the work rolls and stiffen the roll-stack. The stand frame holds the rolls and provides a means to position them. The validation results showed that the exit thickness, strip crown, and rolling load were less than 5% different from the values measured in the test data. The calibrated model was then used to derive strip crown sensitivities to gap, entry crown, work roll crown, and bending force. The 3-D dynamic model was able to predict the strip crown accurately when given calibrated information about the system. This model will be a useful tool for exploring the mechanics of hot rolling in ways that were not previously possible. / Master of Science

Nine independent and four dependent variables are used to build a response surface to calculate strip crown using the difference in the industry standard strip height measurements. The single element response surface in use provides the advantages of continuous derivatives and decouples rolling load from the determination of exit height. The data points to build the response surface are the product of a calibrated finite element model. The rolling dynamics in the finite element model creates a transient that requires nonlinear regression to find the system steady-state values. Weighted-least squares is used to build a response surface using isoparametric interpolation with the non-rectangular domain of the mill stands represented as a single element. The regression statistics, the 1-D projections, comparisons against other response surface models and the comparisons against an existing strip crown model are part the validation of the response surface generated. A four-high mill stand is modeled as a quarter-symmetry 3-D finite element model with an elastic-plastic material model. A comparison of the pressure distribution under the arc of contact with existing research supports the pressure distribution found with experiments conducted by Siebel and Lueg [16] and it also suggests the need for one improvement in the initial velocity for the strip in the finite element model. The strip exit heights show more sensitivity to change than strip exit crown in seven out of the nine independent variables, so a response surface built with the strip exit height is statistically superior to using the derived dependent variable strip exit crown. Sensitivity of strip exit crown and the strip exit heights to changes in work-roll crown are about equal. Backup-roll diameter sensitivity is small enough that oversampling for the mean trend has to be considered or ignore backup-roll altogether. Strip entry velocity is a new independent variable, unless the response surface is built from the derived variable, strip exit crown. A problem found is that the sensitivity of strip entry crown and work-roll crown requires a larger than typical incremental change to get a reliable measure of the change strip exit crown. A narrow choice of high and low strip entry crowns limits the usefulness of the final response surface. A recommendation is to consider the use of the strip cross-section as an exit crown predictor. / Master of Science

Le laminage à chaud de l’acier inoxydable est une étape du procédé de fabrication agissant significativement sur la qualité de surface des produits. Les nuances ferritiques stabilisées sont largement utilisées dans les applications automobiles et de décoration mais sont concernées par le phénomène de collage à chaud. Ces nuances, ayant des résistances élevées en corrosion sèche et en fluage, sont riches en éléments tels que le Cr, le Nb ou le Ti, limitant aussi l’oxydation des brames lors du laminage. Or, l’oxydation maitrisée de la brame permet de protéger la surface du métal de tout contact direct avec les cylindres de laminage.Afin de comprendre l’initiation du collage, une première campagne d’étude sur la topographie et la dégradation de l’état de surface des cylindres a été réalisée. Il a été mis en évidence que l’initiation du collage ne serait pas du à des rayures d’usure en surface des cylindres et dont la profondeur serait supérieure à la couche d’oxyde, car la probabilité que la profondeur d’une rayure de cylindre soit plus importante que la couche d’oxyde est très faible.Ensuite, un banc d’essai reproduisant les conditions tribologiques de l’emprise du laminoir a été conçu afin de mieux appréhender les mécanismes à l’origine du collage. Etant donné l’importance de l’état de surface des brames et des cylindres, ce banc d’essai a permis de travailler sur des échantillons pris en production, permettant d’étudier la surface oxydée des tôles. Cette deuxième campagne d’étude a mis en évidence l’importance des oxydes de silicium sur l’adhérence partielle de la couche d’oxyde, ainsi que la relative hétérogénéité en épaisseur et en composition chimique. / Hot rolling of stainless steel is one of the most important steps in manufacturing process regarding surface quality of the product. Stabilised ferritic stainless steels are widely used in automotive and cosmetic appliances but are also concerned by sticking phenomenon. These grades, having high dry corrosion and creep resistance, are enriched in specific chemical elements such as Cr, Nb or Ti, limiting also slab oxidation during hot rolling. Nevertheless, the mastered oxidation of slab surface is a way to protect metal surface from direct contact with rolls.In order to better understand initiation of sticking, a first campaign was based on topography and rolls surface state wear analysis. This study revealed that sticking initiation is not due to the presence of roll scratches which depth is higher than oxide layer thickness. Indeed, the probability that roll scratches are deeper than oxide layer thickness is very low.In a second time, a pilot was designed, reproducing tribological conditions of a roll bite, to better understand mechanisms that initiate sticking. Keeping in mind the importance of rolls and slab surface state, this pilot is able to use specimen taking from industrial products, having the original oxide layer surface. This second study highlighted the major role of silicium oxides on scale adherence and the high heterogeneity of this scale layer in thickness and in chemical composition.

Este trabalho visa analisar a influência das taxas de resfriamento e temperatura no último passe de laminação a quente de fio máquina de um aço DIN 41Cr4 sobre a microestrutura obtida após a laminação e a sua correlação com aquela resultante do tratamento térmico de esferoidização. Foi avaliado a influência da microestrutura prévia de laminação nos tempos e taxa de resfriamento do tratamento térmico necessários para garantir adequada microestrutura do aço DIN 41Cr4. Foram aplicados conceitos de laminação termomecânica através da variação dos parâmetros: temperatura no último passe de laminação (acabador) e após o último passe de laminação (formador de espiras); velocidade das esteiras do “cooling conveyor” e consequente tempo de permanência das espiras sobre a região de ventiladores e alguns parâmetros no ciclo de tratamento térmico: tempo em patamar e taxa de resfriamento durante a produção de fio máquina redondo de bitolas entre 15,88mm e 16,15mm. A avaliação da resposta às variações impostas ao processo foi realizada através de análises metalográficas de microestrutura no material laminado e no material esferoidizado (grau de esferoidização), descarbonetação e de ensaios de propriedades mecânicas como dureza e tração. A partir dos resultados buscou-se determinar a influência dos parâmetros de laminação na microestrutura do material laminado e a influência desta na resposta ao tratamento térmico de esferoidização. Foram analisadas oito diferentes condições e os resultados mostraram que maiores taxas de resfriamento resultam em microestruturas mais refinadas proporcionando uma simplificação no tratamento térmico, atendendo as propriedades desejáveis no produto final (fio máquina redondo esferoidizado). / This work aims to analyze the influence of the cooling rate, the temperature of the finishing pass and at the coil-forming during hot rolling of DIN 41Cr4 steel wire rod on the microstructure and its influence on plateau times and cooling rate during the spheroidizing heat treatment. For this study thermomechanical rolling concepts were applied through the variation of some hot rolling parameters such as: temperature at the finishing pass and at the coils-former, the cooling conveyor belts velocity and some parameters in the heat treatment cycle: soak temperature and cooling rate during the production of round wire rod with 15,88 mm and 16,15 mm diameter. The evaluation response to changes imposed on the process was evaluated by metallographic analysis of microstructure in hot rolled and spheroidized material (spheroidizing degree), hardness, decarburization and tensile test. Eight rolling conditions and cooling rates were analyzed and results showed that higher cooling rates result in finer microstructures allowing a reduction in heat treatment time to achieve the desirable properties in the final product (spheroidized round wire rod).

The thesis is focussed on improving the material yield in closed-die forging and rolling. The former is restricted to the manufacturing of heavy crown wheels and front axle beams and the latter to the hot rolling of steel slabs. To enhance the yield the commercial FE-codes Form2D and Dyna3D are used. Results from forging simulations are strengthened by full scale experiments. The research is carried out in near contact with the Swedish steel and engineering industry. In closed die forging, two bulk forming problems are treated: How to improve the tool design and how to change the pre-form geometry for decreasing the amount of material exiting the flash gap? In slab rolling, two problems related to material defects are considered: How to eliminate existing surface cracks and how to prevent the formation of voids around macro-inclusions embedded in the steel matrix? Internal voids might be the reason for scrapping the whole workpiece. Considering the forging of crown wheels, a new concept is proposed. For making the central hole of the product, the traditional method was forging a thin circular plate which was then sheared off and scrapped. Using the new technique this operation is replaced by forging a conical tap in the centre, which is then discarded. Doing so, the inner scrap material decreased with about 15%. The idea has been used in production for seven years. – Regarding the forging of front axle beams, a quasi-3D method is used comprising full scale measurements of the axial material flow. Here the theoretical material yield increased 2-7%. Regarding rolling, the initial surface cracks in the simulations are V-shaped with a crack angle of 6o and of depth 5-20 mm. The inclusions are cylindrical and either three times harder or three times softer than the matrix. The behaviour of the cracks and the inclusions are studied as influenced by process parameters. Current industrial input data are used as a reference. – It is concluded that longitudinal cracks cannot be totally eliminated. During rolling their V-shape gradually changes to Y-shape and a remaining oxide flake separates the crack bottom surfaces. For minimizing the detrimental influence of the entrapped oxide, an early closure of the crack bottom is important. Throughout the remaining rolling schedule the entrapped oxide is then torn to pieces with large areas of virgin metal in between ensuring a strong bond. The following recommendations are given for the longitudinal cracks: Light drafts/pass at the beginning of the rolling schedule followed by heavy ones. – Contradictory to longitudinal cracks it is concluded that transversal cracks are possible to eliminate. When eliminated, the initial bottom of the crack coincides with the slab surface. No folds are formed. For the transversal cracks light drafts/pass are proposed together with reversal rolling, the latter for avoiding crack folding. – Voids are easiest formed around hard macro-inclusions in the centre of the slab. Large rolls and heavy drafts are recommended to avoid this. / QC 20100720

ROCHA FILHO, F. A. M. Estudo da perda de ductilidade a quente e resistência à corrosão de vergalhões laminados com e sem a utilização da etapa de reaquecimento. 2017. 97 f. Dissertação (Mestrado em Engenharia e Ciência de Materiais)-Centro de Tecnologia, Universidade Federal do Ceará, Fortaleza, 2017. / Submitted by Pós-Graduação Ciência de Materiais (materiais@metalmat.ufc.br) on 2017-10-02T16:47:29Z No. of bitstreams: 1 2017_dis_famrochafilho.pdf: 4083178 bytes, checksum: da1e3baa97d72a6d875bf5f3a505a3bf (MD5) / Rejected by Marlene Sousa (mmarlene@ufc.br), reason: Prezado Alberto: Existe uma orientação para que normalizemos as dissertações e teses da UFC, em suas paginas pré-textuais e lista de referencias, pelas regras da ABNT. Por esse motivo, sugerimos consultar o modelo de template, para ajudá-lo nesta tarefa, disponível em: http://www.biblioteca.ufc.br/educacao-de-usuarios/templates/ Vamos agora as correções sempre de acordo com o template: 1. 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No. of bitstreams: 1 2017_dis_famrochafilho.pdf: 4081151 bytes, checksum: c897beceaac55281ae4207765bcca1e8 (MD5) Previous issue date: 2017-08-09 / Hot rolling is one of the main forming processes where the semi-finished product obtained in the steel plant, is deformed plastically by compression forces applied between rotating cylinders. During rolling process the hot shortness phenomenon may occur and this may cause of split ends of bar during the sequence of cross section reduction passes. This split ends of bar can be severe and cause disruptions in the production process. The hot shortness may be associated with several factors, such as: the chemical composition of steel; the defects present in the semi-finished product, in the case of billet, produced in the process of continuous casting; the steel temperature during the process; and the conditions of rolling process, where it is worth mentioning the deformation rates, reductions, cylinder diameters, type of rolling stand and pass design. In the process under study, hot shortness generally occurs associated with the production of low carbon steel bars which are rolled from billets with as cast structure, which do not go through the reheating process in natural gas furnaces. Since there is great difficulty in inspecting semi-finished products directly from the continuous casting process, and there are limitations related to modification of rolling process conditions, the objective is to evaluate the influence of billet temperature along its section and the microstructure formed prior to the rolling process, mainly related to grain size and the presence of sulphides. These factors were associated with the hot shotness phenomenon of the steel, through a comparative examination between steels rolled directly from continuous casting and steels that pass through the reheating furnace before being rolled. It was possible to observe that the directly rolled billets had a smaller grain size prior to rolling and higher temperature loss throughout the process, which factors associated with a low Mn / S ratio and high reductions and deformation rates favored the hot shortness of bar during the rolling process. Both production routes provide small microstructural differences in the final product, which did not cause significant differences in the properties of corrosion resistance and mechanical properties of rebar. / A laminação a quente é um dos principais processos de conformação onde o produto semi-acabado obtido na aciaria, é deformado plasticamente através de esforços de compressão aplicados entre cilindros rotativos. Durante o processo de laminação pode ocorrer o fenômeno da perda de ductilidade a quente e este pode causar a separação entre as extremidades da barra durante a sequência de passes de redução da seção transversal do laminado. Essa separação da barra pode ser severa e causar interrupções no processo produtivo. A perda de ductilidade pode estar associada a diversos fatores entre eles estão: a composição química do aço; os defeitos presentes no semi-acabado, no caso tarugo, produzido no processo de lingotamento contínuo; a temperatura do aço durante o processo; e as condições do processo de laminação, onde merece destaque as taxas de deformação, as reduções, diâmetros dos cilindros, tipo de cadeira de laminação e projeto de passes. No processo em estudo, a perda de ductilidade a quente geralmente ocorre associada à produção de barras de aço baixo carbono que são laminadas a partir de tarugos com estrutura bruta de fusão, não passando pelo processo de reaquecimento em fornos a gás natural. Visto que há grande dificuldade de inspeção nos produtos semi-acabados vindos diretamente do processo de lingotamento contínuo, e que há limitações quanto à modificação nas condições do processo de laminação, tem-se como objetivo avaliar a influência da temperatura do tarugo ao longo de sua seção e da microestrutura formada prévia ao processo de laminação, relacionada principalmente com o tamanho de grão e a presença de sulfetos. Esses fatores foram associados com fenômeno de perda de ductilidade a quente do aço, através de um exame comparativo entre os aços laminados diretamente a partir do lingotamento contínuo e os aços que passam pelo forno de reaquecimento antes de serem laminados. Foi possível observar que os tarugos laminados diretamente possuíam menor tamanho de grão prévio a laminação e maior perda de temperatura ao longo do processo, fatores esses que associados a uma baixa relação Mn/S e elevadas reduções e taxas de deformação, favoreceram a perda ductilidade a quente da barra durante o processo de laminação. Ambas as rotas de produção proporcionam pequenas diferenças microestruturais no produto final, que não causaram diferenças significativas nas propriedades de resistência à corrosão e nas propriedades mecânicas dos vergalhões.