No description available.

Recovery, recrystallization, grain growth and precipitation constitute the fundamentals of thermomechanical controlled processing (TMCP) of microalloyed steels. In-depth understanding of these phenomena is indeed needed. In this work, the individual components and some of the potential mutual interactions have been investigated deliberately. The effect of alloying elements of Mn, Si, and Al on recovery and recrystallization has been systematically studied by conducting the stress relaxation tests on binary Fe-0.1%C and ternary Fe-0.1%C-X alloys. The effect of temperature on recovery kinetics was also investigated. The effects were considered by fitting the recovery model through the activation volume term. Higher temperature or lower solute content will accelerate the recovery process and then facilitate the onset of recrystallization. NbC precipitation behavior has been investigated using a nickel-based model alloy, having samples deformed at both room temperature and elevated temperature and subjected to annealing at 700℃ for different times, in order to elucidate the stages of nucleation, growth and coarsening for precipitation. The microstructures preserved by water quenching were examined using transmission electron microscopy (with both metal foil and carbon replica specimens). Results from mechanical response and microstructural evolution are linked and discussed. The precipitate number density and size evolution show good agreements with predictions from a classical strain-induced precipitation model. The in-situ laser-ultrasonics measurement of C-Mn steels provides a unique way to evaluate grain size evolution during TMCP, for different strains of 0.15, 0.25 and 0.35, at 950℃ and 1050℃. Effects of temperature and strain on recovery, recrystallization and grain growth have been covered and elucidated. Higher strains facilitate the onset of recrystallization and grain size refinement. However, higher temperatures only shorten the onset of recrystallization but lead to larger grain size. The effect of microalloying element of Nb on softening kinetics was also investigated by comparing C-Mn/C-Mn-Nb steels at the same conditions. The solute drag effect of Nb can be seen by the onset-delays of recrystallization and larger grain sizes. The laser-ultrasonics results can match well with stress relaxation measurements. The in-situ grain size evolution data has given the possibility to develop robust thermomechanical processing (TMP) models combining deformation, recovery, precipitation, recrystallization and grain growth. The application and validation of the TMP models have been attempted and remain ongoing. / Thesis / Doctor of Philosophy (PhD)

Microalloyed steels are widely used in oil and gas pipelines. They are a class of high strength, low carbon steels containing small additions (in amounts less than 0.1 wt%) of Nb, Ti and/or V. The steels may contain other alloying elements, such as Mo, in amounts exceeding 0.1wt%. Microalloyed steels have good strength, good toughness and excellent weldability, which are attributed in part to the presence of precipitates, especially nano-precipitates with sizes less than 10nm. Nano-precipitates have an important strengthening contribution, i.e. precipitation strengthening. In order to fully understand steel strengthening mechanisms, it is necessary to determine the precipitation strengthening contribution. Because of the fine sizes and low volume fraction, conventional microscopic methods are not satisfactory for quantifying the nano-precipitates. Matrix dissolution is a promising alternative to extract the precipitates for quantification. Relatively large volumes of material can be analyzed, so that statistically significant quantities of precipitates of different sizes are collected. In this thesis, the microstructure features of a series of microalloyed steels are characterized using optical microscopy (OM) and scanning electron microscopy (SEM). Matrix dissolution techniques have been developed to extract the precipitates from the above microalloyed steels. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) are combined to analyze the chemical speciation of these precipitates. Rietveld refinement of the XRD pattern is used to fully quantify the relative amounts of the precipitates. The size distribution of the nano-precipitates (mostly 10 nm) is quantified using dark field imaging (DF) in the TEM. The effects of steel chemistry and processing parameters on grain microstructure and the amount of nano-precipitates are discussed. Individual strengthening contributions due to grain size effect, solid solution strengthening and precipitation strengthening are quantified to fully understand the strengthening mechanisms of the steels. / Materials Engineering

No description available.

<p> The advantages of Thin-Slab Cast Direct-Rolling (TSCDR) process include reduced capital, energy, labour and inventory costs, as well as the ability to roll thinner strip compared to the conventional process of thick slab casting, reheating and hot rolling. There is great interest in utilizing this technology to produce microalloyed steels which can meet American Petroleum Institute (API) standards. However, whereas the conventional approach can produce APIX80, APIXlOO, and even APIX120 steels; the TSCDR process can only produce APIX70 and APIX80. The main obstacles in the way of achieving high API grades are the non-uniform initial as-cast microstructure and the large grains that result from grain growth at high temperature. The production of APIX80 and higher grade steels can only be achieved through a comprehensive research initiative that combines careful control of solidification, homogenization, thermomechanical-processing, cooling and coiling. </p> <p> This contribution examines the solid state microstructure evolution of microalloyed steels under simulated TSCDR conditions. The grain growth kinetics in delta-ferrite and austenite were studied separately using two model alloys. At high temperatures and in the absence of precipitation, the growth kinetics in both delta-ferrite and austenite appeared to follow a simple parabolic growth law. The measured grain growth kinetics was then applied to the problem of grain-size control during the process of TSCDR. Several strategies of controlling and refining the grain size were examined. The kinetics of delta-ferrite to austenite phase transformation was investigated using a quenching dilatometer; the results showed that the austenite phase formed along the original delta grain boundaries, and that the precipitation of austenite at the delta-ferrite grain boundaries effectively pins delta grain growth. The kinetics of the phase transformation was modeled using a local equilibrium model that captures the partitioning of the substitutional elements during the transformation. </p> <p> A novel delta-ferrite/austenite duplex microstructure is proposed to achieve fine and uniform high-temperature microstructure. The grain growth of the matrix phase (delta-ferrite) is controlled by the coarsening mechanism of pinning phase (austenite). The effectiveness of this delta/austenite duplex microstructure was validated experimentally and analyzed in details using a physically-based model. </p> / Thesis / Doctor of Philosophy (PhD)

A elevada competitividade no mundo global de hoje em dia traz à tona diversos desafios à indústria metal-mecânica. Os dois maiores pilares nesta busca pela excelência são os custos e a sustentabilidade, já que a qualidade do produto é competência intrínseca no século XXI. Os aços microligados ganham importância neste cenário, pois sua utilização, geralmente, permite a supressão ou ao menos redução de alguns processos produtivos, o que se traduz em diminuição de custos e, muitas vezes, redução nos aspectos e impactos ambientais. Neste trabalho, buscou-se desenvolver uma composição química e um processo produtivo adequado para parafusos da classe 8.8 que, utilizando-se dos diversos mecanismos de endurecimento dos aços, fosse capaz de eliminar etapas de tratamentos térmicos ao longo do processo. O trabalho contou com o vazamento de uma corrida de um aço microligado ao nióbio, sua posterior conformação mecânica através da laminação, onde vários parâmetros foram avaliados na busca das melhores propriedades mecânicas, o processo de trefilação e conformação a frio dos parafusos e, por fim, o único tratamento térmico da cadeia: o envelhecimento. No decorrer de todas as etapas o material esteve sob avaliação dos parâmetros de processo e de suas propriedades mecânicas, utilizando-se basicamente de ensaios de tração e micrografias. Como resultado do estudo, é afirmada a viabilidade de produção destes parafusos de uma maneira mais sustentável, em função da redução de etapas no projeto e, portanto de seus insumos, possibilitando que o mesmo produto tenha agora um maior retorno financeiro (pelo menor custo de produção), ou mesmo sua oferta facilitada ao mercado de fixadores. Isso pode ser concluído já que, mesmo sem o tratamento de têmpera e revenido, o material atendeu as especificações da ISO 898-1 com folga, apresentando limite de resistência de até 1000MPa, limite de escoamento de 870MPa, alongamento e redução de área de 16 e 60% respectivamente. / The highly competitive global world of today brings many challenges to the metalworking industry. The two major pillars of this quest for excellence are sustainability and costs, since the quality of the product is intrinsic competence in the XXI century. Microalloyed steels have great importance in this scenario, because their use usually allows, the elimination or at least reduction of some production processes, which translates into lower costs and environmental impact. The objective of this work was to develop a chemical composition and a production route suitable for the 8.8 strength class screws using different steel hardening mechanisms to enable the elimination of heat treatment steps in the production process. During this work a continuous casting niobium microalloyed steel was produced followed by subsequent mechanical forming by hot rolling, where several parameters were evaluated to find the best mechanical properties, the process of drawing and cold forming screws, and finally , the single heat treatment of the chain: the aging. During all stages the material was under evaluation of process parameters and mechanical properties, primarily using tensile tests and micrographs. As a result of the study, it affirmed the feasibility of producing these screws in a more sustainable, due to the reduction of steps in the project, and therefore of its inputs, allowing the same product has a higher economic return (at lower cost production), or even easier to market its offering of fasteners. This can be done since, even without the treatment of quenching and tempering, the material met the specifications of ISO 898-1 with clearance, with tensile strength of up to 1000MPa, yield strength of 870MPa, elongation and reduction of area of 16 and 60% respectively.

A elevada competitividade no mundo global de hoje em dia traz à tona diversos desafios à indústria metal-mecânica. Os dois maiores pilares nesta busca pela excelência são os custos e a sustentabilidade, já que a qualidade do produto é competência intrínseca no século XXI. Os aços microligados ganham importância neste cenário, pois sua utilização, geralmente, permite a supressão ou ao menos redução de alguns processos produtivos, o que se traduz em diminuição de custos e, muitas vezes, redução nos aspectos e impactos ambientais. Neste trabalho, buscou-se desenvolver uma composição química e um processo produtivo adequado para parafusos da classe 8.8 que, utilizando-se dos diversos mecanismos de endurecimento dos aços, fosse capaz de eliminar etapas de tratamentos térmicos ao longo do processo. O trabalho contou com o vazamento de uma corrida de um aço microligado ao nióbio, sua posterior conformação mecânica através da laminação, onde vários parâmetros foram avaliados na busca das melhores propriedades mecânicas, o processo de trefilação e conformação a frio dos parafusos e, por fim, o único tratamento térmico da cadeia: o envelhecimento. No decorrer de todas as etapas o material esteve sob avaliação dos parâmetros de processo e de suas propriedades mecânicas, utilizando-se basicamente de ensaios de tração e micrografias. Como resultado do estudo, é afirmada a viabilidade de produção destes parafusos de uma maneira mais sustentável, em função da redução de etapas no projeto e, portanto de seus insumos, possibilitando que o mesmo produto tenha agora um maior retorno financeiro (pelo menor custo de produção), ou mesmo sua oferta facilitada ao mercado de fixadores. Isso pode ser concluído já que, mesmo sem o tratamento de têmpera e revenido, o material atendeu as especificações da ISO 898-1 com folga, apresentando limite de resistência de até 1000MPa, limite de escoamento de 870MPa, alongamento e redução de área de 16 e 60% respectivamente. / The highly competitive global world of today brings many challenges to the metalworking industry. The two major pillars of this quest for excellence are sustainability and costs, since the quality of the product is intrinsic competence in the XXI century. Microalloyed steels have great importance in this scenario, because their use usually allows, the elimination or at least reduction of some production processes, which translates into lower costs and environmental impact. The objective of this work was to develop a chemical composition and a production route suitable for the 8.8 strength class screws using different steel hardening mechanisms to enable the elimination of heat treatment steps in the production process. During this work a continuous casting niobium microalloyed steel was produced followed by subsequent mechanical forming by hot rolling, where several parameters were evaluated to find the best mechanical properties, the process of drawing and cold forming screws, and finally , the single heat treatment of the chain: the aging. During all stages the material was under evaluation of process parameters and mechanical properties, primarily using tensile tests and micrographs. As a result of the study, it affirmed the feasibility of producing these screws in a more sustainable, due to the reduction of steps in the project, and therefore of its inputs, allowing the same product has a higher economic return (at lower cost production), or even easier to market its offering of fasteners. This can be done since, even without the treatment of quenching and tempering, the material met the specifications of ISO 898-1 with clearance, with tensile strength of up to 1000MPa, yield strength of 870MPa, elongation and reduction of area of 16 and 60% respectively.

A elevada competitividade no mundo global de hoje em dia traz à tona diversos desafios à indústria metal-mecânica. Os dois maiores pilares nesta busca pela excelência são os custos e a sustentabilidade, já que a qualidade do produto é competência intrínseca no século XXI. Os aços microligados ganham importância neste cenário, pois sua utilização, geralmente, permite a supressão ou ao menos redução de alguns processos produtivos, o que se traduz em diminuição de custos e, muitas vezes, redução nos aspectos e impactos ambientais. Neste trabalho, buscou-se desenvolver uma composição química e um processo produtivo adequado para parafusos da classe 8.8 que, utilizando-se dos diversos mecanismos de endurecimento dos aços, fosse capaz de eliminar etapas de tratamentos térmicos ao longo do processo. O trabalho contou com o vazamento de uma corrida de um aço microligado ao nióbio, sua posterior conformação mecânica através da laminação, onde vários parâmetros foram avaliados na busca das melhores propriedades mecânicas, o processo de trefilação e conformação a frio dos parafusos e, por fim, o único tratamento térmico da cadeia: o envelhecimento. No decorrer de todas as etapas o material esteve sob avaliação dos parâmetros de processo e de suas propriedades mecânicas, utilizando-se basicamente de ensaios de tração e micrografias. Como resultado do estudo, é afirmada a viabilidade de produção destes parafusos de uma maneira mais sustentável, em função da redução de etapas no projeto e, portanto de seus insumos, possibilitando que o mesmo produto tenha agora um maior retorno financeiro (pelo menor custo de produção), ou mesmo sua oferta facilitada ao mercado de fixadores. Isso pode ser concluído já que, mesmo sem o tratamento de têmpera e revenido, o material atendeu as especificações da ISO 898-1 com folga, apresentando limite de resistência de até 1000MPa, limite de escoamento de 870MPa, alongamento e redução de área de 16 e 60% respectivamente. / The highly competitive global world of today brings many challenges to the metalworking industry. The two major pillars of this quest for excellence are sustainability and costs, since the quality of the product is intrinsic competence in the XXI century. Microalloyed steels have great importance in this scenario, because their use usually allows, the elimination or at least reduction of some production processes, which translates into lower costs and environmental impact. The objective of this work was to develop a chemical composition and a production route suitable for the 8.8 strength class screws using different steel hardening mechanisms to enable the elimination of heat treatment steps in the production process. During this work a continuous casting niobium microalloyed steel was produced followed by subsequent mechanical forming by hot rolling, where several parameters were evaluated to find the best mechanical properties, the process of drawing and cold forming screws, and finally , the single heat treatment of the chain: the aging. During all stages the material was under evaluation of process parameters and mechanical properties, primarily using tensile tests and micrographs. As a result of the study, it affirmed the feasibility of producing these screws in a more sustainable, due to the reduction of steps in the project, and therefore of its inputs, allowing the same product has a higher economic return (at lower cost production), or even easier to market its offering of fasteners. This can be done since, even without the treatment of quenching and tempering, the material met the specifications of ISO 898-1 with clearance, with tensile strength of up to 1000MPa, yield strength of 870MPa, elongation and reduction of area of 16 and 60% respectively.

Made available in DSpace on 2017-07-21T20:43:47Z (GMT). No. of bitstreams: 1 Tamires Brekailo.pdf: 18328712 bytes, checksum: d349535e4e10f491c91783582945c9b3 (MD5) Previous issue date: 2015-01-26 / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The recrystallization study is of fundamental importance in the thermomechanical processing of metals and alloys, since the grain refinement can be obtained affecting the final microstructure. Understanding the behavior of alloys during the mechanical hardening and subsequent annealing is of fundamental importance for the control of their microstructure and final properties. This paper has as main objective a comparative study between the steel SAE 1006 and LNE 380 during processing involving cold rolling and annealing subsequent. And also analyze the influence following the rolling passes in the texture of the material. For this, the two steel samples were cold rolled by 50% and 70% reduction in thickness corresponding to true strain values of 0.7 and 1.2, respectively. Following the samples were annealed in metal bath, and then characterized by optical microscopy, scanning electron microscopy (SEM and FEG), hardness Vickers, texture analysis by x-ray diffraction and EBSD. The LNE 380 steel samples, due to the larger number of particles has undergone further hardening the SAE 1006 steel during cold rolling. In the deformation texture both steels showed partial fiber and fiber , but for the LNE 380 steel fibers are more developed. The LNE 380 steel was laminated with different sequence for the same deformation, and the sequence with more and less passes show texture more intense compared with a sequence of intermediate passes. SAE 1006 steel was recrystallized from shorter times and temperatures that LNE 380 steel, since this has a large number of particles which impede the recrystallization because they act as a barrier to the movement of grain boundaries. / O estudo da recristalização é de fundamental importância no processamento termomecânico de metais e ligas, pois o refino de grão pode ser obtido afetando a microestrutura final. Entender o comportamento das ligas metálicas durante o encruamento e posterior recozimento é de fundamental importância para o controle de suas microestruturas e propriedades finais. Este trabalho tem por objetivo principal realizar um estudo comparativo entre os aços SAE 1006 e LNE 380, durante o processamento envolvendo a laminação a frio e posterior recozimento. E também analisar a influência na sequência de passes da laminação na textura do material. Para isto, amostras dos dois aços foram laminadas a frio até 50% e 70% de redução em espessura, correspondendo a valores de deformação verdadeira de 0,7 e 1,2, respectivamente. Na sequência as amostras foram recozidas em banho metálico, sendo então caracterizadas por microscopia ótica, microscopia eletrônica de varredura (MEV e FEG), dureza Vickers, analise de textura por difração de raios x e EBSD. As amostras do aço LNE 380, devido ao maior número de partículas sofreu um maior encruamento que o aço SAE 1006 durante a laminação a frio. Na textura de deformação ambos os aços apresentaram fibra parcial e fibra, porém para o aço LNE 380 as fibras são mais desenvolvidas. O aço LNE 380 foi laminado com sequência de passes diferentes para a mesma deformação, sendo a sequência com mais e menos passes apresentaram textura mais intensa comparada com uma sequência de passes intermediária. O aço SAE 1006 recristalizou em tempos e temperaturas inferiores que o aço LNE 380, pois este apresenta uma grande quantidade de partículas as quais dificultam a recristalização, pois atuam como uma barreira para a movimentação de contornos e subcontornos de grãos.

This thesis contains the development of a physically-based model describing the microstructural evolution during hot deformation of microalloyed steels. The work is mainly focused on the recrystallization kinetics. During hot rolling, the repeated deformation and recrystallization provides progressively refined recrystallized grains. Also, recrystallization enables the material to be deformed more easily and knowledge of the recrystallization kinetics is important in order to predict the required roll forces. Hot strip rolling is generally conducted in a reversing roughing mill followed by a continuous finishing mill. During rolling in the roughing mill the temperature is high and complete recrystallization should occur between passes. In the finishing mill the temperature is lower which means slower recrystallization kinetics and partial or no recrystallization often occurs. If microalloying elements such as Nb, Ti or V are present, the recrystallization can be further retarded by either solute drag or particle pinning. When recrystallization is completely retarded and strain is accumulated between passes, the austenite grains will be severely deformed, i.e. pancaking occurs. Pancaking of the grains provides larger amount of nucleation sites for ferrite grains upon transformation and hence a finer ferrite grain size is achieved. In this work a physically-based model has been used to describe the microstructural evolution of austenite. The model is built-up by several sub-models describing dislocation density evolution, recrystallization, grain growth and precipitation. It is based on dislocation density theory where the generated dislocations during deformation provide the driving force for recrystallization. In the model, subgrains act as nuclei for recrystallization and the condition for recrystallization to start is that the subgrains reach a critical size and configuration. The retarding effect due to elements in solution and as precipitated particles is accounted for in the model. To verify and validate the model axisymmetric compression tests combined with relaxation were modeled and the results were compared with experimental data. The precipitation sub-model was verified by the use of literature data. In addition, rolling in the hot strip mill was modeled using process data from the hot strip mill at SSAB Strip Products Division. The materials investigated were plain C-Mn steels and Nb microalloyed steels. The results from the model show good agreement with measured data. / QC 20100706