Resumo: Para otimizar as propriedades mecânicas em tração foram realizados tratamentos térmicos específicos para alterar a composição das fases presentes. Para as microestruturas com melhores níveis de resistência, foi avaliado o desempenho em fadiga. Após um tratamento termoquímico de nitrocarbonetação a plasma, os resultados em fadiga são comparados para avaliar a eficácia deste tratamento. A estrutura bainítica apresentou melhor desempenho em fadiga. Ensaios de dureza por microindentação mostraram que a camada de nitretos formada, com cerca de 10μm, apresentaram elevada dureza em torno de 900 HV e com uma camada difusa de 120 μm de profundidade. Foram também realizados testes de fluência nos aços com as microestruturas escolhidas, com e sem a camada de proteção a plasma. A camada de compostos formada produziu uma redução significativa na taxa de fluência. A camada de compostos formada sobre a superfície dos aços mostrou-se eficaz na proteção contra a corrosão. Após duzentas horas de teste em salt-spray não foram observados sinais de corrosão, enquanto nos aços sem esta camada a corrosão mostrou-se intensa logo após as primeiras 24 horas. Foram realizados ensaios de desgaste abrasivo com esferas e solução de pasta de diamante, para esses testes a camada mostrou-se ineficiente, pois a fina camada de nitretos se desgasta e suas partículas agem provocando um desgaste agressivo de três corpos. A condição temperada e revenida exibe mais alta propensão a esse tipo de desgaste / Abstract: In order to optimize tensile strength mechanical properties, specifics termochemical treatments were applied to change the present phases composition. The fatigue life perform was evaluated to the microstructure with better levels of tensile strength. After a nitrocarburizing termochemical treatment, the outcome fatigue life were compared on to evaluate the treatment effectiveness. The isothermal treatment showed better perform in fatigue life. Hardness test by microindentation showed that the layer nitrided formed, around 10μm, exihibited hardness high level, around 900HV and a diffuse layer of 120μm deep. Creep tests has been done on steels with chosen microstructure, with and without nitrocarburizing protection layer. The compound layer formed give rise to a significantly reduction of the creep rate. The compound layer formed on steels surface showed the corrosion protection effectiveness. After 200 hours into salt-spray tests any corrosion signs were observed, whereas without this compound layer the corrosion showed higher into the 24 hours. Abrasion wear with tungsten balls and diamante solution were carried out. The compound layer formed showed to be inefficient, owing to the thin nitrides layer that weared and its hard particles give rise to a higher wear among three bodies. The quenched and tempered condition exhibit a higher propensity for this wear type / Orientador: Antonio Jorge Abdalla / Coorientador: Tomaz Manabu Hashimoto / Banca: Auro Tanaka / Banca: Marcelo dos Santos Pereira / Banca: Carlos de Moura Neto / Banca: Roberto Masato Anazawa / Doutor

Resumo: Os aços multifásicos Complex Phase são aços desenvolvidos para a indústria automobilística que ainda estão sendo utilizados de uma forma bem tímida. Estes aços podem ser usados na parte estrutural das portas, vigas, pilares B, perfis, reforço de estruturas, pára-choques, tetos, capôs e partes do chassi dos carros. O foco deste trabalho foi estudar o comportamento mecânico destes aços após deformação plástica, em diferentes temperaturas e tempos de envelhecimento. O método utilizado é semelhante ao aplicado atualmente pelas indústrias automobilísticas, durante o processo de pintura, chamado Endurecimento ao Forno (efeito Bake Hardening). O efeito visa melhorar a resistência mecânica e manter a conformabilidade, o baixo peso da estrutura e a resistência ao impacto, característicos deste tipo de aço, possibilitando a redução de espessura/peso sem perda da resistência à formação de mossa (dent resistence). Os parâmetros de temperatura e tempo utilizados foram baseados nos processos de envelhecimento conhecidos pelo mercado para os aços Dual Phase e TRIP (Plasticidade Induzida por Transformação) como base de informações, pois para os aços Complex Phase este processo é pouco estudado. Os ensaios foram executados no material como fornecido, e os parâmetros utilizados foram pré-deformação de 2% ± 1, temperaturas de envelhecimento variando de 25 à 180ºC e tempos que variaram de 300 à 9.000.000 segundos. Foram realizadas análises microestruturais e mecânicas dos corpos-de-prova a fim de definir a melhor condição de envelhecimento para o aço Complex Phase, através de ensaios de tração, dureza, microscopia óptica e microscopia eletrônica de varredura observou-se o aumento da resistência para cerca de 890 MPa, sem significativa perda da ductibilidade. Avaliou-se, também, a lei cinética de envelhecimento visando a caracterização do fenômeno ... (Resumo completo, clicar acesso eletrônico abaixo) / Abstract: The Complex Phase steels are developed for the automotive industry although are being timidly used. These steels could be used in the structural part such as doors, beams, B columns, profiles, stringers, bumpers, roofs, hoods and other body parts. The focus of this work was to study the mechanical behavior of these steels after plastic deformation at different temperatures and aging times. The used method is similar to that currently applied by the automobile during painting, called the Bake Hardening. The aim is to improve the mechanical strength, maintaining the formability, low weight of the structure and impact strength, typical of this kind of steel, allowing the reduction in thickness / weight loss without losing dent resistance. The temperature and time parameters used were based on the aging processes known for Dual Phase and TRIP (Transformation Induced Plasticity) steels since information for the Complex Phase is not well reported. The tests were performed on the material as supplied, and the parameters used were pre-strain of 2% ± 1, aging temperatures ranging from 25 to 180 ºC for periods ranging from 300 to 9.000.000 seconds. Microstructure and mechanical properties of material were analyzed to define the best condition for the aging of Complex Phase Steel through tensile and hardness tests, optical microscopy and scanning electron microscopy was observed a increased resistance around 890 MPa without a significant loss of ductility. It was also studied the aging kinetics, aiming to characterize the phenomenon by varying the value Bake Hardening with time and temperature for pre-strain of 2% in tension. We can conclude that the multiphase steel Complex Phase presents the best condition for the Bake Hardening effect at 180 ºC in accordance with the standards and the literature, this condition improves their resistance, keeps their tenacity, and suggests the best condition ... (Complete abstract click electronic access below) / Orientador: Antonio Jorge Abdalla / Coorientador: Marcelo dos Santos Pereira / Banca: Tomaz Manabu Hashimoto / Banca: Milton Sergio Fernandes Lima / Mestre

Radioactive gold has been added to the liquid pool during the continuous casting of mild steel billets, blooms and beam blanks and of a stainless steel slab. The tests were conducted on low-head, curved mold and straight mold, vertical bend type casting machines. From autoradio-graphs of the sections of the steel, observations were made of the flow pattern in the liquid pool, of the uniformity and thickness of the solid shell in the mold and sub-mold regions and of the cast structure of the strand. Pool depths were estimated from the position of tungsten pellets containing radioactive cobalt, dropped into the pool with the gold. One and two-dimensional finite difference heat transfer models were developed to calculate the pool profiles in strands being continuously cast. The predicted pool profiles and pool depths have been compared to profiles measured from autoradiographs and pool depths measured with tungsten pellets. The model-predicted surface temperatures of low carbon steel billets at the mold bottom have been compared to measured values reported in the literature. The pool and surface temperature profiles calculated with the finite difference model have been compared to profiles predicted by an integral profile model. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

Observations have been made of the liquid pool depth and profile in the continuous casting of steel at the Western Canada Steel Co. and at the Premier Works of the Steel Company of Canada. This was done by two techniques. In the first, radioactive gold was added to the mold during a normal casting, and subsequently determining the distribution of gold in the casting by autoradiography. In the second method, tungsten pellets containing a small Co⁶⁰ wire were dropped into the steel during casting in order to obtain direct measurements of the pool depth. The results of the pool profiles and liquid flow patterns as a function of casting conditions will be presented and compared to theoretical predictions based on heat flow considerations. / Applied Science, Faculty of / Materials Engineering, Department of / Graduate

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Thesis (M.B.A.)--Boston University

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