The steel of this work, 0.06C-25Mn-3Al-2Si-1Ni steel, presenting TWIP effect, was hot and cold rolled and then annealed at temperatures between 600 and 850ºC. The microstructure examination was focused in the recrystallization during annealing for different temperatures through optical and scanning electron microscopy. The volume fraction and recrystallized grain size measurements were performed. Tensile tests were conducted at room temperature. A polycrystalline model, based on micromechanics and working hardening theory, developed by Bouaziz et al., to predict the behavior of TWIP steels under different loading paths, was applied to the current steel. The results from the model are in good agreement with mechanical test and show a total elongation above 60%, uniform elongation up to 55% and a tensile strength greater than 600 MPa, which highlights the potential of this steel for its various applications, mainly automotive industry. The model parameters are discussed and their limitations are presented.
The increasing demand, mainly from the automobile industry, for materials which combine high strength, high ductility and low specific weight makes steels with the TWIP (TWinning Induced Plasticity) effect a promising material to meet these requirements. This work aimed to study the kinetics of isothermal recrystallization of a TWIP steel (C-0.06%, Mn-25%, Al-3%, Si-2%, and Ni-1%) after cold rolling. The steel was hot and cold-rolled and then annealed at 700°C with soaking times ranging from 10 to 7200 s. Microstructural analysis was performed using light (LM) and scanning electron microscopy (SEM). Furthermore, quantitative metallography was performed in order to evaluate the recrystallized volume fraction and grain size. A JMAK based model was applied to describe the nucleation grain growth process. The restoration of the steel was also evaluated by microhardness tests. A complete recrystallization after 7200 s at 700°C was observed. It was found that with increasing annealing times, the recrystallized volume fraction also increases, while the nucleation and growth rates decrease, in agreement with the results for plain carbon steels.
Quando deformados plasticamente, os aços baixo carbono microligados apresentam um interessante comportamento. O alongamento uniforme, por exemplo, aumenta com a redução do tamanho de grão ferrítico, enquanto se reduz a razão elástica. O objetivo deste trabalho é acompanhar a evolução microestrutural ao longo do processamento térmico e mecânico para interpretar o desempenho mecânico de um aço microligado baixo carbono (0,11C-1,41Mn-0,028-Nb-0,012Ti) de grão ferrítico ultrafino, obtido por meio de tratamentos térmicos de têmpera, laminação a morno e recozimentos intercrítico e subcrítico. Para isso, verifica-se a evolução do tamanho de grão ferrítico e avalia-se o comportamento mecânico por meio de testes de tração e impacto. A resistência mecânica do aço recozido a 550°C mostra um acréscimo de 25%, comparada com o aço na condição de fornecimento. Para o aço recozido a 800°C, há aumento na resistência mecânica e na energia absorvida no ensaio de impacto. Palavras-chave: Grão ultrafino; Recozimento intercrítico; Laminação a morno; Cementita.
ResumoExiste uma atual necessidade da indústria automobilística por materiais que combinem alta conformabilidade, alta resistência mecânica e baixo peso específico a fim de diminuir o consumo de combustíveis e aumentar a segurança dos passageiros. É neste contexto que surge o aço TWIP (TWinning Induced Plasticity), um aço de alto teor de manganês contendo silício e alumínio (2% a 4%) em sua composição. Sua principal característica é a formação de maclas sob ação de uma tensão. No presente trabalho, investiga-se como as características do encruamento de um aço C-0,06; Mn-25; Al-3; Si-2; Ni-1, com efeito TWIP, laminado a quente e a frio e recozido a temperaturas variáveis entre 600°C e 850°C, influencia as suas propriedades mecânicas. Tal investigação consistiu na obtenção de micrografias ópticas e eletrônicas de varredura (MEV) e na avaliação do tamanho de grão e da fração volumétrica dos grãos recristalizados. Para complementação, foram realizados ensaios de tração. O estudo do encruamento teve a abordagem de Hollomon associada com o critério de Considère. A amostra recozida a 850°C apresenta um expoente de encruamento de 0,55, alongamento total de 66% e limite resistência de 700 MPa. palavras-chave: Efeito TWIP; Laminação a frio; Recristalização; Encruamento. BEhAvIOuR In ThE STRAIn hARDEnInG OF hIGh mn AnD LOw CARBOn TwIp STEEL COLD ROLLInG AnD AnnEALInG AbstractThere is a current need for the automotive industry for materials that combine high formability, high strength and low specific weight in order to reduce fuel consumption and increase passenger safety. In this context, appears the TWIP steel (TWinning Induced Plasticity), defined as a steel with high content of manganese and, yet, silicon and aluminum (2% to 4%) in its composition. Its main feature is the formation of twining under stress. In this study, it was investigated how the characteristics of hardening work of the steel C-0,06, Mn-25, Al-3, Si-2, Ni-1, with TWIP effect, hot and cold rolling and annealing at temperatures varying between 600°C and 850°C, influences their mechanical properties. This research used optical micrography and scanning electron microscopy (SEM) for assessment of grain size and volume fraction of recrystallized grains. To complement this study, tensile tests were performed. The hardening work study uses the approach of Hollomon associated with the Considère criterion. The sample annealing to 850°C shows a work hardening exponent of 0.55, 66% of total elongation and maximum strength of 700 MPa.
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