Medium manganese steels are a grade of the third generation of advanced high strength steels (AHSS) that combine ductility, high strength and toughness for crashing resistance, determining characteristics for application in the automotive industry. During processing these steels are subjected to a hot and cold rolling followed by intercritical annealing in the field of austenite and ferrite phases. On the other side, the warm rolling processing is capable to reduce costs and operating time due to a single operation. The aim of this work was to follow, along warm rolling, the microstructure evolution. This aim was realized by optical and scanning electron microscopy analysis, X-ray diffraction, Vickers microhardness and EBSD technique in an 8Mn-0.08C steel. A very refined and deformed microstructure in warm-rolled condition was obtained, with a higher volume fraction of retained austenite without hot-rolled as previous processing step, which result in high tensile strength and total elongation. The texture shows a greater intensity of the α-fiber components as a function of higher strain in this thermomechanical processing.
Medium manganese steel is typically fabricated from hot-rolling followed by cold-rolling and intercritical annealing processes. However, a singular process, warm-rolling, is an appealing prospect. The microstructure, mechanical properties and texture of a 8 wt-% Mn–0.08 wt-% C steel was investigated by cold-rolling followed by intercritical annealing or warm-rolling. The product from both processes exhibited a highly refined microstructure as well as a large volume fraction of austenite. However, a slightly larger volume fraction as well as a more heterogeneous austenite morphology resulted from the warm-rolling process, associated with a higher work-hardening rate. Microstructural analysis revealed intense α-fibre components due to a higher strain following the warm-rolling process as well as a considerably high-angle grain boundary number which was associated with dislocation accumulation. This paper is part of a Thematic Issue on Medium Manganese Steels.
6Elaine Carballo Siqueira Corrêa 7 Resumo Nas últimas décadas a crescente demanda por carros mais seguros, econômicos e menos poluentes exigiram das montadoras, siderúrgicas e comunidade científica investimentos na pesquisa e no desenvolvimento de novos materiais, tais como os aços avançados de alta resistência. Dentro deste contexto, neste trabalho foi realizada a análise da microestrutura e das propriedades mecânicas de um aço bifásico na condição inicial e submetido a tratamentos térmicos com aquecimento na região intercrítica e resfriamento brusco. Foram analisadas a proporção dos constituintes na microestrutura e as propriedades mecânicas por meio de microscopia óptica e eletrônica de varredura, de difração de raios X, dureza e de tração. Observou-se, em geral, que o aumento da temperatura intercrítica levou a uma elevação da fração de martensita e, consequentemente, influenciou de forma a aumentar os valores de dureza, resistência mecânica e diminuir a ductilidade. Palavras-chave: Aço bifásico; Tratamento intercrítico; Microestrutura; Propriedades. INFLUENCE OF INTERCRITICAL TEMPERATURE IN THE MICROSTRUCTURE AND THE MECHANICAL PROPERTIES OF A DUAL PHASE STEEL AbstractIn recent decades the increasing demand for safer, economical and less polluting cars demanded of automakers, steelmakers and scientific community investments in research and development of new steels, such as advanced high strength steels. The aim of this work was the analysis of the microstructure and the mechanical behavior of a dual phase steel, in the initial condition and subjected to thermal processing with intercritical heating and quenching. The ferrite-martensite fraction in the material was evaluated by optical and scanning electron microscopy techniques and its influence in the mechanical properties was analyzed through hardness and tensile tests. It was observed that the increase of the intercritical temperature raised the fraction of martensite and thus led to an increase in hardness, yield strength and ultimate tensile strength and to a decrease in the elongation.
ResumoOs aços com médio teor manganês são uma classe da terceira geração de aços avançados de alta resistência (AHSS), combinam propriedades mecânicas como ductilidade e resistência mecânica e são assistidos pelo efeito TRIP, características determinantes para sua aplicação na indústria automobilística. Convencionalmente, tais ligas são produzidas por meio de laminação a quente e a frio, seguidas de recozimento intercrítico. No entanto, o trabalho a morno têm ganhado interesse com o intuito de reduzir custos e tempo operacionais. O objetivo deste trabalho foi comparar a influência da laminação a quente e a morno por meio de análises de microscopia óptica, eletrônica de varredura, difração de raios x, microdureza Vickers e técnicas de EBSD em um aço 8Mn-0,08C. Observou-se uma microestrutura mais refinada e deformada no material laminado a morno com um maior percentual de austenita retida, o que derivou uma menor dureza. Na amostra laminada a quente, o valor médio de dureza evidenciou uma microestrutura martensítica de baixo teor de carbono. Em relação à textura verificou-se uma maior intensidade de componentes da fibra α na amostra laminada a morno devido a um maior grau de encruamento neste processamento termomecânico. Palavras-chave: Aço médio manganês; Efeito TRIP; Laminação a quente; Laminação a morno. HOT AND WARM ROLLING EFFECT IN MICROSTRUCTURE, HARDNESS AND TEXTURE OF A MEDIUM MANGANESE STEEL AbstractMedium manganese steels are a grade of the third generation of advanced high strength steels (AHSS) that combine mechanical properties such as ductility and tensile strength, determining characteristics for application in the automotive industry. Conventionally, these steels are subjected to a hot and cold rolling followed by intercritical annealing. However, the warm rolling acquire interest in order to reduce costs and operating time. The aim of this study it is to compare the influence of hot and warm rolling by optical and scanning electron microscopy analysis, X-ray diffraction, Vickers hardness and EBSD in a 8Mn-0,08C steel. A refined and deformed microstructure in warm-rolled sample was observed, with a higher volume fraction of retained austenite, which derived a lower hardness. In the hot-rolled sample, the average hardness value revealed a martensite with a low carbon content microstructure. The texture shows a greater intensity of the α fiber components in warm rolled to warm due to higher strain in this thermomechanical processing.
ResumoAs elevadas taxas de encruamento nos estágios iniciais de deformação plástica observadas para os aços bifásicos constituem uma das principais características que os habilita para aplicação em componentes estruturais na indústria de automóveis. Neste trabalho foi analisada influência de tratamentos intercríticos com aquecimento em temperaturas diversas, seguido de resfriamento brusco, no comportamento em termos de encruamento de um aço bifásico, por meio de dois métodos distintos de determinação do expoente de encruamento. Observou-se que a elevação da temperatura intercrítica influenciou de modo a reduzir o valor do expoente de encruamento, dado que o aumento da temperatura reflete a elevação do percentual de martensita na microestrutura do aço. Além disso, para as diferentes abordagens aplicadas para o cálculo dos valores de n, verificou-se que por meio da técnica de linearização da curva tensão versus deformação foram obtidos valores superiores do expoente de encruamento quando comparada à abordagem utilizando o critério da estricção. Palavras-chave: Aços bifásicos; Tratamento intercrítico; Expoente de encruamento. EFFECT OF THE INTERCRITICAL TEMPERATURE ON THE HARDENING EXPOENT OF DUAL PHASE STEELS AbstractThe high work hardening rates in the early stages of plastic deformation observed in the dual phase steel is one of the most important properties that enables them to be used in structural components in the automobile industry. The aim of this work is to analyze the effects of the intercritical heat treatment at various temperatures followed by quenching on the work hardening behavior of the dual phase steel by different methods for determining the strain hardening exponent n. It was observed that an increase in the intercritical temperature led to the reduction of the strain hardening exponent value. The phenomenon was associated with the increase of the volume fraction of martensite in the steel microstructure. According to the different approaches applied to calculate n value, upper values were found by the curve linearization technique when compared with the approach using the necking criterion.
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