Influência da temperatura de recozimento intercrítico no comportamento mecânico de um aço bifásico carbono
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Dissertação
Data
2009
Autores
FORGAS JUNIOR, A.
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Magnabosco, R.
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Metais-Propriedades mecânicas,Aço
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Esta pesquisa tem como objetivo principal investigar a influência da temperatura de recozimento intercrítico no comportamento mecãnico de um aço bifásico com baixo teor de carbono, analisando principalmente a relação limite de resistência e ductilidade, em virtude de sua emergente importância na indústria automobilística, onde é necessário aliar essas duas propriedades. Para isso, foi adquirida uma chapa de aço bifásico comercial. Com o objetivo de se variar a fração em volume de martensita essa chapa foi tratada termicamente em quatro diferentes temperaturas (715, 755, 780 e 800 C) dentro da faixa de temperaturas da região intercrítica. Realizaram-se ensaios de tração e impacto em amostras do material como recebido e em amostras do material após o tratamento térmico de recozimento intercrítico. Os resultados mostram que a fração em volume de martensita aumenta com a temperatura de recozimento intercrítico, porém o tamanho de grão não apresenta uma variação considerável. Para avaliar a influência da microestrutura no comportamento mecânico do aço bifásico, foram relacionadas as propriedades mecânicas obtidas nos ensaios de tração e impacto em função da fração em volume de martensita obtida, para cada amostra tratada termicamente. Verifica-se que os limites de resistência e de escoamento aumentam com o aumento da fração em volume de martensita, porém com características diferentes. Enquanto que o limite de escoamento aumenta de forma linear, o limite de resistência apresenta duas inclinações, uma mais suave na faixa de 8 a 18% e outra mais significativa na faixa de 18 a 26% de fração em volume de martensita. Isso porque o limite de resistência é afetado diretamente pelo teor de carbono presente na martensita, que diminui com o aumento da temperatura de recozimento intercrítico, ou seja, quanto maior a fração em volume de martensita num aço bifásico menor será o teor de carbono presente na martensita. O limite de escoamento é impactado de maneira menos significativa pelo teor de carbono na martensita, apresentando variação linear, obedecendo, assim, à lei da mistura. A ductilidade, representada pelo alongamento total, diminui de maneira linear com o aumento da fração em volume de martensita, enquanto que o alongamento uniforme, que está relacionado ao processo de estampagem, apresentauma característica não linear, sendo a variação muito pequena quando a fração em volume de martensita aumenta de 18 para 26%, devido à incompatibilidade plástica entre a matriz ferrita e as ilhas de martensita. Por meio do ensaio de impacto observa-se que o material com menor fraçãoem volume de martensita apresenta o maior valor de energia absorvida, e os demais materiais tratados termicamente.
This work has the main purpose to investigate the influence of the intercritical annealing temperature of a dual-phase steel (DP) with low carbon on the mechanical behavior, specially tensile strength and ductility, due to its emergent importance in the automotive industry, where it is necessary to unite these two properties. For this reason a commercial dual-phase steel was obtained. In order to obtain different volume fractions of martensite the steel plate was heat treated at four different temperatures (715, 755, 780 and 780C) inside the intercritical region. Tension and impact tests were performed on this material as received and also on the heat treated specimens. The results show that increasing the annealing temperature lead to increase the volume fraction of martensite, on the other habd the grain size did not have changed. To evaluate the influence of the microstructure on the mechanical behavior of the dual-phase steel, it was related the mechanical properties obtained on tensile and impact tests against to the volume fraction of martensite, for each heat treatment condition. The tensile strength and the yield strength increase when the volume fraction of martensite increases, but in a different way. While the yield strength increases linearly the tensile strength shows two steps, the first on is softer and occurs from 8 to 18%, the second one is more significant and occurs from 18 to 26% volume fraction of martensite. It happens because the tensile strength is strongly affected by the carbon content of the matensite. The yield strength is affected in a different way, showing linearity and obeying the law of the mixture. The ductility, represented by the total elongation, decreases linearly when the volume fraction of martensite increases, while the uniform elongation, related to the stamping process, shows a non linear dependence and its variation is very small when the volume fraction of martensite increases from 18 to 26%, due the plastic incompatibility between the ferrite matrix and the martensite islands. From the impact test it is observed that the specimen heat treated in the lowest temperature shows the higher abosrved energy (upper shelf) than the other specimens. The ductile-to-brittle transition temperature shows no dependence with the volume fraction of martensite and the highest temperature observed was -66C for all specimens. About the work hardening, Hollomon and Ludwick analyzes shows the same effect, the work hardening exponent decreases when the volume fraction of martensite increases. To finalize, it was elaborated a performance evaluation considering the principles mechanical properties, according design and safety requirements, against the heat treatment employed. The specimen heat treated on the lowest temperature (715C), which has the lower volume fraction of martensite, reaches the best performance.
This work has the main purpose to investigate the influence of the intercritical annealing temperature of a dual-phase steel (DP) with low carbon on the mechanical behavior, specially tensile strength and ductility, due to its emergent importance in the automotive industry, where it is necessary to unite these two properties. For this reason a commercial dual-phase steel was obtained. In order to obtain different volume fractions of martensite the steel plate was heat treated at four different temperatures (715, 755, 780 and 780C) inside the intercritical region. Tension and impact tests were performed on this material as received and also on the heat treated specimens. The results show that increasing the annealing temperature lead to increase the volume fraction of martensite, on the other habd the grain size did not have changed. To evaluate the influence of the microstructure on the mechanical behavior of the dual-phase steel, it was related the mechanical properties obtained on tensile and impact tests against to the volume fraction of martensite, for each heat treatment condition. The tensile strength and the yield strength increase when the volume fraction of martensite increases, but in a different way. While the yield strength increases linearly the tensile strength shows two steps, the first on is softer and occurs from 8 to 18%, the second one is more significant and occurs from 18 to 26% volume fraction of martensite. It happens because the tensile strength is strongly affected by the carbon content of the matensite. The yield strength is affected in a different way, showing linearity and obeying the law of the mixture. The ductility, represented by the total elongation, decreases linearly when the volume fraction of martensite increases, while the uniform elongation, related to the stamping process, shows a non linear dependence and its variation is very small when the volume fraction of martensite increases from 18 to 26%, due the plastic incompatibility between the ferrite matrix and the martensite islands. From the impact test it is observed that the specimen heat treated in the lowest temperature shows the higher abosrved energy (upper shelf) than the other specimens. The ductile-to-brittle transition temperature shows no dependence with the volume fraction of martensite and the highest temperature observed was -66C for all specimens. About the work hardening, Hollomon and Ludwick analyzes shows the same effect, the work hardening exponent decreases when the volume fraction of martensite increases. To finalize, it was elaborated a performance evaluation considering the principles mechanical properties, according design and safety requirements, against the heat treatment employed. The specimen heat treated on the lowest temperature (715C), which has the lower volume fraction of martensite, reaches the best performance.