This work describes the steps in the study and development of the high content of C, Si, and Cr in commercial steel with a nanostructured matrix of martensite, bainite and retained austenite. Specimens of this steel were austenitized at 900 °C for 5 min and isothermally heat treated at different times (0.5, 2, 24, 48 h) and temperatures (200, 220, 270 °C). Different mechanical behavior was observed for the samples treated under different conditions due to their microstructural constitution. Specimens treated for 2 h at 200 °C showed lower yield strength of 979 MPa. On the other hand a high tensile strength of 2248 MPa was reached. The sample treated at 270 °C showed higher yield strength of 1363 MPa with the same treatment time. As for the fracture analysis, the brittle fracture mechanism was predominant for the samples treated at 200 °C, while the ductile fracture mechanism was predominant for the samples treated at 270 °C.
The new class of bainitic steels can present toughness at room temperature greater than traditional quenched and tempered martensitic steel. This is because the microstructure of steel with high Si content (≈1.5wt%) submitted to bainitic transformation is compose of fine plates of bainitic ferrite separated by retained austenite. The inhibition of cementite precipitation leads to the improvement of toughness. The presence of cementite facilitates the nucleation of cracks. Moreover, the blocks of retained austenite are undesirable. This morphology is rather unstable and tends to transform into hard and brittle untempered martensite under the influence of small stress, contributing to a low toughness. However, it was observed in this work that the greater the volume fraction of retained austenite, the greater is the toughness (10-24 J) for multi-phase steel. The values of toughness were independent whether the retained austenite is present on film or block forms. The decrease of toughness values was observed by the tempered samples after the bainitic transformation (10-14 J). This occurred because the blocks of retained austenite decomposed into carbides, martensite and/or bainite.
A significant amout of stabilized austenite can be obtained in high carbon steel containing high amounts of manganese and silicon (1.5-2 %). At relatively low temperatures the bainite plates formed are extremely thin, making the material very strong. In this study, the influence of the thermal cycle of austempering on the mechanical behavior of a spring steel 0.56C-1.43Si-0.58Mn-0.47Cr (wt. %), with TRIP effect was investigated. The thermal cycle consisted of heating three groups of hot-rolled wire steel at austenite field of 900°C for 300 s, and quickly transferring those to a metallic bath maintained at 200, 220 or 270°C, respectively, for different heat treatment times. The samples were then tested in tension and their microstructures were examined by scanning and transmission electron microscopy. The samples treated at 220°C showed higher elongation, yield strength and tensile strength than those maintained at 200 or 270°C. The high level of strength and ductility is due to a mixture of martensite and very fine bainitic ferrite with interlath film of retained austenite. The temperature has shown a strong influence on bainite formation kinetics. The fracture behavior of the steel was also evaluated using SEM fractography.
ResumoAços multiconstituídos têm aplicações voltadas principalmente para o setor automobilístico. Isso porque esses apresentam uma boa combinação de elevada resistência, ductilidade e tenacidade. A elevada resistência desses aços é devida a martensita e aos finos feixes de ferrita bainítica, enquanto que a tenacidade e a ductilidade são atribuídas aos filmes finos de austenita retida presentes entre os feixes. Esses filmes contém uma maior concentração de carbono em relação aos blocos de austenita retida. Uma consequência é que estes se tornam mais estáveis mecanicamente e termicamente. Dessa forma, foi realizada a avaliação do desempenho mecânico de um aço tratado isotermicamente abaixo de Mi e revenidos a 400°C/1 h. Os resultados dos ensaios de impacto mostraram que quanto maior a fração volumétrica da austenita retida maior é a energia absorvida, independente da morfologia desse constituinte. Abstract Multi-phase steels have applications primarily focused on the automobile industry. This is because these have a good combination of high strength, ductility and toughness. The high strength of these steels is due to martensite and thin plates of bainitic ferrite, while the toughness and ductility are attributed to thin films of retained austenite present between the plates. These films contain a higher concentration of carbon compared to blocks of retained austenite. The consequence is that these become more mechanically and thermally stable. Thus, the evaluation of the mechanical performance of a treated steel isothermally below Ms and tempered at 400°C/1 h was performed. The results of the impact tests showed that the higher the volume fraction of retained austenite the greater is absorbed energy, independent of the morphology of this constituent.
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