Abstract
A low carbon medium manganese steel with additions of nickel and niobium was designed to further enhance cryogenic impact toughness of medium manganese steels and suppress Lüders deformation. It is interestingly found that the steel containing little retained austenite (around 1.5 vol.%) possesses high strength and excellent cryogenic impact toughness after heat treated at 650 °C for 2 h, whereas conventional medium manganese steels containing little or more unstable retained austenite always show poor cryogenic impact toughness after heat treated at a relatively high temperature. The excellent cryogenic impact toughness is mainly attributed to the formation of fine grains and little retained austenite with high mechanical stability. All steels exhibit continuous yielding behavior, and the yielding behavior is independent of heat treatment temperature. Moreover, the Lüders deformation is suppressed by restraining the formation of Cottrell atmosphere through low carbon design, further reduction in free carbon content in matrix via both the partition of carbon from matrix to austenite and the formation of NbC, some mobile dislocations and the formation of α+α′ mixture.
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