| Five kinds of Fe-C-Mn-Si bainitic steels with different chemical compositions were designed in this paper.After smelting and rolling,samples were taken for isothermal bainitic transformation.Thermal simulation tests were carried out on a Gleeble-1500 hot simulator and microstructure was observed using Scanning Electron Microscopy.According to simulation test results,steel 2 was chosen as tested steel for subsequent experiments.The static continuous cooling transformation curve of steel 2 was drawn using dilatometry and optical metallography.Results show that with cooling rate of 5~20?/s,high temperature transformation is suppressed and bainite can be obtained at temperature in the range of 310~510?.Then,experiments were designed to study the effect of prior austenite deformation on bainitic transformation.The influences of both deformation temperature and strain on bainite transformation kinetics and morphology are examined.The transformation is retarded by deformation at low temperature due to mechanical stabilization of deformed austenite.The inhabiting effect is first weakened,and then enhanced with increase of strain.At intermedium deformation temperature,small strain promotes bainitic transformation,while large strain suppresses bainitic transformation.It is presumed that there exists a critical strain value for intermedium deformation.When the strain is less than the value,the final amount of bainite obtained increases with strain.While the final amount of bainite obtained decreases with strain as the strain is larger than the value.The effect of high temperature deformation on bainitic transformation is similar to intermedium temperature deformation.In addition,bainite sheaves become finer and less directional after deformation compared with undeformed sample.At the same deformation temperature,bainite sheaves are further fined and more chaotic with increasing strain. |
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