Study On Thermodynamics And Kinetics Transformation Of Bainite Steel With Al And Si Elements

Railway track is the basic of the rail service. The quality, performance and service life of the railway directly influence the efficiency and safety of railway transportation. Therefore, the demand on the performance of railway and crossing increase higher and higher, which results in that more researchers start to study and design new rail steels. Bainite steel has got many researchers'attention for a combination of excellent strength and toughness.The bainite steel with Al/Si alloying elements was studied in this paper, and the characterization of thermodynamic and kinetic of phase transformation was investigated. The thermodynamic equilibrium phase diagram of material was calculated by using Thermo-Calc software. According to artificial neural network principle, the CCT curve was predict by using MATLAB software. The effect that element Al replaced part of Si element on the phase transition point and the place of CCT curve was studied by comparing the calculation results. Finally, the microstructure and mechanical properties was predicted according the calculation results.The calculation results showed that the change that the element Al replaced part of Si element resulted in that austenite single phase region was reduced remarkably, the ferrite phase region was extended, A3(Acm), A1b and A1f temperature were improved, the condensing temperature was decreased and the eutectoid point moved right. Both the Bs point and the transformation rate of bainite increased with the increasing Al content. At the same time, the CCT curve moved to the top-left.According to the prediction from the calculated CCT curves, for the specimen of 1#, the transformation temperature of lower bainite was below 379℃, the incubation time was long, the transformation rate was slow and the holding time of isothermal quenching was long. In the case of 2# specimen, the transformation temperature of lower bainite was below 475℃, the transformation was difficult when the isothermal quenching temperature was low while when the temperature was high, the incubation time was short, the transformation rate was fast and the holding time was short.To verify the accuracy and reliability of calculated results and predicted microstructure and mechanical properties, the phase transformation temperature and Ms point was measured by using thermal dilation method. And then, formulate the heat treatment process according to the CCT curves. Finally, the phase composition within the microstructures obtained by various heat treatment processes was detected by optical microscope. The hardness and impact ductility was measured.The study results showed that testing value of phase transition point coincides with calculation value, and testing microstructure and mechanical properties coincides with predicting. The predicting methods are reliably, and are effective study ways that improving efficiency of material design and reducing cost.