Effect of Nb on bainitic transformation kinetics of medium-carbon ultra-fine grained bainitic steels
The effect of niobium(Nb)on the bainitic phase transformation is more complex and there are different opinions,some scholars believe that the addition of Nb hinders the kinetics of the bainitic phase transformation,while others believe that Nb increases the nucleation point of phase deformation and accelerates the kinetics of the bainitic phase transformation.In addition,Nb is generally added in low carbon steels,and is less used in medium and high car-bon steels.The effect of Nb on the phase transformation kinetics of medium carbon steels was investigated using a combination of heat treatment experiments and high-temperature laser confocal microscopy(LSCM)in-situ observa-tion experiments on the popular ultrafine-grained bainitic steels.In addition,field emission scanning electron micros-copy(SEM)and electron backscattering diffraction(EBSD)were used to investigate the effect of Nb on the micro-structure of medium carbon bainitic steel.The results show that Nb refines the original austenite grain of medium car-bon bainitic steel,and the addition of 0.024％Nb refines the original austenite size from(72±2)μm to(51±2)μm at a heating temperature of 1 100 ℃.Theoretical calculations show that most of the Nb is in the precipitation state at this heating temperature,so that the main mechanism of the original austenite refinement is the pinning effect of NbC on the grain boundaries.At the same time,Nb slows down the elongation rate of the bainite bundle,which is caused by its solute drag effect.After adding 0.024％Nb,the average elongation rate of the bainite bundle before 50％vol-ume fraction of the phase transition slows down from(0.43±0.13)μm/s to(0.31±0.16)μm/s.For the medium carbon bainitic steel used in the present experiments,the addition of Nb slows down the overall phase transition kinet-ics of the bainite,mainly due to the grain refinement limiting the growth of the bainite bundle,and the solute drag effect of Nb slows down the growth of the bainite bundle.The main reason is that the grain refinement limits the growth of the bainite bundle and the solute drag effect of Nb slows down the elongation rate of the bainite bundle.At the same time,the addition of Nb refines the bulk M,M/A,and RA,which is beneficial for improving the toughness of the steel.A useful reference for further understanding the mechanism of Nb action in medium carbon bainitic steels is provided.
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