铌对中碳超细晶贝氏体钢相变动力学的影响

Effect of Nb on bainitic transformation kinetics of medium-carbon ultra-fine grained bainitic steels

铌对贝氏体相变的影响较为复杂,且存在不同观点,一部分学者认为铌的添加阻碍贝氏体相变动力学,而另一部分学者认为铌增加相变形核点,加速贝氏体相变动力学。此外,铌一般添加在低碳钢中,在中碳和高碳钢中的应用较少。以当下热门的超细晶贝氏体钢为对象,采用热处理试验与高温激光共聚焦显微镜(LSCM)原位观察试验结合的方法研究铌对中碳钢相变动力学的影响。此外采用场发射扫描电镜(SEM)和电子背散射衍射(EBSD)研究了铌对中碳贝氏体钢显微组织的影响。结果表明,铌细化中碳贝氏体钢原始奥氏体晶粒,加热温度为1100℃时,加入质量分数为0.024%的铌后,使原奥尺寸由(72±2)μm细化至(51±2)μm。通过理论计算发现,在该加热温度下,大部分铌处于析出状态,所以原奥细化的主要机理是NbC对晶界的钉扎作用。同时铌减慢贝氏体束伸长速率,这是其溶质拖曳作用引起的。加入质量分数为0.024%的铌后,相变体积分数50%前贝氏体束平均伸长速率由(0.43±0.13)μm/s减慢至(0.31±0.16)μm/s。对于试验所用中碳贝氏体钢,铌的添加减慢了贝氏体整体相变动力学,主要原因是晶粒的细化限制了贝氏体束的生长,且铌的溶质拖曳作用减慢了贝氏体束的伸长速率。同时铌的添加细化了块状M、M/A和RA,这对提高钢的韧性是有利的。为进一步理解铌在中碳贝氏体钢中的作用机理提供了有益参考。

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 carbon 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 observation experiments on the popular ultrafine-grained bainitic steels.In addition,field emission scanning electron microscopy(SEM)and electron backscattering diffraction(EBSD)were used to investigate the effect of Nb on the microstructure of medium carbon bainitic steel.The results show that Nb refines the original austenite grain of medium carbon 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 1100℃.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% volume 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 kinetics 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.