Effects of Pre-Deformation Temperature on Nanobainite Transformation Kinetics and Microstructure

As a new steel, nanobainite steel has favourable strength and good plasticity, but the bainite transformation needs a longer time, so as to severely slow production efficiency down. So, the research of acceleration methods of bainite transformation is of significance. In this work, in order to accelerate the bainite transformation rate, the 20% compression pre-deformation at 300 similar to 850. and isothermal transformation at 300. were conducted on a thermal simulator. The effects of pre-deformation temperature on nanobainite transformation kinetics and microstructure of the medium carbon nanobainite steel were investigated. The results showed that pre-deformation process obviously shortened the incubation time of bainite transformation. Low pre-deformation temperature could accelerate bainite transformation at whole isothermal region, while high pre-deformation temperature accelerated bainite transformation at the initial stage, and hindered bainite transformation at later stage. Bainite ferrite lath thickness was increased with decreasing pre-deformation temperature. The pre-deformation process increased the frequency of small angle grain boundary of bainite transformation microstructure, and the frequency of small angle grain boundary of low pre-deformation temperature was higher than that of high pre-deformation temperature. The kinetic parameters n of bainite transformation was calculated by the analytical model, the model of nucleation and growth were identified, the pre-deformation process changed the nucleation position of bainite transformation. The crystal corner nucleation was mainly obtained through low pre-deformation temperature, and the crystal edge and the crystal face nucleation were mainly obtained through high pre-deformation temperature.