低碳钢SS400形变强化相变组织演变的动力学

KINETICS OF STRUCTURE EVOLUTION DURING DEFORMATION ENHANCED TRANSFORMATION IN A LOW CARBON STEEL SS400

采用Gleeble1500热模拟机进行压缩实验,研究了Q235级别低碳钢SS400在750和780℃形变强化相变组织演变及动力学的定量特征.过冷奥氏体形变过程的形变强化相变按照其转变动力学的特征可分为3个阶段:前2个阶段的转变动力学方程形式与J—M—A方程的形式相符,而第3阶段的转变动力学与J—M—A方程的形式不相吻合.第1阶段符合Cahn的“位置饱和”机制,动力学参数n值为4,对应于铁素体在原奥氏体晶界及三叉界的形核及快速长大.第2阶段不符合Cahn的“位置饱和”机制,n值在1.0—1.5之间,对应于晶内奥氏体/铁素体前沿畸变区的大量形核.第3阶段对应于剩余少量形核位置时的转变变缓过程.变形提高了晶粒的形核率,同时促进了晶粒的长大速率;形变强化相变铁素体晶粒转变初期的长大速率随应变速率的增加而增大.

Quantitative characterization of microstructural evolution during deformation enhanced transformation in a low carbon steel SS400 was investigated on a Gleeble 1500 machine. General conclusions of the austenite transformation kinetics were formulated. It was shown that the transformation process can be divided into three stages according to the characteristics of transformation kinetics: The kinetics equations of two early stages fitted well in with the J-M-A equation. The kinetics of the first stage obeys Cahn's site saturation mechanism, the kinetics parameter n=4 which indicates the ferrite nucleates at austenite grain boundaries and triple points. Kinetics of the second stage doesn't obey Cahn's theory, the kinetics parameter n=1.0-1.5, corresponding to ferrite nucleating repeatedly at the high stored energy areas in front of the ferrite/austenite interface. The kinetics doesn't obey the J-M-A equation anymore in the final stage, and only few nucleation sites left at this moment. Deformation promotes the growth of ferrite grains, the growing rate of ferrite in the early stage of DEFT was calculated.