Effect of Relaxation Process and Cooling Rate on Intermediate Phase Transformation Structure Refinement

The intermediate transformation microstructure of low-carbon bainite steel was analyzed, which had been deformed in the non-recrystallization temperature range, relaxed for a period of time and cooled by various cooling rates. The influence of relaxation process and cooling rate on the intermediate transformation structure type and microstructure refinement was revealed. The results show that in a slowly cooled sample after relaxation the transformation structure is mainly granular bainite and the effect of relaxation refinement is not marked. But while the cooling rate is fast enough (>10 ℃/s): the main microstructure is lath-like bainite, and relaxation effect can be seen clearly. After relaxation, the lath of bainitic ferrite is more fine and sharp, the morphology of M/A constituent is film-like. Comparing with the non-relaxed sample, it is clearly seen that the relaxation of deformed austenite benefits to obtain ultra-fine lath-like bainite structure. At the same cooling rate, the amount of M/A constituent decreases in the relaxed sample.