Abstract: To investigate the reasons for the low acceptance rate of forgings after air cooling+normalizing+quenching+tempering, the influence of high temperature thermomechanical treatment on the process improvement, the microstructure transformation characteristics and mechanical properties of 20CrMnTiH forgings under two different processes were studied by means of metallographic observation and mechanical properties test. The results show that the insufficient cooling speed of the original process leads to the appearance of granular bainite, the tempered fragments ferrite resulting in insufficient hardness and discrete distribution, and insufficient refinement of grains resulting in low impact property. The improved process ensures sufficient cooling rate and finally obtains a large number of tempered troostite and a small amount of bainite. While a large number of plastic deformation dislocations retained by forging-quenching form sub-crystals during tempering, which greatly refines the grains. In addition, a large number of dispersed θ-Fe₃C particles in α phase pin the dislocations during tensile deformation, properties of the forgings is improved significantly.

Key words: high temperature thermomechanical treatment, 20CrMnTiH steel forging, dislocation, pinning