A finite element analysis of strain induced transformation rolling and an experimental study on the grain refinement potential of severe undercooling thermo-mechanical treatment

A finite element analysis was performed to study the strain induced transformation rolling process through which ultra-fine-grain steels were produced. Within the surface layer of the specimen, where ultra-fined grains were found, high strain and severe undercooling were found to coexist during hot deformation. It is proposed in this paper that the ultra-refinement was caused by the deformation of austenite in a severely undercooled state. To verify the grain refinement potential of severe undercooling thermo-mechanical treatment, hot compression experiments were done on a Gleeble1500 thermo-mechanical simulator. Severe undercooling was obtained by cooling the specimens at very high cooling rate before deformation. Metallographic observations showed that ultra-fine grains could be obtained at a true strain of 0.9 provided that undercooling was higher than 200 K.