On the conditions for achieving ultra-fine grains via thermo-mechanical treatment

A finite element analysis was performed to study a novel hot rolling process by which ultra-fine grains (UFG) were achieved. FE analysis shows that the formation of ultra-fine grains was caused by a combination of severe undercooling and intensive deformation. It is found that undercooling was greater than 180 K within the ultra-fine grained surface layer. It is suggested in this paper that severe undercooling could significantly enhance the efficiency of grain refinement in thermo-mechanical processes. Hence, ultra-fine grains were obtained via a relatively low reduction hot rolling process. Special hot compression tests, during which severe undercooling were introduced before deformation, were done to verify the effect of undercooling on grain refinement. Ultra-fine grains were achieved by compression at a reduction ratio of 0.7. The critical undercooling to achieve ultra-fine grains was about 200 K.