Austenite grain ultrarefinement and the formation of nanocrystallized structure through transformation of low carbon steels

The effects of chemical composition, original microstructure, and heating rate on austenite grain refinement were investigated, and the results indicated that ultrafine austenite with grain size 1m can be obtained from the original microstructure of ferrite/pearlite after being warm rolled and cold deformed, and the ultrafine austenite could also be obtained through combined addition of Nb-V-Ti and by properly increasing heating rate. The transformation of the ultrafine austenite was also investigated, and the results indicated that when the heavy deformation was applied below Ar₃ during cooling, the uniform and equiaxed ferrites with grain size of 0.1 ∼ 0.3μm, close to nanosize, can be obtained. The main mechanism of deformation-induced transformation of ultrafine austenite to ferrite is that the nucleation of ferrite along austenite boundary is enhanced by the boundary slipping of austenite grains.