Abstract: Some observations are reported on the simulation of two thermomechanical routes to produce ultrafine ferrite grain size in steels. One C-Mn grade and Nb, Nb-Ti and Nb-high Ti bearing steels were used in the tests performed on a Gleeble simulator and a laboratory rolling mill. The routes included severe hot deformation of prior grain-refined austenite at the temperature close to Ar3 (DIF) and static recrystallization of fine-grained cold-rolled martensite (SRM). It was observed that the hot deformation induces the formation of ferrite above the Ar3 temperature of the steel, but severe reductions are required for the complete transformation. Strain of 1.2 can result in about 70% of ferrite with the grain size of about 1.4～2 μm in all the studied steels. Similarly, in short annealing of cold-worked martensite, the static recrystallization can also lead to a grain size of about 1.5 μm. The distribution of carbon varies in the microstructures, carbon being in the second phase in the DIF route and in carbide particles in the SRM route, which may have a significant influence on the mechanical properties and the thermal stability of ultrafine grain structure.

Key words: Ultrafine grain size, Steels, Physical simulation, Strain-induced ferrite