Evolution of ultrafine-grained ferrite structure through multi-pass warm caliber-rolling

Ultrafine-grained steel bars with a cross section of 18 mm square were fabricated through a multi-pass warm caliber-rolling for a 0.15%C-0.3%Si-1.5%Mn steel. The average ferrite grain sizes of 0.43 mum, 0.70 mum and 1.2 mum were obtained in the isothermal rolling processes at 773K, 823K and 873K, respectively. Despite of inhomogeneous strain distribution by the caliber-rolling, finally, large plastic strain more than 2 or 3 could be introduced into the whole region of the bar samples. Strain accumulation due to the multi-pass warm deformations was confirmed by comparing microstructural evolution through the multi-pass deformations with that through single-pass deformations. The hardness and the grain size of the ultrafine-grained structures formed through severe warm deformations depend on Zener-Hollomon parameter. These indicate that the formation process of the ultrafine-grained structures is "continuous recrystallization" or "in-situ recrystallization". The similarity to single-pass deformations reveals the multi-pass warm rolling to be an effective method to obtain ultrafine-grained ferrite structures.