Strain(work)-hardening in tensile tests was examined for low carbon steels with various ferrite grain sizes ranged from 0.4 to 16 mm. The steels had microstructures composed of ferrite grains and dispersed cementite particles. They were fabricated through warm caliber bar-rollings with an accumulative area reduction of 93 %.Strain-hardening rate at a given strain increased with an increase in volume fraction of cementite particles. The balance of yield strength and uniform elongation for ultrafine-grained structures could be improved by the dispersion of cementite particles. Effects of the cementite dispersion and the ferrite grain size on the strain-hardening rate can be roughly explained by the work-hardening model with GN-dislocation density. Strain-hardening design using dispersed cementites was proved to be effective in controlling ductility of the ultrafine-grained steels.

Ultrafine grained structure formed dynamically through a severe warm deformation in the temperature range from 773 K to 923 K has been investigated in a 0.16%C-0.4%Si-1.4%Mn steel. The effects of the deformation conditions such as deformation temperature and strain rate on microstructural evolution were examined using a single-pass compression technique with a pair of anvils. A large plastic strain up to 4 was imposed on the specimen interior at a strain rate of 1 or 0.01 s À1 . Ultrafine ferrite grains surrounded by high angle boundaries, whose nominal grain size ranged from 0.26 to 1.1 mm, evolved when the equivalent plastic strain exceeded the critical value about 0.5 to 1, and increased with an increase in strain without any large-scale migration of high angle boundaries. The effects of deformation conditions on microstructural evolution of ultrafine grained structures can be summarized into the Zener-Hollomon(Z-H) parameter dependences. The average size and the volume fraction of newly evolved ultrafine grains depend on the Z-H parameter. Decreasing Z-H parameter enhances the formation of equiaxed ultrafine grains. These indicate that the mechanism forming ultrafine grained structures through the warm severe deformation in the present study is similar to ''continuous recrystallization'' or ''in-situ recrystallization'' and that some activation process during or after the deformation plays an important role in the microstructural evolution.

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