Evolution of Ultrafine-grained Structure through Large Strain-High Z Deformation in a Low Carbon Steel

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Abstract
Ultrafine-grained structure formed dynamically through severe plastic deformation at elevated temperatures has been investigated in a 0.1% C-0.4% Si-1.5% Mn steel. The effects of the strain, the strain rate and the deformation temperature on the microstructural evolution were examined using a compression technique with a pair of anvils. The maximum strain was 4, the deformation temperature was below the Ac1 temperature, and the Zener-Hollomon parameter (Z) ranged between 1012s-1 and 1016s-1. The ultrafine ferrite grains surrounded by the high angle boundaries evolved when the equivalent plastic strain exceeded the critical value of about 1. The number of newly evolved ultrafine grains increased with the strain; however, the average sizes did not depend on the strain. The grain size, d, depended on the strain rate and the deformation temperature. An equation, d(μm)=102.07 Z-0.16, was obtained experimentally. The details of the phenomena are compared with the dynamic discontinuous recrystallization in ferritic steels.