The microstructure with suitable boundary characters for superplasticity is summarized for the steels which consist of two phases, i.e., ferrite (bcc alpha) + austenite (fcc gamma) or ferrite (alpha) + cementite (orthorhombic theta-Fe3C). In (alpha + gamma) duplex alloys, a conventional thermomechanical processing (solution treatment + heavy cold rolling + aging) produces the (alpha + gamma) duplex structure through the competition of recovery/recrystallization of matrix and precipitation. In Fe-Cr-Ni (alpha + gamma) duplex stainless steels with high gamma fractions (40-50%), alpha matrix undergoes recovery to form alpha subgrain boundaries and gamma phase precipitates on a subgrain boundaries with near Kurdjumov-Sachs relationship during aging. By warm deformation, the transition of a boundary structure from low-angle to high-angle type occurs by dynamic continuous recrystallization of a matrix and, simultaneously, coherency across alpha/gamma boundary is lost. Contrarily, alpha phase first precipitates in deformed gamma matrix in Ni-Cr-Fe based alloy during aging. Subsequently discontinuous recrystallization of gamma matrix takes place and the (alpha + gamma) microduplex structure with high-angle gamma boundaries is formed. The formation of those high-angle boundarie in (alpha + gamma) microduplex structure induces the high strain rate superplasticity. In an ultra-high carbon steel, when pearlite was austenitized in the (gamma + theta) region, quenched and tempered at the temperature below A(1), an (alpha + theta) microduplex structure in which most of a boundaries are of high-angle type is formed through the recovery of the fine (alpha' lath martensite + theta) mixture during tempering. Such (alpha + theta) microduplex structure with high angle a boundaries exhibits higher superplasticity than that formed by heavy warm rolling or cold rolling and annealing of pearlite which contains higher fraction of low angle boundaries. (C) 2005 Springer Science + Business Media, Inc.