a b s t r a c tThe effect of cold rolling on the microstructure evolution and mechanical properties of Fe-23Mn-0.3C-1.5Al twinning-induced plasticity (TWIP) steel was studied. The extensive mechanical twinning subdivides the initial grains into nanoscale twin lamellas. In addition, the formation of deformation micro bands at ε440% induces the formation of nanostructured bands of localized shear. It is demonstrated that the mechanical twinning is notably important for dislocation storage within the matrix, as the twin boundaries act as equally effective obstacles to dislocation glide as conventional high-angle grain boundaries. However, the contribution of the grain size strengthening to the overall yield stress (YS) is much smaller than that of the deformation strengthening, which plays a major role in the superior work-hardening behavior of TWIP steels. A very high dislocation density of $ 2 Â 10 15 m À 2 is achieved after plastic deformation with moderate strains. The superposition of deformation strengthening and grain boundary strengthening leads to an increase in the YS from 235 MPa in the initial state to 1400 MPa after 80% rolling.

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The influence of cold biaxial forging and annealing on the microstructure evolution and mechanical properties of Fe-18Mn-0.6C-0.1N TWIP steel was investigated. The microstructure after thermomechanical treatment was examined by means of scanning electron microscopy (SEM) with electron back-scatter diffraction (EBSD) analyzer and transmission electron microscopy (TEM). The microstructure containing a high density of deformation twins was evolved by cold forging. Then, the deformation microstructure was recovered or partially recrystallized during subsequent annealing for 1 h at 673 or 873 K, respectively. The cold biaxial forging followed by partial recrystallization resulted in a very attractive combination of mechanical properties. The yield strength and the ultimate tensile strength of the steel after cold biaxial forging were 1320 and 1540 MPa, respectively, with elongation of 6 %. On the other hand, the yield and ultimate tensile strengths of 770 and 1110 MPa, respectively, and elongation about 40 % were obtained after partial recrystallization. The as-forged and recovery annealed samples showed fatigue limit of 600 and 550 MPa, respectively, and the sample after partial recrystallization showed slightly lower fatigue limit of 500 MPa.K e y w o r d s : TWIP-steel, deformation twinning, microstructure, mechanical properties, fatigue