Abstract: A gradient structure with ferrite-martensite-ferrite layers was obtained in a 0.2%C-2%Mn plain steel by applying ultra-fast cooling during intermediate passes of hot rolling. The large deformation and warm rolling of steel plate with reduction of about 50% is realized， which provides heavily deformed martensite to form ultrafine-grained ferrite with average grain sizes of 0.52μm and 0.66μm by annealing at 450℃ and 530℃， respectively. The microstructure and mechanical properties of the ultrafine-grain steel sheets were studied by scanning electron microscopy(SEM)， electron backscatter diffraction(EBSD)， and quasi-static tensile tests. The results show that the yield strength of the ultrafine-grained steel is 2~3times higher than that of coarse-grain steel annealed at 610℃， arriving at 1475 MPa and 1196 MPa respectively by annealing at 450℃ and 530℃， but the elongation is significantly reduced. Grain boundaries strengthening and dislocations strengthening are the major strengthening mechanisms for improving the strength of ultrofine-grained steel， while the decrease of work hardening rate leads to the decrease of plasticity of ultrafine grained steel.

Key words: gradient structure; ultrafine-grained steel; microstructure; strength; plasticity