Grain refinement of the metallic alloy is one of the important strengthening mechanisms as is the only way to improve both strength and ductility. Cold plastic deformation and subsequent annealing of low carbon steels with martensitic structure, known as the martensite process, is one of the advanced thermomechanical methods to achieve ultrafine grained steels. Nevertheless, the need to apply large thickness reduction during cold rolling, relatively long annealing time and low uniform elongation have made some difficulties i the production of such steels. Since the warm deformation processes such as warm rolling, forging, and extrusion have extensively used in industry, attempts have been made in this study to apply the warm deformation in the martensite process to improve the mechanical properties. For this purpose two plain carbon and Nb steel (0.04%wt) samples were cast. The samples were austenitized at 1000 and 1200 followed by quench at -15 was performed. Microstructural characterization was conducted by color etching and X-ray diffraction (XRD) techniques. Cold rolling up to 50% reduction in thickness was done at room temperature followed by warm rolling up to 50% reduction in thickness at temperature range of 450 to 700 with temperature increment of 50 was carried out on both steel. The results showed a ferritic structure with the mean grain size of 120 nm and 198 nm for the plain carbon steel and the mean grain size of 236 nm and 3.8 µm for the Nb-containing steel at the temperature of 600 and 650 . The lower grain size in the microalloyed steel was attributed to the influence of Nb carbide on grain growth. The hardness results of samples which were warm rolled at different temperatures confirmed the dynamic precipitations of Nb. The promising tensile properties of 846 MPa yield strength and 10% uniform elongation in Nb microalloyed steel and 521 MPa yield strength and 16% uniform elongation in plain carbon steel were achieved. Keywords: Advance thermo-mechanical processing, Ultrafine grained structure, Warm rolling, Martensite treatment, Plain carbon steel, Niobium.
