Ultrafine/nanostructure steels have been attracted many attentions due to their proper mechanical properties. Martensite process is one of the production approaches of such materials which has made some acceptable progresses in this field. By the way, almost long annealing temperature and limited elongation of such steels have made limitations on its industrial productions. The goal of this investigation is evaluating the effect of niobium addition as a microalloying element on production procedure, microstructural evolution and mechanical properties of ultrafine/nanostructure low carbon steel produced by martensite thermomechanical treatment. For this purpose, three steels with same chemical composition and different Nb content (0.00, 0.04 and 0.12 wt%) were cast and prepared. Hot forging operation was carried out at 1100?C In order to lowering the casting grain size and elimination of probable casting defects. The hot deformed steels were austenitized at 1000 ?C and 1225 ?C and quenched to -15 ?C. Cold rolling up to 85% reduction in thickness was done at room temperature for all samples. The cold rolled samples then annealed at temperature range of 450 ?C to 650 ?C and 50 ?C steps. Hardness measurement approved Nb precipitation in microalloyed steels and microstructural investigation pointed out higher effect of secondary precipitates comparison to primary carbides. Results from microscopic images showed addition of Nb on low carbon steel can reduce required annealing time to access nanostructural steel to 300 s using higher temperature. Wholly ferritic structure with mean grain size of 167.1±23.6 nm and 129.4±12.1 nm was achieved after 300 s annealing at 550 ?C and 600 ?C in steels containing 0.04 and 0.12 wt% Nb respectively. Tension test revealed intense effect of Nb precipitates on improvement of mechanical properties. Superior tension properties were 793 MPa yield strength and 14.8% uniform elongation in 0.04 wt% Nb microalloyed steel and 877 MPa yield strength and 10.7% uniform elongation in 0.12 wt% Nb stell. Key words: Ultrafine/nanostrutural steel, martensite treatment, microalloying element, niobium carbide, uniform elongation.
