Dual phase steels (DP) consisting of structures of ductile ferrite and hard martensite islands show superior mechanical properties to commercially available high strength low alloy steels. In this study, the effects of hot rolling parameters such as reduction percentage, rolling and coiling temperature on the microstructure and mechanical properties of dual phase steel with and without niobium were investigated. It is well known that one of the key factors to achieve a dual phase structure is the chemical composition, so, an appropriate chemical composition was chosen in order to produce as-rolled DP steel. Melting process was carried out with an induction furnace. Then the product were forged and homogenized before undergone rolling process. Three different thickness reduction (i.e., 15, 30 and 50%), two different rolling temperatures (950 and 850 °C) and two coiling temperatures (550 and 400 °C) were used. For identify the mechanical properties and microstructural evolution; the tension tests, microhardness measurement and optical microscopic observation were applied. The results showed that all specimens displayed a continuous yield point. Rolling in dual phase region and coiling at low temperature demonstrated elongated grains in the rolling direction. Those elongated grains were not observed as coiling temperature increased. In rolling of the DP steel; with a constant rolling and coiling temperature increasing thickness reduction, increased yield and tensile strength and decreased the total elongation. It was also, observed that the coiling temperature have an important effect on proportion of martensite phase, as this temperature decreased the proportion of martensite was increased. Comparison of specimens with and without Nb showed that specimens containing Nb had higher yield and tensile strength, martensite proportion, ferrite microhardness and lower total elongation and ferrite grain size. It was also observed that in specimens with Nb, bainite and pearlite phases couldn’t form .
