Effect of rolling conditions on ferrite refinement of low-carbon steel

Refinement of the ferrite grains provides a promising approach to simultaneously improving both the strength and the toughness of steels. Among recent techniques to obtain ultrafine ferrite, dynamic strain induced transformation (DSIT) is used. This type of treatment is sensitive to steel composition, deformation temperature, the prior austenite grain size, strain and cooling rate. This work is aiming to improve the mechanical properties of steels using ferrite grain refining through trials on industrial scale using a compact strip production plant (CSP) with hot strip mill (HSM) containing six stands F1-F6 in EZDK Steel Company. The chemical composition of the trials was C (~0.046), Si (0.073) Mn (0.50) and Ti (0.008). The effect of final deformation temperature of 750, 775, 800 and 815°C on the microstructure and mechanical properties was studied. In addition the effect of strain rate variation at the final two rolling stands was correlated with the ferrite refining mechanisms. The ferrite grain size attained after refining varies from 10 μm to be 6 μm. Yield and tensile strength increased slightly through changing deformation temperature, with the advantage of keeping almost the original high ductility. Toughness of the processed steel increased with ferrite grain refining.