Effect of Processing Parameters and Microcontent Nb on the Microstructure and Properties of Medium Carbon Steels

Abstract — Niobium-microalloyed steel has become a standard material in plate and strip for line pipe, automotive and construction use. The effects of controlled rolling parameters and microcontent Nb on the mechanical properties of medium carbon steels were studied in this work. For these purpose two steel grades, with and without niobium, was planed and after steelmaking and continuous casting, rolling process was done. Then, laboratory investigations such as microstructure, mechanical properties and grain size analysis were performed. Tensile and Charpy impact tests specimens were machined out of the central part of the rolled billets. The microstructure of the specimens was examined for each experimental condition using optical microscopy. The results indicate that, increasing the reheating temperature above the dissolution temperature of (Nb)(C, N) improved the impact energy values. By increasing the cooling rate from 0.5 to 1.5°C/s both tensile strength and impact toughness were improved. High elongation percent was also observed on samples reheated at higher temperature and/or cooled with the higher cooling rates. The obtained mechanical properties were related to the characteristics of microstructural components including acicular ferrite, retained austenite, pearlite and ferrite