The synergistic effect of niobium-molybdenum additions on the microstructure of low-carbon bainitic steel

The main purpose of this work was to elucidate the effect of Mo additions on the development of microstructure in hot rolled low-C Nb-containing bainitic steels. Three experimental steels have been investigated; they had the same base composition of 0.05%C-1.7%Mn-0.08%Nb (wt.%); one had no Mo addition and the other two contained 0.1 wt.% and 0.3 wt.%Mo, respectively. The steel strips were manufactured by a combined process of controlled rolling and accelerated cooling. After finish rolling at 900 °C, the steels were treated by accelerated cooling to 650 °C, 550 °C and 450 °C respectively, and immediately held at that temperature for 10 minutes so that the effects on coiling at the different temperatures could be simulated. Tempering treatments at 600 °C for different times ranging from 0.5 to 8 hours were performed to investigate the tempered structures and related properties. Through a series of Optical Microscopy (OM), SEM and TEM examinations and mechanical testing, the results indicated that the steel with the addition of 0.3 wt.%Mo had the advantage of producing a high volume fraction of granular bainite, which gained significant benefits from precipitation hardening whilst achieving increased elongation after the tempering treatment. (AU) © 2015 Companhia Brasileira de Metalurgia e Mineração (CBMM)