Effect of Nb on the microstructure and properties of Ti-Mo microalloyed high-strength ferritic steel

The effects of Nb on the microstructure and mechanical properties of Ti-Mo microalloyed high strength ferritic steel were systematically investigated. It was found that Nb addition inhibited the bainite transformation and was beneficial for obtaining ferritic steel. Further, Nb accelerated the precipitation in austenite and finer ferrite grains were obtained in Ti-Nb-Mo steel. The strengthening mechanism analysis results of experimental steels showed that the precipitation hardening effect of the (Ti, Mo, Nb)C particles was remarkable due to their larger volume fraction, although their average particle size was greater than that of (Ti, Mo)C in Ti-Mo steel. With the decrease of coiling temperature from 650 °C to 550 °C, the grain refinement strengthening and precipitation hardening of experimental steels increased. When the coiling temperature was 550 °C, the ferritic steel with superior mechanical properties was obtained by means of Ti-Nb-Mo complex microalloying. And the tensile strength, yield strength, and elongation of the Ti-Nb-Mo ferritic steel were 755 MPa, 712 MPa, and 22%, respectively.