Abstract
700 MPa grade Ti and Nb-Ti microalloyed steels were developed to study the effect of Nb on the microstructure and mechanical properties in truck beam steels. The microstructure in Ti microalloyed steel mainly consisted of ferrite, bainite, and pearlite. In contrast, the microstructure of Nb-Ti microalloyed steel was characterized by bainite and low volume fraction of ferrite. Moreover, there was a large volume fraction of nano precipitates and coarse TiN inclusions in both the steels because of high Ti content. The TiN inclusion, together with coarse-grained ferrite, is detrimental to the toughness of steels. This influence is weakened by the addition of Nb because of the remarkable effect on grain refinement. The effective grain size in Ti and Nb-Ti steels was 4.2 and 2.2 μm, respectively. As a consequence, although the strength of the two steels was similar, the impact toughness of Nb-Ti steel was significantly better than that of Ti steel. Additionally, the uniformity in tensile properties of Nb-Ti steel was higher than that of Ti steel, which is beneficial for improving product pass rate and processing.

Key words
Nb, truck beam steel, microstructure, precipitation, mechanical properties

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