Multiphase bainite - martensite steels: The significant impact of niobium microalloying on structure and mechanical behavior

We underscore here that microalloying of multiphase steels with niobium (Nb) introduces significant changes in phase transformation through modification of continuous cooling transformation diagram, refinement of phases and consequently mechanical properties, inclusive of crack propagation. Minor additions of niobium enlarged the bainite region, increased the martensite start temperature and refined the bainitic laths. This led to increase in the crack propagation threshold (ΔKth) of Nb-bearing steel to 13.68 MPa m1/2. In a similar manner, tensile strength and impact strength were enhanced. The Nb-induced microstructural changes were consistent with thermal simulations. The study demonstrates that microalloying with Nb is a viable approach to enhance the mechanical properties of multiphase high-strength steels, an aspect not widely explored in multiphase steels.