Role of niobium in a dual-phase steel

Abstract

Two hot-rolled high-strength, low-alloy steels, one microalloyed with niobium, were intercritically annealed at 740°C for 10 min and cooled to produce a dual-phase microstructure. Although both steels showed tensile properties typical of dual-phase microstructures, only the niobium-microalloyed steel could match the mechanical characteristics of a typical commercial dual-phase steel. The role of the niobium would appear to be to provide a suitable level of strength in the initial, as-rolled state of the steel, since the properties in the dual-phase state were found to be a function of the initial state of the steel. A normalizing treatment resulted in grain refinement in both steels, but transmission electron microscopy observations showed that in the niobium-containing steel this was counterbalanced by an increase in Nb(C, N) precipitate size. The normalizing treatment thus equalized the mechanical properties of the initial state of the two steels, leading to similar dual-phase properties in both cases; this observation confirms the importance of the initial state of the steel in obtaining a desirable set of mechanical properties after intercritical treatment. Of course, this initial state could be obtained in a number of ways, and microalloying with niobium represents only one such way.