Effects of Aluminum and Niobium on Impact Properties of Ultra High-Strength TRIP-aided Bainitic Ferrite Steels

The influence of alloying additions on the impact properties of TRIP-aided bainitic ferrite steel (TBF steel) has been investigated extensively. It has been found that the addition of 0.5 mass% aluminum to a 0.2%C-1.5%Si-1.5%Mn (mass%) TBF steel increases the upper shelf Charpy impact absorbed energy while lowering the fracture appearance transition temperature. This is mainly attributed to the localized stress relaxation due to strain-induced transformation of the fine and carbon enriched retained austenite films. It has also been observed that the addition of both 0.5 mass% aluminum and 0.05 mass% niobium to the steel A improves the impact properties, especially when austempered near the martensite start temperature. This is mainly caused by the refined bainitic ferrite lath structure, stable retained austenite and refined prior austenite grain size achieved by the addition of niobium.