Optimum Microalloying of Niobium and Boron in HSLA Steel for Thermomechanical Processing

Abstract

For application to thermomechanical processing (TMP) with acceleated cooling, effect of a combined addition of boron and a grain-refining element such as niobium, titanium or vanadium in very low carbon steels was studied in terms of mechanical properties and microstructures. It was found that, although boron as a single addition has little effect on the properties, the combined addition of niobium and boron improves the strength and toughness balance. A separate addition of boron produces coarse Fe₂₃(CB)₆ precipitates at austenite grain boundaries after rolling and boron is consumed to invalidate the effect of γ-α transformation suppression. Niobium addition to boron steel suppresses the precipitation of Fe₂₃(CB)₆ and strongly retards the γ-α transformation to form a finegrained bainitic structure. Titanium has the same effect as that of niobium, but vanadium does not. The strengthening and toughening mechanism by the combined addition of niobium and boron was also investigated by examining the effect of alloying elements on the minimum recrystallization temperature of austenite during rolling, and γ-α transformation behavior.