Alloying additions of fine precipitate forming elements can be used to refine the grain size of a steel. A fine precipitate distribution in a steel will restrict the growth of austenite grains at high temperature and will retard recrystallisation of deformed austenite grains. If the combination of precipitates and rolling schedule is used then a 'pancake' structure of deformed austenite grains is created during rolling. These deformed grains provide many nucleation sites for subsequent ferrite formation resulting in a fine grained ferritic structure in the final steel product. The choice of alloying elements is important as the precipitates must be stable at the high temperatures of processing in order to pin the grain boundaries. The typical alloying additions used in High Strength Low Alloy steels (HSLA) are Nb, Al, Ti and / or V. They can be added in isolation or in combination with one another.

Using the diagram above which alloying element gives you the most thermodynamically stable precipitate at 1150°C?