Effect of Microalloying strategy on the evolution of austenite grain structures during soaking of Microalloyed HSLA steels

Uniform prior-austenite grain (PAG) structure is essential to achieve a fine and uniform ferrite grain size after processing. However, some microalloyed steels develop bimodal γ grain size distribution during soaking. This study compares the grain growth during reheating in as-cast steels with different combinations of Nb, Ti, V, and Al. Nb-bearing steels exhibit bimodality at 1150–1200°C due to inhomogeneous Nb(C,N) precipitation and differential pinning. Conversely, TiN remains stable and suppresses coarsening, V has minimal influence, and AlN provides uniform grain control up to ∼1150°C. Two established grain-growth formulations and an empirical correlation were evaluated to predict average austenite grain size. The correlation showed the best agreement, while a Bimodality Susceptibility Index (BSI) identified alloy–temperature combinations susceptible to bimodal grain growth.