Phase‐Field Simulation and Ferrite Nucleation Evolution during Banded Structure Formation

20CrMnTi is widely employed in gear manufacturing owing to its excellent cutting performance, toughness, and carburizing ability. Nevertheless, this steel has a tendency to form a banded structure. Herein, thermal expansion experiments are initially conducted. The results reveal that as the austenite grain size increases from 3.5 to 30 μm, the relative difference in the number of ferrite grains between the solute‐poor and solute‐rich regions diminishes from 91.9 to 25.6%. This indicates the similar nucleation behaviors of ferrite between two regions, thereby weakening banded phenomenon. Additionally, a phase‐field model and a nucleation model are established. It is found that a larger austenite grain size reduces carbon concentration in solute‐rich regions, which decreases the difference in the driving force for ferrite nucleation between the solute‐poor and solute‐rich regions. Consequently, the difference in the ferrite nucleation rate between two regions decreases, effectively inhibiting the formation of banded structure. Notably, the calculated results exhibit a high degree of consistency with the experimental data obtained. The innovative point of current findings lies in establishing the intrinsic relationship between austenite grain size and banded structure from the perspective of nucleation, enabling more accurate control over the formation of banded structure.