The microstructure evolution and influence factors of acicular ferrite in low alloy steels

The excellent combination of strength and toughness renders acicular ferrite to be a desirable microstructural contexture in C-Mn and high-strength low-alloy steels. The formation of acicular ferrite in steels is generally affected by factors such as composition, cooling rate, prior austenite grain size (PAGS), and impurity inclusions. Due to the complexity of multi-factor coupling, it is difficult to explore the influence of a single factor through actual experiments. In the current work, the influences of cooling rates, PAGS, and impurity inclusion density on the microstructure evolution of austenite-acicular ferrite transformation in low alloy steels are quantitatively evaluated by multi-phase-field simulation. The numerical results demonstrate that reducing PAGS during austenite-ferrite transformation, increasing cooling rate and increasing inclusion nucleation density can effectively refine acicular ferrite grain size. Moreover, the C element diffuses from acicular ferrite to austenite and accumulates at the phase interface during the phase transformation from austenite to ferrite. The findings in the current work contribute to better regulating and designing acicular ferrite in steels.