Crystallographic and morphological characteristics of acicular ferrite and their formation mechanisms in HSLA steels

The crystallographic and morphological characteristics of acicular ferrite (AF) and their formation mechanisms were investigated in this research. A high-strength low-alloy steel was processed to promote AF formation, and a numerical fitting method was employed to reconstruct the deformed austenite orientations. Comprehensive crystallographic analysis revealed that the crystallographic characteristics of AF are manifested in the selection of variants from multiple close-packed planes (CP) and Bain groups and the near-random variant pairing. This is distinct from those observed in other bainitic microstructures in the literature, exhibiting variants selected from either the same CP or the same Bain group and the preferential variant pairing. These unique crystallographic features arise from multi-variant intragranular nucleation, arrest of lengthening laths and self-accommodation of the transformation shape strain, driven by austenite deformation and proper cooling. Correlative morphology characterization and three-dimensional atom probe results indicate that the boundaries between AF laths become metallographically distinguishable through crystal faceting, martensite/austenite constituent delineating, and surface protrusions induced by carbon segregation at grain boundaries. The morphological features of AF—chaotic grain arrangements and irregular grain shapes—are direct consequences of AF’s distinct crystallographic characteristics and the above boundary revelation mechanisms. These findings advance the understanding and characterization of AF and provide insight into weakening variant selection and forming random variant pairing by austenite deformation and appropriate cooling.