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Coincidence Site Lattice (CSL) grain boundaries are special boundaries that enhance a material’s resistance to hydrogen embrittlement, corrosion, and fracture. Despite their importance, the evolution of CSL grain boundaries in body-centered cubic (bcc) materials, particularly in ferritic steels, has not been systematically explored. This study aims to investigate the formation and evolution of CSL grain boundaries in ferritic steels subjected to varying levels of mechanical deformation and subsequent heat treatments. The study employs a two-step heat treatment process, involving austenitization at 950 °C followed by water quenching, and subsequent tempering at 760 °C. The results indicate that samples with 25% deformation degree, subjected to shorter austenitization times, exhibit the highest CSL fractions, with values exceeding half of the total grain boundary length. However, prolonged austenitization leads to a general decrease in CSL boundary fraction, favoring random grain boundary formation, except for the highest deformed sample, which deviates from this trend, exhibiting a progressive increase in CSL fraction with longer austenitization times. Grain size analysis indicates a strong correlation with CSL fraction, with the highest CSL grain boundaries occurring around . The CSL type distribution analysis confirms as the dominant boundary across all conditions, while higher-order CSL boundaries exhibit no clear correlation with deformation or heat treatment parameters.