<p>The microstructural evolution during friction stir welding (FSW) of martensitic steel is difficult to interpret, primarily due to the austenite-to-martensite transformation that occurs during post-weld cooling. This study utilized electron backscatter diffraction (EBSD) data to reconstruct prior austenite grains (PAGs) from the martensitic microstructure, enabling detailed evaluation of the laser scanning speed’s impact on the FSW process. The results indicate that lower laser scanning speeds lead to larger PAGs and higher aspect ratios of lath martensite in laser-treated steel plates. Compared with conventional FSW (FSW-C), laser pre-treatment reduces the axial force (Fz) exerted on the welding tool. In the stir zone (SZ), PAG size decreases with slower scanning speeds, and the shear texture comprises A and C components. At the highest scanning speed (120&#xa0;cm/min), a thermo-mechanically affected zone (TMAZ) exhibits the lowest ferrite content, and increased carbide precipitation is observed throughout the heat-affected zone (HAZ). Analysis of martensitic variants reveals that severe plastic deformation during FSW suppresses variant selection, leading to a predominance of close-packed (CP) variants. The joint welded at the highest scanning speed exhibits the greatest ultimate tensile strength (UTS), attributed to the minimal ferrite content at the fracture location.</p> Graphical Abstract