<p>In this study, the microstructural characteristics of the tertiary oxide scale on the surface of low-alloy steel were systematically analyzed using electron backscatter diffraction (EBSD) technology, and the local strain fields in its three characteristic regions were quantitatively characterized . The comparison of the grain strain distribution patterns of hematite (Fe₂O₃), magnetite (Fe₃O₄), wüstite (FeO), and ferrite phase revealed a pronounced misorientation gradient characteristic in the region surrounding the crack. The data indicated that the Fe₃O₄ phase in both the oxide layer surface and interfacial transition zone exhibited a lower average misorientation angle compared to the Fe₂O₃ phase. Meanwhile, the nano-sized Fe₃O₄ grain clusters enriched at the steel substrate interface triggered sharp local strain concentrations, closely linked to transformation-induced stress accumulation during their nucleation and growth. Furthermore, the reconfiguration of misorientation distribution induced by interfacial stress relaxation during high-temperature deformation validates the modulation mechanism of phase boundary structures on local plasticity. Thus, this research has elucidated the crystallographic defect characteristics of the oxide layer structure by establishing a model of the microscale plastic strain field in the tertiary oxide scale, providing theoretical frameworks for the precise regulation of phase composition and interfacial behavior of iron oxide scale during thermo-mechanical processing.</p>