<p>This study investigates the high-temperature oxidation behavior of 430F and 430FR1 stainless steels using the weight gain method. Oxidation kinetics, oxide scale morphology, and phase composition were analyzed to understand the oxidation mechanism. The results show that oxidation of 430FR1 steel follows a parabolic rate, controlled by diffusion. The oxide scale is mainly composed of spinel-type oxides containing O, Si, Cr, and Mn, with slight compositional variations. The oxide scale on the 430F steel is significantly thicker than that on the 430FR1 steel, with cracks forming beneath the 430FR1 scale. Silicon oxide is located in the inner layer, while manganese–chromium oxides form the middle and outer layers. Both steels exhibit intergranular oxidation, and Si enhances Cr diffusion, slowing intergranular oxidation in 430FR1. The rapid oxidation of Si forms voids near the surface, where MnS migrates and aggregates. Mn then oxidizes further, migrating to the surface and causing microcracks. The surface oxide scale is composed of SiO₂, MnCr₂O₄, Cr₂O₃, and Fe₂O₃ in sequence.</p>