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Description
Intergranular oxidation during the hot-rolling "laminar cooling-coiling-cooling" process critically impacts surface quality and formability of hot-stamped steel. This study investigates 22MnB5 steel through industrial coil sampling and controlled atmosphere simulations (air/vacuum), establishing formation mechanisms and process optimization strategies. Significant positional variations in oxidation depth were quantified: central regions exhibited 15 μm depth versus 0–5 μm at edges, resulting from differential cooling rates and oxygen accessibility post-coiling. Under air atmosphere, oxidation depth followed a characteristic "C-curve" with temperature, peaking at 15 μm near 700°C. Vacuum conditions triggered FeO decomposition, converting protective oxide scales into internal oxygen sources that increased oxidation depth monotonically from 600°C to 900°C (26 μm at peak). Thermo-Calc thermodynamic simulations elucidated FeO decomposition-driven oxygen diffusion and composite oxide-mediated maintenance of localized low-oxygen partial pressure. Transmission electron microscopy revealed oxidation products with distinct "core-shell architecture": preferential SiO₂ nucleation by Si, Cr-enriched shell precipitation, and rapid Mn diffusion filling grain boundary vacancies to form multicomponent oxides.
This study challenges the conventional wisdom that "low-oxygen environments suppress intergranular oxidation", establishing a novel theoretical framework for understanding post-coiling oxidation behavior under internal low-oxygen conditions. Findings directly correlate with industrial quality control metrics. Based on FeO decomposition-driven oxygen supply mechanisms, this work implements low-temperature coiling (<600°C), accelerated front-section cooling, and forced-air post-coiling cooling to effectively controls oxidation depth below 5 μm. This achieves a closed loop from theoretical discovery to engineering application, providing both fundamental principles and actionable guidelines for high-surface-quality hot-stamped steel production with strong industrial scalability.