Atmosphere-Controlled Solid-State Decarburization and Evolution of Carbon-Gradient Microstructures in Medium-Mn Steel

Abstract

Controlling carbon removal while limiting surface oxidation is essential for constructing composition gradients in high-carbon medium-Mn steel. In this study, solid-state decarburization of Fe-12 wt%Mn-2.7 wt%C alloy sheets was investigated in H2O-H2 and CO2-CO atmospheres by combining thermodynamic calculations with XRD, SEM, OM, EBSD, and GDOES characterization. Thermodynamic analysis showed that, above 1190 K, the critical gas partial-pressure ratio for Fe oxidation is lower in CO2-CO than in H2O-H2, while the competitive-oxidation analysis further indicated a wider selective-oxidation window in the H2O-H2 atmosphere. Experimentally, H2O-H2 produced a relatively uniform oxide layer with a clear interface, whereas PCO2/PCO ≥ 0.29 promoted finger-like MnO growth along grain boundaries in CO2-CO. In both atmospheres, increasing temperature accelerated carbon removal. At 1363 K and 50 min, increasing PH2O/PH2 from 0.47 to 0.51 and further to 0.56 progressively reduced the carbon concentration at a depth of approximately 450 μm from approximately 0.50 to 0.45 and finally to 0.30 at%, demonstrating effective regulation of the through-thickness carbon gradient. EBSD of the specimen treated at 1323 K for 50 min with PH2O/PH2 = 0.47 revealed a near-surface α + γ microstructure and a γ-dominated near-center region, with the number-weighted mean grain size increasing from approximately 14.5 to 59.5 μm. These results establish a processing-microstructure relationship among atmosphere-dependent selective oxidation, carbon removal, and carbon-gradient microstructure formation in medium-Mn steel.