Original Article

Kinetics of solid-state decarburization for 3.5% Si electrical steel in CO₂–CO atmosphere

¹ College of Metallurgy and Energy, North China University of Science and Technology, Tangshan 063210, Hebei, PR China
² University of Science and Technology Beijing, Beijing 100083, PR China

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Received: 30 October 2025
Accepted: 7 January 2026

Abstract

In this study, solid-state decarburization experiments were carried out under a CO₂–CO atmosphere using 1 mm thick Fe–0.18%C–3.5%Si alloy strips, with a focus on adapting to electric furnace-based short-process steelmaking. The results indicate that within the temperature range of 1273–1433 K, the solid-state decarburization process can be divided into three distinct stages: rapid decarburization (0–10 min), sustained decarburization (10—30 min), and decarburization stagnation (30–50 min). The optimal decarburization performance was achieved at 1423 K, where the final carbon content was reduced to 0.009%. However, as the reaction proceeded, a SiO₂ oxide layer gradually thickened on the surface and developed trench-like morphologies, which progressively blocked carbon diffusion pathways and eventually led to decarburization stagnation. Kinetic analysis revealed that during the first 10 min of decarburization at various temperatures, carbon diffusion was the primary rate-limiting step, following an apparent first-order reaction model. Concurrently, the oxide layer thickness increased rapidly during this period, exhibiting a strong correlation with the square root of decarburization time, consistent with a diffusion-controlled growth mechanism. Beyond 10 min, the growth rate of the oxide layer slowed and followed a linear relationship with time, indicating a transition to a mixed control regime governed by both carbon diffusion and oxide layer growth. The evolution of oxide layer thickness and morphology—particularly the formation of trench-like structures—directly altered the resistance to carbon diffusion. In the early stage, a thin oxide layer permitted rapid decarburization dominated by carbon diffusion. In the later stages, however, the thickening of the oxide layer impeded diffusion channels, triggering decarburization stagnation governed by mixed control. As a result, the carbon content versus time relationship gradually shifted toward a linear trend, with a decarburization stagnation line emerging once the oxide layer completely inhibited further reaction. Within the temperature range of 1413–1433 K, the oxide growth rate and carbon diffusion rate achieved a favorable balance, resulting in a dynamic mixed-control mechanism. This synergy minimized resistance while sustaining sufficient diffusion, thereby maximizing the overall decarburization efficiency.