High-Temperature Reactions and Microstructure of Slag–Iron in the Hydrogen Blast Furnace Hearth Based on Liquid Nitrogen-Quenching and Dissection

To curb CO₂ emissions in ironmaking, hydrogen blast furnace (HBF) technology is a key research focus. This study, based on liquid nitrogen quenching and systematic dissection of a 40 m³ HBF, investigated the high-temperature reactions and microstructure of the slag–iron interface within the hearth. Multiscale characterization revealed a distinct stratification of interfacial reaction products that varied with the depth of the slag–iron layer. In the upper (S1 and S2) and lower (S4 and S5) regions, Ti-compound layers were dominant, whereas desulfurization reactions were more prevalent in the central region (S3). Kinetic analysis indicates that the competition between Ti reactions and desulfurization reactions is essentially governed by the partitioning of carbon resources, with oxygen potential exerting synergistic control. Furthermore, the reduced oxygen potential promotes active Mn participation, thereby promoting the Mn desulfurization pathways. This transition is supported by both the observation of (Mn, Ca)S solid solution precipitation upon cooling and thermodynamic calculations using FactSage. Based on these mechanisms, an integrated optimization strategy is proposed for managing Ti–S partitioning in HBF, which combines the control of burden composition, gas injection, slag properties, and dynamic process feedback. This research offers a scientific foundation for optimizing element partitioning and improving hot metal quality in low carbon ironmaking.