Abstract: The use of biochar can reduce fossil CO₂ emissions from direct reduced iron (DRI)-based electric arc furnace (EAF) steelmaking. The effect of slag B2 basicity and FeO concentration on the interaction between pinebark-based biochar and TiO₂-and V₂O₅-bearing DRI-based EAF slag was examined. For this, a carbonaceous material slag module (CSM) was prepared by placing a slag briquette on a biochar substrate, and the CSM underwent optical dilatometry (OD) at 1773 K in an argon atmosphere to assess the slag swelling index. It revealed that the slags with a B2 basicity of 1.5 and an FeO concentration of 40 wt.% exhibited a higher slag swelling index than the others in their respective categories. The swelling index was observed to decline from B2 basicity of 1.5 to 2.5 and prospers as FeO concentration from 20 to 40 wt.%. A similar trend was confirmed with anthracite-based CSMs. Scanning electron microscopy and energy-dispersive spec-troscopy analyses on the reacted slag from biochar-based CSMs revealed a significant reduction in slag FeO concentration and formation of iron solid solution with vanadium in all samples. The thermogravimetry studies showed that the biochar-slag modules exhibited mass loss correlated with the shape changes of the slag in the OD, confirming the characteristics of interaction between the biochar and the slag.