Multi-constraint slag design and dephosphorization modeling for 65Mn steel at a high-carbon end point in a single-slag BOF process

To improve the poor and unstable dephosphorization of 65Mn high-carbon steel in a single-slag basic oxygen furnace (BOF) process with a high-carbon end point, a multi-constraint slag design strategy was established by combining FactSage calculations, industrial trials, and slag microstructural characterization. In the CaO–SiO 2 –FeO–MgO–Al 2 O 3 system, feasible slag compositions were screened by jointly considering liquidus temperature, apparent viscosity, and dephosphorization potential. With MgO and Al 2 O 3 fixed at 8 wt.% and 3 wt.%, respectively, the optimal slag window was determined as 34–38 wt.% CaO, 17–20 wt.% SiO 2 , and 20–24 wt.% FeO, corresponding to a basicity (R = CaO/SiO 2 ) of 1.8–2.2, under the constraints of liquidus temperature below 1350 °C and apparent viscosity at 1400 °C below 0.5 Pa·s. Industrial trials in a 120 t BOF demonstrated that controlling FeO at 20–24 wt.% and basicity at 2.0–2.6 reduced the average end-point phosphorus from 0.026 wt.% to 0.015 wt.% and markedly improved process stability. The dephosphorization rate ( η P ) increased from 70.5% to 85.2%, while the phosphorus partition ratio ( L P ) increased from 62.0 to 78.0. Microstructural analyses revealed that efficient dephosphorization was associated with the preferential formation and continuous distribution of P-enriched 2CaO·SiO 2 –3CaO·P 2 O 5 (C 2 S–C 3 P) solid solutions, whereas the enrichment of FeO–MnO–MgO divalent-oxide solid-solution phases (RO phases) and Fe-rich phases reduced the effective basicity and hindered mass transfer. Finally, a slag prediction model coupling thermodynamic calculations with mass and energy balances was established to output recommended basicity and FeO control windows, a target end-point temperature, and the predicted end-point phosphorus, offering practical guidance for the stable production of high-carbon low-phosphorus steel.