A Unified Phase Field Model for Solidification and Solid-State Transformation in Multi-Component Steels: Application to Nigerian Blast Furnace Products

This study develops a unified phase field model capable of simulating both solidification and subsequent solid-state phase transformations in multi-component low alloy steels relevant to blast furnace products from the Ajaokuta Steel Complex, Kogi State, North Central Nigeria. Existing phase field formulations typically address either solidification or solid-state transformation, and a unified model spanning both regimes would enable prediction of final as-cast and post-rolling microstructures from melt chemistry alone. The model framework extends the grand potential phase field approach to a five-component Fe-C-Mn-Si-S system, incorporating thermodynamic data from the CALPHAD ThermoCalc TCFE database. Numerical implementation employs an adaptive mesh finite element scheme in OpenFAST to manage the disparate length scales of dendritic solidification and austenite decomposition. Model validation is performed against experimental solidification and continuous cooling transformation data generated from Gleeble thermal simulation of blast furnace steel compositions representative of Ajaokuta production heats. Parametric studies investigate the influence of composition and cooling rate on primary solidification mode, microsegregation, and austenite-to-ferrite-pearlite transformation kinetics. Model predictions of pearlite interlamellar spacing as a function of cooling rate show agreement within 8 percent of experimental measurement. The study establishes a computational microstructure prediction tool applicable to optimising Ajaokuta steel rolling schedules for targeted product mechanical properties. Keywords: phase field model, solidification, phase transformation, steelmaking, CALPHAD.