Thermomechanical Processing Design for Microstructural Control in Niobium-Microalloyed Pipeline Steel: An Integrated CALPHAD and FEM Approach

This study develops an integrated computational design methodology combining CALPHAD thermodynamic calculations with finite element modelling of hot rolling mechanics to optimise thermomechanical processing schedules for niobium microalloyed pipeline steel, targeting the mechanical property requirements of API 5L X65 grade for Nigerian deepwater project applications. Niobium microalloying and controlled thermomechanical processing are the primary tools for achieving the combination of high strength, excellent toughness, and weldability required in offshore pipeline steel, and their optimisation through integrated modelling reduces the need for costly and time-intensive experimental trial campaigns. CALPHAD calculations in ThermoCalc quantify the precipitation state of NbC and NbCN as a function of reheating and deformation temperature, providing inputs to a microstructure model for recrystallisation inhibition and grain refinement. The FEM component models rolling pass mechanics and accumulated strain distribution using Forge software, coupled with constitutive equations incorporating recrystallisation and grain growth kinetics fitted to Gleeble thermal simulation experiments. The integrated model predicts final rolled microstructure (ferrite grain size, pearlite fraction) and mechanical properties (yield strength, Charpy energy) as outputs of the processing schedule. Model-guided rolling trials are conducted on Nb-microalloyed steel pilot casts, and final mechanical properties are measured. The model achieves prediction accuracy within 10 percent of experimental values. Keywords: CALPHAD, thermomechanical processing, niobium microalloyed steel, pipeline steel, finite element modelling.