Quantitative modelling of solidification, precipitation and recrystallization behaviour of microalloyed plates recrystallization behaviour of microalloyed plates

Quantitative models have been developed to aid in the design of base chemistry and optimization of process variables for improved strength and toughness of Ti-Nb bearing line pipe steels. The model predicts the precipitate evolution from solidification processing of the slab through to finish rolling of the plate. A quantitative model has been developed based on finite difference method to predict the microsegregation and post-solidification homogenization for a multi-component system typical of line pipe chemistries for a given location in the slab. The model is used to optimize the base chemistry and cooling schedules in the caster to give a homogeneous microstructure. A quantitative thermo-kinetic model has been developed to predict the precipitation and recrystallization behaviour of microalloyed plates, following the approach of Dutta and Sellars. In the case of niobium steel the predictions of strained induced precipitation of NbC and precipitate interaction with static recrystallization are in agreement with the hot torsion experimental results. However, in Ti-Nb bearing steels, particularly wherein TiN particles are well dispersed, epitaxial growth of NbC occurs on preexisting TiN particles. In consequence the retardation of recrystallization sets in at elevated temperature during hot rolling. The kinetics of diffusion controlled growth of NbC for the above case is analyzed quantitatively for continuous cooling rates typical of mill processing schedules to predict solute niobium at finish rolling temperatures. The power of quantitative modelling is used to examine the scientific merits of a design based on low interstitials (C: 0.03, N: 0.003), titanium to the stoichiometric requirement to form TiN (Ti: 0.014) and high niobium (Nb = 0.095) to yield a high solute niobium at finish rolling temperature.