International Journal of Metallurgical Engineering 2013, 2(1): 18-26 DOI: 10.5923/j.ijmee.20130201.03 Process Modeling of Niobium Microalloyed Line Pipe Steels S. V. Subramanian 1,* , Md Kashif Rehman 1 , H. Zurob 1 , J. M. Gray 2 1 Department of Mat. Sci. and Engg., McMaster University, Hamilton, Canada 2 Microalloyed Steel Institute, 5100 Westheimer, St. 540, Houston, Texas 77056, USA Abstract Quantitative modeling of (a) strain induced precipitation of NbC and its interaction with recovery and recrystallization, (b) solute drag effect of Nb on boundary mobility have contributed to understanding the evolution of microstructure during thermo-mechanical rolling and accelerated cooling of plates or strips. While a fully integrated model is yet to be developed based on multi-variants associated with time evolution of local chemistry, deformation effects on precipitation and recrystallization and their mutual interactions influencing the microstructure, a modular approach has been developed to control the microstructural evolution at different stages of processing, based on current metallurgical understanding the underlying phenomena controlling the structure at each stage. The key concept is based on integrating modules of upstream austenite conditioning with downstream austenite transformation to control the morphological microstructure, and density and dispersion of high angle boundaries associated with crystallographic structure to which strength and fracture properties of the steel are related. The microstructural control at each of the various stages of hierarchical evolution is captured and integrated in “Through Process Modeling” based on modular approach. The application of process modeling of microstructure evolution based on modular approach for successful development of higher grade line steels based on high niobium (0.1wt%) is the focus of this paper. Keywords Niobium Microalloying, Process Modeling, Recrystallization, Line Pipe Steel 1. Quantitative Modeling of Solidification Behaviour of Microalloyed Steels (Module-1) The precipitation behavior of microalloyed steel is influenced by solute redistribution from dendritic solidification of steel in the caster and the post-solidification cooling schedule. Dendritic solidification of the steel causes a concentration variation on a local scale, which is of the order of the interdendritic spacing. The interdendritric spacing ranges typically from ten to a few hundred μm, depending on the local solidification rates occurring in industrial slabs. During the solidification of steels, the dendrite spacing is empirically related to the local solidification time (the resident time between the liquidus and solidus) raised to an exponent ranging between 0.5 to 0.33; the faster the cooling rate, the finer the interdendritic spacing. If the steady state dendritic solidification is disturbed by fluid flow, the solute enriched liquid may be transported over a large distance, resulting in macro-segregation, notably centreline segregation. While macrosegregation can be prevented by * Corresponding author: Subraman@mcmaster.ca (S. V. Subramanian) Published online at http://journal.sapub.org/ijmee Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved the control of fluid flow in the caster, microsegregation is inherent in the mechanism of dendritic solidification. A quantitative model has been developed to predict the solute redistribution from dendritic solidification for multi-component micro-alloyed steels[1]. A finite difference method is used to predict the microsegregation at a given location (node point) in the slab. The diffusion equation is represented by the Crank-Nicholson implicit method of a finite difference equation. Local equilibrium is assumed at the solid–liquid interface. Multi-component phase equilibria are rigorously calculated from thermodynamic data for the case of Fe-Mn-C because of the large Mn concentration in the steel. The effects of other alloying elements are taken into account as a first order correction to this base. The slab thermal history used in the analysis is typical of an industrial caster. The algorithm results in the computation of the interdendritic microsegregation during freezing (including the effects of solid state diffusion during freezing) at stage-I and modifies this result by accounting for post-solidification homogenisation in the delta ferrite (stage-II), in the delta and gamma field (stage-III) and in the single phase gamma austenite region (stage-IV), see Fig. 1. Quantitative results of microsegregation in a typical high Nb, Ti bearing line pipe steel (0.03 C, 0.003 N, 0.09 Nb, 0.014Ti in wt%) solidifying at a location 5 mm from the slab surface with an interdendritic spacing of 70 μm are analyzed for the typical cooling schedule of the industrial caster. The solute