On austenite conditioning and recrystallization control of higher grade linepipe steels (X100) with niobium and molybdenum additions

In higher strength linepipe steels (API Grade X100), research on structure-property correlation studies has underscored the importance of control of the density and dispersion of crystallographic high angle boundaries, which are effective as micro-crack arresters to suppress brittle fracture, in addition to morphological microstructure design to impart high strength and fracture toughness associated with resistance to ductile fracture. The control of density and dispersion of high angle boundaries, in turn, requires: (i) austenite grain refinement prior to pancaking, (ii) large strain accumulation by suppressing static recrystallization through strain-induced precipitation of NbC with adequate Zener drag force (NbC precipitate) and solute drag (Nb dissolved in matrix) in order to prevent boundary break away, and (iii) adequate hardenability to promote transformation at low temperature under accelerated cooling conditions to produce a lath tructure with high angle boundaries by a displacive rather than a diffusive mechanism. The role of niobium microalloying on austenite conditioning and recrystallization control will be discussed in the light of quantitative modeling of strain induced precipitation of NbC and its interaction with recovery, and the effect of Zener and solute drag on boundary mobility and recrystallization. In order to promote transformation of pancaked austenite within a low temperature window, alloying with molybdenum is found to be effective, as molybdenum operates synergistically with solute niobium to promote transformation at a low temperature to give a fine lath structure having a high density of high angle boundaries. A combination of techniquesinvolving EBSD, HRTEM and atom probe was used to characterize morphological structure, selection of crystallographic variants, nano-scale precipitates and solute dispersion in API Grade X100 containing niobium and molybdenum addition. The concept of hierarchical control on the evolution of microstructure with high density and dispersion of high angle boundaries will be discussed to achieve the target domain size in higher strength linepipe steels,with emphasis on Nb-Mo design in the base chemistry. (AU) Copyright © 2014 Companhia Brasileira de Metalurgia e Mineração (CBMM) All rights reserved.