Recrystallization and grain coarsening control in processing high niobium microalloyed line pipe steels S.V. Subramanian 1,a , Md Kashif Rehman 1,b , H.Zurob 1,c and C. Shang 2,d 1) Department of Materials Science and Engineering, McMaster University, Hamilton, Canada 2) School of Materials Science and Engg., University of Science and Technology, Beijing, China a subraman@mcmaster.ca, b urrehmmk@mcmaster.ca, c zurobh@mcmaster.ca, d cjshang@mater.ustb.edu.cn Keywords: Coarsening kinetics, recrystallization, niobium microalloyed steels, solute drag Abstract: The effect of solute niobium in retarding coarsening rate of austenite in upstream thermo mechanical controlled rolling of high niobium (0.1wt%Nb) low interstitial steel is analyzed. Solute drag effect of niobium in retarding boundary migration in static recrystallization is examined in thermo-mechanical controlled rolling of high Nb microalloyed steel. The importance of austenite grain refinement prior to pancaking in compact strip rolling of high Nb microalloyed steel as a means to increase surface area to volume ratio of pancaked austenite grain is emphasized. This is to promote adequate nucleation sites for phase transformation even under conditions of reduced strain accumulation resulting from low total rolling reduction below temperature of no recrystallization. 1. Introduction: Control of hierarchical evolution of morphological and crystallographic structure is the key to obtaining high strength and good toughness at low temperature in higher grade line pipe steels. Structure-property correlation studies in high niobium ( 0.1wt%Nb) microalloyed steels have underscored the importance of (i) refining the austenite grain size prior to pancaking, (ii) accumulating adequate strain in the pancaked austenite grains to increase the potential nucleation sites for grain refinement of transformation product through phase transformation, and (iii) promoting high density and dispersion of high angle boundaries by phase transformation through control of accelerated cooling and coiling temperature. Extensive studies have been carried out on strain induced precipitation of NbC and its interaction with recrystallization through Zener pinning of boundary to retard recrystallization. However, the significant role of solute drag by solute niobium on recrystallization and grain coarsening of austenite has not received much attention, which is the focus of this paper. Structure-property correlation of niobium microalloyed line pipe steels involves the integration of three processing modules comprising of grain refinement of austenite during roughing and prevention of grain coarsening between roughing and finish rolling ( Module-1), strain accumulation during pancaking of austenite in finish rolling through preventing recrystallization by pinning of boundary by strain induced precipitation of NbC and solute drag effect due to niobium dissolved in the matrix ( Module-2), and control of density and dispersion of high angle boundaries to prevent brittle fracture nucleation through phase transformation of pancaked austenite by accelerated continuous cooling and interrupted hold at coiling temperature ( Module-3). The role of solute niobium in each of these modules is examined using modeling and available experimental results. Materials Science Forum Vol. 753 (2013) pp 391-396 Online available since 2013/Mar/26 at www.scientific.net © (2013) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.753.391 All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 128.210.126.199, Purdue University Libraries, West Lafayette, USA-01/09/14,09:56:44)