Problems Associated with Modeling Interpass Softening During Industrial Hot Strip Rolling E.I. Poliak* and J.J. Jonas † *Mittal Steel, Research Labs, 3001 E. Columbus Dr., East Chicago, IN, USA † McGill University, Dept. Metallurgical Eng., 3610 University St., Montreal, PQ, Canada *evgueni.poliak@mittalsteel.com , † john.jonas@mcgill.ca Keywords: Hot rolling, interpass softening Abstract. The problems associated with the use of conventional rolling mill models are described. These include the unavoidable variations in temperature and strain rate (rolling speed) during rolling. They are exacerbated by the wide variety of mill types and configurations found in industry and their correspondingly broad ranges of interpass time. Finally, a major limitation arises from the approach currently employed to model the “strain accumulation” attributable to incomplete softening between passes, particularly during the processing of microalloyed steels. Introduction The hot rolling of strip is a most sophisticated rolling process from a metallurgical point of view because, in a single sequence, the speeds can vary by two orders of magnitude and the temperature of the bar can follow complicated paths. Furthermore, a wide variety of types of mill produce steel strips. There are conventional hot strip mills, with either continuous or reversing roughing, followed by multi-stand tandem finishing. In these mills, the two trains are separated by delay tables whose lengths, and hence the related variations in bar temperature, are different for different mills. Some mills of this type utilize zoom rolling, where the finish rolling speed increases along the length of the bar. Many hot strip mills are designed with a coilbox placed in front of the finishing train. In this case, the temperature of the transfer bar is equalized prior to finish rolling. The holding time in the coilbox can vary substantially, depending on steel chemistry, product type and mill performance; thus the bar temperature profile will differ considerably compared to that observed in conventional mills with only a delay table. Hot strip is also produced using single- or two-stand reversing Steckel mills with heated coilboxes installed on both sides of the mill. In contrast to conventional mills, the interpass times in Steckel mills are much longer, but the pass temperatures remain almost constant throughout the entire sequence. Hot strip can also be produced by direct rolling processes using only a few heavy rolling reductions and unique temperature profiles. Because of the wide range of mill parameters described above, many mills continue to encounter problems, particularly when rolling microalloyed grades; these are generally related to appreciable rolling loads, strip shape and profile problems, rolling stability issues, process control, etc. To a large extent, such production difficulties can be attributed to the high sensitivity of microalloyed steels to the mill configuration and to the related thermal routes and timing. These factors have a major influence on the deformation behavior of microalloyed steels and especially on their behavior between deformation operations, making such grades extremely vulnerable to variabilities in the chemistry and processing conditions. In particular, it is the modeling of the interpass softening behavior that so complicates process control during the hot rolling of microalloyed steels. For all types of mill, the on-line process control and off-line computer simulations employ similar models. These are quite cumbersome and are overloaded with a large Materials Science Forum Vols. 500-501 (2005) pp 211-220 Online available since 2005/Nov/15 at www.scientific.net © (2005) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.500-501.211 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-29/08/14,08:18:21)