Evaluation of the Austenite Recrystallization by Multideformation and Double Deformation Tests S. Vervynckt 1) , K. Verbeken 1,2) * , P. Thibaux 3) , and Y. Houbaert 1) 1) Department of Materials Science and Engineering, Ghent University, Technologiepark 903, B-9052 Ghent, Belgium 2) Max-Planck-Institut fu ¨r Eisenforschung, Max-Planck-Strasse 1, 40237 Du ¨sseldorf, Germany 3) OCAS N.V., ArcelorMittal R&D Industry Ghent, J.F. Kennedylaan 3, B-9060 Zelzate, Belgium * Corresponding author; e-mail: Kim.Verbeken@UGent.be A high amount of deformation below the non-recrystallization temperature (T nr ) is a common industrial practice to achieve a good combination of toughness and strength in microalloyed steels. To combine the industrially relevant optimum combination of high productivity and product quality, an accurate knowledge of T nr and the recrystallization kinetics is required. Although a lot of literature data is available on the recrystallization behaviour of microalloyed steels, correlations are often difficult to be made due to the effect of different experimental set-ups and test schedules used to obtain this data. Although it would significantly improve the knowledge about these steels, so far, no systematic comparison has been presented in literature to correlate the different techniques one to another. In this study, different hot rolling simulation techniques and testing schedules were compared, within the experimental constraints of the used equipment, to determine the T nr temperature of two microalloyed steels. Good agreement was found between the results from different test equipment. Furthermore, the results from the multideformation tests under continuous cooling conditions could be correlated with the results from isothermal double deformation tests. Keywords: microalloyed steels, static recrystallization, double deformation, multideformation Submitted on 28 July 2010, accepted on 20 October 2010 Introduction Thermomechanical Controlled Processing (TMCP) is a well known way to provide a good combination of strength, fracture toughness and weldability in microalloyed steels [1]. This process is characterized by slab reheating under well defined temperatures, a high amount of hot deformation below the non-recrystallization temperature (T nr ) and accelerated cooling. The non-recrystallization temperature is defined as the temperature below which no complete static recrystallization occurs between the rolling passes. Deformation below T nr causes an accumulation of the deformation which results in the formation of elongated grains and deformation bands. Since both grain boundaries and deformation bands act as nucleation sites for the austenite–ferrite transformation and since the austenite grain elongation means that the grain boundaries are getting closer to each other, it is obvious that the deformation below T nr increases the nucleation density. In combination with the high nucleation rate caused by accelerated cooling, the process finally leads to a significant smaller ferrite grain [2, 3]. To take advantage of rolling under T nr , 50 to 80% of the reduction has to be given below T nr . If this temperature is low, the roughening plate has to wait until it reaches the right temperature to enter the finishing mill. Waiting, however, implies loss of production and money. A better knowledge of T nr and more in general of the austenite recrystallization kinetics could optimize the process and the best mechanical properties could be reached at lower cost. Improvement of processes or adaptations of equipment or temperature programs to new products are usually tested under industrial conditions. Such experiments, however, are known to be time and money consuming. Consequently, preliminary tests and analysis on laboratory scale are essential in order to obtain the required knowledge on the hot rolling behaviour of these materials. Different experimental approaches are used in literature for the study and quantification of the recrystallization kinetics: (1) direct observation methods such as optical microscopy [4] and electron backscatter diffraction [5] and (2) external mechanical methods, such as multideformation tests [6, 7] and double deformation tests [8–15] which are based on material softening assessment. Direct measurement of the recrystallized fraction in microalloyed steels is very difficult. The material transforms during cooling and only special etching techniques can be used to reveal former austenite grain boundaries (e.g. Be ´chet-Beaujard [16]). Moreover, this procedure is tedious and time consuming and in many cases it appears to be impossible to use this technique due to the low hardenability and/or complexities in observing the quenched austenite. Furthermore, it is often difficult to distinguish between recrystallized and deformed prior austenite grains, con- sequently the analysis method involves a certain subjectiv- ity. Therefore, mechanical testing techniques, which can be divided into two main groups, are mostly preferred. The first group are multideformation tests under continuous cooling conditions and are mainly focused on the determination of the non-recrystallization temperature [17]. These type of tests do not provide any fundamental information on the static recrystallization behaviour of microalloyed steels between two rolling passes. Multideformation tests are mainly performed in torsion due to the possibility of DOI: 10.1002/srin.201000167 steel research int. 82 (2011) No. 4 www.steelresearch-journal.com ß 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 369