Evolution of Austenite Recrystallization and Grain Growth Using Laser Ultrasonics
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TRODUCTION
AUSTENITE grain size is an important microstructural parameter during the hot rolling of steels.[1–3] The austenite grain size influences the softening behavior during rolling and the subsequent phase transformation during cooling and, therefore, affects the final microstructure and mechanical properties. In modern microalloyed low-carbon steels, a careful choice of the thermomechanical processing route and various grain refiners such as niobium (Nb), titanium (Ti), or molybdenum (Mo) can be employed to achieve the desired final microstructures (i.e., after austenite decomposition).[1] Generally, these grain refiners control the austenite grain structure by (1) retarding austenite recrystallization and grain growth and (2) preserving pancaked austenite at the finish mill exit by particle pinning or solute drag.[4–8] In particular, Nb is very effective in delaying recrystallization.[9–12] Given the significance of the austenite microstructure, it is important to have reliable knowledge of the austenite grainsize evolution during reheating and hot rolling. From an experimental perspective, the investigation of the austenite microstructure is challenging, as this microstructure is not present at room temperature. However, over the years, a number of metallographic tools have been developed to measure austenite grain S. SARKAR, Graduate Student, and M. MILITZER and W.J. POOLE, Professors, are with The Centre for Metallurgical Process Engineering, University of British Columbia, Vancouver, BC V6T 1Z4, Canada. Contact e-mail: [email protected] A. MOREAU, Senior Research Officer, is with the Industrial Materials Institute, National Research Council of Canada, Boucherville, QC J4B 6Y4, Canada. Reproduced by permission of the National Research Council of Canada. Manuscript submitted August 2, 2007. Article published onlined February 8, 2008 METALLURGICAL AND MATERIALS TRANSACTIONS A
structures. These tools include conventional metallography, thermal grooving, oxidation, carburization, glass etching, and ferrite/cementite delineation of prior austenite grain boundaries.[13] All these techniques have their advantages and limitations. Conventional metallography involves heat treating the sample in the austenite temperature range, followed by quenching. Provided a martensite microstructure results from the quench, a suitable etching procedure can be employed to reveal the prior austenite grain boundaries. However, for low-carbon steels with low hardenability, an excessive quench rate (>1000 K/s) may be required to produce martensite, and alternative techniques have to be employed. Some other techniques, e.g., the thermal grooving, glass etching, oxidation, or carburization of austenite grain boundaries, is limited to surface grains in which the grain structure may be different from that of the bulk.[13,14] In addition, none of these techniques can be employed for real-time monitoring of the austenite grain size at high temperatures. Laser ultrasonics is an attractive alternative for the real-time monitoring of grain-size
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