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Austenite decomposition during hot strip rolling of microalloyed low carbon steel Tam, Wing Shan


Hot strip rolling is one of the most critical steel manufacturing processes, and hot rolled products are utilized in a wide range of industrial sectors such as construction, automotive, and pipelines. In the final stage of hot rolling, the steels are cooled by water on the runout table; during cooling, the austenite decomposition takes place that determines the final microstructure and thus the final properties of the hot rolled products. To achieve the desired hot band microstructures and required mechanical properties during hot strip rolling, precise control of the cooling path through the runout table is critical. In this study, the austenite decomposition kinetics during continuous cooling of two microalloyed low-carbon steels have been investigated. Austenite grain growth, deformation and austenite decomposition tests were performed using a Gleeble 3500 thermomechanical simulator. The influence of processing parameters including austenite grain size (10μm to 60μm), cooling rate (3℃/s to 100℃/s), and retained strain (0.2, 0.4) on transformation kinetics and the transformation products under no-recrystallized rolling conditions were examined. With contact dilation measurements on a series of continuous cooling tests, transformation temperature was found to increase with decreasing prior austenite grain size, slower cooling rates and higher retained strain. Microstructure characterization was carried out using optical microscopy and EBSD. At higher transformation temperatures, ferrite-pearlite microstructures were observed. As transformation temperatures decrease, refinement of ferrite grains was seen. For sufficiently low transformation temperatures, bainitic structures were obtained. Hardness measurements obtained showed an inverse relationship with transformation temperature. A microstructure model describing these metallurgical phenomena considering the effect of prior austenite grain size, cooling rate and retained strain has been proposed that is applicable to industrially relevant runout table cooling strategies.

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