OBRABOTKAMETALLOV technology Vol. 26 No. 3 2024 Fig. 18. Rolling strategies from conventional rolling to thermomechanical rolling with accelerated cooling [103] High-speed deformation occurs immediately below the γ→α transformation temperature. Due to the heat generated by the deformation, the ferrite turns into austenite for a while before changing back to ferrite. High ferrite stress is used to initiate dynamic recovery. However, this approach cannot result in ultrafine grain size, but ferrite grain size of ~ 3 μm can be achieved. This is due to the fact that further grain refinement in ferrite is very difficult due to the low strain hardening index and higher stacking fault energy of ferrite. Deformation of coarse-grained austenite beyond the critical strain leads to intragranular nucleation of ferrite within the austenite grains, which leads to significant refinement of the ferrite grains. This mechanism is believed to work by forming a layer of ultra-thin ferrite on the surface of a thin strip. Fig. 19. Scheme of thermomechanical rolling and cooling of high-strength sheets or strips [103]
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