Grain Refinement of Low‐Carbon Steel by Controlling Superheat during Rapid Cooling

It is challenging to refine grain size for twin‐roll strip casting steels since very limited deformation of ≈30% during online hot rolling can be performed. In this study, it is proposed to control the casting superheat below 50 K to promote grain refinement. By using droplet solidification test to simulate subrapid solidification, the average grain size is refined by more than a factor of three, from 948 to 285 μm as superheat decreases from 80 to 50 K. To explain this phenomenon, the in situ solidification and phase transformation processes are analyzed using high‐temperature confocal laser scanning microscopy. It is found that the low superheat leads to high undercooling of both L → δ and δ → γ nucleation, contributing to a ≈35% reduction in the nucleation energy barrier. Consequently, the nucleation density of δ during L → δ increases by a factor of ≈2. In addition, γ phase nucleates not only along prior δ grain boundaries but also within δ grains, further contributing to grain refinement.