Abstract <p>This paper systematically investigates the role of naturally deposited film in modulating interfacial heat transfer during sub-rapid solidification of medium-Mn steel. Utilizing the droplet solidification apparatus combined with multiscale characterization methods, the formation mechanism of deposited film and its effect on interfacial heat transfer and sub-rapid solidification structure were revealed. Element mapping confirmed the homogeneous distribution of Mn, Si, O, and Fe elements in the deposited film, with phase components of Mn (55.80 wt%), MnO (40.05 wt%), Fe₃O₄ (1.25 wt%), and SiO₂ (2.90 wt%). With the increasing number of droplet ejection experiments, the peak interfacial heat fluxes decreased firstly (1st–3rd ejection), then increased (3rd–5th ejection), and finally decreased again (&gt; 5th ejection). This is because the presence of the film with low thermal conductivity increased the interfacial thermal resistance at the beginning, but the film started to melt and fill the air gap gradually after the 3rd ejection, causing the decreasing of interfacial thermal resistance. After the gap was filled, the continued deposition of film increased the thermal resistance again and led to the decreasing of the heat flux. With the continued deposition of film, the variation trend of the primary dendrite (close to copper substrate) proportion in the solidification structure of sub-rapid solidified droplet was consistent with that of the peak heat flux, as the formation stage of primary dendrite corresponded to the time period during which the peak heat flux occurred.</p> Graphical Abstract <p></p>