Interfacial heat transfer and solidification structure of sub-rapid solidified silicon steel using a novel droplet solidification apparatus

A novel droplet solidification technique was developed to emulate sub-rapid solidification and facilitate the formation of deposited films during the strip casting of silicon steels (w(Si): 2.5 and 3.5 wt.%). With the increasing number of droplet ejection experiments, the peak heat fluxes between droplet and substrate decreased firstly (1rd–5th ejection), then increased (5th–7th ejection), and finally decreased again (> 7th ejection). In the first five experiments, the interfacial thermal resistance between the droplet and the substrate improved with increasing film thickness. However, at the onset of the 6th droplet ejection experiment, the deposited film initiated its melting process due to the accumulated thermal resistance, which has the potential to eradicate the cavity or air space existing between the droplet and the substrate. Consequently, the interfacial contact condition was improved gradually with the increasing melting area from 5th to 7th droplet ejection experiments, leading to an increase in heat fluxes. Increased SiO₂ content in deposited films for 3.5 wt.% Si steel led to lower peak heat fluxes than for 2.5 wt.% Si steel. The solidification structure of the 2.5 wt.% Si steel droplet sample comprised a fine grain zone at the base, a columnar grain zone in the center, and an equiaxed grain zone at the top. However, the solidification structure of the 3.5 wt.% Si steel droplet only contained columnar grains and equiaxed grains, with a larger average grain size due to the lower interfacial heat flux.