Slag foaming is an important control item in the process of refining molten steel. This study aims to
investigate the factors controlling slag foaming by observing the bubble formation behaviour resulting
from the chemical reaction between iron oxide and Fe-C alloy in the slag. In this study, 0.06 g of Fe-
C alloy was introduced at the bottom of a BN crucible, and 6.0 g of slag (SiO2:CaO:Fe2O3 = 40:40:30)
was placed on top of it. The crucible was positioned inside an infrared image heating furnace, where
the temperature was rapidly raised to 1370°C at a rate of 1000°C/min in a nitrogen stream, and then
held at that temperature for a predetermined time before rapid cooling. Following rapid cooling, the
internal structure of the sample was examined using a high-resolution X-ray CT device. The spherical
equivalent volume was calculated based on the number of observed bubbles and their equivalent
circle diameter. Additionally, the relationship between the volume ratio of small bubbles in the slag
volume and the distance from the bottom of the crucible was determined, along with the calculation
of bubble density and volume ratio. Under various experimental conditions, numerous bubbles with
equivalent circular diameters ranging from 200 to 300 μm were consistently observed. The density
and volume fraction of bubbles with circular equivalent diameters between 200 and 500 μm showed
a tendency to increase as the distance from the bottom of the crucible increased. Bubbles with
equivalent circular diameters of 500 μm or more consistently exhibited lower bubble densities,
regardless of their height within the crucible. The formation of larger bubbles is attributed to a
decrease in iron oxide concentration within the slag as the reaction progresses. However, this is
believed to be due to their higher buoyancy, resulting in shorter residence times within the slag and
a reduction in the number of encapsulated bubbles.