<p>Porosity is one of the major defects in the production of high-nitrogen steel (HNS) billets, severely affecting product quality and productivity, and hindering the broader application of HNS. In this study, the behavior of nitrogen bubbles precipitating at the solidification front is investigated. A physical model for bubble growth and detachment was developed based on the solubility of nitrogen in molten steel. Combined with theoretical derivation, the influence of various factors—such as gas flow rate, orifice diameter, and wall inclination—on bubble growth, detachment, and ascent was analyzed through hydrodynamic experiments. Drawing on prior studies of bubble behavior under subcooled boiling conditions, a force balance analysis was performed to derive a critical detachment condition. The calculated bubble detachment radii under different conditions showed good agreement with experimental results, confirming the validity of the proposed model.</p>