• 3D X-ray microscopy reveals that increasing slag basicity from 0.52 to 1.03 and temperature from 1423 K to 1573 K reduces viscosity and accelerates bubble floating rates.
• Longer holding times (10 to 30 s) increase bubble floating distance and decrease the volume fraction and average equivalent sphere diameter of bubbles solidified in mold flux.
• Higher basicity, temperature, and holding time enhance the removal rate of bubbles, particularly for larger bubbles, improving mold flux performance.
• The findings provide critical insights for optimizing low basicity mold flux formulations for high-Mn high-Al steel continuous casting, potentially reducing defects and improving process stability.