Production method for continuous casting cast billet

A method of manufacturing a continuously-cast slab by continuously casting molten steel in such a manner that a direct current magnetic field zone is applied to the cast slab over the entire width thereof in a direction across the thickness thereof at a position a predetermined distance below the molten metal level to casting direction in a continuously-casting mold and that the molten steel is poured using an immersion nozzle into a molten steel pool in or above the direct current magnetic field zone, the method comprising the steps of providing the immersion nozzle with ejection holes in at least upper and lower two stages;

• disposing at least one lower ejection hole such that it satisfies the following formula (1);
• setting the supply rate Q′ of the molten steel supplied from upper ejection holes smaller than the rate Q consumed by solidification in the molten steel pool above the center of height of the direct current magnetic field zone;
• and adding a particular solute element to the molten steel in or above the direct current magnetic field zone, thereby adjusting the concentration of the solute element in the surface layer portion of the cast slab by increasing the concentration of the solute element as to the molten steel in the upper pool.
• 0<h<(½)·w·tan θ (1)
• where,
• θ:
• downward angle of lower ejection hole(s) (°);
• w:
• length of mold in width direction (m);
• and
• h:
• distance from center of lower ejection hole to center of height of magnetic pole (m)