PROCESS FOR PRODUCING EXTRA-LOW-SULFUR LOW-NITROGEN HIGH-CLEANLINESS STEEL THROUGH MELTING

摘要

A process for producing an extra-low-sulfur low-nitrogen high-cleanliness steel which comprises treating a molten steel through the following steps (1) to (3). Step 1 comprises adding a CaO flux to a molten steel in a ladle at atmospheric pressure. Step 2 comprises setting a ladle lid, bubbling a stirring gas into the molten steel in the ladle to stir the steel while inhibiting the air from coming therein, simultaneously supplying an oxidizing gas to the molten steel, and mixing the resultant oxide with the CaO flux to form a cover slug. Step 3 comprises stopping the supply of the oxidizing gas and bubbling a stirring gas into the molten steel in the ladle to conduct desulfurization and inclusion removal. The lid opening part may be purged with an inert gas. Alternatively, the step (3) may be followed by a step (4) in which the molten steel is subjected to RH vacuum degassing to diminish inclusions contained in the molten steel and conduct denitrification, etc. In this process, the heat treatment of a molten steel, control of a slug composition, stirring of the molten steel and slug, and inert-gas purging of a lid opening part are optimized. Desulfurization and cleaning are hence simultaneously accelerated in a low nitrogen concentration. Thus, an extra-low-sulfur low-nitrogen high-cleanliness steel can be efficiently produced stably.