Process for producing extra-low-sulfur low-nitrogen high-cleanliness steel through melting

Abstract

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 comingtherein, 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.

Description

Technical field [0001] The present invention relates to a method for smelting ultra-low sulfur and low nitrogen steel with high purity. More specifically, it relates to a product with a sulfur content of less than 10 ppm, a total oxygen content of less than 30 ppm, and a nitrogen content of The smelting method of ultra-low sulfur, low nitrogen and high purity steel below 50ppm. Background technique [0002] In recent years, in order to improve the quality of slabs in 9% Ni steel for thick plates and reduce the change in strength characteristics of high-strength line pipe fittings, the demand for ultra-low-sulfur, low-nitrogen, and high-purity steel has increased. In the case of smelting ultra-low sulfur steel, usually in the pre-treatment process of smelting, by adding CaO, Na 2 CO 3 , Metal Mg and other desulfurizers for desulfurization. However, the molten iron desulfurized in the pre-treatment process of molten iron is due to the infiltration of scrap and sulfur (S) from the ...

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Problems solved by technology

That is, (a) when the Al temperature raising method is used to compensate for the temperature drop in the secondary refining process, except for the 2 o 3 In addition to the increase in the amount of inclusions in the molten steel due to the formation of FeO, it also leads to the Al in the slag 2 o 3 The desulfurization ability of slag decreases as the content increases

(b) Operation of the melt-promoted method using fluxes containing fluorite is constrained by fluorine emission restrictions

In addition, in any of the above methods, no consideration is given to low nitriding

(c) In the manufacturing method of low-sulfur and low-nitrogen steel that deoxidizes the molten steel by adding Al after the surface layer slag is generated on the surface of the molten steel, and then adds a desulfurizing agent for desulfurization, it is also difficult to achieve the super Low Sulfur and Low Nitrogen Grades

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