| 摘要 |
PROBLEM TO BE SOLVED: To reduce the harmful inclusion to HIC-resistant characteristic by spreading CaO-containing flux on the molten steel surface in the vacuum vessel of an RH vacuum degassing apparatus, adding Ca and/or Ca alloy into the molten steel in a ladle while keeping the vacuum after regulating total oxygen concn. and sulfur concn. in the molten steel to the specific ranges and regulating the Ca concn. to a specific range. SOLUTION: The flux 8 is spread onto the surface of the molten steel 3 from a top-blown lance 7 in the vacuum vessel 1. The composition of the flux 8 is desirable to be 40-80% CaO and <=40% Al2O3. In this way, the total oxygen concn. in the molten steel is regulated to 5-15 ppm and the sulfur concn. is regulated to <=4 ppm. Successively, a Ca wire 9 is added into the molten steel 3 in the ladle 2 with a wire feeding method. This additional quantity is desirable to 0.07-0.5 kg/t of molten steel as Ca pure content. At this time, the vacuum degree in the vacuum vessel 1 is desirable to be kept to <=80 Torr. In this way, the Ca concn. in the molten steel is regulated to 5-27 ppm.
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