| 摘要 |
PURPOSE: A steel sheet having a thickness of at least 0.30 millimeters and formed of an ultra-low carbon steel having a carbon content of at most 0.010% and which can be subjected to heavy but fine forming with reducing the occurrence of forming defects such as pin hole defects and press forming cracks is provided. CONSTITUTION: The ultra-low carbon steel having a chemical composition comprises, in mass percent, C: at most 0.010%, Si: at most 0.5%, Mn: at most 1.5%, P: at most 0.12%, S: at most 0.030%, Al: at most 0.080%, N: at most 0.0080%, one or both of Ti: at most 0.10% and Nb: at most 0.05%, B: 0-0.0050%, V: 0-0.05%, Ca: 0-0.0050%, and at most 0.1% of each of Cu, Cr, Sn, and Sb as unavoidable components, wherein the total number of non-metallic inclusions observed in 60 fields under a microscope in a sample of the steel prepared in accordance with JIS G0555 is at most 20. The method of manufacturing an ultra-low carbon steel sheet comprises the processes of refining in a converter molten steel having a chemical composition including, in mass percent, C: at most 0.010%, Si: at most 0.5%, Mn: at most 1.5%, P: at most 0.12%, S: at most 0.030%, Al: at most 0.080%, N: at most 0.0080%, one or both of Ti: at most 0.10% and Nb: at most 0.05%, B: 0-0.0050%, V: 0-0.05%, and Ca: 0-0.0050%, secondary refining the primarily refined molten steel after primarily refining molten steel in the converter, continuous casting the secondary refined molten steel, and then hot rolling the continuous cast slab, wherein at the time of the secondary refining, the molten steel is tapped into a refining vessel, a vacuum immersion pipe having an interior that can be adjusted to a negative pressure is immersed in the molten steel in the refining vessel, and a stirring gas is blown into the molten steel.
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