0.005%C-≤0.005% N ultra-low carbon and ultra-low nitrogen steel by a 90 t top and bottom combined blown converter a Wuhan Iron and Steel: single slag process KR desulphuration -LD - RH-KTB - CC and two-step steelmaking process KR desulphuration - LD desiliconisation and dephosphorization-LD combined blown decarbonation - RH-KTB- CC have been analyzed. The practice results showed that during steelmaking with single slag process the average C content in liquid steel in tundish was 25.5×10
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and the average N content was 18.5×10
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and with two-step process the average C content in liquid steel in tundish was 16×10