Strategies for nitrogen control during the production of interstitial-free steel

Interstitial-free (IF) steel is a micro-alloyed steel that contains significantly low concentrations of interstitial elements such as carbon and nitrogen. In this context, nitrogen control in liquid steel is a major issue, particularly in cases where the nitrogen content in hot metal or other charged material is high. This study describes the practice of IF steel production at No.3 steelmaking plant of WISCO using hot metal containing high nitrogen due to hot metal desulphurisation treatment with nitrogen as the carrier gas. By adapting new technology for nitrogen removal in a combined blown converter and an Ruhrstahl Heraeus vessel as well as controlling nitrogen pick-up during transfer operations, final product can be generated with an average nitrogen content of about 18 ppm and with some values as low as 10 ppm.