Hydrogen and nitrogen control in steelmaking at U. S. Steel

The field of steelmaking has seen an increased demand in reducing and controlling the amounts of dissolved gases in steel. Hydrogen and nitrogen are two of the most important gases which, when dissolved in liquid steel, affect its properties significantly. Both of these gases can enter the liquid steel either through steelmaking additions or by reaction of the liquid metal with the atmospheric elements. At United States Steel Corporation (U. S. Steel), empirical evidence has shown that hydrated scrap, lime, and coke additions are major contributors to hydrogen pickup in liquid steel. Similarly, nitrogen impurities in ferroalloys, coke, and scrap are identified sources of nitrogen. In addition, the presence of measurable traces of nitrogen in oxygen gas used at the BOP and Q-BOP has also resulted in elevated levels of nitrogen pickup. There is also an increased likelihood of higher hydrogen and nitrogen in liquid steel from overblow and reblow situations. This additional pickup of hydrogen and nitrogen gases in steel will not only affect the properties of steel; there is significant potential for hydrogen-induced sticker breakouts to occur at the continuous caster, which could result in significant maintenance costs and productivity losses. Therefore, it is imperative to accurately quantify the amounts of hydrogen and nitrogen in liquid steel. Online hydrogen measurement uses measured hydrogen partial pressure in collaboration with equilibrium constants and interaction coefficients relevant for the hydrogen dissolution reaction. To ensure accurate hydrogen readings from the instrument, those thermodynamic values were reviewed, considering the changes in chemistry and temperature in the steelmaking processes. Similarly, precautions dealing with sample preparation to ensure accurate and reproducible nitrogen measurements using optical spectrometric techniques are identified. Discussions on the potential hydrogen-induced breakouts, when uncontrolled and significantly high levels of hydrogen are present in the liquid steel, are also provided in this paper.