Kinetics of Nitrogen Removal from Liquid Third Generation Advanced High-Strength Steel by Tank Degassing

The feasibility of vacuum degassing to remove nitrogen from 3rd Generation Advanced High Strength Steel (AHSS) has been evaluated. AHSS has high Al, Si, and Mn concentrations relative to aluminum-killed steel. The feasibility of nitrogen removal has been evaluated by collating thermodynamic information, collating and measuring interfacial kinetics, and combining these in a tank degasser model. Compared with low-carbon steel, the high alloy content in 3rd Generation AHSS changes nitrogen removal by tank degassing, through effects on both the interfacial reaction rate constant (kN) and the activity coefficient of dissolved nitrogen (fN). The isotope exchange technique (15N to 14N exchange reaction) was applied to measure the interfacial nitrogen reaction rate constant for compositions in the Fe-Si and Fe-Si-Mn systems. For Fe-Si alloys, the nitrogen reaction rate constant decreases slightly with the increased Si levels: from 2.3 × 10−5 to 1.9 × 10−5 mol cm−2 s−1 atm−1 (1.5 wt pct Si), and to 1.4 × 10−5 mol cm−2 s−1 atm−1 (2.5 wt pct Si), at 1650 °C. The calculated activation energy increases from 144 to 147 kJ/mol (1.5 wt pct Si) and finally to 153 kJ/mol (2.5 wt pct Si). For Fe-Si-Mn alloys, the influence of Mn on the interfacial reaction rate constant (at constant Si) is not obvious. By using the measured interfacial rate constant in the tank degasser model, the effects of steel composition have been evaluated and the rate-limiting steps in nitrogen removal assessed.