Effect of nitrogen content and magnesium-cerium treatment on inclusions in interstitial free steel

The electric arc furnace process is beneficial for reducing carbon emission in the steelmaking industry. However, the excessive nitrogen content in electric arc furnace molten steel is unfavourable for producing high-grade interstitial free (IF) steel. In this work, the effect of nitrogen content on inclusions in IF steel and the Mg–Ce treatment modification performance were investigated through thermodynamic calculations and experimental characterisation. The results show that, as the nitrogen content in IF steel rises from 0.0025% to 0.0067%, the precipitation of TiN is promoted, leading to an increase in both the number density and the size of TiN inclusions. The average size of TiN increases from 2.61 to 4.31 μm. After Mg treatment in high-nitrogen IF steel, a significant increase in number density while a noticeable decrease in the size of TiN can be observed. After Ce treatment, the precipitation of TiN clusters embedded in large complex Ce-rich cores causes an increase in the number of large-sized inclusions, which weakens the modification effect. Moreover, after Mg–Ce combined treatment, the TiN clusters are no longer observed, inducing an obvious decrease in the average size of TiN to 2.95 μm. Results of the TiN inclusions statistics and disregistry calculations indicate that plenty of fine dispersed Mg-rich and Ce-rich inclusions can serve as the nuclei for the heterogeneous nucleation of TiN, thereby refining the inclusions.