Effect of sulfur content on precipitation behaviour of MnS-containing inclusions in X70 pipeline steel

The presence of oxide-MnS complex inclusions is beneficial for the improvement of hydrogen resistance in pipeline steel. The effect of S and Mg contents on the generation mechanism of MnS, MgAl2O4, and MgO inclusions, and the heterogeneous nucleation principle of MnS on the surfaces of MgAl2O4 and MgO were revealed using laboratory experiments, thermodynamic calculations, and density functional theory (DFT) computations. The inclusion characteristics in steel were evaluated by using a Scanning Electron Microscope in combination with an Energy Dispersive Spectrometer (SEM + EDS). It has been found that as the Mg content increased, MgAl2O4 transformed into MgO, which can be well predicted by thermodynamic calculations. It was found that MnS exhibited the ability to nucleate heterogeneously on the surfaces of MgO and MgAl2O4 inclusions. Furthermore, proper S content (approximately 30 ppm) can facilitate the formation of oxide-MnS complex inclusions, excess S content was not desired. The smaller the oxide inclusion, the easier to form wrapped MnS on the oxides. The nucleation ability of MnS and MgAl2O4 was identified by DFT calculations and found that they followed the order of (111)MnS//(111)MgAl2O4>(100)MnS//(100)MgAl2O4>(110)MnS//(110)MgAl2O4. Furthermore, the heterogeneous nucleation ability of MnS and MgO followed in the order of (111)MnS//(111)MgO > (100)MnS//(111)MgO > (100)MnS//(100)MgO. This study provides theoretical evidence supporting the heterogeneous nucleation ability of MnS on the surfaces of MgO and MgAl2O4. Moreover, it helps bridge the disparity between the calculation of MnS and MgO nucleation using the two-dimensional mismatch degree and the experimental findings.