Effect of Cooling Rate and Sulfur Content on Inclusion Precipitation and Microstructure in X70 Pipeline Steel

The hydrogen resistance of steel is improved by core-shell inclusions (oxide-MnS); however, the principle of the precipitation behavior of MnS on the oxide surface needs to be studied. This work systematically investigates the effect of cooling rate and sulfur content on the inclusion formation and microstructure in X70 pipeline steels using laboratory experiments, thermodynamic calculations, and density functional theory (DFT) computations. The results show that the typical inclusions are MgAl₂O₄ and MgAl₂O₄MnS inclusions. The size of inclusions increases significantly with the increase of S contents at lower cooling rates. With the increase of S contents, MgAl₂O₄ inclusions can be fully wrapped by MnS inclusions. Both MgAl₂O₄ and MgAl₂O₄-MnS inclusions can induce intragranular acicular ferrite (AF). When the S content increases, the content of AF decreases sharply. Slowing the cooling rate makes the AF wider and gradually causes it to lose the AF characteristics. DFT calculations show that more oxygen vacancies are conducive to the precipitation of MnS on oxides. The small-sized inclusions are conducive to the precipitation of MnS on the oxide surface because small-sized inclusions have more oxygen vacancies. This research provides a better understanding of the factors influencing core-shell type inclusion precipitation and microstructure evolution in X70 pipeline steel.