Investigation of RE-O-S-As Inclusions in High Carbon Steels

The formation of RE-O-S-As inclusions is an inevitable issue when rare earth (RE) elements are used to modify the existing state of residual element arsenic. The evolution process of La-O-S-As inclusions in high carbon steels was investigated under conditions of different La contents. An in-situ sample preparation method was used to prepare film samples. The results show that it is practical to add RE elements to stabilize arsenic by generating As-bearing RE inclusions. A new type of As-bearing RE inclusion was discovered and identified as LaAsO₄. As La content increased from 0.003 to 0.089 wt pct, the dominant inclusions of the La-O-S-As system changed from La₂O₃ to La₂O₂S, then to La₂O₂S-LaAsO₄, and finally to LaAsO₄. A full model of the inclusion formation of the RE-O-S-As system was also established to describe what kind of inclusion to form in conditions of different atomic ratios of RE/O and RE/S. The first As-bearing RE inclusion is RE-O-As or RE-S-As, and RE-O-S-As inclusions can be controlled to form certain inclusions that make O and S useful to modify the existing state of arsenic.