Investigation of change in NMI-landscape during experimental alkali element-containing slag treatment of steel

With ever-rising demands on the mechanical properties of high-strength steels the focus on steel-cleanness is
becoming more and more prominent. Hard and non-deformable non-metallic inclusions can lead to drastically reduced
performance of the material, especially regarding the fatigue resistance. Therefore, many steel grades with high demands
on strength and fatigue properties are deoxidized using Si or Mn instead of Al to avoid Al2O3 inclusions. To further enhance
the steel-cleanness the inclusion landscape can be modified, to achieve lower hardness and better deformability of the
particles. In recent studies this was done by additions of alkali elements to the melt and slag. Direct modification, which is
the incorporation of the alkali elements into the inclusion, could theoretically lower the melting point and the hardness of
the particle, possibly reducing problems during the production and increasing the lifetime of the steel product. During first
trials using alkali oxide-containing slags, a distinct drop in sulfidic NMIs was observed. This could be considered indirect
modification, as no alkali elements could be detected in the NMIs, but the landscape of inclusions shifted drastically. In
further experiments this steel- slag system was quenched after interaction, as thermodynamic calculations using Factsage
8.3 indicated that the reduction in sulfur content could be linked to slow cooling conditions in the first set of experiments.
Quenching was done by suspending the sample in the furnace using Mo-wire. After the steel-slag interaction, the wire was
cut, and the sample was quenched in water or liquid nitrogen, respectively. Samples were then analyzed by automatic and
manual SEM/EDS methods and compared thoroughly. It was found that shifts in the NMI-landscape could be attributed
mainly to the specific cooling conditions in the first set of experiments as calculated by Factsage 8.3 and confirmed by
quenching experiments.