Observed behavior of various oxide inclusions in front of a solidifying low-carbon steel shell

The engulfment and pushing (extrusion) of inclusions during solidification play an important role in the formation of a steel structure and, as a result, for the mechanical properties of the final steel product. The aim of this study is to gain knowledge about the behavior of non-metallic inclusions at the interface between a growing solid front and a liquid phase. The focus is on the effect of the titanium and titanium oxide content on the inclusions and the different phenomena, which occurs at the solid/liquid interface. This was studied in samples of low-carbon steels de-oxidized by different combinations of Al, Ca, and Ti. For this purpose, each metal sample of 0.19 g was melted at a temperature close to 1550 °C in an argon atmosphere and solidified under different solidification rates. A direct observation of inclusion behavior during solidification was made using a confocal scanning laser microscope equipped with an infrared gold image furnace. The alloying elements in the sample varied between: C 0.002–0.044; Si 0.02–1.33; Mn 0.12–1.33; P 0.003–0.016; S 0.003–0.01; Al 0.002–0,033; Ni 0–0.28; Cr 0–0.25; Ti 0.008–0.065; Ca 0.0007–0.002; O 0.002–0.0114 and N 0.0028–0.0056 (mass%). Several types of inclusions with different morphologies were found within the sample. The morphology of the observed inclusions on the molten steel surface varied from round alumina and calcium-oxide-rich inclusion to needle-shaped titanium oxide-rich inclusions. The observed motions of the inclusions at the vicinity of the front of the solidifying steels were classified. At a low solidifying velocity and a small inclusion size, inclusions flowed away from the solidifying front. Furthermore, they also or got pushed a distance and thereafter flowed away from the interface. At a medium velocity and a slightly bigger size, inclusions tend to get pushed in front of the solidifying front. Another observation was that at a high velocity and a large particle size, inclusions tend to get engulfed or pushed and then engulfed by the progressing front. The relationship among the morphology, chemical composition of inclusions and the solidifying velocity is discussed in this article.