Simultaneous Analysis of Soluble and Insoluble Oxygen Contents in Al-Killed Steels of Various C Contents and Supersaturation Phenomena in the Steel

Soluble and insoluble O contents ([S. O] and [I. O], respectively) in Al-killed steels of various C contents (0 to similar to 2 mass pct.) were simultaneously measured using "two-stage" inert gas fusion infrared absorptiometry, where majority of the inclusions was alumina. Several steel specimens were used for the O content analysis where the specimens were either taken from a steel plant or prepared in this laboratory. In addition to this, several pretreatment methods of the specimens were tested in order to provide reliable analysis results. It was found that mechanical grinding of the specimen's surface should be carefully carried out in order not to induce unwanted surface oxidation. This resulted in overestimation of the O content of the specimen. [S. O] and [I. O] in the steel specimens were successfully analyzed using the "two-stage" gas fusion method in the context of inert gas fusion infrared absorptiometry. Considerable portion of total O content ([T. O] = [S. O] + [I. O]) was the [S. O], which was higher than the equilibrium O content of the known Al deoxidation equilibria. Therefore, the supersaturation in the Al deoxidation was confirmed. The analyzed [I. O] was independently validated by measuring area of the exposed alumina inclusions on the polished section of the specimen using SEM. It was also found that increasing C content in the steel lowered the [S. O] while [I. O] hardly changed. It is concluded that C in the steel, mechanical stirring, and remelting, could relieve the supersaturation.