Reoxidation of Molten Stainless Steel and Oxide Dissolution in Tundish Flux

Abstract

Stainless steel is used in various fields thanks to its strong corrosion resistance and shiny appearance. Corrosion does not proceed to the inside due to the dense chromium oxide, and the nano-size chromium oxide can also have the effect of exposing the surface of iron to the outside. The defectless surface of stainless steel products is one of the requirements of the customers, therefore the requirement for the surface quality by the customers are stricter than carbon steel. Accordingly, techniques for controlling the inclusions are required in the stainless steel manufacturing process. Many engineers have developed technologies that can remove inclusions as much as possible by optimizing the AOD (Argon Oxygen Decarburization) and VOD (Vacuum Oxygen Decarburization) processes. Even though molten steel with high cleanliness is produced, other inclusions are regenerated when reoxidation occurs in the casting process. When the steel solidifies without removing the inclusions, the inclusions would cause surface quality issue. Therefore, the present study intended to confirm the oxide generated by reoxidation in the tundish through a series of experiments. Three representative stainless steels (AISI 304, 430, 439) were selected and were oxidized to check what kinds of oxides are formed. Each steel was preliminarily deoxidized by different ways (Si-killed, Al-killed, Al-Ti-killed). The oxide inclusions in each steel sample were found to be different. In the tundish, a flux with a low melting point is used to collect inclusions. If the oxide generated by reoxidation is well dissolved in the tundish flux, it would not be a problem because the number of oxides in the molten steel is reduced. If the oxides do not dissolve in the tundish flux, it continues to stay inside the molten steel and is trapped on the surface during solidification. In this case, surface defects occur during the rolling process. An experiment was conducted to measure the dissolution rate of the oxide inclusion in the tundish flux. CSLM(Confocal Scanning Laser Microscope) was used to measure the dissolution rate. By obtaining time-oxide radius relationship, the dissolution rate was analyzed to elucidate the rate-determining step. Combining the results, it was confirmed whether oxides generated during reoxidation could be captured in the tundish slag. It can help to improve the cleanliness of molten steel in the stainless steel industry by checking the dissolution rates of oxides which were generated during reoxidation for three steel types with different deoxidation methods.