Effect of oxygen blowing during RH treatment on the cleanliness of IF steel

Interstitial-free (IF) steel is widely used in the automobile industry, home appliance industry, etc. Not only very low content of carbon and nitrogen, but also high quality surface of the final product are required for this steel grade. The contents of oxygen and inclusions in the steel have a great influence on the surface quality of the final product. Therefore, it is very important to decrease the carbon content effectively and keep high steel cleanliness at the same time in industrial production. In present work, the effect of oxygen blowing on the cleanliness of IF steel under the forced decarburization by oxygen blowing in the Ruhrstahl Hereaeus (RH) refining process was studied through dense sampling during RH and continuous casting process, and inclusion analysis was carried out with automatic scanning electron microscopy (ASPEX). Oxygen blowing was found to have little effect on the inclusions’ types and morphology throughout the process. The oxygen blowing rate had a great influence on the cleanliness of the molten steel in the early stage of RH refining (within 4 min after adding Al). An increase in the oxygen blowing rate led to an increase in the content of total oxygen (T.O) and the amount of inclusions in the steel, but it had little effect on the steel cleanliness in the subsequent process. Cluster inclusions were mainly found before the vacuum was broken, and finding them in steel after RH refining was difficult. The steel cleanliness in a tundish had little correlation with the oxygen blowing rate during RH treatment, but had a great correlation with the oxygen content in the molten steel before Al deoxidation. The higher the oxygen content before Al deoxidation, the worse the steel cleanliness in the tundish. To improve the cleanliness in the tundish, the oxygen content in molten steel before Al addition should be decreased as much as possible. The T.O and the inclusions amount in the steel showed a downward trend as the production proceeded, which indicates that the steel cleanliness was gradually improved.