Evolution of non-metallic inclusions in ultra low carbon steel after aluminum deoxidization

Abstract

A new technique based on the PDF (Population Density Function) was suggested to investigate the behavior of non-metallic inclusions after deoxidization. In comparison with the traditional analysis using the size histogram of inclusions, the application of PDF can make it possible to attain more useful information to understand the evolution of inclusions such as the period of formation and mechanism for growth. In this study, the PDF of alumina inclusions was investigated for the ultra low carbon grade steel of 20 ppm C. The lollypop samples were taken at three stages after deoxidization using aluminum; at the end of RH degassing process, at tundish during casting operations, and in cast slabs after solidification. For all the samples except for the contaminated ones, the PDF shows a fractal distribution. The exponent in fractal distribution was -3.54 near the end of RH process at 7-13 minutes after deoxidization, which is similar with the previously reported value. In contrast, the exponent was changed to -2.76 in tundish and -2.38 in cast slab, respectively. Different expressions of the fractal distribution can be attributed to the changes in fluid condition of molten steel. The application of PDF also provides the quantitative evaluation for abnormal reoxidization of molten steel. The deviation of PDF from the reference distribution is in accordance with other evidences of reoxidization of molten steel such as the increase of nitrogen pick-up and the decrease of soluble aluminum. © 2014 ISIJ.