Influence of Deformation Degree on the Evaluation of Macroscopic Oxide Cleanness

The demand for cleaner steels requires reducing the number of macroscopic non‐metallic oxide inclusions. A major problem of “clean steel” is the measurement and evaluation of the rare large inclusions which exceed a critical size harmful to the product. To this end, volumetric methods of evaluating macro‐inclusions in the three‐dimensional steel matrix are necessary to qualify steel heats or continuous cast slabs and to check if the refining and casting processes are stable and have been improved. Ultrasonic scans are a powerful tool to assess the quality of the volume. In order to activate macroscopic oxide inclusions for the reflexion of the ultrasound, as‐cast material is deformed by hot rolling. The aim of the present research work was to find out the most effective hot rolling practice with respect to ultrasonic detection of the rare large oxide inclusions. For this purpose, heavily contaminated neighbouring slab samples, corresponding to a severe incidence of ladle slag carryover, were hot rolled in a different manner. The 23‐kg‐samples were either elongated by a ratio λ = 2, 3, 10 or 15, or alternatively cross‐rolled, e.g. spread and stretched by ratios 2 × 5 or 3.2 × 3.2. The best compromise to quantify macroscopic steel cleanliness under consideration of cost and time requirements is based on the pure 10fold elongation or the cross‐rolling by 2fold x 5fold. It likewise became clear that rare large oxides can be detected with good sensitivity, as long as the hot rolling reduction ratio is large enough.