Optical Emission Spectroscopy Pulse Distribution Analysis on Steel Production Samples : Accuracy, sample variation, spread in different samples and steel grades/routes

Abstract: This thesis work aimed to contribute to the development of online characterization techniques for non-metallic inclusions in steelmaking. The study focused on assessing the precision and accuracy of the optical emission spectroscopy pulse distribution analysis (OES/PDA) technique using asreference light optical microscope (LOM) and scanning electron microscope(SEM). Steel samples were collected from three different routes at SSAB Oxelösund steel mill, including vacuum degassing with aluminium or silicon as deoxidizers, and a direct route using magnetic stirring. The precision study compared the inclusion index obtained from each burn spot within the same sample. The precision study includes two parts, the first one compared the dispersion of a set of samples when testing four and eight burn spots, and the second part all the samples were tested to determine the overall dispersion using four burn spots. This study revealed that samples with eight burn spots exhibited higher dispersion and higher average inclusion index, indicating potentially higher inclusion content in the middle section of the sample. Overall, the precision of PDA/OES was found to be satisfactory, with the possibility of further improvement by eliminating outliers. The particle size distribution (PSD) and volume fraction of inclusions obtained from OES/PDA and LOM was compared, and it was observed that OES/PDA yielded higher values in most samples, suggesting limited agreement between the two techniques. Comparing the results of OES/PDA with scanning electron microscopy (SEM) for PSD and volume fraction of inclusions showed relatively better agreement, although accuracy could not be described as entirely accurate. The comparison of the B-factor indicated that OES/PDA generally provided lower values than SEM. SEM was more effective in capturing additions during the steelmaking process, while OES/PDA exhibited lower sensitivity. Finally, the comparison of inclusion chemical composition between OES/PDA and SEM indicated significant disagreement, emphasizing that OES/PDA should not be utilized for this purpose. The limited gathered data from the seven sampled heats indicate that OES/PDA is a precise tool that need more development to be consider accurate to be consider as a monitor tool during secondary metallurgy.