Improving Steel Cleanliness by Optimizing Inclusion Rinse Time at the LMF
Gentle argon stirring in the ladle metallurgy furnace, LMS, after desulfurization is performed to remove inclusions before the ladle is transferred to the caster. It is desired that the stirring time be sufficient to reduce the inclusion density without reintroducing inclusions back into the melt and without resulting in unnecessarily long stirring times. The optimum stirring time will vary depending on the facility. A study was conducted at CMC Steel Texas to determine the effect of argon stirring time at the LMS on the number, type, size and distribution of inclusions in SAE 4330 special bar quality rounds. The tensile and notch toughness as a function of stirring time and inclusion content was determined for specimens quenched and tempered to an average hardness of 40 HRC. The goal was to optimize stirring time in order to ensure the most superior final product in terms of inclusion content and mechanical properties. Inclusion density decreased from 377 to 197 inclusions/mm2 as the stirring time increased to 5 min. However, increasing the stirring time to greater than 9 min increased the inclusion density and this may have been the result of erosion of inclusions from the steel-slag interface back into the melt. The average size of the inclusions ranged between 2 and 3 μ and size generally increased with stirring time. The composition of the respective inclusions were determined using scanning electron microscopy with energy dispersive X-Ray spectroscopy, EDS, and were found to be complex (Mn,Ca)S, MnO-SiO2, spinel, and calcium aluminate inclusions. Tensile properties did not show any correlation to stirring time or final inclusion content Average notch toughness varied from 44 to 59 J and generally decreased with increasing area fraction of inclusion coverage. The decrease in notch toughness was related to clusters of MnS inclusions that debonded from the matrix, creating large voids and low energy ductile crack propagation. Using this data, a final rinse time for the facility was determined to be between 5 and 9 minutes to produce the cleanest steel. Additional rinse time does not improve quality and wastes time and materials.
M. Jennings et al., "Improving Steel Cleanliness by Optimizing Inclusion Rinse Time at the LMF," AIS Tech - Iron and Steel Technology Conference Proceedings, vol. 2, pp. 1255-1264, Association for Iron & Steel Technology, May 2016.
AISTech Iron and Steel Technology Conference (2016: May 16-19, Pittsburgh, PA)
Materials Science and Engineering
Keywords and Phrases
Calcium; Energy dispersive spectroscopy; Ladle metallurgy; Manganese oxide; Scanning electron microscopy; Slags; Tapping (furnace); Energy dispersive X ray spectroscopy; Inclusion content; Ladle metallurgy furnaces; Non-metallic inclusions; Secondary refining; Special bar qualities; Steel cleanliness; Steel slag interface; Iron compounds
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Article - Conference proceedings
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01 May 2016