Ladle Metallurgy & Continuous Casting
This study was undertaken to establish steelmaking parameters to ensure sulfide shape control without undue MnS stringers and oxide inclusions. New tools were used to characterize sulfide and oxide inclusions in steel grades and plate products, and results of trials conducted in steel refining and casting to improve cleanliness and shape control are presented. The trial data were examined with inclusion analysis achieved using an automated scanning electron microscope applied to tundish and plate steel samples. It was shown that to control sulfide shape, it is necessary to desulfurize the steel to very low levels, <10 ppm S in this case, and prevent steel reoxidation during casting. Specific ratios among sulfur, oxygen and calcium were found to be necessary to ensure adequate shape control. Successful implementation of modified practices has not only resulted in improved shape control in certain plate products, but application of these practices to other steel grades has significantly improved overall cleanliness, thus improving overall customer satisfaction.
By use of a multi-depth sampling technique at key steps during the ladle metallurgy furnace process, the evolution of inclusions can be observed up to depths of 1.85 m below the slag layer in both Si-bearing and restricted Si (RSi) grades. This combination of data showed the effects of silicon on the slag/steel interaction. The Si-bearing alumi-num-killed grades had different inclusion distributions, populations, as well as morphologies than the RSi grades. It was also seen that there is a sulfur wt.% threshold that affects the ability for the slag to interact with the inclusions in the steel. In return, the inclusion morphology for RSi heats can mimic that of silicon-bearing heats with low final sulfur wt.%. This threshold was determined to be 0.003 wt.% sulfur in the steel.
The characteristics of inclusions in high-manganese steel samples from laboratory-scale experiments were investigated. This was done by two-dimensional and three-dimensional methods. In the two-dimensional investigations, inclusions on polished cross-sections were observed by using automatic scanning electron microscopy analysis. For the three-dimensional investigations, inclusions collected on a film filter after electrolytic extraction were observed. It was found that electrolytic extraction could be successfully applied to high-manganese steels. Further, automatic analysis, which is a common practice in steel industry, has limitations for the detection and classification of nitride inclusions, whereas it was possible to detect nitrides using the three-dimensional method.
Fluid flow in the mold is known to have a significant impact on the quality of continuously cast steel products. Since it is strongly influenced by the submerged-entry nozzle (SEN) design, particular attention has to be paid on its geometry. In this paper, the main principles of a novel SEN type will be described. Water modeling and computational fluid dynamics are applied to characterize the flow pattern under different conditions. Furthermore, the interaction with electromagnetic stirring (M-EMS) is analyzed by means of a liquid melt model. Finally, actual steel plant performance data and observations are discussed.
In 2011, Nucor Steel Jackson Inc. had severe problems with off-square billets that could not be controlled with the usual operating and maintenance adjustments. A test with a new mold design was conducted that had outstanding results in billet shape and mold life. In addition, significant cost savings were achieved by reducing the amount of manganese and vanadium added to the steel in an effort to control the billet shape. This paper will present infor-mation on the operating, quality and cost-saving results when using the new WAVE mold design.
High-basicity mold powders (CaO/SiO2 = 1.1–1.4) have been used to cast peritectic and medium-carbon steel. However, longitudinal surface cracking can still be an issue. Because softer heat removal in the mold meniscus area is required to prevent this problem, the crystallization properties of mold powder slag with higher basicity (over 1.5) was investigated. A very-high-basicity powder was developed using the results of a unique slag film simulator. This new mold powder provides softer heat removal in the meniscus area. This new technology improved steel surface quality and provided sufficient solidified shell thickness when used in actual casting.