State of the Art in Evaluation and Control of Steel Cleanliness.

Abstract

This paper first reviews the current "state-of-the-art" in the evaluation of steel cleanliness, discussing over 20 different methods. The demand for cleaner steels requires lowering non-metallic oxide inclusions and also controlling their morphology, composition and size distribution. Because no single method can measure all of these aspects accurately, it is best to combine several methods together to quantify steel cleanliness in a given operation. Owing to the cost, time requirements, and sampling difficulties, steel cleanliness is widely inferred using total oxygen, nitrogen pick-up, and other indirect methods. Recent cleanliness values using these indicators are summarized for LCAK at many steel plants around the world. Secondly, this paper reviews operating practices to improve steel cleanliness at the ladle, tundish and continuous caster, emphasizing findings quantified with plant measurements. Inclusions come from many sources, including deoxidation, reoxidation, slag entrapment, refractory wear, and chemical reactions. They generate many defects such as cracks and slivers in the steel product. Steel cleanliness is controlled by attention to a wide range of important operating conditions throughout the steelmaking and casting processes. For ladle operations, FeO and MnO in the slag, ladle treatments, and inclusion modification are discussed. For tundish operations, tundish depth and capacity, casting transitions, refractory lining, tundish flux; gas stirring, and flow controls are discussed. Important transfer operations from ladle to tundish and from tundish to mold, such as argon protection, sealing issues, and SEN clogging are summarized. Caster operations reviewed include the effect of casting speed, fluid flow pattern control, surface level control, and caster curvature.

Nucleation, Growth, Transport, and Entrapment of Inclusions During Steel Casting

“…The current author has reviewed methods to detect inclusions and operating practices to improve steel cleanliness at the ladle, tundish, and continuous caster. 5,[8][9][10][11][12][13][14][15][16][17][18][19][20] Inclusions arise from many sources, including deoxidation, reoxidation, nitride formation, slag entrapment, chemical reactions, and other exogenous sources. Clues to the mechanism of their origin can be discerned from their composition and shape in the final product.…”

Effective Viscosity of Slag and Kinetic Stirring Parameter Applied in Steel Cleanliness During Vacuum Degassing

“…According to the authors, an increase of β s , which is associated with an increase of stirring, can promote slag emulsification phenomenon and new inclusions could be generated. It is possible to obtain new inclusions or "return" inclusions in the liquid steel, such as reoxidation, cleaning of the slag/ steel interface with particles trapping inside the steel and erosion of the refractory ladle wall 41 . For inclusions of the diameter range ≥ 15 µm, a dependence of the inclusionary cleanliness with the parameter β s is not clearly observed, which verifies the smaller values related to the variation of inclusion density, for the greatest diameter range.…”

Erratum to: Oxygen in Molten Iron: Deoxidation with Silicon and Aluminum

“…But first, it is necessary to make a critical assumption: namely that the chemical activity of oxygen in deoxidized steel is the same as that of oxygen in pure iron at the same concentration. For this to be true, it must be known that the oxygen analyzed in the metal is free oxygen, not oxygen in the form of solid inclusions of SiO 2 13 ) noted that in deoxidation with silicon, the precipitation of silica is complete after 60 s, and the silica separates from the melt within 8-12 min, leaving behind only dissolved oxygen in equilibrium with dissolved silicon, and thus free to generate a true emf in the oxygen sensor. It is assumed that the data from Fruehan and co-workers meet these criteria.…”