A Review of Steel Processing Considerations for Oxide Cleanliness
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A Review of Steel Processing Considerations for Oxide Cleanliness B.A. WEBLER and P.C. PISTORIUS Control of non-metallic inclusions is essential for the production of high-quality steel. This review summarizes processes that change inclusion compositions and concentrations during secondary steelmaking—slower changes are limited by reaction between bulk steel and slag or refractory, and faster changes involve direct additions to the steel bath. An example of the former is conversion of alumina inclusions to spinels during ladle treatment, while reoxidation and calcium treatment are typical exemplars of the fast changes. For the slower changes, inclusions approach equilibrium with the liquid steel and conceptually simple kinetic models correctly describe inclusion evolution during ladle treatment. Disequilibrium from faster changes persists for several minutes under typical ladle conditions, with small-scale inhomogeneity in the steel. Fast scanning electron microscopy with microanalysis has facilitated detailed study of these inclusion evolution processes by providing information on inclusion composition, size, and shape. Machine learning methods are likely to be increasingly important in analysis of the results. Such methods have already shown promise to improve classification of inclusions and recognizing inclusion clusters, from analyses of polished sections. Several unresolved issues that require future study are noted. https://doi.org/10.1007/s11663-020-01949-y Ó The Minerals, Metals & Materials Society and ASM International 2020
I.
INTRODUCTION
THE detrimental effects of inclusions on the processing and properties of steel have been well-documented. During processing, solid inclusions can cause clogs that impair flow of liquid steel.[1–3] In products, inclusions affect surface quality[4] and mechanical performance.[5,6] Specific effects on mechanical performance include lower toughness and ductility,[7,8] poorer machinability,[9] nucleation of fatigue cracks,[10] and stress corrosion cracking.[11] The severity of the effects depends on the inclusion concentration (number per unit volume), size, shape, and chemical composition. There have been several reviews of the origin and characterization of inclusions in steel, including relatively recent ones by Cramb,[12] Zhang and Thomas,[13,14] Kaushik et al.,[15] Pretorius et al.,[3] O’Malley,[16] and da Costa e Silva.[17] In this paper, we focus on the time scales at which the chemical
B.A. WEBLER and P.C. PISTORIUS are with the Center for Iron and Steelmaking Research, Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, PA 15213, USA. Contact e-mail: [email protected] Manuscript submitted May 01, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS B
composition of micro-inclusions changes during ladle processing of liquid steel. Understanding of the relevant time scales aids in identifying the causes of observed changes and formulating effective plant practices for inclusion control. The three processes that affect inclusion composition
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