Tracing the Origin of Non-ferrous Oxides in Lamination Defects on Hot-Rolled Coils: Mold Slag Entrainment vs Submerged Entry Nozzle Reaction Products

In this work, lamination defects (slivers) on hot-rolled coils of Ca-treated steel were investigated for microstructure and composition using optical and scanning electron microscopy combined with microanalysis (SEM/EDS). The goal was to identify possible origins for the observed defects which contain a complex assemblage of phases, such as different types of calcium aluminates (CA, CA2, CA6), melilite (C2AS), spinel (MA), and a newly identified phase, CNA2. Mold slag similar to that employed during the cast was absent. Analysis of the bulk composition of some of the defects indicated these to be too rich in alumina to be derived from mold slag through steel-slag redox exchange. In contrast, microstructural observation of the inner side of the submerged entry nozzles (SEN) used during casting showed deposits with compositions comparable to those of the defect material. Based on an estimation of the chemical evolution of mold slag interacting with steel, it is found that the defects are not likely to be entrained mold slag but remobilized SEN deposits, as supported by several microstructural and trace phase criteria. However, it should be noted that extensive reduction of mold slag by steel can lead to compositions rich in sodic-calcic aluminates (CNA2). Therefore, differentiation between specific locations of the defect materials within a casting system requires detailed analysis from the potential sources of origin as well as from the materials found in the defects.