9th ECCC European Continuous Casting Conference – ECCC 2017

We have investigated lamination defects (slivers) on hot rolled coils of Ca-treated steel 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 defect phase, CNA2. Mould slag similar to that employed during the cast was largely absent from the defects. Analysis of the bulk composition of some of the defects indicated these to be too rich in alumina to be derived from mould 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 mould slag interacting with steel, it is found that the defects are not likely to be entrained mould slag but remobilised SEN deposits, as supported by several microstructural and trace phase criteria. We investigated the deposits formed during cast inside the SEN and found that they are similar to the defects in their phase makupp nad composition. However it should be noted that extensive reduction of mould slag by steel can also lead to compositions rich in sodic-calcic aluminates (CNA2), according to exploratory thermodynamic calculations. Therefore differentiation between specific locations of the defect materials within a casting system requires detailed analysis of compositions and microstructures from the potential sources of origin as well as from the materials found in the defects.