Centerline segregation - Slab defects

Centerline segregation - Slab defects

Figure 1: Macroetched cross sections of slabs cast with the original (above) and modified (bottom) roll setting (thickness of slab: 240 mm).

Defect name: Centerline segregation
Record No.: 2705
Type of defect (Internal/Surface): Surface
Defect classification: Slab defects
Steel name: Steel
Steel composition in weight %: No data.
Note: Centerline segregation of slabs relates partly to macrosegregation and partly to the shrinkage of melt, the formation of small shrinkage holes and, occasionally, the formation of inclusions). In continuous casting centerline segregation develops in the middle part of the slab due to solidification and transformation processes, to fluid flow and also to constrained liquid supply, which is necessary for solidification shrinkage compensation. Only enriched melt is present between solid dendrite trunks. Any effect that enhances fluid flow (i.e. cooling conditions, setting of the supporting rolls, bulging between successive rolls, etc.) necessarily results in the flow of the enriched melt, i.e. macrosegregation will form. The possibility of sufficient liquid supply in the mushy area decreases depending on the ratio of solid phase. At the same time the permeability of zigzag channels between dendrite arms also decreases. Lessening the possibility of liquid supply inevitably leads to the formation of shrinkage holes and porosity, which is also typical of centerline segregation formation.
In order to check the inner quality and the calculation results, samples were cut from both strands after casting. The samples (containing the whole cross sectional area) were ground and macroetched by ammonium-persulfate reagent. Pictures of macroetched samples can be seen in Fig. 1.
Reference: Not shown in this demonstration version.