Experimental and Numerical Investigations on Micro- and Macrosegregations in Continuously Cast Steel Slabs
Research output: Thesis › Doctoral Thesis
In the current thesis segregation phenomena in continuously cast steel are examined at a microscopic as well as at a macroscopic scale. On the one hand, a three-dimensional (3D) dendritic solidification structure is reconstructed. The reconstruction is based on two-dimensional (2D) microsegregation patterns which are experimentally determined. On the other hand, the formation of macrosegregation in a continuous casting strand is investigated with numerical simulations. The simulations consider a strand geometry of 25 m length and industrially relevant process conditions. For the 3D reconstruction of the dendritic solidification structure the distribution of the alloying element manganese is captured by microprobe measurements at a sample area of approximately 3 x 3 mm. Removing the sample face and analyzing the manganese distribution repeatedly results in a series of 2D concentration maps which are then combined to a 3D concentration field. After correcting the concentration pattern numerically to consider the influences of curvature and temperature gradient, specified threshold concentrations are connected with surfaces. These iso-surfaces represent the dendritic morphology at a certain moment of solidification. Since the local concentration actually depends on the solidification progress, increasing the concentration threshold enables the visualization of the dendritic growth. The dendrites reconstructed with this method can be used to determine fundamental input parameter for the macroscopic solidification simulation. The formation of macrosegregation in continuous casting of steel slabs is investigated with numerical fluid flow simulations using an Eulerian two-phase model. Each of both phases, melt and solid, comprises of the species iron and carbon. The movement of the solid is predefined analytically. For that purpose the deformation velocity of the dendrites within the mush zone at the strand centre is determined with a simplified thermo-mechanical simulation model. The melt flow inside of the cast strand is influenced by solidification shrinkage and by deformation of the solid caused by undesired periodical strand surface bulging and by well-aimed reduction of the strand cross-section, referred as mechanical softreduction. The performed simulations enable determining the effect of different process parameters on the formation of centreline macrosegregation.
|Translated title of the contribution||Experimentelle und numerische Untersuchungen über Mikro- und Makroseigerungen in stranggegossenen Stahlbrammen|
|Publication status||Published - 2014|