Segregation behaviour in Nb microalloyed steels

Four continuously cast slabs from two sources were characterised by optical microscopy and scanning electron microscope energy dispersive X-ray spectroscopy (SEM-EDS) in the as cast state and after normalisation close to the Ae 3 temperature. For all four slabs the composition of the majority of the slab depth (>80%) was the bulk composition and in this region microsegregation, particularly of Nb, was found to agree with predicted partition ratios from equilibrium thermodynamics. For two of the slabs, which gave final solidification as single phase austenite, equilibrium thermodynamics were also able to predict the compositional centreline macrosegregation. In the other two cases, this simple approach was not successful, which has been ascribed to solidification as a more complex phase mixture (austenite and δ-ferrite) or more complex processing during solidification (e.g. soft reduction).