Influence of Casting Rate on TiNb Microalloyed Steel Slab Surface Area Microstructure

Two slabs of Ti-Nb microalloyed steel were analysed in this work. The first slab was transitional with the initial pulling rate 0.43 m.min-1 and the final pulling rate 0.9 m.min-1. The second slab was cast at the real production pulling rate 1.03 m.min-1. The presence of larger amounts of oscillation marks was observed on the first slab at both pulling rates. The second slab showed no oscillation marks. At the lowest pulling rate, cracks were discovered below the slab surface, often below oscillation marks. Cracks were seldom observed at pulling rate 0.9 m.min-1. In the first slab, especially at the low pulling rate, the presence of cracks and pores was found. Pores were observed often with clusters of aluminum oxides. The microstructure of the slab surface zone was characterised by heterogeneity of ferrite grain sizes at all three pulling rates. This heterogeneity was manifested mainly in the marginal cut-outs at all pulling rates. In the marginal cut-outs the microstructure was granulometrically finer at all three pulling rates than in the central cut-outs. Non-equilibrium microstructure in the marginal cut-outs was also observed.