This work studies the influence of precipitation on branched crack formation in the surface skin of continuously-cast slabs of Ti-Nb HSLA steel. Two slabs were analysed, cast at pulling rates of 0.43 m.min-1 and 1.03 m.min-1. The results show that branched cracks were located on the slab with pulling rate 0.43 m.min-1, which was a transition slab. The branched cracks extended to a depth of 10 mm below the slab surface and occurred mostly below oscillation marks. In the case of the slab cast at pulling rate 1.03 m.min-1 branched cracks did not occur. Evaluation of precipitates obtained by the method of carbon extraction replicas on selected samples confirmed the presence of predominantly spherical or elliptical particles. Square particles were observed in small amounts. Application of selection electron diffraction showed the presence of TiC and NbC carbides. Formation of cracks is influenced by the size of precipitate particles, but also by morphology and distribution of precipitates. It is known that elliptical precipitates have significantly worse effects than spherical precipitates, even when significantly smaller and fewer in number. Globular particles also influence formation of surface cracking, but only when they occur in steel at high volumes or very large size. It is also known that precipitates arranged in rows on grain boundaries cause surface cracking. In this work the slab cast at the lower pulling rate 0.43 m.min-1 showed the presence of adverse precipitates and also rows of excluded precipitates on grain boundaries, which were considered responsible for the observed branched cracks. The formation of branched cracks is conditioned not only by segregation of impurities, but possibly also by the presence of undesirable brittle cementite networks. The presence of cementite was confirmed not only through microstructural analysis, but also diffraction of hard X-ray radiation. This diffraction revealed a branched crack in the sample cast at pulling rate 0.43 m.min-1 including the presence TiC and Fe3O4, the amounts of which were comparable to the amount of Fe3C in the analyzed area. On a sample without occurrence of branched cracks at pulling rate 0.43 m.min-1, there was only Fe3C in detectable amounts. It is known that precipitation of microalloying elements in connection with cementite can lead to higher probability of surface crack formation, as also confirmed in this study.Keywords: slab, branched cracks, oscillation marks, segregation, precipitation
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