Abstract
	An investigation of surface cracks on continuous cast (CC) products of low carbon steels is very important in term of final quality of metallurgical industry rolled production. Fine branched surface cracks are frequently occurred defects of the low carbon steel CC slabs. They are resulted of unsuitable combination of metallurgical and technological CC parameters. Chemical composition of cast steels is mostly important metallurgical parameter, which can has considerable negative effect on surface quality of CC cast slabs. First of all a presence and content of harmful impurities (S, P, Sn, As, Sb, Cu) in the cast steel are the most fundamental.     
	The paper aim is analysis of fine branched surface cracks morphology and explanation of their formation in the CC slab surface zone of low carbon steel enriched by Cu (0.35 %). A lot of the fine branched surface cracks filled by oxides were observed in the structure of slab surface zone. A continual surface margin of intergranular facets arises by connection of microstructurally observed cracks on fracture surface of notch toughness samples from slab surface zone. The Cu-base phases enriched by Fe, Ni and Sn were observed at the interface oxide-steel of analysed cracks and at intergranular fracture surface after crack opening. Initiation and growth of the observed cracks are probably result of the copper and surface-active elements enrichment of inter-dendrite regions and primary grain boundaries and following the liquid Cu-base phase penetration along the primary grain boundaries. Cracks grow from slab surface along weakened boundaries of the granulated primary grains in temperature range 1300 – 1100°C in the slab passing through bottom of mould and upper part of secondary cooling zone in the CC process.