This paper presents the results of microscopic investigations of the structure and chemical composition of the axial zones of continuous casting ingots of 140 x 140 mm cross-section made of G3Si1 steel, comprising the areas of porosityand axial shrinkage defects. The occurrence of shrinkage hollows and non-metallic phase, resulting from chemical reactions of the steel components with oxygen at the solidification front, has been stated in these areas.