Comparative analysis of the structure of solid and hollow steel cast billets

This paper presents the research results of solid structure and hollow steel billets obtained by continuous casting. To substantiate the feasibility of using a hollow billet as an initial one in the production of seamless hot-rolled pipes, a comparative analysis of the distribution of non-metallic inclusions, macro- and microstructure, as well as segregation by structural zones was carried out. When analyzing the macrostructure of a hollow billet, two distinct zones were revealed: equiaxed small and columnar crystals, which distinguishes it, compared with a solid billet, by the absence of a zone of misoriented crystals. This, in turn, helps to eliminate defects such as axial porosity and segregation. The improved quality of the macrostructure during casting of a hollow billet is explained by more favourable conditions for heat removal and a higher rate of solid-phase advance due to bilateral cooling, and less shrinkage of the melt due to its cross-sectional geometry. The distribution of nonmetallic inclusions, consisting of oxide, sulfide and oxysulfide compounds, and liquidation elements, showed that they are concentrated mainly at the boundaries of crystalline zones, and for a solid billet and in the central part. This fact is caused by the development of a zone of intense heat removal. When research the microstructures of solid and hollow workpieces, a ferrite-pearlite mixture is observed in both cases. The microstructure of the hollow billet is more dispersed, which is confirmed by durometric measurements.