Study of the defects of a hot-rolled tube steel sheet and the causes of their creation

The final complex of properties of the pipe-rolled metal products is formed during the entire set of production stages, the most important of which is steelmaking. This is especially true for continuously cast billet, because segregation of cast metal, which is inherited during subsequent rolling, leads to the formation of a heterogeneous structure, and causes a decrease in mechanical properties. A study was carried out on sheet material from steel grade K56, used for the manufacture of electric-welded pipes for oil pipelines. The rejection of the material was carried out on the finished product. As in the base metal of the pipe, and in the heat-affected zone, ultrasonic testing revealed stratifications. To establish the nature of the revealed defects, microstructural studies were carried out using optical and electron microscopy. The results of optical microscopy confirmed the presence of discontinuities in the form of cracks in the axial zone of the samples. The microstructure of the metal in the axial zone has a different from the base metal morphology and is identified as low-healed martensite. While the base metal has a banded ferrite-bainite structure. The presence of martensite is confirmed by microhardness measurements: the base metal has a hardness of about 220 HV, discontinuity in the axial zone of 270 HV. The study of the chemical composition of the structural components along the height of the samples showed an increased number of Si and Mn in the axial line. Also in the central zone, large line-secretions of sulphides and carbides of niobium were found. Thus, it was established that the delaminations found in the axial zone of sheet metal are a consequence of the segregation of the initial continuously cast billet.