Type, number and morphology of non-metallic inclusions left in metal after its deoxidation and modification are among the key factors determining metallurgical quality of large diameter steel tubes of X70 strength class for oil and gas industry. All non-metallic inclusions revealed in segregation strip of tube steel sheet are classified. This component of metallurgical quality has essential effect of forming application properties of tubes, defining the general resource of pipelines. It is stipulated by the fact that non-metallic inclusions in tube steels are considered as one of the causes of cold shortness, low corrosion resis tance, stress cracking and they also have effect on other important metal properties. Multi-phase aluminium oxide based inclusions with forming titanium nitride are analyzed. Behaviour of carbonitrides in solidifying and solid metal depending on cooling temperature and in the central part of a slab (taking into account zonal segregation) is examined. Degree of zonal segregation for sulphur, phosphorus, niobium, titanium, silicon, manganese and copper is investigated using thermodynamic simulation, measured and presented. Variations of composition of carbonitride inclusions with high titanium and niobium content, precipitating from solid metal (average melting composition and central part of a slab) during its cooling are researched. Recommendations for improvement of conditions of steel controlled rolling are formulated on the base of obtained results.

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