Металознавство та обробка металів (Mar 2020)
Features of heat-resistant properties of metal of not-rolled boilertubes produced from continuously cast billets of steel 20
Abstract
For the first time, long-term strength (the main design characteristic of power generating equipment) of metal of hot-rolled boiler tubes manufactured with various reductions on a tube rolling unit with a continuous mill (TPA 30-102) of INTERPIPE NIKO TUBE LLC from non-deformed continuously cast steel 20 billets of a new manufacturer (MZ DNIPROSTAL LLC) was studied. Structure and fracture pattern were studied in tests for heat-resistance of tube samples in various states (after hot rolling and after normalizing). Influence of structural characteristics and technological factors of production of tube billets and tubes on the level of long-term strength of metal of the experimental tubes was established. It was shown that presence of a developed zone of columnar crystals (transcrystallization zone) in the billet macrostructure and the technology of heat treatment of boiler tubes used at the tube producing plant do not enable obtaining of a relatively uniform structure and an optimal level of long-term strength and ductility in the tube metal. This is especially true for the tubes made with low reductions. The need for improving the technology of production of continuously cast billets at MZ DNEPROSTAL LLC and the technology of heat treatment of tubes made from these billets at INTERPIPE NIKO TUBE LLC was established. For optimal combination of long-term strength and long-term ductility of metal of hot-rolled boiler tubes, it is necessary to obtain a developed zone of equiaxed crystals and a limited zone of columnar crystals in macrostructure of initial undeformed continuously cast billets. Necessity of standardizing the macrocrystalline metal structure of undeformed continuously cast boiler billets in TU U 24.1-05757883-216 Round Continuously Cast Steel Billet for the Manufacture of Boiler Tubes was established. Possibility of using steel 20 boiler tubes made from MZ DNIPROSTAL LLC billets on the tube rolling unit TPA 30-102 with reductions not less than 17.0 in a state after hot rolling (without mandatory normalizing from separate heating) was demonstrated at boiler units of thermal power plants (heat electropower stations).
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