Magnitogorsk Iron and Steel Works (Magnitogorsk, Russia):

A. N. Ushakov, Deputy General Director on Production;

Nosov Magnitogorsk State Technical University (Magnitogorsk, Russia):
V. A. Bigeev, Dr. Eng., Prof.
A. N. Stolyarov, Dr. Eng., Prof.
M. V. Potapova, Cand. Eng., Associate Prof., e-mail: marina_potapova8@mail.ru

For the export of Russian gas to Europe, the project has been developed for the building of the gas pipeline “Nord Stream – 2”, part of which must pass under the Baltic Sea. The specificity of operating underwater gas pipeline systems is characterized by a combination of high internal operating pressure of the gas due to the lack of intermediate compressor stations and strong external hydrostatic pressure of the seawater layer, as well as the aggressiveness of the marine environment causing increased metal corrosion. To implement this project, electrofusion longitudinal welded pipes of large diameter with wall thickness up to 41 mm are required. The pipes are made of ultra low sulfur steel grade DNV SAWL 485 FD. The production of such steel was organized in conditions of PJSC “Magnitogorsk Iron and Steel Works”. A thick hot rolled sheet is rolled on the “5000” mill from slabs produced in the oxygen converter shop (BOF Shop). The technological chain of production of steel grade DNV SAWL 485 FD is following: iron desulphurization unit (UID) — oxygen converter — ladle-furnace unit (LFU) — steel vacuumizing unit (USV) RH type — slab curvilinear type CCM with a vertical section. The features of the technology of DNV SAWL 485 FD manufacturing on each unit are considered. Particular attention was paid to the metal desulfurization, carried out in three stages. At the first stage, ladle processing of cast iron is carried out at the UID by blowing fluid-cooled lime and magnesium through the upper lance in a nitrogen stream. The degree of desulfurization of cast iron is equaled to 83–85%, and iron sulfur content after first stage does not exceed 0.005%. At the second stage of desulfurization during metal output from the 370-ton converter a solid slag-forming mixture (SSM) consisted of lime and fluorspar is introduced into the ladle. The degree of desulfurization at this stage is unstable and equaled to 20% approximately. The third final stage of desulfurization is carried out at the LFU and consists of two steps: processing of the metal under “white” slag to reduce the sulfur content in steel to 0.003–0.005% and deep desulfurization by blowing fluid in the argon stream. The developed technology of metal desulfurization allows to produce ultra-low sulfur pipe steel with a sulfur content less than 0.0015%. The overall degree of ladle desulfurization of steel is equaled to 83%.

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