“Metallurgical technologies” Chair, St. Petersburg State Polytechnic University (St. Petersburg, Russia):

Kazakov A. A., Dr. Eng., Prof., Head of the Chair, e-mail: kazakov@thixomet.ru
Kovalev P. V., Cand. Eng., Assist. Prof.
Ryaboshuk S. V., Assistant

Severstal JSC (Cherepovets, Russia):

Zhironkin M. V., Ladle Treatmet Manager, Steel Competence Center
Krasnov A. V., Chief Technologist, Steelmaking Production

Nature of formation and special evolution features of nonmetallic inclusions during ladle refining of converter HSLA steels for pipelines have been studied. Nonmetallic inclusions of the CaO–Al₂O₃–MgO system, close to calcium monoaluminate CaO∙Al₂O₃ with up to 5–6% of MgO, have been found as favorable from morphology point of view. These small inclusions nucleate on endogenous MgO substrates at sufficient high content of calcium in steel melt. Hot rolled plates can be rejected due to the coarse calcium bi- and hexa-aluminate inclusions (CaO∙2Al₂O₃ and CaO∙6Al₂O₃), usually containing exogenous MgO. These coarse inclusions are forming under calcium deficiency conditions, especially in the case of longtime steel holding in a ladle. Critical parameters and technological recommendations providing low level of nonmetallic inclusions in steel have been formulated: calcium/aluminum ratio used for steel deoxidation and modifi cation was increased; interval between additions of calcium and aluminum was decreased; full time of refining was corrected; procedures for lowering of steel reoxidation during pouring were conducted. Twenty five heats were carried out according to recommendations and the specimens from 128 hot-rolled plates were investigated for evaluation of nonmetallic impurity rating of steel. Metallographic analysis revealed that 95% of finished plates had rate up to 2.0 according to GOST 1778-70 (Sh6 method) standard.

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