MORPHOLOGY OF NON-METALLIC INCLUSIONS FORMED DURING REFINING OF STEEL WITH LANTHANUM UNDER VACUUM INDUCTION MELTING CONDITIONS

Abstract

Refining of steels and alloys with rare earth metals (REMs) under vacuum induction melting conditions has become widespread due to the high affinity of these metals for oxygen and sulfur. However, this same high oxygen affinity of REMs also leads to reactions not only with dissolved impurities but also with oxide phases that are in contact with the molten metal, such as the crucible material, slag on the surface of the melt and slag deposits on the walls of the crucible. As a result, a larger amount of REMs must be added than is theoretically required to bind oxygen and sulfur in the melt. Moreover, this introduces uncertainty into the calculation of the optimal mass of REM additions. This uncertainty is further complicated by several other factors, including the initial concentrations of oxygen and sulfur, melt temperature, and the degree of air leakage into the vacuum chamber. Despite the implementation of process control measures, in practice it is not possible to completely eliminate some variation in the residual concentrations of REMs, oxygen, and sulfur in ingots from different heats. However, even such limited variation results in a significant scatter in the characteristics of REM-containing non-metallic inclusions. At the same time, a direct correlation between the morphology of non-metallic inclusions and the total residual REM content is not always observed. Meanwhile, the characteristics of inclusions can have a substantial influence on the microstructure and, consequently, on the properties of the metal. This study establishes the correlation between the content of lanthanum not bound to oxygen and sulfur in ingots of structural steel of the 300M type, produced in laboratory vacuum induction furnaces, and the characteristics of non-metallic inclusions, such as chemical composition, size, and number per unit area of polished section.