Inclusion evolution in 50CrVA spring steel by optimization of refining slag

Abstract

In order to control the CaO-Al₂O₃-SiO₂-MgO system inclusions in 50CrVA spring steel in a lower melting temperature region, high temperature equilibrium experiments between steel and slag were performed in the laboratory, under the conditions of the initial slag basicity within 3–7 and the content of Al₂O₃ between 18–35 mass%, to investigate the formation and evolution of this type of inclusion. The results indicate that the total oxygen content in the steel decreases with the increase of slag basicity and the decrease of Al₂O₃ content in slags, and CaO-Al₂O₃-SiO₂-MgO inclusions tend to deviate from the low melting point region with the increase of Al₂O₃ content in slags. The most favorable composition for the refining slag is composed of 51–56 mass% CaO, 9–13 mass% SiO₂, 20–25 mass% Al₂O₃ and 6 mass% MgO. In this case, the inclusions in 50CrVA spring steel are mostly in the low melting point regions, in which their plasticities are expected to improve during steel rolling. The MgO-based inclusions were observed in the steel matrix and the formation mechanism was theoretically and schematically revealed. It is also found that adding around 11 mass% of MgO into the refining slags is beneficial to reducing the refractory corrosion. Further work should be carried out focusing on the evolution rates of MgO-based inclusions.