Effects of sulphur content and cooling rate on the precipitation characteristics of MnS in high-sulphur peritectic steel

Abstract

In this article, the precipitation characteristics of MnS in typical sulphur-containing peritectic steels at different sulphur contents (0.005% and 0.05%) and cooling rates (3 °C/s, 8 °C/s, 13 °C /s and 18 °C /s) are studied using high-temperature laser confocal microscopy. Thermodynamic calculations and experimental results show that at a cooling rate of 18 °C /s, MnS formation increases from 0.012% to 0.15% as sulphur content in steel rises from 0.005% to 0.05%. This is because when the sulphur content is ≥0.012%, a large number of MnS single-phase inclusions can be precipitated at the end of solidification; when the sulphur content is <0.012%, only a small amount of MnS–Al₂O₃ composite inclusions can precipitate from the solid phase after solidification. Fe–MnS pseudo-binary phase diagram and experimental results indicate that for high-sulphur steel (w_S= 0.05%), rod-like MnS inclusions averaging 8.2 μm form at a cooling rate of 3 °C /s, whereas spherical inclusions averaging ≤3.9 μm occur at cooling rates of ≥8 °C /s. This occurs because the S- and Mn-rich liquid phase from the monotectic reaction can create rod-like MnS inclusions via eutectic reaction at cooling rates ≤3 °C /s, while rapid solidification at ≥8 °C /s leads to spherical inclusions. At the same time, the existence of Mn depleted zone (MDZ) can promote the formation of grain boundary ferrite around MnS single-phase inclusions and acicular ferrite around Al₂O₃–MnS composite inclusions.