Characterization of precipitation, evolution, and growth of MnS inclusions in medium/high manganese steel during solidification process

MnS as a main manganese-containing inclusion in medium/high manganese steel, which can be determined to the mechanical properties of steel. However, the precipitation and evolution mechanism of MnS in medium/high manganese steel is still unclear due to the special solidification properties and element segregation behavior. Hence, the effects of sulfur content, manganese content and cooling rate on the precipitation, evolution, and growth behavior of MnS inclusions in medium/high manganese are various experimentally characterized and thermodynamically elucidated in the present work. The results of 2D and 3D observation indicated that the morphology of MnS in medium manganese steel were transformed from globular and spindle-like (type I MnS) into large-sized rods (type II MnS) and dendritic (type D MnS), when sulfur content was increased from 78 ppm to 1578 ppm. Particularly, the spatial distribution and structure of MnS inclusions are elaborated in detail through X-ray micro-CT observations, which are discussed by defining the quantitative topography parameters and statistical analysis. Based on in situ CSLM observations, MnS precipitation path in high sulfur medium manganese steel was monitored as L → (L + δ + MnS) → (δ + MnS) → (δ + γ + MnS) → (γ + MnS) → (α + γ + MnS) → (α+MnS). Simultaneously, the Scheil-solidification calculations demonstrated that MnS is precipitated at the interdendritic region where the solid-liquid states are coexisted, which is caused by the segregation of sulfur and manganese. Moreover, increasing sulfur content generates the earlier precipitation, and leads to the significant increase in the number and size of MnS inclusions. Besides, the increase of manganese content promotes the transition of MnS from globular to polyhedral shape, which is attributed to the dissociative eutectic reaction of MnS formation preferentially takes place earlier in case of high manganese steel. It is also found that the decreasing of cooling rate and increasing the manganese content tends to increase the number and size of MnS inclusions, which is resulted from the long growth times of high manganese steel induced by large solid-liquid temperature differences. This work provides a systematic understanding of the precipitation and evolution of MnS inclusions in medium/high manganese steel.