A Coupling Model Predicting the Precipitation and Growth of MnS Inclusions in U75V High-Carbon Heavy Rail Steel.

MnS inclusions are the primary non-metallic inclusions in U75V high-carbon heavy rail steel and can be detrimental to the mechanical properties of this material. With the aim of identifying means of controlling the amount and morphology of these inclusions, the present work examined the precipitation thermodynamics and growth kinetics of MnS during the solidification of molten steel using the FactSage 7.0 software package with various modeling calculations. The results indicated that the equilibrium partition coefficients of both Mn (k_Mn) and S (k_S) decreased monotonically with increases in the solid fraction, and the k_Mn and k_S values ranged from 0.6557 to 0.6941 and from 0.0141 to 0.019, respectively. This work also demonstrated that MnS precipitates in the two-phase region during the late stage of solidification to a solid fraction of 0.9263, while increasing the solute concentrations generates earlier precipitation. The Mn and S concentrations were shown to increase rapidly prior to MnS precipitation, after which the Mn concentration continued to increase while the S concentration decreased. Increasing the cooling rate and decreasing the S concentration tended to reduce the size of MnS inclusions but decreasing the Mn concentration had the opposite effect. The sizes of MnS inclusions were calculated using a coupling model and determined to be in the range of 4.4–12.4 μm, in good agreement with values obtained from scanning electronic microscopy observations of a steel billet. Observations of MnS inclusions before and after the rolling process indicated that the deformation ability (defined as the ratio of the length (after the rolling process) to the diameter (before the rolling process) of inclusions) of MnS was in the range of 11.89 to 54.88. This work provides an improved understanding of the precipitation and growth of MnS inclusions in U75V high-carbon heavy rail steel. [ABSTRACT FROM AUTHOR]