Abstract: The effects of different heating and holding process on the MnS inclusions in the bloom and rail of heavy rail steels was investigated. It was found that the large elongated MnS inclusions in rail would split into small particles after heating and holding process. The amount of MnS inclusions larger than 80 μm decreased, while the amount of MnS particles smaller than 5 μm increased when the soaking time exceeded 3.5 hours. However, the phenomenon was not obvious for the MnS inclusions in bloom. The behavior of MnS inclusions followed the Ostwald ripening mechanism, that longer soaking time lead to larger MnS particles. The MnS inclusions were more likely to split at 850℃, at which sulfur was the diffusion controlled element. The three-dimensional morphologies of MnS inclusions in bloom included plate-like, strip-like and irregular, which were then elongated to strip-like along the rolling direction during the subsequent rolling process of bloom into rail. Pure MnS inclusions would start to precipitate only when the solid fraction reached 0.94 on the basis of calculation by Scheil model, the morphology was influenced by the pre-existed grain boundaries. The restrictive factors of the formation of MnS were concentration product of [%Mn]•[%S] and temperature. To reduce the accumulation of Mn and S in the front of solidification and further to reduce the precipitating amount and size of MnS inclusions, decreasing sulphur content and increasing cooling rate were proposed under the condition of constant Mn content in molten steel.

Key words: MnS inclusion, heating and holding process, thermodynamic calculation, non-aqueous electrolyte