Morphological transformation of elongated MnS inclusions in non-quenched and tempered steel during isothermal heating

Elongated MnS inclusions in rolled non-quenched and tempered steel tend to cause the mechanical anisotropy of steel, deteriorate the mechanical properties and degrade the quality and service life of the steel products. To reveal the mechanisms of morphological transformation of strip-shaped MnS inclusions during isothermal heating, the effects of heat treatment time and temperature on the morphology, number density and size distribution of elongated MnS inclusions were systematically studied and discussed. A diffusion couple experiment was also conducted to clarify the diffusion mode of MnS inclusions. The experimental results showed that with the increase in isothermal heating time (from 0 to 10 h at 1473 K) and temperature (from 1173 to 1573 K for 3.0 h), the number density and average aspect ratio of MnS inclusions generally showed an increase and decrease trend, respectively, while the area fraction remained stable and only slightly fluctuated around 0.4%. In the diffusion couple, after the isothermal heating at 1473 K for 3.0 h, the elements Mn and S in the steel near the steel–MnS interface were very stable without any concentration gradient. The morphology change sequence of the elongated MnS inclusions in the rolled non-quenched and tempered steel during the isothermal heating was strip → cylinderization → spindle → spheroidization. Relationship between the diameter of MnS inclusion and the spacing between two MnS inclusions after splitting, and the fitting goodness of different n values under different experimental time and temperature confirmed that the driving force for the transformation of MnS inclusions during the isothermal heating was surface diffusion, instead of volume diffusion.