Characteristics of Genetic Evolution of MnS Inclusions in Special Steel Throughout Production Process

The morphology, number, and distribution characteristics of inclusions influence the application properties of sulfur-alloyed free-cutting steels. Herein, the three-dimensional morphology and distribution characteristics of MnS inclusions as cast and rolled were systemically explored by X-ray micro-CT. Firstly, based on the composition test and applying the Ohnaka model, the element segregation behaviour and its relation to the morphology of MnS inclusions in steel billets were elucidated. Secondly, the growth of MnS in the steel billet was analyzed using the second-phase precipitation theory. The calculation results show that MnS was generated when the solid phase fraction fS=0.56, and the relationship between the content of elements and the morphology and distribution characteristic of MnS was established. Thirdly, the characteristics of MnS inclusions transform with various deformation amounts were investigated, and the relative plasticity of MnS reached the peak value when the deformation amount was 50%. Finally, the genetic characteristics of MnS inclusions as cast and rolled were analyzed. In practice, a series of measures composed of increasing the cooling intensity of continuous casting and rolling the billet in the range where MnS has relatively low plasticity was suggested to optimize the product machinability.