Precipitation and Growth of MnS Inclusions in Non-quenched and Tempered Steel Under the Influence of Solute Micro-segregations During Solidification

Precipitation and growth behaviors of MnS inclusions during the solidification of non-quenched and tempered steel have been experimentally and thermodynamically investigated. Effects of cooling method and cooling rate on the morphologies, size distributions, number densities, and average diameters of MnS inclusions in steels were systematically revealed. Coupled with thermodynamic calculation of MnS precipitation, two solute element micro-segregation models, Clyne–Kurz model and Ohnaka model, were introduced and improved by taking the micro-segregations of silicon, oxygen, and other solute elements into account during the steel solidification. The MnS inclusion growth was estimated according to the model calculation results. A non-linear fitting curve equation for describing the relationship between cooling rate of molten steel v and average inclusion diameter d was acquired by experiments: d = 10.726v^−0.438. At the cooling rate of R_c = 1.26 K s⁻¹, the solidification fraction values for MnS precipitation in the two models were 0.929 and 0.931, respectively. Precipitation and growth of the MnS inclusions during solidification were well predicted by the improved Clyne–Kurz and Ohnaka solute element micro-segregation models by comparison of experimental and calculated results. Both the two models were validated and could be judged as equal in this work.