Thermodynamic and Kinetic Analyses of Inclusion Transformations in Linepipe Steel during Isothermal Heating at 1473 K (1200 °C)

Understanding the thermodynamics and kinetics of solid-state reactions on the characteristic transformations of nonmetallic inclusions in the linepipe steel during isothermal heating is of great importance for achieving good steel quality. Herein, heat treatment experiments are used, which are carried out at 1473 K (1200 °C) between 1 and 10 h in an Ar atmosphere, to investigate the resulting changes in the inclusion characteristics of linepipe steel. The chemical compositions, morphology, and size distribution of the inclusions are characterized. Results indicate that Mn and S in the linepipe steel react with CaS inclusions/phases to form (Ca, Mn)S inclusions/phases during heating at 1473 K (1200 °C), while MnS inclusions are gradually decomposed. Based on the FactSage Macro process, a kinetic model is established to calculate the regional equilibrium at the interface between the inclusion and steel at every time step and predict the extent of the inclusion transformation during isothermal heating. The experimental measurements and predicted results are in reasonable agreement. The diffusion-limited kinetic model verifies that the effects of heating time and inclusion size on inclusion transformation are as predicted for Mn and S diffusion control by observation.