Concepts and characteristic curves for the kinetic transformation of nonmetallic inclusions in liquid steel during solidification and cooling and in solid steel during heating process

The composition of nonmetallic inclusions in the steel varied continuously during the solidification and cooling process of the molten steel and the heating process of the solid steel. To quantitatively evaluate this evolution of inclusion composition, this study proposes an integrated model and discusses the effect of the cooling rate during continuous casting and the holding time during the heating process on the transformation of the inclusion composition. Besides, a concept of transformation fraction of inclusion composition was put forward. Using this concept, several characteristic curves with a significant application value were raised, including the isothermal transformation curve (time-temperature-transformation, TTT), continuous cooling transformation curve (CCT), and equal diameter transformation curve (time diameter transformation, TDT). The integrated model consisted of the fluid flow, heat transfer, solidification and element segregation, thermodynamic equilibrium between the steel and inclusions, mass transfer kinetics in the steel and inclusions, and the variation of the spatial position of the calculation domain. Employing the integrated model, the spatial distribution of inclusion composition in blooms was obtained. Since the transformation of inclusion composition was mainly due to reactions between CaO and CaS, the transformation fraction was used to characterize the extent of the transformation, which was defined as the ratio of the content of CaS in inclusions at a certain time to that in thermodynamic equilibrium at room temperature. The continuous cooling transformation curve of the inclusion composition in the bearing steel was obtained to analyze the effect of the cooling rate on the inclusion composition during the solidification and cooling of liquid steel. At a fixed cooling rate, the transformation fraction of the inclusion composition increased with the reaction time. Simultaneously, the critical cooling rate of different types of steel could be obtained intuitively using these curves. The isothermal transformation curve of the inclusion composition in the heavy rail steel was also acquired to estimate the effect of the heating temperature and holding time on the inclusion composition in solid steels. With the increase of the holding time and heating temperature, the transformation fraction of the inclusion composition had an apparent increase. Moreover, the influence of the steel composition and inclusion size on the transformation of the inclusion composition could be determined using the equal diameter transformation curve in pipeline steel at 1473 K. Inclusions with a small size almost transformed completely within 60 min, while larger inclusions only exhibit a slight change even after heating for several hours. These concepts and characteristic curves can intuitively show the composition transformation of nonmetallic inclusions in steels during the solidification and cooling of liquid steel and heating of solid steel, expanding the control strategy of inclusions in steels from liquid steel to solid steel.