Evolution of inclusions in a pipeline steel during continuous casting and hot rolling process

Industrial trials were performed to study the evolution of inclusions in a pipeline steel during the continuous casting and hot rolling process. The main composition of inclusions changed from Al2O3-CaO in the liquid steel to Al2O3-CaO-CaS in the slab, and then to Al2O3-CaS in the rolled plate. Corresponding area fractions of inclusions increased from 47.0 x 10(-6) to 76.7 x 10(-6), and to 144.3 x 10(-6). It was explained by thermodynamic calculations that the thermodynamic equilibrium between inclusions and the steel varied with the temperature. Element contents of Al, Ca, Mg, S, and O in the steel decreased during the solidification and cooling process. Due to the difference in the cooling rate, the transformation ratio of CaS increased from 61% at the slab surface to 95% in the slab center. It was also affected by the size of inclusions as smaller inclusions provided better kinetic conditions. Critical diameters of inclusions for the composition transformation were calculated. For the hot rolling process, the average aspect ratio of inclusions increased from 1.4 in the slab to 2.8 in the rolled plate. During the hot rolling process, a part of inclusions was crushed into small particles of CaS phase and Al2O3-CaO-MgO cores, leading to a decrease in CaS content of inclusions.