Effect of compression temperature on deformation of CaO–CaS–Al2O3–MgO inclusions in pipeline steel

Abstract

Plain strain deformation trials were carried out on samples of pipeline steels at 1473 K, 1673 K and 1723 K, respectively. The deformation of inclusions in the solid steel at different temperatures and in the semi-solid steel was studied. The composition of inclusions changed from 60.62%Al2O3–16.10%CaO–10.27%MgO–13.01%CaS before deformation to 60.59%Al2O3–13.03%CaO–12.74%MgO–13.63%CaS, 59.69%Al2O3–7.04%CaO–11.51%MgO–21.76%CaS, and 68.26%Al2O3–22.56%CaO–6.68%MgO–2.5%CaS with corresponding deforming temperatures of 1473 K, 1673 K and 1723 K. While the average aspect ratio of inclusions increased from 1.28 to 2.23, 1.32, and 1.35, respectively. Thermodynamic calculations performed by FactSage 7.0 verified the composition transformation from CaO to CaS during the solidification and cooling process of the steel. A kinetic model was used to calculate the dynamic transformation of the inclusion composition at compression temperatures. The inclusion transformation ratio from CaO to CaS increased from 38.28% at 1473 K to 50.50% at 1673 K. For the deformation in the solid steel at the temperature below 1673 K, the thickness of the hard phase CaS increased with the soaking temperature, while the hardness of the steel matrix decreased. The larger hardness difference between inclusions and the steel matrix led to a higher aspect ratio of inclusions after deformation. For the deformation in the semi-solid steel, the small difference of hardness between the soft inclusion phase and the soft steel matrix resulted in a low aspect ratio of inclusions in the semi-solid steel after deformation.