Effect of Rare Earth Metal Alloying on Inclusion Evolution in High-Strength Oil Casing Steel

The type and morphology of inclusions have an important influence on the properties of high strength oil casing steel. In this paper, laboratory experiments were carried out to investigate the effect of cerium (Ce) alloying and holding time on inclusion modification of oil casing steel. The results show that with the increase of Ce content and the holding time, inclusions gradually changed from irregular shape to ellipsoidal shape. During the modification process, inclusions underwent the following transformations: Ca–Al–O + Al₂O₃ + Al–Mn–Si–O → Ca–Ce–Al–O + Ce (Mn, Si)–Al–O → CeAlO₃ → CaO-Ce₂O₃ + Ce₂O₃. When the holding time was 5 minutes, the average size of inclusions decreased from 1.26 to 1.14 μm with Ce content increased from 0 to 77 ppm. Nevertheless, when the holding time was over 20 minutes, the average size of inclusions firstly exhibited a decrease trend while it then showed an increase trend with an increased Ce addition. In addition, the number density of inclusions in molten steel was gradually reduced with Ce content increased from 0 to 77 ppm. For the ingot samples cooled down to room temperature within the furnace, the transformation of inclusions evolved the following characteristics with an increased Ce content in steel: Al₂O₃ + xCaO·yAl₂O₃ → Ca–Ce–Al–O → CeAlO₃ + CeAl₁₁O₁₈→CeAlO₃ → CeAlO₃ + Ce₂O₃. In this study, the optimum modification time and Ce content in Rare Earth Metal (REM) modified high-strength oil casing steel of inclusions was 20 minutes and above 60 ppm, respectively.