Aggregation behavior of Ce–Mg–Mn–O–S composite inclusions in silicon-killed bearing steel and their evolution during heat treatment

In order to mitigate the harm of alumina, calcium aluminate, and magnesium aluminum spinel inclusions on the fatigue performance of bearing steel, the effect of Ce–Mg composite treatment on the cleanliness of silicon-killed GCr15 bearing steel and the evolution of Ce–Mg–Mn–O–S composite inclusions during heat treatment of the steels at 1523 K were investigated in this present work. The results indicate that the cleanliness of S2 with 0.0045 wt% Ce and 0.0016 wt% Mg is better than that in S1 with 0.0020 wt% Ce and 0.0011 wt% Mg, which is attributed to the more effective removal of Ce–Mg–Mn–O–S composite inclusions, with help of good aggregation capacity of Ce-containing inclusions and small volume density of Mg-containing inclusions, due to the increase of rare earth and magnesium content. The aspect ratio and area fraction of the Ce–Mg–Mn–O–S composite inclusions in as-cast steel decrease significantly with isothermal treatment at 1523 K for 150 min, due to the effect of (i) Ostwald aging mechanism of inclusions and (ii) sulfide solid solution during isothermal treatment. With the increase of isothermal treatment time from 150 to 550 min, the aspect ratio and area fraction of the inclusions in S0 (untreated steel) and S1 decrease slightly, because spheroidization and solid solution of inclusions reach their basic equilibrium at 1523 K. However, the aspect ratio and area fraction of the Ce–Mg–Mn–O–S composite inclusions in S2 actually increase, and the inclusions evolve into three-phase (two-phase) inclusions, which is attributed to (iii) transformation of phase compositions in inclusions and interfacial reaction between inclusions and steel matrix during isothermal heating. Therefore, the control of inclusions in silicon-killed bearing steel with Ce and Mg composite treatment should be combined with specific heat treatment processes and appropriate rare earth and magnesium contents, to minimize the damage of inclusions on the fatigue performance of bearing steel products.