Effect of Gadolinium on Inclusions and Microstructure of an Al-Killed High Sulfur Steel Used for the Connecting Rod of Engines

The effect of the total gadolinium (T.Gd) content on the microstructure, inclusions, and the size of austenite grains of the high sulfur steel was investigated. Primary inclusions in the steel without gadolinium addition were MnS with a chain-like shape, distributed along the grain boundary. Inclusions transformed from MnS into MnS–(Gd–O–S) in the steel with 51 and 79 ppm T.Gd, while Gd–O–S inclusions further transformed into (Gd–O–S)–(Gd–S) in the steel with 100 ppm T.Gd. Pure Gd–S inclusions formed in the steel with 192 and 198 ppm T.Gd. When the mass ratio of T.Gd and sulfur (T.Gd/T.S) was higher than 2 in the steel, all sulfur atoms were combined with the gadolinium. Furthermore, the shape of sulfides was altered from chain-like to spherical or spindle-shaped due to the addition of gadolinium alloy, resulting in a more dispersed distribution of inclusions. In the steel with 100 ppm T.Gd, the number density of inclusions reached the minimum value of 55.8 #/mm². The addition of gadolinium led to a reduction in the size of austenite grains from 18.5 to 15.6 μm and the interlamellar spacing of pearlites was decreased from 0.67 to 0.42 μm. Thermodynamic calculations demonstrated that the onset temperature of MnS precipitation was delayed and the volume fraction of MnS precipitation was reduced due to the decrease of the sulfur in the steel.