Effect of Lanthanum Addition on Formation Behaviors of Inclusions in Q355B Weathering Steel.

The effect of lanthanum addition on the formation behaviors of inclusions in Q355B weathering steel was investigated by laboratory experiments and thermodynamic calculations. The results demonstrate that the main inclusions in weathering steel without La addition are large-sized irregular AlO and MnS, with an average size of about 5.35 μm. As La content increases from 0.0075 to 0.0184 wt.%, the dominant inclusions transform from MnS, LaAlO, and AlO-LaAlO into MnS, LaO, and LaAlO-LaO. Meanwhile, the average size of inclusions significantly decreases from 3.4 to 2.48 μm and the distribution is more dispersive. When the La content increases to 0.0425 wt.%, the original MnS and AlO inclusions are completely modified into LaOS and LaO but the inclusions demonstrate serious agglomeration and growth. The thermodynamic calculations indicate that AlO and various lanthanum-containing inclusions are formed in the liquid phase. As the La content in molten steel increases from 0 to 0.0425 wt.%, the AlO inclusion is inclined to be modified into lanthanum oxide and lanthanum oxysulfide and the modification process is AlO → LaAlO → LaO → LaOS, which is very consistent with the experimental observations.