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 Al2O3 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, LaAlO3, and Al2O3-LaAlO3 into MnS, La2O3, and LaAlO3-La2O3. 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 Al2O3 inclusions are completely modified into La2O2S and La2O3 but the inclusions demonstrate serious agglomeration and growth. The thermodynamic calculations indicate that Al2O3 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 Al2O3 inclusion is inclined to be modified into lanthanum oxide and lanthanum oxysulfide and the modification process is Al2O3 → LaAlO3 → La2O3 → La2O2S, which is very consistent with the experimental observations.
Keywords: Q355B weathering steel; lanthanum; nonmetallic inclusion; thermodynamic calculations.