Role of Lanthanum Addition on Acicular Ferrite Transformation in C–Mn Steel

Acicular ferrite formation in C–Mn steel after rare earth La addition and the potency of different La-containing inclusions inducing the formation of acicular ferrite have been investigated. It was found that there would be a large amount of acicular ferrite in quenched steel after 0.020 mass% La additions. The size of effective inclusion for acicular ferrite nucleation is mainly 1–4 µm. The best single La-containing effective inclusion is La2O2S. When MnS attached to the surface of La2O2S forming complex inclusion, the potency of inclusion on nucleating acicular ferrite is enhanced significantly. Low lattice mismatch between α–Fe and La2O2S plays an important role on acicular ferrite nucleation. When patches of MnS precipitated on the surface La2O2S forming the complex inclusion, the main mechanism for that inclusion is the combine of low degree of lattice mismatch and existing of Mn-depleted zone.