Study on the enhancement of corrosion resistance in BT490XT automotive body steel through rare earth La modification of MnS inclusions

This study focuses on analyzing the effects of La on the morphology, elemental distribution, and size evolution of MnS inclusions, as well as their impact on the BT490XT steel's corrosion behavior. The results reveal that in the absence of La, the sulfur-containing inclusions in BT490XT steel primarily consist of MnS and Al 2 O 3 –MnS composite phases, which are numerous and exhibit irregular block-like shapes. When the exposed surfaces of MnS inclusions are the (100) and (110) crystallographic planes, MnS acts as the cathode and is gradually corroded. Conversely, when the exposed surface is the (111) plane, the steel matrix serves as the cathode and undergoes corrosion first. At 1600 °C, the Gibbs free energy of formation for LaAlO 3 and Al 2 O 3 are −3.21265 × 10 5 kJ/mol and −1.11449 × 10 5 kJ/mol, respectively. The Gibbs free energy for the formation of LaAlO 3 is lower than that of Al 2 O 3 . Consequently, with the addition of La, [Al] preferentially combines with La to form LaAlO 3 . Without La addition, the average size of MnS inclusions is 2.22 μm; however, after La addition, the average size of sulfur-containing inclusions decreases to 1.8 μm, and MnS inclusions are modified into smaller LaAlO 3 –MnS composite phases. The potential difference between MnS inclusions and the steel matrix ranges from −1.791 to 1.139 eV, while that between LaAlO 3 and the steel matrix varies from −0.242 to 0.093 eV. Compared to MnS inclusions, the potential difference and fluctuations between LaAlO 3 and the steel matrix are relatively smaller.