Behaviors of Lanthanum and Cerium on Grain Boundaries in Carbon Manganese Clean Steel

The behaviors of La and Ce on grain boundaries in carbon manganese clean steel were investigated by high-resolution transmission electron microscope (HRTEM), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction(XRD) analysis. The existing forms of rare earths (RE) in clean steel were as follows: dissolved in solid solution, forming inclusion or second phase containing RE (RE-Fe-P, La-P, Fe-La eutectic and Fe-Ce phase). The dissolved La or Ce segregated at grain boundaries. The segregation of both S and P at grain boundaries was reduced with suitable RE content. The impact toughness of the steel was improved obviously. La and Ce had effects on purifying molten steel and modifying inclusions in clean steel, whereas with excessive La or Ce, La-Fe-P, La-P and Fe-La eutectic phase or Ce-Fe-P and Fe-Ce intermetallic compound would form along grain boundaries, causing the impact energy to decrease significantly.