Rare earth widely applied to cast iron, ductile iron is used to prevent the spheroidizing interference element damage, the role of spheroidization, grams produce various brands of the ductile cast, such as diesel engine and car crankshaft etc. Rare earth and oxygen, Liu, nitrogen, a strong affinity, join the molten iron can play off oxygen, improve the role of molten iron pure degree, but also with arsenic, zinc, tin, and lead role, such as low melting point impurities generated have lower melting point of compounds, hot insoluble and reduce or eliminate these impurities harmful effect.