Spheroidized elements usually have the following common properties:
(1) The solubility of elements in molten iron is low and the segregation tendency is obvious during solidification.
(2) There are one or two valence electrons on the outermost electron layer of the element, and the secondary inner layer contains eight electrons. This electronic structure will make carbon, sulfur and oxygen have strong affinity with elements, and the properties of the generated compounds are stable, which will significantly reduce the oxygen and sulfur in molten iron.
(3) Although carbon has affinity with spheroidized elements, spheroidized elements are difficult to dissolve in graphite lattice. According to the above characteristics, it can be found that rare earth (heavy rare earth element) and cerium (light rare earth element) in rare earth are effective spheroidizing elements. Rare earth reacts with sulfur, oxygen and other elements to form rare earth oxides and sulfides. These compounds belong to high solubility compounds, which can be used as an external core to promote graphite nucleation.
However, rare earth is generally not used as a spheroidizing agent alone. Research shows that the relative atomic mass of rare earth is generally large, and the consumption of desulfurization and deoxidation is larger than that of some other spheroidizing elements such as magnesium and calcium. At the same time, the density of rare earth compounds generated is large and it is not easy to remove them from molten iron. Therefore, rare earth is generally used as a spheroidizing agent in combination with other elements, such as rare earth magnesium alloy Rare earth magnesium calcium alloy, etc. At the same time, the residual amount of rare earth in iron castings still needs to be controlled, because when the amount of rare earth is too large, it will increase the white tendency of iron castings and inhibit the growth of eutectic grains. In serious cases, it will distort the graphite morphology of iron castings and appear anisotropic graphite. It is pointed out that for thin-walled iron castings, the best residual cerium is 0.006-0.002%; For molten iron with sulfur content of 0.03% – 0.07%, the residual amount of rare earth can be 0.025%; When the cupola is used to produce large-scale ferritic nodular cast iron, the elongation of the thicker part of the cast iron can be increased by 20% if the residual rare earth content is 1.2-2.0 times that of sulfur.