(1) Deep deoxidation and desulfurization: rare earth is easy to react with oxygen and sulfur elements in the molten steel of cast steel. The generated rare earth oxygen (sulfur) compounds have the characteristics of high melting point, small specific gravity and strong stability (as shown in the table). Some float up from the molten steel of cast steel into the slag liquid, and some become nucleation particles of new phase at the initial stage of solidification, which significantly reduces the oxygen and sulfur content in the molten steel of cast steel. Huo Wenxia’s research shows that adding rare earth elements to wear-resistant cast steel can reduce the sulfur content in molten steel by 20% – 40%. Lin Qin and Yan Ying added rare earth elements to heat-resistant steel and T10 steel respectively, and the purity of molten steel was improved. Rare earth elements can play a strong role in purifying molten steel.
|Rare earth compound||Melting point / ℃||Specific gravity / (g · cm-3)||Heat of formation- Δ H298（/ kJ·mol-1）|
(2) Eliminate trace harmful elements: rare earth elements can react with harmful elements such as lead, tin, arsenic and antimony in the molten steel of cast steel to form high melting point rare earth compounds, such as rare earth lead compounds and rare earth arsenides. Most of these high melting point rare earth compounds float up into the slag liquid and are discharged. The part remaining in the molten steel of cast steel can be used as nucleation particles, thus eliminating the harm of trace harmful elements such as lead, tin, arsenic and antimony to cast steel.
(3) Hydrogen and nitrogen fixation: rare earth elements have strong binding ability with hydrogen in molten steel. Take rare earth metal lanthanum as an example: La (s) +h2 (g) =lah (2 s), Δ G=-145070+177.979t, fixed the hydrogen in the cast steel, and then eliminated the phenomenon of hydrogen embrittlement. Due to interaction, rare earth elements can reduce the activity of nitrogen in molten steel and increase its solubility; At the same time, rare earth elements react with nitrogen to produce stable Ren, which plays a role in nitrogen fixation. Huowenxia found that the porosity defects of high manganese cast steel are more serious than that of ordinary cast steel when studying the effect of rare earth on the microstructure and properties of wear-resistant cast steel. After adding rare earth, rare earth forms stable rare earth hydrides and rare earth nitrides with hydrogen and nitrogen, which fixes the gas in high manganese cast steel liquid and effectively reduces the porosity defects of castings.
(4) Purification of grain boundaries: Lin Qin’s research shows that rare earth elements are easy to be enriched at the grain boundaries, which inhibits the segregation of sulfur, phosphorus and other impurity elements to the grain boundaries. The segregation of sulfur and phosphorus at the grain boundaries decreases with the increase of rare earth solid solution. When the rare earth solid solution reaches a certain amount, the segregation of sulfur and phosphorus at the grain boundaries is eliminated, so that the grain boundaries are significantly purified. Chen Suiyuan’s research shows that rare earth element cerium and iron can form fine and evenly distributed fe17ce2 type metal compounds. While refining the structure, it can be used as a second phase strengthening matrix to preferentially segregate at the grain boundary, inhibit the segregation of impurity elements at the grain boundary, purify the grain boundary, and improve the strength of the grain boundary.