deficiency of rare earth metals in the Chinese market, the price of ferrosilicon magnesium alloys recently has risen. To comply with the national sustainable development policy, the Chinese foundry industry must reduce the use of rare earth metals by adopting new technologies.
Rare Earths in Ductile Cast Iron Rare earth metals, magnesium and calcium have
spheroidizing effects in cast iron. Among them, magnesium is the best. However, the addition of a proper percentage of rare earth metals in the spheroidizing agent along with mag- nesium can achieve even better effects. Te two spheroidizing agents work together as follows.
1) Desulfurization and Deoxidization Magnesium, rare earth metals and calcium all have
a strong capacity for desulfurization and deoxidization. Rare earth metals, particularly selenium, and calcium have greater free energy to combine with sulfur and oxy- gen to form sulfide and oxide. The boiling point of mag- nesium is 1,107C, which makes it evaporate easily when it is added to the melt and causes a strong stirring effect. Meanwhile, the gas dissolved into the melt is susceptible to diffusing into bubbles, which float to the melt surface. The oxides and sulfides attach to the bubbles and also float to the surface. Because of this part of the reaction, magnesium has a stronger effect than rare earth metals and calcium in desulfurization and deoxidization.
2) White Iron Formation The rare earth metals dissolved into the melt have
a strong capability to facilitate the formation of car- bides. So, they have a negative effect on graphite for- mation. But, as less rare earth metals are added into the spheroidizing agent with magnesium as the main component, their effect for carbide formation will be reduced. If a large amount of rare earths are added, they will facilitate the formation of white cast iron because they combine easily with sulfur and oxygen. As fewer rare earth metals are added, they will combine with sulfur and oxygen to form large quantities of fine sulfides and oxides, which serve as nuclei that facilitate the formation of graphite. Ten, carbon diffuses to the graphite, so the rare earth metals hinder the formation of white cast iron.
3) Hindering Detrimental Elements In cast ductile iron, some elements work against
spheroidizing. Tey are called detrimental elements and are classified into two categories: Category I: Consumptive detrimental elements, such
as sulfur, oxygen, etc., easily form compounds with spheroidizing agents. When the agents contain rare earth metals, the compounds formed, such as sulfides and oxides, have higher stability and are fine, making it difficult to float to the surface. Meanwhile, the mismatch of these compounds with graphite is small, which makes them act as nuclei for graphite. Magnesium oxide has a high melting point, good stability and low solubility in
二．稀土在球墨铸铁生产中的作用
要想切实地低削减稀土的用量，或尽可能地不用，必 须对其作用有充分的认识。镁、稀土和钙在铸铁中都有 使石墨球化的作用。作为实际应用的球化剂，就它们各 自的综合评价而言，镁是居于首位的。但是，在以镁作 主要球化球化元素的条件下，配合以适当的稀土，可以 得到更好的球化效果。
这里，只能简单地谈到稀土在与镁配合作为球化剂时 的一些作用。
1．脱硫、脱氧
镁、稀土和钙，它们在铁液中都有很强的脱硫、脱 氧作用。就元素与硫、氧作用、生成硫化物和氧化物的 自由能而言，稀土（铈）和钙的脱硫、脱氧能力都比 镁强。但是，镁的沸点为1,107℃，进入铁液后迅速气 化，对铁液有强烈的搅拌作用。同时，溶于铁液中的气 体易于向气泡中扩散、析出，从而被气泡带出。铁液中 的氧化物、硫化物夹杂也易于被气泡吸附并排出。考虑 到反应动力学方面的因素，在铁液中，镁脱氧、脱硫的 作用实际上强于稀土（铈）和钙。
2．对铸铁白口倾向的影响有其两面性
溶于铁液的稀土，促进形成碳化物的作用很强，是反 石墨化元素，但是，在以镁为主的球化剂中配合以少量的 稀土，却可以使铸铁的白口倾向大为减轻；在加入量较多 的情况下，却会增强铸铁的白口倾向。这种看似矛盾的现 象，实际上并不矛盾，其原因是：稀土与硫、氧结合的能 力很强，少量的稀土，很快就与铁液中的硫和氧作用，形 成大量细小的硫化物和氧化物，为铸铁的石墨化提供了大 量异质晶核，使石墨球的数量大幅度增加，便于铁中的碳 向石墨球扩散，从而抑制白口的生成。如果稀土加入量较 多，除与氧和硫作用的以外，还有多余的溶于铁液，这部 分稀土就会增强铸铁的白口倾向。
3．抑制干扰元素的作用
球墨铸铁中，有一些元素起反球化的作用，通常称之 为干扰元素。干扰元素大致可分为两类： 一类，有人称之为消耗型干扰元素，如硫、氧等，都 易于与当前广泛应用的各种球化元素形成化合物。镁、 稀土和钙都可以脱硫、脱氧，消除它们的负面影响，当 然本身也要因此而消耗掉一部分。
球化剂中含有稀土时，其硫化物和氧化物稳定性很 高，粒度细小，在铁液中难以上浮，而且这些化合物 与石墨晶格的失配度很小，可作为石墨析出的异质核 心。球化剂镁的氧化物MgO的熔点高、稳定性好、在 铁液中的溶解度低，但粒度稍大，可以上浮到表面成为
Fall 2011
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