In the early stage, in the aspect of grain refinement of steels by adding rare earth elements, the research mainly focused on Austenitic steels. Some commercial grain refiners were also used for austenitic steels in the market, and their main components were mainly rare earth elements.
At the end of the 20th century, C.Eijk, F.Haakosen and others added ferroalloys containing chromium and cerium (mass fraction: Cr 32%, Si 17%, Ce 8.7%, C 1.24%, Fe remaining) into completely austenitic stainless steels refined by AOD, with good grain refinement effect.The spacing between primary dendrites was reduced by half when observed with light and transmission electron microscopy.
After adding rare-earth elements into the molten steel, the melting points of REO and REO2 in various rare-earth oxides are very high, but they are unstable at high temperatures. It is easy to absorb aluminum oxide particles existing in the molten steel and become CeAlO3.It is confirmed by energy dispersive spectroscopy that there are indeed many CeAlO3 inclusions in steel.
The degree of mismatch between CeAlO3 and austenitic lattice is very small. Therefore, C.Eijk et al. hold that the grain refinement mechanism of austenitic stainless steel is the formation of CeAlO3 inclusions in the molten steel before solidification as heterogeneous nuclei of austenite crystallization and precipitation.Degree of mismatch between cerium-containing compounds and austenitic lattices.
Soon after, C.Eijk et al. applied Cerium-Bearing ferroalloys to austenitic high manganese steel, and the grain refinement effect was closely related to the content of Cerium in the steel.The inclusions CeO 2, CeAlO 3 and Ce2O2S are found in the steel by means of an electron probe microanalysis instrument.The degree of refinement of steel grains depends on the number of such inclusions. To achieve satisfactory results, no less than 157 inclusions per square centimeter should be observed.
It seems that the grain refinement effect of rare earth elements in austenitic steel is affirmative and its mechanism is relatively clear.
Although CeAlO3 and Ce2O3, which are relatively stable in liquid steel, can be used as heterogeneous nuclei for solidification and crystallization of austenitic steels, they are unlikely to act as heterogeneous nuclei in carbon steel and low alloy steels.