From the point of view of lattice mismatch, both CeO2 and CeS can be used as heterostructure nuclei of delta-ferrite crystallization and precipitation, but they often fail to do so. What is the reason?
Tuttle has found in his continuous research that CeO2 and CeS are unstable at liquid steel temperature and their surfaces are easy to absorb oxides of Fe, Al, Si and other elements in liquid steel.CeO2 and CeS, which could be used as heterostructure nuclei during liquid steel crystallization and solidification, lose their function as heterostructure nuclei after absorbing oxides on the surface.
The research work on steel with 0.1%~0.5% carbon content by Shiko and Senoka of Japan shows that the presence of Ce2O3 in the steel can refine the grain.They believe that the mechanism of grain refinement is that the precipitated rare earth oxides exist around the austenite grains and act as pins to restrain the austenite grain growth.However, grain refinement due to pinning can improve the strength of steel, but the plasticity and toughness will deteriorate, which is not the case in their research work.
In previous years, Tuttle also considered that the mechanism of grain refinement might be peritectic transformation of steel during solidification. When transformation from “delta-ferrite + liquid” to “austenite + liquid”, precipitated rare earth compounds act as nucleus of austenite, increasing the number of austenite grains and refining the grains.However, this assumption is questionable considering that the free energy for the formation of oxides and sulfides of rare earth elements is much lower than that of other oxides and sulfides and it is impossible to form after the precipitation of delta-ferrite.
Shortly afterwards, in order to confirm the influence of rare earth elements on primary crystal, Tuttle carried out a comparative test with low carbon steel with high phosphorus content (about 0.06% P). The test result shows that the primary delta-ferrite dendrite can be transformed into equiaxed crystal and the grain is refined when rare earth ferrosilicon is added to the steel.
After repeated consideration, Tuttle concluded that although stable rare earth compounds do not match the delta-ferrite lattice to a great extent, the lattice mismatch along a certain crystal plane and in a certain direction may not be too great.
In Tuttle’s study, rare-earth elements were added to steel using rare-earth ferrosilicon or mixed rare-earth alloys.
Rare earth ferrosilicon is added in stream when steel is tapped after final deoxidization or is flushed into the molten steel at the bottom of the package.The dosage is 0.2%~0.3%, which can be determined according to specific conditions.