In Europe, SiC that is around 90% is used generally as a raw material of carbon and silicon. SiC is effective for improving the properties of molten metal such as reduction of chill depth and improvement of graphite shape, etc. Suggested theories for explaining its effects include the deoxidation theory by SiC and graphite nuclei theory by Cn molecular. European cast engineers think that SiC that is around 90% pure is the most effective for improving the properties of molten metal. As for dendrite, T. Benecke reports that the dendrite arm space becomes short when SiC is added. In this study, we therefore aimed to change SiC purity, investigate the influence of SiC purity on the graphite nodule count (hereafter GNC) and eutectic graphitization ability (hereafter EGA), and review the functions of SiC in spheroidal graphite cast iron. The following results were obtained.

  As the purity of SiC increases, the dissolved speed of SiC becomes fast and the slag generation amount reduces. EGA and GNC also increase. When SiC is more than 95% pure, it is effective for refining the dendrite arm space. The higher the purity is, the higher is its effects, Moreover, the dendrite refining effect of SiC helps increase the graphite nuclei site between dendrite arm space and the GNC. Like SiC, Zr-type inoculant is also effective for refining dendrites. When the (a) dendrite refining effect of SiC that is more than 95% pure and (b) graphite nucleation ability of other inoculants such as Zr, Al, and Ba are combined, GNC increases remarkably..

  In order to clarify the effect of inoculants containing zirconium on spheroidal graphite cast iron, systematic investigation was carried out from the solidification process to the metallographic structure, shrinkage property, and mechanical properties, and the mechanism bringing about the effect was examined from the microstructure.

  The addition of the Fe-Si-Zr inoculant increases the stable eutectic temperature, and the differential curve angle at the end of solidification (θ) becomes small. As the graphite nodules in the microstructure increase, the shrinkage reduces. The mechanism of the improvement of the shrinkage property is considered to be due to the increased amount of graphite crystallized, particularly in the later stage of solidification. As for mechanical properties, the compatibility of tensile strength and elongation improved, and the wall-thickness sensitivity reduced. The mechanism for reducing the wall-thickness sensitivity is considered to be due to the suppression of ferritization around the graphite nodule by graphite crystallization in the late solidification region in which the carbide stabilizing elements are concentrated.

  Though Fe-Si-Ti, Fe-Si-V, and Fe-Si-Nb inoculants also have the effect of increasing graphite nodules, Fe-Si-Zr inoculant has the most stable effect for the difference in molten metal characteristics.

  The effect of zirconium (Zr) inoculant on ductile iron microstructure was investigated. It was found that Zr-containing post inoculant generated high graphite nodule counts and refined the austenite dendritic structure as well. EBSD, a microstructural-crystallographic characterization technique, was utilized to study the mechanism of the high nodule counts by using water-quenched samples. It was found that the effect of Zr on nodule count was not due to the nucleation of graphite but the refinement of austenite dendritic structure. In this study, direct and precise measurement of the austenite grain size was feasible only when higher quenching temperature was used (1150C). This is because the solubility of carbon in austenite increases with higher quenching temperature, bringing the Ms point down below room temperature.

  The angle of cooling rate curve at the end of solidification θ was studied in this research. Results of solidification simulation reveal that latent heat is the reason why θ occurs, and that it affects the size of θ, that is the greater the latent heat or the longer the eutectic solidification time, the smaller is θ.

  θ varies with change in the graphite microstructure. Flake graphite iron has a higher cooling rate and a narrower angle at the end of solidification when compared with spheroidal graphite iron. This is due to the differences in their solidification kinetics and heat transfer coefficient between gray iron and ductile iron. When a D Type graphite or chill structure generates in gray iron, θ increases.

  θ is a good indicator of the shrinkage tendency in ductile iron. That is, a larger angle indicates a higher shrinkage tendency. When shrinkage occurs around the thermo-couple in the shell cup, the cooling rate becomes slower due to the heat transfer deterioration. Finally, the difference in the cooling curves between the ATAS Cup and SP Cup resulted from the difference in the quartz tube diameter used in the two cups.

  In order to elucidate the mechanism of nodule count increase by Bi, we analyzed spheroidal graphite cast iron with increased nodule count by Bi addition. In the Scanning Electron Microscope - Energy Dispersive X-ray Spectrometry (SEM-EDS) analysis, Bi was identified to be present near the center of the spheroidal graphite. The center of this spherical graphite was pretreated by Focused Ion Beam (FIB) micro sampling method to prepare analytical sampls, which were analyzed by Transmission Electron Microscope - Energy Dispersive X-ray Spectrometry (TEM-EDS). As a result, it was found that Bi exists as Bi-La-Ce-Sb oxide on the outer periphery of the graphite core centered on (Mg Ca) S.

  In order to verify whether this Bi-La-Ce-Sb oxide is effective for increasing nodule count, an oxide sample having the same composition as the Bi-La-Ce-Sb oxide analyzed by TEM-EDS was prototyped by mechanical alloying, and the prototype Bi-La-Ce-Sb oxide was added to the molten metal of spheroidal graphite cast iron. As a result, nodule count did not increase with the addition of the prototype Bi-La-Ce-Sb oxide alone, but when the prototype Bi-La-Ce-Sb oxide and Fe-Si inoculant were mixed and added, the nodule count increased more than when only the Fe-Si inoculant was added.

  These results indicate that, Bi addition is effective for increasing nodule count because Bi acts on the graphite nucleus as an oxide.