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The formation mechanism of spheroidal carbide in ultra-low carbon ductile iron fabricated by metal mold casting technique was systematically investigated.The results demonstrated that the spheroidal carbide belongs to eutectic carbide and crystallizes in the isolated eutectic liquid phase area.The formation process of the spheroidal carbide was related to the contact and the intersection between the primary dendrite and the secondary dendrite of austenite.The oxides of magnesium,rare earth and other elements can be acted as heterogeneous nucleation site for the spheroidal carbide.It was also found that the amount of the spheroidal carbide would increase with an increase in carbon content.The cooling rate has an important influence on the spheroidal carbide under the same chemical composition condition.