Abstract: A ductile cast iron having a high tensile strength as well as a high elongation due to its matrix structure of finely mixed ferrite and pearlite grains obtained by the heat treatment. The material is pearlitic ductile cast iron and may be of ordinary ductile cast iron composition with the only exception that the manganese content being substantially less than 1%.A method of heat treatment for obtaining such a ductile cast iron, which is composed substantially of heating the material rapidly to form ferrite, austenite and graphite co-existing structure, and air cooling the same to change that structure into the above mentioned matrix structure.

Abstract: A spheroidal graphite cast iron containing some carbide stabilizing elements to have the maximum strain rate sensitivity factor of more than 0.3 and having very refined grain matrix structure, so that having an improved toughness at room temperature and having a matrix which is composed of fine austenite and fine ferrite grains and superplastic properties at the temperature between the eutectoid transformation temperature range and 50.degree. C higher than that temperature range; and the method of heat treatment to obtain the said spheroidal graphite cast iron.