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Austempered ductile iron having a tensile strength of 1,000 MPa or more and elongation of 12 to 18% has been developed, applied to the spindle, and examined. As a result of optimization of shape, composition, and heat treatment conditions, excellent results have been obtained without any stress concentration or incomplete heat treated portion, and desired functions have been satisfied. Consequently, the spindle which had been an assembly of parts could be made integral, unsprung weight could be reduced, and higher quality became possible. In addition, it has been proved that austempered ductile iron can also be applied to the portion requiring high toughness which so far had been considered impossible with conventional ductile iron, and extending the application, such as taking the place of forgings, can be expected.