PURPOSE: To prevent the generation of the defect of surface fold on the surface of a product by casting the molten metal of cast iron in which the content of C, Si, Mn, S, P and Mg is specified in a water cooled die made of a copper allay and subjecting the obtd. product to heat treatment.

CONSTITUTION: The molten metal whose chemical components are constituted of a compsn. of 3.0 to 3.6% C, 2.0 to 2.6% Si, 0.3 to 0.5% Mn, 0.01 to 0.1% S, <0.05% P and 0.01 to 0.02% Mg, and the balance Fe is cast in a water cooled die made of a copper allay having 0.7 to 0.8cal/cm2.sec°C thermal conductivity. The obtd. pipe joint product is subjected to heat treatment for 2 to 3hr to promote the ferritization of the matrix structure. In this way, the pipe joint imparted with mechanical properties of about ≥40kgf/mm2 strength and about 10 to 15% elongation can be obtd. without generating the defect of surface fold on the surface of the product.