PURPOSE: To prevent the separation of a boundary part and the breakage of a roll neck by joining the outer layer made of cast iron having the prescribed Shore hardness and containing C, Cr, Mn, Ni, etc., in the specified weight % and the core material made of cast steel or forged steel metallurgically.

CONSTITUTION: A glass welded layer 6 is formed in advance on the base metal 5 made of cast steel or forged steel having ≥5.5kg/cm2 tensile strength and ≥1.0% elongation and set inside a fire-proof heating die 1. Then, the cast molten iron containing 2.5W4.0% c, 0.5W1.5% Si, 0.4W1.5% Mn and ≤5% Cr, ≤5% Ni respectively is subjected to welding reaction by pouring it into the gap of the base metal 5 and heating die 1 and the base metal 5 is descended in order. In this way the strength of the boundary part is increased and the defect generation can be prevented because of the outer layer and core material 5 being metallurgically joined. Consequently both of the separation of the boundary part and breakage of the roll neck part can be prevented.