PURPOSE: To manufacture a rust resistant cast steel without deteriorating the castability or reducing the strength by dispersing granular secondary carbide and a little massive primary carbide in the ferrite matrix of a coast steel contg. specified percentages of C, Si, Mn, P, S, Cr, Ni and Mo.

CONSTITUTION: A cast steel is composed of, by weight, 0.5W1.5% C, 2.5W3.5% Si, ≤0.7% Mn, ≤0.05% P, ≤0.1% S, 5W10% Cr, 2W5% Ni, 0.5W1.5% Mo and the balance essentially Fe, and granular secondary carbide and a little massive primary carbide are dispersed in the ferrite matrix of the cast steel. Thus, a rust resistant cast steel forming no rust is obtd. without deteriorating the castability or reducing the strength.