PURPOSE: To develop a heat-resistant cast steel combining high strength with high ductility, by adding and incorporating specific amounts of Al and relatively large amounts of B to a Cr-Ni heat-resistant cast steel containing Nb and Ti.

CONSTITUTION: As a heat-resistant cast steel for heat-treatment installations such as reaction tube for modification furnace, reaction tube for decomposition furnace, etc., used at high temp., a cast steel having a composition containing, by weight, 0.3W0.6% C, <2.0% Si, <2.0% Mn, 20W30% Cr, 20W40% Ni, 0.5W1.5% Nb, 0.05W0.3% Ti, 0.0005W0.02% (not including 0.02%) Al, and 0.004W0.5% (not including 0.004%) B is used. Nb and Ti form carbon borides and improve creep rupture strength of cast steel and B which is capable of being added in relatively large amounts by the coexistence with Al forms carbon borides together with Cr, Nb, and Ti, so that heat-resistant cast steel excellent in strength and ductility at high temp. can be obtained under the coexistence of Cr and Ni.