Heat-resistant, austenitic cast steel and exhaust member made thereof

1. Heat-resistant, austenitic cast steel consisting of, by mass, 0.3-0.6% of C, 1.1-2% of Si, 1.5% or less of Mn, 17.5-22.5% of Cr, 8-13% of Ni, 1.5-4% as (W+2Mo) of at least one of W and Mo, 1-4% of Nb, 0.01-0.3% of N, 0.01-0.5% of S, the balance being Fe and inevitable impurities, and meeting the following formulae (1), (2), (3) and (4):

0.05≦(C—Nb/8)≦0.6  (1),

17.5≦17.5Si−(W+2Mo)  (2),

5.6Si+(W+2Mo)≦13.7  (3), and

0.08Si+(C—Nb/8)+0.015Cr+0.011Ni+0.03W+0.02Mo≦0.96  (4),

wherein the symbol of each element corresponds to the amount (% by mass) of each element in the cast steel, wherein said cast steel is a 20Cr-10Ni based heat resistant austenitic cast steel, wherein a Cr oxide layer is formed in the outermost layer and a blocky Si oxide phase is formed inside the outermost layer, and wherein said steel has a thermal fatigue life of 800 cycles or more, when measured by a thermal fatigue test comprising heating and cooling under the conditions of a heating temperature upper limit of 1000° C., a temperature amplitude of 850° C. or more, and a constraint ratio of 0.25.

2. The heat-resistant, austenitic cast steel according to claim 1 , which has weight loss by oxidation of 15 mg/cm 2 or less when kept at 1000° C. for 200 hours in the air.

3. An exhaust member formed by the heat-resistant, austenitic cast steel recited in claim 1 .

4. The exhaust member according to claim 3 , wherein it is an exhaust manifold, a turbine housing, a turbine housing integral with an exhaust manifold, a catalyst case, a catalyst case integral with an exhaust manifold, or an exhaust outlet.