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

1. A heat-resistant, austenitic cast steel, which consists of by mass

0.4-0.55% of C,

1-2% of Si,

0.5-1.5% of Mn,

18-27% of Cr,

8-22% of Ni,

1.5-2.5% of Nb,

0.01-0.3% of N,

0.1-0.2% of S, and

0.02-0.15% of Al,

the balance being Fe and inevitable impurities;

a machinability index I expressed by the following formula:

I=100×S+75×Al+0.75×Mn−10×C−2×Nb−0.25×Cr−0.15×Ni−1.2×N,

wherein each element symbol represents % by mass of each element in the cast steel, meeting the condition of −3.0≦I≦+14.0.

2. The heat-resistant, austenitic cast steel according to claim 1 , which has a structure, in which the area ratio of sulfide particles having equivalent circle diameters of 2 μm or more to all sulfide particles is 60% or more.

3. The heat-resistant, austenitic cast steel according to claim 1 , wherein when it is milled with a cemented carbide tool at a cutting speed of 150 m/minute, a feed of 0.2 mm/tooth, and a cutting depth of 1.0 mm, under a dry condition without a cutting liquid, a tool life expressed by cutting time until the flank wear of the cemented carbide tool reaches 0.2 mm is 25 minutes or more.

4. An exhaust member made of the heat-resistant, austenitic cast steel recited in claim 1 .