Heat-resistant, austenitic cast steel having excellent thermal fatigue properties, and exhaust member made thereof
A heat-resistant, austenitic cast steel having excellent thermal fatigue properties, comprising by mass 0.3-0.6% of C, 0.5-3% of Si, 0.5-2% of Mn, 15-30% of Cr, 6-30% of Ni, 0.6-5% of Nb, 0.01-0.5% of N, and 0.01-0.5% of S, C/N being 4-7, and the balance being Fe and inevitable impurities; and a ratio A/B of a Cr-carbide-forming index A to a Nb-carbide-forming index B being 0.6-1.7, wherein A and B are expressed by the formula (1) of A=8.5C−Nb+0.05Cr+0.65Ni−5, and the formula (2) of B=7.8Nb.
1. A heat-resistant, austenitic cast steel having excellent thermal fatigue properties, consisting of by mass
0.3-0.6% of C,
0.5-3% of Si,
0.5-2% of Mn,
15-30% of Cr,
6-30% of Ni,
0.6-5% of Nb,
0.01-0.5% of N, and
0.05-0.5% of S,
an amount ratio C/N of C to N being 4-7,
the balance being Fe and inevitable impurities, and
a ratio AB of a Cr-carbide-forming index A to a Nb-carbide-forming index B expressed
by the following formulae (1) and (2):
A=8.5C−Nb+0.05Cr+0.65Ni−5 (1), and
B=7.8Nb (2),
wherein the symbol of an element in each formula represents its content (% by mass), being 0.6-1.7.
2. A heat-resistant, austenitic cast steel having excellent thermal fatigue properties, consisting of by mass
0.3-0.6% of C,
0.5-3% of Si,
0.5-2% of Mn,
15-30% of Cr,
6-30% of Ni,
0.6-5% of Nb,
0.005-0.5% of Zr,
0.01-0.5% of N, and
0.05-0.5% of S,
an amount ratio C/N of C to N being 4-7,
the balance being Fe and inevitable impurities, and
a ratio AB of a Cr-carbide-forming index A to a Nb-carbide-forming index B expressed by the following formulae (1) and (2):
A=8.5C−Nb+0.05Cr+0.65Ni−5 (1), and
B=7.8Nb (2),
wherein the symbol of an element in each formula represents its content (% by mass), being 0.6-1.7.
3. The heat-resistant, austenitic cast steel according to claim 2 , wherein the number of Zr nitride particles having equivalent circle diameters of 1.5 μm or more in the structure is 20-150 per a field area of 0.25 mm 2 .
4. The heat-resistant, austenitic cast steel according to claim 1 , wherein its fatigue life is 1500 cycles or more, when measured by a high-temperature, low-cycle fatigue test, which adds and removes tensile strain and compression strain under the conditions of a test temperature of 900° C., a strain amplitude of 0.5%, a strain-applying speed of 0.1%/second, and compression-keeping time 1 minute.
5. An exhaust member made of the heat-resistant, austenitic cast steel recited in claim 1 .
6. The exhaust member according to claim 5 , which is a turbine housing, an exhaust manifold, a turbine-housing-integrated exhaust manifold, a catalyst case, a catalyst-case-integrated exhaust manifold, or an exhaust outlet.
7. The heat-resistant, austenitic cast steel according to claim 1 , wherein S content is 0.1-0.5% by mass.
8. The heat-resistant, austenitic cast steel according to claim 1 , wherein an amount ratio C/N of C to N is 5-7.
9. The heat-resistant, austenitic cast steel according to claim 2 , wherein its fatigue life is 1500 cycles or more, when measured by a high-temperature, low-cycle fatigue test, which adds and removes tensile strain and compression strain under the conditions of a test temperature of 900° C., a strain amplitude of 0.5%, a strain-applying speed of 0.1%/second, and compression-keeping time 1 minute.
10. An exhaust member made of the heat-resistant, austenitic cast steel recited in claim 2 .
11. The exhaust member according to claim 10 , which is a turbine housing, an exhaust manifold, a turbine-housing-integrated exhaust manifold, a catalyst case, a catalyst-case-integrated exhaust manifold, or an exhaust outlet.
12. The heat-resistant, austenitic cast steel according to claim 2 , wherein S content is 0.1-0.5% by mass.
13. The heat-resistant, austenitic cast steel according to claim 2 , wherein an amount ratio C/N of C to N is 5-7.