JP7740550B2 - Austenitic heat-resistant cast steel and exhaust system parts made of the same - Google Patents

Description

The present invention relates to austenitic heat-resistant cast steel that has excellent shrinkage crack resistance, cold crack resistance, and machinability, as well as good thermal fatigue properties and a reduced content of expensive alloying elements, and to exhaust system components made from the same.

Exhaust system parts used in internal combustion engines, such as automobile engines, particularly the exhaust manifold shown in Figure 1, are manufactured using castings, which allow for a high degree of freedom in shape design, due to their thin walls and complex shapes. Because they are exposed to high-temperature exhaust gases when a vehicle is in operation, exhaust system parts must have excellent heat resistance and durability at high temperatures. Therefore, austenitic heat-resistant cast steel is primarily used as the construction material. Various compositions have been proposed for austenitic heat-resistant cast steel.

For example, International Publication No. 2009/104792 discloses a steel sheet containing, by mass, 0.3 to 0.6% C, 1.1 to 2% Si, 1.5% or less Mn, 17.5 to 22.5% Cr, 8 to 13% Ni, 1.5 to 4% of at least one of W and Mo (W + 2Mo), 1 to 4% Nb, 0.01 to 0.3% N, 0.01 to 0.5% S, the balance being Fe and inevitable impurities, and having the following formulas (1) to (4):
0.05≦(C－Nb/8)≦0.6・・・(1)
17.5≦17.5Si－(W＋2Mo)・・・(2)
5.6Si＋(W＋2Mo)≦13.7・・・(3)
0.08Si＋(C－Nb/8)＋0.015Cr＋0.011Ni＋0.03W＋0.02Mo≦0.96・・・(4)
(where the element symbols in each formula indicate the content (mass %)) has been proposed.

Further, International Publication No. 2016/052750 discloses a steel sheet containing, by mass, 0.3 to 0.6% C, 0.5 to 3% Si, 0.5 to 2% Mn, 15 to 30% Cr, 6 to 30% Ni, 0.6 to 5% Nb, 0.01 to 0.5% N, and 0.01 to 0.5% S, with a C to N content ratio C/N of 4 to 7, the remainder being Fe and unavoidable impurities, and satisfying the following formulas (1) and (2):
A=8.5C-Nb+0.05Cr+0.65Ni-5...(1)
B = 7.8Nb (2)
The present invention proposes a heat-resistant, austenitic cast steel with excellent thermal fatigue properties, characterized in that the ratio A/B of the Cr carbide formation index A to the Nb carbide formation index B, expressed by the formula (where the element symbols in each formula indicate the content (mass%) of the element), is 0.6 to 1.7.

The austenitic heat-resistant cast steels disclosed in WO 2009/104792 and WO 2016/052750 have excellent thermal fatigue life at temperatures around 1000°C or higher, but contain a large amount of expensive alloying elements, resulting in poor cost performance. Furthermore, due to their low ductility at room temperature, thin-walled exhaust manifolds can suffer from cracks (cold cracks) during cooling after casting, potentially reducing product acceptance rates.

JP 2011-219801 A proposes an iron (Fe)-based austenitic heat-resistant cast steel that, when taken as 100% by mass (hereinafter simply referred to as "%"), contains the following: carbon (C): 0.4-0.8%, silicon (Si): 3.0% or less, manganese (Mn): 0.5-2.0%, phosphorus (P): 0.05% or less, sulfur (S): 0.03-0.2%, chromium (Cr): 18-23%, nickel (Ni): 3.0-8.0%, and nitrogen (N): 0.05-0.4%, with the chromium (Cr) to carbon (C) ratio being in the range of 22.5≦Cr/C≦57.5. However, there is a concern that this austenitic heat-resistant cast steel is prone to cracks caused by minute shrinkage cavities (hereinafter also referred to as shrinkage cracks) when cast into exhaust manifolds, which may result in a decrease in the product acceptance rate.

In addition, SCH12, a type of austenitic heat-resistant steel specified in JIS G 5122, is not only prone to shrinkage cracking, but also has difficulty in machinability compared to the materials disclosed in the above-mentioned documents, making it unsuitable for exhaust manifolds, which require machining during manufacturing.

Therefore, the object of the present invention is to provide austenitic heat-resistant cast steel that contains a low amount of expensive alloying elements, is resistant to shrinkage cracking during casting and cold cracking after casting, has good machinability after casting, and has specified thermal fatigue properties at around 800°C, as well as exhaust system parts, particularly exhaust manifolds, made thereof.

That is, the austenitic heat-resistant cast steel of the present invention is, on a mass basis,
C: 0.30-0.50%,
Si: 0.50-2.0%,
Mn: 0.50-2.0%,
S: 0.10-0.40%,
Cr: 16.0-21.0%
Ni: 6.0-12.0%,
Nb: 0.5 to 2.0%, and
Cu: Contains 0.05 to 0.80%
The balance is Fe and unavoidable impurities.

In the austenitic heat-resistant cast steel of the present invention, the S content is preferably 0.15 to 0.37% by mass, and the Nb content is preferably 0.9 to 1.6%. This not only further suppresses shrinkage cracking during casting and cold cracking after casting, but also provides a better balance between machinability and thermal fatigue properties.

In the heat-resistant, austenitic cast steel of the present invention, the number of manganese sulfides having an equivalent circle diameter of 1 μm or more in an arbitrary cross section is preferably 350 to 2550 per mm ^2. This improves machinability while ensuring oxidation resistance.

In the heat-resistant austenitic cast steel of the present invention, the area ratio of niobium carbide in any cross section is preferably 0.5 to 11.0%. This suppresses the occurrence of shrinkage cracks and allows the resulting heat-resistant cast steel to have good high-temperature strength and thermal fatigue properties.

The exhaust system part of the present invention is characterized by being made of the above-mentioned austenitic heat-resistant cast steel.

The exhaust system component is preferably an exhaust manifold.

The austenitic heat-resistant cast steel of the present invention is resistant to shrinkage cracks during casting and cold cracks after casting, has good workability after casting, and possesses good thermal fatigue properties at around 800°C. Furthermore, the content of expensive alloying elements is reduced, achieving low costs. Such austenitic heat-resistant cast steel is suitable for exhaust system components of internal combustion engines, particularly exhaust manifolds.

FIG. 1 is a front view schematically showing an exhaust manifold, which is an example of an exhaust system part. FIG. 1 is a plan view schematically showing a casting from which a test piece for evaluating micro-shrinkage cavities is taken. FIG. 1 is a side view schematically showing a casting from which a test piece for evaluating micro-shrinkage cavities is taken. FIG. 1 is a plan view schematically showing a stepped casting from which a test piece for microstructure observation is taken. FIG. 1 is a side view schematically showing a stepped casting from which a test piece for microstructure observation is taken. 1 is a photograph showing the microstructure of the heat-resistant, austenitic cast steel of Example 4. This is an enlarged photograph of area A in FIG. 4A.

[1] Austenitic heat-resistant cast steel
(A) Composition The content of each element described below is expressed on a mass basis unless otherwise specified.

(1) C (carbon): 0.30-0.50%
Carbon improves the fluidity of molten metal, enhancing castability, and strengthens the austenite matrix by dissolving in the matrix after solidification (solid-solution strengthening). It also forms thermally stable, hard carbides with other alloying elements, such as chromium (Cr) and niobium (Nb), which disperse in the austenite matrix, enhancing high-temperature strength. To effectively utilize these functions, carbon must contain 0.30% or more. However, excessive carbon leads to excessive carbide precipitation, resulting in embrittlement and reduced ductility, as well as poor machinability. Therefore, the carbon content must be kept below 0.50%. Therefore, the carbon content range is set to 0.30-0.50%. The lower limit of carbon content is preferably 0.35%, more preferably 0.37%. The upper limit of carbon content is preferably 0.45%, more preferably 0.44%.

(2) Si (silicon): 0.50-2.0%
Since Si is an element that improves oxidation resistance and, as a result, thermal fatigue life, a content of 0.50% or more is necessary. However, excessive content destabilizes the austenite structure and leads to deterioration of castability, so the upper limit is set to 2.0%. Therefore, the Si content range is set to 0.50 to 2.0%. The lower limit of the Si content is preferably 0.80%, more preferably 0.90%, and most preferably 1.0%. The upper limit of the Si content is preferably 1.5%, more preferably 1.2%, and most preferably 1.1%.

(3) Mn (manganese): 0.50-2.0%
Mn not only stabilizes the austenite structure, but also forms sulfides (MnS) with sulfur (S), which disperse as free-cutting particles in the austenite matrix, improving machinability. To achieve this effect, Mn must be at least 0.50%, but oxidation resistance deteriorates if the Mn content exceeds 2.0%. Therefore, the Mn content range is set to 0.50-2.0%. The lower limit of Mn content is preferably 0.80%, more preferably 0.90%. The upper limit of Mn content is preferably 1.5%, more preferably 1.2%, and most preferably 1.1%.

(4) S (sulfur): 0.10-0.40%
S combines with Mn and Cr to form MnS (sulfide) and (Mn, Cr)S (complex sulfide), which disperse in the austenite matrix. MnS and (Mn, Cr)S can be collectively referred to as manganese sulfides. Manganese sulfides have a lubricating effect, contributing to improved machinability of heat-resistant cast steel. To achieve this effect, the S content is set to 0.10% or more. However, if the S content exceeds 0.40%, high-temperature strength and ductility tend to decrease, and excessive manganese sulfide formation also tends to degrade thermal fatigue properties. Therefore, the S content is set to a range of 0.10 to 0.40%. The lower limit of the S content is preferably 0.12%, more preferably 0.15%, and most preferably 0.16%. The upper limit of the S content is preferably 0.37%, more preferably 0.34%.

(5) Cr (chromium): 16.0-21.0%
Cr dissolves in the matrix to stabilize the austenite structure. It also combines with C to form thermally stable, hard carbides (Cr carbides), which disperse throughout the austenite matrix, improving high-temperature strength. Cr also combines with atmospheric oxygen to form a strong oxide film (passive film ) on the casting surface, improving oxidation resistance at high temperatures. To achieve these effects, the Cr content must be 16.0% or greater. However, if the Cr content exceeds 21.0%, the amount of Cr carbides dispersed in the austenite matrix becomes excessive, promoting crack propagation and actually degrading the thermal fatigue properties of the heat-resistant cast steel. Therefore, the Cr content range is set to 16.0-21.0%. The lower limit of the Cr content is preferably 16.3%, more preferably 16.6%, and most preferably 17.0%. The upper limit of the Cr content is preferably 20.0%, more preferably 19.0%, and most preferably 18.7%.

(6) Ni (nickel): 6.0 to 12.0%
Like Cr, Ni dissolves in the matrix, stabilizing the austenite structure and improving the high-temperature strength and oxidation resistance of heat-resistant cast steel. Ni also improves the castability of thin-walled, complex-shaped exhaust system components, such as exhaust manifolds. To achieve these excellent effects, the Ni content is set to 6.0% or more. On the other hand, increasing the Ni content increases the amount of Ni dissolved in the austenite matrix, but this lowers the solid solubility limit of C and promotes the formation of Cr carbides, which tends to degrade the thermal fatigue properties of the heat-resistant cast steel. Therefore, in order to reduce the use of expensive Ni, it is preferable to limit the Ni content to a level that ensures the desired thermal fatigue properties. A Ni content of 12.0% is sufficient to ensure the required thermal fatigue strength at temperatures around 800°C. Therefore, the Ni content range is set to 6.0 to 12.0%. The lower limit of the Ni content is preferably 6.2%, more preferably 6.3%, and most preferably 6.5%. The upper limit of the Ni content is preferably 10.0%, more preferably 9.0%, and most preferably 8.6%.

(7) Nb (niobium): 0.5-2.0%
Nb preferentially bonds with carbon over chromium to form fine niobium carbides. This suppresses the formation of excess chromium carbides, thereby indirectly improving the high-temperature strength and thermal fatigue properties of heat-resistant cast steel. Furthermore, niobium carbides are eutectic carbides with austenite, and can remain in the molten state until the casting solidifies completely, reducing the formation of minute shrinkage cavities. Therefore, niobium suppresses shrinkage cracking, which is particularly likely to occur when manufacturing thin-walled, complex-shaped castings such as exhaust manifolds. To achieve this effect, the niobium content is set to 0.5% or more. On the other hand, excessive niobium content results in excess niobium carbides, which actually reduces the high-temperature strength and thermal fatigue properties of heat-resistant cast steel. For this reason, the upper limit of the niobium content is set to 2.0%. Therefore, the niobium content range is 0.5-2.0%. The lower limit of the niobium content is preferably 0.9%, more preferably 1.4%. The upper limit of the Nb content is preferably 1.8%, more preferably 1.6%, and most preferably 1.5%.

(8) Cu (copper): 0.80% or less
Even if contained in small amounts, Cu contributes to improving the ductility of heat-resistant cast steel, and is expected to suppress cold cracking after casting. For this reason, the heat-resistant cast steel of the present invention contains Cu. However, if the Cu content exceeds 0.80%, the ductility of the heat-resistant cast steel decreases, so the upper limit of the Cu content is set to 0.80%. The upper limit of the Cu content is preferably 0.50%, more preferably 0.25%, and most preferably 0.20%. On the other hand, the lower limit of the Cu content is not particularly limited (but does not include 0%), and may be 0.05% or 0.10%.

(9) Inevitable Impurities: The austenitic heat-resistant cast steel of the present invention inevitably contains impurities derived from raw materials and/or auxiliary materials (e.g., deoxidizers). Examples of such impurities include P (phosphorus), Al (aluminum), W (tungsten), and Mo (molybdenum). It is preferable to minimize the content of these impurities. For example, P significantly reduces the toughness of heat-resistant cast steel, so its content is preferably 0.06% or less. Al forms slag and slag containing _Al2O3 ( _alumina ) during the melting process, which can become mixed in as inclusions during casting, causing casting defects. It also combines with N (nitrogen) in the atmosphere to form hard and brittle AlN (aluminum nitride), which can be mixed in and reduce the ductility and machinability of the product. For this reason, the Al content is preferably 0.05% or less. Furthermore, both W and Mo form carbides with C, reducing the ductility of the heat-resistant cast steel. They also dissolve in the austenite matrix, reducing the amount of Cr dissolved in the matrix, thereby reducing the oxidation resistance of the matrix. Furthermore, they promote the crystallization of Cr carbides, thereby reducing the thermal fatigue properties. Therefore, it is preferable that W and Mo be 0.60% or less each, and more preferably 0.60% or less in total.

(B) Microstructure Figure 4A is a photograph showing the microstructure of a cut surface of the austenitic heat-resistant cast steel of Example 4, which is an example of the present invention, and Figure 4B is an enlarged photograph of region A in Figure 4A. As shown in Figure 4A, the structure of the austenitic heat-resistant cast steel of the present invention is mainly composed of a gray austenite phase (base) 14, white niobium carbides 15, a eutectic phase 16 of the niobium carbides 15 and the austenite phase 14, and dark gray manganese sulfides 17. The eutectic phase 16 is distributed in a network pattern so as to fill the gaps in the dendritic austenite phase 14.

4B, for clarity, the boundary between the austenite phase 14 and the eutectic phase 16 is indicated by a two-dot chain line. As described above, the eutectic phase 16 exists as a molten liquid until the final stage of solidification, and fills the fine gaps in the dendritic austenite phase 14, which has completed solidification earlier, thereby making it difficult for minute shrinkage cavities to occur.

In the austenitic heat-resistant cast steel of the present invention, the area fraction of niobium carbide 15 in any cross section is preferably 0.5 to 11.0%. If it is less than 0.5%, the effect of suppressing shrinkage cracking will not be sufficient. Furthermore, if it exceeds 11.0%, high-temperature strength and thermal fatigue properties will decrease, and machinability will also decrease. The lower limit of the area fraction of niobium carbide 15 is more preferably 1.0%, even more preferably 1.8%, even more preferably 3.5%, and most preferably 5.0%. On the other hand, the upper limit of the area fraction of niobium carbide 15 is more preferably 10.0%.

4B, it can be seen that many manganese sulfide particles 17 are precipitated in the microstructure of the austenitic heat-resistant cast steel of the present invention. Among these, relatively large manganese sulfide particles 17 with an equivalent circle diameter of 1 μm or more improve machinability, but extremely fine manganese sulfide particles 71 with an equivalent circle diameter of less than 1 μm do not contribute to improving machinability. Manganese sulfide particles 71 with an equivalent circle diameter of less than 1 μm are often present in the eutectic phase 16. The "equivalent circle diameter" refers to the diameter of a circle having an area equal to the area of each particle.

To improve machinability, it is preferable to have a large number of manganese sulfide particles 17 with an equivalent circle diameter of 1 μm or more. However, if the number is too large, the oxidation resistance of the austenitic heat-resistant cast steel decreases. Therefore, the number of manganese sulfide particles 17 with an equivalent circle diameter of 1 μm or more per ^mm2 in any cross section is preferably 350 to 2550. The lower limit of the number of manganese sulfide particles 17 per ^mm2 is more preferably 600, and the upper limit is more preferably 1600, and most preferably 1450.

[2] Exhaust system parts The exhaust system parts of the present invention are made of the above-mentioned austenitic heat-resistant cast steel. Preferred examples of the exhaust system parts are exhaust manifolds, turbine housings, exhaust manifolds integrated with turbine housings, catalyst cases, exhaust manifolds integrated with catalyst cases, and exhaust outlets, with thin-walled exhaust manifolds with complex shapes being particularly preferred.

Figure 1 shows an example of an exhaust manifold. The exhaust manifold 1 has multiple port sections 2, flange sections 3 connected to each port section 2, a collection section 4 for the port sections 2, and a flange section 5 connected to the collection section 4.

The present invention will be described in more detail by the following examples, but the present invention is not limited thereto.

Examples 1 to 7 and Comparative Examples 1 and 2
80 kg of raw materials, consisting of steel scrap, return scrap, and ferroalloys containing predetermined amounts of constituent elements, were melted in air using a high-frequency induction melting furnace (basic lining) with a melting capacity of 100 kg/charge. The melt was tapped at 1650-1700°C and poured into a mold at 1590-1610°C to obtain specimens for composition analysis in Examples 1-7 and Comparative Examples 1 and 2. A simultaneous carbon-sulfur analyzer (CS-444, manufactured by LECO) was used to analyze C (carbon) and S (sulfur), and a solid-state optical emission spectrometer (PDA-8000, manufactured by Shimadzu Corporation) was used to analyze other elements. The results are shown in Table 1.

Furthermore, to prepare 1-inch Y-blocks (as specified in JIS G 5502) for thermal fatigue life testing, test pieces 21 for evaluating micro-shrinkage cavities shown in Figures 2A and 2B, stepped castings 30 for microstructure observation shown in Figures 3A and 3B, and cylindrical test pieces (not shown) for tool life testing with an outer diameter of 100 mm, an inner diameter of 60 mm, and a length of 60 mm, the molten metals of Examples 1 to 7 and Comparative Examples 1 and 2 were poured into each test piece mold under the same conditions as above.

All molds used were _CO2- cured alkaline phenol molds, made by adding 3% by mass of resin (Kao-Step C-840, manufactured by Kao-Quaker Corporation) to silica sand (Nikko silica sand α6) as aggregate.

Notes: (1) Fe and unavoidable impurities.

(1) Evaluation of thermal fatigue properties (thermal fatigue life test)
To evaluate thermal fatigue properties, the following thermal fatigue test (TMF) was conducted. Smooth round bar specimens with a gauge length of 25 mm and a diameter of 10 mm were cut from each 1-inch Y-block and mounted in an electro-hydraulic servo-type materials testing machine (Shimadzu Corporation, Servo Pulser EHF-ED10TF-20L) with a restraint ratio of 1.0. Each specimen was subjected to repeated heating and cooling cycles in air, with a lower cooling limit of 150°C, an upper heating limit of 800°C, and a temperature amplitude of 650°C. Each cycle consisted of a 2-minute heating time, a 1-minute hold time, and a 4-minute cooling time, for a total of 7 minutes. Thermal fatigue was induced under mechanical restraint of expansion and contraction associated with heating and cooling.

The degree of mechanical constraint is expressed by the constraint ratio η, defined as [(free thermal expansion elongation - elongation under mechanical constraint) / (free thermal expansion elongation)]. For example, η = 1.0 refers to mechanical constraint conditions that do not allow any free thermal expansion elongation, while η = 0.5 refers to mechanical constraint conditions that only allow an elongation of 1 mm, whereas the free thermal expansion elongation without mechanical constraint would be, for example, 2 mm. In Examples 1 to 7 and Comparative Examples 1 and 2, thermal fatigue life tests were conducted under a completely constrained condition (η = 1.0), in which no elongation of the test specimen was allowed during either heating or cooling.

When heating and cooling cycles using TMF are repeated under the above mechanical restraint conditions, the tensile load applied to the test specimen decreases due to thermal fatigue. Therefore, in the heating and cooling cycle number vs. tensile load diagram, which records the change in load associated with heating and cooling cycles using TMF, the maximum tensile load in the second cycle (equivalent to the tensile load at the lower limit temperature during the cooling process) is set as the reference (100%). The thermal fatigue life is defined as the number of heating and cooling cycles required until the maximum tensile load measured in each subsequent cycle falls to 75% of this reference (maximum tensile load in the second cycle).

(2) Evaluation of machinability (tool life test)
Machinability was evaluated using a tool life test, a widely used tool for machinability testing. Specifically, a cylindrical test piece (not shown) with an outer diameter of 100 mm, an inner diameter of 60 mm, and a length of 60 mm was machined on an NC lathe. The tool life was measured as the time it took for the flank wear of the tool tip to reach 0.2 mm. The NC lathe used was a TAKISAWA Corporation TAC-510x1000. The tool was an insert (Kennametal KCM25B VBMT160404LF or VBMT331LF) with a carbide substrate and a multilayered TiCN- _{Al2O3 -} TiOCN CVD coating. Cutting was performed under wet conditions using an emulsion-based coolant (Castrol Superedge 6754 manufactured by Castrol Industrial North America) diluted to a concentration of 10–12%, with a tool peripheral speed of 128 m/min, a feed rate of 0.12 mm/tooth, and a depth of cut of 0.3 mm.

(3) Evaluation of shrinkage crack resistance (measurement of the volume fraction of micro-shrinkage cavities)
Focusing on the fact that micro-shrinkage cavities cause shrinkage cracks, the cross section of the micro-shrinkage cavities evaluation specimen 21 shown in Fig. 2A and Fig. 2B was photographed by X-ray, and the obtained image was analyzed to determine the volume fraction of micro-shrinkage cavities, and the shrinkage crack resistance was evaluated based on this.

Figures 2A and 2B show a casting 20 from which a micro-shrinkage cavity evaluation specimen 21 is obtained. The casting 20 includes the micro-shrinkage cavity evaluation specimen portion 21, into which the poured molten metal is finally filled; a weir portion 22 connected to the upstream side of the micro-shrinkage cavity evaluation specimen portion 21; a feeder portion 23 connected to the upstream side of the weir portion 22; a runner portion 24 connected to the upstream side of the feeder portion 23; and a gate portion (not shown) connected to the upstream side of the runner portion 24. The micro-shrinkage cavity evaluation specimen portion 21 is approximately flat, 40 mm wide and 100 mm long, and tapered in thickness, from 13.2 mm at the front end to 20 mm thick at the rear end. The weir portion 22 is 40 mm wide and 12 mm thick. The riser 23 is 50 mm in diameter and is approximately frusto-conical in shape, tapering to about 38.5 mm at a height of 66 mm, with a spherical base that extends downwards by about 20 mm. The runner 24 is 40 mm wide and 13 mm thick.

Molten metal with the same composition as the 1-inch Y-block was poured into a sand mold for producing casting 20, and after cooling to room temperature, the mold was disassembled. Test specimens 21 for evaluating micro-shrinkage cavities were cut out from the resulting casting 20 and subjected to shot blasting.

The cross section of the micro-shrinkage cavity evaluation specimen 21 was imaged using a CT scanner (Nikon Corporation, XTH 450) at a tube voltage of 450 kV. A 3D viewer (Volume Graphics, myVGL) was used to obtain CT tomographic images of an approximately 1.4 mm wide area at the center of the thickness of the micro-shrinkage cavity evaluation specimen 21, with a pitch of 0.1 mm in the thickness direction. Each CT tomographic image was binarized using image processing software (Innotek Corporation, Quick Grain Padplus) into micro-shrinkage cavities (dark areas) and other areas (light areas). The area of the micro-shrinkage cavities (dark areas) per pitch (0.1 mm) was calculated, and this was integrated over the measurement range to determine the volume of the micro-shrinkage cavities (unit: ^mm3 ). The volume fraction of the micro-shrinkage cavities was calculated by dividing the volume of the micro-shrinkage cavities by the measured volume of the micro-shrinkage cavity evaluation specimen 21 (75,000 ^mm3 ).

Table 2 shows the thermal fatigue life, tool life, and volume fraction of micro-shrinkage cavities for Examples 1 to 7 and Comparative Examples 1 and 2.

The thermal fatigue life (TMF) of Examples 1 to 7 was found to be 120 to 255 cycles, meeting the standards required for exhaust manifolds at 800°C. In particular, Example 1 at 220 cycles and Example 5 at 255 cycles exhibited superior thermal fatigue life than Comparative Example 2 at 195 cycles, and Example 5 exhibited a thermal fatigue life 1.1 times longer than Comparative Example 1, which corresponds to the JIS alloy SCH12.

The tool life of Examples 1 to 7 was 47 to 138 minutes, 1.8 to 5.3 times that of Comparative Example 1, and was roughly equivalent to or longer than the 61 minutes of Comparative Example 2 (which has a composition similar to that disclosed in WO 2016/052750) due to its high Cr and Ni content. In particular, Example 6 had a tool life of 138 minutes, 2.3 times that of Comparative Example 2.

The volume fractions of micro-shrinkage cavities in Examples 1 to 7 were 8.27×10 ^-6 to 740×10 ^-6 , all of which were smaller than that of Comparative Example 1. Furthermore, even compared to Comparative Example 2, which had a relatively low volume fraction of micro-shrinkage cavities due to the high Cr and Ni contents, Examples 1, 4, and 5 showed even lower volume fractions of micro-shrinkage cavities, and although Examples 2, 3, 6, and 7 were inferior, all of them were at a level that exhibited shrinkage crack resistance sufficient for use in exhaust manifolds.

From the above comparison, it was found that (a) Examples 1 to 7 have superior machinability (tool life) and shrinkage crack resistance (expressed by the volume fraction of microshrinkage cavities) compared to Comparative Example 1, and (b) Examples 1 to 7 exhibit performance equal to or better than Comparative Example 2, which is expensive because it contains approximately 1.5 to 2.0 times the Ni and approximately 1.3 to 1.5 times the Cr of Examples 1 to 7, despite having lower contents of the expensive Ni and Cr.

The austenitic heat-resistant cast steel of the present invention therefore has a thermal fatigue life that meets the standards required for a material used in exhaust manifolds, has a relatively long tool life, which makes it easy to cut, and has a small volume fraction of micro-shrinkage cavities, which are the cause of shrinkage cracks, making it less likely to develop. Not only is it a well-balanced material for use in exhaust system parts, particularly exhaust manifolds, but it also has a low content of expensive alloying elements, making it economically superior.

(4) Microstructure Observation. A sample cut from the 10 mm-thick section 31 of the stepped casting 30 shown in Figures 3A and 3B was embedded in resin so that the cut surface served as the observation surface, and then mirror-polished to obtain a microstructure observation specimen. The microstructure observation specimen was subjected to microstructure observation and elemental mapping of C, Si, Mn, S, Cr, Ni, and Nb using an electron probe microanalyzer (EPMA-1720, manufactured by Shimadzu Corporation). Microstructure observation was performed using the minimum beam diameter in five randomly selected fields of view magnified 200 times under the following conditions: acceleration voltage 15 kV, beam current 100 mA, pixel count 640 × 480, and integration time per pixel 20 ms/point. Figures 4A and 4B show backscattered electron images (COMPO) of Example 4.

(a) Identification and number determination of manganese sulfide particles
Because sulfur is distributed in high concentrations throughout the manganese sulfide particles, manganese sulfides (MnS and (Mn,Cr)S) were identified by mapping the S content of the microstructure observed at 200x magnification using an EMPA. A mapping image was obtained in which only the manganese sulfides were colored, while the rest of the sample was black. The colored areas corresponding to manganese sulfides ranged from 1500 to 100 in gradation levels depending on their brightness. Using image processing software (Quick Grain Padplus, manufactured by Innotek), the mapping image was binarized into areas with brightness levels above 105 (light areas) and areas with brightness levels below 104 (dark areas). The bright areas were color-inverted to black and the dark areas to white, resulting in an image with a color inversion. The number of black areas (corresponding to manganese sulfides) and the equivalent circle diameter of each black area were measured to determine the number of black areas with an equivalent circle diameter of 1 μm or more per ^mm² .

(b) Measurement of the area ratio of niobium carbide
The area fraction of Nb in the observed field of view was measured using the area fraction measurement function of the EPMA, and the Nb map was superimposed on the microstructure observed at 200x magnification. Since almost all of the Nb existed as NbC, the area fraction of Nb was considered to be equal to the area fraction of NbC.

The number of manganese sulfide particles and the area ratio of niobium carbide for Examples 1 to 7 are shown in Tables 3 and 4, respectively.

Note: (1) The number of manganese sulfide particles with an equivalent circle diameter of 1 μm or more per unit area (1 mm ² ).

As is clear from Table 3-1, the number of manganese sulfide particles per ^mm2 observed in Examples 1 to 7 ranged from 419 to 2525, with an average of 472 to 1537. Furthermore, as is clear from Table 3-2, the maximum circle-equivalent diameter of the manganese sulfide particles observed in Examples 1 to 7 was in the range of 14.0 to 17.7 μm.

As can be seen from Table 4, the area fraction of niobium carbides observed in Examples 1 to 7 ranged from 1.0 to 10.0%, with an average of 1.4 to 9.2%.

Example 8
In this example, the austenitic heat-resistant cast steel of the present invention is used in an exhaust manifold ( exhaust system part) 1 shown in Fig. 1. The dashed line in Fig. 1 indicates the inner surface of the exhaust manifold 1 , and is not visible from the outside.

A 4000 kg raw material mixture containing steel scrap, return scrap, and a predetermined amount of ferroalloy containing predetermined amounts of constituent elements was melted in air using a high-frequency induction melting furnace (basic lining) with a melting capacity of 4800 kg/charge. The molten metal was tapped at 1700-1750°C and poured at 1590-1640°C into a mold having a cavity in the shape of the exhaust manifold shown in Figure 1, thereby obtaining the exhaust system part shown in Figure 1. The other manufacturing conditions were the same as in Example 1. The composition of this exhaust system part is shown in Table 5. The composition was analyzed using the same method as in Example 1.

Notes: (1) Fe and unavoidable impurities.

In the exhaust system part manufactured in Example 8, cold cracks that tend to occur particularly in thin-walled portions and shrinkage cracks that tend to occur during casting were not observed.

The heat-resistant, austenitic cast steel of the present invention is particularly suitable for exhaust manifolds for internal combustion engines, but can also be used for other exhaust system parts , such as turbine housings, turbine housing-integrated exhaust manifolds in which the turbine housing and exhaust manifold are integrally cast, catalyst cases, catalyst case-integrated exhaust manifolds in which the catalyst case and exhaust manifold are integrally cast, and exhaust outlets. Furthermore, the present invention is not limited to these, and can also be used for exhaust system parts that are joined to sheet metal or pipe-shaped members made of other materials. Of course, the use of the heat-resistant, austenitic cast steel of the present invention is not limited to these exhaust system parts .

1 exhaust manifold
20 Casting from which test pieces for evaluating micro-shrinkage cavities are taken
21 Micro-shrinkage cavity evaluation specimen
22 Weir
23 Riser
24 Yudou Department
30 Stepped Casting
31 10 mm thick section
14 Austenite phase
15 Niobium carbide
16 Eutectic phase
17 Manganese sulfide particles
71 Fine manganese sulfide particles

Claims (6)

By mass
C: 0.30-0.50%,
Si: 0.50-2.0%,
Mn: 0.50-2.0%,
S: 0.10-0.40%,
Cr: 16.0-21.0%
Ni: 6.0-12.0%,
Nb: 0.5 to 2.0%, and
Cu: Contains 0.05 to 0.80%
The remainder of the austenitic heat-resistant cast steel is Fe and unavoidable impurities.
2. The austenitic heat-resistant cast steel according to claim 1, wherein the S content is 0.15 to 0.37% by mass and the Nb content is 0.9 to 1.6% by mass. 2. The austenitic heat-resistant cast steel according to claim 1 , wherein the number of manganese sulfides having an equivalent circle diameter of 1 μm or more in an arbitrary cross section is 350 to 2550 per 1 ^mm2 . 2. The heat-resistant, austenitic cast steel according to claim 1 , wherein the area ratio of niobium carbide in any cross section is 0.5 to 11.0%. An exhaust system part made of the heat-resistant austenitic cast steel according to any one of claims 1 to 4. 6. The exhaust system part according to claim 5, wherein the exhaust system part is an exhaust manifold.