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

Abstract

The present invention provides an austenitic heat-resistant cast steel which is characterized by containing, on a mass basis, 0.30-0.50% of C, 0.50-2.0% of Si, 0.50-2.0% of Mn, 0.10-0.40% of S, 16.0-21.0% of Cr, 6.0-12.0% of Ni, 0.5-2.0% of Nb and 0.80% or less of Cu, with the balance being made up of Fe and unavoidable impurities.

Description

[Technical Field]

[0001] The present invention relates to a heat-resistant, austenitic cast steel that has excellent shrinkage crack resistance, cold crack resistance, and machinability, as well as good thermal fatigue properties, and in which the content of expensive alloy elements is reduced, and to an exhaust system part made of the same. [Background technology]

[0002] Exhaust system components used in internal combustion engines, such as automobile engines, especially 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 during vehicle operation, exhaust system components 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.

[0003] For example, International Publication No. 2009 / 104792 discloses a ferromagnetic alloy comprising, 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 unavoidable impurities, and containing 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) (Here, the element symbols in each formula indicate the content (mass%).)

[0004] Furthermore, International Publication No. 2016 / 052750 discloses a ferroelectric ceramic material 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 having 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 (% by mass) of the element), is 0.6 to 1.7.

[0005] 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 many 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 the product acceptance rate.

[0006] JP 2011-219801 A proposes an iron (Fe)-based austenitic heat-resistant cast steel, which, when the total is taken as 100 mass % (hereinafter simply referred to as "%"), contains 0.4 to 0.8% carbon (C), 3.0% or less silicon (Si), 0.5 to 2.0% manganese (Mn), 0.05% or less phosphorus (P), 0.03 to 0.2% sulfur (S), 18 to 23% chromium (Cr), 3.0 to 8.0% nickel (Ni), and 0.05 to 0.4% nitrogen (N), and the ratio of chromium (Cr) to carbon (C) is 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 tiny 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.

[0007] Furthermore, SCH12, a type of austenitic heat-resistant steel specified in JIS G 5122, is not only prone to shrinkage cracking but also has poor machinability compared to the materials disclosed in the above-mentioned documents, making it unsuitable for exhaust manifolds, which require machining during production. Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, an object of the present invention is to provide a heat-resistant, austenitic cast steel which has a low content of expensive alloying elements, is resistant to shrinkage cracking during casting and cold cracking after casting, has good machinability after casting, and has predetermined thermal fatigue properties at around 800°C, and to provide an exhaust system part, in particular an exhaust manifold, made of the same. [Means for solving the problem]

[0009] 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 to 2.0% Mn: 0.50 to 2.0% S: 0.10~0.40%, Cr: 16.0~21.0% Ni: 6.0 to 12.0% Nb: 0.5 to 2.0%, and Cu: 0.05~0.80% Contains The balance is Fe and unavoidable impurities.

[0010] In the heat-resistant, austenitic 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% by mass, which not only further suppresses shrinkage cracking during casting and cold cracking after casting, but also improves the balance between machinability and thermal fatigue properties.

[0011] In the austenitic heat-resistant cast steel of the present invention, the number of manganese sulfides having a circle equivalent diameter of 1 μm or more in an arbitrary cross section is 1 mm 2 It is preferable that the number of particles per particle is 350 to 2550. This ensures oxidation resistance while improving machinability.

[0012] 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%, which can suppress the occurrence of shrinkage cracks and provide a heat-resistant cast steel with good high-temperature strength and thermal fatigue properties.

[0013] The exhaust system part of the present invention is characterized in that it is made of the above-mentioned austenitic heat-resistant cast steel. Let's say .

[0014] Preferably, the exhaust system component is an exhaust manifold. [Effects of the Invention]

[0015] The heat-resistant, austenitic cast steel of the present invention is resistant to shrinkage cracking during casting and cold cracking after casting, has good workability after casting, and has good thermal fatigue properties at around 800°C, and achieves low costs because the content of expensive alloying elements is suppressed. Such heat-resistant, austenitic cast steel is suitable for exhaust system parts of internal combustion engines, in particular exhaust manifolds. [Brief explanation of the drawings]

[0016] [Figure 1] FIG. 1 is a front view schematically showing an exhaust manifold, which is an example of an exhaust system part. [Figure 2A] FIG. 1 is a plan view schematically showing a casting from which a test piece for evaluating micro-shrinkage cavities is taken. [Figure 2B] FIG. 1 is a side view schematically showing a casting from which a test piece for evaluating micro-shrinkage cavities is taken. [Figure 3A] FIG. 1 is a plan view schematically showing a stepped casting from which a test piece for microstructure observation is taken. [Figure 3B] FIG. 1 is a side view schematically showing a stepped casting from which a test piece for microstructure observation is taken. [Figure 4A] 1 is a photograph showing the microstructure of the heat-resistant, austenitic cast steel of Example 4. [Figure 4B] This is an enlarged photograph of area A in FIG. 4A. DETAILED DESCRIPTION OF THE INVENTION

[0017] [1] Austenitic heat-resistant cast steel (A) Composition The content of each element described below is expressed on a mass basis unless otherwise specified.

[0018] (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). Carbon 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 exert these effects, carbon must be present in an amount of 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%.

[0019] (2) Si (silicon): 0.50 to 2.0% Since Si is an element that improves oxidation resistance and consequently improves 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 of the Si content 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%.

[0020] (3) Mn (manganese): 0.50 to 2.0% Mn not only stabilizes the austenite structure, but also forms sulfides (MnS) with sulfur (S) and disperses as free-cutting particles in the austenite matrix, thereby contributing to improved machinability. To achieve this effect, the Mn content must be 0.50% or more, but if it exceeds 2.0%, oxidation resistance deteriorates. Therefore, the Mn content range is set to 0.50 to 2.0%. The lower limit of the Mn content is preferably 0.80%, more preferably 0.90%. The upper limit of the Mn content is preferably 1.5%, more preferably 1.2%, and most preferably 1.1%.

[0021] (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 thermal fatigue properties tend to deteriorate due to excessive formation of manganese sulfides. 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%.

[0022] (5) Cr (chromium): 16.0 to 21.0% Cr dissolves in the matrix to stabilize the austenite structure, and combines with C to form thermally stable hard carbides (Cr carbides), which disperse throughout the austenite matrix, increasing high-temperature strength. Cr also combines with oxygen in the air to form strong oxides on the surface of the casting (passivation). leather This results in a film (a thin film) and improves oxidation resistance at high temperatures. To achieve this effect, the Cr content must be 16.0% or more. 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 deteriorating the thermal fatigue properties of the heat-resistant cast steel. Therefore, the Cr content range is set to 16.0 to 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%.

[0023] (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, from the perspective of reducing 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 around 800°C. Therefore, the Ni content is set to a range of 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%.

[0024] (7) Nb (niobium): 0.5 to 2.0% Nb preferentially bonds with C over Cr to form fine Nb carbides (niobium carbides). This suppresses the formation of excess Cr carbides, thereby indirectly improving the high-temperature strength and thermal fatigue properties of heat-resistant cast steel. Furthermore, Nb carbides are eutectic carbides with austenite and can exist as a molten liquid until the casting solidifies completely after casting, making it less likely to form minute shrinkage cavities. Therefore, Nb suppresses the occurrence of shrinkage cracks caused by shrinkage cavities, which are particularly likely to occur when manufacturing thin-walled, complex-shaped castings such as exhaust manifolds. To achieve this effect, the Nb content is set to 0.5% or more. On the other hand, excessive Nb content results in excess Nb 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 Nb content is set to 2.0%. Therefore, the Nb content range is 0.5 to 2.0%. The lower limit of the Nb 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%.

[0025] (8) Cu (copper): 0.80% or less A trace amount of 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 (however, 0% is not included), but may be 0.05% or 0.10%.

[0026] (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 unavoidable impurities include P (phosphorus), Al (aluminum), W (tungsten), and Mo (molybdenum). It is preferable to minimize the content of these unavoidable impurities. For example, P significantly reduces the toughness of the 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 not only form carbides with C to reduce the ductility of the heat-resistant cast steel, but also dissolve in the austenite matrix, reducing the amount of Cr dissolved in the matrix, thereby reducing the oxidation resistance of the matrix, and further degrading thermal fatigue properties by promoting the crystallization of Cr carbides. Therefore, it is preferable that W and Mo be 0.60% or less each, and more preferably 0.60% or less in total.

[0027] (B) Microstructure Fig. 4A is a photograph showing the microstructure of a cut surface of austenitic heat-resistant cast steel according to Example 4, which is one example of the present invention, and Fig. 4B is an enlarged photograph of region A in Fig. 4A. As shown in Fig. 4A, the structure of the austenitic heat-resistant cast steel according to 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.

[0028] In Fig. 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, and forms minute shrinkage cavities. of This makes it less likely to occur.

[0029] In the austenitic heat-resistant cast steel of the present invention, the area ratio 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 cannot be sufficiently obtained. If it exceeds 11.0%, the high-temperature strength and thermal fatigue properties decrease, and machinability also decreases. The lower limit of the area ratio 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 ratio of niobium carbide 15 is more preferably 10.0%.

[0030] 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. ofRelatively large manganese sulfide particles 17 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.

[0031] To improve machinability, it is better to have a large number of manganese sulfide particles 17 with a circle equivalent diameter of 1 μm or more. However, if there are too many, the oxidation resistance of the austenitic heat-resistant cast steel will decrease. Therefore, it is preferable to have 1 mm of manganese sulfide particles 17 with a circle equivalent diameter of 1 μm or more in any cross section. 2 The number of the manganese sulfide particles 17 per 1 mm 2 is preferably 350 to 2550. 2 The lower limit of the number of winnings is more preferably 600, and the upper limit is more preferably 1600, and most preferably 1450.

[0032] [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. Exhaust A stomanifold is preferred.

[0033] Figure 1 shows an example of an exhaust manifold. The exhaust manifold 1 has multiple ports 2, flanges 3 connected to each port 2, a collection 4 for the ports 2, and a flange 5 connected to the collection 4.

[0034] The present invention will be described in more detail by the following examples, but the present invention is not limited thereto.

[0035] Examples 1 to 7 and Comparative Examples 1 and 2 80 kg of raw materials, which were a mixture 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, and then tapped at 1650-1700°C and poured into a mold at a temperature of 1590-1610°C to obtain test materials for composition analysis in Examples 1-7 and Comparative Examples 1 and 2. A carbon-sulfur simultaneous analyzer (CS-444, manufactured by LECO) was used to analyze C (carbon) and S (sulfur). Use, The analysis of other elements was performed using a solid-state emission spectrometer (Shimadzu Corporation, PDA-8000). The results are shown in Table 1.

[0036] Furthermore, to prepare a 1-inch Y-block (specified in JIS G 5502) for a thermal fatigue life test, a test piece 21 for evaluating micro-shrinkage cavities shown in FIGS. 2A and 2B, a stepped casting 30 for microstructure observation shown in FIGS. 3A and 3B, and a cylindrical test piece (not shown) for a tool life test having 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.

[0037] All molds used were CO2-hardened alkaline phenol molds, and were 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.

[0038] [Table 1] Notes: (1) Fe and unavoidable impurities.

[0039] (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 material 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.

[0040] The degree of mechanical constraint is expressed by the constraint ratio η, which is 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 free thermal expansion elongation at all, and η = 0.5 refers to mechanical constraint conditions that allow only 1 mm of elongation, whereas the free thermal expansion elongation without mechanical constraint is, 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.

[0041] When heating and cooling cycles are repeated using TMF under the above mechanical restraint conditions, the tensile load applied to the test piece decreases due to thermal fatigue. Therefore, in the heating and cooling cycle number vs. tensile load diagram that records the change in load associated with heating and cooling cycles using TMF, the maximum tensile load in the second cycle (corresponding to the tensile load at the lower limit temperature during the cooling process) is set as the reference (100%), and the number of heating and cooling cycles until the maximum tensile load measured in each subsequent cycle decreases to 75% of this reference (maximum tensile load in the second cycle) is defined as the thermal fatigue life.

[0042] (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 machinability of the test piece was evaluated based on the tool life. The NC lathe used was a TAKISAWA Corporation TAC-510x1000 model. 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 of 0.12 mm / tooth, and a depth of cut of 0.3 mm.

[0043] (3) Evaluation of shrinkage crack resistance (measurement of the volume fraction of micro-shrinkage cavities) Focusing on the fact that minute shrinkage cavities cause shrinkage cracks, Fig. 2 A and Figure 2B The cross section of the test piece 21 for evaluating micro-shrinkage cavities shown in FIG. 1 was photographed with an X-ray, and the obtained image was analyzed to determine the volume fraction of micro-shrinkage cavities, from which the shrinkage crack resistance was evaluated.

[0044] 2A and 2B show a casting 20 for extracting a micro-shrinkage cavity evaluation specimen 21. 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.

[0045] Molten metal having the same composition as the 1-inch Y-block was poured into a sand mold for producing the casting 20, and after cooling to room temperature, the mold was disassembled. A test piece 21 for evaluating micro-shrinkage cavities was cut out from the resulting casting 20 and subjected to shot blasting.

[0046] The cross section of the micro-shrinkage cavity evaluation specimen 21 was imaged with a CT device (XTH 450 manufactured by Nikon Corporation) at a tube voltage of 450 kV, and a 3D viewer (myVGL manufactured by Volume Graphics) was used to obtain CT tomographic images of an area approximately 1.4 mm wide at the center of the thickness direction of the micro-shrinkage cavity evaluation specimen 21 at a pitch of 0.1 mm in the thickness direction. Each CT tomographic image was binarized into micro-shrinkage cavities (dark areas) and other areas (light areas) using image processing software (Quick Grain Padplus manufactured by Innotek Corporation), and the area of ​​the micro-shrinkage cavities (dark areas) per pitch (0.1 mm) was calculated. This was integrated over the measurement range to determine the volume of the micro-shrinkage cavities (unit: mm 3 The volume of the micro-shrinkage cavity was calculated by multiplying the measured volume of the micro-shrinkage cavity evaluation specimen 21 (75000 mm 3 ) to determine the volume fraction of micro-shrinkage cavities.

[0047] Table 2 shows the thermal fatigue life, tool life, and volume ratio of micro-shrinkage cavities for Examples 1 to 7 and Comparative Examples 1 and 2.

[0048] [Table 2]

[0049] The thermal fatigue lives (TMF) of Examples 1 to 7 were 120 to 255 cycles, which were found to satisfy 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 lives to 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.

[0050] The tool lives of Examples 1 to 7 were 47 to 138 minutes, which was 1.8 to 5.3 times that of Comparative Example 1, and were almost equal to or longer than the 61 minutes of Comparative Example 2 (having a composition similar to that disclosed in WO 2016 / 052750) which had a relatively long life due to the high Cr and Ni contents. In particular, Example 6 had a tool life of 138 minutes, which was 2.3 times that of Comparative Example 2.

[0051] The volume ratio of micro-shrinkage cavities in Examples 1 to 7 was 8.27 × 10 -6 ~740×10 -6 All of these values ​​were smaller than those 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.

[0052] From the above comparison, it was found that (a) Examples 1 to 7 were superior to Comparative Example 1 in machinability (tool life) and shrinkage crack resistance (expressed by the volume fraction of microshrinkage cavities), and (b) Examples 1 to 7 exhibited performance equal to or better than Comparative Example 2, which is expensive because it contains about 1.5 to 2.0 times as much Ni and about 1.3 to 1.5 times as much Cr as Examples 1 to 7, despite having lower contents of expensive Ni and Cr.

[0053] Therefore, the heat-resistant, austenitic cast steel of the present invention has a thermal fatigue life that satisfies the standard required for a material used in an exhaust manifold, has a relatively long tool life, and therefore has good machinability. In addition, since the volume fraction of micro-shrinkage cavities is small, micro-shrinkage cavities that cause shrinkage cracks are unlikely to occur. Therefore, it can be said that the heat-resistant, austenitic cast steel of the present invention is not only a well-balanced material to be used for exhaust system parts, particularly exhaust manifolds, but also has a low content of expensive alloying elements, and is therefore economically excellent.

[0054] (4) Observation of microstructure A sample cut from a 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 specimen for microstructure observation. The specimen for microstructure observation 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 are backscattered electron images (COMPO) of Example 4.

[0055] (a) Identification and number determination of manganese sulfide particles Since S is distributed in high concentrations throughout the manganese sulfide particles, manganese sulfides (MnS and (Mn, Cr)S) were identified by the S map of the microstructure observed at 200x magnification using an EMPA. A mapping image was obtained in which only the manganese sulfides were colored and the other areas were black. The colored areas corresponding to the manganese sulfides had a gradation range of 1500 to 100 depending on the brightness. The mapping image was binarized using image processing software (Quick Grain Padplus, manufactured by Innotek Co., Ltd.) into areas with a brightness of 105 or higher (light areas) and areas with a brightness of 104 or lower (dark areas), and a color-inverted image was obtained in which the light areas were black and the dark areas were white. The number of black areas (corresponding to manganese sulfides) and the circle-equivalent diameter of each black area were measured, and the area was divided into 1 mm. 2 The number of black areas with a circular equivalent diameter of 1 μm or more per area was counted.

[0056] (b) Measurement of the area ratio of niobium carbide 200x with EPMA in The area fraction of Nb in the observed field of view was measured using the EPMA's area fraction measurement function, and the area fraction of Nb was considered to be equal to the area fraction of NbC, since almost all of the Nb existed as NbC.

[0057] The number of manganese sulfide particles and the area ratio of niobium carbide in Examples 1 to 7 are shown in Tables 3 and 4, respectively.

[0058] [Table 3-1] Note: (1) The unit area (1 mm) of manganese sulfide particles with a circle equivalent diameter of 1 μm or more 2 ) Number of wins.

[0059] [Table 3-2]

[0060] As is clear from Table 3-1, the 1 mm observed in Examples 1 to 72 The number of manganese sulfide particles per particle ranged from 419 to 2525, with an average of 472 to 1537. As is clear from Table 3-2, the maximum equivalent circle diameter of the manganese sulfide particles observed in Examples 1 to 7 was in the range of 14.0 to 17.7 μm.

[0061] [Table 4]

[0062] As is clear from Table 4, the area ratios of niobium carbides observed in Examples 1 to 7 were in the range of 1.0 to 10.0%, with an average of 1.4 to 9.2%.

[0063] Example 8 In this example, the austenitic heat-resistant cast steel of the present invention is used for an exhaust manifold ( exhaust system This is an example of use for part 1. The dashed lines in Figure 1 indicate Exhaust manifold 1 Inside The face This indicates the presence of a mark and is not visible from the outside.

[0064] A raw material weighing 4000 kg, which is a mixture of steel scrap, return scrap, and ferroalloy containing predetermined amounts of constituent elements, is melted in the atmosphere using a high-frequency induction melting furnace (basic lining) with a melting capacity of 4800 kg / charge, and then tapped at 1700-1750°C. The molten metal is poured into a mold having a cavity in the shape of an exhaust manifold shown in Figure 1 at a temperature of 1590-1640°C. exhaust system Other manufacturing conditions were the same as in Example 1. exhaust system The composition of the parts is shown in Table 5. The composition analysis method was the same as in Example 1.

[0065] [Table 5] Notes: (1) Fe and unavoidable impurities.

[0066] Produced in Example 8 exhaust system The parts were free of cold cracks, which tend to occur particularly in thin-walled areas, and shrinkage cracks, which tend to occur during casting. [Industrial Applicability]

[0067] 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 in other exhaust systems. parts For example, it can be used for a turbine housing, a turbine housing-integrated exhaust manifold in which the turbine housing and the exhaust manifold are cast as one unit, a catalyst case, a catalyst case-integrated exhaust manifold in which the catalyst case and the exhaust manifold are cast as one unit, an exhaust outlet, etc. Furthermore, it is not limited to these, and can also be used for an exhaust system in which it is joined to a sheet metal or pipe-shaped member made of other material. parts Of course, the use of the austenitic heat-resistant cast steel of the present invention can be applied to these exhaust systems. parts It is not limited to only. [Explanation of symbols]

[0068] 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

1. 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. 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.

3. In the austenitic heat-resistant cast steel according to claim 1, the number of manganese sulfides having an equivalent circle diameter of 1 μm or more in an arbitrary cross section is 1 mm 2 Austenitic heat-resistant cast steel characterized by having 350 to 2550 particles per 1000 particles.

4. 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%.

5. An exhaust system part made of the heat-resistant austenitic cast steel according to any one of claims 1 to 4.

6. 6. The exhaust system part according to claim 5, wherein the exhaust system part is an exhaust manifold.

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