Heat-resistant cast steel and turbocharger part

Abstract

Heat-resistant cast steel with: 0.55 mass% or more and 1.0 mass% or less C; more than 1.5 mass% and 3.5 mass% or less of Si; more than 0 mass% and 2 mass% or less Mn; 6 mass% or more and 11 mass% or less Ni; 22 mass% or more and 27 mass% or less Cr; more than 0 mass% and 0.6 mass% or less Mo; optionally also more than 0 mass% and 0.2 mass% or less of S; and the rest being Fe and unavoidable impurities.

Description

Technical area

[0001] The present invention relates to a heat-resistant steel casting and a turbocharger part. State of the art

[0002] Turbocharger parts such as turbine housings must be heat-resistant, which is why they are formed using heat-resistant steel castings. A heat-resistant steel casting containing N (nitrogen) is used as such a heat-resistant steel casting in US 2013 / 0 022 488 A1 and US 2016 / 0 130 978 A1.

[0003] The KR 10 2004 0 077 105 A discloses a special steel which contains 0.90 to 1.20 mass% C, 0.50 to 1.50 mass% Si, 0.40 to 1.00 mass% Mn, 6.0 to 7.5 mass% Ni, 23 to 28 mass% Cr, 0.50 to 2.00 mass% Mo and the remainder Fe and unavoidable impurities. Brief description of the inventionTechnical problem

[0004] In heat-resistant steel castings containing N (nitrogen), N is added to stabilize the austenite phase. Heat-resistant steel castings containing N are cast using a die casting process to add N gas. This die casting process requires special casting equipment to perform melting and pouring under pressure. Accordingly, the cost of the equipment for casting heat-resistant steel castings is so high that the manufacturing cost of the heat-resistant steel castings may increase.

[0005] An object of the present invention is to provide a heat-resistant steel casting and a turbocharger part which can further reduce the manufacturing cost of a heat-resistant steel casting. Solution to the problem

[0006] A heat-resistant cast steel contains 0.55 mass% or more and 1.0 mass% or less of C, more than 1.5 mass% and 3.5 mass% or less of Si, more than 0 mass% and 2 mass% or less of Mn, 6 mass% or more and 11 mass% or less of Ni, 22 mass% or more and 27 mass% or less of Cr, more than 0 mass% and 0.6 mass% or less of Mo, optionally also more than 0 mass% and 0.2 mass% or less of S and the balance being Fe and unavoidable impurities.

[0007] In the heat-resistant cast steel according to the present invention, a content of C may be 0.55 mass% or more and 0.8 mass% or less.

[0008] In the heat-resistant cast steel according to the present invention, a content of C may be more than 0.8 mass% and 1.0 mass% or less.

[0009] In the heat-resistant cast steel according to the present invention, a content of Si may be more than 1.5 mass% and 2.5 mass% or less.

[0010] In the heat-resistant cast steel according to the present invention, a content of S may be 0.1 mass% or more and 0.2 mass% or less.

[0011] A turbocharger part according to the present invention is made of one of the heat-resistant steel castings described above.

[0012] Since no N (nitrogen) is added to the heat-resistant steel casting according to the above configuration, there is no need to use the die casting process. Accordingly, the manufacturing cost of the heat-resistant steel casting can be reduced. Short description of the drawings Fig. 1 is a schematic view illustrating a configuration of a turbocharger in an embodiment of the present invention. Fig. 2 is a graph showing relationships between ferrite contents and amounts of C contained in heat-resistant cast steels in the embodiment of the present invention. Fig. 3 is a graph showing relationships between the ferrite contents and amounts of Si contained in the heat-resistant cast steels in the embodiment of the present invention. Fig. 4 is a graph showing relationships between the ferrite contents and amounts of Ni contained in the heat-resistant cast steels in the embodiment of the present invention. Fig. 5 is a graph showing relationships between the ferrite contents and amounts of Cr contained in the heat-resistant cast steels in the embodiment of the present invention. Fig. 6 is a view for explaining a machinability evaluation test method in the embodiment of the present invention. Fig. 7 is a graph showing results of the machinability evaluation test for each heat-resistant cast steel in the embodiment of the present invention. Description of implementation examples

[0013] Below, an embodiment of the present invention will be described in detail using the drawings. A heat-resistant cast steel according to the embodiment of the present invention contains 0.55 mass% or more and 1.0 mass% or less of C, more than 1.5 mass% and 3.5 mass% or less of Si, more than 0 mass% and 2 mass% or less of Mn, 6 mass% or more and 11 mass% or less of Ni, 22 mass% or more and 27 mass% or less of Cr, more than 0 mass% and 0.6 mass% or less of Mo, optionally further more than 0 mass% and 0.2 mass% or less of S, and the remainder being Fe and unavoidable impurities. Next, a description will be given of the reasons why the composition range of each of the alloying components contained in the heat-resistant cast steel is limited.

[0014] C (carbon) is an austenite-forming element and has the function of stabilizing the austenite phase. When the main phase of the heat-resistant steel cast structure is the austenite phase (γ phase), the mechanical properties, such as high-temperature strength, can be better than when the main phase of the structure is the ferrite phase (α phase). It should be noted that the main phase of the structure refers to a phase with the largest volume fraction in the structure. Furthermore, C combines with Cr to form a complex carbide, such as Cr carbide, in the structure, and it has the function of improving mechanical properties, such as high-temperature strength.

[0015] The C content is 0.55 mass% or more and 1.0 mass% or less. When the C content is less than 0.55 mass%, the proportion of ferrite phase increases, and mechanical properties such as high-temperature strength deteriorate. Furthermore, when the C content is less than 0.55 mass%, a σ phase precipitates, making the heat-resistant cast steel brittle. When the C content is greater than 1.0 mass%, the hardness increases due to the precipitation of a large amount of Cr carbide, etc., and processability deteriorates, such as machinability.

[0016] The C content can be 0.55 mass% or more and 0.8 mass% or less. Making the heat-resistant steel casting have a C content of 0.55 mass% or more and 0.8 mass% or less reduces the hardness and can thus improve processability, such as machinability. The C content can be more than 0.8 mass% and 1.0 mass% or less. Making the heat-resistant steel casting have a C content of more than 0.8 mass% and 1.0 mass% or less can further improve mechanical properties, such as high-temperature strength.

[0017] Si (silicon) is a solid solution strengthening element and also has the function of improving oxidation resistance. The Si content is greater than 1.5 mass% and less than 3.5 mass%. When the Si content is less than 1.5 mass%, the oxidation resistance decreases. When the Si content is greater than 3.5 mass%, the amount of ferrite phase increases, and mechanical properties, such as high-temperature strength, deteriorate. Furthermore, when the Si content exceeds 3.5 mass%, the σ phase precipitates, making the heat-resistant cast steel brittle.

[0018] The Si content can be greater than 1.5 mass% and less than 2.5 mass%. Making heat-resistant cast steel have a Si content of greater than 1.5 mass% and less than 2.5 mass% can improve oxidation resistance and mechanical properties such as high-temperature strength.

[0019] Mn (manganese) is an austenite-forming element and has the function of stabilizing the austenite phase. The Mn content is greater than 0 mass% and less than 2 mass%. If the Mn content exceeds 2 mass%, MnS and the like will form, which will impair oxidation resistance.

[0020] Ni (nickel) is an austenite-forming element and has the function of stabilizing the austenite phase. Furthermore, Ni has the function of improving oxidation resistance. The Ni content is 6 mass% or more and 11 mass% or less. When the Ni content is less than 6 mass%, the amount of ferrite phase increases and mechanical properties, such as high-temperature strength, deteriorate. When the Ni content is less than 6 mass%, oxidation resistance also deteriorates. Furthermore, when the Ni content is less than 6 mass%, the σ phase precipitates, and the heat-resistant cast steel becomes brittle.

[0021] However, if the Ni content exceeds 11 mass%, the manufacturing cost of heat-resistant steel castings may increase because Ni is an expensive element. Specifically, the austenite phase in heat-resistant steel castings is stabilized by making the heat-resistant steel castings have a C content of 0.55 mass% or more and 1.0 mass% or less, making the C content larger than that in a conventional heat-resistant steel casting. Accordingly, there is no need to stabilize the austenite phase by making the heat-resistant steel castings have a Ni content of more than 11 mass%. In addition, by making the heat-resistant cast steel have the Si content of more than 1.5 mass% and 3.5 mass% or less and, as described later, have the Cr content of 22 mass% or more and 27 mass% or less, the Si amount and the Cr amount are increased and the oxidation resistance is improved.Since the austenite phase is stabilized by increasing the content of inexpensive C and the oxidation resistance is improved by increasing the content amounts of inexpensive Si and Cr as described above, it is possible to reduce the content amount of expensive Ni and further reduce the manufacturing cost of the heat-resistant cast steel.

[0022] Cr (chromium) has the function of improving oxidation resistance and corrosion resistance. The Cr content is 22 mass% or more and 27 mass% or less. When the Cr content is less than 22 mass%, oxidation resistance and corrosion resistance deteriorate. When the Cr content exceeds 27 mass%, the amount of ferrite phase increases and mechanical properties, such as high-temperature strength, deteriorate. Furthermore, when the Cr content exceeds 27 mass%, the σ phase precipitates, making the heat-resistant cast steel brittle.

[0023] Mo (molybdenum) is a solid solution strengthening element and has the function of improving mechanical properties such as high-temperature strength. The Mo content is greater than 0 mass% and 0.6 mass% or less. When the Mo content is greater than 0.6 mass%, the amount of ferrite phase increases, and mechanical properties such as high-temperature strength deteriorate. Furthermore, when the Mo content is greater than 0.6 mass%, the σ phase precipitates, making the heat-resistant steel cast brittle. Furthermore, the mechanical properties such as high-temperature strength in heat-resistant steel casts are improved by making the heat-resistant steel cast have a C content of 0.55 mass% or more and 1.0 mass% or less, making the C amount larger than that in ordinary heat-resistant steel casts.Accordingly, the content of expensive Mo does not need to be more than 0.6 mass%, and the manufacturing cost of heat-resistant cast steel can be reduced.

[0024] It should be noted that the remainder of heat-resistant cast steel consists of Fe and unavoidable impurities. Unavoidable impurities are impurities that may be mixed into the heat-resistant cast steel but are not intentionally added.

[0025] The heat-resistant cast steel according to the embodiment of the present disclosure may further optionally contain S (sulfur). S (sulfur) has the function of improving the machinability of the heat-resistant cast steel. The S content may be more than 0 mass% and 0.2 mass% or less. When the S content is more than 0.2 mass%, a brittle phase tends to form in a high-temperature environment, and thus mechanical strength, such as high-temperature strength, may deteriorate. The S content may be 0.1 mass% or more and 0.2 mass% or less. Making the heat-resistant cast steel have an S content of 0.1 mass% or more and 0.2 mass% or less can further improve machinability.By making the heat-resistant cast steel have an S content of 0.16 mass% or more and 0.2 mass% or less, the machinability can be improved even further.

[0026] Next, a method for producing heat-resistant cast steel will be described. A raw material for heat-resistant cast steel can be melted in a high-frequency induction furnace or the like and cast to obtain a slab. For example, the raw material for heat-resistant cast steel can be melted in a high-frequency induction furnace and cast by pouring it into a sand mold. As the raw material for the heat-resistant steel casting, an alloy material containing 0.55 mass% or more and 1.0 mass% or less of C, more than 1.5 mass% and 3.5 mass% or less of Si, more than 0 mass% and 2 mass% or less of Mn, 6 mass% or more and 11 mass% or less of Ni, 22 mass% or more and 27 mass% or less of Cr, more than 0 mass% and 0.6 mass% or less of Mo, optionally also more than 0 mass% and 0.2 mass% or less of S and the balance being Fe and unavoidable impurities can be used.

[0027] Since the heat-resistant cast steel according to the embodiment of the present invention can be cast by a general sand casting method and the like for steel castings as described above, casting by the die casting method is unnecessary. Accordingly, a special casting equipment used in the die casting method is unnecessary, and thus the manufacturing cost of the heat-resistant cast steel can be reduced. Furthermore, since the heat-resistant cast steel has excellent oxidation resistance, the heat-resistant cast steel can be cast in an oxidizing atmosphere such as an air atmosphere. In addition, the cast heat-resistant cast steel can be subjected to a homogenization treatment such as annealing and a thermal treatment such as age-hardening.

[0028] Next, the microstructure of heat-resistant cast steel will be described. In the microstructure of heat-resistant cast steel, the main phase of the structure is the austenite phase (γ-phase). The volume fraction of the austenite phase in the microstructure may be 90 vol% or more or 95 vol% or more. The microstructure of heat-resistant cast steel may be such that the ferrite content of the ferrite phase (α-phase) in the microstructure is 1.1% or less when measured by a magnetic induction method (Feritscope) at room temperature (including the case where the ferrite content is zero). The magnetic induction method is a method in which the ferrite content is measured using a magnetic device and utilizes the fact that the ferrite phase is magnetic and the austenite phase, carbides, and the like are non-magnetic.As described above, the heat-resistant cast steel may be an austenitic heat-resistant cast steel in which the main phase of the structure is the austenite phase and the ferrite content is 1.1% or less (including the case where the ferrite content is zero). The structure of the heat-resistant cast steel contains no ferrite phase, or if it contains any, it contains only a small amount, namely 1.1% or less. Accordingly, mechanical properties such as high-temperature strength can be improved. In addition, the ferrite content of the structure of the heat-resistant cast steel may be 0.5% or less (including the case where the ferrite content is zero), or it may be 0.2% or less (including the case where the ferrite content is zero). In addition, the ferrite content of the structure of the heat-resistant cast steel may be zero (containing no ferrite phase).

[0029] A carbide such as Cr carbide may precipitate in the microstructure of heat-resistant cast steel. Ensuring that a carbide such as Cr carbide disperses in the microstructure can improve mechanical properties such as high-temperature strength. The σ phase must not precipitate in the microstructure of heat-resistant cast steel. Preventing the σ phase from being present in the microstructure of heat-resistant cast steel can suppress embrittlement of the heat-resistant cast steel. Furthermore, the microstructure of heat-resistant cast steel contains a large amount of Si and Cr in a solid solution state. When heat-resistant cast steel is exposed to heat in the air atmosphere, protective oxide films composed of silicon oxide such as SiO2 and chromium oxide such as Cr2O3 are formed, which have excellent oxidation resistance, and can improve the oxidation resistance of the heat-resistant cast steel.

[0030] Next, the mechanical properties and the like of the heat-resistant cast steel according to the embodiment of the present invention will be described. For example, at 600°C, the heat-resistant cast steel has the following high-temperature tensile properties: the tensile strength is 378 MPa to 446 MPa, the 0.2% proof strength is 173 MPa to 214 MPa, and the elongation is 9.4% to 14.2%. For example, at 950°C, the heat-resistant cast steel has the following high-temperature tensile properties: the tensile strength is 106 MPa to 131 MPa, the 0.2% proof strength is 55 MPa to 73 MPa, and the elongation is 35.0% to 52.3%. For example, the heat-resistant cast steel has such properties that the Vickers hardness at room temperature is Hv 199 to Hv 234.For example, heat-resistant cast steel has such oxidation resistance properties that the amount of weight loss after an oxidation test (amount of weight loss per unit area) is about 5 mg·cm. -2 up to about 22 mg·cm -2 when a cyclic oxidation test is carried out 200 times in the air atmosphere between temperatures of below 200°C and 980°C. As described above, the heat-resistant cast steel according to the embodiment of the present invention has excellent mechanical properties and excellent oxidation resistance.

[0031] The heat-resistant cast steel according to the embodiment of the present invention can be applied, for example, to turbocharger parts for a vehicle such as an automobile. Specifically, the heat-resistant cast steel can be used as a material of a turbine housing, which is a turbocharger part for a vehicle. Fig. 1 is a schematic view illustrating a configuration of a turbocharger 10. The turbocharger 10 includes a turbine housing 12 and a compressor housing 14. The turbine housing 12 is exposed to exhaust gas at a maximum temperature of approximately 980°C, for example, in an oxidizing atmosphere. When the turbine housing 12 is formed using the heat-resistant cast steel, the turbine housing 12 can have oxidation resistance and mechanical properties such as high-temperature strength. Furthermore, since the heat-resistant cast steel has excellent processability, such as machinability, the heat-resistant cast steel can be easily machined into a predetermined shape of the turbine housing 12. Of course, the heat-resistant cast steel according to the embodiment of the present invention can be applied not only to turbocharger parts such as the turbine housing 12, but also to other mechanical devices and the like.

[0032] As described above, the heat-resistant cast steel according to the embodiment of the present invention contains 0.55 mass% or more and 1.0 mass% or less of C, more than 1.5 mass% and 3.5 mass% or less of Si, more than 0 mass% and 2 mass% or less of Mn, 6 mass% or more and 11 mass% or less of Ni, 22 mass% or more and 27 mass% or less of Cr, more than 0 mass% and 6 mass% or less of Mo, optionally further containing more than 0 mass% and 0.2 mass% or less of S, and the remainder being Fe and unavoidable impurities. Accordingly, there is no need to add N (nitrogen) and stabilize the austenite phase, and the special equipment used for the die casting process is unnecessary. The manufacturing cost of the heat-resistant cast steel can thereby be reduced.

[0033] In the heat-resistant cast steel according to the embodiment of the present invention, it is possible to improve mechanical properties such as high-temperature strength and oxidation resistance. Furthermore, in the heat-resistant cast steel according to the embodiment of the present invention, the content of inexpensive C is increased to stabilize the austenite phase, and the content amounts of inexpensive Si and Cr are increased to improve oxidation resistance. This enables a reduction in the content amount of expensive Ni, and the manufacturing cost of the heat-resistant cast steel can be further reduced.

[0034] When the heat-resistant cast steel according to the embodiment of the present invention contains more than 0 mass% and 0.2 mass% or less of S, the machinability of the heat-resistant cast steel can be further improved while having the above-mentioned effects. Examples-- First Example --

[0035] Heat-resistant steel castings were cast, and their mechanical properties (tensile strength, hardness), ferrite content, and oxidation resistance were evaluated. Table 1 lists the alloy compositions of the heat-resistant steel castings. Table 1

[0036] First, the alloy composition of each heat-resistant steel casting is described. The alloy compositions of the heat-resistant steel castings in Examples 1 to 8 were each configured to contain 0.55 mass% or more and 1.0 mass% or less of C, 1.5 mass% or more and 3.5 mass% or less of Si, more than 0 mass% and 2 mass% or less of Mn, 6 mass% or more and 11 mass% or less of Ni, 22 mass% or more and 27 mass% or less of Cr, more than 0 mass% and 0.6 mass% or less of Mo, and the remainder being Fe and unavoidable impurities.

[0037] In the heat-resistant cast steel of Comparative Example 1, the Si amount was 0.95 mass%, and the effects of decreasing the Si amount were evaluated. Furthermore, the Mn amount in the heat-resistant cast steel of Comparative Example 1 was 3.21 mass%, and the effects of increasing the Mn amount were evaluated. In the heat-resistant cast steel of Comparative Example 2, the Cr amount was 21.92 mass%, and the effects of decreasing the Cr amount were evaluated. In the heat-resistant cast steel of Comparative Example 3, the C amount was 1.13 mass%, and the effects of increasing the C amount were evaluated. In the heat-resistant cast steel of Comparative Example 4, the C amount was 0.3 mass%, and the effects of decreasing the C amount were evaluated. In the heat-resistant cast steel of Comparative Example 5, the Si amount was 4.11 mass%, and the effects of increasing the Si amount were evaluated.In the heat-resistant steel casting of Comparative Example 6, the Ni content was 3.70 mass%, and the effects of reducing the Ni content were evaluated. In the heat-resistant steel casting of Comparative Example 7, the Cr content was 31.30 mass%, and the effects of increasing the Cr content were evaluated. Each heat-resistant steel casting was cast by melting an alloy raw material in a high-frequency induction furnace and pouring it into a sand mold in the air atmosphere. - Evaluation of ferrite content -

[0038] The ferrite content of each heat-resistant steel casting was measured. The ferrite content of each heat-resistant steel casting was measured at room temperature using a magnetic induction method (Feritscope). Table 1 shows the ferrite content of each heat-resistant steel casting.

[0039] Fig. Figure 2 is a graph showing the relationship between the ferrite content and the amount of C contained in the heat-resistant steel castings. In the graph of Fig. 2, the horizontal axis represents the amount of C contained in each heat-resistant steel casting, while the vertical axis represents the ferrite content, and each white circle represents the ferrite content for the corresponding amount of C. It should be noted that Fig. 2 shows the relationships between the ferrite contents and the C amounts in the heat-resistant steel castings of Examples 1 to 8 and Comparative Example 4. The heat-resistant steel casting of Comparative Example 4 (the C amount was 0.3 mass%) had a larger ferrite content than the heat-resistant steel castings of Examples 1 to 8. From this result, it was found that the amount of ferrite phase increases when the C amount is less than 0.55 mass%, while the ferrite content is 1.1% or less (which includes the case where the ferrite content is zero) when the C amount is 0.55 mass% or more, and that it is possible to decrease the amount of ferrite phase and increase the amount of austenite phase.

[0040] Fig. Figure 3 is a graph showing the relationship between the ferrite content and the amount of Si contained in the heat-resistant steel castings. In the graph of Fig. 3, the horizontal axis represents the amount of Si contained in each heat-resistant steel casting, while the vertical axis represents the ferrite content, and each white circle represents the ferrite content for the corresponding amount of Si. It should be noted that Fig. 3 shows the relationships between the ferrite contents and the Si amounts in the heat-resistant steel castings of Examples 1 to 8 and Comparative Example 5. The heat-resistant steel casting of Comparative Example 5 (the Si amount was 4.11 mass%) had a larger ferrite content than the heat-resistant steel castings of Examples 1 to 8. From this result, it was found that the amount of ferrite phase increases when the Si amount is more than 3.5 mass%, while the ferrite content is 1.1% or less (which includes the case where the ferrite content is zero) when the Si amount is 3.5 mass% or less, and that it is possible to decrease the amount of ferrite phase and increase the amount of austenite phase.

[0041] Fig. Figure 4 is a graph showing the relationship between the ferrite content and the amount of Ni contained in the heat-resistant steel castings. In the graph of Fig. 4, the horizontal axis represents the amount of Ni contained in each heat-resistant steel casting, while the vertical axis represents the ferrite content, and each white circle represents the ferrite content for the corresponding amount of Ni. It should be noted that Fig. 4 shows the relationships between the ferrite contents and the Ni amounts in the heat-resistant steel castings of Examples 1 to 8 and Comparative Example 6. The heat-resistant steel casting of Comparative Example 6 (the Ni amount was 3.70 mass%) had a larger ferrite content than the heat-resistant steel castings of Examples 1 to 8. From this result, it was found that the amount of ferrite phase increases when the Ni amount is less than 6 mass%, while the ferrite content is 1.1% or less (which includes the case where the ferrite content is zero) when the Ni amount is 6 mass% or more, and that it is possible to decrease the amount of ferrite phase and increase the amount of austenite phase.

[0042] Fig. Figure 5 is a graph showing the relationship between the ferrite content and the amount of Cr contained in the heat-resistant steel castings. In the graph of Fig. 5, the horizontal axis represents the amount of Cr contained in each heat-resistant steel casting, while the vertical axis represents the ferrite content, and each white circle represents the ferrite content for the corresponding amount of Cr. It should be noted that Fig. 5 shows the relationships between the ferrite contents and the Cr amounts in the heat-resistant steel castings of Examples 1 to 8 and Comparative Example 7. The heat-resistant steel casting of Comparative Example 7 (the Cr amount was 31.30 mass%) had a larger ferrite content than the heat-resistant steel castings of Examples 1 to 8. From this result, it was found that the amount of ferrite phase increases when the Cr amount is more than 27 mass%, while the ferrite content is 1.1% or less (which includes the case where the ferrite content is zero) when the Cr amount is 27 mass% or less, and that it is possible to decrease the amount of ferrite phase and increase the amount of austenite phase. - Oxidation resistance assessment -

[0043] The oxidation resistance of each heat-resistant steel casting was evaluated. Oxidation resistance was evaluated based on the "Method for Elevated Temperature Cyclic Oxidation Testing of Metallic Materials" in JIS Z2282 and the "General Rules for High-Temperature Corrosion Tests of Metallic Materials" in JIS Z2290. Each heat-resistant steel casting was first subjected to 200 cycles of cyclic oxidation testing, in which the heat-resistant steel casting was heated in an atmosphere with an absolute humidity of 10% at a heating temperature of 980°C±5°C for 45 minutes each, followed by cooling at a temperature below 200°C for 30 minutes.After the cyclic oxidation test, an oxidation film was removed (exfoliated), and the weight change before and after the oxidation test was determined to calculate the weight loss after the oxidation test (weight loss per unit area). Table 1 shows the weight loss after the oxidation test for each heat-resistant steel casting.

[0044] The amount of weight loss after the oxidation test for the heat-resistant steel castings of Examples 1 to 8 was about 5 mg·cm -2 up to about 19 mg·cm -2 Meanwhile, the weight loss amount after the oxidation test for the heat-resistant cast steel of Comparative Example 1 (the Si amount was 0.95 mass%, the Mn amount was 3.21 mass%) was about 304 mg·cm -2, the oxidation resistance was lower than that of the heat-resistant cast steels of Examples 1 to 8. From this result, it was found that the oxidation resistance decreases when the Si amount is less than 1.5 mass%, and that the oxidation resistance increases when the Si amount is 1.5 mass% or more. Furthermore, it was found that the oxidation resistance decreases when the Mn amount is more than 2 mass%, and that the oxidation resistance increases when the Mn amount is 2 mass% or less.

[0045] In addition, the weight loss amount after the oxidation test for the heat-resistant cast steel of Comparative Example 2 (the Cr amount was 21.9 mass%) was about 94 mg·cm -2, the oxidation resistance was lower than that of the heat-resistant cast steels of Examples 1 to 8. From this result, it was found that the oxidation resistance decreases when the Cr amount is less than 22 mass%, and that the oxidation resistance increases when the Cr amount is 22 mass% or more. - Hardness Rating -

[0046] The hardness of each heat-resistant cast steel was measured. The hardness was measured by measuring the Vickers hardness at room temperature. The Vickers hardness was determined in accordance with JIS Z2244 "Vickers Hardness Test - Test Method" and JIS Z7725 "Vickers Hardness Test - Verification and Calibration of Testing Machines." A diamond indenter (square pyramid, angle between opposite surfaces: 136°±0.5°) was used as the indenter. The test load was 10 kgf (98 N). The pressing time after stopping was 10 seconds. Table 1 shows the hardness of each heat-resistant cast steel. The hardness of the heat-resistant cast steels of Examples 1 to 8 ranged from Hv 199 to Hv 234. Meanwhile, the hardness of the heat-resistant cast steel of Comparative Example 3 (the amount of C was 1.13 mass%) was Hv 240.From this result, it was found that the hardness is too high and the processability, such as machinability, decreases when the C amount is more than 1.0 mass%, while the processability, such as machinability, increases when the C amount is 1.0 mass% or less.

[0047] Furthermore, the hardness of the heat-resistant steel castings of Examples 1 to 4 and 8 ranged from Hv 199 to Hv 215. The hardness of the heat-resistant steel castings of Examples 5 to 7 ranged from Hv 218 to Hv 234. From this result, it was found that when the C content was 0.55 mass% or more and 0.8 mass% or less, the hardness decreased and the processability, such as machinability, further increased. Meanwhile, when the C content was more than 0.8 mass% and 1.0 mass% or less, the hardness increased and the mechanical properties improved. - Evaluation of tensile properties -

[0048] The high-temperature tensile properties of each heat-resistant steel casting were evaluated based on the "Procedure for Elevated Temperature Tensile Elongation Test" in JIS G0567. The test temperatures were set at 600°C and 950°C, and the tensile strength, 0.2% proof strength, and elongation were measured. Table 1 shows the results of the tensile properties (tensile strength, 0.2% proof strength, elongation) of each heat-resistant steel casting.

[0049] The tensile strength of the heat-resistant steel castings of Examples 1, 2, and 4 to 7 was 378 MPa to 446 MPa at 600°C and 106 MPa to 131 MPa at 950°C. The 0.2% proof strength was 173 MPa to 214 MPa at 600°C and 55 MPa to 73 MPa at 950°C. The elongation was 9.4% to 14.2% at 600°C and 37.6% to 52.3% at 950°C. As described above, each heat-resistant steel casting was found to have excellent high-temperature tensile properties. -- Second example --

[0050] Next, heat-resistant steel castings containing S were cast, and their mechanical properties (tensile strength, hardness), ferrite content, oxidation resistance, and machinability were evaluated. Table 2 shows the alloy compositions of the cast heat-resistant steel castings. Table 2

[0051] First, the alloy composition of each heat-resistant cast steel is described. The alloy compositions of the heat-resistant cast steels in Examples 9 and 10 were each configured to contain 0.55 mass% or more and 1.0 mass% or less of C, 1.5 mass% or more and 3.5 mass% or less of Si, more than 0 mass% and 2 mass% or less of Mn, 6 mass% or more and 11 mass% or less of Ni, 22 mass% or more and 27 mass% or less of Cr, more than 0 mass% and 0.6 mass% or less of Mo, more than 0 mass% and 0.2 mass% or less of S, and the remainder being Fe and unavoidable impurities. The heat-resistant cast steel of Comparative Example 8 was configured to contain no S, and the effects of adding S were evaluated.Each heat-resistant steel casting was cast by melting an alloy raw material in a high-frequency induction furnace and pouring it into a sand mold in the air atmosphere. - Evaluation of ferrite content -

[0052] The ferrite content of each heat-resistant steel casting was measured. The ferrite content of each heat-resistant steel casting was measured using the same method as in Example 1. Table 2 shows the ferrite content of each heat-resistant steel casting. The ferrite content of the heat-resistant steel casting was 0.00% in Example 9, 0.12% in Example 10, and 0.00% in Comparative Example 8. The ferrite content in the heat-resistant steel castings of Examples 9 and 10 was approximately the same as the ferrite content in the heat-resistant steel castings of Examples 1 to 8. Accordingly, it was found that the formation of the ferrite phase was suppressed even when more than 0 mass% and 0.2 mass% or less of S was added. - Oxidation resistance assessment -

[0053] The oxidation resistance of each heat-resistant cast steel was evaluated. The oxidation resistance of each heat-resistant cast steel was evaluated using the same method as in Example 1. Table 2 shows the amount of weight loss after the oxidation test for each heat-resistant cast steel. The amount of weight loss after the oxidation test for the heat-resistant cast steel in Example 9 was 22 mg cm³. -2 , in the example 10 21 mg·cm -2 and in comparative example 8 21 mg·cm -2 The oxidation resistance of the heat-resistant steel castings in Examples 9 and 10 was approximately the same as the oxidation resistance of the heat-resistant steel castings in Examples 1 to 8. - Hardness Rating -

[0054] The hardness of each heat-resistant cast steel was measured. The hardness of each heat-resistant cast steel was measured by measuring the Vickers hardness at room temperature using the same method as in Example 1. Table 2 shows the hardness of each heat-resistant cast steel. The hardness of the heat-resistant cast steel was Hv 229 in Example 9, Hv 228 in Example 10, and Hv 247 in Comparative Example 8. The hardness of the heat-resistant cast steels of Examples 9 and 10 was approximately the same as that of the heat-resistant cast steels of Examples 1 to 8. - Evaluation of tensile properties -

[0055] The high-temperature tensile properties of each heat-resistant cast steel were evaluated. The tensile properties of each heat-resistant cast steel were evaluated using the same method as in the first example. Table 2 shows the results of the tensile properties (tensile strength, 0.2% proof stress, elongation) of each heat-resistant cast steel. The tensile strength of the heat-resistant cast steel at 600°C was 401 MPa in Example 9, 404 MPa in Example 10, and 424 MPa in Comparative Example 8. The 0.2% proof stress at 600°C was 195 MPa in Example 9, 195 MPa in Example 10, and 202 MPa in Comparative Example 8. The elongation of the heat-resistant cast steel at 600°C was 10.0% in Example 9, 10.0% in Example 10, and 9.0% in Comparative Example 8. The tensile strength of the heat-resistant cast steel at 950°C was 118 MPa in Example 9, 119 MPa in Example 10, and 122 MPa in Comparative Example 8.The 0.2% yield strength of the heat-resistant cast steel at 950°C was 65 MPa in Example 9, 67 MPa in Example 10, and 67 MPa in Comparative Example 8. The elongation of the heat-resistant cast steel at 950°C was 38.0% in Example 9, 35.0% in Example 10, and 38.0% in Comparative Example 8. The tensile elongation properties of the heat-resistant cast steel in Examples 9 and 10 were approximately the same as the tensile elongation properties of the heat-resistant cast steel in Examples 1 to 8. - Assessment of machinability -

[0056] The machinability of each heat-resistant steel casting was evaluated. First, a machinability evaluation test method is described. Fig. Figure 6 is a view explaining the machinability evaluation test method. The machinability evaluation test method was performed by conducting a turning test. Each heat-resistant cast steel was formed into a columnar shape to form a specimen. A tool with a carbide-coated edge was used to cut a side surface of the specimen. A lathe rotated at a predetermined speed, and a photograph of the tool was taken every predetermined time to measure the tool wear depth. The peripheral rotation speed (cutting speed) was 50 m / s. Then, the machinability was evaluated based on notch wear (tool wear) for a specified machining time.

[0057] Next, the results of the machining evaluation test for each heat-resistant steel casting are described. Fig. Figure 7 is a graph showing the results of the machining evaluation test for each heat-resistant steel casting. In the graph of Fig.In Figure 7, the horizontal axis represents machining time (T), while the vertical axis represents notch wear (VN). Furthermore, the white circles represent the results for the heat-resistant cast steel of Example 9, the white squares represent the results for the heat-resistant cast steel of Example 10, and the cross symbols represent the results for the heat-resistant cast steel of Comparative Example 8. For the heat-resistant cast steels of Examples 9 and 10, the notch wear was small for each machining time. Meanwhile, for the heat-resistant cast steel of Comparative Example 8, the notch wear was large for each machining time. Furthermore, since the heat-resistant cast steel of Example 10 had smaller notch wear than the heat-resistant cast steel of Comparative Example 9, it was found that the machinability of the heat-resistant cast steel can be further improved when more S is included. Industrial applicability

[0058] According to the present invention, there is no need to use the die casting method, and the manufacturing cost of the heat-resistant steel casting can thus be reduced. Accordingly, the present invention is useful for parts such as turbocharger parts for a vehicle.

Claims

[1] Heat-resistant cast steel with: 0.55 mass% or more and 1.0 mass% or less C; more than 1.5 mass% and 3.5 mass% or less of Si; more than 0 mass% and 2 mass% or less Mn; 6 mass% or more and 11 mass% or less Ni; 22 mass% or more and 27 mass% or less Cr; more than 0 mass% and 0.6 mass% or less Mo; optionally also more than 0 mass% and 0.2 mass% or less of S; and the rest being Fe and unavoidable impurities. [2] The heat-resistant cast steel according to claim 1, wherein a content of C is 0.55 mass% or more and 0.8 mass% or less. [3] The heat-resistant cast steel according to claim 1, wherein a content of C is more than 0.8 mass% and 1.0 mass% or less. [4] The heat-resistant cast steel according to any one of claims 1 to 3, wherein a content of Si is more than 1.5 mass% and 2.5 mass% or less. [5] The heat-resistant cast steel according to any one of claims 1 to 4, wherein a content of S is 0.1 mass% or more and 0.2 mass% or less. [6] A turbocharger part made of the heat-resistant cast steel according to any one of claims 1 to 5.

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