Cast steel alloy, and bearing for supercharger

Abstract

This cast steel alloy contains 0.2-0.7 wt% of C, 22-26 wt% of Cr, 2.0-2.2 wt% of Mn, 15-22 wt% of Ni, 0.0-0.7 wt% of S, 2.2-2.8 wt% of Si, and 38-55 wt% of Fe.

Description

Cast steel alloys and bearings for turbochargers

[0001] This disclosure relates to cast steel alloys and bearings for turbochargers.

[0002] Austenitic stainless steel castings are used as materials for various machine parts. Patent documents 1 to 5 disclose technologies related to austenitic heat-resistant castings. For example, austenitic stainless steel castings are sometimes used as materials for components that make up vehicle turbochargers. As one example, austenitic stainless steel castings are used as materials for the bearings of wastegate valves.

[0003] Japanese Patent No. 4985941, Japanese Unexamined Patent Publication No. 2002-086517, European Patent Publication No. 55312, Japanese Patent Publication No. 2009-541672, Japanese Unexamined Patent Publication No. 2015-514865, International Publication No. 2007 / 147710

[0004] For example, the opening and closing operation of a wastegate valve incorporated into a supercharger involves the phenomenon of parts rubbing against each other. Therefore, the materials used for such parts that rub against each other require attention to their resistance to wear. For example, Patent Document 1 discloses technology relating to an austenitic iron matrix alloy that focuses on wear resistance.

[0005] Incidentally, components used in the exhaust system of a turbocharger are susceptible to corrosion due to the effects of moisture and other substances contained in the exhaust. Therefore, in addition to wear resistance, corrosion resistance is required for components used in the exhaust system of a turbocharger. Accordingly, this disclosure provides a cast steel alloy and a bearing for a turbocharger that can improve both wear resistance and corrosion resistance.

[0006] A cast steel alloy, one embodiment of the present disclosure, contains 0.2 to 0.7 wt% of C, 22 to 26 wt% of Cr, 2.0 to 2.2 wt% of Mn, 15 to 22 wt% of Ni, 0.0 to 0.7 wt% of S, 2.2 to 2.8 wt% of Si, and 38 to 55 wt% of Fe.

[0007] This disclosure provides a cast steel alloy and a bearing for a turbocharger that can improve wear resistance and corrosion resistance.

[0008] This is a cross-sectional view showing a bearing for a turbocharger according to one embodiment. Figure 2(a) is a diagram showing an overview of the test apparatus used for the high-temperature wear test, and Figure 2(b) is an enlarged view of the area around the test piece in Figure 2(a). Figure 3 is a graph showing the relationship between the average friction coefficients of the example and comparative example at 300°C and 900°C. Figure 4 is a graph showing the relationship between the protrusion height, wear depth, and wear volume of the example and comparative example at 300°C. Figure 5 is a graph showing the relationship between the protrusion height, wear depth, and wear volume of the example and comparative example at 900°C. Figure 6 is a graph showing the relationship between the Vickers hardness of the example and comparative example. Figure 7 is a graph showing the relationship between the corrosivity of the example and comparative example.

[0009] A cast steel alloy, one embodiment of the present disclosure, contains 0.2 to 0.7 wt% of C, 22 to 26 wt% of Cr, 2.0 to 2.2 wt% of Mn, 15 to 22 wt% of Ni, 0.0 to 0.7 wt% of S, 2.2 to 2.8 wt% of Si, and 38 to 55 wt% of Fe.

[0010] This cast steel alloy provides excellent wear resistance due to eutectic carbides resulting from major additive elements such as Mn. Furthermore, it offers excellent corrosion resistance due to major additive elements such as Cr and Ni.

[0011] The above-mentioned cast steel alloy may further contain 0.3 to 0.7% by weight of Mo. This composition also provides good wear resistance. In this composition, the content of expensive elements such as Mo in the alloy can be reduced compared to conventional austenitic stainless steel castings, thereby providing good wear resistance and corrosion resistance, as well as improving mass productivity.

[0012] The above-mentioned cast steel alloy may further contain more than 0% by weight of Co and 0.3% by weight or less. This composition allows for good strength even when the alloy is heated to high temperatures. Furthermore, good wear resistance and corrosion resistance can be obtained while utilizing the Co contained in scrap materials, etc., in the alloy.

[0013] The above-mentioned cast steel alloy may further contain Cu in an amount greater than 0% by weight and less than or equal to 0.4% by weight. This composition allows for good strength even when the alloy is heated to high temperatures. Furthermore, it is possible to utilize the Cu contained in scrap materials, etc., in the alloy while obtaining good wear resistance and corrosion resistance.

[0014] In the above-mentioned cast steel alloy, the Mo content may be greater than 0.3% by weight and less than 0.5% by weight. This composition allows for a reduction in the relatively expensive Mo content compared to conventional austenitic stainless steel castings.

[0015] In the above-mentioned cast steel alloy, the amount of sulfur (S) may be greater than 0.5% by weight and less than or equal to 0.7% by weight. This composition allows for an increase in the amount of sulfur compared to conventional austenitic stainless steel castings, thereby improving wear resistance.

[0016] A bearing for a supercharger, as described in this disclosure, is made of a cast steel alloy containing 0.2 to 0.7% by weight of C, 22 to 26% by weight of Cr, 2.0 to 2.2% by weight of Mn, 15 to 22% by weight of Ni, 0.0 to 0.7% by weight of S, 2.2 to 2.8% by weight of Si, and 38 to 55% by weight of Fe.

[0017] This supercharger bearing provides excellent wear resistance due to the eutectic carbides resulting from the crystallization of MnS and Si. Furthermore, the combination of elements in the aforementioned proportions provides excellent corrosion resistance. Therefore, even within a supercharger, where components rub against each other and are affected by moisture in the exhaust gas, this supercharger bearing can suppress deterioration through its excellent wear resistance and corrosion resistance.

[0018] The cast steel alloy and the bearing for the turbocharger according to this embodiment will be described in detail below with reference to the attached drawings. In the description of the drawings, the same elements are denoted by the same reference numerals, and redundant explanations are omitted.

[0019] <Cast Steel Alloy> The composition of the cast steel alloy according to this embodiment will be described in detail below. The cast steel alloy is, for example, an austenitic stainless steel casting. The content (%) of each element is by weight unless otherwise specified. The cast steel alloy according to this embodiment contains 0.2 to 0.7 wt% of C, 22 to 26 wt% of Cr, 2.0 to 2.2 wt% of Mn, 15 to 22 wt% of Ni, 0.0 to 0.7 wt% of S, 2.2 to 2.8 wt% of Si, and 38 to 55 wt% of Fe. The cast steel alloy according to this embodiment may further contain 0.3 to 0.7 wt% of Mo. The cast steel alloy according to this embodiment may further contain more than 0 wt% and 0.3 wt% or less of Co, and more than 0 wt% and 0.4 wt% or less of Cu. The cast steel alloy according to this embodiment may further contain 2 to 3% by weight of W, 1.5 to 1.7% by weight of Nb, and 0 to 0.05% by weight of P. The cast steel alloy according to this embodiment may also contain 0 to 1.0% by weight of unavoidable impurities.

[0020] [C: Carbon] C affects the high-temperature strength of cast steel alloys. C also contributes to suppressing embrittlement and ductility reduction of cast steel alloys. The C content in the cast steel alloy of this embodiment is 0.2% by weight or more and 0.7% by weight or less. Hereinafter, "content" refers to the amount of element added when producing the cast steel alloy. The C content may be greater than 0.2% by weight and 0.7% by weight or less. The C content may be greater than 0.2% by weight and less than 0.4% by weight. Furthermore, the C content may be greater than 0.24% by weight and less than 0.4% by weight. Note that the range of C content in the cast steel alloy of this embodiment may include a portion that is greater than the range of C content for stainless steel castings specified in the Japanese Industrial Standard (JIS G 5121:2003) (0.40% by weight or less). Specifically, the C content in the cast steel alloy of this embodiment may be greater than 0.4% by weight and 0.7% by weight or less. In this embodiment, the carbon content of the cast steel alloy, regardless of the range described above, contributes to suppressing embrittlement and reducing the ductility of the cast steel alloy.

[0021] [Cr: Chromium] Cr is a major element found in cast steel alloys. The Cr content in cast steel alloys is second only to the Fe content. Cr affects the corrosion resistance, oxidation resistance, and workability of cast steel alloys. In this embodiment, the Cr content in the cast steel alloy is 22% by weight or more and 26% by weight or less. The Cr content may be greater than 22% by weight and 26% by weight or less.

[0022] [Mn: Manganese, S: Sulfur] Mn affects Vickers hardness and high-temperature strength. S affects low-temperature wear resistance. Therefore, Mn and S affect the coefficient of friction of cast steel alloys. Specifically, Mn and S crystallize as MnS in cast steel alloys. This crystallized MnS reduces the coefficient of friction. As a result, Mn and S contribute to improving the wear resistance of cast steel alloys. The Mn content in the cast steel alloy of this embodiment is 2.0% by weight or more and 2.2% by weight or less. The Mn content may be greater than 2.0% by weight and 2.2% by weight or less. The Mn content may be greater than 2.0% by weight and less than 2.2% by weight. Also, the ratio of Mn to C may be greater than 2.85 and 11 or less. Mn is an element that has the effect of stabilizing the austenite structure and contributes to improving high-temperature properties and mechanical properties. Furthermore, the S content in the cast steel alloy of this embodiment is 0.0% by weight or more and 0.7% by weight or less. The S content may be greater than 0.0% by weight and 0.7% by weight or less. The S content may be greater than 0.5% by weight and 0.7% by weight or less. Also, the ratio of S to C may be greater than 0.0 and 3.5 or less. The ratio of S to C may be greater than 0.71 and 3.5 or less. Note that the range of S content in the cast steel alloy of this embodiment is greater in all ranges than the range of S content in cast steel alloys specified in the Japanese Industrial Standard (JIS G 5121:2003) (0.04% by weight or less). Regardless of the content and ratio of Mn and S in the cast steel alloy of this embodiment, they contribute to reducing the friction coefficient of the cast steel alloy.

[0023] [Ni: Nickel] Ni is a major element in cast steel alloys. Ni affects the stability and castability of the austenitic structure of cast steel. The Ni content in the cast steel alloy of this embodiment is 15% by weight or more and 22% by weight or less. The Ni content may be greater than 15% by weight and 22% by weight or less.

[0024] [Si: Silicon] Si acts as an oxygen scavenger during casting. Si also contributes to improved oxidation resistance. Si is an element that stabilizes the austenite structure and contributes to improved high-temperature properties and mechanical properties. From this perspective, the Si content in the cast steel alloy of this embodiment is 2.2% by weight or more and 2.8% by weight or less. The Si content may be greater than 2.2% by weight and 2.8% by weight or less. Note that the range of Si content in the cast steel alloy of this embodiment is greater in all ranges than the range of Si content for cast steel alloys specified in the Japanese Industrial Standard (JIS G 5121:2003) (2.0% by weight or less).

[0025] [Mo: Molybdenum] Mo is found, for example, in scrap metal. Mo has the effect of strengthening crystals by solid solution and by forming fine carbides. Mo affects the strength of cast steel alloys in high-temperature environments. Mo may also degrade the oxidation resistance of cast steel alloys. The Mo content in the cast steel alloy of this embodiment is 0.3% by weight or more and 0.7% by weight or less. The Mo content may be greater than 0.3% by weight and 0.7% by weight or less. Also, the Mo content in the cast steel alloy of this embodiment may be greater than 0.3% by weight and less than 0.5% by weight. Furthermore, the ratio of Mo to Mn may be greater than 0.136 and 0.35 or less. The ratio of Mo to Mn may be greater than 0.136 and less than 0.35. Furthermore, the ratio of Mo to S may be greater than 0.42. The ratio of Mo to S may be greater than 0.42 and less than or equal to 1.4. The ratio of Mo to S may be greater than 0.42 and less than 1.4. Regardless of the Mo content and ratio within the above ranges in the cast steel alloy of this embodiment, the strength of the cast steel alloy under high-temperature conditions can be enhanced.

[0026] [Co: Cobalt] Co is found, for example, in scrap metal. Co affects the high-temperature strength, corrosion resistance, ductility, and toughness of cast steel alloys. The Co content in the cast steel alloy of this embodiment is 0.3% by weight or less. The Co content in the cast steel alloy of this embodiment may be greater than 0.0% by weight and less than or equal to 0.3% by weight. Regardless of the Co content in the cast steel alloy of this embodiment, it can contribute to improving the high-temperature strength, corrosion resistance, ductility, and toughness of the cast steel alloy.

[0027] [Cu: Copper] Cu is found, for example, in scrap materials. Cu affects the high-temperature strength, ductility, and toughness of cast steel alloys. The Cu content in the cast steel alloy of this embodiment is 0.4% by weight or less. The Cu content in the cast steel alloy of this embodiment may be greater than 0.0% by weight and less than or equal to 0.4% by weight. Regardless of the Cu content in the cast steel alloy of this embodiment, it can contribute to improving the high-temperature strength, ductility, and toughness of the cast steel alloy.

[0028] [W: Tungsten] W has the effect of strengthening the crystal by solid solution and by forming fine carbides. W affects the strength of cast steel alloys in high-temperature environments. In addition, W may degrade the oxidation resistance of cast steel alloys. The W content in the cast steel alloy of this embodiment is greater than 2.0% by weight and 3.0% by weight or less. The W content in the cast steel alloy of this embodiment may also be greater than 2.0% by weight and less than 2.4% by weight. The W content in the cast steel alloy of this embodiment may also be greater than 2.8% by weight and 3.0% by weight or less.

[0029] [Nb: Niobium] Nb affects the high-temperature strength and thermal fatigue life of cast steel alloys. Furthermore, Nb may also affect the oxidation resistance, machinability, and castability of cast steel alloys. The Nb content in the cast steel alloy of this embodiment is greater than 1.5% by weight and 1.7% by weight or less.

[0030] [P: Phosphorus] Phosphorus improves the fluidity and hardness of cast steel alloys during manufacturing. On the other hand, if the P content in cast steel alloys is too high, the cast steel alloys may become brittle and prone to cracking, potentially reducing their mechanical properties. Taking these factors into consideration, the P content in the cast steel alloy of this embodiment is 0.05% by weight or less. The P content in the cast steel alloy of this embodiment may exceed 0.0% by weight and be 0.05% by weight or less. Furthermore, the range of P content in the cast steel alloy of this embodiment may include a portion that is greater than the range of P content for stainless steel castings specified in the Japanese Industrial Standard (JIS G 5121:2003) (0.040% by weight or less). Specifically, the P content in the cast steel alloy of this embodiment may exceed 0.04% by weight and be 0.05% by weight or less. Regardless of the above range of P content in the cast steel alloy of this embodiment, it improves the fluidity and hardness of the cast steel alloys during manufacturing.

[0031] [Fe: Iron] Fe constitutes the main component of cast steel alloys. In this embodiment, the Fe content in the cast steel alloy is greater than 38% by weight and 55% by weight or less. The Fe content in the cast steel alloy may be 38.45% by weight or more and less than 54.8% by weight. The Fe content in the cast steel alloy may be the remainder obtained by subtracting the total content of each of the above components from the total weight of the cast steel alloy.

[0032] <Bearings for Superchargers> The cast steel alloy of this embodiment is used, for example, in bearings for superchargers. Figure 1 is a cross-sectional view of a supercharger according to an example of an embodiment, including the rotation axis A. The supercharger 1 is applied, for example, to internal combustion engines of ships and vehicles. As shown in Figure 1, the supercharger 1 comprises a turbine 2 and a compressor 3. The turbine 2 comprises a turbine housing 4 and a turbine impeller 6 housed in the turbine housing 4. The turbine housing 4 has a scroll passage 16 extending circumferentially around the turbine impeller 6. The compressor 3 comprises a compressor housing 5 and a compressor impeller 7 housed in the compressor housing 5. The compressor housing 5 has a scroll passage 17 extending circumferentially around the compressor impeller 7.

[0033] The turbine impeller 6 is mounted at one end of the rotating shaft 14, and the compressor impeller 7 is mounted at the other end of the rotating shaft 14. A bearing housing 13 is provided between the turbine housing 4 and the compressor housing 5. The rotating shaft 14 is rotatably supported by the bearing housing 13 via bearings 15, and the rotating shaft 14, turbine impeller 6, and compressor impeller 7 rotate as a single rotating body 12 around the axis of rotation A.

[0034] The rotating shaft 14 can slide against the bearing 15 by rotating. The bearing 15 in this embodiment is a bearing for a supercharger and is not limited to the configuration of the supercharger 1 shown in Figure 1. The bearing 15 is made of, for example, the above-mentioned cast steel alloy. The bearing 15 is made of, for example, a cast steel alloy containing 0.2 to 0.7% by weight of C, 22 to 26% by weight of Cr, 2.0 to 2.2% by weight of Mn, 15 to 22% by weight of Ni, 0.0 to 0.7% by weight of S, 2.2 to 2.8% by weight of Si, and 38 to 55% by weight of Fe.

[0035] Furthermore, the cast steel alloy of this embodiment may be used, for example, as a material for a wastegate valve component incorporated into a turbocharger. For example, the wastegate valve of a vehicle turbocharger adjusts the flow rate of exhaust gas supplied to the turbine by diverting a portion of the exhaust gas flowing through the turbocharger. Therefore, the wastegate valve is exposed to exhaust gas. If the wastegate valve is exposed to exhaust gas for a long period of time, corrosion may occur in the components that make up the wastegate valve due to the corrosive components. Corrosion that occurs in the components can impair the function of the wastegate valve. For example, if the clearance between the shaft and bearing supporting the valve is lost due to corrosion, seizing of the shaft and bearing may occur. Here, since the cast steel alloy of this embodiment has better corrosion resistance than conventional stainless steel castings, even when used as a material for turbocharger components including a wastegate valve, it will be less susceptible to corrosion than conventional turbocharger components.

[0036] <Effects> Components used in the exhaust system of a turbocharger may corrode due to the effects of moisture contained in the exhaust. For example, exhaust (exhaust gas) may contain corrosive components and moisture. Moisture contained in the exhaust may condense on metal components of the exhaust system. Since the condensed water may contain corrosive components contained in the exhaust, the metal components may corrode, mainly at the parts that are in contact with the condensed moisture. Thus, metal components used in the exhaust system of a turbocharger may corrode. Such corrosion can impair the function of the turbocharger's exhaust system. For example, if the clearance between the shaft and the bearing disappears due to corrosion, the shaft and bearing may seize up. For this reason, components used in the exhaust system of a turbocharger are required to have corrosion resistance in addition to wear resistance. Therefore, cast steel alloys and bearings for turbochargers that can improve both wear resistance and corrosion resistance are needed.

[0037] The cast steel alloy according to this embodiment contains 0.2 to 0.7% by weight of C, 22 to 26% by weight of Cr, 2.0 to 2.2% by weight of Mn, 15 to 22% by weight of Ni, 0.0 to 0.7% by weight of S, 2.2 to 2.8% by weight of Si, and 38 to 55% by weight of Fe.

[0038] According to this cast steel-based alloy, good wear resistance can be obtained due to eutectic carbides caused by main additive elements such as Mn. In addition, good corrosion resistance can be obtained by main additive elements such as Cr and Ni. For example, the cast steel-based alloy according to the present embodiment can obtain good wear resistance and good corrosion resistance even in a high temperature range of 700°C or more and 900°C or less. Note that the cast steel-based alloy according to the present embodiment is not limited to the high temperature range in the above range, and can obtain good wear resistance and good corrosion resistance even in a high temperature range.

[0039] The above-mentioned cast steel-based alloy may further contain 0.3 to 0.7% by weight of Mo. Good wear resistance can also be obtained with this composition. In this composition, the content of expensive elements such as Ni and Mo contained in the alloy can be reduced compared with conventional austenitic stainless cast steel, so that good wear resistance and corrosion resistance can be obtained, while reducing manufacturing costs and improving mass productivity (economic efficiency). In addition, this cast steel-based alloy can obtain good wear resistance and corrosion resistance even if it contains Mo that may be contained in scrap scraps and the like, and has a high contribution to recycling.

[0040] The above-mentioned cast steel-based alloy may further contain more than 0% by weight and 0.3% by weight or less of Co. With this composition, good strength can be obtained even when the alloy is heated to a high temperature. In addition, good wear resistance and corrosion resistance can be obtained while utilizing Co contained in scrap scraps and the like for the alloy. That is, this cast steel-based alloy can obtain good wear resistance and corrosion resistance even if it contains Co that may be contained in scrap scraps and the like, can be manufactured at low cost, and has a high contribution to recycling.

[0041] The above-mentioned cast steel-based alloy may further contain more than 0% by weight and 0.4% by weight or less of Cu. With this composition, good strength can be obtained even when the alloy is heated to a high temperature. In addition, good wear resistance and corrosion resistance can be obtained while utilizing Cu contained in scrap scraps and the like for the alloy. That is, this cast steel-based alloy can obtain good wear resistance and corrosion resistance even if it contains Cu that may be contained in scrap scraps and the like, can be manufactured at low cost, and has a high contribution to recycling.

[0042] In the above cast steel-based alloy, Mo may be more than 0.3% by weight and less than 0.5% by weight. This composition makes it possible to reduce the content of relatively expensive Mo compared to conventional austenitic stainless cast steels.

[0043] In the above cast steel-based alloy, S may be more than 0.5% by weight and 0.7% by weight or less. This composition makes it possible to increase the content of S compared to conventional austenitic stainless cast steels, thereby improving wear resistance at low temperatures.

[0044] A bearing for a supercharger according to one aspect of the present disclosure is formed of a cast steel-based alloy containing 0.2 to 0.7% by weight of C, 22 to 26% by weight of Cr, 2.0 to 2.2% by weight of Mn, 15 to 22% by weight of Ni, 0 to 0.7% by weight of S, 2.2 to 2.8% by weight of Si, and 38 to 55% by weight of Fe.

[0045] According to the bearing for a supercharger, excellent wear resistance can be obtained by eutectic carbides derived from main additive elements such as Mn. In addition, excellent corrosion resistance can be obtained by main additive elements such as Cr and Ni. Therefore, even when used inside a supercharger that is affected by mutual friction between components and affected by moisture and the like contained in exhaust gas, the bearing for a supercharger can suppress deterioration due to its excellent wear resistance and corrosion resistance.

[0046] The present disclosure will be described in further detail below with reference to Examples 1 to 4 and Comparative Examples 1 to 5 of the present disclosure, but the present disclosure is not limited to these examples.

[0047] The compositions of the cast steel alloy specimens in Examples 1 to 4 and Comparative Examples 1 to 5 are shown below. The cast steel alloys of Examples 1 to 4 contain 0.45 wt% C, 24 wt% Cr, 2.1 wt% Mn, 18.5 wt% Ni, 0.35 wt% S, 2.5 wt% Si, 0.5 wt% Mo, 0.15 wt% Co, 2.5 wt% W, 1.6 wt% Nb, 0.2 wt% Cu, 0.025 wt% P, 1 wt% unavoidable impurities, and the remainder being Fe. The cast steel alloys of Examples 1 to 4 are composed of elements within the numerical range shown in the claims. Comparative Examples 1 to 3 and 5 are stainless steel castings containing 0.5% by weight of C, 24.5% by weight of Cr, 2% by weight of Mn, 18.5% by weight of Ni, 0.6% by weight of S, 2.65% by weight of Si, 2.75% by weight of W, 2% by weight of Nb, 3% by weight of unavoidable impurities, and the remainder being Fe. Comparative Example 4 is Inconel 713C.

[0048] Figures 2(a) and 2(b) show an overview of the test apparatus 101 used for the high-temperature abrasion test. Figure 2(a) is an overview of the test apparatus used for the high-temperature abrasion test, and Figure 2(b) is an enlarged view of the area around the test specimen in Figure 2(a). First, a flat plate test specimen 102 measuring 35 mm in width x 12 mm in depth x 6 mm in thickness was prepared for each example and each comparative example. Examples 1 and 2 and comparative examples 1 and 2 were prepared as flat plate test specimens 102. In addition, a cylindrical test specimen 103 measuring 12 mm in diameter and 20 mm in height was prepared as a mating material for the flat plate test specimen 102. The material of the cylindrical test specimen 103 was Inconel 713C.

[0049] The temperature of the cylindrical test specimen 103 was measured, and the inside of the furnace 104 was heated so that the cylindrical test specimen 103 reached 300°C or 900°C. Inside this furnace 104, the cylindrical test specimen 103 was pressed vertically against the flat test specimen 102 with a constant load, and then slid back and forth horizontally on the flat test specimen 102. An example of the conditions is as follows: Load: 11N Number of reciprocations: 1000 Sliding speed: 10 cycles / min Stroke width: 10mm

[0050] Example 1 and Comparative Example 1 are flat plate test specimens 102 used when the furnace 104 was heated to 300°C. Example 2 and Comparative Example 2 are flat plate test specimens 102 used when the furnace 104 was heated to 900°C. Hereafter, the flat plate test specimen 102 may be referred to as a "plate," and the cylindrical test specimen 103 may be referred to as a "cylinder."

[0051] <Evaluation 1: High-Temperature Friction Test / Average Friction Coefficient> Figure 3 is a graph showing the relationship between the average friction coefficients of the examples and comparative examples at 300°C and 900°C. The average friction coefficient results shown in Figure 3 are values ​​obtained from the above high-temperature friction test when the temperature inside the furnace 104 was 300°C or 900°C, respectively. The vertical axis represents the average friction coefficient. The average friction coefficient of Example 1, placed in the furnace 104 heated to 300°C, was 0.599. The average friction coefficient of Comparative Example 1, placed in the furnace 104 heated to 300°C, was 0.785. The average friction coefficient of Example 2, placed in the furnace 104 heated to 900°C, was 0.580. The average friction coefficient of Comparative Example 2, placed in the furnace 104 heated to 900°C, was 0.960.

[0052] By comparing Examples 1 and 2, it was confirmed that the average coefficient of friction at 900°C was smaller for the cast steel alloy of the Examples compared to the average coefficient of friction at 300°C. On the other hand, by comparing Comparative Examples 1 and 2, it was confirmed that the average coefficient of friction at 900°C was larger for the stainless steel casting of the Comparative Examples compared to the average coefficient of friction at 300°C. Therefore, it became clear that the cast steel alloy of the Examples maintains a small average coefficient of friction even when applied to a region where high temperatures occur while the exhaust system of a turbocharger is operating steadily. Note that the above-mentioned "high temperature" includes the high temperature range. For example, the above-mentioned "high temperature" includes the high temperature range of 700°C to 900°C. The same applies to the following high temperatures.

[0053] By comparing Example 1 with Comparative Example 1, it was confirmed that at 300°C, the average coefficient of friction of the cast steel alloy of Example 1 was smaller than that of conventional stainless steel casting. Therefore, it was revealed that the cast steel alloy of Example 1 can be applied to areas that become hot during operation, such as the exhaust system of a turbocharger, and that even during the process of rising to a steady operating state, the average coefficient of friction remains lower than that of conventional stainless steel casting.

[0054] By comparing Example 2 with Comparative Example 2, it was confirmed that at 900°C, the average coefficient of friction of the cast steel alloy of the Example was smaller than that of conventional stainless steel castings. Therefore, it became clear that even when the cast steel alloy of the Example is applied to a region where high temperatures occur while the exhaust system of a turbocharger is operating steadily, the average coefficient of friction of the Example remains low compared to conventional stainless steel castings, without increasing with rising temperature.

[0055] <Evaluation 2: High-Temperature Friction Test / Protrusion Height, Wear Depth, and Wear Volume> Figure 4 is a graph showing the relationship between protrusion height, wear depth, and wear volume for the examples and comparative examples at 300°C. The high-temperature wear test results shown in Figure 4 are values ​​obtained from the above high-temperature friction test when the temperature inside the furnace 104 was set to 300°C. The left vertical axis shows the protrusion height (μm) and wear depth (μm), and the right vertical axis shows the wear volume (mm²). ３ The projection height indicates the maximum projection height. The projection height indicates the height at the highest point within the sliding portion between the cylindrical test piece 103 and the flat test piece 102, when the average height calculated from the non-sliding portion between the cylindrical test piece 103 and the flat test piece 102 is set to zero. The wear depth indicates the maximum wear depth. The wear depth indicates the depth at the lowest point within the sliding portion between the cylindrical test piece 103 and the flat test piece 102, when the average height calculated from the non-sliding portion between the cylindrical test piece 103 and the flat test piece 102 is set to zero. The wear volume is calculated using a one-shot 3D measuring instrument and indicates the volume of the recess in the sliding portion when the average height calculated from the non-sliding portion between the cylindrical test piece 103 and the flat test piece 102 is set to the zero plane.

[0056] In the high-temperature wear test A, using Example 1 as the plate (flat test piece 102) and Inconel 713C as the cylinder (cylindrical test piece 103), the protrusion height of the cylinder was 23.28 μm, the protrusion height of the plate was 85.77 μm, the wear depth of the cylinder was 23.37 μm, the wear depth of the plate was 62.15 μm, and the wear volume of the cylinder was 0.047 mm³. ３ The wear volume of the plate was 0.791 mm². ３ That was the case.

[0057] In the high-temperature abrasion test a, when Comparative Example 1 was used as a plate (flat test piece 102) and Inconel 713C as a cylinder (cylindrical test piece 103), the protrusion height of the cylinder was 73.26 μm, the protrusion height of the plate was 43.85 μm, the wear depth of the cylinder was 33.85 μm, the wear depth of the plate was 48.05 μm, and the wear volume of the cylinder was 0.000 mm². ３ The wear volume of the plate was 0.551 mm². ３ That was the case.

[0058] Figure 5 is a graph showing the relationship between protrusion height, wear depth, and wear volume for the examples and comparative examples at 900°C. The high-temperature wear test results shown in Figure 5 are values ​​obtained from the above high-temperature friction test when the temperature inside the furnace 104 was set to 900°C. The left vertical axis shows the protrusion height (μm) and wear depth (μm), and the right vertical axis shows the wear volume (mm²). ３ This indicates that...

[0059] In the high-temperature abrasion test B using Example 2 as the plate (flat test piece 102) and Inconel 713C as the cylinder (cylindrical test piece 103), the protrusion height of the cylinder was 31.47 μm, the protrusion height of the plate was 74.27 μm, the abrasion depth of the cylinder was 12.04 μm, the abrasion depth of the plate was 52.90 μm, and the abrasion volume of the cylinder was 0.041 mm³. ３ The wear volume of the plate was 0.597 mm². ３ That was the case.

[0060] In the test results of the high-temperature wear test b when Comparative Example 2 was used as the plate (flat plate test piece 102) and Inconel 713C was used as the cylinder (cylindrical test piece 103), the projection height of the cylinder was 112.00 µm, the projection height of the plate was 58.50 µm, the wear depth of the cylinder was 10.95 µm, the wear depth of the plate was 110.50 µm, and the wear volume of the cylinder was 0.000 mm ３ , and the wear volume of the plate was 1.331 mm ３ .

[0061] By comparing the test results of Tests A and B, it was confirmed that in the cast steel-based alloy of the example, the projection height, wear depth and wear volume at 900°C are respectively smaller than the projection height, wear depth and wear volume at 300°C. On the other hand, by comparing the test results of Tests a and b, it was confirmed that in the stainless cast steel of the comparative example, the projection height, wear depth and wear volume at 900°C are respectively significantly larger than the projection height, wear depth and wear volume at 300°C. Therefore, it has been clarified that even when the cast steel-based alloy of the example is applied to a region that becomes high temperature while the exhaust system of a supercharger or the like is in steady operation, the projection height, wear depth and wear volume are maintained smaller than those of conventional stainless cast steel.

[0062] <Evaluation 3: Vickers Hardness> Figure 6 is a graph showing the relationship between Vickers hardness of the example and comparative examples. Using Example 3 and Comparative Examples 3 and 4 respectively as test pieces, the Vickers hardness test specified in Japanese Industrial Standards (JIS B 2244:2009) was performed. In the Vickers hardness test results shown in Figure 6, the vertical axis represents Vickers hardness (HV). The Vickers hardness of Example 3 was 299 HV. The Vickers hardness of Comparative Example 3 was 218 HV (1 kgf·20 s), and the Vickers hardness of Comparative Example 4 was 519 HV (1 kgf·20 s).

[0063] As shown in Figure 6, by comparing Example 3 and Comparative Example 3, it was confirmed that the Vickers hardness of the cast steel alloy in Example 3 was greater than that of the stainless steel cast in Comparative Example 3. Therefore, it was suggested that the cast steel alloy in Example 3 has higher wear resistance and maintains dimensional stability compared to the stainless steel cast in Comparative Example 3.

[0064] <Evaluation 4: Corrosiveness Test> Figure 7 is a graph showing the relationship between the corrosiveness of the examples and comparative examples. A natural potential measurement test (hereinafter referred to as the "corrosiveness test") was conducted using Example 4 and Comparative Example 5 as test specimens. A saturated calomel electrode was prepared as the reference electrode, and a 5% by weight sodium chloride aqueous solution at 60°C was prepared as the electrolyte solution. The corrosion potential (Ecorr), etc., was determined from the potential-current curve obtained from the corrosiveness test. The vertical axis of the corrosiveness test results shown in Figure 7 represents a current density of 100 μA / cm². ２ The vertical axis represents the electrode potential relative to the reference electrode, and the horizontal axis represents the open-circuit potential relative to the reference electrode. The dashed line shown in Figure 7 indicates the boundary line where the potential on the vertical axis and the potential on the horizontal axis are equal. At this dashed line, the larger the values ​​of the potential on the vertical axis and the horizontal axis, the better (higher) the corrosion resistance, and the smaller the values ​​of the potential on the vertical axis and the horizontal axis, the worse (lower) the corrosion resistance.

[0065] As shown in Figure 7, by comparing Example 4 and Comparative Example 5, it was confirmed that the corrosion resistance of the cast steel alloy in the example was better than that of the stainless steel casting in the comparative example. In other words, it became clear that the cast steel alloy in the example was less susceptible to corrosion than the stainless steel casting in the comparative example.

[0066] <Modifications> The above describes examples of cast steel alloys and bearings for turbochargers. Cast steel alloys and bearings for turbochargers may be implemented in various forms without being limited to the above examples.

[0067] 1 Supercharger 15 Bearing 101 Test apparatus 102 Flat plate test piece 103 Cylindrical test piece 104 Furnace

Claims

1. A cast steel alloy containing 0.2 to 0.7 wt% C, 22 to 26 wt% Cr, 2.0 to 2.2 wt% Mn, 15 to 22 wt% Ni, 0.0 to 0.7 wt% S, 2.2 to 2.8 wt% Si, and 38 to 55 wt% Fe.

2. The cast steel alloy according to claim 1, further containing 0.3 to 0.7% by weight of Mo.

3. The cast steel alloy according to claim 1 or 2, further containing more than 0% by weight and 0.3% by weight or less of Co.

4. The cast steel alloy according to claim 1 or 2, further containing more than 0% by weight and 0.4% by weight or less of Cu.

5. The cast steel alloy according to claim 2, wherein Mo is greater than 0.3% by weight and less than 0.5% by weight.

6. The cast steel alloy according to claim 1 or 2, wherein S is greater than 0.5% by weight and less than or equal to 0.7% by weight.

7. A bearing for a supercharger, made of a cast steel alloy containing 0.2 to 0.7 wt% C, 22 to 26 wt% Cr, 2.0 to 2.2 wt% Mn, 15 to 22 wt% Ni, 0.0 to 0.7 wt% S, 2.2 to 2.8 wt% Si, and 38 to 55 wt% Fe.

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