Austenitic stainless steel cast steel and method for determining composition of austenitic stainless steel cast steel

Abstract

The austenitic stainless steel cast steel contains 21 to 28% by weight of Cr, 14 to 23% by weight of Ni, 2.0 to 3.5% by weight of W, 1.0 to 3.0% by weight of Nb, 1.5 to 3.5% by weight of Mo, 0 to 3.5% by weight of Cu, 0.6 to 1.1% by weight of C, 0.1 to 0.7% by weight of N, 1.0 to 2.0% by weight of Mn, 0.4 to 0.7% by weight of S, and 2.5 to 4.0% by weight of Si, with the balance being Fe.

Description

Technical Field

[0001] This disclosure relates to austenitic stainless steel cast steel and a method for determining the composition of austenitic stainless steel cast steel. Background Technology

[0002] Austenitic stainless steel cast steel is used as a material for various mechanical components. Patent documents 1-12 disclose technologies related to austenitic heat-resistant cast steel. For example, austenitic stainless steel cast steel is sometimes used as a material for components constituting vehicle turbochargers. As an example, austenitic stainless steel cast steel is used as a material for the bearings of exhaust bypass valves.

[0003] Patent Document 1: German Patent Application Publication No. 102012203569

[0004] Patent Document 2: Japanese Patent Publication No. 2015-514865

[0005] Patent Document 3: German Patent Application Publication No. 102006029121

[0006] Patent Document 4: Japanese Patent No. 5165679

[0007] Patent Document 5: International Publication No. 2005 / 103314

[0008] Patent Document 6: Japanese Patent No. 4985941

[0009] Patent Document 7: Description of Chinese Patent Application Publication No. 111004981

[0010] Patent Document 8: Description of Chinese Patent Application Publication No. 116057187

[0011] Patent Document 9: Japanese Patent Application Publication No. 2022-85613

[0012] Patent Document 10: Description of Chinese Patent Application Publication No. 113862562

[0013] Patent Document 11: Chinese Patent Application Publication No. 114008230

[0014] Patent Document 12: Japanese Patent No. 7269590

[0015] For example, the exhaust bypass valve assembled in the turbocharger opens and closes. This opening and closing action is accompanied by friction between components. Therefore, the materials used for such rubbing components are important for their resistance to wear. For example, Patent Documents 1 and 2 disclose technologies related to austenitic iron matrix alloys for which wear resistance is important. According to the technology disclosed in Patent Documents 1 and 2, manganese sulfide (MnS) is crystallized during the solidification process during casting. As a result, the coefficient of friction is reduced. Summary of the Invention

[0016] In this technical field, there is a desire to further improve the wear resistance of austenitic stainless steel cast steel. Therefore, this disclosure provides austenitic stainless steel cast steel capable of achieving improved wear resistance, and a method for determining the composition of austenitic stainless steel cast steel.

[0017] One aspect of the austenitic stainless steel cast steel disclosed herein contains 21-28 wt% Cr, 14-23 wt% Ni, 2.0-3.5 wt% W, 1.0-3.0 wt% Nb, 1.5-3.5 wt% Mo, 0-3.5 wt% Cu, 0.6-1.1 wt% C, 0.1-0.7 wt% N, 1.0-2.0 wt% Mn, 0.4-0.7 wt% S, and 2.5-4.0 wt% Si, and the remainder contains Fe.

[0018] According to this disclosure, austenitic stainless steel cast steel capable of achieving improved wear resistance and a method for determining the composition of austenitic stainless steel cast steel are provided. Attached Figure Description

[0019] Figure 1 This is a diagram illustrating the concept of a method for predicting wear.

[0020] Figure 2 This is a flowchart illustrating the main steps in determining the composition of austenitic stainless steel cast steel.

[0021] Figure 3 It is a contour map of the predicted sliding wear amount using a predictive formula for sliding wear amount. Figure 3 (a) is a contour plot with Mn added at 1.0 wt%. Figure 3 (b) is a contour plot with Mn added at 2.0 wt%.

[0022] Figure 4 This is a table showing the components of each test piece in the examples, reference examples, and comparative examples.

[0023] Figure 5 This is a graph showing the relationship between the amount of S added and the amount of MnS crystallization.

[0024] Figure 6 (a) is a graph showing the relationship between the amount of Si added and the area ratio of MnS. Figure 6 (b) is a graph showing the relationship between the amount of Si added and the area ratio of the eutectic carbide.

[0025] Figure 7 (a) is a diagram showing the outline of the test apparatus for the high-temperature wear test. Figure 7 (b) is Figure 7 Enlarged view of the vicinity of the test piece in (a).

[0026] Figure 8 (a) is a graph showing the relationship between the amount of MnS crystallization and the amount of wear at 300 degrees Celsius. Figure 8 (b) is a graph showing the relationship between the amount of MnS crystallization and the amount of wear at 900 degrees.

[0027] Figure 9 It is a graph showing the relationship between the amount of eutectic carbide crystallization and the amount of wear at 300 degrees or 900 degrees.

[0028] Figure 10 (a) is a graph showing the relationship between the amount of Si added at 300 degrees or 900 degrees and the wear of the flat plate test piece. Figure 10 (b) is a graph showing the relationship between the amount of Si added at 300 degrees or 900 degrees and the amount of wear on the cylindrical test piece.

[0029] Figure 11 It is a graph showing the relationship between the amount of Si added at 300 degrees or 900 degrees and the total wear of the flat plate and cylinder.

[0030] Figure 12 This is a graph showing the relationship between the amount of MnS crystals and its corrosion resistance.

[0031] Figure 13 This is a graph showing the relationship between the amount of Si added and Vickers hardness. Detailed Implementation

[0032] One aspect of the austenitic stainless steel cast steel disclosed herein contains 21-28 wt% Cr, 14-23 wt% Ni, 2.0-3.5 wt% W, 1.0-3.0 wt% Nb, 1.5-3.5 wt% Mo, 0-3.5 wt% Cu, 0.6-1.1 wt% C, 0.1-0.7 wt% N, 1.0-2.0 wt% Mn, 0.4-0.7 wt% S, and 2.5-4.0 wt% Si, and the remainder contains Fe.

[0033] According to this austenitic stainless steel cast steel, good wear resistance can be obtained through eutectic carbides caused by crystallized MnS and Si.

[0034] The Si content in the aforementioned austenitic stainless steel cast steel can also exceed 3.5% by weight and be less than 4.0% by weight. Good wear resistance can also be obtained with this composition.

[0035] The sulfur content in the aforementioned austenitic stainless steel cast steel can also exceed 0.5% by weight but be less than 0.7% by weight. Good wear resistance can also be obtained with this composition.

[0036] Another method of determining the composition of austenitic stainless steel cast steel according to this disclosure includes the following steps: obtaining the volume fraction of eutectic carbides by using the amount of Si added; obtaining the volume fraction of crystalline MnS by using the amounts of Mn and S added; and obtaining a predicted wear amount by using the volume fraction of eutectic carbides and the volume fraction of MnS.

[0037] According to this method, the wear amount of components made of austenitic stainless steel cast steel can be predicted based on the addition amounts of Si, Mn, and S. Therefore, by repeatedly setting the addition amounts of Si, Mn, and S and predicting the wear amount derived from these addition amounts, it is possible to obtain the addition amounts of Si, Mn, and S that meet the desired target wear amount.

[0038] The following is a reference to the appendix. Figure 1 The austenitic stainless steel cast steel of this embodiment and the method for determining the composition of the austenitic stainless steel cast steel will be described in detail. In the description of the drawings, the same reference numerals are used to denote morphological elements and repeated descriptions are omitted.

[0039] <Austenitic Stainless Steel Cast Steel>

[0040] The composition of the austenitic stainless steel cast steel of this embodiment will be described in detail below. Furthermore, in the following description, the austenitic stainless steel cast steel will be simply referred to as "stainless steel cast steel". Unless otherwise specified, the addition amount (%) of each element is by weight. The stainless steel cast steel of this embodiment contains 21-28 wt% Cr, 14-23 wt% Ni, 2.0-3.5 wt% W, 1.0-3.0 wt% Nb, 1.5-3.5 wt% Mo, 0-3.5 wt% Cu, 0.6-1.1 wt% C, 0.1-0.7 wt% N, 1.0-2.0 wt% Mn, 0.4-0.7 wt% S, and 2.5-4.0 wt% Si, with Fe as the remainder.

[0041] [Cr: Chromium]

[0042] Cr is the main element S contained in stainless steel cast steel. The amount of Cr added to stainless steel cast steel is second only to the amount of Fe. Cr affects the corrosion resistance, oxidation resistance, and workability of stainless steel cast steel. In this embodiment, the amount of Cr added to the stainless steel cast steel exceeds 21% by weight and is less than 28% by weight. The range of Cr added to the stainless steel cast steel in this embodiment has a portion exceeding the range of Cr added to stainless steel cast steel specified in Japanese Industrial Standard (JIS G 5121). Specifically, the amount of Cr added to the stainless steel cast steel in this embodiment can also exceed 27% by weight and is less than 28% by weight.

[0043] [Ni: Nickel]

[0044] Ni is the main element S contained in stainless steel cast steel. Ni affects the stability of the austenitic structure and the castability of the cast steel. In this embodiment, the amount of Ni added to the stainless steel cast steel is more than 14% by weight and less than 23% by weight. The range of Ni added to the stainless steel cast steel in this embodiment has a portion that is greater than the range of Ni added to stainless steel cast steel specified by Japanese Industrial Standard (JIS G 5121). Specifically, the amount of Ni added to the stainless steel cast steel in this embodiment may also be more than 22% by weight and less than 23% by weight.

[0045] [W: tungsten, Mo: molybdenum]

[0046] Both W and Mo affect the strength of stainless steel cast steel under high-temperature conditions. Furthermore, both W and Mo can potentially degrade the oxidation resistance of stainless steel cast steel. In this embodiment, the amount of W added to the stainless steel cast steel exceeds 2.0% by weight and is less than 3.5% by weight. Additionally, the amount of Mo added to the stainless steel cast steel in this embodiment exceeds 1.5% by weight and is less than 3.5% by weight. Moreover, the range of Ni added to the stainless steel cast steel in this embodiment exceeds the range of Mo added to stainless steel cast steel (3% by weight or less) specified in Japanese Industrial Standard (JIS G 5121). Specifically, the amount of Mo added to the stainless steel cast steel in this embodiment can also exceed 3% by weight and be less than 3.5% by weight.

[0047] [Nb: Niobium]

[0048] Nitrogen (Nb) affects the high-temperature strength and thermal fatigue life of stainless steel cast steel. Furthermore, Nb can sometimes also affect the oxidation resistance, machinability, and castability of stainless steel cast steel. In this embodiment, the amount of Nb added to the stainless steel cast steel exceeds 1.0% by weight and is less than 3.0% by weight.

[0049] [Cu: Copper]

[0050] Cu affects the high-temperature strength, ductility, and toughness of stainless steel cast steel. In this embodiment, the amount of Cu added to the stainless steel cast steel is less than 3.5% by weight.

[0051] [C: Carbon]

[0052] C affects the high-temperature strength of stainless steel cast steel. Additionally, C helps to suppress embrittlement and reduce ductility in stainless steel cast steel. In this embodiment, the amount of C added to the stainless steel cast steel exceeds 0.6% by weight and is less than 1.1% by weight. Furthermore, the range of C added to the stainless steel cast steel in this embodiment is greater than the range of C added to stainless steel cast steel specified in Japanese Industrial Standard (JIS G 5121) (less than 0.24% by weight) in all ranges.

[0053] [N: Nitrogen]

[0054] Nitrogen (N) affects the high-temperature strength, ductility, and toughness of stainless steel cast steel. Additionally, N helps suppress embrittlement in stainless steel cast steel. In this embodiment, the amount of N added to the stainless steel cast steel exceeds 0.1% by weight and is less than 0.7% by weight.

[0055] [Mn: Manganese, S: Sulfur]

[0056] Mn and S affect the coefficient of friction of stainless steel cast steel. Specifically, Mn and S crystallize as MnS in stainless steel cast steel. This crystallized MnS reduces the coefficient of friction. As a result, Mn and S contribute to the improvement of the wear resistance of stainless steel cast steel. In this embodiment, the amount of Mn added to the stainless steel cast steel is more than 1.0% by weight and less than 2.0% by weight. Furthermore, the amount of S added to the stainless steel cast steel in this embodiment is more than 0.4% by weight and less than 0.7% by weight. Moreover, the range of S added to the stainless steel cast steel in this embodiment is greater than the range of S added to stainless steel cast steel (less than 0.04% by weight) specified in Japanese Industrial Standard (JIS G5121) in all ranges. Furthermore, the range of Si added to the stainless steel cast steel in this embodiment is also greater than 0.5% by weight and less than 0.7% by weight.

[0057] [Si: Silicon]

[0058] Si acts as a deoxidizer during casting. Furthermore, Si contributes to improved oxidation resistance. On the other hand, Si affects the stability of the austenitic structure. From this perspective, the amount of Si added to the stainless steel cast steel of this embodiment exceeds 2.5% by weight and is less than 4.0% by weight. Moreover, the range of Si added to the stainless steel cast steel of this embodiment is greater than the range of Si added to stainless steel cast steel (2.0% by weight or less) specified in Japanese Industrial Standard (JIS G 5121) in all ranges. Furthermore, the range of Si added to the stainless steel cast steel of this embodiment can also exceed 3.5% by weight and is less than 4.0% by weight.

[0059] [Fe]

[0060] Fe is the main component of stainless steel cast steel. The Fe content in stainless steel cast steel is the residue obtained by subtracting the total amount of the above-mentioned components from the total weight of the stainless steel cast steel.

[0061] <Methods for determining the composition of stainless steel cast steel based on wear resistance>

[0062] However, it has been described that crystallized MnS contributes to improved wear resistance of stainless steel cast steel. On the other hand, crystallized MnS can also become the starting point for pitting corrosion in stainless steel cast steel. Therefore, the amount of crystallized MnS is preferably the minimum amount that achieves the desired wear resistance. However, it is not yet clear what important factors affect the amount of crystallized MnS. For example, although it is a qualitative fact that the amount of crystallized MnS increases with increasing S content, there is no method for quantitatively predicting the amount of crystallized MnS.

[0063] Furthermore, it was predicted that, besides crystallized MnS, other important factors affecting the wear resistance of stainless steel cast steel exist. The wear resistance of the stainless steel cast steel was quantitatively set, and an attempt was made to determine the composition of the stainless steel cast steel to achieve the set target value. In this case, even if a qualitative target such as "to improve wear resistance, the amount of crystallized MnS needs to be increased, therefore the amount of S added should be increased" is set, it is impossible to quantitatively set the amount of S added. Originally, it was unclear whether the composition of the stainless steel cast steel, with wear resistance as the target, could be achieved simply by setting the amounts of Mn and S added.

[0064] Therefore, the inventors of this application conducted several evaluations and careful studies, as described below, and the results show that the wear resistance of stainless steel cast steel can be quantitatively determined by incorporating the amount of Si added in addition to the amounts of Mn and S in the evaluation. According to the method discovered by the inventors for determining the composition of stainless steel cast steel with a focus on wear resistance, the amount of wear can be predicted based on the amounts of Mn, S, and Si added. Therefore, if an acceptable target wear amount is determined, the amounts of Mn, S, and Si added to achieve that target wear amount can be obtained through simple calculation.

[0065] like Figure 1 As shown, in the method for determining the composition of stainless steel cast steel, firstly, the predicted wear amount is treated as a function of the volume fraction of crystalline MnS and the volume fraction of eutectic carbides. Furthermore, the volume fraction of crystalline MnS is treated as a function of the amounts of Mn, S, and Si added, and the volume fraction of eutectic carbides is treated as a function of the amount of Si added. For example, the method for determining the composition of stainless steel cast steel can be as follows: Figure 2 The flowchart shown determines the amount of Si, Mn, and S added to meet the target wear rate.

[0066] First, a target wear amount (S1) is set. For example, the target wear amount can also be defined as the sum of the wear amount of the stainless steel cast steel base material in the embodiment and the wear amount of the object material that rubs against the base material.

[0067] Next, the addition amounts of Si, Mn, and S are set (S2).

[0068] Next, the volume fraction (S3) of the eutectic carbide is obtained. As shown in Equation (1), the volume fraction of the eutectic carbide can be treated as a function of the amount of Si added. The function can also be a quadratic function, for example.

[0069]

[0070] f Carbide Volume fraction (volume %) of eutectic carbides.

[0071] [Si]: Amount of Si added (by weight %).

[0072] Next, the volume fraction (S4) of the crystalline MnS is obtained. As shown in Equation (2), the volume fraction of the crystalline MnS can be treated as a function of the amounts of Mn, S and Si added.

[0073]

[0074] f MnS : Volume fraction of MnS.

[0075] [Mn]: Amount of MnS added (volume %).

[0076] [S]: Amount of S added (by weight %).

[0077] [Si]: Amount of Si added (by weight %).

[0078] Next, the predicted wear amount (S5) is obtained. As shown in Equation (3), the predicted wear amount can be treated as a function of the volume fraction of eutectic carbides and the volume fraction of crystalline MnS.

[0079]

[0080] Wear total : Predicted value of wear amount.

[0081] f Carbide Volume fraction (volume %) of eutectic carbides.

[0082] f MnS : Volume fraction of MnS (volume %).

[0083] Then, it is determined whether the predicted wear amount is below the target wear amount (S6). If the predicted wear amount is below the target wear amount (S6: Yes), the addition amounts of Si, Mn, and S set in step S2 are used. If the predicted wear amount is not below the target wear amount (S6: No), the process returns to step S2 and the addition amounts of Si, Mn, and S are reset again.

[0084] By repeatedly performing the above processes S1 to S6, the amount of Si, Mn, and S added can be set to meet the target wear amount.

[0085] <Effects>

[0086] In Patent Document 1 mentioned above, the improvement in wear resistance brought about by the addition of MnS is qualitatively described for a heat-resistant bearing material made of an austenitic cast iron alloy. However, a quantitative study on the improvement in wear resistance has not been conducted, and the optimal amount of MnS crystals, i.e., the optimal amounts of Mn and S added, is insufficient. Furthermore, in the bearing material described in Patent Document 1, the influence of Si on wear resistance has not been sufficiently studied, and the optimal amount of Si added is also insufficient.

[0087] Therefore, from the viewpoint of wear resistance, it is unclear whether the amount of each element added to the bearing material disclosed in Patent Document 1 is optimal (chemical composition range). Therefore, it is necessary to investigate the optimal chemical composition range and achieve further improvement in wear resistance.

[0088] The austenitic stainless steel cast steel of this embodiment contains 21-28% by weight Cr, 14-23% by weight Ni, 2.0-3.5% by weight W, 1.0-3.0% by weight Nb, 1.5-3.5% by weight Mo, 0-3.5% by weight Cu, 0.6-1.1% by weight C, 0.1-0.7% by weight N, 1.0-2.0% by weight Mn, 0.4-0.7% by weight S, and 2.5-4.0% by weight Si, and contains Fe as the remainder.

[0089] According to this austenitic stainless steel cast steel, good wear resistance can be obtained through eutectic carbides caused by crystallized MnS and Si.

[0090] The Si content in the aforementioned austenitic stainless steel cast steel can exceed 3.5% by weight and be less than 4.0% by weight, and the S content can exceed 0.5% by weight and be less than 0.7% by weight. Good wear resistance can also be obtained with this composition.

[0091] The method for determining the composition of austenitic stainless steel cast steel according to this embodiment includes the following steps: a step of obtaining the volume fraction of eutectic carbides by using the amount of Si added (S3); a step of obtaining the volume fraction of crystalline MnS by using the amount of Mn and S added (S4); and a step of obtaining the predicted wear amount by using the volume fraction of eutectic carbides and the volume fraction of MnS (S5).

[0092] According to this method, the wear amount of components made of austenitic stainless steel cast steel can be predicted based on the addition amounts of Si, Mn, and S. Therefore, by repeatedly setting the addition amounts of Si, Mn, and S and predicting the wear amount derived from these addition amounts, it is possible to obtain the addition amounts of Si, Mn, and S that meet the desired target wear amount.

[0093] Here, based on the above formulas (1), (2), and (3), it is possible to describe Figure 3 (a) and Figure 3 The contour plot shown in (b) shows the weight percentage of Si on the horizontal axis and the weight percentage of S on the vertical axis. Figure 3 (a) is a contour plot with Mn added at 1.0 wt%. Figure 3 (b) is a contour plot with Mn added at 2.0 wt%. In each plot, the wear amount (58.5 μm) in the material described in Patent Document 1 is also marked as a comparative example (G31, G32). Figure 3 (a) and Figure 3 The relationship between the areas H1~H10, which are separated by shading in (b), and the amount of wear is as follows.

[0094] H1: The region where the predicted wear is 90-100 micrometers.

[0095] H2: The region where the predicted wear is 80-90 micrometers.

[0096] H3: The region where the predicted wear is 70-80 micrometers.

[0097] H4: The area with a predicted wear of 60-70 micrometers.

[0098] H5: The area where the predicted wear is 50-60 micrometers.

[0099] H6: The region where the predicted wear is 40-50 micrometers.

[0100] H7: The region where the predicted wear is 30-40 micrometers.

[0101] H8: The area where the predicted wear is 20-30 micrometers.

[0102] H9: The area where the predicted wear is 10-20 micrometers.

[0103] H10: The region where the predicted wear is 0 to 10 micrometers.

[0104] Depend on Figure 3 (a) and Figure 3 As shown in (b), when the amount of S added is less than 0.7 wt% and the amount of Si added is in the range of 0 to 5.0 wt%, the predicted value of sliding wear is the minimum when the amount of Si added is around 3.0 wt%.

[0105] For example, it is known that when the amount of Si added exceeds 2.5 wt% and is less than 4.0 wt%, the amount of S added exceeds 0.4 wt% and is less than 0.7 wt%, thus yielding the predicted wear amount shown in regions B31 and C31. Furthermore, it can be read that by further limiting the range of S added (exceeding 0.5 wt% and less than 0.7 wt%), as shown in regions B32 and C32, the predicted wear amount can be further reduced.

[0106] The present disclosure is further illustrated in detail by way of the following Examples 1 to 7, Reference Examples 1 to 12 and Comparative Example 1, but the present disclosure is not limited to these examples.

[0107] Figure 4 The figures represent the composition of the stainless steel cast steel samples of Examples 1-7, Reference Examples 1-12, and Comparative Example 1. The stainless steel cast steel of Examples 1-7 consists of components included in the numerical ranges shown in the claims.

[0108] In the stainless steel cast steel of Comparative Example 1, the amounts of Mn, S and Si added are constituted by the components illustrated in the aforementioned Patent Document 1.

[0109] <Evaluation 1: Amount of S added and volume fraction of MnS after crystallization>

[0110] Figure 5 This is a graph showing the relationship between the amount of S added and the volume fraction of MnS after crystallization. The horizontal axis represents the mass percentage concentration of S. The vertical axis represents the normalized volume percentage of MnS. Here, the normalized volume percentage of MnS refers to the volume obtained by normalizing the values ​​of the examples and reference examples with the volume percentage of MnS in Comparative Example 1 (labeled C1) as 1.

[0111] In this evaluation 1, under the condition that the amount of Si added was set to a certain value (3.25 mass percentage concentration), the volume fraction of MnS after crystallization was confirmed as a function of the amount of S added.

[0112] Curve G4 is an approximate curve for all markings. It shows that the volume fraction of crystallized MnS increases with increasing S content. Therefore, the volume fraction of crystallized MnS is related to the amount of S added. Thus, it can be confirmed that under the conditions of Si addition of 3.25 wt% and Mn addition of 0.85 wt% or more and 2.0 wt% or less, the volume fraction of crystallized MnS can be expressed as a function of the amount of S added.

[0113] <Evaluation 2: Volume fraction of eutectic carbides crystallized with respect to the amount of S added>

[0114] Under the same conditions as in Evaluation 1, the relationship between the amount of S added and the volume fraction of the crystalline eutectic carbide was determined using the same method as in Evaluation 1. The results showed that no significant change in the volume fraction of the crystalline eutectic carbide was observed with increasing S addition.

[0115] <Evaluation 3: Relationship between the amount of Mn added and the volume fraction of MnS after crystallization>

[0116] Under the same conditions as in Evaluation 1, the relationship between the amount of Mn added and the volume fraction of crystallized MnS was determined using the same method. The results showed that the volume fraction of crystallized MnS did not change significantly with increasing Mn addition.

[0117] <Evaluation 4: Relationship between Mn addition amount and volume fraction of eutectic carbide in crystallization>

[0118] Under the same conditions as in Evaluation 1, the relationship between the amount of Mn added and the volume fraction of the crystalline eutectic carbide was determined using the same method. As a result, no significant change in the volume fraction of the crystalline eutectic carbide was found with increasing Mn addition (refer to...). Figure 5 (marked as M1, M2, M3).

[0119] <Evaluation 5: Relationship between the amount of Si added and the area ratio of MnS after crystallization>

[0120] Figure 6 (a) shows the relationship between the amount of Si added and the area ratio of MnS after crystallization. The horizontal axis represents the weight percentage of Si. The vertical axis represents the normalized area ratio of MnS. Here, the normalized area ratio of MnS refers to the area ratio obtained by normalizing the values ​​of the examples and reference examples with the area ratio of MnS in Comparative Example 1 (labeled C1) being 1. Figure 6 (a) shows that the area ratio of MnS after crystallization decreases with increasing Si addition.

[0121] <Evaluation 6: Relationship between the amount of Si added and the area ratio of the eutectic carbide in crystallization>

[0122] Figure 6 (b) is a graph showing the relationship between the amount of Si added and the area ratio of the crystalline eutectic carbide. The horizontal axis represents the weight percentage of Si. The vertical axis represents the normalized area ratio of the eutectic carbide. Here, the normalized area ratio of the eutectic carbide refers to the area ratio obtained by normalizing the values ​​of the examples and reference examples using the area ratio of the eutectic carbide of Comparative Example 1 (labeled C1) as 1. Figure 6 (b) confirms that the area ratio of the eutectic carbide increases with the increase of the amount of Si added.

[0123] Figure 7 (a) and Figure 7 (b) is a schematic diagram showing the test apparatus 1 used for high-temperature wear testing. First, a flat test piece 2 with a width of 35 mm × depth of 12 mm × thickness of 6 mm was prepared for each embodiment and each reference example. Additionally, a cylindrical test piece 3 with a diameter of 12 mm and a height of 20 mm was prepared as the material for the flat test piece 2. The cylindrical test piece 3 was made of Inconel 718 nickel alloy.

[0124] Both the flat test piece 2 and the cylindrical test piece 3 are placed in a furnace 4 heated to 300 degrees or 900 degrees. In this furnace 4, the cylindrical test piece 3 is pressed vertically against the flat test piece 2 under a certain load, while the flat test piece 2 is slid back and forth horizontally. An example of the conditions is as follows.

[0125] Load: 11N

[0126] Number of round trips: 1000

[0127] Sliding speed: 3.6 mm / sec

[0128] Stroke width: 10mm

[0129] <Evaluation 7: High-Temperature Wear Test / Relationship between Volumetric Ratio of Crystallized MnS and Wear Amount>

[0130] Figure 8 (a) and Figure 8 (b) is a graph showing the relationship between the volume fraction of MnS after crystallization and the amount of wear. Figure 8 (a) shows the volume fraction and wear of MnS at 300 degrees. Figure 8 (b) shows the volume fraction and wear amount of MnS at 900 degrees. The horizontal axis represents the volume percentage of MnS. The vertical axis represents the normalized average wear depth. The average wear depth is the sum of the average wear depths of the flat test piece 2 and the cylindrical test piece 3. The normalized average wear depth refers to the average wear depth obtained by normalizing the values ​​of the examples and reference examples with the average wear depth of Comparative Example 1 (marked C1) as 1. In Evaluation 7, the amount of Si added was set to 3.25% by weight.

[0131] according to Figure 8 (a) It can be confirmed that in Example 1 (marked M1), which is within the range of components shown in the claims, the amount of wear is less compared to Comparative Example 1 (marked C1), which is outside the range of components shown in the claims. That is, it can be confirmed that the stainless steel cast steel of the embodiment improves wear resistance. In addition, it is known that the amount of wear at 300 degrees tends to decrease with increasing MnS addition.

[0132] Figure 8 (b) is a graph showing the relationship between the volume fraction of MnS after crystallization at 900 degrees Celsius and the amount of wear. It can be confirmed that even at 900 degrees Celsius, in Example 1 (marked M1), which falls within the range of components shown in the claims, the amount of wear is less compared to Comparative Example 1 (marked C1), which falls outside the range of components shown in the claims. That is, it can be confirmed that the stainless steel cast steel of the embodiments improves wear resistance. On the other hand, unlike the case at 300 degrees Celsius, the amount of MnS added at 900 degrees Celsius does not significantly contribute to the amount of wear.

[0133] Figure 9 It is a graph showing the relationship between the amount of eutectic carbide crystallization and the amount of wear at 300 degrees or 900 degrees. Figure 9All the notations recorded are evaluation results of test pieces with an Mn addition of 1.5 wt% and an S addition of 0.35 to 0.40 wt%. The horizontal axis represents the weight % of eutectic carbides. The vertical axis represents the standardized average wear depth. That is, in the vertical axis, the average wear depth of each test piece at 300 degrees is standardized such that the average wear depth of Comparative Example 1 at 300 degrees (marked C1) is 1. The average wear depth of each test piece at 900 degrees is standardized such that the average wear depth of Comparative Example 1 at 900 degrees (marked C1) is 1.

[0134] It can be seen that the average wear depth of Examples 4 and 7 (marked M4 and M7) at 300 degrees Celsius is less than that of Comparative Example 1 (marked C1). Furthermore, it can be seen that the average wear depth of Examples 4 and 7 (marked M4 and M7) at 900 degrees Celsius is less than that of Comparative Example 1 (marked C1). Therefore, it can be confirmed that at any temperature between 300 degrees Celsius and 900 degrees Celsius, Examples 4 and 7 exhibit superior wear resistance compared to Comparative Example 1.

[0135] As shown in graph G81, at 300 degrees Celsius, the average wear depth decreases with increasing eutectic carbide content. This confirms that eutectic carbides contribute to improved wear resistance at 300 degrees Celsius. Conversely, as shown in graph G82, at 900 degrees Celsius, the average wear depth increases with increasing eutectic carbide content. This confirms that at 900 degrees Celsius, eutectic carbides may not contribute to improved wear resistance. Therefore, the volume fraction of crystalline eutectic carbides is related to the amount of wear. Furthermore, considering that the contribution of eutectic carbides to wear resistance varies with temperature, the preferred volume fraction of eutectic carbides is approximately 20-30% by volume.

[0136] <Evaluation 8: High-Temperature Wear Test / Relationship between Si Addition Amount and Wear Amount of Flat Plate Test Piece 2 (Part 1)>

[0137] In evaluation 8, the relationship between the amount of Si added and the wear amount of the flat plate test piece 2 and the relationship between the amount of Si added and the wear amount of the cylindrical test piece 3 were confirmed. Figure 10 (a) shows the relationship between the amount of Si added and the amount of wear on the flat plate test piece 2. Figure 10Figure (b) shows the relationship between the amount of Si added and the amount of wear on the cylindrical test piece 3. In Evaluation 8, the amount of Mn added was set to 1.5% by weight, and the amount of S added was set to 0.35~0.40% by weight. The horizontal axis represents the weight percentage of Si. The vertical axis represents the normalized average wear depth of the flat test piece 2. Specifically, the average wear depth of each test piece at 300 degrees was obtained by normalizing the values ​​of the examples and reference examples by taking the average wear depth of Comparative Example 1 at 300 degrees (marked C1) as 1. In addition, in each figure, the circular mark indicates the result at 300 degrees. The diamond mark indicates the result at 900 degrees.

[0138] Figure 10 (a) is the result of test piece 2, which is a flat plate formed from stainless steel cast steel, thus indicating the ease of wear of the stainless steel cast steel itself. According to Figure 10 As shown in (a), the average wear depth of each of Examples 4-7 at 300 degrees Celsius is less than that of Comparative Example 1 (marked C1). Furthermore, it is known that the average wear depth of each of Examples 4-7 at 300 degrees Celsius is less than that of Comparative Example 1 (marked C1). Additionally, it is known that at 300 degrees Celsius, the average wear depth of the plate test piece 2 decreases with increasing Si content. On the other hand, it is known that at 900 degrees Celsius, the average wear depth of the plate test piece 2 increases with increasing Si content.

[0139] Figure 10 (b) is the result of cylindrical test piece 3 made of materials other than stainless steel cast steel, thus indicating the ease with which stainless steel cast steel is susceptible to wear on the target material. Figure 10 As shown in (b), the average wear depth of the cylindrical test piece 3 (the object material) at 300 degrees Celsius is less than that of Comparative Example 1 (marked C1). Furthermore, it is shown that the average wear depth of Examples 4-7 at 900 degrees Celsius is less than that of Comparative Example 1 (marked C1). Additionally, it is shown that at 300 degrees Celsius, the average wear depth of the cylindrical test piece 3 decreases with increasing Si content. It is also shown that at 900 degrees Celsius, the average wear depth of the cylindrical test piece 3 hardly changes with increasing Si content.

[0140] The following can be confirmed in evaluation 8.

[0141] At 300 degrees, with the increase of Si addition, the wear of both the flat plate test piece 2 (austenitic cast steel) and the cylindrical test piece 3 (object material) decreased.

[0142] At 900 degrees, the wear of the flat plate test piece 2 (austenitic cast steel) showed an increasing trend, while the wear of the cylindrical test piece 3 (the material in question) decreased.

[0143] <Evaluation 9: High-Temperature Wear Test / Relationship between Si Addition Amount and Wear Amount of Flat Plate Test Piece 2 (Part 2)>

[0144] In Evaluation 8, the wear amount of the flat plate test piece 2 and the cylindrical test piece 3 was evaluated separately. In Evaluation 9, the wear amount of the flat plate test piece 2 and the cylindrical test piece 3 was evaluated comprehensively.

[0145] Figure 11 This indicates the relationship between the amount of Si added and the total wear amount of the flat test piece 2 and the cylindrical test piece 3. The horizontal axis represents the weight percentage of silicon. The vertical axis represents the standardized total value of the average wear depth of the flat test piece 2 and the cylindrical test piece 3 (total average wear depth). Specifically, the total average wear depth (marked C1) at each temperature (300 degrees or 900 degrees) of Comparative Example 1 was set to 1 for the examples and reference examples.

[0146] Depend on Figure 11 It can be seen that at 300 degrees Celsius, the total average wear depth (marked M4a~7a) of each of Examples 4~7 is less than the total average wear depth (marked C1a) of Comparative Example 1. In addition, it can be seen that at 900 degrees Celsius, the total average wear depth (marked M4b~M7b) of each of Examples 4~7 is also less than the total average wear depth (marked C1b) of Comparative Example 1.

[0147] Therefore, it can be seen that under the two conditions of 300 degrees or 900 degrees, the total average wear depth of each of Examples 4 to 7 is less than the average wear depth of Comparative Example 1.

[0148] Furthermore, it can be seen that the total average wear depth of Examples 4 to 7 at 300 degrees Celsius decreases with increasing Si content (refer to graph G101). On the other hand, the total average wear depth of Examples 4 to 7 at 900 degrees Celsius increases with increasing Si content (refer to graph G102).

[0149] As described above, based on evaluations 1 to 10, it can be confirmed that the austenitic cast steel of the embodiment can improve wear resistance compared to the comparative example.

[0150] Furthermore, the austenitic cast steel of the implementation method was evaluated from a viewpoint different from wear resistance.

[0151] <Evaluation 11: Corrosion Resistance Test>

[0152] As mentioned above, it is known that MnS contributes to improved wear resistance, but it can also be the starting point for pitting corrosion. Therefore, the corrosion resistance of the austenitic cast steel of the embodiment was evaluated.

[0153] In the corrosion resistance test, an anodic polarization test was performed after the spontaneous potential measurement. In the spontaneous potential measurement, the test pieces were first immersed in a 5 wt% (by weight) NaCl aqueous solution, and the spontaneous potential of each test piece was measured for 48 hours. The spontaneous potential measurement was conducted at 60°C under open atmospheric conditions. Next, the same test pieces were used for the anodic polarization test. The anodic polarization test was conducted with a potential scan rate set to 20 mV.

[0154] exist Figure 12 The results of the corrosion resistance test are shown. The horizontal axis represents the volume percentage of MnS, and the vertical axis represents the current density during the anodic polarization test, which is not less than 100 μA / cm. 2 The potential (V). That is, the vertical axis represents the current density exceeding and not less than 100 μA / cm² when the potential is gradually increased during the anodic polarization test. 2 The higher the potential, the better the corrosion resistance.

[0155] It can be seen that the potential indicated by mark M1 in Example 1 is approximately equal to the potential indicated by marks R4 to R7 in Reference Examples 4-7 and mark C1 in Comparative Example 1. Therefore, it can be confirmed that the corrosion resistance of the test piece in Example 1 is not significantly different from that of the test piece in Comparative Example 1. That is, generally speaking, MnS in stainless steel is known to be the starting point of pitting corrosion, but no significant correlation was found between the amount of MnS crystallization and corrosion resistance. Therefore, it can be confirmed that at least within the range of 0.5% to 6.5% by volume of MnS, there is no significant reduction in corrosion resistance.

[0156] <Evaluation 12: Relationship between Si addition amount and Vickers hardness>

[0157] Evaluation 12 evaluated the relationship between the amount of Si added and Vickers hardness. Figure 13 This is a graph showing the relationship between the amount of Si added and Vickers hardness. The horizontal axis represents the weight percentage of Si. The vertical axis represents the normalized Vickers hardness. Here, the normalized Vickers hardness on the vertical axis refers to the Vickers hardness obtained by normalizing the examples and reference examples with the volume percentage of MnS in Comparative Example 1 (marked C1) as 1.

[0158] according to Figure 13 No change was observed in Vickers hardness when the Si addition amount ranged from 0% to 3.0% by weight. However, when the Si addition amount exceeded 3.0% by weight, the Vickers hardness increased. A significant increase in Vickers hardness was observed, particularly at a Si addition amount of 7.0% by weight. This increase in Vickers hardness with increasing Si addition can be attributed to the increase in eutectic carbides with increasing Si addition. Therefore, it can be concluded that when the Si addition amount is 2.5% to 4.0% by weight, no decrease in Vickers hardness was observed with increasing Si addition.

[0159] <Variation Example>

[0160] The above provides examples of austenitic stainless steel cast steel and methods for determining the composition of austenitic stainless steel cast steel. However, austenitic stainless steel cast steel and methods for determining its composition are not limited to the examples described above and can be implemented in various ways.

[0161] The austenitic stainless steel cast steel of this embodiment can also be used as a material for components such as the exhaust bypass valve assembled in a turbocharger.

[0162] The exhaust gas bypass valve of a vehicle's turbocharger regulates the flow of exhaust gas supplied to the turbine by diverting a portion of the exhaust gas flowing within the turbocharger. Therefore, the exhaust gas bypass valve is exposed to the exhaust gas. Sometimes, the exhaust gas contains corrosive components. If the exhaust gas bypass valve is exposed to the exhaust gas for an extended period, these corrosive components can cause corrosion of the components that make up the valve. This corrosion can impair the function of the exhaust gas bypass valve. For example, if the clearance of the shaft and bearings supporting the valve is lost due to corrosion, the shaft and bearings may become stuck.

[0163] The austenitic stainless steel cast steel of this embodiment is suitable as a material for bearings that rotatably support valves. According to the exhaust bypass valve equipped with components using the austenitic stainless steel cast steel of this embodiment, wear resistance to friction generated in the bearings during valve opening and closing as the turbine operates can be improved.

[0164] Explanation of reference numerals in the attached figures

[0165] 1... Test apparatus; 2... Flat plate test piece; 3... Cylindrical test piece; 4... Furnace.

Claims

1. An austenitic stainless steel cast steel characterized by, containing 21 to 28% by weight of Cr, 14 to 23% by weight of Ni, 2.0 to 3.5% by weight of W, 1.0 to 3.0% by weight of Nb, 1.5 to 3.5% by weight of Mo, 0 to 3.5% by weight of Cu, 0.6 to 1.1% by weight of C, 0.1 to 0.7% by weight of N, 1.0 to 2.0% by weight of Mn, 0.4 to 0.7% by weight of S, and 2.5 to 4.0% by weight of Si, and containing Fe as a remainder.

2. The austenitic stainless steel cast steel according to claim 1, characterized by, the content of Si is more than 3.5% by weight and 4.0% by weight or less.

3. The austenitic stainless steel cast steel according to claim 1 or 2, characterized by, the content of S is more than 0.5% by weight and 0.7% by weight or less.

4. A method of determining the composition of an austenitic stainless steel cast steel, characterized in that, having the following steps: a step of obtaining a volume fraction of eutectic carbide using an added amount of Si; a step of obtaining a volume fraction of crystallized MnS using added amounts of Mn and S; and a step of obtaining a predicted abrasion amount using the volume fraction of eutectic carbide and the volume fraction of MnS.

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