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

Abstract

This austenitic stainless cast steel 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, the remainder being Fe.

Description

Austenitic stainless steel casts and method for determining the composition of austenitic stainless steel casts

[0001] The present disclosure relates to cast austenitic stainless steel and a method for determining the composition of cast austenitic stainless steel.

[0002] Austenitic stainless cast steel is used as a material for various machine parts. Patent Documents 1 to 12 disclose techniques related to austenitic heat-resistant cast steel. For example, austenitic stainless cast steel is sometimes used as a material for components that constitute vehicle superchargers. As one example, austenitic stainless cast steel is used as a material for bearings of wastegate valves.

[0003] German Patent Application Publication No. 102012203569, Japanese Patent Publication No. 2015-514865, German Patent Application Publication No. 102006029121, Japanese Patent No. 5165679, International Publication No. 2005 / 103314, Japanese Patent No. 4985941, Chinese Patent Application Publication No. 111004981, Chinese Patent Application Publication No. 116057187, Japanese Patent Application Laid-Open No. 2022-85613, Chinese Patent Application Publication No. 113862562, Chinese Patent Application Publication No. 114008230, Japanese Patent No. 7269590

[0004] For example, a wastegate valve incorporated in a turbocharger performs an opening and closing operation. This opening and closing operation involves the phenomenon of parts rubbing against each other. Therefore, wear resistance is a key consideration for materials used for such rubbing parts. For example, Patent Documents 1 and 2 disclose techniques related to austenitic iron matrix alloys that focus on wear resistance. According to the techniques disclosed in Patent Documents 1 and 2, manganese sulfide (MnS) is crystallized during the solidification process during casting. As a result, the friction coefficient is reduced.

[0005] In the technical field, there is a demand for further improvement in the wear resistance of austenitic stainless cast steel. Therefore, the present disclosure provides austenitic stainless cast steel and a method for determining the composition of austenitic stainless cast steel that can achieve improved wear resistance.

[0006] An austenitic stainless cast steel according to one embodiment of the present disclosure contains 21 to 28 wt % Cr, 14 to 23 wt % Ni, 2.0 to 3.5 wt % W, 1.0 to 3.0 wt % Nb, 1.5 to 3.5 wt % Mo, 0 to 3.5 wt % Cu, 0.6 to 1.1 wt % C, 0.1 to 0.7 wt % N, 1.0 to 2.0 wt % Mn, 0.4 to 0.7 wt % S, 2.5 to 4.0 wt % Si, and the balance being Fe.

[0007] According to the present disclosure, there are provided austenitic stainless cast steel and a method for determining the composition of austenitic stainless cast steel that can improve wear resistance.

[0008] FIG. 1 is a diagram showing the concept of a method for predicting wear loss. FIG. 2 is a flowchart showing the main steps of a method for determining the composition of austenitic stainless cast steel. FIG. 3 is a contour diagram of predicted values ​​of sliding wear loss using a sliding wear loss prediction formula. FIG. 3(a) is a contour diagram when the Mn addition amount is 1.0 wt %, and FIG. 3(b) is a contour diagram when the Mn addition amount is 2.0 wt %. FIG. 4 is a table showing the composition of each test piece of the Examples, Reference Examples, and Comparative Examples. FIG. 5 is a graph showing the relationship between the amount of S added and the amount of MnS crystallization. FIG. 6(a) is a graph showing the relationship between the amount of Si added and the area ratio of MnS. FIG. 6(b) is a graph showing the relationship between the amount of Si added and the area ratio of eutectic carbides. FIG. 7(a) is a diagram showing an outline of the testing equipment used in the high-temperature wear test, and FIG. 7(b) is an enlarged view of the test piece of FIG. 7(a) and its vicinity. FIG. 8(a) is a graph showing the relationship between the amount of MnS crystallized and the wear amount at 300 degrees, and FIG. 8(b) is a graph showing the relationship between the amount of MnS crystallized and the wear amount at 900 degrees. FIG. 9 is a graph showing the relationship between the amount of eutectic carbide crystallized and the wear amount at 300 degrees or 900 degrees. FIG. 10(a) is a graph showing the relationship between the amount of Si added and the wear amount of a flat test piece at 300 degrees or 900 degrees. FIG. 10(b) is a graph showing the relationship between the amount of Si added and the wear amount of a cylindrical test piece at 300 degrees or 900 degrees. FIG. 11 is a graph showing the relationship between the amount of Si added and the total wear amount of a flat test piece and a cylindrical test piece at 300 degrees or 900 degrees. FIG. 12 is a graph showing the relationship between the amount of MnS crystallized and corrosion resistance. FIG. 13 is a graph showing the relationship between the amount of Si added and Vickers hardness.

[0009] An austenitic stainless cast steel according to one embodiment of the present disclosure contains 21 to 28 wt % Cr, 14 to 23 wt % Ni, 2.0 to 3.5 wt % W, 1.0 to 3.0 wt % Nb, 1.5 to 3.5 wt % Mo, 0 to 3.5 wt % Cu, 0.6 to 1.1 wt % C, 0.1 to 0.7 wt % N, 1.0 to 2.0 wt % Mn, 0.4 to 0.7 wt % S, 2.5 to 4.0 wt % Si, and the balance being Fe.

[0010] This austenitic stainless cast steel can provide good wear resistance due to the eutectic carbides resulting from the crystallized MnS and Si.

[0011] The Si content of the above austenitic stainless cast steel may be more than 3.5% by weight and not more than 4.0% by weight, and this composition also provides good wear resistance.

[0012] The austenitic stainless cast steel may contain more than 0.5% by weight and not more than 0.7% by weight of S. This composition also provides good wear resistance.

[0013] A method for determining the composition of austenitic stainless cast steel, which is another embodiment of the present disclosure, includes the steps of obtaining the volume fraction of eutectic carbide using the amount of Si added, obtaining the volume fraction of crystallized MnS using the amounts of Mn and S added, and obtaining a predicted wear amount using the volume fraction of eutectic carbide and the volume fraction of MnS.

[0014] According to this method, it is possible to predict the wear amount of a member made of austenitic stainless cast steel from the added amounts of Si, Mn, and S. Therefore, by repeatedly setting the added amounts of Si, Mn, and S and predicting the wear amount derived from these added amounts, it is possible to obtain the added amounts of Si, Mn, and S that satisfy the desired target wear amount.

[0015] Hereinafter, an austenitic stainless cast steel and a method for determining the composition of an austenitic stainless cast steel according to this embodiment will be described in detail with reference to the accompanying drawings. In the description of the drawings, the same elements are given the same reference numerals, and duplicated explanations will be omitted.

[0016] <Austenitic Stainless Cast Steel> The composition of the austenitic stainless cast steel according to this embodiment will be described in detail below. In the following description, the austenitic stainless cast steel may be simply referred to as "stainless cast steel." The amount (%) of each element added is based on weight unless otherwise specified. The stainless cast steel according to this embodiment contains 21 to 28 wt % Cr, 14 to 23 wt % Ni, 2.0 to 3.5 wt % W, 1.0 to 3.0 wt % Nb, 1.5 to 3.5 wt % Mo, 0 to 3.5 wt % Cu, 0.6 to 1.1 wt % C, 0.1 to 0.7 wt % N, 1.0 to 2.0 wt % Mn, 0.4 to 0.7 wt % S, 2.5 to 4.0 wt % Si, and the balance being Fe.

[0017] [Cr: Chromium] Cr is a major element contained in stainless steel cast. The amount of Cr added to stainless steel cast is second only to the amount of Fe added to stainless steel cast. Cr affects the corrosion resistance, oxidation resistance, and workability of stainless steel cast. The amount of Cr added to the stainless steel cast in this embodiment is greater than 21 wt% and not greater than 28 wt%. The range of the amount of Cr added to the stainless steel cast in this embodiment is greater than the range of the amount of Cr added to stainless steel specified in Japanese Industrial Standards (JIS G 5121). Specifically, the amount of Cr added to the stainless steel cast in this embodiment may be greater than 27 wt% and not greater than 28 wt%.

[0018] [Ni: Nickel] Ni is a major element contained in stainless cast steel. Ni affects the stability and castability of the austenitic structure of the cast steel. The amount of Ni added to the stainless cast steel of this embodiment is greater than 14 wt% and not more than 23 wt%. The range of the amount of Ni added to the stainless cast steel of this embodiment is greater than the range of the amount of Ni added to stainless cast steel specified in Japanese Industrial Standards (JIS G 5121). Specifically, the amount of Ni added to the stainless cast steel of this embodiment may be greater than 22 wt% and not more than 23 wt%.

[0019] [W: Tungsten, Mo: Molybdenum] Both W and Mo affect the strength of stainless steel in high-temperature environments. Furthermore, both W and Mo may degrade the oxidation resistance of stainless steel. The amount of W added to the stainless steel cast steel of this embodiment is greater than 2.0 wt% and less than 3.5 wt%. The amount of Mo added to the stainless steel cast steel of this embodiment is greater than 1.5 wt% and less than 3.5 wt%. The range of the amount of Ni added to the stainless steel cast steel of this embodiment is greater than the range of the amount of Mo added to stainless steel cast steel (3 wt% or less) specified in the Japanese Industrial Standards (JIS G 5121). Specifically, the amount of Mo added to the stainless steel cast steel of this embodiment may be greater than 3 wt% and less than 3.5 wt%.

[0020] [Nb: Niobium] Nb affects the high-temperature strength and thermal fatigue life of stainless cast steel. Furthermore, Nb may also affect the oxidation resistance, machinability, castability, etc. of stainless cast steel. The amount of Nb added to the stainless cast steel of this embodiment is more than 1.0 wt% and not more than 3.0 wt%.

[0021] [Cu: Copper] Cu affects the high-temperature strength, ductility, toughness, etc. of the cast stainless steel. The amount of Cu added to the cast stainless steel of this embodiment is 3.5 wt % or less.

[0022] [C: Carbon] Carbon affects the high-temperature strength of the cast stainless steel. Carbon also contributes to suppressing embrittlement and a decrease in ductility of the cast stainless steel. The amount of carbon added to the cast stainless steel of this embodiment is greater than 0.6 wt % and not more than 1.1 wt %. The range of carbon added to the cast stainless steel of this embodiment is higher than the range of carbon added to the cast stainless steel specified in the Japanese Industrial Standards (JIS G 5121) (not more than 0.24 wt %) in all ranges.

[0023] [N: Nitrogen] N affects the high-temperature strength, ductility, toughness, etc. of the cast stainless steel. N also contributes to suppressing embrittlement of the cast stainless steel. The amount of N added to the cast stainless steel of this embodiment is more than 0.1 wt % and 0.7 wt % or less.

[0024] [Mn: Manganese, S: Sulfur] Mn and S affect the friction coefficient of stainless cast steel. Specifically, Mn and S crystallize as MnS in stainless cast steel. This crystallized MnS reduces the friction coefficient. As a result, Mn and S contribute to improving the wear resistance of stainless cast steel. The amount of Mn added to the stainless cast steel of this embodiment is greater than 1.0 wt% and not more than 2.0 wt%. Furthermore, the amount of S added to the stainless cast steel of this embodiment is greater than 0.4 wt% and not more than 0.7 wt%. Note that the range of the amount of S added to the stainless cast steel of this embodiment is higher than the range of the amount of S added to stainless cast steel specified in the Japanese Industrial Standard (JIS G 5121) (not more than 0.04 wt%) in all ranges. Furthermore, the amount of Si added to the stainless cast steel of this embodiment may be greater than 0.5 wt% and not more than 0.7 wt%.

[0025] [Si: Silicon] Si acts as a deoxidizer during casting. Furthermore, Si contributes to improving 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 cast steel of this embodiment is greater than 2.5 wt.% and not greater than 4.0 wt.%. Note that the range of the amount of Si added to the stainless cast steel of this embodiment is greater than the range of the amount of Si added to stainless cast steel specified in the Japanese Industrial Standards (JIS G 5121) (not greater than 2.0 wt.%) in all ranges. Furthermore, the range of the amount of Si added to the stainless cast steel of this embodiment may be greater than 3.5 wt.% and not greater than 4.0 wt.%.

[0026] [Fe] Fe constitutes the main component of stainless cast steel. The content of Fe in the stainless cast steel may be the remainder obtained by subtracting the total amount of each of the above components from the total weight of the stainless cast steel.

[0027] <Method for Determining the Composition of Stainless Steel Casting Focusing on Wear Resistance> It has already been mentioned that crystallized MnS contributes to improving the wear resistance of stainless steel casting. On the other hand, crystallized MnS can also serve as the starting point for pitting corrosion in stainless steel casting. Therefore, it is desirable to crystallize the minimum amount of MnS necessary to achieve the desired wear resistance. However, it has not been clarified what factors affect the amount of crystallized MnS. For example, although it is qualitatively clear that the amount of crystallized MnS increases with increasing S addition, there has been no method for quantitatively predicting the amount of crystallized MnS.

[0028] Furthermore, it was predicted that there are other factors affecting the wear resistance of cast stainless steel besides the crystallized MnS. The wear resistance of cast stainless steel is quantitatively determined, and an attempt is made to determine the composition of the cast stainless steel that will achieve the set target value. In this case, even if a qualitative target could be set, such as "increasing the amount of S added because it is necessary to increase the amount of crystallized MnS to improve wear resistance," it was not possible to quantitatively determine the amount of S added. In fact, it was unclear whether the composition of cast stainless steel that would achieve the target wear resistance could be determined simply by setting the amounts of Mn and S added.

[0029] Therefore, the inventors of the present application conducted several evaluations, as described below, and conducted extensive research, and as a result, they found that the wear resistance of cast stainless steel can be quantitatively determined by evaluating the amount of Si added in addition to the amounts of Mn and S added. According to the method for determining the composition of cast stainless steel that focuses on wear resistance discovered by the inventors, the amount of wear can be predicted from the amounts of Mn, S, and Si added. Therefore, once an allowable target amount of wear is determined, the amounts of Mn, S, and Si added that will achieve that target amount of wear can be obtained by simple calculation.

[0030] As shown in Figure 1, in the method for determining the composition of stainless steel cast, the predicted wear amount is first treated as a function with the volume fraction of crystallized MnS and the volume fraction of eutectic carbide as variables. Furthermore, the volume fraction of crystallized MnS is treated as a function with the amounts of Mn, S, and Si added as variables, and the volume fraction of eutectic carbide is treated as a function with the amount of Si added as a variable. For example, the method for determining the composition of stainless steel cast can determine the amounts of Si, Mn, and S added that satisfy the target wear amount according to the flowchart shown in Figure 2.

[0031] First, a target wear amount is set (S1). For example, this target wear amount may be defined as the sum of the wear amount of the base material made of stainless cast steel according to the embodiment and the wear amount of the mating material that rubs against the base material.

[0032] Next, the amounts of Si, Mn, and S to be added are set (S2).

[0033] Next, the volume fraction of the eutectic carbide is obtained (S3). The volume fraction of the eutectic carbide can be treated as a function of the amount of Si added as a variable, as shown in formula (1). The function may be, for example, a quadratic function. Ｃａｒｂｉｄｅ =f([Si])…(1) f Ｃａｒｂｉｄｅ : Volume fraction of eutectic carbide (volume %). [Si]: Amount of Si added (weight %).

[0034] Next, the volume fraction of crystallized MnS is obtained (S4). The volume fraction of crystallized MnS can be treated as a function of the amounts of Mn, S, and Si added as variables, as shown in formula (2). ＭｎＳｌ =f([Mn],[S],[Si])…(2) f ＭｎＳ : volume fraction of MnS. [Mn]: amount of MnS added (volume %). [S]: amount of S added (wt %). [Si]: amount of Si added (wt %).

[0035] Next, the predicted wear amount is obtained (S5). The predicted value of the wear amount can be treated as a function of the volume fraction of the eutectic carbide and the volume fraction of the crystallized MnS as variables, as shown in Equation (3). ｔｏｔａｌ = f (f Ｃａｒｂｉｄｅ , f ＭｎＳ )…(3) Wear ｔｏｔａｌ f: predicted wear amount.Ｃａｒｂｉｄｅ f: Volume fraction of eutectic carbide (volume %) ＭｎＳ : Volume fraction of MnS (volume %).

[0036] Then, it is determined whether the predicted wear amount is equal to or less than the target wear amount (S6). If the predicted wear amount is equal to or less than the target wear amount (S6: YES), the amounts of Si, Mn, and S added set in step S2 are adopted. If the predicted wear amount is not equal to or less than the target wear amount (S6: NO), the process returns to step S2 again, and the amounts of Si, Mn, and S added are reset.

[0037] By repeating the above steps S1 to S6, the amounts of Si, Mn, and S to be added that satisfy the target wear amount can be determined.

[0038] <Effects> The above-mentioned Patent Document 1 qualitatively describes the effect of adding MnS to improve the wear resistance of a heat-resistant bearing material made of an austenitic iron cast alloy. However, it does not quantitatively examine the effect of improving the wear resistance, and there is insufficient consideration of the optimal amount of MnS crystallization, i.e., the optimal amounts of Mn and S added. Furthermore, with regard to the bearing material described in Patent Document 1, the effect of Si on wear resistance has not been fully examined, and there has also been insufficient consideration of the optimal amount of Si added.

[0039] For this reason, it was unclear whether the amounts of each element added to the bearing material disclosed in Patent Document 1 were optimal (ranges of chemical composition) from the standpoint of wear resistance, and therefore it was necessary to investigate the optimal ranges of chemical composition and further improve wear resistance.

[0040] The austenitic stainless cast steel according to this embodiment contains 21 to 28 wt% Cr, 14 to 23 wt% Ni, 2.0 to 3.5 wt% W, 1.0 to 3.0 wt% Nb, 1.5 to 3.5 wt% Mo, 0 to 3.5 wt% Cu, 0.6 to 1.1 wt% C, 0.1 to 0.7 wt% N, 1.0 to 2.0 wt% Mn, 0.4 to 0.7 wt% S, 2.5 to 4.0 wt% Si, and the remainder is Fe.

[0041] This austenitic stainless cast steel can provide good wear resistance due to the eutectic carbides resulting from the crystallized MnS and Si.

[0042] The austenitic stainless cast steel may contain more than 3.5% by weight and not more than 4.0% by weight of Si, and more than 0.5% by weight and not more than 0.7% by weight of S. This composition also provides good wear resistance.

[0043] The method for determining the composition of austenitic stainless cast steel according to this embodiment includes a step (S3) of obtaining the volume fraction of eutectic carbide using the amount of Si added, a step (S4) of obtaining the volume fraction of crystallized MnS using the amounts of Mn and S added, and a step (S5) of obtaining a predicted wear amount using the volume fraction of eutectic carbide and the volume fraction of MnS.

[0044] According to this method, it is possible to predict the wear amount of a member made of austenitic stainless cast steel from the added amounts of Si, Mn, and S. Therefore, by repeatedly setting the added amounts of Si, Mn, and S and predicting the wear amount derived from these added amounts, it is possible to obtain the added amounts of Si, Mn, and S that satisfy the desired target wear amount.

[0045] According to the above formulas (1), (2), and (3), the contour diagrams shown in Figures 3(a) and 3(b) can be plotted. The horizontal axis represents the weight percent of Si. The vertical axis represents the weight percent of S. Figure 3(a) is a contour diagram for a case where the Mn content is 1.0 weight percent. Figure 3(b) is a contour diagram for a case where the Mn content is 2.0 weight percent. Each diagram also shows the wear volume (58.5 μm) of the material described in Patent Document 1 as a comparative example (G31, G32). The relationship between the hatched regions H1 to H10 in Figures 3(a) and 3(b) and the wear volume is as follows: H1: Region where the predicted wear volume is 90 to 100 micrometers. H2: Region where the predicted wear volume is 80 to 90 micrometers. H3: Region where the predicted wear volume is 70 to 80 micrometers. H4: Region where the predicted wear volume is 60 to 70 micrometers. H5: Region where the predicted wear amount is 50 to 60 micrometers. H6: Region where the predicted wear amount is 40 to 50 micrometers. H7: Region where the predicted wear amount is 30 to 40 micrometers. H8: Region where the predicted wear amount is 20 to 30 micrometers. H9: Region where the predicted wear amount is 10 to 20 micrometers. H10: Region where the predicted wear amount is 0 to 10 micrometers.

[0046] 3(a) and 3(b) show that when the amount of S added is 0.7 wt % or less and the amount of Si added is in the range of 0 to 5.0 wt %, the predicted value of the amount of sliding wear reaches a minimum when the amount of Si added is around 3.0 wt %.

[0047] For example, when the amount of Si added is more than 2.5 wt% and not more than 4.0 wt%, the predicted wear amount shown in regions B31 and C31 can be obtained by setting the amount of S added to more than 0.4 wt% and not more than 0.7 wt%. Furthermore, it can be seen that by further limiting the range of the amount of S added (more than 0.5 wt% and not more than 0.7 wt%), it is possible to further reduce the predicted wear amount, as shown in regions B32 and C32.

[0048] The present disclosure will be explained in more detail below with reference to Examples 1 to 7, Reference Examples 1 to 12, and Comparative Example 1, but the present disclosure is not limited to these examples.

[0049] The table in Figure 4 shows the compositions of the stainless cast steel specimens of Examples 1 to 7, Reference Examples 1 to 12, and Comparative Example 1. The stainless cast steels of Examples 1 to 7 have compositions falling within the ranges of values ​​set forth in the claims. The stainless cast steel of Comparative Example 1 has a composition in which the amounts of Mn, S, and Si added are exemplified in Patent Document 1 mentioned above.

[0050] <Evaluation 1: Amount of S Added and Volume Fraction of Crystallized MnS> Figure 5 is a graph showing the relationship between the amount of S added and the volume fraction of crystallized MnS. The horizontal axis shows the mass percent concentration of S. The vertical axis shows the normalized volume % of MnS. Here, the normalized volume % of MnS is the value obtained by normalizing the values ​​of the Examples and Reference Examples, with the volume % of MnS in Comparative Example 1 (marker C1) set to 1.

[0051] In this evaluation 1, it was confirmed that the volume fraction of crystallized MnS was expressed as a function of the amount of S added under the condition that the amount of Si added was kept constant (3.25 mass percent concentration).

[0052] Curve G4 is an approximation curve for all markers. It was found that the volume fraction of crystallized MnS increased as the amount of added S increased. This indicates that the volume fraction of crystallized MnS correlates with the amount of added S. Therefore, it was confirmed that under the conditions where the amount of added Si is 3.25 wt.% and the amount of added Mn is 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 added S.

[0053] <Evaluation 2: Amount of S Added and Volume Fraction of Crystallized Eutectic Carbide> Under the same conditions as in Evaluation 1, the relationship between the amount of S added and the volume fraction of crystallized eutectic carbide was confirmed by the same method as in Evaluation 1. As a result, it was not found that the volume fraction of crystallized eutectic carbide changed significantly as the amount of S added increased.

[0054] <Evaluation 3: Relationship between the amount of added Mn and the volume fraction of crystallized MnS> Under the same conditions as in Evaluation 1, the relationship between the amount of added Mn and the volume fraction of crystallized MnS was confirmed by the same method as in Evaluation 1. As a result, it was not observed that the volume fraction of crystallized MnS changed significantly as the amount of added Mn increased.

[0055] <Evaluation 4: Relationship between the amount of added Mn and the volume fraction of crystallized eutectic carbide> Under the same conditions as in Evaluation 1, the relationship between the amount of added Mn and the volume fraction of crystallized eutectic carbide was confirmed by the same method as in Evaluation 1. As a result, it was not found that the volume fraction of crystallized eutectic carbide changed significantly as the amount of added Mn increased (see markers M1, M2, and M3 in Figure 5).

[0056] <Evaluation 5: Relationship between the amount of Si added and the area ratio of crystallized MnS> Figure 6(a) shows the relationship between the amount of Si added and the area ratio of crystallized MnS. The horizontal axis represents the weight percent of Si. The vertical axis represents the normalized area ratio of MnS. Here, the normalized area ratio of MnS is the value obtained by normalizing the values ​​of the Examples and Reference Examples, with the area ratio of MnS in Comparative Example 1 (marker C1) set to 1. From Figure 6(a), it was confirmed that the area ratio of crystallized MnS decreased as the amount of Si added increased.

[0057] <Evaluation 6: Relationship between the amount of added Si and the area ratio of crystallized eutectic carbide> Figure 6(b) is a graph showing the relationship between the amount of added Si and the area ratio of crystallized eutectic carbide. The horizontal axis represents the weight percent of Si. The vertical axis represents the normalized area ratio of eutectic carbide. Here, the normalized area ratio of eutectic carbide is the value obtained by normalizing the values ​​of the examples and reference examples, with the area ratio of eutectic carbide in Comparative Example 1 (marker C1) set to 1. It was confirmed from Figure 6(b) that the area ratio of eutectic carbide increases as the amount of added Si increases.

[0058] 7(a) and 7(b) are diagrams showing an outline of the test apparatus 1 used in the high-temperature wear test. First, for each Example and Reference Example, a flat test piece 2 measuring 35 mm wide x 12 mm deep x 6 mm thick was prepared. Furthermore, a cylindrical test piece 3 measuring 12 mm in diameter and 20 mm in height was prepared as the mating material for the flat test piece 2. The material of the cylindrical test piece 3 was Inconel 718.

[0059] The flat test piece 2 and the cylindrical test piece 3 were both placed in a furnace 4 heated to 300°C or 900°C. In this furnace 4, the cylindrical test piece 3 was pressed vertically against the flat test piece 2 with a constant load, while being slid back and forth horizontally on the flat test piece 2. An example of the conditions is as follows: Load: 11 N Number of reciprocations: 1000 times Sliding speed: 3.6 mm / sec Stroke width: 10 mm

[0060] <Evaluation 7: High-Temperature Wear Test / Relationship Between Volume Fraction of Crystallized MnS and Wear Volume> Figures 8(a) and 8(b) are graphs showing the relationship between the volume fraction of crystallized MnS and wear volume. Figure 8(a) shows the volume fraction of MnS and wear volume at 300 degrees. Figure 8(b) shows the volume fraction of MnS and wear volume at 900 degrees. The horizontal axis shows the volume % of MnS. The vertical axis shows 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 is the value obtained by normalizing the values ​​of the Examples and Reference Examples, with the average wear depth of Comparative Example 1 (Marker C1) set to 1. In Evaluation 7, the amount of Si added was 3.25 wt% in all cases.

[0061] 8(a), it was confirmed that Example 1 (marker M1), which has a composition within the range of the claimed composition, had a smaller wear amount than Comparative Example 1 (marker C1), which has a composition outside the range of the claimed composition. In other words, it was confirmed that the stainless cast steel of the embodiment has improved wear resistance. It was also found that the wear amount at 300°C tended to decrease as the amount of MnS added increased.

[0062] 8(b) is a graph showing the relationship between the volume fraction of crystallized MnS and the wear loss at 900°C. It was confirmed that even at 900°C, Example 1 (marker M1), which is within the composition range set forth in the claims, had a smaller wear loss than Comparative Example 1 (marker C1), which is outside the composition range set forth in the claims. In other words, it was confirmed that the stainless steel cast steel of the embodiment has improved wear resistance. On the other hand, unlike the case of 300°C, the contribution of the amount of MnS added to the wear loss at 900°C was not significant.

[0063] FIG. 9 is a graph showing the relationship between the amount of eutectic carbide crystallized and the wear amount at 300°C or 900°C. All of the markers in FIG. 9 are evaluation results for test specimens with an added amount of Mn of 1.5 wt % and an added amount of S of 0.35 to 0.40 wt %. The horizontal axis shows the weight percent of eutectic carbide. The vertical axis shows the normalized average wear depth. That is, on the vertical axis, the average wear depth of each test specimen at 300°C was normalized so that the average wear depth of Comparative Example 1 at 300°C (marker C1) was 1. Each average wear depth at 900°C was normalized so that the average wear depth of Comparative Example 1 at 900°C (marker C1) was 1.

[0064] It was found that the average wear depths of Examples 4 and 7 (markers M4 and M7) at 300°C were both smaller than that of Comparative Example 1 (marker C1). It was also found that the average wear depths of Examples 4 and 7 (markers M4 and M7) at 900°C were both smaller than that of Comparative Example 1 (marker C1). Therefore, it was confirmed that Examples 4 and 7 had superior wear resistance compared to Comparative Example 1 at both 300°C and 900°C.

[0065] As shown in graph G81, at 300 degrees, the average wear depth tended to decrease as the amount of eutectic carbide increased. In other words, it was confirmed that at 300 degrees, eutectic carbide contributes to improving wear resistance. On the other hand, as shown in graph G82, at 900 degrees, the average wear depth tended to increase as the amount of eutectic carbide increased. In other words, it was confirmed that at 900 degrees, eutectic carbide may not contribute to improving wear resistance. This shows that there is a correlation between the volume fraction of crystallized eutectic carbide and the amount of wear. Furthermore, considering that the contribution of eutectic carbide to wear resistance varies depending on the temperature, it is thought that a volume fraction of eutectic carbide of approximately 20 to 30 volume % is preferable.

[0066] <Evaluation 8: High-Temperature Wear Test / Relationship Between the Amount of Si Added and the Wear Volume of the Flat Test Specimen 2, Part 1> In Evaluation 8, the relationship between the amount of Si added and the wear volume of the flat test specimen 2 and the relationship between the amount of Si added and the wear volume of the cylindrical test specimen 3 were confirmed. FIG. 10(a) shows the relationship between the amount of Si added and the wear volume of the flat test specimen 2. FIG. 10(b) shows the relationship between the amount of Si added and the wear volume of the cylindrical test specimen 3. In Evaluation 8, the amount of Mn added was 1.5 wt %, and the amount of S added was 0.35 to 0.40 wt %. The horizontal axis shows the weight percent of Si. The vertical axis shows the normalized average wear depth of the flat test specimen 2. Specifically, the average wear depth of each test specimen at 300 degrees is normalized by setting the average wear depth at 300 degrees of Comparative Example 1 (marker C1) as 1, and the values ​​of the Examples and Reference Examples are normalized. In each figure, the circular markers indicate the results at 300 degrees. The diamond markers indicate the results at 900 degrees.

[0067] Figure 10(a) shows the results for the flat test specimen 2 made of cast stainless steel, and therefore indicates the susceptibility to wear of the cast stainless steel itself. Figure 10(a) shows that the average wear depth of the flat test specimen 2 at 300 degrees for each of Examples 4 to 7 was smaller than that of Comparative Example 1 (marker C1). It was also found that the average wear depth of each of Examples 4 to 7 at 300 degrees was smaller than that of Comparative Example 1 (marker C1). It was also found that at 300 degrees, the average wear depth of the flat test specimen 2 decreased with increasing amounts of Si added. On the other hand, it was found that at 900 degrees, the average wear depth of the flat test specimen 2 increased with increasing amounts of Si added.

[0068] Figure 10(b) shows the results for cylindrical test specimen 3 formed from a material other than stainless cast steel, and therefore indicates the ease with which stainless cast steel causes wear to the mating material. Figure 10(b) shows that the average wear depth of cylindrical test specimen 3 (mating material) at 300°C was smaller than that of Comparative Example 1 (marker C1). Furthermore, the average wear depth of each of Examples 4 to 7 at 900°C was smaller than that of Comparative Example 1 (marker C1). Furthermore, it was found that at 300°C, the average wear depth of cylindrical test specimen 3 decreased with increasing amounts of Si added. At 900°C, it was found that the average wear depth of cylindrical test specimen 3 hardly changed with increasing amounts of Si added.

[0069] The following was confirmed in Evaluation 8: At 300°C, the wear amount of both the flat test piece 2 (austenitic cast steel) and the cylindrical test piece 3 (mating material) decreased as the amount of Si added increased. At 900°C, the wear amount of the flat test piece 2 (austenitic cast steel) tended to increase, but the wear amount of the cylindrical test piece 3 (mating material) decreased.

[0070] <Evaluation 9: High-Temperature Wear Test / Relationship Between the Amount of Si Added and the Wear Amount of Flat Test Specimen 2, Part 2> In Evaluation 8, the wear amounts of the flat test specimen 2 and the cylindrical test specimen 3 were evaluated. In Evaluation 9, the wear amounts of the flat test specimen 2 and the cylindrical test specimen 3 were evaluated comprehensively.

[0071] 11 shows the relationship between the amount of Si added and the total value of the wear loss of the flat test specimen 2 and the cylindrical test specimen 3. The horizontal axis represents the weight percent of silicon. The vertical axis represents the normalized total value (total average wear depth) of the average wear depths of the flat test specimen 2 and the cylindrical test specimen 3. Specifically, the total average wear depth (marker C1) at each temperature (300°C or 900°C) of Comparative Example 1 is set to 1, and the examples and reference examples are normalized.

[0072] 11, it was found that at 300 degrees, the total average wear depth (markers M4a to 7a) of each of Examples 4 to 7 was smaller than the total average wear depth (marker C1a) of Comparative Example 1. Furthermore, at 900 degrees, it was found that the total average wear depth (markers M4b to M7b) of each of Examples 4 to 7 was also smaller than the total average wear depth (marker C1b) of Comparative Example 1. Therefore, it was found that at both 300 degrees and 900 degrees, the total average wear depth of each of Examples 4 to 7 was smaller than the average wear depth of Comparative Example 1.

[0073] It was also found that the total average wear depth of each of Examples 4 to 7 at 300°C decreased with increasing amounts of Si added (see graph G101), while the total average wear depth of each of Examples 4 to 7 at 900°C increased with increasing amounts of Si added (see graph G102).

[0074] As described above, it was confirmed from the evaluations 1 to 10 that the austenitic cast steel according to the embodiment can improve the wear resistance compared to the comparative example.

[0075] Furthermore, the austenitic cast steel according to the embodiment was evaluated from a viewpoint other than wear resistance.

[0076] <Evaluation 11: Corrosion Resistance Test> As mentioned above, MnS contributes to improving wear resistance, but it is known that it can be the starting point of pitting corrosion. Therefore, the corrosion resistance of the austenitic cast steel according to the embodiment was evaluated.

[0077] In the corrosion resistance test, an anodic polarization test was conducted after measuring the natural potential. In the natural potential measurement, the test specimens were first immersed in a 5 wt% (weight percent) NaCl aqueous solution, and the natural potential of each test specimen was measured for 48 hours. The natural potential measurement was conducted at a temperature of 60°C in an open-air state. Subsequently, an anodic polarization test was conducted using the same test specimens. The anodic polarization test was conducted at a potential sweep rate of 20 mV.

[0078] Figure 12 shows the results of the corrosion resistance test. The horizontal axis shows the volume percentage of MnS, and the vertical axis shows the volume percentage of MnS at a current density of 100 μA / cm in the anodic polarization test. ２That is, the vertical axis represents the potential (V) at which the current density reaches 100 μA / cm when the potential is gradually increased in the anodic polarization test. ２ The higher this potential is, the better the corrosion resistance is.

[0079] It was found that the potential indicated by the marker M1 in Example 1 was almost equivalent to the potential indicated by the markers R4 to R7 in Reference Examples 4 to 7 and the marker C1 in Comparative Example 1. From this, it was confirmed that the corrosion resistance of the test specimen in Example 1 was not significantly different from the corrosion resistance of the test specimen in Comparative Example 1. That is, although it is generally known that MnS in stainless steels becomes the starting point of pitting corrosion, no clear correlation was observed between the amount of MnS crystallization and the corrosion resistance. Therefore, it was confirmed that at least when the volume percent of MnS was in the range of 0.5 to 6.5%, no significant decrease in corrosion resistance occurred.

[0080] <Evaluation 12: Relationship between the amount of Si added and Vickers hardness> In Evaluation 12, the relationship between the amount of Si added and Vickers hardness was evaluated. FIG. 13 is a graph showing the relationship between the amount of Si added and Vickers hardness. The horizontal axis represents the weight percent of Si. The vertical axis represents the normalized Vickers hardness. Here, the normalized Vickers hardness on the vertical axis is obtained by normalizing the examples and reference examples with the volume percent of MnS in Comparative Example 1 (marker C1) set to 1.

[0081] From Figure 13, it can be seen that the Vickers hardness hardly changed when the amount of Si added was in the range of 0 to 3.0 wt%. On the other hand, when the amount of Si added exceeded 3.0 wt%, the Vickers hardness increased. In particular, when the amount of Si added was 7.0 wt%, a significant increase in Vickers hardness was observed. It is believed that the increase in Vickers hardness with increasing Si addition is due to the increase in eutectic carbides that accompanies the increase in Si addition. From the above, it was found that when the amount of Si added was 2.5 to 4.0 wt%, no decrease in Vickers hardness was observed with increasing Si addition.

[0082] <Modifications> Examples of austenitic stainless cast steel and methods for determining the composition of austenitic stainless cast steel have been described above. The austenitic stainless cast steel and methods for determining the composition of austenitic stainless cast steel are not limited to the above examples and may be implemented in various forms.

[0083] The austenitic stainless cast steel of this embodiment may be used, for example, as a material for a wastegate valve part incorporated in a turbocharger.

[0084] 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, which may contain corrosive components. If the wastegate valve is exposed to exhaust gas for a long period of time, these corrosive components may cause corrosion of the components that make up the wastegate valve. Corrosion of the components may impair the function of the wastegate valve. For example, if corrosion causes the clearance between the shaft and bearing that supports the valve to disappear, the shaft and bearing may become stuck.

[0085] The austenitic stainless cast steel according to this embodiment can be suitably used as a material for bearings that rotatably support valves. A wastegate valve equipped with a component using the austenitic stainless cast steel according to this embodiment can improve the wear resistance of the bearings against friction that occurs when the valve opens and closes during turbo operation.

[0086] 1 Test equipment 2 Plate test piece 3 Cylindrical test piece 4 Furnace

Claims

1. Austenitic stainless cast steel containing 21 to 28 weight percent Cr, 14 to 23 weight percent Ni, 2.0 to 3.5 weight percent W, 1.0 to 3.0 weight percent Nb, 1.5 to 3.5 weight percent Mo, 0 to 3.5 weight percent Cu, 0.6 to 1.1 weight percent C, 0.1 to 0.7 weight percent N, 1.0 to 2.0 weight percent Mn, 0.4 to 0.7 weight percent S, 2.5 to 4.0 weight percent Si, and the balance being Fe.

2. The austenitic stainless cast steel according to claim 1, wherein the Si content is greater than 3.5% by weight and not more than 4.0% by weight.

3. Austenitic stainless cast steel according to claim 1 or 2, wherein the S content is more than 0.5% by weight and not more than 0.7% by weight.

4. A method for determining the composition of austenitic stainless cast steel, comprising the steps of: obtaining the volume fraction of eutectic carbide using the amount of Si added; obtaining the volume fraction of crystallized MnS using the amounts of Mn and S added; and obtaining a predicted wear amount using the volume fraction of the eutectic carbide and the volume fraction of MnS.

Images