Austenitic cast steel and castings

Abstract

To provide an austenitic cast steel that has excellent high-temperature strength, oxidation resistance, and machinability, and to provide a casting using the same.SOLUTION: An austenitic cast steel contains 0.2≤C≤0.6 mass%, 0.1≤Si≤2.0 mass%, 0.5≤Mn≤2.0 mass%, 0.04≤S≤0.20 mass%, 21.0≤Ni≤45.0 mass%, 21.0≤Cr≤30.0 mass%, 0.5≤Nb≤3.0 mass%, 0.3≤Al≤0.8 mass%, 0.05≤N≤0.20 mass%, 0≤Mo≤2.0 mass%, and 0≤W≤5.0 mass%, and the balance is Fe and unavoidable impurities. A casting is made of an austenitic cast steel having such a composition.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an austenitic cast steel and a casting, and more particularly to an austenitic cast steel having excellent high-temperature strength, oxidation resistance, and machinability, and a casting using the same. [Background technology]

[0002] In response to recent environmental regulations, designs that increase the exhaust gas temperature in order to improve the thermal efficiency of the engine are becoming mainstream for gasoline turbocharged vehicles. In recent years, there has been a need for exhaust gas temperatures exceeding 1050°C. Ferritic cast steel or austenitic cast steel is primarily used as the housing material for turbochargers for gasoline vehicles, but in response to the recent need for higher temperatures, austenitic cast steel, which has better oxidation resistance and high-temperature strength than ferritic cast steel, is becoming mainstream.

[0003] Austenitic cast steels currently available on the market include DIN standard materials 1.4837, 1.4848, 1.4849, etc. However, even 1.4849, which is said to have the best heat resistance among these existing materials, is concerned about its lack of oxidation resistance and thermal fatigue strength when used at around 1050°C.

[0004] In order to solve this problem, various proposals have been made in the past. For example, Patent Document 1 discloses a high-Cr, high-Ni austenitic heat-resistant cast steel containing, by weight, 0.2-1.0% C, 3% or less Si, 2% or less Mn, 0.5% or less S, 15-30% Cr, 6-30% Ni, 0.5-6% W and / or Mo, 0.5-5% Nb, 0.01-0.5% N, 0.23% or less Al, and 0.07% O, with the balance being Fe and unavoidable impurities. The document states that if the Al content exceeds 0.23%, the high-temperature yield strength, high-temperature tensile strength, oxidation resistance, room-temperature elongation, and machinability decrease.

[0005] Patent Document 2 discloses an austenitic heat-resistant cast steel containing, by mass, 0.4-0.55% C, 1-2% Si, 0.5-1.5% Mn, 18-27% Cr, 8-22% Ni, 1.5-2.5% Nb, 0.01-0.3% N, 0.1-0.2% S, and 0.1-0.15% Al, with the balance being Fe and unavoidable impurities, and satisfying a predetermined relational expression. The document states that if the Al content exceeds 0.15%, casting defects are promoted and machinability, high-temperature strength and ductility are reduced.

[0006] Furthermore, Patent Document 3 discloses a heat-resistant cast steel containing, by mass%, 0.2-1.0% C, 8.0-45.0% Ni, 15.0-30.0% Cr, 10% or less W, and 0.5-3.0% Nb, with the remainder being Fe and unavoidable impurities, and satisfying [C-0.13Nb]: 0.05-0.95%. The document describes that heat-resistant cast steel having such a composition is excellent in heat resistance and thermal fatigue resistance.

[0007] The cast steels described in Patent Documents 1 to 3 all have improved high-temperature strength and thermal fatigue properties by containing large amounts of Cr and Ni and adding several elements selected from Nb, Mo, and W. Furthermore, these cast steels ensure oxidation resistance at high temperatures by adding a relatively large amount of Cr. However, according to research by the present inventors, it has become clear that it is difficult to ensure oxidation resistance at temperatures above 1050° C. simply by adding a large amount of Cr. This is because Cr oxides volatilize and become unstable at temperatures above 1050° C.

[0008] On the other hand, increasing the amount of Ni improves the oxidation resistance in the range above 1050°C. In addition, increasing the amount of Ni reduces the thermal expansion coefficient, which improves the thermal fatigue properties. However, increasing the amount of Ni leads to a decrease in high-temperature strength. In particular, conventional cast steels containing a large amount of Ni cannot be used for components used in temperatures above 1050°C because they lack tensile strength at temperatures above 1000°C. Furthermore, conventional austenitic cast steels are alloys with poor machinability. In particular, austenitic cast steels containing large amounts of alloying elements such as Cr and Ni have problems in that they cause large amounts of tool wear during cutting, and require excessive costs and time for machining. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2005 / 103314 [Patent Document 2] International Publication No. 2013 / 168770 [Patent Document 3] JP 2006-118048 A Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to provide an austenitic cast steel having excellent high-temperature strength, oxidation resistance, and machinability. Another object of the present invention is to provide a casting using such an austenitic cast steel. [Means for solving the problem]

[0011] In order to solve the above problems, the austenitic cast steel according to the present invention comprises: 0.2≦C≦0.6mass%, 0.1≦Si≦2.0mass%, 0.5≦Mn≦2.0mass%, 0.04≦S≦0.20mass%, 21.0≦Ni≦45.0mass%, 21.0≦Cr≦30.0mass%, 0.5≦Nb≦3.0mass%, 0.3≦Al≦0.8mass%, 0.05≦N≦0.20mass%, 0≦Mo≦2.0mass%, and 0≦W≦5.0mass% The remainder consists of Fe and unavoidable impurities. Moreover, a casting according to the present invention is made of the austenitic cast steel according to the present invention. Effect of the Invention

[0012] Adding a certain amount of Cr, Nb, Mo, and W to austenitic cast steel containing a relatively large amount of Ni can improve high-temperature strength while maintaining high oxidation resistance. However, when cast steel containing a large amount of Cr, etc. is cast in an environment where carburization occurs (for example, using a cold box mold), a high-hardness carburized layer is formed on the surface of the casting. As a result, machinability is reduced.

[0013] In contrast, when a certain amount of Al is added to austenitic cast steel, the deterioration of machinability is suppressed while maintaining high high-temperature strength and high oxidation resistance. This is believed to be because a coating mainly composed of aluminum oxide is formed on the surface of the casting during casting, which suppresses the diffusion of carbon from the outside to the inside of the casting. Furthermore, the oxidation resistance of the casting is further improved by adding a certain amount of Al. This is believed to be because a coating mainly composed of aluminum oxide is formed on the surface of the casting during use at high temperatures. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] An embodiment of the present invention will be described in detail below. [1. Austenitic cast steel] [1.1. Ingredients] The austenitic cast steel according to the present invention contains the following elements, with the balance being Fe and unavoidable impurities. The types of added elements, their component ranges, and the reasons for their limitations are as follows:

[0015] (1) 0.2≦C≦0.6mass%: C combines with Cr, Mo, Nb and / or W to form carbides. When carbides are formed, the high-temperature strength of the casting increases and the thermal expansion coefficient decreases. As a result, the thermal fatigue resistance of the casting improves. To obtain such effects, the C content must be 0.2 mass% or more. The C content is preferably 0.26 mass% or more, and more preferably 0.38 mass% or more. On the other hand, if the C content is excessive, the toughness and ductility of the casting may decrease, and the machinability may deteriorate. Therefore, the C content must be 0.6 mass% or less. The C content is preferably 0.50 mass% or less, and more preferably 0.45 mass% or less.

[0016] (2) 0.1≦Si≦2.0mass%: Silicon has the effect of improving the oxidation resistance of castings and the flowability of molten metal, as well as the effect of suppressing the formation of a carburized surface layer. To obtain these effects, the silicon content must be 0.1 mass% or more. The silicon content is preferably 0.5 mass% or more. On the other hand, if the Si content is excessive, the high temperature strength of the casting may decrease. Therefore, the Si content must be 2.0 mass% or less. The Si content is preferably 1.0 mass% or less.

[0017] (3) 0.5≦Mn≦2.0mass%: Mn acts as a deoxidizer. Also, Mn combines with S and Se to form inclusions that improve the machinability of castings. To obtain such effects, the Mn content must be 0.5 mass% or more. The Mn content is preferably 1.0 mass% or more. On the other hand, if the Mn content is excessive, the oxidation resistance of the casting may decrease. Therefore, the Mn content must be 2.0 mass% or less. The Mn content is preferably 1.5 mass% or less.

[0018] (4) 0.04≦S≦0.20mass%: S combines with Mn to form MnS, which improves the machinability of castings. To obtain this effect, the S content must be 0.04 mass% or more. On the other hand, if the S content is excessive, the toughness and ductility of the casting may be significantly reduced. Therefore, the S content must be 0.20 mass% or less. The S content is preferably 0.15 mass% or less, and more preferably 0.10 mass% or less.

[0019] (5) 21.0≦Ni≦45.0mass%: Ni is an element that stabilizes the austenite of the parent phase, and has the effect of increasing the heat resistance and oxidation resistance of the casting. Ni also has the effect of decreasing the thermal expansion coefficient of the casting. In order to obtain such an effect, the Ni content must be 21.0 mass% or more. The Ni content is preferably 23.0 mass% or more, and more preferably 25.0 mass% or more. On the other hand, if the Ni content is excessive, not only will the cost increase, but the machinability of the casting may decrease. Therefore, the Ni content must be 45.0 mass% or less. The Ni content is preferably 32.0 mass% or less, and more preferably 30.0 mass% or less.

[0020] (6) 21.0≦Cr≦30.0mass%: Cr is mainly bonded to C to form M 23It forms C6 type carbides, which helps to improve the high-temperature strength of the casting and reduce the thermal expansion coefficient. Cr in the matrix ensures the oxidation resistance of the casting and has the effect of increasing the heat resistance. To obtain such an effect, the Cr content must be 21.0 mass% or more. The Cr content is preferably 22.5 mass% or more, and more preferably 24.0 mass% or more. On the other hand, if the Cr content is excessive, the brittle σ phase may precipitate, which may reduce the thermal fatigue properties and oxidation resistance of the casting. Therefore, the Cr content must be 30.0 mass% or less. The Cr content is preferably 28.0 mass% or less, and more preferably 26.0 mass% or less.

[0021] (7) 0.5≦Nb≦3.0mass%: Nb combines with C to form MC type carbides, which helps to improve the high-temperature strength of the casting and reduce the thermal expansion coefficient. To obtain such effects, the Nb content must be 0.5 mass% or more. The Nb content is preferably 0.8 mass% or more. On the other hand, if the Nb content is excessive, it may reduce the toughness and ductility of the casting. It may also promote carburization, form coarse primary carbides, and deteriorate the machinability of the casting. Therefore, the Nb content must be 3.0 mass% or less. The Nb content is preferably 2.0 mass% or less, and more preferably 1.5 mass% or less.

[0022] (8) 0.3≦Al≦0.8mass%: Al forms a stable oxide film on the surface of the casting at high temperatures, improving the oxidation resistance of the casting. In addition, when casting is performed in an environment where carburization occurs (for example, when casting is performed using a cold box mold), Al has the effect of suppressing carburization of the casting from the outside and suppressing the deterioration of the machinability of the casting. To obtain such an effect, the Al content must be 0.3 mass% or more. On the other hand, an excessive Al content may deteriorate the weldability of the casting, so the Al content must be 0.8 mass% or less.

[0023] (9) 0.05≦N≦0.20mass%: N has the effect of stabilizing the austenite phase. In addition, N also acts to suppress the coarsening of carbides and the deterioration of the thermal fatigue resistance of castings. To obtain these effects, the N content must be 0.05 mass% or more. On the other hand, if the N content is excessive, nitrides are formed and the toughness and ductility of the casting may decrease. Therefore, the N content must be 0.20 mass% or less.

[0024] (10) 0≦Mo≦2.0mass%: Mo combines with C to form M 23 It forms C6 type carbides, which helps to improve the high temperature strength of the casting and reduce the thermal expansion coefficient. Mo contained in the matrix is ​​also very effective in reducing the thermal expansion coefficient of the casting. Therefore, Mo can be added as necessary. The Mo content is preferably 0.8 mass% or more. On the other hand, if the Mo content is excessive, not only will the cost increase, but the machinability and oxidation resistance of the casting may decrease. Therefore, the Mo content is preferably 2.0 mass% or less. The Mo content is preferably 1.2 mass% or less.

[0025] (11) 0≦W≦5.0mass%: W, like Mo, bonds with C to form M 23 It forms C6 type carbides, which helps to improve the high-temperature strength of the casting and reduce the thermal expansion coefficient. W contained in the matrix is ​​also very effective in reducing the thermal expansion coefficient of the casting. Therefore, W can be added as necessary. The W content is preferably 3.0 mass% or more. On the other hand, if the W content is excessive, not only will the cost increase, but the brittle μ phase will increase, and the thermal fatigue properties of the casting may deteriorate. Therefore, the W content is preferably 5.0 mass% or less.

[0026] (12) Inevitable impurities: In the present invention, the unavoidable impurities include, for example, P, Ti, Cu, etc. The lower the content of these unavoidable impurities, the better.

[0027] Among these, P is a component that contributes to machinability, but if the P content is excessive, the oxidation resistance and toughness / ductility of the casting may be significantly reduced. Therefore, the P content is preferably 0.1 mass% or less, and more preferably 0.05 mass% or less.

[0028] Ti is sometimes added to cast steel to refine the crystal grains. However, Ti has a high affinity with C and N. Therefore, adding excessive Ti may promote carburization, form carbides and nitrides, and deteriorate the machinability of the casting. Therefore, the Ti content is preferably 0.05 mass% or less.

[0029] Cu is an element that is inevitably mixed in from the raw materials. If the Cu content is excessive, the casting may become embrittled. Therefore, the Cu content is preferably 0.1 mass% or less.

[0030] [1.2. Characteristics] [1.2.1. Oxidation Increase] "Oxidation weight gain" refers to the weight increase per unit area when a continuous oxidation test is carried out in air at 1,050°C for 100 hours on a cast steel piece measuring 25 mm x 15 mm x 3 mm. In the austenitic cast steel according to the present invention, when the contents of each element are optimized, the oxidation weight gain is 3.5 g / cm 2 If the content of each element is further optimized, the oxidation weight gain is 3.2 g / cm 2 The result is as follows.

[0031] 1.2.2 Tensile strength "Tensile strength" refers to a value measured in accordance with JIS G 0567. In the austenitic cast steel according to the present invention, when the content of each element is optimized, the tensile strength at 1000° C. is 100 MPa or more. When the content of each element is further optimized, the tensile strength at 1000° C. is 115 MPa or more.

[0032] [1.2.3. Tool life] "Tool life" refers to the total machining time until the wear on the flank of the tool reaches 0.2 mm when dry milling is performed without using cutting fluid using a cemented carbide tool on a casting made of austenitic cast steel under the conditions of cutting speed: 80 mm / min, feed: 0.15 mm / tooth, and cutting depth: 2.0 mm. When the content of each element in the austenitic cast steel according to the present invention is optimized, the tool life is 15 minutes or more even when cutting is performed on a casting cast in an environment where carburization occurs. When the content of each element is further optimized, the tool life is 25 minutes or more.

[0033] 2. Castings [2.1. Materials] The casting according to the present invention is made of the austenitic cast steel according to the present invention. Details of the austenitic cast steel are as described above, and therefore will not be described here.

[0034] [2.2. Shape] In the present invention, the shape of the casting is not particularly limited. Examples of castings produced using the austenitic cast steel according to the present invention include housings of turbochargers for automobiles, exhaust manifolds of engines and their joints, and hearth rolls for continuous annealing furnaces.

[0035] [2.3. Characteristics] [2.3.1. Oxidation Increase] Since the casting according to the present invention is manufactured using the austenitic cast steel according to the present invention, the oxidation weight gain after the continuous oxidation test is small. Details of the oxidation weight gain are as described above, and therefore will not be described here.

[0036] 2.3.2 Tensile strength Since the casting according to the present invention is manufactured using the austenitic cast steel according to the present invention, it has a relatively high tensile strength at 1000° C. Details of the tensile strength at 1000° C. are as described above, and therefore will not be described here.

[0037] [2.2.3. Tool life] Since the casting according to the present invention is manufactured using the austenitic cast steel according to the present invention, it has a long tool life. Details of the tool life are as described above, and therefore will not be described here.

[0038] [2.2.4. Average carbon concentration in the surface layer] Generally, casting can be performed in an environment where carburization does not occur, or in an environment where carburization does occur. For example, when casting is performed using a green mold or sintered mold, the possibility of carburization occurring during casting is low. However, while green molds are inexpensive to manufacture, the dimensional accuracy of the casting is low. Meanwhile, sintered molds are expensive to manufacture, but the dimensional accuracy of the casting is high.

[0039] On the other hand, a cold box mold is a mold in which casting sand is solidified with an adhesive. Therefore, by using a cold box mold, it is possible to manufacture castings with high dimensional accuracy at low cost. However, when casting is performed using a cold box mold, the casting may be carburized by carbon components derived from the adhesive. When austenitic cast steel is cast in an environment where such carburization occurs, a carburized layer is formed on the surface of the casting, and the machinability of the casting may be significantly reduced.

[0040] In contrast, the austenitic cast steel according to the present invention contains relatively large amounts of elements necessary for achieving high high-temperature strength and high oxidation resistance, but the deterioration of the machinability of the casting is suppressed even when the casting is cast in an environment in which carburization occurs. This is believed to be because the carburization resistance of the casting is improved and the formation of a carburized layer during casting is suppressed by further adding a specified amount of Al to the austenitic cast steel having a specified composition.

[0041] When a casting is produced using the austenitic cast steel according to the present invention, by optimizing its components, a casting that includes an as-cast surface and satisfies formula (1) can be obtained even when the casting is produced in an environment in which carburization occurs. The "as-cast surface" includes not only the surface when removed from the mold, but also the surface after removal from the mold and cleaning treatment such as shot blasting or sand blasting. [C i ]<[C s ]≦1.0mass% …(1) however, [C s is the average carbon content in the region from the surface of the as-cast steel to a depth of 0.2 mm, [C i ] is the average carbon content in the center of the casting. The "center of the casting" refers to the area that is not affected by carburization.

[0042] [3. Effect] Adding a certain amount of Cr, Nb, Mo, and W to austenitic cast steel containing a relatively large amount of Ni can improve high-temperature strength while maintaining high oxidation resistance. However, when cast steel containing a large amount of Cr, etc. is cast in an environment where carburization occurs (for example, using a cold box mold), a high-hardness carburized layer is formed on the surface of the casting. As a result, machinability is reduced.

[0043] In contrast, when a certain amount of Al is added to austenitic cast steel, the deterioration of machinability is suppressed while maintaining high high-temperature strength and high oxidation resistance. This is believed to be because a coating mainly composed of aluminum oxide is formed on the surface of the casting during casting, which suppresses the diffusion of carbon from the outside to the inside of the casting. Furthermore, the oxidation resistance of the casting is further improved by adding a certain amount of Al. This is believed to be because a coating mainly composed of aluminum oxide is formed on the surface of the casting during use at high temperatures. EXAMPLES

[0044] (Examples 1 to 8, Comparative Examples 1 to 9) [1. Sample preparation] In a high-frequency induction furnace, cast steel having the composition shown in Table 1 was melted. The resulting molten metal was cast into a cold box mold to obtain a JIS standard Y-type B test material.

[0045] [Table 1]

[0046] 2. Test Method [2.1. High temperature tensile test] Tensile test pieces with a gauge length of 40 mm and a diameter of 8 mm were cut out from the resulting castings. Using the resulting tensile test pieces, a high-temperature tensile test was performed at 1000° C. in accordance with JIS G 0567.

[0047] [2.2. Oxidation Increase] Test pieces measuring 25mm x 15mm x 3mm were cut out from the resulting castings. These were then heated in air at 1050℃ for 100h. The oxidation weight gain was calculated from the weight change before and after the test and the surface area of ​​the test pieces.

[0048] [2.3. Average carbon concentration in the surface layer] The average carbon concentration in the surface layer (region from the casting surface to a depth of 0.2 mm) was measured using an electron probe microanalyzer (EPMA).

[0049] [2.4. Tool life] The obtained castings were milled using a cemented carbide tool under the conditions of cutting speed: 80 mm / min, feed rate: 0.15 mm / tooth, and cutting depth: 2.0 mm. The cutting was performed until the wear on the flank of the tool reached 0.2 mm, and the total machining time was calculated.

[0050] [3. Results] The results are shown in Table 2. From Table 2, the following can be seen. (1) Comparative Examples 1 and 2 have low high-temperature tensile strength, large oxidation mass gain, and high average carbon concentration in the surface layer. This is believed to be due to the small amount of Ni. (2) Comparative Example 3 has low high-temperature strength. This is believed to be due to the small amount of Cr.

[0051] (3) Comparative Examples 4 and 5 have a large oxidation mass gain and a high average carbon concentration in the surface layer. Comparative Example 5 also has a short tool life. This is thought to be due to the absence of Al and the fact that Ti does not provide the same effect as Al. (4) In Comparative Examples 6 and 7, the average carbon concentration in the surface layer was high and the tool life was short. This is believed to be due to the absence of Al. (5) In Comparative Example 8, the average carbon concentration in the surface layer was high and the tool life was short. This is believed to be due to the excessive Cr content.

[0052] (6) Comparative Example 9 had a high average carbon concentration in the surface layer and a short tool life. This is believed to be due to the absence of Al. (7) In all of Examples 1 to 8, the high-temperature tensile strength was high, the oxidation mass gain was small, the average carbon concentration in the surface layer was low, and the tool life was long. This is believed to be because the alloys contain elements necessary for ensuring high-temperature strength and oxidation resistance, and contain an appropriate amount of Al.

[0053] [Table 2]

[0054] Although the embodiment of the present invention has been described in detail above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the gist of the present invention. [Industrial Applicability]

[0055] The austenitic cast steel according to the present invention can be used as a material for automobile turbocharger housings, engine exhaust manifolds and their connecting parts, as well as hearth rolls for continuous annealing furnaces, and the like.

Claims

1. 0.2≦C≦0.6mass%, 0.1≦Si≦2.0mass%, 0.5≦Mn≦2.0mass%, 0.04≦S≦0.20mass%, 21.0≦Ni≦45.0mass%, 21.0≦Cr≦30.0mass%, 0.5≦Nb≦3.0mass%, 0.3≦Al≦0.8 mass%, 0.05≦N≦0.20mass%, 0≦Mo≦2.0 mass%, and 0≦W≦5.0mass% and the balance being Fe and unavoidable impurities.

2. The oxidation mass increase in a continuous oxidation test in air at 1050°C for 100 hours was 3.5 g / cm 2 The austenitic cast steel according to claim 1, wherein:

3. 3. The austenitic cast steel according to claim 1, having a tensile strength at 1000° C. of 100 MPa or more.

4. A casting made of the austenitic cast steel according to any one of claims 1 to 3.

5. 5. The casting according to claim 4, comprising an as-cast surface, the casting satisfying the following formula (1): [C i ]<[C s ]≦1.0mass% …(1) however, [C s is the average carbon content in the region from the surface to a depth of 0.2 mm in the as-cast state, [C i ] is the average carbon content in the center of the casting.

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