Austenitic stainless steel cast and method for manufacturing austenitic stainless steel cast

Abstract

To provide an austenitic stainless cast steel that is low cost and has excellent heat resistance, and to provide a method for manufacturing the same.SOLUTION: An austenitic stainless cast steel of the present disclosure has, in a cross section when heated at 1000°C, an average number of pieces per unit area of carbide Nc of the center part of the austenite crystal grain having a circle equivalent diameter of 500 nm or more of 6.0×10-2 pieces / μm2 or more, and, when setting an average number of pieces per unit area of carbide near the grain boundary of the austenite crystal grain having a circle equivalent diameter of 500 nm or more as Ngb, Ngb / Nc is 1.3 or less.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to cast austenitic stainless steel and a method for producing cast austenitic stainless steel. [Background technology]

[0002] Turbochargers and gas turbines reach high temperatures during use, so the materials used in them must have excellent heat resistance, including oxidation resistance, high strength at high temperatures, and thermal fatigue properties.

[0003] Materials that satisfy the heat resistance requirements include austenitic stainless steel and Ni-based alloys. For example, Patent Document 1 discloses a gas turbine nozzle that is made of a casting containing Ni as the main component, Cr in an amount necessary for high-temperature corrosion resistance, and a carbide-forming element in an amount necessary for solid-solution strengthening, and that has a structure in which eutectic carbides and secondary carbides of a desired size are dispersed in the matrix. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 57-32348 Summary of the Invention [Problem to be solved by the invention]

[0005] However, the gas turbine nozzle disclosed in Patent Document 1 is made of an expensive Ni-based alloy, and lower-cost materials are needed. Also, in turbochargers, exhaust gas temperatures are currently on the rise in order to improve fuel economy, and higher temperature resistance than conventional austenitic stainless cast steel is needed.

[0006] The present disclosure has been made to solve the above-mentioned problems, and aims to provide austenitic stainless cast steel that is low cost and has excellent heat resistance, and a method for manufacturing the same. [Means for solving the problem]

[0007] The austenitic stainless cast steel according to the present disclosure is The chemical composition is, in mass%, C: 0.3% to 0.5%, Mn: 2.0% or less, P: 0.04% or less, S: 0.03% or less, Si: 1.0% to 2.5%, Ni: 36% to 39%, Cr: 18% to 21%, Mo: 0.5% or less, Nb: 1.2 to 1.8%, and the balance being iron and impurities; When heated at 1000°C, the average number of carbides per unit area with a circle equivalent diameter of 500 nm or more, Nc, was 6.0 × 10 -2 pieces / μm 2 When the average number per unit area of ​​the carbides having a circle-equivalent diameter of 500 nm or more in the vicinity of the grain boundaries of austenite grains is Ngb, Ngb / Nc is 1.30 or less. In the cross section of the austenitic stainless cast steel according to the present disclosure when heated at 1000°C, the average number Nc of carbides per unit area having a circle equivalent diameter of 500 nm or more in the center of the austenite grains is 6.0 × 10 -2 pieces / μm 2 When the average number per unit area of ​​the carbides having a circle-equivalent diameter of 500 nm or more in the vicinity of the grain boundaries of austenite grains is defined as Ngb, Ngb / Nc is less than 0.5.

[0008] The method for producing austenitic stainless cast steel according to the present disclosure includes a heating step of heating the cast austenitic stainless cast steel at a heating temperature of 1100°C to 1250°C; a slow cooling step of cooling from the heating temperature to 500°C at an average cooling rate of less than 100°C / hour after the heating step; Equipped with 、 The chemical composition of the austenitic stainless cast steel is, in mass%, C: 0.3% to 0.5%, Mn: 2.0% or less, P: 0.04% or less, S: 0.03% or less, Si: 1.0% to 2.5%, Ni: 36% to 39%, Cr: 18% to 21%, Mo: 0.5% or less, Nb: 1.2 to 1.8%, and the remainder being iron and impurities. The method for producing austenitic stainless cast steel according to the present disclosure includes a heating step of heating the cast austenitic stainless cast steel at a heating temperature of 1100°C to 1250°C, a cooling step of cooling the cast austenitic stainless cast steel from the heating temperature to 500°C at an average cooling rate of 900°C / hour or more after the heating step, an aging step of heating the cast austenitic stainless cast steel in a temperature range of 900°C to 1050°C for one hour or more after the cooling step, and a cooling step of cooling the cast austenitic stainless cast steel from the temperature range of the aging step to 500°C at an average cooling rate of 900°C / hour or more after the cooling step. and a second cooling step of cooling to 0°C at an average cooling rate of 900°C / hour or more, wherein the chemical composition of the austenitic stainless cast steel is, in mass%, C: 0.3% to 0.5%, Mn: 2.0% or less, P: 0.04% or less, S: 0.03% or less, Si: 1.0% to 2.5%, Ni: 36% to 39%, Cr: 18% to 21%, Mo: 0.5% or less, Nb: 1.2 to 1.8%, and the balance being iron and impurities. The method for producing austenitic stainless cast steel according to the present disclosure includes a heating step of heating the cast austenitic stainless cast steel at a heating temperature of 1100°C to 1250°C, and cooling the cast austenitic stainless steel from the heating temperature to 500°C at an average cooling rate of 0.05°C after the heating step. 100 and a slow cooling step of cooling at a rate of less than ° C. / hour. [Effects of the Invention]

[0009] According to the above aspects of the present disclosure, it is possible to provide austenitic stainless cast steel that is low cost and has excellent heat resistance, and a method for manufacturing the same. [Brief explanation of the drawings]

[0010] [Figure 1] 1 is an optical microscope image of the austenitic stainless cast steel according to the first embodiment of the present disclosure after heating. [Figure 2] 10 is an optical microscope image of the austenitic stainless cast steel according to the second embodiment of the present disclosure after heating. [Figure 3] 10 is an optical microscope image of the austenitic stainless cast steel according to the second embodiment of the present disclosure before heating. [Figure 4] 10 is an optical microscope image of the austenitic stainless cast steel according to the third embodiment of the present disclosure before heating. [Figure 5] 1 is an optical microscope image of conventional austenitic stainless cast steel after heating. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present inventors have conducted extensive research into improving heat resistance and have found the following. (1) Repeated thermal stress can cause cracks in conventional austenitic stainless steel casts. (2) In conventional austenitic stainless cast steel, excessive carbides precipitate near the grain boundaries of austenite grains upon heating, as shown in the circled area in Figure 5. (3) In the case of conventional austenitic stainless steel casts, excessive carbides precipitate near the grain boundaries of austenite crystal grains, making the austenitic stainless steel casts embrittlement, and cracks propagate along the carbides at the grain boundaries.

[0012] Based on the above analysis, the present inventors conducted extensive research and obtained the following findings. (A) In a cross section of austenitic stainless cast steel after heating, when the average number of the carbides per unit area in the center of the austenite crystal grains is Nc and the average number of the carbides per unit area in the vicinity of the grain boundaries of the austenite crystal grains is Ngb, if Ngb / Nc is 1.30 or less, embrittlement of the austenitic stainless cast steel can be suppressed. Based on the above findings, the present invention has determined the configuration of the austenitic stainless cast steel of the present disclosure. Note that, in the austenitic stainless cast steel of the present disclosure, since precipitates are controlled by heat treatment, the average number Nc per unit area of ​​carbides with a circle equivalent diameter of 500 nm or more in the center of the austenite crystal grains is 6.0 × 10 -2 pieces / μm 2 That's all. The austenitic stainless cast steel of the present disclosure can achieve high heat resistance due to the above-mentioned effects. Here, the vicinity of the grain boundaries of austenite crystal grains is defined as "the region from the grain boundary of the austenite crystal grain to 10 μm," and the center of the austenite crystal grain is defined as "the region other than the vicinity of the austenite grain boundaries (excluding the precipitate-free region)." In this specification, a numerical range expressed using "to" means a range that includes the numerical values ​​written before and after "to" as the lower and upper limits. In this specification, temperatures such as heating temperatures are defined as the temperatures at the surface of the austenitic cast steel.

[0013] First Embodiment The austenitic stainless cast steel according to the first embodiment will be described below.

[0014] (Nc=6.0×10 -2 pieces / μm 2 (End) In the cross section of the austenitic stainless cast steel according to the first embodiment when heated at 1000 ° C., the average number Nc of carbides having a circle equivalent diameter of 500 nm or more in the center of the austenite grains per unit area is 6.0 × 10 -2 pieces / μm 2The heating time at 1000°C is not particularly limited, and is, for example, 30 minutes. Here, the circle-equivalent diameter refers to the diameter of a circle having an area equal to the projected area of ​​the particle. A more preferable average number of carbides per unit area is 6.5 × 10 -2 pieces / μm 2 More preferably, the average number of carbides per unit area is 7.0×10 -2 pieces / μm 2 In this embodiment, since the precipitation of carbides is controlled by heat treatment, the average number Nc of carbides having a circle equivalent diameter of 500 nm or more in the center of the austenite grain per unit area is 6.0 × 10 -2 pieces / μm 2 In the austenitic cast steel according to the first embodiment, Nc was 6.0 × 10 before heating at 1000 °C. -2 pieces / μm 2 It may be more than that.

[0015] Carbide is a metal element that is M (M: Fe, Cr, Nb) and carbon element that is C. 23 Preferably, it is C6. Carbides can be analyzed, for example, by energy dispersive X-ray spectroscopy (EDX).

[0016] (Method for measuring Nc) The average number of carbides per unit area can be measured by the following method. After heating to 1000°C, austenitic stainless cast steel is cut, the cut surface is etched with picrate acid, and observed under an optical microscope (magnification: 1000x). FIG. 1 is an optical microscope image of austenitic stainless cast steel according to the first embodiment. In the case of FIG. 1, carbides appear as black regions in the center of austenite crystal grains. The number of carbides with a circle equivalent diameter of 500 nm or more within a perfect circle with a diameter of 10 μm is counted at 10 arbitrary locations within the crystal grains in the obtained observation image, and the average number Nc per unit area can be calculated from the obtained number of carbides and the area of ​​the region where the carbides were counted.

[0017] (Ngb / Nc: less than 0.50) In a cross section of the austenitic stainless cast steel according to the first embodiment after heating to 1000°C, when the average number per unit area of ​​the carbides having an equivalent circle diameter of 500 nm or more in the central portions of the austenite grains is Nc and the average number per unit area of ​​the carbides having an equivalent circle diameter of 500 nm or more in the vicinity of the grain boundaries of the austenite grains is Ngb, Ngb / Nc is less than 0.50. Ngb / Nc is more preferably 0.40 or less. Even more preferably, Ngb / Nc is 0.30 or less. Ngb / Nc may be 0.02 or more. In the case of the first embodiment, heating reduces the number of carbides precipitating near the grain boundaries of the austenite grains. This improves the ductility of the metal structure. The heating time at 1000°C is not particularly limited and may be, for example, 30 minutes. For the austenitic cast steel according to the first embodiment, Ngb / Nc may be less than 0.50 before heating to 1000°C.

[0018] (Method for measuring Ngb / Nc) Ngb / Nc can be measured by the following method. Austenitic stainless cast steel is cut after heating to 1000°C, and the cut surface is etched with picrate acid and observed under an optical microscope (magnification 1000x). In the obtained observation image, 10 random locations are selected from the center of the crystal grain and near the grain boundary, and the number of carbides with a circle equivalent diameter of 500 nm or more within a 10 μm circle at each location is counted. Nc is calculated from the obtained number of carbides in the center and the area of ​​the region where the carbides are measured. Ngb can also be calculated from the obtained number of carbides near the grain boundary and the area of ​​the region where the carbides are measured. Ngb / Nc is calculated from the obtained Ngb and Nc. The heating time at 1000°C is not particularly limited, and is, for example, 30 minutes.

[0019] (Average width of precipitate-free region is 1.5μm to 20μm) In a cross section of the austenitic stainless cast steel according to the first embodiment after heating to 1000°C, it is preferable that precipitate-free regions, which are regions where no carbides are observed in austenite grains when observed with an optical microscope at a magnification of 300 times, exist within the austenite grains, and that the width of the precipitate-free regions is 1.5 μm to 20 μm. Deformation of the precipitate-free regions can suppress the propagation of cracks due to thermal stress.

[0020] (Method for measuring the average width of precipitate-free regions) The average width of the precipitate-free region can be measured by the following method. After heating to 1000°C, austenitic stainless cast steel is cut, the cut surface is etched with picrate acid, and observed under an optical microscope (magnification: 300x). In the obtained observation image, 50 carbides with a circle-equivalent diameter of 500 nm or more near the grain boundaries of austenite crystal grains are randomly selected, and the inscribed circle between each carbide and the nearest grain boundary is determined. The diameters of the 50 inscribed circles are averaged, and this average value is used as the average width of the precipitate-free region. The heating time at 1000°C is not particularly limited, and may be, for example, 30 minutes.

[0021] (chemical composition) The chemical composition of the austenitic stainless cast steel according to the first embodiment is, for example, in mass %, C: 0.3% to 0.5%, Mn: 2.0% or less, P: 0.04% or less, S: 0.03% or less, Si: 1.0% to 2.5%, Ni: 36% to 39%, Cr: 18% to 21%, Mo: 0.5% or less, Nb: 1.2 to 1.8%, and the balance being iron and impurities. Each element will be described below.

[0022] C: 0.3% to 0.5% C is an element for forming carbides. If the C content is less than 0.3%, an appropriate amount of carbides may not be formed. Therefore, the C content is preferably 0.3% or more. If the C content is more than 0.5%, excessive carbides are formed. Therefore, the C content is preferably 0.5% or less.

[0023] Mn: 2.0% or less Mn has a deoxidizing effect and is an element that contributes to the stabilization of austenite. However, if the Mn content exceeds 2.0%, the austenitic stainless cast steel may become embrittled. Therefore, the Mn content is preferably 2.0% or less. More preferably, the Mn content is 1.5% or less. The Mn content is further preferably 1.0% or less. There is no need to set a lower limit for the Mn content, but if the Mn content is extremely low, the deoxidizing effect will not be sufficient. Therefore, the Mn content is preferably 0.0001% or more.

[0024] P:0.04% or less P is contained in austenitic stainless cast steel as an impurity. If the P content exceeds 0.04%, ductility decreases. Therefore, the P content is preferably 0.04% or less. The P content is more preferably 0.03% or less, and even more preferably 0.02% or less. Since the P content is an impurity, it is preferable to reduce it as much as possible, but if the P content is reduced too much, the manufacturing cost increases. Therefore, the P content is preferably 0.0001% or more, and more preferably 0.0005% or more.

[0025] S: 0.03% or less S is contained in austenitic stainless cast steel as an impurity. If the S content exceeds 0.03%, the ductility of the austenitic stainless cast steel may decrease. Therefore, the S content is preferably 0.03% or less. A more preferable S content is 0.02% or less. Since S is an impurity, it is preferable to reduce it as much as possible, but if the S content is reduced too much, the manufacturing cost increases. Therefore, the S content is preferably 0.0001% or more. The S content is more preferably 0.0005% or more.

[0026] Si: 1.0% to 2.5% Si has a deoxidizing effect and is an element that contributes to improving corrosion resistance and oxidation resistance at high temperatures. However, if the Si content exceeds 2.5%, the stability of austenite decreases, and toughness may decrease. Therefore, the Si content is preferably 2.5% or less. The Si content is more preferably 2.0% or less. The Si content is even more preferably 1.5% or less. If the Si content is less than 1.0%, the deoxidizing effect may not be sufficient. Therefore, the Si content is preferably 1.0% or more. A more preferred Si content is 1.1% or more.

[0027] Ni: 36% to 39% Ni is an element effective for obtaining austenite and contributes to austenite stability. If Ni is less than 36%, the above effects may not be obtained. Therefore, the Ni content is preferably 36% or more. If Ni is contained in a large amount, the cost increases. Therefore, the Ni content is preferably 39% or less. The Ni content is more preferably 38% or less.

[0028] Cr: 18% to 21% Cr contributes to improving oxidation resistance at high temperatures and is an element necessary for forming carbides. If the Cr content is less than 18%, the above effects may not be obtained. Therefore, the Cr content is preferably 18% or more. However, if the Cr content exceeds 21%, the stability of austenite at high temperatures may decrease. Therefore, the Cr content is preferably 21% or less. A more preferable Cr content is 20% or less.

[0029] Mo: 0.5% or less Mo is a solid solution strengthening element. If the Mo content exceeds 0.5%, the stability of austenite may decrease. Therefore, the Mo content is preferably 0.5% or less. The Mo content is more preferably 0.4% or less. To obtain the effect of Mo, the Mo content is preferably 0.01% or more.

[0030] Nb: 1.2 to 1.8% Nb is an element that forms carbides. If the Nb content is less than 1.2%, appropriate carbides may not be formed. Therefore, the Nb content is preferably 1.2% or more. The Nb content is more preferably 1.3% or more. If the Nb content exceeds 1.8%, a large amount of carbides may precipitate. Therefore, the Nb content is preferably 1.8% or less. The Nb content is more preferably 1.7% or less.

[0031] Remainder: Iron and impurities In the chemical composition of the austenitic stainless cast steel of the present disclosure, the balance is iron and impurities. Here, the impurities are components that are mixed in during the raw materials or manufacturing process when producing the austenitic stainless cast steel. The impurities are allowed within a range that allows the effects of the austenitic stainless cast steel of the present disclosure to be obtained.

[0032] The chemical composition of austenitic stainless cast steel can be analyzed using known methods, such as inductively coupled plasma mass spectrometry.

[0033] "Method of manufacturing austenitic stainless steel castings" The austenitic stainless cast steel according to the first embodiment is produced, for example, by the following method: The components constituting the austenitic stainless cast steel are melted, and the resulting molten metal is poured into a predetermined mold to obtain cast steel.

[0034] (Heating process) Next, the obtained cast steel is subjected to a heating step in which it is heated at a heating temperature of 1100°C to 1250°C. A heating temperature in the range of 1100 to 1250°C is preferable because the chemical components of the austenitic stainless cast steel are uniformly solid-dissolved throughout the crystal grains. A heating time of 5 minutes or more is also preferable because the chemical components of the austenitic stainless cast steel are uniformly solid-dissolved throughout the crystal grains. There is no particular upper limit to the heating time, but since there is little change even if the heating time is 60 minutes or more, it may be set to 60 minutes.

[0035] (Slow cooling process) After the heating process, the cast steel is subjected to a slow cooling process in which it is cooled from the heating temperature to 500°C at an average cooling rate of less than 100°C / hour. During the slow cooling process, elements precipitate and grow as carbides. Carbides grow up to the equilibrium volume, but after reaching the equilibrium volume, relatively small carbides disappear and relatively large carbides grow due to Ostwald ripening. Because coarse carbides exist at grain boundaries, elements near the grain boundaries gather and grow in the coarse carbides at the grain boundaries, thereby reducing the amount of carbides near the grain boundaries. This method is preferable because it reduces the amount of coarse carbides at the grain boundaries when the austenitic stainless steel cast steel is heated to 1000°C and achieves an Ngb / Nc ratio of less than 0.5. Furthermore, the slow cooling process allows carbides to advance to a state close to equilibrium precipitation, improving the stability of the austenitic stainless steel cast steel during high-temperature use.

[0036] <Second embodiment> The austenitic stainless cast steel according to the second embodiment will be described below.

[0037] (Nc=6.0×10 -2 pieces / μm 2 (End) In the cross section of the austenitic stainless cast steel according to the second embodiment when heated to 1000°C, the average number of carbides with a circle equivalent diameter of 500 nm or more in the center of the austenite grains per unit area is 6.0 × 10 -2 pieces / μm 2 The heating time at 1000°C is not particularly limited, and is, for example, 30 minutes. A more preferable average number of carbides per unit area is 6.5 × 10 -2 pieces / μm 2 More preferably, the average number of carbides per unit area is 7.0×10 -2 pieces / μm 2 In this embodiment, since the precipitation of carbides is controlled by heat treatment, the average number per unit area of ​​carbides having a circle equivalent diameter of 500 nm or more in the center of austenite grains is 6.0 × 10 -2 pieces / μm2 That's all.

[0038] Carbide is a metal element that is M (M: Fe, Cr, Ngb) and carbon element that is C. 23 C6 is preferred.

[0039] (Method for measuring Nc) The average number of carbides per unit area can be measured in the same manner as in the first embodiment. After heating at 1000°C, the austenitic stainless cast steel is cut, and the cut surface is etched with picrate acid and observed under an optical microscope (magnification: 1000x). FIG. 2 is an optical microscope image of the austenitic stainless cast steel according to the second embodiment after heating at 1000°C. In the case of FIG. 2, the carbides appear as black areas in the center of the austenite crystal grains. The number of carbides with a circle equivalent diameter of 500 nm or more is counted at 10 arbitrary locations in the obtained observation image, and the average number per unit area can be calculated from the number of carbides obtained and the area of ​​the areas where the carbides were counted.

[0040] (Ngb / Nc:0.50~1.30) In a cross section of the austenitic stainless cast steel according to the second embodiment after heating at 1000°C, when the average number per unit area of ​​the carbides having an equivalent circle diameter of 500 nm or more in the central portions of the austenite grains is defined as Nc, and the average number per unit area of ​​the carbides having an equivalent circle diameter of 500 nm or more in the vicinity of the grain boundaries of the austenite grains is defined as Ngb, the ratio Ngb / Nc is 0.50 to 1.30. In the second embodiment, carbides are uniformly precipitated within the austenite grains, thereby improving the strength and reduction of area of ​​the metal structure. Here, reduction of area refers to the amount of change in the cross-sectional area at the fracture location after the tensile test relative to the cross-sectional area before the tensile test. A more preferred value for Ngb / Nc is 0.70 or more. An even more preferred value for Ngb / Nc is 0.85 or more. A more preferred value for Ngb / Nc is 1.05 or less. A more preferred value for Ngb / Nc is 1.00 or less. The heating time at 1000° C. is not particularly limited, and is, for example, 30 minutes.

[0041] (Method for measuring Ngb / Nc) Ngb / Nc can be measured using the following method. Austenitic stainless steel cast steel is cut after being heated to 1000°C, and the cut surface is etched with picrate acid and observed under an optical microscope (1000x magnification). In the obtained observation image, 10 random locations are selected from the center of the austenite crystal grain and near the grain boundary, and the number of carbides with a circle equivalent diameter of 500 nm or more within a 10 μm circle at each location is counted. Nc is calculated from the number of carbides in the center of the crystal grain and the area of ​​the region where the carbides are measured. Ngb can be calculated from the number of carbides near the grain boundary and the area of ​​the region where the carbides are measured. Ngb / Nc is calculated from the obtained Ngb and Nc.

[0042] (chemical composition) The chemical composition of the austenitic stainless cast steel according to the second embodiment is, for example, in mass %, C: 0.3% to 0.5%, Mn: 2.0% or less, P: 0.04% or less, S: 0.03% or less, Si: 1.0% to 2.5%, Ni: 36% to 39%, Cr: 18% to 21%, Mo: 0.5% or less, Nb: 1.2 to 1.8%, and the balance being iron and impurities.

[0043] "Method of manufacturing austenitic stainless steel castings" The austenitic stainless cast steel according to the second embodiment is produced, for example, by the following method: The components constituting the austenitic stainless cast steel are melted, and the resulting molten metal is poured into a predetermined mold to obtain cast steel.

[0044] (Heating process) Next, the obtained cast steel is subjected to a heating step in which it is heated at a heating temperature of 1100°C to 1250°C. A heating temperature in the range of 1100 to 1250°C is preferable because the chemical components of the austenitic stainless cast steel are uniformly solid-dissolved throughout the crystal grains. A heating time of 5 minutes or more is also preferable because the chemical components of the austenitic stainless cast steel are uniformly solid-dissolved throughout the crystal grains. There is no particular upper limit to the heating time, but since there is little change even if the heating time is 60 minutes or more, it may be set to 60 minutes.

[0045] (cooling process) After the heating step, the cast steel is subjected to a cooling step in which it is cooled from the heating temperature to 500°C at an average cooling rate of 900°C / hour or more. Here, the average cooling rate in the cooling step refers to the average cooling rate from the heating temperature to 500°C. In order to prevent excessive precipitation of carbides during cooling, the average cooling rate is preferably 900°C / hour or more.

[0046] An optical microscope image of the obtained austenitic stainless cast steel according to the second embodiment before heating to 1000°C is shown in Fig. 3. As shown in Fig. 3, when produced by the method for producing austenitic stainless cast steel according to the second embodiment, part of the grain boundary carbides are solid-dissolved, and the structure is homogenized.

[0047] <Third embodiment> The austenitic stainless cast steel according to the third embodiment will be described below.

[0048] (Nc=6.0×10 -2 pieces / μm 2 (End) In the cross section of the austenitic stainless cast steel according to the third embodiment before heating to 1000°C, the average number per unit area of ​​carbides having a circle equivalent diameter of 500 nm or more in the center of the austenite grains is 6.0 × 10 -2 pieces / μm 2 More preferably, the average number of carbides per unit area is 6.5×10 -2 pieces / μm 2 More preferably, the average number of carbides per unit area is 7.0×10 -2 pieces / μm 2 In this embodiment, since the precipitation of carbides is controlled by heat treatment, the average number per unit area of ​​carbides having a circle equivalent diameter of 500 nm or more in the center of austenite grains is 6.0 × 10 -2 pieces / μm 2Furthermore, since carbides are precipitated in the austenitic stainless cast steel before heating, the strength at high temperatures is improved. In the austenitic stainless cast steel of the third embodiment, even after heating to 1000°C, the average number Nc of carbides per unit area with a circle equivalent diameter of 500 nm or more in the central part of the austenite crystal grains is 6.0 × 10 -2 pieces / μm 2 That's all.

[0049] Carbide is a metal element that is M (M: Fe, Cr, Nb) and carbon element that is C. 23 C6 is preferred.

[0050] (Method for measuring Nc) The average number of carbides per unit area can be measured by the following method. Before heating to 1000°C, austenitic stainless cast steel is cut, the cut surface is etched with picrate acid, and observed under an optical microscope (magnification: 1000x). FIG. 4 is an optical microscope image of austenitic stainless cast steel according to the third embodiment. In the case of FIG. 4, carbides appear as black areas in the center of austenite crystal grains. The number of carbides with a circle equivalent diameter of 500 nm or more is counted at 10 arbitrary locations in the obtained observation image, and the average number per unit area can be calculated from the obtained number of carbides and the area of ​​the areas where the carbides were counted.

[0051] (Ngb / Nc:0.50~1.30) In a cross section of the austenitic stainless cast steel according to the third embodiment before heating to 1000°C, when the average number per unit area of ​​the carbides having an equivalent circle diameter of 500 nm or more in the central portions of the austenite grains is defined as Nc, and the average number per unit area of ​​the carbides having an equivalent circle diameter of 500 nm or more in the vicinity of the grain boundaries of the austenite grains is defined as Ngb, the ratio Ngb / Nc is 0.50 to 1.30. In the third embodiment, since the carbides are uniformly precipitated within the austenite grains before heating, the structural stability during high-temperature use is improved, and the reduction in area can be improved. Furthermore, cracking due to grain boundaries caused by repeated thermal stress can be suppressed, and the amount of plastic deformation during the application of thermal stress can be reduced. A more preferable Ngb / Nc is 0.70 or more. An even more preferable Ngb / Nc is 0.85 or more. An even more preferable Ngb / Nc is 1.05 or less. An even more preferable Ngb / Nc is 1.00 or less. In the austenitic stainless cast steel of the third embodiment, even after heating to 1000°C, Ngb / Nc remains in the range of 0.50 to 1.30.

[0052] (Method for measuring Ngb / Nc) Ngb / Nc can be measured using the following method. Austenitic stainless cast steel before heating to 1000°C is cut, the cut surface is etched with picrate acid, and observed under an optical microscope (1000x magnification). In the obtained observation image, 10 random locations are selected from the center of the crystal grain and near the grain boundary, and the number of carbides with a circle equivalent diameter of 500 nm or more within a 10 μm circle at each location is counted. Nc is calculated from the number of carbides in the center and the area of ​​the region where the carbides are counted. Ngb can be calculated from the number of carbides near the grain boundary and the area of ​​the region where the carbides are counted. Ngb / Nc is calculated from the obtained Ngb and Nc.

[0053] (chemical composition) The chemical composition of the austenitic stainless cast steel according to the third embodiment is, for example, in mass %, C: 0.3% to 0.5%, Mn: 2.0% or less, P: 0.04% or less, S: 0.03% or less, Si: 1.0% to 2.5%, Ni: 36% to 39%, Cr: 18% to 21%, Mo: 0.5% or less, Nb: 1.2 to 1.8%, and the balance being iron and impurities.

[0054] "Method of manufacturing austenitic stainless steel castings" The austenitic stainless cast steel according to the third embodiment is produced, for example, by the following method: The components constituting the austenitic stainless cast steel are melted, and the resulting molten metal is poured into a predetermined mold to obtain cast steel.

[0055] (Heating process) Next, the obtained cast steel is subjected to a heating step in which it is heated at a heating temperature of 1100°C to 1250°C. A heating temperature in the range of 1100 to 1200°C is preferable because the chemical components of the austenitic stainless cast steel are uniformly solid-dissolved throughout the crystal grains. A heating time of 5 minutes or more is also preferable because the chemical components of the austenitic stainless cast steel are uniformly solid-dissolved throughout the crystal grains. There is no particular upper limit to the heating time, but since there is little change even if the heating time is 60 minutes or more, it may be set to 60 minutes.

[0056] (cooling process) After the heating step, the cast steel is subjected to a cooling step in which it is cooled from the heating temperature to 500°C at an average cooling rate of 900°C / hour or more. Here, the average cooling rate in the cooling step refers to the average cooling rate from the heating temperature to 500°C. In order to prevent excessive precipitation of carbides during cooling, the average cooling rate is preferably 900°C / hour or more.

[0057] (Aging process) Next, the obtained cast steel is subjected to an aging step in which the steel is heated at an aging temperature of 900°C to 1050°C for one hour or more. An aging temperature in the range of 900°C to 1050°C is preferable because uniform carbides can be precipitated. Furthermore, a heating time of one hour or more is also preferable because uniform carbides can be precipitated.

[0058] (Second cooling step) After the aging step, the cast steel is subjected to a second cooling step in which the steel is cooled from the aging temperature to 500°C at an average cooling rate of 900°C / hour or more. Here, the average cooling rate in the second cooling step refers to the average cooling rate from the aging temperature to 500°C. To prevent excessive precipitation of carbides during cooling, the average cooling rate is preferably 900°C / hour or more.

[0059] In the manufacturing method of austenitic stainless cast steel according to each embodiment described above, known steps may be combined. [Example]

[0060] The present invention will be specifically explained below with reference to examples, but the present invention is not limited to these examples.

[0061] Example 1 An austenitic stainless steel casting having a chemical composition, in mass%, of C: 0.34%, Mn: 0.89%, P: 0.021%, S: 0.007%, Si: 1.13%, Ni: 36.33%, Cr: 18.77%, Mo: 0.02%, Nb: 1.28%, with the remainder being iron and impurities was heated at a heating temperature of 1250°C for 60 minutes, and then cooled from 1250°C to 500°C at an average cooling rate of 65°C / hour to obtain the austenitic stainless steel cast steel of Example 1.

[0062] Example 2 An austenitic stainless steel casting having a chemical composition, in mass %, of C: 0.34%, Mn: 0.89%, P: 0.021%, S: 0.007%, Si: 1.13%, Ni: 36.33%, Cr: 18.77%, Mo: 0.02%, Nb: 1.28%, with the remainder being iron and impurities was heated at a heating temperature of 1250°C for 60 minutes, and then cooled from 1250°C to 500°C at an average cooling rate of 4000°C / hour to obtain the austenitic stainless steel cast steel of Example 2.

[0063] Example 3 Austenitic stainless steel castings with a chemical composition, in mass%, of C: 0.34%, Mn: 0.89%, P: 0.021%, S: 0.007%, Si: 1.13%, Ni: 36.33%, Cr: 18.77%, Mo: 0.02%, Nb: 1.28%, with the remainder being iron and impurities were heated at a heating temperature of 1250 ° C for 60 minutes, and then cooled to 1250 ° C to 500 ° C at an average cooling rate of 4000 ° C / hour. After cooling, the castings were subjected to aging treatment at 950 ° C for 600 minutes, and then cooled to 950 ° C to 500 ° C at an average cooling rate of 3200 ° C / hour to obtain austenitic stainless steel castings of Example 3.

[0064] (Comparative Example 1) An untreated austenitic stainless steel casting having a chemical composition, in mass%, of C: 0.34%, Mn: 0.89%, P: 0.021%, S: 0.007%, Si: 1.13%, Ni: 36.33%, Cr: 18.77%, Mo: 0.02%, Nb: 1.28%, and the remainder being iron and impurities was used as the austenitic stainless steel cast steel of Comparative Example 1.

[0065] (After heating Nc and Ngb / Nc) The Ngb / Nc after heating of the austenitic stainless cast steels of Examples 1 to 3 and Comparative Example 1 was measured by the following method. The austenitic stainless cast steels heated at 1000°C for 30 minutes were cut, and the cut surfaces were etched with picrate acid and observed under an optical microscope (1000x magnification). In the obtained observation image, 10 random locations were selected from the center of the crystal grains and near the grain boundaries, and the number of carbides with a circle equivalent diameter of 500 nm or more within a 10 μm circle was counted at each location. Nc was calculated from the number of carbides in the center and the area of ​​the region where the carbides were counted. Ngb was calculated from the number of carbides near the grain boundaries and the area of ​​the region where the carbides were counted. Ngb / Nc was calculated from the obtained Ngb and Nc. The results are shown in Table 1.

[0066] (Width of precipitate-free region after heating) The average width of the precipitate-free regions of the austenitic stainless cast steel of Example 1 can be measured by the following method. The austenitic stainless cast steel was heated at 1000°C for 30 minutes, then cut, and the cut surface was subjected to electrolytic etching with nitric acid and observed under an optical microscope (magnification: 300x). From the obtained observation image, 50 carbides with a circle-equivalent diameter of 500 nm or more near the grain boundaries of austenite crystal grains were randomly selected, and the inscribed circle between each carbide and the nearest grain boundary was determined. The average value of the diameters of the 50 inscribed circles was calculated, and this average value was taken as the average width of the precipitate-free regions. The results are shown in Table 1. In Table 1, 0.0 indicates that there were no precipitate-free regions.

[0067] (0.2% yield strength) The 0.2% proof stress at high temperatures was measured in accordance with JIS G0567:2012. The test specimen shape was a flanged test specimen as specified in Appendix A.5 of JIS G0567:2012. The test temperature was 1000°C. The results are shown in Table 1.

[0068] (tensile strength) The tensile strength at high temperatures was measured in accordance with JIS G0567:2012. The test specimen shape was a flanged test specimen as described in Appendix A.5 of JIS G0567:2012. The test temperature was 1000°C. The results are shown in Table 1.

[0069] (stretch) The elongation at high temperatures was measured in accordance with JIS G0567:2012. The elongation at break was measured. The test specimen shape was a flanged test specimen as specified in Appendix A.5 of JIS G0567:2012. The test temperature was 1000°C. The results are shown in Table 1.

[0070] (Aperture) The reduction of area at high temperatures was measured in accordance with JIS G0567:2012. The test specimen shape was a flanged test specimen as described in Appendix A.5 of JIS G 0567:2012. The test temperature was 1000°C. The results are shown in Table 1.

[0071] [Table 1]

[0072] From the above, the austenitic stainless cast steels of Examples 1 to 3 according to this embodiment were superior to the austenitic stainless cast steel of Comparative Example 1 in heat resistance.

[0073] In the austenitic stainless cast steel of Example 1, the average number Nc of carbides with a circle equivalent diameter of 500 nm or more per unit area after heating was 6.0 × 10 -2 pieces / μm 2 As a result, the Ngb / Nc ratio was less than 0.50, indicating excellent elongation. Furthermore, observation of the metal structure after high-temperature tensile testing revealed that by preventing excessive precipitation of carbides, the propagation of cracks along the grain boundaries was hardly observed, and cracks propagated within the crystal grains, indicating that embrittlement had been suppressed.

[0074] In the austenitic stainless cast steel of Example 2, the average number Nc of carbides with a circle equivalent diameter of 500 nm or more per unit area after heating was 6.0 × 10 -2 pieces / μm 2As a result, Ngb / Nc was in the range of 0.50 to 1.30, and therefore the 0.2% proof stress, tensile strength, and reduction of area were excellent. After high-temperature tensile testing, the excellent reduction of area indicated improved ductility and suppressed embrittlement.

[0075] In the austenitic stainless cast steel of Example 3, the average number Nc of carbides with a circle equivalent diameter of 500 nm or more per unit area after heating was 6.0 × 10 -2 pieces / μm 2 The results were as above, and Ngb / Nc was in the range of 0.50 to 1.30, so that the 0.2% proof stress, tensile strength, and reduction of area were excellent. The excellent reduction of area after the high-temperature tensile test revealed that ductility was improved and embrittlement was suppressed. Although not shown in Table 1, the austenitic stainless cast steel of Example 3 had an Nc of 6.0 × 10 even in the cross section before heating to 1000°C. -2 pieces / μm 2 The above results indicate that Ngb / Nc is within the range of 0.50 to 1.30.

[0076] As described above, the austenitic stainless cast steel of the present disclosure had excellent heat resistance.

[0077] <Additional Notes> The austenitic stainless cast steel and the method for producing austenitic stainless cast steel described in the above embodiment can be understood as follows.

[0078] (1) In the austenitic stainless cast steel according to the first aspect of the present disclosure, in a cross section when heated at 1000°C, the average number Nc per unit area of ​​carbides having a circle equivalent diameter of 500 nm or more in the center of austenite grains is 6.0 × 10 -2 pieces / μm 2 When the average number per unit area of ​​the carbides having a circle-equivalent diameter of 500 nm or more in the vicinity of the grain boundaries of austenite grains is Ngb, Ngb / Nc is 1.30 or less.

[0079] In this way, embrittlement of the austenitic stainless cast steel can be suppressed.

[0080] (2) The austenitic stainless cast steel according to a second aspect of the present disclosure is the austenitic stainless cast steel according to (1), in which the Ngb / Nc is less than 0.5.

[0081] By doing so, it is possible to suppress embrittlement of the austenitic stainless cast steel, and also to improve the elongation of the austenitic stainless cast steel at high temperatures.

[0082] (3) The austenitic stainless cast steel according to a third aspect of the present disclosure is the austenitic stainless cast steel of (2), which has a precipitate-free region, which is a region where no carbides are observed when observed with an optical microscope at a magnification of 300 times, and the width of the precipitate-free region is 1.5 μm to 20 μm.

[0083] By doing so, the elongation of the austenitic stainless cast steel at high temperatures can be further improved.

[0084] (4) The austenitic stainless cast steel according to a fourth aspect of the present disclosure is the austenitic stainless cast steel of (1), in which the Ngb / Nc is 0.50 to 1.30.

[0085] This makes it possible to suppress embrittlement of the austenitic stainless cast steel, and also to improve the 0.2% yield strength, tensile strength, and reduction of area at high temperatures.

[0086] (5) The austenitic stainless cast steel according to a fifth aspect of the present disclosure is the austenitic cast steel of (1), wherein the average number Nc per unit area of ​​carbides having a circle equivalent diameter of 500 nm or more in the center of austenite crystal grains in a cross section before heating to 1000°C is 6.0 × 10 -2 pieces / μm 2 As described above, when the average number per unit area of ​​the carbides having a circle-equivalent diameter of 500 nm or more in the vicinity of the grain boundaries of austenite grains is Ngb, Ngb / Nc is 0.50 to 1.30.

[0087] This makes it possible to suppress embrittlement of the austenitic stainless cast steel, and also to improve the 0.2% yield strength, tensile strength, and reduction of area at high temperatures.

[0088] (6) The austenitic stainless steel cast steel according to a sixth aspect of the present disclosure is the austenitic stainless steel cast steel according to any one of (1) to (5), wherein the chemical composition of the austenitic stainless steel cast steel is, in mass%, C: 0.3% to 0.5%, Mn: 2.0% or less, P: 0.04% or less, S: 0.03% or less, Si: 1.0% to 2.5%, Ni: 36% to 39%, Cr: 18% to 21%, Mo: 0.5% or less, Nb: 1.2 to 1.8%, and the balance being iron and impurities.

[0089] By doing so, embrittlement of the austenitic stainless cast steel can be further suppressed.

[0090] (7) A method for producing austenitic stainless cast steel according to a seventh aspect of the present disclosure includes a heating step of heating the cast austenitic stainless cast steel at a heating temperature of 1100°C to 1250°C.

[0091] By doing so, the elements can be uniformly dissolved throughout the entire crystal grain.

[0092] (8) A method for producing austenitic stainless cast steel according to an eighth aspect of the present disclosure is the method for producing austenitic stainless cast steel according to (7), further comprising, after the heating step, a slow cooling step of cooling from the heating temperature to 500°C at an average cooling rate of less than 65°C / hour.

[0093] By doing so, it is possible to advance carbide precipitation to a state close to equilibrium, and the stability of the austenitic stainless cast steel during high-temperature use can be improved.

[0094] (9) Austenitic stainless cast steel according to a ninth aspect of the present disclosure is a method for producing austenitic stainless cast steel according to (7), which includes, after the heating step, a cooling step of cooling from the heating temperature to 500°C at an average cooling rate of 900°C / hour or more.

[0095] This can prevent excessive precipitation of carbides during cooling.

[0096] (10) A tenth aspect of the present disclosure relates to a method for producing austenitic stainless cast steel according to (9), which includes, after the cooling step, an aging step of heating in a temperature range of 900°C to 1050°C for one hour or more, and a second cooling step of cooling from the temperature range of the aging step to 500°C at an average cooling rate of 900°C / hour or more.

[0097] By doing so, it is possible to uniformly precipitate carbides within the austenite grains.

[0098] (11) A method for producing austenitic stainless steel according to an eleventh aspect of the present disclosure is a method for producing austenitic stainless steel according to any one of (7) to (10), wherein the chemical composition of the austenitic stainless steel cast steel is, in mass%, C: 0.3% to 0.5%, Mn: 2.0% or less, P: 0.04% or less, S: 0.03% or less, Si: 1.0% to 2.5%, Ni: 36% to 39%, Cr: 18% to 21%, Mo: 0.5% or less, Nb: 1.2 to 1.8%, and the balance being iron and impurities.

[0099] By doing so, embrittlement of the austenitic stainless cast steel can be further suppressed.

Claims

1. The chemical composition, in mass%, is C: 0.3% to 0.5%, Mn: 2.0% or less, P: 0.04% or less, S: 0.03% or less, Si: 1.0% to 2.5%, Ni: 36% to 39%, Cr: 18% to 21%, Mo: 0.5% or less, Nb: 1.2-1.8%, the remainder being iron and impurities; In the cross section when heated at 1000°C, The average number Nc of carbides with a circle equivalent diameter of 500 nm or more in the center of austenite grains per unit area is 6.0 × 10 －２ pieces / μm ２ That's all, When the average number per unit area of ​​the carbides having a circle-equivalent diameter of 500 nm or more in the vicinity of the grain boundaries of austenite crystal grains is Ngb, Ngb / Nc is 1.30 or less.

2. The austenitic stainless cast steel according to claim 1, wherein the Ngb / Nc ratio is less than 0.

5.

3. It has a precipitate-free region, which is a region where no carbide is observed when observed with an optical microscope at a magnification of 300 times, 3. The austenitic stainless cast steel according to claim 2, wherein the width of the precipitate-free region is 1.5 μm to 20 μm.

4. 2. The austenitic stainless cast steel according to claim 1, wherein the Ngb / Nc ratio is 0.50 to 1.

30.

5. In the cross section before heating to 1000°C, The average number Nc of carbides with a circle equivalent diameter of 500 nm or more in the center of austenite grains per unit area is 6.0 × 10 －２ pieces / μm ２ That's all, 2. The austenitic stainless cast steel according to claim 1, wherein Ngb / Nc is 0.50 to 1.30, where Ngb is the average number per unit area of ​​the carbides having an equivalent circle diameter of 500 nm or more in the vicinity of grain boundaries of austenite crystal grains.

6. a heating step of heating the cast austenitic stainless steel at a heating temperature of 1100°C to 1250°C; and a slow cooling step of cooling from the heating temperature to 500°C at an average cooling rate of less than 100°C / hour after the heating step, wherein the chemical composition of the austenitic stainless cast steel is, in mass%, C: 0.3% to 0.5%, Mn: 2.0% or less, P: 0.04% or less, S: 0.03% or less, Si: 1.0% to 2.5%, Ni: 36% to 39%, Cr: 18% to 21%, Mo: 0.5% or less, Nb: 1.2-1.8%, A method for producing austenitic stainless cast steel, the balance of which consists of iron and impurities.

7. a heating step of heating the cast austenitic stainless steel at a heating temperature of 1100°C to 1250°C; a cooling step of cooling from the heating temperature to 500°C at an average cooling rate of 900°C / hour or more after the heating step; an aging step of heating at a temperature range of 900°C to 1050°C for 1 hour or more after the cooling step; a second cooling step of cooling from the temperature range of the aging step to 500°C at an average cooling rate of 900°C / hour or more; The chemical composition of the austenitic stainless cast steel is, in mass%, C: 0.3% to 0.5%, Mn: 2.0% or less, P: 0.04% or less, S: 0.03% or less, Si: 1.0% to 2.5%, Ni: 36% to 39%, Cr: 18% to 21%, Mo: 0.5% or less, Nb: 1.2-1.8%, A method for producing austenitic stainless cast steel, the balance of which consists of iron and impurities.

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