Austenitic stainless steel pipe and method for manufacturing same

Abstract

The present invention provides an austenitic stainless steel pipe having a large diameter, thick walls, high creep strength, outstanding SCC resistance, and outstanding SRC resistance. The austenitic stainless steel pipe according to the present invention has: a chemical composition that contains, in terms of mass%, 0.030% or less of C, 0.10-1.00% of Si, 0.10-2.00% of Mn, 15.00-25.00% of Cr, 6.00-15.00% of Ni, 0.020-0.200% of N, 0.030% or less of sol. Al, 0.001-0.045% of P, 0.010% or less of S, 0.20-1.00% of Nb, etc.; an outer diameter of at least 200 mm; a wall thickness of at least 15 mm; an austenitic crystal grain diameter of 20-300 μm; and a recrystallization rate X of at least 0.90.

Description

Austenitic stainless steel pipe and method of manufacturing the same

[0001] The present invention relates to an austenitic stainless steel pipe and a method for manufacturing the same.

[0002] Steel materials used in chemical plants, such as heating furnace tubes, are exposed to corrosive environments under high temperatures and pressures, and therefore are required to have high creep strength and excellent stress corrosion cracking resistance (hereinafter referred to as "SCC resistance").

[0003] Kudo Takeo et al., "Development of 347AP Stainless Steel for Heating Furnace Tubes with Excellent Polythion SCC Resistance," Sumitomo Metals, Vol. 38 (1986), No. 3, p. 190, discloses an austenitic stainless steel that has excellent weldability and high-temperature strength due to the reduction in carbon content and the addition of appropriate amounts of nitrogen and niobium, and that does not deteriorate in SCC resistance in a polythionic acid environment (hereinafter referred to as "polythionic acid SCC resistance") even after long-term aging without post-weld heat treatment.

[0004] Steel materials used in chemical plants are required to have stress relaxation cracking resistance (hereinafter referred to as "SRC resistance") in addition to creep strength and SCC resistance. To satisfy these properties, it is important to control the grain size. To improve SRC resistance, it is preferable to make the grains fine, but to ensure creep strength, it is preferable to make the grains coarse. Therefore, it is necessary to control the grain size within an appropriate range that satisfies both requirements.

[0005] Takeo Kudo et al., "Development of 347AP Stainless Steel for Heating Furnace Tubes with Excellent Polythion SCC Resistance," Sumitomo Metals, Vol. 38 (1986), No. 3, p. 190

[0006] Chemical plants use equipment of various sizes. Therefore, steel materials of various sizes are used depending on the size of the equipment. On the other hand, when a large-diameter, thick-walled steel pipe is manufactured using austenitic stainless steel containing Nb, good SRC resistance may not be obtained even if the grain size is within the appropriate range.

[0007] An object of the present invention is to provide a large-diameter, thick-walled austenitic stainless steel pipe having high creep strength, excellent SCC resistance, and excellent SRC resistance.

[0008] An austenitic stainless steel pipe according to one embodiment of the present invention has a chemical composition, in mass%, of C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.10 to 2.00%, Cr: 15.00 to 25.00%, Ni: 6.00 to 15.00%, N: 0.020 to 0.200%, sol. Al: 0.030% or less, P: 0.001 to 0.045%, S: 0.010% or less, Nb: 0.20 to 1.00%, and the balance: Fe and impurities. The pipe has an outer diameter of 200 mm or more, a wall thickness of 15 mm or more, an austenite grain size of 20 to 300 μm, and a recrystallization fraction X of 0.90 or more. The recrystallization fraction X is measured as follows. A sample is taken from the center of the wall thickness of the austenitic stainless steel pipe. 10.0 mm of a cross section perpendicular to the tube axis direction ２ The above region is used as the measurement region, and electron beam backscattering measurement is performed in a grid pattern at a scanning interval of 2.0 μm or less. Among the crystal grains in the measurement region, crystal grains with a GOS value of 1.0° or more are classified as non-recrystallized grains, and crystal grains with a GOS value of less than 1.0° are classified as recrystallized grains. The defined angle of the grain boundary is 4.0°. The ratio of the area S of the recrystallized grains to the area S of the measurement region is Ｒ The ratio S Ｒ / S is the recrystallization rate X.

[0009] An austenitic stainless steel pipe according to one embodiment of the present invention has a chemical composition, in mass%, of C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.10 to 2.00%, Cr: 15.00 to 25.00%, Ni: 6.00 to 15.00%, N: 0.020 to 0.200%, sol. Al: 0.030% or less, P: 0.001 to 0.045%, S: 0.010% or less, Nb: 0.20 to 1.00%, and the balance: Fe and impurities. The pipe has an outer diameter of 200 mm or more, a wall thickness of 15 mm or more, an austenite grain size of 20 to 300 μm, and a recrystallization fraction X of 0.90 or more. The recrystallization fraction X is measured as follows. A sample is taken from the center of the wall thickness of the austenitic stainless steel pipe. 10.0 mm of a cross section perpendicular to the tube axis direction２ The above region is used as the measurement region, and electron beam backscattering measurement is performed in a grid pattern at a scanning interval of 2.0 μm or less. Among the crystal grains in the measurement region, crystal grains with a GOS value of 1.0° or more are classified as non-recrystallized grains, and crystal grains with a GOS value of less than 1.0° are classified as recrystallized grains. The defined angle of the grain boundary is 4.0°. The ratio of the area S of the recrystallized grains to the area S of the measurement region is Ｒ The ratio S Ｒ / S is the recrystallization rate X.

[0010] An austenitic stainless steel pipe according to one embodiment of the present invention has a chemical composition, in mass %, of C: 0.030% or less, Si: 0.10 to 1.00%, Mn: 0.10 to 2.00%, Cr: 15.00 to 25.00%, Ni: 6.00 to 15.00%, N: 0.020 to 0.200%, sol. Al: 0.030% or less, P: 0.001 to 0.045%, S: 0.010% or less, Nb: 0.20 to 1.00%, and further containing one or more selected from the group consisting of Mo: 2.00% or less, Cu: 2.50% or less, and B: 0.0050% or less, the balance being Fe and impurities, and having an outer diameter of 200 mm or more, a wall thickness of 15 mm or more, an austenite grain size of 20 to 300 μm, and a recrystallization fraction X of 0.90 or more. The recrystallization fraction X is measured as follows. A sample is taken from the center of the wall thickness of the austenitic stainless steel pipe. A 10.0 mm thick specimen is taken from a cross section perpendicular to the pipe axis direction. ２ The above region is used as the measurement region, and electron beam backscattering measurement is performed in a grid pattern at a scanning interval of 2.0 μm or less. Among the crystal grains in the measurement region, crystal grains with a GOS value of 1.0° or more are classified as non-recrystallized grains, and crystal grains with a GOS value of less than 1.0° are classified as recrystallized grains. The defined angle of the grain boundary is 4.0°. The ratio of the area S of the recrystallized grains to the area S of the measurement region is Ｒ The ratio S Ｒ / S is the recrystallization rate X.

[0011] A method for manufacturing an austenitic stainless steel pipe according to one embodiment of the present invention is a method for manufacturing the above-mentioned austenitic stainless steel pipe, and comprises the steps of: piercing a steel material to manufacture a cup-shaped mother pipe; punching the cup-shaped mother pipe multiple times to manufacture an intermediate product; and heat treating the intermediate product at 1140 to 1190°C. In at least a part of the step of punching multiple times to manufacture the intermediate product, the cup-shaped mother pipe is held at a predetermined heating temperature T1 for a predetermined time, and then held at a heating temperature T2 that is lower than heating temperature T1 for a predetermined time, and then punching is performed.

[0012] According to the present invention, it is possible to obtain a large-diameter, thick-walled austenitic stainless steel pipe having high creep strength, excellent SCC resistance, and excellent SRC resistance.

[0013] Figure 1 is a GOS map of a structure having a recrystallization fraction X of 0.93. Figure 2 shows a frequency distribution of GOS values ​​for the structure of Figure 1. Figure 3 is a GOS map of a structure having a recrystallization fraction X of 0.59. Figure 4 shows a frequency distribution of GOS values ​​for the structure of Figure 3. Figure 5 is a flow chart of an example of a method for manufacturing an austenitic stainless steel pipe. Figure 6 shows an example of a heat pattern for heating in the punching process.

[0014] The present inventors investigated the reason why good SRC resistance may not be obtained when large-diameter, thick-wall steel pipes are manufactured from Nb-containing austenitic stainless steels. As a result, it was found that steel pipes with poor SRC resistance contain many unrecrystallized grains with residual strain in the structure.

[0015] Austenitic stainless steel pipes are generally manufactured by hot working steel material into a tubular shape and then performing a specified heat treatment. Hot working introduces strain into the crystal grains. Heat treatment after hot working causes recrystallization, releasing the strain introduced during hot working. However, if the amount of strain introduced during hot working is insufficient, recrystallization will not occur even after heat treatment, and the strain will remain as unrecrystallized grains.

[0016] Hot rolling (Mannesmann process), hot extrusion (Eugène-Séjournet process), hot punching (Erhardt push bench process), etc. are commonly used as hot working processes for shaping steel material into a tubular shape. Among these, hot punching is used for manufacturing large-diameter, thick-walled steel pipes with an outer diameter of 200 mm or more and a wall thickness of 15 mm or more. While hot extrusion processes impose relatively large working processes, such as a cross-sectional area reduction of 80% or more per extrusion, hot punching processes impose a cross-sectional area reduction of approximately 30% per extrusion at most. Therefore, it is thought that large-diameter, thick-walled steel pipes manufactured by hot punching processes are unlikely to develop uniform working strain in the wall thickness direction, resulting in the tendency for unrecrystallized grains to remain. Working strain can also be introduced by cold working, but when manufacturing large-diameter, thick-walled steel pipes, it is difficult to perform large-deformation cold working after hot working. Furthermore, in steels containing Nb, it is believed that the influence of Nb segregation makes it even more difficult for processing strain to be uniformly applied.

[0017] The inventors have established a method for quantifying the degree of remaining unrecrystallized grains by crystal orientation analysis using electron backscatter diffraction. Specifically, crystal grains with a GOS (Grain Orientation Spread) value, which is an index of the variation in orientation within the same crystal grain, of 1.0° or more are classified as unrecrystallized grains, and crystal grains with a GOS value of less than 1.0° are classified as recrystallized grains. In this case, the defined angle of the grain boundary is 4.0°. The ratio of the area S of the recrystallized grains to the area S of the measurement region is Ｒ The ratio S Ｒ / S is defined as the recrystallization rate X. If this recrystallization rate X is 0.90 or more and the crystal grain size is within an appropriate range (specifically, the austenite crystal grain size is 20 to 300 μm), excellent SRC resistance can be obtained even in a large-diameter, thick-walled steel pipe.

[0018] The present inventors have also investigated methods for obtaining an appropriate structure when manufacturing steel pipes by hot punching, and have found that by performing heating before punching in two stages and optimizing the heating temperature in the heat treatment after punching, it is possible to obtain a structure having a recrystallization rate X of 0.90 or more and an austenite grain size of 20 to 300 μm.

[0019] The present invention has been completed based on the above findings. An austenitic stainless steel pipe according to one embodiment of the present invention will now be described in detail.

[0020] [Chemical Composition] The austenitic stainless steel pipe according to this embodiment has the chemical composition described below. In the following description, "%" for the content of an element means mass %.

[0021] C: 0.030% or less Carbon (C) is inevitably contained. C is present at grain boundaries. ２３ C ６ C forms Cr carbides of this type, which reduces the SCC resistance of the steel. Therefore, the C content is 0.030% or less. The upper limit of the C content is preferably 0.025%, more preferably 0.020%. From the viewpoint of SCC resistance, it is preferable that the C content is as low as possible, but excessive reduction increases the manufacturing cost. The lower limit of the C content is preferably 0.001%, more preferably 0.003%.

[0022] Si: 0.10 to 1.00% Silicon (Si) deoxidizes steel. Si also improves the oxidation resistance and steam oxidation resistance of steel. On the other hand, if the Si content is too high, a σ phase is formed in the steel, reducing the creep strength of the steel. Therefore, the Si content is 0.10 to 1.00%. The lower limit of the Si content is preferably 0.15%, more preferably 0.20%. The upper limit of the Si content is preferably 0.80%, more preferably 0.60%.

[0023] Mn: 0.10 to 2.00% Manganese (Mn) deoxidizes steel. Mn also stabilizes austenite, increasing the creep strength of steel. On the other hand, if the Mn content is too high, a σ phase forms in the steel, reducing the creep strength of the steel. Therefore, the Mn content is 0.10 to 2.00%. The lower limit of the Mn content is preferably 0.30%, more preferably 0.50%, even more preferably 0.80%, and even more preferably 1.00%. The upper limit of the Mn content is preferably 1.80%, even more preferably 1.60%.

[0024] Cr: 15.00 to 25.00% Chromium (Cr) increases the polythionic acid SCC resistance of steel. Cr also increases the oxidation resistance, steam oxidation resistance, and high-temperature corrosion resistance of steel. On the other hand, if the Cr content is too high, the stability of austenite decreases, and the creep strength of the steel decreases. Therefore, the Cr content is 15.00 to 25.00%. The lower limit of the Cr content is preferably 16.00%, more preferably 16.50%. The upper limit of the Cr content is preferably 22.00%, more preferably 20.00%.

[0025] Ni: 6.00 to 15.00% Nickel (Ni) stabilizes austenite and increases the creep strength of steel. However, even if the Ni content is excessively high, the manufacturing cost increases and the above effect saturates. Therefore, the Ni content is 6.00 to 15.00%. The lower limit of the Ni content is preferably 8.00%, and more preferably 9.50%. The upper limit of the Ni content is preferably 14.00%, more preferably 13.00%, and even more preferably 12.00%.

[0026] N: 0.020 to 0.200% Nitrogen (N) dissolves in the matrix to stabilize austenite and improve the creep strength of steel. N also forms fine carbonitrides within grains, increasing the creep strength of steel. On the other hand, if the N content is too high, Cr nitrides are formed at grain boundaries, which reduces the polythionic acid SCC resistance in the weld heat-affected zone when the steel is welded. In addition, the hot workability of the steel is reduced. Therefore, the N content is 0.020 to 0.200%. The lower limit of the N content is preferably 0.040%, more preferably 0.060%. The upper limit of the N content is preferably 0.150%, more preferably 0.100%.

[0027] Sol. Al: 0.030% or less Aluminum (Al) is sometimes contained as a deoxidizer. On the other hand, if the Al content is too high, the hot workability of the steel deteriorates. Therefore, the Al content is 0.030% or less. When it is desired to actively obtain the deoxidizing effect of Al, the Al content may be 0.001% or more. The lower limit of the Al content is more preferably 0.002%. The upper limit of the Al content is preferably 0.025%, and even more preferably 0.020%. In this embodiment, the Al content refers to the content of acid-soluble Al (sol. Al).

[0028] P: 0.001 to 0.045% Phosphorus (P) increases the creep ductility of steel. On the other hand, if the P content is too high, the hot workability and toughness of the steel decrease. The lower limit of the P content is preferably 0.005%, more preferably 0.010%. The upper limit of the P content is preferably 0.040%, more preferably 0.030%.

[0029] S: 0.010% or less Sulfur (S) is an impurity. S reduces the hot workability of steel. Therefore, the S content is 0.010% or less. The upper limit of the S content is preferably 0.008%, more preferably 0.006%. From the viewpoint of hot workability, it is preferable that the S content is as low as possible, but excessive reduction increases the manufacturing cost. The lower limit of the S content is preferably 0.0001%, more preferably 0.0005%.

[0030] Nb: 0.20 to 1.00% Niobium (Nb) forms carbonitrides to fix C, thereby reducing the amount of solute C and improving the polythionic acid SCC resistance of steel. Nb also increases the creep strength of steel by precipitating as carbonitrides and contributes to refinement of grain size, improving SRC resistance. On the other hand, if the Nb content is too high, the amount of coarse carbonitrides increases, which become the starting point of fracture, thereby reducing toughness. The lower limit of the Nb content is preferably 0.25%, more preferably 0.30%. The upper limit of the Nb content is preferably 0.80%, more preferably 0.70%, even more preferably 0.60%, and even more preferably 0.50%.

[0031] The balance of the chemical composition of the austenitic stainless steel pipe according to this embodiment is Fe and impurities, which refer to elements that are mixed in from ores or scrap used as raw materials for steel, or from the environment during the manufacturing process.

[0032] The chemical composition of the austenitic stainless steel pipe according to this embodiment may contain, in place of a portion of Fe, one or more elements selected from the group consisting of 2.00% or less Mo, 2.50% or less Cu, and 0.0050% or less B. Mo, Cu, and B are elements that improve the creep strength of steel, but are all optional elements. In other words, the austenitic stainless steel pipe according to this embodiment may not contain some or all of Mo, Cu, and B.

[0033] Mo: 0 to 2.00% Molybdenum (Mo) increases the creep strength of steel. Mo also suppresses the formation of grain boundary carbides and improves the polythionic acid SCC resistance of steel. Even a small amount of Mo can provide these effects. On the other hand, if the Mo content is too high, the stability of austenite decreases. Therefore, the Mo content is 0 to 2.00%. The lower limit of the Mo content is preferably 0.05%, more preferably 0.10%, and even more preferably 0.20%. The upper limit of the Mo content is preferably 1.50%, more preferably 1.00%, and even more preferably 0.80%.

[0034] Cu: 0 to 2.50% Copper (Cu) precipitates as a fine Cu phase within grains, increasing the creep strength of steel. This effect can be achieved even if even a small amount of Cu is contained. On the other hand, if the Cu content is too high, the creep ductility of the steel decreases. Therefore, the Cu content is 0 to 2.50%. The lower limit of the Cu content is preferably 0.05%, more preferably 0.10%, and even more preferably 0.20%. The upper limit of the Cu content is preferably 2.00%, more preferably 1.80%, even more preferably 1.50%, and even more preferably 1.00%.

[0035] B: 0 to 0.0050% Boron (B) segregates at grain boundaries, increasing grain boundary strength and improving the creep strength and creep ductility of steel. This effect can be achieved even if even a small amount of B is contained. On the other hand, if the B content is too high, the weldability and hot workability of the steel will decrease. Therefore, the B content is 0 to 0.0050%. The lower limit of the B content is preferably 0.0005%, and more preferably 0.0010%. The upper limit of the B content is preferably 0.0040%, and more preferably 0.0030%.

[0036] [Structure] The austenitic stainless steel pipe according to this embodiment has a structure mainly composed of an austenite phase. The volume fraction of the austenite phase in the structure of the austenitic stainless steel pipe according to this embodiment is preferably 90% or more, and more preferably 95% or more.

[0037] [Recrystallization ratio X] The austenitic stainless steel pipe according to this embodiment has a recrystallization ratio X, which will be described below, of 0.90 or more. If the recrystallization ratio X is too small, good SRC resistance cannot be obtained. The recrystallization ratio X is preferably 0.94 or more, more preferably 0.96 or more, and even more preferably 0.98 or more.

[0038] The recrystallization rate X is measured as follows.

[0039] A sample for microstructure observation is taken from the center of the wall thickness of an austenitic stainless steel pipe. Electron backscatter diffraction (EBSD) measurement is performed on a cross section perpendicular to the pipe axis direction. The measurement area is 10.0 mm. ２ If the area of ​​the measurement area is small, the variation in the measurement data will be large. In this case, measurements should be taken in multiple fields of view to ensure that the total area is 10.0 mm or more. ２ For example, the size of one field of view may be 1000 μm×2000 μm (2.0 mm ２ ), measurements may be taken in five or more consecutive fields of view.

[0040] EBSD measurement is performed in a grid pattern with a scan interval of 2.0 μm or less. If the scan interval is too large, an accurate value of the recrystallization ratio X may not be obtained. Specifically, the value tends to be lower than the actual value. On the other hand, if the scan interval is set to 2.0 μm or less, the change in the measurement value due to the size of the scan interval is almost eliminated.

[0041] The grain boundary is determined from the EBSD measurement results. The defined angle of the grain boundary is set to 4.0°. That is, a misorientation of 4.0° or more is considered to be a grain boundary. If the defined angle of the grain boundary (i.e., the threshold misorientation for being considered a grain boundary) is set large, adjacent grains with a misorientation smaller than this defined angle are considered to be a single unrecrystallized grain, resulting in a small value calculated as the recrystallization rate X. On the other hand, if the defined angle of the grain boundary is set small, even subgrain boundaries with a very small driving force for recrystallization are considered to be grains, resulting in a large value calculated as the recrystallization rate X. When the defined angle is set to 4.0°, the most appropriate recrystallization rate X is obtained. Note that this value (4.0°) was determined by creating GOS maps while changing the defined angle of the grain boundary and comparing them with the IQ+IPF map.

[0042] The GOS (Grain Orientation Spread) value of each grain is calculated. The GOS value is the average value of the misorientation between a certain measurement point and other measurement points within the same grain. The GOS value is calculated by multiplying the misorientation between measurement points i and j by α ｉ、ｊ (unit: °) and the number of measurement points in the crystal grain is n, it is expressed by the following formula.

[0043]

[0044] In this embodiment, crystal grains with a GOS value of 1.0° or more are classified as non-recrystallized grains, and crystal grains with a GOS value of less than 1.0° are classified as recrystallized grains. Note that this threshold value (1.0°) was determined by comparing the frequency distribution of GOS values ​​and the GOS map with the IQ+IPF map.

[0045] Area S of recrystallized grains relative to area S of the measurement area Ｒ The ratio S Ｒ / S is defined as the "recrystallization ratio X." In addition, when the size of one visual field is 1000 μm × 2000 μm and EBSD measurement is performed on five or more consecutive visual fields as described above, the recrystallization ratio X may be determined for each visual field and then the average over all the visual fields may be calculated.

[0046] As an example, Fig. 1 shows a GOS map of a structure having a recrystallization fraction X of 0.93, and Fig. 2 shows the frequency distribution of GOS values ​​for the structure of Fig. 1. Similarly, Fig. 3 shows a GOS map of a structure having a recrystallization fraction X of 0.59, and Fig. 4 shows the frequency distribution of GOS values ​​for the structure of Fig. 3. In the GOS maps of Fig. 1 and Fig. 3, the gray areas are crystal grains with a GOS value of less than 1.0° (crystal grains classified as recrystallized grains), and the white areas are crystal grains with a GOS value of 1.0° or more (crystal grains classified as unrecrystallized grains).

[0047] [Austenite grain size] The austenitic stainless steel pipe according to this embodiment has an austenite grain size of 20 to 300 μm. The austenite grain size here is not the grain size used to calculate the GOS value in the EBSD measurement described above, but rather the average grain size calculated based on grain boundaries revealed by general mixed acid corrosion. Specifically, it is measured as follows.

[0048] A sample for microstructural observation is taken from the center of the wall thickness of an austenitic stainless steel pipe. The cross section perpendicular to the pipe axis direction is used as the observation surface, and after mirror polishing the observation surface, it is corroded with a mixed acid of hydrochloric acid and nitric acid in a ratio of 1:1 to reveal the austenite grain boundaries. The grain size number G is determined based on a cutting method in accordance with JIS G 0551 (2013). The grain size number G is converted to the average grain size R (μm) based on the following formula. This average grain size R is defined as the austenite grain size. n = 2 Ｇ＋３ R = 1000 / n １／２

[0049] If the austenite grain size is too small, the total area of ​​the grain boundaries increases, resulting in a decrease in creep strength. On the other hand, if the austenite grain size is too large, the precipitates become non-uniform, also resulting in a decrease in creep strength. The lower limit of the austenite grain size is preferably 30 μm, more preferably 50 μm, even more preferably 60 μm, and even more preferably 70 μm. The upper limit of the austenite grain size is preferably 260 μm, even more preferably 220 μm, and even more preferably 180 μm.

[0050] [Mechanical Properties, etc.] The austenitic stainless steel pipe according to this embodiment preferably has a tensile strength of 515 MPa or more at room temperature.

[0051] The austenitic stainless steel pipe according to this embodiment has an outer diameter of 200 mm or more and a wall thickness of 15 mm or more.

[0052] The lower limit of the outer diameter of the austenitic stainless steel pipe according to this embodiment is preferably 300 mm, more preferably 400 mm, and even more preferably 500 mm. The upper limit of the outer diameter of the austenitic stainless steel pipe according to this embodiment is not particularly limited, but is, for example, 1000 mm.

[0053] The lower limit of the wall thickness of the austenitic stainless steel pipe according to this embodiment is preferably 20 mm, more preferably 25 mm, and even more preferably 30 mm. The upper limit of the wall thickness of the austenitic stainless steel pipe according to this embodiment is not particularly limited, but is, for example, 100 mm.

[0054] The austenitic stainless steel pipe according to this embodiment is preferably a seamless steel pipe.

[0055] [Manufacturing Method] An example of a method for manufacturing an austenitic stainless steel pipe according to this embodiment will be described. The manufacturing method described below is merely an example, and the manufacturing method for an austenitic stainless steel pipe according to this embodiment is not limited to this.

[0056] 5 is a flow diagram of an example of a method for manufacturing an austenitic stainless steel pipe. This manufacturing method includes a piercing step (step S1), multiple punching steps (steps S2-1, S2-2, ..., S2-n), and a heat treatment step (step S3).

[0057] An ingot having the above-described chemical composition is heated to a predetermined temperature and then pierced by a vertical press to produce a bottomed tubular intermediate product (hereinafter referred to as a "cup-shaped mother tube") (piercing step (step S1)). The heating temperature before piercing is, for example, 1200 to 1300°C.

[0058] After heating the cup-shaped mother tube to a predetermined temperature, a mandrel is inserted into the mother tube, and the mother tube is punched through a die (punching step (steps S2-1, S2-2, ..., S2-n)). If the degree of deformation applied in one punching is too great, the bottom of the cup-shaped mother tube will break, making punching impossible. Therefore, multiple punchings are performed to form the mother tube into the desired size. The cross-sectional area reduction rate in one punching is preferably 30.0% or less.

[0059] Here, in at least some of the multiple punching steps (steps S2-1, S2-2, . . . S2-n), punching including two-stage heating, which will be described next, is performed.

[0060] 6 shows an example of a heating pattern for the punching process, in which the cup-shaped blank is first held at a heating temperature T1 for a holding time t1, then held at a heating temperature T2, which is lower than the heating temperature T1, for a holding time t2, and then punching is performed.

[0061] Recrystallization can be promoted by heating the cup-shaped blank to a relatively high temperature T1 before punching. However, since holding the blank at the temperature T1 for a long time causes the crystal grains to become coarse, the heating temperature is lowered to a temperature T2 after holding for the holding time t1.

[0062] The heating temperature T1 is, for example, 1150 to 1250° C. The lower limit of the heating temperature T1 is preferably 1170° C. The upper limit of the heating temperature T1 is preferably 1220° C.

[0063] The retention time t1 is, for example, 60 to 300 minutes. The lower limit of the retention time t1 is preferably 80 minutes, more preferably 120 minutes. The upper limit of the retention time t1 is preferably 240 minutes, more preferably 180 minutes.

[0064] The difference T1-T2 between the heating temperature T1 and the heating temperature T2 is preferably 20°C or more, and more preferably 30°C or more.

[0065] The retention time t2 is, for example, 30 to 300 minutes. The lower limit of the retention time t2 is preferably 40 minutes, more preferably 60 minutes. The upper limit of the retention time t2 is preferably 240 minutes, more preferably 180 minutes.

[0066] As described above, the punching including the two-stage heating described above may be performed in at least some of the multiple punching steps (steps S2-1, S2-2, ..., S2-n). In the other punching steps, the cup-shaped mother tube may be heated to a temperature at which punching is possible (e.g., 1150 to 1250°C), and then punched. However, it is believed that the more times the punching including the two-stage heating described above is performed, the higher the recrystallization rate X can be. Therefore, in the multiple punching steps (steps S2-1, S2-2, ..., S2-n), the punching including the two-stage heating described above is preferably performed two or more times, more preferably three or more times, and even more preferably four or more times.

[0067] The bottom of the cup-shaped raw tube may be cut and removed after the punching step is completed, or may be removed after the heat treatment step (step S3).

[0068] The intermediate product after the punching step is heat treated (step S3). The heating temperature for the heat treatment is 1140 to 1190°C. If the heating temperature for the heat treatment is too low, a structure with a high recrystallization rate X cannot be obtained. On the other hand, if the heat treatment temperature is too low, the crystal grains become coarse. The lower limit of the heat treatment temperature is preferably 1150°C. The upper limit of the heat treatment temperature is preferably 1180°C.

[0069] The holding time of the heat treatment is preferably 15 to 180 minutes. The lower limit of the holding time of the heat treatment is preferably 30 minutes. The upper limit of the holding time of the heat treatment is preferably 120 minutes, and more preferably 60 minutes. After the holding time has elapsed, it is preferable to water-cool the intermediate product.

[0070] It is preferable not to perform cold working after the heat treatment, because if cold working is performed, it becomes difficult to maintain the predetermined size.

[0071] An austenitic stainless steel pipe is manufactured through the above steps. In this manufacturing method, punching including two-stage heating is performed in at least some of the multiple punching steps. After the punching step, heat treatment is performed at 1140 to 1190°C. This results in a structure with a recrystallization ratio X of 0.90 or more and an austenite grain size of 20 to 300 μm.

[0072] The above describes an austenitic stainless steel pipe according to one embodiment of the present invention. According to this embodiment, a large-diameter, thick-walled austenitic stainless steel pipe having high creep strength, excellent SCC resistance, and excellent SRC resistance can be obtained.

[0073] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0074] Steel having the chemical composition shown in Table 1 was melted and produced into an ingot. Piercing, punching and heat treatment were carried out on this ingot under the conditions shown in Tables 2 and 3 to produce steel pipes having the dimensions shown in Table 2.

[0075]

[0076]

[0077]

[0078] Table 3 summarizes the heating patterns for heating before punching. The "Punching 1" column in Table 3 lists the heating conditions before the first punching. The same applies to the "Punching 2," "Punching 3," and so on columns. In these columns, for example, "1250°C (180 minutes)" means that the cup-shaped mother tube was held at 1250°C for 180 minutes before punching. Furthermore, for example, "1200°C (120 minutes) → 1150°C (60 minutes)" means that the cup-shaped mother tube was held at 1200°C for 120 minutes, then held at 1150°C for 60 minutes, and then punching was performed. The recrystallization start temperature of these steels is considered to be approximately 1000°C.

[0079] A sample for microstructure observation was taken from the center of the wall thickness of each produced steel pipe, and the austenite grain size R and recrystallization rate X were measured. Furthermore, a creep rupture test, a tensile test, an SRC test, and an SCC test, which will be described below, were carried out.

[0080] [Creep rupture test] A creep rupture test was carried out in accordance with JIS Z 2271. Specifically, the creep rupture test specimen was heated to 650°C, and then the creep rupture test was carried out. The test stress was set to 200 MPa, and the creep rupture time was determined. The creep rupture time was targeted to be 300 hours or more.

[0081] [Tensile Test] A room temperature tensile test was carried out, with the goal of achieving a tensile strength TS of 515 MPa or more.

[0082] [SRC Test] A stress relaxation test was carried out in accordance with ASTM E328. The test conditions were as follows: Pre-strain: 10% at room temperature (strain rate: 2 mm / min) Temperature: 650°C (heated over 4 hours, held for 2 hours, and then high-temperature strain was applied) Holding time: 1000 hours

[0083] After 1000 hours, the microstructure of the unfractured test specimens was observed using a scanning electron microscope at a cross section perpendicular to the longitudinal direction of the test specimen. If the microscopic observation showed no cracks at the grain boundaries and no creep voids, the SRC resistance was evaluated as "good." If any of the test specimens was fractured, had grain boundary cracks, or had creep voids, the SRC resistance was evaluated as "poor."

[0084] [SCC Test] To evaluate intergranular corrosion resistance, a sulfuric acid / copper sulfate corrosion test was performed in accordance with JIS G 0575. After the boiling test, the test piece was removed from the test solution and subjected to a bending test. The outer apex of the bent surface was then observed under a microscope to check for intergranular corrosion. As a result of the microscopic observation, specimens that did not exhibit intergranular corrosion were evaluated as "good," and specimens that exhibited intergranular corrosion were evaluated as "poor."

[0085] The results are shown in Table 4.

[0086]

[0087] The steel pipes of test numbers 1 to 4 had an austenite grain size R of 20 to 300 μm and a recrystallization ratio of 0.90 or more. These steel pipes had high creep strength and excellent SCC resistance and SRC resistance.

[0088] The steel pipe of test number 5 had poor SRC resistance. This is thought to be due to the low recrystallization rate X. The low recrystallization rate X is thought to be due to the inability to apply sufficient processing strain during the punching process.

[0089] The steel pipe of test number 6 had low creep strength. This is thought to be because the austenite grain size R was too large. The large austenite grain size R is thought to be due to the coarsening of the grains during the punching process.

[0090] The steel pipe of test number 7 had poor SRC resistance. This is thought to be due to the low recrystallization rate X. The low recrystallization rate X is thought to be due to the heating temperature in the heat treatment step being too low.

[0091] The steel pipe of test number 8 had low creep strength. This is thought to be because the austenite grain size R was too large. The large austenite grain size R is thought to be because the heating temperature in the heat treatment step was too high.

[0092] Although the embodiments of the present invention have been described above, the above-described embodiments are merely examples for carrying out the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified and carried out within the scope of the invention.

Claims

1. The chemical composition is by mass%, C: 0.030% or less, Si: 0.10 - 1.00%, Mn: 0.10 - 2.00%, Cr: 15.00 - 25.00%, Ni: 6.00 - 15.00%, N: 0.020 - 0.200%, sol. Al: 0.030% or less, P: 0.001 - 0.045%, S: 0.010% or less, Nb: 0.20 - 1.00%, Mo: 0 - 2.00%, Cu: 0 - 2.50%, B: 0 - 0.0050%, the balance being Fe and impurities, the outer diameter is 200 mm or more, the wall thickness is 15 mm or more, the austenite crystal grain size is 20 - 300 μm, and the recrystallization rate X is 0.90 or more, an austenitic stainless steel pipe. The recrystallization rate X is measured as follows. A sample is taken from the central part of the wall thickness of the austenitic stainless steel pipe. A region of 10.0 mm or more in the cross section perpendicular to the pipe axis direction is used as the measurement region, and electron backscatter diffraction measurement is performed in a grid pattern at a scanning interval of 2.0 μm or less. Among the crystal grains in the measurement region, crystal grains with a GOS value of 1.0° or more are classified as non-recrystallized grains, and crystal grains with a GOS value of less than 1.0° are classified as recrystallized grains. The definition angle of the crystal grain boundary is 4.0°. The ratio S of the area S of the recrystallized grains to the area S of the measurement region is defined as the recrystallization rate X. ２ For the region of 10.0 mm or more in the cross section perpendicular to the pipe axis direction as the measurement region, electron backscatter diffraction measurement is carried out in a grid pattern at a scanning interval of 2.0 μm or less. Among the crystal grains in the measurement region, crystal grains with a GOS value of 1.0° or more are classified as non-recrystallized grains, and crystal grains with a GOS value of less than 1.0° are classified as recrystallized grains. The definition angle of the crystal grain boundary is 4.0°. The ratio S of the area S of the recrystallized grains to the area S of the measurement region is defined as the recrystallization rate X. Ｒ of the recrystallized grains to the area S of the measurement region is defined as the recrystallization rate X. Ｒ of the recrystallized grains to the area S of the measurement region is defined as the recrystallization rate X.

2. The austenitic stainless steel pipe according to Claim 1, wherein the chemical composition is by mass%, Mo: 0.05 - 2.00%, Cu: 0.05 - 2.50%, and B: 0.0005 - 0.0050%, containing one or more selected from the group consisting of, an austenitic stainless steel pipe.

3. The austenitic stainless steel pipe according to Claim 1 or 2, wherein the austenitic stainless steel pipe is a seamless pipe, an austenitic stainless steel pipe.

4. A method for manufacturing the austenitic stainless steel pipe according to Claim 3, comprising a step of piercing a steel material to produce a cup-shaped blank pipe, a step of drawing the cup-shaped blank pipe a plurality of times to produce an intermediate product, and a step of heat-treating the intermediate product at 1140 - 1190 °C, and in at least a part of the step of drawing the cup-shaped blank pipe a plurality of times to produce an intermediate product, after holding the cup-shaped blank pipe at a predetermined heating temperature T1 for a predetermined time, holding it at a heating temperature T2 which is lower than the heating temperature T1 for a predetermined time, and then performing drawing, a method for manufacturing an austenitic stainless steel pipe.