Austenitic stainless steel pipe and method for producing same

Abstract

Provided is an austenitic stainless steel pipe having a large diameter, a thick wall, high creep strength, excellent SCC resistance, and excellent SRC resistance. An austenitic stainless steel pipe which has a chemical composition containing, in 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, and the like, and which has an outer diameter of 200 mm or more, a wall thickness of 15 mm or more, an austenite grain diameter of 20-300 [mu] m, and a recrystallization rate X of 0.90 or more.

Description

Technical Field

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

[0002] Steel materials used in chemical equipment such as heating furnace tubes are exposed to high temperature and high pressure corrosive environments. Therefore, high creep strength and excellent stress corrosion cracking resistance (hereinafter referred to as "SCC resistance") are required.

[0003] Kudo Takeo et al., in "Development of 347AP Stainless Steel for Heating Furnace Tubes with Excellent Polythionic Sulfuric Acid SCC Resistance", Sumitomo Metal, Vol. 38 (1986), No. 3, p. 190, discloses an austenitic stainless steel having excellent weldability and high temperature strength by reducing C and adding appropriate amounts of N and Nb, and having no post-heat treatment after welding, and having SCC resistance in a polythionic acid environment (hereinafter referred to as "polythionic acid SCC resistance") that does not deteriorate after long-term aging.

[0004] Steel materials used in chemical equipment require stress relaxation crack resistance (hereinafter referred to as "SRC resistance") in addition to creep strength and SCC resistance. In order to meet these characteristics, grain control is important. In order to improve SRC resistance, it is preferred to make the grains fine, but in order to ensure creep strength, it is preferred to make the grains coarse. Therefore, it is necessary to control the grain size within an appropriate range that satisfies both.

[0005] Prior art literature

[0006] Patent Literature

[0007] Non-patent document 1: Kudo Takeo et al., "Development of 347AP stainless steel for heating furnace tubes with excellent polythionic acid SCC resistance", Sumitomo Metal, Vol. 38 (1986), No. 3, p. 190 Summary of the invention

[0008] Problem that the invention aims to solve

[0009] Chemical plants use equipment of various sizes. Therefore, steel materials of various sizes are used according to the size of the equipment. On the other hand, when austenitic stainless steel containing Nb is used to manufacture large-diameter and thick-walled steel pipes, good SRC resistance may not be obtained even if the grain size is within an appropriate range.

[0010] An object of the present invention is to provide an austenitic stainless steel pipe having a large diameter and a thick wall and having high creep strength, excellent SCC resistance, and excellent SRC resistance.

[0011] Solution for solving problems

[0012] An austenitic stainless steel pipe according to an embodiment of the present invention has a chemical composition by 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%, balance: Fe and impurities. The outer diameter of the austenitic stainless steel pipe is 200 mm or more, the wall thickness is 15 mm or more, the austenite grain diameter is 20 to 300 μm, and the recrystallization rate X is 0.90 or more. The aforementioned recrystallization rate X is measured as follows. A sample is collected from the central part of the wall thickness of the aforementioned austenitic stainless steel pipe. A region of 10.0 mm 2 or more in the cross section perpendicular to the pipe axis direction is used as the measurement region, and electron backscatter measurement is performed in a grid pattern at a scanning interval of 2.0 μm or less. Among the grains in the aforementioned measurement region, grains with a GOS value of 1.0° or more are classified as non-recrystallized grains, and grains with a GOS value less than 1.0° are classified as recrystallized grains. The defined angle of the grain boundary is set to 4.0°. The area S R of the aforementioned recrystallized grains relative to the area S of the aforementioned measurement region S R / S is used as the recrystallization rate X.

[0013] An austenitic stainless steel pipe according to an embodiment of the present invention has a chemical composition by 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%, balance: Fe and impurities. The outer diameter of the austenitic stainless steel pipe is 200 mm or more, the wall thickness is 15 mm or more, the austenite grain diameter is 20 to 300 μm, and the recrystallization rate X is 0.90 or more. The aforementioned recrystallization rate X is measured as follows. A sample is collected from the central part of the wall thickness of the aforementioned austenitic stainless steel pipe. A region of 10.0 mm 2The above region is used as the measurement region, and electron backscatter diffraction measurement is performed in a grid pattern with a scanning interval of 2.0 μm or less. Among the grains in the measurement region, grains with a GOS value of 1.0° or more are classified as non-recrystallized grains, and grains with a GOS value of less than 1.0° are classified as recrystallized grains. The defined angle of the grain boundary is set to 4.0°. The area S R of the above-mentioned recrystallized grains relative to the area S of the measurement region is S R / S, which is defined as the recrystallization rate X.

[0014] The austenitic stainless steel pipe of one embodiment of the present invention has a chemical composition by mass% of 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%, and further contains 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, with the balance being Fe and impurities. The outer diameter of the austenitic stainless steel pipe is 200 mm or more, the wall thickness is 15 mm or more, the austenite grain diameter is 20 - 300 μm, and the recrystallization rate X is 0.90 or more. The above-mentioned recrystallization rate X is measured as follows. A specimen is collected from the central part of the wall thickness of the austenitic stainless steel pipe. An area of 10.0 mm 2 or more of 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 with a scanning interval of 2.0 μm or less. Among the grains in the measurement region, grains with a GOS value of 1.0° or more are classified as non-recrystallized grains, and grains with a GOS value of less than 1.0° are classified as recrystallized grains. The defined angle of the grain boundary is set to 4.0°. The area S R of the above-mentioned recrystallized grains relative to the area S of the measurement region is S R / S, which is defined as the recrystallization rate X.

[0015] 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 includes: a step of piercing a steel billet to manufacture a cup-shaped tube blank; a step of performing multiple push-bench operations on the cup-shaped tube blank to manufacture an intermediate product; and a step of heat-treating the intermediate product at 1140 - 1190°C. In at least a part of the step of performing the multiple push-bench operations to manufacture the intermediate product, the cup-shaped tube blank is held at a specified heating temperature T1 for a specified time, then held at a temperature lower than the heating temperature T1, i.e., the heating temperature T2, for a specified time, and then push-benched.

[0016] Effect of the Invention

[0017] According to the present invention, an austenitic stainless steel steel pipe having a large diameter and a thick wall, high creep strength, excellent SCC resistance, and excellent SRC resistance can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a GOS diagram of a structure with a recrystallization rate X of 0.93.

[0019] Figure 2 It is Figure 1 The frequency distribution of the GOS values of the structure.

[0020] Figure 3 It is a GOS diagram of a structure with a recrystallization rate X of 0.59.

[0021] Figure 4 It is Figure 3 The frequency distribution of the GOS values of the structure.

[0022] Figure 5 It is a flowchart of an example of a method for manufacturing an austenitic stainless steel steel pipe.

[0023] Figure 6 It is an example of a heating curve for heating in the pipe jacking process. DETAILED DESCRIPTION OF THE INVENTION

[0024] The present inventors investigated the reason why good SRC resistance could not sometimes be obtained when manufacturing a large-diameter and thick-walled steel pipe using austenitic stainless steel containing Nb. As a result, it was found that in a steel pipe with poor SRC resistance, a large amount of unrecrystallized grains with residual strain were contained in the structure.

[0025] An austenitic stainless steel steel pipe is usually manufactured by hot-working a steel billet into a tubular shape and then performing a prescribed heat treatment. Through hot-working, strain is introduced into the grains. Recrystallization occurs by performing heat treatment after hot-working, and the strain introduced during hot-working is released. At this time, it is considered that if the amount of strain introduced during hot-working is insufficient, recrystallization does not occur even if heat treatment is performed, and the strain remains as residual unrecrystallized grains.

[0026] As hot working for making a steel billet into a tube, a hot rolling method (Mannesmann method), a hot extrusion method (high-speed extrusion method with glass lubricant), a hot pipe jacking method (Erhardt pipe jacking method), etc. are generally used. Among them, in the manufacture of steel pipes with a large outer diameter and wall thickness such as an outer diameter of 200 mm or more and a wall thickness of 15 mm or more, the hot pipe jacking method is used. In the hot extrusion method, a relatively large working with a reduction in cross-sectional area of 80% or more is applied in one extrusion. In contrast, in the hot pipe jacking method, the reduction in cross-sectional area of one pipe jacking is at most about 30%. Therefore, it is considered that in the manufacture of such large-diameter and thick-walled steel pipes using the hot pipe jacking method, it is difficult to generate uniform working strain in the wall thickness direction, and unrecrystallized grains tend to remain. Working strain can also be introduced by cold working, but in the case of manufacturing large-diameter and thick-walled steel pipes, it is difficult to perform cold working with a large degree of working after hot working. In addition, in steel containing Nb, it is further difficult to uniformly introduce working strain due to the influence of Nb segregation.

[0027] The present inventors have established a method for quantifying the degree of remaining unrecrystallized grains by using crystal orientation analysis by electron backscatter diffraction. Specifically, grains with a GOS (Grain Orientation Spread) value, which is an index of the deviation of orientations within the same grain, of 1.0° or more are classified as unrecrystallized grains, and grains with a GOS value of less than 1.0° are classified as recrystallized grains. At this time, the defined angle of the grain boundary is set to 4.0°. The area S of the recrystallized grains R The ratio S of the area S of the recrystallized grains to the area S of the measurement region R / S is set as the recrystallization rate X. If this recrystallization rate X is 0.90 or more and the grain size is in an appropriate range (specifically, the austenite grain diameter is 20 to 300 μm), excellent SRC resistance can be obtained even for large-diameter and thick-walled steel pipes.

[0028] The present inventors have also studied a method for obtaining an appropriate structure in the case of manufacturing a steel pipe using the hot pipe jacking method. As a result, it has been found that by heating before pipe jacking in two stages and optimizing the heating temperature of the heat treatment after pipe jacking, a structure with a recrystallization rate X of 0.90 or more and an austenite grain diameter of 20 to 300 μm can be obtained.

[0029] The present invention has been completed based on the above insights. Hereinafter, an austenitic stainless steel pipe according to an embodiment of the present invention will be described in detail.

[0030] [Chemical composition]

[0031] The austenitic stainless steel pipe of the present embodiment has the chemical composition described below. In the following description, "%" of the content of an element means mass %.

[0032] C: 0.030% or less

[0033] Carbon (C) is inevitably contained. C forms M at grain boundaries 23 C6-type Cr carbides, reducing 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 perspective of SCC resistance, the C content is preferably as low as possible, but if it is excessively reduced, the manufacturing cost increases. The lower limit of the C content is preferably 0.001%, more preferably 0.003%.

[0034] Si: 0.10 - 1.00%

[0035] Silicon (Si) deoxidizes the steel. Si further improves the oxidation resistance and steam oxidation resistance of the steel. On the other hand, when the Si content is too high, σ-phase is formed in the steel, reducing the creep strength of the steel. Therefore, the Si content is 0.10 - 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%.

[0036] Mn: 0.10 - 2.00%

[0037] Manganese (Mn) deoxidizes the steel. Mn further stabilizes austenite, increasing the creep strength of the steel. On the other hand, if the Mn content is too high, σ-phase is formed in the steel, reducing the creep strength of the steel. Therefore, the Mn content is 0.10 - 2.00%. The lower limit of the Mn content is preferably 0.30%, more preferably 0.50%, more preferably 0.80%, more preferably 1.00%. The upper limit of the Mn content is preferably 1.80%, more preferably 1.60%.

[0038] Cr: 15.00 - 25.00%

[0039] Chromium (Cr) improves the SCC resistance to polysulfuric acid. Cr further improves the oxidation resistance, steam oxidation resistance, and high-temperature corrosion resistance of the 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 - 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%.

[0040] Ni: 6.00 - 15.00%

[0041] Nickel (Ni) stabilizes austenite, thereby increasing the creep strength of the steel. On the other hand, even if the Ni content is too high, the above effect saturates with respect to the increase in manufacturing cost. Therefore, the Ni content is 6.00 to 15.00%. The lower limit of the Ni content is preferably 8.00%, more preferably 9.50%. The upper limit of the Ni content is preferably 14.00%, more preferably 13.00%, and still more preferably 12.00%.

[0042] N: 0.020 to 0.200%

[0043] Nitrogen (N) dissolves in the matrix to stabilize austenite, improving the creep strength of the steel. N further forms fine carbonitrides within the grains to increase the creep strength of the steel. On the other hand, if the N content is too high, Cr nitrides are formed at the grain boundaries, and when welding the steel, the resistance to polysulfide SCC in the heat-affected zone of the weld decreases. In addition, the hot workability of the steel decreases. 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%.

[0044] sol.Al: 0.030% or less

[0045] Aluminum (Al) is sometimes contained as a deoxidizer. On the other hand, when the Al content is too high, the hot workability of the steel decreases. Therefore, the Al content is 0.030% or less. In the case where it is desired to actively obtain the deoxidation effect achieved by Al, the Al content can 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%, more preferably 0.020%. The Al content in this embodiment refers to the content of acid-soluble Al (sol.Al).

[0046] P: 0.001 to 0.045%

[0047] Phosphorus (P) improves the creep ductility of the steel. On the other hand, when 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%.

[0048] S: 0.010% or less

[0049] Sulfur (S) is an impurity. S reduces the hot workability of the 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 perspective of hot workability, the S content is preferably as low as possible, but if it is desired to reduce it excessively, the manufacturing cost increases. The lower limit of the S content is preferably 0.0001%, more preferably 0.0005%.

[0050] Nb: 0.20 to 1.00%

[0051] Niobium (Nb) fixes C by forming carbonitrides, thereby reducing the amount of solid-solution C and improving the resistance of the steel to SCC by polythionic acid. In addition, Nb precipitates as carbonitrides, thereby increasing the creep strength of the steel and contributing to the refinement of the grain size and the improvement of the SRC resistance. On the other hand, if the Nb content is too high, the amount of coarse carbonitrides increases, becoming the starting point of fracture, and thus the toughness decreases. 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%, more preferably 0.60%, more preferably 0.50%.

[0052] The balance of the chemical composition of the austenitic stainless steel pipe of the present embodiment is Fe and impurities. The impurities mentioned here refer to elements mixed in from ores and scraps used as raw materials for steel, or elements mixed in from the manufacturing environment and the like.

[0053] The chemical composition of the austenitic stainless steel pipe of the present embodiment may contain 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 to replace a part of Fe. Mo, Cu, and B are elements that improve the creep strength of the steel and are all optional elements. That is, the austenitic stainless steel pipe of the present embodiment may not contain some or all of Mo, Cu, and B.

[0054] Mo: 0 to 2.00%

[0055] Molybdenum (Mo) increases the creep strength of the steel. In addition, Mo inhibits the formation of grain boundary carbides and improves the resistance of the steel to SCC by polythionic acid. These effects can be obtained as long as a small amount of Mo is contained. On the other hand, when 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%, more preferably 0.20%. The upper limit of the Mo content is preferably 1.50%, more preferably 1.00%, more preferably 0.80%.

[0056] Cu: 0 to 2.50%

[0057] Copper (Cu) precipitates as fine Cu phases within the grains, enhancing the creep strength of the steel. This effect can be achieved with a small amount of Cu. On the other hand, when 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 further preferably 0.20%. The upper limit of the Cu content is preferably 2.00%, more preferably 1.80%, further preferably 1.50%, and further preferably 1.00%.

[0058] B: 0 to 0.0050%

[0059] Boron (B) segregates at the grain boundaries, enhancing the grain boundary strength and thereby improving the creep strength and creep ductility of the steel. This effect can be achieved with a small amount of B. On the other hand, when the B content is too high, the weldability and hot workability of the steel decrease. Therefore, the B content is 0 to 0.0050%. The lower limit of the B content is preferably 0.0005%, more preferably 0.0010%. The upper limit of the B content is preferably 0.0040%, more preferably 0.0030%.

[0060] [Microstructure]

[0061] The austenitic stainless steel pipe of this embodiment has a microstructure mainly composed of the austenite phase. The volume fraction of the austenite phase in the microstructure of the austenitic stainless steel pipe of this embodiment is preferably 90% or more, more preferably 95% or more.

[0062] [Recrystallization ratio X]

[0063] The recrystallization ratio X described below for the austenitic stainless steel pipe of this embodiment is 0.90 or more. When 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 further preferably 0.98 or more.

[0064] The recrystallization ratio X is measured as follows.

[0065] A specimen for microstructure observation is collected from the central part of the wall thickness of the austenitic stainless steel pipe. The cross-section perpendicular to the pipe axis direction is used as the observation surface, and electron backscatter diffraction (EBSD) measurement is performed. The measurement area is a continuous area with an area of 10.0 mm 2 or more. If the area of the measurement area is small, the deviation of the measurement data becomes large. At this time, it is also possible to perform measurements in multiple fields of view so that the total area is 10.0 mm 2 or more. For example, when the size of one field of view is 1000 μm × 2000 μm (2.0 mm 2 ), it is possible to perform measurements in 5 or more consecutive fields of view.

[0066] The EBSD measurement is carried out in a grid pattern at a scanning interval of 2.0 μm or less. When the scanning interval is too large, an accurate value of the recrystallization rate X may not be obtained. Specifically, there is a tendency for the value to be lower than the original value. On the other hand, if the scanning interval is 2.0 μm or less, there is little change in the measured value due to the size of the scanning interval.

[0067] The grain boundaries are determined from the EBSD measurement results. The defined angle of the grain boundary is set to 4.0°. That is, an orientation difference of 4.0° or more is regarded as a grain boundary. If the defined angle of the grain boundary (i.e., the threshold value of the orientation difference regarded as a grain boundary) is set larger, multiple adjacent grains with an orientation difference smaller than this defined angle are regarded as one unrecrystallized grain, so the value calculated as the recrystallization rate X becomes smaller. On the other hand, if the defined angle of the grain boundary is set smaller, sub-grain boundaries with a very small driving force for recrystallization are also regarded as grains, so the value calculated as the recrystallization rate X becomes larger. When the defined angle is set to 4.0°, the most appropriate recrystallization rate X can be obtained. It should be noted that this value (4.0°) is determined by making a GOS map while changing the defined angle of the grain boundary and comparing it with the IQ + IPF map.

[0068] The GOS (Grain Orientation spread) value of each grain is calculated. The GOS value is the average of the orientation differences between a certain measurement point and other measurement points within the same grain. When the orientation difference between measurement point i and measurement point j is set to α i 、 j (in degrees) and the number of measurement points within the grain is set to n, the GOS value is represented by the following formula.

[0069] [Mathematical formula 1]

[0070]

[0071] In this embodiment, grains with a GOS value of 1.0° or more are classified as unrecrystallized grains, and grains with a GOS value less than 1.0° are classified as recrystallized grains. It should be noted that this threshold value (1.0°) is determined by performing a frequency distribution of the GOS value and comparing the GOS map with the IQ + IPF map.

[0072] The area S of the recrystallized grains R relative to the area S of the measurement region is S R / S as the "recrystallization rate X". It should be noted that as described above, the size of one field of view is 1000 μm × 2000 μm. When the EBSD measurement is performed in 5 or more consecutive fields of view, the recrystallization rate X can be calculated for each field of view, and the average of all fields of view can be obtained.

[0073] As an example,Figure 1 GOS map showing the structure with a recrystallization ratio X of 0.93 Figure 2 indicating Figure 1 the frequency distribution of GOS values of the structure. Similarly, Figure 3 GOS map showing the structure with a recrystallization ratio X of 0.59 Figure 4 indicating Figure 3 the frequency distribution of GOS values of the structure. In the Figure 1 and Figure 3 GOS maps, the gray - shaded part represents grains with GOS values less than 1.0° (grains classified as recrystallized grains), and the white - shaded part represents grains with GOS values of 1.0° or more (grains classified as non - recrystallized grains).

[0074] [Austenite grain diameter]

[0075] The austenite grain diameter of the austenitic stainless - steel steel pipe of this embodiment is 20 - 300 μm. The austenite grain diameter here is not the size of the grains used to calculate the GOS value in the above - mentioned EBSD measurement, but the average grain diameter calculated based on the grain boundaries revealed by general mixed - acid etching. Specifically, the measurement is carried out as follows.

[0076] A specimen for microstructure observation is collected from the central part of the wall thickness of the austenitic stainless - steel steel pipe. Taking the cross - section perpendicular to the pipe axis direction as the observation surface, after mirror - polishing the observation surface, it is etched with a mixed acid with a ratio of hydrochloric acid to nitric acid of 1:1 to reveal the austenite grain boundaries. Based on the intercept method according to JIS G0551(2013), the grain - size grade index G is obtained. Based on the following formula, the grain - size grade index G is converted into the average grain diameter R (μm). This average grain diameter R is taken as the austenite grain diameter.

[0077] n = 2 G+3

[0078] R = 1000 / n 1 / 2

[0079] When the austenite grain diameter is too small, the total area of the grain boundaries becomes large and the creep strength decreases. On the other hand, when the austenite grain diameter is too large, the precipitates become uneven and the creep strength still decreases. The lower limit of the austenite grain diameter is preferably 30 μm, more preferably 50 μm, more preferably 60 μm, more preferably 70 μm. The upper limit of the austenite grain diameter is preferably 260 μm, more preferably 220 μm, more preferably 180 μm.

[0080] [Mechanical properties, etc.]

[0081] The austenitic stainless - steel steel pipe of this embodiment preferably has a tensile strength of 515 MPa or more at room temperature.

[0082] The outer diameter of the austenitic stainless steel pipe of this embodiment is 200 mm or more, and the wall thickness is 15 mm or more.

[0083] The lower limit of the outer diameter of the austenitic stainless steel pipe of this embodiment is preferably 300 mm, more preferably 400 mm, and still more preferably 500 mm. The upper limit of the outer diameter of the austenitic stainless steel pipe of this embodiment is not particularly limited, and is, for example, 1000 mm.

[0084] The lower limit of the wall thickness of the austenitic stainless steel pipe of this embodiment is preferably 20 mm, more preferably 25 mm, and still more preferably 30 mm. The upper limit of the wall thickness of the austenitic stainless steel pipe of this embodiment is not particularly limited, and is, for example, 100 mm.

[0085] The austenitic stainless steel pipe of this embodiment is preferably a seamless pipe.

[0086] [Manufacturing method]

[0087] An example of the manufacturing method of the austenitic stainless steel pipe of this embodiment will be described. The manufacturing method described below is merely an example, and the manufacturing method of the austenitic stainless steel pipe of this embodiment is not limited thereto.

[0088] Figure 5 It is a flowchart of an example of the manufacturing method of the austenitic stainless steel pipe. This manufacturing method includes a piercing process (step S1), a multi-pass pipe drawing process (steps S2-1, S2-2,..., S2-n), and a heat treatment process (step S3).

[0089] After heating an ingot having the above chemical composition to a specified temperature, it is pierced by a vertical press to manufacture a bottomed cylindrical intermediate product (hereinafter referred to as "cup-shaped tube blank") (piercing process (step S1)). The heating temperature before piercing is, for example, 1200 to 1300 °C.

[0090] After heating the cup-shaped tube blank to a specified temperature, a mandrel is inserted and the tube is drawn in a die (pipe drawing process (steps S2-1, S2-2,..., S2-n)). At this time, if the working ratio applied in a single pipe drawing is too large, the bottom of the cup-shaped tube blank will break and pipe drawing cannot be performed. Therefore, multi-pass pipe drawing is performed to form the target size. The cross-sectional shrinkage rate in a single pipe drawing is preferably 30.0% or less.

[0091] Here, in at least a part of the multi-pass pipe drawing process (steps S2-1, S2-2,..., S2-n), pipe drawing including heating in two stages described below is performed.

[0092] Figure 6This is an example of the heating curve for heating in the pipe jacking process. In this heating curve, first, the cup-shaped tube blank is maintained at the heating temperature T1 for only the holding time t1, then maintained at a temperature lower than the heating temperature T1, i.e., the heating temperature T2, for only the holding time t2, and then pipe jacking is performed.

[0093] By heating the cup-shaped tube blank to a relatively high heating temperature T1 before pipe jacking, recrystallization can be promoted. On the other hand, if it is maintained at the heating temperature T1 for a long time, the grain size will coarsen. Therefore, after only maintaining the holding time t1, the heating temperature is reduced to the heating temperature T2.

[0094] The heating temperature T1 is, for example, 1150 - 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.

[0095] The holding time t1 is, for example, 60 - 300 minutes. The lower limit of the holding time t1 is preferably 80 minutes, more preferably 120 minutes. The upper limit of the holding time t1 is preferably 240 minutes, more preferably 180 minutes.

[0096] The difference T1 - T2 between the heating temperature T1 and the heating temperature T2 is preferably 20 °C or more, more preferably 30 °C or more.

[0097] The holding time t2 is, for example, 30 - 300 minutes. The lower limit of the holding time t2 is preferably 40 minutes, more preferably 60 minutes. The upper limit of the holding time t2 is preferably 240 minutes, more preferably 180 minutes.

[0098] As described above, the pipe jacking including the heating in the above two stages only needs to be performed in at least a part of multiple pipe jacking processes (steps S2-1, S2-2, …, S2-n). In other pipe jacking processes, as long as the cup-shaped tube blank is heated to a temperature capable of pipe jacking (for example, 1150 - 1250 °C) and then pipe jacking is performed. However, it is considered that the more times the pipe jacking including the heating in the above two stages is implemented, the higher the recrystallization rate X can be improved. Therefore, in multiple pipe jacking processes (steps S2-1, S2-2, …, S2-n), it is preferably implemented more than 2 times, more preferably more than 3 times, and further preferably more than 4 times for the pipe jacking including the heating in the above two stages.

[0099] The bottom of the cup-shaped tube blank can be cut off and removed after the pipe jacking process is completed, or can be removed after the heat treatment process (step S3).

[0100] The intermediate product after the pipe jacking process is heat-treated (step S3). The heating temperature for the heat treatment is 1140 - 1190 °C. When 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, when the heat treatment temperature is too low, the grain coarsens. The lower limit of the heat treatment temperature is preferably 1150 °C. The upper limit of the heat treatment temperature is preferably 1180 °C.

[0101] The holding time for the heat treatment is preferably 15 - 180 minutes. The lower limit of the holding time for the heat treatment is preferably 30 minutes. The upper limit of the holding time for the heat treatment is preferably 120 minutes, and more preferably 60 minutes. After the holding time, it is preferable to perform water cooling on the intermediate product.

[0102] After the heat treatment, cold working is preferably not performed. This is because if cold working is performed, it is difficult to maintain the specified dimensions.

[0103] Through the above processes, an austenitic stainless steel pipe is manufactured. In this manufacturing method, in at least a part of the multiple pipe jacking processes, pipe jacking including two-stage heating is performed. In addition, after the pipe jacking process, heat treatment is performed at 1140 - 1190 °C. Thereby, a structure with a recrystallization rate X of 0.90 or more and an austenite grain diameter of 20 - 300 μm can be obtained.

[0104] Above, an austenitic stainless steel pipe according to an embodiment of the present invention has been described. According to this embodiment, an austenitic stainless steel pipe with a large diameter and thick wall, having high creep strength, excellent SCC resistance, and excellent SRC resistance can be obtained.

[0105] Examples

[0106] Hereinafter, the present invention will be described more specifically through examples. The present invention is not limited to these examples.

[0107] Steel ingots are melted from steel having the chemical composition shown in Table 1. The steel ingots are pierced, pipe jacked, and heat-treated under the conditions shown in Tables 2 and 3 to manufacture steel pipes with the dimensions shown in Table 2.

[0108] [Table 1]

[0109] Table 1

[0110]

[0111] [Table 2]

[0112] Table 2

[0113]

[0114] [Table 3]

[0115] Table 3

[0116]

[0117] Table 3 is a table summarizing the heating curves before pipe jacking. In the column of "Pipe Jacking 1" in Table 3, the heating conditions before the first pipe jacking are recorded. The same applies to the columns of "Pipe Jacking 2", "Pipe Jacking 3", and so on. In these columns, for example, the notation "1250°C (180 minutes)" means that the cup-shaped tube blank is held at 1250°C for 180 minutes and then pipe jacking is performed. Additionally, for example, the notation "1200°C (120 minutes) → 1150°C (60 minutes)" means that the cup-shaped tube blank is held at 1200°C for 120 minutes, then held at 1150°C for 60 minutes, and then pipe jacking is performed. It should be noted that the recrystallization start temperature of these steels is considered to be approximately 1000°C.

[0118] Specimens for microstructure observation are collected from the central part of the wall thickness of each manufactured steel pipe, and the austenite grain diameter R and the recrystallization rate X are measured. Furthermore, the creep rupture test, tensile test, SRC test, and SCC test described below are carried out.

[0119] [Creep Rupture Test]

[0120] The creep rupture test is carried out in accordance with JIS Z 2271. Specifically, after heating the creep rupture test piece to 650°C, the creep rupture test is carried out. The test stress is set at 200 MPa, and the creep rupture time is obtained. The target is a creep rupture time of 300 hours or more.

[0121] [Tensile Test]

[0122] A room temperature tensile test is carried out. The target is a tensile strength TS of 515 MPa or more.

[0123] [SRC Test]

[0124] The stress relaxation test is carried out in accordance with ASTM E328. The test conditions are as follows.

[0125] Pre-strain: 10% at room temperature (strain rate: 2 mm / minute)

[0126] Temperature: 650°C (heating for 4 hours, holding for 2 hours and then applying high temperature strain)

[0127] Holding time: 1000 hours

[0128] After 1000 hours, a scanning electron microscope was used to observe the microstructure of the cross-section perpendicular to the length direction of the unbroken test piece. As a result of the microscopic observation, when no cracks were confirmed at the grain boundaries and no creep cavities were confirmed either, the SRC resistance was evaluated as "good". When any of the fracture of the test piece, cracks at the grain boundaries, and creep cavities occurred, the SRC resistance was evaluated as "failed".

[0129] [SCC test]

[0130] In the evaluation of intergranular corrosion resistance, a sulfuric acid - copper sulfate corrosion test was carried out in accordance with JIS G 0575. After the boiling test, the test piece was taken out from the test solution, and after the bending test was carried out, the top of the outer side of the bent surface was observed with a microscope to investigate intergranular corrosion. As a result of the microscopic observation, when no intergranular corrosion occurred, it was evaluated as "good", and when intergranular corrosion occurred, it was evaluated as "failed".

[0131] The results are shown in Table 4.

[0132] [Table 4]

[0133] Table 4

[0134]

[0135] CR: Creep test (650 °C, 200 MPa) fracture time

[0136] The austenite grain diameter R of the steel pipes with test numbers 1 to 4 was 20 to 300 μm, and the recrystallization rate was 0.90 or more. These steel pipes had high creep strength, excellent SCC resistance, and SRC resistance.

[0137] The SRC resistance of the steel pipe with test number 5 was poor. It is considered that this is because the recrystallization rate X is low. It is considered that the recrystallization rate X is low because insufficient processing strain was applied in the pipe jacking process.

[0138] The creep strength of the steel pipe with test number 6 was low. It is considered that this is because the austenite grain diameter R is too large. It is considered that the austenite grain diameter R is large because the grains were coarsened in the pipe jacking process.

[0139] The SRC resistance of the steel pipe with test number 7 was poor. It is considered that this is because the recrystallization rate X is low. It is considered that the recrystallization rate X is low because the heating temperature in the heat treatment process was too low.

[0140] The creep strength of the steel pipe with test number 8 was low. It is considered that this is because the austenite grain diameter R is too large. It is considered that the austenite grain diameter R is large because the heating temperature in the heat treatment process was too high.

[0141] As described above, the embodiments of the present invention have been described, but the above-described embodiments are merely examples for implementing the present invention. Therefore, the present invention is not limited to the above-described embodiments, and the above-described embodiments can be appropriately modified within the scope of the invention for implementation.

Claims

1. An austenitic stainless steel steel pipe, the chemical composition of which 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％、 balance: Fe and impurities, the outer diameter of the austenitic stainless steel steel pipe is 200 mm or more, the wall thickness is 15 mm or more, the austenite grain diameter is 20 - 300 μm, the recrystallization rate X is 0.90 or more, The recrystallization ratio X is measured as follows: A specimen is collected from the central part of the wall thickness of the austenitic stainless steel pipe, and a region of 10.0 mm or more in the cross-section perpendicular to the pipe axis direction is taken as the measurement region. Electron backscatter measurement is carried out in a grid pattern at a scanning interval of 2.0 μm or less. Among the grains in the measurement region, grains with a GOS value of 1.0° or more are classified as non-recrystallized grains, and grains with a GOS value less than 1.0° are classified as recrystallized grains. The defined angle of the grain boundary is set to 4.0°. The ratio S 2 of the area S of the recrystallized grains to the area S of the measurement region is taken as the recrystallization ratio X. R R / S is used as the recrystallization ratio X.​ 2. The austenitic stainless steel steel pipe according to claim 1, wherein, the chemical composition contains, by mass%, at least one selected from the group consisting of Mo: 0.05 - 2.00%, Cu: 0.05 - 2.50%, and B：0.0005～0.0050％ more than one kind.

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

4. A method for manufacturing the austenitic stainless steel steel pipe according to claim 3, comprising: a step of piercing a steel billet to manufacture a cup-shaped tube blank; a step of performing multi-pass pipe sinking on the cup-shaped tube blank to manufacture an intermediate product; and a step of heat-treating the intermediate product at 1140 - 1190°C, in at least a part of the step of performing multi-pass pipe sinking to manufacture the intermediate product, after holding the cup-shaped tube blank at a specified heating temperature T1 for a specified time, holding it at a temperature lower than the heating temperature T1, i.e., heating temperature T2, for a specified time, and then performing pipe sinking.

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