Austenitic stainless steel pipe and method for manufacturing same

Abstract

Provided is an austenitic stainless steel pipe that has a thick wall and a large diameter, has a low SCC sensitivity, and exhibits excellent naphthenic acid corrosion resistance. This austenitic stainless steel pipe has a chemical composition containing, in mass%, not more than 0.030% of C, not more than 1.00% of Si, not more than 2.00% of Mn, 15.00-25.00% of Cr, 9.00-17.00% of Ni, 0.020-0.200% of N, not more than 0.050% of sol. Al, not more than 0.045% of P, not more than 0.030% of S, 0.20-1.00% of Nb, more than 2.50% to not more than 5.00% of Mo, etc., and has an outer diameter of not less than 200 mm, and a wall thickness of not less than 15 mm. In the austenitic stainless steel pipe, austenite crystal grains have an average grain diameter of 20-300 µm and exhibit a recrystallization rate X of not less than 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] Austenitic stainless steels are used as steel pipes for chemical plants. With the recent rise in crude oil prices, the proportion of low-priced, low-grade crude oil containing naphthenic acid used in chemical plant facilities such as oil refineries and petrochemical plants is increasing. Therefore, steel materials used in chemical plant facilities are required to have low stress corrosion cracking susceptibility (hereinafter referred to as "SCC susceptibility") as well as excellent naphthenic acid corrosion resistance.

[0003] International Publication No. 2013 / 001956, Japanese Patent Application Laid-Open No. 2015-137419, and Japanese Patent Application Laid-Open No. 2015-137420 disclose an austenitic stainless steel pipe with excellent steam oxidation resistance, an austenitic stainless steel welded joint with excellent steam oxidation resistance, and an austenitic stainless steel pipe with excellent steam oxidation resistance, respectively.

[0004] WO 2017 / 175839 discloses an austenitic stainless steel that combines excellent naphthenic acid corrosion resistance with economical efficiency.

[0005] International Publication No. 2013 / 001956 Japanese Patent Application Laid-Open No. 2015-137419 Japanese Patent Application Laid-Open No. 2015-137420 International Publication No. 2017 / 175839

[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-wall steel pipe is manufactured using austenitic stainless steel containing Nb, it may not be possible to achieve both low SCC susceptibility and excellent naphthenic acid corrosion resistance at the same time.

[0007] An object of the present invention is to provide a large-diameter, thick-walled austenitic stainless steel pipe that has low SCC susceptibility and excellent naphthenic acid corrosion resistance.

[0008] An austenitic stainless steel pipe according to one embodiment of the present invention is an austenitic stainless steel pipe having a chemical composition, in mass%, of C: 0.030% or less, Si: 1.00% or less, Mn: 2.00% or less, Cr: 15.00 to 25.00%, Ni: 9.00 to 17.00%, N: 0.020 to 0.200%, sol. The austenitic stainless steel pipe has an outer diameter of 200 mm or more, a wall thickness of 15 mm or more, an average austenite grain size of 20 to 300 μm, and a recrystallization ratio X of 0.90 or more. The recrystallization ratio 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 sample is taken from a cross section perpendicular to the axial direction of the pipe. ２ The above region 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 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 0.030% or less C, 1.00% or less Si, 2.00% or less Mn, 15.00 to 25.00% Cr, 9.00 to 17.00% Ni, 0.020 to 0.200% N, 0.050% or less sol. Al, 0.045% or less P, 0.030% or less S, 0.20 to 1.00% Nb, more than 2.50% but not more than 5.00% Mo, the balance being Fe and impurities, an outer diameter of 200 mm or more, a wall thickness of 15 mm or more, an average 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. ２ The above region 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 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 is an austenitic stainless steel pipe having a chemical composition, in mass%, of C: 0.030% or less, Si: 1.00% or less, Mn: 2.00% or less, Cr: 15.00 to 25.00%, Ni: 9.00 to 17.00%, N: 0.020 to 0.200%, sol. The austenitic stainless steel pipe has an outer diameter of 200 mm or more, a wall thickness of 15 mm or more, an average 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 axial direction of the pipe. ２ The above region 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 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 that has low SCC susceptibility and excellent naphthenic acid corrosion resistance.

[0013] FIG. 1 is a diagram showing the relationship between soaking conditions and the rate of segregation relaxation. FIG. 2 is a GOS map of a structure with a recrystallization fraction X of 0.93. FIG. 3 is a frequency distribution of GOS values ​​for the structure of FIG. 2. FIG. 4 is a GOS map of a structure with a recrystallization fraction X of 0.59. FIG. 5 is a frequency distribution of GOS values ​​for the structure of FIG. 4. FIG. 6 is an example of a TEM observation image (15 μm × 15 μm × 5 fields of view) of a fine grain portion of an austenitic stainless steel without soaking heat treatment. FIG. 7 is an example of a TEM observation image (15 μm × 15 μm × 5 fields of view) of a coarse grain portion of an austenitic stainless steel without soaking heat treatment. FIG. 8 is a flow diagram of an example of a method for manufacturing an austenitic stainless steel pipe. FIG. 9 is an example of a heat pattern for heating in the punching process.

[0014] The present inventors investigated the cause of the deterioration of naphthenic acid corrosion resistance in large-diameter, thick-wall steel pipes manufactured from Nb-containing austenitic stainless steels. As a result, it was found that steel pipes with reduced naphthenic acid corrosion resistance contain many unrecrystallized grains with residual strain in their microstructures.

[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 average grain size of austenite crystal grains is 20 to 300 μm), excellent naphthenic acid corrosion 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 in which the recrystallization rate X is 0.90 or more and the average grain size of austenite crystal grains is 20 to 300 μm.

[0019] The present inventors have further investigated methods for further reducing the SCC susceptibility of the above-mentioned austenitic stainless steel pipe and for further improving the naphthenic acid corrosion resistance.

[0020] In austenitic stainless steels containing Nb, Nb is prone to microsegregation during solidification. In areas with many Nb precipitates, the pinning force of the Nb precipitates inhibits grain growth, resulting in fine grains. On the other hand, in areas with few Nb precipitates, crystal grains are prone to grain growth. As a result, areas with many Nb precipitates (segregated areas) form a fine-grained structure, while areas with few Nb precipitates (non-segregated areas) form a coarse-grained structure, resulting in a mixed-grain structure overall. Even if the average grain size of the average austenite crystal grains is within the appropriate range, stress corrosion cracking is likely to occur at the grain boundaries of coarse grains in a mixed-grain structure.

[0021] Furthermore, Mo is known as an element that enhances naphthenic acid corrosion resistance, but Mo, like Nb, is also an element that is prone to segregation. If the Mo content becomes uneven, naphthenic acid corrosion resistance decreases. Therefore, in order to reduce SCC susceptibility and improve naphthenic acid corrosion resistance, it is effective to reduce the segregation of Nb and Mo and make the crystal grains uniform.

[0022] The present inventors came up with the idea of ​​performing soaking heat treatment (diffusion annealing, hereinafter simply referred to as "soaking") in which the billet is heated at a high temperature for a long period of time before hot punching, as a method for reducing the segregation of Nb and Mo, and investigated the soaking conditions.

[0023] Specifically, first, an austenitic stainless steel pipe containing, by mass%, 17.09% Cr, 9.88% Ni, 0.323% Nb, etc. was manufactured by hot punching without soaking and by heat treatment after hot punching.

[0024] A micro-test specimen was taken from the center of the wall thickness of this austenitic stainless steel pipe. The center of the wall thickness is a region where segregation is likely to remain and which is prone to forming a duplex grain structure. After mirror-polishing the surface of the micro-test specimen, the structure was observed with a scanning electron microscope to identify the fine grain region (segregated region) and the coarse grain region (non-segregated region). Area analysis was performed using a field emission electron probe microanalyzer (FE-EPMA) on three locations each in the fine grain region and the coarse grain region (six locations in total). Specifically, elemental analysis was performed on a 50 μm × 50 μm micro-square region. The measurement conditions were an acceleration voltage of 15 kV, a probe current of 400 nA, a beam diameter of 2 μm, and an accumulation time of 0.1 seconds. The element to be measured was Nb, and the arithmetic mean of the Nb content in mass% within each micro-square region was calculated. The arithmetic mean of the three micro-square regions was calculated and used as the Nb content in the fine grain region or the coarse grain region.

[0025] As a result, the Nb content in the fine grain portion was 0.36 mass %, and the Nb content in the coarse grain portion was 0.23 mass %.

[0026] Next, the Nb contents in the fine grain portion (segregated portion) and the coarse grain portion (non-segregated portion) when soaking was performed under various conditions were determined by thermodynamic calculation (Nb diffusion calculation). The results are shown in Table 1.

[0027]

[0028] The "segregation relaxation ratio" in Table 1 is a value calculated using the following formula. The closer the segregation relaxation ratio is to 100%, the more segregation is relaxed. Segregation relaxation ratio (%) = (1 - b / a) x 100 a = A - (A + B) / 2 b = C - (C + D) / 2 A: Nb content in the fine grain portion (segregation portion) without soaking B: Nb content in the coarse grain portion (non-segregation portion) without soaking C: Nb content in the fine grain portion (segregation portion) with soaking D: Nb content in the coarse grain portion (non-segregation portion) with soaking

[0029] Fig. 1 is a diagram showing the relationship between soaking conditions and the segregation relaxation rate, in which open marks indicate that the segregation relaxation rate is 75% or more, and solid marks indicate that the segregation relaxation rate is less than 75%.

[0030] From FIG. 1, it can be seen that when the soaking heating temperature is T (° C.) and the soaking holding time is t (hours), if F1=0.348T+2.32t is 450 or more, the segregation relaxation rate can be made 75% or more.

[0031] Based on the results of the above thermodynamic calculations, the inventors actually performed soaking to manufacture austenitic stainless steel pipes and confirmed that Nb segregation was reduced. Furthermore, they confirmed that by reducing Nb segregation and uniformly precipitating Nb precipitates, the structure becomes uniformly granular and SCC susceptibility can be reduced. Specifically, in a cross section perpendicular to the tube axial direction at the center of the wall thickness, the number of Z phases (CrNbN) with a circle equivalent diameter of 500 nm or more is reduced to 1000 μm ２ It was found that SCC susceptibility can be reduced by keeping the number of particles per 1000 or less to 30.0 per 1000. Furthermore, it was confirmed that soaking that satisfies the above conditions reduces Mo segregation and improves naphthenic acid corrosion resistance.

[0032] 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.

[0033] [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 %.

[0034] C: 0.030% or less Carbon (C) is inevitably contained. C is present at grain boundaries. ２３ C ６ This forms Cr carbides of this type, increasing the SCC susceptibility 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 susceptibility, 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%.

[0035] Si: 1.00% or less Silicon (Si) forms a σ phase in steel and reduces the creep strength of the steel. Therefore, the Si content is 1.00% or less. The upper limit of the Si content is preferably 0.80%, more preferably 0.60%. On the other hand, Si has the effect of deoxidizing the steel and improving the oxidation resistance and steam oxidation resistance of the steel. When it is desired to actively obtain these effects, the lower limit of the Si content may be set to 0.10%. The lower limit of the Si content is preferably 0.15%, more preferably 0.20%.

[0036] Mn: 2.00% or less Manganese (Mn) forms a σ phase in steel and reduces the creep strength of the steel. Therefore, the Mn content is 2.00% or less. The upper limit of the Mn content is preferably 1.80%, more preferably 1.60%. On the other hand, Mn has the effect of deoxidizing steel. When this effect is to be actively obtained, the lower limit of the Mn content may be set to 0.10%. 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%.

[0037] Cr: 15.00 to 25.00% Chromium (Cr) reduces the SCC susceptibility of steel. Cr also improves 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%.

[0038] Ni: 9.00 to 17.00% 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 9.00 to 17.00%. The lower limit of the Ni content is preferably 10.00%, more preferably 11.00%, and even more preferably 12.00%. The upper limit of the Ni content is preferably 16.00%, more preferably 15.00%, and even more preferably 14.50%.

[0039] N: 0.020 to 0.200% Nitrogen (N) dissolves in the matrix, stabilizing austenite and improving 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, increasing the SCC susceptibility 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%.

[0040] Sol. Al: 0.050% or less Aluminum (Al) may be contained as a deoxidizer. On the other hand, if the Al content is too high, the hot workability of the steel decreases. Therefore, the Al content is 0.050% 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 0.030%, further preferably 0.025%, and further preferably 0.020%. In this embodiment, the Al content refers to the content of acid-soluble Al (sol. Al).

[0041] P: 0.045% or less Phosphorus (P) reduces the hot workability and toughness of steel. Therefore, the P content is 0.045% or less. The upper limit of the P content is preferably 0.040%, more preferably 0.030%. The lower the P content, the better. However, excessive reduction of the P content increases the cost of the steel. Therefore, taking into account normal industrial production, the lower limit of the P content is preferably 0.001%, more preferably 0.002%.

[0042] S: 0.030% or less Sulfur (S) is an impurity. S reduces the hot workability of steel. Therefore, the S content is 0.030% or less. The upper limit of the S content is preferably 0.020%, more preferably 0.010%, even more preferably 0.008%, and even 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%, and even more preferably 0.0005%.

[0043] Nb: 0.20 to 1.00% Niobium (Nb) forms carbonitrides to fix C, thereby reducing the amount of solute C and lowering the SCC susceptibility of steel. Nb also increases the creep strength of steel by precipitating as carbonitrides. On the other hand, if the Nb content is too high, the amount of coarse carbonitrides increases, which become the starting point of fracture and reduce toughness. Therefore, the Nb content is 0.20 to 1.00%. 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%.

[0044] Mo: more than 2.50% and not more than 5.00% Molybdenum (Mo) combines with S in the service environment to form a sulfide film, contributing to improved naphthenic acid corrosion resistance. Mo also acts as a solid solution strengthening element, increasing the creep strength of steel. On the other hand, if the Mo content is too high, the stability of austenite decreases. Therefore, the Mo content is more than 2.50% and not more than 5.00%. The lower limit of the Mo content is preferably 2.60%, more preferably 2.80%, even more preferably 3.00%, and even more preferably 3.20%. The upper limit of the Mo content is preferably 4.80%, even more preferably 4.50%, even more preferably 4.00%, and even more preferably 3.80%.

[0045] 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.

[0046] 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 Cu: 2.00% or less, B: 0.0050% or less, Ca: 0.0200% or less, and Ti: 0.50% or less. Cu, B, Ca, and Ti are all optional elements. In other words, the austenitic stainless steel pipe according to this embodiment may not contain some or all of Cu, B, Ca, and Ti.

[0047] Cu: 0 to 2.00% 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 2.00%. 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 1.80%, more preferably 1.50%, and even more preferably 1.00%.

[0048] 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%.

[0049] Ca: 0 to 0.0200% Calcium (Ca) improves the hot workability of steel. This effect can be achieved even if even a small amount of Ca is contained. On the other hand, if the Ca content is too high, the toughness of the steel decreases. Therefore, the Ca content is 0 to 0.0200%. The lower limit of the Ca content is preferably 0.0005%, and more preferably 0.0010%. The upper limit of the Ca content is preferably 0.0100%, and more preferably 0.0050%.

[0050] Ti: 0 to 0.50% Titanium (Ti) forms Ti carbides and suppresses the formation of Cr carbides, thereby suppressing sensitization during use of steel at average operating temperatures of over 600°C to 700°C. This effect can be achieved even if even a small amount of Ti is contained. On the other hand, if the Ti content is too high, the stress relaxation cracking resistance of the steel decreases. Therefore, the Ti content is 0 to 0.50%. The lower limit of the Ti content is preferably 0.01%, more preferably 0.02%, and even more preferably 0.03%. The upper limit of the Ti content is preferably 0.40%, more preferably 0.30%, and even more preferably 0.20%.

[0051] [Structure] The austenitic stainless steel pipe according to this embodiment has a structure mainly composed of 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.

[0052] [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, the naphthenic acid corrosion resistance decreases. The recrystallization ratio X is preferably 0.94 or more, more preferably 0.96 or more, and even more preferably 0.98 or more.

[0053] The recrystallization rate X is measured as follows.

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058]

[0059] 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.

[0060] 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.

[0061] As an example, Fig. 2 shows a GOS map of a structure having a recrystallization fraction X of 0.93, and Fig. 3 shows the frequency distribution of GOS values ​​for the structure of Fig. 2. Similarly, Fig. 4 shows a GOS map of a structure having a recrystallization fraction X of 0.59, and Fig. 5 shows the frequency distribution of GOS values ​​for the structure of Fig. 4. In the GOS maps of Fig. 2 and Fig. 4, 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).

[0062] [Average grain size of austenite grains] The austenitic stainless steel pipe according to this embodiment has an average grain size of 20 to 300 μm. The average grain size of austenite grains here is not the grain size used to calculate the GOS value in the EBSD measurement described above, but is the average grain size calculated based on grain boundaries revealed by general mixed acid corrosion. Specifically, it is measured as follows.

[0063] 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 taken as the average grain size of the austenite grains. n = 2 Ｇ＋３ R = 1000 / n １／２

[0064] If the average grain size of austenite grains is too small, the total area of ​​the grain boundaries increases, resulting in a decrease in creep strength. On the other hand, if the average grain size of austenite grains is too large, the precipitates become non-uniform, also resulting in a decrease in creep strength. The lower limit of the average grain size of austenite grains is preferably 30 μm, more preferably 50 μm, even more preferably 60 μm, and even more preferably 70 μm. The upper limit of the average grain size of austenite grains is preferably 260 μm, more preferably 220 μm, and even more preferably 180 μm.

[0065] [Number Density of Nb Precipitates] In the austenitic stainless steel pipe according to the present embodiment, preferably, the number of Z phases having a circle equivalent diameter of 500 nm or more is 1000 μm or less in a cross section perpendicular to the pipe axis direction at the center of the wall thickness. ２ There are 30.0 or less per unit area.

[0066] The number density of Z phases having an equivalent circle diameter of 500 nm or more is measured as follows.

[0067] A sample is taken from the center of the wall thickness of an austenitic stainless steel pipe to prepare a micro-test piece for making an extraction replica. The observation surface is a cross section perpendicular to the pipe axis direction. After mirror polishing the observation surface of the micro-test piece, the micro-test piece is immersed in a 3% nital etching solution for 10 minutes to etch the surface. The etched surface is covered with a carbon vapor deposition film. The micro-test piece with its surface covered with the vapor deposition film is immersed in a 5% nital etching solution for 20 minutes. The vapor deposition film is peeled off from the immersed micro-test piece. The vapor deposition film peeled off from the micro-test piece is washed with ethanol, then scooped up with a Cu sheet mesh and dried.

[0068] This vapor-deposited film (replica film) is observed with a transmission electron microscope (TEM). Specifically, first, the positions of the fine grain portion and the coarse grain portion are identified in advance from the vapor-deposited film with an optical microscope, and then each of the fine grain portion and the coarse grain portion is measured with the TEM. More specifically, the fine grain portion is the portion with the smallest grain size within the region observed with the optical microscope. More specifically, the coarse grain portion is the portion with the largest grain size within the region observed with the optical microscope.

[0069] The acceleration voltage of the TEM is 200 kV. The size of the observation field is 1000 μm in total for each of the fine grain portion and the coarse grain portion. ２ For example, the size of one field of view is 15 μm×15 μm (225 μm ２ ), five or more visual fields are observed for each of the fine grain portion and the coarse grain portion (a total of ten or more visual fields), and the total area of ​​the visual fields is 1000 μm ２ Make sure it is above that.

[0070] In each observation field, particles with a circle-equivalent diameter of 500 nm or more are identified. The particles can be identified from the contrast. In this specification, the term "particle" is not limited to circular (spherical) particles, but may also refer to small pieces having an angular shape or small pieces having an elongated oval shape. The circle-equivalent diameter of a particle can be determined by image analysis of the TEM observation image.

[0071] Figure 6 shows an example of a TEM observation image (15 μm × 15 μm × 5 fields of view) of a fine grain portion of an austenitic stainless steel that was not subjected to soaking heat treatment. Figure 7 shows an example of a TEM observation image (15 μm × 15 μm × 5 fields of view) of a coarse grain portion of an austenitic stainless steel that was not subjected to soaking heat treatment.

[0072] The particles in the steel were identified by EDS and electron beam diffraction, and it was found that particles with a circle equivalent diameter of 500 nm or more were Z phase.

[0073] In the samples shown in Figures 6 and 7, particles with a circle-equivalent diameter of less than 500 nm (approximately 200 nm) were MX. In this example, in the fine grain portion (Figure 6), a total of 38 Z-phase particles and 9 MX particles were found in five visual fields. In the coarse grain portion (Figure 7), a total of 7 Z-phase particles and 39 MX particles were found in five visual fields.

[0074] By performing the soaking heat treatment described later, the number of Z phases having a circle equivalent diameter of 500 nm or more is reduced to 1000 μm ２ The number of pieces per unit can be reduced to 30.0 or less.

[0075] In this embodiment, "the number of Z phases having a circle equivalent diameter of 500 nm or more is 30.0 or less" means that the number of Z phases having a circle equivalent diameter of 500 nm or more is 1000 μm or less, regardless of the location (fine grain portion or coarse grain portion) of the central part of the wall thickness of the austenitic stainless steel pipe. ２ That is, in the austenitic stainless steel pipe of this embodiment, even in the fine grain portion which is the segregation portion (more specifically, the portion with the smallest grain size in the region observed with an optical microscope), the number of Z phases having a circle equivalent diameter of 500 nm or more is 1000 μm or less. ２In other words, in the austenitic stainless steel pipe of this embodiment, there is no location in the wall thickness center where the number of Z phases having an equivalent circle diameter of 500 nm or more exceeds 30.0.

[0076] The number of Z phases having a circle equivalent diameter of 500 nm or more is more preferably 1000 μm ２ More preferably, the number of particles is 25.0 or less per 1000 μm ２ There are 20.0 or less per unit area.

[0077] The austenitic stainless steel pipe according to this embodiment preferably does not contain crystal grains that are twice or more the average grain size.

[0078] In the austenitic stainless steel pipe according to this embodiment, the difference between the maximum (mass%) and minimum (mass%) of the Mo content, as analyzed using a field emission electron probe microanalyzer (FE-EPMA), is preferably 1.10 or less. Note that "1.10 (mass%)" here does not refer to a percentage of the difference based on the maximum or minimum Mo content, but rather refers to the actual difference between the maximum and minimum Mo content values, as expressed in mass%. For example, if the maximum Mo content is 4.00 mass% and the minimum Mo content is 3.00 mass%, the "difference between the maximum and minimum Mo content values" is 1.00 mass%. The difference between the maximum and minimum Mo content values ​​(mass%) as analyzed using FE-EPMA is preferably 1.00 or less.

[0079] The difference between the maximum and minimum values ​​of Mo content analyzed using FE-EPMA is measured as follows: A micro-test specimen is taken from the center of the wall thickness of an austenitic stainless steel pipe. After mirror-polishing the surface of the micro-test specimen, the structure is observed using a scanning electron microscope to identify fine-grained areas (segregated areas) and coarse-grained areas (non-segregated areas). Area analysis is performed using a field emission electron microanalyzer (FE-EPMA) at three locations each in the fine-grained and coarse-grained areas (six locations in total). Specifically, elemental analysis is performed on a micro-square area of ​​50 μm x 50 μm. The measurement conditions are an acceleration voltage of 15 kV, a probe current of 400 nA, a beam diameter of 2 μm, and an accumulation time of 0.1 seconds. The element to be measured is Mo, and the arithmetic mean of the Mo content in mass% is calculated within each micro-square area. The difference between the maximum and minimum values ​​of the Mo content (the arithmetic mean value in each micro square region) in six micro square regions (three in the segregation region and three in the non-segregation region) is determined.

[0080] For example, if the Mo contents of six minute square regions (three segregation regions and three non-segregation regions) are 4.00 mass%, 3.80 mass%, 3.60 mass%, 3.40 mass%, 3.20 mass%, and 3.00 mass%, respectively, the "difference between the maximum and minimum Mo contents" is 4.00 mass% - 3.00 mass% = 1.00 mass%.

[0081] [Dimensions, etc.] 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.

[0082] 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.

[0083] 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.

[0084] The austenitic stainless steel pipe according to this embodiment is preferably a seamless steel pipe.

[0085] [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.

[0086] 8 is a flow diagram of an example of a method for manufacturing an austenitic stainless steel pipe. This manufacturing method includes a soaking heat treatment step (step S0), a piercing step (step S1), multiple punching steps (steps S2-1, S2-2, ..., S2-n), and a heat treatment step (step S3).

[0087] A billet (steel material) having the above-mentioned chemical composition is subjected to a soaking heat treatment (soaking annealing) in which the billet is heated at a high temperature for a long time (step S0). Specifically, the billet is held at a heating temperature T (°C) of 1210 to 1300°C for a holding time t (hours) at which F1, expressed by the following formula, becomes 450 or more: F1 = 0.348T + 2.32t

[0088] The soaking heat treatment diffuses the Nb dissolved in the steel, and Nb precipitates (Z phase and MX) are uniformly precipitated during the subsequent piercing process (step S1), punching process (steps S2-1, S2-2, ..., S2-n) and heat treatment process (step S3).

[0089] If the heating temperature T is too low, it becomes difficult to sufficiently diffuse Nb. On the other hand, if the heating temperature T is too high, the heating furnace will be severely damaged. The lower limit of the heating temperature T is preferably 1230°C. The lower limit of F1 is preferably 455. The upper limit of F1 is preferably 490, more preferably 480.

[0090] The billet that has undergone soaking heat treatment is heated to a predetermined temperature and then pierced using 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.

[0091] 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.

[0092] 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.

[0093] 9 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] 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.

[0098] 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.

[0099] 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, it is preferable to perform the punching including the two-stage heating described above two or more times in the multiple punching steps (steps S2-1, S2-2, ..., S2-n).

[0100] 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).

[0101] 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 high, 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.

[0102] 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.

[0103] 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.

[0104] 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 rate X of 0.90 or more and an average austenite grain size of 20 to 300 μm.

[0105] In the above manufacturing method, a soaking heat treatment step (step S0) is performed before the piercing step (step S1) to sufficiently diffuse the Nb and Mo dissolved in the steel. This makes it possible to reduce the number density of coarse Z phases in the manufactured austenitic stainless steel pipe. Specifically, the number of Z phases having a circle equivalent diameter of 500 nm or more in a cross section perpendicular to the axial direction of the pipe at the center of the wall thickness is reduced to 1000 μm. ２ The number of pieces per unit can be reduced to 30.0 or less.

[0106] The above describes an austenitic stainless steel pipe according to one embodiment of the present invention. According to this embodiment, it is possible to obtain a large-diameter, thick-walled austenitic stainless steel pipe that has low SCC susceptibility and excellent naphthenic acid corrosion resistance.

[0107] The present invention will be described in more detail below with reference to examples, but the present invention is not limited to these examples.

[0108] Steel having the chemical composition shown in Table 2 was melted to produce a billet. This billet was subjected to soaking heat treatment, piercing, punching and heat treatment under the conditions shown in Tables 3 and 4 to produce steel pipes having the dimensions shown in Table 3.

[0109]

[0110]

[0111]

[0112] Table 4 summarizes the heating patterns for heating before punching. The "Punching 1" column in Table 4 lists the heating conditions before the first punching. The same applies to the "Punching 2" and "Punching 3" 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.

[0113] A sample for microstructure observation was taken from the center of the wall thickness of each produced steel pipe, and the average grain size R of austenite crystal grains and the recrystallization rate X were measured. Furthermore, the following were performed: number density analysis of Nb precipitates, evaluation of the presence or absence of a duplex grain structure, evaluation of the presence or absence of Mo segregation, an SCC susceptibility evaluation test, and an evaluation test of naphthenic acid corrosion resistance.

[0114] [Number density analysis of Nb precipitates] The number density of Z phases having a circle equivalent diameter of 500 nm or more was measured by the method described above. ２ Those with 30.0 or less particles per particle were rated as "L" (Low), and those with more than 30.0 particles per particle were rated as "H" (High).

[0115] [Presence or Absence of Duplex Grain Structure] The presence or absence of a duplex grain structure was investigated. Samples that did not contain crystal grains twice or more the average grain size were evaluated as "A" (Absente), and samples that contained crystal grains twice or more the average grain size were evaluated as "P" (Present).

[0116] [Evaluation of the Presence or Absence of Mo Segregation] The presence or absence of Mo segregation was evaluated by the method described above. Samples in which the difference between the maximum and minimum values ​​of the Mo content analyzed using FE-EPMA was 1.10 or less were evaluated as "A" (Absente), and samples in which the difference was more than 1.10 were evaluated as "P" (Present).

[0117] [SCC Susceptibility Evaluation Test] To evaluate SCC susceptibility, a sulfuric acid / copper sulfate corrosion test was performed in accordance with JIS G 0575 using steel that had been subjected to an aging heat treatment at 550°C for 10,000 hours. After the boiling test, the test specimen 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 investigate intergranular corrosion. As a result of the microscopic observation, specimens that did not exhibit intergranular corrosion were rated as "excellent," specimens that exhibited intergranular corrosion with a crack depth of less than 20 μm were rated as "good," and specimens that exhibited intergranular corrosion with a crack depth of 20 μm or more were rated as "unacceptable."

[0118] [Naphthenic Acid Corrosion Resistance Test] A test specimen measuring 10 mm in the circumferential direction, 2 mm in thickness, and 30 mm in length (the longitudinal direction of the test specimen was parallel to the longitudinal direction (rolling direction) of the steel pipe) was taken from the center of the wall thickness of the steel pipe. The taken test specimen was immersed in a 100% cyclohexanecarboxylic acid solution at 200°C under normal pressure for 720 hours. After immersion for 720 hours, it was ultrasonically cleaned using acetone for 3 minutes. The difference in mass of the test specimen before the test and the test specimen after ultrasonic cleaning was calculated as corrosion weight loss. Furthermore, the corrosion rate (mm / year) was calculated from the surface area, specific gravity, and test time of the test specimen. A corrosion rate of 0.01 mm / year or less was evaluated as "excellent," a corrosion rate of more than 0.01 mm / year but less than 0.03 mm / year was evaluated as "good," and a corrosion rate of more than 0.03 mm / year was evaluated as "unacceptable."

[0119] The results are shown in Table 5.

[0120]

[0121] The "Nb precipitates" column in Table 5 shows the evaluation results of the number density of the Z phase. The "duplex grain" column in Table 5 shows the evaluation results of the presence or absence of a duplex grain structure. The "Mo segregation" column in Table 5 shows the evaluation results of Mo segregation. "SCC susceptibility" shows the evaluation results of the SCC susceptibility evaluation test.

[0122] The steel pipes of test numbers 1 to 7 had an average austenite grain size R of 20 to 300 μm and a recrystallization ratio X of 0.90 or more. These steel pipes had low SCC susceptibility and excellent naphthenic acid corrosion resistance. In particular, the steel pipes of test numbers 1 to 4 had an average austenite grain size R of 20 to 300 μm and a recrystallization ratio X of 0.90 or more, and in addition, the number of Z phases having an equivalent circle diameter of 500 nm or more was 1000 μm or more. ２ These steel pipes had a particularly low SCC susceptibility and excellent naphthenic acid corrosion resistance.

[0123] The steel pipe of test number 5 was within the good range, but had a slightly higher SCC susceptibility compared to the steel pipes of test numbers 1 to 4. This is because the number of Z phases with a circle equivalent diameter of 500 nm or more was 1000 μm ２ This is thought to be because the number of Z phases with a circle equivalent diameter of 500 nm or more was greater than 30.0 per 1000 μm, and the structure was a duplex structure containing crystal grains with a size twice or more the average grain size. ２ The fact that the number was greater than 30.0 per particle is thought to be due to insufficient soaking heat treatment.

[0124] The steel pipes of test numbers 6 and 7 were within the good range, but were slightly inferior in naphthenic acid corrosion resistance compared to the steel pipes of test numbers 1 to 4. This is because the number of Z phases having a circle equivalent diameter of 500 nm or more was 1000 μm ２ This is thought to be because the number of crystal grains per unit area was larger than 30.0, the structure was a duplex structure containing crystal grains with a size twice or more the average grain size, and the difference between the maximum and minimum values ​​of the Mo content analyzed using FE-EPMA exceeded 1.10.

[0125] The steel pipes of test numbers 8 and 12 had poor naphthenic acid corrosion 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.

[0126] The steel pipes of test numbers 9 and 13 were highly susceptible to SCC. This is thought to be because the average grain size R of the austenite grains was too large. The large average grain size R of the austenite grains is thought to be due to the grains becoming coarse during the punching process.

[0127] The steel pipes of test numbers 10 and 14 had poor naphthenic acid corrosion 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.

[0128] The steel pipes of test numbers 11 and 15 had high SCC susceptibility. This is thought to be because the average grain size R of the austenite grains was too large. The reason for the large average grain size R of the austenite grains is thought to be because the heating temperature in the heat treatment process was too high.

[0129] The steel pipe of test number 16 had poor naphthenic acid corrosion resistance, which is thought to be due to the Mo content being too low.

[0130] 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. An austenitic stainless steel pipe having a chemical composition, in mass %, of C: 0.030% or less, Si: 1.00% or less, Mn: 2.00% or less, Cr: 15.00 to 25.00%, Ni: 9.00 to 17.00%, N: 0.020 to 0.200%, sol.

1. An austenitic stainless steel pipe having an outer diameter of 200 mm or more, a wall thickness of 15 mm or more, an average austenite grain size of 20 to 300 μm, and a recrystallization fraction X of 0.90 or more, the composition of which is Al: 0.050% or less, P: 0.045% or less, S: 0.030% or less, Nb: 0.20 to 1.00%, Mo: more than 2.50% and 5.00% or less, Cu: 0 to 2.00%, B: 0 to 0.0050%, Ca: 0 to 0.0200%, Ti: 0 to 0.50%, and the balance: Fe and impurities. 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 sample is taken from a cross section perpendicular to the pipe axis direction. ２ The above region 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 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.

2. An austenitic stainless steel pipe according to claim 1, wherein the number of Z phases having a circle equivalent diameter of 500 nm or more is 1000 μm or less in a cross section perpendicular to the pipe axis direction at the center of the wall thickness of the austenitic stainless steel pipe. ２ 30.0 or less per austenitic stainless steel pipe.

3. An austenitic stainless steel pipe according to claim 2, which does not contain crystal grains having a size twice or more of the average grain size.

4. An austenitic stainless steel pipe as claimed in claim 1 or 2, in which the difference between the maximum value (mass%) and the minimum value (mass%) of the Mo content as analysed using a field emission electron microanalyser is 1.10 or less.

5. An austenitic stainless steel pipe according to claim 1 or 2, wherein the austenitic stainless steel pipe is a seamless steel pipe.

6. A method for producing an austenitic stainless steel pipe as set forth in claim 5, comprising the steps of: piercing a steel material to produce a cup-shaped blank; punching the cup-shaped blank multiple times to produce an intermediate product; and heat treating the intermediate product at 1140 to 1190°C, wherein in at least a part of the step of punching multiple times to produce the intermediate product, the cup-shaped blank 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.

7. A method for producing an austenitic stainless steel pipe as set forth in claim 6, further comprising, prior to the step of producing the cup-shaped mother pipe, a step of soaking heat treatment in which the steel material is held at a heating temperature T (°C) of 1210 to 1300°C for a holding time t (hours) at which F1, expressed by the following formula, becomes 450 or more: F1 = 0.348T + 2.32t

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