Highly corrosion-resistant austenitic stainless steel and method for manufacturing the same

Abstract

The highly corrosion-resistant austenitic stainless steel of the present invention contains, by mass%, C: 0.005-0.030%, Si: 0.05-0.30%, Mn: 0.05-0.40%, P: 0.005-0.050%, S: 0.0001-0.0010%, Ni: 22.0-32.0%, Cr: 19.0-28.0%, Mo: 5.0-7.0%, N: 0.18- 0.25%, Al: 0.005-0.100%, Cu: 0.05-0.50%, W: 0.05% or less, Sn: 0.0005-0.0150%, Co: 0.030-0.300%, B: 0.0005-0.0050%, with the balance being Fe and inevitable impurities, satisfying the following formula (1). The area ratio of the σ phase is 1% or less, and as for corrosion resistance, the CPT according to ASTM G48 Method C is 60°C or higher, and a decrease in corrosion resistance can be suppressed even when exposed to the precipitation temperature range of the σ phase. 0.05≤10[%B]+2[%P]+6[%Sn]+0.03[%Si]≤0.20…(1).

Description

Technical Field

[0001] The present invention relates to a highly corrosion-resistant austenitic stainless steel used in environments requiring extremely excellent corrosion resistance, such as chemical plants. Specifically, it relates to a highly corrosion-resistant austenitic stainless steel in which a decrease in corrosion resistance due to the precipitation of a harmful intermetallic compound, σ phase, is delayed, thereby rendering it harmless. Background Art

[0002] Due to its excellent corrosion resistance, highly corrosion-resistant austenitic stainless steel is used in various fields and is applied in environments containing corrosive substances, such as seawater environments, flue gas desulfurization equipment, oil wells, food factories, chemical plants, or various industrial fields such as nuclear power plants. In such environments, when using general-purpose stainless steels such as SUS430 and SUS304, due to insufficient corrosion resistance, it sometimes causes full-surface corrosion or localized corrosion such as pitting, crevice corrosion, and stress corrosion cracking, which greatly restricts its use. Therefore, attempts have been made to improve the corrosion resistance of austenitic stainless steels by adding large amounts of elements such as Cr, Mo, or N that are effective for corrosion resistance.

[0003] For example, Patent Document 1 proposes an austenitic stainless steel with a maximum Cr content of 35%. Patent Document 2 proposes an austenitic stainless steel with a maximum Mo content of 8.0%. Furthermore, Patent Document 3 proposes an austenitic stainless steel with a maximum N content of 0.50%, suitable for severe corrosive environments.

[0004] Highly corrosion-resistant austenitic steel contains large amounts of Cr and Mo to improve corrosion resistance, but these elements also promote the precipitation of the σ phase, which is a harmful intermetallic compound. Therefore, compared with general austenitic stainless steel, the precipitation of the σ phase is extremely fast when exposed to a temperature range of about 700 to 1000°C. If the σ phase precipitates in the steel, the Cr and Mo are lacking around the σ phase, resulting in a decrease in corrosion resistance.

[0005] During the manufacturing process of austenitic stainless steel into plates, strips, or bars, hot forging or hot rolling is performed, and then cold rolling and other processing are performed as needed. Then, so-called solution heat treatment is performed to soften and homogenize the structure. After the heat treatment, rapid cooling such as water cooling is performed to prevent the precipitation of σ phase.

[0006] In contrast, when a high corrosion-resistant austenitic stainless steel plate is used as the raw material and a carbon steel or the like is used as the base material for cladding, or after a structure such as a tank or reactor is made by welding, a heat treatment is performed to remove softening or residual stress. In the former, if the wall thickness becomes thicker due to cladding, the cooling inside the plate becomes slower. In the latter, if a large structure is formed, a portion where the cooling is slower will be generated due to its structure. In either case, it is difficult to avoid the precipitation of the σ phase. In addition, in the manufacturing process of the product, a brazing process using a BA furnace is sometimes performed. This is a temperature range maintained at around 900°C, where the solder is melted and joined, and then cooled in the atmosphere. In this case, the temperature is exposed to around 900°C where the σ phase precipitates for several minutes to several tens of minutes, and the cooling is also slow. If high corrosion-resistant austenitic stainless steel is applied to such a process, it is difficult to obtain the required corrosion resistance.

[0007] Thus, in highly corrosion-resistant austenitic stainless steel, it is desirable to suppress the precipitation of the σ phase as much as possible, and various component compositions and heat treatment conditions have been proposed for this purpose. For example, in Patent Document 4, by suppressing the upper limits of the Cr and Mo contents to 27.00% and 3.20% respectively, and then implementing a solution heat treatment at 1050-1150°C and then performing rapid cooling, an alloy with an area ratio of 0.1% or less and excellent resistance to nitric acid corrosion was developed. However, the method for avoiding the precipitation of the σ phase is to carry out the solution heat treatment in a temperature range of 1050-1100°C where the σ phase does not precipitate, and to achieve this by subsequent rapid cooling. Therefore, the steel in this document is subjected to solution heat treatment, and no consideration is given to the precipitation of the σ phase in the subsequent heat treatment. In addition, in the examples, even if the chemical composition is within the scope of the invention of Patent Document 4, if a heat treatment of 1000°C×3min is performed, a σ phase of 0.4% in area ratio is precipitated, and the prescribed corrosion resistance cannot be obtained.

[0008] Patent Document 5 proposes a high corrosion resistant austenitic stainless steel sheet, which is obtained by converting M, which is determined by the relationship between Cr, Ni, Mo, Mn, Cu, Si, Al, Fe, N, and C. d Value and M dcThe value is adjusted to below the specified value to suppress the precipitation of σ phase in the segregation area of ​​the steel as a whole and in the center of the plate thickness, and by reducing the area ratio of σ phase to less than 1.0%, manufacturability is ensured while achieving thinning. However, in the present invention, the precipitation of σ phase is also suppressed by annealing after cold rolling, and there is no research on the suppression of σ phase precipitation when exposed to the temperature range of σ phase precipitation during subsequent heat treatment and its effect on corrosion resistance. In addition, Patent Document 5 also describes that corrosion resistance can be improved by selectively incorporating from the group of Ti, Nb, Ta, Zr, V, W, Sn, Sb, and Ga. In addition, Ti, Nb, Ta, and Zr improve intergranular corrosion resistance by forming carbonitrides by bonding with C and N, and the addition of V and W particularly improves crevice corrosion resistance. However, while it is described that Sn, Sb, and Ga can be added simply to improve corrosion resistance, there is no mention of their relationship with the σ phase.

[0009] Patent Document 6 describes that by reducing the impurity content within industrially acceptable ranges (O: 50 ppm or less, Al: 50 ppm or less, and Si: 400 ppm or less), the precipitation of the σ phase can be delayed to a usable level even during an aging heat treatment at 650°C for 5000 hours. However, the alloy serving as the base is similar to SUS310S, making it difficult to achieve corrosion resistance useful in factories. Furthermore, in this alloy (20Ni-28Cr), the precipitation of the σ phase is significantly slower than in highly corrosion-resistant stainless steel containing 5% or more Mo.

[0010] Patent Document 7 proposes an austenitic stainless steel that exhibits excellent corrosion resistance even after a final heat treatment in the temperature range of 850 to 980°C by reducing the Mn content in the steel. This makes it excellent for use as a cladding material, which is an inevitable heat treatment in this temperature range during the manufacturing process. However, its basic composition is Fe-0.02C-0.5Mn-14Ni-18Cr-3.2Mo-0.06N, similar to SUS317, and its alloy content is low, making it difficult to achieve corrosion resistance sufficient for use in factories requiring resistance to severe corrosive environments.

[0011] Prior art literature

[0012] Patent Literature

[0013] Patent Document 1: Japanese Patent Application Laid-Open No. 5-247597,

[0014] Patent Document 2: Japanese Patent Application Laid-Open No. 10-060603,

[0015] Patent Document 3: Japanese Patent Application Laid-Open No. 2010-31313,

[0016] Patent Document 4: Japanese Patent Publication No. WO / 2012 / 176802,

[0017] Patent Document 5: Japanese Patent Publication No. WO / 2016 / 076254,

[0018] Patent Document 6: Japanese Patent Application Laid-Open No. 10-140291,

[0019] Patent Document 7: Japanese Patent Application Laid-Open No. 61-223167. Summary of the Invention

[0020] Problems to be solved by the invention

[0021] The present invention has been made in view of the above-mentioned problems in the conventional technology, and its object is to provide a highly corrosion-resistant austenitic stainless steel having excellent corrosion resistance even when exposed to a temperature region where σ phase precipitation occurs, specifically, a temperature range of 700 to 1000°C, and particularly when exposed to around 850°C where the corrosion resistance changes drastically.

[0022] The inventors have conducted extensive research to address the above-mentioned issues. Their findings indicate that, in order to improve corrosion resistance, it is necessary to increase the concentrations of Ni, Cr, Mo, and N in steel. However, among these, the addition of Mo and W, which have a significant effect on the precipitation of the σ phase, should be minimized. Similarly, Mn and Si, elements that induce the precipitation of the σ phase, should be reduced within a range that does not excessively increase production load or affect deoxidation. Furthermore, Ni, N, and Co, elements that stabilize the austenite phase, should be contained within acceptable ranges from the perspectives of cost and weldability. However, it has been determined that these measures alone are insufficient to suppress the σ phase.

[0023] Therefore, in addition to the above, the inventors also explored the effects of further suppressing the precipitation of σ phase. Focusing on the triple points at grain boundaries, where σ phase preferentially precipitates, they investigated various methods to delay the migration of Cr, Mo, and other σ phase-constituting elements there. As a result, they discovered that by appropriately controlling the contents of Sn, B, P, and Si, elements that segregate at grain boundaries, the precipitation of σ phase can be delayed, ensuring excellent corrosion resistance even when exposed to the temperature range where σ phase precipitates.

[0024] Then, the inventor repeatedly studies the condition of giving full play to the delay effect of σ phase precipitation by controlling Sn, B, P amount. As a result, it is found that the control of crystallite diameter is an important factor. In order to make the σ phase solid solution, it is necessary to carry out solution heat treatment at a fully high temperature, so that the crystallite diameter is inevitably coarsened. In this case, the point of the triple key point of the grain boundary of the σ phase preferential precipitation position becomes extremely few, and under the situation of implementing the thermal treatment hereafter, the grain boundary diffusion of Cr, Mo can concentrate to such few position. In this case, even with the effect of above-mentioned elements, the precipitation of σ phase can not be suppressed. In addition, owing to being coarse particle, the sensitization that causes because of carbide also occurs easily, and it is judged that the reduction of corrosion resistance is produced. On the other hand, under the situation that crystallite diameter is too fine, the total area of ​​crystal boundary becomes large, and the distribution of Sn, B, P amount in the grain boundary becomes sparse, and it is judged that the effect of delaying σ phase precipitation can not be fully obtained. From this, it was found that by controlling the crystal grain size in the range of 3.0 to 7.0 according to JIS G0511, Sn, P, B, and Si are appropriately present at the grain boundaries, thereby delaying the precipitation of the σ phase.

[0025] Furthermore, the inventors focused on carbonitrides as a method for controlling crystal grain size. They discovered that by adding one or more of V, Nb, and B, which have high affinity for C and N, within an appropriate range, the grain size according to JIS G0551 can be controlled within the range of 3.0 to 7.0. This also has the effect of making it difficult to precipitate Cr carbides, which can cause sensitization. Furthermore, since Mn increases the solubility of N and inhibits the precipitation of these carbonitrides, adjusting the Mn content was found to be an important factor in controlling the crystal grain size in the present invention.

[0026] The present invention has been completed through the above process. That is, the highly corrosion-resistant austenitic stainless steel of the present invention is characterized by containing, in mass%, C: 0.005-0.030%, Si: 0.05-0.30%, Mn: 0.05-0.40%, P: 0.005-0.050%, S: 0.0001-0.0010%, Ni: 22.0-32.0%, Cr: 19.0-28.0%, Mo: 5.0-7.0%, N: 0.18-0.25%, Al: 0.005-0.100%, Cu: 0.05-0.50%, W: 0.05% or less, Sn: 0.0005-0.0150%, Co: 0.030-0.300%, B: 0.0005-0.0050%, and the balance being Fe and inevitable impurities.

[0027] Furthermore, the following formula (1) is satisfied, the area ratio of the σ phase is 1% or less, and the CPT according to ASTM G48 Method C is 60° C. or higher as corrosion resistance.

[0028] 0.05≤10[%B]+2[%P]+6[%Sn]+0.03[%Si]≤0.20...(1)

[0029] The highly corrosion-resistant austenitic stainless steel of the present invention preferably contains one or two of Nb: 0.005-0.250% and V: 0.005-0.250%, satisfies the following formula (2), and has a base material grain size in the range of 3.0-7.0 according to JIS G0511.

[0030] 1.2≤100{2([%V]+[%Nb])+6[%B]}×([%N]+[%C]-0.1[%Mn])≤5.0...(2)

[0031] The highly corrosion-resistant austenitic stainless steel of the present invention is produced by the following method, wherein the thermal history after the solution heat treatment is such that the temperature range of 700 to 1000° C. is passed through an isothermal holding, cooling, or heating step for 10 to 60 minutes.

[0032] Effects of the Invention

[0033] According to the present invention, even when exposed to a temperature region where σ phase precipitation occurs, a decrease in corrosion resistance can be suppressed, and thus the steel can be suitably used as a joining material for clad steel to be joined to thick-walled carbon steel or the like, or as a highly corrosion-resistant material used in processes passing through a production line furnace for brazing. BRIEF DESCRIPTION OF THE DRAWINGS

[0034] [ Figure 1 ](a) is an electron microscope photograph of the metal sample of the present invention, and (b) is an electron microscope photograph using the EBSD method.

[0035] [ Figure 2 ] is a graph showing the relationship between the σ phase area ratio and CPT at various aging heat treatment temperatures in the present invention.

[0036] [ Figure 3 ] is a graph showing the relationship between the corrosion resistance and formula (1), and the relationship between the number of cracks generated and formula (1) in the present invention.

[0037] [ Figure 4 ] is a graph showing the relationship between corrosion resistance and grain size in the present invention.

[0038] [ Figure 5 ] is a graph showing the relationship between the crystal grain size in the present invention and formula (2).

[0039] [ Figure 6 ] is a graph showing the relationship between corrosion resistance and grain size in the present invention. DETAILED DESCRIPTION

[0040] The present inventors conducted the following <Experiment 1> to <Experiment 3> and conducted research to complete the present invention. This research will be described below.

[0041] Until now, the quantitative evaluation method of the σ phase that causes corrosion resistance degradation has been mainly carried out by the point calculation method represented by ASTM E562. This is a method that evaluates the ratio of the intersection of the grid-shaped markings attached to the microscope and the overlap of the σ phase on the metal structure subjected to etching. Therefore, the evaluation result is affected by the quality of the etching during observation, and there is a risk of an error of several percent relative to the actual amount of σ phase precipitation. Therefore, the inventors used a field emission scanning electron microscope and backscattered electron diffraction method (hereinafter referred to as EBSD method), which is a method that can perform high-precision measurements and obtain high reliability through crystal structure determination, to evaluate the area ratio of the σ phase.

[0042] Previous patents related to σ phase suppression also compared the σ phase suppression effect using the aforementioned "area ratio." However, the relationship between the amount of σ phase precipitation and changes in corrosion resistance when the annealing temperature was varied was unclear. Therefore, the inventors investigated the relationship between the amount of σ phase precipitation and corrosion resistance when the heat treatment temperature and holding time were varied.

[0043] <Experiment 1>

[0044] A high-frequency induction furnace was used to melt steel with a basic composition of Fe-0.01% C-25% Cr-23% Ni-6% Mo-0.20% N-0.4% Cu. The melted mass was 20 kg, and after forming an ingot, it was hot-forged at a heating temperature of 1200°C to form a plate with a thickness of 8 mm and a width of 70 mm. The forged plate was then annealed and pickled, and then cold-rolled to a thickness of 2 mm to produce a cold-rolled plate. The cold-rolled plate was solution-heat treated at 1150°C for 1 minute and cooled by forced air cooling. Furthermore, the cold-rolled plate was subjected to an aging heat treatment at various temperatures and holding times within the range of 700-1100°C for 1-60 minutes. The σ phase area ratio and corrosion resistance of the aging-treated material were measured using the EBSD method.

[0045] The σ phase area ratio was evaluated by electrolytically polishing a small piece cut at right angles to the rolling direction from a heat-treated cold-rolled sheet using a "Tenupol-5" manufactured by Struers Co., Ltd., and then measuring an 80 μm × 240 μm measurement area at a position 1 / 4 of the thickness direction of the cold-rolled sheet using a backscattered electron diffraction device (TSL Solutions Co., Ltd., "EBSD Analysis Software OIM Analysis 7.3") attached to a field emission scanning electron microscope (JEOL Ltd., "JSM-7001F") with a step size of 0.2 μm.

[0046] Corrosion resistance is evaluated by immersing the specimen in an aqueous ferric chloride solution as specified in ASTM G48 Method C and measuring the critical pitting temperature (CPT). 25mm x 50mm specimens are collected from cold-rolled sheets that have undergone aging heat treatment. The entire surface is polished with SiC No. 120 water-resistant abrasive paper and degreased with acetone before use in the test. Each specimen is immersed in 600ml of the test solution for 72 hours, and the minimum temperature (CPT) at which pitting occurs with a depth of 25μm or greater is measured.

[0047] The measurement results of the σ phase area ratio and corrosion resistance are shown in Figure 1 、 2 middle. Figure 1 This is an example of evaluating the area ratio of the σ phase based on the EBSD method. Figure 1 In the secondary electron image of (a), very fine σ phase (white dots) with a particle size of about 0.3 μm precipitated along the grain boundaries are Figure 1 (b) is detected as a white spot in the EBSD image. The results showing the relationship between the area ratio of the fine and trace σ phase and the corrosion resistance in this EBSD method are shown in FIG. Figure 2 The relationship between the σ phase area ratio and CPT when the annealing temperature is maintained for 600 seconds is shown.

[0048] In the steel of the present invention, a decrease in corrosion resistance is observed when the annealing temperature is in the range of 700 to 1000°C, but Figure 2 As shown in the figure, despite the same or slightly higher σ phase area ratio, the temperature at which corrosion resistance is lowest is 850°C. Therefore, to replicate the phenomenon when this alloy is subjected to a temperature range where a decrease in corrosion resistance due to precipitation of the σ phase is particularly pronounced, the alloy was subjected to an aging heat treatment temperature of 850°C with various soaking times, and corrosion resistance was evaluated.

[0049] Experiment 2

[0050] In order to obtain the effect of delaying the deterioration of corrosion resistance caused by the precipitation of σ phase, the delay effect of grain boundary diffusion caused by Sn, B, P and Si, which are elements segregated at the grain boundaries, is considered. Using a high-frequency induction furnace, 20 kg of steel with Fe-25% Cr-23% Ni-6.0% Mo-0.20% N-0.4% Cu as the basic components and various changes in the content of Sn, B, P and Si were melted. Then, forged plates and cold-rolled plates were obtained by the same method as in Experiment 1. At this time, hot workability was evaluated by cracking generated on the side of the forged plate. The cold-rolled plate was subjected to a solution heat treatment of 1150°C×1min and cooled by forced air cooling. Furthermore, the cold-rolled plate was subjected to an aging heat treatment at 850°C. In this experiment, the holding time was varied within 1.5 hours. The corrosion resistance evaluation and grain size measurement were performed on the aging heat-treated material.

[0051] Regarding hot workability, when cracks occurring on the side of the forged plate are visually observed and no cracks exceeding 20 mm are generated, the workability is evaluated as excellent (◎). When there are less than 3 cracks per 100 mm in the longitudinal direction, it is evaluated as good (○). When there are more than 3 cracks but less than 6 cracks, it is evaluated as acceptable (△). When there are more than 6 cracks, it is judged to be unsuitable for processing and is evaluated as poor (×).

[0052] Corrosion resistance was evaluated by measuring the critical pitting temperature (CPT) in the same manner as in Experiment 1. When the CPT exceeded 60°C even after a soaking time of more than 1.5 hours, the test was rated as excellent (◎) because the degradation of pitting resistance during aging was particularly well suppressed. When the soaking time before the CPT reached 60°C was from 1.2 hours to less than 1.5 hours, the test was rated as good (○). When the soaking time was from 1 hour to less than 1.2 hours, the test was rated as acceptable (△). When the CPT dropped to 60°C after soaking for less than 1 hour, the test was rated as poor (×).

[0053] The grain size of the steel was measured based on JIS G0551 using a cold-rolled sheet heat-treated at 1150° C. for 1 minute.

[0054] [Table 1]

[0055]

[0056] The test results are shown in Table 1. Figure 3 This is a graph plotting the test results in Table 1. It shows the range of 1 hour or more of soaking time (left vertical axis) required before the CPT reaches 60°C when heat treatment is performed at 850°C in the corrosion resistance test, using the relationship between the contents of B, P, Sn, and Si (horizontal axis) in formula (1).

[0057] 0.05≤10[%B]+2[%P]+6[%Sn]+0.03[%Si]≤0.20...(1)

[0058] according to Figure 3 , No. 1 to 18 with a soaking time of more than 1 hour before the CPT is reduced to 60°C, showing a good effect of delaying the degradation of corrosion resistance. In addition, it is judged that B is effective even when added in small amounts compared to P and Sn. In addition, for No. 1 to 18 with a soaking time of more than 1 hour before the CPT is reduced to 60°C, it was found that the area ratio of the σ phase was less than 1.0% and the particle size of the σ phase was less than 2μm when the aging heat treatment was carried out at 850°C for a holding time of 60 minutes. It should be noted that it was confirmed that the particle size of the σ phase increased with the increase in the area ratio of the σ phase.

[0059] However, although the ranges of B, P, Sn, and Si are appropriate, the time taken for No. 11 to 14 and No. 17 to drop to 60°C is shorter than that for No. 1 to 10, and is barely sufficient for 1 hour ( Figure 3 The grain size of the steel was measured to be 7.0 to 7.5, and the grains were all fine, indicating that even if the addition amounts of Sn, B, and P were controlled, the corrosion resistance degradation inhibition effect could not be fully exerted. In addition, in No. 17 and 18, where the formula (1) exceeded 0.20, more than 6 cracks occurred on the side, indicating that they could not be used for high-temperature processing. No. 19 and 20, where the formula (1) was less than 0.05, had a CPT of less than 1 hr before it dropped to 60°C. Therefore, it is necessary to control the formula (1) within the range of 0.05 to 0.20.

[0060] In Table 1 and Figure 3 Among them, since No. 11 to No. 14 and No. 17, which have low corrosion resistance degradation suppression effects, are all fine particles, it is thought that the crystal grain size is related to the effects of Sn, B, P, and Si. Therefore, for Steel 6 in Table 1, the temperature and time of the solution heat treatment were varied to investigate the correlation between the crystal grain size and the effect of delaying corrosion resistance degradation. The results are shown in Tables 2 and 3. Figure 4 middle.

[0061] [Table 2]

[0062]

[0063] As shown in Table 2 and Figure 4 As shown, the soaking time before the CPT drops to 60°C varies depending on the grain size, suggesting an appropriate range. No. 6-c, which was solution heat treated at 1080°C for 1 minute, had a fine grain size of 7.5. On the other hand, No. 6-b, which had a high soaking temperature of 1150°C and a long soaking time of 30 minutes, had a coarse grain size of 2.5.

[0064] In the present invention, the present invention relates to a kind of heat treatment of the present invention, and the present invention relates to a kind of heat treatment of the present invention.Therefore, in order to prevent the residual of σ phase, need to carry out solution heat treatment at high temperature, but crystal grain will inevitably become coarse, accelerate the precipitation of σ phase, in addition, also can make corrosion resistance reduce because of the precipitation of carbide.But in the thermal treatment of low temperature, crystal grain size becomes minute, can't fully obtain the effect of the precipitation of the delay σ phase caused by containing above-mentioned P, B, Sn, Si.In addition, there is the worry that can't make σ phase disappear completely.Therefore, even in the thermal treatment of high temperature, also demonstrate the necessity of the technology that crystal grain size is controlled in suitable scope.

[0065] <Experiment 3>

[0066] According to the results of Experiment 2 above, by appropriately adjusting the contents of B, P, Sn, and Si, it is possible to delay the degradation of corrosion resistance when exposed to the temperature range where σ phase precipitates. However, in order to fully realize this effect, it is known that further control of the crystal grain size is necessary. Therefore, methods for controlling this have been repeatedly studied.

[0067] 20 kg of steel composed primarily of Fe, 0.2% Si, 25% Cr, 23% Ni, 6.0% Mo, 0.4% Cu, 0.003% Sn, and 0.020% P was melted in a high-frequency induction furnace. During this melting process, the contents of V, Nb, B, C, N, and Mn were varied to control the grain size by utilizing the pinning effect and to induce precipitation of carbonitrides. The value of equation (1) at this point ranged from 0.05 to 0.10. A 2 mm thick cold-rolled sheet was obtained in the same manner as in Experiment 1, except that a solution heat treatment was performed at 1150°C for 1 minute. A aging heat treatment was then performed at 850°C with varying holding times within 1.5 hours. The resulting material was used as a test material, and corrosion resistance evaluation and grain size measurement were performed using the same methods as above. The test results are shown in Table 3. Figure 5 This is a graph showing the relationship between the crystal grain size and the following formula (2).

[0068] 1.2≤100{2([%V]+[%Nb])+6[%B]}×([%N]+[%C]-0.1[%Mn])≤5.0...(2)

[0069] [Table 3]

[0070]

[0071] exist Figure 5 It was found that the larger the value of formula (2), the finer the crystal grains, and the more controllable the grain size. When the value of this formula is in the range of 1.2 to 5.0, the grain size is in the range of 3.0 to 7.0.

[0072] also, Figure 6This is a graph showing the relationship between the soaking time range of 1 hour or more until the CPT reaches 60°C and the crystal grain size based on JISG 0511. Figure 6 If the crystal grain size is smaller than 3.0, that is, coarse particles, the CPT reaches 60°C before less than 1 hour. If the crystal grain size is in the range of 3.0 to 7.0, it exceeds 1 hour before reaching 60°C, and a good effect of inhibiting corrosion resistance degradation is obtained. In particular, the particle size in the range of 4 to 6 is the best. However, if the crystal grain size is larger than 7, that is, fine particles, it is again less than 1 hour. It can be seen from this that in order to fully delay the degradation of corrosion resistance, it is preferably to adjust the crystal grain size within an appropriate range, and the particle size needs to be controlled within the range of 3.0 to 7.0.

[0073] Next, the reasons for limiting the composition of each element and the relationship formula in the present invention will be described.

[0074] C: 0.005~0.030%

[0075] C is an element that is effective in stabilizing the austenite phase and suppresses the precipitation of the σ phase. It is also an important element for forming carbonitrides for controlling the grain size. Therefore, it is necessary to add at least 0.005%. However, if it is excessively contained, the grain size becomes fine due to the pinning effect of the carbonitrides, and the effect of delaying the precipitation of the σ phase cannot be obtained. In addition, the precipitation of Cr carbides becomes easy during welding, etc., which deteriorates the corrosion resistance. Therefore, the upper limit is set to 0.030%. The preferred lower limit of the content is 0.007%, the more preferred lower limit is 0.009%, the preferred upper limit is 0.025%, and the more preferred upper limit is 0.020%.

[0076] Si: 0.05-0.30%

[0077] Si is an important element of the present invention that has a deoxidizing effect. It exists at grain boundaries along with Sn, B, and P and is an indispensable element for delaying the precipitation of the σ phase. However, excessive Si content accelerates the precipitation of the σ phase and also facilitates scale formation, which deteriorates wettability during brazing. Therefore, the Si content is set to 0.05-0.30%. The preferred lower limit of the content is 0.07%, and the more preferred lower limit is 0.09%. The preferred upper limit is 0.25%, and the more preferred upper limit is 0.23%.

[0078] Mn: 0.05~0.40%

[0079] Mn is an element added as a deoxidizer. It stabilizes the austenite phase and increases the solubility of nitrogen. Therefore, it is an essential element for controlling the particle size using carbonitrides. Therefore, it is necessary to contain 0.05% or more of Mn. However, excessive addition promotes the precipitation of σ phase, reducing corrosion resistance. In addition, it forms MnS, which becomes the starting point of pitting corrosion and deteriorates corrosion resistance. Therefore, the Mn content is set to 0.05-0.40%. The preferred lower limit of the content is 0.06%, and the more preferred lower limit is 0.07%. The preferred upper limit is 0.30%, and the more preferred upper limit is 0.25%.

[0080] P: 0.005~0.050%

[0081] P is an element that inevitably mixes into steel as an impurity, but in the present invention, it is an essential element that exists at grain boundaries and delays the precipitation of σ phase. To achieve this effect, it is necessary to add at least 0.005%. However, if the content exceeds 0.050%, corrosion resistance and hot workability will be significantly deteriorated. Therefore, the P content is set to 0.005-0.050%. The preferred lower limit of the content is 0.010%, and the more preferred lower limit is 0.012%. The preferred upper limit is 0.040%, and the more preferred upper limit is 0.035%.

[0082] S: 0.0001~0.0010%

[0083] S is an impurity element that inevitably enters steel. It reduces hot workability and forms sulfides, which serve as the starting point for pitting corrosion, thus negatively affecting corrosion resistance. In this experiment, no effect was observed to delay the deterioration of corrosion resistance due to the precipitation of σ phase, as with P. Therefore, a very low S content is preferred, with the upper limit ideally being 0.0010%. However, since S increases the fluidity of the melt during melting, it also improves weldability. To achieve good weldability, a content of 0.0001% or more is preferred. The preferred lower limit of the content is 0.0002%, with a more preferred lower limit being 0.0003%. The preferred upper limit is 0.0008%, with a more preferred upper limit being 0.0007%.

[0084] Ni: 22.0-32.0%

[0085] Nickel is an element that stabilizes the austenite phase and is an important element for suppressing the precipitation of intermetallic compounds such as the σ phase, improving pitting corrosion resistance and overall corrosion resistance. However, if the Ni content exceeds 32.0%, it will lead to increased hot deformation resistance and increased costs. Therefore, the Ni content is set to 22.0-32.0%. The preferred lower limit of the content is 23.0%, and the more preferred lower limit is 23.5%. The preferred upper limit is 31.5%, and the more preferred upper limit is 30.0%.

[0086] Cr: 19.0~28.0%

[0087] Cr is an essential element for improving not only pitting resistance but also crevice corrosion resistance and intergranular corrosion resistance. However, excessive Cr content promotes the precipitation of σ phase, which in turn deteriorates corrosion resistance. Therefore, the Cr content is set to 19.0-28.0%. The preferred lower limit of the content is 21.0%, and the more preferred lower limit is 22.0%. The preferred upper limit is 27.0%, and the more preferred upper limit is 25.0%.

[0088] Mo: 5.0~7.0%

[0089] Mo, like Cr and N, is an element that improves pitting and crevice corrosion resistance. However, excessive Mo content significantly promotes the precipitation of σ phase, deteriorating corrosion resistance. Therefore, the Mo content is set within a range of 5.0 to 7.0%. The preferred lower limit is 5.1%, and the more preferred lower limit is 5.2%. The preferred upper limit is 6.7%, and the more preferred upper limit is 6.5%.

[0090] N: 0.18~0.25%

[0091] N is an element that stabilizes the austenite phase and is effective in suppressing the precipitation of the σ phase. In addition, like Cr and Mo, it significantly improves pitting corrosion resistance and crevice corrosion resistance. Furthermore, like C, it is an element that forms carbonitrides for controlling the grain size. Therefore, it is necessary to add at least 0.18%. However, if the N content is excessive, a large amount of carbonitrides will precipitate, the grain size will become fine, and the effect of delaying the precipitation of the σ phase cannot be obtained. Therefore, it cannot exceed 0.25%. The preferred lower limit of the content is 0.19%, the more preferred lower limit is 0.20%, the preferred upper limit is 0.24%, and the more preferred upper limit is 0.23%.

[0092] Al: 0.005~0.100%

[0093] Al is added as a deoxidizer. Furthermore, in the presence of a CaO-SiO2-Al2O3-MgO slag, it promotes desulfurization by deoxidation, making it an important element for stabilizing the boron yield during refining. However, excessive Al content can easily lead to scale formation, which can deteriorate brazing wettability. Therefore, the Al content is set to 0.005-0.100%. The preferred lower limit is 0.008%, more preferably 0.010%, and the preferred upper limit is 0.080%, more preferably 0.070%.

[0094] Cu: 0.05-0.50%

[0095] Cu is an element that stabilizes the austenite phase and helps improve acid resistance. To achieve this effect, it is necessary to contain 0.05% or more. However, since excessive addition increases costs and deteriorates hot workability, the upper limit is set to 0.50% or less. Therefore, its content is set to 0.05-0.50%. The preferred lower limit of the content is 0.07%, and the more preferred lower limit is 0.08%. The preferred upper limit is 0.45%, and the more preferred upper limit is 0.40%.

[0096] Sn: 0.0005~0.0150%

[0097] Sn exists in the grain boundaries together with B and P in the present invention, thus becoming an important element for delaying the precipitation of the σ phase. In order to obtain this effect, it is necessary to add at least 0.0005%. However, when containing more than 0.0150%, Sn itself has the effect of promoting the precipitation of the σ phase. Therefore, the content of Sn is set to 0.0005-0.0150%. The preferred lower limit of the content is 0.0010%, the more preferred lower limit is 0.0012%, the preferred upper limit is 0.0100%, and the more preferred upper limit is 0.0090%.

[0098] Co: 0.030~0.300%

[0099] Co, like Ni, has the effect of stabilizing the austenite phase and suppressing the precipitation of the σ phase. Moreover, it is a useful element with a higher σ phase suppression effect per unit weight than Ni. In order to achieve this effect, it is necessary to contain at least 0.030% or more. However, Co is an element more expensive than Ni, and excessive addition leads to high costs. Therefore, the upper limit is set to 0.300%. The preferred lower limit of the content is 0.040%, the more preferred lower limit is 0.050%, the preferred upper limit is 0.295%, and the more preferred upper limit is 0.290%.

[0100] B: 0.0005~0.0050%

[0101] B is an important element constituting the present invention. It exists in the grain boundaries together with P and Sn, and plays an effect of delaying the precipitation of the σ phase. In addition, it properly controls the grain size of the steel together with V and Nb, which also plays an important role in delaying the precipitation of the σ phase. Therefore, it is necessary to add at least 0.0005%. However, if B is contained excessively, carbonitrides will precipitate in large quantities, and the grain size will become fine due to the excessive pinning effect, and the delay effect of the precipitation of the σ phase cannot be obtained. In addition, the hot workability is significantly deteriorated. Therefore, the upper limit is set to 0.0050%. The preferred lower limit of the content is 0.0007%, the more preferred lower limit is 0.0008%, the preferred upper limit is 0.0035%, and the more preferred upper limit is 0.0032%.

[0102] 0.05≤10[%B]+2[%P]+6[%Sn]+0.03[%Si]≤0.20...(1)

[0103] By containing B, P, Sn, and Si as the above-mentioned constituent elements within the specified ranges and satisfying the above-mentioned relationship, Sn, B, and P segregate at the grain boundaries, further delaying the deterioration of corrosion resistance due to precipitation of the σ phase. The preferred lower limit is 0.06, and the more preferred lower limit is 0.08. The preferred upper limit is 0.18, and the more preferred upper limit is 0.16.

[0104] σ phase area ratio is less than 1.0%

[0105] When heat treatment at 850°C is performed, where the corrosion resistance is particularly significantly reduced, the area ratio of the σ phase is less than 1.0% when the time before the CPT drops to 60°C is more than 1 hour. This is clearly shown by the precise quantification of the area ratio of the σ phase based on EBSD and its corrosion test. Therefore, the σ area ratio needs to be less than 1.0%. It is preferably less than 0.8%, and more preferably less than 0.7%. In addition, due to the large amount of precipitation of the σ phase, the degree of the Cr and Mo deficiency layer formed around the σ phase becomes worse. Therefore, in order to delay the reduction in corrosion resistance, the particle size of the σ phase is preferably small. In the present invention, the upper limit of its size is less than 2.0μm. It is preferably 1.8μm, and more preferably less than 1.6μm.

[0106] Nb, V: 0.005~0.250%

[0107] Nb and V are important elements constituting the present invention. Nb, together with V and B, combines with C and N to form carbides, nitrides or carbonitrides to control the grain size, thereby having the effect of delaying the precipitation of the σ phase. In order to obtain this effect, it is necessary to contain more than 0.005% of any one of them. However, even if it is any one of them, when Nb and V contain more than 0.250%, the precipitation of intermetallic compounds is promoted, resulting in a decrease in corrosion resistance. Therefore, it is set as the upper limit. The preferred lower limit of the content is 0.006%, the more preferred lower limit is 0.007%, the preferred upper limit is 0.230%, and the more preferred upper limit is 0.210%.

[0108] Note that the particle size control effect of these Nb and V can be achieved regardless of whether they are contained alone or in combination. Therefore, the effect can be exhibited by selectively containing any one or more of them.

[0109] 1.2≤100{2([%V]+[%Nb])+6[%B]}×([%N]+[%C]-0.1[%Mn])≤5.0...(2)

[0110] By adding one or two of the constituent elements C, N, and B, as well as V and Nb, within appropriate ranges and satisfying the aforementioned relationship regarding carbonitride precipitation, a suitable pinning effect can be achieved, and the grain size according to JIS G0551 can be controlled within the range of 3.0 to 7.0, thereby retarding the precipitation rate of the σ phase. The preferred lower limit is 1.3, and the more preferred lower limit is 1.4. The preferred upper limit is 4.5, and the more preferred upper limit is 4.2.

[0111] The crystal grain size of the base material according to JIS G0511 is 3.0 to 7.0

[0112] Because the precipitation speed of σ phase is affected by crystal grain size, it is necessary to control it. When the crystal grain size based on JIS G0577 exceeds 3.0 and is coarse, that is, the particle size number decreases, the point of the grain boundary triple point as the σ phase preferential precipitation position decreases, and the grain boundary diffusion of Cr and Mo is concentrated, which accelerates the growth of σ phase. On the other hand, when the crystal grain size is finer than 7.0, that is, the particle size number is large, the total area of ​​the grain boundary increases, and the distribution of Sn, B, and P in the grain boundary becomes sparse, and the effect of delaying the precipitation of σ phase cannot be fully obtained. Therefore, the scope of crystal grain size is set to 3.0~7.0. The preferred lower limit is 3.5, and the more preferred lower limit is 4.0. The preferred upper limit is 6.5, and the more preferred upper limit is 6.0.

[0113] The balance of the highly corrosion-resistant austenitic stainless steel of the present invention, other than the aforementioned components, consists of Fe and inevitable impurities. The inevitable impurities herein refer to components that inevitably enter the stainless steel for various reasons during industrial production and are permitted to be present within a range that does not adversely affect the effects of the present invention.

[0114] Next, a method for producing the highly corrosion-resistant austenitic stainless steel according to the present invention will be described.

[0115] The method for manufacturing the stainless steel of the present invention is not particularly limited, but is preferably manufactured by the following method. First, raw materials such as iron scraps or stainless steel scraps, ferrochrome, ferronickel, pure nickel, and metallic chromium are melted in an electric furnace. Then, in an AOD furnace or VOD furnace, while blowing oxygen and argon for decarburization and refining, quicklime, fluorite, Al, Si, etc. are added for desulfurization and deoxidation. The slag composition during this treatment is preferably adjusted to the CaO-Al2O3-SiO2-MgO-F system. In addition, in order to effectively perform desulfurization, the slag preferably satisfies CaO / Al2O3≥2 and CaO / SiO2≥3. In addition, the refractory material of the AOD furnace or VOD furnace is preferably a magnesium-chromium alloy or dolomite. After refining using the above-mentioned AOD furnace, the composition and temperature are adjusted through the LF process, and then continuous casting is performed to produce slabs. Thereafter, hot rolling is performed and, as required, cold rolling is performed to produce thick plates or thin plates such as hot-rolled steel sheets and cold-rolled steel sheets.

[0116] Example

[0117] The present invention is described in more detail below using examples. However, the present invention is not limited to these examples, provided it does not exceed its scope. First, raw materials such as iron scraps, stainless steel scraps, and ferrochrome were melted in a 60-ton electric furnace. Then, in the AOD process, oxygen and argon were blown in for decarburization and refining. Quicklime, fluorite, Al, and Si were then added for desulfurization and deoxidation. Ingots were then cast using a continuous casting machine to obtain slabs (Samples 1 to 45) having the chemical compositions shown in Table 4.

[0118] [Table 4]

[0119]

[0120] The mark *() indicates a departure from the scope of the invention

[0121] Chemical components other than C, S, and N were analyzed by fluorescent X-ray analysis. N was analyzed by inert gas pulse heating fusion, and C and S were analyzed by combustion in an oxygen stream and infrared absorption.

[0122] Then, the slab was hot rolled according to a conventional method to obtain a hot rolled steel sheet with a thickness of 8.0 mm. At this time, the hot workability was evaluated by the cracks that had already occurred on the side of the hot rolled steel sheet. Next, the hot rolled steel sheet was solution heat treated and then cold rolled. After product annealing and pickling, a cold rolled strip with a thickness of 2.0 mm was obtained. Product annealing was carried out under conditions of holding at 1150°C for 1 minute and then water cooling. Furthermore, the cold rolled strip was subjected to an aging heat treatment at 850°C with the holding time varied within a range of no more than 1.5 hours. For the aging heat treated material, the corrosion resistance evaluation described below was carried out, and the grain size was measured based on JIS G0551. Furthermore, the EBSD method similar to <Experiment 1> was used to quantitatively evaluate the area ratio of the σ phase and the grain size of the σ phase.

[0123] <Hot workability evaluation test>

[0124] The cracks generated on the side surfaces of the hot-rolled steel sheets were visually observed. If no cracks larger than 40 mm were generated, the workability was evaluated as excellent (◎). If there were fewer than 3 cracks per 100 mm in the longitudinal direction, the sheet was evaluated as good (○). If there were more than 3 cracks but fewer than 6 cracks, the sheet was evaluated as acceptable (△). If there were more than 6 cracks, the sheet was judged to be unworkable and evaluated as poor (×).

[0125] <Corrosion Resistance Evaluation Test>

[0126] The cold-rolled strip subjected to the aging heat treatment was subjected to a ferric chloride solution immersion test in accordance with ASTM G48 (Method C) under the following conditions, and the critical pitting temperature (CPT) was measured to evaluate the corrosion resistance.

[0127] Test piece: width 25mm × length 50mm × thickness 2mm;

[0128] Test solution: 6 mass% FeCl3 + 1 mass% HCl aqueous solution;

[0129] .Test liquid volume: 600ml per test piece;

[0130] Surface grinding: Use #120 SiC grinding paper for wet grinding of the entire surface;

[0131] Test temperature: 55-100°C;

[0132] Immersion time: 100 hours;

[0133] .Number of test pieces (n): 2 for each condition;

[0134] Evaluation Criteria: The pitting depth of the test specimens was measured, and the critical pitting temperature (CPT) at which a pitting depth of 25 μm or greater was determined and evaluated. During the aging heat treatment, if the CPT exceeded 60°C even after a soaking time of 1.5 hours, this was evaluated as excellent (◎) due to the particularly excellent suppression of pitting resistance degradation during aging. If the soaking time before the CPT reached 60°C was 1.2 hours to less than 1.5 hours, this was evaluated as good (○). If the soaking time was 1 hour to less than 1.2 hours, this was evaluated as acceptable (△). If the CPT dropped to 60°C after soaking for less than 1 hour, this was evaluated as poor (×).

[0135] <Measurement of σ Phase Area Ratio>

[0136] The area ratio of the σ phase was measured by the same EBSD method as in <Experiment 1> for the cold-rolled strip subjected to aging heat treatment at 850° C. with a holding time of 60 minutes.

[0137] Test piece collection direction: Collect from a direction perpendicular to the rolling method;

[0138] Sample grinding: Electrolytic grinding was performed using "Tenupol-5" manufactured by Strauers.

[0139] EBSD measurement: Field emission scanning electron microscope (manufactured by JEOL Ltd., “JSM7001F”) with attached backscattered electron diffraction device (manufactured by TSL Solutions Ltd., “EBSD analysis software OIM Analysis 7.3”);

[0140] Measurement area: 80 μm × 240 μm;

[0141] Step size: 0.2 μm.

[0142] <Crystal System Determination of σ Phase>

[0143] For the same sample as the sample for which the σ phase area ratio was determined, the grain size of the σ phase was determined from a 5000-times magnification composition image of a scanning electron microscope.

[0144] The evaluation results are shown in the following Table 5. Table 5 shows the judgment based on the following relationship formula in the present invention, that is, the relationship formula for suppressing corrosion resistance degradation,

[0145] 0.05≤10[%B]+2[%P]+6[%Sn]+0.03[%Si]≤0.20...(1)

[0146] The relationship between the control of crystal grain size is:

[0147] 1.2≤100{2([%V]+[%Nb])+6[%B]}×([%N]+[%C]-0.1[%Mn])≤5.0...(2)

[0148] The case where the relationship is satisfied is indicated by a circle, and the case where it is not satisfied is indicated by an x.

[0149] [Table 5]

[0150]

[0151] The mark *() indicates a departure from the scope of the invention

[0152] As shown in Table 5, in Tests 1 to 18, where the components met the ranges of the present invention, the time until the CPT reached 60°C was 1 hour or longer, demonstrating a good delay in corrosion resistance degradation. Furthermore, the crystal grain sizes were all within the range of 3.0 to 7.0. In Tests 19 to 30, where the components met the ranges of the present invention but the formula (2) was less than 1.2 or exceeded 5.0, the crystal grain sizes all deviated from the range of 3.0 to 7.0, and the CPT remained at 60°C or below for slightly longer than 1 hour.

[0153] In contrast, in Test Nos. 31 to 33, which satisfy the composition range of the present invention but have formula (1) below 0.05, the CPT takes less than 1 hr to reach 60°C. In these cases, the σ phase area ratio exceeds 1% and precipitates, with a particle size exceeding 2 μm.

[0154] In addition, in test number 31, the formula (2) exceeded 5.0, the crystal grain size was 9.0, and it was too fine, and the time until the CPT reached 60° C. was only 0.3 hr.

[0155] In test No. 32, the formula (2) was less than 1.2, the crystal grain size was 2.0, and it was too coarse. The time until the CPT reached 60° C. was only 0.4 hr.

[0156] In addition, in test numbers 34 to 37 where the expression (1) exceeded 0.20, the CPT took more than 1 hour to reach 60°C, showing a good effect of delaying the deterioration of corrosion resistance. However, six or more cracks had already occurred on the side of the hot-rolled steel sheet, indicating that the steel sheet could not be subjected to high-temperature processing.

[0157] Furthermore, in Test Nos. 38 to 41, where the contents of any of Sn, B, P, and Si were below the inventive range, the effect of delaying σ phase precipitation due to these substances was not sufficiently achieved, and the CPT reached 60°C for less than 1 hr. In all cases, the σ phase area ratio exceeded 1%, and the particle size exceeded 2 μm.

[0158] In addition, in test numbers 42 to 44, in which any one of Sn, B, and P exceeded the range of the invention, the CPT took more than 1 hour to reach 60°C, showing a good effect of delaying corrosion resistance degradation. However, six or more cracks had already occurred on the side of the hot-rolled steel sheet, indicating that it was not suitable for high-temperature processing.

[0159] In Test No. 45, in which Si exceeded the range of the invention, the CPT took less than 1 hour to reach 60° C. The area ratio of the σ phase at this time exceeded 1%, and the particle size exceeded 2 μm.

[0160] Industrial applicability

[0161] According to the present invention, even when exposed to a temperature region where σ phase precipitation occurs, a decrease in corrosion resistance can be suppressed, and thus the steel can be suitably used as a joining material for clad steel to be rolled and joined with thick-walled carbon steel, or as a highly corrosion-resistant material used in processes passing through a production line furnace for brazing.

Claims

1. Highly corrosion-resistant austenitic stainless steel, characterized by: In mass %, Contains C: 0.005-0.030%, Si: 0.05-0.30%, Mn: 0.05-0.40%, P: 0.005-0.050%, S: 0.0001-0.0010%, Ni: 22.0-32.0%, Cr: 19.0-28.0%, Mo: 5.0-7.0%, N: 0.18-0.25%, Al: 0.005-0.100%, Cu: 0.05-0.50%, W: 0.05% or less, Sn: 0.0005-0.0150%, Co: 0.030-0.300%, B: 0.0005-0.0050%, and the balance is Fe and inevitable impurities. The following formula (1) is satisfied, the area ratio of the σ phase is 1% or less, and the corrosion resistance is such that the CPT according to ASTM G48 Method C is 60°C or higher. 0.05≤10[%B]+2[%P]+6[%Sn]+0.03[%Si]≤0.20...(1).

2. The highly corrosion-resistant austenitic stainless steel according to claim 1, characterized in that: Containing one or two of Nb: 0.005 to 0.250% and V: 0.005 to 0.250%, and satisfying the following formula (2), the base material has a crystal grain size in the range of 3.0 to 7.0 based on JIS G0511, 1.2≤100{2([%V]+[%Nb])+6[%B]}×([%N]+[%C]-0.1[%Mn])≤5.0...(2).

3. A method for producing highly corrosion-resistant austenitic stainless steel, the method for producing highly corrosion-resistant austenitic stainless steel according to claim 1 or 2, characterized in that: As the thermal history after the solution heat treatment, the temperature range of 700 to 1000° C. is maintained or cooled or heated for 10 to 60 minutes.

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