Duplex stainless steel with excellent resistance to discoloration by crude phosphoric acid and structure for crude phosphoric acid using the same

Abstract

To provide a duplex stainless steel that has economic superiority coupled with a degree of corrosion resistance, exhibiting high resistance to blackening due to crude phosphoric acid.SOLUTION: A duplex stainless steel includes a predetermined component, with the DI value represented by formula 1 being 300 or more, leading to the brightness L* of 65 or more, which can suppress the blackening to a non-discernible level. For example, the duplex stainless steel includes, as the main elements, C: 0.001-0.100%, Si: 0.05-1.50%, Mn: 0.10-2.00%, Cr: 23.0-30.0%, Ni: 4.00-10.00%, Mo: 0.50-3.00%, Cu: 0.01-3.00%, N: 0.10-0.25%, Ca: 0.0010-0.0100%, P: 0-0.050%, and S: 0-0.0040%, with the balance being Fe and inevitable impurities. (Formula 1) DI value=12Cr-Ni+30Mo+2Cu+10N.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to duplex stainless steel, and more particularly to a duplex stainless steel having excellent resistance to discoloration (blackening) caused by crude phosphoric acid, and a structure for crude phosphoric acid using the same. [Background technology]

[0002] International logistics involves a wide range of goods, including corrosive substances such as acidic liquids and gases. The transportation of such chemicals is generally carried out by specially designed vessels such as chemical tankers, and corrosion-resistant steel, especially stainless steel, is used for the storage tanks.

[0003] For example, Patent Document 1 proposes an alloy-saving duplex stainless steel that maintains corrosion resistance and toughness while being cost-effective, as an alternative to the austenitic stainless steel conventionally used as a storage tank steel material. Patent Document 2 proposes a duplex stainless steel that combines a ferrite phase with good sulfuric acid corrosion resistance and an austenite phase with good toughness and workability, as a steel material for high-concentration sulfuric acid storage tanks in sulfuric acid production plants, chemical tankers for transporting sulfides, and the like.

[0004] Crude phosphoric acid is one of the products transported by chemical tankers. Crude phosphoric acid is a basic raw material for fertilizers, detergents, feed, pharmaceuticals, and other products. However, Japan has limited phosphorus resources and relies almost entirely on imports via sea transport. It is known that transporting crude phosphoric acid causes the interior surfaces of tanker tanks to discolor black (blackening). Therefore, after transporting the crude phosphoric acid, the discolored interior surfaces of the tanks are cleaned to prevent contamination, and new chemicals are loaded. This cleaning process is laborious and requires frequent cleaning, which poses a problem in terms of transportation costs and repair time for chemical transport. In response to this issue, for example, Patent Document 3 focuses on the components of crude phosphoric acid and finds that Cr, Ni, Mo, and Cu are effective in preventing corrosion, including blackening, caused by crude phosphoric acid. It proposes an austenitic stainless steel containing primarily these elements. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] International Publication No. 2009 / 119895 [Patent Document 2] Japanese Patent Application Publication No. 11-269612 [Patent Document 3] Japanese Patent Application Laid-Open No. 2002-121655 Summary of the Invention [Problem to be solved by the invention]

[0006] The inner surfaces of storage tanks in chemical tankers that transport crude phosphoric acid have traditionally been made of austenitic stainless steels with good corrosion resistance, such as SUS316L and 329J3L. However, even SUS316L can become blackened when storing crude phosphoric acid, and there is an increasing demand for alternative materials.

[0007] Patent Documents 1 and 2 are more cost-effective than austenitic stainless steels containing a lot of Ni, but are inferior to SUS316L in corrosion resistance to sulfides, crude phosphoric acid, and the like, and therefore cannot be expected to be effective as a measure against blackening.

[0008] Patent Document 3 found that Cr, Ni, Mo, and Cu are effective against corrosion, including blackening, of crude phosphoric acid, and introduced the GI value shown by the following formula, proposing an austenitic stainless steel with a GI value within a certain range (28% to 40%). GI value = -Cr + 0.94Ni + 4.61Mo + 7.74Cu

[0009] However, although the austenitic stainless steel of Patent Document 3 is effective to a certain extent against blackening caused by crude phosphoric acid, it has been confirmed that blackening actually occurs on the inner surface of the storage tank of a chemical tanker that transports crude phosphoric acid. Therefore, there is a demand for further improvement in the blackening resistance and for improvement from an economical viewpoint because the stainless steel contains a large amount of expensive elements such as Ni. Therefore, the present invention focuses on duplex stainless steels, which are superior to austenitic stainless steels from an economical standpoint and have a certain level of corrosion resistance, and aims to obtain duplex stainless steels that have even better resistance to blackening by crude phosphoric acid. [Means for solving the problem]

[0010] In order to achieve the above object, the present inventors have conducted extensive research and have obtained the following findings. (a) First, we considered the mechanism of blackening of the inner surface of the storage tank during storage of crude phosphoric acid. The blackening of the inner surface of the storage tank did not occur in the crude phosphoric acid solution part, but on the inner surface of the tank above the liquid surface. In other words, the inner surface of the storage tank that comes into contact with the gas phase turned black. From this, we considered that corrosion occurs when the components contained in the crude phosphoric acid solution evaporate and condense on the inner surface of the storage tank, and we continued our investigation.

[0011] Crude phosphoric acid is produced by dissolving phosphate rock in sulfuric acid, and is loaded onto chemical tankers as a liquid for transport. At this time, it has been discovered that the crude phosphoric acid contains F- (fluoride ions) and CL- (chloride ions). During transport by ship, the temperature inside the tank reaches 40-50°C, causing the crude phosphoric acid liquid to evaporate, with the gas phase containing F- and Cl-. As a result, it has been discovered that when the gas phase condenses on the inside of the tank, it becomes HF (hydrofluoric acid) and HCl (hydrochloric acid), corroding the steel inside the tank and turning it black.

[0012] Furthermore, the crude phosphoric acid solution contains fine particles generated during the dissolution of phosphate rock in sulfuric acid, which settle in the storage tank and form sediments (sludge). When the storage tank is roughly cleaned with water after unloading the crude phosphoric acid solution, F- and Cl- in the sludge diffuse into the water. The evaporated F- and Cl- condense on the inner surface of the tank, forming HF and HCl, causing black discoloration. It has been confirmed that black discoloration is particularly severe when sludge is present. While the cause has not been identified, it is thought to be as follows: In the absence of sludge, the F- concentration in the crude phosphoric acid decreases due to the volatilization of silicofluoride gas, which is expected to reduce discoloration (corrosion). However, in the presence of sludge, fluoride ions equivalent to the volatilized silicofluoride gas are continuously replenished from the sludge, so corrosion and discoloration are likely to continue. The black discoloration observed when the stainless steel described in Patent Document 3 was used on the inner surface of an actual chemical tank is thought to be due to the presence of sludge.

[0013] (b) Based on the above findings, we investigated stainless steels that are effective against HF and HCl corrosion, which affect blackening. In addition to Cr, Ni, Mo, and Cu, which were investigated in Patent Document 3, we also added N (nitrogen), which is effective in neutralizing acidic condensation water, and conducted a systematic analysis of these elements. As a result, we analyzed the blackening resistance of each element and derived a new index, the Discoloration Resistance Index (DI), which is calculated using the following formula 1. DI value = 12Cr-Ni + 30Mo + 2Cu + 10N Formula 1 As a result of numerous experimental studies, it was found that blackening could be suppressed by increasing the DI value, even when exposed to crude phosphoric acid solution containing sludge.

[0014] (c) When evaluating the lightness L* (JIS Z 8781-4) of the stainless steel surface as an index for evaluating blackening resistance, it was confirmed that if L* is 65 or higher, there will be no problems with not only corrosion resistance but also visual aesthetics. The relationship between the above index DI value and lightness L* was confirmed through numerous experiments, and it was confirmed that if DI is 300 or higher, lightness L* of 65 or higher can be ensured. In other words, it was discovered that duplex stainless steel with excellent blackening resistance can be obtained if the content of each element satisfies the following formula. DI value = 12Cr-Ni + 30Mo + 2Cu + 10N ≥ 300 The present invention was made based on the above findings, and the gist of the present invention is as follows.

[0015] [1] The components are in mass%: C: 0.001 to 0.100%, Si: 0.05 to 1.50% Mn: 0.10 to 2.00%, Cr: 23.0~30.0%, Ni: 4.00~10.00%, Mo: 0.50-3.00%, Cu: 0.01 to 3.00%, N: 0.10~0.25%, Ca: 0.0010 to 0.0100%, P: 0.050% or less, S: 0.0040% or less, Co: 0-3.00%, V: 0~1.00%, Nb: 0 to 0.200%, Ta: 0 to 0.200%, Al: 0 to 0.100%, Ti: 0 to 0.050% Zr: 0 to 0.090% Hf: 0 to 0.100%, B: 0~0.0050%, Mg: 0 to 0.0050%, W: 0~1.00%, REM: 0 to 0.100%, and Sn: 0 to 0.100% The DI value shown by the following formula 1 is 300 or more, A duplex stainless steel with excellent resistance to discoloration by crude phosphoric acid, characterized in that the balance is Fe and unavoidable impurities. DI value = 12Cr-Ni + 30Mo + 2Cu + 10N Formula 1 However, the element symbols in formula 1 represent the content (mass%) of each element contained in the steel, and 0 is substituted if the element is not contained. [2] The components are in mass%: Co: 0.01 to 3.00%, V: 0.01 to 1.00%, Nb: 0.005 to 0.200%, Ta: 0.005 to 0.200%, Al: 0.003 to 0.100%, Ti: 0.001 to 0.050%, Zr: 0.001 to 0.090%, Hf: 0.001 to 0.100%, B: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, W: 0.01 to 1.00%, REM: 0.005 to 0.100%, and Sn: Contains one or more selected from 0.005 to 0.100% The duplex stainless steel having excellent resistance to discoloration by crude phosphoric acid described in [1] above. [3] The duplex stainless steel having excellent resistance to discoloration by crude phosphoric acid according to [1] or [2] above, wherein the lightness L* is 65 or more. [4] The duplex stainless steel having excellent resistance to discoloration by crude phosphoric acid according to [1] or [2] above, wherein the DI value is 320 or more. [5] The duplex stainless steel having excellent resistance to discoloration by crude phosphoric acid according to [4] above, wherein the lightness L* is 70 or more. [6] A structure for crude phosphoric acid production using the duplex stainless steel according to any one of [1] to [5] above. [7] The structure for crude phosphoric acid according to [6] above is a phosphoric acid manufacturing plant, a phosphoric acid storage tank, or a phosphoric acid transport pipe. [Effects of the Invention]

[0016] According to the present invention, it is possible to obtain a duplex stainless steel having resistance to blackening, which can suppress blackening, and in particular, a duplex stainless steel having resistance to blackening caused by crude phosphoric acid. [Brief explanation of the drawings]

[0017] [Figure 1] FIG. 1 is a diagram showing an example of the relationship between the DI value and the lightness L*. DETAILED DESCRIPTION OF THE INVENTION

[0018] Hereinafter, embodiments of the present invention will be described. In this specification, unless otherwise specified, "%" for components indicates % by mass.

[0019] C: 0.001 to 0.100% C is an element that dissolves in the austenite phase to increase strength, and therefore the C content should be 0.001% or more, and preferably 0.003% or more, 0.005% or more, 0.007% or more, 0.009% or more, 0.0011% or more, 0.0013% or more, 0.015% or more, 0.17% or more, 0.19% or more, or 0.021% or more. On the other hand, C promotes the precipitation of Cr carbides, so a high C content causes intergranular corrosion and deteriorates corrosion resistance. Therefore, the C content is set to 0.100% or less. The C content is preferably 0.095% or less, 0.090% or less, 0.085% or less, 0.080% or less, 0.075% or less, 0.070% or less, 0.065% or less, or 0.060% or less.

[0020] Si: 0.05 to 1.50% Si is sometimes used as a deoxidizing element or added to improve oxidation resistance, so the Si content is preferably 0.05% or more. On the other hand, a high Si content hardens the steel sheet and deteriorates its toughness and workability. Therefore, the Si content is set to 1.50% or less. The Si content is preferably 1.45% or less, 1.40% or less, 1.35% or less, 1.30% or less, 1.25% or less, 1.20% or less, 1.15% or less, 1.10% or less, 1.05% or less, 1.00% or less, 0.95% or less, or 0.90% or less.

[0021] Mn: 0.10 to 2.00% Mn has the effect of increasing the austenite phase and increasing the solid solubility of nitrogen, thereby suppressing defects such as bubbles during manufacturing. Therefore, the Mn content should be 0.10% or more. The Mn content is preferably 0.15% or more, 0.20% or more, 0.25% or more, 0.30% or more, 0.35% or more, or 0.40% or more. On the other hand, a large amount of Mn reduces corrosion resistance and hot workability. Therefore, the Mn content should be 2.00% or less. The Mn content is preferably 1.80% or less, 1.60% or less, 1.50% or less, 1.40% or less, 1.30% or less, 1.25% or less, 1.20% or less, 1.15% or less, 1.10% or less, 1.05% or less, 1.00% or less, or 0.95% or less.

[0022] Cr: 23.0~30.0% Cr is an important element for ensuring corrosion resistance against crude phosphoric acid, strengthening the passive film and repairing the passive film after Fe dissolution. It is particularly effective in suppressing the formation of Fe fluorides when the steel sheet surface comes into contact with HF. Therefore, the Cr content should be 23.0% or more, preferably 23.5% or more, 24.0% or more, 24.5% or more, 25.0% or more, or 25.5% or more. On the other hand, a large amount of Cr increases the amount of chromium nitride precipitated, increases the risk of hot work cracking, and deteriorates toughness. Therefore, the Cr content should be 30.0% or less, and preferably 29.8% or less, 29.6% or less, 29.4% or less, 29.2% or less, or 29.0% or less.

[0023] Ni: 4.00 to 10.00% Ni is an austenite-stabilizing element and has the effect of improving corrosion resistance. To ensure a certain amount of austenite phase in duplex stainless steel, the Ni content should be 4.00% or more, preferably 4.10% or more, 4.20% or more, 4.30% or more, 4.40% or more, or 4.50% or more. On the other hand, Ni contributes little to blackening resistance and is an expensive element, so the content should be as low as possible. Therefore, the Ni content should be 10.00% or less, preferably 9.50% or less, 9.00% or less, 8.50% or less, 8.00% or less, 7.50% or less, or 7.00% or less.

[0024] Mo: 0.50 to 3.00% Mo has the effect of improving corrosion resistance and repairing the passive film after Fe dissolution. In particular, it is effective in suppressing discoloration (especially blackening) caused by the dissolution of Fe on the steel sheet surface. For this reason, the Mo content should be 0.50% or more, and preferably 0.60% or more, 0.70% or more, 0.80% or more, 0.90% or more, 1.00% or more, 1.20% or more, 1.40% or more, 1.60% or more, 1.80% or more, 2.00% or more, or 2.10% or more. On the other hand, Mo is an expensive element, so it is desirable to keep its content as low as possible. Therefore, the Mo content should be 3.00% or less, and preferably 2.95% or less, 2.90% or less, 2.85% or less, or 2.80% or less.

[0025] Cu: 0.01 to 3.00% Cu is an element that improves acid resistance, particularly sulfuric acid resistance, and has the effect of suppressing the dissolution of Fe from the steel sheet surface when acidic condensation water adheres to the steel sheet. Therefore, the Cu content is preferably 0.01% or more, and more preferably 0.03% or more, 0.05% or more, 0.07% or more, 0.10% or more, 0.15% or more, 0.20% or more, 0.25% or more, 0.30% or more, 0.35% or more, 0.40% or more, 0.45% or more, 0.50% or more, 0.55% or more, 0.60% or more, 0.65% or more, 0.70% or more, 0.75% or more, 0.80% or more, 0.85% or more, 0.90% or more, 0.95% or more, 1.00% or more, or 1.10% or more. On the other hand, a large amount of Cu increases the raw material cost and deteriorates the hot workability, so the Cu content should be 3.00% or less, preferably 2.95% or less, 2.90% or less, 2.85% or less, or 2.80% or less. N: 0.10~0.25%,

[0026] N: 0.10 to 0.25% When dissolved, N becomes NH4+ (ammonium ion), which increases the pH of acidic condensation water and inhibits the dissolution of Fe. It also dissolves in the austenite phase, improving strength and corrosion resistance and contributing to reduced alloying. Therefore, it is desirable for N to contain 0.10% or more. The N content should more preferably be 0.12% or more, 0.14% or more, or 0.16% or more. On the other hand, N is also an element that significantly affects the precipitation of chromium nitrides, and if it is contained in large amounts, the amount of chromium nitride precipitates increases, resulting in a deterioration in toughness. Therefore, the N content should be 0.25% or less, preferably 0.23% or less, or 0.22% or less.

[0027] Ca: 0.0010 to 0.0100% Ca is an element that enhances corrosion resistance, and it is believed that Ca modifies inclusions that are the starting point of corrosion in environments where HF or HCl is present, thereby suppressing the elution of Fe. Therefore, the Ca content should be 0.0010% or more, and preferably 0.0015% or more, 0.0020% or more, 0.0025% or more, or 0.0030% or more. On the other hand, a large amount of Ca makes the steel more susceptible to hot work cracking and reduces corrosion resistance, so the Ca content should be 0.0100% or less, and preferably 0.0080% or less, 0.0060% or less, 0.0050% or less, or 0.0045% or less.

[0028] By including the above elements in the specified amounts, a duplex stainless steel with good resistance to blackening by crude phosphoric acid can be obtained. In addition to the above elements, one or more elements selected from Co, V, Nb, Ta, Al, Ti, Zr, Hf, B, Mg, Sn, W, and REM may be included as needed. These elements do not necessarily have to be included, but by including them, the additional effects of each element can be obtained. These elements will be explained below.

[0029] Co: 0-3.00% Co may be contained because it is an austenite stabilizing element like N. However, since it is an expensive element, the Co content should be 3.00% or less, preferably 2.50% or less, 2.00% or less, 1.50% or less, 1.00% or less, or 0.50% or less.

[0030] V: 0 to 1.00% V is an element that improves corrosion resistance, so it may be contained as needed. However, if a large amount of V is contained, the load during rolling increases, making manufacturing defects more likely to occur. Therefore, the V content should be 1.00% or less, and preferably 0.80% or less, or 0.50% or less. There is no particular lower limit for the V content, but to obtain the above effects, the V content is preferably 0.01% or more.

[0031] Nb: 0 to 0.200% Nb may be added as needed because it has the effect of suppressing the precipitation of chromium nitrides by forming a compound with N. However, a large amount of Nb reduces the workability of the steel sheet. Therefore, the Nb content should be 0.200% or less, preferably 0.170% or less, or 0.150% or less. There is no particular lower limit for the Nb content, but to obtain the above effect, the Nb content is preferably 0.001% or more.

[0032] Ta: 0 to 0.200% Ta may be added as needed because it forms carbides and nitrides in steel and has the effect of additionally enhancing corrosion resistance. However, if Ta is added in large amounts, the excess carbides and nitrides formed will impair toughness. Therefore, the Ta content should be 0.200% or less, preferably 0.170% or less, or 0.150% or less. There is no particular lower limit for the Ta content, but to obtain the above effect, the Ta content is preferably 0.001% or more.

[0033] Al: 0 to 0.100% Al may be added as needed for desulfurization and deoxidation. However, adding a large amount of Al increases raw material costs and manufacturing costs. Therefore, the Al content should be 0.100% or less. There is no particular lower limit for the Al content, but to obtain the above effects, the Al content is preferably 0.001% or more.

[0034] Ti: 0 to 0.050% Like Nb, Ti prevents coarsening of the weld heat-affected zone and furthermore has the effect of finely crystalline equiaxed solidification, so it may be added as needed. However, a large amount of Ti reduces uniform elongation and local elongation. Therefore, the Ti content should be 0.050% or less, preferably 0.040% or less, and more preferably 0.030% or less. There is no particular lower limit for the Ti content, but to obtain the above effects, the Ti content is preferably 0.001% or more.

[0035] Zr: 0 to 0.090% Zr may be added as needed because it has the effect of forming carbides and nitrides in steel and refining crystal grains. However, if Zr is added in large amounts, the excess carbides and nitrides formed will impair toughness. Therefore, the Zr content should be 0.090% or less, and preferably 0.050% or less, 0.010% or less, or 0.004% or less. There is no particular lower limit for the Zr content, but to obtain the above effect, the Zr content is preferably 0.001% or more.

[0036] Hf: 0 to 0.100% Hf may be added as needed because it has the effect of forming carbides and nitrides in steel and refining crystal grains. However, if a large amount of Hf is added, the excess carbides and nitrides formed will impair toughness. Therefore, the Hf content should be 0.100% or less, and preferably 0.050% or less, or 0.010% or less. There is no particular lower limit for the Hf content, but to obtain the above effect, the Hf content is preferably 0.001% or more.

[0037] B: 0 to 0.0050% B has the effect of improving hot workability and may be added as needed. However, if a large amount of B is added, corrosion resistance will be significantly deteriorated. Therefore, the B content should be 0.0050% or less, preferably 0.040% or less, or 0.030% or less. There is no particular lower limit for the B content, but to obtain the above effect, the B content is preferably 0.0003% or more.

[0038] Mg: 0 to 0.0050% Mg not only deoxidizes but also has the effect of refining the solidification structure, so it may be added as needed. However, adding a large amount of Mg increases the cost of the steelmaking process. Therefore, the Mg content is set to 0.0050% or less. There is no particular lower limit for the Mg content, but to obtain the above effects, the Mg content is preferably 0.0001% or more.

[0039] W: 0 to 1.00% W is an element that improves corrosion resistance, so it may be added as needed. However, if a large amount of W is added, the load during rolling increases, making manufacturing defects more likely to occur. Therefore, the W content should be 1.00% or less, and preferably 0.80% or less, or 0.50% or less. There is no particular lower limit for the W content, but to obtain the above effects, the W content is preferably 0.01% or more.

[0040] Sn: 0 to 0.100% Sn is an element that improves corrosion resistance, so it may be contained as needed. However, a large amount of Sn content deteriorates hot workability. Therefore, the Sn content should be 0.100% or less. There is no particular lower limit for the Sn content, but to obtain the above effects, the Sn content is preferably 0.001% or more.

[0041] REM: 0 to 0.100% REM (rare earth elements) have the effect of improving hot workability and may be added as needed. However, adding a large amount of REM impairs manufacturability and increases costs. Therefore, the REM content should be 0.100% or less, 0.090% or less, 0.080% or less, or 0.070% or less. There is no particular lower limit for the REM content, but to obtain the above effect, the REM content is preferably 0.001% or more, 0.005% or more, 0.010% or more, or 0.020% or more.

[0042] REM is a general term for 17 elements, including Sc, Y, and 15 elements from La to Lu (lanthanoids), and the REM content refers to the total content of these elements. Industrially, lanthanoids are added in the form of misch metals.

[0043] The balance other than the above elements is Fe and impurities. Here, "impurities" refers to components that are mixed in during industrial steel production due to raw materials such as ore and scrap, and various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. Of the impurities, it is advisable to limit the contents of P, S, and O.

[0044] P:0.050% or less P is an element that inevitably gets mixed into steel and is also contained in raw materials such as Cr. However, a large amount of P reduces formability. Therefore, the lower the P content, the better, and it is recommended to set it to 0.050% or less, 0.040% or less, 0.030% or less, 0.020% or less, or 0.010% or less. There is no particular lower limit, but because removing P requires very high refining costs, it may be set to 0.001% or more from an economical standpoint.

[0045] S: 0.0040% or less S is an element that inevitably gets mixed into steel. It combines with Mn to form inclusions that can become the starting point for rusting. Since the lower the S content, the better the corrosion resistance, so it is recommended to keep the S content at 0.0040%, 0.0030%, 0.0020%, or 0.0010%. There is no particular lower limit, but because removing S requires very high refining costs, it may be set to 0.0001% or more from an economical standpoint.

[0046] [Organization] Since the stainless steel according to the present invention containing the above elements is a dual-phase steel, its structure has an austenite phase and a ferrite phase. The ratio of the austenite phase to the ferrite phase is not particularly limited. For example, in any observation surface (for example, in the case of a steel plate, a cross section in the thickness direction), the austenite phase may account for 20 to 80% by area, with the remainder being the ferrite phase. Also, a martensite phase may be partially contained. If a martensite phase is contained, it is desirable that the area ratio in any observation surface is 10% or less, preferably 8% or less, 6% or less, 5% or less, 4% or less, or 3% or less.

[0047] [Lightness L*] As an objective index of black resistance, we applied lightness L*, defined in the L*a*b* color space (JIS Z 8781-4). Lightness L* is an index that expresses the brightness of a color as a number ranging from 0 to 100, with a higher value indicating a brighter (whitish) appearance and a lower value indicating a darker (blackish) appearance.

[0048] The inventors compared samples that had actually been blackened by crude phosphoric acid and confirmed that there is no problem with visual aesthetics if L* is 65 or higher. Since blackening cannot be visually confirmed when L* is at a level of 65 or higher, it is easy to assume that the sample also has sufficient corrosion resistance. From the viewpoint of suppressing blackening, the larger the L* value, the better, and it is preferable that L* is 67 or more, 70 or more, 75 or more, 80 or more, or 85 or more.

[0049] [DI value] As mentioned above, the blackening of stainless steel caused by crude phosphoric acid occurs when F- (fluoride ions) and CL- (chloride ions) in the crude phosphoric acid solution vaporize and condense on the stainless steel surface that forms the inside of the tank, turning into HF (hydrofluoric acid) and HCl (hydrochloric acid), which corrode the steel inside the tank.Furthermore, F- and Cl- are also present in the sludge that accumulates in the crude phosphoric acid solution, and these F- and Cl- are supplied from the sludge, which similarly corrodes the steel inside the tank.

[0050] Therefore, we investigated stainless steels that are effective against HF and HCl corrosion, which affect blackening. In addition to Cr, Ni, Mo, and Cu investigated in Patent Document 3, we also added N (nitrogen), which is effective in neutralizing acidic condensation water, and conducted a systematic analysis of these elements. At this time, we systematically analyzed the relationship between the lightness L*, which was introduced as an objective indicator of discoloration (blackening) of stainless steel, and the amount of each element, and found that the DI value shown in the following equation 1 can be used as an indicator. DI value = 12Cr-Ni + 30Mo + 2Cu + 10N Formula 1 Here, the element symbols in formula 1 represent the content (mass%) of each element contained in the steel, and 0 is substituted if the element is not contained.

[0051] It has been found that increasing the DI value increases the lightness L* and also suppresses discoloration (blackening) of stainless steel. An example of the relationship between DI value and L* is shown in Figure 1. As can be seen from Figure 1, a DI value of 300 or higher results in a lightness L* of 65 or higher, even when exposed to a crude phosphoric acid solution, thereby suppressing blackening. The higher the DI value, the better, and preferably it is 320 or higher, 330 or higher, 340 or higher, 350 or higher, or 360 or higher. There is no particular upper limit to the DI value; it can be determined appropriately based on the content range of each element. Furthermore, even if the DI value is increased, L* will only approach the limit of 100. As can be seen from Figure 1, there is no problem with the upper limit of the DI value being 380, 390, or 400.

[0052] [Manufacturing method] The method for producing the duplex stainless steel according to the present invention is not particularly limited. As described above, if the steel contains the predetermined elements in the predetermined amounts, it can be manufactured by applying a conventional manufacturing method. From the viewpoint of ensuring corrosion resistance, it is recommended to process the steel into a steel plate or the like and then subject it to a solution treatment. The solution treatment can also be performed according to a conventional method, for example, by holding the steel in an environment of 1050°C for 20 minutes.

[0053] [Structure] The duplex stainless steel according to the present invention is effective in preventing discoloration (blackening) caused by crude phosphoric acid, and is therefore suitable for use in structures for crude phosphoric acid. The structure for crude phosphoric acid is not particularly limited as long as it is a facility that handles crude phosphoric acid, and examples thereof include phosphoric acid manufacturing plants, phosphoric acid storage tanks (including storage tanks on ships), and phosphoric acid transport pipes. Furthermore, although the duplex stainless steel according to the present invention is effective for crude phosphoric acid, its use is not limited to crude phosphoric acid production facilities. For example, it can be used in facilities that handle solutions containing F- or Cl-, as well as in the same manner as general duplex stainless steels. [Example]

[0054] Examples of the present invention will be described below, but the present invention is not limited to the conditions used in the following examples.

[0055] [Preparation of test materials] Steel with the chemical composition shown in Table 1 was melted in an MgO crucible in a 50 kg vacuum induction furnace in the laboratory and cast into a flat steel ingot approximately 100 mm thick. The main body of the steel ingot was processed into a hot-rolling blank, heated to 1180°C, held at that temperature for 1 hour, and then hot-rolled into a 12 mm-thick hot-rolled steel plate. The final solution heat treatment was performed at 1050°C for 20 minutes, followed by water cooling. From the resulting hot-rolled plate, several strips (at least four), each 50 mm long, 25 mm wide, and 4 mm thick, were cut and wet-polished on all sides using No. 400 grit. As comparative materials, SUS316 and SUS316L were prepared as test strips of the same shape.

[0056] [Test solution] A test liquid was prepared by mixing 10 cc of crude phosphoric acid with 10 g of sludge that had actually accumulated in a crude phosphoric acid storage layer and placing the mixture in an acid-resistant container.

[0057] [Immersion test] The container containing the test solution was kept at 40°C, and one test piece was immersed (partially immersed) by half its length in the test solution, and the immersion was continued for 200 hours while maintaining the temperature at 40°C. This reproduced the environment in which crude phosphoric acid is actually transported by sea. After 200 hours, the test piece was removed from the container, and the lightness L* of the gas phase (the portion not immersed) was measured using a color difference meter (e.g., CR-400 manufactured by Konica Minolta). L* was measured at any 10 points on the portion of the test piece that was in contact with the gas phase, and the arithmetic average of these measurements was used as the lightness L* value of the test piece.

[0058] [Test Results] The evaluation results for lightness L* for each test material are shown in Table 1. The presence or absence of blackening was also confirmed by visual inspection. In Table 1, blackening was visually observed and marked with an "X", no blackening was observed and the color was light grayish, marked with an "◎", and no blackening was observed but the color was grayish but not quite light grayish, marked with an "◯". As shown in Table 1 and Figure 1, if the DI value was 300 or more, the lightness L* was 65 or more and no blackening was observed visually. Furthermore, if the DI value was 300 or more, the lightness L* was 65 or more and no blackening was observed.

[0059] [Table 1] [Industrial Applicability]

[0060] The present invention can be used in plant equipment and industrial equipment such as equipment, storage tanks, pipelines, etc., which require corrosion resistance against acidic substances. In particular, it can be used in places where corrosion resistance and blackening resistance against crude phosphoric acid are required.

Claims

1. The components are in mass%: C: 0.001-0.100%, Si: 0.05-1.50% Mn: 0.10-2.00%, Cr: 23.0-30.0%, Ni: 4.00-10.00%, Mo: 0.50-3.00%, Cu: 0.01-3.00%, N: 0.10-0.25%, Ca: 0.0010-0.0100%, P: 0.050% or less, S: 0.0040% or less, Co: 0-3.00%, V: 0 to 1.00%, Nb: 0 to 0.200%, Ta: 0-0.200%, Al: 0-0.100%, Ti: 0 to 0.050%, Zr: 0 to 0.090%, Hf: 0-0.100%, B: 0 to 0.0050%, Mg: 0 to 0.0050%, W: 0-1.00%, REM: 0 to 0.100%, and Sn: 0.001 to 0.100%; The DI value shown in Formula 1 is 300 or more, A duplex stainless steel having excellent resistance to discoloration by crude phosphoric acid, the balance being Fe and unavoidable impurities. DI value = 12Cr - Ni + 30Mo + 2Cu + 10N ... Formula 1 However, the element symbols in formula 1 represent the content (mass %) of each element contained in the steel.

2. The components are in mass%: Co: 0.01 to 3.00%, V: 0.01 to 1.00%, Nb: 0.005-0.200%, Ta: 0.005-0.200%, Al: 0.003-0.100%, Ti: 0.001 to 0.050%, Zr: 0.001 to 0.090%, Hf: 0.001-0.100%, B: 0.0001 to 0.0050%, Mg: 0.0001 to 0.0050%, W: 0.01 to 1.00%, and REM: Contains one or more selected from 0.005 to 0.100%; 2. A duplex stainless steel having excellent resistance to discoloration by crude phosphoric acid according to claim 1.

3. A structure for crude phosphoric acid production using the duplex stainless steel according to claim 1 or 2.

4. 4. The structure for producing crude phosphoric acid according to claim 3, wherein the structure for producing crude phosphoric acid is a phosphoric acid production plant, a phosphoric acid storage tank, or a phosphoric acid transport pipe.

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