EP4667612A1

EP4667612A1 - Ferritic-austenitic duplex stainless steel material, production method therefor, and structure for crude phosphoric acid

Info

Publication number: EP4667612A1
Application number: EP24823259.7A
Authority: EP
Inventors: Satoshi NAKATSUJI; Akinori Kawano
Original assignee: Nippon Steel Corp
Current assignee: Nippon Steel Corp
Priority date: 2023-06-15
Filing date: 2024-06-03
Publication date: 2025-12-24
Also published as: JPWO2024257639A1; WO2024257639A1; EP4667612A4; CN121127617A

Abstract

A ferrite-austenite duplex stainless steel material having a composition including, on a mass basis, C: 0.100% or less, Si: 0.05 to 1.50%, Mn: 0.05 to 2.00%, Ni: 4.00 to 9.00%, P: 0.050% or less, S: 0.0040% or less, Cr : 23.0 to 30.0%, N: 0.100 to 0.250%, Cu: 0.01 to 2.00%, Mo: 0.50 to 2.50%, Al: 0.100% or less, Nb: 0.200% or less, the balance being Fe and impurities,
wherein a value of DF represented by the following equation (1) is 45.0 to 70.0:

DF = 7.2 (Cr + 0.8Mo + 0.78Si) - 8.9 (Ni + 0.03Mn + 0.72Cu + 22C + 21N) - 44.9 ··· (1)

in which each of the element symbols represents a content (% by mass) of each element; and
wherein an austenite phase satisfies a relational expression of the following equation (2):

{(0.8 Cr)}^{2} + 2 Ni + 7 Mo + 30 Cu + 30 N \geq 340

in which each of the element symbols represents a content (% by mass) of each element.

Description

[Technical Field]

The present invention relates to a ferrite-austenite duplex stainless steel material and a method for producing the same, and a structure for crude phosphoric acid.

[Background Art]

Articles involved in international logistics are diverse, and some of them include chemicals such as liquids and gases that contain corrosive substances, e.g., acids. Transport of these chemical is performed by chemical tankers and other vessels of special specifications, and their storage tanks are made of corrosion-resistant steel, especially stainless steel. For example, Patent Literature 1 proposes that a ferrite-austenite duplex stainless steel having a certain composition is used in place of austenitic stainless steel materials conventionally used as storage tank steel materials. In addition, Patent Literature 2 proposes that a ferritic austenitic duplex stainless steel having a certain composition is used for sulfuric acid plants that handle crude sulfuric acid, and for sulfuric acid storage tanks, and the like.
As a stainless steel material applied to line pipes used for the transport of oil and natural gas, Patent Literature 3 proposes a ferrite-austenite duplex stainless steel material having good corrosion resistance in environments containing a trace amount of hydrogen sulfide.
By the way, one of the articles transported by chemical tankers is crude phosphoric acid. The crude phosphoric acid is a basic raw material for fertilizers, detergents, feeds, pharmaceuticals, and the like. However, Japan has few phosphorus resources and most of them are imported by marine transport. Since the crude phosphoric acid contains corrosive substances such as F^- and Cl^- derived from phosphate ore, it is known that transporting crude phosphoric acid causes corrosion (including discoloration such as blackening) on inner surfaces of storage tanks of the tankers. Therefore, after transporting the crude phosphoric acid, maintenance such as repairs and cleaning is performed to the inner surfaces of the storage tanks from the viewpoint of preventing contamination, and new chemicals are loaded. Such maintenance is very time-consuming and frequent, which is problematic in terms of cost and time for chemical transport.
The austenitic stainless steel materials having good corrosion resistance such as SUS 316 L and 329 J3 L have been conventionally applied to the inner surfaces of storage tanks of chemical tankers that transport crude phosphoric acid. However, when the crude phosphoric acid is stored, corrosion occurs even in SUS 316 L, and there is increasing demand for alternative materials day by day.
Therefore, the ferrite-austenite duplex stainless steel materials described in Patent Literatures 1 to 3 may also be applied to the inner surfaces of storage tanks of chemical tankers that transport crude phosphoric acid.
However, the ferrite-austenite duplex stainless steel materials described in Patent Literature 1 to 3 have lower corrosion resistance against sulfides and crude phosphoric acid than SUS 316 L, although they are more cost significant than austenite stainless steel materials with high Ni content.
Therefore, Patent Literature 4 proposes an austenite stainless steel material with adjusted contents of Cr, Ni, Mo, and Cu, which would otherwise affect the corrosion resistance against crude phosphoric acid.

[Citation List]

[Patent Literatures]

[PTL 1] WO 2009/119895 A1
[PTL 2] Japanese Patent Application Publication No. H11-269612 A
[PTL 3] Japanese Patent Application Publication No. 2018-59157 A
[PTL 4] Japanese Patent Application Publication No. 2002-121655 A

[Summary of Invention]

[Technical Problem]

Although the austenite stainless steel material described in Patent Literature 4 has a certain effect on corrosion against crude phosphoric acid, corrosion (especially, blackening) has been observed on the inner surfaces of storage tanks of chemical tankers that transported actual crude phosphoric acid. Therefore, there is a need for further improvement of corrosion resistance against crude phosphoric acid and improvement from an economic standpoint by reducing the contents of expensive elements such as Ni.
This invention targets a ferrite-austenite duplex stainless steel material that is superior to austenitic stainless steel materials from an economic standpoint, and an object of this invention is to provide a ferrite-austenite duplex stainless steel material having improved corrosion resistance against crude phosphoric acid and a method for producing the same.
Also, an object of this invention is to provide a structure for crude phosphoric acid that has improved corrosion resistance against crude phosphoric acid and can reduce the cost and time of maintenance.

[Solution to Problem]

As a result of intensive studies for ferrite-austenite duplex stainless steel materials to solve the above problems, the inventors have obtained the following findings.
The crude phosphoric acid is produced by dissolving phosphate ore with sulfuric acid and loaded as a liquid on chemical tankers and transported. The crude phosphoric acid contains low concentrations of F^- (fluorine ions) and Cl^- (chlorine ions), and these ions are corrosive substances that preferentially dissolve the austenite phase, which thus causes a decrease in the corrosion resistance of crude phosphoric acid.
Based on the above findings, the inventors have examined the elements related to the preferential dissolution of the austenite phase to crude phosphoric acid and found that amounts of five elements (Cr, Ni, Mo, Cu, and N) in the austenite phase are closely related to the preferential dissolution of the austenite phase.
Therefore, the inventors have found that preferential dissolution of the austenite phase can be suppressed by controlling the amount of each of the above elements in the austenite phase in a well-balanced manner. The inventors have also found that corrosion resistance against crude phosphoric acid can be improved by controlling the composition of the ferrite-austenite duplex stainless steel itself in a well-balanced manner. The present invention was completed on the basis of these findings.
Thus, this invention relates to a ferrite-austenite duplex stainless steel material having a composition comprising, on a mass basis, C: 0.100% or less, Si: 0.05 to 1.50%, Mn: 0.05 to 2.00%, Ni: 4.00 to 9.00%, P: 0.050% or less, S: 0.0040% or less, Cr : 23.0 to 30.0%, N: 0.100 to 0.250%, Cu: 0.01 to 2.00%, Mo: 0.50 to 2.50%, Al: 0.100% or less, Nb: 0.200% or less, the balance being Fe and impurities,

wherein a value of DF represented by the following equation (1) is 45.0 to 70.0: DF = 7.2 (Cr + 0.8Mo + 0.78Si) - 8.9 (Ni + 0.03Mn + 0.72Cu + 22C + 21N) - 44.9 ··· (1) in which each of the element symbols represents a content (% by mass) of each element; and
wherein an austenite phase satisfies a relational expression of the following equation (2): ${(0.8 Cr)}^{2} + 2 Ni + 7 Mo + 30 Cu + 30 N \geq 340$ in which each of the element symbols represents a content (% by mass) of each element.

Also, this invention relates to a method for producing a ferrite-austenite duplex stainless steel material, the method comprising:

a soaking step of a rolled material at a soaking temperature of 950 to 1150°C for more than one minute, the rolled material having a composition comprising, on a mass basis, C: 0.100% or less, Si: 0.05 to 1.50%, Mn: 0.05 to 2.00%, Ni: 4.00 to 9.00%, P: 0.050% or less, S: 0.0040% or less, Cr : 23.0 to 30.0%, N: 0.100 to 0.250%, Cu: 0.01 to 2.00%, Mo: 0.50 to 2.50%, Al: 0.100% or less, Nb: 0.200% or less, the balance being Fe and impurities;
a first cooling step of slowly cooling the rolled material obtained in the soaking step to a rapid cooling initiation temperature at a cooling rate of 5°C/s or more and less than 10°C/s; and
a second cooling step of rapidly cooling the rolled material obtained in the first cooling step from the rapid cooling initiation temperature,
wherein the rapid cooling initiation temperature is from 950 to 990°C and is lower than the soaking temperature.

Further, this invention relates to a structure for crude phosphoric acid, the structure comprising the ferrite-austenite duplex stainless steel material.

[Advantageous Effects of Invention]

According to this invention, it is possible to provide a ferrite-austenite duplex stainless steel having improved corrosion resistance against crude phosphoric acid and a method for producing the same.
Also, according to this invention, it is possible to provide a structure for crude phosphoric acid that has improved corrosion resistance against crude phosphoric acid and can reduce the cost and time of maintenance.

[Description of Embodiments]

Hereinafter, embodiments of the present invention will be specifically described. The invention is not limited to these embodiments; modifications or improvements made by those skilled in the art that do not depart from the spirit of the invention are also included within its scope.
It should be noted that, as used herein, the expression "%" in relation to any component means "% by mass", unless otherwise specified.
The ferrite-austenite stainless steel material according to this invention (hereafter simply abbreviated as a "duplex stainless steel material") has a composition including, on a mass basis, C: 0.100% or less, Si: 0.05 to 1.50%, Mn: 0.05 to 2.00%, Ni: 4.00 to 9.00%, P: 0.050% or less, S: 0.0040% or less, Cr : 23.0 to 30.0%, N: 0.100 to 0.250%, Cu: 0.01 to 2.00%, Mo: 0.50 to 2.50%, Al: 0.100% or less, Nb: 0.200% or less, the balance being Fe and impurities.
Also, the term "stainless steel material" as used herein means a material formed of stainless steel, and a shape of the material is not particularly limited. Examples of the shape of the material include a sheet shape (including a strip shape), a rod shape, and a tubular shape. Further, the material may be various shaped steels having cross-sectional shapes such as T-shape and I-shape.
The term "ferrite-austenite" as used herein means that the metallographic structure is mainly made of two phase of a ferrite phase and an austenite phase at ordinary temperature. Therefore, the "ferritic-austenite" includes those containing minor amounts of phases other than the ferrite phase and the austenite phase (for example, martensite phases, etc.).
Further, the term "impurities" as used herein refers to components which are contaminated by raw materials such as ores and scraps, and various factors in the production steps, when the stainless steel materials are industrially produced, and which are permissible in a range that does not adversely affect the present invention. For example, the impurities include unavoidable impurities.
In addition, with respect to the content of each element as used herein, containing or comprising "xx % or less" means that it contains xx % or less but contains an amount more than 0% (especially, more than the impurity level).
The duplex stainless steel material according to an embodiment of this invention may optionally contains one or more selected from: Ti: 0.050% or less, B: 0.0050% or less, Ca: 0.0010 to 0.0100%, Mg: 0.0001 to 0.0020%, Zr: 0.090% or less, Co: 3.00% or less, V: 1.000% or less, Ta: 0.200% or less, Sn: 0.100% or less, O: 0.0050% or less, W: 1.000% or less, and REM: 0.100% or less.
Each component will be described in detail below.

<C: 0.100% or less>

C is preferably reduced as much as possible. The lower limit of the C content is not particularly limited, but since the refining costs are higher for decarburization of steel types containing a relatively large amount of Cr, such as duplex stainless steel materials, it is sufficient to reduce the C content to, for example, 0.001%. On the other hand, C promotes the deposition of Cr carbides, so that if the C content is too high, intergranular corrosion occurs and the corrosion resistance decreases. Therefore, from the point of view for reducing the corrosion resistance, the Cr content should be 0.100% or less. From the viewpoint of stably ensuring this effect, the C content is preferably 0.095% or less, 0.090% or less, 0.085% or less, or 0.080% or less.

<Si: 0.05 to 1.50%>

Si is an element that is used as a deoxidizing element and is added to improve oxidation resistance. From the viewpoint of obtaining these effects, the Si content should be 0.05% or more. From the viewpoint of stably ensuring these effects, the Si content is preferably 0.10% or more, 0.20% or more, or 0.30% or more. On the other hand, if the Si content is too high, the duplex stainless steel material becomes hard, and the toughness and workability decrease. Therefore, from the viewpoint of suppressing the deterioration of toughness and workability, the Si content should be 1.50% or less. From the viewpoint of stably ensuring this effect, the Si content is preferably 1.45% or less, 1.40% or less, 1.35% or less, or 1.30% or less.

<Mn: 0.05 to 2.00%>

Mn increases austenite, enhances nitrogen solubility, and helps prevent bubble defects during production. From the viewpoint of obtaining these effects, the Mn content should be 0.05% or more. From the viewpoint of stably ensuring these effects, the Mn content is preferably 0.10% or more, or 0.20% or more. On the other hand, if the Mn content is too high, the corrosion resistance and hot workability decrease. Therefore, from the viewpoint of suppressing deterioration of corrosion resistance and hot workability, the Mn content should be 2.00% or less. From the viewpoint of stably ensuring this effect, the Mn content is preferably 1.90% or less, 1.80% or less, 1.70% or less, or 1.60% or less.

<Ni: 4.00 to 9.00%>

Ni is an austenite stabilizing element and has an effect of improving corrosion resistance by suppressing the progress of corrosion. In order to ensure a certain amount of austenite phase as a duplex stainless steel material, the Ni content should be 4.00% or more. From the viewpoint of stably ensuring this effect, the Ni content is 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 is an expensive element, and it is, therefore, preferable that the Ni content is as low as possible. Therefore, the Ni content should be 9.00% or less. From the viewpoint of cost reduction, the Ni content is preferably 8.50% or less, 8.00% or less, or 7.80% or less.

<P: 0.050% or less>

P is an element that is inevitably mixed into duplex stainless steel materials, and is also contained in raw materials such as Cr. If the P content is too high, formability decreases. Therefore, a lower P content is more preferable, so that the P content should be 0.050% or less. Moreover, from the viewpoint of stably suppressing the deterioration of formability, the P content is preferably 0.045% or less, or 0.040% or less. On the other hand, the lower limit of the P content is not particularly limited, but removal of P requires a very high refining cost. Therefore, considering cost efficiency, the P content may be 0.001% or more.

<S: 0.0040% or less>

S is an element that is inevitably mixed into duplex stainless steel materials, and may be combined with Mn to form inclusions that may become the starting points for rusting. Furthermore, a lower S content more improves the corrosion resistance. Therefore, the S content should be 0.0040% or less. From the viewpoint of stably ensuring this effect, the S content is preferably 0.0035% or less, or 0.0030% or less. On the other hand, the lower limit of the S content is not particularly limited, but removal of S requires a very high refining cost. Therefore, in view of economic efficiency, the S content may be 0.0001% or more.

<Cr: 23.0 to 30.0%>

Cr can improve corrosion resistance against crude phosphoric acid by strengthening the passive film. Cr also has an effect of restoring the passive film after Fe is dissolved. From the viewpoint of obtaining these effects, the Cr content should be 23.0% or more. From the viewpoint of stably ensuring the effects, the Cr content is 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, if the Cr content is too high, an amount of chromium nitrides deposited increases, the risk of hot working cracks increases, and toughness decreases. From the view point of controlling them, the Cr content should be 30.0% or less. From the viewpoint of stably controlling them, the Cr content is preferably 29.8% or less, 29.6% or less, 29.4% or less, 29.2% or less, or 29.0% or less.

<N: 0.100 to 0.250%>

When N dissolves, it forms NH₄ ⁺ (ammonium ion), which raises the pH of acidic condensation water and helps prevent Fe dissolution. Nitrogen (N) is also an element that is dissolved into the austenite phase to enhance strength and corrosion resistance. Its contribution also helps reduce the need for other alloying elements. From the viewpoint of obtaining these effects, the N content should be 0.100% or more. From the viewpoint of stably ensuring these effects, the N content is preferably 0.120% or more, 0.130% or more, or 0.140% or more. Nitrogen is a significant factor in the deposition of chromium nitrides. If the nitrogen content is high, the quantity of deposited chromium nitrides increases, which can result in reducing toughness and corrosion resistance. From the view point of controlling them, the N content should be 0.250% or less. For more stable effects, the N content is preferably 0.230% or less, or even 0.220% or less.

<Cu: 0.01 to 2.00%>

Cu enhances resistance against acid corrosion, especially in crude phosphoric acid with sulfuric acid, and helps prevent Fe dissolution from duplex stainless steel surfaces exposed to acidic condensed water. From the viewpoint of obtaining this effect, the Cu content should be 0.01% or more. From the viewpoint of stably ensuring this effect, the Cu content is preferably 0.05% or more, 0.10% or more, 0.20% or more, or 0.25% or more. On the other hand, Higher Cu content raises material costs and reduces hot workability. From the viewpoint of controlling them, the Cu content should be 2.00% or less. For more stable effects, the Cu content is preferably 1.80% or less, 1.50% or less, 1.00% or less, or 0.80% or less.

<Mo: 0.50 to 2.50%>

Mo has effects of improving corrosion resistance and restoring the passive film after Fe is dissolved. In particular, it has an effect of suppressing corrosion (particularly blackening) caused by the dissolution of Fe on the surface of the duplex stainless steel material. From the viewpoint of obtaining these effects, the Mo content should be 0.50% or more. From the viewpoint of stably ensuring this effect, the Mo content is preferably 0.80% or more, 0.90% or more, or 1.00% or more. On the other hand, since Mo is an expensive element, it is desirable to reduce the content as much as possible. Therefore, the Mo content should be 2.50% or less. From the viewpoint of cost reduction, the Mo content is preferably 2.30% or less, or 2.20% or less.

<Al: 0.100% or less>

Al is an element included for desulfurization and deoxidation. However, since a large amount of Al increases raw material costs and production costs, the Al content should be 0.100% or less. Moreover, from the viewpoint of stably reducing increases in raw material costs and production costs, the Al content is preferably 0.090% or less or 0.080% or less. On the other hand, the lower limit of the Al content is not particularly limited, but from the viewpoint of obtaining the above effects, the Al content is preferably 0.001% or more.

<Nb: 0.200% or less>

Nb has an effect of suppressing deposition of chromium nitride by forming a compound with N. However, if the Nb content is too high, the workability of the duplex stainless steel material decreases. From the viewpoint of suppressing deterioration of workability, the Nb content should be 0.200% or less. From the viewpoint of stably ensuring this effect, the Nb content is preferably 0.190% or less, or 0.180% or less. On the other hand, the lower limit of the Nb content is not particularly limited, but from the viewpoint of obtaining the above effects, the Nb content is preferably 0.001% or more.

<Ti: 0.050% or less>

Ti has effects of preventing coarsening of the heat-affected zone during welding of the duplex stainless steel material and of forming fine equiaxed crystals in the solidified structure, and therefore can be contained as necessary. However, if the Ti content is too high, the uniform elongation and local elongation decrease. From the viewpoint of suppressing decreases in uniform elongation and local elongation, the Ti content should be 0.050% or less. From the viewpoint of stably ensuring this effect, the Ti content is preferably 0.045% or less, or 0.040% or less. On the other hand, the lower limit of the Ti content is not particularly limited, but from the viewpoint of ensuring the above effects, the Ti content is preferably 0.001% or more.

<B: 0.0050% or less>

B has an effect of improving hot workability and can optionally be added. However, an excessively high B content will significantly deteriorate the corrosion resistance. From the viewpoint of suppressing the deterioration of the corrosion resistance, the B content should be 0.0050% or less. From the viewpoint of stably ensuring this effect, the B content is preferably 0.0040% or less, or 0.0030% or less. On the other hand, the lower limit of the B content is not particularly limited, but from the viewpoint of obtaining the above effects, the B content is preferably 0.0003% or more.

<Ca: 0.0010 to 0.0100%>

Ca is an element that enhances corrosion resistance, and is presumed to reduce inclusions that serve as starting points for corrosion in an environment where crude phosphoric acid is present, and to suppress the dissolution of Fe, and therefore, Ca can optionally be contained. From the viewpoint of ensuring this effect, the Ca content should be 0.0010% or more. From the viewpoint of stably ensuring this effect, the Ca content is preferably 0.0015% or less, or 0.0020% or less. On the other hand, an excessively high content of Ca tends to generate heat work cracks and deteriorates the corrosion resistance. From the view point of controlling them, the Ca content should be 0.0100% or less. From the viewpoint of stably suppressing them, the Ca content is preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less.

<Mg: 0.0001 to 0.0020%>

Mg has effects of not only deoxidizing but also refining the solidification structure, and can be thus added as needed. However, since a higher Mg content leads to an increase in costs in the steelmaking step, the Mg content should be 0.0020% or less. From the viewpoint of cost reduction, the Mg content is preferably 0.0019% or less, or 0.0018% or less. On the other hand, from the viewpoint of obtaining the above effects, the Mg content should be 0.0001% or more.

<Zr: 0.090% or less>

Zr has an effect of forming carbides and nitrides in the duplex stainless steel material to refine the crystal grains, and therefore can optionally be added. However, if the Zr content is too high, the toughness decreases due to the excessively formed carbides and nitrides. Therefore, from the viewpoint of suppressing a decrease in toughness, the Zr content should be 0.090% or less. From the viewpoint of stably ensuring this effect, the Zr content is preferably 0.085% or less. On the other hand, the lower limit of the Zr content is not particularly limited, but from the viewpoint of obtaining the above effects, the Zr content is preferably 0.001% or more, or 0.005% or more.

<Co: 3.00% or less>

As with N, Co is an austenite stabilizing element and can optionally be contained. However, since Co is an expensive element, the Co content should be 3.00% or less. From the viewpoint of cost reduction, the Co content is preferably 2.80% or less, or 2.60% or less. On the other hand, the lower limit of the Co content is not particularly limited, but from the viewpoint of obtaining the above effects, the Co content is preferably 0.01% or more.

<V: 1.000% or less>

V is an element that improves corrosion resistance and can optionally be contained. However, if the V content is too high, the load during rolling increases and manufacturing defects are more likely to occur, so the V content should be 1.000% or less. Moreover, from the viewpoint of stably suppressing the formation of manufacturing defects, the V content is preferably 0.950% or less. On the other hand, the lower limit of the V content is not particularly limited, but from the viewpoint of obtaining the above effects, the V content is preferably 0.010% or more, or 0.050% or more.

<Ta: 0.200% or less>

Ta has an effect of forming carbides and nitrides in the duplex stainless steel material to additionally enhance the corrosion resistance, so it can optionally be contained. However, if the Ta content is too high, the toughness decreases due to the excessively formed carbides and nitrides, so the Ta content should be 0.200% or less. From the viewpoint of stably ensuring this effect, the Ta content is preferably 0.170% or less, or 0.150% or less. On the other hand, the lower limit of the Ta content is not particularly limited, but from the viewpoint of obtaining the above effects, the Ta content is preferably 0.001% or more, or 0.005% or more.

<Sn: 0.100% or less>

Sn is an element that improves corrosion resistance and can optionally be contained. However, if the Sn content is too high, the hot workability decreases, so the Sn content should be 0.100% or less. From the viewpoint of stably ensuring this effect, the Sn content is preferably 0.050% or less, 0.030% or less, or 0.010% or less. On the other hand, the lower limit of the Sn content is not particularly limited, but from the viewpoint of obtaining the above effects, the Sn content is preferably 0.0001% or more, or 0.0003% or more.

<O: 0.0050% or less >

As with S, O is an element that affects hot workability, and so it is desirable that an amount of O is as low as possible. Therefore, the O content should be 0.0050% or less. From the viewpoint of stably ensuring this effect, the O content is preferably 0.0048% or less. On the other hand, the lower limit of the O content is not particularly limited, but reduction of the O content leads to higher production costs. Therefore, the O content is preferably 0.001% or more.

<W: 1.000% or less>

W is an element that improves corrosion resistance and can optionally be contained. However, if the W content is too high, the load during rolling increases and manufacturing defects are easily generated, so the W content should be 1.000% or less. From the viewpoint of stably ensuring this effect, the W content is preferably 0.900% or less, or 0.800% or less. On the other hand, the lower limit of the W content is not particularly limited, but from the viewpoint of obtaining the above effects, the lower limit of W is 0.005% or more, or 0.010% or more.

<REM: 0.100% or less>

REM (rare earth elements) has an effect of improving hot workability and can optionally be contained. However, a higher REM content impairs producibility and increases costs, so the REM content should be 0.100% or less. From the viewpoint of stably ensuring this effect, the REM content is preferably 0.095% or less, or 0.090% or less. On the other hand, the lower limit of the REM content is not particularly limited, but from the viewpoint of obtaining the above effects, the REM content is 0.001% or more, 0.005% or more, or 0.010% or more.
REM is the generic term for 17 elements in total: Sc, Y, and 15 elements (lanthanoides) from La to Lu, and the REM content means the total content of these elements. These elements can be used alone or in combination of two or more. Moreover, the lanthanides are industrially added in the form of misch metals.
The duplex stainless steel material according to an embodiment of this application has a value of DF represented by the following equation (1) of 45.0 to 70.0, preferably 48.0 to 69.5, and more preferably 50.0 to 69.0: DF = 7.2 (Cr + 0.8Mo + 0.78Si) - 8.9 (Ni + 0.03Mn + 0.72Cu + 22C + 21N) - 44.9 ··· (1)
In the above equation (1), the symbol for each element represents a content (% by mass) of each element.
Here, the DF is an index representing the amount of the ferrite phase. Therefore, 100-DF means the amount of the austenite phase. It should be noted, however, that the DF is an index determined based on the contents of the elements, and it has a possibility which does not correspond to the amount of the austenite phase actually measured. If the value of the DF is less than 45.0, it will lead to excessively high strength and deteriorate the producibility. On the other hand, when the value of DF is more than 70.0, the proportion of the austenite phase decreases, and therefore the amount of solid solution of N in the austenite phase decreases. As a result, an amount of chromium nitride deposited increases, resulting in decreases in toughness and corrosion resistance.
In the duplex stainless steel material according to an embodiment of this invention, it is preferable that the austenite phase satisfies a relational expression of the following equation (2): ${(0.8 Cr)}^{2} + 2 Ni + 7 Mo + 30 Cu + 30 N \geq 340$
In the above equation (2), the symbol for each element represents a content (% by mass) of each element.
Here, the equation (2) represents the relationship between the contents of five elements (Cr, Ni, Mo, Cu, and N) that affect the preferential dissolution of the austenite phase in crude phosphoric acid. By controlling the contents of the elements in the austenite phase so as to satisfy the relationship of the equation (2), the preferential dissolution of the austenite phase in the crude phosphoric acid can be suppressed, and corrosion of the duplex stainless steel material can be suppressed. From the viewpoint of stably ensuring this effect, the value of the left side of the equation (2) is preferably 345 or more, more preferably 350 or more, and even more preferably 355 or more. The upper limit of the value on the left side of the equation (2) is not particularly limited, but it is, for example, 800 or 700.
Here, the content of each element in the austenite phase, which is used herein to calculate the relational expression of the equation (2), can be measured by EPMA (electron probe micro-analyzer). Specifically, qualitative analysis is carried out by EPMA using a specimen obtained by mirror-polishing a cross section of the duplex stainless steel material in the thickness direction parallel to the rolling direction. By performing qualitative Ni mapping on the entire cross section, portions that can be clearly determined to be the austenite phase based on the Ni content are identified. Then, Cr, Ni, Mo, Cu and N are quantitatively analyzed at the approximate center of the identified austenite phase. The quantitative analysis is performed at ten or more points, and an average value is determined to be the results of the content of each element.
The austenite phase preferably contains 21.0 to 30.0% of Cr, 7.5 to 12.0% of Ni, 0.30 to 2.00% of Mo, 0.3 to 2.1% of Cu, and 0.10 to 0.35% of N. By controlling the content of each element to such a range, the relationship of the equation (2) is easily satisfied. In the austenite phase, the Cr content is preferably 22.0 to 25.5%, the Ni content is preferably 8.0 to 12.0%, the Mo content is preferably 1.20 to 1.80%, the Cu content is preferably 0.4 to 0.9%, and the N content is preferably 0.25 to 0.33%.
The duplex stainless steel material according to an embodiment of this invention preferably satisfies a relational expression of the following equation (3): $Expected CPT - Measured CPT \leq 10 ° C$
In the equation, the expected CPT is a critical pitting temperature (°C) calculated by 2.5Cr + 7.6Mo + 3.19N - 26 in which Cr, Mo and N represent the Cr, Mo and N contents (% by mass) in the duplex stainless steel material, respectively, and the measured CPT is a critical pitting temperature (°C) measured in a pitting corrosion resistance test in accordance with ASTM G48E.
The measured CPT tends to decrease when chromium nitrides are deposited, and so if the CPT difference (expected CPT - measured CPT) is 10°C or less, it can be considered that the amounts of chromium nitrides deposited are low. That is, when the relational expression of the equation (3) as described above is satisfied, the deposition of chromium nitride is suppressed, so that the corrosion resistance can be sufficiently ensured. On the other hand, if the relational expression of the equation (3) as described above is not satisfied, there is a risk of the corrosion resistance decreasing due to the deposition of chromium nitrides.
The duplex stainless steel material according to the embodiment of the present invention may be either a hot rolled material or a cold rolled material, but when it is used for storage tanks of chemical tankers and the like, the hot-rolled material is preferable.
The thickness of the duplex stainless steel material according to the embodiment of the present invention may be adjusted as needed depending on applications, and is not particularly limited, but it may generally be 20.0 mm or less, preferably 15.0 mm or less, and more preferably 10.0 mm or less. If the duplex stainless steel material is in a form of a bar, the thickness means the circular equivalent diameter of the cross section. If the duplex stainless steel material is a shaped steel, the thickness means the thickness at any point in the cross section.
The duplex stainless steel according to this embodiment has improved corrosion resistance against crude phosphoric acid and can be used for various components that will be in contact with crude phosphoric acid. Among them, the duplex stainless steel material according to an embodiment of this invention is suitable for use in structures for crude phosphoric acid.
The term "structures for crude phosphoric acid" as used herein refers to various facilities that handle crude phosphoric acid. Examples of the structures for crude phosphoric acid include phosphoric acid production plants, phosphoric acid storage tanks (including storage tanks on ships), and pipes for transporting phosphoric acid.
The method for producing the duplex stainless steel material according to the embodiment of the present invention is not particularly limited as long as it is a method that can produce a duplex stainless steel sheet having the above features.
Hereinafter, an example of methods for producing duplex stainless steel materials is described.
The method for producing the duplex stainless steel material according to an embodiment of this invention includes a soaking step, a first cooling step, and a second cooling step.
The soaking step is to soak the rolled material having the above composition at a soaking temperature of 950 to 1150°C for more than one minute.
If the soaking temperature is lower than 950°C, it cannot sufficiently achieve the solid solution of the chromium nitride. Furthermore, if the soaking temperature is higher than 1150°C, the amount of the ferrite phase will increase and it will be difficult to control the composition of the austenite phase within a predetermined range. From the viewpoint of stably suppressing these problems, the soaking temperature is preferably 1000 to 1100°C.
The soaking time may be more than 1 minute, but it is preferably less than 10 minutes in view of production efficiency and production costs.
In addition, the rolled material can be produced by a conventional method. For example, stainless steel having the above composition may be produced by vacuum melting to form a steel slab, which may then be hot rolled.
The first cooling step is to slowly cool the rolled material obtained in the soaking step to a rapid cooling initiation temperature at a cooling rate of 5°C/s or more and less than 10°C/s.
If the cooling rate in the first cooling step is 10°C/s or more, it will be difficult to control the composition of the austenite phase within a predetermined range. From the viewpoint of stably suppressing this problem, the cooling rate in the first cooling step is preferably 9°C/s or less, more preferably 8°C/s or less, and even more preferably 7°C/s or less. Furthermore, if the cooling rate in the first cooling step is less than 5°C/s, the amount of chromium nitride deposited increases, and in some cases, the σ phase may be deposited, resulting in a decrease in corrosion resistance.
The cooling method in the first cooling step is not particularly limited, but it may be, for example, air cooling.
The second cooling step is to rapidly cool (at a cooling rate of 10°C/s or more) the rolled material obtained in the first cooling step from the rapid cooling initiation temperature.
The rapid cooling initiation temperature is 950 to 990°C, and is lower than the soaking temperature as described above. If the rapid cooling initiation temperature is lower than 950°C, the amount of the ferrite phase produced increases, resulting in a decrease in corrosion resistance. Furthermore, if the rapid cooling initiation temperature is higher than 990°C, it will be difficult to control the composition of the austenite phase within a predetermined range. From the viewpoint of stably suppressing these problems, the rapid cooling initiation temperature is preferably 960 to 980°C. In addition, the rapid cooling termination temperature is not particularly limited, and it can be, for example, room temperature.
The cooling method in the second cooling step is not particularly limited, but it may be, for example, water cooling.
The structure for crude phosphoric acid according to an embodiment of this invention includes the duplex stainless steel material described above. The structure for crude phosphoric acid may further include members other than the duplex stainless steel material described above.
The structure for crude phosphoric acid according to an embodiment of this invention can be produced by processing the above duplex stainless steel into a predetermined shape and then assembling it by welding or the like.
The structure for crude phosphoric acid includes, but not limited to, preferably phosphoric acid production plants, phosphoric acid storage tanks, or phosphoric acid transport pipes.

[Examples]

The content of the present invention will be described below in detail with reference to Examples, but the present invention is not construed as being limited thereto.
Stainless steels having the compositions shown in Tables 1-1 and 1-2 were smelted in a MgO crucible in a 50 kg vacuum induction furnace in a laboratory and cast into flat steel ingots each having a thickness of about 100 mm. The main body of each flat steel ingot was processed into a material for hot rolling, heated at 1180°C, maintained at that temperature for 1 hour, and then hot rolled into a hot-rolled sheet having a thickness of 12 mm. In Tables 1-1 and 1-2, the values of DF and expected CPT were calculated based on the content of each element.

The soaking step, the first cooling step and the second cooling step were then carried out under the conditions shown in Table 2 to obtain a duplex stainless steel sheet. The cooling in the first cooling step was carried out by air cooling, and the cooling rate was controlled by controlling the flow rate of the cooling gas. In the second cooling step, the material was rapidly cooled to room temperature by water cooling.

[Table 2]

Nos.	Soaking Step		First Cooling Step	Second Cooling Step
Nos.	Temperature (°C)	Time (m)	Cooling Rate (° C/s)	Rapid Cooling Initiation Temperature (°C)
1	1050	4	5	960
2	1050	5	7	960
3	990	3	6	970
4	970	6	6	960
5	1000	7	5	970
6	980	5	6	980
7	960	2	6	960
8	970	4	7	970
9	1040	6	7	950
10	1060	7	5	960
11	1040	5	5	960
12	1120	8	6	960
13	1030	3	5	990
14	1070	6	5	960
15	970	7	7	990
16	980	8	6	980
17	1010	7	5	950
18	1050	5	6	970
19	980	6	6	960
20	1040	5	6	980
21	960	5	5	960
22	1050	4	6	980
23	1070	4	6	990
24	1050	4	7	960
25	1050	4	6	950
26	1050	4	7	960
27	1200	4	6	960
28	1050	4	13	960
29	1050	4	5	1030
30	1050	4	3	980
31	1050	4	4	980
32	1050	4	6	960
33	1050	6	6	960
34	1050	4	6	960
35	1050	4	6	960
36	1050	4	6	960
37	1050	4	6	960
38	1050	4	6	960
39	1050	4	6	960
40	1050	4	6	960
41	1050	4	6	960
The underlines indicate that they are outside the scope of this invention.

The content of each element in the austenite phase was be measured by EPMA (electron probe micro-analyzer). Specifically, qualitative analysis was carried out by EPMA using a specimen obtained by mirror-polishing a cross section of the duplex stainless steel material in the thickness direction parallel to the rolling direction. The EPMA was performed at an acceleration voltage of 15 kV, and a 300 µm×300 µm region was measured in a lattice pattern at 0.76 µm intervals to obtain data for a total of 140,625 points. In the qualitative analysis, Ni qualitative mapping was performed on the entire cross section to identify portions that could be clearly determined to be the austenite phase based on the Ni content. Then, Cr, Ni, Mo, Cu and N were quantitatively analyzed at the approximate center of the identified austenite phase. The quantitative analysis was performed at ten or more points, and an average value is determined to be the results of the content of each element. The obtained content of each element was used to calculate the value on the left side of the relational expression of the equation (2).

A sample having 10 mm × 80 mm × 2 mm was taken from the duplex stainless steel sheet, and the entire surface was wet-polished with a No. 600 grindstone. An aqueous solution simulating crude phosphoric acid, which contained 70% by mass of phosphoric acid, 0.4% by mass of F ions, and 0.04% by mass of Cl ions, was then prepared. The container containing the aqueous solution was maintained at 50°C, and half of the sample was immersed therein for 6 hours. The corrosion rate (mm/y) was then calculated from the change in mass (corrosion weight loss) before and after immersion. If the corrosion rate is 0.15 mm/y or less, it can be determined that the corrosion resistance is improved. Since the corrosion generated in both the gas and liquid phases, the corrosion rate was calculated using the entire area of the sample.
Furthermore, the sample after the above test was visually evaluated for discoloration (blackening).

The measured CPT was determined in accordance with ASTM G48E by taking a sample having 50 mm x 25 mm x 2 mm thickness (thickness from a position of a depth of 1 mm from the outermost surface) from the duplex stainless steel sheet, and then conducting a pitting corrosion test on this sample at an aqueous solution temperature of 30°C, and measuring the CPT (Critical Pitting Temperature). In measuring the CPT, pitting corrosion generated on the end face (cross section in the thickness direction) of the sample was not counted, and only pitting corrosion generated on a surface of 50 mm x 25 mm was counted.
The CPT difference (expected CPT - measured CPT) was calculated using the measured CPT thus obtained and the expected CPT calculated above. The CPT difference is expressed as circle if it was 10°C or lower, and as × if it was higher than 10° C.

Table 3 shows the above evaluation results.

[Table 3]

Nos.	Composition of Austenite Phase (% by mass)						Equation (2)	Corrosion Rate (mm/y)	Discoloration	CPT Difference (°C)	Classification
Nos.	Fe	Cr	Ni	Mo	Cu	N	Equation (2)	Corrosion Rate (mm/y)	Discoloration	CPT Difference (°C)	Classification
1	64.3	24.0	9.3	1.44	0.6	0.30	424	0.09	Absent	○	Examples
2	65.3	23.5	8.6	1.73	0.6	0.30	409	0.11	Absent	○	Examples
3	68.5	21.4	7.5	1.45	0.9	0.28	353	0.14	Absent	○	Examples
4	63.4	24.8	9.2	1.39	0.9	0.36	459	0.07	Absent	○	Examples
5	70.7	22.0	5.8	0.88	0.3	0.26	346	0.12	Absent	○	Examples
6	61.7	26.1	9.7	0.68	1.6	0.23	515	0.07	Absent	○	Examples
7	65.1	24.6	7.8	1.30	0.9	0.41	449	0.11	Absent	○	Examples
8	63.4	25.5	8.8	0.44	1.6	0.24	493	0.08	Absent	○	Examples
9	61.1	25.8	11.1	1.29	0.4	0.23	478	0.08	Absent	○	Examples
10	63.0	25.5	9.7	0.53	1.0	0.28	478	0.09	Absent	○	Examples
11	61.6	24.6	11.4	1.42	0.6	0.31	448	0.10	Absent	○	Examples
12	67.7	23.7	7.2	0.94	0.1	0.36	394	0.11	Absent	○	Examples
13	67.4	23.6	7.2	0.91	0.6	0.26	403	0.10	Absent	○	Examples
14	63.9	25.4	6.9	1.12	2.2	0.44	515	0.07	Absent	○	Examples
15	62.6	24.6	9.6	1.25	1.7	0.33	474	0.10	Absent	○	Examples
16	62.5	26.3	7.6	0.94	2.2	0.42	543	0.07	Absent	○	Examples
17	69.0	22.4	7.2	0.88	0.3	0.21	357	0.14	Absent	○	Examples
18	63.3	24.5	11.2	0.53	0.3	0.19	424	0.12	Absent	○	Examples
19	67.2	23.0	7.7	1.15	0.7	0.35	393	0.10	Absent	○	Examples
20	64.5	24.6	8.8	1.42	0.4	0.29	435	0.09	Absent	○	Examples
21	61.3	25.0	10.7	1.29	1.4	0.33	482	0.08	Absent	○	Examples
22	67.7	22.7	7.6	1.40	1.2	0.29	399	0.10	Absent	○	Examples
23	66.8	22.7	7.9	0.44	1.9	0.27	413	0.08	Absent	○	Examples
24	62.0	25.9	11.1	0.52	0.3	0.24	470	0.05	Absent	○	Examples
25	70.5	21.8	7.5	0.44	0.6	0.29	348	0.13	Absent	○	Examples
26	63.1	25.6	9.0	0.49	1.5	0.28	494	0.06	Absent	○	Examples
27	68.7	21.6	7.8	1.62	0.1	0.19	334	0.28	Present	○	Comp.
28	71.0	20.1	6.8	0.54	0.5	0.23	317	0.68	Present	○	Comp.
29	72.0	22.1	5.2	0.37	0.1	0.20	336	0.34	Present	○	Comp.
30	68.3	21.1	8.0	1.84	0.4	0.24	332	0.23	Present	×	Comp.
31	69.2	21.2	7.1	1.55	0.4	0.36	337	0.21	Present	×	Comp.
32	61.6	26.1	9.4	1.11	1.5	0.26	516	0.50	Present	×	Comp.
33	70.0	22.3	5.1	0.56	1.7	0.28	392	0.59	Present	○	Comp.
34	67.5	25.9	5.6	0.64	0.0	0.45	457	0.73	Present	○	Comp.
35	63.8	24.5	9.6	1.33	0.6	0.23	436	0.69	Present	○	Comp.
36	68.5	23.6	5.6	0.64	1.3	0.45	423	0.42	Present	○	Comp.
37	59.5	27.3	11.1	1.33	0.6	0.23	534	0.42	Present	○	Comp.
38	72.2	19.8	5.6	1.39	0.8	0.20	302	0.47	Present	○	Comp.
39	64.4	24.9	9.6	0.30	0.6	0.21	443	0.58	Present	○	Comp.
40	67.4	25.9	5.6	0.64	0.0	0.49	460	0.47	Present	○	Comp.
41	65.6	22.7	9.6	1.33	0.6	0.13	381	0.28	Present	○	Comp.
The underlines indicate that they are outside the scope of this invention.

As shown in Table 3, it was confirmed that Nos. 1 to 26 (Examples) had a lower corrosion rate and improved corrosion resistance against crude phosphoric acid, because the duplex stainless steel sheets had the predetermined composition and DF, and the austenite phase satisfied the relational expression of the equation (2).
In contrast, No. 27 (Comparative Example) could not control the element concentrations in the austenite phase and did not satisfy the relational expression of the equation (2), because the soaking temperature was too high. As a result, corrosion resistance against crude phosphoric acid was not sufficient.
No. 28 (Comparative Example) could not control the element concentrations in the austenite phase and did not satisfy the relational expression of the equation (2), because of the excessively high cooling rate in the first cooling step. As a result, corrosion resistance against crude phosphoric acid was not sufficient.
No. 29 (Comparative Example) could not control the element concentrations in the austenite phase and did not satisfy the relational expression of the equation (2), because the rapid cooling initiation temperature in the second cooling step was too high. As a result, corrosion resistance against crude phosphoric acid was not sufficient.
Nos. 30 and 31 (Comparative Examples) could not control the element concentrations in the austenite phase and did not satisfy the relational expression of the equation (2), because of the excessively low cooling rate in the first cooling step. In addition, the corrosion resistance against crude phosphoric acid was not sufficient, because chromium nitrides, the σ phases and the like were deposited.
No. 32 (Comparative Example) had the larger amount of chromium nitride deposited and insufficient corrosion resistance against crude phosphoric acid, because of the higher DF value and excessive ferrite phase.
No. 33 (Comparative Example) had insufficient corrosion resistance against crude phosphoric acid as a result of reduced resistance against the acid because of the higher Mn content.
No. 34 (Comparative Example) did not have sufficient corrosion resistance against crude phosphoric acid, because it did not contain Cu.
No. 35 (Comparative Example) did not have sufficient corrosion resistance against crude phosphoric acid, because it did not contain Nb.
No. 36 (Comparative Example) had insufficient corrosion resistance against crude phosphoric acid, because of the lower DF value and the excess of austenite phase, which increased the amount of the austenite phase that would preferentially be corroded.
No. 37 (Comparative Example) had the higher Cr content, resulting in the larger amount of chromium nitride deposited and insufficient corrosion resistance against crude phosphoric acid.
No. 38 (Comparative Example) could not maintain the passive film in the crude phosphoric acid environment, resulting in insufficient corrosion resistance against crude phosphoric acid, because of the excessively low Cr content.
No. 39 (Comparative Example) could not maintain the passive film in the crude phosphoric acid environment, resulting in insufficient corrosion resistance against crude phosphoric acid, because of the excessively low Mo content.
No. 40 (Comparative Example) had insufficient corrosion resistance against crude phosphoric acid, because a larger amount of chromium nitride was deposited in the ferrite phase as a result of the excessively high N content.
No. 41 (Comparative Example) did not obtain a sufficient effect of suppressing Fe elution due to the generation of NH₄ ⁺ in the crude phosphoric acid environment, resulting in insufficient corrosion resistance against crude phosphoric acid, because of the excessively low N content.
As can be seen from the above results, according to this invention, it is possible to provide a ferrite-austenite duplex stainless steel having improved corrosion resistance against crude phosphoric acid and a method for producing the same. Also, according to this invention, it is possible to provide a structure for crude phosphoric acid that has improved corrosion resistance against crude phosphoric acid and can reduce the cost and time of maintenance.
Moreover, based on the above results, the present invention can be the following aspects:

(Aspect 1) A ferrite-austenite duplex stainless steel material having a composition comprising, on a mass basis, C: 0.100% or less, Si: 0.05 to 1.50%, Mn: 0.05 to 2.00%, Ni: 4.00 to 9.00%, P: 0.050% or less, S: 0.0040% or less, Cr : 23.0 to 30.0%, N: 0.100 to 0.250%, Cu: 0.01 to 2.00%, Mo: 0.50 to 2.50%, Al: 0.100% or less, Nb: 0.200% or less, the balance being Fe and impurities,
- wherein a value of DF represented by the following equation (1) is 45.0 to 70.0: DF = 7.2 (Cr + 0.8Mo + 0.78Si) - 8.9 (Ni + 0.03Mn + 0.72Cu + 22C + 21N) - 44.9 ··· (1) in which each of the element symbols represents a content (% by mass) of each element; and
- wherein an austenite phase satisfies a relational expression of the following equation (2): ${(0.8 Cr)}^{2} + 2 Ni + 7 Mo + 30 Cu + 30 N \geq 340$ in which each of the element symbols represents a content (% by mass) of each element.
(Aspect 2) The ferrite-austenite duplex stainless steel material according to Aspect 1, further comprising one or more selected from, on a mass basis, Ti: 0.050% or less, B: 0.0050% or less, Ca: 0.0010 to 0.0100%, Mg: 0.0001 to 0.0020%, Zr: 0.090% or less, Co: 3.00% or less, V: 1.000% or less, Ta: 0.200% or less, Sn: 0.100% or less, O: 0.0050% or less, W: 1.000% or less, and REM: 0.100% or less.
(Aspect 3) The ferrite-austenite duplex stainless steel material according to Aspect 1 or 2, wherein the ferrite-austenite duplex stainless steel material satisfies a relational expression of the following equation (3): $Expected CPT - Measured CPT \leq 10 ° C$ in which the expected CPT is a critical pitting temperature (°C) calculated by 2.5Cr + 7.6Mo + 3.19N - 26 in which Cr, Mo and N represent the Cr, Mo and N contents (% by mass) in the ferrite-austenite duplex stainless steel material, respectively, and the measured CPT is a critical pitting temperature (°C) measured in a pitting corrosion resistance test in accordance with ASTM G48E.
(Aspect 4) The ferrite-austenite duplex stainless steel material according to any one of Aspects 1 to 3, wherein the austenite phase comprises, on a mass basis, 21.0 to 30.0% of Cr, 7.5 to 12.0% of Ni, 0.30 to 2.00% of Mo, 0.3 to 2.1% of Cu, and 0.10 to 0.35% of N.
(Aspect 5) The ferrite-austenite duplex stainless steel material according to any one of Aspects 1 to 4, wherein the ferrite-austenite duplex stainless steel material is used for a structure for crude phosphoric acid.
(Aspect 6) A method for producing a ferrite-austenite duplex stainless steel material, the method comprising:
- a soaking step of soaking a rolled material at a soaking temperature of 950 to 1150°C for more than one minute, the rolled material having a composition comprising, on a mass basis, C: 0.100% or less, Si: 0.05 to 1.50%, Mn: 0.05 to 2.00%, Ni: 4.00 to 9.00%, P: 0.050% or less, S: 0.0040% or less, Cr : 23.0 to 30.0%, N: 0.100 to 0.250%, Cu: 0.01 to 2.00%, Mo: 0.50 to 2.50%, Al: 0.100% or less, Nb: 0.200% or less, the balance being Fe and impurities;
- a first cooling step of slowly cooling the rolled material obtained in the soaking step to a rapid cooling initiation temperature at a cooling rate of 5°C/s or more and less than 10°C/s; and
- a second cooling step of rapidly cooling the rolled material obtained in the first cooling step from the rapid cooling initiation temperature,
- wherein the rapid cooling initiation temperature is from 950 to 990°C and is lower than the soaking temperature.
(Aspect 7) The method for producing a ferrite-austenite duplex stainless steel material according to Aspect 6, wherein the rolled material further comprises one or more selected from, on a mass basis, Ti: 0.050% or less, B: 0.0050% or less, Ca: 0.0010 to 0.0100%, Mg: 0.0001 to 0.0020%, Zr: 0.090% or less, Co: 3.00% or less, V: 1.000% or less, Ta: 0.200% or less, Sn: 0.100% or less, O: 0.0050% or less, W: 1.000% or less, and REM: 0.100% or less.
(Aspect 8) A structure for crude phosphoric acid, the structure comprising the ferrite-austenite duplex stainless steel material according to any one of Aspects 1 to 5.
(Aspect 9) The structure for crude phosphoric acid according to Aspect 8, wherein the structure for crude phosphoric acid is a phosphoric acid production plant, a tank for phosphoric acid storage, or a pipe for transporting phosphoric acid.

Claims

A ferrite-austenite duplex stainless steel material having a composition comprising, on a mass basis, C: 0.100% or less, Si: 0.05 to 1.50%, Mn: 0.05 to 2.00%, Ni: 4.00 to 9.00%, P: 0.050% or less, S: 0.0040% or less, Cr : 23.0 to 30.0%, N: 0.100 to 0.250%, Cu: 0.01 to 2.00%, Mo: 0.50 to 2.50%, Al: 0.100% or less, Nb: 0.200% or less, the balance being Fe and impurities,
wherein a value of DF represented by the following equation (1) is 45.0 to 70.0: DF = 7.2 (Cr + 0.8Mo + 0.78Si) - 8.9 (Ni + 0.03Mn + 0.72Cu + 22C + 21N) - 44.9 ··· (1)
in which each of the element symbols represents a content (% by mass) of each element; and
wherein an austenite phase satisfies a relational expression of the following equation (2): ${(0.8 Cr)}^{2} + 2 Ni + 7 Mo + 30 Cu + 30 N \geq 340$ in which each of the element symbols represents a content (% by mass) of each element.
The ferrite-austenite duplex stainless steel material according to claim 1, further comprising one or more selected from, on a mass basis, Ti: 0.050% or less, B: 0.0050% or less, Ca: 0.0010 to 0.0100%, Mg: 0.0001 to 0.0020%, Zr: 0.090% or less, Co: 3.00% or less, V: 1.000% or less, Ta: 0.200% or less, Sn: 0.100% or less, O: 0.0050% or less, W: 1.000% or less, and REM: 0.100% or less.
The ferrite-austenite duplex stainless steel material according to claim 1 or 2, wherein the ferrite-austenite duplex stainless steel material satisfies a relational expression of the following equation (3): $Expected CPT - Measured CPT \leq 10 ° C$ in which the expected CPT is a critical pitting temperature (°C) calculated by 2.5Cr + 7.6Mo + 3.19N - 26 in which Cr, Mo and N represent the Cr, Mo and N contents (% by mass) in the ferrite-austenite duplex stainless steel material, respectively, and the measured CPT is a critical pitting temperature (°C) measured in a pitting corrosion resistance test in accordance with ASTM G48E.
The ferrite-austenite duplex stainless steel material according claim 1 or 2, wherein the austenite phase comprises, on a mass basis, 21.0 to 30.0% of Cr, 7.5 to 12.0% of Ni, 0.30 to 2.00% of Mo, 0.3 to 2.1% of Cu, and 0.10 to 0.35% of N.
The ferrite-austenite duplex stainless steel material according to claim 1 or 2, wherein the ferrite-austenite duplex stainless steel material is used for a structure for crude phosphoric acid.
A method for producing a ferrite-austenite duplex stainless steel material, the method comprising:
a soaking step of soaking a rolled material at a soaking temperature of 950 to 1150°C for more than one minute, the rolled material having a composition comprising, on a mass basis, C: 0.100% or less, Si: 0.05 to 1.50%, Mn: 0.05 to 2.00%, Ni: 4.00 to 9.00%, P: 0.050% or less, S: 0.0040% or less, Cr : 23.0 to 30.0%, N: 0.100 to 0.250%, Cu: 0.01 to 2.00%, Mo: 0.50 to 2.50%, Al: 0.100% or less, Nb: 0.200% or less, the balance being Fe and impurities;

a first cooling step of slowly cooling the rolled material obtained in the soaking step to a rapid cooling initiation temperature at a cooling rate of 5°C/s or more and less than 10°C/s; and

a second cooling step of rapidly cooling the rolled material obtained in the first cooling step from the rapid cooling initiation temperature,

wherein the rapid cooling initiation temperature is from 950 to 990°C and is lower than the soaking temperature.
The method for producing a ferrite-austenite duplex stainless steel material according to claim 6, wherein the rolled material further comprises one or more selected from, on a mass basis, Ti: 0.050% or less, B: 0.0050% or less, Ca: 0.0010 to 0.0100%, Mg: 0.0001 to 0.0020%, Zr: 0.090% or less, Co: 3.00% or less, V: 1.000% or less, Ta: 0.200% or less, Sn: 0.100% or less, O: 0.0050% or less, W: 1.000% or less, and REM: 0.100% or less.
A structure for crude phosphoric acid, the structure comprising the ferrite-austenite duplex stainless steel material according to claim 1 or 2.
The structure for crude phosphoric acid according to claim 8, wherein the structure for crude phosphoric acid is a phosphoric acid production plant, a tank for phosphoric acid storage, or a pipe for transporting phosphoric acid.