WO2024257639A1 - Ferritic-austenitic duplex stainless steel material, production method therefor, and structure for crude phosphoric acid - Google Patents

Abstract

This ferritic-austenitic duplex stainless steel material has a composition containing, in terms of mass, at most 0.100% of C, 0.05-1.50% of Si, 0.05-2.00% of Mn, 4.00-9.00% of Ni, at most 0.050% of P, at most 0.0040% of S, 23.0-30.0% of Cr, 0.100-0.250% of N, 0.01-2.00% of Cu, 0.50-2.50% of Mo, at most 0.100% of Al, and at most 0.200% of Nb, with the remainder consisting of Fe and impurities, wherein: a DF value represented by expression (1) of DF=7.2(Cr+0.8Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9 (in the expression, the element symbols represent the contents (mass%) of the respective elements) is 45.0-70.0; and an austenite phase satisfies relational expression (2) of (0.8Cr)²+2Ni+7Mo+30Cu+30N≥340 (in the expression, the element symbols represent the contents (mass%) of the respective elements).

Description

Ferritic-austenitic duplex stainless steel material, its manufacturing method, and structure for crude phosphoric acid

The present invention relates to a ferritic-austenitic duplex stainless steel material, a manufacturing method thereof, and a structure for crude phosphoric acid.

　International logistics involves a wide variety of goods, including chemical products such as liquids and gases containing corrosive substances, such as acids. Such chemical products are generally transported by specially designed ships such as chemical tankers, and corrosion-resistant steel materials, particularly stainless steel materials, are used for the storage tanks. For example, Patent Document 1 proposes using a ferritic-austenitic duplex stainless steel material with a specified composition instead of the austenitic stainless steel material that has traditionally been used as a storage tank steel material. Patent Document 2 also proposes using a ferritic-austenitic duplex stainless steel material with a specified composition for sulfuric acid plants that handle crude sulfuric acid and sulfuric acid storage tanks.

In addition, Patent Document 3 proposes a ferritic-austenitic duplex stainless steel material with excellent corrosion resistance in a trace hydrogen sulfide environment as a stainless steel material to be used for line pipes used in transporting oil and natural gas.

By the way, crude phosphoric acid is one of the chemicals transported by chemical tankers. Crude phosphoric acid is a basic raw material for fertilizers, detergents, feed, medicines, etc., but Japan has few phosphorus resources and relies on imports by sea transport for most of it. It is known that crude phosphoric acid contains corrosive substances such as F ^- and Cl ^- derived from phosphate rock, and therefore corrosion (including discoloration such as blackening) occurs on the inner surface of the tank when crude phosphoric acid is transported. Therefore, after the transportation of crude phosphoric acid, maintenance such as repair and cleaning is performed on the inner surface of the tank in order to prevent contamination, and new chemicals are loaded. Such maintenance is very laborious and frequent, and is therefore a problem in terms of cost and time in chemical transportation.

Conventionally, austenitic stainless steel materials with good corrosion resistance, such as SUS316L and 329J3L, have been used for the inner surface of the tank of a chemical tanker that transports crude phosphoric acid. However, even SUS316L corrodes when crude phosphoric acid is stored, and the demand for alternative materials is growing day by day.
Therefore, it is considered that the ferritic-austenitic duplex stainless steel materials described in Patent Documents 1 to 3 may be applied to the inner surfaces of storage tanks of chemical tankers that transport crude phosphoric acid.
However, although the ferritic-austenitic duplex stainless steel materials described in Patent Documents 1 to 3 are more cost effective than Ni-rich austenitic stainless steels, their corrosion resistance to sulfides, crude phosphoric acid, etc. is inferior to that of SUS316L.

Patent Document 4 proposes an austenitic stainless steel material with adjusted contents of Cr, Ni, Mo, and Cu, which affect corrosion resistance to crude phosphoric acid.

International Publication No. 2009/119895 Japanese Patent Application Publication No. 11-269612 JP 2018-59157 A JP 2002-121655 A

The austenitic stainless steel material described in Patent Document 4 is effective to a certain extent against corrosion caused by crude phosphoric acid, but it has been confirmed that corrosion (particularly blackening) occurs on the inner surface of the tank of a chemical tanker that actually transports crude phosphoric acid. Therefore, there is a demand for further improvement in corrosion resistance to crude phosphoric acid, as well as improvement from an economical perspective by reducing the content of expensive elements such as Ni.

The present invention is directed to a ferritic-austenitic duplex stainless steel material, which is superior from an economical point of view to an austenitic stainless steel material, and aims to provide a ferritic-austenitic duplex stainless steel material having excellent corrosion resistance to crude phosphoric acid and a method for producing the same.
Another object of the present invention is to provide a structure for crude phosphoric acid which has excellent corrosion resistance against crude phosphoric acid and can reduce maintenance costs and time.

The present inventors have conducted extensive research into ferritic-austenitic duplex stainless steel materials in order to solve the above problems, and as a result have made the following discoveries.
Crude phosphoric acid is produced by dissolving phosphate rock with sulfuric acid, and is transported as a liquid on a chemical tanker. Crude phosphoric acid contains low concentrations of F ^- (fluorine ion) and Cl ^- (chlorine ion), which are corrosive substances that preferentially dissolve the austenite phase, causing a decrease in the corrosion resistance of crude phosphoric acid.
Based on the above findings, elements related to the preferential dissolution of the austenite phase in crude phosphoric acid were investigated, and it was found that the amounts of five elements (Cr, Ni, Mo, Cu, and N) contained in the austenite phase are closely related to the preferential dissolution of the austenite phase.
The present inventors have found that preferential dissolution of the austenite phase can be suppressed by controlling the amounts of the above elements contained in the austenite phase in a well-balanced manner. The present inventors have also found that corrosion resistance to crude phosphoric acid can be improved by controlling the composition of the ferritic-austenitic duplex stainless steel material itself in a well-balanced manner. The present invention has been completed based on the above background.

That is, the present invention has a composition, on a mass basis, of 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, with the balance being Fe and impurities;
The following formula (1):
DF=7.2(Cr+0.8Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9... (1)
(wherein each element symbol represents the content (mass%) of each element) is 45.0 to 70.0;
The austenite phase has the following formula (2):
(0.8Cr) ² +2Ni+7Mo+30Cu+30N≧340... (2)
(wherein the element symbols represent the contents (mass%) of each element)

The present invention also relates to a soaking process in which a rolled material containing, by mass, 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, and the balance being Fe and impurities, is soaked at a soaking temperature of 950 to 1150 ° C. for more than 1 minute;
A first cooling step of slowly cooling the rolled material obtained in the soaking step to a quenching start temperature at a cooling rate of 5 ° C./sec or more and less than 10 ° C./sec;
A second cooling step of rapidly cooling the rolled material obtained in the first cooling step from the rapid cooling start temperature,
The present invention relates to a method for producing a ferritic-austenitic duplex stainless steel material, wherein the quenching start temperature is a temperature of 950 to 990°C and is a temperature lower than the soaking temperature.

Furthermore, the present invention relates to a structure for crude phosphoric acid comprising the above-mentioned ferritic-austenitic duplex stainless steel material.

According to the present invention, it is possible to provide a ferritic-austenitic duplex stainless steel material having excellent corrosion resistance to crude phosphoric acid and a method for producing the same.
Furthermore, according to the present invention, it is possible to provide a structure for crude phosphoric acid which has excellent corrosion resistance against crude phosphoric acid and can reduce maintenance costs and time.

The following is a detailed description of the embodiments of the present invention. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements to the following embodiments, which are made based on the ordinary knowledge of those skilled in the art, fall within the scope of the present invention, without departing from the spirit of the present invention.
In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

The ferritic-austenitic duplex stainless steel material according to an embodiment of the present invention (hereinafter simply referred to as "duplex stainless steel material") has a composition containing C: 0.100% or less, Si: 0.05-1.50%, Mn: 0.05-2.00%, Ni: 4.00-9.00%, P: 0.050% or less, S: 0.0040% or less, Cr: 23.0-30.0%, N: 0.100-0.250%, Cu: 0.01-2.00%, Mo: 0.50-2.50%, Al: 0.100% or less, Nb: 0.200% or less, with the balance being Fe and impurities.

Here, in this specification, the term "stainless steel material" refers to a material formed from stainless steel, and the shape of the material is not particularly limited. Examples of the material shape include a plate shape (including a strip shape), a rod shape, a tube shape, and the like. In addition, the material may be various shaped steels having a cross-sectional shape such as a T-shape or an I-shape.
In this specification, the term "ferritic-austenitic" refers to a metal structure that is mainly composed of two phases, ferritic and austenitic, at room temperature. Therefore, the term "ferritic-austenitic" also includes metal structures that contain small amounts of phases other than ferritic and austenitic phases (e.g., martensite phase, etc.).
Furthermore, in this specification, the term "impurities" refers to components that are mixed in due to various factors in raw materials such as ores and scraps and in the manufacturing process during industrial production of stainless steel materials, and are acceptable within a range that does not adversely affect the present invention. For example, impurities also include unavoidable impurities.
With regard to the content of each element, "xx% or less" means that the content is xx% or less, but includes an amount exceeding 0% (particularly, above the impurity level).

The duplex stainless steel material according to the embodiment of the present invention may further contain 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, as necessary.
Each component will be described in detail below.

<C: 0.100% or less>
It is desirable to reduce C as much as possible. There is no particular lower limit for the C content, but the refining costs are high for decarburizing steels that contain a relatively large amount of Cr, such as duplex stainless steels. Therefore, it is sufficient to reduce the C content to, for example, 0.001%. On the other hand, since C promotes the precipitation of Cr carbides, if the C content is too high, intergranular corrosion occurs and the corrosion resistance decreases. Therefore, From the viewpoint of suppressing the deterioration of corrosion resistance, the C content is set to 0.100% or less. From the viewpoint of stably securing this effect, the C content is set to 0.095% or less, 0.090% or less. It is preferably 0.085% or less, or 0.080% or less.

<Si: 0.05-1.50%>
Silicon 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 silicon content is set to 0.05% or more. From the viewpoint of stably securing these effects, the amount of Si 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 two-phase The stainless steel material becomes hard, and the toughness and workability are reduced. Therefore, from the viewpoint of suppressing the reduction in toughness and workability, the Si content is set to 1.50% or less. From the viewpoint of stably securing it, it is preferably 1.45% or less, 1.40% or less, 1.35% or less, or 1.30% or less.

<Mn: 0.05-2.00%>
Mn has the effect of increasing the austenite phase and increasing the solid solubility of nitrogen to suppress bubble defects during manufacturing. From the viewpoint of obtaining this effect, the Mn content is set to 0.05% or more. In order to stably ensure this effect, 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 are deteriorated. Therefore, from the viewpoint of suppressing the deterioration of corrosion resistance and hot workability, the Mn content is set to 2.00% or less. % or less, preferably 1.80% or less, 1.70% or less, or 1.60% or less.

<Ni: 4.00-9.00%>
Ni is an austenite stabilizing element and has the effect of improving corrosion resistance by suppressing the progress of corrosion. In order to ensure a certain amount of austenite phase in duplex stainless steel materials, the Ni content is set to 4.00% or more. In order to stably ensure this effect, the Ni content is 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, since Ni is an expensive element, it is preferable that the Ni content is as small as possible. Therefore, the Ni content is set to 9.00% or less. From the viewpoint of cost reduction, the Ni content is set to It 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, so the lower the P content, the better. The P content is set to 0.050% or less. From the viewpoint of stably suppressing the deterioration of formability, the P content is preferably set to 0.045% or less or 0.040% or less. The lower limit of P is not particularly limited, but removing P requires a very high refining cost, so the P content may be set to 0.001% or more in consideration of economic efficiency.

<S: 0.0040% or less>
S is an element that is inevitably mixed into duplex stainless steel materials, and it can combine with Mn to form inclusions that can become the starting point for rusting. The lower the S content, the better the corrosion resistance. Therefore, the S content is set to 0.0040% or less. From the viewpoint of stably ensuring this effect, the S content is preferably set to 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 removing S requires a very high refining cost. Therefore, from the viewpoint of economy, the S content is set to 0.0001% or more. Good too.

<Cr:23.0~30.0%>
Cr strengthens the passive film, thereby improving the corrosion resistance against crude phosphoric acid. Cr also has the effect of repairing the passive film after Fe is dissolved. From the viewpoint of stably securing these effects, the Cr content is set to 23.0% or more. In addition, the Cr content is set to 23.5% or more, 24.0% or more, or 24.5% or more. On the other hand, if the Cr content is too high, the amount of chromium nitride precipitated increases, and the risk of hot working cracking also increases. In order to suppress these, the Cr content is set to 30.0% or less. In order to stably suppress these, the Cr content is set to 29.8% or less, % or less, 29.4% or less, 29.2% or less, or 29.0% or less is preferable.

<N: 0.100-0.250%>
When N dissolves, it becomes _NH4 ⁺ (ammonium ion), which has the effect of raising the pH of acidic condensation water and suppressing the dissolution of Fe. It also dissolves in the austenite phase, improving strength and corrosion resistance, and reducing the amount of alloying. In order to obtain these effects, the N content is set to 0.100% or more. In order to stably secure these effects, the N content is set to 0.120% or more. On the other hand, N is an element that greatly affects the precipitation of chromium nitrides, and if it is contained in a large amount, the amount of chromium nitrides precipitated increases, In order to suppress these, the N content is set to 0.250% or less. In order to stably suppress these, the N content is set to 0.230% or less or 0. It is preferable that the ratio is 220% or less.

<Cu: 0.01-2.00%>
Cu is an element that improves acid resistance, particularly corrosion resistance against crude phosphoric acid containing sulfuric acid, and has the effect of suppressing the dissolution of Fe from the surface of the duplex stainless steel material when acidic condensed water adheres to the material. From the viewpoint of obtaining this effect, the Cu content is set to 0.01% or more. From the viewpoint of stably ensuring this effect, the Cu content is set to 0.05% or more, 0.10% or more, 0.20% or more, % or more, or 0.25% or more. On the other hand, if the Cu content is too high, the raw material cost increases and the hot workability deteriorates. From the viewpoint of suppressing these, the Cu content is From the viewpoint of stably suppressing these, the Cu content is set to 1.80% or less, 1.50% or less, 1.00% or less, or 0.80% or less. preferable.

<Mo: 0.50-2.50%>
Mo has the effect of improving corrosion resistance and also has the effect of repairing the passive film after Fe dissolution. In particular, it prevents Fe from dissolving on the surface of duplex stainless steel material and causing corrosion (especially blackening). From the viewpoint of obtaining this effect, the Mo content is set to 0.50% or more. From the viewpoint of stably securing this effect, the Mo content is set to 0.80% or more, 0.90% or more. % 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 is set to 2.50% or less. From the viewpoint of reducing costs, the Mo content is preferably 2.30% or less or 2.20% or less.

<Al: 0.100% or less>
Al is an element contained for desulfurization and deoxidization. However, if a large amount of Al is contained, it will lead to an increase in raw material costs and manufacturing costs, so the Al content is set to 0.100% or less. The Al content is preferably 0.090% or less or 0.080% or less from the viewpoint of stably reducing increases in raw material costs and manufacturing costs. On the other hand, the lower limit of the Al content is not particularly limited. From the viewpoint of obtaining the above-mentioned effects, the Al content is preferably 0.001% or more.

<Nb: 0.200% or less>
Nb has the effect of suppressing the precipitation of chromium nitrides by forming a compound with N. However, if the Nb content is too high, the workability of the duplex stainless steel material decreases. From this viewpoint, the Nb content is set to 0.200% or less. Furthermore, 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-mentioned effects, the Nb content is preferably 0.001% or more.

<Ti: 0.050% or less>
Ti has the effect of preventing coarsening of the heat-affected zone during welding of 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 are reduced. From the viewpoint of suppressing the reduction in the uniform elongation and local elongation, the Ti content is set to 0.050% or less. The Ti content is From the viewpoint of stably securing this effect, it is preferable that the Ti content is 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 securing the above effect, The Ti content is preferably 0.001% or more.

<B: 0.0050% or less>
B has the effect of improving hot workability, and therefore can be contained as necessary. However, if the B content is too high, the corrosion resistance is significantly reduced. From the viewpoint of suppressing the reduction in corrosion resistance, The B content is set to 0.0050% or less. From the viewpoint of stably ensuring this effect, the B content is preferably set to 0.0040% or less or 0.0030% or less. The lower limit of the content is not particularly limited, but from the viewpoint of obtaining the above-mentioned effects, the B content is preferably 0.0003% or more.

<Ca: 0.0010-0.0100%>
Ca is an element that enhances corrosion resistance. It is believed that Ca reduces inclusions that are the starting point of corrosion in an environment where crude phosphoric acid is present, and inhibits the elution of Fe. Therefore, it is included as necessary. From the viewpoint of ensuring this effect, the Ca content is set to 0.0010% or more. From the viewpoint of stably ensuring this effect, the Ca content is set to 0.0015% or more or 0. It is preferable that the Ca content is 0.0020% or more. On the other hand, if the Ca content is too high, hot work cracks are likely to occur and the corrosion resistance is also reduced. From the viewpoint of suppressing these, the Ca content is set to 0.0100% or less. From the viewpoint of stably suppressing these, the Ca content is preferably 0.0090% or less, 0.0080% or less, or 0.0070% or less.

<Mg: 0.0001-0.0020%>
Mg can be added as necessary since it has the effect of not only deoxidizing but also refining the solidified structure. However, a high Mg content increases the cost in the steelmaking process. The Mg content is set to 0.0020% or less. From the viewpoint of reducing costs, the Mg content is preferably set to 0.0019% or less or 0.0018% or less. Therefore, the Mg content is set to 0.0001% or more.

<Zr: 0.090% or less>
Zr can be added as necessary because it has the effect of forming carbides and nitrides in duplex stainless steel materials and refining crystal grains. However, if the Zr content is high, excessive formation of carbides and nitrides occurs. The toughness is reduced by the carbides and nitrides formed. Therefore, from the viewpoint of suppressing the reduction in toughness, the Zr content is set to 0.090% or less. In addition, the Zr content is set to 0.090% or less in order to stably secure this effect. From this viewpoint, it is preferable that the Zr content is 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-mentioned effects, the Zr content is preferably 0.001% or more or 0.005% or more. It is preferable that:

<Co: 3.00% or less>
Co, like N, is an austenite stabilizing element and can be contained as necessary. However, since Co is an expensive element, the Co content is set to 3.00% or less. From the viewpoint of reducing costs, 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-mentioned 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 be added as necessary. However, if the V content is too high, the load during rolling increases and manufacturing defects are more likely to occur. The V content is preferably 0.950% or less from the viewpoint of stably suppressing the formation of manufacturing defects. From the viewpoint of obtaining the above-mentioned effects, the V content is preferably, but not limited to, 0.010% or more, or 0.050% or more.

<Ta: 0.200% or less>
Ta forms carbides and nitrides in duplex stainless steel materials and has the effect of additionally increasing corrosion resistance, so it can be added as necessary. However, if the Ta content is high, excessive formation The Ta content is set to 0.200% or less because the toughness is reduced by the carbides and nitrides formed. From the viewpoint of stably securing this effect, the Ta content is set to 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-mentioned 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 therefore may be contained as necessary. However, if the Sn content is high, hot workability is reduced, so the Sn content is set to 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. The Sn content is not particularly limited, but from the viewpoint of obtaining the above-mentioned effects, the Sn content is preferably 0.0001% or more, or 0.0003% or more.

<O: 0.0050% or less>
Like S, O is an element that affects hot workability, so it is desirable to have as little O as possible. Therefore, the O content is set to 0.0050% or less. From the viewpoint of stably securing the O content, 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 reducing the O content leads to an increase in manufacturing costs. The content is preferably 0.0001% or more.

<W: 1.000% or less>
W is an element that improves corrosion resistance, and can be added as necessary. However, if the W content is too high, the load during rolling is increased, and manufacturing defects are more likely to occur. The W content is set to 1.000% or less. From the viewpoint of stably ensuring this effect, the W content is preferably set to 0.900% or less or 0.800% or less. The lower limit is not particularly limited, but from the viewpoint of obtaining the above-mentioned effects, the W content is preferably 0.005% or more, or 0.010% or more.

<REM: 0.100% or less>
REM (rare earth elements) have the effect of improving hot workability, and therefore may be added as necessary. However, if the REM content is too high, manufacturability is impaired and costs are increased. The REM content is set to 0.100% or less. From the viewpoint of stably ensuring this effect, the REM content is preferably set to 0.095% or less or 0.090% or less. The lower limit of the content is not particularly limited, but from the viewpoint of obtaining the above-mentioned effects, the REM content is preferably 0.001% or more, 0.005% or more, or 0.010% or more.
REM is a general term for 17 elements, including Sc, Y, and 15 elements from La to Lu (lanthanoids), and the REM content means the total content of these elements. In addition, lanthanides are industrially added in the form of misch metals.

In the duplex stainless steel material according to the embodiment of the present invention, the value of DF represented by the following formula (1) is 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 formula (1), the element symbols represent the contents (mass%) of each element.
Here, DF is an index representing the amount of ferrite phase. Therefore, 100-DF means the amount of austenite phase. However, since DF is an index determined based on the content of elements, it should be noted that it may not match the amount of austenite phase actually measured. If the value of DF is less than 45.0, the strength will be excessively high and manufacturability will decrease. On the other hand, if the value of DF exceeds 70.0, the proportion of austenite phase will decrease, and the amount of N dissolved in the austenite phase will decrease. As a result, the amount of chromium nitride precipitates will increase, and toughness and corrosion resistance will decrease.

In the duplex stainless steel material according to the embodiment of the present invention, the austenite phase preferably satisfies the following relational expression (2).
(0.8Cr) ² +2Ni+7Mo+30Cu+30N≧340... (2)
In formula (2), the element symbols represent the contents (mass%) of each element.
Here, formula (2) is a formula expressing the relationship between the contents of five elements (Cr, Ni, Mo, Cu, and N) that affect the preferential dissolution of the austenite phase in the crude phosphoric acid. By controlling the contents of the elements in the austenite phase so as to satisfy the relationship of formula (2), the preferential dissolution of the austenite phase in the crude phosphoric acid can be suppressed, and the corrosion of the duplex stainless steel material can be suppressed. From the viewpoint of stably securing this effect, the value of the left side of formula (2) is preferably 345 or more, more preferably 350 or more, and even more preferably 355 or more. The upper limit of the value of the left side of formula (2) is not particularly limited, but is, for example, 800 or 700.

Here, the content of each element in the austenite phase used to calculate the relational expression (2) can be measured by EPMA (electron probe microanalyzer). Specifically, a mirror-polished sample of a thickness-wise cross section of a duplex stainless steel material parallel to the rolling direction is used for qualitative analysis by EPMA. A portion that can be clearly determined to be an austenite phase based on the Ni content is identified by qualitative mapping of Ni for the entire cross section. Then, Cr, Ni, Mo, Cu and N are quantitatively analyzed at approximately the center of the identified austenite phase. Quantitative analysis is performed at 10 or more points, and the average value is taken as the content result for each element.

The austenite phase preferably contains 21.0-30.0% Cr, 7.5-12.0% Ni, 0.30-2.00% Mo, 0.3-2.1% Cu, and 0.10-0.35% N. By controlling the content of each element within such ranges, the relationship of formula (2) is more likely to be satisfied. In the austenite phase, the Cr content is preferably 22.0-25.5%, the Ni content is 8.0-12.0%, the Mo content is 1.20-1.80%, the Cu content is 0.4-0.9%, and the N content is 0.25-0.33%.

The duplex stainless steel material according to the embodiment of the present invention preferably satisfies the relational expression of the following formula (3).
Expected CPT - Actual CPT ≦ 10°C ... (3)
In the formula, the expected CPT is the pitting corrosion initiation temperature (°C) calculated by 2.5Cr + 7.6Mo + 3.19N - 26 (where Cr, Mo and N represent the Cr, Mo and N contents (mass%) in the duplex stainless steel material, respectively), and the measured CPT is the pitting corrosion initiation temperature (°C) measured by a pitting corrosion resistance test in accordance with ASTM G48E.
Since the measured CPT tends to decrease when chromium nitrides precipitate, if the CPT difference (expected CPT - measured CPT) is 10°C or less, it can be considered that the amount of chromium nitrides precipitated is small. In other words, when the relational expression (3) is satisfied, the precipitation of chromium nitrides is suppressed, and corrosion resistance can be sufficiently ensured. On the other hand, when the relational expression (3) is not satisfied, there is a risk that the corrosion resistance will decrease due to the precipitation of chromium nitrides.

The duplex stainless steel material according to the embodiment of the present invention may be either hot-rolled or cold-rolled, but when used for storage tanks of chemical tankers, etc., it is preferable to use hot-rolled material.

The thickness of the duplex stainless steel material according to the embodiment of the present invention is not particularly limited and may be adjusted appropriately depending on the application, but is generally 20.0 mm or less, preferably 15.0 mm or less, and more preferably 10.0 mm or less. When the duplex stainless steel material is rod-shaped, the thickness refers to the circle-equivalent diameter of the cross section. When the duplex stainless steel material is a steel section, the thickness refers to the thickness at any point on the cross section.

The duplex stainless steel material according to the embodiment of the present invention has excellent corrosion resistance against crude phosphoric acid, and therefore can be used for various members that may come into contact with crude phosphoric acid. Among them, the duplex stainless steel material according to the embodiment of the present invention is suitable for use in a structure for crude phosphoric acid.
Here, in this specification, the term "crude phosphoric acid structure" refers to various facilities that handle crude phosphoric acid. Examples of the crude phosphoric acid structure include a phosphoric acid manufacturing plant, a phosphoric acid storage tank (including a storage tank of a ship), a phosphoric acid transport pipe, etc.

The method for producing a duplex stainless steel material according to the embodiment of the present invention is not particularly limited as long as it is a method capable of producing a duplex stainless steel material having the above-mentioned characteristics.
Hereinafter, an example of a method for producing a duplex stainless steel material according to an embodiment of the present invention will be described.
A method for producing a duplex stainless steel material according to an embodiment of the present invention includes a soaking step, a first cooling step, and a second cooling step.

The soaking step is a step of soaking 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 less than 950°C, the chromium nitride cannot be sufficiently dissolved. If the soaking temperature exceeds 1150°C, the amount of ferrite phase increases and it becomes 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 is preferably less than 10 minutes in consideration of production efficiency and production costs.
The rolled material can be produced by a conventional method, for example, by vacuum melting a stainless steel having the above composition to form a steel slab, which is then hot rolled.

The first cooling step is a step of slowly cooling the rolled material obtained in the soaking step to a quenching start temperature at a cooling rate of 5° C./sec or more and less than 10° C./sec.
If the cooling rate in the first cooling step is 10° C./sec or more, it becomes 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./sec or less, more preferably 8° C./sec or less, and even more preferably 7° C./sec or less. If the cooling rate in the first cooling step is less than 5° C./sec, the amount of chromium nitride precipitated increases, and in some cases, the σ phase precipitates, resulting in a decrease in corrosion resistance.
The cooling method in the first cooling step is not particularly limited, but may be, for example, air cooling.

The second cooling step is a step of rapidly cooling (at a cooling rate of 10° C./sec or more) the rolled material obtained in the first cooling step from the rapid cooling start temperature.
The quenching start temperature is a temperature of 950 to 990°C, and is a temperature lower than the above-mentioned soaking temperature. If the quenching start temperature is less than 950°C, the amount of ferrite phase generated increases, resulting in a decrease in corrosion resistance. If the quenching start temperature exceeds 990°C, it becomes difficult to control the composition of the austenite phase within a predetermined range. From the viewpoint of stably suppressing these problems, the quenching start temperature is preferably 960 to 980°C. The quenching end temperature is not particularly limited, and may be, for example, room temperature.
The cooling method in the second cooling step is not particularly limited, but may be, for example, water cooling.

The structure for producing crude phosphoric acid according to the embodiment of the present invention includes the above-mentioned duplex stainless steel material. This structure for producing crude phosphoric acid may further include a member other than the above-mentioned duplex stainless steel material.
The structure for producing crude phosphoric acid according to the embodiment of the present invention can be manufactured by processing the above-mentioned duplex stainless steel material into a predetermined shape and then assembling it by welding or the like.
The structure for crude phosphoric acid is not particularly limited, but is preferably a phosphoric acid production plant, a phosphoric acid storage tank, or a phosphoric acid transport pipe.

The present invention will be explained in detail below with reference to examples, but the present invention should not be construed as being limited to these.

Stainless steels having the compositions shown in Tables 1-1 and 1-2 were melted in MgO crucibles in a 50 kg vacuum induction furnace in a laboratory and cast into flat steel ingots with a thickness of about 100 mm. The main body of the flat steel ingot was processed into a material for hot rolling, heated to 1180°C, and then held at that temperature for 1 hour, and then hot rolled into a hot-rolled sheet with 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.
Next, a duplex stainless steel sheet was obtained by performing a soaking step, a first cooling step, and a second cooling step under the conditions shown in Table 2. The first cooling step was performed by air cooling, and the cooling rate was controlled by controlling the flow rate of the cooling gas. The second cooling step was performed by water cooling to room temperature.

<Content of each element in the austenite phase>
The content of each element in the austenite phase was measured by EPMA (electron probe microanalyzer). Specifically, a sample was mirror-polished in the thickness direction of a duplex stainless steel material parallel to the rolling direction, and a qualitative analysis was performed by EPMA. The EPMA was performed at an acceleration voltage of 15 kV, and a lattice-like measurement was performed in an area of 300 μm x 300 μm at intervals of 0.76 μm, resulting in a total of 140,625 pieces of data. In the qualitative analysis, a portion that can be clearly determined to be an austenite phase from the Ni content was identified by qualitative mapping of Ni for the entire cross section. Then, Cr, Ni, Mo, Cu, and N were quantitatively analyzed at approximately the center of the identified austenite phase. The quantitative analysis was performed at 10 or more points, and the average value was used as the result of the content of each element. In addition, the value of the left side of the relational expression of formula (2) was calculated using the obtained content of each element.

<Corrosion test>
A test piece of 10 mm x 80 mm x 2 mm was taken from the duplex stainless steel plate and wet-polished over the entire surface using a No. 600 grindstone. Next, an aqueous solution simulating crude phosphoric acid containing 70% by mass phosphoric acid, 0.4% by mass F ^-ions , and 0.04% by mass Cl ^-ions was prepared. Next, the container containing this aqueous solution was kept at 50°C, and half of the test piece was immersed in it for 6 hours. Then, the corrosion rate (mm/y) was calculated from the mass change (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 excellent. Since the corroded area is both the gas phase and the liquid phase, the corrosion rate was calculated using the entire area of the test piece.
Furthermore, the test pieces after the above tests were visually evaluated for discoloration (blackening).

<Actual CPT and CPT difference>
The measured CPT was determined in accordance with ASTM G48E by taking a test piece of 50 mm x 25 mm x 2 mm thickness (thickness from a position 1 mm deep from the outermost surface) from a duplex stainless steel plate, carrying out a pitting test on the test piece at an aqueous solution temperature of 30° C., and measuring the CPT (Critical Pitting Temperature). Note that when measuring the CPT, pitting that occurred on the end face (cross section in the thickness direction) of the test piece was not counted, and only pitting that occurred on the 50 mm x 25 mm surface was counted.
The CPT difference (expected CPT - actual CPT) was calculated using the actual CPT thus obtained and the expected CPT calculated above. The CPT difference was represented as ◯ when it was 10°C or less, and × when it was more than 10°C.

The results of each of the above evaluations are shown in Table 3.

As shown in Table 3, in Nos. 1 to 26 (invention examples), the duplex stainless steel plate had a predetermined composition and DF, and the austenite phase satisfied the relational expression of formula (2), so it was confirmed that the corrosion rate was slow and the corrosion resistance to crude phosphoric acid was excellent.
In contrast, in No. 27 (Comparative Example), the soaking temperature was too high, so that the element concentrations in the austenite phase could not be controlled, and the relational expression (2) was not satisfied. As a result, the corrosion resistance against crude phosphoric acid was insufficient.
In No. 28 (Comparative Example), the cooling rate in the first cooling step was too fast, so that the element concentrations in the austenite phase could not be controlled and the relational expression (2) was not satisfied. As a result, the corrosion resistance against crude phosphoric acid was insufficient.
In No. 29 (Comparative Example), the quenching start temperature in the second cooling step was too high, so that the element concentrations in the austenite phase could not be controlled and the relational expression (2) was not satisfied. As a result, the corrosion resistance against crude phosphoric acid was insufficient.
In Nos. 30 and 31 (comparative examples), the cooling rate in the first cooling step was too slow, so the element concentrations in the austenite phase could not be controlled, and the relational expression of formula (2) was not satisfied. In addition, chromium nitrides and σ phases were precipitated, so the corrosion resistance to crude phosphoric acid was insufficient.
No. 32 (Comparative Example) had a high DF value and an excessive amount of ferrite phase, so that the amount of chromium nitride precipitated was large and the corrosion resistance to crude phosphoric acid was insufficient.
No. 33 (Comparative Example) contained a large amount of Mn, which resulted in a decrease in acid resistance and insufficient corrosion resistance to crude phosphoric acid.
No. 34 (Comparative Example) did not contain Cu, and therefore had insufficient corrosion resistance against crude phosphoric acid.
No. 35 (Comparative Example) did not contain Nb, and therefore had insufficient corrosion resistance against crude phosphoric acid.
No. 36 (Comparative Example) had a low DF value and an excessive amount of austenite phase, and therefore the amount of austenite phase that corroded preferentially increased, and therefore the corrosion resistance to crude phosphoric acid was insufficient.
No. 37 (Comparative Example) had a high Cr content, and therefore precipitates of chromium nitrides were large, and the corrosion resistance to crude phosphoric acid was insufficient.
No. 38 (Comparative Example) had an excessively low Cr content, and therefore was unable to maintain a passive film in the crude phosphoric acid environment, resulting in insufficient corrosion resistance to the crude phosphoric acid.
No. 39 (Comparative Example) had an excessively low Mo content, and therefore was unable to maintain a passive film in the crude phosphoric acid environment, resulting in insufficient corrosion resistance to the crude phosphoric acid.
No. 40 (Comparative Example) had an excessively high N content, which resulted in the precipitation of a large amount of chromium nitride in the ferrite phase, and therefore the corrosion resistance to crude phosphoric acid was insufficient.
In No. 41 (Comparative Example), the N content was too low, so the effect of suppressing Fe elution due to the generation of NH ⁴⁺ in a crude phosphoric acid environment was not sufficiently obtained, and as a result, the corrosion resistance to crude phosphoric acid was insufficient.

As can be seen from the above results, the present invention can provide a ferritic-austenitic duplex stainless steel material with excellent corrosion resistance to crude phosphoric acid, and a manufacturing method thereof. Furthermore, the present invention can provide a structure for crude phosphoric acid that has excellent corrosion resistance to crude phosphoric acid and can reduce maintenance costs and time.

Furthermore, based on the above results, the present invention can be configured as follows.

(Aspect 1) On a mass basis, it has a composition including 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, and the balance being Fe and impurities;
The following formula (1):
DF=7.2(Cr+0.8Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9... (1)
(wherein each element symbol represents the content (mass%) of each element) is 45.0 to 70.0;
The austenite phase has the following formula (2):
(0.8Cr) ² +2Ni+7Mo+30Cu+30N≧340... (2)
(wherein the chemical symbols represent the content (mass%) of each element)

(Aspect 2) The ferritic-austenitic duplex stainless steel material according to aspect 1, further comprising one or more selected from, by mass, Ti: 0.050% or less, B: 0.0050% or less, Ca: 0.0010-0.0100%, Mg: 0.0001-0.0020%, 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 ferritic-austenitic duplex stainless steel material according to aspect 1 or 2, which satisfies the following relational expression (3).
Expected CPT - Actual CPT ≦ 10°C ... (3)
In the formula, the expected CPT is the pitting corrosion initiation temperature (°C) calculated by 2.5Cr + 7.6Mo + 3.19N - 26 (wherein Cr, Mo and N represent the Cr, Mo and N contents (mass%) in the ferritic-austenitic duplex stainless steel material, respectively), and the actual CPT is the pitting corrosion initiation temperature (°C) measured by a pitting corrosion resistance test in accordance with ASTM G48E.

(Aspect 4) The ferritic-austenitic duplex stainless steel material according to any one of aspects 1 to 3, wherein the austenite phase contains, by mass, 21.0 to 30.0% Cr, 7.5 to 12.0% Ni, 0.30 to 2.00% Mo, 0.3 to 2.1% Cu, and 0.10 to 0.35% N.

(Aspect 5) A ferritic-austenitic duplex stainless steel material according to any one of aspects 1 to 4, used in a structure for crude phosphoric acid.

(Aspect 6) A soaking process in which a rolled material containing, by mass, 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, and the balance being Fe and impurities, is soaked at a soaking temperature of 950 to 1150 ° C. for more than 1 minute;
A first cooling step of slowly cooling the rolled material obtained in the soaking step to a quenching start temperature at a cooling rate of 5 ° C./sec or more and less than 10 ° C./sec;
A second cooling step of rapidly cooling the rolled material obtained in the first cooling step from the rapid cooling start temperature,
The method for producing a ferritic-austenitic duplex stainless steel material, wherein the quenching start temperature is a temperature of 950 to 990°C and is a temperature lower than the soaking temperature.

(Aspect 7) The method for producing a ferritic-austenitic duplex stainless steel material according to aspect 6, wherein the rolled material further contains, by mass, one or more selected from Ti: 0.050% or less, B: 0.0050% or less, Ca: 0.0010-0.0100%, Mg: 0.0001-0.0020%, 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 comprising a ferritic-austenitic 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 phosphoric acid storage tank, or a phosphoric acid transport pipe.

Claims (9)

The composition includes, 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, and the balance being Fe and impurities;
The following formula (1):
DF=7.2(Cr+0.8Mo+0.78Si)-8.9(Ni+0.03Mn+0.72Cu+22C+21N)-44.9... (1)
(wherein each element symbol represents the content (mass%) of each element) is 45.0 to 70.0;
The austenite phase has the following formula (2):
(0.8Cr) ² +2Ni+7Mo+30Cu+30N≧340... (2)
(wherein the chemical symbols represent the content (mass%) of each element) The ferritic-austenitic duplex stainless steel material according to claim 1, further comprising at least one selected from, by mass, Ti: 0.050% or less, B: 0.0050% or less, Ca: 0.0010-0.0100%, Mg: 0.0001-0.0020%, 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 ferritic-austenitic duplex stainless steel material according to claim 1 or 2, which satisfies the following relational expression (3):
Expected CPT - Actual CPT ≦ 10°C ... (3)
In the formula, the expected CPT is the pitting corrosion initiation temperature (°C) calculated by 2.5Cr + 7.6Mo + 3.19N - 26 (wherein Cr, Mo and N represent the Cr, Mo and N contents (mass%) in the ferritic-austenitic duplex stainless steel material, respectively), and the actual CPT is the pitting corrosion initiation temperature (°C) measured by a pitting corrosion resistance test in accordance with ASTM G48E. The ferritic-austenitic duplex stainless steel material according to claim 1 or 2, wherein the austenite phase contains, by mass, 21.0-30.0% Cr, 7.5-12.0% Ni, 0.30-2.00% Mo, 0.3-2.1% Cu, and 0.10-0.35% N. The ferritic-austenitic duplex stainless steel material according to claim 1 or 2, used in a structure for crude phosphoric acid. a soaking step of soaking a rolled material containing, by mass, 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, with the balance being Fe and impurities, at a soaking temperature of 950 to 1150°C for more than 1 minute;
A first cooling step of slowly cooling the rolled material obtained in the soaking step to a quenching start temperature at a cooling rate of 5 ° C./sec or more and less than 10 ° C./sec;
A second cooling step of rapidly cooling the rolled material obtained in the first cooling step from the rapid cooling start temperature,
The method for producing a ferritic-austenitic duplex stainless steel material, wherein the quenching start temperature is a temperature of 950 to 990°C and is a temperature lower than the soaking temperature. The method for producing a ferritic-austenitic duplex stainless steel material according to claim 6, wherein the rolled material further contains, by mass, one or more selected from Ti: 0.050% or less, B: 0.0050% or less, Ca: 0.0010-0.0100%, Mg: 0.0001-0.0020%, 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 comprising the ferritic-austenitic duplex stainless steel material according to claim 1 or 2. The structure for crude phosphoric acid according to claim 8, which is a phosphoric acid production plant, a phosphoric acid storage tank, or a phosphoric acid transport pipe.