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

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

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

[0001] The present invention relates to a ferritic-austenitic duplex stainless steel material, a method for producing the same, and a structure for crude phosphoric acid.

[0002] International logistics involves a wide variety of goods, including chemical products such as liquids and gases containing corrosive substances, e.g., acids. Such chemical products are generally transported by specially designed vessels, such as chemical tankers, whose storage tanks are typically made of corrosion-resistant steel, particularly stainless steel. For example, Patent Document 1 proposes using a ferritic-austenitic duplex stainless steel material having a predetermined composition in place of the austenitic stainless steel material conventionally used as a storage tank steel material. Patent Document 2 also proposes using a ferritic-austenitic duplex stainless steel material having a predetermined composition in sulfuric acid plants that handle crude sulfuric acid, sulfuric acid storage tanks, and the like.

[0003] Furthermore, Patent Document 3 proposes a ferritic-austenitic duplex stainless steel material that has excellent corrosion resistance in a trace hydrogen sulfide environment as a stainless steel material to be applied to line pipes used for transporting oil and natural gas.

[0004] Crude phosphoric acid is one of the products transported by chemical tankers. Crude phosphoric acid is a basic raw material for fertilizers, detergents, feed, medicines, etc. However, Japan has few phosphorus resources and is almost entirely dependent on imports by sea. Crude phosphoric acid contains F derived from phosphate rock. - and Cl - It is known that transporting crude phosphoric acid causes corrosion (including discoloration such as blackening) on ​​the inner surface of the tank of a tanker because crude phosphoric acid contains corrosive substances such as phosphate. Therefore, after transporting crude phosphoric acid, maintenance such as repair and cleaning of the inner surface of the tank is carried out to prevent contamination before new chemicals are loaded. This type of maintenance is very time-consuming and requires frequent maintenance, which is a problem in terms of cost and time when transporting chemicals.

[0005] Conventionally, austenitic stainless steel materials with good corrosion resistance, such as SUS316L and 329J3L, have been used for the inner surfaces of storage tanks of chemical tankers transporting crude phosphoric acid. However, even SUS316L is subject to corrosion when crude phosphoric acid is stored, and the demand for alternative materials is growing day by day. Therefore, it is considered to use the ferritic-austenitic duplex stainless steel materials described in Patent Documents 1 to 3 for the inner surfaces of storage tanks of chemical tankers transporting crude phosphoric acid. However, although the ferritic-austenitic duplex stainless steel materials described in Patent Documents 1 to 3 are more cost-effective than austenitic stainless steels with high Ni content, they have inferior corrosion resistance to sulfides, crude phosphoric acid, and the like compared to SUS316L.

[0006] Therefore, Patent Document 4 proposes an austenitic stainless steel material in which the contents of Cr, Ni, Mo, and Cu, which affect the corrosion resistance to crude phosphoric acid, are adjusted.

[0007] International Publication No. 2009 / 119895 Japanese Patent Application Laid-Open No. 11-269612 Japanese Patent Application Laid-Open No. 2018-59157 Japanese Patent Application Laid-Open No. 2002-121655

[0008] Although the austenitic stainless steel material described in Patent Document 4 has a certain effect on corrosion resistance against crude phosphoric acid, it has been confirmed that corrosion (particularly blackening) occurs on the inner surface of a storage tank of a chemical tanker that actually transports crude phosphoric acid. Therefore, there is a demand for further improvement in corrosion resistance against crude phosphoric acid and improvement from an economical viewpoint by reducing the content of expensive elements such as Ni.

[0009] The present invention is directed to ferritic-austenitic duplex stainless steel materials, which are superior to austenitic stainless steel materials from an economical standpoint, and aims to provide a ferritic-austenitic duplex stainless steel material that has excellent corrosion resistance to crude phosphoric acid, and a method for producing the same. Another aim of the present invention is to provide a structure for crude phosphoric acid that has excellent corrosion resistance to crude phosphoric acid and can reduce maintenance costs and time.

[0010] The present inventors have conducted extensive research into ferritic-austenitic duplex stainless steel materials to solve the above-mentioned problems, and as a result have discovered the following: Crude phosphoric acid is produced by dissolving phosphate rock in sulfuric acid, and is transported as a liquid on a chemical tanker. - (fluorine ion) and Cl - The present inventors have found that the preferential dissolution of the austenite phase can be suppressed by controlling the amounts of the above elements in a well-balanced manner. Furthermore, the present inventors have also found that the corrosion resistance of ferritic-austenitic duplex stainless steels to crude phosphoric acid can be improved by controlling the composition of the ferritic-austenitic duplex stainless steel itself in a well-balanced manner. The present invention was developed based on the above background.

[0011] That is, the present invention provides a steel sheet having a composition, by mass, containing 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, which is expressed by 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 the element symbols represent the content (mass%) of each element) is 45.0 to 70.0, and the austenite phase is represented by the following formula (2): (0.8Cr) 2+2Ni+7Mo+30Cu+30N≧340 (2) (wherein the element symbols represent the contents (mass %) of each element).

[0012] The present invention also provides a rolling 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, a soaking step of soaking the rolling material at a soaking temperature of 950 to 1150°C for more than 1 minute, and a first cooling step of slowly cooling the rolling 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, wherein the rapid cooling start temperature is a temperature of 950 to 990°C and is a temperature lower than the soaking temperature.

[0013] Furthermore, the present invention relates to a structure for crude phosphoric acid production comprising the above-mentioned ferritic-austenitic duplex stainless steel material.

[0014] 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, a method for producing the same, and a structure for crude phosphoric acid that has excellent corrosion resistance to crude phosphoric acid and can reduce maintenance costs and time.

[0015] Hereinafter, embodiments of the present invention will be described in detail. The present invention is not limited to the following embodiments, and it should be understood that modifications and improvements to the following embodiments, as appropriate, based on the ordinary knowledge of those skilled in the art, without departing from the spirit of the present invention, also fall within the scope of the present invention. In this specification, the "%" designation for components means "% by mass" unless otherwise specified.

[0016] A 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 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.

[0017] Here, in this specification, "stainless steel material" refers to a material formed from stainless steel, and its shape is not particularly limited. Examples of the shape include plate (including strip), rod, and tube. It may also have various cross-sectional shapes such as T-shaped and I-shaped. Furthermore, in this specification, "ferritic-austenitic" refers to a material whose metal structure at room temperature is primarily two-phase, consisting of ferrite and austenite. Therefore, "ferritic-austenitic" also includes materials containing small amounts of phases other than ferrite and austenite (e.g., martensite). Furthermore, in this specification, "impurities" refer to components that are mixed in during the industrial production of stainless steel materials due to raw materials such as ores and scrap, or various factors in the manufacturing process, and are acceptable within a range that does not adversely affect the present invention. For example, impurities also include unavoidable impurities. Regarding the content of each element, "including xx% or less" means that the content is xx% or less, but greater than 0% (particularly, greater than the impurity level).

[0018] The duplex stainless steel material according to the embodiment of the present invention may further contain, as necessary, 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.

[0019] <C: 0.100% or Less> It is desirable to reduce C as much as possible. Although there are no particular limitations on the lower limit of the C content, it is sufficient to reduce it to, for example, 0.001% because decarburization of steel types containing a relatively high Cr content, such as duplex stainless steel, increases refining costs. On the other hand, C promotes the precipitation of Cr carbides, so if the C content is too high, intergranular corrosion occurs and corrosion resistance decreases. Therefore, from the viewpoint of suppressing a decrease in corrosion resistance, the C content is set to 0.100% or less. Furthermore, 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.

[0020] <Si: 0.05 to 1.50%> Si is an element used as a deoxidizing element and added to improve oxidation resistance. From the viewpoint of obtaining these effects, the Si content is set to 0.05% or more. Furthermore, 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 its toughness and workability decrease. Therefore, from the viewpoint of suppressing the decrease in toughness and workability, the Si content is set to 1.50% or less. Furthermore, 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.

[0021] <Mn: 0.05 to 2.00%> Mn has the effect of increasing the austenite phase and increasing the solid solubility of nitrogen, thereby suppressing bubble defects and the like that occur during manufacturing. From the viewpoint of achieving this effect, the Mn content is set to 0.05% or more. Furthermore, from the viewpoint of stably ensuring 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, corrosion resistance and hot workability decrease. Therefore, from the viewpoint of suppressing the decrease in corrosion resistance and hot workability, the Mn content is set to 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.

[0022] <Ni: 4.00 to 9.00%> Ni is an austenite-stabilizing element and has the effect of improving corrosion resistance by suppressing corrosion progression. In order to ensure a certain amount of austenite phase in a duplex stainless steel material, the Ni content is set to 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, since Ni is an expensive element, its content is preferably as low as possible. Therefore, the Ni content is set to 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.

[0023] <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. Since an excessively high P content reduces formability, the lower the P content, the better, and the P content is set to 0.050% or less. Furthermore, from the viewpoint of stably suppressing the reduction in 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 removing P requires very high refining costs. Therefore, the P content may be set to 0.001% or more in consideration of economic efficiency.

[0024] <S: 0.0040% or less> S is an element that is inevitably mixed into duplex stainless steel materials. It combines with Mn to form inclusions, which may become the starting point for rusting. Furthermore, the lower the S content, the better the corrosion resistance. Therefore, the S content is set to 0.0040% or less. Furthermore, 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 removing S requires very high refining costs. Therefore, the S content may be set to 0.0001% or more in terms of economic efficiency.

[0025] <Cr: 23.0 to 30.0%> Cr strengthens the passive film, thereby improving corrosion resistance against crude phosphoric acid. Cr also has the effect of repairing the passive film after Fe dissolution. From the viewpoint of obtaining these effects, the Cr content is set to 23.0% or more. Furthermore, from the viewpoint of stably ensuring these 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, the amount of chromium nitride precipitation increases, and the risk of hot work cracking increases, resulting in a decrease in toughness. From the viewpoint of suppressing these, the Cr content is set to 30.0% or less. From the viewpoint of stably suppressing these, 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.

[0026] <N: 0.100-0.250%> When N is dissolved, it becomes NH4 +(ammonium ions), which have the effect of increasing the pH of acidic condensation water and suppressing the dissolution of Fe. N is also an element that dissolves in the austenite phase to improve strength and corrosion resistance, thereby contributing to alloy saving. From the viewpoint of achieving these effects, the N content is set to 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. On the other hand, N is also an element that greatly affects the precipitation of chromium nitrides, and if contained in large amounts, the amount of chromium nitride precipitation increases, resulting in a decrease in toughness and corrosion resistance. From the viewpoint of suppressing these effects, the N content is set to 0.250% or less. From the viewpoint of stably suppressing these effects, the N content is preferably 0.230% or less or 0.220% or less.

[0027] <Cu: 0.01 to 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 it. From the viewpoint of achieving this effect, the Cu content is set to 0.01% or more. Furthermore, 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, a high Cu content increases raw material costs and deteriorates hot workability. From the viewpoint of suppressing these, the Cu content is set to 2.00% or less. From the viewpoint of stably suppressing these, the Cu content is preferably 1.80% or less, 1.50% or less, 1.00% or less, or 0.80% or less.

[0028] <Mo: 0.50 to 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 has the effect of suppressing corrosion (particularly blackening) due to the elution of Fe on the surface of the duplex stainless steel material. From the viewpoint of obtaining this effect, the Mo content is set to 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 keep its content as low as possible. Therefore, the Mo content is set to 2.50% or less. Furthermore, from the viewpoint of reducing costs, the Mo content is preferably 2.30% or less or 2.20% or less.

[0029] <Al: 0.100% or less> Al is an element contained for desulfurization and deoxidation. However, a large amount of Al increases raw material costs and manufacturing costs, so the Al content is set to 0.100% or less. Furthermore, from the viewpoint of stably reducing increases in raw material costs and manufacturing costs, the Al content is preferably 0.090% or less or 0.080% or less. On the other hand, although 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.

[0030] <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 the viewpoint of suppressing the decrease in workability, 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, although there is no particular limitation on the lower limit of the Nb content, from the viewpoint of obtaining the above effect, the Nb content is preferably 0.001% or more.

[0031] <Ti: 0.050% or less> Ti has the effect of preventing coarsening of the weld heat-affected zone when welding a duplex stainless steel material and of forming fine equiaxed crystals in the solidification structure, and therefore can be added as needed. However, if the Ti content is too high, uniform elongation and local elongation decrease. From the viewpoint of suppressing the decrease in uniform elongation and local elongation, the Ti content is set to 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, although there is no particular limitation on the lower limit of the Ti content, from the viewpoint of ensuring the above effect, the Ti content is preferably 0.001% or more.

[0032] <B: 0.0050% or less> B has the effect of improving hot workability and can be contained as needed. However, if the B content is too high, corrosion resistance will be significantly reduced. From the viewpoint of suppressing the reduction in corrosion resistance, the B content is set to 0.0050% or less. Furthermore, 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, although the lower limit of the B content is not particularly limited, from the viewpoint of obtaining the above effect, the B content is preferably 0.0003% or more.

[0033] <Ca: 0.0010 to 0.0100%> Ca is an element that enhances corrosion resistance. It is believed that Ca reduces inclusions that serve as corrosion initiation sites in an environment where crude phosphoric acid is present and suppresses the elution of Fe, and therefore can be added as needed. From the viewpoint of ensuring this effect, the Ca content is set to 0.0010% or more. Furthermore, from the viewpoint of stably ensuring this effect, the Ca content is preferably 0.0015% or more or 0.0020% or more. On the other hand, a high Ca content makes hot work cracking more likely to occur and also reduces corrosion resistance. From the viewpoint of suppressing these, the Ca content is set to 0.0100% or less. Furthermore, 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.

[0034] <Mg: 0.0001 to 0.0020%> Mg not only has the effect of deoxidizing but also of refining the solidified structure, and therefore can be added as necessary. However, a high Mg content increases costs in the steelmaking process, so the Mg content is set to 0.0020% or less. Furthermore, from the viewpoint of reducing costs, 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 is set to 0.0001% or more.

[0035] <Zr: 0.090% or less> Zr has the effect of forming carbides and nitrides in duplex stainless steel materials and refining crystal grains, and therefore can be added as needed. However, if the Zr content is high, the toughness decreases due to the excess carbides and nitrides formed. Therefore, from the viewpoint of suppressing the decrease in toughness, the Zr content is set to 0.090% or less. Furthermore, from the viewpoint of stably ensuring this effect, the Zr content is preferably 0.085% or less. On the other hand, although there is no particular limitation on the lower limit of the Zr content, from the viewpoint of obtaining the above-mentioned effect, the Zr content is preferably 0.001% or more or 0.005% or more.

[0036] <Co: 3.00% or less> Co, like N, is an austenite stabilizing element and can be contained as needed. However, since Co is an expensive element, the Co content is set to 3.00% or less. Furthermore, from the viewpoint of reducing costs, the Co content is preferably 2.80% or less or 2.60% or less. On the other hand, although there is no particular limitation on the lower limit of the Co content, from the viewpoint of obtaining the above-mentioned effects, the Co content is preferably 0.01% or more.

[0037] <V: 1.000% or less> V is an element that improves corrosion resistance, and therefore can be contained as needed. However, if the V content is too high, the load during rolling increases, making it easier for manufacturing defects to occur, so the V content is set to 1.000% or less. In addition, from the viewpoint of stably suppressing the occurrence of manufacturing defects, the V content is preferably 0.950% or less. On the other hand, although the lower limit of the V content is not particularly limited, from the viewpoint of obtaining the above-mentioned effects, the V content is preferably 0.010% or more or 0.050% or more.

[0038] <Ta: 0.200% or less> Ta forms carbides and nitrides in duplex stainless steel materials and has the effect of additionally enhancing corrosion resistance, so it can be added as needed. However, if the Ta content is high, the toughness decreases due to the excess carbides and nitrides formed, so the Ta content is set to 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, although there is no particular limitation on the lower limit of the Ta content, from the viewpoint of obtaining the above effect, the Ta content is preferably 0.001% or more or 0.005% or more.

[0039] <Sn: 0.100% or less> Sn is an element that improves corrosion resistance, so it can be contained as needed. However, if the Sn content is high, hot workability will decrease, so the Sn content is set to 0.100% or less. Furthermore, 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, although the lower limit of the Sn content is not particularly limited, from the viewpoint of obtaining the above effect, the Sn content is preferably 0.0001% or more or 0.0003% or more.

[0040] <O: 0.0050% or less> Like S, O is an element that affects hot workability, so it is desirable that its content be as low as possible. Therefore, the O content is set to 0.0050% or less. Furthermore, from the viewpoint of stably ensuring this effect, the O content is preferably 0.0048% or less. On the other hand, although the lower limit of the O content is not particularly limited, reducing the O content leads to an increase in manufacturing costs. Therefore, the O content is preferably 0.0001% or more.

[0041] <W: 1.000% or less> W is an element that improves corrosion resistance, and therefore may be contained as needed. However, if the W content is too high, the load during rolling increases, making it easier for manufacturing defects to occur, so the W content is set to 1.000% or less. In addition, 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, although the lower limit of the W content is not particularly limited, from the viewpoint of obtaining the above-mentioned effect, the W content is preferably 0.005% or more or 0.010% or more.

[0042] <REM: 0.100% or Less> REM (rare earth elements) have the effect of improving hot workability and can be added as needed. However, a high REM content impairs manufacturability and increases costs, so the REM content is set to 0.100% or less. Furthermore, to stably ensure this effect, the REM content is preferably 0.095% or less or 0.090% or less. While there is no particular lower limit for the REM content, to obtain the above-mentioned effect, the REM content is preferably 0.001% or more, 0.005% or more, or 0.010% or more. REM is a collective term for 17 elements, including Sc, Y, and 15 elements (lanthanoids) from La to Lu, and the REM content refers to the total content of these elements. These elements can be used alone or in combination of two or more. Furthermore, lanthanoids are industrially added in the form of misch metal.

[0043] In the duplex stainless steel material according to the embodiment of the present invention, the DF value, as expressed 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 content (mass%) of each element. Here, DF is an index representing the amount of ferrite phase. Therefore, 100 - DF represents the amount of austenite phase. However, it should be noted that because DF is an index determined based on the element content, it may not match the amount of austenite phase actually measured. If the DF value is less than 45.0, the strength will be excessively high, resulting in reduced manufacturability. On the other hand, if the DF value exceeds 70.0, the proportion of the austenite phase decreases, and the amount of N dissolved in the austenite phase decreases, resulting in an increase in the amount of chromium nitride precipitated, which reduces toughness and corrosion resistance.

[0044] In the duplex stainless steel material according to the embodiment of the present invention, it is preferable that the austenite phase satisfies the following relational expression (2): (0.8Cr) 2 +2Ni+7Mo+30Cu+30N≧340 (2) In formula (2), the element symbols represent the content (mass%) of each element. Formula (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 these elements in the austenite phase so as to satisfy the relationship of formula (2), the preferential dissolution of the austenite phase in crude phosphoric acid can be suppressed, thereby suppressing corrosion of the duplex stainless steel material. From the viewpoint of stably ensuring 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 may be, for example, 800 or 700.

[0045] 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 direction cross section of a duplex stainless steel material parallel to the rolling direction is used, and a qualitative analysis is performed by EPMA. Ni qualitative mapping of the entire cross section is performed to identify a portion that can be clearly determined to be an austenite phase based on the Ni content. Cr, Ni, Mo, Cu, and N are then 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 used as the content of each element.

[0046] The austenite phase preferably contains 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. By controlling the content of each element within these ranges, the relationship of formula (2) is more likely to be satisfied. In the austenite phase, the Cr content is preferably 22.0 to 25.5%, the Ni content is 8.0 to 12.0%, the Mo content is 1.20 to 1.80%, the Cu content is 0.4 to 0.9%, and the N content is 0.25 to 0.33%.

[0047] The duplex stainless steel material according to the embodiment of the present invention preferably satisfies the following relational expression (3): Expected CPT - Measured CPT ≦ 10°C (3) In the expression, 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, a CPT difference (expected CPT - measured CPT) of 10°C or less can be considered to indicate a small amount of chromium nitride precipitation. In other words, when the relational expression (3) is satisfied, precipitation of chromium nitrides is suppressed, thereby ensuring sufficient corrosion resistance. On the other hand, if the relational expression (3) is not satisfied, there is a risk that the corrosion resistance will decrease due to the precipitation of chromium nitrides.

[0048] 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 is preferably a hot-rolled material when used for storage tanks of chemical tankers, etc.

[0049] 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 in the form of a rod, the thickness refers to the equivalent circle diameter of the cross section. When the duplex stainless steel material is in the form of a shaped steel, the thickness refers to the thickness at any point on the cross section.

[0050] 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 components that may come into contact with crude phosphoric acid. Among these, the duplex stainless steel material according to the embodiment of the present invention is suitable for use in structures for crude phosphoric acid. Here, in this specification, "structures for crude phosphoric acid" refers to various facilities that handle crude phosphoric acid. Examples of structures for crude phosphoric acid include phosphoric acid manufacturing plants, phosphoric acid storage tanks (including storage tanks on ships), and phosphoric acid transport pipes.

[0051] The method for producing a duplex stainless steel material according to an 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-described characteristics. An example of a method for producing a duplex stainless steel material according to an embodiment of the present invention will be described below. The 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.

[0052] The soaking step is a step in which a rolled material having the above composition is soaked 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. Furthermore, 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. While the soaking time may be longer than one minute, it 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, a stainless steel having the above composition may be melted by vacuum melting to form a steel slab, which may then be hot-rolled.

[0053] 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 / s or more but less than 10°C / s. If the cooling rate in the first cooling step is 10°C / s or more, it becomes difficult to control the austenite phase composition 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 precipitation increases, and in some cases, the σ phase precipitates, resulting in reduced corrosion resistance. The cooling method in the first cooling step is not particularly limited, but air cooling, for example, may be used.

[0054] 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 quenching start temperature. The quenching start temperature is a temperature of 950 to 990°C, and is a temperature lower than the soaking temperature. If the quenching start temperature is less than 950°C, the amount of ferrite phase produced increases, resulting in reduced corrosion resistance. Furthermore, if the quenching start temperature exceeds 990°C, it becomes difficult to control the austenite phase composition 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 for the second cooling step is not particularly limited and may, for example, be water cooling.

[0055] The structure for producing crude phosphoric acid according to the embodiment of the present invention comprises the above-mentioned duplex stainless steel material. This structure for producing crude phosphoric acid may further comprise 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 producing 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.

[0056] The present invention will be described in detail below with reference to examples, but the present invention should not be construed as being limited to these examples.

[0057] Stainless steels with the compositions shown in Tables 1-1 and 1-2 were melted in MgO crucibles in a laboratory 50 kg vacuum induction furnace and cast into flat steel ingots approximately 100 mm thick. The main body of the flat steel ingot was processed into a hot-rolling material, heated to 1180°C, held at that temperature for 1 hour, and then hot-rolled into a 12 mm-thick hot-rolled sheet. In Tables 1-1 and 1-2, the DF and expected CPT values ​​were calculated based on the content of each element. Next, a soaking step, a first cooling step, and a second cooling step were performed under the conditions shown in Table 2 to obtain a duplex stainless steel sheet. 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.

[0058]

[0059]

[0060]

[0061] <Content of Each Element in the Austenite Phase> The content of each element in the austenite phase was measured using an electron probe microanalyzer (EPMA). Specifically, a mirror-polished sample of a thickness-wise cross section of a duplex stainless steel material parallel to the rolling direction was used for qualitative analysis using the EPMA. The EPMA was performed at an acceleration voltage of 15 kV, measuring a 300 μm × 300 μm area in a grid pattern with 0.76 μm intervals, resulting in a total of 140,625 data points. In the qualitative analysis, Ni qualitative mapping was performed on the entire cross section to identify areas that could be clearly determined to be austenite phase based on their Ni content. Then, Cr, Ni, Mo, Cu, and N were quantitatively analyzed at approximately the center of the identified austenite phase. The quantitative analysis was performed on 10 or more points, and the average values ​​were used to determine the content of each element. The obtained content of each element was used to calculate the value of the left side of the relational expression (2).

[0062] <Corrosion Test> A test piece measuring 10 mm x 80 mm x 2 mm was taken from a duplex stainless steel plate, and the entire surface was wet-polished using a No. 600 grinding stone. Then, the test piece was polished with a wet polishing agent containing 70% by mass of phosphoric acid, 0.4% by mass of F. - ions and 0.04% by mass of Cl - An aqueous solution simulating crude phosphoric acid containing ions was prepared. Next, a container containing this aqueous solution was kept at 50°C, and half of the test specimen was immersed in it for 6 hours. The corrosion rate (mm / y) was calculated from the mass change (corrosion weight loss) before and after immersion. A corrosion rate of 0.15 mm / y or less can be considered to be excellent corrosion resistance. Since the corrosion occurred in both the gas phase and the liquid phase, the corrosion rate was calculated using the entire area of ​​the test specimen. Furthermore, the test specimens after the above test were visually evaluated for discoloration (blackening).

[0063] <Measured CPT and CPT Difference> The measured CPT was determined in accordance with ASTM G48E by taking a 50 mm x 25 mm x 2 mm thick test piece (thickness from a position 1 mm deep from the outermost surface) from a duplex stainless steel plate, then performing a pitting test on this test piece at an aqueous solution temperature of 30°C, and measuring the CPT (pitting initiation temperature, critical pitting temperature). Note that when measuring the CPT, pitting that occurred on the end surface (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 - measured CPT) was calculated using the measured CPT thus obtained and the expected CPT calculated above. A CPT difference of 10°C or less was represented by ○, and a CPT difference of more than 10°C was represented by ×.

[0064] The results of the above evaluations are shown in Table 3.

[0065]

[0066] As shown in Table 3, in Nos. 1 to 26 (inventive examples), the duplex stainless steel plates had the specified composition and DF, and the austenite phase satisfied the relationship of formula (2). Therefore, it was confirmed that they had a slow corrosion rate and excellent corrosion resistance against crude phosphoric acid. In contrast, in No. 27 (comparative example), the soaking temperature was too high, so the element concentrations in the austenite phase could not be controlled, and the relationship of formula (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 the element concentrations in the austenite phase could not be controlled, and the relationship of formula (2) was not satisfied. As a result, the corrosion resistance against crude phosphoric acid was insufficient. In No. 29 (comparative example), the rapid cooling start temperature in the second cooling step was too high, so the element concentrations in the austenite phase could not be controlled, and the relationship of formula (2) was not satisfied. As a result, the corrosion resistance against crude phosphoric acid was insufficient. In No. 30 and No. 31 (Comparative Examples), the cooling rate in the first cooling step was too slow, making it impossible to control the element concentrations in the austenite phase, and thus the relationship in formula (2) was not satisfied. Furthermore, chromium nitrides, σ phases, and the like precipitated, resulting in insufficient corrosion resistance to crude phosphoric acid. No. 32 (Comparative Example) had a high DF value and an excessive amount of ferrite phase, resulting in a large amount of chromium nitride precipitation and insufficient corrosion resistance to crude phosphoric acid. No. 33 (Comparative Example) contained a large amount of Mn, which reduced acid resistance and resulted in insufficient corrosion resistance to crude phosphoric acid. No. 34 (Comparative Example) did not contain Cu, so its corrosion resistance to crude phosphoric acid was insufficient. No. 35 (Comparative Example) did not contain Nb, so its corrosion resistance to crude phosphoric acid was insufficient. No. 36 (Comparative Example) had a low DF value, resulting in an excess austenite phase, and therefore an increased amount of austenite phase that corroded preferentially, resulting in insufficient corrosion resistance to crude phosphoric acid. No. 37 (Comparative Example) had a high Cr content, resulting in significant precipitation of chromium nitrides, resulting in insufficient corrosion resistance to crude phosphoric acid. No. 38 (Comparative Example) had an insufficient Cr content, resulting in an inability to maintain a passive film in the crude phosphoric acid environment, resulting in insufficient corrosion resistance to crude phosphoric acid.No. 39 (Comparative Example) had too little Mo content, which meant that a passive film could not be maintained in a crude phosphoric acid environment, and as a result, the corrosion resistance to crude phosphoric acid was insufficient. No. 40 (Comparative Example) had too much N content, which meant that a large amount of chromium nitride precipitated in the ferrite phase, and therefore the corrosion resistance to crude phosphoric acid was insufficient. No. 41 (Comparative Example) had too little N content, which meant that NH in a crude phosphoric acid environment was not maintained. 4+ As a result, the corrosion resistance to crude phosphoric acid was insufficient.

[0067] As can be seen from the above results, the present invention can provide a ferritic-austenitic duplex stainless steel material having excellent corrosion resistance to crude phosphoric acid and a method for producing the same. 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.

[0068] Furthermore, based on the above results, the present invention can be embodied in the following aspects.

[0069] (Aspect 1) A composition 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, and is expressed by 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 the element symbols represent the content (mass%) of each element) is 45.0 to 70.0, and the austenite phase is represented by the following formula (2): (0.8Cr) 2 +2Ni+7Mo+30Cu+30N≧340 (2) (wherein the element symbols represent the content (mass%) of each element).

[0070] (Aspect 2) The ferritic-austenitic duplex stainless steel material according to Aspect 1, further comprising, by mass, 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.

[0071] (Aspect 3) A ferritic-austenitic duplex stainless steel material according to Aspect 1 or Aspect 2, which satisfies the following relational expression (3): Expected CPT - Measured CPT ≦ 10°C (3), where expected CPT is the pitting corrosion initiation temperature (°C) calculated according to 2.5Cr + 7.6Mo + 3.19N - 26 (where Cr, Mo, and N represent the Cr, Mo, and N contents (mass%) in the ferritic-austenitic duplex stainless steel material, respectively), and measured CPT is the pitting corrosion initiation temperature (°C) measured by a pitting corrosion resistance test in accordance with ASTM G48E.

[0072] (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.

[0073] (Embodiment 5) The ferritic-austenitic duplex stainless steel material according to any one of embodiments 1 to 4, used in a structure for crude phosphoric acid.

[0074] (Aspect 6) 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. A soaking step in which a rolled material is soaked at a soaking temperature of 950 to 1150 ° C. for more than 1 minute; and A first cooling step in which the rolled material obtained in the soaking step is slowly cooled to a quenching start temperature at a cooling rate of 5 ° C. / second or more but less than 10 ° C. / second. a second cooling step of rapidly cooling the rolled material obtained in the first cooling step from the rapid cooling start temperature, wherein the rapid cooling start temperature is a temperature of 950 to 990°C and is a temperature lower than the soaking temperature.

[0075] (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 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.

[0076] (Embodiment 8) A structure for crude phosphoric acid, comprising the ferritic-austenitic duplex stainless steel material according to any one of embodiments 1 to 5.

[0077] (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

1. A composition 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, and 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 the element symbols represent the contents (mass%) of each element) is 45.0 to 70.0, and the austenite phase is represented by the following formula (2): (0.8Cr) 2 +2Ni+7Mo+30Cu+30N≧340 (2) (wherein the element symbols represent the content (mass%) of each element).

2. The ferritic-austenitic duplex stainless steel material according to claim 1, further comprising, by mass, 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.

3. The ferritic-austenitic duplex stainless steel material according to claim 1 or 2, which satisfies the following relational expression (3): Expected CPT - Measured CPT ≦ 10°C ... (3) In the expression, 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 measured CPT is the pitting corrosion initiation temperature (°C) measured by a pitting corrosion resistance test in accordance with ASTM G48E.

4. The ferritic-austenitic duplex stainless steel material according to claim 1 or 2, 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.

5. The ferritic-austenitic duplex stainless steel material according to claim 1 or 2, which is used for a structure for crude phosphoric acid.

6. A soaking step of soaking a rolled material containing, by mass, 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, at a soaking temperature of 950-1150°C for more than one minute; and 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 quenching the rolled material obtained in the first cooling step from the quenching start temperature, wherein the quenching start temperature is a temperature of 950 to 990°C and is a temperature lower than the soaking temperature.

7. A method for producing a ferritic-austenitic duplex stainless steel material as set forth in 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 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.

8. A structure for crude phosphoric acid comprising the ferritic-austenitic duplex stainless steel material according to claim 1 or 2.

9. 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.

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