Duplex stainless steel and its use

Abstract

This disclosure relates to a corrosion-resistant duplex stainless steel (ferritic-austenite alloy) suitable for use in a urea manufacturing plant, and its use. This disclosure also relates to objects made of said duplex stainless steel. Furthermore, this disclosure also relates to a method for manufacturing urea, a urea manufacturing plant including one or more components made of said duplex stainless steel, and a method for modifying an existing urea manufacturing plant.

Description

Technical Field

[0001] The present disclosure relates to a corrosion-resistant duplex stainless steel (ferrite-austenite alloy) suitable for use, for example, in a plant for urea production. The present disclosure also relates to an object of the duplex stainless steel, and more particularly, to a bar, a formed object made from the bar, or a forging.

Background Art

[0002] Duplex stainless steel refers to a ferrite-austenite alloy. Such an alloy has a microstructure including a ferrite phase and an austenite phase. Examples of background documents regarding this point include, for example, International Publication No. 95 / 00674, U.S. Patent No. 7,347,903, and International Publication No. 2017 / 013180. The duplex stainless steels described in these documents have high corrosion resistance and can therefore be used even in highly corrosive environments such as, for example, urea production plants.

[0003] However, although the above-disclosed duplex stainless steels are useful for most applications, like all duplex stainless steels, they have limitations in the applications in which they can be used due to the transition temperature. The transition temperature is the temperature at which a material changes from ductility to brittleness. This means that these steels are not suitable for specific applications because they do not meet the ductility criteria. Therefore, there is still a need for duplex stainless steels with higher impact toughness, that is, higher strength and ductility, in order to avoid brittle fracture defined by the requirements for impact strength at the minimum design temperature.

[0004] In addition, the stability of the microstructure of duplex stainless steel depends on the composition, and in order to ensure appropriate corrosion resistance and sufficient mechanical properties, it is important to have a material with a stable microstructure. Therefore, there is still a need for duplex stainless steels with a stable microstructure.

[0005] Furthermore, there is still a need for further improvements to duplex stainless steel materials used in components exposed to low temperatures and high pressures, such as high-pressure vessels, in urea production plants, for example. [Overview of the Initiative]

[0006] Accordingly, one aspect of the present disclosure is to provide a corrosion-resistant duplex stainless steel with improved impact toughness, particularly when used in or exposed to low-temperature, high-pressure environments.

[0007] Another aspect of this disclosure is to provide a duplex stainless steel with improved structural stability.

[0008] Another aspect of this disclosure is to provide a duplex stainless steel having a transition temperature that makes the steel ductile even at low temperatures.

[0009] To address one or more of the aforementioned aspects, this disclosure provides: C 0.030 or less; Si 0.5 or less; Mn 1.5 or less; Cr over 28.0 to 29.0; Ni 7.1 to 7.8; Mo 2.0 to less than 3.0; N 0.30 to 0.36; Cu 1.0 or less; S 0.02 or less; P 0.03 or less; The remainder is Fe and inevitably occurring impurities. We provide duplex stainless steel including [specific component].

[0010] Through thorough research, the inventors have discovered a specific duplex stainless steel composition that provides high impact toughness at low design temperatures, i.e., below approximately 20°C. The duplex stainless steel of the present invention will also have improved ductility and improved structural stability. Furthermore, the duplex stainless steel of the present invention provides that objects made from this stainless steel have adequate corrosion resistance and sufficient mechanical properties. These aforementioned properties are particularly useful when objects made from the duplex stainless steel are used, for example, in applications exposed to high pressure and low temperatures.

[0011] In addition, this disclosure relates to duplex stainless steel objects as defined above or below and their uses. This disclosure also relates to duplex stainless steel bars as defined above or below. Examples of objects include machined parts, pressure bearing components, large diameter pipes or tubes or plates. Additional examples of objects include, but are not limited to, parts, e.g., molded or machined parts; valves, e.g., valve bodies or ejectors; tubes or pipes, e.g., seamless tubes or large diameter pipes; pressurized components, e.g., containers, tubes or bearing components; or plates. Objects may be manufactured from the duplex stainless steel bars of the present invention or forged.

[0012] In addition, this disclosure relates to the use of duplex stainless steel as defined above or below in a carbamate environment.

[0013] The disclosure also relates to a method for producing urea, wherein at least part of the apparatus is made from a duplex stainless steel object as defined above or below, and to a urea production plant comprising one or more components including duplex stainless steel as defined above or below.

[0014] Furthermore, the disclosure also provides a method for modifying an existing urea production plant, and a method for reducing the risk of undesirable precipitate formation during the manufacture of pressure bearing components by using parts or objects made of duplex stainless steel as defined above or below. [Modes for carrying out the invention]

[0015] Therefore, this disclosure relates to duplex stainless steel, in weight % (wt%), C 0.030 or less; Si 0.5 or less; Mn 1.5 or less; Cr over 28.0 to 29.0; Ni 7.1 to 7.8; Mo 2.0 to less than 3.0; N 0.30 to 0.36; Cu 1.0 or less; S 0.02 or less; P 0.03 or less; The remainder is Fe and inevitably occurring impurities. This relates to duplex stainless steel, including...

[0016] Broadly speaking, this disclosure is based on the prudent insight that better impact toughness and structural stability can be obtained by using duplex stainless steel as defined above or below. This is particularly important in applications and / or areas where the material is exposed to high pressure and low temperatures. Therefore, the duplex stainless steel as defined above or below is particularly useful in the manufacture of objects that require high impact strength at low temperatures (below about 20°C), such as pressurized components and / or pressurized vessels, or valves in piping systems, for example.

[0017] Although the super duplex stainless steel described in the above literature has good impact toughness, there is room for improvement in its impact toughness, especially in large parts with a particularly coarse microstructure, especially at the minimum design temperature of the object. The duplex stainless steel defined above or below exhibits significantly high impact toughness and structural stability at low temperatures. Therefore, one of the advantages of the duplex stainless steel of the present invention is to expand the design possibilities, enable the reduction of the design temperature, and improve the availability and productivity of large parts. The inventors have made a surprising discovery that by manufacturing an object such as a pressure vessel, a pressure tube, or a pressure pipe from a bar of the duplex stainless steel defined above or below, the impact toughness of the object is dramatically improved. Furthermore, the structural stability is also improved, and the duplex stainless steel of the present invention will be less sensitive to the formation of unnecessary chromium nitride during the manufacture of the material, especially during the manufacture of the object. The reduction of the formation of unnecessary chromium nitride means that the object of the duplex stainless steel of the present invention can be used in low-temperature and high-pressure applications. According to an embodiment, in order to more surely satisfy the above aspects and especially to ensure that the impact toughness is sufficiently high, the duplex stainless steel of the present invention can also satisfy the following requirements: 28.4 - [Cr] - 29 * [N] + 0.44 * [Ni] - 1.66 * [Si] + 0.027 * [Cr] 2 *[N] is greater than 0 (1) [where the values of each element are given in wt%].

[0018] <{ Furthermore, in order to more surely satisfy the above aspects, for example, to ensure that the impact toughness is sufficiently high, the duplex stainless steel of the present invention can also satisfy the following requirements:[[ID=]11] [Cr] * [N] is less than 10, [where the values of each element are given in wt%]. The above requirements ensure that the content of chromium nitride in the matrix is low, which is an advantage because these particles are present in the ferrite phase and the phase boundary and reduce the impact toughness. Therefore, a high impact toughness is ensured by having a low content of chromium nitride or substantially no chromium nitride.

[0019] The present disclosure also relates to the use of the duplex stainless steel defined above or below in a carbamate environment, such as an ammonium carbamate environment.

[0020] Therefore, the present disclosure relates to the use of the duplex stainless steel defined above or below in a carbamate environment, such as an ammonium carbamate environment.

[0021] In addition, the duplex stainless steel of the present invention defined above or below will have sufficient ductility to avoid brittle fracture when tested at a temperature below the minimum design temperature, which is the recommended minimum service temperature, and is therefore very suitable for use as a building material.

[0022] In the following, the importance of different alloying elements of duplex stainless steel will be briefly discussed. All percentages of chemical composition are given in weight % (wt%) unless otherwise specified. The upper and lower limits of the individual elements of the composition can be freely combined within the broadest range set in the claims unless otherwise specified.

[0023] Carbon (C) is an element that should preferably be avoided, but it cannot be avoided in order to prevent the generation of scrap. Therefore, the C content needs to be kept as low as possible because its solubility is limited in both the ferrite and austenite phases. This limited solubility means that if the precipitation rate of carbides is too high, there is a risk of a resulting decrease in corrosion resistance. Therefore, the C content needs to be limited to 0.030 wt% or less, such as 0.020 wt% or less, such as 0.017 wt% or less, such as 0.015 wt% or less, such as 0.010 wt% or less. According to an embodiment, the content can be from 0.005 to 0.030 wt%, such as from 0.005 to 0.020 wt%.

[0024] Silicon (Si) is used as a deoxidizing additive in steel production. However, if the Si content is too high, the tendency for intermetallic phase precipitation increases, and the solubility of nitrogen decreases. Surprisingly, Si has been shown to have a significant impact on impact toughness, and therefore should be kept as low as possible. For this reason, the Si content needs to be limited to 0.5% by weight or less. According to the embodiment, the Si content is in the range of 0.05 to 0.5% by weight, for example, in the range of 0.1 to 0.4% by weight.

[0025] Manganese (Mn) is added to increase the solubility of nitrogen and to substitute for nickel as an alloying element, as Mn is thought to stabilize austenite. However, its content should be 1.5% by weight or less, as it may adversely affect structural stability. According to the embodiment, the Mn content is in the range of 0.5 to 1.5% by weight.

[0026] Chromium (Cr) is the most effective element for obtaining resistance to most types of corrosion. Since Cr content is crucial for corrosion resistance, it should be as high as possible. However, there is a balance between high chromium content and good structural stability. Surprisingly, the inventors found that ductility improved by reducing the Cr content as much as possible. Furthermore, the Cr content cannot be too low, as it would negatively impact corrosion resistance. Therefore, the inventors found a range of Cr content that balances excellent impact toughness with good corrosion resistance.

[0027] Therefore, in order to achieve sufficient corrosion resistance and ensure structural stability, the chromium content must be in the range of more than 28.0% to 29.0% by weight. According to the embodiments, the chromium content is greater than 28.00%. According to the embodiments, the chromium content is less than 29.00% by weight.

[0028] Nickel (Ni) is primarily used as an austenite-stabilizing element. The advantage of using Ni is that it does not negatively affect structural stability. A Ni content of at least 7.1% by weight is necessary to ensure structural stability, as chromium nitride may form during heat treatment if the Ni content is less than 7.1% by weight. However, since Ni can form strong complexes with ammonium, the Ni content should be kept as low as possible. Therefore, the Ni content is typically in the range of 7.1 to 8.0% by weight, for example, 7.1 to 7.8% by weight.

[0029] Molybdenum (Mo) is used to improve the passivation of duplex stainless steel, so the Mo content should be 2.0% by weight or more. However, if the Mo content is too high, there is a risk of intermetallic phase precipitation. Therefore, the Mo content should be 3.0% by weight or less. To obtain the best possible corrosion resistance, the Mo content needs to be as high as possible while avoiding excessive susceptibility to the sigma phase. If the Mo content exceeds 3.0% by weight, the driving force of the sigma phase becomes very high, making it difficult to manufacture parts without the sigma phase. According to one embodiment, the Mo content is in the range of 2.1 to 2.7% by weight.

[0030] Nitrogen (N) is a powerful austenite-forming agent and promotes the reconstruction of austenite. In addition, N influences the distribution of Cr, Mo, and Ni in the austenite and ferrite phases. Therefore, the higher the N content, the higher the relative proportion of Cr and Mo in the austenite phase. This means that the corrosion resistance of the austenite is improved and that high content of Cr and Mo can be incorporated into duplex stainless steel while maintaining structural stability. The inventors have found that the N content needs to be at least 0.32 wt% to have good structural stability and ductility. However, there is a limit to the solubility of nitrogen, and if the nitrogen level is too high, the risk of chromium nitride formation increases, which will affect corrosion resistance. Therefore, the N content should be between 0.32 and 0.36 wt%.

[0031] Copper (Cu) is an optional element in this disclosure and, if included, improves general corrosion resistance in acidic environments such as sulfuric acid. However, high Cu content will reduce pitting and crevice corrosion resistance. Therefore, the Cu content should be limited to 1.0% by weight or less, for example, 0.8% by weight or less, for example, 0.4% by weight or less. According to embodiments, the Cu content is between 0.01 and 0.8% by weight, for example, 0.01 and 0.4% by weight.

[0032] Sulfur (S) negatively affects corrosion resistance by forming easily soluble sulfides. Therefore, the S content should be limited to 0.02% by weight or less, for example, 0.01% by weight or less.

[0033] Phosphorus (P) is a common impurity element. When present in amounts exceeding approximately 0.03% by weight, it can adversely affect properties such as hot ductility, weldability, and corrosion resistance. The amount of P in alloys should be limited to 0.03% by weight or less, for example, 0.02% by weight or less.

[0034] The ferrite content of the duplex stainless steel according to this disclosure is important for corrosion resistance. Therefore, the ferrite content is preferably in the range of, for example, 30 to 70 volume%, for example, 35 to 65 volume%, or for example, 40 to 60 volume%.

[0035] In addition, duplex stainless steel as defined above or below may optionally contain other elements during the manufacturing process to improve workability, such as hot workability and machinability. Examples of such elements include, but are not limited to, calcium (Ca), aluminum (Al), cerium (Ce), and boron (B). When these elements are added, they are added in a total amount of a maximum of 0.5% by weight. The alloy as defined above or below may optionally contain, for example, defined amounts of defined elements C, Si, Mn, Cr, Ni, Mo, N, Cu, S, and P, with the remainder being Fe and unavoidable impurities, and further may contain the defined elements in the amounts described above, and further contain up to 0.5% by weight of optional additive elements such as Ca, Al, Ce, and B, which are added for workability, with the remainder being Fe and unavoidable impurities.

[0036] In this disclosure, commonly present impurities are considered to be impurities originating from the manufacturing process and / or the scrap materials used. Generally, duplex stainless steel contains a maximum of approximately 1% by weight, for example, a maximum of 0.8% by weight, of commonly present impurities. Examples of impurities include titanium (Ti), niobium (Nb), hafnium (Hf), barium (Ba), vanadium (V), cobalt (Co), and tungsten (W). The permissible content will differ depending on which of these impurity elements is present in the steel. For example, W and Co may be present up to 1.0% by weight as impurities from the use of scrap without affecting the properties of the steel, while the permissible content of Ti, Hf, and V may be, for example, 0.1% by weight or less each.

[0037] Duplex stainless steel as defined above or below can be manufactured by conventional methods, namely melting, casting, subsequent hot working and / or cold working, and optional additional heat treatments. Examples of hot working include forging and hot rolling.

[0038] This disclosure also relates to formed objects comprising duplex stainless steel as described above or below.

[0039] According to one embodiment, the duplex stainless steel object of the present invention can be manufactured using ingot casting. According to another embodiment, the object may be a bar that can be manufactured by forging or rolling.

[0040] This disclosure also relates to the use of duplex stainless steel as defined above or below in a urea synthesis process in any one of the embodiments described above or below. This use of duplex stainless steel as defined above or below is for the purpose of improving the impact toughness of one or more components of the apparatus used in the process, for example, one or more components of a high-pressure urea synthesis section, for example, components that come into contact with a carbamate solution.

[0041] The disclosure also relates to a plant for producing urea, the plant comprising one or more components comprising duplex stainless steel as defined above or below. According to one embodiment, one or more pressurized components comprises or are manufactured from duplex stainless steel as defined above or below. The duplex stainless steel can be used in a method of modifying an existing urea production plant, the plant comprising one or more components selected from the group of valves, valve bodies, ejectors, and high-pressure piping, the method characterized by replacing one or more components with components comprising duplex stainless steel as defined above or below.

[0042] This disclosure also relates to the use of duplex stainless steel in a carbamate environment as defined above or below.

[0043] This disclosure also relates to the use of duplex stainless steel, as defined above or below, in a urea synthesis process to improve the impact toughness of one or more components of a high-pressure urea synthesis section that come into contact with an ammonium carbamate solution.

[0044] The disclosure also relates to a method for producing urea, wherein at least one component of the apparatus is made from duplex stainless steel as defined above or below, the method preferably comprises forming ammonium carbamate and dehydrating the ammonium carbamate to obtain urea.

[0045] This disclosure also relates to a plant for the production of urea, the plant comprising one or more components including duplex stainless steel as defined above or below.

[0046] The disclosure also relates to a method for modifying an existing urea production plant, the plant comprising one or more components selected from a group of valves, e.g., valve bodies or ejectors; tubes and pipes, e.g., seamless tubes or large-diameter pipes; pressurized components, e.g., containers, tubes or bearing components; or plates, the method characterized in that one or more of these objects are replaced with corresponding objects or modified objects comprising duplex stainless steel as defined above or below.

[0047] The disclosure also relates to a method for modifying an existing urea production plant, the plant comprising one or more components, the method characterized by replacing one or more pressurized components or containers with components or containers comprising duplex stainless steel as defined above or below.

[0048] Duplex stainless steel, as defined above or below, can also be used in other applications of equipment where good corrosion resistance is required. Some examples of possible applications of duplex stainless steel include use as a component material for process chemical parts intended for use in nitric acid environments, melamine production, the paper and pulp industry, for example, in white liquor environments. This steel can be used, for example, in the manufacture of seamless tubes, welded tubes, flanges, couplings, and sheet metal.

[0049] This disclosure is further illustrated by the following non-limiting embodiments. [Examples]

[0050] The example compositions were obtained from full-scale heat production, manufactured by melting in an electric arc furnace (EAF), followed by argon oxygen decarburization (AOD) refining and ingot casting. The chemical compositions are given in Table 1. All samples had identical final dimensions and were produced using a similar manufacturing route involving hot working followed by solution treatment and annealing by rapid quenching in water.

[0051] Impact toughness was evaluated at 20°C and 0°C in accordance with ISO 148-1. Full-scale test specimens (10 × 10 × 55 mm) with V-notches were taken from the half-radius in accordance with ISO 377. The lateral direction was tested.

[0052] The claimed composition improves the impact toughness predicted by the standard. TIFF2026522726000001.tif135170

[0053] As shown in Table 1, all heats in this invention have high average impact toughness at both 0°C and 20°C and also satisfy the requirements of this disclosure.

Claims

1. Duplex stainless steel, in weight % (wt%), the following: C 0.030 or less; Si 0.5 or less; Mn 1.5 or less; Cr over 28.0 to 29.0; Ni 7.1 to 7.8; Mo 2.0 to less than 3.0; N 0.30 to 0.36; Cu 1.0 or less; S 0.02 or less; P 0.03 or less; The remainder is Fe and inevitably occurring impurities. Duplex stainless steel, including

2. The aforementioned duplex stainless steel, 28.4-[Cr]-29*[N]+0.44*[Ni]-1.66*[Si]+0.027*[Cr] ２ * [N] is greater than 0 (1) [In the formula, the values ​​of each element are given in weight percent.] The duplex stainless steel according to claim 1, which satisfies the requirements.

3. The aforementioned duplex stainless steel, (2) [Cr] * [N] is less than 10 [In the formula, the values ​​of each element are given in weight percent.] A duplex stainless steel according to claim 1 or claim 2 that satisfies the requirements.

4. Duplex stainless steel according to any one of claims 1 to 3, wherein the carbon content is 0.020% by weight or less.

5. A duplex stainless steel according to any one of claims 1 to 4, wherein the Si content is in the range of 0.05 to 0.5% by weight.

6. A duplex stainless steel according to any one of claims 1 to 5, wherein the Si content is in the range of 0.1 to 0.4% by weight.

7. A duplex stainless steel according to any one of claims 1 to 6, wherein the Mn content is in the range of 0.5 to 1.5% by weight.

8. A duplex stainless steel according to any one of claims 1 to 7, wherein the Cr content exceeds 28.

00.

9. Duplex stainless steel according to any one of claims 1 to 8, wherein the Mo content is in the range of 2.1 to 2.7% by weight.

10. Duplex stainless steel according to any one of claims 1 to 9, wherein the Cu content is in the range of 0.8% by weight or less, for example, 0.4% by weight or less.

11. An object comprising duplex stainless steel according to any one of claims 1 to 10.

12. The duplex stainless steel or object according to any one of claims 1 to 10 or claim 11, wherein the duplex stainless steel or object has an average impact toughness of at least 50 J at 0°C according to ISO 148-1.

13. The object according to claim 11 or claim 12, wherein the object is selected from a valve, for example, a valve body or ejector; a tube and pipe, for example, a seamless tube or a large-diameter pipe; a pressurized component, for example, a container, tube or bearing component; or a plate.

14. Use of the object according to any one of claims 11 to 13 in a carbamate environment.

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