A duplex stainless steel and use thereof

Abstract

The present disclosure relates to a corrosion resistant duplex stainless steel (ferritic austenitic alloy) which is suitable for use in a plant for the production of urea; and uses thereof. The disclosure also relates to objects made of said duplex stainless steel. Furthermore, the present disclosure also relates to a method for the production of urea and to a plant for the production of urea comprising one or more parts made from said duplex stainless steel, and to a method of modifying an existing plant for the production of urea.

Description

[0001] A DUPLEX STAINLESS STEEL AND USE THEREOF

[0002] Technical field

[0003] The present disclosure relates to a corrosion resistant duplex stainless steel (ferritic austenitic alloy) suitable for use for example in a plant for the production of urea. The disclosure also relates to an object of said duplex stainless steel, more specifically to a bar or a formed object made of said bar or to a forged object.

[0004] Duplex stainless steel refers to ferritic-austenitic alloy. Such alloys have a microstructure comprising ferritic and austenitic phases. Background references in this respect include for example WO 95 / 00674 and US 7,347,903 and WO 2017 / 013180. The duplex stainless steels described therein are highly corrosion resistant and can therefore be used, e.g., in the highly corrosive environment of a urea manufacturing plant.

[0005] However, even though the duplex stainless steels disclosed above work for most applications, they have, as all duplex stainless steels, limitations in which applications they can be used due to their transition temperature. The transition temperature is the temperature where the material will change from ductile to brittle. This means that for certain applications these steels are not suitable as they do not fulfill the criteria of ductility. Hence, there is still a need for a duplex stainless steel having higher impact toughness, i.e., higher strength and ductility, in order to avoid brittle fracture as per definition by the requirement of impact strength at the minimum design temperature.

[0006] Additionally, the microstructural stability of the duplex stainless steel is dependent on the composition, and it is important to have a material with a stable microstructure in order to assure proper corrosion resistance as well as sufficient mechanical properties. Thus, there is thus still a need for duplex stainless steels having a stable microstructure.

[0007] Further, there still exists a need for a further improvement of the duplex stainless steel materials used in for example the plants for the production of urea, for example for parts which are exposed to high pressure at low temperatures, such as high pressure vessels.

[0008] Summary of the disclosure

[0009] An aspect of the present disclosure is therefore to provide a corrosion resistant duplex stainless steel having an improved impact toughness, especially when used in an environment of high pressures at low temperatures or exposed to said conditions.

[0010] Another aspect of the present disclosure is to provide a duplex stainless steel with improved structure stability.

[0011] Another aspect of the present disclosure is to provide a duplex stainless steel which will have a transitional temperature which will allow the steel to be ductile at low temperatures.

[0012] In order to address one or more of the foregoing aspects, the present disclosure provides a duplex stainless steel comprising

[0013] C < 0.030;

[0014] Si < 0.5;

[0015] Mn <1.5;

[0016] Cr more than 28.0 to 29.0;

[0017] Ni 7.1 to 7.8;

[0018] Mo 2.0 to less than 3.0;

[0019] N 0.30 to 0.36;

[0020] Cu < 1.0;

[0021] S < 0.02;

[0022] P < 0.03; balance Fe and unavoidable occurring impurities.

[0023] The present inventors have by thorough research found a specific duplex stainless steel composition which will provide for a high impact toughness at low design temperatures, i.e., temperatures less than about 20 °C. The present duplex stainless steel will additionally have improved ductility and improved structure stability. Furthermore, the present duplex stainless steel will provide for that an object of said stainless steel will have proper corrosion resistance as well as sufficient mechanical properties. These before-mentioned properties are very useful when an object of said duplex stainless steel is to be used in applications where it is for example exposed to high pressures and low temperatures.

[0024] Additionally, the present disclosure relates to an object of the hereinabove or hereinafter defined duplex stainless steel and to the use thereof. The present disclosure also relates to a bar of the hereinabove or hereinafter defined duplex stainless steel. Examples of an object is a formed object, such as a machined component, a pressure bearing component, a large pipe or a tube or a plate. Additional examples but not limited of an object are a part, such as a formed or a machined component; a valve, such as a valve body or an ejector; a tube or a pipe, such as a seamless tube or a large pipe; a pressurized component such as a vessel, a tube or a bearing component; or a plate. The object may be manufactured from a bar of the present duplex stainless steel or may be forged.

[0025] Additionally, the present disclosure relates to the use of the duplex stainless steel as defined hereinabove or hereinafter in carbamate environment.

[0026] The present disclosure also relates to a method for producing urea wherein at least one part of the equipment is made from an object of a duplex stainless steel as defined hereinabove or hereinafter and to a plant for the production of urea comprising one or more parts comprising a duplex stainless steel as defined hereinabove or hereinafter.

[0027] Further, the present disclosure also provides a method of modifying an existing plant for the production of urea and a method for reducing the risk of forming unwanted precipitates during fabrication of pressure bearing parts by using a part or an object made from the duplex stainless steel as defined the hereinabove or hereinafter. Detailed description

[0028] Hence, the present disclosure relates to a duplex stainless steel comprising in weight% (wt%):

[0029] C < 0.030;

[0030] Si < 0.5;

[0031] Mn <1.5;

[0032] Cr more than 28.0 to 29.0;

[0033] Ni 7.1 to 7.8;

[0034] Mo 2.0 to less than 3.0;

[0035] N 0.30 to 0.36;

[0036] Cu < 1.0;

[0037] S < 0.02;

[0038] P < 0.03; balance Fe and unavoidable occurring impurities.

[0039] In a broad sense, the present disclosure is based on the judicious insight that an even better impact toughness and structural stability is obtained with the duplex stainless steel as defined hereinabove or hereinafter. This is specifically important for applications and / or areas where materials are exposed to high pressure and low temperature. Thus, said duplex stainless steel as defined hereinabove or hereinafter is especially useful for manufacturing an object which is in need of high impact strength at low temperature (less than about 20 °C), such as for example a pressurized part and / or a pressurized vessel, or for example a valve in a piping system.

[0040] Even though the super duplex stainless steel as described in the documents mentioned above has good impact toughness, the impact toughness, especially for large components with coarse microstructure, leaves room for improvement, especially at the minimum design temperature for an object. The duplex stainless steel as defined hereinabove or hereinafter shows remarkably high impact toughness and structural stability at low temperatures. Thus, one of the advantages of the present duplex stainless steel is that it will provide for better design possibilities and will enable low design temperatures, increase availability and producibility of large components. The inventors have come to the surprising finding that by manufacturing an object such as a pressurized vessel or a pressurized tube or a pressurized pipe from a bar of the duplex stainless steel as defined hereinabove or hereinafter, the impact toughness of the object will be drastically improved. Further, the structure stability will also be improved, making the present duplex stainless steel less sensitive to form unwanted chromium nitrides during production of the material and especially during the fabrication of an object. The reduced formation of unwanted chromium nitrides means that an object of the present duplex stainless steel will be able to be used in applications of low temperature and high pressure. According to embodiments, and in order to further ensure that the aspects mentioned above are fulfilled, especially ensuring that the impact toughness is high enough, the present duplex stainless steel may also fulfill the following requirement: 28.4-[Cr]-29*[N]+0.44*[Ni]-1.66*[Si]+0.027*[Cr]2*[N] is greater than 0 (1). wherein the values of the elements are given in weight%.

[0041] Furthermore, in order to further ensure that the aspects mentioned above are fulfilled, such as to ensure that the impact toughness is high enough, the present duplex stainless steel may also fulfill the following requirement: [Cr]*[N] is less than 10, wherein the values of the elements are given in weight%.The above requirements will ensure that the content of chromium nitrides will be low in the matrix and this is an advantage as these particles will reduce the impact toughness as they will be found in the ferrite phase and in the phase boarders. Thus, having a low content or essentially no content at all of chromium nitrides will ensure a high impact toughness.

[0042] The present disclosure also relates to the use in carbamate environment, such as ammonium carbamate environment, of the duplex stainless steel as defined hereinabove or hereinafter.

[0043] Thus, the present disclosure relates to the use in carbamate environment, such as ammonium carbamate environment, of the duplex stainless steel as defined hereinabove or hereinafter.

[0044] Additionally, the present duplex stainless steel as defined hereinabove or hereinafter will be very suitable for being used as construction material as it will be be ductile enough to avoid brittle fracture, when tested at a temperature not warmer than the minimum design temperature which is the recommended lowest temperature for use.

[0045] In the following, the importance of the different alloying elements of the duplex stainless steel will be briefly discussed. All percentages for the chemical composition are given in weight-% (wt-%), unless explicitly disclosed otherwise. Upper and lower limits of the individual elements of the composition can be freely combined within the broadest limits set out in the claims, unless explicitly disclosed otherwise.

[0046] Carbon (C) is an element which preferable should be avoided but due to scrap it is not possible. Thus, the content of C should be as low as possible as it has a limited solubility in both ferrite and austenite phase. This limited solubility implies that there is a risk for carbide precipitations at too high percentages with a decreased corrosion resistance as a consequence. Therefore, the C-content should be restricted to < 0.030 wt%, such as < 0.020 wt%, such as < 0.017 wt%, such as < 0.015 wt%, such as < 0.010 wt%. According to embodiments, the content of may be 0.005 to 0.0030 wt%, such as 0.005 to 0.020 wt%

[0047] Silicon (Si) is used as a deoxidation additive in steel manufacture. However, too high Si content increases the tendency for precipitations of intermetallic phases and decreases the solubility of N. It has been shown that Si surprisingly has a great impact on the impact toughness and should therefore be as low as possible. For this reason, the Si content should be restricted to < 0.5 wt%. According to embodiments, the content of Si is in the range of 0.05 to 0.5 wt%, such as in the range of 0.1 to 0.4 wt%.

[0048] Manganese (Mn) is added to increase the solubility of N and for replacing Ni as an alloying element as Mn is considered to be austenite stabilizing. However, Mn may have a negative impact on the structure stability and therefore the content is < 1.5 wt%. According to embodiments, the content of Mn is in the range of 0.5 to 1.5 wt%.

[0049] Chromium (Cr) is the most active element for obtaining resistance against most types of corrosion. Cr content is of great importance for the corrosion resistance and should therefore be as high as possible. However, there is a balance between high chromium content and good structure stability. The inventors have surprisingly found that by having the Cr content as low as possible, the ductility will be improved. Further, the Cr content cannot be too low as this will have a negative impact on the corrosion resistance. Hence, the present inventors have found the range of Cr for which both an excellent impact toughness is provided as well as a good corrosion resistance. Therefore, in the present disclosure, in order to attain sufficient corrosion resistance and also ensure structural stability, the Cr content should be in the range of more than 28.0 to 29.0 wt%. According to embodiments, the chromium content is more than 28.00. According to embodiments, the chromium content is lower than 29.00 weight%.

[0050] Nickel (Ni) is mainly used as an austenite stabilizing element. The advantage with Ni is that it has no negative effect on the structure stability. A Ni content of at least 7.1 wt% is required to ensure the structural stability because if the Ni content is below 7.1 wt%, chromium nitrides may be formed during heat treatment. However, Ni may form a strong complex with ammonium, therefore the Ni-content should be kept as low as possible. Thus, the Ni content is in the range of 7.1 to 8.0 wt%, such as from 7.1 to 7.8 wt%.

[0051] Molybdenum (Mo) is used to improve the passivity of the duplex stainless steel and therefore the Mo content is higher than or equal to 2.0 wt%. However, too high content of Mo involves the risk of precipitations of intermetallic phases. Therefore, Mo is less than or equal to 3.0 wt%. To obtain as good corrosion properties as possible, the content of Mo should be as high as possible without having the sensitivity for sigma phase to be unreasonably high. If the content of Mo is higher than 3.0 wt%, the driving force for sigma phase will be so high that it will be difficult to produce components without sigma phase. According to one embodiment, the Mo content is in the range of 2.1 to 2.7 wt%.

[0052] Nitrogen (N) is a strong austenite former and enhances the reconstitution of austenite. Additionally, N influences the distribution of Cr and Mo and Ni in the austenitic phase and ferritic phase. Thus, higher content of N increases the relative share of Cr and Mo in the austenitic phase. This means that the austenite becomes more resistant to corrosion, also that higher contents of Cr and Mo may be included into the duplex stainless steel while the structure stability is maintained. The inventors have found that in order to have a good structural stability and also ductility, the N content should be at least 0.32 wt%. However, the solubility of nitrogen is limited, and a too high level of nitrogen will increase the risk of forming chromium nitrides which in turn will affect the corrosion resistance. Therefore, N should be between 0.32 to 0.36 wt%.

[0053] Copper (Cu) is an optional element in the present disclosure and if included it will improve the general corrosion resistance in acid environments, such as sulfuric acid. However, high content of Cu will decrease the pitting and crevice corrosion resistance. Therefore, the content of Cu should be restricted to < 1.0 wt%, such as < 0.8 wt%, such as < 0.4 wt%. According to embodiments, the content of Cu is between 0.01 to 0.8 wt%, such as 0.01 to 0.4 wt%.

[0054] Sulfur (S) influences the corrosion resistance negatively by the formation of easily soluble sulfides. Therefore, the content of S should be restricted to < 0.02 wt%, such as < 0.01 wt%.

[0055] Phosphorus (P) is a common impurity element. If present in amounts greater than approximately 0.03 wt%, it can result in adverse effects on e.g., hot ductility, weldability, and corrosion resistance. The amount of P in the alloy should be restricted to < 0.03 wt%, such as < 0.02 wt%.

[0056] The ferrite content of the duplex stainless steel according to the present disclosure is important for the corrosion resistance. Therefore, the ferrite content is preferably in the range of from such as in the range of from 30 to 70 vol.%, such as in the range of from 35 to 65 vol.%, such as in the range of from 40 to 60 vol.%.

[0057] Additionally, other elements may optionally be added to the duplex stainless steel as defined hereinabove or hereinafter during the manufacturing process in order to improve the processability, e.g. the hot workability, the machinability etc. Examples, but not limiting, of such elements are calcium (Ca), aluminium (Al), cerium (Ce) and boron (B). If added, these elements are added in an amount of max 0.5 wt% in total. Optionally, e.g., it is possible for the alloy, as defined hereinabove or hereinafter, comprising the defined elements C, Si, Mn, Cr, Ni, Mo, N, Cu, S, and P in the amounts specified, with balance Fe and unavoidable impurities, to consist of said defined elements in said amounts, plus max. 0.5 wt% of added optional elements, such as added for processability, such as Ca, Al, Ce and B, with balance Fe and unavoidable impurities.

[0058] In the present disclosure, normally occurring impurities are considered to be impurities resulting from the manufacturing process and / or the scrap material used. In general, duplex stainless steels do not comprise more than about usually at most about 1 wt% in total, such as at most 0.8 wt% of normally occurring impurities. Examples of impurities are titanium (Ti), niobium (Nb), hafnium (Hf), barium (Ba), vanadium (V) and cobalt (Co) and tungsten (W). Depending on which of these impurity elements may be present in the steel, the content which will be tolerated will be different. For example, W and Co may be present as impurities due to the scrap used up to 1.0 wt% without affecting the properties of steel while the tolerated content of Ti, Hf, and V will for example be not more than 0.1 wt% each.

[0059] The duplex stainless steel as defined hereinabove or hereinafter may be manufactured according to conventional methods, i.e. melting, casting followed by hot working and / or cold working and optional additional heat treatment. Examples of hot working is forging and hot rolling.

[0060] The present disclosure also relates to a formed object comprising the duplex stainless steel as mentioned hereinabove or hereinafter.

[0061] According to embodiments, an object of the present duplex stainless steel may be manufactured using ingot casting. According to embodiments, the object may also be a bar which may have been manufactured by using forging or rolling.

[0062] The present disclosure also relates to the use of a duplex stainless steel as defined hereinabove or hereinafter, in any one of the embodiments described hereinbefore and hereinafter, in a urea synthesis process. This use of the duplex stainless steel as defined hereinabove or hereinafter is for improving impact toughness of one or more parts of the equipment used in said process, such as of one or more parts of a high pressure urea synthesis section, such as of parts that come in contact with carbamate solution.

[0063] The present disclosure also relates to a plant to produce urea, wherein said plant comprises one or more parts comprising the duplex stainless steel as defined hereinabove or hereinafter. According to one embodiment, one or more of the pressurized parts comprises, or is made from, the duplex stainless steel as defined hereinabove or hereinafter. Said duplex stainless steel can be used in a method of modifying an existing plant for the production of urea, said plant comprising one or more components selected from the group of valves, valve bodies, ejectors, and high pressure piping, wherein said method is characterized in that one or more parts are replaced by a part comprising the duplex stainless steel as defined hereinabove or hereinafter.

[0064] The present disclosure also relates to the use of the duplex stainless steel as defined hereinabove or hereinafter in carbamate environments.

[0065] The present disclosure also relates to the use of a duplex stainless as defined hereinabove or hereinafter in a urea synthesis process for improving impact toughness of one or more parts of a high pressure urea synthesis section in contact with ammonium carbamate solution.

[0066] The present disclosure also relates to a method for producing urea wherein at least one part of the equipment is made from a duplex stainless steel as defined hereinabove or hereinafter the method preferably comprising forming ammonium carbamate, and dehydrating ammonium carbamate to provide urea.

[0067] The present disclosure also relates to a plant for the production of urea, wherein said plant comprising one or more parts comprising a duplex stainless steel as defined hereinabove or hereinafter. The present disclosure also relates to a method of modifying an existing plant for the production of urea, said plant comprising one or more components selected from the group of a valve, such as a valve body or an ejector; a tube and a pipe, such as a seamless tube or large pipe; a pressurized component such as a vessel, a tube or a bearing component; or a plate, wherein said method is characterised in that one or more of these objects is / are replaced by the corresponding or a modified object comprising a duplex stainless steel as defined hereinabove or hereinafter.

[0068] The present disclosure also relates a method of modifying an existing plant for the production of urea, said plant comprising one or more construction components, wherein said method is characterised in that one or more pressurized parts or vessels is replaced by a part or a vessel comprising a duplex stainless steel as defined hereinabove or hereinafter.

[0069] The duplex stainless steel as defined hereinabove or hereinafter may be used for other applications, wherein good corrosion resistance is required for the equipment. Some examples of possible uses of the duplex stainless steel include use as a construction material in process chemistry components which are intended to be used in nitric acid environments, melamine production, use in the paper and pulp industry, such as in white liquor environment. The steel may be used for example for manufacturing seamless tubes, welded tubes, flanges, couplings, and sheet-metal.

[0070] The present disclosure is further illustrated by the following non-limiting examples.

[0071] Examples

[0072] The example compositions have been obtained from full-scale heats produced 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.

[0073] All the samples had the same final dimension and were produced using similar production routes via hot working followed by solution annealing with rapid quenching in water. The impact toughness was evaluated per ISO 148-1 at 20 °C and 0 °C. Full size specimens (10x10x55 mm) with V-notch were taken from the half-radius in accordance with ISO 377. The transverse direction was tested.

[0074] With the claimed compositions, the impact toughness improves, which is predicted by the criteria.

[0075] Table 1 - The heats within the present disclosure are marked with a The balance of each heat is Fe and unavoidable impurities.

[0076]

[0077] As can be seen from Table 1, all the heats within the present invention have a high average impact toughness at both 0 and 20 °C and are also fulfilling the requirements of the present disclosure.

Claims

Claims1. A duplex stainless steel comprising in weight% (wt%):C < 0.030;Si < 0.5;Mn <1.5;Cr more than 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;S < 0.02;P < 0.03; balance Fe and unavoidable occurring impurities.

2. The duplex stainless steel according to claim 1, wherein said duplex stainless steel fulfills the following requirement:28.4-[Cr]-29*[N]+0.44*[Ni]-1.66*[Si]+0.027*[Cr]2*[N] is greater than 0 (1) wherein the values of the elements are given in weight%.

3. The duplex stainless steel according to claim 1 or claim 2, wherein said duplex stainless steel fulfills the following requirement:[Cr] * [N] is less than 10 (2), wherein the values of the elements are given in weight%.

4. The duplex stainless steel according to any one of claims 1 to 3, wherein the content of C is < 0.020 wt%.

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

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

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

8. The duplex stainless steel according to any one of claims 1 to 7, wherein the content of Cr is more than 28.00.

9. The duplex stainless steel according to claims 1 to 8, wherein the content of Mo is in the range of 2.1 to 2.7 wt%.

10. The duplex stainless steel according to claims 1 to 9, wherein the content of Cu is in the range of Cu is < 0.8 wt%, such as < 0.4 wt%.

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

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

13. The object according to claim 11 or claim 12, wherein said object is selected from a valve, such as a valve body or an ejector; a tube and a pipe, such as a seamless tube or large pipe; a pressurized component such as a vessel, a tube or a bearing component; or a plate.

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

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