Ferritic stainless steel sheet and related production method

Abstract

The present invention relates to a ferritic stainless steel sheet, and its composition is the content represented by weight: C ≦ 0.03%, 0.25% ≦ Mn ≦ 1%, preferably 0.3% ≦ Mn ≦ 0.5%, 0% < Si ≦ 0.20%, preferably Si ≦ 0.15%, accompanied by Mn / Si ≧ 1.2, S ≦ 0.005%, P ≦ 0.04%, 19.0% ≦ Cr ≦ 24.0%, Ni ≦ 0.5%, Mo ≦ 0.10%, N ≦ 0.03%, Cu ≦ 0.20%, 0.40% ≦ Nb ≦ 1.0%, 0.05% ≦ Ti ≦ 0.2%, preferably 0.05% ≦ Ti ≦ 0.15%, Zr ≦ 0.02%, Al ≦ 0.02%, V ≦ 0.2%, Co ≦ 0.05%, Sn ≦ 0.05%, rare earths ≦ 800 ppm, The following is understood: V + Zr + Al ≦ 0.2%, Ti + V + Zr + Al ≦ 0.30%, Ti + Nb ≦ 1.0%, Ni + Cu + Co ≦ 0.60%, 2×Nb - 7×C ≧ 0.8%, 0% ≦ Ti - 4×N ≦ 0.15%, 0.2 ppm ≦ Ca ≦ 20 ppm, 1 ppm ≦ O ≦ 60 ppm, and includes the remainder of the composition consists of iron and inevitable process impurities, the sheet is an annealed and pickled sheet, The sheet relates to a ferritic stainless steel sheet containing a volume fraction of Fe2Nb Laves phase less than 0.2%.

Description

[Technical Field]

[0001] The present invention relates to ferritic stainless steel sheet and related production methods. [Background technology]

[0002] The development of high-temperature electrochemical applications (fuel cells or solid oxide electrolysis cells) for converting hydrogen compounds to electricity, or conversely for producing hydrogen products from carbon-free electricity, requires the use of novel materials capable of operating for tens of thousands of hours at temperatures between 500°C and 1000°C, with dozens of stop-start cycles. The components that form the cells of these systems, the anode, cathode, and electrolyte, are made from ceramics or cermets, the type of which can vary depending on the design. These cells are supported by or connected to metal parts called interconnectors, whose role is to distribute reactive gases and collect electrons within the cell. The low thermal expansion coefficient (10.5 x 10) of cells, whose electrolyte is often based on yttria-stabilized zirconia, allows for the use of high-temperature materials. -6 K -1 ) requires the use of high-chromium ferritic stainless steels, which have a very similar coefficient of thermal expansion at these high temperatures and are able to withstand oxidation. Furthermore, the interconnects must not cause degradation of cell performance by contamination of the anode and cathode, specifically by chromium diffusion or excessively strong electrical resistivity of the oxides formed on their surfaces.

[0003] Currently used ferritic stainless steels have a controlled very high chromium content and very low contents of some residual elements such as silicon, and rare earth elements (specifically lanthanum). Therefore, their vacuum production from pure raw materials is expensive. The competitiveness of these electrolytic cells is affected, as the interconnects constitute a large proportion by weight in these systems. Therefore, the idea is to use more conventional ferritic stainless steels produced in electric furnaces, such as AISI 441 or 444 steels, which have excellent heat-resistant properties. However, under some conditions of use, experiments have shown that the performance of these steels deteriorates more rapidly, despite oxide layers of comparable composition and thickness. The cause of this faster deterioration of performance is the increase in the temperature of 1000°C at 850°C. 6 The segregation of silicon at the metal-oxide interface forms a thin, more or less continuous and electrically highly resistive silica (silicon dioxide) film with a resistivity in the Ω-cm range. This value is comparable to the resistivity of chromium oxide, Cr2O3, which is in the 100 Ω-cm range at 850°C. The silica film has a very limited thickness, typically on the order of a few tens of nanometers at 850°C after 1000 hours of operation. However, given the electrical resistivity of silica, it contributes strongly to increasing the area-specific resistance (ASR). However, silicon cannot be completely eliminated from current, affordable production methods. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Faria, Geraldo Lucio de; Melo, Denilson Pereira de; Moreira, Paulo Sérgio, “UTILIZAÇÃO DA METODOLOGIA SAG TEST PARA AVALIAR O COMPORTAMENTO EM FLUÊNCIA DOS AÇOS INOXIDÁVEIS AISI 321 E AISI 441”, p.34 - 44, 75° Congresso Anual da ABM, São Paulo, 2022, ISSN: 2594 - 5327, DOI 10.5151 / 2594 - 5327 - 34135

Summary of the Invention

Problems to be Solved by the Invention

[0005] Therefore, one object of the present invention is to propose a ferritic stainless steel sheet having electrical conductivity characteristics equivalent to those of steels having a very high chromium content, even after use at high temperatures for a long time.

[0006] Preferably, since the system is also subject to thermo - mechanical loads that are likely to cause deformations leading to modifications in the contact and function of the interconnector, it is also required to improve the creep resistance of the sheet.

Means for Solving the Problems

[0007] For this purpose, the subject of the present invention is a ferritic stainless steel sheet having the following composition, where the contents are expressed by weight: C ≦ 0.03%, 0.25% ≦ Mn ≦ 1%, preferably 0.3% ≦ Mn ≦ 0.5%, 0% < Si ≦ 0.20%, preferably Si ≦ 0.15%, With Mn / Si ≧ 1.2, S ≦ 0.005%, P ≦ 0.04%, 19.0% ≦ Cr ≦ 24.0%, Ni ≦ 0.5%, Mo≦0.10% N≦0.03%, Cu≦0.20%, 0.40%≦Nb≦1.0%, 0.05%≦Ti≦0.2%, preferably 0.05%≦Ti≦0.15%, Zr≦0.02%, Al≦0.02%, V≦0.2%, Co≦0.05%, Sn≦0.05% Rare earths≦800ppm, It is understood that: V+Zr+Al≦0.2% Ti+V+Zr+Al≦0.30% Ti+Nb≦1.0% Ni+Cu+Co≦0.60% 2×Nb-7×C≧0.8% 0%≦Ti-4×N≦0.15%, 0.2 ppm≦Ca≦20 ppm, 1 ppm ≦ O ≦ 60 ppm, The remainder of the composition consists of iron and unavoidable process impurities; The sheet is annealed and pickled, The sheet has a volume fraction of Fe2Nb Laves phase of less than 0.2%.

[0008] The ferritic steel sheet of the present invention may have one or more of the following characteristics, taken alone or in any technically possible combination: - the alloy has a rare earth content between 50 ppm and 800 ppm; - the niobium content of the alloy satisfies the following relationship: Nb-10×(C+N)≧0%; - The sheet has a thickness between 0.1mm and 2.5mm, - when the sheet has a thickness between 1.2 mm and 2.5 mm, the sheet has an average grain size between 30 micrometers and 80 micrometers, and when the sheet has a thickness of 0.1 mm or more but less than 1.2 mm, the sheet has an average grain size between 15 micrometers and 80 micrometers; - The sheet is a cold-rolled and annealed sheet; - when subjected to a heat treatment at a temperature of 850 ° C for 1000 hours, the sheet contains a volume fraction of Fe2Nb Laves phase of 0.8% or more, when subjected to a heat treatment at a temperature of 850 ° C for 1000 hours, the sheet contains a volume fraction of Fe3Nb3X cubic phase of less than 0.05%; - when subjected to a heat treatment at a temperature of 850 °C for 1000 hours, the sheet on each of its surfaces comprises an oxide layer and silicon oxide precipitates at the interface between the steel of the sheet and the oxide layer, such that the surface fraction of silicon oxide precipitates at the interface between the steel of the sheet and the oxide layer is less than or equal to 0.35; and The oxide layer has a thickness of 10 μm or less.

[0009] The present invention also relates to a method for producing a ferritic stainless steel sheet, comprising the steps of: - a steel having the composition described above is produced; - blanks are cast from this steel, - the blank is held at a temperature of 1150°C to 1260°C for a time period of between 40 and 60 minutes, and the blank is hot rolled to obtain a hot rolled sheet having a thickness of between 2.5 mm and 6 mm; - The hot-rolled sheet is annealed, - The hot-rolled and annealed sheet is pickled; - in a single step or in several steps separated by intermediate annealing operations, the hot-rolled sheet is cold-rolled at a temperature between ambient temperature and 300°C; - Final annealing of the cold rolled sheet is carried out at a temperature between 1000°C and 1100°C for a time between 10 seconds and 6 minutes to obtain a fully recrystallized structure.

[0010] The production method of the invention may also comprise one or more of the following features, taken alone or in any technically possible combination: - Annealing of the hot-rolled sheet is performed at a temperature between 1000 °C and 1100 °C for a time of 30 seconds to 6 minutes. - The intermediate annealing operation is performed at a temperature between 950 °C and 1100 °C for a time of 30 seconds to 6 minutes, and - The final annealing is performed at a temperature between 1050 °C and 1090 °C.

[0011] The present invention will be better understood and other aspects and advantages will become apparent by reading the following detailed description of the exemplary embodiments given with reference to the accompanying drawings.

Brief Description of the Drawings

[0012] [Figure 1] FIG. 1 schematically illustrates the sheet of the present invention in a cross-sectional view after the aging heat treatment. [Figure 2] FIG. 2 schematically illustrates the determination of the surface fraction of silica precipitates at the metal-oxide interface after the aging treatment.

Modes for Carrying Out the Invention

[0013] The present invention relates to a ferritic stainless steel sheet having the following composition, with the contents expressed by weight: C ≤ 0.03%, 0.25% ≤ Mn ≤ 1%, preferably 0.3% ≤ Mn ≤ 0.5%, 0% < Si ≤ 0.20%, preferably Si ≤ 0.15%, accompanied by Mn / Si ≥ 1.2, S ≤ 0.005%, P ≤ 0.04%, 19.0% ≤ Cr ≤ 24.0%, Ni ≤ 0.5%, Mo ≤ 0.10%, N ≤ 0.03%, Cu ≤ 0.20%, 0.40% ≤ Nb ≤ 1.0%, 0.05% ≤ Ti ≤ 0.2%, preferably 0.05% ≤ Ti ≤ 0.15%, Zr ≤ 0.02%, Al≦0.02%, V≦0.2%, Co≦0.05%, Sn≦0.05% Rare earths≦800ppm, It is understood that: V+Zr+Al≦0.2% Ti+V+Zr+Al≦0.30% Ti+Nb≦1.0% Ni+Cu+Co≦0.60% 2×Nb-7×C≧0.8% 0%≦Ti-4×N≦0.15%, 0.2 ppm≦Ca≦20 ppm, 1 ppm ≦ O ≦ 60 ppm, The remainder of the composition is made up of iron and unavoidable process impurities.

[0014] Regarding the chemical composition of steel, carbon increases the mechanical properties at high temperatures, specifically creep resistance. However, due to its very low solubility in ferrite, carbon forms carbides M at temperatures below about 900 °C. 23 Carbon tends to precipitate in the form of C6 or M7C3. These precipitates, generally located at grain boundaries, predispose to chromium depletion near these boundaries and therefore to intergranular corrosion. Specifically, this susceptibility can arise in welds with heat-affected zones heated to very high temperatures. Therefore, to obtain satisfactory resistance to intergranular corrosion and to avoid a decrease in workability, the carbon content must be limited to at most 0.03%. Furthermore, the carbon content must satisfy the relationship with niobium, as explained below.

[0015] Chromium is an essential element for stabilizing the ferrite phase and increasing its resistance to oxidation. To obtain a ferrite structure at any temperature and excellent resistance to repeated oxidation, particularly when the sheet has a limited thickness (less than 0.5 mm) that limits the chromium reserves available for oxidation, its minimum content, together with the other elements of the composition, must be 19.0% or more. During aging, and as illustrated in FIG. 1, an oxide layer 2, comprising an inner layer 4 of chromium oxide Cr2O3 and an optional outer layer 5 of manganese-rich chromium oxide, forms on the surface of the metal substrate 1, protecting the steel for very long periods and at high temperatures. However, the maximum chromium content should not exceed 24.0%, otherwise it may excessively increase the mechanical strength at ambient temperatures, causing substantial embrittlement and deteriorating workability.

[0016] In the context of this invention, the terms "internal" and "external" are used in relation to proximity to the metal substrate 1, with the internal layers being closer to the metal substrate 1 than the external layers.

[0017] The alloy has a manganese content between 0.25% and 1% by weight. At these contents, manganese increases the alloy's mechanical properties and also allows the formation of an outer layer 5, possibly containing iron, of manganese-rich chromium oxide and spinel type (Mn,Fe)Cr2O4. The excellent thermodynamic stability of these oxides allows for limited chromium evaporation at high temperatures in the presence of water vapor. This manganese-rich chromium oxide outer layer 5 also promotes excellent adhesion of protective coatings, such as LSM-type (strontium-doped lanthanum manganite) or MCO-type (manganese cobalt oxide, spinel) coatings. These spinels have very good electrical conductivity, with resistivities in the 20 Ω-cm range at 850°C. However, above 1% by weight, the oxidation kinetics under heating becomes excessively fast, resulting in the formation of a thick, adherent oxide layer, which makes pickling difficult when producing sheet. Manganese, like Ni, Cu, and Co, is also a gammagenic element that must be limited in ferritic steels. Therefore, the manganese content is limited to 1%.

[0018] Preferably, the manganese content is between 0.3% and 0.5%.

[0019] Silicon is a very effective element for increasing resistance to oxidation. However, the silicon oxide (silica)3 formed at the metal-oxide interface is 1×10, ten times lower than that of the base metal and chromium oxide Cr2O3. -6 K -1The oxide layer 2 has a very low coefficient of expansion in this region, reducing the adhesion of the oxide layer 2 as a whole. The poor adhesion of the oxide layer 2 leads to poor conductivity. Furthermore, silicon has a high electrical resistivity, which is very detrimental to the target application, since the oxidized metal must have good electrical conductivity. Silicon is also a hardening element of ferrite, reducing its ductility and cold workability when cooled. Therefore, the silicon content must be limited to a minimum amount, not exceeding 0.20% by weight, with a preferred maximum of 0.15%. Since the presence of trace amounts of silicon is unavoidable, the silicon content is higher than 0%. The silicon content generally remains above 0.05%. Reducing the Si content below this value may require expensive processes.

[0020] Furthermore, the Mn / Si ratio must be greater than or equal to 1.2 in order to promote the formation of the outer layer 5 of manganese-rich chromium oxide of the (Mn,Fe)Cr2O4 type mentioned above, as opposed to the adverse effects of silicon oxide.

[0021] Sulfur and phosphorus are impurities that reduce hot ductility and workability. Phosphorus easily segregates at grain boundaries, reducing their cohesion. Sulfur also contributes to oxidation by segregating at metal-oxide interfaces and reducing adhesion to the metal. In this regard, the sulfur and phosphorus contents must be less than 0.005 wt.% and 0.04 wt.%, respectively.

[0022] Nickel is a gammagenic element that increases the ductility of steel. Its content is limited to maintain the single-phase ferrite structure. Furthermore, nickel does not improve the desired properties, and its intentional addition would increase production costs, given the high price of nickel. The nickel content should also be as low as possible, not exceeding 0.5% by weight.

[0023] Molybdenum not only increases resistance to high temperatures but also to oxidation. However, in high-chromium steels containing titanium and niobium, particularly in hot-rolled strips between 2.5 mm and 6 mm thick, it weakens the ferrite matrix. Molybdenum excessively reduces ductility and workability and is an expensive additive. Its content should be less than 0.10%, preferably strictly less than 0.10%.

[0024] Similar to carbon, nitrogen increases mechanical properties. However, nitrogen tends to precipitate at grain boundaries in the form of nitrides, thereby reducing corrosion resistance. To limit susceptibility to intergranular corrosion, the nitrogen content should be below 0.03%.

[0025] Copper has a hardening effect under heat. However, in excessive amounts, it reduces ductility during hot rolling. Like nickel, it is also a gammagenic element that should be limited. Therefore, in this respect, the copper content should be less than 0.20% by weight.

[0026] Niobium is an important element in the present invention. Often, this element can be used as a stabilizing element in ferritic stainless steels, and the phenomenon of susceptibility to intergranular corrosion mentioned above can be prevented through the addition of elements that form thermally very stable carbides or carbonitrides. In this way, carbon and nitrogen in the solution are reduced as much as possible, thereby avoiding the subsequent precipitation of chromium carbides and nitrides. Thus, niobium, as well as titanium, and to a lesser extent zirconium and vanadium, provide stable fixation of carbon and nitrogen.

[0027] However, in the range of 650°C to 950°C, niobium also combines with iron to form several intermetallic compounds: the inventors have therefore demonstrated that the grain boundary precipitation of hexagonal FeNb that appears at high temperatures can be advantageously used to increase the heat, creep, and, specifically, mechanical properties under the intended service conditions.

[0028] Furthermore, the inventors have discovered that the precipitation of Fe2Nb compounds in a hexagonal crystal structure (called the Laves phase) traps a portion of the silicon, thus minimizing the formation of silicon oxide, which is desirable for this application. Specifically, compared to the Fe3Nb3X cubic phase, where X represents nitrogen, oxygen, or carbon, which also tends to form, the Fe2Nb Laves phase contains approximately four times more silicon by weight. Adjusting the initial silicon content and trapping a portion of the silicon by this phase significantly reduces silicon segregation at the metal-oxide interface and the formation of highly resistive silica. Furthermore, the type and spatial distribution of these precipitates, i.e., at the grain boundaries, significantly improves creep resistance up to 1000°C.

[0029] To form the Fe2Nb phase under the intended service conditions, several conditions must be met: the niobium content must be between 0.40% and 1.0% such that 2 × Nb - 7 × C ≥ 0.8%, and the composition must also include a titanium content between 0.05% and 0.2% such that 0% ≤ Ti - 4 × N ≤ 0.15%. Preferably, the niobium, carbon, and nitrogen contents must also be such that Nb - 10 × (C + N) ≥ 0%.

[0030] If the total niobium content in the steel is less than 0.40%, the steel will be insufficiently stabilized, and the amount of FeNb precipitates formed at high temperatures may be insufficient to obtain the desired high-temperature properties. To obtain this preferred niobium precipitate, the inventors have also demonstrated the importance of the effective niobium content: effective niobium refers to the amount of niobium in solid solution available for precipitation with iron, assuming that carbon and nitrogen are fully precipitated with niobium and titanium in the form of carbonitrides TiN, TiC, and Nb(C,N). To ensure a sufficient effective niobium content, the niobium content must satisfy the relationship 2 × Nb − 7 × C ≥ 0.8%, and preferably Nb − 10 × (C + N) ≥ 0%.

[0031] Conversely, a large excess of Nb causes weakening, particularly in ferrite with very high chromium contents. This excess significantly increases hardness, slows recrystallization, and can also limit the cold workability of the metal. Therefore, the niobium content is limited to Nb≦1.0%.

[0032] The titanium content must be greater than or equal to 0.05% to favor the formation of the FeNb phase at the expense of the FeNbX phase, but must be less than or equal to 0.2% to limit excessive internal oxidation at high temperatures. Preferably, the titanium content is greater than or equal to 0.05% and / or less than or equal to 0.15%.

[0033] Also, titanium must satisfy the relationship with nitrogen of 0%≦Ti-4×N≦0.15%, otherwise niobium may precipitate in the form of cubic Fe3Nb3X compounds, which are less efficient at trapping silicon, rather than in the form of hexagonal Fe2Nb above 650°C.

[0034] Furthermore, to avoid making the metal excessively brittle and to ensure satisfactory toughness of the weld, the sum of the contents of Nb and Ti must be less than or equal to 1.0%.

[0035] Vanadium, zirconium, and aluminum are nitrogen-stabilizing elements that increase the mechanical strength at high temperatures; however, the total content of these elements should be limited to at most 0.2%, otherwise workability may be reduced.

[0036] In addition, aluminum is easily oxidized to form alumina, which has high electrical resistance, so the aluminum content is limited to at most 0.02%. Specifically, at 850°C, the electrical resistivity of alumina is 10 7 in the Ωcm range, which means that alumina has a resistivity ten times greater than silica (SiO2).

[0037] The zirconium content is at most 0.02% to avoid a decrease in workability and to limit the risk of surface damage.

[0038] The vanadium content is at most 0.2% to avoid a decrease in workability.

[0039] Furthermore, the inventors have demonstrated that the contents of titanium, aluminum, vanadium and zirconium must be jointly limited to limit metal embrittlement: their sum must be such that Ti+Al+V+Zr≦0.30%.

[0040] Cobalt is a hardening element under heat, but it also reduces workability, so its content must be less than 0.05% by weight.

[0041] Also, when added to those of copper and nickel, the cobalt content must be as low as possible to prevent an increase in electrical resistivity, which would be detrimental to the present application. For this purpose, the total content of Ni+Cu+Co must not exceed 0.60%.

[0042] To avoid problems associated with hot forging, the tin content must be less than 0.05%.

[0043] The alloy also contains calcium at a content between 0.2 ppm and 20 ppm. Calcium acts to reduce the sulfur and oxygen content of the steel during the production process. The calcium concentration should be limited to avoid internal oxidation of the metal when used at high temperatures.

[0044] The oxygen content of the steel is between 1 ppm and 60 ppm, for example about 20 ppm. As with calcium, its concentration should be limited to avoid the presence of oxide inclusions and internal oxidation of the metal when used at high temperatures.

[0045] Rare earth elements (REEs) can be added to improve the adhesion of the oxide layer, making the steel resistant to corrosion. However, the rare earth content should not exceed 800 ppm. Above this content, metal production can be difficult due to reactions of the rare earths with the refractories present in the casting ladle. These reactions can lead to the formation of REE oxides, which can reduce the inclusion cleanliness of the steel. Furthermore, the efficacy of the REEs is sufficient at the proposed content; further increases may only increase unnecessary production costs, given the high price of REEs, and may lead to accelerated refractory wear. If rare earths are added, the content should preferably be at least 50 ppm.

[0046] Preferably, the rare earth is selected from cerium, lanthanum, and yttrium or a combination thereof. Specifically, the rare earth includes a mixture of cerium and lanthanum. Preferably, the rare earth is a mixture of cerium and lanthanum.

[0047] The steel of the invention is generally in the form of cold-rolled and annealed sheet, the thickness of which is generally between 0.1 mm and 2.5 mm.

[0048] In the present invention, the structure of the steel in the delivery state, i.e. the cold rolled and annealed sheet, is fully recrystallized.

[0049] The bending creep of a sheet at elevated temperatures is a function of the thickness of the sheet: for the same mechanical load and metallurgical condition, a sheet of limited thickness can undergo large deformation in bending.

[0050] When the thickness of the cold-rolled and annealed sheet is between 1.2 mm and 2.5 mm, the average grain size of the steel is preferably between 30 and 80 micrometers, i.e., between 4 and 7 ASTM numbers (standard ASTM E-112). Within this range of strip thickness, a grain size of 30 micrometers or more (ASTM ≦ 7) is advantageous because it ensures slight deformation of the sheet by bending under creep at high temperatures for that thickness. Also within this range of strip thickness, the grain size is preferably 80 micrometers or less (ASTM ≧ 4). Grain sizes above 80 micrometers (ASTM < 4) lead to the appearance of an unattractive surface anomaly known as "orange skin" when working at ambient temperatures, which adversely affects the good adhesion of protective coatings.

[0051] When the thickness of the cold-rolled and annealed sheet is equal to or greater than 0.1 mm and less than 1.2 mm, the average grain size of the steel is preferably between 15 and 80 micrometers, i.e., between ASTM numbers 4 and 9. Within this strip thickness range, a grain size of 15 micrometers or greater (ASTM ≦ 9) is advantageous because it ensures slight deformation of the sheet by bending under creep at high temperatures relative to its thickness. Also within this strip thickness range, the grain size is preferably equal to or less than 80 micrometers (ASTM ≧ 4). Grain sizes greater than 80 micrometers (ASTM < 4) lead to the appearance of an unattractive surface anomaly known as "orange skin" when worked at ambient temperatures, which adversely affects the good adhesion of protective coatings.

[0052] The microstructure of the cold-rolled and annealed sheet in the as-received state contains precipitates consisting essentially of intragranular titanium and niobium carbonitrides. The annealing treatment carried out on the sheet has the effect of dissolving the hexagonal structured Fe2Nb Laves phase type and the cubic structured Fe3Nb3X type intermetallic precipitates contained in the microstructure.

[0053] Specifically, the volume fraction of the Fe2Nb Laves phase in the cold rolled and annealed sheet in the as-received state is less than 0.2%.

[0054] The volume fraction of the Fe2Nb Laves phase is determined as follows:

[0055] In the first stage, conventional polishing is carried out using abrasives with a particle size of up to 1 μm, followed by an electrolytic attack in 60% nitric acid on the cross section of the specimen taken perpendicular to the rolling direction.

[0056] Observations are made under an electron microscope in backscattered electron mode. A minimum magnification of 1000x is used to obtain a complete picture and accurately determine particle size. Five images are taken for each specimen and condition. Backscattered electron mode produces contrast in chemical composition across a 256-level scale, called the grayscale, ranging from white (255) to black (0).

[0057] The volume fractions of the Fe2Nb and Fe3Nb3X intermetallic phases are determined via image analysis of the resulting images using, for example, Image J software.

[0058] First, the obtained images are processed using a thresholding method to retain only the whitest precipitates, which correspond to the Fe2Nb and Fe3Nb3X intermetallic phases. In backscattered electron mode, niobium and iron-rich precipitates appear as the compounds with the lightest shadows in the image due to their atomic numbers. As a result, the Fe2Nb and Fe3Nb3X intermetallic phases can be distinguished from the niobium and titanium carbonitrides by their chemical contrast and the appearance of lighter shadows. In this thresholding step, for each image, a threshold is chosen that allows the Fe2Nb and Fe3Nb3X intermetallic precipitates to be distinguished from the rest of the image. After thresholding, the intermetallic precipitates appear white in a black matrix.

[0059] The image is then manually filtered to remove artifacts such as holes or impurities.

[0060] The surface fractions of the Fe2Nb and Fe3Nb3X intermetallic phases are then determined from this thresholded and filtered image. In this context, it is accepted that the volume fraction is equal to the surface fraction. Thus, the volume fractions of the Fe2Nb and Fe3Nb3X intermetallic phases are obtained.

[0061] Furthermore, in each of the images, a measurement of the Fe / Nb ratio is performed by energy dispersive spectroscopy (EDS). Since this ratio is different in each of these intermetallic phases, it makes it possible to distinguish between cubic intermetallic Fe3Nb3X and hexagonal Fe2Nb, thereby determining the volume fraction of the Fe2Nb Laves phase from the total volume fraction of the Fe2Nb and Fe3Nb3X intermetallic phases.

[0062] In the cold rolled and annealed sheet, niobium is mostly in solid solution, specifically, in the cold rolled and annealed sheet, the weight content of Nb in solid solution in the as-received state is at least 0.3%.

[0063] Furthermore, the sheet of the present invention is characterized in that when subjected to a heat treatment at a temperature between 650°C and 1000°C for a time of 30 minutes or more, in addition to the above-mentioned titanium and niobium carbonitrides, the structure of the sheet contains Fe2Nb compounds of hexagonal structure (Laves phase) and similar and grain boundary precipitates. The structure of the sheet subjected to the above-mentioned heat treatment may also contain grain boundary precipitates of Fe2Nb compounds, which are reduced as the heat treatment time increases.

[0064] Specifically, after aging heat treatment in air at 850° C. for 1000 hours, the volume fraction of Fe2Nb Laves phases in the sheet is at least 0.8%. These Fe2Nb Laves phases are mainly located at grain boundaries.

[0065] An ageing heat treatment in air at 850°C for a period of 1000 hours is considered representative of the service conditions of the steel and is commonly used for qualification of the steel.

[0066] In the present invention, FeNb precipitates dominate the grain boundary precipitates. As mentioned above, these precipitates have the advantage of capturing some of the silicon to reduce their content in solid solution, so that silicon is less likely to segregate at the metal-oxide interface.

[0067] Specifically, after the heat treatment, the volume fraction of Fe3Nb3X precipitates remains below 0.05%.

[0068] Furthermore, the type and distribution of these Fe2Nb precipitates, together with the grain size, are highly favorable for creep resistance up to 1000°C.

[0069] Due to the formation of Fe2Nb precipitates, silica segregation at the metal-oxide interface is very limited compared to prior art steels.

[0070] Specifically, after the above-described aging treatment at 850° C. for 1000 hours in air, the surface fraction of silica segregating at the metal-oxide interface remains limited.

[0071] Specifically, as illustrated in Figure 1, after the aging treatment, the sheet on each of its surfaces comprises an oxide layer 2 over the base metal 1. The oxide layer 2 includes an inner layer 4 of chromium oxide Cr2O3 (chromia) and an outer layer 5 comprising manganese-rich chromium oxide and iron, possibly in the spinel form (Mn,Fe)Cr2O4.

[0072] At the interface between the oxide layer 2 and the steel of the sheet, also called base metal 1, the sheet contains silicon oxide 3 or silica precipitates. In the sheet of the present invention, after the above-mentioned aging treatment, the surface fraction of silica precipitates at this metal-oxide interface is less than 0.35, meaning that silica covers less than 35% of the metal-oxide interface. -6 Scm -1 The electrical conductivity of silica in this region is about 10 at 850°C. -2 Scm -1It is 10,000 times smaller than that of chromia, and its thermal expansion coefficient is 10 times smaller than that of chromia, the base metal, and zirconia, which forms the electrolyte of electrochemical cells. The surface fraction of silica at this metal-oxide interface is directly related to the resistivity, area specific resistance, and conductivity of the metal-oxide interface. Thus, when the surface fraction of silica at the interface is 35%, the electrical conductivity of the interface is reduced by 35% compared to the silica-free configuration, and the conductivity of the interface is reduced by 10% when silica is absent. -2 Scm -1 From 6.5 x 10 -3 Scm -1 or the interfacial resistivity increases by about 50%, from 100 Ω cm to 150 Ω cm. The interfacial area-specific resistance, which is equal to the resistivity times the thickness of the silica film, also increases.

[0073] The surface fraction of segregation of silicon oxide 3 can be determined from an image of the cross section of the specimen taken under a scanning electron microscope in backscattered electron mode with a magnification of 10,000 times, in a direction perpendicular to the rolling direction.

[0074] The horizontal edges of the image are parallel to the surface of the sheet. The observed cross-sectional length L is 12 μm in the cross section of the sheet.

[0075] The backscattered electron mode produces contrast in chemical composition on a 256-level scale, called the grayscale, ranging from white (255) to black (0). In the resulting image, silica, due to its composition and the atomic numbers of its constituent elements, appears as the darkest phase, compared to the base metals, which appear in very light shades, and chromium oxide, which is a medium shade of gray.

[0076] Therefore, the image obtained corresponds to the image shown as (a) in FIG.

[0077] The images were analyzed using image analysis software, for example using Image J software.

[0078] More specifically, the image, which is first transformed via an automated process using image analysis software to increase the contrast between elements in the image, shows the segregation of base metal 1, oxide layer 2, and silicon oxide 3. The goal of this step is that all acquired images should have the same contrast, allowing the segregation of silicon oxide 3 to be verified independently of the observed metal-oxide interface and the analyzed oxidized metal. After this processing, the image shown in Figure 2(b) is obtained.

[0079] The image is then thresholded to distinguish between the silicon oxide 3 segregation and the rest of the image. Thus, only two levels are kept: black for the silicon oxide 3 segregation and white for the rest of the image, and an adapted threshold is set to distinguish the silicon oxide 3 segregation. For example, a threshold value of 70 is chosen, and pixels with a greyscale higher than 70 are replaced with white 255, and pixels with a greyscale lower than 70 are replaced with black. Afterwards, any artifacts are filtered and the process quality of each image is manually varied.

[0080] From this image, the sum of the estimated lengths Li on the vertical axis of the interface areas containing silicon oxide 3 precipitates in the area of ​​this measurement is determined, as illustrated in image (c) in FIG.

[0081] The surface fraction is then calculated as the ratio ΣLi / L between the sum of the estimated lengths and the length of the measurement range L. In the example illustrated in Figure 2, the surface fraction is 77%.

[0082] In the present invention, after heat treatment in air at 850° C. for 1000 hours, this ratio ΣLi / L remains below 0.35, ie below 35%.

[0083] Also, given the chromium content of the steel according to the invention, after the heat treatment described above the total thickness of the oxide layer 2 generally remains below 10 μm.

[0084] Specifically, the sheet of the present invention can be obtained using the following method: - making a steel having the aforementioned composition, - blanks are cast from this steel, - the blank is held at a temperature of 1150°C to 1260°C for a time period of between 40 and 60 minutes, and the blank is hot-rolled to obtain a hot-rolled sheet having a thickness of between 2.5 mm and 6 mm; The hot-rolled sheet is annealed, for example, at a temperature between 1000°C and 1100°C for a time between 30 seconds and 6 minutes. - The hot-rolled and annealed sheet is pickled; - the hot-rolled sheet is cold-rolled in a single step or in several steps at temperatures between ambient temperature and 300°C, after each step the sheet is annealed and pickled. By the term "step" it may be understood here to mean that the cold rolling comprises either a single pass or a series of several passes (e.g. five passes) not separated by any intermediate annealing; for example, it may be envisaged that the cold-rolling sequence comprises a first series of five passes followed by an intermediate annealing, then a second sequence of five passes, typically with the intermediate annealing separating the steps carried out between 950°C and 1100°C for 30 seconds to 6 minutes. - A final annealing is carried out on the cold-rolled sheet at a temperature between 1000°C and 1100°C, preferably between 1050°C and 1090°C, for a time between 10 seconds and 6 minutes, in order to obtain a fully recrystallized structure with an ASTM mean grain size of preferably between 4 and 7 when the sheet is between 1.2 mm and 2.5 mm thick, and between 4 and 9 when the sheet is between 0.1 mm and less than 1.2 mm thick. This heat treatment allows the niobium to go into solid solution.

[0085] The volume fraction of the Laves phase, i.e. the Fe2Nb compound with hexagonal structure, is very low in the structure of the sheet, being less than 0.2% in the as-received state, i.e. after this final annealing. [Example]

[0086] A series of experiments demonstrating the advantages of the present invention will now be described. Laboratory castings were examined and chemical analyses were performed as shown in Table 1.

[0087] [Table 1]

[0088] For each of the steels in Table 1, the remainder is iron and unavoidable process impurities.

[0089] The cast specimens were deformed using the following method: - The cast blanks were held at a temperature of 1220°C for 40 minutes and hot rolled to obtain sheets with a thickness of 5 mm. - The sheet was annealed at 1080°C for 6 minutes and pickled. - The hot-rolled sheet was cold-rolled at ambient temperature to obtain a sheet with a thickness of 1.5 mm. - Final annealing was carried out on the sheet at a temperature of 1080°C for 4 minutes.

[0090] The volume fraction of the Laves phase, i.e., the Fe2Nb compound with hexagonal structure, was determined in the structure of each sheet in the as-received state, i.e., after annealing of the cold-rolled sheets. Table 2, under the column "Fe2Nb Laves phase in as-received state," gives the volume fraction of the Laves phase determined in the sheet in the as-received state.

[0091] The sheets were then subjected to a heat treatment at 850°C for 1000 hours.

[0092] After this heat treatment, the precipitates in the structure were determined. Table 2 below, under the column "Fe2Nb Laves phase after 1000 hours at 850°C," gives the volume fraction of the Laves phase after this heat treatment.

[0093] The volume fraction of Fe3Nb3X intermetallic precipitates in the sheet after this heat treatment was also determined and this fraction is given in Table 2 under the column "Fe3Nb3X cubic phase after treatment at 850°C for 1000 hours."

[0094] Then, for each of the sheets, the surface fraction of segregated silicon oxide 3 was determined from images obtained under an electron microscope as described above. The determined surface fractions are given in Table 2 under the column "Silica surface fraction after treatment at 850°C for 1000 hours."

[0095] The toughness of the annealed and pickled hot strip obtained after hot rolling to a thickness of 5 mm at 1220 °C was also determined using a Charpy impact test on notched specimens according to the NF EN ISO 148-1 (March 2017 edition) standard, for which the energy absorbed by the specimen was measured as a function of temperature (between -10 °C and 80 °C) when impacted to the breaking point using a three-point bending test. At a temperature of 20 °C (the temperature closest to ambient temperature), the toughness was 30 J / cm. 2 A strip is considered ductile if it has an impact strength of greater than 0.15.

[0096] [Table 2]

[0097] In Table 2, comparative studies are underlined.

[0098] Regarding the tests relating to the inventions (E1 to E3), the following is observed: - The niobium and titanium contents satisfy the above-mentioned conditions, i.e.: - Nb between 0.40% and 1.0%, and 2 × Nb-7 × C ≥ 0.8%; - Ti between 0.05% and 0.2% and 0%≦Ti-4×N≦0.15%; - Mn / Si ratio is greater than 1.2, and - The sum of Ni+Cu+Co is between 0 and 0.60%. After treatment at 850°C for 1000 hours, the volume fraction of hexagonal Fe2Nb intermetallic precipitates is 0.8% or more, and the surface fraction of silicon oxide 3 after treatment at 850°C for 1000 hours is 35% or less.

[0099] In this way, a sheet is obtained with excellent performance in terms of electrical conductivity under use conditions, in particular comparable to that of steels with very high chromium contents.

[0100] In contrast, in comparative tests E5 to E8, the titanium and / or niobium content does not satisfy the above condition. In these tests, the volume fraction of hexagonal FeNb intermetallic precipitates after 1000 hours of treatment at 850°C is less than 0.8%. Therefore, silicon is less likely to be trapped by the FeNb Laves phase, and therefore silica segregation is less likely to be limited, even when the silicon content is less than 0.2%, as in alloy K5. Furthermore, in these comparative tests, the Mn / Si ratio does not satisfy the above condition. It can be noted that in these tests, the surface fraction of silicon oxide 3 after 1000 hours of treatment at 850°C shows a surface fraction of more than 35%, leading to deteriorated performance in terms of electrical conductivity, with an interfacial resistivity increased by more than 50%.

[0101] Furthermore, in tests E1 to E3, it was 30 J / cm 2 In contrast, in comparative tests E4 and E5 of steels with molybdenum contents exceeding the limits described in this invention, the hot strip obtained was ductile, since it exhibited an impact strength of 30 J / cm. 2 It has an impact strength less than 1000kJ / cm2 and is therefore not ductile.

[0102] As explained above, according to one optional aspect: - when the thickness of the cold-rolled and annealed sheet is between 1.2 mm and 2.5 mm, the average grain size of the steel is between 30 micrometers and 80 micrometers, i.e., ASTM number between 4 and 7; - When the thickness of the cold-rolled and annealed sheet is greater than or equal to 0.1 mm and less than 1.2 mm, the average grain size of the steel is between 15 micrometers and 80 micrometers, i.e., an ASTM number between 4 and 9.

[0103] To confirm the technical effect of this optional property, the inventors planned to subject sheets having the composition of the invention to creep testing under their own weight at 850° C. for 200 hours.

[0104] These sheets, made using the method described above, were obtained from hot-rolled and pickled sheets, cold-rolled at ambient temperature to the final thickness given in Table 3 for each test, and then final annealed under the annealing conditions given in Table 3. The sheets were then pickled and the grain size was measured using the circular intercept procedure described in the ASTM E112 standard. Table 3 also specifies the average grain size for each test.

[0105] Creep was measured using a creep test known as the "Sag test." The Sag test is not normalized but is used to characterize creep. This test is described in the paper by Faria, Geraldo Lucio de; Melo, Denilson Pereira de; Moreira, Paulo S▲e▼rgio, "Using the Sag Test Method to Validate or Improve the Fluid Composition of AISI 321 and AISI 441 Inoxides," pp. 34-44, 75° Congresso Anual da ABM, S▲a▼o Paulo, 2022, ISSN: 2594-5327, DOI 10.5151 / 2594-5327-34135.

[0106] For the creep test, strips of metal are cut from the sheet in the as-delivered state (cold-rolled, annealed and pickled), the strips having a length of 205 mm and cut in the rolling direction to a width of 25 mm and a thickness corresponding to the final thickness of the strip (1.5 mm or 0.5 mm depending on the test under consideration).

[0107] These flat strips were suspended on two supports with a 200 mm gap in a furnace at 850 °C for a determined time. Deflection measurements, which characterize the deformation under creep, were carried out regularly at 1 hour, 25 hours, 50 hours, 100 hours, and 200 hours. Measurements of the specimen deflection were carried out at ambient temperature on a flat surface, typically marble, using a comparator with an accuracy and resolution of less than 0.05 mm. For each test, three specimens were tested.

[0108] Table 3 below gives the average deflection values ​​of the test specimens after 200 hours of exposure time.

[0109] [Table 3]

[0110] These tests show that a grain size of less than 30 μm for a sheet thickness of 1.5 mm reduces creep properties. After 200 hours at 850°C, deflections of more than 3 mm were observed. Similarly, an average grain size of less than 15 μm for a sheet thickness of 0.5 mm reduces creep properties. After 200 hours at 850°C, deflections of more than 9 mm were observed.

[0111] Therefore, embodiments in which the average grain size satisfies the conditions specified above are particularly advantageous in terms of creep resistance. [Explanation of symbols]

[0112] 1. Base Metals 2. Oxide layer 3. Silicon oxide 4 Inner layer 5. Outer Layer

Claims

1. A ferritic stainless steel sheet, Its composition, expressed by weight, contains: C≦0.03%, 0.25%≦Mn≦1%, preferably 0.3%≦Mn≦0.5%, 0%<Si≦0.20%, preferably Si≦0.15%, With Mn / Si≧1.2, S≦0.005%, P≦0.04%, 19.0%≦Cr≦24.0%, Ni≦0.5%, Mo≦0.10%, N≦0.03%, Cu≦0.20%, 0.40%≦Nb≦1.0%, 0.05%≦Ti≦0.2%, preferably 0.05%≦Ti≦0.15%, Zr≦0.02%, Al≦0.02%, V≦0.2%, Co≦0.05%, Sn≦0.05%, Rare earths≦800 ppm, It is understood that: V+Zr+Al≦0.2%, Ti+V+Zr+Al≦0.30%, Ti+Nb≦1.0%, Ni+Cu+Co≦0.60%, 2 × Nb − 7 × C ≧ 0.8%, 0%≦Ti−4×N≦0.15%, 0.2 ppm≦Ca≦20 ppm, 1 ppm≦O≦60 ppm, the remainder of the composition consists of iron and unavoidable process impurities; The sheet is an annealed and pickled sheet, The sheet contains less than 0.2% Fe ２ A ferritic stainless steel sheet containing a volume fraction of Nb Laves phase.

2. 2. The ferritic stainless steel sheet according to claim 1, wherein the rare earth content is 50 ppm≦rare earth≦800 ppm.

3. The ferritic stainless steel sheet according to claim 1 or 2, wherein Nb-10×(C+N)≧0%.

4. The ferritic stainless steel sheet according to any one of claims 1 to 3, characterized in that the sheet has a thickness of 0.1 mm or more and 2.5 mm or less.

5. The sheet is When the sheet has a thickness of 1.2 mm or more and 2.5 mm or less, an average particle size of 30 micrometers or more and 80 micrometers or less; and when the sheet has a thickness of 0.1 mm or more but less than 1.2 mm, an average grain size of 15 micrometers or more and 80 micrometers or less; The ferritic stainless steel sheet according to claim 4, characterized in that it has

6. The ferritic stainless steel sheet according to any one of claims 1 to 5, characterized in that the sheet is a cold-rolled and annealed sheet.

7. When subjected to a heat treatment at a temperature of 850°C for 1000 hours, the sheet contains 0.8% or more Fe. ２ 7. A ferritic stainless steel sheet according to claim 1, characterized in that it contains a volume fraction of Nb Laves phase.

8. When subjected to a heat treatment at 850°C for 1000 hours, the sheet contains less than 0.05% Fe. ３ Nb ３ A ferritic stainless steel sheet according to any one of claims 1 to 7, characterized in that it contains a volume fraction of X phase.

9. 9. The ferritic stainless steel sheet according to any one of claims 1 to 8, characterized in that when subjected to a heat treatment at a temperature of 850 °C for a period of 1000 hours, the sheet on each of its surfaces comprises an oxide layer (2) such that the surface fraction of precipitates of silicon oxide (3) at the interface between the sheet steel and the oxide layer (2) is less than or equal to 0.

35.

10. 10. The ferritic stainless steel sheet according to claim 9, characterized in that the oxide layer (2) has a thickness of 10 μm or less.

11. 1. A method for producing ferritic stainless steel sheet, comprising: - a steel having a composition according to any one of claims 1 to 3 is produced; - blanks are cast from this steel; - the blank is held at a temperature of between 1150°C and 1260°C for a period of between 40 and 60 minutes, and the blank is hot rolled to obtain a hot rolled sheet having a thickness of between 2.5mm and 6mm; - the hot-rolled sheet is annealed; the hot-rolled and annealed sheet is pickled; - the hot-rolled sheet is cold-rolled, in a single step or in several steps separated by intermediate annealings, at a temperature above ambient temperature and up to 300°C; a final annealing of the cold-rolled sheet at a temperature between 1000°C and 1100°C for a period of between 10 seconds and 6 minutes in order to obtain a fully recrystallized structure.

12. 12. The method of claim 11, wherein the annealing of the hot rolled sheet is carried out at a temperature of 1000°C to 1100°C for a time of 30 seconds to 6 minutes.

13. 13. A method according to claim 11 or 12, characterized in that the intermediate annealing operation or operations are carried out at a temperature of 950°C to 1100°C for a time of 30 seconds to 6 minutes.

14. 14. The method according to any one of claims 11 to 13, characterized in that the final annealing is carried out at a temperature of ≥ 1050°C and ≤ 1090°C.

Images