Ferritic stainless steel sheet and associated production method

Abstract

The invention relates to a ferritic stainless steel sheet, the composition of which comprises, the contents being 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 ≤ 800 ppm, it being understood that: V + Zr + Al ≤ 0.2% Ti + V + Zr + Al ≤ 0.30% Ti + Nb ≤ 1.0% Ni + Cu + Co ≤ 0.60% 2xNb-7xC ≥ 0.8% 29 0% ≤ Ti - 4xN ≤ 0.15% 0.2 ppm ≤ Ca ≤ 20 ppm 1 ppm ≤ O ≤ 60 ppm, the remainder of the composition consisting of iron and unavoidable impurities resulting from the preparation, the sheet being an annealed and pickled sheet, the sheet comprising a volume fraction of Fe2Nb Laves phases of less than 0.2%.

Description

[0001] Ferritic stainless steel sheet and associated manufacturing process

[0002] The present invention relates to a ferritic stainless steel sheet and a method of manufacturing the same.

[0003] The development of high-temperature electrochemical applications (fuel cells or solid oxide electrolyzers) to transform hydrogenated compounds into electricity or, conversely, produce hydrogenated products from decarbonized electricity requires the use of new materials that must operate between 500°C and 1000°C for tens of thousands of hours with a few dozen on-off cycles. The anodes, cathodes and electrolytes constituting the cell of these systems are made from ceramics or cermet, the nature of which can vary depending on the design. These cells are supported or linked to metal parts called interconnectors, the role of which is to distribute the reactive gases in the cell and to collect the electrons. The low coefficient of thermal expansion of cells, whose electrolyte is often based on yttria-containing zirconia (10.5x10" 6 K -1) requires the use of high-chromium ferritic stainless steels whose coefficient of thermal expansion is very close and which are capable of resisting oxidation at these high temperatures. Furthermore, the interconnectors must not cause degradation of the cell's performance by contamination of the anode and cathode, in particular by diffusion of chromium or by excessively high electrical resistivity of the oxide forming on its surface.

[0004] The ferritic stainless steels used today are very high in chromium, with certain residuals such as silicon kept very low and contain rare earths (especially lanthanum). Their production under vacuum from pure raw materials is therefore expensive. Since the weight share of the interconnectors is very high in these systems, the competitiveness of these electrolysers is affected. The idea is therefore to use more conventional ferritic stainless steels produced by electric furnace such as AISI 441 or 444 steels which have good hot properties. However, experiments show that, under certain operating conditions, the performance of such steels deteriorates more quickly despite an oxide layer of comparable composition and thickness.The origin of this faster degradation of performance is a segregation of silicon at the metal-oxide interface which forms a more or less continuous thin film of silica (silicon oxide) which is very electrically resistive, with a resistivity of the order of 10. 6 Q.cm at 850°C. This value should be compared to the resistivity of chromine CrsOs, which is around 100 Q.cm at 850°C. The silica film is very thin in thickness, and typically has a thickness of around a few tens of nanometers at 850°C after 1000 hours of operation. However, given the electrical resistivity of silica, it contributes to a significant increase in the area specific resistance (ASR). However, silicon cannot be completely eliminated by current production processes at a reasonable cost.

[0005] An aim of the invention is therefore to provide a ferritic stainless steel sheet having conductivity properties comparable to those of very high chromium steels, even after use for long periods at high temperature.

[0006] Preferably, we also seek to improve the creep resistance of the sheet metal because the system is also subject to thermomechanical loads which can generate deformations modifying the contacts and functions of the interconnectors.

[0007] To this end, the invention relates to a ferritic stainless steel sheet, the composition of which comprises, the contents being expressed by weight:

[0008] C ≤ 0.03%

[0009] 0.25% ≤ Mn ≤ 1%, preferably 0.3% ≤ Mn ≤ 0.5% 0% ≤ Si ≤ 0.20%, preferably Si ≤ 0.15% with Mn / Si > 1.2

[0010] S ≤ 0.005%

[0011] P ≤ 0.04%

[0012] 19.0% ≤ Cr ≤ 24.0%

[0013] 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%

[0014] V ≤ 0.2%

[0015] Co ≤ 0.05%

[0016] Sn ≤ 0.05%,

[0017] Rare earths ≤ 800 ppm, it being understood that:

[0018] V + Zr +AI ≤ 0.2% Ti + V + Zr +AI ≤ 0.30%

[0019] Ti + Nb ≤ 1.0%

[0020] Ni + Cu + Co s 0.60%

[0021] 2xNb-7xC > 0.8%

[0022] 0% ≤ Ti - 4xN ≤ 0.15%

[0023] 0.2 ppm ≤ Ca ≤ 20 ppm

[0024] 1 ppm ≤ O ≤ 60 ppm the remainder of the composition being made up of iron and unavoidable impurities resulting from the production, the sheet being an annealed and pickled sheet, the sheet comprising a volume fraction of Laves Fe2Nb phases of less than 0.2%.

[0025] The ferritic steel sheet according to the invention may also comprise one or more of the following characteristics, taken individually or in any technically possible combination:

[0026] - the alloy has a rare earth content of between 50 ppm and 800 ppm.

[0027] - the niobium content of the alloy respects the following relationship: Nb - 10x(C+N) > 0%.

[0028] - the sheet metal has a thickness between 0.1 mm and 2.5 mm;

[0029] - the sheet has an average grain size of between 30 micrometers and 80 micrometers when the sheet has a thickness of between 1.2 mm and 2.5 mm and an average grain size of between 15 micrometers and 80 micrometers when the sheet has a thickness greater than or equal to 0.1 mm and less than 1.2 mm;

[0030] - the sheet is a cold-rolled and annealed sheet;

[0031] - when subjected to heat treatment at a temperature of 850°C for a period of 1000 hours, the sheet comprises a volume fraction of Laves Fe2Nb phases greater than or equal to 0.8%;

[0032] - when subjected to heat treatment at a temperature of 850°C for a period of 1000 hours, the sheet comprises a volume fraction of cubic phases Fe3Nb3X of less than 0.05%;

[0033] - when subjected to a heat treatment at a temperature of 850°C for a period of 1000 hours, the sheet comprises, on each of its faces, a layer of oxides and, at the interface between the steel of the sheet and the layer of oxides, silicon oxide precipitates, such that the surface fraction of the silicon oxide precipitates at the interface between the steel of the sheet and the layer of oxides is less than or equal to 0.35; and - the layer of oxides has a thickness less than or equal to 10 μm.

[0034] The invention also relates to a method of manufacturing a ferritic stainless steel sheet comprising the following steps:

[0035] - a steel is produced having the composition as described above;

[0036] - a semi-finished product is cast from this steel;

[0037] - the semi-finished product is heated to a temperature greater than or equal to 1150°C and less than or equal to 1260°C for a period of between 40 minutes and 60 minutes and the semi-finished product is hot-rolled to obtain a hot-rolled sheet with a thickness of between 2.5 mm and 6 mm;

[0038] - the hot-rolled sheet is annealed,

[0039] - the hot-rolled and annealed sheet is stripped,

[0040] - said hot-rolled sheet is cold-rolled, at a temperature between room temperature and 300°C, in a single stage or in several stages separated by intermediate annealing;

[0041] - a final annealing of the cold-rolled sheet is carried out at a temperature between 1000°C and 1100°C and for a duration between 10 seconds and 6 minutes to obtain a completely recrystallized structure.

[0042] The manufacturing method according to the invention may also comprise one or more of the following characteristics, taken in isolation or in any technically possible combination:

[0043] - the annealing of the hot-rolled sheet is carried out at a temperature between 1000°C and 1100°C, for a period of 30 seconds to 6 minutes;

[0044] - the intermediate annealing(s) are carried out at a temperature between 950°C and 1100°C for a period of 30 seconds to 6 minutes; and

[0045] - the final annealing is carried out at a temperature between 1050°C and 1090°C.

[0046] The invention will be well understood and other aspects and advantages will appear more clearly on reading the following detailed description of an exemplary embodiment given with reference to the appended figures, in which:

[0047] Figure 1 schematically illustrates a sheet according to the invention seen in a cross-section after an aging heat treatment;

[0048] Figure 2 schematically illustrates the determination of the surface fraction of silica precipitates at the metal-oxide interface after such an aging treatment.

[0049] The invention relates to a ferritic stainless steel sheet, the composition of which comprises, the contents being expressed by weight: C ≤ 0.03%

[0050] 0.25% ≤ Mn ≤ 1%, preferably 0.3% ≤ Mn ≤ 0.5%

[0051] 0% ≤ Si ≤ 0.20%, preferably Si ≤ 0.15% with Mn / Si > 1.2

[0052] S ≤ 0.005%

[0053] P ≤ 0.04%

[0054] 19.0% ≤ Cr ≤ 24.0%.

[0055] When ≤ 0.5%

[0056] For ≤ 0.10%

[0057] N ≤ 0.03%

[0058] Cu ≤ 0.20%

[0059] 0.40% ≤ Nb ≤ 1.0%.

[0060] 0.05% ≤ Ti ≤ 0.2%, preferably 0.05% ≤ Ti ≤ 0.15%

[0061] Zr ≤ 0.02%

[0062] At ≤ 0.02%

[0063] V ≤ 0.2%

[0064] With ≤ 0.05%

[0065] Sn ≤ 0.05%,

[0066] T rare earths ≤ 800 ppm being understood to be:

[0067] V + Zr +AI ≤ 0.2%.

[0068] Ti + V + Zr +AI ≤ 0.30%

[0069] Ti + Nb ≤ 1.0%

[0070] Ni + Cu + Co ≤ 0.60%

[0071] 2xNb–7xC ≥ 0.8%.

[0072] 0% ≤ Ti − 4xN ≤ 0.15%

[0073] 0.2 ppm ≤ Ca ≤ 20 ppm

[0074] 1 ppm ≤ O ≤ 60 ppm the rest of the composition being made up of iron and unavoidable impurities resulting from the production.

[0075] Regarding the chemical composition of steel, carbon increases the mechanical characteristics at high temperatures, in particular the creep resistance. However, due to its very low solubility in ferrite, carbon tends to precipitate in the form of M23C6 or M7C3 carbides at a temperature below about 900°C. This precipitation, generally located at the grain boundaries, can lead to a depletion of chromium in the vicinity of these boundaries and therefore to sensitization to intergranular corrosion. This sensitization can be found in particular in Heat Affected Zones in welding which have been reheated to very high temperatures. The carbon content must therefore be limited to at most 0.03% to obtain satisfactory resistance to intergranular corrosion as well as not to reduce formability. In addition, the carbon content must satisfy a relationship with niobium, as will be explained later.

[0076] Chromium is an essential element for stabilizing the ferritic phase and for increasing resistance to oxidation. In conjunction with the other elements in the composition, its minimum content must be greater than or equal to 19.0% in order to obtain a ferritic structure at any temperature and good resistance to cyclic oxidation, particularly when the thickness of the sheet is thin (less than or equal to 0.5 mm) and this thickness limits the reservoir of chromium available during oxidation. Indeed, during aging, and as illustrated in Figure 1, an oxide layer 2 comprising an internal layer 4 of chromia Cr2O3 and possibly an external layer 5 of manganese-rich chromium oxide forms on the surface of the metal substrate 1 and protects the steel over very long periods and at high temperatures.The maximum chromium content must not, however, exceed 24.0%, otherwise the mechanical resistance at room temperature will be excessively increased, generating significant brittleness and degrading the ability to be shaped.

[0077] In the context of this invention, the terms "internal" and "external" are used in relation to proximity to the metal substrate 1, an internal layer being closer to the metal substrate 1 than an external layer.

[0078] The alloy has a manganese content of between 0.25% by weight and 1% by weight. At these contents, manganese increases the mechanical characteristics of the alloy and also allows the formation of an external layer 5 of manganese-rich chromium oxide which may contain spinel-type iron (Mn,Fe)Cr2O4. The good thermodynamic stability of these oxides makes it possible to limit the evaporation of chromium at high temperatures in the presence of water vapor. This external layer 5 of manganese-rich chromium oxide is also favorable to the good adhesion of protective coatings, for example a coating of the LSM type (strontium-doped lanthanum manganite) or of the MCO type (manganese cobalt oxide spinel). These spinels also have very good electrical conductivity, with a resistivity of the order of 20 Q.cm at 850°C.However, beyond 1% by weight, the hot oxidation kinetics become too rapid and a thick, highly adherent oxide layer develops, making pickling operations difficult during sheet metal production. Manganese is also a gamma-generating element that should be limited, like Ni, Cu, Co in ferritic steels. The manganese content is therefore limited to 1%.

[0079] Preferably the manganese content is between 0.3% and 0.5%.

[0080] Silicon is a very effective element for increasing oxidation resistance. However, silicon oxide (silica) 3, which forms at the metal-oxide interface, has a very low coefficient of expansion, of the order of 1.10 -6 K' 1, ten times lower than that of the base metal and chromium oxide Cr2O3, reducing the adhesion of the oxide layer 2 as a whole. However, poor adhesion of the oxide layer 2 leads to degraded conductivity properties. In addition, silicon has a high electrical resistivity, which is very detrimental in the intended application since the oxidized metal must have good electrical conductivity. Silicon is also a cold-hardening element of ferrite, reducing its ductility and its suitability for cold forming. The silicon content must therefore be limited to a minimum and must not exceed 0.20% by weight with a preference for a maximum of 0.15%. The silicon content is greater than 0%, due to the unavoidable presence of silicon as a trace. The silicon content generally remains greater than or equal to 0.05%. Indeed, reducing the Si content below this value requires expensive production processes.

[0081] Furthermore, the Mn / Si ratio must be greater than or equal to 1.2, to promote the formation of the external layer 5 of manganese-rich chromium oxide of the type (Mn,Fe)Cr2C>4 described above to the detriment of silicon oxide.

[0082] Sulfur and phosphorus are impurities that reduce hot ductility and formability. Phosphorus readily segregates at grain boundaries and reduces their cohesion. Sulfur is also detrimental to oxidation by segregating at the metal-oxide interface and reducing its adhesion to the metal. As such, the sulfur and phosphorus contents must be less than or equal to 0.005% and 0.04% by weight, respectively.

[0083] Nickel is a gamma-generating element that increases the ductility of steel. In order to maintain a single-phase ferritic structure, its content is limited. Furthermore, nickel does not improve the desired properties, and its intentional addition would increase the production cost due to its high price. Its content should be as low as possible and less than or equal to 0.5% by weight.

[0084] Molybdenum not only increases high-temperature strength but also oxidation resistance. However, in high-chromium steels containing titanium and niobium, it causes brittleness of the ferritic matrix, particularly in hot-rolled strips with a thickness of between 2.5 mm and 6 mm. Molybdenum excessively reduces ductility and formability, and is an expensive addition element. Its content should be less than or equal to 0.10%, preferably strictly less than 0.10%.

[0085] Like carbon, nitrogen improves mechanical properties. However, nitrogen tends to precipitate at grain boundaries in the form of nitrides, thus reducing corrosion resistance. To limit intergranular corrosion sensitization problems, the nitrogen content should be less than or equal to 0.03%.

[0086] Copper has a heat-hardening effect. However, in excessive quantities, it reduces ductility during hot rolling. Like nickel, it is also a gamma-generating element that must be limited. As such, the copper content must be less than or equal to 0.20% by weight.

[0087] Niobium is an important element of the invention. Usually, this element can be used as a stabilizing element in ferritic stainless steels: in fact, the phenomenon of sensitization to intergranular corrosion mentioned above can be avoided by the addition of elements forming carbides or carbonitrides which are very thermally stable. In this way, carbon and nitrogen in solution are reduced as much as possible and thus a subsequent precipitation of chromium carbides and nitrides is avoided. Niobium, as well as titanium and, to a lesser extent, zirconium and vanadium, therefore stably fixes carbon and nitrogen.

[0088] But niobium also combines with iron to form certain intermetallic compounds in the 650°C-950°C range: the inventors have demonstrated that an intergranular precipitation of hexagonal Fe2Nb occurring at high temperature could be used to increase the mechanical properties when hot, particularly in creep, therefore in the targeted conditions of use.

[0089] Furthermore, the inventors discovered that precipitates of Fe2Nb compounds with a hexagonal structure (called Laves phases) capture part of the silicon, and therefore minimize the formation of silicon oxides, which is sought for the application. In particular, compared to the cubic Fe3Nb3X phases, where X denotes nitrogen, oxygen or carbon, which are also likely to form, the Fe2Nb Laves phases contain approximately four times more silicon by mass percentage. Adjusting the initial silicon content and capturing part of the silicon by such phases makes it possible to very significantly reduce the segregation of silicon at the metal-oxide interface and the formation of highly resistive silica. In addition, the nature and spatial distribution of these precipitates in the intergranular region, i.e. at the grain boundaries, are very favorable for resisting creep up to 1000°C.In order to form Fe2Nb phases under the intended conditions of use, several conditions must be met: the niobium content must be between 0.40% and 1.0% and such that 2xNb-7xC > 0.8%, and the composition further comprises titanium at a content between 0.05% and 0.2%, and such that 0% ≤ Ti - 4xN ≤ 0.15%. Preferably, the niobium, carbon and nitrogen contents are further such that Nb - 10x(C+N) > 0%.

[0090] If the total Nb content of the steel is less than 0.40%, the steel is insufficiently stabilized and the amount of Fe2Nb precipitates formed at high temperature is insufficient to obtain the desired high temperature properties. To obtain this favorable precipitation of niobium, the inventors have also highlighted the importance of the effective niobium content: effective niobium refers to the amount of niobium in solid solution available to precipitate with iron, assuming that carbon and nitrogen have completely 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 respect the relationship 2xNb-7xC ≥ 0.8%, and preferably respect the relationship Nb - 10x(C+N) > 0%.

[0091] Conversely, a significant excess of Nb is embrittling, particularly in a very high chromium ferrite. This excess greatly increases hardness and slows recrystallization, which also limits the metal's ability to be cold worked. The niobium content is therefore limited to Nb ≤ 1.0%.

[0092] To promote the formation of Fe2Nb phases, to the detriment of Fe3Nb3X phases, the titanium content must be greater than or equal to 0.05%, but less than or equal to 0.2% to limit excessive internal oxidation at high temperature. Preferably, the titanium content is greater than or equal to 0.05% and / or less than or equal to 0.15%.

[0093] Furthermore, titanium must respect a relationship with nitrogen 0% ≤ Ti-4xN ≤ 0.15%. Otherwise, niobium precipitates, not in the form of Fe2Nb with a hexagonal structure from 650°C, but in the form of cubic Fe3Nb3X compounds, which are less effective in trapping silicon.

[0094] Furthermore, the sum of the Nb and Ti contents must be less than or equal to 1.0% to avoid excessive brittleness of the metal and guarantee satisfactory resilience of the welds.

[0095] Vanadium, zirconium and aluminum are nitrogen stabilizing elements that increase mechanical strength at high temperatures, but the total content of these elements should be limited to at most 0.2% so as not to reduce formability. Furthermore, the aluminum content is limited to at most 0.02% because aluminum is susceptible to oxidation to form alumina, which is highly electrically resistive. In particular, the electrical resistivity of alumina is around 10 7 Q.cm at 850°C, alumina is therefore ten times more resistive than silica (SiO2).

[0096] The zirconium content is at most 0.02% so as not to reduce formability and limit the risk of surface defects.

[0097] The vanadium content is at most 0.2% so as not to reduce formability.

[0098] Furthermore, the inventors have highlighted that the contents of titanium, aluminum, vanadium and zirconium must be jointly limited in order to limit the brittleness of the metal. The sum of their contents must be such that: Ti+AI+V+Zr ≤ 0.30%.

[0099] Cobalt is a heat-hardening element but degrades formability. Therefore, its content must be less than or equal to 0.05% by weight.

[0100] In addition, the content of cobalt added to that of copper and nickel must be as low as possible to avoid an increase in electrical resistivity detrimental to the application. To this end, the total Ni+Cu+Co content must be less than or equal to 0.60%.

[0101] To avoid hot forgeability problems, the tin content should be less than or equal to 0.05%.

[0102] The alloy also contains calcium at a level of between 0.2 ppm and 20 ppm. Calcium is used in the steelmaking process to reduce the sulfur and oxygen content of the steel. Its concentration should be limited to prevent internal oxidation of the metal during use at high temperatures.

[0103] The oxygen content of steel ranges from 1 ppm to 60 ppm, with an average oxygen content of around 20 ppm. As with calcium, its concentration should be limited to avoid the presence of oxide inclusions or internal oxidation of the metal when used at high temperatures.

[0104] Rare earth elements (REE) can be added to improve the adhesion of the oxide layers that make the steel corrosion-resistant. However, the rare earth content should not exceed 800 ppm. Beyond this content, metal production could be made difficult due to the reactions of the rare earths with the refractories coating the ladle. These reactions would lead to the significant formation of REE oxides that would degrade the inclusion cleanliness of the steel. In addition, the effectiveness of REE is sufficient at the proposed levels, and going beyond this would only unnecessarily increase the production cost due to the high price of REE, and also the accelerated wear of the refractories that this would cause. When rare earths are added, their content is preferably at least 50 ppm. Preferably, the rare earths are chosen from cerium, lanthanum and yttrium or their combinations.In particular, rare earths comprise a mixture of cerium and lanthanum. Preferably, the rare earths consist of a mixture of cerium and lanthanum.

[0105] The steel according to the invention is generally in the form of cold-rolled and annealed sheet and the thickness of the sheet is generally between 0.1 mm and 2.5 mm.

[0106] According to the invention, the structure of the steel in the delivery state, i.e. of the cold-rolled and annealed sheet, is completely recrystallized.

[0107] The flexural creep of sheet metal at high temperatures is a function of sheet thickness. For the same mechanical load and metallurgical condition, a thinner sheet metal deforms more in bending.

[0108] 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 micrometers and 80 micrometers, i.e., an ASTM index (ASTM E-112 standard) of between 4 and 7. In this strip thickness range, a grain size greater than or equal to 30 micrometers (ASTM ≤ 7) is advantageous, since it ensures low creep deformation in bending of the sheet at high temperature relative to its thickness. Furthermore, in this strip thickness range, the grain size is preferably less than or equal to 80 micrometers (ASTM > 4). Indeed, a grain size greater than 80 micrometers (ASTM ≤ 4) leads to the appearance of unsightly surface irregularities, known as "orange peel" appearance, during shaping at room temperature and detrimental to the good adhesion of the protective coating.

[0109] 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 preferably between 15 micrometers and 80 micrometers, i.e., an ASTM index of between 4 and 9. In this strip thickness range, a grain size greater than or equal to 15 micrometers (ASTM ≤ 9) is advantageous, since it ensures low creep deformation of the sheet at high temperature relative to its thickness. Furthermore, in this strip thickness range, the grain size is preferably less than or equal to 80 micrometers (ASTM > 4). Indeed, a grain size greater than 80 micrometers (ASTM ≤ 4) leads to the appearance of unsightly surface irregularities, known as an “orange peel” appearance, during shaping at room temperature, and is detrimental to the good adhesion of the protective coating.

[0110] The microstructure of the cold-rolled and annealed sheet in the as-delivered condition includes precipitates that consist essentially of intragranular titanium and niobium carbonitrides. Indeed, the annealing carried out on the sheet has the effect of dissolving the intermetallic precipitates of the Laves Fe2Nb phase type with hexagonal structure and of the Fe3Nb3X type with cubic structure present in the microstructure.

[0111] In particular, the volume fraction of Laves Fe2Nb phases in the cold-rolled and annealed sheet in the as-delivered condition is less than 0.2%.

[0112] The volume fraction of Fe2Nb Laves phases is determined as follows.

[0113] First, standard polishing is carried out with abrasives with grain sizes up to 1 pm, followed by electrolytic etching in 60% nitric acid on a cross-section of a sample, taken in a direction orthogonal to the rolling direction.

[0114] Observation is made using an electron microscope in backscattered electron mode. A minimum magnification of x1000 is used to obtain an overall view and accurately determine particle size. Five images are taken per sample and condition. The backscattered electron mode generates a chemical composition contrast on a scale of 256 levels, called grayscale, ranging from white (255) to black (0).

[0115] The volume fraction of the intermetallic phases Fe2Nb and Fe3Nb3X is determined by image analysis, for example using Image J software, from the images thus obtained.

[0116] First, the images obtained are processed using a thresholding method to keep only the whitest precipitates corresponding to the intermetallic phases Fe2Nb and Fe3Nb3X. Indeed, in backscattered electron mode, the precipitates rich in niobium and iron, due to their atomic number, appear as the lightest compounds in the images. Consequently, the intermetallic phases Fe2Nb and Fe3Nb3X are distinguished from the niobium and titanium carbonitrides in the images obtained by chemical contrast and appear lighter. During this thresholding step, for each image, the threshold is chosen to distinguish the intermetallic precipitates Fe2Nb and Fe3Nb3X from the rest of the image. After thresholding, the intermetallic precipitates are represented in white in a black matrix.

[0117] Manual filtering of the images is then carried out to eliminate artifacts such as holes or impurities.

[0118] The surface fraction of intermetallic phases Fe2Nb and Fe3Nb3X is then determined from this thresholded and filtered image. In this context, it is assumed that the volume fraction is equal to the surface fraction. This gives the volume fraction of intermetallic phases Fe2Nb and Fe3Nb3X. Furthermore, the Fe / Nb ratio is measured by energy dispersive spectroscopy (EDS) in each of the images. Since this ratio is different in each of these intermetallic phases, it makes it possible to distinguish between cubic intermetallic Fe3Nb3X and hexagonal intermetallic Fe2Nb, and thus to determine the volume fraction of Laves phases Fe2Nb from the total volume fraction of intermetallic phases Fe2Nb and Fe3Nb3X.

[0119] In cold-rolled and annealed sheet, niobium is predominantly in solid solution. In particular, the mass content of Nb in solid solution in cold-rolled and annealed sheet in the as-delivered condition is at least 0.3%.

[0120] The sheet according to the invention is further characterized in that, when subjected to a heat treatment at a temperature between 650°C and 1000°C for a time greater than or equal to 30 minutes, the structure of the sheet comprises, in addition to the titanium and niobium carbonitrides mentioned above, a homogeneous and intergranular precipitation of Fe2Nb compounds of hexagonal structure (Laves phases). The structure of the sheet subjected to such a heat treatment may further comprise an intragranular precipitation of Fe2Nb compounds, which however decreases when the duration of the heat treatment increases.

[0121] In particular, after an aging heat treatment in air at a temperature of 850°C for a period of 1000 hours, the volume fraction of Laves Fe2Nb phases in the sheet is at least 0.8%. Furthermore, these Laves Fe2Nb phases are overwhelmingly intergranular.

[0122] The aging heat treatment in air at a temperature of 850°C for a period of 1000 hours is considered representative of the conditions of use of the steel and is generally used for the qualification of steels.

[0123] According to the invention, Fe2Nb precipitates are the vast majority among the intergranular precipitates. As mentioned above, these precipitates have the advantage of capturing part of the silicon to reduce its content in the solid solution, which is then less likely to segregate at the metal-oxide interface.

[0124] In particular, after such heat treatment, the volume fraction of Fe3Nb3X precipitates remains less than 0.05%.

[0125] Moreover, together with the grain size, the nature and distribution of these Fe2Nb precipitates are very favorable for resisting creep up to 1000°C.

[0126] Due to the formation of Fe2Nb precipitates, silica segregation at the metal-oxide interface is very limited compared to prior art steels. In particular, after such an aging treatment in air at a temperature of 850°C for 1000 hours, the surface fraction of silica segregation at the metal-oxide interface remains limited.

[0127] In particular, at the end of such an aging treatment, as illustrated in Figure 1, the sheet comprises, on each of its faces, a layer of oxides 2 above the base metal 1. The layer of oxides 2 comprises an internal layer 4 of chromium oxide Cr2O3 (chromine) and an external layer 5 of chromium oxide rich in manganese and which may contain spinel-type iron (Mn,Fe)Cr2O4.

[0128] At the interface between the oxide layer 2 and the steel of the sheet, also called base metal 1, the sheet comprises precipitates of silicon oxide 3 or silica. In the sheets according to the invention, after such an aging treatment, the surface fraction of the silica precipitates at this metal-oxide interface is less than or equal to 0.35, which means that the silica covers at most 35% of the metal-oxide interface. The electrical conductivity of the silica, of the order of 10 -6 S.cm -1at 850°C, is 10,000 times weaker than that of chromine, of the order of 10 -2 S.cm -1 at 850°C, and its coefficient of thermal expansion is 10 times lower than those of chromia, the base metal, and zirconia, which makes up the electrolyte of the electrochemical cell. This surface fraction of silica at the metal-oxide interface is directly related to the resistivity, specific surface resistance, and conductivity of the metal-oxide interface. Thus, when the surface fraction of silica at the interface is equal to 35%, the electrical conductivity of the interface is reduced by 35% compared to a configuration without silica, going from 10 -2 S.cm -1 in the absence of silica at 6.5.10 -3 S.cm -1or the resistivity of the interface is increased by approximately 50%, from 100 Q.cm to 150 Q.cm. Furthermore, the specific surface resistance of the interface, equal to the resistivity multiplied by the thickness of the silica film, is increased in an equivalent manner.

[0129] The surface fraction of silicon oxide segregation 3 can be determined from an image of a cross-section of a sample, in a direction orthogonal to the rolling direction, obtained by scanning electron microscope with a magnification of x10,000 in backscattered electron mode.

[0130] The horizontal edges of the image are parallel to the surface of the sheet metal. The length L of the observed cross-section, in a plane transverse to the sheet metal, is equal to 12 pm.

[0131] The backscattered electron mode generates a chemical composition contrast on a scale of 256 levels called gray levels ranging from white (255) to black (0). In the images obtained, silica, by its composition and the atomic number of the elements constituting it, appears as the darkest phase in contrast with the base metal which appears very light and the chromium oxide of intermediate gray shade.

[0132] The image thus obtained corresponds to the image noted (a) in Figure 2.

[0133] This image is then analyzed using image analysis software, for example using Image J software.

[0134] More specifically, the image is first transformed by the image analysis software via automated processing in order to increase the contrast between the elements of the image: the base metal 1, the oxide layer 2 and the silicon oxide segregations 3. The objective of this step is that all the acquired images have an identical contrast and highlight the silicon oxide segregations 3 independently of the observed metal-oxide interface and the analyzed oxidized metal. At the end of this processing, the image noted (b) in Figure 2 is obtained.

[0135] The image is then thresholded to distinguish the 3 silicon oxide segregations from the rest of the image. Thus, only two levels are kept, black for the 3 silicon oxide segregations and white for the rest of the image, by setting a threshold suitable for distinguishing the 3 silicon oxide segregations. For example, the threshold is chosen equal to 70, the pixels with a gray level greater than 70 being represented in white 255 and the pixels with a gray level less than or equal to 70 being represented in black. Then, any artifacts are filtered and the processing quality of each image is manually checked.

[0136] From this image, we then determine the sum of the projected lengths Li, on a longitudinal axis, of the regions of the interface in which the silicon oxide precipitates 3 are present in this measurement field, as illustrated in image (c) of Figure 2.

[0137] The area fraction is then calculated as the ratio between the sum of the projected lengths and the length L of the measuring field Z Li / L. In the example illustrated in Figure 2, the area fraction is equal to 77%.

[0138] According to the invention, after heat treatment in air at 850°C for 1000 hours, this Z Li / L ratio remains less than or equal to 0.35, i.e. 35%.

[0139] Furthermore, taking into account the chromium content of the steel according to the invention, the total thickness of the oxide layer 2 generally remains less than or equal to 10 μm after such heat treatment.

[0140] The sheet metal according to the invention can in particular be obtained by the following process:

[0141] - a steel with the above composition is produced;

[0142] - a semi-finished product is cast from this steel; - the semi-finished product is heated to a temperature greater than or equal to 1150°C and less than or equal to 1260°C for a period of between 40 minutes and 60 minutes, and the semi-finished product is hot-rolled to obtain a hot-rolled sheet with a thickness of between 2.5 mm and 6 mm;

[0143] - the hot-rolled sheet is annealed, for example at a temperature between 1000°C and 1100°C for a period of 30 seconds to 6 minutes;

[0144] - the hot-rolled and annealed sheet is stripped;

[0145] - said hot-rolled sheet is cold-rolled, at a temperature between ambient and 300°C, in a single step or in several steps, the sheet being annealed and pickled following each step. It should be understood that, by the term "step", here is meant cold rolling comprising either a single pass or a succession of several passes (for example five passes) which are not separated by any intermediate annealing; it is possible, for example, to envisage a cold rolling sequence comprising a first series of five passes, then an intermediate annealing, then a second sequence of five passes; typically, the intermediate anneals separating the steps are carried out between 950°C and 1100°C for 30 seconds to 6 minutes;

[0146] - a final annealing of the cold-rolled sheet is carried out, at a temperature between 1000°C and 1100°C, preferably between 1050°C and 1090°C, and for a duration between 10 seconds and 6 minutes, to obtain a completely recrystallized structure with an average ASTM grain size preferably between 4 and 7 if the sheet has a thickness between 1.2 mm and 2.5 mm and preferably between 4 and 9 if the sheet has a thickness greater than or equal to 0.1 mm and less than 1.2 mm. This heat treatment allows the niobium to be put into solid solution.

[0147] The volume fraction of Laves phases, i.e. Fe2Nb compounds with hexagonal structure, in the structure of the sheet is very low and less than 0.2% in the delivery state, i.e. after this final annealing.

[0148] We will now describe a series of experiments demonstrating the interest of the invention. We studied laboratory castings whose chemical analyses are given in Table 1. (&ÿns) \ neafqei

[0149] For each of the steels in Table 1, the remainder is iron, as well as unavoidable impurities resulting from production.

[0150] The cast samples were processed according to the following procedure:

[0151] - the cast semi-finished products were heated to a temperature of 1220°C for 40 minutes, and hot rolled to obtain a sheet 5 mm thick;

[0152] - the sheets were annealed at 1080°C for 6 minutes and pickled;

[0153] -hot rolled sheets were cold rolled at room temperature to obtain a sheet of thickness 1.5 mm;

[0154] - a final annealing was carried out on the sheets at a temperature of 1080°C for 4 minutes.

[0155] The volume fraction of Laves phases, i.e. Fe2Nb compounds with hexagonal structure, in the structure of each sheet was determined in the as-delivered condition, i.e. after annealing of the cold-rolled sheet. In Table 2, the volume fraction of Laves phases thus determined in the sheet in the as-delivered condition is indicated in the column "Fe2Nb Laves phases in the as-delivered condition".

[0156] The sheets were then subjected to heat treatment at 850°C for a period of 1000 hours.

[0157] Following this heat treatment, the precipitates present in the structure were determined. In Table 2 below, in the column "Fe2Nb lava phases after treatment at 850°C for 1000 hours", the volume fraction of Laves phases at the end of this heat treatment.

[0158] The volume fraction of intermetallic Fe3Nb3X precipitates in the sheet was also determined after this heat treatment, and this fraction was reported in the column “Cubic phases Fe3Nb3X after treatment at 850°C for 1000 hours” in Table 2.

[0159] The surface fraction of the silicon oxide segregation 3 was then determined for each of the sheets from an image obtained by electron microscope, as described above. The surface fraction thus determined is reported in the column “Surface fraction of silica after treatment at 850°C for 1000 hours” in Table 2.

[0160] The impact strength of the hot, annealed and pickled strip obtained after hot rolling at 1220°C up to a thickness of 5 mm was also determined by an impact strength test on a KCV specimen using the Charpy test in accordance with standard NF EN ISO 148-1 (March 2017 version) during which the energy absorbed by the specimen broken by bending impact is measured as a function of temperature (between -10°C and 80°C). The strip is considered ductile if its impact strength is greater than 30 J / cm 2 for the temperature of 20°C (temperature closest to room temperature). Table 2

[0161] In Table 2, the comparative trials are highlighted.

[0162] It is noted that, for the tests according to the invention (E1 to E3), in which:

[0163] - the niobium and titanium contents meet the conditions described above, namely:

[0164] - Nb between 0.40% and 1.0% and 2xNb-7xC > 0.8%, and

[0165] - Ti between 0.05% and 0.2%, and 0% ≤ Ti - 4xN ≤ 0.15%, and

[0166] - the Mn / Si ratio is greater than 1.2, and

[0167] - the sum Ni+Cu+ Co is between 0 and 0.60%, the volume fraction of intermetallic precipitates Fe2Nb with hexagonal structure is greater than or equal to 0.8% after treatment at 850°C for 1000 hours and the surface fraction of silicon oxide 3 after treatment at 850°C for 1000 hours represents a surface fraction less than or equal to 35%.

[0168] This results in sheets with good performance in terms of electrical conductivity under operating conditions, and in particular comparable to those of very high chromium steels.

[0169] On the contrary, in the case of comparative tests E5 to E8, the titanium and / or niobium contents do not meet the above conditions. In these tests, the volume fraction of intermetallic Fe2Nb precipitates with hexagonal structure is less than 0.8% after treatment at 850°C for 1000 hours. Silicon is therefore less likely to be trapped by the Fe2Nb Laves phases and to limit silica segregation even when silicon is less than 0.2% as in alloy K5. Furthermore, in these comparative tests, the Mn / Si ratio does not meet the above condition. It is noted that, in these tests, the surface fraction of silicon oxide 3 after treatment at 850°C for 1000 hours represents a surface fraction greater than 35%, which leads to degraded performance in terms of electrical conductivity, with an interface resistivity increased by more than 50%.

[0170] Furthermore, in tests E1 to E3, the hot strip is ductile, since it has a resilience greater than 30 J / cm 2 . On the contrary, in the case of comparative tests E4 and E5, in which the steel has a molybdenum content higher than the limits described in the context of the invention, the hot strip obtained is not ductile, since it has a resilience of less than 30 J / cm 2 .

[0171] As explained above, according to an optional aspect:

[0172] - the average grain size of the steel is between 30 micrometers and 80 micrometers, i.e. ASTM index between 4 and 7, when the thickness of the cold-rolled and annealed sheet is between 1.2 mm and 2.5 mm; and

[0173] - the average grain size of the steel is between 15 micrometers and 80 micrometers, i.e. ASTM index between 4 and 9, 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.

[0174] In order to confirm the technical effect of this optional property, the inventors subjected sheets having compositions according to the invention to a creep test under their own weight at 850°C for 200 hours.

[0175] These sheets were obtained from the hot-rolled and pickled sheets obtained by the process described above by cold rolling at room temperature to a final thickness, reported in Table 3 for each test, followed by final annealing under annealing conditions reported in Table 3. The sheets were then pickled and the grain sizes measured by the circular intercept method as described in ASTM E112. Table 3 also specifies the average grain size for each test.

[0176] Creep was measured using a creep test called “Sag Test”. The Sag Test is not standardized but is used to characterize creep. This essay is described in the article Faria, Geraldo Lucio de; Melo, Denilson Pereira of; Moreira, Paulo Sérgio. UTILIZAÇÀO DA METODOLOGIA SAG TEST PARA AVALIAR O COMPORT AMENTO EM FLUÊNCIA DOS AÇOS INOXIDÀVEIS AISI 321 E AISI 441, p. 34-44. In: 75° Congresso Anual da ABM, Sâo Paulo, 2022. ISSN: 2594-5327, DOI 10.5151 / 2594-5327-34135.

[0177] For the creep test, strips of metal 205 mm long, 25 mm wide and of a thickness corresponding to the final thickness of the strip (1.5 mm or 0.5 mm depending on the test considered) are cut from the sheets in the delivered condition (cold rolled, annealed and pickled).

[0178] These flat strips are then suspended in the furnace at 850°C on two supports with a 200 mm air gap for a given period. Deflection measurements, which characterize the creep deformation, are carried out regularly at 1 hour, 25 hours, 50 hours, 100 hours and 200 hours. The measurement of the deflection of the specimen is carried out at room temperature on a flat surface, typically a marble, using a comparator with a precision and resolution of less than 0.05 mm. For each test, three specimens were tested.

[0179] The average values ​​of the specimen deflection after 200 hours of exposure are reported in Table 3 below.

[0180] Table 3

[0181] These tests demonstrate that a grain size of less than 30 μm for 1.5 mm thick sheets degrades creep properties. Indeed, a deflection greater than 3 mm is observed after 200 hours at 850°C. Similarly, an average grain size of less than 15 μm for 0.5 mm thick sheets degrades creep properties. Indeed, a deflection greater than 9 mm is observed after 200 hours at 850°C.

[0182] The embodiment in which the average grain size meets the conditions specified above is therefore particularly advantageous in terms of creep resistance.

Claims

CLAIMS 1. Ferritic stainless steel sheet, the composition of which includes, the contents being 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 ≤ 800 ppm, it being understood that: V + Zr +AI ≤ 0.2% Ti + V + Zr +AI ≤ 0.30% Ti + Nb ≤ 1 .0% Ni + Cu + Co s 0.60% 2xNb-7xC > 0.8% 0% ≤ Ti - 4xN ≤ 0.15% 0.2 ppm ≤ Ca ≤ 20 ppm 1 ppm ≤ O ≤ 60 ppm the remainder of the composition being made up of iron and unavoidable impurities resulting from the production, the sheet being an annealed and pickled sheet, the sheet comprising a volume fraction of Laves Fe2Nb phases of less than 0.2%.

2. Ferritic stainless steel sheet according to claim 1, wherein: 50 ppm ≤ Rare earths ≤ 800 ppm.

3. Ferritic stainless steel sheet according to one of claims 1 or 2, in which: Nb - 10x(C+N) > 0%.

4. Ferritic stainless steel sheet according to any one of claims 1 to 3, characterized in that the sheet has a thickness of between 0.1 mm and 2.5 mm.

5. Ferritic stainless steel sheet according to claim 4, characterized in that the sheet has an average grain size of between 30 micrometers and 80 micrometers when the sheet has a thickness of between 1.2 mm and 2.5 mm and an average grain size of between 15 micrometers and 80 micrometers when the sheet has a thickness greater than or equal to 0.1 mm and less than 1.2 mm.

6. 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. Ferritic stainless steel sheet according to any one of claims 1 to 6, characterized in that, when subjected to a heat treatment at a temperature of 850°C for a period of 1000 hours, the sheet comprises a volume fraction of Laves Fe2Nb phases greater than or equal to 0.8%.

8. Ferritic stainless steel sheet according to any one of claims 1 to 7, characterized in that, when subjected to a heat treatment at a temperature of 850°C for a period of 1000 hours, the sheet comprises a volume fraction of Fe3Nb3X phases of less than 0.05%.

9. 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 comprises, on each of its faces, a layer of oxides (2) and, at the interface between the steel of the sheet and the layer of oxides (2), silicon oxide precipitates (3), such that the surface fraction of the silicon oxide precipitates (3) at the interface between the steel of the sheet and the layer of oxides (2) is less than or equal to 0.

35.

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

11. Process for manufacturing a ferritic stainless steel sheet characterized in that: - a steel is produced having the composition according to any one of claims 1 to 3; - a semi-finished product is cast from this steel; - the semi-finished product is heated to a temperature greater than or equal to 1150°C and less than or equal to 1260°C for a period of between 40 minutes and 60 minutes and the semi-finished product is hot-rolled to obtain a hot-rolled sheet with a thickness of between 2.5 mm and 6 mm; - the hot-rolled sheet is annealed, - the hot-rolled and annealed sheet is stripped, - said hot-rolled sheet is cold-rolled, at a temperature between ambient and 300°C, in a single stage or in several stages separated by intermediate annealing; - a final annealing of the cold-rolled sheet is carried out at a temperature between 1000°C and 1100°C and for a duration between 10 seconds and 6 minutes to obtain a completely recrystallized structure.

12. Method according to claim 11, characterized in that the annealing of the hot-rolled sheet is carried out at a temperature between 1000°C and 1100°C, for a period of 30 seconds to 6 minutes.

13. Method according to one of claims 11 or 12, characterized in that the intermediate annealing or annealings are carried out at a temperature between 950°C and 1100°C for a period of 30 seconds to 6 minutes.

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

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