Ferritic stainless steel plate and related preparation method

Abstract

The invention relates to a ferritic stainless steel plate. The ferritic stainless steel plate comprises the following components in percentage by weight: less than or equal to 0.03% of C; 0.25% < = Mn < = 1%, preferably 0.3% < = Mn < = 0.5%, 0% lt; si < = 0.20%, preferably < = 0.15%, and Mn / Si > = 1.2; s < = 0.005%; p is less than or equal to 0.04%; 19.0% < = Cr < = 24.0%; ni < = 0.5%; 0.10% or less of Mo; less than or equal to 0.03% of N; 0.20% or less of Cu; 0.40% < = Nb < = 1.0%; 0.05% < = Ti < = 0.2%, preferably 0.05% < = Ti < = 0.15%; zr is less than or equal to 0.02%; less than or equal to 0.02% of Al; v < = 0.2%; less than or equal to 0.05% of Co; less than or equal to 0.05% of Sn; rare earth is less than or equal to 800ppm; v + Zr + Al is less than or equal to 0.2%; 0.30% or less of Ti, V, Zr and Al; ti + Nb < = 1.0%; 0.60% or less of Ni + Cu + Co; 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 processing impurities, the sheet being an annealed and pickled sheet comprising less than 0.2% by volume of Fe2Nb Laves phase.

Description

Technical Field

[0001] The present invention relates to a ferritic stainless steel sheet and a related preparation method. Background Art

[0002] The development of high-temperature electrochemical applications (fuel cells or solid oxide electrolyzer cells) that convert hydrogen-containing compounds into electrical energy, or conversely produce hydrogen-containing products from carbon-free electricity, requires the use of new materials that can operate for tens of thousands of hours at temperatures between 500 °C and 1000 °C and have dozens of start-stop cycles. The components of the cells forming these systems, the anode, cathode, and electrolyte, are made of ceramics or cermets, and their types can vary according to the designed functions. These cells are supported or connected to by metal components called interconnects, which have the function of distributing reactive gases within the cells and collecting electrons. The lower coefficient of thermal expansion of the cells, where the electrolyte is usually based on yttria-stabilized zirconia (10.5×10 -6 K -1 ), requires the use of high-chromium ferritic stainless steels that have very close coefficients of thermal expansion and can resist oxidation at these high temperatures. In addition, the interconnects must not cause a decrease in cell performance due to contamination of the anode and cathode, especially due to the diffusion of chromium or due to the too-high resistivity of the oxide formed on their surfaces.

[0003] Currently used ferritic stainless steels have very high chromium contents, along with very low and controllable contents of certain residues (such as silicon), and also contain rare earths (especially lanthanum). Therefore, vacuum production using pure raw materials is expensive. Since the interconnects account for a major weight ratio in these systems, the competitiveness of these electrolyzers is affected. Therefore, it is desirable to use more conventional ferritic stainless steels produced in electric furnaces, such as AISI 441 steel or AISI 444 steel, which have good heat-resistant properties. However, experiments have shown that under certain operating conditions, although the composition and thickness of the oxide layer are comparable, the performance of the above steels degrades faster. The reason for this rapid performance degradation is the segregation of silicon at the metal-oxide interface, which forms a thin film of silicon dioxide (silicon oxide), and this film is more or less continuous and has a high resistivity, with a resistivity of about 10 6 Ω·cm at 850 °C. This value should be compared with the resistivity of chromium oxide Cr2O3, which has a resistivity of about 100 Ω·cm at 850 °C. This silicon dioxide film has a very narrow thickness and usually has a thickness of about dozens of nanometers at 850 °C after operating for 1000 hours. However, considering the resistivity of silicon dioxide, it contributes significantly to increasing the area specific resistance (ASR). However, current production methods cannot completely eliminate silicon at a reasonable cost. Summary of the Invention

[0004] Accordingly, an object of the present invention is to provide a ferritic stainless steel sheet which has electrical conductivity comparable to that of a steel with an extremely high chromium content even after long-term use at high temperatures.

[0005] Preferably, it is also necessary to improve the creep resistance of the sheet because the system is also subjected to thermo-mechanical loads which may produce deformations leading to changes in the contact and function of the interconnects.

[0006] To this end, the subject of the present invention is a ferritic stainless steel sheet having the following composition, expressed in terms of weight percentages:

[0007] C ≤ 0.03%;

[0008] 0.25% ≤ Mn ≤ 1%, preferably 0.3% ≤ Mn ≤ 0.5%,

[0009] 0% < Si ≤ 0.20%, preferably Si ≤ 0.15%, and

[0010] Mn / Si ≥ 1.2;

[0011] S ≤ 0.005%;

[0012] P ≤ 0.04%;

[0013] 19.0% ≤ Cr ≤ 24.0%;

[0014] Ni ≤ 0.5%;

[0015] Mo ≤ 0.10%;

[0016] N ≤ 0.03%;

[0017] Cu ≤ 0.20%;

[0018] 0.40% ≤ Nb ≤ 1.0%;

[0019] 0.05% ≤ Ti ≤ 0.2%, preferably 0.05% ≤ Ti ≤ 0.15%;

[0020] Zr ≤ 0.02%;

[0021] Al ≤ 0.02%;

[0022] V ≤ 0.2%;

[0023] Co ≤ 0.05%;

[0024] Sn ≤ 0.05%;

[0025] Rare earths ≤ 800 ppm;

[0026] It is necessary to satisfy:

[0027] V + Zr + Al ≤ 0.2%;

[0028] Ti + V + Zr + Al ≤ 0.30%;

[0029] Ti + Nb ≤ 1.0%;

[0030] Ni + Cu + Co ≤ 0.60%;

[0031] 2×Nb - 7×C ≥ 0.8%;

[0032] 0% ≤ Ti - 4×N ≤ 0.15%;

[0033] 0.2 ppm ≤ Ca ≤ 20 ppm;

[0034] 1 ppm ≤ O ≤ 60 ppm;

[0035] The remaining part of the composition consists of iron and inevitable processing impurities;

[0036] The sheet is an annealed pickled sheet;

[0037] The sheet contains less than 0.2% by volume of the Fe2Nb Laves phase.

[0038] The ferritic steel sheet of the present invention may further have one or more of the following features individually or in any technically possible combination:

[0039] The alloy has rare earths with a content between 50 ppm and 800 ppm;

[0040] The niobium content of the alloy satisfies the following relationship: Nb - 10×(C + N) ≥ 0%.

[0041] The sheet has a thickness between 0.1 mm and 2.5 mm;

[0042] When the sheet has a thickness between 1.2 mm and 2.5 mm, the sheet has an average grain size between 30 μm and 80 μm; when the sheet has a thickness greater than or equal to 0.1 mm and less than 1.2 mm, the sheet has an average grain size between 15 μm and 80 μm;

[0043] The sheet is a cold-rolled annealed sheet;

[0044] When heat-treated at a temperature of 850 °C for 1000 hours, the sheet contains greater than or equal to 0.8% by volume of the Fe2Nb Laves phase;

[0045] When heat-treated at a temperature of 850 °C for 1000 hours, the sheet contains a Fe3Nb3X cubic phase in a volume fraction of less than 0.05%;

[0046] When heat-treated at a temperature of 850 °C for 1000 hours, the sheet contains an oxide layer on each of its surfaces, and at the interface between the sheet steel and the oxide layer, precipitates including silicon oxide are present such that the surface fraction of the silicon oxide precipitates at the interface between the sheet steel and the oxide layer is less than or equal to 0.35; and

[0047] The oxide layer has a thickness of less than or equal to 10 μm.

[0048] The present invention also relates to a method for preparing a ferritic stainless steel sheet, comprising the following steps:

[0049] Preparing a sheet having the above composition;

[0050] Casting a semi-finished product from the sheet;

[0051] Heating the semi-finished product to a temperature greater than or equal to 1150 °C and less than or equal to 1260 °C, holding for a period between 40 minutes and 60 minutes, and hot-rolling the semi-finished product to obtain a hot-rolled sheet having a thickness between 2.5 mm and 6 mm;

[0052] Annealing the hot-rolled sheet;

[0053] Pickling the hot-rolled and annealed sheet;

[0054] The hot-rolled sheet is cold-rolled at a temperature between ambient temperature and 300 °C in a single step or in multiple steps separated by intermediate annealing operations;

[0055] The cold-rolled sheet is finally annealed at a temperature between 1000 °C and 1100 °C for a period between 10 seconds and 6 minutes to obtain a fully recrystallized structure.

[0056] The preparation method according to the present invention may also have one or more of the following features, individually or in any technically possible combination:

[0057] The annealing of the hot-rolled sheet is carried out at a temperature between 1000 °C and 1100 °C for a period between 30 seconds and 6 minutes;

[0058] The intermediate annealing operation is carried out at a temperature between 950 °C and 1100 °C for a period between 30 seconds and 6 minutes; and

[0059] The final annealing is carried out at a temperature between 1050 °C and 1090 °C. Description of the Drawings

[0060] The present invention will be well understood by reading the following detailed description of the embodiments given with reference to the accompanying drawings, and other aspects and advantages will become apparent, wherein:

[0061] Figure 1 A cross-sectional view of the sheet material according to the present invention after age heat treatment is schematically shown:

[0062] Figure 2 The determination of the surface fraction of precipitates of silicon dioxide at the metal-oxide interface after the age treatment is schematically shown. Detailed Description of the Invention

[0063] The present invention relates to a ferritic stainless steel sheet having the following composition, including the contents expressed by weight:

[0064] C ≤ 0.03%;

[0065] 0.25% ≤ Mn ≤ 1%, preferably 0.3% ≤ Mn ≤ 0.5%,

[0066] 0% < Si ≤ 0.20%, preferably Si ≤ 0.15%, and

[0067] Mn / Si ≥ 1.2;

[0068] S ≤ 0.005%;

[0069] P ≤ 0.04%;

[0070] 19.0% ≤ Cr ≤ 24.0%;

[0071] Ni ≤ 0.5%;

[0072] Mo ≤ 0.10%;

[0073] N ≤ 0.03%;

[0074] Cu ≤ 0.20%;

[0075] 0.40% ≤ Nb ≤ 1.0%;

[0076] 0.05% ≤ Ti ≤ 0.2%, preferably 0.05% ≤ Ti ≤ 0.15%;

[0077] Zr ≤ 0.02%;

[0078] Al ≤ 0.02%;

[0079] V ≤ 0.2%;

[0080] Co ≤ 0.05%;

[0081] Sn ≤ 0.05%;

[0082] Rare earth ≤ 800 ppm;

[0083] It is required to meet:

[0084] V + Zr + Al ≤ 0.2%;

[0085] Ti + V + Zr + Al ≤ 0.30%;

[0086] Ti + Nb ≤ 1.0%;

[0087] Ni + Cu + Co ≤ 0.60%;

[0088] 2 × Nb - 7 × C ≥ 0.8%;

[0089] 0% ≤ Ti - 4 × N ≤ 0.15%;

[0090] 0.2 ppm ≤ Ca ≤ 20 ppm;

[0091] 1 ppm ≤ O ≤ 60 ppm;

[0092] The remaining part of the composition consists of iron and inevitable processing impurities.

[0093] In terms of the chemical composition of the steel, carbon improves the mechanical properties at high temperatures, especially the creep resistance. However, due to the extremely low solubility of carbon in ferrite, at temperatures below about 900 °C, carbon tends to precipitate in the form of carbides M 23 C6 or M7C3. Such precipitates, usually located at grain boundaries, can cause chromium depletion near these grain boundaries, making them vulnerable to intergranular corrosion. Especially in the heat-affected zone of welding heated to very high temperatures, this susceptibility can be encountered. Therefore, the carbon content must be limited to at most 0.03% to obtain satisfactory intergranular corrosion resistance and avoid reducing workability. In addition, the carbon content must satisfy the relationship with niobium, which will be explained below.

[0094] Chromium is an essential element for stabilizing the ferrite phase and improving oxidation resistance. Together with other elements in the composition, its minimum content must be greater than or equal to 19.0% to obtain a ferrite structure and good cyclic oxidation resistance at any temperature, especially when the above-mentioned sheet has a narrow thickness (less than or equal to 0.5 mm), and this thickness limits the storage of chromium available for oxidation. During the aging process, as Figure 1 shown, an oxide layer 2 forms on the surface of the metal substrate 1, which includes an inner layer 4 of chromium oxide Cr2O3 and an optional outer layer 5 of manganese-rich chromium oxide, and protects the steel at high temperatures for a long time. However, the maximum content of chromium shall not exceed 24.0%, otherwise this will excessively increase the mechanical strength at ambient temperature, generate a large amount of brittleness and reduce workability.

[0095] In the context of the present invention, the terms "inner" and "outer" are used in relation to the proximity to the metallic substrate 1, with the inner layer being closer to the metallic substrate 1 than the outer layer.

[0096] The above alloy has a manganese content between 0.25 wt% and 1 wt%. At these contents, manganese improves the mechanical properties of the alloy and is also capable of forming a manganese-rich chromia outer layer 5, which may contain the spinel-type iron (Mn,Fe)Cr2O4. The good thermodynamic stability of these oxides can limit the evaporation of chromium at high temperatures in the presence of water vapor. This manganese-rich chromia outer layer 5 also promotes good adhesion of the protective coating, such as an LSM-type (strontium-doped lanthanum manganite) or MCO-type (cobalt manganese oxide, spinel) coating. These spinels have very good electrical conductivity, with a resistivity of approximately 20 Ω.cm at 850 °C. However, when exceeding 1 wt%, the oxidation kinetics under heat action become too fast, forming a thick and strongly adherent oxide layer, making the pickling operation difficult during the production of sheets. Similar to Ni, Cu, Co, manganese is also a γ element, which must be limited in ferritic steels. Therefore, the manganese content is limited to 1%.

[0097] Preferably, the above manganese content is between 0.3% and 0.5%.

[0098] Silicon is an element that is very effective in improving oxidation resistance. However, the silicon oxide (silicon dioxide) 3 formed at the metal-oxide interface has a very low coefficient of thermal expansion, approximately 1.10 -6 K -1 , which is one-tenth of the coefficient of thermal expansion of the base metal and chromium sesquioxide Cr2O3, reducing the overall adhesion of the oxide layer 2. The low adhesion of the oxide layer 2 leads to a decrease in electrical conductivity. In addition, silicon has a high resistivity, which is very disadvantageous for the target application because the oxidized metal must have good electrical conductivity. When cold, silicon is also a hardening element in ferritic iron, reducing the ductility and cold workability of ferritic iron. Therefore, the silicon content must be limited to a minimum and shall not exceed 0.20 wt%, with a maximum preferably of 0.15 wt%. Due to the inevitable presence of trace amounts of silicon, the silicon content is greater than 0%. The silicon content generally remains greater than or equal to 0.05%. Reducing the silicon content below this value requires expensive processing techniques.

[0099] Furthermore, the Mn / Si ratio must be greater than or equal to 1:2 to promote the formation of the manganese-rich chromia outer layer 5 of the above (Mn,Fe)Cr2O4 type and to be unfavorable for the formation of silicon oxide.

[0100] Sulfur and phosphorus are impurities that reduce the thermoplasticity and workability. Phosphorus tends to segregate at grain boundaries and reduce their cohesion. Sulfur segregates at the metal-oxide interface, reducing the adhesion to the metal and thus being harmful to oxidation. In this regard, the contents of sulfur and phosphorus must be less than or equal to 0.005 wt% and 0.04 wt%, respectively.

[0101] Nickel is a γ element that can improve the ductility of steel. To maintain a single-phase ferritic structure, the content of nickel is restricted. In addition, nickel cannot improve the target properties, and considering the high price of nickel, deliberately adding nickel will increase the production cost. The content of nickel must also be as low as possible and less than or equal to 0.5 wt%.

[0102] Molybdenum not only improves the high-temperature resistance but also the oxidation resistance. However, in high-chromium steels containing titanium and niobium, especially in hot-rolled strip steels with a thickness between 2.5 mm and 6 mm, it can cause weak parts to appear in the ferritic matrix. Molybdenum excessively reduces the ductility and workability and is an expensive additive element. Its content must be less than or equal to 0.10%, preferably strictly less than 0.10%.

[0103] Like carbon, nitrogen improves the mechanical properties. However, nitrogen tends to precipitate at grain boundaries in the form of nitrides, thus reducing the corrosion resistance. To limit the susceptibility to intergranular corrosion, the nitrogen content must be less than or equal to 0.03%.

[0104] Copper has a hardening effect under heat. However, an excess reduces the ductility during hot rolling. Like nickel, it is also a γ element that must be restricted. Therefore, in this regard, the copper content must be less than or equal to 0.20 wt%.

[0105] Niobium is an important element of the present invention. Generally, this element can be used as a stabilizing element in ferritic stainless steels: the above-mentioned phenomenon susceptible to intergranular corrosion can be prevented by adding elements that form carbides or carbonitrides, and these carbides or carbonitrides have good thermal stability. In this way, the carbon and nitrogen in the solution are reduced as much as possible, thus avoiding the subsequent precipitation of chromium carbides and chromium nitrides. Therefore, niobium and titanium, and to a lesser extent zirconium and vanadium, provide stable carbon and nitrogen fixation.

[0106] However, niobium also forms certain intermetallic compounds in combination with iron in the range of 650 °C to 950 °C: the inventors have demonstrated that the intergranular precipitation of the hexagonal Fe2Nb occurring at high temperature is beneficial to improving the mechanical properties of the material when heated, especially beneficial to the mechanical properties under creep conditions, and thus beneficial to the mechanical properties under the target use conditions.

[0107] In addition, the inventors have found that precipitates of the Fe2Nb compound with a hexagonal structure (referred to as the Laves phase) trap some of the silicon, thus minimizing the formation of silicon oxide, which is desirable in this application. In particular, the Fe2Nb Laves phase contains approximately four times as much silicon by weight as the cubic phase Fe3Nb3X (where X represents nitrogen, oxygen, or carbon) that may also form. Adjustment of the initial silicon content and the trapping of some of the silicon by these phases can significantly reduce the segregation of silicon at the metal-oxide interface and the formation of high-resistance silicon oxide. In addition, the type and intergranular (i.e., at the grain boundaries) spatial distribution of these precipitates result in a significant increase in creep resistance up to 1000 °C.

[0108] To form the Fe2Nb phase under the target use conditions, the following conditions must be noted: the niobium content must be between 0.40% and 1.0% and such that 2×Nb - 7×C ≥ 0.8%, and the above composition must also contain titanium, with the titanium content between 0.05% and 0.2%, and such that 0% ≤ Ti - 4×N ≤ 0.15%. Preferably, the contents of niobium, carbon, and nitrogen also satisfy Nb - 10×(C + N) ≥ 0%.

[0109] If the total content of Nb in the above steel is less than 0.40%, the steel is not stable enough, and the number of Fe2Nb precipitates formed at high temperatures is insufficient to obtain the target high-temperature properties. To obtain such favorable niobium precipitates, the inventors have also demonstrated the importance of the effective niobium content: the effective niobium refers to the amount of niobium that can precipitate with iron in the solid solution assuming that carbon and nitrogen have completely precipitated with niobium and titanium in the form of carbides and nitrides 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 also satisfy the relationship Nb - 10×(C + N) ≥ 0%.

[0110] Conversely, a large excess of niobium results in weakening, especially in ferrites with a very high chromium content. This excess significantly increases the hardness and slows down recrystallization, which also limits the cold working properties of the metal. Therefore, the niobium content is limited to Nb ≤ 1.0%.

[0111] To promote the formation of the Fe2Nb phase (while suppressing the formation of the Fe3Nb3X phase), the titanium content must be greater than or equal to 0.05%, 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%.

[0112] At the same time, titanium and nitrogen must satisfy the relationship: 0% ≤ Ti-4×N ≤ 0.15%. Otherwise, at 650°C and above, niobium will not precipitate in the form of hexagonal Fe2Nb, but in the form of cubic Fe3Nb3X compounds with lower silicon capture efficiency.

[0113] Furthermore, the sum of the contents of Nb and Ti must be less than or equal to 1.0% to avoid excessive embrittlement of the metal and to ensure satisfactory toughness of the weld.

[0114] Vanadium, zirconium and aluminum are elements that stabilize nitrogen and increase mechanical strength at high temperatures, but the total content of these elements should be limited to a maximum of 0.2%, otherwise the processing properties will be reduced.

[0115] Furthermore, the aluminum content is limited to at most 0.02% because aluminum readily oxidizes to form highly resistive aluminum oxide. In particular, the resistivity of aluminum oxide at 850°C is approximately 10 7 Ω.cm, which means that the resistivity of aluminum oxide is 10 times that of silicon dioxide (SiO2).

[0116] The zirconium content is at most 0.02% in order to avoid a reduction in workability and to limit the risk of surface defects.

[0117] The vanadium content is a maximum of 0.2% to avoid a reduction in workability.

[0118] Furthermore, the inventors have demonstrated that the contents of titanium, aluminum, vanadium and zirconium must be limited together to limit the brittleness of the metal. The sum of their contents must satisfy: Ti+Al+V+Zr≤0.30%.

[0119] Cobalt is a hardening element when heated, but it reduces workability. Therefore, its content must be less than or equal to 0.05% by weight.

[0120] At the same time, the cobalt content, when added to the copper and nickel contents, must be as low as possible to prevent an increase in resistivity that would be detrimental to the application. For this purpose, the total content of Ni+Cu+Co must be less than or equal to 0.60%.

[0121] To avoid problems associated with hot forging, the tin content must be less than or equal to 0.05%.

[0122] The alloys also contain calcium, ranging between 0.2 ppm and 20 ppm. During production, calcium reduces the sulfur and oxygen levels in the steel. When used at high temperatures, calcium concentrations should be limited to prevent internal oxidation of the metal.

[0123] The oxygen content in the steel is between 1 ppm and 60 ppm, for example, about 20 ppm. As in the case of calcium, its concentration should be limited when used at high temperatures to prevent the appearance of oxide inclusions or internal oxidation of the metal.

[0124] Rare earth elements (REE) can be added to improve the adhesion of the oxide layer and make the steel corrosion-resistant. However, the content of rare earth elements shall not exceed 800 ppm. Exceeding this content will make the production of the metal difficult due to the reaction of rare earth with the refractory material of the ladle lining for casting. These reactions will lead to the formation of REE oxides, which will reduce the purity of inclusions in the steel. In addition, at the recommended content, the efficacy of REE is sufficient. Considering the high price of REE, increasing its content will only unnecessarily increase the production cost and will cause accelerated wear of the refractory material. If rare earth is added, its content is preferably at least 50 ppm.

[0125] Preferably, the above rare earth is selected from cerium, lanthanum and yttrium or a combination thereof. In particular, the rare earth includes a mixture of cerium and lanthanum. Preferably, the above rare earth is a mixture of cerium and lanthanum.

[0126] The steel of the present invention is usually in the form of cold-rolled annealed sheets, and the thickness of the sheets usually ranges between 0.1 mm and 2.5 mm.

[0127] In the present invention, the structure of the steel (i.e., cold-rolled annealed sheets) under transportation conditions is fully recrystallized.

[0128] The bending creep of the sheet at high temperature is a function of the sheet thickness. For the same mechanical load and metallurgical state, the sheet with a narrower thickness will experience greater deformation when bent.

[0129] When the thickness of the cold-rolled annealed sheet ranges between 1.2 mm and 2.5 mm, the average grain size of the above steel is preferably between 30 microns and 80 microns, that is, the ASTM grade (ASTM E-112 standard) is between 4 and 7. Within this strip thickness range, a grain size greater than or equal to 30 microns (ASTM ≤ 7) is advantageous because such a grain size can ensure slight deformation caused by bending of the strip under high-temperature creep conditions relative to the thickness of the strip. At the same time, within this strip thickness range, the above grain size is preferably less than or equal to 80 microns (ASTM ≥ 4). A grain size greater than 80 microns (ASTM < 4) will cause unsightly surface irregularities called "orange peel" during processing at ambient temperature, which is harmful to the good adhesion of the protective coating.

[0130] When the thickness of the cold-rolled 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 μm and 80 μm, that is, the ASTM grade is between 4 and 9. In this strip thickness range, a grain size greater than or equal to 15 μm (ASTM ≤ 9) is advantageous because such a grain size can ensure a slight deformation caused by bending of the strip under high-temperature creep conditions relative to the thickness of the strip. At the same time, in this strip thickness range, the above-mentioned grain size is preferably less than or equal to 80 μm (ASTM ≥ 4). A grain size greater than 80 μm (ASTM < 4) will cause an unsightly surface unevenness called the "orange peel" phenomenon during processing at ambient temperature, which is harmful to the good adhesion of the protective coating.

[0131] The microstructure of the cold-rolled annealed sheet under transport conditions includes precipitates mainly composed of carbonitrides of intragranular titanium and niobium. The annealing treatment performed on the sheet has the effect of dissolving the intermetallic precipitates of the Fe2Nb Laves phase type with a hexagonal structure and the Fe3Nb3X type of intermetallic precipitate with a cubic structure contained in the microstructure.

[0132] In particular, the volume fraction of the Fe2Nb Laves phase of the cold-rolled annealed sheet under transport conditions is less than 0.2%.

[0133] The volume fraction of the Fe2Nb Laves phase is determined as follows.

[0134] In the first stage, standard polishing is carried out using an abrasive with a grain size of at most 1 μm, and then the cross-section of the test sample is electrolytically etched with 60% nitric acid in a direction orthogonal to the rolling direction.

[0135] Observation is carried out using an electron microscope in the backscattered electron mode. A minimum magnification of ×1000 is used to obtain an overall view and accurately determine the particle size. Five images are taken for each test sample under each condition. The backscattered electron mode produces 256 levels of chemical component contrast, called gray levels, ranging from white (255) to black (0).

[0136] The volume fractions of the Fe2Nb and Fe3Nb3X intermetallic phases are determined by image analysis of the obtained images, for example, using Image J software.

[0137] First, the obtained images are processed by a thresholding method to retain only the whitest precipitates corresponding to the Fe2Nb and Fe3Nb3X intermetallics. In the backscattered electron mode, the niobium- and iron-rich precipitates appear as the brightest-shaded compounds in the image due to their atomic numbers. Thus, the Fe2Nb and Fe3Nb3X intermetallic phases can be distinguished from the niobium and titanium carbonitrides by chemical contrast and appear brighter in the shade. In this thresholding step, for each image, a threshold is selected to enable the discrimination of the Fe2Nb and Fe3Nb3X intermetallic precipitates from the rest of the image. After thresholding, the above-mentioned intermetallic precipitates appear white in the black matrix.

[0138] Then the images are manually filtered to remove artifacts such as holes or impurities.

[0139] Then, the surface fraction of the Fe2Nb and Fe3Nb3X intermetallic phases is determined from this thresholded and filtered image. In this case, it is acceptable that the volume fraction is equal to the surface fraction. Thereby, the volume fractions of the Fe2Nb and Fe3Nb3X intermetallic phases are obtained.

[0140] In addition, the measurement of the Fe / Nb ratio is carried out in each image by energy-dispersive spectroscopy (EDS). This ratio varies in each intermetallic phase and enables the discrimination between the cubic intermetallic compound Fe3Nb3X and the hexagonal Fe2Nb, thus enabling the determination of the volume fraction of the Fe2Nb Laves phase from the total volume fractions of the Fe2Nb and Fe3Nb3X intermetallic phases.

[0141] In the cold-rolled annealed sheet, niobium mainly exists in solid solution. In particular, in the cold-rolled annealed sheet under transportation conditions, the weight content of Nb in solid solution is at least 0.3%.

[0142] Another feature of the sheet of the present invention is that when heat-treated at a temperature between 650 °C and 1000 °C for a period of at least 30 minutes, the structure of the sheet, in addition to the above-mentioned titanium and niobium carbonitrides, also includes a homogeneous intergranular precipitation of the Fe2Nb compound with a hexagonal structure (Laves phase). The structure of the sheet after the above heat treatment may additionally include intergranular precipitates of the Fe2Nb compound. However, when the heat treatment period increases, such precipitates decrease.

[0143] In particular, after aging heat treatment at 850 °C in air for 1000 hours, the volume fraction of the Fe2Nb Laves phase in the sheet is at least 0.8%. These Fe2Nb Laves phases are mainly intergranular phases.

[0144] An aging heat treatment at a temperature of 850 °C in air for 1000 hours is considered representative of the service conditions of the steel and is commonly used to qualify the steel.

[0145] In the present invention, Fe2Nb precipitates are dominant among the intergranular precipitates. As described above, these precipitates have the advantage of trapping part of the silicon to reduce its content in solid solution, so that silicon is less likely to segregate at the metal-oxide interface.

[0146] In particular, after the heat treatment, the volume fraction of Fe3Nb3X precipitates remains less than 0.05%.

[0147] Furthermore, together with the grain size, the type and distribution of these Fe2Nb precipitates are very beneficial to creep resistance up to 1000 °C.

[0148] Due to the formation of Fe2Nb precipitates, the segregation of silica at the metal-oxide interface is very limited compared to the steel of the prior art.

[0149] In particular, after the aging treatment at a temperature of 850 °C in air for 1000 hours, the surface fraction of silica segregation at the metal-oxide interface remains limited.

[0150] In particular, after the aging treatment as Figure 1 shown, the above-mentioned sheet includes an oxide layer 2 located above the base metal 1 on each of its surfaces. The oxide layer 2 includes an inner layer 4 (the inner layer 4 is chromium oxide Cr2O3 (chromium sesquioxide)) and an outer layer 5 (the outer layer 5 is manganese-rich chromium oxide and may contain spinel-type iron (Mn,Fe)Cr2O4).

[0151] At the interface between the oxide layer 2 and the sheet steel (also called the base metal 1), the above-mentioned sheet includes precipitates of silicon oxide or silica 3. In the sheet of the present invention, after the aging treatment, the surface fraction of silica precipitates at this metal-oxide interface is less than or equal to 0.35, which means that the metal-oxide interface covered by silica does not exceed 35%. At 850 °C, the conductivity of silica is about 10 -6 S.cm -1 which is the conductivity of chromium sesquioxide (about 10 -2 S.cm -1)One ten-thousandth; and its coefficient of thermal expansion is one-tenth of the coefficients of thermal expansion of chromium sesquioxide, the base metal forming the electrolyte of the electrochemical cell, and zirconia. The surface fraction of silica at the metal-oxide interface is directly related to the resistivity, sheet resistance, and conductivity of the metal-oxide interface. Therefore, when the surface fraction of silica at the interface is 35%, the conductivity of this interface is reduced by 35% compared to the interface structure without silica, changing from 10 -2 S.cm -1 to 6.5×10 -3 S.cm -1 , or the resistivity of the interface is increased by approximately 50%, from 100 Ω.cm to 150 Ω.cm. At the same time, the sheet resistance of the interface (equal to the resistivity multiplied by the thickness of the silica film) increases in the same manner.

[0152] The surface fraction of the segregates of silica 3 can be determined from a cross-sectional image of the test sample in a direction orthogonal to the rolling direction, which is obtained by a scanning electron microscope with a magnification of ×10000 in the backscattered electron mode.

[0153] The horizontal edge of the image is parallel to the surface of the sheet. The length L of the cross-section observed in the plane across the sheet is 12 μm.

[0154] The backscattered electron mode produces 256 levels of chemical composition contrast, called gray levels, ranging from white (255) to black (0). In the obtained image, compared with the base metal presented as very light shade and chromium oxide presented as medium gray shade, due to its composition and the atomic numbers of its constituent elements, silica appears as the darkest phase.

[0155] The image thus obtained corresponds to Figure 2 the image shown in (a) in

[0156] Analyze this image using image analysis software (such as using Image J software).

[0157] More specifically, the image is initially transformed by the image analysis software through automatic processing to increase the contrast between the elements of the image, showing: the base metal 1, the oxide layer 2, and the segregates of silica 3. The purpose of this step is to make all the acquired images have the same contrast and to be able to demonstrate the segregates of silica 3, independent of the metal-oxide interface observed and the oxidized metal analyzed. After this processing, the image shown in (b) in Figure 2 is obtained.

[0158] Then, the image is thresholded to distinguish the segregates of silicon oxide 3 from the rest of the image. Thus, only two levels are retained: black represents the segregates of silicon oxide 3, and white represents the rest of the image. A threshold suitable for distinguishing the segregates of silicon oxide 3 is set. For example, the threshold is selected as 70, pixels with a gray level higher than 70 are represented by white 255, and pixels with a gray level lower than or equal to 70 are represented by black. Then, any artifacts are filtered, and the processing quality of each image is manually verified.

[0159] From this image, it can be seen that the sum of the projection lengths Li of the regions containing the precipitates of silicon oxide 3 within the measurement field in the interface along the longitudinal axis is determined, as shown in Figure 2 image (c) in

[0160] Then, the surface fraction is calculated as the ratio between the sum of the projection lengths and the length L of the measurement field, i.e., ΣLi / L. In the Figure 2 example shown, the surface fraction is 77%.

[0161] In the present invention, after heat treatment in air at 850 °C for 1000 hours, the ratio ΣLi / L remains less than or equal to 0.35, i.e., 35%.

[0162] Meanwhile, considering the chromium content of the steel of the present invention, the total thickness of the oxide layer 2 after the said heat treatment generally remains less than or equal to 10 μm.

[0163] The sheet of the present invention can be obtained particularly by the following method:

[0164] Prepare a steel having the above composition;

[0165] Cast a semi-finished product from the steel;

[0166] Heat the semi-finished product to a temperature greater than or equal to 1150 °C and less than or equal to 1260 °C, hold for a period of 40 minutes to 60 minutes, and hot-roll the semi-finished product to obtain a hot-rolled sheet with a thickness between 2.5 mm and 6 mm;

[0167] Anneal the said hot-rolled sheet, for example, at a temperature between 1000 °C and 1100 °C for a period of 30 seconds to 6 minutes;

[0168] Pickle the hot-rolled and annealed sheet;

[0169] The hot-rolled sheet is cold-rolled in one or more steps at a temperature between ambient temperature and 300 °C, and the sheet is annealed and pickled after each step. It should be understood that the term "step" here refers to cold rolling, which includes single-pass cold rolling or continuous multi-pass (e.g., five passes) cold rolling not separated by any intermediate annealing; for example, a conceivable cold rolling sequence includes performing a first series of five passes, followed by intermediate annealing, and then a second sequence of five passes; generally, the intermediate annealing separating the steps is carried out at a temperature between 950 °C and 1100 °C for 30 seconds to 6 minutes;

[0170] The cold-rolled sheet is finally annealed for a period between 10 seconds and 6 minutes at a temperature between 1000 °C and 1100 °C (preferably between 1050 °C and 1090 °C) to obtain a fully recrystallized structure. If the thickness of the sheet is between 1.2 mm and 2.5 mm, the ASTM average grain size of the fully recrystallized structure is preferably between 4 and 7. If the thickness of the sheet is greater than or equal to 0.1 mm and less than 1.2 mm, the ASTM average grain size of the fully recrystallized structure is preferably between 4 and 9. This heat treatment can keep niobium in a solid solution state.

[0171] In the sheet structure, the volume fraction of the Laves phase (i.e., the Fe2Nb compound with a hexagonal structure) is very low and is less than 0.2% under the transportation conditions (i.e., after this final annealing).

[0172] A series of experiments will now be described to demonstrate the advantages of the present invention. Laboratory castings were examined, which included the chemical analysis shown in Table 1.

[0173] Table 1

[0174] Steel C Mn Si S P Cr Ni Mo N Cu Nb Ti Zr Al v I#1 0.009 0.34 0.16 0.001 0.002 21.9 0.20 0.002 0.018 0.10 0.69 0.120 0.002 0.009 0.11 I#2 0.015 0.36 0.14 0.001 0.023 21.3 0.24 0.026 0.010 0.07 0.54 0.089 0.002 0.008 0.12 I#3 0.014 0.34 0.16 0.001 0.002 19.1 0.20 0.002 0.015 0.05 0.69 0.130 0.002 0.009 0.13 K#1 0.018 0.34 0.15 0.001 0.004 19.2 0.20 <![CDATA 1.920 > 0.015 0.05 0.70 0.140 0.002 0.009 0.12 K#2 0.017 0.34 <![CDATA 0.39 > 0.001 0.004 19.0 0.20 <![CDATA 1.910 > 0.015 0.10 0.59 <![CDATA 0.001 > 0.001 0.005 0.14 K#3 0.017 0.29 <![CDATA 0.45 > 0.001 0.003 21.0 0.20 0.020 0.023 <![CDATA 0.40 > <![CDATA 0.39 > 0.130 0.001 0.010 0.10 K#4 0.016 0.27 <![CDATA 0.26 > 0.0003 0.028 20.2 0.19 0.006 0.020 <![CDATA 0.42 > 0.42 0.089 0.001 0.007 0.12 K#5 0.015 <![CDATA 0.23 > 0.18 0.0003 0.03 <![CDATA 17.0 > 0.20 0.006 0.020 <![CDATA 0.31 > <![CDATA 0.30 > 0.080 0.001 0.007 0.11

[0175] Table 1 (continued)

[0176] Steel Co Sn Ca O Ce + La + Y Mn / Si V + Zr + Al Ti + V + Al + Zr Ni + Cu + Co 2Nb - 7C Ti - 4N I#1 0.03 0.006 2 21 0 2.13 0.12 0.24 0.33 1.32 0.05 I#2 0.03 0.005 1 25 0 2.57 0.13 0.21 0.33 0.98 0.05 I#3 0.02 0.005 2 19 0 2.13 0.14 0.27 0.27 1.28 0.07 K#1 0.02 0.005 2 18 0 2.27 0.13 0.27 0.27 1.27 0.08 K#2 0.03 0.005 3 19 0 <![CDATA 0.87 > 0.15 0.15 0.33 1.06 <![CDATA -0.06 > K#3 0.01 0.005 2 20 0 <![CDATA 0.64 > 0.11 0.24 <![CDATA 0.61 > <![CDATA 0.66 > 0.04 K#4 0.03 0.005 2 21 0 <![CDATA 1.05 > 0.13 0.22 <![CDATA 0.64 > <![CDATA 0.74 > 0.01 K#5 0.03 0.005 2 22 0 1.28 0.13 0.21 0.54 <![CDATA 0.49 > 0.01

[0177] For each steel in Table 1, the remainder is iron and unavoidable processing impurities.

[0178] The cast test samples were rolled using the following method:

[0179] The cast semi-finished product was heated to a temperature of 1220 °C, held for 40 minutes, and hot-rolled to obtain a sheet with a thickness of 5 mm;

[0180] The above sheet was annealed at 1080 °C for 6 minutes and pickled;

[0181] The above hot-rolled sheet was cold-rolled at room temperature to obtain a sheet with a thickness of 1.5 mm;

[0182] The above-mentioned sheet is finally annealed at a temperature of 1080 °C for 4 minutes.

[0183] Determine the volume fraction of the Laves phase (i.e., the Fe2Nb compound with a hexagonal structure) in the structure of each sheet under the transportation conditions (i.e., after annealing the cold-rolled sheet). The column "Fe2Nb Laves phase under transportation conditions" in Table 2 gives the volume fraction of the Laves phase measured in the above-mentioned sheets under the transportation conditions.

[0184] Then these sheets are heat-treated at 850 °C for a period of 1000 hours.

[0185] After this heat treatment, the precipitates in the structure are measured. The column "Fe2Nb Laves phase after treatment at 850 °C for 1000 hours" in Table 2 below gives the volume fraction of the Laves phase after this heat treatment.

[0186] The volume fraction of the Fe3Nb3X intermetallic precipitates in the sheets after this heat treatment is also measured, and this fraction is given in the column "Fe3Nb3X cubic phase after treatment at 850 °C for 1000 hours" in Table 2.

[0187] Then, as described above, for each sheet, the surface fraction of the segregation of silicon oxide 3 is determined from the images obtained by electron microscopy. It is given in the column "Surface fraction of silicon dioxide after treatment at 850 °C for 1000 hours" in Table 2.

[0188] A Charpy impact test is carried out on the notched specimens in accordance with standard NF EN ISO 148-1 (March 2017 edition), and the toughness of the hot-rolled strip annealed and pickled is also measured. The hot-rolled strip is obtained by hot-rolling at 1220 °C to a thickness of 5 mm: in this test, by means of a three-point bending test, the energy absorbed by the specimen at the moment of impact on the fracture point is measured, and this energy is a function of the temperature (between -10 °C and 80 °C). If the impact strength of the strip at a temperature of 20 °C (the temperature closest to the ambient temperature) is greater than 30 J / cm 2 , then the strip is considered to be tough.

[0189] Table 2

[0190]

[0191]

[0192] In Table 2, the comparative tests have been underlined.

[0193] It can be seen that for the tests according to the present invention (E1 to E3), where:

[0194] The contents of niobium and titanium satisfy the above conditions, i.e.:

[0195] Nb is between 0.40% and 1.0% and 2×Nb - 7×C ≥ 0.8%, and

[0196] Ti is between 0.05% and 0.2% and 0% ≤ Ti - 4×N ≤ 0.15%, and

[0197] The Mn / Si ratio is greater than 1:2, and

[0198] The sum of Ni + Cu + Co is between 0 and 0.60%,

[0199] The volume fraction of the hexagonal-structured Fe2Nb intermetallic precipitate after treatment at 850°C for 1000 hours is greater than or equal to 0.8%, and the surface fraction of silicon dioxide 3 after treatment at 850°C for 1000 hours is less than or equal to 35%.

[0200] In this way, sheets with good performance in terms of electrical conductivity under service conditions are obtained, particularly comparable to steels with extremely high chromium contents.

[0201] Conversely, in comparative tests E5 to E8, the contents of titanium and / or niobium do not satisfy the above conditions. In these tests, the volume fraction of the hexagonal-structured Fe2Nb intermetallic precipitate after treatment at 850°C for 1000 hours is less than 0.8%. Therefore, silicon is less likely to be trapped by the Fe2Nb Laves phase, and even if the silicon content is less than 0.2% as in the K5 alloy, the segregation of silicon dioxide is still restricted. In addition, in these comparative tests, the Mn / Si ratio does not satisfy the above conditions. It should be noted that: in these tests, the surface fraction of silicon dioxide 3 after treatment at 850°C for 1000 hours represents a surface fraction greater than 35%, which leads to a decline in performance in terms of electrical conductivity and an increase in the interface resistivity by more than 50%.

[0202] In addition, in tests E1 to E3, the hot strip steel is ductile because it exhibits an impact strength greater than 30 J / cm 2 of. Conversely, in comparative tests E4 and E5, in which the steel has a molybdenum content greater than the limit value as described in the present invention, the obtained hot strip steel is not ductile because it has an impact strength less than 30 J / cm 2 of.

[0203] As described above, according to an optional aspect:

[0204] 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 μm and 80 μm, i.e., the ASTM grade is between 4 and 7; and

[0205] When the thickness of the cold-rolled 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 microns and 80 microns, that is, the ASTM grade is between 4 and 9.

[0206] To confirm the technical effects of this optional feature, the inventors carried out a creep test on the sheet having the composition described in the present invention at 850 °C under its own weight for 200 hours.

[0207] These sheets were obtained from hot-rolled pickled sheets prepared according to the above method, which includes cold rolling to the final thickness shown in Table 3 for each test at ambient temperature and then final annealing under the annealing conditions shown in Table 3. Subsequently, the sheets were pickled, and the grain size was measured using the circle intercept method specified in standard ASTM E112. Table 3 also indicates the average grain size of each test.

[0208] The creep characteristics were determined by a creep test known as the "deflection test". This deflection test is non-standard but is used to characterize the creep characteristics. This test is described in the article Faria, Geraldo Lucio de; Melo, Denilson Pereira de; Moreira, Paulo Sérgio, DA METODOLOGIA SAG TEST PARA AVALIAR OCOMPORTAMENTO EM DOS AISI 321E AISI 441, p. 34-44. It is recorded in the 75th Annual Congress of ABM, Paulo, 2022. ISSN: 2594-5327, DOI 10.5151 / 2594-5327-34135.

[0209] For the above creep test, the metal strip was cut from the sheet in the as-transported condition (cold-rolled, annealed and pickled). The strip had a length of 205 mm and was cut along the rolling direction, 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 considered).

[0210] These flat strips were suspended on two supports with a 200 mm air gap and held in a furnace at 850 °C for a defined time. The measurement of the deflection was carried out regularly at 1 hour, 25 hours, 50 hours, 100 hours and 200 hours to characterize the deformation under creep. At ambient temperature, the deflection of the test sample was measured on a flat surface (usually marble) using a comparator with a precision and resolution of less than 0.05 mm. For each test, three samples were tested.

[0211] Table 3 below gives the average deflection values of the test samples after 200 hours of exposure.

[0212] Table 3

[0213]

[0214] These tests show that for a sheet with a thickness of 1.5 mm, a grain size less than 30 μm reduces the creep performance. After holding at 850 °C for 200 hours, a creep amount exceeding 3 mm was observed. Similarly, for a sheet thickness of 0.5 mm, an average grain size less than 15 μm reduces the creep performance. After holding at 850 °C for 200 hours, a creep amount exceeding 9 mm was observed.

[0215] Therefore, embodiments in which the average grain size meets the above specified conditions are particularly advantageous in terms of creep resistance.

Claims

1. A ferritic stainless steel plate, comprising the following components expressed by weight: C≤0.03％； 0.25%≤Mn≤1%, preferably 0.3%≤Mn≤0.5%, 0% <Si≤0.20%,优选Si≤0.15%,且 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 earth ≤800ppm; Need to meet: 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%; 0.2ppm≤Ca≤20ppm; 1ppm≤O≤60ppm; The remainder of the said composition consists of iron and unavoidable processing impurities; The plate is an annealed and pickled plate; The plate comprises Fe2Nb Laves phase in an amount of less than 0.2% by volume.

2. The ferritic stainless steel sheet according to claim 1, wherein, 50ppm≤rare earth≤800ppm.

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 material has a thickness between 0.1 mm and 2.5 mm.

5. The ferritic stainless steel sheet according to claim 4, wherein, When the plate has a thickness between 1.2 mm and 2.5 mm, the plate has an average grain size between 30 microns and 80 microns; when the plate has a thickness greater than or equal to 0.1 mm and less than 1.2 mm, the plate has an average grain size between 15 microns and 80 microns.

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

7. The ferritic stainless steel sheet according to any one of claims 1 to 6, characterized in that: When heat treated at a temperature of 850° C. for a period of 1000 hours, the plate comprises a Fe 2 Nb Laves phase in an amount greater than or equal to 0.8% by volume.

8. The ferritic stainless steel sheet according to any one of claims 1 to 7, characterized in that When heat treated at a temperature of 850° C. for a period of 1000 hours, the plate comprises less than 0.05% by volume of the Fe 3 Nb 3 X phase.

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

35.

10. The 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. A method for preparing a ferritic stainless steel plate, characterized in that: Prepare a plate having a composition according to any one of claims 1 to 3; Casting a semi-finished product from the plate; heating the semi-finished product 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 hot rolling the semi-finished product to obtain a hot-rolled plate having a thickness between 2.5 mm and 6 mm; annealing the hot-rolled plate; Pickling of hot rolled annealed plates; The hot-rolled sheet is cold-rolled in a single step or in multiple steps separated by intermediate annealing at a temperature between the ambient temperature and 300 °C; The cold-rolled sheet is finally annealed for a period between 10 seconds and 6 minutes at a temperature between 1000 °C and 1100 °C to obtain a fully recrystallized structure.

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

13. The method according to any one of claims 11 or 12, characterized in that, The intermediate annealing operation is carried out for a period between 30 seconds and 6 minutes at a temperature between 950 °C and 1100 °C.

14. The 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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