STRIP MADE WITH FERRITIC STAINLESS STEEL AND RELATED PRODUCTION PROCEDURE

Description

FERRITIC STAINLESS STEEL AND RELATED PROCEDURE PRODUCTION OF A TAPE MADE WITH SUCH STEEL The present invention relates to the production of a ferritic stainless steel, 5 in particular in the form of strips for making hot components of the exhaust system of vehicles, and the related production process, with high productivity and cost-effectiveness of the system and high product quality. It is known that in the steel industry, taking into account both the increases in the costs of raw materials and energy used as well as the greater competitiveness required by the 10 global market, as well as increasingly restrictive regulations in terms of pollution from exhaust gases, there is a particular need for a manufacturing method of High quality stainless steel strips for hot exhaust system components of the automotive industry, which requires lower investment and production costs and greater production flexibility. This allows for greater competitiveness. 15 also to the final product transformation industry with lower energy consumption, so as to also minimize the negative impact on the environment. In fact, the reduction of the vehicle weight to help reduce fuel consumption has become an environmental sustainability requirement that car manufacturers must meet require materials with ever-improving resistance to corrosion and heat for the 20 hot exhaust system components, such as manifold, pipes, particulate filter, catalyst, EGR (Exhaust Gas Recirculation) system. The traditional austenitic stainless steel AISI 321 (EN 1.4541 according to standard EN 10088-2) has excellent resistance to high temperatures, but it is also known that shows a weak resistance to cyclic oxidation, i.e. to the deterioration of the layer 25 oxide after alternating heating and cooling treatments, typical of many conditions of use. Recently, for the hot part of the exhaust system, High-Cr ferritic stainless steels were also introduced, but these steels were not have the properties necessary to be used above 850°C, and therefore cannot be intended for use in the parts closest to the engine where the material must have 30 excellent resistance to temperature, thermal fatigue, cyclic oxidation and corrosion. For example, the exhaust manifold, which is directly connected to the engine, is subjected to temperatures up to 1050°C. Once the resistance to high temperatures has been ensured, the choice of a steel ferritic compared to an austenitic steel is due in particular to technical advantages for use at high temperatures thanks to its lower thermal expansion coefficient 5 compared to austenitic alloys, which in turn increases oxidation resistance cyclic during thermal cycles, and to economic advantages for the price of ferritic alloys which is significantly lower and less volatile than that of austenitic alloys thanks to to the lower amount of expensive alloying elements such as nickel. Some patents describe solutions to increase the resistance of steels 10 ferritic at high temperatures, even up to 1000°C. For example, EP 1818422 describes a ferritic stainless steel for applications such as automotive exhaust manifolds, burners, heat exchangers, turbocharger casings and boilers, in which the issues related to parts subject to a higher periodic operating temperature at 950°C are resolved by ensuring that the intergranular precipitation state is 15 consisting of at least 80% of cubically structured FeNb intermetallic phase, so as to 2 3 slow down the phenomenon of hot viscous flow or creep. Furthermore, since a larger grain size means higher creep resistance, while the well recrystallized but fine grain ensures greater formability, called size of the grain is declared to be less than 60µm to obtain a compromise between strength 20 creep and formability at room temperature. However, in order for this invention to have a precipitation of the type described above, limits the presence of other elements that favor the precipitation of carbonitrides, such as Al, Zr and Ti. In fact, the following conditions are required (% by weight): Zr ≤ 0.02%, Al ≤ 0.02%, and Ti + Al + Zr ≤ 0.03%. 25 EP 2060650 describes ferritic steels for use in motor vehicle exhaust systems, in where the use of Nb as an element capable of precipitating carbonitrides is combined with the combination of high levels of Cr, Cu, Mo and W, in order to obtain adequate yield strength values ​​at high temperatures and increased creep resistance. In particular, the following conditions are required (% by weight): 1 ≤ Cu ≤2%; 1 ≤ W≤ 2.5% 30 with 1.2Nb+5Mo+6Cu≥11.5% and 15Nb+25Mo+0.5Cu≥10.5%. Other patents describe ferritic steels with Nb stabilization, but in general the contextual addition of Cr, Mo, Ti, Al, and occasionally Zr, is intended to improve the corrosion resistance and weldability of the alloys, while their effect in the improving high temperature resistance is only in terms of limiting the swelling 5 of the grain due to the pinning effect of the precipitates, i.e. the limitation of growth of the grains themselves thanks to the blocking effect of the grain edges favored by the presence of such precipitates in fine form. In particular, EP 2280090 describes how the addition of Nb in a stainless steel ferritic eliminates grain coarsening by the edge locking effect given 10 from the presence of precipitates (pinning effect). At the same time, however, it also comes emphasized how the beneficial effect of such mechanisms decreases more and more with the increase in temperature and at high temperatures, in particular for systems of exhaust fumes, the steels manufactured with the teachings of this patent are not still effective in controlling the creep of steel. 15 EP 2224030 describes a ferritic stainless steel used for EGR coolers, heat exchange equipment or aqueous urea tanks in catalyst systems SCR (Selective Catalytic Reduction), where Ti and Al are the key elements for maintain the wettability of the molten braze at satisfactory levels. Furthermore, also in this case, the addition of Nb, which favors the precipitation of carbonitrides of 20 niobium, is used to reduce grain enlargement during welding thanks to to the pinning effect of the precipitates. CN 102099500 describes a ferritic stainless steel used for tanks aqueous urea, in which adequate resistance to salt corrosion is achieved by controlling the Ti and / or Nb content of the alloy so that the phenomenon of 25 sensitization, i.e. the reduction of corrosion resistance due to precipitation of chromium carbides at the grain boundaries of the crystalline structure which, in its at times, it locally lowers the chromium content essential for the formation of the film passive. The Al content is less than 0.5% by weight while Ti and / or Nb is used to improve corrosion resistance. 30 CN 109563596 describes a ferritic stainless steel used in devices heat recovery or EGR coolers, in which the concentration of certain active elements which Si, Cr, Mo and Ni is checked to have adequate corrosion resistance for pitting caused by condensation water. Furthermore, proper weldability for brazing is ensured by adding Nb, and optionally Zr, to inhibit the phenomenon of sensitization. The Al content is between 0.001 and 0.15% and C + N ≤ 0.03%. 5 From CN 105296860 and CN 103459636 a suitable ferritic stainless steel is known for a biofuel feed system or for the heat recovery unit of exhaust, in which to ensure adequate corrosion resistance in the presence of biofuels at temperatures up to 100°C or salts, while maintaining high conductivity thermal, a chemical composition and a specific heat treatment are proposed which 10 conditions the surface to have a stable passive film with a fraction cationic Cr, Si, Nb, Ti and Al content greater than 30%. The Al content is between 0.002 and 0.5%, and this film has adequate resistance to corrosion even in the presence of fatty acids. The composition of the alloy is further enhanced by adding Nb and Ti to avoid sensitization after welding, as well as Al and optionally Zr 15 to improve corrosion resistance. CN 111593266 describes a ferritic stainless steel for mufflers, exhaust pipes discharge of household appliances, kitchen equipment and cars, where the problem of pitting resistance in humid or acidic environments was solved by setting adequate concentrations of Cr and Mo. In addition the total oxygen content is 20 kept below 35 ppm. Summarizing the teachings of the above-mentioned prior art, it can be stated that: • Nb is added to exert the pinning effect of Nb carbonitrides on grain growth during heating cycles; • Al, which is a deoxidizing agent used in the steelmaking process, 25 is kept at a low level to avoid the formation of an Al-rich passive layer during welding operations; • Zr is sometimes used to indirectly improve corrosion resistance inhibiting the sensitization phenomenon; • only EP 1818422 addresses the problem of high temperature, and its teaching 30 refers to an alloy optimized for creep by preferential precipitation of the intermetallic phase Fe Nb, while the resistance to cyclic oxidation is guaranteed by the 2 3 following relationships: Cr+5Si≥20% by weight and V+10Ti ≥0.06% by weight. The following effects of alloying elements are also known: C: high strength, precipitate fraction. Carbon is always present in 5 stainless steels and heat-resistant alloys, it is a very strong austenitizing element strong and increases the strength of the steel. In austenitic, ferritic and duplex, is usually kept at low levels, typically 0.005-0.03% by weight. If combined with chromium to form chromium carbide, may have a harmful effect on corrosion resistance by removing chromium from the solid solution in the alloy and 10 reducing the amount of available chromium to ensure corrosion resistance (awareness raising). N: high resistance, fraction of precipitates. Nitrogen is an element capable of making precipitate in the form of nitride or carbonitride elements similar to it such as aluminum, zirconium, niobium, etc. Furthermore, these nitrides are particularly stable and their presence 15 can ensure excellent microstructure resistance to high temperatures. Cr: ferrite stabilizer, oxidation resistance, element capable of form carbides. Chromium is a highly reactive element and is therefore the basis of properties that distinguish stainless steels from other corrosion-resistant materials Corrosion. The resistance of stainless steels to the chemical effects of corrosive agents 20 is determined by their ability to protect themselves through the formation of a passive film adherent and insoluble reaction products that shield the metal substrate from a both uniform and localized corrosion attack. This layer is effective in protecting the alloy in both corrosive and high-temperature oxidizing environments. In order for the passivation occurs and remains stable, the Fe-Cr alloy must have a minimum content 25 of chromium of approximately 11% by weight. Mn: solid solution strengthening, austenite stabilizing. The properties of the manganese allows it to act as an alloying and deoxidizing element in steel. When added to molten steel, manganese reacts with oxygen to form oxide of manganese (MnO). Furthermore, manganese combines preferentially with sulfur 30 to form manganese sulfide (MnS), thus preventing the formation of sulfides low iron fluxes. Yes: solid solution reinforcement, ferrite stabilizer. In small quantities, the Silicon gives steels a slight hardenability and is used in the iron and steel industry primary as a deoxidizer. Small amounts of silicon are commonly added to stainless steels to improve their resistance to oxidation and to stabilize the 5 ferrites. Mo: solid solution strengthening, element capable of forming carbides, ferrite stabilizer, improving wet corrosion resistance, resistance to pitting and crevice corrosion. Molybdenum is used in stainless steels in quantities up to 8% and more, commonly up to 4%. Also 10 relatively small percentages of molybdenum have powerful effects in improving the resistance to pitting in chloride-containing environments and corrosion resistance interstitial. Generally strengthens the passivity of the surfaces by decreasing the tendency to rupture of previously formed passive films. Nb: creep resistance, stabilizing interstitial elements by means of 15 formation of carbides and nitrides. In stainless steels, as regards resistance to corrosion, it is known that the stabilization of the grade by additions of niobium prevents the risk of intergranular corrosion in thermally stressed areas. To avoid this, typically in ferritic steels, niobium is added in large quantities sufficient depending on the levels of carbon and nitrogen in the alloy composition. 20 minimum theoretical amount of niobium required for complete stabilization, based on stoichiometric calculations, is described by the relationship: Nb≥ 0.2% + 5(C + N)%. In steels ferritic stainless steels with the addition of niobium, in a quantity between 0.4% and 0.7%, It is one of the most effective methods for improving thermal fatigue resistance. Al: element capable of forming nitrides, enhances resistance to oxidation. 25 Aluminum is known to stabilize the ferritic phase, but above all it is an element that enhances resistance to hot oxidation by being able to form a very strong oxide thin and close-fitting leather. Its importance is also linked to its high affinity with nitrogen, which makes it an element that, when properly dosed, is able to form nitrides stable at high temperatures. 30 Zr: element capable of forming nitrides, enhances resistance to oxidation. zirconium is an element capable of forming nitrides and carbonitrides stable at high temperatures. temperatures; therefore, it enhances creep resistance. It is also known to take part in the formation of the surface oxide layer improving the resistance to oxidation heat. Ti: an element capable of forming carbonitrides. Titanium, like niobium, is also 5 an element capable of precipitating carbides and nitrides preferentially with respect to chromium carbides, maintaining the necessary percentages of chromium in the matrix. however, the tendency is to form coarse carbonitrides, which are not in the microstructure able to perform the function of making it extremely resistant to high temperatures. For this reason it is not added and is not present, except in traces, in the composition of the 10 present invention. The aim of the present invention is therefore the production of ferritic steel strips. suitable for the above needs and with improved performance compared to technical products note. This purpose is achieved by means of a tape made of a steel having the composition and the characteristics according to claim 1 and the related production process, 15 while further advantageous features are reported in the dependent claims. The main advantages of the steel according to the present invention can be summarized as follows: • high formability and weldability, • excellent creep resistance at high temperatures up to 20 1050°C, • excellent resistance to corrosion and cyclic oxidation up to 1050°C, • excellent resistance to oxidation in environments containing exhaust fumes such as in combustion engines, • better thermal conductivity than austenitic steels, 25 • reduced manufacturing costs compared to austenitic steels. The ferritic stainless steel of the present invention has a chemical composition comprising the following elements (expressed in % by weight), the remainder being iron and inevitable impurities: C: ≤0.03% 30 N: ≤0.03% Cr: 18-25% Mo: 1.0-2.5% Nb: 0.4-0.7% Yes: 0.2-1% Al: ≤0.10% 5 Zr: ≤0.05% Ti: ≤0.03% in addition to the following elements that are not intentionally added, but are the limits according to the norm of typical ferritic steels: Mn: ≤1% 10 Cu: ≤0.5% Ni: ≤0.5% S: ≤0.015% P: ≤0.05% Furthermore, the following further compositional constraints are foreseen (concentrations 15 expressed in % by weight): a) Nb > 0.27% + 7·(C+N)% which guarantees complete stabilization of the elements interstitial carbon and nitrogen, counteracting the phenomenon of sensitization and thus ensuring excellent resistance to corrosion; b) 0.04% ≤ Al + Zr + Ti ≤ 0.10% which expresses the minimum and maximum quantity of elements 20 capable of forming nitrides; c) 0.5 ≤ C / N ≤ 1.6 which expresses the condition for controlling the distribution of the fraction precipitated between carbides and nitrides, ensuring the type of precipitates responsible for the creep resistance. Note that the condition C + N ≤ 0.06% sets the upper limit to the fraction 25 total of precipitable carbides and nitrides, and that the preferential composition of the The present invention provides in constraint b) only the presence of Al without the addition of Zr and Ti. Furthermore, the grain size of the resulting steel strip microstructure from the composition and the process according to the invention, it is preferably in the 20-80µm range which guarantees maximum creep resistance and a structure 30 homogeneous recrystallized which does not penalize the ductility and formability characteristics of the product at room temperature. The improvement of creep resistance is achieved by an adequate addition of aluminum and / or zirconium respecting the constraint b), without reducing the formability properties and at the same time enhancing the resistance to cyclic oxidation. The addition of just aluminum in the above-mentioned preferential composition has advantages in terms of 5 production process, as the addition of zirconium in the production phases before casting is a complex step, and also in terms of costs as it Zirconium is notoriously an expensive alloying element. Titanium alone, in the absence of aluminum and / or zirconium, does not guarantee the same results as adding the other two elements, which are highly affine with nitrogen to form nitrides. 10 Usually, the stabilization of interstitial elements, and especially nitrogen, is carried out exclusively by adding niobium, while in the present invention Stabilization is supported by the addition of Al and / or Zr. Since the stability thermodynamics of AlN and ZrN greater than that of NbN, the available nitrogen is precipitated preferentially in the form of the first two, producing the effect of 15 increase the mole fraction of carbon in niobium carbonitrides of general formula Nb(Cx,N1-x), formed during the last stages of solidification as well as during treatment heating of the slab before hot rolling. Furthermore, the reduction of the amount of niobium involved in the high-pressure precipitation temperature of nitrides of the NbN type, increases the availability of this element for the 20 precipitation in Fe Nb C type carbides (called MC where M stands for Metals) favoring 3 3 6 thus the precipitation of a second phase which turns out to be extremely favorable for resistance to temperatures well above 1000°C. It has been observed, however, that an excess of Al (> 0.1%), thanks to its excellent affinity for nitrogen, can give rise to the unwanted precipitation of coarse aluminum nitrides, especially in leather (not 25 linked to stabilization), for nitrogen capture from the atmosphere during the processes of annealing. In parallel, an excess of Zr (>0.05%), in addition to the useful precipitation of ZrN, can compete with niobium for carbon, also precipitating as ZrC and reducing the formation of “useful” precipitates of the MC type. The Nb range between 0.4% and 0.7% by weight, prescribed in the present invention, is 30 capable of ensuring maximum resistance to viscous flow (creep) at temperatures very high (up to 1050°C), having an ideal content to promote the precipitation of Fe Nb C type phases, according to the above-mentioned differential precipitation mechanisms 3 3 of nitrogen with Al and Zr. Furthermore, the addition of Al and / or Zr is also beneficial for the resistance to oxidation, thanks to the high affinity for oxygen of these elements alloy. 5 Even in limited quantities within the range specified above, aluminum interferes with the formation mechanisms of the scale, i.e. the oxide layer surface that forms during exposure to high temperatures and therefore in particular during the operation of the component, spreading towards the surface of the tape and going undergo selective oxidation. It therefore promotes the formation of a scale 10 stable, hard and adherent which, not subject to sudden fragmentation phenomena, limits the weight loss typical of thermal oxides. Another advantage of the present invention is the economic aspect, in fact steel ferritic stainless steel described in the present invention is capable of ensuring adequate resistance to creep and oxidation at high temperatures 15 temperatures, as specified above, at lower costs than austenitic steels which are used for similar high-temperature applications. Compared to steels austenitic, ferritic steels are cheaper and have a less variable cost, thanks to the absence of very expensive alloying elements such as nickel. The production process according to the invention involves numerous phases 20 traditional and some specific innovative phases as described in detail below. Liquid steel is produced using an electric arc furnace (EAF), then it is subjected to a decarburization process with oxygen and argon in a AOD (Argon Oxygen Decarburization) converter and possibly under vacuum in a VOD converter (Vacuum Oxygen Decarburization). 25 It is then cast with conventional continuous casting machines, in slabs of normal thickness or in thin slabs, subsequently hot rolled, annealed, pickled and finally cold rolled to the final thickness. It is then subjected to a recrystallization annealing treatment so that the final size of the grains in the microstructure are at least 20µm. 30 The heating of slabs, typically between 190 and 250 mm thick, before rolling it is preferably carried out at temperatures in the range between 1180 and 1250°C for times between 150 and 240 minutes. Hot rolling is carried out at temperatures between 1000 and 1050°C at the finisher entrance and between 850 and 950°C at the finisher exit finisher, up to thicknesses between 1.5 and 6 mm. A first difference compared to the production process of a stainless steel 5 traditional ferritic is that, after hot rolling, the steel of this invention must be cooled in the range of 550-650°C on the roller conveyor before of the winding in the coil, or in special cooling tanks after the winding into the coil. This cooling phase is necessary in order to inhibit the precipitation of embrittled phases, while it is not necessary in the standard process of 10 production for stainless steels which are usually wound at temperatures of at least 800°C without being cooled after hot rolling. The steel of the present invention, in fact, due to its high content of both Cr which of Nb is subject, in the range of 650-800°C, to the precipitation of phases intermetallics with a hexagonal structure (Laves phases), which are by their nature embrittled 15 and which could give rise to difficulties in the subsequent phases of the production process. Furthermore, they would subtract Nb and therefore interfere with the correct balancing of the precipitates useful for the purpose of improving creep resistance. Annealing of hot rolled strip is carried out at temperatures of 950-980°C for about 60 seconds, with a standard process. The subsequent cold rolling 20 allows you to obtain tapes with thicknesses from 0.5 to 3 mm, depending on the application desired. Finally, another innovative aspect of this process is the final annealing of the cold rolled strip which allows the recrystallization of the microstructure in the final tape with the correct grain size, so as to also guarantee the characteristics 25 mechanics and the formability of the material. Given the particularity of the composition steel chemistry of the present invention, such annealing must be such as to shape the correct microstructure without interfering with the stability of the precipitates that have formed formed in the upstream phases and which are responsible for the excellent creep resistance. The final annealing is then carried out using for the rolled material 30 cold a specific time-temperature curve: a very slow rise in temperature, reaching 880°C in about 240s, and finally annealing between 880°C and 1020°C very fast, for about 60 seconds. This curve is very different from the process applied to steels standard ferritic stainless steels, for which it is typical to minimize the rise times in temperature and increase the time spent at the treatment temperature (soaking). Confirming the best creep and oxidation resistance performances 5 cyclic of a steel of the present invention, the applicant has carried out numerous tests on different types of steel which are illustrated below with the help of the attached drawings, in which: Fig.1 is a schematic of the test setup used to evaluate the creep resistance; 10 Figs.2 and 3 are diagrams illustrating the results of the tests carried out according to the diagram of Fig.1 on four samples according to the invention (S1-S4) and three samples comparatives (C1-C3) which do not fall within the scope of the invention; and Fig.4 is a diagram illustrating the results of the oxidation resistance tests cyclic tests carried out on two samples according to the invention (S5-S6) and two comparative samples 15 (C1-C2) which do not fall within the scope of the invention, in addition to a traditional austenitic steel. Referring to Fig.1, it can be seen that to evaluate the creep resistance to At high temperatures a sag test was used in which the samples pieces of tape with a length of 220 mm, a width of 20 mm and different thicknesses, considering that the standard thickness is 2 mm. These pieces are placed 20 horizontally in a kiln on a pair of wedge-shaped ceramic supports A with both the ends approximately 20 mm above the bottom of the oven itself. The distance between the supports A is L=200 mm and the piece is centered so that it protrudes 10 mm from each support A. The sample is kept in the oven at a constant temperature of 1050°C±5°C for 100 hours in air and, due to the creep phenomenon that occurs at high temperatures, 25 undergoes a deflection caused by its own weight, the extent of which depends mainly on from its thickness. The deflection of the sample is measured at the beginning of the test (d ) and at the end of the test (d ), is expressed in millimeters as D = d - d with a precision 1 1 0 typical of ±0.5 mm. The test result depends on the length L, the thickness t of the sample and the test temperature. 30 From the metallurgical point of view the main factors are the quantity of niobium and the grain size, through the Nabarro-Herring diffusive creep mechanism, since the larger the grain size, the less tendency it has to undergo deflection for viscous flow (creep). When performing a sag test with a fixed geometry of the sample holder, an almost constant Nb content and at a sufficiently high temperature high enough to ensure that the ferrite grain size is so large that it does not affect 5 the process, the only relevant parameter is the thickness t of the tape. Assuming that the sample is a simply supported beam, the maximum shear stress in the midpoint between supports A is directly proportional to L and inversely proportional to the thickness t. To estimate the outcome of the sag test with the geometric arrangement illustrated above, we do 10 reference to a limit deflection of 10 mm on a length L=200 mm, corresponding at 5% of the length, for a strip of thickness t = 2 mm at a temperature of 1050°C. ref This deflection is considered the maximum allowable deflection, and for tapes of different thickness the equivalent deflection D* referred to 2 mm thick tapes is calculated according to the relation D*=D(t)*(t / t), where D(t) is the deflection measured at t t . Also in this case, ref ref 15 the maximum permissible deflection for the equivalent deflection D* is equal to 10 mm. The test results are shown in Figs.2 and 3 in terms of percentage. of deflection relative to the maximum allowable deflection, for which a sample fails the test if this percentage exceeds 100%. For the evaluation of resistance to cyclic oxidation, the test method 20 used is intended to measure the weight loss of the oxide flake of a material subject to heating and subsequent cooling cycles. For applications of interest in the exhaust system sector, 25-minute heating cycles are applied at working temperature followed by a 5 minute cooling, so as to carry out two cycles every hour. The test lasts for 1000 cycles in total and the weight of the samples is 25 monitored periodically. The weight loss indicates that the oxide layer that forms during the heating is lost during cooling, due to the difference in thermal contraction coefficient between the metal and the oxide itself which generates a strong stress at the interface. When the scale that forms the oxide layer is made up of 30 preferably from an aluminum oxide and / or zirconium base, is generally of the type very thin, stable, hard and adherent and does not undergo sudden phenomena of crushing resulting in very little or no weight loss. The size of the Samples are 60 x 60 mm and typical test temperatures are: 900-1000-1020-1050°C. The beneficial results obtained through the teachings of the present invention are illustrated below with reference to the following examples. EXAMPLE 1 (S1) A steel according to the present invention having the following composition in terms of weight percentages: C=0.013; N=0.017; Cr=19; Si=0.4; Mo=1.96; Al=0.075; Ti=0.008; Nb=0.57; Zr=0. This example corresponds to the preferred embodiment which contains 10 aluminum but not zirconium, and its chemical composition satisfies the constraints mentioned previously: a) Nb > 0.27% + 7(C+N)%= 0.48%; b) 0.04% ≤ Al+Ti+Zr = 0.083% ≤ 0.10%.; c) 0.5 ≤ C / N = 0.76 ≤ 1.6. Sample S1 was industrially produced by conventional casting in slabs of 215 mm thickness and hot rolling, after heating the slabs at 15 1200°C, up to a thickness of 5 mm. The finishing mill process was carried out between 1000°C and 880°C and the tape was wound on reel at temperature in the range 550- 650°C. The strip was then annealed and pickled using a standard process, and subsequently cold rolled to a thickness of 2 mm and subjected to annealing final at a maximum temperature of 1000°C according to the specific procedure described in 20 precedence. EXAMPLE 2 (S2) A steel according to the present invention having the following composition in terms of weight percentages: C=0.011; N=0.016; Cr=19; Si=0.5; Mo=1.85; Al=0.045; Ti=0.001; 25 Nb=0.60; Zr=0. This example corresponds to the preferred embodiment which contains aluminum but not zirconium, and its chemical composition satisfies the constraints mentioned previously: a) Nb > 0.27% + 7(C+N)% = 0.459; b) 0.04% ≤ Al+Ti+Zr = 0.046% ≤ 0.10%.; c) 0.5 ≤ C / N = 0.69 ≤ 1.6. Sample S2 was produced on a laboratory scale using a high-pressure melting furnace. 30 vacuum induction with which 80 kg ingots were cast and then hot rolled using a laboratory rolling mill, annealed, pickled, and then rolled cold to a final thickness of 2 mm. The final annealing was always performed using laboratory equipment, and all stages of the process in the laboratory have respected the same specifications as industrial treatments. 5 EXAMPLE 3 (S3) A steel according to the present invention having the following composition in terms of weight percentages: C=0.011; N=0.015; Cr=19; Si=0.5; Mo=1.85; Al=0.025; Ti=0.001; Nb=0.59; Zr=0.02. In this example derived from S2 some of the aluminum has been replaced with zirconium, while the other elements are substantially unchanged, and its 10 chemical composition satisfies the constraints mentioned above: a) Nb > 0.27% + 7(C+N)% = 0.452%; b) 0.04% ≤ Al+Ti+Zr = 0.046% ≤ 0.10%.; c) 0.5 ≤ C / N = 0.73 ≤ 1.6. Sample S3 was produced on a laboratory scale in the same way as the sample S2. EXAMPLE 4 (S4) A steel according to the present invention having the following composition in terms of weight percentages: C=0.011; N=0.016; Cr=19; Si=0.5; Mo=1.85; Al=0; Ti=0.001; Nb=0.59; Zr=0.042. In this example derived from S2 the aluminum has been replaced with 20 zirconium, while the other elements are substantially unchanged, and its composition chemistry satisfies the constraints mentioned above: a) Nb > 0.27% + 7(C+N)% = 0.459%; b) 0.04% ≤ Al+Ti+Zr = 0.043% ≤ 0.10%.; c) 0.5 ≤ C / N = 0.69 ≤ 1.6. Sample S4 was produced on a laboratory scale in the same way as the sample S2. EXAMPLE 5 (S5) A steel according to the present invention having the following composition in terms of weight percentages: C=0.013; N=0.015; Cr=19; Si=0.4; Mo=1.83; Ti=0.008; Nb=0.60; Zr=0; Al=0.042. This example corresponds to the preferred embodiment which 30 contains aluminum but no zirconium and its chemical composition meets the constraints mentioned above: a) Nb > 0.27% + 7(C+N)% = 0.466%; b) 0.04% ≤ Al+Ti+Zr = 0.05% ≤ 0.10%.; c) 0.5 ≤ C / N = 0.87 ≤ 1.6. Sample S5 was industrially produced in the same way as sample S1. EXAMPLE 6 (S6) 5 A steel according to the present invention with the following composition in terms of weight percentages: C=0.015; N=0.015; Cr=19; Si=0.4; Mo=1.83; Ti=0.008; Nb=0.59; Zr=0; Al=0.062. This example corresponds to the preferred embodiment which contains aluminum but no zirconium and its chemical composition meets the constraints mentioned above: a) Nb > 0.27% + 7(C+N)%= 0.48%; b) 0.04% ≤ Al+Ti+Zr = 10 0.07% ≤ 0.10%.; c) 0.5 ≤ C / N = 1 ≤ 1.6. Sample S6 was industrially produced in the same way as sample S1. COMPARATIVE EXAMPLE 1 (C1) A conventional steel 441LI (1.4509 according to EN 10088-2) with the following 15 composition in terms of weight percentages: C=0.014; N=0.017; Cr=18; Si=0.5; Al=0.01; Ti=0.14; Nb=0.37. This example, compared to the intervals of the present invention, it turns out to have an excess of Ti and a deficiency of Nb and Mo, and does not respect the constraints a) because Nb < 0.27% + 7(C+N)% = 0.487% and b) because Al+Ti+Zr = 0.15% > 0.10%. 20 Sample C1 was produced with a conventional continuous casting cycle in slabs, hot rolling after heating the slab to 1200°C, coiling without cooling, annealing, pickling, cold rolling to 2 mm thickness and final annealing at a maximum temperature of 1000°C according to the traditional process. 25 COMPARATIVE EXAMPLE 2 (C2) A steel acquired from the market of measured chemical composition, in terms of percentages by weight: C=0.0163; N=0.017; Cr=19; Si=0.4; Mo=1.97; Al=0.002; Ti=0.003; Nb=0.60; Zr=0. The chemical composition is similar to the steel of the present invention, but does not respect constraint b) because Al+Ti+Zr = 0.005% < 0.04%. 30 The balance of elements capable of stabilizing the interstitials, in particular nitrogen, is so everything moved towards the precipitation of niobium nitrides, not favoring instead the precipitation of second phases that are much more favorable to creep resistance. they know the production details. COMPARATIVE EXAMPLE 3 (C3) 5 A steel produced with the following composition in terms of weight percentages: C=0.012; N=0.015; Cr=19; Si=0.5; Mo=1.85; Al=0.065; Zr=0.075; Ti=0.001; Nb=0.59. This example, compared to the ranges of the present invention, appears to have an excess of Zr, and does not respect constraint b) because Al+Ti+Zr = 0.141% > 0.10%. Sample C3 was produced on a laboratory scale in the same way as the 10 sample S2. The creep resistance test (sag test) was performed on samples S1 and C1-C2 as described above. The results are reported in the following table 1 and represented in Fig.2. Sample Deflection at 1000°C (% of limit) Deflection at 1050°C (% of limit) C1 130 - C2 52 110 S1 35 80 15 Table 1 It is clear that the sample S1 according to the invention remains well within the limit of deflection expected in both the 1000°C and 1050°C tests, the comparative sample C2 resists 1000°C but fails the test at 1050°C and the comparative sample C1 fails the test was already carried out at 1000°C, so the test at 1050°C was not even carried out. 20 The results of a similar test on samples S2-S4 and C3 are reported in the following table 2 and represented in Fig.3, with the difference that the test was performed only at 1050°C and that the 100% limit threshold is defined in relation to the deflection of the sample S2 taken as a reference, rather than with respect to the absolute deflection value. Sample Deflection at 1050°C (% of limit) S2 100 S3 84 S4 92 C3 111 Table 2 This test shows how the percentages of Al and Zr affect the performance of creep resistance of steel, taking as reference the sample S2 which contains only Al in a quantity of 0.045% by weight (Ti is equal to 0.001% by weight in all 5 samples, so it does not affect the results). In the variant of sample S3, with a 0.02% reduction in the amount of Al compensated by an equal increase in Zr, the performance improves compared to S2 and the same occurs, albeit to a lesser extent, in the variant of sample S4 containing only Zr in quantity similar to the Al content of S2 and in any case such as to satisfy requirement b) 10 because Al+Zr+Ti > 0.04%. On the contrary, the performance worsens compared to S2 in the variant of sample C3 containing Al and Zr in quantities such as not to satisfy the requirement b) because Al+Zr+Ti >0.10%. As regards the resistance to cyclic oxidation, the results of the tests carried out according to the modality described above, at a temperature of 1050°C, are shown 15 in the graph of Fig.4, from which an improvement in performance can be seen as the Al content increases. In fact, the two samples S5 and S6 differ mainly for the Al content which goes from 0.042% in S5 to 0.062% in S6, while samples C1 and C2 comparative examples involve much lower amounts of Al, respectively 0.01% and 0.02%. 20 The behavior of an austenitic steel AISI 321 is also shown which is notoriously resistant to hot creep up to about 1000°C, but in general it is very little resistance to cyclic oxidation. In fact, the data for this steel refer to a test carried out at a temperature of 1000°C, which is even lower than the 1050°C used for testing other samples.

Claims

1. Ferritic stainless steel characterised by having a chemical composition comprising the following elements expressed as a percentage by weight, the remainder being iron and inevitable impurities: C: <0.03% N: <0.03% Cr: 18-25% Mo: 1.0-2.5% Nb: 0.4-0.7% Si: 0.2-1% Al: <0.10% Zr: <0.05% Ti: <0.03% Mn: <1% Cu: <0.5% Ni: <0.5% S: <0.015% P: <0.05% and by having the additional compositional constraints, with concentrations expressed as a percentage by weight: a) Nb > 0.27% + 7-(C+N)%; b) 0.04% < Al + Zr + Ti < 0.10%; c) 0.5 < C / N < 1.

6.

2. Ferritic steel according to claim 1, characterized in that the grain size of its microstructure is in the range 20-80 μm.

3. A production process for a strip made of a ferritic steel according to claim 1 or 2, comprising the following steps: a) continuous casting into slab; b) hot rolling, with possible preheating, with an exit temperature of at least 850°C; SIB - 20 BI3276M-TE c) coil winding; d) annealing; e) pickling; f) cold rolling; 5 g) recrystallization annealing; characterized in that it also comprises a cooling step to 550-650°C performed immediately before or immediately after step c), and in that step g) is performed with a very slow temperature rise, reaching 880°C in approximately 240 seconds, and a very rapid annealing between 880°C and 1020°C, for approximately 60 seconds. 10 4.Production process according to claim 3, characterized in that the phases include the following parameters: a) continuous casting into slabs with a thickness of between 190 and 250 mm; b) heating between 1180 and 1250°C for times between 150 and 240 minutes before hot rolling conducted at temperatures between 1000 and 1050°C at the finisher entrance and between 15 850 and 950°C at the finisher exit, with thicknesses of 1.5-6 mm; d) annealing at temperatures of 950-980°C for approximately 60 seconds; f) cold rolling to thicknesses of 0.5-3 mm.