Strip made of ferritic stainless steel and related production process

Abstract

A strip made of ferritic stainless steel having a specific composition, in particular with reference to the quantities of C, N, Nb, Al, Zr and Ti, and with additional compositional constraints relating to these elements in order to ensure complete stabilization of the interstitial elements C and N, counteracting the phenomenon of sensitization and thus ensuring excellent corrosion resistance, as well as controlling the distribution of the precipitated fraction between carbides and nitrides, ensuring the type of precipitates responsible for creep resistance, while improving cyclic oxidation resistance.

Description

[0001] STRIP MADE OF FERRITIC STAINLESS STEEL AND RELATED PRODUCTION PROCESS

[0002] The present invention relates to the production of a strip made of ferritic stainless steel, in particular for manufacturing hot components of vehicle exhaust systems, and the related production process, with high productivity and cost-effectiveness of the plant and high product quality.

[0003] It is known that in the steel industry, due to the increases in the costs of raw materials and energy used, and to the greater competitiveness required by the global market, as well as the increasingly restrictive regulations in terms of exhaust gas pollution, there is a particularly pressing need for a method of manufacturing high-quality stainless steel strips for hot components of the exhaust system in the automotive industry, which requires lower investment and production costs and greater production flexibility. This will also make the end product manufacturing industry more competitive with lower energy consumption, thereby minimizing the negative impact on the environment. In fact, reducing vehicle weight to help lower fuel consumption has become an environmental sustainability requirement and to meet this requirement car manufacturers are demanding materials with ever-improving corrosion and heat resistance for hot components in the exhaust system, such as manifolds, pipes, particulate filters, catalytic converters, and EGR (Exhaust Gas Recirculation) systems.

[0004] Traditional AISI 321 austenitic stainless steel (EN 1.4541 according to EN 10088- 2) has excellent resistance to high temperatures, but it is also known to show weak resistance to cyclic oxidation, i.e., deterioration of the oxide layer after alternating heating and cooling treatments, typical of many conditions of use. Recently, ferritic stainless steels with a high Cr content have also been introduced for the hot part of the exhaust system, but these steels do not have the properties necessary for use above 850°C and therefore cannot be used in parts closest to the engine where the material must have 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 of up to 1050°C. Once resistance to high temperatures has been ensured, the choice of ferritic steel over austenitic steel is due in particular to technical advantages for use at high temperatures thanks to its lower thermal expansion coefficient compared to austenitic alloys, which in turn increases resistance to cyclic oxidation during thermal cycles, and economic advantages due to the price of ferritic alloys, which is significantly lower and less volatile than that of austenitic alloys thanks to the lower quantity of expensive alloying elements such as nickel.

[0005] Some patents describe solutions for increasing the resistance of ferritic steels to high temperatures, up to 1000°C. For example, EP 1818422 describes a ferritic stainless steel for applications such as automotive exhaust manifolds, burners, heat exchangers, turbocharger housings, and boilers, where issues related to parts subject to periodic operating temperatures above 950°C are resolved by ensuring that the intergranular precipitation state consists of at least 80% Fe2Nb3 intermetallic phase with a cubic structure, in order to slow down the phenomenon of hot viscous flow or creep. Furthermore, given that a larger grain size means greater creep resistance, while a well- recrystallized but fine grain ensures greater formability, said grain size is stated to be less than 60 pm in order to achieve a compromise between creep resistance and formability at room temperature.

[0006] However, in order to achieve the type of precipitation described above, this invention limits the presence of other elements that promote 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%.

[0007] EP 2060650 describes ferritic steels for use in automotive exhaust systems, in which the use of Nb as an element capable of precipitating carbonitrides is combined with 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% with 1.2Nb+5Mo+6Cu > 11.5% and 15Nb+25Mo+0.5Cu > 10.5%.

[0008] Other patents describe Nb-stabilized ferritic steels, but in general, the simultaneous 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 improving high-temperature resistance is only in terms of limiting grain growth due to the pinning effect of the precipitates, i.e., limiting the growth of the grains themselves thanks to the phenomenon of grain edge blocking favored by the presence of these precipitates in fine form.

[0009] EP 2280090 describes how the addition of Nb in ferritic stainless steel counteracts grain growth through a dual effect: hindering its growth by accumulating this element in solid solution (i.e., free and unprecipitated) at the boundary of the grain itself (drag effect) and by blocking the movement of the boundaries due to the presence of precipitates (pinning effect) .

[0010] This document specifies that these two phenomena together are capable of strengthening the structure of ferritic steel as the temperature rises when it is subjected to a brazing welding operation. In general, however, the beneficial effect of these mechanisms decreases more and more with rising temperature and at high temperatures, particularly for fumes exhaust systems, the same mechanisms are still not effective in controlling the viscous flow (creep) of steel.

[0011] US 2015 / 020933 describes a ferritic steel suitable for applications in exhaust systems but, similar to EP 2280090, its resistance to high temperatures is based on the increase in alloying elements such as Cr, Mo and Nb, and therefore on the usual mechanisms of grain boundary blocking or the strengthening effect of Nb in solid solution. The objective of the patent is not creep resistance at extremely high temperatures but the production of highly formable strips, made possible by the addition of Mn (>0.6%), B (0.005-0.5%) and by process conditions (e.g., coil winding temperature <500°C) that ensure the development of crystallographic textures that improve press formability and optimize the strength of the steel in intermediate temperature ranges.

[0012] Also in other patents describing ferritic steels for exhaust systems, such as JP 3427502B2 or US 2021 / 032731, the mechanisms of high-temperature resistance are based on the presence of Mo and Nb, with Nb in solid solution. However, it is also specified that, at higher temperatures, the simultaneous presence of silicon (Si>0.6%) or antimony (Sb 0.002-0.5%) in solid solution is required, when the contribution of Nb tends to decay. As regards oxidation resistance above 950°C, for the material described in JP 3427502B2, this is based on compounds of the type sulfides of Zr or Ca or rare earths such as Y, La, or Ce. These elements, being more similar to sulfur than manganese, tend to precipitate sulfur in the form of sulfides that contribute to the adhesiveness of the scale, preventing catastrophic oxidation (i.e., the fragmentation of the scale itself).

[0013] EP 2224030 describes a ferritic stainless steel used for EGR radiators, heat exchange equipment, or aqueous urea tanks in SCR (Selective Catalytic Reduction) catalytic systems, in which Ti and Al are the key elements for maintaining the wettability of the molten braze at satisfactory levels. Furthermore, in this case too, the addition of Nb, which promotes the precipitation of niobium carbonitrides, is exploited to reduce grain growth during welding thanks to the pinning effect of the precipitates.

[0014] CN 102099500 describes a ferritic stainless steel used for aqueous urea tanks, 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 sensitization does not occur, i.e., the reduction in corrosion resistance due to the precipitation of chromium carbides at the boundaries of the grains of the crystal structure, which in turn locally lowers the chromium content essential for the formation of the passive film. The Al content is less than 0.5% by weight, while Ti and / or Nb are used to improve corrosion resistance.

[0015] CN 109563596 describes a ferritic stainless steel used in heat recovery devices or EGR radiators, in which the concentration of certain active elements such as Si, Cr, Mo and Ni is controlled to provide adequate resistance to pitting corrosion caused by condensation water. In addition, proper braze weldability is ensured by adding Nb, and optionally Zr, to inhibit sensitization. The Al content is between 0.001 and 0.15% and C + N < 0.03%.

[0016] CN 105296860 and CN 103459636 describe a ferritic stainless steel suitable for a biofuel supply system or exhaust heat recovery unit, in which, to ensure adequate corrosion resistance in the presence of biofuels at temperatures up to 100°C or salts, while maintaining high thermal conductivity, a chemical composition and a specific heat treatment are proposed that conditions the surface so as to have a stable passive film with a cationic fraction of Cr, Si, Nb, Ti and Al greater than 30%. The Al content is between 0.002 and 0.5%, and said film has adequate corrosion resistance even in the presence of fatty acids. The composition of the alloy is further enhanced by adding Nb and Ti to prevent sensitization after welding, as well as Al and optionally Zr to improve corrosion resistance. CN 111593266 describes a ferritic stainless steel for mufflers, household appliances, and kitchen equipment, with the aim of improving resistance to pitting corrosion in humid or acidic environments. To this end, the teaching provides for setting appropriate concentrations of Cr and Mo and also controlling the percentages of Cu and Mn to increase corrosion resistance in such environments. In addition, the total oxygen content is kept below 35 ppm.

[0017] EP 4234770 describes a ferritic steel with excellent resistance to red oxide (iron oxide-based scale) in a water vapor atmosphere and also for exhaust system components. This document refers to temperatures of 300-900°C and is not dedicated to controlling the phenomenon of creep resistance, as confirmed by the fact that it does not require the presence of Mo, except as an option. Its teaching, for the purposes of red oxide resistance, focuses on defining an appropriate combination of elements (essentially Cr, Al, Si) that give a particular type and grain size of oxides on the surface.

[0018] Corrosion resistance, particularly in environments containing sulfides, is the subject of US 2010 / 0139818, which describes a Nb -stabilized ferritic stainless steel. The teaching does not specifically address creep resistance at high temperatures but describes the effect of controlling grain growth in the microstructure by means of fine precipitation (maximum particle size 1 pm) of Nb carbides of the specific type NbC. The composition does not include the addition of Mo except as an option. This patent also describes a production process, with particular attention to the cooling phase of the strip after hot rolling, which is recommended to be at a controlled speed.

[0019] Other patents describe ferritic steels with a chemical composition similar to that of the present invention, but their teachings are dedicated to maintaining adequate mechanical and / or corrosion resistance characteristics of the weld metal after resolidification of the molten zone and in the heat-affected zone after the welding process, and in no case is the phenomenon of creep resistance mentioned.

[0020] In JP 2007290033A, for example, it is essential to control the oxygen content (above 20 ppm in the molten zone) in order to achieve correct weld penetration, and to do this, Al and Ca are also controlled within a specific analytical range; in addition, Ti must be > 0.1%.

[0021] In JP H0570899A, on the other hand, adequate corrosion resistance of the weld zone, verified by corrosion tests in corrosive environments at 80°C, is ensured by the appropriate combination of alloying elements so that they work to prevent corrosion. For example, it is required that Ti > 0.5% and Ti+Nb > 0.15 + 7(C+N) to prevent intergranular corrosion; Cr + 3(Mo+Cu) > 23.5% to prevent corrosion in the molten area; 5(Ti+Zr) + 20(Al-0.01) > 1.5% to prevent corrosion in the heat-affected zone.

[0022] Summarizing the teachings of the above-mentioned prior art, it can be stated that in the case of patents describing steels for exhaust systems with resistance to hot viscous flow at high temperatures (creepy.

[0023] • Nb is added to exert the pinning effect of Nb carbonitrides on grain growth during heating cycles, or to exert, together with Mo, a microstructure reinforcing function (obstacle to grain boundary movements) at high temperatures, if present in solid solution (drag effect),'

[0024] • high contents of Cu and W and / or V are added together with Nb to reinforce the microstructure at high temperatures thanks to their ability to remain in solid solution and accumulate at the grain boundaries;

[0025] • the same mechanism of microstructure reinforcement at high temperatures can be achieved by adding Si and / or Sb, as they are able to remain in solid solution when the effect of Nb tends to decay;

[0026] • creep control can be achieved through the precipitation of specific second phases (Fe2Nb3or NbC);

[0027] • in the case of patents describing ferritic steels of similar composition, but other uses that do not require creep resistance: Ti and / or Nb and / or Zr are added to prevent sensitization, i.e., the reduction in corrosion resistance due to the precipitation of chromium carbides at the boundaries of the grains of the crystalline structure, which in turn locally lowers the chromium content essential for the formation of the passive film;

[0028] • Al, Ti, Zr, Ca, Cu and Mo are used in various combinations to obtain surface layers (on steel or welded areas) with adequate corrosion resistance in various environments.

[0029] The following effects of alloying elements are also known:

[0030] C: high strength, fraction of precipitates. Carbon is always present in stainless steels and heat-resistant alloys, is a very strong austenitizing element and increases the strength of steel. In austenitic, ferritic, and duplex stainless steels, it is usually kept at low levels, typically 0.005-0.03% by weight. When combined with chromium to form chromium carbide, it can have a detrimental effect on corrosion resistance by removing chromium from the solid solution in the alloy and reducing the amount of chromium available to provide corrosion resistance (sensitization).

[0031] N: high strength, fraction of precipitates. Nitrogen is an element capable of precipitating elements similar to it, such as aluminum, zirconium, niobium, etc., in the form of nitride or carbonitride. Furthermore, these nitrides are particularly stable, and their presence can ensure excellent microstructure resistance at high temperatures.

[0032] Cr: ferrite stabilizer, oxidation resistance, element capable of forming carbides. Chromium is a highly reactive element and is therefore the basis of the properties that distinguish stainless steels from other corrosion-resistant materials. The resistance of stainless steels to the chemical effects of corrosive agents is determined by their ability to protect themselves through the formation of an adherent and insoluble passive film of reaction products that shields the metal substrate from both uniform and localized corrosion attack. This layer is effective in protecting the alloy in both corrosive and high- temperature oxidizing environments. For passivation to occur and remain stable, the Fe- Cr alloy must have a minimum chromium content of approximately 11% by weight.

[0033] Mn: solid solution strengthening, austenite stabilizer. The properties of manganese allow it to act as an alloying and deoxidizing element in steel. When added to molten steel, manganese reacts with oxygen to form manganese oxide (MnO). In addition, manganese preferentially combines with sulfur to form manganese sulfide (MnS), thus preventing the formation of low-melting iron sulfides.

[0034] Si: solid solution strengthening, ferrite stabilizer. In small quantities, silicon imparts mild hardenability to steels and is used in primary steelmaking as a deoxidizer. Small amounts of silicon are commonly added to stainless steels to improve oxidation resistance and stabilize ferrite.

[0035] Mo: solid solution strengthening, carbide-forming element, ferrite stabilizer, improvement of wet corrosion resistance, pitting resistance, and crevice corrosion resistance. Molybdenum is used in stainless steels in quantities of up to 8% and more, commonly up to 4%. Even relatively small percentages of molybdenum have powerful effects in improving pitting resistance in chloride-containing environments and crevice corrosion resistance. It generally strengthens the passivity of surfaces by decreasing the tendency of previously formed passive films to break down.

[0036] Nb: creep resistance, stabilizer of interstitial elements through the formation of carbides and nitrides. In stainless steels, with regard to corrosion resistance, it is known that stabilizing the grade by adding niobium prevents the risk of intergranular corrosion in thermally stressed areas. To avoid this, typically in ferritic steels, niobium is added in sufficient quantities depending on the carbon and nitrogen levels in the alloy composition. The 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 ferritic stainless steels, the addition of niobium, in an amount between 0.4% and 0.7%, is one of the most effective methods for improving thermal fatigue resistance.

[0037] Ak an element capable of forming nitrides, it enhances oxidation resistance. Aluminum is known to stabilize the ferritic phase, but above all, it is an element that enhances hot oxidation resistance by forming a very thin and adherent oxide layer. Its importance is also linked to its high affinity with nitrogen, which makes it an element that, when properly dosed, is capable of forming stable nitrides at high temperatures.

[0038] Zr: an element capable of forming nitrides, it enhances oxidation resistance. Zirconium is an element capable of forming nitrides and carbonitrides that are stable at high temperatures; therefore, it enhances creep resistance. It is also known to participate in the formation of the surface oxide layer, improving hot oxidation resistance.

[0039] Ti: an element capable of forming carbonitrides. Like niobium, titanium is also an element capable of precipitating carbides and nitrides preferentially over chromium carbides, preserving the necessary percentages of chromium in the matrix. However, it tends to form coarse carbonitrides, which are unable to make the microstructure resistant to high temperatures. For this reason, it is added in very small quantities (<0.03%) to the composition of the present invention.

[0040] The objective of the present invention is to produce ferritic steel strips suitable for the above requirements and with improved performance compared to prior art products. This objective is achieved by means of a strip made of steel having the composition and characteristics according to claim 1 and the related production process, while further advantageous characteristics are set out in the dependent claims. The main advantages of the steel strip according to the present invention can be summarized as follows:

[0041] • high formability and weldability,

[0042] • excellent resistance to viscous flow (creep) at high temperatures up to 1050°C,

[0043] • excellent resistance to corrosion and cyclic oxidation up to 1050°C,

[0044] • excellent resistance to oxidation in environments containing exhaust fumes such as in combustion engines,

[0045] • better thermal conductivity than austenitic steels,

[0046] • lower manufacturing costs compared to austenitic steels.

[0047] The ferritic stainless steel used to make the strip of the present invention has a chemical composition comprising the following elements (expressed in % by weight): C: <0.03%

[0048] N: <0.03%

[0049] Cr: 18-25%

[0050] Mo: 1.0-2.5%

[0051] Nb: 0.4-0.7%

[0052] Si: 0.2-1%

[0053] Al: <0.10%

[0054] Zr: <0.05%

[0055] Ti: <0.03% in addition to the following elements, which are not intentionally added but are the standard limits for typical ferritic steels:

[0056] Mn: <1%

[0057] Cu: <0.5%

[0058] Ni: <0.5%

[0059] S: <0.015%

[0060] P: <0.05% the remainder being iron and unavoidable impurities.

[0061] In addition, the following further compositional constraints apply (concentrations expressed in % by weight): a) Nb > 0.27 + 7(C+N), which guarantees the complete stabilization of the interstitial elements carbon and nitrogen, counteracting the phenomenon of sensitization and thus ensuring excellent corrosion resistance; b) 0.04 < Al + Zr + Ti < 0.10, which expresses the minimum and maximum quantity of elements capable of forming nitrides; c) 0.5 < C / N < 1.6, which expresses the condition for controlling the distribution of the precipitated fraction between carbides and nitrides, ensuring the type of precipitates responsible for creep resistance.

[0062] Finally, the precipitation state of second phases obtained on the finished product based on the composition described above and the process according to the invention differs from the prior art in that it consists of at least 80% by number of particles in which the atomic fraction of Nb in their stoichiometry is not greater than 56% among the elements Fe, Cr and Nb only.

[0063] As is well known, precipitates (second phases) form in metals. These have compositions and crystal structures that differ from the matrix (main phase) and are formed by diffusion and segregation phenomena during cooling or heat treatment. The term "precipitation state" refers to the set of crystalline phase particles (carbides, nitrides, intermetallic compounds) separated from the metal matrix after reaching the thermodynamic solubility limit. The mechanical properties of the metal (strength, hardness, toughness) and corrosion resistance depend greatly on this. In heat treatments, the precipitation state of the second phases is controlled in order to obtain the desired characteristics.

[0064] Note that the condition C + N < 0.06% sets the upper limit on the total fraction of precipitable carbides and nitrides, and that the preferred composition of the present invention also includes in constraint b) the presence of Al alone without the addition of Zr and Ti. Furthermore, the average grain size in the steel strip resulting from the composition and process according to the invention is preferably in the range of 20-80 pm, which guarantees maximum creep resistance and a homogeneous recrystallized structure that does not compromise the ductility and formability characteristics of the product at room temperature.

[0065] The improvement in creep resistance is achieved by adding an appropriate amount of aluminum and / or zirconium in accordance with constraint b), without reducing the formability properties and simultaneously enhancing the resistance to cyclic oxidation. The addition of aluminum alone to the above-mentioned preferred composition has advantages in terms of the production process, since the addition of zirconium in the production stages prior to casting involves a complex procedure, and this alloying element is also expensive. Titanium alone, in the absence of aluminum and / or zirconium, does not guarantee the same results as the addition of the other two elements, which are highly compatible with nitrogen to form nitrides.

[0066] Usually, the stabilization of interstitial elements, and especially nitrogen, is carried out exclusively by adding niobium, while in the present invention it is supported by the addition of Al and / or Zr. Since the thermodynamic stability of AIN and ZrN is greater than that of NbN, the available nitrogen is preferentially precipitated in the form of the first two, producing the effect of increasing the molar fraction of carbon in niobium carbonitrides of the general formula Nb(Cx,Ni-x), formed during the final stages of solidification and during the heat treatment of the slab prior to hot rolling.

[0067] Furthermore, reducing the amount of niobium involved in the high-temperature precipitation of NbN-type nitrides increases the availability of this element for subsequent preferential precipitation (>80% in numerical fraction) of particles characterized by an atomic fraction of Nb in their metallic component not exceeding 56%. These precipitates are essentially carbides of the type (Fe,Cr)3Nb3C (of the type MeC where M indicates a metal) and Laves phases of the type (Fe,Cr)2Nb. This precipitation state is extremely favorable for resistance to temperatures well above 1000°C.

[0068] Conversely, a precipitation state preferentially consisting of compounds with high atomic fractions of Nb (>56%) in their stoichiometry, e.g., Nb (C,N), induces lower resistance to hot creep in the case of long exposures to temperatures above 1000°C.

[0069] It has also been observed that an excess of Al (>0.1%), due to its high affinity for nitrogen, can lead to the undesirable precipitation of coarse aluminum nitrides, especially in the subsurface region of the strip (not related to stabilization), due to nitrogen uptake from the atmosphere during high-temperature treatments in air. At the same time, an excess of Zr (> 0.05%), in addition to the useful precipitation of ZrN, can compete with niobium for carbon, precipitating also as ZrC, reducing the formation of "useful" precipitates of the MeC type. The Nb range between 0.4% and 0.7% by weight, prescribed in the present invention, is able to guarantee maximum resistance to viscous flow (creep) at very high temperatures (up to 1050°C), having an ideal content to promote the precipitation of phases of the type (Fe,Cr)3Nb3C, and in any case phases with a low Nb content in their stoichiometry, according to the above-mentioned mechanisms of differential precipitation of nitrogen with Al and Zr. Furthermore, the addition of Al and / or Zr is also advantageous for oxidation resistance, thanks to the high affinity for oxygen of these alloying elements.

[0070] Even in limited quantities within the range specified above, aluminum interferes with the mechanisms of scale formation, i.e., the surface oxide layer that forms during exposure to high temperatures and therefore in particular during component operation, diffusing towards the surface of the strip and undergoing selective oxidation. Aluminum therefore promotes the formation of a stable, hard, and adherent scale which, by not undergoing fragmentation, limits the weight loss typical of thermal oxides.

[0071] Another advantage of the present invention is the economic aspect. In fact, the ferritic stainless steel described in the present invention is able to guarantee adequate resistance to viscous flow (creep) and oxidation at high temperatures, as specified above, at lower costs than austenitic steels used for similar applications at high temperatures. Compared to austenitic steels, ferritic steels are more economical and have a less variable cost, thanks to the absence of very expensive alloying elements such as nickel.

[0072] The production process according to the invention involves numerous traditional steps and some specific innovative steps, as described in detail below.

[0073] The liquid steel is produced in an electric furnace (Electric Arc Furnace = EAF), then undergoes a decarburization process with oxygen and argon in an AOD (Argon Oxygen Decarburization) converter and, if necessary, under vacuum in a VOD (Vacuum Oxygen Decarburization) converter.

[0074] It is then cast into slabs using conventional continuous casting machines or into 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 grain size is at least 20 pm, and preferably in the range of 20-80 pm.

[0075] The possible heating of conventional slabs, typically between 190 and 250 mm thick, prior to rolling is preferably carried out at temperatures between 1180 and 1250°C for periods between 150 and 240 minutes. Hot rolling is carried out at finisher inlet temperatures between 1000 and 1050°C and finisher outlet temperatures between 850 and 950°C, up to thicknesses between 1.5 and 6 mm.

[0076] A first difference with respect to the production process of a traditional ferritic stainless steel is that, after hot rolling, the steel of the present invention must have a coiling temperature in the range of 575-650°C obtained by adequate cooling on the roller table immediately before coiling. This coiling temperature range is necessary in order to inhibit the precipitation of embrittling phases, whereas it is not necessary in the standard production process for stainless steels, which are usually wound at temperatures of at least 800°C without being cooled after hot rolling.

[0077] In fact, due to its high Cr and Nb content, the steel of the present invention may be subject, in the range of 650-800°C, to excessive precipitation of hexagonal intermetallic phases (Laves phases), which are inherently embrittling and could cause difficulties in subsequent steps of the production process. Furthermore, they would remove Nb and therefore interfere with the correct balance of precipitates useful for improving creep resistance. Conversely, below 550°C, ferritic steels can undergo the so-called embrittlement phenomenon due to the precipitation of brittle phases containing Cr stable between 425 and 525°C, so it is preferable to set the lower limit of the coiling temperature well above this range.

[0078] The annealing of the hot-rolled strip is carried out at temperatures of 950-980°C for about 60 seconds, using a standard process. The subsequent cold rolling allows strips with thicknesses from 0.5 to 3 mm to be obtained, depending on the desired application.

[0079] Finally, another innovative aspect of this process is the final annealing of the cold- rolled strip, which allows the microstructure in the final strip to recrystallize with the correct grain size, thus also guaranteeing the mechanical characteristics and formability of the material. Given the particular chemical composition of the steel strip of the present invention, this annealing must be such as to produce the correct microstructure and at the same time ensure the stability of the precipitates that have formed in the preceding steps and which are responsible for the excellent creep resistance.

[0080] The final annealing is therefore carried out using a specific time-temperature curve for the cold-rolled material: a very slow rise in temperature, reaching 880°C in about 240 seconds, and finally a very fast annealing between 880°C and 1000°C for about 60 seconds. This curve is very different from the process applied to standard ferritic stainless steels, for which it is typical to minimize the temperature rise time and increase the time at the treatment temperature (soaking).

[0081] To confirm the improved creep and cyclic oxidation resistance of a steel of the present invention, the applicant has carried out numerous tests on different types of steel strips, which are illustrated below with the aid of the attached drawings, in which:

[0082] Fig.l is a diagram of the test configuration used to evaluate creep resistance;

[0083] Figs.2 and 3 are diagrams illustrating the results of the tests carried out according to the diagram in Fig.l on four samples according to the invention (S1-S4) and four comparative samples (C1-C4) that do not fall within the scope of the invention;

[0084] Fig, 4 is a diagram illustrating the results of cyclic oxidation resistance tests carried out on two other samples according to the invention (S5-S6) and two of the comparative samples (C1-C2) that do not fall within the scope of the invention, as well as a traditional austenitic steel strip; and

[0085] Figs.5-12 are ternary Nb-Fe-Cr diagrams illustrating the compositions of the various possible phases of the precipitates.

[0086] With reference to Fig.l, it can be seen that a deflection test (sag test) was used to evaluate creep resistance at high temperatures, in which pieces of strip 220 mm long, 20 mm wide, and of various thicknesses were used as samples, considering that the standard thickness is 2 mm. These pieces are placed horizontally in an oven on a pair of wedge- shaped ceramic supports "A" with both ends approximately 20 mm above the bottom of the oven. The distance between supports "A" is L=200 mm and the piece is centered so that it protrudes 10 mm from each support.

[0087] 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, undergoes a deflection caused by its own weight, the extent of which depends mainly on its thickness. The deflection of the sample is measured at the beginning of the test (do) and at the end of the test (di), and is expressed in millimeters as D = di- do with a typical accuracy of ±0.5 mm. The result of the test depends on the length L, the thickness t of the sample, and the test temperature. From a metallurgical point of view, the main factors controlling deflection are the amount of niobium and the grain size, through the Nabarro-Herring diffusive creep mechanism, since the larger the grain size, the lower the tendency to undergo deflection due to viscous flow (creep). When performing a sag test with a fixed sample holder geometry, the only relevant parameter is the thickness t of the strip, assuming with good approximation a fixed Nb content and a temperature high enough to ensure that the ferrite grain size is large enough not to influence the process. Assuming that the sample is a simply supported beam, the maximum shear stress at the midpoint between supports "A" is directly proportional to L2and inversely proportional to thickness t.

[0088] To estimate the outcome of the sag test with the geometric arrangement illustrated above, reference is made to a limit sagging of 10 mm over a length L=200 mm, corresponding to 5% of the length, for a strip with a thickness tref=2 mm at a temperature of 1050°C. This sagging is considered the maximum permissible deflection, and for strips of different thicknesses, the equivalent sagging D* referring to 2 mm thick strips is calculated according to the relationship D*=D(t)*(trei) / t), where D(t) is the sagging measured at t tref. In this case too, the maximum permissible deflection for the equivalent sagging D* is 10 mm.

[0089] Characterization of the precipitation state

[0090] To clarify the metallurgical mechanism responsible for high-temperature creep resistance, metallographic sections of numerous industrially produced cold-rolled and annealed strips were observed using a scanning electron microscope (SEM). A semi- quantitative chemical microanalysis of the precipitates was performed using energy- dispersive spectroscopy (EDS). The precipitates were characterized considering only the substitutional elements Fe, Cr, and Nb and expressing the composition in terms of normalized atomic fraction (x) so that XFe+xcr+xNb=l.

[0091] For each sample, SEM-EDS analysis was performed considering 10 fields at 2500X magnification, corresponding to an area of 2000 pm2each, to determine the chemical composition of at least 25 particles smaller than approximately 0.5 pm.

[0092] Based on thermodynamic considerations, the following four stable and metastable precipitation phases can be found in this steel, each characterized by its own chemical composition and crystal structure:

[0093] • Nb(C,N) - Niobium carbonitride: defective face-centered cubic lattice structure, with atomic fraction of Nb >80%;

[0094] • (Fe,Cr,Nb)2(N,C) - Nb-Cr carbonitride with trigonal lattice structure and atomic fraction of Nb between 56% and 80%;

[0095] • (Fe,Cr)3Nb3C - Cubic carbide: MeC type face-centered cubic lattice structure with atomic fraction of Nb between 40% and 56%;

[0096] • (Fe,Cr)2Nb - Laves phase: intermetallic compound with hexagonal lattice structure and atomic fraction of Nb between 27% and 40%.

[0097] Their stoichiometry is not fixed but extends over a continuous range of chemical compositions, as the phases can be considered as solid solutions. All particles always contain a certain amount of Cr from the ferritic matrix. The three selected elements Fe, Cr and Nb are the minimum information required to univocally identify all possible precipitate phases that can form in the system. Carbon was not included because its quantitative determination using the EDS technique does not guarantee sufficient accuracy compared to heavier elements.

[0098] The nominal composition ranges of all possible precipitates are shown graphically in Fig.5 as areas with different hatching patterns. Starting from the Nb comer, the first phase is Nb(C,N), followed by (Fe,Cr,Nb)2(N,C), (Fe,Cr)3Nb3C and Laves phase (Fe,Cr)2Nb. The limit of the atomic fraction of Nb not exceeding 56%, identified as a distinctive feature in claim 1, is at the upper limit of the composition range of (Fe,Cr)3Nb3C.

[0099] The dotted line represents a constant Fe / Cr atomic ratio and a variable Nb content, a characteristic of all precipitates in this steel, with an increase in Cr content in the precipitates as the Nb content decreases, and on average all particle chemical compositions will fall along this dotted line.

[0100] On this basis, the results of the EDS analysis on the analyzed precipitates can be plotted on a ternary Fe-Cr-Nb diagram similar to that in Fig.5. Each particle is represented by a point indicating the measured composition. In this way, it is possible to evaluate their distribution with respect to the theoretical composition of the phases listed above.

[0101] The ternary diagrams in Figures 6-12 show the distribution of the chemical composition of the precipitates of selected samples of cold-rolled and annealed steel strips, expressed as the atomic fraction of the main metal components Fe, Cr and Nb, obtained by SEM-EDS analysis.

[0102] For the evaluation of cyclic oxidation resistance, the test method used aims to measure the weight loss of the oxide scale of a material subjected to cycles of heating and subsequent cooling. For applications of interest in the exhaust system sector, heating cycles of 25 minutes at working temperature are applied, followed by 5 minutes of cooling, so as to perform two cycles per hour. The test comprises a total of 1000 cycles and the weight of the samples is monitored periodically.

[0103] The weight loss indicates that the oxide layer formed during the heating phase is lost during cooling, due to the difference in the coefficient of thermal expansion between the metal and the oxide itself, which generates strong tension at the interface. When, on the other hand, the scale forming the oxide layer consists mainly of an aluminum oxide and / or zirconium oxide base, it is generally very thin, stable, hard, and adherent and does not undergo sudden fracture phenomena, resulting in very limited or no weight loss. The dimensions of the samples are 60 x 60 mm and the typical test temperatures are: 900- 1000-1020-1050°C.

[0104] EXAMPLE US

[0105] A steel strip according to the present invention with 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 containing aluminum but no zirconium, and its chemical composition satisfies the constraints mentioned above: 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.

[0106] Sample SI was produced industrially by conventional casting into 215 mm thick slabs and hot rolling, after heating the slabs to 1200°C, to a thickness of 5 mm. The finishing mill process was carried out between 1000°C and 880°C and the strip was coiled at a temperature in the range 575-650°C. The strip was then annealed at temperatures of 950-980°C for about 60 seconds and pickled using a standard process, then cold rolled to a thickness of 2 mm and subjected to a final annealing at a maximum temperature of 1000°C according to the specific procedure described above.

[0107] SEM analyses of the precipitation state in the final product showed that most of the precipitates fall within the compositional range of (Fe,Cr)3Nb3C with the presence of some Laves phases (Fig.6). Rare Nb carbonitrides were observed towards the lower left corner of the ternary diagram, the presence of which did not compromise the outcome of the sag test.

[0108] EXAMPLE 2 (S2)

[0109] A steel strip according to the present invention with 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; Nb=0.60; Zr=0. This example corresponds to the preferred embodiment containing aluminum but not zirconium, and its chemical composition satisfies the constraints mentioned above: 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.

[0110] The S2 sample was produced on a laboratory scale using a vacuum induction melting furnace with which 80 kg ingots were cast, then hot rolled using a laboratory rolling mill, annealed, pickled, and subsequently cold rolled to a final thickness of 2 mm. The final annealing was also carried out using laboratory equipment, and all steps of the laboratory process complied with the same specifications as industrial treatments.

[0111] SEM analyses of the precipitation state in the final product showed that most of the precipitates fall within the compositional range of (Fe,Cr)3Nb3C with the presence of some Laves phases (Fig.7). Rare Nb carbonitrides were observed towards the lower left corner of the ternary diagram, the presence of which did not compromise the outcome of the sag test.

[0112] EXAMPLE 3 (S3)

[0113] A steel strip according to the present invention with 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, part of the aluminum has been replaced with zirconium, while the other elements are essentially unchanged, and its 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.

[0114] Sample S3 was produced on a laboratory scale in the same way as sample S2, and SEM analyses (Fig.8) produced the same result as S2.

[0115] EXAMPLE 4 (S4)

[0116] A steel strip according to the present invention with 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, aluminum has been replaced with zirconium, while the other elements are essentially unchanged, and its chemical composition 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.

[0117] Sample S4 was produced on a laboratory scale in the same way as sample S2, and SEM analyses (Fig.9) produced the same result as S2.

[0118] EXAMPLE 5 (S5)

[0119] A steel strip according to the present invention with 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 containing aluminum but no zirconium, and its chemical composition satisfies 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.

[0120] Sample S5 was produced industrially in the same way as sample SI, and SEM analyses (Fig.10) produced the same result as SI.

[0121] EXAMPLE 6 (S6)

[0122] A steel strip 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 containing aluminum but no zirconium, and its chemical composition satisfies the constraints mentioned above: a) Nb > 0.27 + 7(C+N) = 0.48; b) 0.04% < Al+Ti+Zr = 0.07% < 0.10%; c) 0.5 < C / N = 1 < 1.6. Sample S6 was produced industrially in the same way as sample SI.

[0123] SEM analyses of the precipitation state in the final product showed that almost all of the precipitates fall within the compositional range of (Fe,Cr)3Nb3C with the presence of some Laves phases (Fig.l 1). Only one Nb carbonitride was observed in the lower left corner of the ternary diagram.

[0124] COMPARATIVE EXAMPLE 1 (C

[0125] A strip of conventional steel 441LI (1.4509 according to EN 10088-2) with the following composition in terms of weight percentages: C=0.014; N=0.017; Cr=18; Si=0.5; ADO.01 ; Ti=0.14; Nb=0.37. This example, compared to the ranges of the present invention, has an excess of Ti and a deficiency of Nb and Mo, and does not meet the constraints a) because Nb < 0.27 + 7(C+N) = 0.487 and b) because Al+Ti+Zr = 0.15% > 0.10%.

[0126] Sample Cl was produced using a conventional cycle of continuous casting into slabs, hot rolling after heating the slab to 1200°C, coiling without cooling, annealing, pickling, cold rolling to a thickness of 2 mm, and final annealing at a maximum temperature of 1000°C according to the traditional process.

[0127] COMPARATIVE EXAMPLE 2 (C2)

[0128] A steel strip purchased on the market with a measured chemical composition, in terms of weight percentages: C=0.0163; N=0.017; Cr=19; Si=0.4; Mo=1.85%; ADO.002; TD0.003; Nb=0.60; Zr=0. The chemical composition is similar to the steel of the present invention but does not comply with constraint b) because Al+Ti+Zr = 0.005% < 0.04%. The balance of elements capable of stabilizing interstitial elements, in particular nitrogen, is therefore shifted entirely towards the precipitation of niobium nitrides, rather than favoring the precipitation of second phases that are much more conducive to creep resistance. The production details are unknown.

[0129] COMPARATIVE EXAMPLE 3 (C3)

[0130] A steel strip produced with the following composition in terms of weight percentages: C=0.012; N=0.015; Cr=19; Si=0.5; Mo=1.85; AD0.065; Zr=0.075; Ti=0.001; Nb=0.59. This example, compared to the ranges of the present invention, has an excess of Zr and does not comply with constraint b) because Al+Ti+Zr = 0.141% > 0.10%.

[0131] Sample C3 was produced on a laboratory scale in the same way as sample S2.

[0132] COMPARATIVE EXAMPLE 4 (C4)

[0133] A steel strip with the following composition in terms of weight percentages: C=0.013; N=0.015; Cr=19; Si=0.4; Mo=1.8; Al=0.045; Ti=0.008; Nb=0.60; Zr=0 which corresponds to the preferred embodiment containing aluminum but no zirconium, and its chemical composition also satisfies the constraints mentioned above: a) Nb > 0.27 + 7(C+N) = 0.47; b) 0.04% < Al+Ti+Zr = 0.053% < 0.10%; c) 0.5 < C / N = 0.87 < 1.6.

[0134] Sample C4 was produced industrially by conventional casting into 215 mm thick slabs and hot rolling, after heating the slabs to 1200°C, to a thickness of 5 mm. The finishing mill process was carried out at an exit temperature of 720°C and the strip was coiled at a temperature in the range 500-530°C. The strip was then annealed, pickled, cold rolled to a thickness of 2 mm, and subjected to a final annealing at a maximum temperature of 1000°C according to the specific procedure described above with reference to S 1.

[0135] SEM analyses of the precipitation state illustrated in the ternary diagram in Fig.12 showed that almost all precipitates have a chemical composition with atomic fraction of Nb above the 56% limit.

[0136] Table 1 below lists the numerical fraction of particles with an atomic content of Nb (among the elements Fe, Cr, Nb only) not exceeding 56% for the six samples S1-S6 according to the invention and for the comparative sample C4, which falls within the chemical composition ranges but differs in terms of the production process.

[0137] Table 1

[0138] The creep resistance test (sag test) was performed on samples SI, Cl, C2, and C4 as described above. The results are shown in Table 2 below and represented in Fig.2 in terms of percentage deflection relative to the maximum permissible deflection, whereby a sample fails the test if this percentage exceeds 100%.

[0139] Table 2

[0140] It is clear that sample SI according to the invention remains well within the deflection limit expected in both the 1000°C and 1050°C tests, the comparative samples C2 and C4 withstand 1000°C but fail the test at 1050°C, and the comparative sample Cl fails the test already at 1000°C, so the test at 1050°C was not performed.

[0141] The results of a similar test on samples S2-S4 and C3 are shown in Table 3 below and represented in Fig.3, with the difference that the test was only performed at 1050°C and that the 100% threshold limit is defined in relation to the deflection of sample S2 taken as a reference, rather than in relation to the absolute deflection value.

[0142] Table 3

[0143] This test shows how the percentages of Al and Zr affect the creep resistance performance of steel, taking as a reference the S2 sample, which contains only 0.045% Al by weight (Ti is equal to 0.001% by weight in all samples, so it does not affect the results).

[0144] In the S3 sample variant, with a 0.02% reduction in the amount of Al compensated by an equal increase in Zr, performance improves compared to S2, and the same occurs, albeit to a lesser extent, in the S4 sample variant containing only Zr in an amount similar to the Al content of S2 and in any case such as to satisfy requirement b) because 0.04% < Al + Zr + Ti < 0.10%. On the contrary, performance worsens compared to S2 in the C3 sample variant containing Al and Zr in quantities that do not satisfy requirement b) because Al + Zr + Ti > 0.10%.

[0145] With regard to cyclic oxidation resistance, the results of the tests carried out as described above, at a temperature of 1050°C, are shown in the graph in Fig.4, which shows an improvement in performance as the Al content increases. In fact, the two samples S5 and S6 differ mainly in their Al content, which increases from 0.042% in S5 to 0.062% in S6, while the samples Cl and C2 of the comparative examples contain much lower amounts of Al, 0.01% and 0.02% respectively.

[0146] The behavior of an AISI 321 austenitic steel strip is also shown, which is known to be resistant to hot viscous flow up to approximately 1000°C, but in general is very little resistant to cyclic oxidation. In fact, the data for this strip refer to a test carried out at a temperature of 1000°C, which is even lower than the temperature of 1050°C used for testing the other samples.

Claims

CLAIMS1. Strip made of ferritic stainless steel having a chemical composition comprising the following elements expressed as a percentage by weight: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% the rest being iron and unavoidable impurities, and having the following additional compositional constraints, with concentrations expressed as weight percentage: a) Nb > 0.27 + 7-(C+N); b) 0.04 < Al + Zr + Ti < 0.10; c) 0.5 < C / N < 1.6; characterized in that the precipitation state of second phases obtained on the finished strip by cold rolling and recrystallization annealing consists of at least 80% particles in which the atomic fraction of Nb in their stoichiometry is not greater than 56% among the elements Fe, Cr and Nb only.

2. Strip according to claim 1, characterized in that the average grain size is in the range of 20-80 pm.

3. Production process of a strip made of a ferritic stainless steel according to claim 1 or 2, comprising the following steps:a) continuous casting into slabs with a thickness between 190 and 250 mm; b) hot rolling, with possible preheating, carried out at finisher inlet temperatures between 1000 and 1050°C and finisher outlet temperatures between 850 and 950°C, with thicknesses of 1.5-6 mm; c) coil winding; d) annealing at temperatures of 950-980°C for approximately 60 seconds; e) pickling; f) cold rolling to thicknesses of 0.5-3 mm; g) recrystallization annealing; characterized in that it also includes a cooling step performed immediately before step c) so that the coiling takes place in the temperature range 575-650°C, and in that step g) is performed with a very slow temperature rise, reaching 880°C in approximately 240 seconds, and a very fast annealing between 880°C and 1000°C for approximately 60 seconds.

4. Production process according to claim 3, characterized in that step b) includes preheating between 1180 and 1250°C for times between 150 and 240 minutes before hot rolling.

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