EP0735153B1 - Stainless ferritic steel, particularly suitable as catalyst substrate - Google Patents

Abstract

A ferritic steel alloy, used as a catalyst support in a car exhaust filter, comprises 12-25% Cr, 4-7% Al, less than 0.08% in total of one or more of Ce, La, Nd, Pr and Y, pref. less than 0.3% in total of Zr and/or Nb and less than the following of other constituents 0.03% C, 0.02% N, 0.22% Ni, 0.002% S, 0.6% Si and 0.4% Mn. The actual amounts of Zr and/or Nb are defined mathematically in terms of the C and N levels.

Description

The present invention relates to a ferritic stainless steel, resistant to oxidation at high temperature, usable, especially for catalyst support structures, such as, for example, structures contained in motor vehicle exhausts.

The catalyst support structures made with strips of iron-chromium-aluminum steel are intended to resist oxidation and deformations at high temperature.

The steels used must be capable of being produced as part of a industrial production, for example, in continuous casting followed by transformations to obtain wide steel strips and low thickness for making strips.

It is known from German patent C 632 657 an alloy of iron, chromium, FeCrAl aluminum with up to 30% chromium, from 0.1 to 11.5% of aluminum, 0.05 to 2% of rare earths such as, for example, cerium, and may contain zirconium and titanium.

European patent EP 0429 793 is also known, which describes FeCrAl alloys containing rare earths, active elements, such as cerium, lanthanum, praseodymium and stabilizers, titanium or niobium. The bill active elements at high contents is proposed. A minimum content in phosphorus is recommended in order to improve the hot fragility of alloys with regard to the high contents of active elements. The contents phosphorus levels thus specified are lower than those usually encountered during the development of stainless steels. The addition of stabilizer such as titanium is carried out to improve the hot brittleness of alloys. Oxidation tests, which have been carried out, are carried out at a temperature of 1170 ° C.

US patent 4,414,023 also describes FeCrAl alloys containing the active elements cerium, lanthanum, praseodymium and stabilizers such as zirconium and / or niobium. Addition of active elements is carried out to prevent chipping of the oxide layer.

Addition of zirconium as a stabilizer under the condition Zr ≤ 91 (% C / 12 +% N / 14 + 0.03) is performed to trap the carbon and nitrogen in the form of carbides and nitrides.

The addition of niobium under the condition Nb ≤ 93 (% C12 +% N / 14 + 0.0075) is performed to improve the creep resistance.

This patent mentions very high stabilizer contents and claims stabilization with Zr as being preferable for resistance to oxidation. It also indicates that the addition of several stabilizers is not recommended because it leads to behavior similar to that alloys with a single stabilizer having the worst resistance to oxidation.

In addition, the range of zirconium contents is wide and does not allow not meet the dimensional stability conditions of the supports catalyst. Similarly, the Niobium contents do not allow a resistance optimum in oxidation.

Patent application EP 0 480 461 is also known. relating to a ferritic steel containing aluminum and having a good resistance to oxidation, patent application in which it is specified that the presence of niobium improves the creep resistance of the supports. This behavior is defined according to the nitrogen contents, which is not justified due to the presence of aluminum and / or zirconium, because it forms compounds of aluminum nitride and zirconium so preferable to niobium nitride.

The object of the invention is to present a ferritic stainless steel, usable in particular for support structures of catalysts subjected to a temperature variation cycle, and having improved behavior in oxidation and deformation in elongation at high temperature.

de 12 à 25 % de chrome

de 4 à 7 % d'aluminium

moins de 0,03 % de carbone

moins de 0,02 % d'azote

moins de 0,22 % de nickel

moins de 0,002 % de soufre

moins de 0,6 % de silicium

moins de 0,4 % de manganèse,

des éléments actifs choisis parmi le cerium, le lanthane, le néodyme, le praséodyme, l'ytrium, pris seuls ou en combinaison, à une teneur inférieure à 0,08 %, au moins un élément stabilisant choisi parmi le zirconium et/ou le niobium, le reste étant constitué par le fer et les impuretés inévitables,
   les teneurs en zirconium et/ou niobium satisfaisant aux conditions suivantes :
   pour le zirconium, 91 (C %/12 + N %/14) - 0,1 ≤ Zr ≤ 91 (C %/12 + N %/14) + 0,1    pour le niobium, 93×0,8. (C %/12 ) - 0,1 ≤ Nb ≤ 93×0,8 (C %/12) + 0,15 et Nb < 0,3 %.    pour le zirconium et le niobium, 91 (N %/14) - 0,05 ≤ Zr ≤ 91 (N %/14) + 0,05, et 93×0,8 (C %/12) - 0,05 ≤ Nb ≤ 93×0,8 (C %/12) + 0,10. The subject of the invention is a stainless steel comprising in its weight composition:

12 to 25% chromium

4 to 7% aluminum

less than 0.03% carbon

less than 0.02% nitrogen

less than 0.22% nickel

less than 0.002% sulfur

less than 0.6% silicon

less than 0.4% manganese,

active elements chosen from cerium, lanthanum, neodymium, praseodymium, ytrium, taken alone or in combination, at a content of less than 0.08%, at least one stabilizing element chosen from zirconium and / or niobium, the rest being iron and unavoidable impurities,
the zirconium and / or niobium contents satisfying the following conditions:
for zirconium , 91 (C% / 12 + N% / 14) - 0.1 ≤ Zr ≤ 91 (C% / 12 + N% / 14) + 0.1 for niobium , 93 × 0.8. (C% / 12) - 0.1 ≤ Nb ≤ 93 × 0.8 (C% / 12) + 0.15 and Nb <0.3%. for zirconium and niobium , 91 (N% / 14) - 0.05 ≤ Zr ≤ 91 (N% / 14) + 0.05, and 93 × 0.8 (C% / 12) - 0.05 ≤ Nb ≤ 93 × 0.8 (C% / 12) + 0.10.

The other characteristics of the invention are:

the active elements are chosen from cerium, lanthanum, neodymium, praseodymium, taken alone or in combination, and contained in a compound called "mischmetal".

The sum of the zirconium and niobium contents is less than 0.300%.

The sum of the carbon and nitrogen contents is less than 0.04%.

Silicon and manganese contents satisfy relationship If / min ≥1.

For the zirconium stabilizing element, used alone in the composition, the minimum aluminum content satisfies the following condition: 4% + 6 Zr% - 91 (C% / 12 + N% / 14).

For the niobium stabilizing element, used alone in the composition, the minimum aluminum content satisfies the following condition: 4% + 5 Nb% - 93 (C% / 12 + N% / 14).

For combined zirconium and niobium stabilizers, the minimum aluminum content satisfies the following condition: 4% + 5 (Zr + Nb) - 92 (C% / 12 + N% / 14).

When zirconium is introduced alone into the composition, the content of active elements satisfies the following relationship: 0.03 - 0.2 (Zr% - 91 N% / 14) ≤ (Ce + La + Nd + Pr + Y) ≤ 0.08 - 0.2 (Zr% - 91 N% / 14)

When niobium is introduced alone into the composition, the content of active elements satisfies the following relationship: 0.03 - 0.025 (Nb%) ≤ (Ce + La + Nd + Pr + Y) ≤ 0.08 - 0.025 (Nb%)

When zirconium and niobium are introduced into the composition in combination, the content of active elements satisfies the following relationship: 0.03 - 0.2 (Zr% - 91 N% / 14) - 0.025 (Nb%) ≤ (Ce + La + Nd + Pr + Y) ≤ 0.08 - 0.2 (Zr% - 91 N% / 14) - 0.025 (Nb%).

The following description and the attached drawings, all given as non-limiting example, will make the invention well understood.

Figure 1 group of resilience characteristics by measuring the transition temperature for steels with different grades in selected stabilizers.

Figure 2 presents a series of evolution characteristics of oxidation kinetics constants as a function of temperature for different stabilizers.

Figure 3 shows a series of elongation curves as a function the content of active elements.

Figure 4 shows a series of features in elongation for different zirconium and niobium contents in compositions having a defined content of active elements.

The ferritic stainless steel, according to the invention, resistant to oxidation at high temperature, has the following weight composition:
Cr: (12-25)%; Al: (4-7)%; C ≤ 0.03% N ≤ 0.02%; S ≤ 0.002%; If ≤ 0.6%; Mn ≤ 0.4%; active elements chosen from cerium, lanthanum, praseodymium, neodymium, ytrium, taken alone or in combination at a content ≤ 0.08%, stabilizers chosen from zirconium, niobium, taken alone or in combination , at a content ≤ 0.300%.

Preferably, the active elements are chosen from cerium, lanthanum, praseodymium, neodymium, taken alone or in combination, these elements being the constituents of the mixture called "mischmetal".

Lanthanum can be replaced by ytrium which has close chemical properties.

The steel intended, in particular for the manufacture of a catalyst support structure produced with a strip whose thickness is generally less than 200 μm, must have an oxidation resistance at temperatures generally below 1150 ° C. for several hundred d 'hours. The support structure must have good aptitude for hot and cold transformation and also satisfy the elongation deformation characteristics during oxidation.
According to the invention, it has been demonstrated precise conditions concerning the contents of stabilizing elements and active elements which must be respected for the production of steel in the form of rolled strips and for an improvement of the resistance to oxidation and elongation of said steel.
From the point of view of hot processing and transformation, the beneficial effect of the addition of stabilizers which allows the reduction of the ductile / brittle transition temperatures has been highlighted. However, the excess of stabilizer is harmful. According to the invention, it is highlighted that it is imperative to control the stabilizer contents so as to comply with the following conditions:

For a steel, according to the invention, stabilized with zirconium: 91 (C% / 12 + N% / 14) - 0.1 ≤ Zr ≤ 91 (C% / 12 + N% / 14) + 0.1

For a steel, according to the invention, stabilized with niobium: 93 × 0.8. (C% / 12) - 0.1 ≤ Nb ≤ 93 × 0.8 (C% / 12) + 0.15 and Nb <0.3%.

For a steel, according to the invention, stabilized with zirconium and niobium: 91 (N% / 14) - 0.05 ≤ Zr ≤ 91 (N% / 14) + 0.05, and 93 × 0.8 (C% / 12) - 0.05 ≤ Nb ≤ 93 × 0.8 (C% / 12) + 0.10.

The coefficient 0.8 is a factor imposed by the analysis of the stochiometry of the niobium-based compounds precipitated in the matrix.

Figure 1 group of resilience characteristics measured at average transition temperatures of steels with different contents of stabilizers chosen from zirconium and niobium.

• la teneur en zirconium libre ΔZr telle que ΔZr satisfait à la relation suivante: ΔZr % = Zr % - 91(C %/ 12 + N %/ 14),
• la teneur en niobium libre ΔNb telle que ΔNb satisfait à la relation suivante : ΔNb % = Nb % - 93 × 0,8 (C % / 12 )

It is represented on the abscissa:

• the free zirconium content ΔZr such that ΔZr satisfies the following relationship: ΔZr% = Zr% - 91 (C% / 12 + N% / 14),
• the content of free niobium ΔNb such that ΔNb satisfies the following relationship: ΔNb% = Nb% - 93 × 0.8 (C% / 12)

It can be seen that an excess, such as a lack of stabilizer in the composition of steel, is harmful.

It is therefore necessary to control the contents of zirconium and / or niobium so as to give the steel transition temperatures lowest possible ductile / brittle. Control of the elements stabilizers is important, given the casting production process keep on going. Uncontrolled stabilization can lead to embrittlement slabs, incompatible with industrial production.

From the point of view of the choice of stabilizers, steels containing in their composition of zirconium or niobium or titanium, have been tested in oxidation at different temperatures chosen between 900 ° C and 1400 ° C.

The oxidation test consists in measuring a mass gain ΔM reported to an area unit S.

The mass gain, corresponding to an oxidation, obeys a law of the type (ΔM / S) ² = Kp ^t , Kp being a constant called parabolic law, of exponential type, function of the temperature and the energy of activation of the oxidation reaction and t being the duration of the test.

• les variations de Kp (g^2/m⁴/sec) en fonction de l'inverse de la température absolue 1/T, pour des aciers stabilisés au titane ou au zirconium ou au niobium. Les vitesses de réaction d'oxydation sont exprimées par les valeurs de la constante parabolique Kp. Lorsque ces valeurs sont faibles, les cinétiques sont plus lentes et l'oxydation moins importante. Le bon comportement à l'oxydation est obtenu pour les valeurs de Kp les plus faibles possibles. D'après cette figure, nous pouvons remarquer que quel que soit l'acier, les constantes paraboliques augmentent avec la température. Les cinétiques d'oxydation augmentent donc aussi logiquement avec la température.

In Figure 2, are plotted:

• variations in Kp (g ^{2 /} m ⁴ / sec) as a function of the inverse of the absolute temperature 1 / T, for steels stabilized with titanium or zirconium or niobium. The oxidation reaction rates are expressed by the values of the parabolic constant Kp. When these values are low, the kinetics are slower and the oxidation less important. The good oxidation behavior is obtained for the lowest possible Kp values. From this figure, we can notice that whatever the steel, the parabolic constants increase with temperature. The oxidation kinetics therefore also logically increase with temperature.

This figure also shows that the nature of the stabilizers modifies these kinetics and that, surprisingly, they can have a beneficial or harmful influence depending on the temperature of use. Thus, at temperatures above 1150 ° C, it is titanium which has the best protective character against oxidation. At temperatures below 1150 ° C, on the other hand, the addition of titanium exerts a detrimental influence compared to the addition of niobium or zirconium. The extreme temperature of use of metallic catalyst support structures is generally below 1150 ° C. We see from this figure, and taking into account the temperatures of use of the catalyst support structures, that the best stabilizers are niobium and / or zirconium. The addition of titanium does not give good results in the envisaged temperature range. In addition, the combined addition of zirconium and niobium does not, contrary to what is mentioned in the prior art, lead to degradation of the shade in the proportions defined according to the invention. From Figure 2, we can notice that the addition of stabilizers leads to significant differences in the kinetics of oxidation.
The amount of aluminum necessary to resist oxidation at a given temperature and time therefore depends on the nature of the stabilizers. So to withstand 1,100 ° C for 400 hours, we have established the minimum amounts of aluminum necessary, depending on the stabilizers and the carbon and nitrogen content.
For zirconium: Al% minimum = 4% + 6 Zr% - 91 (C% / 12 + N% / 14). For niobium: Al% minimum = 4% + 5 Nb% - 93 × 0.8 (C% / 12).

We note that the quantity of aluminum necessary for stabilization with titanium corresponds to the following relationship: Al% minimum = 4% + 20 Ti% - 48 (C% / 12 + N% / 14).

For the combined addition of zirconium and niobium, we have: Al% minimum = 4% + 6 Zr% + 5 Nb% - 91 (N% / 14) - 93 × 0.8 (C% / 12).

The addition of titanium leads to minimum aluminum values which are not compatible with industrial production.

The formation of the oxide layer during the oxidation treatment generates constraints. These constraints are not negligible and can deform the catalyst support structure. The support structure of catalyst follows variations in elongation as a function of time, at a given temperature. These variations are manifested by a strong lengthening for a relatively short period of time and then, by stability of the elongation over time corresponding to a plateau and finally, by strong elongations for a period of time relatively long. The strong elongations occurring during a long period, are linked to the formation of chromium oxide diffusing in the alumina layer. This type of elongation has been identified, it is linked to the depletion of the aluminum content of the strip composition.

FIG. 3 shows elongations at the bearing as a function of the content of active elements. The elongation at the bearing depends, in this example, the content of active elements Ce, La, Pr, Nd entering the composition of "mischmetal" but also, and surprisingly, of the stabilizing element used. For example, the "mischmetal" content depends the zirconium content, because it is an active element from the point of view from oxidation. So the best deformation behaviors by elongation are obtained for "mischmetal" contents included between 0.02 and 0.04% for zirconium stabilization and between 0.04 and 0.075% for niobium stabilized steel. The addition of these elements, in trapping sulfur, improves the resistance to oxidation of steels. These additions must be controlled to optimize the properties of the steel. The simultaneous addition of Zr and Nb gives an opportunity to increase the range of content of active elements, content between 0.02 and 0.075%.

FIG. 4 presents a diagram of behavior in deformation by elongation, at the level, for different contents of zirconium and niobium, the contents of zirconium and niobium being adjusted as a function of the contents of carbon and nitrogen.
With regard to the elongation values at the plateau, the best results are obtained with the addition of niobium. The addition of zirconium has higher values. The origin of this phenonene is linked to the reactivity of the oxygen stabilizers. The reactivity of these stabilizers is greatly limited when these are added in a controlled amount in relation to the proportions of carbon and nitrogen.
Table 1, below, gives the different compositions of the alloys A, B1, B2, B3, C1, C2, shown in the figure. AT B1 B2 B3 C1 C2 VS 0.019 0.009 0.018 0.037 0.014 0.017 Yes 0.296 0.319 0.386 0.560 0.350 0.340 Mn 0.285 0.299 0.428 0.295 0.288 0.290 Or 0.195 0.215 0.150 0.196 0.216 0.214 Cr 20.10 20.19 20.18 22.10 20.03 20.11 Mo 0.033 0.033 0.041 0.018 0.031 0.028 Cu 0.036 0.039 0.035 0.012 0.035 0.043 S <5 ppm 2 ppm 9 ppm 4 ppm <10 ppm <10 ppm P 0.020 0.020 0.020 0.011 0.018 0.021 Al 5.03 4.7 5.18 4.6 5.2 5.4 NOT 0.007 0.004 0.008 0.012 0.006 0.006 This 0.0351 0.0133 0.0177 0.0111 0.0339 0.023 The 0.0151 0.0064 0.0082 0.0050 0.0155 0.010 Zr - 0.083 0.191 0.284 0.006 - Nb - - - - 0.205 0.285 This figure shows that the elongation at the plateau increases linearly with the stabilizer content. In order to obtain the best elongation behaviors, the stabilizer contents, and consequently, the carbon and nitrogen contents must be limited to very low contents: (C + N) ≤ 0.04% Zr and / or Nb ≤ 0.300%.

The simultaneous addition of Zr and Nb traps carbon and nitrogen to essentially form compounds of the ZrN and NbC type. This reduced choice considerably the amount of free stabilizers available for oxidation due to the thermodynamic stability of ZrN nitrides. The NbC formation develops during the thermal cycle due to the lower chemical affinity of niobium for oxygen.

Carbon and nitrogen are inevitable elements entering the composition of steels. These elements cause very strong reductions in hot ductility and lead to problems of steel processing.

The presence of zirconium and / or niobium stabilizers, by trapping the carbon and or nitrogen improve the hot ductility of the alloy. However, high carbon and nitrogen contents result in counterpart to stabilizers contents also very important. The significant precipitation of carbides and nitrides in the alloy decreases resistance to oxidation of the product by weakening the oxide layer.

Thus, in order to limit the presence of too many precipitates, the carbon content must be less than 0.03%, nitrogen content must be less than 0.02% and the carbon and nitrogen content must be preferably less than 0.04%.

According to the invention, it is preferable to limit the nitrogen contents to less than 0.01% so as to reduce the zirconium contents, for improve the characteristics of steel in elongation.

Zirconium and / or nobium are elements of addition volunteers planned to sequester carbon and / or nitrogen and thus improve the hot ductility of the grade. These elements, called stabilizers, must be controlled taking into account the production process envisaged in casting keep on going. In fact, insufficient stabilization would lead to a excessive embrittlement of slabs, incompatible with production industrial. Significant stabilization would lead to a deterioration of the resistance to oxidation of steel in the form of strip.

The combined addition of zirconium and niobium stabilizers allows both good resistance to oxidation and good cohesion of supports. Indeed, in addition to the properties of niobium as a stabilizer, it allows bonding between the spirally wound sheets of the supports. Thus, niobium can allow the elimination of solder tracks, by example based on nickel and possible contamination due to metal of the solder.

However, niobium can modify the kinetics of oxidation and must not be added in a proportion greater than 0.3%.
The product must withstand several hundred hours at very high temperature, that is, up to 1100 ° C. To meet this condition, the alloy must contain at least 4% aluminum. This content is necessary to form a protective oxide layer on the surface and to prevent premature depletion of the aluminum content in the strip. The aluminum content must be less than 7% in order to avoid the problems of transformation of the shade following an excessive degradation of the ductility when hot.
For alloys containing these aluminum contents, aluminum nitrides are preferably formed rather than niobium nitrides.

• ces éléments restent à la surface et éventuellement s'oxydent si l'activité chimique de l'élément est suffisante, c'est, plus particulièrement, le cas du silicium. Dans le cas d'aciers contenant beaucoup d'aluminium, l'oxydation du silicium est impossible. Cet élément reste en surface et participe efficacement à la protection en exerçant le rôle de barrière à la diffusion d'autres éléments.
• ces éléments migrent vers la surface et se subliment. C'est, plus particulièrement, le cas du manganèse, qui se trouve en grande quantité sur les parois des fours lors des traitements sous vide. Ce phénomène est néfaste du point de vue déformation à l'allongement car l'évaporation du manganèse libère la surface du métal et provoque l'oxydation des éléments ayant une grande affinité chimique pour l'oxygène.

Pour ces deux raisons, il est important pour conserver de bonnes propriétés en résistance à l'oxydation de maintenir le rapport Si/Mn

1.Silicon and manganese are highly oxidizable elements and also play a non-negligible role in elongation behavior. These two elements, under the influence of a treatment at high temperature, tend to migrate to the surface of the metal. There are then two possibilities:

• these elements remain on the surface and possibly oxidize if the chemical activity of the element is sufficient, this is, more particularly, the case of silicon. In the case of steels containing a lot of aluminum, oxidation of silicon is impossible. This element remains on the surface and participates effectively in protection by exercising the role of barrier to the diffusion of other elements.
• these elements migrate to the surface and sublimate. This is, more particularly, the case of manganese, which is found in large quantities on the walls of ovens during vacuum treatments. This phenomenon is harmful from the point of view of elongation deformation because the evaporation of manganese frees the surface of the metal and causes the oxidation of the elements having a great chemical affinity for oxygen.

For these two reasons, it is important to maintain good oxidation resistance properties to maintain the Si / Mn ratio

1.

Concerning the other elements contained in the composition of the steel according to the invention:

Phosphorus and sulfur are unavoidable impurities entering in the manufacture of stainless steels.

Phosphorus is usually found in steels stainless to a content of about 0.02%. This element plays a role neutral or slightly beneficial on the product's resistance to oxidation trapping excess cerium in the form of phosphides. Sulfur is also encountered in stainless steels with a content of approximately 0.005%. Sulfur has a detrimental influence on the behavior oxidation, it reduces the adhesion of the oxide to the strip and promotes chipping of this layer. For this reason, sulfur must be maintained at the lowest possible contents: less than 0.002%.

The chromium content of the steel must be sufficient, i.e. greater than 12%, to present the good properties with respect to the corrosion and promote the formation and behavior of the oxide layer at high temperature. The chromium content should also not be too high, i.e. less than 25%, to avoid processing problems steel.

According to the invention, preferably, the chromium content is included between 14 and 22%, which corresponds to an interval in concentration in chromium optimized for corrosion and steel processing.

The copper introduced into the composition is a residual element that found in the products used at the base in the development of steel.

The product resulting from the invention is intended for the manufacture of metal support structures for calalyzers, from strips including the thickness is less than 200 µm, and more commonly equal to 50 µm +/- 10 µm.

Claims (12)

1. Stainless ferritic steel, resisting oxidation at high temperature and particularly suitable as a catalyst substrate, comprising in its composition by weight:

   between 12 and 25% chromium

   between 4 and 7% aluminium

   less than 0.03% carbon

   less than 0.02% nitrogen

   less than 0.22% nickel

   less than 0.002% sulphur

   less than 0.6% silicon

   less than 0.4% manganese,

   active elements chosen from cerium, lanthanum, neodymium, praseodymium, yttrium, taken on their own or in combination, with a content less than 0.08%, at least one stabilising element chosen from zirconium and/or niobium, the remainder being constituted by iron and the inevitable impurities,
      the content of zirconium and/or niobium satisfying the following conditions:
      for zirconium, 91 (C %/12 + N %/14) - 0.1≤Zr≤91 (C %/12 + N %/14) + 0.1    for niobium, 93x0.8. (C %/12) - 0.1≤Nb≤93x0.8 (C %/12) + 0.15 and Nb < 0.3 %.    for zirconium and niobium, 91 (N %/14) - 0,05≤Zr≤91 (N %/14) + 0.05, and 93x0.8 (C %/12) - 0.05≤Nb≤93x0.8 (C %/12) + 0.1.
2. Steel according to claim 1, characterised in that the active elements are chosen from cerium, lanthanum, neodymium, praseodymium, yttrium, taken on their own or in combination, and contained in a compound called Amischmetal®.
3. Steel according to claim 1, characterised in that the sum of the contents of zirconium and niobium is lower than 0.300%.
4. Steel according to claim 1, characterised in that the sum of the contents of carbon and nitrogen is lower than 0.04%,
5. Steel according to claim 1, characterised in that the contents of silicon and manganese satisfy the relationship Si/Mn≥1.
6. Steel according to claim 1, characterized in that for the stabilising element zirconium used on its own in the composition, the minimum content of aluminium satisfies the following condition: 4 % + 6 Zr % - 91 (C %/12 + N %/14).
7. Steel according to claim 1, characterized in that for the stabilising element niobium used on its own in the composition, the minimum content of aluminium satisfies the following condition: 54 % + 5 Nb % - 93 (C %/12 + N %/14).
8. Steel according to claim 1, characterised in that for the stabilising elements zirconium and niobium combined, the minimum content of aluminium satisfies the following condition: 4 % + 5 (Zr + Nb) - 92 (C %/12 + N %/14).
9. Steel according to claims 1 and 6, characterised in that the content of active elements satisfies the following relationship: 0.03 - 0.2(Zr% - 91 N%/14)≤(Ce + La + Nd + Pr + Y) ≤0.08 - 0.2(Zr% - 91 N%/14)
10. Steel according to claims 1 and 7, characterised in that the content of active elements satisfies the following relationship: 0.03 - 0.025(Nb%)≤(Ce + La + Nd + Pr + Y)≤0.08 - 0.025(Nb%)
11. Steel according to claims 1 and 8, characterised in that the content of active elements satisfies the following relationship: 0.03 - 0.2(Zr% - 91 N%/14) - 0.025(Nb%)≤(Ce + La + Nd + Pr + Y)≤0.08 - 0.2(Zr% - 91 N%/14) - 0.025(Nb%).
12. Steel according to claims 1 to 11, characterised in that it additionally includes in its composition less than 0.5% copper.

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