EP0735153A1 - 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, in particular for catalyst support structures, such as, for example, structures contained in the exhausts of motor vehicles.

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

The steels used must be capable of being produced in the context of industrial production, for example, in continuous casting followed by transformations to obtain steel strips of large width and thin for the production of strips.

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

It is also known from European patent EP 0429 793 which describes FeCrAl alloys containing rare earths, active elements, such as cerium, lanthanum, praseodymium and stabilizers, titanium or niobium. The addition of active elements at high contents is proposed. A minimum phosphorus content is recommended in order to improve the hot fragility of the alloys with regard to the high contents of active elements. The minimum phosphorus contents thus specified are lower than those usually encountered during the production of stainless steels. The addition of stabilizer such as titanium is carried out to improve the hot brittleness of the alloys. The 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. The addition of active elements is carried out to avoid flaking of the oxide layer.

The addition of zirconium as a stabilizer under the condition Zr ≤ 91 (% C / 12 +% N / 14 + 0.03) is carried out to trap 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 carried out to improve the creep resistance.

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

In addition, the range of zirconium contents is wide and does not make it possible to satisfy the conditions of dimensional stability of the catalyst supports. Similarly, the Niobium contents do not allow optimum resistance to oxidation.

It is also known patent application EP 0 480 461 concerning 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 supports. This behavior is defined as a function of the nitrogen contents, which is not justified due to the presence of aluminum and / or zirconium, because aluminum nitride and zirconium compounds are preferably formed with niobium nitride.

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

   pour le niobium, $$\text{93×0,8. (C \%/12 ) - 0,1 ≤ Nb ≤ 93×0,8 (C \%/12) +}\mathit{\text{0,15}}$$

et Nb < 0,3 %.
   pour le zirconium et le niobium, $$\text{91 (N \%/14) - 0,05 ≤ Zr ≤ 91 (N \%/14) + 0,05,}$$

et $$\text{93×0,8 (C \%/12) - 0,05 ≤ Nb ≤ 93×0,8 (C \%/12) +}\mathit{\text{0,10.}}$$

the zirconium and / or niobium contents satisfying the following conditions:
for zirconium , $$\text{91 (C\% / 12 + N\% / 14) - 0.1 ≤ Zr ≤ 91 (C\% / 12 + N\% / 14) + 0.1}$$

for niobium , $$\text{93 × 0.8. (C\% / 12) - 0.1 ≤ Nb ≤ 93 × 0.8 (C\% / 12) +}\mathit{\text{0.15}}$$

and Nb <0.3%.
for zirconium and niobium , $$\text{91 (N\% / 14) - 0.05 ≤ Zr ≤ 91 (N\% / 14) + 0.05,}$$

and $$\text{93 × 0.8 (C\% / 12) - 0.05 ≤ Nb ≤ 93 × 0.8 (C\% / 12) +}\mathit{\text{0.10.}}$$

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%.

The silicon and manganese contents satisfy the Si / min relationship ≥1.

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

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

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

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

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

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

The following description and the accompanying drawings, all given by way of non-limiting example, will make the invention clear.

FIG. 1 groups resilience characteristics by measuring the transition temperature for steels having different contents of selected stabilizers.

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

FIG. 3 presents a series of elongation curves as a function of the content of active elements.

FIG. 4 presents a series of elongation characteristics for different contents of zirconium and niobium 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 similar 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 specific 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 in 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: $$\text{91 (C\% / 12 + N\% / 14) - 0.1 ≤ Zr ≤ 91 (C\% / 12 + N\% / 14) + 0.1}$$

et Nb < 0,3 %. For a steel, according to the invention, stabilized with niobium: $$\text{93 × 0.8. (C\% / 12) - 0.1 ≤ Nb ≤ 93x0.8 (C\% / 12) +}\mathit{\text{0.15}}$$

and Nb <0.3%.

et $$\text{93×0,8 (C \%/12) - 0,05 ≤ Nb ≤ 93×0,8 (C \%/12) +}\mathit{\text{0,10.}}$$

For a steel, according to the invention, stabilized with zirconium and niobium: $$\text{91 (N\% / 14) - 0.05 ≤ Zr ≤ 91 (N\% / 14) + 0.05,}$$

and $$\text{93 × 0.8 (C\% / 12) - 0.05 ≤ Nb ≤ 93 × 0.8 (C\% / 12) +}\mathit{\text{0.10.}}$$

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

FIG. 1 groups the resilience characteristics measured by means of the transition temperatures of steels having different stabilizer contents chosen from zirconium and niobium.

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

It is represented on the abscissa:

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

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

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

From the point of view of the choice of stabilizers, steels containing in their composition 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 related to a surface 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 required, depending on stabilizers and carbon and nitrogen content.

For zirconium: $$\text{Al\% minimum = 4\% + 6 Zr\% - 91 (C\% / 12 + N\% / 14).}$$

For niobium: $$\text{Al\% minimum = 4\% + 5 Nb\% - 93 × 0.8 (C\% / 12).}$$

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

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

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

The formation of the oxide layer during the oxidation treatment generates stresses. These constraints are not negligible and can deform the catalyst support structure. The catalyst support structure follows variations in elongation as a function of time, at a given temperature. These variations are manifested by a strong elongation for a relatively short period of time, then by a stability of the elongation over time corresponding to a plateau and finally, by strong elongations for a relatively long period of time. The strong elongations occurring over 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 level depending on the content of active elements. The elongation at the level depends, in this example, on the content of active elements Ce, La, Pr, Nd used in the composition of the "mischmetal" but also, and surprisingly, on the stabilizing element used. For example, the "mischmetal" content depends on the zirconium content, since this is an active element from the point of view of oxidation. Thus, the best deformation behavior by elongation is obtained for "mischmetal" contents of between 0.02 and 0.04% for stabilization with zirconium and between 0.04 and 0.075% for steel stabilized with niobium. The addition of these elements, by trapping the sulfur, improves the oxidation resistance of the steels. These additions must be controlled in order to optimize the properties of the steel. The simultaneous addition of Zr and Nb gives a possibility of increasing the range of the 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 stretching behavior, the stabilizer contents, and by Consequently, the carbon and nitrogen contents must be limited to very low contents: $$\text{(C + N) ≤ 0.04\% Zr and / or Nb ≤ 0.300\%.}$$

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

Carbon and nitrogen are inevitable elements used in the composition of steels. These elements cause very large reductions in hot ductility and cause problems with steel transformation.

The presence of zirconium and / or niobium stabilizers, by trapping carbon and / or nitrogen, improves the hot ductility of the alloy. However, high levels of carbon and nitrogen lead in return to very high stabilizer contents. The significant precipitation of carbides and nitrides in the alloy reduces the 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%, the nitrogen content must be less than 0.02% and the carbon and nitrogen content must preferably be lower. at 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, in order to improve the characteristics of the steel in elongation.

Zirconium and / or nobium are voluntary addition elements intended to trap carbon and / or nitrogen and thus improve the hot ductility of the grade. These elements, called stabilizers, must be checked taking into account the production process envisaged in continuous casting. In fact, insufficient stabilization would lead to excessive embrittlement of the slabs, incompatible with industrial production. Significant stabilization would lead to degradation of the oxidation resistance of the steel in the form of strip.

The combined addition of zirconium and niobium stabilizing elements allows both good resistance to oxidation and good cohesion of the 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 the solder tracks, for example based on nickel and a possible contamination due to the filler 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 inevitable impurities used in the manufacture of stainless steels.

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

The chromium content of the steel must be sufficient, that is to say greater than 12%, to present the good properties with respect to corrosion and to favor the formation and the behavior of the oxide layer. at high temperature. The chromium content should also not be too high, that is to say less than 25%, in order to avoid the problems of steel transformation.

According to the invention, preferably, the chromium content is between 14 and 22%, which corresponds to a chromium concentration range optimized with respect to corrosion and the transformation of steel.

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

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

Claims (12)

Ferritic stainless steel resistant to oxidation at high temperature, usable, in particular for catalyst support, 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,
Zirconium and / or niobium contents satisfying the following conditions:
for zirconium , $$\text{91 (C\% / 12 + N\% / 14) - 0.1 ≤ Zr ≤ 91 (C\% / 12 + N\% / 14) + 0.1}$$

for niobium , $$\text{93 × 0.8. (C\% / 12) - 0.1 ≤ Nb ≤ 93 × 0.8 (C\% / 12) +}\mathit{\text{0.15}}$$

and Nb <0.3%.
for zirconium and niobium , $$\text{91 (N\% / 14) - 0.05 ≤ Zr ≤ 91 (N\% / 14) + 0.05,}$$

and $$\text{93 × 0.8 (C\% / 12) -0.05 ≤ Nb ≤ 93 × 0.8 (C\% / 12) +}\mathit{\text{0.1.}}$$

Steel according to claim 1, characterized in that the active elements are chosen from cerium, lanthanum, neodymium, praseodymium, taken alone or in combination, and contained in a compound called "mischmetal". Steel according to claim 1, characterized in that the sum of the zirconium and niobium contents is less than 0.300%. Steel according to claim 1, characterized in that the sum of the carbon and nitrogen contents is less than 0.04%. Steel according to claim 1, characterized in that the contents of silicon and manganese satisfy the Si / Mn relationship ≥ 1. Steel according to claim 1, characterized in that for the stabilizing element zirconium used alone in the composition, the content aluminum minimum meets the following condition: $$\text{4\% + 6 Zr\% - 91 (C\% / 12 + N\% / 14).}$$

Steel according to claim 1, characterized in that, for the niobium stabilizing element used alone in the composition, the minimum aluminum content satisfies the following condition: $$\text{4\% + 5 Nb\% - 93 (C\% / 12 + N\% / 14).}$$

Steel according to claim 1, characterized in that, for the combined zirconium and niobium stabilizing elements, the minimum aluminum content satisfies the following condition: $$\text{4\% + 5 (Zr + Nb) - 92 (C\% / 12 + N\% / 14).}$$

Steel according to claims 1 and 6, characterized in that the content of active elements satisfies the following relationship: $$\text{0.03 - 0.2 (Zr\% - 91 N\% / 14) ≤ (Ce + La + Nd + Pr + Y) ≤ 0.08 - 0.2 (Zr\% - 91 N\% / 14)}$$

Steel according to claims 1 and 7, characterized in that the content of active elements satisfies the following relationship: $$\text{0.03 - 0.025 (Nb\%) ≤ (Ce + La + Nd + Pr + Y) ≤ 0.08 - 0.025 (Nb\%)}$$

Steel according to claims 1 and 8, characterized in that the content of active elements satisfies the following relationship: $$\text{0.03 - 0.2 (Zr\% - 91 N\% / 14) - 0.025 (Nb\%) ≤ (Ce + La + Nd + Pr + Y) ≤ 0.08 - 0.2 (Zr\% - 91 N\% / 14) - 0.025 (Nb\%).}$$

Steel, according to claims 1 to 11, characterized in that it further comprises in its composition less than 0.5% copper.

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