EP2027300B1 - Steel compositions for special uses - Google Patents

Abstract

The invention concerns steels having excellent resistance over time, in a corrosive atmosphere due to oxidizing environments such as, for example, fumes or water vapor, under high pressure and/or temperature. The invention concerns a steel composition for special applications, said composition containing, by weight, about 1.8 to 11% of chromium (and preferably between about 2.3 and 10% of chromium), less than 1% of silicon, and between 0.20 and 0.45% of manganese. It has been found that it is possible to adjust the contents of the composition based on a predetermined model, selected to obtain substantially optimal properties with respect to corrosion in specific conditions of high temperature performances. Said model can involve as additive of as residue at least one element selected among molybdenum, tungsten, cobalt, and nickel.

Description

The invention relates to a new steel composition for special purposes, in particular high performance in the presence of corrosion by oxidizing media such as, for example, fumes or water vapor, under high pressure and / or temperature.

High atmospheres of pressure and temperature in the presence of water vapor exist in particular in the industrial production of electricity. The generation, conditioning (including overheating and reheating) and transport of water vapor are done using steel elements, especially seamless tubes. Despite a long history of solutions envisaged or implemented, on which we will return, serious problems remain in terms of holding in the environment concerned, as well as in time.

These problems are particularly difficult to solve, in particular because of the significant variability of the properties of the steels according to their constituents, and the heaviness of the hot corrosion tests over a long period.

In the rest of this document, the term "corrosion" or "hot corrosion" will be used to denote the phenomena of loss of metal by hot oxidation.

The present invention improves the situation.

The invention generally provides a steel composition for special applications, which is in the zone comprising, in terms of weight content, about 1.8 to 11% of chromium (and preferably between about 2.3 and 10% of chromium ), less than 1% of silicon, and between 0.20 and 0.45% of manganese. It has been found possible to adjust the contents of the composition according to a predetermined pattern chosen to obtain substantially optimal corrosion characteristics under given high performance conditions. temperature. This model can involve as addition or as residual at least one element selected from molybdenum, tungsten, cobalt, and nickel.

More particularly, the composition has a silicon content by weight of between about 0.20 and 0.50%, preferably between about 0.30 and 0.50%. It may also comprise a manganese content by weight of between about 0.25 and 0.45%, and more preferably between about 0.25 and 0.40%.

According to the invention, said model comprises at least one contribution term of chromium, and a contribution term of manganese alone. The manganese contribution term alone may include a second degree polynomial function of the manganese content. The term chromium contribution may include a quadratic term in inverse of the chromium content, and a term in inverse of a quantity containing the chromium content.

• la composition d'acier comporte entre 2,3 et 2,6 % en poids de Chrome, environ.
• la composition d'acier comporte entre 8,9 et 9,5 % à 10 %, en poids de Chrome, environ.

According to embodiments, which will be described in more detail:

• the steel composition comprises between 2.3 and 2.6% by weight of chromium, approximately.
• the steel composition comprises from about 8.9 to about 9.5% to about 10% by weight of chromium.

The invention also covers a seamless tube or its accessory, consisting essentially of a proposed steel composition, the application of the steel composition to seamless tubes and accessories, intended to generate, convey or condition water vapor under high pressure and temperature, as well as the technique described for optimizing the properties of special steel compositions, in particular for their application to seamless tubes and accessories, intended to generate, convey or condition water vapor under high pressure and temperature.

• La figure 1 illustre schématiquement le déroulement dans le temps d'un premier mécanisme d'oxydation, dit ici de <type 1> ;
• La figure 2 illustre schématiquement le déroulement dans le temps d'un second mécanisme d'oxydation, dit ici de <type 2> ;
• La figure 3 est un graphe illustratif de propriétés de compositions d'acier ;
• La figure 4 est un tableau de compositions d'acier, ayant fait l'objet de mesures de corrosion à long terme à 650°C, lesquelles figurent dans la dernière colonne du tableau ;
• La figure 5 est un graphe représentant une correspondance entre des données mesurées et des données calculées ; et
• La figure 6 est un graphe formant détail partiel de la figure 5.

Other characteristics and advantages of the invention will appear better on reading the detailed description below, made with reference to the appended drawings, in which:

• The figure 1 schematically illustrates the course in time of a first oxidation mechanism, here called <type 1>;
• The figure 2 schematically illustrates the course in time of a second oxidation mechanism, here called <type 2>;
• The figure 3 is an illustrative graph of properties of steel compositions;
• The figure 4 is a table of steel compositions that have been subjected to long-term corrosion measurements at 650 ° C, which are shown in the last column of the table;
• The figure 5 is a graph representing a correspondence between measured data and calculated data; and
• The figure 6 is a graph forming partial detail of the figure 5 .

The drawings, the description below and its annexes contain, for the most part, elements of a certain character. They can therefore not only serve to better understand the present invention, but also contribute to its definition, if any.

The conditions under which the invention can be applied are now examined.

Consider for example the case of a fossil fuel power plant, which includes a power boiler delivering superheated steam to a steam turbine coupled to an alternator. We know the good thermal efficiency of this kind of thermal power plants, which we also try to make less and less polluting, by limiting the emissions of both fumes and harmful gases such as SO ₂ , NO _x and CO ₂ , the latter being more particularly responsible for the greenhouse effect. But the reduction in the relative amount of CO ₂ produced during the combustion passes through the increase of the efficiency of the boiler, which is related to the temperature and the pressure of the steam delivered to the turbine.

Since water vapor is essentially confined in seamless steel tubes, attempts have been made for many years to improve the long-term strength characteristics of the tubes at the high temperature internal pressure of the fluid. by improving their resistance to creep and in particular their resistance to creep rupture in 100,000 hours.

• la spécification A213, intitulée "Standard Specification for Seamless Ferritic and Austenitic Alloy-Steel Boiler, Superheater and Heat-Exchanger Tubes", et
• la spécification A335 : "Standard Specification for Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service".

The so-called American Society for Testing and Materials ("ASTM") has established standards or specifications from which the skilled person draws for the selection of their steels. As special steels for high temperature use, these are:

• Specification A213, entitled "Standard Specification for Seamless Ferritic and Austenitic Alloy-Steel Boiler, Superheater and Heat-Exchanger Tubes", and
• Specification A335: "Standard Specification for Seamless Ferritic Alloy-Steel Pipe for High-Temperature Service".

Boilers from the 1960s used unalloyed steels for boiler screen panels and grades of 2.25% Cr and 1% Mo (ASTM A313 ASTM A213 and P22 grades T22) for the hot parts of the superheater tubes and the superheated steam lines (160 bar - 560 ° C).

Austenitic stainless steels at 18% Cr and 10% Ni intrinsically have better creep characteristics than the lower alloyed grades with a ferritic structure, but have serious drawbacks because the same boiler must then comprise parts made of carbon steel. austenitic structure and others ferritic structure: it follows on the one hand differences in coefficients of thermal expansion, and on the other hand the need to make welded joints between tubes of different metallurgical structure.

The trend has therefore been to improve materials with a ferritic structure.

The steel X 20 Cr Mo V 12 - 1 to 12% Cr according to the German standard DIN 17.175 is not very fashionable because its implementation is very delicate and its creep characteristics are exceeded.

The 1980s saw the emergence in the standards of microalloyed 9% Cr grades (T91 and P91, T92 and P92 according to ASTM A213 and A335) with both good creep resistance and excellent processing properties. .

In parallel, appeared in the 1990s, grades to 2.25% Cr microallied (T23, P23, T24, P24) to improve the performance of the display panels and / or parts of the superheaters.

Then there were problems of resistance to hot oxidation, especially in the case of 9% Cr steels in comparison with X 20 Cr Mo V 12 - 1 steel containing 12% Cr. It is known that Cr and also Si and Al are elements that reduce hot oxidation.

• l'oxydation par les fumées oxydantes, et
• l'oxydation par la vapeur d'eau.

The term <hot oxidation> includes two types of phenomena:

• oxidation by oxidizing fumes, and
• oxidation by water vapor.

Oxidation on the outer surface of the tubes

The oxidation phenomena by the oxidizing fumes occur outside the tubes and more particularly outside the superheater tubes taking into account the flow of fumes that see these tubes.

They result in a loss of thickness of metal and thus by an increase of the tangential stress σ in the tube that can be written by the attached relation [11], where D is the outside diameter, e is the thickness and P internal vapor pressure inside the tubes.

The kinetics of oxidation is even faster than the oxide layer (or calamine) is thin. One might think that it is self-limiting with the growth of the calamine layer. Unfortunately, when the calamine layer is thick, it loses adhesion and breaks off into leaves (exfoliation). It follows that the oxidation resumes at high speed where the metal is exposed.

A metal having slow oxidation kinetics and capable of forming fine and adherent calamines is therefore highly desirable.

Oxidation on the inner surface of the tubes

The same is true for other reasons for the phenomena of oxidation by water vapor which are manifested inside the tubes and which have been more recently studied. Indeed, the scale formed inside the superheater tubes is a thermal insulator between the fumes (heat source) and the water vapor to overheat. And a thick calamine on the steam side (inside the tube) results in a higher temperature of the metal than when the calamine is thin. However, the negative influence of temperature on creep resistance is exponential.

A characteristic of identical creep resistance, a steel tube resistant to oxidation by steam can thus superheat steam at a higher temperature than a steel tube less resistant to oxidation by steam.

• dans le cas des tubes dé surchauffeurs une accumulation de la calamine exfoliée dans les épingles des serpentins de surchauffeurs, entravant la circulation de vapeur et pouvant causer des éclatements de tubes surchauffeurs par surchauffe catastrophique,
• un entraînement de la calamine exfoliée, issue tant des tubes surchauffeurs que des collecteurs de vapeur ou des conduites de vapeur, dans les aubes de la turbine avec un risque d'érosion et/ou abrasion et de destruction de celles-ci.

In addition, in case of calamine thick and / or little adherent, exfoliation thereof may result in:

• in the case of superheater tubes, an accumulation of exfoliated calamine in the pins of the superheater coils, hindering the flow of steam and causing superheater tubes to burst due to catastrophic overheating,
• entrainment of the exfoliated calamine, resulting from both superheater tubes and steam collectors or steam lines, in the blades of the turbine with a risk of erosion and / or abrasion and destruction thereof.

State of the art

For the time being, the boiler calculation codes do not take into account the characteristics of resistance to hot oxidation (empirical rules are used which define too pessimistically an extra thickness for hot oxidation by both smoked only by water vapor).

Applicant's approach

In WO 02/081766 , the Applicant has proposed a seamless tube steel composition having very good properties in terms of both creep rupture strength and resistance to hot oxidation.

This composition is commercially designated VM12. It has surprised the inventors with regard to the resistance to hot oxidation by steam at 600 ° C. and 650 ° C., which is much greater than that of the 9% Cr steels, equal to or even greater than that of the X 20 Cr Mo V12-1 steel also containing 12% Cr and almost as good as that of TP 347 FG austenitic grade containing 18% Cr.

Experimental results obtained at the Ecole des Mines de Douai were presented at the conference "High Temperature Corrosion and Protection of Materials 6, the Embiez 2004, and were published in Materials Science Forum, Vol 461-464 (2004) p. 1039-1046, under the title "Steam Corrosion Resistance of New 12% Ferrite Boiler Steels ".

The authors (V. Lepingle et al.) Have observed that it is difficult to predict in a quantitative way the kinetics of hot oxidation, the elements of the chemical composition of steel being able to have a non-linear influence, or even to work with synergy.

In particular, they showed the existence of two distinct types of growth mechanisms involved in hot oxidation, illustrated on the Figures 1 and 2 .

The figure 1 illustrates the mechanism conventionally governing the hot oxidation of 9-12% Cr steels. As can be seen, the oxide germs homogeneously over the entire surface.

The mechanism of figure 2 relates to the VM 12 grade, to certain X20 Cr Mo V 12-1 steel compositions and to TP 347 FG fine-grained austenitic grade: here, the oxide is produced in the form of isolated seeds which must develop on the surface before forming a layer and developing in depth. This mechanism leads to slow oxidation kinetics and adherent calamines.

Other studies have also focused on predicting the kinetics of hot oxidation by water vapor.

dans laquelle Δm est l'accroissement de masse par oxydation et t le temps, tandis que z est généralement pris égal à 1/2. La constante Kp présentant une décroissance brutale au-delà d'une certaine teneur en chrome.A paper by Zurek et al. was also presented at the conference Embiez and published in " Materials Sciences Forum ", Vol 461-464 (2004) pp 791-798 . It qualitatively shows the influence of various chemical elements on the variation of the constant Kp of the empirical oxidation law $$\mathrm{Dm}\mathrm{=}\mathrm{<figure-callout\; id="2"\; label="Kp\; t"\; filenames="imgf0004.png"\; state="\{\{state\}\}">Kp\; </figure-callout>}{\mathrm{t}}^{}{\mathrm{}}^{\mathrm{2}}$$

where Δm is the oxidation mass increase and t is the time, while z is usually 1/2. The constant Kp exhibiting a sudden decrease beyond a certain chromium content.

• L'ajout de manganèse déplace vers la droite la zone de forte décroissance de Kp en fonction de la teneur en chrome ; Selon ce travail, addition de Mn tend à contrarier l'effet bénéfique du Cr ;
• L'ajout de silicium ou de cobalt déplace au contraire vers la gauche la zone de forte décroissance de Kp en fonction de la teneur en chrome. Selon ce travail, Si et Co ont une influence bénéfique qui étend le domaine d'action du Cr.

The main conclusions that can be drawn from Zurek et al. are the following (see figure 3 ):

• The addition of manganese shifts to the right the zone of strong decay of Kp as a function of the chromium content; According to this work, addition of Mn tends to thwart the beneficial effect of Cr;
• The addition of silicon or cobalt displaces on the contrary to the left the zone of strong decay of Kp as a function of the chromium content. According to this work, Si and Co have a beneficial influence that extends the field of action of Cr.

It is understood that it is difficult to draw precise indications on the properties of any particular alloy.

Osgerby et al. ( S. Osgerby, A. Fry "Assessment of the oxidation behavior of high temperature plant materials" Proceedings from the 4th International EPRI Conference, October 25-28, 2004 - Hilton Head Island, South Carolina - pp 388-401 ) have also studied the oxidation of various steels and alloys of Ni by water vapor. They performed on the results a treatment using neural networks. They resulted in equations which in the case of ferritic steels at 9-12% Cr show quantitatively a positive influence of Cr, Si, Mn and Mo and a negative influence of W.

Overall, the conclusions of this work are diverse, and even opposite as regards the case of manganese in ferritic steels.

The Applicant has sought to do better, and in particular to obtain quantitative elements to improve existing steels, including those with 9% Cr whose resistance to oxidation is considered until now insufficient and those to 2.25 % Cr.

Experiments of the Applicant

The Ecole des Mines de Douai first developed, on the occasion of a study contract with the Applicant, a formula for predicting the loss of metal thickness (determined after etching of the oxide formed without metal attack) over one year from a modeling of the influence of all elements of the chemical composition.

This so-called LPL (Lower Protective Layer of Scale) is not public and the terms are unknown to the Applicant.

The Applicant has simply been able to note significant differences between the experimental results and the results obtained by application of the LPL formula, which were communicated to him.

The Applicant has therefore resumed measurements of the hot oxidation kinetics by water vapor at 650 ° C presented at the conference Les Embiez 2004 (see above) on 16 samples of steels with a ferritic structure (ferrite + perlite, bainite revenue, martensite revenue) whose Cr content ranges from 2.25% (T22 - T23) to 13%. The figure 4 is a table of composition of the steels tested with, in last column, the values of the corrosion measurements corresponding to the loss of thickness of metal over a year (corrosion rate Vcor) for these steels.

The term "ND" in the table of the figure 4 means "not available".

The Applicant has performed on these experimental results a multidimensional statistical analysis. It is based on a plurality of terms reflecting a reasoned empirical approach of certain mechanisms or influences, which determine the Vcor corrosion rate.

After several tests, the Applicant has obtained the formula [21] attached, which expresses the corrosion rate Vcor at 650 ° C, in the long term, that is to say over a period of about a year .

The formula [21] gives the average loss of metal thickness (in mm) over one year of exposure to water vapor at 650 ° C. This average loss of thickness is it even deduced from a weight loss of the metal after selective etching of the oxide, under standard conditions. Formula [21] has various terms as follows: Term Influence represented 1 / Cr ² mainly represents the influence of the chromium content, here a inverse dependence on the square of the chromium content 1 / A mainly represents the influence of molybdenum, tungsten, nickel and cobalt contents, given an interaction with the chromium content B mainly represents the influence of the silicon content, also in view of an interaction with the chromium content VS mainly represents the influence of the manganese content, taking into account interactions with the tungsten and nickel contents

The contents of formula [21] are expressed as% by weight (or by weight).

The coefficients α (alpha), β (beta) and δ (delta) and those involved in the expressions B and C have substantially the values indicated in appendix 1, section 3, expressions [31] to [36].

• une fonction de la teneur en chrome qui comprend un terme en 1/Cr² avec un terme d'allure en 1/Cr (terme 1/A), et un terme correctif en Cr (terme B),
• une fonction polynomiale (ici du second degré) de la teneur en Manganèse (terme C),
• une contribution conjointe (notée q) de W + Ni (tungstène + nickel) qui est d'une part en 1/-q dans le terme A, et d'autre part en q dans le terme C.
• les autres teneurs n'interviennent qu'une seule fois, d'une manière qui se lit directement sur la formule.

Besides this, if we examine the formula [21] globally, it appears that it includes in particular:

• a function of the chromium content which comprises a term in 1 / Cr ² with a term of appearance in 1 / Cr (term 1 / A), and a corrective term in Cr (term B),
• a polynomial function (here of the second degree) of the Manganese content (term C),
• a joint contribution (denoted q) of W + Ni (tungsten + nickel) which is on the one hand in 1 / -q in the term A, and on the other hand in q in the term C.
• the other grades only occur once, in a way that reads directly on the formula.

• en figure 5 (partie de droite), que la correspondance est excellente pour des teneurs en Chrome voisines de 2,25 %,
• en figure 5 (partie de gauche), ainsi qu'en figure 6, qui est un détail de la partie de gauche de la figure 5, que la correspondance est également excellente pour des teneurs en Chrome voisines de 9 % et 12 %.

The figures 5 and 6 illustrate how this new Vcor-ordinate formula (Vcor predicted) compares with the experimental results known to the Applicant in the abscissa (Vcor measured). It follows:

• in figure 5 (right part), that the correspondence is excellent for chromium contents close to 2.25%,
• in figure 5 (left part), as well as figure 6 , which is a detail of the left part of the figure 5 , that the correspondence is also excellent for chromium contents close to 9% and 12%.

In short, modeling and experience yield remarkably consistent results. Obviously, the invention is not limited to the expression of the formula [21], which is known to write equivalents of different appearance. One can also write simplified equivalents, of more local use (in terms of ranges of contents), taking into account the properties of variation of each of the terms, or their elements. Finally, if the formula [21] was set at 650 ° C, it is naturally valid for other temperatures, lower or higher. For example, a steel grade having a rather high corrosion rate at 650 ° C may be acceptable at lower temperatures, if it has interesting properties from any point of view, including a lower manufacturing cost.

More finely, the Applicant has found a strong detrimental influence of the Mn content above about 0.25%, according to the indications of the formula [21] (grade range studied: 0.2 - 0.53% ). It also found that the Si content plays little when Si is greater than or equal to 0.20% (grade range studied: 0.09-0.47%). It also noted the absence of significant influence of the carbon content within the limits studied (0.1-0.2%).

The Applicant was then interested in searching among the ferritic performance grades of the specifications ASTM, A213 and A335 for use in boilers (T91, P91, T92, P92, T23, P23, T24, P24) particular areas of chemical composition that lead with thin and very adherent calamines to make the tubes work better at steam temperatures of the order of 600 ° or 650 ° C and vapor pressures of the order of 300 bar.

In general, pipe manufacturers have so far ordered their steel at the lower chromium range, given the cost of this element and its alphagenic nature of this element. For example, for a theoretical range of 8.00 to 9.50% for ASTM A213 grade T91, tube manufacturers control a steel containing around 8.5% Cr, which minimizes the risk of ferrite delta on product.

As for the manganese, it is known that it makes it possible to fix the sulfur of the steel, and that this fixation avoids problems of forgeability (burning of the steel). Thus, while the range of ASTM A213 is 0.30-0.60% for T91 grade, it is usual to develop steels for high temperature use with manganese contents close to 0.50%. , so at the top of this range.

In general, the steel grades proposed here for seamless tubes for conveying water vapor under high pressure and temperature include (by weight) 1.8 to 13% chromium (Cr), less than 1% silicon (Si) and between 0.10 and 0.45% manganese (Mn). As an option, the steel comprises an addition of at least 1 element chosen from molybdenum (Mo), tungsten (W), cobalt (Co), vanadium (V), niobium (Nb), titanium ( Ti), boron (B) and nitrogen (N).

• premier groupe les aciers à 2,25 % Cr : grades T/P22, T/P23, T/P24
• second groupe les aciers à 9 %. Cr : grades T/P91, T/P92

In view of the experience acquired, the Applicant has focused on two groups of performance shades in creep because alloyed with Mo or W and microalliés (Nb, V, N and possibly B and Ti) and oxidation improvable hot. Those are :

• first group steels at 2.25% Cr: grades T / P22, T / P23, T / P24
• second group the 9% steels. Cr: T / P91, T / P92 grades

This resulted in the identification of special grades of steels particularly advantageous in terms of corrosion rate, as will be seen now.

Embodiment E10: T22 and P22 steels

• 0,30 à 0,60 % Mn
• au plus 0,50 % Si
• 1,90 à 2,60 % Cr
• 0,87 à 1,13 % Mo
• 0,05 à 0,15 % C
• au plus 0,025 % S
• au plus 0,025 % P

ASTM standards A213 and A335 define grades T22 and P22 respectively as containing:

• 0.30 to 0.60% Mn
• at most 0.50% Si
• 1.90 to 2.60% Cr
• 0.87 to 1.13% Mo
• 0.05 to 0.15% C
• not more than 0.025% S
• not more than 0.025% P

Since they are old grades, they do not contain microadditions of Ti, Nb, V and B.

In Table T10 below, columns 2 to 7 specify the compositions for a domain reference steel, and for three other proposed steels (designated in column 1). In the measured Vcor column, "ND" means unavailable. It will be understood that the tests required to determine a reliable and accurate corrosion rate at high temperature over one year are particularly long, delicate and expensive.

For the reference steel (R10), we see that the measured value and the value predicted by the formula [21] correspond almost exactly. Since the formula [21] is thus verified, it gives indications on other steel grades of this embodiment E10. These other grades are represented by three examples, denoted E10-max, E10-med, and E10-min, according to the corrosion rate obtained. <b> Table T10 </ b> mn Yes Cr MB W Or Co Vcor measured Calculated Vcor Reference (R10) 0.46 0, 23 2.06 1 0.014 0.15 - 1,035 1 , 04 E10-max 0.45 0.20 2.30 1.0 - 0.2 - ND 0.86 E10-min 0.30 0.45 2.60 0.9 - 0.1 - ND 0.61 E10-med1 0.40 0.20 2.30 1.0 - 0.2 - ND 0.83 E10-MED2 0.35 0.30 2.45 0.95 - 0.15 - ND 0.70

The selection of grades E10 allows a gain of between 18% (for E10-ax) and 42% (for E10-min), relative to the corrosion rate of the composition "reference" R10.

In this mode E10, the steel has between 2.3 and 2.6% Cr.

Preferably, the steel of mode E10 comprises an Si content of between 0.20 and 0.50% and very preferably between 0.30 and 0.50%. Preferably, the steel comprises an Mn content of between 0.30 and 0.45%.

The steel according to this mode E10 preferably comprises between 0.87 and 1% Mo. It does not include a voluntary addition of W, tungsten being a residual steel and its content of the order of 0.01% .

Very preferably, the steel according to the mode E10 has contents of Cr, Mn, Si, Mo, W, Ni, Co whose Vcor value calculated according to the equation [21] is at most equal to about 0.9 mm / year, preferably 0.85 mm / year. Better results are obtained for Vcor at most equal to about 0.7 mm / year.

Embodiment E11: T23 and P23 steels

• 0,10 à 0,60 % Mn
• au plus 0,50 % Si
• 1,90 à 2,60 % Cr
• 0,05 à 0,30 % Mo
• 1,45 à 1,75 % W
• 0,04 à 0,10 % C
• au plus 0,030 % P
• au plus 0,010 % S
• 0,20 à 0,30 % V
• 0,02 à 0,08 % Nb
• 0,0005 à 0,006 % B
• au plus 0,030 % de N
• au plus 0,030 % d'Al

ASTM standards A213 and A335 define grades T23 and P23 respectively as containing:

• 0.10 to 0.60% Mn
• at most 0.50% Si
• 1.90 to 2.60% Cr
• 0.05 to 0.30% Mo
• 1.45 to 1.75% W
• 0.04 to 0.10% C
• not more than 0.030% P
• not more than 0.010% S
• 0.20 to 0.30% V
• 0.02 to 0.08% Nb
• 0.0005 to 0.006% B
• not more than 0.030% of N
• not more than 0.030% of Al

The replacement of a large proportion of molybdenum by tungsten and microadditions gives these grades very improved creep characteristics compared to T / P22 grades. Such an improvement does not, on the other hand, make it possible to increase the upper temperature resistance limit with respect to hot oxidation.

In Table T11 below, columns 2 to 7 specify the compositions for a reference steel of the domain, and for three other proposed steels (designated in column 1). For the reference steel, we see that the measured value and the value predicted by the formula [21] correspond exactly. The formula [21] being thus verified, it gives indications on the other three steel grades of this embodiment E11, denoted E11-max, E11-med, and E11-min, according to the corrosion rate obtained. Table T11 mn Yes Cr MB W Or Co Vcor measured Calculated Vcor Reference (R11) 0.48 0.24 2, 07 0.10 1.54 0.05 - 1.43 1.43 E11-max 0.45 0.20 2.30 0.20 1.60 0.10 - ND 1.26 E11-min 0.25 0.50 2.60 0.05 1.45 0.02 - ND 0.70 E11-med1 0.40 0.20 2.30 0.10 1.60 0.10 - ND 1.12 E11-MED2 0.30 0.30 2.45 0.10 1.50 0.05 - ND 0.84

The selection of grades E11 allows a gain of between 12% (for E11-max) and 51% (for E11-min), compared to the corrosion rate of the composition "reference".

In this mode E11, the steel comprises between 2.3 and 2.6% Cr.

Preferably, the mode E11 steel has an Si content between 0.20 and 0.50% and very preferably between 0.30 and 0.50%. Preferably, the steel comprises an Mn content of between 0.25 and 0.45%.

The steel according to this mode E11 preferably comprises between 1.45% and 1.60% W and between 0.05 and 0.20% Mo.

Very preferably, the steel according to the mode E11 has contents of Cr, Mn, Si, Mo, W, Ni, Co whose Vcor value calculated according to the equation [21] is less than about 1.4 mm / year, preferably at most equal to about 1.25 mm / year. Better results are obtained for Vcor at most equal to about 0.9 mm / year.

Embodiment E12: T24 / P24 steels

• 0,30 à 0,70 % Mn
• 0,15 à 0,45 % Si
• 2,20 à 2,60 % Cr
• 0,70 à 1,10 % Mo
• 0,04à0,10%C
• au plus 0,020 % P
• au plus 0,010 % S
• 0,20 à 0,30 % V
• 0,06 à 0,10 % Ti
• 0,0015 à 0,0020 % B
• au plus 0,012 % N
• au plus 0,020 % Al

These steels contain according to ASTM A213

• 0.30 to 0.70% Mn
• 0.15 to 0.45% Si
• 2.20 to 2.60% Cr
• 0.70 to 1.10% MB
• 0,04à0,10% C
• not more than 0.020% P
• not more than 0.010% S
• 0.20 to 0.30% V
• 0.06 to 0.10% Ti
• 0.0015 to 0.0020% B
• not more than 0.012% N
• not more than 0.020% Al

Table T12 below is constructed similarly to Tables T10 and T11. <b> Table T12 </ b> mn Yes Cr MB W Or Co Vcor measured Calculated Vcor Reference (R12) 0.50 0.25 2.30 0.85 - 0.05 - ND 0.83 E12 - max 0.45 0.25 2.40 0.90 - 0.10 - ND 0.76 E12 - min 0.30 0.45 2.60 0.70 - 0.02 - ND 0.58 E12 - med 0.40 0.30 2.50 0.80 - 0.05 - ND 0.67

The gain is more limited on the selection according to the invention: from 9% (E12-max) to 30% (E12-min). It is believed that this is mainly because the margin on the Cr content is lower than for the embodiments E10 or E11.

According to this mode E12, the steel comprises between 2.4 and 2.6% Cr. Preferably, the steel has an Si content of between 0.20 and 0.45% and very preferably between 0.30 and 0.45%. Preferably, the steel comprises an Mn content of between 0.30 and 0.45%.

The steel according to this mode E12 does not include any addition of W (residual tungsten content of the order of 0.01%); its Mo content is preferably between 0.70 and 0.9%.

Very preferably, the steel according to this mode E12 has contents of Cr, Mn, Si, Mo, W, Ni, Co whose Vcor value calculated according to the equation [21] is at most equal to about 0.8 mm / and preferably at most equal to about 0.75 mm / year. Better results are obtained for Vcor at most equal to about 0.7 mm / year.

It will be observed that the modes E10, E11 and E12 (generally noted E1) are quite similar in terms of chromium, manganese and silicon content. Thus, other contents Cr, Mn and / or Si of one of these modes E1 can be applied at least partially to another mode E1.

Execution mode E20: T9 and P9 steels

• 0,30 à 0,60 % Mn
• 0,25 à 1,00 % Si
• 8,00 à 10,00 % Cr
• 0,90 à 1,10% Mo
• au plus 0,15 % C
• au plus 0,025 % P
• au plus 0,025 % S

ASTM standards A213 and A335 define grades T9 and P9, respectively, as containing:

• 0.30 to 0.60% Mn
• 0.25 to 1.00% Si
• 8.00 to 10.00% Cr
• 0.90 to 1.10% MB
• not more than 0.15% C
• not more than 0.025% P
• not more than 0.025% S

With respect to the embodiments E21 and E22 discussed later in the text, the steels according to embodiment E20 do not contain microadditions of V, Nb, N or B.

In Table T20 below, columns 2 to 7 specify the compositions for a reference steel of the domain, and for three other proposed steels (designated in column 1). In the measured Vcor column, "ND" means unavailable. It will be understood that the tests required to determine a reliable corrosion rate and Precise at high temperatures over a year are particularly long, delicate and expensive.

Formula [21] has been derived from the indications for different grades of steel of this embodiment E20. These grades are represented by three examples, denoted E20-max, E20-med, and E20-min, according to the corrosion rate obtained. Table T20 mn Yes Cr MB W Or Co Vcor measured Calculated Vcor Reference (R20) 0.50 0.30 8.50 0.95 0, 01 0.15 - ND 0137 E20 - max 0.45 0.25 9.20 1.00 0.01 0.2 - ND 0.089 E20-min 0.30 0.45 10.00 0.90 0.01 0.02 - ND 0.012 E20-med1 0.35 0.40 9.60 0.95 0.01 0.15 - ND 0,034 E20 - med2 0.40 0.35 9.40 0.95 0.01 0.15 - ND 0,060

The selection of grades E20 allows a gain of between 16% (for E20-max) and 89% (for E20-min), compared to the corrosion rate of the "reference" composition R20.

In this mode E20, the steel has between 9.2 and 10.00% Cr.

Preferably, the steel of the mode E20 has an Si content of between 0.25 and 0.50% and very preferably between 0.30 and 0.40%. Preferably, the steel comprises an Mn content of between 0.30 and 0.45%.

The steel according to this mode E20 preferably comprises between 0.90 and 1.00% Mo. It does not include a voluntary addition of W, the tungsten being a residual of the steel and its content of the order of 0, 01%.

Very preferably, the steel according to the mode E20 has contents of Cr, Mn, Si, Mo, W, Ni, Co whose Vcor value calculated according to the equation [21] is at most equal to about 0.09 mm / year, preferably 0.06 mm / year. Better results are obtained for Vcor at most equal to about 0.04 mm / year.

Embodiment E21: T91 / P91 steels

• 0,30 à 0,60 % Mn
• 0,20 à 0,50 % Si
• 8,00 à 9,50 % Cr
• 0,85 à 1,05 % Mo
• au plus 0,40 % Ni
• 0,08 à 0,12 % C
• au plus 0,020 % P
• au plus 0,010 % S
• 0,18 à 0,25 % V
• 0,06 à 0,1 % Nb
• 0,030 à 0,070 % N
• au plus 0,040 % Al

These steels contain according to ASTM A213 and A335:

• 0.30 to 0.60% Mn
• 0.20 to 0.50% Si
• 8.00 to 9.50% Cr
• 0.85 to 1.05% MB
• not more than 0.40% Ni
• 0.08 to 0.12% C
• not more than 0.020% P
• not more than 0.010% S
• 0.18 to 0.25% V
• 0.06 to 0.1% Nb
• 0.030 to 0.070% N
• not more than 0.040% Al

Table T21 below is constructed similarly to Table T10. <b> Table T21 </ b> mn Yes Cr MB W Or Co Vcor measured Vcor calculated Reference (R21) 0.46 0.31 8.73 0.99 0.01 0.26 - 0.094 0.106 E21-max 0.45 0.3 8.90 0.95 - 0.20 - ND 0,095 E21- min 0.30 0.50 9.50 0.85 - 0.02 - ND 0,021 E21 - med 0.40 0.35 9.00 0.90 - 0.05 - ND 0.066

The gain on the selection of these embodiments E21 ranges from 10% (E21-max) to 80% (E21-min). It is remarkable that for E21-min, the value obtained is five times lower than the reference value.

According to this mode E21, the steel comprises between 8.9 and 9.5% Cr.

Preferably, the steel comprises an Si content of between 0.20 and 0.50% and very preferably between 0.30 and 0.50%.

Preferably, the steel comprises an Mn content of between 0.30 and 0.45%. It preferably comprises between 0.85% and 0.95% Mo.

Preferably, the steel according to embodiment E21 comprises at most 0.2% Ni (and very preferably at most 0.1%), and practically no tungsten (residual of the order of 0.01%).

Very preferably, the steel according to the mode E21 has contents of Cr, Mn, Si, Mo, W, Ni, Co whose Vcor value calculated according to the equation [21] is less than about 0.1 mm / year. Better results are obtained for Vcor at most equal to about 0.07 mm / year.

Embodiment E22: T92 / P92 steels

• au plus 0,30 à 0,60 % Mn
• au plus 0,50 % Si
• 8,50 à 9,50 % Cr
• 0,30 à 0,60 % Mo
• 1,50 à 2,00 % W
• au plus 0,40 % Ni
• 0,07 à 0,13 % C
• au plus 0,020 % P
• au plus 0,010 % S
• 0,15 à 0,25 % V
• 0,04 à 0,09 % Nb
• 0,001 à 0,006 % B
• 0,030 à 0,070 % N
• au plus 0,040 % Al

These steels contain according to ASTM A213 and A335:

• not more than 0.30 to 0.60% Mn
• at most 0.50% Si
• 8.50 to 9.50% Cr
• 0.30 to 0.60% Mo
• 1.50 to 2.00% W
• not more than 0.40% Ni
• 0.07 to 0.13% C
• not more than 0.020% P
• not more than 0.010% S
• 0.15 to 0.25% V
• 0.04 to 0.09% Nb
• 0.001 to 0.006% B
• 0.030 to 0.070% N
• not more than 0.040% Al

Table T22 below is constructed similarly to Table T10. <b> Table T22 </ b> mn Yes Cr MB W Or Co Vcor measured Vcor calculated Reference (R21) 0.41 0.22 8.51 0.44 1.69 0.13 - 0.113 0.113 E22 - max 0.40 0.25 8.90 0.45 1.70 0.20 - ND 0.11 E22 - min 0.30 0.50 9.50 0.30 1.50 0.02 - ND 0,055 E22 - med 0.35 0.30 9.20 0.40 1.70 0.1 - ND 0.082

Here, the gain on the selection of these embodiments E22 ranges from 2% (E22-max) to 52% (E22-min).

According to this embodiment E22, the steel comprises between 8.9 and 9.5% Cr.

Preferably, the steel of mode E22 has an Si content of between 0.20 and 0.50% and very preferably between 0.30 and 0.50%.

Preferably, the steel of mode E22 comprises an Mn content of between 0.30 and 0.45% and more preferably between 0.30 and 0.40%.

The steel according to the mode E22 preferably comprises between 0.30% and 0.45% Mo. It comprises between 1.50 and 1.75% W.

Preferably, the steel according to the mode E22 comprises at most 0.2% Ni and very preferably at most 0.1%.

Very preferably, the steel according to the mode E22 has contents of Cr, Mn, Si, Mo, W, Ni, Co which, according to the equation [21], give a value Vcor at most equal to about 0.11 mm / year. Better results are obtained for Vcor at most equal to about 0.08 mm / year.

It will be observed that the modes E21 and E22 (generally noted E2) are quite similar in terms of chromium, manganese and silicon content. Thus, the other contents of Cr, Mn and / or Si of one of these modes E2 can be applied at least partially to the other.

We will now consider an intermediate situation.

The model used leads to increasing the content of certain alphagenic elements such as Cr, Si and to reducing the content of certain gamma-containing elements such as Mn and Ni, which may favor the appearance of delta ferrite.

If the reduction in Mo and / or W content (alphagenic elements) is insufficient to compensate for the increase in Cr, Si content and the reduction of that in Mn and Ni from the point of view of the appearance of delta ferrite, it It will be necessary to adjust the content of gamma-like elements such as N and C that are not involved in the present model. In this respect, it will be possible to use known formulas for prediction of delta ferrite as a function of the contents of equivalent chromium and equivalent nickel.

The proposed technique for optimizing special steels includes the following elements. It starts from a known steel grade or grade with known properties other than hot corrosion, which we want to optimize from the point of view of hot corrosion. A long-term corrosion property is calculated according to a model such as that of formula [21] on a reference composition. In the vicinity of the known steel, a particular range of composition of the steel grade is sought, leading to a better value of the corrosion property according to the same model.

• éviter de fabriquer des aciers inhabituels seulement pour des tests de corrosion,
• éviter les délicats et coûteux tests de corrosion à long terme et haute température.

Since the model is highly reliable, this technique has many advantages, including:

• avoid making unusual steels only for corrosion tests,
• avoid the delicate and costly tests of long-term and high temperature corrosion.

This technique makes it possible especially to use targeted and not outrageously pessimistic data for the design of boilers or piping systems. steam and thereby minimize the extra thickness of corrosion taken into account in the design calculations.

It also makes it possible to increase the vapor temperature at a given metal temperature and to avoid calamine exfoliations by promoting the heterogeneous and discontinuous germination of the oxide on the surface of the steel on the vapor side.

The steel according to the invention can also be used without the list being exhaustive as sheet to manufacture welded tubes, fittings, reactors, boiler parts, as molded part for manufacturing turbine bodies or valve bodies as forging for making turbine shafts and rotors, fittings, as metal powder for making various components in powder metallurgy, as solder metal and other similar applications.

Annex 1 Section 1

$$\sigma =P\frac{\left(D-e\right)}{2{}_{}\mathit{e}}$$

Section 2

$${\mathrm{V}}_{\mathit{HORN}}^{650\mathit{°\; C}}=\alpha \frac{1}{\mathrm{<figure-callout\; id="2"\; label="VS\; \; r"\; filenames="imgf0004.png"\; state="\{\{state\}\}">VS\; </figure-callout>}{r}^{}{}^2}+\beta \frac{1}{\mathrm{AT}}+\mathit{\delta \; B}+\mathrm{VS}$$

Section 3

$$\mathit{Alpha}=2828$$

$$\mathit{Beta}=0237$$

$$\mathrm{AT}=\mathit{Cr}-\left(\mathit{MB}\mathit{+}\mathit{W}\mathit{+}\mathit{Or}\mathit{+}\mathit{Co}\right)$$

$$\mathit{Delta}=0091$$

$$B=1.40-0.12*\mathit{Cr}+0007/\mathit{Yes}$$

$$\mathrm{VS}=1.2*\mathit{mn}*\mathit{mn}-0.53*\mathit{mn}+0.02*\left(W+\mathit{Or}\right)-0012$$

Claims (16)

1. Steel composition according to specifications ASTM A213 and A335 defining respectively grades T23 and P23 displaying high hot corrosion performance due to oxidising environments such as fumes or water vapour, characterised in that it comprises, by weight, about 2.3 to 2.6 % of chromium, at most 0.5 % of silicon, between 0.20 and 0.45 % of manganese, between 1.45 and 1.6 % of tungsten, and between 0.05 and 0.2 % of molybdenum, the contents of the steel composition being adjusted based on a predetermined model, selected to obtain substantially optimum properties for resistance to hot oxidation under given conditions for high-temperature performance over a long period, the model being: $${\mathrm{V}}_{\mathit{COR}}^{\mathit{650}\mathit{°}C}=\alpha \frac{\mathit{1}}{\mathit{C}{\mathit{r}}^{\mathit{2}}}\mathit{+}\mathit{\beta }\frac{\mathit{1}}{\mathit{A}}\mathit{+}\mathit{\delta \; B}\mathit{+}\mathit{C}$$

   

   $$\mathit{Alpha}\mathit{=}\mathit{2.828}$$

   

   $$\mathit{Beta}=\mathit{0.237}$$

   

   $$\mathit{A}\mathit{=}\mathit{Cr}\mathit{-}\left(\mathit{Mo}\mathit{+}\mathit{W}\mathit{+}\mathit{Ni}\mathit{+}\mathit{Co}\right)$$

   

   $$\mathit{Delta}=\mathit{0.091}$$

   

   $$\mathit{B}\mathit{=}\mathit{1.40}\mathit{-}\mathit{0.12}\mathit{*}\mathit{Cr}\mathit{+}\mathit{0.007}\mathit{/}\mathit{Si}$$

   

   $$\mathit{C}\mathit{=}\mathit{1.2}\mathit{*}\mathit{Mn}\mathit{*}\mathit{Mn}\mathit{-}\mathit{0.53}\mathit{*}\mathit{Mn}\mathit{+}\mathit{0.02}\mathit{*}\left(\mathit{W}\mathit{+}\mathit{Ni}\right)\mathit{-}\mathit{0.012}$$

   

   
   contents by weight of Cr, Mn, Si, Mo, W, Ni, Co being such that the corrosion value Vcor is less than 1.4.
2. Steel composition according to claim 1, characterised by contents by weight of Cr, Mn, Si, Mo, W, Ni, Co being such that the corrosion value Vcor is at most equal to 1.25.
3. Steel composition according to specifications ASTM A213 and A335 defining respectively grades T22 and P22 displaying high hot corrosion performance due to oxidising environments such as fumes or water vapour, characterised in that it comprises, by weight, about 2.3 to 2.6 % of chromium, at most 0.5 % of silicon, between 0.20 and 0.45 % of manganese, between 0.87 and 1 % of molybdenum and very little tungsten, the contents of the steel composition being adjusted based on a predetermined model, selected to obtain substantially optimum properties for resistance to hot oxidation under given conditions for high-temperature performance over a long period, the model being: $${\mathrm{V}}_{\mathit{COR}}^{\mathit{650}\mathit{°}C}=\alpha \frac{\mathit{1}}{\mathit{C}{\mathit{r}}^{\mathit{2}}}\mathit{+}\mathit{\beta }\frac{\mathit{1}}{\mathit{A}}\mathit{+}\mathit{\delta \; B}\mathit{+}\mathit{C}$$

   

   $$\mathit{Alpha}\mathit{=}\mathit{2.828}$$

   

   $$\mathit{Beta}=\mathit{0.237}$$

   

   $$\mathit{A}\mathit{=}\mathit{Cr}\mathit{-}\left(\mathit{Mo}\mathit{+}\mathit{W}\mathit{+}\mathit{Ni}\mathit{+}\mathit{Co}\right)$$

   

   $$\mathit{Delta}=\mathit{0.091}$$

   

   $$\mathit{B}\mathit{=}\mathit{1.40}\mathit{-}\mathit{0.12}\mathit{*}\mathit{Cr}\mathit{+}\mathit{0.007}\mathit{/}\mathit{Si}$$

   

   $$\mathit{C}\mathit{=}\mathit{1.2}\mathit{*}\mathit{Mn}\mathit{*}\mathit{Mn}\mathit{-}\mathit{0.53}\mathit{*}\mathit{Mn}\mathit{+}\mathit{0.02}\mathit{*}\left(\mathit{W}\mathit{+}\mathit{Ni}\right)\mathit{-}\mathit{0.012}$$

   

   
   contents by weight of Cr, Mn, Si, Mo, W, Ni, Co being such that the corrosion value Vcor is at most equal to 0.9.
4. Steel composition according to claim 3, characterised by contents by weight of Cr, Mn, Si, Mo, W, Ni, Co being such that the corrosion value Vcor is at most equal to 0.85.
5. Steel composition according to specifications ASTM A213 and A335 defining respectively grades T24 and P24 displaying high hot corrosion performance due to oxidising environments such as fumes or water vapour, characterised in that it comprises, by weight, about 2.4 to 2.6 % of chromium, between 0.15 to 0.45 % of silicon, between 0.30 and 0.45 % of manganese, between 0.7 and 0.9 % of molybdenum and almost no tungsten, the contents of the steel composition being adjusted based on a predetermined model, selected to obtain substantially optimum properties for resistance to hot oxidation under given conditions for high-temperature performance over a long period, the model being: $${\mathrm{V}}_{\mathit{COR}}^{\mathit{650}\mathit{°}C}=\alpha \frac{\mathit{1}}{\mathit{C}{\mathit{r}}^{\mathit{2}}}\mathit{+}\mathit{\beta }\frac{\mathit{1}}{\mathit{A}}\mathit{+}\mathit{\delta \; B}\mathit{+}\mathit{C}$$

   

   $$\mathit{Alpha}\mathit{=}\mathit{2.828}$$

   

   $$\mathit{Beta}=\mathit{0.237}$$

   

   $$\mathit{A}\mathit{=}\mathit{Cr}\mathit{-}\left(\mathit{Mo}\mathit{+}\mathit{W}\mathit{+}\mathit{Ni}\mathit{+}\mathit{Co}\right)$$

   

   $$\mathit{Delta}=\mathit{0.091}$$

   

   $$\mathit{B}\mathit{=}\mathit{1.40}\mathit{-}\mathit{0.12}\mathit{*}\mathit{Cr}\mathit{+}\mathit{0.007}\mathit{/}\mathit{Si}$$

   

   $$\mathit{C}\mathit{=}\mathit{1.2}\mathit{*}\mathit{Mn}\mathit{*}\mathit{Mn}\mathit{-}\mathit{0.53}\mathit{*}\mathit{Mn}\mathit{+}\mathit{0.02}\mathit{*}\left(\mathit{W}\mathit{+}\mathit{Ni}\right)\mathit{-}\mathit{0.012}$$

   

   
   contents by weight of Cr, Mn, Si, Mo, W, Ni, Co being such that the corrosion value Vcor is at most equal to 0.8.
6. Steel composition according to claim 5, characterised by contents by weight of Cr, Mn, Si, Mo, W, Ni, Co being such that the corrosion value Vcor is at most equal to 0.75.
7. Steel composition according to specifications ASTM A213 and A335 defining respectively grades T91 and P91 displaying high hot corrosion performance due to oxidising environments such as fumes or water vapour, characterised in that it comprises, by weight, about 8.9 to 9.5 % of chromium, between 0.2 to 0.5 % of silicon, between 0.85 and 0.95 % of molybdenum, between 0.3 and 0.45 % of manganese and substantially an absence of W, the contents of the steel composition being adjusted based on a predetermined model, selected to obtain substantially optimum properties for resistance to hot oxidation under given conditions for high-temperature performance over a long period, the model being: $${\mathrm{V}}_{\mathit{COR}}^{\mathit{650}\mathit{°}C}=\alpha \frac{\mathit{1}}{\mathit{C}{\mathit{r}}^{\mathit{2}}}\mathit{+}\mathit{\beta }\frac{\mathit{1}}{\mathit{A}}\mathit{+}\mathit{\delta \; B}\mathit{+}\mathit{C}$$

   

   $$\mathit{Alpha}\mathit{=}\mathit{2.828}$$

   

   $$\mathit{Beta}=\mathit{0.237}$$

   

   $$\mathit{A}\mathit{=}\mathit{Cr}\mathit{-}\left(\mathit{Mo}\mathit{+}\mathit{W}\mathit{+}\mathit{Ni}\mathit{+}\mathit{Co}\right)$$

   

   $$\mathit{Delta}=\mathit{0.091}$$

   

   $$\mathit{B}\mathit{=}\mathit{1.40}\mathit{-}\mathit{0.12}\mathit{*}\mathit{Cr}\mathit{+}\mathit{0.007}\mathit{/}\mathit{Si}$$

   

   $$\mathit{C}\mathit{=}\mathit{1.2}\mathit{*}\mathit{Mn}\mathit{*}\mathit{Mn}\mathit{-}\mathit{0.53}\mathit{*}\mathit{Mn}\mathit{+}\mathit{0.02}\mathit{*}\left(\mathit{W}\mathit{+}\mathit{Ni}\right)\mathit{-}\mathit{0.012}$$

   

   
   contents by weight of Cr, Mn, Si, Mo, W, Ni, Co being such that the corrosion value Vcor is less than 0.1.
8. Steel composition according to claim 7, characterised by contents by weight of Cr, Mn, Si, Mo, W, Ni, Co being such that the corrosion value Vcor is at most equal to 0.07.
9. Steel composition according to specifications ASTM A213 and A335 defining respectively grades T91 and P91 displaying high hot corrosion performance due to oxidising environments such as fumes or water vapour, characterised in that it comprises, by weight, about 8.9 to 9.5 % of chromium, at most 0.5 % of silicon, between 0.3 and 0.45 % of manganese, between 1.5 and 1.75 % of tungsten, and between 0.3 and 0.45 % of molybdenum, the contents of the steel composition being adjusted based on a predetermined model, selected to obtain substantially optimum properties for resistance to hot oxidation under given conditions for high-temperature performance over a long period, the model being: $${\mathrm{V}}_{\mathit{COR}}^{\mathit{650}\mathit{°}C}=\alpha \frac{\mathit{1}}{\mathit{C}{\mathit{r}}^{\mathit{2}}}\mathit{+}\mathit{\beta }\frac{\mathit{1}}{\mathit{A}}\mathit{+}\mathit{\delta \; B}\mathit{+}\mathit{C}$$

   

   $$\mathit{Alpha}\mathit{=}\mathit{2.828}$$

   

   $$\mathit{Beta}=\mathit{0.237}$$

   

   $$\mathit{A}\mathit{=}\mathit{Cr}\mathit{-}\left(\mathit{Mo}\mathit{+}\mathit{W}\mathit{+}\mathit{Ni}\mathit{+}\mathit{Co}\right)$$

   

   $$\mathit{Delta}=\mathit{0.091}$$

   

   $$\mathit{B}\mathit{=}\mathit{1.40}\mathit{-}\mathit{0.12}\mathit{*}\mathit{Cr}\mathit{+}\mathit{0.007}\mathit{/}\mathit{Si}$$

   

   $$\mathit{C}\mathit{=}\mathit{1.2}\mathit{*}\mathit{Mn}\mathit{*}\mathit{Mn}\mathit{-}\mathit{0.53}\mathit{*}\mathit{Mn}\mathit{+}\mathit{0.02}\mathit{*}\left(\mathit{W}\mathit{+}\mathit{Ni}\right)\mathit{-}\mathit{0.012}$$

   

   
   contents by weight of Cr, Mn, Si, Mo, W, Ni, Co being such that the corrosion value Vcor is at most equal to 0.11.
10. Steel composition according to claim 9, characterised by contents by weight of Cr, Mn, Si, Mo, W, Ni, Co being such that the corrosion value Vcor is at most equal to 0.08.
11. Steel composition according to one of claims 7 to 10, characterised in that it comprises less than 0.2 % of nickel.
12. Steel composition according to specifications ASTM A213 and A335 defining respectively grades T9 and P9 displaying high hot corrosion performance due to oxidising environments such as fumes or water vapour, characterised in that it comprises, by weight, about 9.2 to 10 % of chromium, between 0.25 and 1 % of silicon, between 0.9 and 1 % of molybdenum, between 0.3 and 0.45 % of manganese, and substantially an absence of W, the contents of the steel composition being adjusted based on a predetermined model, selected to obtain substantially optimum properties for resistance to hot oxidation under given conditions for high-temperature performance over a long period, the model being: $${\mathrm{V}}_{\mathit{COR}}^{\mathit{650}\mathit{°}C}=\alpha \frac{\mathit{1}}{\mathit{C}{\mathit{r}}^{\mathit{2}}}\mathit{+}\mathit{\beta }\frac{\mathit{1}}{\mathit{A}}\mathit{+}\mathit{\delta \; B}\mathit{+}\mathit{C}$$

    

    $$\mathit{Alpha}\mathit{=}\mathit{2.828}$$

    

    $$\mathit{Beta}=\mathit{0.237}$$

    

    $$\mathit{A}\mathit{=}\mathit{Cr}\mathit{-}\left(\mathit{Mo}\mathit{+}\mathit{W}\mathit{+}\mathit{Ni}\mathit{+}\mathit{Co}\right)$$

    

    $$\mathit{Delta}=\mathit{0.091}$$

    

    $$\mathit{B}\mathit{=}\mathit{1.40}\mathit{-}\mathit{0.12}\mathit{*}\mathit{Cr}\mathit{+}\mathit{0.007}\mathit{/}\mathit{Si}$$

    

    $$\mathit{C}\mathit{=}\mathit{1.2}\mathit{*}\mathit{Mn}\mathit{*}\mathit{Mn}\mathit{-}\mathit{0.53}\mathit{*}\mathit{Mn}\mathit{+}\mathit{0.02}\mathit{*}\left(\mathit{W}\mathit{+}\mathit{Ni}\right)\mathit{-}\mathit{0.012}$$

    

    
    contents by weight of Cr, Mn, Si, Mo, W, Ni, Co being such that the corrosion value Vcor is at most equal to 0.09.
13. Steel composition according to one of claims 1 to 4 and 7 to 11, characterised in that it comprises a content of silicon between 0.20 and 0.50 %, preferably between 0.30 and 0.50 %.
14. Steel composition according to one of claims 1 to 4, characterised in that it comprises a content of manganese between 0.25 and 0.45 %.
15. Seamless or accessory tube, basically consisting of a steel composition according to one of the preceding claims.
16. Application of the steel composition to seamless and accessory tubes, intended to generate, to convey or to condition water vapour under elevated pressure and temperature.

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