EP0626460A1 - Austenitic stainless steel with high resistance against corrosion in chloride and sulfuric environments, and uses of this steel - Google Patents

Abstract

The steel contains, in proportions by weight, from 20 to 30 % of chromium, 25 to 32 % of nickel, from 3 to 7 % of molybdenum, from 0.35 to 0.8 % of nitrogen, from 0.5 to 5.4 % of manganese, up to 0.06 % of carbon and up to 1 % of silicon. As a result of its great versatility in corrosion resistance, the steel can be employed in particular for the manufacture of equipment for decontamination of fumes, the paper pulp industry, for the chemical industry or for oil exploration, seawater plants and for the manufacture of tankers for transporting corrosive products. The steel has a very high structural stability.

Description

The present invention relates to an austenitic stainless steel with high mechanical strength and high corrosion resistance.

To manufacture equipment intended in particular for the purification of fumes from thermal electric power stations or intended for oil platforms working in contact with sea water and environments containing acid gases or also for the manufacture of paper pulp or for the chemical industry, stainless steels with high mechanical strength and high corrosion resistance are used in chlorinated and sulfuric media or even in both chlorinated and sulfuric media. These stainless steels are superaustenitic steels, austenoferritic steels or superaustenitic steels with a high nitrogen content. Superaustenitic steels with a high nitrogen content are, for these applications, the steels which offer the best performance in terms of combination of mechanical characteristics and corrosion resistance. They are described by two European patents: EP-A-0.438.992 and EP-A-0.342.574.

However, these steels (described by EP-A-0.438.992 and EP-A-0.342.574) have drawbacks. On the one hand if the improvement of the corrosion resistance in chlorinated medium of these steels is effective, the resistance to corrosion in concentrated sulfuric media polluted or not of these new grades is average so that the suitability of these steels to versatile use in various corrosive environments or containing different corrosive agents, for example chlorides and sulfurics is less good than that of previously known steels which on the other hand had weaker mechanical characteristics.

On the other hand, when the type of steel described by patent EP-A-0,438,992 is used to manufacture thick parts, it appears during the manufacture of these parts, phenomena of segregation or precipitation of intermetallic phases which very significantly deteriorate the mechanical properties including resilience and corrosion resistance.

The object of the present invention is to remedy these drawbacks by proposing a stainless steel with high mechanical characteristics and in particular with an elastic limit greater than 400 MPa and with high resistance to corrosion in chlorinated media and pure or polluted sulfuric media. and in particular having a puncture corrosion resistance index PREN =% Cr + 3.3% Mo + 16% N greater than 50, which has a very good aptitude for versatile use in various corrosive environments containing different corrosive agents for example chlorinated and sulfuric and which makes it possible to manufacture thick parts which have a very good resilience and resistance to corrosion at heart.

To this end, the subject of the present invention is an austenitic stainless steel with a high mechanical characteristic and high corrosion resistance, the chemical composition of which comprises by weight:
20% ≦ Cr ≦ 30%
25% ≦ Ni ≦ 32%
3% ≦ MB ≦ 7%
0.35% ≦ N ≦ 0.8%
0.5% ≦ Mn ≦ 5.4%
C ≦ 0.06%
if ≦ 1%,
the rest, with the exception of iron, being constituted by impurities resulting from the production.

Preferably, the carbon content is less than 0.04%. To improve its resistance to corrosion, this steel can also comprise from 0.5% to 3% of copper.

It is preferable that this steel contains less than 0.010% sulfur.

It can also contain at least one element taken from B, Nb, V, Al in contents which are: B from 0.0001 to 0.003%, Nb from 0.001 to 0.3%, V from 0.001 at 0.3%, Al from 0.001 to 0.1%.

Preferably this steel contains:
23% ≦ Cr ≦ 28%
25% ≦ Ni ≦ 28%
4.5% ≦ MB ≦ 7%.

Even more preferably, the composition of the steel according to the invention is as follows:
25% ≦ Cr ≦ 26%
25% ≦ Ni ≦ 26%
6% ≦ MB ≦ 7%
0.4% ≦ N ≦ 0.5%
2.5% ≦ Mn ≦ 3.5%
C ≦ 0.03%
If ≦ 0.3%
1% ≦ Cu ≦ 2%,
the rest, with the exception of iron, being constituted by impurities resulting from the production.

Finally, the chemical composition of the steel must preferably satisfy the following relationships:
95 <=% Cr + 0.3% Ni + 9% Si + 27% Mo + 130% P - 8% N <232
and preferably:
95 <kP <210
and
319 <kC = 3.3% Cr + 10% Ni +% Mo + 1.5% Cu <432
and preferably:
355 <kC <432.

The invention also relates to the use of the steel according to the invention for the manufacture of equipment for decontaminating smoke from thermal power plants and from plants for the incineration of household waste, in particular gas washing towers. or smoke, gas or smoke pipes and chimneys; for the manufacture of delignification equipment, in particular by the bisulfite process, of filtration and bleaching of paper pulp; for the manufacture of equipment for the chemical industry in chlorinated or acidic environments and in particular for the manufacture of tanks, reservoirs, reactors, pump casing tubes and pump shafts; for the manufacture of off-shore platform equipment subject to corrosion by seawater and / or hydrocarbons and in particular flare supports, heat exchangers, separators, tubular plates, piping for transporting seawater, piping used for the transport of hydrocarbons, elements for protecting pylon areas located in the vicinity of the free surface of the sea, rods, pump shafts, flanges connection, well heads, manifolds (risers) and risers; for the manufacture of road or rail transport tanks of highly corrosive chlorinated or acidic products.

The invention will now be described in a more precise but non-limiting manner.

• du chrome : plus de 20 % pour obtenir une bonne résistance à la corrosion localisée et inférieure à 30 % pour avoir une cinétique de précipitation des carbures et/ou phases intermétalliques pas trop rapide de préférence, on choisira une teneur en chrome comprise entre 23 % et 28 % et de manière plus préférentielle entre 25 % et 26 %.
• du nickel : plus de 25 % pour obtenir une résistance à la corrosion dans des milieux très divers et notamment dans les milieux sulfuriques purs ou pollués et/ou les gaz acides et moins de 32 % pour ne pas trop diminuer la solubilité de l'azote ; on choisira de préférence une teneur en nickel comprise entre 25 % et 28 % et de manière encore plus préférentielle entre 25 % et 26 %.
• du molybdène : plus de 3 % pour améliorer la résistance à la corrosion localisée et moins de 7 % pour limiter les ségrégations dans les produits épais qui détériorent la résilience et la tenue à la corrosion ; de préférence on choisira un molybdène supérieur à 4,5 % et de manière plus préférentielle une teneur supérieure à 6 %.
• de l'azote : plus de 0,35 % pour obtenir un niveau de caractéristiques mécaniques élevées, améliorer la stabilité structurale et augmenter la résistance à la corrosion et moins de 0,8 % pour éviter de trop détériorer la résilience par précipitation de nitrures ; de préférence on choisira une teneur en azote comprise entre 0,4 % et 0,5 %.
• du manganèse : plus de 0,5 % pour améliorer la solubilité de l'azote et moins de 5,4 % car une trop forte teneur en manganèse détériore la stabilité structurale de l'acier et dégrade les réfractaires d'aciérie pendant l'élaboration.

The austenitic stainless steel according to the invention must contain (contents expressed in% by weight):

• chromium: more than 20% to obtain a good resistance to localized corrosion and less than 30% to have a kinetics of precipitation of carbides and / or intermetallic phases not too fast preferably, a chromium content of between 23% will be chosen and 28% and more preferably between 25% and 26%.
• nickel: more than 25% to obtain resistance to corrosion in very diverse media and in particular in pure or polluted sulfuric media and / or acid gases and less than 32% so as not to reduce the nitrogen solubility too much ; preferably, a nickel content of between 25% and 28% and even more preferably between 25% and 26% will be chosen.
• molybdenum: more than 3% to improve resistance to localized corrosion and less than 7% to limit segregation in thick products which deteriorate resilience and corrosion resistance; preferably a molybdenum greater than 4.5% will be chosen and more preferably a content greater than 6%.
• nitrogen: more than 0.35% to obtain a high level of mechanical characteristics, improve structural stability and increase resistance to corrosion and less than 0.8% to avoid damaging the resilience by precipitation of nitrides; preferably, a nitrogen content of between 0.4% and 0.5% will be chosen.
• manganese: more than 0.5% to improve the solubility of nitrogen and less than 5.4% because too high a manganese content deteriorates the structural stability of the steel and degrades the steelworks refractories during production .

Such steel should contain less than 0.06% carbon to avoid precipitation of carbides at the grain boundaries which deteriorate the corrosion resistance and it is preferable to limit this content to 0.04% and even better to 0.03 %.

Steel always contains a little sulfur which is favorable to machinability but which favors pitting corrosion, so it is preferable to have a sulfur content of less than 0.01%.

To improve the resistance to corrosion in sulfuric medium and in acid chlorinated medium, copper can be added between 0.5% and 3% and preferably between 1% and 2%; copper also has the advantage of improving machinability.

To improve the mechanical characteristics, between 0.001% and 0.3% of niobium or vanadium can be added.

To improve forgeability and thus facilitate hot rolling or hot forging operations, it is preferable to add from 0.001% to 0.1% of aluminum and possibly from 0.0001% to 0.003% of boron.

A steel is thus obtained which contains in its most general form:
20% ≦ Cr ≦ 30%
25% ≦ Ni ≦ 32%
3% ≦ MB ≦ 7%
0.35% ≦ N ≦ 0.8%
0.5% ≦ Mn ≦ 5.4%
C ≦ 0.06%
If ≦ 1%,
the rest, with the exception of iron, being constituted by impurities resulting from the production.

• une résistance mécanique élevée et notamment une limite d'élasticité supérieure à 400 MPa,
• une bonne résilience en particulier lorsqu'il est utilisé pour réaliser des pièces épaisses ou massives telles que des tôles fortes ou des pièces forgées, du fait en particulier d'une teneur en molybdène limitée à 7 % maximum,
• une bonne résistance à la corrosion localisée en milieu chloruré, il est notamment caractérisé par un indice de piqure P.R.E.N. = % Cr + 3,3 % Mo + 16 % N > 50
• une bonne résistance à la corrosion dans les milieux à la fois chlorurés et sulfuriques, du fait de sa teneur en nickel élevée (> 25 %).

Preferably, it contains less than 0.04% carbon. This steel has the advantage of having simultaneously:

• high mechanical strength and in particular an elastic limit greater than 400 MPa,
• good resilience, in particular when it is used to produce thick or massive parts such as heavy plates or forgings, due in particular to a molybdenum content limited to a maximum of 7%,
• good resistance to localized corrosion in a chlorinated environment, it is in particular characterized by a bite index PREN =% Cr + 3.3% Mo + 16% N> 50
• good corrosion resistance in both chlorinated and sulfuric media, due to its high nickel content (> 25%).

By adding 0.5% to 3% copper to this steel, a steel is obtained whose corrosion resistance and machinability are improved.

When 0.001% to 0.3% of niobium or 0.001% to 0.3% of vanadium is added to the steels previously defined, a steel is obtained whose mechanical characteristics are improved.

With an additional addition of 0.001% to 0.1% of aluminum and / or from 0.0001% to 0.003% of boron, a steel is obtained whose forgeability is improved.

Additions of copper, vanadium, niobium, boron, aluminum are optional and can be done alone or in combination.

The main alloying elements have, for certain properties, effects all the more favorable as their content is high and for other properties, effects all the less unfavorable as the content is lower; therefore, it is preferable to choose the chemical composition in a composition range which is not too wide. It is therefore preferable, in all cases, to limit the areas of chromium, nickel and molybdenum to:
23% ≦ Cr ≦ 28%
25% ≦ Ni ≦ 28%
4.5% ≦ MB ≦ 7%.

The inventors have found that the best results are obtained with a steel whose composition is as follows:
25% ≦ Cr ≦ 26%
25% ≦ Ni ≦ 26%
6% ≦ MB ≦ 7%
0.4% ≦ N ≦ 0.5%
2.5% ≦ Mn ≦ 3.5%
1% ≦ Cu ≦ 2%
C ≦ 0.03%
If ≦ 0.3%.
and preferably,
S ≦ 0.01%,
the rest, with the exception of iron, being constituted by impurities resulting from the production.

This steel may also contain Nb, V, B or Al as indicated above.

• pour garantir une faible ségrégation et peu de précipitation de carbures et/ou phases intermétalliques :
     95 < kP = % Cr + 0,3 % Ni + 9 % Si + 27 % Mo + 130 % P - 8 % N < 232
  et de préférence 95 < kP < 210
• pour avoir une bonne polyvalence de la résistance à la corrosion (en particulier en milieux sulfuriques purs ou pollués et en milieux chlorurés) :
     319 < kC = 3,3 % Cr + 10 % Ni + % Mo + 1,5 % Cu < 432
  et de préférence : 355 < kC < 432.

The inventors have also found that for these steels to have optimal properties, their chemical compositions had to satisfy the following relationships:

• to guarantee a low segregation and little precipitation of carbides and / or intermetallic phases:
  95 <kP =% Cr + 0.3% Ni + 9% Si + 27% Mo + 130% P - 8% N <232
  and preferably 95 <kP <210
• to have good versatility in corrosion resistance (in particular in pure or polluted sulfuric media and in chlorinated media):
  319 <kC = 3.3% Cr + 10% Ni +% Mo + 1.5% Cu <432
  and preferably: 355 <kC <432.

By way of example, a steel of the following composition was produced:
Cr = 25%
Ni = 25.5%
Mo = 6.5%
N = 0.45%
Cu = 1.5%
C = 0.020%
If = 0.25%
S = 0.001%,
the rest, with the exception of iron, being constituted by impurities resulting from the production.

This steel was produced in the form of a bar 500 mm in diameter obtained after air cooling. The mechanical characteristics were as follows:
Re = 490 MPa
R = 890 MPa
A = 57%
Kcv at 20 ° C = 285 Joules
Kcv at - 50 ° C = 280 Joules
Kcv at 20 ° C at the heart of the circle = 250 Joules.

This latter value is a sign of very good structural stability.

A localized corrosion test in a chlorinated medium according to the American standard ASTM G 48 gave a pitting temperature greater than or equal to the boiling point.

A generalized corrosion test in sulfuric medium at 10% H₂SO₄ at 80 ° deaerated polluted up to 500 ppm of chlorine did not show any corrosion after 96 hours (O mdj = O mg / dm² / day) whereas for the nuances of the prior art, under the same conditions, corrosion is of the order of 100 mdj.

This steel has an additional advantage which comes from the fact that the product A x R of the elongation at break by the breaking limit is very high (approximately twice as much as for the steels of the prior art used for transport) so that the impact resistance of the walls produced with this steel is very high and in particular much higher than for steels of the prior art.

This characteristic has the advantage of making it possible to produce tanks, receptacles or pipes for the transport of corrosive products which are much safer in the event of impact than equivalent equipment produced with steels according to the prior art.

The properties of this steel make it particularly suitable for the manufacture of reactors (scrubbers, washing tower, filter tanks, attack tanks), tubes (welded and seamless), chimneys, junction pieces such as flanges, manifolds (manifolds), pipelines (flow lines), separators and tanks for road or rail transport, for industries in which these equipments are subjected to very severe corrosions by pure chlorinated and / or sulfuric media or polluted and in particular for off-shore platforms of petroleum exploitations, for pollution abatement facilities from combustion fumes from thermal power stations or from incineration of household waste, for the preparation of paper pulp in particular by the process called "bisulfite", and in particular for filtration, bleaching and delignification equipment, for the chemical industry and more particularly for equipment nts of hydrometallurgy and fertilizer industry using attack on ores by concentrated sulfuric media.

• dans les plate-formes off-shore pour l'exploitation de champs pétroliers ou gaziers sous-marins, l'acier selon l'invention est utilisé pour réaliser des équipements de process soumis à la corrosion par l'eau de mer notamment les supports de torchères, les échangeurs de chaleur et les séparateurs, et notamment, les plaques tubulaires, les tuyauteries de transport d'eau de mer et les tuyauteries utilisées pour le traitement du pétrole ou du gaz, les protections de la zone des pylones qui est au voisinage de la surface libre de la mer, les masses-tiges, les arbres de pompe et les brides de raccordement soumises à la corrosion par l'eau de mer, les têtes de puits, les collecteurs (manifolds), les colonnes montantes (risers).
• dans les industries de la dépollution pour réaliser les équipements soumis à une corrosion soit par l'acide chlorhydrique soit par l'acide sulfurique soit par des mélanges de ces acides avec parfois présence d'acide fluorhydrique et notamment pour la réalisation des tours de lavage des gaz ou fumées de combustion des centrales thermiques et des usines d'incinération d'ordure ainsi que pour la fabrication des conduites menant aux cheminées ; dans le cas particulier des tours de lavage des gaz d'une centrale thermique, les équipements sont en particulier, le réacteur, le présaturateur, la structure interne de l'absorbeur et la cheminée.
• dans l'industrie de la pâte à papier pour la fabrication des équipements de délignification en particulier par le procédé au bisulfite et des équipements de filtration et de blanchiement par des composés chlorés très oxydants tels que Cl₂ et ClO₂ et également par des composés du type peroxyde d'hydrogène et ozone ; pour la délignification il s'agit notamment des préchauffeurs, des lessiveurs, des imprégnateurs et des lessiveurs continus ; pour le lavage et le blanchiement, il s'agit notamment du laveur, du bac de filtration, de la tour de blanchiement au chlore et au dioxyde de chlore ainsi que ses équipements de diffusion, lavage et filtration et la tour de blanchiement à l'hypochlorite avec son laveur et son bac de filtration.
• dans l'industrie chimique, l'acier selon l'invention peut être utilisé avantageusement pour réaliser notamment des bacs, des réservoirs, des réacteurs, des tubes, des corps de pompe, des arbres de pompe qui sont en contact avec des milieux fortement chlorurés ou des milieux acides.

More specifically:

• in off-shore platforms for the exploitation of underwater oil or gas fields, the steel according to the invention is used to produce process equipment subjected to corrosion by sea water, in particular the supports of flares, heat exchangers and separators, and in particular, tube plates, piping for seawater transport and piping used for the treatment of oil or gas, protections for the pylon area which is at in the vicinity of the free sea surface, the drill rods, the pump shafts and the connection flanges subjected to corrosion by sea water, the well heads, the manifolds (risers), the risers (risers) ).
• in the depollution industries to produce equipment subject to corrosion either by hydrochloric acid or by sulfuric acid or by mixtures of these acids with sometimes the presence of hydrofluoric acid and in particular for the realization of washing towers for gases or fumes from combustion of thermal power stations and refuse incineration plants as well as for the manufacture of pipes leading to chimneys; in the particular case of the gas washing towers of a thermal power plant, the equipment is in particular, the reactor, the presaturator, the internal structure of the absorber and the stack.
• in the pulp industry for the manufacture of delignification equipment, in particular by the bisulfite process and of filtration and bleaching equipment with highly oxidizing chlorine compounds such as Cl₂ and ClO₂ and also with compounds of the peroxide type hydrogen and ozone; for delignification, these are in particular preheaters, digesters, impregnators and continuous digesters; for washing and bleaching, these are in particular the washer, the filtration tank, the chlorine and chlorine dioxide bleaching tower as well as its diffusion, washing and filtration equipment and the bleaching tower. hypochlorite with its washer and its filtration tank.
• in the chemical industry, the steel according to the invention can be advantageously used to produce in particular tanks, reservoirs, reactors, tubes, pump bodies, pump shafts which are in contact with highly chlorinated or acidic media.

This steel also makes it possible to produce any part subject to abrasion / corrosion in chlorinated and / or acidic environments.

For all these applications, in fact, the person skilled in the art is constantly looking for steel which has the highest possible mechanical characteristics and the highest possible corrosion resistance without, however, its price being exorbitant in order to produce the most reliable equipment. possible with the longest possible service life and at a price compatible with their industrial operation. From this point of view, the steel according to the invention, by virtue of its chemical composition and its properties, is much more advantageous than nickel-based superalloys.

The applications described are not limiting and those skilled in the art will be able to choose this steel when they deem it useful.

Claims (13)

1.- Austenitic stainless steel with high mechanical strength and high corrosion resistance characterized in that its chemical composition comprises by weight:
20% ≦ Cr ≦ 30%
25% ≦ Ni ≦ 32%
3% ≦ MB ≦ 7%
0.35% ≦ N ≦ 0.8%
0.5% ≦ Mn ≦ 5.4%
C ≦ 0.06%
if ≦ 1%,
the rest, with the exception of iron, being constituted by impurities resulting from the production. 2. Austenitic stainless steel according to claim 1, characterized in that:
C ≦ 0.04%. 3.- Austenitic stainless steel according to claim 1 or claim 2, characterized in that it further comprises from 0.5% to 3% of copper. 4.- Austenitic stainless steel according to any one of claims 1 to 3, characterized in that it contains less than 0.010% sulfur. 5. Austenitic stainless steel according to any one of claims 1 to 4, characterized in that it also contains at least one of the following elements: B, Nb, V, Al in the following contents: B 0, 0001 to 0.003%, Nb: 0.001 to 0.3%, V: 0.001 to 0.3%, Al: 0.001 to 0.1%. 6. Austenitic stainless steel according to any one of the preceding claims, characterized in that it preferably contains:
23% ≦ Cr ≦ 28%
25% ≦ Ni ≦ 28%
4.5% ≦ MB ≦ 7%. 7.- Austenitic stainless steel according to claim 6, characterized in that it preferably contains:
25% ≦ Cr ≦ 26%
25% ≦ Ni ≦ 26%
6% ≦ MB ≦ 7%
0.4% ≦ N ≦ 0.5%
2.5% ≦ Mn ≦ 3.5%
C ≦ 0.03%
If ≦ 0.3%
and further
1% ≦ Cu ≦ 2%, the rest, with the exception of iron, being made up of impurities resulting from processing. 8. Austenitic stainless steel according to any one of the preceding claims, characterized in that the chemical composition satisfies the following relationships:
95 <kP =% Cr + 0.3% Ni + 9% Si + 27% Mo + 130% P - 8% N <232
and preferably: 95 <kP <210
319 <kC = 3.3% Cr + 10% Ni +% Mo + 1.5% Cu <432
and preferably: 355 <kC <432. 9.- Use of an austenitic stainless steel according to any one of claims 1 to 8, for the manufacture of equipment for decontaminating smoke from thermal power plants and from household waste incineration plants, in particular washing towers for gas or smoke, gas or smoke pipes and chimneys. 10.- Use of an austenitic stainless steel according to any one of claims 1 to 8 for the manufacture of delignification equipment, in particular by the bisulfite process, filtration and bleaching of the paper pulp. 11.- Use of an austenitic stainless steel according to any one of claims 1 to 8 for the manufacture of equipment for the chemical industry in chlorinated or acidic medium and in particular the manufacture of tanks, reservoirs, reactors, tubes, body pump and pump shafts. 12.- Use of an austenitic stainless steel according to any one of claims 1 to 8 for the manufacture of equipment for offshore platforms subjected to marine corrosion and in particular flare supports, heat exchangers , separators, tubular plates, piping for transporting seawater, piping used for the oil or gas treatment process, protections for pylon areas located near the free surface of the sea, drill rods, pump shafts, connection flanges, well heads, manifolds (manifolds) and risers. 13.- Use of an austenitic stainless steel according to any one of claims 1 to 8 for the manufacture of road or rail transport tanks of chlorides or highly corrosive acids.