EP1867748A1 - Duplex stainless steel - Google Patents

Abstract

Stainless steel duplex, comprises (in wt.%) a composition of carbon (= 0.05), chromium (21-25), nickel (1-2.95), nitrogen (0.16-0.28), manganese (= 2), molybdenum (= 0.45), tungsten (= 0.15), silicon (= 1.4), aluminum (= 0.05), copper (0.11-0.5), sulfur (= 0.01), phosphorous (0.04), boron (= 0.0005), cobalt (= 0.5), rare earth metals (= 0.1), vanadium (= 0.5), titanium (0.1), niobium (= 0.3), magnesium (= 0.1) and iron (rest), and impurities resulting from development, where the sum of molybdenum and tungsten/2 is >= 0.5. The microstructure is constituted by austenite and ferrite. Independent claims are included for: (1) preparation of the steel sheet comprising providing an ingot or a slab of the steel of the composition and hot roll of the ingot or slab at 1150-1280[deg] C to obtain a plate, a strip or a reel; (2) hot rolled steel quarto sheet, obtained by the process and having a thickness of 5-100 mm; (3) cold rolled steel band obtained by cold rolling a hot rolled reel obtained by the process; (4) preparation of a bar or a wire i.e. hot rolled from the given steel, comprising providing an ingot or a bloom of continuous casting of a steel, hot roll of the ingot or the bloom at 1150-1280[deg] C to obtain a bar i.e. cooled with air or a coil of wire i.e. cooled with water, then optionally thermal treatment at 900-1100[deg] C and cooling bar or the crown by hardening; (5) a hot rolled bar obtained by the process and having a diameter of 18-250 mm and hot rolled wire obtained by the process and having a diameter of 4-30 mm; (6) a cold drawn bar obtained by the process and having a diameter of 4-60 mm and drawn wire obtained by the process and having a diameter of 0.01-20 mm; (7) preparation of a steel section, comprising cold shaping of a hot rolled bar obtained by the process; (8) steel section obtained by the process; (9) preparation of a forged steel part, comprising passing the hot rolled steel bar into slug and forging the slug at 1100-1280[deg] C; (10) forged steel part obtained by the process; and (11) molded part obtained by molding of the steel.

Description

The present invention relates to a duplex stainless steel, more particularly intended for the manufacture of structural elements for production plants (chemical, petrochemical, paper, offshore) or energy production, without being there limited, and the method of manufacturing a sheet, strip, bars, son, or profiles of this steel.

This steel can more generally be used in substitution of a type 304L stainless steel in many applications, for example, in previous industries or in the food industry, including parts made from formed son (welded grids ..) profiles (strainers ..), axes ... One could also make molded parts and forged parts.

For this purpose, stainless steel grades 304 and 304L are known, the microstructure of which in the annealed state is essentially austenitic; when cold-worked, they may also contain a variable proportion of martensite. These steels, however, have high additions of nickel, the cost is generally prohibitive. In addition, these grades may pose a problem from a technical point of view for certain applications because they have low tensile characteristics in the annealed state, especially with regard to the yield strength, and a low resistance to stress corrosion.

Also known are austenitic-ferritic stainless steels, which are composed mainly of a mixture of ferrite and austenite, such as 1.4362, 1.4655, 1.4477, 1.4462, 1.4507, 1.4410, 1.4501 and 1.4424 steels of the standard. EP10088 , all of which contain more than 3.5% nickel. These steels are particularly resistant to corrosion and stress corrosion.

Also known are ferritic or ferrito-martensitic stainless steel grades, the microstructure of which, for a defined range of heat treatments, is composed of two constituents, ferrite and martensite, preferably in a ratio of 50/50, such as grade 1.4017 of EN10088. These grades, with a chromium content generally less than 20%, have high mechanical tensile properties, but do not exhibit satisfactory corrosion resistance.

In addition, a simplification of the manufacturing process of sheets, strips, bars, wires or steel profiles is also sought.

The object of the present invention is to overcome the disadvantages of the steels and manufacturing processes of the prior art by providing a stainless steel having good mechanical characteristics and in particular a tensile yield strength greater than 400 or 450MPa to the annealed or dissolved state, a high resistance to corrosion and in particular greater than or equal to that of 304L, good microstructural stability and good resilience of the welded zones, without the addition of expensive additive elements, as well as a process for producing sheets, strips, bars, wires, or profiles made of this steel that is simplified to use.

• C ≤ 0,05 %
• 21 % ≤ Cr ≤ 25 %
• 1 % ≤ Ni ≤ 2,95 %
• 0,16 % ≤ N ≤ 0,28 %
• Mn ≤ 2,0 %
• Mo + W/2 ≤ 0,50 %
• Mo ≤ 0,45 %
• W ≤ 0,15 %
• Si ≤ 1,4 %
• Al ≤ 0,05 %
• 0,11 % ≤ Cu ≤ 0,50 %
• S ≤ 0,010 %
• P ≤ 0,040 %
• B ≤ 0,0005 %
• Co ≤ 0,5 %
• REM ≤ 0,1 %
• V ≤ 0,5 %
• Ti ≤ 0,1 %
• Nb ≤ 0,3 %
• Mg ≤ 0,1 %

le reste étant du fer et des impuretés résultant de l'élaboration et la microstructure étant constituée d'austénite et de ferrite.For this purpose, the invention firstly relates to a duplex stainless steel, the composition of which consists of, in% by weight:

• C ≤ 0.05%
• 21% ≤ Cr ≤ 25%
• 1% ≤ Ni ≤ 2.95%
• 0.16% ≤ N ≤ 0.28%
• Mn ≤ 2.0%
• Mo + W / 2 ≤ 0.50%
• Mo ≤ 0.45%
• W ≤ 0.15%
• If ≤ 1.4%
• Al ≤ 0.05%
• 0.11% ≤ Cu ≤ 0.50%
• S ≤ 0.010%
• P ≤ 0.040%
• B ≤ 0.0005%
• Co ≤ 0.5%
• REM ≤ 0.1%
• V ≤ 0.5%
• Ti ≤ 0.1%
• Nb ≤ 0.3%
• Mg ≤ 0.1%

the remainder being iron and impurities resulting from the preparation and the microstructure consisting of austenite and ferrite.

• la proportion de ferrite est comprise entre 35 et 65% en volume, et de préférence entre 35 et 55% en volume.
• les pourcentages en poids en chrome, molybdène, silicium, nickel, carbone, azote, cuivre et manganèse respectent la relation suivante :
  40 ≤ IF ≤ 70 et de préférence 40 ≤ IF ≤ 60
  avec $$\mathrm{IF}=6\times \left(\%\mathrm{Cr}+1,32\times \%\mathrm{Mo}+1,27\times \%\mathrm{Si}\right)\mathrm{-}\mathrm{10}\mathrm{\times }\mathrm{(}\mathrm{\%}\mathrm{Ni}\mathrm{+}\mathrm{24}\mathrm{\times }\mathrm{\%}\mathrm{C}\mathrm{+}\mathrm{16}\mathrm{,}\mathrm{15}\mathrm{\times }\mathrm{\%}\mathrm{N}\mathrm{+}\mathrm{0.5}\mathrm{\times }\mathrm{\%}\mathrm{Cu}\mathrm{+}\mathrm{0}\mathrm{,}\mathrm{4}\mathrm{\times }\mathrm{\%}\mathrm{Mn}\mathrm{)}\mathrm{-}\mathrm{6}\mathrm{,}\mathrm{17}$$
  
• les pourcentages en poids en chrome, molybdène, azote et nickel et manganèse respectent la relation suivante :
  - IRCL ≥ 30,5 et de préférence IRCL ≥ 32
  avec $$\mathrm{IRCL}=\%\mathrm{Cr}+3,3\mathrm{\times }\mathrm{\%}\mathrm{Mo}\mathrm{+}\mathrm{16}\mathrm{\times }\mathrm{\%}\mathrm{N}\mathrm{+}\mathrm{2}\mathrm{,}\mathrm{6}\mathrm{\times }\mathrm{\%}\mathrm{Ni}\mathrm{-}\mathrm{0}\mathrm{,}\mathrm{7}\mathrm{\times }\mathrm{\%}\mathrm{Mn}$$
  
• la teneur en chrome est comprise entre 22 et 24% en poids,
• la teneur en manganèse est inférieure à 1,5% en poids,
• la teneur en calcium est comprise entre 0,0005 et 0,03% en poids.

The steel according to the invention may also comprise the following characteristics, taken separately or in combination:

• the proportion of ferrite is between 35 and 65% by volume, and preferably between 35 and 55% by volume.
• the percentages by weight of chromium, molybdenum, silicon, nickel, carbon, nitrogen, copper and manganese respect the following relation:
  40 ≤ IF ≤ 70 and preferably 40 ≤ IF ≤ 60
  with $$\mathrm{IF}=6\times \left(\%\mathrm{Cr}+1,32\times \%\mathrm{MB}+1,27\times \%\mathrm{Yes}\right)\mathrm{-}\mathrm{10}\mathrm{\times }\mathrm{(}\mathrm{\%}\mathrm{Or}\mathrm{+}\mathrm{24}\mathrm{\times }\mathrm{\%}\mathrm{VS}\mathrm{+}\mathrm{16}\mathrm{,}\mathrm{15}\mathrm{\times }\mathrm{\%}\mathrm{NOT}\mathrm{+}\mathrm{0.5}\mathrm{\times }\mathrm{\%}\mathrm{Cu}\mathrm{+}\mathrm{0}\mathrm{,}\mathrm{4}\mathrm{\times }\mathrm{\%}\mathrm{mn}\mathrm{)}\mathrm{-}\mathrm{6}\mathrm{,}\mathrm{17}$$
  
• the percentages by weight of chromium, molybdenum, nitrogen and nickel and manganese respect the following relation:
  - IRCL ≥ 30.5 and preferably IRCL ≥ 32
  with $$\mathrm{SWIR}=\%\mathrm{Cr}+3,3\mathrm{\times }\mathrm{\%}\mathrm{MB}\mathrm{+}\mathrm{16}\mathrm{\times }\mathrm{\%}\mathrm{NOT}\mathrm{+}\mathrm{2}\mathrm{,}\mathrm{6}\mathrm{\times }\mathrm{\%}\mathrm{Or}\mathrm{-}\mathrm{0}\mathrm{,}\mathrm{7}\mathrm{\times }\mathrm{\%}\mathrm{mn}$$
  
• the chromium content is between 22 and 24% by weight,
• the manganese content is less than 1.5% by weight,
• the calcium content is between 0.0005 and 0.03% by weight.

• on approvisionne un lingot ou une brame d'un acier de composition conforme à l'invention,
• on lamine ledit lingot ou ladite brame à chaud, à une température comprise entre 1150 et 1280 °C pour obtenir une tôle, une bande ou une bobine.

A second object of the invention is constituted by a method of a sheet, a strip or a hot-rolled steel coil according to the invention, according to which:

• supplying an ingot or a slab of a steel composition according to the invention,
• said billet or said slab is rolled at a temperature of between 1150 and 1280 ° C to obtain a sheet, a strip or a coil.

In a particular embodiment, said ingot or hot slab is rolled at a temperature of between 1150 and 1280 ° C. to obtain a so-called quarto sheet, and then a heat treatment is carried out at a temperature of between 900 and 1100 ° C. and said sheet is cooled by quenching in air.

• on approvisionne un lingot ou un bloom de coulée continue d'un acier de composition l'invention,
• on lamine à chaud ledit lingot ou ledit bloom, depuis une température comprise entre 1150 et 1280°C pour obtenir une barre que l'on refroidit à l'air ou une couronne de fil que l'on refroidit à l'eau,

puis, facultativement :

• on effectue un traitement thermique à une température comprise entre 900 et 1100°C, et
• on refroidit ladite barre ou ladite couronne par trempe.

A third object of the invention is constituted by a method for manufacturing a hot-rolled bar or wire made of steel according to the invention, according to which:

• supplying an ingot or a continuous casting bloom of a composition steel the invention,
• said ingot or said bloom is hot-rolled from a temperature of between 1150 and 1280 ° C. to obtain a bar which is cooled in air or a ring of wire which is cooled with water,

then, optionally:

• a heat treatment is carried out at a temperature between 900 and 1100 ° C, and
• said bar or said ring is cooled by quenching.

In a particular embodiment, it is also possible to perform a cold drawing of said bar or a drawing of said wire, after cooling.

The invention also covers a method of manufacturing a steel section, according to which a cold forming of a hot-rolled bar obtained according to the invention is carried out, as well as a method of manufacturing a forged part made of steel. steel, according to which a hot-rolled bar obtained according to the invention is fed in pieces, and then forging said billet between 1100 ° C. and 1280 ° C.

• les tôles d'acier laminée à chaud, dite quarto, et présentant une épaisseur comprise entre 5 et 100 mm, et les bandes et bobines, qui peuvent être utilisées pour la fabrication d'éléments de structures pour des installations de production de matière ou de production d'énergie, en particulier, pour des installations de productions de matière et d'énergie fonctionnant entre -100 et 300°C.
• les bandes d'acier laminées à froid pouvant être obtenues par laminage à froid d'une bobine laminée à chaud,
• les barres laminées à chaud présentant un diamètre de 18mm à 250 mm et les barres étirées à froid présentant un diamètre de 4 mm à 60 mm, ces produits pouvant être utilisés pour la fabrication de pièces mécaniques telles que des pompes, des axes de vannes, des axes de moteurs et des raccords fonctionnant dans des milieux corrosifs,
• les fils laminés à chaud présentant un diamètre de 4 à 30 mm et les fils tréfilés présentant un diamètre de 0,010 mm à 20 mm, ces produits pouvant être utilisés pour la fabrication d'assemblages formés à froid, pour l'industrie agro-alimentaire, l'extraction du pétrole et des minerais, ou pour la fabrication de tissus et tricots métalliques pour filtration de produits chimiques, de minerai ou de matières alimentaires,
• les profilés,
• les pièces forgées pouvant être utilisées pour la fabrication de brides ou de raccords,
• les pièces moulées pouvant être obtenues par moulage d'un acier selon l'invention.

The invention furthermore covers various products that can be obtained by the methods according to the invention as well as their uses, such as:

• hot-rolled steel plate, known as quarto, having a thickness of between 5 and 100 mm, and strips and coils, which may be used for the manufacture of structural elements for production plants of material or energy production, in particular, for material and energy production installations operating between -100 and 300 ° C.
• cold rolled steel strips obtainable by cold rolling a hot rolled coil,
• hot rolled bars having a diameter of 18mm to 250mm and cold drawn bars having a diameter of 4mm to 60mm, these products can be used for the manufacture of mechanical parts such as pumps, valve shafts, motor shafts and connectors operating in corrosive environments,
• hot-rolled yarns with a diameter of 4 to 30 mm and wire drawn with a diameter of 0.010 mm to 20 mm, these products may be used for the manufacture of cold-formed assemblies for the food industry, the extraction of petroleum and ores, or for the manufacture of fabrics and metal knits for the filtration of chemicals, minerals or food materials,
• the profiles,
• forgings that can be used for the manufacture of flanges or fittings,
• the molded parts that can be obtained by molding a steel according to the invention.

Other features and advantages of the invention will appear on reading the description which follows, given solely by way of example.

The duplex stainless steel according to the invention comprises the contents defined below.

The carbon content of the grade is less than or equal to 0.05% and preferably less than 0.03% by weight.

The chromium content of the grade is between 21 and 25% by weight, preferably between 22 and 24% by weight in order to obtain a good resistance to corrosion, which is at least equivalent to that obtained with the shades of the type 304 or 304L.

The nickel content of the grade is between 1 and 2.95% by weight. This austenite forming element is voluntarily maintained at a low level because of its cost. It is added in order to obtain good properties of resistance to the formation of corrosion cavities and to obtain a good compromise resilience / ductility. It has indeed the advantage of translating the transition curve of the resilience to low temperatures, which is particularly advantageous for the manufacture of thick quarto plates for which the properties of resilience are important. As the nickel content is limited, in the steel according to the invention, it has been found that, in order to obtain a suitable austenite content after heat treatment between 900 ° C. and 1100 ° C., other elements must be added. austenite formers in unusually high amounts and to limit the contents of ferrite-forming elements.

The nitrogen content of the grade is between 0.16 and 0.28%, which generally implies that nitrogen is added to the steel during processing. This austenite forming element first makes it possible to obtain a two-phase ferrite + austenite duplex steel containing a proportion of austenite suitable for good resistance to stress corrosion, and also to obtain high mechanical characteristics for the metal. It still allows to have a good microstructural stability in the heat-affected zone of the welded zones. Its maximum content is limited because, beyond 0.28%, solubility problems can be observed: formation of blowholes during the solidification of slabs, blooms, ingots, moldings or welds.

The manganese content, also austenite-forming element below 1150 ° C, is maintained below 2.0% by weight, and preferably below 1.5% by weight, because of the adverse effects of this element on many points. Thus, it poses problems in the development and refinement of the grade, because it attacks some refractories used for the pockets, which requires a more frequent replacement of these expensive elements and therefore more frequent interruptions of the process. The ferro-manganese additions normally used to make up the grade also contain significant levels of phosphorus, and also selenium, which are not desired to be introduced into the steel and which are difficult to remove when refining the shade. Manganese disrupts this refining by limiting the possibility of decarburization. It is also a problem further downstream in the process because it deteriorates the corrosion resistance of the grade due to the formation of MnS manganese sulfides, and oxidized inclusions. This element was traditionally added to the nuances that one wished to enrich with nitrogen, in order to increase the solubility of this element in the shade. Without a sufficient manganese content, it was therefore not possible to achieve such a level of nitrogen in the steel. The present inventors, however, have found that it is possible to limit the addition of manganese in the steel according to the invention, while adding enough nitrogen to obtain the desired effect on the ferrite-austenite equilibration of the metal of the invention. base and stabilization of thermally affected areas of welded areas.

Molybdenum, a ferrite-forming element, is maintained at less than 0.45% by weight, as tungsten is maintained at less than 0.15% by weight. Moreover, the contents of these two elements are such that the sum Mo + W / 2 is less than 0.50% by weight, preferably less than 0.4% by weight and particularly preferably less than 0.3%. in weight. Indeed, the present inventors have found that by keeping these two elements, as well as their sums, below the values indicated, there was no observing intermetallic precipitation embrittlement, which allows in particular to de-constrain the manufacturing process steel sheets or strips in allowing air cooling of the sheets and strips after heat treatment or hot work. In addition, they observed that by controlling these elements within the limits claimed, the weldability of the grade was improved.

Copper, an austenite forming element, is present in a content of between 0.11 and 0.50% by weight, and preferably between 0.15 and 0.40% by weight. This element improves the resistance to corrosion in a reducing acid medium. However, its content is limited to 0.50% by weight to avoid the formation of epsilon phases that it is desired to avoid, because they cause hardening of the ferritic phase and embrittlement of the duplex alloy.

The oxygen content is preferably limited to 0.010% by weight in order to improve its forging ability.

Boron is an optional element that can be added to the grade according to the invention to the extent of 0.003% by weight, in order to improve its transformation when hot. In another embodiment, however, it is preferred to limit the boron content to less than 0.0005% by weight to limit the risks of cracking on welding and continuous casting.

Silicon, a ferrite-forming element, is present in a content of less than 1.4% by weight. Aluminum, a ferrite-forming element, is present at a content of less than 0.05% by weight and preferably of between 0.005% and 0.040% by weight in order to obtain inclusions of calcium aluminates with a low melting point . The maximum aluminum content is also limited in order to avoid excessive formation of aluminum nitrides. The action of these two elements silicon and aluminum is essentially to ensure good deoxidation of the steel bath during the preparation.

Cobalt, an austenite forming element, is maintained at a content of less than 0.5% by weight, and preferably less than 0.3% by weight. This element is a residual brought by the raw materials. It is limited particularly because of the handling problems it can pose after irradiation of parts in nuclear facilities.

The rare earths (designated REM) may be added to the composition in an amount of 0.1% by weight and preferably less than 0.06% by weight. These include cerium and lanthanum. We limit the contents in these elements as they are likely to form unwanted intermetallics.

Vanadium, a ferrite-forming element, may be added in the grade of up to 0.5% by weight and preferably less than 0.2% by weight in order to improve the cavernous corrosion resistance of the steel.

Niobium, a ferrite-forming element, may be added in the grade of 0.3% by weight and preferably less than 0.050% by weight. It improves the tensile strength of the grade, thanks to the formation of fine niobium nitrides. Its content is limited to limit the formation of coarse niobium nitrides.

Titanium, a ferrite-forming element, may be added in the grade of 0.1% by weight and preferably less than 0.02% by weight to limit the formation of titanium nitrides formed in the liquid steel in particular.

Calcium may also be added to the grade according to the invention to obtain a calcium content of less than 0.03% by weight, and preferably greater than 0.0005% by weight, in order to control the nature of the inclusions of calcium. oxides and improve machinability. The content of this element is limited because it is likely to form with sulfur calcium sulphides which degrade the properties of corrosion resistance.

The sulfur is maintained at a content of less than 0.010% by weight and preferably less than 0.003% by weight. As we saw earlier, this element forms sulphides with manganese or calcium, sulphides whose presence is detrimental to the resistance to corrosion. It is considered an impurity.

Magnesium addition up to a final content of 0.1% can be made to modify the nature of the sulfides and oxides.

The selenium is preferably maintained at less than 0.005% by weight because of its detrimental effect on the corrosion resistance. This element is generally added to the grade as impurities in ferro-manganese ingots.

Phosphorus is maintained at less than 0.040% by weight and is considered an impurity.

The rest of the composition consists of iron and impurities. In addition to those already mentioned above, mention may also be made of zirconium, tin, arsenic, lead or bismuth. The tin may be present in a content of less than 0.100% by weight and preferably less than 0.030% by weight to avoid welding problems. The arsenic may be present in a content of less than 0.030% by weight and preferably less than 0.020% by weight. The lead may be present in a content of less than 0.002% by weight and preferably less than 0.0010% by weight. The bismuth may be present in a content of less than 0.0002% by weight and preferably less than 0.00005% by weight. Zirconium may be present at 0.02%.

Moreover, the present inventors have found that, when the grades according to the invention are such that their percentages by weight of chromium, molybdenum, nitrogen, nickel and manganese respect the relationship below, they have a good resistance to localized corrosion. , ie formation of bites or caves: $$\mathrm{IFCL}=\%\mathrm{Cr}+3,3\mathrm{\times }\mathrm{\%}\mathrm{MB}\mathrm{+}\mathrm{16}\mathrm{\times }\mathrm{\%}\mathrm{NOT}\mathrm{+}\mathrm{2}\mathrm{,}\mathrm{6}\mathrm{\times }\mathrm{\%}\mathrm{Or}\mathrm{-}\mathrm{0}\mathrm{,}\mathrm{7}\mathrm{\times }\mathrm{\%}\mathrm{mn}\mathrm{\ge }\mathrm{30},5\mathrm{and\; preference}\ge 32$$

The microstructure of the steel according to the invention, in the annealed state, is composed of austenite and ferrite, which are preferably, after treatment of 1 hour at 1000 ° C., in a proportion of 35 to 65% by weight. ferrite volume and more preferably from 35 to 55% by volume of ferrite.

The present inventors have also found that the following formula conveniently accounts for the ferrite content at 1100 ° C: $$\mathrm{IF}=6\times \left(\%\mathrm{Cr}+1,32\times \%\mathrm{MB}+1,27\times \%\mathrm{Yes}\right)\mathrm{-}\mathrm{10}\mathrm{\times }\mathrm{(}\mathrm{\%}\mathrm{Or}\mathrm{+}\mathrm{24}\mathrm{\times }\mathrm{\%}\mathrm{VS}\mathrm{+}\mathrm{16}\mathrm{,}\mathrm{15}\mathrm{\times }\mathrm{\%}\mathrm{NOT}\mathrm{+}\mathrm{0}\mathrm{,}\mathrm{5}\mathrm{\times }\mathrm{\%}\mathrm{Cu}\mathrm{+}\mathrm{0}\mathrm{,}\mathrm{4}\mathrm{\times }\mathrm{\%}\mathrm{mn}\mathrm{)}\mathrm{-}\mathrm{6}\mathrm{,}\mathrm{17}$$

Thus, to obtain a ferrite content of between 35 and 65% at 1100 ° C., the IF number must be between 40 and 70.

In the annealed state, the microstructure does not contain other phases which would be harmful for its mechanical properties in particular, such as the sigma phase and other intermetallic phases. In the cold worked state, a part of the austenite may have been converted to martensite, depending on the effective deformation temperature and the amount of cold deformation applied.

In general, the steel according to the invention can be prepared and manufactured in the form of hot-rolled sheets, also called quarto plates, but also in the form of hot-rolled strips, from slabs or ingots and also under Cold rolled strip form from hot rolled strip. It can also be hot rolled into bars or wire-machines or into profiles or forged; these products can then be hot-formed by forging or cold-formed into drawn bars or profiles or into drawn wires. The steel according to the invention can also be used by molding followed or not by heat treatment.

In order to obtain the best possible performance, use will preferably be made of the process according to the invention which firstly comprises the supply of an ingot, slab or bloom of steel having a composition in accordance with the invention.

This ingot, this slab or this bloom are generally obtained by melting the raw materials in an electric furnace, followed by a vacuum reflow of the AOD or VOD type with decarburization. The grade can then be cast in the form of ingots, or in the form of slabs or blooms by continuous casting in a bottomless mold. It could also be envisaged to cast the shade directly in the form of thin slabs, in particular by continuous casting between counter-rotating rolls.

After supplying the ingot or slab or bloom, it is optionally heated to reach a temperature between 1150 and 1280 ° C, but it is also possible to work directly on the slab that has just been continuously cast, in the hot casting.

In the case of the manufacture of metal sheets, the slab or the slab is then hot-rolled to obtain a so-called quarto sheet which generally has a thickness of between 5 and 100 mm. The reduction rates generally used at this stage vary between 3 and 30%. This sheet is then subjected to a heat treatment of resuspension of precipitates formed at this stage by reheating at a temperature between 900 and 1100 ° C, and then cooled.

The method according to the invention provides an air quenching cooling which is easier to implement than the cooling conventionally used for this type of shade, which is a faster cooling, using water. However, it remains possible to cool with water if desired.

This slow cooling, in air, is made possible thanks to the particular balancing of the composition according to the invention which is not subject to the precipitation of compounds harmful to its properties of use.

At the end of the hot rolling, the quarto plate can be glued, cut and stripped, if it is desired to deliver it in this state.

This bare steel can also be rolled on a band train at thicknesses between 3 and 10 mm.

In the case of the production of long products from ingots or blooms, it is possible to hot roll in a single hot mill on a multi-cage mill, in corrugated rolls, at a temperature of between 1150 and 1280 ° C., to obtain a bar or a ring of wire rod or laminate. The section ratio between the initial bloom and the final product is preferably greater than 3, so as to ensure the internal health of the rolled product.

When a bar has been manufactured, it is cooled at the rolling outlet by simple air spreading.

When laminated wire with a diameter greater than 13 mm has been manufactured, it can be cooled by quenching in a ring of water at the outlet of the rolling mill.

When wire of diameter less than or equal to 13 mm has been manufactured, it can be cooled by quenching with water in turns spread on a conveyor after passing them on the conveyor in 2 to 5 minutes through a furnace. solution at a temperature between 850 ° C and 1100 ° C.

Subsequent heat treatment in the oven, between 900 ° C and 1100 ° C, can be performed optionally on these bars or rings already treated in the hot rolling, if it is desired to complete the recrystallization of the structure and slightly lower the mechanical characteristics in traction.

After the cooling of these bars or son rings, we can proceed to various hot forming treatments or cold, depending on the final use of the product. Thus, it will be possible to cold drawing bars or wire drawing son, after cooling.

It will also be possible to cold profile the hot-rolled bars, or even to manufacture parts after having cut the bars into billets and forged them.

• Figure 1: Corrélation entre % de ferrite après traitement à 1100°C et indice IF pour des produits bruts
• Figure 2 : Ecart diamétral relatif Delta ∅ en fonction de la température de déformation
• Figure 3 : Potentiels de piqûres E1 et E2 déterminés sur barreaux forgés en fonction de l'indice IRCL
• Figure 4 : Vitesse de corrosion uniforme V déterminée sur barreaux forgés en fonction de l'indice IRCL
• Figure 5 : Températures critiques CCT et CPT déterminées sur barreaux forgés en fonction de l'indice IRCL

In order to illustrate the invention, tests have been carried out and will be described, in particular with reference to FIGS. 1 to 5 which represent:

• Figure 1: Correlation between% ferrite after treatment at 1100 ° C and IF index for raw products
• Figure 2: Relative diameter difference Delta ∅ as a function of the deformation temperature
• Figure 3: Pitting potentials E1 and E2 determined on bars forged according to the IRCL index
• Figure 4: Uniform corrosion rate V determined on forged bars according to the IRCL index
• Figure 5: CCT and CPT critical temperatures determined on bars forged according to the IRCL index

Examples

25 kg laboratory ingots were made by vacuum induction melting of raw materials and pure ferroalloys, followed by the addition of nitrogen by adding nitrided ferroalloys under partial nitrogen pressure and cast in a pressurized metal mold. external 0.8 bar nitrogen. Of these, only tests 14441 and 14604 are in accordance with the invention.

An industrial casting according to the invention of 150 tons referenced 8768 was performed. This grade was developed by melting in the electric oven, then refined under vacuum with decarburization to reach the target carbon level. It was then continuously cast into slabs of section 220 x 1700 mm, then hot rolled after heating to 1200 ° C in so-called quarto plates of thickness 7, 12 and 20mm. The sheets thus obtained are then subjected to a heat treatment at about 1000 ° C. in order to put in solution the different precipitates present at this stage. At the end of the heat treatment, the sheets are cooled with water and then planed, cut and stripped.

The compositions in percentages by weight of the various grades developed in the laboratory or in an industrial way are collated in Table 1, as well as those of various industrial products or semi-finished products elaborated in electric furnace, refining with the AOD, casting in ingot or in continuous, mentioned for comparison.

1. Ferrite contents 1.1 Ferrite content on raw products

On pieces of 1 to 8 cm ³ cut in these casting laboratory in the raw state of casting or on industrial products in the raw state of casting, it was carried out in salt bath, with quenching with water in end of treatment, heat treatments of 30 minutes at variable temperature, to determine the proportion of ferrite at high temperature. Ferrite being magnetic, unlike austenite, carbides and nitrides may be present, a metering method was used to measure the saturation magnetization. The ferrite contents thus determined are reported in Table 2 and reported in FIG.

If we consider FIG. 1, a good correlation is found between the IF index and the ferrite contents measured on the base metal after treatments at 1100 ° C.

It is furthermore noted that the casting according to the invention, No. 14441, has, below 1300 ° C., a ferrite content suitable for the hot conversion into a duplex structure. In addition, after treatment in the range of 950 ° C to 1100 ° C, it has a ferrite content suitable for resistance to stress corrosion. <b> Table 2 </ b> cast 14382 14383 14441 14426 14422 14425 14424 140301 436002 517077 533054 150091 Product Ingot Ingot Ingot Ingot Ingot Ingot Ingot BCC BCC BCC BCC BCC Gross state 55.6 50.5 52.6 50.3 25.4 + 900 ° C 45.6 89.5 54.4 71.2 98.7 100 91.9 45 51.0 47.2 20.5 + 950 ° C 48.7 87.1 51.7 71.1 98.8 99.6 94.6 42.8 48.9 46.1 25.4 + 1000 ° C 50.9 90.0 54.5 71.8 99.4 99.4 93.4 50.8 42.1 50.7 46.0 28.8 + 1050 ° C 55.7 81.0 53.0 77.8 98.6 99.1 78.8 54 44.2 54.6 48.3 33.7 + 1100 ° C 60.8 84.6 55.5 82.0 99.0 87.4 75.4 58.6 47.6 59.4 51.3 36.1 + 1150 ° C 65.2 88.6 59.0 88.1 98.9 75.6 78.1 64.6 52.7 66.7 57.9 41.1 + 1200 ° C 76.6 94.2 64.0 95.4 98.8 78.4 71.6 59.3 75.5 64.8 46.7 + 1250 ° C 92.3 98.1 67.7 100 99.2 81.0 86.2 81.5 67.4 86.0 73.2 55.1 + 1300 ° C 95.2 97.7 72.6 99.4 98.7 85.9 93.5 100 78.3 99.0 85.0 66.4 BCC = continuous casting bloom

1.2. Ferrite content on finished products

The ferrite content was also measured by the grid method (according to ASTM E 562) on forged bars after heat treatment at 1030 ° C and on thermally affected areas of electrode deposited weld seams with constant energy. leading to cooling rates of 20 ° C / sec at 700 ° C. The results (ferrite contents of base metal and heat-affected zone) are given in Table 3. It can be seen that the flows 14441 and 14604 according to the invention have a ferrite content in the base metal and in the affected zone. thermally favoring resistance to localized and stressed corrosion, as well as resilience (see Table 5). <b> Table 3 - Ferrite contents </ b> REF Product α MB (%) α ZAT (%) 14441 * Forged bar 48 70 14604 * Forged bar 54 65 14382 Forged bar 49 80 14383 Forged bar 79 88 14660 Forged bar 48 72 UNS S32101 LAC sheet 45 67 UNS S32304 LAC sheet 47 75 *: according to the invention
LAKE: Hot rolled; NA: not applicable
α MB (%): ferrite content measured on the base metal
α ZAT: ferrite content measured on the thermally affected zone

2. Flowability

The 14439 ingot has blistered and is unusable. To avoid this phenomenon during airflows at atmospheric pressure, it is therefore necessary to limit the nitrogen content of the castings according to the invention to less than 0.28% by weight.

3. Hot processing capacity

The hot deformation capacity was evaluated using hot tensile tests carried out on specimens whose calibrated portion, 8 mm in diameter and 5 mm in length, is heated by Joule effect for 80 seconds at 1280. ° C, then cooled to 2 ° C per second until the test temperature varies between 900 and 1280 ° C. When this temperature is reached, the rapid pull is immediately triggered at a speed of 73 mm / s; after breaking, the necking diameter is measured at the fracture.

Tableau 4 : écarts diamétraux relatifs (essais de traction à chaud) Température d'essai (°C) Delta ∅ (en %) coulée14382 coulée 14383 coulée 14441* 1280 85,0 100,0 96,7 1250 98,3 86,7 1200 75,0 98,3 76,7 1150 70,0 95,0 61,7 1100 63,3 93,3 56,7 1050 51,7 75,0 44,2 1010 45,0 1000 65,0 40,0 980 36,7 960 58,3 950 35,8 900 35,0 51,7 36,7 * : selon l'invention The relative diametric difference (Table 4), as defined below, accounts for the heat distortion capability: $$\mathrm{Delta\; \varnothing }=100\times \left(1-\left(\mathrm{final\; diameter}/\mathrm{initial\; diameter}\right)\right)$$

<b> Table 4 </ b>: relative diametric deviations (hot tensile tests) Test temperature (° C) Delta ∅ (in%) coulée14382 casting 14383 casting 14441 * 1280 85.0 100.0 96.7 1250 98.3 86.7 1200 75.0 98.3 76.7 1150 70.0 95.0 61.7 1100 63.3 93.3 56.7 1050 51.7 75.0 44.2 1010 45.0 1000 65.0 40.0 980 36.7 960 58.3 950 35.8 900 35.0 51.7 36.7 *: according to the invention

It can be seen from Table 4 and Figure 2 which represents the data in the form of curves that casting 14441 according to the invention has a hot deformation capacity comparable to that of the comparative reference casting No. 14382.

4. Mechanical properties

The tensile properties Re _0.2 and R _m were determined according to the NFEN 10002-1 standard. The KV resilience was determined at different temperatures according to the NF EN 10045 standard. <b> Table 5 - Mechanical characteristics </ b> REF Product Re _0.2 (MPa) R _m (MPa) KV 20 ° C (J) KV -50 ° C (J) 14441 * Forged bar 477 716 334 51 14604 * Forged bar 477 691 288 18 14382 Forged bar 436 664 > 339 339 14383 Forged bar 458 604 79 9 14660 Forged bar 493 701 293 31 304L LAC sheet 218 523 312 301 316L LAC sheet 232 537 307 298 UNS S32101 LAC sheet 466 720 101 60 UNS S32304 LAC sheet 438 663 268 153 8768 * LAC sheet 519 743 *: according to the invention
LAKE: Hot rolled; NA: not applicable
Re _0.2 : yield strength at 0.2% deformation
R _m : breaking strength.

The results of laboratory flows 14441 and 14604 and industrial casting 8768, all three according to the invention, show that a yield strength greater than 450 MPa, twice that obtained for austenitic steels of the AISI 304L type, can be used. obtained.

The resilience values determined at 20 ° C. for laboratory flows 14441 and 14604 and industrial casting 8768, all three according to the invention, are all greater than 200 J, which is satisfactory. given the level of the elastic limit of these grades. For casting 14383 outside the invention, low nitrogen content and high ferrite content in the annealed state, the resilience values at 20 ° C are less than 100 J. This confirms the need for a sufficient nitrogen addition for to obtain a satisfactory level of tenacity.

5. Corrosion resistance

Corrosion resistance tests were carried out both on the bars forged from laboratory laps and on coupons taken from hot-rolled sheets from industrial castings.

5.1 Resistance to localized corrosion

The resistance to pitting corrosion is evaluated by plotting the curves potential intensities and determination of the pitting potential for i = 100μA / cm ² . This parameter was measured in a strongly chlorinated neutral medium (pH = 6.4) ([CI-] = 30g / l) at 50 ° C (E ₁ ), representative of the brines encountered in water desalination plants. sea, and in a slightly acidic medium (pH = 5.5) weakly chlorinated ([Cl-] = 250ppm) at room temperature (E ₂ ), representative of a drinking water. The critical pitting temperature in medium ferric chloride (FeCl ₃ 6%) was also determined according to ASTM G48-00 method C.

In another series of tests, the pitting resistance was determined in deaerated neutral medium at 0.86 Moles / liter NaCl, corresponding to 5% by weight NaCl, at 35 ° C. A measurement of the abandonment potential for 900 seconds is carried out. Then, a potentiodynamic curve is plotted at a speed of 100 mV / min from the abandonment to the pitting potential. The pitting potential (E ₃ ) is determined for i = 100 μA / cm ² . Samples according to the invention were tested under these conditions, as well as reference samples in grade 304L and austenitic-ferritic duplex grades type 1.4362 and others.

Resistance to crevice corrosion was studied by measuring the critical cavern temperature in the highly chlorinated neutral medium (pH = 6.4) ([CI-] = 30g / l). The mounting to promote crevice corrosion is in accordance with the recommendations made in ASTM G78-99. The critical cave temperature is the minimum temperature for which cavities deeper than 25 μm have been observed.

The values obtained are shown in Table 6. The comparison between the results obtained on the UNS sheet S32304 and the bar resulting from casting 14382, both of similar chemical composition, indicates that the corrosion resistance of a bar is more weak than that of a hot-rolled sheet of the same composition.

(teneurs en Cr, Mo, N , Ni et Mn en % en poids)
rend bien compte du classement de l'ensemble de compositions à moins de 6% de nickel en tenue à la corrosion localisée (voir figures 3, 4 et 5).The present inventors have found that the index of resistance to localized corrosion, ie formation of pits or caverns, abbreviated by IRCL and defined by: $$\mathrm{SWIR}=\mathrm{Cr}+3,3\times \mathrm{MB}+16\times \mathrm{NOT}+2,6\times \mathrm{Or}-0,7\times \mathrm{mn}$$

(Cr, Mo, N, Ni and Mn contents in% by weight)
gives a good account of the classification of the set of compositions containing less than 6% nickel in localized corrosion resistance (see FIGS. 3, 4 and 5).

Castings 14383 and 14660 outside the invention, IRCL indices equal to 28.7 and 29.8, behave less well in corrosion than a steel type AISI 304L. Castings 14604 and 14441, according to the invention, of IRCL 30,9 and 33, behave at least as well as type 304L steel. To obtain a corrosion resistance at least equal to that of the grade AISI 304L, we have found that the steels according to the invention should preferably have an IRCL greater than 30.5 and preferably greater than 32.

5.2 Uniform corrosion resistance

Uniform corrosion was characterized by evaluating the loss of mass corrosion rate after immersion for 72 hours in a 2% sulfuric acid solution heated to 40 ° C.

The comparison of the corrosion rates for the experimental flows at 2.5% Ni and 0.2% N (14441, according to the invention, and 14660, excluding the invention) also shows the negative effect of a high Mn content on the resistance to uniform corrosion in sulfuric medium. <b> Table 6 </ b> localized and uniform corrosion resistance data REF Product SWIR E ₁ (V / ECS) E ₂ (V / ECS) E ₃ (V / ECS) CPT (° C) CCT (° C) V (mm / year) 14441 * Forged bar 33.0 0,165 1,058 0,320 7.5 50 0.73 14604 * Forged bar 30.9 0,159 0.802 5 45 1.8 14382 Forged bar 35.8 0.302 1,323 0,420 15 60 0.24 14383 Forged bar 28.7 0.049 0.595 0,050 0 35 4.95 14660 Forged bar 29.8 0,094 0.707 7.5 45 1.11 304L LAC sheet N / A 0.188 0.834 0.210 5 65 316L LAC sheet N / A 0.266 0.865 7.5 75 UNS S32101 LAC sheet 26.4 0.163 0.855 12.5 UNS S32304 LAC sheet 35.7 0.413 1,330 ¹ 17.5 95 517077 Rolled bar 34.6 0.415 140301 Rolled bar 47.1 1,200 ¹ 8768 * LAC sheet 33.1 0.227 1,273 ¹ *: according to the invention
¹ : potential for oxidation of the solvent, no sting observed
LAKE: hot rolled; NA: not applicable
E ₁ : pitting potential in neutral medium (pH = 6.4) and strongly chlorinated (30 g / l Cl ^- ) at 50 ° C.
E ₂ : pitting potential in slightly acid (pH = 5.5) and weakly chloride (250 ppm Cl ^- ) at 25 ° C
E ₃ : pitting potential in neutral and chlorinated medium (NaCl 5%) at 35 ° C.
CPT: critical temperature of puncture in ferric chloride medium
CCT: critical cavern temperature in neutral (pH = 6.4) and strongly chlorinated (30g / l Cl ^- )
V: uniform corrosion rate in sulfuric acid medium 2% at 40 ° C

Claims (28)

Duplex stainless steel, the composition of which consists of, in% by weight: C ≤ 0.05% 21% ≤ Cr ≤ 25% 1% ≤ Ni ≤ 2.95% 0.16% ≤ N ≤ 0.28% Mn ≤ 2.0% Mo + W / 2 ≤ 0.50% Mo ≤ 0.45% W ≤ 0.15% If ≤ 1.4% Al ≤ 0.05% 0.11% ≤ Cu ≤ 0.50% S ≤ 0.010% P ≤ 0.040% B ≤ 0.0005% Co ≤ 0.5% REM ≤ 0.1% V ≤ 0.5% Ti ≤ 0.1% Nb ≤ 0.3% Mg ≤ 0.1% the remainder being iron and impurities resulting from the preparation and the microstructure consisting of austenite and ferrite. Steel according to claim 1, further characterized in that the proportion of ferrite is between 35 and 65% by volume. Steel according to claims 1 or 2, further characterized in that the percentages by weight of chromium, molybdenum, silicon, nickel, carbon, nitrogen, copper and manganese respect the following relationship:
40 ≤ IF ≤ 70
with $$\mathrm{IF}=6\times \left(\%\mathrm{Cr}+1,32\times \%\mathrm{MB}+1,27\times \%\mathrm{Yes}\right)\mathrm{-}\mathrm{10}\mathrm{\times }\mathrm{(}\mathrm{\%}\mathrm{Or}\mathrm{+}\mathrm{24}\mathrm{\times }\mathrm{\%}\mathrm{VS}\mathrm{+}\mathrm{16}\mathrm{,}\mathrm{15}\mathrm{\times }\mathrm{\%}\mathrm{NOT}\mathrm{+}\mathrm{0}\mathrm{,}\mathrm{5}\mathrm{\times }\mathrm{\%}\mathrm{Cu}\mathrm{+}\mathrm{0}\mathrm{,}\mathrm{4}\mathrm{\times }\mathrm{\%}\mathrm{mn}\mathrm{)}\mathrm{-}\mathrm{6}\mathrm{,}\mathrm{17}$$

Steel according to claims 2 or 3, further characterized in that the proportion of ferrite is between 35 and 55% by volume. Steel according to claims 2 or 3, further characterized in that
40 ≤ IF ≤ 60 Steel according to any one of claims 1 to 5, further characterized in that the percentages by weight of chromium, molybdenum, nitrogen and nickel and manganese respect the following relationship:
IRCL ≤ 30.5
with $$\mathrm{SWIR}\mathrm{=}\mathrm{\%}\mathrm{Cr}\mathrm{+}\mathrm{3}\mathrm{,}\mathrm{3}\mathrm{\times }\mathrm{\%}\mathrm{MB}\mathrm{+}\mathrm{16}\mathrm{\times }\mathrm{\%}\mathrm{NOT}\mathrm{+}\mathrm{2}\mathrm{,}\mathrm{6}\mathrm{\times }\mathrm{\%}\mathrm{Or}\mathrm{-}\mathrm{0}\mathrm{,}\mathrm{7}\mathrm{\times }\mathrm{\%}\mathrm{mn}$$

Steel according to claim 6, further characterized in that IRCL ≥ 32. Steel according to any one of claims 1 to 7, further characterized in that the chromium content is between 22 and 24% by weight. Steel according to any one of claims 1 to 8, further characterized in that the manganese content is less than 1.5% by weight. Steel according to any one of claims 1 to 9, further characterized in that the calcium content is between 0.0005 and 0.03% by weight. A method of manufacturing a hot-rolled steel sheet, strip or coil according to any one of claims 1 to 10, wherein: an ingot or slab of a composition steel is supplied according to any one of claims 1 to 10 said slug or slab is rolled while hot at a temperature of between 1150 and 1280 ° C. in order to obtain a sheet, a strip or a coil. A method of manufacturing a hot rolled steel sheet according to claim 11, wherein: said billet or hot slab is rolled at a temperature of between 1150 and 1280 ° C. to obtain a so-called quarto sheet, and then a heat treatment is carried out at a temperature of between 900 and 1100 ° C., and said sheet is cooled by quenching in air. A hot-rolled steel sheet, called quarto, obtainable by the process according to claim 12 and having a thickness of between 5 and 100 mm. Use of a so-called quarto sheet according to claim 13 or a hot rolled coil obtained by the process according to claim 11 for the manufacture of structural elements for material production or energy production installations. Use according to claim 14, wherein said material and energy production facilities operate between -100 and 300 ° C. A cold rolled steel strip obtainable by cold rolling a hot rolled coil obtained by the method of claim 11. A method of manufacturing a hot rolled steel bar or wire according to any one of claims 1 to 10, wherein: supplying an ingot or bloom of continuous casting of a steel of composition according to any one of claims 1 to 10, said ingot or said bloom is hot-rolled from a temperature of between 1150 and 1280 ° C. to obtain a bar which is cooled in air or a ring of wire which is cooled with water, then, optionally: a heat treatment is carried out at a temperature of between 900 and 1100 ° C., and said bar or said ring is cooled by quenching. A hot rolled bar obtainable by the method of claim 17 and having a diameter of 18mm to 250mm and hot rolled wire obtainable by the method of claim 17 and having a diameter of 4 to 30mm. Manufacturing method according to claim 17, wherein is carried out a cold drawing of said bar or a drawing of said wire, after cooling. A cold drawn bar obtainable by the process according to claim 19 having a diameter of 4 mm to 60 mm and drawn wire obtainable by the process according to claim 19 having a diameter of 0.010 mm to 20 mm. Use of a bar according to claims 18 or 20 for the manufacture of mechanical parts such as pumps, valve shafts, motor shafts and connectors operating in corrosive environments. Use of a yarn according to claims 18 or 20 for the manufacture of cold-formed assemblies, for the food industry, the extraction of petroleum and ores, or for the manufacture of fabrics and knits of metal for filtration purposes. chemicals, ore or food materials. A method of manufacturing a steel profile, in which a cold roll forming of a hot rolled bar obtained by the method according to claim 17 is carried out. Steel profile obtainable by the method of claim 23. A method of manufacturing a steel forging, wherein a hot-rolled bar obtained by the process according to claim 17 is slugged into billets and then forging said billet between 1100 ° C and 1280 ° C. Forging made of steel obtainable by the process according to claim 25. Use of a forging according to claim 26 for the manufacture of flanges or couplings. A molded part obtainable by molding a steel according to any one of claims 1 to 10.

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