EP2038445A2 - Duplex stainless steel - Google Patents

Abstract

The invention relates to a duplex stainless steel composition consisting, in percent by weight, of: 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%; Co = 0.5%; REM = 0.1%; V = 0.5%; Ti = 0, 1%; Nb = 0.3%; Mg < 0.1%, the balance being iron and impurities resulting from the smelting, and the microstructure consisting of austenite and 35 to 65% ferrite by volume, the composition furthermore satisfying the following relationships: 40 = IF < 70 with IF = 6´(%Cr + 1.32´%Mo + 1.27´%Si) - 10´(%Ni + 24´%C + 16.15´%N + 0.5´%Cu + 0.4´%Mn) - 6.17 and IRCL > 30.5, where IRCL = %Cr +3.3´%Mo +16´%N + 2.6´%Ni - 0.7´%Mn, and to a process for manufacturing plates, sheets, coils, bars, wires, sections, forgings and castings made of this steel.

Description

 Duplex stainless steel

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 forgings.

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 the steels 1.4362, 1.4655, 1.4477, 1.4462, 1.4507, 1.4410, 1.4501 and 1.4424 of the EP10088 standard, which all 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 purpose of the present invention is to overcome the drawbacks 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 method of manufacturing sheets, strips, bars, wires, or profiles made of this steel which is of simplified implementation.

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% 0.11% ≤ Cu <0.50% S <0.010% P <0.040% Co <0.5% REM <0.1%

V ≤ 0.5% Ti ≤ 0.1% Nb ≤ 0.3% Mg ≤ 0.1% the rest being iron and impurities resulting from the preparation and the microstructure consisting of austenite and 35 to 65 % ferrite by volume, the composition also respecting the following relationships:

40 ≤ IF ≤ 70 and preferably 40 ≤ IF <60 with

IF = 6 x (% Cr + 1, 32 x% Mo + 1, 27x% Si) - 10x (% Ni + 24 x% C

+ 16.15 x% N + 0.5 x% Cu + 0.4 x% Mπ) - 6.17 and

IRCL> 30.5 and preferably> 32 with

IRCL -% Cr +3.3 x% Mo + 16 x% N + 2.6 x% Ni - 0.7 x% Mn

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

the proportion of ferrite is between 35 and 55% by volume,

the chromium content is between 22 and 24% by weight,

the manganese content is less than 1.5% by weight;

the calcium content is less than 0.03% by weight; the molybdenum content is greater than 0.1% by weight; 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 of composition in accordance with the invention,

said billet or hot 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. ₁ and said sheet is cooled by quenching in air.

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 according to the invention,

said billet or said bloom is hot-rolled from a temperature of between 1150 and 128 ^° 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.

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 5 obtained in accordance with the invention is fed in pieces, then forging said billet between 1100 ^β C and 128O ⁰ C.

The invention also covers various products that can be obtained by the processes according to the invention as well as their uses, such as: - hot-rolled steel sheets, called quarto, and having a thickness of between 5 and 100 mm, and strips and coils, which may be used for the manufacture of structural elements for material production or energy production installations, in particular, for material and energy production installations operating between

-100 and 300 ^° C. and preferably between -50 and 300 ^° C.,

- cold rolled steel strips obtainable by cold rolling a hot rolled coil,

- hot rolled bars with a diameter of 18mm to 250mm and cold drawn bars with a diameter of 4mm to 60mm, these products can be used for the manufacture of mechanical parts such as pumps, valve spindles , motor shafts and connectors operating in corrosive media, - hot-rolled wire 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 cold-formed assemblies for the food industry, petroleum and mineral extraction, or for the manufacture of metal fabrics and knits for the filtration of chemicals, minerals or foodstuffs. - 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. In fact, an excessively high content of this element degrades the resistance to localized corrosion by increasing the risk of precipitation of chromium carbides in the heat-affected zones of the welds.

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, and is preferably less than or equal to 2.7, or even 2.5% by weight. This austenite forming element is added in order to obtain good resistance properties to the formation of corrosion cavities. At levels of greater than 1% and preferably greater than 1.2% by weight, it has a favorable effect in combating the initiation of pitting corrosion. However, its content is limited because beyond 2.95% by weight, degradation of the resistance to the propagation of these pits is observed. Its addition also provides a good compromise resilience / ductility. It has the advantage of translating the transition curve from resilience to low temperatures, which is particularly advantageous for the manufacture of thick quarto plates for which the properties of resilience are important.

The nickel content is limited, in the steel according to the invention, it was found that it was appropriate to obtain an appropriate austenite content after heat treatment between 900 ° C and 1100 ⁰ C ₁ to add other elements austenite formers in usually 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 115O ⁰ 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 area of corrosion. 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 sheets or strips of steel by 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. It is however preferred to maintain a minimum molybdenum content of 0.1% in order to improve the heat forgeability of the grade. In addition, the development of a grade having less than 0.1% molybdenum would greatly limit the use of recycling scrap for this grade, which poses implementation problems, in particular requiring the use of a 100% filler consisting of pure ferroalloys.

Copper, austenite-forming element, is present in a content of between 0.11 and 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 which may be added to the grade according to the invention in a content of between 0.0005% and 0.01% by weight, preferably between 0.0005% and 0.005% and more particularly preferably between 0.0005% and 0.003% by weight, in order to improve its heat conversion. 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. It is 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 0.0002% or even greater than 0.0005% by weight, in order to control the nature of oxide inclusions 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. In a preferred embodiment, the calcium content is limited to less than 0.0005% by weight and preferably less than 0.0002%.

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 due to its corrosion. This The element is usually 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%.

Furthermore, the present inventors have found that, when the percentages by weight of chromium, molybdenum, nitrogen, nickel and manganese respect the relationship below, the grades concerned have a good resistance to localized corrosion, ie to the formation of bites or caves:

IRCL =% Cr + 3.3 X% Mo + 16 x% N + 2.6 x% Ni - 0.7 x% Mn> 30.5

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 gives a good account of the ferrite content at 1100 ^° C.: IF = 6 X (% Cr + 1.32 x% M + 1.27 x% Si) - 10 x (% Ni + 24 x% C + 16.15 x% N + 0.5 x% Cu + 0.4 x% Mn) - 6.17

Thus, to obtain a proportion of ferrite between 35 and 65% at 1100 ⁰ C ₁ 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 iπtermétaϋiques phases. In the cold worked state, some 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 implemented 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. One could also consider casting 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 ⁰ 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 ⁰ C, and then cooled. The method according to the invention provides cooling by air quenching 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 limited contents of nickel and molybdenum of the composition according to the invention which is not subject to the precipitation of intermetallic phases, harmful for its properties of use. This cooling can in particular be carried out at speeds ranging from 0.1 to 2.7 ^o C / s. 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 between 1150 and 128O ⁰ C, for me of wire rod or the mine. 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 dissolution at a temperature between 850 ⁰ C and 1100 ⁰ C.

Subsequent heat treatment in the oven, between 900 * C and 1100 ⁰ C ₁ may 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 wire rings, we can proceed to different shaping treatments hot 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.

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: FIG. 1: Correlation Between% of Ferrite After Treatment with

1100 ⁶ C and IF index for raw products - Figure 2: Relative diameter difference Delta 0 as a function of the deformation temperature

- Figure 3: Pitting potential Ë1 and E2 determined on forged bars according to the IRCL index - Figure 4: uniform corrosion rate V determined for bars forged 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 220 x 1700 mm section, then hot rolled after heating to 1200 ⁰ C in so-called quarto plates of thickness 7, 12 and 20 mm. The sheets thus obtained are then subjected to heat treatment at 1000 ^° C. in order to put into solution the various 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 different grades developed in the laboratory or in an industrial way are collated in Table 1, as well as those of different products or developed industrial semi-finished products in an electric furnace, refining I ^AOD, casting into ingots or continuously referred to 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 hot conversion into a duplex structure. Further, after treatment in the field ^D 950 C to 1100 C ^β, it has an appropriate ferrite content in resistance to stress corrosion.

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 1030X and on thermally affected areas of electrode-deposited weld seams with constant energy leading to cooling rates from 20 ^β C / s to 700X. 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).

Table 3 - Ferrite contents

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 128 ° C. ⁰ C, then cooled to 2 ° C per second to the test temperature which varies between 900 and 128O ⁰ 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.

The relative diametric difference (Table 4), as defined below, accounts for the heat distortion capability:

Delta 0 = 100 x (1 - (final diameter / initial diameter)

Table 4: Relative Diameter Deviations (Hot Tensile Testing)

* according to the invention

It can be seen from Table 4 and FIG. 2, which represents the data in the form of curves, that the casting 14441 according to the invention has a hot deformation capacity comparable to that of the comparative reference casting n ^β 14382 .

4. Mechanical properties

The tensile properties Re _Oι 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.

Table 5 - Mechanical characteristics

according to the invention

LAKE: Hot rolled;

Re _{o, 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 the laboratory flows 14441 and 14604 and the industrial casting 8768, all three according to the invention, are all greater than 200 J, which is satisfactory in view of the level of the limit of elasticity of these shades. For casting 14383 outside the invention, with a low nitrogen content and high ferrite content in the annealed state, the 20 ^β C resistivity values are less than 100 J. This confirms the necessity of a sufficient nitrogen addition for to obtain a satisfactory level of tenacity.

5. Corrosion resistance

Corrosion resistance tests were carried out both on forging bars 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 ([CI-] = 250ppm) at room temperature (E2), representative of a drinking water. The critical temperature of pitting in ferric chloride medium (FeCl ₃ 6%) was also determined according to ASTM G48-00 method C.

In another series of tests, the resistance was determined to corrosion in neutral medium bite deaerated 0.86 Moles / liter NaCl, corresponding to 5% by weight NaCl ₁ to 35 ⁰ C. A measure of the potential abandonment for 900 seconds is performed. Then, a potentiodynamic curve is plotted at a speed of 100 mV / min from the abandonment to the pitting potential. The pitting potential (E3) is determined for i = 100 μA / cm ² . Samples according to the invention were tested under these conditions, as well as reference samples in 304L grade and austenitic-ferritic duplexes 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 greater than 25 μm 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.

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:

IRCL = Cr + 3.3 x Mo + 16 x N + 2.6 x Ni - 0.7 x Mn (contents of Cr, Mo ₁ N, Ni and Mn 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, of IRCL indices equal to 28.7 and 29.8, behave less well in corrosion than a steel of the type

AISI 304L. The 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 AISI grade

304L ₁ 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 rate of mass loss corrosion 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. Table 6 - Localized and Uniform Corrosion Resistance Data

according to the invention

1. solvent oxidation potential, no sting observed LAC: hot rolled; NA: not applicable

E ₁ pxtentiel puncture in neutral medium (pH = 6.4) and strongly chlorinated

(30 g / l CI) at 50 ^° C.

-2 • pitting potential in a slightly acid medium (pH = 5.5) and weakly chlorinated (250 ppm CI ^' ) at 25 ^° C.

E ₃ : pitting potential in neutral and chlorinated medium (NaCl 5%) at 35 ° C CPT: critical puncture temperature in ferric chloride medium CCT: critical cavern temperature in a neutral medium (pH = 6.4) and strongly chlorinated ( 30 g / l of CI ^" ) V: uniform corrosion rate in 2% sulfuric acid medium at 40 ^° C. 5.3 Repassivation Potential

The steel samples are polished under water with SiC papers until 1200, and then aged for 24 hours in the air.

The cyclic polarization test carried out in a chlorinated medium is carried out starting with a measurement of the abandoning potential for 15 minutes, followed by a cyclic dynamic polarization at 100 mV / min from the abandon potential to the potential for which the current reaches the intensity of 300μA / cm ² and the return to the potential for which the current 0 is zero.

The values of the puncture potentials (Vpit) and the repassivation potentials (Vrepassivation) of the pits previously formed are thus determined. The results obtained are collated in Table 7.

Table 7 - Repassivation by nickel level

Based on the NaCl ₁ repassivation potential tests, the higher the nickel level, the greater the difference between the pitting potential and the repassivation potential, showing that nickel is not beneficial. repassivation of a shade according to the invention having previously undergone a puncture attack.

Claims

1. 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% Co ≤ 0.5% REM <0.1% V <0.5% Ti ≤ 0.1% Nb ≤ 0.3% Mg <0.1% the rest being iron and impurities resulting from the preparation and the microstructure consisting of austenite and 35 to 65% of ferrite by volume, said composition also respecting the following relationships: 40 <IF ≤ 70 with IF = 6 x (% Cr + 1.32 x% Mo + 1.27x% Si) - 10x (% Ni + 24 x% C + 16.15 x% N + 0.5 x% Cu + 0.4x % Mn) - 6.17 and IRCL> 30.5 with IRCL =% Cr +3.3 x% Mo + 16 x% N + 2.6 x% Ni - 0.7 x% Mn The steel of claim 1, further characterized in that: IRCL> 32. 3. Steel according to claims 1 or 2, further characterized in that the proportion of ferrite is between 35 and 55% by volume. 4. The steel according to any one of claims 1 to 3, further characterized in that 40 <IF <60 5. Steel according to any one of claims 1 to 4, further characterized in that the chromium content is between 22 and 24% by weight. Steel according to any one of claims 1 to 5, further characterized in that the manganese content is less than 1.5% by weight. Steel according to any one of claims 1 to 6, further characterized in that the calcium content is less than 0.03% by weight. Steel according to any one of claims 1 to 7, further characterized in that the molybdenum content is greater than 0.1% by weight. A method of manufacturing a sheet, strip or hot rolled steel coil according to any one of claims 1 to 8, wherein: - supplying an ingot or a slab of a composition steel according to any one of claims 1 to 10 - said slab or said slab hot rolled, at a temperature between 1150 and 1280 ⁰ C to obtain a sheet, a tape or a reel. The method of manufacturing a hot rolled steel sheet according to claim 9, 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. 11. Hot rolled steel sheet, called quarto, obtainable by the process according to claim 10 and having a thickness of between 5 and 100 mm. 12. Use of a so-called quarto sheet according to claim 11 or a hot rolled coil obtained by the process according to claim 9 for the manufacture of structural elements for material production or production plants. energy. 13. Use according to claim 12, wherein said production facilities of material and energy operate between -100 and 300 ⁰ C and preferably between -50 and 300 ⁰ C. 14. A cold-rolled steel strip obtainable by cold rolling a hot-rolled coil obtained by the process according to claim 9. A method of manufacturing a hot rolled steel bar or wire according to any one of claims 1 to 8, wherein: - supplying an ingot or bloom of continuous casting of a composition steel according to any one of claims 1 to 8, said billet or said bloom is hot-rolled from a temperature of between 1150 and 128 ^° 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 process of claim 15 and having a diameter of 18mm to 250mm and hot rolled wire obtainable by the process of claim 15 having a diameter of 4 to 30mm. 17. The manufacturing method according to claim 15, 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 17 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. 19. Use of a bar according to claims 16 or 18 for the manufacture of mechanical parts such as pumps, valve shafts, motor shafts and connectors operating in corrosive environments. 20. Use of a wire according to claims 16 or 18 for the manufacture of cold-formed assemblies, for the food ^• industry, the extraction of oil and ores, or for the manufacture of fabrics and metal knits for filtration chemicals, minerals or food materials. 21. A method of manufacturing a steel profile, according to which a cold roll forming of a hot rolled bar obtained by the method according to claim 15 is carried out. 22. A steel profile obtainable by the method of claim 21. 23. A method of manufacturing a steel forging according to which a hot-rolled bar obtained by the process according to claim 15 is slugged into billets and forging said slug between HOO ° 0 and 128 ° C. Steel forging piece obtainable by the method of claim 23. 25. Use of a forging according to claim 24 for the manufacture of flanges or fittings. 26. Molded part obtainable by molding a steel according to any one of claims 1 to 8.