BRPI0713673A2 - duplex stainless steel. - Google Patents

Abstract

DUPLEX STAINLESS STEEL. The present invention relates to a duplex stainless steel composition, the composition of which comprises by weight percentage: C <0.005% 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 remainder being iron and impurities resulting from the preparation and the microstructure consisting of austenite and 35 to 65% ferrite by volume, the composition also having the following ratios: 40 <IF <70 with IF = 6 X (% Cr + 1.32 X% Mo + 1.27 X% Si) - 10 X (% Ni + 24 x% C + 16.15 X% N + 0.5 X% Cu + 0.4 X % 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 as well as a process of manufacture of sheets, straps, coils, bars, wires, sections, forgings, castings of this steel.

Description

Report of the Invention Patent for "DUPLEX STAINLESS STEEL".

The present invention relates to a duplex stainless steel, more particularly for the manufacture of structural elements for chemical (petrochemical, petrochemical, paper, offshore) or energy production plants, but not for that purpose. as well as the process of making a sheet, strap, rod, wire or profile of such steel.

This steel can most commonly be used in place of a 304L stainless steel in numerous applications, for example in the preceding or the food industry, including parts made from formed wire (welded grids ...) of profiles (finishes). ..), axes ... One could also make molded and forged parts.

For this purpose, the stainless steel nuances of type 304 and 304L are known, whose annealed state microstructure is essentially austenitic; in the cold hammered state they may also contain a variable proportion of martensite. These steels, however, carry strong additions of nickel, the cost of which is generally prohibitive. In addition, these shades may present a technical problem for certain applications as they have small tensile characteristics in the annealed state, notably with respect to the yield strength, and a strength low to high stress corrosion.

Austenitic ferritic stainless steels, which are mainly composed of a mixture of ferrite and austenite, are also known, such as EP 10088 1.4362, 1.4655, 1.4477, 1.4462, 1.4507, 1.4410, 1.4501 and 1.4424 steels. of 3.5% nickel. These steels are particularly resistant to corrosion and stress corrosion.

Also known as ferritic or ferrite-martensitic stainless steel shades, whose microstructure is, for a defined range of heat treatments, composed of two constituents, ferrite and martensite, preferably in a ratio of 50/50, such as nuances 1.4017 of EN 10088. These nuances, with chrome content generally below 20%, have high tensile mechanical characteristics but do not have satisfactory corrosion resistance.

On the other hand, a simplification of the manufacturing process of steel plates, straps, bars, wires or profiles is also sought.

The purpose of the present invention is to prevent the drawbacks of prior art steels and manufacturing processes by providing a stainless steel having good mechanical characteristics and in particular a tensile strength limit of greater than 400 to even at 450MPa in the annealed or in solution state, high corrosion resistance and in particular greater than or equal to that of 304L, good microstructural stability and good resilience of welded areas, without the addition of costly addition elements, as well. as a process of making plates, straps, bars, wires or profiles in this steel that is of simplified use.

To this end, the invention first relates to a duplex stainless steel, the composition of which consists of a percentage 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%

Si <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 remainder being iron and impurities resulting from the elaboration and the microstructure consisting of austenite and 35 to 65% ferrite by volume, the composition also having the following ratios: 40 ≤ IF ≤ 70 and preferably , 40 ≤ IF ≤ 60

with

IF = 6 × (% Cr + 1.32 ×% Mo + 1.27 ×% Si) - 10 × (% Ni + 24 ×% C + 16.15 ×% N + 0.5 ×% Cu + 0.4 ×% Μη) - 6.17

and

IRCL ≥ 30.5 and preferably ≥ 32

with

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

Steel according to the invention may also comprise the following optional features, considered alone 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 a process of a hot-rolled steel plate, belt or coil according to the invention according to which:

- provides an ingot or ingot of a composition steel according to the invention;

- This ingot or hot ingot is rolled at a temperature of between 1150 and 1280 ° C to produce a plate, belt or coil.

In a particular embodiment, this ingot or ingot is hot rolled at a temperature of between 1150 or 1280 ° C to obtain a fourth plate, then heat treated at a temperature of between 900 and 1100 °. C, and cool this plate by air quenching.

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

- provides a continuous casting ingot or magnifying glass of a composite steel according to the invention;

- this ingot or loupe is hot-rolled from 1150 to 1280 ° C to provide an air-cooling bar or a water-cooled wire crown; then, optionally:

- heat treatment is carried out at a temperature of between 900 and 1100 ° C; and

- quenches this bar or crown by temper.

In a particular embodiment, furthermore, a cold drawing of this bar or drawing of that wire may be carried out at the end of cooling.

The invention also encompasses a manufacturing process of a steel section, whereby a cold forming of a hot-rolled bar obtained according to the invention is carried out, as well as a manufacturing process of a forged steel part, whereby a hot-rolled bar obtained according to the invention is broken into pieces, then the piece is forged between 1100 ° C and 1280 ° C.

The invention also encompasses different products obtainable by the processes according to the invention, as well as their uses, such as:

- the hot-rolled steel plates, said fourth, having a thickness of between 5 and 100 mm, and straps and coils, which may be used for the fabrication of structural elements for material producing installations. or energy, in particular for energy and matter production installations operating between -100 and 300 ° C e. preferably between -50 and 300 ° C;

- cold rolled steel straps obtainable by cold rolling from a hot rolled coil;

- hot rolled bars having a diameter of 18 mm to 250 mm and cold drawn bars having a diameter of 4 mm to 60 mm, these products may be used for the manufacture of mechanical parts such as pumps, shafts valves, engine shafts, and fittings that work in corrosive media; - hot rolled yarn having a diameter of 4 to 30 mm and drawn wire having a diameter of 0.010 mm to 20 mm, these products may be used for the manufacture of cold formed connections for the agri-food industry, extraction of oil and minerals, or for the manufacture of metal fabrics and knitting for filtering chemicals, ores or foodstuffs;

- the profiles;

- Forged parts may be used for the manufacture of flanges or fittings;

molded parts which can be obtained by casting a steel according to the invention.

Other features and advantages of the invention will appear upon reading the following description given by way of example only.

Duplex stainless steel according to the invention comprises the contents defined below.

The carbon content of the nuance is less than or equal to 0.05% and preferably less than 0.03% by weight. Indeed, a very high content of this element degrades the resistance to localized corrosion, increasing the risk of chromium carbide precipitation in thermally affected weld zones.

The chrome content of the nuance is 21 to 25 wt%, preferably 22 to 24 wt%, in order to obtain a good corrosion resistance which is at least equivalent to that obtained with type 304 nuances. or 304L.

The nickel content of the nuance is from 1 to 2.95 wt% and is preferably less than or equal to 2.7, even 2.5 wt%. This austenite forming element is added in order to achieve good corrosion resistance properties. For contents greater than 1%, and preferably greater than 1.2% by weight, it has a favorable effect to combat the onset of injection corrosion. However, their content is limited to 2.95% by weight, but there is a degradation of the resistance to spread of these injections. Increasing it also allows for a good resilience / ductility compromise. It is in fact of interest to translate the transition curve from resilience to low temperatures, which is particularly advantageous for the manufacture of thick sheet metal for which resilience properties are important.

The nickel content being limited in steel according to the invention has been found to be suitable for achieving an appropriate austenite content after heat treatment between 900 and 110 ° C, adding other forming elements. austenite in unusually high quantities and limit the levels in ferrite forming elements.

The nitrogen content of nuance is between 0.16 and 0.28%, which generally implies that nitrogen is added to the steel when it is made. This austenite forming element allows initially to achieve a biphasic ferrite + austenite steel containing a proportion of austenite suitable for good resistance to stress corrosion and also to achieve high mechanical characteristics for the metal. It also provides good microstructural stability in the thermally affected zone of welded zones. Its maximum content is limited because, in addition to 0.28%, solubility problems can be observed: failure formation when solidification of ingots, magnifiers, ingots, castings or welds.

The manganese content, also an austenite-forming element below 1150 ° C, is maintained below 2.0% by weight and preferably below 1.5% by weight due to the harmful effects of that element. on numerous points. Thus, it presents problems when elaborating and tuning the nuance, as it attacks certain refractories used for the crucibles, which requires a more frequent replacement of these costly elements and, therefore, the most frequent interruptions of the process. Iron manganese supplies, which are commonly used for nuance, also contain notable phosphorus and selenium contents, which are not desired for introduction into steel and are difficult to remove. , when tuning the nuance. On the other hand, manganese disturbs this tuning, limiting the possibility of decarburization. However, it presents the problem more to the corrosion of nuance due to the formation of MnS manganese sulfides, and oxidized inclusions. This element was traditionally added to the nuances that one wanted to enrich with nitrogen in order to increase the solubility of this element in the nuance. In the absence of sufficient manganese content, it was therefore not possible to achieve this level of nitrogen in steel. However, the present inventors have found that it was possible to limit the addition of manganese to steel according to the invention by adding sufficient nitrogen to achieve the desired effect on the base metal ferrite-austenite balance and zone stabilization. thermally affected areas of the welded areas.

Molybdenum, the ferrite forming element, is kept below 0.45% by weight, in the same way as tungsten is kept below 0.125% by weight. On the other hand, the contents of these two elements are such that the sum Mo + W / 2 is less than 0.50 wt.%, Preferably less than 0.4 wt.% And particularly preferably less than 0.3. % by weight. Indeed, the present inventors have found that by keeping these two elements, as well as their sums, under the indicated values, no brittle intermetallic precipitations were observed, which makes it possible to pre-compress the manufacturing process of steel sheets or belts. allowing air cooling of the plates and belts after heat treatment or hot use. In addition, they observed that controlling these elements within the claimed limits improved nuance welding capability. However, it is preferred to maintain a minimum molybdenum content of 0.1% in order to improve the hot forging of nuance. Furthermore, the elaboration of a nuance with less than 0.1% molybdenum from recycling hardware for this nuance presents problems of use, in particular requiring the use of a 100% ferroalloy load. pure.

Copper, austenite-forming element, is present in a content of from 0.11 to preferably from 0.15 to 0.40% by weight. This element improves corrosion resistance in reducing acid medium. However, their content is limited to 0.50% by weight to avoid the formation of epsilon phases to be avoided as 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 suitability.

Boron is an optional element which may be added nuance according to the invention in a content of from 0.0005% to 0.01% by weight, preferably from 0.0005 to 0.005 and more particularly. 0.0005% to 0.0003% by weight in order to improve its hot processing. In another embodiment, however, it is preferred to limit the boron content to less than 0.0005% by weight to limit the risk of cracking to welding and continuous casting.

Silicon, a ferrite forming element, is present in a content of less than 1.4% by weight. Ferrite forming aluminum is present in a content of less than 0.05% by weight and preferably from 0.005% to 0.040% by weight in order to achieve low calcium aluminate inclusions. fusion point. The maximum aluminum content is also limited to prevent excessive formation of aluminum nitrides. The action of these two silicon and aluminum elements is essentially to ensure a good deoxidation of the steel bath upon preparation. Cobalt, the austenite forming element, is maintained at a content of less than 0.5 wt%, and preferably less than 0.3 wt%. This element is a residual provided by the raw materials. This is noticeably limited because of the maintenance problems it may have after irradiation of parts in nuclear installations.

Rare earths (referred to as REM) may be added in the composition at a height of 0.1 wt% and preferably less than 0.06 wt. Lanthan. The contents of these elements are limited as they are capable of forming unwanted intermetallic.

Vanadium, a ferrite forming element, may be added nuanced at a height of 0.5 wt% and preferably less than 0.2 wt% in order to improve the cavernous corrosion maintenance of the steel.

Niobium, a ferrite forming element, may be added at a height of 0.3 wt% and preferably less than 0.050 wt%. It allows to improve the mechanical tensile strength of the nuance, thanks to the formation of niobium nitride ends. Its content is limited to limit the formation of coarse niobium nitrides.

Titanium, a ferrite forming element, may be added nuanced at a height of 0.1 wt%, and preferably less than 0.02 wt% to limit the formation of titanium nitrides formed in liquid steel notably.

It may also be added to the nuance of the invention according to the invention to obtain a calcium content of less than 0.03% by weight, and preferably greater than 0.0002% even. greater than 0.0005% by weight in order to control the nature of oxide inclusions and to improve machinability. The content of this element is limited because it is capable of forming as sulfur calcium sulfides that degrade corrosion resistance properties. In a preferred embodiment, the calcium content is limited to less than 0.0005% by weight and preferably less than 0.0002%.

Sulfur is maintained at a content of less than 0.010% by weight and preferably less than 0.003% by weight. As has been seen earlier, this element forms sulfides with manganese or calcium, sulfides whose presence is harmful to corrosion resistance. It is considered as an impurity.

A concurrent addition of magnesium with a final content of 0.1% may be made to modify the nature of sulfides and oxides.

Selenium is preferably kept at less than 0.005% in

weight due to its corrosion. This element is usually supplied in the nuance as impurities of iron manganese ingots.

Phosphorus is kept below 0.040% by weight and is considered as an impurity. The rest of the composition consists of iron and impurities.

In addition to those already mentioned above, zirconium, tin, arsenic, lead and bismuth will also be mentioned. Tin may be present in a content of less than 0.100 wt.% And preferably less than 0.030 wt.% To avoid welding problems. Arsenic may be present in a content of less than 0.030% by weight and preferably less than 0.020% by weight. Lead may be present in a content of less than 0.002 wt% and preferably less than 0.0010 wt%. Bismuth may be present in a content of less than 0.0002 wt.% And preferably less than 0.00005 wt.%. Zirconium may be present, corresponding to 0.02%.

On the other hand, the present inventors found that when the percentages by weight of chromium, molybdenum, nitrogen, nickel and manganese respect the relationship below, the mentioned nuances have a good resistance to localized corrosion, that is, in the formation of injections. or caves.

<formula> formula see original document page 11 </formula> The microstructure of the steel according to the invention in the annealed state is composed of austenite and ferrite, which are preferably after 1 hour treatment at 1000 °. C, in a proportion of 35 to 65% by volume of ferrite and, more particularly preferably, of 35 to 55% by volume of ferrite. The present inventors have also found that the following formula conveniently considers the ferrite content at 1100 ° C.

IF = 6 χ (% Cr + 1.32 χ% Mo + 1.27 χ% Si) - 10 χ (% Ni + 24 χ% C + 16.15 χ% N + 0.5 χ% Cu + 0, 4 χ% Mn) - 6.17.

Thus, in order to obtain a ferrite ratio of 35 to 65% at 1100 ° C, the IF index must be between 40 and 70.

In the annealed state, the microstructure contains no other phases that would be detrimental to its mechanical properties notably, such as the sigma phase and other intermetallic phases. In the cold hammered state, a part of the austenite may have been converted to martensite, depending on the effective strain temperature and the amount of strain and the amount of cold strain applied.

In general, steel according to the invention can be made and manufactured in the form of hot-rolled sheets, still called quarter sheets, but also in the form of hot-rolled straps, from ingots. or ingots and also in the form of cold-rolled belts from hot-rolled belts. It can also be hot rolled into bars or wire rods or in profiles or forged; These products may then be hot forged or cold processed into drawn bars or sections or into drawn wire. Steel according to the invention may also be used by casting followed or not by use of casting followed or not by heat treatment.

In order to obtain the best possible performance, preferably the process according to the invention, which initially comprises the provision of an ingot or a steel loupe, having a composition according to the invention. with the invention.

This ingot or lupe is generally obtained by melting the raw materials in an electric furnace, followed by decarburizing AOD or VOD vacuum remelting. It can then be cast in the nuance of the ingots, or in the form of magnifying glasses by continuous casting in a bottomless ring. One could also consider casting the nuance directly in the form of thin ingots, in particular by continuous casting between counter-rotating cylinders.

After ingot or magnifying glass is supplied, the heat may be heated to a temperature of between 1150 and 1280 ° C, but it is also possible to work directly on the newly cast ingot in the heat of the casting. .

In the case of sheet metal fabrication, the ingot is then hot rolled to give a fourth plate generally having a thickness of between 5 and 100 mm. The reduction rates generally employed at this stage range from 3 to 30%. This plate is then subjected to a solution heat treatment of the precipitates formed at this stage by heating to a temperature of 900 to 1100 ° C, then cooled.

The process according to the invention provides for an air quench cooling that is easier to apply than the classically used cooling for this kind of nuance, which is faster cooling as a water aid. It is, however, possible to cool the water if desired.

Such slow air cooling is notably made possible thanks to the limited nickel and molybdenum contents of the composition according to the invention which is not subject to precipitation of intermetallic phases detrimental to their properties of use. This cooling can in particular be done at speeds ranging from 0.1 to 2.7 ° C / s.

At the end of hot rolling, the fourth plate can be planed, cut and pickled if it is to be delivered in this state.

This uncoated steel may also be rolled onto an endless mat with thicknesses of between 3 and 10 mm.

In the case of the manufacture of long products from ingots or magnifying glasses, it may be hot-rolled by heating them on a multi-compartment laminator in corrugated cylinders at a temperature between 1150 and 1280 ° C to wire. machine or laminate. The section ratio between the initial magnifying glass and the final product is preferably greater than 3 to ensure the internal preservation of the laminated product.

When a bar is manufactured, it is cooled at the Iamination outlet by simple air gauging.

When the rolled wire of diameter greater than 13 mm has been manufactured, it may be cooled by corona tempering in a water container at the outlet of the rolling mill.

When a wire of 13 mm or less in diameter has been manufactured, it may be cooled by quenching the water in endless turns on a mat after passing them on a mat of 2 to 5 mn through a solution furnace. at a temperature between 850 and 1100 ° C.

Aftermath heat treatment in the oven at 900 to 1100 ° C may optionally be practiced on these bars or crowns already treated in the lamination heating if the recrystallization of the structure is to be completed and the tensile mechanical characteristics slightly decreased.

At the end of the cooling of these bars or wire crowns, different hot and cold forming treatments may be carried out depending on the end use of the product. This will allow cold drawing of the bars or wire drawing at the end of cooling.

Hot-rolled bars may also be cold-formed, or parts may be manufactured after the bars have been turned into pieces and forged.

In order to illustrate the invention, tests have been made and will be described, notably with reference to Figures 1 to 5, which represent:

- Figure 1: Correlation between% of ferrite after treatment at 1100 0C and IF index for raw products.

- Figure 2: Relative delta 0 diametric deviation as a function of the deformation temperature.

- Figure 3: Potentials of injections E1 and E2 determined on forged bars as a function of the IRCL index.

- Figure 4: Uniform corrosion velocity V determined on forged bars against IRCL index

- Figure 5: Critical CCT and CPT temperatures determined on forged bars as a function of the IRCL index.

25kg laboratory ingots were made by fusion by

vacuum induction of raw materials and pure ferroalloys after supply of nitrogen by the addition of nitrided ferroalloys under partial nitrogen pressure and cast in a metal mold under external pressure of 80 KPa (0.8 bar) nitrogen. Of these, only tests 14441 and 14604 are in accordance with the invention.

An industrial foundry according to the invention of 150 tons referenced 8768 was made. This nuance was elaborated by melting in the electric furnace, then tuned under vacuum with decarburization to achieve the target carbon level. It was then continuously cast into 220 χ 1790 mm section ingots, then hot rolled after heating to 1200 ° C on said 7, 12 and 20 mm quarter thickness sheets. The sheets thus obtained are then heat treated to 1000 ° C in order to settle the different precipitates present at this stage. At the end of the heat treatment, the plates are cooled with water, then flattened, cut and pickled.

The compositions in percent by weight of the different grades prepared in the laboratory or in an industrial form are combined in Table 1, as well as those of different industrial products or semi-products made in an electric furnace, AOD tuning, ingot cast or in continuous, mentioned by way of comparison. <table> table see original document page 16 </column> </row> <table> <table> table see original document page 17 </column> </row> <table> .1. Ferrite contents

.1.1. Ferrite content of crude products. Pieces of 1 to 8 cm3 cut out in these laboratory foundries in the raw foundry state or industrial products in the raw foundry state were carried out in a salt bath, with water immersion for the purpose of treatment, heat treatment of 30 minutes at variable temperature to determine the proportion of ferrite at high temperature. The magnetic tool, unlike austenite, carbides and nitrite, which was present, applied a measurement method by measurement of saturation suspension. The ferrite grades thus determined are reported in Table 2 and reported in Figure 1.

If Figure 1 is considered, a good correlation is found between the IF index and the ferrite grades measured on the base metal after treatments at 1100 0C.

On the other hand, it is found that the foundry according to the invention, No. 14441, present below 1300 ° C, has a ferrite content suitable for 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. <table> table see original document page 19 </column> </row> <table> 1.2. Ferrite content of finished products

Ferrite content was also measured by the grid method (according to ASTM E 562) on forged bars, after heat treatment at 1030 0C and on thermally affected areas of weld beads deposited by electrode-coated electrode. constant energy at a cooling rate of 20 ° C / s to 700 ° C. The results (ferrite contents in base metal and in thermally affected zone) are given in table 3. It is found that foundries 14441 and 14604, according to the invention, have a ferrite content in the base metal and in the thermally affected zone favoring localized and stressed corrosion resistance as well as resilience (as per table 5)

Table 3 - Ferrite contents

<table> table see original document page 20 </column> </row> <table>

*: according to the invention LAC: hot rolled

α Μ.B (%): ferrite content measured on base metal α Z.A.T .: ferrite content measured on thermally affected zone

2. Casting

Ingot 14439 failed and was unusable. In order to avoid this phenomenon, when casting under air at atmospheric pressure, it was therefore necessary to limit the nitrogen content of foundries according to the invention to less than 0,28% by weight.

3. Hot Processing Capacity

The hot deformation capacity was evaluated as an aid to hot tensile tests carried out on samples whose calibrated part, 8 mm in diameter and 5 mm in length, is heated by Joule effect for 80 seconds at 1280 °. C, then cooled to 20 ° C per second until the test temperature ranges from 900 to 1280 ° C. When this temperature is reached, rapid traction is immediately activated at a speed of 73 mm / s; after rupture, the stress diameter at the rupture level is measured.

Relative diametric deviation (table 4) as defined below considers the hot deformation capability:

Delta φ = 100 χ (1 - (final diameter / initial diameter) Table 4: Relative diametric deviations (hot tensile tests)

<table> table see original document page 21 </column> </row> <table>

* according to the invention

It can be seen from reading Table 4 and Figure 2 that the data in the form of curves show that the cast 14441 according to the invention has a hot deformation capacity comparable to that of the reference cast. Comparative No. 14382.

4. Mechanical Properties

Tensile properties Re0,2 and Rm were determined according to NFEN 10002-1. KV resilience was determined at different temperatures according to NF EN 10045. Table 5 - Mechanical characteristics

<table> table see original document page 22 </column> </row> <table> *: according to the invention

LAC: Hot Rolled;

Re0,2: yield strength at 0,2% deformation Rm: breaking strength

The results of laboratory castings 144412 and 14602 of industrial cast 8768, all three according to the invention, show that an elastic limit greater than 450 MPa is double that obtained for austenitic steels of type AISI 304L, can be obtained.

The resilience values determined at 20 ° C for laboratory foundries 14441 and 14604 and industrial foundry 8768, all three according to the invention, are all greater than 200 J, which is satisfactory considering the level of the elastic limit of these nuances. For the 14383 foundry with the exception of the invention, the low nitrogen 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 a satisfactory level of toughness is obtained. 5. Corrosion Maintenance

Corrosion resistance tests were performed at the same time on forged laboratory bars and on coupons taken from hot-rolled plates from industrial foundries. .5.1 Localized Corrosion Maintenance

Resistance to corrosion by injection is evaluated by plotting potential intensity curves and determining the injection potential i = 100 μΑ / cm2. This parameter was measured in a very chlorinated neutral medium (pH = 6.4) ([Cl-] = 30 g / l) at 50 ° C (Ei), representative of the brines found in seawater desalting plants. and in a slightly acidic medium (pH = 5.5) slightly chlorinated ([Cl-] = 250 ppm) at ambient temperature (E2) representative of drinking water. The critical injection temperature in ferric chloride medium (FeCl3 6%) was also determined according to ASTM G48-00 method C.

In another series of tests, the resistance to injection corrosion in deaerated neutral medium at 0.86 moles / liter in NaCI1 corresponding to 5 wt% in NaCI at 35 ° C was determined. A measure of the abandonment potential for 900 seconds is made. Then a potentiometric curve is drawn at a speed of 100 mV / min from abandonment to injection potential. The injection potential (E3) is determined for i = 100 μΑ / cm2. Under these conditions, samples were tested according to the invention, as well as reference samples in 304L nuances and 1.4362 austene-ferritic duplex nuances and others.

The resistance to cavernous corrosion was studied by measuring the critical cave temperature in the neutral (pH = 6.4) heavily chlorinated environment ([Cl-] = 30 g / l). Mounting that allows for cavernous corrosion is in accordance with the recommendations made in ASTM G78-99. The critical cave temperature is the minimum temperature for which caves deeper than 25 pm on.

The values obtained are shown in Table 6. Comparison between the results obtained on the UNS S32304 sheet and the bar from the 14382 cast, all two of similar chemical composition, indicates that the corrosion resistance of a bar is lower. than that of a hot-rolled plate of the same composition. The present inventors have found that the localized corrosion resistance index, ie, injection or cave formation, summarized by IRCL and defined by: <formula> formula see original document page 24 </formula> (Cr, Mo, N contents , Ni and Mn in% by weight)

considers the composition set rating to less than 6% nickel for localized corrosion maintenance (see figures 3, 4 and 5)

Castings 14383 and 14660 with the exception of the invention, of IRCL indices of 28.7 and 29.8, behave worse in corrosion than an AISI 304L type steel. The foundries 14604 and 14441 according to the invention of IRCL 30.9 and 33 behave at least as well as type 304L steel. In order to obtain corrosion maintenance at least equal to that of AISI 304L nuance, it has been found that steels according to the invention should preferably have an IRCL of greater than 30.5 and preferably greater than 32.

5.2 Maintenance of uniform corrosion

Uniform corrosion was characterized by assessing the rate of corrosion by mass loss after soaking 72 hours in a 2% sulfuric acid solution brought to 40 ° C. Comparison of corrosion speeds for foundries

Experimental studies at 2.5% Ni and 0.2% N (14441 according to the invention and 14660 with the exception of the invention) also show the negative effect of a high Mn content on uniform corrosion resistance in sulfur medium. - co.

Table 6 - Localized and Uniform Corrosion Resistance Data

<table> table see original document page 24 </column> </row> <table> <table> table see original document page 25 </column> </row> <table>

*: according to the invention

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

Ei: injection potential in neutral (pH = 6.4) and strongly and highly chlorinated (30 g / l Cl) at 5 ° C

E2: injection potential in slightly acidic (pH = 5.5) and slightly chlorinated (250 ppm Cl) medium at 25 ° C

E2: Neutral chlorinated injection potential (5% NaCl) at 35 ° C CPT: Critical injection temperature in ferric chloride medium CCT: Critical cave temperature in neutral (pH = 6.4) and highly chlorinated (30 g / l Cl ·)

V: uniform corrosion rate in sulfuric acid medium 2% at 40 ° C 5.3 Repassivation potential

The steel samples are polished under water with SiC paper up to 1200, then aged 24 hours in the air.

The cyclic polarization test in chlorinated media is performed starting with a measurement of the abandonment potential for 15 minutes, followed by a cyclic dynamic polarization at 100 mV / min from the abandonment potential to the potential for which the current reaches the intensity of 300 μΑ / cm2 and the return to the potential for which the current is zero.

In this way the values of injection potentials (Vpit) and repassivation potentials (Vrepassivation) of the previously formed injections are determined. The results are gathered in table 7.

Table 7. Nickel rate repassivation <table> table see original document page 26 </column> </row> <table> From the tests of repassivation potentials in NaCI medium the higher the nickel rate, the more The deviation between the injection potential and the repassivation potential will be high, which shows that nickel is not beneficial for the repassivation of a nuance according to the invention, having previously been attacked by injection.

Claims (26)

1. Duplex stainless steel, the composition of which is as a percentage by weight: <formula> formula see original document page 27 </formula> Steel according to claim 1, characterized by the fact that IRCL ≥32. Steel according to one of claims 1 or 2, characterized in that the proportion of ferrite is between 35 and 55% by volume. Steel according to any one of Claims 1 to 3, characterized in that 40 <IF <60. Steel according to any one of Claims 1 to 4, characterized in that, in addition, the chromium content is between 22 and 24% by weight. Steel according to any one of claims 1 to 5, characterized in that, furthermore, the manganese content is less than 1.5% by weight. Steel according to one of Claims 1 to 6, characterized in that the calcium content is less than 0.03 by weight. Steel according to any one of claims 1 to 7, characterized in that the molybdenum content is more than 0.1% by weight. A process for the manufacture of a hot-rolled steel plate, belt or bonnet as defined in any one of claims 1 to 8, wherein: - a steel ingot or ingot is supplied of composition as defined in any one of claims 1 to 10; - this ingot or hot ingot is rolled at a temperature of from 1150 to 1280 ° C to obtain a plate, belt or coil. A process for the manufacture of a hot-rolled steel sheet according to claim 9, wherein: - this ingot or hot ingot is rolled at a temperature of from 1150 to 1280 ° C to obtain a fourth plate, then - heat treatment is carried out at a temperature of from 900 to 1100 ° C; and - this plate is cooled by air quenching. Hot-rolled steel plate, said fourth, obtainable by the process according to claim 10 and having a thickness of between 5 and 100 mm. Use of a said sheet as defined in claim 11 or a hot-rolled coil obtained by the process as defined in claim 9 for the fabrication of frame elements for material production plants or of energy production. Use according to claim 12, wherein such matter and energy production facilities operate at -100 to 300 ° C and preferably at -50 to 300 ° C. Cold-rolled steel belt which may be obtained by cold-rolling of a hot-rolled coil obtained by the process as defined in claim 9. A process for the manufacture of a hot-rolled steel bar or wire as defined in any one of claims 1 to 8, wherein: - a continuous casting ingot or magnifying glass is provided for a composition steel as defined in any one of claims 1 to 8; - this ingot or loupe is hot-rolled from a temperature of between 1150 and 1280 ° C to produce an air-cooling bar or a water-cooled wire crown; then optionally: - heat treatment is carried out at a temperature of from 900 to 1100 ° C; and - quenches this bar or crown by quenching. Hot rolled bar obtainable by the process as defined in claim 15 and having a diameter of 18 mm to 250 mm and hot rolled wire obtainable by the process as defined in claim 15 and having a diameter of 4 to 30 mm. Process for manufacturing according to claim 15, whereby cold drawing of this bar or drawing of this yarn from cooling is carried out. Cold drawn bar obtainable by the process as defined in claim 17 having a diameter of 4 mm to 60 mm and drawn wire obtainable by the process as defined in claim 9 having a diameter of 0.010 mm to 20 mm. Use of a bar as defined in claim 16 or 18 for the manufacture of mechanical parts such as pumps, valve shafts, motor shafts and fittings that work in corrosive media. Use of a yarn according to claim 16 or -18 for the manufacture of cold formed bonds, for the food industry, the extraction of oil and minerals, or for the manufacture of fabric and metal knitting by filtering chemicals, ores or foodstuffs. Process for the manufacture of a steel profile, whereby cold forming of a hot rolled bar obtained by the process as defined in claim 15 is carried out. The steel profile obtainable by the process according to claim 21. Process for the fabrication of a forged steel part, whereby a hot-rolled bar obtained by the process as defined in claim 15 is cut into pieces, then a forge of that piece is made between 1100 ° C and 1280 ° C. ° C. A steel forged part obtainable by the process as defined in claim 23. Use of a forged part as defined in claim 24 for the manufacture of flanges or fittings. Molded part obtainable by casting a steel as defined in any one of claims 1 to 8.