CA2656946C - 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 facilities (chemical, petrochemical, paper, offshore) or energy production, without being limited to the manufacturing process of a sheet, strip, bar, wire, or of profiles of this steel.
This steel can more generally be used in substitution of a type 304L stainless steel in many applications, by example, in previous industries or in the agri-food, including parts made from formed yarn (grids welded, ..) profiles (strainers ..), axes ... We could also realize castings and forgings.
To this end, stainless steel grades of type 304 are known and 304L whose microstructure 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 of which is generally prohibitive. In addition, these nuances may be problematic from a technical point of view for some applications because they have low tensile characteristics in the state annealing, 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 that the steels 1.4362, 1.4655, 1.4477, 1.4462, 1.4507, 1.4410, 1.4501 and 1.4424 of EP10088, all of which contain more than 3.5% of nickel. These steels are particularly resistant to corrosion and stress corrosion.

2 Stainless steel grades known as ferritic or ferritic-martensitic materials, whose microstructure is, for a defined range of heat treatments, consisting of two constituents, ferrite and martensite, preferably in a ratio of 50150, such as grade 1.4017 of EN10088. These shades, with chromium content generally less than 20%, have characteristics mechanical high tensile, but do not exhibit resistance to satisfactory corrosion.
In addition, a simplification of the sheet manufacturing process, bands, bars, wires or steel profiles, is also sought.
The object of the present invention is to overcome the disadvantages steels and manufacturing processes of the prior art by a stainless steel with good characteristics mechanical devices and in particular a yield strength greater than 400 or 450MPa in the annealed or dissolved state, resistance to corrosion and in particular greater than or equal to that of 304L, a good microstructural stability and good resilience of areas soldered, without the addition of expensive additives, and a process for manufacture of sheets, strips, bars, wires, or profiles of this steel which is of simplified implementation.

For this purpose, the invention firstly relates to a stainless steel duplex, the composition of which consists of, in% by weight:
C: 5 0.05%
21%: 5 Cr: 5 25%
1% sNi: 5 2.95%
0.16% SN50,28%
Mn <2.0%
Mo + W / 2E 0.50%
Mo: 5 0.45%
W <0.15%
If 5 1.4%
AI 5 0.05%

3 0.11% sCu 4 0.50%
S <_0,010%
P! 5 0.040%
Co: 0.5%
REM <0.1%
VS 0.5%
0.1%
Nb 5 0.3%
Mgs0,1%
the rest being iron and impurities resulting from the elaboration and the microstructure consisting of austenite and 35 to 65% ferrite voiume, the composition further respecting the following relationships:

40 SiF 5 70 and preferably 40: 5 IF: 9 60 with IF = 6x (% Cr + 1.32x / aMo + 1.27x% Si) - 10x (/ aNi + 24x / aC) + 16.15x% N + 0.5x% Cu + 0.4 x% Mn) -6.17 and IRCL? 30.5 and preferably> 32 with IRCL ~% Cr + 3,3x% Mo + 16x% Nfi2,6x% Ni-0,7x% Mn The steel according to the invention can also comprise the optional features, individually or in isolation 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.

4 A second object of the invention is constituted by a method of a sheet, strip or hot-rolled steel coil according to the invention, according to which:
supplying an ingot or a slab of a composition steel according to the invention, said slug or slab is rolled while hot at a temperature between 1150 and 1280 C to obtain a plate, a band or a coil.

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

A third object of the invention is constituted by a method of manufacture of hot-rolled steel bar or wire according to the invention, according to which:
supplying an ingot or a continuous casting bloom of a composition steel the invention, said bullion or said bloom is hot-rolled for a temperature between 1150 and 1280 C to obtain a bar cooled in air or a wire crown that is cooled to the 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 carry out a cold drawing of said bar or drawing of said wire, at the end of cooling.

The invention also covers a method of manufacturing a profile made of steel, according to which cold profiling of a rolled bar is carried out obtained according to the invention, as well as a manufacturing process of a steel forging, according to which one slugs a bar

5 hot rolled obtained according to the invention, then carries a forging said slug between 1100 C and 1280 C, The invention furthermore covers different products that can be obtained by the methods according to the invention as well as their uses, such as:
- hot-rolled steel plate, called quarto, and having a thickness between 5 and 100 mm, and strips and coils, which can be used for the manufacture of structures for production facilities of material or energy production, in particular, for material and energy productions running between -100 and 300 C and preferably between -50 and 300 C, - cold rolled steel strip obtainable by cold rolling of a hot-rolled coil, ~ hot-rolled bars with a diameter of 18mm to 250 mm and cold drawn bars with a diameter of 4 mm to 60 mm, these products can be used for the manufacture mechanical parts such as pumps, valves, motor shafts and couplings operating in corrosive environments, - hot-rolled wires with a diameter of 4 to 30 mm and drawn wire having a diameter of 0.010 mm to 20 mm, these products that can be used to manufacture assemblies cold-formed, for the food and agriculture industry, oil and ores, or for the manufacture of fabrics and knits for the filtration of chemicals, ore or food materials, WO 2007/14451

6 PCT / FR2007 / 000994 - the profiles, - forgings that can be used for the manufacture of flanges or fittings, ~ castings that can be obtained by molding a steel according to the invention.

Other features and advantages of the invention will become apparent reading of the description which will follow, given only for for 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. Indeed, a content too elevated in this element degrades localized corrosion resistance in increasing the risk of precipitation of chromium carbides in heat affected areas of the welds.
The chromium content of the grade is between 21 and 25%
weight, preferably between 22 and 24% by weight in order to obtain a good resistance to corrosion, at least equivalent to that obtained with type 304 or 304L grades.
The nickel content of the grade is between 1 and 2.95% in weight, and is preferably less than or equal to 2.7, or even 2.5 weight. This austenite forming element is added in order to obtain good resistance properties to the formation of corrosion caverns.
At contents greater than 1% and preferably greater than 1.2% by weight weight, it has a favorable effect to combat the initiation of corrosion by pitting. However, its content is limited because, beyond 2.95% by weight, A deterioration in resistance to the propagation of these pits is observed.
Its addition also allows to obtain a good compromise resilience /
ductility. It has indeed the advantage of translating the curve of transition from resilience to low temperatures, which is particularly

7 advantageous for the manufacture of thick quarto plates for which Resilience properties are important.
Since the nickel content is limited, in the steel according to the invention, found it appropriate, to obtain a suitable austenite content after heat treatment between 900 C and 1100 C, to add other forming elements of austenite 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 usually implies that nitrogen is added to the steel when development. This austenite forming element first makes it possible to obtain two-phase ferrite + austenite duplex steel containing a proportion austenite suitable for good resistance to stress corrosion, and also to obtain high mechanical characteristics for the metal.
It still allows to have good microstructural stability in the area thermally affected welded areas. It limits its maximum content because, beyond 0.28%, one can observe problems of solubility:
formation of blisters during the solidification of the slabs, blooms, ingots, castings or welds.
The manganese content, also forming austenite element below 1150 ° C., is kept below 2.0% by weight, and preferably less than 1.5% by weight, because of the detrimental effects of this element on many points. Thus, it poses problems when Elaboration and refinement of the nuance because it attacks some refractories used for the pockets, which requires a replacement more frequent of these costly elements and therefore more interruptions frequent process. The ferro-manganese inputs that are used Normally to make composition of the shade, furthermore contain noticeable levels of phosphorus, and also of selenium, which can not be not want the introduction into the steel and that are difficult to remove when of refining the shade. Manganese disrupts this ripening by limiting the possibility of decarburization. It is also a problem further downstream in the process, because it deteriorates the corrosion resistance

8 of the grade due to the formation of manganese sulphides MnS, and oxidized inclusions. This element was traditionally added to nuances that one wished to enrich in nitrogen, in order to increase the solubility of this element in the nuance. In the absence of sufficient content manganese, it was therefore not possible to achieve such a level of nitrogen in steel. The present inventors, however, found that it was possible to limit the addition of manganese in the steel according to the invention, while by adding enough nitrogen to achieve the desired effect on ferrite-austenite balancing of the base metal and the stabilization of heat affected areas of welded areas.
Molybdenum, a ferrite-forming element, is maintained at a less than 0.45% by weight, just as tungsten is maintained at a content of 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% by weight. Indeed, present inventors have found that by maintaining these two elements, as well as their sums, under the values indicated, no intermetallic precipitation, which makes it possible de-constrain the manufacturing process of steel sheets or strips by allowing air-cooling of plates and strips after treatment thermal or hot work. In addition, they observed that controlling these elements within the limits claimed, it was improved the welding ability of the grade. However, we prefer to maintain minimum molybdenum content of 0.1% in order to improve the forgeability to hot from the shade. In addition, the development of a nuance presenting less than 0.1% molybdenum would the use of recycling scrap for this grade, which poses problems of implementation, in particular by requiring the use of a 100% filler consisting of pure ferroalloys.
Copper, the austenite forming element, is present in one between 0.11 and 0.50% by weight, and preferably between

9 0.15 and 0.40% by weight. This element improves corrosion resistance in reducing acidic medium. However, its content is limited to 0.50% in weight to avoid the formation of epsilon phases that one wishes 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, to improve its forging ability.
Boron is an optional element that can be added to the nuance according to the invention in a content of between 0.0005% and 0.01% in weight, preferably between 0.0005% and 0.005% and more particularly preferably between 0.0005% and 0.003% by weight, so to improve its hot transformation. In another embodiment, it is preferable, however, to limit the boron content to less than 0.0005% by weight to reduce the risk of cracking during welding and casting keep on going.
Silicon, a ferrite-forming element, is present in one 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 between 0.005% and 0.040% by weight in order to obtain inclusions low melting calcium aluminates. We also limit the content maximum aluminum to prevent excessive formation of nitrides aluminum. The action of these two elements silicon and aluminum is essential to ensure good deoxidation of the steel bath when development.
Cobalt, the austenite forming element, is maintained at a less than 0.5% by weight, and preferably less than 0.3% by weight.
This element is one. residual brought by the raw materials. It is limited in particular because of the handling problems that he can post irradiation of parts in nuclear facilities.
Rare earths (referred to as REM) can be added to the composition at a level 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, can be added to the grade up to 0.5% by weight and preferably less than 0.2% by weight 5 weight, to improve the resistance to crevice corrosion of steel.
Niobium, a ferrite-forming element, can be added to the grade at 0.3% by weight and preferably below 0.050%
in weight. It improves the mechanical tensile strength of the nuance, thanks to the formation of fine niobium nitrides. We limit his

10 to limit the formation of coarse niobium nitrides.
Titanium, a ferrite-forming element, can be added to the nuance at a level of 0.1% by weight and preferably less than 0.02% by weight to limit the formation of titanium nitrides formed in liquid steel especially.
We can also add to the nuance according to the invention of calcium, to obtain a calcium content of less than 0.03% by weight, and preferably greater than 0.0002% or even greater than 0.0005% in weight, in order to master the nature of the inclusions of oxides and to improve machinability. We limit the content of this element because it is likely to form with sulfur calcium sulphides which degrade properties corrosion resistance. In a preferred embodiment, limits the calcium content 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 at a content of less than 0.003% by weight. As we have seen above, this element forms sulphides with manganese or calcium, sulphides whose presence is harmful to the resistance to corrosion. It is considered an impurity:
An addition of magnesium up to a final 0.1% can be made to modify the nature of sulphides and oxides.
The selenium is preferably maintained at less than 0.005% by weight because of its detrimental effect on corrosion resistance. This

11 element is usually brought into the nuance as impurities of the ferro-manganese ingots.
Phosphorus is maintained at a content of less than 0.040% by weight and is considered an impurity.
The rest of the composition consists of iron and impurities. Outraged those already mentioned above, mention may also be made of zirconium, tin, arsenic, lead or bismuth. Tin can be present in one content less than 0.100% by weight and preferably less than 0.030% by weight weight 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 at a level below 0.002% by weight and preferably less than 0.0010% by weight. The bismuth may be present at a level of less than 0.0002% by weight and preferably less than 0.00005% by weight. Zirconium can be present at 0.02%.
In addition, the present inventors have found that when percentages by weight of chromium, molybdenum, nitrogen, nickel and manganese respect the relationship below, the relevant nuances have a good resistance to localized corrosion, ie to the formation of picks or caves:

IRCL =% Cr + 3.3 x% Mo + 16 x% N + 2.6 x% Ni - 0.7x% 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 volume ferrite and more particularly preferably from 35 to 55% by volume of ferrite.

The present inventors have also found that the following formula correctly reflects the ferrite content at 11 00 C:

12 1F = 6x (% Cr + 1.32x% Mo + 1.27x% Si) -10x (% Ni + 24x% 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 C, the IF index should be between 40 and 70.

In the annealed state, the microstructure does not contain other phases which would be harmful for its particular mechanical properties, such as 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 actual deformation temperature and the amount of deformation to cold applied.
In general, the steel according to the invention can be produced and manufactured in the form of hot-rolled sheets, also called sheets quarto, but also in the form of hot-rolled strips, from slabs or ingots and also in the form of cold-rolled strip from hot-rolled strips. It can also be hot rolled in bars or wire-rods or in profiles or forged; these products can be then hot-forged or cold-formed into bars or profiles drawn or drawn wire. The steel according to the invention can also be by molding followed or not heat treatment.
In order to obtain the best possible performances, we will use preferably the method according to the invention which comprises firstly the supply of a steel ingot, slab or bloom having a composition according to the invention.
This ingot, this slab or this bloom are generally obtained by fusion of raw materials in an electric oven, followed by a remelting Vacuum type AOD or VOD with decarburization. We can then cast the grade in the form of ingots, slabs or blooms by continuous casting in a bottomless mold. We could also consider casting the shade directly in the form of

13 thin slabs, in particular by continuous casting between contra-cylinders rotatable.
After supplying the ingot or slab or bloom, possibly reheat to reach a temperature between 1150 and 1280 C, but it is also possible to work directly on the slab just poured continuously, in the hot casting.
In the case of the manufacture of metal sheets, the slab or ingot to obtain a so-called quarto sheet which presents generally a thickness of between 5 and 100 mm. Rates of reduction usually employed at this stage vary between 3 and 30%. This sheet is then subjected to a solution heat treatment precipitates formed at this stage by reheating at a temperature between 900 and 1100 C, then cooled.
The process according to the invention provides quench cooling to the air which is easier to put in. work that cooling classically used for this type of grade, which is a = cooling faster, using water. However, it remains possible to proceed to a water cooling if desired.
This slow cooling, in the air, is made possible thanks to the limited nickel and molybdenum contents of the composition according to the invention which is not subject to the precipitation of phases intermetallic, harmful for its properties -use. This cooling can in particular be carried out at speeds ranging from 0.1 to 2.7 C / s.
At the end of the hot rolling, the quarto plate can be flattened, cut and stripped, if it is desired to deliver it in this state.
This bare steel can also be rolled on a belt train at thicknesses between 3 and 10 mm.
In the case of the manufacture of long products from ingots or of blooms, you can hot roll in one hot on a rolling mill multi-cages, in fluted cylinders, at a temperature between 1150 and 1280 C, to obtain a bar or a ring of wire rod or

14 the mine. The section relationship between the initial bloom and the final product is of preference greater than 3, so as to ensure the internal health of the product the mine.
When a bar has been manufactured, it is cooled out of rolling by simple air stranding.
When laminated wire with a diameter larger than 13 mm, it can be cooled by quenching in a tray of water at the output 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 coiled turns after passing them on the conveyor in 2 to 5 minutes through a furnace of solution dissolution at a temperature between 850 C and 1100 C.
Subsequent heat treatment in an oven, between 900 ° C. and 1100 ° C., can be practiced optionally on these bars or crowns already treated in the hot rolling, if one wishes to complete the recrystallization of the structure and slightly lower the characteristics mechanical traction.

After the cooling of these bars or crowns of son, it will be possible to carry out various treatments of formatting hot or cold, depending on the final use of the product. So, we can cold drawing the bars or drawing the wires, at the end cooling.
It will also be possible to cold profile hot rolled bars, or even make parts after having debited the bars plots and forging 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:
Figure 1: Correlation between% ferrite after-treatment at 1100 C and IF index for raw products - Figure 2: Relative diameter difference Delta 0 according to the deformation temperature - Figure 3: El puncture potential and E2 determined on bars forged according to the IRCL index 5 - Figure 4: Uniform corrosion rate V determined on bars forged according to the IRCL index - Figure 5: CTC and CPT critical temperatures determined on bars forged according to the IRCL index 10 Examples 25 kg laboratory ingots were made by melting vacuum induction of raw materials and pure ferroalloys, then nitrogen supply by addition of nitrided ferroalloys under pressure

15 of nitrogen and cast into a metal mold under external pressure of 0.8 bar of nitrogen. Of these, only tests 14441 and 14604 are according to the invention.
An industrial casting according to the invention of 150 tons referenced 8768 has been completed. This nuance was developed by fusing in the oven electric, then refined under vacuum with decarburization to reach the target carbon level. It was then continuously poured into slabs of section 220 x 1700 mm, then hot rolled after reheating to 1200 C in so-called quarto plates of thickness 7, 12 and 20mm. Sheet metal thus obtained are then subjected to a heat treatment to 1000 C in order to put in solution the various precipitates present at this Stadium. At the end of the heat treatment, the sheets are cooled to. the water then glided, cut and stripped.

The compositions in percentages by weight of the different shades developed in the laboratory or industrially are in Table 1, as well as those of different products or WO 2007/1445

16 PCT / FR2007 / 000994 industrial semi-finished products in electric furnace, refining at the OD, ingot or continuous casting, mentioned for comparison.

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1. Ferrite contents 1.1 Ferrite content on raw products On pieces of 1 to 8 cm3 cut from these laboratory in the raw state of casting or on industrial products in the state gross casting, was carried out in a salt bath, with quenching with water at the end of treatment, heat treatments of 30 minutes at temperature variable, to determine the proportion of ferrite at high temperature. The ferrite being magnetic, unlike austenite, carbides and any nitrides present, a method of measurement of saturation magnetization. The ferrite contents as well determined in Table 2 and reported in Figure 1.

If we consider Figure 1, we find a good correlation between the IF index and the ferrite contents measured on the base metal after treatments at 1104 C.

It is also noted that casting according to the invention, n 14441, present, below 1300 C, a ferrite content appropriate to the hot transformation 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.

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(c) 6 (d) (d) (d) (D .. ~ UUUUUUUUU ~
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U to cd rn ô 4 ~ NN ce) F + + + + tt F + + p ~

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1.2. Ferrite contents on finished products The ferrite content was also measured by the method of grid (according to ASTM E 562) on forged bars after treatment thermal at 1030 C and on thermally affected areas of cords weld electrode deposited with constant energy leading to cooling rates of 20 C / s at 700 C.
results (ferrite contents in base metal and in affected area thermally) are given in Table 3. It can be seen that the 14441 and 14604 according to the invention have a ferrite content in the base metal and in the thermally affected area favoring resistance to localized corrosion and * under tension, as well as resilience (see Table 5).

Table 3- Ferrite contents a MB to ZAT
REF Product (%) (%) 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 Sheet metal LAC 45 67 UNS S32304 Sheet metal LAC 47 75 according to the invention LAKE: Hot rolled;
a MB (%): ferrite content measured on the base metal a ZAT: ferrite content measured on the thermally affected zone 2. Flowability The 14439 ingot has blistered and is unusable. For avoid this phenomenon when casting under air pressure atmospheric, it is therefore necessary to limit the nitrogen content castings according to the invention less than 0.28% by weight.

3. Hot processing capacity The hot deformation capacity was evaluated using tests of hot traction, carried out on specimens whose calibrated portion, diameter 8 mm and length 5 mm, is heated by Joule effect during 80 seconds at 1280 C, then cooled to 2 C per second until test temperature which varies between 900 and 1280 C. When this temperature is reached, immediate pulling is initiated immediately, the speed of 73 mm / s; after rupture, the necking diameter is measured at the level of the break.

The relative diameter difference (Table 4), as defined below, makes account of the hot deformation capacity:

Delta 0 = 100 x (1 - (final diameter 1 initial diameter) Table 4: Relative Diameter Deviations (Hot Tensile Testing) Temperature Delta 0 (in%) Test (C) Casting 43382 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 x: according to the invention We can see from Table 4 and Figure 2 which in represents the data in the form of curves, that the casting 14441 according to the invention has a heat distortion capability comparable to that of comparative reference casting No. 14382.

4. Mechanical properties The tensile properties Reo, 2 and Rm were determined according to the NFEN 10002-1 standard. KV resilience has been determined at different temperatures according to standard NF EN 10045.

Table 5 - Mechanical characteristics KV KV
REF Product Re 0.2 Rm 20 C -50 C
(MPa) (MPa) (J) (J) 14441 * Forged bar 477 716 334 51 14604 * Forged bar 477 691 288 18 14382 Barreau fo.rgé 436 664> 339 339 14383 Forged bar 458 604 79 9 14660 Forged bar 493 701 293 31 304L Sheet Metal 218 523 312 301 316L Sheet metal 232 232 537 307 298 UNS S32101 Sheet Metal LAC 466 720 101 60 UNS S32304 Sheet Metal 438 663 268 153 8768 * Sheet metal LAC 519 743 according to the invention LAKE: Hot rolled;
Reo, 2: yield strength at 0.2% deformation R.: breaking strength.
The results of the 14441 and 14604 laboratory flows and the industrial casting 8768, all three according to the invention, show that a elastic limit greater than 45OMPa, twice that obtained for austenitic steels of type AISI 304L, can be obtained.
Resilience values determined at 20 C for laboratories 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 the casting 14383 outside the invention, low in nitrogen and high in ferrite in the annealed state, the resilience values at 20 C are lower 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 forged bars from laboratory flows and on coupons taken from hot-rolled sheets from castings, industrial.

5.1 Resistance to localized corrosion The resistance to pitting corrosion is evaluated by plotting curves potential intensities and determination of the pitting potential for i = 100Pa / cm2. This parameter was measured in a neutral medium (pH =
6.4) strongly chlorinated ([CI-] = 30g / l) at 50 ° C (El), representative of 10 brines encountered in water desalination plants sea, and in a slightly acidic medium (pH - 5.5) weakly chlorinated ([C! -] = 250ppm) at room temperature (E2), representative of a water drinking. The critical temperature of pitting in ferric chloride medium (FeCl3 6%) was also determined according to ASTM G48-00 Method C.
In another series of tests, the resistance to pitting corrosion in deaerated neutral medium at 0.86 Moles / liter in NaCl, corresponding to 5% by weight in NaCl, at 35 C. A measure of the potential abandonment for 900 seconds is performed. Then a curve 20 potentio-dynamic is plotted at a speed of 100 mV / min from abandonment to the potential of sting. The pitting potential (E3) is determined for i = 100 pA / cmZ. Under these conditions, samples according to the invention, as well as reference samples in grade 304L and in austenitic-ferritic duplex type 1.4362 and 25 others.
Cavernous corrosion resistance was studied by measuring the critical cave temperature in the neutral medium (pH = 6.4) strongly chlorinated ([CI-] = 30g / 1). The assembly to promote the cavernous corrosion is consistent with the recommendations made in the ASTM G78-99 standard. The critical cave temperature is Is minimum temperature for which caverns a depth greater than 25 pm were observed.

The values obtained are shown in Table 6. Comparison between the results obtained on the sheet in UNS S32304 and the resulting bar of casting 14382, both of similar chemical composition, indicate that the resistance to corrosion of a bar is lower than that a hot-rolled sheet of the same composition.
The present inventors have found that the resistance index to the localized corrosion, ie formation of bites or caves, abbreviated by IRCL and defined by:
SWIR = Cr + 3,3xMo + + 16xN 2,6xNi-0,7xMn (Cr, Mo, N, Ni and Mn contents in% by weight) gives a good account of the ranking of the set of compositions to less than 6% nickel in localized corrosion resistance (see Figures 3, 4 and 5).
Castings 14383 and 14660 outside the invention, with IRCL indices equal to 28.7 and 29.8, behave less well in corrosion than a steel 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. For obtain a corrosion resistance at least equal to that of the grade AISI
304L, we have found that the steels according to the invention must have preferably an IRCL greater than 30.5 and preferably greater than 32.

5.2 Uniform corrosion resistance Uniform corrosion was characterized by evaluating the speed of mass loss corrosion after immersion 72 hours in a 2% sulfuric acid solution brought to 44 C.

Comparison of corrosion rates for castings Experimental at 2.5% Ni and 0.2% N (14441, according to the invention, and 14660, 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 REF Product IRCL El E2 E3 CPT CCT V
(V / ECS) (V / ECS) (VIECS) (C) (C) (mm / yr) 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 Sheet metal LA NA 0.188 0.834 0.210 5 65 316L Sheet metal NA NA 0.266 0.865 7.5 75 UNS S32101 Sheet metal LAC 26.4 0.163 0.855 12.5 UNS S32304 Sheet metal LAC 35.7 0.413 1.330 '17.5 95 517077 Rolled bar 34.6 0.415 140301 Laminated bar 47,1 1,200 ' 8768 * Sheet metal LAC 33.1 0.227 1.2731 according to the invention : potential for oxidation of the solvent, no sting observed LAKE: hot soaked; NA - not applicable El: pitting potential in a neutral medium (pH = 6.4) and strongly chlorinated (30 g / l CI 2) at 50 ° C
E2: pitting potential in slightly acidic medium (pH = 5.5) and weakly chlorinated (250ppm CI) at 25 ° C
E3: 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 a neutral medium (pH = 6.4) and strongly chlorinated (30g / 1 CI) V: uniform corrosion rate in sulfuric acid medium 2% at 40 ° C

5.3 Potential for repassivation Steel samples are polished under water using SiC papers until 1200, then aged 24 hours in the air.
The cyclic polarization test carried out in a chlorinated medium is carried out starting with a measure of the abandonment potential for 15 minutes, followed by cyclic dynamic polarization at 100mV / min from abandonment potential up to the potential for which current is reached the intensity of 300pA / cmZ and the return to the potential for which the current, is zero.
The values of the puncture potentials (Vpit) and repassivation potentials (Vrepassivation) of the bites previously formed. The results obtained are collated in Table 7.

Table 7 - Repassivation by nickel level Casting% Ni Vpit - Vrepassivation (mV / FCS) 14382 4.5460 14441 2.5 361 14383 1.5 227 Based on the NaCi repassivation potential tests, more the nickel level is high, the greater the difference between the pitting potential and the repassivation potential is high, which shows that nickel is not beneficial to the repassivation of a shade according to the invention having previously suffered a puncture attack.

Claims (27)

1. Duplex stainless steel, the composition of which consists of, in%
weight:
C <= 0.05%
21%: <= Cr <= 25%
1%: 5 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%
Ca <0.03%

the rest being iron and impurities resulting from the elaboration and the microstructure being consisting of austenite and 35 to 65% ferrite by volume, said composition respecting in in addition to the following relationships:

40 <= IF <= 70 with IF = 6x (% Cr +% Mo + 1,32x 1,27x% Si) -10x (% Ni + 24x% C +% N + 0.5x 16,15x% 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. Steel according to claim 1, characterized in that:

IRCL> = 32. 3. Steel according to claims 1 or 2, characterized in that the proportion ferrite is between 35 and 55% by volume. 4. Steel according to any one of claims 1 to 3, characterized in that than 40 <= IF <= 60. 5. Steel according to any one of claims 1 to 4, characterized in that that the chromium content is between 22 and 24% by weight. Steel according to one of Claims 1 to 5, characterized in that that the Manganese content is less than 1.5% by weight. Steel according to one of claims 1 to 6, characterized in that that the Molybdenum content is greater than 0.1% by weight. 8. A method of manufacturing a sheet, strip or coil rolled at hot in steel according to any one of claims 1 to 7, wherein:

supplying an ingot or a slab of a composition steel according to Moon any of claims 1 to 7, and said billet or said slab is rolled while hot, at a temperature of enter 1150 and 1280 ° C to obtain a sheet, strip or coil. 9. Process for manufacturing a hot rolled steel sheet according to the claim 8, according to which:

said billet or said slab is rolled while hot, at a temperature of enter 1150 and 1280 ° C to obtain a sheet called quarto, 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. 10. Hot-rolled steel sheet, known as quarto, obtainable by process according to claim 9 and having a thickness of between 5 and 100 mm. 11. Use of a so-called quarto sheet according to claim 10, or a rolled coil to heat obtained by the process according to claim 8, for the manufacture of elements of structures for material production or production facilities energy. The use of claim 11, wherein said facilities of Material and energy productions operate between -100 and 300 ° C. 13. Use according to claim 12, wherein said facilities of Matter and energy productions operate between -50 and 300 ° C. 14. Cold-rolled steel strip obtainable by cold rolling a reel hot rolled obtained by the process according to claim 8. 15. Process for manufacturing a steel hot-rolled bar or wire according to one any of claims 1 to 7, wherein:

supplying an ingot or bloom of continuous casting of a steel of composition according to any one of claims 1 to 7, hot rolled said ingot or said bloom, from a temperature of between 1150 and 1280 ° C for get a bar that is air-cooled or a wire crown that one cool to 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. 16. Hot rolled bar obtainable by the process according to claim 15, and having a diameter of 18 mm to 250 mm and hot rolled wire can to be obtained by the process according to claim 15 and having a diameter of 4 to 30 mm. 17. Manufacturing process according to claim 15, according to which stretching cold of said bar or a drawing of said wire, at the end of cooling. 18. 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 17 having a diameter of 0.010 mm to 20 mm. 19. Use of a bar according to claims 16 or 18, for the parts manufacturing mechanical. 20. Use of a bar according to claim 19, characterized in that rooms are pumps, valve shafts, motor shafts and fittings operating in corrosive environments. 21. Use of a thread according to claims 16 or 18, for the manufacturing cold formed assemblies, for the food industry, extraction oil and ores, or for the manufacture of metal fabrics and knits for product filtration chemical, mineral or food materials. 22. A method of manufacturing a steel profile, according to which a profiling to of a hot-rolled bar obtained by the process according to claim 15. 23. Steel profile obtainable by the method according to the claim 22. 24. Process for manufacturing a steel forgings according to which debiting in lopins a hot rolled bar obtained by the process according to claim 15, then we perform forging said billet between 1100 ° C and 1280 ° C. 25. Steel forgings obtainable by the process according to claim 24. 26. Use of a forging according to claim 24, for the manufacture of flanges or fittings. 27. Molded part obtainable by molding a steel according to one of any Claims 1 to 8.