MX2008016172A - 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 Description of the Invention The present invention relates to a duplex stainless steel, more particularly it is proposed for the manufacture of structural components for material production facilities (chemical, petrochemical, paper and maritime industries) or energy production facilities, without that is limited in some way to these, and also to the process for. manufacture, plates, strips, bars, rods, wire or sections of this steel. More generally, this steel can be used as a substitute for a type 304L stainless steel in many applications, for example in the previous industries or in the agri-food industry, including parts produced from wire or formed bar (welded grills, etc.), sections (strainers, etc.), axes, etc. It is also possible to produce castings and slabs. For this purpose, grades of type 304 and type 304L stainless steel are known, which, in the non-annealed state, have an essentially austenitic microstructure. In the cold worked state they may also contain a variable proportion of martensite. However, these steels include large additions of nickel, the cost of which is generally prohibitive. In addition, these Ref. 199002 grades may have problems from a technical point of view in certain applications, since they have poor tensile properties in the annealed state, especially with respect to the yield strength, and a rather low resistance to stress corrosion. Also known are austenitic-ferritic stainless steels, which are mainly composed of a mixture of ferrite and austenite, such as steels 1.4362, 1.4655, 1.4462, 1.4507, 1.4410, 1.4501 and 1.4424 according to the standard EP10088, which contain more than 3.5% nickel. These steels are particularly resistant to corrosion and to corrosion under tension. The grades of ferritic or ferritic-martensitic stainless steel are also known, the microstructure of which, for a defined range of heat treatments, is composed of two constituents ferrite and martensite - preferably in a 50/50 ratio, such as the degree 1.4017 in accordance with the standard EN10088. These grades, with a chromium content generally less than 20%, have high traction mechanical properties but have no satisfactory corrosion resistance. In addition, it is also desirable to simplify the process for manufacturing plate, strip, bar, rod, wire or steel sections.

The object of the present invention is to remedy the disadvantages of the steels and manufacturing processes of the prior art by providing a stainless steel exhibiting good mechanical properties and in particular a tensile yield limit greater than 400 or even 450 MPa in the state treated with solution or annealing, a high resistance to corrosion, in particular the same or better than that of 304L, good microstructural stability and good tenacity of the welded areas, without adding expensive addition elements, and also providing a process for manufacturing plate , strip, bar, rod, wire or sections of this steel that is simpler to implement. For this purpose, the first object of the invention is 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% or + 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 is iron and impurities resulting from the fusion, and the microstructure consists of austenite and 35 to 65% by volume of ferrite, the composition also satisfies the following ratios: 40 < IF < 70, preferably 40 < IF < 60 where IF = 6x (% Cr + 1.32x% Mo + 1.27x% Si) - 10x (% Ni + 24x% C + 16.15x% N + 0.5x% Cu + 0.4x% Mn) - 6.17 and ILCR 30.5, preferably = 32 where ILCR =% Cr + 3.3x% Mo + 16x% N + 2.6x% Ni - 0.7x% n. The steel according to the invention can also include the following optional features, taken individually or in combination: the proportion of ferrite is between 35 and 55% in 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; and the molybdenum content is greater than 0.1% by weight. A second object of the invention consists of a process for hot-rolled plate, strip or spiral made of steel according to the invention, in which: a ingot or plate of a steel of composition according to the invention is provided; and the ingot or plate is hot rolled at a temperature between 1150 and 1280 ° C to obtain plate, strip or spiral. In a particular method of implementation, the ingot is hot-rolled at a temperature between 1150 and 1280 ° C to obtain what is called a fourth plate; then a heat treatment is carried out at a temperature between 900 and 1100 ° C; and the plate is cooled by an annealing with air. A third object of the invention consists of a process for manufacturing hot-rolled rod or rod made of steel according to the invention, in which: a steel ingot or billet is continuously cast of the composition of the invention; the billet is rolled hot from a temperature between 1150 and 1280 ° C to obtain a bar, which is cooled with air, or a roll of wire material which is cooled with water; and then, optionally; a heat treatment is carried out at a temperature between 900 and 1100 ° C; and the bar or a roll of wire material is cooled by quenching. In a particular method of implementation, a cold drawing operation is performed on the bar or a die stretching operation is performed on the bar, after being cooled. The invention also covers a process for manufacturing a steel section, in which a cold forming operation is carried out in a hot-rolled bar obtained according to the invention, and also a process for manufacturing a steel forging, in the which a hot-rolled bar obtained according to the invention is cut into blanks and then a forging operation is performed on the blanks between 1100 ° C and 1280 ° C. The invention also covers several products that are they can be obtained by the processes according to the invention and also their uses, such as: fourth plate of hot-rolled steel, having a thickness between 5 and 100 mm, and strip and roll, which can be used for the manufacture of components structural for installations for the production of energy or production of material, in particular for installations for the production of energy and material operating between -100 and 300 ° C, preferably between -50 and 300 ° C; cold rolled steel strip that can be obtained by cold rolling a hot rolled roll; Hot rolled bar having a diameter of 18mm to 250mm and cold drawn bar having a diameter of 4mm to 60mm, the products can be used for the manufacture of mechanical parts such as pumps, valve shafts, axles of engine and propeller and couplings operating in corrosive media; - hot rolled bar having a diameter of 4 to 30 mm and bar or wire stretched in die that has a diameter of 0.010 mm to 20 mm, the products can be used for the manufacture of cold formed assemblies, for the agri-food industry, for oil extraction and mineral, or for the manufacture of woven or knitted woven wire cloth for the filtration of chemicals, minerals or food; sections; forgings that can be used for the manufacture of clamps or couplings; and castings that can be obtained by casting a steel according to the invention. Other features and advantages of the invention will become apparent upon reading the following description, given only by way of example. The duplex stainless steel according to the invention comprises the contents defined below. The carbon content of the grade is equal to or less than 0.05%, preferably less than 0.03%, by weight. This is because too high a content of this element degrades the resistance to localized corrosion increasing the risk of chrome carbides precipitating in the heat-affected weld zones. The chromium content of the grade is between 21 and 25% by weight, preferably between 22 and 24% by weight, to obtain good corrosion resistance, which is at least equivalent to that obtained with grades 304 or 304L.

The nickel content of the grade is between 1 and 2.95%, preferably equal to or less than 2.7%, or even 2.5% by weight. This austenite-forming element is added to obtain good resistance to interstitial corrosion. At contents of more than 1% and preferably more than 1.2% by weight, it has a favorable effect in combating the initiation of pitting corrosion. However, its content is limited since above 2.95% by weight there is a degradation in the resistance to sting propagation. The addition of nickel also makes it possible to obtain a good tenacity / ductility arrangement since it has the benefit of transferring the toughness transition curve towards low temperatures, this is particularly advantageous for the manufacture of thick plate, for which the properties of Tenacity are important. Since the content of nickel in the steel according to the invention is limited, it has been found that it is necessary, in order to obtain an appropriate austenite content after the 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% by weight, which generally means that nitrogen is added to the steel during melting. This element Austenite former first makes it possible to obtain two-phase ferrite-austenite duplex steel which contains an appropriate ratio of austenite which exhibits good corrosion resistance under tension, and also to obtain metal with high mechanical properties. It also makes it possible to have good microstructural stability in weld areas affected by heat. Its maximum content is limited since, above 0.28%, a problem of solubility can be observed: formation of blisters during the solidification of plates, billets, ingots, castings or welds. The manganese content, which is also an austenite-forming element below 1150 ° C, remains below 2.0% by weight, and preferably below 1.5% by weight, due to the harmful effects of this element in many counts. Consequently, it has problems during the melting and refining of the grade since it attacks certain refractories used for the spoons. This requires that these costly items be replaced more frequently, and therefore causes the process to be interrupted more frequently. The additions of ferro-manganese normally used to get the grade to the composition also contain appreciable amounts of phosphorus and also selenium, the elements are not desirable to introduce into the steel and are difficult to remove when the grade is refined. Manganese also alters this refinement, limiting the possibility of decarburization. It also has an additional problem downstream in the process, since it deteriorates the corrosion resistance of the grade due to the formation of manganese sulphides (MnS) and oxidized inclusions. This element is conventionally added to degrees that it is desired to enrich with nitrogen, to increase the solubility of this element in the degree. Without sufficient manganese content, it is therefore not possible to achieve such a nitrogen level in the steel. However, the inventors have found that it is possible to limit the addition of manganese in the steel according to the invention, while still adding sufficient nitrogen to obtain the desired effect in the ferrite-austenite balance of the base metal and to stabilize the zones. of welds affected by heat. Molybdenum, a ferrite-forming element, is maintained at a content of less than 0.45% by weight, while tungsten is maintained at a content of less than 0.15% by weight. In addition, 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 even more preferably less than 0.3% by weight. This is because the present inventors found that by keeping these two elements, and with the sum of their contents below the indicated values, no precipitates intermetallic precipitation is observed, making it possible, in particular, to relax the process to manufacture plate or steel strip allowing the plate or strip to be cooled with air after heat treatment or hot processing. In addition, they have observed that, by controlling these elements within the claimed limits, the weldability of the grade is improved. However, it is preferred to maintain a minimum molybdenum content of 0.1% to improve the hot forgeability of the grade. In addition, the melting of a grade that has less than 0.1% molybdenum could imply greatly limiting the use of recirculation waste for this grade, having processing problems, in particular requiring that a change consisting of 100% ferric pure alloys. Copper, an austenite-forming element, is present in an amount of between 0.11 and 0.50% by weight, and preferably between 0.15 and 0.40% by weight. This element improves the resistance to corrosion in an acid reducing medium. However, its content is limited to 0.50% by weight to prevent the formation of epsilon phases, which is desirable to avoid, since these result. in the hardening of the ferritic phase and brittleness of the duplex alloy.

The oxygen content is preferably limited to 0.010% by weight, to improve its forgeability. Boron is an optional element that can be added to the degree according to the invention in an amount between 0.0005% and 0.01% by weight, preferably between 0.0005% and 0.005%. and even more preferably between 0.0005% and 0.003% by weight, to improve its hot conversion. However, in another embodiment, it is preferred to limit the boron content to less than 0.0005% by weight to limit the risk of cracking during welding and continuous casting. Silicon, a ferrite-forming element, is present with a content of less than 1.4% by weight. Aluminum, a ferrite-forming element, is present with a content of less than 0.05% by weight and preferably between 0.005% and 0.040% by weight, to obtain inclusions of calcium aluminate of low melting point. The maximum aluminum content is also limited, to avoid the excessive formation of aluminum nitrides. The action of these two elements - silicon and aluminum - is essentially to ensure the correct deoxidation of the steel bath during melting. The cobalt, an austenite forming element, is maintained at a content of less than 0.5% by weight, preferably less than 0.3% by weight. This element is a residual element provided by the raw material. In In particular, it is limited due to the handling problems it may have after the irradiation of components in nuclear power plants. Rare earth metals (denoted by REM) can be added to the composition in an amount of 0.1% by weight, preferably less than 0.06% by weight. Mention may be made, in particular, of cerium and lanthanum. The contents of these two elements are limited, since they are responsible for forming undesirable intermetallics. Vanadium, a ferrite forming element, can be added to the grade in an amount of 0.5% by weight and preferably less than 0.2%, to improve the interstitial corrosion resistance of the steel. Niobium, a ferrite-forming element, may be added to the degree in an amount of 0.3% by weight and preferably less than 0.050% by weight. It helps to improve the tensile strength of the grade, thanks to the formation of fine niobium nitrides. Its content is limited, to limit the formation of thick niobium nitrides. Titanium, a ferrite forming element, may be added to the degree in an amount of 0.1% by weight and preferably less than 0.02% by weight, to limit the formation of titanium nitrides formed, in particular, in liquid steel. It is also possible to add calcium to the degree of according to the invention, to obtain a calcium content of less than 0.03% by weight and preferably more than 0.0002% or even more than 0.0005% by weight, to control the nature of the oxide inclusions and improve the machinability. The content of this element is limited, since it is responsible for forming, with sulfur, calcium sulfides which degrade the properties of resistance to corrosion. In a preferred embodiment, the calcium content is limited to less than 0.0005%, preferably less than 0.0002% by weight. The sulfur is maintained at a content of less than 0.010% by weight and preferably with a content of less than 0.003% by weight. As mentioned above, this element forms sulphides with manganese or calcium, the presence of sulfides is harmful to the corrosion resistance. Sulfur is considered an impurity. An addition of magnesium in an amount with a final content of 0.1% can be done to modify the nature of the sulfides and oxides. Selenium is preferably kept less than 0. 005% by weight due to its harmful effect on corrosion resistance. This element is, in general, introduced into the grade as ferrite impurities and manganese ingots. Phosphorus is maintained with a content of less than 0.040% by weight, and is considered as an impurity.

The rest of the composition consists of iron and impurities. Different from those mentioned above, these impurities can also be zirconium, tin, arsenic, lead or bismuth. The tin may be present with a content of less than 0.100% by weight and preferably less than 0.030% by weight to prevent welding problems. The arsenic may be present with a content of less than 0.030% by weight and preferably less than 0.020% by weight. The lead may be present with a content of less than 0.002% by weight and preferably less than 0.0010% by weight. The bismuth may be present with a content of less than 0.0002% by weight and preferably less than 0.00005% by weight. Zirconium can be present in an amount of 0.02%. In addition, the present inventors have found that, when the percentages by weight of chromium, molybdenum, nitrogen, nickel and manganese satisfy the subsequent relationship, the grades in question exhibit good resistance to localized corrosion, ie resistance to the formation of pits or interstices ILCR =% Cr + 3.3x% Mo + 16x% N + 2.6x% 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, the phases are preferably, after treatment for 1 h at 1000 ° C, present with a proportion of ferrite from 35 to 65% by volume and more particularly preferably from 35 to 55% by volume. The present inventors have also found that the following formula adequately describes the ferrite content at 1100 ° C: IF = 6x (% Cr + 1.32x% Mo + 1.27x% Si) - 10x (% Ni + 24x% C + 16.15x % N + 0.5x% Cu + 0.4x% Mn) - 6.17. Therefore, to obtain a ferrite content between 35 and 65% at 1100 ° C, the ferrite index IF should be between 40 and 70. In the annealed state, the microstructure does not contain other phases that could be harmful to its mechanical properties , especially such as the sigma phase and other intermetallic phases. In the cold worked state, part of the austenite may have been converted to martensite, depending on the current deformation temperature and the amount of cold deformation applied. In general, the steel according to the invention can be melted and manufactured in the form of a hot-rolled plate, again called a fourth plate, but also in the form of a hot-rolled strip, of plates or ingots, and also in the form of cold rolled strip of hot rolled strip. It can also be hot rolled in the form of rod or bar material or sections or slabs. These products can then be converted into hot by Forged or cold converted into bar or stretched sections or die-stretched wire. The steel according to the invention can also be processed by casting, if it is followed by heat treatment or not. To obtain the best possible performance characteristic, it will be preferred to use the process according to the invention which first includes the provision of a steel ingot, plate or billet having a composition according to the invention. This ingot, plate or billet is generally obtained by melting the raw materials in an electric furnace followed by vacuum remelting of the AOD or VOD type with decarburization. The grade can then be cast in the form of ingots, or in the form of plates or billets by continuous casting in a bottomless mold. It is also conceivable to strain the grade directly in the form of thin plates, in particular by continuous casting between counter-rotating rolls. After having provided the ingot, plate or billet, it can optionally be reheated to reach a temperature between 1150 and 1280 ° C, but it is also possible to work directly on the plate immediately after being cast continuously, while it is still hot.

In the case of plate manufacture, the plate or ingot is then hot rolled to obtain a fourth plate which generally has a thickness between 5 and 100 mm. The reduction ratios generally used in this stage vary between 3 and 30%. This plate then undergoes a heat treatment to put the precipitates, formed in this stage, back into solution by reheating at a temperature between 900 and 1100 ° C, and then cooled. The process according to the invention provides quenching cooling with air, which is easier to implement than the conventionally used cooling for this type of grade, cooling is faster, using water. However, it remains possible to perform a cooling operation with water, if desired. This cooling with slow air is particularly possible thanks to the limited nickel and molybdenum content of the composition according to the invention, which is not subject to the precipitation of intermetallic phases harmful to its properties of use. This cooling, in particular, can be carried out at a speed that varies from 0.1 to 2.7 ° C / s. After being hot rolled, the plate fourth can be leveled, cut and stripped if you wish to supply it in this state. It is also possible to laminate this bare steel in a strip mill below a thickness of between 3 and 10 mm. In the case of manufacturing long products of billets or billets, hot rolling in a single hot passage in a multiple box mill, between grooved rollers, at a temperature between 1150 and 1280 ° C is possible to obtain a bar or roll of wire material, or laminate. The cross-sectional relationship between the initial billet and the final product is preferably greater than 3, to ensure the strength of the rolled product. When a bar has been manufactured, it is cooled leaving the laminator, by simple dispersion of air. When the rolled rod with a diameter of more than 13 mm is manufactured, it can be cooled, leaving the laminator, by tempering in a roll form in a water tank. When the rod with a diameter of 13 mm or less is manufactured, it can be cooled by tempering with water in the form of extended turns on a conveyor after the turns have passed to along a conveyor for 2 to 5 minutes through a treatment furnace with solution at a temperature between 850 ° C and 1100 ° C. A subsequent furnace heat treatment between 900 ° C and 1100 ° C can optionally be carried out on this bar or roll, already treated after hot rolling, if desired to complete the recrystallization of the structure and slightly reduce the tensile properties . After this bar or rod roll has cooled, several hot forming or cold forming treatments can be carried out, depending on the final use of the product. The bar may undergo a cold drawing operation or the rod may undergo a die stretching operation, after being cooled. The hot rolled bar can also be cold formed or parts can be manufactured after the bar has been cut into blanks and then forged. To illustrate the invention, tests were made and these will be described, in particular, with reference to figures 1 to 5 which show: Figure 1: A correlation between% ferrite after treatment at 1100 ° C and IF for products as they processed; Figure 2: A relative diametral change? 0 as a function of the deformation temperature; Figure 3: Pitting potentials El and E2, determined in forged bars, as a function of the index I LCR r Figure 4: The uniform corrosion rate V, determined in forged bars, as a function of the index - Figure 5: Critical temperatures TCc and TCp, determined in forged bars, as a function of the index EXAMPLES Laboratory ingots of 25 kg were produced by vacuum induction melting of ferro-pure alloy raw materials, followed by introduction of nitrogen by the addition of nitrided ferro-alloys under a partial pressure of nitrogen and casting in a metal mold under a external nitrogen pressure of 0.8 bar. Among these, only trials 14441 and 14604 were in accordance with the invention. An industrial melting charge according to the invention of 150 tons referenced as 8768 was produced. This grade was melted by melting in a The electric furnace was then refined in vacuum with decarburization to achieve the proposed carbon level. Then it was continuously cast in plates measuring 220x1700 mm cross section before being hot rolled, after reheating to 1200 ° C, in fourth plates with thicknesses of 7, 12 and 20 mm. The plates thus obtained then underwent a heat treatment at around 1000 ° C to place the various precipitates present in this stage again in solution. After the heat treatment, the plates were cooled with water, then leveled, cut and pickled. The compositions in percentages by weight of the various grades melted on a laboratory scale or an industrial scale are given in table 1, together with the compositions of the various industrial products or semi-finished products melted in an electric furnace, followed by refining AOD and cast in ingots or continuous casting, these are mentioned for comparison.

Table 1 barga and fusion No. 14441 14604 8768 14382 14383 14439 14426 14422 14425 14424 14660 Product 25 kg 25 kg 1501 25 kg 25 kg 25 kg 25 kg 25 kg 25 kg 25 kg 25 kg At 0.014 0.012 0.0042 0.010 0.015 0.014 < 0.002 < 0.002 0.024 C 0.016 0.028 0.020 0.020 0.02O 0.017 0021 0.022 0 019 0.020 0.024 Cr 23.07 22.60 22.83 23.03 23.01 23.05 26.67 26.58 26.68 26.61 22.79 Cu 0.301 0.300 0.15 0.304 0.297 0.299 0.279 0.280 0.280 0.208 0.284 5 Mn 1,282 1,284 1.25 1,288 1,277 1,309 0.724 0.706 0.723 0.705 4.780 Mo 0.24 »0.249 0.35 0261 0.250 0.251 1.322 1.337 1.327 1 328 0.296 N 0.212 0.239 0.21 0.110 0.110 0.280 0.119 0.117. 0.300 0.237 0.199 NI 2.539 1.692 2.50 4.249 1.552 1.485 4.632 1.419 1.541 2 549 2.470 0 0.0049 0.0038 0.0042 0.0031 0.0039 0.0052 0.0316 0.0284 0.0205 0.0221 0.0033 P 0.023 0.023 0.024 0.024 0.024 0.022 0.025 0.022 0025 0.022 0.025 S 0.0009 0.0010 0.0005 0.0008 0.0008 0.0009 0.0209 0.0203 0.0210 0.0203 0.0014 SI 0.430 0.35T 0.44 0.399 0.455 0.403 0.424 0.391 0.407 0.408 0.49 V 0.121 0.061 0.064 0.123 0.122 0.120 0.106 0.102 0.109 0.107 0.013 W < 0.010 < 0.010 o.o-is < 0.010 < 0.010 < 0.010 < 0.010 < 0.010 < 0010 < 0.010 < 0.010 Ti 0.0046 0.0017 0.007 0.0027 0.0039 0.0027 0.0041 0.0059 0.0047 0.0050 0.0011 Zr 0.0048 0.0052 0.0042 0.0049 0.0055 0.0064 0.0055 00060 0.0058 0.0072 0.0083 Colt; 0.002 < 0.002 0.041 < 0.002 < 0.0O2 < 0.002 0.002 < 0.002 < 0.002 < 0.002 < 0.002 Ca < 0.0005 < 0.0005 0.0003 < 0.0005 < 0.0005 0.005 < 0.0005 «0.005 < 0.0005 < 0.0005 O.0002 Nb < 0.002 < 0.002 0.0008 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 It < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.OO2 15 As < 0.002 < 0.002 < 0.002 < O.002 < 0002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 < 0.002 Ce- »La < 0.0OO2 O.0002 0.0DO2 O 0002 < O.O0O2 < 0.0002 < 0.0002 < 0.0002 O.0002 0 0002 < 0.0002 Mg < 0.0005 < 0.OOD5 0.0004 < 0.0005 < 0.0005 < 0 OO05 < 0.0005 < 0.0005 < 0.0005 < 0.0005 < 0.0005 1 B < 0.0005 < 0.0005 0.0024 < 0.0005 < 0.0005 < 0.0005 < 0.0005 < 0.0005 < 0.0005 < 0.0005 < 0.0005 Table 1 - (continued) 5 *: Plate or billet or laminated bar 1. Ferrite contents 1.1 Ferrite contents in products as processed Specimens ranging from 1 to 8 cm3 were cut from these laboratory melt loads in the cast state or from industrial products in the cast state, and heat treatments were performed for 30 days. minutes at various temperatures in these specimens, in a salt bath, followed by a tempering with water at the end of treatment, to determine the proportion of ferrite at high temperature. Since the ferrite is magnetic, different from the austenite, carbides and nitrides possibly present, a test method was used in which the saturation magnetization was measured. The ferrite contents thus determined are given in table 2 and plotted in figure 1. Figure 1 shows that there is a good correlation between the IF index and the ferrite contents measured in the base metal after the treatments at 1100 ° C. It has also been shown that the melt charge 14441 according to the invention has, below 1300 ° C, an appropriate ferrite content for hot transformation to a duplex structure. In addition, after heat treatment in the range of 950 ° C to 1100 ° C, it has an appropriate ferrite content for resistance to stress corrosion.

Table 2 5 CCB = Continuously cast slurry 15 1. 2. Ferrite content in final products The ferrite content was also measured by the grid method (according to the ASTM E 562 standard) in forged bars after heat treatment at 1030 ° C and in areas affected by the heat of welding cords deposited by a coated electrode with a constant energy that results in cooling speeds at 700 ° C of 20 ° C / s. The results (ferrite contents of the base metal and of the zone affected with heat) are given in Table 3. This shows that the melt charges 14441 and 14604 according to the invention have a ferrite content in the base metal and in the affected area with the heat that is favorable to localized corrosion resistance and resistance to corrosion under tension, and also favorable to tenacity (see table 5).

Table 3 - Ferrite contents *: according to the invention; HR: hot rolled; OIBM (%): ferrite content measured in the base metal; (???? '· Ferrite content measured in the affected area with heat. 2. Collability Ingot 14439 exhibited blisters and was unusable. To avoid this phenomenon during casting in air at atmospheric pressure, it was necessary to limit the nitrogen content of the melt charges according to the invention to less than 0.28% by weight. 3. Hot transformation capacity Hot deformability was evaluated using hot tensile tests performed on test specimens, the calibrated part of which, having a diameter of 8 mm and a length of 5 mm, was heated by heating Joule by 80 seconds at 1280 ° C and then cooled to 2 ° C per second below the test temperature, which varied between 900 and 1280 ° C. When this temperature was reached, the rapid traction test was started immediately, at the speed of 73 mm / s; After the fracture, the diameter of the crack in the fracture was measured. The relative diametral change (table 4), as defined later, reflects hot deformability. ? 0 = 100x (l - (final diameter / initial diameter)).

Table 4: Relative diametral changes (hot traction test) *: according to the invention. In the examination of Table 4 and Figure 2, which present the data in the form of curves, it can be seen that the melt charge 14441 according to the invention has a heat deformability comparable to that of the melt charge 14382 of the comparative reference.

The traction properties determined in accordance with standard NFEN 10002-1. Kv tenacity was determined at various temperatures in accordance with the NF EN 10045 standard.

Table 5 - Mechanical Properties *: according to the invention; HR: hot rolled; Reo.2: at 0.2% deformation of the elasticity limit Rm: tensile strength.

The results of the laboratory fusion loads 14441 and 14604 and of the industrial fusion load 8768, all three according to the invention, show that an elasticity limit greater than 450 MPa can be obtained, that is, twice that one obtained by austenitic steels of type AISI 304L.

The tenacity values determined at 20 ° C for the laboratory fusion loads 14441 and 14604 and for the industrial fusion load 8768, all three according to the invention, are all greater than 200 J, this is satisfactory taking into account the level of elasticity limit of these grades. For the melt charge 14383 not according to the invention, which has a low nitrogen content and a high ferrite content in the annealed state, the toughness values at 20 ° C are below 100 J. This confirms the need of a sufficient nitrogen addition to obtain a satisfactory level of toughness. 5. Corrosion resistance Corrosion resistance tests were performed both on the forged bars of the laboratory fusion loads and on samples removed from hot-rolled plates that come from the industrial fusion loads. 5. 1 Resistance to localized corrosion The resistance to pitting corrosion was evaluated by plotting the current-potential curves and determining the pitting potential for i = 100 μ? / Cm2. This parameter was measured in a neutral medium (pH = 6.4) with a high concentration of chloride ([Cl "] = 30 g / 1) at 50 ° C (Ei), representative of the brine found in seawater desalination plants, and in a slightly acidic medium (pH = 5.5) with a low concentration of chloride ([Cl ~] = 250 ppm) at room temperature (E2) representative of water to drink. The critical pit temperature in a ferric chloride medium (6% FeCl3) was also determined in accordance with the ASTM G48-00 standard, method C. In another series of tests, the resistance to pitting corrosion was determined in a medium neutral deaerated containing 0.86 mol / liter of NaCl, corresponding to 5% NaCl by weight, at 35 ° C. A potential flotation measurement was performed for 900 seconds. Next, a potentiodynamic curve was plotted at a rate of 100 mV / min from the flotation potential to the pitting potential. The pitting potential (E3) was determined for i = 100 μ? / Cm2. Under these conditions, the specimens according to the invention and reference specimens of grade 304L and austenitic-ferritic grades of type 1.4362 and others were tested. The resistance to interstitial corrosion was studied by measuring the critical interstitial temperature in the neutral medium (pH = 6.4) with a high concentration of chloride ([Cl ~] = 30 g / 1). The arrangement that favors the interstitial flotation corrosion was, according to the recommendations given in the ASTM G78-99 standard. The Critical interstitial temperature is the minimum temperature at which interstices with a depth greater than 25 μp are observed. The values obtained are given in Table 6. The comparison between the results obtained in the plate made of UNS S32304 and the bar obtained from the heat 14382, these two are 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 index of localized corrosion resistance, ie resistance to the formation of pits or interstices, abbreviated as ILCR and defined by: ILCR = Cr + 3.3xMo + 16xN + 2.6xNi - 0.7xMn (contents of Cr, Mo, N, Ni and Mn in% by weight) adds the classification of all compositions that contain less than 6% nickel in terms of resistance to localized corrosion (see Figures 3, 4 and 5). The melt charges 14383 and 14660 not according to the invention, which have ILCR indices equal to 28.7 and 29.8, exhibit a lower corrosion behavior than an AISI 304L type steel. Melt charges 14604 and 14441 according to the invention, having an ILCR of 30.9 and 33, behave at least as steel type 304L. To get A corrosion resistance at least equal to that of the AISI 304L grade, it has been found that the steels according to the invention should preferably have an ILCR greater than 30.5 and preferably greater than 32. 5. 2 Uniform corrosion resistance Uniform corrosion was characterized by evaluating the corrosion rate by weight loss after immersion for 72 hours in a 2% diluted sulfuric acid solution heated to 40 ° C. The comparison of the corrosion rates for the experimental melt charge containing 2.5% Ni and 0.2% N (14441 according to the invention and 14660 not according to the invention) clearly shows the negative effect of a high content of n in the resistance to uniform corrosion in a sulfuric medium.

Table 6 - Uniform localized corrosion resistance data Ei E2 E3 Tcc V Ref. Product ILCR (V / ECS) (V / ECS) (V / ECS) (° C) (° C) (mm / y) rod 14441 * 33.0 0.165 1.058 0.320 7.5 50 0.73 forged 14604 * bar 30.9 0.159 0.802 5 45 1.8 forged Ei E2 E3 Tcc V Ref. Product ILCR (V / ECS) (V / ECS) (V / ECS) (° C) (° C) (mm / y) bar 14382 35.8 0.302 1.323 0.420 15 60 0.24 forged bar 14383 28.7 0.049 0.595 0.050 0 35 4.95 forged ada bar 14660 29.8 0.094 0.707 7.5 45 1.11 forged 304L plate HR NA 0.188 0.834 0.210 5 65 316L plate HR NA 0.266 0.865 7.5 75 UNS plate HR 26.4 0.163 0.855 12.5 S32101 UNS plate HR 35.7 0.413 1.3301 17.5 95 S32304 bar 517077 34.6 0.415 laminated bar 140301 47.1 1.2001 laminated 8768 * plate HR 33.1 0.227 1.2731 *: according to the invention; 1: oxidation potential of the solvent, no stings observed; HR: hot rolled; NA: not applicable; ??: pitting potential in neutral medium (pH = 6.4) having a high chloride concentration (30 g / 1 Cl ~) at 50 ° C; E2: pitting potential in slightly acidic environment (pH = 5.5) having a low concentration of chloride (250 ppm Cl ") at 25 ° C; E3: pitting potential in neutral chloride medium (5%) NaCl) at 35 ° C; critical interstitial temperature in a ferric chloride medium; critical interstitial temperature in neutral medium (pH = 6.4) with a high chloride concentration (30 g / 1 of Cl "), uniform corrosion rate in 2% sulfuric acid medium at 40 ° C. 5. 3 Potential for repassivation Steel specimens were polished under water using SIC paper up to 1200 and then matured for 24 hours in air. The cyclic polarization test in a chloride medium was carried out starting with the measurement of the flotation potential for 15 min., Followed by cyclic dynamic polarization at 100 mV / min starting from the flotation potential to the potential for which the current reached an intensity of 300 μ? / cm2, followed by return to the potential for which the current is zero. Therefore, the pitting potential (PPiC) and the repassivation potentials (Prepassing) of the previously formed holes were determined. The results obtained are given in table 7.

Table 7 - Repassivation as a function of the nickel content From the repassivation potential tests in NaCl medium, the higher the nickel content, the greater the difference between the pitting potential and the repassivation potential. This shows that nickel is not beneficial for the repassivation of a degree according to the invention that has previously been subjected to pitting corrosion. It is noted that in relation to this date, the best method known to the applicant to carry out the aforementioned invention, is that which is clear from the present description of the invention.

Claims (26)

1. CLAIMS Having described the invention as above, the contents of the following claims are claimed as property: 1. Duplex stainless steel, characterized in that the composition of this 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% Yes < 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 is iron and impurities resulting from the fusion, and the microstructure consists of austenite and 35 to 65% by volume of ferrite, the composition also satisfies the following ratios: 40 < IF < 70 where IF = 6x (% Cr + 1.32x% Mo + 1.27x% Si) - 10x (% Ni + 24x% C + 16.15x% N + 0.5x% Cu + 0.4x% Mn) - 6.17 and ILCR = 30.5 where ILCR =% Cr + 3.3x% Mo + 16x% N + 2.6x% Ni - 0.7x% Mn. 2. Steel according to claim 1, characterized in that, in addition: ILCR = 32. 3. Steel according to claim 1 or 2, characterized in that in addition the proportion of ferrite is between 35 and 55% by volume. 4. Steel according to any of claims 1 to 3, characterized in that in addition: 40 < IF < 60. Steel according to any of claims 1 to 4, characterized in that in addition the chromium content is between 22 and 24% by weight. 6. Steel according to any of claims 1 to 5, characterized in that in addition the manganese content is less than 1.5% by weight. 7. Steel according to any of claims 1 to 6, characterized in that in addition the calcium content is less than 0.03% by weight. 8. Steel according to any of claims 1 to 7, characterized in that in addition the molybdenum content is greater than 0.1% by weight. Process for manufacturing plate, strip or hot-rolled roll made of steel according to any of claims 1 to 8, characterized in that: an ingot or plate of a steel is provided.A composition according to any of claims 1 to 8; and the ingot or plate is hot rolled at a temperature between 1150 and 1280 ° C to obtain plate, strip or spiral. Process for manufacturing hot-rolled plate made of steel according to claim 9, characterized in that: the ingot is hot-rolled at a temperature between 1150 and 1280 ° C to obtain what is called a fourth plate; a heat treatment is carried out at a temperature between 900 and 1100 ° C; and the plate is cooled by an annealing with air. 11. Hot-rolled steel fourth plate, characterized in that it can be obtained by the process according to claim 10 and has a thickness between 5 and 100 mm. 12. Use of a fourth plate in accordance with claim 11 or a hot-rolled roll obtained by the process according to claim 9, for the manufacture of structural components for installations for energy production or production of material. 13. Use according to claim 12, wherein the energy and material production facilities operate between -100 and ^ 300 ° C, preferably between -50 and 300 ° C. 14. Cold rolled steel strip, characterized in that a hot rolled roll obtained by the process according to claim 9 can be obtained by cold rolling. 15. Process for manufacturing hot rolled rod or rod made of steel in accordance with any of claims 1 to 8, characterized in that: a continuously cast steel billet or billet of the composition according to any one of claims 1 to 8 is provided; the billet is rolled hot from a temperature between 1150 and 1280 ° C to obtain a bar, which is cooled with air, or a roll of wire material which is cooled with water; and then, optionally; - a heat treatment is carried out at a temperature between 900 and 1100 ° C; and the bar or a roll of wire material is cooled by quenching. 16. Hot rolled bar, characterized in that it can be obtained by the process according to claim 15 and has a diameter of 18 mm to 250 mm and hot rolled rod that can be obtained by the process according to claim 15 and which has a diameter of 4 to 30 mm. 17. Manufacturing process according to claim 15, characterized in that a cold drawing operation is carried out on the bar or a drawing operation is carried out on the rod, after being cooled. 18. Cold drawn bar that can be obtained by the process according to claim 17, characterized in that it has a diameter of 4 mm to 60 ram, and rod or die-cut wire that can be obtained by the process according to claim 17, which has a diameter of 0.010 mm at 20 mm 19. Use of a bar according to claim 16 or 18, for the manufacture of mechanical parts such as pumps, valve shafts, motor and propeller shafts and couplings operating in corrosive media. 20. Use of a rod or wire according to claim 16 or 18, for the manufacture of cold-formed assemblies, for the agri-food industry, for extraction of oil and ore, or for the manufacture of woven or knitted woven wire cloths for the filtration of chemicals, minerals or food. 21. Process for manufacturing a steel section, characterized in that a cold forming operation is performed on a hot-rolled bar obtained by the process according to claim 15. 22. Steel section, characterized in that it can be obtained by the process according to claim 21. 23. Process for manufacturing a steel slab, characterized in that a hot-rolled bar obtained by the process according to claim 15 is It is cut into blanks and then a forging operation is carried out on the blanks between 1100 ° C and 1280 ° C. 24. Steel slab, characterized in that it can be obtained by the process according to claim 23. 25. Use of a slab according to claim 24, for the manufacture of clamps or couplings. 26. Casting, characterized in that it can be obtained by casting a steel according to any of claims 1 to 8.