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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
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  • C21D9/525—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for wires; for strips ; for rods of unlimited length for wire, for rods
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  • B21B—ROLLING OF METAL
  • B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
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  • B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
  • B21B3/02—Rolling special iron alloys, e.g. stainless steel
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  • B21C1/00—Manufacture of metal sheets, metal wire, metal rods, metal tubes by drawing
  • B21C1/003—Drawing materials of special alloys so far as the composition of the alloy requires or permits special drawing methods or sequences
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  • B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
  • B21J—FORGING; HAMMERING; PRESSING METAL; RIVETING; FORGE FURNACES
  • B21J1/00—Preparing metal stock or similar ancillary operations prior, during or post forging, e.g. heating or cooling
  • B21J1/02—Preliminary treatment of metal stock without particular shaping, e.g. salvaging segregated zones, forging or pressing in the rough
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  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
  • C21D1/60—Aqueous agents
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  • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
  • C21D1/56—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering characterised by the quenching agents
  • C21D1/613—Gases; Liquefied or solidified normally gaseous material
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  • C21D6/00—Heat treatment of ferrous alloys
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0221—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the working steps
  • C21D8/0226—Hot rolling
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  • C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
  • C21D8/0247—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment
  • C21D8/0263—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips characterised by the heat treatment following hot rolling
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  • C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
  • C21D8/06—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires
  • C21D8/065—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of rods or wires of ferrous alloys
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  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/46—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for sheet metals
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
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  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/002—Ferrous alloys, e.g. steel alloys containing In, Mg, or other elements not provided for in one single group C22C38/001 - C22C38/60
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  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/005—Ferrous alloys, e.g. steel alloys containing rare earths, i.e. Sc, Y, Lanthanides
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  • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
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  • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/46—Ferrous alloys, e.g. steel alloys containing chromium with nickel with vanadium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
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  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
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  • C22C38/60—Ferrous alloys, e.g. steel alloys containing lead, selenium, tellurium, or antimony, or more than 0.04% by weight of sulfur
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/001—Austenite
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  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/005—Ferrite

Definitions

• the present invention relates to an austenitic ferritic stainless steel more particularly intended for the manufacture of structural elements for production plants of matter (chemistry, petrochemistry, paper, offshore) or of energy production, without however be limited.
• This steel can more generally be used in substitution of a type 4301 stainless steel in many applications, for example, in previous industries or in the food industry, including parts made from formed son (welded grids ,. .) profiles (strainers ..), axes ... One could also make molded parts and forgings.
• grades of stainless steel of type 1.4301 and 1.4307 are known, the annealed microstructure of which is essentially austenitic; in the cold worked state, they may further contain a variable proportion of hardening martensite.
• These steels however, have high additions of nickel, the cost is generally prohibitive.
• these grades may pose a problem from a technical point of view for certain applications because they have low tensile characteristics in the annealed state, especially with regard to the yield strength, and a low resistance to stress corrosion.
• these austenitic grades have high thermal conductivity coefficients which, when used as reinforcement of concrete structures, prevent good thermal insulation.
• Ferritic or ferritic-martensitic stainless steel grades are also known, the microstructure of which, for a defined range of heat treatments, is composed of ferrite and martensite, such as the 1.4017 grade of the EN10088 standard. These grades, with a chromium content generally less than 20%, have high mechanical tensile properties, but do not exhibit satisfactory corrosion resistance.
• the object of the present invention is to overcome the disadvantages of the steels and manufacturing processes of the prior art by providing a stainless steel having, without excessive addition of expensive alloying elements such as nickel and molybdenum:
• the invention firstly relates to an austenitic-ferritic stainless steel, the composition of which comprises, in% by weight: 0.01% ⁇ C ⁇ 0.10%
• the remainder being iron and impurities resulting from the preparation and the microstructure consisting of austenite and 35 to 65% of ferrite by volume, preferably 35 to 55% ferrite by volume, the composition further respecting the following relationships:
• IRCGCU % Cr + 3.3% Mo + 2% Cu + 16% N + 2.6% Ni - 0.7% Mn and 0 ⁇ IU ⁇ 6.0
• the steel according to the invention has:
• a second subject of the invention consists of a method for manufacturing a sheet, strip or hot-rolled steel coil according to the invention according to which:
• said slug or said slab is rolled while hot at a temperature of between 150 and 1280 ° C. in order to obtain a sheet, a strip or a coil.
• the method of manufacturing a hot-rolled steel sheet according to the invention comprises the steps of:
• the method of manufacturing a steel hot-rolled bar or wire according to the invention comprises the steps of:
• the method according to the invention further comprises the following characteristics, taken alone or in combination:
• a hot-rolled bar obtained according to the invention is debited in pieces, then forging said billet between 1100 ° C. and 1280 ° C.
• duplex stainless steel according to the invention comprises the contents defined below.
• the carbon content of the grade is between 0.01% and 0.10%, and preferably less than 0.05% by weight. In fact, an excessively high content of this element degrades the resistance to localized corrosion by increasing the risk of precipitation of chromium carbides in the heat-affected zones of the welds.
• the chromium content of the grade is between 20.0 and 24.0% by weight, and preferably between 21.5 and 24% by weight in order to obtain a good resistance to corrosion, which is at least equivalent to that obtained with the type 304 or 304L grades.
• the nickel content of the grade is between 1.0 and 3.0% by weight, and is preferably less than or equal to 2.8% by weight.
• This austenite forming element is added in order to obtain good resistance properties to the formation of corrosion cavities. Its addition also provides a good compromise resilience / ductility. It has indeed the advantage of translating the transition curve of the resilience to low temperatures, which is particularly advantageous for the manufacture of large bars or thick quarto plates for which the properties of resilience are important. Its content is limited to 3.0% because of its high price.
• the nitrogen content of the grade is between 0.12% and 0.20%, and preferably between 0.12% and 0.18%, which generally implies that nitrogen is added to the steel. during the elaboration.
• This austenite-forming element first participates in obtaining a two-phase ferrite / austenite steel containing a proportion of austenite suitable for good resistance to stress corrosion, but also to obtain high mechanical characteristics. It also makes it possible to limit the formation of ferrite in the thermally affected zone of the welded zones, which avoids the risk of embrittlement of these zones. Its maximum content is limited because, beyond 0.16% of nitrogen, defects appear on the continuous casting blooms. These defects consist of longitudinal depressions which in turn generate surface defects on the rolled bars which can be troublesome in some cases. Above 0.18%, the longitudinal depressions are very marked and there is also blistering related to exceeding the maximum amount of nitrogen that can remain in solution in the structure of this grade.
• the manganese content of the grade is between 0.5% and 2.0% by weight, preferably between 0.5 and 1.9% by weight and more preferably between 0.5 and 1.8% by weight. in weight.
• This element is austenite forming but only below 1150 ° C. At higher temperatures, it delays the formation of austenite upon cooling, resulting in excessive ferrite formation in the thermally affected areas of the welds, making them too resilient.
• manganese if it is present in an amount greater than 2.0% in the grade, poses problems during the preparation and the refining of the grade, because it attacks certain refractories used for the pockets, which necessitates a more frequent replacement of these expensive elements and therefore more frequent interruptions of the process.
• ferromanganese which are normally used to make up the composition, contain, in addition, notable levels of phosphorus, and also of selenium, which are not desired to be introduced into the steel and which are difficult to remove during refining the nuance.
• Manganese disrupts this refining by limiting the possibility of decarburization. It also poses a problem further downstream in the process, since it deteriorates the corrosion resistance of the grade due to the formation of MnS manganese sulfides, and oxidized inclusions. It is preferred to limit it to less than 1, 9, or even less than 1, 8% by weight and more preferably less than 1, 6% by weight, since tests have shown that forgeability and more generally heat processing improved when its content was lowered. In particular, it has been possible to observe the formation of cracks rendering the grade unfit for hot rolling, for a content greater than 2.0%.
• the copper an austenite-forming element, is present in a content of between 1.6 and 3.0% by weight, and preferably between 2.0 and 2.8% by weight, or even between 2.2 and 2 , 8% by weight. It participates in obtaining the desired two-phase austenitic-ferritic structure, making it possible to obtain better resistance to generalized corrosion without having to raise the nitrogen content of the grade to a level that is too high.
• copper in solid solution improves the resistance to corrosion in a reducing acid medium. Below 1.6%, the nitrogen level required to have the desired two-phase structure begins to become too great to avoid the surface quality problems of the continuous casting blooms described above. Above 3.0%, segregation and / or copper precipitations begin to be risked, which can lead to localized corrosion resistance and loss of resilience during prolonged use (beyond one year) at the end of the year. above 200 ° C.
• Molybdenum a ferrite-forming element
• Molybdenum is an element which is present in the grade in a content of between 0.05 and 1.0%, or even between 0.05 and 0.5% by weight
• tungsten is an optional element that can be added at a content of less than 0.15% by weight.
• the contents of these two elements are such that the sum Mo + W / 2 is less than 1.0% by weight, preferably less than 0.5%, or even less than 0.4% by weight, and so particularly preferred less than 0.3% by weight.
• the present inventors found that by keeping these two elements, as well as their sums, below the values indicated, we did not observe any weakening intermetallic precipitations, which makes it possible in particular to de-constrain the manufacturing process of the steel sheets or strips by allowing an air cooling of the sheets and strips after heat treatment or hot implementation. In addition, they observed that by controlling these elements within the limits claimed, the weldability of the grade was improved.
• Silicon a ferrite-forming element, is present in a content of between 0.2% and 1.5% by weight, preferably less than 1.0% by weight. It is added to ensure a good deoxidation of the steel bath during the preparation, but its content is limited because of the risk of sigma phase formation in case of poor quenching after hot rolling.
• Aluminum, a ferrite-forming element is an optional element which can be added to the grade in a content of less than 0.05% by weight and preferably of between 0.005% and 0.040% by weight in order to obtain inclusions of calcium aluminates with a low melting point. Its maximum content is limited in order to avoid excessive formation of aluminum nitrides.
• Vanadium a ferrite-forming element
• Vanadium is an optional element which may be present in the grade in an amount ranging from 0.02% to 0.5% by weight and preferably less than 0.2% by weight, so that to improve the resistance to crevice corrosion of steel. It may also be present as a residual element added when adding chromium.
• Niobium a ferrite-forming element
• Niobium is an optional element that may be present in the grade in an amount ranging from 0.001% to 0.5% by weight. It makes it possible to improve the mechanical tensile strength of the grade and its machinability via a better fractionation of the machining chips, thanks to the formation of fine niobium nitrides of type NbN or niobium and chromium type NbCrN (Phase Z). Its content is limited to limit the formation of coarse niobium nitrides.
• Titanium a ferrite-forming element
• Titanium is an optional element which may be present in the grade in an amount ranging from 0.001% to 0.5% by weight and preferably in an amount ranging from 0.001% to 0.3% by weight. weight. It improves the mechanical strength of the grade and its machinability through a better fractionation of machining chips, thanks to the formation of fine nitride titanium. Its content is limited in order to avoid the formation of clusters of titanium nitrides formed in liquid steel in particular.
• Boron is an optional element that may be present in the grade according to the invention in an amount ranging from 0.0001% to 0.003% by weight, in order to improve its heat conversion.
• Cobalt, austenite forming element is an optional element that may be present in the grade in an amount of from 0.02 to 0.5% by weight. This element is a residual brought by the raw materials. It is limited particularly because of the handling problems it can pose after irradiation of parts in nuclear facilities.
• Rare earths are optional elements that may be present in the grade up to 0.1% by weight. These include cerium and lanthanum. The contents in these elements are limited because they are capable of forming unwanted intermetallics.
• Calcium may also be present in the grade according to the invention in an amount ranging from 0.0001 to 0.03% by weight, and preferably greater than 0.0005% by weight, in order to control the nature of the inclusions. of oxides and improve machinability.
• the content of this element is limited because it is likely to form with sulfur calcium sulphides which degrade the properties of corrosion resistance.
• Magnesium addition up to a final content of 0.1% can be made to modify the nature of the sulfides and oxides.
• the selenium is preferably maintained at less than 0.005% by weight because of its detrimental effect on the corrosion resistance.
• This element is generally added to the grade as impurities in the ferromanganese ingots.
• the oxygen content is preferably limited to 0.01% by weight in order to improve its forging ability and the resilience of its welds.
• the sulfur is maintained at a content of less than 0.030% by weight and preferably less than 0.003% by weight.
• this element forms sulphides with manganese or calcium, sulphides whose presence is detrimental to the resistance to corrosion. It is considered an impurity.
• Phosphorus is maintained at less than 0.040% by weight and is considered an impurity.
• the rest of the composition consists of iron and impurities.
• zirconium, tin, arsenic, lead or bismuth may be present in a content of less than 0.100% by weight and preferably less than 0.030% by weight to avoid welding problems.
• the arsenic may be present in a content of less than 0.030% by weight and preferably less than 0.020% by weight.
• the lead may be present in a content of less than 0.002% by weight and preferably less than 0.0010% by weight.
• the bismuth may be present in a content of less than 0.0002% by weight and preferably less than 0.00005% by weight.
• Zirconium may be present at 0.02%.
• 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 1050 ° C., in a proportion of 35 to 65% by weight. ferrite volume and more preferably from 45 to 55% by volume of ferrite.
• the IF number must be between 40 and 65.
• the microstructure does not contain other phases which would be harmful for its mechanical properties in particular, such as the sigma phase and other intermetallic phases.
• some of the austenite may have been converted to martensite, depending on the effective deformation temperature and the amount of cold deformation applied.
• IRCGU> 32.0 and preferably> 34.0 with IRCGU % Cr + 3.3% Mo + 2% Cu + 16% N + 2.6% Ni - 0.7% Mn
• the steel according to the invention can be prepared and manufactured in the form of hot-rolled sheets, also called quarto plates, but also in the form of hot-rolled strips, from slabs or ingots and also under Cold rolled strip form from hot rolled strip. It can also be hot rolled into bars or wire-machines or into profiles or forged; these products can then be hot-formed by forging or cold-formed into drawn bars or profiles or into drawn wires.
• the steel according to the invention can also be implemented by molding followed or not by heat treatment.
• This ingot, this slab or this bloom are generally obtained by melting the raw materials in an electric furnace, followed by a vacuum reflow of the AOD or VOD type with decarburization.
• the grade can then be cast in the form of ingots, or in the form of slabs or blooms by continuous casting in a bottomless mold. It could also be envisaged to cast the shade directly in the form of thin slabs, in particular by continuous casting between counter-rotating rolls.
• the ingot or slab or bloom After supplying the ingot or slab or bloom, it is optionally heated to reach a temperature between 1150 and 1280 ° C, but it is also possible to work directly on the slab that has just been continuously cast, in the hot casting.
• the slab or the slab is then hot-rolled to obtain a so-called quarto sheet which generally has a thickness of between 5 and 100 mm.
• the reduction rates generally used at this stage vary between 3 and 30%.
• This sheet is then subjected to a solution heat treatment precipitates formed at this stage by reheating at a temperature between 900 and 1100 ° C, and then cooled.
• the method according to the invention provides cooling by air quenching which is easier to implement than the cooling conventionally used for this type of shade, which is a faster cooling, using water. However, it remains possible to cool with water if desired.
• This slow cooling, in air, is made possible thanks to the limited contents of nickel and molybdenum of the composition according to the invention which is not subject to the precipitation of intermetallic phases, harmful for its properties of use.
• This cooling can in particular be carried out at speeds ranging from 0.1 to 2.7 ° C / s.
• the quarto plate can be glued, cut and stripped, if it is desired to deliver it in this state.
• This bare steel can also be rolled on a band train at thicknesses between 3 and 10 mm.
• one or several hot rolls can be hot rolled on a multi-cage mill, in corrugated rolls, at a temperature of between 1150 and 1280 ° C. obtain a bar or a ring of wire rod or laminate.
• the section ratio between the initial bloom and the final product is preferably greater than 3, so as to ensure the internal health of the rolled product.
• laminated wire When laminated wire has been manufactured, it can be cooled by quenching in a ring of water at the outlet of the rolling mill or by quenching with water in coils spread on a conveyor after passing them. on a conveyor through a solution furnace at a temperature of between 850 ° C. and 1100 ° C.
• Subsequent heat treatment in the oven may be optionally performed on these bars or crowns already treated in the hot rolling, if it is desired to complete the recrystallization of the structure and slightly lower the mechanical characteristics in traction.
• the tensile properties Rp 0 , 2 and R m were determined according to the NFEN 10002-1 standard.
• the KV resilience was determined at different temperatures according to the NF EN 10045 standard.
• the test consists in finding the turning speed which generates 0.15 mm of undercut wear in 15 minutes of actual machining.
• the test is made in regular turning passes with a coated carbide insert.
• the frozen parameters are:
• draft wear is measured by an optical system coupled to a camera at a magnification of * 32. This measurement is the area of the worn zone relative to the apparent length of this zone. If a notch wear greater than 0.45mm (3 times the VB value) occurs or a tip collapse occurs before 0.15mm wear is obtained, the value of the VB is considered 15/0, 15 is not accessible; then the maximum speed for which there is no flanking wear of 0.45mm or tip collapse in 15min will be determined and the result will be indicated that the VB-15 / 0.15 is greater than this value.
• Vc m i n The determination of Vc m i n is done by a turning pass at increasing speed. It starts with a very low cutting speed V c (40m / min), and one goes up to a speed higher than Vb-i 5 / o, i5 regularly during the pass.
• the cutting conditions are:
• Vc m i n The curve obtained is monotonous decreasing in most cases.
• the value of Vc m i n is that corresponding to an inflection of the curve.
• the chips obtained are evaluated by comparing them with chip shapes predefined in the ISO 3685 "COM turning" standard.
• the CFZ is the table area grouping the conditions in f and a p. for which the chips are well fragmented, which is quantified by counting the number of satisfactory combinations. In the context of the present invention, it is considered that a value of ZFC less than 15, measured under the conditions described above, is not in accordance with the invention.
• the critical dissolution or activity current expressed in ⁇ / cm 2 in sulfuric acid medium at 2 mol / liter at 23 ° C. was determined.
• a measurement of the abandonment potential for 900 seconds is first made; then, a potentiodynamic curve is plotted at a speed of 10 mV / min from -750 mV / ECS to + 1V / ECS.
• the critical current corresponds to the maximum current of the peak highlighted before the passivity domain.
• the comparative grades 6 to 8 and 12 show a formation of longitudinal depressions on the continuous casting blooms, while the grades 1 to 5 according to the invention were free, thus demonstrating the good flowability of the shade according to the invention.
• the tensile yield strength of the tests according to the invention is much higher than 450 MPa, unlike what is observed for the comparative grade 9, for example.
• Resilience values on sheets and bars of high thicknesses at 20 ° C. and -46 ° C. are also satisfactory and in particular better than that of comparative grades 6 and 7, for example.
• the shades according to the invention all furthermore have good machinability both in terms of cutting speed and chip splitting zone.
• the comparative grades 6 and 7, as well as 11 and 12, whose UI numbers are negative do not have a sufficient cutting speed
• the comparative grade 10 whose UI index is greater than 6, 0 have an insufficient chip fractionation zone.
• the generalized corrosion resistance of the shades according to the invention is very satisfactory, and in particular better than that of the comparative grade 8.
• the shades according to the invention are the only ones to combine all the desired properties, namely a good flowability, a tensile yield strength greater than 400 or 450MPa in the annealed or dissolved state, good resilience on high thickness plates and bars, preferably greater than 100 J at 20 ° C and greater than 20 J at -46 ° C, high generalized corrosion resistance, and good machinability.

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