EP3472363A1 - Steel composition - Google Patents

Steel composition

Info

Publication number
EP3472363A1
EP3472363A1 EP17737334.7A EP17737334A EP3472363A1 EP 3472363 A1 EP3472363 A1 EP 3472363A1 EP 17737334 A EP17737334 A EP 17737334A EP 3472363 A1 EP3472363 A1 EP 3472363A1
Authority
EP
European Patent Office
Prior art keywords
steel
advantageously
composition according
content
carburizing
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP17737334.7A
Other languages
German (de)
French (fr)
Inventor
Jacques Bellus
Atman BENBAHMED
Johanna Andre
Fredrik SANDBERG
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Aubert and Duval SA
Original Assignee
Aubert and Duval SA
Erasteel SAS
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Aubert and Duval SA, Erasteel SAS filed Critical Aubert and Duval SA
Publication of EP3472363A1 publication Critical patent/EP3472363A1/en
Pending legal-status Critical Current

Links

Classifications

    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D8/005—Modifying the physical properties by deformation combined with, or followed by, heat treatment of ferrous alloys
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/06—Surface hardening
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/004—Heat treatment of ferrous alloys containing Cr and Ni
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/005—Heat treatment of ferrous alloys containing Mn
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/007—Heat treatment of ferrous alloys containing Co
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D6/00—Heat treatment of ferrous alloys
    • C21D6/008—Heat treatment of ferrous alloys containing Si
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/36—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for balls; for rollers
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
    • C21D9/40—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for rings; for bearing races
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/02—Making ferrous alloys by powder metallurgy
    • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/001—Ferrous alloys, e.g. steel alloys containing N
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/02—Ferrous alloys, e.g. steel alloys containing silicon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/04—Ferrous alloys, e.g. steel alloys containing manganese
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/06—Ferrous alloys, e.g. steel alloys containing aluminium
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • 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
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C38/00—Ferrous alloys, e.g. steel alloys
    • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
    • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
    • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/20—Carburising
    • C23C8/22—Carburising of ferrous surfaces
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/08—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases only one element being applied
    • C23C8/24—Nitriding
    • C23C8/26—Nitriding of ferrous surfaces
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/06—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases
    • C23C8/28—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals using gases more than one element being applied in one step
    • C23C8/30—Carbo-nitriding
    • C23C8/32—Carbo-nitriding of ferrous surfaces
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C8/00—Solid state diffusion of only non-metal elements into metallic material surfaces; Chemical surface treatment of metallic material by reaction of the surface with a reactive gas, leaving reaction products of surface material in the coating, e.g. conversion coatings, passivation of metals
    • C23C8/80—After-treatment
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22F—WORKING METALLIC POWDER; MANUFACTURE OF ARTICLES FROM METALLIC POWDER; MAKING METALLIC POWDER; APPARATUS OR DEVICES SPECIALLY ADAPTED FOR METALLIC POWDER
    • B22F2998/00—Supplementary information concerning processes or compositions relating to powder metallurgy
    • B22F2998/10—Processes characterised by the sequence of their steps
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D1/00—General methods or devices for heat treatment, e.g. annealing, hardening, quenching or tempering
    • C21D1/18—Hardening; Quenching with or without subsequent tempering
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D2211/00—Microstructure comprising significant phases
    • C21D2211/001—Austenite
    • C—CHEMISTRY; METALLURGY
    • C21—METALLURGY OF IRON
    • 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
    • C21D2241/00—Treatments in a special environment
    • C21D2241/01—Treatments in a special environment under pressure
    • C21D2241/02—Hot isostatic pressing

Definitions

  • the present invention relates to a new type of low carbon 20CrMoCo steel for thermochemical treatment particularly for the field of transmissions such as bearings and gears.
  • Bearings are mechanical devices to ensure relative movements and constraints in orientation and direction between two parts.
  • the bearings comprise several components: inner ring, outer ring as well as rolling bodies (ball or cylinder) arranged between these two rings. To ensure reliability and performance over time, it is important that these elements have good properties in rolling fatigue, wear, etc., ...
  • Gears are mechanical power transmission devices. To ensure a favorable power density (power ratio transmitted by the size of the gears) and the reliability of operation, the gears must have good structural fatigue properties (tooth foot) and contact fatigue (tooth blanks).
  • Conventional techniques for making these metal components use electrical steelmaking processes followed by possible remelting operations, or single or multiple vacuum rejections.
  • the ingots thus produced are then shaped by hot transformation processes such as rolling or forging in the form of bar, tube or ring. There are two types of metallurgy to ensure the final mechanical properties.
  • 1st type the chemical composition of the component makes it possible to obtain the mechanical properties directly after suitable heat treatment.
  • 2nd type the component requires a thermochemical treatment to enrich the surface with interstitial elements such as carbon and nitrogen. This enrichment in general superficial chemical elements then allows to obtain high mechanical properties after heat treatment on depths of a few millimeters maximum. These steels generally have better ductility properties than the first type steels.
  • thermochemical processes applied to steels of the first type aimed at enriching the surface with nitrogen to obtain very high mechanical properties.
  • Type 1 and Type 2 steels generally have surface hardness levels greater than 58 HRC.
  • the most common grades known as M50 (0.8% C-4% Cr-4.2% Mo-1% V), or 50NL (0.12% C-4% Cr-4.2%) Mo-3.4% Ni-1% V) do not exceed after possible thermochemical treatment and heat treatment adapted a surface hardness of 63 HRC.
  • GB2370281 discloses a valve seat steel by powder metallurgy technology compacted from powder mixtures of an iron base and harder particles whose matrix has the following composition, in percentages by weight of the composition Total:
  • Molybdenum 1.0-9.0;
  • Silicon 0.1-1.0; Tungsten: 1.0-3.0;
  • Vanadium 0.1-1.0
  • Nickel + Cobalt + Copper 3.0-15.0;
  • this matrix comprises from 5 to 40% by volume of perlite, which results in a lack of ductility of this matrix and therefore embrittlement.
  • Chromium 2.5-5.0
  • Molybdenum 4-6;
  • Tungsten 2-4.5;
  • Vanadium 1-3;
  • Nickel 2-4;
  • Niobium 0-2;
  • Nitrogen 0-0.5;
  • composition grade MIX5 (0.18% C-3.45% Cr-4.93% Mo-3.05% W-2.09% V-0.30% Si-2.89% Ni -5.14% Co-0.27% Mn) which is the most interesting because it has the greatest surface hardness.
  • This grade makes it possible to reach a surface hardness after solution treatment at 1150 ° C. and returned at 560 ° C. to a maximum hardness level of about 800 HV, ie an equivalent of 64 HC maximum (comparative example 1).
  • the inventors have surprisingly found that, by lowering the tungsten content of the steel described in application WO2015 / 082342, the steel obtained had, after thermochemical treatment, in particular carburizing and / or nitriding, a surface hardness. very high and even greater than or equal to 64 HRC after solution heat treatment at a temperature in the range 1100 ° C - 1160 ° C and returned at a temperature greater than or equal to 475 ° C.
  • thermodynamic equilibrium of the steel described in this document is significantly different from that of the application WO2015 / 082342 or that according to the present invention.
  • M6C carbide is not prohibited in the context of the present invention.
  • the skilled person will therefore not seek in view of the teaching of this document to reduce the amount of tungsten steel steel WO2015 / 082342 application. On the contrary, it tends to increase it to improve the hardness of this steel.
  • the present invention therefore relates to a steel composition, advantageously cementable and / or nitrurable, more advantageously cementable, comprising, advantageously consisting essentially of, in particular consisting of, in percentages by weight of the total composition: Carbon: 0.05-0.40, preferably 0.10-0.30;
  • Chromium 2.50-5.00, preferably 3.00-4.50;
  • Molybdenum 4.00-6.00;
  • Tungsten 0.01 - 1.80, preferably 0.02-1.50;
  • Vanadium 1.00-3.00, preferably 1.50-2.50;
  • Cobalt 2.00-8.00, preferably 3.00-7.00;
  • Niobium ⁇ 2.00
  • Nitrogen ⁇ 0.50, preferably ⁇ 0.20;
  • Silicon ⁇ 0.70, preferably 0.05-0.50;
  • Manganese ⁇ 0.70, preferably 0.05-0.50;
  • Aluminum ⁇ 0.15, preferably ⁇ 0.10;
  • Niobium + Vanadium content being in the range 1.00-3.50;
  • a particularly advantageous composition comprises, advantageously consists essentially of, in particular consists of, in percentages by weight of the total composition:
  • Carbon 0.10-0.30, preferably 0.15-0.25;
  • Chromium 3.00 - 4.50, preferably 3.50 - 4.50;
  • Molybdenum 4.00-6.00, preferably 4.50-5.50;
  • Vanadium 1.50-2.50; preferably 1.70-2.30; Nickel: 2.00-4.00, preferably 2.50-3.50;
  • Cobalt 3.00-7.00, preferably 4.00-6.00;
  • Silicon 0.05-0.50, preferably 0.05-0.30;
  • Manganese 0.05-0.50, preferably 0.05-0.30;
  • Niobium ⁇ 2.00
  • Niobium + Vanadium content being in the range 1.00-3.50;
  • the unavoidable impurities chosen in particular from Titanium (Ti), Sulfur (S), Phosphorus (P), Copper (Cu), Tin (Sn), Lead (Pb), Oxygen ( O) and their mixtures are kept at the lowest level.
  • These impurities are generally due mainly to the manufacturing process and the quality of charging.
  • the composition according to the invention comprises at most 1% by weight of unavoidable impurities, advantageously at most 0.75% by weight, still more advantageously at most 0.50% by weight, relative to the total weight of the composition.
  • the carbide forming elements which also have a stabilizing effect on ferrite, so-called alphagenes, are essential to the steel composition according to the invention so as to provide sufficient hardness, resistance to heat and wear.
  • stabilizing elements of the austenite so-called gammagenic elements.
  • the steel composition according to the invention therefore comprises carbon (C) in a content in the range 0.05-0.40%, preferably 0.10- 0.30%, even more preferably 0.15- 0.25%, more advantageously 0.18-0.20% by weight relative to the total weight of the composition.
  • the Carbon (C) stabilizes the austenitic phase of the steel at the heat treatment temperatures and is essential for the formation of carbides which bring the mechanical properties in general in particular the mechanical resistance, the high hardness, the resistance to the heat and to wear.
  • the presence of a small amount of carbon in a steel is beneficial to avoid the formation of unwanted and fragile intermetallic particles and to form small amounts of carbides to prevent excessive grain growth during quenching.
  • the initial carbon content should not be too high since it is possible to increase the surface hardness of the components formed from the steel composition by carburizing.
  • the carbon is implanted in the surface layers of the component, so as to obtain a hardness gradient.
  • Carbon is the main element for controlling the hardness of the martensitic phase formed after carburizing and heat treatment. In case-hardened steel, it is essential to have a strong core with a low carbon content while having a hard surface with a high carbon content after thermochemical carburizing treatment.
  • the steel composition according to the invention also comprises chromium (Cr) in a content in the range 2.50-5.00%, preferably 3.00-4.50%, even more preferred 3.50 4.50%, still more advantageously 3.80-4.00% by weight relative to the total weight of the composition.
  • Cr chromium
  • Chrome contributes to the formation of carbides in steel and is, after carbon, the main element that controls the hardenability of steels.
  • chromium can also favor ferrite and residual austenite.
  • increasing the chromium content reduces the maximum quenching temperature.
  • the chromium content of the steel composition according to the invention should not be too high.
  • the steel composition according to the invention also comprises molybdenum (Mo) in a content in the range 4.00-6.00%, preferably 4.50-5.50%, even more preferably 4.80- 5.20%, by weight relative to the total weight of the composition.
  • Mo molybdenum
  • Molybdenum improves the resistance to wear, wear resistance and hardness of steel. However, molybdenum has a strong stabilizing effect on the ferrite phase and must not be present in excessive amounts in the steel composition according to the invention.
  • the steel composition according to the invention further comprises Tungsten (W) in a content in the range 0.01-1.80%, preferably 0.02-1.50%, even more preferably 0.03-1.40%, advantageously 0.04-1.30%, still more preferably 0.05-1.30%, in particular 0.1- 1.30% by weight relative to the total weight of the composition.
  • W Tungsten
  • Tungsten is a ferrite stabilizer and a strong carbide-forming element. It improves resistance to heat treatment and wear and hardness by carbide formation. It is, however, very expensive and as a ferrite stabilizer also lowers the surface hardness of the steel and especially the ductility and toughness properties. For this element to play its full role, it is necessary to carry out high temperature dissolution.
  • the steel composition according to the invention further comprises vanadium (V) in a content in the range of 1.00-3.00%, preferably 1.50-2.50%, even more preferred 1.70 -2.30%, advantageously 2.00-2.30%, in particular 2.00-2.20%, by weight relative to the total weight of the composition.
  • V vanadium
  • Vanadium stabilizes the ferrite phase and has a strong affinity for carbon and nitrogen. Vanadium provides resistance to wear and tear by forming hard vanadium carbides. Vanadium may be partly substituted by niobium (Nb), which has similar properties.
  • the combined Niobium + Vanadium content must therefore be in the range 1.00-3.50% by weight relative to the total weight of the composition.
  • the steel composition according to the invention does not comprise Niobium.
  • the steel composition according to the invention also comprises nickel (Ni) in a content in the range 2.00-4.00%, preferably 2.50-3.50%, more preferably 2.70- 3.30%, advantageously 3.00-3.20%, by weight relative to the total weight of the composition.
  • the steel composition according to the invention also comprises cobalt (Co) in a content in the range 2.00-8.00%, preferably 3.00-7.00%, even more preferred 4.00 -6.00%, advantageously 4.50-5.50%, more preferably 4.90-5.40%, more particularly 4.90-5.20%, by weight relative to the total weight of the composition.
  • Co cobalt
  • Cobalt is a highly stabilizing element of austenite that prevents the formation of undesirable ferrite. Unlike Nickel, Cobalt increases the Ms temperature, which in turn decreases the amount of residual austenite. Cobalt, in combination with nickel, allows the presence of ferrite stabilizers such as the carbide-forming elements Mo, W, Cr and V.
  • the carbide-forming elements are essential for the steel according to the invention because of their effect on hardness, resistance to heat and wear. Cobalt has a small effect of increasing hardness on steel. However, the increase in hardness is correlated with the decrease in toughness. It is therefore not necessary for the steel composition according to the invention to contain an excessive amount of cobalt.
  • the steel composition according to the invention may further comprise silicon (Si) in a content ⁇ 0.70%, by weight relative to the total weight of the composition.
  • Si silicon
  • it comprises silicon, in particular in a content in the range 0.05-0.50%, preferably 0.05-0.30%, advantageously 0.07-0.25%, more advantageously 0 , 10-0.20%, by weight relative to the total weight of the composition.
  • the steel composition according to the invention may further comprise manganese (Mn) in a content ⁇ 0.70%, by weight relative to the total weight of the composition.
  • Mn manganese
  • it comprises manganese, in particular in a content in the range 0.05-0.50%, preferably 0.05-0.30%, advantageously 0.07-0.25%, even more advantageously 0 , 10-0.22%, more particularly 0.10-0.20% by weight relative to the total weight of the composition.
  • Manganese stabilizes the austenite phase and decreases the MS temperature in the steel composition.
  • Manganese is generally added to steels during their manufacture so as to attach to Sulfur by formation of manganese sulfide during solidification. This eliminates the risk of formation of iron sulphides which have an adverse effect on the hot machining of steels.
  • Manganese is also part of the deoxidation step like Silicon. The combination of manganese with silicon gives deoxidation more efficient than each of these elements alone.
  • the steel composition according to the invention may comprise nitrogen (N3 ⁇ 4) in a content ⁇ 0.50%, preferably ⁇ 0.20%, by weight relative to the total weight of the composition.
  • Nitrogen promotes the formation of austenite and lowers the transformation of austenite to martensite. Nitrogen can to a certain extent replace the carbon in the steel according to the invention. However, the carbon + nitrogen content must be in the range 0.05-0.50% by weight relative to the total weight of the composition.
  • the steel composition according to the invention may comprise aluminum (Al), in a content of ⁇ 0.15%, preferably ⁇ 0.10%, by weight relative to the total weight of the composition. .
  • Aluminum (Al) can indeed be present during the steel manufacturing process according to the invention and contributes very effectively to the deoxidation of the liquid steel. This is particularly the case during reflow processes such as the VIM-VAR process.
  • the aluminum content is generally higher in the steels produced using the VIM-VAR process than in the steels obtained by the powder technology.
  • Aluminum causes difficulties during the atomization by obstruction of the casting nozzle by oxides.
  • Low oxygen content is important for good micro-cleanliness as well as good mechanical properties such as fatigue resistance and mechanical strength.
  • the oxygen content obtained by ingot is typically less than 15 ppm.
  • the composition according to the present invention is cementable, that is to say it can undergo a cementation treatment, and / or nitrurable, that is to say it can undergo nitriding treatment and even advantageously it can undergo a thermochemical treatment, in particular chosen from carburizing, nitriding, carbonitriding and cementation followed by nitriding.
  • nitriding is the nitrogen content that increases on the surface of the steel, and therefore also its surface hardness.
  • the steel composition according to the invention after a thermochemical treatment, advantageously carburizing or nitriding or carbonitriding or carburizing and then nitriding, followed by a heat treatment, a superficial hardness superior or equal to 64 HRC, advantageously greater than or equal to 65 HRC, still more preferably greater than or equal to 66 HRC, measured according to ASTM E18 or equivalent standard.
  • the steel composition obtained by these treatments advantageously has a surface carbon concentration of between 1 and 1.25% by weight relative to the total weight of the composition.
  • Said heat treatment may comprise:
  • a first cooling in particular under a neutral gas at, for example, a pressure of 2 bar, advantageously up to room temperature (this phase makes it possible to obtain a mainly martensitic microstructure with residual austenite
  • This residual austenite is a function of the cooling temperature: the content decreases with the cooling temperature
  • - (5) then advantageously a second cooling at a temperature below -40 ° C, more preferably below -60 ° C, even more preferably about -75 ° C, in particular for 2 hours (this phase allows to reduce the residual austenite content), - (6) and advantageously one or more revenues, more preferably at least three revenues, preferably at a temperature greater than or equal to 475 ° C, more preferably greater than or equal to 500 ° C, in particular greater than or equal to 550 ° C, more particularly about 560 ° C., even more particularly for 1 hour each (this or these incomes allow the precipitation of carbides and the partial or total decomposition of the residual austenite, which makes it possible to obtain ductility properties).
  • the advantage of the steel according to the invention is therefore to obtain high levels of hardness with a limited heat treatment (temperature between 1090 ° C-1160 ° C, preferably between 1100 ° C-1160 ° C / more preferably between 1100 ° C - 1155 ° C, in particular between 1100 ° C - 1150 ° C, more particularly 1150 ° C).
  • the steel composition according to the invention after a thermochemical treatment, advantageously carburizing or nitriding or carbonitriding or carburizing and then nitriding, followed by a heat treatment, a martensitic structure having a residual austenite content of less than 10% by weight and free from ferrite and pearlite, known phases to reduce the surface hardness of the steel.
  • the present invention furthermore relates to a method for manufacturing a steel blank having the composition according to the invention, characterized in that it comprises:
  • step d) of the process according to the present invention is as described above.
  • thermochemical treatment of step c) of the process according to the present invention consists of a carburizing or nitriding or carbonitriding or cementation and then nitriding treatment, advantageously it is a cementation treatment.
  • step b) of the process according to the present invention consists of a rolling, forging and / or spinning step.
  • step a) of developing the process according to the present invention is carried out by a conventional process for producing a furnace with refining and conductive slag refining (ESR), or by a method VI -VAR, possibly with a step of remelting under conductive slag (ESR) and / or under vacuum (VAR), or by metallurgy of the powders such as atomization by gas and compression by hot isostatic compaction (HIP).
  • ESR refining and conductive slag refining
  • VI -VAR possibly with a step of remelting under conductive slag (ESR) and / or under vacuum (VAR), or by metallurgy of the powders such as atomization by gas and compression by hot isostatic compaction (HIP).
  • the steel according to the present invention can be produced by a VIM-VAR process.
  • This process makes it possible to obtain a very good inclusion cleanliness and improves the chemical homogeneity of the ingot. It is also possible to carry out a conductive slag remelting pathway (ESR: Electro Slag Remelting) or combine ESR and VAR (vacuum remelting) operations.
  • ESR Electro Slag Remelting
  • VAR vacuum remelting
  • This steel can also be obtained by metallurgy powders. This process makes it possible to produce high purity metal powder by atomization, preferably gas atomization, which makes it possible to obtain very low oxygen contents.
  • the powder is then compressed using, for example, hot isostatic compaction (HIP).
  • HIP hot isostatic compaction
  • the present invention also relates to a steel blank that can be obtained by the process according to the invention.
  • This blank is made of steel having the composition according to the present invention and as described above. It further relates to the use of a blank according to the invention or a steel composition according to the invention for the manufacture of a mechanical member, advantageously in the field of transmission such as gears, shafts transmission and bearings.
  • a mechanical member advantageously a transmission member or a gear, in particular a gear, a transmission shaft or a bearing, more particularly a bearing, made of steel having the composition according to the invention or obtained from a steel blank according to the invention.
  • the steel obtained is inexpensive, in particular because of the low tungsten content, although it has a surface hardness level after high thermochemical treatment, with a martensite structure free of massive phases of the austenite or ferrite or perlite type.
  • FIG. 1 represents the profile of surface hardness (microhardness in HV0.5 as a function of the depth of steel (in mm) of two examples according to the invention (grades B and C) and a comparative example (grade A ) according to the application WO2015 / 082342 having the composition indicated in Table 1 below as well as a comparative example 50NCL (0.12% C-4% Cr-4.2% Mo-3.4% Ni-1 % V), obtained after carburization and heat treatment comprising the following steps: (1) heating at 1150 ° C, (2) holding for 15 min at 1150 ° C for austenitization, (3) cooling under a neutral gas at pressure of 2 bar, (4) a period at room temperature, (5) cooling at -75 ° C for 2 hours, and (6) 3 revenues at 550 ° C for grade C and 560 ° C for grades A and B for 1 hour each.
  • FIG. 2 represents the superficial hardness profile (microhardness in HV0.5 as a function of the steel depth (in mm) of example 2 (grade C) according to the invention having the composition indicated in table 1 ci below as well as a comparative example 50 Nil (0.12% C-4% Cr-4.2% Mo-3.4% / 1 N-1% V), obtained after cementation and heat treatment comprising the following steps: (1) heating at 1100 ° C, (2) holding for 15 min at 1100 ° C for austenitization, (3) cooling under a neutral gas at a pressure of 2 bar, (4) a period at room temperature, ( 5) cooling at -75 ° C for 2 hours, and (6) 3 tempering at 475 ° C or 500 ° C or 550 ° C or 575 ° C for grade C or 560 ° C for Comparative Example 50Nil for 1 hour each.
  • FIG. 3 represents the profile of surface hardness (microhardness in HV0.5 as a function of the steel depth (in mm) of Example 2 (grade C) according to the invention having the composition indicated in Table 1 ci below as well as a comparative example 50NiL (0.12% C-4% Cr-4.2% Mo-3.4% Ni-1% V), obtained after cementation and heat treatment comprising the following steps: 1) heating at 1150 ° C, (2) holding for 15 min at 1150 ° C for austenitization, (3) cooling under a neutral gas at a pressure of 2 bar, (4) a period at room temperature, (5) ) at -75 ° C for 2 hours, and (6) 3 at a temperature of 475 ° C or 500 ° C or 550 ° C or 575 ° C for grade C or 560 ° C for Comparative Example 50NIL for 1 hour each.
  • compositions according to the invention having a low W content have higher hardness levels, of the order of 860 HV corresponding to 66 HRC. It should also be noted that the drop in the W content relative to the prior art does not significantly affect the hardness level of the base metal which is of the order of 540 HV corresponding to 51 HRC.
  • the steel having the composition according to the invention (low W content) thus makes it possible to obtain higher levels of hardness with a heat treatment limited to 1150 ° C. compared to that of the prior art having a higher content. in W.
  • a recovery temperature of 500 ° C. is particularly advantageous since the hardness level reaches 66-67 HRC (with treatment in solution at 1100 ° C. and 1150 ° C.) (FIGS. 2 and 3). At 575 ° C, the results are still very interesting with values above 64 HRC after dissolution only at 1150 ° C ( Figure 3).

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Abstract

The present invention relates to a case-hardenable and/or nitridable steel composition comprising, in percentages by weight of the total composition: carbon: 0.05-0.40, preferably 0.10-0.30; chromium: 2.50-5.00, preferably 3.00-4.50; molybdenum: 4.00-6.00; tungsten: 0.01-1.80, preferably 0.02-1.50; vanadium: 1.00-3.00, preferably 1.50-2.50; nickel: 2.00-4.00; cobalt: 2.00-8.00, preferably 3.00-7.00; iron: balance and also the inevitable impurities, optionally additionally comprising one or more of the following elements: niobium: ≤ 2.00; nitrogen: ≤ 0.50, preferably ≤ 0.20; silicon: ≤ 0.70, preferably 0.05-0.50; manganese: ≤ 0.70, preferably 0.05-0.50; aluminum: ≤ 0.15, preferably ≤ 0.10; the combined content of niobium + vanadium being within the range 1.00-3.50; and the content of carbon + nitrogen being within the range 0.05-0.50. It also relates to the process for the manufacture thereof, to the steel blank obtained and to a mechanical member comprising same.

Description

COMPOSmON D'ACIER  STEEL COMPONENT
La présente invention concerne un nouvel acier de type 20CrMoCo à faible teneur en carbone pour traitement thermochimique en particulier destiné au domaine des transmissions telles que les roulements et les engrenages.  The present invention relates to a new type of low carbon 20CrMoCo steel for thermochemical treatment particularly for the field of transmissions such as bearings and gears.
Les roulements sont des organes mécaniques permettant d'assurer des mouvements relatifs et contraints en orientation et direction entre deux pièces. Les roulements comprennent plusieurs composants : bague interne, bague externe ainsi que des corps roulants (bille ou cylindre) disposés entre ces deux bagues. Pour assurer une fiabilité et des performances dans le temps, il est important que ces différents éléments aient de bonnes propriétés en fatigue de roulement, usure, etc.,... Bearings are mechanical devices to ensure relative movements and constraints in orientation and direction between two parts. The bearings comprise several components: inner ring, outer ring as well as rolling bodies (ball or cylinder) arranged between these two rings. To ensure reliability and performance over time, it is important that these elements have good properties in rolling fatigue, wear, etc., ...
Les engrenages sont des organes mécaniques de transmission de puissance. Pour assurer une densité de puissance favorable (ratio de puissance transmise par l'encombrement des engrenages) et la fiabilité de fonctionnement, les engrenages doivent présenter des bonnes propriétés en fatigue structurale (pied de dent) et fatigue de contact (flans de dent). Les techniques conventionnelles pour réaliser ces composants métalliques ont recours à des procédés d'élaboration d'aciérie électrique suivies d'opérations éventuelles de refusion, ou de refusions sous vide simples ou multiples. Les lingots ainsi réalisés sont ensuite mis en forme par des procédés de transformation à chaud comme le laminage ou le forgeage sous forme de barre, de tube ou de bague. II existe deux types de métallurgie pour assurer les propriétés mécaniques finales. Gears are mechanical power transmission devices. To ensure a favorable power density (power ratio transmitted by the size of the gears) and the reliability of operation, the gears must have good structural fatigue properties (tooth foot) and contact fatigue (tooth blanks). Conventional techniques for making these metal components use electrical steelmaking processes followed by possible remelting operations, or single or multiple vacuum rejections. The ingots thus produced are then shaped by hot transformation processes such as rolling or forging in the form of bar, tube or ring. There are two types of metallurgy to ensure the final mechanical properties.
1er Type : la composition chimique du composant permet d'obtenir les propriétés mécaniques directement après traitement thermique adapté. 2ème Type : le composant requiert un traitement thermochimique permettant d'enrichir la surface en éléments interstitiels comme le carbone et l'azote. Cet enrichissement en général superficiel en éléments chimiques permet alors d'obtenir de hautes propriétés mécaniques après traitement thermique sur des profondeurs de quelques millimètres maximum. Ces aciers présentent en général de meilleures propriétés en ductilité que les aciers du 1er Type. 1st type: the chemical composition of the component makes it possible to obtain the mechanical properties directly after suitable heat treatment. 2nd type: the component requires a thermochemical treatment to enrich the surface with interstitial elements such as carbon and nitrogen. This enrichment in general superficial chemical elements then allows to obtain high mechanical properties after heat treatment on depths of a few millimeters maximum. These steels generally have better ductility properties than the first type steels.
Il existe aussi des procédés thermochimiques appliqués aux aciers du 1er type visant à enrichir la surface en azote pour obtenir de très hautes propriétés mécaniques.  There are also thermochemical processes applied to steels of the first type aimed at enriching the surface with nitrogen to obtain very high mechanical properties.
La première des propriétés requises dans le domaine du roulement ou des engrenages est l'obtention d'un très haut niveau de dureté. Ces aciers de type 1 et de type 2 présentent généralement des niveaux de dureté superficielles supérieures à 58 HRC. Les nuances les plus répandues et connues sous le vocable M50 (0,8%C-4%Cr-4,2%Mo-l%V), ou 50NÎL (0,12%C-4%Cr-4,2%Mo-3,4%Ni-l%V) ne dépassent pas après traitement thermochimique éventuel et traitement thermique adapté une dureté superficielle de 63 HRC. The first of the properties required in the field of rolling or gears is obtaining a very high level of hardness. These Type 1 and Type 2 steels generally have surface hardness levels greater than 58 HRC. The most common grades known as M50 (0.8% C-4% Cr-4.2% Mo-1% V), or 50NL (0.12% C-4% Cr-4.2%) Mo-3.4% Ni-1% V) do not exceed after possible thermochemical treatment and heat treatment adapted a surface hardness of 63 HRC.
La demande GB2370281 décrit un acier pour siège de soupape par la technologie de la métallurgie des poudres compactée à partir de mélanges de poudre d'une base fer et de particules plus dures dont la matrice a la composition suivante, en pourcentages en poids de la composition totale : GB2370281 discloses a valve seat steel by powder metallurgy technology compacted from powder mixtures of an iron base and harder particles whose matrix has the following composition, in percentages by weight of the composition Total:
Carbone : 0,2-2,0;  Carbon: 0.2-2.0;
Chrome : 1,0-9,0;  Chrome: 1.0-9.0;
Molybdène : 1,0-9,0;  Molybdenum: 1.0-9.0;
Silicium : 0,1-1,0; Tungstène : 1,0-3,0; Silicon: 0.1-1.0; Tungsten: 1.0-3.0;
Vanadium : 0,1-1,0;  Vanadium: 0.1-1.0;
Nickel+ Cobalt + Cuivre : 3,0-15,0;  Nickel + Cobalt + Copper: 3.0-15.0;
Fer : solde  Iron: balance
Toutefois cette matrice comprend de 5 à 40% en volume de perlite, ce qui a pour conséquence un manque de ductilité de cette matrice et donc une fragilisation. However, this matrix comprises from 5 to 40% by volume of perlite, which results in a lack of ductility of this matrix and therefore embrittlement.
La demande de brevet WO2015/082342 décrit un acier pour roulement ayant la composition suivante, en pourcentages en poids de la composition totale :  The patent application WO2015 / 082342 describes a rolling steel having the following composition, in percentages by weight of the total composition:
Carbone : 0,05-0,5;  Carbon: 0.05-0.5;
Chrome : 2,5-5,0;  Chromium: 2.5-5.0;
Molybdène : 4-6;  Molybdenum: 4-6;
Tungstène : 2-4,5;  Tungsten: 2-4.5;
Vanadium : 1-3;  Vanadium: 1-3;
Nickel : 2-4;  Nickel: 2-4;
Cobalt : 2-8;  Cobalt: 2-8;
Fer : solde  Iron: balance
ainsi que les impuretés inévitables, optionnellement comprenant en outre, un ou plusieurs des éléments suivants : and unavoidable impurities, optionally further comprising one or more of the following:
Niobium : 0-2 ;  Niobium: 0-2;
Azote : 0-0,5;  Nitrogen: 0-0.5;
Silicium : 0-0,7;  Silicon: 0-0.7;
Manganèse : 0-0,7;  Manganese: 0-0.7;
Aluminium : 0-0,15;  Aluminum: 0-0.15;
et en particulier la nuance MIX5 de composition (0,18%C-3,45%Cr- 4,93%Mo-3,05%W-2,09%V-0,30%Si-2,89%Ni-5,14%Co-0,27%Mn) qui est la plus intéressante car présentant la plus grande dureté superficielle. Cette nuance permet d'atteindre une dureté superficielle après traitement de mise en solution à 1150°C et revenu à 560°C à un niveau maximal de dureté d'environ 800 HV, soit un équivalent de 64 H C maximum (exemple comparatif 1). and in particular composition grade MIX5 (0.18% C-3.45% Cr-4.93% Mo-3.05% W-2.09% V-0.30% Si-2.89% Ni -5.14% Co-0.27% Mn) which is the most interesting because it has the greatest surface hardness. This grade makes it possible to reach a surface hardness after solution treatment at 1150 ° C. and returned at 560 ° C. to a maximum hardness level of about 800 HV, ie an equivalent of 64 HC maximum (comparative example 1).
L'obtention de duretés superficielles supérieures à 64 HRC, en particulier à l'aide d'un traitement thermique de mise en solution à une température inférieure ou égale à 1160°C, est donc difficile à obtenir alors qu'elles permettraient d'améliorer significativement les propriétés du composant. Obtaining surface hardnesses greater than 64 HRC, in particular using a solution heat treatment at a temperature of less than or equal to 1160 ° C, is therefore difficult to obtain whereas it would make it possible to improve significantly the properties of the component.
Les inventeurs se sont aperçus de façon surprenante qu'en abaissant la teneur en tungstène de l'acier décrit dans la demande WO2015/082342, l'acier obtenu présentait, après traitement thermochimique, en particulier de cémentation et/ou nitruration, une dureté superficielle très élevée et même supérieure ou égale à 64 HRC après un traitement thermique de mise en solution à une température comprise dans la gamme 1100°C - 1160°C et revenu à une température supérieure ou égale à 475°C. The inventors have surprisingly found that, by lowering the tungsten content of the steel described in application WO2015 / 082342, the steel obtained had, after thermochemical treatment, in particular carburizing and / or nitriding, a surface hardness. very high and even greater than or equal to 64 HRC after solution heat treatment at a temperature in the range 1100 ° C - 1160 ° C and returned at a temperature greater than or equal to 475 ° C.
Ceci n'était pas du tout évident au vu de ce document qui incitait à utiliser une teneur élevée en tungstène telle que dans la nuance MIX5 (3% de tungstène) qui est considérée comme la composition présentant la meilleure dureté. This was not at all obvious in view of this document which encouraged the use of a high tungsten content such as in the MIX5 grade (3% tungsten) which is considered as the composition having the best hardness.
La demande de brevet US2004/0187972 décrit un acier ayant une teneur en tungstène comprise entre 0,5 et 2%. Toutefois un tel acier a une haute teneur en carbone (0,5-0,75%) et est donc difficilement cémentable et/ou nitrurable. Il n'appartient donc pas au même domaine technique que les aciers de la demande WO2015/082342 ni que les aciers selon la présente invention. En outre ce document justifie la fourchette en tungstène comprise entre 0,5 et 2% selon le paragraphe [0035] de la façon suivante: The patent application US2004 / 0187972 describes a steel having a tungsten content of between 0.5 and 2%. However, such a steel has a high carbon content (0.5-0.75%) and is therefore difficult to harden and / or nitrurable. It therefore does not belong to the same technical field as the steels of the application WO2015 / 082342 nor the steels according to the present invention. In addition, this document justifies the tungsten range between 0.5 and 2% according to paragraph [0035] as follows:
0,5% : contribution à la dureté à chaud par mise en solution dans la matrice  0.5%: contribution to hot hardness by dissolving in the matrix
- 2% : maximum pour limiter très fortement la formation de carbure M6C stables à haute température. - 2%: maximum to very strongly limit the formation of carbide M6C stable at high temperature.
Il enseigne donc bien que le tungstène est connu de l'homme de l'art pour son action favorable concernant l'augmentation de dureté non seulement à chaud mais aussi à température ambiante. La seule raison pour limiter sa teneur dans ce document est donc d'éviter la formation de carbure M6C stables à haute température.  It therefore teaches that tungsten is known to those skilled in the art for its favorable action on increasing hardness not only hot but also at room temperature. The only reason for limiting its content in this document is therefore to avoid the formation of stable M6C carbides at high temperature.
Or l'équilibre thermodynamique de l'acier décrit dans ce document est notablement différent de celui de la demande WO2015/082342 ou de celui selon la présente invention.  However, the thermodynamic equilibrium of the steel described in this document is significantly different from that of the application WO2015 / 082342 or that according to the present invention.
Ainsi la présence de carbure M6C n'est pas proscrit dans le cadre de la présente invention. L'homme du métier ne cherchera donc pas au vu de l'enseignement de ce document à diminuer la quantité de tungstène de l'acier de la demande WO2015/082342. Il aurait au contraire tendance à l'augmenter pour améliorer la dureté de cet acier. Thus the presence of M6C carbide is not prohibited in the context of the present invention. The skilled person will therefore not seek in view of the teaching of this document to reduce the amount of tungsten steel steel WO2015 / 082342 application. On the contrary, it tends to increase it to improve the hardness of this steel.
Ainsi le fait que la baisse du niveau de tungstène de l'acier de la demande WO2015/082342 entraîne une augmentation de la dureté superficielle est donc totalement inattendu pour l'homme du métier. Thus the fact that the decrease of the tungsten level of the steel of application WO2015 / 082342 causes an increase in the surface hardness is therefore totally unexpected for the skilled person.
La présente invention concerne donc une composition d'acier, avantageusement cémentable et/ou nitrurable, plus avantageusement cémentable, comprenant, avantageusement constituée essentiellement de, en particulier constituée de, en pourcentages en poids de la composition totale: Carbone : 0,05-0,40, de préférence 0,10-0,30 ; The present invention therefore relates to a steel composition, advantageously cementable and / or nitrurable, more advantageously cementable, comprising, advantageously consisting essentially of, in particular consisting of, in percentages by weight of the total composition: Carbon: 0.05-0.40, preferably 0.10-0.30;
Chrome : 2,50-5,00, de préférence 3,00-4,50;  Chromium: 2.50-5.00, preferably 3.00-4.50;
Molybdène : 4,00-6,00;  Molybdenum: 4.00-6.00;
Tungstène : 0,01 - 1,80, de préférence 0,02-1,50 ;  Tungsten: 0.01 - 1.80, preferably 0.02-1.50;
Vanadium : 1,00-3,00, de préférence 1,50-2,50;  Vanadium: 1.00-3.00, preferably 1.50-2.50;
Nickel : 2,00-4,00;  Nickel: 2.00-4.00;
Cobalt : 2,00-8,00, de préférence 3,00-7,00;  Cobalt: 2.00-8.00, preferably 3.00-7.00;
Fer : solde  Iron: balance
ainsi que les impuretés inévitables, as well as unavoidable impurities,
optionnellement comprenant en outre, un ou plusieurs des éléments suivants : optionally further comprising one or more of the following:
Niobium : < 2,00 ;  Niobium: <2.00;
Azote : < 0,50, de préférence < 0,20 ;  Nitrogen: <0.50, preferably <0.20;
Silicium : < 0,70, de préférence 0,05-0,50;  Silicon: <0.70, preferably 0.05-0.50;
Manganèse :≤ 0,70, de préférence 0,05-0,50;  Manganese: ≤ 0.70, preferably 0.05-0.50;
Aluminium : <0,15, de préférence < 0,10 ;  Aluminum: <0.15, preferably <0.10;
la teneur combinée en Niobium + Vanadium étant comprise dans la gamme 1,00-3,50 ; the combined Niobium + Vanadium content being in the range 1.00-3.50;
et la teneur en Carbone + Azote étant comprise dans la gamme 0,05-0,50. and the carbon + nitrogen content being in the range 0.05-0.50.
Une composition particulièrement intéressante comprend, avantageusement est constituée essentiellement de, en particulier est constituée de, en pourcentages en poids de la composition totale: A particularly advantageous composition comprises, advantageously consists essentially of, in particular consists of, in percentages by weight of the total composition:
Carbone : 0,10-0,30, de préférence 0,15-0,25 ;  Carbon: 0.10-0.30, preferably 0.15-0.25;
Chrome : 3,00-4,50, de préférence 3,50-4,50;  Chromium: 3.00 - 4.50, preferably 3.50 - 4.50;
Molybdène : 4,00-6,00, de préférence 4,50-5,50;  Molybdenum: 4.00-6.00, preferably 4.50-5.50;
Tungstène 0,02 -1,50, de préférence 0,03 -1,40 ;  Tungsten 0.02 - 1.50, preferably 0.03 - 1.40;
Vanadium : 1,50-2,50; de préférence 1,70-2,30 ; Nickel : 2,00-4,00, de préférence 2,50-3,50 ; Vanadium: 1.50-2.50; preferably 1.70-2.30; Nickel: 2.00-4.00, preferably 2.50-3.50;
Cobalt : 3,00- 7,00, de préférence 4,00-6,00;  Cobalt: 3.00-7.00, preferably 4.00-6.00;
Silicium : 0,05-0,50, de préférence 0,05-0,30;  Silicon: 0.05-0.50, preferably 0.05-0.30;
Manganèse : 0,05-0,50, de préférence 0,05-0,30;  Manganese: 0.05-0.50, preferably 0.05-0.30;
Fer : solde  Iron: balance
ainsi que les impuretés inévitables, as well as unavoidable impurities,
optionnellement elle comprend en outre, un ou plusieurs des éléments suivants : optionally, it further comprises one or more of the following:
Niobium : < 2,00 ;  Niobium: <2.00;
Azote : < 0,20 ;  Nitrogen: <0.20;
Aluminium : <0,10 ;  Aluminum: <0.10;
la teneur combinée en Niobium + Vanadium étant comprise dans la gamme 1,00-3,50 ; the combined Niobium + Vanadium content being in the range 1.00-3.50;
et la teneur en Carbone + Azote étant comprise dans la gamme 0,05-0,50. and the carbon + nitrogen content being in the range 0.05-0.50.
En particulier les impuretés inévitables, notamment choisies parmi le Titane (Ti), le Soufre (S), le Phosphore (P), le Cuivre (Cu), l'Etain (Sn), le Plomb (Pb), l'Oxygène (O) et leurs mélanges, sont maintenues au plus bas niveau. Ces impuretés sont généralement dues essentiellement au procédé de fabrication et à la qualité de l'enfournement. De façon avantageuse la composition selon l'invention comprend au plus 1% en poids d'impuretés inévitables, avantageusement au plus 0,75% en poids, encore plus avantageusement au plus 0,50% en poids, par rapport au poids total de la composition. In particular the unavoidable impurities, chosen in particular from Titanium (Ti), Sulfur (S), Phosphorus (P), Copper (Cu), Tin (Sn), Lead (Pb), Oxygen ( O) and their mixtures are kept at the lowest level. These impurities are generally due mainly to the manufacturing process and the quality of charging. Advantageously, the composition according to the invention comprises at most 1% by weight of unavoidable impurities, advantageously at most 0.75% by weight, still more advantageously at most 0.50% by weight, relative to the total weight of the composition.
Les éléments formateurs de carbures, qui ont aussi un effet stabilisant sur la ferrite, éléments dits alphagènes, sont essentiels à la composition d'acier selon l'invention de façon à fournir suffisamment de dureté, de résistance à la chaleur et à l'usure. Afin d'obtenir une microstructure exempte de ferrite qui fragiliserait le composant, il est nécessaire d'ajouter des éléments stabilisateurs de l'austénite, éléments dits gammagènes. Une combinaison correcte d'éléments stabilisateurs de l'austénite (Carbone, Nickel, Cobalt et Manganèse) et d'éléments stabilisateurs de la ferrite (Molybdène, Tungstène, Chrome, Vanadium et Silicium) permet d'obtenir une composition d'acier selon l'invention ayant des propriétés supérieures, en particulier après traitement thermochimique tel que la cémentation. The carbide forming elements, which also have a stabilizing effect on ferrite, so-called alphagenes, are essential to the steel composition according to the invention so as to provide sufficient hardness, resistance to heat and wear. In order to obtain a ferrite-free microstructure which would weaken the component, it is necessary to add stabilizing elements of the austenite, so-called gammagenic elements. A correct combination of austenite stabilizing elements (carbon, nickel, cobalt and manganese) and stabilizing elements of ferrite (molybdenum, tungsten, chromium, vanadium and silicon) makes it possible to obtain a steel composition according to the invention. invention having superior properties, especially after thermochemical treatment such as carburizing.
La composition d'acier selon l'invention comprend donc du carbone (C) en une teneur comprise dans la gamme 0,05-0,40%, de préférence 0,10- 0,30%, encore plus préférée 0,15-0,25%, encore plus avantageusement 0,18-0,20% en poids par rapport au poids total de la composition. En effet le Carbone (C) stabilise la phase austénitique de l'acier aux températures de traitement thermique et est essentielle pour la formation de carbures qui apportent les propriétés mécaniques en générale notamment la résistance mécanique, la haute dureté, la résistance à la chaleur et à l'usure. La présence d'une petite quantité de carbone dans un acier est bénéfique pour éviter la formation de particules intermétalliques indésirables et fragiles et pour former de petites quantités de carbures pour éviter la croissance excessive des grains pendant la trempe. La teneur initiale en carbone ne devra toutefois pas être trop élevée puisqu'il est possible d'augmenter la dureté superficielle des composants formés à partir de la composition d'acier par cémentation. Pendant la cémentation, le carbone est implanté dans les couches de surface du composant, de façon à obtenir un gradient de dureté. Le carbone est le principal élément pour le contrôle de la dureté de la phase martensitique formée après cémentation et traitement thermique. Dans un acier cémenté, il est essentiel d'avoir un cœur solide avec une faible teneur en carbone tout en ayant une surface dure avec une forte teneur en carbone après traitement thermochimique de cémentation. The steel composition according to the invention therefore comprises carbon (C) in a content in the range 0.05-0.40%, preferably 0.10- 0.30%, even more preferably 0.15- 0.25%, more advantageously 0.18-0.20% by weight relative to the total weight of the composition. Indeed the Carbon (C) stabilizes the austenitic phase of the steel at the heat treatment temperatures and is essential for the formation of carbides which bring the mechanical properties in general in particular the mechanical resistance, the high hardness, the resistance to the heat and to wear. The presence of a small amount of carbon in a steel is beneficial to avoid the formation of unwanted and fragile intermetallic particles and to form small amounts of carbides to prevent excessive grain growth during quenching. The initial carbon content, however, should not be too high since it is possible to increase the surface hardness of the components formed from the steel composition by carburizing. During cementation, the carbon is implanted in the surface layers of the component, so as to obtain a hardness gradient. Carbon is the main element for controlling the hardness of the martensitic phase formed after carburizing and heat treatment. In case-hardened steel, it is essential to have a strong core with a low carbon content while having a hard surface with a high carbon content after thermochemical carburizing treatment.
La composition d'acier selon l'invention comprend en outre du Chrome (Cr) en une teneur comprise dans la gamme 2,50-5,00%, de préférence 3,00-4,50%, encore plus préférée 3,50-4,50%, encore plus avantageusement 3,80-4,00% en poids par rapport au poids total de la composition. The steel composition according to the invention also comprises chromium (Cr) in a content in the range 2.50-5.00%, preferably 3.00-4.50%, even more preferred 3.50 4.50%, still more advantageously 3.80-4.00% by weight relative to the total weight of the composition.
Le Chrome contribue à la formation de carbures dans l'acier et est, après le carbone, l'élément principal qui contrôle la trempabilité des aciers.  Chrome contributes to the formation of carbides in steel and is, after carbon, the main element that controls the hardenability of steels.
Toutefois le Chrome peut aussi favoriser la ferrite et l'austénite résiduelle. De plus, augmenter la teneur en Chrome réduit la température maximale de trempe. La teneur en Chrome de la composition d'acier selon l'invention ne doit donc pas être trop élevée. However, chromium can also favor ferrite and residual austenite. In addition, increasing the chromium content reduces the maximum quenching temperature. The chromium content of the steel composition according to the invention should not be too high.
La composition d'acier selon l'invention comprend également du Molybdène (Mo) en une teneur comprise dans la gamme 4,00-6,00%, de préférence 4,50-5,50%, encore plus préférée 4,80-5,20%, en poids par rapport au poids total de la composition. The steel composition according to the invention also comprises molybdenum (Mo) in a content in the range 4.00-6.00%, preferably 4.50-5.50%, even more preferably 4.80- 5.20%, by weight relative to the total weight of the composition.
Le Molybdène améliore la tenue au revenu, la résistance à l'usure et la dureté de l'acier. Toutefois, le Molybdène a un fort effet stabilisant sur la phase ferrite et ne doit donc pas être présent en trop grande quantité dans la composition d'acier selon l'invention.  Molybdenum improves the resistance to wear, wear resistance and hardness of steel. However, molybdenum has a strong stabilizing effect on the ferrite phase and must not be present in excessive amounts in the steel composition according to the invention.
La composition d'acier selon l'invention comprend de plus du Tungstène (W) en une teneur comprise dans la gamme 0,01-1,80%, de préférence 0,02-1,50 %, encore plus préférée 0,03-1,40 %, avantageusement 0,04-1,30%, encore plus avantageusement 0,05-1,30%, en particulier 0,1-1,30% en poids par rapport au poids total de la composition. The steel composition according to the invention further comprises Tungsten (W) in a content in the range 0.01-1.80%, preferably 0.02-1.50%, even more preferably 0.03-1.40%, advantageously 0.04-1.30%, still more preferably 0.05-1.30%, in particular 0.1- 1.30% by weight relative to the total weight of the composition.
Le tungstène est un stabilisant de ferrite et un élément fortement formateur de carbures. Il améliore la résistance au traitement thermique et à l'usure et la dureté par formation de carbures. Il est cependant très coûteux et en tant que stabilisateur de la ferrite abaisse également la dureté superficielle de l'acier et surtout les propriétés de ductilité et ténacité. Pour que cet élément joue pleinement son rôle, il est nécessaire de procéder à des mises en solution à haute température. Tungsten is a ferrite stabilizer and a strong carbide-forming element. It improves resistance to heat treatment and wear and hardness by carbide formation. It is, however, very expensive and as a ferrite stabilizer also lowers the surface hardness of the steel and especially the ductility and toughness properties. For this element to play its full role, it is necessary to carry out high temperature dissolution.
La composition d'acier selon l'invention comprend en outre du Vanadium (V) en une teneur comprise dans la gamme 1,00-3,00%, de préférence 1,50-2,50 %, encore plus préférée 1,70-2,30 %, avantageusement 2,00- 2,30%, en particulier 2,00-2,20%, en poids par rapport au poids total de la composition. The steel composition according to the invention further comprises vanadium (V) in a content in the range of 1.00-3.00%, preferably 1.50-2.50%, even more preferred 1.70 -2.30%, advantageously 2.00-2.30%, in particular 2.00-2.20%, by weight relative to the total weight of the composition.
Le Vanadium stabilise la phase ferrite et a une forte affinité avec le carbone et l'azote. Le Vanadium apporte la résistance à l'usure et au revenu par formation de carbures de vanadium durs. Le Vanadium peut être en partie substitué par le niobium (Nb), qui a des propriétés similaires.  Vanadium stabilizes the ferrite phase and has a strong affinity for carbon and nitrogen. Vanadium provides resistance to wear and tear by forming hard vanadium carbides. Vanadium may be partly substituted by niobium (Nb), which has similar properties.
La teneur combinée en Niobium + Vanadium doit donc être comprise dans la gamme 1,00-3,50 % en poids par rapport au poids total de la composition.  The combined Niobium + Vanadium content must therefore be in the range 1.00-3.50% by weight relative to the total weight of the composition.
Si le Niobium est présent, sa teneur doit être ≤ 2,00 % en poids par rapport au poids total de la composition. Avantageusement, la composition d'acier selon l'invention ne comprend pas de Niobium. La composition d'acier selon l'invention comprend également du Nickel (Ni) en une teneur comprise dans la gamme 2,00-4,00%, de préférence 2,50-3,50 %, encore plus préférée 2,70-3,30 %, avantageusement 3,00-3,20%, en poids par rapport au poids total de la composition. If the Niobium is present, its content must be ≤ 2.00% by weight relative to the total weight of the composition. Advantageously, the steel composition according to the invention does not comprise Niobium. The steel composition according to the invention also comprises nickel (Ni) in a content in the range 2.00-4.00%, preferably 2.50-3.50%, more preferably 2.70- 3.30%, advantageously 3.00-3.20%, by weight relative to the total weight of the composition.
Le Nickel favorise la formation d'austénite et donc inhibe la formation de ferrite. Un autre effet du Nickel est de décroître la température Ms, c'est- à-dire la température à laquelle la transformation d'austénite en martensite commence lors du refroidissement. Cela peut empêcher la formation de martensite. La quantité de Nickel doit donc être contrôlée de façon à éviter la formation d'austénite résiduelle dans les composants cémentés. La composition d'acier selon l'invention comprend de plus du Cobalt (Co) en une teneur comprise dans la gamme 2,00-8,00%, de préférence 3,00- 7,00%, encore plus préférée 4,00-6,00%, avantageusement 4,50-5,50%, plus avantageusement 4,90-5,40%, plus particulièrement 4,90-5,20%, en poids par rapport au poids total de la composition.  Nickel promotes the formation of austenite and therefore inhibits the formation of ferrite. Another effect of nickel is to decrease the temperature Ms, that is the temperature at which transformation from austenite to martensite begins during cooling. This can prevent the formation of martensite. The amount of nickel must therefore be controlled so as to avoid the formation of residual austenite in the cemented components. The steel composition according to the invention also comprises cobalt (Co) in a content in the range 2.00-8.00%, preferably 3.00-7.00%, even more preferred 4.00 -6.00%, advantageously 4.50-5.50%, more preferably 4.90-5.40%, more particularly 4.90-5.20%, by weight relative to the total weight of the composition.
Le Cobalt est un élément fortement stabilisateur de l'austénite qui empêche la formation de ferrite indésirable. Contrairement au Nickel, le Cobalt augmente la température Ms, ce qui à son tour diminue la quantité d'austénite résiduelle. Le Cobalt, en association avec le Nickel, permet la présence de stabilisateurs de ferrite tels que les éléments formateurs de carbures Mo, W, Cr et V. Les éléments formateurs de carbures sont essentiels pour l'acier selon l'invention en raison de leur effet sur la dureté, la résistance à la chaleur et à l'usure. Le Cobalt a un petit effet d'augmentation de la dureté sur l'acier. Toutefois, l'augmentation de la dureté est corrélée à la décroissance de la ténacité. Il ne faut donc pas que la composition d'acier selon l'invention contienne une quantité trop importante de Cobalt. La composition d'acier selon l'invention peut en outre comprendre du Silicium (Si) en une teneur < 0,70%, en poids par rapport au poids total de la composition. Avantageusement, elle comprend du Silicium, en particulier en une teneur comprise dans la gamme 0,05-0,50%, de préférence 0,05-0,30%, avantageusement 0,07-0,25%, encore plus avantageusement 0,10-0,20%, en poids par rapport au poids total de la composition. Cobalt is a highly stabilizing element of austenite that prevents the formation of undesirable ferrite. Unlike Nickel, Cobalt increases the Ms temperature, which in turn decreases the amount of residual austenite. Cobalt, in combination with nickel, allows the presence of ferrite stabilizers such as the carbide-forming elements Mo, W, Cr and V. The carbide-forming elements are essential for the steel according to the invention because of their effect on hardness, resistance to heat and wear. Cobalt has a small effect of increasing hardness on steel. However, the increase in hardness is correlated with the decrease in toughness. It is therefore not necessary for the steel composition according to the invention to contain an excessive amount of cobalt. The steel composition according to the invention may further comprise silicon (Si) in a content <0.70%, by weight relative to the total weight of the composition. Advantageously, it comprises silicon, in particular in a content in the range 0.05-0.50%, preferably 0.05-0.30%, advantageously 0.07-0.25%, more advantageously 0 , 10-0.20%, by weight relative to the total weight of the composition.
Le Silicium stabilise fortement la ferrite, mais est souvent présent lors du procédé de fabrication de l'acier lors de la désoxydation de l'acier liquide. Des teneurs faibles en oxygène sont en effet également importantes pour obtenir de faibles niveaux d'inclusions non-métalliques et de bonnes propriétés mécaniques telles que la résistance à la fatigue et la résistance mécanique.  Silicon strongly stabilizes ferrite, but is often present during the steelmaking process during the deoxidation of liquid steel. Low oxygen contents are indeed also important for obtaining low levels of non-metallic inclusions and good mechanical properties such as fatigue resistance and mechanical strength.
La composition d'acier selon l'invention peut en outre comprendre du Manganèse (Mn) en une teneur < 0,70%, en poids par rapport au poids total de la composition. Avantageusement, elle comprend du Manganèse, en particulier en une teneur comprise dans la gamme 0,05-0,50%, de préférence 0,05-0,30%, avantageusement 0,07-0,25%, encore plus avantageusement 0,10-0,22%, encore plus particulièrement 0,10-0,20% en poids par rapport au poids total de la composition. The steel composition according to the invention may further comprise manganese (Mn) in a content <0.70%, by weight relative to the total weight of the composition. Advantageously, it comprises manganese, in particular in a content in the range 0.05-0.50%, preferably 0.05-0.30%, advantageously 0.07-0.25%, even more advantageously 0 , 10-0.22%, more particularly 0.10-0.20% by weight relative to the total weight of the composition.
Le Manganèse stabilise la phase austénite et décroit la température Ms dans la composition d'acier. Le Manganèse est en général ajouté dans les aciers lors de leur fabrication de façon à s'attacher au Soufre par formation de sulfure de Manganèse pendant la solidification. Cela supprime le risque de formation de sulfures de Fer qui ont un effet défavorable sur l'usinage à chaud des aciers. Le Manganèse fait aussi partie de l'étape de désoxydation comme le Silicium. La combinaison du Manganèse avec le Silicium donne une désoxydation plus efficace que chacun de ces éléments seuls. Manganese stabilizes the austenite phase and decreases the MS temperature in the steel composition. Manganese is generally added to steels during their manufacture so as to attach to Sulfur by formation of manganese sulfide during solidification. This eliminates the risk of formation of iron sulphides which have an adverse effect on the hot machining of steels. Manganese is also part of the deoxidation step like Silicon. The combination of manganese with silicon gives deoxidation more efficient than each of these elements alone.
Optionnellement, la composition d'acier selon l'invention peut comprendre de l'Azote (N¾), en une teneur < 0,50%, de préférence < 0,20 %, en poids par rapport au poids total de la composition. Optionally, the steel composition according to the invention may comprise nitrogen (N¾) in a content <0.50%, preferably <0.20%, by weight relative to the total weight of the composition.
L'Azote favorise la formation d'austénite et abaisse la transformation d'austénite en martensite. L'Azote peut dans une certaine mesure remplacer le Carbone dans l'acier selon l'invention. Toutefois la teneur en Carbone + Azote doit être comprise dans la gamme 0,05-0,50% en poids par rapport au poids total de la composition.  Nitrogen promotes the formation of austenite and lowers the transformation of austenite to martensite. Nitrogen can to a certain extent replace the carbon in the steel according to the invention. However, the carbon + nitrogen content must be in the range 0.05-0.50% by weight relative to the total weight of the composition.
De façon optionnelle, la composition d'acier selon l'invention peut comprendre de l'Aluminium (Al), en une teneur < 0,15%, de préférence < 0,10 %, en poids par rapport au poids total de la composition. Optionally, the steel composition according to the invention may comprise aluminum (Al), in a content of <0.15%, preferably <0.10%, by weight relative to the total weight of the composition. .
L'Aluminium (Al) peut en effet être présent lors du procédé de fabrication de l'acier selon l'invention et contribue de manière très efficace à la désoxydation de l'acier liquide. C'est en particulier le cas lors des procédés de refusion tels que le procédé VIM-VAR. La teneur en Aluminium est en générale plus élevée dans les aciers produits en utilisant le procédé VIM- VAR que dans les aciers obtenus par la technologie des poudres. L'Aluminium génère des difficultés au cours de l'atomisation par obstruction de la busette de coulée par des oxydes. Une faible teneur en Oxygène est importante pour obtenir une bonne micro-propreté ainsi que de bonnes propriétés mécaniques telles que la résistance à la fatigue et la résistance mécanique. Les teneurs en Oxygène obtenues par voie lingot sont typiquement inférieures à 15 ppm. Avantageusement, la composition selon la présente invention est cémentable, c'est-à-dire qu'elle peut subir un traitement de cémentation, et/ou nitrurable, c'est-à-dire qu'elle peut subir un traitement de nitruration et même avantageusement elle peut subir un traitement thermochimique, en particulier choisi parmi la cémentation, la nitruration, la carbonitruration et la cémentation suivie de la nitruration. Aluminum (Al) can indeed be present during the steel manufacturing process according to the invention and contributes very effectively to the deoxidation of the liquid steel. This is particularly the case during reflow processes such as the VIM-VAR process. The aluminum content is generally higher in the steels produced using the VIM-VAR process than in the steels obtained by the powder technology. Aluminum causes difficulties during the atomization by obstruction of the casting nozzle by oxides. Low oxygen content is important for good micro-cleanliness as well as good mechanical properties such as fatigue resistance and mechanical strength. The oxygen content obtained by ingot is typically less than 15 ppm. Advantageously, the composition according to the present invention is cementable, that is to say it can undergo a cementation treatment, and / or nitrurable, that is to say it can undergo nitriding treatment and even advantageously it can undergo a thermochemical treatment, in particular chosen from carburizing, nitriding, carbonitriding and cementation followed by nitriding.
Ces traitements permettent d'améliorer la dureté superficielle de l'acier, en ajoutant des éléments carbones et/ou azotes. Ainsi, si la cémentation est utilisée, la teneur en Carbone de la surface de l'acier augmente et donc sa dureté superficielle. La surface est ainsi avantageusement enrichie en Carbone avec un enrichissement en particulier compris entre 0,5% - 1,7% en poids, par rapport au poids total de la composition.  These treatments make it possible to improve the surface hardness of the steel by adding carbon and / or nitrogen elements. Thus, if carburization is used, the carbon content of the surface of the steel increases and thus its surface hardness. The surface is thus advantageously enriched with carbon with an enrichment in particular of between 0.5% - 1.7% by weight, relative to the total weight of the composition.
Si la nitruration est utilisée, c'est la teneur en Azote qui augmente à la surface de l'acier, et donc également sa dureté superficielle. If nitriding is used, it is the nitrogen content that increases on the surface of the steel, and therefore also its surface hardness.
Si la carbonitruration ou la cémentation suivie de la nitruration sont utilisées, ce sont les teneurs en Carbone et Azote à la surface de l'acier qui sont augmentées et donc également sa dureté superficielle. If carbonitriding or cementation followed by nitriding are used, it is the carbon and nitrogen contents on the surface of the steel which are increased and therefore also its surface hardness.
Ces procédés sont bien connus de l'homme du métier. These methods are well known to those skilled in the art.
Dans un mode de réalisation avantageux, la composition d'acier selon l'invention présente, après un traitement thermochimique, avantageusement de cémentation ou de nitruration ou de carbonitruration ou de cémentation puis de nitruration, suivi d'un traitement thermique, une dureté superficielle supérieure ou égale à 64 HRC, avantageusement supérieure ou égale à 65 HRC, encore plus avantageusement supérieure ou égale à 66 HRC, mesurée selon la norme ASTM E18 ou norme équivalente. La composition d'acier obtenue grâce à ces traitements a avantageusement une concentration en carbone en surface comprise entre 1 et 1,25% en poids par rapport au poids total de la composition. Ledit traitement thermique peut comprendre : In an advantageous embodiment, the steel composition according to the invention, after a thermochemical treatment, advantageously carburizing or nitriding or carbonitriding or carburizing and then nitriding, followed by a heat treatment, a superficial hardness superior or equal to 64 HRC, advantageously greater than or equal to 65 HRC, still more preferably greater than or equal to 66 HRC, measured according to ASTM E18 or equivalent standard. The steel composition obtained by these treatments advantageously has a surface carbon concentration of between 1 and 1.25% by weight relative to the total weight of the composition. Said heat treatment may comprise:
- (1) une mise en solution de l'acier à une température comprise entre 1090°C-1160°C, avantageusement entre 1100°C-1160°C, plus avantageusement entre 1100 et 1155°C, en particulier entre 1100 et 1150°C, plus particulièrement de 1150°C,  (1) dissolving the steel at a temperature of between 1090 ° C. and 1160 ° C., advantageously between 1100 ° C. and 1160 ° C., more advantageously between 1100 and 1155 ° C., in particular between 1100 and 1150 ° C .; ° C, more particularly 1150 ° C,
- (2) suivi avantageusement d'un maintien à cette température jusqu'à austénitisation complète, en particulier pendant une durée de 15 minutes (trempe), (ces 2 phases (1) et (2) permettent la mise en solution totale ou partielle des carbures initialement présents),  - (2) advantageously followed by keeping at this temperature until complete austenitization, in particular for a period of 15 minutes (quenching), (these 2 phases (1) and (2) allow the total or partial solution solution carbides initially present),
- (3) puis éventuellement un premier refroidissement (trempe), en particulier sous gaz neutre à, par exemple, une pression de 2 bars, avantageusement jusqu'à la température ambiante, (cette phase permet d'obtenir une microstructure principalement martensitique avec de l'austénite résiduelle. Cette austénite résiduelle est fonction de la température de refroidissement : la teneur diminue avec la température de refroidissement), - (3) then optionally a first cooling (quenching), in particular under a neutral gas at, for example, a pressure of 2 bar, advantageously up to room temperature (this phase makes it possible to obtain a mainly martensitic microstructure with residual austenite This residual austenite is a function of the cooling temperature: the content decreases with the cooling temperature),
- (4) suivi éventuellement d'un maintien à la température ambiante, - (4) possibly followed by a maintenance at ambient temperature,
- (5) puis avantageusement d'un deuxième refroidissement à une température inférieure à -40°C, plus avantageusement inférieure à -60°C, encore plus avantageusement d'environ -75°C, en particulier pendant 2 heures (cette phase permet de diminuer la teneur en austénite résiduelle), - (6) et avantageusement un ou plusieurs revenus, plus avantageusement au moins trois revenus, avantageusement à une température supérieure ou égale à 475°C, plus avantageusement supérieure ou égale à 500°C, en particulier supérieure ou égale à 550°C, plus particulièrement d'environ 560°C, encore plus particulièrement pendant 1 heure chacun (ce ou ces revenus permettent la précipitation de carbures et la décomposition partielle ou totale de l'austénite résiduelle. Cela permet d'obtenir des propriétés de ductilité). L'intérêt de l'acier selon l'invention est donc d'obtenir de hauts niveaux de dureté avec un traitement thermique limité (température comprise entre 1090°C-1160°C, avantageusement entre 1100°C-1160°C/ plus avantageusement entre 1100°C - 1155°C, en particulier entre 1100°C - 1150°C, plus particulièrement de 1150°C). - (5) then advantageously a second cooling at a temperature below -40 ° C, more preferably below -60 ° C, even more preferably about -75 ° C, in particular for 2 hours (this phase allows to reduce the residual austenite content), - (6) and advantageously one or more revenues, more preferably at least three revenues, preferably at a temperature greater than or equal to 475 ° C, more preferably greater than or equal to 500 ° C, in particular greater than or equal to 550 ° C, more particularly about 560 ° C., even more particularly for 1 hour each (this or these incomes allow the precipitation of carbides and the partial or total decomposition of the residual austenite, which makes it possible to obtain ductility properties). The advantage of the steel according to the invention is therefore to obtain high levels of hardness with a limited heat treatment (temperature between 1090 ° C-1160 ° C, preferably between 1100 ° C-1160 ° C / more preferably between 1100 ° C - 1155 ° C, in particular between 1100 ° C - 1150 ° C, more particularly 1150 ° C).
Dans un mode de réalisation particulièrement avantageux, la composition d'acier selon l'invention présente, après un traitement thermochimique, avantageusement de cémentation ou de nitruration ou de carbonitruration ou de cémentation puis de nitruration, suivi d'un traitement thermique, une structure martensitique ayant une teneur d'austénite résiduelle inférieure à 10% en poids et exempte de ferrite et de perlite, phases connues pour diminuer la dureté superficielle de l'acier. In a particularly advantageous embodiment, the steel composition according to the invention, after a thermochemical treatment, advantageously carburizing or nitriding or carbonitriding or carburizing and then nitriding, followed by a heat treatment, a martensitic structure having a residual austenite content of less than 10% by weight and free from ferrite and pearlite, known phases to reduce the surface hardness of the steel.
Ledit traitement thermique peut être tel que décrit ci-dessus. La présente invention concerne en outre un procédé de fabrication d'une ébauche en acier ayant la composition selon l'invention, caractérisé en ce qu'il comprend : Said heat treatment may be as described above. The present invention furthermore relates to a method for manufacturing a steel blank having the composition according to the invention, characterized in that it comprises:
a) une étape d'élaboration de l'acier ; b) une étape de transformation de l'acier ; (a) a steel making step; (b) a step of processing the steel;
c) un traitement thermochimique; c) a thermochemical treatment;
d) et un traitement thermique. Avantageusement le traitement thermique de l'étape d) du procédé selon la présente invention est tel que décrit ci-dessus. d) and a heat treatment. Advantageously, the heat treatment of step d) of the process according to the present invention is as described above.
De façon avantageuse, le traitement thermochimique de l'étape c) du procédé selon la présente invention consiste en un traitement de cémentation ou de nitruration ou de carbonitruration ou de cémentation puis de nitruration, avantageusement il s'agit d'un traitement de cémentation. Advantageously, the thermochemical treatment of step c) of the process according to the present invention consists of a carburizing or nitriding or carbonitriding or cementation and then nitriding treatment, advantageously it is a cementation treatment.
En particulier, l'étape b) du procédé selon la présente invention consiste en une étape de laminage, de forgeage et/ou de filage. Ces procédés sont bien connus de l'homme du métier. In particular, step b) of the process according to the present invention consists of a rolling, forging and / or spinning step. These methods are well known to those skilled in the art.
Dans un mode de réalisation avantageux, l'étape a) d'élaboration du procédé selon la présente invention est mise en œuvre par un procédé d'élaboration conventionnelle four à arc affinage et refusion sous laitier conducteur (ESR), ou par un procédé VI -VAR, avec éventuellement une étape de refusion sous laitier conducteur (ESR) et/ou sous vide (VAR), ou par Métallurgie des poudres telle que atomisation par gaz et compression par compaction isostatique à chaud (HIP). In an advantageous embodiment, step a) of developing the process according to the present invention is carried out by a conventional process for producing a furnace with refining and conductive slag refining (ESR), or by a method VI -VAR, possibly with a step of remelting under conductive slag (ESR) and / or under vacuum (VAR), or by metallurgy of the powders such as atomization by gas and compression by hot isostatic compaction (HIP).
Ainsi l'acier selon la présente invention peut être élaboré par un procédé VIM-VAR. Ce procédé permet d'obtenir une très bonne propreté inclusionnaire et améliore l'homogénéité chimique du lingot. Il est aussi possible de procéder à une voie de refusion sous laitier conducteur (ESR : Electro Slag Remelting) ou de combiner des opérations ESR et VAR (refusion sous vide). Thus the steel according to the present invention can be produced by a VIM-VAR process. This process makes it possible to obtain a very good inclusion cleanliness and improves the chemical homogeneity of the ingot. It is also possible to carry out a conductive slag remelting pathway (ESR: Electro Slag Remelting) or combine ESR and VAR (vacuum remelting) operations.
Cet acier peut être aussi obtenu par Métallurgie des poudres. Ce procédé permet de produire de la poudre métallique de grande pureté par atomisation, de préférence atomisation par gaz permettant d'obtenir de très faibles teneurs en oxygène. La poudre est ensuite comprimée par recours par exemple à une compaction isostatique à chaud (HIP).  This steel can also be obtained by metallurgy powders. This process makes it possible to produce high purity metal powder by atomization, preferably gas atomization, which makes it possible to obtain very low oxygen contents. The powder is then compressed using, for example, hot isostatic compaction (HIP).
Ces procédés sont bien connus de l'homme du métier. La présente invention concerne également une ébauche d'acier susceptible d'être obtenue par le procédé selon l'invention. Cette ébauche est faite à base d'acier ayant la composition selon la présente invention et telle que décrite ci-dessus. Elle concerne de plus l'utilisation d'une ébauche selon l'invention ou d'une composition d'acier selon l'invention pour la fabrication d'un organe mécanique, avantageusement dans le domaine de la transmission tels que les engrenages, les arbres de transmission et les roulements. Elle concerne enfin un organe mécanique, avantageusement un organe de transmission ou un engrenage, en particulier un engrenage, un arbre de transmission ou un roulement, plus particulièrement un roulement, en acier ayant la composition selon l'invention ou obtenu à partir d'une ébauche d'acier selon l'invention. These methods are well known to those skilled in the art. The present invention also relates to a steel blank that can be obtained by the process according to the invention. This blank is made of steel having the composition according to the present invention and as described above. It further relates to the use of a blank according to the invention or a steel composition according to the invention for the manufacture of a mechanical member, advantageously in the field of transmission such as gears, shafts transmission and bearings. Finally, it relates to a mechanical member, advantageously a transmission member or a gear, in particular a gear, a transmission shaft or a bearing, more particularly a bearing, made of steel having the composition according to the invention or obtained from a steel blank according to the invention.
En effet, avec la composition d'acier selon l'invention, il est possible de combiner la haute dureté superficielle et la résistance à l'usure de surface avec un cœur ayant une haute résistance à la fatigue et une haute résistance mécanique. Indeed, with the steel composition according to the invention, it is possible to combine the high surface hardness and the surface wear resistance. with a heart having a high resistance to fatigue and a high mechanical strength.
Ces aciers sont donc utilisables dans des domaines exigeants tels que les roulements pour l'aérospatiale.  These steels are therefore usable in demanding areas such as bearings for aerospace.
En outre l'acier obtenu est peu coûteux en raison en particulier de la faible teneur en tungstène, bien que présentant un niveau de dureté superficielle après traitement thermochimique élevé, avec une structure martensite exempte de phases massives de type austénite ou ferrite ou perlite. In addition, the steel obtained is inexpensive, in particular because of the low tungsten content, although it has a surface hardness level after high thermochemical treatment, with a martensite structure free of massive phases of the austenite or ferrite or perlite type.
L'invention sera mieux comprise à la lecture des exemples et des figures qui suivent qui sont donnés à titre indicatif non limitatif. The invention will be better understood on reading the examples and figures which follow, which are given by way of non-limiting indication.
Dans les exemples, sauf indication contraire, tous les pourcentages sont exprimés en poids, la température est exprimée en degré Celsius et la pression est la pression atmosphérique. In the examples, unless otherwise indicated, all percentages are by weight, the temperature is expressed in degrees Celsius and the pressure is atmospheric pressure.
La figure 1 représente le profil de dureté superficielle (microdureté en HV0,5 en fonction de la profondeur de l'acier (en mm) de deux exemples selon l'invention (nuances B et C) et d'un exemple comparatif (nuance A) selon la demande WO2015/082342 ayant la composition indiquée dans le tableau 1 ci-dessous ainsi que d'un exemple comparatif 50NÏL (0,12%C- 4%Cr-4,2%Mo-3,4%Ni-l%V), obtenus après cémentation et traitement thermique comprenant les étapes suivantes : (1) un chauffage à 1150°C, (2) un maintien pendant 15 min à 1150°C pour austénitisation, (3) un refroidissement sous gaz neutre à une pression de 2 bars, (4) une période à température ambiante, (5) un refroidissement à -75°C pendant 2 heures, et (6) 3 revenus à 550°C pour la nuance C et 560°C pour les nuances A et B pendant 1 heure chacun. FIG. 1 represents the profile of surface hardness (microhardness in HV0.5 as a function of the depth of steel (in mm) of two examples according to the invention (grades B and C) and a comparative example (grade A ) according to the application WO2015 / 082342 having the composition indicated in Table 1 below as well as a comparative example 50NCL (0.12% C-4% Cr-4.2% Mo-3.4% Ni-1 % V), obtained after carburization and heat treatment comprising the following steps: (1) heating at 1150 ° C, (2) holding for 15 min at 1150 ° C for austenitization, (3) cooling under a neutral gas at pressure of 2 bar, (4) a period at room temperature, (5) cooling at -75 ° C for 2 hours, and (6) 3 revenues at 550 ° C for grade C and 560 ° C for grades A and B for 1 hour each.
La figure 2 représente le profil de dureté superficielle (microdureté en HV0,5 en fonction de la profondeur de l'acier (en mm) de l'exemple 2 (nuance C) selon l'invention ayant la composition indiquée dans le tableau 1 ci-dessous ainsi que d'un exemple comparatif 50Nil_ (0,12%C-4%Cr- 4,2%Mo-3,4°/oNi-l%V), obtenu après cémentation et traitement thermique comprenant les étapes suivantes : (1) un chauffage à 1100°C, (2) un maintien pendant 15 min à 1100°C pour austénitisation, (3) un refroidissement sous gaz neutre à une pression de 2 bars, (4) une période à température ambiante, (5) un refroidissement à -75°C pendant 2 heures, et (6) 3 revenus à une température de 475°C ou de 500°C ou de 550°C ou de 575°C pour la nuance C ou de 560°C pour l'exemple comparatif 50Nil_ pendant 1 heure chacun.  FIG. 2 represents the superficial hardness profile (microhardness in HV0.5 as a function of the steel depth (in mm) of example 2 (grade C) according to the invention having the composition indicated in table 1 ci below as well as a comparative example 50 Nil (0.12% C-4% Cr-4.2% Mo-3.4% / 1 N-1% V), obtained after cementation and heat treatment comprising the following steps: (1) heating at 1100 ° C, (2) holding for 15 min at 1100 ° C for austenitization, (3) cooling under a neutral gas at a pressure of 2 bar, (4) a period at room temperature, ( 5) cooling at -75 ° C for 2 hours, and (6) 3 tempering at 475 ° C or 500 ° C or 550 ° C or 575 ° C for grade C or 560 ° C for Comparative Example 50Nil for 1 hour each.
La figure 3 représente le profil de dureté superficielle (microdureté en HV0,5 en fonction de la profondeur de l'acier (en mm) de l'exemple 2 (nuance C) selon l'invention ayant la composition indiquée dans le tableau 1 ci-dessous ainsi que d'un exemple comparatif 50NiL (0,12%C-4%Cr- 4,2%Mo-3,4%Ni-l%V), obtenu après cémentation et traitement thermique comprenant les étapes suivantes : (1) un chauffage à 1150°C, (2) un maintien pendant 15 min à 1150°C pour austénitisation, (3) un refroidissement sous gaz neutre à une pression de 2 bars, (4) une période à température ambiante, (5) un refroidissement à -75°C pendant 2 heures, et (6) 3 revenus à une température de 475°C ou de 500°C ou de 550°C ou de 575°C pour la nuance C ou de 560°C pour l'exemple comparatif 50NÏL pendant 1 heure chacun. Exemples 1 et 2 : FIG. 3 represents the profile of surface hardness (microhardness in HV0.5 as a function of the steel depth (in mm) of Example 2 (grade C) according to the invention having the composition indicated in Table 1 ci below as well as a comparative example 50NiL (0.12% C-4% Cr-4.2% Mo-3.4% Ni-1% V), obtained after cementation and heat treatment comprising the following steps: 1) heating at 1150 ° C, (2) holding for 15 min at 1150 ° C for austenitization, (3) cooling under a neutral gas at a pressure of 2 bar, (4) a period at room temperature, (5) ) at -75 ° C for 2 hours, and (6) 3 at a temperature of 475 ° C or 500 ° C or 550 ° C or 575 ° C for grade C or 560 ° C for Comparative Example 50NIL for 1 hour each. Examples 1 and 2:
Trois coulées laboratoires de 110 Kg chacune environ(deux exemples selon l'invention : exemple 1 et exemple 2 et un exemple comparatif selon la demande WO2015/082342 : exemple comparatif 1) ont été élaborées par le procédé VIM-VAR selon la composition figurant au tableau 1 ci- dessous :  Three laboratory flows of about 110 Kg each (two examples according to the invention: example 1 and example 2 and a comparative example according to the application WO2015 / 082342: comparative example 1) were developed by the VIM-VAR method according to the composition shown in FIG. Table 1 below:
Tableau 1  Table 1
Ces trois compositions sont très similaires. La principale différence tient à la teneur en W. These three compositions are very similar. The main difference is the W content.
Ces trois coulées laboratoire ont été transformées en barre de diamètre 40 mm par un procédé de forgeage à chaud sous presse de 2000T. Des barreaux de diamètre 30 mm ont été usinés dans la barre et cémentés. Les barreaux cémentés ont été traités par (1) un chauffage à 1100°C ou 1150°C, (2) un maintien pendant 15 min à cette température pour austénitisation, (3) un refroidissement sous gaz neutre à une pression de 2 bars, (4) une période à température ambiante, (5) un refroidissement à -75°C pendant 2 heures, et (6) 3 revenus à une température entre 475°C et 560°C pendant 1 heure chacun. These three laboratory flows were converted into a 40 mm diameter bar by a 2000T press hot forging process. Bars of diameter 30 mm were machined in the bar and case hardened. The cemented bars were treated by (1) heating at 1100 ° C or 1150 ° C, (2) holding for 15 min at this temperature for austenitization, (3) cooling under a neutral gas at a pressure of 2 bar, (4) a period at room temperature, (5) cooling at -75 ° C for 2 hours, and (6) 3 incurring at a temperature between 475 ° C and 560 ° C for 1 hour each.
Les profils de dureté superficielle en HV obtenus mesurés selon la norme ASTM E384 sont reportés dans les figures 1 à 3 avec ceux obtenus avec l'acier 50NÏL (0,12%C-4%Cr-4,2%Mo-3,4%Ni-l%V) ayant subi le même traitement d'austénitisation et de refroidissement à l'ambiante puis au froid, et 3 revenus à 560°C.  The profiles of surface hardness in HV obtained measured according to the ASTM E384 standard are reported in FIGS. 1 to 3 with those obtained with 50NIL steel (0.12% C-4% Cr-4.2% Mo-3.4 % Ni-1% V) having undergone the same austenitization and cooling treatment at ambient and then cold, and 3 incomes at 560 ° C.
Les compositions selon l'invention ayant une faible teneur en W présentent des niveaux de duretés plus élevés, de l'ordre de 860 HV correspondant à 66 HRC. Il faut aussi noter que la baisse de la teneur en W par rapport à l'art antérieur n'affecte pas significativement le niveau de dureté du métal de base qui est de l'ordre de 540 HV correspondant à 51 HRC. The compositions according to the invention having a low W content have higher hardness levels, of the order of 860 HV corresponding to 66 HRC. It should also be noted that the drop in the W content relative to the prior art does not significantly affect the hardness level of the base metal which is of the order of 540 HV corresponding to 51 HRC.
L'acier ayant la composition selon l'invention (faible teneur en W) permet donc d'obtenir de plus hauts niveaux de dureté avec un traitement thermique limité à 1150°C par rapport à celui de l'art antérieur ayant une plus forte teneur en W.  The steel having the composition according to the invention (low W content) thus makes it possible to obtain higher levels of hardness with a heat treatment limited to 1150 ° C. compared to that of the prior art having a higher content. in W.
Il est à remarquer également qu'une température de revenu de 500°C est particulièrement intéressante puisque le niveau de dureté atteint les 66-67 HRC (avec traitement en solution à 1100°C et 1150°C) (figures 2 et 3). A 575°C, les résultats restent encore très intéressants avec des valeurs supérieurs à 64 HRC après mise en solution à 1150°C uniquement (figure 3).  It should also be noted that a recovery temperature of 500 ° C. is particularly advantageous since the hardness level reaches 66-67 HRC (with treatment in solution at 1100 ° C. and 1150 ° C.) (FIGS. 2 and 3). At 575 ° C, the results are still very interesting with values above 64 HRC after dissolution only at 1150 ° C (Figure 3).

Claims

REVENDICATIONS
1. Composition d'acier cémentable et/ou nitrurable comprenant, avantageusement constituée essentiellement de, en pourcentages en poids de la composition totale: 1. Composition of cementable and/or nitridable steel comprising, advantageously consisting essentially of, in percentages by weight of the total composition:
Carbone : 0,05-0,40, de préférence 0,10-0,30 ; Carbon: 0.05-0.40, preferably 0.10-0.30;
Chrome : 2,50-5,00, de préférence 3,00-4,50; Chromium: 2.50-5.00, preferably 3.00-4.50;
Molybdène : 4,00-6,00; Molybdenum: 4.00-6.00;
Tungstène : 0,01-1,80, de préférence 0,02-1,50 ; Tungsten: 0.01-1.80, preferably 0.02-1.50;
Vanadium : 1,00-3,00, de préférence 1,50-2,50; Vanadium: 1.00-3.00, preferably 1.50-2.50;
Nickel : 2,00-4,00; Nickel: 2.00-4.00;
Cobalt : 2,00-8,00, de préférence 3,00-7,00; Cobalt: 2.00-8.00, preferably 3.00-7.00;
Fer : solde Iron: balance
ainsi que les impuretés inévitables, as well as the inevitable impurities,
optionnellement comprenant en outre, un ou plusieurs des éléments suivants : optionally further comprising one or more of the following elements:
Niobium :≤ 2,00 ; Niobium: ≤ 2.00;
Azote : < 0,50, de préférence < 0,20 ; Nitrogen: <0.50, preferably <0.20;
Silicium : < 0,70, de préférence 0,05-0,50; Silicon: <0.70, preferably 0.05-0.50;
Manganèse :≤ 0,70, de préférence 0,05-0,50; Manganese: ≤ 0.70, preferably 0.05-0.50;
Aluminium : < 0,15, de préférence < 0,10 ; Aluminum: <0.15, preferably <0.10;
la teneur combinée en Niobium + Vanadium étant comprise dans la gamme 1,00-3,50 ; the combined Niobium + Vanadium content being in the range 1.00-3.50;
et la teneur en Carbone + Azote étant comprise dans la gamme 0,05-0,50. and the Carbon + Nitrogen content being in the range 0.05-0.50.
2. Composition d'acier selon la revendication 1, caractérisée en ce qu'elle comprend, avantageusement en ce qu'elle est constituée essentiellement de, en pourcentages en poids de la composition totale: Carbone : 0,10-0,30, de préférence 0,15-0,25 ; 2. Steel composition according to claim 1, characterized in that it comprises, advantageously in that it consists essentially of, in percentages by weight of the total composition: Carbon: 0.10-0.30, preferably 0.15-0.25;
Chrome : 3,00-4,50, de préférence 3,50-4,50; Chromium: 3.00-4.50, preferably 3.50-4.50;
Molybdène : 4,00-6,00, de préférence 4,50-5,50; Molybdenum: 4.00-6.00, preferably 4.50-5.50;
Tungstène 0,02-1,50, de préférence 0,03-1,40 ; Tungsten 0.02-1.50, preferably 0.03-1.40;
Vanadium : 1,50-2,50; Vanadium: 1.50-2.50;
Nickel : 2,00-4,00, de préférence 2,50-3,50 ; Nickel: 2.00-4.00, preferably 2.50-3.50;
Cobalt : 3,00-7,00, de préférence 4,00-6,00; Cobalt: 3.00-7.00, preferably 4.00-6.00;
Silicium : 0,05-0,50, de préférence 0,05-0,30; Silicon: 0.05-0.50, preferably 0.05-0.30;
Manganèse : 0,05-0,50, de préférence 0,05-0,30; Manganese: 0.05-0.50, preferably 0.05-0.30;
Fer : solde Iron: balance
ainsi que les impuretés inévitables, as well as the inevitable impurities,
optionnellement comprenant en outre, un ou plusieurs des éléments suivants : optionally further comprising one or more of the following elements:
Niobium : < 2,00 ; Niobium: <2.00;
Azote : < 0,20 ; Nitrogen: <0.20;
Aluminium : <0,10 ; Aluminum: <0.10;
la teneur combinée en Niobium + Vanadium étant comprise dans la gamme 1,00-3,50 ; the combined Niobium + Vanadium content being in the range 1.00-3.50;
et la teneur en Carbone + Azote étant comprise dans la gamme 0,05-0,50. and the Carbon + Nitrogen content being in the range 0.05-0.50.
3. Composition d'acier selon l'une quelconque des revendications 1 ou3. Steel composition according to any one of claims 1 or
2, caractérisée en ce qu'elle comprend au plus 1% en poids d'impuretés inévitables, avantageusement au plus 0,5% en poids par rapport au poids total de la composition. 2, characterized in that it comprises at most 1% by weight of unavoidable impurities, advantageously at most 0.5% by weight relative to the total weight of the composition.
4. Composition d'acier selon l'une quelconque des revendications 1 à4. Steel composition according to any one of claims 1 to
3, caractérisée en ce que les impuretés inévitables sont choisis parmi le Titane, le Soufre, le Phosphore, le Cuivre, l'Etain, le Plomb, l'Oxygène et leurs mélanges. 3, characterized in that the inevitable impurities are chosen from the Titanium, Sulfur, Phosphorus, Copper, Tin, Lead, Oxygen and their mixtures.
5. Composition d'acier selon l'une quelconque des revendications 1 à 4, caractérisée en ce que la teneur en tungstène est comprise dans la gamme 0,03-1,40, de préférence dans la gamme 0,04-1,30 en pourcentages en poids de la composition totale. 5. Steel composition according to any one of claims 1 to 4, characterized in that the tungsten content is in the range 0.03-1.40, preferably in the range 0.04-1.30 in percentages by weight of the total composition.
6. Composition d'acier selon l'une quelconque des revendications 1 à 5, caractérisée en ce qu'elle présente, après un traitement thermochimique, avantageusement de cémentation ou de nitruration ou de carbonitru ration ou de cémentation puis de nitruration, suivi d'un traitement thermique, une dureté superficielle supérieure ou égale à 64 HRC, avantageusement supérieure ou égale à 65 HRC. 6. Steel composition according to any one of claims 1 to 5, characterized in that it presents, after a thermochemical treatment, advantageously carburizing or nitriding or carbonitriding or carburizing then nitriding, followed by heat treatment, a surface hardness greater than or equal to 64 HRC, advantageously greater than or equal to 65 HRC.
7. Composition d'acier selon l'une quelconque des revendications 1 à7. Steel composition according to any one of claims 1 to
6, caractérisée en ce qu'elle présente, après un traitement thermochimique, avantageusement de cémentation ou de nitruration ou de carbonitru ration ou de cémentation puis de nitruration, suivi d'un traitement thermique, une structure martensitique ayant une teneur d'austénite résiduelle inférieure à 10% et exempte de ferrite et de perlite. 6, characterized in that it presents, after a thermochemical treatment, advantageously carburizing or nitriding or carbonitriding or carburizing then nitriding, followed by a heat treatment, a martensitic structure having a lower residual austenite content at 10% and free of ferrite and pearlite.
8. Composition d'acier selon l'une quelconque des revendications 6 ou8. Steel composition according to any one of claims 6 or
7, caractérisée en ce le traitement thermique comprend une mise en solution à une température comprise entre 1090°C-1160°C suivi d'une trempe avec éventuellement un refroidissement à une température inférieure à -40°C et plusieurs revenus, avantageusement au moins trois revenus, à une température supérieure ou égale à 475°C. 7, characterized in that the heat treatment comprises a solution at a temperature between 1090°C-1160°C followed by quenching with possibly cooling to a temperature below -40°C and several temperings, advantageously at least three tempers, at a temperature greater than or equal to 475°C.
9. Procédé de fabrication d'une ébauche en acier ayant la composition selon l'une quelconque des revendications 1 à 8, caractérisé en ce qu'il comprend : 9. Process for manufacturing a steel blank having the composition according to any one of claims 1 to 8, characterized in that it comprises:
a) une étape d'élaboration de l'acier ; a) a steel production stage;
b) une étape de transformation de l'acier ; b) a steel processing stage;
c) un traitement thermochimique; c) thermochemical treatment;
d) et un traitement thermique. d) and heat treatment.
10. Procédé de fabrication selon la revendication 9, caractérisé en ce que l'étape c) consiste en un traitement de cémentation ou de nitruration ou de carbonitru ration ou de cémentation puis de nitruration, avantageusement il s'agit d'un traitement de cémentation. 10. Manufacturing process according to claim 9, characterized in that step c) consists of a carburizing or nitriding treatment or carbonitriding or carburizing then nitriding, advantageously it is a carburizing treatment .
11. Procédé de fabrication selon l'une quelconque des revendications 9 ou 10, caractérisé en ce que l'étape d) comprend une mise en solution à une température comprise entre 1090°C-1160°C, avantageusement comprise entre 1100°C-1150°C, suivi d'un maintien à cette température jusqu'à austénitisation complète avec éventuellement un refroidissement à une température inférieure à -40°C, avantageusement de -75°C, et plusieurs revenus, avantageusement au moins trois revenus, à une température supérieure ou égale à 475°C, avantageusement supérieure ou égale à 500°C. 11. Manufacturing process according to any one of claims 9 or 10, characterized in that step d) comprises dissolution at a temperature between 1090°C-1160°C, advantageously between 1100°C- 1150°C, followed by maintaining at this temperature until complete austenitization with possibly cooling to a temperature below -40°C, advantageously -75°C, and several tempers, advantageously at least three tempers, at a temperature greater than or equal to 475°C, advantageously greater than or equal to 500°C.
12. Procédé de fabrication selon l'une quelconque des revendications 9 à 11, caractérisé en ce que l'étape b) consiste en une étape de laminage, de forgeage et/ou de filage. 12. Manufacturing method according to any one of claims 9 to 11, characterized in that step b) consists of a rolling, forging and/or spinning step.
13. Procédé de fabrication selon l'une quelconque des revendications 9 à 12, caractérisé en ce que l'étape a) d'élaboration est mise en œuvre par un procédé d'élaboration conventionnelle four à arc affinage et refusion sous laitier conducteur (ESR), ou par un procédé VIM-VAR, avec éventuellement une étape de refusion sous laitier conducteur (ESR) et/ou sous vide (VAR), ou par Métallurgie des poudres telle que atomisation par gaz et compression par compaction isostatique à chaud (HIP). 13. Manufacturing method according to any one of claims 9 to 12, characterized in that step a) of development is implemented by a conventional arc furnace production process refining and remelting under conductive slag (ESR ), or by a VIM-VAR process, possibly with a remelting step under conductive slag (ESR) and/or under vacuum (VAR), or by powder metallurgy such as gas atomization and compression by hot isostatic compaction (HIP ).
14. Ebauche d'acier susceptible d'être obtenu par un procédé selon l'une quelconque des revendications 9 à 13. 14. Steel blank capable of being obtained by a process according to any one of claims 9 to 13.
15. Utilisation d'une ébauche selon la revendication 14 ou d'une composition d'acier selon l'une quelconque des revendications 1 à 8 pour la fabrication d'un organe mécanique, avantageusement d'un roulement. 15. Use of a blank according to claim 14 or of a steel composition according to any one of claims 1 to 8 for the manufacture of a mechanical member, advantageously a bearing.
16. Organe mécanique, avantageusement un roulement ou un engrenage, en acier ayant la composition selon l'une quelconque des revendications 1 à 8 ou obtenu à partir d'une ébauche d'acier selon la revendication 14. 16. Mechanical member, advantageously a bearing or a gear, made of steel having the composition according to any one of claims 1 to 8 or obtained from a steel blank according to claim 14.
EP17737334.7A 2016-06-17 2017-06-16 Steel composition Pending EP3472363A1 (en)

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US10865457B2 (en) 2020-12-15
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CN117867408A (en) 2024-04-12
CN109790594A (en) 2019-05-21
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US20190338383A1 (en) 2019-11-07
WO2017216500A1 (en) 2017-12-21

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