Classifications

• • 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
  • 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
  • C21D9/00—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor
  • C21D9/32—Heat treatment, e.g. annealing, hardening, quenching or tempering, adapted for particular articles; Furnaces therefor for gear wheels, worm wheels, or the like
• • 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
  • 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
  • C22C38/02—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
  • C22C38/12—Ferrous alloys, e.g. steel alloys containing tungsten, tantalum, molybdenum, vanadium, or niobium
• • 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
  • C22C38/48—Ferrous alloys, e.g. steel alloys containing chromium with nickel with niobium or tantalum
• • 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
  • 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
  • C21D2211/00—Microstructure comprising significant phases
  • C21D2211/008—Martensite

Definitions

• the present invention relates to a new steel of grade 10CrMoNiVCo with low carbon content and high cobalt content for thermochemical treatment in particular intended for the field of transmissions such as bearings and gears.
• the alloy according to the invention is also usable for other applications requiring high surface hardness combined with good core toughness, for example in the case of injection systems.
• Bearings are mechanical devices allowing relative movements, constrained in orientation and direction, between two components.
• Bearings comprise several components: inner race, outer race as well as rolling bodies (balls or rollers) arranged between these two races. To ensure reliability and performance over time, it is important that these various elements have good properties of rolling fatigue, wear, etc.
• Gear trains are mechanical devices for power transmission. To ensure a favorable power density (ratio of power transmitted to overall dimensions of the gear train) and operational reliability, gear trains must have good properties of structural fatigue (tooth root) and contact fatigue (tooth flank).
• the conventional techniques for producing these metallic components employ production methods of electric steelmaking followed by optional operations of remelting, or single or multiple vacuum remelting.
• the ingots thus produced are then formed by methods of hot working such as rolling or forging in the form of bar, tube or rings.
• 2nd Type the component requires a thermochemical treatment to enrich the surface with interstitial chemical elements such as carbon and/or nitrogen. This enrichment, generally at the surface, then allows high mechanical properties to be obtained after heat treatment to depths of some millimeters at most. These steels generally have better properties of ductility than the steels of the 1st type.
• thermochemical methods applied to steels of the 1st type with the aim of enriching the surface with nitrogen to obtain very high mechanical properties.
• the first of the properties required in the field of bearings or gear trains is obtaining a very high level of hardness.
• These steels of type 1 and of type 2 generally have levels of surface hardness above 58 HRC.
• the grades used most widely and known by the term M50 (0.8% C-4% Cr-4.2% Mo-1% V) or 50NiL (0.12% C-4% Cr-4.2% Mo-3.4% Ni-1% V) do not exceed, after optional thermochemical treatment and suitable heat treatment, a surface hardness of 63 HRC. It is now necessary to obtain hardnesses above 64 HRC for significant improvement of the properties of the component.
• Application GB2370281 describes a valve seat steel produced by powder metallurgy technology starting from mixtures of powder with an iron base and harder particles.
• the matrix which only constitutes one part of the steel, has the following composition, in percentages by weight of the total composition:
• this matrix comprises from 5 to 40 vol % of pearlite, with consequent lack of ductility of this matrix and therefore embrittlement.
• the material also contains porosity (up to 10%), which does not allow good properties of mechanical strength and fatigue strength to be achieved.
• this document does not suggest using a low copper content and on the contrary indicates that its content may be up to 15 wt %.
• a high copper content is undesirable for the applications of the present invention as copper is known to cause embrittlement and its content should not exceed 0.5 wt % relative to the total weight of the composition of the steel.
• Patent application WO2015/082342 describes a rolling bearing steel having the following composition, in percentages by weight of the total composition:
• MIX5 of 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), which is the most interesting as it has the highest surface hardness.
• This grade makes it possible to reach a surface hardness after solution heat treatment at 1150° C. and tempering at 560° C. at a maximum hardness level of about 800 HV, or the equivalent of max. 64 HRC.
• this application states that the Co content must be limited to at most 8% and it is even preferable for it to be at most 7% and even more preferably at most 6% as Co increases the level of hardness of the base material, which leads to a decrease in toughness.
• the grade MIX5 that is preferred thus has a Co content of 5.14%.
• Patent application WO2017216500 describes a rolling bearing steel having the following composition, in percentages by weight of the total composition:
• the combined niobium+vanadium content being in the range 1.00-3.50; and the carbon+nitrogen content being in the range 0.05-0.50.
• grade C of composition (0.18-0.20% C-3.90-4.00% Cr-5.00-5.20% Mo-0.10-0.20% W-2.10-2.30% V-0.14-0.16% Si-3.05-3.09% Ni-5.00-5.40% Co-0.18-0.22% Mn-0.03-0.05% Al) is preferred, as it has the highest surface hardness.
• This grade makes it possible to reach a surface hardness after solution heat treatment at 1100° C.-1150° C. and tempering at 500° C. at a maximum level of hardness of about 66-67 HRC, which is well above the surface hardness obtained with a grade according to application WO2015/082342 (grade A: FIG. 1).
• the Co content must be limited to at most 8% and it is even preferable for it to be at most 7% and even more preferably at most 6% as it increases the level of hardness of the base material, which leads to a decrease in toughness.
• the grade C that is preferred thus has a Co content of 5.00-5.40%.
• the inventors found, surprisingly, that by increasing the cobalt content of the steel described in applications WO2015/082342 and WO2017216500 to a content between 9 and 12.5%, while maintaining the carbon content at a level less than or equal to 0.2% (new carbon/cobalt balance), the steel obtained had, after thermochemical treatment, in particular carburizing and/or nitriding, a very high surface hardness, even above 67 HRC, in particular greater than or equal to 68 HRC and a hardness at 1 mm greater than 860 HV (which corresponds to about 66 HRC according to standard ASTME140-12b published in May 2013) after solution heat treatment at a temperature in the range 1100° C.-1160° C. and tempering at a temperature greater than or equal to 475° C., while displaying a level of hardness of the base material between 400 and 650 HV.
• U.S. Pat. No. 8,157,931 describes a steel of type Ni—Co having a cobalt content between 9.9 and 10% and a carbon content between 0.1 and 0.12% and having a high surface hardness of the order of 68-69 HRC.
• said steel has a high chromium content (5.3-5.4%), a low content of vanadium (0.20-0.21%) and molybdenum (2.5-2.52%) and does not contain tungsten.
• This grade balancing leads, after thermochemical treatment and associated quality treatment (comprising quenching at 1110° C.
• Patent application JPH11-210767 describes a class of steel for aeronautical rolling bearing application with an improved service life having the following composition, in percentages by weight of the total composition:
• This grade is submitted to carburizing or carbonitriding.
• this application does not describe the core hardness (reflecting the mechanical strength) of this grade, and in view of the very low level of carbon, this is expected to degrade the mechanical strength.
• this application does not describe any carburizing profile to a deep layer.
• high hardness in the full depth as far as 400 microns from the surface, which corresponds to the so-called Hertz zone, a zone subjected to very high shear stresses.
• High hardness throughout this depth also provides more tolerance when it comes to removing material for repair or grinding during machining, and this is all the more useful for the power transmission application, which is not mentioned in JPH11-210767.
• the inventors realized that it was possible to obtain balancing different from that proposed by JPH11-210767 with a higher carbon content, at least 0.06 wt %, and a range of cobalt between 9.0 and 12.5 wt %, making it possible (a) to obtain a good compromise between core hardness and toughness, in other words a good compromise between mechanical strength and toughness, and (b) to allow more vanadium in its composition without degrading the toughness, which is favorable for wear resistance.
• the present invention therefore relates to a steel composition, advantageously carburizable and/or nitridable, more advantageously carburizable, comprising, advantageously consisting essentially of, in particular consisting of, in percentages by weight of the total composition:
• the combined niobium+vanadium content being in the range 1.0-3.5; and the carbon+nitrogen content being in the range 0.06-0.50.
• a particularly interesting composition comprises, advantageously consists essentially of, in particular consists of, in percentages by weight of the total composition:
• the combined niobium+vanadium content being in the range 1.00-3.50; and the carbon+nitrogen content being in the range 0.06-0.50.
• the inevitable impurities notably selected from titanium (Ti), sulfur (S), phosphorus (P), copper (Cu), tin (Sn), lead (Pb), oxygen (O) and mixtures thereof, are kept at the lowest level.
• these impurities are generally due essentially to the method of manufacture and the quality of the charge.
• the composition according to the invention comprises at most 1 wt % of inevitable impurities, advantageously at most 0.75 wt %, even more advantageously at most 0.50 wt %, relative to the total weight of the composition.
• the carbide formers which also have a stabilizing effect on ferrite, so-called alpha-forming elements, are essential to the steel composition according to the invention so as to provide sufficient hardness, heat resistance and wear resistance.
• austenite stabilizers so-called gamma-forming elements.
• austenite stabilizers carbon, nickel, cobalt and manganese
• ferrite stabilizers mobdenum, tungsten, chromium, vanadium and silicon
• the steel composition according to the invention therefore comprises carbon (C) at a content in the range 0.06-0.20%, preferably 0.07-0.20%, in particular 0.08-0.20%, more particularly 0.08-0.18%, by weight relative to the total weight of the composition.
• carbon (C) stabilizes the austenitic phase of the steel at the heat treatment temperatures and is essential for formation of carbides, which supply the mechanical properties in general, notably mechanical strength, high hardness, heat resistance and wear resistance.
• the presence of a small amount of carbon in a steel is beneficial for avoiding formation of undesirable, brittle intermetallic particles and for forming small amounts of carbides to avoid excessive grain growth during solution treatment before the quenching operation.
• the initial carbon content need not, however, be too high, since it is possible to increase the surface hardness of the components formed from the steel composition by carburizing. It is also known that, generally, increasing the carbon content makes it possible to increase the level of hardness significantly, which is generally detrimental with respect to the ductility properties. That is why the carbon content is limited to max. 0.20% to obtain a level of core hardness of the material of at most 650 HV.
• carbon is introduced into the surface layers of the component, so as to obtain a hardness gradient. Carbon is the principal element for controlling the hardness of the martensitic phase formed after carburizing and heat treatment. In a case-hardened steel, it is essential to have a core portion of the material with a low carbon content while having a hard surface with a high carbon content after carburizing thermochemical treatment.
• the steel composition according to the invention further comprises chromium (Cr) at a content in the range 2.5-5.0%, preferably 3.0-4.5%, even more preferably 3.5-4.5%, even more advantageously 3.8-4.0 wt % relative to the total weight of the composition.
• Cr chromium
• Chromium contributes to the formation of carbides in steel and is one of the main elements controlling the hardenability of steels.
• chromium may also promote the appearance of ferrite and residual austenite. Therefore the chromium content of the steel composition according to the invention must not be too high.
• the steel composition according to the invention also comprises molybdenum (Mo) at a content in the range 4.0-6.0%, preferably 4.5-5.5%, even more preferably 4.8-5.2%, by weight relative to the total weight of the composition.
• Mo molybdenum
• Molybdenum improves tempering resistance, wear resistance and the hardness of steel. However, molybdenum has a strong stabilizing effect on the ferrite phase and therefore should not be present in an excessive amount in the steel composition according to the invention.
• the steel composition according to the invention further comprises tungsten (W) at a content in the range 0.01-3.0%, preferably 0.01-1.5%, even more preferably 0.01-1.4%, advantageously 0.01-1.3%, by weight relative to the total weight of the composition.
• W tungsten
• Tungsten is a ferrite stabilizer and a strong carbide former. It improves resistance to heat treatment and to wear as well as hardness by forming carbides. However, it may also lower the surface hardness of the steel and especially the properties of ductility and toughness. For this element to perform its role fully, it is necessary to apply solution treatment at high temperature.
• the steel composition according to the invention further comprises vanadium (V) at a content in the range 1.0-3.0%, preferably 1.5-2.5%, even more preferably 1.7-3.0%, advantageously 1.7-2.5%, more advantageously 1.7-2.3%, even more advantageously 2.00-2.3%, in particular 2.0-2.2%, 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 wear resistance and tempering resistance by forming hard vanadium carbides. Vanadium may be replaced partly with niobium (Nb), which has similar properties.
• the combined niobium+vanadium content must therefore be in the range 1.0-3.5 wt % relative to the total weight of the composition, advantageously in the range 1.7-3.5 wt % 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) at a content in the range 2.0-4.0%, preferably 2.5-3.5%, even more preferably 2.7-3.3%, advantageously 3.0-3.2%, by weight relative to the total weight of the composition.
• Nickel promotes the formation of austenite and therefore inhibits the formation of ferrite. Another effect of nickel is to lower the temperature Ms, i.e. the temperature at which the transformation of austenite to martensite begins during cooling. This may prevent martensite formation. The amount of nickel must therefore be controlled so as to avoid formation of residual austenite in the carburized components.
• the steel composition according to the invention further comprises cobalt (Co) at a content in the range 9.0-12.5%, preferably 9.5-12.5%, advantageously 9.5-11.0%, more advantageously 9.5-10.5%, by weight relative to the total weight of the composition.
• cobalt content is measured according to standards ASTM-E1097-12 published in June 2017 and ASTM E1479_16 published in December 2016.
• the error in measurement of the cobalt content of the steel according to the invention is thus about ⁇ 2.5% relative, and is evaluated according to standards IS05724-1 (December 1994), ISO5725-2 (December 1994), ISO5725-3 (December 1994), ISO5725-4 (December 1994), ISO5725-5 (December 1994), ISO5725-6 (December 1994) and standard NF ISO/CEI Guide 98-3 of 11 Jul. 2014.
• Cobalt is a strong austenite stabilizer that prevents the formation of undesirable ferrite. In contrast to nickel, cobalt increases the temperature Ms, which in its turn decreases the amount of residual austenite. Cobalt, in combination with nickel, allows the presence of ferrite stabilizers such as the carbide formers Mo, W, Cr and V.
• the carbide formers are essential for the steel according to the invention on account of their effect on hardness, heat resistance and wear resistance. Cobalt has a small effect on the steel of increasing the hardness. However, this increase in hardness is correlated with a decrease in toughness. Therefore the steel composition according to the invention should not contain an excessive amount of cobalt.
• Addition of Co makes it possible to limit the content of C, avoiding the promotion of ferrite for a composition according to the invention (containing the contents of Cr, Mo, V, Ni and W as described above). This limitation of carbon makes it possible to compensate for the increase in hardness associated with the addition of Co.
• the steel composition according to the invention may further comprise silicon (Si) in a content ⁇ 0.70 wt % relative to the total weight of the composition.
• Si silicon
• it comprises silicon, in particular at a content in the range 0.05-0.50%, preferably 0.05-0.30%, advantageously 0.07-0.25%, even more advantageously 0.10-0.20%, by weight relative to the total weight of the composition.
• Silicon is a strong ferrite stabilizer, but is often present during steelmaking, during deoxidation of the molten steel. Low oxygen contents are in fact also important for obtaining low levels of nonmetallic inclusions and good mechanical properties such as fatigue strength and mechanical strength.
• the steel composition according to the invention may further comprise manganese (Mn) in a content ⁇ 0.70 wt % relative to the total weight of the composition.
• Mn manganese
• it comprises manganese, in particular at 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%, even more particularly 0.10-0.20% by weight relative to the total weight of the composition.
• Manganese stabilizes the austenite phase and decreases the temperature Ms in the steel composition.
• Manganese is generally added to the steels during their manufacture owing to its affinity for sulfur, there is thus formation of manganese sulfide during solidification. This eliminates the risk of formation of iron sulfides, which have an unfavorable effect on hot machining of the steels.
• Manganese also forms part of the deoxidation step, like silicon. The combination of manganese and silicon gives more effective deoxidation than each of these elements alone.
• the steel composition according to the invention may comprise nitrogen (N), in a content ⁇ 0.50%, preferably ⁇ 0.20%, by weight relative to the total weight of the composition.
• Nitrogen promotes austenite formation and lowers the transformation of austenite to martensite. Nitrogen may to a certain extent replace carbon in the steel according to the invention, forming nitrides. However, the carbon+nitrogen content must be in the range 0.06-0.50 wt % relative to the total weight of the composition.
• the steel composition according to the invention may comprise aluminum (Al), in a content ⁇ 0.15%, preferably ⁇ 0.10%, by weight relative to the total weight of the composition.
• Aluminum (Al) may in fact be present during steelmaking according to the invention and contributes very effectively to deoxidation of the molten steel. This is the case in particular in remelting processes, such as the VIM-VAR process.
• the aluminum content is in general higher in the steels produced by the VIM-VAR process than in the steels obtained by powder metallurgy. Aluminum gives rise to difficulties during atomization by obstructing the pouring spout with oxides.
• a low oxygen content is important for obtaining good micro-cleanness as well as good mechanical properties such as fatigue strength and mechanical strength.
• the oxygen contents obtained by the ingot route are typically below 15 ppm.
• the composition according to the present invention is carburizable, i.e. it can undergo a carburizing treatment, and/or nitridable, i.e. it can undergo a nitriding treatment and even advantageously it can undergo a thermochemical treatment, in particular selected from carburizing, nitriding, carbonitriding and carburizing followed by nitriding.
• a carburizing treatment i.e. it can undergo a nitriding treatment
• nitriding treatment i.e. it can undergo a nitriding treatment and even advantageously it can undergo a thermochemical treatment, in particular selected from carburizing, nitriding, carbonitriding and carburizing followed by nitriding.
• a thermochemical treatment in particular selected from carburizing, nitriding, carbonitriding and carburizing followed by nitriding.
• the surface is thus advantageously enriched with carbon to obtain a final carbon content (final surface carbon content) of 0.5%-1.7 wt %, more particularly of 0.8%-1.5 wt %, more advantageously of at least 1 wt %, in particular of 1-1.3 wt %, even more advantageously >1.1 wt %, even more particularly between 1.2 and 1.5 wt %.
• the surface carbon content will be understood to have been determined by sampling from a surface layer to a depth of 100 microns.
• nitriding is used, it is the nitrogen content that increases at the surface of the steel, and therefore also the surface hardness.
• carbonitriding or carburizing followed by nitriding is used, it is the contents of carbon and nitrogen at the surface of the steel that are increased and therefore also the surface hardness.
• the steel composition according to the invention has, after a thermochemical treatment, advantageously of carburizing or of nitriding or of carbonitriding or of carburizing and then nitriding, followed by a heat treatment, a surface hardness above 67HRC, in particular greater than or equal to 68 HRC, measured according to standard ASTM E18 published in July 2017 or an equivalent standard.
• It also has, advantageously, a surface hardness greater than or equal to 910 HV (about 67.25 HRC according to standard ASTM E140-12b published in May 2013), advantageously greater than or equal to 920 HV, in particular greater than or equal to 940 HV, measured according to standard ASTM E384 published in August 2017 or an equivalent standard, in particular after a solution treatment at a temperature of 1100° C.
• It also has, advantageously, a surface hardness greater than or equal to 930 HV (corresponding to about 67.75 HRC according to standard ASTM E140-12b published in May 2013), advantageously greater than or equal to 940 HV (corresponding to 68 HRC according to standard ASTM E140-12b published in May 2013), in particular greater than or equal to 950 HV, measured according to standard ASTM E384 published in August 2017 or an equivalent standard after a solution treatment at a temperature of 1150° C.
• 930 HV corresponding to about 67.75 HRC according to standard ASTM E140-12b published in May 2013
• 940 HV corresponding to 68 HRC according to standard ASTM E140-12b published in May 2013
• 950 HV measured according to standard ASTM E384 published in August 2017 or an equivalent standard after a solution treatment at a temperature of 1150° C.
• a hardness at a depth of 1 mm greater than or equal to 860 HV (which corresponds to about 66 HRC according to standard ASTM E140-12b published in May 2013), advantageously greater than or equal to 870 HV, in particular greater than or equal to 880 HV, measured according to standard ASTM E384 published in August 2017 or an equivalent standard, in particular after a solution treatment at a temperature of 1100° C.
• It also has, advantageously, a level of hardness of the base material (core material hardness) between 440 and 650 HV, advantageously between 440 and 630 HV, measured according to standard ASTM E384 published in August 2017 or an equivalent standard.
• the steel composition obtained as a result of these treatments advantageously has a surface concentration of carbon (final surface content) of 1-1.3 wt %.
• Said heat treatment may comprise:
• the advantage of the steel according to the invention is therefore that of obtaining high levels of hardness with a limited heat treatment (temperature between 1090° C.-1160° C., advantageously between 1100° C.-1160° C., more advantageously between 1100° C.-1155° C., in particular between 1100° C.-1150° C., more particularly of 1150° C.).
• the steel composition according to the invention has, after thermochemical treatment, advantageously of carburizing or of nitriding or of carbonitriding or of carburizing and then nitriding, followed by a heat treatment, a martensitic structure having a residual austenite content below 10 wt %, more advantageously below 0.5 wt %, and free from ferrite and pearlite, phases that are known to decrease the surface hardness of steel.
• Said heat treatment may be as described above.
• the present invention further relates to a method of manufacturing a steel blank having the composition according to the invention, characterized in that it comprises:
• step d) of the method according to the present invention is as described above.
• thermochemical treatment in step c) of the method according to the present invention consists of a treatment of carburizing or of nitriding or of carbonitriding or of carburizing and then nitriding, advantageously it is a carburizing treatment, more particularly allowing carbon enrichment of the surface, leading to a final surface carbon content of at least 1 wt %, even more advantageously >1.1 wt %.
• step b) of the method according to the present invention consists of a step of rolling, forging and/or extrusion, advantageously forging.
• the steelmaking step a) of the method according to the present invention is carried out by a conventional steelmaking process in an arc furnace with refining and remelting under conductive slag (ESR, electroslag remelting), or by a VIM or VIM-VAR process, optionally with a step of remelting under conductive slag (ESR, electroslag remelting) and/or under vacuum (VAR), or by powder metallurgy such as gas atomization and compaction by hot isostatic pressing (HIP).
• ESR conductive slag
• VAR vacuum
• powder metallurgy such as gas atomization and compaction by hot isostatic pressing (HIP).
• the steel according to the present invention may be produced by a VIM-VAR process.
• This process makes it possible to obtain very good cleanness with respect to inclusions, and improves the chemical homogeneity of the ingot. It is also possible to proceed by a route of remelting under conductive slag (ESR: ElectroSlag Remelting) or to combine ESR and VAR (vacuum remelting) operations.
• ESR ElectroSlag Remelting
• This steel may also be obtained by powder metallurgy. This method makes it possible to produce metal powder of great purity by atomization, preferably gas atomization to obtain low oxygen contents. The powder is then compacted for example by hot isostatic pressing (HIP).
• HIP hot isostatic pressing
• the present invention also relates to a steel blank obtainable by the method according to the invention.
• This blank is made on the basis of steel having the composition according to the present invention and as described above.
• a blank according to the invention or of a steel composition according to the invention for making a mechanical device or an injection system advantageously a transmission component such as a gear train, a transmission shaft and/or a rolling bearing and in particular a rolling bearing.
• a mechanical device advantageously a transmission component, in particular a gear train, a transmission shaft or a bearing, more particularly a bearing or a gear train, even more particularly a bearing, made of steel having the composition according to the invention or obtained from a steel blank according to the invention.
• Example 1 0.18 3.1 3.9 5.1 2.1 1.18 10.0 0.2 0.18 0.023 0.005 GRADE A
• Example 2 0.20 3.1 3.9 5.1 2.2 2.96 10.1 0.18 0.21 0.02 0.009 GRADE B
• Example 3 0.16 3.1 3.9 5.1 2.1 1.19 10.0 0.21 0.18 0.02 0.009 GRADE C
• Example 4 0.16 3.0 4.0 5.1 2.1 2.92 10.1 0.22 0.25 0.016 0.005 GRADE
• Example 5 0.16 3.1 3.9 5.0 2.1 0.01 10.0 0.123 0.2 0.042 0.005 GRADE
• Example 6 0.17 3.1 4.0 5.2 2.2 0.01 12.4 0.17 0.2 0.038 0.006 GRADE F Comparative 0.14 3.1 2.1 2.7 1.2 1.32 10.0 0.222 0.16 0.022 0.004 example 1: GRADE G
• the Nb content is below the limit of detection. Nb ⁇ 0.005% for all the examples.
• compositions are very similar, with the exception of comparative example 1.
• the notable main differences between comparative example 1 and example 1 relate to the content of V, Mo and Cr.
• the carburized rods were treated by (1) a solution treatment at 1100° C. or 1150° C., (2) holding at this temperature for 15 min for austenitization, (3) cooling under neutral gas at a pressure between 2 and 6 bar (2 ⁇ 10 5 and 6 ⁇ 10 5 Pa), (4) a period at room temperature, (5) cooling to ⁇ 70° C. for 2 hours, and (6) 3 tempering operations at a temperature of 500° C. for 1 hour each.
• the surface hardness after carburizing exceeds 920 HV for a temperature of solution treatment of 1100° C. and exceeds 930 HV for a temperature of solution treatment of 1150° C.
• the hardness at a depth of 1 mm is always above 860 HV for a temperature of solution treatment of 1100° C. and is always above 880 HV for a temperature of solution treatment of 1150° C. for all the examples except comparative example 1 (effect of the lack of alloying elements).
• the hardnesses of the base materials are all below 650 HV.
• Comparative example 2 has delta ferrite after heat treatment, at a low level but sufficient to decrease the toughness properties.
• Example 7 very close to comparative example 2 at the level of its composition apart from W, does not have delta ferrite and makes it possible to obtain toughness values almost doubled relative to comparative example 2 while maintaining good mechanical strength (Rm) of about 1500 MPa, which was determined according to standard ASTM E399-17 published in February 2018, equivalent to a core hardness of 450 HV according to standard ASTM E384 published in August 2017.

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