Steel composition

Abstract

The present invention relates to a steel composition comprising, in percentages by weight of the total composition: carbon: 0.09-0.17; chromium: 3.00-6.00; molybdenum: 4.00-6.00; tungsten: ≤ 1.00; vanadium: ≤ 0.30; nickel: 2.00-4.00; cobalt: 6.00-10.00; iron: the remainder, as well as unavoidable impurities, the combined content of niobium + vanadium being ≤ 0.30. The invention also relates to the method for manufacturing same, to the steel blank obtained and to a mechanical member or an injection system comprising same.

Description

STEEL COMPOSITION Technical Field

[0001] The present invention relates to a new low-carbon, low-vanadium 1OCrMoNiCo type steel for thermochemical treatment, particularly intended for use in transmission systems such as bearings and gears. The alloy according to the invention is also suitable for other applications requiring high surface hardness combined with good core toughness, for example, in injection molding systems. Previous technique

[0002] Bearings are mechanical components that allow relative and constrained movement in orientation and direction between two parts, thus transmitting motion. Bearings comprise several components: an inner ring, an outer ring, and rolling elements (balls or cylinders) positioned between these two rings. To ensure reliability and long-term performance, it is important that these various elements have good properties in terms of rolling fatigue, wear, etc.

[0003] Gears are mechanical components for transmitting power. To ensure a favorable power density (ratio of power transmitted to the size of the gears) and reliable operation, gears must exhibit good structural fatigue and contact fatigue properties.

[0004] Conventional techniques for manufacturing these metal components involve electric arc furnace processes followed by possible remelting or single or multiple vacuum remelting operations. The resulting ingots are then shaped by hot forming processes such as rolling or forging into bars, tubes, or rings.

[0005] There are two types of metallurgy to ensure the final mechanical properties. - 1st Type: the chemical composition of the component allows the mechanical properties to be obtained directly after appropriate heat treatment. - 2nd Type: The component requires a thermochemical treatment to enrich the surface with interstitial chemical elements such as carbon and / or nitrogen. This generally superficial enrichment allows for high mechanical properties after heat treatment to depths of a few millimeters maximum. These steels generally exhibit better ductility properties than 1st Type steels.

[0006] There are also thermochemical processes applied to first-type steels aimed at enriching the surface with nitrogen to obtain very high mechanical properties.

[0007] The primary requirement for bearings and gears is a very high level of hardness. Type 1 and Type 2 steels typically exhibit surface hardness levels exceeding 58 HRC. The most common grades, known as M50 (0.8%C-4%Cr-4.2%Mo-1%V) or M50NiL (0.12%C-4%Cr-4.2%Mo-3.4%Ni-1%V), do not exceed a surface hardness of 63 HRC after any thermochemical and appropriate heat treatment. Achieving hardness levels above 64 HRC is now required to significantly improve the component's properties. It is possible to significantly increase the surface hardness level beyond 66 HRC by applying a nitriding treatment to M50, or a duplex treatment consisting of a case hardening step followed by a nitriding treatment with M50NiL. However, the implementation process is very complex.

[0008] However, the core toughness (K1C or Kq) of these steels is also an important property. Indeed, good core toughness makes the steel less brittle, which provides greater safety for large bearings (for example, tapered roller bearings in the axle boxes of the TGV train).

[0009] It is therefore important to find a good compromise between surface hardness and toughness.

[0010] The applicant has already described in patent application WO2015 / 082342 a bearing steel having the following composition, in weight percentages of the total composition: Carbon: 0.05-0.5; Chrome: 2.5-5.0; Molybdenum: 4-6; Tungsten: 2-4.5; Vanadium: 1-3; Nickel: 2-4; Cobalt: 2-8; Iron: balance, along with unavoidable impurities, optionally including one or more of the following: Niobium: 0-2; Nitrogen: 0-0.5; Silicon: 0-0.7; Manganese: 0-0.7; Aluminum: 0-0.15; and in particular the MIX5 grade with the 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 because it exhibits the highest surface hardness. This grade, also known as 65NiL®, achieves a surface hardness after solution treatment at 1150°C and tempering at 560°C, reaching a maximum hardness of approximately 800 HV, equivalent to a maximum of 64 HRC. However, this requirement specifies that the vanadium content must be at least 1%, and preferably at least 1.5%, as all examples have higher vanadium contents (between 1.94% and 2.06%) because vanadium provides resistance to wear and tempering through the formation of vanadium carbide. Furthermore, the K1C or Kq toughness of these steels is a maximum of 45 MPa·m 1 / 2 .

[0011] Patent application WO2017 / 216500 describes a bearing steel having the following composition, expressed as percentages by weight of the total composition: Carbon: 0.05-0.40, preferably 0.10-0.30; Chrome: 2.50-5.00, preferably 3.0-4.5; Molybdenum: 4.00-6.00; Tungsten: 0.01 - 1.8, 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 as well as unavoidable impurities, optionally including in addition one or more of the following: 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; Aluminium: <0.15, preferably < 0.10; the combined content of Niobium + Vanadium being in the range of 1.00-3.50; and the content of Carbon + Nitrogen being in the range of 0.05-0.50.

[0012] In particular, in the examples, 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-0.180-0.220%Mn-0.03-0.05%Al) is preferred as it exhibits the greatest surface hardness. This grade achieves a surface hardness of approximately 66-67 HRC after solution treatment at 1100°C-1150°C and tempering at 500°C, significantly exceeding the surface hardness obtained with a grade conforming to WO201 5 / 082342 (Grade A: Figure 1). However, this specification also stipulates that the vanadium content must be at least 1%, and preferably at least 1.50%, with a minimum of 2.00% being even more desirable. Indeed, all examples contain a minimum content of 2.00%.As with application WO2015 / 082342, this application teaches that vanadium provides resistance to wear and tempering by forming vanadium carbide.

[0013] Patent application WO2019 / 186016 describes a bearing steel having the following composition, expressed as percentages by weight of the total composition: Carbon: 0.06-0.20 preferably 0.08-0.18; Chrome: 2.5-5.0, preferably 3.0-4.5; Molybdenum: 4.0-6.0; Tungsten: 0.01-3.0; Vanadium: 1.0-3.0, preferably 1.5-2.5; Nickel: 2.0-4.0; Cobalt: 9.0-12.5, preferably 9.5-11.0; Iron: balance as well as unavoidable impurities, optionally including in addition one or more of the following: Niobium: < 2.0; Nitrogen: < 0.50, preferably < 0.20; Silicon: < 0.70, preferably 0.05-0.50; Manganese: < 0.70, preferably 0.05-0.50; Aluminium: <0.15, preferably < 0.10; the combined Niobium + Vanadium content being in the range of 1.0-3.5; and the Carbon + Nitrogen content being in the range of 0.06-0.50.

[0014] In particular, among the examples, the grade in Example 6 (Grade F) is preferred because it exhibits the highest surface hardness. This grade achieves a surface hardness of 980 HV with a 1 mm hardness exceeding 900 HV. This gives the material excellent surface fatigue and undercoat properties. However, this application also specifies that the vanadium content must be at least 1%, and even more so, at least 1.50%, and even more preferably at least 2.00%. Indeed, all the examples contain a minimum content of 2.00%, and the preferred example even contains 2.2%. As with application WO2015 / 082342, this application indicates that vanadium provides resistance to wear and tempering through the formation of vanadium carbides.

[0015] Thus, steels with increasingly higher surface hardnesses could be developed. However, inventors realized that the toughness of these various steels, which could achieve a surface hardness exceeding 64 HRC after case hardening, was generally less than 55 MPa·m 1 / 2 , reference value of M50NIL. To date, steels are either limited in surface hardness after case hardening to a content below 64 HRC, or limited in toughness with a K1C or Kq lower than the reference value of M50NIL steel.

[0016] It is therefore important to find a good compromise between surface hardness after case hardening which is greater than 64 HRC (800HV) while maintaining a level of core toughness equivalent to that of M50NIL steel.

[0017] The inventors were surprised to discover that such a compromise could be achieved by reducing the vanadium content, while maintaining a limited cobalt content (due to potential supply issues) through carbon / cobalt balancing, and by keeping the tungsten content low, advantageously below the detection limit. Reducing the vanadium content does not require increasing the niobium content, but at least some of the vanadium can be substituted with niobium, given that these elements have similar properties. Description of the invention

[0018] The present invention therefore relates to a steel composition comprising, (advantageously consisting essentially of, in particular consisting of), in weight percentages of the total composition: Carbon: 0.09-0.17, preferably 0.09 - 0.14; Chrome: 3.00-6.00, preferably 3.50-6.00; Molybdenum: 4.00-6.00, preferably 4.90-5.20; Tungsten: <1.00, preferably <0.01; Vanadium: < 0.30, preferably <0.25; Nickel: 2.00-4.00, preferably 3.00-3.80; Cobalt: 6.00-10.00, preferably 7.00-8.00; Iron: balance and unavoidable impurities, optionally including in addition one or more of the following: Niobium: < 0.90, advantageously < 0.01; the combined content of Niobium + Vanadium being < 0.9, advantageously < 0.30 Nitrogen: < 0.50, preferably < 0.20; Silicon: < 0.70, preferably < 0.200; Manganese: < 0.70, preferably < 0.200; Aluminium: <0.15, preferably <0.10.

[0019] Advantageously this steel composition is case-hardenable and / or nitridable, more advantageously case-hardenable.

[0020] A particularly interesting composition includes, (advantageously consists mainly of, in particular consists of), in weight percentages of the total composition: Carbon: 0.095-0.15, preferably 0.095-0.14; Chrome: 3.00-6.00, preferably 3.50-6.00; Molybdenum: 4.50-5.50, preferably 4.90-5.20; Tungsten: <1.00, preferably <0.01; Vanadium: < 0.25, preferably <0.05; Nickel: 3.00-4.00, preferably 3.30-3.80; Cobalt: 7.00-8.00, preferably 7.20-7.80; Iron: balance, along with unavoidable impurities, optionally including in addition one or more of the following: Niobium: < 0.01; the combined content of Niobium + Vanadium being < 0.25, advantageously < 0.05 Nitrogen: < 0.50, preferably < 0.20; Silicon: < 0.70, preferably < 0.200; Manganese: < 0.70, preferably < 0.200; Aluminium: <0.15, preferably <0.10.

[0021] In particular, unavoidable impurities, notably selected from Titanium (Ti), Sulfur (S), Phosphorus (P), Copper (Cu), Tin (Sn), Lead (Pb), Oxygen (O), and mixtures thereof, are kept to the lowest possible level. These impurities are generally due primarily to the manufacturing process and the quality of the furnace loading. Advantageously, the composition according to the invention comprises at most 1% by weight of unavoidable impurities, advantageously at most 0.75% by weight, and even more advantageously at most 0.50% by weight, relative to the total weight of the composition.

[0022] The carbide-forming elements, known as alpha-forming elements (which promote solidification in delta ferrite), are essential to the steel composition according to the invention in order to provide sufficient hardness, heat resistance, and wear resistance. To obtain a microstructure free of ferrite, which would weaken the component, it is necessary to add austenite-stabilizing elements, known as gamma-forming elements.

[0023] A correct combination of gamma-generating elements (Carbon, Nickel, Cobalt) and alpha-generating elements (Molybdenum, Tungsten, Chromium, Vanadium) makes it possible to obtain a steel composition according to the invention having superior properties, in particular after thermochemical treatment such as carburizing.

[0024] Advantageously, the alpha-stabilizing elements do not contain tungsten and / or vanadium and / or niobium, or only very low or even extremely low levels. Advantageously, the tungsten and / or vanadium and / or niobium content is at the limit of detection.

[0025] The steel composition according to the invention therefore comprises carbon (C) in a content within the range of 0.09-0.17%, preferably 0.09-0.15%, more preferably 0.09-0.14%, in particular 0.09-0.14%, more particularly 0.09-0.13%, even more preferably 0.095-0.15%, particularly preferably 0.095-0.14%, in particular 0.095-0.14%, more particularly 0.095-0.13%, by weight relative to the total weight of the composition. Indeed, carbon (C) stabilizes the austenitic phase of the steel at heat treatment temperatures and is essential for the formation of carbides, which provide the mechanical properties in general, notably mechanical strength, high hardness, and resistance to heat and wear.The presence of a small amount of carbon in steel is beneficial for preventing the formation of undesirable and brittle intermetallic particles and for forming small amounts of carbides to prevent excessive grain size growth during solution heating prior to quenching. However, the initial carbon content should not be too high, as it is possible to increase the surface hardness of components formed from the steel composition by case hardening. It is also generally known that increasing the carbon content leads to increased hardness. This significantly reduces the hardness level, which is generally detrimental to ductility. For this reason, the carbon content is limited to a maximum of 0.17% to achieve a core hardness of no more than 650 HV. During carburizing, carbon is implanted into the surface layers of the component to create a hardness gradient. Carbon is the primary element for controlling the hardness of the martensitic phase formed after carburizing and heat treatment. In a carburized steel, it is essential to have a core with a low carbon content and a hard surface with a high carbon content after the thermochemical carburizing treatment.

[0026] The steel composition according to the invention further comprises Chromium (Cr) in a content in the range of 3.00-6.00%, preferably 3.20-6.00%, even more preferably 3.50-6.00%, by weight relative to the total weight of the composition.

[0027] Chromium contributes to the formation of carbides in steel and is one of the main elements that controls the hardenability of steels.

[0028] However, chromium can also promote the formation of ferrite as an alpha-stabilizing element and residual austenite. Therefore, the chromium content of the steel composition according to the invention should not be too high. Chromium is also a hardening element, either through precipitation of carbides or in solid solution. A very high chromium content generally results in a decrease in toughness.

[0029] The steel composition according to the invention also comprises Molybdenum (Mo) in a content in the range of 4.00-6.00%, preferably 4.50-5.50%, even more preferably 4.80-5.20%, in particular 4.90-5.20%, by weight relative to the total weight of the composition.

[0030] Molybdenum improves the tempering strength, wear resistance, and hardness of steel. However, molybdenum has a strong stabilizing effect on the ferrite phase as an alpha-stabilizing element and should therefore not be present in excessive quantities in the steel composition according to the invention, as this could lead to the formation of embrittling phases and a decrease in toughness.

[0031] The steel composition according to the invention may further comprise tungsten (W). However, if present, its content is <1.00%, advantageously <0.50%, more advantageously <0.02%, and even more advantageously <0.01% by weight relative to the total weight of the composition, in particular <0.01%. Advantageously, the steel composition according to the invention does not comprise tungsten except for unavoidable impurities, i.e., its content is below the detection limit (i.e., <0.01% by weight relative to the total weight of the composition).

[0032] Tungsten is an alpha-stabilizing element and a strong carbide-forming element. It improves resistance to heat treatment and wear, as well as hardness, through carbide formation. However, it can also lower the surface hardness of steel and, especially, its ductility and toughness. Therefore, it is preferable to avoid it in order to maintain good core toughness. For this element to fully play its role, high-temperature solution treatment is necessary.

[0033] The steel composition according to the invention further comprises Vanadium (V) in a content <0.30%, advantageously <0.25%, more advantageously <0.23%, in particular <0.10%, more particularly <0.05%, even more advantageously <0.02% by weight relative to the total weight of the composition.

[0034] Vanadium is an alpha-stabilizing element and has a strong affinity for carbon and nitrogen. Vanadium provides resistance to wear and tempering through the formation of hard vanadium carbides. Vanadium can be partially substituted by niobium (Nb), which has similar properties. The inventors observed that vanadium had a negative impact on toughness, particularly at concentrations above 0.9%.

[0035] The combined content of Niobium + Vanadium is < 0.9%, advantageously <0.86%, more advantageously <0.88%, advantageously <0.87%, even more advantageously <0.07%, in particular <0.06%, more particularly <0.30%, advantageously <0.25%, more advantageously <0.23%, in particular < 0.10%, more particularly <0.05%, even more particularly <0.04%, particularly <0.03%, even more advantageously <0.02%, by weight relative to the total weight of the composition.

[0036] If niobium is present, its content must be < 0.90%, advantageously < 0.86%, more advantageously < 0.05%, even more advantageously < 0.02%, in particular < 0.01%, more particularly < 0.005%, by weight relative to the total weight of the composition. Advantageously, the steel composition according to the invention does not include niobium.

[0037] The steel composition according to the invention also comprises Nickel (Ni) in a content in the range of 2.00-4.00%, preferably 2.50-3.80%, even more preferably 3.00-3.75%, in particular 3.00-3.60%, advantageously 3.04-3.52%, by weight relative to the total weight of the composition.

[0038] Nickel is a gamma-stabilizing element and therefore limits or even inhibits the formation of delta ferrite during solidification. Another effect of nickel is to lower the Ms temperature, that is, the temperature at which the transformation of austenite to martensite begins during cooling. This can prevent the complete formation of austenite to martensite. The amount of nickel must therefore be controlled to avoid the formation of residual austenite in cemented components. However, nickel has a positive effect on toughness.

[0039] The steel composition according to the invention further comprises Cobalt (Co) in a content in the range of 6.00-10.00%, preferably 7.00-8.00%, advantageously 7.20-7.80%, more advantageously 7.30-7.70%, by weight relative to the total weight of the composition.

[0040] Cobalt is a gamma-stabilizing element and therefore limits or even inhibits the formation of delta ferrite during solidification. Unlike nickel, cobalt increases the melting temperature (Ms), which in turn reduces the amount of residual austenite. Cobalt, in combination with nickel, limits or even inhibits the formation of delta ferrite despite the presence of alpha-stabilizing elements such as carbide-forming elements Mo, Cr, and V, and possibly W. Carbide-forming elements are essential for the steel according to the invention due to their effect on hardness, heat resistance, and wear resistance. Cobalt has a slight hardness-increasing effect on steel. However, this increase in hardness is correlated with a decrease in toughness. Therefore, the steel composition according to the invention must not contains an excessive amount of cobalt. The addition of cobalt limits the carbon content and thus the formation of delta ferrite during solidification with a composition according to the invention (containing the levels of chromium, molybdenum, vanadium, nickel, and woad as described above). Furthermore, current supply difficulties with cobalt may arise due to its use in electric batteries.

[0041] The steel composition according to the invention may further comprise Silicon (Si) in a content < 0.70%, advantageously < 0.200%, more advantageously < 0.160%, in particular < 0.100%, in particular < 0.01000%, by weight relative to the total weight of the composition.

[0042] Silicon is a highly alpha-stabilizing element, but it is often present during the steelmaking process when deoxidizing liquid steel. Low oxygen levels are also important for achieving low levels of non-metallic inclusions and good mechanical properties such as fatigue resistance and tensile strength.

[0043] The steel composition according to the invention may further comprise Manganese (Mn) in a content < 0.70%, advantageously < 0.200%, more advantageously < 0.190%, in particular < 0.100%, in particular < 0.01000%, by weight relative to the total weight of the composition.

[0044] Manganese is a gamma-stabilizing element that lowers the melting point (Ms) in steel. Manganese is generally added to steels during their manufacture because of its affinity for sulfur; manganese sulfide forms during solidification. This eliminates the risk of iron sulfide formation, which negatively impacts the hot forgeability of steels. Manganese, like silicon, is also involved in the deoxidation process. The combination of manganese and silicon results in more effective deoxidation than either element alone.

[0045] Optionally, the steel composition according to the invention may include Nitrogen (N), in a content < 0.50%, preferably < 0.20%, advantageously < 0.01%, in particular < 0.02% by weight relative to the total weight of the composition.

[0046] Nitrogen promotes the formation of austenite and lowers the transformation temperature (Ms) of austenite to martensite. Nitrogen can, to some extent, replace carbon in the steel according to the invention to form nitrides.

[0047] Optionally, the steel composition according to the invention may include Aluminium (Al), in a content < 0.15%, preferably < 0.10%, advantageously < 0.05%, in particular < 0.04% by weight relative to the total weight of the composition.

[0048] Aluminum (Al) can indeed be present during the steelmaking process according to the invention and contributes very effectively to the deoxidation of liquid steel. This is particularly true during remelting processes such as the VIM-VAR (Vacuum Induction Melting - Vacuum Arc Remelting) process. The aluminum content is generally higher in steels produced using the VIM-VAR process than in steels obtained by powder technology. Aluminum causes difficulties during atomization by clogging the molten metal nozzle with oxides.

[0049] A low oxygen content is important for achieving good microcleanliness and mechanical properties such as fatigue resistance and strength. Oxygen contents obtained via ingot molding are typically less than or equal to 15 ppm.

[0050] Advantageously, the composition according to the present invention is cementable, that is to say that it can undergo a cementation treatment, and / or nitrurable, that is to say that it can undergo a nitriding treatment and even advantageously it can undergo a thermochemical treatment, in particular chosen from cementation, nitriding, carbonitriding and cementation followed by nitriding.

[0051] These treatments improve the surface hardness of steel by adding carbon and / or nitrogen. Thus, if case hardening is used, the carbon content of the steel surface increases, thereby increasing surface hardness. The surface is advantageously enriched with carbon to obtain a final carbon content (final surface carbon content) of 0.5%–1.7% by weight, more particularly 0.8%–1.5% by weight, and more advantageously at least 1% by weight. particularly of 1-1.3% by weight, even more advantageously > 1.1% by weight, and even more particularly between 1.2 and 1.5% by weight. In this document, surface carbon content is understood to have been determined using a sample of a surface layer to a depth of 200 microns.

[0052] If nitriding is used, it is the nitrogen content that increases on the surface of the steel, and therefore also the surface hardness.

[0053] If carbonitriding or carburizing followed by nitriding is used, the carbon and nitrogen content on the surface of the steel is increased, and therefore also the surface hardness.

[0054] These processes are well known to those skilled in the art.

[0055] In an advantageous embodiment, the steel composition according to the invention, after thermochemical treatment, advantageously carburizing, nitriding, carbonitriding, or carburizing followed by nitriding, and then heat treatment, exhibits a surface hardness greater than 64 HRC, in particular greater than or equal to 65 HRC, measured according to ASTM E18 published in July 2017 or an equivalent standard. It further advantageously exhibits a surface hardness greater than or equal to 800 HV (approximately 64 HRC according to ASTM E140-12b published in May 2013), advantageously greater than or equal to 825 HV, measured according to ASTM E384 published in August 2017 or an equivalent standard, in particular after solution treatment at a temperature of 1050°C.

[0056] It also advantageously exhibits a hardness at 1 mm depth greater than or equal to 750 HV (which corresponds to approximately 62HRC according to ASTM E140-12b published in May 2013), advantageously, greater than or equal to 780 HV, in particular greater than or equal to 800 HV, measured according to ASTM E384 published in August 2017 or equivalent standard, in particular after solution treatment at a temperature of 1050°C.

[0057] It also advantageously presents a level of hardness of the base material (core material hardness) between 440 and 600 HV, advantageously between 470 and 550 HV, measured according to the ASTM E384 standard published in August 2017 or equivalent standard.

[0058] In an advantageous embodiment, the steel composition according to the invention, after thermochemical treatment, advantageously carburizing, nitriding, carbonitriding, or carburizing followed by nitriding, followed by heat treatment, exhibits a K1C or Kq toughness greater than or equal to 50 MPa.m 1 / 2 advantageously greater than or equal to 55 MPa.m 1 / 2 , more advantageously greater than or equal to 60 MPa.m 1 / 2 , measured according to ASTM E399-23 published in July 2023 or equivalent standard.

[0059] In an advantageous embodiment, the steel composition according to the invention has, after thermochemical treatment, advantageously carburizing or nitriding or carbonitriding or carburizing then nitriding, followed by heat treatment, a core tensile strength Rm of between 1000 and 1600 MPa, advantageously between 1380 and 1500 MPa, measured according to the standard NFEN ISO 6892-1 published in December 2019 or equivalent standard.

[0060] The steel composition obtained through these treatments advantageously has a surface carbon concentration (final surface content) of 0.9 - 1.50% by weight.

[0061] This heat treatment may include: - (1) a solution treatment of the steel at a temperature between 1025°C and 1100°C, advantageously between 1030°C and 1050°C, more particularly 1050°C, - (2) advantageously followed by maintaining 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 dissolution of the carbides initially present), - (3) then possibly an initial cooling (quenching), in particular under a neutral gas at, for example, a pressure of 2 bars (2x10 5 Pa), advantageously down to room temperature, (this phase allows obtaining a predominantly martensitic microstructure with residual austenite. This residual austenite is a function of the temperature reached during cooling: the content decreases with the temperature reached during cooling), - (4) possibly followed by maintenance at room temperature, - (5) then advantageously a second cooling to a temperature less than -40°C, more advantageously less than -60°C, even more advantageously around -70°C, especially for 2 hours (this phase allows the residual austenite content to decrease), - (6) and advantageously one or more tempers, more advantageously at least three tempers, advantageously at a temperature greater than or equal to 475°C, more advantageously between 475°C and 550°C, in particular 500 or 525°C, even more particularly for 1 hour each (this or these tempers allow the precipitation of carbides and the partial or total decomposition of the residual austenite. This allows obtaining ductility properties), the temper(s) being possibly followed and / or preceded by one or more coolings, advantageously at a temperature less than -40°C, more advantageously less than -60°C, even more advantageously about -70°C, in particular for 2 hours.

[0062] The advantage of the steel according to the invention is therefore to obtain high levels of hardness with limited heat treatment, that is to say with a lower solution temperature than in the prior art (temperature between 1025°C-1100°C, advantageously between 1030°C-1050°C, more particularly 1050°C) while maintaining good toughness.

[0063] In a particularly advantageous embodiment, the steel composition according to the invention has, after thermochemical treatment, advantageously of carburizing or nitriding or carbonitriding or carburizing then nitriding, followed by heat treatment, a martensitic or bainito-martensitic structure having a residual austenite content of less than 10% by weight, more advantageously less than 0.5% by weight, and free from ferrite and pearlite, phases known to decrease the surface hardness of steel.

[0064] The said heat treatment can be as described above.

[0065] The present invention further relates to a method for manufacturing a steel blank having the composition according to the invention, characterized in that it comprises: a) a steel production step; b) a steel transformation step; c) a thermochemical treatment; d) and a heat treatment.

[0066] Advantageously the heat treatment of step d) of the process according to the present invention is as described above.

[0067] Advantageously, the thermochemical treatment of step c) of the process according to the present invention consists of a cementation or nitriding or carbonitriding or cementation followed by nitriding treatment, advantageously it is a cementation treatment, more particularly allowing an enrichment in surface carbon resulting in a final surface carbon content of at least 1% by weight, even more advantageously > 1.1% by weight.

[0068] In particular, step b) of the process according to the present invention consists of a rolling, forging and / or spinning step, advantageously forging. These processes are well known to those skilled in the art.

[0069] In an advantageous embodiment, step a) of the development of the process according to the present invention is carried out by a conventional arc furnace development process with refining and remelting under conductive slag (ESR), or by a special vacuum development VIM process, optionally with a conductive slag (ESR) and / or vacuum (VAR) remelting step, or by Powder Metallurgy such as gas atomization and hot isostatic pressing (CIC), advantageously by a special development VIM-VAR process and vacuum remelting.

[0070] Thus, the steel according to the present invention can be produced by a VIM-VAR process. This process provides very good inclusion cleanliness and improves the chemical homogeneity of the ingot. It is also possible to use an electroslag remelting (ESR) route or to combine ESR and VAR (vacuum remelting) operations.

[0071] This steel can also be obtained through powder metallurgy. This process produces high-purity metal powder by atomization, preferably gas atomization to achieve low oxygen levels. The powder is then compressed, for example, by hot isostatic pressing (HIP).

[0072] These processes are well known to those skilled in the art.

[0073] The present invention also relates to a steel blank that can be obtained by the process according to the invention. This blank is made from steel having the composition according to the present invention and as described above.

[0074] It also 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 component or an injection system, advantageously of a power or motion transmission component, more advantageously of a transmission element, advantageously of a transmission element such as a gear, a transmission shaft and / or a bearing and therefore in particular of a bearing.

[0075] It thus relates to a mechanical organ, advantageously for transmitting power or motion, more advantageously a transmission element, in particular a gear, a transmission shaft or a bearing, more particularly a bearing or a gear, 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.

[0076] Finally, it relates to a steel injection system having the composition according to the invention or obtained from a steel blank according to the invention.

[0077] Indeed, with the steel composition according to the invention, it is possible to combine high surface hardness and surface wear resistance after thermochemical treatment with a core part of the material having both high fatigue resistance, high mechanical strength and good toughness.

[0078] These steels are therefore suitable for demanding applications such as bearings and gears for the aerospace industry, particularly aircraft engines, and injection systems. Manufacturing these components requires a process that delivers high-level metallurgical quality, combining excellent surface properties and good core toughness, at least equivalent to the M50NiL reference grade.

[0079] The invention will be better understood by reading the following examples, which are given by way of non-limiting illustration, and by reading the description of the figures that follow.

[0080] In the examples, unless otherwise stated, all percentages are expressed by weight, temperature is expressed in degrees Celsius, and pressure is atmospheric pressure. Brief description of the drawings

[0081] Metal blanks of various shapes, rectangular (20 x 25 mm) or round (18 mm diameter), were taken from the forged bars lengthwise. Case hardening treatments were carried out on an industrial low-pressure case hardening plant. Heat treatments were performed under vacuum to avoid altering the surface of the products. Hardness curves (HV0.3) as a function of distance from the surface were established for each grade under two solution heating conditions: 1050°C and 1100°C before quenching. Gas quenching was followed by a treatment at -75°C for two hours. The triple tempering temperature of 525°C was identical for all samples.

[0082] [Fig. 1] Figure 1 represents the hardness profile (HV) as a function of depth (distance to surface) in pm of comparative example 2 after carburizing under the conditions of the examples for a solution temperature of 1050°C or 1100°C.

[0083] [Fig. 2] Figure 2 represents the hardness profile (HV) as a function of depth (distance to surface) in pm of grade A according to the invention after carburizing under the conditions of the examples for a solution temperature of 1050°C or 1100°C.

[0084] [Fig. 3] Figure 3 represents the hardness profile (HV) as a function of depth (distance to surface) in pm of grade B according to the invention after carburizing under the conditions of the examples for a solution temperature of 1050°C or 1100°C.

[0085] [Fig. 4] Figure 4 represents the hardness profile (HV) as a function of depth (distance from the surface) in pm of grade C according to the invention after cementation under the conditions of the examples for a solution temperature of 1050°C or 1100°C.

[0086] [Fig. 5] Figure 5 represents the hardness profile (HV) as a function of depth (distance to surface) in pm of grade D according to the invention after carburizing under the conditions of the examples for a solution temperature of 1050°C or 1100°C. EXAMPLES

[0087] Table 1 shows the composition of seven grades obtained in two different production furnaces. Grades A and E according to the invention and comparative examples 1 and 2 correspond to castings of 150 kg each, and grades B to D according to the invention correspond to castings of approximately 6 to 9 kg each. These castings were produced using the VIM process. The 150 kg ingots were remelted using the VAR process.

[0088] [Table 1]

[0089] Comparative example 1 corresponds to the reference grade C65NIL, i.e. to an example according to application WO2017 / 216500 containing more vanadium, more carbon and less cobalt than the compositions according to the invention, and comparative example 2 to an example according to application WO2017 / 216500 containing more vanadium than the compositions according to the invention.

[0090] The 150 kg ingots produced using VIM followed by VAR remelting were transformed into bars with a diameter of less than 90 mm by a hot forging process under a 2000 T or 2500 T press. The smaller ingots (6 to 9 kg) produced using VIM were transformed by drop forging into bars with a maximum dimension of 30 mm (round or square).

[0091] Square or cylindrical samples from wrought products were case-hardened and then heat-treated with (1) solution heating at 1100°C or 1050°C, (2) holding at this temperature for 30 min, (3) cooling under a neutral gas at a pressure between 2 and 6 bar (2 x 10 5 and 6X10 5 (Pa), (4) a period at room temperature, (5) a cooling to -75°C for 2 hours, and (6) 3 returns to a temperature of 525°C for 1 hour each.

[0092] For grade E, square or cylindrical type samples from wrought products were case hardened and then heat treated with (1) solution heating at 1050°C, (2) holding for 30 min at this temperature, (3) oil quenching, (4) cooling to -75°C for 2 hours, and (5) 3 temperings at a temperature of 525°C for 1 hour each.

[0093] The surface hardness profiles HV0.3 in HV obtained after carburizing and heat treatment of quality according to the invention were measured according to the ASTM E384 standard published in August 2017 for grades A, Grades B, C, and D, and comparative example 2, are shown in Figures 1 to 5 as a function of the solution temperature (1050°C or 1100°C). Grades A to D achieve a minimum surface hardness of 800 HV0.3 (between 850 and 900 HV). The reduced vanadium content improves carbon diffusion. The depth required to obtain a surface hardness greater than 800 HV is approximately 1.4 mm minimum for the grades according to the invention and a maximum of 1.2 mm for the grades in comparative example 2.

[0094] Mechanical tests were performed in two stages on comparative examples 1 and 2 and on grade A: tests with a solution temperature of 1100°C and tests with a solution temperature of 1050°C. The tensile test data—Rm, Rp, A%, and Z%—were measured according to NF EN ISO 6892-1, published in December 2019, under test condition B, and the KV at room temperature was measured according to ISO 148-1, published in March 2017. The core hardness (HRC) was measured according to ASTM E384, published in August 2017. K1C or Kq results were obtained on two types of specimens, CT10 and CT15, according to ASTM E399, depending on the solution temperature. The results are presented in Tables 2 and 3 below and compared with the M50NIL reference:

[0095] [Table 2]

[0096] * : Kq results

[0097] [Table 3] Solution temperature of 1050°C

[0098] Grade A according to the invention therefore exhibits a K1C or Kq value higher than that of the comparative examples for a similar core hardness. The K1C or Kq level is thus higher than that of the reference grade M50NIL. Grade A therefore offers a better compromise between surface hardness after case hardening and toughness properties than the reference M50NIL.

[0099] For grade E, we obtain a good result in K1 C with values ​​greater than 50 MPa.m 1 / 2 , for a mechanical strength level of 1566 MPa. The high Rm level of this grade reflects a high surface hardness, typically greater than 850 HV.

Claims

Demands

1. Steel composition comprising, in weight percentages of the total composition: Carbon: 0.09-0.17, preferably 0.09 - 0.14; Chrome: 3.00-6.00, preferably 3.50-6.00; Molybdenum: 4.00-6.00, preferably 4.90-5.20; Tungsten: <1.00, preferably <0.01; Vanadium: < 0.30, preferably <0.25; Nickel: 2.00-4.00, preferably 3.00-3.80; Cobalt: 6.00-10.00, preferably 7.00-8.00; Iron: balance, along with unavoidable impurities, optionally including in addition one or more of the following: Niobium: < 0.90, advantageously < 0.01; the combined content of Niobium + Vanadium being < 0.9, advantageously <0.86; Nitrogen: < 0.50, preferably < 0.20; Silicon: < 0.70, preferably < 0.200; Manganese: < 0.70, preferably < 0.200; Aluminium: <0.15, preferably <0.

10.

2. Steel composition according to claim 1, characterized in that it comprises at most 1% by weight of unavoidable impurities, advantageously at most 0.5% by weight.

3. Steel composition according to any one of claims 1 or 2, characterized in that the unavoidable impurities are selected from Titanium, Sulfur, Phosphorus, Copper, Tin, Lead, Oxygen and mixtures thereof.

4. Steel composition according to any one of claims 1 to 3, characterized in that it is case-hardenable and / or nitridable.

5. A steel composition according to any one of claims 1 to 4, characterized in that, after thermochemical treatment, advantageously carburizing or nitriding or carbonitriding or carburizing followed by nitriding, and then heat treatment, it has a surface hardness greater than or equal to 64 HRC, advantageously greater than or equal to 65 HRC, and a K1C or Kq toughness greater than or equal to 50 MPa·m 1 / 2 advantageously greater than or equal to 60 MPa.m 1 / 2 .

6. Steel composition according to any one of claims 1 to 5, characterized in that it has, after thermochemical treatment, advantageously carburizing or nitriding or carbonitriding or carburizing then nitriding, followed by heat treatment, a core tensile strength Rm of between 1000 and 1600 MPa, advantageously between 1380 and 1500 MPa.

7. Steel composition according to any one of claims 1 to 6, characterized in that it exhibits, after thermochemical treatment, advantageously carburizing or nitriding or carbonitriding or carburizing followed by nitriding, followed by heat treatment, a martensitic or bainito-martensitic structure having a residual austenite content of less than 10% and free from ferrite and pearlite.

8. Steel composition according to any one of claims 5 to 7, characterized in that the heat treatment comprises solution treatment at a temperature between 1025°C-1100°C, advantageously between 1030 and 1050°C, followed by quenching with optional cooling, advantageously at a temperature below -40°C, and several temperings, advantageously at least three temperings, at a temperature between 475°C and 550°C, in particular 500°C, the tempering(s) being optionally followed and / or preceded by one or more coolings, advantageously at a temperature below -40°C.

9. A method for manufacturing a steel blank having the composition according to any one of claims 1 to 8, characterized in that it comprises: a) a steelmaking step; b) a steel processing step; c) a thermochemical treatment; d) and a heat treatment.

10. A manufacturing process according to claim 9, characterized in that step c) consists of a carburizing or nitriding or carbonitriding treatment or carburizing followed by nitriding, advantageously it is a carburizing treatment

11. A manufacturing process according to any one of claims 9 or 10, characterized in that step c) consists of a carburizing treatment enabling surface carbon enrichment resulting in a final surface carbon content of at least 1% by weight, even more advantageously > 1.1% by weight.

12. A manufacturing process according to any one of claims 9 to 11, characterized in that step d) comprises a solution treatment at a temperature between 1025°C-1100°C, advantageously between 1030°C-1050°C, followed by holding at this temperature until complete austenitization with optionally cooling to a temperature below -40°C, advantageously -70°C, and several temperings, advantageously at least three temperings, at a temperature between 475°C and 550°C, in particular 500°C, the tempering(s) being optionally followed and / or preceded by one or more coolings, advantageously below -40°C.

13. A manufacturing process according to any one of claims 9 to 12, characterized in that step b) consists of a rolling, forging and / or spinning step.

14. A manufacturing process according to any one of claims 9 to 13, characterized in that step a) of processing is carried out by a conventional arc furnace processing method refining and remelting under conductive slag (ESR), or by a special vacuum VIM process, possibly with a conductive slag (ESR) and / or vacuum (VAR) remelting step, or by Powder Metallurgy such as including gas atomization and hot isostatic compaction (HIC), advantageously by a special VIM-VAR process and vacuum remelting.

15. Steel blank capable of being obtained by a process according to any one of claims 9 to 14.

16. Use of a blank according to claim 15 or of a steel composition according to any one of claims 1 to 8 for the manufacture of a mechanical component or of an injection system, advantageously of a bearing.

17. Mechanical component, advantageously for transmitting power or motion, more advantageously a transmission element, in particular a bearing or 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 15.

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