EP3765646B1 - Steel composition - Google Patents

Description

The present invention relates to a new steel of the 10CrMoNiVCo type with a low carbon content and a 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 can also be used for other applications requiring high surface hardness combined with good core toughness, for example in the case of injection systems.

Bearings are mechanical components that ensure relative and constrained movements in orientation and direction between two parts. Bearings include 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 different elements have good properties in terms of rolling fatigue, wear, etc.,...

Gears are mechanical power transmission devices. To ensure favorable power density (ratio of power transmitted by the size of the gears) and operating reliability, the gears must have good structural fatigue (tooth root) and contact fatigue (tooth blank) properties. The conventional techniques for producing these metallic components have recourse to electric steelmaking processes followed by possible remelting operations, or single or multiple vacuum remeltings. The ingots thus produced are then shaped by hot transformation processes such as rolling or forging in the form of bars, tubes or rings.

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 chemical elements such as carbon and/or nitrogen. This generally superficial enrichment then makes it possible to obtain high mechanical properties after heat treatment to depths of a few millimeters maximum. These steels generally have better ductility properties than Type 1 steels.

There are also thermochemical processes applied to steels of the 1st type aimed at enriching the surface with nitrogen to obtain very high mechanical properties.

The first of the properties required in the field of bearings or gears is obtaining a very high level of hardness. These type 1 and type 2 steels generally have surface hardness levels above 58 HRC. The most common grades known as 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 any thermochemical treatment and appropriate heat treatment, a surface hardness of 63 HRC. Obtaining hardnesses above 64 HRC is now required to significantly improve the properties of the component.

• Carbone : 0,2-2,0;
• Chrome : 1,0-9,0;
• Molybdène : 1,0-9,0;
• Silicium : 0,1-1,0;
• Tungstène : 1,0-3,0;
• Vanadium : 0,1-1,0;
• Nickel+ Cobalt + Cuivre : 3,0-15,0;
• Fer : solde

Requirement GB2370281 describes a valve seat steel by powder metallurgy technology compacted from powder mixtures of an iron base and harder particles. The matrix, which constitutes only part of the steel, has the following composition, in percentages by weight of the total composition:

• Carbon: 0.2-2.0;
• Chromium: 1.0-9.0;
• 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;
• Iron: balance

However, this matrix comprises from 5 to 40% by volume of pearlite, which results in a lack of ductility of this matrix and therefore embrittlement. In addition, the material also contains porosity (up to 10%) which does not allow good mechanical strength and fatigue properties to be achieved. Finally, this document does not suggest using a low copper content and on the contrary indicates that its content can be up to 15% by weight. However, a high copper content is not sought for the applications of the present invention because copper is a known embrittling agent, the content of which should not exceed 0.5% by weight relative to the total weight of the steel composition. .

• Carbone : 0,05-0,5;
• Chrome : 2,5-5,0;
• Molybdène : 4-6;
• Tungstène : 2-4,5;
• Vanadium : 1-3;
• Nickel : 2-4;
• Cobalt : 2-8;
• Fer : solde

• ainsi que les impuretés inévitables, optionnellement comprenant en outre, un ou plusieurs des éléments suivants :
  • Niobium : 0-2 ;
  • Azote : 0-0,5;
  • Silicium : 0-0,7;
  • Manganèse : 0-0,7;
  • Aluminium : 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 HRC maximum. Toutefois cette demande indique que la teneur en Co doit être limitée à au plus 8% et même il est préférable qu'elle soit au plus de 7% et même encore plus préféré d'au plus 6% car le Co augmente le niveau de dureté du matériau de base qui entraîne une décroissance de la ténacité. La nuance MIX5 qui est préférée a ainsi une teneur en Co de 5,14%.

The patent application WO2015/082342 describes a bearing steel having the following composition, in percentages by weight of the total composition:

• Carbon: 0.05-0.5;
• Chromium: 2.5-5.0;
• Molybdenum: 4-6;
• Tungsten: 2-4.5;
• Vanadium: 1-3;
• Nickel: 2-4;
• Cobalt: 2-8;
• Iron: balance

• as well as unavoidable impurities, optionally further comprising 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 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 achieve a surface hardness after solution treatment at 1150°C and tempered at 560°C to a maximum level of hardness of approximately 800 HV, ie an equivalent of 64 HRC maximum. However, this application indicates that the Co content must be limited to at most 8% and even it is preferable that it be at most 7% and even more preferred by at most 6% because Co increases the level of hardness. of the base material which leads to a decrease in toughness. The preferred grade MIX5 thus has a Co content of 5.14%.

• Carbone : 0,05-0,40, de préférence 0,10-0,30 ;
• Chrome : 2,50-5,00, de préférence 3,0-4,5;
• Molybdène : 4,0-6,0;
• Tungstène : 0,01 - 1,8, de préférence 0,02-1,5 ;
• Vanadium : 1,0-3,0, de préférence 1,5-2,5;
• Nickel : 2,0-4,0;
• Cobalt : 2,0-8,0, de préférence 3,0-7,0;
• Fer : solde

• ainsi que les impuretés inévitables,
• optionnellement comprenant en outre, un ou plusieurs des éléments suivants :
  • Niobium : ≤ 2,0;
  • Azote : ≤ 0,50, de préférence ≤ 0,20 ;
  • Silicium : ≤ 0,70, de préférence 0,05-0,50;
  • Manganèse : ≤ 0,70, de préférence 0,05-0,50;
  • Aluminium : ≤0,15, de préférence ≤ 0,10 ;
• la teneur combinée en Niobium + Vanadium étant comprise dans la gamme 1,00-3,50 ;
• et la teneur en Carbone + Azote étant comprise dans la gamme 0,05-0,50. En particulier dans les exemples, la nuance C de 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) est la préférée car présentant la plus grande dureté superficielle. Cette nuance permet d'atteindre une dureté superficielle après traitement de mise en solution à 1100°C-1150°C et revenu à 500°C à un niveau maximal de dureté d'environ 66-67 HRC qui est bien supérieur à la dureté superficielle obtenue avec une nuance selon la demande WO2015/082342 (nuance A : figure 1). Toutefois cette demande indique également que la teneur en Co doit être limitée à au plus 8% et même il est préférable qu'elle soit au plus de 7% et même encore plus préféré d'au plus 6% car il augmente le niveau de dureté du matériau de base qui entraîne une décroissance de la ténacité. La nuance C qui est préférée a ainsi une teneur en Co de 5,00-5,40%.

The patent application WO2017216500 describes a bearing steel having the following composition, in percentages by weight of the total composition:

• Carbon: 0.05-0.40, preferably 0.10-0.30;
• Chromium: 2.50-5.00, preferably 3.0-4.5;
• Molybdenum: 4.0-6.0;
• Tungsten: 0.01-1.8, preferably 0.02-1.5;
• Vanadium: 1.0-3.0, preferably 1.5-2.5;
• Nickel: 2.0-4.0;
• Cobalt: 2.0-8.0, preferably 3.0-7.0;
• Iron: balance

• as well as the inevitable impurities,
• optionally further comprising 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;
  • Aluminum: ≤0.15, preferably ≤0.10;
• the combined content of Niobium + Vanadium being in the range 1.00-3.50;
• and the Carbon + Nitrogen content being in the range 0.05-0.50. In particular in the examples, composition grade C (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 the preferred because it has the greatest surface hardness. This grade makes it possible to reach a surface hardness after solution treatment at 1100°C-1150°C and tempered at 500°C to a maximum level of hardness of around 66-67 HRC which is much higher than the surface hardness obtained with a shade according to demand WO2015/082342 (shade A: figure 1). However, this application also indicates that the Co content must be limited to at most 8% and even it is preferable that it be at most 7% and even more preferably at most 6% because it increases the level of hardness of the base material which leads to a decrease in toughness. Grade C which is preferred thus has a Co content of 5.00-5.40%.

Obtaining surface hardnesses greater than 67 HRC, in particular using a solution heat treatment at a temperature less than or equal to 1160° C., is therefore difficult to obtain even though they would make it possible to improve significantly the properties of the component. The inventors realized surprisingly that by increasing the cobalt content of the steel described in the applications WO2015/082342 And WO2017216500 at a content of 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 presented, after thermochemical treatment, in particular cementation and / or nitriding, a very high surface hardness and even greater than 67 HRC, in particular greater than or equal to 68 HRC and a hardness at 1mm greater than 860 HV (which corresponds to approximately 66HRC according to the standard ASTME140-12b published in May 2013) after a solution heat treatment at a temperature in the range 1100°C - 1160°C and returned to a temperature greater than or equal to 475°C, while presenting a level of hardness of the base material between 400 and 650 HV.

This was not at all obvious in view of these documents which encouraged the use of a low cobalt content such as in the MIX5 grade (5.14% cobalt) and in the C grade (5.00-5.40 % cobalt) which are considered to be the compositions with the best hardness.

The patent US8157931 describes a steel of the Ni-Co type having a cobalt content of between 9.9 and 10% and a carbon content of between 0.1 and 0.12% and having a high surface hardness of the order from 68-69 HRC. However, such a steel has a high chromium content (5.3-5.4%), a low vanadium (0.20-0.21%) and molybdenum (2.5-2.52%) content and does not contain tungsten. This shade balancing leads after thermochemical treatment and associated quality treatment (including quenching at 1110°C and tempering at 482°C) to an interesting surface hardness but which decreases very rapidly with depth, it is thus from 600 µm of depth already identical to that of the base metal (figure 1). This is probably due to the lower carbon content in the cemented layer that the grade can withstand in order to avoid any risk of the formation of an embrittling graphite phase. Claim 1 of this patent thus stipulates a carbon content in the cemented layer limited to approximately 0.8%. In fact, graphite could appear from 1% by weight of C in the cemented layer (surface layer obtained after cementation).

It is therefore not easy to find the right balance of the grade (including Cr, Mo, V, W, C) in view of this document to arrive at an optimization of both the surface hardness, the hardness profile ( depth) and tenacity (of which we have an idea by the hardness at heart). In addition, it was not easy in view of this document to achieve a deep cementation layer that would allow the introduction of much more carbon than the grades of the state of the art (up to 1.5 % by weight of C) while limiting the risk of graphite appearing.

• Carbone : maxi 0,05;
• Chrome : 2,5-5,5;
• Tungstène équivalent (2xMo+W): 12,5-20;
• Vanadium : maxi 1,5;
• Nickel : maxi 5,0;
• Cobalt : maxi 20,0;
• Silicium : 0,15 - 1,0
• Manganèse : 0,15-1,5
• Fer : solde

The patent application JPH11-210767 describes a family of steel for aeronautical bearing application with an improved service life having the following composition, in percentages by weight of the total composition:

• Carbon: max 0.05;
• Chromium: 2.5-5.5;
• Tungsten equivalent (2xMo+W): 12.5-20;
• Vanadium: max 1.5;
• Nickel: max 5.0;
• Cobalt: max 20.0;
• Silicon: 0.15 - 1.0
• Manganese: 0.15-1.5
• Iron: balance

This grade is subjected to case hardening or carbonitriding. However, this application only describes the surface hardness properties of 66-69 HRC and only describes the toughness qualitatively. Balancing this very low carbon grade, ≤ 0.05% by weight, requires limiting the vanadium content to ≤ 1.5% by weight so as not to degrade the toughness, yet vanadium is an interesting element to improve wear resistance.

This application also does not describe the core hardness (translating the mechanical resistance) of this grade, and given the very low level of carbon it is expected that this degrades the mechanical resistance.

Also, this application does not describe any deep layer carburizing profile. However, it would be interesting to have a high hardness throughout the depth up to 400 microns from the surface which corresponds to the so-called Hertz zone, a zone subjected to very high shear stresses. A high hardness throughout this depth also allows more tolerance when it comes to removing material for repair or rectification during machining, and it is all the more useful for the transmission application. of power which is not mentioned in JPH11-210767 .

The inventors noticed that it was possible to obtain a balance different from that proposed by JPH11-210767 with a higher carbon content, at least 0.06% by weight, and a range of cobalt between 9.0 and 12.5% by weight which allow (a) to obtain a good compromise between core hardness and toughness , in other words a good compromise between mechanical resistance and toughness, and (b) to admit more vanadium in its composition without degrading the toughness, which is favorable for the wear resistance.

The present invention therefore relates to a steel composition, as defined in the claims.

• Carbone : 0,06-0,20 de préférence 0,08-0,18;
• Chrome : 2,5-5,0, de préférence 3,0-4,5;
• Molybdène : 4,0-6,0;
• Tungstène : 0,01-3,0;
• Vanadium : 1,0-3,0, de préférence 1,50-2,50;
• Nickel : 2,0-4,0;
• Cobalt : 9,0-12,5, de préférence 9,5-11,0;
• Fer : solde

• ainsi que les impuretés inévitables en une teneur d'au plus 1%,
• optionnellement comprenant en outre, un ou plusieurs des éléments suivants :
  • Niobium : ≤ 2,0;
  • Azote : ≤ 0,50, de préférence ≤ 0,20;
  • Silicium : ≤ 0,70, de préférence 0,05-0,50;
  • Manganèse : ≤ 0,70, de préférence 0,05-0,50;
  • Aluminium : ≤0,15, de préférence ≤ 0,10;
• la teneur combinée en Niobium + Vanadium étant comprise dans la gamme 1,0-3,5;
• et la teneur en Carbone + Azote étant comprise dans la gamme 0,06-0,50. Une composition particulièrement intéressante comprend, ou est constituée essentiellement de, ou est constituée de, en pourcentages en poids de la composition totale:
  • Carbone : 0,06-0,20, de préférence 0,08-0,18;
  • Chrome : 3,0-4,5, de préférence 3,5-4,5;
  • Molybdène : 4,0-6,0, de préférence 4,5-5,5;
  • Tungstène 0,01 - 3,0;
  • Vanadium : 1,5-2,5, de préférence 2,0-2,3;
  • Nickel : 2,0-4,0, de préférence 2,5-3,5;
  • Cobalt : 9,5-12,5, de préférence 9,5-10,5;
  • Fer : solde
• ainsi que les impuretés inévitables en une teneur d'au plus 1%,
• optionnellement comprenant en outre, un ou plusieurs des éléments suivants :
  • Niobium : ≤ 2,0;
  • Azote : ≤ 0,20;
  • Silicium : ≤ 0,70, de préférence 0,05-0,50;
  • Manganèse : ≤ 0,70, de préférence 0,05-0,50;
  • Aluminium : ≤0,10;
• la teneur combinée en Niobium + Vanadium étant comprise dans la gamme 1,00-3,50 ;
• et la teneur en Carbone + Azote étant comprise dans la gamme 0,06-0,50.

Advantageously, the composition is cementable and/or nitridable, more advantageously cementable, comprising, or consisting essentially of, or consisting of, in percentages by weight of the total composition:

• Carbon: 0.06-0.20 preferably 0.08-0.18;
• Chromium: 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 1.50-2.50;
• Nickel: 2.0-4.0;
• Cobalt: 9.0-12.5, preferably 9.5-11.0;
• Iron: balance

• as well as the unavoidable impurities in a content of at most 1%,
• optionally further comprising 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;
  • Aluminum: ≤0.15, preferably ≤0.10;
• the combined content of Niobium + Vanadium being in the range 1.0-3.5;
• and the Carbon + Nitrogen content being in the range 0.06-0.50. A composition of particular interest comprises, or consists essentially of, or consists of, in percentages by weight of the total composition:
  • Carbon: 0.06-0.20, preferably 0.08-0.18;
  • Chromium: 3.0-4.5, preferably 3.5-4.5;
  • Molybdenum: 4.0-6.0, preferably 4.5-5.5;
  • Tungsten 0.01 - 3.0;
  • Vanadium: 1.5-2.5, preferably 2.0-2.3;
  • Nickel: 2.0-4.0, preferably 2.5-3.5;
  • Cobalt: 9.5-12.5, preferably 9.5-10.5;
  • Iron: balance
• as well as the unavoidable impurities in a content of at most 1%,
• optionally further comprising one or more of the following:
  • Niobium: ≤ 2.0;
  • Nitrogen: ≤ 0.20;
  • Silicon: ≤ 0.70, preferably 0.05-0.50;
  • Manganese: ≤ 0.70, preferably 0.05-0.50;
  • Aluminum: ≤0.10;
• the combined content of Niobium + Vanadium being in the range 1.00-3.50;
• and the Carbon + Nitrogen content being in the range 0.06-0.50.

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 mainly due to the manufacturing process and the quality of the charging. The composition according to the invention comprises at most 1% by weight of unavoidable impurities, advantageously at most 0.75% by weight, even 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 the ferrite, so-called alphagenic elements, 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 microstructure free of ferrite which would weaken the component, it is necessary to add austenite stabilizing elements, so-called gammagenic elements. A correct combination of austenite stabilizing elements (Carbon, Nickel, Cobalt and Manganese) and ferrite stabilizing elements (Molybdenum, Tungsten, Chromium, Vanadium and Silicon) makes it possible to obtain a steel composition according to the invention having superior properties, particularly after thermochemical treatment such as carburizing.

The steel composition according to the invention therefore comprises carbon (C) in a content comprised 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. Indeed 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 heat and to wear. The presence of a small amount of carbon in a steel is beneficial for avoiding the formation of undesirable and brittle intermetallic particles and for forming small amounts of carbides to prevent excessive grain size growth during solution treatment prior to quenching operation. The initial carbon content should 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, in general, increasing the carbon content makes it possible to significantly increase the level of hardness, which is generally penalizing with respect to the ductility properties. It is for this reason that the carbon content is limited to a maximum of 0.20% to obtain a level of core hardness of the material of a maximum of 650 HV. During carburizing, the carbon is implanted in the surface layers of the component, so as to obtain a gradient of hardness. Carbon is the main 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 part of the material 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 further comprises Chromium (Cr) in a content comprised 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% by weight relative to the total weight of the composition.

Chromium contributes to the formation of carbides in steel and is one of the main elements which controls the hardenability of steels.

However, Chromium can also promote the appearance of ferrite and residual austenite. The chromium content of the steel composition according to the invention must therefore not be too high.

The steel composition according to the invention also comprises Molybdenum (Mo) in a content comprised 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.

Molybdenum improves the resistance to tempering, the wear resistance and the hardness of the steel. However, molybdenum has a strong stabilizing effect on the ferrite phase and should therefore not be present in too large a quantity in the steel composition according to the invention.

The steel composition according to the invention further comprises Tungsten (W) in a content comprised 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.

Tungsten is a ferrite stabilizer and a strong carbide-forming element. It improves the resistance to heat treatment and wear and the hardness by formation of carbides. However, it can also lower the surface hardness of steel and especially the properties of ductility and toughness. For this element to fully play its role, it is necessary to carry out high-temperature solution treatments.

The steel composition according to the invention further comprises Vanadium (V) in a content comprised 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.

Vanadium stabilizes the ferrite phase and has a strong affinity with carbon and nitrogen. Vanadium provides resistance to wear and tempering by forming hard vanadium carbides. Vanadium can be partly substituted by niobium (Nb), which has similar properties.

The combined content of Niobium + Vanadium must therefore be in the range 1.0-3.5% by weight relative to the total weight of the composition, advantageously in the range 1.7-3.5% by weight relative to the total weight of the composition.

If Niobium is present, its content must be ≤ 2.0% by weight relative to the total weight of the composition. Advantageously, the steel composition according to the invention does not include niobium.

The steel composition according to the invention also comprises Nickel (Ni) in a content comprised 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 decrease the Ms temperature, that is to say the temperature at which the transformation of austenite into martensite begins during cooling. This can prevent the formation of martensite. The quantity 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 further comprises Cobalt (Co) in a content comprised in the range 9.0-12.5%, preferably 9.5-12.5%, advantageously 9.5-11 0.0%, more preferably 9.5-10.5%, by weight relative to the total weight of the composition. The Cobalt content is measured according to the ASTM-E1097-12 standards published in June 2017 and ASTM E1479_16 published in December 2016. The error in measuring the Cobalt content of the steel according to the invention is thus ±2, 5% relative approximately and evaluated according to ISO5724-1 (December 1994), ISO5725-2 (December 1994), ISO5725-3 (December 1994), ISO5725-4 (December 1994), ISO5725-5 (December 1994), ISO5725 -6 (December 1994) and the NF ISO/CEI Guide 98-3 standard of July 11, 2014.

Cobalt is a strong stabilizing element of austenite which prevents the formation of undesirable ferrite. Unlike Nickel, Cobalt increases the Ms temperature, which in turn decreases the amount of residual austenite. Cobalt, in association 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, heat resistance and wear resistance. Cobalt has a small hardness increasing effect on steel. However, this increase in hardness is correlated with the decrease in toughness. The steel composition according to the invention must therefore not contain too large a quantity of cobalt. The addition of Co makes it possible to limit the C content while avoiding the promotion of ferrite for a composition according to the invention (containing the Cr, Mo, V, Ni and W contents as described above). This limitation in carbon makes it possible to compensate for the increase in hardness linked to the addition of Co.

The steel composition according to the invention may also 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 comprised 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 strongly stabilizes ferrite, but is often present during the steelmaking process during the deoxidation of liquid steel. Low oxygen contents are indeed also important to obtain low levels of non-metallic inclusions and good mechanical properties such as fatigue strength and mechanical strength.

The steel composition according to the invention may also 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 comprised 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 Ms temperature in the steel composition. Manganese is generally added to steels during their manufacture because of its affinity for sulfur, so manganese sulphide is formed 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 more effective deoxidation than each of these elements alone.

Optionally, 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 the formation of austenite and lowers the transformation of austenite into martensite. Nitrogen can to a certain extent replace carbon in the steel according to the invention to form nitrides. However, the carbon+nitrogen content must be in the range 0.06-0.50% by weight relative to the total weight of the composition.

Optionally, 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) can in fact 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 steels produced using the VIM-VAR process than in steels obtained by powder technology. Aluminum generates difficulties during atomization by obstruction of the casting nozzle by oxides.

A low oxygen content is important to obtain good micro-cleanliness as well as good mechanical properties such as fatigue resistance and mechanical strength. The oxygen contents obtained by the ingot route are 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 nitriding, that is to say it can undergo a nitriding treatment and even advantageously it can undergo a thermochemical treatment, in particular chosen from carburizing, nitriding, carbonitriding and carburizing followed by nitriding.

These treatments make it possible to improve the surface hardness of the steel, by adding carbon and/or nitrogen elements. Thus, if carburizing is used, the carbon content of the steel surface increases and therefore leads to an increase in surface hardness. The surface (surface layer advantageously having a thickness of 100 microns) is thus advantageously enriched with carbon to obtain a final carbon content (final surface carbon content) of 0.5% - 1.7% by weight, more particularly of 0 .8% - 1.5% by weight, more preferably at least 1% by weight, in particular 1-1.3% by weight, even more preferably > 1.1% by weight, even more particularly between 1 .2 and 1.5% by weight. In the remainder of this document, the superficial carbon content will be understood to have been determined using sampling of a superficial layer to a depth of 100 microns.

If nitriding is used, it is the Nitrogen content which increases on the surface of the steel, and therefore also the surface hardness.

If carbonitriding or case hardening followed by nitriding are used, it is the Carbon and Nitrogen contents at the surface of the steel which are increased and therefore also the surface hardness.

These methods are well known to those skilled in the art.

In an advantageous embodiment, the steel composition according to the invention has, after a thermochemical treatment, advantageously carburizing or nitriding or carbonitriding or carburizing then nitriding, followed by a heat treatment, a higher surface hardness at 67HRC, in particular greater than or equal to 68 HRC, measured according to the ASTM E18 standard published in July 2017 or equivalent standard. It also advantageously has a surface hardness greater than or equal to 910HV (approximately 67.25 HRC according to the ASTM E140-12b standard published in May 2013), advantageously greater than or equal to 920 HV, in particular greater than or equal to 940HV, measured according to the ASTM E384 standard published in August 2017 or equivalent standard, in particular after solution treatment at a temperature of 1100°C. It also advantageously has a surface hardness greater than or equal to 930 HV (corresponding to approximately 67.75 HRC according to the ASTM E140-12b standard published in May 2013), advantageously greater than or equal to 940 HV (corresponding to 68 HRC according to the 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 equivalent standard after solution treatment at a temperature of 1150°C.

It more advantageously has a hardness at 1 mm depth greater than or equal to 860 HV (which corresponds to approximately 66 HRC according to the ASTM E140-12b standard published in May 2013), advantageously greater than or equal to 870 HV, in particular greater than or equal to 880 HV, measured according to the ASTM E384 standard published in August 2017 or equivalent standard, in particular after solution treatment at a temperature of 1100°C. It also advantageously has a hardness at 1 mm depth greater than or equal to 880 HV, advantageously, greater than or equal to 890 HV, in particular greater than or equal to 900 HV, measured according to the ASTM E384 standard published in August 2017 or equivalent standard

It also advantageously has a level of hardness of the base material (material core hardness) of between 440 and 650 HV, advantageously between 440 and 630 HV, measured according to the ASTM E384 standard published in August 2017 or equivalent standard.

The steel composition obtained by virtue of these treatments advantageously has a surface carbon concentration (final surface content) of 1-1.3% by weight.

• (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,
• (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),
• (3) puis éventuellement un premier refroidissement (trempe), en particulier sous gaz neutre à, par exemple, une pression de 2 bars (2×10⁵ Pa), 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),
• (4) suivi éventuellement d'un maintien à la température ambiante,
• (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 -70°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 comprise entre 475°C et 530°C, en particulier de 500°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).

Said heat treatment may include:

• (1) solution treatment of the steel at a temperature between 1090°C-1160°C, advantageously between 1100°C-1160°C, more advantageously between 1100 and 1155°C, in particular between 1100 and 1150° C, more particularly 1150°C,
• (2) advantageously followed by maintenance 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 a first cooling (quenching), in particular under neutral gas at, for example, a pressure of 2 bars (2×10 ⁵ Pa), advantageously down to ambient 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) optionally followed by holding at room temperature,
• (5) then advantageously a second cooling to a temperature below -40°C, more advantageously below -60°C, even more advantageously about -70°C, in particular for 2 hours (this phase makes it possible to decrease the residual austenite content),
• (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 530°C, in particular 500°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 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 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 1150°C).

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

Said heat treatment may be as described above.

1. a) une étape d'élaboration de l'acier;
2. b) une étape de transformation de l'acier;
3. c) un traitement thermochimique;
4. d) et un traitement thermique.

The present invention further relates to a process for manufacturing a steel blank having the composition according to the invention, characterized in that it comprises:

1. a) a steelmaking step;
2. b) a steel transformation step;
3. c) a thermochemical treatment;
4. d) and heat treatment.

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

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

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

In an advantageous embodiment, step a) for producing the process according to the present invention is implemented by a conventional production process in an arc furnace with refining and remelting under conductive slag (ESR), or by a VIM or VIM-VAR process, possibly with a conductive slag remelting step (ESR) and/or under vacuum (VAR), or by Powder Metallurgy such as gas atomization and compression by hot isostatic compaction (HIP).

Thus, the steel according to the present invention can be produced by a VIM-VAR process. This process makes it possible to obtain very good inclusion cleanliness and improves the chemical homogeneity of the ingot. It is also possible to carry out a remelting process under conductive slag (ESR: Electro Slag Remelting) or to combine ESR and VAR (vacuum remelting) operations.

This steel can also be obtained by Powder Metallurgy. This process makes it possible to produce metal powder of high purity by atomization, preferably gas atomization making it possible to obtain low oxygen contents. The powder is then compressed using, for example, hot isostatic compaction (HIP).

These methods are well known to those skilled in the art.

The present invention also relates to a steel blank capable of being 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.

It also relates to the use of a blank according to the invention or of a steel composition according to the invention for the manufacture of a mechanical component or of an injection system, advantageously of a transmission such as a gear, a transmission shaft and/or a bearing and therefore in particular a bearing.

It thus relates to a mechanical component, 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. 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. Indeed, with the steel composition according to the invention, it is possible to combine the high surface hardness and the resistance to surface wear after thermochemical treatment with a core part of the material having a high resistance to fatigue and a high mechanical resistance.

These steels can therefore be used in demanding fields such as bearings for the aerospace industry or injection systems.

The invention will be better understood on reading the following examples which are given by way of non-limiting indication.

In the examples, unless otherwise indicated, all percentages are expressed by weight, the temperature is expressed in degrees Celsius and the pressure is atmospheric pressure.

^1st series of examples : Seven laboratory castings of approximately 9 kg each (6 examples according to the invention and a comparative example of composition close to that of the patent US8157931 : comparative example 1) were prepared by the VIM process according to the composition shown in Table 1 below (in % by weight relative to the total weight of the composition), the balance being Fe: <u>Table 1</u> Element VS Neither CR Mo v W Co Whether min Al NOT Example 1: GRADE A 0.18 3.1 3.9 5.1 2.1 1.18 10.0 0.2 0.18 0.023 0.005 Example 2: GRADE B 0.20 3.1 3.9 5.1 2.2 2.96 10.1 0.18 0.21 0.02 0.009 Example 3: 0.16 3.1 3.9 5.1 2.1 1.19 10.0 0.21 0.18 0.02 0.009 C-GRADE Example 4: GRADE D 0.16 3.0 4.0 5.1 2.1 2.92 10.1 0.22 0.25 0.016 0.005 Example 5: GRADE E 0.16 3.1 3.9 5.0 2.1 0.01 10.0 0.123 0.2 0.042 0.005 Example 6: GRADE F 0.17 3.1 4.0 5.2 2.2 0.01 12.4 0.17 0.2 0.038 0.006 Comparative Example 1: GRADE G 0.14 3.1 2.1 2.7 1.2 1.32 10.0 0.222 0.16 0.022 0.004

The Nb content is below the detection limit. Count <0.005% for all examples.

These compositions are very similar with the exception of Comparative Example 1. The main notable differences between Comparative Example 1 and Example 1 relate to the V, Mo and Cr content.

These laboratory castings were transformed into bars with a diameter of 40 mm by a hot forging process under a 2000 T press. Bars with a diameter of 20 mm were machined in the bar and case-hardened. The cemented bars were treated by (1) solution treatment at 1100°C or 1150°C, (2) holding for 15 min at this temperature for austenitization, (3) cooling under neutral gas at a pressure between 2 and 6 bars (2×10 ⁵ and 6×10 ⁵ Pa), (4) a period at room temperature, (5) cooling at -70°C for 2 hours, and (6) 3 tempering at a temperature of 500°C for 1 hour each. The surface hardness profiles in HV measured according to the ASTM E384 standard published in August 2017 of examples 1 to 6 and of comparative example 1 are indicated in tables 2 and 3. <u>Table 2 (dissolution at 1100°C)</u> Example hardness core material Hardness at 1 mm depth Surface hardness Example 1: GRADE A 522 888 936 Example 2: GRADE B 485 863 927 Example 3: GRADE C 542 890 938 Example 4: GRADE D 495 878 934 Example 5: GRADE E 554 880 942 Example 6: GRADE F 567 927 976 Comparative Example 1: GRADE G 576 835 847 Example hardness core material Hardness at 1 mm depth Surface hardness Example 1: GRADE A 550 888 949 Example 2: GRADE B 543 888 943 Example 3: GRADE C 603 933 957 Example 4: GRADE D 552 904 957 Example 5: GRADE E 612 934 940 Example 6: GRADE F 627 936 988 Comparative Example 1: GRADE G 585 868 878

For all the chemical compositions except for comparative example 1, the surface hardness after carburizing exceeds 920 HV for a solution treatment temperature of 1100° C. and exceeds 930 HV for a solution treatment temperature of 1150° C. The hardness at 1 mm depth is always greater than 860 HV for a solution temperature of 1100°C and is always greater than 880 HV for a solution temperature of 1150°C for all the examples except the example comparative 1 (effect of the lack of alloying elements).

The hardnesses on the base materials are all less than 650 HV.

^2nd series of examples : 2 castings of 100 kg each (an example according to the invention and a comparative example 2) were produced by the VIM process according to the composition shown in Table 4 below (in % by weight relative to the total weight of the composition), the balance being Fe: <u>Table 4</u> Element VS Neither CR MB v W Co Whether min Al NOT Example 7: GRADE H 0.06 3.2 3.9 4.8 2.1 1.1 10.2 0.16 0.14 Comparative Example 2: GRADE 1 0.05 3.1 3.8 5.0 2.1 2.8 10.0 0.17 0.14

These laboratory castings were transformed into bars with a diameter of 40 mm by a hot forging process under a 2000 T press. Bars with a diameter of 20 mm were machined in the bar and case-hardened. The case-hardened bars were treated according to the same process as for the first series of tests except for the dissolution which was carried out at 1100° C. and the triple tempering which was carried out at 525° C. for 1 hour. Table 5 below gives the results of the toughness tests carried out on CT10 specimens according to the ASTM E399-17 standard published in February 2018. <u>Table 5</u> Example Tenacity (MPa.√m) Mechanical resistance (MPa) Example 7 44-60 1400-1700 Comparative example 2 35 1500

Comparative example 2 presents delta ferrite after heat treatment, in a small quantity but sufficient to reduce the toughness properties.

Example 7, very close to comparative example 2 in terms of its composition to the nearest W, does not present delta ferrite and makes it possible to obtain toughness values almost doubled compared to comparative example 2 while maintaining a good mechanical resistance (Rm) of approximately 1500 MPa, which was determined according to the ASTM E399-17 standard published in February 2018, equivalent to a core hardness of 450 HV according to the ASTM E384 standard published in August 2017.

Claims (14)

1. A steel composition comprising, in percentages by weight of the total composition:

   Carbon: 0.06-0.20;

   Chromium: 2.5-5.0;

   Molybdenum: 4.0-6.0;

   Tungsten: 0.01-3.0;

   Vanadium: 1.0-3.0;

   Nickel: 2.0-4.0;

   Cobalt: 9.0-12.5;

   Iron: remainder

   as well as the inevitable impurities in an amount of at most 1 wt%,

   optionally further comprising one or more of the following elements:

   Niobium: ≤ 2.0;

   Nitrogen: ≤ 0.50;

   Silicon: ≤ 0.70;

   Manganese: ≤ 0.70;

   Aluminum: ≤0.15;

   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.
2. The steel composition as claimed in claim 1, characterized in that it comprises, in percentages by weight of the total composition:

   Carbon: 0.06-0.20, preferably 0.08-0.18;

   Chromium: 3.0-4.5, preferably 3.5-4.5;

   Molybdenum: 4.0-6.0, preferably 4.5-5.5;

   Tungsten 0.01-3.0;

   Vanadium: 1.5-2.5, preferably 2.0-2.3;

   Nickel: 2.0-4.0, preferably 2.5-3.5;

   Cobalt: 9.5-12.5, preferably 9.5-10.5;

   Iron: remainder

   as well as the inevitable impurities,

   optionally further comprising one or more of the following elements:

   Niobium: ≤ 2.0;

   Nitrogen: ≤ 0.20;

   Silicon: ≤ 0.70, preferably 0.05-0.50;

   Manganese: ≤ 0.70, preferably 0.05-0.50;

   Aluminum: ≤0.10;

   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.
3. The steel composition as claimed in either one of claims 1 or 2, characterized in that it comprises at most 0.5 wt% of inevitable impurities, the inevitable impurities being advantageously selected from titanium, sulfur, phosphorus, copper, tin, lead, oxygen and mixtures thereof.
4. The steel composition as claimed in any one of claims 1 to 3, characterized in that it 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 67 HRC, in particular greater than or equal to 68 HRC and advantageously a hardness greater than or equal to 66 HRC at a depth of 1 mm.
5. The steel composition as claimed in any one of claims 1 to 4, characterized in that it 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 martensitic structure having a residual austenite content below 0.5 wt% and free from ferrite and pearlite.
6. The steel composition as claimed in either one of claims 4 or 5, characterized in that the thermal treatment comprises a solution treatment at a temperature between 1090°C-1160°C followed by quenching optionally with cooling, advantageously to a temperature below -40°C, and several tempering operations, advantageously at least three tempering operations, at a temperature between 475°C and 530°C, in particular of 500°C.
7. A method of making a steel blank having the composition as claimed in any one of claims 1 to 6, characterized in that it comprises:

   a) a steelmaking step, advantageously carried out by a conventional steelmaking process in an arc furnace and with refining and remelting under conductive slag (ESR), or by a VIM or VIM-VAR process, optionally with a step of remelting under conductive slag (ESR) and/or under vacuum (VAR), or by powder metallurgy such as gas atomization and compaction by hot isostatic pressing (HIP);

   b) a step of transformation of the steel, advantageously which consists of a step of rolling, forging and/or extrusion;

   c) a thermochemical treatment;

   d) and a heat treatment.
8. The method of manufacture as claimed in claim 7, characterized in that step c) consists of a treatment of carburizing or of nitriding or of carbonitriding or of carburizing and then nitriding, advantageously it is a carburizing treatment.
9. The method of manufacture as claimed in either one of claims 7 or 8, characterized in that step c) consists of a carburizing treatment allowing carbon enrichment of the surface leading to a final surface carbon content of at least 1 wt%, even more advantageously > 1.1 wt%.
10. The method of manufacture as claimed in any one of claims 7 to 9, characterized in that step d) comprises a solution treatment at a temperature between 1090°C-1160°C, advantageously between 1100°C-1150°C, followed by holding at this temperature until completion of austenitization optionally with cooling to a temperature below -40°C, advantageously -70°C, and several tempering operations, advantageously at least three tempering operations, at a temperature between 475°C and 530°C, in particular of 500°C.
11. A steel blank obtainable by a method as claimed in any one of claims 7 to 10.
12. The use of a blank as claimed in claim 11 or of a steel composition as claimed in any one of claims 1 to 6 for making a mechanical device or an injection system, advantageously a bearing.
13. A mechanical device, advantageously a transmission component, in particular a bearing or a gear train, made of steel having the composition as claimed in any one of claims 1 to 6 or obtained from a steel blank as claimed in claim 11.
14. An injection system made of steel having the composition as claimed in any one of claims 1 to 6 or obtained from a steel blank as claimed in claim 11.