EP3765646A1

EP3765646A1 - Steel composition

Info

Publication number: EP3765646A1
Application number: EP19742812.1A
Authority: EP
Inventors: Jacques Bellus; Atman BENBAHMED; Johanna Andre; Fredrik SANDBERG
Original assignee: Aubert and Duval SA
Current assignee: Aubert and Duval SA
Priority date: 2018-03-14
Filing date: 2019-03-14
Publication date: 2021-01-20
Anticipated expiration: 2039-03-14
Also published as: PL3765646T3; EP3765646B1; US20210010116A1; FR3078978A1; WO2019186016A1; FR3078978B1; ES2960814T3

Abstract

The present invention relates to a steel composition containing, 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 1.5-2.5; nickel: 2.0-4.0; cobalt: 9.0-12.5, preferably, 9.5-11.0; iron: the remainder, as well as inevitable impurities, optionally further comprising one or more of the following elements: 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; with the combined content of niobium + vanadium ranging from 1.0 to 3.5; and the carbon + nitrogen content ranging from 0.06 to 0.50. The invention further relates to a method for manufacturing the above composition, the steel blank obtained therewith, and a mechanical body or injection system comprising said blank.

Description

STEEL COMPOSITION

The present invention relates to a new low carbon and high cobalt type lOCrMoNiVCo type steel for thermochemical treatment, particularly 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 a high surface hardness combined with a good tenacity at heart, for example in the case of injection systems.

Bearings are mechanical devices to ensure relative movements and constraints in orientation and direction between two parts. The bearings comprise several components: inner ring, outer ring as well as rolling bodies (ball or cylinder) arranged between these two rings. To ensure reliability and performance over time, it is important that these elements have good properties in rolling fatigue, wear, etc., ...

Gears are mechanical power transmission devices. To ensure a favorable power density (power ratio transmitted by the size of the gears) and the reliability of operation, the gears must have good structural fatigue properties (tooth foot) and contact fatigue (tooth blanks). Conventional techniques for making these metal components use electrical steelmaking processes followed by possible remelting operations, or single or multiple vacuum rejections. The ingots thus produced are then shaped by hot transformation processes such as rolling or forging in the form of bar, tube or ring. There are two types of metallurgy to ensure the final mechanical properties.

1st type: the chemical composition of the component makes it possible to obtain the mechanical properties directly after suitable heat treatment. 2nd type: the component requires a thermochemical treatment to enrich the surface with interstitial chemical elements such as carbon and / or nitrogen. This enrichment in general superficial then allows to obtain high mechanical properties after heat treatment on depths of a few millimeters maximum. These steels generally have better ductility properties than the first type steels.

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

The first of the properties required in the field of rolling or gears is obtaining a very high level of hardness. These Type 1 and Type 2 steels generally have surface hardness levels greater than 58 HRC. The most widespread shades 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 possible thermochemical treatment and heat treatment adapted a surface hardness of 63 HRC. Obtaining hardnesses greater than 64 HRC is now required to significantly improve the properties of the component.

GB2370281 discloses a valve seat steel by powder metallurgy technology compacted from powder mixtures of an iron base and harder particles. The matrix, which constitutes a part of the steel, having 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 perlite, 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 achieve good properties in mechanical strength and fatigue. Finally, this document does not suggest using 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 desired for the applications of the present invention because copper is a known embrittler whose content should not exceed 0.5% by weight relative to the total weight of the composition of the steel. .

The patent application WO2015 / 082342 describes a rolling 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

and 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 composition grade MIX5 (0.18% C-3.45% Cr-4.93% Mo-3.05% W-2.09% V-0.30% Si-2.89% Ni -5.14% Co-0.27% Mn) which is the most interesting because it has the greatest surface hardness. This grade makes it possible to reach a surface hardness after solution treatment at 1150 ° C. and returned at 560 ° C. to a maximum hardness level of about 800 HV, ie an equivalent of 64 HRC maximum. However, this request indicates that the content of Co must be limited to at most 8% and even it is preferable that it is at most 7% and even more preferred at most 6% because the Co increases the level of hardness base material which causes a decrease in toughness. The MIX5 grade which is preferred thus has a Co content of 5.14%.

The patent application WO2017216500 describes a rolling 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 unavoidable 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 Niobium + Vanadium content 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, the 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-O, 14-0.16% Si-3, 05-3, 09% Ni-5, 00-5, 40% Co-O, 18-0, 22% Mn-O, 03-0, 05% AI) is the preferred because having the greatest surface hardness. This grade makes it possible to reach a surface hardness after solution treatment at 1100 ° C.-1150 ° C. and returned at 500 ° C. to a maximum hardness level of about 66-67 HRC, which is much greater than the surface hardness. obtained with a nuance according to the application WO2015 / 082342 (grade A: FIG. 1). However, this request also indicates that the content of Co must be limited to at most 8% and even it is preferable that it be at most 7% and even more preferred at most 6%. because it increases the level of hardness of the base material which leads to a decrease in toughness. The preferred C grade thus has a Co content of 5.00-5.40%.

The obtaining of superficial hardnesses greater than 67 HRC, in particular using a solution heat treatment at a temperature below or equal to 1160 ° C, is therefore difficult to obtain whereas they would make it possible to improve significantly the properties of the component. The inventors have surprisingly found that by increasing the cobalt content of the steel described in the applications WO2015 / 082342 and WO2017216500 to a content between 9 and 12.5%, while maintaining the carbon content at a level less than or equal to 0.2% (new balancing carbon / cobalt), the steel obtained had, after thermochemical treatment, in particular carburizing and / or nitriding, a very high surface hardness and even greater than 67 HRC, in particular greater than or equal to 68 HRC and 1mm hardness greater than 860 HV (corresponding to approximately 66HRC according to the ASTME140-12b standard published in May 2013) after a solution heat treatment at a temperature in the range 1100 ° C - 1160 ° C and returned at a temperature greater than or equal to 475 ° C, while having a level of hardness of the base material between 400 and 650 HV.

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

US Pat. No. 815,791 discloses a Ni-Co type steel 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 content of chromium (5.3- 5.4%), a low content of vanadium (0.20-0.21%) and of molybdenum (2.5-2.52%) and does not contain tungsten. This equilibrium balancing leads after thermochemical treatment and associated quality treatment (including quenching at 1110 ° C. and tempering at 482 ° C.) at an interesting surface hardness but which decreases very rapidly with depth, so it is from 600 pm 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 can withstand the shade to avoid any risk of forming fragile graphite phase. Claim 1 of this patent thus stipulates a carbon content in the cemented layer limited to about 0.8%. Indeed graphite could appear from 1% by weight of C in the cemented layer (surface layer obtained after cementation).

It is therefore not obvious to find the right balance of the shade (including Cr, Mo, V, W, C) in view of this document to achieve an optimization of both the surface hardness, the hardness profile ( depth) and tenacity (of which one has an idea by hardness at heart). In addition it was not obvious from this document to achieve a deep carburizing layer that allows to introduce much more carbon than the nuances of the state of the art (up to 1.5 % by weight of C) while limiting the risk of occurrence of graphite.

The patent application JPH11-210767 describes a family of steel for aeronautical rolling 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 carburizing or carbonitriding. However this application only describes the surface hardness properties of 66-69 HRC and only describes the tenacity qualitatively. Balancing this very low carbon content, <0.05% by weight, requires limiting the vanadium content to <1.5% by weight so as not to degrade the tenacity, and vanadium is an interesting element. to improve the wear resistance.

This request does not describe either the hardness at heart (reflecting the mechanical strength) of this grade, and given the very low level of carbon it is expected that it degrades the mechanical strength.

Also, this application does not describe any deep layer carburizing profile. However, it would be interesting to have a high hardness in all the depth up to 400 microns of the surface which corresponds to the so-called Hertz zone, area stressed by very high shear stresses. A high hardness in all this depth also allows to have more tolerance when it comes to removing material for repair or grinding during a machining, and it is all the more useful for the application transmission of power that is not mentioned in JPH11-210767. The inventors have realized that it is possible to obtain a different balance from that proposed by JPH 11-210767 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 allows (a) to obtain a good compromise between hardness and toughness, that is to say a good compromise between mechanical strength and toughness, and (b) to admit more vanadium in its composition without degrade toughness, which is favorable for wear resistance.

The present invention therefore relates to a steel composition, advantageously cementable and / or nitrurable, more advantageously cementable, comprising, advantageously consisting essentially of, in particular consisting of, in percentages by weight of the total composition:

Carbon: 0.06-0.20, 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 unavoidable 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 Niobium + Vanadium content being in the range 1, 0-3.5;

and the carbon + nitrogen content being in the range 0.06-0.50.

A particularly advantageous composition comprises, advantageously consists essentially of, in particular consists of, in percentages by weight of the total composition:

Carbon: 0.06-0.20, preferably 0.08-0.18;

Chromium: 3.0-4.4, 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 unavoidable impurities,

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 Niobium + Vanadium content 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 due mainly to the manufacturing process and the quality of charging. Advantageously, the composition according to the invention comprises at most 1% by weight of unavoidable impurities, advantageously at most 0.75% by weight, still more advantageously at most 0.50% by weight, relative to the total weight of the composition.

The carbide-forming elements, which also have a stabilizing effect on ferrite, so-called alphagenes, are essential to the steel composition according to the invention so as to provide sufficient hardness, resistance to heat and wear. . In order to obtain a ferrite-free microstructure which would weaken the component, it is necessary to add stabilizing elements of the austenite, so-called gammagenic elements.

A correct combination of austenite stabilizing elements (carbon, nickel, cobalt and manganese) and stabilizing elements of ferrite (molybdenum, tungsten, chromium, vanadium and silicon) makes it possible to obtain a steel composition according to the invention. invention having superior properties, especially after thermochemical treatment such as carburizing.

The steel composition according to the invention therefore comprises carbon (C) in a content in the range 0.06-0.20%, preferably 0.07-0.20%, in particular 0.08-0. , 20%, more particularly 0.08-0.18%, by weight relative to the total weight of the composition. Carbon (C) stabilizes the austenitic phase of the steel at the treatment temperatures thermal and is essential for the formation of carbides which provide the mechanical properties in general including mechanical strength, high hardness, resistance to heat and wear. The presence of a small amount of carbon in a steel is beneficial to avoid the formation of undesirable and fragile intermetallic particles and to form small amounts of carbides to prevent excessive growth of grain size during solution dissolution prior to quenching operation. The initial carbon content, however, should not be too high since it is possible to increase the surface hardness of the components formed from the steel composition by carburizing. It is also known that in general the increase in the carbon content makes it possible to significantly increase the level of hardness, which is generally disadvantageous with respect to ductility properties. It is for this reason that the carbon content is limited to 0.20% maximum to obtain a hardness level at the core of the material of at most 650 HV. During cementation, the carbon is implanted in the surface layers of the component, so as to obtain a hardness gradient. Carbon is the main element for controlling the hardness of the martensitic phase formed after cementation and heat treatment. In case-hardened steel, it is essential to have a core portion of the material with a low carbon content while having a hard surface with a high carbon content after thermochemical carburizing treatment.

The steel composition according to the invention further comprises chromium (Cr) in a content in the range of 2.5-5.0%, preferably 3.0-4.5%, even more preferred 3.5. -4.5%, still more preferably 3.8-4.0% by weight relative to the total weight of the composition. Chrome contributes to the formation of carbides in steel and is one of the main elements that 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 should not be too high.

The steel composition according to the invention also comprises molybdenum (Mo) in a content in the range of 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 wear, wear resistance and hardness of steel. However, molybdenum has a strong stabilizing effect on the ferrite phase and must not be present in excessive amounts in the steel composition according to the invention.

The steel composition according to the invention further comprises Tungsten (W) in a content in the range 0.01-3.0%, preferably 0.01-1.5%, even more preferred 0.01 -1.4%, advantageously 0.01-1.3%, by weight relative to the total weight of the composition.

Tungsten is a stabilizer for ferrite and a strong carbide-forming element. It improves resistance to heat treatment and wear and hardness by carbide formation. However, it can also lower the surface hardness of the steel and especially the properties of ductility and toughness. For this element to play its full role, it is necessary to carry out high temperature dissolution. The steel composition according to the invention further comprises vanadium (V) in a content in the range of 1.0-3.0%, preferably 1.5-2.5%, even more preferably 1, 7. -3.0%, advantageously 1.7-2.5%, more preferably 1.7-2.3%, still more preferably 2.00-2.3%, especially 2.0-2.2%, by weight relative to the total weight of the composition.

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

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

If the 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 comprise Niobium.

The steel composition according to the invention also comprises nickel (Ni) in a content in the range 2.0-4.0%, preferably 2.5-3.5%, even more preferably 2, 7- 3, 3%, advantageously 3, 0-3, 2%, by weight relative to the total weight of the composition.

Nickel promotes the formation of austenite and therefore inhibits the formation of ferrite. Another effect of Nickel is to decrease the temperature Ns, ie the temperature at which transformation from austenite to martensite begins during cooling. This can prevent the formation of martensite. The quantity of nickel must therefore be controlled to avoid the formation of residual austenite in cemented components.

The steel composition according to the invention further comprises cobalt (Co) in a content within the range of 9.0-12.5%, preferably 9.5-12.5%, advantageously 9.5-11. , 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 standards ASTM-E1097-12 published in June 2017 and ASTM E1479_16 published in December 2016. The measurement error of the cobalt content of the steel according to the invention is thus ± 2, About 5% relative and evaluated according to IS05724-1 (December 1994), IS05725-2 (December 1994), IS05725-3 (December 1994), IS05725-4 (December 1994), IS05725-5 (December 1994), IS05725 -6 (December 1994) and the standard NF ISO / IEC Guide 98-3 of July 11, 2014.

Cobalt is a highly stabilizing element of austenite that prevents the formation of undesirable ferrite. Unlike Nickel, Cobalt increases the Ms temperature, which in turn decreases the amount of residual austenite. Cobalt, in combination with nickel, allows the presence of ferrite stabilizers such as the carbide-forming elements Mo, W, Cr and V. The carbide-forming elements are essential for the steel according to the invention because of their effect on hardness, resistance to heat and wear. Cobalt has a small effect of increasing hardness on steel. However, this increase in hardness is correlated with the decrease in toughness. It is therefore not necessary for the steel composition according to the invention to contain an excessive amount of cobalt. The 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 contents of Cr, Mo, V, Ni and W as described above). This limitation in carbon makes it possible to compensate the increase in hardness related to the addition of Co.

The steel composition according to the invention may further comprise silicon (Si) in a content <0.70%, by weight relative to the total weight of the composition. Advantageously, it comprises silicon, in particular in a content in the range 0.05-0.50%, preferably 0.05-0.30%, advantageously 0.07-0.25%, more advantageously 0 , 10-0.20%, by weight relative to the total weight of the composition.

Silicon strongly stabilizes ferrite, but is often present during the steelmaking process during the deoxidation of liquid steel. Low oxygen contents are indeed also important for obtaining low levels of non-metallic inclusions and good mechanical properties such as fatigue resistance and mechanical strength.

The steel composition according to the invention may further comprise manganese (Mn) in a content <0.70%, by weight relative to the total weight of the composition. Advantageously, it comprises manganese, in particular in a content in the range 0.05-0.50%, preferably 0.05-0.30%, advantageously 0.07-0.25%, even more advantageously 0 , 10-0.22%, more particularly 0.10-0.20% by weight relative to the total weight of the composition.

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. it eliminates the risk of formation of iron sulphides which have an adverse effect on the hot machining of steels. Manganese is also part of the deoxidation step like Silicon. The combination of manganese with silicon gives deoxidation more efficient than each of these elements alone.

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 to martensite. Nitrogen can to a certain extent replace the 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 of <0.15%, preferably <0.10%, by weight relative to the total weight of the composition. .

Aluminum (Al) can indeed be present during the steel manufacturing process according to the invention and contributes very effectively to the deoxidation of the liquid steel. This is particularly the case during reflow processes such as the VIM-VAR process. The aluminum content is generally higher in the steels produced using the VIM-VAR process than in the steels obtained by the powder technology. Aluminum causes difficulties during the atomization by obstruction of the casting nozzle by oxides. A low oxygen content is important for good micro-cleanliness as well as good mechanical properties such as fatigue resistance and mechanical strength. The oxygen content obtained by ingot is typically less than 15 ppm.

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

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

If nitriding is used, it is the nitrogen content that increases on the surface of the steel, and therefore also the surface hardness. If carbonitriding or cementation followed by nitriding are used, it is the carbon and nitrogen contents on the surface of the steel which are increased and thus 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, after a thermochemical treatment, advantageously carburizing or nitriding or carbonitriding or carburizing and then nitriding, followed by a heat treatment, a superficial hardness superior at 67HRC, in particular greater than or equal to 68 HRC, measured according to ASTM E18 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 dissolution 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 depending on the ASTM E140-12b standard published in May 2013), in particular greater than or equal to 950 HV, measured according to the ASTM E384 standard published in August 2017 or equivalent standard after dissolution at a temperature of 1150 ° C.

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

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

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

Said heat treatment may comprise:

- (1) a set of steel solution at a temperature between 1090 ° C-1160 ° C, preferably between 1100 ^° C-1160 ° C, more preferably between 1100 and 1155 ° C, in particular between 1100 and 1150 ° C, more particularly 1150 ° C,

- (2) advantageously followed by keeping at this temperature until complete austenitization, in particular for a period of 15 minutes (quenching), (these 2 phases (1) and (2) allow the total or partial solution solution carbides initially present),

- (3) then optionally a first cooling (quenching), in particular under a neutral gas at, for example, a pressure of 2 bar (2 × 10 ⁵ Pa), advantageously up to room temperature (this phase makes it possible to obtain a mainly martensitic microstructure with residual austenite. This residual austenite is a function of the cooling temperature: the content decreases with the cooling temperature),

- (4) possibly followed by a maintenance at ambient temperature,

- (5) then advantageously a second cooling at a temperature below -40 ° C, more preferably below -60 ° C, even more preferably about -70 ° C, in particular for 2 hours (this phase allows to reduce the residual austenite content),

- (6) and advantageously one or more incomes, more preferably at least three revenues, preferably at a temperature greater than or equal to 475 ° C, more preferably between 475 ° C and 530 ° C, in particular 500 ° C, still more particularly for 1 hour each (this or these incomes allow the precipitation of carbides and the partial or total decomposition of the residual austenite, which makes it possible to obtain ductility properties).

The steel of interest according to the invention is therefore to obtain high levels of hardness with a limited thermal treatment (temperature between 1090 ° C-1160 ^° C, preferably between 1100 ° C-1160 ° C, more preferably between 1100 ° C-1155 ° C, in particular between 1100 ° C-1150 ° C, more particularly 1150 ° C).

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

Said heat treatment may be as described above.

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

(a) a steel making step;

(b) a step of processing the steel;

c) a thermochemical treatment;

d) and a 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 cementation and then nitriding treatment, advantageously it is a case hardening treatment, more particularly allowing a surface carbon enrichment resulting in a final carbon content of at least 1% by weight, still more preferably> 1.1% by weight.

In particular, step b) of the method according to the present invention consists of a rolling step, forging and / or spinning, advantageously forging. These methods are well known to those skilled in the art. In an advantageous embodiment, the step a) of developing the process according to the present invention is carried out by means of a conventional arc furnace refining and conductive slag remelting (ESR) process, or by a VIM or VIM-VAR process, possibly with a conductive slag (ESR) and / or vacuum (VAR) remelting step, or by powder metallurgy such as gas atomization and hot isostatic compaction compression (HIP) ).

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

This steel can also be obtained by metallurgy powders. This process makes it possible to produce high purity metal powder by atomization, preferably gas atomization, 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 that can be obtained by the process according to the invention. This blank is made of steel having the composition according to the present invention and as described above.

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

It thus relates to a mechanical member, advantageously a transmission element, in particular a gear, a transmission shaft or a bearing, more particularly a bearing or a gear, 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. In fact, with the steel composition according to the invention, it is possible to combine the high surface hardness and the surface wear resistance after thermochemical treatment with a core portion of the material having a high resistance to fatigue and a high mechanical strength.

These steels can be used in demanding areas such as bearings for aerospace or injection systems.

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

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

the series of examples ^era Seven laboratory castings 9 Kg each of (six 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 method the composition contained in Table 1 below (in% by weight relative to the total weight of the composition), the balance being Fe:

Table 1

The Nb content is below the limit of detection. Nb <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 are the content of V, Mo and Cr.

These laboratory flows were transformed into 40 mm diameter bars by a hot forging process in a 2000 T press. Bars of 20 mm diameter were machined in the bar and case hardened.

The cemented bars were treated with (1) solution at 1100 ° C or 1150 ° C, (2) hold for 15 min at this temperature for austenitization, (3) cooling under a neutral gas at a pressure between 2 and 6 bar (2 x 10 ⁵ and 6 x ¹⁰ ⁵ Pa), (4) a period at room temperature, (5) cooling at -70 ° C for 2 hours, and (6) 3 incurring at a temperature of 500 ° C for 1 hour each. The HV hardness profiles measured according to ASTM E384 published in August 2017 of Examples 1 to 6 and Comparative Example 1 are shown in Tables 2 and 3.

Table 2 (dissolution at 1100 ° 0

Table 3 (dissolution at 1150 ° C.

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

The hardnesses on basic materials are all less than 650 HV.

2 ^nd series of examples: 2 castings of 100 kg each (an example according to the invention and a comparative example 2) were developed by the VIM method according to the composition shown in Table 4 below (in% by weight relative to to the total weight of the composition), the balance being Fe:

Table 4

The cemented bars were treated according to the same procedure as for the first test series except the dissolution which was made at 1100 ° C and the triple income which was operated at 525 ° C for 1 hour. Table 5 below gives the results of the tenacity tests carried out on CT10 test pieces according to the ASTM E399-17 standard published in February 2018.

Table 5

Comparative Example 2 shows delta ferrite after heat treatment, in a small amount but sufficient to decrease toughness properties.

Example 7, very close to Comparative Example 2 at the level of its W-composition, does not have delta ferrite and makes it possible to obtain tenacity values almost doubled compared with Comparative Example 2 while maintaining a good mechanical strength (Rm) of about 1500 MPa, which was determined according to ASTM E399-17 published in February 2018, is equivalent to a hardness of 450 HV according to ASTM E384 published in August 2017.

Claims

A steel composition comprising, advantageously consisting essentially 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 1.5-2.5;

Nickel: 2.0-4.0;

Cobalt: 9.0-12.5, preferably 9.5-11.0;

Iron: balance

as well as unavoidable impurities,

optionally 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 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. A steel composition according to claim 1, characterized in that it comprises, advantageously in that it consists essentially of, in percentages by weight of the total composition:

Carbon: 0.06-0.20, preferably 0.08-0.18; Chromium: 3.0-4.4, 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 unavoidable impurities,

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 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. A steel composition according to any one of claims 1 or 2, characterized in that it comprises at most 1% by weight of unavoidable impurities, preferably at most 0.5% by weight.

4. A steel composition according to any one of claims 1 to 3, characterized in that the unavoidable impurities are selected from titanium, sulfur, phosphorus, copper, tin, lead, oxygen and their mixtures.

5. A steel composition according to any one of claims 1 to 4, characterized in that it is cementable and / or nitrurable.

6. A steel composition according to any one of claims 1 to 5, characterized in that it has, after a thermochemical treatment, preferably carburizing or nitriding or carbonitriding or carburizing and nitriding, followed by a heat treatment, a surface hardness greater than 67 HRC, in particular greater than or equal to 68 HRC and advantageously a hardness to 1 mm deep greater than or equal to 66 HRC.

7. A steel composition according to any one of claims 1 to 6, characterized in that it has, after a thermochemical treatment, preferably carburizing or nitriding or carbonitriding or carburizing and nitriding, followed by a heat treatment, a martensitic structure having a residual austenite content of less than 0.5% by weight and free from ferrite and pearlite.

8. Steel composition according to any one of claims 6 or 7, characterized in that the heat treatment comprises a dissolution at a temperature between 1090 ° C-1160 ° C followed by quenching with possibly cooling , advantageously at a temperature below -40 ° C, and several incomes, advantageously at least three incomes, at a temperature between 475 ° C and 530 ° C, in particular 500 ° C.

9. A method of manufacturing a steel blank having the composition according to any one of claims 1 to 8, characterized in that it comprises:

(a) a steel making step;

(b) a step of processing the steel;

c) a thermochemical treatment;

d) and a heat treatment.

10. The manufacturing method according to claim 9, characterized in that step c) consists of a carburizing treatment or nitriding or carbonitriding or carburizing and nitriding, advantageously it is a cementation treatment.

11. The manufacturing method according to any one of claims 9 or 10, characterized in that step c) consists of a cementation treatment for a carbon enrichment on the surface resulting in a final carbon content of at least 1 % by weight, still more preferably> 1.1% by weight.

12. The manufacturing method according to any one of claims 9 to 11, characterized in that step d) comprises dissolving at a temperature between 1090 ° C-1160 ° C, preferably between 1100 ° C- 1150 ° C, followed by keeping at this temperature until complete austenitization with possible cooling at a temperature below -40 ° C, preferably -70 ° C, and several incomes, preferably at least three incomes, at a temperature between 475 ° C and 530 ° C, in particular 500 ° C.

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

14. Manufacturing process according to any one of claims 9 to 13, characterized in that the step a) of elaboration is implemented by a conventional method of elaboration arc furnace and with refining and remelting slag conductor (ESR), or by a VIM or VIM-VAR process, possibly with a conductive slag (ESR) and / or vacuum (VAR) remelting step, or by powder metallurgy such as gas atomization and Hot isostatic compaction compression (HIP).

15. A steel blank obtainable by a process according to any one of claims 9 to 14.

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

17. Mechanical member, advantageously transmission element, in particular rolling 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.

Steel injection system having the composition according to any one of claims 1 to 8 or obtained from a steel blank according to claim 15.