STEEL COMPOSITION
The present invention relates to a steel composition comprising, 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: remainder as well as the 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; 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. It further relates to method of manufacture thereof, the steel blank obtained and a mechanical device or an injection system comprising same.
The present invention relates to a new steel of grade 10CrMoNiVCo with low carbon content and high cobalt content for thermochemical treatment in particular intended for the field of transmissions such as bearings and gears. The alloy according to the invention is also usable for other applications requiring high surface hardness combined with good core toughness, for example in the case of injection systems.
Bearings are mechanical devices allowing relative movements, constrained in orientation and direction, between two components. Bearings comprise several components: inner race, outer race as well as rolling bodies (balls or rollers) arranged between these two races. To ensure reliability and performance over time, it is important that these various elements have good properties of rolling fatigue, wear, etc.
Gear trains are mechanical devices for power transmission. To ensure a favorable power density (ratio of power transmitted to overall dimensions of the gear train) and operational reliability, gear trains must have good properties of structural fatigue (tooth root) and contact fatigue (tooth flank).
The conventional techniques for producing these metallic components employ production methods of electric steelmaking followed by optional operations of remelting, or single or multiple vacuum remelting. The ingots thus produced are then formed by methods of hot working such as rolling or forging in the form of bar, tube or rings.
There are two types of metallurgy for providing 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, generally at the surface, then allows high mechanical properties to be obtained after heat treatment to depths of some millimeters at most. These steels generally have better properties of ductility than the steels of the 1st type.
There are also thermochemical methods applied to steels of the 1st type with the aim of enriching the surface with nitrogen to obtain very high mechanical properties.
The first of the properties required in the field of bearings or gear trains is obtaining a very high level of hardness. These steels of type 1 and of type 2 generally have levels of surface hardness above 58 HRC. The grades used most widely and known by the term M50 (0.8% C-4% Cr-4.2% Mo-1% V) or 50NiL (0.12% C-4% Cr-4.2% Mo-3.4% Ni-1% V) do not exceed, after optional thermochemical treatment and suitable heat treatment, a surface hardness of 63 HRC. It is now necessary to obtain hardnesses above 64 HRC for significant improvement of the properties of the component.
Application GB2370281 describes a valve seat steel produced by powder metallurgy technology starting from mixtures of powder with an iron base and harder particles. The matrix, which only constitutes one part of the steel, has the following composition, in percentages by weight of the total composition:
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- 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: remainder
However, this matrix comprises from 5 to 40 vol % of pearlite, with consequent lack of ductility of this matrix and therefore embrittlement. Furthermore, the material also contains porosity (up to 10%), which does not allow good properties of mechanical strength and fatigue strength to be achieved. Finally, this document does not suggest using a low copper content and on the contrary indicates that its content may be up to 15 wt %. Now, a high copper content is undesirable for the applications of the present invention as copper is known to cause embrittlement and its content should not exceed 0.5 wt % relative to the total weight of the composition of the steel.
Patent application WO2015/082342 describes a rolling bearing steel having the following composition, in percentages by weight of the total composition:
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- 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: remainder
as well as the inevitable impurities, optionally further comprising one or more of the following elements:
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- Niobium: 0-2;
- Nitrogen: 0-0.5;
- Silicon: 0-0.7;
- Manganese: 0-0.7;
- Aluminum: 0-0.15;
and in particular grade MIX5 of composition (0.18% C-3.45% Cr-4.93% Mo-3.05% W-2.09% V-0.30% Si-2.89% Ni-5.14% Co-0.27% Mn), which is the most interesting as it has the highest surface hardness. This grade makes it possible to reach a surface hardness after solution heat treatment at 1150° C. and tempering at 560° C. at a maximum hardness level of about 800 HV, or the equivalent of max. 64 HRC. However, this application states that the Co content must be limited to at most 8% and it is even preferable for it to be at most 7% and even more preferably at most 6% as Co increases the level of hardness of the base material, which leads to a decrease in toughness. The grade MIX5 that is preferred thus has a Co content of 5.14%.
Patent application WO2017216500 describes a rolling bearing steel having the following composition, in percentages by weight of the total composition:
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- 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: remainder
as well as the inevitable impurities,
optionally further comprising one or more of the following elements:
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- 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: ≤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, grade C of composition (0.18-0.20% C-3.90-4.00% Cr-5.00-5.20% Mo-0.10-0.20% W-2.10-2.30% V-0.14-0.16% Si-3.05-3.09% Ni-5.00-5.40% Co-0.18-0.22% Mn-0.03-0.05% Al) is preferred, as it has the highest surface hardness. This grade makes it possible to reach a surface hardness after solution heat treatment at 1100° C.-1150° C. and tempering at 500° C. at a maximum level of hardness of about 66-67 HRC, which is well above the surface hardness obtained with a grade according to application WO2015/082342 (grade A: FIG. 1). However, this application also states that the Co content must be limited to at most 8% and it is even preferable for it to be at most 7% and even more preferably at most 6% as it increases the level of hardness of the base material, which leads to a decrease in toughness. The grade C that is preferred thus has a Co content of 5.00-5.40%.
Obtaining surface hardnesses above 67 HRC, in particular using a solution heat treatment at a temperature less than or equal to 1160° C., is therefore difficult to achieve, whereas they would allow significant improvement of the properties of the component.
The inventors found, surprisingly, that by increasing the cobalt content of the steel described in applications WO2015/082342 and WO2017216500 to a content between 9 and 12.5%, while maintaining the carbon content at a level less than or equal to 0.2% (new carbon/cobalt balance), the steel obtained had, after thermochemical treatment, in particular carburizing and/or nitriding, a very high surface hardness, even above 67 HRC, in particular greater than or equal to 68 HRC and a hardness at 1 mm greater than 860 HV (which corresponds to about 66 HRC according to standard ASTME140-12b published in May 2013) after solution heat treatment at a temperature in the range 1100° C.-1160° C. and tempering at a temperature greater than or equal to 475° C., while displaying a level of hardness of the base material between 400 and 650 HV.
This was not at all obvious in view of these documents, which suggested using a lower cobalt content such as in grade MIXS (5.14% of cobalt) and in grade C (5.00-5.40% of cobalt), which are regarded as the compositions giving the best hardness.
U.S. Pat. No. 8,157,931 describes a steel of type Ni—Co having a cobalt content between 9.9 and 10% and a carbon content between 0.1 and 0.12% and having a high surface hardness of the order of 68-69 HRC. However, said steel has a high chromium content (5.3-5.4%), a low content of vanadium (0.20-0.21%) and molybdenum (2.5-2.52%) and does not contain tungsten. This grade balancing leads, after thermochemical treatment and associated quality treatment (comprising quenching at 1110° C. and tempering at 482° C.), to a surface hardness that is interesting, but decreases very quickly with depth, thus starting from 600 μm of depth it is already identical to that of the base metal (FIG. 1). This is probably due to the lower carbon content in the cemented layer that the grade can support to avoid any risk of formation of embrittling graphite phase. Claim 1 of that patent thus stipulates a carbon content in the cemented layer limited to about 0.8%. In fact, graphite could appear starting from 1 wt % of C in the cemented layer (surface layer obtained after carburizing).
It is therefore not obvious to find good balancing of the grade (including Cr, Mo, V, W, C) in view of this document to achieve simultaneous optimization of surface hardness, hardness profile (depth) and toughness (for which we have an idea from the core hardness). Furthermore, it was not obvious in view of this document to produce a deep carburizing layer that would allow much more carbon to be introduced than the grades of the prior art (up to 1.5 wt % of C) while limiting the risk of appearance of graphite.
Patent application JPH11-210767 describes a class of steel for aeronautical rolling bearing application with an improved service life having the following composition, in percentages by weight of the total composition:
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- Carbon: max. 0.05;
- Chromium: 2.5-5.5;
- Tungsten equivalent (2×Mo+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: remainder
This grade is submitted to carburizing or carbonitriding.
However, this application only describes the properties of surface hardness of 66-69 HRC and only describes the toughness qualitatively. Balancing of this grade at very low carbon, 0.05 wt %, necessitates limiting the vanadium content to 1.5 wt % so as not to degrade the toughness, vanadium being an interesting element allowing wear resistance to be improved.
Moreover, this application does not describe the core hardness (reflecting the mechanical strength) of this grade, and in view of the very low level of carbon, this is expected to degrade the mechanical strength.
Furthermore, this application does not describe any carburizing profile to a deep layer. Now, it would be interesting to have high hardness in the full depth as far as 400 microns from the surface, which corresponds to the so-called Hertz zone, a zone subjected to very high shear stresses. High hardness throughout this depth also provides more tolerance when it comes to removing material for repair or grinding during machining, and this is all the more useful for the power transmission application, which is not mentioned in JPH11-210767.
The inventors realized that it was possible to obtain balancing different from that proposed by JPH11-210767 with a higher carbon content, at least 0.06 wt %, and a range of cobalt between 9.0 and 12.5 wt %, making it possible (a) to obtain a good compromise between core hardness and toughness, in other words a good compromise between mechanical strength and toughness, and (b) to allow more vanadium in its composition without degrading the toughness, which is favorable for wear resistance.
The present invention therefore relates to a steel composition, advantageously carburizable and/or nitridable, more advantageously carburizable, comprising, advantageously consisting essentially of, in particular consisting of, in percentages by weight of the total composition:
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- 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: remainder
as well as the inevitable impurities,
optionally further comprising one or more of the following elements:
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- 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 interesting composition comprises, advantageously consists essentially of, in particular consists of, in percentages by weight of the total composition:
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- 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:
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- 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 inevitable impurities, notably selected from titanium (Ti), sulfur (S), phosphorus (P), copper (Cu), tin (Sn), lead (Pb), oxygen (O) and mixtures thereof, are kept at the lowest level. These impurities are generally due essentially to the method of manufacture and the quality of the charge. Advantageously, the composition according to the invention comprises at most 1 wt % of inevitable impurities, advantageously at most 0.75 wt %, even more advantageously at most 0.50 wt %, relative to the total weight of the composition.
The carbide formers, which also have a stabilizing effect on ferrite, so-called alpha-forming elements, are essential to the steel composition according to the invention so as to provide sufficient hardness, heat resistance and wear resistance. In order to obtain a microstructure free from ferrite, which would weaken the component, it is necessary to add austenite stabilizers, so-called gamma-forming elements.
A correct combination of austenite stabilizers (carbon, nickel, cobalt and manganese) and ferrite stabilizers (molybdenum, tungsten, chromium, vanadium and silicon) makes it possible to obtain a steel composition according to the invention having superior properties, in particular after thermochemical treatment such as carburizing.
The steel composition according to the invention therefore comprises carbon (C) at a content in the range 0.06-0.20%, preferably 0.07-0.20%, in particular 0.08-0.20%, more particularly 0.08-0.18%, by weight relative to the total weight of the composition. In fact carbon (C) stabilizes the austenitic phase of the steel at the heat treatment temperatures and is essential for formation of carbides, which supply the mechanical properties in general, notably mechanical strength, high hardness, heat resistance and wear resistance. The presence of a small amount of carbon in a steel is beneficial for avoiding formation of undesirable, brittle intermetallic particles and for forming small amounts of carbides to avoid excessive grain growth during solution treatment before the quenching operation. The initial carbon content need not, however, be too high, since it is possible to increase the surface hardness of the components formed from the steel composition by carburizing. It is also known that, generally, increasing the carbon content makes it possible to increase the level of hardness significantly, which is generally detrimental with respect to the ductility properties. That is why the carbon content is limited to max. 0.20% to obtain a level of core hardness of the material of at most 650 HV. During carburizing, carbon is introduced into the surface layers of the component, so as to obtain a hardness gradient. Carbon is the principal element for controlling the hardness of the martensitic phase formed after carburizing and heat treatment. In a case-hardened steel, it is essential to have a core portion of the material with a low carbon content while having a hard surface with a high carbon content after carburizing thermochemical treatment.
The steel composition according to the invention further comprises chromium (Cr) at a content in the range 2.5-5.0%, preferably 3.0-4.5%, even more preferably 3.5-4.5%, even more advantageously 3.8-4.0 wt % relative to the total weight of the composition.
Chromium contributes to the formation of carbides in steel and is one of the main elements controlling the hardenability of steels.
However, chromium may also promote the appearance of ferrite and residual austenite. Therefore the chromium content of the steel composition according to the invention must not be too high.
The steel composition according to the invention also comprises molybdenum (Mo) at a content in the range 4.0-6.0%, preferably 4.5-5.5%, even more preferably 4.8-5.2%, by weight relative to the total weight of the composition.
Molybdenum improves tempering resistance, wear resistance and the hardness of steel. However, molybdenum has a strong stabilizing effect on the ferrite phase and therefore should not be present in an excessive amount in the steel composition according to the invention.
The steel composition according to the invention further comprises tungsten (W) at a content in the range 0.01-3.0%, preferably 0.01-1.5%, even more preferably 0.01-1.4%, advantageously 0.01-1.3%, by weight relative to the total weight of the composition.
Tungsten is a ferrite stabilizer and a strong carbide former. It improves resistance to heat treatment and to wear as well as hardness by forming carbides. However, it may also lower the surface hardness of the steel and especially the properties of ductility and toughness. For this element to perform its role fully, it is necessary to apply solution treatment at high temperature.
The steel composition according to the invention further comprises vanadium (V) at a content in the range 1.0-3.0%, preferably 1.5-2.5%, even more preferably 1.7-3.0%, advantageously 1.7-2.5%, more advantageously 1.7-2.3%, even more advantageously 2.00-2.3%, in particular 2.0-2.2%, by weight relative to the total weight of the composition.
Vanadium stabilizes the ferrite phase and has a strong affinity for carbon and nitrogen. Vanadium provides wear resistance and tempering resistance by forming hard vanadium carbides. Vanadium may be replaced partly with niobium (Nb), which has similar properties.
The combined niobium+vanadium content must therefore be in the range 1.0-3.5 wt % relative to the total weight of the composition, advantageously in the range 1.7-3.5 wt % relative to the total weight of the composition.
If niobium is present, its content must be 2.0 wt % 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) at a content in the range 2.0-4.0%, preferably 2.5-3.5%, even more preferably 2.7-3.3%, advantageously 3.0-3.2%, by weight relative to the total weight of the composition.
Nickel promotes the formation of austenite and therefore inhibits the formation of ferrite. Another effect of nickel is to lower the temperature Ms, i.e. the temperature at which the transformation of austenite to martensite begins during cooling. This may prevent martensite formation. The amount of nickel must therefore be controlled so as to avoid formation of residual austenite in the carburized components.
The steel composition according to the invention further comprises cobalt (Co) at a content in the range 9.0-12.5%, preferably 9.5-12.5%, advantageously 9.5-11.0%, more advantageously 9.5-10.5%, by weight relative to the total weight of the composition. The cobalt content is measured according to standards ASTM-E1097-12 published in June 2017 and ASTM E1479_16 published in December 2016. The error in measurement of the cobalt content of the steel according to the invention is thus about ±2.5% relative, and is evaluated according to standards IS05724-1 (December 1994), ISO5725-2 (December 1994), ISO5725-3 (December 1994), ISO5725-4 (December 1994), ISO5725-5 (December 1994), ISO5725-6 (December 1994) and standard NF ISO/CEI Guide 98-3 of 11 Jul. 2014.
Cobalt is a strong austenite stabilizer that prevents the formation of undesirable ferrite. In contrast to nickel, cobalt increases the temperature Ms, which in its turn decreases the amount of residual austenite. Cobalt, in combination with nickel, allows the presence of ferrite stabilizers such as the carbide formers Mo, W, Cr and V. The carbide formers are essential for the steel according to the invention on account of their effect on hardness, heat resistance and wear resistance. Cobalt has a small effect on the steel of increasing the hardness. However, this increase in hardness is correlated with a decrease in toughness. Therefore the steel composition according to the invention should not contain an excessive amount of cobalt. Addition of Co makes it possible to limit the content of C, avoiding the promotion of ferrite for a composition according to the invention (containing the contents of Cr, Mo, V, Ni and W as described above). This limitation of carbon makes it possible to compensate for the increase in hardness associated with the addition of Co.
The steel composition according to the invention may further comprise silicon (Si) in a content ≤0.70 wt % relative to the total weight of the composition. Advantageously, it comprises silicon, in particular at a content in the range 0.05-0.50%, preferably 0.05-0.30%, advantageously 0.07-0.25%, even more advantageously 0.10-0.20%, by weight relative to the total weight of the composition.
Silicon is a strong ferrite stabilizer, but is often present during steelmaking, during deoxidation of the molten steel. Low oxygen contents are in fact also important for obtaining low levels of nonmetallic inclusions and good mechanical properties such as fatigue strength and mechanical strength.
The steel composition according to the invention may further comprise manganese (Mn) in a content ≤0.70 wt % relative to the total weight of the composition. Advantageously, it comprises manganese, in particular at a content in the range 0.05-0.50%, preferably 0.05-0.30%, advantageously 0.07-0.25%, even more advantageously 0.10-0.22%, even more particularly 0.10-0.20% by weight relative to the total weight of the composition.
Manganese stabilizes the austenite phase and decreases the temperature Ms in the steel composition. Manganese is generally added to the steels during their manufacture owing to its affinity for sulfur, there is thus formation of manganese sulfide during solidification. This eliminates the risk of formation of iron sulfides, which have an unfavorable effect on hot machining of the steels. Manganese also forms part of the deoxidation step, like silicon. The combination of manganese and silicon gives more effective deoxidation than each of these elements alone.
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 austenite formation and lowers the transformation of austenite to martensite. Nitrogen may to a certain extent replace carbon in the steel according to the invention, forming nitrides. However, the carbon+nitrogen content must be in the range 0.06-0.50 wt % relative to the total weight of the composition.
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) may in fact be present during steelmaking according to the invention and contributes very effectively to deoxidation of the molten steel. This is the case in particular in remelting processes, such as the VIM-VAR process. The aluminum content is in general higher in the steels produced by the VIM-VAR process than in the steels obtained by powder metallurgy. Aluminum gives rise to difficulties during atomization by obstructing the pouring spout with oxides.
A low oxygen content is important for obtaining good micro-cleanness as well as good mechanical properties such as fatigue strength and mechanical strength. The oxygen contents obtained by the ingot route are typically below 15 ppm.
Advantageously, the composition according to the present invention is carburizable, i.e. it can undergo a carburizing treatment, and/or nitridable, i.e. it can undergo a nitriding treatment and even advantageously it can undergo a thermochemical treatment, in particular selected from carburizing, nitriding, carbonitriding and carburizing followed by nitriding. These treatments make it possible to improve the surface hardness of the steel, by adding the elements carbon and/or nitrogen. Thus, if carburizing is used, the carbon content of the surface of the steel 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 wt %, more particularly of 0.8%-1.5 wt %, more advantageously of at least 1 wt %, in particular of 1-1.3 wt %, even more advantageously >1.1 wt %, even more particularly between 1.2 and 1.5 wt %. In the rest of this document, the surface carbon content will be understood to have been determined by sampling from a surface layer to a depth of 100 microns.
If nitriding is used, it is the nitrogen content that increases at the surface of the steel, and therefore also the surface hardness.
If carbonitriding or carburizing followed by nitriding is used, it is the contents of carbon and nitrogen at the surface of the steel that are increased and therefore also the surface hardness.
These methods are familiar to a person skilled in the art.
In an advantageous embodiment, the steel composition according to the invention has, after a thermochemical treatment, advantageously of carburizing or of nitriding or of carbonitriding or of carburizing and then nitriding, followed by a heat treatment, a surface hardness above 67HRC, in particular greater than or equal to 68 HRC, measured according to standard ASTM E18 published in July 2017 or an equivalent standard. It also has, advantageously, a surface hardness greater than or equal to 910 HV (about 67.25 HRC according to standard ASTM E140-12b published in May 2013), advantageously greater than or equal to 920 HV, in particular greater than or equal to 940 HV, measured according to standard ASTM E384 published in August 2017 or an equivalent standard, in particular after a solution treatment at a temperature of 1100° C. It also has, advantageously, a surface hardness greater than or equal to 930 HV (corresponding to about 67.75 HRC according to standard ASTM E140-12b published in May 2013), advantageously greater than or equal to 940 HV (corresponding to 68 HRC according to standard ASTM E140-12b published in May 2013), in particular greater than or equal to 950 HV, measured according to standard ASTM E384 published in August 2017 or an equivalent standard after a solution treatment at a temperature of 1150° C.
It also has, advantageously, a hardness at a depth of 1 mm greater than or equal to 860 HV (which corresponds to about 66 HRC according to standard ASTM E140-12b published in May 2013), advantageously greater than or equal to 870 HV, in particular greater than or equal to 880 HV, measured according to standard ASTM E384 published in August 2017 or an equivalent standard, in particular after a solution treatment at a temperature of 1100° C. It also has, advantageously, a hardness at a depth of 1 mm 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 standard ASTM E384 published in August 2017 or an equivalent standard.
It also has, advantageously, a level of hardness of the base material (core material hardness) between 440 and 650 HV, advantageously between 440 and 630 HV, measured according to standard ASTM E384 published in August 2017 or an equivalent standard.
The steel composition obtained as a result of these treatments advantageously has a surface concentration of carbon (final surface content) of 1-1.3 wt %.
Said heat treatment may comprise:
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- (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 of 1150° C.,
- (2) advantageously followed by holding at this temperature until completion of austenitization, in particular for 15 minutes (quenching), (these 2 steps (1) and (2) allow complete or partial solution of the carbides initially present),
- (3) and then optionally a first cooling (quenching), in particular under neutral gas, for example at a pressure of 2 bar (2×105 Pa), advantageously to room temperature (this step 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) and then advantageously a second cooling to a temperature below −40° C., more advantageously below −60° C., even more advantageously of about −70° C., in particular for 2 hours (this step makes it possible to decrease the content of residual austenite),
- (6) and advantageously one or more tempering operations, more advantageously at least three tempering operations, advantageously at a temperature greater than or equal to 475° C., more advantageously between 475° C. and 530° C., in particular of 500° C., even more particularly for 1 hour each (this or these tempering operation(s) allow precipitation of carbides and partial or complete decomposition of the residual austenite. This makes it possible to obtain properties of ductility).
The advantage of the steel according to the invention is therefore that of obtaining high levels of hardness with a limited heat treatment (temperature between 1090° C.-1160° C., advantageously between 1100° C.-1160° C., more advantageously between 1100° C.-1155° C., in particular between 1100° C.-1150° C., more particularly of 1150° C.).
In a particularly advantageous embodiment, the steel composition according to the invention has, after thermochemical treatment, advantageously of carburizing or of nitriding or of carbonitriding or of carburizing and then nitriding, followed by a heat treatment, a martensitic structure having a residual austenite content below 10 wt %, more advantageously below 0.5 wt %, and free from ferrite and pearlite, phases that are known to decrease the surface hardness of steel. Said heat treatment may be as described above.
The present invention further relates to a method of manufacturing a steel blank having the composition according to the invention, characterized in that it comprises:
a) a steelmaking step;
b) a step of transformation of the steel;
c) a thermochemical treatment;
d) and a heat treatment.
Advantageously the heat treatment in step d) of the method according to the present invention is as described above.
Advantageously, the thermochemical treatment in step c) of the method according to the present invention consists of a treatment of carburizing or of nitriding or of carbonitriding or of carburizing and then nitriding, advantageously it is a carburizing treatment, more particularly allowing carbon enrichment of the surface, leading to a final surface carbon content of at least 1 wt %, even more advantageously >1.1 wt %.
In particular, step b) of the method according to the present invention consists of a step of rolling, forging and/or extrusion, advantageously forging. These methods are familiar to a person skilled in the art.
In an advantageous embodiment, the steelmaking step a) of the method according to the present invention is carried out by a conventional steelmaking process in an arc furnace with refining and remelting under conductive slag (ESR, electroslag remelting), or by a VIM or VIM-VAR process, optionally with a step of remelting under conductive slag (ESR, electroslag remelting) and/or under vacuum (VAR), or by powder metallurgy such as gas atomization and compaction by hot isostatic pressing (HIP).
Thus, the steel according to the present invention may be produced by a VIM-VAR process. This process makes it possible to obtain very good cleanness with respect to inclusions, and improves the chemical homogeneity of the ingot. It is also possible to proceed by a route of remelting under conductive slag (ESR: ElectroSlag Remelting) or to combine ESR and VAR (vacuum remelting) operations.
This steel may also be obtained by powder metallurgy. This method makes it possible to produce metal powder of great purity by atomization, preferably gas atomization to obtain low oxygen contents. The powder is then compacted for example by hot isostatic pressing (HIP).
These methods are familiar to a person skilled in the art.
The present invention also relates to a steel blank obtainable by the method according to the invention. This blank is made on the basis of steel having the composition according to the present invention and as described above.
It further relates to the use of a blank according to the invention or of a steel composition according to the invention for making a mechanical device or an injection system, advantageously a transmission component such as a gear train, a transmission shaft and/or a rolling bearing and in particular a rolling bearing.
It thus relates to a mechanical device, advantageously a transmission component, in particular a gear train, a transmission shaft or a bearing, more particularly a bearing or a gear train, even more particularly a bearing, made of steel having the composition according to the invention or obtained from a steel blank according to the invention.
It finally relates to an injection system made of steel 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 high surface hardness and resistance to surface wear after thermochemical treatment with a core portion of the material having a high fatigue strength and a high mechanical strength.
These steels are therefore usable in demanding fields such as bearings for aerospace applications or injection systems.
The invention will be better understood on reading the following examples, which are given as a guide and are nonlimiting.
In the examples, unless stated otherwise, all the percentages are expressed by weight, the temperature is expressed in degrees Celsius and the pressure is atmospheric pressure.
1ST SERIES OF EXAMPLESSeven laboratory heats of about 9 kg each (6 examples according to the invention and a comparative example with a composition similar to that in U.S. Pat. No. 8,157,931: comparative example 1) were produced by the VIM process according to the composition shown in Table 1 below (in wt % relative to the total weight of the composition), the remainder being Fe:
The Nb content is below the limit of detection. Nb<0.005% for all the examples.
These compositions are very similar, with the exception of comparative example 1. The notable main differences between comparative example 1 and example 1 relate to the content of V, Mo and Cr.
These laboratory heats were transformed into bars with a diameter of 40 mm by hot forging with a 2000 T press. Rods with a diameter of 20 mm were machined from the bar and carburized.
The carburized rods were treated by (1) a solution treatment at 1100° C. or 1150° C., (2) holding at this temperature for 15 min for austenitization, (3) cooling under neutral gas at a pressure between 2 and 6 bar (2×105 and 6×105 Pa), (4) a period at room temperature, (5) cooling to −70° C. for 2 hours, and (6) 3 tempering operations at a temperature of 500° C. for 1 hour each.
The profiles of surface hardness in HV measured according to standard ASTM E384 published in August 2017 for examples 1 to 6 and comparative example 1 are presented in Tables 2 and 3.
For all the chemical compositions except comparative example 1, the surface hardness after carburizing exceeds 920 HV for a temperature of solution treatment of 1100° C. and exceeds 930 HV for a temperature of solution treatment of 1150° C. The hardness at a depth of 1 mm is always above 860 HV for a temperature of solution treatment of 1100° C. and is always above 880 HV for a temperature of solution treatment of 1150° C. for all the examples except comparative example 1 (effect of the lack of alloying elements).
The hardnesses of the base materials are all below 650 HV.
2ND SERIES OF EXAMPLES2 heats of 100 kg each (one 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 wt % relative to the total weight of the composition), the remainder being Fe:
These laboratory heats were transformed into bars with a diameter of 40 mm by hot forging with a 2000 T press. Rods with a diameter of 20 mm were machined from the bar and carburized.
The carburized rods were treated by the same method as for the first test series apart from the solution treatment, 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 performed on test specimens CT10 according to standard ASTM E399-17 published in February 2018.
Comparative example 2 has delta ferrite after heat treatment, at a low level but sufficient to decrease the toughness properties.
Example 7, very close to comparative example 2 at the level of its composition apart from W, does not have delta ferrite and makes it possible to obtain toughness values almost doubled relative to comparative example 2 while maintaining good mechanical strength (Rm) of about 1500 MPa, which was determined according to standard ASTM E399-17 published in February 2018, equivalent to a core hardness of 450 HV according to standard ASTM E384 published in August 2017.
Claims
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,
- 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 30 nitrogen content being in the range 0.06-0.50.
2. The steel composition as claimed in claim 1, comprising, in percentages by weight of the total composition:
- Carbon: 0.06-0.20;
- Chromium: 3.0-4.5;
- Molybdenum: 4.0-6.0;
- Tungsten 0.01-3.0;
- Vanadium: 1.5-2.5;
- Nickel: 2.0-4.0;
- Cobalt: 9.5-12.5;
- 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;
- Manganese: ≤0.70;
- 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 claim 1, comprising at most 1 wt % of inevitable impurities.
4. The steel composition as claimed in claim 1, wherein the inevitable impurities are selected from titanium, sulfur, phosphorus, copper, tin, lead, oxygen and mixtures thereof.
5. The steel composition as claimed in claim 1, which is carburizable and/or nitridable.
6. The steel composition as claimed in claim 1, which has, after a thermochemical treatment, followed by a heat treatment, a surface hardness above 67 HRC.
7. The steel composition as claimed in claim 1, which has, after a thermochemical treatment, followed by a heat treatment, a martensitic structure having a residual austenite content below 0.5 wt % and free from ferrite and pearlite.
8. The steel composition as claimed in claim 6, wherein the thermal treatment comprises a solution treatment at a temperature between 1090° C.-1160° C. followed by quenching optionally with cooling and several tempering operations at a temperature between 475° C. and 530° C.
9. A method of making a steel blank having the composition as claimed in claim 1, comprising:
- a) a steelmaking step;
- b) a step of transformation of the steel;
- c) a thermochemical treatment;
- d) and a heat treatment.
10. The method of making as claimed in claim 9, wherein step c) consists of a treatment of carburizing or of nitriding or of carbonitriding or of carburizing and then nitriding.
11. The method of making as claimed in claim 9, wherein 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 %.
12. The method of making as claimed in claim 9, wherein step d) comprises a solution treatment at a temperature between 1090° C.-1160° C. followed by holding at this temperature until completion of austenitization optionally with cooling to a temperature below −40° C., and several tempering operations at a temperature between 475° C. and 530° C.
13. The method of making as claimed in claim 9, wherein step b) consists of a step of rolling, forging and/or extrusion.
14. The method of making as claimed in claim 9, wherein the steelmaking step a) is carried out by a conventional steelmaking process in an arc furnace and with refining and remelting under conductive slag (ESR, electroslag remelting), or by a VIM or VIM-VAR process, optionally with a step of remelting under conductive slag (ESR, electroslag remelting) and/or under vacuum (VAR), or by powder metallurgy such as gas atomization and compaction by hot isostatic pressing (HIP).
15. A steel blank obtainable by a method as claimed in claim 9.
16. (canceled)
17. A mechanical device made of steel having the composition as claimed in claim 1.
18. An injection system made of steel having the composition as claimed in claim 1.
19. A mechanical device as claimed in claim 17 which is a transmission component.
20. A mechanical device as claimed in claim 17 which is a transmission component, a bearing, or a gear train.
Type: Application
Filed: Mar 14, 2019
Publication Date: Jan 14, 2021
Inventors: Jacques BELLUS (Saint Genest-Lerpt), Atman BENBAHMED (Cormeilles-en-Parisis), Johanna ANDRE (Uppsala), Fredrik SANDBERG (Uppsala)
Application Number: 16/980,168
