U.S. patent application number 16/980168 was filed with the patent office on 2021-01-14 for steel composition.
The applicant listed for this patent is AUBERT & DUVAL, ERASTEEL. Invention is credited to Johanna ANDRE, Jacques BELLUS, Atman BENBAHMED, Fredrik SANDBERG.
Application Number | 20210010116 16/980168 |
Document ID | / |
Family ID | 1000005164916 |
Filed Date | 2021-01-14 |
United States Patent
Application |
20210010116 |
Kind Code |
A1 |
BELLUS; Jacques ; et
al. |
January 14, 2021 |
STEEL COMPOSITION
Abstract
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: .ltoreq.2.0; Nitrogen: .ltoreq.0.50,
preferably .ltoreq.0.20; Silicon: .ltoreq.0.70, preferably
0.05-0.50; Manganese: .ltoreq.0.70, preferably 0.05-0.50; Aluminum:
.ltoreq.0.15, preferably .ltoreq.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.
Inventors: |
BELLUS; Jacques; (Saint
Genest-Lerpt, FR) ; BENBAHMED; Atman;
(Cormeilles-en-Parisis, FR) ; ANDRE; Johanna;
(Uppsala, SE) ; SANDBERG; Fredrik; (Uppsala,
SE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
AUBERT & DUVAL
ERASTEEL |
Paris
PARIS |
|
FR
FR |
|
|
Family ID: |
1000005164916 |
Appl. No.: |
16/980168 |
Filed: |
March 14, 2019 |
PCT Filed: |
March 14, 2019 |
PCT NO: |
PCT/FR2019/050573 |
371 Date: |
September 11, 2020 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C21D 8/005 20130101;
C22C 38/06 20130101; C21D 9/40 20130101; C23C 8/26 20130101; C22C
38/52 20130101; C23C 8/22 20130101; C21D 2211/001 20130101; C22C
38/12 20130101; C22C 38/001 20130101; C21D 9/32 20130101; C21D
2211/008 20130101; C23C 8/32 20130101; C22C 38/02 20130101 |
International
Class: |
C22C 38/52 20060101
C22C038/52; C22C 38/12 20060101 C22C038/12; C22C 38/00 20060101
C22C038/00; C22C 38/02 20060101 C22C038/02; C22C 38/06 20060101
C22C038/06; C21D 9/40 20060101 C21D009/40; C21D 9/32 20060101
C21D009/32; C21D 8/00 20060101 C21D008/00; C23C 8/22 20060101
C23C008/22; C23C 8/26 20060101 C23C008/26; C23C 8/32 20060101
C23C008/32 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 14, 2018 |
FR |
1852202 |
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: .ltoreq.2.0; Nitrogen:
.ltoreq.0.50; Silicon: .ltoreq.0.70; Manganese: .ltoreq.0.70;
Aluminum: .ltoreq.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:
.ltoreq.2.0; Nitrogen: .ltoreq.0.20; Silicon: .ltoreq.0.70;
Manganese: .ltoreq.0.70; Aluminum: .ltoreq.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.degree. C.-1160.degree. C. followed by quenching optionally
with cooling and several tempering operations at a temperature
between 475.degree. C. and 530.degree. 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.degree. C.-1160.degree. C. followed by holding at this
temperature until completion of austenitization optionally with
cooling to a temperature below -40.degree. C., and several
tempering operations at a temperature between 475.degree. C. and
530.degree. 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.
Description
[0001] 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.
[0002] 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.
[0003] 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).
[0004] 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.
[0005] There are two types of metallurgy for providing the final
mechanical properties.
[0006] 1st Type: the chemical composition of the component makes it
possible to obtain the mechanical properties directly after
suitable heat treatment.
[0007] 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.
[0008] 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.
[0009] 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.
[0010] 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: [0011] Carbon:
0.2-2.0; [0012] Chromium: 1.0-9.0; [0013] Molybdenum: 1.0-9.0;
[0014] Silicon: 0.1-1.0; [0015] Tungsten: 1.0-3.0; [0016] Vanadium:
0.1-1.0; [0017] Nickel+Cobalt+Copper: 3.0-15.0; [0018] Iron:
remainder
[0019] 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.
[0020] Patent application WO2015/082342 describes a rolling bearing
steel having the following composition, in percentages by weight of
the total composition: [0021] Carbon: 0.05-0.5; [0022] Chromium:
2.5-5.0; [0023] Molybdenum: 4-6; [0024] Tungsten: 2-4.5; [0025]
Vanadium: 1-3; [0026] Nickel: 2-4; [0027] Cobalt: 2-8; [0028] Iron:
remainder
[0029] as well as the inevitable impurities, optionally further
comprising one or more of the following elements: [0030] Niobium:
0-2; [0031] Nitrogen: 0-0.5; [0032] Silicon: 0-0.7; [0033]
Manganese: 0-0.7; [0034] Aluminum: 0-0.15;
[0035] 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.degree. C. and tempering at
560.degree. 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%.
[0036] Patent application WO2017216500 describes a rolling bearing
steel having the following composition, in percentages by weight of
the total composition: [0037] Carbon: 0.05-0.40, preferably
0.10-0.30; [0038] Chromium: 2.50-5.00, preferably 3.0-4.5; [0039]
Molybdenum: 4.0-6.0; [0040] Tungsten: 0.01-1.8, preferably
0.02-1.5; [0041] Vanadium: 1.0-3.0, preferably 1.5-2.5; [0042]
Nickel: 2.0-4.0; [0043] Cobalt: 2.0-8.0, preferably 3.0-7.0; [0044]
Iron: remainder
[0045] as well as the inevitable impurities,
[0046] optionally further comprising one or more of the following
elements: [0047] Niobium: .ltoreq.2.0; [0048] Nitrogen:
.ltoreq.0.50, preferably .ltoreq.0.20; [0049] Silicon:
.ltoreq.0.70, preferably 0.05-0.50; [0050] Manganese: .ltoreq.0.70,
preferably 0.05-0.50; [0051] Aluminum: .ltoreq.15, preferably
.ltoreq.0.10;
[0052] 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.
[0053] 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.degree. C.-1150.degree. C.
and tempering at 500.degree. 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%.
[0054] Obtaining surface hardnesses above 67 HRC, in particular
using a solution heat treatment at a temperature less than or equal
to 1160.degree. C., is therefore difficult to achieve, whereas they
would allow significant improvement of the properties of the
component.
[0055] 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.degree.
C.-1160.degree. C. and tempering at a temperature greater than or
equal to 475.degree. C., while displaying a level of hardness of
the base material between 400 and 650 HV.
[0056] 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.
[0057] 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.degree. C. and tempering at
482.degree. C.), to a surface hardness that is interesting, but
decreases very quickly with depth, thus starting from 600 .mu.m 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).
[0058] 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.
[0059] 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: [0060] Carbon: max. 0.05; [0061]
Chromium: 2.5-5.5; [0062] Tungsten equivalent (2.times.Mo+W):
12.5-20; [0063] Vanadium: max. 1.5; [0064] Nickel: max. 5.0; [0065]
Cobalt: max. 20.0; [0066] Silicon: 0.15-1.0 [0067] Manganese:
0.15-1.5 [0068] Iron: remainder
[0069] This grade is submitted to carburizing or
carbonitriding.
[0070] 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.
[0071] 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.
[0072] 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.
[0073] 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.
[0074] 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: [0075] Carbon: 0.06-0.20
preferably 0.08-0.18; [0076] Chromium: 2.5-5.0, preferably 3.0-4.5;
[0077] Molybdenum: 4.0-6.0; [0078] Tungsten: 0.01-3.0; [0079]
Vanadium: 1.0-3.0, preferably 1.50-2.50; [0080] Nickel: 2.0-4.0;
[0081] Cobalt: 9.0-12.5, preferably 9.5-11.0; [0082] Iron:
remainder
[0083] as well as the inevitable impurities,
[0084] optionally further comprising one or more of the following
elements: [0085] Niobium: .ltoreq.2.0; [0086] Nitrogen:
.ltoreq.0.50, preferably .ltoreq.0.20; [0087] Silicon:
.ltoreq.0.70, preferably 0.05-0.50; [0088] Manganese: .ltoreq.0.70,
preferably 0.05-0.50; [0089] Aluminum: .ltoreq.0.15, preferably
.ltoreq.0.10;
[0090] 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.
[0091] A particularly interesting composition comprises,
advantageously consists essentially of, in particular consists of,
in percentages by weight of the total composition: [0092] Carbon:
0.06-0.20, preferably 0.08-0.18; [0093] Chromium: 3.0-4.5,
preferably 3.5-4.5; [0094] Molybdenum: 4.0-6.0, preferably 4.5-5.5;
[0095] Tungsten 0.01-3.0; [0096] Vanadium: 1.5-2.5, preferably
2.0-2.3; [0097] Nickel: 2.0-4.0, preferably 2.5-3.5; [0098] Cobalt:
9.5-12.5, preferably 9.5-10.5; [0099] Iron: remainder
[0100] as well as the inevitable impurities,
[0101] optionally further comprising one or more of the following
elements: [0102] Niobium: .ltoreq.2.0; [0103] Nitrogen:
.ltoreq.0.20; [0104] Silicon: .ltoreq.0.70, preferably 0.05-0.50;
[0105] Manganese: .ltoreq.0.70, preferably 0.05-0.50; [0106]
Aluminum: .ltoreq.0.10;
[0107] 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.
[0108] 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.
[0109] 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.
[0110] 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.
[0111] 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.
[0112] 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.
[0113] Chromium contributes to the formation of carbides in steel
and is one of the main elements controlling the hardenability of
steels.
[0114] 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.
[0115] 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.
[0116] 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.
[0117] 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.
[0118] 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.
[0119] 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.
[0120] 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.
[0121] 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.
[0122] 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.
[0123] 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.
[0124] 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.
[0125] 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.
[0126] 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.
[0127] The steel composition according to the invention may further
comprise silicon (Si) in a content .ltoreq.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.
[0128] 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.
[0129] The steel composition according to the invention may further
comprise manganese (Mn) in a content .ltoreq.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.
[0130] 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.
[0131] Optionally, the steel composition according to the invention
may comprise nitrogen (N), in a content .ltoreq.0.50%, preferably
.ltoreq.0.20%, by weight relative to the total weight of the
composition.
[0132] 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.
[0133] Optionally, the steel composition according to the invention
may comprise aluminum (Al), in a content .ltoreq.0.15%, preferably
.ltoreq.0.10%, by weight relative to the total weight of the
composition.
[0134] 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.
[0135] 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.
[0136] 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.
[0137] If nitriding is used, it is the nitrogen content that
increases at the surface of the steel, and therefore also the
surface hardness.
[0138] 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.
[0139] These methods are familiar to a person skilled in the
art.
[0140] 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.degree. 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.degree. C.
[0141] 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.degree. 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.
[0142] 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.
[0143] 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 %.
[0144] Said heat treatment may comprise: [0145] (1) solution
treatment of the steel at a temperature between 1090.degree.
C.-1160.degree. C., advantageously between 1100.degree.
C.-1160.degree. C., more advantageously between 1100 and
1155.degree. C., in particular between 1100 and 1150.degree. C.,
more particularly of 1150.degree. C., [0146] (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), [0147] (3) and then optionally a first
cooling (quenching), in particular under neutral gas, for example
at a pressure of 2 bar (2.times.10.sup.5 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), [0148] (4) optionally
followed by holding at room temperature, [0149] (5) and then
advantageously a second cooling to a temperature below -40.degree.
C., more advantageously below -60.degree. C., even more
advantageously of about -70.degree. C., in particular for 2 hours
(this step makes it possible to decrease the content of residual
austenite), [0150] (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.degree. C., more advantageously between 475.degree. C. and
530.degree. C., in particular of 500.degree. 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).
[0151] 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.degree. C.-1160.degree.
C., advantageously between 1100.degree. C.-1160.degree. C., more
advantageously between 1100.degree. C.-1155.degree. C., in
particular between 1100.degree. C.-1150.degree. C., more
particularly of 1150.degree. C.).
[0152] 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.
[0153] 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:
[0154] a) a steelmaking step;
[0155] b) a step of transformation of the steel;
[0156] c) a thermochemical treatment;
[0157] d) and a heat treatment.
[0158] Advantageously the heat treatment in step d) of the method
according to the present invention is as described above.
[0159] 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 %.
[0160] 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.
[0161] 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).
[0162] 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.
[0163] 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).
[0164] These methods are familiar to a person skilled in the
art.
[0165] 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.
[0166] 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.
[0167] 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.
[0168] 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.
[0169] 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.
[0170] These steels are therefore usable in demanding fields such
as bearings for aerospace applications or injection systems.
[0171] The invention will be better understood on reading the
following examples, which are given as a guide and are
nonlimiting.
[0172] 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 EXAMPLES
[0173] Seven 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:
TABLE-US-00001 TABLE 1 Element C Ni Cr Mo V W Co Si Mn Al N Example
1: 0.18 3.1 3.9 5.1 2.1 1.18 10.0 0.2 0.18 0.023 0.005 GRADE A
Example 2: 0.20 3.1 3.9 5.1 2.2 2.96 10.1 0.18 0.21 0.02 0.009
GRADE B Example 3: 0.16 3.1 3.9 5.1 2.1 1.19 10.0 0.21 0.18 0.02
0.009 GRADE C Example 4: 0.16 3.0 4.0 5.1 2.1 2.92 10.1 0.22 0.25
0.016 0.005 GRADE D Example 5: 0.16 3.1 3.9 5.0 2.1 0.01 10.0 0.123
0.2 0.042 0.005 GRADE E Example 6: 0.17 3.1 4.0 5.2 2.2 0.01 12.4
0.17 0.2 0.038 0.006 GRADE F Comparative 0.14 3.1 2.1 2.7 1.2 1.32
10.0 0.222 0.16 0.022 0.004 example 1: GRADE G
[0174] The Nb content is below the limit of detection. Nb<0.005%
for all the examples.
[0175] 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.
[0176] 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.
[0177] The carburized rods were treated by (1) a solution treatment
at 1100.degree. C. or 1150.degree. 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.times.10.sup.5 and
6.times.10.sup.5 Pa), (4) a period at room temperature, (5) cooling
to -70.degree. C. for 2 hours, and (6) 3 tempering operations at a
temperature of 500.degree. C. for 1 hour each.
[0178] 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.
TABLE-US-00002 TABLE 2 (solution treatment at 1100.degree. C.) Core
Hardness material at depth Surface Example hardness of 1 mm
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: 576 835 847 GRADE G
TABLE-US-00003 TABLE 3 (solution treatment at 1150.degree. C.) Core
Hardness material at depth Surface Example hardness of 1 mm
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: 585 868 878 GRADE G
[0179] 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.degree. C. and exceeds
930 HV for a temperature of solution treatment of 1150.degree. C.
The hardness at a depth of 1 mm is always above 860 HV for a
temperature of solution treatment of 1100.degree. C. and is always
above 880 HV for a temperature of solution treatment of
1150.degree. C. for all the examples except comparative example 1
(effect of the lack of alloying elements).
[0180] The hardnesses of the base materials are all below 650
HV.
2ND SERIES OF EXAMPLES
[0181] 2 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:
TABLE-US-00004 TABLE 4 Element C Ni Cr Mo V W Co Si Mn Al N Example
7: 0.06 3.2 3.9 4.8 2.1 1.1 10.2 0.16 0.14 GRADE H Comparative 0.05
3.1 3.8 5.0 2.1 2.8 10.0 0.17 0.14 example 2: GRADE I
[0182] 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.
[0183] 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.degree. C. and the triple tempering, which was
carried out at 525.degree. 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.
TABLE-US-00005 TABLE 5 Toughness Mechanical Example (MPa m)
strength (MPa) Example 7 44-60 1400-1700 Comparative example 2 35
1500
[0184] Comparative example 2 has delta ferrite after heat
treatment, at a low level but sufficient to decrease the toughness
properties.
[0185] 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.
* * * * *