Technical Field The present invention relates generally to the field of metallurgy and to a bearing component made from a steel composition having relatively low carbon content. The steel composition is through-hardenable.

Background

Bearings are devices that permit constrained relative motion between two parts. Rolling element bearings comprise inner and outer raceways and a plurality of rolling elements (balls or rollers) disposed there-between. For long-term reliability and performance it is important that the various elements have resistance to rolling contact fatigue, wear and creep. An important characteristic of bearing steels is the hardenability, i.e. the depth up to which the alloy is hardened after putting it through a heat-treatment process.

A known bearing steel comprises 0.97 wt.% C, 0.32 wt.% Si, 0.31 wt.% Mn, 1 .43 wt.% Cr, the balance being Fe and any unavoidable impurities. Another commercial through- hardenable steel comprises 0.65 wt.% C, 1 .50 wt.% Si, 1.40 wt.% Mn, 1 .10 wt.% Cr, 0.25 wt.% Mo, the balance being Fe and any unavoidable impurities.

Summary The present invention aims to provide a bearing component made from a low carbon bearing steel that can be through-hardened by a simple heat-treatment. In particular, the present invention provides a bearing component made from a steel alloy comprising: from 0.5 to 0.7 wt.% carbon,

from 1 .4 to 1.8 wt.% silicon,

from 1 .7 to 2.1 wt.% manganese,

from 0.05 to 0.4 wt.% molybdenum,

from 1 .1 to 1 .5 wt.% chromium,

from 50 to 200 ppm nitrogen, from 0 to 0.2 wt.% vanadium,

from 0 to 0.25 wt.% nickel,

from 0 to 0.3 wt.% copper,

from 0 to 0.2 wt.% cobalt,

from 0 to 0.05 wt.% aluminium,

from 0 to 0.1 wt.% niobium,

from 0 to 0.2 wt.% tantalum, from 0 to 0.025 wt.% phosphorous,

from 0 to 0.015 wt.% sulphur,

from 0 to 0.075 wt.% tin,

from 0 to 0.075 wt.% antimony,

from 0 to 0.04 wt.% arsenic,

from 0 to 0.002 wt.% lead, up to 15 ppm oxygen,

up to 50 ppm calcium,

up to 20 ppm boron,

up to 30 ppm titanium, the balance iron, together with any unavoidable impurities.

The steel alloy preferably has a martensitic microstructure and may, for example, comprise tempered martensite. The microstructure may optionally include carbides, nitrides and/or carbonitrides. The microstructure may optionally include untransformed "retained" austenite.

The present invention will now be further described. In the following passages different aspects of the invention are defined in more detail. Each aspect so defined may be combined with any other aspect or aspects unless clearly indicated to the contrary. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

In the bearing component according to the present invention, the steel alloy composition comprises from 0.5 to 0.7 wt.% carbon, preferably from 0.55 to 0.65 wt.% carbon, more preferably approximately 0.6 wt.% carbon. In combination with the other alloying elements, this results in the desired martensitic microstructure. The relatively low carbon content impacts positively on the hardenability of the steel, making it particularly suitable for bearing components with relatively large wall thicknesses. In addition, the high hardenability of the steel during austenitisation reduces or eliminates the likelihood of forming the slack-quench phase "Troostite" upon quenching the bearing component. It is believed that Troostite may trigger premature bearing component failure during service, if present. The relatively lower carbon content also means that the steel will possess good toughness and as such good defect-tolerance, and that it is particularly suitable for continuous casting and induction hardening and tempering processes.

In the bearing component according to the present invention, the steel alloy composition comprises from 1.4 to 1.8 wt.% silicon, preferably from 1 .5 to 1 .7 wt.% silicon, more preferably approximately 1 .6 wt.% silicon. In combination with the other alloying elements, this results in the desired microstructure with a minimum amount of retained austenite. Silicon helps to suppress the precipitation of cementite and carbide formation. Also, silicon strengthens the structure and improves resistance to softening caused by tempering.

However, too high a silicon content may result in undesirable surface oxides and a poor surface finish. For this reason, the maximum silicon content is 1 .8 wt.%. Steels with high silicon content tend to retain more austenite in their hardened and tempered structures due to the carbide-suppressing characteristics of the element, which will lower the hardness and have a negative impact on dimensional stability. It follows that the steel concentration of silicon can be reduced to lower the retained austenite content. The relatively high silicon content in the steel alloy used in the present invention has also been found to contribute to good tempering resistance.

In the bearing component according to the present invention, the steel alloy composition comprises from 1.7 to 2.1 wt.% manganese, preferably from 1.8 to 2.0 wt.% manganese, more preferably approximately 1 .9 wt.% manganese. Manganese acts to increase the stability of austenite relative to ferrite. Manganese also acts to improve hardenability.

Manganese may also act to lower the stacking fault energy of the austenite. For these reasons, and in combination with the other alloying elements, the steel alloy according to the present invention comprises at least 1 .7 wt.% manganese and no more than 2.1 wt.% manganese. ln the bearing component according to the present invention, the steel composition comprises from 0.05 to 0.4 wt.% molybdenum, preferably from 0.05 to 0.3 wt.%

molybdenum, more preferably from 0.1 to 0.3 wt.% molybdenum, still more preferably from 0.1 to 0.2 wt.% molybdenum, still more preferably approximately 0.15 wt.% molybdenum. Molybdenum may act to avoid grain boundary embrittlement and has a strong positive impact on hardenability. However, molybdenum is expensive and hence it is desirable, if possible, to limit its content in the alloy. In combination with the other alloying elements, it is possible to limit the molybdenum content to a maximum of 0.4 wt.%, more preferably 0.2 wt.%.

In the bearing component according to the present invention, the steel composition comprises from 1.1 to 1.5 wt.% chromium. Apart from its positive effect on hardenability, the content of chromium was found during thermodynamic calculations to greatly impact the type of carbide obtainable during hardening in that if the concentration is too low, the relatively undesirable cementite phase is stabilised. The alloy therefore comprises at least 1 .1 wt.% chromium, preferably at least 1.2 wt.% chromium. On the other hand, the chromium content must be restricted, for example, to ensure sufficient carbon in solid solution in the austenite phase during hardening. For the austenite to transform into a sufficiently hard structure at lower temperatures it must possess sufficient dissolved carbon and nitrogen. The steel alloy therefore comprises a maximum of 1 .5 wt.% chromium. The steel composition preferably comprises from 1.1 to 1.4 wt.% chromium, more preferably from 1 .2 to 1 .4 wt.% chromium, still more preferably approximately 1 .3 wt.% chromium.

With a lower steel carbon content, the overall percentage of carbides, particularly cementite, which is retained (undissolved) during hardening, is typically low. This has the benefit that there are fewer sites in the alloy microstructure where damage may initiate. The steel toughness is higher leading to more tolerance to micro-defects such as non-metallic inclusions, for example. On the other hand, with fewer carbides retained during

austenitisation, the risk of austenite grain growth, which is detrimental to mechanical properties and fatigue, is higher. However, this aspect can be addressed by, for example, control of the aluminium and nitrogen contents of the steel.

To prevent any possible excessive austenite grain growth during hardening, it is believed beneficial to add micro-alloying additions, and nitrogen, such that small, very fine precipitates that pin the prior austenite grain boundaries are formed. For example, the steel alloy may comprise up to 0.2 wt.% vanadium, for example 0.05 to 0.15 wt.% vanadium, preferably approximately 0.1 wt.% vanadium. Vanadium may form carbides, nitrides and/or carbonitrides. In addition, one or more of the optional elements Ta and/or Nb may be added to the alloy to form carbides, nitrides and/or carbonitrides.

In some embodiments, nitrogen is added such that the steel alloy comprises from 50 to 200 ppm nitrogen, preferably from 75 to 200 ppm nitrogen. In other embodiments, there is no deliberate addition of nitrogen. Nevertheless, the alloy necessarily still comprises at least 50 ppm nitrogen due to exposure to the atmosphere during melting. Nitrogen in an amount of 50 to 200 ppm, in conjunction with the other alloying elements, has been found to be beneficial in terms of achieving the desired microstructure and properties. Preferably, the steel alloy comprises no more than 0.05 wt.% aluminium. Aluminium may be present in the steel in very small amounts due to de-oxidation. However, its content must be controlled to ensure high cleanliness consistent with bearing application requirements.

As noted, the steel composition may also optionally include one or more of the following elements: from 0 to 0.2 wt.% vanadium (for example 0.05 to 0.2 wt.% vanadium)

from 0 to 0.25 wt.% nickel (for example 0.02 to 0.2 wt.% nickel)

from 0 to 0.3 wt.% copper (for example 0.02 to 0.2 wt.% copper)

from 0 to 0.2 wt.% cobalt (for example 0.05 to 0.2 wt.% cobalt)

from 0 to 0.05 wt.% aluminium (for example 0.025 wt.% aluminium)

from 0 to 0.1 wt.% niobium (for example 0.05 to 0.1 wt.% niobium)

from 0 to 0.2 wt.% tantalum (for example 0.05 to 0.2 wt.% tantalum) It will be appreciated that the steel alloy referred to herein may contain unavoidable impurities, although, in total, these are unlikely to exceed 0.3 wt.% of the composition. Preferably, the alloys contain unavoidable impurities in an amount of not more than 0.1 wt.% of the composition, more preferably not more than 0.05 wt.% of the composition. In particular, the steel composition may also include one or more impurity elements. A non- exhaustive list of impurities includes, for example: from 0 to 0.025 wt.% phosphorous

from 0 to 0.015 wt.% sulphur

from 0 to 0.04 wt.% arsenic

from 0 to 0.075 wt.% tin

from 0 to 0.075 wt.% antimony

from 0 to 0.002 wt.% lead

from 0 to 0.002 wt.% boron The steel alloy composition preferably comprises little or no sulphur, for example from 0 to 0.015 wt.% sulphur.

The steel alloy composition preferably comprises little or no phosphorous, for example from 0 to 0.025 wt.% phosphorous.

The steel composition preferably comprises < 15 ppm oxygen. Oxygen may be present as an impurity. The steel composition preferably comprises < 30 ppm titanium. Titanium may be present as an impurity. The steel composition preferably comprises < 20 ppm boron. Boron may be present as an impurity at, for example, 1 -5 ppm.

The steel composition preferably comprises < 50 ppm calcium. Calcium may be present as an impurity but may also be added intentionally in very small amounts, for example 1 -10 ppm or 1 -3 ppm. The steel alloy composition may consist essentially of the recited elements. It will therefore be appreciated that in addition to those elements that are mandatory other non-specified elements may be present in the composition provided that the essential characteristics of the composition are not materially affected by their presence. The bearing component comprises the steel alloy as herein described. Examples of bearing components include a rolling element (e.g. ball, cylinder, tapered or specially-profiled rolling element), an inner ring, and an outer ring. The present invention also provides a bearing comprising a bearing component as herein described. The present invention will now be described further with reference to a suitable heat treatment for the steel alloy, provided by way of example.

After hot-working of the steel bar or ring, for example, hot-rolling and/or hot-forging, the steel microstructure may harden by transforming into martensite during subsequent cooling to room temperature. It follows that a subsequent tempering step may be applied resulting in a tough-tempered structure that is particularly suitable for induction hardening and tempering processes - at a great cost-saving - at least due to not needing to carry out the expensive spheroidise-annealing process. The resulting tough core is beneficial from a performance standpoint. Alternatively, the steel may be spheroidise-annealed from the as hot-worked state, to improve machinability. Afterwards, the steel composition may be austenitised at a temperature in the range of from 850 to 905 ^e C, preferably 875 to 895 ^e C, for example approximately 885 ^e C. The alloy composition favours the formation of austenite that has a specific concentration of the recited alloying elements and this is believed to favour the formation of certain crystal defects, such as twins and stacking faults, once the alloy is quenched/transformed. The alloy may be tempered following quenching. The microstructure of the final alloy typically comprises martensite as the main phase and optionally some retained austenite, metal carbides, nitrides and/or carbonitrides. The microstructure in one embodiment comprises tempered martensite. A noted the microstructure may also include a small amount of unavoidable retained austenite. The microstructure may also include a small fraction of bainitic-ferrite (bainite).

The alloy may also be snap-tempered after the first quench and cooling to room

temperature. Afterwards, the steel may be deep-frozen, or sub-zero treated, followed by a single temper and then air-cooling.

For large bearing components, where the wall thickness is relatively high, a bainitic heat treatment may be adopted. In this case, instead of martensite, bainitic-ferrite is the main phase in the resulting microstructure.

Alternatively, to reduce thermal stresses in large size components, the steel articles may be quenched to temperatures just above the martensite start temperature (M _s ), equilibrated at temperature, then quenched to transform into martensite. During equilibration, a small amount of austenite may transform into bainite (bainitic-ferrite) prior to the final quench; with little or no adverse effects on mechanical properties. The microstructure in such a case may be a mixed bainite and martensite. In any case, as-quenched martensite is preferably always followed by tempering.

The invention will now be explained with reference to the following non-limiting examples and non-limiting drawings, in which.

Brief Description of the Drawings

Figures 1 a) and b) are optical micrographs showing the microstructure of a steel alloy for use in a bearing component according to the present invention.

Figures 2 a) and b) represent calculated continuous cooling transformation (CCT) diagrams for: a) a steel alloy for use in a bearing component according to the present invention; and b) a commercial through-hardenable steel.

Example 1

Steel, comprising in wt.%

C: 0.6

Si: 1 .6

Mn: 1 .9

Mo: 0.1

Cr: 1.35

N:≥ 50 ppm

V: max 0.2

Ni: max 0.25

Cu: max 0.30

P: max 0.025

S: max 0.015

As+Sn+Sb: max 0.075

Pb: max 0.002

Al: max 0.050

Fe: Balance

Oxygen level should be less than 10 ppm, Ti level less than 30 ppm, and Ca level less than The novel steel composition, once austenitised at approximately 885 ^e C, quenched and tempered, has a martensitic microstructure and aims at strengthening the matrix, with significant increase in hardness, by means of facilitating the formation of nanoscale crystal defects, such as, for example, planar defects, such as twins and stacking faults.

The increase in hardness may be achieved without the need for expensive alloying elements, or special heat treatments.

Example 2

An experimental steel melt was prepared by Vacuum Induction Melting (VIM) followed by casting the molten steel into a 100 kg ingot. After discarding the head and the bottom of the ingot, ca. 55 kg were then used. The chemical composition can be seen below: Steel, comprising in wt.%

C: 0.6

Mn: 1 .88

Si: 1 .53

Cr: 1.28

Cu: 0.22

Ni: 0.1

Mo: 0.07

N: 0.0051

Al: 0.0079

P: 0.0045

S: 0.0014

Ti: 0.0007

O: 0.0032

Fe: Balance

The ingot was then sectioned and reheated at 1200 ^° C then hot rolled to a final thickness of 20 mm according to the rolling schedule shown in the table below: Table: Hot rolling schedule

After finish rolling, the plates were then air-cooled to 650 ^° C before being placed in a furnace that was heated to 650 ^° C, where they were allowed to cool in the furnace.

Some of the plate material was then soft-annealed (spheroidising-annealing process) by heating to the temperature of 800 ^° C, soaking at temperature, then the steel was slowly cooled to room temperature (furnace-cooled). This heat treatment process ensures a structure which is sufficiently soft for machinability.

More important perhaps is that the soft-annealing process is also carried out to control the starting microstructure prior to hardening (austenitisation).

Indeed the morphology and size of carbide particles found in the microstructure after the aforementioned heat treatment process determine the dissolution response of said particles during austenitisation, which will determine the matrix chemical composition. In turn, the chemical composition of the austenite matrix during austenitisation will determine the final properties of the bearing steel microstructure. Following the spheroidising-annealing process, four specimens were sectioned and hardened as follows:

1 - Austenitising at 900 ¾ for 45 min at temperature

2 - Oil quench (60 °C) for 15 min

3 - Air-cooling

4 - Freezing at -80 °C for a minimum of 1 h

5 - Immediately tempering at 160°C for 2 h 6 - Cooling to room temperature

7 - Freezing at -80 °C for 1 h minimum

8 - Immediately tempering at 160°C for 2 h

9 - Air-cooling

The resulting microstructure was tempered martensite with very little carbides that could be resolved under the visible light optical microscope, see Figures 1 a) and b).

Figures 2 a) and b) represent the calculated CCT diagrams of the steel composition of: Example 2 in a); and that of a commercial, through-hardenable steel in b), the composition of which is provided below.

Commercial through-hardenable steel comprising in wt.%. Figure 2 b)

C: 0.65

Mn: 1 .4

Si: 1 .5

Cr: 1.1

Mo: 0.25

Fe: 95.1

Both CCT diagrams show similarities, however, the steel composition of Example 2, Figure 2a), contains a higher Mn content, but far less molybdenum, which is an expensive element.

The Transitions ( ^e C) in Figure 2a) (Example 2) are as follows:

Pearlite: 757.9

Bainite: 436.7

Ferrite: 739.7

Martensite:

Start: 190.2

50%: 150.0

90%: 56.5

The Transitions ( ^e C) in Figure 2b) (commercial through-hardenable steel) are as follows:

Pearlite: 767.8

Bainite: 451 .8 Ferrite: 749.6

Martensite:

Start: 194.6

50%: 154.6

90%: 61 .6

The hardness measured at room temperature was 63.0±0.4 HRC for one specimen and 63.4±0.1 HRC for the second. The load used during the measurement was 150 kgf. The steel alloy microstructure also exhibits good tempering resistance and withstands service at high temperatures. Thanks to the steel's relatively low carbon concentration, it could be austenitised at somewhat high temperatures without negatively impacting its toughness. The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art, and remain within the scope of the appended claims and their equivalents.