Technical Field The present invention relates generally to the field of metallurgy. More specifically, the present invention relates to a steel alloy and a method of heat- treating a steel alloy, which may be used in the manufacture of, for example, bearings. 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 (for example balls and/or rollers) disposed therebetween. For long-term reliability and performance it is important that the various elements have a high resistance to rolling fatigue, wear and creep.

Conventional techniques for manufacturing metal components involve hot-rolling or hot-forging to form a bar, rod, tube or ring, followed by a soft forming process to obtain the desired component. Surface hardening processes are well known and are used to locally increase the hardness of surfaces of finished or semifinished components so as to improve, for example, wear resistance and fatigue resistance. A number of surface or case hardening processes are known for improving rolling contact fatigue resistance.

An alternative to case-hardening is through-hardening. Through-hardened components differ from case-hardened components in that the hardness is uniform or substantially uniform throughout the component. Through-hardened components are also generally cheaper to manufacture than case-hardened components because they avoid the complex heat-treatments associated with carburizing, for example.

For through-hardened bearing steel components, two heat-treating methods are available: martensite hardening or austempering. Component properties such as toughness, hardness, microstructure, retained austenite content, and dimensional stability are associated with or affected by the particular type of heat- treatment employed. The martensite through-hardening process involves austenitising the steel prior to quenching below the martensite start temperature. The steel may then be low- temperature tempered to stabilize the microstructure.

The bainite through-hardening process involves austenitising the steel prior to quenching above the martensite start temperature. Following quenching, an isothermal bainite transformation is performed. Bainite through-hardening is sometimes preferred in steels instead of martensite through-hardening. This is because a bainitic structure may possess superior mechanical properties, for example toughness and crack propagation resistance.

Bainitic steel structures are produced by the transformation of austenite to bainitic-ferrite at intermediate temperatures of typically from 190 to 500°C. The cooling of the austenite leads to a microstructure comprising ferrite, carbides and retained austenite. Bainite itself comprises a structure of supersaturated ferrite containing particles of carbide, the dispersion of the latter depending on the formation temperature. The hardness of bainite is usually somewhere intermediate between that of pearlite and martensite.

The steel known as SP10 has the following chemical composition: Fe-0.8C-1.5Si- 2Mn-1AI-1Cr-0.25Mo-1.5Co in wt. %. Austenitisation followed by bainite hardening (200 °C, 72 hours) results in a fine microstructure comprising retained austenite and bainitic ferrite. However, the hardness and dimensional stability of this alloy are deemed too low for bearing applications. It is an object of the present invention to address some of the problems associated with the prior art, or at least to provide a commercially useful alternative thereto. Summary of the Invention

According to a first aspect, there is provided a steel alloy comprising: from 0.5 to 1.2 wt. % carbon,

from 1 to 2 wt. % silicon,

from 0.25 to 2.2 wt. % manganese,

from 0.85 to 2 wt. % chromium,

from 0.5 to 5 wt. % cobalt,

from 0.1 to 0.6 wt. % molybdenum,

from 100 to 350 ppm nitrogen, optionally up to 0.3 wt.% vanadium, and/or

up to 2 wt. % aluminium, and/or

up to 250 ppm zirconium, and/or

up to 0.1 wt. % niobium, and/or

up to 0.2 wt. % tantalum, and/or

up to 0.005 wt. % calcium, and the balance iron together with unavoidable impurities.

The steel alloy may exhibit high hardness and/or dimensional stability. This means that the steel alloy can usefully find application in the manufacture of, for example, a bearing component such as, for example, an inner or outer raceway. The steel alloy is typically a bearing steel alloy.

Without being bound by theory, it is considered that a number of particles may form during austenitisation of the steel such as, for example, carbides (e.g. vanadium carbide), nitrides (e.g. aluminium nitride) and carbonitrides (e.g. vanadium carbonitride). However, vanadium-rich precipitates, for example, usually form at lower temperatures, which dictates proper control of the hot- working schedule of the steel. Such particles may act to refine the grain size of the resulting austenite grains.

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. The term "martensite start temperature" as used herein refers to the temperature at which the transformation from austenite to martensite begins on cooling. The martensite start temperature is typically denoted M _s .

The term "bainite start temperature" as used herein refers to the highest temperature at which ferrite can transform by a displacive transformation. The bainite start temperature is typically denoted B _s .

The steel alloy composition comprises from 0.5 to 1 .2 wt. % carbon, preferably from 0.7 to 0.9 wt. % carbon, more preferably about 0.8 wt. % carbon. In combination with the other alloying elements, this results in the desired fine bainitic structure. Carbon acts to lower the bainite transformation temperature so that a fine structure is achievable. The presence of carbon may result in the formation of carbides and/or carbonitrides during austenitisation, which may act as austenite grain refiners. When the carbon content is higher than 1 .2 wt. % there is a reduction in the maximum volume fraction of the bainitic ferrite portion of the microstructure. When the carbon content is lower than 0.5 wt. % the alloys have a higher martensite start temperature.

The steel alloy composition comprises from 1 to 2 wt. % silicon, preferably from 1 .3 to 1 .7 wt. % silicon, more preferably about 1 .5 wt. % silicon. In combination with the other alloying elements, this results in the desired fine bainitic structure with a minimum amount of retained austenite. Silicon helps to suppress the precipitation of cementite and carbide formation. 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 2 wt. %. Silicon may also promote the formation of finely dispersed vanadium carbides.

The alloy composition comprises from 0.25 to 2.2 wt. % manganese, preferably from 1.8 to 2.2 wt. % manganese, more preferably about 2 wt. % manganese. Manganese acts to increase the stability of austenite relative to ferrite. However, manganese levels above 2.2 wt. % may serve to increase the amount of retained austenite and to decrease the rate of transformation to bainite. The steel alloy composition comprises from 0.85 to 2 wt. % chromium, preferably from 0.9 to 1.1 wt. % chromium, more preferably about 1 wt. % chromium. Chromium acts to increase hardenability and reduce the bainite start temperature. Chromium may also be beneficial in terms of corrosion resistance. The steel alloy composition comprises from 0.5 to 5 wt. % cobalt. In one embodiment, the steel alloy composition preferably comprises from 1.3 to 1.7 wt. % cobalt, more preferably about 1.5 wt. % cobalt. In an alternative embodiment, the steel alloy composition preferably comprises from 3.5 to 4.5 wt. % cobalt, more preferably about 4 wt. % cobalt. Cobalt has been found to improve the corrosion resistance of the steel alloy. This is very important if the steel alloy is used in bearing components for wind turbines or marine pods, for example. Such bearings may become contaminated by sea water, which can drastically reduce the service life of the bearing. The addition of cobalt further increases the rate of superbainite formation.

The steel composition comprises from 0.1 to 0.6 wt. % molybdenum, preferably from 0.2 to 0.3 wt. % molybdenum, more preferably about 0.25 wt. % molybdenum. Molybdenum acts to avoid austenite grain boundary embrittlement owing to impurities such as, for example, phosphorus. Molybdenum also acts to increase hardenability and reduce the bainite start temperature. The molybdenum content in the alloy is preferably no more than about 0.6 wt. %, otherwise the austenite transformation into bainitic ferrite may cease too early, which can result in significant amounts of austenite being retained in the structure. The steel alloy comprises from 100 to 350 ppm nitrogen. The presence of from 100 to 350 ppm nitrogen, together with the other alloying elements in their recited ranges, may result in the formation of metal nitrides during austenitisation. Such metal nitrides may act as austenite grain refiners. Examples of metal nitrides include, for example, vanadium carbide, aluminium nitride and/or vanadium carbonitride.

When the steel alloy comprises aluminium, the nitrogen content is preferably from 250 to 300 ppm. Such a nitrogen level may lead to the formation of nitrides (e.g. AIN) in a particularly suitable size distribution, fraction and space distribution to result in enhanced austenite grain refinement.

When the steel alloy is substantially aluminium free or contains very low levels of aluminium (for example, less than 0.004 wt. % aluminium or less than 0.002 wt. % aluminium), the nitrogen content is preferably from 100 to 300 ppm nitrogen, more preferably from 150 to 300 ppm nitrogen. Such a nitrogen level may lead to the formation of nitrides in a particularly suitable size distribution, fraction and space distribution to result in enhanced austenite grain refinement.

The steel alloy optionally comprises up to 0.3 wt. % vanadium. Preferably, the steel alloy comprises from 0.02 to 0.2 wt. % vanadium, more preferably from 0.05 to 0. 5 wt. % vanadium, even more preferably about 0.1 wt. % vanadium. The presence of vanadium in the recited ranges may result in increased austenite grain refinement (for example, due to the formation of vanadium carbides and/or vanadium carbonitrides). Vanadium may also result in further reduction of the grain boundary cementite network and therefore an improvement of the mechanical properties of the steel alloy. The steel alloy composition may optionally comprise up to 2 wt. % aluminium. Preferably the steel alloy composition comprises from 0.9 to 1.1 wt. % aluminium, more preferably about 1 wt. % aluminium. Aluminium has been found to improve the intrinsic toughness of the bearing component, possibly due to it suppressing carbide formation. Aluminium may also serve as a deoxidiser. However, the use of aluminium requires stringent steel production controls to ensure cleanliness and this increases the processing costs. In one embodiment, the steel alloy composition comprises substantially no aluminium. In another embodiment, the steel alloy contains very low levels of aluminium such as, for example, 0.005 wt. % or less, 0.004 wt. % or less, 0.003 wt. % or less, or 0.002 wt. % or less. Aluminium may be added in such low levels merely for the purpose of de- oxidation.

The steel alloy optionally comprises zirconium. Preferably, the steel alloy comprises from 50 to 200 ppm zirconium, more preferably from 100 to 200 ppm zirconium. The presence of zirconium in the recited ranges may result in the introduction of homogeneously distributed nucleation sites (for example, fine zirconium oxides) prior to nucleation of metal nitride and/or vanadium rich precipitates. Accordingly, refinement of the austenite grains may be further increased. Zirconium should be added after the main de-oxidation of the steel alloy when only approximately from 20 to 40 ppm oxygen remains in the steel alloy. Typically, the steel does not contain both vanadium and zirconium.

The steel alloy composition may comprise up to 0.1 wt. % niobium, preferably from 0.001 to 0.05 wt. % niobium, more ^" preferably from 0.001 to 0.01 wt. % niobium; and/or up to 0.2 wt. % tantalum, preferably from 0.001 to 0.2 wt. % tantalum. Niobium and tantalum may act to control the austenite grain size. Accordingly, niobium and/or tantalum may be added to the steel alloy to at least partially replace vanadium. Therefore, when the steel alloy comprises niobium and/or tantalum, the sum of vanadium, niobium and tantalum is preferably not greater than 0.2 wt. %.

The steel alloy may comprise up to 0.005 wt. % calcium, for example from 0.001 to 0.005 wt. % calcium, or from 0.001 to 0.0035 wt. % calcium. Calcium may serve as a deoxidiser and/or desulphuriser. The addition of calcium to the steel alloy after the sulphur content has been reduced to a level substantially of the same order as the oxygen content, may reduce the total number of sulphide inclusions remaining in the steel and it will modify the shape of the remaining inclusions into one that is less detrimental to mechanical properties. As a result, the addition of calcium may serve to provide favourable fatigue properties and may also provide the steel alloy with more uniform properties in all directions, thereby reducing directional anisotropy in the steel alloy.

It will be appreciated that the steel alloy referred to herein may contain unavoidable impurities, although, in total, these are unlikely to exceed 0.5 wt. % of the composition. Preferably, the alloys contain unavoidable impurities in an amount of not more than 0.3 wt. % of the composition, more preferably not more than 0.1 wt. % of the composition. Phosphorous and sulphur are preferably kept to a minimum.

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.

In a preferred embodiment the steel alloy composition may comprise:

from 0.7 to 0.9 wt. % carbon,

from 1.3 to 1 .7 wt. % silicon,

from 1.8 to 2.2 wt. % manganese,

from 0.9 to 1.1 wt. % chromium,

from 1 .3 to 1.7 wt. % cobalt,

from 0.2 to 0.3 wt. % molybdenum,

from 100 to 350 ppm nitrogen,

optionally from 0.05 to 0.15 wt. % vanadium,

optionally from 0.9 to 1.1 wt. % aluminium,

optionally from 0.001 to 0.05 wt. % niobium,

and the balance iron together with unavoidable impurities. In a preferred embodiment the steel alloy composition may comprise:

from 0.7 to 0.9 wt. % carbon,

from 1.3 to 1.7 wt. % silicon,

from 1.8 to 2.2 wt. % manganese,

from 0.9 to 1.1 wt. % chromium, from 3.5 to 4.5 wt. % cobalt,

from 0.2 to 0.3 wt. % molybdenum,

from 100 to 350 ppm nitrogen,

optionally from 0.05 to 0.15 wt. % vanadium,

optionally from 0.9 to 1.1 wt. % aluminium,

optionally from 0.001 to 0.05 wt. % niobium,

and the balance iron together with unavoidable impurities.

The microstructure of the resulting steel alloy typically comprises nano-structured bainitic ferrite and retained austenite. The microstructure is typically substantially carbide-free. The microstructure may optionally contain some tempered martensite.

In greater detail, the microstructure of the resulting steel alloy typically comprises at least 60 vol.% bainlte, more typically at least 80 vol.% bainite, still more typically at least 90 vol.% bainite (bainitic-ferrite). The bainite is preferably lower bainite and preferably has a very fine structure. In particular, the material preferably has a microstructure comprising plates of bainitic-ferrite of less than 200 nm, typically from 10 to 100 nm, more typically from 20 to 80 nm. The plates of bainitic-ferrite are typically interspersed with retained austenite. The bainite typically forms at least 60% of the microstructure (by volume), more typically at least 80%, still more typically at least 90%. The microstructure may also contain small carbide, nitride and/or carbo-nitride precipitates, for example nano-scale precipitates, typically 5 -30 nm average size. Any such precipitates typically make up no more than 5 vol%, more typically no more than 3 vol% of the microstructure, for example from 0.5 to 3 vol%.

The structure of the steel alloys may be determined by conventional microstructural characterisation techniques such as, for example, optical microscopy, TEM, SEM, AP-FIM, and X-ray diffraction, including combinations of two or more of these techniques.

According to a further aspect, there is provided a method of heat-treating a steel alloy comprising: (i) providing a steel alloy composition comprising:

from 0.5 to 1.2 wt. % carbon,

from 1 to 2 wt. % silicon,

from 0.25 to 2.2 wt. % manganese,

from 0.85 to 2 wt. % chromium,

from 0.5 to 5 wt. % cobalt,

from 0.1 to 0.6 wt. % molybdenum,

from 100 to 350 ppm nitrogen, optionally up to 0.3 wt, % vanadium, and/or

up to 2 wt. % aluminium, and/or

up to 250 ppm zirconium, and/or

up to 0.1 wt. % niobium, and/or

up to 0.2 wt. % tantalum, and/or

up to 0.005 wt. % calcium, and the balance iron together with unavoidable impurities.

(ii) heating the composition to a temperature of from 800 to 950 °C to at least partially austenise the composition;

(iii) quenching the composition to a first temperature T1 , wherein 0.7M _S ≤ T1 < 1.3M _S , M _s being the martensite start temperature of the austenite composition; and optionally (iv) heating the composition to a second temperature T2 below the bainite start temperature of the austenite composition B _s .

The resulting alloy exhibits high hardness and/or dimensional stability. This means that it can usefully find application in the manufacture of, for example, a bearing component such as, for example, an inner or outer raceway. The microstructure of the resulting steel alloy typically comprises nano-structured bainitic ferrite and retained austenite. Steps (iii) and (iv) of the method of the present invention typically result in bainite transformation. This bainite transformation is typically carried out at a temperature of less than 300 °C, more typically less than 280 °C. One result of the low transformation temperature is that the plates of bainitic-ferrite are very fine. In particular, the material preferably has a microstructure comprising plates of bainitic-ferrite of less than 200 nm, typically from 10 to 100 nm, more typically from 20 to 80 nm.

Following steps (i) to (iii), the steel alloy composition is heated to a second temperature T2 below the bainite start temperature of the austenite composition B _s . This heating step (iv) results in an acceleration of the bainite transformation kinetics. As a result of this acceleration, for the same transformation time at temperature, the final steel alloy typically contains less retained austenite. This results in increased strength and hardness, and better dimensional stability. The dimensional stability is critical when the steel alloy is in the form of a bearing component, which operate at warm-to-elevated temperatures, typically 80°C and above. The amount of- retained austenite is typically less than 20 vol.%, more typically less than 10 vol.%, even more typically less than 8 vol%. In one embodiment the amount of retained austenite is about 6 vol%.

In addition, the acceleration of the bainite transformation kinetics may result in a shorter transformation time for a given retained austenite content in the final alloy structure. For example, in comparison to a conventional heat treatment (for example, austenitisation followed by heating at 200 °C for 72 hours), the overall bainite transformation time of the method of the present invention may be reduced by at least 12 hours. This may result in significant cost and time savings. In step (ii), the composition is heated to a temperature of 800 to 950 °C to at least partially austenise the composition. In a typical embodiment, the composition may be heated to a temperature of from 850 to 925 °C, more typically from 880 to 920 °C. The composition is typically held at such temperatures for at least 15 minutes, typically less than 60 minutes, more typically for about 30 minutes. In one embodiment, T1 is above the martensite start temperature. This may result in deformation of the residual austenite, i.e. the induction of internal stresses. During the subsequent step (iv), the bainite transformation may be substantially accelerated. Accordingly, in comparison to a conventional bainite transformation step of heating at 200 °C for 72 hours, the overall bainite transformation time of the method described herein may be particularly shortened. In this embodiment, T1 is preferably from 190 to 210 °C, more preferably about 200 °C. Such temperatures are suitable for deforming the retained austenite as well as ensuring sufficiently fine bainitic structure.

In this embodiment, during step (iii), the composition is preferably held at T1 for at least 5 hours, more preferably from 12 to 36 hours, even more preferably from 12 to 24 hours, still even more preferably from 12 to 16 hours. The time at which the composition is held at T1 is preferably minimised in view of cost. Holding the composition at T1 for at least 5 hours, preferably at least 12 hours, may result in particularly advantageous levels of retained austenite deformation.

In an alternative embodiment, T1 is below the martensite start temperature. This may result in the presence of small amounts of martensite in the final steel alloy, thereby increasing the strength and hardness. In addition, the martensitic transformation may result in an increase in austenite deformation. Since the martensitic transformation is immediate, it is not necessary to hold the alloy composition at T1 for long periods of time. Accordingly, the composition is typically held at T1 for less than 30 minutes, preferably about 15 minutes or less. In this embodiment, the microstructure of the resulting steel alloy preferably comprises from 10 to 50 vol% martensite, more preferably from 15 to 40 vol% martensite, the remainder being bainitic ferrite and retained austenite.

T2 at its upper limit may be just below the bainite start temperature. T2 is preferably from 50 to 150 °C below the bainite start temperature, more preferably from 90 to 110 °C below the bainite start temperature. T2 is preferably from 200 to 280 °C, more preferably from 210 to 260 °C, even more preferably about 250 °C. Lower temperatures may result in only a minimal reduction in the retained austenite levels of the resulting steel alloys. Higher temperatures are preferably avoided in view of cost and the somewhat weaker structure obtained. It should be noted that the bainite start temperature for the second step of transformation may change as the austenite gets enriched in carbon during the first bainite transformation step.

During step (iv) the composition is typically heated isothermally.

The method may further comprise (v) cooling the composition to room temperature.

Preferably the method further comprises (vi) cooling the composition to a temperature of less than 0°C. This may reduce the austenite content of the resulting steel alloy, thereby increasing its strength, hardness and dimensional stability.

Preferably the- method further comprises (vii) tempering at a temperature of from 100 to 200 °C for at least one hour. Such tempering may serve to reduce the occurrence of cracking in the resulting steel alloy. Preferably, such tempering is carried out after step (vi). In a preferred embodiment, the composition is double or triple tempered with freezing (step (vi)) in between tempering steps. When both steps (vi) and (vii) are carried out, the steel alloy composition is typically allowed to cool to room temperature before subsequent freezing. In addition, the final tempering step is typically followed by air cooling to room temperature.

The method preferably further comprises (viii) subjecting the steel alloy to a surface finishing technique. The hardened bearing steel components may optionally be burnished, especially the raceways, followed by tempering and air- cooling. Afterwards, the bearing steel components are finished by means of hard- turning and/or grinding operations such as lapping and honing. The burnishing and tempering operations may cause the yield strength of the affected areas to increase dramatically with significant improvement in hardness, compressive residual stress and better resistance to rolling contact fatigue. The steel alloy composition may be a bearing steel alloy. The steel alloy may be in the form of a bearing component, preferably at least one of a rolling element, an inner ring, and an outer ring.

In a further aspect, the present invention provides a steel alloy produced according to the method described herein.

The invention will now be described with reference to the following non-limiting Figures, in which: is a flowchart of an embodiment of the method of the present invention.

is a plot of mass fraction of AIN with temperature for steel alloys of the present invention.

is a plot of mass fraction of VC with temperature for steel alloys of the present invention.

is a plot of mass fraction of AIN with temperature for steel alloys of the present invention.

is a plot of mass fraction of VC with temperature for steel alloys of the present invention.

is a plot of mass fraction of V(C,N) with temperature for steel alloys of the present invention.

is a plot of mass fraction of AIN with temperature for a steel alloy of the present invention.

is a plot of mass fraction of VC with temperature for a steel alloy of the present invention.

is a plot of mass fraction of V(C,N) with temperature for a steel alloy of the present invention.

is a visible light optical micrograph of a steel according to the prior art. Figure 11 is a visible light optical micrograph of a steel alloy according to the present invention.

Figure 12 is visible light optical micrographs of two steel alloys according to the present invention.

Figure 13 shows HRC hardness (150 kgf) values of two steel alloys according to the present invention and a reference steel alloy.

Figure 14 visible light optical micrographs of two steel alloys according to the present invention and a reference steel alloy.

Referring to Figure 1 , the following steps were carried out:

(i) providing a steel alloy composition comprising:

from 0.5 to 1.2 wt. % carbon,

from 1 to 2 wt. % silicon,

from 0.25 to 2.2 wt. % manganese,

from 0.85 to 2 wt. % chromium,

from 0.5 to 5 wt. % cobalt,

from 0.1 to 0.6 wt. % molybdenum,

from 100 to 350 ppm nitrogen, optionally up to 2 wt. % aluminium, and/or

up to 0.3 wt. % vanadium, and/or

up to 250 ppm zirconium, and/or

up to 0.1 wt. % niobium, and/or

up to 0.2 wt. % tantalum, and/or

up to 0.005 wt. % calcium, and the balance iron together with unavoidable impurities,

(ii) heating the composition to a temperature of from 800 to 950 °C to at least partially austenise the composition; (iii) quenching the composition to a first temperature T1 , wherein 0.7M _S ≤ T1 < 1.3M _S , Ms being the martensite start temperature of the composition; and

(iv) heating the composition to a second temperature T2 below the bainite start temperature of the composition B _s .

The invention will now be described with reference to the following non-limiting Examples. Example 1

Steel alloys A to 1 D were prepared having the following compositions:

0.8 wt. % C

1.5 wt. % Si

2.0 wt. % Mn

1.0 wt. % Al

1.0 wt. % Cr

1.5 wt. % Co

0.25 wt. % Mo

150 ppm N (Steel 1 A) or

50 ppm N and 0.05 wt. % V (Steel 1 B) or

150 ppm N and 0.1 wt % V (Steel 1C) or

150 ppm N and 0.2 wt. % V (Steel 1 D). Figures 2 and 3 show plots of the mass fractions of AIN and VC, respectively with temperature. (Figure 2 - 1A: circles, 1 B: squares, 1C: line, 1 D: crosses; Figure 3 - 1 B: squares, 1C: triangles, 1 D: circles). The wt. %s of AIN and VC were calculated at an austenitisation temperature of 900 °C (Thermo-Calc, TCFE6) and the results were as follows:

Steel 1A: 0.044 wt. % AIN;

Steel 1 B: 0.044 wt. % AIN and 0.02 wt. % VC;

Steel 1C: 0.044 wt. % AIN and 0.10 wt. % VC;

Steel 1 D: 0.044 wt. % AIN and 0.27 wt. % VC. Steel alloys 1 E to 1 H were prepared having the following compositions:

0.8 wt. % C

1.5 wt. % Si

2.0 wt. % Mn

1.0 wt. % Al

1.0 wt. % Cr

1.5 wt. % Co

0.25 wt. % Mo

300 ppm N (Steel 1 E) or

300 ppm N and 0.05 wt. % V (Steel 1 F) or

300 ppm N and 0.1 wt. % V (Steel 1G) or

300 ppm N and 0.2 wt. % V (Steel 1 H).

Figures 4 and 5 show plots of the mass fractions of AIN and VC, respectively with temperature. (Figure 4 - 1 E: circles, 1 F: squares, 1G: line, 1 H: crosses; Figure 5 - 1 F: squares, 1G: triangles, 1 H: circles). The wt. %s of AIN and VC were calculated at an austenitisation temperature ^' of 900 ° ^' C (Thermo-Calc, TCFE6) and the results were as follows:

Steel I E: 0.088 wt. % AIN;

Steel 1 F: 0.088 wt. % AIN and 0.02 wt. % VC;

Steel 1G: 0.088 wt. % AIN and 0.10 wt. % VC;

Steel 1 H: 0.088 wt. % AIN and 0.27 wt. % VC.

Example 2

Steel alloys 2A to 2C were prepared having the following compositions: 0.8 wt. % C

1.5 wt. % Si

2.0 wt. % Mn

1.0 wt. % Cr

1.5 wt. % Co 0.25 wt. % Mo

200 ppm N and 0.05 wt. % V (Steel 2A) or

200 ppm N and 0.1 wt. % V (Steel 2B) or

200 ppm N and 0.2 wt. % V (Steel 2C).

Figure 6 shows a plot of the mass fractions of V(C,N), with temperature. (Figure 6 - 2A: squares, 2B: triangles, 2C: circles). The wt. %s of V(C,N) were calculated at an austenitisation temperature of 900 °C (Thermo-Calc, TCFE6) and the results were as follows:

Steel 2A: 0.06 wt. % V(C,N);

Steel 2B: 0.12 wt. % V(C,N);

Steel 2C: 0.25 wt. % V(C,N). Example 3

Steel alloy 3A was prepared having the following composition:

0.8 wt. % C

1.5 wt. % Si

2.0 wt. % Mn

1.0 wt. % Al

1.0 wt. % Cr

4.0 wt. % Co

0.25 wt. % Mo

300 ppm N and

0.1 wt. % V

Figures 7 and 8 show plots of the mass fractions of AIN and VC, respectively with temperature. (Figure 7 - 3A: circles; Figure 8 - 3A: squares). The wt. %s of AIN and VC were calculated at an austenitisation temperature of 900 °C (Thermo- Calc, TCFE6) and the results were as follows:

Steel 3A: 0.088 wt. % AIN and 0.11 wt. % VC. Example 4

Steel alloy 4A was prepared having the following composition: 0.8 wt. % C

1.5 wt. % Si

2.0 wt. % Mn

1.0 wt. % Cr

4.0 wt. % Co

0.25 wt. % Mo

200 ppm N and

0.1 wt. % V (Steel 4A)

Figure 9 shows a plot of the mass fractions of V(C,N), with temperature. (Figure 9 - 4A: triangles). The wt. %s of V(C,N) were calculated at an austenitisation. temperature of 900 °C (Thermo-Calc, TCFE6) and the results were as follows:

Steel 4A: 0,12 wt. % V(C,N) _; Example 5

Two steels melts, coded 5A and 5B were vacuum induction melted, the chemical composition of which can be seen in Table 1. As a reference steel material (representing the prior art), SP10 (steel code 14MR0002) was also vacuum induction melted.

Table 1 : Steel chemical compositions in wt. %. The balance is iron. All the steels were hot-rolled with area reduction ratio of 10:1.

After hot-rolling, the Nb and V micro-alloyed steels were air-cooled to 650 °C, placed in a furnace at this temperature where they were furnace-cooled to room temperature.

To examine the response of the steels to austenitisation, specimens were austenitised at 900 °C for an extended soaking time of 1 hour, oil quenched, then rinsed in cold water for 10 minutes and immediately tempered.

The optical micrographs of steel 14MR0002 and steel 5A are shown in Figures 10 and 11 , respectively. It is clear that steel 5A exhibits a much finer microstructure than steel 14MR0002. In addition, steel 14MR0002 exhibited a number of micro-cracks (indicated by arrows). The presence of cracked martensite plates is a clear evidence of excessively large austenlte grains.

As shown in Figure 12, the steel 5B (bottom) exhibited a structure that was comparable to that of the 5A (top) in terms of fineness.

Specimens from the steels in Table 1 were austenitised at 900 °C for 30 min followed by quenching into a salt bath kept at 200 °C for 72 h then air-cooled. The measured HRC hardness values can be seen in Figure 13. It is clear that the finer structures of the steel alloys 5A and 5B led to significant hardness improvement over the prior art steel SP10/14MR0002. The bainitic structures are demonstrated in Figure 14 (steel alloy 5A: top, steel alloy 5B: middle, and steel alloy 14MR0002: bottom; left hand side: pixel size = 55.8 nm, Mag = 2.00 K X; right hand side: pixel size = 11.2 nm, Mag 10.00 K X) showing, in all cases, at the shown magnifications, carbide-free bainite microstructures.

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.