GB2513881A - Steel Alloy - Google Patents

Abstract

A steel comprising (by weight): 0.8 - 1.2 % carbon, 1.8 - 2.2 % silicon, 1.6 - 2.2 % chromium, 0.2 - 0.8 % molybdenum, 0 - 0.8 % manganese, 0 - 0.6 % vanadium, 0 - 0.2 % niobium, 0 - 0.3 % tantalum, 0 - 0.4 % nickel, 0 - 0.5 % copper, 0 - 0.05 % nitrogen, 0 - 0.04 % phosphorus, 0 - 0.04 % sulphur and the balance iron and unavoidable impurities. The steel comprises at least 60 volume % bainitic ferrite. The steel may also comprise 0-10 volume % carbide, nitride or carbo-nitride precipitates of mean diameter 1-50 nm, 0-10 volume % retained austenite and/or 10-20 volume % tempered martensite. The steel is transformed to bainite by cooling at least partially austenised steel to a temperature in the range 110-325 0C where it is held for a time in the range 2 hours to 80 days. The steel can be used to make rolling elements, inner rings or outer rings of bearings.

Description

Steel Alloy

Technical Field

The present invention relates generally to the field of metallurgy and to an improved steel alloy and a method for producing the same. The steel has high tensile strength and toughness and may be used in a number of applications, including, 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 semi-finished 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 5OOC. 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.

An existing bearing steel grade (EN ISO 683-17:1999 lOOCrMnMoSi8-4-6 / 1.3539) comprises: from 0.9-1.05 wt% carbon, from 0.4-0.6 wt% silicon, from 0.8-1.1 wt% manganese, from 1.8-2.05 wt% chromium, from 0.5-0.6 wt% molybdenum, max. 0.025 wt% phosphorus, max. 0.015 wt% sulphur, and the balance iron, together with any unavoidable impurities.

For bearing applications there is a desire for steel alloys with a finer micrstructure There is also a desire to decrease production times.

It is an object of the present invention to provide a steel alloy and to address some of the problems associated with the prior art, or at least to provide a commercially useful alternative thereto. The steel alloy may be used in the manufacture of a bearing component, for example a raceway or rolling element.

Summary of the Invention

According to a first aspect, there is provided a steel alloy comprising: from 0.8-1.2 wt% carbon, from 1.8-2.2 wt% silicon, from 1.6-2.2 wt% chromium, from 0.2-0.8 wt% molybdenum, optionally one or more of from 0-0.8 wt% manganese, from 0 -0.6 wt% vanadium, from 0-0.2 wt% niobium, from 0 -0.3 wt% tantalum, from 0 -0.4 wt% nickel, from 0-0.5 wt% copper, from 0 -0.05 wt% nitrogen, from 0 -0.04 wt% phosphorus, from 0 -0.04 wt% sulphur, and the balance iron, together with any unavoidable impurities.

The steel alloy structure preferably comprises bainite as the main phase. The microstructure preferably comprises bainitic ferrite and carbide precipitates. The microstructure may further comprise some retained austenite. In one embodiment, the microstructure comprises bainitic ferrite, martensite and carbide precipitates and optionally some 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.

The steel composition preferably comprises from 0.8 to 1.15 wt% carbon, more preferably from 0.85 to 1.1 wt% carbon, still more preferably from 0.9 to 1.0 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. 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 0.8 wt% the alloys have a higher martensite start temperature.

The steel composition preferably comprises from 1.8 to 2.1 wt% silicon, more preferably from 1.8 to 2 wt% silicon, still more preferably from 1.8 tol.9 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 2.2 wt%.

The steel composition preferably comprises from 1.7 to 2.1 wt% chromium, more preferably from 1.8 to 2 wt% chromium, still more preferably from 1.8 to 1.9 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 composition preferably comprises from 0.3 to 0.7 wt% molybdenum, more preferably from 0.4 to 0.6 wt% molybdenum, still more preferably from 0.5 to 0.6 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 0.8 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.

If desired, the alloy may also comprise from 0 to 0.6 wt% manganese, preferably from 0.1 to 0.5 wt% manganese, more preferably from 0.4 to 0.5 wt% manganese. Manganese acts to increase the stability of austenite relative to ferrite. However, it is generally preferred to keep Mn to a minimum in order to reduce the amount of retained austenite and to increase the rate of transformation to bainite. For this reason, the manganese content is preferably no more than 0.15 wt%, more preferably no more than 0.1 wt%, still more preferably no more than 0.05 wt%.

If desired, the alloy may also comprise from 0 to 0.6 wt% vanadium. In one embodiment, the steel composition comprises from 0.05 to 0.4 wt% vanadium, more preferably from 0.1 to 0.3 wt% vanadium. Vanadium can be useful during austenitisation because it helps to control the austenite grain size.

In a preferred embodiment there is provided a steel alloy comprising: from 0.8-1 wt% carbon, from 1.8-1.9 wt% silicon, from 1.7-1.9 wt% chromium, from 0.4-0.6 wt% molybdenum, from 0.3 -0.5 wt% manganese, from 0.1 -0.3 wt% vanadium, optionally one or more of from 0-0.2 wt% niobium, from 0 -0.3 wt% tantalum, from 0 -0.4 wt% nickel, from 0 -0.5 wt% copper, from 0-0.05 wt% nitrogen, from 0 -0.04 wt% phosphorus, from 0 -0.04 wt% sulphur, and the balance iron, together with any unavoidable impurities.

It will be appreciated that the steel alloys according to the present invention 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 alloys according to the present invention 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 microstructure of the steel alloys according to the present invention typically comprises, in the as-hardened condition, nano-structured bainitic ferrite and retained austenite with nanoprecipitates. The as-hardened microstructure may optionally contain some tempered martensite.

In greater detail, the microstructure typically comprises at least 60 vol.% bainite, more typically at least 80 vol.% bainite, still more typically at least 90 vol.% bainite. The bainite is preferably lower bainite and preferably has a very fine structure. Bainite can be obtained by carrying out a transformation at a relatively low temperature, typically less than 350°C, more typically from 110 to 325°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. 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 retained austenite content is kept to a minimum to improve strength, hardness and, in particular, dimensional stability. The amount of retained austenite is typically less than 10 vol.%, more typically less than 5 vol.%. 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%. In one embodiment, the structure is free or at least essentially free of carbides, nitrides and/or carbo-nitrides.

In one embodiment, the structure advantageously comprises 1 to 10 vol% retained austenite, optionally up to 10 vol% carbides, nitrides and/or carbo-nitrides, and the balance comprising bainitic ferrite. More preferably, the structure comprises 1 to 5 vol% retained austenite, optionally up to 5 vol% carbides, nitrides and/or carbo-nitrides, and the balance comprising bainitic ferrite.

In another embodiment, the microstructure includes martensite, for example from to 20 vol% martensite. That is, the microstructure comprises both bainitic ferrite and martensite. An example of such a microstructure is one that comprises 1 to 10 vol% retained austenite, optionally up to 10 vol% carbides, nitrides and/or carbo-nitrides, and the balance comprising bainitic ferrite and martensite. In the case where the structure comprises mixed martensite and bainite, the martensite is typically formed first by quenching the austenite below the martensite-start temperature, followed by bainite transformation at a higher temperature.

As noted above, it is advantageous for the retained austenite content to be kept to a minimum to improve strength, hardness and, in particular, dimensional stability. The dimensional stability is critical for bearing components operating at warm-to-elevated temperatures, typically 80C and above.

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.

The steel alloys according to the present invention typically transform to bainite at a temperature of from 110 to 325t, more typically from 115 to 250t. The transformation time for the bainite formation to cease is typically from 2 hours to days, more typically from 6 hours to 60 days. The transformation time depends on the transformation temperature: the time is longer for lower temperatures. The amount of bainite that is formed depends on the transformation temperature: more bainite is formed at lower temperatures. The amount of retained austenite increases for higher transformation temperatures.

The process for the manufacture of the steel alloys avoids rapid cooling so that residual stresses can be avoided in large component pieces.

If desired, various mechanical properties can be improved by carrying out any of the conventional post-bainite transformation steps. For example, in some cases, the yield strength can be improved by carrying out a post-bainite transformation deformation step followed by tempering.

Additionally, the strength and hardness of the steel structure may be improved further by cooling the alloy to a temperature below room temperature such that the austenite content is reduced. The cooling step may be applied during the heat-treatment, or hardening-process, or after the final stage of transformation into bainite, once the hardened alloy has cooled down to room temperature. In any case, the martensite that is formed during the cooling step is preferably tempered immediately so as to reduce cracking.

According to a second aspect of the present invention, there is provided a bearing component comprising a steel alloy as herein described. Examples of bearing components where the steel can be used include a rolling element (ball, cylinder or taper), an inner ring, and an outer ring. The bearing component could also be part of a linear bearing such as ball and roller screws. The present invention also provides a bearing comprising a bearing component as herein described.

According to a third aspect, there is provided a method for preparing a steel alloy, the method comprising: (i) providing a steel alloy composition as herein described; (H) heating the steel composition to at least partially austenitise the composition; and -10 - (Hi) maintaining the composition at a bainite formation temperature until bainite formation ceases.

The step of heating the composition to at least partially austenitise the composition will typically involve raising the temperature of the composition to at least about 860CC, preferably at least about 885CC. For example, working in a temperature range of from 860 to 950C, preferably from 885 to 925CC.

The step of maintaining the composition at a bainite formation temperature until a bainite structure is formed will typically comprise adjusting the temperature of the hot austenitised composition to a temperature in the range of from 150 to 300C.

The preferred heat-treatment is from 200 to 250t. The treatment is preferably, but not necessarily, isothermal. This leads to the most consistent structure. The processing time is typically from 6 to 72 hours, although longer times are possible. The greater the temperature, the shorter the treatment time that will be required, but the less refined the final structure.

Examples of heat-treatments The following heat-treatments are provided by way of example. First, an alloy composition is prepared and cast. The alloy may then be subjected to a conventional high-temperature soaking step, followed by hot-rolling, typically at a starting temperature of about 1150°C. Several hot-rolling passes may be applied as necessary. The hot-rolled steel, which can be in a bar or plate form, is then allowed to cool slowly to room temperature to avoid the formation of high-carbon martensite. A typical preferred structure in the as hot-rolled condition, at room temperature, is pearlite.

The hot-rolled material may then optionally be homogenised in a homogenisation step, such as an approx. 1 2OOC treatment for about 24 to 48 hours in vacuum.

The material may then, optionally, be furnace-cooled to allow it to cool down slowly to room temperature, also under vacuum.

-fl_i -If desired, the material may be machined in a machining step to near-net-shape components, for example bearing components. The material or machined shapes are subsequently fully or partially austenitised, for example using an approx. 900°C austenitising heat-treatment for about 30 to 60 minutes. For large bearing components, it may be preferable that heating is carried out in steps with equalisation time at each intermediate temperature step.

The austenitisation may be full or partial, depending on, primarily, the desired carbon content in the austenite phase.

Immediately after austenitisation, the material (or component) is cooled down to the bainite transformation regime and allowed to transform isothermally. Bainite transformation can be carried out at a fixed transformation temperature, for example at about 200°C for about 24 to 72 hours. A multi-step transformation temperature schedule may also be adopted to tailor the phase fractions in the structure.

After the isothermal treatment step, the material (or component) may be air-cooled and then finished by machining and/or grinding to the required final dimensions. Finally, an optional step of honing, lapping or polishing can be performed.

Preferred aspects of the heat-treatment are described below, again by way of

example.

Example of austenitisation The steel composition according to the present invention is designed to be austenitised at temperatures typically in the range of from 800 to 9502C for durations of typically 10 to 60 minutes. A specific example is an austenitising heat-treatment at about 900°C for about 30 minutes. In contrast to conventional carbide-free very fine bainitic structures (known as superbainite"), austenite may -12 -not be the only phase that is present, or stable, at the chosen austenitisation temperature. That is, immediately prior to quenching for the subsequent bainite transformation, austenite may not be the only phase that is present in the alloys according to the present invention. In particular, in addition to the austenite that forms during austenitisation, numerous very small particles that refine the austenite grains may also exist.

Example of bainitic transformation io After austenitisation and subsequent quenching, such that all the reconstructive transformation products are avoided during cooling, the following bainitic transformation schedule may take place.

1. Holding the component (for example a bearing component) isothermally, typically in a salt bath, at a temperature just above the martensite-start (MS) temperature for a time that is sufficient for at least partial bainitic transformation.

Typical hardening parameters at this stage are holding at about 2OOC for durations of up to about 3 days.

2. The second step is to heat the component from the first bath temperature.

This may be achieved by transferring the charge to a second salt bath kept at a temperature below the bainite-start temperature (BS) and holding isothermally until the bainitic transformation has ceased.

3. The bainite transformed component is then air-cooled and subsequently cleaned. Step 2 may be repeated at a different temperature above about 200 C and below the BS temperature; this has been found to reduce the austenite content in the hardened structure even further. This can be followed by air-cooling to room temperature.

4. Optionally, when the bainite transformation has ceased and the component has been cooled to room temperature, the component may then be -13 -refrigerated or frozen to sub-zero temperatures, followed immediately by tempering to further reduce the content of retained austenite.

The resultant bainitically hardened microstructure results in good mechanical properties. Also, given its very fine scale, the microstructure ensures significant total grain boundary areas, which is advantageous in terms of hydrogen trapping.

This means that the steel alloys according to the present invention are also capable of resisting hydrogen embrittlement.

Examples of mixed structures A variation of the heat treatment is to quench the component to a temperature just below the MS temperature to form a small amount of martensite. This is then followed by transforming the residual austenite into bainite in a second salt bath.

Beneficial martensite content is within the range 10% to 50%, more preferably 15% to 40%, by volume. The remainder being the residual austenite and the prior austenite grain refining particles.

For the same fraction of bainite and martensite formed in the first salt bath, the martensite transformation is immediate, thus no prolonged holding is necessary.

Furthermore, the martensitic transformation will be accompanied by an increase in austenite deformation that is deemed beneficial as it accelerates the bainite transformation.

This acceleration of the bainite transformation kinetics may be utilised as follows.

First, for a given retained austenite content in the final hard bearing steel structure, the transformation time is shorter. Second, for the same transformation time at temperature, the final hard bearing steel structure will exhibit less retained austenite, resulting in improved dimensional stability, increased strength and hardness.

-14 -The transformed martensite may be tempered, preventing cracking, during the second step where bainite is allowed to form. A bainitic bearing steel structure that is partially martensitic ensures even better strength and hardness.

In the second step, where the residual austenite transforms into bainite (bainitic-ferrite), the isothermal transformation temperature should be below the BS temperature.

The present invention will now be described further, by way of example, with reference to the accompany drawings in which: Figure 1 is the calculated phase diagram for the alloy described in the Examples.

Examples

In this example, the following steel composition was provided: 0.9wt%C 1.Swt% Si 0.4wt% Mn 1.8 wt% Cr 0.5 wt% Mo 0.2wt%V ppm N and the balance Fe, together with any unavoidable impurities.

The phase fraction versus temperature calculated with MatCalc version 5.50, release 1.001, is presented in Figure 1.

In equilibrium, cementite dissolves above about 810°C and M7C3 dissolves at about 925°C. The vanadium-rich precipitates are very stable and will be retained at even very high austenitisation temperatures exceeding 1000°C. Therefore, for -15 -an example austenitisation temperature of 885°C, the following phases exist at equilibrium: austenite, M7C3, and the vanadium-rich precipitates.

The composition of the austenite phase at 885°C for the alloy composition at equilibrium was found using Thermo-CaIc, database TCFE6: FCC_A1#1 Status ENTERED Driving force O.0000E+OO Moles 9.7861 E-O1, Mass 5.2226E+01, Volume fraction 9.81 22E-01 Mass fractions: FE 9.50077E-01, CR 1.64122E-02, MN 4.00358E-03, V io 2.07802E-04, SI 1.79975E-02, C 7.87672E-03, MO 3.41 301 E-03. and N 1.201 33E-05.

The approximate 0.79 wt% carbon in solid solution in the austenite at high temperature (885°C) was sufficient to ensure a good hard structure in the is hardened condition. The calculated MS temperature was estimated to be about 159°C, whereas the BS temperature was about 300°C.

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.

Claims (13)

1. -16 -CLAIMS: 1. A steel alloy comprising from 0.8-1.2 wt% carbon, from 1.8-2.2 wt% silicon, from 1.6-2.2 wt% chromium, from 0.2-0.8 wt% molybdenum, optionally one or more of from 0 -0.8 wt% manganese, from 0 -0.6 wt% vanadium, from 0-0.2 wt% niobium, from 0-0.3 wt% tantalum, from 0 -0.4 wt% nickel, from 0 -0.5 wt% copper, from 0 -0.05 wt% nitrogen, from 0 -0.04 wt% phosphorus, from 0-0.04 wt% sulphur, and the balance iron, together with any unavoidable impurities.
2. 2. A steel alloy as claimed in claim 1 comprising from 0.8 to 1.15 wt% carbon, preferably from 0.85 to 1.1 wt% carbon, more preferably from 0.9 to 1.0 wt% carbon.
3. 3. A steel alloy as claimed in claim 1 or claim 2 comprising from 1.8 to 2.1 wt% silicon, preferably from 1.8 to 2 wt% silicon.
4. 4. A steel alloy as claimed in any one of the preceding claims comprising from 1.7 to 2.1 wt% chromium, preferably from 1.8 to 2 wt% chromium.

   -17 -
5. 5. A bearing steel alloy as claimed in any one of the preceding claims comprising from 0.3 to 0.7 wt% molybdenum, preferably from 0.4 to 0.6 wt% molybdenum, more preferably from 0.5 to 0.6 wt% molybdenum.
6. 6. A steel alloy as claimed in any one of the preceding claims comprising from 0.05 to 0.6 wt% manganese, preferably from 0.1 to 0.5 wt% manganese, more preferably from 0.4 to 0.5 wt% manganese.io
7. 7. A steel alloy as claimed in any one of the preceding claims comprising from 0.05 to 0.5 wt% vanadium, preferably from 0.1 to 0.4 wt% vanadium, more preferably from 0.1 to 0.3 wt% vanadium.
8. 8. A steel alloy as claimed in any one of the preceding claims having a microstructure comprising bainitic ferrite and carbide precipitates.
9. 9. A steel alloy as claimed in claim 8, wherein the microstructure further comprises retained austenite and optionally tempered martensite.
10. 10. A steel alloy as claimed in claim 8 or claim 9, wherein the carbide precipitates have a mean diameter of from 1 to 50 nm, preferably from 1 to 30 nm, more preferably from 5 to 25 nm.
11. 11. A bearing component comprising a steel alloy as defined in any of claims 1 to 10.
12. 12. A bearing component as claimed in claim 11, which is at least one of a rolling element, an inner ring, and/or an outer ring.
13. 13. A bearing comprising a bearing component as claimed in claim 11 or claim 12.