Patent Application
20060070685
The invention relates to a thin-walled preferably stainless, bearing component, in particular a rolling bearing component, such as a bearing ring, sleeve or bush, that is produced without material-removing machining.
Bearing components of this type, which are used, for example, in needle bearings or the like, are usually deep-drawn from a cold-rolled strip. A precondition for the use of a deep-drawing process is that internal and external diameters of constant wall thickness with a correspondingly low level of roughening of the running surfaces, and which require no further remachining, be achieved. This requires a uniform, isotropic deformability of the cold-rolled strip. In the prior art, for example, a cold-rolled strip of deep-drawing steel containing approximately 0.10% of carbon as well as manganese, chromium and nickel is used for this purpose. Typical steels in this respect are DC04 modified, C15 modified, SAE1012 or SAE1015, or alternatively case-hardening steels, such as 16MnCr5 and 17Cr3. The drawn component is then case-hardened in order to achieve the desired wear resistance and load-bearing capacity. Since the deep-drawing cold-rolled strip is of corrodible steel, it is still necessary to apply a corrosion-resistant coating.
Bearing components of this type are very often also used in environments in which over long term use they are exposed to corrosive attack. One example application is in the automotive sector, where bearing components of this type are exposed to a very wide range of weathering conditions. The bearing components can be used in a very wide range of applications, as a known rolling bearing or, for example, in the form of universal joint bushes or the like. However, particularly in the automotive sector, the demands imposed on the corrosion resistance of the assembled elements are constantly rising. Thus, hot-galvanized body components or aluminum components which are protected against corrosion for many years or are inherently not corrodible, are being used. Bearing components which are produced according to the prior art cannot always match these increased demands, since the corrosion-resistant coating is insufficient to satisfy the requirement that they last a number of years. Consequently, corrosion can occur in particular at components which are directly exposed to the aggressive media, for example in the region of uncovered universal joints, or the like.
The invention is therefore based on the problem of providing a bearing component which can be used in such situations and satisfies the demands imposed with regard to corrosion resistance.
To solve this problem, the invention provides a bearing component which is deep-drawn from a cold-rolled strip with a carbon content of 0.15-0.25% and a chromium alloying content of at least 12% and which has a martensitic microstructure of which at least at the edge side is enriched with dissolved nitrogen.
The bearing component according to the invention is advantageously produced from a cold-rolled strip which can be deep-drawn but at the same time is substantially corrosion-resistant. It has emerged that a cold-rolled strip with a carbon content of 0.15-0.25% and a high chromium alloying content of at least 12% satisfies this condition. On the one hand, it can be deep-drawn, i.e., it is highly deformable and has a low yield strength of at most approx. 320 MPa, as well as a high elongation at break of at least 25% with a maximum tensile strength of 565 N.mm2. Furthermore, in addition to good mechanical properties, the material also has quasi-isotropic properties during cold-forming. These isotropic properties allow thin-walled components with a wall thickness of between 0.5 and 2.5 mm to be drawn and at the same time a homogeneous wall thickness to be obtained over the circumference, with minimal distortion wedge formation. The high chromium content means that the cold-rolled strip is unlikely to corrode. The cold-rolled strip steel according to the invention should be a CrMo or CrMnMo steel, with the carbon content in the range indicated.
A defined rolling and annealing sequence during production of the cold-rolled strip starting from hot-rolled strip is crucial to achieving this profile of properties in the cold-rolled strip used. For this purpose, it is customary in the steelworks for the material to be melted in an electric arc furnace then finish-treated by secondary metallurgy in a converter and then cast on a continuous-casting installation. The slabs produced in this way are then hot-rolled, pickled and split. At the start of the production of the cold-rolled strip, recrystallization of the hot-rolled strip is carried out in a bell-type furnace. After blasting and pickling, the strip is cold-rolled to the desired strip thickness. Before final skin-rolling to set the final thickness, a further recrystallization anneal is carried out. The grain sizes of approx. 8 mm and coarser in accordance with ASTM which can be achieved in this way promote good deformability. Although this runs contrary to the usual requirement in the bearings industry for fine-grained steels, in combination with the chromium carbides which form, it is possible to control the grain size, since the carbides are thermally stable and have a fine-grain stabilizing action during the heat treatment. On account of the isotropic properties, it is possible to produce very homogenous thin-walled precision components in the deep-drawing process. The orange peel formation which is often observed during deep-drawing in the prior art and leads to a deterioration in the running wall quality, is compensated for and avoided on account of the grain-size influencing action of carbide described, despite the larger ferrite grains.
Although the use of this high-alloy steel allows the production of a markedly corrosion-resistant component by precision deep-drawing, the bearing component does not yet satisfy the demands imposed with regard to load-bearing capacity. The cold-rolled strip used is considerably softer than the deep-drawing but not corrosion-resistant steels used in the prior art. However, high demands in terms of load-bearing capacity are imposed on the mechanically stressed surfaces on account of the forces which are active in operation. Consequently, it is necessary to harden the deep-drawn component.
For this purpose, according to the invention, the bearing component is nitrided. For this purpose, nitrogen atoms are fed to the steel, which is in the austenitic state at the prevailing temperature, in a physical-thermochemical hardening process. The nitrogen is incorporated in the austenitic microstructure and remains present in dissolved form in the martensitic microstructure too. The heat treatment process is controlled in such a way that there is high or maximum nitrogen solubility in the steel, but precipitation of embrittling nitrides, in particular at the grain boundaries, is avoided by controlling the atmosphere and in particular the cooling conditions. On account of the atomic radius of nitrogen, a specific diffusion rate at which the nitrogen diffuses into the steel is predetermined. After cooling, what is known as nitrogen martensite is formed.
The introduction of the nitrogen in dissolved form first has the advantage of allowing surface hardening, achieving values of at least 58 HRC or 650 HV. This surface hardness is more than sufficient with respect to the mechanical properties required. However, the nitrogen which is introduced also improves the corrosion resistance of the bearing component to an even greater extent than that which has already been achieved by the high chromium alloying content. It is possible to achieve a corrosion resistance of at least 96 h in the salt spray test in accordance with ASTM B117.
The nitrogen content in the edge zone should be 0.05-1.5%, in particular 0.1-0.5%. An excessively high nitrogen content leads to embrittlement of the material on account of nitride precipitations, which is undesirable. Consequently, it is necessary to control the quantity of nitrogen with respect to the maximum nitrogen solubility, in order to remain below the limit value above which nitride precipitation occurs.
The depth of the edge zone (case) should be at least 0.05 mm, in particular at least 0.1 mm, preferably at least 0.2 mm, in which context it is sufficient for it to be limited to at most 0.5 mm. In the case of thin-walled components, for example with a wall thickness of up to approx. 1 mm, with relatively great case depths, it is possible for the nitrogen to be sufficiently enriched over the entire cross section, since the nitrogen diffusion takes place from both sides. The extent to which the nitrogen diffuses in of course depends on the conditions used in the heat treatment, in particular the temperature, the nitrogen pressure and of course also the nitriding duration. In the case of relatively thick components, exclusively dissolved carbon is present in the core, whereas a nitrogen-rich martensitic edge zone with dissolved carbon fractions is present in the edge region. On account of these conditions, the critical quenching rates are also dilatometrically different, which causes internal compressive stresses in the edge zone, which are favorable for the component properties.
With a view to maximizing nitrogen solubility in the austenite, it is expedient if the cold-rolled strip is nickel-free. The absence of nickel is also advantageous in that it is also possible to use the bearing component in the food industry sector. The high resistance to water and cleaning agents required there is achieved by the component according to the invention. If possible, there should also be no silicon, since this element reduces the nitrogen solubility. An excessive carbon content is also disadvantageous in this respect. Furthermore, the vanadium, niobium and titanium contents should be relatively low, since these elements increase the risk of precipitation of grain boundary nitrides, which lead to embrittlement.
As has been described, the component hardness on the nitrogen-rich component side should be at least 58 HRC, corresponding to 650 HV. When using the cold-rolled strip according to the invention in combination with the nitriding, the heat resistance can likewise be set to a very high level and is at least 300° C. The chromium content in combination with the nitrogen content which has diffused in and the maximum level of which is material-dependent up to the solubility limit should also be selected in such a way that the component has a corrosion resistance of at least 96 h in the salt spray test.
In addition to the bearing component, the invention also relates to a process for producing such a component, in which an austenitic cold-rolled strip with a carbon content of 0.15-0.25% and a chromium alloying content of at least 12% is used, the component is initially deep-drawn from this strip and is then enriched with nitrogen at the edge sides as part of a heat treatment in a nitrogen-rich atmosphere, after which the component is cooled in order to form a martensitic microstructure in which the nitrogen is present in dissolved form.
The nitrogen enrichment itself should be carried out at a temperature of 1000-1200° C. The duration of the heat treatment depends on the degree of nitriding, but may be at least 15 min up to several hours. Furthermore, it is dependent on the starting steel used, since the maximum nitrogen solubility is dependent on the composition of this steel. The nitrogen pressure during the treatment should be 0.1-3 bar.
It has also been determined to be positive for the high alloying content to allow slow cooling, cooling rates of £50° C./s being possible.
This has a highly beneficial effect on the dimensional stability of the components. Consequently, there is no need for sudden quenching, which in the prior art at times leads to changes in dimensions and shape.
Furthermore, it is expedient for the cooling to be followed by a low-temperature treatment in order to continue the formation of martensite. In addition, a further heat treatment for tempering the microstructure can be carried out after the tempering or after any low-temperature treatment which is carried out.
Other features and advantages of the present invention will become apparent from the following description of the invention which refers to the accompanying drawings.
Further advantages, features and details of the invention will emerge from the exemplary embodiment described below and with reference to the drawings, in which:
The universal joint bush 2 is drawn from a cold-rolled strip which has a carbon content of 0.15-0.25% with a chromium alloying content of at least 12%. Known cold-rolled strips which can be used and have these contents as well as isotropic deformation properties, which ensure a homogeneous wall thickness in the drawn component, include, for example, X20Cr13 or X22Cr13. Both are high-alloy steels with a chromium content of around 13% and a carbon content of 0.20% in the case of cold-rolled strip X20Cr13 or 0.22% in the case of cold-rolled strip X22Cr13.
As part of the production, after deep-drawing, a heat treatment, during which nitrogen is introduced into the surfaces, is carried out in order to harden the surfaces of the universal joint bush 2 so as to achieve the required load-bearing capacity in particular in the region of the bush inner wall 6, which is required for use and the mechanical loads which are then present. For this purpose, the component is heated to a temperature of between 1000° and 1200° C. in a nitrogen-rich atmosphere and is held at this temperature for a certain time. The nitrogen which is present diffuses into the austenitic microstructure, the diffusion depth being dependent on the prevailing nitrogen pressure, the duration of the heat treatment and the cold-rolled strip material used. After this diffusion step has ended, cooling is carried out in order to form martensite, in which the nitrogen is present in the dissolved form. This means that the quantity of nitrogen introduced is to be selected in such a way, taking account of the material-dependent maximum nitrogen solubility, and ultimately the parameters during the diffusion treatment are to be set in such a way, that no nitrogen precipitation, which has an embrittling action, takes place.
After cooling, which can take place relatively slowly at a cooling rate of a few 10° C./s , to continue the martensite formation and make it more intensive, a low-temperature treatment can be carried out, during which the universal joint bush 2 is cooled to temperatures of below −50° C., for example −80° C. This cooling is followed by a tempering step for relieving stresses, in which the component is heated, for example, to 100° C. or 150° C.
The high chromium alloy in combination with the dissolved nitrogen leads to an excellent corrosion resistance in the component obtained, the composition of the starting material of the cold-rolled strip nonetheless being selected in such a way that it has very good deep-drawing properties, including with regard to dimensional stability, and the dimensional stability is also maintained during the subsequent nitriding.
The hardness curve over the cross section can be seen in
The following hardness values were found:
It was found that on account of the nitriding, nitrogen-rich martensite is present at the two edge regions and is considerably harder than the martensite in the core. The curve and also the table above demonstrate a significant drop in hardness the closer the measurement points lie to the center, with the curve also flattening significantly toward the center. This hardness curve correlates with the nitrogen and carbon distribution in the edge region. In this respect,
The curve C represents the carbon content. This rises slightly in the edge region, which is attributable to the fact that the nitrogen which has diffused in partially displaces the carbon into the depth, producing a type of “bow wave”, leading to slightly higher carbon contents near the edge. Therefore, dissolved carbon and nitrogen are present in the edge region, whereas the nitrogen content decreases with increasing depth toward the core, and in the core only carbon in dissolved form is present.
The curve denoted by Cr also shows the chromium content. It can be seen that in the edge region tested, this is substantially unaffected, remaining over 13.8%. On account of this unaffected Cr content and the absence of carbide and nitride precipitations from the microstructure resulting from the carbon and nitrogen contents given, the corrosion resistance is excellent, to the extent that this corrosion resistance is attributable to the alloying element chromium in combination with the nitrogen. The corrosion resistance is well over 96 hours in the salt spray test.
Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2004 048 172.5 | Oct 2004 | DE | national |
