EP2737097B1 - Steel, component and method for producing steel - Google Patents

Description

Embodiments of the present invention relate to steel, a component comprising the steel, and a method for producing the steel.

Many machines and systems have high mechanical loads in various shapes and forms. Often, but by no means exclusively, machine parts are affected here that perform relative movements to one another, for example translatory or rotary movements. In addition to permanent loads, short-term and / or local loads can also occur, which can lead to fatigue or static overloading of the machine parts in question.

Steel is used particularly for such machine parts due to its extensive assembly, its strength, its deformability, its suitability for welding and many other properties. One of the most important technological properties is that steel can be fully or partially hardened. If, for example, only an edge hardening or surface hardening is carried out, the correspondingly treated surface becomes harder and thus mechanically and tribologically more resistant, while lower-lying areas of the machine part have a lower hardness and thus a greater toughness, as a result of which the machine part as a whole is subjected to higher cyclical and static loads.

Dynamic loads are often introduced into the surface of the machine parts, so they have a strong impact on the typically hardened areas. These can cause dislocation movements in the material, which can ultimately lead to plastic deformations inside the machine part. These often just occupy the area of grain boundaries in the structure of steel, which can lead to an intercrystalline fracture before the strength in the volume is reached if the material is susceptible.

To counteract this, preference is given to using steels which have a large number of grain boundaries, i.e. which have small crystals in their structure, as this means that the mechanical loads and coating (segregation) with weakening elements (e.g. phosphorus, sulfur) or compounds (e.g. carbides) are distributed over a larger number and thus area of grain boundaries and the stress on the individual grain boundaries decreases.

However, the hardening of the steel in the edge region typically takes place thermally, with the steel of the machine part being heated correspondingly strongly. This can take place, for example, by flame or induction hardening, that is to say, for example, by inductive heating of the workpiece.

Hardening is a thermally induced process, so that, especially with larger edge hardening depths, undesired heating above the austenite temperature of the steel and thus overheating of the structure can occur. As a result, there is increased grain growth, i.e. coarse grain formation. In later use, this can lead to a reduced strength due to a weakening of the grain boundaries (e.g. due to segregation, i.e. accumulation of harmful elements), with a tendency to intergranular fracture.

So far, steels with a maximum carbon content of 1.1% by weight (% by weight) and sufficient surface hardenability have been used, which is influenced by the alloy composition. It also tries to guide the hardening process so that excessive grain growth is avoided.

JP 2000 026939 teaches the production of steel with a carbon content greater than 1.2%, in which excessive hardness (approximately 62 in HRC) is avoided. US 2004/0202567 A1 teaches a steel for use in high strength pinion shaft for manufacturing a pinion shaft in automotive steering systems and a manufacturing method therefor. DE 1608155 teaches the use of a steel for high-performance chains, in particular for round link chains of planing and conveying systems in mining companies, which are exposed to strong corrosion influences and unusually high permanent and alternating loads.

Proceeding from this, the object of the present invention is therefore to create a steel which has improved hardenability and / or improved grain stability and / or static strength.

This object is achieved by a steel according to claim 1 or a method for producing steel according to claim 6.

One embodiment of a steel has tantalum with a proportion of at least 0, 1 and 2 percent by weight and a carbon proportion that is at least 0.25 percent by weight and at most 1.1 percent by weight.

One embodiment of a method of making steel includes providing a base alloy of the steel and alloying the base alloy with tantalum such that the steel has tantalum in a proportion of at least 0.1 and 2 percent by weight and a carbon content that is at least 0.25 percent by weight and is at most 1.1 percent by weight.

Embodiments of the invention are based on the knowledge that improved hardenability and / or improved grain stability and / or static strength can be achieved by adding tantalum to a steel with a carbon content of at least 0.25 percent by weight and at most 1.55 percent by weight as a (micro) alloying element. Studies have shown that even very low tantalum contents of about 0.01 percent by weight lead to improved hardenability or to improved grain stability. With increasing tantalum content in the range between 0.01 and 2.0 percent by weight, a corresponding improvement in the abovementioned properties compared to unalloyed steel is achieved. The addition of tantalum enables an increase in the austenitizing temperature during the hardening. hereby The accessible process temperatures can be increased to over 1000 ° C, for example up to 1150 ° C, while conventional steels require significantly lower temperatures.

Embodiments of the present invention are also based on the object of creating a component which has improved fatigue resistance in at least one section.

This object is achieved by a component according to claim 8.

An embodiment of the present invention in the form of a component thus has the aforementioned steel at least in one section, the section extending from a surface of the component into an interior of the component, and the component having an edge hardening in the section.

In some embodiments of the present invention, this can extend, for example, at least 5 mm into the interior of the component, while in other embodiments, smaller or greater edge hardening depths can also be achieved.

The edge hardening depth is to be understood as the vertical distance to the surface until a hardness of 550 HV1 (52.5 HRC) is reached. Embodiments of the present invention with regard to this aspect are based on the knowledge that by using the steel described above, improved hardenability and / or static strength and / or improved grain stability can be achieved during the heat treatment during hardening, or that a process for hardening a component can thereby be achieved can be simplified. In some exemplary embodiments of the present invention, an edge hardening with a greater edge hardening depth of at least 5 mm in at least one section of the component can be achieved by (e.g. inductive) hardening without that increased coarse grain formation occurs here, which could adversely affect the fatigue resistance and static strength of the material. However, exemplary embodiments of the present invention are by no means restricted to components with an edge hardening depth of 5 mm or more. Embodiments of the present invention also include components with a shallower edge hardening depth than 5 mm, for example, those with edge hardening depths of at least 100 µm, at least 200 µm, at least 500 µm, at least 1 mm or at least 2 mm.

In the context of the present description, nominal weight percentages are used with respect to the individual proportions. Steels can deviate from the nominal proportions due to the production-related processes involved in steel production. The proportion of an alloy element or another component (e.g. carbon) of a steel in a specific exemplary embodiment can therefore fall below or exceed the nominal values within the scope of the usual manufacturing and manufacturing tolerances.

• Fig. 1 zeigt einen Querschnitt durch ein Kegelrollenlager mit Bauteilen gemäß Ausführungsbeispielen der vorliegenden Erfindung;
• Fig. 2 zeigt eine schematische Querschnittsdarstellung durch ein Bauteil gemäß einem Ausführungsbeispiel der vorliegenden Erfindung während der Randhärtung; und
• Fig. 3 illustriert einen Verlauf einer Härte durch ein Bauteil gemäß einem Ausführungsbeispiel der vorliegenden Erfindung als Funktion eines Abstands von der Oberfläche des Bauteils.

Embodiments of the present invention are explained below with reference to the accompanying figures.

• Fig. 1 shows a cross section through a tapered roller bearing with components according to embodiments of the present invention;
• Fig. 2 shows a schematic cross-sectional view through a component according to an embodiment of the present invention during edge hardening; and
• Fig. 3 illustrates a course of hardness through a component according to an exemplary embodiment of the present invention as a function of a distance from the surface of the component.

Below, with reference to the Figures 1 to 3 Exemplary embodiments of the present invention are described and explained in more detail.

As already described in the introduction, strong mechanical loads occur in many applications, which lead to fatigue of the component in question. These loads are often short-term, intermittent or periodic dynamic loads that often only affect small areas of the component, for example via a point or line contact.

In order to make the corresponding components more resistant to fatigue, hardened steels are often used, especially surface-hardened steels. As was also described at the beginning, steels which have small grain sizes are preferred in particular in order to distribute mechanical loads and debilitating segregation or excretion assignments over a large number and thus area of grain boundaries.

In induction hardening or flame hardening, a workpiece is therefore often locally heated to a temperature of more than 1000 ° C by heating it from the outside (flame hardening) or by eddy currents generated in an outer layer of the workpiece near the surface (induction hardening). Typically, temperatures of up to 1150 ° C. are reached locally. However, this can easily lead to overheating of the structure, which in turn can lead to the coarse grain formation described at the beginning and frequently associated grain boundary segregations and thus to a reduction in the fatigue resistance and static strength of the material and component.

According to embodiments of the present invention, however, an improvement in the grain stability and / or the hardenability in the heat treatment can be achieved by using a steel which, in addition to a carbon (C) content of at least 0.25 percent by weight and at most 1.55 percent by weight has a proportion of tantalum (Ta) which is between 0.01 and 2 percent by weight.

By using this steel as a material, it may also be possible, for example, to achieve an edge hardening depth of more than 5 mm and to keep the structure more grain-stable during edge hardening, which can be advantageous, for example, in the rolling bearing area for large bearing rings, but also for rolling elements. Depending on the concrete composition of a steel according to an embodiment of the present invention, edge hardening depths of 10 mm and possibly above can be achieved.

However, exemplary embodiments of the present invention are not limited to such components with an edge hardening depth of at least 5 mm. According to exemplary embodiments of the present invention, components with smaller or greater edge hardening depths can also be realized, that is, for example, at least 100 μm, at least 1 mm or at least one of the further possible edge hardening depths mentioned here in the context of the present description. If, in exemplary embodiments of the present invention, the carbon content of the steel is reduced to values of at least 0.25 percent by weight and at most 1.1 percent by weight or even at most 0.6 percent by weight, this can possibly lead to a further improved edge hardenability, which allows greater edge hardening depths and the danger melting of the structure is reduced.

A variation in the carbon content in the range between approximately 0.4 and approximately 0.55% by weight, that is to say for example approximately 0.45% by weight, can additionally impart advantageous properties to the steel, such as improved inductive solderability.

Exemplary embodiments of the present invention make it possible to increase the temperatures used for edge hardening and thus to reduce the aforementioned risk of coarse grain formation. By using steels in accordance with exemplary embodiments In the present invention, the temperature required for curing can be increased in the range above 1000 ° C. if necessary.

Depending on the amount of tantalum added, one speaks of an alloy or a microalloy. In the area of the so-called microalloy, the proportion of the alloy metal is in a range that is typically not specified. In the case of tantalum, this is generally the range below 0.1% by weight. In the range of the tantalum content of 0.1% by weight and above, it is usually specified so that the addition of the alloying element in this range is called alloying. Independently of this, however, in the context of the present description, the term “alloying” is understood to mean both that of microalloying and that of alloying.

Embodiments of the present invention may further comprise an alloy element with a weight fraction between 0.1 and 5 percent by weight, which is molybdenum (Mo), nickel (Ni), silicon (Si), manganese (Mn) or chromium (Cr) can. In embodiments of the present invention, the weight fraction is often in the range between 0.8% by weight and 1.5% by weight. For example, steel can be produced according to an embodiment of the present invention based on 50CrMo4 or 43CrMo4 as the base alloy. As some of these base alloys also show, a plurality of the aforementioned alloy elements can also be used in exemplary embodiments.

In further exemplary embodiments of the present invention, the steel furthermore has a further alloy element with a weight fraction between 0.01 and 2 percent by weight, the further alloy element being niobium (Nb), titanium (Ti) or vanadium (V). The weight fraction of this further alloy element is often in the range between 0.1 and 1 weight percent. One example is steels according to exemplary embodiments of the present invention based on 50CrV 4.

Frequently, in the case of steels with a niobium content (Nb), the weight proportion of the tantalum (Ta) is at least 1 times, at least 5 times, at least 10 times, at least 20 times, at least 50 times or at least 100 times. times the proportion by weight of niobium (Nb). These ratios of the masses of niobium (Nb) and tantalum (Ta) show that niobium very often occurs as a contamination or additional element of tantalum. Especially with regard to the improved hardenability and the possible positive effects on the steel and the components comprising it, studies indicate that tantalum (Ta) leads to better hardenability of the steel than niobium. Even though niobium does not appear to have any serious negative effects, the studies indicate that tantalum could lead to better hardenability compared to niobium. In this context, the possibility of whether this could possibly be due to the formation of tantalum carbide (TaC) in the structure is discussed. First of all, there is an effort to limit the niobium content compared to the tantalum content. In particularly high-quality applications, a tantalum content of at least 120 times, at least 150 times or at least 200 times the niobium content can therefore correspond, these ratios already being located at the limit of the technical separation possibility of the two elements. Due to the apparently not very detrimental effect of niobium (Nb), it may therefore be more economical to tolerate a certain proportion of niobium for economic and procurement reasons, as well as for the actual steel production, in order to simplify the process of steel production and / or to keep it technically more stable ,

With regard to chromium (Cr), it may be advisable in the case of exemplary embodiments to limit the chromium content to a maximum of 5 percent by weight, to a maximum of 3.8 percent by weight or to a maximum of 2.1 percent by weight. This may result in a competitive situation in the formation of carbides or other carbon-containing compounds, complexes or deposits between Tantalum and chrome can be prevented. It may also be advisable to add molybdenum (Mo) with a weight fraction of up to 0.6 weight percent.

With regard to further elements, a steel according to an exemplary embodiment of the present invention frequently comprises no further alloy elements (ie excluding carbon) with a respective proportion of more than 0.2% by weight apart from iron (Fe), with a total proportion of the further elements as a whole Does not exceed 10% by weight. In the case of purer types of steels, these limits can also be 0.1% by weight or 5% by weight.

For the sake of completeness, it makes sense to point out that a steel is a material whose mass fraction of iron (Fe) is greater than that of any other element, whose carbon content is generally less than 2% by weight. % is and may contain other elements. Embodiments of the steel have a carbon content that is at least 0.25 percent by weight but does not exceed 1.55 percent by weight. If necessary, a sufficient amount of carbon can be provided for carbide formation. In particular, an increase in the carbon content can thus be avoided in the case of steels according to one exemplary embodiment. Regardless of this, however, it may also be advisable in the case of exemplary embodiments to limit the carbon content to a maximum of 1.1 percent by weight or to 0.6 percent by weight. This may result in more targeted carbide formation.

A steel according to one exemplary embodiment can be designed as roller bearing steel, as defined, for example, in ISO 683-17: 1999. Such an induction hardenable steel based on a 43CrMo4 base alloy according to an exemplary embodiment can, for example, achieve one or more hardness values as listed in Table 1 below. Table 1 shows a minimum HRC value (HRC _min ) for a typical steel alloy and a maximum HRC value (HRC _max ) for a typical steel alloy a distance d from a quenched end face in mm. The hardenability can be measured by end-quenching tests according to Jominy at different end-face distances. Table 1: d [mm] HRC _max HRC _min 1.5 61 53 3 61 53 5 61 52 7 60 51 9 60 49 11 59 43 13 59 40 15 58 37 20 56 34 25 53 32 30 51 31 35 48 30 40 47 30 45 46 29 50 45 29

The Rockwell hardness values are given on the HRC scale. Depending on the precise alloy composition, deviating HRC values can also be achieved. For example, HRC values that are 2 or 3 HRC values above or below the minimum and / or maximum HRC values HRC _max or HRC _min mentioned can also be achieved on the basis of the 43CrMo4 base alloy mentioned.

Depending on the steel used in accordance with exemplary embodiments of the present invention, hardnesses starting from 20 HRC in the case of a steel with a carbon content of 0.25% by weight up to 70 HRC in the case of a steel with a carbon content of 1.55% by weight be achieved.

In the case of a steel according to one exemplary embodiment, for example based on a 50CrMo4 base alloy, one or more hardness values according to the following minimum and maximum HRC values HRC _min or HRC _max given in Table 2, depending on the distance d from the quenched end face be achievable in mm. Table 2: d [mm] HRC _max HRC _min 1.5 65 60 3 65 60 5 64 59 7 64 58 9 63 57 11 63 56 13 63 55 15 62 53 20 61 50 25 60 47 30 58 45 35 57 44 40 55 43 45 54 42 50 54 42

Here too, depending on the exact alloy composition and also, where appropriate, on the basis of other base alloys corresponding minimum and maximum values HRC HRC _min - _max values achieved or HRC be that 2 or 3 HRC levels or values above or below the in Table 2 values are. The hardenability can also be measured here by end-quenching tests according to Jominy at different end-face distances.

As already explained above, steels according to exemplary embodiments of the present invention have a reduced grain growth during the heat treatment and thus (generally) a small grain size. According to ASTM standard E 112 (ASTM = American Society for Testing Materials) they have structures with grains with a code number of 5, 6, 7 or above. After the preparation of a cut, a photo with a magnification of 100: 1 is compared with various standard images. Class 5 corresponds to grains with an average diameter of about 60 microns, those of class 6 with an average diameter of about 45 microns, those of class 7 with an average diameter of about 35 microns and those of class 8 with one average diameter of about 22 microns. The grain size and grain boundaries of steels are the size or grain boundaries of the former austenite grains.

With regard to the microstructure of non-metallic inclusions, exemplary embodiments of steels can be quantified, for example, using the measurement methods defined in the standards ASTM E45 and ISO 4967 and DIN 50602: 1985. With these processes, too, sections are made and compared in a 100: 1-times magnification with standard images. Embodiments can be provided according to the following microstructure classes or better. Table 3 shows non-metallic inclusions according to ASTM E45 and ISO 4967, which can achieve exemplary embodiments. Table 3: Jernkontret standard diagram A (fine or thin) 2.5 A (thick) 1.5 B (fine or thin) 2.0 B (thick) 1.0 C (fine or thin) 0.5 C (thick) 0.5 D (fine or thin) 1.0 D (thick) 1.0

Correspondingly, Table 4 shows non-metallic inclusions in accordance with DIN 50602: 1985, which exemplary embodiments can achieve. Here bar diameters d in mm are compared to characteristic cumulative K values. Table 4: d [mm] K factor 200 <d K4 ≤ 20 140 <d ≤ 200 K4 ≤ 18 100 <d ≤140 K4 ≤ 16 70 <d ≤ 100 K4 ≤ 14 35 <d ≤ 70 K4 ≤ 12 17 <d ≤ 35 K3 ≤ 15 8 <d ≤ 17 K3 ≤ 10 d ≤ 8 K2 ≤ 12

Steels according to exemplary embodiments of the present invention are produced in a process which comprises two steps which can be carried out simultaneously or in succession. In a first step, a base alloy of the steel is provided, which is then alloyed with tantalum in a second step.

Basically, steels according to exemplary embodiments of the present invention can be produced using all methods, even if individual methods are more likely to be used in peripheral areas due to economic and / or process-related properties. It is thus possible to carry out both process steps, for example in the case of tonnages which are only to be produced to a lesser extent, as part of the crucible steel process or the electrical steel process, the tantalum being added in metallic form, for example in powder form or as granules (pieces), or as a chemical compound. As a chemical compound, tantalum can be added, for example, as tantalum carbide (TaC), tantalum boride (TaB ₂ ), tantalum silicide (TaSi ₂ ) or tantalum oxide (Ta ₂ O ₅ ).

However, the method steps can also be carried out sequentially. For example, the step of providing the base alloy can include the provision of pig iron in a blast furnace route, but also by other methods. The pig iron can then be further processed into steel using a blow molding process (e.g. LD process or Linz-Donawitz process) or an oven refurbishment process (e.g. Siemens-Martin process). As an optional further step, the provision of the base alloy can further comprise a refining which leads to an adjustment (generally a reduction) in the content of elements such as silicon (Si), manganese (Mn), sulfur (S) or else phosphorus (P) , The properties of the base alloy produced in this way can also optionally be changed further by adding further elements. Examples include vanadium (V), chromium (Cr), calcium (Ca), silicon (Si), niobium (Nb), titanium (Ti), nickel (Ni) and molybdenum (Mo). Deoxidation can be achieved, for example, by adding aluminum (A1), silicon (Si), calcium (Ca) or calcium compounds.

In the second step of alloying with tantalum, the tantalum can then be added to the base alloy in metallic or chemically bonded form. The tantalum can be added to the base alloy in the form of the chemical compounds mentioned above, for example. The step of alloying with tantalum is a typical step of secondary metallurgy, which can be carried out, for example, in a ladle furnace process after the refining. Additional alloying elements can optionally be added, such as vanadium (V), chromium (Cr), calcium (Ca), silicon (Si), niobium (Nb), titanium (Ti), nickel (Ni), molybdenum (Mo) or other elements, insofar as this is still necessary or desired. Chromium, tantalum and molybdenum, for example, can have a corrosion-inhibiting effect. The step of alloying with tantalum takes place here at a temperature of less than 1600 ° C., the temperature optionally also being able to be limited to values below 1550 ° C. or below 1500 ° C.

In the context of secondary metallurgy, further process steps can be included, such as degassing by vacuum degassing or by other processes. This can optionally be followed by further mechanical, thermal or other processing steps, for example rolling the steel.

Fig. 1 shows a cross section through a tapered roller bearing 100 for a large installation, for example a wind power installation, a tidal power installation, a rolling mill or a construction machine. The tapered roller bearing 100 includes an outer ring 110 and an inner ring 120, which in FIG Fig. 1 are shown with respect to a line of symmetry 130, wherein the line of symmetry 130 coincides with the axis of the tapered roller bearing 100. A plurality of frustoconical rolling elements 140 are arranged between the inner ring 120 and the outer ring 110 and are guided by an optional cage 150.

Due to a relative movement of the inner ring 120 to the outer ring 110, the rolling elements 140 roll on a running surface 160 of the inner ring 120 and a running surface 170 of the outer ring 110. The tapered roller bearing 100 has guide rims for lateral guidance of the rolling bodies 140. The inner ring thus comprises a first flange 180 and a second flange 190, while the outer ring shown here has no lateral guide ribs.

The outer ring 110 and the inner ring 120 are made entirely of steel in accordance with an exemplary embodiment of the present invention. Starting from the running surfaces 160, 170 of the two roller bearing rings 110, 120, these each have an edge hardening area 200, 210 in which the steel of the two roller bearing rings 110, 120 has been subjected to edge hardening. Both edge hardening areas 200, 210 extend here from the surfaces of the two components, that is to say the two running surfaces 160, 170, to an edge hardening depth which can be predetermined by the process parameters and the material properties of the steel used, into the component. In exemplary embodiments of the present invention, the edge hardening areas can extend from the surfaces of the components, for example, at least 5 mm into them, but can also have smaller or greater edge hardening depths. ever If required, a thicker edge hardening area of 10 mm or more can be created, which counteracts premature fatigue, particularly in the case of components subject to high loads, such as the roller bearing rings of large machines, and also the higher allowance for hard machining in the case of larger roller bearing rings (e.g. due to warpage ) Takes into account. But also in the area of rolling elements and other components or machine parts, edge hardening areas with edge hardening depths to the extent described are often very welcome for various reasons.

Both the outer ring 110 and the inner ring 120 therefore represent exemplary embodiments of a component according to the present invention.

Fig. 2 illustrated using the example of in Fig. 1 outer ring 110 shown the generation of the edge hardening region 200 by means of induction hardening. For this purpose, an inductor 220 is guided over the surface to be hardened, that is to say in the present case over the running surface 170, while the inductor 220 generates eddy currents in the workpiece (component or outer ring 110) via an alternating magnetic field. For this purpose, inductor 220 includes an in Fig. 2 Coil, not shown, through which an alternating current can be switched on with a predetermined, adjustable or programmable frequency. By setting the current intensity of the current, i.e. essentially the power of the inductor 220, by determining the frequency of the alternating current and by determining the duration of the action of the alternating field, a certain amount of heat is introduced into a surface layer of the workpiece 110 by means of electrical current flow and heat conduction , which leads to heating of the workpiece. In order to avoid damage to the inductor 220 by high temperatures, this can optionally also be an in Fig. 2 Cooling system, not shown, include, for example, water cooling.

Based on the skin effect, the penetration depth of the alternating field and thus a thickness of the edge hardening area to be generated can be determined via the frequency. As a result, a quantity of heat can thus be introduced into the workpiece (outer ring 110) below its surface (running surface 170), through which it closes the thermally induced hardening of the steel. Due to the effect of heat, a new structure then occurs in the edge hardening area 170, but due to the use of the steel according to exemplary embodiments of the present invention, coarse grain formation in austenite with large martensite needles or laths formed during quenching and corresponding former austenite grain boundaries is substantially prevented, at least however can be reduced so that the aforementioned thicknesses of the edge hardening areas can be achieved at least without excessive material damage (e.g. coarse grain with grain boundary segregation).

The inductor 220 can, for example, generate temperature increases of more than 100 ° C./s and above. Investigations have shown here that with temperature increases between 100 ° C./s and 300 ° C./s, grains with an ASTM parameter of 5 or above, possibly even 6 or 7 and above, can be obtained in the edge hardening region 200.

Steels in accordance with the exemplary embodiments of the present invention can thus be inductively hardened and can be designed to be high-strength at least in the edge region by such hardening. They are therefore suitable as steels for rolling bearings and for other areas of application in which the components made from them are subjected to strong dynamic and / or static loads. Rolling bearings are typically cyclic and / or static compressive loads, with the improved hardenability and grain stability counteracting wear, fatigue and spontaneous failure. The lack of critical tensile stresses and the high hydrostatic pressure in the Hertzian contact allow the hardened steel to flow microplastically and lead to a service life that is orders of magnitude longer than that of a comparable cyclic tensile / compressive or circular bending stress in the event of rolling fatigue due to rollover.

A multi-frequency process can optionally be used for induction hardening. In addition, the in Fig. 2 Not shown receptacle for the workpiece (outer ring 110) may optionally be grounded to improve the heating by the eddy currents thrown into the workpiece. The inductor 220 can be operated, for example, at a frequency in the range between 1 kHz and 5 kHz at a distance of less than 20 mm from the surface to be hardened (raceway or running surface 170 and 160). The current or power to be used depends heavily on the geometry and size of the component.

Fig. 3 shows a schematically simplified representation of a hardness curve 230 (H = hardness) as a function of a depth (d = depth) from the surface, that is to say in the case of FIGS Figures 1 and 2 Outer ring 110 shown from the tread 170 starting from the edge to the core. Starting from the surface of the outer ring (d = 0), the hardness initially has a first constant value in the edge hardening area 200 before it decreases outside the edge hardening area 200 and strives for a second value in the interior of the workpiece. The first value of the hardness H is higher than that of the second value, the first value being due to the edge hardening carried out and the second value being due to the properties of the underlying steel. The first value is determined by the hardenability of the steel.

The Rockwell hardness can be specified according to the HRC scale, for example. Depending on the steel used in accordance with exemplary embodiments of the present invention, hardnesses starting from 20 HRC in the case of a steel with a carbon content of 0.25% by weight up to 70 HRC in the case of a steel with a carbon content of 1.55% by weight be achieved. The hardenability can be measured by end-quenching tests according to Jominy at different end-face distances.

The resulting hardness is mainly caused by the carbon content, the range from 0.25% by weight to 1.55% by weight for induction hardening and flame curing is suitable. The other alloy metals improve hardenability and prevent excessive grain growth, a small tantalum content of 0.01% by weight and above, which is attributable to the microalloy, acting to stabilize the grain. This also applies to higher tantalum fractions in the range between 0.1% by weight and 2% by weight. Through the interaction of carbon and the alloying element tantalum (Ta), the structure of the steel can be kept largely stable with a high resulting hardness even with a longer and / or higher temperature treatment. In further exemplary embodiments, the tantalum content can also be at least 0.2 percent by weight or 0.25 percent by weight.

In exemplary embodiments, the tantalum content can also be increased beyond the aforementioned values. If this is increased to values of at least 0.5 percent by weight, for example, in addition to the fine grain formation, the carbide formation or carbide formation can also be positively influenced, if necessary.

A suitable (micro) alloying with further alloying elements, for example with niobium (Nb), titanium (Ti) or vanadium (V), can (further) hinder the grain growth during the heat treatment. Typical levels of these alloying elements are between 0.01 and 2% by weight. According to the invention, conventional induction hardenable steels, e.g. B. 50CrMo4 or 43CrMo4, as base alloys in the manner described additionally (micro) alloyed so as to. B. 50CrMo4 + Ta or 43CrMo4 + Ta.

It is assumed that embedded carbides hinder grain growth, as these act as obstacles to the movement of the grain boundaries and as germs during the transformation into austenite and can thus support grain formation. Therefore, with increasing alloy addition, these steels are less sensitive to overheating than unalloyed steels with the same carbon content. The stored carbides prevent excessive grain growth. Tantalum carbide (TaC) in particular is particularly advantageous here, since this is not the case in comparison to other carbides seems to dissolve quickly. Via microalloying with an alloying element, the austenite grain growth can be slowed down when heated to the forging temperature by the formation of metal carbides. The forging temperature is often in the range of the relevant austenitizing temperatures for induction hardening with short-term austenitizing.

Overall, the material becomes more tolerant to overheating during induction hardening through microalloying. This extends the available process window and makes the process less sensitive to deviations.

The exemplary embodiments of the present invention for components have so far been shown in FIG Fig. 1 also shown inner ring 110 and the outer ring 120 of a single row tapered roller bearing. However, exemplary embodiments of the present invention are by no means restricted to this. In addition to multi-row (e.g. two-row) rolling bearing rings, rolling elements (including a cylindrical roller, ball and tapered roller) and other rolling bearing components to be hardened inductively, the exemplary embodiments also include other types of rolling bearing designs, such as cylindrical roller bearings, barrel roller bearings, ball bearings, four-point bearings and needle bearings as well, such as plain bearings, intermediate rings and other components and machine parts of rotary and linear bearing technology. Components of other disciplines of vehicle and mechanical engineering can also be implemented as exemplary embodiments of the present invention. This basically includes all components that have at least one area that is subjected to an increased load, so that it makes sense to carry out a corresponding edge hardening there starting from the edge of the component in question.

Even if only components that have been made entirely of steel according to an exemplary embodiment of the present invention have been described hitherto, exemplary embodiments are also not limited in this respect to a complete design made of one material. A component according to an exemplary embodiment of the present invention thus has at least one section Steel according to an embodiment of the present invention. The section extends from a surface of the component into an interior of the component, the component having an edge hardening in the section. In some embodiments of the present invention, the edge hardening extends at least 5 mm into the interior of the component. However, larger or smaller edge hardening depths can also be implemented according to exemplary embodiments of the present invention, as has already been described above. Examples of this are composite components which have a correspondingly shaped section in which steel is implemented according to an exemplary embodiment of the present invention together with a corresponding edge hardening area. Other areas of the component in question can be made, for example, from a different metal, a different alloy or a different steel. In these cases, a connection can be made cohesively in the form of a soldered or welded connection. However, other connection methods are also conceivable here, for example a non-positive or positive connection or an adhesive bond as a further form of the material connection. These methods can be used, for example, if soldering or welding is out of the question, i.e. if the substances involved cannot be soldered or welded. Examples of this can be plastics or glass fiber reinforced materials. In embodiments that are not completely made of steel according to one embodiment, but at least have a section, this can extend from the surface along a straight line in such a way that the straight line runs completely in the steel until it touches a (further) Surface section emerges from the component.

In the case of exemplary embodiments, the component or its section which comprises the steel can optionally be comprised of further materials. Such a component can also be made entirely or in a corresponding section from a material which comprises the steel. Such a material can comprise, for example, a fiber-reinforced steel or another hybrid material combination in which steel is used in accordance with one exemplary embodiment.

A steel according to one exemplary embodiment can thus be a steel for flame or induction hardening, for example. As an alternative or in addition to this, it can also be a roller bearing steel. In the case of a steel according to one exemplary embodiment, it can have no further elements with a respective proportion of more than 0.2% by weight apart from iron and carbon and the aforementioned alloying substances, with a total proportion of the further elements not exceeding 10% by weight. By implementing an exemplary embodiment, an improved and / or easier edge hardening can possibly be achieved.

The possible areas of application of exemplary embodiments of the present invention initially include all large-scale systems in which individual components are subjected to a corresponding mechanical load, which makes edge hardening advisable. This includes wind and tidal power plants as well as generators, construction machines, cranes, excavators, transporters, trains, planes, rolling mills and other machines. In principle, it can also be advisable to use exemplary embodiments in smaller systems and their components, since high loads can also occur in such systems, for example due to sudden shock loads. Microalloyed, grain-stable steels for inductive heat treatment according to exemplary embodiments of the present invention, and corresponding components can therefore be used in a wide range of applications.

LIST OF REFERENCE NUMBERS

100

Tapered roller bearings

110

outer ring

120

inner ring

130

line of symmetry

140

rolling elements

150

Cage

160

tread

170

tread

180

first shelves

190

second shelves

200

Randhärtungsbereich

210

Randhärtungsbereich

220

inductor

230

hardness profile

Claims (8)

1. Steel including tantalum in a proportion of at least 0.1 per cent by weight and at most 2 per cent by weight inclusive and a carbon content of at least 0.25 per cent by weight and at most 1.1 per cent by weight,
   optionally further including at least one alloy element in a proportion by weight between 0.1 and 5 per cent by weight, where the alloy element is molybdenum, nickel, silicon, manganese or chromium, optionally also including at least one further alloy element in a proportion by weight between 0.01 and 2 per cent by weight, where the further alloy element is niobium, titanium or vanadium,
   including tantalum in a proportion by weight greater than 10 times the proportion by weight of niobium and a balance of iron.
2. Steel according to Claim 1, in which the tantalum content is higher than 0.1 per cent by weight or higher than 0.5 per cent by weight, and/or wherein the carbon content is not more than 0.6 per cent by weight.
3. Steel according to either of the preceding claims, including nonmetallic inclusions according to ASTM E45 and ISO 4967 that correspond at most to the values defined below: -Jernkontoret standard diagram- A (fine or thin) 2.5 A (thick) 1.5 B (fine or thin) 2.0 B (thick) 1.0 C (fine or thin) 0.5 C (thick) 0.5 D (fine or thin) 1.0 D (thick) 1.0, or including nonmetallic inclusions according to DIN 50602:1985 which, for a bar diameter d in mm 200 < d, have a K factor K4 ≤ 20;

   for 140 < d ≤ 200 a K factor K4 ≤ 18;

   for 100 < d ≤ 140 a K factor K4 ≤ 16;

   for 70 < d ≤ 100 a K factor K4 ≤ 14;

   for 35 < d ≤ 70 a K factor K4 ≤ 12;

   for 17 < d ≤ 35 a K factor K3 ≤ 15;

   for 8 < d ≤ 17 a K factor K3 ≤ 10; and

   for d ≤ 8 a K factor K2 ≤ 12.
4. Steel according to any of the preceding claims, including at least sections of a microstructure having grains having an index of 5 or higher according to ASTM and/or have a hardenability with at least one hardness value at at least a distance value d from a surface, wherein the at at least one hardness value is within an interval given by an HRC_max value and an HRC_min value, where the subsequent HRC_max and HRC_min values respectively assigned to a distance value d may be up to 3 HRC grades higher or lower than the HRC_max and HRC_min values: d [mm] HRC_max HRC_min 1.5 65 60 3 65 60 5 64 59 7 64 58 9 63 57 11 63 56 13 63 55 15 62 53 20 61 50 25 60 47 30 58 45 35 57 44 40 55 43 45 54 42 50 54 42
5. Steel according to any of the preceding claims, which is a steel for flame hardening or induction hardening, and/or wherein the steel is a roller bearing steel.
6. Process for producing steel, comprising:

   providing a base alloy for the steel; and

   alloying the base alloy with tantalum,

   such that the steel includes tantalum in a proportion of at least 0.1 per cent by weight and at most 2 per cent by weight inclusive and a carbon content of at least 0.25 per cent by weight and at most 1.1 per cent by weight, and optionally further includes at least one alloy element in a proportion by weight between 0.1 and 5 per cent by weight, where the alloy element is molybdenum, nickel, silicon, manganese or chromium, and optionally further includes at least one further alloy element in a proportion by weight between 0.01 and 2 per cent by weight, where the further alloy element is niobium, titanium or vanadium, and includes tantalum in a proportion by weight greater than 10 times the proportion by weight of niobium and

   a balance of iron.
7. Process according to Claim 6, in which the alloying comprises adding tantalum carbide, tantalum boride, tantalum silicide, tantalum oxide or tantalum to the base alloy.
8. Component (110; 120) including, at least in one section, steel according to any of Claims 1 to 5, wherein the section extends from a surface (160; 170) of the component (110; 120) into an interior of the component (110; 120), and wherein the component (110; 120) has edge hardening (200; 210) in the section.

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