WO2013014280A1

WO2013014280A1 - Steel, component and method for producing steel

Info

Publication number: WO2013014280A1
Application number: PCT/EP2012/064806
Authority: WO
Inventors: Jesko-Henning Tanke; Thilo Von Schleinitz
Original assignee: Aktiebolaget Skf
Priority date: 2011-07-28
Filing date: 2012-07-27
Publication date: 2013-01-31
Also published as: DE102011079955A1; EP2737097B1; DE102011079955B4; EP2737097A1

Abstract

Steel according to an exemplary embodiment comprises tantalum in a fraction of between 0.01 and 2 percent by weight and a carbon fraction, which is at least 0.25 percent by weight and at most 1.55 percent by weight. A component (110; 120) according to an exemplary embodiment of the present invention comprises this steel at least in one portion, the portion extending from a surface (160; 170) of the component (110; 120) into an interior of the component (110; 120), and wherein the component (110; 120) is case-hardened (200; 210) in the portion. This may allow improved and/or easier case hardening to be achieved.

Description

Steel, component and method of making steel

Embodiments of the present invention relate to steel, a structural member having the steel, and a method of manufacturing the steel.

In many machines and systems, high mechanical loads occur in different shapes and forms. Often, but by no means exclusively, machine parts are affected here, which perform relative movements to each other, so for example, translational or rotational movements. In addition to continuous loads, short-term and / or local loads can occur which can lead to fatigue or static overstressing of the relevant machine parts.

Especially for such machine parts, steel is used on account of its far-reaching manufacturability, its strength, its deformability, its weldability and many other properties. One of the most important technological features is that steel can be completely or partially hardened. If, for example, only edge hardening or surface hardening is carried out, then the correspondingly treated surface becomes harder and thus more mechanically and tribologically resistant, while deeper areas of the machine part have lower hardness and thus greater toughness, as a result of which the machine part as a whole becomes cyclically and statically more resilient. Dynamic loads are often introduced via the surface of the machine parts in this, so have a strong impact on the typically hardened areas. This can cause dislocation movements in the material, which can ultimately lead to plastic deformation in the interior of the machine part. These often claim the range of grain boundaries in the microstructure of steel, which can lead to an intercrystalline fracture before reaching the strength in volume under susceptible material state.

To counteract this, preference is given to using steels which have a large number of grain boundaries, ie have small crystals in the microstructure, since in this way the mechanical stresses and segregation with weakening elements (eg phosphorus, sulfur) or even compounds (For example, carbides) are distributed over a larger number and thus area of grain boundaries and decreases the stress of the individual grain boundaries.

However, the hardening of the steel in the edge region typically takes place thermally, the steel of the machine part being heated accordingly. This can be done for example by flame or induction hardening, so for example by an inductive heating of the workpiece.

Hardening represents a thermally induced process, so that undesired heating above the austenitic temperature of the steel and thus overheating of the microstructure can occur, especially in the case of relatively large edge hardening depths. As a result, an increased grain growth, so a coarse grain formation occurs. This can lead to reduced strength in the subsequent insert structure due to weakening of the grain boundaries (eg, by segregation, i.e., attachment of noxious elements), with a tendency for intergranular fracture.

To date, steels having a maximum carbon content of 1, 1% by weight (wt%) and a sufficient surface hardenability have been used, which is influenced by the alloy composition. Furthermore, seeks to guide the hardening process to avoid excessive grain growth.

Based on this, the object of the present invention is therefore to provide 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 8.

An exemplary embodiment of a steel comprises tantalum in a proportion of between 0.01 and 2 percent by weight and a carbon content of at least 0.25 percent by weight and at most 1.55 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 at a level of between 0.01 and 2 percent by weight and a carbon content that is at least 0.25 percent by weight; at most 1.55 weight percent.

Embodiments of the invention are based on the finding that an improved hardenability and / or improved grain stability and / or static strength can be achieved by using tantalum as a (carbon) steel having a carbon content of at least 0.25% by weight and at most 1.55% by weight. ) Alloying element is added. Here, investigations have shown that even very low tantalum levels of about 0.01 percent by weight lead to improved hardenability or to improved grain stability. With increasing proportion of tantalum, a corresponding improvement of the abovementioned properties compared to unalloyed steel is achieved in the range between 0.01 and 2.0 per cent by weight. The addition of tantalum makes it possible to increase the austenitizing temperature during edge hardening. hereby For example, the accessible process temperatures can be increased to over 1000 ° C, for example up to 150 ° C, while conventional steels require significantly lower temperatures.

Embodiments of the present invention further have for its object to provide a component which has improved fatigue resistance in at least one section.

This object is achieved by a component according to claim 10.

An embodiment of the present invention in the form of a component thus comprises in at least a portion of the aforementioned steel, the portion extending from a surface of the component to an interior of the component, and wherein the component has edge hardening in the portion.

For example, in some embodiments of the present invention, it may extend at least 5 mm into the interior of the component, while in other embodiments, lower or greater edge cure depths may be realized.

As edge hardening depth, the vertical distance to the surface should be understood until reaching an ultimate hardness of 550 HV1 (52.5 HRC).

Embodiments of the present invention with respect to this aspect are based on the finding that improved hardenability and / or static strength and / or improved grain stability during the heat treatment during curing can be achieved by the use of the steel described above, or that thereby a process for curing a component can be simplified. Thus, in some embodiments of the present invention, edge hardening having a greater edge hardening depth of at least 5 mm can be achieved in at least a portion of the component by (eg, inductive) hardening. ne that here an increased coarse grain formation sets in, which could adversely affect the fatigue resistance and static strength of the material. However, embodiments of the present invention are by no means limited to components having an edge hardening depth of 5 mm or more. Embodiments of the present invention also include components with a lower edge hardening depth than 5 mm, so for example those with Randhärtungstiefen of at least 100 μηι, at least 200 μηι, at least 500 μηι, at least 1 mm or at least 2 mm.

For the purposes of the present description, nominal weight percentages are used with respect to the individual components. In the case of steels, deviations from the nominal proportions may occur due to the production-related processes of steel production. The proportion of an alloying element or another component (eg carbon) of a steel of a specific embodiment may thus be below or exceeding the nominal values within the scope of the usual manufacturing and manufacturing tolerances.

Embodiments of the present invention will be explained below with reference to the accompanying drawings.

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 exemplary embodiment of the present invention during edge hardening; FIG. and

3 illustrates a graph of hardness through a component according to an embodiment of the present invention as a function of a distance from the surface of the component. In the following, embodiments of the present invention will be described and explained in more detail with reference to FIGS.

As described in the introduction, in many applications strong mechanical loads occur which lead to fatigue of the relevant component. Frequently, these loads are short-term, intermittent or periodic dynamic loads, which often act only on 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 frequently used, in particular surface-hardened steels. As has also been described at the outset, in particular steels are preferred which have small particle sizes in order to distribute occurring mechanical loads and weakening segregation or precipitation assignments to a large number and thus area of grain boundaries.

In induction hardening or flame hardening, therefore, a workpiece is locally heated to a temperature of more than 1000 ° C frequently by being heated from outside (flame hardening) or by eddy currents (induction hardening) generated in an outer layer of the workpiece near the surface. Typically, locally temperatures of up to 1 150 ° C can be achieved. However, this can easily lead to overheating of the structure, which in turn can lead to the coarse grain formation described above and thus frequently associated grain boundary segregation 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 the use of a steel which in addition to a proportion of carbon (C) of at least 0.25 weight percent and at most 1.55 weight percent zent a proportion of tantalum (Ta), which is between 0.01 and 2 percent by weight.

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

However, embodiments of the present invention are not limited to such components having an edge hardening depth of at least 5 mm. It can be realized in accordance with embodiments of the present invention as well components with lower or larger Randhärtungstiefen, ie, for example, at least 100 μηι, at least 1 mm or at least one of the other, here mentioned in the present description possible Randhärtungstiefen. In embodiments of the present invention, when the carbon content of the steel is reduced to values of at least 0.25 weight percent and at most 1.1 weight percent or even at most 0.6 weight percent, this may eventually result in further improved edge hardenability allowing greater edge cure depths and risk the melting of the microstructure is reduced.

Variation of the carbon content in the range of between about 0.4 and about 0.55 wt%, for example, about 0.45 wt%, can add additional beneficial properties to the steel, such as improved inductive solderability.

Thus, embodiments of the present invention make it possible to increase the temperatures used for edge curing and thus to reduce the aforementioned risk of coarse grain formation. Due to the use of steels according to As a result of the present invention, if necessary, the temperature required for curing can be increased in the range above 1000 ° C.

Depending on the amount of added tantalum one speaks of an alloy or a micro alloy. In the area of so-called microalloying, the proportion of the alloying metal is within 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 region is referred to as alloying. Regardless of this, however, in the context of the present description, the term alloying is understood to mean both that of microalloying and of alloying.

Embodiments of the present invention may further comprise an alloying element having a weight fraction between 0, 1 and 5 percent by weight, which may be 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% and 1.5% by weight. For example, steel according to an embodiment of the present invention may be produced based on 50CrMo4 or 43CrMo4 as a base alloy. As some of these base alloys also show, in embodiments also a plurality of the aforementioned alloying elements can be used.

In further embodiments of the present invention, the steel further comprises another alloying element having a weight fraction between 0.01 and 2 percent by weight, the further alloying element being niobium (Nb), titanium (Ti) or vanadium (V). The proportion by weight of this further alloying element is often in the range between 0, 1 and 1 percent by weight. One example is steels according to embodiments of the present invention based on 50CrV4. In the case of steels with niobium content (Nb), the proportion by weight of tantalum (Ta) is frequently 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 weight fraction of niobium (Nb). In these proportions of the niobium (Nb) and tantalum (Ta) masses, it is found that niobium very often occurs as a tantalum contaminant. Especially with regard to the improved hardenability and the resulting positive effects on the steel and its components, studies suggest that tantalum (Ta) leads to better hardenability of the steel than niobium. Although niobium does not appear to have any seriously negative effects, the studies indicate that tantalum niobium could lead to better hardenability. In this context, the possibility is discussed whether this could possibly be due to the formation of tantalum carbide (TaC) in the microstructure. Thus, there is an initial desire to limit the niobium content in comparison to the tantalum content. For particularly high-quality applications, therefore, a tantalum content of at least 120 times, at least 150 times or at least 200 times the niobium content may also correspond, with these ratios already being very close to 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 suitable for economic and procurement reasons, and for reasons of actual steel production, to tolerate a certain niobium content 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 embodiments, to limit the chromium content to a maximum of 5% by weight, to a maximum of 3.8% by weight or to a maximum of 2.1% by weight. Thus, this may possibly result in a competitive situation in the formation of carbides or other carbon-containing compounds, complexes or deposits between see tantalum and chrome are suppressed. It may also be advisable to add molybdenum (Mo) by weight up to 0.6% by weight.

With regard to further elements, a steel according to an embodiment of the present invention often comprises no other alloying elements (ie excluding carbon) other than iron (Fe) in a respective proportion of more than 0.2 wt.%, Wherein a total proportion of the further elements as a whole 10 wt .-% does not exceed. In purer versions of steels, these limits may also be 0.1 wt .-% and 5 wt .-%.

For the sake of completeness, it is worth pointing 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 can contain other elements. Embodiments of the steel have a carbon content that is at least 0.25 weight percent, but does not exceed 1.55 weight percent. Thus, if necessary, a sufficient amount of carbon can be provided for carbide formation. In particular, in steels according to an embodiment, an increase of the carbon content can be avoided if necessary. Regardless, in embodiments, however, it may also be advisable to limit the carbon content to at most 1.1 percent by weight or 0.6 percent by weight. As a result, if appropriate, a more targeted carbide formation can be effected.

A steel according to an embodiment may be designed as bearing steel, as defined for example in ISO 683-17: 1999. Such an induction hardenable steel based on a 43CrMo4-base alloy according to one embodiment can achieve, for example, one or more hardness values, as listed in Table 1 reproduced below. Table 1 shows a minimum H C value (HRC _m j _n ) 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 opposite each other. The hardenability can be measured here by Jominy forehead quenching tests at different end face distances.

Table 1 :

d [mm] HRC _max HRC _m j "

1.5 61 53

3 61 53

5 61 52

7 60 51

9 60 49

1 1 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 hardness values are given here according to Rockwell according to the HRC scale. Depending on the exact alloy composition, deviating HRC values may also be achievable. Thus, HRC values which are 2 or 3 HRC values above or below the stated minimum and / or maximum HRC values HRC _max or HRC _min can optionally also be achieved on the basis of the abovementioned base alloy 43CrMo4.

Depending on the steel used according to embodiments of the present invention, hardnesses starting from 20 HRC in the case of a steel having a carbon content of 0.25% by weight up to 70 HRC in a steel having a carbon content of 1.55% by weight can be used. be achieved. Thus, in the case of a steel according to one exemplary embodiment-for example based on a 50CrMo4-based alloy - one or more hardness values can be determined according to the following minimum and maximum HRC values HRC _m j _n or HRC _max given in Table 2 as a function of the distance d from the Quenched face in mm can be achieved.

Table 2:

d [mm] HRC _max HRC _m j "

1.5 65 60

3 65 60

5 64 59

7 64 58

9 63 57

1 1 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

Depending on the exact alloy composition and also, if appropriate, on the basis of other base alloys, corresponding minimum and maximum HRC values HRC _min and HRC _max values can be achievable, which are 2 or 3 HRC levels or values above or below the values given in Table 2 are given. The hardenability can also be measured here by Jominy forehead quenching tests at different end face distances.

As explained above, steels according to embodiments of the present invention have reduced grain growth in the heat treatment and thus (generally) a small grain size. According to ASTM standard E 1 12 (ASTM = American Society for Testing Materials) they have microstructures with grains with a score of 5, 6, 7 or more. After the preparation of a cut, a photo is compared at a magnification of 100: 1 with different standard images. The class 5 grains here correspond with a mean diameter of about 60 μηι, those of class 6 such with a mean diameter of about 45 μηι, those of class 7 such with a mean diameter of about 35 μηι and those of class 8 such with a average diameter of about 22 μηι. In the case of steels, the size or grain boundaries of the former austenite grains are referred to as grain size and grain boundaries.

With regard to the microstructure of non-metallic inclusions, exemplary embodiments of steels can be quantified using, for example, the measurement methods defined in the standards ASTM E45 and ISO 4967 and DIN 50602: 1985. Even with these methods, cuts are made, and compared with standard images in a magnification of 100: 1. Embodiments may be provided according to the following microstructure classes or better. Table 3 shows nonmetallic inclusions according to ASTM E45 and ISO 4967, which can achieve exemplary embodiments.

Table 3:

Jernkontret

default chart

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 Accordingly, Table 4 shows non-metallic inclusions according to DIN 50602: 1985, which can achieve embodiments. In this case, bar diameter d in mm is compared with 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 embodiments of the present invention are made in a process comprising two steps that may be performed simultaneously or sequentially. In this case, a base alloy of the steel is provided in a first step, which is then alloyed with tantalum in a second step.

In principle, steels according to embodiments of the present invention can be produced using all methods, even if due to economic and / or process-related properties, individual methods are used more in peripheral areas. Thus, it is possible to carry out both process steps, for example in the case of only small tonnages to be produced in the context of the crucible steel process or the electrical steel process, wherein the tantalum is added in metallic form, for example in powder form or as granules (pieces), or as a chemical compound. As a chemical compound, tantalum may 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 process steps can also be carried out sequentially. For example, the step of providing the base alloy may include providing pig iron in a blast furnace route, but also by other methods. The pig iron can then be further processed into steel by means of a blowing process (eg LD process or Linz-Donawitz process) or a fresh hearth process (eg Siemens-Martin process). As an optional further step, the provision of the base alloy may further comprise refining resulting in an adjustment (generally a decrease) in the content of elements such as silicon (Si), manganese (Mn), sulfur (S), or even phosphorus (P) , Also, by adding further elements, the properties of the base alloy thus produced can optionally be further changed. Examples include vanadium (V), chromium (Cr), calcium (Ca), silicon (Si), niobium (Nb), titanium (Ti), nickel (Ni), and molybdenum (Mo). For example, deoxidation can be achieved by adding aluminum (AI), 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 bound form. The tantalum can in this case be added, for example, in the form of the abovementioned chemical compounds of the base alloy. The step of alloying with tantalum is hereby a typical step of secondary metallurgy, which can be carried out, for example, in a ladle furnace process after refining. Optionally, further alloying elements may be added, such as vanadium (V), chromium (Cr), calcium (Ca), silicon (Si), niobium (Nb), titanium (Ti), nickel (Ni), molybdenum (Mo) or other elements, as far as necessary or desired. For example, chromium, tantalum and molybdenum can act to inhibit corrosion. The step of alloying with tantalum takes place here at a temperature of less than 1600 ° C, where appropriate, the temperature can also be limited to values of below 1550 ° C or below 1500 ° C.

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

Fig. 1 shows a cross section through a tapered roller bearing 100 for a large plant, such as a wind turbine, a tidal power plant, a rolling mill or a construction machine. The tapered roller bearing 100 comprises an outer ring 110 and an inner ring 120, which are shown in FIG. 1 with respect to a line of symmetry 130, wherein the line of symmetry 130 coincides with the axis of the tapered roller bearing 100. Between the inner ring 120 and the outer ring 1 10 a plurality of frustoconical rolling elements 140 is arranged, which are guided by an optional cage 150.

By a relative movement of the inner ring 120 to the outer ring 1 10, the rolling elements 140 roll on a running surface 160 of the inner ring 120 and a running surface 170 of the outer ring 1 10 from. For lateral guidance of the rolling elements 140, the tapered roller bearing has 100 guide rims. The inner ring thus comprises a first board 180 and a second board 190, while the outer ring shown here has no lateral guide rims.

The outer ring 1 10 and the inner ring 120 are in this case made entirely of a steel according to an embodiment of the present invention. Starting from the running surfaces 160, 170 of the two roller bearing rings 1 10, 120, these each have an edge hardening region 200, 210, in which the steel of the two rolling bearing rings 1 10, 120 has been subjected to edge hardening. Both edge hardening regions 200, 210 extend from the surfaces of the two components, that is to say the two running surfaces 160, 170, to a peripheral hardening depth that can be predetermined by the process parameters and the material properties of the steel used, into the component. For example, in embodiments of the present invention, the edge hardening regions extending from the surfaces of the components may extend at least 5 mm into them, but may also have lower or greater edge hardening depths. ever If necessary, a thicker edge hardening range of 10 mm or more can be produced, which counteracts premature fatigue, especially in the case of heavily loaded components, such as the rolling bearing rings of large machines, and also for higher machining allowances for larger rolling bearing rings (eg because of distortion ). But also in the field of rolling elements and other construction or machine parts Randhärtungsbereiche with Randhärtungstiefen in the extent described are often very welcome for various reasons.

Both the outer ring 1 10 and the inner ring 120 therefore represent embodiments of a component according to the present invention.

FIG. 2 illustrates the production of the edge hardening region 200 by means of induction hardening using the example of the outer ring 110 shown in FIG. 1. For this purpose, an inductor 220 is guided over the surface to be hardened, that is in the present case over the running surface 170, while the inductor 220 in the workpiece (component or outer ring 1 10) generates eddy currents via an alternating magnetic field. For this purpose, the inductor 220 comprises a coil, not shown in FIG. 2, by means of which an alternating current with a predetermined, adjustable or programmable frequency can be added. By adjusting the current intensity of the current, ie substantially 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 by electrical current flow and heat conduction in a surface layer of the workpiece 1 10th introduced, which leads to a heating of the workpiece. In order to avoid damaging the inductor 220 by high temperatures, this may optionally also include a cooling system, not shown in FIG. 2, that is to say a water cooling system.

Due to the skin effect, a 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 10) under its surface (tread 170), through which it can be introduced the thermally induced hardening of the steel. Due to the effect of heat, formation of the structure then occurs in the edge hardening region 170, but because of the use of the steel according to embodiments of the present invention, coarse grain formation in the austenite with large martensite needles or slats and corresponding former austenite grain boundaries arising during quenching is essentially 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 (eg coarse grain with grain boundary segregation).

By the inductor 220, for example, temperature increases of more than 100 ° C / s and above can be generated. Studies have shown that with temperature increases between 100 ° C / s and 300 ° C / s grains with an ASTM characteristic of 5 or more, possibly even 6 or 7 and more in the edge hardening range 200 can be achieved.

Steels according to the embodiments of the present invention are so inductively curable and can be made high strength by such a cure at least in the edge region. They are therefore suitable as steels for rolling bearings and for other applications 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, with improved hardenability and grain stability counteracting wear, fatigue and spontaneous failure. The absence of critical tensile stresses and the high hydrostatic pressure in Hertzian contact cause the hardened steel to flow in a microplastic manner and lead to an order of magnitude greater fatigue life due to rolling over orders of magnitude than a comparable cyclic tensile, compressive or circumferential bending stressing. In induction hardening, a multi-frequency method can optionally also be used. In addition, the receptacle for the workpiece (outer ring 110), not shown in FIG. 2, can optionally be grounded in order to improve the heating by the eddy currents thrown in the workpiece. The inductor 220 can in this case be operated, for example, with 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 strongly 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, ie in the case of the outer ring 110 shown in FIGS. 1 and 2, starting from the running surface 170 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 region 200 before it decreases outside of the edge hardening region 200 and strives for a second value in the interior of the workpiece. The first value of the hardness H is hereby higher than that of the second value, whereby the first value is due to the edge hardening carried out and the second value to the properties of the underlying steel. The first value is therefore determined by the hardenability of the steel.

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

The resulting hardness is mainly caused by the carbon content, the range of 0.25 wt .-% to 1.55 wt .-% for the induction hardening tion and the flame hardening is suitable. The other alloying metals improve hardenability and prevent excessive grain growth, even a small proportion of tantalum of 0.01% by weight and above, which is attributable to the micro-alloying, has a grain-stabilizing effect. This also applies to higher tantalum contents in the range between 0.1 wt .-% and 2 wt .-%. As a result of 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 in the case of a longer and / or higher-temperature heat treatment. In other embodiments, the tantalum level may also be at least 0.2 weight percent or 0.25 weight percent.

In embodiments, the tantalum component can also be increased beyond the aforementioned values. For example, if this is increased to values of at least 0.5 percent by weight, in addition to grain refinement, carbide formation or carbide formation may also be positively influenced.

Even by suitable (micro) alloying with other alloying elements, for example with niobium (Nb), titanium (Ti) or vanadium (V), the grain growth in the heat treatment (further) is hindered. Typical contents of these alloying elements are between 0.01 and 2% by weight. In accordance with the invention, conventional inductively hardenable steels, eg. B. 50CrMo4 or 43CrMo4, as base alloys in the manner described in addition (micro) alloyed to z. To obtain 50CrMo4 + Ta or 43CrMo4 + Ta.

It is believed that embedded carbides impede grain growth as they act as barriers to grain boundary motion and austenite transformation as seeds, thus helping to grain formation. Therefore, these steels are less susceptible to overheating with increasing alloying addition than unalloyed steels with the same carbon content. The stored carbides prevent excessive grain growth. Especially tantalum carbide (TaC) is particularly advantageous here, since this is not so in comparison to other carbides seems to dissolve quickly. By means of micro alloying with an alloying element, austenite grain growth during heating to forging temperature can be slowed down by the formation of metal carbides. The forging temperature is often within the range of the relevant austenitizing temperatures for induction hardening with short time austenitizing.

Overall, the material becomes more tolerant to overheating during induction hardening due to microalloying. This expands the available process window and makes the process less susceptible to deviations.

As embodiments of the present invention for components hitherto, the inner ring 1 10 and the outer ring 120 of a single row tapered roller bearing shown in Fig. 1 have been described. However, embodiments of the present invention are by no means limited thereto. In addition to multi-row (eg, double-row) rolling bearing rings, rolling elements (including a cylindrical roller, ball and tapered roller) and other inductively hardened rolling bearing components, the embodiments also include other types of rolling bearings, such as cylindrical roller bearings, roller bearings, ball bearings, four-point bearings and needle roller bearings as well, such as slide bearings, intermediate rings and other construction and machine parts of rotary and linear bearing technology. Also components of other disciplines of vehicle and mechanical engineering can be implemented as embodiments of the present invention. In principle, this includes all components which have at least one region which is subjected to an increased load, so that it makes sense to carry out a corresponding edge hardening starting from the edge of the relevant component.

Although hitherto only components have been described which are made entirely of steel according to an embodiment of the present invention, embodiments in this respect are not limited to a complete embodiment of a 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 portion extends from a surface of the component into an interior of the component, the component having edge hardening in the portion. In some embodiments of the present invention, edge hardening extends at least 5 mm into the interior of the component. However, according to embodiments of the present invention, larger or smaller edge hardening depths may also be implemented, as previously described. Examples thereof are composite components having a correspondingly shaped portion in which steel according to an embodiment of the present invention is implemented together with a corresponding edge hardening area. Other areas of the component in question can be made, for example, from another metal, another alloy or another steel. A connection can be made in these cases, for example, cohesively in the form of a solder or welded connection. But other connection methods are conceivable here, for example, a non-positive or positive connection or a bond as another form of cohesive connection. These methods can be used, for example, when soldering or welding is out of the question, ie if the substances involved are not solderable or weldable. Examples of this may be plastics or glass fiber reinforced materials. In embodiments, which are not completely made of a steel according to an embodiment, but at least have a section, this may extend from the surface along a straight line such that the straight line runs completely in the steel until it reaches a (further) Surface section emerges from the component.

In embodiments, the component or its section comprising the steel may optionally comprise further materials. Thus, such a component can also be made entirely or in a corresponding section of a material which comprises the steel. Such a material may include, for example, a fiber reinforced steel or other hybrid material combination using steel according to one embodiment. A steel according to one embodiment may thus be, for example, a steel for flame or induction hardening. It may alternatively or additionally be a rolling bearing steel. In the case of a steel according to an embodiment, this can have no further elements with a respective proportion of more than 0.2 percent by weight, except for iron and carbon and the abovementioned alloying materials, with a total proportion of the further elements not exceeding 10 percent by weight. By implementing an exemplary embodiment, an improved and / or lighter edge hardening may optionally be achievable.

Among the possible fields of application of exemplary embodiments of the present invention are, first of all, all large plants in which individual components are subjected to a corresponding mechanical stress which renders edge hardening advisable. These include wind and tidal power plants as well as generators, construction machinery, cranes, excavators, vans, trains, aircraft, rolling mills and other machinery. In principle, it may also be advisable to use embodiments in smaller systems and their components, since even in such systems, for example, by sudden shock loads high stresses may occur. Micro-alloyed grain stable steels for inductive heat treatment according to embodiments of the present invention, as well as corresponding components, are therefore applicable in a wide field of applications.

LIST OF REFERENCE NUMBERS

100 tapered roller bearings

1 10 outer ring

120 inner ring

130 symmetry line

140 rolling elements

150 cage

160 tread

170 tread

180 first shelves

190 second shelves

200 edge hardening area

210 edge hardening area

220 inductor

230 hardness profile

Claims

Pententanspr che steel, component and method for producing steel

A steel comprising tantalum at a level of between 0.01 and 2% by weight and a carbon content of at least 0.25% by weight and at most 1.55% by weight.

2. A steel according to claim 1, wherein the tantalum content is higher than 0.1 weight percent or higher than 0.5 weight percent, and / or wherein the carbon content is at most 1, 1 weight percent or at most 0.6 weight percent.

3. Steel according to one of the preceding claims, further comprising an alloying element with a weight fraction between 0, 1 and 5 weight percent, wherein the alloying element is molybdenum, nickel, silicon, manganese or chromium.

4. Steel according to one of the preceding claims, which has tantalum with a weight fraction which is greater than 1 times the weight fraction of niobium.

Steel according to any one of the preceding claims, having non-metallic inclusions in accordance with ASTM E45 and ISO 4967, which corresponds at most to the values defined in Table 3, or which has non-metallic inclusions in accordance with DIN 50602: 1985, not exceeding the values defined in Table 4. corresponds.

6. Steel according to one of the preceding claims, which has an at least partially having a structure with grains having a ratio of 5 or more according to ASTM, and / or a hardenability having at least one hardness value at least one distance value from a surface, wherein the at least one hardness value is in a by a HC _max value and _min value HRC given interval, wherein the HRC _max - _min and HRC - values versus the indicated in Table 2 HRC _max - and

HRC _min values may be exceeded or exceeded by up to 3 HRC levels.

A steel according to any one of the preceding claims, which is a steel for flame or induction hardening, and / or wherein the steel is a rolling bearing steel.

8. A method of producing steel, comprising:

Providing a base alloy of the steel; and

Alloying the base alloy with tantalum,

so that the steel has tantalum in a proportion of between 0.01 and 2% by weight and a carbon content of at least 0.25% by weight and at most 1.55% by weight.

The method of claim 8, wherein said alloying comprises adding tantalum boride, tantalum boride, tantalum silicide, tantalum oxide or tantalum to the base alloy.

A component (1 10; 120) comprising at least a portion of steel according to any one of claims 1 to 7, wherein the portion of a surface (160; 170) of the component (1 10; 120) in an interior of the component (1 10; 120) and wherein the member (1 10; 120) has edge hardening (200; 210) in the portion.