阅读说明：本技术 基础材料组分、由该基础材料制造用于传动带的横向件的方法及包括由此制造的横向件的传动带 (Base material composition, method for manufacturing a transverse member for a drive belt from the base material and drive belt comprising a transverse member manufactured thereby ) 是由 B·彭宁斯 H·J·W·兰德瑞克 于 2019-07-25 设计创作，主要内容包括：本发明涉及一种构成用于在无极变速器中使用的传动带(3)的横向件(32),所述传动带(3)包括相对于其环形的张紧元件(31)的周向可滑动地并入的多个所述横向件(32)。横向件(32)由碳钢制成,所述碳钢包含0.60-1.2重量％的碳和0.30-0.60重量％的铬。根据本发明,所述碳钢还包含相当少量的0.05-0.15重量％的钒,从而显著地且有利地提高横向件(32)的疲劳强度。(The invention relates to a transverse member (32) constituting a drive belt (3) for use in a continuously variable transmission, said drive belt (3) comprising a plurality of said transverse members (32) slidably incorporated with respect to the circumferential direction of its endless tension element (31). The cross-piece (32) is made of carbon steel containing 0.60-1.2% by weight of carbon and 0.30-0.60% by weight of chromium. According to the invention, the carbon steel also contains a considerable amount of vanadium, between 0.05 and 0.15% by weight, so as to significantly and advantageously increase the fatigue strength of the cross-piece (32).)

1. A base material for transverse members (32) of a drive belt (3), which drive belt (3) has an endless tension element (31) and has a plurality of said transverse members (32) slidably mounted on the tension element (31), which base material is a carbon steel containing 0.60-1.2% by weight of carbon and 0.30-0.60% by weight of chromium, characterized in that the base material also contains 0.05-0.15% by weight of vanadium, preferably 0.10% by weight of vanadium.

2. The base material of claim 1, further comprising 0.01 to 0.03 weight percent niobium.

3. The base material according to claim 1 or 2, characterized in that the base material further comprises 0.005-0.015 wt.% nitrogen.

4. The base material of claim 1, 2 or 3, further comprising 0.50-0.80 wt.% manganese and 0.25-0.50 wt.% silicon.

5. The base material according to the preceding claim, characterized in that it additionally contains only iron and possibly traces of known contaminating substances, such as phosphorus, sulphur and oxygen.

6. A method for manufacturing a transverse member (32) for a drive belt (3), which drive belt (3) has an endless tension element (31) and has a plurality of said transverse members (32) slidably mounted on the tension element (31), wherein a transverse member (32) is made of a basic material according to any one of the preceding claims, wherein the basic material, i.e. the transverse member (32) made thereof, is subjected to a quench hardening heat treatment comprising a first process step (I) of austenitization, a second process step (II) of quenching and a third process step (III) of tempering, characterized in that in the third process step (III) of tempering the transverse member (32) is heated to a temperature above 250 ℃, preferably to about 300 ℃.

7. Method for manufacturing a transverse member (32) according to claim 6, characterized in that the first process step (I) of austenitizing is carried out in a process gas containing a carbon-containing gas, such as carbon monoxide, in such an amount that the carbon partial pressure in the process gas corresponds to the carbon content in weight-% of the basic material.

8. Method for manufacturing a transverse member (32) according to claim 6, characterized in that the first process step (I) of austenitizing is carried out in a process gas containing a carbon-containing gas, such as carbon monoxide, in such an amount that the carbon partial pressure in the process gas exceeds the carbon content of the basic material in% by weight.

9. Method for manufacturing a transverse member (32) according to claim 8, characterized in that after the completion of the first process step (I) of austenitizing, the carbon content in the vicinity of the surface of the transverse member (32) is 0.1-0.25% by weight higher than the carbon content of the basic material.

10. Method for manufacturing a transverse member (32) according to claim 7, 8 or 9, characterized in that the first process step (I) of austenitizing is carried out in a process gas additionally containing ammonia.

11. Drive belt (3) with an endless tension element (31) and with a number of transverse members (32) which are slidably mounted on the tension element (31), characterized in that the transverse members (32) of the drive belt (3) are transverse members (32) manufactured by means of a manufacturing method according to any one of claims 6-10.

Technical Field

The invention relates to a transverse member for a drive belt comprising an endless tension element and a plurality of such transverse members slidably arranged on the tension element in its circumferential direction. The drive belt is (a/o) used in variable belt and pulley transmissions known in the drive trains of motor vehicles. This particular type of drive belt and its cross member members is well known in the art and may be known, for example, from PCT application published as WO2017/108206 a 1.

Background

In order for the drive belt to function properly and permanently in the transmission, the transverse members must be resistant to both wear and metal fatigue. In this respect, it is known that the fatigue strength of the transverse members is determined by their shape, which is generally optimized with regard to the stress level and stress amplitude occurring during operation of the drive belt. Furthermore, the cross pieces may have residual compressive stresses in their surface layers, for example, as a result of subjecting them to the known stone-roll deburring process after cutting out from the base material. By such residual compressive stress, it is known that the initiation and/or growth of microcracks caused by, in particular, surface defects, improves the fatigue strength thereof.

In order to limit the wear rate of the contact surfaces of the cross-piece to a level suitable or at least acceptable in typical automotive applications of transmissions, it is known to provide the cross-piece with a material hardness of at least 58 on the rockwell C scale (HRC). The hardness values are achieved by producing the cross-members from carbon containing steel and quench hardening the cross-members. The steel composition of the basic material of the transverse member has a carbon content of between 0.60 and 1.2 wt.%, typically 0.75 wt.% +/-0.05 wt.%, and further comprises at least 0.30-0.60 wt.% chromium. Typically, but not necessarily, the base material of the cross-piece also comprises 0.50-0.70 wt.% manganese and 0.25-0.50 wt.% silicon.

The steel standards actually used in this respect are JIS SKS95 and DIN 1.2003 (also known as 75Cr 1). For example, DIN 1.2003 steel specifies a composition containing 0.70 to 0.80 wt.% carbon, 0.60 to 0.80 wt.% manganese, 0.25 to 0.50 wt.% silicon, 0.30 to 0.40 wt.% chromium, and the balance iron, and unavoidable contaminants, with the presence of phosphorus and sulfur generally being limited explicitly to 0.030 wt.%.

European patent publication EP- ｃA-1233207 provides an example of ｃA known heat treatment applied when manufacturing cross-pieces. This conventional quench hardening heat treatment comprises a step of heating the transverse member above the so-called austenitizing temperature of the steel (for example above ± 780 ℃ in the case of DIN 1.2003 steel compositions) in order to transform its crystal structure from ferrite to austenite, and a subsequent quenching step of cooling the transverse member sufficiently rapidly to a sufficiently low temperature, for example 110 ℃, in order to transform the austenite phase into the martensite phase at least in the bulk. Thereafter, the transverse member is subjected to a further tempering process step, i.e. it is heated to an intermediate temperature of around 200 ℃, for example 185 ℃, for about 40 minutes, in order to increase its ductility and toughness and thereby its fatigue strength to the required level. As a result of the tempering process step, the material hardness of the steel is also reduced compared to the hardness directly after the quenching process step. The microstructure or crystalline structure of the quench hardened steel is mainly martensite, typically with some austenite present (so-called "retained austenite").

In the above-described known optimization of conventional quench hardening heat treatment, it is known to carburize or carbonize the transverse member to provide residual compressive stresses in its surface layer. By such residual compressive stress, it is known that initiation and/or growth of micro-cracks, particularly caused by surface defects, can be suppressed, thereby improving the fatigue strength of the transverse member. For example, WO2017/108206 describes a known carburizing heat treatment applied to transverse members. The latter heat treatment comprises the process step of heating the transverse member in a carbon-containing gas atmosphere to a temperature above the so-called austenitizing temperature of the base material concerned (for example above ± 780 ℃ in the case of DIN 1.2003 steel). In particular, in such a heat treatment, the carbon potential of the gas atmosphere exceeds the carbon content of the relevant base material. By the latter feature of the known heat treatment, the surface layer of the transverse member is enriched with carbon. More specifically, a carbon potential of 0.9 is applied or, in general, a carbon content of 0.1 to 0.25 is higher than that in% by weight with respect to the base material.

In the case of carbonitriding, a nitrogen-containing gas is also added to the gas atmosphere, so that the surface layer of each cross piece is enriched not only with carbon but also with nitrogen.

Disclosure of Invention

The above-described known process provides the transverse element with a considerable wear resistance and a considerable fatigue strength. However, there is still a constant desire in the art to further reduce wear and/or to further improve the fatigue strength of the transverse members. On the one hand, the robustness and the service life of the transmission as a whole can thereby be increased, and on the other hand, the drive force transmitted by the transmission can be increased and/or the size of the transmission can be reduced.

The basis of the present invention is the finding that the fatigue strength of the transverse member can be further optimized by adding a surprisingly small amount of vanadium, in the order of 0.10 wt.%, in an optimized amount of about 0.05-0.15 wt.%, to the steel composition of the known base material. It has been observed that by adding the above-mentioned small amount of vanadium to the base material, a grain size refining effect is obtained, which not only improves the fatigue strength of the base material but also improves its workability. In particular, due to the presence of vanadium at the grain boundaries, the growth of austenite grains in austenitization is suppressed. This refinement of the grain size effectively reduces the size of the defects formed on the cutting surfaces of the cross-piece in blanking (so-called "galling" defects).

Furthermore, to a certain extent, a precipitation hardening effect can also be advantageously achieved in the quench hardening heat treatment of the transverse elements after blanking thereof. This precipitation hardening occurs by the formation of very fine vanadium carbides and/or nitrides dispersed throughout the cross-piece. However, when less than 0.05 wt.% vanadium is added, this effect is hardly noticeable, whereas when more than 0.15 wt.% vanadium, undesirable side effects begin to become relevant, such as increased brittleness.

Furthermore, according to the invention, the above-mentioned positive effects of vanadium are advantageously enhanced by adding a very small amount of niobium, less than 0.03% by weight, to the base material steel composition. This surprisingly small amount of niobium was found to support and enhance the grain refinement formation of vanadium and also to form niobium precipitates, i.e. niobium carbides and/or niobium nitrides, also dispersed throughout the transverse member.

Further according to the invention, the quench hardening heat treatment itself is finely but correlatively fine-tuned in a surprising manner in order to achieve an optimized grain size refinement and/or precipitation hardening effect in relation to the vanadium and/or niobium addition. In particular, according to the present invention, the tempering of the quench hardening process step is carried out at a temperature of 250-375 ℃, preferably about 300 ℃. At such relatively high tempering temperatures, the vanadium and/or niobium precipitates nucleate and grow to their optimal size within the ranges herein. Furthermore, it is highly advantageous according to the present invention that neither the austenitizing nor the tempering process step of the quench hardening heat treatment need be extended in duration to allow such precipitates to form. For example, the duration of the tempering process step may be kept close to about 40 minutes for conventional applications, i.e. may have a value between 30 and 60 minutes depending on the specific composition of the base material. Preferably, the tempering process step thus modified is preferably carried out in a protective gas atmosphere, which is in particular free of oxygen.

Further according to the invention, vanadium and/or niobium precipitates naturally form more abundantly and/or coarser near the surface of the transverse member than towards its core, due to the local abundance of nitrogen and/or carbon originating from the surrounding process gas during the process steps of austenitizing and/or tempering. In order to enhance the formation of said deposits in the volume region of the entire cross-piece, a minimum presence of 0.005 wt.% of nitrogen in the base material steel composition is provided. In order to avoid brittleness, the content of nitrogen is at most 0.015 wt.%, also in view of the relatively high tempering temperature according to the invention. In this way, the fatigue strength of the transverse element is optimally increased by the vanadium and/or niobium precipitates.

Furthermore, particularly with regard to the above-mentioned heat treatment for carburization and carbonitriding, it was found that the formation of an iron carbide network is also advantageously suppressed by adding the above-mentioned small amount of vanadium to the known base material steel composition. These iron carbide networks are mainly formed near the surface of the transverse member, because carbon atoms are relatively abundant locally due to the inward diffusion from said surface. These iron carbide networks are detrimental to the fatigue strength of the transverse member, and in particular these networks promote intergranular fatigue cracking.

It is evident that vanadium has a higher affinity for binding to carbon than iron, so that vanadium carbide advantageously replaces or at least prioritizes carbide formation of iron. Vanadium carbide was found to be much less detrimental to the fatigue strength of the transverse member than the iron carbide network at the grain boundaries, particularly because these vanadium carbides do not form such a network, but rather form dispersed, nano-sized precipitates.

In further accordance with the present disclosure, and in the case of relatively mild carburization or mild carbonitriding, which is typically applied in the manufacture of transverse members, very small amounts of vanadium, in the range of 0.05 to 0.15 wt.%, in the base material, are sufficient in this respect.

Drawings

The principle and features of the novel transverse member and the proposed manufacturing method thereof will now be further explained by way of non-limiting example with reference to the accompanying drawings.

Fig. 1 provides a schematic illustration of a well-known continuously variable transmission provided with two pulleys and a drive belt.

Figure 2 provides a schematically illustrated cross-sectional view of a known drive belt comprising steel transverse members and tension elements.

Fig. 3 schematically shows three stages of a conventional quench hardening process applied as part of the overall manufacturing method of the transverse member, the process comprising the steps of austenitizing, quenching and tempering.

FIG. 4 provides a graph showing carbon activity a at three austenitizing temperatures in the austenitizing process step_CAnd a graph of the relationship between the equilibrium carbon content, ECC, in weight%.

Fig. 5 provides a diagram illustrating in the form of a so-called north chart the positive influence of grain size refinement, defect size reduction and precipitation hardening effects of a transverse member according to the invention on its fatigue strength.

Fig. 6 is a photographic cross-sectional view of a steel sample showing (white) iron carbide at grain boundaries of the microstructure.

Detailed Description

Fig. 1 shows a central part of a known continuously variable transmission or CVT applied to the drive train of a motor vehicle, typically between the engine and the driven wheels of the motor vehicle. The transmission comprises two pulleys 1, 2 each provided with a pair of conical pulley discs 4, 5 mounted on a pulley shaft 6 or 7, defining between the pulley discs 4, 5 a circumferential pulley groove of substantially V-shape. At least one pulley disc 4 of each pair of pulley discs 4, 5, i.e. of each pulley 1, 2, is axially movable along a pulley shaft 6, 7 of the respective pulley 1, 2. A drive belt 3 is wound on the pulleys 1, 2 and is located in its pulley grooves for transmitting rotational movement and accompanying torque between the pulley shafts 6, 7.

The transmission usually also comprises an actuating device which during operation exerts an axially directed clamping force on said pulley discs 4 of each pulley 1, 2 which are axially movable, which clamping force is directed towards the respective other pulley disc 5 of that pulley 1, 2, so that the drive belt 3 is clamped between these discs 4, 5 of the pulley 1, 2. These clamping forces determine not only the friction between the drive belt 3 and the respective pulley 1, 2, but also the radial position R of the drive belt 3 between its respective pulley discs 4, 5 at each pulley 1, 2, which determines the variator speed ratio between its pulley axles 6, 7.

An example of a known drive belt 3 is shown in more detail in figure 2 in a sectional view towards its circumferential direction. The drive belt 3 comprises an endless tension element 31 in the form of two sets of flat and thin, i.e. band-shaped, flexible metal rings 44. The drive belt 3 further comprises a plurality of transverse members 32 mounted on the tension element 31 along its circumference. In this particular example, each set of rings 44 is received in a respective recess or groove 33 defined by the cross-piece 32 on either side thereof, i.e., on either axial side of the central portion 35 of the cross-piece 32. The groove 33 of the transverse member 32 is located between a bottom part 34 and a top part 36 of the transverse member 32, seen in radial direction with respect to the whole of the drive belt 3.

On the axial side of said bottom part 34 of the cross-piece, the cross-piece 32 is provided with a contact surface 37 for frictional contact with the pulley discs 4, 5. The contact surfaces 37 of each cross member 32 are oriented at an angle Φ to each other that substantially matches the angle of the V-pulley groove. The cross-members 32 thus take up the clamping force such that when an input torque is applied to the so-called drive pulley 1, the friction between the pulley sheaves 4, 5 and the belt 3 causes the rotation of the drive pulley 1 to be transmitted to the so-called driven pulley 2 via the likewise rotating drive belt 3, or vice versa.

During operation of the CVT, the cross member 32 member of the drive belt 3 is intermittently clamped between the respective pair of pulley discs 4, 5 of the pulleys 1, 2. Although this clamping obviously presses the bottom part 34 of the cross-piece 32, a tensioning force is also generated therein, in particular in the transition region between the bottom part 34 and the central part 35 thereof. Thus, the cross-piece 32 is not only subject to wear, but also to metal fatigue loading due to the aforementioned intermittent clamping thereof.

As is known, the transverse elements 32 are usually manufactured from a basic material of steel, such as 75Cr1(DIN 1.2003) steel, a blanking process being usually employed and the steel being quench-hardened as part of the overall production process of the drive belt 3. The quench hardening heat treatment includes three process steps I, II and III schematically shown in fig. 3. In a first process step I, a batch of transverse elements 32 is heated in the oven chamber 60 to a temperature substantially above the austenitizing temperature of the steel concerned, in order to bring the components into an austenitic crystalline structure, so-called austenitizing. In this first process step I, the cross piece 32 is typically placed in a neutral process gas, for example a mixture of nitrogen, hydrogen and a carbon-containing gas, such as carbon monoxide. The amount, i.e. the partial volume, of the carbon-containing gas in the process gas is selected such that the so-called carbon potential of the process gas is substantially equal to the carbon content of the steel to be treated. In this case, the cross member 32 is neither enriched with carbon nor depleted of carbon at its surface. The hydrogen advantageously promotes the decomposition of carbon monoxide, while at the same time it ensures that the process gas remains non-oxidizing by reacting with oxygen to form water vapor:

CO+H2→C+H2O [1]

the equilibrium constant K1 of the decomposition reaction [1] is defined as:

K₁＝(a_C·P_H2O)/(P_CO·P_H2) [2]

wherein, P_xRepresents the partial pressure, a, in% by volume of the corresponding gas "x" in the process gas_CRepresenting the so-called carbon activity of the process gas. The above decomposition reaction [1]The equilibrium constant K1 of (a) can be approximately expressed as: .

¹⁰log(K₁)＝-7.494+7130/T [3]

Wherein T represents the austenitizing temperature in kelvin. The carbon activity a of the process gas thus determined_CMay be related to the equilibrium carbon content at the surface of the cross-piece 32, i.e. the (surface) carbon content in equilibrium with the process gas. The graph of FIG. 4 provides the carbon activity a at three austenitizing temperatures_CThis relationship with the equilibrium carbon content in% by weight ECC. As noted above, in conventional austenitization, the cross-piece 32 is placed in a neutral process gas with the carbon activity a_CDefined such that the equilibrium carbon content ECC according to the graph of fig. 4 is substantially equal to the carbon content of the base material of the cross piece 32.

In a second process step II, the batch of transverse members 32 is quenched, i.e. rapidly cooled, to form a (meta-stable) microstructure consisting predominantly of supersaturated martensite crystals. In this second process step II, the cooling of the crossmembers 32 is usually effected by immersing them in an oil bath 70. Thereafter, in a third process step III, the batch of transverse elements 32, after austenitizing and quenching, is reheated in an oven 80 to increase its ductility and toughness, the so-called tempering. The treatment temperature applied in this third process step III, i.e. the tempering temperature, is much lower than the treatment temperature applied in the first process step I, i.e. the austenitizing temperature. For example, the tempering temperature may be as low as 185 degrees celsius, so that it can be performed in air.

In order to further reduce wear during operation and/or to further improve the fatigue strength of the cross-piece 32, it is currently proposed to add vanadium and/or niobium to the steel base material of the cross-piece 32. In particular, according to the invention, by adding a relatively small amount of 0.05-0.15 wt.% vanadium and/or less than 0.03 wt.% but preferably more than 0.01 wt.% niobium to the base material of the transverse element 32, a finer grain size is advantageously obtained after quenching of the transverse element. In addition, the precipitation hardening effect of the cross-piece 32 can be achieved, in particular, by the third, tempering process step III, in which the quench hardening heat treatment is carried out at a temperature of 250-375 ℃.

Included in fig. 5 is a so-called north view showing the improvement in fatigue strength of the cross-piece 32 that can be achieved when applying the technical teachings of the present invention. In the north graph, the defect size DS in the tested part is correlated on a log-log scale with the critical fatigue load CFL, i.e. the fatigue load FL that ultimately leads to fatigue fracture of the tested part. In fig. 5, the dashed lines show the critical fatigue loads CFLc of the conventional cross-piece 32, while the solid lines show the critical fatigue loads CFLn of the novel cross-piece 32, i.e., the cross-piece 32 embodying the technical teachings of the present invention. In fig. 5:

the arrow (r) shows the increase in fatigue strength irrespective of the defect size DS in relation to the increase in the surface residual compressive stress of the cross-piece 32, whereby the entire critical fatigue load line moves to the right in the north-china diagram;

arrow (c) shows an additional fatigue strength increase, mainly for relatively small defects, related to an increase in the material hardness of the cross-piece 32, whereby the bending point of the critical fatigue load CFL moves up and to the right in the north-china diagram; and

arrow (c) shows the indirect increase in fatigue strength through the reduction in defect size associated with the refinement of the grain size of the base material to improve its workability.

In known variations of the quench hardening heat treatment described above, the transverse members 32 are further toughened by carburizing or carbonitriding the transverse members 32. In these cases, the amount, i.e. the partial volume, of the carbon-containing gas in the austenitizing process gas is selected such that the resulting carbon activity a_CCorresponds to a base material higher than steelEquilibrium carbon content of the material ECC (see fig. 4), thereby enriching the cross member 32 with carbon at its surface. In particular, the equilibrium carbon content ECC is set to be 0.1 to 0.25 higher than the carbon content in weight percent of the base material, for example, 0.9 for a steel base material containing 0.75% (in weight percent) carbon.

It has been found that an iron carbide network may form at the grain boundaries of the steel base material during the above-mentioned carburizing heat treatment or during carbonitriding. Fig. 6 provides a rather extreme example of such a microstructure, with iron carbide precipitates appearing white at the grain boundaries. According to the invention, such iron carbide network promotes intergranular fatigue cracking, which should preferably be avoided by adding vanadium to the steel base material in the amounts specified above. Vanadium replaces iron and is combined with carbon, so that the formation of iron carbide is effectively inhibited. These vanadium carbides advantageously form dispersed, nano-sized precipitates rather than a larger iron carbide network at grain boundaries.

In addition to all of the details of the foregoing description and accompanying drawings, the present disclosure also relates to and includes all of the features of the claims. Reference signs in the claims do not limit their scope but are provided merely as a non-limiting example of corresponding features. Depending on the circumstances, the claimed features may be applied individually in a given product or in a given method, but any combination of two or more such features may also be applied therein.

The present aspects are not limited to the embodiments and/or examples explicitly mentioned herein, but also include modifications, adaptations and practical applications thereof, especially those that may occur to one skilled in the art.