EP3830308A1

EP3830308A1 - Basic material composition, method for manufacturing a transverse member for a drive belt from such basic material and a drive belt comprising a thus manufactured transverse member

Info

Publication number: EP3830308A1
Application number: EP19752108.1A
Authority: EP
Inventors: Bert Pennings; Hermen Jan Willem LENDERINK
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2018-07-27
Filing date: 2019-07-25
Publication date: 2021-06-09
Also published as: WO2020020491A1; CN112400032A; JP2021533256A

Abstract

The invention relates to a transverse element (32) component of a drive belt (3) for use in a continuously variable transmission, which drive belt (3) comprises a number of such transverse elements (32) that are slideably included therein relative to the circumference of an endless tensile element (31) thereof. The transverse element (32) is made from carbon steel including between 0.60 and 1.2% by weight carbon and between 0.30 and 0.60 % by weight chromium. According to the invention the carbon steel further includes a relatively small amount of between 0.05 and 0.15 % by weight vanadium for remarkably and favourably enhancing the fatigue strength of the transverse element (32).

Description

BASIC MATERIAL COMPOSITION, METHOD FOR MANUFACTURING A TRANSVERSE MEMBER FOR A DRIVE BELT FROM SUCH BASIC MATERIAL AND A DRIVE BELT COMPRISING A THUS MANUFACTURED TRANSVERSE MEMBER

The present invention relates to a transverse member component of a drive belt comprising an endless tensile element and several such transverse members that are arranged on tensile element, slideably along the circumference thereof. The drive belt is a/o applied in the well-known, variable belt-and-pulley type transmission in the powertrain of a motor vehicle. This particular type of drive belt and its transverse member component are well known in the art, for example from the PCT application published as WO 2017/108206 A1 .

For the correct and durable functioning of the drive belt in the transmission it is imperative that the transverse members are resistant to both wear and to metal fatigue. In this respect it is known that the fatigue strength of the transverse member is determined by the shape thereof that is generally optimized in terms of the stress levels and stress amplitudes that occur during operation of the drive belt. Furthermore, the transverse members may be provided with a compressive residual stress in a surface layer thereof, for example by subjecting these to the known deburring process of stone tumbling after they have been cut from basic material. By such compressive residual stress, the initiation and/or growth of micro cracks from, in particular, surface defects are known to be suppressed, thus improving the fatigue strength thereof.

For limiting a wear rate of the contact faces of the transverse member to a level that is suitable or at least acceptable in the typical automotive application of the transmission, it is known to provide the transverse members with a material hardness of at least 58 on the Rockwell hardness C-scale (HRC). This hardness value is realized by producing the transverse members from a carbon containing steel and by quench hardening the transverse members. The carbon content in the steel composition of the transverse member basic material lies in the range between 0.60 and 1 .2% by weight, typically 0.75% +/- 0.05% by weight and further includes at least between 0.30 and 0.60% by weight chromium. Typically, however non-essentially, the basic material of the transverse members also includes between 0.50 and 0.70% by weight manganese and between 0.25 and 0.50% by weight silicon.

Practically applied steel standards are in this respect are J IS SKS95 and DIN 1 .2003 (also known as: 75Cr1 ). DIN 1.2003 steel, for example, is specified to include between 0.70 and 0.80% by weight carbon, between 0.60 and 0.80% by weight manganese, between 0.25 and 0.50% by weight silicon and between 0.30 and 0.40% by weight chromium with balance iron and with inevitable contaminations whereof the presence of phosphorous and sulfur is typically explicitly limited to 0.030% by weight each.

An example of a known heat treatment applied in the manufacturing of the transverse members is provided by the European patent publication EP-A-1 233 207. Such conventional quench hardening heat treatment includes the steps of heating the transverse member to above the so-called austenitizing temperature of the steel (e.g. above ±780 °C in case of DIN 1 .2003 steel composition), for transforming the crystalline structure thereof from ferrite into austenite, and of subsequent quenching, i.e. of cooling the transverse member sufficiently rapidly and to a sufficiently lower temperature, for example 1 10 °C, for, at least predominantly, transforming the austenitic phase into a metastable martensitic phase. Thereafter, the transverse member is subjected to the further process step of tempering, i.e. of heating it to a moderate temperature of around 200 °C, for example 185 °C, for about 40 minutes to increase the ductility and toughness thereof and thus to bring its fatigue strength to the required level. As a result of tempering process step also the material hardness of the steel is reduced as compared to such hardness immediately after the process step of quenching. The quench hardened steel has a micro or crystalline structure of mainly martensite, typically with some austenite remaining present as well (so-called“retained austenite”).

In a known optimization of the above conventional quench hardening heat treatment, it is known to carburize or carbo-nitride the transverse members, whereby these are provided with a compressive residual stress in a surface layer thereof. By such compressive residual stress the initiation and/or growth of micro cracks from, in particular, surface defects are known to be suppressed, thus improving the fatigue strength of the transverse members. For example, WO2017/108206 describes the known carburizing heat treatment applied to the transverse members. This latter heat treatment includes the process step of heating the transverse members to above the so-called austenitizing temperature of the basic material concerned (e.g. above ±780 °C in case of DIN 1 .2003 steel) in a carbon-containing gas atmosphere. In particular in such heat treatment, the carbon potential of the gas atmosphere exceeds the carbon content of the basic material concerned. By this latter feature of the known heat treatment, the surface layer of the transverse members is enriched with carbon. More specifically, a carbon potential of 0.9 is applied, or, generally speaking, between 0.1 and 0.25 higher than the carbon content in %-by-weight of the basic material concerned.

In case of carbo-nitriding, also a nitrogen-containing gas is added to the gas atmosphere, such that the surface layer of every transverse member is enriched not only with carbon, but also with nitrogen.

The above known processes provide the transverse members with a considerable resistance against wear, as well as a considerable fatigue strength. Still, it remains an ever present desire in the art to further reduce wear and/or to further increase fatigue strength of the transverse members. On the one hand, the robustness and service life of the transmission as a whole may be improved thereby, on the other hand the driving power to be transferred by the transmission may be improved and/or the transmission may be reduced in size.

Underlying the present invention is the discovery that the fatigue strength of the transverse member can be further optimized by adding a surprisingly small amount of vanadium of between 0.05 and 0.15 % by weight to the known basic material steel composition with an optimum of about 0.10% by weight. It has been observed that already by the said small amount of vanadium to the basic material, a grain size refinement effect is obtained that improves not only the fatigue strength of the basic material, but also the workability thereof. In particular, a growth of the austenite grains in the austenitizing is suppressed by the presence of vanadium at the grain boundaries. Such grain size refinement effectively reduces the size of defects formed on the cut surfaces of the transverse members in blanking (so-called“galling” defect).

Moreover, to a certain extend also a precipitation hardening effect can be favorably obtained in the quench hardening heat treatment of the transverse members after the blanking thereof. Such precipitation hardening occurs by the formation of very fine vanadium carbides and/or vanadium nitrides dispersed throughout the transverse member. However, when adding less than 0.05% by weight vanadium such effect is hardly noticeable and above 0.15% by weight vanadium unwanted side-effects start to become relevant, such as an increase in brittleness.

Further according to the present invention, the aforementioned positive effects of vanadium are favorably enhanced by adding a minimal amount of niobium of less than 0.03% by weight to the basic material steel composition. This surprisingly small amount of niobium was found to support and enhance the grain refinement forming effect of vanadium and to form niobium precipitates, i.e. niobium carbides and/or niobium nitrides as well, dispersed throughout the transverse member as well. Further according to the present invention, for achieving the optimum grain size refinement and/or precipitation hardening effect in relation to the amount of vanadium and/or niobium added, the quench hardening heat treatment itself is delicately but relevantly fine-tuned in a surprising manner. In particular, according to the present invention, the quench hardening process step of tempering is carried out at a temperature of between 250 and 375 °C, preferably around 300 °C. At such relative high tempering temperature the vanadium and/or niobium precipitates nucleate and grow to their optimum size within the present context. Moreover according to the present invention, it is highly advantageous that the duration of neither the austenitizing nor the tempering process step of the quench hardening heat treatment needs to be extended to allow for such precipitate formation. For example, the duration of the tempering process step can remain close to the conventionally applied 40 minutes or so, i.e. can have a value between 30 and 60 minutes depending on the specific composition of the basic material. Preferably the thus modified process step of tempering is carried out in protective gas atmosphere that is, in particular free of oxygen.

Further according to the present invention, the vanadium and/or niobium precipitates are naturally formed more abundantly and/or coarser closer to the surface of the transverse member than towards its core, because of the local abundance of nitrogen and/or carbon originating from the surrounding process gas in the process steps of austenitizing and/or of tempering. For enhancing the said precipitate formation throughout the bulk of the transverse member, a minimum presence of nitrogen of 0.005% by weight is specified for the basic material steel composition. The nitrogen content is at most 0.015% by weight to avoid brittleness, also in view of the relatively high tempering temperature according to the present invention. In this way, the fatigue strength of the transverse member is optimally enhanced by the vanadium and/or niobium precipitates.

Moreover and specifically in relation to the said heat treatments of carburizing and carbo-nitriding, it was discovered that by adding the said small amount of vanadium to the known basic material steel composition, also the formation of iron carbide networks is favorably suppressed. These iron carbide networks mainly form near the surface of the transverse members due to the relative local abundance of carbon atoms by the inward diffusion thereof from the said surface. These iron carbide networks are detrimental to the fatigue strength of the transverse members, in particular these networks facilitate intergranular fatigue fracture. Apparently vanadium possesses a higher affinity to bond with carbon than iron, such that vanadium carbides are favorably formed instead of, or at least in preference to the iron carbides. The vanadium carbides were found to be considerably less detrimental to the fatigue strength of the transverse members than the said iron carbide networks at the grain boundaries, in particular because these do not form such networks but instead form as scattered, nanometer sized precipitates.

Further according to the present disclosure and in the context of the relatively mild carburizing or mild carbo-nitriding that is typically applied in the manufacturing of the transverse members, already a very small amount of vanadium of between 0.05 and 0.15% by weight in the basic material suffices in this respect.

The above discussed principles and features of the novel transverse member and its proposed manufacturing method will now be elucidated further by way of a non limiting example, with reference to the accompanying drawings.

Figure 1 provides a schematically depicted example of the well-known continuously variable transmission provided with two pulleys and a drive belt.

Figure 2 provides a schematically depicted cross-section of the known drive belt incorporating steel transverse members and a tensile element.

Figure 3 schematically indicates the three stages of the conventional quench hardening process that is applied as part of the overall manufacturing method of the transverse member and that includes the steps of austenitizing, quenching and tempering.

Figure 4 provides a graph illustrating the relationship in between a carbon activity ac and an equilibrium carbon content ECC in %-by-weight in the process step of austenitizing for three austenitizing temperatures.

Figure 5 provides a graph in the form of a so-called Kitagawa-diagram illustrating the positive influence of a grain size refinement, a defect size reduction and precipitation hardening effect of the transverse members in accordance with the present invention on the fatigue strength thereof.

Figure 6 is a photographed cross-section of a steel sample showing (white) iron carbides at the grain boundaries of the micro-structure.

Figure 1 shows the central parts of a known continuously variable transmission or CVT that is commonly applied in the drive-line of motor vehicles between the engine and the driven wheels thereof. The transmission comprises two pulleys 1 , 2 that are each provided with a pair of conical pulley discs 4, 5 mounted on a pulley shaft 6 or 7, between which pulley discs 4, 5 a predominantly V-shaped circumferential pulley groove is defined. At least one pulley disc 4 of each pair of pulley discs 4, 5, i.e. of each pulley 1 , 2, is axially moveable along the pulley shaft 6, 7 of the respective pulley 1 , 2. A drive belt 3 is wrapped around the pulleys 1 , 2, located in the pulley grooves thereof for transmitting a rotational movement and an accompanying torque between the pulley shafts 6, 7.

The transmission generally also comprises activation means that -during operation- impose on the said axially moveable pulley disc 4 of each pulley 1 , 2 an axially oriented clamping force that is directed towards the respective other pulley disc 5 of that pulley 1 , 2, such that the drive belt 3 is clamped between these discs 4, 5 of the pulleys 1 , 2. These clamping forces not only determine a friction force between the drive belt 3 and the respective pulleys 1 , 2, but also a radial position R of the drive belt 3 at each pulley 1 , 2 between the pulley discs 4, 5 thereof, which radial position(s) R determine a speed ratio of the transmission between the pulley shafts 6, 7 thereof.

An example of a known drive belt 3 is shown in more detail in figure 2, in a cross- section thereof facing in its circumference direction. The drive belt 3 incorporates an endless tensile element 31 in the form of two sets of flat and thin, i.e. of ribbon-like, flexible metal rings 44. The drive belt 3 further comprises a number of transverse members 32 that are mounted on the tensile element 31 along the circumference thereof. In this particular example, each set of rings 44 is received in a respective recess or slot 33 defined by the transverse members 32 on either lateral side thereof, i.e. on either axial side of a central part 35 of the transverse members 32. The slots 33 of the transverse member 32 are located in-between a bottom part 34 and a top part 36 of the transverse member 32, as seen in radial direction relative to the drive belt 3 as a whole.

On the axial sides of the said bottom part 34 thereof, the transverse members 32 are provided with contact faces 37 for arriving in friction contact with the pulley discs 4, 5. The contact faces 37 of each transverse member 32 are mutually oriented at an angle f that essentially matches an angle of the V-shaped pulley grooves. Thus, the transverse members 32 take-up the said clamping force, such that when an input torque is exerted on the so-called driving pulley 1 , friction between the discs 4, 5 and the belt 3 causes a rotation of the driving pulley 1 to be transferred to the so-called driven pulley 2 via the likewise rotating drive belt 3 or vice versa.

During operation in the CVT the transverse member 32 components of the drive belt 3 are intermittently clamped between the respective pairs of pulley discs 4, 5 of the pulleys 1 , 2. Although such clamping obviously results in a compression of the bottom part 34 of the transverse members 32, tensile forces are generated therein as well, in particular in a transition region between the bottom part 34 and the central part 35 thereof. Thus, the transverse members 32 are not only subjected to wear, but due the said intermittent clamping thereof also to metal fatigue loading.

It is well-known and generally applied to manufacture the transverse members 32 from steel basic material, such as 75Cr1 (DIN 1 .2003) steel, typically by means of a blanking process, and to quench harden the steel as part of the overall production process of the drive belt 3. The heat treatment of quench hardening comprises three process steps I, II and III that are schematically illustrated in figure 3. In a first process step I a batch of the transverse members 32 are heated in an oven chamber 60 to a temperature substantially above the austenitizing temperature of the steel in question in order to provide these with a crystalline structure of austenite, i.e. so-called austenitizing. In this first process step I, the transverse members 32 are typically placed in a neutral process gas, such as 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 chosen such that the so-called carbon potential of the process gas is essentially equal to the carbon content of the steel to be processed. In this case, transverse members 32 are neither enriched with nor depleted from carbon at the surface thereof. The hydrogen favorably promotes the decomposition of the carbon monoxide, while ensuring that the process gas remains non-oxidizing by reacting with oxygen forming water vapor:

CO + H2 C + H20 [1 ]

An equilibrium constant K1 of the above decomposition reaction [1 ] is defined by:

Kl = (ac · PH2O) / (Pco · PH2) [2] with Px representing the partial pressure in %-by-volume/100 in the process gas of a respective gas“x” and ac representing the so-called carbon activity of the process gas. The equilibrium constant Ki of the above decomposition reaction [1 ] can be approximated by:

¹⁰log(Ki) = -7.494 + 7130 / T [3] with T representing the austenitizing temperature in Kelvin. The thus determined carbon activity ac of the process gas can be related to an equilibrium carbon content at the surface of the transverse members 32, i.e. a (surface) carbon content that is in equilibrium with the process gas. The graph of figure 4 provides such relationship between carbon activity ac and equilibrium carbon content ECC in %-by-weight for three austenitizing temperatures. As mentioned above, in conventional austenitizing, the transverse members 32 are placed in a neutral process gas, whereof the carbon activity ac is defined such that the equilibrium carbon content ECC according to the graph of figure 4 is essentially equal to the carbon content of the basic material of the transverse members 32.

In a second process step II, the batch of transverse members 32 are quenched, i.e. are rapidly cooled to form a (meta-stable) microstructure largely composed of supersaturated martensitic crystals. In this second process step II, the cooling of the transverse members 32 is typically realized by immersing these in an oil bath 70. Thereafter, in a third process step III, the batch of transverse members 32 are re heated in an oven chamber 80 after being austenitized and quenched, in order to increase the ductility and toughness thereof, i.e. so-called tempering. The processing temperature applied in this third process step III, i.e. the tempering temperature, is much lower than the processing temperature applied in the first process step I, i.e. the austenitizing temperature. For example, the tempering temperature can be as low as 185 degrees Centigrade, such that it can take place in air.

In order to further reduce wear during operation and/or to further increase fatigue strength of the transverse members 32, it is presently proposed to add vanadium and/or niobium to the steel basic material of the transverse members 32. In particular according to the present invention, by adding a relatively small amount of between 0.05 and 0.15 % by weight for vanadium and/or of less than 0.03% by weight, but preferably of more than 0.01% by weight for niobium to the basic material of the transverse member 32, a finer grain size is favorably obtained after the quench hardening thereof. Moreover, in particular by carry out the third, tempering process step III of the quench hardening heat treatment at a temperature of between 250 and 375 °C, also a precipitation hardening effect is obtained for the transverse members 32.

In figure 5 a so-called Kitagawa-diagram is included that illustrates the improvement in the fatigue strength of the transverse member 32 that can be realized when applying the technical teaching of the present invention. In a Kitagawa-diagram a defect size DS in a tested part is correlated to a critical fatigue load CFL, i.e. a fatigue load FL that ultimately leads to the fatigue fracture of the tested part, on a double logarithmic scale. In figure 5, the dashed line illustrates the critical fatigue load CFLc of the conventional transverse member 32, whereas the solid line illustrates the critical fatigue load CFLn of the novel transverse member 32, i.e. a transverse member 32 embodying the technical teaching of the present invention. In figure 5:

- the arrow © illustrates a fatigue strength improvement irrespective of the defect size DS related to the said increase in residual compressive stress at the surface of the transverse member 32, whereby the complete critical fatigue load line shifts to the right in the Kitagawa-diagram;

- the arrow © illustrates an additional fatigue strength improvement mainly for relatively small defects related to the said increase in material hardness of the transverse member 32, whereby the bend-point in the critical fatigue load CFL shifts upwards and to the right in the Kitagawa-diagram; and

- the arrow ® illustrates an indirect fatigue strength improvement by a reduction in defect size related to the said grain size refinement of the basic material improving the workability thereof.

In a known modification of the above-described quench hardening heat treatment, the transverse members 32 are toughened further by carburizing or carbo- nitriding these. In these cases the amount, i.e. the partial volume of the carbon containing gas in the process gas in austenitizing is chosen such that the resulting carbon activity ac corresponds to an equilibrium carbon content ECC (see figure 4) that is higher than the carbon content of the steel basic material, such that the transverse members 32 are enriched with carbon at the surface thereof. In particular, the equilibrium carbon content ECC is set 0.1 to 0.25 higher than the carbon content in %-by-weight of the basic material, e.g. at 0.9 for a steel basic material containing 0.75% by weight carbon.

It was discovered that in the said heat treatments of carburizing or during carbo- nitriding iron carbide networks may form at the grain boundaries of the steel basic material. Figure 6 provides a rather extreme example of such micro-structure with iron carbide precipitates appearing in white at the grain boundaries. According to the present invention, such iron carbide networks facilitate intergranular fatigue fracture and should preferably be avoided by including vanadium in the steel basic material in the above-defined amount. The vanadium bonds with carbon instead of iron, effectively suppressing the formation of iron carbides. These vanadium carbides are favorably formed as scattered, nanometer sized precipitates, rather than as larger networks of iron carbides at the grain boundaries.

The present invention, in addition to the entirety of the preceding description and all details of the accompanying figures, also concerns and includes all the features of the appended set of claims. Bracketed references in the claims do not limit the scope thereof, but are merely provided as non-binding examples of the respective features. The claimed features can be applied separately in a given product or a given process, as the case may be, but it is also possible to apply any combination of two or more of such features therein.

The invention(s) is (are) not limited to the embodiments and/or the examples that are explicitly mentioned herein, but also encompasses amendments, modifications and practical applications thereof, in particular those that lie within reach of the person skilled in the relevant art.

Claims

1 . A basic material for a transverse member (32) for a drive belt (3) with an endless tensile element (31 ) and with a number of such transverse members (32), slideably mounted on the tensile element (31 ), which basic material is a carbon steel including between 0.60 and 1.2 % by weight carbon and between 0.30 and 0.60 % by weight chromium, characterized in that the basic material further includes between 0.05 and 0.15 % by weight vanadium and preferably includes 0.10 % by weight vanadium.

2. The basic material according to claim 1 , characterized in that it further includes between 0.01 and 0.03 % by weight niobium.

3. The basic material according to claim 1 or 2, characterized in that it further includes between 0.005 and 0.015% by weight nitrogen.

4. The basic material according to claim 1 , 2 or 3, characterized in that it further includes between 0.50 and 0.80 % by weight manganese and between 0.25 and 0.50 % by weight silicon.

5. The basic material according to a preceding claim, characterized in that it further includes only iron possibly with traces of known contaminations such as phosphorous, sulfur and oxygen.

6. A method for manufacturing a transverse member (32) for a drive belt (3) with an endless tensile element (31 ) and with a number of such transverse members (32) that are slideably mounted on the tensile element (31 ), wherein the transverse member (32) is made from the basic material according to a preceding claim, wherein the basic material, that is to say the transverse member (32) made therefrom, is subjected to a quench hardening heat treatment comprising a first process step (I) of austenitizing, 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 of 250 °C or more and preferably is heated to about 300 °C.

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

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

9. The method for manufacturing the transverse member (32) according to claim 8, characterized in that after the first process step (I) of austenitizing is completed the carbon content near the surface of the transverse member (32) is between 0.1 and 0.25 % by weight higher than the carbon content of the basic material.

10. The method for manufacturing the 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 comprising ammonia gas.

1 1 . A drive belt (3) with an endless tensile element (31 ) and with a number of such transverse members (32) that are slideably mounted on the tensile element (31 ), characterized in that the transverse members (32) of the drive belt (3) are manufactured by means of the manufacturing method according to one of the claims 6 to 10.