NL1042940B1

NL1042940B1 - 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: NL1042940B1
Application number: NL1042940A
Authority: NL
Inventors: Pennings Bert; Jan Willem Lenderink Hermen
Original assignee: Bosch Gmbh Robert
Priority date: 2018-07-27
Filing date: 2018-07-27
Publication date: 2020-01-31

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 5 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. 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 10 enhancing the fatigue strength of the transverse element (32). 1042940

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 disclosure 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 under No. 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.

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 normally lies in the range between 0.60 and 1.2% by weight. Furthermore, between 0.50 and 0.70% by weight manganese, between 0.25 and 0.50% by weight silicon and between 0.30 and 0.60% by weight chromium is typically present in such steel composition.

Typically applied steels in this respect are JIS SKS95 and DIN 1.2003 (also known as: 75Cr1). In addition to between 0.70 and 0.80% carbon by weight, the DIN 1.2003 steel composition further includes 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 such a known production process is provided by the European patent publication EP-A-1 233 207. The 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 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”).

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.

According to the present disclosure it was discovered 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 disclosure, 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 disclosure, 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 disclosure, 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 disclosure, 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 disclosure, 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 disclosure. In this way, the fatigue strength of the transverse member is optimally enhanced by the vanadium and/or niobium precipitates.

The above discussed principles and features of the novel transverse member and its proposed manufacturing method will now be elucidated further along a drawing in which:

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

Figure 4 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 disclosure on the fatigue strength thereof.

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 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 crosssection 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 φ 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, 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. Hydrogen is added to promote the decomposition of the carbon monoxide, while ensuring that the process gas remains non-oxidizing by reacting with oxygen forming water vapor.

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 reheated 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 disclosure, 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 4 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 disclosure. 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 4, 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 disclosure. In figure 4:

- the arrow Φ 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.

The present disclosure, in addition to the entirety of the preceding description and all details of the accompanying figures, also concerns and includes all of 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 can also be applied simultaneously therein in any combination of two or more of such features therein.

The invention(s) represented by the present disclosure 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

CONCLUSIONS

A base material for a transverse element (32) for a drive belt (3) with an endless tension element (31) and a number of slidably arranged transverse elements (32), in particular for transferring between two pulleys (1, 2) driving power, which base material is a carbon steel with 0.60 to 1.2 weight% of carbon, characterized in that the base material consists of at least 0.05 weight% and at most 0.15 weight% and preferably about 0.10 weight% of vanadium .

The base material according to claim 1, characterized in that it contains between 0.01 and 0.03% by weight of niobium.

The base material according to claim 1 or 2, characterized in that it contains at least 0.005% by weight and at most 0.015% by weight of nitrogen.

The base material according to claim 1,2 or 3, characterized in that it further comprises at least 0.50% by weight and at most 0.80% by weight of manganese, at least 0.25% by weight and at most 0.50% by weight silicon, at least 0.30% by weight and at most 0.60% by weight of chromium.

The base material according to claim 4, characterized in that it further consists exclusively of iron with the exception of known impurities such as phosphorus, sulfur and oxygen.

A method for manufacturing a transverse element (32) for a drive belt (3) from the base material according to a preceding claim, with an endless tension element (31) and a number of transversely arranged transverse elements (32) arranged thereon, in particular for transferring two pulleys (1, 2) from a driving power, in which method the base material, or the transverse element (32) produced therefrom, is subjected to a curing process in which the base material is agitated in a first process step (I), in a second process step ( II) is quenched and annealed in a third process step (III), characterized in that in the third annealed process step (III) the base material is heated to 250 degrees Celsius or more, preferably about 300 degrees Celsius.

The method for manufacturing a transverse element (32) from the base material according to claim 6, characterized in that the first austenitic process step (I) is carried out in a process gas comprising a carbon-containing gas, such as

5 carbon monoxide, in an amount such that the partial carbon pressure in the process gas is at least substantially equal to the proportion of carbon in the base material in weight%.

8. A drive belt (3) with an endless tension element (31) and a number of slidably arranged transverse elements (32) thereon, in particular for transferring a driving power between two pulleys (1, 2), characterized in that the transverse elements are manufactured with the aid of the method according to claim 6, 7 or 8.