阅读说明：本技术 用于无级变速器的传动带的环构件 (Ring member for a drive belt for a continuously variable transmission ) 是由 B·彭宁斯 于 2020-11-27 设计创作，主要内容包括：本发明涉及一种用在用于无级变速器的传动带(3)中的柔性金属带(41),所述柔性金属带(41)具有氮化表面层,并且由马氏体时效钢合金制成,所述钢合金包含：15至20质量％的镍,10至14质量％的钴,4至6质量％的钼,高达2.5质量％的铬和高达2.0质量％的铝,其中钴(Co)含量和铝(Al)含量满足条件：19质量％<(Co+6*Al)<21质量％。(The invention relates to a flexible metal strip (41) for use in a drive belt (3) for a continuously variable transmission, said flexible metal strip (41) having a nitrided surface layer and being made of a maraging steel alloy comprising: 15 to 20 mass% nickel, 10 to 14 mass% cobalt, 4 to 6 mass% molybdenum, up to 2.5 mass% chromium and up to 2.0 mass% aluminum, wherein the cobalt (Co) content and the aluminum (Al) content satisfy the condition: 19 mass% < (Co +6 Al) <21 mass%.)

1. A flexible metal belt (41) for use in a drive belt (3) for a continuously variable transmission having two pulleys (1, 2) and a drive belt (3), the flexible metal belt (41) being made of a steel alloy comprising:

-15 to 20 mass% of nickel (Ni),

-4 to 18 mass% of cobalt (Co),

-at least 4 mass% molybdenum (Mo),

at least 7% by mass in total of molybdenum (Mo), chromium (Cr) and/or aluminum (Al), and

-the balance iron (Fe),

the strip (41) is provided with a nitrided surface layer, characterized in that the steel alloy more particularly comprises 9.0 to 14 mass% cobalt, 4.0 to 6.0 mass% molybdenum and at most 2.5 mass% chromium, the contents of cobalt and aluminium satisfying the following condition:

19 mass% < (Co +6 Al) <21 mass% (1),

where Co represents the cobalt content in mass% and Al represents the aluminum content in mass%.

2. Flexible metal strip (41) according to claim 1, characterized in that said steel alloy more particularly comprises 1.0 to 1.5 mass% of aluminium, wherein the cobalt and aluminium contents satisfy the following condition:

(Co +6 × Al) ═ 20 mass% (2).

3. Flexible metal strip (41) according to claim 1 or 2, characterized in that the steel alloy more particularly comprises 13 mass% cobalt, 1.1 mass% aluminium, 5.0 mass% molybdenum and 1.0 mass% chromium.

4. Flexible metal strip (41) according to any of the preceding claims, characterized in that the nitrided surface layer of the strip (41) has a thickness comprised between 12.5 and 17.5 microns.

5. Flexible metal strip (41) according to any of the preceding claims, characterized in that said strip (41) has a nominal thickness of 185 microns.

Technical Field

The present invention relates to an endless flexible metal belt for use as a ring member in a drive belt for power transmission between two adjustable pulleys of a well-known continuously variable transmission or in motor vehicles. In the drive belt, a plurality of such annular elements are incorporated in at least one but usually two laminated, i.e. radially embedded, groups thereof. The known drive belt also comprises a plurality of transverse segments which are slidably mounted on such a ring set and are usually also made of metal.

Background

Maraging steel is commonly used as a base material for rings, since this material has a high resistance to wear and to bending and/or tensile stress fatigue, at least after it has been subjected to a suitable heat treatment, including precipitation hardening, i.e. ageing and nitriding, in particular so-called gas soft nitriding. The basic alloying elements of maraging steel are iron, nickel, cobalt and molybdenum and may vary within a wide range, however, in particular for the currently considered drive belt applications of the endless element, international patent application publication WO 2018/122397a1 currently discloses the basic alloying ranges of maraging steel:

-15 to 20 mass% of nickel (Ni),

-4 to 18 mass% of cobalt (Co),

-at least 4 mass% molybdenum (Mo),

at least 7% by mass in total of molybdenum (Mo), cobalt (Cr) and/or aluminum (Al), and

-the balance iron (Fe).

In the application of endless drive belts, not only the yield strength of the endless element, but also its surface hardness and surface residual compressive stress are important product properties, which have a significant influence on the load-bearing capacity and the life of the drive belt. In particular, these latter product characteristics determine to a large extent the fatigue strength and wear resistance of the annular parts. In fact, these product characteristics of the surface of the annular element depend not only on the composition of the maraging steel, in particular on the abundance therein of the alloying elements forming the precipitates, but also on the process parameters of its nitriding heat treatment.

In the heat treatment of nitriding, nitrogen atoms are introduced into the microstructure by lattice diffusion from the surface of the ring-shaped member. In the surface layer of the annulus, these nitrogen atoms react with the available molybdenum, aluminum and/or chromium to form (Mo, Al, Cr) type nitrides, the formation of which is enhanced by the presence of cobalt, which reduces the solubility of other alloying elements in the iron-nickel matrix. By nitriding these nitrides in the surface layer, said surface residual compressive stress is achieved, which creates and balances the residual tensile stress in the core of the annular element. This core residual tensile stress in turn effectively limits the (additional) tension that the endless element can withstand during operation of the drive belt without exceeding its yield strength. Thus, even if increasing the thickness of its nitrided surface layer can generally support the fatigue strength and wear resistance of the ring, it will reduce its yield strength. In practice, therefore, in ring nitriding, a balance is maintained between the thickness of the nitrided surface layer, in particular the remaining thickness of the core of the ring, and the achievable surface hardness and surface residual compressive stress.

Disclosure of Invention

Based on the above insight, the present invention aims to maximize the surface residual compressive stress in the annular piece, i.e. to maximize the nitride concentration in the annular piece close to the surface of the annular piece, for a given nitride layer thickness obtained in said nitriding heat treatment. In the teaching of WO 2018/122397a1, this increase in nitride concentration would require that the amount of molybdenum, chromium and aluminium in the maraging steel component be increased to significantly above 7 mass%, while the amount of cobalt is oriented towards the top of the disclosed cobalt range. However, this known solution not only increases the cost of the maraging steel, but it has also been found that the thickness of the nitrided surface layer is disadvantageously increased. However, according to the present invention, a more advantageous option of maximizing the surface compressive stress is available.

According to the present invention it has surprisingly been found that by correlating the amount of aluminium and the amount of cobalt in the maraging steel alloy, the formation of nitrides can be significantly enhanced, in particular while not substantially increasing the nitrided surface layer thickness. Thus, high surface compressive stresses can be achieved in the annulus with a relatively low cobalt content. In particular, according to the invention, this advantageous result is achieved by the basic composition of the maraging steel comprising 9.0 to 14 mass% of cobalt and satisfying the following conditions:

19 mass% < (Co +6 Al) <21 mass% (1)

With this specific relationship between the cobalt content and the aluminum content according to condition (1), a balance is achieved between the formation of aluminum-type nitrides in the nitrided surface layer and the formation of aluminum-type intermetallic precipitates (Ni3Al) in the ring core. In this respect, optimum results are obtained with a basic composition of maraging steel comprising 1.0 to 1.5 mass% of aluminium and satisfying the following conditions:

(Co +6 Al) ═ 20 mass% (2)

In particular, in the range of the cobalt-aluminum ratio derived from the above conditions (1) and (2), nitrogen is captured much more in the nitride formed in the nitriding heat treatment than outside the range, which can be derived from a nitrogen concentration-depth dependence obtained by glow discharge light emission spectroscopy, for example. In particular, the reduction in the nitride concentration near the ring surface in relation to the depth below the ring surface (nitriding) is less severe than in maraging steels with alloy compositions outside the ranges of conditions (1) or (2). Thus, a high residual compressive stress is achieved in the nitrided surface layer of the ring, while a relatively small increase in residual tensile stress in the core of the ring is achieved. In particular, such an increase in the core residual tensile stress is smaller than the increase in the surface residual compressive stress, since this is mainly achieved by an increase in the concentration of nitride close to the ring surface, rather than an increase in the thickness of the nitrided surface layer.

Additionally, according to the present invention, the contents of molybdenum and chromium are also preferably set within a narrow range, preferably 4.0 to 6.0 mass% of molybdenum and up to 2.5 mass% of chromium, respectively. In the latter case, the nitrided surface layer advantageously has a mixture of nitrides of the (Al, Mo, Cr) type and of the (Al, Mo) type close to the ring surface and a mixture of nitrides of the (Al, Mo, Cr) type and intermetallic precipitates of Ni3Al at the boundary between the nitrided surface layer and the ring core. Thereby, an advantageous transition from compressive residual stress in the nitrided surface layer to tensile stress in the ring core is obtained.

Although the presently contemplated maraging steel composition may include certain amounts of other alloying elements, such as titanium, this is not required herein. In this case, only traces of other elements, such as unavoidable phosphorus and silicon contamination, are present therein.

It should be noted that, within the scope of the present invention, the nitriding heat treatment itself may be carried out as is conventional in the art. The above-mentioned advantageous effects are almost completely achieved by the specific maraging steel composition according to the invention, and therefore no special or specific setting of the ageing and nitriding heat treatments is relied upon or required. In fact, the currently considered series of maraging steels is even suitable for a combined heat treatment of simultaneous ageing and nitriding, which is described in european patent EP 1753889B 1 for a conventional maraging steel consisting of 18 mass% of nickel, 5 mass% of molybdenum, 16.5 mass% of cobalt and the balance iron.

Drawings

The drive belt described above, its ring member and its method of manufacture will now be described in more detail by means of non-limiting, illustrative embodiments thereof, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a known transmission including two variable pulleys and a drive belt;

figure 2 shows, in schematic cross-section, two known types of drive belts, each provided with a set of embedded flexible metal rings and a plurality of metal transverse segments slidably mounted on such a ring set along the circumferential direction of the ring set;

FIG. 3 provides a schematic representation of a currently relevant portion of a known integral method of manufacturing a belt loop member, including heat treatments of precipitation hardening and gas nitrocarburizing, wherein:

fig. 4 is a graph relating nitrogen content in the material of the annular member to distance relative to the outer surface of the annular member (i.e., a plot of nitrogen amount as a function of depth).

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 applicable friction between the drive belt 3 and the respective pulley 1, 2, but also the radial position R of the drive belt 3 between the pulley 1, 2 and its respective pulley disc 4, 5. These radial positions R determine the variator speed ratio. Transmissions of this type, their manner of activation and their operation are known per se.

In fig. 2, two known examples of the drive belt 3 are schematically shown in a cross-sectional view of the drive belt towards its circumferential direction. The known drive belt 3 comprises transverse segments 32 arranged in a row along the circumference of an endless carrier in the form of one or two ring sets 31 of metal rings 41. The thickness of the transverse segments 32 is small in relation to the circumferential length of such a ring set 31, in particular so small that hundreds of transverse segments 32 are comprised in the row of transverse segments. In either example of the drive belt 3 of fig. 2, the ring set 31 is laminated, i.e. comprises a plurality of flat, thin and flexible individual annular members 41 that are embedded in one another. Although the ring set 31 is shown in the drawings as comprising 5 embedded rings 41, in practice, 6, 9, 10 or 12 ring members 41 each 185 microns thick are used in most cases in such ring sets 31.

On the left side of fig. 2, an embodiment of a drive belt 3 is shown, which comprises two such ring sets 31, each accommodated in a respective laterally oriented recess of a transverse segment 32, which recess opens towards its respective (i.e. left and right) axial side. Such a lateral opening is defined between the body portion 33 and the head portion 35 of the transverse section 32 on either side of a relatively narrow neck portion 34 provided between and interconnecting the body portion 33 and the head portion 35.

On the right side of fig. 2, an embodiment of the drive belt 3 is shown, which comprises only a single ring set 31. In this case, the ring set 31 is accommodated in a centrally located recess of the transverse section 32 which opens out radially outside the drive belt 3. Such a central opening is defined between a base portion 39 of the transverse section 32 and two cylindrical portions 36, said cylindrical portions 36 extending in a radially outward direction from either axial side of the base portion 39, respectively. In this radially outward direction, the central opening is partially closed by a corresponding axially extending hook portion 37 of the cylindrical portion 36.

The transverse segments 32 of both drive belts 3 are provided on each of their two sides with a contact surface 38 for frictional contact with the pulley discs 4, 5. The contact faces 38 of each transverse segment 32 are mutually oriented at an angle alpha that substantially matches the angle of the V-pulley groove. The transverse section 32 is also typically made of metal.

As is well known, during operation of the transmission, the individual annular members 41 of the drive belt 3 are tensioned by (a/o) radially directed reaction forces with respect to said clamping force. The resulting ring tension is, however, not constant but varies, which variation is not only related to the torque to be transmitted by the transmission, but also to the rotation of the drive belt 3 in the transmission. Thus, in addition to the yield strength and wear resistance of the ring 41, fatigue strength is also an important property and design parameter thereof. Thus, maraging steel is used as a base material for the ring 41, which steel can be hardened by precipitation forming (ageing) to increase its overall strength and additionally case hardened by nitriding (gas soft nitriding) to improve wear resistance and in particular fatigue strength.

Figure 3 shows the relevant parts of a known manufacturing method of a belt loop member 41, as is commonly applied in the field relating to the production of metal drive belts 3 for automotive applications. The individual process steps of the known manufacturing method are indicated by roman numerals.

In a first process step I a sheet or plate 20 of maraging steel base material having a thickness of about 0.4mm is bent into a cylindrical shape and meeting plate ends 21 are welded together in a second process step II to form a hollow cylinder or tube 22. In a third process step III, the tube 22 is annealed in the furnace chamber 50. Thereafter, in a fourth process step IV, the tube 22 is cut into a plurality of annular ring-shaped elements 41, which ring-shaped elements 41 are then rolled in a fifth process step V to reduce 1/2 their thickness while being elongated. The thus elongated ring 41 is subjected to a further, ring annealing process step VI to eliminate the work hardening effect of the previous rolling process step by recovery and recrystallization of the ring material in the furnace chamber 50 at temperatures well above 600 degrees celsius (e.g., about 800 ℃). At such high temperatures, the microstructure of the ring material is entirely composed of austenite crystals. However, when the temperature of the annular element 41 is again lowered to room temperature, this microstructure transforms back to martensite as desired.

After annealing VI, in a seventh process step VII, the ring 41 is calibrated by mounting the ring 41 around two rotating rollers and stretching the ring 41 to a predetermined circumferential length by forcing the rollers apart. In this ring calibration of the seventh process step VII, internal stresses are also exerted on the ring 41. Thereafter, the ring 41 is heat treated in a combined ageing treatment, i.e. bulk precipitation hardening, and nitriding, i.e. case hardening, in an eighth process step VIII. In particular, such combined heat treatment involves maintaining the ring 41 at a controlled temperature in a furnace chamber 50 containing a controlled process atmosphere of a mixture of ammonia, hydrogen and nitrogen. It is known in the prior art to control the volume concentration of ammonia in the process atmosphere to a value of 5 to 25%, to control the volume concentration of hydrogen in the process atmosphere to a value of 5 to 15%, and to control the temperature of the process atmosphere to a value of 450 to 525 ℃. In this respect, practical values are about 10% by volume of ammonia, about 5% by volume of hydrogen and 470 ℃.

In the furnace chamber, the ammonia molecules are decomposed at the surface of the ring 41 into hydrogen and nitrogen atoms, which can enter the crystal structure of the ring 41. By these interstitial nitrogen atoms, it is known that the resistance to wear and fatigue fracture can be significantly improved. Typically, the combined ring piece ageing and nitriding of the eighth process step VIII is performed until the nitrided layer or nitrogen diffused area formed at the outer surface of the ring piece 41 reaches a desired thickness, for example 25 microns.

It is especially noted that the aging heat treatment may alternatively be performed in an ammonia-free process gas after or before such a combined heat treatment (i.e. without nitriding at the same time). This separate aging heat treatment is applied when the duration of the nitriding treatment is too short to complete the precipitation hardening process simultaneously.

In a ninth process step IX, a plurality of ring segments 41 thus machined are assembled to form the ring stack 31 by radial stacking, i.e. selected ring segments 41 are concentrically fitted to achieve a minimum radial play or clearance between each pair of adjacent ring segments 41. It is especially noted that it is also known in the art to alternatively assemble the ring stack 31 directly after ring calibration in the seventh process step VII, i.e. before ring ageing and ring nitriding in the eighth process step VIII.

In fig. 4, the two dashed lines DL1, DL2 and one solid line SL1 each represent the measured (by so-called glow discharge optical emission spectroscopy or GDOES) nitrogen content N in mass% as a function of the depth D below the outer surface of the respective annular element 41. The dashed area in fig. 4 indicates the range where GDOES measurements cannot provide accurate results due to start-up limitations and/or unavoidable contamination of the test sample surface. The locally measured nitrogen content N plotted in fig. 4 represents the amount of nitride locally present in the corresponding ring 41, i.e., locally formed in the ring-shaped nitrided portion. Further, such a local nitride amount indicates a local compressive residual stress in the ring member 41.

The three annular elements 41 which produce the measurement data DL1, DL2 and SL1 of fig. 4 were all produced by means of the process steps I to VIII of the above-described production method, but they started from different substrates. The two dashed lines DL1 and DL2 represent two conventional maraging steels consisting of 18 mass% nickel, 16.5 mass% cobalt, 5 mass% and 7 mass% molybdenum, respectively, and the balance iron. As can be seen from fig. 4, by increasing the molybdenum content from 5 mass% (line DL1) to 7 mass% (line DL2), the measured nitrogen content N and thus the amount of nitrides will decrease, at least until a depth D of about 15 microns below the surface of the annular element 41. This is expected because molybdenum is an alloying element that forms nitrides. However, the effect of the additional molybdenum on nitrogen absorption is limited.

Instead of or in addition to increasing the molybdenum content, the amount of nitride in the nitrided surface layer may be further increased by adding aluminum and/or chromium to the base material. For this purpose, a wide and open range of alloy compositions with 15 to 20 mass% nickel, 4 to 18 mass% cobalt, a total of at least 7 mass% molybdenum, chromium and/or aluminum, and the balance iron is proposed in the prior art. However, excellent results are obtained in accordance with the present invention within a relatively narrow subrange within this known open-ended range of alloy compositions. In particular, according to the invention, in the presence of aluminium in the alloy composition, the specific ratio between the aluminium content and the cobalt content provides a very effective nitriding heat treatment, in terms of the amount of nitrides formed therein close to the outer surface of the annular element 41:

19 mass% < (Co +6 Al) <21 mass% (1)

The solid line SL1 in fig. 4 represents a specific maraging steel composition within this narrow range of alloy compositions according to the invention, consisting of 19 mass% nickel, 13 mass% cobalt, 5.0 mass% molybdenum, 1.0 mass% chromium, 1.1 mass% aluminium and the balance iron. It can be seen from figure 4 that in the latter maraging steel composition the amount of nitrides, determined by the measured nitrogen content N, is significantly and advantageously increased over substantially the entire thickness of the nitrided surface layer, advantageously without increasing the thickness at the same time, relative to the two conventional maraging steels. For example, in the range of the depth D from 5 to 10 micrometers, such an increase in the nitrogen content N is measured more than 2-fold. Furthermore, a nitrogen content [ N ] of more than 4.0 mass%, for example 4.5 mass% or more than 4.5 mass% can be achieved after nitriding at a level of 2 to 3 μm below the surface of the maraging steel component SL 1. The latter value is even outside the permissible range of nitrogen contents [ N ] of the conventional maraging steel components DL1 and DL2, since in these cases this indicates the formation of a so-called composite layer consisting of FexN, which is known to be detrimental to the fatigue strength of the annular piece 41.

This relatively increased nitrogen content [ N ] in the range of the currently considered maraging steel alloy compositions also means that a reduced nitrided surface layer thickness can be applied upon nitriding, advantageously reducing the core residual tensile stress at comparable surface compressive residual stresses. In particular, according to the invention, the nitrided surface layer can be reduced from the conventional thickness of 22.5 to 27.5 μm to a thickness of 12.5 to 17.5 μm.

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.