JP2016505092A - Heat treatment process in manufacturing method of ring set for drive belt - Google Patents

Abstract

A method for producing a flexible steel ring (32), in particular a method for producing a ring set (31) for a drive belt (3) with a plurality of said flexible steel rings (32) stacked one upon the other, said method Includes a processing step (IX) of applying a nitrided surface layer to the ring (32) by heat treatment. Herein, the thickness of the nitrided surface layer of the ring (32) is actively controlled by controlling the duration of the nitridation heat treatment.

Description

  The present specification relates to a heat treatment process in a method for manufacturing a ring set for a drive belt described in the superordinate concept of claim 1. The drive belt is mainly used as a power transmission means between two adjustable pulleys of a known continuously variable transmission mainly applied to automobiles.

  Such types of drive belts are generally known and consist of a number of transverse elements or segments. These transverse elements or segments are slidably incorporated in one or more ring sets, each consisting of a number of stacked flexible belt rings. In this type of drive belt, the transverse elements are not coupled to the ring set, but rather are slid along the circumference of the ring set during operation of the CVT. The individual belt rings of the ring set also slide relative to each other during operation.

  The belt ring of the drive belt is alternatively referred to as a strip, loop or endless belt, with the mechanical characteristics of particularly high tensile strength and (bending) fatigue strength and the desired steel from the basic material in sheet form. Manufactured from steel, in particular maraging steel, which combines a relatively good processability to form and the material properties of the final product belt ring. The desired material properties include the appropriate hardness of the ring core material to be able to bend in the longitudinal direction of the belt ring in combination with high tensile strength and sufficient elasticity and ductility characteristics, and resistance to the belt ring. A much harder outer or surface layer is included to provide wear. In addition, considerable compressive residual stress is applied to the surface layer of the belt ring to provide high resistance to metal fatigue. This latter feature is particularly important. This is because the belt ring experiences numerous loads and bending cycles during the service life of the drive belt in the transmission.

  Such drive belt manufacturing methods are generally known in the art. One important part of such an overall manufacturing method is the heat treatment of the belt ring, which includes a precipitation hardening treatment step and a nitridation treatment step.

  Precipitation hardening, also known as aging, is achieved by heating the belt ring to a temperature above 400 degrees Celsius (° C.). Fine metal precipitates are cultured at this temperature and grow at a plurality of arbitrary locations of the ring material. As the precipitate grows, the hardness of the ring material generally increases until a maximum hardness value is reached, after which the hardness of the ring material generally begins to decrease again (so-called “overaging”). In order to prevent (serious) oxidation of the belt ring surface, precipitation hardening is usually carried out in an inert or reduced processing atmosphere, such as nitrogen gas or nitrogen gas mixed with some hydrogen gas. .

  The nitriding treatment applies a surface layer that is additionally hardened and also subjected to a compression (pre) load to the belt ring. In nitriding, a gas soft nitriding that is at least a commonly applied variation of nitriding, the belt ring is also maintained at a temperature above 400 ° C. in a processing atmosphere containing ammonia gas (NH 3). . At such a temperature, ammonia molecules dissociate on the surface of the belt ring to form hydrogen gas and nitrogen atoms, and the nitrogen atoms enter the crystal lattice of the ring material. As the nitriding process continues, the nitrogen atoms are diffused at a rate determined by the processing temperature, leaving the surface and entering the ring material so that the belt ring has an increased thickness of the nitrided surface layer. Will be given. The thickness of the nitride layer is also closely related to the material properties such as hardness and compressive residual stress at the ring surface obtained / obtained in the nitriding process, so that the composition of the nitriding atmosphere, particularly in the nitriding atmosphere It depends on the concentration of ammonia and hydrogen, as well as the temperature and duration of the nitriding process. Thus, the three parameters of the process atmosphere composition, process temperature, and process time / duration are all representative of the main process settings for the nitriding process.

  For practical use with a drive belt applied to a given transmission, the thickness of the belt ring nitride layer is primarily important in determining the long life of the belt. In particular, if the nitride layer is extremely thin, the wear and fatigue properties of the belt ring will be insufficient, or if the nitride layer is extremely thick, the ring material will be too brittle and the ring stress level will be The elastic limit will be exceeded. In either case, the belt ring and thus the entire drive belt will be damaged early. Thus, once a desired or target value is specified for nitride layer thickness, it is important to achieve such target value accurately and consistently in (mass) production of drive belts. It is.

  In mass production of drive belts, many external factors can affect the nitriding process and even interfere with the nitriding process. For example, in large-scale mass production, the processing temperature of an industrial furnace can vary significantly. Moreover, the furnace is not always charged uniformly with a belt ring. The charge therefore determines the rate at which ammonia gas is consumed, i.e. dissociates at the ring surface, and thus the required supply of process gas. Accordingly, actively controlling the process settings of the nitriding process is important for the required accuracy and consistency of the nitriding process.

  In practice this means that the belt ring is removed from the normal process flow at regular intervals. The nitride layer thickness of the selected belt ring is measured. For this purpose, several measuring methods are effective, for example as described in EP 1869434. Depending on the magnitude and sign of the difference between the measured actual nitride layer thickness and the target nitride layer thickness, the ammonia concentration in the nitriding atmosphere can be adjusted. In particular, when the actual nitride layer does not reach the target value, the supply amount of ammonia gas is usually increased, or when the actual nitride layer exceeds the target value, the supply is normally performed. The amount will be reduced.

  This known nitriding process control has been successfully applied for many years, but does not take into account the recent evolution of ring set manufacturing methods. A particularly recent evolution relates to assembling the ring set before heat treatment instead of assembling the ring set from individual belt rings after heat treatment. This recent evolution regarding the setting of the manufacturing method of the ring set is described in EP 1815160, which greatly improves the utilization rate of the heat treatment furnace. This is because a ring set with typically 6-12 belt rings occupies roughly the same space as one individual belt ring in the furnace. Obviously, in the ring set, most of the belt ring surfaces are close to each other, and the treatment atmosphere there is more easily the ammonia gas than on the outside, i.e. the ring surface that forms the outer surface of the entire ring set. Decrease from. Thus, for example, based on the above-mentioned European Patent Application Publication No. 1815160, the optimal process settings for the ring set nitridation process take into account, among other things, the continuous nitridation process failures and / or deviations that may occur in mass production. Even if it is effective to provide a specified nitride layer thickness on each individual ring surface of the belt ring of the ring set by the active control of the nitriding process described above required to Actual process settings will deviate or deviate somewhat from the optimal settings for nitriding in practice. In other words, deviations from the specified nitride layer thickness are almost inevitable in existing settings for mass production, at least for some ring surfaces of the belt rings of the ring set.

  The present specification aims to improve the existing manufacturing method of the drive belt ring set, in particular in terms of the processing steps of the nitriding treatment of the ring set. According to the specification, such an improvement is found in the production method according to claim 1. In particular, the novel manufacturing method allows ring set nitridation to affect the nitride layer thickness widely corresponding to both the ring surfaces facing each other inside the ring set and the ring surfaces facing the outside of the ring set. Active control of the processing steps of the process is provided. In other words, if the process settings are changed to compensate for the actual nitride layer thickness below the target value and increase the nitride layer thickness actually delivered to the belt ring, this nitridation will result. The increase in material layer thickness is preferably similar for the entire ring surface of all belt rings in the ring set. In particular, in the novel manufacturing method according to the present invention, if the actual (ie, measured) nitride layer thickness is less than the target nitride layer thickness, the ring set nitridation process steps are extended, and If the actual (ie, measured) nitride layer thickness is greater than the target value, the ring set nitridation process steps are shortened. At the same time, the supply amount of the processing gas, in particular ammonia gas, and the processing temperature preferably remain unchanged at least in accordance with the thickness deviation as described above.

It is the figure which showed roughly an example of the well-known continuously variable transmission with which the drive belt was provided. It is a perspective view which shows the cross section of a drive belt. FIG. 3 schematically shows a portion of the known manufacturing method of a ring set component of a drive belt that is relevant to the present invention. It is a figure showing the step of the gas soft nitriding process in the manufacturing method shown in FIG.

  In the following, embodiments of the present invention will be described in detail with reference to the drawings.

  FIG. 1 shows the central part of a known continuously variable transmission or CVT that is applied to the driveline of an automobile, generally between the engine and the drive wheels of the automobile. This transmission has two pulleys 1 and 2, and each of the pulleys 1 and 2 is provided with two conical pulley disks 4 and 5, and between these pulley disks 4 and 5. A predetermined V-shaped groove is defined, and one of the disks 4 is disposed on the pulley shafts 6 and 7, and is movable in the axial direction along the pulley shafts 6 and 7. A drive belt 3 is wound around the pulleys 1 and 2 in order to transmit the rotational motion ω and the accompanying torque T from one pulley 1 and 2 to the other pulley 2 and 1. The transmission generally also has actuating means, which act on at least one of the discs 4 with an axially directed clamping force Fax towards the other pulley disc 5 respectively. The belt 3 is tightened between the disks 4 and 5. This also determines the (speed) transmission ratio between the rotational speed of the driven pulley 2 and the rotational speed of the driving pulley 1.

  An example of a known drive belt 3 is shown in detail in a sectional view in FIG. The belt 3 incorporates two ring sets 31, each ring set 31 comprising (in this embodiment) six thin flat or belt-like flexible belt rings 32. The belt 3 further comprises a number of plate-like metal transverse elements 33, which are held together by ring sets 31 arranged in respective recesses of the transverse element 33. . The transverse element 33 receives the tightening force Fax when the input torque Tin is applied to the so-called drive pulley 1, whereby the friction generated between the disks 4, 5 and the belt 3 rotates the drive pulley 1. The rotation is transmitted to the so-called driven pulley 2 via the driving belt 3 that similarly rotates.

  During operation, the CVT applies periodically changing tensile and bending stresses, i.e. fatigue loads, to the drive belt 3 and in particular to its belt ring 32. Thus, typically the resistance to metal fatigue of the ring 32, ie the fatigue strength, determines the functional service life of the drive belt 3 at a given torque T to be transmitted by the drive belt 3. Therefore, in developing a ring set manufacturing method, it has been a long-standing problem to achieve the required ring fatigue strength with a minimum of composite materials and processing costs.

  FIG. 3 shows the relevant part of the method for producing a known drive belt 3, i.e. the method for producing the ring set 31 of the drive belt 3, in this case the individual processing steps are It is represented by Roman numerals.

  In a first processing step I, a thin sheet or sheet 11 of basic material, typically having a thickness in the range of 0.4 mm to 0.5 mm, is bent into a cylindrical shape and is abutted in a second processing step II. The thin plate ends 12 are welded together to form an open hollow cylinder or tube 13. In the third processing step III, the tube 13 is annealed. Next, in the fourth processing step IV, the tube 13 is cut into a plurality of annular strips 14. Then, in a fifth processing step V, these annular strips 14 are rolled to reduce their thickness to a value of 0.150 to 0.200 mm, typically to about 185 microns, while long. Extend the length. The strip 14 after rolling is usually called a belt ring 32. The belt ring 32 is then further annealed in a sixth processing step VI, and the previous rolling process (by recovery and recrystallization of the ring material at a temperature substantially above 600 degrees Celsius (“° C.”), eg, about 800 ° C. That is, the work hardening effect of the fifth step V) is removed. Then, in a seventh processing step VII, the belt ring 32 is calibrated, i.e. the belt ring 32 is mounted around two rotating rollers and these rollers are separated to extend to a predetermined circumference. In the seventh processing step VII, the internal stress in the belt ring 32 is also dispersed.

  Next, in the eighth processing step VIII, each ring set 31 is assembled by superimposing these belt rings 32 on each other's circumferential surface from a plurality of belt rings 32 having circumferential lengths that are appropriately adapted to each other. Finally, the ring set 31 is heat-treated in the precipitation hardening or aging IX-A and gas soft nitriding treatment IX-N in the ninth treatment step IX. In particular, for aging and nitriding treatments, the ring set 31 is set to 400-500 ° C. in a furnace having a controlled gas atmosphere typically consisting of nitrogen, hydrogen and ammonia gas, generally about 10% by volume ammonia gas. Heating to a temperature of typically 485 ° C is included.

  The exact processing settings for the heat treatment are selected according to the basic material of the belt ring 32 (ie, the alloy composition of the maraging steel) and selected according to the desired mechanical properties for the belt ring 32. Of note regarding the latter are generally targeted core hardness values of at least 500 HV1.0, surface hardness values of at least 800 HV0.1, and nitrided surface layer or nitrogen diffusion region thicknesses of 25 to 35 microns. Is Rukoto. The duration of the heat treatment is obtained on the basis of the mechanical properties, the processing temperature, and the processing atmosphere composition, and actually has a value within the range of 30 to 90 minutes.

  The above-mentioned processing steps are considered to realize the respective processing steps, that is, the part related to the present invention of the entire manufacturing method of the drive belt 3 very effectively, but only after the belt ring 32 is heat-treated. It is not uncommon to not assemble the ring set 31 from these belt rings 32, that is, to change the order of the eighth processing step VIII and the ninth processing step IX.

FIG. 4 schematically shows a nitriding portion of the ninth processing step IX, that is, a heat treatment for nitriding the ring set 31. For simplicity, only two belt rings 32 of the ring set 31 are shown (in cross section). Furthermore, the gap 34 between these two rings 32 is very exaggerated (ie, in practice, the width of such a gap 34 is much smaller than the thickness of the belt ring 32, reaching only a few microns). Not required). In order to provide the nitrided surface layer on the belt ring 32, the ring set 31 is immersed in a processing atmosphere containing gaseous ammonia molecules schematically represented by four circles in FIG. The large circle represents the nitrogen atom of the ammonia molecule, and the three smaller circles represent the hydrogen atom of the ammonia molecule. At least some of the ammonia molecules are dissociated at the surface of the belt ring 32, in which case three hydrogen atoms are released allowing one nitrogen atom to penetrate into the crystal lattice of the belt ring 32. This ammonia dissociation reaction is schematically shown in an ellipse surrounded by a broken line in FIG. As part of the ammonia dissociation reaction, the released hydrogen atoms combine with each other to form hydrogen gas. In other words, the ammonia dissociation reaction is represented by the following formula:
2NH ₃ → 2 (N) + 3H ₂ (1)
In general, it has been found that the rate at which this ammonia dissociation reaction (1) occurs is proportional to the treatment temperature and the ammonia concentration in the treatment atmosphere, and inversely proportional to the hydrogen concentration in the treatment atmosphere.

  Based on FIG. 4, the supply of ammonia to the surface of the belt ring 32 is (most) unstable inside the gap 34 between each pair of adjacent belt rings 32 in the ring set. That is, ammonia is usually supplied to the inside of the gap 34 by (gas) diffusion, whereas ammonia is supplied to the outside of the gap 34 by circulation and / or forced circulation of the processing atmosphere. (And hydrogen is removed). In this way, dissociated ammonia molecules on the surface of the belt ring 32 are replaced. This means that when the ammonia concentration in the processing atmosphere changes, the influence on the rate at which the ammonia dissociation reaction (1) occurs is considerably less outside the gap 34 than inside the gap 34. I mean. This is simply because the supply of ammonia is unstable inside the gap 34, and in the supply determined by the (gas) diffusion rate determined by the ammonia concentration outside the gap 34, the supply amount of ammonia is large. Based on the fact that it will fluctuate. Therefore, the change in the ammonia concentration greatly affects the thickness of the nitride layer on the surface of the belt ring 32 facing the inside of the gap 34.

  On the other hand, when the ammonia concentration in the treatment atmosphere is sufficiently high, the ammonia dissociation reaction (1) outside the gap 34 is on the right side (in the reaction formula (1)) regardless of the (small) change in the ammonia concentration. It will go even further. As a result, the change in ammonia concentration has a relatively small effect on the thickness of the nitride layer at the surface of the belt ring 32 that does not face the inside of the gap 34, ie, forms the outer surface of the ring set 31 as a whole. Will be affected.

  Furthermore, even when the processing temperature changes, the rate at which the ammonia dissociation reaction (1) occurs on the surface of the belt ring 32 is affected differently between the inside and outside of the gap 34. As mentioned above, the supply of ammonia molecules is unstable inside the gap 34, and the rate of the ammonia dissociation reaction varies less than outside the gap 34 in relation to the processing temperature. Outside the gap 34, ammonia molecules are relatively abundant, so ammonia dissociation is mainly determined by the processing temperature, especially when the ammonia concentration in the processing atmosphere is sufficiently high and the processing temperature is sufficiently low.

  Therefore, the change in any one of the parameters of the ammonia concentration in the processing atmosphere of the ninth processing step IX in which the aging and the nitriding processing are combined and the processing temperature is caused by the belt ring 32 facing the gap 34. It has been found that the thickness of the nitride layer has a different effect between the surface and the ring surface which forms the outer surface of the ring set 31 as a whole. This means that the thickness of the nitride layer on the entire surface of the belt ring 32 in the ring set 31 is uncontrollable by at least one of the two parameters in an unequal amount. doing.

  Therefore, in the present specification, the active control of the ninth processing step IX in which aging and nitriding treatment are combined, particularly the thickness of the nitride layer supplied to the belt ring 32 processed in the ninth processing step IX. This active control is selected to be realized based on the duration parameter of the ninth processing step IX. In other words, the composition of the processing gas supplied to the processing atmosphere applied to the ninth processing step IX and the processing temperature are provided to provide the belt ring 32 with desired material characteristics such as a desired nitride layer thickness. In particular, if there is a deviation in processing results, that is, if the actual nitride layer thickness deviates from the desired nitride layer thickness, it is proportional to such deviation. Offset the deviation by adapting the process duration.

  In addition, the duration of the ninth processing step IX, in order to support the controllability of this ninth processing step IX, the processing temperature applied there (at least currently in practice in general application) It is set to a relatively low value of less than 475 ° C. (preferably about 470 ° C.). Further, similarly, in order to support the controllability of the ninth processing step IX by the duration of the ninth processing step IX, the ammonia concentration in the processing gas supplied to the processing atmosphere applied there is set to 12 It is set to a relatively high value above the volume%, preferably about 15% by volume (at least compared to the concentration currently in common use at present). Furthermore, these unique process settings have been found to be very suitable. This is because these process settings are applied to the nitriding process of the individual belt rings and also to the nitriding process of the ring set described above.

  In addition to the entire description above and the accompanying detailed drawings, the specification relates to and encompasses all features recited in the claims. Reference signs in parentheses in the claims do not limit the scope of the claims, but are merely described as non-restrictive examples of the respective features. The features recited in the claims can optionally be applied individually to a given product or process, but two or more of these features can also be applied in combination.

  The invention described in the present specification is not limited to the embodiments and / or examples described in detail in the specification, and in particular, modifications, changes, and the like within the scope conceived by those skilled in the related art. And actual applications.

Claims (8)

A method for producing a flexible steel ring (32), in particular a method for producing a ring set (31) for a drive belt (3) with a plurality of said flexible steel rings (32) stacked one upon the other, said ring (32) in a method comprising a processing step (IX) of applying a nitrided surface layer by heat treatment,
The method of manufacturing a ring set (31) for the drive belt (3), characterized in that the method is set to change the duration of the heat treatment.   The method of claim 1, wherein in the heat treatment, the ring (32) is placed in a heated gas atmosphere containing ammonia gas.   The method of claim 2, wherein in the heat treatment, the temperature and amount of ammonia in the gas atmosphere is kept at least essentially constant.   The method according to claim 2 or 3, wherein in the heat treatment, the temperature and amount of ammonia in the gas atmosphere is more than 12% by volume, preferably about 15% by volume.   3. In the heat treatment, the gas atmosphere further comprises nitrogen gas and hydrogen gas in addition to ammonia, and the amounts of ammonia, nitrogen and hydrogen in the gas atmosphere are kept at least essentially constant. 3. The method according to 3 or 4.   The method according to any one of claims 1 to 5, wherein, in the heat treatment, the temperature of the gas atmosphere has a value within a range of 400 to 475 ° C, and preferably about 470 ° C.   After the heat treatment, the actual thickness of the nitride surface layer of one ring (32) selected from the plurality of rings (32) is measured, and the measured nitride layer thickness is determined as a desired nitride layer thickness. If the desired value is smaller than the actual thickness of the measured nitrided surface layer, the duration of the heat treatment is shortened, or the desired value is the actual value of the measured nitrided surface layer. The method according to any one of claims 1 to 6, wherein if the thickness is larger than the thickness, the duration of the heat treatment is extended.   Before the heat treatment, a plurality of flexible steel rings (32) are stacked on each other to form a ring set (31) for the drive belt (3), and the heat treatment is applied to the entire ring set (31), The method according to any one of the preceding claims, wherein a nitrided surface layer is applied to the ring (32) of the ring set (31).

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