JP2015502457A - Heat treatment process in manufacturing method of drive belt metal ring element - Google Patents

Abstract

The present invention relates to a heat treatment process in a method for producing a metal ring (14) for use in a drive belt (1). The ring (14) is manufactured in two steps. In the first step, the ring (14) is kept in an atmosphere containing no ammonia gas at a temperature in the range of 450 ° C. to 500 ° C. for 3 hours or more, and in the second step, the ring (14) is 425 ° C. Maintained in an atmosphere containing at least 2 volume percent but less than 10 volume percent ammonia at a temperature in the range of ~ 475 ° C.

Description

  The present invention relates to a method of manufacturing a metal ring used for a drive belt, and more particularly to a heat treatment process part of the manufacturing method described in the superordinate concept of claim 1. The drive belt is typically used as a means for power transmission between two adjustable pulleys of a widely known and continuously variable transmission mainly applied to automobiles.

  One well-known type of drive belt is described in detail in EP 1403551. The drive belt is formed from a number of relatively thin transverse metal elements. The transverse metal elements are slidably assembled to two stacked endless tensioning means, each of the tensioning means being one of the mutually integrated ones, alternatively called an endless band or hoop. It consists of a set of flat metal rings. Such rings are typically made from precipitation hardened steel such as maraging steel. Precipitation hardened steels have outstanding tensile strength, bending fatigue, and good resistance to tensile stress, a relatively favorable possibility of producing steel from sheet material into the desired shape, and material properties of the final product ring (preferred Should not vary along the circumference of the ring). The present invention specifically relates to 17 to 19 weight percent nickel, 4 to 6 weight percent molybdenum, 8 to 18 weight percent cobalt, less than 1 weight percent titanium, and optionally a small amount (e.g., (Less than 1 percent by weight) relates to a range of maraging steel alloys having a basic composition containing other alloy components and / or impurities, the balance being iron.

  The aforementioned desired material properties are to obtain the proper hardness and wear resistance of the ring core material in order to combine outstanding tensile strength properties with sufficient elasticity to allow longitudinal bending of the ring. And a very hard outer surface layer of the ring. In addition, the outer surface layer has a residual compressive stress in order to obtain a high resistance to metal fatigue. This is a key feature of the ring based on the numerous loads and bending cycles that the ring is exposed to during the life of the belt.

  The principles of known manufacturing methods for such rings are widely known in the art and are described, for example, in EP-A-1753889. The ring is formed from a sheet material. The sheet material is bent and welded to a cylindrical shape or tube, which is heat treated, i.e. annealed, to return to its original material properties, i.e. the changes introduced by bending and welding are sufficiently eliminated. The tube is then cut into multiple hoops. The hoop is then rolled and stretched to a desired thickness that is typically about 0.185 mm in the final product. After roll processing, the hoop is usually referred to as a ring or band. The ring is subjected to a further annealing step to remove internal stresses introduced during roll processing. The ring is then sized, i.e. the ring is mounted around two rotating rollers and stretched to a predetermined circumference.

  Finally, according to EP-A-1753889, the ring is subjected to precipitation hardening, i.e. heat treatment of aging or core hardening, and gas soft nitriding, i.e. surface hardening by insertion of nitrogen atoms into the steel lattice of the ring. Exposed to a combined single process step including both heat treatment. According to EP-A-1753889, in such a combined aging and nitriding process step, the ring is kept in ammonia gas containing a process atmosphere of 440 ° C. to 480 ° C. for 45 minutes to 65 minutes. There is a need.

  Over the years, the chemistry of nitriding heat treatment has been extensively studied and increasingly understood. PCT / EP2010 / 007783, which has not yet been published, is based on such newly gained detailed understanding, whereby the effectiveness of the known process steps combined with aging and nitridation is still significant. It is pointed out that it can be improved. In particular, it has been proposed that the ammonia gas content of the process atmosphere can be reduced well below 10 volume percent under conditions where such a process atmosphere is maintained at 500 ° C. or higher. The advantage of the latter process setting as such is that a very small amount of ammonia is used, so that the costs associated with the nitriding heat treatment and its surrounding effects are advantageously reduced.

  As recognized in PCT / EP2010 / 007783, the new process settings described above must be precisely controlled, otherwise an unexpected increase in process temperature or ammonia concentration, for example, will occur. Sometimes a so-called iron nitride compound layer is formed on the surface of the ring, which unacceptably reduces the (metal) fatigue strength of the ring. This phenomenon of compound layer formation relates in particular to the current range of maraging steel ring materials that do not contain non-metal inclusions such as titanium nitride or contain very little, and this material is Instead, it tends to eventually break from metal fatigue due to cracking from surface defects. In fact, it has proved extremely difficult to reliably control the actual process settings within a narrow range where compound layers and / or other surface defects are not formed, especially in a mass production environment.

  The following were found to be further limitations on this new process setting: That is, this new process setting is only suitable for a limited range of maraging steel ring material composition, i.e. has a relatively high cobalt and / or molybdenum content to achieve the required rate of precipitation hardening. Only suitable for maraging steel material composition. This type of steel is relatively expensive due to the high cobalt and / or molybdenum content mentioned above.

  Therefore, in order to reduce the risk of (i) compound layer formation, while the above process settings preferably maintain a small ammonia content of the process atmosphere in nitriding, (ii) the boundary range of the maraging steel composition It is considered suitable to be able to change to fit.

  According to the present invention, the object of the present invention is realized by a heat treatment process comprising separate aging and nitriding steps. In this case, the aging heat treatment requires keeping the ring in the nitriding gas at a temperature of 450 ° C. to 500 ° C. for 180 minutes or more, and the nitriding heat treatment is performed at a temperature of 425 ° C. to 475 ° C. for 45 to 90 minutes. However, it is necessary to keep the ring in a process atmosphere containing less than 12 volume percent, preferably less than 10 volume percent ammonia gas. In particular, good results have been obtained with an aging heat treatment performed at 490 ° C. for 200 minutes and / or with a nitridation heat treatment performed at 450 ° C. for 75 minutes in a process atmosphere containing 4 to 8 volume percent ammonia. It was.

  The above-described basic characteristics of the present invention will be described with reference to the accompanying drawings and the illustrated examples.

It is the schematic which shows the transmission to which the drive belt and drive belt in this invention were applied. FIG. 2 is a schematic diagram showing the stacked tensioning means and transverse elements oriented relative to each other in the drive belt. 1 is a diagram representing a known manufacturing method of a metal ring applied to an endless tensioning means of a drive belt, including combined aging and nitriding process steps. FIG. 4 is a diagram representing the aging and nitriding heat treatment of the ring element of the drive belt tensioning means according to the invention.

  In the drawings, the separate process steps of the known manufacturing method and the new manufacturing method are represented by Roman numerals.

  FIG. 1 schematically shows a continuously variable transmission (CVT; continuously variable transmission) comprising a drive belt 1 wound around two pulleys 4 and 5. The belt 1 is composed of two sets of interleaved thin flat metal rings 14 (FIG. 2) (alternatively referred to as bands 14), laminated endless pulling means 2 and A plurality of transverse members 3 (alternatively referred to as transverse elements). The transverse members 3 are mounted in a substantially continuous row along the circumference of the pulling means 2 and are slidable along the circumference. Such continuously variable transmissions are well known per se.

  FIG. 2 shows a cross section of a known belt 1, showing a front view of the transverse member 3 and a cross section of the tension means 2. The lateral members 3 each have side surfaces 6 on the sides, by which the lateral members 3 engage the pulley disks of the two transmission pulleys 4, 5. The ring 14 of the tensioning means 2 is made from high quality maraging steel. The typical thickness of the ring 14 is in the range of 0.15 mm to 0.25 mm, its typical width is in the range of 8 mm to 35 mm, and its typical circumference is 500 mm to 1000 mm. Is in range.

FIG. 3 shows a portion of the above-described known manufacturing method of the belt 1, particularly the ring 14 of the belt 1, which has been implemented from the beginning of the metal push belt manufacturing, and which relates to the present invention. In the first process step I, the sheet of material 11 is bent into a cylindrical shape, with the sheet ends 12 abutting against each other being welded together in the second process step II to form the tube 13. Is done. In the third step III of the process, the tube 13 is annealed at a temperature above 800 ° C. in an inert process atmosphere such as vacuum or nitrogen gas N ₂ . Thereafter, in a fourth process step IV, the tube 13 is cut into a number of hoops 14. The hoop 14 is subsequently rolled and stretched to the desired thickness in process step V. After rolling, the wheel 14 is usually referred to as the ring 14 or band 14. Ring 14 is exposed to further annealing process step VI to remove internal stresses introduced during roll processing. Thereafter, in a seventh process step VII, the ring 14 is sized, i.e. the ring 14 is mounted around two rotating rollers and stretched to a predetermined circumferential length. In this seventh process step VII, internal stress distribution is also produced in the ring 14. The internal stress distribution defines the so-called curl radius of each ring 14. Finally, in the eighth step VIII of the known manufacturing method, the ring 14 is subjected to two heat treatments, precipitation hardening or aging and gas soft nitriding, ie surface hardening by insertion of nitrogen atoms into the steel lattice of the ring. . It is known to perform two heat treatments, such as aging and nitriding, separately, ie in two separate sequential process steps, or simultaneously in a single process step.

  In nitriding, the ring 14 is kept in an ammonia gas containing process atmosphere. Recent developments have successfully used less than 10 volume percent of unconventional ammonia in nitriding. Such small amounts of ammonia content in nitriding are suitable per se, but with the disadvantage that the tolerances to variations in process parameters (ie temperature, time and atmosphere) are narrow (less resistant), and ring materials (ie It has been found that more stringent requirements are imposed on the aging steel composition).

  The present invention, shown diagrammatically in FIG. 4, provides a method for using such a small amount of ammonia in nitriding, less than 10 volume percent, for a wider range of ring material compositions, while controlling process parameters. There is no need for accuracy. According to the present invention, the aging and nitriding heat treatment of the ring 14 comprises two separate or sequential process steps VIII-A, VIII-B. In the first process step VIII-A according to the present invention, the ring 14 is free of ammonia gas at a process temperature of 450 ° C. to 500 ° C. for a process duration of 180 minutes or more and is single or at least Maintained in a process atmosphere consisting primarily of nitrogen gas. In the second process step VIII-B according to the present invention, the ring 14 is in the range of 2 volume percent to less than 12 volume percent ammonia gas at a process temperature of 425 ° C. to 475 ° C. for a process elapsed time of 45 minutes to 90 minutes. Kept in a process atmosphere containing

  The present invention further relates to all the details of the following claims, in addition to the above description and all the details of the drawings which have not been explained but are readily apparent to the expert.

Claims (6)

A heat treatment process in a method for producing a metal ring (14) for use in a drive belt (1),
Ring (14) is produced in two steps, aging and nitriding, respectively, and in the aging process step, ring (14) is kept in an atmosphere free from ammonia gas at a temperature in the range of 450 ° C. to 500 ° C., In the nitriding process step, the ring (14) is kept in an atmosphere containing ammonia gas at a temperature in the range of 425 ° C. to 475 ° C.,
In the heat treatment process,
Treat ring (14) for more than 3 hours with aging process steps,
Treating ring (14) in an atmosphere containing at least 2 volume percent, but less than 12 volume percent ammonia, in a nitriding process step;
A heat treatment process characterized by that.   The heat treatment process according to claim 1, wherein the nitriding process step treats the ring (14) in an atmosphere containing 4 to 8 volume percent ammonia.   The heat treatment process according to claim 1, wherein the ring (14) is treated at 450 ° C for 75 minutes in a nitriding process step.   The heat treatment process according to any one of claims 1 to 3, wherein the ring (14) is treated at 490 ° C for 200 minutes in an aging process step.   The ring (14) comprises 17 weight percent to 19 weight percent nickel, 4 weight percent to 6 weight percent molybdenum, 8 weight percent to 18 weight percent cobalt, less than 1 weight percent titanium, the balance being iron 5. A heat treatment process according to any one of claims 1 to 4, comprising a maraging steel alloy in the range of an aging steel alloy.   A laminated endless pulling means (2) and a plurality of metal elements (3) slidably provided on the pulling means (2), the pulling means (2) comprising: A drive belt (1) for a continuously variable transmission for an automobile, comprising a set of metal rings (14) superposed on each other exposed to the heat treatment method according to any one of the preceding claims.

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