KR101546561B1 - Heat treatment process for a drive belt metal ring component - Google Patents

Abstract

The present invention relates to a heat treatment method in a method for manufacturing a metal ring (14) for a drive belt (1), wherein the heat treatment method comprises a treatment step (VIII-O) for oxidizing the ring (14) (VIII-N) for nitriding the ring (14) in an atmosphere containing oxygen and a subsequent processing step (VIII-N) for nitriding the ring (14) And / or for a few minutes or more.

Description

TECHNICAL FIELD [0001] The present invention relates to a heat treatment method for a metal ring component for a drive belt,

The present invention relates to a method of manufacturing a metal ring for a drive belt, and more particularly to a heat treatment method in a metal ring manufacturing method for a drive belt as defined by the preamble of claim 1 below. The drive belt is commonly used as a means for transmitting power between two adjustable pulleys of known continuously variable transmissions which are predominantly applied to automobiles.

One known type of single-drive belt is described in detail in EP-A-1 403 551, which consists of a plurality of relatively thin transverse metal elements which are slidably incorporated into two stacked endless tensioning means And each of the two stacked endless tensioning means consists of a set of mutually nested flat metal rings, otherwise referred to as bands or hoops. These rings are produced from precipitation hardening steels such as maraging steels, and precipitation hardening steels are produced from steel in the form of a ring, preferably of a ring, which is the end product, Shape, and material characteristics, it possesses considerably high tensile strength characteristics and good resistance properties to tensile stress and bending fatigue. In particular, the present invention relates to a titanium alloy having a basic composition of 17 to 19 wt.% Nickel, 4 to 6 wt.% Molybdenum, 8 to 18 wt.% Cobalt and balance iron and a small amount of titanium, i.e. less than 1 wt. To the range of maraging steel alloys that can be used.

These predetermined material properties include a ring core material of appropriate hardness to combine properties of sufficient elasticity and significantly high tensile strength to allow an extremely hard outer surface layer of the ring to provide longitudinal bending and abrasion resistance of the ring. In addition, the outer surface layer is provided with residual compressive stress which provides a high resistance to metal fatigue, and the residual compressive strength is dependent on the important characteristics of the ring due to the infinite load and bending cycle the ring receives during its service life to be.

The rationale for the known manufacturing method of such a ring is well known in the art, and is described, for example, in JP-A-2004-043962. The ring is formed from a sheet base material, which is bent and welded to a cylindrical shape or a tube and is heat treated to restore the original properties of the material, i. E. To substantially eliminate the internal changes caused by bending and welding I.e., annealed. The tube is then cut into a plurality of hoops, which are then rolled and stretched to the required thickness of typically about 0.185 mm in the final product. After the rolling step, the hoops are generally referred to as rings or bands. The ring undergoes a further annealing step to remove the internal stress caused during the rolling step. Thereafter, the ring is calibrated, that is, the ring is mounted around the two rotating rollers and stretched to a predetermined peripheral length.

Finally, the ring is subjected to a heat treatment method involving three separate steps, in which a different composition of the processing gas is applied. First, the ring is precipitation hardened, i.e. aged, in an environment predominantly containing nitrogen (N ₂ ), and secondly, in an environment containing a considerable amount of oxygen (O ₂ ), for example, Is oxidized in the atmosphere, and thirdly the ring is nitrided, ie it is case hardened in an environment containing significant amounts of ammonia (NH ₃ ).

The joint purpose pursued for a long period of time in the further development and / or improvement of the ring manufacturing method described above is not only in view of improving the material properties realized in the finally produced ring in consideration of the field of use of the drive belt But to improve the efficiency of the ring manufacturing process in terms of providing such good material properties in the most cost effective manner possible. According to the present invention, especially the cost efficiency aspect can be improved according to the known ring making method.

To this end, the present invention provides a new specification for the heat treatment method which is part of the overall manufacturing process for the metallic ring component of the drive belt. According to the present invention, by increasing the strength of the oxidation step of the ring, the above-mentioned known heat treatment method can be remarkably improved and even simplified, while at the same time shortening the aging step of the preceding ring. Thereby, the oxidation strength of the ring is determined by either or both of the temperature and the duration applied to the inside.

In view of the above, according to JP-A-2004-043962, the duration and temperature of the oxidation step of the ring are each severely limited to a time of less than 15 minutes and a temperature of less than 450 degrees Celsius respectively, It is currently thought to be related to some degree of aging of the ring embodied in JP-A-2004-043962. According to the present invention, a considerable loss of the elongation strength of the finally produced ring reported in JP-A-2004-043962 is believed to be due to over-aging phenomenon. The growth of the ring during the step of nitriding the ring and the step of oxidizing the ring at the high temperature while the material is maintained at a sufficiently high temperature and the intermetallic precipitate formed in the maraging steel matrix of the ring, Over-aging occurs when growing larger than the enemy size. Therefore, the total duration of the heat treatment of the ring must not exceed a certain limit, at least independently of the treatment temperature applied therein, but each of the individual heat treatment methods consisting of the ring aging step, the ring oxidation step and the ring nitriding step It is thought that it can be changed to some extent between the duration.

An advantage of the novel process specification according to the invention is that the degree of freedom of variation between the respective durations can be as evenly distributed as possible with the processing capability of each heat treatment process step, It is possible to realize better efficiency and cost efficiency with respect to the manufacturing method of the ring. Each of these capabilities is determined by, for example, the size of the oven or oven chamber required for each step in the manufacturing chain or production line.

In a preferred embodiment of the present invention, the individual aging step of the known ring preceding the oxidation step of the ring in the conventional heat treatment method of the ring component for the drive belt may be omitted. Obviously, this will significantly improve the cost effectiveness of the overall ring manufacturing method, since no separate aging oven or aging chamber is needed anymore.

In view of the latter, those skilled in the art will recognize that by increasing the duration of the nitridation step of the ring and simultaneously reducing the ammonia concentration of the process gas, i. E. The aging process and the nitriding process It is anticipated that similar results may be achieved by enabling simultaneous completion, i. E. Simultaneous completion of a single step combining the ring aging step and the ring nitriding step. However, Applicants have found that the material properties for the final produced ring are not optimized through this particular method of manufacturing the ring. Unexpectedly, it has been found that discontinuous precipitation phenomena, which can reduce the (metal) fatigue strength of the ring, are caused through this manufacturing method.

To reliably avoid this phenomenon, additional criteria are now introduced. According to the present invention, this additional criterion entails that the (core) hardness value of the ring, which is the intermediate product in the heat treatment process after the oxidation step is completed and before the nitriding step of the ring is started, has a hardness value of 400 HV 1.0 or higher. More specifically, in order to further reliably avoid the phenomenon of over-aging, the (core) hardness value of the ring, which is an intermediate product, reaches at least 500 HV1.0.

BRIEF DESCRIPTION OF THE DRAWINGS The above-described basic features of the present invention will be described by way of example with reference to the accompanying drawings.

1 is a schematic view of a drive belt according to the present invention and a transmission to which such a drive belt is applied.
Figure 2 is a diagram showing how the stacked tensioning means and the transverse elements are mutually oriented within the drive belt.
Fig. 3 schematically shows a known method of manufacturing a metal ring applied to an endless tensioning means of a drive belt.
Figure 4 shows a heat treatment process which is part of the above-described manufacturing method optimized according to the present invention.
Figure 5 shows a novel method of the part of the heat treatment process according to the invention.
6 is a photograph showing an enlarged section of a ring including a nitrided surface layer in which discontinuous precipitation occurs.

In the figures, in the known production methods and the novel production methods, individual processing steps are indicated in the form of roman numerals.

Fig. 1 schematically shows a continuously variable transmission CVT in which the drive belt 1 is wound around two pulleys 1 and 2. Fig. The belt 1 comprises a laminate tensioning means 2 in the form of two sets of mutually nested endless thin metal rings 14 (otherwise referred to as bands 14), a transverse element 3, (Alternatively referred to as transverse elements 3), which are mounted along the periphery of the tension means 2 and are freely slidable therefrom. Such a continuously variable transmission is known per se.

Fig. 2 shows a cross section of the front side of the transverse element 3 and the laminate tension means 2. The lateral side 6 of the transverse element 3 is shown in the lateral direction, with the lateral side resting on the conical surface of one of the drive pulleys or the driven pulley. The ring 14 of the tensioning means 2 is made of high quality steel, for example maraging steel. The thickness of the normal ring 14 is in the range of 0.15 mm to 0.25 mm, the normal ring width is in the range of 8 mm to 35 mm, and the normal ring circumferential length is in the range of 500 mm to 1000 mm.

Fig. 3 shows the relevant part of the above-described belt 1, particularly known to the ring 14 of the belt, as has been done since the initial metal push belt was produced. The sheet 11 of the base material is bent into a cylinder shape in the first treatment step I and the end portions 12 of the sheet which are mutually abutted in the second treatment step II are welded to each other to form the tube 13. [ In the third treatment step (III), the tube 13 is annealed. Next, in the fourth treatment step (IV), the tube 13 is cut into a plurality of FOUPs 14, and the cut FOUPs 14 are rolled and stretched to a predetermined thickness in a subsequent fifth processing step (V) . After the rolling step, hoop 14 is commonly referred to as ring 14 or band 14. The ring 14 is subjected to a further annealing step (VI) to remove the internal stress caused during the rolling step. Next, in the seventh processing step (VII), the ring 14 is calibrated, that is, the ring is mounted on two rotating rollers and stretched to a predetermined peripheral length. Further, in the seventh processing step (VII), an internal stress distribution is imposed on the ring 14, which forms a so-called curling radius of each ring 14.

Finally, in an eighth process step (VIII) of the known production process more specifically shown in FIG. 4 below, the ring 14 is heat treated in three separate stages, and in each step a different composition of the process gas Is applied. First, the ring 14 are precipitation hardened, i.e. mostly nitrogen (N ₂₎ environment and treated aged at composed (step Ⅷ-A), the second to the ring is waiting, that is, a significant amount of oxygen (O ₂₎ (Step VIII-O), and thirdly the ring is nitrided, i.e. it is surface hardened in an environment containing significant amounts of ammonia (NH ₃ ) (Step VIII-N).

In the known process step (VIII-A) for aging the ring, the ring 14 is heat-treated for several hours at a temperature of 430 ° C to 480 ° C, and the duration of the treatment is mainly dependent on the composition of the ring material used It is known. For example, an increase in the amount of cobalt (Co) by weight in the basic maraging steel alloy composition results in a significant increase in the nucleating (i.e., formation) of iron molybdenum (Fe _x Mo _y ) and nickel molybdenum (Ni _x Mo _y ) . In fact, it is known that by including at least 8 to 18 weight percent cobalt in the maraging steel alloy, a treatment duration of less than 2 hours, even a low treatment duration of 45 to 90 minutes, can be achieved. During the ring aging step (VIII-A), the hardness of the ring material increases because intermetallic precipitation continues to form and grow in the steel matrix. It is also known to accelerate the aging step (VIII-A) of the ring by applying a treatment temperature up to 500 ° C.

In the known oxidation treatment step (VIII-O) of the ring, the ring 14 is heated to a temperature of 330 ° C to 450 ° C for 5 to 15 minutes. During the oxidation step (VIII-O) of the ring, the surface of the ring 14 is cleaned and prepared or "activated" for the nitriding treatment.

In the known nitriding treatment step (VIII-N) of the ring, the ring 14 is heated to a temperature between 420 [deg.] C and 500 [deg.] C for 35-80 min, and the treatment duration is known to vary mainly depending on the treatment temperature . During the nitriding step (VIII-N) of the ring, the ring 14 is provided with a nitrided diffusion region or surface layer, usually 25 to 35 micrometers, with extreme hardness and a significant compressive stress is applied.

By arranging a plurality of rings 14 intentionally selected from the plurality of rings 14 thus treated, that is, by arranging one ring 14 concentrically around the other ring 14 as shown in Fig. 3 A tensioning means 2 is formed whereby a positive or negative play between the adjacent rings 14 of the tensioning means 2 is permitted.

According to the present invention, the above-described known heat treatment method can be significantly improved by increasing the duration and / or temperature of the oxidation step (VIII-O) of the ring, The duration can be significantly reduced as required by the maraging steel alloy composition of the ring 14. Thus, according to the present invention, the ring 14 is oxidized at a temperature higher than 450 DEG C and / or for a time longer than 15 minutes (treatment step VIII-O). Thus, with respect to the maraging steel alloy composition described above, the aging step (VIII-A) of the preceding ring is advantageously reduced to less than 30 minutes.

In a preferred embodiment of the present invention, the ring 14 is oxidized (temperature step VIII-O) at a temperature between 400 ° C and 575 ° C for 20 to 90 minutes. According to the present invention, in particular with respect to the maraging steel alloy composition described above, the preceding individual aging treatment step (VIII-A) of the ring, in the latter case, Can be completely and advantageously omitted from the heat treatment method of the component. In the latter novel and simple heat treatment method according to the present invention, the ring 14 is preferably oxidized (treatment step VIII-O) at a temperature between 440 ° C and 480 ° C for 30 minutes to 60 minutes.

Finally, the Applicant has found that the combined strength of the individual treatment steps of the ringing stage (VIII-A) and the ring oxidation stage (VIII-O) (if included in the heat treatment process) preferably meets the minimum requirements . Otherwise, a discontinuous precipitation phenomenon may occur in the subsequent nitriding treatment step (VIII-N) of the ring, and this phenomenon is illustrated in FIG. 6 and may result in a reduction of the (metal) fatigue strength of the ring 14 have.

6 is a photograph of an enlarged section of a ring 14 comprising a nitrided surface layer (NSL). It can be seen from the photographs that the material near the outer surface of the ring 14 has a relatively rough course structure associated with the discontinuous deposition (DP), since the alloying elements of the maraging steel, such as molybdenum, Implies locally forming a nitride at the crystal boundary instead of an intermetallic compound.

In particular, only the nitriding step (VIII-N) of the ring after the (core) hardness value of the intermediate product ring 14 has reached a hardness value of at least 400 HV1.0 in the oxidation step (VIII- It was found that it would be desirable to begin to be performed. In this case, the discontinuous precipitation (DP) does not occur. On the other hand, in order to avoid said over-aging in the subsequent nitriding step (VIII-N) of the ring, said (core) hardness value of the ring which is the intermediate product preferably does not exceed the hardness value of 500 HV1.0.

The present invention also relates to the details of the following claims, apart from the foregoing and all of the details of the drawings, which are not so obvious to those skilled in the art, which are not described.

Claims (10)

A process step (VIII-O) of placing a metal ring (14) in an oxygen containing environment to oxidize the metal ring (14), and a process step A method of manufacturing a metal ring (14) having a thickness between 0.150 mm and 0.250 mm for a drive belt (1), comprising at least a subsequent processing step (VIII-N)
Wherein the treatment step (VIII-O) for oxidizing the metal ring (14) has a duration between 30 minutes and 60 minutes, and the oxygen-containing environment is maintained at a temperature between 440 ° C and 480 ° C. Process according to claim 1, characterized in that the metal ring (14) is made of a maraging steel alloy and the process step (VIII-O) for oxidizing the metal ring (14) is carried out at a temperature between 430 [deg.] C and 500 [deg.] C (VIII-A) after aging the metal ring (14) in the step (VIII-A). 4. The method of claim 1 or 2, wherein the metal ring (14) comprises 17 to 19 weight percent nickel, 4 to 6 weight percent molybdenum, 8 to 18 weight percent cobalt, 0 weight percent and less than 1 weight percent Titanium and a maraging steel alloy comprising iron as the remainder and wherein the method of heat treatment in the method of making a metal ring comprises only the oxidation treatment step (VIII-O) and the nitriding treatment step (VIII-N) Heat treatment method. The metal ring according to claim 1 or 2, wherein the metal ring (14) comprises 17 to 19% by weight of nickel, 4 to 6% by weight of molybdenum, 8 to 18% by weight of cobalt, Wherein the heat treatment method in the metal ring manufacturing method comprises only an oxidation treatment step (VIII-O) and a nitriding treatment step (VIII-N). 3. The process according to claim 1 or 2, characterized in that the intermediate product, the metal ring (14) between the oxidizing treatment step (VIII-O) and the nitriding treatment step (VIII-N) is between 400 HV 1.0 and 500 HV 1.0 Of the hardness value of the heat treatment. delete delete delete delete delete

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