JP3784618B2 - Manufacturing method of laminated ring - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method of manufacturing a laminated ring constituting a continuously variable transmission belt used for power transmission of a continuously variable transmission.
[0002]
[Prior art]
A continuously variable transmission belt used for power transmission of a continuously variable transmission is configured by a stacked ring in which a plurality of metal rings are stacked and a number of metal elements assembled to the stacked rings.
[0003]
Conventionally, the laminated ring is manufactured by the following method. First, a cylindrical drum by welding the ends of the thin plate of maraging steel is ultra high strength steel, homogeneous partially hardened hardness unevenness by the heat during the welding with respect to the drum In order to make it, the first solution treatment is performed. Next, the solution-treated drum is cut into a predetermined width to form a ring, and the ring is rolled. Next, after the second solution treatment is performed on the rolled ring, circumference correction is performed to correct the ring after the solution formation to a predetermined circumference. Then, after aging and nitriding treatment are performed on the ring subjected to the peripheral length correction to improve the internal and surface hardness, a plurality of rings having different peripheral lengths are stacked on each other to form a stacked ring. .
[0004]
In the conventional manufacturing method, in order to recrystallize the rolled structure and restore the shape of the metal structure deformed by rolling, the second solution treatment for the rolled ring is generally performed at a recrystallization temperature of maraging steel. In a temperature range of about 760 ° C. or more and less than 850 ° C. Further, according to the study by the present inventors, the second solution treatment is performed at about 780 ° C. or higher, which is the recrystallization completion temperature of maraging steel, in order to prevent occurrence of uneven nitriding in the subsequent nitriding treatment. It is preferable to carry out in a temperature range of less than ° C.
[0005]
When the second solution treatment is carried out at a temperature within the above range, a subsequent aging treatment and nitriding treatment can provide excellent tensile strength, surface hardness, internal hardness, and a nitride layer having an appropriate depth to obtain the physical properties. Can be obtained. However, the ring subjected to the second solution treatment at the temperature in the above range cannot be sufficiently stretched by pulling, and the laminated ring formed as described above is used as a continuously variable transmission belt. There is a problem that if a notch, a crack, or the like is formed, it develops and is easily broken.
[0006]
On the other hand, when maraging steel is heated at a temperature of about 500 to 750 ° C., it is known that a reverse transformation austenite phase is generated. The reverse-transformed austenite phase is the one in which intermetallic compounds such as Ni ₃ Ti precipitated at the grain boundaries of the maraging steel are partly dissolved in the matrix, and the portion with a high Ni concentration is locally austenitic. It is. The reverse transformed austenite phase has increased toughness and elongation due to the formation of the reverse transformed austenite phase. In view of this, it is conceivable to perform a heat treatment to generate the reverse transformed austenite phase instead of the second solution.
[0007]
However, when the amount of the reverse-transformed austenite phase increases, age hardening does not occur in the subsequent aging treatment, and it becomes difficult to form a uniform and stable nitride layer even in the nitriding treatment, and sufficient tensile strength, surface hardness, internal hardness There is a disadvantage that it becomes difficult to obtain.
[0008]
[Problems to be solved by the invention]
An object of the present invention is to provide a method for manufacturing a laminated ring that eliminates such inconveniences and has a tensile strength, surface hardness, and internal hardness equivalent to those of the prior art, and that does not easily break even if a notch, a crack, or the like occurs. And
[0009]
[Means for Solving the Problems]
The inventors of the present invention can impart to the maraging steel ring toughness and elongation that are difficult to break even if notches, cracks, etc. occur, and to the belt for continuously variable transmissions by aging treatment and nitriding treatment. Various studies were conducted on heat treatment conditions that can provide the necessary tensile strength, surface hardness, and internal hardness. As a result, the present inventors have found that the amount of reverse transformed austenite phase produced can be controlled by subjecting the ring after rolling to a heat treatment in a specific temperature range, thereby completing the present invention.
[0010]
Therefore, in order to achieve the above object, the laminated ring manufacturing method of the present invention forms a ring by cutting a cylindrical drum formed by welding end portions of maraging steel thin plates to a predetermined width. A step of rolling, a step of rolling the ring, a step of heat-treating the rolled ring, a step of subjecting the ring subjected to the heat treatment to aging treatment and nitriding treatment, and a ring subjected to nitriding treatment Among these, in the method of manufacturing a laminated ring comprising a step of forming a laminated ring by laminating a plurality of rings having different circumferential lengths little by little , the heat treatment on the rolled ring is performed at 550 to 620 ° C. By carrying out at a temperature within the range, 15 to 25% by volume of a reverse-transformed austenite phase is generated with respect to the maraging steel constituting the ring .
[0011]
According to the manufacturing method of the present invention, the rolled ring is subjected to a heat treatment at a temperature in the above range, thereby imparting toughness and elongation that makes it difficult to break even if a notch, a crack, or the like occurs in the ring. Therefore, it is possible to generate a reverse-transformed austenite phase in an amount sufficient to prevent the formation of a stable and uniform nitrided layer by age hardening and nitriding by the subsequent aging treatment.
[0012]
If the temperature of the heat treatment is less than 550 ° C., the ring may be excessively pulled or may break when a notch, a crack, or the like occurs. If the temperature of the heat treatment exceeds 620 ° C., the tensile strength, surface hardness, and internal hardness required for the continuously variable transmission belt cannot be obtained by the subsequent aging treatment and nitriding treatment.
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Next, embodiments of the present invention will be described in more detail with reference to the accompanying drawings. FIG. 1 is a process diagram schematically showing a method for manufacturing a laminated ring of the present embodiment, FIG. 2 is a diagram showing the relationship between the heat treatment temperature and the expansion rate of maraging steel, and FIG. 3 is the heat treatment temperature and reverse transformation. FIG. 4 is a diagram showing the relationship between the austenite phase generation amount, FIG. 4 is a diagram showing the relationship between the elongation ratio of the ring obtained by the manufacturing method of this embodiment and the tensile strength, and FIG. FIG. 6 is a diagram showing the relationship between the heat treatment temperature, the surface hardness and the internal hardness, and FIG. 7 is a diagram showing the relationship between the heat treatment temperature and the tensile strength.
[0014]
In the manufacturing method of this embodiment, first, as shown in FIG. 1, the maraging steel thin plate 1 is bent to form a loop, and then the ends are welded to form a cylindrical drum 2. At this time, since the maraging steel exhibits age hardening by the heat of welding, high hardness portions appear on both sides of the welded portion of the drum 2.
[0015]
Therefore, next, the drum 2 is accommodated in the vacuum furnace 3 and heated to perform a solution treatment, thereby removing hardness unevenness. When the solution treatment is completed, the drum 2 is unloaded from the vacuum furnace 3 and cut into a predetermined width to form the ring 4.
[0016]
The ring 4 formed as described above is then rolled at a rolling reduction of 40 to 50%. A rolled structure is formed on the rolled ring 4 with a thickness of about 30 μm from the surface. Accordingly, the rolled ring 4 is accommodated in the heating furnace 5 and subjected to heat treatment at a temperature in the range of 550 to 620 ° C., thereby generating a reverse transformed austenite phase.
[0017]
After cooling, the ring 4 subjected to the heat treatment is corrected in circumference, and further subjected to aging treatment and nitriding treatment. Then, among the rings 4 that have been subjected to nitriding treatment, a plurality of rings 4 having different circumferential lengths are laminated one by one to form a laminated ring (not shown).
[0018]
Next, the heat treatment in the heating furnace 5 for the rolled ring 4 will be described in detail.
[0019]
When the maraging steel constituting the ring 4 is heated, the crystal structure changes, but this change can be grasped as a change in the expansion coefficient of the ring 4 as shown in FIG. Referring to FIG. 2, in ring 4, when the processing temperature exceeds 500 ° C., an intermetallic compound such as Ni ₃ Ti that has precipitated at the grain boundaries of the maraging steel starts to partially dissolve in the matrix. . As a result, a portion with a high Ni concentration is locally austenitized to produce a reverse transformed austenite phase. The formation of the reverse transformed austenite phase continues until the processing temperature exceeds about 740 ° C. and recrystallization of the maraging steel begins. The recrystallization of the maraging steel includes austenitization, which is completed when the processing temperature reaches about 780 ° C.
[0020]
The reverse-transformed austenite phase is generated in the range of 500 to 740 ° C. as described above. At this time, the ratio (volume%) of the amount of the reverse-transformed austenite phase to the entire maraging steel is as shown in FIG. Change. Therefore, in the present embodiment, the heat treatment in the heating furnace 5 for the rolled ring 4 is performed in the range of 550 to 620 ° C., so that the reverse of 15 to 25% by volume with respect to the maraging steel constituting the ring 4. A transformation austenite phase is generated.
[0021]
Next, after performing heat treatment at each temperature within the range of the present embodiment, ring 4 (Examples 1 to 3) subjected to nitriding by immersing in a salt bath at 530 ° C. for 20 minutes and nitriding by a tuftride method. For comparison, after performing heat treatment at 500 ° C., which is a temperature lower than the lower limit of the temperature range, the ring 4 (Comparative Example 1) subjected to nitriding under the same conditions as described above and simultaneously aged is stretched. Then, the tensile strength with respect to the elongation rate was measured, and the elongation rate to the elongation limit was measured. The results are shown in FIG.
[0022]
From FIG. 4, the ring 4 (Comparative Example 1) that was heat-treated at 500 ° C. had an elongation of 2.1% and did not obtain sufficient stretchability. The ring 4 (Example 1) that was subjected to heat treatment was 2.8%, and the ring 4 (Example 2) that was heat-treated at 580 ° C. was 3.8%, and the ring 4 (Example 1) that was heat-treated at 620 ° C. 3) is 3.0% and does not break, and it is clear that remarkably excellent stretchability is imparted to the comparative example.
[0023]
Next, for comparison with each of the rings 4 of Examples 1 to 3, after heat treatment at 700 ° C., which is higher than the upper limit of the temperature range, nitriding is performed at the same time as the above, and aging is performed simultaneously For the treated ring 4 (Comparative Example 2), the depth of the nitrided layer was measured visually. The results are shown in FIG.
[0024]
From FIG. 5, all of the rings 4 of Examples 1 to 3 are provided with a nitrided layer having a depth of 20 μm or more, which is necessary for constituting a laminated ring of a continuously variable transmission belt, In the ring 4 of Comparative Example 2, it is clear that the thickness of the nitride layer does not reach 20 μm. As is apparent from FIG. 3, the ring 4 of Comparative Example 2 has a larger amount of the reverse transformed austenite phase than the rings 4 of Examples 1 to 3, so that a stable and uniform nitride layer is formed. It is thought that it is not done.
[0025]
Next, the surface hardness and the internal hardness of the ring 4 subjected to heat treatment at a temperature in the range of 550 to 700 ° C., and further subjected to nitriding by immersing in a salt bath at 530 ° C. for 20 minutes by a tuftride method and simultaneously aging treatment. Was measured. The results are shown in FIG.
[0026]
From FIG. 6, when the heat treatment is performed in the range of 550 to 620 ° C. of the present embodiment, the surface hardness of 750 (HMV) or more required for constituting the laminated ring of the continuously variable transmission belt is obtained. Although it is obtained, it is clear that when the temperature of the heat treatment exceeds 620 ° C. which is the upper limit of the temperature in the range of the present embodiment, the surface hardness becomes less than 750 (HMV). Further, regarding the internal hardness, it is clear that the hardness sharply decreases when the temperature of the heat treatment exceeds 620 ° C. which is the upper limit of the temperature in the range of the present embodiment.
[0027]
Next, after performing heat treatment at a temperature in the range of 550 to 700 ° C., the tensile strength was measured for the ring 4 that was further nitrided by immersing in a salt bath at 530 ° C. for 20 minutes by the tuftride method and simultaneously aging treatment. did. The results are shown in FIG. From FIG. 7, it is clear that when the temperature of the heat treatment exceeds 620 ° C. which is the upper limit of the temperature in the range of the present embodiment, the tensile strength sharply decreases.
[0028]
From the above results, by performing the heat treatment on the rolled ring 4 at a temperature in the range of 550 to 620 ° C., a ring 4 having excellent tensile strength, surface hardness, and internal hardness is obtained along with excellent stretchability. It is clear. The laminated ring formed from such a ring 4 can be suitably used for a continuously variable transmission belt due to the tensile strength, surface hardness, and internal hardness, and for the continuously variable transmission due to the stretchability. Even if a notch, a crack, or the like occurs when used for a belt, it is difficult to break.
[0029]
The ring 4 of the present embodiment is subjected to nitriding treatment by the tuftride method at the same time as the nitriding treatment following the heat treatment. However, after aging treatment by heating in a heating furnace, gas nitriding treatment or gas soft nitriding treatment is performed. A treatment may be applied, or an aging treatment may be performed simultaneously with gas nitriding treatment or gas soft nitriding treatment by heating in a heating furnace.
[Brief description of the drawings]
FIG. 1 is a process diagram schematically showing a method for producing a laminated ring of the present invention.
FIG. 2 is a diagram showing the relationship between the heat treatment temperature and the expansion rate of maraging steel.
FIG. 3 is a diagram showing the relationship between the heat treatment temperature and the amount of reverse transformed austenite phase formed.
FIG. 4 is a diagram showing the relationship between the elongation percentage and tensile strength of the ring obtained by the production method of the present invention.
FIG. 5 is a diagram showing the relationship between the heat treatment temperature and the depth of the nitride layer.
FIG. 6 is a diagram showing the relationship between heat treatment temperature, surface hardness and internal hardness.
FIG. 7 is a diagram showing the relationship between heat treatment temperature and tensile strength.
[Explanation of symbols]
1 ... thin plate of maraging steel, 2 ... drum, 4 ... ring.

Claims (1)

A step of forming a ring by cutting a cylindrical drum formed by welding ends of thin sheets of maraging steel to a predetermined width, a step of rolling the ring, and a heat treatment for the rolled ring Laminating by laminating a plurality of rings with slightly different circumferential lengths among the ring subjected to aging treatment and nitriding treatment and the ring subjected to nitriding treatment. A method of manufacturing a laminated ring comprising a step of forming a ring ,
The heat treatment for the rolled ring is performed at a temperature in the range of 550 to 620 ° C. to generate 15 to 25% by volume of a reverse transformed austenite phase with respect to the maraging steel constituting the ring. A method for manufacturing a laminated ring.

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