JP3630299B2 - Method for manufacturing endless ring for metal belt of continuously variable transmission - Google Patents

Description

【００２７】
表１に示すように、１８％Ｎｉ系マルエージング鋼からなる無端リングに、３時間の時効熱処理、および窒化処理を施すことによって、内部硬さが５００Ｈｖ以上であると共に、表面窒素濃度が０．５％以上、窒化深さが板厚中心までの距離の２０〜５０％の窒化層を備え、さらに６００ＭＰａ以上の圧縮残留応力を有するようにした本発明Ａ，Ｂ，Ｃ，Ｄに係わる金属ベルト用無端リングにおいては、いずれも極めて良好な疲労強度を備えていることが確認された。
【００２８】
これに対して、窒化処理時間が８分と短く、表面窒素濃度が低く、窒化深さが浅い比較例Ｅ、逆に窒化処理時間が７５分と長く、窒化深さが深い比較例Ｆ、および時効処理時間が１時間と短いために時効硬さが５００Ｈｖに満たない比較例Ｇの無端リングにおいては、いずれも十分な疲労強度が得られず、時効処理を窒化処理と同時（窒化しながら時効）に行った比較例Ｈにおいては、比較的高い圧縮残留応力が得られるものの、疲労強度はあまり高くないことが判明した。この理由は、必ずしも明確になっていないが、結晶粒界の母材中のチタンが窒素との析出物を作り、粒界強度が低下したことによるものと考えられる。また、窒化処理を行わず、時効処理のみを施した比較例Ｉの無端リングにおいては、圧縮残量応力が低く、比較例の中でも最低レベルの疲労強度を示すことが確認された。
【図面の簡単な説明】
【図１】本発明に係わる無端リングを用いた無段変速機用金属ベルトの形状および構造を示す概略説明図である。
【図２】本発明に係わる無端リングの製造工程の一例を示す説明図である。
【図３】（ａ）時効処理におけるヒートパターンの一例を示す説明図である。
（ｂ）窒化処理におけるヒートパターンの一例を示す説明図である。
（ｃ）時効処理と窒化処理を同時に行う場合のヒートパターンを示す説明図である。
【図４】無端リングの疲労試験方法を示す概略説明図である。
【図５】無端リングの窒化深さと疲労強度の関係を示すグラフである。
【図６】白色層形成に及ぼす窒化時間と窒化温度の関係を示すグラフである。
【図７】無端リングの窒化深さと周長変化量の関係を示すグラフである。
【符号の説明】
Ｒ 金属ベルト用無端リング[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for manufacturing an endless ring for a metal belt used as a component part of a metal belt in a belt-type continuously variable transmission used in, for example, an automobile.
[0002]
[Prior art]
The metal belt used in the belt-type continuously variable transmission as described above has a shape as shown in FIG. 1, for example, two sets of metal endless layers in which about nine metal blocks B are laminated, for example. Since the ring R is connected endlessly, the endless ring R is required to have high tensile strength associated with power transmission and fatigue strength that can withstand repeated bending stress caused by winding around the pulley. For the endless ring R, maraging steel known as super-strength steel is used, and nitriding treatment is further performed.
[0003]
This maraging steel has a problem that, when nitriding, a passive film is formed by oxidation, so that nitrogen intrusion is hindered and uniform nitriding is difficult. JP-A-11-229113 proposes a uniform nitriding method.
[0004]
On the other hand, such an endless ring for a metal belt is used by stacking around 10 rings as described above, and therefore, it is necessary to reduce the variation in the circumferential elongation before and after the nitriding treatment of the ring. For this reason, conventionally, the nitriding treatment of maraging steel described in Japanese Patent Application Laid-Open No. 59-80772, in which the ammonia concentration is reduced during gas nitriding to reduce the amount of nitrogen penetration and the nitriding depth is shallow. The method was considered good.
[0005]
[Problems to be solved by the invention]
In recent years, there has been a growing need for higher engine output and smaller and lighter weights. To meet these demands, metal belts are required to have higher strength. Above all, since maraging steel having the highest strength is used, it is necessary to consider the limit in terms of material, and it can be said that it is desirable to increase the strength by surface hardening treatment such as nitriding.
[0006]
However, only the nitriding treatment with a shallow nitriding depth has been studied due to the above-mentioned circumferential length variation problem. To cope with higher torque, the thickness of the endless ring is increased or the number of used sheets is increased. There was a problem that they had to take countermeasures.
[0007]
OBJECT OF THE INVENTION
The present invention has been made in view of the above-described problems in conventional endless rings for metal belts, and has excellent fatigue strength and less perimeter variation based on nitriding treatment. It aims at providing the manufacturing method of.
[0008]
[Means for Solving the Problems]
The manufacturing method of an endless ring for a metal belt of a continuously variable transmission according to the present invention is an aging heat treatment of an endless ring made of 18% Ni maraging steel and having a thickness of 0.1 to 0.3 mm to a hardness of 500 Hv or more. After that, the ring is subjected to a nitriding treatment in which the nitrogen concentration on the surface is 0.5% or more, the nitrogen penetration depth is 20 to 50% of the distance to the center of the plate thickness, and the compressive residual stress value is 600 MPa or more. In the manufacturing method according to claim 2 as an embodiment of the manufacturing method of the endless ring for metal belt, the nitriding treatment is gas nitriding or gas soft nitriding using ammonia gas. Such a configuration in the method for manufacturing an endless ring for a metal belt is a means for solving the above-described conventional problems.
[0009]
[Effects of the Invention]
An endless ring for a metal belt of a continuously variable transmission according to the present invention is manufactured by a process as shown in FIG.
[0010]
That is, for example, 18% Ni-based maraging steel rolled to a thickness of about 0.3 mm is joined annularly by laser beam or plasma beam welding, and then heated to 800 to 850 ° C. for solution treatment. And then cut to a plate width of 12.5 mm. Then, after barrel polishing, it is ring-rolled to a plate thickness of about 0.18 mm and a circumferential length of 700 mm, heated to 800 to 850 ° C., and subjected to a solid solution treatment again. And while performing dimension correction and performing an age hardening process by heating to 480-510 degreeC, the nitriding process is performed for about 10 to 60 minutes at the temperature of about 450 degreeC.
[0011]
The 18% Ni-based maraging steel used in the present invention is, by mass ratio, C: 0.03% or less, Si: 0.1% or less, Mn: 0.1% or less, Ni: 17 to 19 %, Mo: 3 to 5.5%, Co: 7 to 9.5%, Al: 0.05 to 0.2%, Ti: 0.15 to 0.8%.
[0012]
FIG. 5 shows a plate thickness of 0.18 mm, a width of 12.5 mm, and a ring formed by plasma welding, solution heat treatment and rolling using 18% Ni maraging steel having a plate thickness of 0.3 mm as described above. After processing into an endless ring with a circumference of 700 mm, as shown in FIG. 3A, an aging treatment of heating temperature: 480 ° C. and holding time: 3 hours was performed to adjust the hardness to Hv570, and then FIG. As shown in b), the fatigue strength of the endless ring when the temperature is fixed at 450 ° C. and the nitriding treatment is performed by changing the holding time in various ways is the distance to the center of the plate thickness (1/1 of the plate thickness). 2) shows the result of arrangement by the ratio (%) of the nitriding depth with respect to 2), and the fatigue strength is about 28% of the nitriding depth, that is, the nitrided layer has a depth of about 0.025 mm from both surfaces of the endless ring. In the ring (processing time 15 minutes) A peak value each, nitride depth to the thickness center was found to have an optimum value in the range of 20% to 50%.
[0013]
In FIG. 5, the nitriding depth of 0% is not subjected to nitriding treatment, whereas the nitriding depth of 60% has a brittle compound layer called a white layer formed on the outermost surface. It can be seen that the fatigue strength decreases.
[0014]
In the above nitriding treatment, in order to remove the passive film, the temperature inside the furnace was raised to 450 ° C. and the atmosphere was maintained at a residual ammonia concentration of 40%, and then the ring was placed in the furnace, and methylene chloride was removed. Nitriding was performed by injecting about 10 to 40 ml. Furthermore, as shown in FIG. 4, the fatigue strength is applied to the endless ring obtained by attaching a large roller L1 having a diameter of 160 mm and a small roller L2 having a diameter of 80 mm, and a constant tensile load of 5000 N is applied. Evaluation was made based on the number of repetitions (number of rotations) until breakage when rotating at 1000 rpm.
[0015]
FIG. 6 shows the nitriding temperature and processing time of the white layer formation region in the gas nitriding treatment with a residual ammonia concentration of 40%, based on the above result that the fatigue strength is reduced by the formation of the white layer. For example, it is understood that when the processing temperature is 450 ° C., processing for 80 minutes or longer should be avoided, and when the processing temperature is 500 ° C., processing for 50 minutes or longer should be avoided. Note that this graph is a result when the residual ammonia concentration is 40%, and it should be noted that the formation region of the white layer is shifted depending on the ammonia concentration.
[0016]
FIG. 7 illustrates the relationship between the nitriding depth and the circumferential length change amount in each endless ring obtained as described above, and the circumferential length change amount increases as the nitrogen penetration amount increases. Even if the nitrogen concentration is increased, the variation in ring circumference after nitriding does not increase, and the ring circumference elongation after nitriding has a strong correlation with the nitrogen content. It became clear that the problem of variation in ring circumference could be solved.
[0017]
In the method for manufacturing an endless ring for a metal belt according to the present invention, the hardness after aging treatment is 500 Hv or more because the internal hardness or the aging hardness is less than 500 Hv, that is, the aging sufficiently proceeds. If the nitriding treatment is performed in a state in which the nitriding treatment is not performed, the nitrided layer becomes brittle and a sufficient aging strength cannot be obtained.
[0018]
In addition, the surface nitrogen concentration of the nitrided layer is set to 0.5% or more because when the surface nitrogen concentration is less than 0.5%, the surface hardness and compressive residual stress for obtaining sufficient fatigue strength are increased. By not getting.
[0019]
The reason why the compressive residual stress value is set to 600 MPa or more is that sufficient fatigue strength cannot be obtained when the compressive residual stress value is less than 600 MPa.
[0020]
【The invention's effect】
In the method of manufacturing an endless ring for a metal belt of a continuously variable transmission according to claim 1 of the present invention, an endless ring made of 18% Ni-based maraging steel is subjected to aging heat treatment to a hardness of 500 Hv or more, and then the surface nitrogen concentration Is 0.5% or more, the nitrogen penetration depth is 20 to 50% of the distance to the center of the plate thickness, and the compressive residual stress value is 600 MPa or more. Without increasing the variation, the tensile and fatigue strength of the endless ring for metal belt can be greatly improved. In the manufacturing method of the endless ring for metal belt according to claim 2 as an embodiment of the manufacturing method, As the nitriding treatment, gas nitriding treatment using ammonia gas or gas soft nitriding treatment is performed. As well as a manner easy, since it can relatively low temperature process in a gas nitrocarburizing treatment, leading to excellent effect that it is possible to suppress the distortion.
[0021]
【Example】
Hereinafter, the present invention will be described more specifically based on examples.
[0022]
As shown in FIG. 2, by applying plasma welding, solution heat treatment and rolling to 18% Ni-based maraging steel having a plate thickness of 0.3 mm, plate thickness: 0.18 mm, width: 12.5 mm, ring Peripheral length: processed into an endless ring of 700 mm.
[0023]
Next, aging treatment and nitriding treatment are performed under various conditions combined with the heat patterns shown in FIGS. 3A to 3C, and the aging hardness of the obtained endless ring and compression after nitriding treatment are performed. Residual stress, surface hardness, internal hardness, surface nitrogen concentration and nitriding depth were measured, and the fatigue strength of each endless ring was investigated by the method shown in FIG. 4 (tensile load: 5000 N, rotational speed: 1000 rpm). .
[0024]
In the nitriding treatment, in order to remove the passive film on the ring surface, after raising the temperature in the furnace, adjusting the atmosphere (residual ammonia concentration 40%) and charging the ring, about 10 ml to 40 ml of methylene chloride is put in the furnace. Injected into.
[0025]
These results are also shown in Table 1.
[0026]
[Table 1]

[0027]
As shown in Table 1, by subjecting an endless ring made of 18% Ni-based maraging steel to an aging heat treatment and nitriding treatment for 3 hours, the internal hardness is 500 Hv or more and the surface nitrogen concentration is 0.1. A metal belt according to the present invention A, B, C, D having a nitrided layer having a nitriding depth of 20 to 50% of the distance to the center of the plate thickness and having a compressive residual stress of 600 MPa or more. It was confirmed that all endless rings for use had extremely good fatigue strength.
[0028]
In contrast, Comparative Example E, in which the nitriding time is as short as 8 minutes, the surface nitrogen concentration is low, and the nitriding depth is shallow, on the contrary, Comparative Example F in which the nitriding time is as long as 75 minutes and the nitriding depth is deep, and In the endless ring of Comparative Example G in which the aging hardness is less than 500 Hv because the aging treatment time is as short as 1 hour, sufficient fatigue strength is not obtained, and the aging treatment is performed simultaneously with nitriding treatment (aging while nitriding) In Comparative Example H performed in (1), it was found that although a relatively high compressive residual stress was obtained, the fatigue strength was not so high. Although this reason is not necessarily clear, it is considered that titanium in the base material of the crystal grain boundary forms a precipitate with nitrogen and the grain boundary strength is lowered. Further, it was confirmed that the endless ring of Comparative Example I in which only the aging treatment was performed without performing the nitriding treatment had a low compressive residual stress and exhibited the lowest level of fatigue strength among the Comparative Examples.
[Brief description of the drawings]
FIG. 1 is a schematic explanatory view showing the shape and structure of a metal belt for a continuously variable transmission using an endless ring according to the present invention.
FIG. 2 is an explanatory view showing an example of a manufacturing process of an endless ring according to the present invention.
FIG. 3A is an explanatory diagram showing an example of a heat pattern in an aging treatment.
(B) It is explanatory drawing which shows an example of the heat pattern in a nitriding process.
(C) It is explanatory drawing which shows a heat pattern in the case of performing an aging treatment and a nitriding treatment simultaneously.
FIG. 4 is a schematic explanatory diagram showing a fatigue test method for an endless ring.
FIG. 5 is a graph showing the relationship between the nitriding depth of an endless ring and the fatigue strength.
FIG. 6 is a graph showing the relationship between nitriding time and nitriding temperature affecting white layer formation.
FIG. 7 is a graph showing the relationship between the nitriding depth of an endless ring and the amount of change in circumference.
[Explanation of symbols]
R Endless ring for metal belt

Claims (2)

An endless ring made of 18% Ni maraging steel with a thickness of 0.1 to 0.3 mm is subjected to an aging heat treatment to a hardness of 500 Hv or more, and then the nitrogen concentration on the surface is 0.5% or more and nitrogen is infiltrated. A method for producing an endless ring for a metal belt of a continuously variable transmission, characterized in that a nitriding treatment is performed such that the depth is 20 to 50% of the distance to the center of the plate thickness and the compressive residual stress value is 600 MPa or more. 2. The method for producing an endless ring for a metal belt of a continuously variable transmission according to claim 1, wherein the nitriding treatment is gas nitriding or gas soft nitriding using ammonia gas.

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