JPH06228734A - Production of steel for clutch diaphragm spring - Google Patents

Abstract

PURPOSE:To provide a steel excellent in warm settling resistance, strength, and fatigue strength and used for a clutch diaphragm spring to be exposed to high temp. atmosphere. CONSTITUTION:A steel which has a composition containing, by weight, 0.4-0.8% C, 1.0-2.5% Si, 0.5-2.0% Mn, 0.1-1.5% Cr, and 0.1-0.5% Mo and further containing, as necessary one or >=2 kinds among 0.05-0.5% V, 0.05-0.5% Nb, and 0.05-0.5% Ti is hardened and tempered and then nitrided so that hardness in the central part is regulated to >=HV400. Shot peening can be done after nitriding treatment. It is possible to apply nitriding treatment directly to an as-hardened steel and eliminate tempering.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a steel for a clutch diaphragm spring which is exposed to a high temperature atmosphere corresponding to a metallurgically warm temperature.

[0002]

2. Description of the Related Art With the increase in output of various devices, the parts constituting the devices have come to be used in a state of being exposed to a harsh environment as compared with the prior art. For example,
Conventionally, the components incorporated in the clutch of the automobile have been used at or near room temperature, but in the design aiming at high output, they are exposed to a high temperature atmosphere that is metallurgically warm. A disc spring such as a clutch diaphragm spring incorporated in a clutch is required to have excellent settling resistance even in a warm state according to the severer use atmosphere.

Further, in order to reduce energy consumption,
Consideration is being given to reducing the weight and size of various parts incorporated in automobiles. In this respect as well, high strength is required for the clutch diaphragm spring. Furthermore, it is necessary to have high fatigue strength as the applied stress increases. Due to such changes in the operating environment, spring steels for clutch diaphragm springs are desired to simultaneously satisfy warm fatigue resistance, high strength and high fatigue strength. Warm settling resistance is described in, for example, Japanese Patent Application Laid-Open No. 4-52224.
It can be improved by the methods introduced in Japanese Laid-Open Patent Publication No. 4-246124. However, according to these methods, although the fatigue resistance against warming is improved, the obtained fatigue strength does not exceed the generally known value depending on the tensile strength.

[0004]

As a method for improving fatigue strength, there are high strength and surface treatment. Fatigue strength is generally improved by increasing strength, and hardness HV4
Up to about 00, the hardness is improved. However, HV4
When the hardness exceeds 50, the fatigue strength shows a substantially constant value. It is considered that this is because the notch sensitivity increases as the hardness increases, and the material deterioration due to the presence of surface scratches and inclusions. In some cases, the higher strength may rather reduce the fatigue strength. It is known that fatigue strength is improved by performing surface treatment such as carburizing, nitriding, and shot peening. However, in the method by carburization, the steel material is heated to a high temperature of about 930 ° C. during carburization, so that the toughness is reduced due to the coarsening of the austenite grains. In addition, manufacturing costs are high, quenching strain is large, grain boundary oxidation and quenching defects in the surface layer are likely to occur,
On the contrary, the fatigue strength may decrease.

On the other hand, the nitriding treatment can be carried out by any method in the eutectoid temperature (592) in the Fe-N binary phase diagram.
℃) or less. Therefore, it has no quenching distortion due to transformation and is widely used in various industrial parts. But,
Commonly used SCM435, SNCM43
With steel materials such as 1, SACM645 and the like, relatively high fatigue strength can be obtained, but the hardness of the central portion does not exceed HV400. Therefore, these nitriding steel materials cannot be used as parts subjected to high stress.

In shot peening, by appropriately selecting the tempering temperature, a high compressive stress can be applied to the surface layer portion while maintaining a high hardness in the central portion, and the fatigue strength is improved. However, as the central hardness increases, the plastic strain applied by shot peening decreases and the compressive residual stress decreases. As a result, the notch sensitivity becomes high, and the fatigue strength may deteriorate due to the deterioration of the surface roughness.

As described above, depending on the conventional strengthening, surface treatment, etc., it has the warm settling resistance and strength necessary for a clutch diaphragm spring used in a high temperature atmosphere, and has a value not less than a value depending on the tensile strength. It is not possible to obtain spring steel with fatigue strength. The present invention has been devised in order to solve such a problem, and a steel having a specified composition and composition is subjected to quenching and tempering and then subjected to a nitriding treatment to obtain a high resistance to warm settling and Fatigue strength and HV4
It is an object of the present invention to provide a steel for a clutch diaphragm spring which has three hardnesses of the central part of 00 or more at the same time.

[0008]

In order to achieve the object, the method for producing a steel for clutch diaphragm springs according to the present invention has a C content of 0.4 to 0.8% by weight and a Si content of 1.0 to 2.
5% by weight, Mn: 0.5 to 2.0% by weight, Cr: 0.1
After quenching and tempering a steel containing .about.1.5 wt% and Mo: 0.1 to 0.5 wt%, nitriding treatment is performed so that the central portion hardness HV400 or more is maintained. The steel used in the present invention may have V: 0.05 to 0.
5% by weight, Nb: 0.05 to 0.5% by weight and Ti:
It may contain 0.05 to 0.5% by weight of one kind or two or more kinds. The nitrided steel can be shot peened to further improve the fatigue strength.
It is also possible to omit tempering, subject the as-quenched steel to a nitriding treatment, and use the tempering together with the heating during the nitriding treatment.

[0009]

[Operation] The present inventors conducted detailed research and research on the component system and treatment conditions so that excellent warm fatigue resistance, high strength, and high fatigue strength were simultaneously satisfied. As a result, when the steel having the specified components and composition is subjected to quenching and tempering and then subjected to nitriding treatment, a steel exhibiting excellent properties in terms of warm fatigue resistance, high strength and high fatigue strength can be obtained. I found a thing. In the steel used in the present invention, S
By adding i, the tempering softening resistance can be increased and the tempering temperature at which the required strength is obtained can be set to a high temperature equal to or higher than the nitriding temperature, and the decrease in the center hardness after the nitriding treatment can be suppressed. Therefore, nitriding, which is not possible with conventional steel, provides high fatigue strength as well as high strength required for parts to which high stress is applied.

By the way, the term "sagging" is a phenomenon in which the spring undergoes plastic deformation as time passes with a slight strain being applied. From a microscopic point of view, dislocations introduced by the applied strain and dislocations existing before the strain is applied move, and settling occurs. The addition of Si is effective from the viewpoint of the mechanism of the occurrence of fatigue. That is, the addition of Si increases the temper softening resistance, and the tempering required to obtain the strength required for the spring can be performed at a higher temperature. As a result, the dislocation density after tempering is reduced, and a thermally stabilized metallographic structure is obtained.
Further, Mo added together with Si precipitates as fine carbides during tempering. The carbide Mo ₂ C acts as an inhibitor that prevents the movement of dislocations. Further, since Cr and Ti are added as necessary, the diffusion layer is formed in the surface layer portion by the nitriding treatment. The formation of the diffusion layer increases hardness and imparts high compressive residual stress.
As a result, the fatigue strength is remarkably improved.

In the steel for clutch diaphragm springs according to the present invention, in such a thermally stable state, the density of introduced dislocations is low and the movement of dislocations is effectively prevented. As a result, a very excellent warm settling resistance can be obtained. When shot peening is performed after the nitriding treatment, compressive residual stress is applied to the steel surface, and the hard compound layer is removed from the surface layer portion. As a result, the fatigue strength is further improved. The nitriding treatment may be performed directly on the as-quenched steel without omitting tempering. In this case, tempering is simultaneously performed by heating during the nitriding treatment.

The alloying elements and their contents contained in the steel for clutch diaphragm springs used in the present invention will be described below. C: An alloying element necessary for increasing the strength of steel. In order to obtain the strength required as a steel for clutch diaphragm springs by quenching and tempering, it is necessary to contain 0.4% by weight or more of C. But 0.8
When a large amount of C in excess of wt% is contained, quench cracking is likely to occur and the toughness also decreases. Therefore, in the present invention, the C content is set in the range of 0.4 to 0.8% by weight.

Si: An important alloying element for increasing the temper softening resistance. By adding Si, it becomes possible to perform tempering at a temperature higher than the nitriding temperature. Si is
The compound layer produced by the nitriding treatment is made thin, and it effectively acts to reduce the adverse effect of the hard compound layer on the fatigue strength. Such an effect becomes remarkable when the Si content is 1.0% by weight or more. However, when a large amount of Si is contained in excess of 2.5% by weight, harmful internal oxidation and decarburization as a steel for clutch diaphragm springs are likely to occur. In addition, excessive Si content promotes graphitization during hot rolling or annealing, and the quenching heating time becomes long and the toughness deteriorates. Therefore, in the present invention, 1.0 to 2.5
The Si content was set in the range of% by weight.

Mn: Effective for deoxidizing molten steel,
Improves the hardenability of steel. In order to obtain these effects, it is necessary to contain 0.5% by weight or more of Mn. However, if the Mn content exceeds 2.0% by weight, the toughness after quenching and tempering is significantly reduced. Therefore, in the present invention, the Mn content is set in the range of 0.5 to 2.5% by weight. Cr: It is an effective alloying element that suppresses graphitization and internal oxidation promoted by the inclusion of Si and improves hardenability like Mn. In addition, a compound is formed with N in the diffusion layer formed by the nitriding treatment to increase hardness and generate high compressive residual stress. As a result, fatigue strength increases. These effects are obtained by adding 0.1% by weight or more of C.
It becomes remarkable with r content. However, if the Cr content exceeds 1.5% by weight, the toughness after quenching and tempering significantly decreases. Therefore, in the present invention, the Cr content is set in the range of 0.1 to 1.5% by weight.

Mo: It forms a solid solution in the austenite phase during heating by quenching and precipitates as fine carbides during tempering. The precipitated carbide acts as an inhibitor that blocks the movement of dislocations and improves the warm settling resistance. Further, the addition of Mo enhances the resistance to temper softening, and the hardness of the central portion after the nitriding treatment is maintained at a high level. These effects are
It becomes remarkable when the Mo content is 0.1% by weight or more. But,
Inclusion of a large amount of Mo in excess of 0.5% by weight increases the amount of relatively coarse undissolved carbide that is not solid-soluted in the austenite phase during quenching and heating, and causes a decrease in fatigue strength as with non-metallic inclusions. Becomes Therefore, in the present invention, the Mo content is set in the range of 0.1 to 0.5% by weight.

V, Nb: These are optional components added as necessary, and are present as carbides in the steel and partly dissolve in the austenite phase by heating during quenching. V and Nb, which are solid-solved, are precipitated as fine carbides during tempering similarly to Mo, and improve the cold set resistance. On the other hand, undissolved V and Nb carbides that are not solid-soluted in the austenite phase during quenching prevent coarsening of austenite grains. In order to obtain such an effect, it is necessary to contain 0.1% by weight or more of V and / or Nb. But,
When a large amount of V and / or Nb exceeding 0.5% by weight is contained, the amount of relatively coarse undissolved carbide that is not solid-dissolved in the austenite phase at the time of quenching increases, and the fatigue strength is increased like non-metallic inclusions. It causes to decrease. Therefore,
When V and / or Nb is contained, its content should be 0.
It is set in the range of 1 to 0.5% by weight.

Ti: A selective component added as necessary, which increases the hardness and imparts a high compressive residual stress by forming a compound with N in the diffusion layer formed by the nitriding treatment. As a result, fatigue strength increases. In order to obtain these effects, it is necessary to contain 0.1% by weight or more of Ti. However, 0.5% by weight
Even if a large amount of Ti exceeding the above amount is contained, the effect of improving the properties commensurate with the increase in the amount cannot be obtained, and the cost of the steel material increases. Therefore, when Ti is added, its content should be 0.1-0.1%.
Set in the range of 0.5% by weight.

The steel in which the alloy components and contents are adjusted in this way is subjected to heat treatment of quenching and tempering in order to obtain desired strength. Steels whose components / compositions fall within the ranges specified in the present invention have high temper softening resistance, and the tempering temperature required to obtain the strength required as a clutch diaphragm spring is higher than that of conventional spring steels ( For example, it becomes 500 to 600 ° C.). The high temperature tempering reduces the dislocation density in the tempered martensite. In addition, Mo precipitates as fine carbide. The reduction in dislocation density and the precipitation of Mo-based carbides are combined with each other, and the warm settling resistance is remarkably improved.

By the nitriding treatment, Cr, Ti and the like form a compound with N to form a diffused layer which is harder than the hardness of the central portion and imparts a compressive residual stress. as a result,
Fatigue strength is improved. Further, in the steel according to the present invention, the temper softening resistance is high, and the tempering temperature is higher than the nitriding temperature. Therefore, the nitriding treatment does not reduce the hardness of the central portion, and further increases the fatigue strength and increases the strength. Will be possible. As the nitriding treatment, although not limited to the present invention, salt bath nitriding, gas nitriding, gas soft nitriding, ion nitriding or the like can be adopted. By either method, a diffusion layer is formed on the surface of the steel material and a compressive residual stress is applied.

After the nitriding treatment, shot peening is performed if necessary. The compressive residual stress is further imparted by the shot peening, the hard compound layer having poor workability is removed from the surface layer, and the fatigue strength is further increased.
Further, since the diffusion layer harder than the central portion is formed in the surface layer by the nitriding treatment, the deterioration of the surface roughness due to shot peening is smaller than that in the case of shot peening a steel material that has been quenched and tempered. Become. Therefore, in order to make the shot peening more effective, more hard conditions can be adopted.

[0021]

[Example] Steel having the components and compositions shown in Table 1 was melted in a converter,
Continuously cast into a slab. The steel types A to E in Table 1 are steels according to the present invention, and the steel types F to I are comparative steels.

[Table 1]

The slab is subjected to normal hot rolling to obtain a plate thickness of 3.5.
After being annealed, a cold-rolled sheet having a thickness of 35 mm was used and cold-rolled at a rolling rate of 35%. The cold rolled sheet was annealed again. As the annealing conditions, an annealing temperature of 710 ° C. and a soaking time of 10 hours were adopted in all cases. The annealed material was heated to 880 ° C. for 10 minutes, oil-quenched, and tempered to a hardness of HV450. A test piece was cut out from the quenched and tempered material and subjected to gas nitriding. As the nitriding condition, the test piece was heated at a temperature of 5 in an NH ₃ gas atmosphere.
The condition of holding at 20 ° C. for 35 hours was adopted. Table 2 shows the change in hardness of the central part before and after the nitriding treatment.

[Table 2]

Since the comparative steels F to I have small temper softening resistance, it was necessary to set the tempering temperature to 500 ° C. or lower in order to obtain the strength HV450 required for the clutch diaphragm spring. Therefore, when subjected to a nitriding treatment that is heated to a high temperature for a long time, the center hardness becomes
It fell to 0 or less, and the strength required for the clutch diaphragm spring could not be obtained. On the other hand, in the steels A to E according to the present invention, since the tempering softening resistance was high, the decrease in hardness due to the nitriding treatment was very slight. Shot peening is applied to nitriding steel,
The effects of nitriding and shot peening on fatigue strength were investigated. In addition, the shot peening is hardness HV
Shot particles having a diameter of 0.6 mm at 700 were used under the conditions of an arc height of 0.20 mmA and a coverage of 200%. In addition, the fatigue test was performed by both plane bending.

[Table 3]

As is clear from Table 3 showing the survey results,
In the steels A to E according to the present invention, high compressive residual stress and center hardness are obtained by shot peening after nitriding and nitriding. Further, since the hard diffusion layer is formed by the nitriding treatment, the increase in surface roughness due to shot peening is suppressed to a low level. Therefore, 1000N
A high fatigue limit of around / mm ² is obtained. On the other hand, in the case where the nitriding treatment is not applied, neither the steel A according to the present invention nor the comparative steel F has a very high fatigue strength. Further, in the case where the comparative steel F is subjected to the nitriding treatment, the hardness of the central portion is remarkably reduced, and the low fatigue limit is shown.
Quenched and tempered steels and as-quenched steels were subjected to nitriding treatment, and their fatigue resistance was evaluated by a relaxation test. Relaxation test, test temperature 350
℃, initial strain 1.0% and holding time 12 hours,
The reduction rate of the load before and after holding was measured as the relaxation rate. The measurement results are shown in Table 4.

[Table 4]

As is clear from Table 4, comparative steels F to I
Indicates a high relaxation rate. This is because the comparative steels F to I have low temper softening resistance and high dislocation density remaining after tempering. On the other hand, the steels A to E in which the alloy components, the quenching temperature and the tempering temperature are all within the ranges specified in the present invention exhibit a much lower relaxation rate than the comparative steels F to I, and are excellent in warm settling resistance. You can see that This is because the tempering softening resistance of the steels A to E is high, and the fine Mo-based carbides precipitated during tempering or nitriding prevent movement of dislocations.

[0026]

As described above, in the present invention, the alloy design that increases the temper softening resistance is adopted to prevent the softening due to the heating during the nitriding treatment. The obtained steel for clutch diaphragm springs has very excellent warm settling resistance, which is the most important characteristic, and exhibits high strength and high fatigue strength. Therefore, when it is incorporated in a clutch of an automobile that is exposed to a high temperature and harsh atmosphere as the output increases, it maintains stable characteristics for a long period of time.

Front Page Continuation (72) Inventor Toshiro Yamada 11-1 Showa-cho, Kure-shi, Hiroshima Inside Steel Research Laboratory, Nisshin Steel Co., Ltd. In-house (72) Inventor Haruki Ito 2-1, Asahi-cho, Kariya city, Aichi Aisin Seiki Co., Ltd.

Claims (4)

[Claims] 1. C: 0.4 to 0.8% by weight, Si: 1.
0-2.5% by weight, Mn: 0.5-2.0% by weight, C
Hardness HV after quenching and tempering steel containing r: 0.1 to 1.5 wt% and Mo: 0.1 to 0.5 wt%
A method for producing a steel for clutch diaphragm springs, which comprises nitriding treatment so as to maintain 400 or more. 2. C: 0.4 to 0.8% by weight, Si: 1.
0-2.5% by weight, Mn: 0.5-2.0% by weight, C
r: 0.1 to 1.5% by weight and Mo: 0.1 to 0.5% by weight, further V: 0.05 to 0.5% by weight, Nb:
After quenching and tempering steel containing one or more of 0.05 to 0.5% by weight and Ti: 0.05 to 0.5% by weight, the center hardness HV of 400 or more is maintained. A method for manufacturing a steel for a clutch diaphragm spring, characterized by nitriding. 3. A method for producing a steel for a clutch diaphragm spring, which comprises shot peening after the nitriding treatment according to claim 1 or 2. 4. A method for manufacturing a steel for clutch diaphragm springs, wherein the nitriding treatment according to any one of claims 1 to 3 is directly applied to as-quenched steel.