JP2005139508A - Method for producing valve spring - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a nitridation method capable of obtaining a surface nitriding hardness and a nitriding depth equal to or more than heretofore in a shorter period of time than heretofore while suppressing the production of iron-nitrogen compound. <P>SOLUTION: A coil spring consisting of a steel containing, by weight, 0.60-0.80% C, 1.20-2.20% Mn is quenched and tempered, and then is nitrided at 480-500°C for 30-110 min by using a treating gas mainly consisting of ammonia gas. The steel may further contain one or more elements selected among 0.40-1.40% Cr, 0.05-0.25% Mo and 0.05-0.60% V. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a valve spring for an internal combustion engine of an automobile and a method for manufacturing the same, and more particularly to a surface nitriding treatment.

In order to increase the fatigue strength of the valve spring, it is effective to increase the surface hardness. Conventionally, nitriding has been performed as a surface treatment for the purpose of improving fatigue strength. In order to link the nitriding treatment to the fatigue strength improvement of the valve spring, it is necessary to secure the necessary hardness by sufficiently nitriding the surface and to secure a sufficient nitriding depth.
Japanese Patent Laid-Open No. 9-12614 Japanese Unexamined Patent Publication No. 5-156351 JP 2003-193197 A Japanese Patent No. 3142689 Japanese Patent No. 3006432

Currently, gas soft nitriding using a processing gas mainly composed of ammonia gas, which is easy to process, is mainly used for nitriding a valve spring. In order to obtain a sufficient surface hardness and nitriding depth in this gas soft nitriding treatment, it is said that nitriding must be performed for at least 2 to 4 hours (Patent Document 3). However, conventionally, in such a nitriding treatment, it has been a problem that a compound layer is formed on the surface. The compound layer is a layer made of an iron-nitrogen compound such as Fe _2-3 N. Iron-nitrogen compounds are very hard materials and are very brittle. If such an iron-nitrogen compound layer exists on the surface of the valve spring, the layer will crack very early when the valve spring is used, and it will propagate to the base material (spring steel layer) under the layer. This causes a problem that the valve spring breaks early (Patent Document 2).

  In order to prevent the formation of such iron-nitrogen compounds on the surface, a method of changing the nitrogen concentration of the atmosphere during the treatment in two stages is known (Patent Document 5). That is, first, nitriding is performed in a high-concentration ammonia atmosphere to allow a sufficient amount of nitrogen to enter the steel, and then the ammonia concentration in the atmosphere is lowered to prevent the formation of iron-nitrogen compounds on the outermost surface. The iron-nitrogen compound already produced is diffused toward the inside.

  However, since such a process naturally takes a long time, adopting it in a normal production process greatly reduces the production efficiency at that point.

  The present invention provides a nitriding treatment method capable of obtaining a surface nitriding hardness and nitriding depth that are the same as or higher than conventional ones in a shorter time than the conventional one while suppressing the formation of such iron-nitrogen compounds. It is.

  The spring nitriding method according to the present invention made to solve the above-mentioned problems contains C: 0.60 to 0.80%, Si: 1.20 to 2.20%, and Mn: 0.30 to 0.90% in weight ratio. A coil spring made of steel is subjected to nitriding with a processing gas mainly composed of ammonia gas at a temperature between 480 ° C. and 500 ° C. for a time between 30 minutes and 110 minutes.

  The material steel may be steel containing C: 0.60 to 0.75%, Si: 1.20 to 2.20%, and Mn: 0.30 to 0.90% in weight ratio.

  Further, the steel may contain one or more of Cr: 0.40 to 1.40%, Mo: 0.05 to 0.25%, V: 0.05 to 0.60%.

  Alternatively, the steel may further contain one or more of Cr: 0.40 to 1.00%, Mo: 0.05 to 0.25%, and V: 0.05 to 0.25%.

  After the nitriding treatment, it is desirable to perform shot peening so that the surface compressive residual stress becomes 1600 MPa or more.

  The shot peening is desirably performed three times or more.

  In the first shot peening, a hard shot ball of φ0.5 to 0.7 mm is used, and in the second shot peening, a hard shot ball of φ0.2 to 0.4 mm is used. For peening, it is desirable to use a shot ball of φ0.15mm or less.

Further, the valve spring according to the present invention is made of steel containing C: 0.60 to 0.80%, Si: 1.20 to 2.20%, Mn: 0.30 to 0.90% in weight ratio, after coiling / nitriding / shot peening, Ammonia gas nitriding treatment is performed so that the hardness of 0.02 mm from the surface is Hv 750 or more and the average thickness of the nitrogen compound layer on the surface is 0.1 to 3 μm.

  Similarly, the material steel may be steel containing C: 0.60 to 0.75%, Si: 1.20 to 2.20%, and Mn: 0.30 to 0.90% in weight ratio.

  The steel may further contain one or more of Cr: 0.40 to 1.40%, Mo: 0.05 to 0.25%, and V: 0.05 to 0.60%.

  Alternatively, the steel may further contain one or more of Cr: 0.40 to 1.00%, Mo: 0.05 to 0.25%, and V: 0.05 to 0.25%.

  Any of the above valve springs is preferably shot peened so that the surface compressive residual stress becomes 1600 MPa or more after nitriding.

  According to the method for manufacturing a valve spring according to the present invention, the generation of a nitrogen compound layer on the outermost surface of the valve spring is suppressed to a minimum, and the generation of cracks therefrom is suppressed during use. On the other hand, since the surface hardness and sufficient nitrogen diffusion layer depth required to ensure the predetermined durability can be obtained, a valve spring having high durability (fatigue life) can be obtained by combining both effects.

BEST MODE FOR CARRYING OUT THE INVENTION

  The reason why the respective components of steel, which is the material of the valve spring according to the present invention, is defined as described above is as follows.

C: 0.60 ~ 0.80%
Carbon is an element that has the greatest influence on giving strength to steel, and in order to give strength sufficient to have sag resistance and durability as a high strength valve spring intended by the present invention, 0.60 is required. Must contain at least%. However, if the content exceeds 0.80%, cold forming becomes difficult. Further, a decrease in durability due to a decrease in toughness is also a problem.
Even when the upper limit is set to 0.75%, the necessary surface hardness can be ensured by nitriding, and the internal hardness can be lowered slightly to ensure higher toughness.

Si: 1.20-2.20%
Silicon, like carbon, is an element that imparts strength to steel, but in the case of a spring, the effect of imparting sag resistance is important. When the silicon content is less than 1.20%, the sag resistance necessary for the high-strength valve spring intended by the present invention cannot be imparted. However, silicon is also an element that promotes surface decarburization during heating. In the case of a spring that is used in a state where the maximum stress is applied to the surface, the most attention must be paid to the generation of surface decarburization during heat treatment. If the silicon content exceeds 2.20%, ferrite decarburization may occur on the surface during heat treatment (particularly quenching heating).

Mn: 0.30-0.90%
Manganese has a remarkable effect of improving hardenability in addition to increasing the strength of the matrix by solid solution in steel. However, since it also has the effect of stabilizing austenite, the amount of retained austenite after quenching increases. If the manganese content is less than 0.30%, the strength required for the high-strength valve spring intended by the present invention may not be obtained. However, if the content exceeds 0.90%, the amount of retained austenite after heat treatment increases, which may adversely affect sag resistance.

Cr: 0.40 to 1.40%
Chromium, like manganese, dissolves in steel to increase its strength and improve hardenability. Moreover, when it dissolves in steel, it is known to have an effect of increasing its toughness. However, this is also one of the elements that stabilize austenite, and when added excessively, the amount of retained austenite after heat treatment increases. In the valve spring according to the present invention, chromium is added particularly in order to utilize the effect of improving the toughness. However, if the content is less than 0.40%, the intended durability may not be obtained. On the other hand, even if added in excess of 1.40%, no further improvement in toughness can be expected, and only the cost is increased. Further, the amount of retained austenite after the heat treatment increases, which may adversely affect the sag resistance as described above. When the content of other elements (for example, carbon and manganese) is high, the effect is sufficiently exhibited even if the upper limit is set to 1.00%.

Mo: 0.05-0.25%
Molybdenum increases the strength of steel by itself and has a great effect of improving hardenability. These qualitative effects are the same as those of manganese and chromium, but the amount added may be sufficiently smaller than those. The reason why the molybdenum content in the material steel of the present invention is 0.05% or more is that if it is less than that, sufficient strength intended by the present invention cannot be obtained. This is because the effect is saturated and only increases costs. In addition, like manganese and chromium, the adverse effect of stabilizing the retained austenite after heat treatment was also considered.

V: 0.05 ~ 0.60%
Vanadium combines with carbon in the steel to form vanadium carbide, but this carbide is very fine and does not dissolve in the steel up to high temperatures. Accordingly, there is an effect of preventing the austenite crystal grains from growing during heating for heat treatment (quenching during oil tempering) and preventing the toughness of the steel from being lowered. In the present invention, paying attention to the effect of preventing the austenite crystal grains of vanadium from becoming coarse, 0.05 to 0.60% thereof is added. If it is less than this, a sufficient amount of carbide is not generated, and such a crystal grain growth preventing effect can hardly be expected. On the other hand, the effect of preventing crystal grain growth is sufficiently exhibited at about this upper limit value, and when it is added beyond this upper limit, the vanadium carbide itself grows and becomes larger, which may adversely affect durability (fatigue resistance). Note that chromium, molybdenum and vanadium may be only one of them, but when a plurality of them are added, the upper limit of vanadium may be 0.25%.

  In the present invention, nitriding treatment using a processing gas mainly composed of ammonia gas is used, and the nitriding treatment temperature is set to 480 ° C. to 500 ° C., which is higher than the conventional temperature, in order to shorten the processing time. This ensures a sufficient nitrogen diffusion layer depth and suppresses formation of the compound layer on the outermost surface. Specifically, the average thickness of the compound layer is 5 μm or less, and the surface hardness (the hardness within 0.02 mm from the outermost surface) can be Hv750 or more. In addition, since a part of the compound layer on the surface disappears by performing shot peening treatment on the valve spring thus nitrided, the average thickness of the compound layer on the surface finally becomes 3 μm or less. . By performing the shot peening treatment three times or more as described above, it is possible to secure the necessary compressive residual stress (1600 MPa or more) to ensure the required durability performance after sufficiently removing the compound layer on the surface. become. When performing shot peening three times or more, by setting the type of shot sphere used each time as described above, in addition to the effects of removing the surface compound layer and applying compressive residual stress, the durability of the shot sphere itself Thus, the manufacturing cost of the valve spring can be reduced.

  The characteristic of the valve spring which is one Example of this invention was investigated. FIG. 1 shows the main chemical components (mass%) of the material steels of the invention products and the conventional products tested. A valve spring was manufactured by the process shown in FIG. 2 using a wire having such a component. In this step, the nitriding treatment was performed at 4 minutes at 480 ° C. for 60 minutes, 90 minutes, 120 minutes and 240 minutes using a nitriding gas having an ammonia gas fraction (volume) of 75%. The shot peening is performed three times, the hard shot ball of φ0.6mm is used in the first stage, the hard shot ball of φ0.3mm is used in the second stage, and φ0.10mm is used in the third stage shot peening. A shot ball was used.

  The hardness distribution of the nitrided layer after the nitriding treatment of each invention product and the conventional product is shown in FIG. In any case, the surface hardness exceeds Hv850, and the depth (nitride layer depth) of Hv700 or more exceeds 0.03 mm (30 μm). In the nitriding of the invention product at 480 ° C. × 60 minutes to 120 minutes, the hardness at 0.02 mm from the surface is Hv780 or more, but the conventional product is Hv700 or less.

  FIG. 4 shows the results of measuring the average thickness of the outermost surface compound layer after shot peening by varying the nitriding time in the invention. When the nitriding time is 90 minutes, the average thickness of the compound layers is 3 μm or less. If it exceeds 120 minutes, it becomes 3 μm or more, and it is about 5 μm in 240 minutes. FIG. 5 shows micrographs of the surfaces of the nitriding treatment time of 90 minutes (invention product) and 240 minutes (conventional product). In order to set the thickness of the compound layer after shot peening to 0.1 to 3 μm in this way, it is necessary to set the thickness of the compound layer before shot peening to 2 to 5 μm.

  FIG. 6 shows the results of measuring the surface compressive residual stress values after performing the three-step shot peening on the inventions having the compound layers having various thicknesses. As the compound layer becomes thicker, the surface compressive residual stress increases. When there is no compound layer, the surface compressive residual stress is also low.

  Finally, the durability test of the invention having various compound layer thicknesses was performed. The result is shown in FIG. When the average thickness of the compound layer is 3 μm or more, the durability tends to decrease. Further, sufficient durability cannot be obtained even when there is no compound layer.

The chemical composition of the material of the inventive and conventional valve springs that are embodiments of the present invention. Manufacturing process drawing of invention product and conventional product valve spring. Surface hardness distribution after nitriding treatment of inventive and conventional valve springs. The graph which shows the relationship between nitriding treatment time and surface compound layer thickness of an invention valve spring. The micrograph of the surface compound layer of an invention product and a conventional product valve spring. The graph which shows the relationship between the surface compound layer thickness of the invention material valve spring and the surface compressive residual stress after shot peening. The graph which shows the relationship between surface compound layer thickness and durability of an invention valve spring.

Claims (12)

  Coil spring made of steel containing C: 0.60-0.80% by weight, Si: 1.20-2.20%, Mn: 0.30-0.90%, at a temperature between 480 ° C.-500 ° C. for 30 minutes-110 A method for manufacturing a valve spring, characterized in that nitriding with a processing gas mainly composed of ammonia gas is performed for a time of minutes.   Coil springs made of steel containing C: 0.60 to 0.75% by weight, Si: 1.20 to 2.20%, Mn: 0.30 to 0.90% at a temperature between 480 ° C. and 500 ° C. for 30 minutes to 110 A method for manufacturing a valve spring, characterized in that nitriding with a processing gas mainly composed of ammonia gas is performed for a time of minutes.   3. The steel according to claim 1, wherein the steel further contains one or more of Cr: 0.40 to 1.40%, Mo: 0.05 to 0.25%, and V: 0.05 to 0.60%. Manufacturing method for the valve spring.   3. The steel according to claim 1, further comprising one or more of Cr: 0.40 to 1.00%, Mo: 0.05 to 0.25%, and V: 0.05 to 0.25%. Manufacturing method for the valve spring.   5. The method for manufacturing a valve spring according to claim 1, wherein shot peening is performed after the nitriding treatment so that the surface compressive residual stress is 1600 MPa or more.   6. The method for manufacturing a valve spring according to claim 5, wherein the shot peening is performed three or more times.   The first shot peening uses a hard shot ball of φ0.5-0.7mm, the second shot peening uses a hard shot ball of φ0.2-0.4mm, and the third shot peening uses φ0 7. The method for manufacturing a valve spring according to claim 6, wherein a shot ball of .15 mm or less is used.   Made of steel containing C: 0.60-0.80%, Si: 1.20-2.20%, Mn: 0.30-0.90% by weight, and after coiling / nitriding / shot peening, 0.02mm hardness from surface is Hv750 or more The valve spring is characterized in that ammonia gas nitriding is performed so that the average thickness of the nitrogen compound layer on the surface is 0.1 to 3 μm.   Made of steel containing C: 0.60-0.75%, Si: 1.20-2.20%, Mn: 0.30-0.90% in weight ratio, 0.02mm hardness from surface is Hv750 or more after coiling / nitriding / shot peening The valve spring is characterized in that ammonia gas nitriding is performed so that the average thickness of the nitrogen compound layer on the surface is 0.1 to 3 μm.   10. The steel according to claim 8 or 9, further comprising one or more of Cr: 0.40 to 1.40%, Mo: 0.05 to 0.25%, and V: 0.05 to 0.60%. Valve spring.   10. The steel according to claim 8 or 9, further comprising one or more of Cr: 0.40 to 1.00%, Mo: 0.05 to 0.25%, and V: 0.05 to 0.25%. Valve spring.   The valve spring according to any one of claims 8 to 11, wherein shot peening is performed so that the surface compressive residual stress becomes 1600 MPa or more after nitriding.