JP3847350B2 - Spring with excellent fatigue resistance and surface treatment method for producing the spring - Google Patents

Description

【００８６】
ここで、このコイルばねにかける最大平均応力を６９０ＭＰａと想定して、前記（１）式、（２）のところで説明したように、平均応力と振幅応力の互換性を考慮すると、平均応力τｍ＝６９０−ｘに対し、上記参考例３のばねの疲労限振幅応力τａは、τａ≧４４０．６＋ｘ／５と表現できる。線径、線の引張強さ、鋼種などを勘案して、下記（３）式を満たすばねを参考例３のばねとし、ごく表層の圧縮残留応力を550ＭＰａ以上とする。
【００８７】
平均応力τｍ＝６９０−ｘのとき、繰返し数５×１０⁷回における疲労限振幅応力τａ≧４２２＋ｘ／５…（３）
ここで、ｘ：０〜１４０
【００８８】
これらのばね（比較例は（５）のみ）の残留応力分布を示す図７より、最表面から深さ５０μｍまでの表層部の残留応力がＳＳ処理によって大きく改善されたことがわかる。また、これらのばねの表面粗さRmaxは０．６ｍｍ粒子投射のままでは１３．２μｍ、０．６ｍｍ粒子投射後全粒子平均径３７μｍ投射後の参考例３によるばねでは９．２μｍであった。
【００８９】
上記の試験で、参考例３のばねの疲労試験応力が高い場合、ばねのへたりがやや大きくなった。このへたり防止のため、ピアノ線に代えてケイ素及び／またはクロムなどの耐へたり性を富ます元素を添加したパーライト組織冷間伸線タイブの鋼線を使用することやホットセッティングの実施が対策として考えられる。
【００９０】
（参考例４）
３．２ｍｍ径のＪＩＳ ＳＷＯＳＣ−Ｖ、弁ばね用オイルテンパー線を用いて弁ばねを試作した。この弁ばねは、窒化処理せずにＳＳ処理を施して製造した。
【００９１】
この弁ばねの製造工程は次のとおりである。すなわち、ばねコイリング、４００℃・２０分の低温焼鈍、０．６ｍｍ径の鉄系ラウンドカットワイヤの速度７０ｍ／ｓｅｃでの投射、高炭素鋼微粒子ＳＳ処理（速度１０７ｍ／ｓｅｃ、全粒子平均径４０μｍ、最大粒子平均径７５μｍ）、さらに２２０℃で２０分の低温焼鈍、最後に冷間セッティングを施した。このばねのごく表層の圧縮残留応力は１０１０ＭＰａであった。このばねの疲労試験を実施したところ、平均応力τｍ＝５８８ＭＰａＮ繰返し数Ｎ＝５×１０⁷回での疲労限振幅応力は４６６ＭＰａとなった。この応力は、平均応力を６９０−ｘと置くと、振幅応力τａはτａ＝４４５．６＋ｘ／５と表現できる。ＳＷＯＳＣ−Ｖオイルテンパー線の引張強さばらつき、線径範囲、などを考慮して、ごく表層の鉄地の圧縮残留応力を９００ＭＰａ以上とし、次の（４）式のようにばねの疲労強度を定める。
【００９２】
平均応力τｍ＝６９０−ｘ、繰返し応力τａとして、５×１０⁷回における疲労限応力が、τａ≧４４０＋ｘ／５…（４）[0001]
BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a fine hard metal particle projection for a valve spring for an internal combustion engine, a clutch spring for a transmission such as an automobile, a high-strength thin plate spring, and the like. The present invention relates to a surface treatment method that improves the performance by the method and a high-performance spring manufactured by this surface treatment method.
[0002]
[Background]
The following are conventional techniques related to the present invention.
1. Japanese Patent Publication No.2-17607 “Surface processing heat treatment method for metal products”
This technology relates to a surface processing heat treatment method in which a shot of 40 to 200 μm having a hardness equal to or higher than the product hardness is injected at a speed of 100 m / sec or more, and the temperature near the surface is raised to the A3 transformation point or higher. is there.
This method is a method of causing the austenite of the workpiece to be transformed by heat generated by the collision of the workpiece surface layer and the transformation of the metal structure by rapid cooling, and differs from the technical idea and content of this patent.
[0003]
2. Japanese Patent Laid-Open No. 9-279229 “Surface Treatment Method for Steel Workpiece”
In the technique according to this publication, a large number of hard metal particles having a diameter of 20 to 100 μm are collided with a steel workpiece surface at a speed of 80 m / sec or more, and the temperature rise limit on the workpiece surface is 150 ° C. or more, and the recovery / recrystallization temperature is exceeded. The main content is to control to low temperature.
This patent does not mention nitridation. In addition, in this patent, there are almost no restrictions on the material of the metal particles, such as specific gravity and hardness, and there is a limitation that the collision speed is 80 m / sec or more, but it is clear where the optimum speed is. is not. In the embodiment described in this patent, only 180 m / sec is described and it can be seen that there is an effect, but it can be said that it is unclear whether or not it is the best condition.
[0004]
3. JP-A-10-118930 “Spring shot peening method and spring product”
0.64% C-Si-Mn-Cr-Mo-V steel spring is nitrided, and after shot peening with a shot with a diameter of 0.5 to 1.0 mm, the specific gravity of the projection material is 12 to 16, Surface peening with a diameter of 0.05 to 0.2 mm and a hardness of Hv 1200 to 1600, surface residual stress σR = −1950 MPa, repetition rate 5 × 10⁷The fatigue limit is 700 ± 620 MPa. This fatigue limit stress is the claim of this patent.4The fatigue strength of is not reached.
[0005]
The purpose and method of this patent are similar to the present invention, but this patent uses cemented carbide particles with dimensions of 0.05 to 0.2 mm, specific gravity of 12 to 16, high hardness, high cost, and limited manufacturer. On the other hand, in the present invention, metal particles such as iron-based particles having a diameter of 0.01 to 0.08 mm, which are cheaper and easier to obtain, are used. Further, the fatigue strength obtained as a result of the present condyle invention can obtain an excellent effect as compared with this conventional patent.
[0006]
4). Japanese Patent No. 2613601 (Japanese Patent Laid-Open No. 1-83644) "High-strength spring" C0.6-0.7%, Si 1.2-1.6%, Mn 0.5-0.8%, Cr0. 5 to 0.8%, a total of 0.05 to 0.2% of one or more of V, Mo, Nb, and Ta, the balance iron and impurities, and the size of the nonmetallic inclusions is 15 μm or less at maximum, Surface roughness Rmax 15 μm or less, maximum compressive residual stress near the surface is 85 to 110 kgf / mm²A spring that is (833-1079 MPa) is described. In this patent, the maximum compressive residual stress near the surface layer is 110 kgf / mm.²It is described that when it exceeds (= 1079 MPa), the production becomes difficult and the surface roughness is lowered, and the fatigue strength is lowered. A spring manufactured by this inventor and other researchers at the Spring Technology International Conference hosted by ESF (European Spring Federation) in Dusseldorf, Germany, on April 3, 1990 after the filing of this patent. The performance of is described in detail. The title of this paper is A High Strength Spring for Automotive Engine, whose authors are M.Abe, K.Saitoh, N.Takamura and H.Yamamoto. The maximum compressive residual stress on the surface of the spring corresponding to the invention of Japanese Patent No. 2613601 described in this paper is about 950 MPa from the FIG. 9 of the same paper, about 820 MPa on the outermost surface, the surface roughness of this spring from the second table of the same paper Is Rmax 10.6 μm. The fatigue limit is 5 × 10, as shown in FIG.⁷Τm = 588 MPa and τa = ± (450 to 480) MPaThe
On the other hand, in the present invention, even if the maximum value of the compressive residual stress of the surface layer exceeds 1079 MPa, the surface roughness of the spring is not increased, and the residual stress is maximized at the outermost surface or very close to the surface, and from the surface. Can effectively prevent fatigue failure, so even without nitriding,The following formula (2)A spring satisfying the fatigue limit can be obtained.
Cyclic stress τm ± τa, number of repetitions: 5 × 10 ⁷ When τm = 690−x,
τa ≧ 470 + x / 5 (2)
Here, x: 0 to 183, unit: MPa
[0007]
5). Japanese Patent Laid-Open No. 5-337963 “Method for Manufacturing Coil Spring”
The surface roughness is kept low by shot peening, and after descaling, nitriding, and then shot peening with a 0.8 mm diameter cut wire, the surface roughness Rmax of the spring product is 5 μm or less.⁷60 ± 57 kgf / mm at the number of stress cycles²There is a description that fatigue strength of (588 ± 559 MPa) was obtained. However, in the data described in the examples obtained by this method, the fatigue strength is claimed in the present application.4(1) is not satisfied. Further, this method patent does not disclose fine particle projection as in the present invention.
[0008]
6). Japanese Patent Application Laid-Open No. 7-214216 “Method for Manufacturing High-Strength Spring”
The steel wire spring is subjected to electrolytic polishing, and then subjected to nitriding treatment. Further, particles having a hardness of Hv 600 to 800 and a diameter of 0.6 to 1.0 mm are used as the first stage shot, and subsequently, 0.02 as the second stage shot. Although there is a description that it is preferable to use particles having a diameter of about 05 to 0.2 mm and a hardness in the range of Hv 700 to 900, further analysis and analysis on the particle size of 0.05 mm to 0.2 mm and There is no consideration. In the example, a steel ball having a particle diameter of 0.15 mm and a hardness Hv of 800 is used as the second stage shot, and the number of repetitions is 5 × 10.⁷As for the fatigue limit of the spring in the rotation, an average stress of 637 MPa and an amplitude stress of ± 560 MPa have been reported.4The expression (1) representing the described spring fatigue limit is not satisfied. Further, the definition of the second stage particle projection condition is different from that of the present invention.
[0009]
7). Japanese Patent Application Laid-Open No. 5-177544 "Method for Manufacturing Coil Spring"
This patent is a method of performing nitriding after spring forming and further performing shot peening. The shot peening is a method in which first-stage shot peening, low-temperature annealing, and second-stage shot peening using shots smaller than the first-stage shot peening are sequentially performed. In the detailed explanation column of the present invention, as the second stage shot peening, one having a size of about 0.05 to 0.20 mm and a hardness of Hv 700 to 900 is used. It is marked as preferred. However, no further detailed analysis or explanation has been made on the difference in the effects between the particle projection of 0.05 mm diameter and the particle projection of 0.1 mm diameter or 0.2 mm diameter. , Hv800, projection pressure 5kgf / cm²Second-stage shot peening is performed under the conditions of The resulting fatigue limit is 5 × 10 repetitions.⁷The average stress τm = 686 MPa and the amplitude stress τA = ± 567 MPa per revolution, and the compressive residual stress of the very surface layer does not reach 1400 MPa from FIG. 3, and none of the values satisfy the eighth aspect of the present invention.
[0010]
DISCLOSURE OF THE INVENTION
The above-mentioned prior art column has already pointed out problems for each individual technology. In the prior art, as a shot projection method for a surface nitrided spring having a relatively high surface hardness, super hard particle projection with a diameter of 50 μm or more and 200 μm or less (Conventional Technology 3), and 20-20 for improving the fatigue characteristics of a steel workpiece. Although 100 μm particle projection is mentioned and the particle size and the like are roughly limited (same as in prior art 2, 6 and 7), the relationship between the true effective and appropriate projection method and the performance of the projected spring Was quite ambiguous.
[0011]
In addition, in the patent of Prior Art 3, it is estimated that the price of the cemented carbide particles to be used is high, and there is a problem in the economical efficiency of the projected particles. Further, since the spring fatigue strength of the examples is lower than that of the present invention, it cannot be considered that the technical problem has been sufficiently elucidated and solved.
[0012]
Conventionally, there has been a strong demand for reducing the size and weight of valve springs for internal combustion engines and other various springs for automobiles. In light of these demands, the present invention provides a spring processing method and a spring that can increase the fatigue strength of various springs more than before, thereby improving the running performance of an automobile and improving fuel efficiency by reducing the size and weight. It aims to be realized. In order to realize such an excellent performance spring, generation of microcracks from the spring surface layer under repeated high stress, prevention of growth, and microscopic cracks from non-metallic inclusions existing directly under the spring surface layer Preventing the growth of cracks is a technical challenge. Claims 1 and 2 (with nitriding step))A high-performance spring produced by this technology is a technology corresponding to this technical problem.4It is. These claims provide a relatively economical answer to the above two technical challenges..
[0013]
The projection speed referred to in the present invention is the speed immediately before the collision of the projected particles on the spring surface. As the particle projecting method, the present invention employs an impeller method and a honing method using a gas such as air as a carrier. In addition, even if so-called stress peening is applied in which external stress is applied in a static or constant strain state and the particles are projected onto the spring, the effect of projecting fine particles and other particles is not impaired, but rather the surface compressive residual stress is further improved. Since there is an effect of preventing fatigue breakage, a method of projecting particles with a stress load is also included in the method of the present invention. However, the stress peening requires a special dedicated jig or apparatus, resulting in an increase in cost. Claims of the invention4 isThis is not a stress peening, but a spring subjected to particle projection without applying stress or strain, and is a high fatigue strength spring obtained without relying on stress peening.
[0014]
In addition, even if the fine particle projection of the present invention is performed by heating the spring to a temperature of about 100 to 250 ° C. in advance, the effect is not lost and is included in the method of the present invention. Similarly, between the particle projection described in the claims of the present application and the next finer particle projection, and after the final particle projecting step, a strain age hardening treatment or low-temperature annealing at 150 to 250 ° C. is performed or warm after the particle projection. / Cold setting is also included in the content of the present invention.
[0015]
When the stress received by the spring is increased, a large stress is applied to the spring surface layer, and the surface layer portion cannot withstand repeated stress and causes a fine crack. In order to prevent this fine crack, first, it is necessary to make the residual stress of the spring surface layer in a compressed state and to increase its absolute value as much as possible. Although compressive residual stress cannot be applied beyond its elastic limit, the present invention overcomes this problem by simultaneously improving the elastic limit by work hardening of the spring surface layer by fine particle projection, thereby raising the compressive residual stress to a high level. At the same time, by increasing the yield point and hardness as much as possible without impairing the toughness of the spring surface layer, slip deformation due to repeated stress is prevented, and the generation and growth of surface cracks are prevented. In addition, if fine dents or cracks are generated on the spring surface layer due to particle projection, this will cause fatigue cracks, so it is necessary to consider and project conditions that do not create such surface defects on the surface layer due to particle projection. . In order to satisfy such a requirement, in the present invention, fine metal particles having an optimum shape and physical properties of a diameter of 10 μm or more and less than 100 μm, more preferably 10 to 80 μm, are projected under optimum speed conditions. In particular, in the present invention, if the projection speed and the projection density are increased without exceeding the A3 transformation point in the spring surface layer, even if no recovery / recrystallization occurs, the surface of the spring layer is extended by fine cracks or strong processing. It was found that toughness degradation occurred and the fatigue strength was lower than in the case of lower speed projection. Work hardening with work hardening or strain aging on the surface so as not to cause such fine cracks on the spring surface, at a temperature lower than the A3 transformation point, and at a temperature lower than the iron base undergoes recovery recrystallization. A spring having excellent characteristics can be obtained by projecting fine particles under appropriate conditions so as not to cause deterioration of ductility and fine cracks. For springs with a wire diameter of about 2 mm or more or a plate thickness of about 1.5 mm to 2 mm or more, iron-based particles with a diameter of 0.2 to 0.9 mm are projected on a non-nitrided or non-nitrided spring to give a deep residual stress inside. It is necessary to perform the above-described fine particle projection after application. At this time, first, the above-mentioned particle projection of 0.2 to 0.9 mm diameter includes firstly projecting particles of 0.5 to 0.9 mm diameter, and subsequently performing particle projection of 0.2 to 0.4 mm diameter.The
[0016]
Next, there are roughly the following three methods for preventing fatigue breakage due to non-metallic inclusions inside the spring surface layer. One of them is a reduction in the size of non-ductile non-metallic inclusions contained in the spring material. The minimum dimension (critical dimension) of harmful inclusions becomes smaller as the hardness of the spring becomes higher, and when the hardness of the iron ground around the inclusion is about Hv 520 to 580, it is about 20 to 15 μm. However, in Hv580-630, it is about 10 micrometers. Therefore, if the dimension of the nonmetallic inclusion existing inside the spring material is equal to or larger than the critical dimension, it is necessary to regulate the internal hardness of the spring material according to the maximum dimension. The second method keeps the residual stress around the place where harmful non-metallic inclusions are present, thereby preventing the growth of microcracks around the inclusions. For this purpose, a round cut wire having a relatively large diameter of 0.5 to 0.90 mmφ to 1.0 mmφ is projected at a speed of 40 to 90 m / sec, and a depth of 0.2 mm to 0.5 mm from the spring surface. Conventionally, compressive residual stress has been applied. When the wire diameter or plate thickness of the spring is 1.5 to 2.0 mm to 2.5 mm, a round cut wire having a diameter of 0.2 to 0.4 mm is projected at a speed of 40 to 90 m / sec. It is also necessary to prevent the breakage from inclusions by applying compressive residual stress to a depth position of about 06 to 0.13 mm. In these cases, if the projection speed is increased, a locally uneven deformation region is generated in the spring material surface layer, and fine dents and cracks are generated in the surface layer to easily cause fatigue breakage from the spring surface layer. It is necessary to project so that there is no such defect. In order to prevent the occurrence of such fine cracks, it is necessary to determine a specific maximum projection speed for each spring with an upper limit of the projection speed of 90 m / sec. Further, when the projection speed is less than 40 m / sec, the residual stress imparting effect is reduced and cannot be imparted sufficiently deep, so the lower limit speed is set to 40 m / sec.
[0017]
The third method is to reduce the hardness of the spring material including inclusions. However, if the hardness is reduced excessively, the sag, which is one of the important characteristics of the spring, increases and the spring performance is impaired. Therefore, this method cannot be adopted unnecessarily. Therefore, the claims of the present invention4Then, it is made for the hardness in the position of 0.2-0.5 mm depth to become Hv520 or more at least. Normally, fatigue failure of a spring starting from inclusions occurs at a depth of 0.2 to 0.5 mm from the spring surface, and the hardness and fatigue strength of iron in this depth region are closely related. Carbides in inclusion control in steelmaking plants and heat treatment in wire-making plants so that the average dimension of fractured fracture surfaces of inclusions containing harmful carbides, nitrides, borides, etc. is less than 20 μm to about 15 μm or less. If the hardness in this depth region is Hv 520 to 580, fatigue failure due to inclusions can be prevented by controlling the dimensions such as. ThroughIf it can be controlled so that the average dimension of the natural object at the spring fracture surface is 10 μm or less, the hardness at the depth of 0.2 to 0.5 mm from the surface is Hv 630 or less, and fatigue praying due to inclusions can be prevented. Therefore, the present invention claims4Then, the hardness at a 0.2-0.5 mm depth position is limited to Hv630 or less..
[0018]
The inclusion inclusion state also varies depending on the type of spring material. That is, generally, an increase in the amount of alloy such as Si, Cr, Mo, V, Nb, W, and Al may deteriorate the level of non-ductile non-metallic inclusions in the spring steel material. In the case of a piano wire, there are almost no inclusions of 10 μm or more with current technology. In the case of alloy mesh oil temper wire for valve springs, Al as a harmful inclusion₂O_Three(Alumina), MgO / Al₂O_Three(Spinel), SiO₂(Silica). These hard non-ductile oxide inclusions can be rendered harmless by controlling the form of the ductile inclusions during steelmaking. On the other hand, since carbides or nitrides or carbonitrides such as VC, NbC, TiC, and TiN maintain a spherical or angular shape, this countermeasure is necessary in the case of a spring steel material that contains a relatively large amount of elements such as V, Nb, and Ti. Therefore, it is necessary to detoxify or prevent its formation by measures such as examination of heating conditions for rolling and annealing and prevention of mixing of Ti and the like from raw materials at the steelmaking stage. In order to prevent spring breakage due to the presence of harmful inclusions, it is desirable to reduce the content of V, Nb, Ti, etc. contained in the spring steel as much as possible.4Component steel(1)Then, addition of 0.03 to 0.60% and 0.02 to 0.20%, respectively, of V and / or Nb is effective for refining crystal grains and improves the ductility of the spring and promotes nitriding. Steel component(1)Ni added to the steel has an effect of improving the toughness of the spring steel, and is considered to be effective for preventing fatigue damage and preventing fatigue crack propagation of a spring tempered with high strength. However, if it exceeds 0.5%, retained austenite is likely to be generated in the processing of the wire and the wire, and the ductility of the spring steel in the process of production is reduced, so the upper limit was made 0.5%. Also component steel(1)Co addition to the steel reduces the transformation time when cooling from high temperatures such as pearlite transformation, and brings the effects of making the microstructure during wire production into fine pearlite with good cold workability, making wire production easier To. However, even if added over 3.0%, it is an economically expensive element and its effect is reduced for the cost. Therefore, the upper limit of the amount added is set to 3.0%.
[0019]
Claim4Component steel(1)Or(2)On the other hand, the addition of Mo, Cr and Al all promotes nitrogen penetration during spring nitriding. If any of these elements is added in an excessive amount, a nitrogen compound is deposited on the very surface of the spring, preventing diffusion and penetration in the depth direction inside the spring, and the effect of improving the fatigue durability of the spring is reduced. For this reason, in this invention, the addition upper limit of Mo, Cr, and Al was 0.6%, 1.8%, and 0.5% by mass%, respectively. W increases heat resistance and is effective in preventing decarburization of springs.(1)Or(2)If the amount exceeds 0.5%, the hardenability becomes excessive and the number of annealing increases, and the manufacturing complexity and cost increase become significant. Therefore, the upper limit is made 0.5%. Ingredient class(1)Thus, C increases the strength of the steel and is also necessary for fatigue strength. Since the effect is reduced when the content is less than 0.5%, the lower limit is set to 0.5%.
[0020]
On the other hand, if C exceeds 0.8%, the strength improving effect becomes small and brittleness is exhibited, so the upper limit was made 0.8%. Even if there is a decarburized layer on the surface layer, if the degree is not extreme, the hardness is compensated for by nitriding, and therefore the method of the present invention can be applied to such decarburized material. Si exhibits a good effect on spring strength and sag resistance. In addition, in the spring strengthened by quenching and tempering, the effect is small if the amount is less than 1.2%, and if it exceeds 2.5%, problems are likely to occur in workability due to decarburization promotion and ductility deterioration during production. Lower and upper limits are ingredient classes(1)And 1.2% and 2.5%.
[0021]
In addition, the present invention claims4of(4)Even maraging steel with these components has an effect of improving fatigue strength.(4)As included in the claims.
[0022]
Maraging steel becomes alloyed element solution and austenite (solution) treatment by heating at a high temperature of about 800-900 ° C, and becomes a relatively soft martensite by cooling, and this is subjected to cold drawing and work hardening. Spring molding from. Thereafter, an aging treatment is performed at around 500 ° C. to obtain strength and springiness. After this, nitriding treatment can be applied to increase the fatigue strength by the method according to claim 1 or 2.The
[0023]
Since maraging steel springs have superior sag resistance compared to low alloy steel wire springs, their tensile strength after aging is 1900 MPa or more and their performance is demonstrated.TheIt is particularly suitable for applications that require sag resistance and fatigue resistance. The solution treatment is a heat treatment applied to high-alloy steels such as stainless steel and high-manganese steel, and is rapidly cooled from a state in which carbide or the like is dissolved at a high temperature (dissolved in the steel structure). Thus, the heat treatment brings the precipitate to room temperature without reprecipitation.
[0024]
The present invention(1)Spring for nitriding (including low-temperature carbonitriding, the main purpose of which is nitrogen addition) in the machining process (Claims)4And its manufacturing method (Claim 1,2, 3)It is made up of.
[0025]
In the spring for nitriding of (1), pickling, electropolishing, shot peening and the like are conventionally known as a descaling method performed before nitriding. Pickling has problems such as generation of fine cracks due to hydrogen embrittlement on the spring surface and is not suitable for the present invention. Since electropolishing has problems such as large-scale equipment for mass production, shot peening (particle projection) has been taken up as a descaling prior to nitriding in the present invention. In addition, it is necessary to adjust the projection speed, the projected particle diameter, and the like so as not to generate a local shear deformation band. Such surface layer defects due to particle projection before nitriding remain without disappearing even after nitriding. When performing particle projection for descaling prior to nitriding, to promote nitriding to a relatively deep temperature at a relatively low temperature, the material is a rope system, and relatively large particles of 0.3 to 0.8 mm are spring-loaded. It is good to project so as not to damage the surface layer at any speed of 40 to 90 m / sec. Also, when stress is applied to the spring, adjacent lines near the end of the spring are likely to contact each other, but sufficient descaling occurs at such a line-to-line contact portion to promote nitrogen ingress during nitriding. In order to prevent fatigue failure from the vicinity of the spring end, it has been found that fine particle projection having a diameter of 10 μm or more and less than 100 μm, more desirably 10 to 80 μm, is effective after the above-described 0.3 to 0.8 mm particle projection. . As a projection condition at this time, in order to project fine particles without causing fine cracks and local deformation bands harmful to fatigue on the surface layer, the projection speed is 50 to 160 m / sec, more preferably 60 to 140 m / sec. Furthermore, it was found that controlling the surface temperature of the spring at the time of fine particle projection to a lower temperature than causing recovery and recrystallization is effective in preventing surface layer defects. When the nitriding temperature is 500 ° C. or less and about 450 ° C. or more, the depth of the surface plastic deformation region due to the fine particle projection is relatively shallow, but nitrogen penetrates to a depth comparable to the particle projection of 0.3 to 0.8 mm diameter. Therefore, it is also effective to perform only fine particle projection without projecting particles of 0.3 to 0.8 mm. The second aspect limits the projection conditions for such a purpose and reason.
[0026]
Nitriding treatment or low-temperature carbonitriding treatment is performed at a temperature of about 500 ° C. or less, and is mainly a treatment for adding nitrogen and, in some cases, a portion of carbon and introducing it into the spring surface layer portion. As a result of the small amount) intrusion, a high compressive residual stress is imparted to the surface layer portion. The fine particle projection of the present invention is also effective for a relatively hard spring having a spring surface hardness of about Hv 800 to 1100 after nitriding. The particle projection of 0.2 to 0.9 mm diameter after nitriding brings the depth of compressive residual stress to a deeper position than that of nitriding. For this reason, the effect which prevents the fatigue fracture from the nonmetallic inclusion and fine crack in the 0.5 mm depth position from the surface vicinity is exhibited.
[0027]
After the relatively large 0.2-0.9 mm diameter iron-based particles are projected as described above, the fatigue breakage from the surface layer and from the internal non-metallic inclusions is further achieved by the projection under the optimum conditions of the fine gold bent particles of the present invention. Can be prevented even under repeated loading at high stress.
[0028]
The particle projection after nitriding has a hardness Hv of 500 to 800 and is softer than the outermost layer hardness (micro Vickers hardness at a depth of about 5 μm from the outermost surface) of the treated spring. Whether hard metal particles such as steel having a diameter of 200 to 900 μm are projected at 40 m / sec to 90 m / sec, thereby preventing the formation of harmful microcracks on the surface layer and applying compressive residual stress to the relatively inner part of the spring ( The particle projection of the hardness Hv500-800 having a diameter of 0.5 to 0.9 mm and a hardness of Hv500 to 800 having a diameter of 0.2 to 0.4 mm is performed, and the surface layer is harmful. Applying high internal compressive residual stress, including relatively close to the surface, while preventing microcracking etc.3).
[0029]
Subsequently, the hardness is Hv600 or more and Hv1100 or less, and the hardness of the outermost layer of the spring before nitriding is equal to or less than the hardness of the outermost layer of the spring, the average diameter of all projected particles is 80 μm or less, and the average diameter of individual particles 10 μm or more and less than 100 pm, more desirably, the average diameter of all particles is 65 μm or less, the average particle diameter is 10 to 80 μm, the specific gravity is 7.0 to 9.0, and the shape of the metal particles is spherical or relatively close to a spherical shape at a speed of 50 to 190 m. The projection is performed at a speed of / sec, and more desirably at a speed of 60 m / sec to 140 m / sec (such a fine hard metal particle projection technique of the present invention is hereinafter referred to as an SS process).
[0030]
FIG. 1 shows C: 0.60%,. After nitriding a spring steel containing Si: 1.45%, Mn: 0.68%, Ni: 0.28%, Cr: 0.85%, V: 0.07% (unit: mass%), 0 The impact velocity by the projected fine particles on the surface of the spring of surface hardness Hv930, which is obtained by projecting 6mm diameter high carbon steel particles (hardness Hv550) at a speed of 70m / sec, affects the compressive residual stress near the surface after the projection. It is the experimental result which calculated | required the influence, and both compressive residual stress in the outermost layer and the surface layer of 10 micrometer depth is 1900 (N / mm²) It can be seen that the impact velocity with the above high stress is optimum at around 95 m / sec. Here, the nominal diameter of the projected particles is 50 μm, the average particle diameter is about 63 μm in the initial product (new product) with n = 60 measurements, the average diameter of the maximum particles is 80 μm or less, the minimum particle average diameter is 50 μm, individual Spherical or nearly spherical ellipsoidal particles that have a maximum / minimum diameter ratio of 1.1 or less and a minority of 1.5 or more particles but no angular sharp edges, and have an average hardness Hv860 and specific gravity 8.2. As for temperature control, the instantaneous temperature rise limit of the iron surface (excluding the nitrogen compound layer) of the spring surface nitrided layer due to collision causes effective work hardening under the interaction with nitrogen atoms. The projectile was controlled at a lower temperature than the softening due to the recovery recrystallization of the layer. Confirmation that such temperature control is performed is performed by a technique such as micro Vickers hardness measurement of the sample work surface layer after the shot or high magnification structure observation using an electron microscope.
[0031]
As can be seen from FIG. 1 showing the results of the experiment, the maximum compressive residual stress value in the vicinity of the surface layer (the outermost layer to 10 μm depth) exceeds 1800 MPa with a velocity of v = 90 to 152 m / sec, and has a good distribution. Indicated. In particular, under the condition of v = 90 m / sec, the compressive residual stress on the outermost surface is almost 2000 MPa, the distribution is good, and it can be seen that the effect of improving fatigue strength is great. That is, in high-speed steel particle projection with v ≦ 152 m / sec and total particle average diameter of 63 μm, defects such as local adiabatic shear bands and cracks in the nitride compound layer near the workpiece surface may hinder fatigue life. Hardly occurs.
[0032]
However, when the velocity exceeds 170 to 190 m / sec even with the same particle projection, fine cracks and strong deformation bands appear in the vicinity of the surface, and the residual stress is also lower than when the velocity is lower. For this reason, in the present invention, the upper limit of the fine particle projection speed is set to 190 m / sec. Here, when the fine particle projection speed is higher than 190 m / sec, fine cracks are formed on the nitrided surface, or the fatigue durability improving effect is reduced due to surface embrittlement.
[0033]
Further, the effect of this fine particle size on the spring fatigue strength is that if there are angular and sharp square-shaped particles in the projected particles, the effect of improving the fatigue strength is reduced, and the average diameter is as large as 100 μm or more. When particles are mixed, the effect of improving fatigue strength is impaired. Furthermore, the shot speed at the point where the outermost surface layer and the stress curve of 10 μm depth intersect is 95 m / sec, but the surface layer compressive residual stress is 1800 MPa or more at a shot speed of 20% before and after this intersection (76 to 114 m / sec). Thus, it can be seen that a large compressive residual stress can be formed in a relatively thick surface layer range. The fatigue strength is expected to be improved at a lower speed than the condition for obtaining the maximum value of the compressive residual stress of the surface layer up to a depth of 10 μm, and a good fatigue test result with a residual stress of about 1700 MPa or more at a projection speed of 60 m / sec or more. Is obtained. Further, since particularly good results are expected in the fatigue characteristics even when the projection speed is 130 to 150 m / sec and the average is 140 m / sec or less, the desirable speed is set to 60 to 140 m / sec.
[0034]
FIG. 2 shows the residual stress distribution when the fine particle projection speed of the above average particle diameter of 63 μm is 90 m / sec and 190 m / sec.
[0035]
Next, the hardness of the particles was slightly reduced to Hv700, and the same experiment as described above was performed using steel particles having an average particle size of 50 μm, a substantial particle size of 40 μm, and a maximum particle size of about 75 μm. As a result, when the speed was 190 m / sec, generation of microcracks and partial peeling of the compound layer were observed as in the case of high-speed steel particle projection. In addition, when the velocity v = 60 m / sec to 140 m / sec, the maximum compressive residual stress in the vicinity of the surface layer is slightly smaller than that in the high-speed steel particle projection, but shows a value exceeding 1700 MPa, which has a great effect on durability improvement. I understood that I can expect it. The surface hardness of the test nitriding spring used at this time is about Hv930. Although the surface hardness of the spring after completion of the fine particle projection was only slightly increased to about Hv 950, as described above, large compressive residual stress was formed on the workpiece surface by the particle projection with the convenience equal to or less than the surface hardness of the workpiece. It was confirmed that FIG. 3 shows the same spring as that used in the test of FIG. 1 after nitriding the oil-tempered wire for a high-strength valve spring and then performing high-carbon particle projection of 0.6 mm. It is the figure which took the initial particle diameter (the nominal diameter displayed on a bag new article) on the axis, and arranged the compressive residual stress on the surface on the vertical axis. In both cases, the material of the projecting particles is a high-speed steel with a specific gravity of 8.2. The initial average hardness of the particles is 50 μm in nominal diameter and Hv860 (the initial average average particle diameter is approximately 63 μm), which decreases as the nominal diameter increases. The nominal diameter is 200 μm and Hv770. In addition, the number in a figure is the collision speed (unit: m / sec) of the particle | grains to the spring surface. From this figure, it is clear that the effect of imparting compressive residual stress on the surface is greatly reduced in the case of particle projection with a nominal diameter of 100 μm compared to the case of 50 μm. Of the new particles having a nominal diameter of 100 μm, the average diameter of the maximum particles was 125 μm, and the average average diameter of the new particles having a nominal diameter of 50 μm was 80 μm (all measured results of n = 60). None of the particles had sharp corners, was mainly spherical, and partly an elliptical sphere shape that was relatively close to a sphere.
[0036]
Fine particles having sharp edges are undesirable because they tend to inhibit fatigue. Further, for example, when the particle size variation of individual fine particles having an average diameter of 44 μm is large and particles having a size of 90 to 105 μm are mixed in several% or more, the fatigue strength improving effect is the average particle size of 44 μm and the maximum particle The diameter is smaller than that when the diameter is about 75 μm. As described above, the effect of improving the fatigue strength of the spring also affects the average diameter of all the projected particles. In addition, the mixture of particles having a large maximum particle diameter inhibits the fatigue strength. Therefore, in this patent, the mixture of particles substantially larger than 80 μm has the effect of improving the fatigue strength, but the degree of the effect is lowered. Therefore, the upper limit dimension is set to less than 100 μm, more preferably 80 μm. In addition, the particle | grains whose average diameter of each projection particle is smaller than the average diameter or nominal diameter of all the particles are not angular, the specific gravity is 7.0-9.0, the hardness is Hv700 or more, 1100 or less spherical shape. Or when it is close to it, a projection effect is not inhibited. Rather, when the average particle size of each particle is smaller than 50 μm, it is effective for increasing the hardness of the spring pole surface layer and the compressive residual stress. However, since the influence thickness of hardness and residual stress decreases as the particle size decreases, the treatment method of the present invention (claim 1 and2)Then, it is preferable that the average particle diameter is 20 μm or more. In addition, even if a relatively small amount of fine particles of 10 μm or less are mixed, the projection and the effect of the shape, specific gravity, and the like are similar to those of the claims.Yes.In addition, as the nominal diameter of the projected particle becomes smaller, it is generally difficult to produce or use the particle without variation in its size. Therefore, even if the nominal diameter is determined, the particle size actually has a distribution, and a good effect cannot be obtained unless the particle is selected in consideration of this distribution.
[0037]
When the hardness of the surface layer is about Hv850 or more due to nitriding, even if the particle has the same or less hardness, a part of the kinetic energy of the particle at the time of collision is spent on the deformation of the spring surface layer. The surface temperature also rises momentarily. As a result, the yield and plastic deformation of the nitrided spring surface layer progress, and it is considered that the promotion of dislocation growth due to the interaction between the dissolved nitrogen atoms and the kinetic dislocations and the hardening due to dislocation fixation proceed.
[0038]
When the hardness of the fine particles is lower than Hv600, the residual stress generation efficiency in the spring surface layer is reduced, so the lower limit is set to Hv600. However, since deformation of the spring surface and formation of compressive residual stress are possible with Hv 500 to 600, the lower limit hardness may be Hv 500 or more depending on the case. If the hardness of the projecting particles is higher than the hardness of the nitrided spring surface, it will tend to generate microcracks from the spring surface and impair the spring fatigue strength.Therefore, the upper limit of particle hardness is set here. Or less.
[0039]
Here, “work hardening under interaction with nitrogen atoms” will be described. An iron-based nitrogen compound such as epsilon iron nitride may be formed on the surface of the nitrided spring steel material. Furthermore, a relatively fine iron nitride is formed inside the steel by a part of the nitrogen atoms diffused and permeated into the steel, which increases the hardness. However, in addition to these, there is solid solution nitrogen in the iron ground, and this solid solution nitrogen itself contributes to the increase in hardness and the same compressive residual stress. This solute nitrogen provides resistance to mold deformation during SS treatment, but when the workpiece surface layer begins plastic deformation, the dislocations move and are affected by heat generation, increasing the diffusion rate of nitrogen atoms in iron. In the process, at least a part of the dislocation is fixed, and the dislocation cell (subgrain) is refined by promoting dislocation growth. This is considered to prevent the occurrence of a slip deformation band due to the repeated stress of the surface layer when the spring is used, and as a result, prevent the formation of microcracks due to fatigue failure. Nitrogen has a much higher solid solubility than carbon, and due to the coexistence with manganese, silicon, etc. in the steel, the solid solubility is much higher than that of the iron-nitrogen binary system. It is considered to be. From this point, it can be said that the nitriding of the spring steel and the subsequent SS treatment are very effective for improving the spring characteristics.
[0040]
Based on the influence of the above projecting particle size, in the present invention, the initial total particle average diameter is 80 μm or less, and each particle is 10 μm or more and less than 100 μm. The specific gravity is 7.0 to 9.0 mainly considering readily available steel materials, the hardness is Hv 600 to 1100, and the hardness is equal to or less than the hardness of the spring surface layer before particle projection. More preferably, the initial total particle average diameter is 65 to 50 to 20 μm, and the average diameter of each individual particle is 80 μm or less.
[0041]
next,(2)The means of the method of the present invention concerning the improvement of the fatigue strength of a spring not subjected to nitriding (and low-temperature carbonitriding) will be described.
[0042]
In order to increase the compressive residual stress on the spring surface without using conventional nitriding or low temperature carbonitriding, (i) Is it possible to improve or devise shot peening using a material with higher strength than before? ii) Shot peening may be improved or devised using the same material as before. The improvement of the shot peening method of (i) and (ii) includes a method in which stress is applied to the spring in advance to perform particle projection (stress peening), or particle projection is performed in two or three stages, and the projected particle diameter is sequentially increased. There are known a method of reducing the size of the particles, a method of performing particle projection while the spring is heated warmly, and the like. As the spring becomes stronger, its elastic limit is improved, so that a higher residual stress can be applied.
[0043]
However, for example, as described in the above-mentioned prior art 4 and Japanese Patent Application Laid-Open No. 64-83644 "High Strength Spring", a silicon chrome steel oil temper for a valve spring defined in JIS standard G3561 (1994). For high-strength oil tempered wires that have a tensile strength higher than the tensile strength of the wire and whose chemical composition is different from the JIS standard, the compressive residual stress near the surface layer is 1079 MPa (110 kgf / mm²) If it is given above, the reliability of the spring characteristics also decreases because it is related to the formation of microcracks on the surface in addition to the residual stress.The
[0044]
(A) Hard metal particles having a hardness Hv of 350 to 900 and a particle diameter of 200 to 900 μm are applied to the surface of the spring that has been molded and tempered and has a hardness of the surface layer of Hv 400 to 750, and a velocity of 40 m / sec to 90 m. projecting at / sec, thereby preventing the occurrence of harmful fine cracks in the surface layer, and applying a compressive residual stress to the relatively inner part of the spring; (B) the spring surface after the step (A) above The surface treatment method according to claim 3, wherein the surface layer has a hardness and compression residue of 30 to 50 μm or less from the surface without generating harmful microcracks that inhibit fatigue strength. Surface treatment method for improving the durability of a spring that particularly increases stressIn step (B), the material of the projecting fine particles is high carbon steel or high speed steel, which is a material similar to the spring, and therefore has an elastic coefficient equivalent to that of the spring, so that the elastic deformation is simultaneously distributed to the spring surface layer and the projecting particles. It is thought that the fact that the particle shape and the particle shape are not square and fine is one cause of suppressing the generation of fine cracks that inhibit fatigue strength and excessive surface layer processing. In this way, the compressive residual stress on the surface is greatly increased by fine particle projection because the introduction of dislocations due to large plastic deformation in the surface layer and the fixation of many introduced dislocations by carbon atoms repeatedly proceeds with each particle projection. To be related. In other words, the supply of carbon atoms, which originally existed in the spring material in the form of soot carbide, became thermodynamically unstable due to the high pressure and temperature rise for a very short time due to the fine particle projection, and a part of it was short. The carbon atoms, which decompose in time and become free as a result, diffuse around the dislocations, relax the elastic stress field of the dislocations, and provide resistance to dislocation movement, thereby promoting dislocation growth. For this reason, the dislocation cell structure is refined, and surface hardening and high compressive residual stress are imparted without impairing toughness. However, claims4of(4)In maraging steels that contain almost no carbon, the increase in compressive residual stress and hardness near the surface due to fine particle projection is mainly attributed to the increase in dislocation density rather than the decomposition of iron carbide mentioned above (nitriding In this case, it is considered that the dislocation mobility decreases due to the decomposition of the nitrogen compound and the dislocation fixing, and the dislocation density increases and the dislocation fixing proceeds.
[0045]
FIG. 4 shows a fine pearlite structure composed of C: 0.57%, Si: 1.5%, Mn: 0.7%, Cr: 0.68% (the unit is mass%), the remaining impurities and iron. Name of 50 μm diameter (n = 60 actual measurements, initial effect of high carbon steel particles on the bending fatigue strength of cold drawn wire, then cold rolled finish thickness 0.97 mm, average surface hardness Hv 537-589 Average hardness Hv 865, specific gravity 7.5, total particle average diameter 37 μm, individual particles are distributed in an average diameter of 10 to 75 μm, all of which are spherical or close to each other and have no sharp edges. The initial average hardness Hv860, specific gravity 8.2, total particle average diameter 63μm, maximum particle average diameter 80μm, minimum particle average diameter 50μm) and the effect of iron-based fine particle projection speed on the fatigue strength after projection. is there. In this case, it can be seen that there is an optimum projection speed around a collision speed of 100 m / sec. In the high carbon steel particle projection with the impact velocity by the particle projection of 107 m / sec and 183 m / sec, the compressive residual stress on the outermost surface was 950 MPa. Nevertheless, the fact that the former fatigue strength is higher than the latter indicates that the occurrence of fine cracks in the surface layer or the toughness of the spring surface is involved in addition to the residual stress. That is, when the projection speed is 183 m / sec, it is considered that the generation of fine cracks on the spring surface layer and deterioration of ductility were caused. Thus, when the speed is 183 m / sec, the fatigue strength improvement effect is recognized, but the effect is smaller than the case where the projection speed is 160 m / sec or less.TheWhen the projection speed is less than 50 m / sec, the fatigue strength improvement effect becomes small, so this was made the lower limit. More desirably, the lower limit speed was set to 60 m / sec. In addition, the average particle diameter of the projected particles was changed, and the same material projection as that described above was performed on the same spring as the spring to be processed in FIG. As a result, the fatigue strength of the spring after the particle projection decreased significantly as the nominal diameter of the new projection particle increased to 100 μm, 200 μm, and 300 μm (FIG. 5). It is considered that the fatigue strength improvement effect decreases as the particle size increases due to a decrease in the compressive residual stress imparting effect on the surface layer and a decrease in the hardness increase. For this reason, in this invention, the total average diameter of a projection particle shall be 80 micrometers or less, and the average diameter of each particle shall be less than 100 micrometers. Beyond this, although effective, the effectiveness decreases.
[0046]
NitroThe minimum average particle diameter of the projected metal particles on the spring surface that does not become 10 μm is below that, the depth of compressive residual stress due to projection is several μm or less, and the depth at which sufficient compressive residual stress is obtained is shallow By becoming. However, even if particles having a diameter of 10 μm or less are mixed, there is no problem in quality as long as the amount is small. The reason why the maximum average particle size is less than 100 μm is that the residual stress and hardness improvement effect of the surface layer become smaller at a particle size larger than that.
[0047]
The reason why the maximum average size of all the projected particles is set to 80 μm is that the effect of improving the durability is greater than that in the case of the total average particle size of 100 μm. The specific gravity of 7.0 to 9.0 is intended to utilize particles made of steel materials that are relatively inexpensive and easily available. About 196 GN / m of elastic modulus of steel spring²Compared to 450 to 650GN / m for cemented carbide²The elastic deformation and the mold deformation are concentrated on the surface layer of the projected spring rather than the projected particles. For this reason, in the cemented carbide, surface irregularities are relatively large, and non-uniform deformation such as adiabatic shear deformation band is relatively likely to occur.. ExcessiveThe purpose is to avoid concentration of deformation on the spring, which is the workpiece, and the density is set to 7.0 to 9.0 with the intention of using iron-based particles.
[0048]
Moreover, although the hardness lower limit of the projection particle | grain with respect to the spring which is not nitrided was set to Hv350, as the hardness of the workpiece spring surface, there are many springs of Hv400-600, but it is a particle projection a little softer than workpiece | work material hardness.Also effectiveThis is because the fruit is exhibited.
[0049]
Moreover, the upper limit of the projected particle hardness is set to Hv1100 because the Hv1100 can be set as the upper limit of the hardness of steel particles that can be obtained relatively inexpensively, and when the hardness is Hv1100 or less, the fatigue resistance improvement effect is sufficient. It is because it is accepted.
[0050]
The reason why the lower limit of the projection speed of hard metal particles having a particle diameter of 10 to less than 100 μm, a specific gravity of 7.0 to 9.0, and a hardness of Hv 350 to 1100 is 50 m / sec. This is because sufficient durability cannot be improved. In addition, the upper limit of the particle projection speed is set to 160 m / sec. The projection energy / particle projection area becomes excessive at a speed exceeding the upper limit, and the compressive residual stress of the spring surface layer is lower than the lower speed. This is because the generation of microcracks on the surface layer is promoted, and the effect of improving the durability of the spring is reduced for the consumed energy.
[0051]
A non-nitrided thin leaf spring sample corresponding to FIGS. 4 and 5 described above, high-carbon steel particles having an average particle diameter of 37 μm and a hardness of Hv865 are projected at a speed of 90 m / sec. A sag test was carried out at 160 ° C. on the springs that had been subjected to the same low-temperature annealing and the springs that had been subjected to the final low-temperature annealing in the same processing step. As a result, the spring sag without the final low-temperature annealing at 230 ° C. was equivalent to the spring in which it was performed, and had excellent sag resistance. On the other hand, in the spring sample in which a steel shot having a diameter of 0.3 mm was projected at a speed of 100 m / sec, the final low temperature annealing had better sag resistance than the non-executed sample.
The cause of this is that in the former, carbide deformation in the steel occurs more severely than in the latter, and there are relatively many free carbon atoms decomposed with the help of this, and this free carbon is effective in preventing dislocation migration during the creep test at 160 ° C. This is thought to be due to the effective use of. However, if two types of springs with or without low temperature annealing at 230 ° C are subjected to short-time setting at room temperature under the same stress condition, the springs that are not subjected to low-temperature annealing are larger than the springs that have been subjected to that setting. It was.
From this, it is understood that the dislocations generated on the spring surface layer by the projection are not sufficiently fixed only by the projection of the fine hard metal particles. In addition, the 160 ° C. sag test does not depend on whether or not the 230 ° C. low-temperature annealing is performed in advance, because the fine hard metal particle projection is more than the 0.3 mm diameter metal particle projection. This means that deformation and annihilation of the iron carbide and cementite are promoted, and strain aging by carbon atoms decomposed when heated to 160 ° C. proceeds in a short time. However, it is estimated that the temperature rise due to instantaneous heat generation of the spring surface layer due to particle projection is almost inversely proportional to the diameter of the projected particle at the same projection speed. This is because if the same particle hardness and the same spring material are used, the time required for deformation of the spring surface layer by collision is proportional to the particle diameter, but as the particle diameter becomes smaller, the time required for deformation becomes shorter and deformation during deformation This is thought to be because the time for heat to escape out of the deformation region is shortened, resulting in an increase in the temperature of the deformation region (Bowden Taber, translated by Noriyoshi Hamada, Solid Friction and Lubrication, 4th edition, Maruzen, Showa Issued 50 years, see description on page 256 and equation (8) Here, there is an explanation that the contact time of the collision object is proportional to the square root of (mass M / particle radius r), √ (M / r). According to √ (M / r) ∝r, the contact time is proportional to r after all.)
[0052]
In the spring surface layer by the fine particle projection of the present invention, it is considered that heat generation due to collision and deformation and strain age hardening by carbon and nitrogen atoms proceed better than particles having a diameter of 0.3 mm. Also, the deformation of cementite is considered to be due to the fact that cementite has a characteristic that its deformation resistance decreases as the temperature rises. When the fine particle projection speed is about 180 m / sec, the cementite is deformed and partially disappears, and the fragmentation is promoted. Cementite fragmentation reduces the effect of preventing dislocation movement in iron that is generated and moved by deformation, and is considered to contribute to a decrease in surface residual stress along with the projection speed. It should be noted that when the variation in the size of the average particle size of the projection fine particles used in the present invention increases and the ratio of particles having a larger size increases, the effect of improving the durability decreases. For this reason, the maximum average particle diameter needs to be substantially less than 100 μm, and desirably 80 μm or less.
[0053]
As another operational effect of the fine particle projection of the present invention, it is possible to realize a reduction in spring deformation due to the fine particle projection, and as a result, it has been found that the occurrence of dimensional variation of the spring can be reduced in mass production. This is because the influence layer of the fine particle projection of the present invention is relatively thin, which suppresses the large deformation of the spring, and because the present invention is based on a relatively low-speed particle collision at the time of the fine particle projection, the higher speed projection is achieved. It can be estimated that the variation in the projection speed can be made smaller than that (FIG. 6).
[0054]
When the surface layer of the high carbon steel spring treated in this way is observed with a transmission electron microscope, a very fine and curved microstructure (subgrain) develops in the deformation zone due to surface deformation, and cementite precipitates. Although some fragmentation of particles and refinement of the interval and increase of dislocations in iron are observed, when fine particles are projected at the optimum projection speed of the present invention, cementite fragmentation hardly occurs. In addition, a clear microstructure (polygonalized structure) due to recovery recrystallization was not observed at all. Also, no supercooled structures such as martensite and paynite were found.
[0055]
A spring having a relatively large wire diameter or plate thickness, specifically, a wire spring having a wire diameter of 1.5 to 2.0 mm or more, can apply compressive residual stress to the inside of the surface layer by multistage shot peening. It is effective and widely used in applications such as valve springs for internal combustion engines such as automobiles.[0044] lines 1-7As shown in (A) above, projecting particles having a diameter of 0.2 to 0.9 mm at a speed of 40 to 90 m / sec imparts compressive residual stress to the inside relatively and causes fatigue fracture from non-metallic inclusions. It is for preventing. However, in the case of a spring having a wire diameter larger than 2.0 to 2.5 mm, the particle projection of 0.2 to 0.4 mm diameter is performed after the particle projection of 0.5 to 0.9 mm diameter.so,The residual stress of the calendar can be relatively increased to prevent cracking from the inside and the vicinity of the surface to some extent. After such 0.2-0.9 mm diameter particle projection, the compressive residual stress on the surface is still insufficient, and this causes defects such as microcracks harmful to fatigue failure due to the fine particle projection of the present invention. Enhance.
[0056]
To overcome the disadvantages of relatively large particle projectionsOnA hard metal having a specific gravity of 7.0 to 9.0 having a diameter of less than 10 to 100 μm, an average particle diameter of 20 to 80 μm, a spherical shape or the like, and a hardness of Hv 350 to 1100 By sufficiently projecting the fine particles at a speed of 50 to 160 m / sec, a highly worked layer is uniformly formed and high compressive residual stress is imparted to the surface layer without causing micro cracks and large concaves that are harmful to fatigue strength.
[0057]
The coverage of the particle projection of less than 10 to 100 μm or preferably 10 to 80 μm in the present invention is desirably 100% or more with respect to a site where the durability improvement of the target spring is required, and the above-mentioned sufficient projection The meaning of this corresponds to this.
[0058]
The initial hardness lower limit of particles having a diameter of 0.2 to 0.9 mm was set to Hv350 because the particles whose hardness is lower than that of the spring surface are repeatedly deformed and repeatedly work-hardened due to repeated particle collisions. Rises. Further, even if the hardness is low, if it is equal to or higher than Hv 350, a part of the energy of collision is used for deformation of the spring surface layer.
[0059]
Thus, it has been found that even in the case of a non-nitrided spring having a surface hardness lower than that of the nitrided spring, good results can be obtained under conditions similar to those of the nitrided spring.
[0060]
The initial hardness of the projected particle of the present invention is a new value, and the claims hardness and other values are those of a new product. In the present invention, since the projected particles are gradually worn and worn by repeated use, particles that are smaller than the new dimensions are actually used, and particles that have sharp angular edges due to fracture during use. It is necessary not to change. Also, low temperature annealing for removing residual stress at a temperature of about 250 to 500 ° C. of the cold-formed spring in the manufacturing process of the spring of the present invention, after coil spring molding or after residual stress removal annealing after coil spring molding Or heated to a temperature of about 200 to 250 ° C. for improving sag resistance after polishing of the bearing surface such as after nitriding, after fine particle projection or after 0.2 to 0.9 mm diameter particle projection in the previous process. Steps such as low-temperature annealing and warm or cold setting for the same purpose are included in the spring production of the present invention.
The effect of the hard metal particle projection of the patent of this application is that fatigue stress is imparted to the spring surface layer by imparting high compressive residual stress without causing microcrack formation harmful to fatigue fracture or excessive ductility of the spring surface layer due to excessive plastic working. It is to prevent fatigue cracks from propagating from the defects on the surface of the spring and the vicinity of the surface layer, which cause destruction, and improve fatigue durability. The hard fine particle projection of the present invention realizes work hardening by work deformation of the metal structure of the spring surface layer without damaging damage to the spring surface layer, and as a result, imparts extremely high compressive residual stress. Due to the instantaneous heat generation and high pressure caused by this fine particle projection, Fe in spring steel_ThreeDislocation fixation and dislocation growth by solid solution C atoms generated by strong deformation of C and disappearance by partial decomposition are promoted. In the nitrided spring surface layer, solid solution nitrogen causes dislocation fixation and proliferation due to instantaneous deformation and heat generation during the projection of fine particles, like the above-mentioned C atoms. By these, refinement | miniaturization and work hardening of the cell structure of a spring surface layer are accelerated | stimulated especially. These aspects were clarified by transmission electron micrographs tens of thousands of times. It is considered that the large work hardening of the surface layer improves the elastic limit of the surface layer and, as a result, contributes to the improvement of residual stress remaining within the elastic limit. The highest effect can be exhibited when the impact speed to the spring by fine particle projection is 60 to 140 m / sec. Although it is effective at a higher speed than this, especially the nitrided spring has a residual due to processing gradually along with the impact speed. As the stress decreases, microcracks and embrittlement of the material due to processing appear, and as a result, the effect of improving fatigue strength also decreases. For nitriding springs, the speed is 190 m / sec, and more strictly speaking, those damages are particularly noticeable when the speed exceeds 170 m / sec. For non-nitrided springs, the effect exceeds 160 m / sec. This is far from the optimal condition. Moreover, if the impact speed | velocity | rate by projection falls from 60 m / sec or 50 m / sec, the depth processed by the impact will become small and a residual stress will also become low. For this reason, although there is an effect of improving fatigue strength, it is clearly inferior from the optimum condition.
[0061]
[Brief description of the drawings]
FIG. 1 is a relational curve diagram of a compression residual stress on a surface of a high tension spring in which steel particles having a diameter of 0.6 mm are projected after nitriding, and fine steel particles (new average diameter of 63 μm) are further projected, and a projection speed.
2 is the same as FIG. 1 and shows a compressive residual stress curve when the average particle diameter of 63 μm high-speed steel fine particles is projected to 90 m / sec and 190 m / sec onto a nitride spring after 0.6 mm particle projection.
FIG. 3 is a relationship curve diagram of a compressive residual stress and a projected particle diameter by second-stage particle projection onto a high-strength spring subjected to the same nitriding and 0.6 mm particle projection as in FIG.
FIG. 4 is a diagram showing the effect of the impact speed on a spring by two types of copper particle projection having a nominal diameter of 50 μm on the fatigue limit amplitude stress of the spring after projection. In this figure, a part of the data in FIG. 5 is extracted and rearranged.
FIG. 5 is a result of investigating the influence of hard metal particle projection on a spring thin plate spring, and shows the average diameter of the projected particles whose materials are high carbon steel and high speed steel and the fatigue limit amplitude stress after the particle projection (average Stress, 786 N / mm²(Constant). The numbers in the figure are the collision speed of the particles.
FIG. 6 shows a result of measuring a reduction in height of a thin leaf spring by hard metal particle projection.
This figure is taken from measurements in the same test as the data of FIGS. The numbers attached to the plot points indicate the nominal particle diameter.
FIG. 7 is an iron ground residual stress distribution curve by X-rays of a surface layer portion of a valve spring manufactured with a 4.0 mm diameter piano wire.
[0062]
BEST MODE FOR CARRYING OUT THE INVENTION
Embodiments of the present invention will be described below.
[0063]
(Embodiment 1)
In order to improve durability of valve springs, clutch springs, etc., particularly fatigue resistance, by nitriding, the following processes have been conventionally employed.
[0064]
Alloy steel oil tempered wire (hereinafter referred to as OT wire) → Spring molding (cold coiling) → Residual stress removal annealing → Seat surface polishing → Surface scale removal → Nitriding treatment → Shot peening → Low temperature annealing Here, shot after nitriding As peening, in the case of a one-step shot, a large number of hard metal particles such as Hv500 to 800 steel balls having a particle diameter of about 0.5 to 0.9 mm or cut wires are usually projected. In the case of a two-stage shot, a large number of metal particles having a particle size of about 0.2 to 0.4 mm are projected after a shot of a large number of steel balls having a particle size of about 0.5 to 0.9 mm.
[0065]
In the present invention, a method of shot peening after nitriding is provided, and after the first stage or the second stage following the first stage, the total average particle diameter is 80 μm or less and 20 μm or more, and the individual particle average diameter is 10 μm or more and less than 100 μm, Metal particles having a specific gravity of 7.0 to 9.0, a hardness of Hv 600 or more and Hv 1100 or less, and a hardness equal to or less than the spring surface hardness after nitriding or carbonitriding, without a spherical shape or an angular portion close to it as a shape. Projection is carried out at 50 to 190 m / sec, effectively effecting work hardening of the spring surface layer and prevention of formation of fine cracks, and imparting high residual stress and hardness to the outermost surface layer.
[0066]
Furthermore, after these steps, by ensuring dislocation fixing in the shot influence layer (surface layer 150-200 μm) by low-temperature annealing, fatigue resistance and stickiness can be obtained only by conventional methods. A spring having very good durability that could not be obtained could be obtained.
[0067]
Further, examples of the descaling method before nitriding include pickling, electropolishing, and metal particle projection. In the present invention, a descaling method before nitriding is provided in claim 2. This method is intended to obtain high fatigue durability after nitriding by fine iron-based particle projection.
[0068]
The manufacture and performance of the spring of Embodiment 1 will be described below.
[0069]
According to the method of claim 2, descaling prior to nitriding, followed by nitriding treatment and subsequent particle projection,4High performance springs can be manufactured.
High-strength valve spring oil with a diameter of 3.2 mm containing C: 0.59%, Si: 1.90%, Mn: 0.84%, Ni: 0.27%, Cr: 0.96%, V: 0.09% (units are weight%) Temper wire (Claims)4of(2)Material), cold coiling, 420 ° C. stress relief annealing, bearing surface polishing, and as a descaling process, the average particle diameter is 37 μm, the average particle diameter is 75 to 10 μm, and the maximum / minimum diameter ratio of each particle Particles having a specific gravity of 7.5 and a hardness of Hv865 were projected at a speed of 107 m / sec and then nitrided to obtain a hardness Hv910 of the surface layer (depth 3 to 5 μm position). Further, a round cut wire having a diameter of 0.6 mm and a hardness of Hv550 was sufficiently projected at a speed of 70 m / sec to apply a compressive residual stress to a relatively inside. The surface hardness at this time was Hv930. Following this, the average diameter of all particles is 37 μm, and among the individual particles, the average diameter of the largest particles is 75 μm or less, the individual particle minimum diameter is approximately 10 μm, the major axis is 1.2 mm or less, and the non-angular abbreviated specific gravity. 7.6 High carbon steel particles having an average hardness of Hv865 were sufficiently projected at an average speed of 107 m / sec. Then, low temperature annealing was implemented at 220 degreeC. The surface hardness at this time was Hv975.
[0070]
The compressive residual stress of the outermost spring layer at this time was 2010 MPa. Moreover, the hardness of the spring at the 0.2 mm depth position and 0.5 mm depth position from the surface at this time was Hv570 and Hv545, respectively. Moreover, the nonmetallic inclusion in steel was 15 micrometers or less, and the carbonitride was smaller than 10 micrometers. The hardness of the outermost surface of this spring as it is nitrided is Hv910, the hardness of the projected 0.6 mm diameter carbon steel particles is Hv550, the average initial hardness of the high carbon steel fine particles is Hv865, The average hardness of the particles was Hv960. The spring was subjected to an average stress of 686 MPa and the amplitude stress was changed to 1000 times / min. The fatigue test was performed at a speed of. As a result, 5 × 10⁷The fatigue limit was ± 677 MPa or more in terms of amplitude stress, and n = 6 springs were not broken. This spring is claimed4In addition, the manufacturing method corresponds to claims 1 and 2.
[0071]
Next, as a descaling process, after projecting a cut wire having a diameter of 0.6 mm and a hardness of Hv550 onto a spring at a speed of 70 m / sec, high carbon steel particles having an average particle diameter of 37 μm are projected at a speed of 107 m / sec. Fatigue durability similar to the above could be ensured with a spring in which the steps after nitriding were the same as those of the first embodiment. At this time, when only the 0.6 mm cut wire having a hardness of Hv550 is projected as a descaling method, N = 5 × 10 5 even if the two-stage projection of the present invention is applied after nitriding.⁷The fatigue limit at the time was 686 MPa ± 647 MPa. The fatigue strength of the coil spring for valve spring can be expressed by average stress τm and amplitude stress ± τa when the stress repetition number N is determined to be a certain value. Here, N = 5 × 10⁷Decide on times. In the prior art, when τm = 686 MPa, a value of about 610 to 620 MPa was achieved as τa. However, as in the present invention, high fatigue strength such as τm = 686 MPa and τa ≧ 677 MPa has not been achieved conventionally. In the case of springs of the same quality and shape, it has been known that the fatigue limit stress amplitude τa decreases as the average stress τm increases. It has been found that the fatigue limit τa is approximately reduced by x / 5 as τm increases by xMPa. Therefore, the fatigue limit τm ± τa can be expressed as (constant 1−x) ± (constant 2 + x / 5). Now, if 800 MPa is taken as the constant 1, the fatigue limit can be expressed as (800−x) ± (constant 2 + x / 5). When the above fatigue limit of 686 MPa ± 647 MPa is applied to this equation, the constant 2 is 624.2 MPa. Therefore, in the present invention, the fatigue limit stress is claimed as4In the claims, a spring satisfying the following expression (1) is included.
[0072]
That is, when τm = 800−x, τa ≧ 620 + x / 5 (1)
Here, unit; both MPa, x: Variable, 0 or more and 150 or less
The above-mentioned spring descaled by 0.6 mm diameter iron-based particle projection before nitriding barely satisfied the formula (1), but with a high stress repetition of an average stress of 686 MPa and an amplitude stress of ± 677 MPa, It was sporadic to cause contact failure between the lines. However, if the SS treatment of the present invention is sufficiently performed following the 0.6 mm diameter particle projection as a descaling method, the fatigue fracture of such a line contact portion can be improved. Descaling by step shots is also included in the present invention.
[0073]
-Comparative spring of Embodiment 1(1)When(2)
In addition, with the above-mentioned spring, the comparison spring which omitted the fine grain projection of the second stage(1)Has an average stress of 686 MPa and an amplitude stress at the fatigue limit of ± 510 MPa.4The fatigue strength of is not satisfied. Further, only the second stage is changed, and steel particles having a total particle average diameter of about 72 μm, a maximum particle average diameter of about 200 μm, and a minimum particle diameter of about 7 μm are projected at an air pressure of 0.5 MPa (the collision speed of particles having an average diameter of 72 μm is about 130 m). / Sec) comparative spring(2)Prototyped. The fatigue limit stress of this spring has the same average stress as that of the spring of the first embodiment, the amplitude stress becomes ± 530 MPa, and a little effect is recognized.4Not satisfied.
[0074]
In the experiment, SS treatment was performed after projecting steel particles having a diameter of 0.6 mm and a hardness of Hv550 after nitriding, but especially for workpieces having a wire diameter and a thickness of 1.5 to 2 mm or less. There are few advantages even if it is projected. Rather, it is more advantageous to perform SS treatment immediately after nitriding in terms of performance and cost including fatigue resistance.The
[0075]
(Reference example 1)
Non-nitriding springNiheiA large number of hard metal particles having an average diameter of 10 μm or more and less than 100 μm, a specific gravity of 7.0 to 9.0, and a hardness of Hv 350 to 1100 are projected to keep the surface roughness of the spring as low as possible, and local excessive deformation (local The surface of the spring without nitriding by generating a highly uniform hard working layer on the surface of the spring pole and applying a high residual stress as much as possible. This is a spring processing method aimed at preventing fatigue breakage from the layer.
[0076]
Hard metal particles having a hardness Hv of 350 to 1100, a specific gravity of 7.0 to 9.0, an average particle size of 10 μm or more and less than 100 μm, preferably 10 to 80 μm on the surface of the spring are preferably 50 m / sec or more and 160 m / sec or less, preferably By projecting at 60 m / sec to 140 m / sec, the compressive residual stress of the extreme surface layer is increased without generating microcracks or non-uniform shear deformation bands that are detrimental to durability near the surface layer, and the spring from the surface layer Prevent fatigue breakage. This improves the fatigue strength and durability of small springs and various thin leaf springs manufactured from thin piano wires and thin oil temper wires.
[0077]
ThrowExceeding the A3 transformation point as in Japanese Examined Patent Publication No. 2-17607, “Surface processing heat treatment method for metal products”, where the impact of the spray velocity has been investigated and studied in detail, and the fine particle projection velocity v has been defined as 100 m / sec or higher. In addition, the projection is performed at a velocity V> 160 m / sec and the surface layer is not excessively deformed, and the projection is performed at a velocity V ≦ 160 m / sec, preferably 60 m / sec ≦ V ≦ 140 m / sec. It is characterized in that higher durability is obtained by controlling the temperature rise to a temperature lower than causing recovery recrystallization and avoiding excessive deformation of the surface layer.
[0078]
As described above, the test spring has a cross-sectional shape of 0.97 mm, a width of 5.1 mm, a hardness of Hv 537 to 589, and chemical components of 0.55% C, 1.47% Si and others. With spring steel that has been patented, drawn, and cold-rolled, the spring machining process is in the order of spring forming → stress relief annealing → fine particle projection → low temperature annealing (230 ° C.).(1)Carbon steel fine particles having a total particle average diameter of 37 μm (new), hardness Hv865, specific gravity 7.6, and(2)High-speed steel fine particles having a total particle average diameter of 63 μm (new), hardness Hv860, and specific gravity of 8.2 were used. The fine particles were sufficiently projected onto the spring at various speeds. Thereafter, a spring fatigue test was performed to determine the relationship between the fine particle projection speed and the fatigue strength. The result is shown in FIG. The fatigue limit stress at this time is an average stress of 785 MPa and a repetition rate of 10⁷Amplitude stress that does not break at a time is taken. As a result, both the carbon steel particles and the high-speed steel particles were found to have the best fatigue strength improvement effect at a collision speed of 60 to 140 m / sec.(2)In the high-speed steel particle projection, the collision velocity v is considered to be 50 m / sec to 140 m / sec and the fatigue limit amplitude stress exceeds 700 MPa. Also,(1)In the high carbon steel particle projection, the fatigue limit amplitude stress is considered to exceed 700 MPa when the collision velocity V is about 60 m / sec to about 160 m / sec, and a very good improvement effect is recognized.
[0079]
aboveReference example 1As a comparative example, a spring without a shot has a fatigue limit amplitude stress of 440 MPa and a low fatigue limit. In addition, the fatigue limit amplitude stress is ± 300 MPa for a spring that has sufficiently projected a 0.3 mm diameter steel shot at a velocity V = 100 m / sec (this sample is replaced with a 0.3 mm diameter steel shot, The process isReference example 1The effect of particle projection cannot be found.
[0080]
(Reference example 2)
For high-strength springs with relatively large cross-sectional dimensions, such as non-nitriding springs with a wire diameter of 2 mm, FineAs a pretreatment of the particle projection treatment, steel-based particles having a diameter of 0.2 to 0.9 mm are projected at v = 40 to 90 m / sec to give a compressive residual stress to a relatively inside. As a result, the compressive residual stress reaches the highest value at a location several + μm or more from the surface, but the extreme surface layer has a lower value than the internal maximum value. For this reason, in this state, fatigue breakage starting from the vicinity of the spring surface cannot be sufficiently prevented. In order to improve this point, after the above-mentioned 0.2-0.9 mm diameter particle projection, the velocity v = 50-160 m / sec, more preferably, v = 60-140 m / sec, the particle size is 10 to less than 100 μm, More desirably, hard metal particles having a particle size of 10 to 80 μm, a specific gravity of 7.0 to 9.0, and a hardness of Hv 350 to 1100 are projected.
[0081]
・Reference example 2Spring
Oil tempered wire for high-strength valve springs (Chemical component C: 0.61%, Si: 1.46) with a wire diameter of 3.2 mm, a tensile strength of 2070 MPa higher than JIS, SWOSC-V, and a surface layer hardness of about Hv620 %, Mn: 0.70%, Ni: 0.25%, Cr: 0.85%, V: 0.06%, all in mass%)Is formed into a coil spring with cold, low temperature annealing at 400 ° C. for 20 minutes to remove residual stress generated by coiling, bearing surface polishing, 0.6 mm diameter specific gravity about 7.8, hardness Hv550 of steel particles Following projection at a speed of 70 m / sec, the nominal particle size is 50 μm, the measured average particle size of all new particles is 37 μm, the maximum / minimum diameter ratio of individual particles is 1.2 or less, there is no angularity, the specific gravity is about 7.5, and the average Particles having a hardness of Hv865 and having an average diameter of 10 to 75 μm (however, n = 60 measured values) were sufficiently projected at a collision speed of 107 m / sec. Further, after low-temperature annealing for fixing dislocation at 220 ° C., it was finished by cold setting. Made in this wayReference example 2The compressive residual stress of the iron ground due to X-rays on the outermost surface of the spring was 1350 MPa, and the residual stress became smaller as it entered the spring. Similarly, the hardness of the very surface layer was Hv 690, and the hardness at a depth of 0.2 mm to 0.5 mm from the surface layer was Hv 600 to 580 on the spring inner diameter side. As a result of the fatigue test of this spring, the number of repetitions was 5 × 10⁷The fatigue limit of the rotation was n = 10 test springs without breakage, and the average stress was 588 MPa and the amplitude stress was ± 510 MPa. Assuming that the average stress applied to the coil spring is 690 MPa at the maximum, and the average stress τm = 690−x, the number of repetitions N = 5 × 10⁷The fatigue limit amplitude stress τa in the rotation can be set to τa = 489.6 + x / 5 based on the concept of conversion of τm and τa described in the first embodiment. However, since this formula is only a formula of the above one test result, considering the tensile strength, steel type, wire diameter, etc. of the steel wire,
When the average stress τm = 690−x,
Fatigue limit amplitude stress τa = ± (470 + x / 5) (2)
Was. This springMay have 1200 to 1600 MPa as the residual stress of the very surface layer (outermost layer)Many.
[0082]
・Reference example 2Comparison spring(3), (4)
aboveReference example 2This is an oil tempered wire of the same lot as the spring of, and it is almost the same process as this, but only a 50μm diameter iron-based fine particle projection is omitted.(3)Was made. At this time, the maximum compressive residual stress of the surface layer portion was generated in a place about 40 μm from the surface, and the value was about 820 MPa. The compressive residual stress on the very surface is 630 MPa.is there.This fatigue test result is 5 × 10⁷The fatigue limit of the rotation is an average stress of 588 MPa, and the amplitude stress is ± 440 MPa.TheIn addition, as the second stage projection, high carbon steel particles having a name of 100 μm, an actual average particle diameter of 97 μm, a maximum particle diameter of 130 μm, a minimum particle diameter of about 35 μm, and a maximum / minimum diameter ratio of individual particles of 1.2 or less. Project at a speed of about 85m / sec, then furtherReference example 2Comparison spring finished at 220 ° C with low temperature annealing and cold setting(4)created. SoNumber of repetitions of 5 × 10⁷The fatigue test results at the time are the average stress of 588 MPa and the amplitude stress of ± 461 MPa.The
[0083]
The relationship between the hardness of the spring surface and the hardness of the projected particles before the particles having a diameter of less than 10 to 100 μm are projected. However, if the spring is not nitrided, the spring surface layer has a lower hardness than the case of nitriding. Even in the case of steel particle projection having a high ductility and a hardness higher than the spring surface hardness, it is difficult to generate fine cracks or the like if the projection speed is 160 m / sec or less. On the other hand, even if the projected particle hardness is lower than the spring surface, the surface layer reforming effect is recognized. In particular, when the workpiece spring has a high hardness of Hv550 to 600 or higher with a relatively high-speed projection exceeding 100 to 140 m / sec, the projection is performed with fine particles having a hardness equal to or lower than that of the workpiece. However, the unevenness of the surface is reduced, and the residual stress is relatively high inside. In addition, when the hardness of the projected particle is low, work hardening occurs more remarkably in the projected particle itself than in the workpiece spring due to repeated projection, but when the new hardness of the particle falls below Hv350, the surface layer of the workpiece spring is modified. The efficiency of the quality effectTheFurther, the finely projected particles made of carbon steel or alloy steel can be obtained relatively inexpensively and are economical, and their hardness is Hv 1100 or less, and the surface roughness of the spring is harmful to such economy and durability. The upper limit hardness of new fine particles was set to Hv1100 in order to avoid the increase in the thickness and the fine cracks on the surface layer.
[0084]
(Reference example 3)
A spring manufactured from a steel wire that is processed and strengthened by wire drawing mainly made of fine pearlite is subjected to normal shot peening of a relatively large size without nitriding treatment, and then fine particle projection with a nominal diameter of 50 μm is performed. Manufactured by the methodTavernIs described below. A valve spring for an automobile internal combustion engine was prototyped using a piano wire having a diameter of 4.0 mm, a tensile strength, σB = 1,735 MPa, and an average hardness Hv of about 450. After coiling the piano wire in the cold, it was annealed for 15 minutes at 350 ° C. to remove the residual stress on the inner surface of the coil, and then the seat surface was polished. This was sufficiently projected with a cut wire having a diameter of 0.6 mm and a hardness of Hv550, followed by low-temperature annealing at 220 ° C. Subsequently, high-carbon steel particles having an average particle diameter of 37 μm, a maximum particle diameter of about 75 μm, a specific gravity of about 7.6, a hardness of Hv865, a maximum / minimum diameter ratio of 1.2 or less, and a non-angular shape are added at a speed of 107 m / sec. Projected enough. Subsequently, this was subjected to low-temperature annealing at 220 ° C., and further cold setting was performed. The outermost layer compressive residual stress of this spring was 590 MPa (FIG. 7).
[0085]
Comparison spring at this time(5)As aboveReference example 3A spring (other than that, the same material and process) was prepared by omitting only the 50 μm diameter fine particle projection of the spring. The compressive residual stress of the outermost layer is 430 MPa (FIG. 7)Is. Another comparison spring(6)As aboveReference example 3Instead of the second-stage projection of the spring, particles having a name of 100 μm were projected under the same conditions as the second-stage projection of Comparative Example 2.
Prototype in this wayReference example 3Fatigue tests of the valve spring and comparative spring were conducted. The test was performed at a rate of 1000 times / minute, and n = 15 springs were tested for each stress level. The result is as followsReference example 3The improvement effect of this spring over the comparative spring was clear. The former is, Cyclic stress τm ± τa, number of cycles: 5 × 10 ⁷ When τm = 690−x, τa ≧ 422 + x / 5 (3)
Here, unit: MPa, x: 0 to 140
(3) is satisfied, but the comparison spring(5)When(6)Does not meet it.

[0086]
Here, assuming that the maximum average stress applied to the coil spring is 690 MPa, and considering the compatibility between the average stress and the amplitude stress as described in the above equations (1) and (2), the average stress τm = For 690-x,Reference example 3The fatigue limit amplitude stress τa of this spring can be expressed as τa ≧ 440.6 + x / 5. Considering wire diameter, wire tensile strength, steel grade, etc.,underA spring that satisfies the formula (3)Reference example 3The compression residual stress of the very surface layer is 550 MPa or more.The
[0087]
When the average stress τm = 690−x, the number of repetitions is 5 × 10⁷Fatigue limit amplitude stress τa ≧ 422 + x / 5 (3)
Where x: 0 to 140
[0088]
These springs (comparative examples are(5)FIG. 7 showing the residual stress distribution of (only) shows that the residual stress of the surface layer portion from the outermost surface to the depth of 50 μm is greatly improved by the SS treatment. In addition, the surface roughness Rmax of these springs is 13.2 μm with 0.6 mm particle projection, and the average particle size after projection of 0.6 mm particle is 37 μm after average particle diameter.Reference example 3It was 9.2 μm in the spring by.
[0089]
In the above test,Reference example 3When the spring fatigue test stress was high, the spring sag increased slightly. In order to prevent this sagging, it is possible to use a steel wire with a cold-drawn pearlite structure with an element with high sag resistance, such as silicon and / or chromium, instead of a piano wire, and hot setting. As a countermeasureBe.
[0090]
(Reference example 4)
A valve spring was prototyped using a 3.2 mm diameter JIS SWOSC-V and an oil temper wire for the valve spring. This valve spring was manufactured by performing SS treatment without nitriding treatment.
[0091]
The manufacturing process of this valve spring is as follows. Spring coiling, low temperature annealing at 400 ° C. for 20 minutes, projection of iron-based round cut wire with a diameter of 0.6 mm at a speed of 70 m / sec, high carbon steel fine particle SS treatment (speed 107 m / sec, total particle average diameter 40 μm) , Maximum particle average diameter 75 μm), further low temperature annealing at 220 ° C. for 20 minutes, and finally cold setting. The compressive residual stress of the very surface layer of this spring was 1010 MPa. When the fatigue test of this spring was carried out, the average stress τm = 588 MPaN the number of repetitions N = 5 × 10⁷The fatigue limit amplitude stress at the time was 466 MPa. This stress can be expressed as τa = 445.6 + x / 5 when the average stress is set to 690−x. Considering the tensile strength variation, wire diameter range, etc. of SWOSC-V oil temper wireAndThe compression residual stress of the iron surface of the very surface layer is set to 900 MPa or more, and the fatigue strength of the spring is determined as in the following equation (4).The
[0092]
Average stress τm = 690−x, cyclic stress τa is 5 × 10⁷Fatigue limit stress in rotation is τa ≧ 440 + x / 5 (4)

Claims (4)

(A) nitriding the surface layer of the spring;
(B) Softer than the surface hardness of the nitrided spring (micro Vickers hardness at a depth of 5 μm from the outermost surface), hardness Hv 500 to 800, particle size 200 a step of the hard metal particles of ~900μm projected at 40m / sec~90m / sec, projection to prevent the surface layer of ChoHoso crack by (shot peening) is applied to the inside of it compressive residual stress Woba,
(C) On the spring surface after the step (B), the average diameter of all particles is 80 μm or less, and each particle has an average diameter of 10 μm or more and less than 100 μm, and there is no sphere or a sphere-like square shape as a shape. , Specific gravity 7.0 to 9.0, hardness Hv600 or more and Hv1100 or less, and the outermost layer hardness of the spring after nitriding or low temperature carbonitriding (micropickers hardness at a depth of 5 μm from the outermost surface) ) A large number of fine metal particles having the following hardness are projected at a speed of 50 to 190 m / sec, and the instantaneous temperature rise limit of the iron surface of the spring surface nitrided layer (excluding the nitrogen compound layer) due to the collision is determined. It causes work hardening of the spring surface layer, but by projecting it while controlling it at a lower temperature than softening due to recovery recrystallization, it effectively works to prevent the work hardening of the surface layer and prevent the occurrence of microcracks, resulting in high compressive residual stress and hardness. Give The surface treatment method of the spring and having a that step. (A) A specific gravity of 7.0-9. Which is a large number of spheres having a diameter of 10 μm or more and less than 100 μm , an average particle diameter of 80 μm or less, and an individual particle average diameter of 10 to 80 μm, or an angular portion close thereto. 0, hardness Hv 350-900 of metal particles such as iron-based metal at a collision speed of 50 m / sec or more and 160 m / sec or less, and the temperature rise limit of the spring surface due to the collision causes work hardening of the spring iron ground Projecting so as to control the temperature to be lower than causing recovery recrystallization and not causing fine cracks, and the like,
(B) nitriding the surface layer of the spring after the step (A);
(C) The surface of the nitrided spring is softer than the nitrided outermost layer hardness (micro Vickers hardness at a depth of 5 μm from the outermost surface), and has a hardness Hv of 500 to 800 and a particle size of 200 a step of the hard metal particles of ~900μm projected at 40m / sec~90m / sec, projection prevents fine cracks in the surface layer by (shot peening) is applied to the inside of it compressive residual stress Woba,
(D) On the spring surface after the step (C), the average diameter of the main particles is 80 μm or less, and each individual particle has an average diameter of 10 μm or more and less than 100 μm, and there are no spheres or horns near the sphere as the shape. , Specific gravity 7.0-9.0, hardness Hv600 or more and Hv1100 or less, and the outermost layer hardness of the spring after nitriding or low temperature carbonitriding (micropickers hardness at a depth of 5 μm from the outermost surface) ) A large number of fine metal particles having the following hardness are projected at a speed of 50 to 190 m / sec, and the instantaneous boundary temperature limit of the iron surface of the spring surface nitrided layer (excluding the nitrogen compound layer) due to the collision is determined by the spring. Although it causes work hardening of the surface layer, high compression residual stress and hardness are achieved by effectively preventing the work hardening of the surface layer and preventing the occurrence of microcracks by projecting while controlling to a lower temperature than softening due to recovery recrystallization. Grant The surface treatment method of the spring and having a that step. Oite to as engineering of (B) the claim 1, the first stage of the large 0.5~0.9mm diameter of the particles projection of dimension projection of hard metal particles 0.9mm diameter from 0.2 the surface treatment method of the spring which comprises carrying out separately the second stage projection of small 0.2~0.4mm diameters dimensions. A spring manufactured from a circular cross-section line or a modified cross-section line, the process of claim 1 as an essential process, and the manufactured chemical composition steel coil spring of the following (1), the compressive residual stress of iron due to the X-ray of the very surface layer Is larger than 1700 MPa, and the dimensions of the hard non-metallic inclusions, carbides, carbonitrides and nitrides that cause fatigue breakage of the spring and the hardness of the matrix satisfy X or Y of the following claims. A high fatigue strength spring having a fatigue strength at a repetition rate of 5 × 10 ⁷ that satisfies the following formula (1).
That is, when the cyclic stress is τm ± τa and τm = 800−x,
τa ≧ (620 + x / 5) (1)
Where τm: average stress, τa: amplitude stress,
x: Variable, 0 or more and 150 or less Unit: Both MPa
X: 0.2 mm or more from the spring surface when the dimension of hard non-metallic inclusions, carbides, carbonitrides and nitrides which are present in the spring and cause fatigue breakage of the spring is less than 20 μm to 15 μm or less The hardness of the base metal at a depth position up to 5 mm is controlled to Hv520 or more and 580 or less.
Y: 0.2 mm or more from the spring surface when the dimensions of hard non-metallic inclusions, carbides, carbonitrides and nitrides present in the spring that cause fatigue breakage of the spring can be controlled to 10 pm or less. The hardness of the base metal at the depth position up to 5 mm is controlled to Hv520 or more and 630 or less.
Here, the component steel (1) is as follows.
(1) C: 0.50 to 0.80%, Si: 1.20 to 2.5%, Mn: ≤ 1.20, Ni: ≤ 0.5%, Cr: ≤ 1.80 V to root if the balance being iron and impurities: spring with the addition of 0.03 to 0.60 percent.
(Chemical composition unit: Both mass%)

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