Compression coil spring and method for manufacturing the same

Abstract

The problem addressed by the present invention is to provide a compression coil spring the fatigue resistance of which is improved by decreasing the quantity of coarsened pro-eutectoid ferrite grains that can become fracture origins.　The present invention is a compression coil spring for which a steel rod with an equivalent circle diameter of 1.5–10.0 mm is used, wherein: a depth from the surface at which the value of the unloaded compressive residual stress is zero in roughly the direction of maximum principal stress when a compressive load is applied to the spring on the inner-diameter side of an arbitrary horizontal cross-section of an effective section is deemed a crossing point; and the compression coil spring has the following physical characteristics at the depth of the crossing point. (1) In a metallurgical microscope image captured at a magnification of 1000 after the metallic structure has been made to appear using an etching fluid, standardization of brightness values of an obtained observation gray-scale image is performed; and thereafter, if the mean brightness value is μ and the standard deviation for brightness values is σ, the equivalent circle diameter of the largest white structure after binarization processing has been performed with μ + 2σ as a threshold value for binarization is 10 μm or less. (2) The hardness is 500–700 HV.

Description

Technical Field

[0001] The present invention relates to a compression coil spring used, for example, in an automobile engine or clutch, and more particularly to a technique for improving durability by including ferrite while restricting its size.

Background Art

[0002] In recent years, against the backdrop of environmental problems, the demand for lower fuel consumption in automobiles has been growing stricter year by year, and there is an even stronger demand than ever for the miniaturization and weight reduction of automobile parts. In response to this demand for miniaturization and weight reduction, in compression coil springs such as valve springs used in engines and clutch torsion springs used in clutches, efforts have been made to improve fatigue resistance and sag resistance, which are important characteristics of coil springs.

[0003] Patent Document 1 proposes a high-tensile steel material with a strength of 1200 MPa or more and superior fatigue characteristics compared to conventional steel materials. In Patent Document 1, when the non-uniform structure is mainly composed of lath martensite, it is stated that by setting the area fraction of ferrite to 3% or less, it becomes difficult for the non-uniform structure to act as a starting point for internal fracture (paragraph 0020).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] However, even if the area fraction of ferrite is reduced to 3% or less, the presence of large ferrite grains cannot be avoided as they become the starting point for internal structural cracking. In hypoeutectoid steel, protereminate ferrite, which precipitates first from austenite, tends to coarseen depending on the heating conditions. However, the case where protereminate ferrite constitutes the majority of the heterogeneous structure has not been studied.

[0006] Against this backdrop, the present invention aims to improve fatigue resistance by reducing large protereminate ferrites that serve as fracture initiation points. [Means for solving the problem]

[0007] The inventors investigated the relationship between heating temperature during coil spring forming and durability, and found that lower heating temperatures resulted in superior durability. Furthermore, they discovered that when formed cold, fracture originates from the surface of the coil spring, while when formed hot, fracture originates from internal structural cracking within the coil spring.

[0008] When the inventors investigated the fracture site of a coil spring formed at a hot temperature, they found that the higher the heating temperature, the larger the microstructure at the fracture initiation point. This large microstructure was a soft structure with a hardness of approximately 470 HV, while the surrounding area was a hard structure with a hardness of approximately 610 HV. When the microstructure was etched with an etching solution, the large microstructure remained unetched and appeared white, indicating that it was proecution ferrite, while the surrounding microstructure was tempered martensite.

[0009] Based on the above, the inventors obtained a histogram with luminance values ​​on the x-axis from grayscale image data (raw data) of the tissue in order to quantify ferrite, which is a white tissue, from the tissue. This histogram was standardized to a normal distribution, and when the mean value of the luminance values ​​in the normal distribution was μ and the standard deviation was σ, the inventors performed binarization processing with μ + 2σ as the threshold to extract only the white tissue. From the extracted white tissue, the area ratio and the maximum equivalent diameter were determined, and it was found that the fracture rate in fatigue tests decreased significantly when the maximum equivalent diameter was 10 μm or less.

[0010] The present invention is based on the above findings and is a compression coil spring made of steel wire with an equivalent circle diameter of 1.5 to 10.0 mm, containing by mass 0.45 to 0.8% C, 0.15 to 3.0% Si, and 0.3 to 1.2% Mn, and also containing one or more of the following as optional components: 0.5 to 1.9% Cr, 1.5% or less Ni, 0.5% or less V, 1.5% or less Mo, and 0.5% or less W, with the remainder being iron and unavoidable impurities. The compression coil spring has the following physical properties at the crossing point depth, where the value of the unloaded compressive residual stress in the direction of approximately the maximum principal stress when a compressive load is applied to the spring is zero, on the inner diameter side of an arbitrary cross-section of the effective part of the compression coil spring.

[0011] (1) After revealing the metal structure using an etching solution, the luminance values ​​of the obtained grayscale image obtained from the metal microscope image taken at a magnification of 1000x were standardized, and when the mean value of the luminance values ​​was μ and the standard deviation of the luminance values ​​was σ, μ + 2σ was used as the threshold for binarization. The equivalent circle diameter of the largest white tissue after chemical treatment is 10 μm or less. (2) The average hardness is 500-700 HV.

[0012] The reasons for limiting the numerical ranges specified in this invention are explained below. First, the reasons for limiting the chemical composition of the steel wire used in this invention are explained. In this invention, a steel wire containing at least 0.45 to 0.8% C, 0.15 to 3.0% Si, and 0.3 to 1.2% Mn is used. In the following description of the components, "%" means "mass%".

[0013] (1) Material components C: 0.45~0.8% Carbon (C) contributes to increased strength. If the C content is less than 0.45%, the effect of increasing strength is not sufficient, resulting in insufficient fatigue resistance and deformation resistance. On the other hand, if the C content exceeds 0.8%, toughness decreases, making it more prone to cracking. Therefore, the C content should be between 0.45% and 0.8%.

[0014] Si: 0.15~3.0% Si is effective in deoxidizing steel and contributes to improved strength and resistance to tempering softening. These effects are not fully achieved when the Si content is less than 0.15%. On the other hand, if the Si content exceeds 3.0%, toughness decreases, making cracking more likely, and it also promotes decarburization, leading to a decrease in wire surface strength. Therefore, the Si content should be between 0.15% and 3.0%.

[0015] Mn: 0.3~1.2% Mn contributes to improved hardenability. If the Mn content is less than 0.3%, it becomes difficult to ensure sufficient hardenability, and the effect of sulfur adhesion (MnS formation), which is detrimental to ductility, is also diminished. On the other hand, if the Mn content exceeds 1.2%, ductility decreases, and cracks and surface scratches become more likely to occur. For this reason, the Mn content should be between 0.3% and 1.2%.

[0016] These additive elements are the minimum necessary elements for the present invention and do not limit the addition of other elements. In other words, in the present invention, one or more of the elements commonly used in the composition of spring steel—Cr at 0.5-1.9%, Ni at 1.5% or less, V at 0.5% or less, Mo at 1.5% or less, and W at 0.5% or less—can be added as appropriate depending on the purpose. As a result, it becomes possible to manufacture coil springs with higher performance or those more suitable for specific applications. For example, the case of adding Cr is described below.

[0017] Cr: 0.5~1.9% Cr is effective in preventing decarburization, and also contributes to improved strength and resistance to tempering softening, thus improving fatigue resistance. It is also effective in improving resistance to deformation at warm temperatures. For this reason, in this invention, it is preferable to further contain 0.5 to 1.9% Cr. If the Cr content is less than 0.5%, these effects cannot be fully obtained. On the other hand, if the Cr content exceeds 1.9%, toughness decreases, and cracks and surface scratches become more likely to occur.

[0018] (2) Crossing point depth Hot-formed compression coil springs fracture on the inner diameter side of the coil where repeated tensile stress acts. According to the inventor's research, the crossing point is defined as the depth from the surface at which the value of the unloaded compressive residual stress in the direction of approximately the maximum principal stress when a compressive load is applied to the spring, and it has been found that many fracture initiations occur at the crossing point depth on the inner diameter side of the coil on an arbitrary cross section of the effective portion of the compression coil spring. The crossing point depth is preferably 0.10 mm or more from the surface when using wire with a diameter of 1.5 mm or more and less than 3.0 mm, 0.2 mm or more from the surface when using wire with a diameter of 3.0 mm or more and less than 5.0 mm, and 0.25 mm or more from the surface when using wire with a diameter of 5.0 mm or more and less than 10.0 mm. In this invention, the following physical characteristics are defined at the crossing point depth on the inner diameter side of the coil.

[0019] (3) Equivalent diameter of the largest white tissue: 10 μm or less The white structure consists of ferrite that appeared white without being corroded by the etching solution. If ferrite grains with an equivalent circular diameter exceeding 10 μm are present, these grains become the starting point for fracture in the compression coil spring, reducing its fatigue resistance. Therefore, the equivalent circular diameter of the largest ferrite grain was limited to 10 μm or less.

[0020] (4) Average hardness: 500~700HV For valve springs, clutch torsion springs, and other coil springs used under high load stress, the strength of the wire itself is important in order to satisfy the required fatigue resistance and sag resistance. Specifically, the crossing point is defined as the depth from the surface where the value of the unloaded compressive residual stress in the direction of approximately the maximum principal stress when a compressive load is applied to the inner diameter side of the coil spring is zero. The average Vickers hardness at the depth of the crossing point from the inner diameter side surface of the coil in any cross section of the effective part of the compression coil spring must be in the range of 500 to 700 HV. If it is less than 500 HV, sufficient fatigue resistance and sag resistance cannot be obtained due to the low material strength.

[0021] Also, when it exceeds 700 HV, due to the increase in notch sensitivity associated with the decrease in toughness, the risk of early fracture due to surface scratches generated by rubbing against tools during coiling or cracks starting from the valleys of the wire surface roughness formed by shot peening increases, making it unsuitable for use as automotive parts where reliability is important. Therefore, on the inner diameter side of the coil spring, the depth from the surface where the value of the compressive residual stress at no load in the direction of the substantially maximum principal stress when a compressive load is applied to the spring is zero is defined as the crossing point, and the average Vickers hardness at the crossing point depth position from the inner diameter side surface of an arbitrary cross-section of the effective part of the compression coil spring is set to 500 - 700 HV.

[0022] (4) Area ratio of white structure: 3.0 - 5.0% The present invention has a structure containing a white structure (ferrite grains). Ferrite grains have the effect of improving the toughness of the compression coil spring, and it is necessary to have 3.0 area% or more to obtain this effect. On the other hand, when the area ratio of ferrite exceeds 5.0%, a hardness of 500 HV or more cannot be obtained. Therefore, the area ratio of ferrite is preferably 3.0 - 5.0%. As the structure other than ferrite, all may contain tempered martensite or 0.1 - 8.0 area% of retained austenite.

[0023] (5) Spring index: 3 - 8 The present invention is a hot-formed compression coil spring, and the equivalent circle diameter of the wire (the diameter when calculated as a perfect circle from the cross-sectional area of the wire, including non-circular cross-sections such as square and oval) is 1.5 - 10.0 mm, and the spring index is, for example, 3 - 8, and it can be suitably applied to generally cold-formed compression coil springs.

[0024] Among them, it can be suitably applied to compression coil springs with an equivalent circle diameter of 1.5 - 10.0 mm used in valve springs, clutch torsion springs, etc., where the degree of processing during coiling is large (that is, in cold forming, the tensile residual stress on the inner diameter side of the coil generated by coiling is large) and high fatigue resistance is required.

[0025] Furthermore, the coil spring shape in this invention can be applied to various shapes of coil springs, including the cylindrical shape, which is typical for coil springs as the outer diameter of the coil remains virtually unchanged throughout all turns. For example, it is possible to form irregularly shaped springs such as conical, bell-shaped, drum-shaped, and barrel-shaped springs.

[0026] (6) Surface roughness Rz: 20 μm or less For valve springs, clutch torsion springs, and other components used under high load stress, surface roughness, along with the compressive residual stress distribution mentioned above, is important to satisfy the required fatigue resistance. Our inventors have conducted fracture mechanics calculations and verification experiments, and have found that the effect of crack initiation and propagation originating from the surface can be neutralized by setting the depth of surface scratches (i.e., surface roughness Rz (maximum height)) to 20 μm or less. Therefore, a surface roughness Rz of 20 μm or less is preferable. If Rz exceeds 20 μm, the valleys in the surface roughness become sources of stress concentration, making it easier for cracks to initiate and propagate from these valleys, leading to premature fracture.

[0027] The present invention can utilize hard-drawn wires such as carbon steel wire, hard steel wire, piano wire, and spring steel wire used as spring materials, as well as oil-tempered wires such as carbon steel oil-tempered wire, chromium-vanadium steel oil-tempered wire, silicon-chromium steel oil-tempered wire, and silicon-chromium-vanadium steel oil-tempered wire.

[0028] (7) Manufacturing method The present invention relates to a method for manufacturing a compression coil spring, comprising: heating a steel wire rod with an equivalent diameter of 1.5 to 10.0 mm, which contains, by weight 0.45 to 0.8% C, 0.15 to 3.0% Si, and 0.3 to 1.2% Mn, and as an optional component, one or more of the following: 0.5 to 1.9% Cr, 1.5% or less Ni, 0.5% or less V, 1.5% or less Mo, and 0.5% or less W, with the remainder being iron and unavoidable impurities, from room temperature to 900 to 950°C in 2.5 seconds or less using a high-frequency heating coil, holding for 0.5 to 2.0 seconds, then coiling using a coiling tool, and performing quenching and tempering to impart the following physical properties.

[0029] (1) After revealing the metal structure using an etching solution, the brightness values ​​of the obtained grayscale images were standardized in the metal microscope images taken at a magnification of 1000x, When the mean value of the brightness is μ and the standard deviation of the brightness is σ, then μ + 2σ is the threshold for binarization. The equivalent circle diameter of the largest white tissue after binarization is 10 μm or less. (2) The hardness is 500-700 HV.

[0030] In this invention, a steel wire is heated from room temperature to 900-950°C in 2.5 seconds or less using a high-frequency heating coil, held for 0.5-2.0 seconds, then coiled using a coiling tool and hardened. In particular, by limiting the heating temperature to 900-950°C, the coarsening of proejecta ferrite can be suppressed, and the maximum circular diameter of the ferrite structure can be limited to 10 μm or less. [Effects of the Invention]

[0031] According to the present invention, a compression coil spring with improved fatigue resistance can be obtained by reducing the proterecution ferrite structure, which is the starting point for fracture. [Brief explanation of the drawing]

[0032] [Figure 1] (A) is a micrograph of the compression coil spring of an embodiment of the present invention at a depth of 0.2 mm from the inner diameter side surface of the coil, and (B) is a histogram of the brightness in (A). [Figure 2] (A) is a standardized tissue photograph of Figure 1(A), and (B) is a histogram of brightness in (A). [Figure 3] (A) is a binarized tissue image of Figure 2(A), and (B) is a histogram of the brightness in (A). [Examples]

[0033] (1) Manufacturing of compression coil springs Oil-tempered wire with a diameter of 4.1 mm and consisting of the chemical components listed in Table 1 was prepared. Then, a compression coil spring with a spring index of 6.00, an effective number of turns of 3.25, and a total number of turns of 5.75 was fabricated from the oil-tempered wire by hot coiling.

[0034] [Table 1]

[0035] In hot coiling, the material was heated to the temperatures shown in Table 2 within 2.5 seconds using a coiling device equipped with a high-frequency heating coil, and held for the time shown in Table 2. After coiling, it was quenched by immersion in quenching oil. Subsequently, tempering was performed at the temperatures listed in Table 2 (Examples 1-5, Comparative Examples 1 and 2).

[0036] [Table 2]

[0037] (2) Physical properties The physical properties of the samples obtained in this manner were investigated as follows. The results are shown in Table 2.

[0038] 1. Equivalent circle diameter and area ratio of the largest white tissue. Each sample was cut, an arbitrary cross-section of the effective portion of the coil spring was polished, and it was corroded with Nital solution. The area 0.2 mm deep from the surface on the inner diameter side of the coil was observed under a metallurgical microscope at 1000x magnification. Here, the 0.2 mm depth is the crossing point depth, where the value of the unloaded compressive residual stress in the direction of approximately the maximum principal stress when a compressive load is applied to the spring is zero, on the inner diameter side of a 4.1 mm wire spring. The observed image is shown in Figure 1(A). In Figure 1(A), the white areas are ferrite that was not corroded with Nital solution, and the surrounding area is tempered martensite. Figure 1(B) is a histogram obtained by image analysis of the observed grayscale image, with brightness on the horizontal axis and frequency on the vertical axis.

[0039] Next, the raw data shown in Figure 1(A) was standardized using the following procedure. The original data with brightness values ​​between 0 and 255 was standardized so that the mean μ of the brightness values ​​was 0 and the standard deviation σ was 1. In this process, values ​​that were outside ±3σ of the mean μ were treated as ±3σ.

[0040] Next, 3 was added to the entire brightness value so that the minimum brightness value would be 0. This resulted in a minimum brightness value of 0 and a maximum brightness value of 6. Then, the entire value was divided by 6 and multiplied by 255. This resulted in brightness values ​​ranging from 0 to 255.

[0041] Figure 2(A) shows the tissue image standardized as described above, and Figure 2(B) shows the standardized tissue image after image analysis, with brightness on the horizontal axis. frequency This is a histogram with the vertical axis representing the value of the data.

[0042] Next, the standardized data was binarized using the following procedure. When the mean value of the standardized luminance values ​​was μ and the standard deviation was σ, μ+2σ was set as the threshold (vertical line in Figure 2(B)). Then, the area where the luminance value is less than μ+2σ (left side of the vertical line) was set to black (luminance value 0), and the area where the luminance value is μ+2σ or greater (right side of the vertical line) was set to white (luminance value 255), and binarization was performed. Figure 3(A) is the tissue image after binarization and consists only of white and black areas. Figure 3(B) is a histogram obtained by image analysis of the binarized tissue image, with luminance on the horizontal axis and frequency on the vertical axis.

[0043] Next, the binarized tissue images were analyzed to determine the area ratio of the white areas (ferrite), and a histogram of the equivalent circle diameter of the white areas was obtained. This allowed us to obtain the equivalent circle diameter of the largest white area. Table 2 shows the ferrite grain area ratio and the equivalent circle diameter of the largest ferrite grain for each sample.

[0044] 2. Hardness Using a Vickers hardness tester (FutureTech FM-600), measurements were taken at a load of 200 gf at five locations 0.2 mm from the surface on the inner diameter side of any cross-section of the effective portion of the coil spring, and the average value was calculated. The calculation results are shown in Table 2.

[0045] (3) Fatigue resistance (fracture rate) Fatigue tests were conducted at room temperature (in air) using a hydraulic servo-type fatigue testing machine (Sagimiya Seisakusho). The test stress was set to 735±686 MPa for steel grades A and B, 735±711 MPa for steel grades C and D, and 735±637 MPa for steel grade E. The test frequency was 20 Hz, and eight samples were tested for each grade. Fatigue resistance was evaluated by the fracture rate (number of fractures / number of samples) after 30 million vibration cycles.

[0046] As shown in Table 2, in Invention Examples 1 to 5, the maximum ferrite grain circle equivalent diameter at a depth of 0.2 mm from the surface on the inner diameter side of any cross-sectional area of ​​the effective portion of the coil spring was 10 μm or less, the ferrite grain area ratio was 3.0 to 5.0%, and the hardness was 500 to 700 HV. As a result, the fracture rate was 0 / 8.

[0047] In contrast, in Comparative Example 1, the maximum ferrite grain circle equivalent diameter at a depth of 0.2 mm from the inner diameter surface of the coil was 10.4 μm, and the hardness was 470 HV. As a result, two out of eight compression coil springs broke.

[0048] In Comparative Example 2, the maximum ferrite grain circle equivalent diameter at a depth of 0.2 mm from the inner diameter surface of the coil was 11.5 μm, and as a result, 5 out of 8 compression coil springs broke.

[0049] Furthermore, in Comparative Examples 1 and 2, the quenching temperature (heating temperature by the high-frequency heating coil) exceeded 950°C, causing the proteolytic ferrite to coarseen, resulting in the maximum ferrite grain diameter exceeding 10 μm, which led to the fracture rate described above. [Industrial applicability]

[0050] This invention can be used for valve springs used in engines, and compression coil springs including clutch torsion springs used in clutches.

Claims

1. A compression coil spring made of steel wire with an equivalent diameter of 1.5 to 10.0 mm, containing, by weight percent, 0.45 to 0.8% C, 0.15 to 3.0% Si, and 0.3 to 1.2% Mn, and also containing one or more of the following optional components: 0.5 to 1.9% Cr, 1.5% or less Ni, 0.5% or less V, 1.5% or less Mo, and 0.5% or less W, with the remainder being iron and unavoidable impurities. A compression coil spring having the following physical characteristics at the crossing point, where, on the inner diameter side of an arbitrary cross-section of the effective portion of the compression coil spring, the depth from the surface at which the value of the unloaded compressive residual stress in the direction of approximately the maximum principal stress when a compressive load is applied to the spring is zero. (1) After revealing the metal structure using an etching solution, the brightness values ​​of the obtained observation images were standardized in the metal microscope images taken at a magnification of 1000x, When the mean value of the brightness is μ and the standard deviation of the brightness is σ, the equivalent circle diameter of the largest white tissue corresponding to ferrite in the tissue image after binarization processing with μ + 2σ as the binarization threshold is 10 μm or less. (2) The hardness is 500 to 700 HV.

2. The compression coil spring according to claim 1, wherein the area ratio of white tissue in the binarized tissue image is 3.0 to 5.0%.

3. The compression coil spring according to claim 2, wherein the remainder of the tissue image after binarization, other than the white tissue, is tempered martensite or retained austenite contained in tempered martensite in an area percentage of 0.1 to 8.0%.

4. A compression coil spring according to any one of claims 1 to 3, wherein the spring index is 3 to 8.

5. A compression coil spring according to any one of claims 1 to 3, wherein the surface roughness Rz is 20 μm or less.

6. A method for manufacturing a compression coil spring, comprising: heating a steel wire rod with an equivalent diameter of 1.5 to 10.0 mm, which is made up of 0.45 to 0.8% by weight of C, 0.15 to 3.0% of Si, and 0.3 to 1.2% of Mn, and also containing one or more of the following as optional components: 0.5 to 1.9% of Cr, 1.5% or less of Ni, 0.5% or less of V, 1.5% or less of Mo, and 0.5% or less of W, with the remainder being iron and unavoidable impurities, from room temperature to 900 to 950°C in 2.5 seconds or less using a high-frequency heating coil, holding for 0.5 to 2.0 seconds, then coiling using a coiling tool, quenching and tempering, and defining the crossing point as the depth from the surface at which the value of the unloaded compressive residual stress in the direction of approximately the maximum principal stress when a compressive load is applied to the spring is zero, on the inner diameter side of an arbitrary cross-section of the effective part of the compression coil spring, wherein the following physical properties are imparted at the crossing point depth position. (1) After revealing the metal structure using an etching solution, the brightness values ​​of the obtained observation images were standardized in the metal microscope images taken at a magnification of 1000x, When the mean value of the brightness is μ and the standard deviation of the brightness is σ, the equivalent circle diameter of the largest white tissue corresponding to ferrite in the tissue image after binarization processing with μ + 2σ as the binarization threshold is 10 μm or less. (2) The hardness is 500 to 700 HV.

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