Compression coil spring

Abstract

To provide a compression coil spring preventing a breakage starting from an inclusion through reducing a distortion between crystal grains and a distortion in a crystal grain so as to improve a fatigue resistance.SOLUTION: A compression coil spring has, at locations 0.2 mm deep and d / 4 deep (d being a wire diameter) from a surface on a coil inner diameter side of an arbitrary cross section in an effective part of the compression coil spring, physical characteristics as below: (1) in a measurement of a GOS (Grain Oriented Spread) map by a SEM / EBSD (Electron Back Scatter Diffraction) method, an area ratio of grains with a GOS value less than 3° is 85% or more; (2)in a measurement of a KAM (Karnel Average Misorientation) map by the SEM / EBSD method, an area ratio of pixels with a KAM value less than 3° is 95% or more; and (3) hardness is 580 to 700 HV.SELECTED DRAWING: None

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 compression coil spring having excellent fatigue resistance even in a usage environment under high stress. [Background technology]

[0002] In recent years, with the backdrop of environmental issues, the demand for improved fuel efficiency in automobiles has been getting stricter every year, and there is a stronger demand than ever for automobile parts to be made smaller and lighter. In response to this demand for smaller and lighter parts, for example, for compression coil spring parts such as valve springs used in engines and clutch torsion springs used in clutches, there has been active research into increasing the strength of materials and strengthening the surfaces through surface treatments, and as a result, efforts have been made to improve fatigue resistance and settling resistance, which are important properties of coil springs.

[0003] With the advancement of high-strength materials and surface strengthening technologies such as nitriding and shot peening, the initiation point of fatigue fracture has shifted from the surface to the interior. It is also known that most internal initiation points are caused by inclusions.

[0004] To prevent internally initiated fracture, it is effective to impart compressive residual stress. For example, Patent Document 1 claims that by heating a wire rod to the austenite region by high-frequency heating and then coiling it, it is possible to prevent the generation of residual stress due to processing and obtain excellent fatigue resistance. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Patent No. 5064590 Summary of the Invention [Problem to be solved by the invention]

[0006] However, it has been found that internal fracture originating from inclusions cannot be reduced to zero even if compressive residual stress is applied as in the technology described in Patent Document 1. In other words, it has been found that even if compressive residual stress is applied, the probability of inclusion breakage increases when the strain between crystal grains or within crystal grains is large.

[0007] Therefore, an object of the present invention is to provide a compression coil spring that reduces strain between and within crystal grains, thereby suppressing breakage originating from inclusions and improving fatigue resistance.

[0008] The inventors have learned from the above phenomenon that when the strain between crystal grains and the strain within crystal grains are small, the probability of inclusion breakage decreases, and have found that it is possible to reduce the strain between crystal grains and the strain within crystal grains by combining appropriate high-frequency heating conditions, coiling conditions, and quenching and tempering conditions. [Means for solving the problem]

[0009] The present invention has been made based on the above findings, and is a compression coil spring using a steel wire having a circle equivalent diameter d of 1.5 to 10.0 mm containing, by weight, 0.45 to 0.8% C, 0.15 to 3.0% Si, 0.3 to 1.2% Mn, and one or more of 0.5 to 2.0% Cr, 1.5% or less Ni, 0.5% or less V, 1.5% or less Mo, and 0.5% or less W as optional components, with the balance being iron and unavoidable impurities, wherein the compression coil spring has the following physical properties at a depth of 0.2 mm and a depth of d / 4 (d is the wire diameter) from the surface of the inner diameter side of the coil in an arbitrary cross section of an effective portion of the compression coil spring. (1) When a GOS (Grain Oriented Spread) map is measured using the SEM / EBSD (Electron Back Scatter Diffraction) method, the area ratio of crystal grains with a GOS value of less than 3° is 85% or more. (2) When measuring the KAM (Kernel Average Misorientation) map using the SEM / EBSD method, the area ratio of pixels with a KAM value of less than 3° is 95% or more. (3) Hardness is 580 to 700 HV.

[0010] Here, the GOS value is an index that represents the strain gradient between grains, where the average misorientation between all pixels in each grain is the value of that grain. <3° In the present invention, GOS <3° The area ratio of the crystal grains in the observation field to the whole observation field is regulated to be 85% or more. <3° When the area ratio of the crystal grains to the entire observation field is 85% or more, it means that a structure with little distortion between crystal grains has been obtained.

[0011] The KAM value is an index that represents the strain gradient within a crystal grain, calculated by calculating the average misorientation between the pixel of interest and the six adjacent pixels and setting this value as the value of the central pixel. <3° In the present invention, KAM <3° The area ratio of the pixels in the entire observation field of view to be 95% or more. <3° When the area ratio of the pixels to the entire observation field is 95% or more, this means that crystal grains with reduced strain within the crystal grains have been obtained.

[0012] The reasons for limiting the numerical ranges stipulated in the present invention will be explained below. First, the reasons for limiting the chemical components of the steel wire rod used in the present invention will be explained. In the present invention, a steel wire rod containing at least 0.45-0.8% C, 0.15-3.0% Si, and 0.3-1.2% Mn is used. In the following explanation, "%" means "% by weight".

[0013] (1) Material composition C: 0.45-0.8% C contributes to improving strength. If the C content is less than 0.45%, the effect of improving strength is not sufficient, and fatigue resistance and sag resistance become insufficient. On the other hand, if the C content exceeds 0.8%, toughness decreases and cracks are more likely to occur. For this reason, the C content is set to 0.45 to 0.8%.

[0014] Silicon: 0.15 to 3.0% Silicon is effective in deoxidizing steel and also contributes to improving strength and temper softening resistance. If the silicon content is less than 0.15%, these effects are not fully obtained. On the other hand, if the silicon content exceeds 3.0%, toughness decreases, cracks become more likely to occur, and decarburization is promoted, resulting in a decrease in wire surface strength. For this reason, the silicon content is set to 0.15 to 3.0%. The silicon content is preferably 0.5% or more, and more preferably 1.0% or more.

[0015] Mn: 0.3-1.2% Mn contributes to improving hardenability. If the Mn content is less than 0.3%, it becomes difficult to ensure sufficient hardenability, and the effect of fixing S (MnS formation), which is harmful to ductility and toughness, becomes poor. On the other hand, if the Mn content exceeds 1.2%, ductility decreases and cracks and surface scratches are likely to occur. For this reason, the Mn content is set to 0.3 to 1.2%.

[0016] These added elements are the minimum elements necessary to constitute the present invention, and do not limit the addition of other elements. That is, in the present invention, one or more of the generally used components of spring steel, Cr 0.5 to 2.0%, Ni 1.5% or less, V 0.5% or less, Mo 1.5% or less, and W 0.5% or less, can be appropriately added according to the purpose, and as a result, it is possible to manufacture a coil spring with higher performance or more suitable for the application. For example, the case of adding Cr will be described below.

[0017] Cr: 0.5~2.0% Cr is effective in preventing decarburization, and also contributes to improving strength and temper softening resistance, and is effective in improving fatigue resistance. It is also effective in improving sag resistance in warm conditions. For this reason, in the present invention, it is preferable to further contain 0.5 to 2.0% Cr. If the Cr content is less than 0.5%, these effects cannot be sufficiently obtained. On the other hand, if the Cr content exceeds 2.0%, the toughness decreases, and cracks and surface scratches are likely to occur.

[0018] (2) GOS <3° In the present invention, the GOS map was measured by the SEM / EBSD method at a depth of 0.2 mm and a depth of d / 4 (d is the wire diameter) from the inner diameter surface of the coil of an arbitrary cross section of the effective part of a compression coil spring. <3° The area ratio of the crystal grains is 85% or more. The GOS value is small, less than 3°, that is, the strain between the crystal grains is small, so the inclusion sensitivity is reduced and compressive residual stress is easily generated. Therefore, since the area ratio of such crystals is 85% or more, fatigue resistance can be improved.

[0019] (3) KAM <3° In the present invention, the KAM map was measured by the SEM / EBSD method at a depth of 0.2 mm and a depth of d / 4 from the inner diameter surface of the coil of an arbitrary cross section of the effective part of a compression coil spring. <3° The area ratio of pixels with this angle is 95% or more. The KAM value is small, less than 3°, i.e., the distortion between pixels is small, which reduces inclusion sensitivity and makes it easier for compressive residual stress to occur. Therefore, since the area ratio of such pixels is 95% or more, fatigue resistance can be improved.

[0020] (4) Hardness For valve springs, clutch torsion springs, and the like used under high load stress, the strength of the wire itself is also important in order to satisfy the required fatigue resistance and sag resistance. That is, the hardness at a depth of 0.2 mm and a depth of d / 4 from the coil inner diameter side surface of any cross section of the effective part of the compression coil spring must be 580 to 700 HV. If the hardness is in this range, sufficient compressive residual stress can be obtained at a depth of 0.1 to 0.4 mm, which is the fracture origin. Therefore, fracture at the internal origin is prevented, and high fatigue resistance can be obtained.

[0021] If the hardness is less than 580HV, sufficient fatigue resistance and settling resistance cannot be obtained due to the low material strength. On the other hand, if the hardness exceeds 700HV, the notch sensitivity increases due to the decrease in toughness, and the risk of early breakage due to cracks originating from surface scratches caused by rubbing with tools during coiling or valleys in the wire surface roughness formed by shot peening increases, making it unsuitable for use as an automobile part where reliability is important. It is more preferable that the hardness is 580HV to 690HV.

[0022] (5) Average grain size It is preferable that the average grain size is 2.0 μm or less when an IPF map (boundaries with an orientation angle difference of 3° or more are regarded as grain boundaries) is measured at a depth of 0.2 mm and / or a depth of d / 4 from the inner diameter surface of the coil in any cross section of the effective portion of the compression coil spring by the SEM / EBSD method. If the average grain size exceeds 2.0 μm, it becomes difficult to obtain sufficient fatigue resistance. And, a small average grain size, i.e., blocks and laths in prior γ grains are fine, is favorable for improving fatigue resistance because it has a high resistance to crack propagation.

[0023] (6) Surface roughness For valve springs, clutch torsion springs, and the like used under high load stress, in order to satisfy the required fatigue resistance, the above-mentioned compressive residual stress distribution as well as surface roughness are important. As a result of the inventor's fracture mechanics calculation and verification experiment, it has been found that the effect of cracks originating from the surface and their propagation can be neutralized by making the depth of the surface scratches (i.e., surface roughness Rz (maximum height)) 20 μm or less. For this reason, it is preferable that the surface roughness Rz is 20 μm or less. If Rz exceeds 20 μm, the valleys of the surface roughness become a source of stress concentration, and cracks are likely to occur and propagate with the valleys as the surface origin, which makes it easier for early breakage to occur.

[0024] Surface roughness is formed by friction with tools during coiling and by shot peening. The magnitude of surface roughness formed by shot peening is determined by a combination of conditions such as the hardness of the wire and the grain size, hardness, and projection speed of the shot. Therefore, it is necessary to set the shot peening conditions appropriately so that Rz does not exceed 20μm relative to the hardness of the wire.

[0025] (7) Residual stress Fatigue resistance can be improved by imparting compressive residual stress to the vicinity of the wire surface of the spring. The compressive residual stress is in the direction of the approximate maximum principal stress when a compressive load is applied to the spring, i.e., in the direction at +45° to the axial direction of the wire at the above-mentioned specified depth from the inner diameter side surface of the coil.

[0026] The compressive residual stress distribution from the wire surface to the inside of the spring is given by shot peening and, if necessary, setting. Shot peening is performed to make the wire uneven from the surface to a depth of, for example, 20 μm or less, and the compressive residual stress is imparted to the deep part by the plastic processing. In particular, it is important to further increase the compressive residual stress in the range of about 0.1 to 0.4 mm deep, which is the actual starting point of the fracture. As an index showing the magnitude of the compressive residual stress inside the wire, the crossing point is preferably 0.10 mm or more deep from the surface when a wire with a wire diameter of 1.5 mm or more and less than 3.0 mm is used, 0.20 mm or more deep from the surface when a wire with a wire diameter of 3.0 mm or more and less than 5.0 mm is used, and 0.25 mm or more deep from the surface when a wire with a wire diameter of 5.0 mm or more is used. The crossing point is the depth from the surface at which the compressive residual stress becomes zero when no load is applied inside the wire, and a deep crossing point suggests that the compressive residual stress penetrates deep from the surface. In either case, the maximum compressive residual stress on the surface of the inner diameter side of the spring when no load is applied value In the case of valve springs and clutch torsion springs, which are subjected to high load stress, the compressive residual stress is preferably 620 MPa or more. By setting the compressive residual stress distribution in this manner, a compression coil spring with excellent fatigue resistance can be obtained.

[0027] When multiple shot peening is performed in the shot peening process, the sphere-equivalent diameter of the shot used in the subsequent shot peening is preferably smaller than the sphere-equivalent diameter of the shot used in the previous shot peening. Specifically, the shot peening process is preferably a multiple shot peening process consisting of a first shot peening process using shots with a particle size of 0.6 to 1.2 mm, a second shot peening process using shots with a particle size of 0.2 to 0.8 mm, and a third shot peening process using shots with a particle size of 0.02 to 0.50 mm. This allows the surface roughness increased by the previous shot peening process to be reduced by the subsequent shot peening process.

[0028] The shot diameter and number of stages in the shot peening treatment are not limited to those mentioned above, and may be any value that provides the required residual stress distribution, surface roughness, etc., depending on the required performance. Therefore, the shot diameter, material, number of stages, etc. are appropriately selected. In addition, the distribution of compressive residual stress induced also differs depending on the projection speed and projection time, so these are also appropriately set as necessary.

[0029] (8) Coil spring shape The present invention is suitable for compression coil springs with the following specifications, which require a large degree of working during coiling and require both high fatigue resistance and sag resistance: The present invention can be used for compression coil springs that are generally cold-formed and have a wire equivalent circle diameter (the diameter calculated from the wire cross-sectional area when the wire is treated as a perfect circle, including non-circular cross sections such as rectangular and oval) of 1.5 to 10 mm and a spring index of 3 to 20.

[0030] In particular, it is suitable for compression coil springs with a circle equivalent diameter of 1.5 to 10.0 mm and a spring index of 3 to 8 that are used in valve springs, clutch torsion springs, etc., which require a large degree of processing during coiling (i.e., in cold forming, the tensile residual stress generated on the inner diameter side of the coil due to the coiling process is large) and high fatigue resistance.

[0031] Also, unlike conventional hot forming methods, the coiling process of the present invention does not require a core bar because a coil spring forming machine, which will be described later, is used. Therefore, there is a high degree of freedom in the shape of the spring that can be formed. In other words, the shape of the coil spring in the present invention can be applied to coil springs of other shapes, including cylindrical shapes, which are typical of coil springs and have almost no change in the coil outer diameter over the entire turn. For example, it is also possible to form irregularly shaped springs, such as conical, bell, drum, and barrel shapes.

[0032] In addition, the compression coil spring of the present invention is desirably formed by a coil spring forming machine having a feed roller for continuously supplying steel wire material, a coiling section for forming the steel wire material into a coil shape, and cutting means for coiling the steel wire material a predetermined number of turns and then separating the steel wire material continuously supplied from the rear, the coiling section including a wire guide for guiding the steel wire material supplied by the feed roller to an appropriate position, a coiling tool consisting of a coiling pin or coiling roller for processing the steel wire material supplied via the wire guide into a coil shape, and a pitch tool for imparting a pitch, and further including heating means for raising the temperature of the steel wire material to the austenite region within 2.5 seconds between the outlet of the feed roller and the coiling tool, and then subjected to a shot peening treatment.

[0033] In the compression coil spring of the present invention, it is preferable that the heating means is high-frequency heating, and a high-frequency heating coil is arranged concentrically with the steel wire on the passage path of the steel wire in the wire guide or on the passage path of the steel wire in the space between the outlet end of the steel wire in the wire guide and the coiling tool. Note that since it is sufficient to heat the steel wire to the austenite range in a short time, heating may be performed by electrical heating or laser heating other than high-frequency heating.

[0034] The compression coil spring of the present invention can be manufactured by a manufacturing method that sequentially performs the following steps: a coiling step of forming the steel wire using a coil spring forming machine having a feed roller for continuously supplying the steel wire, a coiling section for forming the steel wire into a coil shape, and cutting means for coiling the steel wire by a predetermined number of turns and then separating the steel wire continuously supplied from the rear, the coiling section including a wire guide for guiding the steel wire supplied by the feed roller to an appropriate position, a coiling tool consisting of a coiling pin or a coiling roller for processing the steel wire supplied via the wire guide into a coil shape, and a pitch tool for providing a pitch, and further including heating means for raising the temperature of the steel wire to the austenite range within 2.5 seconds between the outlet of the feed roller and the coiling tool, a quenching step of quenching the coil that has been separated after coiling and whose temperature is still in the austenite range, a tempering step of tempering the coil, a shot peening step of imparting compressive residual stress to the surface of the wire, and a setting step.

[0035] In the present invention, the original purpose of the tempering process is to temper the coil hardened by the quenching process to a coil having appropriate hardness and toughness. In the present invention, the tempering process is performed to reduce the distortion between crystal grains and the distortion within the crystal grains. For this reason, the tempering temperature is set to 440 to 460°C. If the tempering temperature is less than 440°C, the distortion between the crystal grains remains and the distortion within the crystal grains is not sufficiently reduced. On the other hand, if the tempering temperature exceeds 460°C, the hardness at a depth of 0.2 mm and a depth of d / 4 from the inner diameter side surface of the coil does not reach 580HV, and the fatigue resistance cannot be improved.

[0036] Low temperature aging treatment for the purpose of recovering the surface elastic limit may be combined as necessary. Here, the low temperature aging treatment can be performed after the shot peening process or between each stage of the multi-stage shot peening. When the shot peening with a grain size of 0.02 to 0.30 mm is performed as the final stage of the multi-stage shot peening, it is preferable to perform the low temperature aging treatment as a pretreatment in order to further increase the compressive residual stress of the outermost surface. In addition, there are various methods such as cold setting and hot setting as the setting performed on the coil as a settling prevention treatment in the setting process, and the setting method is appropriately selected according to the desired characteristics.

[0037] By hot coiling as in the above manufacturing method, it is possible to prevent the generation of residual stress due to processing. And by heating the steel wire rod to the austenite region within 2.5 seconds, it is possible to prevent the crystal grains from becoming coarse, and it is possible to obtain excellent fatigue resistance. In addition, because hot coiling is performed, it is possible to obtain excellent fatigue resistance without carrying out a surface strengthening treatment such as a nitriding treatment, and it is possible to reduce manufacturing costs.

[0038] The present invention can be applied to hard-drawn wires used as spring materials, such as carbon steel wire, hard steel wire, piano wire, and spring steel wire, as well as oil-tempered wires, such as carbon steel oil-tempered wire, chrome vanadium steel oil-tempered wire, silicon chrome steel oil-tempered wire, and silicon chrome vanadium steel oil-tempered wire. In particular, the present invention is suitable for application to inexpensive hard-drawn wires. This is because, by using hot forming using high-frequency heating, even if inexpensive wire material is used, it is possible to obtain a spring with better fatigue resistance than a conventional cold-formed spring that uses an expensive oil-tempered wire to which high-grade elements are added. Effect of the Invention

[0039] According to the present invention, the strain between crystal grains and the strain within crystal grains can be reduced, so that breakage originating from inclusions can be suppressed and fatigue resistance can be improved. [Brief description of the drawings]

[0040] [Figure 1] 1A to 1C are diagrams illustrating an example of a manufacturing process for a coil spring. [Diagram 2] FIG. 2 is a schematic diagram of a coiling machine forming section for carrying out an embodiment of the present invention. [Diagram 3] FIG. 2 is an enlarged schematic view of a main part of a forming section of a coiling machine for carrying out an embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0041] An embodiment of the present invention will be specifically described below. In the present invention, in order to perform heating during coiling, a hot forming method as shown in Fig. 1(A) is used. In this embodiment, a steel wire is supplied to a coiling machine by a wire drawer, and the steel wire is rapidly heated in the coiling machine, formed into a coil, quenched in a quenching tank, and further tempered.

[0042] An outline of the coiling machine forming section is shown in Fig. 2. As shown in Fig. 2, the coiling machine forming section 1 includes a feed roller 10 for continuously supplying the steel wire material M, a coiling section 20 for forming the steel wire material M into a coil shape, a cutting means 30 having a cutting blade 30a and an inner die 30b for cutting off the steel wire material M continuously supplied from the rear after coiling a predetermined number of turns, and a high-frequency heating coil 40 for heating the steel wire material M between the outlet of the feed roller 10 and the coiling tool 22. The coiling section 20 includes a wire guide 21 for guiding the steel wire material M supplied by the feed roller 10 to an appropriate position, a coiling tool 22 consisting of a coiling pin (or coiling roller) 22a for processing the steel wire material M supplied via the wire guide 21 into a coil shape, and a pitch tool 23 for providing a pitch.

[0043] Rapid heating in the coiling machine is performed by a high-frequency heating coil 40, and the temperature of the steel wire material is raised to the austenite region within 2.5 seconds. The installation position of the high-frequency heating coil is shown in Fig. 3. The high-frequency heating coil 40 is installed near the wire guide 21, and the coiling section 20 is provided so that the steel wire material M can be shaped immediately after being heated. The installation position of the high-frequency heating coil may be other than that shown in this embodiment, as long as the steel wire material M can be shaped immediately after being heated.

[0044] In the coiling section 20, the steel wire M that has passed through the wire guide 21 is brought into contact with the coiling pin 22a and bent with a predetermined curvature, and is then brought into contact with another downstream coiling pin 22a and bent with a predetermined curvature. The steel wire M is then brought into contact with the pitch tool 23 to give a pitch so as to form a desired coil shape. When the desired number of turns is reached, the steel wire M is cut by shearing between the cutting blade 30a of the cutting means 30 and the straight portion of the inner mold 30b, and the steel wire M supplied from the rear is separated from the spring-shaped steel wire M.

[0045] First, a steel wire M containing, by weight, at least 0.45-0.8% C, 0.15-3.0% Si, and 0.3-1.2% Mn and having a circle equivalent diameter of 1.5-10.0 mm is prepared. The steel wire M is fed to a feed roller 10 by a wire feeder (not shown), and the steel wire M is heated to the austenite region by a high-frequency heating coil 40 within 2.5 seconds, after which it is coiled in a coiling section 20 (coiling process).

[0046] Next, the coils separated after coiling and still in the austenite range are quenched (the quenching solvent is, for example, oil at about 60°C) in a quenching tank (not shown) (quenching process), and then tempered (for example, at 440 to 460°C) (tempering process). Quenching results in a high-hardness structure made of martensite, and further tempering can result in a tempered martensite structure with excellent toughness. Here, the quenching and tempering processes can be performed by using a general method, and the heating temperature of the wire before quenching, the type and temperature of the quenching solvent, and the tempering temperature and time are appropriately set depending on the material of the steel wire M.

[0047] Furthermore, by subjecting the steel wire rod M to a shot peening treatment (shot peening process) or a setting treatment (setting process), it is possible to obtain the desired fatigue resistance. Since the coiling is performed in a state where the wire is heated to the austenite region, it is possible to prevent the generation of residual stress due to processing. Therefore, compared with conventional coil springs produced by a cold forming method, the coil spring of the present invention is more easily imparted with compressive residual stress by shot peening, and it is possible to effectively impart deep and large compressive residual stress from the surface on the inner diameter side of the spring. Furthermore, by performing the setting treatment, a deeper compressive residual stress distribution is formed in the maximum principal stress direction when used as a spring, thereby improving fatigue resistance.

[0048] In this embodiment, a multi-stage shot peening process is performed, which includes a first shot peening process using shots with a particle size of 0.6 to 1.2 mm, a second shot peening process using shots with a particle size of 0.2 to 0.8 mm, and a third shot peening process using shots with a particle size of 0.02 to 0.30 mm. Since the shot peening process performed later uses shots smaller than those used in the shot peening process performed earlier, the surface roughness of the wire can be smoothed.

[0049] The shots used in shot peening can be steel cut wires, steel beads, high hardness particles such as FeCrB, etc. Also, the compressive residual stress can be adjusted by the sphere equivalent diameter of the shot, projection speed, projection time, and multi-stage projection method.

[0050] In addition, in this embodiment, hot setting is performed as the setting treatment, and the wire is heated to 100 to 300°C, and plastic strain is applied to the spring-shaped steel material so that the amount of shear strain acting on the wire surface is equal to or greater than the amount of shear strain at the acting stress when actually used as a spring.

[0051] The compression coil spring of the present invention manufactured by the above-mentioned processes has the following physical properties at a depth of 0.2 mm and a depth of d / 4 from the inner diameter side surface of the coil in any cross section of the effective portion of the compression coil spring.

[0052] In other words, the GOS map was measured using the SEM / EBSD method. <3° The area ratio of the crystal grains is 85% or more, and the KAM map measurement by the SEM / EBSD method shows that the KAM <3° The area ratio of these pixels is 95% or more, and the hardness is 580 to 700 HV. Therefore, the compression coil spring of the present invention has excellent fatigue resistance. EXAMPLES

[0053] [Manufacturing of compression coil springs] Compression coil springs were manufactured using a coiling machine (see Figures 2 and 3) equipped with a high-frequency heating coil. An oil-tempered wire made of the chemical components listed in Table 1 was prepared, and the wire was heated to 900°C using a coiling machine to manufacture compression coil springs using the steel types and manufacturing process shown in Figure 1 and Table 2. The specifications of the manufactured compression coil springs were as follows: shape: cylindrical, uniform pitch, closed end (end faces ground), wire diameter: 3.5 mm, spring index: 6.00, total (effective) number of turns: 5.75 turns.

[0054] [Table 1]

[0055] [Table 2]

[0056] [Test Method] (1) GOS <3° Area ratio of crystal grains A GOS map was created using the SEM / EBSD method with a JEOL JSM-7000F (TSL Solutions OIM Analysis ver7.2.1), and the GOS value was less than 3° (GOS <3° The area ratio of crystal grains in the effective cross section of the coil spring was calculated. Here, measurements were performed at depths of 0.2 mm and d / 4 mm from the surface on the inner diameter side of the effective cross section of the coil spring, over an area of ​​10 μm × 30 μm, with an observation magnification of 5000 times and a measurement step size of 0.05 μm, and boundaries with an orientation angle difference of 5° or more were defined as grain boundaries.

[0057] (2) KAM <3° Pixel area ratio of Using the SEM / EBSD method, a KAM map was created using a JEOL JSM-7000F (TSL Solutions OIM Analysis ver7.2.1), and the KAM value was less than 3° (KAM <3° The area ratio of pixels where the pixel area is greater than the effective area of ​​the coil spring was calculated. Here, measurements were taken at depths of 0.2 mm and d / 4 mm from the surface on the inner diameter side of the cross section of the effective part of the coil spring, over an area of ​​10 μm x 30 μm, with an observation magnification of 5000x and a measurement step size of 0.05 μm.

[0058] (3) Hardness Using a Vickers hardness tester (Future Tech FM-600), a measurement load of 200 gf was used to measure at five arbitrary positions at depths of 0.2 mm and d / 4 mm from the surface on the inner diameter side of the effective cross section of the coil spring, and the average value was calculated.

[0059] (4) Average grain size (d GS ) The average grain size was measured by SEM / EBSD method using JEOL JSM-7000F (TSL Solutions OIM Analysis ver7.2.1). Here, the measurement was performed at a depth of 0.2 mm and d / 4 mm from the surface of the wire cross section of the coil spring, and the average grain size was calculated using an observation magnification of 5000 times in a range of 10 μm x 30 μm, a measurement step size of 0.05 μm, and boundaries with an orientation angle difference of 3° or more as grain boundaries.

[0060] (5) Surface roughness (Rz (maximum height)) The surface roughness was measured using a non-contact three-dimensional shape measuring device (MITAKA NH-3) in accordance with JIS 0601. The measurement conditions were: measurement magnification: 100 times, measurement distance: 4 mm, measurement pitch: 0.002 mm, cutoff value: 0.8 mm.

[0061] (6) Fatigue resistance (breakage rate, inclusion breakage rate) Fatigue tests were conducted using a parallel compression spring endurance testing machine. The test stress was 735±686MPa for steel types A and B, and 735±711MPa for steel types C and D. The test frequency was 20Hz, and the number of repetitions was 5×10 7 The number of times was set as the number of times, the breakage rate was evaluated as the number of breaks / number of tests, and the inclusion breakage rate was evaluated as the number of breaks originating from inclusions / number of tests. The test results are shown in Table 3.

[0062] [Table 3]

[0063] [Evaluation Results] In Examples 1 to 5, the GOS at a depth of 0.2 mm and at d / 4 from the inner diameter side surface of the coil <3° The area ratio of the crystal grains is 85% or more, KAM <3° The area ratio of the pixels was 95% or more, and it was confirmed that the strain between and within the crystal grains was reduced. In addition, the hardness at the same position as above was 580 to 700 HV, which confirmed that the breakage rate and the inclusion-originating breakage rate were low, and the fatigue resistance properties originating from the inclusions were improved.

[0064] In addition, in Examples 1 to 5, the average grain size is 1.1 μm or less, which fully satisfies the desired average grain size of 2.0 μm or less. Such fine grains are obtained by rapid heating using high-frequency heating. In other words, heating in a short time using high-frequency heating suppresses the coarsening of prior austenite grains or leads to finer grains, which contributes to improved fatigue resistance.

[0065] In Comparative Example 1, coiling was performed by cold forming, and the subsequent annealing alone resulted in large distortion between crystal grains, resulting in GOS. <3° When the area ratio of the grains becomes less than 85%, the strain in the grains is not eliminated and the KAM <3° The area ratio of pixels with this property was less than 95%. As a result, it was confirmed that the breakage rate and the inclusion-originating breakage rate were high, and that the fatigue resistance properties originating from the inclusions had deteriorated.

[0066] In Comparative Example 2, cold coiling and annealing are performed, but since heating by nitriding is performed, the distortion within the crystal grains is reduced compared to Comparative Example 1, and the KAM <3° The area ratio of the pixels was more than 95%. However, the distortion between the crystal grains was large, and the GOS <3° Since the area ratio of the crystal grains was less than 85%, the breakage rate and the rate of breakage originating from inclusions were high, and it was confirmed that the fatigue resistance properties originating from inclusions were reduced.

[0067] In Comparative Example 3, the steel type and manufacturing process are the same as those in Inventive Example 3, but the tempering temperature is 400°C. As a result, the distortion between the crystal grains remains and the reduction in the distortion within the crystal grains is insufficient, resulting in the GOS <3° The area ratio of the crystal grains in the KAM is less than 85%. <3° The area ratio of pixels with this property was less than 95%. As a result, it was confirmed that the breakage rate and the inclusion-originating breakage rate were high, and that the fatigue resistance properties originating from the inclusions had deteriorated.

[0068] In Comparative Example 4, the steel type and manufacturing process were the same as those in Invention Example 3, but the tempering temperature exceeded 460°C, so the hardness at a depth of 0.2 mm and at a depth of d / 4 from the coil inner diameter side surface did not reach 580 HV, and fatigue resistance could not be improved. For this reason, it was confirmed that the breakage rate and inclusion-originating breakage rate were high, and fatigue resistance properties originating from inclusions were deteriorated.

[0069] As described above, according to the present invention, it is possible to reduce the strain between crystal grains as well as the strain within crystal grains, thereby suppressing breakage originating from inclusions and obtaining a compression coil spring with excellent fatigue resistance. [Industrial Applicability]

[0070] The present invention can be suitably used for compression coil springs that require high fatigue resistance, such as compression coil springs used in automobile engines and clutches. [Explanation of symbols]

[0071] 1... coiling machine forming section, 10... feed roller, 20... coiling section, 21... wire guide, 22... coiling tool, 22a... coiling pin, 23... pitch tool, 30... cutting means, 30a... cutting blade, 30b... inner mold, 40... high frequency heating coil, M... steel wire material.

Claims

1. A compression coil spring using a steel wire having a circle equivalent diameter d of 1.5 to 10.0 mm, containing, by weight, 0.45 to 0.8% C, 0.15 to 3.0% Si, 0.3 to 1.2% Mn, and one or more of 0.5 to 2.0% Cr, 1.5% or less Ni, 0.5% or less V, 1.5% or less Mo, and 0.5% or less W as optional components, with the balance being iron and unavoidable impurities, wherein the compression coil spring has the following physical properties at a depth of 0.2 mm and a depth of d / 4 (d is the wire diameter) from the surface of the inner diameter side of the coil in an arbitrary cross section of an effective portion of the compression coil spring. (1) When a grain oriented spread (GOS) map is measured by a SEM / EBSD (Electron Back Scatter Diffraction) method, the area ratio of crystal grains having a GOS value of less than 3° is 85% or more. (2) In a KAM (Kernel Average Misorientation) map measured by the SEM / EBSD method, the area ratio of pixels having a KAM value of less than 3° is 95% or more. (3) The hardness is 580 to 700 HV.

2. 2. The compression coil spring according to claim 1, wherein an average crystal grain size is 2.0 μm or less when an IPF map (boundaries with an orientation angle difference of 3° or more are defined as grain boundaries) is measured by a SEM / EBSD method at a depth of 0.2 mm from the coil inner diameter side surface of any cross section of the effective portion of the compression coil spring.

3. 3. The compression coil spring according to claim 1, wherein an average crystal grain size is 2.0 μm or less when an IPF map is measured by a SEM / EBSD method at a depth of d / 4 from the inner diameter side surface of the coil in any cross section of the effective portion of the compression coil spring.

4. 4. The compression coil spring according to claim 1, wherein the spring index is 3 to 8.

5. The compression coil spring according to any one of claims 1 to 4, wherein the surface roughness Rz is 20 µm or less.

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