JP7549459B2 - Compression spring - Google Patents

Description

The present invention relates to a compression coil spring used, for example, in an automobile engine or clutch, and in particular to a compression coil spring that has excellent fatigue resistance even in a high-stress operating environment.

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

Advances in material strength and surface strengthening technologies such as nitriding and shot peening have resulted in the initiation of fatigue breakage moving from the surface to the interior. It is also known that most internal initiation points originate from inclusions.

To prevent internally initiated fracture, it is effective to impart compressive residual stress. For example, Patent Document 1 claims that by heating the wire to the austenite region using 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.

Patent No. 5064590

However, it has been found that even if compressive residual stress is applied as in the technology described in Patent Document 1, internal fracture originating from inclusions cannot be reduced to zero. 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.

Therefore, the present invention aims to provide a compression coil spring that reduces strain between and within crystal grains, thereby suppressing breakage caused by inclusions and improving fatigue resistance.

The inventor learned from the above phenomenon that the probability of inclusion breakage decreases when the strain between and within crystal grains is small, and discovered that it is possible to reduce the strain between and within crystal grains by combining appropriate high-frequency heating conditions, coiling conditions, and quenching and tempering conditions.

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 , 0.5 to 2.0% Cr , and one or more of 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.

Here, the GOS value is an index representing the strain gradient between crystal grains, where the average value of the orientation difference between all pixels in each crystal grain is the value of that crystal grain. "GOS value less than 3°" is represented as "GOS _{< 3°} ". In the present invention, the area ratio of crystal grains with GOS _{< 3°} to the entire observation field is specified to be 85% or more. When the area ratio of crystal grains with GOS _{< 3°} to the entire observation field is 85% or more, it means that a structure with little strain between crystal grains is obtained.

The KAM value is an index representing the strain gradient within a crystal grain, calculated as the average value of the orientation differences between a pixel of interest and six adjacent pixels, and expressed as the value of the central pixel. "KAM value less than 3°" is represented as "KAM _{< 3°} ". In the present invention, the area ratio of pixels with KAM _{< 3°} to the entire observation field is specified as 95% or more. When the area ratio of pixels with KAM _{< 3°} to the entire observation field is 95% or more, it means that a crystal grain with reduced strain within the crystal grain has been obtained.

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, 0.3-1.2% Mn , and 0.5-2.0% Cr is used. In the following explanation, "%" means "weight %".

(1) Material component 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 sufficiently obtained, and fatigue resistance and sag resistance become insufficient. If the C content exceeds 0.8%, the toughness decreases and cracks tend to occur. Therefore, the C content is set to 0.45 to 0.8%.

Si:0.15~3.0%
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 cannot be sufficiently obtained. If the content exceeds 3.0%, the toughness decreases, making cracks more likely to occur, and it also promotes decarburization, resulting in a decrease in the wire rod surface strength. The Si content is preferably 0.5% or more, and more preferably 1.0% or more.

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 detrimental to ductility and toughness, is suppressed. On the other hand, if the Mn content exceeds 1.2%, the ductility decreases and cracks and surface scratches tend to occur. For this reason, the Mn content is set to 0.3 to 1.2%. Let us assume that.
Cr:0.5~2.0%
Cr is effective in preventing decarburization, and also contributes to improving strength and tempering 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, the Cr content is set to 0.5 to 2.0%. If the Cr content is less than 0.5%, these effects cannot be obtained sufficiently. If the amount exceeds 2.0%, the toughness decreases, and cracks and surface scratches tend to occur.

These added elements are the minimum elements necessary to constitute the present invention, and do not limit the addition of other elements. In other words, in the present invention, it is possible to appropriately add one or more of the following elements generally used as the composition of spring steel: Ni 1.5% or less, V 0.5% or less, Mo 1.5% or less, and W 0.5% or less, depending on the purpose, and as a result, it is possible to manufacture coil springs with higher performance or more suitable for the application .

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

(3) KAM _{< 3°}
In the present invention, in a KAM map measurement by SEM/EBSD method at a depth of 0.2 mm and a depth of d/4 from the inner diameter surface of the coil in an arbitrary cross section of the effective portion of the compression coil spring, the area ratio of pixels with KAM _{< 3°} is 95% or more. Since the KAM value is small, less than 3°, that is, the distortion between pixels is small, the inclusion sensitivity is reduced and compressive residual stress is easily introduced. Therefore, since the area ratio of such pixels is 95% or more, fatigue resistance can be improved.

(4) Hardness For valve springs, clutch torsion springs, etc. 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 inner diameter side surface of the coil in 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.

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

(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 preferable for improving fatigue resistance because it has a high resistance to crack propagation.

(6) Surface Roughness For valve springs, clutch torsion springs, etc. used under high load stress, in order to satisfy the required fatigue resistance, the above-mentioned compressive residual stress distribution as well as the surface roughness are important. As a result of the inventor's fracture mechanics calculation and its 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., the 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 stress concentration source, and cracks are likely to occur and propagate with the valleys as the surface origin, which leads to early breakage.

Surface roughness is formed by friction with tools during coiling and by shot peening. The magnitude of the surface roughness formed by shot peening is determined by a combination of the hardness of the wire and conditions such as 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 20 μm relative to the hardness of the wire.

(7) Residual Stress By applying compressive residual stress to the vicinity of the wire surface of the spring, the fatigue resistance can be improved. 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 of +45° to the axial direction of the wire at the above-mentioned predetermined depth from the inner diameter side surface of the coil.

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 given 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, for example, 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 inside the wire when no load is applied, and a deep crossing point suggests that the compressive residual stress penetrates deep from the surface. In either case, the maximum compressive residual stress value on the surface on the inner diameter side of the spring when no load is applied is preferably 620 MPa or more in the case of a valve spring or a clutch torsion spring that are subjected to high load stress. By setting the compressive residual stress distribution in this manner, a compression coil spring with excellent fatigue resistance can be obtained.

When multiple shot peening is performed in the shot peening process, it is preferable that the sphere-equivalent diameter of the shot used in the later shot peening is smaller than the sphere-equivalent diameter of the shot used in the earlier shot peening. Specifically, it is preferable that the shot peening process is a multi-stage 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 earlier shot peening to be reduced by the later shot peening.

The shot diameter and number of stages in the shot peening process are not limited to those mentioned above, and it is sufficient if the required residual stress distribution and surface roughness, etc., can be obtained according to the required performance. Therefore, the shot diameter, material, number of stages, etc. are selected appropriately. In addition, the induced compressive residual stress distribution also differs depending on the projection speed and projection time, so these are also set appropriately as necessary.

(8) Coil Spring Shape The present invention is suitable for compression coil springs with the following specifications, which require a large degree of processing during coiling and require both high fatigue resistance and sag resistance: The present invention can be used for generally cold-formed compression coil springs with 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.

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, which are used in valve springs and clutch torsion springs, etc., which require a large degree of processing during coiling (i.e., the tensile residual stress generated on the inner diameter side of the coil due to the coiling process during cold forming) and high fatigue resistance.

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. This allows for a high degree of freedom in the spring shapes that can be formed. In other words, the coil spring shapes of the present invention can be applied to coil springs of other shapes, including cylindrical shapes, which are typical of coil springs and in which the outer diameter of the coil remains almost constant throughout all turns. For example, it is also possible to form irregularly shaped springs, such as conical, bell, drum, and barrel shapes.

The compression coil spring of the present invention is preferably formed by a coil spring forming machine having a feed roller for continuously supplying steel wire, a coiling section for forming the steel wire into a coil, and a cutting means for cutting off the steel wire continuously supplied from the rear after coiling the steel wire a predetermined number of turns, and the coiling section is equipped with 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 coiling roller for processing the steel wire supplied via the wire guide into a coil shape, and a pitch tool for applying a pitch, and further includes a heating means for heating the steel wire to the austenite region within 2.5 seconds between the outlet of the feed roller and the coiling tool, and is then subjected to a shot peening treatment.

In the compression coil spring of the present invention, the heating means is high-frequency heating, and it is preferable that 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.

The compression coil spring of the present invention has a feed roller for continuously supplying steel wire, a coiling section for forming the steel wire into a coil, and a cutting means for cutting off the steel wire continuously supplied from the rear after coiling the steel wire by a predetermined number of turns. The coiling section has 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 coiling roller for processing the steel wire supplied via the wire guide into a coil shape, and a pitch tool for pitching. The coil spring can be manufactured by a manufacturing method that sequentially performs the following steps: a coiling process for forming the steel wire using a coil spring forming machine having a heating means for heating the steel wire to the austenite range within 2.5 seconds between the outlet of the feed roller and the coiling tool; a quenching process for quenching the coil that has been separated after coiling and is still in the austenite range; a tempering process for tempering the coil; a shot peening process for imparting compressive residual stress to the surface of the wire; and a setting process.

In the present invention, the original purpose of the tempering process is to temper the coil hardened by the quenching process into a coil with appropriate hardness and toughness. In the present invention, the tempering process is performed to reduce the distortion between the crystal grains as well as 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 fatigue resistance cannot be improved.

It may be combined with low-temperature aging treatment to recover the surface elastic limit, if necessary. Here, low-temperature aging treatment can be performed after the shot peening process or between each stage of multi-stage shot peening. When shot peening with shots having a grain size of 0.02 to 0.30 mm is performed as the final stage of multi-stage shot peening, it is preferable to perform it as a pretreatment in order to further increase the compressive residual stress of the outermost surface. Note that there are various methods, such as cold setting and hot setting, for setting the coil as a treatment to prevent sagging in the setting process, and the method is selected appropriately depending on the desired characteristics.

By using hot coiling as in the manufacturing method described above, it is possible to prevent the generation of residual stress due to processing. Furthermore, by heating the steel wire to the austenite region within 2.5 seconds, it is possible to prevent the crystal grains from becoming coarse, and excellent fatigue resistance can be obtained. In addition, because hot coiling is performed, excellent fatigue resistance can be obtained without the need for surface strengthening treatment such as nitriding, and manufacturing costs can be reduced.

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. It is particularly suitable for application to inexpensive hard-drawn wires. This is because hot forming using high-frequency heating makes it possible to obtain springs with superior fatigue resistance, even when inexpensive wire material is used, compared to conventional cold-formed springs that use expensive oil-tempered wire with added high-grade elements.

According to the present invention, it is possible to reduce the strain between crystal grains and within crystal grains, thereby suppressing breakage originating from inclusions and improving fatigue resistance.

1A to 1C are diagrams illustrating an example of a manufacturing process for a coil spring. FIG. 2 is a schematic diagram of a coiling machine forming section for carrying out an embodiment of the present invention. 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.

One embodiment of the present invention will be described in detail below. In the present invention, a hot forming method as shown in FIG. 1(A) is used to heat the wire during coiling. In this embodiment, the steel wire is fed to a coiling machine by a wire feeder, where it is rapidly heated and then formed into a coil shape, quenched in a quenching tank, and further tempered.

Figure 2 shows an outline of the coiling machine forming section. As shown in Figure 2, the coiling machine forming section 1 has 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 equipped with a cutting blade 30a and an inner mold 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 has 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 applying a pitch.

Rapid heating in the coiling machine is performed by a high-frequency heating coil 40, which raises the temperature of the steel wire material to the austenite region within 2.5 seconds. Figure 3 shows the installation position of the high-frequency heating coil. 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 heating. 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 heating.

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 at a predetermined curvature, and then brought into contact with the downstream coiling pin 22a and bent at a predetermined curvature. The steel wire M is then brought into contact with the pitch tool 23 to impart a pitch to the desired coil shape. Once the desired number of turns is reached, the steel wire M is cut by shearing between the straight portion of the inner mold 30b and the cutting blade 30a of the cutting means 30, separating the steel wire M supplied from the rear and the spring-shaped steel wire M.

First, prepare 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 circular equivalent diameter of 1.5-10.0 mm. The steel wire M is fed to the feed roller 10 by a wire feeder (not shown), and the steel wire M is heated to the austenite region by the high-frequency heating coil 40 within 2.5 seconds, after which it is coiled in the coiling section 20 (coiling process).

Next, the coil that has been separated after coiling and whose temperature is still in the austenite range is 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 440-460°C) (tempering process). Quenching creates a high-hardness structure made of martensite, and further tempering creates a tempered martensite structure with excellent toughness. Here, the quenching and tempering processes can be performed using general methods, 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.

Furthermore, by subjecting the steel wire M to a shot peening treatment (shot peening process) or a setting treatment (setting process), the desired fatigue resistance can be obtained. Since the coiling is performed while the wire is heated to the austenite region, the generation of residual stress due to processing can be prevented. Therefore, compared to conventional coil springs produced by cold forming, the coil spring of the present invention is more easily given compressive residual stress by shot peening, and deep and large compressive residual stress can be effectively given 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, improving fatigue resistance.

In this embodiment, a multi-stage shot peening process is performed, 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 0.30 mm. The latter shot peening process uses smaller shots than the earlier shot peening process, so the surface roughness of the wire can be smoothed.

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

In addition, in this embodiment, hot setting is performed as the setting process, heating to 100 to 300°C and applying plastic strain 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.

The compression coil spring of the present invention produced by the above process has the following physical properties at a depth of 0.2 mm and 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.

That is, in a measurement of a GOS map by the SEM/EBSD method, the area ratio of crystal grains with GOS _{< 3°} is 85% or more, in a measurement of a KAM map by the SEM/EBSD method, the area ratio of pixels with KAM _{< 3°} 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.

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

[Test Method]
(1) Area ratio of crystal grains with GOS _{< 3°} A GOS map was created using a JEOL JSM-7000F (TSL Solutions OIM Analysis ver7.2.1) by the SEM/EBSD method, and the area ratio of crystal grains with a GOS value of less than 3° (GOS _{< 3°} ) was calculated. Here, the measurement was performed at a depth 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, in a range of 10 μm x 30 μm, with an observation magnification of 5000 times and a measurement step size of 0.05 μm, and the boundaries with an orientation angle difference of 5° or more were defined as grain boundaries.

(2) Area ratio of pixels with KAM _{< 3°} A KAM map was created using a JEOL JSM-7000F (TSL Solutions OIM Analysis ver7.2.1) by the SEM/EBSD method, and the area ratio of pixels with a KAM value of less than 3° (KAM _{< 3°} ) was calculated. Here, measurements were performed at positions 0.2 mm deep and d/4 mm deep from the surface on the inner diameter side of the effective part cross section of the coil spring, over a range of 10 μm x 30 μm, with an observation magnification of 5000 times and a measurement step size of 0.05 μm.

(3) Hardness Using a Vickers hardness tester (Future Tech FM-600), a measurement load of 200 gf was used to measure five arbitrary positions 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 portion of the coil spring, and the average value was calculated.

(4) Average grain size ( _dGS )
The average grain size was measured by the SEM/EBSD method using a JEOL JSM-7000F (TSL Solutions OIM Analysis ver7.2.1). Here, the measurement was performed at 0.2 mm from the surface and d The observation was performed at a depth of 10 μm×30 μm at a magnification of 5000 times, a measurement step size of 0.05 μm, and boundaries with an orientation angle difference of 3° or more were defined as grain boundaries to calculate the average crystal grain size.

(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.

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

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

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

In Comparative Example 1, coiling was performed by cold forming, and then annealing was performed only. The strain between crystal grains was large, and the area ratio of crystal grains with GOS _{< 3°} was less than 85%, and the strain within the crystal grains was not eliminated, and the area ratio of pixels with KAM _{< 3°} was less than 95%. As a result, the breakage rate and the inclusion-originating breakage rate were high, and it was confirmed that the fatigue resistance properties originating from inclusions were deteriorated.

In Comparative Example 2, cold coiling and annealing were performed, but heating by nitriding was performed, so that the distortion within the crystal grains was reduced and the area ratio of pixels with KAM _{< 3°} was 95% or more compared to Comparative Example 1. However, the distortion between the crystal grains was large and the area ratio of crystal grains with GOS _{< 3°} was less than 85%, so that the breakage rate and the inclusion-originating breakage rate were high, and it was confirmed that the fatigue resistance properties originating from inclusions were reduced.

In Comparative Example 3, the steel type and manufacturing process were the same as those of Invention Example 3, but the tempering temperature was 400° C. As a result, strain remained between crystal grains and strain within the crystal grains was not sufficiently reduced, so that the area ratio of crystal grains with GOS _{< 3°} was less than 85% and the area ratio of pixels with KAM _{< 3°} was less than 95%. As a result, it was confirmed that the breakage rate and the inclusion-originating breakage rate were high, and the fatigue resistance properties originating from inclusions were deteriorated.

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

As described above, according to the present invention, it is possible to reduce the strain between crystal grains as well as the strain within the crystal grains, thereby suppressing breakage caused by inclusions and obtaining a compression coil spring with excellent fatigue resistance.

The present invention can be suitably used for compression coil springs that require high fatigue resistance, such as those used in automobile engines and clutches.

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 (5)

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, 0.5 to 2.0% Cr, and one or more of 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. The compression coil spring according to claim 1, in which 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 considered to be grain boundaries) is measured by the SEM/EBSD method at a depth of 0.2 mm from the inner diameter side surface of the coil in any cross section of the effective part of the compression coil spring. The compression coil spring according to claim 1 or 2, in which an average crystal grain size is 2.0 μm or less when an IPF map is measured by the SEM/EBSD method at 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. A compression coil spring according to any one of claims 1 to 3, having a spring index of 3 to 8. A compression coil spring according to any one of claims 1 to 4, having a surface roughness Rz of 20 μm or less.

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