Steel material for spring, and coil spring

Abstract

A steel material for a spring according to the present invention contains C at 0.30 mass% to 0.60 mass%, Si at 2.20 mass% to 2.80 mass%, Mn at 0.05 mass% to 1.50 mass%, Cr at 0.05 mass% to 1.00 mass%, Cu at 0.05 mass% to 1.00 mass%, Ni at 0.05 mass% to 1.00 mass%, and Ti at 0.01 mass% to 0.20 mass%, with the balance made up by Fe and inevitable impurities. The sphere-equivalent radius of ε-carbides (ε-Fe2-3C) measured by an X-ray small-angle scattering method is 4.5 nm or below.

Description

Spring steel and coil springs

[0001] The present invention relates to a spring steel material and a coil spring formed by spirally winding a wire made of this spring steel material.

[0002] In recent years, environmental issues have led to increasingly stringent demands for improved fuel economy in automobiles, resulting in ever stronger demands for smaller and lighter automobile parts. To meet this demand for smaller and lighter automobile parts, there is an increasing demand for products that can withstand higher stresses and have excellent resistance to fatigue, settling, and delayed fracture, for example, in the area of ​​coil spring parts such as suspension springs.

[0003] A spring steel material having excellent durability and settling resistance contains, by weight, 0.35 to 0.55% C, 1.80 to 3.00% Si, 0.50 to 1.50% Mn, 0.50 to 3.00% Ni, 0.10 to 1.50% Cr, 0.01 to 0.05% Al, 0.010 to 0.025% N, with the remainder being substantially Fe, and when quenched and tempered to a hardness of HRC 55, has an impact value of 4.45 kgf / cm in a Charpy impact test. ２ Furthermore, the residual shear strain ΓR obtained in the sag test after forming the coil spring is 5.2 × 10 －４ The following spring steel has been proposed (see, for example, Patent Document 1).

[0004] Furthermore, as a spring steel material having excellent sag resistance, it contains C: 0.5 to less than 0.8% (meaning mass %, the same applies hereinafter), Si: 1.2 to 2.5%, Mn: 0.5 to 1.5%, Cr: 0.05 to 1.5%, and V: 0.05 to 0.25%, and the ferrite in the pearlite structure contains V and Cr carbides, carbonitrides, and V and Cr composite carbides and composite carbonitrides with a circle equivalent diameter of 50 nm or less at a total density of 10 particles / μm 2 Spring steels that satisfy the above criteria have been proposed (see, for example, Patent Document 2).

[0005] Patent No. 2839900 Patent No. 3940263

[0006] In the technology of Patent Document 1, 130 kgf / mm ２Although the sag resistance at 1400 MPa has been confirmed, it does not satisfy the sag resistance requirement at 1400 MPa that is currently required for suspension springs for automobiles. Furthermore, the technology of Patent Document 2 aims to produce V and Cr carbides, carbonitrides, and V and Cr composite carbides and composite carbonitrides with a circle equivalent diameter of 50 nm or less in ferrite at a density of 10 particles / μm in total. 2 Although it has been confirmed that the formation of these compounds improves the resistance to sag, they are not sufficient to withstand use under high stress.

[0007] The present invention has been made in view of the above, and has an object to provide a spring steel material that is excellent in sag resistance, and a coil spring made of this spring steel material.

[0008] In order to solve the above-mentioned problems and achieve the object, the spring steel according to the present invention contains C in an amount of 0.30 mass% or more and 0.60 mass% or less, Si in an amount of 2.20 mass% or more and 2.80 mass% or less, Mn in an amount of 0.05 mass% or more and 1.50 mass% or less, Cr in an amount of 0.05 mass% or more and 1.00 mass% or less, Cu in an amount of 0.05 mass% or more and 1.00 mass% or less, Ni in an amount of 0.05 mass% or more and 1.00 mass% or less, Ti in an amount of 0.01 mass% or more and 0.20 mass% or less, and the balance being Fe and inevitable impurities, and contains ε carbides (ε-Fe ２―３ C) The equivalent sphere radius is 4.5 nm or less.

[0009] Further, the spring steel material according to the present invention is characterized in that, in the above-mentioned invention, the amount of ε carbide (ε-Fe ２―３ C) is characterized in that the volume ratio thereof is 1.5 vol % or more.

[0010] Furthermore, the coil spring according to the present invention contains C in an amount of 0.30 mass% or more and 0.60 mass% or less, Si in an amount of 2.20 mass% or more and 2.80 mass% or less, Mn in an amount of 0.05 mass% or more and 1.50 mass% or less, Cr in an amount of 0.05 mass% or more and 1.00 mass% or less, Cu in an amount of 0.05 mass% or more and 1.00 mass% or less, Ni in an amount of 0.05 mass% or more and 1.00 mass% or less, Ti in an amount of 0.01 mass% or more and 0.20 mass% or less, and the balance being Fe and inevitable impurities, and has a content of ε carbide (ε-Fe ２―３ C) is characterized in that the equivalent sphere radius is 4.5 nm or less.

[0011] Further, the coil spring according to the present invention is the above-mentioned invention, wherein the ε carbide (ε-Fe ２―３ C) is characterized in that the volume ratio thereof is 1.5 vol % or more.

[0012] The present invention has the effect of providing a spring steel material that has excellent resistance to settling even under high stress, and a coil spring made of this spring steel material.

[0013] FIG. 1 is a diagram illustrating the manufacturing process of a hot spring and a cold spring. FIG. 2 is a diagram illustrating the ε-carbide sphere-equivalent radius of a spring steel according to an example. FIG. 3 is a diagram illustrating the ε-carbide volume fraction of a spring steel according to an example. FIG. 4 is a diagram illustrating the results of a delayed fracture resistance test of a coil spring according to an example. FIG. 5 is a diagram illustrating the relationship between stress and sag in a make-up test of a coil spring according to an example. FIG. 6 is a diagram illustrating the ε-carbide sphere-equivalent radius of a coil spring according to an example. FIG. 7 is a diagram illustrating the ε-carbide volume fraction of a coil spring according to an example. FIG. 8 is a diagram illustrating a method for calculating the sphere-equivalent radius of carbides in a coil spring using small-angle X-ray scattering.

[0014] Hereinafter, embodiments for carrying out the present invention (hereinafter referred to as "embodiments") will be described with reference to the accompanying drawings. Note that the drawings are schematic, and the relationship between the thickness and width of each part, the thickness ratio of each part, etc. may differ from the actual ones, and the drawings may also include parts with different dimensional relationships and ratios.

[0015] (Embodiment) The coil spring according to the present invention has a low content of ε carbide (ε-Fe) measured by small angle X-ray scattering. ２―３ C) is formed using a spring steel material having a sphere-equivalent radius of 4.5 nm or less.

[0016] <Material Composition> The spring steel according to the present invention contains C in a proportion of 0.30% by mass or more and 0.60% by mass or less. C contributes to improving the strength of the spring steel. If the C content is less than 0.30% by mass, the effect of improving strength is not sufficiently obtained, resulting in insufficient fatigue resistance and sag resistance. Furthermore, if the C content exceeds 0.60% by mass, toughness decreases and cracking becomes more likely to occur. From the above viewpoints, it is preferable that C be contained in a proportion of 0.44% by mass or more and 0.50% by mass or less.

[0017] The spring steel according to the present invention contains Si in a proportion of 2.20% by mass or more and 2.80% by mass or less. Si is effective in deoxidizing the steel material and contributes to improving strength and temper softening resistance. If the Si content is less than 2.20% by mass, the above effects are not sufficiently obtained, and carbides generated during tempering become coarse. If the Si content exceeds 2.80% by mass, toughness decreases, cracking becomes more likely, and decarburization is promoted, resulting in a decrease in wire surface strength. Furthermore, by setting the Si content within the above range, the yield ratio is improved, making plastic deformation less likely to occur and improving sag resistance. From the above viewpoints, it is preferable that Si be contained in a proportion of 2.20% by mass or more and 2.60% by mass or less.

[0018] The spring steel according to the present invention contains Mn in a proportion of 0.05% by mass or more and 1.50% by mass or less. Mn contributes to improving hardenability. If the Mn content is less than 0.05% by mass, it becomes difficult to ensure sufficient hardenability, and the effect of fixing S (MnS formation), which is harmful to ductility and toughness, becomes poor. Furthermore, if the Mn content exceeds 1.50% by mass, ductility decreases, and cracks and surface scratches tend to occur. From the above viewpoints, it is preferable that the Mn content be 0.75% by mass or more and 1.25% by mass or less.

[0019] The spring steel according to the present invention contains Cr in a proportion of 0.05% by mass or more and 1.00% by mass or less. Cr is effective in preventing decarburization, and contributes to improving strength and temper softening resistance, and is effective in improving fatigue resistance and corrosion resistance. Cr is also effective in improving warm sag resistance. If the Cr content is less than 0.05% by mass, the above effects cannot be fully obtained. If the Cr content exceeds 1.00% by mass, toughness decreases, and cracks and surface scratches tend to occur. From the above viewpoints, it is preferable that the Cr content be 0.25% by mass or more and 0.55% by mass or less.

[0020] The spring steel according to the present invention contains Cu in a proportion of 0.05% by mass or more and 1.00% by mass or less. Cu is effective in improving hardenability, and can improve fatigue strength and corrosion resistance by dissolving in ferrite. If the Cu content is less than 0.05% by mass, the above effects cannot be sufficiently obtained. Furthermore, if the Cu content exceeds 1.00% by mass, cracks may occur during hot working. From the above viewpoints, it is preferable that Cu be contained in a proportion of 0.10% by mass or more and 0.45% by mass or less.

[0021] The spring steel according to the present invention contains Ni in a proportion of 0.05% by mass or more and 1.00% by mass or less. Ni is effective in improving hardenability, suppressing the formation of carbides, and improving fatigue strength and corrosion resistance. If the Ni content is less than 0.05% by mass, the effect of improving hardenability becomes insufficient. If the Ni content exceeds 1.00% by mass, not only will cost be a problem, but the amount of retained austenite will increase, reducing fatigue life. From the above viewpoints, it is preferable that Ni be contained in a proportion of 0.10% by mass or more and 0.45% by mass or less.

[0022] The spring steel according to the present invention contains Ti in a proportion of 0.01% by mass or more and 0.20% by mass or less. Ti combines with C and N to form carbides and nitrides, which act as hydrogen trapping sites, thereby suppressing hydrogen diffusion into the steel, improving corrosion resistance and delayed fracture resistance, and improving strength and toughness through grain refinement and precipitation strengthening. If the Ti content is less than 0.01% by mass, the above effects cannot be fully achieved. Furthermore, if the Ti content exceeds 0.20% by mass, a large amount of TiN is formed, resulting in a decrease in fatigue strength. From the above viewpoints, it is preferable that the Ti content be 0.080% by mass or more and 0.130% by mass or less.

[0023] The spring steel according to the present invention must contain C, Si, Mn, Cr, Cu, Ni, and Ti in the above proportions, but may contain elements other than those listed above as inevitable impurities. Here, "unavoidable impurities" refer to components that are mixed in during the industrial production of spring steel due to various factors such as raw materials and production processes, and whose content does not adversely affect the properties of the spring steel according to the present invention. Examples of inevitable impurities include the following elements. When the content of the following elements is below the upper limit, such elements are treated as "unavoidable impurities" in the spring steel according to the present invention. P: 0.025 mass% or less S: 0.025 mass% or less Al: 0.030 mass% or less O: 0.003 mass% or less N: 0.010 mass% or less

[0024] <Carbide> The spring steel according to the present invention has a low ε carbide content (ε-Fe ２―３ C) has a spherical equivalent radius of 4.5 nm or less. When the spherical equivalent radius of the ε carbide is 4.5 nm or less, dislocation pinning and dissolved hydrogen trapping become possible, and the sag resistance and delayed fracture resistance of a coil spring formed using this spring steel are improved.

[0025] The spherical equivalent radius of ε carbide can be calculated by small-angle X-ray scattering. Small-angle X-ray scattering is a method for evaluating the structure of a substance by irradiating a sample with X-rays and determining the scattering angle and intensity of the X-rays that have passed through the sample. In the present invention, the radius of ε carbide (ε-Fe２－３ The size distribution of carbides (C, TiC) was evaluated. Fig. 8 is a diagram for explaining a method for calculating the spherical equivalent radius of carbides in a coil spring by small-angle X-ray scattering. Fig. 8(A) shows scattering profiles obtained by small-angle X-ray scattering for a spring steel according to the present invention (Example 2-4 described below) and a conventional steel (Comparative Example 2-2). The vertical axis represents the scattering intensity (log), and the horizontal axis represents the scattering vector q (= 4π sin θ / λ).

[0026] The dotted line in Figure 8(A) represents the background, and the presence of precipitates (carbides) increases the strength of the background. In Figure 8(A), the presence of at least two types of particles with different sizes (first particles and second particles) was confirmed by changes in the slope of the scattering profile. The two types of particles were estimated based on the results of scattering profiles obtained by changing the tempering temperature of a single spring steel.

[0027] Fig. 8(B) shows the scattering profile obtained by small-angle X-ray scattering for the spring steel material according to the present invention (Examples 2-4, tempering temperatures: 300°C, 440°C). In the figure, the area surrounded by the dotted line (corresponding to the second particle in Fig. 8(A)) shows a difference in scattering intensity, i.e., a difference in the amount of precipitation. This indicates that the amount of precipitation of iron carbide, i.e., ε carbide (ε-Fe ２－３ Based on the results of the estimation of the second particles, it was estimated that the first particles were TiC.

[0028] For the small-angle X-ray scattering method, a thin sheet of spring steel approximately 50 μm thick was prepared, and a scattering profile was obtained using synchrotron radiation (SPring-8). The scattering vector q (= 4π sin θ / λ) was measured at approximately 0.006 to 3, which includes at least a range of 0.04 to 2.0. Data processing involved converting the measurement data into absolute intensities and combining data from different q ranges. Elements required for absolute intensity conversion include, for example, the type of steel (composition), transmittance, instrument constant (calibration factor), and BG measurement data.

[0029] In addition, as a one-dimensional processing, the scattering angle was integrated from the origin of the integrated scattering angle in the scattering angle 2θ (radius) direction, the origin of the scattering angle was determined, camera length calibration (conversion from detector coordinates to diffraction angle) and streak (noise) processing were performed.

[0030] For background processing, data measured without the sample was subtracted as background, and the intensity derived from the sample was calculated based on the following formula: (I_obs-I_dark) / Tr=I_s+I_bg-I_dark, where I_obs is the measured data, Tr is the transmittance, I_s is the intensity derived from the sample, I_bg, I_dark are the backgrounds derived from the device.

[0031] For normalization, the scattering cross section per unit volume was converted into an absolute value (corrected for exposure time, sample thickness, and instrument coefficient).

[0032] After processing, parameter fitting was performed using parameters such as particle size, shape factor, size distribution width, scaling factor (relative volume fraction), and BG (intercept, slope) to determine the ε carbide (ε-Fe ２－３ C) The equivalent sphere radius was calculated.

[0033] When fitting the scattering intensity I (q, s1, r1, s2, r2), the following assumptions were made: The sum of the volume fractions (1st + 2nd) was set to 10% or less in the following formula (1).

[0034] The volume-weighted frequency distribution was assumed to be a log-normal distribution of the following formula (2). In addition, the equatorial radius refers to the radius of the axis of rotation of the original ellipse, out of the two diameters (major diameter and minor diameter).

[0035] The shape was assumed to be a sphere (ε=1) or a spheroid of revolution represented by the following formula (3).

[0036] The initial parameter values ​​were as follows: (s1, r1, σ1, ε1) = (0.05, 20, 1, 0.5) (s2, r2, σ2, ε2) = (0.1, 10, 0.25, 0.5) s1, s2, r1, and r2 were calculated (fitted) within the range of qmin≦q≦qmax by the least squares method (iterative solution) until the results converged. Here, qmin is the lower limit of the fitting range and is the maximum value among the plots between 0.065 and 0.075, and qmax is the upper limit of the fitting range and is the minimum value among the plots between 0.9 and 1.1. The fitting accuracy is preferably 0.001 or less in terms of the fitting index MSLE (mean square logarithmic error). It is preferable that the number of plots included in the fitting range be 40 or more. In the present invention, ε carbide (ε-Fe ２－３ C) The spherical equivalent radius is calculated using the median value of the size distribution of the measured particles.

[0037] The spring steel material according to the present invention is a steel containing ε carbide (ε-Fe ２―３ C) is 1.5 vol% or more. ２―３ When the volume fraction of C) is 1.5 vol % or more, dislocation pinning and dissolved hydrogen trapping become possible, and the settling resistance and delayed fracture resistance of a coil spring formed using this spring steel material are improved.

[0038] ε carbide (ε-Fe ２―３ The volume fraction of C) can be calculated by X-ray diffraction. A cylindrical spring steel with a diameter of 0.2 mm was prepared from the steel material and measured and analyzed using an X-ray diffraction (XRD) device (Spring-8 BL19B2 multipurpose high-throughput diffractometer Polaris) under the following conditions: Energy: Approximately 25 keV Wavelength: 0.49596 Å Detector: One-dimensional semiconductor detector Exposure time: 120 seconds exposure x 4 times (8 minutes exposure) Temperature: Room temperature, 2θ: 2.095° to 84.845°

[0039] <Physical Properties> The coil spring according to the present invention has a residual shear strain of 8.5×10 after a 96-hour tightening test at a compressive stress of 1400 MPa and a temperature of 80°C. －４It is preferable that the hardness is less than or equal to 0.05 mm. The hardness of the coil spring used in the tightening test is HRC 54. The residual shear strain was measured by compressing the coil spring between flat plates at a predetermined pressure and holding it in that state at 80°C for 96 hours. The residual shear strain was calculated using the average value ΔP of the load loss at deflection rates of 30% and 60% using the following formula: γ = 8DΔP / πGd3 × 100 (%), where d is the wire diameter, D is the coil center diameter, and G is the modulus of transverse elasticity (nominal value according to JIS). The residual shear strain was measured at different compressive stresses, and the residual shear strain at a compressive stress of 1,400 MPa was calculated using an approximate formula.

[0040] The coil spring according to the present invention has a residual shear strain of 8.5 × 10 after a tightening test at a stress of 1400 MPa, a temperature of 80°C, and 96 hours. －４ In the coil spring according to the present invention, the residual shear strain after a tightening test at a shear stress of 1400 MPa, a temperature of 80°C, and a duration of 96 hours is 8.0 × 10 －４ More preferably, it is 7.0 × 10 or less. －４ It is even more preferable that:

[0041] Furthermore, the spring steel according to the present invention preferably has a fracture time of 330 hours or more in a four-point bending delayed fracture test in which a bending stress of 1500 MPa is applied under the JASO M609 CCT cycle while in an HRC55 state. The JASO M609 CCT cycle involves repeated cycles of salt spray (5% aqueous sodium chloride solution) at 35°C for 2 hours, drying at 60°C for 4 hours at a relative humidity of 20-30%, and wetting at 50°C for 2 hours at a relative humidity of 95% or higher. Here, "an HRC55 steel state" refers to a steel obtained by heating a raw material, quenching, and tempering it without coiling in the case of hot treatment, or a steel obtained by rapidly quenching and tempering the raw material in the case of cold treatment.

[0042] The spring steel according to the present invention exhibits high delayed fracture resistance by having a fracture time of 330 hours or more in a four-point bending delayed fracture test in which a bending stress of 1500 MPa is applied under a JASO M609 CCT cycle while the steel is in a state of HRC 55. It is more preferable that the coil spring according to the present invention has a fracture time of 350 hours or more in a four-point bending delayed fracture test in which a bending stress of 1500 MPa is applied under a JASO M609 CCT cycle while the steel is in a state of HRC 55.

[0043] <Manufacturing Method> The coil spring according to the present invention can be manufactured by either hot treatment or cold treatment. Figure 1 is a diagram illustrating the manufacturing processes for the hot spring and cold spring. The coil spring according to the present invention can be manufactured by the hot treatment or cold treatment shown in Figure 1 from a green material (spring steel) containing 0.30 mass% to 0.60 mass% C, 2.20 mass% to 2.80 mass% Si, 0.05 mass% to 1.50 mass% Mn, 0.05 mass% to 1.00 mass% Cu, 0.05 mass% to 1.00 mass% Ni, 0.01 mass% to 0.20 mass% Ti, with the balance being Fe and unavoidable impurities.

[0044] Hot springs can be manufactured by heating and forming raw materials (coiling for coil springs), then performing heat treatments such as quenching and tempering, followed by shot peening and setting, and then painting.

[0045] Cold springs can be manufactured by quenching and tempering raw material, then forming (coiling for coil springs), shot peening, setting, and painting.

[0046] The coil spring according to the present invention has excellent resistance to settling and delayed fracture under high stress, and therefore can be suitably used for automobile parts, such as suspension springs.

[0047] Hereinafter, examples of the spring steel material and coil spring according to the present invention will be described, but the present invention is not limited to these examples.

[0048] <Spring Steel> (Example 1-1) A base material containing 0.46 mass% C, 2.50 mass% Si, 0.95 mass% Mn, 0.40 mass% Cr, 0.25 mass% Cu, 0.25 mass% Ni, and 0.09 mass% Ti was rapidly quenched at 900°C or higher and rapidly tempered at 300 to 600°C to produce a spring steel. Rapid quenching and rapid tempering refer to quenching and tempering processes in which the temperature is increased at a rate of 50°C / sec or higher and the holding time is 10 seconds or less. The tensile strength of the spring steel was adjusted to 1900 to 2100 MPa. The ε-carbide equivalent radius and ε-carbide volume fraction of this spring steel were calculated.

[0049] Example 1-2 The ε carbide sphere equivalent radius and the ε carbide volume fraction were calculated for a steel material containing 0.48 mass% C, 2.30 mass% Si, 0.95 mass% Mn, 0.45 mass% Cr, 0.20 mass% Cu, 0.30 mass% Ni, and 0.09 mass% Ti, and produced in the same manner as in Example 1-1.

[0050] Comparative Example 1-1 For a steel material containing 0.55 mass% C, 1.40 mass% Si, 0.70 mass% Mn, and 0.70 mass% Cr and produced in the same manner as in Example 1-1, the ε carbide sphere equivalent radius and the ε carbide volume fraction were calculated.

[0051] Comparative Example 1-2 For a steel material containing 0.41 mass% C, 2.10 mass% Si, 0.93 mass% Mn, 0.36 mass% Cr, 0.26 mass% Cu, 0.24 mass% Ni, and 0.10 mass% Ti, and produced in the same manner as in Example 1-1, the ε carbide sphere equivalent radius and the ε carbide volume fraction were calculated.

[0052] (Evaluation method) -ε carbide (ε-Fe ２―３C) spherical equivalent radius - For each of the spring steel materials according to Examples 1-1 to 1-2 and Comparative Examples 1-1 to 1-2, thin plate-shaped samples of 60 μm were prepared, and scattering profiles were obtained using synchrotron radiation (SPring-8). The scattering vector q (= 4π sin θ / λ) was measured at 0.006 to 3. Data processing involved converting the measurement data into absolute intensities, and combining data from different q ranges. Elements required for absolute intensities included, for example, the type of steel (composition), transmittance, equipment constant (calibration factor), and BG measurement data. Parameter fitting was performed on the processed data using parameters such as particle size / shape factor (assuming ellipsoidal shape), size distribution width, scaling factor (relative volume fraction), and BG (intercept, slope), and the like, to obtain the ε carbide (ε-Fe ２―３ The spherical equivalent radius of ε carbide (ε-Fe C) was calculated. FIG. 2 is a diagram for explaining the spherical equivalent radius of ε carbide of the spring steel according to the example. As shown in FIG. 2, ２―３ C) The equivalent sphere radius was 3.68 nm for Example 1-1, 4.10 nm for Example 1-2, 5.01 nm for Comparative Example 2-1, and 4.65 nm for Comparative Example 2.

[0053] -ε carbide (ε-Fe ２―３ C) Volume Fraction - From the steel material (raw steel material that was rapidly quenched and rapidly tempered to HRC 55) in the manufacturing process of Examples 1-1 to 1-2 and Comparative Examples 1-1 to 1-2, cylindrical spring steel with a diameter of 0.2 mm was prepared, and measured and analyzed under the following conditions using an X-ray diffraction (XRD) device (Spring-8 BL19B2 multipurpose high-throughput diffractometer Polaris). Energy: Approximately 25 keV Wavelength: 0.49596 Å Detector: One-dimensional semiconductor detector Exposure time: 120 seconds exposure x 4 times (8 minutes exposure) Temperature: Room temperature, 2θ: 2.095° to 84.845° Figure 3 is a diagram for explaining the ε carbide volume fraction of the spring steel according to the example. As shown in FIG. 3, the volume fraction of ε carbide was 2.0 vol % in Example 1-1, 2.2 vol % in Example 1-2, 2.2 vol % in Comparative Example 1-1, and 1.5 vol % in Comparative Example 1-2.

[0054] ε carbide (ε-Fe ２―３The spring steel of the present invention, in which the equivalent sphere radius of C) is 4.5 nm or less, is ε-Fe ２ It is expected that the fine carbides will be able to pin dislocations and trap dissolved hydrogen, and that these properties will improve the elongation, reduction of area, toughness, hydrogen embrittlement resistance, and sag resistance of molded products made from spring steel.

[0055] - Delayed fracture resistance - Bending test pieces (parallel portion φ8 mm × length 100 mm) made of steel materials (steel materials obtained by rapid quenching and rapid tempering of green materials to HRC 55) produced in the manufacturing process of Examples 1-1 to 1-2 and Comparative Examples 1-1 to 1-2 were subjected to a four-point bending delayed fracture test in which a bending stress of 1500 MPa was applied under the JASO M609 CCT cycle, and the fracture time was measured. The test was performed twice. The results are shown in Figure 4 (values ​​are average values). The fracture time was 267 hours for Example 1-1, 401.5 hours for Example 1-2, 137 hours for Comparative Example 1-1, and 282 hours for Comparative Example 1-2.

[0056] <Coil spring> (Example 2-1) A steel material containing 0.46 mass% C, 2.50 mass% Si, 0.95 mass% Mn, 0.40 mass% Cr, 0.25 mass% Cu, 0.25 mass% Ni, and 0.09 mass% Ti was used, heated, coiled, quenched at 900°C or higher, tempered at 300 to 600°C, set, shot peened, set again, and painted to produce a coil spring (hot treated, HRC54).

[0057] Example 2-2 A steel material containing 0.48 mass% C, 2.30 mass% Si, 0.95 mass% Mn, 0.45 mass% Cr, 0.20 mass% Cu, 0.30 mass% Ni, and 0.09 mass% Ti was used, heated, coiled, quenched at 900°C or higher, tempered at 300 to 600°C, set, shot peened, set again, and painted to produce a coil spring (hot treated, HRC54).

[0058] Example 2-3 A steel material (Example 1-1) containing 0.46 mass% C, 2.50 mass% Si, 0.95 mass% Mn, 0.40 mass% Cr, 0.25 mass% Cu, 0.25 mass% Ni, and 0.09 mass% Ti was used, and the steel material was rapidly quenched at 900°C or higher, rapidly tempered at 300 to 600°C, coiled, stress relief annealed, set, shot peened, set again, and painted to produce a coil spring (cold treatment, HRC54).

[0059] Example 2-4 A steel material (Example 1-2) containing 0.48 mass% C, 2.30 mass% Si, 0.95 mass% Mn, 0.45 mass% Cr, 0.20 mass% Cu, 0.30 mass% Ni, and 0.09 mass% Ti was used, and the steel material was rapidly quenched at 900°C, rapidly tempered at 300 to 600°C, coiled, stress relief annealed, set, shot peened, set again, and painted to produce a coil spring (cold treatment, HRC54).

[0060] Comparative Example 2-1 A steel material (Comparative Example 1-1) containing 0.55 mass% C, 1.40 mass% Si, 0.70 mass% Mn, and 0.70 mass% Cr was used, and the steel material was rapidly quenched at 900°C, rapidly tempered at 300 to 600°C, coiled, stress relief annealed, set, shot peened, set again, and painted to produce a coil spring (cold treatment, HRC 54).

[0061] (Comparative Example 2-2) A steel material (Comparative Example 1-2) containing 0.41 mass% C, 2.10 mass% Si, 0.93 mass% Mn, 0.36 mass% Cr, 0.26 mass% Cu, 0.24 mass% Ni, and 0.10 mass% Ti was used, and the steel material was rapidly quenched at 900°C or higher, rapidly tempered at 300 to 600°C, coiled, subjected to stress relief annealing, setting, shot peened, set again, and painted to produce a coil spring (cold treatment, HRC54).

[0062] (Evaluation Method) - Settling Resistance - The coil springs obtained in Examples 2-1 to 2-4 and Comparative Examples 2-1 to 2-2 were compressed and clamped between flat plates at a predetermined pressure, and held in this clamped state at 80°C for 96 hours. The residual shear strain was calculated using the average value ΔP of the load loss at deflection rates of 30% and 60% according to the following formula: γ = 8DΔP / πGd3 × 100 (%), where d is the wire diameter, D is the coil center diameter, and G is the modulus of transverse elasticity (nominal value according to JIS). The results are shown in Figure 5. The residual shear strain at a compressive stress of 1,400 MPa was obtained by calculating an approximate formula from the residual shear strain at different compressive stresses. The settling resistance was 7.4 × 10 for Example 2-1. －４ , Example 2-2 is 7.2 × 10 －４ , and Example 2-3 is 6.3 × 10 －４ , and Example 2-4 is 6.7 × 10 －４ , and Comparative Example 2-1 is 12.1 × 10 －４ , Comparative Example 2-2 is 8.8 × 10 －４ It was.

[0063] -ε carbide (ε-Fe ２―３ C) spherical equivalent radius - A 60 μm thin plate-shaped sample was prepared from the coil springs obtained in Examples 2-3 to 2-4 and Comparative Examples 2-1 to 2-2, and a scattering profile was obtained using synchrotron radiation (SPring-8). The scattering vector q (= 4π sin θ / λ) was measured at 0.006 to 3. Data processing involved converting the measurement data into absolute intensities and combining data from different q ranges. Elements required for absolute intensities include the steel type (composition), transmittance, equipment constant (calibration factor), and BG measurement data. Parameter fitting was performed on the processed data using parameters such as particle size / shape factor (assuming ellipsoidal shape), size distribution width, scaling factor (relative volume fraction), and BG (intercept, slope), and the like, to obtain the ε carbide (ε-Fe ２―３ C), the sphere equivalent radius was calculated. FIG. 6 is a diagram for explaining the volume fraction of ε carbide in the coil spring according to the example. As shown in FIG. 6, the volume fraction of ε carbide (ε-Fe ２―３ C) The equivalent sphere radius was 3.67 nm for Example 2-3, 4.12 nm for Example 2-4, 4.96 nm for Comparative Example 2-1, and 4.59 nm for Comparative Example 2-2.

[0064] -ε carbide (ε-Fe ２―３ C) Volume Ratio—Cylindrical spring steel specimens with a diameter of 0.2 mm were prepared from the coil springs obtained in Examples 2-3 to 2-4 and Comparative Examples 2-1 to 2-2, and were measured and analyzed using an X-ray diffraction (XRD) device (Spring-8 BL19B2 Multipurpose High-Throughput Diffractometer Polaris) under the following conditions. Energy: Approximately 25 keV Wavelength: 0.49596 Å Detector: One-dimensional semiconductor detector Exposure time: 120-second exposure x 4 times (8-minute exposure) Temperature: Room temperature, 2θ: 2.095° to 84.845° FIG. 7 is a diagram illustrating the volume ratio of ε carbide in the coil springs according to the examples. As shown in FIG. 7, the volume ratio of ε carbide was 1.6 vol% in Example 2-3, 2.1 vol% in Example 2-4, 2.6 vol% in Comparative Example 2-1, and 1.7 vol% in Comparative Example 2-2.

[0065] From the above, it is clear that ε carbide (ε-Fe ２―３ C) The coil spring manufactured from the spring steel of the present invention, in which the equivalent sphere radius is 4.5 nm or less, is ２ It was confirmed that the C is refined, and these fine carbides enable dislocation pinning and the trapping of dissolved hydrogen, resulting in excellent sag resistance and delayed fracture resistance. ２―３ It has been confirmed that a coil spring made of a spring steel material having a volume fraction of C) of 1.5 vol % or more (more preferably 2.0 vol % or more) is also excellent in sag resistance and delayed fracture resistance.

Claims

1. Contains C in an amount of 0.30 mass% or more and 0.60 mass% or less, Si in an amount of 2.20 mass% or more and 2.80 mass% or less, Mn in an amount of 0.05 mass% or more and 1.50 mass% or less, Cr in an amount of 0.05 mass% or more and 1.00 mass% or less, Cu in an amount of 0.05 mass% or more and 1.00 mass% or less, Ni in an amount of 0.05 mass% or more and 1.00 mass% or less, Ti in an amount of 0.01 mass% or more and 0.20 mass% or less, with the balance being Fe and inevitable impurities, and the content of ε carbides (ε-Fe) measured by X-ray ２―３ C) A spring steel material having a sphere-equivalent radius of 4.5 nm or less.

2. ε-carbide (ε-Fe) measured by X-ray ２―３ 2. The spring steel according to claim 1, wherein the volume fraction of C) is 1.5 vol % or more.

3. Contains C in an amount of 0.30% by mass or more and 0.60% by mass or less, Si in an amount of 2.20% by mass or more and 2.80% by mass or less, Mn in an amount of 0.05% by mass or more and 1.50% by mass or less, Cr in an amount of 0.05% by mass or more and 1.00% by mass or less, Cu in an amount of 0.05% by mass or more and 1.00% by mass or less, Ni in an amount of 0.05% by mass or more and 1.00% by mass or less, Ti in an amount of 0.01% by mass or more and 0.20% by mass or less, with the balance being Fe and inevitable impurities, and the content of ε carbides (ε-Fe) measured by X-ray ２―３ C) A coil spring having a sphere-equivalent radius of 4.5 nm or less.

4. ε-carbide (ε-Fe) measured by X-ray ２―３ 4. The coil spring according to claim 3, wherein the volume fraction of C) is 1.5 vol % or more.

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