Coil spring

Abstract

A coil spring according to the present invention contains 0.30 mass% to 0.60 mass% inclusive of C, 2.20 mass% to 2.80 mass% inclusive of Si, 0.05 mass% to 1.50 mass% inclusive of Mn, 0.05 mass% to 1.00 mass% inclusive of Cr, 0.05 mass% to 1.00 mass% inclusive of Cu, 0.05 mass% to 1.00 mass% inclusive of Ni, 0.01 mass% to 0.20 mass% inclusive of Ti, with the balance being made up of Fe and inevitable impurities. The coil spring has a residual shear strain of 8.5 × 10-4 or less after a fastening test of 96 hours under a shear stress of 1400 MPa and a temperature of 80°C.

Description

Coil spring

[0001] The present invention relates to a coil spring formed by winding a wire in a spiral shape.

[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 with 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 (for example, Patent Document 1).

[0004] Patent No. 2839900

[0005] In the technology of Patent Document 1, 130 kgf / mm ２ Although the sag resistance at 1400 MPa has been confirmed, it does not satisfy the sag resistance requirement at 1400 MPa currently required for suspension springs for automobiles.

[0006] The present invention has been made in view of the above, and has an object to provide a coil spring that is excellent in resistance to sag.

[0007] In order to solve the above-mentioned problems and achieve the object, a 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 wherein the coil spring has a residual shear strain of 8.5×10 after a clamping test at a shear stress of 1400 MPa, a temperature of 80°C, and for 96 hours. －４ The following is the result.

[0008] Furthermore, in the coil spring according to the present invention, in the above invention, the time to fracture in a four-point bending test in which a bending stress of 1500 MPa is applied under a JASO M609 CCT cycle in a state of a steel material of HRC 55 is 330 hours or more.

[0009] The present invention has the effect of providing a coil spring that has excellent resistance to settling even under high stress.

[0010] FIG. 1 is a diagram illustrating the manufacturing process of a hot spring and a cold spring. FIG. 2 is a diagram illustrating the relationship between stress and sag in a tightening test of a coil spring according to an embodiment of the present invention. FIG. 3 is a diagram illustrating the results of a delayed fracture resistance test of a coil spring according to an embodiment of the present invention. FIG. 4 is a TEM observation photograph of a spring steel according to the conventional technology (Comparative Example 1). FIG. 5 is a TEM observation photograph of a spring steel according to the conventional technology (Comparative Example 2). FIG. 6 is a TEM observation photograph of a spring steel according to an embodiment of the present invention (Example 3). FIG. 7 is a TEM observation photograph of a spring steel according to an embodiment of the present invention (Example 4).

[0011] 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.

[0012] (Embodiment) A coil spring according to the present invention 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, and the balance being Fe and inevitable impurities. －４ The following is the result.

[0013] <Material Components> The coil spring 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 coil spring. 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 cracks become 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 0.48% by mass or less.

[0014] The coil spring 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 steel 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 formed during tempering become coarse. If the Si content exceeds 2.80% by mass, toughness decreases, making cracks more likely to occur, and decarburization is promoted, resulting in a decrease in wire rod 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 perspectives, it is preferable that Si be contained in a proportion of 2.3% by mass or more and 2.5% by mass or less.

[0015] The coil spring according to the present invention contains Mn in a ratio 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 0.85% by mass or more and 1.15% by mass or less.

[0016] The coil spring 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. It 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. Furthermore, 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 0.35% by mass or more and 0.45% by mass or less.

[0017] The coil spring 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 the Cu content be 0.20% by mass or more and 0.35% by mass or less.

[0018] The coil spring 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. Furthermore, 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 0.20% by mass or more and 0.35% by mass or less.

[0019] The coil spring according to the present invention preferably 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 material, 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, the Ti content is preferably 0.080% by mass or more and 0.120% by mass or less.

[0020] The coil spring according to the present invention is required to contain C, Si, Mn, Cr, Cu, Ni, and Ti in the above-mentioned proportions, but may contain elements other than those mentioned above.The steel according to the present invention may contain, in addition to C, Si, Mn, Cr, Cu, Ni, and Ti, S in proportions of 0.025 mass% or less and P in proportions of 0.025 mass% or less.

[0021] S forms MnS in the steel material and improves machinability. If the S content exceeds 0.025% by mass, MnS may segregate at grain boundaries, resulting in a decrease in grain boundary strength. Therefore, the S content is preferably 0.025% by mass or less.

[0022] If the P content is high, it segregates at the austenite grain boundaries, reducing the grain boundary strength. The P content is preferably 0.025 mass% or less.

[0023] <Fine Carbides> The coil spring according to the present invention has fine carbides such as titanium carbide and iron carbide. Figures 6 and 7 are transmission electron microscope (TEM) photographs of the steel material constituting the coil spring according to the embodiment of the present invention. As shown in Figures 6 and 7, the titanium carbide is spherical and has a size of 10 to 50 nm, and the iron carbide is a fine carbide with a long edge of 1 nm or more and 100 nm or less. Refining the carbides enables dislocation pinning and the trapping of dissolved hydrogen, improving the sag resistance and delayed fracture resistance of the coil spring.

[0024] <Physical Properties> The coil spring according to the present invention has a compressive stress of 1400 MPa, a temperature of 80°C, and a residual shear strain of 8.5 × 10 after a 96-hour tightening test.－４ The hardness of the coil spring used in the tightening test was HRC 54. The residual shear strain was measured by compressing and tightening the coil spring between flat plates at a specified pressure and holding it in that tightened state at 80°C for 96 hours. The residual shear strain (γ) was calculated using the average load loss ΔP at deflection rates of 30% and 60% using the following formula (1): γ = 8DΔP / πGd3 × 100 (%) (1) d: wire diameter, D: coil center diameter, G: modulus of transverse elasticity (nominal value according to JIS) The residual shear strain was determined at different compressive stresses, and the residual shear strain at a compressive stress of 1400 MPa was calculated using the approximate formula.

[0025] The coil spring according to the present invention has a residual shear strain of 8.5 × 10 after a clamping test at a shear 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 clamping test at a shear stress of 1400 MPa, a temperature of 80°C, and a duration of 96 hours is 8.0 × 10 －４ Preferably, it is 7.0 x 10 or less. －４ It is even more preferable that:

[0026] Furthermore, the coil spring according to the present invention preferably has a time to fracture of 330 hours or more in a four-point bending test in which a bending stress of 1500 MPa is applied under the JASO M609 CCT cycle when the steel is 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, followed by 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, "HRC55 steel" refers to steel obtained by heating, quenching, and tempering the raw material without coiling in the case of hot treatment, or steel obtained by rapidly quenching and tempering the raw material in the case of cold treatment.

[0027] The coil spring according to the present invention exhibits high resistance to delayed fracture by having a time to fracture of 330 hours or more in a four-point bending test in which a bending stress of 1500 MPa is applied under a CCT cycle of JASO M609 when the steel material is in the state of HRC 55. It is more preferable that the coil spring according to the present invention has a time to fracture of 350 hours or more in a four-point bending test in which a bending stress of 1500 MPa is applied under a CCT cycle of JASO M609 when the steel material is in the state of HRC 55.

[0028] <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 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, and the balance being Fe and unavoidable impurities.

[0029] Hot springs can be manufactured by heating and coiling raw material, followed by quenching and tempering, setting, shot peening, setting again, and painting.

[0030] Cold springs can be manufactured by rapid quenching and rapid tempering of raw material, followed by coiling, stress relief annealing, setting, shot peening, setting again, and painting.

[0031] 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.

[0032] Example 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, and after heating and coiling, the steel material was 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).

[0033] Example 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, and after heating and coiling, the steel material was 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).

[0034] Example 3 A coil spring (cold treatment, HRC 54) was manufactured using 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, which 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. In the present invention, rapid quenching and rapid tempering refer to quenching and tempering processes in which the temperature is increased at a rate of 50°C / second or higher and the holding time is 10 seconds or less, respectively.

[0035] Example 4 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, 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).

[0036] Comparative Example 1 A steel material 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 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, HRC 54).

[0037] Comparative Example 2 A steel material containing 0.41 mass % of C, 2.10 mass % of Si, 0.93 mass % of Mn, 0.36 mass % of Cr, 0.26 mass % of Cu, 0.24 mass % of Ni, and 0.10 mass % of 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, HRC 54).

[0038] (Evaluation Method) - Settling Resistance - The coil springs obtained in Examples 1 to 4 and Comparative Examples 1 and 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 γ was calculated using the average value ΔP of the load loss at deflection rates of 30% and 60% according to the following formula (1): γ = 8DΔP / πGd3 × 100 (%) (1) d: wire diameter, D: coil center diameter, G: modulus of transverse elasticity (nominal value according to JIS) The results are shown in Figure 2. 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 1. －４ , and Example 2 is 7.2 × 10 －４ , and Example 3 is 6.7 × 10 －４ , and Example 4 is 6.3 × 10 －４ , and Comparative Example 1 is 12.1 × 10 －４ , and Comparative Example 2 is 8.8 × 10 －４ From the above, it can be seen that the values ​​indicating the sag resistance of Comparative Examples 1 and 2 are lower than those of Examples because the Si content is outside the range of the present invention.

[0039] - Delayed fracture resistance - The steel materials (raw materials that had been rapidly quenched and rapidly tempered to HRC 55) produced in the manufacturing process of Examples 3 and 4 and Comparative Examples 1 and 2 were subjected to a four-point bending test in which a bending stress of 1500 MPa was applied under the JASO M609 CCT cycle, and the time to fracture was measured. The test was performed twice. The results are shown in Figure 3 (values ​​are average values). The time to fracture was 267 hours for Example 3, 401.5 hours for Example 4, 137 hours for Comparative Example 1, and 282 hours for Comparative Example 2.

[0040] - Fine Carbides - The structures of the steel materials (steel materials in which raw materials were rapidly quenched and rapidly tempered to HRC 55) in the manufacturing process of Examples 3 and 4 and Comparative Examples 1 and 2 were observed using a transmission electron microscope (200 kV-field emission transmission electron microscope, JEM-2100F (manufactured by JEOL Ltd.)), and fine carbides were observed. Fig. 4 is a TEM photograph of Comparative Example 1, Fig. 5 is that of Comparative Example 2, Fig. 6 is that of Example 3, and Fig. 7 is that of Example 4. In Examples 3 and 4, as shown in Figs. 6 and 7, spherical titanium carbides of 10 to 50 nm and iron carbides having lengths of several tens of nm or more and 200 nm or less were observed. On the other hand, iron carbides having lengths of several tens of nm or more and 300 nm or less were observed in Comparative Example 1. Furthermore, in Comparative Example 2, spherical titanium carbides of 100 to 200 nm (coarse titanium carbides) and iron carbides having lengths of several tens of nm or more and 200 nm or less were observed.

[0041] The coil spring of 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 unavoidable impurities. It has been confirmed that the coil spring of the present invention has fine titanium carbides and iron carbides, and these carbides are capable of pinning dislocations and trapping dissolved hydrogen, and therefore has excellent sag resistance and delayed fracture resistance.

[0042] As described above, the coil spring according to the present invention is suitable for obtaining a coil spring that is excellent in resistance to sag.

Claims

1. A coil spring containing 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, and the remainder being Fe and unavoidable impurities, wherein the residual shear strain after a 96-hour tightening test at a shear stress of 1400 MPa and a temperature of 80°C is 8.5 x 10 －４ Below is a coil spring.

2. A coil spring as claimed in claim 1, in which the time to fracture in a four-point bending test in which a bending stress of 1500 MPa is applied under a JASO M609 CCT cycle in the state of HRC 55 steel is 330 hours or more.

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