Steel wire and spring

Abstract

The invention relates to a steel wire and a spring. A steel wire is made of steel containing 0.7% by mass to 1.0% by mass (inclusive) of carbon, 0.12% by mass to 0.32% by mass (inclusive) of silicon, and 0.3% by mass to 0.9% by mass (inclusive) of manganese, with the remainder being iron and unavoidable impurities. In a cross-section perpendicular to the longitudinal direction of the steel wire, the total decarburized layer depth is 0.5% or less of the cross-section diameter. In a cross section perpendicular to the longitudinal direction of the steel wire, the difference between the maximum value and the minimum value of hardness on a straight line passing through the center of gravity is 50 HV or less.

Description

[0001] This application is a divisional application of the Chinese patent application with application date of December 5, 2017 and application number 201780089135.1. Technical Field

[0002] The present invention relates to a steel wire and a spring, and more particularly, to a steel wire suitable for manufacturing a spring and a spring made of the steel wire. This application claims priority based on Japanese Patent Application No. 2017-062172 filed on March 28, 2017, the disclosure of which is incorporated herein by reference in its entirety. Background Art

[0003] Steel wires for spring production are required to have improved permanent deformation resistance and strength. Steel wires with suppressed central carbon segregation have been proposed as high-strength spring steel wires excellent in permanent deformation resistance and strength (see, for example, Patent Document 1).

[0004] Prior art literature

[0005] Patent Literature

[0006] Patent Document 1: Japanese Patent Application Laid-Open No. H06-184696 Summary of the invention

[0007] The steel wire according to the present invention is made of steel containing 0.7 mass % or more and 1.0 mass % or less of carbon, 0.12 mass % or more and 0.32 mass % or less of silicon, and 0.3 mass % or more and 0.9 mass % or less of manganese, with the remainder being iron and inevitable impurities. In a cross section perpendicular to the longitudinal direction of the steel wire, the total decarburized layer depth is 0.5% or less of the cross section diameter, and in a cross section perpendicular to the longitudinal direction of the steel wire, the difference between the maximum value and the minimum value of the hardness on a straight line passing through the center of gravity is 50 HV or less. BRIEF DESCRIPTION OF THE DRAWINGS

[0008] Figure 1 It is a schematic cross-sectional view showing a cross section perpendicular to the longitudinal direction of the steel wire.

[0009] Figure 2 It is a schematic cross-sectional view showing the vicinity of a surface of a cross section perpendicular to the longitudinal direction of the steel wire.

[0010] Figure 3 A schematic perspective view showing the spring structure.

[0011] Figure 4 A flow chart briefly illustrating a method for manufacturing a steel wire.

[0012] Figure 5 It is a schematic cross-sectional view showing a cross section perpendicular to the longitudinal direction of a raw material wire material.

[0013] Figure 6 It is a schematic cross-sectional view for explaining the wire drawing step.

[0014] Figure 7 Schematic diagram for illustrating the experimental method for resistance to permanent deformation.

[0015] Figure 8 Schematic diagram for illustrating the experimental method for resistance to permanent deformation.

[0016] Fig. 9 Schematic diagram for illustrating the experimental method for resistance to permanent deformation.

[0017] Fig.10 Shows the results of the permanent deformation resistance test.

[0018] Fig.11 The hardness distribution in the cross section perpendicular to the longitudinal direction of the wire is shown.

[0019] Fig.12 Shows the results of the permanent deformation resistance test.

[0020] Fig.13 Shows the relationship between total decarburization depth and surface hardness. DETAILED DESCRIPTION

[0021] [Problems to be Solved by the Present Disclosure]

[0022] As described above, steel wires for making springs are required to have improved permanent deformation resistance and strength. Therefore, an object of the present invention is to provide a steel wire having excellent permanent deformation resistance and strength and a spring made of the steel wire and having excellent permanent deformation resistance and strength.

[0023] [Beneficial Effects of the Present Disclosure]

[0024] As described above, a steel wire excellent in permanent deformation resistance and strength can be provided.

[0025] [Description of Embodiments of the Invention]

[0026] First, each aspect of the present invention is described one by one. The steel wire of the present application is made of steel, and the steel contains: carbon (C) of more than 0.7 mass% and less than 1.0 mass%, silicon (Si) of more than 0.12 mass% and less than 0.32 mass%, and manganese (Mn) of more than 0.3 mass% and less than 0.9 mass%, and the remainder is iron (Fe) and unavoidable impurities. In a cross section perpendicular to the longitudinal direction of the steel wire, the total decarburized layer depth is less than 0.5% of the cross-sectional diameter. In a cross section perpendicular to the longitudinal direction of the steel wire, the difference between the maximum and minimum hardness on a straight line passing through the center of gravity is less than 50HV.

[0027] According to the inventor's research, the permanent deformation resistance and strength can be improved by selecting an appropriate component composition, having a decarburized layer depth at a relatively low level compared with the past, and reducing the change in radial hardness. For the steel wire of the present application, a steel having a component composition that enables the obtained spring to have the necessary strength is used. In addition, in order to improve the permanent deformation resistance and strength, the total decarburized layer depth is at a relatively low level, that is, less than 0.5% of the diameter, and the difference between the maximum and minimum values ​​of the hardness in the depth direction (radial direction) is set to less than 50 HV. As a result, the steel wire of the present application with excellent permanent deformation resistance and strength can be provided.

[0028] In the steel wire, the inevitable impurities preferably include 0.025 mass % or less of phosphorus (P), 0.025 mass % or less of sulfur (S), and 0.2 mass % or less of copper (Cu). The contents of each of P, S, and Cu contained in the component composition as inevitable impurities may be within a specific range, thereby improving properties such as toughness.

[0029] In the steel wire, the diameter of the cross section perpendicular to the longitudinal direction may be 0.5 mm or more and 7 mm or less. The steel wire of the present application is particularly suitable for a steel wire having a diameter within a specific range. If the cross section perpendicular to the longitudinal direction has a shape other than a circle, the "diameter of the cross section perpendicular to the longitudinal direction" herein refers to the diameter of a circle surrounding the cross section.

[0030] The steel wire may have a tensile strength of 1900 MPa or more and 2450 MPa or less. A tensile strength of 1900 MPa or more can ensure sufficient strength. A tensile strength of 2450 MPa or less can easily ensure sufficient toughness.

[0031] In the steel wire, the cross-sectional reduction ratio in the tensile test may be 40% or more and 60% or less. A cross-sectional reduction ratio of 40% or more can improve the processability of the wire material into a spring and improve the fatigue strength of the spring formed by processing the wire material. A cross-sectional reduction ratio of 60% or less can easily ensure sufficient strength.

[0032] In the steel wire, the residual shear strain in the permanent deformation resistance test may be preferably 0.2% or less. Therefore, excellent permanent deformation resistance may be more reliably ensured.

[0033] In the steel wire, the cross-sectional reduction ratio during wire drawing can be 80% or more and 90% or less. Therefore, excellent permanent deformation resistance and excellent strength can be more reliably ensured.

[0034] The spring of the present application is made of the steel wire mentioned above. The spring of the present application is made of the steel wire having excellent resistance to permanent deformation and strength, so a spring having excellent resistance to permanent deformation and strength can be provided.

[0035] In the spring, the compressive residual stress on the surface can be 300 MPa or more, thereby extending the fatigue life of the spring.

[0036] In the spring, the hardness of the surface of the spring may be 450 HV or more, thereby extending the fatigue life of the spring.

[0037] [Details of Embodiments of the Invention]

[0038] Next, one embodiment of the steel wire according to the present invention is described below with reference to the accompanying drawings. In the following drawings, the same or corresponding parts are represented by the same reference numerals, and the description thereof will not be repeated.

[0039] refer to Figure 1 and 2 The steel wire 1 in this embodiment has a circular cross section perpendicular to the longitudinal direction. The steel wire 1 is made of steel containing 0.7 mass % or more and 1.0 mass % or less of carbon, 0.12 mass % or more and 0.32 mass % or less of silicon, and 0.3 mass % or more and 0.9 mass % or less of manganese, and the remainder is iron and inevitable impurities. As steel having such a component composition, for example, SWRS87A, SWRS87B, etc. specified in JIS G3502 can be used. The steel wire 1 has a metal structure including a pearlite structure, not a metal structure produced by quenching and tempering.

[0040] refer to Figure 1 , the difference between the maximum and minimum hardness on the straight line L passing through the center (center of gravity) G is 50 HV or less. Specifically, the hardness is measured along the straight line L. Then, the difference between the maximum and minimum hardness is 50 HV or less. The hardness can be measured, for example, using a Vickers hardness tester (micro Vickers hardness tester).

[0041] refer to Figure 1 and 2 , in a cross section perpendicular to the longitudinal direction of the steel wire 1, the total decarburized layer depth d is less than 0.5% of the cross section diameter. The total decarburized layer depth d can be measured according to JIS G0558. More specifically, refer to Figure 2, the decarburized portion 4 is formed near the outer peripheral surface 2 of the steel wire 1. In the base metal 5, the decarburized portion 4 extends from the outer peripheral surface 2 to the inside along the grain boundary in the steel. In the steel wire 1 of the present embodiment, ferrite decarburization as decarburization in the entire crystal grain does not occur, but partial decarburization (grain boundary decarburization) occurs along the grain boundary to form the decarburized portion 4. The depth of the area where partial decarburization occurs is the total decarburized layer depth d in the present embodiment. The total decarburized layer depth d is preferably 0.5% or less of the diameter of the steel wire 1, and more preferably 0.3% or less.

[0042] For the steel wire 1 of the present embodiment, the steel having the above-mentioned component composition which can ensure the strength required for the spring is used. In order to improve the permanent deformation resistance and strength, the total decarburized layer depth d is at a relatively low level of 0.5% or less of the diameter, and the difference between the maximum value and the minimum value of the hardness in the depth direction (radial direction) is 50 HV or less. As a result, the steel wire 1 is a steel wire excellent in permanent deformation resistance and strength.

[0043] In the steel forming the steel wire 1, it is preferred that inevitable impurities contain 0.025 mass % or less of P, 0.025 mass % or less of S, and 0.2 mass % or less of Cu. The respective contents of P, S, and Cu contained in the component composition as inevitable impurities may be within a specific range so as to improve properties such as toughness.

[0044] For example, the steel wire 1 has a diameter of 0.5 mm or more and 7 mm or less. The diameter of the steel wire 1 may be 2 mm or more and 6.5 mm or less. The diameter of the steel wire 1 may be 2 mm or more and 5 mm or less.

[0045] The steel wire 1 preferably has a tensile strength of 1900 MPa or more and 2450 MPa or less. A tensile strength of 1900 MPa or more can ensure sufficient strength. A tensile strength of 2450 MPa or less can easily ensure sufficient toughness. In order to achieve higher strength, the tensile strength of the steel wire 1 is more preferably 1950 MPa or more. In order to more reliably ensure sufficient toughness, the tensile strength of the steel wire 1 is more preferably 2160 MPa or less.

[0046] In the steel wire 1, the cross-sectional reduction ratio in the tensile test is preferably 40% or more and 60% or less. A cross-sectional reduction ratio of 40% or more can improve the processability of the wire into a spring and the fatigue strength of the spring formed by processing the wire. A cross-sectional reduction ratio of 60% or less can easily ensure sufficient strength. In order to improve the processability of the wire into a spring and the fatigue strength of the spring formed by processing the wire, the cross-sectional reduction ratio is more preferably 50% or more.

[0047] The cross-sectional reduction ratio when drawing the steel wire 1 is preferably 80% or more and 90% or less. Therefore, excellent permanent deformation resistance and excellent strength can be more reliably achieved. The cross-sectional reduction ratio when drawing the steel wire 1 is more preferably 83% or more and 87% or less.

[0048] refer to Figure 3 The spring 100 in this embodiment is made of the steel wire 1 of this embodiment as described above. The spring 100 is the steel wire 1 that has been processed into a coil spring shape. The spring 100 is made of the steel wire 1 having a coil shape. The spring 100 is a coil spring. The spring 100 is made of the steel wire 1 of this embodiment that has excellent resistance to permanent deformation and strength, so the spring 100 is a spring that has excellent resistance to permanent deformation and strength.

[0049] In the spring 100, the compressive residual stress in the surface (the surface corresponding to the outer peripheral surface of the steel wire 1) is preferably 300 MPa or more. Therefore, the fatigue life of the spring 100 is extended. Basically, a larger compressive residual stress in the surface is preferred. However, in order to achieve a compressive residual stress greater than 800 MPa, the time required for shot peening, for example, increases, which may lead to problems such as reduced production efficiency. Therefore, the compressive residual stress in the surface may be 800 MPa or less.

[0050] In the spring 100, the hardness of the surface is preferably 450 HV or more. Therefore, the fatigue life of the spring is extended. Basically, a higher hardness in the surface is preferred. However, achieving a hardness greater than 650 HV may lead to problems such as reduced production efficiency. Therefore, the hardness of the surface may be 650 HV or less.

[0051] Next, an example of a method for manufacturing the steel wire 1 and the spring 100 will be described. Figure 4 In the method for manufacturing the steel wire 1 in the present embodiment, first, a raw material wire material preparation step is performed as a step (S10). Figure 5 In step (S10), a raw material wire 10 made of steel containing 0.7 mass % or more and 1.0 mass % or less of carbon, 0.12 mass % or more and 0.32 mass % or less of silicon, and 0.3 mass % or more and 0.9 mass % or less of manganese, with the remainder being iron and inevitable impurities, is prepared. Specifically, for example, a raw material wire 10 made of SWRS87B specified in JIS G3502 is prepared. The shape of the cross section perpendicular to the longitudinal direction of the raw material wire 10 is circular.

[0052] Next, a step of removing the surface layer is performed as step (S20). In step (S20), the surface layer including the outer peripheral surface of the raw material wire 10 prepared in step (S10) is removed along the entire circumference. For example, the surface layer can be removed by cutting. Although step (S20) is not an essential step for manufacturing the steel wire 1, it can be performed to remove the decarburized layer of the raw material wire 10 prepared in step (S10).

[0053] Next, a Padun treatment is performed as step (S30). In step (S30), the raw material wire 10 treated in step (S20) is subjected to a Padun treatment. Specifically, the raw material wire 10 is subjected to the following heat treatment: the raw material wire 10 is heated to a temperature range above the austenitizing temperature (point A1), and then quenched to a temperature above M s The temperature range of the raw material wire 10 is set to the A1 point and maintained within the temperature range. As a result, the metal structure of the raw material wire 10 becomes a fine pearlite structure with a small interlamellar spacing. In the above-mentioned Payton treatment, in order to suppress the occurrence of decarburization, the raw material wire 10 is heated to a temperature range of A1 point or higher in an inert gas atmosphere. In order to suppress the occurrence of decarburization, it is preferred to heat the raw material wire 10 to a temperature range of A1 point or higher within the shortest required time.

[0054] Next, a wire drawing step is performed as step ( S40 ). In step ( S40 ), the raw material wire material 10 is drawn (stretched).

[0055] Figure 6 The cross section of the workpiece in the advancing direction α in the stretching device for performing step (S40) is shown. Figure 6 , the stretching device for performing step (S40) includes a die 50. In the die 50, a through hole is formed that passes through the die 50 in the advancing direction α of the workpiece. The wall surface surrounding the through hole is a processing surface 51 to be in contact with the workpiece. The raw material wire 10 that has been advanced to step (S30) advances in the longitudinal direction (the advancing direction α of the workpiece) to enter the through hole in the die 50. When the raw material wire 10 reaches the entrance 58, the outer peripheral surface 11 of the raw material wire 10 contacts the processing surface 51 of the die 50. Thus, the raw material wire 10 is processed, that is, plastically deformed in such a manner that the shape of its cross section perpendicular to the longitudinal direction corresponds to the shape of the processing surface 51 in the cross section perpendicular to the advancing direction α of the workpiece. For the area of ​​the through hole in the die 50 in the cross section perpendicular to the advancing direction α of the workpiece, the area at the exit 59 is smaller than the area at the entrance 58. When the raw material wire 10 reaches the exit 59 of the die 50, the stretching by the processing surface 51 of the die 50 is completed, and the steel wire 1 is obtained thereby.

[0056] The through hole of the die 50 has a tapered region. In the tapered region, the cross section of the through hole perpendicular to the advancing direction α of the workpiece becomes smaller from the inlet 58 toward the outlet 59. The die half angle θ, which is the taper angle of the tapered region, can be made smaller to reduce the difference between the maximum and minimum values ​​of the hardness on the straight line passing through the center of gravity in the cross section perpendicular to the longitudinal direction of the obtained steel wire 1. For the method of manufacturing the steel wire 1 of the present embodiment, a die 50 having a die half angle θ that is about 30% smaller than that of the die used in the method of manufacturing ordinary spring steel wire is used. In step (S40), the cross-sectional reduction ratio (S0-S) / S0 is preferably 80% or more and 90% or less, and more preferably 83% or more and 87% or less, wherein S0 is the cross-sectional area of ​​the cross section of the raw material wire 10 perpendicular to the longitudinal direction, S0-S is the difference between S0 and the cross-sectional area S of the cross section of the steel wire 1 perpendicular to the longitudinal direction, and the cross-sectional reduction ratio refers to the ratio of the difference (S0-S) to the cross-sectional area S0. The wire drawing in step (S40) may include a plurality of drawing steps performed using a plurality of dies.

[0057] The steel wire 1 of the present embodiment is obtained through the above steps. In the method for manufacturing the steel wire 1 of the present embodiment, step (S20) is performed to remove the decarburized layer. In addition, in step (S30), heating in the Paedon treatment is performed in an inert atmosphere to suppress the occurrence of decarburization in the raw wire 10. Therefore, the total decarburized layer depth in the cross section perpendicular to the longitudinal direction of the steel wire 1 can be less than 0.5% of the cross-sectional diameter. In addition, in step (S40) of the method for manufacturing the steel wire 1 of the present embodiment, wire drawing is performed using a die 50 having a small die half angle θ. Thus, in the cross section perpendicular to the longitudinal direction of the steel wire 1, the difference between the maximum and minimum hardness values ​​on the straight line passing through the center of gravity can be reduced to less than 50HV. Therefore, by the method for manufacturing the steel wire 1 of the present embodiment, a steel wire 1 having excellent resistance to permanent deformation and strength can be manufactured.

[0058] Next, refer to Figure 4 , a spring forming step is performed as step (S50), in which the steel wire 1 obtained through steps (S10) to (S40) is processed into a spring shape. Specifically, in step (S50), cold winding is performed to process the steel wire 1 into a spiral spring shape having a spiral shape (see Figure 3 ).

[0059] Next, a strain relief step is performed as step (S60). In step (S60), a process of reducing the strain (processing strain) introduced into the spring when performing step (S50) is performed. Specifically, the spring obtained by performing step (S50) is annealed: for example, the spring is heated to a temperature of 200°C or more and 400°C or less, and maintained for 10 minutes or more and 60 minutes or less.

[0060] Next, a shot peening step is performed as step (S70). In step (S70), the spring whose strain has been reduced in step (S60) is ground on its end surface and then shot peened. Thereafter, the spring is annealed, cold-set, etc. as needed.

[0061] Through the above steps, the spring 100 of the present embodiment is obtained. In the method for manufacturing the spring 100 of the present embodiment, the steel wire 1 having excellent resistance to permanent deformation and strength is processed to manufacture the spring 100. Therefore, the spring 100 having excellent resistance to permanent deformation and strength can be manufactured. In addition, for example, the conditions of shot peening in step (S70) and the annealing conditions, the wire drawing conditions in step (S40) and the composition of the steel can be adjusted to adjust the size and hardness of the compressive residual stress in the surface of the spring 100 so that the size and hardness of the compressive residual stress are each within a preferred range (the compressive residual stress is more than 300MPa and the hardness is more than 450HV). In step (S70), shot peening can include multiple shot peening steps, such as two shot peening steps. In this case, the first shot peening step and the second shot peening step can be different in the size of the shot peening particles. The second shot peening can use shot peening particles that are smaller than the shot peening particles of the first shot peening. The average value of the diameter of the shot peening particles of the first shot peening and the diameter of the shot peening particles of the second shot peening may be, for example, about 0.3 mm. The diameter of the shot peening particles of the first shot peening may be, for example, 0.3 mm or more and 0.8 mm or less, and the diameter of the shot peening particles of the second shot peening may be, for example, 0.05 mm or more and 0.3 mm or less.

[0062] Example

[0063] Example 1

[0064] An experiment was conducted to confirm the permanent deformation resistance of the steel wire of the present invention. The experimental steps are as follows. First, a raw wire made of SWRS87B specified in JIS G3502 is prepared, and a steel wire 1 is manufactured by a method similar to the method in the above-mentioned embodiment. Steel wires 1 (Examples A, B, C and D) whose cross-sectional reduction ratios in wire drawing in step (S40) are 78.2%, 83.0%, 84.9% and 86.4%, respectively, are manufactured. For comparison, a general oil-tempered wire (Comparative Example A) and a general piano wire (Comparative Example B) are also prepared. As a general oil-tempered wire, an oil-tempered wire for valve springs (SWOS C-V specified in JIS G3561) is used. As a general piano wire, a piano wire made by subjecting a raw wire made of SWRS82A specified in JIS G3522 to a Paedon treatment and wire drawing is used.

[0065] The steel wires 1 prepared in Examples A to D are each processed into Figure 7 A test piece 70 of the shape shown in the figure is provided, wherein the test piece 70 has a straight long side 72 and a pair of straight short sides 71, wherein the pair of straight short sides 71 are connected to opposite ends of the long side 72 and extend in a direction perpendicular to the long side 72 in a manner opposite to each other. Next, the test piece 70 is subjected to a heat treatment of heating to 350° C. and maintaining it for 20 minutes. Then, the test piece 70 is placed on the main surface 81A of the substrate 81 in a manner in which the long side 72 and the pair of short sides 71 are in contact with the main surface 81A. Figure 8 As shown in FIG. 1 , while the test piece 70 is heated to 120° C., one short side 71 is held by a holding member 82 so that the short side 71 remains in contact with the main surface 81A, and the other short side 71 is raised by a clamp 83 while the connecting portion of the long side 72 remains in contact with the main surface 81A. The test piece 70 is held for 24 hours under the condition of generating a shear stress of 1000 MPa. Thereafter, as Fig. 9 As shown, the jig 83 is removed, and the angle φ between the main surface 81A and the short side 71 maintained in the raised state is measured. Based on the obtained angle φ, the residual shear strain is calculated. Similar experiments are performed on Comparative Examples A and B to calculate the shear strain. The experimental results are shown in Fig.10 middle.

[0066] refer to Fig.10 , it can be confirmed that: compared with Comparative Example B which is a general piano wire, Examples A to D which are steel wires of the present application each have a reduced residual shear strain and thus have excellent resistance to permanent deformation. In addition, compared with Comparative Example A which is an oil-tempered wire with high manufacturing cost due to quenching and tempering treatment, Examples B, C and D each having a cross-sectional reduction rate during wire drawing of 80% or more and 90% or less have a permanent deformation resistance equivalent to that of Comparative Example A. In view of the above, it is confirmed that: a cross-sectional reduction rate during wire drawing of 80% or more and 90% or less (83% or more and 87% or less) is preferred. In particular, the residual shear strain of Example C is 0.18% or less, and the resistance to permanent deformation of Example C is particularly excellent.

[0067] From the above experimental results, it can be confirmed that the steel wire of the present invention can achieve excellent resistance to permanent deformation.

[0068] Example 2

[0069] Experiments were conducted to confirm the effect of the die half-angle θ during wire drawing on the properties of the steel wire. According to the method for manufacturing the steel wire in the above-mentioned embodiment, the steel wire was manufactured under the condition that the die half-angle θ during wire drawing was within the range used for manufacturing ordinary spring steel wire (Comparative Example C), and the steel wire was manufactured under the condition of using a die half-angle θ that was 30% smaller than the die half-angle θ of Comparative Example C (the above-mentioned Example B). Tensile tests were performed on these steel wires. In addition, in a cross section perpendicular to the longitudinal direction of each steel wire of Example B and Comparative Example C, the hardness distribution from the position directly below the surface to the center on a straight line passing through the center of the cross section was measured. The results of the tensile test are shown in Table 1. The measurement results of the hardness distribution are shown in Tables 2 and Fig.11 The diameter of each steel wire is 3.5 mm.

[0070]

[0071]

[0072] Referring to Table 1, the steel wire of Example B is greater than the steel wire of Comparative Example C in both tensile strength and area reduction. From the above, it is confirmed that setting the die half angle to a smaller angle is effective in improving area reduction while ensuring high hardness.

[0073] Refer to Table 2 and Fig.11 The hardness difference in the radial direction of the steel wire of Comparative Example C is 69 HV, and the hardness difference in the radial direction of the steel wire of Example B is 43 HV. From the above, it is confirmed that setting the die half angle to a smaller angle is also effective in reducing the hardness difference.

[0074] Example 3

[0075] An experiment was conducted to confirm the effect of surface hardness and the magnitude of compressive stress in the surface on the durability of the spring. The experimental steps are as follows.

[0076] First, a raw wire made of SWRS87B specified in JIS G3502 is prepared, and a steel wire 1 is manufactured by performing steps (S10) to (S40) similar to the above-mentioned embodiment. The steel wire 1 is cold-wound (average diameter of the coil: 21.6 mm, number of turns: 5.75 turns, effective number of turns: 3.25 turns), and then annealed by heating to 300° C. and holding for 25 minutes to eliminate strain. Thereafter, the end surface of the obtained spring is ground and shot peening is performed. Shot peening includes a first shot peening and a second shot peening. The average diameter of the shot peening particles of the first shot peening and the shot peening particles of the second shot peening is 0.3 mm. Thereafter, a low-temperature annealing of heating the spring to 230° C. and holding for 10 minutes is performed, and cold setting is also performed to manufacture a spring 100. In shot peening, the size of the shot peening particles and the treatment time of the first shot peening and the second shot peening were adjusted to obtain seven samples (samples A to G) with different surface hardness and compressive residual stress in the surface. The surface hardness was measured according to JIS Z2244. The residual stress was measured by X-ray diffraction. Then, under the conditions of an average stress of 588 MPa and a stress amplitude of 534 MPa, a fatigue test was performed on the spring 100 of samples A to G by repeated compression in the axial direction. Eight springs were manufactured for each sample A to G and the experiment was performed. After repeated application of stress for 10 6 Repeated stress application 10 times 7 The number of unbroken springs was counted to evaluate the durability. Table 3 shows the surface hardness and compressive residual stress in the surface of each sample and the experimental results.

[0077]

[0078] In the column "Durability Evaluation" in Table 3, A indicates particularly high durability, B indicates high durability, C indicates sufficient durability, and D indicates insufficient durability. The conditions of the above experiment are stricter than the normal use conditions of the spring. Therefore, a spring evaluated as D is not necessarily unusable, but can still be used under milder conditions.

[0079] Referring to Table 3, samples with a surface hardness of 450 HV or more (further 500 HV or more) and samples with a compressive residual stress of 300 MPa or more in the surface have high durability. From the above, it can be seen that for the spring of the present application, a surface hardness of 450 HV or more is preferred, and a surface hardness of 500 HV or more is more preferred. It is also confirmed that the spring of the present application with a compressive residual stress of 300 MPa or more in the surface is preferred.

[0080] Example 4

[0081] Experiments were conducted to confirm the resistance of the spring of the present invention to permanent deformation. Specifically, the springs 100 of samples E, F and A were subjected to the following treatment: the springs heated to 120°C were compressed in the axial direction and maintained for 48 hours. For comparison, a general oil-tempered wire (sample H, the same type of oil-tempered wire as in Example 1) and a general piano wire (sample I, the same type of piano wire as in Example 1) were processed into similar springs and subjected to similar experiments. Thereafter, the compression was released, and the residual shear strain was calculated from the change in the axial length of the spring from before the test to after the test. The shear stress value when compressing the spring was changed to confirm the relationship between the shear stress value and the shear strain amount. The experimental results are shown in Fig.12 middle.

[0082] refer to Fig.12 , for each spring, an increase in shear stress is accompanied by an increase in residual shear strain. For each stress, the shear strain amount of samples E, F, and A, which are springs of the present invention, is smaller than the shear strain amount of sample I, which is a spring made of a general piano wire. In addition, the shear strain amount of samples E, F, and A is equivalent to the shear strain amount of sample H, which is a spring made of a general oil-tempered wire with a high manufacturing cost due to the cost of heat treatment. From the above, it is confirmed that the spring of the present application is a spring with excellent resistance to permanent deformation.

[0083] Example 5

[0084] An experiment was conducted to confirm the influence of the total decarburized layer depth on the surface hardness of the steel wire. In the method for manufacturing the steel wire of the embodiment described above, steel wires having different total decarburized layer depths were prepared by changing whether to perform step (S20) and the heat treatment conditions in step (S30). The surface hardness of each steel wire was then measured. Regarding the surface hardness, the hardness in the area from the surface of the cross section to 25 μm was measured according to JIS Z2244. The experimental results are shown in Fig.13 In. Fig.13 In the figure, the horizontal axis represents the ratio of the total decarburized layer depth in the cross section perpendicular to the longitudinal direction to the diameter of the cross section of the steel wire. Fig.13 In the figure, the vertical axis represents the surface hardness.

[0085] refer to Fig.13 , the greater the total decarburization depth, the smaller the surface hardness. For a total decarburization depth of 0.5% or less of the wire diameter, the surface hardness (surface hardness) has a preferred value of 450 HV or more. From the above, it was confirmed that in a cross section perpendicular to the longitudinal direction of the wire, it is preferred that the total decarburization depth be 0.5% or less of the diameter.

[0086] It should be understood that the embodiments and examples disclosed herein are illustrative and not restrictive in all aspects. The scope of the present invention is defined by the claims rather than the above description, and includes any modifications within the meaning and scope equivalent to the claims.

[0087] Description of symbols

[0088] 1: steel wire; 2: peripheral surface; 4: decarburized portion; 5: base metal; 10: raw wire; 11: peripheral surface; 50: die; 51: processed surface; 58: inlet; 59: outlet; 70: test piece; 71: short side; 72: long side; 81: substrate; 81A: main surface; 82: holding member; 83: fixture; 100: spring.

Claims

1. A steel wire for a spring, the steel wire being made of steel containing: 0.7 mass % or more and 1.0 mass % or less of carbon, 0.12 mass % or more and 0.32 mass % or less of silicon, and 0.3 mass % or more and 0.9 mass % or less of manganese, the remainder being iron and unavoidable impurities, The metal structure of the steel is a pearlite structure, In the cross section perpendicular to the longitudinal direction of the steel wire, the total decarburized layer depth is less than 0.5% of the cross section diameter. In the cross section perpendicular to the longitudinal direction of the steel wire, the difference between the maximum and minimum hardness values ​​on the straight line passing through the center of gravity is less than 50 HV, The diameter of the cross section perpendicular to the longitudinal direction is 0.5 mm or more and 6.5 mm or less, The steel wire is produced by heating a raw material wire rod to a temperature in the range of A1 point or higher, which is an austenitizing temperature, in an inert gas atmosphere. 2 . The steel wire for a spring according to claim 1 , wherein the unavoidable impurities include 0.025 mass % or less of phosphorus, 0.025 mass % or less of sulfur, and 0.2 mass % or less of copper. 3 . The steel wire for a spring according to claim 1 , wherein a diameter of a cross section perpendicular to a longitudinal direction is 0.5 mm or more and 5 mm or less. 4 . The steel wire for a spring according to claim 1 , wherein the steel wire has a tensile strength of 1900 MPa or more and 2450 MPa or less. 5 . The steel wire for a spring according to claim 1 , wherein a reduction in area in a tensile test is 40% or more and 60% or less.

6. The steel wire for spring according to claim 1 or 2, wherein the residual shear strain in a permanent deformation resistance test is 0.2% or less. 7 . The steel wire for spring according to claim 1 , wherein a reduction in area during wire drawing is 80% or more and 90% or less.

8. A spring made of the steel wire for a spring according to any one of claims 1 to 7. 9 . The spring according to claim 8 , wherein the compressive residual stress in the surface of the spring is 300 MPa or more. 10 . The spring according to claim 8 , wherein the hardness of the surface of the spring is 450 HV or more.

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