Spring member

Abstract

A spring member according to the present invention is formed by deforming a wire material. The wire material contains, in terms of mass%, 0.39% to 0.60% inclusive of carbon (C) and 1.80% to 2.80% inclusive of silicon (Si), with the balance being made up of iron (Fe) and unavoidable impurities. The parameter P calculated using formula (1) is 9.0 or more.　(1): P = (carbide area ratio) - ((HRC - 49) × 0.9) In the formula, HRC is the Rockwell hardness of a formed article.

Description

Spring material

[0001] The present invention relates to a spring member.

[0002] Conventionally, hot forming and cold forming have been adopted in the process of manufacturing spring members such as coil springs. Of these, cold forming has high processing accuracy but higher deformation resistance than hot forming, making it unsuitable for forming relatively large members or wire rods with large wire diameters. On the other hand, hot forming has lower processing accuracy than cold forming, but lower deformation resistance, making it possible to form large members or wire rods with large wire diameters. For this reason, hot forming is used for forming large members or wire rods with large wire diameters. As a member formed by hot forming, a spring member formed by spirally winding wire rod is known (see, for example, Patent Document 1). Here, in conventional hot forming, for example, after hot forming, quenching is performed in an oil bath and then tempering is performed in a furnace.

[0003] Special table 2017-532450 publication

[0004] In order to adapt spring members to various usage environments, it has been required that they be capable of forming large members, such as those formed by hot forming, and wire rods with large wire diameters, which are conventionally used in hot forming, and that they have high toughness.

[0005] The present invention has been made in view of the above, and has as its object to provide a spring member having higher toughness than members formed by conventional hot forming.

[0006] In order to solve the above-mentioned problems and achieve the object, the spring member according to the present invention is a spring member formed by deforming a wire rod, characterized in that the wire rod contains, by mass%, 0.39% to 0.60% carbon (C), 1.80% to 2.80% silicon (Si), and the balance being iron (Fe) and unavoidable impurities, and has a parameter P calculated using the following formula (1) of 9.0 or greater: P = carbide area ratio - ((HRC - 49) x 0.9) (1) where HRC is the Rockwell hardness of the molded product.

[0007] Furthermore, in the spring member according to the present invention, in the above invention, the wire rod further contains 0.05% or more and 1.50% or less of manganese (Mn), 0.05% or more and 1.50% or less of chromium (Cr), 0.025% or less of phosphorus (P), 0.025% or less of sulfur (S), 1.00% or less of copper (Cu), 1.00% or less of nickel (Ni), and 0.20% or less of titanium (Ti).

[0008] In addition, the spring member according to the present invention is characterized in that, in the above invention, the carbide area ratio is calculated based on a binarized image obtained by binarizing an image of an area where carbides are precipitated from a scanning electron microscope (SEM) image.

[0009] In addition, the spring member of the present invention is characterized in that, in the above invention, the binary image is generated by extracting an area in which the aspect ratio of the longitudinal length to the lateral length of the area identified as the carbide is 2 or more.

[0010] Furthermore, in the spring member according to the present invention, the wire rod has an equivalent circle diameter of 8 mm or more and 24 mm or less.

[0011] In addition, the spring member of the present invention is characterized in that, in the above invention, the difference between the maximum and minimum values ​​of the average Vickers hardness in the depth direction from the surface of the wire on the inner diameter side, outer diameter side, and inter-wire side of the spring member toward the center of the wire is 30 HV or less in the depth range of 0.5 mm or more and 3.0 mm or less.

[0012] Furthermore, the spring member according to the present invention is characterized in that, in the above invention, in a residual stress distribution curve in the direction of maximum principal stress generated when a compressive load is applied to the inner diameter side of the spring member, the vertical axis represents compressive residual stress and the horizontal axis represents depth from the surface, the integrated value in the range of depth from the wire surface of 0.3 mm to 0.5 mm is 10 MPa mm or more.

[0013] According to the present invention, it is possible to obtain a spring member having higher toughness than members formed by conventional hot forming.

[0014] FIG. 1 is a diagram illustrating a configuration of a coil spring according to an embodiment of the present invention. FIG. 2 is a diagram illustrating an example of a manufacturing method of a coil spring according to an embodiment of the present invention. FIG. 3 is a diagram illustrating an example of an SEM image. FIG. 4 is a flowchart illustrating a flow of a parameter calculation process. FIG. 5 is a diagram illustrating acquisition of a region where precipitation occurs perpendicular to an observation surface. FIG. 6 is a diagram (part 1) illustrating an example of binarization of a carbide precipitation region of a tempered molded product produced by the manufacturing method shown in FIG. 2. FIG. 7 is a diagram (part 2) illustrating an example of binarization of a carbide precipitation region of a tempered molded product produced by the manufacturing method shown in FIG. 2. FIG. 8 is a diagram (part 3) illustrating an example of binarization of a carbide precipitation region of a tempered molded product produced by the manufacturing method shown in FIG. 2. FIG. 9 is a diagram (part 1) illustrating an example of binarization of a carbide precipitation region of a molded product after tempering in hot forming. FIG. 10 is a diagram (part 2) illustrating an example of binarization of a carbide precipitation region of a molded product after tempering in hot forming. Fig. 11 is a diagram (part 3) for explaining an example of binarization of a carbide precipitation region of a molded product after tempering in hot forming. Fig. 12 is a diagram for explaining an example of the relationship between hardness and parameters. Fig. 13 is a diagram for explaining an example of the relationship between the depth from the surface of a wire rod and the hardness of a molded product after tempering produced by the manufacturing method shown in Fig. 2. Fig. 14 is a diagram for explaining an example of the relationship between the depth from the surface of a wire rod and the hardness in cold forming. Fig. 15 is a diagram for explaining calculation of an integrated value in a compressive residual stress distribution curve.

[0015] 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.

[0016] (Embodiment) FIG. 1 is a diagram illustrating the configuration of a coil spring according to one embodiment of the present invention. The coil spring 1 is produced by spirally winding a wire rod. The coil spring 1 is produced using, for example, a wire rod made of a metal or alloy. Specifically, the wire rod contains, by mass, 0.39% to 0.60% carbon (C), 1.80% to 2.80% silicon (Si), and the remainder iron (Fe) and inevitable impurities. The wire rod has a wire diameter of, for example, 8 mm to 24 mm. This wire diameter is the diameter of the wire rod used primarily for hot forming. Note that the coil spring 1 shown in FIG. 1 is an example of a spring member, and other shapes, such as a zigzag shape, a stabilizer bar, or a leaf spring, may also be used.

[0017] The wire rod preferably further contains, by mass%, 0.05% to 1.50% manganese (Mn), 0.05% to 1.50% chromium (Cr), 0.025% to 0.025% phosphorus (P), 0.025% to 0.025% sulfur (S), 1.00% to 1.00% copper (Cu), 1.00% to 1.00% nickel (Ni), and 0.20% to 0.20% titanium (Ti). When manganese (Mn) is contained within the above range, hardenability can be improved. When chromium (Cr) is contained within the above range, hardenability and corrosion resistance can be improved. When copper (Cu) is contained within the above range, strength due to precipitation hardening can be improved. When nickel (Ni) is contained within the above range, hardenability, toughness, and corrosion resistance can be improved. When titanium (Ti) is contained within the above range, toughness can be improved. Phosphorus (P) and sulfur (S) are components that are mixed in during steelmaking, but it is preferable that they are contained within the above ranges.

[0018] Next, an example of a method for manufacturing the coil spring 1 will be described with reference to Fig. 2. Fig. 2 is a diagram for explaining the method for manufacturing the coil spring according to the present embodiment. The coil spring 1 is manufactured by processing a base material.

[0019] First, a base material 100 (see FIG. 2(a)) made of a wire is subjected to a wire drawing process to obtain a drawn wire material 101 (see FIG. 2(b)). At this time, the base material 100 (drawn wire material 101) is not subjected to a heat treatment, and a wire drawing machine is used to reduce the diameter of the wire material by, for example, passing it through a die, thereby obtaining a wire material (drawn wire material 101) with a designed diameter.

[0020] Thereafter, the drawn wire material 101 is shaped by cold forming (see FIG. 2(c)). Specifically, the drawn wire material 101 is wound using a winding machine 200. This winding machine 200 includes, for example, a winding pin and a cutting tool, and shapes the drawn wire material 101 by bringing it into contact with the winding pin, and cuts the drawn wire material 101 to a predetermined length using the cutting tool.

[0021] The formed material 102 obtained by winding and cutting the drawn wire material 101 is subjected to electrical heating (see (d) of FIG. 2). In electrical heating, a first current-carrying member 211 is attached to one end of the formed material 102, and a second current-carrying member 212 is attached to the other end, and electricity is passed through the first current-carrying member 211 and the second current-carrying member 212 to pass electricity through the formed material 102. Heat is generated by this current flow, and the formed material 102 is heated.

[0022] After the formed material 102 is electrically heated, the formed material 102 is quenched (see FIG. 2(e)). The formed material 102 is immersed in a tank 221 containing a water-soluble quenchant 222. The temperature and concentration of the water-soluble quenchant are controlled to ensure appropriate heat treatment quality. Immersion of the formed material 102 in the water-soluble quenchant 222 results in a quenched formed material 103. Note that oil may be used instead of the water-soluble quenchant 222.

[0023] After quenching, the formed material 103 is subjected to electrical heating (electrical tempering) for tempering (see FIG. 2(f)). In electrical tempering, a first electrical conducting member 231 is attached to one end of the formed material 103 and a second electrical conducting member 232 is attached to the other end, and current is passed through the first electrical conducting member 231 and the second electrical conducting member 232 to pass current through the formed material 103. This current passage generates heat, which heats the formed material 103. In electrical tempering, electrical conditions are set for reheating the formed material 103 to a predetermined hardness.

[0024] The coil spring 1 shown in Fig. 1 is produced by processing the base material 100 according to the above-described process. Here, annealing may be performed before or after the wire drawing process. Furthermore, if the wire diameter is as designed in the base material state, it is possible to form the base material 100 without performing the wire drawing process. Note that various controls such as energization are performed by a control device (not shown).

[0025] Next, the characteristics of the tempered coil spring produced by the above-described forming process will be described with reference to Fig. 3 to Fig. 13. Fig. 3 is a diagram showing an example of a scanning electron microscope (SEM) image of the surface of a tempered molded product. Fig. 3 is a diagram showing an example of an SEM image of a tempered molded product (coil spring) produced by the manufacturing method shown in Fig. 2.

[0026] When the molded product is tempered, transition carbides are precipitated. 2～3 C. Hardening due to the precipitation of this carbide makes it possible to improve or adjust the hardness and toughness of the molded product.

[0027] In the SEM images of various molded products shown in FIG. 3, the linear objects observed in the center correspond to carbides.

[0028] 4 is a flowchart showing the flow of a parameter calculation process. This calculation method is executed using a computer. The computer is configured using hardware such as a central processing unit (CPU), a graphics processing unit (GPU), a field programmable gate array (FPGA), an application specific integrated circuit (ASIC), a complex programmable logic device (CPLD), and a memory in which data such as various programs is installed. An example of how a processing unit of a computer executes the calculation process will be described below.

[0029] First, the processing unit acquires an SEM image of the molded product after tempering (step S101). The acquired SEM image is, for example, a secondary electron image. The processing unit acquires the SEM image of the molded product, for example, via a communication network or a recording medium. In this case, the molded product is corroded using nital or the like, which makes it possible to more clearly distinguish the image from carbides in the subsequent binarization process.

[0030] The processing unit then acquires a region where carbide is precipitated perpendicular to the observation surface (step S102). The observation location of the SEM image at this time is a region with a depth of 0.3 mm or more and 0.8 mm or less on the outer diameter side of the coil spring. In accordance with instructions input by the user, the processing unit acquires, for example, six different fields of view, each consisting of a region where carbide is precipitated, by cutting out the six fields of view from the SEM image. For example, the processing unit cuts out the region where carbide is precipitated from the SEM image.

[0031] FIG. 5 is a diagram for explaining how to obtain the region where carbide has precipitated perpendicular to the observation surface. FIG. 5 shows an image of the surface of a molded product observed by SEM at 10,000x magnification. The user specifies the position of carbide precipitation by increasing the observation magnification while changing the observation conditions such as magnification. The user can specify the region where carbide has precipitated (regions R1, R2, R3, R4, R5, R6, R7, R8, R9, R10, R11, R12, R13, R14, R15, R16, R17, R18, R19, R20, R21, R22, R23, R24, R25, R26, R27, R28, R29, R30, R31, R32, R33, R34, R35, R36, R37, R38, R39, R40, R41, R42, R43, R44, R45, R46, R47, R48, R49, R50, R51, R52, R53, R54, R55, R56, R57, R58, R59, R59, R60, R61, R62, R63, R64, R65, R66, R67, R68, R69, R69, R70, R71, R72, R73, R74, R75, R76, R77, R78, R79, R79, R79, R79, R79, R80, R79, R81, R79, R79, R71, R72, R73, R74, R75, R76, R77, R78, R79, R79, R79, R79, R79, R79, R79 10 ) in the region R where the precipitates are parallel to the observation surface. 10 The region R1 where carbides are precipitated perpendicular to the observation surface is cut out and obtained, excluding the region R1. Here, since carbides have a flat shape, they appear linear when viewed perpendicular to the observation surface.

[0032] In this case, the characteristics of the region may be determined using an inverse pole figure (IPF) map obtained by electron backscattered diffraction pattern (EBSD). The IPF map represents a crystal orientation map defined by crystal planes, which is displayed in color based on the direction (plane index) relative to the observation surface. From the IPF map, the user identifies the region where carbides are precipitated perpendicular to the observation surface, and specifies the position (observation range or cut-out range). Note that the region may also be mechanically cut out using image processing or the like.

[0033] Returning to FIG. 4 , the processing unit binarizes the image of each region (step S103). In binarization, a preset signal value (e.g., brightness value) is used as a threshold, and positions having a signal value equal to or greater than the threshold are colored white, and positions having a signal value less than the threshold are colored black, thereby binarizing the image. In the binarization process, carbides can be colored white and other components can be colored black by adjusting the threshold. In this case, from the viewpoint of reliably extracting carbides precipitated perpendicular to the observation surface, it is preferable that the processing unit removes carbides having an aspect ratio, which is the ratio of the longitudinal length to the lateral length of the carbide region, of less than 2, and generates a binarized image in which only carbides in regions having an aspect ratio of 2 or greater are extracted. Note that noise removal may be performed simultaneously during binarization. Furthermore, the settings of white and black may be reversed during binarization.

[0034] FIGS. 6 to 8 are diagrams illustrating an example of binarization of the carbide precipitation region of a molded product after tempering produced by the manufacturing method shown in FIG. 2 . FIGS. 9 to 11 are diagrams illustrating an example of binarization of the carbide precipitation region of a molded product after tempering in hot forming. The molded products shown in FIGS. 6 to 11 are each manufactured using a wire containing C, Si, Mn, Cr, P, S, Cu, and Ni in the mass percent ranges described above, with the balance being iron (Fe) and unavoidable impurities. FIG. 6 shows the carbide extraction region of a molded product with a Rockwell hardness (HRC) of 49.2. FIG. 7 shows the carbide extraction region of a molded product with an HRC of 51.1. FIG. 8 shows the carbide extraction region of a molded product with an HRC of 53.4. FIG. 9 shows the carbide extraction region of a molded product with an HRC of 50.9. FIG. 10 shows the carbide extraction region of a molded product with an HRC of 54.2. 11 shows the extracted area of ​​carbides in a compact with an HRC of 56.4. The HRC was calculated by measuring the Vickers hardness (HV) of each compact and converting it into Rockwell hardness.

[0035] In Figures 6 to 11, (a) shows an image of the region in the SEM image where carbides precipitated perpendicular to the observation surface, and (b) shows a binarized image of each region after noise removal and extraction of the region with an aspect ratio of 2 or more. As shown in Figures 6 to 11, binarized images are obtained for each region of the tempered molded product produced by the manufacturing method shown in Figure 2 and the molded product produced by hot forming, in which only carbides with an aspect ratio of 2 or more are represented. Note that Figures 6(a) to 11(a) show samples that were etched as a pretreatment for SEM observation. Specifically, the molded product was etched by exposing it to a mixture of 0.5 ml of 60% nitric acid and 20 ml of ethanol for 3 seconds.

[0036] Then, the processing unit calculates the ratio of the area occupied by carbides in the image (carbide area ratio) using the binarized image (step S104).

[0037] After calculating the carbide area ratio, the processing unit calculates a parameter (step S105). In this embodiment, the parameter P is calculated using the following formula (1): P=Carbide Area Ratio−((HRC−49)×0.9) (1), where HRC is the Rockwell hardness of the molded product.

[0038] The parameter is calculated by the calculation process described above. If this parameter is 9.0 or more, it can be said that a high toughness can be obtained at the same hardness as a hot-formed product.

[0039] Here, the relationship between parameters and hardness will be described with reference to FIG. 12. FIG. 12 is a diagram for explaining an example of the relationship between hardness and parameters. In FIG. 12, ● indicates the measured value of the product formed by the manufacturing method shown in FIG. 2, and ◇ indicates the measured value of the product formed by hot forming. The material used for forming is a wire rod having the above-mentioned composition, and the composition is the same. As shown in FIG. 12, the parameters calculated by the above formula (1) for the tempered product produced by the manufacturing method shown in FIG. 2 are larger than those for the hot forming, and the broken line L1 connecting these values ​​remains at a value of 9.0 or more. On the other hand, for the product formed by hot forming, the broken line L1 connecting the parameters calculated by the above formula (1) remains at a value of 9.0 or more. 10 However, the value has remained below 9.0.

[0040] Furthermore, in the spring member according to this embodiment, it is preferable that the difference between the maximum and minimum values ​​of the average Vickers hardness in the range of 0.5 mm or more and 3.0 mm or less in the depth direction from the surface of the wire on the inner diameter side, outer diameter side, and between the wires (between the inside and outside) of the spring member toward the center of the wire is 40 HV or less.

[0041] Fig. 13 is a diagram illustrating an example of the relationship between the depth from the surface of a wire rod and the hardness of a formed product after tempering produced by the manufacturing method shown in Fig. 2 . Fig. 14 is a diagram illustrating an example of the relationship between the depth from the surface of a wire rod and the hardness in cold forming. The results shown in Figs. 13 and 14 are for wire rods having the above-mentioned composition, which were subjected to the manufacturing method shown in Fig. 2 and cold forming, respectively, using the same wire rod. In the graphs shown in Figs. 13 and 14 , the solid line indicates the hardness of the outer diameter side, the dashed line indicates the hardness of the inner diameter side, the dashed line indicates the hardness of wire gap 1, and the dotted line indicates the hardness of wire gap 2, which is located at a different position in the wire rod from wire gap 1. Note that wire gaps 1 and 2 were measured at the same winding position of the wire rod in the manufacturing method and cold forming shown in Fig. 2 .

[0042] In the tempered molded product (see FIG. 13) produced by the manufacturing method shown in FIG. 2, the average Vickers hardness values ​​at depths of 0.5 mm to 3.0 mm were 566 HV on the inner diameter side, 569 HV on the outer diameter side, 571 HV between wires 1, and 572 HV between wires 2, and the difference between the maximum and minimum values ​​was 6 HV.

[0043] In contrast, for the cold-formed product (see Figure 14), the average Vickers hardness values ​​at depths of 0.5 mm to 3.0 mm were 584 HV on the inner diameter side, 595 HV on the outer diameter side, 555 HV at line spacing 1, and 557 HV at line spacing 2, with a difference between the maximum and minimum values ​​of 40 HV. From these results, it can be seen that the spring material produced by the manufacturing method shown in Figure 2 has a hardness difference of, for example, 30 HV or less, and thus a spring member can be obtained in which the difference in hardness depending on the location on the wire material is smaller than that produced by cold forming.

[0044] Furthermore, in the spring member according to this embodiment, in the direction of maximum principal stress that occurs when a compressive load is applied to the inner diameter side, in a residual stress distribution curve with the vertical axis representing compressive residual stress and the horizontal axis representing depth from the surface (wire radius), it is preferable that the integrated value in the range of depth from the wire surface of 0.3 mm or more to 0.5 mm or less is 10 MPa mm or more.

[0045] 15 is a diagram for explaining the calculation of the integral value in the compressive residual stress distribution curve. The integral value is the difference in area between the regions (hatched regions in FIG. 15 ) formed by depths of 0.3 mm and 0.5 mm from the surface, zero compressive residual stress, and the compressive residual stress distribution curve, and corresponds to the value (difference) obtained by subtracting the area of ​​the region where the compressive residual stress is negative from the area where the compressive residual stress is positive. In the example shown in FIG. 15 , the integral value is negative.

[0046] Here, for molded products subjected to shot peening according to the method described in Japanese Patent No. 6,318,048, the tempered molded product produced by the manufacturing method shown in FIG. 2 had a compressive strength of 32 MPa·mm, while the cold-formed molded product had a compressive strength of 4 MPa·mm. Furthermore, for molded products subjected to shot peening according to the method described in Japanese Patent No. 5,393,280, the tempered molded product produced by the manufacturing method shown in FIG. 2 had a compressive strength of 28 MPa·mm, while the cold-formed molded product had a compressive strength of -13 MPa·mm. This is because spring members can experience delayed fracture originating from the bottom of corrosion pits, and compressive residual stress at a depth of 0.3 to 0.5 mm is thought to be effective in preventing delayed fracture. In this case, durability can be improved by setting the integral value to 10 MPa·mm or more.

[0047] In the embodiment of the present invention described above, by setting the parameter of the spring member calculated by the above formula (1) to 9.0 or more, it is possible to obtain a spring member having higher toughness than members formed by conventional hot forming.

[0048] Although the embodiments for carrying out the present invention have been described above, the present invention should not be limited to only the above-described embodiments.

[0049] As described above, the present invention can include various embodiments not described here, and various design changes can be made within the scope of the technical idea specified by the claims. For example, in the above-mentioned embodiment, an example of manufacturing a spring member by forming a green material and then conducting electric tempering has been described, but in a spring member that has undergone hot forming and electric tempering, which is different from conventional hot forming, if the parameter calculated by the above formula (1) is 9.0 or more, a product formed by a manufacturing method including hot forming is also included as a spring member according to the present invention.

[0050] As described above, the spring member according to the present invention is suitable for obtaining a spring member having higher toughness than members formed by conventional hot forming.

[0051] REFERENCE SIGNS LIST 1 Coil spring 100 Base material 101 Drawn wire material 102, 103 Formed material 200 Winding machine 211, 231 First current-carrying member 212, 232 Second current-carrying member 221 Tank 222 Water-soluble quenching agent

Claims

1. A spring member formed by deforming a wire rod, wherein the wire rod contains, by mass, 0.39% to 0.60% carbon (C), 1.80% to 2.80% silicon (Si), and the remainder iron (Fe) and inevitable impurities, and wherein a parameter P calculated using the following formula (1) is 9.0 or greater: P = carbide area ratio - ((HRC - 49) x 0.9) ... (1) where HRC is the Rockwell hardness of the formed product.

2. The spring member according to claim 1, characterized in that the wire rod further contains 0.05% or more and 1.50% or less of manganese (Mn), 0.05% or more and 1.50% or less of chromium (Cr), 0.025% or less of phosphorus (P), 0.025% or less of sulfur (S), 1.00% or less of copper (Cu), 1.00% or less of nickel (Ni), and 0.20% or less of titanium (Ti).

3. The spring member according to claim 1, characterized in that the carbide area ratio is calculated based on a binarized image obtained by binarizing an image of an area where carbides are precipitated from a scanning electron microscope (SEM) image.

4. The spring member according to claim 3, characterized in that the binary image is generated by extracting an area identified as the carbide where the aspect ratio between the longitudinal length and the lateral length is 2 or more.

5. The spring member according to claim 1, wherein the wire has an equivalent circle diameter of 8 mm or more and 24 mm or less.

6. A spring member according to claim 1, characterized in that the difference between the maximum and minimum average Vickers hardness values ​​in the depth direction from the wire surface on the inner diameter side, outer diameter side, and between-wire sides of the spring member toward the center of the wire is 30 HV or less within a depth range of 0.5 mm to 3.0 mm.

7. A spring member according to claim 1, characterized in that, in the direction of maximum principal stress that occurs when a compressive load is applied to the inner diameter side of the spring member, in a residual stress distribution curve with the vertical axis representing compressive residual stress and the horizontal axis representing depth from the surface, the integrated value in the range of depth from the wire surface of 0.3 mm to 0.5 mm is 10 MPa·mm or more.

Images