JP2011149036A - Method for manufacturing coil spring for automotive suspension, and coil spring for automotive suspension - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a technology for manufacturing a coil spring superior in setting resistance, durability and corrosion-fatigue resistance, for an automotive suspension. <P>SOLUTION: A method for manufacturing the coil spring for the automotive suspension includes: heat-treating a formed coil; subjecting the heat-treated coil to warm-shot-peening treatment; and subjecting the coil after the warm-shot-peening treatment to hot-setting treatment. The method for manufacturing the coil spring for the automotive suspension may use steel as a raw material, which includes, by mass ratio, 0.35-0.55% C, 1.60-3.00% Si, 0.20-1.50% Mn, 0.10-1.50% Cr, further one or more from among 0.40-3.00% Ni, 0.05-0.50% Mo and 0.05-0.50% V, and the balance Fe with unavoidable impurities. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a manufacturing method of a coil spring for automobile suspension and a coil spring for automobile suspension.

  Non-Patent Document 1 discloses a conventional method of manufacturing a coil spring for automobile suspension. In this manufacturing method, first, a wire is hot-formed into a coil shape, and heat treatment (quenching and tempering) is performed on the formed coil. Next, hot setting is performed on the coil after heat treatment, then warm shot peening is performed, and then setting and coating are performed. In this manufacturing method, the sag resistance of the coil is improved by hot setting, and the durability is improved by warm shot peening.

The Spring Society of Japan "Spring" 4th edition, page 508

  Automotive suspension coil springs are required to have high sag resistance and durability. In the manufacturing method of Non-Patent Document 1, although improvement in sag resistance and durability is achieved, higher sag resistance and durability are required.

  An object of the present invention is to provide a technique for manufacturing a coil spring for automobile suspension having higher sag resistance and durability than the prior art.

  As described above, in the conventional manufacturing method of a coil spring for automobile suspension, hot setting is performed after heat treatment, and then warm shot peening is performed. As a result of intensive studies by the present inventors, when hot shot peening, which is non-directional plastic processing, is performed after hot setting, the directional compressive residual stress imparted to the coil by hot setting becomes warm shot peening. Turned out to be countered. Then, as a result of conducting experiments with various manufacturing processes being changed, the shot is subjected to warm shot peening, and then hot setting is performed on the coil after molding, thereby compressing the coil surface by warm shot peening. It has been found that both the residual stress and the directional compressive residual stress applied to the coil by hot setting remain effectively.

  The manufacturing method of the automotive suspension coil spring of the present invention was created based on the above findings. In this manufacturing method, the material is formed into a coil shape, the formed coil is heat-treated, the heat-treated coil is subjected to warm shot peening, and the hot shot peened coil is subjected to hot setting. I do. According to this manufacturing method, a car suspension coil spring excellent in sag resistance and durability can be obtained.

  In the manufacturing method of the coil spring for automobile suspension, the mass ratio is C: 0.35 to 0.55%, Si: 1.60 to 3.00%, Mn: 0.20 to 1.50%, Cr: 0.10 to 1.50%, Ni: 0.40 to 3.00%, Mo: 0.05 to 0.50%, V: 0.05 to 0.50% Steels containing more than one kind and the balance being Fe and inevitable impurities can be used as a raw material. By determining the component ranges of each element contained in the steel used as a raw material as described above, it is possible to further improve sag resistance, durability, and corrosion fatigue resistance.

  The present invention also provides a novel automobile suspension coil spring excellent in durability and sag resistance. That is, in the automotive suspension coil spring of the present invention, a compressive residual stress is applied to at least a part of the spring element wire. The ratio of the sum of the compressive residual stresses in the 135 ° and 315 ° directions to the sum of the compressive residual stresses in the 45 ° and 225 ° directions from the axial direction of the strand at the position where the compressive residual stress is applied is ( Compressive residual stress in the direction of 135 ° + compressive residual stress in the direction of 315 °) / (compressive residual stress in the direction of 45 ° + compressive residual stress in the direction of 225 °). The ratio of the sum of 135 ° and 315 ° compressive residual stresses to the sum of 45 ° and 225 ° compressive residual stresses at the distance of 0.2 mm is the 135 ° and 315 ° directions on the wire surface. And the ratio of the sum of compressive residual stresses in the direction of 45 ° and 225 ° to the sum of compressive residual stresses.

  Generally, when using a coil spring for automobile suspension, it is known that the direction of tensile stress applied to the wire of the coil spring is a diagonal direction of 135 ° and 315 ° from the axial direction of the wire. For this reason, if the strands of the coil spring have a high compressive residual stress in the diagonal direction of 135 ° and 315 °, the tensile stress is effectively canceled by this compressive residual stress, and the durability and resistance of the coil spring are reduced. Increases drooping. In particular, when the compressive residual stress in the diagonal direction of 135 ° and 315 ° is larger than the compressive residual stress in the diagonal direction of 45 ° and 225 ° perpendicular to each other, the compression is performed to increase the durability and sag resistance of the coil spring. Residual stress is effectively applied. Furthermore, in the coil spring for automobile suspension, the surface of the wire is scraped by corrosion or the like according to the use environment of the coil spring. Therefore, in order to improve the durability and sag resistance of the coil spring for automobile suspension, compressive residual stress is applied not only on the surface of the wire of the coil spring but also at a predetermined depth from the surface of the wire. It is extremely important that Specifically, the coil spring for automobile suspension has high compressive residual stress in the diagonal directions of 135 ° and 315 ° described above at a position where the distance from the surface of the wire toward the center of the wire diameter is 0.2 mm. If so, the compressive residual stress of the coil spring remains effectively even if the surface of the wire of the coil spring is corroded or scraped according to the use environment of the coil spring. As a result, the durability and sag resistance of the coil spring are higher than when the compressive residual stress is provided only on the wire surface.

  The coil spring for automobile suspension according to the present invention has a compressive residual in the directions of 135 ° and 315 ° at a position where the distance from the surface of the strand toward the center of the wire diameter is 0.2 mm at the position where the compressive residual stress is applied. The ratio of the sum of stresses to the sum of compressive residual stresses in the 45 ° and 225 ° directions is the sum of the compressive residual stresses in the 135 ° and 315 ° directions and the compressive residual stresses in the 45 ° and 225 ° directions on the strand surface. Greater than the ratio with the sum. That is, effective compressive residual stress is effectively applied to improve durability and sag resistance at a position where the distance from the surface of the strand toward the center of the wire diameter is 0.2 mm. Therefore, the automotive suspension coil spring of the present invention can exhibit high sag resistance and durability.

  This automotive suspension coil spring has a compressive residual stress in the 135 ° direction and a compressive residual stress in the 315 ° direction at a position where the distance from the surface of the strand toward the center of the wire diameter is 0.2 mm. It is preferably 1200 MPa. This automotive suspension coil spring has a high compressive residual stress in the range of 800 to 1200 MPa in the 135 ° and 315 ° directions at a position where the distance from the surface of the strand toward the center of the wire diameter is 0.2 mm. Therefore, the automotive suspension coil spring of the present invention can exhibit high sag resistance and durability.

  This automobile suspension coil spring preferably has a spring hardness of HRC 50 to 56. This automobile suspension coil spring has a standard hardness required for an automobile suspension coil spring. Therefore, the automotive suspension coil spring of the present invention can exhibit high sag resistance and durability.

The table | surface which shows the mass ratio of each substance contained in the raw material of the coil spring for motor vehicle suspension. The table | surface which shows the specification of the coil spring for motor vehicle suspension of this invention example 1 and the comparative example 1. FIG. The table | surface which shows the item of the coil spring for motor vehicle suspension of this invention example 2 and the comparative example 2. FIG. Table showing the tightening test results. The table | surface which shows an endurance test result. The table | surface which shows a corrosion fatigue test result. The graph which shows the relationship between the distance from the surface of a coil spring, and compressive residual stress. The graph which shows the compressive residual stress of each measurement angle in the surface of a coil spring. The graph which shows the compressive residual stress of each measurement angle | corner in depth 0.1mm. The graph which shows the compressive residual stress of each measurement angle | corner in depth 0.2mm. The graph which shows the compressive residual stress of each measurement angle | corner in depth 0.3mm. The table | surface which shows the calculation result of (135 degree compressive residual stress +315 degree compressive residual stress) / (45 degree compressive residual stress +225 degree compressive residual stress). Compressive residual stress at each measurement angle for each distance from the surface of the coil spring and (135 ° compressive residual stress + 315 ° compressive residual stress) / (45 ° compressive residual stress + 225 ° compressive residual stress) The table which shows the ratio. The side view which shows the measuring method of the compressive residual stress of a coil spring by the X-ray stress measuring method. The top view which shows the measuring method of the compressive residual stress of a coil spring by a X-ray stress measuring method.

  A method of manufacturing a coil spring for automobile suspension according to an embodiment embodying the present invention will be described. In the manufacturing method of this embodiment, a coil spring for automobile suspension is manufactured by performing coil molding, heat treatment, warm shot peening, hot setting, cold shot peening, and cold setting in this order. Hereinafter, in this specification, the automobile suspension coil spring may be simply referred to as a coil spring.

  In the coil forming step, the wire is formed into a coil shape. The wire may be formed hot (temperature above the recrystallization temperature of the wire) or warm (temperature lower than the recrystallization temperature of the wire) or depending on the wire diameter, coil spring size, etc. You may carry out by cold (room temperature). In the case of hot molding, for example, it can be performed at 800 to 1000 ° C. In the case of warm molding, for example, it can be performed at 50 to 400 ° C. Moreover, conventionally well-known various methods can be used for the method of shape | molding in a coil shape, For example, you may shape | mold using a coiling machine and may shape | mold by the method of winding around a metal core.

  Examples of the material used as the wire include C: 0.35 to 0.55%, Si: 1.60 to 3.00%, Mn: 0.20 to 1.50%, and Cr: 0.0. 10 to 1.50%, Ni: 0.40 to 3.00%, Mo: 0.05 to 0.50%, V: 0.05 to 0.50%, any one or more A steel material containing Fe and the balance of Fe and inevitable impurities can be suitably used. By using such a steel material, sag resistance, durability, and corrosion fatigue resistance can be improved. The reason why specific upper and lower limits are set for each element is as follows.

  C: 0.35-0.55%. C is an essential element for increasing the strength of the steel wire, but if it is less than 0.35%, sufficient strength cannot be obtained, and conversely if it exceeds 0.55%, formability and toughness deteriorate. .

  Si: 1.60 to 3.00%. Si is an element necessary for ensuring the strength, hardness and sag resistance of the spring, and when it is small, the necessary strength and sag resistance are insufficient, so 1.60% was made the lower limit. Moreover, since it will not only harden | cure a material but to embrittle if it adds too much, 3.00% was made the upper limit.

  Mn: 0.20 to 1.50%. Mn improves the hardenability of the steel and fixes S in the steel to prevent its damage. However, if it is less than 0.20%, the effect is not obtained. On the other hand, if it exceeds 1.50%, the toughness is lowered and the moldability may be impaired.

  Cr: 0.10 to 1.50%. Cr improves hardenability and imparts temper softening resistance. If it is less than 0.10%, the effect is not sufficient, and if it exceeds 1.50%, solid solution of the carbide is suppressed and the strength is lowered, so that it is not effective.

  Containing any one or more of Ni: 0.40 to 3.00%, Mo: 0.05 to 0.50%, and V: 0.05 to 0.50%. Ni, Mo, and V are all elements that increase the softening resistance during tempering. For this reason, the softening resistance at the time of tempering can be increased by containing one or more of these elements. Note that the amount required to obtain the above-mentioned effect varies depending on each element. That is, Ni is 0.40 to 3.00%, Mo is 0.05 to 0.50%, and V is 0.05 to 0.50%. If each element is less than the lower limit, the necessary softening resistance cannot be obtained. On the other hand, Ni: 3.00%, Mo: 0.50%, V: 0.50% are amounts that saturate the softening resistance increasing effect by these elements, and it is useless to contain more than this, There is a possibility that moldability may be reduced due to an excessive increase in strength.

  In addition to the steel materials described above, various materials (for example, spring steel and the like) that have been conventionally used as a coil spring material can be used as the wire material used as the coil material.

  In the heat treatment step, heat treatment is performed on the coil formed into a coil shape by the above-described forming step. The heat treatment performed in this step differs depending on whether the above molding step is performed hot, warm or cold. That is, when the above molding process is performed hot, quenching and tempering are performed. Quenching and tempering imparts strength and toughness to the coil. The temperature condition for quenching can be 800 to 1000 ° C. The temperature condition of tempering can be 300-500 degreeC. On the other hand, when the above molding process is performed warm or cold, low temperature annealing is performed. The low temperature annealing can remove harmful residual stress (residual tensile stress) inside and on the coil. The conditions for low-temperature annealing can be 20 to 60 minutes at 300 to 500 ° C. The method of quenching and tempering the coil and low-temperature annealing of the coil can be performed by any conventionally known method.

  In the warm shot peening process, the coil subjected to the above heat treatment is warm shot peened. Warm shot peening imparts a large compressive residual stress to the coil surface, improving the durability and corrosion fatigue resistance of the coil. Here, the temperature at which shot peening is performed can be appropriately set within a temperature range that is equal to or lower than the recrystallization temperature of the wire and higher than room temperature. For example, the coil temperature can be set to 150 to 400 ° C., and more preferably 250 to 350 ° C. By performing shot peening in such a temperature range, a larger compressive residual stress can be applied to the coil surface. For shot peening, a steel ball (shot) having a diameter of 0.6 to 1.2 mm can be used. By using such a steel ball, compressive residual stress can be suitably imparted without causing harmful damage to the coil surface. Moreover, the projection speed of a steel ball can be 50-100 m / s. Moreover, the coverage can be in the range of 80% or more, and the number of processings can be 1 to 2 times. Various conventionally known methods can be used for the steel ball shot method.

In the hot setting process, setting is performed in a state where the temperature of the coil is warm. By hot setting, a directional compressive residual stress is applied to the coil to improve durability, and a relatively large plastic deformation occurs in the coil, thereby improving the sag resistance of the coil. Here, the temperature at which hot setting is performed can be appropriately set within a temperature range that is equal to or lower than the recrystallization temperature of the wire and higher than room temperature. For example, the coil temperature can be in the range of 150 to 400 ° C. By performing setting in such a temperature range, the amount of plastic deformation imparted to the coil can be increased, and the sag resistance can be improved. Note that it is preferable to perform setting at a temperature lower than the temperature at which the warm shot peening is performed because it is not necessary to heat the coil after the warm shot peening. Moreover, it is preferable to perform setting in the range where the residual shear strain γ of the setting is 10 × 10 ⁻⁴ to 40 × 10 ⁻⁴ . By performing setting within this range, it is possible to improve the sag resistance without reducing the durability. For the setting, various conventionally known methods can be used.

  In the cold shot peening process, shot peening is performed with the coil temperature at room temperature. By performing cold shot peening in addition to warm shot peening, the durability of the coil can be further improved. For cold shot peening, a steel ball (shot) having a diameter of 0.1 to 1.0 mm can be used. In addition, it is preferable to make the diameter of the steel ball used for cold shot peening smaller than the diameter of the steel ball used for warm shot peening. For example, when the diameter of the steel ball used for warm shot peening is 1.2 mm, the diameter of the steel ball used for cold shot peening is 0.8 mm. By performing warm shot peening and cold shot peening, a large compressive residual stress is imparted to the coil by the warm shot peening performed earlier, and the surface roughness of the coil is improved by the cold shot peening performed later, Coil durability and corrosion fatigue resistance are further improved. The conditions for cold shot peening, that is, the projection speed can be 50 to 100 m / s, the coverage can be 80% or more, and the number of treatments can be 1 to 2.

In the cold setting process, setting is performed in a state where the temperature of the coil is set to room temperature. By performing cold setting in addition to the hot setting, the sag resistance of the coil is further improved. The cold setting is preferably performed in the range of residual shear strain γ of 1 × 10 ⁻⁴ to 10 × 10 ⁻⁴ . By performing setting in this range, the sag resistance can be further improved.

  In addition, each process of said cold shot peening and cold setting can be abbreviate | omitted, and only warm shot peening and hot setting can also be performed. Moreover, you may include other processes other than said each process. For example, a water cooling process may be performed after hot setting. In addition, the spring hardness of the coil spring manufactured by the manufacturing method of the present embodiment is in the range of HRC50 to 56. The spring hardness of the coil spring is more preferably in the range of HRC53 ± 2.

(First embodiment)
An example in which a tightening test, a durability test, and a corrosion fatigue test are performed using the coil spring manufactured by the manufacturing method of the present embodiment will be described. In this test example, each test was performed using Invention Examples 1 and 2 produced by the production method of the present invention and Comparative Examples 1 and 2 produced by a conventional production method.

  FIG. 1 is a table showing the mass ratio of each element contained in the steel materials A and B, which are the materials of the coil spring used in the test.

  Steel A has a mass ratio of C: 0.47%, Si: 2.00%, Mn: 0.7%, P: 0.005%, S: 0.005%, Ni: 0.55%, It is a steel containing Cr: 0.2%, V: 0.2%, the balance being Fe and inevitable impurities. In this example, after dissolving each substance so that the mass ratio is as described above, the material of the coil springs of the present invention example 1 and the comparative example 1 is obtained by carrying out ingot rolling and wire rod rolling. It was.

  Steel material B has a mass ratio of C: 0.47%, Si: 2.18%, Mn: 0.44%, P: 0.007%, S: 0.006%, Ni: 0.53%, Cu: 0.01%, Cr: 0.29%, Mo: 0.09%, V: 0.1%, Ti: 0.023%, B: 0.0021%, the balance being Fe and inevitable Steel made of impurities. In this example, after melting each substance so that the mass ratio is as described above, the material of the coil springs of the present invention example 2 and the comparative example 2 is obtained by carrying out the ingot rolling and wire rolling. It was.

[Invention Example 1]
The coil spring of Example 1 of the present invention is made of the wire material made of the steel material A described above. The specific manufacturing method is cold forming, temper treatment (low temperature annealing), warm shot peening, hot setting, water cooling, cold shot peening, cold setting for the wire tempered wire made of steel A The steps were performed in this order.

Each condition in the manufacturing process of Example 1 of the present invention is as follows. The conditions for the low-temperature annealing were set at 350 ° C. for 30 minutes. The diameter of the steel ball (shot) used for warm shot peening was 1.2 mm. The coil temperature during warm shot peening was set to 300 ° C. The coil temperature at the time of hot setting was 200 ° C., and the allowance was 21 mm (residual shear strain γ = 13.7 × 10 ⁻⁴ ). The diameter of the steel ball used for cold shot peening was 0.8 mm. The allowance for cold setting was 3 mm (residual shear strain γ = 2 × 10 ⁻⁴ ).

  The coil spring of Example 1 of the present invention manufactured as described above has a spring hardness of HRC53 and has the specifications shown in FIG. That is, the coil spring of Example 1 of the present invention is a cylindrical single-sided pig type, and has a wire diameter of 12.6 mm, a coil average diameter of 162.6 mm, a free length of 318.5 mm, an effective winding of 3.12 windings, and a spring constant of 24.0 N / mm.

[Comparative Example 1]
The coil spring of Comparative Example 1 was manufactured by using the wire made of the steel material A as a material in the same manner as Example 1 of the present invention. Specifically, for the wire tempered wire made of steel A, cold forming, tempering (low temperature annealing), hot setting, warm shot peening, water cooling, cold shot peening, cold setting, Each process is performed in this order to form a coil spring. The point which performs hot setting before warm shot peening differs from the manufacture procedure of the example 1 of this invention. The manufacturing procedure of Comparative Example 1 is a procedure according to a conventional manufacturing method of a suspension coil for automobile suspension.

  Each condition in the manufacturing process of Comparative Example 1 is as follows. The temperature for performing warm shot peening was 200 ° C. The temperature for performing hot setting was 300 ° C. Other conditions are the same as those in the manufacturing process of Example 1 of the present invention.

  The coil spring of Comparative Example 1 also has a spring hardness of HRC53 as in Example 1 of the present invention, and has the specifications shown in FIG.

[Invention Example 2]
The coil spring of the present invention example 2 is made of the wire material made of the steel material B described above. A specific manufacturing method includes quenching heating, hot forming, quenching (oil cooling), tempering, warm shot peening, hot setting, water cooling, cold shot peening, cold setting for the wire made of steel B. Each process is performed in this order to form a coil spring.

Each condition in the manufacturing process of Example 2 of the present invention is as follows. The quenching heating conditions were 990 ° C., and the tempering heating conditions were 370 ° C. The diameter of the shot used for warm shot peening was 1.0 mm. The coil temperature during warm shot peening was set to 350 ° C. The coil temperature at the time of hot setting was 180 ° C., and the allowance was 36 mm (residual shear strain γ = 26 × 10 ⁻⁴ ). The diameter of the shot used for cold shot peening was 0.6 mm. The allowance for cold setting was 4 mm (residual shear strain γ = 2 × 10 ⁻⁴ ).

  The coil spring of Example 2 of the present invention manufactured as described above has a spring hardness of HRC54 and has the specifications shown in FIG. That is, the coil spring of Example 2 of the present invention has a cylindrical shape with a wire diameter of 12.4 mm, an average coil diameter of 110.9 mm, a free length of 323.0 mm, an effective volume of 5.55, and a spring constant of 39.1 N / mm. It was.

[Comparative Example 2]
The coil spring of Comparative Example 2 was manufactured using the wire made of the steel material B as a material in the same manner as Example 2 of the present invention. Specifically, each process of quenching heating, hot forming, quenching (oil cooling), tempering, hot setting, warm shot peening, water cooling, cold shot peening, cold setting is performed on the wire made of steel material B. In this order. The point which performs hot setting before warm shot peening differs from the manufacture procedure of the example 2 of this invention. The manufacturing procedure of Comparative Example 2 is also a procedure according to a conventional manufacturing method of a suspension coil for automobile suspension.

  Each condition in the manufacturing process of Comparative Example 2 is as follows. The coil temperature during warm shot peening was 230 ° C. The coil temperature during hot setting was set to 330 ° C. Other conditions are the same as those in the manufacturing process of Example 2 of the present invention.

  The coil spring of Comparative Example 2 also had the spring hardness HRC54 as in Example 2 of the present invention, and had the specifications shown in FIG.

(1. Tightening test)
The tightening sagging test was conducted on Example 1 of the present invention and Comparative Example 1. This tightening sagging test is a test in which a coil spring is tightened at a height at the maximum load and is put into a constant temperature bath for a predetermined time. The change in load at the mounting height before and after the test was measured, and the amount of residual shear strain was calculated. Here, the tightening load was 5472N. The thermostat temperature was 80 ° C. and the test time was 96 hours. In this tightening test, two inventive examples 1 and two comparative examples 1 were prepared, and the same test was performed for each. The result of the test is shown in FIG. In FIG. 4, “SP → HS” in the processing order column indicates that “hot setting was performed after warm shot peening”. Similarly, “HS → SP” indicates that “warm shot peening was performed after hot setting” (the same applies to the following drawings). Moreover, the numerical value in a figure shows the amount of sag (residual shear strain amount ( ^x10-4 )), respectively. As shown in FIG. 4, the amount of sag of each coil spring of Example 1 of the present invention was smaller than the amount of sag of Comparative Example 1.

  As the above test results show, the coil spring of Example 1 of the present invention manufactured by performing hot setting after warm shot peening is higher than the coil spring of Comparative Example 1 manufactured by performing warm shot peening after hot setting. Has sag resistance.

(2. Endurance test)
The endurance test was conducted on Example 2 of the present invention and Comparative Example 2. In this durability test, the load acting on the coil spring was periodically changed, and the number of times of vibration (the number of durability) until the coil spring was damaged was measured. Here, the stress applied to the coil spring was 735 ± 550 MPa as the main stress. Also in this durability test, two coil springs of Example 2 and Comparative Example 2 were prepared, and the same test was performed for each. The test results are shown in FIG. The numerical values in the figure indicate the number of endurance (10,000 times), respectively. As shown in FIG. 5, the durability of the coil spring of Example 2 of the present invention was remarkably greater than the durability of Comparative Example 2 (about twice).

  As the above test results show, the coil spring of Example 2 of the present invention manufactured by hot setting after warm shot peening is higher than the coil spring of Comparative Example 2 manufactured by performing warm shot peening after hot setting. Durable.

(3. Corrosion fatigue test)
The corrosion fatigue test was conducted on Example 2 of the present invention and Comparative Example 2. It is assumed that the coil spring of Example 2 of the present invention and the coil spring of Comparative Example 2 used in this corrosion fatigue test are pre-coated. This corrosion fatigue test was performed by the following method. First, the coil spring is dropped on basalt JIS No. 7 crushed stone and rotated by 90 ° four times to scratch the coil spring coating (chipping). Next, 60 cycles of corrosion treatment with {salt spray (5% NaCl, 35 ° C.) 6 h + dry (60 ° C., RH 20%) 6 h + wet (50 ° C., RH 95%) 12 h} for one cycle are performed on the coil spring. . Shake 3000 times every 5 cycles of corrosion treatment. After the end of 60 cycles, the coil spring was further vibrated, and the durability was measured. The method for exciting the coil spring is the same as in the above durability test. The stress applied to the coil spring was 735 ± 550 MPa as the main stress. Also in this corrosion fatigue test, two coil springs of Invention Example 2 and Comparative Example 2 were prepared, and the same test was performed for each. The test results are shown in FIG. The numerical values in the figure indicate the number of endurance (10,000 times), respectively. As shown in FIG. 6, the durability of the coil spring of Example 2 of the present invention was higher than that of Comparative Example 2.

  As the above test results show, the coil spring of Example 2 of the present invention manufactured by hot setting after warm shot peening is higher than the coil spring of Comparative Example 2 manufactured by performing warm shot peening after hot setting. Has corrosion fatigue resistance.

(Second embodiment)
The example which measured the residual stress with respect to the coil spring manufactured with the manufacturing method of this embodiment is demonstrated. In this test example, for each of the coil spring of Invention Example 3 manufactured by performing hot setting after warm shot peening and the coil spring of Comparative Example 3 manufactured by performing warm shot peening after hot setting, the coil One location on the outer surface of the coil of the spring was determined as a measurement point, and the residual stress was measured while changing the X-ray irradiation angle with respect to the measurement point by 45 °. Specifically, as shown in FIGS. 14 and 15, a part of the wire of the coil spring is cut out and X-rays are irradiated obliquely from above to the spring wire (sample) and diffracted from the coil spring. By measuring X-rays, the compressive residual stress applied to the coil spring was measured (the compressive residual stress was measured by the so-called X-ray stress measurement method). Further, as shown in FIG. 15, the direction in which the X-ray irradiation direction and the axial direction of the coil spring wire coincide with each other is defined as a measurement angle of 0 °, and the coil spring wire is rotated counterclockwise with respect to 0 °. The direction of rotation was the positive direction. Further, the same measurement was repeatedly performed while changing the distance from the surface of the strand toward the radial center without changing the position of the measurement point. Note that the direction in which the wire of the coil spring is rotated counterclockwise with respect to 0 ° is the positive direction when the coil spring is clockwise. On the other hand, when the above-described coil spring is left-handed, the direction in which the wire of the coil spring is rotated clockwise with respect to 0 ° is the positive direction. Hereinafter, the distance from the surface of the wire toward the center in the radial direction may be simply referred to as “distance from the surface” or “depth”. In addition, the X-ray irradiation angle with respect to the measurement point may be simply referred to as “measurement angle”. In addition, in the present invention example 3 and comparative example 3 which are measurement objects, cold shot peening and cold setting are both omitted in order to clearly grasp the comparison result. The other production conditions of Invention Example 3 and Comparative Example 3 were the same as those of Invention Example 2 described above.

  FIG. 7 is a graph showing the relationship between the distance from the surface of the coil spring and the compressive residual stress. The horizontal axis represents the distance (mm) from the surface, and the vertical axis represents the compressive residual stress (MPa). The compressive residual stress on the vertical axis indicates the average value of the measurement results for each X-ray irradiation angle from the eight different directions with respect to the measurement point at the same distance from the surface. As shown in FIG. 7, the compressive residual stress of Example 3 of the present invention was higher than the compressive residual stress of Comparative Example 3 in all regions from a shallow region to a deep region from the surface.

  8 to 11 show the distribution of compressive residual stress for each X-ray irradiation angle with respect to the measurement point, according to the distance (depth) from the surface. FIG. 8 shows the surface of the coil spring, FIG. 9 shows the compression residual stress at each measurement angle at each measurement point determined at a depth of 0.1 mm, FIG. 10 shows a depth of 0.2 mm, and FIG. 11 shows a depth of 0.3 mm. Each is shown. 8 to 11, the eight axes radially expressed at intervals of 45 ° from the center indicate the compressive residual stress (MPa) in each case where the measurement angles are 45 °, 90 °,. Show. As shown in FIG. 8, on the surface of the coil spring, Example 3 of the present invention shows high compressive residual stress in the diagonal direction with the measurement angles of 135 ° and 315 °. Compared with Comparative Example 3, the difference is remarkable. The diagonal directions of 135 ° and 315 ° are the same as the directions of the tensile stress applied to the coil springs of Example 3 and Comparative Example 3. For this reason, if there is a high compressive residual stress in the diagonal direction of 135 ° and 315 °, the tensile stress is effectively canceled out by this compressive residual stress. As a result, the coil spring of Example 3 of the present invention in which high residual stress appears in the 135 ° and 315 ° directions has higher durability and sag resistance than the coil spring of Comparative Example 3.

  The 135 ° and 315 ° direction compressive residual stresses that have a favorable influence on durability and sag resistance are effectively applied to the coil spring of Example 3 of the present invention. It is possible to quantitatively evaluate the compression residual stress by obtaining the ratio of the 45 ° and 225 ° directions orthogonal to the 135 ° and 315 ° directions. Therefore, the ratio of the compressive residual stress in the 135 ° and 315 ° directions to the compressive residual stress in the 45 ° and 225 ° directions is expressed as (135 ° compressive residual stress + 315 ° compressive residual stress) / (45 ° compressive residual stress). + 225 ° compressive residual stress). The obtained calculation results are shown in FIG. As shown in FIG. 12, the calculated value of Example 3 of the present invention was greater than 1 at all depths. This also reveals that the coil spring of Example 3 of the present invention has a high compressive residual stress in the 135 ° and 315 ° directions.

  As shown in the above measurement results, the coil spring of Example 3 of the present invention manufactured by performing hot setting after warm shot peening is the same as the coil spring of Comparative Example 3 manufactured by performing warm shot peening after hot setting. Compared to high sag resistance and durability.

(Third embodiment)
Another example in which the residual stress is measured for the coil spring manufactured by the manufacturing method of this embodiment will be described. In this test example, the coil springs of Examples 4 and 5 of the present invention manufactured by performing hot setting after warm shot peening, and the coil springs of Comparative Examples 4 and 5 manufactured by performing warm shot peening after hot setting, respectively. The compression residual stress at each of the measurement angles of 45 °, 135 °, 225 °, and 315 ° was measured. Moreover, the same measurement was repeatedly performed, changing the depth from the strand surface, without changing the position of a measurement point. In addition, in the present invention examples 4 and 5 and comparative examples 4 and 5 which are measurement objects, cold shot peening and cold setting are both omitted in order to clearly grasp the comparison results. The other production conditions of Invention Examples 4 and 5 and Comparative Examples 4 and 5 were the same as those of Invention Example 2 described above. The compressive residual stress was measured in the same manner as in the second embodiment.

  FIG. 13 shows the compressive residual stress (MPa) at each measurement angle and (135 ° compressive residual stress + 315) for each depth (distance from the surface toward the radial center) (mm) from the surface of the strand. It is a table | surface which shows the ratio of the compressive residual stress calculated | required by (compressive residual stress of (degree)) / (compressive residual stress of 45 degree + compressive residual stress of 225 degree). In FIG. 13, the values of the compressive residual stresses corresponding to the measurement directions “135 °, 315 °” (for example, the values of the compressive residual stress “607,648” of the surface of Example 4 of the present invention) are the former (“ 607 ") indicates the value of compressive residual stress in the 135 ° direction, and the latter (" 648 ") indicates the value of compressive residual stress in the 315 ° direction. Similarly, the value of each compressive residual stress corresponding to “45 °, 225 °” also indicates the value of the compressive residual stress in the 45 ° direction for the former, and the value of the compressive residual stress in the 225 ° direction for the latter.

  As described above, when the coil spring has a high compressive residual stress in the diagonal direction of 135 ° and 315 °, the tensile stress is effectively canceled by this compressive residual stress, and the durability and sag resistance of the coil spring are reduced. Increases nature. Further, if the coil spring has a high compressive residual stress in the diagonal direction of 135 ° and 315 ° at a position where the distance from the surface of the wire to the center of the wire diameter is 0.2 mm, the wire of the coil spring Even if the surface is corroded, the compressive residual stress remains effectively, so the durability and sag resistance of the coil spring are higher than when only a high compressive residual stress is provided on the wire surface. .

  In the present example, as shown in FIG. 13, the ratio of the compressive residual stress in Examples 4 and 5 of the present invention is greater at the depth of 0.2 mm than at the surface. Moreover, the ratio values at a depth of 0.2 mm are all 1 or more. That is, the coil springs of Examples 4 and 5 of the present invention have higher compressive residual stresses in the diagonal directions of 135 ° and 315 ° effective for tensile stress. On the other hand, the ratio of the compressive residual stress of Comparative Examples 4 and 5 is 1 or less at any depth. That is, the coil springs of Comparative Examples 4 and 5 are not effectively provided with compressive residual stress in the diagonal directions of 135 ° and 315 ° effective against tensile stress.

  Furthermore, as shown in FIG. 13, the coil springs of Examples 4 and 5 of the present invention have both values of compressive residual stress in the direction of 135 ° and compressive residual stress in the direction of 315 ° at a depth of 0.2 mm. Greater than 950 MPa. On the other hand, in Comparative Examples 4 and 5, the value of the compressive residual stress in the 135 ° direction and the value of the compressive residual stress in the 315 ° direction at a depth of 0.2 mm are both about 700 to 750 MPa. That is, the coil springs of Examples 4 and 5 of the present invention have higher compressive residual stress in the effective direction (135 °, 315 °) with respect to the tensile stress than those of Comparative Examples 4 and 5.

  From the above, it is clear that the coil springs of Invention Examples 4 and 5 have higher compressive residual stresses in the 135 ° and 315 ° directions at a depth of 0.2 mm compared to Comparative Examples 4 and 5. It becomes. As described above, the coil springs of Examples 4 and 5 of the present invention having high compressive residual stress in the 135 ° and 315 ° directions at a depth of 0.2 mm are higher than the coil springs of Comparative Examples 4 and 5. Durability and sag resistance.

  As shown in the above measurement results, the coil springs of Examples 4 and 5 of the present invention manufactured by performing hot setting after warm shot peening are Comparative Examples 4 and 5 manufactured by performing warm shot peening after hot setting. Compared to coil springs, it has higher sag resistance and durability.

Specific examples of the present invention have been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and changes of the specific examples illustrated above.
In addition, the technical elements described in the present specification or the drawings exhibit technical usefulness alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. In addition, the technology exemplified in this specification or the drawings achieves a plurality of objects at the same time, and has technical utility by achieving one of the objects.

Claims (5)

  The material is formed into a coil shape, heat-treated on the coil after molding, warm shot peening is performed on the coil after heat treatment, and hot setting is performed on the coil after warm shot peening A manufacturing method of a coil spring for automobile suspension.   C: 0.35 to 0.55%, Si: 1.60 to 3.00%, Mn: 0.20 to 1.50%, Cr: 0.10 to 1.50% in terms of mass ratio Along with Ni: 0.40 to 3.00%, Mo: 0.05 to 0.50%, V: 0.05 to 0.50%, with the balance being Fe and inevitable The manufacturing method of the coil spring for automobile suspension according to claim 1, wherein steel made of impurities is used as a raw material. A coil spring for automobile suspension,
Compressive residual stress is applied to at least a part of the spring wire,
The ratio of the sum of the compressive residual stresses in the 135 ° and 315 ° directions and the sum of the compressive residual stresses in the 45 ° and 225 ° directions at the position where the compressive residual stress is applied is expressed as (compressive residual stress in the 135 ° direction + 315 Compressive residual stress in the direction of °) / (Compressive residual stress in the direction of 45 ° + Compressive residual stress in the direction of 225 °)
The ratio of the sum of compressive residual stresses in the 135 ° and 315 ° directions to the sum of compressive residual stresses in the 45 ° and 225 ° directions at a position where the distance from the surface of the strand toward the center of the wire diameter is 0.2 mm is ,
A coil spring for automobile suspension characterized in that it is larger than the ratio of the sum of compressive residual stresses in the 135 ° and 315 ° directions and the sum of compressive residual stresses in the 45 ° and 225 ° directions on the surface of the strand.   The compressive residual stress in the 135 ° direction and the compressive residual stress in the 315 ° direction at a position where the distance from the surface of the strand toward the center of the wire diameter is 0.2 mm is 800 to 1200 MPa. The automobile suspension coil spring according to claim 3.   5. The automobile suspension coil spring according to claim 3, wherein the spring hardness is HRC 50 to 56. 6.

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