JP4674843B2 - Coil spring manufacturing method - Google Patents

Description

The present invention relates to a method for manufacturing a coil spring .

  2. Description of the Related Art In recent years, automobile engines have been pursued for higher rotation and higher output for the purpose of improving fuel efficiency and reducing weight. In order to cope with this, there is an increasing demand for higher fatigue strength of coil springs. Currently, the surface strengthening method for coil springs includes nitriding treatment, which is a chemical surface hardening method, that is, a treatment method in which nitrogen is diffused and penetrated from the steel surface to harden the steel surface, and a shot is a physical surface hardening method. A combination of peening treatment is common. In particular, shot peening prevents the occurrence of fatigue fracture starting points by applying compressive residual stress in the vicinity of the coil spring surface.

With such high performance of the coil spring, in the shot peening process, it is necessary to apply an extremely high compressive residual stress to a portion close to the surface of the coil spring. Therefore, in general, as the hardness of the spring material is increased and the surface hardness is increased during nitriding treatment, the conventional projection material has a higher hardness such as cemented carbide having a higher stress imparting ability (Patent Document 1) or round. It has been common to use cut wires.
JP-A-8-323626

However, if a conventional high-hardness projection material such as the above-mentioned carbide is used for shot peening, the surface roughness of the coil spring becomes rough, and it becomes easy for fatigue cracks due to the surface notch effect, Due to excessive plastic deformation, the compressive residual stress at the outermost surface portion may be reduced, and the fatigue strength may be reduced contrary to the purpose.
In addition, a round cut wire (hereinafter referred to as RCW), which is a general material, is a method of drawing a hard steel wire, cutting the wire, and then projecting it onto a rigid wall to round the edge. From the upper limit, it was difficult to produce a product having a particle size of 0.25 mm or less and a high hardness at low cost, and none of them could achieve the peening effect as a problem.
The present invention has been made in view of the above problems, and an object of the present invention is to provide a method of manufacturing a coil spring that can suppress the surface roughness to a low level and can impart a high compressive residual stress in the vicinity of the surface. And

The manufacturing method of the coil spring of the present invention made to achieve the above object is as follows: C: 0.5 to 0.7%, Si: 1.2 to 2.5%, Mn: 0.3 in mass%. -1.0%, Cr: 0.4-2.0%, P: 0.025% or less, S: 0.025% or less, and Mo: 0.05- 2.0% V: containing one or 0.05 to 0.3% Kizuto machining a spring wire material the balance being iron and unavoidable impurities, hot rolling, peeling, annealing After coiling, cold drawing, oil tempering, coiling, and descaling, the coil spring was subjected to nitriding, and then the hard steel wire rod was used as the projection material in the first stage. The Vickers hardness Hv is 800 to 1100, the Young's modulus is 100 GPa or less, and the average particle diameter of the particles is φ0. 35~0.200mm a is an amorphous particles using a shot material in the second stage Shottopi - characterized by a hardening treatment.

The coil spring manufactured by the shot peening process using the amorphous particles of the present invention has high strength, excellent fatigue strength, and good delayed fracture resistance, and compared with a conventional manufacturing method using carbide or the like. And extremely low cost.
In addition, according to the manufacturing method of the present invention, it is possible to easily manufacture this high-quality coil spring. In recent years, with the aim of improving the fuel consumption and reducing the weight of automobile engines, higher rotation and higher output are achieved. On the other hand, it is possible to respond to the market needs of coil springs at a low cost, and the industrial utility value is high.

In the present invention, C: 0.5 to 0.7% by mass, Si: 1.2 to 2.5%, Mn: 0.3 to 1.0%, Cr: 0.4 to 2.0 P: 0.025% or less, S: 0.025% or less, and Mo: 0.05-2.0%, V: 0.05-0.3% as necessary It contains one or two, Kizuto machining a spring wire material the balance being iron and unavoidable impurities, hot rolling, peeling, after each process annealing, and cold drawing, oil tempering After the treatment, coiling is performed and a descaling process is performed to obtain a coil spring. The obtained coil spring is further subjected to nitriding treatment, and then the first stage shot peening treatment is performed using the hard steel wire as the projection material, and further the second stage shot peening treatment is performed using the amorphous particles as the projection material. . As the second-stage projection material, amorphous particles having a Vickers hardness Hv of 800 to 1100, a Young's modulus of 100 GPa or less, and an average particle diameter of φ0.035 to 0.200 mm are used.

Thus, in the two-stage shot peening process, an excellent effect can be obtained when amorphous particles are used for the projection material in the second stage.
The results of performing various types of shot peening on the coil springs molded under certain conditions and measuring the compressive residual stress and the surface roughness for each of the examples will be described below.

  Production example of coil spring: In this production example, C: 0.64%, Si: 2.02%, Mn: 0.30%, Cr: 0.86%, P: 0.010, S: Using a high-strength oil tempered wire for springs of 0.005%, V: 0.10%, and tensile strength σB = 2122MPa, by performing cold coiling forming corresponding to the coiling process, the wire diameter is φ3.2mm, coil center A coil spring material having a diameter of 20.0 mm, a total number of turns of 6, an effective number of turns of 4, a free length of 47 mm, and a spring constant of 32 N / mm was obtained.

  Thereafter, low temperature tempering at 440 ° C. for 15 minutes was performed to remove harmful residual stress. Next, both seat surfaces were ground to obtain a coil spring having a free length of 47.0 mm. At this time, the spring constant was 32 N / mm 2.

  Thereafter, a gas nitriding treatment corresponding to the nitriding step was performed at 480 ° C. for 4 hours in an ammonia atmosphere, and the Vickers hardness Hv was 600 to 850 at a depth of 0.050 mm of the nitride layer near the surface.

Next, the shot peening process referred to in the present invention is performed. Shot peening was performed as a first-stage shot peening treatment using a hard steel wire having a particle diameter of φ0.6 mm, which will be described later.

  The first stage shot peening process was performed using an impeller-type shot peening machine (manufactured by Shinto Kogyo Co., Ltd.) for a projection speed of 70 m / sec × projection time of 400 sec. At this time, the arc height was 0.45 mmA or more and the coverage was 99% or more.

  The shot ball is obtained by drawing a hard steel wire rod, cutting the drawn wire, and then projecting it onto a rigid wall to round the edge. Vickers hardness Hv560 was used.

  Next, processing conditions for the second stage shot peening are shown. The projection material used in the second stage uses a projection material having a smaller size and higher hardness than the first stage in order to give an appropriate residual stress to the outermost surface of the product. Therefore, this time, amorphous particles having a hardness of Hv and higher than 800 using an iron-based material were employed.

  Amorphous particles are characterized by a low Young's modulus compared to crystalline projection materials of the same hardness. For sample A, amorphous particles having a Vickers hardness Hv of 930, a Young's modulus of 83 GPa, and a particle size of 0.1 mm were used as the projection material. For sample B, a super hard shot having a Vickers hardness Hv of 1400, a Young's modulus of 210 GPa, and a particle size of 0.1 mm was used as a projection material. For sample C, RCW having a Vickers hardness Hv of 800, a Young's modulus of 210 GPa, and a particle size of 0.25 mm was used as a projection material. For sample D, amorphous particles having a Vickers hardness Hv of 930, a Young's modulus of 83 GPa, and a particle size of 0.03 mm were used as the projection material.

  The processing conditions for the second stage shot peening were as follows: projection was performed for a projection pressure of 0.2 MPa × projection time of 420 sec using an air-type shot peening machine (manufactured by Shinto Kogyo). The arc height at this time was 0.25 mmN for sample A, 0.31 mmN for sample B, 0.24 mmN for sample C, and 0.17 mmN for sample D. Samples A, B, C, and D all had a coverage of 99% or more.

  Next, the coil spring subjected to the second stage treatment was heated by an electric furnace, and a low temperature annealing at 200 ° C. for 15 minutes was performed to remove harmful residual stress.

  Next, the compressive residual stress was measured for each of the samples A to C. As a measurement method, a general X-ray residual stress measurement method was used as a non-destructive method. The result is shown in the graph of FIG.

  From the graph of FIG. 1, when a carbide shot (sample B) and amorphous particles (sample A) other than RCW (sample C) are used as the projection material, the compressive residual stress near the surface is high and the depth is sufficient. It can be seen that a good compressive residual stress distribution can be obtained. Furthermore, the carbide shot (sample B) and the amorphous particles (sample A) have a peak of compressive residual stress at a position very close to the surface of the workpiece and have sufficient depth, and are effective in improving the fatigue strength of the coil spring. I can confirm.

  However, Sample A, which is an amorphous particle having a particle size of 0.03 mm, has a maximum compressive stress on the workpiece surface, but its distribution from the surface to 50 μm is not sufficient. For this reason, no special effect was obtained in improving the fatigue strength of the coil spring.

  In addition, it is difficult to obtain particles having a size of 0.200 mm or more at low cost because of the limitation on the manufacturing method that requires the melted metal to be rapidly solidified at a very high cooling rate. It is not economically effective to use as.

  Next, Table 1 shows the results of measuring the surface roughness of the coil spring of the example (sample A) and the coil springs of the comparative examples (sample B, sample C, sample D).

  From Table 1, it can be seen that when amorphous particles (sample A) are used as a projection material, the surface roughness is 5.0 μm or less in Rz, and an extremely smooth surface is created. In comparison, RCW (sample C) and carbide shot (sample B) have a surface roughness of Rz of 5.0 μm or more and are difficult to finish smoothly.

  Next, FIG. 2 shows an amorphous particle, and FIG. From the SEM observation of the surface of the projection material in FIGS. 2 and 3, it can be confirmed that the surface of the carbide shot (FIG. 3) has sharp irregularities, but the surface of the amorphous particles (FIG. 2) is extremely smooth.

  Next, FIG. 4 shows a surface SEM photograph of the valve spring of sample A and FIG. 4 and 5, the surface (FIG. 4) of the valve spring (sample A) peened with amorphous particles creates a very smooth surface compared to the surface of the carbide shot (sample B) (FIG. 5). I was able to confirm that it was born.

  From this, it is clear that amorphous particles can be peened without imparting sharp irregularities that are harmful from the viewpoint of fatigue life of the valve spring.

  Of the four types of springs whose surfaces are hardened by nitriding, the spring (sample A) having a smooth surface can be expected to have improved impact resistance and delayed fracture resistance compared to other springs.

  From Table 1 above, Sample A using amorphous particles as a projection material created a smooth peened surface compared to carbide shot (Sample B) and RCW (Sample C). In addition, as shown in FIG. 1, a compressive residual stress equivalent to that of the cemented carbide shot (sample B) is obtained. Therefore, when amorphous particles are used as a projection material for pinning treatment (sample A), a coil spring having a long fatigue life can be obtained as a result of obtaining a smooth treated surface and high compressive stress. .

  Table 2 shows the fatigue strength test results of the coil spring of the example (sample A) and the coil springs of the comparative examples (sample B and sample D).

The fatigue strength represents a value at 1 × 10 ⁷ times of each sample. Sample A and sample B were equivalent in fatigue strength, and sample D clearly had a lower fatigue strength than sample A and sample B. It is considered that samples A and B have a long fatigue life due to a high compressive residual stress near the surface.

From Table 1 above, Sample A using amorphous particles as a projection material created a smooth peened surface compared to carbide shot (Sample B) and RCW (Sample C). In addition, as shown in FIG. 1, a compressive residual stress equivalent to that of the cemented carbide shot (sample B) is obtained.
Therefore, when amorphous particles are used as a projection material for peening treatment (Sample A), it is possible to obtain a coil spring with excellent delayed fracture resistance as a result of obtaining a smooth treated surface and high compressive residual stress. It becomes.

  Table 3 shows the results of the delayed fracture resistance test of the coil spring of the example (sample A) and the coil spring of the comparative example (sample B).

* The symbol “×” indicates breakage, and “○” indicates unbreakage.

  In the delayed fracture resistance test, each sample was compressed under a stress condition of 1300 MPa and then immersed in a corrosive solution adjusted to a concentration of 0.1 mass% sulfuric acid and 0.1 mass% carbon disulfide to break the coil spring. It was evaluated by the time to do. The temperature of the corrosive liquid at that time was 10 to 15 ° C., and the number of tests was N = 6.

  In sample A, four coil springs are not broken after 24 hours, while sample B is broken except for one coil spring, and sample A is clearly superior in delayed fracture resistance. I understand.

  In general, it is said that the mechanism of local corrosion growth is that the micro-concave of the material promotes metal elution to increase the corrosion current density, and as a result, local corrosion progresses at a high rate and causes pitting. Compared to sample B, sample A has the same compressive residual stress, but as shown in Table 1, the surface roughness Rz has a characteristic of smoothness of 5 μm or less, and as a result, it is hard to be corroded and pitting corrosion. It is considered that the delayed fracture resistance was excellent due to the slow progress.

It is a graph which shows the residual-stress measurement result of the coil spring (sample A) of an Example, and the coil spring (sample B, sample C, sample D) of a comparative example. It is a projection material surface SEM photograph of an amorphous particle. It is a projection material surface SEM photograph of a super hard shot. 4 is a surface SEM photograph of a coil spring of Sample A. It is a surface SEM photograph of the coil spring of sample B.

Claims (1)

C: 0.5 to 0.7% by mass%, Si: 1.2 to 2.5%, Mn: 0.3 to 1.0%, Cr: 0.4 to 2.0%, While limiting to P: 0.025% or less, S: 0.025% or less, Mo: 0.05-2.0%, V: 0.05-0.3% 1 type or 2 as needed containing seeds, Kizuto machining a spring wire material the balance being iron and unavoidable impurities, hot rolling, peeling, after each process annealing, and cold drawing, after oil tempering treatment, and coiling The coil spring obtained by performing the descaling treatment is subjected to nitriding treatment, and then the first stage shot peening treatment is performed using a hard steel wire as a projection material, and the Vickers hardness Hv is 800 to 1100 and the Young's modulus is 100 GPa In the following, further, amorphous particles having an average particle diameter of φ0.035 to 0.200 mm are used as a projection material. Eye Shottopi - method of manufacturing a coil spring, which comprises the hardening treatment.