JP2557052B2 - Method for manufacturing spring steel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for manufacturing a round bar-shaped steel material for springs, which is used, for example, in coil springs and torsion bars.

[Prior Art] Spring steel materials used for coil springs, torsion bars, etc. as suspension springs for automobiles are strongly desired to be lightweight from the viewpoint of resource saving and energy saving. It tends to be used in stress. Conventionally, as a method for increasing the strength of a steel material for springs, a thermomechanical treatment method called ausforming is known.

This method is a method in which a material is heated to an austenite region and then subjected to strong working in a supercooled austenite state where it is rapidly cooled (in a plate material, usually 50% or more per pass) and then quenching. According to this method, it is said that a high-strength material in which the ductility is maintained to some extent can be obtained by the synergistic action of the miniaturization of martensite by working and the working effect.

[Problems to be solved by the invention]

In order to obtain a desired high-strength material by ausforming, it is said that it is necessary to carry out 50% or more strong working per pass. If such strong working is performed, an extremely large forming load is required in the work reduction process,
The energy consumption required for processing is large, the strain region reaches the axial center of the material, and the effect of processing is given to the entire cross section. However, under the general usage conditions of spring materials, they are often used mainly by bending or twisting, and therefore exhibit a stress distribution that causes maximum stress on the surface. In other words, the core has an unnecessarily high strength, and it cannot be said that the energy and the like spent for the high strength are effectively utilized.

Further, as an example of application to the manufacturing method of the coil spring, there are a method of giving a bending strain due to coiling during cooling, a method of giving a torsional strain by twisting a material around an axis, and a method of giving a peening. However, as a drawback of these methods, for example, when a bending strain is applied by coiling, residual stress of compression remains on the outside of the bending and tensile remains on the inside of the bending, so that there is a problem in material uniformity. Further, even in the method of applying torsional strain or the method of applying peening, it is extremely difficult to apply strain evenly to the surface of a round rod-shaped material.

[Means for solving problems]

The method of the present invention, after heating the material consisting of spring steel in the shape of a round bar to an austenite temperature, and then cooling to a supercooled austenite region of Ms point (300 ° C.) or higher below the austenite temperature, and for drawing in this supercooled austenite region The material is drawn using a die, and the die is used so that the ratio (h / L) of the length L of the processing area to the average material thickness h of the processed portion is 8.7 or more during this drawing. The feature is that the processing area has a surface reduction ratio that does not reach the axis of the material.

[Action]

According to the above method, the ausforming effect is uniformly concentrated on the surface layer portion of the material drawn in the supercooled austenite region. That is, since the crystal grains in the surface layer of the material are extended and refined, and work hardening is included in addition to the extension and refinement of the crystal grains, the strength can be further enhanced. In the conventional ausforming method, a strong reduction is required at the time of working, but in the present invention, it is only necessary to locally concentrate the strain, and the forming load at the time of drawing is compared with the conventional ausforming. And significantly reduced. Moreover, since the spring steel material obtained by the method of the present invention is reinforced only in the surface layer portion, it has a high strength for use under stress conditions such as a coil spring or a torsion bar to which bending or torsion is mainly applied. The portion (surface layer) that has been worked effectively works, and the energy spent for strengthening can be effectively used.

〔Example〕

FIG. 1 shows an example of an apparatus for producing the steel material 1 for a coil spring. 2 shows an example of the manufacturing process of the present invention. In FIG. 1, a round bar-shaped material 1'is passed through a die 5 to be reduced in diameter. The material 1'is, for example, spring steel such as SUP7. A heating means 6 capable of heating the material 1 ′ to an austenitizing temperature and a cooling means 7 for cooling the material 1 ′ to a supercooled austenite temperature are provided on the inlet side of the die 5. The heating means 6 is composed of, for example, a power source 10 and a roller-shaped electrode 11 connected to the power source 10, and conducts resistance heating by energizing the material 1 '.

On the outlet side of the die 5, a cooling means 13 and a heating means 14 for reheating are provided. The heating means 14 also includes a roller-shaped electrode 15 connected to the power supply 10. Cooling means 7,13
Is for cooling the material 1'by ejecting a fluid from a nozzle, and atomized water or air can be adopted as the ejected fluid. The heating means 6, 14 may be a high-frequency induction heating device. In the illustrated example, the heating means 14 for reheating is continuously provided on the carry-out side of the cooling means 13, but this heating means 14 is provided at a position different from that shown in the drawing in order to process the reheating in a batch in a separate process. It does not matter even if it is provided in.

The material 1'is heat-treated and processed through the temperature history shown in FIG. First, the material 1 ′ is heated by the heating means 6.
Is rapidly heated to a uniform austenite region (850 ° C or higher for spring steel). It is not necessary to keep the temperature isothermal over time. The heating rate is not particularly limited,
The rapid heating method promotes fine dispersion of carbides and refinement of crystal grains, so that a more excellent high strength / high ductility material can be obtained. Therefore, a heating rate of 50 ° C./sec or more by resistance heating or high frequency induction heating is desirable.

The material 1 ′ heated in the above step is rapidly cooled by the cooling means 7 to a predetermined temperature between the austenite temperature and the Ms point (about 300 ° C. in the case of spring steel) at which martensitic transformation starts. The temperature at this time is appropriately set according to the composition of the material 1 '. Cooling temperature shall be 80 ℃ / sec or more.
However, when it is desired to positively perform the pearlite transformation, cooling may be performed at a rate close to air cooling.

And when the temperature reaches the desired Ms point or higher, the die 5
Then, the diameter of the surface layer is reduced. By this processing, the diameter of the material 1'is uniformly reduced in the circumferential direction, and a processing area 20 is formed on the surface side as conceptually shown in FIG. In the deeper layer than the processed region 20, there is an unprocessed region 21 having coarse crystal grains.
The raw area 21 may be a normal tempered tissue.

The workability (reduction ratio r) at the time of diameter reduction is the outer diameter before diameter reduction d
₀ (see FIG. 1), where d ₁ is the outer diameter after diameter reduction, r = 1− (d ₁ / d ₀ ) ² .

When the ratio h / L between the length L of the working area by the die 5 and the depth h is Δ, as shown in FIG. If it is set, the deformation area does not reach the shaft center. For example, die half-angle α =
In the case of 15 ° (0.2618rad), When the value of the area reduction rate r that can satisfy the condition of Δ ≧ 8.7 is obtained from the relational expression, the area reduction rate r for keeping only the surface layer portion is 0.11 (11%) or less. When the die half angle α = 11 °, the area reduction ratio r satisfying Δ ≧ 8.7 is about 8.5% or less, and when α = 7 °, the r is about 5.5% or less. However, if it is less than 5%, the desired effect cannot be obtained. The seventh
As shown in the figure, there is a correlation between the half-angle of Tice and the reduction ratio of the limit at which the deformation does not penetrate. Many of the dies used in industrial applications have a half-angle of 7 to 30 deg, and when the half-angle of the die increases, the surface pressure applied to the die increases, leading to increased die wear and shortened life. As the heat treatment increases and the desired effect cannot be obtained, the area reduction ratio r is in the range of 5 to 20%. As shown in FIG. 4, the condition that the deformation area does not reach the axis is Δ = h / L ≧ 8.7. On the other hand, if the penetration depth of the deformation area is δ, Therefore, the ratio of penetration depth to thickness is That is, it is about 16%, but since the thickness h is the average thickness of the processed portion, it is set to about 20% with a margin.

When the surface layer portion is processed as described above, the crystal grains in the processing area are extended in the axial direction and the circumferential direction of the steel material 1 to be refined. Moreover, in the surface layer portion, work hardening by plastic working is included in addition to the refinement of the crystal grains, so that the strength more than that obtained by simply refining the crystal grains is vaporized.

Note that shot peening is performed on ordinary spring steel materials for the purpose of improving fatigue resistance. The general state of the residual stress distribution in the cross-section of shot peened steel exhibits a pressure residual stress field at the surface layer and a tensile stress field toward the center. Since the material depth that shifts from compression to tension is usually about 0.1 to 0.2 mm,
A sufficient effect can be expected if the depth of the working area in this embodiment is set to that extent. In the present example, the diameter d ₂ of the unprocessed region 21 was set to approximately 0.8 d ₁ or more with respect to the outer diameter d _{1 after the} diameter reduction.

In this way, the material 1'is processed only on the surface layer,
It is not necessary to heat the entire cross section when heating. Therefore, if local heating of only the surface layer portion, that is, so-called pattern baking is performed using high frequency induction heating, further energy saving in the heating step can be achieved.

After the diameter reduction processing described above, the cooling means 13 further cools the tissue to freeze it. However, in order to positively utilize the transformation promoting effect by working, a method of giving some air cooling time and then rapidly cooling may be adopted. In this case, transformation of the outermost surface progresses and a soft layer is formed on the outermost surface. More specifically, when a microhardness material having a micro Vickers hardness (HMV) of 550 or more is used, the notch sensitivity is increased, and even a small surface scratch is likely to cause early fatigue fracture.
As a countermeasure against this, the hardness of the outermost surface in the surface layer portion is slightly lowered to improve notch susceptibility and fatigue strength.

FIG. 6 shows the hardness distribution after quenching of the steel material 1 for springs manufactured by the method of the above embodiment. The working area depth is about 20% of the outer diameter. In this example, the Vickers hardness of the surface layer is
Although it is HMV585, the hardness is slightly lowered on the outermost surface, and the notch sensitivity is improved. In this spring steel material 1,
The double flexural fatigue strength σwb of 10 ⁷ times could be more than ± 80 Kg f / mm ² .

By the way, in the case of the conventional material in which the drawing of the surface layer is not performed, when heat treated to the same hardness as in the above example, 10
The double bending fatigue strength σwb of ⁷ times stays within the range of ± 75 to ± 80 Kg f / mm ² . That is, the fatigue strength of this example is equal to or higher than that of the conventional material.

The spring steel material 1 is reheated by the heating means 14 and tempered if necessary. After that, the spring steel material 1 is formed by a coiling machine (not shown) to form a coil spring. Note that tempering may be performed by a batch process in another process.

〔The invention's effect〕

According to the present invention, a high-strength spring steel material can be obtained without performing strong working such as conventional ausforming, energy required for heat treatment and processing can be reduced, and heating of only the surface layer portion such as pattern baking can be performed. It is also possible to finish. Further, strain is applied uniformly over the entire circumference of the round bar-shaped material. Further, the strength is further improved by work hardening of the surface layer portion, and it is extremely effective particularly for springs mainly subjected to torsional stress such as coil springs and torsion bars. In this way, not only a high-strength steel material for springs can be obtained, but also a great effect is achieved in energy saving.

[Brief description of drawings]

1 is a schematic side view for explaining an example of an apparatus for carrying out the method of the present invention, FIG. 2 is a process explanatory view showing an example of the method of the present invention, FIG. 3 is a view showing a temperature history of a material, FIG. FIG. 4 is a diagram showing the relationship between the length and depth of the deformation zone and the yield pressure. FIG. 5 is a sectional view of the steel material for springs obtained by the present invention, FIG. 6 is a diagram showing the distribution of hardness of the steel material for springs shown in FIG. 5, and FIG. 7 is a die half angle and a surface reduction ratio. It is a figure which shows the relationship of. 1 ... Spring steel material, 1 '... material, 5 ... Die, 6 ...
… Heating means, 7,13 …… Cooling means.

Claims (1)

(57) [Claims] 1. A rod-shaped spring steel material is heated to an austenite temperature and then heated at a temperature below the austenite temperature.
The material is cooled to a supercooled austenite area above the Ms point (300 ° C), and the above material is drawn using a drawing die in this supercooled austenite area. Manufacture of spring steel material characterized by reducing the area reduction so that the processing area does not reach the axis of the material by using a die whose ratio (h / L) to the material average thickness h is 8.7 or more. Method.