JPS6232250B2 - - Google Patents

Description

[Detailed description of the invention]

(a) Technical Field The present invention relates to a method for producing a spring steel wire with excellent fatigue resistance used in vehicle suspension springs and the like. (b) Technical Background There are issues with steel wires for vehicle suspension springs, such as fatigue resistance and relaxation value. Here, ``settling'' refers to permanent deformation of the coil spring during use.
This is a phenomenon in which the free height changes. Conventionally, such spring steel wires are JIS SUP6~7.
Although steel wires of steel grades such as and SAE9254 are used, there is a need to reduce the weight of vehicles to save energy, and one of these demands is a steel wire for springs with excellent fatigue resistance. That is, if a spring steel wire with excellent resistance to fatigue could be developed, it would be possible to further increase the design stress, and as a result, the weight of the coil spring could be reduced. In order to improve this resistance to sagging, attempts have been made to increase the strength through various heat treatments, but if the strength is increased unnecessarily, the problem is that the increased strength results in a lack of toughness such as elongation and drawing. There is. In addition, increasing the strength of the steel wire increases its sensitivity to minute scratches on the surface of the steel wire and internal defects, resulting in practical problems such as a lack of reliability such as easy breakage during spring processing or use. The inventors of the present invention have conducted various studies aimed at obtaining a spring steel wire with excellent resistance to fatigue without increasing the strength, and as a result, they have succeeded in achieving their objective. (C) Disclosure of the Invention A feature of the present invention is that in a spring steel wire manufactured by quenching and tempering, after the tempering treatment, the steel wire is heated again below the tempering temperature, and the steel wire is reduced in the temperature range. A method for producing a spring steel wire with excellent resistance to fatigue, which is characterized by applying plastic working with an area ratio of 10% to 30% or bending with a bending strain of 1% to 3%, and the above-mentioned method. When the steel wire manufactured by the method is evaluated as a spring under the following conditions, it has a residual shear strain of 2.0 × 10 ^-4 or less and is a spring steel wire with excellent fatigue resistance. . In other words, the spring is processed by cold forming, and the spring is subjected to a constant load of 115 kg/ ^mm2 at room temperature for 100 min.
Measures residual shear strain when loaded continuously over time. Here, the residual shear strain is calculated from the amount of decrease ΔP in load at a constant spring height before and after applying a constant load to the spring, using the following equation. Residual shear strain γ = 8・D/π・d ³・G・ΔP d: Wire diameter D: Coil center diameter G: Transverse elastic modulus Here, the reason why the carbon content of this steel wire is limited to 0.5 to 0.7% is because the vehicle If the aim is to reduce the weight of a steel wire and improve its resistance to set, it is necessary to give sufficient strength to the steel wire, but if the carbon content is less than 0.5%, sufficient strength will not be obtained, and if it is more than 0.7%, the toughness will decrease. This is because it is inhibited. The spring steel wire of the present invention is manufactured as follows. FIG. 1 is a schematic diagram showing the process. The wire 2 supplied from the supply 1 is sent in the direction of the arrow by a feeding pinch roller 3, straightened into a substantially straight line by a straightening machine 4, and then sent to a heating device 6 by a second feeding pinch roller 5. Here, the wire rod is heated to a quenching temperature determined by the steel type of the steel product, and is cooled in a cooling tank 7 to complete the quenching process.
Thereafter, the steel wire is immediately sent to a heating device 9 by a third feeding pinch roller 8, heated to a tempering temperature, and then cooled to room temperature by a cooling bath 10. The feature of the present invention is that after the quenching and tempering treatments are completed, the steel wire is heated to below the tempering temperature by the heating device 13, and in that temperature range, the steel wire is processed by the processing device 14 to achieve a reduction in area of 10%. Plastic working of 30% or more or bending of 1% or more and 3% or less of distortion is applied, and then the cooling device 15 is used to rapidly cool down to room temperature, and the feeding pinch rollers 16 are used to wind up the material. It is taken up by machine 17. It has been found that the steel wire 18 manufactured in this manner is a spring steel wire with significantly improved resistance to fatigue. In the above, the heating device 13, the processing device 14, and the cooling device 15 may be used in tandem with the previous quenching and tempering process, or the quenching and tempering process may be heated and processed separately. It's okay. Here, the reason why the temperature at which the steel wire is reheated by the heating device 13 is set to be lower than the tempering temperature is that heating the steel wire above the tempering temperature will reduce the strength of the steel wire, and the strength of the steel wire will decrease due to the initial quenching and tempering treatment. This is because the strength properties obtained by using the same method may no longer be obtained. In addition, the area reduction rate applied by wire drawing was set to 10% or more and 30% or less because the processing amount was 10% or less.
If it is less than 30%, it is insufficient for the amount of processing required to suppress the settling phenomenon, and in order to add processing larger than 30%, it requires extremely large power and is difficult to implement on the equipment. It's for a reason. Furthermore, the amount of strain applied by bending was set at 1% to 3% because if the amount of strain applied is less than 1%, it is insufficient for the amount of strain required to suppress the sagging phenomenon. , and it is difficult to apply a strain larger than 3% on the equipment. Depending on the processing method and amount of processing, the steel wire may undergo work hardening and its strength will significantly increase, so heat treatment conditions should be adjusted appropriately to obtain the specified strength depending on the steel type, processing method, and amount of processing. I have to decide. Although the heating processing apparatus 12 has a processing apparatus 14 after heating in FIG. 1, the steel wire of the present invention can also be obtained by heating-processing-heating. Next, an example will be explained. (d) Best Mode for Carrying Out the Invention Example 1 SAE9254 (C: 0.56wt%, Si:
1.37wt%, Mn: 0.7wt%, Cr: 0.59wt%), and a wire drawing die was used as the processing device 14 to give processing strain. The quenching temperature at this time is 970℃, the tempering temperature is 465℃, and the area reduction rate of the steel wire is 20%, and the heating temperature during processing is 450℃.The wire diameter after processing is 9.5mm.
The tensile strength of the steel wire after processing was 185Kg/ ^mm2 . The steel wire produced in this manner was processed into a coil spring and subjected to a static test for resistance to fatigue.
For comparison, coil springs with the same specifications were made using 9.5 mmφ SAE9254 and SUP7 steel wires that had been subjected to normal quenching and tempering treatments (no processing) to have a tensile strength of 185 Kg/mm ² . The specifications of the coil spring are as follows. Wire diameter: 9.5mmφ Coil average diameter: 60mmφ Free height: 260mm Effective number of turns: 4.25 Total number of turns: 6.25 After setting with a stress of 120Kg/mm ² using this, the test stress was set to 115Kg/mm ² Table 1 shows the results of a sag test at room temperature in which a constant load was applied continuously for 100 hours.

[Table] As shown in Table 1, the steel wire for springs according to the present invention was found to have extremely superior fatigue resistance compared to conventional products. Example 2 The same SAE9254 as in Example 1 was used as a test material, and strain was applied while processing using a straight roll as a processing device. At this time, the quenching temperature is 970℃, the tempering temperature is 435℃, the maximum bending strain is 2.1%, and the heating temperature during processing is 410℃.
℃, the wire diameter after processing is 9.5 mmφ, and the tensile strength is 200 Kg/mm ² . A coil having the same specifications as in Example 1 was prepared using this steel wire, and a test for resistance to settling at room temperature was conducted. Again, similar steel wires SAE9254 and SUP7 are used for comparison.
A 9.5 mmφ steel wire with a tensile strength of 200 Kg/mm ² was prepared by normal quenching and tempering treatment, and a coil of the same shape was fabricated. After setting with a stress of 125Kg/ ^mm2 , a constant load was applied continuously for 100 hours to give a test stress of 115Kg/ ^mm2 , and a fatigue test was performed at room temperature.The results are shown in Table 2. show.

[Table] As shown in the table, it can be seen that the product of the present invention is more than twice as good in resistance to settling as the comparative product.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram showing a method for manufacturing the product of the present invention. 1: Supply, 2: Wire, 3, 5, 8, 11,
16: Feeding pinch roll, 4: Straightening machine, 6,
9, 13: heating device, 7, 10, 15: cooling tank,
14: processing device, 12: heating processing device, 17: winding machine, 18: steel wire of the present invention.

Claims (1)

[Claims] 1. In a spring steel wire manufactured by quenching and tempering with a carbon content of 0.5 to 0.7% by weight, after the quenching and tempering treatment, the steel wire is heated again to a temperature below the tempering temperature, and the steel wire is heated in that temperature range. Add plastic working to the wire with an area reduction rate of 10% to 30%, or apply bending to a bending strain of 1% to 3%,
After that, it was rapidly cooled to room temperature, and when a constant load of 115 kg/mm ² was continuously applied at room temperature for 100 hours to a cold-formed spring using the spring steel wire produced in this way, the residual shear strain of the spring A method for producing a spring steel wire with excellent resistance to fatigue, characterized in that the amount is 2×10 ^-4 or less.