JPS61217525A - Manufacture of coil spring - Google Patents

Abstract

PURPOSE:To manufacture coil spring having long service life and superior fatigue resistance, by heating, cooling rapidly straight rod shaped material for coil spring to carry out pattern quenching, then tempering the material at a specified temp. and applying coiling and cooling thereto. CONSTITUTION:Straight rod shaped material of steel for spring contg. 0.6% C is heated rapidly to quenching temp. by high frequency induction heating, etc., then cooled rapidly to carry out pattern quenching. In this case, thickness of a hardened layer 1 is controlled to 40-75% by quenching. Next, said material is reheated to 250-600 deg.C to temper it and warm coiled at the tempering temp. range to form into coil spring. Further if necessary, the spring is annealed at a low temp. of about 180 deg.C to manufacture coil spring superior in toughness and fatigue resistance.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for manufacturing a coil spring that can provide a coil spring with excellent fatigue resistance and long life.

[Conventional technology]

The conventional manufacturing method for coil springs is to coil spring steel, then quench and temper it. It is known that one way to increase strength is to lower the tempering temperature and perform low-temperature tempering. ing. However, in this case, there is a problem that the toughness of the spring steel decreases as the tempering temperature decreases.

In addition, as another means of increasing strength, new steel types containing precipitation-strengthening elements are being developed, but in this case the material cost is considerably high and it is not suitable for general use.

Therefore, the present inventors have proposed a manufacturing method for coil springs that applies processing heat treatment, that is, application of tempering warm coiling, as an effective method for increasing strength, and has filed an application for example as Utility Model Application No. 57-90360. has been done. According to this method, the tempered steel exhibits high ductility when warm worked at the tempering temperature, which not only improves workability and improves the working limit, but also improves workability through dynamic strain aging, etc. It has excellent strength at room temperature and residual ductility at room temperature, etc.
This method can be expected to have great effects as a method of strengthening steel.

(Problems to be Solved by the Invention) However, conventional tempering warm coiling employs a method in which the coil spring material is hardened down to the core of the steel material, that is, completely hardened, and then coiled while being tempered. Coiling involves applying bending and twisting to the coil spring material while forming it, so the outer surface of the coil, that is, the outer end of the bend (hereinafter referred to as the outer end of the coil), is affected by springback after processing. Compressive residual stress also occurs at the inner bending end of the coil (hereinafter referred to as
A tensile residual stress is generated at the inner end of the coil (called the inner end of the coil). For example, the wire diameter is 98. Average coil diameter is 90m (D/d=10
), a residual stress of -65UKfjf/1rvn2 was generated at the outer end of the coil, and a residual stress of +62.8Kgf/ytm was generated at the inner end of the coil.

If a large tensile stress is generated on the inner end side of the coil as described above, it tends to become a starting point for fatigue failure when a compressive load is applied, and this adversely affects fatigue resistance.

(Means for Solving the Problems) The present invention rapidly heats a coil spring material made of straight bar-shaped steel, then rapidly cools it and performs pattern hardening.
This method of manufacturing a coil spring is characterized by reheating and tempering to a temperature of 0° C. to 600° C., performing warm coiling in this temperature range, and then cooling.

[Effect]

In the present invention, pattern hardening refers to heat treatment in which the core of the spring material is not hardened, that is, incomplete hardening, as opposed to complete hardening. What is the hardened state here?
The metal structure is in a state where 50% or more of the metal structure is martensitic, and at this time, the hardness of the metal structure exhibits a substantially constant value depending on the amount of carbon contained.

- As an example, the Rockwell hardness of 0.6C% steel after quenching is approximately HRC45. Therefore, quenching is performed so that the core has a hardness below this level. Note that if the carbon content decreases, the lower limit of quenching hardness will be lower than in the above example.

In the straight rod-shaped spring material pattern-hardened as described above, surface compressive residual stress is generated due to transformation strain. For this reason,
The tensile residual stress on the inner end side of the coil caused by hot tempering coiling can be significantly reduced or eliminated, and can be prevented from becoming a starting point for fatigue failure.

Moreover, since warm coiling is performed, it is possible to give a large degree of workability to the hardened product without breaking it, and it is also possible to generate deep compressive residual stress.
It has a great effect on improving lifespan and fatigue resistance.

〔Example〕

A spring equivalent to 0.6C%! l1lI steel material (J Is, G
Pattern hardening is performed by rapidly heating a coil spring material (see 4801), that is, a straight rod-shaped SUP material, to a predetermined hardening temperature by high-frequency induction heating or high-frequency resistance heating, and then rapidly cooling it.

High frequency heating is performed continuously while moving the spring material in the axial direction. However, it is also possible to heat the spring material to the quenching temperature using other heating means while keeping it fixed.

FIG. 1 schematically shows a cross section of a pattern-hardened coil spring material. , for 0.8 C%,
The quenching hardness HRC> is about 45 to 60. The outer surface of the hardened layer 1 becomes martensite (one phase) with an HRC of 60 or more, but the closer it gets to the core part 2, the less martensite becomes, and in the core part 2 it becomes low carbon martensite + troostite (two phases). becomes. In the hardened state, martensite accounts for over 50%, and the lower limit of hardness is HRC45.
(at 0.6 C%).

Then, the quenching temperature, cooling rate, etc. are set in relation to the steel type so that the hardening depth by quenching is 50 to 75%.

The hardening depth is defined as (
r/R) xlOO%.

Note that when a steel type with a low carbon content (for example, 0.3 C%) is used, the limit of quenching hardness will be smaller than HRC45.

When the above-described pattern hardening is performed and the surface side becomes a martensitic structure, surface compressive residual stress is generated due to transformation strain, resulting in a residual stress distribution as shown by the two-dot chain line A in FIG.

Next, the pattern-hardened straight rod-shaped coil spring material is reheated and tempered at a temperature of 250° C. to 600° C., and warm coiling is performed in this tempering temperature range. It is preferable to use high frequency induction heating or high frequency resistance heating for heating for tempering.

When the above-mentioned tempered warm coiling is performed, the conventional completely hardened material has compressive residual stress at the outer end of the coil and tensile residual stress at the inner end of the coil, as shown by broken line B in Figure 3. arise. However, in this example, compressive residual stress is generated in advance on the inner end of the coil by pattern hardening, so as shown by solid line C in FIG. 3, tensile stress remains on the inner end of the coil during warm coiling. Stress can be reduced or eliminated.

After performing the above warm coiling, low-temperature annealing is performed as necessary. The annealing temperature is, for example, 180° C. and the time is about 40 minutes.

If shot peening is then applied in the same manner as with ordinary coil springs, further improvement in fatigue resistance can be expected. Further, setting, rust prevention treatment, etc. may be performed as necessary.

According to this embodiment described above, by utilizing surface compressive residual stress due to transformation strain during quenching, the generation of tensile residual stress on the inner end side of the coil, which is the starting point of fatigue fracture, is suppressed, and fatigue resistance is improved. can.

Moreover, by performing warm coiling after tempering,
A spring with good warm workability and high strength can be obtained. Deep residual stress can be generated by appropriately selecting the temperature conditions (low temperature side) for warm coiling.

Note that if the hardening depth due to pattern hardening is 40% or less, fatigue deformation occurs from the inside of the cross section. Further, if the ratio is 75% or more, the dangerous cross section of fatigue failure approaches the vicinity of the surface, making it easier to break mechanically. Therefore, the hardening depth is preferably in the range of 40 to 75%, and may be adjusted within this range depending on the desired material strength.

In addition, the steel type targeted by the present invention is 8300 with a reduced carbon content.
It is also applicable to a class (0.3C%).

〔Effect of the invention〕

As described above, according to the present invention, it is possible to exhibit great effects in improving the life span and the fatigue resistance.

[Brief explanation of drawings]

FIG. 1 is a diagram schematically showing a cross section of the pattern hardened material, FIG. 2 is a diagram showing the quench hardening depth of the pattern hardened material, and FIG. 3 is a diagram showing the distribution of residual stress. Applicant's agent Patent attorney Takehiko Suzue Figure 1 Figure 2 Figure 3

Claims (3)

[Claims] (1) After rapidly heating a coil spring material made of straight rod-shaped steel, it is rapidly cooled and pattern hardened, then reheated to a temperature of 250°C to 600°C that is suitable for coiling, and warm coiled in this temperature range. 1. A method for manufacturing a coil spring, characterized by performing the following steps and cooling the spring. (2) The method for manufacturing a coil spring according to claim 1, wherein the hardening depth by the pattern hardening is 40 to 75%. (3) The method for manufacturing a coil spring according to claim 1 or 2, characterized in that low-temperature annealing is performed after the warm coiling.