JPS58213825A - Reinforcing method of spring steel - Google Patents

Abstract

PURPOSE:To obtain spring steel which is increased in strength by tempering the spring steel which is subjected to a hardening treatment in a prescribed temp. range, tempering the same in the temp. region where the change in the 3rd stage arises or the temp. below said region and subjecting the steel to shot peening simultaneously in the tempering process. CONSTITUTION:Electricity is conducted through electrodes 2, 3 to the spring steel A which is subjected beforehand to coiling and a hardening treatment to heat the steel quickly in a short time, for example, for about 30sec, and the steel is tempered at the temp. at which the change in the 3rd stage of the tempering arises or the temp. below said temp. The temp. in this case is <=350 deg.C, more preferably about 200-250 deg.C with middle carbon steel. The heating speed, heating temp., holding time, etc. in this stage are controlled with a computer 3 for control in accordance with the surface temp. of the steel A while aid temp. is detected with a temp. measuring device 4. Shot peening devices 5, 5 are operated simultaneously in the above-described tempering stage to apply warm peening to the steel with the prescribed coverage, arc height and projection time controlled with the computer 3, whereby the intended spring steel is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for increasing the strength of spring steel used in coil springs, leaf springs, automobile stabilizer bars, and the like.

Conventionally, it has been known to subject spring steel to shot peening for the purpose of improving its fatigue resistance.

Generally speaking, this type of peening process is: ■ peening quenched and tempered spring steel with a sorbite structure at room temperature, and ■ peening quenched and tempered spring steel with a sorbite structure. There is a method of adding peening in a warm range of 200 to 400°C after reconstitution.

All of these have a sorbite structure as a matrix, and are hard and strong, so they have sufficient properties for general springs1. Higher strength spring steel is required for the purpose of reducing waste, etc., and conventional spring steel has problems with mechanical properties such as proportionality limit and fatigue resistance in order to obtain high strength springs. There were some difficulties.

By the way, when tempering hardened steel, if you measure the mechanical properties by varying the tempering temperature, you will notice that in addition to the two-way change due to temperature, there are also changes in the first stage, second stage, eight third stage, and so on. It is known that unique changes called so-called For example, in medium carbon steel, the temperature at which each of these stages occurs is 200°C or less in the first stage, and 200°C or less in the second stage.
The third stage occurs at around 300°C and above 250°C.

That is, in the first stage, rupture stress occurs due to fixation of transformation dislocations by carbon atoms and precipitation of carbides.

Yield stress and -F increase in proportional limit are observed. In the second stage, retained austenite decomposes into low carbon martensite and hexacarbide (7, accompanied by hardening).

Further, in the third stage, C carbide is dissolved and cementite is precipitated. At this stage, the elastic limit reaches its maximum.

Figure 1 is an actual measurement example showing the relationship between the tempering temperature and each mechanical property for medium carbon steel, where a is the breaking stress, b is the yield stress, 0 is the proportional limit, and d is the plastic elongation at break water. I'm doing it. As is clear from this measurement example, the proportional limit C of the yield stress, which is important for copper for springs, becomes high when the tempering temperature exceeds 150°C, and when the tempering temperature is 250°C, the second stage of It can be seen that the plastic elongation at break d, which has a maximum value in the temperature range and has an adverse effect on fatigue strength, becomes small in the third stage.

On the other hand, the relationship between temperature and residual stress when shot peening is actually measured is as shown in FIG. 2.

That is, it can be seen that the residual stress e (the curve shown by the solid line) is the largest when shot peening is performed at room temperature, and the peening effect gradually decreases as the temperature rises. (Curve f... 9 curves g at 300℃... 1 curve h at 350℃...
- At 400°C) In particular, at temperatures above 350°C, the effectiveness is considerably lower than that of peening under Murobuchi. That is, in order to generate compressive residual stress on the steel surface, it is necessary to perform peening at a temperature as close to room temperature as possible, preferably at least 350°C, and preferably 300°C or less. In addition to improving the surface strength due to the residual stress of this peening, the present inventors focused on the influence of the above-mentioned tempering temperature structure change on mechanical properties,
By setting the tempering temperature to the temperature at which the third stage change occurs, and performing peening at the same time as tempering in this tempering process, the surface layer is strongly worked, promoting fine precipitation of carbides and fixing of dislocations. It was discovered that the strength of copper for springs can be increased through the interaction of

The present invention has been developed based on the above-mentioned circumstances, and its purpose is to provide a tempered martensitic matrix (K) with high proportional limit, tensile strength, and yield stress, excellent fatigue resistance, and notch sensitivity, as well as excellent fatigue resistance. It is an object of the present invention to provide a method for strengthening copper for springs by which a high-strength spring with excellent properties can be obtained.

That is, in the method of the present invention, spring copper that has been hardened is tempered at an ffr of 1 degree and at a temperature not exceeding 350°C until the third stage of tempering occurs, and at the same time, shot peening is carried out for 1r during this tempering. : This is a method of strengthening copper for springs.

An embodiment of the present invention will be described below with reference to the drawings. FIG. 3 shows an example of an apparatus for carrying out the method of the present invention, in which A indicates a coiled spring copper. Electrodes 2.2 constituting the heating device 1'fr are attached to both ends of the spring copper A, so that a desired electrical current can be provided via a power transformer (not shown). 3 is a control computer that can control heating speed, heating temperature, heating holding time, etc. Further, a temperature measuring device 4 for detecting the surface temperature of the spring copper A is provided. This temperature measuring device 4
A well-known radiation thermometer is used, and the detected temperature signal is input to the control computer 3.

Further, shot peening devices 5, 5 are provided for peening the spring copper A. This shot peening device 5.5 may be of a well-known general type, and is preferably one in which the capereno, arc height, projection time, etc. can be controlled by the above-mentioned control combination.

In order to strengthen the spring copper A using the above embodiment device, the spring copper A, which has been coiled and hardened in advance, is energized through the electrodes 2, 2, and rapidly heated for a short period of time, for example, about 30 seconds. Then, tempering is carried out at a temperature that causes the change in the third stage of tempering or a temperature lower than this temperature (350° C. or lower for medium carbon steel, preferably about 200° C. to 250° C.). The heating rate, heating temperature, holding time, etc. at this time are as follows:
The temperature of the surface 6 of the spring copper A is detected by the temperature measuring device 4 and controlled by the control converter 3 based on the temperature signal.

And at the same time in the above tempering process.

The hot peening apparatuses 5, 5 are operated, and warm peening is carried out at a predetermined capa-reso, arc height, and projection time controlled by the control conbeater 3. By adding this shot peening process, spring copper A
The surface layer of the steel is subjected to severe processing, and dislocations generated on the surface of the old material are fixed and fine precipitation is promoted. In other words, deformation-induced precipitation occurs, and since the temperature of spring copper A at this time is maintained at a temperature that causes age hardening through tempering to the third stage of hardened martensite, this mud 1 is Peening during tempering is extremely effective in obtaining fine precipitates, and high-strength copper for springs using tempered martensite as a matrix can be obtained.

Moreover, as mentioned above, if the temperature is below 350°C, it is possible to apply compressive residual stress that is almost the same as that of conventional room temperature shot peening, as described above (Fig. 2). Almost the same effect can be obtained.

Moreover, in this temperature range, as mentioned above (see Figure 1), the proportional limit and yield stress are at their maximum, so the fatigue resistance and fatigue strength are improved, and the synergistic effect with the shot peening effect makes it excellent. A high strength spring can be obtained.

In this example, shot peening was performed in this temperature range after tempering at 150°C to 250°C, and the best results were obtained especially when the tempering temperature was around 200°C.

Moreover, according to this embodiment, since the tempering process and the shot peening process are performed at the same time, shot peening is performed after tempering as in the conventional method. Side netting can be done in about tens of seconds, greatly improving work efficiency.

Furthermore, when the restoring temperature exceeds the temperature at which the third stage of change occurs, the proportional limit and yield stress gradually decrease, and the fatigue strength decreases, and the plastic elongation at break becomes minimum, which has an adverse effect on fatigue strength. It will fall short of that. Furthermore, if the temperature exceeds 350°C, the sheet beaning effect will be significantly reduced, making it impossible to obtain the desired compressive residual stress. Therefore, in the present invention, the upper limit of the tempering temperature is set to a temperature range that causes the third stage change, and the temperature is set not to exceed 350°C. Furthermore, if the tempering temperature is lower than 150°C, as mentioned above (see Figure 1), an increase in yield stress and proportional limit cannot be expected;
The vorticity is set to be L7 vorticity above ℃.

The temperature at which the change occurs in the third stage of tempering is generally about 250° C., but since it changes depending on the composition of the spring copper, the specific value is not limited to this example.

Further, the present invention can of course achieve the intended purpose even if a heating furnace other than the direct current heating device 1 used in the Kamimachi 1 is used.

Furthermore, the present invention can be similarly applied to various types of spring copper used in coil springs, leaf springs, torsion bars, vehicle stabilizers, and the like.

Because it is tempered at a temperature that does not exceed the temperature range, spring copper made from strong and hard tempered martensite is obtained by precipitation of carbides due to age hardening of the quenched martensite.
Yield stress 1 tensile strength is improved. Since shot peening is carried out in the above temperature range during this tempering process, it is possible to impart compressive residual stress to the surface of the spring steel that is almost equivalent to that of conventional room temperature peening.
This warm peening promotes the interaction between the fine precipitation of carbides and the fixation of dislocations, significantly improving fatigue strength and fatigue resistance.

Furthermore, shot peening performed under the above conditions significantly reduces surface flaws in spring copper, makes the texture finer, and imparts compressive residual stress, improving so-called notch sensitivity and improving fatigue resistance. Through these synergistic effects, a high-strength spring with excellent fatigue resistance can be obtained.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the relationship between the mechanical properties of spring steel and tempering temperature, Fig. 2 is a diagram showing the relationship between the sheet beaning effect and temperature, and Fig. 3 is a diagram showing the relationship between the mechanical properties of spring steel and tempering temperature. FIG. 1 is a schematic diagram of an example device. 1... Heating device, 5... Topeening device,
A: Steel for springs. Applicant's agent Patent attorney Suzue Takehiko 11- Figure 1 Smoke Moto'L! 7ij (C')

Claims (1)

[Claims] The hardened spring steel is tempered within the temperature range of 150°C to 350°C and at or below the temperature range where the third stage of tempering occurs, and simultaneously shot peened using the over-tempered rod. A method for strengthening spring steel, characterized by subjecting it to.