JPS63491B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to cold-formed coil springs. Conventionally, coil springs are formed by either cold forming or hot forming. Small diameter springs are mainly formed by cold forming, and large diameter springs of 10 mmφ or more are mainly formed by hot forming. This is because the thicker the diameter of the material and the higher the strength, the more difficult it becomes to cold form the material. To explain the situation in more detail, conventionally, large-diameter coil springs of 10 mmφ or more are produced by hot forming a material into a coil spring shape, and then applying quenching and tempering to give the required high strength. Manufactured. According to the hot forming method, it is easy to form a coil, but on the other hand, the decarbonization of the spring surface that occurs during heating,
The quality of the finished product may deteriorate due to the fact that the strength of the material decreases due to hot treatment, which tends to cause surface scratches, and because the heat treatment is performed on coiled springs, the strength tends to vary, and the surface roughness and deformation occur during heat treatment. Specifically, defects are more likely to occur compared to cold-formed products. According to the cold forming method, which manufactures coil springs by cold forming wire rods such as oil-tempered wires with a predetermined strength and a relatively small diameter, the strength of the wire rods remains the same because the wire rods are not heated during forming. In this respect, it is superior to the hot forming method because it retains its properties and does not cause surface roughness. However, if the material is a high-strength wire rod, there is a problem that forming becomes difficult as the wire diameter increases. The present invention aims to provide an excellent cold-formed coil spring that combines the advantages of the conventional coil springs obtained by the above-mentioned hot forming method and cold forming method, and eliminates the defects of each. It is something to do. More specifically, the present invention uses heat treatment consisting of rapid heating by induction heating and rapid cooling to improve the crystal grain size of the base material.
Has a fine grain structure with ASTM number 9 or higher, and has tensile strength
It has mechanical properties that combine high strength, ductility and toughness of 150Kgf/ ^mm2 or more, elongation of 9% or more, and reduction of area of 49% or more, and the fatigue limit after 2 million cycles is the torsional shear stress τw = wire diameter of 10mmφ or more. It is an object of the present invention to provide a cold-formed coil spring that has a strength of 60±45 Kgf/mm ² or more and has very little set. The present invention will be explained in detail below with reference to FIGS. 1 to 3. For example, after drawing a wire rod made of carbon steel, it is rapidly heated by high-frequency induction heating, etc., and then kept in the austenite region at 1000℃ or less for a minimum time that can be set depending on the chemical composition of the wire rod, and then water quenched. The grains of the steel are extremely fine, ASTM no. 9 to 12, and the microstructure has a non-uniform carbon concentration, which results in higher strength and toughness during the quenching stage compared to normal heat treatment. The ability to manufacture wire rods is already known as a feature of rapid heating quenching such as high-frequency induction heating, but the present inventor has demonstrated that wire rods with a carbon content of 0.3% or more are subjected to the above quenching process. After heating the obtained high-strength wire rod to a temperature of 300° to 600°C, which is relatively higher than the conventional tempering temperature, the heating is stopped. Tensile strength can be improved by maintaining the temperature for an extremely short period of 60 seconds or less (compared to 30 to 60 minutes with conventional methods) and cooling.
It has been found that a wire rod having a strength of 150 Kgf/mm ² or more, high toughness, and excellent cold workability can be obtained. As mentioned above, this method departs from the conventional concept of tempering and uses rapid heating at a relatively high temperature and for an extremely short period of time to gradually heat up the decomposition of martensite containing supersaturated interstitial atoms such as carbon and the precipitation of carbides. The first stage of tempering, which occurs more rapidly than when the tempering is carried out and is held for a short time and then rapidly cooled, provides insufficient thermal energy to reach the generally considered sufficient tempering state at that temperature. This is presumed to be due to the fact that the reaction is temporarily stopped in a state where the tempering is insufficient, including the distribution and shape of the carbides. In other words, by quenching by rapid heating, short-time holding, and rapid cooling, the steel material is made into a martensitic structure with fine grains, thereby giving the steel material the base necessary for having high strength and high ductility. The first stage of high-temperature and short-time tempering provides a strength of over 150Kgf/ ^mm2 , and although the dislocation density is generally high, the yield point is kept relatively low and sufficient elongation is achieved after yielding. A state in which the ratio of dynamic dislocations necessary to obtain the following properties is high, that is, a state in which the resulting properties are high in ductility, and specifically, in a state in which workability is high. Of course, at this stage, the tempering is insufficient, so it can be said that the resistance to settling is not favorable. Some of the experimental results to prove this are shown in the table below. Experimental example 1 (1) Specimen material SAE 1552 Chemical composition C=0.51, Mn=1.56 Diameter 12mm Quenching hardness Hv=800 (2) Relationship between tempering conditions and mechanical properties

[Table] (3) Experimental results According to the experimental results above, quenching by rapid heating, short-term holding, and rapid cooling was performed, and after
When rapidly heated to 300-600℃, held for a short time of less than 60 seconds, and then rapidly cooled and tempered,
It has been found that a wire rod with high strength and high workability, with a tensile strength of 150 Kgf/mm ² or more, an elongation of 9% or more, and a reduction of area of 49% or more, can be obtained. Note that FIG. 1 shows the relationship between the tempering time and temperature of the specimen in this experimental example, and FIG. 2 shows the relationship between the holding time and the hardness and toughness of the wire at a tempering temperature of 500°C. In Figure 1, the vertical axis is the tempering temperature, the horizontal axis is the tempering time (logarithm), and in Figure 2 the horizontal axis is the tempering time (logarithm).
Curves a, b and c show changes in tensile strength, area of area and elongation, respectively. As is clear from the above experimental example, when carbon steel with a carbon content of 0.51% is used as a material by the above-mentioned first-stage tempering process, a wire rod with high strength of 150 Kgf/mm ² or more and excellent workability can be obtained. It can be grown. Furthermore, according to other experimental examples of the present inventor, for example,
Carbon content 0.3% as chemical composition shown in JIS SWRH 62B, SAE9254, SUP6, 7, 9
If the above carbon steel is used as the material, 150
Kgf/mm ² or more, 220Kg depending on steel type or composition system
It has also been found that a wire rod with high strength of about f/mm ² and excellent workability can be obtained. The above-mentioned wire rod has high strength and high workability, so it has a diameter of 5 mm compared to the conventional wire diameter for cold forming, such as 10 to 16 mmφ.
A large diameter coil spring that is 60% thicker can be easily cold-formed. That is, it is possible to easily cold-form a high-strength, large-diameter coil spring using a known molding machine. After cold-forming the above-mentioned wire rod into a coil spring, the second stage of low-temperature tempering, in which the wire is heated to 300-500℃ for 30-60 minutes, gives the coil spring excellent creep resistance and extremely little set-up. You can get a spring. In other words, the wire rod that maintains an extremely high dislocation density through the first stage of tempering, which is characterized by rapid heating and short-time holding, undergoes plastic processing through cold forming, thereby further multiplying its dislocations. In addition to this, tempering is generally performed at a temperature range of 300 to 500°C for a certain period of time of 30 to 60 minutes, with an upper limit lower than the above-mentioned first stage tempering temperature, so as not to reduce the high strength obtained in the first stage tempering. By performing the second stage of tempering in an electric furnace, etc., a sticking phenomenon occurs between dislocations and solute atoms or carbides, making the dislocations immobile and forming so-called immobile dislocations, resulting in a type of strain aging effect. As a result, the elastic limit, yield point, and relaxation characteristics of the cold-formed coil spring are improved, and the decomposition of martensite, precipitation, distribution, and shape of carbides during the heat treatment stage before cold forming of the wire rod are improved. It is presumed that the insufficient tempering phenomenon described above will be stabilized, and that the coil spring will ultimately be given high creep resistance while maintaining high strength. It goes without saying that the second stage of low-temperature tempering adds the general effect of removing residual stress due to cold working. The inventor conducted an experiment to confirm the effect of the second stage of stabilizing tempering. Figure 3 shows part of the experimental results. Experimental Example 2 (1) Experimental Conditions (1) Specimen Same as in Experimental Example 1. (2) Heat treatment conditions Heat treatment was performed under the same conditions as in Experimental Example 1. (3) One part of the heat-treated specimen underwent plastic deformation due to torsion (processed material), and the other part did not undergo plastic deformation due to torsion (unprocessed material), and both were subjected to the second stage of stability under the same conditions (described above). A chemical tempering treatment was applied. (2) Experimental results As shown in Figure 3, the vertical axis is the tensile load P.
, the horizontal axis shows the strain ε, m is the processed material, n
shows the tensile load-strain curve of the unprocessed material. From FIG. 3, it is clear that the processed material has a higher elastic limit than the unprocessed material, and has higher creep resistance. The inventor further conducted the following experiment to compare the specific mechanical properties of the coil spring according to the present invention with those of a coil spring obtained by conventional hot forming. Experimental example 3 (1) Specimen Raw material Wire diameter 14mmφ Material A: SAE in which the present invention was implemented
1552 B: SUP using conventional method
6 Manufacturing process

【table】

[Table] ↓

[Table] Both A and B have a strength level of 160Kgf/ ^mm2 , and are completed compression coil spring products as shown below. D/d......6 Effective number of turns Na...5 turns Total number of turns Nt...7 turns Free height H...220 mm (2) Experimental method Both A and B were subjected to a constant strain fatigue tester and 200
The fatigue limit after 10,000 repetitions was determined. (3) Experimental results The results were as follows.

[Table] In other words, the fatigue limit of the cold-formed coil spring according to the present invention is 60±45Kg, whereas that of the conventional hot-formed coil spring is 60±38Kgf/ ^mm2 .
It showed a fatigue limit of f/mm ² or more. In the above embodiments, the case where high frequency induction heating is mainly used as the rapid heating means has been described, but substantially the same effect can be obtained even if a direct energization method or the like is used instead. According to the present invention, the tensile strength of the base material can be increased to 150 kg by heat treatment consisting of rapid heating and rapid cooling by induction heating.
Although it has high strength of f/mm ² or more, it has high workability of elongation of 9% or more and reduction of area of 49% or more, so it can be used for wires with a diameter of 10 mmφ or more, which conventionally had to be hot formed. It is possible to easily provide cold-formed coil springs made of large-diameter, high-strength wire rods, and as a result, the decarbonization of the spring surface and material strength, which were unavoidable with conventional hot-formed coil springs, can be easily provided. It is possible to realize a large-diameter, high-strength coil spring that does not suffer from deterioration, strength variations, surface roughness, deformation, etc. Furthermore, the product has a fatigue limit of 60±45Kgf/mm ² after 2 million cycles using a constant strain fatigue tester.
Since it has the above-mentioned high durability and low flattening property, it is possible to provide a coil spring which has the above-mentioned effects and extremely superior spring characteristics compared to conventional ones.

[Brief explanation of the drawing]

FIGS. 1 to 3 are diagrams showing experimental results of the present invention, respectively.

Claims (1)

[Claims] 1 Through heat treatment consisting of rapid heating and rapid cooling by induction heating, the base material has a fine martensitic structure with a grain size of ASTM number 9 or higher, and a tensile strength of 150 kg.
It has mechanical properties that combine high strength with f/mm ² or more, elongation of 9% or more, and reduction of area of 49% or more, as well as ductility and toughness.
Wire diameter 10mmφ or more, fatigue limit after 2 million cycles is torsional shear stress τw=60±45Kgf/mm2 or ^more , and
Cold-formed coil spring with extremely low settling.