KR820001821B1 - High strenght spring steel and its manufacturing process - Google Patents

Abstract

This method is related to the high strength spring steel with the surface layer having the super microscopic structure. The heating temp. of the steel surface is between the higher AC3 transformation point and the lower Ar1 transformation point. At that temp. the steel is alteted repeatedly in a short period of time. When the steel material is heated, the temp. of the steel center becomes more intense. After that, the steel material is cooled at the short time. So, the high strength spring steel having the detailed crystal grain is manufactured.

Description

Manufacturing Method of Steel for High Tenacity Spring

The accompanying drawings are diagrams for explaining the thermal cycle of the present invention.

In the present invention, the surface heating of the steel is applied to the steel and subjected to surface heating that periodically changes the temperature between the Ac ₃ transformation point and the Ar ₁ transformation point for a short time in a short time. The present invention relates to a steel material for high toughness springs and a method for producing the same, characterized in that the grain size is refined as it is heat-treated to the surface layer from the center, and the grain size of the surface layer is an ultrafine structure.

More specifically, the high-strength spring mounting material according to the present invention, after heating the steel surface temperature above the Ac ₃ transformation temperature point by surface heating, stops heating to lower the surface temperature of the steel material below the Ar ₁ transformation point temperature. The heat treatment cycle of the quenching cooling is produced by continuously heat-treating and then repeatedly heating until it becomes a heating state or the entire heating.

Elastic materials such as coil springs and torsion bars require high fatigue strength, in particular high torsional fatigue strength, as the most necessary characteristic. When this type of spring is analyzed for deformation in use, stresses such as squeezing and twisting applied to the spring increase toward the surface centered on the neutral axis of the elastic wire rod, so that the maximum stress in the surface layer is increased. It is evident that the main purpose of the present invention is to form a steel structure corresponding to this stress distribution.

However, conventionally, for example, in manufacturing a coil spring, after extracting the steel for spring, an oil temper treatment is performed to form a high strength wire rod in cold, coiled, or spring steel It is based on a method of imparting strength by performing heat treatment after molding into a coil, and in any case, it was only to obtain a uniform heat treatment structure over the entire cross section by normal heat treatment.

Therefore, in the conventional method for producing steel for springs, it is not possible to form a dense structure corresponding to the stress distribution given when the spring is used. Moreover, in the conventional method of manufacturing a coil spring or the heat treatment process of the spring steel wire, the entire cross-section up to the core portion of the steel was heated only once after the Ac ₃ transformation point and immediately heat-treated, thus making it impossible to form fine grains.

Although a treatment method for miniaturizing metal grains has been studied outside of the field of manufacturing springs, Elaine U.S. Patent No. 3,178,324 discloses that the martensite structure is heated by forcibly cooling the metal body to at least Ac _{3 points} throughout its cross section. It is a repetitive method of miniaturizing the whole tissue by forming two or more treatments, and has nothing to do with the direct improvement of fatigue strength. In order to improve the strength of spring steels, it is necessary to decide how much the structure of the entire steel cross section should be decided, and to achieve the above-mentioned highly improved improvement. The invention of forming a dense distribution of corresponding tissue crystals is novel and unprecedented.

An object of the present invention is a high strength, high toughness, high fatigue, high fatigue, having an ultra-fine structure in the outermost surface layer having a strength distribution suitable for stress distribution such as squeezing, torsion, etc. given to the spring when the spring is used. To provide a spring steel with strength.

Spring steel, which is a high steel according to the present invention, is subjected to surface heating using a high frequency induction heating method, the steel surface stops heating after the degree is higher than Ac ₃ transformation point, and the surface temperature of the steel is Ar using the thermal conductivity of the steel. It is obtained by continuously giving a thermal cycle of rapid cooling to lower than _one transformation point, and then bringing it into the entire heating or near heating state, and then heat treating it.

The present invention will be described in detail below with reference to the accompanying drawings. As described above, the strength imparting method in the conventional coil spring manufacturing method aims to obtain a uniform heat treatment structure over the entire cross-section of the steel by the usual heat treatment. It is difficult to produce a spring having a strength distribution suitable for the distribution, and furthermore, since the heat treatment is to heat the entire cross-section up to the core portion of the steel only once with an Ac ₃ point, and then immediately heat-treat it, the crystal grains are not refined.

On the other hand, as described above, although not used in the method for producing the spring, the repeated heat treatment method as a method for miniaturizing the crystal grains is also well known.

This method is composed of the following configuration, that is, by heating the steel throughout the cross-section of the metal to the Ac ₃ transformation point or more, and forced cooling, and repeating the rapid cycle of rapid quenching to lower the temperature of the steel to room temperature The finer grain size of the steel is advanced to increase the strength and fatigue strength of the steel.

Features of the present invention with respect to the known art are as follows.

1) In the above-described spring manufacturing technique and also in the grain refining technique of the steel, the temperature is higher than the Ac ₃ transformation point over the entire cross section of the steel by quenching and quenching. However, in the present invention, only the surface layer of the steel is not less than the Ac ₃ transformation point. Heat.

2) In the above known technique, that is, the crystal grain refinement technique of steel, the cooling is performed during repeated heating only by performing repeated surface heating in the present invention, while repeating heat treatment is applied to the entire cross section of the steel to obtain refinement of crystal grains. when Ar ₁ transformation point is lower than that by the thermal conduction of the steel itself.

3) In the above known technique, i.e., the spring manufacturing technique or the forced grain refinement technique, the temperature at which the Ac ₃ transformation point or more is reduced to room temperature by forced cooling over the entire cross-section, but in the present invention is repeated heating. The temperature of the surface layer portion of the steel except for the heating of the final stage is equal to or higher than Ac ₃ transformation point. Therefore, the temperature of the surface portion of the steel layer is reduced to Ar by a short time without using forced cooling after heating. _It falls below ₁ transformation point.

That is, in the above known technique, after the rapid stop, the whole cross section is lowered to room temperature by forced cooling, but in the present invention, after the sudden stop, the temperature of the surface layer portion is lower than or equal to Ar ₁ transformation point in a short time without using forced cooling. The core part is gradually heated up while repeating the heat cycle of lowering the temperature, and the quenching and heat treatment are performed after the heating is completed or near the entire heating.

Accordingly, the above known technique, that is, the grain refinement technique of steel, is characterized in that the grain refinement effect is uniformly imparted to the entire cross section of the steel, but the present invention provides the grain size as it goes from the deep portion of the steel to the surface layer portion. It is remarkable as a manufacturing method of steel materials for springs in that it provides a strength distribution that is heterogeneous and suitable for spring use conditions in terms of providing a steel material with a finer, finer grain size of the surface layer portion.

In addition, it is apparent to those skilled in the art that the above-described Ac ₃ transformation point and Ar ₁ transformation point are determined according to the type of steel and the composition component. According to the present invention, a short time surface heating by a high frequency power is repeatedly applied to a material with a predetermined rest time, so that the entire heating of the entire cross section of the material or a heating state close thereto is performed, followed by heat treatment, followed by a high frequency induction heating method, and the like. Tempering by a heating method makes it a summary.

The present invention will be described in detail in accordance with the illustrated embodiment. The figure shows the change of the temperature of the steel surface layer and core part in the case of implementing this invention using the high frequency induction heating method, The vertical axis | shaft shows temperature and the horizontal axis | shaft shows time.

L _{1 to} L ₄ are high frequency induction heating coils arranged in series with a predetermined interval in connection with the transfer passage of the steel wire, and wires W fed in the direction of the arrow pass through the high frequency induction heating coils L _{1 to} L ₄ . In the process, the heat cycle according to the present invention is repeated for the wire W. Of course, the relationship between the dimensions of the steel wire and the frequency of the applied high-frequency power should be appropriately selected, but the number of induction heating coils L _{1 to} L ₄ given to the transfer path in relation to the feed rate of the heated steel W and the respective coils Induction heating coils L _{1 to} L of wire rod W by appropriately setting specifications such as length l _{1 to} l ₄ , adjacent induction heating coil spacing d _{1 to} d ₄ , and input power density P _{1 to} P ₄ to the induction heating coil. _As shown in FIG. ₄ , the heating cycle by ₄ can display the temperature rising characteristic as shown by the surface layer surface of the wire rod W as A, and the temperature rising characteristic as shown by the core part of the wire rod W as B is shown.

That is, the wire rod W is heated by t ₁ second in the first heat cycle by the induction heating coil L ₁ , and the temperature of the surface layer becomes higher than the AC ₃ transformation point, but leaves the induction heating coil L ₁ to induce the heating coil. by air-cooling of the t ₁ 'seconds between until it is in L _2, the temperature of the surface layer side is lowered below the Ar ₁ transformation point.

Induction heating coil L the surface layer by the heating of the _second of t seconds, in a second thermal cycle by ₂ again AC is more than ₃ transformation point, and then the wire W is leaving the induction heating coil L _2, t ₂ 'seconds and air cooling The temperature of the surface layer portion decreases below the Ar ₁ transformation point.

The same heat cycle is repeated in the following order. On the other hand, the core portion of the wire W is still close to room temperature in the first heat cycle by the induction heating coil L ₁ , but is heated up sequentially by repeating the heat cycle, for example, heat by the induction heating coil L ₄ in the drawing. At the end of the cycle, the temperature becomes more than the AC ₃ transformation point. On the other hand, the temperature of the surface layer portion is not lowered below the Ar ₃ transformation point by the air cooling of t ₄ 'seconds. .

The wire rod is hardened by quenching at this stage. When hardening is completed, if the material to be heated is a wire rod, and if it is a rod of a certain size, it is tempered by high frequency flow heating or other well-known heating method continuously with hardening or on another line. It imparts mechanical properties required to. The present inventors have tried various experiments to confirm the effect of the present invention.

A part of the explanation is as follows.

Experimental Example 1

1) Experimental conditions

(1) test piece

10mm in diameter

Chemical composition JIS SUP6 was used. The standard is as follows.

C 0.55 to 0.65 P 0.35 or less

Si 1.50-1.80 S 0.335 or less

Mn 0.70 to 1.00

(2) Arrangement of induction heating coil

As shown, induction heating coils L _{1 to} L ₄ were arranged at predetermined intervals in the wire feed passage.

a) Coil lengths of induction heating coils L ₁ , L ₂ , L ₃ and L ₄ respectively

L _1- L ₃ : 30 mm, L ₄ : 180 mm

b) spacing between each induction heating coil d ₁ , d ₂ , d ₃ : 120 mm d ₄ : 360 mm

(3) heating condition

a) Input power to each induction heating coil L _{1 to} L ₄

b) feedrate of wire 120mm / sec

2) Experiment Method

The heat cycle according to the present invention was repeated according to the above experimental conditions, and at the same time as the end of the fourth heat cycle, quenching was performed using cooling water. The heating time by induction heating in each thermal cycle was 0.25 second at t ₁ to t ₃ and 1.5 second at t ₄ , during which the surface temperature of the test piece rose to 880 ° C. to 900 ° C.

After hardening, the said test piece was continuously subjected to the tempering treatment of 500 degreeC for 2 second by high frequency heating.

3) Experiment result

Thus obtained cross-sectional crystal grain size and the same diameter of the wire rod with the same chemical composition as the test piece was hardened by high-frequency induction heating at 880 ° C to 900 ° C for 3 seconds and then hardened, and then tempered as described above. The results compared with those treated were as shown in the first table.

TABLE 1

Experimental Example 2

1) Experimental conditions

(1) test piece

10 mm diameter

Chemical composition As in Experimental Example 1.

2) Experiment Method

As in Experiment 1, the test piece was repeatedly heated in four heat cycles, followed by hardening and tempering.

Tensile sensitivity and alternating fatigue strength of this test piece were subjected to high-frequency induction heating at 80 ° C to 900 ° C for 3 seconds in the same chemical composition as the test piece, followed by hardening, followed by the same tempering treatment. Compared with oil-tempered one. The results are as shown in the second table.

TABLE 2

According to other experimental results conducted by the present inventors, even if the content of C is 0.3% or more, or the content of C is 0.3 or less, for example, as is known, the content of Mn and B is 1% and 0.001% or more, respectively. It has been confirmed that the same effect as in the above embodiment can be obtained by repeating the heat cycle in the present invention using a hardening-enabled wire rod as illustrated in the one or the third table.

TABLE 3

In addition, the number of thermal cycles is not limited to four times as in the above-described embodiment, and of course, the wire rod is heated or not heated at least twice by a plurality of heat cycles depending on the chemical composition of the wire rod used and the heating temperature of the wire rod surface. All cases of quenching after being brought into close proximity are included.

Of course, when the heating cycle is interrupted, it is a matter of course that a method of obtaining an appropriate surface temperature using an auxiliary means such as air cooling from the outside may be used in parallel.

As can be seen from the above experimental results, according to the present invention, the short-term surface heating by the high frequency power is repeated for a predetermined rest time, and thus the transformation of the crystal structure of the wire rod is repeated in a short time. Since the grain size can be made fine and the surface layer can be made into an ultrafine structure, it has a strength distribution suitable for stresses such as curling and torsion applied to the spring when the spring is used, and cannot be obtained by a conventional heat treatment method. It is possible to realize spring steel with high toughness.

In addition, in the application of the present invention, a plurality of induction heating coils are arranged at predetermined intervals, and the steel coils are sent in the heating coils to repeat the heat cycle, or the steel coils are fixedly fixed to the heating coils. The same thermal cycle may be repeated, and any method may be used as long as the yaw can be given a thermal cycle as described above.

In the application of the repeated heat cycle according to the present invention, since the inner surface of the heating surface utilizes the temperature drop caused by the heat conduction of the steel itself, the thermal energy input is not lost at all, and in the fine method described at the beginning of the present specification, As can be seen, the difference in the energy required is obvious compared with the case of repeating the thermal cycle by forced cooling from the outside, so that about 1/3 of the power is sufficient when the present example is compared with the conventional method. The present invention can be said to be an extremely economical manufacturing method.

Claims (1)

The heating temperature of the steel surface is repeatedly changed between the temperature above the AC ₃ transformation point and the temperature below the Ar ₁ transformation point for a short time, and during the period in which the vertical change of the heating temperature continues, the forced center temperature rises to completely or completely A method for producing a steel for high strength springs having crystal grain size that is gradually refined toward the surface layer from a deep portion prepared by hardening hardening after bringing it to a near-to-full heating state.

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