KR20110111021A - Manufacturing method for high strength bolt having excellent fatigue property - Google Patents

Abstract

The present invention is to manufacture a bolt having a high strength and at the same time excellent in fatigue characteristics with low energy and low cost, in the method of manufacturing a bolt using a cold extrusion die device including a processing portion and a pressing member,
A raw material input step of inserting a bolt preform into the processing unit;
An extrusion step of pressing the bolt preform with the pressing member to produce a bolt head and a body; And
It relates to a high-strength bolt manufacturing method having excellent fatigue characteristics, characterized in that it comprises the step of heat-treating the bolt manufactured through the extrusion at a temperature of 250 ~ 300 ℃ more than 2 hours.

Description

MANUFACTURING METHOD FOR HIGH STRENGTH BOLT HAVING EXCELLENT FATIGUE PROPERTY

The present invention relates to a method of manufacturing bolts by cold working, and more particularly, to a method capable of securing high strength at low energy and low cost and at the same time manufacturing a bolt having excellent fatigue characteristics.

In recent years, in order to save time and money in bolt manufacturing, research on the technology of manufacturing bolts and steels for bolts has been continued without omitting heat treatment. This bolt manufacturing technology is excellent in economics and productivity because the heat treatment can be omitted, but since the strength before processing is directly related to the strength of the finished product, the bolt forming apparatus and dies are subjected to a large load during the cold press process, resulting in a shortened die life. In the end, there is a problem that leads to a decrease in productivity. In particular, the problem is more prominent in the manufacture of high strength bolts with high bolt strength.

Conventionally, when manufacturing a high-strength bolt, a method of lowering the strength of a raw material through a spheroidization heat treatment before cold rolling to enable cold rolling, and then heat-treating again to satisfy the strength required for the product. Specifically, after the wire for sizing purpose is carried out through a wire heat treatment in a wire state, a bolt having a microstructure of the final tempered martensite is subjected to a process of nodular heat treatment, bolt forming, hardening, and soaking. In many cases, the method was produced. Therefore, the strength of the bolt was determined by the composition, quenching, and heat treatment process. In order to secure sufficient strength, about two heat treatments were generally performed to the final product in the bolt manufacturing step.

However, the bolt manufacturing process that has to go through this heat treatment step does not meet the trend aimed at cost reduction through the omission of the process, and the wire rods thus manufactured are also rapidly made of high strength, so that the high-strength bolt products can be stably cold processed. There is a need for technology.

In accordance with the demands of the times described above, Korean Patent Registration No. 10-0931200 provides a die that can manufacture bolts in an extruded form without heat treatment in order to increase the strength, thereby securing a high strength bolt. . The patented technology is manufactured by cold-processing the body portion in the form of an extrusion rather than a technique of hitting a conventional bolt head, and has the effect of increasing the strength of the bolt after processing and at the same time a very high strength of the raw material It is possible to directly cold work without softening heat treatment (heat treatment for lowering the strength to facilitate cold working).

The patented technology is to increase the strength in accordance with the amount of processing is added as the body portion is processed in the extrusion form, which is called work hardening or stress hardening.

According to the patented technology, a high-strength bolt can be manufactured without a separate heat treatment, but a substantial amount of residual stress remains in the body portion of the bolt through an extrusion method, and the residual stress is the above-described processing method. It is a very natural phenomenon which is obtained when using the and is inevitable.

The residual stress is related to the generation of fatigue cracks at repeated loads applied to the bolts, and the appropriate residual stresses have the effect of fixing the residual stresses so that they do not grow anymore even if fatigue cracks occur. That is, if the residual stress remaining in the bolt body is excessive, there is a problem of causing fatigue crack growth during repeated loads applied to the bolt, and if the residual stress is insufficient, fatigue crack is not fixed and the problem of showing low fatigue characteristics is insufficient. have.

Therefore, when manufacturing bolts through cold working, there is an urgent need for a technology capable of securing excellent fatigue properties while securing excellent strength.

According to one aspect of the present invention, when manufacturing the bolt through cold working, by using a work hardening phenomenon to secure the improvement of the strength, and at the same time provides a method for producing a bolt excellent in fatigue characteristics .

The present invention relates to a method for manufacturing a bolt using a die extrusion apparatus of cold extrusion method including a processing portion and a pressing member,

A raw material input step of inserting a bolt preform into the processing unit;

An extrusion step of pressing the bolt preform with the pressing member to produce a bolt head and a body; And

It is to provide a high-strength bolt manufacturing method having excellent fatigue characteristics, comprising the step of heat-treating the bolt produced through the extrusion at a temperature of 250 ~ 300 ℃ more than 2 hours.

In the case of manufacturing the bolt according to the present invention, it is possible to secure the most excellent fatigue strength while improving the strength through work hardening. In other words, the bolt manufactured according to the present invention can manufacture a bolt capable of securing the optimum fatigue characteristics through the stress relaxation heat treatment while maintaining excellent strength.

1 is a schematic diagram of a die apparatus used in the method of the present invention.
2 is a schematic diagram of a die apparatus for manufacturing a bolt by a conventional pressure mill method.
Figure 3 is a graph showing the change in tensile strength and cross-sectional reduction rate according to the heat treatment conditions obtained in the embodiment.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

In the present invention, the wire applied to manufacture the bolt is a wire that is omitted after the heat treatment, and is often referred to as an unstructured wire. Products having these characteristics can be manufactured directly by sizing purposes after wire fabrication, and only through bolt forming and final processing.

The biggest feature of wire rods that omit the heat treatment is to reduce the energy consumed in the heat treatment process to lower the product cost, and because the heat treatment is not performed, quench crack, bending and other defects due to heat treatment occurs There is an advantage that does not. However, the strength is not secured by heat treatment, but it is a form of obtaining strength through cold working such as drawing, so that ductility and toughness may be reduced. In the case of bolts through the extrusion process, residual stress on the fatigue strength of the product Is very important, it is very important to adjust it properly.

As a result of the study, the inventors have found that the surface residual stress is distributed about 2 to 3 mm in thickness directly below the surface of the bolt body, which is due to the friction force generated while the material passes through the die.

When the bolt is manufactured by using the extrusion process, the work hardening effect obtained by the extrusion can be used, confirming that the strength of the body and the head of the bolt is increased, It was confirmed that the surface residual stress caused by, and to devise a technology that can control this led to the present invention.

The invention applies only if the bolt is manufactured using an extrusion process. That is, the present invention relates to a technique of utilizing the residual stress remaining in the bolt body portion to obtain the fatigue characteristics through proper heat treatment, and thus does not apply to bolts manufactured through the normal head hitting, not the extrusion process. The bolt produced by hitting the head is not subject to the present invention because the above-described residual stress does not remain.

In the bolt manufacturing method of the present invention in manufacturing a bolt using a die extrusion apparatus of the cold extrusion method including a processing portion and a pressing member,

A raw material input step of inserting a bolt preform into the processing unit;

An extrusion step of pressing the bolt preform with the pressing member to produce a bolt head and a body; And

It includes the step of heat-treating the bolt produced through the extrusion at a temperature of 250 ~ 300 ℃ for 2 hours or more.

The cold extrusion die apparatus used in the present invention will be described in detail with reference to FIG. However, FIG. 1 is an example of the die apparatus and is not limited thereto.

As shown in FIG. 1, the die of the cold extrusion method applied to the present invention includes a die member 102 having a processing unit 104 on which a material is processed, and a reinforcement housing and enclosing the die member. A ring 101 and a pressing member 103 that serves to pressurize the raw material introduced into the processing unit.

The processing part 104 has a cylindrical shape suitable for bolt formation, and the diameter of the processing part 104 includes two different diameters. Of the two diameters, a relatively large diameter is referred to as a first diameter and a relatively small diameter as a second diameter. Of the two diameters, the diameter of the raw material input side, that is, the punch member side, should be larger. This is because a large diameter raw material input side should be able to be processed into the head of the bolt when forming the bolt.

In addition, the distal end of the processing unit 104 may be an open type processing part, which is for the die apparatus of the present invention using only extrusion, unlike a closed processing part for a conventional pressure type method. Because the bolt length limit is removed, bolts of various lengths can be manufactured.

The die apparatus applied to the present invention has the following differences as compared with the conventional die-casting apparatus shown in FIG.

Although the diameter of the die part 202 of the conventional pressurization method is constant, the die part 202 of the die apparatus used in the present invention has a cylindrical shape suitable for bolt molding, and the diameters of the die 104 are different from each other. Two other diameters.

In addition, since the die apparatus used in the present invention does not need to be subjected to the impact as in the prior art, the die apparatus used in the present invention does not need to be subjected to the impact as in the conventional pressurization method. ) Is longer than the conventional punch member 203, and the lifetime of the die apparatus is also increased as a whole.

In order to manufacture the bolt using the extrusion process, the bolt preform is put into the raw material input portion of the processing unit 104. In this case, unlike the conventional bolt manufacturing method, the bolt preform uses a diameter larger than the second diameter and smaller than the first diameter.

The injected bolt preform is pressed by the pressing member 103 and extruded from the machining portion having the first diameter to the machining portion having the second diameter. Unlike the punch member 203 used in the conventional pressing method, the pressing member 103 does not have a bolt head forming groove, and has a pressing protrusion capable of pressing the preform into the processing unit 104. It is characteristic.

The bolt manufactured through the above process obtains a work hardening effect by extrusion, so that even after molding the bolt using a material having a low basic strength, the bolt with increased strength can be easily manufactured.

However, the bolt manufactured as described above has a residual stress on the surface of the bolt body due to the effect of hardening, in the present invention, by heating the bolt manufactured through the extrusion, the stress is adjusted to the body portion of the bolt. Such heat treatment is referred to as stress relieving.

Since the stress remaining on the bolt body surface may cause breakage or cracking during machining due to surface defects or local stress concentration on the surface during extrusion or cold forging, stress relief through stress relaxation heat treatment is required. This stress relaxation heat treatment is very sensitive to the heat treatment temperature and time, and when the heat treatment at the appropriate temperature and time, all residual stress is resolved to facilitate the processing, and not only prevent cracks or fractures in advance, but also during processing The beautiful surface can also be obtained.

Since the residual stress remaining on the surface of the bolt body is the highest level immediately after machining, and the stress difference between the central part and the surface part of the bolt is very large, which causes a decrease in the fatigue strength, the heat treatment performs the extrusion to manufacture the bolt. It is preferable to carry out immediately after.

It is preferable to perform the said heat processing at 250-300 degreeC for 2 hours or more. If the temperature is less than 250 ° C., there is a lack of diffusion temperature at which locally concentrated stresses can be resolved, and sufficient stress is not easily resolved, and any residual stresses are not removed, which may cause cracks. When the temperature exceeds 300 ℃, the pearlite heat treatment temperature is high, there is a problem that the structure is weakly transformed to cause a decrease in tensile strength.

On the other hand, when the heat treatment time is less than 2 hours, it is not sufficient time to relieve stress through diffusion, and thus it is preferable to carry out for 2 hours or more because the stress cannot be solved.

On the other hand, it is more preferable that the heat treatment time does not exceed 4 hours. If the heat treatment is performed for an excessively longer time, a natural tissue transformation caused by the aging phenomenon is caused, resulting in a problem of tissue nonuniformity, and considering the economics of production, it is more preferable not to exceed 4 hours.

Hereinafter, embodiments of the present invention will be described in detail. However, the present invention is not limited to the following examples.

(Example)

Stress relaxation heat treatment was performed on the bolts manufactured through the extrusion process, fatigue strength was measured, and the results are shown in Table 1.

This embodiment was carried out for the bolt manufactured through the extrusion process, the material used was a piano wire called JIS-SWRS82BC. The bolt was manufactured by the extrusion process using the die apparatus shown in FIG.

Immediately after the bolt was manufactured, the measured tensile strength of the bolt was 1656 MPa, and the strength of the wire immediately before extrusion was 1350 MPa.

For the bolt manufactured as described above, after the stress relaxation heat treatment was performed by varying the temperature and time of Table 2, the tensile strength and the cross-sectional reduction rate of each specimen were measured, and the fatigue strength was measured. Shown in On the other hand, it is shown as a graph in Figure 3 to easily evaluate the change in tensile strength and cross-sectional reduction rate according to the heat treatment conditions.

The fatigue strength was evaluated using a fracture life (number of repeated loads until the specimen breaks) under a constant cyclic load measured, and the test method is as follows. Bolts manufactured and stress-treated through the extrusion process are subjected to a tension-free load using a universal tensile tester to make 1000 MPa → 0 MPa in one cycle, and one cycle is repeated once within 10 seconds (10 Hz). ) Was tested. This repeated load was continuously added until the bolt break, and the number of cycles at break was recorded three or more times, and the fatigue strength was evaluated through the average value.

Psalter Heat treatment temperature
(℃) Heat treatment time
(hr) The tensile strength
(MPa) Section reduction rate
(%) Fatigue strength
(time) Remarks One 0 0 1656 46 43972 Conventional example 2 200 2 1677 47 64347 Comparative Example 1 3 200 4 1676 46 59496 Comparative Example 2 4 200 6 1673 48 71653 Comparative Example 3 5 250 2 1684 43 79856 Inventive Example 1 6 250 4 1696 46 84165 Inventive Example 2 7 250 6 1687 47 112568 Inventory 3 8 300 2 1695 49 195394 Honorable 4 9 300 4 1686 48 163391 Inventory 5 10 300 6 1699 39 59841 Comparative Example 4 11 350 2 1646 37 49683 Comparative Example 5 12 350 4 1657 34 46481 Comparative Example 6 13 350 6 1644 35 2238 Comparative Example 7

As shown in the results of Table 1, it can be seen that the conventional example without the stress relief heat treatment has a high tensile strength, but the fatigue characteristics are weak.

The most suitable stress relaxation heat treatment condition was that the invention examples have excellent tensile strength and cross-sectional reduction rate, but the fatigue strength was improved due to the change of residual stress due to the stress relaxation heat treatment.

When the stress relaxation heat treatment is performed, the time is increased when the temperature is increased based on the temperature and time. On the contrary, even if the temperature is lowered, it is known that the same effect is obtained when the time is taken for a long time.

When the heat treatment temperature is too low (Comparative Examples 1 to 3) it does not have a sufficient heat treatment effect, there is no effect of stress relaxation, in particular, even if a long time treatment (Comparative Example 3) it was confirmed that the fatigue properties do not improve, In the case where is too high (Comparative Examples 5 to 7), it can be seen that the residual stress is so insufficient that the fatigue strength is lowered, and the tensile strength and the cross-sectional reduction rate are also lowered.

In addition, in the case where the desired temperature is heat-treated for less than 2 hours, the temperature range proposed by the present invention is relatively low, and it is insufficient to raise the temperature to the desired temperature in the whole material, and there is no effect of heat treatment and it is not uniform. . In addition, when it is processed for a long time at high temperature (comparative example 4), it turns out that a cross-sectional reduction rate falls and the improvement effect of fatigue strength is also insignificant.

101 ..... Reinforcement ring 102 ..... Dice absent
103 ..... Pressure member 104 ..... Processing part
202 ..... Dice member 203 ..... Punch member

Claims (5)

In the method of manufacturing a bolt using a die extrusion apparatus of cold extrusion method including a processing portion and a pressing member,
A raw material input step of inserting a bolt preform into the processing unit;
An extrusion step of pressing the bolt preform with the pressing member to produce a bolt head and a body; And
Heat-treating the bolt manufactured through the extrusion at a temperature of 250-300 ° C. for at least 2 hours.
Method for producing a high strength bolts having excellent fatigue characteristics, characterized in that it comprises a.
The method according to claim 1,
The heat treatment is an excellent high strength bolt manufacturing method performed for 2 to 4 hours.
The method according to claim 1,
A diameter of the bolt preform is smaller than the first diameter and larger than the second diameter, the method of producing a high strength bolt having excellent fatigue characteristics, characterized in that.
The method according to claim 1,
The cold extrusion die apparatus includes a die member including a cylindrical machining portion having a first diameter and a two-stage diameter of a second diameter smaller than the first diameter;
A reinforcing ring surrounding and receiving the die member; And
Pressing member for pressurizing the bolt preform injected into the processing unit
Method for producing a high strength bolts having excellent fatigue characteristics, characterized in that it comprises a.
The method of claim 4,
The processing unit is a manufacturing method of high strength bolts having excellent fatigue characteristics, characterized in that the processing unit is open in the longitudinal direction.

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