KR102269865B1 - Method for manufacturing seamless tube for hollow spring steel, seamless tube for spring steel produced by this and automobile suspension spring steel using same - Google Patents

Abstract

The present invention relates to a method for manufacturing a seamless tube for hollow spring steel, a seamless tube for spring steel manufactured by the method, and a vehicle suspension spring using the same, through a mandrel plug to have durability of 300,000 cycles or more, which is the level of a vehicle suspension spring that can be applied to a real vehicle. Downsizing the seamless tube to the actual vehicle spring size by piercing the center of the round bar, but if there is a flaw in the inner diameter of the seamless tube, the flaw depth is 30 μm or less to prevent the cause of premature failure due to concentration of stress It relates to a method for manufacturing a seamless tube for hollow spring steel, characterized in that, a seamless tube for spring steel manufactured thereby, and a vehicle suspension spring using the same.

Description

A method for manufacturing a seamless tube for hollow spring steel, a seamless tube for spring steel manufactured thereby, and a vehicle suspension spring using the same

The present invention relates to a method for manufacturing a seamless tube for hollow spring steel, a seamless tube for spring steel manufactured by the method, and a vehicle suspension spring using the same, and more particularly, to have durability of 300,000 cycles or more, which is the level of a vehicle suspension spring that can be applied to a real vehicle. Downsize the seamless tube to the actual vehicle spring size by piercing the center of the round bar through the mandrel plug, but if there is a flaw in the inner diameter of the seamless tube, the stress is concentrated to prevent premature failure. It relates to a method for manufacturing a seamless tube for hollow spring steel, characterized in that is less than or equal to 30 μm, a seamless tube for spring steel manufactured thereby, and a vehicle suspension spring using the same.

Recently, environmental regulations and fuel efficiency regulations for automobiles have become very important issues around the world to control greenhouse gas emissions, and weight reduction of vehicle parts and materials for improving fuel efficiency of automobiles has become an essential factor, not an option.

According to the IRS Global report (ISBN 9788998207427), the US raised the fuel efficiency standard from 36.2 mpg in 2015 to 44.8 mpg in 2020, and Europe raised the greenhouse gas emission standard from 130 g/km in 2015 to 95 g/km in 2020. China, which cannot be left out, is also strengthening its regulatory standards by about 28% from 6.9ℓ/100km in 2015 to 5ℓ/100km in 2020.

Korea is concentrating on technology development with the goal of achieving a fuel efficiency standard of 24.3 km/ℓ by 2020. When the vehicle weight is reduced by approximately 10%, fuel efficiency is improved by 6%, exhaust gas is reduced (4.5% CO, 8.8% reduction in NOx), and acceleration is achieved. It is very urgent and urgent to develop a technology to reduce the weight of vehicle parts and materials as it has effects such as improvement of steering performance and shortening of braking distance.

Among the weight reduction parts of automobiles, the drive unit (power transmission) part has been mass-produced as a hollow tube from a solid bar, but suspension in a suspension system that requires a high weight reduction and high functional characteristics It is technically very difficult to apply hollow parts for weight reduction of spring steel for devices. Currently, there are no cases of actual vehicle application in advanced overseas companies such as Japan and Europe, and they are actively conducting research by setting the goal of practical application through technology development within the next few years.

Coil springs applied to automobiles absorb shocks or vibrations generated from the road surface while driving and minimize transmission to the vehicle body. This is a key part of the suspension device, and it has been reported that there has been no successful case of applying a hollow tube to date.

Coil springs are generally manufactured by winding a solid bar of spring steel, and although they require a link connection mechanism or a shock absorber, and have a complex structure, they are widely used parts because their energy absorption is larger and more flexible than that of a leaf spring.

The strength is mainly determined by the thickness of the round bar, and the more the number of turns and the larger the diameter of the coil, the more flexible it is. In order to change the strength and flexibility of the spring according to the load or the degree of impact on the road surface, it is manufactured by changing the thickness or diameter of the round bar.

As described above, these coil springs have been generally manufactured through coiling using round bars and are still being applied to all vehicles. Efforts to develop technology are being attempted.

As a result of examining various overseas patent data, seamless tubes were applied to manufacture hollow springs. Most of them used seamless tubes manufactured by hot extrusion, and the main contents were about the material composition of spring steel, high strength and heat treatment conditions. .

In the patent document (Patent Publication No. 2010-0015369), a seamless steel pipe capable of reducing the depth of continuous flaws to about 50 μm or less is mentioned, but detailed flaw control technology is not clearly described, and the seamless tube is manufactured by an extrusion process. By manufacturing, there was a problem that it was difficult to apply the actual vehicle due to weak durability.

Japanese Patent Laid-Open No. 2007-125588

The present invention has been devised in view of the above problems, and the first object of the present invention is to pierce the center of a round bar through a mandrel plug so as to have a durability of 300,000 cycles or more, which is the level of an automobile suspension spring that can be applied to a real vehicle. An object of the present invention is to provide a method for manufacturing a seamless tube for hollow spring steel in which a tube is downsized to the size of a real vehicle spring, a seamless tube for spring steel manufactured thereby, and a vehicle suspension spring using the same.

Furthermore, the second object of the present invention is to achieve a down-sizing to a minimum depth of 6 to 11 μm or less through pilger rolling after inner diameter grinding of a seamless tube in a cold process, after secondary drawing A method for manufacturing a seamless tube for hollow spring steel that allows manufacturing of a seamless tube for hollow spring steel having a flaw depth of 30 μm or less on the inner diameter surface by downsizing to the actual vehicle spring size, and the seamless tube for spring steel manufactured thereby and using the same It is to provide an automobile suspension spring.

According to a feature for achieving the above object, the first invention relates to a method for manufacturing a seamless tube for hollow spring steel. For this, the round bar is heat-treated and then the center of the round bar is pierced through a mandrel plug to form a seamless tube with a dense structure. (S100) and a state in which the depth of the inner diameter surface flaw of the seamless tube is minimized through primary drawing, grinding the inner diameter surface, and pilger rolling (S100) and the hot process seamless tube A cold process of completing a seamless tube sized down with suspension spring steel through secondary drawing processing and surface polishing in a raw down sizing state; (S200), but the pilger rolling is performed with large-diameter grooves on the surface of the rotating mold and A small-diameter groove is provided to downsize the inner diameter surface flaw depth of the seamless tube to 6 to 11 μm or less, and the secondary drawing is downsized to the actual vehicle spring size in which the internal diameter surface flaw depth is 30 μm or less. characterized in that

The second invention, in the first invention, the hot process (S100) is the step of forming a center hole in the round bar (S110), and heat-treating in a temperature range of 1050 ° C. to 1200 ° C. to pierce the round bar (S120) And, it is characterized in that it consists of a step (S130) of forming a hollow seamless tube with dense tissue by attaching the mandrel plug to the center hole of the round bar that moves while rotating by two piercing rolls.

In the third invention, in the second invention, step S120 is a first heat treatment step (S121) of induction heating to a temperature range of 1050 ° C to 1100 ° C to pierce the round bar, and to minimize the temperature deviation inside and outside the round bar It is characterized in that it consists of a secondary heat treatment step (S121) of heating for 450 seconds to 550 seconds in a heating furnace in a temperature range of 1150 ° C to 1200 ° C.

The fourth invention, in the second invention, after step S130, compensating for the decrease in the temperature of the pierced seamless tube, and reheating in a temperature range of 1150 ° C to 1200 ° C for downsizing (S140), and two rotating rollers It characterized in that it further comprises a sizing rolling step (S150) to pass the seamless tube between.

In the fifth invention, in the first invention, the cold process (S200) is a step of spheroidizing heat treatment to improve the cold drawing workability of a seamless tube having a very high hardness through a hot process (S210), and a seamless tube for downsizing After the first drawing process (S220), the inner diameter surface polishing step (S230) of removing flaws and scratches by grinding the inner diameter surface of the first drawn seamless tube, and fixing the inner diameter polished seamless tube A pilger rolling step (S240) of downsizing the inner diameter surface flaw depth of the seamless tube to 6 to 11 μm or less by moving a rotating mold provided with large-diameter grooves and small-diameter grooves on the surface of which the tube diameter is reduced, and the pilger-rolled seamless Secondary drawing processing (S250) so that the depth of the flaw (flaw) on the inner diameter surface of the tube is downsized to 30㎛ or less, the step of correcting the straightness of the seamless tube (S260), and used for hollow spring steel after cutting It is characterized in that it consists of a step (S270) of completing the seamless tube.

In the sixth invention, in the fifth invention, step S210 is a step (S211) of charging a seamless tube into a furnace having a temperature range of 750 to 800° C., and then maintaining it for 50 to 70 minutes (S211), and the temperature of the furnace It is characterized in that it consists of a step (S212) of furnace cooling for 25 to 35 hours after lowering the range to 400 to 500°C, and a step (S213) of air cooling by taking out the seamless tube.

A seventh invention, in any one of the first to fourth inventions, is characterized in that the seamless tube has a surface flaw of 100 μm or less after the hot process.

The eighth invention relates to a seamless tube for spring steel manufactured by a method for manufacturing a seamless tube for hollow spring steel, characterized in that it is manufactured by any one of the first to seventh inventions.

A ninth invention is an automobile suspension spring using a seamless tube for spring steel manufactured by the method for manufacturing a seamless tube for hollow spring steel of the eighth invention.

According to the method for manufacturing a seamless tube for hollow spring steel of the present invention, by piercing the center of a round bar through a mandrel plug and downsizing the seamless tube to the actual vehicle spring size, durability of more than 300,000 cycles, which is the level of a vehicle suspension spring applicable to a real vehicle, is achieved. There is an achievable effect.

In addition, according to the seamless tube for spring steel manufactured by the method for manufacturing a seamless tube for hollow spring steel and the spring using the same, by applying a seamless tube having a flaw depth of 30 μm to the inner diameter surface, stress is concentrated through the flaw, It is effective in preventing the cause of breakage.

1 is a flowchart of a method for manufacturing a seamless tube for hollow spring steel according to the present invention;
2 is a detailed flowchart of step S100 of FIG. 1;
3 is a detailed flowchart of step S200 of FIG. 1;
4 is a photograph showing the pilger equipment showing step S240 of FIG. 3;
5 is a photograph of measuring the inner diameter surface state of the seamless tube after step S240 using an optical microscope;
6 is a photograph of measuring the inner diameter surface state of the seamless tube for spring steel manufactured by the method for manufacturing a seamless tube for hollow spring steel of the present invention;
7 is a distribution diagram showing the depth of a surface flaw in the inner diameter of the seamless tube of FIG. 6 .
8 is a photograph of a vehicle suspension spring using a seamless tube for spring steel manufactured by the method for manufacturing a seamless tube for hollow spring steel according to the present invention;
9 is a photograph showing a spring manufactured by an extrusion processing method rather than a piercing processing method.

The following objects, other objects, features and advantages of the present invention will be readily understood through the following preferred embodiments in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments described herein and may be embodied in other forms.

Rather, the embodiments introduced herein are provided so that the disclosed subject matter may be thorough and complete, and that the spirit of the present invention may be sufficiently conveyed to those skilled in the art.

The embodiments described and illustrated herein also include complementary embodiments thereof.

As used herein, the singular also includes the plural unless specifically stated otherwise in the phrase. As used herein, the terms 'comprise' and/or 'comprising' do not exclude the presence or addition of one or more other components.

Hereinafter, the present invention will be described in detail with reference to the drawings. In describing the specific embodiments below, various specific contents have been prepared to more specifically explain and help the understanding of the invention. However, a reader having enough knowledge in this field to understand the present invention can recognize that it can be used without these various specific details. In some cases, it is mentioned in advance that parts that are commonly known and not largely related to the invention in describing the invention are not described in order to avoid confusion in describing the invention.

1 is a flowchart related to a method for manufacturing a seamless tube for hollow spring steel according to the present invention, FIG. 2 is a detailed flowchart of step S100 of FIG. 1, FIG. 3 is a detailed flowchart of step S200 of FIG. 1, and FIG. 4 is FIG. It is a picture showing the pilger equipment showing the step S240 of.

1 to 3, the present invention pierces the center of a round bar and downsizing a seamless tube to the actual vehicle spring size, thereby securing durability of 300,000 cycles or more, which is the level of a suspension spring applicable to a real vehicle. It relates to a method for manufacturing a seamless tube for hollow spring steel.

In addition, the present invention relates to a method for manufacturing a seamless tube for hollow spring steel that can prevent the cause of premature failure due to concentration of stress through the flaw by manufacturing a seamless tube having a flaw depth of 30 μm on the inner diameter surface.

In addition, the present invention is a method for manufacturing a seamless tube for hollow spring steel, in which a round bar is formed into a seamless tube through a mandrel plug, and then flaws are removed through inner diameter grinding, and the depth of flaws can be minimized in the drawing process for downsizing. is about

The method for manufacturing a seamless tube for hollow spring steel of the present invention is largely manufactured in two steps, which is a hot process of forming a round bar into a seamless tube (step S100) and downsizing the flaw depth of the hot-processed seamless tube to 30 μm or less It is composed of a cold process (step S200).

Step S100 is a step to secure a dense seamless tube with a seamless structure by piercing the center of the round bar through a mandrel plug after heat-treating the round bar with a hot process.

To this end, in step S100, as shown in FIG. 2, first, a center hole for attaching a mandrel plug for piercing a round bar in step S110 is formed.

And in step S120, before piercing the round bar, it is a process of heat treatment in a temperature range of 1050° C. to 1200° C., and passes through the first and second heat treatment steps.

Here, the primary heat treatment step (S121) is induction heating to a temperature range of 1050 ° C. to 1100 ° C. to pierce the round bar. If it is less than 1050 ° C, the internal and external temperature of the round bar is low or non-uniform, so that the secondary heat treatment time is long. not.

And the secondary heat treatment step (S121) is heated for 450 seconds to 550 seconds in a heating furnace in a temperature range of 1150 ° C to 1200 ° C in order to minimize the temperature deviation of the inside and outside of the round bar subjected to the first heat treatment, if it is 1150 ° C or less, the piercing process It is undesirable because the equipment is overloaded and the dimensional deviation of the pierced seamless tube is severe, and the temperature is 1200℃ or higher, and the piercing tool (mandrel plug) melts or deteriorates, resulting in a large dimensional deviation of the seamless tube. It is not preferable because

In step S130, a hollow seamless tube with dense tissue is formed by attaching the mandrel plug to the center hole of the round bar that moves while rotating by the two piercing rolls. This piercing method can obtain a seamless seamless tube with a denser metal structure compared to drilling or extrusion, so it has the durability of an automobile suspension spring that can be applied to a real vehicle.

At this time, in order to manufacture a flaw depth of about 90 μm or less inside the pierced tube, it is preferable that the number of rotations of the piercing roll is 8.5 times the number of rotations of the round bar. This is because, if the piercing speed is too slow or too high, the plug surface will melt or become rough, which greatly increases the occurrence of deep flaws or scratches inside the tube.

On the other hand, the above-described step S100 is a hot process, and consists of steps S110 to S130.

However, in the hot process of step S100, when the diameter of the applied suspension spring is small, the sizing reduction width also increases, selectively compensating for the temperature decrease of the pierced seamless tube, and reheating in the temperature range of 1150 ° C to 1200 ° C for downsizing. Step; (S140) and a sizing rolling step of down sizing (section reduction ratio 1.5% to 2.0%) by passing a seamless tube between them with two rotating rollers; (S150) may be further included.

In this case, the seamless tube should be preceded by a flaw of 100 μm or less on the inner diameter surface after the hot process. It is not preferable to have a thickness of 100 μm or more because a problem occurs in durability due to an increase in cost and a decrease in the thickness of the seamless tube in the cold fixing inner diameter surface polishing step, which will be described later.

In addition to the above-described durability problem, when removing flaws in a polishing process to be described later, when polishing 100 μm or more, it is not preferable because the manufacturing cost increases.

Step S200 is a cold process, first drawing the hot-processed seamless tube, grinding the inner diameter surface, and then downsizing the inner diameter surface of the seamless tube through pilger rolling to minimize the depth of the flaw In this state, a seamless tube sized down with suspension spring steel is completed through secondary drawing and surface polishing.

In this step S200, first, as shown in FIG. 3, in order to improve the cold drawing workability of the seamless tube having a very high hardness through the hot process in step S210, spheroidizing heat treatment should be preceded.

The spheroidizing heat treatment is a state in which the hardness of the seamless tube manufactured by the hot process is very high (HrC40 or more), which decreases the reduction in area of the tube in the drawing process included in the subsequent cold process, and the post-treatment process This is to improve the drawing processability included in the cold process because it can cause various quality and productivity problems.

The above-mentioned spheroidizing heat treatment step (S210) is a step (S211) of charging the seamless tube into a furnace of a temperature range of 750 to 800° C., then maintaining it for 50 to 70 minutes, and the temperature range of the furnace to 400 After lowering to 500° C., furnace cooling for 25 to 35 hours (S212), and air cooling by taking out the seamless tube (S213).

In step S220, the first drawing of the seamless tube is performed in order to downsize the spheroidized heat-treated seamless tube (section reduction rate 28% to 30%). Although the above-described drawing process has an advantage in that the cross-sectional reduction ratio of the seamless tube is very large, there is a problem in that the depth of the flaw generated on the inner diameter surface of the seamless tube becomes large. However, since the depth of the flaw on the inner diameter surface in the hot process is 50 μm or less, even if the depth of the flaw is increased during the primary drawing process, it is possible to minimize the loss of the polishing thickness after polishing, which will be described later.

In step S230, the inner diameter surface of the first drawn-processed seamless tube is polished to remove flaws and scratches generated on the inner diameter surface.

Step S240 is a pilger rolling process. As shown in FIG. 4, after fixing a seamless tube with a polished inner diameter, move the rotating mold provided with large-diameter grooves and small-diameter grooves on the surface of which the tube diameter is reduced to form the inner diameter surface of the seamless tube. ) Downsizing (reduction of section by 57 to 60%) while minimizing the depth to 6 ~ 12㎛ or less.

In step S250, the pilger-rolled seamless tube is subjected to secondary drawing to downsize the inner diameter surface to a depth of 30 μm or less (section reduction rate of 40 to 60%).

In step S260, the straightness of the secondary drawn seamless tube is corrected.

Finally, in step S270, after cutting to a certain size, a seamless tube used for hollow spring steel is completed.

5 is a photograph of measuring the inner diameter surface state of the seamless tube after step S240 using an optical microscope.

As shown in FIG. 5 , no flaws of 12 μm or more were observed on the inner diameter surface of the seamless tube subjected to pilger rolling, and very small flaws of about 10 μm were observed. Therefore, it can be seen that the surface flaws of the inner diameter of the tube are similar to the state before the pilger rolling process without significant change, despite the reduction in sizing to about 58% with the reduction in area through the pilger rolling process.

6 is a photograph of the inner diameter surface state of the seamless tube for spring steel manufactured by the method for manufacturing a seamless tube for hollow spring steel of the present invention, and FIG. 7 is a distribution diagram showing the surface flaw depth of the seamless tube inner diameter of FIG. 6 .

As an example, a round bar with an outer diameter of 55Ø was expanded to 57Ø×5.0t through piercing, and then a seamless tube for spring steel of 15.9Ø×3.3t was manufactured through a series of seamless tube manufacturing methods for spring steel.

As described in FIG. 6 , it was found that the seamless tube manufactured by the method for manufacturing a seamless tube for hollow spring steel had an inner diameter surface flaw depth of about 20 μm.

These flaws are judged to be formed during the secondary drawing process after the pilger rolling process, and as a result, the flaw depth of the seamless tube downsized to the actual vehicle spring size is observed to be around 20㎛, which is a seamless structure in which the structure is dense through the piercing process. It is considered that the tube plays a large role.

7 is a graph analyzed after measuring the entire inner diameter surface of the seamless tube at 200 magnification under an optical microscope, the number of measured flaws is 219, the size range of flaws is 7 to 29 μm, and the average flaw depth is 14.4 μm Could know.

The seamless tube manufactured by the seamless tube manufacturing method for hollow spring steel can be manufactured in the range of 12 ~ 18Ø × 3 ~ 3.5t for passenger cars, and 45 ~ 70Ø × 7 ~ 15t for large vans. is of course

8 is a photograph of a vehicle suspension spring using a seamless tube for spring steel manufactured by the method for manufacturing a seamless tube for hollow spring steel according to the present invention, and FIG. 9 is a photograph showing a spring manufactured by an extrusion processing method, not a piercing processing method. .

As shown in FIG. 8, the automobile suspension spring using the seamless tube for spring steel manufactured by the method for manufacturing the seamless tube for hollow spring steel of the present invention secures the flaw depth on the inner diameter surface mentioned above to 30 μm or less, so that the actual vehicle application Durability of more than 300,000 cycles can be secured at this possible level.

However, in Figure 9, a 55㎛ flaw was observed on the inner diameter surface of the spring, which is expected to be due to the extrusion processing method that is not dense in structure, and the stress is concentrated (notch effect) by these deep flaws in 50,000 cycles or less It was found that fatigue failure occurred.

The embodiments described in this specification and the configurations shown in the drawings are only the most preferred embodiment of the present invention and do not represent all the technical spirit of the present invention, so various equivalents that can be substituted for them at the time of the present application It should be understood that there may be variations and examples.

Claims (9)

A hot process (S100) of heat-treating the round bar and then piercing the center of the round bar through a mandrel plug to form a dense seamless tube with no seams;
After the hot-processed seamless tube is first drawn, the inner diameter surface is polished, and large and small diameter grooves are provided on the surface of the rotary mold through pilger rolling to reduce the depth of the inner diameter surface flaw of the seamless tube. Downsizing to 6 ~ 11㎛ or less, secondary drawing processing, downsizing to the actual vehicle spring size with a flaw depth of 30㎛ or less on the inner diameter surface, and secondary drawing processing and surface polishing in the downsizing state, followed by suspension spring It is made including a cold process (S200) of completing a seamless tube sized down with steel,
The hot process (S100) is,
Forming a center hole for attaching a mandrel plug for piercing a round bar (S110);
The first heat treatment step (S121) of induction heating the round bar to a temperature range of 1050 ° C. to 1100 ° C. and 450 seconds to 550 seconds in a heating furnace in a temperature range of 1150 ° C to 1200 ° C to minimize the temperature deviation inside and outside the round bar A second heat treatment step (S122) of heating to perform heat treatment to pierce the round bar (S120) and;
It consists of a step (S130) of forming a hollow seamless tube with dense tissue by attaching the mandrel plug to the center hole of the round bar that moves while rotating by two piercing rolls (S130),
Rotation of the piercing roll,
The number of rotations of the piercing roll is made to be 8.5 times that of the round bar,
After the step (S130) of molding into the hollow seamless tube,
Compensating for the temperature decrease of the pierced seamless tube, and reheating in a temperature range of 1150 ° C to 1200 ° C for downsizing (S140);
Further comprising a sizing rolling step (S150) of passing a seamless tube between the two rotary rollers,
The cold process (S200) is,
In order to improve the cold drawing workability of a seamless tube having a very high hardness through a hot process, the seamless tube is charged into a furnace in a temperature range of 750 to 800° C. and then maintained for 50 to 70 minutes (S211), the furnace ( After lowering the temperature range of the furnace to 400 ~ 500 ° C. and then performing furnace cooling for 25 to 35 hours (S212) and taking out the seamless tube and cooling the tube with air (S213), spheroidizing heat treatment (S210)
A step of first drawing a seamless tube for downsizing (S220), and
An inner diameter surface polishing step (S230) of removing flaws and scratches by grinding the inner diameter surface of the first drawn seamless tube;
Pilger rolling that downsizes the inner diameter surface flaw depth of the seamless tube to 6 ~ 11㎛ or less by moving a rotating mold with large diameter grooves and small diameter grooves on the surface of which the tube diameter is reduced after fixing the polished seamless tube Step (S240) and,
A step of performing secondary drawing processing so that the flaw depth of the inner diameter surface of the pilger-rolled seamless tube is downsized to 30 μm or less (S250);
A step of correcting the straightness of the seamless tube (S260),
After cutting, it consists of a step (S270) of completing a seamless tube used for hollow spring steel,
The seamless tube is a method of manufacturing a seamless tube for hollow spring steel, characterized in that the flaw on the inner diameter surface after the hot process is 100㎛ or less.
delete delete delete delete delete delete A seamless tube for spring steel manufactured by the method for manufacturing a seamless tube for hollow spring steel of claim 1.
A vehicle suspension spring using a seamless tube for spring steel manufactured by the method for manufacturing a seamless tube for hollow spring steel of claim 8.

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