KR20150078772A - Prevention method of delayed fracture by cold trim for hot stamped parts - Google Patents

Abstract

The present invention relates to a method to prevent delayed fracture occurring in a trip part after the cold trim of a hot stamping component. According to an embodiment of the present invention, the method includes a step (a) of forming a forming body by a hot stamping process; a step (b) of cutting a side of the forming body by a trip process; and a step (c) of removing residual stress by applying a current to the trip part of the forming body.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention [0001] The present invention relates to a method of preventing delayed fracture due to cold-

The present invention relates to a method for preventing delayed fracture, and more particularly, to a method for preventing delayed fracture generated in a trim portion after a cold trim of a hot stamped molded component.

In recent years, the automobile industry has been focusing on weight reduction for improving fuel efficiency, but has been gradually using ultra-high strength body parts to secure sufficient rigidity.

In other words, there is an increasing tendency to use lightweight yet high strength materials to form products that have a relatively low weight and a stronger rigidity.

However, these super high strength materials have considerably higher strength than ordinary steel alloys or aluminum alloys, and are difficult to industrially utilize because of limited formability in a room temperature environment.

In recent years, hot stamping molding has attracted much attention. Hot stamping molding is performed by heating a blank of a material having hardenability, forming a shape by press molding in a heated state, To thereby obtain a high-strength molded body.

Since press molding requires a portion to be supported by the mold, the blank is inevitably included in a portion that must be removed through later cutting. In addition, when precise dimensional accuracy is required, a trim process is required in a cured state after molding.

However, since the rapid-cooling molded article after hot forming has a high strength, the trim process using the press causes the mold life to be reduced due to wear of the mold.

Therefore, the trim is mainly performed by using a laser or a water jet. Such a trim method has a long processing time and a high processing cost, which causes an increase in the production cost of the product.

Particularly, in the case of the laser trim, since the cut portion is heated, there is a problem that the heat affected portion changing the physical properties of the original material adversely affects the quality of the product.

Therefore, in recent years, researches for cold-pressing hot stamping parts by pressing, such as development of a new material for cold-trim die and change of process conditions, have been continued.

At this time, in order to put the cold stamping process of hot stamping parts into practical use. It is necessary to solve the problem that the delayed fracture occurs due to the residual stress generated in the trim part of the cold stamping part of the hot stamping part.

On the other hand, the 'electroplasticity effect', which has been continuously studied recently, indicates a phenomenon in which the flow stress in the metal material is lowered when current is passed through the metal material. Although the principle of electrification and firing is not clearly understood, it can not be explained by the effect of heat and temperature rise in view of the fact that the flow stress is lowered at a temperature not exceeding the hot working temperature as well as the melting point of the material (See Non-Patent Document 1).

KR 10-2013-0043310 A (2013.04.30 open)

 Roth, J. T., Loker, I., Mauck, D., Warner, M., Golovashchenko, S. F., Krause, A., 2008. Enhanced formability of 5754 aluminum sheet metal using electric pulsing. Trans. NAMRI / SME 36, 405-412.

SUMMARY OF THE INVENTION The present invention has been conceived to solve the problems as described above, and an embodiment of the present invention is to provide a hot stamping- It is intended to provide a method that can be used.

According to a preferred embodiment of the present invention, there is provided a method of manufacturing a semiconductor device, comprising: (a) forming a formed body by a hot stamping process; (b) cutting one side of the molded body by a trim process; And (c) applying a current to the trim portion of the molded body to remove residual stress at room temperature. The present invention also provides a method for preventing delayed fracture of a hot stamping component according to a cold trim.

The step (b) is characterized in that the step of trimming is performed in a cold state by a press.

The step (c) includes the steps of: (c-1) arranging electrode pairs having mutually different polarities on the upper side and the lower side of the trim portion in the thickness direction, the electrode pairs being spaced apart from each other along the longitudinal direction of the trim portion And (c-2) applying a current to the electrode pair to remove residual stress of the trim portion.

At this time, the pair of electrodes may be arranged closely to the stress concentration portion of the trim portion.

It is also possible that a larger current is applied to the electrode pair disposed in the stress concentration portion of the trim portion.

It is also possible that a current is applied to the electrode pair disposed in the stress concentration portion of the trim portion for a longer period of time.

At this time, electric current may be applied to the trim unit by the combination of the upper die and the lower die provided on the upper and lower sides of the molded body, respectively.

At this time, it is preferable that the electrode pairs are provided on one side of the insulation coated portion formed in the upper die and the lower die to prevent current leakage.

Also, following step (b), step (c) may be performed using the press and the mold used in the trim step of step (b).

Meanwhile, in the step (c), it is preferable that the current is applied in a current density of 5 to 100 A / mm 2 for a time of more than 0 but less than 3 seconds.

In the step (c), a pulse current may be applied to the trim unit.

According to a preferred embodiment of the present invention, the residual stress in the trim portion is removed by current conduction after the cold-trim of the hot stamping component, thereby preventing generation of delayed fracture.

Therefore, even if the hot stamping part is finally molded by the cold-trim method, it is possible to provide a product free from defects due to generation of delayed breakage as in the conventional art.

1 is a perspective view of a B-pillar shown as an example of a hot stamping component;
2 is a flowchart of a method for preventing delayed fracture according to a cold trim of a hot stamping component according to an embodiment of the present invention.
3 is a schematic view of a mold for cold trim according to an embodiment of the present invention.
4 is a schematic view of a current-applying mold according to an embodiment of the present invention;
5 is a schematic view of a mold for cold trim equipped with an electrode according to another embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of a delayed fracture prevention method according to a cold trim of a hot stamping part of the present invention will be described with reference to the accompanying drawings. In this process, the thicknesses of the lines and the sizes of the components shown in the drawings may be exaggerated for clarity and convenience of explanation.

In addition, the terms described below are defined in consideration of the functions of the present invention, which may vary depending on the intention or custom of the user, the operator. Therefore, definitions of these terms should be made based on the contents throughout this specification.

In addition, the following embodiments are not intended to limit the scope of the present invention, but merely as exemplifications of the constituent elements set forth in the claims of the present invention, and are included in technical ideas throughout the specification of the present invention, Embodiments that include components replaceable as equivalents in the elements may be included within the scope of the present invention.

Example

1 is a perspective view of a B-pillar shown as an example of a hot stamping component.

The B-pillar 10 refers to a portion connecting the roof and the bottom surface of the vehicle body between the front door and the rear door of the vehicle. Since the B-pillar 10 needs to maintain its shape with high strength in the event of a vehicle collision, ≪ / RTI >

Hereinafter, the present invention will be described by way of example of the B-pillar 10, but the present invention is not limited to this, and it is disclosed in advance that the present invention is applicable to a manufacturing process of all parts requiring a cold trim process after hot stamping Leave.

Conventionally, one of the reasons for not performing a trim operation using a press with respect to a component formed by a hot stamping process is due to delayed fracture of the trim portion.

This delayed fracture is caused by the generation of residual stress during the trim work by the press. Therefore, when the residual stress generated in the trim portion is removed, it is possible to prevent delayed fracture from occurring in advance.

The present invention is characterized in that residual stress is removed by applying a current to a trim portion after a cold trim by a press. Hereinafter, the present invention will be described in detail with reference to FIG. 2 to FIG.

2 is a flowchart of a method of preventing delayed fracture according to a cold trim of a hot stamping component according to an embodiment of the present invention.

Hot stamping  step( S10 ):

The hot stamping step 10 may be performed in various ways by conventionally known hot stamping processes. The hot stamping process described below is only an example that can be applied to the hot stamping step (S10) of the present invention, and the present invention is not limited thereto.

First, the material is heated to a forming temperature in a heating device (not shown). For example, since a material of 1 GPa or more in grade is difficult to be processed by ordinary press molding at room temperature, it is heated to a molding temperature at which moldability of the material can be secured and supplied to a press apparatus (not shown).

In the hot stamping process, it is preferable that the press apparatus is provided with a cooling channel therein so that the mold can be rapidly cooled by the refrigerant supplied to the cooling channel.

On the other hand, after the heated material is loaded on the press apparatus, the press apparatus is operated to mold the formed article S. At this time, since the formed article S still maintains a high temperature, unexpected deformation may occur when the press apparatus is immediately opened and the formed article S is cooled. Therefore, it is preferable to perform the quenching step in a state in which the molded body S is constrained while the press apparatus is closed.

Trim phase ( S20 ):

3 is a schematic view of a mold for cold trim according to an embodiment of the present invention.

The formed body S formed through the hot stamping step S10 is transferred to a press apparatus provided with the cold-forging mold 100, and a trim process for cutting unnecessary portions is performed. In the case of the B-pillar 10 according to the embodiment of the present invention, portions corresponding to both side edges 11 correspond to this.

At this time, the trim process is performed by cold pressing. Here, 'cold' refers to the temperature at which the internal structure of the material does not change, that is, the temperature of the material is relatively lower than the temperature required for the heat treatment or hot working.

According to an embodiment of the present invention, the upper mold 200 or the lower mold 300 provided in the press can raise and lower the molded body S to trim.

For example, as shown in FIG. 3, the trim cutter 400 provided in the upper die 200 of the press, in a state in which the hot-stamped formed article S is supported by the lower die 300 of the press, A trim process can be performed in such a manner that one side of the molded body S is cut.

At this time, a portion of the formed body S where the trim cutter 400 is cut off is referred to as a trim portion. The trim cutter 400 during the trim process digs the formed body S and cuts the trim S Residual stress is generated.

By current application Residual stress  Removal step ( S30 ):

4 is a schematic view of a current-applying mold according to an embodiment of the present invention.

It is necessary to remove the residual stress generated in the trim portion of the molded body S in order to prevent the delayed break generated in the trim portion of the product after the trimming step S20.

An embodiment of the present invention is characterized in that a current is applied to the trim portion (S31) for eliminating the residual stress of the trim portion (S32).

However, the current application in the present invention is for the purpose of utilizing the electroplasticity effect, and is not for the purpose of changing the microstructure such as recrystallization due to the temperature rise of the material.

That is, when the current is applied to the molded body S at the time of energization, the temperature of the molded body S may be partially raised due to the resistance heat, but the temperature required for heat treatment, hot forming or warm forming is not reached. Therefore, in the present specification, the 'room temperature' refers to a temperature that is relatively lower than a heat treatment temperature or a hot forming temperature or a warm forming temperature, respectively, which are different depending on the material of the formed body.

For example, the hot stamping temperature of steel is 930 DEG C or higher, and in the case of warm forming, it is heated to 600 to 900 DEG C through atmospheric heating. On the other hand, in the case of energization according to an embodiment of the present invention The temperature is less than 300 占 폚.

Also, while the temperature during warm forming of aluminum is 200 to 300 ° C, the temperature during electrification and firing is less than 200 ° C and the warm forming temperature of the magnesium alloy is 300 to 400 ° C. The temperature at the time of energization is less than 200 ° C.

The current application dies 100 'according to the embodiment of the present invention are arranged such that at least one pair of electrodes having polarities different from each other are arranged on both sides in the width direction of the molded product S, It is preferable to arrange at least one electrode pair having different polarities on the upper side and the lower side of the trim portion in the thickness direction so that the current is made to flow in the thickness direction of the trim portion.

This is because it is not necessary to supply a current to the entire area of the molded body S, but also to energize the current around the trim portion where the residual stress is generated to maximize the effect of removing the residual stress.

At this time, it is preferable that at least one or more electrode pairs are arranged to be spaced apart from each other along the longitudinal direction of the trim portion, and the electrode pairs are arranged to be denser in the portion where the residual stress is concentrated due to the shape or thickness change.

This is for the purpose of supplying more electric energy to the stress concentration portion, and as another example, it is possible to allow a larger current to be applied to the pair of electrodes arranged at the stress concentration portion and / or its peripheral portion than the other pair of electrodes, It is also possible to apply a current.

On the other hand, in the present invention, the current is not intended to raise the temperature due to the generation of the resistance heat and accordingly to change the microstructure, but to eliminate the residual stress, The physical properties of the material may be changed.

Therefore, it is preferable that the current is applied through the electrode pair for a time of more than 0 and less than 3 seconds to prevent generation of resistance heat, and the current density at this time is preferably 5 to 100 A / mm < 2 >. If the current density is less than 5 A / mm < 2 >, the effect of removing the residual stress is insignificant. If the current density exceeds 100 A / mm < 2 >

On the other hand, it is also possible that the current is applied in the form of a pulse current. At this time, the intensity, cycle, and holding time of the pulse current can be appropriately selected according to the kind of the material and the magnitude of the residual stress, and the pulse current can be appropriately selected in a controller (not shown) provided between the power supply device (not shown) Lt; / RTI >

The application of the current may be performed by attaching each of the electrodes 500 individually to the trim portion of the formed body S, and preferably, as shown in FIG. 4, A plurality of electrodes 500 having different polarities are provided in the lower mold 300.

The upper mold 200 is provided with a plurality of electrodes 500 of positive polarity along the longitudinal direction of the trim portion and the lower mold 300 is provided with a positive electrode 500 of the upper mold 200, A plurality of (-) electrodes 500 may be provided so as to be opposed to each other. The electrodes 500 of the upper mold 200 and the lower mold 300 may be assembled to the upper mold 200 or the lower mold 300 in a detachable manner so that the electrodes 500 can be easily replaced. Are preferably opposed to each other.

When the lower mold 300 rises or the upper mold 200 descends and is brought into contact with the molded body S in a state where the molded body S is supported by the lower mold 300, The electrode 500 of the mold 500 and the electrode 500 of the lower mold 300 are electrically connected to each other through the molded body S so that a current is applied to the trimmed portion.

At this time, in order to prevent the current applied to the trim portion from leaking to the upper mold 200 or the lower mold 300 through the portion other than the trim portion of the molded body S, the upper mold 200 and the lower mold 300 It is preferable that the insulating coating portion 600 is formed.

The insulating coating portion 600 may be formed by coating an insulating coating material on the upper surface of the upper mold 200 facing the molded body S and the lower surface of the lower mold 300.

The electrode 500 is provided on one side of the insulating coating portion 600 adjacent to the trim portion and is prevented from leaking current to portions other than the trim portion by the insulating coating portion 600. In addition, the insulated coating portion 600 also serves to induce current to flow mainly in the thickness direction of the trim portion.

In the case where the upper mold 200 and the lower mold 300 are brought into contact with the molded body S and a current is applied by using the press as described above, the press and the mold used in the above-mentioned trim step S20 can be used as they are There is an advantage.

In other words, it is also possible that the molded body S is transferred to another press apparatus provided for current application after the trim to carry out the residual stress removing step. However, after the trim step S20, The residual stress removing step S30 may be performed using the press device and the mold used in the step S20).

FIG. 5 is a schematic view of a mold for cold trim equipped with an electrode according to another embodiment of the present invention. FIG.

5, a cold trim mold 100 '' having an electrode according to another embodiment of the present invention includes a trim cutter 400 provided on one side of a top mold 200, (500) is installed on one side of the lower side of the lower mold (200) and one side of the upper side of the lower mold (300).

Accordingly, the cold stamping process and the residual stress removing process by current application can be sequentially performed using the same mold in one press apparatus.

At this time, in the trimming step S20, no power is supplied to the electrode 500 from the power supply device, and the current is not leaked through the trim cutter 400 at the time of current application, as well as between the electrode 500 and the trim cutter 400 It is preferable that the insulating coating portion 600 is formed.

10: B-filler
S: molded article
100: Trim device
200: Prize money type
300: Lower mold
400: trim cutter
500: electrode
600: insulated coating part

Claims (11)

(a) forming a formed body by a hot stamping process;
(b) cutting one side of the molded body by a trim process; And
(c) applying a current to the trim portion of the molded body to remove residual stress at room temperature, thereby preventing delayed fracture of the hot stamping component in accordance with the cold trim.
The method of claim 1, wherein the step (b)
Wherein the step of trimming is performed in a cold state by pressing.
The method of claim 1, wherein the step (c)
(c-1) arranging electrode pairs having mutually different polarities on the upper side and the lower side in the thickness direction of the trim portion, wherein the electrode pairs are disposed at least one apart from each other along the longitudinal direction of the trim portion;
(c-2) applying a current to the electrode pair to remove the residual stress of the trim portion, thereby preventing delayed fracture of the hot stamping component due to the cold trim.
4. The method of claim 3, wherein the step (c-1)
Wherein the pair of electrodes are arranged closely to a stress concentration portion of the trim portion.
4. The method of claim 3, wherein the step (c-2)
Wherein a larger current is applied to the electrode pair disposed in the stress concentration portion of the trim portion.
4. The method of claim 3, wherein the step (c-2)
Wherein a current is applied to an electrode pair disposed in a stress concentration portion of the trim portion for a longer period of time.
4. The method of claim 3, wherein the step (c-2)
Wherein a current is applied to the trim part by a combination of a top mold and a bottom mold provided on the upper and lower sides of the molded body, respectively, and the electrode pair.
The method of claim 7, wherein the step (c-2)
Wherein the pair of electrodes are provided on one side of the insulation coated portion formed on the upper die and the lower die, respectively, so that leakage of current is prevented, thereby preventing delayed breakage of the hot stamping component.
The method of claim 7,
Wherein the step (c) is performed using the press and the mold used in the step of trimming in the step (b) following the step (b).
The method of claim 1, wherein the step (c)
Wherein a current is applied in a current density of 5 to 100 A / mm < 2 > for a time of less than 3 seconds but less than 3 seconds.
The method of claim 1, wherein the step (c)
And a pulse current is applied to the trim portion.

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