KR102098271B1 - Hot press forming method of Al alloy sheet - Google Patents

Abstract

According to one perspective of the present invention, provided is a hot press forming method of an aluminum alloy sheet. The hot press forming method of the aluminum alloy sheet comprises: a step of heating the sheet at a temperature wherein a solution heat treatment of the alloy is conducted; a first cooling step to insert the heated sheet into a mold of the hot press forming apparatus and to cool the sheet to the range of forming temperatures; a step of forming the sheet within the range of forming temperatures; a second cooling step to cool the formed sheet to room temperature; a step of rapidly cooling the formed sheet to the range between -50°C and -200°C to perform a treatment of extremely low temperatures; and a step of performing the up-hill quenching on the formed sheet on which the treatment of extremely low temperatures is performed. The present invention aims to provide the hot press forming method of the aluminum alloy sheet, which is able to reduce damage by heat on the mold and remove residual stress existing in the materials.

Description

Hot press forming method of aluminum alloy sheet {Hot press forming method of Al alloy sheet}

The present invention relates to a method for hot press forming an aluminum alloy plate, and more specifically, aluminum that can prevent damage due to heat of a mold frequently generated in a hot press forming process and remove residual stress due to hot forming quenching. It relates to a hot press forming method of an alloy plate.

In recent years, in accordance with the trend of light weight in the automobile field, the case of steel materials constituting the vehicle body is increasingly being replaced by aluminum alloys, which are lightweight materials. It is desirable that components used in automobiles are made of a small number of parts to suit the end use. As a technology that meets these requirements, there is a technology of forming a part from a single metal plate by a press process using a mold. In the field of manufacturing aluminum alloy parts for automobiles, a cold press forming method using a plate material (T4 treatment) that has been subjected to natural aging treatment after solution treatment is mainly used. However, in the case of such cold press molding, since the molding is performed at room temperature, the difficulty of molding is often high, so it is often subjected to a multi-stage cold pressing step, and therefore, it is disadvantageous in terms of formability to be applied to a high-strength aluminum alloy. In order to solve this problem, a hot press molding method is proposed in which an aluminum alloy is heated to a high temperature to increase formability and then press-molded. In general, the hot press forming process of an aluminum alloy is performed by heating the solution-treated sheet material to a molding temperature and then inserting it into a mold of a press device at a high temperature and molding it to room temperature. In the case of hot press molding, a large amount of heat is transferred from the workpiece to the mold because the workpiece is molded in a high temperature state, and repeated heat cycles may lead to faster deterioration and wear of the mold, thereby designing the mold, Problems may arise that complicate operation and maintenance. In addition, a high level of residual stress may be present in the molded product due to the volume change according to the rapid temperature change in the step of molding at a high temperature after solution treatment and quenching it to room temperature. Residual stress present in these products can act as a factor that hinders product quality. For example, in the case of an aluminum thick plate, a material may be unable to withstand the residual stress and bend during the subsequent cutting process due to the residual stress generated in the hot press forming process. Therefore, it is necessary to develop a new process that can remove residual stress inside the product after such hot press molding.

The present invention is to solve a number of problems, including the problems as described above, to reduce the damage caused by heat of the mold and to provide a hot press forming method of an aluminum alloy plate that can remove the residual stress existing inside the material The purpose. However, these problems are exemplary, and the scope of the present invention is not limited thereby.

According to one aspect of the present invention, there is provided a method for hot press forming an aluminum alloy sheet.

The hot press forming method of the aluminum alloy plate includes: heating the plate to a temperature at which the solution treatment of the alloy is performed; A first cooling step of charging the plated material to a mold temperature range after charging the heated plated material into a mold of a hot press forming apparatus; Forming the plate material in the forming temperature range; A second cooling step of cooling the molded plate material to room temperature; Quenching the molded plate material in a range of -50 ° C to -200 ° C to perform cryogenic treatment; And up-hill quenching the molded sheet material subjected to cryogenic treatment.

In the hot press forming method of the aluminum alloy plate, it may further include an artificial aging treatment after the step of the up-hill quenching.

In the hot press forming method of the aluminum alloy plate material, the first cooling step may include performing a direct injection of a cooling medium to the plate material in the mold.

In the hot press forming method of the aluminum alloy plate, the first cooling step may include spraying a lubricant together with the cooling medium.

In the hot press forming method of the aluminum alloy plate, the second cooling step, by supplying a cooling medium to the channel provided in the mold to cool the mold, and heat transfer by directly contacting the molded plate with the mold It may include the step of cooling the molded plate material through.

In the hot press forming method of the aluminum alloy plate material, the up-hill quenching step comprises: rapidly heating the mold by supplying a heating medium to a channel provided in the mold, and heating the mold into the mold. And directly heating the molded plate to heat the molded plate through heat transfer.

In the hot press forming method of the aluminum alloy plate, the heating medium may include high temperature oil or water vapor.

In the hot press forming method of the aluminum alloy plate material, the up-hill quenching may include heating the molded plate material by energizing heating.

In the hot press forming method of the aluminum alloy plate material, the cryogenic treatment may include cooling the molded plate material using liquid nitrogen.

In the hot press forming method of the aluminum alloy plate material, the up-heel quenching step may have an end temperature of the up-heel quenching range of 100 ° C to 200 ° C.

In the hot press forming method of the aluminum alloy plate material, the up-forming temperature range may have a range of 200 ℃ to 400 ℃.

According to the embodiment of the present invention made as described above, it is possible to reduce the damage to the mold generated in the process of forming a plate material heated to a high temperature as in the conventional hot press molding. In addition, it can have an effect of effectively removing or reducing the residual stress generated by hot forming quenching included in the hot press forming process of the aluminum alloy. Of course, the scope of the present invention is not limited by these effects.

1 is a view schematically showing a hot press forming system according to an embodiment of the present invention.
2 is a process flow chart schematically illustrating a hot press forming method of an aluminum alloy sheet according to an embodiment of the present invention.
3 is a graph comparing and analyzing residual stress values of aluminum alloy sheet samples according to experimental and comparative examples of the present invention.
4 is a graph analyzing Vickers hardness values and tensile strength values of aluminum alloy sheet samples according to Experimental Examples and Comparative Examples of the present invention.

Hereinafter, various embodiments of the present invention will be described in detail with reference to the accompanying drawings. The embodiments of the present invention are provided to more fully describe the present invention to those of ordinary skill in the art, and the following embodiments can be modified in various other forms, and the scope of the present invention is as follows. It is not limited to the Examples. Rather, these embodiments are provided to make the present disclosure more faithful and complete, and to fully convey the spirit of the present invention to those skilled in the art.

In general, in the case of an aluminum alloy, there is a problem in that residual stress is generated in the plate material when forming and rapidly cooling at a high temperature after solution treatment. In order to solve this, the present invention provides a method for hot press forming of an aluminum alloy plate having a reduced residual stress compared to the prior art through cryogenic treatment and up-heel quenching when molding at a high temperature and then rapidly cooling.

1 is a diagram illustrating a hot press forming system according to an embodiment of the present invention. Specifically, Figure 1 (a) is a view schematically showing the structure of the hot press forming system 1000, Figure 1 (b) and (c) schematically illustrates the hot press forming apparatus 300 It is a drawing.

1 (a), the hot press forming system 1000 includes a temperature control unit 100, a medium supply line 200, a hot press forming apparatus 300, a media circulation tank 400 and a cooler / heater It may include 500.

Referring to (b) and (c) of FIG. 1, the hot press forming apparatus 300 includes a lower mold 310, an upper mold 320 and a cooling medium injection device 330.

The lower mold 310 includes a punch portion 315 that is formed by pressing the lower surface of the object to be molded 600. The upper mold 320 has a receiving space that can accommodate the punch portion 315 when the mold is molded, and includes a holding portion 325 for holding the molded object 600 together with the punch portion 315.

The cooling medium injection device 330 is a device that directly injects the cooling medium onto the upper surface of the object to be formed, so that the object to be cooled can be cooled by the injection cooling method. The cooling medium, for example, may include air (air), it is also possible to input a separate cooling medium with air. As shown in (b) of FIG. 1, the lower mold 310 and the upper mold 320 are disposed outside the space between the lower mold 310 and the upper mold 320 in the opened state, and the molded material Cooling medium from the upper side of (600) is injected directly to the molded object (600).

The cooling medium injection device 330 may further include a device that supplies a lubricant injected with the cooling medium. Therefore, it is possible to selectively jet the lubricating oil together with the cooling medium to the molding material 600 through the cooling medium injection device 330. Through this, by applying a lubricant to the surface of the object to be molded, it is possible to prevent damage to the molded surface of the object to be molded 600 and perform efficient molding.

In addition, the inside of the upper mold 320 and the lower mold 310 may include a channel 340 for flowing a fluid to a portion in contact with the molded object 600, respectively. In addition, in order to measure the temperature of the molded object 600, it may further include a temperature sensor (not shown) contacting the lower surface of the molded object 600.

The channel 340 is a medium that flows depending on the purpose of use of the hot press forming apparatus 300. Cooling media such as cooling water or liquid nitrogen may be used, or heating such as high temperature / high pressure steam or oil may be used. Media can be used. That is, the cooling medium and the heating medium share the channel 340 and are supplied into the hot press forming apparatus 300 through the medium supply line 200.

The medium that has passed through the hot press forming apparatus 300 is supplied to the temperature control unit 100 through the medium circulation tank 400 to continuously heat or cool the molded object 600. At this time, in order to efficiently circulate the medium, the medium that has passed through the medium circulation tank 400 is cooled / heated by using the cooler / heater 500 and supplied to the medium temperature control unit 100.

Hereinafter, a hot press forming method of an aluminum sheet will be described using the hot press forming system 1000 illustrated in FIG. 1. 2 is a process flow chart schematically illustrating a hot press forming method of an aluminum alloy plate according to an embodiment of the present invention.

Referring to FIG. 2, in the hot press forming method of an aluminum alloy plate, a step (A) of heating the plate at a temperature at which the solution treatment of the alloy is performed and a step (B) of maintaining at the solution treatment temperature are performed. . The temperature for the solution treatment is a temperature at which all of the precipitates in the aluminum alloy can be dissolved in the aluminum base, for example, may have a range of 480 to 550 ° C. The aluminum alloy may include, for example, 2000-based, 6000-based, and 7000-based alloys. The solution treatment can be carried out in a conventional heating furnace.

After the solution treatment is completed, the plate material is charged into the mold of the hot press forming apparatus, and then the first cooling step (C) of cooling the plate material to a molding temperature range, forming the plate material in the molding temperature range (D), and The second cooling step (E) of cooling the molded sheet material to room temperature is continuously performed.

According to an embodiment of the present invention, the aluminum plate material introduced into the mold is molded after the first cooling step of cooling the solution treatment temperature below a predetermined temperature. For example, the molding temperature may range from 200 to 400 ° C.

As the aluminum sheet is cooled before molding by the first cooling step, it is possible to secure a relatively good formability compared to the cold press process while preventing the thermal damage problem of the mold that occurs when molding a high-temperature workpiece. There will be. When the molding temperature exceeds 400 ° C, hot forming problems may occur. On the other hand, if it is less than 200 ° C., a problem that moldability deteriorates due to a low molding temperature may appear.

In the first cooling step, as shown in (b) of FIG. 1, the molded material 600 is used by using the cooling medium spraying device 330 while the upper mold 320 and the lower mold 310 are opened. Cooling is performed by spraying the cooling medium directly onto the aluminum plate. For example, compressed air can be injected directly into an aluminum plate with lubricant. At this time, it can be confirmed whether the aluminum plate has reached a desired temperature range using a temperature sensor, and when it is confirmed that the temperature range is reached, injection is stopped to complete the first cooling step.

After the first cooling step is completed, the upper mold 320 and the lower mold 310 are molded to plastically deform the plate material as shown in FIG.

After the molding of the plate is completed, the second cooling step (E) is started. In the second cooling step, the cooling medium is cooled by inputting a cooling medium into the channel 340 in a state where the upper mold 320 and the lower mold 310 are mixed, thereby cooling the aluminum sheet that is in direct contact with the mold.

As another example, after the molding is completed, the upper mold 320 and the lower mold 310 are opened, and then the cooling medium is directly injected into the molded plate using a cooling medium injection device 330 to cool to room temperature.

In the first cooling step and the second cooling step, it is important to cool the aluminum alloy matrix at a rapid cooling rate such that precipitates are not precipitated. Precipitation in the first cooling step can affect the shaping of the plate material, and precipitation in the second cooling step can affect mechanical properties in the subsequent artificial aging step. The cooling rate may be determined according to the type, composition, material thickness and size of aluminum to be processed. For example, it may be cooled at a cooling rate of 10 ° C / sec to 50 ° C / sec, more preferably at a cooling rate of 20 ° C / sec to 30 ° C / sec.

The aluminum plate molded by the above-described method may have residual stress inside the plate molded by rapid cooling before and after molding, and it is necessary to reduce the residual stress. To this end, after the second cooling step, a step of reducing the residual stress of the molded plate is performed.

Specifically, the step of reducing the residual stress includes the step (F) of treating the molded sheet material at a cryogenic temperature in a range of -50 ° C to -200 ° C and up-heel the molded sheet material at a temperature higher than room temperature. -hill quenching).

The step of treating the molded sheet material at a cryogenic temperature in the range of -50 ° C to -200 ° C may be typically performed using liquid nitrogen. For example, a cryogenic treatment may be performed by cooling the molded sheet material that has been sealed in the mold and then cooled to room temperature by directly introducing it into a liquid nitrogen bath, or by spraying liquid nitrogen directly into the molded sheet material.

After the cryogenic treatment is completed, a step (G) of performing up-hill quenching to rapidly heat it back to a temperature above room temperature is performed. Residual stress can be eliminated by using the fast heating rate of the material when up-hill? The up-heel quench heating rate may have a range of 10 ° C / sec to 50 ° C / sec, more preferably 20 ° C / sec to 30 ° C / sec. The end temperature of the up-hill quenching may range from 100 to 200 ° C.

The residual stress existing inside the molded plate material by the above-described cryogenic treatment and up-heel quenching can be significantly reduced, and the reason is as follows. In the cryogenic treatment step, the material is quenched from room temperature to cryogenic temperature, and in this case, a rapid contraction of volume occurs, which causes a strong compressive stress inside. However, in the up-heel quenching step performed later, the temperature of the material rises rapidly, and the internal compressive stress is relaxed by rapid volume expansion. In order for this up-heel quenching to work effectively, there must be a temperature difference with a level that can be a driving force for fast temperature rise and residual stress relaxation. In order to realize this temperature difference, it is necessary to lower the starting temperature of the up-heel quenching as much as possible, so that the step of lowering the temperature to cryogenic temperature before the up-heel quenching is performed.

Heating for up-heel quenching can be performed, for example, by directly spraying a heating medium such as hot water vapor to a cryogenically treated member.

As another example, it may be performed using a mold having a channel through which the fluid flows. That is, by supplying a heating medium such as high temperature / high pressure oil or water vapor to a channel provided in a mold, the mold is rapidly heated, and the molded plate is directly pressed into the mold to heat the molded plate through heat transfer. Steps can be performed. For example, as shown in Fig. 1 (c), after charging the molded plate material into a mold, and then heating the mold by flowing a heating medium through a channel of the mold, heat from the mold to the molded plate material Up-hill quenching can be performed by allowing it to be delivered. The mold for up-heel quenching may be the same as the mold used for molding, but it may also be a mold provided separately as a separate device.

As another example, after the cryogenic treatment, energizing heating may be performed for up-heel quenching. The energization heating is a method of heating the aluminum member to be energized by resistance heat by connecting an anode and a cathode to the aluminum member and applying electric power. In the case of energized heating, it is possible to rapidly heat to the desired temperature by controlling the power input. Therefore, after the molded sheet material subjected to the cryogenic treatment is moved to the energizing heating device, up-heel quenching can be performed through energizing heating.

After the cryogenic treatment and the up-hill quenching are completed, additional artificial aging is performed to precipitate precipitates on the aluminum base, thereby improving mechanical properties such as hardness and strength of the molded plate. The temperature of artificial aging can be performed, for example, in the range of 110 to 200 ° C.

Hereinafter, experimental examples for understanding the present invention will be described. However, the following experimental examples are only to help understanding of the present invention, and the present invention is not limited to the following experimental examples.

<Experimental Example>

An aluminum alloy plate sample was prepared using the molding method of the aluminum alloy plate according to the experimental example of the present invention. The aluminum alloy used in the experiment was an Al2014 alloy.

Referring to FIG. 1, a sample in the form of a plate was introduced into a heating furnace to perform solution treatment at 500 ° C. After the solution treatment was completed, the sample was introduced into a mold of a hot press apparatus, and then a first cooling step was performed up to 300 ° C. In this case, compressed air and lubricant were directly sprayed onto the sample using a cooling medium injection device to cool. After the first cooling step, press molding was performed using a mold at a molding temperature, and then cooling water was flowed through a cooling channel provided inside the mold to perform a second cooling step to room temperature. After the second cooling step was completed, the press-molded sample was taken out of the mold and then poured into liquid nitrogen to perform a cryogenic treatment at a temperature of about -80 ° C. After the cryogenic treatment was completed, the member was taken out of liquid nitrogen and then supplied with hot water vapor (10.9bar, 175 ° C), heated to and maintained at 175 ° C, and then cooled to room temperature. Finally, the sample was artificially aged at 175 ° C. for 10 hours to prepare Experimental Example 1 of the present invention.

In addition, it was performed in the same manner as described above, and after repeating the cryogenic treatment and the up-heel quenching treatment three times, artificial aging was performed to prepare Experimental Example 2 of the present invention.

On the other hand, for comparison, a solution treatment was performed on the same material, and a first cooling step was performed up to a molding temperature. After molding at a molding temperature and performing a second cooling step to room temperature, natural aging was performed to prepare Comparative Example 1. In addition, the process was performed in the same manner as in Comparative Example 1, except that the second cooling step was terminated and artificial aging was performed instead of natural aging to prepare Comparative Example 2.

Residual stress, Vickers hardness, and tensile strength results were compared and analyzed for each experimental example and comparative examples.

3 is a graph analyzing residual stress values of samples according to experimental and comparative examples of the present invention. Residual stress was measured by selecting 5 regions in one sample.

Referring to Figure 3, it can be seen that the residual stresses in Experimental Example 1 and Experimental Example 2 of the present invention were significantly lower than Comparative Example 1 and Comparative Example 2. Through this, it can be seen that the stress remaining in the molded sheet after high temperature molding by the cryogenic treatment and ear-heel quenching of the present invention is significantly resolved.

On the other hand, when comparing Experimental Example 1 and Experimental Example 2, Experimental Example 2, which repeated three times of cryogenic treatment and up-heel quenching, exhibited a lower residual stress value than Experimental Example 1. If the cryogenic treatment and the up-heel quenching were repeatedly performed, it was confirmed that the residual stress was more efficient.

4 is a graph comparing and analyzing Vickers hardness values and tensile strength values of samples according to Experimental Examples and Comparative Examples of the present invention.

Referring to Figure 4, it can be seen that both the Vickers hardness value and the tensile strength value of Experimental Example 1 and Experimental Example 2 are larger than Comparative Example 1 and Comparative Example 2. Comparative Example 2 and Experimental Example 1, the Vickers hardness value was measured similarly, but comparing the tensile strength value, it can be seen that Experimental Example 1 is higher than Comparative Example 2. In addition, it can be seen that the mechanical properties of Experimental Example 2 in which cryogenic treatment and up-heel quenching were repeated three times were greater than in Experimental Example 1.

The present invention has been described with reference to the embodiment shown in the drawings, but this is only exemplary, and those skilled in the art will understand that various modifications and equivalent other experimental examples are possible therefrom. Therefore, the true technical protection scope of the present invention should be determined by the technical spirit of the appended claims.

100: temperature control unit
200: medium supply line
300: hot press forming device
310: lower mold
315: punch
320: upper mold
325: holding unit
330: cooling medium injection device
340: channel
400: medium circulation tank
500: cooler / heater
600: molded material
1000: hot press forming system

Claims (11)

As a method of hot press forming an aluminum alloy sheet,
Heating the plate to a temperature at which the solution treatment of the alloy is performed;
A first cooling step of charging the plated material to a mold temperature range after charging the heated plated material into a mold of a hot press forming apparatus;
Forming the plate material in the forming temperature range;
A second cooling step of cooling the molded plate material to room temperature;
Quenching the molded plate material in a range of -50 ° C to -200 ° C to perform cryogenic treatment; And
Up-hill quenching the cryogenic molded sheet;
Including,
The first cooling step,
Including the step of performing a direct injection of a cooling medium and a lubricant simultaneously on the entire surface of the plate in the mold,
Hot press forming method of aluminum alloy plate. According to claim 1,
Further comprising the step of artificial aging after the step of up-hill quenching (up-hill quenching),
Hot press forming method of aluminum alloy plate. delete delete According to claim 1,
The second cooling step,
Cooling the mold by supplying a cooling medium to a channel provided in the mold, and cooling the molded plate through heat transfer by directly contacting the molded plate with the mold,
Hot press forming method of aluminum alloy plate. According to claim 1,
The step of up-hill quenching,
A step of rapidly heating the mold by supplying a heating medium to a channel provided in the mold, and directly heating the molded plate with the mold to heat the molded plate through heat transfer.
Hot press forming method of aluminum alloy plate. The method of claim 6,
The heating medium comprises high temperature oil or water vapor,
Hot press forming method of aluminum alloy plate. According to claim 1,
The step of up-hill quenching,
Comprising the steps of heating the molded plate by energizing heating,
Hot press forming method of aluminum alloy plate. According to claim 1,
The cryogenic treatment step,
Cooling the molded plate using liquid nitrogen,
Hot press forming method of aluminum alloy plate. According to claim 1,
The step of calling up-hill,
The end temperature of the up-hill quenching ranges from 100 ° C to 200 ° C,
Hot press forming method of aluminum alloy plate. According to claim 1,
The molding temperature range,
Having a range of 200 ° C to 400 ° C,
Hot press forming method of aluminum alloy plate.

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