Hot stamping process using hot stamping dies and hot stamping presses

Abstract

The invention relates to a hot stamping die (2, 3) comprising a die body (11) having a working surface (9) in contact with a blank during a hot stamping operation, and at least one porous die part (4) having a corresponding porous working surface portion (7), said porous die body part being in contact with a reservoir (6, 40) containing a cooling medium (8), said porous die body part comprising a plurality of egress channels (5) extending from said reservoir (6, 40) to said porous working surface portion, The ejection channels (5) are configured to eject the cooling medium (8) from a reservoir (6, 40) towards the porous work surface portion (7) when the pressure of the cooling medium rises above a threshold ejection pressure, and the dies (2, 3) do not include exhaust channels for discharging excess ejected coolant from the die after hot stamping.

Description

[Technical Field]

[0001] The present invention relates to hot stamping in the metallurgical industry, and more particularly to a hot stamping die and a hot stamping press. The invention also relates to a process for hot stamping metal blanks, such as steel blanks, using a hot stamping press. [Background Art]

[0002] It is known that the process of hot stamping pressing of high-temperature metal blanks, which means metal blanks at a temperature of about 900 degrees Celsius or above, enables hot press forming of metal blanks having high tensile strength and yield strength into complex shapes.

[0003] An important step in the hot stamping process is the rapid cooling of the freshly formed blank. Cooling methods are already known, for example, from EP 3 045 236 A1 and EP 1 671 715 A1, which describe a hot press stamping cooling method using a hot press stamping apparatus comprising a die body in which a plurality of channels are arranged, said channels are connected to a refrigerant tank and lead to ejection holes that distribute refrigerant to a formed blank placed in the hot stamping press.

[0004] However, with this cooling process, it remains difficult to ensure uniform cooling of the metal blank. In addition, the amount of refrigerant ejected is large, and a discharge channel for discharging the refrigerant from the working surface of the die is required. This results in additional time and refrigerant required for cooling each pressed blank. [Prior Art Documents] [Patent Documents]

[0005] [Patent Document 1] European Patent No. 3045236 Specification [Patent Document 2] European Patent No. 1671715 [Overview of the project] [Problems that the invention aims to solve]

[0006] Therefore, the object of the present invention is to improve upon the shortcomings of the prior art by providing a hot stamping die and a hot stamping press that enhance the efficiency and speed of cooling of hot-formed metal blanks.

[0007] The present invention also provides a hot stamping process using the hot stamping die and press of the present invention. [Means for solving the problem]

[0008] For this purpose, a first object of the present invention is a hot stamping die comprising a die body having a work surface that contacts a blank during a hot stamping operation, and at least one porous die portion having a corresponding porous work surface portion, wherein the porous die body portion is in contact with a reservoir, the reservoir contains a cooling medium, the porous die body portion comprises a plurality of ejection channels extending from the reservoir to the porous work surface portion, the ejection channels are configured to discharge the cooling medium from the reservoir toward the porous work surface portion when the pressure on the cooling medium rises above a threshold ejection pressure, and the die does not have an ejection channel for discharging excess coolant ejected from the die after hot stamping.

[0009] The hot stamping dies according to the present invention may also have any of the following optional features, which may be considered individually or in combination. -Each ejection channel in the porous die section has a cylindrical portion. - The cylindrical portion of each ejection channel terminates at a frustoconical portion on the porous work surface, with its widest part located on the porous work surface. - The diameter of the cylindrical portion of the ejection channel is between 0.1 and 0.5 millimeters. - The porous die portion is made of steel or stainless steel. - The porous die portion is located within a controlled cavity inside the die body, which opens onto the hot stamping work surface. - The die is designed for use in a stamping press, which closes in accordance with the stamping direction during a hot stamping operation, and at each point on its surface, the angle between the direction perpendicular to the die working surface and the stamping direction is an angle α consisting of 0° to 90°, and the die has a porous die portion in all areas where α is at least between 45° and 90°. -Each of the upper and lower dies comprises at least one porous die portion located within a cavity that opens onto the corresponding hot stamping die work surface. - The porous die portion occupies the entire die body.

[0010] A second object of the present invention is that the die consists of the aforementioned die, and the porous portion of the die is manufactured by additive manufacturing.

[0011] A third object of the present invention is a hot stamping process using a hot stamping press equipped with an upper die and a lower die, wherein at least one of the dies comprises a work surface that contacts a blank during the hot stamping operation and a die body that contacts a reservoir, the reservoir contains a cooling medium, and the work surface comprises a porous work surface portion. The lower die comprises a porous die body portion having a plurality of ejection channels extending from the reservoir to the porous work surface, the ejection channels being capable of ejecting the cooling medium from the reservoir toward the porous work surface during the hot stamping process, the reservoir being equipped with a pressurizing device that can be operated to raise the pressure of the cooling medium in the reservoir above a threshold ejection pressure and can be stopped to release additional pressure on the cooling medium, and the hot stamping process is -(i) The step of operating a pressurizing device to completely fill the ejection channel until the cooling medium reaches the porous work surface portion, -(ii) A step of heating the steel blank, -(iii) The step of transferring the steel blank to a hot stamping press, -(iv) The step of stopping the pressurizing device, -(v) The step of positioning the blank within the hot stamping press, -(vi) a step of hot stamping the steel blank by clamping an upper die and a lower die, wherein step i can be performed simultaneously with steps ii and iii, and step iv can be performed after step i and before step vi.

[0012] Preferably, the cooling medium is an aqueous solution.

[0013] Most preferably, the cooling medium is water.

[0014] Other features and advantages of the present invention will be described in more detail in the following description.

[0015] The present invention will be better understood by reading the following description. The following description is provided for illustrative purposes only and is not intended to be limiting in any way. [Brief explanation of the drawing]

[0016] [Figure 1] This figure shows a cross-sectional view of the hot stamping press of the present invention, which comprises a hot stamping lower die and a hot stamping upper die. [Figure 2] Figure 1 is a cross-sectional view of a hot stamping press having a hot blank between dies. [Figure 3] Figure 1 is a cross-sectional view of a hot stamping press having a blank formed between dies. [Figure 4]It is a cross-sectional view of a part of a die body including a porous die portion provided with an ejection channel connected to a coolant reservoir. [Figure 5] It is a perspective view of a porous die portion of the hot stamping die of the present invention. [Figure 5a] It is a detailed perspective view of the working surface of the porous die portion in Fig. 5. [Figure 5b] It is a view showing the cross-section A-A of Fig. 5a of the first embodiment of the ejection channel of the porous die portion in Fig. 5. [Figure 5c] It is a view showing the cross-section A-A of Fig. 5a of the second embodiment of the ejection channel of the porous die portion in Fig. 5. [Figure 6] It is a cross-sectional view of a part of the die body of Fig. 4 while the ejection channel of the porous die portion ejects coolant onto the working surface of the porous die portion. [Figure 7] It is the same view as Fig. 4, showing a blank moving toward the working surface. [Figure 8] It is a view showing a blank on the working surface of the porous portion of the sweating die of Fig. 6. [Figure 9] It is a perspective view of an omega-shaped hot stamping die of the present invention. [Figure 10] It is a perspective view of a lower hot stamping die of the present invention designed for the automotive industry. Mode for Carrying Out the Invention

[0017] It should be noted that the terms “lower,” “upper,” “above,” “below,” “lowest,” “highest,” “top,” “bottom,” “left,” and “right” used in this application refer to the position and orientation when the reinforcing carrier device, battery pack, and different parts of the vehicle are positioned perpendicular to the ground. The term “perpendicular” defines an angle of 90°+ / -15°, and the term “parallel” defines an angle of 0°+ / -15°. Hot stamping is a forming technique that includes heating a blank to a temperature (typically about 900°C) at which the microstructure of the steel transforms at least partially into austenite, forming the blank at high temperatures by stamping the blank, and quenching the formed part to obtain a microstructure with very high strength. Hot stamping makes it possible to obtain parts with complex shapes and very high strength without springback. To bring about the aforementioned advantages of hot stamping, the material used is known as a press-curing material, which has a chemical composition that allows it to form the desired cured microstructure when subjected to the hot stamping process described above.

[0018] Additive manufacturing is a manufacturing process that uses computer-aided design (CAD) data to instruct hardware to deposit material, often layer by layer, in the precise geometric shape that reproduces the CAD data.

[0019] As shown in Figures 1 to 5, the present invention discloses a hot stamping press 1 comprising at least a lower die 3 and an upper die 2. As shown in Figures 1 to 3, the lower die 3 and the upper die 2 are formed complementaryly to allow deformation of a high-temperature metal blank 10 into a metal part having a desired volumetric shape. Advantageously, the lower die 3 and the upper die 2 are made of steel or stainless steel.

[0020] In the following description and accompanying drawings, the present invention is described in detail only for the case of the lower die 3 for the sake of simplification. However, it should be understood that this may also apply to the upper die 2, and that it may apply to both the upper die 2 and the lower die 3 of the same hot stamping press 1.

[0021] The lower die 3 comprises a die body 11 having a work surface 9 provided to contact the blank 10 during the hot stamping operation. The lower die 3 also comprises a plurality of porous die sections 4 managed within the lower die body 11. Each porous die section 4 has a porous work surface section 7 and a plurality of ejection channels 5 leading to the porous work surface section 7 of the porous die section 4 under consideration. The ejection channels 5 of each porous die section 4 are in fluid contact with a reservoir 6 of a cooling medium 8, also called "coolant." The reservoir 6 is shown in Figure 4 as managed within the cavity of the lower die body 11. However, this is only one specific embodiment. Generally speaking, the reservoir 6 may be located away from the die body 11, and the coolant 8 can be distributed to the porous die sections 4 via pipes. Also, if there are multiple porous die sections 4, there may be multiple reservoirs 6, provided that each porous die section 4 is adequately supplied with coolant 8. In other words, each porous die section 4 is fluidly connected to the reservoir 6. In certain embodiments, as shown in Figures 1 to 3, the coolant 8 is supplied to the porous die portion 4 using the cooling circuit 40 of the die itself. In fact, to cool the dies 2 and 3 during the hot stamping process, the dies 2 and 3 are equipped with a cooling circuit 40, which typically does not come into contact with the surface of the die but simply circulates inside the die body 11. The coolant 8 is constantly circulated within the cooling circuit 40 during the hot stamping process to limit the temperature rise of the dies 2 and 3, which can ultimately reduce the quenching efficiency of the dies 2 and 3.

[0022] The coolant 8 may be an aqueous solution such as salt water or unsalted water, or any other liquid refrigerant adapted to cool the high-temperature metal blank 10 to be stamped.

[0023] As shown in Figures 5 and 5a, the ejection channels 5 are regularly arranged within the porous die portion 4 under consideration according to a matrix pattern. In the first modification shown in Figure 5b, the ejection channels 5 are cylindrical. The diameter of each channel 5 is in the range of 0.1 to 0.5 millimeters, preferably 0.2 to 0.4 millimeters. In the second modification shown in Figure 5c, each ejection channel 5 has a cylindrical portion terminating at a frustoconical portion 20, the largest portion of which is located at the porous work surface portion 7 of the porous die portion 4 under consideration. Following this second modification, the liquid coolant 8 spreads more favorably on the porous work surface portion 7 of the porous die portion 4 under consideration and more broadly on the work surface 9 of the lower die 3. The diameter of the cylindrical portion of each channel 5 is in the range of 0.1 to 0.5 millimeters, preferably 0.2 to 0.4 millimeters.

[0024] In both modifications, the ejection channel 5 is configured to eject the coolant 8 from the reservoir 6 toward the porous work surface portion 7 when the pressure of the coolant 8 rises above a threshold ejection pressure. To enable this, a pressurizing device (not shown) is connected to the reservoir 6, and the pressurizing device is controlled by control means to raise the pressure of the coolant 8 above a threshold ejection pressure. The threshold ejection pressure depends on the coolant 8 used and the specific configuration of the ejection channel 5 being used. A higher threshold ejection pressure is required for a viscous coolant 8. A higher threshold ejection pressure is required for a narrow ejection channel 5.

[0025] In addition, the amount of coolant 8 released is small enough to completely vaporize on the working surface 9 of the lower die 3 while the pressed blank 10 is cooling, so the lower die 2 and upper die 3 do not need to have discharge channels to remove any excess coolant 8 that is ejected after hot stamping.

[0026] Thanks to the diameter of the ejection channel 5 and their ability to eject a fixed amount of coolant 8 that has been completely vaporized, the hot stamping dies 2 and 3 function as "sweating" dies in relation to the natural sweating phenomenon.

[0027] As shown in Figure 1, the lower die 3 comprises a plurality of porous die portions 4, which may be located within the bottom wall of the die body 11, preferably within the side walls of the die body 11, because the precision of contact between the blank 10 and the work surface 9 is lowest with respect to the side walls, thereby allowing for a lower quenching rate at the side walls. More precisely, considering that the hot stamping press 1 is closed according to the stamping direction X, and considering the angle α between the direction perpendicular to the die work surface 9 and the stamping direction X, the angle α is in the range of 0° to 90°, and the porous die portions are preferentially located in the region of the lower die 3 where α is in the range of 45° to 90°. The advantage of such a configuration is that the blank 10 pressed between dies 2 and 3 receives lower contact pressure in the region of the lower die 3 where α is in the range of 45° to 90°. Furthermore, the lower the contact pressure, the worse the heat transfer between the blank 10 and dies 2 and 3. Therefore, in order to ensure a more rapid and uniform cooling of the pressed blank 10, it is important to position the porous die portion 4 in the region of the lower die 3 where heat transfer with the blank 10 is worst, i.e., the region of the lower die 3 where α is in the range of 45° to 90°.

[0028] Following this configuration, Figure 9 shows an omega-shaped lower die 37 having a porous die portion 4 positioned on a side surface 39 opposite to the die 37, where the angle α between the stamping direction X and the direction N perpendicular to the porous work surface portion 7 of the porous die portion 4 is in the range of 45° to 90°.

[0029] Figure 10 shows an industrial lower die 38 controlled to press a steel blank to form an automotive component (a B-pillar in Figure 10). As shown in Figure 10, the porous die portion 4 is controlled on the side surface of the industrial die 38 in a region where α is in the range of 45° to 90°.

[0030] In another embodiment (not shown), the entire lower die body 11 is made of a porous die portion 4.

[0031] On the other hand, in certain embodiments, the entire die (lower die and upper die) can be fabricated from the porous portion 4, which can have industrial advantages, for example, in terms of the simplicity of tool design.

[0032] In certain embodiments, the porous die portion 4 is manufactured using additive manufacturing. Advantageously, additive manufacturing allows for the production of complex metal shapes with very precise dimensions, such as the porous die portion 4, which has dense, narrow ejection channels. Other, more traditional manufacturing methods may be used to produce the porous die portion 4, such as directly manufacturing a metal casting of the final shape or casting a block of metal into which the ejection channels 5 are subsequently perforated (also known as "subtractive manufacturing").

[0033] In a particular embodiment, the porous die portion 4 is manufactured by an additive manufacturing process having the following characteristics: - Additive manufacturing process: Powder bed fusion method - Printing under inert gas, using argon as the inert gas. -50 micron layer thickness - Lasers used: 4 x 500W Yb lasers - Powder used: 316L stainless steel powder having the following typical chemical composition and particle size distribution:

[0034] [Table 1]

[0035] [Table 2]

[0036] Figures 2 to 8 illustrate the hot stamping process using the hot stamping press 1 of the present invention.

[0037] In the first step, the control means activates the pressurizer above a threshold ejection pressure to completely fill the ejection channel 5 until the coolant 8 reaches the porous work surface portion 7 of the porous die portion 4 (Figure 6) into which the coolant 8 is considered.

[0038] In the second step, the blank 10, preferably a steel blank 10, is heated to a desired temperature, typically about 900 or 1000 degrees Celsius.

[0039] In the third step, the high-temperature steel blank 10 is transferred to the hot stamping press 1, as shown in Figure 7.

[0040] In the fourth step, the control means stops the pressurizer, which means that the pressure in the reservoir 6 slowly decreases until it falls below the threshold ejection pressure. More precisely, once the pressurizer is stopped, the pressure in each reservoir 6 decreases naturally due to the vaporization of the coolant 8 by the transfer of the hot steel blank during the third step. The diameter of the ejection channel 5 is small enough to prevent the coolant 8 from returning to the reservoir 6. The stopping of the pressurizer may be performed immediately after the first step of the process and always before hot stamping the steel blank 10 by clamping the upper die 2 and the lower die 3.

[0041] In the fifth step, as shown in Figures 8 and 2, the blank 10 is positioned on the work surface 9 of the corresponding hot stamping lower die 3.

[0042] In the final sixth step, as shown in Figure 3, the hot stamping press 1 is closed and the upper die 2 and lower die 3 are clamped in order to hot stamp the steel blank 10.

[0043] To improve the speed of the process, the first, second, and third steps may be performed simultaneously. In addition, the stopping of the pressurizing device, which always occurs before the sixth step of the process, allows for the complete vaporization of the coolant 8 in the area between the work surface 9 and the blank 10. The coolant 8 on the porous work surface portion 7 does not return to the reservoir 6 through the ejection channel 5.

Claims

1. A hot stamping process using a hot stamping press (1) equipped with an upper die (2) and a lower die (3), wherein at least one of the dies comprises a work surface (9) that contacts a blank during the hot stamping operation and a die body (11) that contacts reservoirs (6, 40), the reservoirs (6, 40) contain a cooling medium (8), the work surface (9) has a porous work surface portion (7), and the lower die (3) is a porous die comprising a plurality of ejection channels (5) extending from the reservoirs (6, 40) to the porous work surface (7). The main body comprises, the ejection channel (5) is capable of ejecting the cooling medium from the reservoir (6, 40) toward the porous work surface (7) during the hot stamping process, the reservoir (6, 40) is equipped with a pressurizing device that can be operated to raise the pressure of the cooling medium (8) in the reservoir above a threshold ejection pressure and can be stopped to release the additional pressure on the cooling medium (8), the die (2, 3) does not have an ejection channel for discharging excess coolant ejected from the die after hot stamping, and the hot stamping process is - (i) The step of operating a pressurizing device to completely fill the ejection channel (5) until the cooling medium (8) reaches the porous work surface portion (9), - (ii) A step of heating the steel blank (10), - (iii) The step of transferring the steel blank (10) to the hot stamping press (1), - (iv) The step of stopping the pressurizing device, - (v) The step of positioning the blank (10) inside the hot stamping press (1), - (vi) A step of hot stamping the steel blank (10) by clamping the upper die (2) and the lower die (3), wherein step i can be performed simultaneously with steps ii and iii, and step iv is performed after step i and before step vi, A hot stamping process that includes the following features.

2. The process according to claim 1, wherein the cooling medium (8) is an aqueous solution.

3. The process according to claim 1, wherein the cooling medium (8) is water.

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