An ultra-high strength steel hot stamping die and a method of using the same

Abstract

The application discloses a kind of ultra-high strength steel hot stamping die and its using method, it is related to hot stamping forming die field, the hot stamping die includes: upper die holder, lower die holder, upper die insert, lower die insert, main pressure mechanism, pre-pressing mechanism, cooling system and timing control mechanism.Main pressure mechanism is used to drive the upper die holder and the upper die insert relative to the lower die holder carry out closing, pressure maintaining and opening die action;Pre-pressing mechanism is used to before the main pressure mechanism is completely closed, pre-pressing force is applied to the sheet metal located in the edge region of the cavity;Timing control mechanism is configured as: in the first stage of closing stroke triggers the pre-pressing mechanism action, and in closing and pressure maintaining process, control the main pressure mechanism at least two different sizes of pressure maintaining force is applied.The application hot stamping die can effectively inhibit part rebound, improve edge forming quality, and adopt mechanical hydraulic timing control, structure is reliable, cost controllable.

Description

Technical Field

[0001] This invention relates to the field of hot stamping forming die technology, and more particularly to a die set, specifically a hot stamping die made of ultra-high strength steel and its method of use. Background Technology

[0002] Hot stamping is one of the core processes for manufacturing ultra-high-strength steel automotive structural components. Boron steels, such as 22MnB5 and 30MnB5, are heated to 900-950℃ to achieve complete austenitization, then rapidly transferred to a die for simultaneous stamping and in-die quenching, ultimately yielding a fully martensitic structure with a tensile strength exceeding 1500 MPa. This process effectively solves the problems of poor plasticity, high springback, and difficulty in forming complex shapes with ultra-high-strength steel at room temperature, and is therefore widely used in the mass production of safety structural components such as automotive A-pillars, B-pillars, and crash beams.

[0003] In hot stamping processes, sheet metal has extremely low yield strength at high temperatures, typically only 50-100 MPa, and is relatively thin. After being removed from the furnace, the sheet metal is highly susceptible to displacement due to its own weight or slight contact during transfer to the lower die surface and placement. Traditional dies rely solely on the final clamping of the upper and lower die inserts when the main slide closes to fix the sheet metal. During the downward movement of the upper die, the sheet metal may experience lateral movement or edge warping under the impact of the die closing, leading to dimensional differences in the edge contour of the formed part, folding or wrinkling in the flange area, and for plates of unequal thickness or welded plates, movement can also cause the weld to deviate from the design position, thus causing cracking.

[0004] Furthermore, after sheet metal is formed at high temperatures, austenite undergoes a phase transformation to martensite during the holding and cooling process, accompanied by a volume expansion of approximately 2-4%. Simultaneously, the part undergoes thermal shrinkage at the end of the cooling phase. The interaction between these volume changes and the rigid constraints of the mold cavity causes the final shape of the part to deviate from the theoretical profile, resulting in springback. Traditional methods for addressing springback often rely on mold profile bending compensation, such as the sheet metal springback compensation swing block assembly and the mold and stamping press with this assembly disclosed in Chinese Utility Model Patent Publication No. CN209531864U, and the stamping springback compensation control method disclosed in Chinese Invention Patent Publication No. CN115270426A. However, traditional springback compensation strategies are highly sensitive to process fluctuations, such as the initial sheet metal temperature, transfer time, and cooling rate, and require repeated mold modifications for different parts, limiting their applicability and failing to fundamentally solve the dimensional deviation problem caused by the alternating effects of phase transformation expansion and thermal shrinkage.

[0005] Therefore, it is necessary to improve upon the shortcomings of existing technologies in order to solve the above problems. Summary of the Invention

[0006] This invention overcomes the shortcomings of the prior art and provides an ultra-high strength steel hot stamping die and its usage method.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: Firstly, the present invention provides an ultra-high strength steel hot stamping die, comprising:

[0008] Upper mold base and lower mold base;

[0009] The upper mold insert is fixed to the lower surface of the upper mold base;

[0010] The lower mold insert is fixed to the upper surface of the lower mold base. The upper mold insert and the lower mold insert together form a cavity that matches the shape of the part when the mold is closed.

[0011] The main clamping mechanism is used to drive the upper mold base and the upper mold insert to perform mold closing, pressure holding and mold opening actions relative to the lower mold base;

[0012] A pre-clamping mechanism is provided on the upper mold base and the upper mold insert, and is used to apply a pre-clamping force to the sheet material located in the edge area of ​​the cavity before the main clamping mechanism is fully closed;

[0013] The timing control mechanism is linked with the main clamping mechanism and the pre-clamping mechanism. The timing control mechanism is configured to trigger the pre-clamping mechanism in the first stage of the mold closing stroke, and control the main clamping mechanism to apply at least two different holding pressures during the mold closing and pressure holding process.

[0014] In a preferred embodiment of the present invention, the pre-tightening mechanism includes: a hydraulic cylinder, a clamping head, and a guide sleeve;

[0015] The cylinder body of the hydraulic cylinder is fixed to the upper surface of the upper mold base, and its piston rod extends vertically downward through the through hole opened in the upper mold base and the upper mold insert; the pressing head is fixedly connected to the end of the piston rod and located below the upper mold insert, and is used to directly press the edge of the sheet metal; the guide sleeve is disposed in the through hole on the upper mold insert and slides with the piston rod.

[0016] In a preferred embodiment of the present invention, the timing control mechanism includes: a pre-clamping trigger switch, a telescopic contact rod, a main clamping pressure switching valve, and a delay valve;

[0017] The pre-clamping trigger switch is a mechanical limit switch, installed on the side of the lower mold base. The telescopic contact rod is fixed to the bottom of the upper mold base. When the upper mold base descends to a preset height, the telescopic contact rod touches the pre-clamping trigger switch to activate the pre-clamping mechanism. The main clamping pressure switching valve is connected in the hydraulic circuit of the main clamping mechanism and is used to switch the output pressure of the main clamping mechanism between a first holding pressure and a second holding pressure, wherein the second holding pressure is greater than the first holding pressure. The delay valve is connected to the control terminal of the main clamping pressure switching valve and is used to control the main clamping pressure switching valve to switch from the first holding pressure to the second holding pressure after the mold is closed and a preset delay has elapsed.

[0018] In a preferred embodiment of the present invention, the timing control mechanism is further configured to: control the hydraulic cylinder to depressurize at the same time as or after the delay valve is activated, so that the pressing head retracts under the action of the return mechanism.

[0019] In a preferred embodiment of the present invention, the lower surface of the pressing head is a plane or a curved surface that matches the shape of the edge of the sheet metal; as the main pressing mechanism continues to descend, the pressing head maintains a constant pressing force on the edge of the sheet metal through the floating or following function of the hydraulic cylinder.

[0020] In a preferred embodiment of the present invention, a cooling system is further included, the cooling system comprising: a cooling water channel, an inlet main pipe, a return main pipe, and an electromagnetic control valve;

[0021] The cooling water channel is formed inside the upper mold insert and / or the lower mold insert; the main water inlet pipe and the main water return pipe are connected to the cooling water channel; the electromagnetic control valve is on the main water inlet pipe and is used to control the flow of the cooling medium.

[0022] In a preferred embodiment of the present invention, the timing control mechanism further includes a cooling start / stop switch, which is electrically connected to the electromagnetic control valve and is used to start cooling when the mold is closed.

[0023] In a preferred embodiment of the present invention, the distance between the cooling water channel of the upper mold insert and the lower mold insert and the cavity surface is 8-15 mm, and the diameter of the water channel is 6-10 mm.

[0024] Secondly, the present invention provides a method for using an ultra-high strength steel hot stamping die, comprising the following steps:

[0025] S1. Transfer the ultra-high strength steel plate material heated to the austenitizing temperature and place it on the lower mold insert;

[0026] S2. Start the main clamping mechanism to drive the upper die base downward. When the upper die base moves down to a distance of 50-80 mm from the lower die base, the timing control mechanism triggers the pre-clamping mechanism to clamp the edge area of ​​the sheet metal with a pressure of 0.5-1.0 MPa.

[0027] S3. The main clamping mechanism continues to descend until the mold is fully closed, so that the sheet metal is formed in the cavity. At the same time, the timing control mechanism controls the main clamping mechanism to apply the first holding pressure and starts the cooling system to cool the mold.

[0028] S4. Maintain the first holding pressure for the first 4-6 seconds during the pressure holding and cooling process;

[0029] S5. After holding pressure and cooling for 4-6 seconds, the timing control mechanism controls the main pressing mechanism to switch the holding pressure from the first holding pressure to the second holding pressure, wherein the second holding pressure is greater than the first holding pressure.

[0030] S6. After the total holding and cooling time reaches 8-12 seconds, stop cooling, open the mold and remove the formed part.

[0031] In a preferred embodiment of the present invention, the first holding pressure is 800-1000 kN and the second holding pressure is 1200-1500 kN; the timing for switching the holding pressure in step S5 is set according to the time point when the plate temperature drops below 300 ℃ or when the martensitic phase transformation is basically completed.

[0032] This invention addresses the shortcomings of the prior art and has the following beneficial effects:

[0033] (1) The present invention provides a hot stamping die for ultra-high strength steel and its usage method. By setting a pre-clamping mechanism, the timing control mechanism triggers the pre-clamping action before the main clamping mechanism is fully closed, and applies a pre-clamping force to the sheet metal in the edge area of ​​the cavity. This can prevent the sheet metal from shifting position and the edge from leaving the mold surface, effectively avoiding edge contour size difference, flange area folding or wrinkling caused by sheet metal movement. At the same time, it can also prevent the weld from deviating from the design position for welded plates, thereby providing a stable geometric boundary for suppressing springback in the subsequent pressure holding stage, and improving the edge forming quality and springback control.

[0034] (2) The present invention controls the main clamping mechanism to apply at least two specific pressures during the mold closing and pressure holding process through a timing control mechanism. The lower first pressure and the higher second pressure can avoid the inability of the traditional constant pressure holding method to take into account the volume change requirements. The present invention uses a segmented pressure holding strategy to flexibly accommodate the volume increment during the expansion stage and rigidly follow and fit during the contraction stage, effectively suppressing the final springback of the part. At the same time, since the pre-clamping has ensured the accurate edge position, the segmented pressure holding further locks the edge geometric accuracy, thereby improving the springback suppression and edge control effect. Attached Figure Description

[0035] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments recorded in the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0036] Figure 1 This is a half-sectional structural diagram of the hot stamping die of the present invention in the open state;

[0037] Figure 2 This is a schematic diagram of the half-section structure and the enlarged part of the upper mold base and upper mold insert of the present invention;

[0038] Figure 3 This is a half-sectional structural diagram of the hot stamping die of the present invention in the closed state;

[0039] In the diagram: 1. Upper mold base; 11. Upper mold insert; 2. Lower mold base; 21. Lower mold insert; 3. Cavity; 4. Hydraulic cylinder; 41. Piston rod; 42. Pressing head; 43. Guide sleeve; 5. Pre-pressing trigger switch; 6. Telescopic contact rod; 7. Cooling water channel; 8. Inlet water main pipe; 9. Return water main pipe. Detailed Implementation

[0040] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0041] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein. Therefore, the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0042] In the description of this application, it should be understood that the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Therefore, a feature defined with "first," "second," etc., may explicitly or implicitly include one or more of that feature. In the description of this invention, unless otherwise stated, "a plurality of" means two or more.

[0043] In the description of this application, it should be noted that, unless otherwise expressly specified and limited, the terms "installation," "connection," and "linking" should be interpreted broadly. For example, they can refer to fixed connections, detachable connections, or integral connections; they can refer to mechanical connections or electrical connections; they can refer to direct connections or indirect connections through an intermediate medium; and they can refer to the internal communication between two components. Those skilled in the art will understand the specific meaning of the above terms in this application based on the specific circumstances.

[0044] Application Overview:

[0045] During the in-mold quenching process of ultra-high strength steel, the austenite-to-martensite phase transformation induces volume expansion, with an expansion rate of approximately 2-4%. However, in the later cooling stage, as the overall temperature of the part drops to near room temperature, significant thermal shrinkage occurs. Traditional hot stamping dies apply only a constant closing pressure throughout the holding pressure stage. The applicant has found that the constant pressure holding method has shortcomings during the phase transformation expansion stage:

[0046] If the mold constraint force is too large, the volume expansion cannot be released, resulting in excessive compressive stress inside the part, which may even cause local expansion and cracking of the mold or crushing of the part. If the constraint force is insufficient, the part will undergo slight expansion deformation driven by volume expansion, causing local areas of the cavity to bear additional tensile stress and changing the shape of the part. In the subsequent heat shrinkage stage, the volume of the part decreases, and a small gap is generated between it and the cavity. As a result, it cannot actively follow the shrinkage process, causing the part to shrink freely in a state of lack of constraint, resulting in a large amount of springback.

[0047] To address the aforementioned problems, this invention proposes an ultra-high strength steel hot stamping die and its usage method. Before die closing, a pre-clamping mechanism pre-fixes the edges of the sheet metal to prevent movement and warping. During the holding pressure process, a timing control mechanism achieves segmented pressure switching between low-pressure and high-pressure holding. Specifically, a lower first holding pressure is used during the phase transformation expansion stage, allowing for slight elastic deformation of the cavity to absorb volume expansion. During the heat shrinkage stage, the pressure automatically switches to a higher second holding pressure, forcing the upper die insert to press down slightly further, tightly fitting the shrunken part, thereby eliminating shrinkage gaps, effectively suppressing part springback, and avoiding sheet metal edge defects, thus improving the dimensional accuracy and forming quality of the hot stamped parts.

[0048] Example 1

[0049] like Figure 1As shown, a hot stamping die for ultra-high strength steel includes: an upper die base 1 and a lower die base 2; an upper die insert 11 fixed to the lower surface of the upper die base 1; a lower die insert 21 fixed to the upper surface of the lower die base 2, wherein the upper die insert 11 and the lower die insert 21 together form a cavity 3 matching the shape of the part in the mold-closed state; a main clamping mechanism for driving the upper die base 1 and the upper die insert 11 to perform mold-closing, pressure-holding, and mold-opening actions relative to the lower die base 2; a pre-clamping mechanism disposed on the upper die base 1 and the upper die insert 11 for applying a pre-clamping force to the sheet metal located in the edge area of ​​the cavity 3 before the main clamping mechanism fully closes the mold; and a timing control mechanism linked with the main clamping mechanism and the pre-clamping mechanism, wherein the timing control mechanism is configured to trigger the action of the pre-clamping mechanism in the first stage of the mold-closing stroke, and control the main clamping mechanism to apply at least two different holding forces during the mold-closing and pressure-holding processes.

[0050] It is understood that the core of this invention lies in integrating the pre-clamping mechanism and the timing control mechanism into the hot stamping die. Before the die is closed, the pre-clamping mechanism fixes the edge of the sheet metal in advance to prevent it from shifting and warping during the subsequent forming process. During the holding and cooling process, the timing control mechanism switches the holding pressure of the main clamping mechanism from a lower first holding pressure to a higher second holding pressure according to the phase transformation and shrinkage characteristics of ultra-high strength steel. This can work together to solve the problems of poor sheet metal edge forming quality and difficulty in suppressing part springback in traditional dies.

[0051] In this embodiment, the upper die insert 11 is fixed to the lower surface of the upper die base 1 by bolts or positioning pins, and its lower surface, i.e. the profile, is consistent with the shape of the lower surface of the part to be formed; correspondingly, the lower die base 2 is positioned directly below the upper die base 1, and the upper die base 1 can move up and down along the guide mechanism of the stamping machine. The lower die insert 21 is fixed to the upper surface of the lower die base 2 by bolts or positioning pins, and its upper surface, i.e. the profile, is consistent with the shape of the lower surface of the part to be formed; when the upper die base 1 drives the upper die insert 11 down to completely close with the lower die insert 21, the space between the two forms a cavity 3 that perfectly matches the final shape of the part.

[0052] Furthermore, the main clamping mechanism is the main slide of the stamping machine and its drive system, used to drive the upper die holder 1 to perform die closing, pressure holding, and die opening actions; such as Figure 1 and Figure 2 As shown, the cooling system includes cooling water channels 7 opened inside the upper mold insert 11 and the lower mold insert 21, a main water inlet pipe 8 and a main water return pipe 9 connected to the cooling water channels 7, and an electromagnetic control valve installed on the main water inlet pipe 8 for rapidly quenching the formed parts during the pressure holding stage.

[0053] In this embodiment, to address the issue of the sheet metal edges easily shifting and warping during mold closing, a pre-clamping mechanism is provided on the upper mold base 1 and the upper mold insert 11, such as... Figure 2As shown, the pre-tightening mechanism includes a hydraulic cylinder 4, a clamping head 42, and a guide sleeve 43. The cylinder body of the hydraulic cylinder 4 is fixed to the upper surface of the upper mold base 1 by bolts. The piston rod 41 of the hydraulic cylinder 4 extends vertically downward and passes through the through holes coaxially opened on the upper mold base 1 and the upper mold insert 11 in sequence. In order to ensure the vertical accuracy and sealing of the piston rod 41 movement, a guide sleeve 43 is press-fitted in the through hole of the upper mold insert 11, and the piston rod 41 and the guide sleeve 43 form a sliding fit.

[0054] It is understandable that the hydraulic cylinder 4 is controlled by an external hydraulic control system. The hydraulic control system is based on the power provided by the motor, uses a hydraulic pump to convert mechanical energy into pressure, pushes hydraulic oil, controls various valves to change the flow direction of hydraulic oil, thereby pushing the hydraulic cylinder 4 to make different strokes and different directions of action. The specific structure is a general standard part or a component known to those skilled in the art, which will not be described in detail here.

[0055] Furthermore, the clamping head 42 is fixed to the end of the piston rod 41 by means of threaded connection or flange connection, and the clamping head 42 can rise and retract to the bottom of the upper mold insert 11. When the clamping head 42 descends and extends, it is located below the upper mold insert 11. The lower surface of the clamping head 42 is a smooth plane to accommodate sheet metal with straight edges. In terms of position, the clamping head 42 is located directly above the edge flange area of ​​the cavity 3 in the mold width direction. When the sheet metal is placed on the lower mold insert 21, the edge pressing surface of the sheet metal that needs to be clamped is directly below the clamping head 42.

[0056] In this embodiment, the timing control mechanism includes a pre-clamping trigger switch 5, a telescopic contact rod 6, a main clamping pressure switching valve, and a delay valve; wherein, the pre-clamping trigger switch 5 is a mechanical limit switch, which is fixedly installed on the side of the lower mold base 2, and its trigger roller extends vertically upward; the telescopic contact rod 6 is fixed at the bottom of the upper mold base 1, and its position is aligned vertically with the roller of the pre-clamping trigger switch 5.

[0057] Furthermore, the main clamping pressure switching valve is a hydraulically controlled directional valve, connected in series in the control oil circuit of the main hydraulic cylinder of the stamping machine tool. The main clamping pressure switching valve has two preset pressure output ports. The first output port corresponds to the first holding pressure, and the second output port corresponds to the second holding pressure, wherein the second holding pressure is greater than the first holding pressure. The delay valve is a pneumatic or hydraulic delay device. Its input end is connected to the signal that the main slide has closed to the mold position, such as a pressure relay or another limit switch. Its output end is hydraulically connected to the control end of the main clamping pressure switching valve. The delay time of the delay valve can be adjusted by a manual knob.

[0058] The working process of this embodiment will be explained in detail below with reference to the above structure:

[0059] S1. Transfer the ultra-high strength steel plate material heated to the austenitizing temperature and place it on the lower mold insert 21.

[0060] S2. Start the main clamping mechanism, that is, the main slide of the stamping machine tool drives the upper die holder 1 to start descending. When the upper die holder 1 descends to a distance of 50-80 mm from the lower die holder 2, the telescopic contact rod 6 fixed at the bottom of the upper die holder 1 contacts the roller of the pre-clamping trigger switch 5 installed on the side of the lower die holder 2. The contact action causes the circuit inside the pre-clamping trigger switch 5 to close and send an electrical signal. This signal controls the hydraulic reversing valve of the oil cylinder 4 to open. The high-pressure hydraulic oil then enters the rodless chamber of the oil cylinder 4, driving the piston rod 41 to drive the clamping head 42 to extend downward. The clamping head 42 first contacts the edge flange area of ​​the sheet metal and presses it against the corresponding surface of the lower die insert 21 with a pressure of 0.5-1.0 MPa, so that the edge of the sheet metal is fixed.

[0061] Furthermore, the main slider did not stop and continued to descend. During the process, the hydraulic circuit of the cylinder 4 was switched to a floating state, so that the piston rod 41 could be passively retracted upward according to the relative position change between the upper mold insert 11 and the sheet metal, but the internal hydraulic pressure remained constant, thereby ensuring that the pressing head 42 always maintained a constant pressing force on the edge of the sheet metal, thus preventing the sheet metal from moving around and not damaging the sheet metal due to excessive squeezing.

[0062] S3. The main slider continues to descend, causing the upper mold insert 11 and the lower mold insert 21 to complete mold closing (e.g., Figure 3 As shown, the sheet metal is stamped into the final part shape within the closed cavity 3. At the moment the mold closes, an independent mold closing signal, such as that issued by the pressure relay of the main hydraulic circuit, is sent to the electromagnetic control valve of the cooling system. The electromagnetic control valve immediately opens, and cooling water enters the cooling water channel 7 inside the upper mold insert 11 and the lower mold insert 21 through the water inlet main pipe 8, starting the in-mold quenching and cooling of the sheet metal. The timing control mechanism controls the main clamping mechanism to apply the first holding pressure F1, which is between 800-1000 kN. This holding pressure is sufficient to maintain the tight closure of the mold, while reserving a small elastic space for the cavity 3 to accommodate the upcoming material phase change volume expansion.

[0063] S4. In the first 4-6 seconds of holding and cooling, the sheet temperature drops rapidly from 900 ℃ to about 300 ℃. This temperature range is the main range where the austenite to martensite transformation occurs, accompanied by a 2-4% volume expansion. Since the main clamping mechanism applies a relatively low first holding pressure F1 at this time, the mold closing stiffness is relatively soft. When the sheet expands in volume, the huge internal stress generated inside will force the upper mold insert 11 and the lower mold insert 21 to produce a small amount of elastic deformation, thereby effectively absorbing the volume increase brought about by the phase transformation expansion. This can avoid the pressure inside the cavity 3 from rising sharply due to excessive rigid constraint, which could damage the mold or crush the parts. At the same time, the clamping head 42 of the pre-clamping mechanism continues to maintain constant clamping on the edge of the sheet during this stage, ensuring that the edge will not detach from the mold surface and warp during the expansion process.

[0064] S5. From the moment the mold is closed and pressure holding begins, the delay valve starts timing. After 4-6 seconds of pressure holding and cooling, the delay valve outputs a control oil pressure, which drives the main clamping pressure switching valve to switch the pressure supply of the main hydraulic cylinder from the first output port to the second output port. The holding pressure applied by the main clamping mechanism then switches from F1 to the second edge F2, with a range of 1200-1500 kN. At this time, the temperature of the sheet metal has dropped below 300 ℃, the martensitic phase transformation is basically completed, and the part enters the thermal shrinkage stage.

[0065] Furthermore, as the temperature continues to drop, the part begins to shrink, inevitably creating a tiny gap between it and the surface of the cavity 3. At this point, the suddenly increased second holding pressure F2 forces the upper mold insert 11 to move slightly downwards, closely fitting against the shrinking part surface. This actively compensates for the gaps caused by shrinkage, thereby ensuring that the part is always precisely constrained by the cavity 3 throughout the entire cooling process from high temperature to low temperature, thus minimizing the deviation between the final shape and the theoretical surface.

[0066] S6. After the total holding and cooling time reaches 8-12 seconds, the sheet temperature has dropped below 200℃, which is lower than the martensitic transformation completion temperature, and the microstructure transformation is complete. At this time, the cooling system closes the electromagnetic control valve according to the instruction of the timing control mechanism to stop cooling. The main slide moves the upper die holder 1 upward to open the mold. At the same time, the hydraulic circuit of the oil cylinder 4 is switched to the return state, and the piston rod 41 drives the pressing head 42 to retract upward to the initial position. Finally, the formed part is taken out by the robot or manually, completing a complete stamping cycle.

[0067] Example 2

[0068] This embodiment further optimizes and supplements the timing control mechanism based on Embodiment 1. In Embodiment 1, the clamping head 42 of the pre-clamping mechanism maintains a clamping force of 0.5-1.0 MPa on the edge of the sheet metal throughout the entire pressure holding process. However, for some parts with complex edge shapes or extremely high surface quality requirements, prolonged pressure holding by the clamping head 42 may leave slight indentations on the surface of the part. To solve this problem, this embodiment provides an improved timing control strategy, namely, controlling the hydraulic cylinder 4 to retract after the high-pressure holding stage begins.

[0069] Specifically, the timing control mechanism, based on the pre-tightening trigger switch 5, the main tightening pressure switching valve, and the delay valve, adds a pressure relief control branch connected to the hydraulic circuit of the oil cylinder 4. This pressure relief control branch is controlled by the output signal of the delay valve. The key difference lies in step S5:

[0070] When the preset time of the delay valve is reached, its output is divided into two paths: one path controls the oil pressure to push the main clamping pressure switching valve, switching the holding pressure of the main clamping mechanism from F1 to F2; the other path simultaneously or with a slight delay acts on the pressure relief control branch of the cylinder 4, causing its reversing valve to operate. At this time, the rodless chamber of the cylinder 4 no longer receives pressurized oil, but is connected to the oil tank for pressure relief. Under the action of the return spring inside the cylinder 4 or the constant back pressure from the rod chamber, the clamping head 42 quickly retracts upward, releasing the clamping of the edge of the part. Subsequently, under a higher F2 pressure, the main clamping mechanism performs a comprehensive and powerful contraction and follow-up pressing on the part by the entire surface of the upper mold insert 11.

[0071] In this embodiment, by removing the clamping head 42 before the start of the high-pressure holding stage, the slight marks left on the surface of the part can be effectively eliminated. At the same time, after the part enters the heat shrinkage stage, the overall surface of the upper mold insert 11 is sufficient to provide uniform and strong clamping force. The withdrawal of the pre-clamping mechanism will not affect the effect of suppressing springback, thereby improving the process adaptability of the mold and the quality of the finished part.

[0072] Example 3

[0073] Based on Examples 1 and 2, this embodiment optimizes and supplements the arrangement of the cooling water channel 7 and its coordination with pressure switching.

[0074] Specifically, to achieve more uniform and efficient cooling of the sheet metal, especially during complex phase transition and shrinkage stages, the cooling channels 7 in this embodiment adopt conformal cooling. Specifically, the cooling channels 7 inside the upper mold insert 11 and the lower mold insert 21 are no longer limited to straight holes. They are processed by additive manufacturing (3D printing) technology. The distance between the channel and the surface of the cavity 3 is 8-15 mm, and the channel diameter is 6-10 mm. The direction of the cooling channels 7 conforms to the complex three-dimensional curved surface contour of the part. For example, in areas where heat is concentrated, such as corners, bulges, or deep grooves of the part, the cooling channels 7 are densified or close to the surface, thereby achieving rapid and uniform heat extraction.

[0075] Furthermore, the cooling start / stop switch not only controls the on / off of cooling water, but its signal also participates in the fine-tuning control of the main clamping pressure switching valve. For example, when the cooling water starts circulating, the temperature sensor or flow sensor arranged on the inlet main pipe 8 or return main pipe 9 monitors the cooling intensity in real time. This signal is fed back to the programmable timing controller. In this embodiment, it can be a simple industrial timer, which still does not require a complex PLC.

[0076] Furthermore, based on the preset cooling curve, the timing controller issues a brief pressure release command in the middle of the pressure holding cooling process, such as when the part temperature is expected to drop to the martensitic phase transformation point. This causes the main clamping pressure switching valve to briefly switch back to the first holding pressure F1, lasting for 0.5-1 seconds, before switching back to the second holding pressure F2. This brief pressure breathing action can further release the residual stress during the phase transformation expansion process, preventing the part from developing microcracks or rebounding due to stress concentration during subsequent shrinkage.

[0077] For example, its working process is as follows: after the mold closing and cooling starts, the main clamping mechanism first holds pressure with F1. When the cooling system runs for 3 seconds, the timing controller sends a pulse signal to control the main clamping pressure switching valve to temporarily reduce the holding pressure to 500 kN. After maintaining it for 0.5 seconds, it is then switched to F2 for high-pressure contraction.

[0078] The conformal cooling channel 7 in this embodiment can ensure that the sheet metal obtains a uniform temperature field during the phase transformation cooling stage, so that the martensitic transformation occurs synchronously throughout the entire part. This avoids additional thermal stress and uneven deformation caused by temperature differences. The pressure breathing setting can simulate the dynamic balance between the volume change of the material and the mold constraint during the phase transformation process, resulting in lower internal residual stress and higher dimensional stability of the final part. Therefore, it has extremely important value for manufacturing high-end automotive safety parts with extremely complex shapes or high dimensional accuracy requirements.

[0079] Example 4

[0080] This embodiment provides another specific implementation of the pre-compression mechanism, particularly for sheet metal with complex curved edges, such as the inner panel of a car door or a B-pillar with a three-dimensional flange edge.

[0081] The pressing head 42 described in Examples 1-3 has a flat lower surface, which is suitable for pressing the edge of a sheet material that is flat or has little curvature variation. However, for sheet materials with complex three-dimensional curvature at the edge, a flat pressing head cannot achieve uniform and stable pre-pressing, which may lead to excessive local pressure or failure to fit effectively.

[0082] To solve this problem, this embodiment improves the pre-pressing mechanism. The overall mold structure based on this embodiment is the same as that of the previous embodiment, except that the shape of the pressing head 42 and the linkage method with the oil cylinder 4 are different.

[0083] Specifically, the lower surface of the pressing head 42 is a curved surface that matches the three-dimensional shape of the edge area of ​​the sheet metal. It is reverse-processed based on the negative shape of the corresponding edge area in the part's digital model. For example, if the edge of the sheet metal is an upward-curving flange, then the lower surface of the pressing head 42 will be correspondingly machined with a groove so that it can perfectly wrap around and fit the flange when pressing.

[0084] Furthermore, in order to adapt to complex contact surfaces, the connection between the piston rod 41 of the hydraulic cylinder 4 and the clamping head 42 can adopt a ball joint or universal joint structure, so that the clamping head 42 can adaptively swing slightly when it contacts the sheet metal, so as to ensure that its lower curved surface and the edge curved surface of the sheet metal achieve maximum area contact, thereby distributing the pre-clamping force evenly on the edge of the curved surface.

[0085] Furthermore, the timing control logic of this embodiment has also been adjusted. Since the curved surface of the pressing head 42 fits the edge of the sheet metal very precisely, in order to avoid interference or collision during pre-pressing, the triggering time of the pre-pressing trigger switch 5 is set earlier. When the upper mold base 1 descends to a distance of 280-100 mm from the lower mold base, the pre-pressing action is started, which allows the pressing head 42 to have more time to extend at a slower speed and sit on the curved edge of the sheet metal.

[0086] This embodiment can greatly expand the application scope of the mold of the present invention. For sheet metal with complex three-dimensional curved edges that are difficult to handle by traditional molds, the curved pressing head 42 provided in this embodiment, in combination with ball joint connection, can realize conformal pre-pressing. This not only prevents sheet metal from shifting and warping, but also ensures that the material flow of complex edges is controlled during the forming process, thereby avoiding local thinning or cracking caused by uneven pressing. It can provide a technical basis for applying hot stamping technology to more complex and lighter automotive structural parts and body panels.

[0087] Example 5:

[0088] This embodiment provides a verification experiment of the ultra-high strength steel hot stamping die as described in any one of Embodiments 1-4, to illustrate the technical effects of the present invention. The material used in the experiment is 22MnB5 ultra-high strength steel sheet, whose chemical composition by mass percentage is: C: 0.22-0.25%, Si: 0.20-0.30%, Mn: 1.20-1.40%, Cr: 0.15-0.25%, B: 0.002-0.005%, Ti: 0.03-0.05%, with the balance being Fe and unavoidable impurities. The sheet thickness is 1.5 mm. The above sheet is cut into rectangular pieces with dimensions of 300 mm × 200 mm, and the target part for hot stamping is a simplified model of a B-pillar reinforcement with a trapezoidal cross-section and edge flanges.

[0089] Furthermore, the main clamping mechanism is a hydraulic hot stamping machine with a nominal pressure of 5000 kN, and is equipped with an ultra-high strength steel hot stamping die as described in Example 1; wherein, the pre-clamping mechanism includes two sets of symmetrically arranged oil cylinders 4 and clamping heads 42, the lower surface of the clamping heads 42 is a plane; the cooling water channel 7 of the cooling system is 12 mm away from the surface of the cavity 3, and the water channel diameter is 8 mm; the delay valve of the timing control mechanism is an adjustable pneumatic delay device.

[0090] It should be noted that all experimental examples and comparative examples were performed according to the following basic process steps, with differences only made to the key process parameters in each step.

[0091] The basic process includes the following steps:

[0092] S1. Place the 22MnB5 steel plate in an atmosphere-protected heating furnace, heat it to 930℃, hold it for 5 minutes to make the plate completely austenitized, and then transfer the plate to the lower mold insert 21 of the mold.

[0093] S2. The main slide of the stamping machine drives the upper die holder 1 to descend. When the upper die holder 1 descends to 70 mm, it triggers the pre-clamping mechanism to clamp the edge flange area of ​​the sheet metal with the set pre-clamping pressure.

[0094] S3. The main slide continues to descend until the mold is fully closed. The sheet metal is formed in cavity 3. At the moment the mold is closed, the cooling system is activated. Cooling water at 20°C begins to circulate at a flow rate of 30 L / min. At the same time, the main clamping mechanism applies the set first holding pressure F1.

[0095] S4, maintain the first holding pressure F1 for 5 seconds.

[0096] S5. After holding pressure and cooling for 5 seconds, according to the settings, the timing control mechanism switches the holding pressure of the main pressing mechanism from the first holding pressure F1 to the second holding pressure F2, or keeps it unchanged.

[0097] S6. After a total holding and cooling time of 10 seconds, cooling is stopped, and the main slide moves the upper mold base 1 upward to open the mold and remove the formed part.

[0098] Experimental Example 1

[0099] This experimental example uses the standard working mode and basic process steps as described in Example 1, with the difference being:

[0100] S2, the hydraulic cylinder 4 drives the clamping head 42 to clamp the edge of the sheet metal with a pressure of 0.8 MPa;

[0101] S3. After the mold is closed, the main clamping mechanism applies a first holding pressure F1 of 935 kN.

[0102] S5. After holding pressure and cooling for 5 seconds, the timing control mechanism switches the holding pressure of the main pressing mechanism to the second holding pressure F2 of 1310 kN. The pressing head 42 remains pressed throughout the entire holding pressure process.

[0103] Experiment Example 2

[0104] This experimental example is basically the same as Experiment 1. The difference is that the steps in S5 are as follows: after holding the pressure and cooling for 5 seconds, the timing control mechanism sends two signals at the same time: the first signal controls the main clamping pressure switching valve to switch the holding pressure from 935 kN to 1350 kN; the second signal controls the pressure relief branch of the oil cylinder 4, so that the clamping head 42 immediately retracts under the action of the return spring, releasing the clamping on the edge of the part.

[0105] Experimental Example 3

[0106] This experimental example is basically the same as Experiment 1, except that the pre-compression pressure is different. The specific steps of S2 are: the oil cylinder 4 drives the clamping head 42 to press the edge of the plate with a pressure of 0.5 MPa.

[0107] Experiment Example 4

[0108] This experimental example is basically the same as Experiment 1, except that the pre-compression pressure is different. The specific steps of S2 are: the oil cylinder 4 drives the clamping head 42 to press the edge of the plate with a pressure of 1.0 MPa.

[0109] Experimental Example 5

[0110] This experimental example is basically the same as Experiment 1, except that the first holding pressure is different. The specific steps of S3 are as follows: after the mold is closed, the main clamping mechanism applies the first holding pressure F1 of 800 kN.

[0111] Experimental Example 6

[0112] This experimental example is basically the same as Experiment 1, except that the first holding pressure is different. The specific steps of S3 are as follows: after the mold is closed, the main clamping mechanism applies the first holding pressure F1 of 1000 kN.

[0113] Comparative Example 1

[0114] This comparative example is basically the same as Experimental Example 1, except that no pre-compression action was performed, that is, the pre-compression head 42 remained in the retracted state throughout the entire process.

[0115] Comparative Example 2

[0116] This comparative example is basically the same as Experimental Example 1, except that the pre-compression pressure is different. The specific steps of S2 are: the hydraulic cylinder 4 drives the clamping head 42 to press the edge of the sheet material with a pressure of 0.4 MPa.

[0117] Comparative Example 3

[0118] This comparative example is basically the same as Experimental Example 1, except that the pre-compression pressure is different. The specific steps of S2 are: the hydraulic cylinder 4 drives the clamping head 42 to press the edge of the sheet material with a pressure of 1.15 MPa.

[0119] Comparative Example 4

[0120] This comparative example is basically the same as Experimental Example 1. The difference is that segmented pressure holding is not performed. Constant pressure and low pressure are used. The specific steps of S3 are: after the mold is closed, the main clamping mechanism applies a holding pressure of 935 kN; the specific steps of S5 are: after holding pressure and cooling for 5 s, the holding pressure is maintained at 935 kN, and the clamping head 42 keeps the pressure pressed throughout the entire holding pressure process.

[0121] Comparative Example 5

[0122] This comparative example is basically the same as Experimental Example 1. The difference is that segmented pressure holding is not performed. Constant pressure and high pressure are used. The specific steps of S3 are: after the mold is closed, the main clamping mechanism applies a holding pressure of 1310 kN; the specific steps of S5 are: after holding pressure and cooling for 5 seconds, the holding pressure is maintained at 1310 kN, and the clamping head 42 keeps the pressure pressed throughout the entire holding pressure process.

[0123] Comparative Example 6

[0124] This comparative example is basically the same as Experimental Example 1. The difference is that the first holding pressure is different. The specific steps of S3 are: after the mold is closed, the main clamping mechanism applies the first holding pressure F1 of 750 kN.

[0125] Comparative Example 7

[0126] This comparative example is basically the same as Experimental Example 1. The difference is that the first holding pressure is different. The specific steps of S3 are as follows: after the mold is closed, the main clamping mechanism applies the first holding pressure F1 of 1050 kN.

[0127] Performance testing: The formed parts obtained in Experimental Examples 1-6 and Comparative Examples 1-7 were subjected to performance tests on springback amount and edge defect rate, respectively. The results are shown in Table 1.

[0128] Springback amount: The entire surface of the part is scanned using the ATOS Core 3D optical scanner to obtain point cloud data. The scan data is then best fitted and aligned with the CAD theoretical model of the target part. Five measurement points are selected at the bottom center and the middle of the side wall of the trapezoidal part to measure the absolute value of the deviation between the actual position and the theoretical position. The average value of the deviation of the five points is taken as the springback amount of the part.

[0129] Edge defect rate: Visually inspect the flange area at the edge of the part and use a surface defect measuring instrument to count the total length of the defect area with obvious wrinkles, folds or material stacking, and calculate its percentage of the total perimeter of the part edge (1000 mm in this example), as the edge defect rate.

[0130] Table 1:

[0131] Performance testing Preload pressure (MPa) F1 (kN) F2 (kN) Springback (mm) Edge defect rate (%) Experimental Example 1 0.8 935 1310 0.15 2.1 Experiment Example 2 0.8 935 1310 0.19 1.8 Experimental Example 3 0.5 935 1310 0.18 3.5 Experiment Example 4 1.0 935 1310 0.14 2.6 Experimental Example 5 0.8 800 1310 0.20 3.1 Experimental Example 6 0.8 1000 1310 0.18 2.9 Comparative Example 1 / 935 1310 0.42 15.8 Comparative Example 2 0.4 935 1310 0.38 11.2 Comparative Example 3 1.15 935 1310 0.25 8.5 Comparative Example 4 0.8 935 / 0.88 3.8 Comparative Example 5 0.8 1310 / 0.65 4.2 Comparative Example 6 0.8 750 1310 0.41 3.5 Comparative Example 7 0.8 1050 1310 0.39 3.2

[0132] As shown in Table 1:

[0133] A comparison between Experiment 1 and Experiment 2 shows that in Experiment 2, the clamping head 42 was retracted before the start of the high-pressure shrinkage stage, resulting in a springback of 0.19 mm and an edge defect rate of 1.8%, which is basically the same as in Experiment 1. This is because before the start of the high-pressure shrinkage stage, the sheet temperature had already dropped below 300 ℃, and the martensitic phase transformation was basically completed. At this time, the part already had sufficient strength and the edge area had been fully formed. Removing the clamping head 42 would not cause edge instability or increased springback. At the same time, retracting the clamping head 42 would eliminate any slight indentations that might be left on the surface of the part, which would help improve the surface quality. The results indicate that the hot stamping die of the present invention can flexibly select the timing of the clamping head 42 according to the shape and surface quality requirements of the part without affecting the suppression of springback and edge defects.

[0134] A comparison between Experimental Example 1 and Comparative Example 1 reveals that Comparative Example 1, which did not undergo pre-clamping, exhibited a springback of 0.42 mm and an edge defect rate of 15.8%. During mold closing, the sheet metal was at a high temperature with extremely low yield strength. Furthermore, the surface of the lower mold insert 21 was smooth, resulting in a low coefficient of friction between the sheet metal and the mold. Without pre-clamping force, the sheet metal was prone to lateral movement under the downward impact of the upper mold, causing the edge contour of the part to deviate from its designed position. Simultaneously, the edge of the sheet metal might be lifted upwards due to airflow or contact impact during mold closing, and subsequently pressed into the cavity 3 after mold closing, forming folds or wrinkles. The movement also caused the overall position of the part to shift, leading to an increase in springback during the subsequent heat shrinkage stage.

[0135] A comparison of Experimental Examples 1 and 3-4 with Comparative Examples 2-3 reveals that Experimental Examples 1 and 3-4, employing specific pre-compression pressures, achieve better springback and edge defect rates than all comparative examples without damaging the sheet metal. Comparative Example 2 uses excessively low pre-compression pressure, resulting in a springback of 0.38 mm and an edge defect rate of 11.2%. This insufficient pressure fails to overcome the inadequate friction between the sheet metal and the lower die, allowing the sheet metal to still shift under minor impacts. Furthermore, the contact pressure between the pressure head and the sheet metal edge is insufficient to suppress edge warping, causing some edge areas to detach from the lower die surface before mold closing. Comparative Example 3 uses excessively high pre-compression pressure, resulting in a springback of 0.25 mm and an edge defect rate of 8.5%. While this excessively high pre-compression pressure firmly secures the sheet metal, it creates localized indentations on the sheet metal surface and even causes minor plastic deformation at the sheet metal edges, altering the material flow characteristics of the edge areas and consequently affecting the final formed profile and increasing springback.

[0136] A comparison of Experiment 1 and Comparative Examples 4-5 reveals that: Comparative Example 4, employing a constant-pressure low-pressure holding strategy, exhibits a springback of 0.88 mm, while Comparative Example 5, employing a constant-pressure high-pressure holding strategy, shows a springback of 0.65 mm. This is because ultra-high-strength steel undergoes two stages during in-mold quenching: phase transformation expansion and thermal contraction. The constant-pressure low-pressure strategy allows for slight elastic deformation of the mold cavity 3 during the phase transformation expansion stage to absorb volume expansion. However, during the thermal contraction stage, the part begins to shrink after its temperature drops below 300 ℃. At this point, the low pressure cannot allow the upper mold insert 11 to move further downwards to fit the shrunken part, resulting in a gap between the part and the cavity 3. The part shrinks freely without constraint, ultimately leading to significant springback. The constant-pressure high-pressure strategy effectively compensates for the shrinkage gap during the thermal contraction stage. However, during the phase transformation expansion stage, excessively high holding pressure rigidly constrains volume expansion, causing excessive pressure stress inside the part, potentially leading to local crushing or mold damage. Furthermore, the inability to release expansion stress causes irreversible expansion deformation of the part's shape under expansion drive, also resulting in springback. This invention employs a segmented pressure-holding strategy, using a specific lower first pressure to absorb volume expansion during the phase change expansion stage, and automatically switching to a specific higher second pressure to actively adhere to the shrinking parts during the heat shrinkage stage. This simultaneously solves the size control problem in both stages, keeping the springback within 0.2 mm.

[0137] A comparison of Experimental Examples 1, 5-6, and Comparative Examples 6-7 reveals that Experimental Examples 1 and 5-6, by employing an appropriate first holding pressure, can induce a slight elastic deformation of approximately 0.05-0.10 mm in the mold cavity 3 during the phase transformation expansion stage. This precisely accommodates the 2-4% volume expansion increment, preventing a sharp increase in pressure within the cavity 3 while avoiding excessive expansion of the part due to weak constraint. Comparative Example 6 uses an excessively low first holding pressure, resulting in a springback of 0.41 mm and an edge defect rate of 3.5%. This low holding pressure fails to maintain the mold's closing stiffness, and under the stress of phase transformation expansion, the upper mold insert 11 may experience significant elastic springback, causing excessive increase in the volume of the cavity 3. The part undergoes significant expansion deformation under the expansion drive, deviating from its theoretical shape. Furthermore, the excessively low pressure may also lead to minute gaps on the mold parting surface, allowing cooling water to seep into the cavity 3 and affecting the uniformity of quenching. Comparative Example 7 uses an excessively high first holding pressure, resulting in a springback of 0.39 mm and an edge defect rate of 3.2%. Excessive holding pressure restricts phase transformation expansion, resulting in high residual compressive stress inside the part. The expansion cannot be effectively absorbed, which may lead to local material crushing or excessive internal expansion force on the mold, increasing the risk of mold cracking. At the same time, this over-constraint state will change the stress distribution of the part, causing additional deformation during the subsequent heat shrinkage stage and increasing the springback.

[0138] The above description is based on the preferred embodiments of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or essential characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects. The scope of the invention is defined by the appended claims rather than the foregoing description, and all variations falling within the meaning and scope of equivalents of the claims are intended to be included within the present invention. No reference numerals in the claims should be construed as limiting the scope of the claims.

[0139] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. An ultra-high strength steel hot stamping die, characterized by, include: Upper mold base and lower mold base; The upper mold insert is fixed to the lower surface of the upper mold base; The lower mold insert is fixed to the upper surface of the lower mold base. The upper mold insert and the lower mold insert together form a cavity that matches the shape of the part when the mold is closed. The main clamping mechanism is used to drive the upper mold base and the upper mold insert to perform mold closing, pressure holding and mold opening actions relative to the lower mold base; A pre-clamping mechanism is provided on the upper mold base and the upper mold insert, and is used to apply a pre-clamping force to the sheet material located in the edge area of ​​the cavity before the main clamping mechanism is fully closed; The timing control mechanism is linked with the main clamping mechanism and the pre-clamping mechanism. The timing control mechanism is configured to trigger the pre-clamping mechanism in the first stage of the mold closing stroke, and control the main clamping mechanism to apply at least two different holding pressures during the mold closing and pressure holding process.

2. The hot stamping die for ultra-high strength steel according to claim 1, characterized by The pre-tightening mechanism includes: a hydraulic cylinder, a clamping head, and a guide sleeve; The cylinder body of the hydraulic cylinder is fixed to the upper surface of the upper mold base, and its piston rod extends vertically downward through the through hole opened in the upper mold base and the upper mold insert; the pressing head is fixedly connected to the end of the piston rod and located below the upper mold insert, and is used to directly press the edge of the sheet metal; the guide sleeve is disposed in the through hole on the upper mold insert and slides with the piston rod.

3. The hot stamping die for ultra-high strength steel according to claim 2, characterized by The timing control mechanism includes: a pre-clamping trigger switch, a telescopic contact rod, a main clamping pressure switching valve, and a delay valve; The pre-clamping trigger switch is a mechanical limit switch, installed on the side of the lower mold base. The telescopic contact rod is fixed to the bottom of the upper mold base. When the upper mold base descends to a preset height, the telescopic contact rod touches the pre-clamping trigger switch to activate the pre-clamping mechanism. The main clamping pressure switching valve is connected in the hydraulic circuit of the main clamping mechanism and is used to switch the output pressure of the main clamping mechanism between a first holding pressure and a second holding pressure, wherein the second holding pressure is greater than the first holding pressure. The delay valve is connected to the control terminal of the main clamping pressure switching valve and is used to control the main clamping pressure switching valve to switch from the first holding pressure to the second holding pressure after the mold is closed and a preset delay has elapsed.

4. The hot stamping die for ultra-high strength steel according to claim 3, characterized by The timing control mechanism is further configured to control the hydraulic cylinder to depressurize at the same time as or after the delay valve is activated, so that the pressing head retracts under the action of the return mechanism.

5. The hot stamping die for ultra-high strength steel according to claim 2, wherein The lower surface of the clamping head is a plane or a curved surface that matches the shape of the sheet metal edge; as the main clamping mechanism continues to descend, the clamping head maintains a constant clamping force on the sheet metal edge through the floating or follow-up function of the hydraulic cylinder.

6. The hot stamping die of ultra-high strength steel according to claim 1, wherein, It also includes a cooling system, which comprises: a cooling water channel, an inlet main pipe, a return main pipe, and an electromagnetic control valve; The cooling water channel is formed inside the upper mold insert and / or the lower mold insert; the main water inlet pipe and the main water return pipe are connected to the cooling water channel; the electromagnetic control valve is on the main water inlet pipe and is used to control the flow of the cooling medium.

7. The hot stamping die of an ultra-high strength steel according to claim 6, characterized in that, The timing control mechanism also includes a cooling start / stop switch, which is electrically connected to the electromagnetic control valve and is used to start cooling when the mold is closed.

8. The hot stamping die of an ultra-high strength steel according to claim 6, wherein The distance between the cooling water channel of the upper mold insert and the lower mold insert and the cavity surface is 8-15 mm, and the diameter of the water channel is 6-10 mm.

9. A method of using an ultra-high strength steel hot stamping die based on any one of claims 1-8, characterized in that, Includes the following steps: S1. Transfer the ultra-high strength steel plate material heated to the austenitizing temperature and place it on the lower mold insert; S2. Start the main clamping mechanism to drive the upper die base downward. When the upper die base moves down to a distance of 50-80 mm from the lower die base, the timing control mechanism triggers the pre-clamping mechanism to clamp the edge area of ​​the sheet metal with a pressure of 0.5-1.0 MPa. S3. The main clamping mechanism continues to descend until the mold is fully closed, so that the sheet metal is formed in the cavity. At the same time, the timing control mechanism controls the main clamping mechanism to apply the first holding pressure and starts the cooling system to cool the mold. S4. Maintain the first holding pressure for the first 4-6 seconds during the pressure holding and cooling process; S5. After holding pressure and cooling for 4-6 seconds, the timing control mechanism controls the main pressing mechanism to switch the holding pressure from the first holding pressure to the second holding pressure, wherein the second holding pressure is greater than the first holding pressure. S6. After the total holding and cooling time reaches 8-12 seconds, stop cooling, open the mold and remove the formed part.

10. The method of using an ultra-high strength steel hot stamping die according to claim 9, characterized in that, The first holding pressure is 800-1000 kN, and the second holding pressure is 1200-1500 kN; the timing for switching the holding pressure in step S5 is based on the time when the plate temperature drops below 300 ℃ or when the martensitic phase transformation is basically completed.

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