Method for forming flow channels in bipolar plates based on variable roll gap multi-pass rolling

Abstract

The application belongs to the bipolar plate flow channel processing technical field, and discloses a bipolar plate flow channel forming method based on variable roll gap multi-pass rolling, which is characterized in that: firstly, a concave die and a convex die are respectively assembled on an upper rolling roller and a lower rolling roller; a metal slab is driven by a traction device to be rolled in multiple passes, wherein the roll gap of each subsequent pass is gradually reduced relative to the roll gap of the previous pass, and the rotation directions of the adjacent passes and the feeding direction of the metal slab are opposite; after the final pass rolling is completed, the bipolar plate flow channel with the required depth and shape is formed on the metal slab; the application has the advantages that: under the premise of ensuring the flow channel forming quality, the production cost and equipment investment are significantly reduced, the production efficiency and product flatness are improved, the quality-cost-efficiency is taken into account, and the economic benefit is greatly improved.

Description

Technical Field

[0001] This invention belongs to the field of bipolar plate flow channel processing technology, and relates to a bipolar plate flow channel forming method based on variable roll gap multi-pass rolling. Background Technology

[0002] Metal bipolar plates are the core component of proton exchange membrane fuel cells, and the densely distributed microchannels on them directly affect the cell's performance. To reduce the weight and cost of the fuel cell stack, bipolar plates are becoming increasingly thinner, with channel depth-to-width ratios reaching over 0.6, making their forming extremely difficult. Roll forming, due to its continuous production capability, high efficiency, and low forming load, is considered one of the most promising processes for the large-scale manufacturing of metal bipolar plates.

[0003] However, existing roll forming processes generally employ a fixed roll gap and a single-pass, one-time forming method, which has the following drawbacks: (1) In one forming process, the waist of the runner (the transition area between the rounded corners of the die and the punch) is subjected to bending and stretching deformation at the same time, forming a local high equivalent plastic strain concentration area. The thickness reduction rate is very likely to exceed the material forming limit, resulting in toughness damage and developing into macroscopic cracks. (2) All deformation is completed within seconds, local strain concentration cannot be relieved, the material has no chance to redistribute, and the risk of cracking is further aggravated.

[0004] To address the aforementioned problems, existing technologies mainly employ two improvement schemes, each with significant drawbacks: The first is multi-pass progressive rolling, which uses multiple sets of dies with different geometric parameters for separate forming, decomposing the total deformation. While this method can reduce the deformation per pass, it requires manufacturing a dedicated die for each pass, resulting in high die costs, large production line footprint, and poor flexibility. The second is multi-pass stamping, which utilizes the controllable motion curve of a servo press to achieve multi-pass forming. However, this method has high equipment costs, high tonnage requirements (typically ≥2500kN), and cannot support continuous production, leading to low efficiency.

[0005] In summary, existing deep runner forming processes for metal bipolar plates face a technical bottleneck where it is difficult to simultaneously achieve a balance between quality, cost, and efficiency. Therefore, there is an urgent need for a new forming process that can effectively avoid runner cracking, eliminate the need for mold replacement, and offer low cost and high efficiency. Summary of the Invention

[0006] The present invention aims to provide a bipolar plate flow channel forming method based on variable roll gap multi-pass rolling, so as to improve the forming quality of bipolar plates, reduce production costs, and improve production efficiency.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The bipolar plate flow channel forming method based on variable roll gap multi-pass rolling includes the following steps: S1. The die and punch for forming bipolar plate flow channels are respectively assembled on the upper and lower rolls of the wedge cross rolling mill. S2. Place the metal slab between the upper and lower rolls, set the roll gap to the first roll distance H1, control the rotation of the upper and lower rolls, and drive the metal slab to move along the first feed direction through the traction device to complete the first rolling pass. S3. After the first rolling pass is completed, the roll gap is reduced to the second roll gap H2, that is: H2 < H1. Then, the upper and lower rolls are controlled to rotate in opposite directions, and the metal slab moves along the second feed direction opposite to the first feed direction to complete the second rolling pass. S4. Following the method of step S3, at least one more subsequent rolling pass is performed, wherein the roll pitch of each subsequent pass gradually decreases relative to the roll pitch of the previous pass, and the rotation direction of the rolls and the feeding direction of the metal slab in adjacent passes are opposite, so that the metal slab undergoes multiple alternating progressive rolling on a single die. S5. After the final rolling pass is completed, bipolar plate channels of the required depth and shape are formed on the metal slab.

[0008] Furthermore, the number of rolling passes n≥2. When the target depth of the runner is ≤0.3mm, n=2; when the target depth of the runner is 0.3~0.5mm, n=3; when the target depth of the runner is >0.5mm, n≥4.

[0009] Furthermore, given the thickness of the metal slab as t, determine the first... i Roll gap H per pass i The decrease in roll gap between adjacent passes ΔH ≥ 0.05 mm, and the roll gap H of the final pass... n ≤1.2t.

[0010] Furthermore, a pause time of 0.1 to 2 seconds is set between the end of each rolling pass and the beginning of the next pass for adjusting the roll gap and relaxing the stress on the metal slab.

[0011] Furthermore, the tension applied by the traction device at both ends of the metal slab is 5% to 15% of the yield strength of the metal slab.

[0012] Furthermore, the rotational speed of the upper and lower rolls is 1 to 10 rpm.

[0013] Furthermore, the aspect ratio of the bipolar plate channel is ≥0.6.

[0014] Compared with the prior art, the advantages of the present invention are: (1) This invention decomposes the traditional single-pass one-time forming process into a progressive deformation process with multiple passes of decreasing roll gap and alternating rolling direction, so that the local equivalent plastic strain of the runner waist accumulates step by step along each pass. The strain increment of each pass is lower than the forming limit of the material. At the same time, the material flow redistribution between passes and the beneficial Bauschinger effect and strain path switching introduced by the reversal of rolling direction effectively reduce the stress triaxiality and damage accumulation rate, fundamentally avoiding cracking of the runner waist due to excessive thinning, and significantly improving the yield. (2) The present invention adopts small reduction and multi-pass reciprocating rolling. The peak rolling force of each pass is much lower than that of single-pass one-time forming. The overall load distribution is stable, and the requirements for mill rigidity, motor power and transmission system are significantly reduced. Thus, smaller tonnage rolling equipment can be used, greatly reducing equipment manufacturing costs and energy consumption, and significantly reducing production costs. (3) The present invention adopts a combination of reverse rolling and progressive reduction of roll gap, so that the buckling amount of material in each pass is limited to a small range. At the same time, reverse rolling activates reverse dislocation movement and reduces residual stress. Subsequent passes have a flattening effect on plate shape. Excellent flatness is conducive to the precise assembly of bipolar plates and membrane electrode assembly, reducing contact resistance and improving the overall performance and reliability of fuel cell. (4) This invention requires only one set of flow channel rolling die. By gradually reducing the gap between the rolls and switching the rolling direction and the feed direction, the same set of die can achieve the progressive forming effect equivalent to multiple sets of dies with different geometric parameters, saving more than 60% of the die manufacturing cost. At the same time, it reduces the die change time and production line floor area, significantly improving production flexibility. Attached Figure Description

[0015] Figure 1 This is a rolling process flow diagram of the present invention; Figure 2 This is a diagram illustrating the deformation process of the metal slab in this invention. Figure 3 This is a comparison diagram of the radial rolling force time history curves of the present invention and a comparative example; Figure 4 This is a comparison of the time history curves of the maximum equivalent plastic strain of the present invention and the comparative example; Figure 5 Equivalent plastic strain contour plots of the formed parts obtained by comparative rolling; Figure 6 This is an equivalent plastic strain contour plot of the rolled part obtained by the present invention. Figure 7 This is a comparison diagram of the flatness of the molded parts of the present invention and the comparative example. Detailed Implementation

[0016] The preferred embodiments of the present invention will now be described with reference to the accompanying drawings. It should be understood that the preferred embodiments described herein are for illustrative and explanatory purposes only and do not constitute a limitation thereof.

[0017] A bipolar plate flow channel forming method based on variable roll gap multi-pass rolling is used to form 0.1mm thick 316L stainless steel bipolar plates with a target flow channel depth of 0.5mm and a depth-to-width ratio of 0.8. The specific forming steps are as follows: S1. The concave die 1 and convex die 2 for forming bipolar plate flow channels are precisely assembled onto the upper roll 3 and lower roll 4 of the wedge cross rolling mill, respectively. The diameter of the upper roll 3 and lower roll 4 is 100mm. The geometric accuracy of the grooves on the concave and convex dies is ±0.005mm. The material is Cr12MoV tool steel with a hardness of HRC58~62. The metal slab 5 is made of 316L austenitic stainless steel with a thickness of 0.10mm and a width of 50~500mm. The length is as required by the downstream process and can be continuously supplied as a fixed-length slab or coil. The surface is clean and the flatness is ≤0.5%. S2. Set the gap between the upper roll 3 and the lower roll 4 to the first roll gap H1 = 0.30 mm. This gap is much larger than the thickness of the metal slab 5. Only the top tip of the punch 2 extends into the area of ​​the die 1. The upper roll 3 and the lower roll 4 rotate at a speed of 3 rpm. The metal slab 5 is placed between the upper roll 3 and the lower roll 4. The traction device clamps the metal slab 5 and feeds it from left to right (i.e., the first feed direction). A constant tension of about 10% of the slab yield strength is applied to the metal slab 5 to perform the first pass of rolling. In the first rolling pass mentioned above, after the metal slab 5 is bitten in, the deformation mode is mainly bending under a large gap of 0.30mm, and the flow channel only forms shallow grooves. This is the initial pre-forming stage of the flow channel, that is, the initial stage of strain accumulation decomposition, which triggers material flow redistribution. S3: After the first rolling pass is completed, the wedge mill pauses for 0.3s. During the pause, the metal slab 5 briefly leaves the contact area of ​​the rolls. The stress gradient between the deformed area and the undeformed area drives a small redistribution of the material, which initially alleviates the local strain concentration in the waist area. S4: The gap between the upper roll 3 and the lower roll 4 is precisely reduced to the second roll gap H2 = 0.20 mm, and the roll gap reduction between adjacent passes is ΔH = 0.10 mm (≥ 0.05 mm). The roll drive mechanism drives the upper roll 3 and the lower roll 4 to rotate in opposite directions at a speed of 3 rpm. The traction device clamps the metal slab 5 and feeds it from right to left (i.e., the second feed direction), and applies a constant tension of about 10% of the slab's yield strength to the metal slab 5 for the second pass rolling. This is the intermediate depth forming stage of the flow channel, where strain accumulation continues and material flow redistribution deepens. S5: After the second rolling pass is completed, the wedge mill pauses again for 0.3 seconds; S6: Reduce the gap between the upper roll 3 and the lower roll 4 to the final value H. n =0.10mm, punch 2 is fully embedded in die 1, upper roll 3 and lower roll 4 rotate in opposite directions again at a speed of 3rpm, traction device clamps metal slab 5 and resumes feeding from left to right and applies a constant tension of about 10% of the slab yield strength to metal slab 5, and performs the third rolling pass on metal slab 5 to form a bipolar plate flow channel with a depth of 0.5mm; this is the final shaping stage of the flow channel. The main function of this pass is no longer large plastic flow, but to complete the final geometric shaping and precision calibration of the flow channel.

[0018] The core principle of the forming method of this invention lies in the following: by decreasing the roll gap with each pass, the total equivalent plastic strain required to complete the final flow channel is decomposed along the pass direction (strain accumulation decomposition mechanism), so that the local strain increment of each pass is lower than the material forming limit; at the same time, in conjunction with the reversal of the rolling direction, the Bauschinger effect and strain path switching are used to activate more slip systems, promoting dislocation rearrangement and plastic recovery inside the material; the short pauses between passes allow the material to undergo gradual flow redistribution, allowing the strain concentration area in the waist to spread to a wider area; in addition, the small downward pressure makes the hydrostatic stress in the deformation zone dominant, and the stress triaxiality is significantly reduced, thereby greatly slowing down the accumulation rate of toughness damage; the above mechanisms work together to finally achieve crack-free, high-precision, and low-cost bipolar plate flow channel forming.

[0019] The underlying principle of the forming method of the present invention to avoid cracking is as follows: This method fundamentally reconstructs the spatial and temporal distribution of plastic deformation. Through the synergistic effect of five coupled mechanisms, it ensures that the local strain in the waist of the flow channel remains within the material's safe forming limit. These five mechanisms can be summarized into three levels: deformation decomposition (mechanism 1 and mechanism 2), which solves the strain distribution problem through the synergistic effect of these two mechanisms; stress regulation (mechanism 3 and mechanism 4), which is used to solve the damage suppression problem; and equipment realization (mechanism 5), which is used to solve the cost control problem.

[0020] (i) Deformation decomposition: Decompose large deformations into incremental accumulations of small increments.

[0021] Mechanism 1: Strain cumulative decomposition.

[0022] This method will achieve the total equivalent plastic strain required to complete the final flow channel geometry. Decomposed along the track direction: , i =1,2,…,n, by controlling the decrease in roll gap for each pass (e.g., ΔH = 0.1 mm / pass), the local strain increment of the metal slab in each pass is made... Always below the material forming limit Taking the formation of a 0.5mm deep runner from a 0.1mm thick 316L slab as an example: the strain increment of a single-pass process exceeds the safety threshold of 18%; while in a three-pass process, the strain increment of each pass is about 1 / 3 to 1 / 2 of the total strain, all below the safety threshold. This principle transforms the target geometry that cannot be achieved by a one-time large deformation into one that can be achieved by the accumulation of multiple small deformations.

[0023] Mechanism 2: Gradual material flow redistribution.

[0024] During the brief separation period between the end of each rolling pass and the beginning of the next, the stress gradient between the deformed and undeformed areas of the metal slab drives a slight redistribution of the material: The first pass (roller gap 0.3mm) only forms a shallow groove, the deformation is mainly bending, the flange area remains flat, and the material has not been significantly pulled into the flow channel. In the second pass (roll gap 0.2mm), since the sidewall of the metal slab already has an initial inclination angle, the punch presses further into the die. The metal slab is no longer bending from zero, but continues to flow plastically on the existing curved surface. More peripheral material is pulled into the runner, and the waist of the runner is thinned and dispersed to a wider contact arc. The third pass (roll gap 0.1mm) is mainly responsible for shaping, with the smallest local strain increment; This progressive material replenishment effect means that the waist of the flow channel is no longer the only strain concentration area, and adjacent straight areas work together to share plastic deformation, achieving a more uniform thickness distribution.

[0025] (ii) Stress control: suppress damage from two dimensions: stress path and stress triaxiality.

[0026] Mechanism 3: The switching effect of reverse rolling on strain path.

[0027] This method employs an alternating pattern of opposite rolling directions in adjacent passes (clockwise → counterclockwise → clockwise), introducing two key effects. First, it leverages the Bauschinger effect: after the metal slab undergoes positive deformation in the first pass, continuing loading along the original direction requires higher yield stress (strain hardening). However, if loading is changed to reverse, although the principal strain direction remains unchanged, the shear strain direction changes, and the reverse yield stress decreases. This means that during the second pass of reverse rolling, the initial plastic flow of the material is more easily initiated, avoiding local stress concentration caused by a sharp increase in stress at the moment of biting in. Second, the strain path switching activates more slip systems: continuous unidirectional rolling easily forms dislocation pile-up and shear bands (precursors to local instability), while reverse rolling activates reverse dislocation movement, promoting dislocation rearrangement and mutual annihilation, which macroscopically manifests as plastic recovery. This mechanism allows the formability of the material at the beginning of each pass to be regenerated.

[0028] Mechanism 4: Active control of stress triaxiality.

[0029] According to GTN damage theory, the material damage accumulation rate strongly depends on the stress triaxiality η, and the relationship is as follows: , Where: D represents the damage variable of the material, usually between 0 and 1, the symbol d represents the derivative, and dD / dε represents the material damage accumulation rate; when η > 0 (tensile stress is dominant), damage accumulates rapidly; when η < 0 (compression stress is dominant), damage hardly occurs.

[0030] This method modulates stress triaxiality on two levels: Overall: The contact arc length Ld of each rolling pass is relatively short, and the relationship is as follows: Where: R is the roll diameter, the hydrostatic pressure stress in the deformation zone is significantly negative, the waist of the flow channel is in a state dominated by compressive stress, and η < 0; At the local level: Although there is a tensile stress component on the outer surface of the waist of the flow channel due to bending, the absolute value of this tensile stress is much lower than the level under a single-pass process because the forming depth of each pass is small. Even if η is positive, it is significantly reduced. Estimation shows that, under the same total strain, the cumulative damage from a multi-pass small deformation process can be reduced by more than an order of magnitude compared to a single-pass large deformation process. This is the most critical damage mechanics basis for this method to avoid cracking.

[0031] (III) Equipment implementation: Replace multiple sets of hardware in space with changes in timing parameters.

[0032] Mechanism 5: Equivalent multi-station forming effect under a single mold.

[0033] Traditional multi-station roll forming processes require the use of four sets of roll dies with different geometries to form the material sequentially. However, this method cleverly utilizes the same set of roll dies (i.e., concave and convex dies) by varying the roll gap to achieve equivalent progressive forming: at a large roll gap (0.3 mm), only the tip of the convex die extends into the concave die area, and the metal slab only perceives the "shallow groove" geometry; when the roll gap is reduced to a medium size (0.2 mm), the convex die and concave die fit more tightly, and the metal slab perceives the "medium-depth groove"; when the roll gap is reduced to its final value (0.1 mm), the convex die is fully embedded in the concave die, and the final flow channel geometry is formed.

[0034] The continuous change in the spatial dimension of the roll gap effectively replaces the discrete switching of multiple sets of dies, and the alternating pattern of opposite rolling directions in adjacent passes greatly saves process time compared with the traditional variable roll gap rolling process.

[0035] The combined effect of the above five mechanisms ensures that the cumulative total strain meets the target geometric requirements, but the strain and damage at the waist are both below the forming limit / critical damage value throughout the process, resulting in no cracking defects.

[0036] The technical effects of the present invention are verified by comparing it with comparative examples.

[0037] The metal slab material, die specifications, and runner geometry parameters used in this invention and the comparative example are the same, as shown in the table below: slab material 316L austenitic stainless steel slab thickness 0.10mm slab elastic modulus 193 GPa Poisson's ratio of slab 0.30 slab yield strength ≈285MPa slab tensile strength ≈590MPa Number of flow channels 5 parallel flow channels Target depth of flow channel 0.50mm (depth-to-width ratio ≈ 0.8) Die and dent materials Tool steel (Cr12MoV) Roll diameter 100mm Die and socket fit accuracy Groove tolerance ±0.005mm Comparative example: Using a single-pass fixed roll gap process, the metal slab is fed into the roll meshing area by a traction device. The upper and lower rolls maintain a fixed roll gap of 0.10mm (i.e. the original thickness of the metal slab). All five flow channels are formed in one rotation in one direction.

[0038] The metal slab of the present invention undergoes three rolling passes in sequence, with the rotation directions of the rolls in adjacent passes being opposite and the feeding directions of the metal slab being reversed, and the roll pitch decreasing with each pass.

[0039] The invention and the comparative example were compared using the finite element numerical simulation method. By quantitatively analyzing key indicators such as rolling force, maximum equivalent plastic strain, and slab flatness, the technical effects of the invention in reducing forming load, suppressing strain concentration, and improving surface quality were verified.

[0040] I. Analysis of rolling force and equivalent plastic strain.

[0041] The simulation was performed using a finite element method (FEM) platform. Figure 3 The figure shows the radial rolling force time history curve. As can be seen from the figure, the peak radial rolling force of the comparative single-pass rolling method can reach 4.26kN, and the peak value is steep with violent stress fluctuations. However, the peak radial rolling force of the three-pass variable-pitch reciprocating rolling method of the present invention is reduced to 3.64kN, which occurs in the final forming stage of the third pass. The peak radial rolling force of the first and second passes is only about 0.5kN and 0.6kN, respectively. The overall force load distribution is stable. This indicates that the bite conditions are mild under small reduction and the rolling force is stable. At the same time, it means that the requirements of the three-pass variable-pitch reciprocating rolling process on the mill rigidity, motor power and transmission system are significantly reduced, which is the mechanical basis for the reduction of equipment cost.

[0042] Figure 4 The figure shows the time history curve of the maximum equivalent plastic strain. It can be seen from the figure that the maximum equivalent plastic strain of the three-pass rolling method of the present invention accumulates progressively over the three passes in a manner of approximately 0.20 → 0.34 → 0.69, with the strain increment for each pass being... , , The values ​​are all significantly smaller than the maximum equivalent plastic strain of 0.77 generated in a single pass of rolling in the comparative example, with an absolute value reduction of 10.4%. At the same time, the strain path in the upper and middle sections of the single pass of rolling in the comparative example exceeds the safe forming threshold of 316L stainless steel (the unidirectional tensile forming limit of metal slabs is about 18% to 20% of the thickness reduction rate, after which toughness damage will occur). This indicates that the three-pass variable roll gap reciprocating rolling method of the present invention decomposes the total strain along the pass direction, effectively reduces the local strain accumulation rate in each pass, and fundamentally weakens the cracking driving force.

[0043] The aforementioned strain accumulation decomposition reduced the absolute value of the maximum equivalent plastic strain by 10.4%. However, the strain reduction alone is insufficient to fully explain the underlying mechanism by which this method avoids cracking. According to the Rice–Tracey toughness damage model, the damage accumulation rate of the material is exponentially related to the stress triaxiality η, and the damage amount is a time-history integral. In the comparative single-pass rolling, all deformation is concentrated within seconds, resulting in a high strain rate. The stress triaxiality in the runner waist remains consistently at a high positive value, leading to an exponential increase in the damage accumulation rate. The actual accumulated damage amount is far greater than the degree reflected by the ratio of the absolute strain values ​​(0.77 / 0.69)—this is the fundamental reason why single-pass rolling is prone to cracking in the runner waist. In contrast, the three-pass variable roll gap reciprocating rolling of the present invention disperses the high stress triaxiality peak originally concentrated in a single pass into multiple low stress triaxiality small deformation segments through pass decomposition and reverse rolling, which greatly reduces the damage increment of each pass and reduces the total cumulative damage by more than an order of magnitude compared with single-pass rolling, thereby fundamentally suppressing cracking at the level of damage mechanics.

[0044] Figure 5 and Figure 6 The equivalent plastic strain contour plots of the comparative example and the final molded part of the present invention are shown below. Figure 5 It can be seen that the strain of the single-pass rolled part in the comparative example forms a distinct narrow band of high strain concentration along the waist of the runner, as shown by the red dashed box in the figure; while the high strain area of ​​the three-pass variable-pitch reciprocating rolled part of the present invention ( Figure 6 (As shown in the red dashed box) the maximum equivalent strain value was significantly reduced, and the adjacent flat zone participated in part of the plastic deformation; this indicates that during the interval between passes, the metal slab underwent material flow redistribution, and the waist no longer independently bore all the strain.

[0045] II. Flatness Result Analysis.

[0046] Twenty-five points were randomly selected on the surface of the bipolar plates formed in both the comparative example and the present invention, and their deviations relative to the reference plane were measured. The results are as follows. Figure 7As shown in the figure, the three-pass variable-pitch reciprocating rolling of the present invention, through the roll leveling effect after multiple reverse rolling, reduces the flatness error of the formed part from 0.577 in the comparative single-pass rolling to 0.143, a reduction of 75.2%. This flatness improvement mechanism is closely related to the rolling path switching and the cumulative forming effect of multiple passes. Specifically, on the one hand, reverse rolling activates the reverse dislocation motion, reducing the residual stress inside the slab in each pass, and playing a leveling role on the plate shape and flange edge in subsequent rolling passes, significantly improving the overall forming quality of the material; on the other hand, the gradual reduction of the roll pitch limits the buckling amount of the material in each pass to a small range, thereby avoiding local buckling instability that is prone to occur under large reduction; the combined effect of both significantly improves the flatness of the formed part.

[0047] In summary, simulation results demonstrate that the three-pass rolling method of this invention fundamentally avoids cracking of the runner waist due to excessive thinning, greatly improving forming quality while reducing production costs.

[0048] It should be noted that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still modify the technical solutions described in the above embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A bipolar plate flow channel forming method based on variable roll gap multi-pass rolling, characterized in that... Includes the following steps: S1. The die and punch for forming bipolar plate flow channels are respectively assembled on the upper and lower rolls of the wedge cross rolling mill. S2. Place the metal slab between the upper and lower rolls, set the roll gap to the first roll distance H1, control the rotation of the upper and lower rolls, and drive the metal slab to move along the first feed direction through the traction device to complete the first rolling pass. S3. After the first rolling pass is completed, the roll gap is reduced to the second roll gap H2, that is: H2 < H1. Then, the upper and lower rolls are controlled to rotate in opposite directions, and the metal slab moves along the second feed direction opposite to the first feed direction to complete the second rolling pass. S4. Following the method of step S3, at least one more subsequent rolling pass is performed, wherein the roll pitch of each subsequent pass gradually decreases relative to the roll pitch of the previous pass, and the rotation direction of the rolls and the feeding direction of the metal slab in adjacent passes are opposite, so that the metal slab undergoes multiple alternating progressive rolling on a single die. S5. After the final rolling pass is completed, bipolar plate channels of the required depth and shape are formed on the metal slab.

2. The bipolar plate flow channel forming method based on variable roll gap multi-pass rolling according to claim 1, characterized in that: The number of rolling passes n≥2. When the target depth of the runner is ≤0.3mm, n=2; when the target depth of the runner is 0.3~0.5mm, n=3; when the target depth of the runner is >0.5mm, n≥4.

3. The bipolar plate flow channel forming method based on variable roll gap multi-pass rolling according to claim 2, characterized in that: The thickness of the metal slab is t. Determine the first... i Roll gap H per pass i The decrease in roll gap between adjacent passes ΔH ≥ 0.05 mm, and the roll gap H of the final pass... n ≤1.2t.

4. The bipolar plate flow channel forming method based on variable roll gap multi-pass rolling according to claim 1, characterized in that: After each rolling pass is completed and before the next pass begins, a pause time of 0.1 to 2 seconds is set between passes for adjusting the roll gap and relaxing the stress on the metal slab.

5. The bipolar plate flow channel forming method based on variable roll gap multi-pass rolling according to claim 1, characterized in that: The tension applied by the traction device at both ends of the metal slab is 5% to 15% of the yield strength of the metal slab.

6. The bipolar plate flow channel forming method based on variable roll gap multi-pass rolling according to claim 5, characterized in that: The rotational speed of the upper and lower rollers is 1 to 10 rpm.

7. The bipolar plate flow channel forming method based on variable roll gap multi-pass rolling according to claim 1, characterized in that: The aspect ratio of the bipolar plate flow channel is ≥0.6.

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