Bending test method for rolling forming limit

Abstract

The invention discloses a bending test method for roll forming limit, which is characterized by comprising the following steps of: firstly, designing and manufacturing a set of multi-pass bending test die in which the opening spacing of a concave die is not changed and the fillet radius of a convex die is gradually reduced; the set of die is adopted to respectively carry out a limit condition test of one-time bending and a limit condition test of multiple times of bending according to the radius of the convex die fillet from large to small to obtain the ratio of the limit bending radiuses of multiple times of bending to the limit bending radius of one-time bending; and then carrying out a one-time bending limit condition test on a plate which needs to be tested and is made of the same material to obtain a one-time bending limit bending radius, and multiplying the one-time bending limit bending radius by the specific value to obtain a rolling forming limit. The method has the beneficial effects that the rolling forming limit of the high-strength steel plate is tested through a simple bending experiment, the multi-pass dispersion continuous deformation characteristic of the rolling process can be considered, the method is simple and easy to implement, economical and efficient, and the test result is more accurate.

Description

Technical Field

[0001] The invention relates to the technical field of metal plastic forming, and in particular to a bending test method for roll forming limit. Background Art

[0002] Roll Forming, also known as roll bending, cold bending, etc., is a plastic processing method for forming metal plates into slender profile structures. During roll forming, metal coils or single sheets are gradually bent transversely through multiple passes of forming rollers with specific profiles configured in sequence to form long strips of profiles with specific cross-sections. For open or closed thin-walled tube beam type slender structures, roll forming is the most suitable method.

[0003] In recent years, with the increasing demand for lightweight in the automotive and other fields, high-strength steel tube beam type roll-formed parts have been increasingly used. Due to the poor plasticity of high-strength steel, it is easy to crack during forming, resulting in forming failure. For specific high-strength steel sheets and roll-formed shapes, it is necessary to understand the roll-forming performance or forming limit of the sheet in detail before formulating the roll-forming process, which is generally expressed by the minimum bending radius. Due to the special deformation mode of the roll-forming process, a smaller bending radius can be obtained than bending. Therefore, it is difficult to accurately evaluate the roll-forming performance of high-strength steel sheets using conventional sheet forming performance test methods such as unidirectional stretching, bending, and cupping tests.

[0004] The fundamental reason for the above phenomenon is that the rolling process has the characteristics of "multi-pass" and "dispersed" deformation. Studies have shown that dispersed forming can improve the plastic deformation ability of metals to a certain extent. Since the commonly used unidirectional stretching, bending and cupping test methods do not take into account the "multi-pass" dispersed deformation characteristics of rolling, it is impossible to accurately test the rolling forming limits of various high-strength steel plates, and it is impossible to meet the needs of rolling process design and on-site production quality control. Therefore, it is necessary for technical personnel in this field to develop a sheet metal forming performance test method for the rolling process to better meet the needs of rolling processing design. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, the technical problem to be solved by the present invention is: how to provide a bending test method for the rolling forming limit that can indirectly obtain the rolling forming limit data of metal sheets, so as to better understand the material properties and meet the rolling processing design requirements.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A bending test method for roller forming limit, characterized in that a set of bending test dies with multiple passes in which the opening spacing of the concave dies remains unchanged and the radius of the punch corner gradually decreases is designed and manufactured, and the set of dies is used to perform a single bending limit test and a multiple bending limit test according to the punch corner radius from large to small on the plate, respectively, to obtain the ratio of the respective limit bending radii of multiple bending and single bending; then, for the plate of the same material to be tested, a single bending limit test is performed on it to obtain the limit bending radius of the single bending, and then the ratio is multiplied to obtain the forming limit of roller pressing.

[0008] In this way, the method uses multiple sets of bending test dies with gradually decreasing fillet radii, and uses a step-by-step stamping bending test to simulate the deformation mode instead of roll forming, and measures the limit bending radius of a single bend and the limit bending radius of multiple bends. In this way, according to their ratio, for the same material sheet to be tested, only one bending limit case test is required, and the limit bending radius of a single bend multiplied by the ratio can be converted to obtain its roll forming limit. In this way, the test process is greatly simplified by the simulation conversion method, which is convenient for obtaining the roll forming limit of the metal sheet before the roll forming process, so as to better understand the material properties and meet the roll forming process design requirements.

[0009] As an optimization, the present invention comprises the following steps:

[0010] (1) Perform a standard tensile test on a plate (high-strength steel plate) with a certain material grade and thickness t to obtain stress-strain data and elongation δ value;

[0011] (2) Using numerical simulation methods, the fracture strain ε of the studied sheet is obtained according to the stress-strain data and elongation δ obtained from the tensile test. k ;

[0012] Step (2) specifically includes the following steps:

[0013] 1) Establish a finite element analysis model consistent with the tensile test, use the ductile damage model, and input the fracture strain ε k The initial value of is used to simulate the tensile test process, obtain the simulation result of the tensile deformation of the sample until it breaks, and obtain the elongation δ1 at this time;

[0014] 2) Compare the simulated elongation δ1 with the experimental elongation δ; if δ1>δ, it means ε k is set too large and needs to be reduced; on the contrary, if δ1<δ, it means ε k The setting is too small and needs to be increased;

[0015] 3) According to the analysis results of step 2), ε kThe numerical value is adjusted, the finite element analysis model of the tensile test is input, and the simulation calculation of the tensile test is repeated to obtain ε k The elongation δ1 after numerical adjustment is compared with δ. If the difference between the two values ​​is greater than 10%, ε is adjusted again according to the method described in step 2). k Make adjustments and re-simulate the tensile test process;

[0016] 4) Repeat steps 2) and 3) until the error between the simulated elongation δ1 and the elongation δ of the tensile test is less than 10%, and the material fracture strain ε at this time is k As the final value. In this way, the approximation method can more quickly simulate and obtain the fracture strain value of the plate.

[0017] (3) According to the grade and thickness t of the plate, the structure and size of the bending test mold are designed, and the initial value r0 of the punch corner radius and the initial value W0 of the die opening spacing are determined according to the following formula:

[0018]

[0019] Among them, δ is the material elongation obtained from the tensile test, and t is the sheet thickness;

[0020] (4) The initial values ​​of the punch corner radius and the die spacing obtained by formula (1) are adjusted to obtain the dimensional data basis for mold design, ensuring that the bending test can produce a fracture phenomenon and thus determine the forming limit;

[0021] This is because r0 in formula (1) is the limit bending radius estimated according to the elongation of the material, and W0 is designed to be larger to ensure that a punch with a larger corner radius can be used without changing the die. Therefore, the obtained W0 and r0 may not meet the requirements of material bending fracture. It is necessary to further adjust the two parameters of the die spacing and punch corner radius of the bending test mold to ensure that fracture can occur in the experiment, so as to determine the forming limit.

[0022] As an optimization, the adjustment steps of the parameters of the initial value r0 of the punch fillet radius and the initial value W0 of the die spacing include:

[0023] 1) Establish a finite element analysis model of the bending test mold and use the determined fracture strain ε k , simulate the bending process and obtain the simulation results of the bending deformation of the specimen;

[0024] 2) If the sheet metal does not crack when the bending angle reaches 90°, gradually reduce the radius of the punch corner or the distance between the die and re-simulate the bending process;

[0025] 3) When the bending angle reaches 90°, the sheet metal cracks and the die opening spacing W0 is increased. ′ As the opening spacing of the final series of concave dies, the radius of the punch corner r at this time ′ 0 is used as the dimensional data basis for the design of the series punch corner radius.

[0026] (5) Determine the number of bending times N (i.e., the number of punches) of the multi-pass bending test die, and design the bending angle and the fillet radius of each pass; when determining the number of multi-pass bending times, the target bending angle is uniformly set to 90°, and the number of multi-pass bending times N and the bending angle of each pass are determined according to the strength index and thickness t of the sheet material and the number of rolling times designed by the rolling process, and finally design and determine the fillet radius of the corresponding N punches;

[0027] As an optimization, step (5) specifically includes the following steps:

[0028] The number of bending times N for multiple passes is determined as follows:

[0029] For high-strength steel materials with a yield strength of 1180MPa or less, the number of multi-pass bending i is 5 to 8 times; for high-strength steel materials with a yield strength between 1180 and 1300MPa, the number of multi-pass bending i is 7 to 9 times; for high-strength steel materials with a yield strength between 1300 and 1500MPa, the number of multi-pass bending i is 8 to 10 times; for high-strength steel materials with a yield strength of 1500MPa and above, the number of multi-pass bending i is 9 to 12 times;

[0030] Bending angle θ for each pass i Calculate as follows:

[0031]

[0032] Where n is the change index;

[0033] set up (Round up), the punch corner radius r' of the Lth pass L Set as r0', the radius of the punch corner of other passes is r i 'Press-down design:

[0034]

[0035] Among them, r i is the radius of the punch corner of each pass, m is the correction coefficient (the value range of m is 0.5~2), i is the number of bending passes, θ i is the bending angle of the i-th pass, and r0 is the punch corner radius obtained by the final simulation in step (4).

[0036] In this way, the number of passes and the bending angle of each pass are first obtained according to the existing roll forming design method, and then the punch size that will definitely crack in a single bending calculated previously is used as the intermediate value, and the punch size of each pass is adjusted according to the change ratio of the bending angle of different passes, so as to ensure the rationality of the punch size design to the greatest extent, so that in the extreme case test of multiple bendings, it can produce cracks before the last punch, obtain the test results, and ensure that the test has a wider feasibility.

[0037] (6) Process and manufacture test molds for multi-pass bending, and prepare sheet metal specimens with double the number of passes for bending tests;

[0038] (7) Conduct single-pass bending and multi-pass bending physical experiments. The specific steps are as follows:

[0039] Multi-pass bending: Use a designed series of punches to perform multiple continuous bends on the same sheet metal specimen in order from large to small according to the fillet radius (i.e., the sheet metal bent in the last bend is used as the initial sheet metal for the next bending test), and the bending angle of each pass is confirmed according to the designed value; until cracks just appear on the outer surface of the deformation zone of the specimen, the fillet radius of the punch in the last pass is taken as the limit bending radius r of the multi-pass bending 2min ;

[0040] Single bending: Starting from the punch with cracks in the multiple bending tests, use punches with different fillet radii from small to large to perform a 90° single bending test on different plate samples until no cracks are generated on the sample. The fillet radius of this punch is the limit bending radius r of the single bending. 1min ; or use punches with different fillet radii from large to small to bend the sheet metal specimen 90° at a time. When cracks appear on the outer surface of the specimen deformation zone, stop the test and take the bending radius of the last test as the limit bending radius r of the first bend. 1min .

[0041] (8) The maximum bending radius r of multiple bends 2min The limit bending radius r of a single bend 1min By comparison, we can get the deviation coefficient k:

[0042] k=r 2min / r 1min (4);

[0043] For high-strength steel plates with a certain material grade and thickness t, the designed set of dies can be used for actual tests or computer simulation tests to obtain the ultimate bending radius r of a single bend. 1min , and then multiply by the coefficient to get the roller's limit bending radius r 2min ,Right now:

[0044] r2min =k*r 1min (5).

[0045] Compared with the existing methods, the beneficial effects of the present invention are: the test of the roll forming limit of high-strength steel plates is realized through a simple bending experiment, and the multi-pass dispersed continuous deformation characteristics of the rolling process can be taken into account. It is simple, easy, economical and efficient, and the test results are more accurate. BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Figure 1 Schematic diagram of the process steps of the method of the present invention.

[0047] Figure 2 It is a schematic diagram of the simulation of the tensile fracture of a plate sample used in the implementation of the present invention.

[0048] Figure 3 It is a schematic structural diagram of a bending test mold used in the implementation of the present invention. DETAILED DESCRIPTION

[0049] The present invention is further described in detail below in conjunction with specific implementation modes.

[0050] Specific implementation: A bending test method for roll forming limit, characterized in that a set of bending test dies with a constant opening spacing of the concave die and a gradually decreasing radius of the convex die are first designed and manufactured, see Figure 3 The set of dies is used to carry out the limit case test of single bending and the limit case test of multiple bending according to the radius of the punch corner from large to small on the plate specimen (the standard specimen for bending test in the shape of a rectangular block as a whole), and the ratio of the limit bending radius of multiple bending and single bending is obtained; then, for the plate of the same material to be tested, the limit case test of single bending is carried out to obtain the limit bending radius of single bending, and then the forming limit of rolling is obtained by multiplying the ratio.

[0051] In this way, the method uses multiple sets of bending test dies with gradually decreasing fillet radii, and uses a step-by-step stamping bending test to simulate the deformation mode instead of roll forming, and measures the limit bending radius of a single bend and the limit bending radius of multiple bends. In this way, according to their ratio, for the same material sheet to be tested, only one bending limit case test is required, and the limit bending radius of a single bend multiplied by the ratio can be converted to obtain its roll forming limit. In this way, the test process is greatly simplified by the simulation conversion method, which is convenient for obtaining the roll forming limit of the metal sheet before the roll forming process, so as to better understand the material properties and meet the roll forming process design requirements.

[0052] When implemented, the present invention includes the following steps, see Figure 1 :

[0053] (1) Perform a standard tensile test on a plate (high-strength steel plate) with a certain material grade and thickness t to obtain stress-strain data and elongation δ value;

[0054] (2) Using numerical simulation methods, the fracture strain ε of the studied sheet is obtained according to the stress-strain data and elongation δ obtained from the tensile test. k ;

[0055] Step (2) specifically includes the following steps:

[0056] 1) Establish a finite element analysis model consistent with the tensile test, use the ductile damage model, and input the fracture strain ε k The initial value of the tensile test process is simulated to obtain the simulation results of the tensile deformation of the specimen until it breaks. Figure 2 , and the elongation δ1 is obtained at this time;

[0057] 2) Compare the simulated elongation δ1 with the experimental elongation δ; if δ1>δ, it means that ε k The setting is too large and needs to be reduced; on the contrary, if δ1<δ, it means that ε k The setting is too small and needs to be increased;

[0058] 3) According to the analysis results of step 2), ε k The numerical value is adjusted, the finite element analysis model of the tensile test is input, and the simulation calculation of the tensile test is repeated to obtain ε k The elongation δ1 after numerical adjustment is compared with δ. If the difference between the two values ​​is greater than 10%, ε is adjusted again according to the method described in step 2). k Make adjustments and re-simulate the tensile test process;

[0059] 4) Repeat steps 2) and 3) until the error between the simulated elongation δ1 and the elongation δ of the tensile test is less than 10%, and the material fracture strain ε at this time is k As the final value. In this way, the approximation method can more quickly simulate and obtain the fracture strain value of the plate.

[0060] (3) According to the grade and thickness t of the plate, the structure and size of the bending test mold are designed, and the initial value r0 of the punch corner radius and the initial value W0 of the die opening spacing are determined according to the following formula:

[0061]

[0062] Among them, δ is the material elongation obtained from the tensile test, and t is the sheet thickness;

[0063] (4) The initial values ​​of the punch corner radius and the die spacing obtained by formula (1) are adjusted to obtain the dimensional data basis for mold design, ensuring that the bending test can produce a fracture phenomenon and thus determine the forming limit;

[0064] This is because r0 in formula (1) is the limit bending radius estimated according to the elongation of the material, and W0 is designed to be larger to ensure that a punch with a larger corner radius can be used without changing the die. Therefore, the obtained W0 and r0 may not meet the requirements of material bending fracture. It is necessary to further adjust the two parameters of the die spacing and punch corner radius of the bending test mold to ensure that fracture can occur in the experiment, so as to determine the forming limit.

[0065] During implementation, the adjustment steps of the parameters of the initial value r0 of the punch fillet radius and the initial value W0 of the die spacing include:

[0066] 1) Establish a finite element analysis model of the bending test mold and use the determined fracture strain ε k , simulate the bending process and obtain the simulation results of the bending deformation of the specimen;

[0067] 2) If the sheet metal does not crack when the bending angle reaches 90°, gradually reduce the radius of the punch corner or the distance between the die and re-simulate the bending process; (During implementation, it can be reduced by 1mm each time)

[0068] 3) When the bending angle reaches 90°, the sheet metal cracks and the die opening spacing W0 is increased. ′ As the opening spacing of the final series of concave dies, the radius of the punch corner r at this time ′ 0 is used as the dimensional data basis for the design of the series punch corner radius.

[0069] (5) Determine the number of bending times N (i.e., the number of punches) of the multi-pass bending test die, and design the bending angle and the fillet radius of each pass; when determining the number of multi-pass bending times, the target bending angle is uniformly set to 90°, and the number of multi-pass bending times N and the bending angle of each pass are determined according to the strength index and thickness t of the sheet material and the number of rolling times designed by the rolling process, and finally design and determine the fillet radius of the corresponding N punches;

[0070] When implemented, the specific steps of step (5) are as follows:

[0071] The number of bending times N for multiple passes is determined as follows:

[0072] For high-strength steel materials with a yield strength of 1180MPa or less, the number of multi-pass bending i is 5 to 8 times; for high-strength steel materials with a yield strength between 1180 and 1300MPa, the number of multi-pass bending i is 7 to 9 times; for high-strength steel materials with a yield strength between 1300 and 1500MPa, the number of multi-pass bending i is 8 to 10 times; for high-strength steel materials with a yield strength of 1500MPa and above, the number of multi-pass bending i is 9 to 12 times; refer to the existing roller pressing pass design method [Meng Yifan. Research on the relationship between roll forming process and microstructure and properties of ultra-high strength steel [D]. North China University of Technology, 2023; Liu Xiaoli. Research on springback prediction and control of continuous roll forming of complex section ultra-high strength steel [D]. Beijing University of Science and Technology, 2018; J Cheng, J Cao, Z Wei, et al. Modelling and prediction of twist springback for UHSS thin-walled component with asymmetric complex section in roll forming process [J]. Int J Adv Manuf Tech, 2023, 129(9-10): 4257-4274.】obtained.

[0073] Bending angle θ for each pass i Calculate as follows:

[0074]

[0075] Wherein, n is the variation index; it is obtained by referring to the existing [Xiao Naihong, Liu Jiying. Cold bending forming technology [M]. Chemical Industry Press, 2008].

[0076] set up (Round up), the punch corner radius r' of the Lth pass L Set as r0', the punch corner radius of other passes is r' i Push-to-type design:

[0077]

[0078] Among them, r i ' is the radius of the punch corner of each pass, m is the correction coefficient (0°<θ i ≤90°, So m>0.5. So m<2. So the value range of m is 0.5~2. In specific implementation, the lower the strength of the plate, the smaller the value of m, and vice versa). i is the number of bending passes, θ iis the bending angle of the i-th pass, and r0 is the punch corner radius obtained by the final simulation in step (4).

[0079] In this way, the number of passes and the bending angle of each pass are first obtained according to the existing roll forming design method, and then the punch size that will definitely crack in a single bending calculated previously is used as the intermediate value, and the punch size of each pass is adjusted according to the change ratio of the bending angle of different passes, so as to ensure the rationality of the punch size design to the greatest extent, so that in the extreme case test of multiple bendings, it can produce cracks before the last punch, obtain the test results, and ensure that the test has a wider feasibility.

[0080] (6) Process and manufacture test molds for multi-pass bending, and prepare sheet metal specimens with double the number of passes for bending tests;

[0081] (7) Conduct single-pass bending and multi-pass bending physical experiments. The specific steps are as follows:

[0082] Multi-pass bending: Use a designed series of punches to perform multiple continuous bends on the same sheet metal specimen in order from large to small according to the fillet radius (i.e., the sheet metal bent in the last bend is used as the initial sheet metal for the next bending test), and the bending angle of each pass is confirmed according to the designed value; until cracks just appear on the outer surface of the deformation zone of the specimen, the fillet radius of the punch in the last pass is taken as the limit bending radius r of the multi-pass bending 2min ;

[0083] Single bending: Starting from the punch with cracks in the multiple bending tests, use punches with different fillet radii from small to large to perform a 90° single bending test on different plate samples until no cracks are generated on the sample. The fillet radius of this punch is the limit bending radius r of the single bending. 1min ; or use punches with different fillet radii from large to small to bend the sheet metal specimen 90° at a time. When cracks appear on the outer surface of the specimen deformation zone, stop the test and take the bending radius of the last test as the limit bending radius r of the first bend. 1min .

[0084] (8) The maximum bending radius r of multiple bends 2min The limit bending radius r of a single bend 1min By comparison, we can get the deviation coefficient k:

[0085] k=r 2min / r 1min (4);

[0086] For high-strength steel plates with a certain material grade and thickness t, the designed set of dies can be used for actual tests or computer simulation tests to obtain the ultimate bending radius r of a single bend. 1min; Multiply by the coefficient to get the roller's limit bending radius r 2min ;Right now:

[0087] r 2min =k*r 1min (5).

Claims

1. A bending test method for roll forming limit, characterized in that: Firstly, a set of multi-pass bending test dies with a constant opening spacing of the concave dies and a gradually decreasing radius of the punch corner is designed and manufactured. The set of dies is used to carry out a limit case test of a single bending and a limit case test of multiple bendings according to the radius of the punch corner from large to small, respectively, to obtain the ratio of the limit bending radii of multiple bending and single bending. Then, for the plates of the same material to be tested, a limit case test of a single bending is carried out to obtain the limit bending radius of a single bending, and then the forming limit of rolling is obtained by multiplying the ratio.

2. The bending test method of the roll forming limit according to claim 1, characterized in that: The present invention comprises the following steps: (1) Perform a standard tensile test on a plate with a certain material grade and thickness t to obtain stress-strain data and elongation Numeric value; (2) Using numerical simulation methods, the stress-strain data and elongation obtained from the tensile test are , to obtain the fracture strain of the studied plate ; (3) According to the grade and thickness t of the plate, design the structure and size of the bending test die, and determine the initial value of the punch corner radius according to the following formula: , Initial value of the die opening spacing : （1） in, is the material elongation obtained from the tensile test, and t is the sheet thickness; (4) The initial values ​​of the punch corner radius and the die spacing obtained by formula (1) are adjusted to obtain the dimensional data basis for mold design, ensuring that fracture can occur in the bending test and thus determining the forming limit; (5) Determine the number of bending times N of the multi-pass bending test die, and design the bending angle and the corner radius of each pass; when determining the number of multi-pass bending times, the target bending angle is uniformly set to 90°, and the number of multi-pass bending times N and the bending angle of each pass are determined according to the strength index and thickness t of the plate and the number of rolling times of the rolling process, and finally the corner radius of the corresponding N punches is designed and determined; (6) Process and manufacture test molds for multi-pass bending, and prepare sheet metal specimens with double the number of passes for bending tests; (7) Conduct single-pass bending and multiple-pass bending physical experiments to obtain their respective limit bending radii; (8) The maximum bending radius of multiple bends The maximum bending radius of a single bend By comparison, we can get the deviation coefficient k: （4）； For high-strength steel plates with a certain material grade and thickness t, the designed set of dies is subsequently used for actual testing or computer simulation testing to obtain the ultimate bending radius of a single bend. , and then multiply by the coefficient to get the limit bending radius of the roller ,Right now: （5）。 3. The bending test method of the roll forming limit according to claim 2, characterized in that: Step (2) specifically includes the following steps: 1) Establish a finite element analysis model consistent with the tensile test, use the ductile damage model, and input the fracture strain The initial value of the tensile test process is simulated to obtain the simulation results of the tensile deformation of the sample until it breaks, and the elongation at this time is obtained. ; 2) The elongation obtained by simulation The elongation obtained from the experiment For comparison; if ,illustrate The setting is too large and needs to be reduced; on the contrary, if ,illustrate The setting is too small and needs to be increased; 3) Based on the analysis results of step 2), The numerical value is adjusted, the finite element analysis model of the tensile test is input, and the simulation calculation of the tensile test is repeated to obtain Elongation after numerical adjustment ; and If the difference between the two values ​​is greater than 10%, repeat the method described in step 2). Make adjustments and re-simulate the tensile test process; 4) Repeat steps 2) and 3) until the simulated elongation is Elongation of tensile test The error is less than 10%, and the fracture strain of the material at this time as the final value.

4. The bending test method of the roll forming limit according to claim 2, characterized in that: In step (4), the initial value of the punch corner radius is Initial value of the distance between the die and the die The steps to adjust the parameters include: 1) Establish a finite element analysis model of the bending test mold and use the determined fracture strain , simulate the bending process and obtain the simulation results of the bending deformation of the specimen; 2) If the sheet metal does not crack when the bending angle reaches 90°, gradually reduce the radius of the punch corner or the distance between the die and re-simulate the bending process; 3) When the bending angle reaches 90°, the sheet metal cracks and the die opening spacing is As the opening spacing of the final series of concave dies, the radius of the punch corners is Serves as the dimensional data basis for the design of series punch corner radius.

5. The bending test method of roll forming limit according to claim 2, characterized in that: Step (5) specifically includes the following steps: The number of bending times N for multiple passes is determined as follows: For high-strength steel materials with a yield strength of 1180MPa or less, the number of multi-pass bending i is 5 to 8 times; for high-strength steel materials with a yield strength between 1180 and 1300MPa, the number of multi-pass bending i is 7 to 9 times; for high-strength steel materials with a yield strength between 1300 and 1500MPa, the number of multi-pass bending i is 8 to 10 times; for high-strength steel materials with a yield strength of 1500MPa and above, the number of multi-pass bending i is 9 to 12 times; Bending angle of each pass Calculate as follows: （2） in, is the change index; set up If there is a decimal, round it up. Punch corner radius of each pass Set as , the punch corner radius of other passes Push-to-type design: （3） in, is the punch corner radius of each pass, is the correction factor, The value range is 0.5~2. is the number of bending passes, For the The bending angle of each pass, is the punch corner radius obtained by the final simulation in step (4).

6. The bending test method of roll forming limit according to claim 2, characterized in that: Step (7) specifically includes the following steps: Multi-pass bending test: Use a designed series of punches to perform multiple continuous bends on the same sheet metal specimen in order from large to small corner radius, and the bending angle of each pass is confirmed according to the designed value; until cracks just appear on the outer surface of the deformation zone of the specimen, the corner radius of the punch of the previous pass is taken as the limit bending radius of the multi-pass bending ; Single bending test: Starting from the punch with cracks in multiple bending tests, use punches with different fillet radii from small to large to perform a 90° single bending test on different plate samples until no cracks appear on the sample. The fillet radius of this punch is the limit bending radius of the single bending. Or use punches with different fillet radii from large to small to bend the sheet metal specimen 90° at a time. When cracks appear on the outer surface of the specimen deformation zone, stop the test and take the bending radius of the last test as the limit bending radius of the first bend. .

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