A method of bend testing a roll formed limit

Abstract

The application discloses a bending test method for a roll forming limit, characterized in that a set of multi-pass bending test dies with a constant opening distance of a female die and gradually decreasing round corner radii of a male die is designed and manufactured, the set of dies is used to respectively perform a limit condition test of one-time bending and a limit condition test of multi-time bending according to the round corner radii of the male die from large to small, and a ratio of the limit bending radii of the multi-time bending and the one-time bending is obtained; then, the same material plate to be tested is subjected to the limit condition test of one-time bending to obtain the limit bending radius of the one-time bending, and then the limit bending radius is multiplied by the ratio to obtain the roll forming limit. The application has the beneficial effects that the roll forming limit of a high-strength steel plate is tested through simple bending experiments, the multi-pass dispersed continuous deformation characteristics of the roll forming process can be considered, the method is simple and easy to implement, is economic and efficient, and the test result is more accurate.

Description

TECHNICAL FIELD

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

[0002] Roll forming, also known as roll bending or cold bending forming, is a plastic processing method for forming a metal plate into an elongated profile structure. During roll forming, the metal coil or single sheet is gradually bent transversely through a plurality of passes of forming rollers with specific profiled surfaces to produce a long profiled section with a specific cross section. For open or closed thin-walled tubular beam type elongated structural members, roll forming is the most suitable method.

[0003] In recent years, with the increasing demand for lightweight in the field of automobiles and the like, high-strength steel tubular beam profiled parts have been increasingly applied. Due to the poor plasticity of high-strength steel, cracking easily occurs during forming, resulting in forming failure. For specific high-strength steel sheet and roll forming shape, the roll forming performance or forming limit of the sheet, generally represented by the minimum bending radius, needs to be understood in detail before the roll forming process is developed. Due to the special deformation mode of the roll forming process, a smaller bending radius than bending can be obtained. Therefore, it is difficult to accurately evaluate the roll forming performance of high-strength steel sheets by using conventional sheet forming performance test methods such as uniaxial tension, bending and cupping test.

[0004] The root cause of the above phenomenon is that the roll forming process has the characteristics of "multi-pass" and "dispersion" deformation. Research shows that dispersion forming can improve the plastic deformation ability of metal to a certain extent. Since the commonly used uniaxial tension, bending and cupping test methods do not consider the "multi-pass" and "dispersion" deformation characteristics of roll forming, the roll forming limit of various high-strength steel sheets cannot be accurately tested, and the needs of roll forming process design and on-site production quality control cannot be met. Therefore, it is necessary for those skilled in the art to develop a sheet forming performance test method for roll forming process to better meet the roll forming processing design needs. SUMMARY

[0005] In view of the above problems of the prior art, the technical problem to be solved by the present application is to provide a bending test method for roll forming limit, which can indirectly obtain the roll forming limit data of metal sheets, so as to better know the material performance and meet the roll forming processing design needs.

[0006] To solve the above technical problems, the present application adopts the following technical solutions:

[0007] A bending test method for the forming limit of roll forming, characterized in that a set of bending test dies with a plurality of passes and a constant opening distance of the female die and gradually decreasing round corner radius of the male die is designed and manufactured, the set of dies is used to respectively perform a limit condition test of one-time bending and a limit condition test of multiple-time bending according to the round corner radius of the male die from large to small, and a ratio of the limit bending radius of multiple-time bending to the limit bending radius of one-time bending is obtained; then, the same material plate to be tested is subjected to a limit condition test of one-time bending to obtain the limit bending radius of one-time bending, and then multiplied by the ratio to obtain the forming limit of roll forming.

[0008] In this way, the method adopts a plurality of sets of bending test dies with gradually decreasing round corner radius of the male die, uses a step-by-step stamping bending test to simulate and replace the deformation mode of roll forming, measures the limit bending radius of one-time bending and the limit bending radius of multiple-time bending, and according to the ratio, the limit bending radius of one-time bending of the same material plate to be tested is multiplied by the ratio to obtain the forming limit of roll forming. In this way, the test process is greatly simplified through simulation and conversion, and the forming limit of the metal plate of roll forming is obtained before roll forming, so that the material performance is better known and the design requirements of roll forming are met.

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

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

[0011] (2) using a numerical simulation method, obtaining the fracture strain ε k of the plate under study according to the stress-strain data and the elongation δ obtained from the tensile test;

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

[0013] 1) establishing a finite element analysis model consistent with the tensile test, using a ductile damage model, inputting an initial value of the fracture strain ε k , simulating the tensile test process to obtain a simulation result of the tensile deformation of the sample until fracture, and obtaining the elongation δ1 at this time;

[0014] 2) comparing the simulation- obtained elongation δ1 with the experiment- obtained elongation δ; if δ1> δ, it means that ε ε is set too large and needs to be reduced; on the contrary, if δ1< δ, it means that ε k is set too small and needs to be increased;

[0015] 3) according to the analysis result of step 2), adjusting the fracture strain ε kThe numerical value is adjusted, the finite element analysis model of the tensile test is input, the simulation calculation of the tensile test is re-performed, and ε k The numerical value of the elongation δ1 after adjustment is compared with δ, and if the difference between the two numerical values is greater than 10%, then the method described in step 2) is used to adjust ε k to 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 k as the final value. In this way, the approximation method can be used to more quickly simulate 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 round corner radius and the initial value W0 of the die opening distance are determined according to the following formula:

[0018]

[0019] Where δ is the elongation of the material obtained by the tensile test, and t is the thickness of the plate;

[0020] (4) Adjust the initial values of the punch round corner radius and the die opening distance obtained by formula (1) to obtain the size data basis of the mold design, and ensure that the bending test can occur fracture phenomenon to 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 round corner radius can be used without changing the die, so W0 and r0 obtained do not necessarily meet the requirements of material bending fracture, and further adjustment of the die opening distance and the punch round corner radius of the bending test mold is required to ensure that the fracture phenomenon occurs in the experiment, thereby determining the forming limit.

[0022] As an optimization, the adjustment steps of the initial punch round corner radius r0 and the initial die opening distance W0 parameters include:

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

[0024] 2) If the plate does not crack when the bending angle reaches 90°, then gradually reduce the punch round corner radius or the die opening distance, and re-simulate the bending process;

[0025] 3) until the bending angle reaches 90°, the plate material cracks, the opening distance W0 of the die at this time ′ the opening distance of the final series die, the die radius r at this time ′ 0 as the size data basis for designing the series die radius.

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

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

[0028] The multi-pass bending number N is determined in the following manner:

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

[0030] The bending angle θ of each pass i is calculated according to the following formula:

[0031]

[0032] Wherein, n is the variation index;

[0033] Let (ceiling), the punch radius r' of the Lth pass L is r0', and the punch radius r' of other passes is designed according to the following formula: i

[0034]

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

[0036] ​Thus, the number of passes and the bending angle of each pass are obtained according to the existing roll forming design method, and then the punch size of each pass is adjusted according to the change ratio of the bending angle of each pass, with the punch size of the single bending calculated previously as an intermediate value, so as to maximize the rationality of the punch size design, so that the test can produce a crack before the last punch in the extreme case of multiple bending, and the test result is obtained, so as to ensure the wider feasibility of the test.

[0037] (6) Process and manufacture the test die of multiple-pass bending, and prepare the plate specimen for bending test with double number of passes;

[0038] (7) Perform the physical experiment of single bending and multiple-pass bending, and the specific steps are as follows:

[0039] Multiple-pass bending: adopt the designed series of punches to perform multiple-pass continuous bending on the same plate specimen from large to small according to the round corner radius (i.e. take the plate of the last bending as the initial plate to continue the next bending experiment), and the bending angle of each pass is confirmed according to the designed value; until the crack appears on the outer surface of the deformation zone of the specimen, and the round corner radius of the punch of the last pass is taken as the limit bending radius r 2min of multiple-pass bending.

[0040] Single bending: take the punch of the multiple-pass bending test as the starting point, and then perform the single bending test of 90° on different plate specimens by using the punch with different round corner radii from small to large, until the specimen does not produce a crack, and the round corner radius of the punch is taken as the limit bending radius r 1min of single bending; or perform the single bending test of 90° on the plate specimen by using the punch with different round corner radii from large to small, and stop the experiment when the crack appears on the outer surface of the deformation zone of the specimen, and the bending radius of the last experiment is taken as the limit bending radius r 1min of single bending.

[0041] (8) Compare the limit bending radius r 2min of multiple-pass bending with the limit bending radius r 1min of single bending, and obtain the deviation coefficient k:

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

[0043] For the high-strength steel plate with a certain material grade and thickness t, the limit bending radius r 1min of single bending can be obtained by actual test or computer simulation test using the designed die set, and then the limit bending radius r 2min of roll forming is obtained by multiplying the coefficient, i.e.

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

[0045] Compared with the prior method, the present application has the advantages that the roll forming limit of high-strength steel plate is tested through simple bending experiment, and the multi-pass dispersed continuous deformation characteristics of the roll forming process are considered, which is simple, easy to operate, economical and efficient, and the test result is more accurate. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 The figure is a flow chart of the method of the present application.

[0047] Figure 2 The figure is a schematic diagram of the simulation of the tensile fracture of the plate sample in the implementation of the present application.

[0048] Figure 3 The figure is a schematic diagram of the structure of the bending test die used in the implementation of the present application. DETAILED DESCRIPTION

[0049] The present application will be further described in detail below in combination with specific embodiments.

[0050] In the implementation, a bending test method for roll forming limit is characterized in that a set of bending test dies with a multi-pass concave die opening distance unchanged and a convex die round corner radius gradually decreasing is designed and manufactured, as shown in Figure 3 The set of dies is used to respectively perform a one-time bending limit condition test and a multi-time bending limit condition test according to the convex die round corner radius from large to small on the plate sample (a standard sample for bending test in the form of a whole rectangular block), and a ratio of the limit bending radius of the multi-time bending to the limit bending radius of the one-time bending is obtained; then the same material plate to be tested is subjected to a one-time bending limit condition test to obtain the limit bending radius of the one-time bending, and then multiplied by the ratio to obtain the roll forming limit.

[0051] In this way, the present method uses a set of bending test dies with a convex die round corner radius gradually decreasing, and uses a step-by-step stamping and bending test to simulate the deformation mode of roll forming, and measures the limit bending radius of the one-time bending and the limit bending radius of the multi-time bending. According to the ratio, for the same material plate to be tested, only a one-time bending limit condition test is needed to obtain the limit bending radius of the one-time bending, and then multiplied by the ratio to obtain the roll forming limit. In this way, the test process is greatly simplified through simulation and conversion, and the roll forming limit of the metal plate before roll forming is obtained to better know the material performance and meet the design requirements of roll forming.

[0052] In the implementation, the present application includes the following steps, as shown in Figure 1 :

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

[0054] (2) Obtain the fracture strain ε of the plate under study by using numerical simulation method according to the stress-strain data and elongation δ obtained from the tensile test; k ;

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

[0056] 1) Establish a finite element analysis model consistent with the tensile test, adopt a ductile damage model, input the initial value of fracture strain ε k , simulate the tensile test process, obtain the simulation result of the sample tensile deformation until fracture, refer to Figure 2 , and obtain the elongation δ1 at this time;

[0057] 2) Compare the simulation obtained elongation δ1 with the experiment obtained elongation δ; if δ1> δ, it means that ε k is set too large and needs to be reduced; otherwise, if δ1< δ, it means that ε k is set too small and needs to be increased;

[0058] 3) Adjust the value of ε k according to the analysis result of step 2), input the finite element analysis model of the tensile test, and re-simulate the tensile test to obtain the elongation δ1 after adjusting the value of ε k ; compare it with δ, if the difference between the two values is greater than 10%, adjust ε k according to the method described in step 2) and re-simulate the tensile test process;

[0059] 4) Repeat steps 2) and 3) until the error between the simulation obtained elongation δ1 and the elongation δ of the tensile test is less than 10%, and take the fracture strain ε k at this time as the final value. In this way, the fracture strain value of the plate can be obtained more quickly by using the approximation method.

[0060] (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 r0 of the round corner radius of the convex die and the initial value W0 of the opening distance of the concave die according to the following formula:

[0061]

[0062] Wherein, δ is the elongation of the material obtained from the tensile test, and t is the thickness of the plate;

[0063] (4) Adjust the initial values of the punch round radius and the die gap obtained from formula (1) to obtain the size data basis of the die design, and ensure that the bending experiment can occur fracture phenomenon to 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 large to ensure that a punch with a larger round radius can be used without replacing the die, so the obtained W0 and r0 do not necessarily meet the requirements of material bending fracture, and further adjustment of the bending test die punch round radius and die gap is required to ensure that the experiment can occur fracture phenomenon, thereby determining the forming limit.

[0065] In implementation, the adjustment step of the initial value of the punch round radius r0 and the initial value of the die gap W0 parameters includes:

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

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

[0068] 3) Until the sheet metal cracks during the bending angle reaching 90°, the die opening gap W0 ′ at this time is taken as the opening gap of the final series of dies, and the punch round radius r ′ 0 at this time is taken as the size data basis of the design of the series of punch round radii.

[0069] (5) Determine the bending times N (i.e. the number of punches) of the multi-pass bending test die, and design the bending angle and punch round radius of each pass; When determining the multi-pass bending times, the target angle of the bending is uniformly set to 90°, according to the strength index and thickness t of the sheet metal, the bending times N of the multi-pass bending and the bending angle of each pass are determined according to the roll pressing process design, and the corresponding N punch round radii are finally designed and determined;

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

[0071] The multi-pass bending times N are determined in the following manner:

[0072] The number of multi-pass bending i is 5-8 for high-strength steel material with yield strength of 1180 MPa or less; the number of multi-pass bending i is 7-9 for high-strength steel material with yield strength of 1180-1300 MPa; the number of multi-pass bending i is 8-10 for high-strength steel material with yield strength of 1300-1500 MPa; the number of multi-pass bending i is 9-12 for high-strength steel material with yield strength of 1500 MPa or more; and the number of multi-pass bending i is obtained by referring to the existing roll pressing pass number design method

Meng Yifan. Relationship between process and microstructure of ultra-high strength steel in roll forming [D]. North University of Science and Technology, 2023; Liu Xiaoli. Research on springback prediction and control of complex section ultra-high strength steel continuous roll forming [D]. University of Science and Technology of Beijing, 2018; J Cheng, J Cao, Z Wei, et al. Modeling 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.

[0073] The bending angle θ of each pass i is calculated by the following formula:

[0074]

[0075] wherein n is a variable index; and the existing

Xiaona Hong, Liu Jiyin. Cold bending forming technology [M]. Chemical Industry Press, 2008

[0076] Let be the rounding radius r' of the punch of the Lth pass (rounded up), and the rounding radius r' of the punch of other passes L is r0', and the rounding radius r' of the punch of other passes i is designed by the following formula:

[0077]

[0078] wherein r' i is the rounding radius of the punch of each pass, and m is a correction coefficient (0°<θ i ≤90°, Therefore, m>0.5. Therefore, m<2. Therefore, the value range of m is 0.5-2, and 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, and θ iis the bending angle of the i-th pass, and r0 is the punch round corner radius obtained in step (4).

[0079] In this way, the number of passes and the bending angle of each pass are obtained according to the existing roll forming design method, and then the punch size that will definitely crack in single bending calculated previously is taken as an 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 maximize the rationality of the design of the punch size, so that it can produce a crack before the last punch in the extreme case test of multiple bending, and the test result is obtained, and the test has wider feasibility.

[0080] (6) Process and manufacture a test die for multiple-pass bending, and prepare a plate specimen for bending test with double the number of passes;

[0081] (7) Perform a single bending and multiple-pass bending physical experiment, and the specific steps are as follows:

[0082] Multiple-pass bending: A series of punches designed are used to perform multiple-pass continuous bending on the same plate specimen from large to small according to the round corner radius (i.e. the plate after the last bending is taken as the initial plate for the next bending experiment), and the bending angle of each pass is confirmed according to the designed value; until a crack appears on the outer surface of the deformation zone of the specimen, and the round corner radius of the punch of the last pass is taken as the limit bending radius r 2min of multiple-pass bending.

[0083] Single bending: Starting from the punch that produces a crack in the multiple-pass bending test, different punches with different round corner radii are used to perform 90° single bending on different plate specimens from small to large, and the test is stopped when the specimen does not produce a crack, and the round corner radius of the punch is taken as the limit bending radius r 1min of single bending; or punches with different round corner radii are used to perform 90° single bending on plate specimens from large to small, and the test is stopped when a crack appears on the outer surface of the deformation zone of the specimen, and the bending radius of the last experiment is taken as the limit bending radius r 1min of single bending.

[0084] (8) Compare the limit bending radius r 2min of multiple-pass bending with the limit bending radius r 1min of single bending to obtain 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 limit bending radius r 1min; multiply by a factor to get the limit bending radius r of the roll 2min ; i.e.

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

Claims

1. A bending test method for roll forming limit, characterized in that: First, a set of multi-pass bending test dies with a constant opening spacing of the die and a gradually decreasing punch corner radius is designed and manufactured. The die is used to perform a single bending limit test on the sheet metal and a multiple bending limit test with the punch corner radius increasing from large to small, obtaining the ratio of the limit bending radius of multiple bending to that of a single bending. Then, a single bending limit test is performed on the sheet metal of the same material to be tested to obtain the limit bending radius of a single bending. The forming limit of the roller is then obtained by multiplying the ratio. The method comprises the following steps: (1) Perform a standard tensile test on the plate with the determined 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 , 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 mold, 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) Adjust the initial values ​​of the punch corner radius and the die spacing obtained by formula (1) to obtain the dimensional data basis for mold design, ensuring that the bending test can produce fracture phenomena and thus determine 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 fillet radius of each pass; when determining the number of multi-pass bending times, the target bending angle is uniformly set to 90°, and according to the strength index and thickness t of the plate, the number of multi-pass bending times N and the bending angle of each pass are determined in accordance with the rolling process design method, and finally the fillet 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 multi-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 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）。 2. The bending test method of the roll forming limit according to claim 1, 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) According to 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 ; compare it with Compare the two values. If the difference 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.

3. The bending test method of the roll forming limit according to claim 1, characterized in that: In step (4), the initial value of the punch radius is Initial value of the distance between die and cavity 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 punch corner radius or the die spacing and re-simulate the bending process; 3) When the bending angle reaches 90°, the sheet metal cracks and the die opening spacing is adjusted. As the opening spacing of the final series of dies, the punch corner radius at this time Serves as the dimensional data basis for the design of series punch corner radius.

4. The bending test method of the roll forming limit according to claim 1, wherein: Step (5) specifically includes the following steps: The number of bends N for multiple passes is determined as follows: For high-strength steel materials with a yield strength of 1180 MPa and below, the number of multi-pass bending i is 5 to 8 times; for high-strength steel materials with a yield strength between 1180 and 1300 MPa, the number of multi-pass bending i is 7 to 9 times; for high-strength steel materials with a yield strength between 1300 and 1500 MPa, the number of multi-pass bending i is 8 to 10 times; for high-strength steel materials with a yield strength of 1500 MPa 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 up. Punch corner radius of each pass Determined , the punch corner radius of other passes Push-to-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 the pass, is the punch corner radius obtained by the final simulation in step (4).

5. The bending test method of roll forming limit according to claim 1, wherein: Step (7) specifically includes the following steps: Multi-pass bending test: Using a designed series of punches, perform multiple continuous bends on the same sheet metal specimen in descending order of fillet 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 specimen deformation zone, the fillet 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 that cracked after multiple bending tests, perform a 90° bending test on different plate samples using punches with different fillet radii from small to large, until the sample does not crack. 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 bending. .

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