A method for evaluating edge cracking in high-strength steel forming using hole expansion limit strain

Abstract

This invention provides a method for evaluating edge cracking in high-strength steel during forming using the hole expansion limit strain, belonging to the technical field of stamping forming of high-strength steel for automobiles. The method includes the following steps: by optimizing the stopping criteria of the hole expansion test and through repeated iterations, determining the punch limit stroke under the condition that hole expansion of high-strength steel does not produce microcracks; obtaining the hole expansion limit strain Δεultra of the high-strength steel based on the average of the hole expansion values ​​of n high-strength steel samples corresponding to the punch limit strokes without microcracks; comparing this value with the maximum equivalent strain occurring at the edge of the high-strength steel during actual stamping forming; or comparing this value with the maximum equivalent strain occurring at the edge during simulated stamping forming of high-strength steel in CAE analysis software to obtain the evaluation result. This method is unaffected by the working environment, is fast, simple, easy to master, and highly accurate, and can be widely applied to stamping production lines.

Description

Technical Field

[0001] This invention relates to the technical field of stamping high-strength steel for automobiles, and more particularly to a method for evaluating edge cracking of high-strength steel forming using hole expansion limit strain. Background Technology

[0002] With the rise and rapid development of new energy vehicles in China, lightweight vehicle bodies represent a significant challenge and competitive opportunity for automakers. High-strength steel is widely used in safety structural components of vehicle bodies, such as longitudinal beams, sills, bumpers, and B-pillars, to achieve lightweight manufacturing goals. However, as the strength of high-strength steel increases, its plasticity decreases, and edge cracking is prone to occur during stamping after shearing, such as bending cracking of the sheared section, edge cracking of roll bending, and cracking of expanded and flanged edges, resulting in a significant increase in stamping scrap rates. The edge cracking problem in high-strength steel stamping differs from that of traditional mild steel stamping. High-strength steel has a high yield strength ratio and a low work hardening index (n-value), meaning it fractures quickly after localized plastic deformation. Therefore, its local forming performance has a more significant impact on fracture. Furthermore, the intense plastic deformation that occurs during shearing of high-strength steel sheets leads to significant changes, with more pronounced localized non-uniform hardening at the sheared edges, making edge cracking more likely during subsequent forming processes. Therefore, the edge cracking problem of high-strength steel cannot be accurately predicted based solely on the overall forming performance indicators such as the material's yield strength, tensile strength, and elongation.

[0003] In the forming process of sheet metal, the Forming Limit Diagram (FLD) is one of the most important failure evaluation methods. However, the FLD, based on the necking instability theory, cannot accurately predict edge cracking in high-strength steel, and edge cracking occurs earlier than predicted. To address the edge cracking problem in high-strength steel, the hole expansion test is widely used to test its edge forming performance. The hole expansion test is a test to detect the edge forming performance of a material. Material with a central hole is placed under a hole expansion punch, and pressure is applied to increase the diameter of the central hole until cracks appear. The hole expansion rate reflects the edge forming performance of the material. The results of the hole expansion test can well reflect the edge's resistance to cracking, making it a good method for studying the edge cracking problem of advanced high-strength steel. Therefore, accurate measurement of the hole expansion rate of high-strength steel is key to evaluating edge cracking in high-strength steel forming. In standard hole expansion tests, the hole expansion rate is based on observing signs that the sample hole edge is about to crack. The movement speed of the cone punch should be slowed down immediately to accurately capture the instant when the sample hole edge cracks, that is, to ensure that the sample hole edge deformation state is exactly at the cracking moment when the test is stopped. Therefore, two hole expansion test stopping standards have been derived: (1) Stop the test by visually observing the instant when the sample hole edge cracks. (2) Stop the test immediately when the force value of the testing machine suddenly drops during the hole expansion test, which is considered to be the moment when the sample hole edge cracks. The first observation method has a large error and often results in the problem of measuring an excessively large hole expansion rate; the second control method has lag or ineffectiveness for hole expansion tests of high-strength steel. Through a large number of tests, it has been verified that the force value detection method often fails to detect the change in force value at the moment when the hole edge cracks, leading to the failure of the test. Furthermore, both of the above methods yielded results of through-cracks, meaning the test was stopped when the first crack corresponding to the thickness of the sample appeared. Such through-cracks indicate that the high-strength steel has reached the fracture failure mode, and there is a large error in the evaluation of edge cracking in high-strength steel.

[0004] Therefore, there is an urgent need for an accurate and rapid evaluation method for edge cracking in high-strength steel forming, providing a quantitative evaluation index. On the one hand, this index can guide the actual stamping process and ensure the stamping pass rate and stability; on the other hand, it can serve as a threshold parameter for evaluating whether edge cracking occurs during the simulation of high-strength steel stamping in CAE software. Summary of the Invention

[0005] In view of the technical problems mentioned in the background section, this invention provides a method for evaluating edge cracking in high-strength steel during forming using the hole-expansion ultimate strain. This method utilizes the hole-expansion performance of high-strength steel to evaluate edge cracking during the forming process, linking the application performance and mechanical properties of high-strength steel, thus combining the stamping performance of high-strength steel with the hole-expansion ultimate strain.

[0006] The technical means employed in this invention are as follows:

[0007] A method for evaluating edge cracking in high-strength steel forming using hole expansion limit strain includes the following steps:

[0008] Step 1: By optimizing the stopping criteria of the hole expansion test and through repeated iterations, determine the limit stroke of the punch under the condition that hole expansion of high-strength steel does not produce microcracks;

[0009] Step 2: Based on the average value of the hole expansion values ​​of the high-strength steel that do not produce microcracks corresponding to the punch limit stroke of the n samples, obtain the hole expansion limit strain Δεultra of the high-strength steel.

[0010] Step 3: The maximum equivalent strain occurring at the edge of high-strength steel during the actual stamping process. Comparisons are made to obtain evaluation results; or the maximum equivalent strain occurring at the edge during the high-strength steel stamping process is simulated using CAE analysis software. By making comparisons, evaluation results can be obtained;

[0011] like or If this high-strength steel is found to be prone to edge cracking during the stamping of automotive parts, the stamping result will be deemed unqualified; if or If the high-strength steel is used to ensure the safety of the edge forming results when stamping automotive parts, and no edge cracking occurs, then the stamping result is considered qualified.

[0012] Furthermore, in step 1, the diameter of the circular hole of the sample punched in the optimized hole expansion test is 10 mm; and the circular hole of the sample is located at the center of the sample, and the distance between the center of the circular hole of the sample and any edge of the sample is not less than 45 mm.

[0013] Furthermore, in step 1, during the optimization of the hole enlargement test, the direction of the burrs formed after punching the sample is consistent with the direction of the hole to be enlarged, and the center of the sample's circular hole is aligned with and perpendicular to the axis of the hole enlargement punch; and the lower surface of the sample and the upper surface of the punch are on the same horizontal line, defining the stroke of the hole enlargement punch as zero point.

[0014] Furthermore, the speed of the punch is ≤1mm / s.

[0015] Furthermore, the stopping criterion is the appearance of a through crack or microcrack at the circular hole of the sample.

[0016] Furthermore, in step 1, determining the punch limit stroke without microcracks in the high-strength steel hole expansion includes the following steps:

[0017] Step 11: After the test stops according to the stopping standard, record the stroke of the punch on the testing machine as X0;

[0018] Step 12: Perform a hole enlargement test with a stroke of X0-1mm to determine whether microcracks appear in the sample hole; if no microcracks exist in the sample hole, proceed to step 13; if microcracks exist in the sample hole, continue iterating with a stroke of -1mm based on the X0-1mm stroke, i.e., X0-2mm, X0-3mm, ..., until no microcracks exist in the sample hole, and then proceed to step 13.

[0019] Step 13: Record the stroke of the punch on the testing machine, denoted as X1; record the previous stroke X1+1mm as the stroke from which the microcrack appeared in the round hole, and then proceed to step 14:

[0020] Step 14: Perform a hole enlargement test with a stroke of X1+0.5mm to determine whether microcracks appear in the sample hole; if there are no microcracks in the sample hole, proceed to step 15; if there are microcracks in the sample hole, proceed to step 16.

[0021] Step 15: Select the hole enlargement to be between X1+0.5mm and X1+1mm, and then proceed to step 17;

[0022] Step 16: Select a hole enlarging stroke between X1 and X1+0.5mm, and then proceed to step 17;

[0023] Step 17: Continue iterating with stroke increments of 0.2mm and 0.1mm respectively until microcracks appear in adjacent strokes and microcracks do not appear. Record the strokes without microcracks as X2, and the strokes with microcracks are X2+0.1mm.

[0024] The limit stroke of the punch under the condition that the high-strength steel hole expansion does not produce microcracks is X2.

[0025] Furthermore, in step 2, the average value of the hole expansion values ​​of n high-strength steel holes that do not produce microcracks is calculated as follows:

[0026] Take n samples, n≥3, and set the punch stroke to X2 to conduct a hole expansion test. Record the hole expansion values ​​obtained from the test as l1, l2, ..., ln. Then the corresponding hole expansion limit strains εultra1, εultra2, ..., εultraran are respectively:

[0027]

[0028] The average value Δεultra of the hole expansion values ​​of n high-strength steels without producing microcracks is:

[0029] Δεultra=(εultra1+εultra2+…+εultran) / n.

[0030] Compared with the prior art, the present invention has the following advantages:

[0031] (1) This invention uses the limit expansion value that does not produce microcracks in the expansion test instead of the expansion value for through cracks, which is more consistent with the situation where cracking occurs during the actual stamping process of high-strength steel, thus evaluating the stamping as unqualified. Furthermore, by using the punch stroke to control the test stop, the limit punch stroke that does not produce microcracks at the hole edge is determined through repeated iterations, thereby obtaining the limit expansion value for high-strength steel. The obtained limit expansion value is more accurate, eliminating the error caused by excessively large cracks observed manually in traditional methods, which leads to excessively high expansion values.

[0032] (2) The hole expansion limit strain of high-strength steel obtained by this invention based on the limit hole expansion value is an inherent performance index of high-strength steel material. The data is easy to obtain and is not affected by external factors such as stamping process and mold condition. Furthermore, hole expansion limit strains of different grades and thicknesses can be obtained, thereby establishing a database, reducing the amount of repeated experiments, and ensuring data consistency. At the same time, this invention provides a quantitative evaluation index, which can be used in CAE software to simulate the threshold parameter for evaluating whether the edge cracks during the stamping process of high-strength steel, effectively avoiding the error caused by using the high-strength steel FLC curve to evaluate whether the edge cracks during the forming process.

[0033] (3) This invention is not affected by the working environment, is quick and simple to operate, easy to master and highly accurate, and can be widely applied to the stamping production line. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 This is a schematic diagram of the overall process of the present invention.

[0036] Figure 2 The present invention describes a through-crack state that appears at the circular hole of the sample during hole enlargement.

[0037] Figure 3 The present invention describes the appearance of microcracks at the circular hole of the sample during hole enlargement.

[0038] Figure 4This is a high-strength steel CP780-1.60mm stamped car seat component with a square grid printed on its surface, as described in Embodiment 1 of the present invention.

[0039] Figure 5 This is a high-strength steel DP980-1.40mm stamped automotive chassis component with a square grid printed on its surface, as described in Embodiment 2 of the present invention. Detailed Implementation

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

[0041] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in orders other than those illustrated or described herein. Furthermore, the terms "comprising" and "having," and any variations thereof, are intended to cover a non-exclusive inclusion; for example, a process, method, system, product, or apparatus that comprises a series of steps or units is not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to such processes, methods, products, or apparatus.

[0042] like Figure 1 As shown, this invention provides a method for evaluating edge cracking in high-strength steel forming using the hole expansion limit strain, comprising the following steps:

[0043] Step 1: By optimizing the stopping criteria for the hole expansion test (in this application, the stopping criteria is the appearance of a through crack or micro-crack at the circular hole of the specimen), and through repeated iterations, the limit stroke of the punch is determined when the hole expansion of high-strength steel does not produce micro-cracks; the diameter of the circular hole of the specimen punched in the hole expansion test is optimized to be 10 mm; and the circular hole of the specimen punched is located in the center of the specimen, and the distance between the center of the circular hole of the specimen punched and any edge of the specimen is not less than 45 mm.

[0044] In a preferred embodiment, during the optimized hole-expanding test, the direction of the burrs formed after punching the sample is consistent with the direction of the hole to be expanded, and the center of the sample's circular hole is aligned with and perpendicular to the axis of the hole-expanding punch; and the lower surface of the sample and the upper surface of the punch are on the same horizontal line, defining the stroke of the hole-expanding punch as zero point.

[0045] Preferably, in this application, the speed of the punch is ≤1 mm / s.

[0046] As a preferred method, determining the punch limit stroke without microcracks during high-strength steel hole expansion includes the following steps:

[0047] Step 11: After the test stops according to the stopping standard, record the stroke of the punch on the testing machine as X0;

[0048] Step 12: Perform a hole enlargement test with a stroke of X0-1mm to determine whether microcracks appear in the sample hole; if no microcracks exist in the sample hole, proceed to step 13; if microcracks exist in the sample hole, continue iterating with a stroke of -1mm based on the X0-1mm stroke, i.e., X0-2mm, X0-3mm, ..., until no microcracks exist in the sample hole, and then proceed to step 13.

[0049] Step 13: Record the stroke of the punch on the testing machine, denoted as X1; record the previous stroke X1+1mm as the stroke from which the microcrack appeared in the round hole, and then proceed to step 14:

[0050] Step 14: Perform a hole enlargement test with a stroke of X1+0.5mm to determine whether microcracks appear in the sample hole; if there are no microcracks in the sample hole, proceed to step 15; if there are microcracks in the sample hole, proceed to step 16.

[0051] Step 15: Select the hole enlargement to be between X1+0.5mm and X1+1mm, and then proceed to step 17;

[0052] Step 16: Select a hole enlarging stroke between X1 and X1+0.5mm, and then proceed to step 17;

[0053] Step 17: Continue iterating with stroke increments of 0.2mm and 0.1mm respectively until microcracks appear in adjacent strokes and microcracks do not appear. Record the strokes without microcracks as X2, and the strokes with microcracks are X2+0.1mm.

[0054] The limit stroke of the punch when expanding the hole in high-strength steel without producing microcracks is X2.

[0055] Step 2: Obtain the ultimate strain Δεultra of the high-strength steel by averaging the hole expansion values ​​of the n high-strength steel samples that do not produce microcracks, corresponding to the ultimate punch stroke of the n samples. The calculation method for the average of the n high-strength steel hole expansion values ​​that do not produce microcracks in Step 2 is as follows:

[0056] Take n samples, n≥3, and set the punch stroke to X2 to conduct a hole expansion test. Record the hole expansion values ​​obtained from the test as l1, l2, ..., ln. Then the corresponding hole expansion limit strains εultra1, εultra2, ..., εultraran are respectively:

[0057]

[0058] The average value Δεultra of the hole expansion values ​​of n high-strength steels without producing microcracks is:

[0059] Δεultra=(εultra1+εultra2+…+εultran) / n.

[0060] Step 3: The maximum equivalent strain occurring at the edge of high-strength steel during the actual stamping process. Comparisons are made to obtain evaluation results; or the maximum equivalent strain occurring at the edge during the high-strength steel stamping process is simulated using CAE analysis software. By making comparisons, evaluation results can be obtained;

[0061] like or If this high-strength steel is found to be prone to edge cracking during the stamping of automotive parts, the stamping result will be deemed unqualified; if or If the high-strength steel is used to ensure the safety of the edge forming results when stamping automotive parts, and no edge cracking occurs, then the stamping result is considered qualified.

[0062] Example 1

[0063] High-strength steel CP780-1.60mm stamped automotive seat parts.

[0064] (1) Punch a round hole in high-strength steel according to GB / T 15825.4-2008 “Metallic Sheet Forming Properties and Test Methods Part 4: Hole Expansion Test”. Prepare a square specimen with a length and width of 100 mm and punch a round hole with a diameter of 10 mm at the center of the specimen. The inner diameter of the punching die is 10.4 mm.

[0065] (2) When placing the specimen to be expanded on the reaming test machine, ensure that the direction of the burrs formed after punching the specimen is consistent with the reaming direction, and ensure that the center of the round hole is aligned with and perpendicular to the axis of the reaming punch (cone). Finally, ensure that the lower surface of the specimen to be expanded is on the same horizontal line as the upper surface of the reaming punch (cone), and define the stroke of this reaming punch (cone) on the reaming test machine as the zero point. Then start the test machine and press the punch (cone) into the round hole on the specimen at a speed of 0.8 mm / s. When a crack appears at the round hole of the specimen, stop the test immediately. Record the stroke traveled by the punch (cone) on the test machine at this time, which is 8.1 mm.

[0066] (3) First, a hole-expanding experiment was conducted with a punch stroke of 8.1mm - 1mm = 7.1mm, and microcracks were still found. Then, a hole-expanding experiment was conducted with a punch stroke of 7.1mm - 1mm = 6.1mm, and no microcracks were found. Then, an iteration of 0.5mm was performed between the 6.1mm and 7.1mm strokes, and a hole-expanding experiment was conducted with a punch stroke of 6.6mm, and no microcracks were found. Then, an iteration of 0.2mm was performed between the 6.6mm and 7.1mm strokes, and a hole-expanding experiment was conducted with a punch stroke of 6.8mm, and microcracks were found. Finally, an iteration of 0.1mm was performed between the 6.6mm and 6.8mm strokes, and a punch stroke of 6.7mm was observed, and no microcracks were found. Therefore, 6.7mm is the limit stroke of the punch (cone) when hole-expanding of high-strength steel CP780-1.6mm does not produce microcracks.

[0067] (4) Take 5 specimens and set the punch (cone) stroke to 6.7 mm to conduct hole enlargement tests. Record the hole enlargement values ​​obtained from these 5 tests as 11.45 mm, 11.46 mm, 11.45 mm, 11.48 mm, and 11.45 mm. Then, the hole enlargement limit strain ε of these specimens is... ultra1 ε ultra2 ... ε ultran They are respectively:

[0068] The average value is denoted as Δε. ultra =(0.1354+0.1363+0.1354+0.1380+0.1354) / 5=0.1361, then Δε ultra =0.1361 is the ultimate strain for reaming high-strength steel CP780-1.60mm.

[0069] (5) Before the high-strength steel CP780-1.60mm is stamped, a square grid with a size of 2.00mm×2.00mm is printed on the edge area of ​​the high-strength steel plate that has been blanked by electrochemical etching.

[0070] (6) The high-strength steel plate with a 2.00mm × 2.00mm square grid printed on the edge area is stamped into shape, such as... Figure 3 As shown, the size of the square grid changes. Taking the square grid as a unit, let's take a unit grid size change of 2.11mm × 2.03mm as an example.

[0071] The strains in its three directions are ε1, ε2, and ε3, respectively, then we can obtain:

[0072]

[0073] Furthermore, combining this with the principle of constant volume during plastic deformation: ε1 + ε2 + ε3 = 0,

[0074] We can obtain ε3 = -(ε1 + ε2) = -(0.054 + 0.015) = -0.069. Then, using the equivalent strain calculation formula:

[0075]

[0076] The equivalent strain of this element is calculated to be 0.0726. Furthermore, the maximum equivalent strain of the edge mesh region after stamping deformation can be calculated. The maximum equivalent strain in the edge area of ​​this CP780-1.60mm stamped automotive seat component.

[0077] (7) The maximum equivalent strain of high-strength steel in the edge forming area of ​​actual stamped automotive parts, 0.0913, is compared with 90% of the hole expansion limit strain of high-strength steel, 0.1391, i.e., 0.1252. It should be noted that a 10% safety threshold is given here to ensure the stability and safety of the actual stamping process. Therefore, 0.0913 < 0.1252, i.e. This indicates that the high-strength steel is safe for edge forming during the stamping of automotive parts, and that edge cracking will not occur.

[0078] Example 2

[0079] High-strength steel DP980-1.40mm stamped automotive chassis parts.

[0080] (1) Punch a round hole in high-strength steel according to GB / T 15825.4-2008 “Metallic Sheet Forming Properties and Test Methods Part 4: Hole Expansion Test”. Prepare a square specimen with a length and width of 100 mm and punch a round hole with a diameter of 10 mm at the center of the specimen. The inner diameter of the punching die is 10.3 mm.

[0081] (2) When placing the specimen to be expanded on the reaming test machine, ensure that the direction of the burrs formed after punching the specimen is consistent with the reaming direction, and ensure that the center of the round hole is aligned with and perpendicular to the axis of the reaming punch (cone). Finally, ensure that the lower surface of the specimen to be expanded is on the same horizontal line as the upper surface of the reaming punch (cone), and define the stroke of this reaming punch (cone) on the reaming test machine as the zero point. Then start the test machine and press the punch (cone) into the round hole on the specimen at a speed of 0.8 mm / s. When a crack appears at the round hole of the specimen, stop the test immediately. Record the stroke traveled by the punch (cone) on the test machine at this time, which is 9.7 mm.

[0082] (3) First, a hole-expanding experiment was conducted with a punch stroke of 9.7mm-1mm=8.7mm, and no microcracks were found. Then, the hole-expanding experiment was conducted with a punch stroke of 9.2mm, iterating in increments of 0.5mm between the 8.7mm and 9.7mm strokes. Microcracks appeared. Then, the hole-expanding experiment was conducted with a punch stroke of 8.9mm, iterating in increments of 0.1mm between the 8.7mm and 8.9mm strokes. Microcracks appeared at the punch stroke of 8.8mm. Therefore, 8.7mm is the limit stroke of the punch (cone) for hole expansion without microcracks in high-strength steel DP980-1.40mm.

[0083] (4) Take 4 specimens and set the punch (cone) stroke to 8.7 mm for hole enlargement test. Record the hole enlargement values ​​obtained from these 4 tests as 12.15 mm, 12.16 mm, 12.16 mm, and 12.16 mm. Then, the hole enlargement limit strains εultra1, εultra2, εultra3, and εultra4 of these specimens are respectively:

[0084]

[0085] The average value Δεultra = (0.1947 + 0.1956 + 0.1956 + 0.1956) / 4 = 0.1954

[0086] Then Δεultra=0.1954 is the ultimate strain of the 1.40mm diameter hole expansion of high-strength steel DP980.

[0087] (5) Before the high-strength steel DP980-1.40mm is stamped, a square grid with a size of 2.50mm×2.50mm is printed on the edge area of ​​the high-strength steel plate that has been blanked by electrochemical etching.

[0088] (6) The high-strength steel plate with a 2.50mm × 2.50mm square grid printed on the edge area is stamped into shape, such as... Figure 4As shown, the size of the square grid changes. Taking the square grid as a unit, let's take a unit grid size change of 2.73mm × 2.55mm as an example.

[0089] The strains in its three directions are ε1, ε2, and ε3, respectively, then we can obtain:

[0090]

[0091] Furthermore, combining this with the principle of constant volume during plastic deformation: ε1 + ε2 + ε3 = 0,

[0092] We can obtain ε3 = -(ε1 + ε2) = -(0.088 + 0.020) = -0.108. Then, using the equivalent strain calculation formula:

[0093]

[0094] The equivalent strain of this element is calculated to be 0.1149. Furthermore, the maximum equivalent strain of the edge mesh region after stamping deformation can be calculated. The maximum equivalent strain in the edge area of ​​this DP980-1.40mm stamped automotive seat component

[0095] (7) The maximum equivalent strain of high-strength steel in the edge forming area of ​​actual stamped automotive parts, 0.1698, is compared with 90% of the hole expansion limit strain of high-strength steel, 0.1954, i.e., 0.1759. It should be noted that a 10% safety threshold is given here to ensure the stability and safety of the actual stamping process. Therefore, 0.1698 < 0.1759, i.e. This indicates that the high-strength steel is safe for edge forming during the stamping of automotive parts, and that edge cracking will not occur.

[0096] The sequence numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments. In the above embodiments of the present invention, the descriptions of each embodiment have their own emphasis; parts not described in detail in a certain embodiment can be referred to in the relevant descriptions of other embodiments. It should be understood that the disclosed technical content in the several embodiments provided in this application can be implemented in other ways.

[0097] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for evaluating edge cracking in high-strength steel forming using hole expansion ultimate strain, characterized in that, Includes the following steps: Step 1: By optimizing the stopping criteria of the hole expansion test and through repeated iterations, determine the limit stroke of the punch when the hole expansion of high-strength steel does not produce microcracks; optimize the diameter of the circular hole of the sample punched in the hole expansion test to 10mm; the stopping criterion is the appearance of microcracks at the circular hole of the sample. Step 1, determining the punch limit stroke under the condition that the high-strength steel hole expansion does not produce microcracks, includes the following steps: Step 11: After the test stops according to the stopping standard, record the travel distance of the punch on the testing machine as X0; Step 12: Perform a hole enlargement test with a stroke of X0-1mm to determine whether microcracks appear in the sample hole; if no microcracks exist in the sample hole, proceed to step 13; if microcracks exist in the sample hole, continue iterating with a stroke of -1mm based on the X0-1mm stroke, i.e., X0-2mm, X0-3mm, ..., until no microcracks exist in the sample hole, and then proceed to step 13. Step 13: Record the stroke of the punch on the testing machine, denoted as X1; record the previous stroke X1+1mm as the stroke from which the microcrack appeared in the round hole, and then proceed to step 14: Step 14: Perform a hole enlargement test with a stroke of X1+0.5mm to determine whether microcracks appear in the sample hole; if there are no microcracks in the sample hole, proceed to step 15; if there are microcracks in the sample hole, proceed to step 16. Step 15: Select the hole enlargement to be between X1+0.5mm and X1+1mm, and then proceed to step 17; Step 16: Select a hole enlarging stroke between X1 and X1+0.5mm, and then proceed to Step 17; Step 17: Continue iterating with stroke increments of 0.2mm and 0.1mm respectively until microcracks appear in adjacent strokes and microcracks do not appear. Record the strokes without microcracks as X2, and the strokes with microcracks are X2+0.1mm. The limit stroke of the punch under the condition that the high-strength steel hole expansion does not produce microcracks is X2; Step 2: According to n The limit stroke of the punch corresponding to a sample n The average value of the hole expansion values ​​of high-strength steel without microcracks is used to obtain the hole expansion limit strain Δεultra of the high-strength steel; in step 2, n The method for calculating the average value of hole expansion for high-strength steel without producing microcracks is as follows: Take n samples, n≥3, and set the punch stroke to X2 to conduct a hole expansion test. Record the hole expansion values ​​obtained from the test as l1, l2, ..., ln. Then the corresponding hole expansion limit strains εultra1, εultra2, ..., εultraran are respectively: ； ； …… ； but n The average value Δεultra of the hole expansion value that does not produce microcracks in high-strength steel is: Δεultra=(εultra1+εultra2+……+εultran) / n; Step 3: The maximum equivalent strain occurring at the edge of high-strength steel during the actual stamping process. max Comparisons are made to obtain evaluation results; or the maximum equivalent strain occurring at the edge during the high-strength steel stamping process is simulated using CAE analysis software. max By making comparisons, we can obtain evaluation results; like max or max If the high-strength steel has an edge cracking rate of ≥90%Δεultra, it is considered unqualified because the edge of the steel is prone to cracking during the stamping of automotive parts. max or max If the value is less than 90%Δεultra, the high-strength steel is considered safe for edge forming when stamping automotive parts, and edge cracking will not occur. The stamping result is considered qualified.

2. The method for evaluating edge cracking of high-strength steel forming using the hole expansion limit strain according to claim 1, characterized in that, In step 1, the punched sample hole is located at the center of the sample, and the distance between the center of the punched sample hole and any edge of the sample is not less than 45 mm.

3. A method for evaluating edge cracking of high-strength steel forming using the hole expansion limit strain according to claim 1 or 2, characterized in that, In step 1, during the optimization test, the direction of the burrs formed after punching the sample is consistent with the direction of the hole to be expanded, and the center of the sample's circular hole is aligned with and perpendicular to the axis of the expanding punch; and the lower surface of the sample and the upper surface of the punch are on the same horizontal line, and the stroke of the expanding punch is defined as zero point.

4. The method for evaluating edge cracking of high-strength steel forming using the hole expansion limit strain according to claim 3, characterized in that, The speed of the punch is ≤1mm / s.

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