A method for calculating an index for evaluating the overall formability of a metal sheet

Abstract

This invention discloses a method for calculating indices to evaluate the overall formability of sheet metal. The method involves conducting mechanical property tests on the sheet metal, followed by: A1: calculating the true stress-strain curve of the material based on the test data; A2: obtaining the true stress-strain curve over a large strain range through model extrapolation, simulation benchmarking, or other large strain testing methods; A3: calculating the change of the instantaneous strain hardening index with plastic strain based on the true stress-strain data; A4: calculating and plotting the forming limit curve based on the obtained limiting strain; A5: calculating the forming strain homogenization capability index; A6: setting evaluation criteria to classify and rate the forming capability indices; and A7: comprehensively evaluating the forming performance of the sheet metal based on the evaluation results. This method, by considering the true stress-strain curve over a large strain range and the forming limit strain, can more comprehensively and accurately evaluate the overall formability of sheet metal.

Description

Technical Field

[0001] This invention relates to the field of metal processing, and in particular to a method for calculating an index to evaluate the overall formability of metal sheets. Background Technology

[0002] Sheet metal is widely used in industrial production, and its formability directly affects product quality and production efficiency. Especially with increasing industry demands for environmental protection and safety, more and more new materials are being developed and applied. Applying these materials to vehicle bodies requires a comprehensive consideration of formability and performance, demanding accurate understanding of the material's mechanical properties and forming characteristics during the design phase. New materials often introduce complex microstructures, leading to complex deformation mechanisms, which presents difficulties and challenges for evaluating formability and for successful forming applications.

[0003] Traditional methods for evaluating the formability of sheet metal suffer from problems such as inconsistent evaluation standards, complex testing procedures, and imprecise evaluation results. For sheet metal materials used in forming, the strain hardening index (n-value) and the forming limit diagram are commonly used indicators for testing and evaluating formability. The n-value is usually calculated using stress-strain curves obtained from uniaxial tensile tests; a higher n-value indicates a stronger strain homogenization ability of the material. However, research has found that, except for traditional sheet metal, the instantaneous strain hardening index of new materials and most sheet metal undergoes significant changes during the strain development stage. Therefore, using traditional methods, selecting an average n-value over a certain strain range to represent the overall performance, is no longer applicable to evaluating the formability of all sheet metal. Furthermore, traditional methods cannot directly compare the formability of sheet metal with different elongation rates, nor do they consider the deformation range after the equivalent plastic strain exceeds the uniform elongation rate in a uniaxial tensile test.

[0004] Forming limit curves (FLCs) can be used for forming simulations or part forming experiments to analyze the risk of instability or cracking. FLD0 is defined as the lowest point of the principal strain in the forming limit curve, i.e., the forming limit value when the material is in or near a plane strain state. For traditional materials, FLC0 is typically used to compare the forming capabilities of different materials laterally. A comparison between new and traditional materials reveals that while the FLC0 values ​​and limit points are approximately the same for both materials, there are significant differences in the ultimate height of the crown (LDH) for specimens of different widths. The results indicate that, at the same forming height, materials with higher n-values ​​achieve a more uniform strain distribution in the formed part. Although the ultimate strain at instability is similar, the material exhibits better part forming capability. Therefore, comparing only FLD0 or forming limit curves cannot fully reflect the forming performance of different materials.

[0005] In the published documents, patent application number CN202311265567.7 proposes a method and system for evaluating the bending performance of longitudinally sheared beams. The method includes obtaining the mechanical response curves of the characteristic regions of the longitudinal shearing of non-standard tensile specimens; determining the failure strain of the longitudinal shear surface by inversely comparing the mechanical response curves of the non-standard tensile specimens; establishing a bending finite element model, setting boundary conditions and bending angles, and calculating the plastic strain required for beam forming; establishing the relationship between the surface hardness and formability of the longitudinal shear surface based on the failure strain of the longitudinal shear surface and the plastic strain required for beam forming, and evaluating the formability of the longitudinal shear surface through surface hardness. Based on this method, an evaluation system is also proposed.

[0006] The patent document with application number CN202211734012.8 proposes a method for evaluating the local formability of high-strength steel, which uses the rate of change of fracture thickness (fracture thinning rate) of thin plate uniaxial tensile specimens before and after fracture in conventional mechanical property testing to predict the local formability of the material.

[0007] The patent document with application number CN202311644113.0 proposes a method for measuring the edge crack resistance of dual-phase steel. The method involves measuring the cross-sectional area of ​​a uniaxial tensile specimen to construct the true fracture strain (TFS). The method also involves obtaining the true uniform strain (εu) by measuring the uniform elongation Ag during the tensile process. Finally, the method constructs a formability index (F0) using the true fracture strain and the true uniform strain. The larger the F0, the stronger the material's resistance to edge cracking.

[0008] Patent application CN202311284713.0 proposes a method for determining the edge forming capability of a thin metal sheet: preparing a hole-expanding sample, conducting hole-expanding experiments and DIC experiments, screening sample data, selecting characteristic points, measuring the strain data of the characteristic points, calculating the data, and performing results and analysis. This method predicts the edge forming capability of a material by measuring its real-time comprehensive strain rate.

[0009] Patent application number CN202010619705.7 discloses a rapid testing mold for the forming performance of cold-rolled steel sheets and its usage method. The device includes an upper stamping mold and a lower forming mold, which fit together. The upper and lower stamping molds are equipped with multiple conical cups of varying shapes and depths, designed to simulate the forming process of stamping steel sheets for automotive and home appliance expansion parts. By comparing the cracking conditions of a standard sample with those of the steel sheet under test, the forming performance of the steel sheet under test can be quickly evaluated.

[0010] Patent application CN202310430012.7 discloses a method for predicting and evaluating the overall formability and local formability of high-strength steel. Uniaxial tensile tests are conducted, and the work hardening index n or the true uniform strain εu obtained from the tests is used as an evaluation index for overall formability. The effective strain range centered on the necking point is approximately 20 mm, and the fracture strain based on the thickness of a specimen with an intact fracture surface is measured as an evaluation index for local formability.

[0011] The patent search reveals that existing patents related to formability assessment generally cover two main areas: overall formability assessment and local formability assessment, as well as some simulated forming assessment experiments. Specifically, the overall formability assessment of sheet metal primarily relies on a single work hardening index (n) value and uniform elongation strain, which is not significantly different from traditional methods using n values ​​and forming limit curves.

[0012] Existing n-values ​​and forming limit curves cannot adequately reflect the forming performance of sheet metal. The applicability of conventional forming performance evaluation methods and indices to sheet metal should be assessed through the analysis of experimental data. Furthermore, new forming performance evaluation methods and indices should be established through theoretical analysis to guide part selection and processing performance evaluation. Therefore, developing an efficient, accurate, and easy-to-operate method for testing and evaluating the forming capability of sheet metal is of significant practical importance. Summary of the Invention

[0013] The purpose of this invention is to provide a method for calculating the index of the overall formability of sheet metal, and to propose an efficient, accurate and easy-to-operate method for testing and evaluating the formability of sheet metal, which can be used to guide the selection of parts and the evaluation of processing performance.

[0014] The above-mentioned technical objective of this invention has been achieved by the following technical solutions:

[0015] A method for calculating the index of evaluating the overall formability of metal sheets, which includes the following steps before calculating the index: Mechanical property tests are first conducted on the metal sheet.

[0016] A1: The true stress-strain curve of the material is calculated based on the mechanical property test data;

[0017] A2: Obtain the true stress-strain curves over a large strain range through model extrapolation, simulation benchmarking, or other large strain test methods;

[0018] A3: Calculate the change of instantaneous strain hardening index with plastic strain based on real stress-strain data;

[0019] A4: Based on the limit strain obtained in the forming limit test, calculate and plot the forming limit curve FLC, and obtain the principal strain value FLD0 at the lowest point of the forming limit curve.

[0020] A5: Calculate the forming strain homogenization capability index (SHC); specifically, the formula for calculating SHC is as follows:

[0021]

[0022] Wherein, n' is the instantaneous value of n, which is obtained by fitting and extrapolating the true plastic stress-strain curve;

[0023] A6: Set evaluation criteria to classify and rate the forming capability indicators;

[0024] A7: Based on the evaluation results, a comprehensive evaluation of the forming performance of the metal sheet is conducted.

[0025] The mechanical property test specifically includes the following steps:

[0026] Step 1: Prepare the metal sheet sample to be tested;

[0027] Step 2: Set specific test parameters for mechanical and forming properties;

[0028] Step 3: Conduct mechanical and forming property tests on the samples and record the data during the tests;

[0029] Step 4: Record the data during the performance testing of the metal sheet sample using strain measurement tools to obtain experimental data and provide data support for subsequent calculations.

[0030] Step A1 specifically includes the following steps:

[0031] The calculated stress and strain values ​​are mapped one-to-one, and the actual stress-strain curve is plotted in a coordinate system with strain as the abscissa and stress as the ordinate. This curve is used to evaluate the mechanical properties of the material.

[0032] Step A2 specifically includes the following steps:

[0033] The real stress-strain curve data of materials with a large strain range can be obtained by fitting the test data with a preset constitutive model and extrapolating, repeatedly calculating by establishing a finite element simulation model of the test, or designing a corresponding material large strain test scheme.

[0034] Step A3 specifically includes the following steps:

[0035] From the stress-strain data obtained in step A2, each pair of strain values ​​and the corresponding stress values ​​are extracted. These data will be used for subsequent calculations.

[0036] Step A4 specifically includes the following steps:

[0037] The collected limiting strain data are processed, and the processed limiting strain data points are plotted on a coordinate system. The data points are connected into a smooth curve, namely the forming limit curve (FLC), by interpolation. The lowest point of the curve is found, and the principal strain value is read or calculated at the lowest point of the forming limit curve. This value is FLD0.

[0038] In step A5, SHC is the instantaneous strain hardening exponent (n' value) at plastic strain (ε). p )from to within the interval for ε p The ratio of the integral value to the strain range; n'(ε) p () represents the strain hardening index of the material at a specified strain; and ε represents the upper and lower limits of the calculated strain range; u This represents the true plastic strain value corresponding to uniform elongation. The instability limit of a material under plane strain can be obtained through a forming limit experiment, specifically the principal strain value FLD0 at the lowest point of the forming limit curve. For FLD0, 0.1ε u .

[0039] In step A5, the extrapolation model formulas for different types of steel profiles can use either the Swift model or the MPL model.

[0040] Step A6 specifically includes the following steps: Based on the actual application needs and industry standards of metal sheets, formulate detailed forming capability evaluation index standards, and classify the collected forming capability indexes according to different attributes, such as mechanical performance indexes and plastic deformation capability indexes, to represent the role and influence of each index in the metal sheet forming process.

[0041] Step A7 specifically includes the following steps: summarizing the evaluation results of various forming capability indicators to form an evaluation report, conducting a comprehensive analysis of various forming capability indicators, and evaluating the performance of metal sheets under different forming conditions.

[0042] In summary, the present invention has the following beneficial effects:

[0043] 1. Improve the accuracy of overall formability assessment: This method, by considering the true stress-strain curve of the material over a large strain range and the forming limit strain, can more comprehensively and accurately assess the overall formability of metal sheets.

[0044] 2. Adaptable to more material types: This method uses the instantaneous n-value variation trend within a large strain range to evaluate the strain homogenization ability of materials. It is applicable to different steel types and facilitates comparison between different steel types.

[0045] 3. Guiding practical applications: This method can guide the application of sheet metal in industrial fields such as automobiles, providing detailed and accurate data support for simulation, forming and processing. Attached Figure Description

[0046] Figure 1 This is a schematic diagram illustrating the principle of the indicator calculation method of the present invention.

[0047] Figure 2 These are the tensile stress-strain curves of each material in Example 1.

[0048] Figure 3 This is a forming limit curve diagram of each material in Example 1.

[0049] Figure 4 This is a true plastic stress-strain curve of each material in the uniform deformation segment of Example 1.

[0050] Figure 5 It is the actual plastic stress-strain curve of each material in Example 1, fitted and extrapolated to the principal strain value corresponding to the lowest point of the forming limit curve.

[0051] Figure 6 This is a graph showing the instantaneous n-values ​​of each material in Example 1. Detailed Implementation

[0052] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below with reference to the figures and specific embodiments.

[0053] This invention proposes a method for calculating the index of evaluating the overall formability of metal sheets. Before calculating the index, the metal sheet is first subjected to mechanical property tests, including the following steps:

[0054] Step 1: Prepare the metal sheet sample to be tested; preferably, process the metal sheet sample by using a suitable cutting tool to cut the metal sheet into the required size and shape, and remove oil, dust and other impurities from the sample surface to ensure that the sample surface is clean.

[0055] Step 2: Set specific test parameters for mechanical and forming properties; including specific parameters such as tensile speed, blank holder force, and temperature;

[0056] Step 3: Conduct mechanical and forming property tests on the samples and record the data during the tests; preferably, conduct uniaxial tensile tests and forming limit tests.

[0057] The uniaxial tensile test specifically includes the following steps: placing the specimen in the clamp of the tensile testing machine, setting the tensile speed and other parameters, starting the loading until the specimen breaks;

[0058] The forming limit test specifically includes the following steps: placing the specimen between the die and the blank holder of the testing machine, and using the blank holder force to press the specimen material until the specimen breaks.

[0059] Step 4: Record the data during the performance testing of the metal sheet sample using strain measurement tools to obtain experimental data. During the uniaxial tensile test, simultaneously record the load and corresponding displacement data of the metal sheet sample to provide data support for subsequent calculations;

[0060] In forming limit tests, the limiting strain is derived from the collected sample image data using digital image correlation, providing data support for subsequent calculations.

[0061] like Figure 1 As shown, a method for calculating an index to evaluate the overall formability of metal sheets includes the following steps:

[0062] A1: The true stress-strain curve of the material is calculated based on the mechanical property test data;

[0063] Step A1 specifically includes the following steps:

[0064] The calculated stress and strain values ​​are mapped one-to-one, and the actual stress-strain curve is plotted in a coordinate system with strain as the abscissa and stress as the ordinate. This curve is used to evaluate the mechanical properties of the material.

[0065] A2: Obtain the true stress-strain curves over a large strain range through model extrapolation, simulation benchmarking, or other large strain test methods;

[0066] Step A2 specifically includes the following steps:

[0067] The real stress-strain curve data of materials with a large strain range can be obtained by fitting the test data with a preset constitutive model and extrapolating, repeatedly calculating by establishing a finite element simulation model of the test, or designing a corresponding material large strain test scheme.

[0068] A3: Based on the stress-strain data in step A2, calculate the change of the instantaneous strain hardening index with plastic strain;

[0069] Step A3 specifically includes the following steps:

[0070] From the stress-strain data obtained in step A2, each pair of strain values ​​and corresponding stress values ​​are extracted. These data points will be used for subsequent calculations.

[0071] A4: Based on the limiting strain obtained from the forming test data, calculate and plot the forming limit curve FLC, and obtain the principal strain value FLD0 at the lowest point of the forming limit curve.

[0072] Step A4 specifically includes the following steps:

[0073] The collected limiting strain data are processed, and the processed limiting strain data points are plotted on a coordinate system. Using interpolation, the data points are connected to form a smooth curve, known as the forming limit curve (FLC). The lowest point of the curve is identified; this lowest point typically indicates the location where the metal sheet is most likely to fracture during forming. At the lowest point of the forming limit curve, the principal strain value is read or calculated. This value is FLD0, representing the maximum principal strain the metal sheet can withstand during forming without fracture.

[0074] A5: Calculate the forming strain homogenization capability index (SHC); Step A5 specifically includes the following steps:

[0075] The collected strain data is processed to remove outliers and noise, and the strain values ​​at various locations of the sheet metal during the forming process are calculated. Specifically, the calculation formula for SHC is as follows:

[0076]

[0077] Wherein, SHC is the instantaneous strain hardening exponent (n' value) at plastic strain (ε). p )from to within the interval for ε p The ratio of the integral value to the strain range; n'(ε) p () represents the strain hardening index of the material at a specified strain; and ε represents the upper and lower limits of the calculated strain range; u This represents the true plastic strain value corresponding to uniform elongation. The instability limit of a material under plane strain can be obtained through a forming limit experiment, specifically the principal strain value FLD0 at the lowest point of the forming limit curve. For FLD0, 0.1ε u .

[0078] Wherein, n' is the instantaneous value of n, which is obtained by fitting the extrapolated true plastic stress-strain curve. The extrapolation model formula for different types of steel can be the Swift model or the MPL model.

[0079] A6: Set evaluation criteria to classify and rate the forming capability indicators;

[0080] Step A6 specifically includes the following steps:

[0081] Based on the actual application needs and industry standards of sheet metal, detailed evaluation index standards for forming capability are formulated. The collected forming capability indexes are classified according to different attributes, such as mechanical property indexes (e.g., tensile strength, yield strength, etc.) and plastic deformation capability indexes (e.g., elongation, bending performance, etc.), to represent the role and influence of each index in the forming process of sheet metal.

[0082] A7: Based on the evaluation results, a comprehensive evaluation of the forming performance of the metal sheet is conducted.

[0083] Step A7 specifically includes the following steps:

[0084] The evaluation results of various forming capability indicators are summarized to form an evaluation report. The evaluation report comprehensively analyzes the various forming capability indicators, evaluates the performance of metal sheets under different forming conditions, and gives clear evaluation conclusions and recommendations.

[0085] Example 1

[0086] In this embodiment, four types of automotive thin steel sheets—DC06, HC220B, DP980, and QP980—are used as examples to illustrate the calculation method for evaluating the overall formability of metal sheets proposed in this invention. The method specifically includes the following steps:

[0087] like Figure 2 As shown, tensile specimens of four materials, DC06, HC220B, DP980 and QP980, were processed, quasi-static tensile tests were conducted, data were collected, and uniaxial tensile stress-strain curves of each material were plotted.

[0088] like Figure 3 As shown, forming limit specimens of four materials, DC06, HC220B, DP980 and QP980, were processed, forming limit tests were conducted, data were collected, and forming limit curves of each material were plotted.

[0089] like Figure 4 As shown, the uniform deformation segments of the tensile stress-strain curves for DC06, HC220B, DP980, and QP980 materials are extracted. Substituting these segments into the true plastic stress conversion formula yields the true plastic stress-strain curves for each material in the uniform deformation segment. The true plastic stress conversion formula is as follows:

[0090] σ T =σ E *(1+ε E (2a);

[0091] ε T=Ln(1+ε E )-σ T / E. (2b);

[0092] like Figure 5 As shown, the true plastic stress-strain curves of DC06, HC220B, DP980, and QP980 materials were fitted and extrapolated according to the specified constitutive model. Specifically, DC06 and HC220B were fitted and extrapolated using the Swift model, while DP980 and QP980 were fitted and extrapolated using the MPL model. The true plastic stress-strain curves of DC06, HC220B, DP980, and QP980 materials were extrapolated to the principal strain value FLD0 at the lowest point of their respective forming limit curves. The determined FLD0 values ​​for DC06, HC220B, DP980, and QP980 were 0.371, 0.255, 0.111, and 0.181, respectively. The fitting extrapolation formula for the Swift model is shown in (3a), and the fitting extrapolation formula for the MPL model is shown in (3b), as detailed below:

[0093]

[0094] like Figure 6 As shown, the instantaneous n-value (n') curves for each material are calculated based on the extrapolated stress-strain curves of DC06, HC220B, DP980, and QP980. Among them, the SHC (Self-Strain Homogenization Capability) index used to calculate the forming strain homogenization capability index for each material is... ε u , As shown in the table below:

[0095]

[0096] Substitute the data into the SHC calculation formula to calculate the forming strain homogenization ability index of DC06, HC220B, DP980, and QP980 materials. The index is 0.2489 for DC06, 0.2152 for HC220B, 0.06803 for DP980, and 0.0957 for QP980.

[0097] This method provides a complete process from experimental design to index calculation. Compared with traditional evaluation methods, this method is more efficient in experimental design and execution, significantly shortens the testing cycle, and accurately reflects the overall formability of sheet metal. The high-precision test results help to more accurately predict and evaluate the performance of sheet metal in the stamping process, reducing production risks. This method can calculate the formability index of different samples and make horizontal comparisons, which helps to identify performance differences under different materials or process conditions. Through horizontal comparison, it is easier to screen out materials with better performance. In engineering applications, it can effectively evaluate the overall formability of sheet metal. Combined with traditional indicators, it can more comprehensively evaluate the formability of advanced high-strength steel sheets, providing accurate references for stamping process design and showing good application prospects.

[0098] In the description of this invention, it should be noted that the terms "upper," "lower," "inner," "outer," "left," and "right," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings, or the orientation or positional relationship commonly used when the product of this invention is in use, or the orientation or positional relationship commonly understood by those skilled in the art. These terms are used only for the convenience of describing this invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this invention. Furthermore, the terms "first," "second," etc., are used only to distinguish descriptions and should not be construed as indicating or implying relative importance. In the description of this invention, it should also be noted that, unless otherwise explicitly specified and limited, terms such as "set" and "connect" should be interpreted broadly. For example, "connection" can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection or an indirect connection through an intermediate medium; it can be a connection within two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances. In this document, the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, which includes not only the elements listed but also other elements not expressly listed.

[0099] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the present invention as claimed. The scope of protection of this invention is defined by the appended claims and their equivalents.

Claims

1. A method for calculating an index to evaluate the overall formability of a metal sheet, wherein, before calculating the index, a mechanical property test is first conducted on the metal sheet, characterized in that, Includes the following steps: A1: The true stress-strain curve of the material is calculated based on the mechanical property test data; A2: Obtain the true stress-strain curves over a large strain range through model extrapolation, simulation benchmarking, or other large strain test methods; A3: Calculate the change of instantaneous strain hardening index with plastic strain based on real stress-strain data; A4: Based on the limit strain obtained in the forming limit test, calculate and plot the forming limit curve FLC, and obtain the principal strain value FLD0 at the lowest point of the forming limit curve. A5: Calculate the forming strain homogenization capability index (SHC); specifically, the formula for calculating SHC is as follows: Where n' is the instantaneous value of n, and SHC is the instantaneous strain hardening exponent (n' value) at plastic strain (ε). p )from to within the interval for ε p The ratio of the integral value to the strain range; n'(ε) p () represents the strain hardening index of the material at a specified strain; and ε represents the upper and lower limits of the calculated strain range; u This represents the true plastic strain value corresponding to uniform elongation. The instability limit of the material under plane strain is obtained through forming limit experiments, which is the principal strain value FLD0 at the lowest point of the forming limit curve of the material. A6: Set evaluation criteria to classify and rate the forming capability indicators; A7: Based on the evaluation results, a comprehensive evaluation of the forming performance of the metal sheet is conducted.

2. The method for calculating the index for evaluating the overall formability of metal sheets according to claim 1, characterized in that, The mechanical property test specifically includes the following steps: Step 1: Prepare the metal sheet sample to be tested; Step 2: Set specific test parameters for mechanical and forming properties; Step 3: Conduct mechanical and forming property tests on the samples and record the data during the tests; Step 4: Record the data during the performance testing of the metal sheet sample using strain measurement tools to obtain experimental data and provide data support for subsequent calculations.

3. The method for calculating the index for evaluating the overall formability of metal sheets according to claim 1, characterized in that, Step A1 specifically includes the following steps: The calculated stress and strain values ​​are mapped one-to-one, and the actual stress-strain curve is plotted in a coordinate system with strain as the abscissa and stress as the ordinate. This curve is used to evaluate the mechanical properties of the material.

4. The method for calculating the index for evaluating the overall formability of metal sheets according to claim 1, characterized in that, Step A2 specifically includes the following steps: The real stress-strain curve data of materials with a large strain range can be obtained by fitting the test data with a preset constitutive model and extrapolating, repeatedly calculating by establishing a finite element simulation model of the test, or designing a corresponding material large strain test scheme.

5. The method for calculating the index for evaluating the overall formability of metal sheets according to claim 1, characterized in that, Step A3 specifically includes the following steps: From the large-strain range real stress-strain data obtained in step A2, extract each pair of strain values ​​and the corresponding stress values. These data will be used for subsequent calculations.

6. The method for calculating the index for evaluating the overall formability of metal sheets according to claim 1, characterized in that, Step A4 specifically includes the following steps: The collected limiting strain data are processed, and the processed limiting strain data points are plotted on a coordinate system. The data points are connected into a smooth curve, namely the forming limit curve (FLC), by interpolation. The lowest point of the curve is found, and the principal strain value is read or calculated at the lowest point of the forming limit curve. This value is FLD0.

7. The method for calculating the index for evaluating the overall formability of metal sheets according to claim 1, characterized in that, In step A5, SHC is the instantaneous strain hardening exponent (n' value) at plastic strain (ε). p )from to within the interval for ε p The ratio of the integral value to the strain range; n'(ε) p () represents the strain hardening index of the material at a specified strain; and These are the upper and lower limits of the strain range for calculation; ε u This represents the true plastic strain value corresponding to uniform elongation. The instability limit of a material under plane strain can be obtained through a forming limit experiment, specifically the principal strain value FLD0 at the lowest point of the forming limit curve. For FLD0, 0.1ε u .

8. The method for calculating the index for evaluating the overall formability of metal sheets according to claim 1, characterized in that, In step A5, the extrapolation model formulas for different types of steel profiles can use either the Swift model or the MPL model.

9. The method for calculating the index for evaluating the overall formability of metal sheets according to claim 1, characterized in that, Step A6 specifically includes the following steps: Based on the actual application needs and industry standards of sheet metal, detailed evaluation index standards for forming capability are formulated. The collected forming capability indexes are classified according to different attributes, such as mechanical performance indexes and plastic deformation capability indexes, to represent the role and influence of each index in the sheet metal forming process.

10. The method for calculating the index for evaluating the overall formability of metal sheets according to claim 1, characterized in that, Step A7 specifically includes the following steps: The evaluation results of various forming capability indicators are summarized to form an evaluation report. A comprehensive analysis of the various forming capability indicators is conducted to evaluate the performance of metal sheets under different forming conditions.

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