Method for predicting plastic deformation capacity of automobile structural steel hot-rolled plate under static stretching condition

Abstract

The invention relates to a method for predicting the plastic deformation capacity of an automobile structural steel hot-rolled plate under a static stretching condition. The method comprises the following steps: step 1, measuring the thickness reduction rate through static stretching; step 2, micro grain size characterization; and step 3, forming limit determination, contrast and evaluation. According to the method provided by the invention, the critical point of possible plastic deformation and necking deformation of the hot-rolled plate under the static stretching condition can be quickly simulated and obtained, and a theoretical reference is provided for failure formation reasons of material stamping in combination with microcosmic grain deformation degree analysis.

Description

Technical Field

[0001] The present invention relates to the field of plastic deformation analysis of hot-rolled sheets, and uses experimental analysis methods to quickly characterize the plastic deformation ability of hot-rolled sheets so as to predict the stamping performance of materials, belonging to the field of plastic deformation detection and analysis of steel materials. Background Art

[0002] With the underlying demands of various automotive industry users, core technologies such as quality control, and the short-process development trend, in order to achieve deep processing and high added value of hot-rolled products, it is necessary for the producers of hot-rolled sheet raw materials to continuously optimize the variety structure of automotive structural steel and upgrade the quality. How to quickly and effectively predict the plastic deformation ability of materials at the hot-rolled raw material stage. Usually, the experimental means we use mainly include the determination of elongation and plastic strain ratio in tensile properties; FLD and hole expansion rate in forming experiments; and the characterization of the anisotropy of the mechanical properties of steel sheets through texture analysis by an X-ray diffractometer. Among these, since hot-rolled sheets do not have a strong texture orientation, it is often impossible to obtain an obvious r value or a favorable texture, and there is no good predictability for whether the material has good stamping formability. Moreover, the forming test also has certain limitations on the thickness and strength of the specimen, and it may be difficult to conduct tests such as hole expansion and FLD for high-strength steel sheets with a thickness greater than 5 mm. The purpose of the present invention is to quickly simulate and obtain the critical point of possible plastic deformation and necking deformation of hot-rolled sheets under static tensile conditions through experimental analysis methods, and combine the analysis of the degree of microscopic grain deformation to provide a theoretical reference for the reasons for the failure of materials during stamping.

[0003] Regarding patents on the determination of plastic deformation and thickness deformation of hot-rolled sheets, most of them are applied to thickness measurement devices for various materials, or the prediction of hot-rolled sheet rolling process parameters, or the transformation of tensile measurement devices, etc. For example, the Chinese patent application with the publication number CN116078831A discloses "a method for calculating rolling force during the dynamic thickness reduction production process of strip cold rolling". The method relates to the field of rolling technology, and accurately predicts the rolling force during the dynamic thickness reduction production process of strip cold rolling by comprehensively considering various process parameters in the thickness reduction rolling process, and solves the problem of predicting the real-time rolling force during the thickness reduction rolling process under different production conditions. The Chinese patent application with the publication number CN116046568A discloses "a method for analyzing the plastic deformation law of laser shock metal materials". The method is a method for analyzing the plastic deformation law of laser shock metal materials. The Chinese patent application with the publication number CN116067772A discloses "a method for predicting the high-temperature yield strength during the creep aging process of aluminum alloy under the condition of first force and then heat". The method is a method for predicting the high-temperature yield strength during the creep aging process of aluminum alloy under the condition of first force and then heat, belonging to the field of alloy processing technology. Summary of the Invention

[0004] The object of the present invention is to provide a method for predicting the plastic deformation ability of hot-rolled sheets of automotive structural steel under static tensile conditions, mainly to solve the problem of failure analysis such as cracking during the stamping process of hot-rolled sheets of automotive structural steel at downstream users, and to quickly carry out the prediction and evaluation of the plastic deformation ability of hot-rolled sheets under laboratory conditions, so as to provide theoretical data support for improving the product quality of hot-rolled sheets of automotive structural steel.

[0005] The present invention is a method for predicting the plastic deformation ability of hot-rolled sheets of automotive structural steel under static tensile conditions. By obtaining the thickness reduction during the plastic deformation stage of the material under static tensile conditions, selecting a reasonable strain range, and analyzing and calculating through a mathematical model; taking the surface of the thinnest area after stretching for metallographic sample preparation, and using an optical microscope and an electron microscope EBSD for grain size characterization and analysis; combining with the comparison of the measured values of the forming limit of the material, a prediction and evaluation of the plastic deformation ability of hot-rolled sheets of automotive structural steel are formed. The present invention mainly includes the following steps: (1) Measuring the thickness reduction rate under static tension; (2) Microscopic grain size characterization; (3) Forming limit measurement and control evaluation, which are described in detail as follows:

[0006] 1 Measuring the thickness reduction rate under static tension:

[0007] 1.1 Take a P5 standard tensile sample on the hot-rolled sheet and pull it to fracture on a tensile testing machine.

[0008] 1.2 Analyze the tensile curve, analyze the maximum force elongation rate Agt (%) of the tensile sample to obtain the plastic deformation strain range of the material, and determine the tensile strain control value of the thickness data.

[0009] 1.3 Conduct a tensile test under the condition of the strain control value on a tensile testing machine to obtain the deformed tensile sample. Refer to the Ae value and the Ag value to obtain the plastic strain interval of the material. Mainly examine the plastic deformation of the grains before the maximum force and the degree of grain deformation, and stop when selecting a suitable strain level.

[0010] 1.4 Calculate the thickness reduction rate in two ways: manual measurement and automatic measurement of data.

[0011] Obtaining the thickness deformation value is a difficult point. Since the change in thickness is usually relatively small and not easy to measure accurately, during the static tensile test, the engineering strain in the longitudinal and transverse directions of the sample is mainly obtained by tracking with longitudinal and transverse extensometers, and the engineering strain e in the width direction of the sample b , according to the principle of volume invariance, the deformation in the length direction of the sample is converted into the deformation in the thickness direction.

[0012] (1) In the elastic stage and the uniform plastic deformation stage before necking occurs during the tensile test of the sample, formula (1) can be satisfied.

[0013] a0b0L = abL………………………(1)

[0014] Wherein:

[0015] a0—the original thickness of the specimen, in millimeters (mm); b0—the original width of the specimen, in millimeters (mm);

[0016] L0—the original gauge length of the specimen, in millimeters (mm);

[0017] a—the thickness of the specimen after the agreed engineering strain, in millimeters (mm);

[0018] b—the width of the specimen after the agreed engineering strain, in millimeters (mm);

[0019] L—the length of the specimen after the agreed engineering strain, in millimeters (mm);

[0020] (2) During the tensile process before necking occurs, the change rate of the instantaneous thickness reduction of the specimen is consistent, and satisfies formula (2):

[0021]

[0022] (3) Calculate formulas (3) and (4) according to the engineering strain in the length direction and the width direction:

[0023]

[0024]

[0025]

[0026]

[0027] Substitute formula (5) and formula (6) into formula (2), and then the following can be obtained:

[0028]

[0029] 2 Microcrystalline grain characterization:

[0030] 2.1 Sampling preparation: Take a sample from the thinnest area of the fixed-strain tensile sample. The observation surface is the surface. After embedding, grinding, polishing, and drying with alcohol solution, it is made into a metallographic sample for standby;

[0031] 2.2 Optical microscope observation: The metallographic sample is corroded with a 3-4% nitric acid alcohol solution. The observation surface is placed under an upright optical microscope to observe the microstructure. Focus on observing the size and shape of ferrite grains, and record pictures under 100-fold and 500-fold magnifications. Use an image analyzer to carry out quantitative analysis, and use the metallographic secant method to conduct grain size characterization analysis on the grain metallographic pictures.

[0032] 2.3 Vibration polishing treatment: After polishing the metallographic sample, place it on a vibration polishing machine, add polishing liquid and conduct vibration polishing treatment for 6 hours, and thoroughly clean the polished surface with alcohol.

[0033] 2.4 Electron microscope observation: Place the vibration-polished metallographic sample diffusely under a scanning electron microscope for EBSD analysis of the scanning electron microscope. Focus on observing the morphology and size of ferrite grains on the observed surface, and use the HKL Channel5 software to perform grain size characterization analysis by the EBSD grain reconstruction method.

[0034] 2.5 Grain size characterization

[0035] The grain size characterization technology is similar to the traditional metallographic secant method and the secant method analysis provided by EBSD. Basically, it calculates the length of the secant between adjacent grain boundaries. The difference is that the EBSD secant method requires an artificially given setting value to define the grain boundary angle (i.e., the critical misorientation), so as to define the grain boundary. For carbon microalloyed steel, this setting value is generally taken as 2°. Grain boundaries below this setting value are ignored. The grain reconstruction method of EBSD also needs to first give a setting value of the critical misorientation. Combining with the characteristics of the steel grade, it is also set as 2°. This method is relatively less affected by orientation noise and is more suitable for characterizing the grain size in the annealed state.

[0036] 3 Forming limit determination

[0037] Perform FLD determination on similar materials. Use wire cutting to obtain a group of specimens with different widths, and perform bulging on a group of specimens through a rigid punch. The middle part of the specimen generates bulging deformation under the action of the punch to form a convex bulge. The grid circles printed on the surface of the specimen before pretreatment are distorted. Stop the test when necking or cracking occurs in a certain part of the convex bulge, measure the major and minor axis dimensions of the grid circles in the necking area or near it or in the cracking area, and calculate the surface limit principal strains (e1, e2). Analyze and establish the correlation between the thickness reduction rate, grain size and forming limit value.

[0038] The technical effects of the present invention are mainly reflected in the following aspects:

[0039] (1) Through static tensile thickness reduction rate + grain size characterization analysis, comparing with the forming performance, quickly judge the plastic deformation ability of the hot-rolled sheet through thickness deformation measurement. At the same time, combining with the grain size characterization under the corresponding strain, predict the plastic deformation ability of the hot-rolled sheet of automotive structural steel. Currently, most of the research focuses on the simulation or forming test of formed parts, which is restricted by various factors such as the test equipment capacity and test time, and it is impossible to quickly predict and evaluate at the hot-rolled sheet stage.

[0040] (2) By using the static stretching process, combined with the plastic strain characteristics of the steel grade, a stretching with a set strain value is given to cause plastic deformation of the specimen. Through calculation, a clear thickness reduction rate is obtained, and an intuitive evaluation of the thickness deformation ability is given.

[0041] (3) This method provides a quick simulation to obtain the critical point of possible plastic deformation and necking deformation of the hot-rolled sheet under static stretching conditions, and combined with the analysis of the degree of microscopic grain deformation, it provides a theoretical reference for the reasons for the failure of the material during stamping. Description of the Drawings

[0042] Figure 1 is the metallographic structure of the longitudinal specimen,

[0043] Figure 2 is the metallographic structure of the longitudinal specimen,

[0044] Figure 3 is the EBSD grain size measurement,

[0045] Figure 4 is the FLD of the hot-rolled sheet. Detailed Implementation Manner

[0046] To deepen the understanding of the present invention, the following detailed description of this embodiment will be made in conjunction with the accompanying drawings.

[0047] Example 1: A method for predicting the plastic deformation ability of a hot-rolled sheet of automotive structural steel under static stretching conditions. By obtaining the thickness reduction during the plastic deformation stage of the material under static stretching conditions, selecting a reasonable strain range, and analyzing and calculating through a mathematical model; taking the surface of the thinnest area after stretching for metallographic sample preparation, and using an optical microscope and an electron microscope EBSD for grain size characterization and analysis; combining with the comparison of the measured values of the forming limit of the material, a predictive evaluation of the plastic deformation ability of the hot-rolled sheet of automotive structural steel is formed. The present invention mainly includes the following steps: (1) Static stretching to measure the thickness reduction rate; (2) Microscopic grain size characterization; (3) Forming limit measurement and control evaluation, which are described in detail as follows:

[0048] 1 Static stretching to measure the thickness reduction rate:

[0049] 1.1 Take a P5 standard tensile sample on the hot-rolled sheet and pull it to fracture on a tensile testing machine.

[0050] 1.2 Analyze the tensile curve, analyze the maximum force elongation rate Agt (%) of the tensile sample to obtain the plastic deformation strain range of the material, and measure the tensile strain control value of the thickness data.

[0051] 1.3 Conduct a tensile test under strain control conditions on a tensile testing machine to obtain the deformed tensile specimen. Refer to the Ae value and Ag value to obtain the plastic strain range of the material. Mainly investigate the plastic deformation of the grains before the maximum force and the degree of grain deformation, and stop when selecting an appropriate strain level.

[0052] 1.4 Calculate the thickness reduction rate in two ways: manual measurement and automatic measurement of data.

[0053] Obtaining the thickness deformation value is a difficult point. Since the change in thickness is usually relatively small and not easy to measure accurately, during the static tensile test, the longitudinal and transverse extensometers are mainly used to track the engineering strain in the specimen length direction and the engineering strain e in the specimen width direction b , and according to the principle of volume invariance, the deformation in the specimen length direction is converted into the deformation in the thickness direction.

[0054] (1) In the elastic stage and the uniform plastic deformation stage before necking occurs in the specimen tensile process, formula (1) can be satisfied.

[0055] a0b0L = abL ………………………(1)

[0056] In the formula:

[0057] a0 —— The original thickness of the specimen, in millimeters (mm); b0 —— The original width of the specimen, in millimeters (mm);

[0058] L0 —— The original gauge length of the specimen, in millimeters (mm);

[0059] a —— The thickness of the specimen after the agreed engineering strain, in millimeters (mm);

[0060] b —— The width of the specimen after the agreed engineering strain, in millimeters (mm);

[0061] L —— The length of the specimen after the agreed engineering strain, in millimeters (mm);

[0062] (2) Before necking occurs, the change in the instantaneous thickness reduction rate of the specimen during the tensile process is consistent and satisfies formula (2):

[0063]

[0064] (3) Calculate formulas (3) and (4) according to the engineering strain in the length direction and the width direction:

[0065]

[0066]

[0067]

[0068]

[0069] Substituting Formula (5) and Formula (6) into Formula (2), we can obtain:

[0070]

[0071] 2 Microcrystalline grain size characterization:

[0072] 2.1 Sample preparation: Take a sample from the thinnest area of the fixed-strain tensile sample. The observation surface is the surface. After embedding, grinding, and polishing, it is dried with an alcohol solution to make a metallographic sample for use.

[0073] 2.2 Optical microscope observation: The metallographic sample is corroded with a 3-4% nitric acid alcohol solution. The observation surface is placed under an upright optical microscope to observe the microstructure. Focus on observing the size and shape of ferrite grains, and record pictures at magnifications of 100 times and 500 times. Quantitative analysis is carried out using an image analyzer, and the grain size of the grain metallographic pictures is characterized and analyzed by the metallographic secant method.

[0074] 2.3 Vibration polishing treatment: After polishing the metallographic sample, place it on a vibration polishing machine, add polishing liquid, and carry out vibration polishing treatment for 6 hours. Thoroughly clean the polished surface with alcohol.

[0075] 2.4 Electron microscope observation: Place the metallographic sample after vibration polishing under a scanning electron microscope for EBSD analysis of the scanning electron microscope. Focus on observing the morphology and size of ferrite grains on the observation surface, and use the EBSD grain reconstruction method in the HKL Channel5 software for grain size characterization and analysis.

[0076] 2.5 Grain size characterization

[0077] The grain size characterization technology is similar to the traditional metallographic secant method and the secant method provided by EBSD. Basically, it calculates the length of the secant between adjacent grain boundaries. The difference is that the EBSD secant method requires an artificially given setting value to define the grain boundary angle (i.e., the critical misorientation), so as to define the grain boundary. For carbon microalloyed steel, this setting value is generally taken as 2°. Grain boundaries with values lower than this setting value are ignored. The grain reconstruction method of EBSD also needs to first give a setting value of the critical misorientation. Combining with the characteristics of the steel grade, it is also set as 2°. This method is relatively less affected by orientation noise and is more suitable for characterizing the grain size of the annealed state.

[0078] 3 Forming limit determination

[0079] Perform FLD measurements on similar materials. Use wire cutting to obtain a set of specimens with different widths. Perform bulging on a set of specimens using a rigid punch. The middle part of the specimen undergoes bulging deformation under the action of the punch to form a convex bulge. The grid circles printed on the surface of the specimen for pretreatment are distorted. Stop the test when necking or cracking occurs in a certain part of the convex bulge, and measure the major and minor axis dimensions of the grid circles in the necking area or nearby or in the area near the crack. Calculate the surface limit principal strains (e1, e2). Analyze and establish the correlation between the thickness reduction rate, grain size, and forming limit value.

[0080] Example 2: The specimen is automotive structural steel SAPH440. Take transverse, longitudinal, 45° standard tensile specimens, a set of forming FLD specimens, and metallographic block specimens on the hot-rolled plate. Conduct tests according to the steps of (1) static tensile test to determine the thickness reduction rate; (2) microscopic grain size characterization; (3) forming limit comparison, and establish the correlation between the thickness reduction rate, grain size, and forming limit value.

[0081] 1 Static tensile test to determine the thickness reduction rate:

[0082] 1.1 Take P5 standard tensile specimens in three directions on the hot-rolled plate and pull them to fracture on a tensile testing machine. Obtain the tensile data of the hot-rolled plate. See Table 1.

[0083] Table 1 Tensile data of hot-rolled plate

[0084]

[0085]

[0086] 1.2 Analyze the tensile curve, analyze the maximum force elongation rate Agt (%) of the tensile specimen to obtain the plastic deformation strain range of the material, and determine the tensile strain control value of the thickness data. Referring to the Ae value and Ag value, the test strain value range is controlled between 3% and 14%. Mainly investigate the plastic deformation of grains in each direction before the maximum force and the degree of grain deformation, and consider stopping at a strain level of 13%.

[0087] 1.3 Conduct a tensile test under the condition of a strain control value of 13% on a tensile testing machine to obtain the deformed tensile specimen.

[0088] 1.4 Calculate the thickness deformation value in two ways: manual measurement and automatic measurement data. Substitute the manual measurement data and automatic measurement data into the formula for calculation, and the calculated data of the reduction rate are shown in Table 2.

[0089] Table 2 Calculated values of maximum force plastic reduction rate

[0090]

[0091] 2 Microscopic grain size characterization:

[0092] 2.1 Sampling: The ductility and thickness reduction of the longitudinal specimen are obvious. After fixing the strain of the longitudinal tensile specimen, a sample is taken from the thinnest area, and the observation surface is the surface. After embedding, grinding, polishing, it is dried with alcohol solution to make a metallographic sample for use;

[0093] 2.2 Optical microscope observation: The metallographic sample is corroded with 3 - 4% nitric acid alcohol solution. The observation surface is placed under an upright optical microscope to observe the microstructure. The focus is on observing the size and shape of ferrite grains, and pictures are recorded at magnifications of 100 times and 500 times.

[0094] 2.3 Vibratory polishing treatment: After polishing the metallographic sample, it is placed on a vibratory polishing machine, and polishing fluid is added for 6 hours of vibratory polishing treatment. The polished surface is thoroughly cleaned with alcohol.

[0095] 2.4 Electron microscope observation: The metallographic sample after vibratory polishing is diffusely observed under a scanning electron microscope for EBSD analysis of the scanning electron microscope. The focus is on observing the morphology and size of ferrite grains on the observation surface. The grain size is characterized and analyzed using the EBSD grain reconstruction method with HKL Channel5 software.

[0096] 2.5 Grain size characterization

[0097] Quantitative analysis is carried out using an image analyzer. The grain size of the grain metallographic pictures is characterized and analyzed using the metallographic secant method, and the average grain size is measured to be 5.74 μm; the grain size is measured using EBSD, and the average grain size calculated by the software using the grain reconstruction method is 5.60 μm, and the grain count is statistically 837, and the maximum and minimum grain sizes are found to be 10 μm.

[0098] 3 Forming limit determination

[0099] The same type of material is measured, the FLD forming limit diagram is plotted, and the surface engineering (limit) principal strain is obtained. The correlation between the thickness reduction rate, grain size, and forming limit value is established.

[0100] 4 Conclusion

[0101] Through static tensile specimens, the thickness reduction rate of SAPH440 material is obtained to be about 6%, corresponding to the grain size of the material. In the plastic deformation stage of the fine-grained material, the deformation degree of the length and width grains is not obvious. Comparing with the level of the forming principal strain of 0.40, when the material thickness reduction during user stamping exceeds 6%, the material is likely to enter the necking stage. By measuring the correlation between the thickness reduction rate and the grain size, it can provide an optimization direction for on-site process adjustment, improve the grain size, appropriately optimize the plastic properties, quickly predict and evaluate at the hot-rolled plate stage, and at the same time provide reference data for the downstream user forming process.

[0102] It should be noted that the above embodiments are not intended to limit the protection scope of the present invention. Any equivalent transformation or substitution made on the basis of the above technical solutions falls within the protection scope of the claims of the present invention.

Claims

1. A prediction method for the plastic deformation ability of hot-rolled sheets of automotive structural steel under static stretching conditions, characterized in that, The method includes the following steps: Step 1: Static stretching to measure the thickness reduction rate; Step 2: Microcrystalline grain size characterization; Step 3: Forming limit measurement for control and evaluation.

2. The prediction method for the plastic deformation ability of a hot-rolled sheet of automotive structural steel under static stretching conditions according to claim 1, characterized in that Step 1: Static stretching to measure the thickness reduction rate, specifically as follows: 1.1 Take a P5 standard tensile sample on the hot-rolled sheet and pull it to fracture on a tensile testing machine. 1.2 Analyze the tensile curve, analyze the maximum force elongation rate Agt(%) of the tensile sample to obtain the plastic deformation strain range of the material, and measure the tensile strain control value of the thickness data. 1.3 Conduct a tensile test under the condition of the strain control value on the tensile testing machine to obtain the deformed tensile sample, refer to the Ae value and Ag value, and obtain the plastic strain range of the material. 1.4 Calculate the thickness reduction rate in two ways: manual measurement and automatic measurement of data. During the static tensile test, the engineering strain in the length direction of the specimen and the engineering strain e in the width direction of the specimen are mainly obtained by tracking with longitudinal and transverse extensometers. b , according to the principle of volume constancy, the deformation in the length direction of the specimen is converted into the deformation in the thickness direction. (1) In the elastic stage and the uniform plastic deformation stage before necking occurs in the sample tensile process, formula (1) can be satisfied. a0b0L = abL ………………………(1) In the formula: a0—the original thickness of the sample, in millimeters (mm); b0—the original width of the sample, in millimeters (mm); L0—the original gauge length of the sample, in millimeters (mm); a—the thickness of the sample after the agreed engineering strain, in millimeters (mm); b—the width of the sample after the agreed engineering strain, in millimeters (mm); L—the length of the sample after the agreed engineering strain, in millimeters (mm); (2) Before necking occurs, the change in the instantaneous thickness reduction rate of the sample during the tensile process is consistent, satisfying formula (2): In the formula: η—the thickness reduction rate, in percentage (%); (3) Calculate formulas (3) and (4) according to the engineering strains in the length direction and the width direction: Substitute formula (5) and formula (6) into formula (2), and then the following can be obtained: In the formula: e b —— Engineering strain in the width direction of the specimen, in percentage (%); e L —— Engineering strain in the specimen length direction, unit: percentage (%).

3. The prediction method for the plastic deformation ability of the hot-rolled sheet of automotive structural steel under static stretching conditions according to claim 1, wherein, Step 2 Microcrystalline grain size characterization, specifically as follows: 2.1 Sample preparation: Take a block sample from the thinnest area of the fixed-strain tensile sample, with the observation surface being the surface. After embedding, grinding, polishing, and drying with an alcohol solution, make a metallographic sample for use. 2.2 Optical microscope observation: The metallographic sample is corroded with a 3 - 4% nitric acid alcohol solution, and the observation surface is placed under an upright optical microscope to observe the microstructure. Focus on observing the size and shape of ferrite grains, and record pictures under 100-fold and 500-fold magnification states. Use an image analyzer to carry out quantitative analysis, and use the metallographic secant method to conduct grain size characterization analysis on the grain metallographic pictures. 2.3 Vibration polishing treatment: After polishing the metallographic sample, place it on a vibration polishing machine, add polishing liquid for 6 hours of vibration polishing treatment, and thoroughly clean the polished surface with alcohol. 2.4 Electron microscope observation: Place the vibration-polished metallographic sample on a scanning electron microscope for EBSD analysis of the scanning electron microscope. Focus on observing the morphology and size of ferrite grains on the observation surface, and use the HKL Channel5 software to conduct grain size characterization analysis using the EBSD grain reconstruction method. 2.5 Grain size characterization: for carbon microalloyed steel, the set value is taken as 2°. Grain boundaries below this set value are ignored. For the grain reconstruction method of EBSD, a set value of critical orientation difference also needs to be given first. Considering the characteristics of the steel grade, it is also set as 2°. This method is relatively less affected by orientation noise and is more suitable for characterizing the grain size in the annealed state.

4. The prediction method for the plastic deformation ability of the hot-rolled sheet of automotive structural steel under static tensile conditions according to claim 1, characterized in that Step 3: Forming limit determination, specifically as follows. Conduct FLD determination on similar materials. Use wire cutting to obtain a set of specimens with different widths. Perform bulging on a set of specimens with a rigid punch. Bulging deformation occurs in the middle of the specimen under the action of the punch to form a convex bulge. The grid circles printed on the surface of the specimen for pretreatment are distorted. Stop the test when necking or cracking occurs in a certain part of the convex bulge. Measure the major and minor axis dimensions of the grid circles in the necking area or nearby or in the area near the crack. Calculate the surface limit principal strains (e1, e2), and analyze and establish the correlation between the thickness reduction rate, grain size, and forming limit value.

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