A method for quantitatively characterizing the embrittlement degree of high-strength steel

Abstract

This invention discloses a quantitative characterization method for the degree of brittleness in high-strength steel. The method is characterized by integrating three factors—tensile-compressive asymmetry, elongation after fracture, and fracture morphology—to construct a brittleness evaluation index for the material. This quantitative characterization method for the degree of brittleness in high-strength steel better reflects the physical properties of high-strength steel, quantitatively describing the ratio of toughness to brittleness, and providing a more accurate and reliable characterization of brittleness. This allows for better application in fracture analysis and judgment of high-strength steel, improving the accuracy of the assessment.

Description

Technical Field

[0001] This invention relates to the field of physical property testing technology for metallic structural materials, specifically to a quantitative characterization method for the degree of brittleness in high-strength steel. Background Technology

[0002] The concept of achieving lightweight products using high-strength materials such as high-strength steel has become widely accepted in transportation, aerospace, and construction in recent decades, leading to the rapid development of related technologies. However, due to the inherent contradiction between material strength and toughness, the formability (or plasticity and toughness) decreases significantly with increasing strength, which is the biggest obstacle to the application of various high-strength steels. To obtain the ideal "strength-plasticity" combination (high strength-plasticity product), the industry has developed three generations of high-strength steels: duplex (DP), martensitic (MS), transformation-induced plasticity (TRIP), and quenched partitioning (QP). However, even when strictly adhering to the specifications stipulated in the design verification stage during common sheet metal forming processes such as stamping and roll forming, there is still a significant risk of cracking in mass production. While adopting a conservative design is relatively safe, it results in a significant waste of material properties. Heating can improve plasticity, but it has inherent shortcomings in terms of production cost, efficiency, and quality. In this context, accurately evaluating the formability of high-strength steel and predicting potential cracking failures during production becomes crucial for product and process design.

[0003] In structural fracture analysis, classical fracture mechanics classifies materials into two main types: "brittle" and "ductile," and has established corresponding theoretical models for each. These models have been widely applied in engineering practice with great success. To date, extensive research has been conducted on the forming cracking of high-strength steel, resulting in various theoretical proposals. However, practice shows that high-strength steel exhibits many deformation and fracture behaviors different from ordinary steel, such as the absence of obvious necking before fracture, shear cracking, and edge cracking, making it difficult to accurately predict forming cracking using existing methods. One important reason is the intrinsic inversion between the "strength" and "ductility" of materials, making "brittleness" an inevitable characteristic of high-strength materials. The aforementioned special mechanical behaviors of high-strength steel are essentially related to its brittle properties. Therefore, actual high-strength steel possesses a "quasi-brittle" material characteristic combining "ductility + brittleness." To accurately analyze and predict the forming cracking of high-strength steel, it is necessary to consider its quasi-brittle material properties and the special characteristics of brittle fracture. However, the existing analysis of forming cracks in high-strength steel usually follows the analysis method of ordinary steel, and is mostly based on the "toughness" damage fracture assumption, without considering the brittle characteristics of high-strength materials and their effects.

[0004] Quantitative characterization of the degree of material brittleness is a prerequisite for introducing brittle properties and their effects into the fracture analysis of high-strength steel. Although ductile and brittle materials differ significantly in macroscopic deformation behavior, fracture mechanisms, and fracture morphology, existing theories, including fracture mechanics, do not provide a rigorous definition or distinction between the two. Since the evolution of material properties from "ductile" to "brittle" is a gradual process, it is difficult to give a clear boundary between the two. Due to the complexity of the problem, the degree of brittleness of materials cannot be assessed solely by a single indicator (such as elongation). In fields such as civil engineering and coal mining, the concept of a "brittleness index (BI)" has been proposed for typical brittle materials such as rocks and coal, but it is not suitable for quasi-brittle metallic materials such as high-strength steel, which exhibit a combination of "toughness and brittleness." CN201810734778.3 proposed a brittleness testing method for metallic materials, which used a "brittleness index" model to characterize the static and dynamic brittleness indices of metallic materials. However, it still adopted the definition method for ideal brittle materials such as soil and rock, did not consider the quasi-brittle characteristics of high-strength steel materials, and could not quantitatively explain the ratio of "toughness + brittleness". Therefore, it could not be used to analyze the cracking problem in the forming process of high-strength steel.

[0005] Therefore, how to provide a quantitative characterization method for the degree of brittleness of high-strength steel that takes into account both toughness and brittleness, is more consistent with the physical properties of high-strength steel, and is more accurate and reliable, so that it can be better applied to the fracture analysis and judgment of high-strength steel, is a problem that needs to be solved by those skilled in the art. Summary of the Invention

[0006] In view of the shortcomings of the prior art, the technical problem to be solved by the present invention is: how to provide a quantitative characterization method for the degree of brittleness of high-strength steel that is more in line with the physical properties of high-strength steel, can quantitatively describe the ratio of toughness and brittleness, and can be more accurate and reliable, so that it can be better applied to the fracture analysis and judgment of high-strength steel and improve the accuracy of the judgment.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution:

[0008] A quantitative characterization method for the degree of brittleness of high-strength steel is characterized by the fact that this method integrates three factors, namely tensile-compressive asymmetry, elongation after fracture, and fracture morphology, to construct a brittleness evaluation index for high-strength steel materials, thereby achieving a quantitative characterization of the degree of brittleness of high-strength steel materials.

[0009] Because the evolution of material properties from "tough" to "brittle" is a gradual process, it is difficult to give a clear boundary between the two. Using elongation after fracture as a criterion... For example, it is generally believed that It belongs to the category of tough materials. This indicates a brittle material. Due to the complexity of real-world applications, the degree of brittleness cannot be assessed using a single indicator; a multi-scale, multi-dimensional analysis of material deformation and fracture behavior is necessary. For example, tensile yield strength... Compressive yield strength The ratio of tensile and compressive strengths can effectively reveal the internal asymmetry of a material, which is closely related to brittle behavior. For ductile metallic materials, due to their good dislocation slip capability, the tensile and compressive yield strengths are basically equivalent. However, as the degree of brittleness increases, the sensitivity of internal micro-defects, second-phase particles, and interfaces to compressive stress decreases, but their sensitivity to tensile stress increases. This makes them more prone to local stress concentration and early damage during tension, resulting in a significantly lower tensile yield strength than compressive yield strength. Generally, the tensile and compressive yield strengths of ductile materials are basically symmetrical, while the compressive strength of brittle materials is higher than their tensile strength. The greater the difference between tensile and compressive strengths, the higher the degree of brittleness of the material is usually. However, compared to ordinary steel materials, high-strength steel has a more complex macro- and micro-deformation and failure mechanism. For example, the deformation of DP steel depends more on the coordination between the two phases, which makes it unlike cast iron or tool steel, which suffer from significant tensile-compressive asymmetry due to internal defects or brittle phases. Based on this, this scheme simultaneously considers three factors: tension-compression asymmetry, elongation after fracture, and fracture morphology to construct a brittleness evaluation index for high-strength steel materials, which can more effectively reflect the degree of brittleness of high-strength steel.

[0010] Furthermore, this method specifically includes the following steps:

[0011] (1) Perform uniaxial tensile and uniaxial compression physical tests on the high-strength steel material to be evaluated, repeat the experiment three times and take the average value of the measured data to obtain the tensile yield strength. Compressive yield strength Elongation after fracture Metallographic observation of the fracture morphology was performed, and the microstructure of dimples, transgranular (cleavage), and intergranular fracture was analyzed using metallographic software to obtain the proportion of dimples relative to the fracture surface area. (average value);

[0012] (2) Determine the normalized brittleness index of the material by the tensile-compressive strength ratio. Specifically, it can be obtained through the following formula:

[0013]

[0014] (3) Based on the elongation after fracture Determining the elongation factor of a material and its normalized brittleness index Specifically, it can be obtained through the following formula:

[0015]

[0016] (4) Determine the normalized brittleness index of the material based on the fracture surface geometry characteristics. Specifically, it is obtained through the following formula:

[0017]

[0018] (5) , , After combining and weighting, the "brittleness index" is obtained from the following formula. :

[0019]

[0020] in, These are the weighting coefficients obtained through experiments.

[0021] Step (2) of this method considers that the tension-compression asymmetry stems from the different response mechanisms of the material's internal microstructure under tensile and compressive stresses. As the degree of brittleness increases, the sensitivity of internal defects and the relative stress state of brittleness increases, leading to a significantly higher compressive yield strength than the tensile yield strength. Therefore, the tension-compressive strength ratio... The decrease in the value directly reflects the intensification of the material's brittleness. Therefore, in this technical solution, the normalized brittleness index determined by the tensile-compressive strength ratio is calculated using formula (1). .if ,Right now , The material is a typical tough material; The smaller, The larger the value, the more severe the brittleness from the perspective of tensile-compressive strength ratio.

[0022] In step (3) of this method, the post-fracture elongation is considered to be a traditional macroscopic indicator for measuring the plastic deformation capacity of a material, and also a major basis for distinguishing between tough and brittle materials in the engineering field. Practice shows that when The material exhibits typical toughness characteristics when It exhibits typical brittle characteristics. Based on this engineering practice, in this technical solution, a monotonic correspondence is established between the elongation after fracture and the degree of brittleness. Therefore, formula (2) is used to obtain the result from the elongation after fracture. Determined normalized fragility index .if , The material is a typical tough material; , The material is a typical brittle material. The larger the elongation after fracture, the greater the elongation. The more severe the brittleness, the better.

[0023] The consideration in step (4) of this method is because long-term research in the field of fracture geology has shown that when the dimple area accounts for a certain percentage of the total area... This is a typical ductile fracture, in which... Since this is a typical brittle fracture, the fracture surface morphology is transformed into a quantitative standard based on this objective standard. The normalized brittleness index determined by the fracture surface geometry is calculated using formula (3). Therefore, if , The material is a typical tough material. The larger the value, the more severe the degree of brittleness from the perspective of fracture geometry.

[0024] In step (5) of this method, formula (4) is used to... By combining and weighting the data, we obtain the "fragility index". .in, These are weighting coefficients obtained experimentally, reflecting the contribution weight of each sub-indicator to the degree of brittleness. Because steels of different strength grades and different degrees of brittleness have varying measured values ​​of formability (such as ultimate arch height)... The dimensions and numerical ranges of the data differ significantly, making it impossible to directly compare the brittleness of different materials using the raw data, nor can it be compared with the individual brittleness indices. (All are) The dimensionless quantities within the interval are coupled for modeling. Therefore, normalization is needed to eliminate the influence of dimensions and uniformly map the original forming performance data to... Intervals are used to construct objectively comparable fragile reference benchmarks. This is achieved by establishing an independent... Furthermore, a reference standard that reflects the material's formability is used to calibrate the weighting coefficients. .

[0025] Therefore, more specifically, the weighting coefficients The experimental calibration method includes the following steps:

[0026] ① Select at least 10 different strength grades of high-strength steel and determine their respective brittleness indices according to steps (2)-(4). ;

[0027] ② Measure the molding properties of each sample to obtain a molding property dataset. ;

[0028] ③ Due to the ultimate arch height It is negatively correlated with the degree of brittleness. The larger it is, the more resilient it is. The smaller the value, the greater the brittleness. To establish a reference standard positively correlated with the degree of brittleness, normalization is performed using a reverse mapping method according to formula (5):

[0029]

[0030] in, This represents the measured ultimate arch height of the material under test. The minimum ultimate arch height among all materials involved in the calibration; The maximum value of the ultimate arch height among all materials involved in the calibration;

[0031] ④ with As the independent variable, Establish a multiple linear regression model with the variable as the dependent variable:

[0032]

[0033] in, Let be the regression coefficients to be determined;

[0034] ⑤ Use the least squares method to solve for the regression coefficients, with the goal of minimizing the total sum of squared errors:

[0035]

[0036] in, It is the sum of squared residuals; The total number of materials involved in the calibration; For the first The reference value of the brittleness index of the material is calculated by formula (5); The first The brittleness index of the material is calculated by formulas (1), (2), and (3);

[0037] By solving the system of equations and performing multiple linear regression, the regression coefficients can be obtained. The value;

[0038] ⑥ Normalize the regression coefficients:

[0039]

[0040] Normalized This is the final weighting coefficient used, which satisfies... .

[0041] Thus, through three steps—Min-Max extreme value method data standardization (dimension removal), combined weighting method weight allocation, and weighted average summarization—the data is... Normalization is performed to make . This indicates that the material is an ideally tough material. This indicates that the material is completely brittle. Generally speaking, between.

[0042] Therefore, this invention has the following beneficial effects: The "brittleness index" obtained by this invention is based on multi-dimensional and cross-scale comprehensive analysis of experimental data, possessing strong physical background and operability. It not only enables quantitative characterization of the degree of brittleness in high-strength materials but also provides a connecting link for introducing the influence of brittle characteristics into fracture analysis. Therefore, the quantitative characterization method for the degree of brittleness in high-strength steel of this invention is more consistent with the physical characteristics of high-strength steel, quantitatively explaining the ratio of toughness and brittleness, and providing a more accurate and reliable characterization of brittleness. This allows it to be better applied to fracture analysis and judgment of high-strength steel, improving the accuracy of the judgment. Attached Figure Description

[0043] Figure 1 This is a schematic diagram of the method steps of the present invention.

[0044] Figure 2 This is a schematic diagram of the gauge length dimensions of a uniaxial tensile specimen of DP1180 high-strength steel.

[0045] Figure 3 The stress-strain curve of DP1180 high-strength steel under tension.

[0046] Figure 4 The dimensions of the uniaxial compression specimen for DP1180 high-strength steel are shown.

[0047] Figure 5 The stress-strain curve of DP1180 high-strength steel under compression.

[0048] Figure 6 The original microstructure of the fracture surface of DP1180 high-strength steel.

[0049] Figure 7 The microstructure of DP1180 steel after statistical cleavage characteristics.

[0050] Figure 8 This is a schematic diagram of the gauge length dimensions of a uniaxial tensile specimen of low-carbon steel.

[0051] Figure 9 The stress-strain curve of low-carbon steel under tension.

[0052] Figure 10 The dimensions are for a uniaxial compression test specimen made of low-carbon steel.

[0053] Figure 11 The stress-strain curve of low-carbon steel under compression.

[0054] Figure 12 The original microstructure of the fracture surface of low carbon steel.

[0055] Figure 13 The microstructure of low-carbon steel after statistical cleavage features. Detailed Implementation

[0056] The present invention will now be described in further detail with reference to the accompanying drawings and specific embodiments.

[0057] In specific implementation: This method requests protection for a quantitative characterization method of the brittleness degree of high-strength steel. Its feature is that it integrates three factors, namely tensile-compressive asymmetry, elongation after fracture, and fracture morphology, to construct a brittleness evaluation index for high-strength steel materials, thereby realizing a quantitative characterization of the brittleness degree of high-strength steel materials.

[0058] See Figure 1 This method specifically includes the following steps:

[0059] (1) Perform uniaxial tensile and uniaxial compression physical tests on the high-strength steel material to be evaluated, repeat the experiment three times and take the average value of the measured data to obtain the tensile yield strength. Compressive yield strength Elongation after fracture Metallographic observation of the fracture morphology was performed, and the microstructure of dimples, transgranular (cleavage), and intergranular fracture was analyzed using metallographic software to obtain the proportion of dimples relative to the fracture surface area. (average value);

[0060] (2) Determine the normalized brittleness index of the material by the tensile-compressive strength ratio. Specifically, it can be obtained through the following formula:

[0061]

[0062] (3) Based on the elongation after fracture Determining the elongation factor of a material and its normalized brittleness index Specifically, it can be obtained through the following formula:

[0063]

[0064] (4) Determine the normalized brittleness index of the material based on the fracture surface geometry characteristics. Specifically, it is obtained through the following formula:

[0065]

[0066] (5) After combining and weighting, the "brittleness index" is obtained from the following formula. :

[0067]

[0068] in, These are the weighting coefficients obtained through experiments.

[0069] Step (2) of this method considers that the tension-compression asymmetry stems from the different response mechanisms of the material's internal microstructure under tensile and compressive stresses. As the degree of brittleness increases, the sensitivity of internal defects and the relative stress state of brittleness increases, leading to a significantly higher compressive yield strength than the tensile yield strength. Therefore, the tension-compressive strength ratio... The decrease in the value directly reflects the intensification of the material's brittleness. Therefore, in this technical solution, the normalized brittleness index determined by the tensile-compressive strength ratio is calculated using formula (1). .if ,Right now , The material is a typical tough material; The smaller, The larger the value, the more severe the brittleness from the perspective of tensile-compressive strength ratio.

[0070] In step (3) of this method, the post-fracture elongation is considered to be a traditional macroscopic indicator for measuring the plastic deformation capacity of a material, and also a major basis for distinguishing between tough and brittle materials in engineering practice. Practice shows that when The material exhibits typical toughness characteristics when It exhibits typical brittle characteristics. Based on this engineering practice, in this technical solution, a monotonic correspondence is established between the elongation after fracture and the degree of brittleness. Therefore, formula (2) is used to obtain the result from the elongation after fracture. Determined normalized fragility index .if , The material is a typical tough material; , The material is a typical brittle material. The larger the elongation after fracture, the greater the elongation. The more severe the brittleness, the better.

[0071] The consideration in step (4) of this method is because long-term research in the field of fracture geology has shown that when the dimple area accounts for a certain percentage of the total area... This is a typical ductile fracture, in which... Since this is a typical brittle fracture, the fracture surface morphology is transformed into a quantitative standard based on this objective standard. The normalized brittleness index determined by the fracture surface geometry is calculated using formula (3). Therefore, if , The material is a typical tough material. The larger the value, the more severe the degree of brittleness from the perspective of fracture geometry.

[0072] In step (5) of this method, formula (4) is used to... By combining and weighting the data, we obtain the "fragility index". .in, These are weighting coefficients obtained experimentally, reflecting the contribution weight of each sub-indicator to the degree of brittleness. Because steels of different strength grades and different degrees of brittleness have varying measured values ​​of formability (such as ultimate arch height)... The dimensions and numerical ranges of the data differ significantly, making it impossible to directly compare the brittleness of different materials using the raw data, nor can it be compared with the individual brittleness indices. (All are) The dimensionless quantities within the interval are coupled for modeling. Therefore, normalization is needed to eliminate the influence of dimensions and uniformly map the original forming performance data to... Intervals are used to construct objectively comparable fragile reference benchmarks. This is achieved by establishing an independent... Furthermore, a reference standard that reflects the material's formability is used to calibrate the weighting coefficients. .

[0073] In practical implementation, the weighting coefficient The experimental calibration method includes the following steps:

[0074] ① Select at least 10 different strength grades of high-strength steel and determine their respective brittleness indices according to steps (2)-(4). ;

[0075] ② Measure the molding properties of each sample to obtain a molding property dataset. ;

[0076] ③ Due to the ultimate arch height It is negatively correlated with the degree of brittleness. The larger it is, the more resilient it is. The smaller the value, the greater the brittleness. To establish a reference standard positively correlated with the degree of brittleness, normalization is performed using a reverse mapping method according to formula (5):

[0077]

[0078] in, This represents the measured ultimate arch height of the material under test. The minimum ultimate arch height among all materials involved in the calibration; The maximum value of the ultimate arch height among all materials involved in the calibration;

[0079] ④ with As the independent variable, Establish a multiple linear regression model with the variable as the dependent variable:

[0080]

[0081] in, Let be the regression coefficients to be determined;

[0082] ⑤ Use the least squares method to solve for the regression coefficients, with the goal of minimizing the total sum of squared errors:

[0083]

[0084] in, It is the sum of squared residuals; The total number of materials involved in the calibration; For the first The reference value of the brittleness index of the material is calculated by formula (5); The first The brittleness index of the material is calculated by formulas (1), (2), and (3);

[0085] By solving the system of equations and performing multiple linear regression, the regression coefficients can be obtained. The value;

[0086] ⑥ Normalize the regression coefficients:

[0087]

[0088] Normalized This is the final weighting coefficient used, which satisfies... .

[0089] Thus, through three steps—Min-Max extreme value method data standardization (dimension removal), combined weighting method weight allocation, and weighted average summarization—the data is... Normalization is performed to make . This indicates that the material is an ideally tough material. This indicates that the material is completely brittle. Generally speaking, Between 0 and 1.

[0090] To further verify the effectiveness of the present invention, the applicant, under the premise of meeting the above method steps, used a high-strength steel and a low-carbon steel as actual examples, respectively, and calculated the degree of brittleness of the high-strength steel for comparison.

[0091] Example 1: Taking the experimental results of DP1180 steel from the paper "A Girard, V Grolleau, D Mohr. Using miniature cuboidal speccimens to determine compression Stress–strain curve of sheet metal understatic and dynamic loading. International Journal of Solids and Structures 320 (2025) 113527" as an example, calculate its brittleness index. number.

[0092] (1) The parameter values ​​required for this method have been disclosed in the above-mentioned papers. The specific values ​​are as follows.

[0093] ① According to section "3. Experimental procedures" of the paper, the uniaxial tensile specimen was machined by wire cutting, and the gauge length of the specimen is: length Sample structure as follows Figure 2 As shown.

[0094] ② Quasi-static mechanical tests were conducted at room temperature using an electronic universal testing machine. (Based on the image in the paper...) Extracting the tensile stress-strain curve as follows: Figure 3 As shown. The tensile yield strength of DP1180 steel is extracted from this curve. Elongation after fracture .

[0095] ③ The compression specimen is a miniature cuboid with a cross-sectional side length of 1.05 mm (consistent with the original thickness of the DP1180 steel plate) and a total height of 1.57 mm. Figure 4 As shown. The sample was machined by wire cutting, and the surface roughness is... , Parallelism tolerance of upper and lower end faces This ensures that the force is evenly distributed during loading.

[0096] ④ According to the image in the paper " Extracting the compressive stress-strain curve as follows: Figure 5 As shown. The compressive yield strength of DP1180 steel is extracted from this curve. .

[0097] ⑤ The microstructure of the fracture surface was obtained using scanning electron microscopy, as shown in the figure. Figure 6 As shown. Using ImageJ image processing software, the cleavage area in the field of view was calculated after preprocessing the image. Total area . Figure 7 The microstructure after statistical cleavage characteristics is obtained, and finally the proportion of dimples to cross-sectional area is obtained. .

[0098] (2) Calculate the normalized brittleness index determined by the tensile-compressive strength ratio using formula (1) of this method. The tensile yield strength obtained in step (1) and compressive yield strength Substituting into formula (1), we obtain the normalized brittleness index. for:

[0099] .

[0100] (3) The elongation after fracture is obtained by formula (2) of this method. Determined normalized fragility index ; Due to the elongation after fracture obtained in step (1) In Substituting the interval into formula (2), we obtain the normalized brittleness index. for:

[0101] .

[0102] (4) Calculate the normalized brittleness index determined by the fracture geometry using formula (3) of this method. ; Due to the proportion of the dimple portion relative to the cross-sectional area obtained in step (1) Substituting into formula (3), we obtain the normalized brittleness index. :

[0103] (5) After combining and weighting, the "brittleness index" is obtained from formula (4) of this method. Among them, the weighting coefficient The calibration method uses the Limiting Dome Height (LDH) test to determine the formability of the material, and the test results are used as a reference value for the brittleness index. .

[0104] ① The test was conducted in accordance with GB / T15825.3-2008 standard:

[0105] A 100 mm diameter square sheet metal specimen is placed between a die (100 mm inner diameter) and a blank holder, and a blank holder force of 50 kN is applied to prevent wrinkling. A 50 mm diameter hemispherical punch is used to lift the specimen upwards at a speed of 1 mm / min. When the specimen breaks, the test is stopped immediately, and the punch travel height H (mm) at the moment of breakage is recorded. The test is repeated 3 times for each material, and the average value is taken as the ultimate camber height for that material. .

[0106] All materials involved in the calibration (total) Repeat the above experiment to obtain the ultimate arch height dataset. Find the maximum value among them. and minimum value .

[0107] ② Brittleness index reference value Normalization is performed according to formula (5) of this invention.

[0108] ③ Summarize the measurement results of all materials to form a complete list. Dataset of group samples:

[0109]

[0110] ④ with As the independent variable, Establish a multiple linear regression model with the variable as the dependent variable:

[0111]

[0112] In the formula, denoted as the regression coefficient to be determined.

[0113] ⑤ Use the least squares method to solve for the regression coefficients, with the goal of minimizing the total sum of squared errors:

[0114]

[0115] By solving the system of equations and performing multiple linear regression, the regression coefficients can be obtained. The value of .

[0116] ⑥ Normalize the regression coefficients:

[0117]

[0118] Normalized This is the final weighting coefficient used, which satisfies... .

[0119] The data is standardized (dimensionless) using the Min-Max extreme value method, weighted by a combined weighting method, and then summarized by a weighted average. Normalization is performed to make . This indicates that the material is an ideally tough material. This indicates that the material is completely brittle. Generally speaking, exist between.

[0120] The weights are determined using a combination weighting method: .Will Substituting into formula (4), we finally obtain the "brittleness index". for:

[0121] .

[0122] Example 2: Taking the experimental results of low-carbon steel in the literature "T Koizumi, M Kuroda. Evaluation of tension-compressionasymmetry of a low-carbon steel sheet using a modified classical compressiontest method. IOP Conf. Series: Journal of Physics: Conf. Series 1063 (2018)012167" as an example, this method is used to calculate its brittleness index. .

[0123] (1) The parameter values ​​required for this method have been disclosed in the above-mentioned papers. The specific values ​​are as follows.

[0124] ① According to section "2.3. Testing conditions, equipments and evaluation of SDE" in the paper, the gauge length of the uniaxial tensile specimen is: length ,like Figure 8 As shown.

[0125] ② Based on the images "Figure 3(a)" and "Table 1." in the paper, extract the tensile stress-strain curve as follows: Figure 9 As shown. The tensile yield strength of low-carbon steel is extracted from this curve. Elongation after fracture .

[0126] ③ The compression specimen is a cylinder with a diameter and height of 10 mm, such as Figure 10 As shown.

[0127] ④ Based on the images “Figure 3(a)” and “Table 1” in the paper, extract the compressive stress-strain curve as follows: Figure 11 As shown. The compressive yield strength of low-carbon steel is extracted from this curve. .

[0128] ⑤ The microstructure of the fracture surface was obtained using scanning electron microscopy, as shown in the figure. Figure 12 As shown. Using ImageJ image processing software, the cleavage area in the field of view was calculated after preprocessing the image. Total area . Figure 13 The microstructure after statistical cleavage characteristics is obtained, and finally the proportion of dimples to cross-sectional area is obtained. .

[0129] (2) The tensile yield strength obtained in step (1) and compressive yield strength Substituting into formula (1), we obtain the normalized brittleness index. for:

[0130] .

[0131] (3) Due to the elongation after fracture obtained in step (1) In Substituting into formula (2), we obtain the normalized brittleness index. .

[0132] (4) Due to the proportion of the dimple portion relative to the cross-sectional area obtained in step (1) In Substituting the interval into formula (3), we obtain the normalized brittleness index. .

[0133] (5) Determine the weights using the combined weighting method: .Will Substituting into formula (4), we obtain the "brittleness index". for:

[0134]

[0135] The calculation results from Examples 1 and 2 above show that the brittleness index of DP1180 high-strength steel is... 0.323. Brittleness index of low-carbon steel. : 0.082.

[0136] The above results verify that the brittleness characterization method of the present invention can effectively distinguish materials with different degrees of brittleness, and the brittleness index... The larger the value, the higher the degree of brittleness of the material. This method comprehensively considers three dimensions: "tensile-compression asymmetry", "elongation after fracture" and "fracture morphology", combining the "toughness + brittleness" of high-strength steel, effectively avoiding the limitations of assessing the degree of brittleness of materials based on a single indicator.

[0137] As can be seen from the specific process and principle of the present invention described above, when implemented, the quantitative characterization method of the degree of brittleness of high-strength steel of the present invention can be directly applied to the prediction of cracking risk and material selection guidance in the cold forming of high-strength steel.

[0138] Quantitative assessment of crack risk level:

[0139] Calculate the individual fragility indices according to the present invention. Combined with the calibrated weighting coefficients The brittleness index of the material under test was calculated. The cracking risk level of the tested material should be determined according to the following grading criteria: If If it is low risk, then it can be judged as low risk; if If so, it can be determined as medium risk; if If so, it can be judged as high risk; if If so, it can be determined as an extremely high risk.

[0140] Taking the two materials in Examples 1 and 2 as examples: the brittleness index of DP1180 high-strength steel The risk level is deemed high, as brittle cracking along the edges is prone to occur during bending and deep drawing processes in actual cold forming; the brittleness index of low-carbon steel is... The material was determined to be low-risk, with few instances of cracking occurring during conventional stamping and rolling processes. This assessment is highly consistent with the cracking behavior of both materials in actual cold forming operations.

[0141] Quantitative material selection guidelines for structural components:

[0142] Based on the forming complexity and service safety requirements of the target structural component, and combined with the risk assessment results from step 3, targeted material selection is carried out:

[0143] For structural components that are simple to deform and mass-produced (such as automotive body panels): prioritize low-risk options. Materials should be selected to ensure zero defects in the forming process while balancing production efficiency and manufacturing costs.

[0144] For structural components with complex deformation and high safety requirements (such as automotive crash beams, door frame reinforcement plates, and load-bearing components of engineering machinery): low-risk or medium-risk components should be prioritized. Materials; if high-risk materials are selected Simultaneously, process compatibility verification must be carried out to ensure molding safety.

Claims

1. A quantitative characterization method for the degree of brittleness in high-strength steel, characterized in that, This method integrates three factors—tensile-compression asymmetry, post-fracture elongation, and fracture morphology—to construct a brittleness evaluation index for high-strength steel materials, thereby achieving a quantitative characterization of the degree of brittleness in high-strength steel materials.

2. The quantitative characterization method for the degree of brittleness of high-strength steel according to claim 1, characterized in that, Specifically, the following steps are included: (1) Perform uniaxial tensile and uniaxial compression physical tests on the high-strength steel material to be evaluated, repeat the experiment three times and take the average value of the measured data to obtain the tensile yield strength. Compressive yield strength Elongation after fracture Metallographic observation of the fracture morphology was performed, and the microstructure of dimples, transgranular and intergranular fractures was analyzed using metallographic imaging software to obtain the proportion of dimples relative to the fracture surface area. ; (2) Determine the normalized brittleness index of the material by the tensile-compressive strength ratio. Specifically, it can be obtained through the following formula: ； (3) Based on the elongation after fracture Determining the elongation factor of a material and its normalized brittleness index Specifically, it can be obtained through the following formula: ； (4) Determine the normalized brittleness index of the material based on the fracture surface geometry characteristics. Specifically, it is obtained through the following formula: ； (5) After combining and weighting, the "fragility index" is obtained from the following formula. : ； in, These are the weighting coefficients obtained through experiments.

3. The quantitative characterization method for the degree of brittleness of high-strength steel according to claim 2, characterized in that, Weighting coefficients The test calibration method includes the following steps: ① Select at least 10 different strength grades of high-strength steel and determine their respective brittleness indices according to steps (2)-(4). ; ② Measure the molding properties of each sample to obtain a molding property dataset. ; ③ Because the ultimate arch height H is negatively correlated with the degree of brittleness ( The larger it is, the more resilient it is. The smaller the value, the greater the brittleness. To establish a reference standard positively correlated with the degree of brittleness, normalization is performed using a reverse mapping method according to formula (5): ； in, This represents the measured ultimate arch height of the material under test. The minimum ultimate arch height among all materials involved in the calibration; The maximum value of the ultimate arch height among all materials involved in the calibration; ④ with As the independent variable, Establish a multiple linear regression model with the variable as the dependent variable: ； in, Let be the regression coefficients to be determined; ⑤ Use the least squares method to solve for the regression coefficients, with the goal of minimizing the total sum of squared errors: ； in, It is the sum of squared residuals; The total number of materials involved in the calibration; For the first The reference value of the brittleness index of the material is calculated by formula (5); The first The brittleness index of the material is calculated by formulas (1), (2), and (3); By solving the system of equations and performing multiple linear regression, the regression coefficients can be obtained. The value; ⑥ Normalize the regression coefficients: ； Normalized This is the final weighting coefficient used, which satisfies... .

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