Method for evaluating stamping forming fracture defect of automobile steel through continuous forming capability

Abstract

The invention provides a method for evaluating automobile steel stamping forming fracture defects through continuous forming capacity, and belongs to the technical field of automobile steel stamping forming. According to the method, a strain analysis mode is adopted, the application performance and the strain of the automobile steel are combined together, then the deformation history in stamping forming is a linear or approximately linear path, the principle that the plastic forming volume is not changed is combined, and two quantifiable indexes including the continuous forming capacity and the safe forming margin are used; and evaluating the stamping forming fracture defect of the steel for the automobile. According to the method, a macroscopic plate is converted into countless square grid units through grid strain analysis, then the strain states of the square grid units are expressed through equivalent strain according to an equivalent strain calculation formula in the classical plasticity theory, and finally, the continuous forming capacity and the safe forming margin are utilized for quantitative indexes, namely, the continuous forming capacity and the safe forming margin. And evaluating the stamping forming fracture defect of the steel for the automobile. The evaluation method has sufficient theoretical basis support, and accuracy and precision are guaranteed.

Description

Technical Field

[0001] The present invention relates to the technical field of automobile steel stamping and forming, and more particularly to a method for evaluating automobile steel stamping and forming cracking defects based on continued forming capability. Background Art

[0002] In recent years, my country's automotive industry has developed rapidly, with production and sales ranking first in the world for many consecutive years. The automotive industry plays an increasingly important role in my country's economy. Approximately 70% of automotive parts require stamping. Since the advent of stamping, especially in the production of automotive parts, it has been accompanied by various stamping defects, the most common of which is stamping cracking. Stamping cracking refers to the cracking phenomenon that occurs when the tensile stress exceeds a critical value due to insufficient strength or plasticity of the material. Stamping cracking reduces the stability of stamping production, increases the scrap rate, and causes significant economic losses. Therefore, how to accurately evaluate whether cracking defects will occur in the stamping of automotive steel has become a common concern and direction of effort for the automotive and steel industries.

[0003] Traditional evaluation methods for crack defects in stamping of automotive steel rely mainly on two aspects. The first is the mechanical properties of the automotive steel itself, such as yield strength, tensile strength, elongation, n value, r value, etc. If these parameters meet national standards or enterprise-defined standards, it is considered that the automotive steel stamping will not cause crack defects and meets the enterprise's stamping requirements. The second is the feedback results of actual on-site stamping. If the automotive steel does not crack or show hidden cracks during the stamping process, the stamping is considered qualified. Otherwise, the mold needs to be debugged or the stamping material needs to be replaced. Traditional evaluation methods have great limitations and uncertainties. There is no quantitative evaluation index. Problems such as "not meeting the standards but not being suitable for use" of stamping materials, excessive forming performance, and excessive thinning often occur. In addition, a large amount of mold debugging work is required, which can easily lead to an increase in stamping scrap rate. It cannot meet the high standards of modern automobile manufacturers for stamping qualification rate, economic efficiency of stamping material selection, and stamping stability.

[0004] With a precise evaluation method, we can maximize the use of resources, using the "right material in the right place," significantly improving stamping pass rate and stability, and creating positive economic value for the company. Conversely, this can easily lead to an increase in stamping defect rates and increased costs due to excess stamping performance. There is an urgent need for a high-precision, easy-to-use, and quantifiable evaluation method for cracking defects in automotive steel stamping. This method could, on the one hand, guide the actual stamping process and ensure stamping pass rate and stability; on the other hand, serve as a threshold parameter for cracking defects in CAE software simulations of automotive steel stamping processes. Summary of the Invention

[0005] In response to the technical problems mentioned in the above background technology, a method for evaluating crack defects in stamped automotive steel using continuous forming capability is provided. The present invention provides a method for evaluating crack defects in stamped automotive steel using continuous forming capability. Through room temperature tensile testing and Erichsen cupping tests, a polynomial fitting method is used to derive the ultimate strain polynomial of the automotive steel material itself. Subsequently, during the actual stamping process of the automotive steel, the continuous forming capability (i.e., the difference between the equivalent strain at the ultimate strain state corresponding to the maximum stamping degree of the automotive steel and the equivalent strain at that strain state) is compared with the safety forming margin (the equivalent strain of the ultimate forming capability achievable by automotive steel at 8%-10%) to evaluate whether the automotive steel stamped automotive parts will produce crack defects. This method overcomes the limitations and inaccuracies of traditional methods for evaluating crack defects in stamped automotive steel. Furthermore, the method utilizes two quantifiable indicators, continuous forming capability and safety forming margin, for evaluation. The method is simple to operate, easy to master, and not limited by operator experience. It has high consistency and can be widely adopted in production lines such as steel mills, OEMs, and parts manufacturers. At the same time, the evaluation method provided by the present invention can be applied to commercial stamping simulation software to evaluate the threshold parameters of whether crack defects occur in stamping of automobile steel.

[0006] The technical means adopted in the present invention are as follows:

[0007] The method for evaluating the fracture defects of stamping steel for automobiles by continuous forming capability adopts strain analysis, including the following steps:

[0008] Step 1: Obtain the anisotropy coefficient of automotive steel through room temperature tensile testing;

[0009] Step 2: Obtain the limit strain polynomial of automotive steel through the Erichsen cupping test, and express the formability of automotive steel through quantifiable equivalent strain;

[0010] Step 3: Based on the intersection of the linear stamping path corresponding to the strain of a certain unit in the process of stamping automobile parts made of automobile steel materials and the curve fitted by the limit strain polynomial, the limit strain that can be achieved by this unit is obtained;

[0011] Step 4: Obtain the equivalent strain of the unit. The difference between the equivalent strain corresponding to the ultimate strain state of the unit and the equivalent strain corresponding to the strain state that has occurred in the unit is the unit's continued forming capability.

[0012] Step 5: Obtain the continued forming capability of the unit with the maximum stamping degree during the stamping process of automobile steel materials for automobile parts, compare it with the safety forming margin, and evaluate whether the automobile parts stamped with automobile steel will have cracking defects.

[0013] Furthermore, in step 1, obtaining the anisotropy coefficient of automobile steel through a room temperature tensile test includes the following steps:

[0014] Step 11: Perform a room temperature tensile test on the automotive steel material to obtain the plastic strain ratio r value;

[0015] Step 12: Since the plastic strain ratio r value is often directional, the values in different directions of the material plane are often different; therefore, samples with angles of 0°, 45°, and 90° along the rolling direction are taken to obtain the plastic strain ratio r values in different directions, which are recorded as r0, r 45 、r 90 ;

[0016] Step 13: Obtain the anisotropy coefficient of the automotive steel material for:

[0017]

[0018] Furthermore, in the Erichsen cupping test, N specimens are prepared, each having a length L, 160mm≤L≤180mm, and a width B, 20mm≤B≤180mm, where N≥8; and a square grid of size A0×A0, 0.1mm<A0≤4.0mm, is printed on the surface of the N specimens by electrochemical corrosion or hand-painting.

[0019] Furthermore, in step 2, by measuring the size change of the critical grid near the necking area or the rupture area, recorded as A1×A2, and taking A1>A2, the surface limit strain of the sample is (ε 11 , ε 12 );

[0020] Among them, ε 11 represents the principal strain; ε 12 represents the secondary strain; and ε 11 =ln(A1 / A0),ε 12 =ln(A2 / A0);

[0021] The apparent limit strain (ε 11 , ε 12 )、(ε 21 , ε 22 ),……、(ε n1 , ε n2 ) are plotted in the strain coordinate system with the primary strain ε1 on the ordinate and the secondary strain ε2 on the abscissa, according to the trend and distribution characteristics of the data;

[0022] Perform polynomial fitting, the degree of the polynomial is m, m ≥ 2, that is, the fitted polynomial ε1 is:

[0023]

[0024] Among them, ε, b, c, and C are all constants;

[0025] The fitting polynomial ε1 is the ultimate strain polynomial of automobile steel.

[0026] Furthermore, the step 3 includes the following steps:

[0027] Step 31: Print a square grid of size B0×B0 on the blanked sheet by electrochemical etching or hand-drawing, with 0.1mm≤B0≤5.0mm;

[0028] Step 32: The sheet with the printed square grid is subjected to the first-order OP10 stamping. The size of a certain square grid becomes B1×B2, B1>B2. The square grid is regarded as a unit. After stamping, the principal strain ε1=ln(B1 / B0), the secondary strain ε2=ln(B2 / B0), and the thickness strain is recorded as ε t ;

[0029] Step 33: According to the principle of constant volume of metal plastic deformation ε1+ε2+ε t =0, we know that ε t =-(ε1+ε2);

[0030] Step 34: According to the equivalent strain ε in classical plasticity theory e for:

[0031]

[0032] Furthermore, in step 4, since most of the stamping paths are linear stamping paths, and the stamping paths are started when the major and minor strains are 0, the linear stamping path of the major and minor strains (ε1, ε2) of a certain unit satisfies the formula:

[0033] ε1=Aε2;

[0034] Where A represents a constant;

[0035] If the unit is continuously loaded to the necking state, that is, the intersection of the linear punching path and the curve fitted by the limit strain polynomial, the achievable limit strain (ε 1limit , ε 2limit ), so the equivalent strain ε corresponding to the ultimate strain elimit for:

[0036]

[0037] The ultimate strain state that this unit can reach is the equivalent strain ε corresponding to the necking state.elimit The equivalent strain ε corresponding to the strain state that has occurred in this unit e The difference is the continued forming ability of this unit, recorded as ε erest ,but:

[0038] ε erest =ε elimit -ε e .

[0039] Furthermore, the step 5 includes the following steps:

[0040] Step 51: Measure the mesh changes in the mesh area of the automobile parts stamped from automobile steel materials to obtain the strain of each unit;

[0041] Step 52: Obtain the maximum value of the equivalent strain of each unit, that is, find the unit with the maximum stamping forming degree, and the corresponding equivalent strain is recorded as ε' e ;

[0042] Step 53: Obtain the equivalent strain ε' of the limit strain that can occur in the unit with the maximum stamping forming degree elimit ;

[0043] Step 54: Obtain the continuous forming capability ε' of the unit where the maximum stamping forming degree occurs erest , that is, ε' erest =ε' elimit -ε' e .

[0044] Furthermore, the safety forming margin ε' esafe =(8%-10%)·ε' elimit .

[0045] Furthermore, in step 5, according to the continued forming capability ε' erest and safety forming margin ε' esafe , the evaluation of whether automobile parts stamped from automobile steel have cracking defects is as follows:

[0046] (1) If ε' erest ≥4(ε' esafe ), then the result of stamping the automotive steel into automotive parts is safe and will not cause stamping cracking. However, the automotive steel has a large continuous forming capacity, resulting in excessive material properties and is not suitable for stamping the automotive parts;

[0047] (2) If ε' esafe <ε' erest <4(ε' esafe), then the result of stamping the automobile parts with the automobile steel is evaluated to be safe and suitable for stamping the automobile parts;

[0048] (3) If ε' erest ≤ε' esafe , it can be evaluated that the result of stamping automobile parts with this automobile steel is unsafe and is not suitable for stamping this automobile part.

[0049] Compared with the prior art, the present invention has the following advantages:

[0050] (1) This invention uses grid strain analysis to convert a macroscopic sheet material into numerous square grid cells. Based on the equivalent strain calculation formula in classical plasticity theory, the strain state of the square grid cells is expressed as equivalent strain. Finally, two quantifiable indicators, continued forming capability and safe forming margin, are used to evaluate the cracking defects in automotive steel stamping. This evaluation method is supported by sufficient theoretical basis, ensuring accuracy and precision.

[0051] (2) The ultimate strain polynomial for automotive steel derived by the present invention through room temperature tensile testing, Erichsen cupping testing, and polynomial fitting is the inherent forming property of the material itself, and the required data is easy to obtain. Therefore, the ultimate strain polynomial for automotive steel of different grades and thickness specifications can be obtained, and then a database can be established, reducing the number of repeated tests, ensuring data consistency, and improving work efficiency.

[0052] (3) The evaluation method provided by the present invention is simple to operate and is not affected by external factors such as operator experience, stamping process, and mold status. It can be widely promoted and applied to production lines such as steel mills, main engine factories, and parts and components supporting factories.

[0053] (4) The evaluation method provided by the present invention can be adapted to commercial stamping simulation software and can be used as a threshold parameter for evaluating whether automobile steel stamping produces cracking defects in commercial stamping simulation software. The calculation is relatively simple and the time required is short. The stamping performance can be evaluated quickly with high accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0054] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative labor.

[0055] Figure 1 It is a schematic diagram of the overall process of the present invention.

[0056] Figure 2This is the surface limit strain distribution after the DC06-0.70mm Erichsen cupping test in Example 1 of the present invention.

[0057] Figure 3 This is the limit strain fitting curve of DC06-0.70mm in Example 1 of the present invention when the secondary strain is less than 0.

[0058] Figure 4 This is the ultimate strain fitting curve of DC06-0.70mm in Example 1 of the present invention when the secondary strain is ≥0.

[0059] Figure 5 This is the surface limit strain distribution after the DC04-0.75mm Erichsen cupping test in Example 2 of the present invention.

[0060] Figure 6 This is the limit strain fitting curve of DC04-0.75mm in Example 2 of the present invention when the secondary strain is less than 0.

[0061] Figure 7 This is the ultimate strain fitting curve of DC04-0.75mm in Example 2 of the present invention when the secondary strain is ≥0. DETAILED DESCRIPTION

[0062] In order to enable those skilled in the art to better understand the solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0063] It should be noted that the terms "first", "second", etc. in the description and claims of the present invention and the above-mentioned drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that the numbers used in this way can be interchanged where appropriate, so that the embodiments of the present invention described herein can be implemented in an order other than those illustrated or described herein. In addition, the terms "including" and "having" and any variations thereof are intended to cover non-exclusive inclusions. For example, a process, method, system, product or device that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or inherent to these processes, methods, products or devices.

[0064] like Figure 1 As shown, the present invention provides a method for evaluating the fracture defects of automobile steel stamping by continuous forming ability, which adopts the method of strain analysis and includes the following steps:

[0065] Step 1: Obtaining the anisotropy coefficient of automotive steel through room temperature tensile testing; including the following steps:

[0066] Step 11: Perform room temperature tensile tests on automotive steel materials to obtain the plastic strain ratio r value in accordance with GB / T 228.1-2010 "Tension tests on metallic materials - Part 1: Room temperature test methods";

[0067] Step 12: Since the plastic strain ratio r value is often directional, the values in different directions of the material plane are often different; therefore, samples with angles of 0°, 45°, and 90° along the rolling direction are taken to obtain the plastic strain ratio r values in different directions, which are recorded as r0, r 45 、r 90 ;

[0068] Step 13: Obtain the anisotropy coefficient of the automotive steel material for:

[0069]

[0070] Step 2: Obtain the limit strain polynomial of automotive steel through the Erichsen cupping test, and express the forming ability of automotive steel through quantifiable equivalent strain.

[0071] Preferably, in the present application, in the Erichsen cupping test, N specimens with a length of L, 160mm≤L≤180mm, and a width of B, 20mm≤B≤180mm are prepared in accordance with GB / T 4156-2007 "Erichsen cupping test for thin plates and strips of metallic materials", N≥8; and a square grid with a size of A0×A0 is printed on the surface of the N specimens by electrochemical corrosion or hand-painting, 0.1mm<A0≤4.0mm.

[0072] By measuring the size change of the critical grid near the necking area or the rupture area, recorded as A1×A2, and taking A1>A2, the surface limit strain of the specimen is (ε 11 , ε 12 );

[0073] Among them, ε 11 represents the principal strain; ε 12 represents the secondary strain; and ε 11 =ln(A1 / A0),ε 12 =ln(A2 / A0);

[0074] The apparent limit strain (ε 11 , ε 12 )、(ε 21 , ε 22 ),……、(εn1 , ε n2 ) are plotted in the strain coordinate system with the primary strain ε1 on the ordinate and the secondary strain ε2 on the abscissa, according to the trend and distribution characteristics of the data;

[0075] Perform polynomial fitting, the degree of the polynomial is m, m ≥ 2, that is, the fitted polynomial ε1 is:

[0076]

[0077] Among them, a, b, c, and C are all constants;

[0078] The fitted polynomial ε1 is the ultimate strain polynomial for automotive steel. The following two points should be noted: (1) The higher the degree m of the polynomial, the higher the fitting accuracy, but it is also more prone to overfitting. (2) Based on the trend and distribution characteristics of the data, the fitted polynomial is generally a two-stage polynomial, namely, ε2 ≥ 0 and ε2 < 0.

[0079] Step 3: The ultimate strain that can be achieved by a unit is obtained based on the intersection of the linear stamping path corresponding to the strain of the unit in the process of stamping automobile parts made of automobile steel materials and the curve fitted by the ultimate strain polynomial; the following steps are included:

[0080] Step 31: Print a square grid of size B0×B0 on the blanked sheet by electrochemical etching or hand-drawing, with 0.1mm≤B0≤5.0mm;

[0081] Step 32: The sheet with the printed square grid is subjected to the first-order OP10 stamping. The size of a certain square grid becomes B1×B2, B1>B2. The square grid is regarded as a unit. After stamping, the principal strain ε1=ln(B1 / B0), the secondary strain ε2=ln(B2 / B0), and the thickness strain is recorded as ε t ;

[0082] Step 33: According to the principle of constant volume of metal plastic deformation ε1+ε2+ε t =0, we know that ε t =-(ε1+ε2);

[0083] Step 34: According to the equivalent strain ε in classical plasticity theory e for:

[0084]

[0085] Step 4: Obtain the equivalent strain of a unit. The difference between the equivalent strain corresponding to the ultimate strain state of a unit and the equivalent strain corresponding to the strain state that has occurred in a unit is its continued forming capability.

[0086] Since most of the stamping paths are linear stamping paths, and the stamping paths start when the major and minor strains are 0, the linear stamping path of the major and minor strains (ε1, ε2) of a certain unit satisfies the formula:

[0087] ε1=Aε2;

[0088] Where A represents a constant;

[0089] If the unit is continuously loaded to the necking state, that is, the intersection of the linear punching path and the curve fitted by the limit strain polynomial, the achievable limit strain (ε 1limit , ε 2limit ), so the equivalent strain corresponding to the ultimate strain ε elimit for:

[0090]

[0091] The ultimate strain state that this unit can reach is the equivalent strain ε corresponding to the necking state. elimit The equivalent strain ε corresponding to the strain state that has occurred in this unit e The difference is the continued forming ability of this unit, recorded as ε erest ,but:

[0092] ε eres t=ε elimit -ε e .

[0093] Step 5: Obtain the continued forming capability of the unit with the maximum stamping degree during the stamping process of automobile steel materials for automobile parts, and compare it with the safety forming margin to evaluate whether the automobile parts stamped with automobile steel will have cracking defects. Safety forming margin ε' esafe =(8%-10%)·ε' elimit . The following steps are included:

[0094] Step 51: Measure the mesh changes in the mesh area of the automobile parts stamped from automobile steel materials to obtain the strain of each unit;

[0095] Step 52: Obtain the maximum value of the equivalent strain of each unit, that is, find the unit with the maximum stamping forming degree, and the corresponding equivalent strain is recorded as ε' e ;

[0096] Step 53: Obtain the equivalent strain ε' of the limit strain that can occur in the unit with the maximum stamping forming degree elimit ;

[0097] Step 54: Obtain the continuous forming capability ε' of the unit where the maximum stamping forming degree occurserest , that is, ε' erest =ε' elimit -ε' e .

[0098] As a preferred embodiment, in this application, according to the continued forming ability ε' erest and safety forming margin ε' esafe , the evaluation of whether automobile parts stamped from automobile steel have cracking defects is as follows:

[0099] (1) If ε' erest ≥4(ε' esafe ), it can be concluded that the automotive steel used in stamping automotive parts is safe and will not cause stamping cracking. However, the automotive steel has a large continuous forming capacity, resulting in excessive material properties, making it unsuitable for stamping this automotive part. A material replacement is recommended.

[0100] (2) If ε' esafe <ε' erest <4(ε' esafe ), it can be evaluated that the results of stamping automobile parts made of this automobile steel are safe, stamping cracking will not occur, and the forming capacity of the material can be fully utilized.

[0101] (3) If ε' erest ≤ε' esafe , it can be evaluated that the results of automobile parts stamped by this automobile steel are unsafe and are prone to stamping defects such as stamping cracks and hidden cracks.

[0102] Example 1

[0103] DC06-0.70mm stamped automotive exterior side panels using interstitial atomic steel.

[0104] 1. According to GB / T 228.1-2010 "Tension test of metallic materials Part 1: Room temperature test method", DC06-0.70mm was subjected to room temperature tensile test to obtain its plastic strain ratio r0, r45, and r90 values. It can be obtained that r0 = 2.51, r45 = 2.59, and r90 = 3.22, and the anisotropy coefficient of DC06-0.70mm can be known.

[0105] 2. According to GB / T 4156-2007, "Erichsen Cupping Test for Metallic Sheet and Strip," nine specimens were prepared, each with a length of 180 mm and widths of 30 mm, 60 mm, 80 mm, 90 mm, 100 mm, 120 mm, 140 mm, 160 mm, and 180 mm. A 2.5 mm × 2.5 mm square grid was printed on the surfaces of the eight specimens by electrochemical etching or hand-painting, and the Erichsen cupping test was then conducted.

[0106] 3. After the Erichsen cupping test, the mesh on the sample surface changes. Taking a sample as an example, by measuring the size change of the critical mesh near the necking area or the rupture area, the surface limit strain of the sample can be determined. The surface limit strains of the above 9 samples are (0.644, -0.400), (0.632, -0.335), (0.613, -0.287), (0.556, -0.210), (0.496, -0.112), (0.361, 0.000), (0.401, 0.125), (0.440, 0.328), (0.445, 0.380), and are plotted in the strain coordinate system with the ordinate as the primary strain ε1 and the abscissa as the secondary strain ε2, as shown in the figure. Figure 1 As shown. According to the trend and distribution characteristics of the data, they are fitted with polynomials, that is, the fitted curve is as follows Figure 2 、 Figure 3 As shown, it can be seen that the fitted polynomial is:

[0107]

[0108] Moreover, the fitting degree value R2=0.9966 between the fitting curve with ε2<0 and the actual data, and the fitting degree value R2=1 between the fitting curve with ε2≥0 and the actual data, are both greater than 0.99, indicating a good fitting degree.

[0109] 4. Before stamping the side panel components using interstitial-free atomic steel DC06-0.70mm, a 2.5mm×2.5mm square grid is printed on the blanked sheet by electrochemical etching or hand-drawing. The sheet with the printed square grid undergoes OP10 first-order stamping, and the size of a certain square grid becomes 2.83mm×2.62mm. Considering the square grid as a unit, the principal strain after stamping is ε1=ln(2.83 / 2.5)=0.124, the secondary strain ε2=ln(2.62 / 2.5)=0.047, and the thickness strain is recorded as ε t According to the principle of constant volume of metal plastic deformation ε1+ε2+ε t =0, we know that ε t=-(ε1+ε2)=-0.171. Then, according to the equivalent strain calculation formula in classical plasticity theory, it is as follows:

[0110]

[0111] Calculate the equivalent strain ε of this unit e =0.458.

[0112] 5. Most of the stamping paths are linear stamping paths. Therefore, the major and minor strains (0.124, 0.047) of this square unit are loaded along the linear path, satisfying the linear formula, and we can get:

[0113] ε1=2.638ε2,

[0114] If the unit is continuously loaded to the necking state, that is, the intersection of the linear punching path and the curve fitted by the limit strain polynomial, that is,

[0115] We can get ε 2limit = 0.155, then the ultimate major and minor strains (ε 1limit , ε 2limit )=(0.409, 0.155), ultimate thickness strain ε tlimit =-0.564. It can be seen that the equivalent strain ε corresponding to this limit strain is elimit =1.510.

[0116] 6. Continue to obtain the forming capability, the equivalent strain ε corresponding to the ultimate strain state (i.e., necking state) that this unit can reach elimit = 1.510 Equivalent strain ε corresponding to the strain state that has occurred in this unit e =0.458, which is the continued forming ability of this unit, recorded as ε erest , then ε erest =ε elimit -ε e =1.510-0.458=1.052.

[0117] 7. By measuring the grid changes in the grid area of automobile parts made of stamped steel materials, the strain of each unit can be obtained, and the maximum value of the equivalent strain of each unit can be obtained, that is, the unit with the maximum stamping degree can be found, and the corresponding equivalent strain is recorded as ε' e =1.311.

[0118] Then obtain the equivalent strain ε' of the limit strain that can occur in the unit with the maximum stamping forming degree elimit =1.608, the continuous forming capability ε' of the unit with the maximum stamping forming degree can be obtainederest , that is, ε' erest =ε' elimiy -ε' e =1.608-1.311=0.297.

[0119] 8. According to the requirements of the stamping forming qualification rate and stability of the side outer panel of automobile parts, 10% of the ultimate forming capacity is taken as the safety forming margin ε' esafe , we know that ε' esafe =10%×ε' elimit =0.161. It can be seen that (ε' esafe =0.161)<(ε' erest =0.297)<(4×ε' esafe =0.644)

[0120] It can be evaluated that the results of stamping automobile parts made of this automobile steel are safe, no stamping cracking will occur, and the forming capacity of the material can be fully utilized.

[0121] Example 2

[0122] DC04-0.75mm stamped interstitial atomic steel for automobile exterior panels and fender outer panels.

[0123] 1. According to GB / T 228.1-2010 "Tension test of metallic materials Part 1: Room temperature test method", a room temperature tensile test was conducted on DC04-0.75mm to obtain its plastic strain ratio r0, r45, and r90 values. It can be obtained that r0 = 2.11, r45 = 2.19, and r90 = 2.85, and the anisotropy coefficient of DC04-0.75mm can be known.

[0124] 2. According to GB / T 4156-2007, "Erichsen Cupping Test for Metallic Sheet and Strip," prepare 10 specimens, each 180 mm in length and 20 mm, 40 mm, 60 mm, 80 mm, 90 mm, 100 mm, 120 mm, 140 mm, 160 mm, and 180 mm in width. Then, electrochemically etch or hand-paint a 2.0 mm × 2.0 mm square grid on the surface of each of the 10 specimens and conduct the Erichsen cupping test.

[0125] 3. After the Erichsen cupping test, the mesh on the sample surface changes. Taking a sample as an example, by measuring the size change of the critical mesh near the necking area or the rupture area, the surface limit strain of the sample can be determined. The surface limit strains of the 10 samples measured above are (0.64, -0.503), (0.581, -0.376), (0.533, -0.288), (0.47, -0.193), (0.435, -0.156), (0.368, -0.082), (0.001, 0.289), (0.123, 0.372), (0.244, 0.429), (0.354, 0.459), and are plotted in the strain coordinate system with the primary strain ε1 as the ordinate and the secondary strain ε2 as the abscissa, as shown in the figure. Figure 4 As shown. According to the trend and distribution characteristics of the data, they are fitted with polynomials, that is, the fitted curve is as follows Figure 5 、 Figure 6 As shown, it can be seen that the fitted polynomial is:

[0126]

[0127] Moreover, the fitting degree value R2=0.9997 between the fitting curve with ε2<0 and the actual data, and the fitting degree value R2=1 between the fitting curve with ε2≥0 and the actual data, are both greater than 0.99, indicating a good fitting degree.

[0128] 4. Before stamping the fender outer panel components using interstitial-free atomic steel DC04-0.75mm, a 2.0mm×2.0mm square grid is printed on the blanked sheet by electrochemical etching or hand-drawing. The sheet with the printed square grid undergoes OP10 first-order stamping, and the size of a certain square grid becomes 2.53mm×2.32mm. Considering the square grid as a unit, the principal strain after stamping is ε1=ln(2.53 / 2.0)=0.235, the secondary strain ε2=ln(2.32 / 2.0)=0.148, and the thickness strain is recorded as ε t According to the principle of constant volume of metal plastic deformation ε1+ε2+ε t =0, we know that ε t =-(ε1+ε2)=-0.383. Then, according to the equivalent strain calculation formula in classical plasticity theory, it is as follows:

[0129]

[0130] Calculate the equivalent strain ε of this unit e =0.907.

[0131] 5. Most of the stamping paths are linear stamping paths. Therefore, the major and minor strains (0.235, 0.148) of this square unit are loaded along the linear path, satisfying the linear formula, and we can get:

[0132] ε1=1.588ε2,

[0133] If the unit is continuously loaded to the necking state, that is, the intersection of the linear punching path and the curve fitted by the limit strain polynomial, that is,

[0134] We can get ε 2limit = 0.277, then the ultimate major and minor strains (ε 1limit , ε 2limit )=(0.440, 0.277), ultimate thickness strain ε tlimit =-0.717. It can be seen that the equivalent strain ε corresponding to this limit strain is eli,it =1.510.

[0135] 6. Continue to obtain the forming capability, the equivalent strain ε corresponding to the ultimate strain state (i.e., necking state) that this unit can reach elimit = 1.510 Equivalent strain ε corresponding to the strain state that has occurred in this unit e =0.907, which is the continued forming capability of this unit, denoted as ε erest , then ε erest =ε elimit -ε e =1.510-0.907=0.603.

[0136] 7. By measuring the grid changes in the grid area of automobile parts made of stamped steel materials, the strain of each unit can be obtained, and the maximum value of the equivalent strain of each unit can be obtained, that is, the unit with the maximum stamping forming degree can be found, and the corresponding equivalent strain is recorded as ε' e =1.435.

[0137] Then obtain the equivalent strain ε' of the limit strain that can occur in the unit with the maximum stamping forming degree elimit =1.569, the continuous forming capability ε' of the unit with the maximum stamping forming degree can be obtained erest , that is, ε' erest =ε' elimit -ε' e =1.569-1.435=0.134.

[0138] 8. According to the requirements of the stamping forming qualification rate and stability of the side outer panel of automobile parts, 10% of the ultimate forming capacity is taken as the safety forming margin ε' esafe , we know that ε'esafe =10%×ε' elimit =0.157. It can be seen that (ε' erest =0.134)≤(ε' esafe =0.157)

[0139] It can be evaluated that the results of stamping automobile parts made of this automobile steel are unsafe and are prone to stamping defects such as stamping cracks and hidden cracks.

[0140] The serial numbers of the above embodiments of the present invention are for descriptive purposes only and do not represent the superiority or inferiority of the embodiments. In the above embodiments of the present invention, the description of each embodiment has its own focus. For parts not described in detail in a particular embodiment, please refer to the relevant description of other embodiments. In the several embodiments provided in this application, it should be understood that the disclosed technical content can be implemented by other means.

[0141] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that they can still modify the technical solutions described in the above embodiments, or replace some or all of the technical features therein with equivalents. However, these modifications or replacements do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the embodiments of the present invention.

Claims

1. A method for evaluating the cracking defects of automobile steel stamping by continuous forming capability, using strain analysis, characterized in that: The following steps are involved: Step 1: Obtain the anisotropy coefficient of automotive steel through room temperature tensile testing; Step 2: Obtain the limit strain polynomial of automotive steel through the Erichsen cupping test, and express the formability of automotive steel through quantifiable equivalent strain; Step 3: Based on the intersection of the linear stamping path corresponding to the strain of a certain unit in the process of stamping automobile parts made of automobile steel materials and the curve fitted by the limit strain polynomial, the limit strain that can be achieved by this unit is obtained; Step 4: Obtain the equivalent strain of the unit. The difference between the equivalent strain corresponding to the ultimate strain state of the unit and the equivalent strain corresponding to the strain state that has occurred in the unit is the unit's continued forming capability. Step 5: Obtain the continued forming capability of the unit with the maximum stamping degree during the stamping process of automobile steel materials for automobile parts, compare it with the safety forming margin, and evaluate whether the automobile parts stamped with automobile steel will have cracking defects.

2. The method for evaluating automobile steel stamping cracking defects by continuous forming capability according to claim 1, characterized in that: In step 1, obtaining the anisotropy coefficient of automobile steel through a room temperature tensile test comprises the following steps: Step 11: Perform a room temperature tensile test on the automotive steel material to obtain the plastic strain ratio r value; Step 12: Since the plastic strain ratio r value is often directional, the values in different directions of the material plane are often different; therefore, samples with angles of 0°, 45°, and 90° along the rolling direction are taken to obtain the plastic strain ratio r values in different directions, which are recorded as r0, r 45 、r 90 ; Step 13: Obtain the anisotropy coefficient of the automotive steel material for:

3. The method for evaluating automobile steel stamping cracking defects by continuous forming capability according to claim 1, characterized in that: In the Erichsen cupping test, N specimens are prepared, each having a length L, 160 mm ≤ L ≤ 180 mm, and a width B, 20 mm ≤ B ≤ 180 mm, where N ≥ 8; and a square grid of size A0 × A0, 0.1 mm < A0 ≤ 4.0 mm, is printed on the surface of the N specimens by electrochemical etching or hand-painting.

4. The method for evaluating automobile steel stamping cracking defects by continuous forming capability according to claim 1, characterized in that: In step 2, by measuring the size change of the critical grid near the necking area or the rupture area, record it as A1×A2, and take A1>A2, the surface limit strain of the sample is (ε 11 , ε 12 ); Among them, ε 11 represents the principal strain; ε 12 represents the secondary strain; and ε 11 =ln(A1 / A0),ε 12 =ln(A2 / A0); The apparent limit strain (ε 11 , ε 12 )、(ε 21 , ε 22 ),……、(ε n1 , ε n2 ) are plotted in the strain coordinate system with the primary strain ε1 on the ordinate and the secondary strain ε2 on the abscissa, according to the trend and distribution characteristics of the data; Perform polynomial fitting, the degree of the polynomial is m, m ≥ 2, that is, the fitted polynomial ε1 is: Among them, a, b, c, and C are all constants; The fitting polynomial ε1 is the ultimate strain polynomial of automobile steel.

5. The method for evaluating automobile steel stamping cracking defects by continuous forming capability according to claim 1, characterized in that: The step 3 includes the following steps: Step 31: Print a square grid of size B0×B0 on the blanked sheet by electrochemical etching or hand-drawing, with 0.1mm≤B0≤5.0mm; Step 32: The sheet with the printed square grid is subjected to the first-order OP10 stamping. The size of a certain square grid becomes B1×B2, B1>B2. The square grid is regarded as a unit. After stamping, the principal strain ε1=ln(B1 / B0), the secondary strain ε2=ln(B2 / B0), and the thickness strain is recorded as ε t ; Step 33: According to the principle of constant volume of metal plastic deformation ε1+ε2+ε t =0, we know that ε t =-(ε1+ε2); Step 34: According to the equivalent strain ε in classical plasticity theory e for:

6. The method for evaluating automobile steel stamping cracking defects by continuous forming capability according to claim 1, characterized in that: In step 4, since most of the stamping paths are linear stamping paths, and the stamping paths are started when the major and minor strains are 0, the linear stamping path of the major and minor strains (ε1, ε2) of a certain unit satisfies the formula: ε1=Aε2; Where A represents a constant; If the unit is continuously loaded to the necking state, that is, the intersection of the linear punching path and the curve fitted by the limit strain polynomial, the achievable limit strain (ε 1limit , ε 2limit ), so the equivalent strain ε corresponding to the ultimate strain elimit for: The ultimate strain state that this unit can reach is the equivalent strain ε corresponding to the necking state. elimit The equivalent strain ε corresponding to the strain state that has occurred in this unit e The difference is the continued forming ability of this unit, recorded as ε erest ,but: e erest =e elimit -e e 。 7. The method for evaluating automobile steel stamping cracking defects by continuous forming capability according to claim 1, characterized in that: The step 5 includes the following steps: Step 51: Measure the mesh changes in the mesh area of the automobile parts stamped from automobile steel materials to obtain the strain of each unit; Step 52: Obtain the maximum value of the equivalent strain of each unit, that is, find the unit with the maximum stamping forming degree, and the corresponding equivalent strain is recorded as ε' e ; Step 53: Obtain the equivalent strain ε' of the limit strain that can occur in the unit with the maximum stamping forming degree elimit ; Step 54: Obtain the continuous forming capability ε' of the unit where the maximum stamping forming degree occurs erest , that is, ε' erest =ε' elimit -ε' e .

8. The method for evaluating automobile steel stamping cracking defects by continuous forming capability according to claim 1, characterized in that: The safety forming margin ε' esafe =(8%-10%)·ε' elimit .

9. The method for evaluating automobile steel stamping cracking defects by continuous forming capability according to claim 1, characterized in that: In step 5, according to the continued forming capability ε' erest and safety forming margin ε' esafe , the evaluation of whether automobile parts stamped with automobile steel have cracking defects is as follows: (1) If ε' erest ≥4(ε' esafe ), then the result of stamping the automotive steel into automotive parts is safe and will not cause stamping cracking. However, the automotive steel has a large continuous forming capacity, resulting in excessive material properties and is not suitable for stamping the automotive parts; (2) If ε' esafe <ε' erest <4(ε' esafe ), then the result of stamping the automobile parts with the automobile steel is evaluated to be safe and suitable for stamping the automobile parts; (3) If ε' erest ≤ε' esafe , it can be evaluated that the result of stamping automobile parts with this automobile steel is unsafe and is not suitable for stamping this automobile part.

Images