Method for accurately evaluating local forming performance of pre-deformed material

Abstract

The invention discloses a method for accurately evaluating the local forming performance of a pre-deformed material, which comprises the following steps of: (1) sampling on a plate, and processing into a large-size sample; (2) stretching the large-size sample for pre-deformation, and obtaining a deformed sample by taking the target deformation amount as the pre-strain amount; (3) processing a small-size sample in a uniform plastic deformation area of the deformed sample; (4) measuring the width and thickness of the parallel section of the small-size sample, and calculating to obtain the sectional area; (5) stretching the small-size sample until the fracture fails, so as to obtain two sections of snapped samples; (6) respectively measuring the length and width sizes of the cross sections of the two sections of tensile samples, and calculating the cross section area of the tensile samples after fracture; and (7) the real fracture strain is calculated through the percentage reduction of area, and the real fracture strain is used as the characterization of the local forming performance index of the material when the pre-deformation amount is used. The method is simple and convenient, less in human intervention, high in test precision, not limited by the strength and thickness of the steel grade, and wide in application range.

Description

Technical Field

[0001] The invention relates to a plate forming performance testing technology, in particular to a method for accurately evaluating the local forming performance of a material after pre-deformation. Background Art

[0002] With the increasingly stringent regulations on energy conservation, emission reduction and lightweight collision safety, higher requirements are placed on the development of the automotive industry. High-strength steel and advanced high-strength steel have both high strength and good plasticity, which can meet the dual goals of lightweight and safety. Therefore, they account for an increasing proportion of white bodies, and the stamped parts are becoming more and more complex. As a result, more and more quality problems are exposed, especially edge cracking caused by poor local forming performance, such as flanging cracking, hole expansion, hole flanging cracking, bending cracking, etc.

[0003] In actual production, the forming process of stamping parts often requires continuous operation of multiple processes rather than single forming. For example, the front longitudinal beam usually needs to go through five or six processes such as drawing, flanging, and shaping to finally produce a product that meets the specifications. In the early forming process, the sheet metal undergoes different degrees of pre-deformation, and then undergoes local forming such as bending / flanging. The early pre-deformation will affect the local forming performance of the sheet metal, and accurate evaluation is required to ensure the quality of the produced parts.

[0004] At present, domestic and foreign research generally believes that the hole expansion rate is the most direct and effective way to characterize the local forming performance of materials. However, the hole expansion rate usually measured is an indicator to characterize the local forming performance of the original plate, which has little engineering guidance significance for parts that have been pre-deformed and then bent / flanged. Therefore, it is urgent to develop a method that can accurately evaluate the local forming performance of materials after pre-deformation, so as to provide guidance for material selection recommendations and engineering applications. Summary of the invention

[0005] The technical problem to be solved by the present invention is to provide a simple and convenient method for accurately evaluating the local forming performance of a material after pre-deformation.

[0006] In order to solve the above technical problems, the technical solution adopted by the present invention includes the following steps: (1) sampling from the plate and processing it into a larger size sample;

[0007] (2) stretching a larger size specimen for pre-deformation, with the target deformation amount being the pre-strain amount e, to obtain a deformed specimen;

[0008] (3) Processing small-sized specimens in the uniform plastic deformation zone of the deformed specimen;

[0009] (4) Measure the width and thickness of the parallel section of the small-size specimen and calculate the cross-sectional area A0;

[0010] (5) Stretching the small-size specimen until it breaks and fails, obtaining two-section tensile specimens;

[0011] (6) Measure the length and width of the two sections of the broken specimens respectively, and calculate the cross-sectional area A of the broken specimens. f ;

[0012] (7) Calculate the true fracture strain ε by the cross-sectional shrinkage, ε = ln(A0 / A f ), the true fracture strain ε is used as the characterization of the local forming performance index of the material when the pre-deformation is e.

[0013] Furthermore, in the step (1), sampling is performed according to the local forming target direction of the sample; if the local forming target direction cannot be determined, sampling is performed in three directions of 0°, 45°, and 90°.

[0014] Furthermore, in the step (7), when sampling is performed in three directions, the minimum value of the true fracture strain ε in the three directions is taken as the local forming performance index of the material.

[0015] Furthermore, the method further includes the following steps: if it is desired to characterize the loss of local forming performance of the material under the pre-deformation amount e, a sample is taken from the plate and processed into a small-sized specimen, and steps (4) to (7) are used to obtain the fracture strain ε0 of the material when no pre-deformation occurs, and the loss rate of local forming performance Δe is obtained by calculation, Δe=(ε0-ε) / ε0×100%.

[0016] The beneficial effect of adopting the above technical solution is that: the present invention uses a larger-sized tensile specimen to apply a certain degree of pre-deformation to it, processes it into a small-sized specimen in its uniform plastic deformation zone, re-measures the width and thickness of the specimen, stretches it until it breaks and fails, and characterizes its local forming performance by measuring the cross-sectional shrinkage rate of the small-sized specimen before and after stretching, thereby obtaining the local forming performance of the material under the pre-deformation. The method of the present invention is simple and convenient, with less human intervention, high test accuracy, small sample requirements, no need for additional equipment and fixtures, and is not limited by the strength and thickness of steel grades. It has a wide range of applications and strong targeting of engineering guidance applications. The local forming performance of all parts obtained by bending, flanging, punching and other deformation methods after drawing can be evaluated by the method of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The present invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0018] Figure 1 It is a schematic diagram of the evaluation process of the present invention;

[0019] Figure 2 It is a schematic diagram of the structure of the larger size sample of the present invention;

[0020] Figure 3 It is a schematic diagram of the structure of the small-size sample of the present invention;

[0021] Figure 4 This is a cross-sectional view of the broken specimen described in Example 1. DETAILED DESCRIPTION

[0022] Figure 1 As shown, the method for accurately evaluating the local forming performance of the material after pre-deformation includes the following steps:

[0023] (1) Analyze the forming process of the steel plate to be tested when it is subsequently processed into a part, and clarify the pre-deformation amount e and the local forming target direction of the steel type to be tested, which are the deformation amount and forming direction during part processing. Figure 2 As shown, samples are taken from the plate according to the local forming target direction and processed into larger-sized specimens. The forming direction of the larger-sized specimens during stretching must be consistent with the local forming target direction. If hole expansion or hole flanging is performed after pre-deformation, the local forming target direction cannot be clearly determined, then larger-sized specimens in the directions of 0°, 45°, and 90° are taken respectively.

[0024] (2) A tensile testing machine is used to stretch the larger sample. The forming direction is consistent with the local forming target direction to achieve pre-deformation of the sample. The target deformation of the larger sample is the pre-strain value e, and a deformed sample is obtained. The pre-strain value is controlled by an extensometer on the tensile testing machine. The spacing between the extensometers needs to be greater than the parallel section length of the subsequent processing of the small sample to ensure that the uniform plastic deformation zone can process the small sample.

[0025] (3) Remove the deformed specimen from the tensile testing machine. Figure 3 As shown, in the uniform plastic deformation zone of the deformed specimen, that is, the uniform deformation section of the deformed specimen, wire cutting is used to process a small-sized specimen.

[0026] (4) Measure the width and thickness of the parallel section of the small-size specimen and calculate the cross-sectional area A0.

[0027] (5) The small-sized specimen is stretched until the specimen breaks, and two-section tensile specimens are obtained.

[0028] (6) Measure the length and width of the cross-section of the two broken specimens under a stereo microscope, and calculate the cross-sectional area A of the two broken specimens after breaking. f1 and A f2 , take the average cross-sectional area of ​​the two broken specimens as the cross-sectional area after fracture A f =(A f1 +A f2 ) / 2.

[0029] (7) Calculate the true fracture strain ε by the cross-sectional shrinkage, ε = ln(A0 / A f ), and the true fracture strain ε is used as the characterization of the local forming performance index of the material when the pre-deformation amount is e. If larger-sized specimens are taken in three directions, the three specimens are respectively subjected to steps (2)-(7), and the minimum value of the true fracture strain ε in the three directions is taken as the local forming performance index of the material.

[0030] (8) If the loss of local forming performance of the material under the pre-deformation amount e is to be characterized, a sample is taken from the plate without pre-deformation and processed into a small-sized specimen. Steps (4) to (7) are used to obtain the fracture strain ε0 of the material without pre-deformation, ε0 = ln(A 10 / A 1f ), A 10 is the cross-sectional area of ​​the small-size specimen without pre-deformation, A 1f is the cross-sectional area of ​​the sample after fracture without pre-deformation; and the loss rate of local formability is calculated as Δe = (ε0-ε) / ε0×100%. If the local formability of the material is characterized by the hole expansion rate, the loss rate of local formability obtained by this method is also applicable.

[0031] In the above process, the size requirements of the small-sized specimen must meet the size requirements of the tensile test, and the size requirements of the larger-sized specimen must meet the requirement that the length of the uniform deformation section that can be covered by the extensometer is greater than the parallel section length of the small-sized specimen, so as to ensure that a certain amount of uniform deformation has occurred in the small-sized specimen before stretching.

[0032] Example 1: Take a 1.25mm thick 780DP longitudinal beam of a certain main engine factory as an example to illustrate the deformed part which is bent along the parallel rolling direction after drawing.

[0033] (1) Through the analysis of the part forming process, it is known that the deformation of the part in the drawing sequence is about 3%, and then it is deformed by flanging and bending along the parallel rolling direction to obtain a qualified part, but very slight cracks appear on the edge of the part. The elongation of the deformation zone caused by flanging and bending is 22%, and the true strain of the edge of the bending area is 0.457; the target 780DP sample is taken and processed into a larger size specimen parallel to the rolling direction;

[0034] (2) Apply 3% pre-strain to the larger size specimen on the tensile testing machine. The pre-strain amount is controlled by the extensometer on the tensile testing machine, and the extensometer spacing is set to 100 mm;

[0035] (3) Unloading, removing the deformed specimen, and processing it into small-sized specimens by wire cutting;

[0036] (4) The width and thickness of the small-size specimen after pre-deformation were measured to be 12.5 mm and 1.245 mm respectively, and the cross-sectional area A0 was calculated to be 15.5625 mm 2 ;

[0037] (5) Stretching a small-size specimen until it breaks;

[0038] (6) Figure 4 As shown in the figure, the length and width of the cross-section of the two broken specimens were measured under a stereo microscope, and the cross-sectional areas of the two broken specimens were calculated, which were 9.88 mm 2 、9.71mm 2 , take the average cross-sectional area of ​​the two broken specimens as the cross-sectional area after fracture A f 9.795mm 2 ;

[0039] (7) The true fracture strain ε = ln(A0 / A f )=0.46, it can be obtained that the local forming performance index ε of the material is 0.46 when the pre-deformation is 3%.

[0040] (8) Characterize the loss of local formability of the material under 3% pre-deformation. Take the sample without pre-deformation in the same direction, directly wire cut it into a small-sized sample, repeat the above steps (4) to (7), and get the local formability index ε0 of the material when no pre-deformation occurs to be 0.51. The loss rate of local formability Δe=(ε0-ε) / ε0×100% is calculated to be 9.8%; if the hole expansion rate is used to characterize the local formability, it can also be estimated that the hole expansion rate loss is also about 9.8% when the pre-deformation is 3%. After testing, it is found that the hole expansion rate of the material without pre-deformation is 25%. According to this method, when the pre-deformation is 3%, the hole expansion rate of the material is reduced to 22.55%, which is consistent with the result that slight cracks are generated when the edge elongation reaches 22% after the part is pre-deformed by 3%.

[0041] Example 2: Take a 1.6mm thick 980QP door sill part with expanded hole after drawing from a certain OEM as an example.

[0042] (1) Through the analysis of the part forming process, it is known that the deformation of the part in the drawing sequence is about 5%, and then the hole expansion is performed to obtain a qualified part, but micro cracks that have not penetrated the edge of the hole of the part have already occurred. The hole expansion rate of the hole edge area in the hole expansion process is 16.5%, and the true strain of the hole edge area is 0.58; the target 980QP sample is taken, and three larger size specimens with 0°, 45° and 90° in opposite directions are processed;

[0043] (2) Apply 5% pre-strain to the larger size specimens on the tensile testing machine. The pre-strain amount is controlled by the extensometer on the tensile testing machine, and the extensometer spacing is set to 100 mm;

[0044] (3) Unloading, removing the deformed specimen, and processing it into small-sized specimens by wire cutting;

[0045] (4) After pre-deformation, the width of the small-size specimens is 12.5 mm, and the thickness in the three directions is 1.558 mm, 1.552 mm, and 1.546 mm, respectively. The calculated cross-sectional areas A0 are 19.428 mm 2 、19.353mm 2 、19.279mm 2 .

[0046] (5) Stretching a small-size specimen until it breaks;

[0047] (6) Measure the length and width of the cross-section of the two broken specimens under a stereo microscope, and calculate the cross-sectional area of ​​the two specimens after breaking. The three directions are 10.405 mm 2 and 10.161mm 2 , 10.926mm 2 and 10.542mm 2 、10.605mm 2 and 10.633mm 2 ; Take the average of the cross-sectional areas of the two specimens as the cross-sectional area after fracture A f , 10.283mm in three directions 2 、10.734mm 2 、10.619mm 2 ;

[0048] (7) The true fracture strain ε = ln(A0 / A f ), the three directions are 0.636, 0.589, and 0.596 respectively, among which the true fracture strain value ε in the 45° direction is the smallest, and it can be obtained that the local forming performance index ε of the material is 0.589 when the pre-deformation is 5%.

[0049] (8) Characterize the loss of local formability of the material under 5% pre-deformation. Take a sample without pre-deformation in the same direction, directly wire cut it into a small-sized sample, repeat the above steps (4) to (7), and obtain the local formability index ε0 of the material when no pre-deformation occurs, which is 0.693. The loss rate of local formability is Δe = (ε0-ε) / ε0×100%, which is 15% by calculation. If the hole expansion rate is used to characterize the local formability, it can also be estimated that the hole expansion rate loss is also about 15% when the pre-deformation is 3%. After testing, it is found that the hole expansion rate of the material without pre-deformation is 20%. According to the method of the present invention, when the pre-deformation is 5%, the hole expansion rate of the material is reduced to 17%, which is consistent with the result that non-penetrating cracks are generated when the hole expansion rate near the hole edge reaches 16.5% after the part is pre-deformed by 5%.

Claims

1. A method for accurately evaluating the local forming performance of a material after pre-deformation, characterized in that: The method comprises the following steps: (1) sampling the plate and processing the sample into a larger size sample; (2) Stretching a larger size specimen for pre-deformation, with the target deformation amount being the pre-strain amount e, to obtain a deformed specimen; (3) Processing a small-sized specimen in the uniform plastic deformation zone of the deformed specimen; (4) Measure the width and thickness of the parallel section of the small-size specimen and calculate the cross-sectional area A0; (5) Stretching the small-sized specimen until it breaks and fails, and obtaining two-section tensile specimens; (6) Measure the length and width of the two sections of the broken specimens respectively, and calculate the cross-sectional area A of the broken specimens. f ; (7) Calculate the true fracture strain ε by the cross-sectional shrinkage, ε=ln(A0 / A f ), the true fracture strain ε is used as the characterization of the local forming performance index of the material when the pre-deformation is e.

2. A method for accurately evaluating the local forming performance of a material after pre-deformation according to claim 1, characterized in that: In the step (1), sampling is performed according to the local forming target direction of the sample; if the local forming target direction cannot be determined, sampling is performed in three directions: 0°, 45°, and 90°.

3. A method for accurately evaluating the local forming performance of a material after pre-deformation according to claim 2, characterized in that: In the step (7), when sampling is performed in three directions, the minimum value of the true fracture strain ε in the three directions is taken as the local forming performance index of the material.

4. A method for accurately evaluating the local forming performance of a material after pre-deformation according to claim 1, 2 or 3, characterized in that: The method further includes the following steps: if it is desired to characterize the loss of local formability of the material under the pre-deformation amount e, a sample is taken from the plate and processed into a small-sized specimen, and steps (4) to (7) are used to obtain the fracture strain ε0 of the material when no pre-deformation occurs, and the loss rate of local formability Δe is obtained by calculation, Δe=(ε0-ε) / ε0×100%.

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