A method for evaluating global and local formability of advanced high-strength steel
By combining DIC technology with nonlinear fitting, the global and local deformation areas of high-strength steel are separated, which solves the problem of incomplete formability evaluation in existing technologies and achieves high-precision, low-cost forming performance evaluation and product matching.
Patent Information
- Application Number
- CN202210995783.6
- Authority / Receiving Office
- CN · China
- Patent Type
- Patents(China)
- Current Assignee / Owner
- Filing Date
- 2022-08-18
- Publication Date
- 2025-10-03
- Estimated Expiration
- 2042-08-18
AI Technical Summary
Existing technologies make it difficult to simultaneously evaluate the global and local formability of advanced high-strength steel, resulting in an incomplete assessment of the material's forming performance and an inability to accurately match stamping products.
The DIC technique is combined with nonlinear fitting to separate the global and local deformation regions through uniaxial tensile tests. The parameters of global and local formability are determined using the equivalent plastic strain distribution diagram and Gaussian curve fitting.
It enables a thorough evaluation of the forming properties of advanced high-strength steel, precise matching of stamping products, and high measurement accuracy at low cost.
Smart Images
Figure CN115389318B_ABST
Abstract
Description
Technical Field
[0001] The invention relates to a method for evaluating the global and local formability of advanced high-strength steel, and belongs to the technical field of metal material stamping forming. Background Art
[0002] As global regulations on automotive fuel efficiency and safety standards become increasingly stringent, advanced high-strength steel is increasingly being used in automotive body structural parts due to its excellent strength, plasticity and crash resistance, which can simultaneously meet the dual goals of lightweighting the vehicle body and improving safety.
[0003] Based on the deformation characteristics of the material during stamping, formability is divided into global formability and local formability. Global formability is typically characterized by coordinated deformation of the material over a large area, with necking or cracking caused by excessive thinning. Forming methods primarily include stretching, planar forming, and bulging, and are generally characterized by the work hardening index (n value) and uniform elongation Ag obtained from uniaxial tensile testing. In contrast, local formability refers to a relatively concentrated deformation location with no significant necking at the cracking location. It is primarily related to the material's ability to resist crack propagation. Forming methods primarily include flanging, hole expansion, and bending, and are commonly characterized by hole expansion ratio and limiting bend ratio. Currently, most researchers use FLC to evaluate the global formability of materials and hole expansion tests to evaluate local formability, which cannot comprehensively evaluate the global and local formability of high-strength steel. The uniform deformation stage of the uniaxial tensile test is the global deformation stage of the material, while the stage from necking to cracking is the local deformation stage of the material. Therefore, uniaxial tensile testing can be used to comprehensively evaluate the global and local formability of a material. Summary of the Invention
[0004] The purpose of the present invention is to provide a method for evaluating the global and local formability of advanced high-strength steel. By combining DIC technology with nonlinear fitting, the plastic deformation of the material within a 50mm gauge length segment is divided into global and local deformation regions, and parameters used to characterize the global and local formability characteristics of the material are determined, thereby more thoroughly understanding the forming performance of the material and accurately matching the material for the stamping product. The method has high measurement accuracy, simple operation, and low cost, effectively solving the above-mentioned problems existing in the background technology.
[0005] The technical solution of the present invention is: a method for evaluating the global and local formability of advanced high-strength steel, comprising the following steps: using DIC technology to obtain the equivalent plastic strain distribution cloud map of the uniaxial tensile specimen before fracture, establishing a functional relationship between the equivalent plastic strain and the position, and taking the first-order derivative of the equivalent plastic strain with respect to the position as 10 -4The point of maximum equivalent plastic strain in the global deformation area is taken as the dividing point between global and local deformation, the point of maximum equivalent plastic strain in the global deformation area is taken as the maximum strain that can be tolerated during global deformation, and the point of maximum equivalent plastic strain in the local deformation area is taken as the maximum strain that can be tolerated during local deformation, thereby determining the parameters that characterize the quality of the global and local formability of the material.
[0006] During uniaxial tension, the material strain is uniformly distributed within the gauge length before necking, demonstrating global formability. When the material reaches its ultimate tensile strength through work hardening, necking occurs within a small area, and the material near the neck exhibits uneven deformation. Therefore, the material near the neck in the fractured specimen exhibits local formability, while the material away from the neck exhibits global formability.
[0007] The DIC technique is used to obtain an equivalent plastic strain cloud map of a frame before fracture. The equivalent plastic strain of each point is extracted every 1 mm along the length direction at the center position of the specimen gauge section, and a relationship diagram between the equivalent plastic strain and position is established. The functional relationship curve between the two is obtained by Gaussian curve fitting.
[0008] The definition of the dividing point between global deformation and local deformation is based on the characteristics that the equivalent plastic strain of the material is uniformly distributed in the uniform deformation stage and the strain distribution at the necking position is extremely uneven. The first-order derivative of the equivalent plastic strain with respect to the position is taken and the first-order derivative is equal to 10 -4 The point is defined as the dividing point between global and local deformation.
[0009] The material within the gage length segment from the left endpoint to the left demarcation point and from the right endpoint to the right demarcation point exhibits global deformation. The maximum equivalent plastic strain within this range represents the maximum strain the material can withstand during global deformation. The material within the left and right demarcation points exhibits local deformation. The maximum equivalent plastic strain within this range represents the maximum strain the material can withstand during local deformation. The greater the strain a material can withstand during deformation, the better its formability.
[0010] The curve shape of the local deformation area can also reflect the local formability of the material. The narrower the curve, the stronger the coordinated deformation ability of the various phases of the material, the more helpful it is to resist the generation and expansion of holes after necking, and the better the local formability.
[0011] The specific steps are as follows:
[0012] (1) Use wire cutting to process a uniaxial tensile specimen with a gauge length of A50 and a specimen size of 20 mm*200 mm. Spray uneven speckles within the gauge length. The more irregular the size and distribution of the speckles, the better.
[0013] (2) Draw a straight line along the length direction at the center of the test segment, define the center position of the gauge length on the line segment as 0, the left vertex position of the gauge length segment as -25 mm, and the right vertex position as +25 mm;
[0014] (3) The uniaxial tensile specimen was mounted on the fixture of a Zwick / Roell tensile testing machine, ensuring that the tensile direction of the specimen was parallel to the movement direction of the testing machine, and the tensile test was performed at a speed of 0.067 mm / s;
[0015] (4) Using DIC technology at a frequency of 50 Hz, the equivalent plastic strain distribution cloud map within the gauge section and the fracture moment of the specimen during the entire deformation process were recorded, and the equivalent plastic strain distribution map of the last frame before fracture was extracted;
[0016] (5) On the line segment drawn in step (2), extract the equivalent plastic strain at that position every 1 mm, and draw a distribution diagram of the equivalent plastic strain versus position, with the position as the horizontal coordinate and the equivalent plastic strain as the vertical coordinate;
[0017] (6) Using Gaussian formula x~N(μ, σ 2 ), where μ and σ 2 is a constant, ε is the equivalent plastic strain, and x is the distance between this point and point 0. The function curve of the equivalent plastic strain ε and position x is obtained by fitting;
[0018] (7) Taking the first-order derivative of ε with respect to x, we get Establish the curve of the first-order derivative y versus position x, and set y=10 -4 The point is defined as the starting point of the transformation from global to local deformation, x∈(0~10 -4 ) is the global deformation area, and the other positions are local deformation areas;
[0019] (8) The maximum equivalent plastic strain in the global deformation region is the maximum strain that the material can withstand during global deformation, and the maximum equivalent plastic strain in the local deformation region is the maximum strain that the material can withstand during local deformation. The shape of the curve in the local deformation region also reflects the local forming ability of the material. The narrower the curve, the stronger the coordinated deformation ability of the various phases of the material, the more helpful it is to resist the generation and expansion of holes after necking, and the better the local formability.
[0020] The beneficial effects of the present invention are: using a method combining DIC technology with nonlinear fitting, the plastic deformation of the material within a 50mm gauge length segment is divided into global and local deformation areas, and the parameters used to characterize the global and local formability characteristics of the material are determined, thereby providing a more thorough understanding of the material's forming performance and accurately matching materials for stamping products. The measurement has high accuracy, simple operation, and low cost. BRIEF DESCRIPTION OF THE DRAWINGS
[0021] Figure 1 The A50 uniaxial tensile specimen of the embodiment of the present invention;
[0022] Figure 2 is an equivalent plastic strain distribution diagram of a frame before fracture according to an embodiment of the present invention;
[0023] Figure 3 is a graph showing the relationship between equivalent plastic strain and position according to an embodiment of the present invention;
[0024] Figure 4 is a curve showing the relationship between the equivalent plastic strain and the position according to an embodiment of the present invention;
[0025] Figure 5 is a first-order derivative curve of equivalent plastic strain versus position according to an embodiment of the present invention;
[0026] Figure 6 is a parametric diagram of the global and local formability characteristics of the material of the embodiment of the present invention. DETAILED DESCRIPTION
[0027] In order to make the purpose, technical solutions and advantages of the invention implementation cases clearer, the technical solutions in the invention implementation cases will be clearly and completely described below in conjunction with the drawings in the implementation cases. Obviously, the implementation cases described are only a small part of the implementation cases of the present invention, rather than all the implementation cases. Based on the implementation cases in the present invention, all other implementation cases obtained by ordinary technicians in this field without making creative work are within the scope of protection of the present invention.
[0028] Example
[0029] Comprehensive evaluation of the global and local formability of advanced high-strength steel DP980-1.2mm.
[0030] The experimental steps are:
[0031] (1) A sheet of DP980 material with a thickness of 1.2 mm was selected and processed into a uniaxial tensile specimen with a gauge length of A50 on the wire cutting machine. The specimen size was 20 mm*200 mm. The speckle was sprayed at the gauge length. The more uneven the size and distribution of the speckle, the better.
[0032] (2) At the center of the specimen, draw a straight line along the length direction, and define the center of the gauge length as 0 point. The coordinates of the left endpoint of the gauge length are -25 mm, and the coordinates of the right endpoint are +25 mm. Figure 1 As shown;
[0033] (3) Install the uniaxial tensile specimen on the fixture of the Zwick / Roell tensile testing machine, adjust the specimen direction to ensure that the deformation direction of the specimen is parallel to the movement direction of the testing machine, and install the DIC data acquisition equipment;
[0034] (4) A uniaxial tensile test was performed at a speed of 0.067 mm / s to ensure that the material was in a quasi-static deformation state. The equivalent plastic strain distribution diagram of each frame during the test was recorded at a frequency of 50 Hz using a DIC device, and the strain distribution cloud diagram of the frame before fracture was extracted, such as Figure 2 As shown;
[0035] (5) In the range of -25mm and +25mm, extract the equivalent plastic strain at each point along the length of the specimen at intervals of 1mm. With the position as the horizontal coordinate and the equivalent plastic strain as the vertical coordinate, draw a graph of the relationship between the equivalent plastic strain and the position, as shown in the following example: Figure 3 As shown;
[0036] (6) Using Gaussian formula x~N(μ, σ 2 ), Fitting the functional relationship between equivalent plastic strain and position, the fitting results in constant μ=0, constant σ 2 =0.949, the relationship between equivalent plastic strain and position is: like Figure 4 As shown;
[0037] (7) The first-order derivative of the equivalent plastic strain ε with respect to position x Establish a curve of the first-order derivative y versus position x, and find the first-order derivative of 10 -4 There are two points A and B, A and B are the dividing points between the global and local deformation areas. The distance between point A and the zero point is -7.88mm, and the distance between point B and the zero point is 9.13mm. The deformation of the material at the positions x∈(-25mm~-7.88mm) and x∈(9.13mm~25mm) within the gauge length section is global deformation, and the deformation of the material at the position x∈(-7.88mm~9.13mm) is local deformation, as shown in the following example: Figure 5 As shown;
[0038] (8) The maximum equivalent plastic strain in the global deformation region is 0.102, so the maximum equivalent plastic strain that DP980-1.2mm can withstand in the global deformation stage is 0.102; (9) The maximum equivalent plastic strain value in the local deformation region is the highest point of the curve, which is 0.428, so the maximum equivalent plastic strain that DP980-1.2mm can withstand in the local deformation stage is 0.428. The narrow shape of the curve in the local deformation region indicates that the martensite and ferrite phases in the DP980 material can be deformed in a coordinated manner, specifically good local formability, such as Figure 6 .
Claims
1. A method for evaluating the global and local formability of advanced high-strength steel, characterized in that The following steps are included: using DIC technology to obtain the equivalent plastic strain distribution cloud map of the uniaxial tensile specimen before fracture, establishing the relationship curve between equivalent plastic strain and position, and converting the first-order derivative of equivalent plastic strain to position, which is equal to 10 -4 The point is used as the dividing point between global deformation and local deformation. The maximum equivalent plastic strain in the global deformation area is the maximum strain that can be tolerated during global deformation, and the maximum equivalent plastic strain in the local deformation area is the maximum strain that can be tolerated during local deformation, thereby determining the parameters that characterize the global and local deformation capabilities of the material. The specific steps are as follows: (1) Use wire cutting to process a uniaxial tensile specimen with a gauge length of A50, and spray uneven speckles within the gauge length. The more irregular the size and distribution of the speckles, the better. (2) Draw a straight line along the length direction at the center of the test, define the center position of the gauge length on the line segment as 0 point, the left vertex position of the gauge length segment as -25 mm, and the right vertex position as +25 mm; (3) Install the uniaxial tensile specimen on the fixture of the Zwick / Roell tensile testing machine, ensuring that the tensile direction of the specimen is parallel to the movement direction of the testing machine, and perform the tensile test at a speed of 0.067 mm / s; (4) Using DIC technology at a frequency of 50 Hz, the equivalent plastic strain distribution cloud map within the gauge section and the fracture moment of the specimen during the entire deformation process were recorded, and the equivalent plastic strain distribution map of the last frame before fracture was extracted; (5) On the line segment drawn in step (2), extract the equivalent plastic strain at each position every 1 mm, and draw a distribution diagram of the equivalent plastic strain versus position with the position as the horizontal coordinate and the equivalent plastic strain as the vertical coordinate; (6) Using Gaussian formula , , where μ and σ 2 is a constant, ε is the equivalent plastic strain, and x is the distance between this point and point 0. The function curve of the equivalent plastic strain ε and position x is obtained by fitting; (7) Taking the first-order derivative of ε with respect to x, we get , establish the curve of the first-order derivative y changing with position x, and Point is defined as the starting point of the transformation from global to local deformation. The deformation of the position is the global deformation area, and the other positions are the local deformation areas; (8) The maximum equivalent plastic strain in the global deformation region is the maximum strain that the material can withstand during global deformation, and the maximum equivalent plastic strain in the local deformation region is the maximum strain that the material can withstand during local deformation. The shape of the curve in the local deformation region also reflects the local forming ability of the material. The narrower the curve, the stronger the coordinated deformation ability of the various phases of the material, the more helpful it is to resist the generation of holes and the expansion of microcracks after necking, and the better the local formability. During uniaxial tensile deformation of the high-strength steel, the material strain is uniformly distributed within the gauge length section before necking, exhibiting global formability. When the stress value to which the material is subjected reaches the tensile strength, the material necks within a small range, and the necking to fracture stage exhibits local formability. Using DIC technology, an equivalent plastic strain cloud map is obtained for the frame before fracture. At the center position of the gauge length section of the specimen, the equivalent plastic strain of each point is extracted every 1 mm along the length direction. A relationship diagram between the equivalent plastic strain and position is established, and a functional relationship curve between the two is obtained using Gaussian curve fitting. The dividing point between global deformation and local deformation is defined by the following method: the equivalent plastic strain of the material in the gauge section is uniformly distributed during the uniform deformation stage; the strain distribution at the necking position is extremely non-uniform after necking; the first-order derivative of the equivalent plastic strain with respect to the position is taken, and the first-order derivative is equal to 10 -4 The point is defined as the dividing point between global deformation and local deformation; the material within the range from the left end point to the left dividing point and from the right end point to the right dividing point shows global deformation, and the maximum equivalent plastic strain is the maximum strain that the material can withstand during global deformation; the material within the left and right dividing points shows local deformation, and the maximum equivalent plastic strain within this range is the maximum strain that the material can withstand during local deformation; the maximum strain value that the material can withstand during deformation indicates better formability of the material; the curve shape of the local deformation area can also reflect the local formability of the material. The narrower the curve, the stronger the coordinated deformation ability of the various phases of the material, the more helpful it is to resist the generation and expansion of holes after necking, and the better the local formability.