A method for predicting and evaluating the overall formability and the local formability of high-strength steel

Abstract

The application discloses a method for predicting and evaluating the overall formability and local formability of high-strength steel. u The uniaxial tensile test is carried out, and the strain is uniformly distributed before the material reaches the tensile strength or necking instability, which reflects the overall formability of the material. The effective strain range with the necking as the center is about 20 mm, the thickness-based fracture strain of a sample with a complete fracture is determined as the evaluation index of the local formability. The method can well predict and evaluate the overall formability and local formability of advanced high-strength steel, and has the advantages of simple operation, reliable result, wide application range and low cost.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of sheet metal stamping forming, and particularly relates to a method for predicting and evaluating the overall formability and local formability of high-strength steel. BACKGROUND

[0002] With the development and application of high-strength steel, especially the development of the third generation of advanced high-strength steel, new forming performance challenges and fracture problems related to crack resistance have been brought, such as edge cracking, limited flanging and hole expansion performance and bending performance. Generally, these fracture behaviors are inconsistent with the conventional formability indicators characterized by tensile tests or forming limit curves (FLC). As a typical example, compared with CP steel of the same strength level, even if DP steel has greater uniform elongation and total elongation and higher limit strain in FLC, the DP steel usually exhibits lower flanging and hole expansion performance and edge formability than the CP steel. The inconsistency between the fracture resistance and the conventional ductility definition will inevitably promote the development of new formability standards for advanced high-strength steel. The general use of formability or ductility indicators to evaluate or predict the forming capacity of such materials is not objective and accurate enough, so more accurate indicators need to be defined, thereby more terms are defined: such as overall formability, local formability and related fracture toughness, bending performance and edge cracking sensitivity. The overall formability is the most traditional explanation of formability, mainly including deep drawing forming, tensile and plane strain, which refers to the ability of a material to plastically deform without forming local necking to uniformly distribute strain, and can also be explained as the resistance to the beginning of necking instability, wherein a larger material area deforms at the same time, so it is called overall formability. On the other hand, the local formability is related to the damage tolerance and crack resistance (hole expansion performance, bending performance, edge cracking, etc.) of the material, which refers to the ability of a material to plastically deform in a local area without cracking, and can also be explained as the anti-fracture ability of the concentrated deformation, so it is called local formability.

[0003] Generally, the elongation and fracture strain of steel sheet decrease with the increase of material strength, and lower elongation and fracture strain will lead to cracking due to local strain exceeding the maximum strain allowed by the material. Therefore, the strength, elongation and fracture strain of the material are the key mechanical properties of the material that determine its fracture resistance. The elongation of the material is positively correlated with the strain hardening capacity, and higher strain hardening capacity can improve the uniform elongation and delay the occurrence of necking, thereby improving the overall formability. Higher fracture strain can delay or inhibit the formation and expansion of cracks caused by local strain concentration after necking, i.e. improve the fracture resistance of the material, thereby improving the local formability. As mentioned earlier, there is no clear correspondence between local formability and tensile strength / ductility. Therefore, relevant test methods have been developed to evaluate the local formability of advanced high-strength steel, such as the hole expansion test, which has become a standard method for evaluating the flange flanging performance of advanced high-strength steel sheet, and the hole expansion rate has become an important forming parameter for advanced high-strength steel products. However, the hole expansion rate is not a material property, but depends on many external factors that can cause a large amount of data dispersion and affect its reliability, such as the preparation method of the hole, the edge quality, the operator, the crack detection method, etc. In order to overcome these uncertainties and improve the accuracy of local formability prediction, an electrochemical etching grid method for measuring material fracture strain has been developed to predict local formability. However, due to the size of the grid and the nature of non-continuous measurement, it is difficult to conveniently and accurately measure the fracture strain. The digital image correlation technique developed twenty years ago is a non-contact tool for full-field strain measurement and has been widely used in material characterization. It can continuously measure large strains at any deformation stage, so it can well measure the fracture strain of the material and predict the local formability, but this method is tedious and time-consuming, and is not suitable for continuous measurement in industrial scenarios. The method based on tensile test and optical strain measurement can well balance the advantages and disadvantages of the above methods, and this method is simple and reliable, and can well predict and evaluate the overall formability and local formability of high-strength steel.

[0004] In summary, with the increasing application of advanced high-strength steel and the increasing strength, the concepts of overall formability and local formability will become crucial, and it is necessary to thoroughly understand the inherent overall formability and local formability characteristics of high-strength steel materials. The purpose of the present application is to predict and evaluate the overall formability and local formability of high-strength steel using a simple and effective method. SUMMARY

[0005] The present application provides a method for predicting and evaluating the overall formability and local formability of high-strength steel, which uses uniaxial tensile test and key formability parameters obtained from the test to characterize the overall formability and local formability of the material. This method has high measurement accuracy and simple operation, and can not only predict and evaluate the formability of advanced high-strength steel to guide the selection of materials for specific applications, but also help to establish future performance indicators for advanced high-strength steel.

[0006] To achieve the above object, the present application is realized by the following technical solutions.

[0007] A method for predicting and evaluating the overall formability and local formability of high-strength steel, characterized in that: a uniaxial tensile test is performed, and before the material reaches the tensile strength or necking instability, the strain is uniformly distributed, which reflects the overall formability of the material, and the work hardening index n or the true uniform strain ε u is obtained by the test as an evaluation index of the overall formability; and when the material necks to instability cracking, the strain is concentratedly distributed, which reflects the local formability of the material, and the effective strain range with the necking as the center point is about 20 mm, and the thickness-based fracture strain of the sample with a complete fracture is measured as an evaluation index of the local formability;

[0008] The work hardening index n can be measured by a uniaxial tensile test.

[0009] The true uniform strain can be calculated according to the formula: In the formula, UE is the percentage of uniform elongation, which is calculated; and the true uniform strain is also equal in value to the work hardening index n under the uniform elongation.

[0010] The thickness-based fracture strain refers to the true thickness strain or the engineering thickness strain.

[0011] The true thickness strain calculation method has two kinds:

[0012] (1) In the formula, a0 is the initial thickness within the gauge length of the sample, a u is the minimum thickness of the cross section of the fracture after the sample is fractured;

[0013] (2) In the formula, a g is the thickness of the uniform extension section within the gauge length of the sample, a u is the minimum thickness of the cross section of the fracture after the sample is fractured, and this calculation method removes the influence of the uniform strain.

[0014] The engineering thickness strain, also known as the fracture thinning rate, has a calculation formula as follows: In the formula, a0 is the initial thickness within the gauge length of the sample, a u is the minimum thickness of the cross section of the fracture after the sample is fractured.

[0015] The method for predicting and evaluating the overall formability and local formability of high-strength steel comprises the following steps:

[0016] (1) Prepare standard thin sheet tensile specimens according to ISO 6892-1, record the initial thickness a0 within the gauge length of the specimen, carry out uniaxial tensile test, the number of parallel tests is not less than three times, measure the strain hardening index n or the percentage of uniform elongation UE;

[0017] (2) Take the complete fracture specimen under the body microscope, measure the thickness a g of the uniform extension segment within the gauge length of the specimen u , and consider removing outliers;

[0018] (3) Calculate the true uniform strain, true thickness strain and engineering thickness strain:

[0019] (4) Take the average value of the true uniform strain of the three groups of specimens as the evaluation index of the overall formability;

[0020] (5) Take the average value of the true thickness strain or / and engineering thickness strain of the three groups of specimens as the evaluation index of the local formability.

[0021] The higher the work hardening index and the true uniform strain, the stronger the ability of the material to plastically deform to uniformly distribute strain without forming local necking, and the better the overall formability of the material.

[0022] The higher the true thickness strain and engineering thickness strain, the stronger the ability of the material to plastically deform in local areas without breaking, and the better the local formability of the material.

[0023] The technical principle adopted by the present application is that before the start of local necking, the material plastically deforms under the action of work hardening and forces the strain to be uniformly distributed as the load increases, until necking occurs, and the process ends. Therefore, the strain hardening index n and the true uniform strain value are suitable as evaluation indexes of the overall formability.

[0024] Advanced high strength steels usually exhibit a local formability reduction due to damage during forming, which leads to part fracture in the form of edge cracking or plane strain failure (e.g. slight necking fracture or small radius draw bending fracture) during forming. During the fracture process of metallic materials, the plastic deformation at the fracture (crack tip) varies greatly due to the different microstructure and stress state of the material, which leads to different macroscopic sizes of the fracture surface. Fracture is a highly localized behavior, and the ductile fracture describes the deformation capacity of the material before fracture, and the ductile fracture failure mechanism depends largely on the local microstructure. The fracture strain is derived from the reduction of the fracture area of the sample in uniaxial tension test, which refers to the equivalent plastic strain of the material at the crack initiation, and a method for quantifying the reduction of formability is to measure the fracture strain of the material, so the fracture strain is a good evaluation index of ductile fracture. The fracture surface or fracture line obtained, especially the horizontal plane, can be easily used as a measure of local formability. After local necking occurs, the material flows from the thickness in the loading direction (first principal strain), while the width direction strain (second principal strain) is relatively small due to the influence of the geometry of the sheet sample, which is only affected by the overall formability, combined with the definition of local formability, the ability of the material to plastically deform in the local area without cracking, and considering that the thickness strain (third principal strain) does not change its direction during loading, therefore, the third principal strain, i.e. the thickness-based fracture strain, is more suitable as a measure of local formability than the area-based fracture strain.

[0025] The thickness-based fracture strain includes the thickness strain before necking and the thickness strain after necking, and the thickness strain before necking, i.e. the thickness strain of the uniform elongation section, is mainly affected by the overall formability, so the thickness strain after necking is calculated, including the engineering thickness strain and the true thickness strain, and the results obtained may be closer to the actual value.

[0026] The engineering thickness strain, also known as nominal thickness strain (ε 3f ), does not consider the thickness change of the material itself. It is very useful in practical use, such as easy calculation and use in testing, prediction of whether different materials will crack under the same load during sheet forming, etc. However, it also has a disadvantage, which is to mask the strain hardening effect of the material due to the neglect of the actual size change of the sample during the test. Therefore, if the analyzed material does not produce large deformation, the difference between the engineering value and the true value of the thickness strain is not large.

[0027] The true thickness strain (ε f)considering the fracture thickness variation of the material itself. Using the true thickness strain value as the evaluation index is more accurate. Considering that the uniform strain is affected by the overall formability, sometimes removing the true thickness strain under the true uniform strain can more accurately predict and evaluate the local formability of high-strength steel materials.

[0028] The beneficial effects of the present application relative to prior art methods are:

[0029] Based on damage and failure analysis, the work hardening index n or true uniform strain ε u As an evaluation index for measuring overall formability, the thickness-based fracture strain (engineering thickness strain or / and true thickness strain) is used as an evaluation index for measuring local formability; the true thickness strain is also suitable for predicting and evaluating fracture toughness, edge crack sensitivity and bendability. This method is simple to operate, reliable in results, widely applicable and low in cost. BRIEF DESCRIPTION OF DRAWINGS

[0030] Figure 1 is a thickness measurement diagram of the uniform extension section within the gauge length of the sample;

[0031] Figure 2 is a measurement diagram of the minimum thickness of the cross section of the fracture after the sample is broken;

[0032] Figure 3 is a fracture macroscopic morphology diagram of samples with different thicknesses (width-thickness ratio). DETAILED DESCRIPTION

[0033] In order to make the purpose, technical scheme and advantages of the present application clearer, further detailed description of the present application will be made below in combination with the drawings and examples. It should be understood that the specific examples described herein are only used to explain the present application, and are not used to limit the present application. Within a reasonable variation range, they are used to explain the content of the present application.

[0034]

Example 1

[0035] Prediction and evaluation of the overall formability and local formability of DP980 with thicknesses of 1.0 mm and 1.8 mm respectively

[0036] The method comprises the following steps:

[0037] (1) Prepare a standard sheet tensile sample according to ISO 6892-1 (use a vertical CNC machining center to prepare A80 tensile samples perpendicular to the rolling direction and parallel to the rolling direction), record the initial thickness a0 within the gauge length of the sample, carry out uniaxial tensile test (Zwick 100KN electronic universal material testing machine), the number of parallel tests is not less than three times, and the uniform elongation percentage UE is measured;

[0038] (2) Take the complete fracture sample under the body microscope, measure the thickness a of the uniform extension section in the sample gauge length g (Actual thickness of the side away from the fracture in the parallel length range of the post-fracture sample, as shown in Figure 1 ) and the minimum thickness a of the fracture cross section after the sample breaks u (Determine the final fracture minimum thickness by taking the minimum value of multiple point measurements, as shown in Figure 2 ), and consider removing outliers;

[0039] (3) Calculate the true uniform strain ε u , true thickness strain ε 3f , and ε 3f *, engineering thickness strain (fracture thinning rate) e 3f ;

[0040] (4) Take the average of the true uniform strain of the three groups of samples as the evaluation index of overall formability;

[0041] (5) Take the average of the true thickness strain or / and engineering thickness strain of the three groups of samples as the evaluation index of local formability.

[0042] The measurement and calculation results are shown in Table 1.

[0043] Table 1 Overall formability and local formability index

[0044]

[0045]

[0046] From the calculation results, it is known that as the sample thickness increases, i.e. as the width-to-thickness ratio decreases, the true uniform strain ε u , true thickness strain ε 3f , and ε 3f *, engineering thickness strain (fracture thinning rate) e 3f of the material decrease, and the local formability decreases. This is because when the sample thickness is small, the sample near the crack tip is in a plane stress state. As the sample thickness increases, the plane stress state transitions to a plane strain state. When the sample exceeds a certain thickness, the material will be completely in a plane strain state. Generally, within a certain range, thinner samples have higher fracture toughness, and as the sample thickness increases, the fracture toughness value of the material will gradually decrease, eventually tending to a constant lower limit value. That is, the fracture toughness value increases with the increase of thickness, at this time, due to the pure shear fracture of the thinner thickness sample, the fracture toughness value decreases with the increase of thickness, and the fracture is mixed type, at a certain thickness of the sample, the fracture toughness value is stable at the same level, at this time, the sample is mainly flat end port, it is generally believed that the plane strain condition is realized at this time, as shown in Figure 3The results show that the plastic deformation and stress concentration in the material are different when the stress state in the material is different, which is reflected in the change of the macroscopic morphology of the material fracture. Therefore, when predicting and evaluating the local formability of advanced high-strength steel using the thickness strain of the fracture of the tensile test sample, the thickness or width-thickness ratio of the sample should be clear. In addition, the local formability of the material along the rolling direction is generally better than that along the direction perpendicular to the rolling direction.

[0047] Example 2

[0048] Prediction and evaluation of the overall formability and local formability of conventional DP780 with a thickness of 1.4 mm and high ductility DH780 with a thickness of 1.5 mm

[0049] The procedure of Example 1 was followed, and the results are shown in Table 2.

[0050] Table 2 Overall formability and local formability indicators

[0051]

[0052] From the calculation results, it can be seen that the DH780 has a higher true uniform strain ε u , i.e. the overall formability of DH780 is better than that of DP780, which is mainly due to the contribution of the "TRIP" effect of the appropriate amount of residual austenite in DH780 during loading. However, it can be seen from the thickness strain indicators that DP780 is generally higher than DH780, i.e. DP780 exhibits more excellent local formability than DH780, which is mainly due to the fact that DP780 usually does not contain residual austenite, and the presence of residual austenite, especially island-shaped or granular residual austenite, generates martensite after TRIP effect, which is not conducive to resisting local strain concentration distribution. It can be predicted that the local formability of the DP780 is better than that of the DH780, and the actual measured hole expansion ratio results are also consistent with the results, the average value of the measured hole expansion ratio is DP780: 30.5%, DH780: 22.3%.

[0053] Example 3

[0054] Prediction and evaluation of the overall formability and local formability of DP780 and CP780 with a thickness of 2.0 mm

[0055] The procedure of Example 1 was followed, and the results are shown in Table 3.

[0056] Table 3 Overall formability and local formability indicators

[0057]

[0058] It can be seen that under the premise of the same strength level, the local formability of CP780 is significantly better than that of DP780, which is mainly related to the microstructure of the material matrix. The matrix structure of DP780 is mainly composed of ferrite and martensite. The soft phase ferrite provides plasticity, and the hard phase martensite provides strength, which results in a higher hardness difference between the two phases. Therefore, the initial density of dislocation accumulation is low, and it finally shows higher uniform strain performance, that is, higher overall formability. The matrix structure of CP780 is mainly ferrite-bainite matrix, containing a small amount of martensite, pearlite and residual austenite. Due to the lower hardness difference between the phases and the better overall uniformity of the microstructure, it finally shows higher fracture resistance.

[0059] Those skilled in the art will easily understand that the above description is only the preferred embodiment of the present application, and is not intended to limit the present application. Any modification, equivalent replacement and improvement made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for predicting and evaluating the overall formability and local formability of high-strength steel, characterized in that: Standard sheet metal tensile specimens were prepared according to ISO 6892-1 standard, and uniaxial tensile tests were performed. The strain distribution was uniform before the material reached its tensile strength or necking instability, reflecting the overall formability of the material. The work hardening index n or the true uniform strain ε obtained from the test was used as the basis for the determination. u As an evaluation index of overall formability; the strain concentration distribution from necking to instability and cracking of the material reflects the local formability of the material. The effective strain range with the necking as the center point is 20 mm. The fracture strain based on the thickness of the specimen with a complete fracture surface is measured and used as an evaluation index of local formability. The work hardening index n can be measured by a uniaxial tensile test; The true uniform strain is defined by the following formula: , where UE is the percentage of uniform elongation, calculated; or obtained based on the fact that the actual uniform strain is numerically equal to the work hardening index n under uniform elongation; The fracture strain based on thickness refers to the actual thickness strain; The method for calculating the true thickness strain is as follows: In the formula: a g It is the thickness of the uniformly extended section within the gauge length of the specimen, a u It is the minimum thickness of the cross-section of the fracture surface after the specimen breaks. This calculation method can remove the influence of uniform strain.

2. The method for predicting and evaluating the overall formability and local formability of high-strength steel according to claim 1, characterized in that, The method includes the following steps: (1) Prepare standard thin plate tensile specimens according to ISO 6892-1, record the initial thickness a0 within the gauge length of the specimen, carry out uniaxial tensile tests, and conduct parallel tests no less than three times. Measure the work hardening index n or uniform elongation percentage UE. (2) Take a specimen with a complete fracture surface and measure the thickness a of the uniformly extended section within the gauge length of the specimen under a stereomicroscope. g The minimum thickness 'a' of the fracture surface cross-section after the specimen breaks u And remove outliers; (3) Calculate the true uniform strain and the true thickness strain; the method for calculating the true thickness strain is as follows: In the formula: a g It is the thickness of the uniformly extended section within the gauge length of the specimen, a u It is the minimum thickness of the cross-section of the fracture surface after the specimen breaks. This calculation method can remove the influence of uniform strain. (4) The average value of the true uniform strain of the three groups of specimens is used as the evaluation index of overall formability; (5) The average value of the true thickness strain of the three groups of specimens is used as the evaluation index of local formability.

3. The method for predicting and evaluating the overall formability and local formability of high-strength steel according to claim 1, characterized in that, The higher the work hardening index or true uniform strain, the stronger the material's ability to undergo plastic deformation to uniformly distribute strain without forming local necking, and the better the overall formability of the material.

4. The method for predicting and evaluating the overall formability and local formability of high-strength steel according to claim 1, characterized in that, The higher the actual thickness strain, the stronger the material's ability to undergo plastic deformation in local areas without fracturing, and the better the material's local formability.

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