A test method for delayed cracking performance of ultra-high strength steel parts

Abstract

The present application provides a method for testing the delayed cracking performance of ultra-high-strength steel parts, which can prepare ultra-high-strength steel plates into characteristic specimens with different bending radii through a specific preforming process, simulate the different stress and strain states of ultra-high-strength steel sheets in the pre-blanking forming and post-forming edge punching processing processes, and test the delayed cracking characteristics of ultra-high-strength steel sheets under different bending radii and punching section quality conditions to achieve the purpose of testing the effect of the part processing process on the delayed cracking performance of ultra-high-strength steel sheets. This method can not only truly reflect the delayed cracking risk generated by the ultra-high-strength steel parts processing process, but is also easy to operate.

Description

Technical Field

[0001] The invention relates to the field of steel performance testing, and in particular to a method for testing the delayed cracking performance of an ultra-high-strength steel part. Background Art

[0002] In recent years, ultra-high strength steel (abbreviated as GIP steel) with a tensile strength of more than 980MPa has gradually been used in China. Its application range not only includes bumpers and door anti-collision beams, but also extends to body parts such as A-pillars, B-pillars, C-pillars and some seat parts, etc., to meet the increasingly stringent requirements for body lightweight and collision safety performance. However, with the continuous increase in the requirements for tensile strength, especially when the tensile strength reaches more than 1000MPa, the delayed cracking sensitivity of steel increases significantly. Among them, the high sensitivity to hydrogen-induced delayed cracking has become an important factor restricting the promotion and application of ultra-high strength steel.

[0003] The delayed cracking of ultra-high strength steel occurs under the combined action of materials, environment and stress, and is closely related to the characteristics of the material, the stress state and the service environment. The main processing technologies of ultra-high strength steel include bending, hole expansion, flanging, shallow drawing, trimming, punching, etc. Different processing methods will leave defects such as micropores, microcracks and dislocations on the material. These defect positions will become hydrogen capture traps or provide channels for rapid transmission of hydrogen atoms; under the action of external forces, stress concentration will also form at the defect positions, which will have a great impact on the hydrogen-induced delayed cracking behavior of the material. In addition, the material itself will also produce different residual strain states due to different processing processes, and the existence of these processing residual strains will promote the occurrence of delayed fracture. The latest research believes that the residual strain generated during the processing process is the third most important factor causing delayed fracture of high-strength steel besides the external stress and the diffusible hydrogen content in the material. Therefore, reducing the defects and residual strains generated during the processing stage of ultra-high strength steel is an effective way and measure to reduce the risk of delayed cracking of ultra-high strength steel parts.

[0004] At present, domestic and foreign research institutions have carried out research on the evaluation method of delayed cracking sensitivity of high-strength steel. However, since the delayed cracking performance of high-strength steel is affected by many factors, a universal evaluation method has not yet been formed, and the comparability between them is not strong. Commonly used test methods include U-shaped bending beam test, three-point bending test, punch cup test, slow tensile test, constant load test and prefabricated crack test. These tests are usually used to evaluate the delayed cracking sensitivity of the material itself, and do not fully consider the influence of actual part deformation and blanking section quality on delayed cracking performance.

[0005] For example, the patent application with publication number CN1344932A discloses "a method for setting the shape, working stress and use environment of steel parts", and proposes to find the relationship between the critical diffusible hydrogen amount (HC) and stress, find the diffusible hydrogen amount (HE) that invades the steel during the use of the parts, and then find the stress of the critical diffusible hydrogen amount (HC) equal to the above-mentioned invading diffusible hydrogen amount (HE), and set the shape and working stress of the steel parts to be lower than the above-mentioned stress, so as to achieve the purpose of preventing delayed fracture. The patent application with publication number CN102692373A discloses "a stress corrosion sensitivity evaluation device based on a small punch test technology", and proposes a stress corrosion sensitivity evaluation device based on a small punch test technology, which, in conjunction with a specific sampling device, can effectively solve the problem of difficult stress corrosion sensitivity evaluation of small samples and single-sided processed samples. The patent application with publication number CN103344488A discloses "a fixture for slow tensile stress corrosion test", and proposes a fixture for slow tensile stress corrosion test, which can effectively prevent galvanic corrosion caused by different materials between the sample and the fixture in high-temperature and high-pressure water environment, and can make a more comprehensive and accurate performance evaluation of the material. The patent application with publication number CN104502196A discloses "a device and method for detecting hydrogen embrittlement sensitivity of high-strength steel surface treatment process", and proposes a device and method for detecting hydrogen embrittlement sensitivity of high-strength steel surface treatment process, which can determine in which surface treatment process of high-strength steel is prone to hydrogen embrittlement, and provide guidance for high-strength steel surface treatment process.

[0006] The above patent cannot truly reflect the risk of delayed cracking caused by the processing of ultra-high-strength steel parts. Therefore, it is necessary to design special specimens to test the impact of the processing process on the delayed cracking performance of ultra-high-strength steel. Summary of the invention

[0007] In order to solve the above problems, the present application intends to provide a delayed cracking performance test method for ultra-high strength steel parts, which can truly reflect the delayed cracking risk generated during the processing of ultra-high strength steel parts. The test method includes the following steps:

[0008] Sample preparation steps: use a preforming process to process the ultra-high strength steel plate, and punch the processed ultra-high strength steel plate into n punched samples of the same size, where n is an integer, and n≥6; the punched samples are pressed to obtain n characteristic samples, each of the characteristic samples has an intermediate indentation, and each of the intermediate indentations has a different bending fillet radius, and each bending fillet radius is R1~Rn, Rn>R1, and R1≥4 millimeters (mm), Rn≤16 millimeters.

[0009] Delayed cracking performance test steps: The delayed cracking performance evaluation test of ultra-high strength steel thin plates is used to test the delayed cracking performance of characteristic samples, and the delayed cracking fracture time and the number of cracks of each characteristic sample are recorded respectively.

[0010] The above technical solution can process the ultra-high-strength steel plate through any preforming process, and then prepare the processed ultra-high-strength steel into characteristic samples with different bending fillet radii. By testing the delayed cracking characteristics of ultra-high-strength steel plates under different bending fillet radii and blanking section quality, the different stress and strain states of ultra-high-strength steel under different processing technologies can be simulated, thereby achieving the purpose of testing the influence of the part processing process on the delayed cracking performance of ultra-high-strength steel thin plates. This method can not only truly reflect the delayed cracking risk generated by the processing of ultra-high-strength steel parts, but is also easy to operate.

[0011] Furthermore, in the sample preparation step, two or more preforming processes are used to process the ultra-high strength steel plates respectively, and corresponding characteristic sample groups are obtained respectively, that is, each process corresponds to a characteristic sample group, each characteristic sample group has n characteristic samples, and the middle indentations of each characteristic sample of each characteristic sample group are respectively R1 to Rn, and the sizes of R1 to Rn of each characteristic sample group are the same. The preforming process can be a variety of different processes, including but not limited to a pre-blanking forming process and a post-forming edge blanking process.

[0012] Further, the punched sample is rectangular, the length of the punched sample is 200-300 mm, and the width of the punched sample is 80-150 mm, including the end points. For example, the size of the punched sample can be 200×80 mm.

[0013] Furthermore, each of the n characteristic samples includes two side indentations, the radius of the curved corners of the two side indentations is R0, and the two side indentations are symmetrically arranged with the middle indentation of each characteristic sample as the midline, and 6mm≤R0≤10mm. In this technical solution, when the middle indentation is deformed, the two side indentations play the role of pressing the material to hinder the flow of the material on both sides, so as to ensure that the middle indentation has sufficient deformation.

[0014] Furthermore, the tensile strength of the ultra-high strength steel plate is ≥980MPa.

[0015] Furthermore, in the delayed cracking performance test step, the characteristic specimens are immersed in a 0.1 mol / L (or expressed as 0.1N) HCL solution, and the delayed cracking fracture time and the number of cracks of each characteristic specimen are recorded respectively. In other words, in this technical solution, the method for evaluating the delayed cracking performance of ultra-high strength steel sheets is to immerse each characteristic specimen in a 0.1 mol / L hydrochloric acid solution at room temperature and then observe it.

[0016] Furthermore, in the present application, the thickness of the "ultra-high-strength steel plate" or "ultra-high-strength steel thin plate" is preferably 0.6 to 3 mm.

[0017] The technical solution of the present application is used to test the influence of different processing techniques on the delayed cracking performance of ultra-high-strength steel parts. Characteristic specimens are made by processing ultra-high-strength steel plates used to produce automotive parts, and then the characteristic specimens are tested in combination with the delayed cracking performance evaluation standard. The results show that the sample preparation is operational, the test results have good repeatability, the simulation test is close to the actual processing state of ultra-high-strength steel parts, and the influence of processing factors on the delayed cracking performance of ultra-high-strength steel sheets is fully considered. The delayed cracking performance test method for ultra-high-strength steel parts provided by the present application makes the characteristic specimens close to the actual processing state of ultra-high-strength steel parts, improves the level of delayed cracking performance detection technology for ultra-high-strength steel plates, and helps to establish technical specifications for delayed cracking prevention and control when ultra-high-strength steel is used as components. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 is a schematic diagram of a characteristic sample of an embodiment of the present application;

[0019] Figure 2 yes Figure 1 A schematic flow chart of the testing method of the illustrated embodiment. DETAILED DESCRIPTION

[0020] The following specific embodiments illustrate the implementation of the present invention, and those skilled in the art can easily understand other advantages and effects of the present invention from the contents disclosed in this specification. Although the description of the present invention will be introduced in conjunction with the preferred embodiment, this does not mean that the features of this invention are limited to this implementation. On the contrary, the purpose of introducing the invention in conjunction with the implementation is to cover other options or modifications that may extend based on the claims of the present invention. In order to provide a deep understanding of the present invention, the following description will include many specific details. The present invention can also be implemented without using these details. In addition, in order to avoid confusion or blurring the focus of the present invention, some specific details will be omitted in the description. It should be noted that the embodiments of the present invention and the features in the embodiments can be combined with each other without conflict.

[0021] The numerical range in this application should be understood as all ranges consisting of the upper limit or preferred value or better value of the range and the lower limit or preferred value or better value of the range specifically disclosed herein, regardless of whether these ranges are disclosed separately. In addition, if a numerical range is mentioned herein, unless otherwise specified, the range should include its endpoints and all integers and fractions within the range.

[0022] In order to make the objectives, technical solutions and advantages of the present invention more clear, the technical solutions of the present application will be described in detail below in conjunction with the accompanying drawings and specific implementation methods.

[0023] The present application provides a method for testing the delayed cracking performance of an ultra-high-strength steel part, comprising a sample preparation step and a delayed cracking performance testing step.

[0024] Sample preparation steps:

[0025] The ultra-high-strength steel plate is processed by a preforming process, and the processed ultra-high-strength steel plate is punched into n punching specimens of the same size, wherein n is an integer, and n≥6; the punching specimens are press-formed to obtain n characteristic specimens, each of the characteristic specimens has an intermediate indentation, each of the intermediate indentations has a different bending fillet radius, and the bending fillet radii are R1 to Rn, Rn>R1, and R1≥4mm, Rn≤16mm.

[0026] Specifically, in any embodiment of the present application, two or more preforming processes are used to process ultra-high strength steel plates respectively to obtain corresponding characteristic sample groups, that is, each process corresponds to a characteristic sample group, which is used to simulate the different stress and strain states of ultra-high strength steel under different processing processes. Among them, ultra-high strength steel plates refer to steel plates with a tensile strength of ≥980MPa and a thickness of, for example, 0.6 to 3mm.

[0027] Furthermore, each characteristic sample includes two side indentations, the radius of the curved corners of the two side indentations is R0, and the two side indentations are symmetrically arranged with the middle indentation of each characteristic sample as the midline, and 6mm≤R0≤10mm. When the middle indentation is deformed, the two side indentations play the role of pressing the material to hinder the flow of the material on both sides, or in other words, hinder the flow of the two sides of the characteristic sample to the middle indentation position, so as to ensure that the middle indentation has sufficient deformation.

[0028] In the above embodiment, each characteristic sample group has n characteristic samples to simulate the characteristics of the steel plate under different bending fillet radii and punching section quality conditions, and the bending fillet radii of each group of characteristic samples are respectively R1 to Rn.

[0029] In any embodiment of the present application, the punched sample is rectangular, the length of the punched sample is 200 to 300 mm, and the width of the punched sample is 80 to 150 mm.

[0030] Delayed cracking performance test steps:

[0031] The delayed cracking performance evaluation test of ultra-high strength steel thin plates was used to test the delayed cracking performance of the characteristic samples, and the delayed cracking fracture time and the number of cracks of each characteristic sample were recorded respectively. Specifically, each characteristic sample was immersed in a 0.1 mol / L (i.e., 0.1 N) hydrochloric acid (HCL) solution, and the delayed cracking fracture time and the number of cracks of each characteristic sample were recorded respectively.

[0032] The present application is used to test the influence of different processing technologies on the delayed cracking performance of ultra-high-strength steel parts. Ultra-high-strength steel plates used for the production of automotive parts are processed to make characteristic specimens, which are then tested in combination with a delayed cracking performance evaluation test. The influence of processing factors on the delayed cracking performance of ultra-high-strength steel sheets is fully considered, so that the characteristic specimens are close to the actual processing state of ultra-high-strength steel parts, the level of delayed cracking performance detection technology is improved, and it is helpful to establish technical specifications for delayed cracking prevention and control when ultra-high-strength steel is used as components.

[0033] Example 1

[0034] This embodiment uses the delayed cracking performance test method of ultra-high strength steel parts provided in this application to evaluate the delayed cracking risk generated during the processing, combined with Figure 2 As shown, the specific steps are as follows:

[0035] Sample preparation steps:

[0036] The ultra-high-strength steel plates are processed by the pre-blanking forming process and the post-forming edge punching process respectively. The pre-blanking forming process refers to first using a press, shearing machine, sawing machine, flame cutting, plasma cutting, laser cutting and other processing methods to cut the sheet material into a specific blank shape, and then directly forming and processing the part features. The feature edges do not need to be processed again to meet the product edge size requirements. The post-forming edge punching process refers to first forming and processing the part features, and then using a die punching process to cut off excess edge material from the feature edges. After processing, the feature edges meet the product edge size requirements. Although the above two pre-forming processes cover a variety of processing methods, in this application, the processing method or combination of processing methods used for each feature sample in each feature sample group must be the same.

[0037] Type I characteristic specimens: The DP1180 steel plate with a thickness of 1.4 mm was processed by pre-blanking forming process, and the processed ultra-high strength steel plate was punched into 6 rectangular punched specimens of 200 mm × 80 mm. The punched specimens were pressed to obtain 6 characteristic specimens, which were numbered 1, 2, 3, 4, 5, and 6 respectively. Figure 1As shown, each characteristic sample has a middle indentation, and each middle indentation has a different bending fillet radius. The bending fillet radius of the middle indentation of characteristic samples No. 1 to 6 is R1 to R6, R1 = 4mm, R2 = 5mm, R3 = 6mm, R4 = 7mm, R5 = 8mm, R6 = 9mm. There are two side indentations on both sides of the middle indentation. In this embodiment, the bending fillet radius R0 of the two side indentations is uniformly 6mm. This group of characteristic samples is used to simulate the pre-blanking and profiling process of ultra-high-strength steel sheets, and is also referred to as Class I characteristic samples in this application.

[0038] Type II characteristic specimens: The DP1180 steel plate with a thickness of 1.4 mm was processed by the post-forming edge blanking process. The processed ultra-high strength steel plate was also punched into 6 rectangular punched specimens of 200 mm × 80 mm. The punched specimens were pressed to obtain 6 characteristic specimens, which were numbered 7, 8, 9, 10, 11, and 12 respectively. Figure 1 As shown, each characteristic sample has a middle indentation, and each middle indentation has a different bending fillet radius. The bending fillet radius of the corresponding middle indentation of characteristic samples No. 7 to No. 12 is also R1 to R6. The characteristic sample No. 7 has R1 = 4mm, the characteristic sample No. 8 has R2 = 5mm, the characteristic sample No. 9 has R3 = 6mm, the characteristic sample No. 10 has R4 = 7mm, the characteristic sample No. 11 has R5 = 8mm, and the characteristic sample No. 12 has R6 = 9mm. Similarly, there are two side indentations on both sides of the middle indentation. In this embodiment, the bending fillet radius R0 of the two side indentations is uniformly 6mm. Different from the characteristic sample of type I, as shown in FIG. Figure 1 As shown, the type II characteristic sample has a side blanking edge line (shown in a). This group of characteristic samples is also referred to as type II characteristic samples in this application.

[0039] Delayed cracking performance test steps:

[0040] The type I characteristic specimens and type II characteristic specimens were immersed in 0.1N HCl solution, respectively, and the delayed cracking and fracture time and the number of cracks of type I characteristic specimens No. 1 to 6, as well as the delayed cracking and fracture time and the number of cracks of type II characteristic specimens No. 7 to 12 were recorded respectively.

[0041] The delayed cracking and fracture time of the above-mentioned type I characteristic specimens and type II characteristic specimens are shown in Table 1, where "-" represents that no fracture occurs for more than 30 days; the number of cracks is shown in Table 2. It can be seen from the table that the DP1180 steel plate is prone to delayed cracking when the bending fillet radius is small. As the bending fillet radius of the characteristic specimen increases, the delayed cracking and fracture time of the specimen section becomes later and the number of cracks becomes smaller. The delayed cracking and fracture time of the type II characteristic specimen is generally earlier than that of the type I characteristic specimen. When the bending fillet radius is the same, the number of delayed cracking cracks of the type II characteristic specimen is more than that of the type I characteristic specimen.

[0042] Table 1 Delayed cracking and fracture time of type I characteristic specimens and type II characteristic specimens

[0043] Bending corner radius Type I characteristic specimen Delayed cracking and rupture time / day Class II characteristic specimen Delayed cracking and rupture time / day R1=4mm No. 1 13 No.7 9 R2=5mm No. 2 15 No. 8 12 R3=6mm No.3 - No. 9 20 R4=7mm No. 4 - No. 10 - R5=8mm No. 5 - No. 11 - R6=9mm No. 6 - No. 12 -

[0044] Table 2 Number of delayed cracking cracks of type I characteristic specimens and type II characteristic specimens

[0045] Bending corner radius Type I characteristic specimen Number of delayed cracking cracks Class II characteristic specimen Number of delayed cracking cracks R1=4mm No. 1 2 No.7 3 R2=5mm No. 2 1 No. 8 2 R3=6mm No.3 0 No. 9 1 R4=7mm No. 4 0 No. 10 0 R5=8mm No. 5 0 No. 11 0 R6=9mm No. 6 0 No. 12 0

[0046] Test conclusion:

[0047] By analyzing the delayed cracking test data of type I characteristic specimens and type II characteristic specimens, it can be found that with the increase of the bending fillet radius, the stress value of the specimen section becomes smaller, the delayed cracking occurs later, and the number of cracks is also smaller; the fracture quality of the specimen using the expected forming process (type I characteristic specimen) under the same bending fillet radius is better than that of the specimen punched after forming (type II characteristic specimen), the stress value of the specimen section is smaller, the delayed cracking occurs later, and the number of cracks is less.

[0048] In the above embodiment, the sample preparation is operable, the test results have good repeatability, the simulation test is close to the actual processing state of ultra-high-strength steel parts, and the influence of processing factors on the delayed cracking performance of ultra-high-strength steel parts is fully considered. Through this test, the sensitivity of ultra-high-strength steel sheets to part feature design, processing methods and processes can be evaluated, and the purpose of testing the influence of part processing on the delayed cracking performance of ultra-high-strength steel sheets is achieved. It makes up for the defects of the prior art that only the delayed cracking sensitivity of the material itself can be evaluated, and the influence of actual part deformation and blanking section quality on delayed cracking performance can be fully considered.

[0049] Although the present invention has been illustrated and described with reference to certain preferred embodiments of the present invention, it should be understood by those skilled in the art that the above is a further detailed description of the present invention in conjunction with specific embodiments, and it cannot be determined that the specific implementation of the present invention is limited to these descriptions. Those skilled in the art may make various changes in form and details, including making several simple deductions or substitutions, without departing from the spirit and scope of the present invention.

Claims

1. A method for testing the delayed cracking performance of ultra-high strength steel parts, characterized in that: The following steps are included: Sample preparation steps: using a preforming process to process the ultra-high strength steel plate, punching the processed ultra-high strength steel plate into n punching samples of the same size, with a thickness of 0.6~3mm, wherein n is an integer, and n≥6; pressing the punching samples to obtain n characteristic samples, each of the characteristic samples has a middle indentation, each of the middle indentations has a different bending fillet radius, each of the bending fillet radii is R1~Rn, Rn>R1, and R1≥4mm, Rn≤16mm; each of the characteristic samples includes two side indentations, the bending fillet radius of the two side indentations is R0, the two side indentations are symmetrically arranged with the middle indentation of each characteristic sample as the midline, and 6mm≤R0≤10mm; Delayed cracking performance test steps: The delayed cracking performance test of the ultra-high strength steel sheet is adopted to carry out the delayed cracking performance test on the characteristic samples, and the delayed cracking fracture time and the number of cracks of each characteristic sample are recorded respectively.

2. The testing method according to claim 1, characterized in that: In the sample preparation step, two or more of the preforming processes are used to process the ultra-high strength steel plates respectively, and characteristic sample groups having n characteristic samples are obtained respectively, and the R1~Rn of each characteristic sample group are the same in size.

3. The testing method according to claim 2, characterized in that: The preforming process includes a pre-blanking forming process and a post-forming edge punching process.

4. The testing method according to claim 1, characterized in that: The punching specimen is rectangular, the length of the punching specimen is 200-300 mm, and the width of the punching specimen is 80-150 mm.

5. The testing method according to claim 1, characterized in that: The tensile strength of the ultra-high strength steel plate is ≥980MPa.

6. The testing method according to claim 1, characterized in that: In the delayed cracking performance test step, the characteristic samples are immersed in a 0.1 mol / L HCL solution, and the delayed cracking fracture time and the number of cracks of each characteristic sample are recorded respectively.

7. The testing method according to claim 1, characterized in that: The thickness of the ultra-high strength steel plate is 0.6-3 mm.

8. The testing method according to claim 3, characterized in that: The characteristic specimen obtained by the post-forming edge blanking process has an edge blanking edge line.

9. The testing method according to claim 1, characterized in that: n=6, R1=4mm, R2=5mm, R3=6mm, R4=7mm, R5=8mm, R6=9mm.

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