A method for evaluating improvement effect of heat treatment process of high-strength steel

Abstract

The application discloses a method for evaluating the improvement effect of a high-strength steel heat treatment process, which can quantize the relationship between the heat treatment process improvement effect and residual stress, so as to evaluate the heat treatment process effect of reducing residual stress to improve the delayed cracking performance, and is beneficial to precisely regulating and controlling the heat treatment process, and can improve the delayed cracking performance and achieve the energy-saving and carbon-reducing effect.

Description

Technical Field

[0001] This invention relates to the field of material-sensitive fracture technology, and more specifically, to a method for evaluating the improvement effect of heat treatment processes on high-strength steel. Background Technology

[0002] Environmentally sensitive fracture is a common corrosion-induced fracture problem in the materials field, especially for high-strength steel and ultra-high-strength steel, which are more susceptible to hydrogen embrittlement and have a higher risk of delayed fracture during production and use. Residual stress generated during the processing and use of high-strength automotive steel is one of the important reasons for delayed cracking in this steel. Adopting appropriate heat treatment processes is an important means to reduce residual stress in high-strength automotive steel.

[0003] Current methods for reducing residual stress through heat treatment cannot obtain a quantitative relationship between residual stress and the heat treatment process. This is not conducive to precisely controlling the heat treatment process for different high-strength automotive steels to improve residual stress and reduce the risk of delayed cracking. In addition, residual stress testing has always been a major challenge in the industry. Existing residual stress testing equipment generally suffers from large measurement errors, is greatly affected by external environmental conditions, and requires precision, making it difficult to meet the needs of rapid response in on-site production.

[0004] In view of the above, there is a need to develop a method that is suitable for on-site production and can quantify the changes in residual stress of high-strength steel before and after heat treatment, so as to meet the needs of on-site production and improve the delayed cracking performance of materials. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for evaluating the improvement effect of heat treatment processes on high-strength steel. This method can quantify the relationship between the improvement effect of heat treatment processes and residual stress, thereby evaluating the effect of heat treatment processes in reducing residual stress and improving delayed cracking performance. This is beneficial for precise control of heat treatment processes, achieving both improved delayed cracking performance and energy saving and carbon reduction.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] This invention provides a method for evaluating the improvement effect of heat treatment process on high-strength steel, comprising the following steps:

[0008] S1, Sample preparation;

[0009] S2, perform surface treatment on the sample;

[0010] S3, use loading bolts to load the surface-treated sample onto the U-bending tester, then attach the stress sheet to the top of the sample at the center position, connect the stress sheet to the stress-strain tester, and then perform the first stress correction;

[0011] S4, remove the loading bolts from step S3, perform the first residual stress measurement, and obtain the residual stress of the sample before heat treatment.

[0012] S5, heat-treat the sample after testing the residual stress in step S4.

[0013] S6. Repeat steps S3 to S4 to perform a second stress correction and a second residual stress measurement on the heat-treated sample to obtain the residual stress of the sample after heat treatment.

[0014] S7. Based on the residual stress of the sample before heat treatment obtained in step S4 and the residual stress of the sample after heat treatment obtained in step S6, calculate the improvement effect of the heat treatment process on the sample.

[0015] Preferably, in step S1, multiple parallel samples of high-strength steel are cut using a shearing method, ensuring that the sample surface is flat and without bending, and loading holes are symmetrically provided at both ends of the sample.

[0016] Preferably, in step S2, acetone is used for ultrasonic treatment to ensure that the sample surface is clean.

[0017] Preferably, in step S3, the sample is bent on a U-bending tester using loading bolts until the two sides of the sample are parallel and the middle is bent into a U shape with a bending radius of 3 to 20 mm. The difference between the distance D between the two ends of the sample and the bending diameter of the sample is not higher than 5% of the bending diameter of the sample.

[0018] Preferably, in step S3, the stress gauge is attached to the center of the top of the bent section of the sample to ensure that the center of the strain gauge is aligned with the center of the sample.

[0019] Preferably, in steps S3 and S6, during the first stress correction and the second stress correction, the stress is ensured to be zero after the correction is balanced.

[0020] Preferably, in step S5, the heat treatment of the sample is carried out according to the heat treatment process of high-strength steel.

[0021] Preferably, in step S7, the improvement effect of the heat treatment process of the sample is as follows:

[0022] Calculate the residual stress difference ΔN before and after heat treatment of the sample. After taking the average value, remove the ΔN corresponding to deviations greater than 10%, and recalculate the average value of the residual stress difference before and after heat treatment of the sample. This value is the improvement effect of the heat treatment process of the sample.

[0023] The beneficial effects of this invention are:

[0024] 1. This invention indirectly obtains the residual stress change value of high-strength steel samples from the side using strain gauges and stress-strain measuring instruments. Compared with using residual stress measuring equipment to measure the residual stress of samples, this method is simpler and more effective, suitable for rapid evaluation of heat treatment processes on the production site, facilitating timely adjustment of heat treatment processes and improving production efficiency. At the same time, this method quantifies the relationship between the improvement effect of heat treatment process and residual stress, making it more intuitive and accurate.

[0025] 2. This invention includes, but is not limited to, applications in measuring residual stress during delayed cracking of high-strength steel. It can also be applied to testing changes in residual stress in other materials. This invention can be used to evaluate the improvement effect of heat treatment processes based on reducing residual stress to improve delayed cracking performance. It is beneficial to precisely control the heat treatment process to achieve both improved delayed cracking performance and energy saving and carbon reduction effects. This invention is beneficial to improving the accuracy of delayed cracking tests on high-strength steel, enriching test methods, and providing test support for the safety evaluation of high-strength steel. Attached Figure Description

[0026] Figure 1 This is a flowchart illustrating the method of evaluating the improvement effect of heat treatment process on high-strength steel according to the present invention.

[0027] Figure 2 This is a schematic diagram of a sample of the present invention;

[0028] Figure 3 This is a schematic diagram of the present invention where a sample is loaded onto a U-bending testing machine;

[0029] Figure 4 This is a schematic diagram of the sample after bending according to the present invention;

[0030] Figure 5 The strain gauge of this invention is installed on Figure 3 A schematic diagram of the sample;

[0031] Figure 6 This is a schematic diagram of the first stress correction in this invention;

[0032] Figure 7 This is a schematic diagram of the first residual stress measurement of the present invention;

[0033] Figure 8 This is a schematic diagram of the second residual stress measurement of the present invention. Detailed Implementation

[0034] To better understand the above-mentioned technical solutions of the present invention, the technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments.

[0035] This invention primarily focuses on high-strength steel, but it is also applicable to other metallic materials. Based on the delayed cracking test standard for high-strength automotive steel sheets, and combined with the characteristics of stress changes, this invention utilizes relevant laws to develop a quantifiable stress testing method suitable for on-site production to meet the needs of on-site production and improve delayed cracking performance.

[0036] Combination Figure 1 As shown, the present invention provides a method for evaluating the improvement effect of heat treatment process on high-strength steel, comprising the following steps:

[0037] S1, Sample preparation;

[0038] Prepare multiple parallel samples (e.g., 5 parallel samples). Shear the high-strength steel material to a size of 120mm x 20mm, ensuring the sample surface is flat and free of bending. Loading holes are symmetrically placed at both ends of the sample. (See [link to documentation]). Figure 2 As shown.

[0039] S2, the sample is surface treated by sonication with acetone for 15 minutes to ensure surface cleanliness;

[0040] S3, use loading bolts to load the surface-treated sample onto the U-bending tester, then attach the stress sheet to the top of the sample at the center position, connect the stress sheet to the stress-strain tester, and then perform the first stress correction;

[0041] Sample bending: The sample is bent using a U-bending tester. The bending radius is set according to relevant standards, and the bending radius ranges from 3mm to 20mm.

[0042] Sample loading: Load the sample onto the U-bending tester using loading bolts until both sides are parallel and the middle bends into a U-shape. The difference between the distance D between the two ends of the sample and the sample diameter Y should not exceed 5% of the diameter. See the operating procedure for details. Figure 3 As shown, the sample bends as follows Figure 4 As shown;

[0043] Strain gauge installation: Use strong adhesive to attach strain gauge 2 to the top center of the bent section B of the sample, ensuring that the center of strain gauge 2 is aligned with the center of the sample. Figure 5 As shown.

[0044] First stress correction: Connect strain gauge 2 to stress-strain tester 3, and balance it after correction to ensure that the stress is zero at this time. Figure 6 As shown,

[0045] S4, remove the loading bolts from step S3, perform the first residual stress measurement, and obtain the residual stress of the sample before heat treatment.

[0046] First residual stress measurement: After removing the loading bolts from the sample, the reading of the stress-strain tester 3 is the residual stress of the sample at this point, which is the residual stress of the sample before heat treatment, denoted as N1. Figure 7 As shown;

[0047] S5, heat-treat the sample after testing the residual stress in step S4.

[0048] Sample heat treatment process: Remove the strain gauge on the top of the sample, then place the sample into the heat treatment furnace, set the temperature, heating rate and holding time and other parameters according to the heat treatment process of high-strength steel, and then start the heat treatment.

[0049] S6. Repeat steps S3 to S4 to perform a second stress correction and a second residual stress measurement on the heat-treated sample to obtain the residual stress of the sample after heat treatment.

[0050] Second stress correction: Repeat step S3 to perform a second stress correction on the heat-treated sample. After correction, the sample is balanced to ensure that the stress is zero at this time.

[0051] Second residual stress measurement: Repeat step S4 to perform a second residual stress measurement on the heat-treated sample, obtaining the residual stress N2 at this time, which is the residual stress of the sample after heat treatment. See [link to relevant documentation]. Figure 8 As shown;

[0052] S7. Based on the residual stress of the sample before heat treatment obtained in step S4 and the residual stress of the sample after heat treatment obtained in step S6, calculate the improvement effect of the heat treatment process on the sample.

[0053] The improvement effect of the heat treatment process on the sample is as follows: Calculate the residual stress difference ΔN before and after heat treatment, that is, the residual stress reduction ΔN = N1 - N2. Calculate the residual stress difference ΔN of multiple parallel samples and obtain the ΔN of multiple parallel samples. After taking the average value, calculate the deviation. ΔN with a deviation greater than 10% is removed. Then, recalculate the average value of the residual stress difference ΔN before and after heat treatment. This average value A is the improvement effect of the heat treatment process on the sample.

[0054] The above method indirectly obtains the residual stress change value of the sample from the side using strain gauges and stress-strain measuring instruments, quantifying the relationship between the improvement effect of heat treatment process and residual stress, which is more intuitive and accurate. Compared with using residual stress measuring equipment to measure the residual stress of the sample, this method is simple and effective, suitable for rapid evaluation of heat treatment process on the production site, facilitating timely adjustment of heat treatment process and improving production efficiency.

[0055] Example 1

[0056] The method for evaluating the improvement effect of high-strength steel heat treatment process in this embodiment is as follows:

[0057] (1) Sample preparation: The DP1180 was used as the test object. The sample was cut to a size of 120mm*20mm by shearing to ensure that the sample surface was flat and without bending. Five parallel samples were prepared for each test group. The sample dimensions are shown in [reference]. Figure 2 As shown;

[0058] (2) Sample surface treatment: Sonicate with acetone for 15 minutes to ensure surface cleanliness;

[0059] (3) Sample bending: The sample is bent using a U-bending tester. The bending radius is set according to the SEP1970 standard, and the bending radius range is 5mm.

[0060] (4) Sample loading: Load the material using the loading bolts until both sides are parallel. The difference between the distance D between the two ends and the sample diameter Y should not exceed 5% of the diameter. See the operation procedure below. Figure 3 As shown, the sample bends as follows Figure 4 As shown;

[0061] (5) Strain gauge installation: Use strong adhesive to attach the strain gauge to the center of the top of the sample, ensuring that the center of the strain gauge is aligned with the center of the sample. Figure 5 As shown.

[0062] (6) First stress correction: Connect the strain gauge to the stress testing instrument, and balance it after correction to ensure that the stress is zero at this time. Figure 6 As shown;

[0063] (7) First residual stress measurement: Remove the bolts from the sample. The reading on the stress strain gauge at this time is the residual stress of the sample, which is the residual stress of the sample before heat treatment, denoted as N1. Figure 7 As shown;

[0064] (8) Sample heat treatment process: Remove the strain gauge on the top of the sample, then place the sample into the heat treatment furnace, set the temperature to 170℃, the heating rate to 10℃ / min, and the holding time to 20min.

[0065] (9) Second stress correction: Repeat steps (5) and (6), and balance after correction to ensure that the stress is zero at this time;

[0066] (10) Second residual stress measurement: Repeat step (7) to obtain the residual stress N2 at this time. See the process below. Figure 8 As shown;

[0067] (11) Calculation of the improvement effect of heat treatment process: Calculate the residual stress difference ΔN before and after heat treatment of the sample, that is, N1-N2 is the residual stress reduction value. Test 5 groups of samples and obtain ΔN of 5 groups of samples. Then take the average value and calculate the deviation. ΔN with a deviation greater than 10% is removed. Then recalculate its average value. The average value A is the measured value of the improvement effect of heat treatment process. See Table 1 for details.

[0068] Table 1

[0069]

[0070] Example 2

[0071] The method for evaluating the improvement effect of high-strength steel heat treatment process in this embodiment is as follows:

[0072] (1) Sample preparation: The DP1180 was used as the test object. The sample was cut to a size of 120mm*20mm by shearing to ensure that the sample surface was flat and without bending. Five parallel samples were prepared for each test group. The sample dimensions are shown in [reference]. Figure 2 As shown;

[0073] (2) Sample surface treatment: Sonicate with acetone for 15 minutes to ensure surface cleanliness;

[0074] (3) Sample bending: The sample is bent using a U-bending tester. The bending radius is set according to the SEP1970 standard, and the bending radius range is 5mm.

[0075] (4) Sample loading: Load the material using the loading bolts until both sides are parallel. The difference between the distance D between the two ends and the sample diameter Y should not exceed 5% of the diameter. See the operation procedure below. Figure 3 As shown, the sample bends as follows Figure 4 As shown;

[0076] (5) Strain gauge installation: Use strong adhesive to attach the strain gauge to the center of the top of the sample, ensuring that the center of the strain gauge is aligned with the center of the sample. Figure 5 As shown.

[0077] (6) First stress correction: Connect the strain gauge to the stress testing instrument, and balance it after correction to ensure that the stress is zero at this time. Figure 6 As shown;

[0078] (7) First residual stress measurement: Remove the bolts from the sample. The reading on the stress strain gauge at this time is the residual stress of the sample, which is the residual stress of the sample before heat treatment, denoted as N1. Figure 7 As shown;

[0079] (8) Sample heat treatment process: Remove the strain gauge on the top of the sample, then place the sample into the heat treatment furnace, set the temperature to 200℃, the heating rate to 10℃ / min, and the holding time to 10min.

[0080] (9) Second stress correction: Repeat steps (5) and (6), and balance after correction to ensure that the stress is zero at this time;

[0081] (10) Second residual stress measurement: Repeat step (7) to obtain the residual stress N2 at this time. See the process below. Figure 8 As shown;

[0082] (11) Calculation of the improvement effect of heat treatment process: Calculate the residual stress difference ΔN before and after heat treatment of the sample, that is, N1-N2 is the residual stress reduction value. Test 5 groups of samples and obtain ΔN of 5 groups of samples. Then take the average value and calculate the deviation. ΔN with a deviation greater than 10% is removed. Then recalculate its average value. The average value A is the measured value of the improvement effect of heat treatment process. See Table 2 for details.

[0083] Table 2

[0084]

[0085] Example 3

[0086] The method for evaluating the improvement effect of high-strength steel heat treatment process in this embodiment is as follows:

[0087] (1) Sample preparation: The DP980 was used as the test object. The sample was cut to a size of 120mm*20mm by shearing to ensure that the sample surface was flat and without bending. Five parallel samples were prepared for each test group. The sample dimensions are shown in [reference]. Figure 2 As shown;

[0088] (2) Sample surface treatment: Sonicate with acetone for 15 minutes to ensure surface cleanliness;

[0089] (3) Sample bending: The sample is bent using a U-bending tester. The bending radius is set according to the SEP1970 standard, and the bending radius range is 5mm.

[0090] (4) Sample loading: Load the material using the loading bolts until both sides are parallel. The difference between the distance D between the two ends and the sample diameter Y should not exceed 5% of the diameter. See the operation procedure below. Figure 3 As shown, the sample bends as follows Figure 4 As shown;

[0091] (5) Strain gauge installation: Use strong adhesive to attach the strain gauge to the center of the top of the sample, ensuring that the center of the strain gauge is aligned with the center of the sample. Figure 5 As shown.

[0092] (6) First stress correction: Connect the strain gauge to the stress testing instrument, and balance it after correction to ensure that the stress is zero at this time. Figure 6 As shown;

[0093] (7) First residual stress measurement: Remove the bolts from the sample. The reading on the stress strain gauge at this time is the residual stress of the sample, which is the residual stress of the sample before heat treatment, denoted as N1. Figure 7 As shown;

[0094] (8) Sample heat treatment process: Remove the strain gauge on the top of the sample, then place the sample into the heat treatment furnace, set the temperature to 170℃, the heating rate to 10℃ / min, and the holding time to 20min.

[0095] (9) Second stress correction: Repeat steps (5) and (6), and balance after correction to ensure that the stress is zero at this time;

[0096] (10) Second residual stress measurement: Repeat step (7) to obtain the residual stress N2 at this time. See the process below. Figure 8 As shown;

[0097] (11) Calculation of the improvement effect of heat treatment process: Calculate the residual stress difference ΔN before and after heat treatment of the sample, that is, N1-N2 is the residual stress reduction value. Test 5 groups of samples and obtain ΔN of 5 groups of samples. Then take the average value and calculate the deviation. ΔN with a deviation greater than 10% is removed. Then recalculate its average value. The average value A is the measured value of the improvement effect of heat treatment process. See Table 3 for details.

[0098] Table 3

[0099]

[0100] Those skilled in the art should recognize that the above embodiments are merely illustrative of the present invention and are not intended to limit the present invention. Any variations or modifications to the above embodiments that are within the spirit and essence of the present invention will fall within the scope of the claims of the present invention.

Claims

1. A method for evaluating the improvement effect of heat treatment process on high-strength steel, characterized in that, Includes the following steps: S1, Sample preparation In step S1, multiple parallel samples of high-strength steel are cut using a shearing method, ensuring that the sample surface is flat and without bending. Loading holes are symmetrically provided at both ends of the sample. S2, perform surface treatment on the sample; S3, use loading bolts to load the surface-treated sample onto the U-bending tester, then attach the stress sheet to the top of the sample center position, connect the stress sheet to the stress-strain tester, and then perform the first stress correction; S4, Remove the loading bolts from step S3, perform the first residual stress measurement, and obtain the residual stress of the sample before heat treatment. In step S3, the sample is bent using a loading bolt on a U-bending testing machine until both sides of the sample are parallel and the middle is bent into a U-shape. The bending radius is 3-20 mm, and the difference between the distance D between the two ends of the sample and the bending diameter of the sample is not higher than 5% of the bending diameter of the sample. S5, heat-treat the sample after testing the residual stress in step S4. S6. Repeat steps S3 to S4 to perform a second stress correction and a second residual stress measurement on the heat-treated sample to obtain the residual stress of the sample after heat treatment. S7. Based on the residual stress of the sample before heat treatment obtained in step S4 and the residual stress of the sample after heat treatment obtained in step S6, the improvement effect of the heat treatment process on the sample is obtained. In step S7, the improvement effect of the heat treatment process of the sample is as follows: Calculate the residual stress difference ΔN before and after heat treatment of the sample. After taking the average value, remove the ΔN corresponding to deviations greater than 10%, and recalculate the average value of the residual stress difference before and after heat treatment of the sample. This value is the improvement effect of the heat treatment process of the sample.

2. The method for evaluating the improvement effect of heat treatment process on high-strength steel according to claim 1, characterized in that: In step S2, acetone is used for ultrasonic treatment to ensure that the sample surface is clean.

3. The method for evaluating the improvement effect of heat treatment process on high-strength steel according to claim 1, characterized in that, In step S3, the stress gauge is attached to the center of the top of the bent section of the sample to ensure that the center of the strain gauge is aligned with the center of the sample.

4. The method for evaluating the improvement effect of heat treatment process on high-strength steel according to claim 1, characterized in that, In steps S3 and S6, during the first stress correction and the second stress correction, the stress is ensured to be zero after the correction is balanced.

5. The method for evaluating the improvement effect of heat treatment process on high-strength steel according to claim 1, characterized in that: In step S5, the heat treatment of the sample is carried out according to the heat treatment process of high-strength steel.

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