Preparation method of sample for reinforcing steel bar low-temperature tensile test and reinforcing steel bar low-temperature tensile test method

Abstract

The invention discloses a preparation method of a sample for a low-temperature tensile test of a reinforcing steel bar and a low-temperature tensile test method of the reinforcing steel bar. The preparation method comprises the following steps: taking a steel bar sample E1 to be tested, collecting the diameter D1 and the length L1 of the steel bar sample E1, and testing the yield strength Y1 and the tensile strength T1 of the steel bar sample E1 at normal temperature; additionally taking a steel bar to be measured as a strip steel bar E2, and controlling the length L2lt of the E2; l1; a clamping section steel bar E3 is selected, and the diameter D3 of the E3 is equal to D1; the length L3 is equal to a + L2 / 2, and a is equal to 100-150mm; the yield strength Y3 at normal temperature is greater than or equal to Y1 + 100 * b, and b is equal to 0.5-1.5; the tensile strength T3 at normal temperature is greater than or equal to Y2 + 140 * c, and c is equal to 1-2; the percentage elongation after fracture A3 is greater than or equal to 15%; e1 and E3 are used as main reinforcements, a strip reinforcing steel bar E2 is used as a welding supporting structure, and strip welding is conducted on E1 and E3; and performing surface notch processing on the E1 to obtain the sample for the low-temperature tensile test of the reinforcing steel bar.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of steel material test sample preparation, and particularly relates to a preparation method of a sample for a steel bar low-temperature tensile test and a low-temperature tensile test method of a steel bar. BACKGROUND

[0002] In recent years, LNG storage tank projects have developed rapidly, and the use amount of LNG storage tanks has increased, and the use amount of steel bars has also increased. Since LNG storage tanks are mainly used in low-temperature environments, high requirements are put forward for the low-temperature performance of steel bars.

[0003] The low-temperature tensile test is an important test method for characterizing the low-temperature performance of steel bars. The strength of the steel bar increases as the temperature decreases. During the low-temperature tensile test, the test section is located in the test box, and the clamp of the tensile testing machine is located outside the test box. The clamping section of the sample is clamped by the clamp, and thus the clamping section of the sample is located outside the test box and is in an environment with a higher temperature than the test box. During the low-temperature tensile test, the clamping section often breaks, resulting in invalid test data of the tensile test.

[0004] However, the steel bar is not allowed to be mechanically processed during the low-temperature tensile test to avoid damaging the surface structure. At present, the problem is mainly solved by modifying the clamp of the tensile testing machine during the low-temperature tensile test of the steel bar. However, modifying the clamp has the problems of complex process and poor effect, which limits the development of the low-temperature tensile test of the steel bar. SUMMARY

[0005] The application aims to provide a preparation method of a sample for a steel bar low-temperature tensile test and a low-temperature tensile test method of a steel bar.

[0006] To achieve one of the above purposes, an embodiment of the application provides a preparation method of a sample for a steel bar low-temperature tensile test, which comprises the following steps: Taking a steel bar sample E1 to be tested, collecting the diameter D1 and the length L1 of the sample, and testing the yield strength Y1 and the tensile strength T1 of the sample at room temperature; Taking another steel bar to be tested as a help strip steel bar E2, and controlling the length L2 of the E2 to be less than the length L1 of the sample; Selecting a clamping section steel bar E3, and controlling the diameter D3 of the E3 to be equal to the diameter D1 of the sample, the length L3 of the E3 to be a+L2 / 2, a=100-150 mm, the yield strength Y3 of the E3 at room temperature to be greater than or equal to Y1+100×b, b=0.5-1.5, the tensile strength T3 of the E3 at room temperature to be greater than or equal to Y2+140×c, c=1-2, and the elongation A3 after fracture to be greater than or equal to 15%; Welding, abutting the E3 to the two ends of the E1 respectively, taking the E1 and the E3 as the main bars, and taking the E2 as the welding support structure to help weld the joint of the E1 and the E3; Surface cutting processing is performed on E1 to obtain the sample for low-temperature tensile test of the reinforcing bar.

[0007] As a further improvement of the embodiment of the application, during welding, E3 is butted to the two ends of E1 respectively, E1 and E3 are used as the main reinforcement, and the help strip reinforcing bar E2 is used as the welding support structure, and the help strip welding is performed at the two ends of E1 respectively.

[0008] As a further improvement of the embodiment of the application, during welding, E1 and E3 are coaxially arranged, and the end surface spacing between E1 and E3 is 2-3 mm.

[0009] As a further improvement of the embodiment of the application, double help strip welding is used for welding.

[0010] As a further improvement of the embodiment of the application, the tensile strength T4 of the welding rod used for welding is greater than or equal to T1.

[0011] As a further improvement of the embodiment of the application, the current during welding is 100-150 A, and the voltage is 20-30 V.

[0012] As a further improvement of the embodiment of the application, L2=d×D2, d=6-8, wherein D2 is the diameter of the help strip reinforcing bar E2.

[0013] As a further improvement of the embodiment of the application, 200+6×D1+L2≤L1≤H+L2, wherein H is the height of the inner cavity of the test box used for low-temperature tensile test, and the height direction of the inner cavity of the test box is consistent with the length direction of E1.

[0014] As a further improvement of the embodiment of the application, the diameter D1 of the sample E1 to be tested is 16-40 mm.

[0015] As a further improvement of the embodiment of the application, b is inversely proportional to the test temperature during low-temperature tensile test; and c is inversely proportional to the test temperature during low-temperature tensile test.

[0016] To achieve one of the above application purposes, the embodiment of the application further provides a low-temperature tensile test method of a reinforcing bar, which comprises the following steps: A sample is prepared, which is prepared by using the sample preparation method for low-temperature tensile test of a reinforcing bar as described above; The clamps of a tensile testing machine are used to clamp the reinforcing bars E3 at the clamping sections of the sample respectively away from one end of the sample E1 to be tested, so that the sample E1 to be tested is located in a test box used for low-temperature tensile test, and the clamps are located outside the test box. The test box is cooled to a preset temperature and then kept at the temperature. The tensile test is performed until the sample is broken, and the tensile property index of the steel test sample E1 at low temperature is measured.

[0017] As a further improvement of an embodiment of the present application, the test method further comprises the steps of: After the sample is broken, the maximum force total elongation rate Agt and the elongation rate A after breaking of the steel bar are measured manually according to GB / T228.1-2021 Metal Materials Tensile Test Part 1 Ambient Temperature Test Method after the temperature returns to room temperature.

[0018] As a further improvement of an embodiment of the present application, the cooling of the test chamber to the preset temperature and the subsequent holding specifically comprises: The test chamber is cooled by liquid nitrogen, and the holding time t after the test chamber is cooled to the preset temperature is 20-40 min, and the holding time t is proportional to the diameter D1 of the steel test sample E1. The preset temperature is -196℃-0℃.

[0019] As a further improvement of an embodiment of the present application, the strain rate before yielding is set to be ≤0.00015 / s, and the strain rate after yielding is set to be ≤0.002 / s during the tensile test.

[0020] Compared with the prior art, the beneficial effects of the present application include: (1) The sample preparation method for the steel bar low-temperature tensile test of the present application can not only improve the connection strength between the clamping section steel bar and the steel test sample, but also strengthen the clamping section steel bar, avoid the problem that the clamping section steel bar E3 is in a relatively high temperature environment, while the steel test sample E1 is in a lower temperature environment, and ensure that the breaking position of the low-temperature tensile test is in the steel test sample section, thereby improving the test precision. The method can be applied to low-temperature tensile test of steel bars of different strength grades. Moreover, after welding, the welded joint does not need to be heat treated after welding, and can be directly used for low-temperature tensile test. The method is simple, easy to operate, and widely applicable, and does not need to modify the existing low-temperature tensile testing machine.

[0021] (2) The low-temperature tensile test method of the steel bar of the application, when the low-temperature tensile test of the steel bar is performed, the steel bar sample E1 to be tested is in the test box at low temperature, and the clamping section steel bar E3 is outside the test box at relatively high temperature. When the tensile test is performed, it can ensure that the fracture position of the low-temperature tensile test is in the steel bar sample E1 to be tested, and the temperature uniformity of the steel bar sample E1 to be tested along the length direction is improved, so as to ensure the effective test of the tensile test and improve the test precision. Moreover, the method can be applied to the low-temperature tensile test of the steel bar at different strength levels and different test temperatures. Moreover, the existing low-temperature tensile testing machine does not need to be modified. BRIEF DESCRIPTION OF DRAWINGS

[0022] Figure 1 is a structural schematic diagram of a preparation method of a sample for low-temperature tensile test of a steel bar according to an embodiment of the application. DETAILED DESCRIPTION

[0023] The technical solutions of the application will be further described in combination with specific embodiments, but the scope of protection is not limited to the description.

[0024] It is found through research that the strength of the steel bar increases with the decrease of temperature. When the temperature decreases, the atomic thermal motion in the steel bar weakens, the diffusion coefficient of the atoms decreases, the grain boundary migration becomes difficult, and the dislocation movement is blocked, so that the strength of the steel bar increases. Moreover, the lower the temperature, the more significant the increase of the strength of the steel bar.

[0025] When the low-temperature tensile test of the steel bar is performed, the fracture position often appears in the clamping section.

[0026] In view of this, the application provides a preparation method of a sample for low-temperature tensile test of a steel bar to solve the problem that the steel bar sample always breaks from the clamping section located outside the test box during the low-temperature tensile test.

[0027] Reference Figure 1 The preparation method comprises the following steps: S11. Take a steel bar sample E1 to be tested, collect the diameter D1 and the length L1 thereof, and test the yield strength Y1 and the tensile strength T1 thereof at normal temperature.

[0028] S12. Take another steel bar to be tested as a help steel bar E2, and control the length L2 of the help steel bar E2 to be less than the length L1 of the steel bar sample E1 to be tested.

[0029] That is, the help steel bar E2 and the steel bar sample E1 to be tested are made of the same material and have the same diameter, and only the lengths of the two are different.

[0030] S13. Select the clamping section steel bar E3, control the diameter D3 of E3 = D1; the length L3 = a + L2 / 2, a = 100-150 mm; the yield strength Y3 at normal temperature ≥ Y1 + 100 x b, b = 0.5-1.5; the tensile strength T3 at normal temperature ≥ Y2 + 140 x c, c = 1-2; the elongation A3 after fracture ≥ 15%.

[0031] S14. Welding Butt joint E3 at both ends of E1, take E1 and E3 as the main reinforcement, take the help strip steel bar E2 as the welding support structure, and perform help strip welding on the joint of E1 and E3.

[0032] S15. Perform surface notch processing on E1 to obtain the steel bar sample for low-temperature tensile test.

[0033] Specifically, according to GB / T 228.3-2019 Metal Materials Tensile Test Part 3: Low Temperature Test Method and YB / T4641-2018 Low Temperature Steel Bar for Liquefied Natural Gas Storage Tank, surface notch processing is performed on the steel bar sample E1 to be tested.

[0034] In this way, by performing help strip welding on the clamping section, in combination with the relationship control among the performance and parameters of the steel bar sample E1 to be tested, the help strip steel bar E2, and the clamping section steel bar E3, not only can the connection strength between the clamping section steel bar and the steel bar to be tested be improved, but also the clamping section steel bar can be strengthened, thereby avoiding the problem that the clamping section steel bar E3 is in a relatively high temperature environment while the steel bar sample E1 to be tested is in a lower temperature environment, and thus the clamping section is prone to fracture during low-temperature tensile test, ensuring that the fracture position of the low-temperature tensile test is in the steel bar sample section to be tested, and improving the test accuracy; moreover, the method can be applied to low-temperature tensile test of steel bars of different strength grades; and after welding, the welded joint does not need to be subjected to post-weld heat treatment, and can be directly used for low-temperature tensile test, so that the method is simple, easy to operate, and widely applicable, and does not need to modify the existing low-temperature tensile testing machine.

[0035] Among them, the yield strength Y1 and the tensile strength T1 of the steel bar sample E1 to be tested at normal temperature, and the yield strength Y3, the tensile strength T3, and the elongation A3 after fracture of the clamping section steel bar E3 at normal temperature are all tested according to GB / T228.1-2021 Metal Materials Tensile Test Method Part 1: Room Temperature Test Method.

[0036] The diameter D1 of the steel bar sample E1 to be tested, the diameter D2 of the help strip steel bar E2, and the diameter D3 of the clamping section steel bar E3 are all nominal diameters.

[0037] The help strip welding is an arc welding process for steel bar connection in building engineering. During construction, the steel bars to be welded are placed in head-to-head and a certain end gap is kept, and a help strip steel bar is arranged on the side of the joint as a welding support structure, and then welding is performed.

[0038] Preferably, in step S14, the steel test sample E1 and the clamping section steel bar E3 are coaxially arranged during welding, and the end gap between E1 and E3 is 2-3 mm. In this way, the joint end of E1 and E3 can be avoided after welding. The dislocation ensures the connection strength of the joint.

[0039] Before welding, the cross section of the steel test sample E1 and the clamping section steel bar E3 needs to be mechanically polished to ensure smooth and flat end surface to improve the connection strength of the joint.

[0040] Preferably, in step S14, double help strip welding is used for welding. That is, a help strip steel bar E2 is arranged on both sides of the joint of E1 and E3 as a welding support structure, and then welding is performed. In this way, the connection strength of the joint can be improved, and the uniformity of the strength around the joint can be improved.

[0041] Preferably, in step S14, the tensile strength T4 of the welding rod used for welding is greater than the tensile strength T1 of the steel test sample E1 at room temperature, so as to ensure that the strength of the welded joint is not lower than the strength of the steel test sample E1, so as to avoid breaking at the weld position during the tensile test.

[0042] Specifically, the welding rod needs to be selected according to the chemical composition design scheme and the carbon equivalent of the steel test sample E1, the help strip steel bar E2 and the clamping section steel bar E3.

[0043] Preferably, in step S14, the current during welding is 100-150 A and the voltage is 20-30 V. By controlling the current and voltage during welding, the welding rod and the steel bar can be efficiently melted. If the current and voltage during welding are too small, the efficiency is not enough, and the welding rod and the steel bar cannot be melted. If the current and voltage during welding are too large, it is easy to burn through and splash, which affects the quality of the welded joint and leads to insufficient strength of the welded joint.

[0044] Preferably, in step S12, the length L2 of the help strip steel bar E2 is controlled to be d x D2, wherein d = 6-8. By controlling the proportional relationship between the length L2 and the diameter D2 of the help strip steel bar E2, the welding joint can be ensured to be firm. If the length of the help strip steel bar E2 is too small, the strength of the welding joint is not enough and the joint is easy to crack during stretching. If the length of the help strip steel bar E2 is too large, the length of the clamping end is not enough, which affects the normal performance of the test.

[0045] Further, 200+6xD1+L2L1H+L2, wherein H is the height of the inner cavity of the test chamber used for the low-temperature tensile test, the height direction of the inner cavity of the test chamber is consistent with the length direction of the steel test sample E1 to be tested. In this way, the steel test sample E1 to be tested is ensured to be in the test chamber used for the low-temperature tensile test, so as to ensure that the fracture position of the low-temperature tensile test is located in the steel test sample E1 to be tested, and the data validity of the low-temperature tensile test is ensured. In addition, the fracture position is located in the steel test sample E1 to be tested, and the steel test sample E1 after fracture still has sufficient length for manual measurement of the maximum force total elongation rate Agt.

[0046] Preferably, in step S11, the diameter D1 of the steel test sample E1 to be tested is 16-40 mm.

[0047] Preferably, in step S13, b is inversely proportional to the test temperature during the low-temperature tensile test. That is, the lower the test temperature during the low-temperature tensile test, the greater the value of b, and when selecting the clamping section steel E3, the greater the yield strength Y3 of the clamping section steel E3 at room temperature needs to be controlled.

[0048] Correspondingly, in step S13, c is inversely proportional to the test temperature during the low-temperature tensile test. That is, the lower the test temperature during the low-temperature tensile test, the greater the value of c, and when selecting the clamping section steel E3, the greater the tensile strength T3 of the clamping section steel E3 at room temperature needs to be controlled.

[0049] The application also provides a low-temperature tensile test method of steel, which is carried out according to GB / T 228.3-2019 Metal Materials Tensile Test.

[0050] Specifically, the low-temperature tensile test method of the steel comprises the following steps: S21. Preparing a test sample The test sample is prepared by the above-mentioned method for preparing a test sample for low-temperature tensile test of steel.

[0051] S22. Using the clamps of the tensile testing machine to clamp the ends of the clamping section steels E3 of the test sample respectively away from the steel test sample E1 to be tested, so that the steel test sample E1 to be tested of the test sample is located in the test chamber used for the low-temperature tensile test, and the clamps are located outside the test chamber.

[0052] S23. Cooling the test chamber until the preset temperature is reached and then keeping the temperature.

[0053] S24. Carrying out the tensile test until the test sample is broken, and measuring the tensile performance index of the steel test sample E1 to be tested at low temperature.

[0054] Therefore, when the sample prepared by the above preparation method is subjected to the low-temperature tensile test of the steel bar, the steel bar sample E1 to be tested is in the test box at low temperature, and the clamping section steel bar E3 is outside the test box at relatively high temperature. During the tensile test, the fracture position of the low-temperature tensile test can be ensured to be in the steel bar sample E1 to be tested, and the temperature uniformity of the steel bar sample E1 to be tested along the length direction is improved, so as to ensure the effective test of the tensile test and improve the test precision. Moreover, the method can be applied to the low-temperature tensile test of the steel bar at different strength levels and different test temperatures, and the existing low-temperature tensile testing machine does not need to be modified.

[0055] The tensile performance index at low temperature includes, but is not limited to, yield strength Y and tensile strength T.

[0056] Further, the low-temperature tensile test method of the steel bar further comprises the steps of: S25. After the sample is fractured, after the temperature returns to room temperature, the maximum force total elongation Agt and the elongation after fracture A of the steel bar are measured according to GB / T228.1-2021 Metal Materials Tensile Test Part 1 Room Temperature Test Method.

[0057] Preferably, step S23 specifically comprises: The test box is cooled by liquid nitrogen, and the holding time t after the test box is cooled to the preset temperature is 20-40 min, and the holding time t is proportional to the diameter D1 of the steel bar sample to be tested.

[0058] The test box is cooled by liquid nitrogen, and the holding time t after the test box is cooled to the preset temperature is 20-40 min, and the holding time t is proportional to the diameter D1 of the steel bar sample to be tested.

[0059] The preset temperature is -196℃-0℃. That is, the low-temperature tensile test can be applied to the temperature detection range of -196℃-0℃, that is, the application can be used for testing the low-temperature tensile performance of the steel bar in the temperature range of -196℃-0℃.

[0060] Specifically, the test box is cooled from the outside of the test box by liquid nitrogen. In this way, the temperature uniformity in the test box can be improved, thereby improving the temperature uniformity of the steel bar sample E1 to be tested along the length direction.

[0061] In step S23, a temperature testing device is arranged in the test chamber, and the distance between the temperature testing device and the surface of the steel sample E1 to be tested is 2-3 mm. In this way, the temperature in the test chamber, especially the temperature around the steel sample E1 to be tested, can be monitored in real time, so as to improve the accuracy of temperature control.

[0062] In step S24, when the tensile test is performed, the strain rate before yielding is ≤0.00015 / s, and the strain rate after yielding is ≤0.002 / s. In this way, the law of dislocation movement in the steel at low temperature is fully considered, and the strength and plasticity index of the steel at low temperature can be more accurately obtained.

[0063] In summary, the low-temperature tensile test method of the steel provided by the embodiment of the present application solves the problems of unable to test some indexes and large fluctuation of test values compared with the related art, has high test efficiency and high precision, has wide application range, and does not need to modify the equipment.

[0064] The detailed description listed above is only a specific description of the feasible embodiments of the present application, and is not used to limit the protection scope of the present application. Equivalent embodiments or changes made without departing from the spirit of the present application should be included in the protection scope of the present application.

[0065] The beneficial effects of the present application will be further illustrated by specific examples and comparative examples. Of course, the examples are only a part of the numerous changed embodiments of the present application, not all.

[0066] Embodiment The present embodiment uses the above-mentioned sample preparation method for low-temperature tensile test of steel, which specifically includes the following steps: S11a. Take the steel sample E1 to be tested.

[0067] The diameter D1 of the steel sample E1 to be tested is 20 mm.

[0068] According to GB / T228.1-2021 Metallic Materials-Tensile Testing-Part 1: Method of Test at Room Temperature, the yield strength Y1 of the steel sample E1 to be tested at room temperature is 454 MPa, and the tensile strength T1 is 577 MPa.

[0069] S12a. Take another steel to be tested as the help steel E2.

[0070] The help steel E2 and the steel sample E1 to be tested are made of the same material and have the same diameter, that is, the diameter D2 of the help steel E2 is D1=20 mm. The length L2 of the help steel E2 is 7xD2=140 mm.

[0071] The height H of the inner cavity of the test chamber used in the low-temperature tensile test is 400 mm, so the length L1 of the steel test sample E1 to be measured is between 460 mm and 540 mm, and the length L1 of the steel test sample E1 to be measured is 500 mm.

[0072] S13a. Select the clamping section steel bar E3.

[0073] The diameter D3 of the clamping section steel bar E3 is D1=20 mm; the length L3 is 190 mm; the clamping section steel bar E3 is tested according to GB / T 228.1-2021 Metallic Materials-Tensile Testing-Part 1: Method of Test at Room Temperature to test the yield strength and tensile strength of the clamping section steel bar E3 at room temperature, and the clamping section steel bar E3 needs to meet the yield strength Y3≥549 MPa, the tensile strength T3≥759 MPa, and the elongation A3≥18% at room temperature.

[0074] S14a. Welding Before welding, the cross section of the steel test sample E1 to be measured and the clamping section steel bar E3 is mechanically polished to ensure that the end face is smooth and flat.

[0075] During welding, the welding rod is made of low alloy steel, and the tensile strength T4 of the welding rod is ≥577 MPa. The clamping section steel bar E3 is butted at both ends of the steel test sample E1 to be measured, and the steel test sample E1 to be measured and the clamping section steel bar E3 are used as the main reinforcement, and the steel test sample E1 to be measured and the clamping section steel bar E3 are coaxially arranged, and the end face distance between E1 and E3 is controlled to be 2-3 mm, and the steel bar E2 is used as the welding support structure, and the joint of E1 and E3 is welded with double-sided welding. The current during welding is 140 A, and the voltage is 25 V. After welding, no heat treatment is needed.

[0076] S15a. According to GB / T 228.3-2019 Metallic Materials-Tensile Testing-Part 3: Low-Temperature Test Method and YB / T 4641-2018 Low-Temperature Steel Reinforcement for Liquefied Natural Gas Storage Tanks, the surface of the steel test sample E1 is cut to obtain the steel test sample for low-temperature tensile test.

[0077] Further, the low-temperature tensile test of the aforementioned sample is carried out by using the low-temperature tensile test method of the steel bar described in the foregoing. The specific steps are as follows: S21a. Preparation of sample The sample is prepared according to the preparation method of the steel bar low-temperature tensile test sample described in the foregoing.

[0078] S22a. The clamping section steel bar E3 of the sample is clamped by the clamp of the tensile testing machine away from the end of the steel test sample E1 to be measured, and the steel test sample E1 to be measured of the sample is located in the test chamber for low-temperature tensile test.

[0079] S23a. Adopt liquid nitrogen to cool the test box from the outside of the test box to the preset temperature -100℃, and keep the temperature for 25 minutes.

[0080] The test box is provided with three temperature testing devices, which are uniformly arranged along the length direction of the steel test sample E1. The distance between each temperature testing device and the surface of the steel test sample E1 is 2mm. When the temperatures measured by the three temperature testing devices all reach the preset temperature ±1℃, the timing and temperature keeping start.

[0081] S24a. The strain rate before yield is 0.0001 / s, and the strain rate after yield is 0.001 / s. The tensile test is carried out until the sample is broken, and the yield strength Y of the steel test sample E1 at the preset temperature -100℃ is measured as 542MPa, and the tensile strength T is 722MPa.

[0082] S25a. Open the test box, take out the broken sample, and after the temperature returns to room temperature, according to GB / T228.1-2021 metal material tensile test part 1 room temperature test method, manual measurement is used to measure the maximum force total elongation rate Agt of the sample is 7.5%, and the elongation after breaking A is 10%.

[0083] Comparative example The preparation method of the steel test sample of the comparative example includes the following steps: S11b. Take the steel test sample E1.

[0084] The diameter D1 of the steel test sample E1 is 20mm.

[0085] According to GB / T228.1-2021 metal material tensile test method part 1: room temperature test method, the yield strength Y1 of the steel test sample E1 at room temperature is measured as 454MPa, and the tensile strength T1 is 577MPa.

[0086] S12b. Take another steel test sample as the help bar steel E2.

[0087] The help bar steel E2 and the steel test sample E1 are made of the same material and have the same diameter, that is, the diameter D2 of the help bar steel E2 is D1=20mm. The length L2 of the help bar steel E2 is 7xD2=140mm.

[0088] The height H of the inner cavity of the test box used for low temperature tensile test is 400mm, so the length L1 of the steel test sample E1 is between 460mm and 540mm, and the length L1 of the steel test sample E1 is 500mm.

[0089] S13b. Select the clamping section steel E3.

[0090] The diameter D3 of the clamping section steel bar E3 is D1=20 mm; the length L3 is 190 mm; the yield strength Y3 of the clamping section steel bar E3 is 498 MPa, the tensile strength T3 is 658 MPa, and the elongation A3 after fracture is ≥18% according to GB / T 228.1-2021 Metallic materials-Tensile testing-Part 1: Method of test at room temperature.

[0091] S14b. Welding Before welding, the cross sections of the steel test sample E1 to be tested and the clamping section steel bar E3 are mechanically polished to ensure that the end faces are smooth and flat.

[0092] During welding, a low alloy steel electrode is used, and the tensile strength T4 of the electrode is ≥577 MPa. The clamping section steel bar E3 is butted at both ends of the steel test sample E1 to be tested, and the steel test sample E1 to be tested and the clamping section steel bar E3 are used as the main reinforcement. The steel test sample E1 to be tested and the clamping section steel bar E3 are coaxially arranged, and the end-to-end distance between E1 and E3 is controlled to be 2-3 mm. The steel bar E2 is used as a welding support structure to perform double-sided welding on the joint between E1 and E3. The current during welding is 140 A, and the voltage is 25 V. No heat treatment is required after welding.

[0093] S15b. According to GB / T 228.3-2019 Metallic materials-Tensile testing-Part 3: Method of test at low temperatures and YB / T 4641-2018 Low-temperature steel bars for liquefied natural gas storage tanks, the surface of E1 is cut to obtain a steel bar sample for low-temperature tensile testing.

[0094] Further, the aforementioned sample is subjected to a low-temperature tensile test. The specific steps are as follows: S21b. Preparing a sample The sample is prepared according to the aforementioned method of preparing a steel bar sample.

[0095] S22b. The clamping section steel bar E3 of the sample is clamped by the clamp of the tensile testing machine away from the steel test sample E1 to be tested, and the steel test sample E1 to be tested of the sample is located in the test box for low-temperature tensile testing.

[0096] S23b. The test box is cooled by liquid nitrogen from the outside of the test box to a preset temperature of -100°C, and then kept at the temperature for a time t of 25 min.

[0097] The test box is provided with three temperature testing devices, which are uniformly arranged along the length direction of the steel test sample E1 to be tested, and the distance between each temperature testing device and the surface of the steel test sample E1 to be tested is 2 mm. When the temperatures measured by the three temperature testing devices all reach the preset temperature ±1°C, the time-keeping for keeping the temperature is started.

[0098] S24b. Set the strain rate before yield as 0.0001 / s and the strain rate after yield as 0.001 / s. The tensile test was carried out. However, during the tensile process, the sample fractured at the yield stage and the fracture position was at the clamping section steel E3. The measured yield strength and tensile strength data were invalid.

[0099] It can be seen that the preparation method of the steel sample for low temperature tensile test of the present application can effectively avoid the problem that the steel sample is easy to fracture at the clamping section during the low temperature tensile test, ensure that the fracture position of the low temperature tensile test is at the sample section to be tested, and improve the test effectiveness and test accuracy.

[0100] It should be understood that although the present specification is described in terms of embodiments, each embodiment does not necessarily contain only one independent technical solution, and the description of the specification is only for the sake of clarity, and those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that those skilled in the art can understand.

Claims

1. A method for preparing a sample for a low-temperature tensile test of a steel bar, characterized in that: The preparation method comprises: Take the steel bar sample E1 to be tested, collect its diameter D1 and length L1, and test its yield strength Y1 and tensile strength T1 at room temperature; Take another steel bar to be tested as the auxiliary steel bar E2, and control the length L2 of E2 <L1; Select the clamping section steel bar E3, control the diameter of E3 D3 = D1; length L3 = a + L2 / 2, a = 100 ~ 150mm; yield strength at room temperature Y3 ≥ Y1 + 100 × b, b = 0.5 ~ 1.5; tensile strength at room temperature T3 ≥ Y2 + 140 × c, c = 1 ~ 2; elongation after fracture A3 ≥ 15%; Welding: connect E3 at both ends of E1, use E1 and E3 as main reinforcement, use the auxiliary steel bar E2 as the welding support structure, and perform auxiliary bar welding on the joints of E1 and E3; The surface of E1 was cut to obtain the steel bar low-temperature tensile test specimen.

2. The method for preparing a steel bar low-temperature tensile test specimen according to claim 1, wherein: During welding, E1 and E3 are coaxially arranged, and the end face distance between E1 and E3 is 2~3mm.

3. The method for preparing a steel bar low-temperature tensile test specimen according to claim 1, wherein: The welding adopts double-sided bar welding.

4. The method for preparing a steel bar low-temperature tensile test specimen according to claim 1, wherein: The tensile strength of the welding rod used is T4 ≥ T1.

5. The method for preparing a steel bar low-temperature tensile test specimen according to claim 1, wherein: The current during welding is 100~150A and the voltage is 20~30V.

6. The method for preparing a steel bar low-temperature tensile test specimen according to claim 1, wherein: L2=d×D2, d=6~8, where D2 is the diameter of the reinforcement bar E2.

7. The method for preparing a steel bar low-temperature tensile test specimen according to claim 6, characterized in that: 200+6×D1+L2≤L1≤H+L2, where H is the height of the inner cavity of the test box used for the low-temperature tensile test, and the height direction of the inner cavity of the test box is consistent with the length direction of E1.

8. The method for preparing a steel bar low-temperature tensile test specimen according to claim 1, wherein: b is inversely proportional to the test temperature during the low-temperature tensile test; c is inversely proportional to the test temperature during the low-temperature tensile test.

9. A low temperature tensile test method for steel bars, characterized in that: The test method comprises the steps of: preparing a sample, wherein the sample is prepared by the method for preparing a sample for a low-temperature tensile test of steel bars according to any one of claims 1 to 8; Use the clamps of the tensile testing machine to clamp the clamping sections of the steel bars E3 of the sample, each of which is away from one end of the steel bar sample E1 to be tested, so that the steel bar sample E1 to be tested of the sample is located in the test chamber used for the low-temperature tensile test, and the clamps are located outside the test chamber; Cool the test chamber to the preset temperature and then keep it warm; The tensile test was carried out until the sample broke, and the tensile performance index of the steel bar sample E1 to be tested at low temperature was measured.

10. The low-temperature tensile test method for steel bars according to claim 9, characterized in that: The cooling of the test chamber to a preset temperature and then keeping the temperature thereafter specifically includes: Liquid nitrogen is used to cool the test chamber to the preset temperature, and the holding time t is 20 to 40 minutes, and the holding time t is proportional to the diameter D1 of the steel bar sample E1 to be tested; Among them, the preset temperature is -196℃~0℃.

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